Comprehensive Studies on Dual Excitation Behavior of Double Proton versus Charge
Transfer in 4-(N-Substituted amino)-1H-pyrrolo[2,3-b]pyridines
Chung-Chih Cheng,† Chen-Pin Chang,† Wei-Shan Yu,‡ Fa-Tsai Hung,§Yun-I Liu,| Guo-Ray Wu,| and Pi-Tai Chou*,‡
Department of Chemistry, Fu-Jen Catholic UniVersity, 242, Taipei, Taiwan, Republic of China, Department of Chemistry, National Taiwan UniVersity, 106, Taipei, Taiwan, Republic of China, The National Hu-Wei Institute of Technology, Yunlin, Taiwan, Republic of China, and Department of Chemistry and Biochemistry, National Chung Cheng UniVersity, 621, Chia-Yi, Taiwan, Republic of China
ReceiVed: May 21, 2002; In Final Form: December 13, 2002
Comprehensive spectroscopic and dynamical studies on the dual excitation behavior of proton vs charge transfer for 4-(dimethylamino)-1H-pyrrolo[2,3-b]pyridine (DPP) and its related derivatives are reported. In cyclohexane, DPP dimer and/or dual hydrogen-bonded complex are formed with association constants Kaas
high as∼4.2 × 103and 5.2× 104M-1 (e.g., the DPP/acetic acid complex) at 298 K, respectively, which upon electronic excitation undergo ultrafast rate (.6.7× 1010s-1) of double-proton transfer, resulting in a
unique tautomer emission. Dual fluorescence was observed in polar, aprotic solvents, in which the large Stokes shifted emission band originates from the charge-transfer species incorporating a dimethylamine and pyridine ring as electron donor and acceptor, respectively. Detailed solvent-polarity and temperature-dependent studies in combination with theoretical approaches have been performed to determine the excited-state charge-transfer properties such as dipole moment, orbital configuration, etc. Supplementary support for the dual charge/proton-transfer behavior was provided by the comparative spectroscopy and dynamics of various DPP-related derivatives. Further time-resolved measurements conclude that dual emissions share a common Franck-Condon excited state but undergo two independent relaxation channels. In protic solvents, such as ethanol, following fast solvent relaxation dynamics, the excited charge-transfer state further undergoes a solvent (i.e. alcohol) assisted proton-transfer reaction. The charge versus proton-transfer emission can be distinguished via the temporal spectral evolution. The results demonstrate DPP to be a unique model among 7-azaindole analogues in which the interplay between charge and proton-transfer reactions is operative in the excited state.
1. Introduction
In their seminal studies, Taylor et al.1 found that the first
excited singlet state of 7-azaindole (7AI) dual hydrogen-bonded (HB) dimer undergoes an excited-state double proton transfer (ESDPT), resulting in a large Stokes-shifted tautomer emission (e.g., λmax ∼ 480 nm in cyclohexane). This dual
hydrogen-bonding system has ever since been recognized as a simplified model for the base pair of DNA. At the molecular level, the ESDPT process provides one possible mechanism for the mutation due to a “misprint” induced by the proton-transfer tautomerism of a specific DNA base pair during replication, recording an error message.2-5 On this basis, much research
has focused on the spectroscopy and dynamics1-3,6-17as well
as theoretical approaches18-20 of ESDPT in the 7AI dimer.
Conversely, via the formation of a 7AI(host)/guest HB complex, the dynamics of ESDPT incorporating various types of guest molecules has also received considerable attention through both experimental21-26 and theoretical approaches.27-30 Studies of
7AI complexed to single molecules of carboxylic acids and lactams have revealed rapid ESDPT reaction,26b-d especially
for the catalytic type of reaction where the guest molecule (e.g., acetic acid) remains unchanged during the proton-transfer reaction. In bulk alcohols and water the dynamics of solvent assisted ESDPT in 7AI and its analogue 7-azatryptophan have been successfully applied to probe the solvation and/or protein dynamics.21-25During the past few years, focus on the chemical
modification of 7AI has received particular interest to study the substituent effect on the proton transfer reaction. It has been shown that the number as well as the position of the nitrogen atom in either a five- or six-member ring system of 7AI alters the relative ESDPT thermodynamics, especially in the noncata-lytic type of proton-transfer reaction.31 The generalization of
amine-imine type of tautomerism in 7AI and its corresponding analogues has been verified via the hydrogenation of the C2
-C3 double bond in 7AI, forming 7-azaindoline.32The results
open up a study of ESDPT extended to a DNA base such as adenine derivatives possessing similar type of tautomerism proposed in the mutation process.33On the other hand, based
on the design of 3-cyano-7-azaindole, a steady state tautomer emission in aqueous solution has been unambiguously resolved and successfully applied as a model to closely examine various proposed mechanisms of excited-state proton-transfer reactions for 7AI in pure water.34
In another approach, in the previous communication35we
reported the dual excitation behavior of excited-state biprotonic * To whom correspondence should be addressed.
†Department of Chemistry, Fu-Jen Catholic University. ‡Department of Chemistry, National Taiwan University. §The National Hu-Wei Institute of Technology.
|Department of Chemistry and Biochemistry, National Chung Cheng University.
10.1021/jp021243b CCC: $25.00 © 2003 American Chemical Society Published on Web 02/15/2003
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transfer vs charge transfer in 4-(dimethylamino)-1H-pyrrolo-[2,3-b]pyridine (DPP, see Figure 1). The results make DPP a unique model among hydrogen-bonded complexes to study the interplay between two fundamental mechanisms fine-tuned by solvent polarity as well as hydrogen-bonding perturbation. In this paper we have further performed comprehensive spectro-scopic and dynamic studies on DPP and its relative derivatives, aimed at gaining a detailed insight into the electronic properties and corresponding relaxation dynamics. Ground-state thermo-dynamics as well as excited-state proton-transfer thermo-dynamics for DPP dimer and DPP/guest complexes have been deduced in nonpolar solvents. Detailed solvent-polarity-dependent studies in combination with semiempirical calculations led us to derive the excited-state charge-transfer properties such as dipole moment, orbital configuration, etc. Finally, both temperature and time dependent fluorescence spectroscopy and dynamics were performed to resolve the excited-state charge-versus-proton-transfer dynamics.
2. Experimental Section
2.1. Materials. The synthesis of DPP was performed accord-ing to procedures used in previous reports with slight modification.36-38Brief methodology and spectral
characteriza-tion in each step for synthesizing DPP and related derivatives are described below. For clarity, structures of DPP and its corresponding analogues are depicted in Figure 1.
7-Hydroxy-1H-pyrrolo[2,3-b]pyridinium m-Chlorobenzoate (C14H11ClN2O3) (1). A 1.2 g (5.9 mmol) sample of 85% m-chloroperoxybenzoic acid was added to 0.5 g (4.23 mmol) of 1H-pyrrolo[2,3-b]pyridine dissolved in 15 mL of redistilled 1,2-dimethoxyethane The resulting solution was stirred at room temperature for 1.5 h and during this period the product gradually precipitated. The mixture was then cooled and the light yellow product was isolated by filtration and washed with diethyl ether to give 1 (0.45 g,∼75%).
1H-Pyrrolo[2,3-b]pyridine-7-oxide (C7H6N2O) (2). A 1.0 g (3.4 mmol) sample of 1 in 10 mL of water was basified to pH ∼ 9 with saturated K2CO3solution, followed by the extraction
of chloroform. The solution was then concentrated to obtain 2.
