Fluorescence Enhancement of
trans
-4-Aminostilbene by
N
-Phenyl Substitutions: The “Amino Conjugation Effect”
Jye-Shane Yang,* Shih-Yi Chiou, and Kang-Ling Liau Contribution from the Department of Chemistry, National Central UniVersity,
Chung-Li, Taiwan 32054
Received June 14, 2001. Revised Manuscript Received October 31, 2001
Abstract:The synthesis, structure, and photochemical behavior of the trans isomers of 4-(N-phenylamino)-stilbene (1c), 4-(N-methyl-N-phenylamino)4-(N-phenylamino)-stilbene (1d), 4-(N,N-diphenylamino)4-(N-phenylamino)-stilbene (1e), and 4-(N-(2,6-dimethylphenyl)amino)stilbene (1f) are reported and compared to that of 4-aminostilbene (1a) and 4-N,N-dimethylaminostilbene (1b). Results for the corresponding 3-styrylpyridine (2) and 2-styrylnaphthalene analogues (3) are also included. The introduction ofN-phenyl substituents to 4-aminostilbenes leads to a more planar ground-state geometry about the nitrogen atom, a red shift of the absorption and fluorescence spectra, and a less distorted structure with a larger charge-transfer character for the fluorescent excited state. Consequently, theN-phenyl derivatives 1c-e have low photoisomerization quantum yields and high
fluorescence quantum yields at room temperature, in contrast to the behavior of 1a, 1b, and most unconstrained monosubstitutedtrans-stilbenes. The isomerization of 1c and 1d is a singlet-state process, whereas it is a triplet-state process for 1e, presumably due to a relatively higher singlet-state torsional barrier. The excited-state behavior of 1f resembles 1a and 1b instead of 1c-e as a consequence of the
less planar amine geometry and weaker orbital interactions between theN-phenyl and the aminostilbene groups. Such anN-phenyl substituent effect is also found for 2 and 3 and thus appears to be general for stilbenoid systems. The nature of this effect can be described as an “amino conjugation effect”.
Introduction
Studies on substituent effects continue to provide insights into the excited states of stilbenes and related systems.1-7Two decay
processes, fluorescence and trans f cis isomerization, account for the excited-state behavior of trans-stilbene and most substituted stilbenes in solution. Isomerization can occur via either the singlet or the triplet state, depending on the nature of substituents. The triplet-state mechanism is important for some halogen-, nitro-, and carbonyl-substituted stilbenes, but the singlet-state mechanism is dominant for trans-stilbene and other substituted stilbenes. Double bond torsion for singlet-state isomerization involves a thermal barrier between the planar (1t*)
and the perpendicular (1p*) states. The barrier height and the
temperature thus determine the quantum yields of fluorescence vs isomerization when intersystem crossing is negligible in the
excited-state decay. For example, in the case of trans-stilbene a torsional barrier of∼3.5 kcal/mol corresponds to a fluores-cence yield of 3-5% and a nearly maximum of trans f cis yield of ∼50% at room temperature.3 Substituents can, in
principle, raise the torsional barrier and thus the fluorescence quantum yield by lowering the energy of the fluorescent1t*
state more than that of the1p* state. However, the undefined
nature of the 1p* state precludes effective prediction of the
substituent effect on the torsional barrier and the fluorescence efficiency of trans-stilbenes. A literature survey reveals that, except forπ-substituents such as phenyl8or styryl9groups that
extend the conjugation length, neither electron-withdrawing nor electron-donating substituents can lead to fluorescence as the dominant mode of excited-state decay (>50%) for unconstrained and 4-substituted trans-stilbenes at room temperature.1-3
It has recently been recognized that N-phenyl vs N-alkyl substitution of aromatic amines can lead to superior performance in materials chemistry.10-14Examples include improvement in
thermal stability,10molecular hyperpolarizability,10-12and hole * To whom correspondence should be addressed. E-mail: jsyang@
cc.ncu.edu.tw.
(1) Go¨rner, H.; Kuhn, H. J. AdV. Photochem. 1995, 19, 1-117. (2) Waldeck, D. H. Chem. ReV. 1991, 91, 415-436.