1H NMR (200 MHz, DMSO-d 6)δ 6.58 (d, J ) 3.0 Hz, 1H), 7.10 (d, J ) 6.0 Hz, 1H), 7.48 (d, J ) 3.0 Hz, 1H), 7.67 (d, J ) 8.0 Hz, 1H), 8.18 (d, J ) 6.0 Hz, 1H), 13.50 (s, 1H, pyrrole NH). 4-Chloro-1H-pyrrolo[2,3-b]pyridine (C7H5ClN2) (3). A 0.5 g sample of 2 was added in portions to the cooled phosphoric trichloride solution (5 mL), and the mixture was refluxed for 5 h. Phosphoric trichloride was then distilled off under reduced pressure followed by the dilution of water (5 mL). The solution was basified with sodium carbonate to allow the precipitation for∼1 h. The precipitate was filtered off to obtain 3.1H NMR
(200 MHz, DMSO-d6)δ 6.55 (d, J ) 3.4 Hz, 1H), 7.22 (d, J )
5.1 Hz, 1H), 7.63 (d, J ) 3.4 Hz, 1H), 8.23 (d, J ) 5.1 Hz, 1H), 10.25 (s, 1H, pyrrole NH).
4-(Dimethylamino)-1H-pyrrolo[2,3-b]pyridine (C9H11N3, DPP). A mixture of 3 (2 mmol) and dimethylammonium chloride (10 mmol) was heated in an oil bath at 180°C for 5 h. After cooling, the dark brown reaction mixture was worked up by flash column chromatography (eluent: 10-20% methanol by weight in dichloromethane) to obtain DPP.1H NMR (200 MHz, CDCl 3) δ 3.27 (s, 6H), 6.18 (d, J ) 6.0 Hz, 1H), 6.64 (d, J ) 3.8 Hz, 1H), 7.12 (d, J ) 3.8 Hz, 1H), 7.96 (d, J ) 6.0 Hz, 1H), 11.8 (s, 1H, pyrrole NH). 1-Methyl-4-(dimethylamino)-1-H-pyrrolo[2,3-b]pyridine (C10H13N3, 1MDPP). 1MDPP was synthesized by adding sodium hydride (100 mg) to the THF solution containing DPP (0.15 g), followed by the addition of methyl iodide (100 mg). The resulting product, 1MDPP, was purified by column chroma-tography (eluent 1:1 v/v hexanes: ethyl acetate).1H NMR (200
MHz, CDCl3)δ 3.28 (s, 6H), 3.89 (s, 3H), 6.20 (d, J ) 6.0 Hz,
1H), 6.62 (d, J ) 3.6 Hz, 1H), 6.94 (d, J ) 3.6 Hz, 1H), 8.03 (d, J ) 6 Hz, 1H).
7-Methyl-4-(dimethylamino)-1H-pyrrolo[2,3-b]pyridine (C10H13N3, 7MDPP). 7MDPP was synthesized by the reaction of DPP (0.1 g) and methyl iodide (0.5 g) in THF under N2
atmosphere. NaOH (2.5N, 10 mL) was then added, and the mixture was stirred for∼20 min to obtain crude 7MDPP. The product was further purified by column chromatography (elu-ent: ethyl acetate and then methanol). 1H NMR(CDCl
3, 400
MHz)δ 3.38 (s, 6H), 4.08 (s, 3H), 6.05 (d, J ) 7.2 Hz, 1H),
6.76 (d, J ) 2.8 Hz, 1H), 7.35 (d, J ) 7.2 Hz, 1H), 7.47 (d, J ) 2.8 Hz, 1H).
4-Amino-1H-pyrrolo[2,3-b]pyridine (C7H7N3, APP). APP was prepared according to a previously reported procedure.39Briefly,
a mixture of 0.2 g (1.11 mmol) of 4-nitro-1H-pyrrolo[2,3-b]-pyridine-7-oxide40,41and 0.8 g of Fe powder in 8 mL of glacial
acetic acid was heated with stirring at 100°C for 2 h. The cooled mixture was diluted with 20 mL of water, adjusted to pH∼ 10-11 with NaOH, and then continuously extracted with diethyl ether (∼250 mL). The extract was dried over anhydrous Na2
-SO4and then evaporated on a rotary evaporator to obtain APP. 1H NMR (200 MHz, DMSO-d
6)δ 6.48 (d, J ) 7.0 Hz, 1H),
6.82 (d, J ) 3.0 Hz, 1H), 7.28 (d, ) J 3.0 Hz, 1H), 7.87 (d, J ) 7.0 Hz, 1H), 12.03(s, 1H, pyrrole NH).
Various solvents are of spectrograde quality (Merck, Inc.) and used right after received. 2-Methyltetrahydrofuran (2MTHF) and acetonitrile showed traces of fluorescence impurities and were fractionally distilled prior to use.
2.2. Measurements. Steady-state absorption and emission spectra were recorded by a Varian (Cary 3E) UV-vis spectro-photometer and a Hitachi (F4500) fluorimeter, respectively. Both wavelength-dependent excitation and emission response of the fluorimeter have been calibrated according to a previously reported method.31bRoom-temperature fluorescence quantum
yields were measured using quinine sulfate/1.0 N H2SO4 as Figure 1. Structures of DPP and its corresponding
proton/charge-transfer isomers, methylated derivatives, and analogues.
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reference, assuming a yield of 0.564 with 365-nm excitation.42
For the temperature-dependent emission measurement a 3rd harmonic of the Nd:YAG laser (355 nm, 8 ns duration, Continuum Surlite II) pumped optical parametric oscillator (OPO) served as a light source. The output of the fundamental laser frequencies was then frequency doubled by a BBO crystal to obtain tunable excitation frequencies between 270 and 320 nm. The emission was collected at a right angle with respect to the excitation source and detected by an intensified charge coupled detector (ICCD, Princeton Instrument, Model 576G/ 1). Typically, 100 laser shots were collected and averaged in each data acquisition period. Low-temperature measurements were performed in an Oxford cryostat (Model CCC1104) with temperature regulator (Model ITC502).
Pico-nanosecond lifetime measurements were achieved by using either a second (380-420 nm) or third (260-275 nm) harmonic of the femtosecond Ti-Sapphire oscillator (82 MHz, Spectra Physics) as an excitation source coupled with a pulse picker (NEOS, model N17389). This combination gives an output repetition rate of 8 MHz to avoid the accumulation of residue from the previous excitation pulse due to the longer fluorescence decay components. An Edinburgh OB 900-L time-correlated single photon counter was used as a detecting system. Since the fwhm of the excitation pulse is typically∼100 fs, the resolution is limited by the detector response of∼30 ps. The fluorescence decays were analyzed by the sum of expo-nential functions with an iterative convolution method reported previously.42 This procedure should allow partial removal of
the instrument time broadening and consequently renders a temporal resolution of∼15 ps.
Detailed methods of the theoretical approach (e.g. 6-31G(d, p) and higher levels) for the thermodynamics of HB complexes in the ground state have been described previously.43 For
calculating the physical parameters such as dipole moment, cavity radius, etc., of DPP, various methods including AM1, PM3, or MP2/6-31G(d, p) were applied. The complexity of molecular structure makes an ab initio approach in the excited state extremely time consuming and in some cases even not feasible. Instead, the properties of various low-lying excited singlet states were estimated by an INDO/S-CI calculation based on an AM1 geometry optimized ground-state structure. All singly excited configurations from the 15 highest occupied to the 15 lowest unoccupied (225 configurations) molecular orbitals were involved in the computation.
The following sections are organized according to a sequence of steps where we first performed detailed absorption and fluorescence titration experiments to determine the thermody-namics and ESDPT properties of DPP self-dimerization and hydrogen-bonding complexes in nonpolar solvents. Subse-quently, dual emission properties of DPP in polar, aprotic solvents were investigated and discussed on the basis of the excited-state charge-transfer property. Finally, the differentiation between solvent diffusive reorganization and solvent relaxation to affect the solvent assisted ESDPT dynamics in alcohols was elaborated.
3. Results
3.1. Hydrogen-Bonding Association in Nonpolar Solvents. When the concentration was prepared to be as low as 9.2× 10-6M, DPP exhibits the lowest singletπf π* absorption band
maximum at 303 nm (303 ∼ 6750 M-1cm-1). The spectral
feature is similar to that of 1MDPP (see Figure 2) generally treated as a nonproton-transfer model due to the lack of the pyrrolic N-H proton. Thus, the assignment of the 303-nm band
to the amine-like monomer species is unambiguous. Upon increasing the concentration, changes in the spectral features were observed in which the spectra reveal red shift with the appearance of a new 0-0 onset peak maximum at 310 nm (see Figure 2a-e). In comparison, the absorption features of 1MDPP in cyclohexane reveal concentration independence. Since the difference between DPP and 1MDPP lies in the dual hydrogen-bonding sites in DPP, the results unambiguously conclude the formation of DPP dimer and/or higher-order aggregates in cyclohexane through the dual hydrogen-bonding effect. Similar to that proposed in the 7AI self-association,3,26b-d we first
assume a dominant self-dimerization form possibly possessing a cyclic type of hydrogen bonding configuration where the dual hydrogen-bonding sites, i.e., the N(1)H hydrogen and the pyridinic nitrogen in DPP (i.e., N(7)), act as a proton donor and acceptor, respectively. Consequently, a competitive equi-librium between DPP monomer and its corresponding self-association is established, and the dimerization constant Kacan
be expressed as
where C0 is the initial concentration of DPP, and Cpdenotes
the concentration of the DPP dimeric form. In the case of 7AI and its corresponding analogues, significant red shift of the absorption spectrum near the onset of the monomer S0f S1
(ππ*) absorption is commonly observed upon
dimeri-zation.1-3,31-34For instance, the absorbance at wavelengths of
>310 nm can be exclusively attributed to the dimeric absorption of 7AI.3On the the basis of the Beer-Lambert law, eq a can
thus be rewritten as
where A and Ddenote the absorbance (in the 1 cm path-length
cell) and the molar extinction coefficient of the dimer, respec-tively, at a selected wavelength. According to the plot of C0/A
as a function ofx1/A, Kacan then be deduced by the ratio for
the intercept versus the square of the slope.
For the case of DPP, however, significant overlaps between monomeric and dimeric species were observed throughout the concentration-dependent UV-vis absorption measurement. As Figure 2. (s) The concentration-dependent absorption spectra of DPP in cyclohexane, in which DPP (C0) was prepared at (a) 1.1× 10-4, (b) 2.1× 10-4, (c) 4.3× 10-4, (d) 8.6× 10-4, (e) 1.7× 10-3M. The absorption spectra are normalized at 305 nm. (- -) The absorption spectrum of 1MDPP (1.0× 10-5M) in cyclohexane. Insert: Plot of A0/(A - A0) versus
x
1/(A - A0) at 310 nm for b-e and a best linear least-squares fitting curve using eq d. See section 3.1 for the definition of A0and A. Ka) Cp (C0- 2Cp)2 (a) C0 A )x
1 Dx
1 Kax
1 A+ 2D (b)Downloaded by NATIONAL TAIWAN UNIV on July 27, 2009
a result, upon considering the absorbance attributed to both monomer and dimer at any selected wavelength, modification of eq b is necessary. A revised equation of b with an arbitrary cell path length ofλ has been deduced and expressed as
No assumption is made in eq c except that the value of the molar extinction coefficient of the monomer M,at the analyzed
wavelength needs to be obtained prior to the deduction of Ka
value. Multiplying M on both sides, eq c can be further
simplified to
where A0in eq d simply denotes the absorbance of the monomer
at the selected wavelength, assuming that no dimer is formed at the prepared concentration. A0can be obtained by performing
the concentration dependent study at a sufficiently low con-centration so that monomer exists predominantly (e.g., 9.2× 10-6M in the case of DPP). By knowing the proportionality of the dilution, A0values can thus be obtained in each prepared
concentration. In this study, the wavelength at 310 nm was selected due to the relatively higher absorptivity upon dimer-ization (vide infra). Under a sufficiently low concentration where only monomer exists, M
310
was measured to be ca. 2835 M-1cm-1. A plot of A0/(A - A0) at 310 nm as a function of
x
1/(A - A0) shown in insert of Figure 2 reveals sufficientlylinear behavior, supporting the assumption of dimeric formation in concentrated DPP. Accordingly, by fixing the Mand l()
1.0 cm) values, the best linear least-squares fit using eq d to the insert of Figure 2 gives D310and Kavalues to be (1.4 ( 0.2)
× 104 M-1cm-1 and (4.2 ( 0.4) × 103 M-1, respectively.
Knowing that one dimer actually consists of two DPP chro-mophores, the extinction coefficient for each DPP in the dimeric form is thus deduced to be D310/2 ∼ 7100 M-1 cm-1. In comparison to that of the 7AI dimer (Ka∼ 2.2 × 103M-1in
cyclohexane26d), the higher dimerization constant in DPP may
be rationalized by increasing the basicity of the pyridinic nitrogen resonantly enhanced by the electron donating property of the para-dimethylamino functional group (vide infra). This viewpoint can be further supported by adding acetic acid as the guest molecule, forming a DPP/acetic acid dual hydrogen-bonded complex.
Figure 3 shows the absorption spectra of DPP as a function of the acetic acid concentration in cyclohexane, in which the initial concentration of DPP C0was prepared to be as low as
1.2× 10-5M to avoid self-dimerization. The formation of DPP/ acetic acid hydrogen-bonded complexes is clearly shown by the growth of an absorption band at ∼310 accompanied by the appearance of an isosbestic point at ca. 295 nm throughout the titration, indicating the existence of equilibrium with a common intermediate. In comparison, although 1MDPP provides two hydrogen-bonding sites, i.e., the pyridinic and dimethylamino nitrogen, negligible spectral changes were observed upon adding acetic acid up to 10-3M. The result unambiguously supports the formation of a 1:1 DPP/acetic acid complex incorporating cyclic dual hydrogen bonds at N(1)H and N(7) positions. Similar to that observed in the concentration-dependent study the acetic
acid absorption titration spectra reveal significant overlap between uncomplexed DPP monomer and 1:1 DPP/acetic acid hydrogen-bonded complex. In consideration of self-dimerization for acetic acid the competitive equilibrium can be expressed as
In the above expression, the possibility of a single hydrogen bonding formation between DPP(N(CH3)2) site and acetic acid
is neglected due to its rather small association constant. Since DPP is complexed by stoichiometrically equivalent guest molecules (e.g., acetic acid), while acetic acid is in excess, the relationship between the measured absorbance as a function of the free acetic acid concentration Cgcan be expressed by
where Mand Cin eq e denote molar extinction coefficients of
DPP monomer and the hydrogen-bonded complex, respectively, at a specific wavelength. On the basis of the negligible consumption of acetic acid upon forming the DPP/acetic acid complex, Cg can be deduced independently by the
self-association equation of the acetic acid; namely,
where Cg0 is the initially prepared acetic acid concentration.
The self-association constant K′a of acetic acid has been reported to be 3.7× 104M-1 44in n-heptane, which was used
in the case of cyclohexane due to their similar solvent polarity. A straight-line plot of A0/(A - A0) as a function of 1/Cgat a selected wavelength of 310 nm (see insert of Figure 3) supports the validity of the assumption of a 1:1 DPP/acetic acid complex formation. Consequently, a best linear least-squares fit using eq e deduces Ka to be (5.2 ( 0.5) × 104 M-1 which is
approximately twice as large as that of the 7AI/acetic acid Figure 3. The absorption spectra of DPP (8.0 × 10-6 M in cyclohexane) as a function of free acetic acid concentration Cgof (a) 0, (b) 8.3× 10-6, (c) 1.2× 10-5, (d) 1.5× 10-5, (e) 2.0× 10-5, (f) 2.5× 10-5M. Insert: The plot of A0/(A - A0) at 310 nm as a function of 1/Cgin curves b-f and a best least-squares fitting curve using eq e.
A0 A - A0) M M- C
(
1 KaCg+ 1)
(e) Cg) Cg0-[
(4K′aCg 0+ 1)-x
8K′aCg0+ 1 4K′a]
C0l A - MC0l)x
l D- 2M‚x
1 Kax
1 A - MC0l+ 2 D- 2M (c) A0 A - A0)x
lM2 D- 2Mx
1 Ka‚x
1 A - A0+ 2M D- 2M (d)Downloaded by NATIONAL TAIWAN UNIV on July 27, 2009
complex (∼2.2 × 104M-1),26dfurther supporting the proposed
stronger proton-accepting property on the pyridinic nitrogen. The fluorescence spectra of DPP in cyclohexane reveal strong concentration dependence (see Figure 4A). At sufficiently low concentrations where the monomer mainly exists in the equi-librium, a very weak, normal Stokes shifted emission maximum at∼327 nm was observed (the FNband,Φf∼ 9.24 × 10-4).
Upon increasing the concentration, a large Stokes-shifted emission maximum at 440 nm (the FTband) gradually appears.