(3) (a) Saltiel, J.; Waller, A. S.; Sears, D. F., Jr.; Hoburg, E. A.; Zeglinski, D. M.; Waldeck, D. H. J. Phys. Chem. 1994, 98, 10689-10698. (b) Saltiel, J.; Waller, A. S.; Sears, D. F., Jr.; Garrett, C. Z. J. Phys. Chem. 1993, 97, 2516-2522. (c) Saltiel, J., Sun, Y.-P. Photochromism, Molecules and Systems; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; pp 64-164. (d) Saltiel, J.; Charlton, J. L. Rearrangements in Ground and Excited States, de Mayo, P., Ed.; Academic Press: New York, 1980; Vol. 3, pp 25-89.
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61, 2242-2246. (b) Verbiest, T.; Burland, D. M.; Jurich, M. C.; Lee, V. Y.; Miller, R. D.; Volksen, W. Science 1995, 268, 1604-1606. (c) Gilmour, S.; Montgomery, R. A.; Marder, S. R.; Cheng, L.-T.; Jen, A. K.-Y.; Cai, Y.; Perry, J. W.; Dalton, L. R. Chem. Mater. 1994, 6, 1603-1604. (d) Marder, S. R.; Perry, J. W. Science 1994, 263, 1706-1707.
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mobility13,14of arylamines when applied for nonlinear optics
or in organic electroluminescent devices. On the basis of theoretical studies, the influence of N-phenyl substitutions has been attributed to a more planar amine structure and to a nonconventional “conjugation” between the aryl and the intro-duced phenyl groups.11,13In comparison to these ground-state
properties, the N-phenyl substituent effect on the excited-state behavior of arylamines is less investigated.15In particular, while
the excited states of aminostilbenes and their N,N-dimethyl derivatives have been extensively investigated,4-7,16-18 little
attention has been paid to their N-phenyl-substituted counter-parts. Moreover, several N,N-diaryl derivatives of trans-4-aminostilbenes have been investigated as solid-state electro-luminescent materials and shown to have high electro-luminescent efficiency;19however, no fluorescence quantum yield data were
reported. This behavior is in complete contrast to the inherently weak fluorescence of both trans-4-aminostilbene (1a)5and
trans-4-N,N-dimethylaminostilbene (1b).16In this context, we have
investigated the N-phenyl substituent effect on the excited-state behavior of aminostilbenes. We report herein that the fluores-cence of 1a and 1b in solutions is dramatically increased by more than 1 order of magnitude upon N-phenyl (1c,d) or N,N-diphenyl (1e) substitutions. We attribute this to an “amino conjugation effect” on the basis of the absence of fluorescence enhancement in the case of 1f, where the “conjugation” between the stilbene and the phenyl group is interrupted or diminished. Similar effects are also observed for the trans-1,2-diarylethyl-enes 2 and 3 and thus appears to be general for stilbenoid systems. The origin of this “amino conjugation effect” on fluorescence as well as other excited-state properties will be elucidated and discussed.
Results and Discussion
Molecular Structure. The recent development of
palladium-catalyzed aromatic carbon-nitrogen bond formation20provides
a feasible method for the synthesis of N-phenylamino- and N,N-diphenylamino-substituted stilbenes and 1,2-diarylethylenes.21
As is shown in Scheme 1, the Pd2(dba)3/(()-BINAP/NaOBut
catalyst system22was employed for the coupling of
trans-4-bromostilbene (1x) and corresponding amines in toluene solution to afford 1c, 1d, and 1f. Compound 1e was prepared under slightly different conditions, in which the ligand (()-BINAP was replaced by P(t-Bu)3.23The same method was also applied
to the formation of compound series 2 and 3 by replacing 1x with trans-3-(4-bromostyryl)pyridine (2x) and trans-2-(4-bromo-styryl)naphthalene (3x), respectively. The precursors 1x-3x were in turn prepared according to the standard Horner-Wadsworth-Emmons procedures for olefin synthesis.24
The ground-state structures of 1a-f have been investigated by MOPAC-AM1 calculations.25Compounds 1c, 1d, and 1f
are structurally unsymmetrical and can adopt either one of two planar conformations as a consequence of rotation about the styrene-diphenylamine single bond. As is depicted for 1c (eq 1), the conformer 1c-syn, which has both the N-phenyl and
styrene groups located on the same side with respect to the long molecular axis, is slightly lower in energy by 0.29 kcal/mol than the other conformer 1c-anti. The syn conformers of both
1d and 1f were also calculated to have a lower energy than the
corresponding anti conformers with energy differences similar to that for 1c (Table 1).