Different excitation spectral features were obtained upon monitoring at FNand FTbands where the excitation spectrum
of the FTband is red shifted by∼10 nm with respect to that of
the FNband. The rise time of both emission bands cannot be
resolved by our current photon counting system with a response limit of ∼15 ps. The results in combination with resolvable, different relaxation dynamics for FN(τobs∼ 70 ps) and FT(τobs
∼ 2.8 ns) bands unambiguously conclude that they originate from different ground-state species and one cannot be the precursor of the other. In comparison, 1MDPP, which is considered a nonproton-transfer model compound, exhibits a concentration independent, normal Stokes-shifted emission maximum at 328 nm (τf∼ 85 ps; Φf∼ 1.36 × 10-3). On the
other hand, 7MDPP, the analogue of proton-transfer tautomer for DPP, reveals a single fluorescence band maximum at 470 nm in cyclohexane (τf∼ 4.1 ns; Φf∼ 9.8 × 10-2). It is thus
conceivable to conclude that the FNband is ascribed exclusively
to the monomer emission, while the FTband with a Stokes shift
(peak-to-peak) of∼7850 cm-1can be unambiguously assigned to the tautomer emission resulting from ESDPT of the DPP dimer.
Upon exciting the DPP/acetic acid complex, a similar ESDPT reaction was observed, resulting in a proton-transfer tautomer emission (λmax∼ 440 nm) of which the excitation spectrum is
red shifted by ∼10 nm with respect to the uncomplexed monomer (see Figure 4B). The emission monitored at > 430 nm exhibits a single-exponential decay rate (τobs ∼ 5.2 ns)
accompanied by system response limited rise dynamics (i.e., kpt> 6.7 × 1010s-1). The results unambiguously demonstrate
that in nonpolar solvents both catalytic (acetic acid) and
noncatalytic (dimer) types26cof ultrafast ESDPT take place in
DPP dual hydrogen-bonded complexes.
3.2. In Polar, Aprotic Solvents. In contrast to the large self-association for DPP in cyclohexane, both absorption and emission spectra reveal concentration independence within the studied concentrations of 10-6-10-3 M in all polar, aprotic solvents. The results can be rationalized by the large (polar) solvent stabilization energy for the solvated DPP monomer, preventing self-association or even higher-order aggregation. However, unlike the single, solvent-polarity-dependent Stokes-shifted emission observed in 7AI,45remarkable dual emission
was observed for DPP in polar, aprotic solvents (Figure 5). These two emission bands, specified as FNand FC (the
long-wavelength band) originating from a common ground-state species, are ascertained by the same fluorescence excitation spectra of DPP in, e.g., ethyl acetate (see Figure 6). The excitation spectra, within experimental error, are also identical with the absorption spectrum, indicating that both emissions result from a common Franck-Condon excited state. In contrast to the nearly solvent-independent absorption maximum, peak frequencies for both FNand FCbands reveal solvent-polarity
dependence, decreasing as the empirical parameter of solvent-polarity ET(30) increases (see Figure 5 and Table 1). For
example, the peak frequency of the FNband is red shifted by∼ Figure 4. A. The fluorescence spectra of DPP in cyclohexane where
DPP was prepared at (a) 4.2× 10-5and (b) 6.5× 10-5M. Spectra was normalized at 330 nm. The excitation spectra (6.5× 10-5M) monitored at (o o) 340 and (‚ ‚) 500 nm. (- -) Absorption and emission spectra of 7MDPP in cyclohexane. B. The fluorescence spectra of DPP (8.0 × 10-6 M in cyclohexane) as a function of the acetic acid concentration of (a) 2.0× 10-6, (b) 8.30× 10-6, (c) 1.20× 10-5, (d) 1.33× 10-5, (e) 1.65× 10-5M. The excitation spectra of c monitored at (o o) 340 and (‚ ‚) 500 nm.
Figure 5. The fluorescence spectra of DPP in various solvents: (a) ethyl ether (34.5), (b) ethyl acetate (38.1), (c) dichloromethane (40.7), (d) acetonitrile (45.6), (e) ethanol (51.9). ET(30) value in kcal/mol for each solvent is specified in parentheses. The excitation wavelength is 290 nm.
Figure 6. The excitation spectra of DPP (1.5× 10-5M) in ethyl acetate monitored at (a) (- -) 500 nm, (b) (- ‚) 340 nm, and (c) (s) the absorption spectrum.
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550 cm-1from ethyl ether to acetonitrile, while it is as large as 1745 cm-1for the FCband. In a comparative study the
proton-transfer tautomer analogue 7MDPP exhibits a normal fluores-cence band, of which the peak frequency reveals slight solvent-polarity dependence, being 21,187 cm-1(470 nm) in cyclohexane and 21 500 cm-1 (465 nm) in acetonitrile (see Table 1). The drastic difference in spectral properties between 7MDPP emis-sion and the FCband of DPP in combination with the lack of
solvent proton donating site to assist the proton-transfer reaction lead us to exclude the assignment of the FCband in polar, aprotic
solvents to a proton-transfer tautomer emission.
Perhaps the strongest support for this viewpoint is given by 1MDPP; a nonproton- transfer model for DPP. 1MDPP revealed similar dual emission and solvent-polarity-dependent spectral-shifted properties (see Figure 7 and Table 1) although the relative intensity between FN and FC bands is different from
that observed in DPP (see Figures 5 and 7 for comparison). In consideration of the low ionization energy for the dimethylamino substituents, the results lead us to propose that the FC band
originates from an excited-state intramolecular charge transfer (ESICT) reaction incorporating dimethylamine (electron donor) and pyridine ring (electron acceptor) in DPP. Further verification of the charge transfer emission can also be given by the synthesis of APP (see Figure 1) in which only the FNband was observed
in various polar, aprotic solvents studied. For instance, APP exhibited single, normal Stokes shifted emission band maxima at∼335 and 343 nm in CH2Cl2and CH3CN, respectively (not
shown here). Theoretically, removing an additional methyl group leads to an increase of ionization energy of∼0.4 eV for simple aliphatic amines.46 Assuming a similar trend holds in the
aromatic system, APP is expected to possess higher ionization energy of∼0.8 eV than DPP, rationalizing its lack of charge-transfer property in the excited state.
The solvent-polarity dependent emission property can be specified more quantitatively according to the theory derived from dielectric polarization, specifying that the spectral shift of the fluorescence upon increasing the solvent polarity depends
on the difference in permanent dipole moments between ground and excited states. The magnitude of the excited-state dipole moments can thus be estimated by a method incorporating the fluorescence solvatochromic shift.47-51If the dipole moments
of the solute are approximated by a point dipole in the center of a spherical cavity with a radius a0, on the basis of nearly
solvent independent absorption properties and negligence of the solute polarizability one obtains
where v˜fand v˜fvacin eq f are the spectral position (in terms of
TABLE 1: Physical Parameters of Solvents as Well as Photophysical Properties of DPP, 1MDPP, and 7MDPP in Various Solvents at Room Temperature
(× 104cm-1)(× 10-3) (× 10-3) (× 10-2) solventa ET(30) kcal/mol ∆f v˜abs v˜FN v˜FC v˜FT ΦFN ΦFC ΦFT τFN (ns) τFC (ns) τFT (ns) DPP CHE 30.9 0 3.300 3.054 2.272 0.92 10.31 0.070 0.45 2.80 ether 34.5 0.1684 3.286 3.042 2.500 2.19 1.10 0.102 0.31 EA 38.1 0.1997 3.278 3.037 2.457 1.31 2.30 0.080 0.35 DCM 40.7 0.2184 3.250 2.998 2.392 2.68 6.70 0.120 0.51 ACN 45.6 0.3062 3.248 2.985 2.325 2.37 20.8 0.110 2.30 EtOH 51.9 0.2890 3.234 2.990 2.183 2.08 6.67 0.050 0.22b 3.60 MeOH 55.4 0.3095 3.242 2.982 2.175 1.01 2.74 0.042 0.17b 3.17 1MDPP CHE 30.9 0 3.229 3.043 NA 1.36 0.085 ether 34.5 0.1684 3.223 3.038 2.514 1.81 0.70 0.095 0.15 EA 38.1 0.1997 3.219 3.001 2.471 2.60 1.00 0.128 0.18 DCM 40.7 0.2184 3.190 2.984 2.405 3.11 1.50 0.131 0.22 ACN 45.6 0.3062 3.200 2.980 2.331 3.07 6.50 0.162 1.18 EtOH 51.9 0.2890 3.179 2.981 2.262 2.50 6.80 0.055 1.33 MeOH 55.4 0.3095 3.183 2.972 2.212 1.70 4.90 0.048 1.05 7MDPP CHE 30.9 0 2.759 2.119 98.0 4.065 EA 38.1 0.1997 2.813 2.146 97.0 4.258 DCM 40.7 0.2184 2.829 2.148 137 4.618 ACN 45.6 0.3062 2.837 2.150 108 4.121 EtOH 51.9 0.2890 2.845 2.174 87.0 3.405
aCHE: cyclohexane. EA: ethyl acetate. DCM: dichloromethane. ACN: acetonitrile.bThe rise time.
Figure 7. The fluorescence spectra of 1MDPP in various solvents: (a) ethyl ether, (b) ethyl acetate, (c) dichloromethane, (d) acetonitrile, and (e) ethanol. The excitation wavelength is 290 nm. * denotes the Rayleigh scattering peak.
ν˜f) ν˜f
vac-2(µbe- µbg) 2
hca03 ∆f (f)
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wavenumber) of the solvation equilibrated fluorescence maxima and the value extrapolated to the diluted gas-phase, respectively,
µ
bg andµbe are the dipole moment vectors of the ground and
excited states, and∆f is the Lippert solvent polarity parameter47
which is generally expressed as
where and n denote the static dielectric constant and the refractive index of the solvent, respectively. The plot of spectral maxima for both FNand FCbands as a function of∆f is shown
in Figure 8A and B for DPP and 1MDPP, respectively. As predicted by eq f, a linear relationship is found for both FNand
FCbands from ethyl ether to acetonitrile. For DPP the calculated
slope of -1.24 × 104 cm-1 for the F
C band is much steeper
than that of -2.02× 103cm-1for the F
Nband. Similar slopes
of -2.60× 103and -1.3× 104cm-1 were obtained for F N
and FC bands, respectively, in 1MDPP. Apparently, for both
DPP and 1MDPP the change of excited-state dipole moment with respect to the ground state is expected to be larger in the FCband, consistent with its assignment of the charge-transfer
emission. Furthermore, the proton-transfer tautomer emission, i.e., the FTband, in cyclohexane can be clearly distinguished
from the FCband by its far deviation from the Lippert’s plot in
the aprotic, polar solvents (see Figure 8A), further supporting the different spectral properties between proton-transfer (FT) and
charge transfer (FC) emission.
Time-resolved measurements have also been performed for both DPP and 1MDPP in various polar, aprotic solvents, and
the results are summarized in Table 1. Both FNand FCbands
show single-exponential decay dynamics with system response limited rise time (i.e. < 15 ps). The results lead us to conclude that the FNband cannot be a precursor for the FCband. This in
combination with the same excitation spectra for the dual emission indicates that both FNand FCbands originate from a
common Franck-Condon excited state followed by two inde-pendent relaxation pathways. More details regarding the quan-titative analysis of the excited-state dipole moment as well as relaxation dynamics are elaborated in the discussion section. 3.3. In Protic Solvents (Alcohols). Intriguing excited-state properties were observed in alcohols. In the steady-state measurement, for instance, DPP exhibited dual emission maxima at 335 nm (FN) and 460 nm in ethanol (see Figure 5). While ν˜maxof the FNband lies on the linear plot, the correlation for ν˜maxof the long-wavelength band versus ∆f deviates
signifi-cantly from the linear behavior of the FCband in aprotic solvents
(see Figure 8A). The result on one hand may indicate ESICT is operative, while the charge transfer species is subject to the specific hydrogen-bonding interactions with protic solvents. On the other hand, it could reflect different photophysical phenom-ena other than the charge-transfer event that occurred in the polar, aprotic solvents. Although the former proposed mecha-nism may be partially supported by a similar dual emission for 1MDPP, where the deviation of the Lippert’s plot was also observed for the long wavelength band in ethanol (or methanol, see Figure 8B), some supplementary evidence leads us to discount this proposed mechanism. First, the apparent quantum yield of the long wavelength band (Φf∼ 6.67 × 10-2) for DPP
in ethanol is higher than that of the FCband in the polar, aprotic
solvents by∼ 1 order of magnitude. In comparison, for 1MDPP, the yield in ethanol is about the same as those of the FC band
in aprotic solvents (see Table 1). Second, the peak maximum of the long-wavelength emission band for DPP (460 nm) is red shifted by∼ 15 nm from that for 1MDPP in ethanol (445 nm). In comparison, the difference in ν˜max for the FC band is
negligible between DPP and 1MDPP in polar, aprotic solvents (see Table 1). It thus seems likely that the large Stokes shifted emission band observed in ethanol for DPP and 1MDPP may exhibit intrinsically different characteristics.
Perhaps the strongest support for the above viewpoint was given by the time-resolved measurement. Drastically different excited-state dynamics were observed between DPP and 1MDPP in ethanol. By monitoring at > 410 nm, a single-exponential decay component with a lifetime of 1.3 ns was resolved for 1MDPP in ethanol, while the rise time is beyond the system response of 15 ps. In contrast, for DPP the time-dependent fluorescence can be well fitted by two components expressed as
Independent of the monitored wavelengths at >410 nm, k1
and k2were deduced to be 220 ( 20 ps-1and 3.6 ( 0.2 ns-1,
respectively. However, instead of the positive values for the fitted a2throughout the monitored wavelengths the sign of a1is
wavelength dependent. When monitoring at >450 nm a negative a1was obtained as indicated by a rise component of 224 ( 30
ps (see Figure 9a), which gradually shifts to a positive value, i.e., the appearance of a faster decay component, upon monitor-ing at shorter emission wavelength (e.g., 410 nm, see Figure 9b). Further analyses revealed that|a1/a2| was not equal to 1.0
and was found to be wavelength dependent. When the time-dependent fluorescence was monitored at the steady-state emission maximum of ∼460 nm this value is deduced to be Figure 8. The fluorescence maxima of (A) DPP, (B) 1MDPP (in terms
of wavenumber) as a function of ∆f in various solvents of (1) cyclohexane, (2) ethyl ether, (3) ethyl acetate, (4) dichloromethane, (5) acetonitrile, (6) ethanol, and (7) methanol. (O) The high-frequency band. (b) The low-frequency band.
∆f ) -1 2 + 1- 12 n2- 1 2n2+ 1 F(t) ) a1e-k1t+ a 2e -k2t (g)
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∼0.5, indicating that ∼50% of the fluorescence promptly exists, while the remaining 50% has not yet been populated. The results confirm the existence of two species at >410 nm. Due to its negligible emission intensity at >410 nm, the possibility of the observed decay dynamics originating, in part, from the residue of the normal emission (i.e., the FNband,λmax∼ 335 nm) can
be excluded. DPP possesses a similar parent structure to 7AI where the pyridinic nitrogen and pyrrolic hydrogen provide proton donating and accepting sites, respectively. In the previous section (3.1), DPP has been shown to undergo ESDPT in its dimer and complexed form with acetic acid in cyclohexane. Knowing that 7AI also undergoes alcohol catalyzed proton transfer in the excited state,21-25,26a it is thus reasonable to
propose the occurrence of ESDPT for DPP in ethanol, resulting in a proton-transfer tautomer emission (denoted by the FTband).
Evidence to support this viewpoint is given by 7MDPP, a proton-transfer analogue of DPP, which exhibits a normal Stokes shifted fluorescence with spectral and dynamic features (λmax
∼ 460 nm, τf∼ 3.4 ns, and Φf∼ 8.7 × 10-2) similar to that of
the long-wavelength band of DPP in ethanol. Furthermore, an
interesting comparison can be made by plottingν˜maxversus∆f
for the FTband of DPP in cyclohexane (i.e. the dimeric form),
ethanol and methanol, giving rise to a sufficiently linear correlation (see Figure 8A, dashed line), supporting their similar spectral properties.