(12) Moylan, C. R.; Twieg, R. J.; Lee, V. Y.; Swanson, S. A.; Betterton, K. M.; Miller, R. D. J. Am. Chem. Soc. 1993, 115, 12599-12600. (13) Sakanoue, K.; Motoda, M.; Sugimoto, M.; Sakaki, S. J. Phys. Chem. A
1999, 103, 5551-5556.
(14) (a) Pacansky, J.; Waltman, R. J.; Seki, H. Bull. Chem. Soc. Jpn. 1997, 70, 55-59. (b) Kitamura, T.; Yokoyama, M. J. Appl. Phys. 1991, 69, 821-826.
(15) Rumi, M.; Ehrlich, J. E.; Heikal, A. A.; Perry, J. W.; Barlow, S.; Hu, Z.; McCord-Maughon, D.; Parker, T. C.; Ro¨ckel, H.; Thayumanavan, S.; Marder, S. R.; Beljonne, D.; Bre´das, J.-L. J. Am. Chem. Soc. 2000, 122, 9500-9510.
(16) Le´tard, J.-F.; Lapouyade, R.; Rettig, W. J. Am. Chem. Soc. 1993, 115, 2441-2447.
(17) Papper, V.; Pines, D.; Likhtenshtein, G.; Pines, E. J. Photochem. Photobiol. A Chem. 1997, 111, 87-96.
(18) (a) Lewis, F. D.; Kalgutkar, R. S. J. Phys. Chem. A 2001, 105, 285-291. (b) Il’ichev, Y. V.; Ku¨hnle, W.; Zachariasse, K. A. Chem. Phys. 1996, 211, 441-453. (c) Lapouyade, R.; Kuhn, A.; Letard, J.-F.; Rettig, W. Chem. Phys. Lett. 1993, 208, 48-58. (d) Gruen, H.; Go¨rner, H. J. Phys. Chem. 1989, 93, 7144-7152.
(19) (a) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1990, 56, 799-801. (b) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1990, 56, 531-533. (c) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phys. Lett. 1989, 55, 1489-1491.
(20) (a) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc. Chem. Res. 1998, 31, 805-818. (b) Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046-2067.
(21) See Supporting Information for details.
(22) Wolfe, J. P.; Buchwald, S. L. J. Org. Chem. 2000, 65, 1144-1157. (23) Yamamoto, T.; Nishiyama, M.; Koie, Y. Tetrahedron Lett. 1998, 39,
2367-2370.
(24) Wadsworth, W. S., Jr. Org. React. 1977, 25, 73-253.
(25) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902-3909.
Scheme 1
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The structural differences in the amine groups among 1a-f can be described by two parameters: the sum of bond angles (θ) about the N atom for all cases and the dihedral angle (R) between the two N-phenyl planes of the amine groups for 1c-f (Table 1). We would expect a larger orbital overlap among the amine nitrogen, the N-phenyl, and the stilbene groups when the θ value is closer to 360°and the R value to 0°. AM1-calculations suggest that theθ value for aminostilbenes increases upon both N-alkyl and N-phenyl substitutions, but this effect is in the order N-phenylation > N-alkylation (i.e., 1e > 1d > 1c > 1b). Similar results have been observed for aniline deriva-tives,13but theθ values are aminostilbenes > anilines with the
same amine substituents (e.g., 1a (342.9°) > aniline (335.2°) and 1d (357.3°) > methyldiphenylamine (351.8°)) (Table 1). However, both 1e and triphenylamine11attain completely planar
structures about the N atom (θ ) 360°)(i.e., a sp2character for
the N atom). These results indicate that the more of the larger aryl groups the amine contains, the more planar structure about the N atom and thus the better the delocalization of the amine lone pair electrons to the π-systems. The calculated R value for 1c is 50°. Slightly larger values of R are obtained for 1d and 1e, presumably due to the bulkier amine groups (Table 1). In comparison to 1c, the introduction of methyl substituents at the ortho positions of the N-phenyl group (1f) not only increases the dihedral angle R but also decreases theθ value. Results from X-ray crystallography for related N-phenyl-substituted aminostilbenes that form single crystals indeed agree with the AM1-predicted syn conformations andθ and R values.26
Absorption Spectra. The absorption spectra of 1c-f in
hexane are shown in Figure 1. For comparison, the spectra of
1a5 and 1b16 in hexane are included. Except for 1e, which