As shown in Table 1, the lifetime of normal emission (i.e. the FNband) was∼50 ps in ethanol, while the rise time of the
proton-transfer tautomer emission was well resolved to be 220 ps. It is thus very unlikely that the precursor of ESDPT originates from the normal form of DPP (i.e. the species attributed to the FNemission), but more plausibly through its
charge transfer species. Consequently, a mechanism incorporat-ing charge/proton-transfer coupled reaction was tentatively proposed in Scheme 1c. Upon Franck Condon excitation, the charge-transfer reaction takes place, accompanied by fast solvent relaxation dynamics, to reach a stabilized charge-transfer state. The equilibrated charge-transfer state further undergoes solvent (ethanol) assisted, irreversible ESDPT, resulting in a proton-transfer tautomer emission. The rise kinetics of the tautomer emission depend on the types of short-carbon chain monoal-cohols used, being 170, 220, and 250 ps-1in methanol, ethanol, and propanol, respectively. In addition, a deuterium isotope effect was observed in the rise dynamics. For example, kptDwas
measured to be∼648 ps-1in ethanol-d, giving kptH/kptDto be
2.9. It is thus believed that the mechanism of alcohol catalyzed ESDPT in DPP should be similar to a two-step mechanism proposed for 7AI and its corresponding analogues,29,34
incor-porating equilibrium solvation followed by a rapid proton transfer, which is possibly governed by a tunneling mechanism shown in Scheme 1c. For this case kptcan be further expressed
by
where kpt0and∆G, respectively, denote the proton tunneling
rate and difference in free energy between cyclic and neighbor-Figure 9. The logarithm plot of the time-dependent fluorescence of
DPP in ethanol and its corresponding best-fitted curve. The emission wavelength was monitored at (a) 465 nm and (b) 410 nm.λex) 266 nm.
SCHEME 1: The Proposed Photophysics of DPP in (a) Nonpolar Solvents (e.g., Cyclohexane); (b) Polar, Aprotic Solvents (e.g., Acetonitrile); and (c) Protic Solvents (e.g., Ethanol).
kpt) k1 k-1kpt 0) k pt 0 e-∆G/RT
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bonded structures.29A distinct difference in mechanism between
DPP and 7AI is that the precursor of the ESDPT in DPP is a charge-transfer species, while the normal (neutral) species is operative in the case of 7AI.
On the basis of the prompt population (<15 ps) of the charge-transfer species followed by ESDPT, the time-dependent signal for FC(t)(charge-transfer emission) and FT(t) (proton-transfer
emission) in ethanol can be expressed as
where R denotes the instrument factor, including sensitivity, alignment, etc., of the detecting system, [FC]0 is the initial
population of the charge-transfer species in the excited state, and krC(ν˜) and krT(ν˜) are radiative decay rates of charge-transfer
and proton-transfer tautomer emission, respectively. kC is the
decay rate of the FCband excluding the rate of proton-transfer
reaction, and kTdenotes the observed decay rate of the tautomer
emission. It is reasonable to assume kptto be .kC+ kTin protic
solvents. As a result, the sum of FC(t) and FT(t) at a monitored
specific wavelength (e.g. > 400 nm) can be expressed as
Since krC(ν˜) and krT(ν˜) incorporate the Franck-Condon factor, krC(ν˜) - krT(ν˜) is expected to be wavelength dependent,
rationalizing the variation of preexponential factor a1 in both
value and sign.
As predicted from the FC band of 1MDPP (see Figure 7),
the charge-transfer emission is expected to exhibit a peak maximum at∼445 nm in ethanol. In the steady-state measure-ment, although the fluorescence yield of the charge-transfer emission is obscured in ethanol due to the dominant rate of ESDPT in DPP, an attempt has been made to resolve the individual FC and FT bands by the time-dependent
spectral-evolution technique. Figure 10a shows the emission spectrum obtained by integrating the photon-counting signal in a time delay interval of 0-150 ps. It is apparent that the resulting spectrum which has a maximum at∼440 nm is blue shifted versus that of∼460 nm obtained at a time delay of >1.0 ns (see Figure 10b). The time-resolved spectral evolution substanti-ates our proposal of two emitting species in the long wavelength
band, ascribed to the charge-transfer precursor and its subsequent proton-transfer tautomer emission.
4. Discussion
4.1. Excited-State Charge-Transfer Properties. Much of the evidence presented in previous sections has ambiguously drawn the conclusion that the excited-state charge transfer reaction is operative for DPP in polar solvents. In this section, we have made an attempt to further explore the excited-state charge transfer properties in DPP. One key issue is to see if the ESICT mechanism incorporates the lock of a charge separation through the rotation of the dimethyl amino group, namely the twist intramolecular charge transfer (TICT).52-55 However,
excited-state proton transfer is prohibited in N,6N6
-dimethyl-adenosine since the pyrrolic nitrogen has been attached to a sugar moiety. Experimentally, a direct verification of such a proposed mechanism may require the design of a rigid alkyl amino substituent so that the rotation is sterically hindered with respect to the parent 7AI, prohibiting the TICT dynamics. Unfortunately such a synthetic goal has not yet been achieved in our laboratory at this stage. Alternatively, the possibility of a TICT mechanism operated in DPP was herein examined by theoretical approaches.
According to the ab initio approach (MP2/6-31G(d, p)) the geometry optimized structure of DPP in the ground state revealed two carbon atoms in the dimethylamino group lying on the same symmetry plane with respect to the parent 7AI moiety (Φ∼ 0°, structure N in Figure 1). Conversely, a DPP structure with the dimethyl group being perpendicular to the plane of 7AI moiety (Φ∼ 90°, structure C) is calculated to be higher in energy than that of the in-plane one by 4.5 kcal/mol. The difference is more or less the same upon incorporating semiempirical PM3-SMx solvation models57 to obtain the
solvation free energy in various aprotic solvents, which is then added to the calculated “gas phase” energies. Thus, structure N was concluded to be the most stable conformer in the ground state, of which the magnitude of dipole moment is deduced to be 3.01 D with its orientation 165°in plane relative to the C8
-C9bond counted positive counterclockwise. The vector of dipole
moment was also calculated by various computation methods including AM1, PM3, and Hartree-Fock methods incorporating different basis sets. The results listed in Table 2 indicate that Figure 10. The time-dependent spectral evolution of DPP in ethanol
acquired at (a) (O) 0-150 ps, (b) (b) > 1.0 ns. λex) 266 nm.
FC(t) ) RkrC(ν˜)[FC]0e -(kC+kpt)t FT(t) )Rkr T( ν˜)kpt[FC]0 kT- kC- kpt[e -(kC+kpt)t- e-kTt] FC(t) + FT(t) ) R[FC]0[(krC(ν˜) - kr T (ν˜))e-kptt+ k r T (ν˜)e-kTt] ) a1e-kptt+ a 2e -kTt
TABLE 2: The Magnitude (Unit: Debye) and Angle (Unit: Degree) of Dipole Moment for DPP Calculated by Various Theoretical Approaches
methoda magnitude angle
Ground State MP2/6-31G(d, p) 3.0147 164.65b HF/6-31G(d, p) 4.0238 164.16 AM1 2.727 159.80 PM3 2.595 157.76 INDO/S-CI 1.688 143.13 Excited State (Φ ) 0°) 6.1d 28.0c Excited State (Φ ) 90°) 13.2d 36.7c aThe angle is specified as the orientation of dipole moment vector
relative to the C8-C9bond (see Figure 1) counted positive clockwise.
bAll semiempirical calculations were performed by HyperChem 6.03
on Win2000 professional at Intel PIII 733 computer.cThe angle is
specified as the orientation of dipole moment vector relative to the ground state (i.e., structure N, see Figure 1) using INDO/S-CI method.
dValues were obtained by the best fit of Figure 8A in polar, aprotic
solvents using eq f where magnitude of dipole moment in the ground state and angles between ground and excited states were calculated by MP2/6-31(d, p) and INDO/S-CI methods, respectively (see text).
Downloaded by NATIONAL TAIWAN UNIV on July 27, 2009
depending on the method applied, the magnitudes of the calculated dipole moment diversify, whereas the orientation of dipole moment is less affected.