possesses a second absorption band at shorter wavelengths, these
molecules have a single intense long wavelength band, resem-bling those of trans-stilbene and other 4-substituted stilbenes.27
The introduction of N-phenyl substituents in 1a results in considerable bathochromic and hyperchromic shifts, suggesting substantial interactions between the N-phenyl and aminostilbene groups. Indeed, it has been suggested for triphenylamine and its derivatives that there is conjugation between the nitrogen lone pair electrons and the phenylπ-electrons, and that the whole molecule is a new chromophore with characteristic absorption and emission spectra.28The second band near 300 nm for 1e is
a consequence of an electronic transition mainly localized in the triphenylamine moiety,29which is supported by the results
of semiempirical INDO/S-SCF-CI (ZINDO) calculations (vide infra).30
The absorption maxima of compound series 1-3 in hexane and acetonitrile are reported in Table 2. It is interesting to note that the absorption maxima of 1b and 1c are essentially identical, and that the extent of the red shift of the absorption maxima for 1a-f (i.e., 1a < 1f < 1b∼ 1c < 1d < 1e) increases with increased planarization of the system, as is indicated by theθ values (Table 1). The red shifts of absorption maxima in compound series 2 and 3 also follow the order e > c > f. In all
(26) Yang, J.-S.; Chiou, S.-Y. Unpublished results.
(27) Gegiou, D.; Muszkat, K. A.; Fischer, E. J. Am. Chem. Soc. 1968, 90, 3907-3918.
(28) (a) Sander, R.; Stu¨mpflen, V.; Wendorff, J. H.; Greiner, A. Macromolecules 1996, 29, 7705-7708. (b) Janic´, I.; Kakasˇ, M. J. Mol. Struct. 1984, 114, 249-252.
(29) Subrayan, R. P.; Kampf, J. W.; Rasmussen, P. G. J. Org. Chem. 1994, 59, 4341-4345.
(30) Zerner, M. C.; Leow, G. H.; Kirchner, R. F.; Mueller-Westerhoff, U. T. J. Am. Chem. Soc. 1980, 102, 589-599.
Table 1. AM1-Calculated Heats of Formation (∆Hf), Sum of Bond
Angles (θ) about the N Atom, and Dihedral Angles (R) between
theN-Phenyl Planes for 1a-f
1a 1b 1c 1d 1e 1f ∆Hf(kcal/mol) 59.01 69.54 95.35 (95.64)a 103.91 (104.19)a 137.30 83.80 (84.11)a θ (degree) 342.9 351.6 353.9 357.3 360.0 347.8 R (degree) 50 53 56 (59)b 71 aValue in parentheses is for the anti conformer (eq 1).bValue in
parentheses is for the second N-phenyl substituent vs N-phenyl of stilbene group.
Figure 1. UV-vis absorption spectra of 1a-f in hexane.
Table 2. Maxima of UV Absorption (λabs) and Fluorescence (λfl), Fluorescence-Band Half-Width (∆ν1/2), 0,0 Transition (λ0,0), and Stokes Shifts (∆νst) of Aminostilbenes 1-3 in Hexane and Acetonitrile
compd solvent λabs(nm)a λfl(nm)b ∆ν1/2 (cm-1) λ 0,0(nm) ∆νst (cm-1)c 1ad hexane 316 (361) 380 3580 354 5330 acetonitrile 318 423 3418 378 7806 1be hexane 347 379 (400) 3425 370 2433 acetonitrile 351 440 3306 394 5763 1c hexane 346 378 (397) 2813 369 2447 acetonitrile 351 436 3263 389 5554 1d hexane 352 385 (404) 2619 378 2435 acetonitrile 354 450 3753 395 6062 1e hexane 362 (295) 395 (417) 2475 387 2307 acetonitrile 362 (297) 455 3368 402 5646 1f hexane 341 375 (395) 3381 365 2659 acetonitrile 347 424 3370 384 5233 2af cyclohexane 336 390 4121 acetonitrile 342 446 6818 2c hexane 352 385 (405) 2729 376 2435 acetonitrile 358 456 3217 402 6003 2e hexane 358 (297) 403 (425) 2433 396 3119 acetonitrile 367 (294) 476 3370 414 6239 2f hexane 346 383 (402) 3169 373 2792 acetonitrile 351 450 3149 396 6268 3af cyclohexane 342 392 3730 acetonitrile 348 451 6562 3c hexane 355 388 (407) 3274 380 2396 acetonitrile 361 455 3353 406 5723 3e hexane 371 (301) 405 (426) 2781 396 2263 acetonitrile 371 (298) 489 3300 416 6504 3f hexane 350 385 (403) 3360 378 2597 acetonitrile 356 450 3526 398 5868
aThe second absorption band in parentheses.bThe second vibronic band
in parentheses.c∆ν
st) νabs- νfl.dFrom ref 5, except values for∆ν1/2 and∆νst.eFrom ref 16.fFrom ref 4.