Based on the results of INDO/S-CI calculation (see Experi-mental Section), Figure 11 depicts the two lowest unoccupied and three highest occupied frontier molecular orbitals calculated for DPP in the planar (Φ ) 0°) and twisted (Φ ) 90°) conformations, where Φ specifies the rotational angle of N(CH3)2relative to the parent 7AI plane. Except for the second
highest occupied orbital (HOMO-1) all rest frontier molecular orbitals possess the same configuration between N and C conformers. The π-symmetry configuration was found for
LUMO+1, LUMO and HOMO orbitals, while HOMO-2 is mainly attributed to the lone pair orbital of the pyridinic nitrogen. Instead of theπ-symmetry for the HOMO-1 orbital
in N conformer, the HOMO-1 orbital in structure C possesses aσ-symmetry where the charge density is largely localized on
the dimethylamino nitrogen. The lowest Franck-Condon singlet excitation in the planar form was dominated by HOMO f LUMO+1 and HOMO-1 f LUMO (32 206 cm-1) transition
possessing a 1ππ* configuration (see Table 3). In contrast, it
was altered to HOMO-1 (σ-symmetry) f LUMO (π-symmetry)
in the case of the C conformer (∼27 030 cm-1), indicating that the lowest excited singlet state for structure C, to a certain extent, possesses a charge transfer character where the electron Figure 11. The electron-density distribution of the two lowest unoccupied and three highest occupied frontier molecular orbitals calculated (INDO/ S-CI) for DPP in the planar (Φ ) 0°) and perpendicular (Φ ) 90°) conformations.
TABLE 3: The Lowest Singlet Electronic Transitions Calculated for DPP in the Planar (Φ ) 0°) and Twisted (Φ
) 90°) Conformations Using the INDO/S-CI Method (See Experimental Section for the Detailed Description)
transition ν˜ (cm-1) fa dominant configurationsb Φ ) 0° S0f S1(ππ*) 32206 0.038 H f L+1, H-1 f L S0f S2(nπ*) 33772 0 H-2 f L S0f S3(ππ*) 34916 0.3177 H f L Φ ) 90° S0f S1(σπ*) 27030 0.001 H-1 f L S0f S2(ππ*) 32258 0.086 H f L, H f L+1 S0f S3(nπ*) 32680 0 H-2 f L
aOscillator strength.bH: HOMO. L: LUMO.
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density flows from dimethylamino substituent to the ring, particularly the pyridinic nitrogen. In summary, theoretical approaches for both ground and lowest-lying excited states of DPP are qualitatively in accordance with a TICT model originally proposed by Grabowski and co-workers.52 The
calculated charge-transfer emission of 27 030 cm-1in a vacuum is consistent with the extrapolation ofν˜maxfor the FC band to
∆f ) 0 (Figure 8A), which gives rise to an intercept of 26 950 cm-1for DPP in the gas phase. Further solvent relaxation must take place to stabilize the point dipole of the charge-transfer species, resulting in a larger Stokes shifted emission upon increasing the solvent polarity. It should also be noted that analyses have not been performed in consideration of various twisting angles. Thus, the possibility of TICT taking place at a certain small but critical angle rather than the Φ ) 90° conformer cannot be ruled out at this stage. Further studies on the molecular design of an intrinsically twisted DPP analogue are crucial to resolve this issue.
The excited-state dipole moments for the solvated DPP structures attributed to either FNor FCbands could be estimated
from the slopes depicted in Figure 8. Because the ground state dipole moment for DPP has not yet been determined experi-mentally, we adopted the value of 3.01 D deduced from the MP2/6-31G(d, p) method incorporating the electron correlation. As mentioned in the previous section, the orientation of the dipole moment is much more reliable than its magnitude, especially in the excited state, among various computational methods. On the basis of the INDO/S-CI method, the angles of dipole moment between ground (structure N) and FN(the first 1ππ* state of structure N) or F
C(the first1σπ* state of structure C) state were estimated to be 28.0°and 36.7°, respectively (see Table 2). If charge transfer from the dimethylamino group to the pyridinic nitrogen makes a key contribution, we would expect a short-axis (C8-C9)-oriented excited-state dipole
mo-ment of the C state. This viewpoint is supported by the nearly parallel orientation (∼ 180°) ofbµC*relative to the C8-C9bond
(see INDO/S-CI method in Table 2). Employing these calculated angles as fixed parameters, magnitudes of excited-state dipole moment for both FNand FCstates can thus be estimated. In eq
f, the radius of spherical cavity aowas calculated to be 4.57 Å
based on a geometry optimized structure of DPP (HF/6-31G(d, p)). Accordingly, a best fit of Figure 8 using eq f gives the magnitude of dipole moment to be 6.1 and 13.2 D for N* and C*, respectively (see Table 2). The combination of experimental and theoretical approaches predicts a relative trend of|µC*| >
|µN*| > |µg| for DPP. A larger solvatochromic shift is then
expected for the charge-transfer state (i.e., the C* state) in comparison to the normal state (N*), rationalizing the difference in solvent-polarity dependence of FNand FCemissions (i.e.,
theν˜maxvalue in Figure 8).
4.2. Relaxation Dynamics in Aprotic Solvents. The time-resolved fluorescence studies reveal that the rise time of the FC
emission band is irresolvable (i.e. < 15 ps), while a finite lifetime of 30-160 ps was resolved for the FNband in various
solvents studied. Careful analyses also conclude that preexpo-nential factors of the fitted decay dynamics for both FNand FC
bands are positive throughout the spectral region of 300-600 nm. No negative preexponential values can be resolved in our current time correlated photon-counting measurement. This in combination with identical excitation spectra suggests that FN
and FC states share a common Franck-Condon state. Upon
excitation, this initially prepared electronic state rapidly relaxes to FNand FCstates which become decoupled without
intercon-version. Consequently, these two states decay independently, and the FNstate cannot be the precursor of the FCstate.
A strong support for this model was given by the temperature-dependent fluorescence spectra of DPP in 2MTHF shown in Figure 12. The FN emission revealed strong temperature
dependence. Its apparent quantum yield Φapp increased from
1.8× 10-3at 298 K to a plateau (Φapp∼ 0.47) at ∼90 K and
remained constant at lower temperatures. There is a sharp increase of the FNintensity in the range between 150 and 100
K where a turning point at∼115 K was observed. In contrast, the intensity of the FCband was nearly temperature independent,
and only increased by∼ 1.5-fold from 298 down to 80 K (not shown here). The steady-state results were further compared with the temperature-dependent relaxation dynamics. The lifetime monitored at the FNband (e.g., 340 nm) follows the
steady-state pattern where it increased significantly from 70 ps at 298 K to 3.2 ns at 80 K. Conversely, the FCband increased
only slightly from 1.5 to 2.1 ns. Under a system response limit of 15 ps, rise dynamics cannot be extracted for either the FNor
FC band throughout the temperature-dependent study, further
indicating that at the studied temperatures, the FNspecies cannot
be the precursor of the FC band. Assuming a
temperature-independent radiative decay rate krfor the FNband, the observed
temperature-dependent decay rate kobscan be expressed as kobs
) kr+ knr+ knr(T). Accordingly, the apparent quantum yield
is deduced to
where knr denotes the temperature independent radiationless
decay rate constant, possibly involving internal conversion, intersystem crossing, etc. The temperature-dependent radiation-less decay rate constant knr(T) can be further expressed as an
Arrhenius type of thermally deactivated pathway of knr(T) ) Ae-Ea/RT, which is then plugged into eq h to obtain
As indicated by steady-state and time-resolved measurements, the lifetime and intensity of the FNband at <90 K are nearly
temperature independent. Thus, knr(T) is assumed to be
negli-gible, andΦappexpressed in eq h can be simplified to kr/(knr+ kr), which was measured to be∼0.47 at 80 K. On the other
hand, kobswas resolved to be 3.1× 108s-1at 80 K. Accordingly kr and knr were deduced to 1.5 × 108 and 1.6 × 108 s-1, Figure 12. The plot of apparent quantum yield (Φapp) for the FNband of DPP as a function of the temperature (T). Insert: The plot ln{(1/ Φapp- 1 - knr/kr)kr}versus 1/T from 298 to 80 K and a best linear least-squares fitting curve using eq i.
Φapp) kr kr+ knr+ knr(T) (h) ln
{(
1 Φapp - 1 -knr kr)
kr}
) ln A -Ea RT (i)Downloaded by NATIONAL TAIWAN UNIV on July 27, 2009
respectively. With all parameters provided, the plot of ln{(1/ Φapp - 1 - knr/kr)kr} as a function of reciprocal of the
temperature reveals a straight line and Ea was deduced to be
2.1 kcal/mol with a frequency factor A of 3.6× 1012s-1(see
insert of Figure 12).