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cases, the absorption spectra undergo only small shifts with changing solvent polarity, indicating a small difference between the dipole moments of the ground and Franck-Condon (FC) excited states.
The electronic structure and spectra of 1a-f have been further investigated by means of ZINDO calculations using the algorithm developed by Zerner and co-workers.30Ground-state
molecular structures from the AM1 calculations were adopted. The calculated energies and oscillator strengths of the lowest excited singlet states (S1) are reported in Table 3. Our
calcula-tions can reproduce the literature data for 1a.5In the cases of 1c, 1d, and 1f, data are reported for both syn and anti
conformers. The results indicate that both conformers have virtually the same electronic character. The relative ZINDO-derived S1transition energies (1a < 1f < 1b < 1c < 1d < 1e)
and oscillator strengths (1a∼ 1e < 1b < 1d < 1f < 1c) agree well with the observed absorption spectra (Figure 1), although the calculated transition energies for 1b are somewhat overes-timated. Similar discrepancies between the observed and ZINDO calculated transition energies have been reported11for other
N,N-dimethylaminoarenes.
The ZINDO-derived highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for
1a, 1c, and 1e are shown in Figure 2. More detailed orbitals of 1a have been documented,5and those of 1c and 1e are provided
in the Supporting Information. In all three cases, the HOMO is localized primarily on the aminostyrene moiety, but there is a progressive change on going from 1a to 1c to 1e, wherein the charge density is increased at the nitrogen atom but decreased at the central double bond.31On the other hand, the LUMO is
localized on the stilbene moiety with nearly the same appearance in all three cases. These results suggest that the HOMO f LUMO transition has an increased amine f styrene charge-transfer character and a decreased antibonding character for the central double bond on going from 1a to 1c to 1e. In addition, the introduction of N-phenyl substituents results in more extensive configuration interaction. Consequently, the contribu-tion of the HOMO f LUMO configuracontribu-tion to the descripcontribu-tion of S1is reduced from >95% in 1a to∼85% in 1e (Table 3).
The other configurations that contribute to S1in 1c and 1e are
transitions mainly localized in the diphenylamine or
triphenyl-amine moiety. The higher energy absorption band of 1e is calculated to be a consequence of S0f S4with configurations
all localized in the triphenylamine moiety (Table 3). The frontier orbitals of 1b and 1f resemble those of 1a, and those of 1d resemble those of 1c (figures not shown). Unlike the cases of
1c-e,31the N-phenyl group in 1f plays nearly no role in the
frontier orbitals. This can be attributed to the poor orbital overlap between the N-phenyl and the aminostilbene groups.
Fluorescence Spectra. The fluorescence maxima (λfl), the
half-bandwidth (∆ν1/2), the 0,0 transitions (λ0,0), and the Stokes
shift (∆νst) of aminostilbenes 1-3 in hexane and acetonitrile are reported in Table 2. The 0,0 transitions were estimated from the intersection of normalized absorption and fluorescence spectra. No discernible change in the shape of the fluorescence spectra of 1c-f was found when the excitation wavelength was varied from 290 to 370 nm.21In conjunction with the
single-exponential fluorescence decay (vide infra), it is concluded that either one conformer predominates or both conformers have the same excited state behavior in the cases of structurally unsym-metrical aminostilbenes 1c, 1d, and 1f. The same electronic structures are calculated by ZINDO for both syn and anti conformers of 1c, 1d, and 1f, in accord with the latter situation. While the fluorescence spectra of 2 are essentially independent of the excitation wavelength, small differences in the fluores-cence spectra can be observed in the cases of compound series
3.21On the basis of the well-established rotational isomerism
in the parent hydrocarbon of 3 (2-styrylnaphthalene),32this can
be attributed to the different spectra of conformers that differ in naphthyl-vinyl conformation.