The fast depopulation of the FN state may be tentatively
rationalized by the proximity of the nπ* state which has been
calculated to be in the second excited state and was estimated to be only∼4.4 kcal/mol above the lowest singlet ππ* state (see Table 3). Note this value is on the same magnitude as the activation energy (i.e., 2.1 kcal/mol) of the temperature-dependent radiationless pathway for DPP in 2MTHF. As a result, the Franck-Condon excited state may undergo a fast thermally activated1ππ* f1nπ* internal conversion. The vibronic mixing
between nπ* and ππ* states may explain the extremely low
fluorescence yield of the FN state. In an extreme case, the
corresponding pseudo Jahn-Teller distortion may be incorpo-rated, enhancing a nonradiative decay channel. Such a mech-anism requires the molecule to be distorted along a nontotally symmetric (i.e., out-of-plane) coordinate.58-60 Twisting the
exocyclic dimethylamino group possibly induces a nontotally symmetric distortion. Accordingly, the relationship between movement into the charge-transfer state and nonradiative decay of the FN state might be considered an efficient deactivation
pathway. This proposed mechanism, however, is discounted via studying the spectral properties of APP. Although ESICT is prohibited in APP due to the high ionization energy of the amino substituent, the florescence (i.e., the FNband) yield is still low
(∼10-3) in all polar solvents studied, indicating that radiationless pathways other than the charge-transfer reaction dominate the decay dynamics of the FNband.
Conclusions
In summary, we have reported dual photophysical properties, i.e., the excited-state proton transfer versus charge-transfer reactions, on DPP in different environments. To summarize the viewpoints made in previous sections, Scheme 1a-c depicts the photophysical pathways of DPP categorized by nonpolar, polar, aprotic, and protic solvents. Several remarks can be pointed out on the basis of Scheme 1: (1) The electron donating property of the dimethylamino group on DPP enhances the dual hydrogen bonding strength in both DPP dimer and DPP/acetic acid complex, which upon excitation undergo ultrafast double proton transfer (kpt> 6.7 × 1010s-1), resulting in an
imine-like tautomer emission in nonpolar solvents. (2) The excited-state intramolecular charge transfer takes place in polar, aprotic solvents, of which the relaxation dynamics are independent of the normal emission (i.e., the FNband). As supported by the
theoretical approach, the mechanism incorporating TICT may be operative, resulting in a large change of the dipole moment. (3) In protic solvents such as in ethanol, following the fast excited-state intramolecular charge transfer and solvent relax-ation, the solvent catalyzed proton-transfer reaction takes place (e.g.,∼220 ps-1in ethanol), resulting in an imine-like tautomer emission. Thus, DPP provides an ideal model to study the dual excitation behavior of proton transfer vs charge transfer, which is unique among the 7AI analogues.
Although the spectroscopy and dynamics have been studied comprehensively, several perplexities remain unsolved. The main question lies in relation to the branching ratio as well as the corresponding dynamics for the population of the FN and
FCspecies. While TICT seems to be operative in the case of
DPP, definitive proof and correlation with the twisting angles remain to be explored. It should be noted that to explain the spectroscopy and dynamics of dual emission for N,6N6
-di-methyladenosines, Albinsson and co-workers56have proposed
a similar nonprecursor-sater type of TICT mechanism. In contrast, Andreas et al.61 recently claimed a precursor-sater
relation for the dual emission of N,6N6-dimethyladenine
deriva-tives. For the case of DPP, the independent relaxation dynamics between FN and FC bands leads us to tentatively adopt the
nonprecursor-sater TICT mechanism at this stage. Focus on design and synthesis of DPP derivatives so that the electron-donating site can be geometrically locked to test the proposed TICT/ESDPT coupled mechanism is currently in progress. It is believed that this approach coupled with further femtosecond time-resolved measurements will gain more insight on this issue. Finally, several derivatives of DPP and APP have been proposed as potential antitumor agents.62,63 It is thus of
considerable interest to investigate whether the ESDPT reaction catalyzed by, e.g., the carboxylic group will induce the antitumor activity. Many amino acid side chains have abnormal pKavalues
in proteins.64Therefore, unionized carboxylic acids might be
involved in the hydrogen-bond interaction under biological conditions. Formation of cyclic dual hydrogen bonds allows an amino acid side chain incorporating carboxylic acid to discrimi-nate between different nucleic acid bases, which serves an important role in the selective recognition of nucleic acid bases by proteins.65Therefore, it is of great importance to probe the
dual hydrogen bonding protein-nucleic acid interaction65on the
basis of the excited-state proton-transfer tautomerism of DPP and APP.
Acknowledgment. This work was supported by the National Science Council, Taiwan, R.O.C. (NSC89-2113-M-194-009). References and Notes
(1) Taylor, C. A.; El-Bayoumi, M. A.; Kasha, M. Proc. Natl. Acad. Sci. U.S.A. 1969, 63, 253.
(2) Ingham, K. C.; Abu-Elgheit, M.; El-Bayoumi, M. A. J. Am. Chem. Soc. 1971, 93, 5023.
(3) Ingham, K. C.; El-Bayoumi, M. A. J. Am. Chem. Soc. 1974, 96, 1674.
(4) Watson, J. D.; Crick, F. H. C. Nature (London) 1953, 171, 964. (5) Watson, D. G.; Sweet, R. M.; Marsh, R. E. Acta Crystallogr. 1965, 19, 573.
(6) Hetherington, W. M., III; Micheels. R. H.; Eisenthal, K. B. Chem. Phys. Lett. 1979, 66, 230.
(7) Fuke, K.; Yoshiuchi, H.; Kaya, K. J. Phys. Chem. 1984, 88, 5840. (8) Waluk, J.; Herbich, J.; Oelkrug, D.; Uhl, S. J. Phys. Chem. 1986, 90, 3866.
(9) Fuke, K.; Kaya, K. J. Phys. Chem. 1989, 93, 614.
(10) Tokumura, K.; Watanabe, Y.; Udagawa, M.; Itoh, M. J. Am. Chem. Soc. 1987, 109, 1346.
(11) Catala´n, J.; Kasha, M. J. Phys. Chem. A 2000, 104, 10812. (12) Douhal, A.; Kim, S. K.; Zewail, A. H. Nature 1995, 378, 260. (13) Folmer, D. E.; Poth, L.; Wisniewski, E. S.; Castleman, A. W., Jr. Chem. Phys. Lett. 1998, 287, 1.
(14) Chachisvilis, M.; Fiebig, T.; Douhal, A.; Zewail, A. H. J. Phys. Chem. A 1998, 102, 669.
(15) Takeuchi, S.; Tahara, T. J. Phys. Chem. A 1998, 102, 7740. (16) Fiebig, T.; Chachisvilis, M.; Manger, M.; Zewail, A. H.; Douhal, A.; Garcia-Ochoa, I.; de La Hoz Ayuso, A. J. Phys. Chem. A 1999, 103, 7419.
(17) Chou, P. T.; Liao, J. H.; Wei, C. Y.; Yang, C. Y.; Yu, W. S.; Chou, Y. H. J. Am. Chem. Soc. 2000, 122, 986.
(18) Douhal, A.; Guallar, V.; Moreno, M.; Lluch, J.-M. Chem. Phys. Lett. 1996, 256, 370.
(19) Guallar, V.; Batista, V. S.; Miller, W. H. J. Chem. Phys. 1999, 110, 9922.
(20) Catala´n, J.; Del Valle, J. C.; Kasha, M. Proc. Natl. Acad. Sci. U.S.A.,
1999, 96, 8338.
(21) McMorrow, D.; Aartsma, T. J. Chem. Phys. Lett. 1986, 125, 581. (22) Moog, R. S.; Bovino, S. C.; Simon, J. D. J. Phys. Chem. 1988, 92, 6545.
(23) Konijnenberg, J.; Huizer, A. H.; Varma, C. A. G. O.J. Chem. Soc., Faraday Trans. 2 1988, 84 (8), 1163.
(24) (a) Moog, R. S.; Maroncelli, M. J. Phys. Chem. 1991, 95, 10359. (b) Chapman, C. F.; Maroncelli, M. J. Phys. Chem. 1992, 96, 8430.
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