The fluorescence spectra of 1c-f in hexane, which are shown in Figure 3 along with the spectra of 1a5 and 1b,16 display (31) The contribution of charge density of the N-phenyl substituent(s) to the
HOMO is∼10% in 1c and ∼25% in 1e, but their contribution to the LUMO is negligible for both cases.
(32) Saltiel, J.; Sears, D. F., Jr.; Choi, J.-O.; Sun, Y.-P.; Eaker, D. W. J. Phys. Chem. 1994, 98, 35-46 and references therein.
Table 3. Results of the ZINDO Calculations for the Lowest Singlet
States of 1a-f
compd excited state ∆E (nm) fa descriptionb
1ac S1 316 1.328 37:38 (0.95) 1b S1 319.4 1.335 43:44 (0.94) 1c S1 324.5 (324.4)d 1.443 (1.525)d 51:52 (0.89) 1d S1 329.5 (329.8)d 1.405 (1.478)d 54:55 (0.88) 1e S1 342.6 1.326 65:66 (0.85) 65:69 (0.07) S4 295.8 0.315 64:67 (0.10) 65:67 (0.49) 65:71 (0.17) 65:72 (0.05) 1f S1 319.3 (319.0)d 1.435 (1.478)d 57:58 (0.93) aOscillator strength.bOrbital numbers of the pure configurations are
given and the weight of each configuration is given in parentheses. Only configurations with 5% or greater contribution are included.cFrom ref 5. dValues in parentheses are for the anti conformer (eq 1).
Figure 2. ZINDO-derived HOMO and LUMO for 1a, 1c, and 1e. Only atomic charge densities with 5% or larger contribution are included.
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vibrational structure. The vibrational structure in all six amino-stilbenes is 1300 ( 80 cm-1, which is similar to that of trans-stilbene and can be attributed to the CdC stretching mode.2
The spectra in Figure 3 are normalized according to the 0,0 band and arranged in an order of increased intensity ratio of 0,0 vs 0,1 band. According to the Franck-Condon principle, compounds with a less extent of structural distortion on going from the ground state to the excited state show a more intense 0,0 band.33Thus, the increased transition probability of the 0,0
bands in 1c-e when compared with the other three species suggests that the structure of the fluorescent state, particularly in the region of the central double bond, is less distorted in
1c-e. Along this line, the fluorescence half-bandwidth is
narrower (Table 2). These spectral features are consistent with the results of ZINDO calculations, which suggest a reduced role for the central double bond in the lowest excited state of 1c-e. This might be associated with their higher torsional barriers and lower quantum yields for trans f cis photoisomerization (vide infra). Similar variations in the 0,0 vs 0,1 bands are also found for compound series 2 and 3 in hexane.21
Figure 4 shows the normalized fluorescence spectra of 1c in solvents of different polarity, which are typical for all amino-stilbenes. Whereas vibrational structure can be observed in hexane, the spectra are broadened in toluene and are completely structureless in more polar solvents. Unlike the absorption
spectra, the fluorescence maxima show a considerable red shift on going from hexane to acetonitrile, providing evidence for the charge-transfer character of the fluorescent singlet (CT*) state. The solvent-dependent shifts34can be used to determine
the dipole moment of the excited state using the Lippert-Mataga equation (eq 2).35
where
whereνabsandνflare the absorption and fluorescence maxima,
µgandµeare the ground and excited-state dipole moments, a is the solvent cavity radius in Å, is the solvent dielectric, and
n is the solvent refractive index. The ground-state dipole
moments (µg) are calculated using the ZINDO algorithm.30The
values of the solvatochromic slopes for 1c-f are reported in Table 4 along with the literature data for 1a and 1b. The excited-state dipole moments of 1a-f, calculated from eq 2, are summarized in Table 4. Whereas the N-phenyl derivatives 1c-f have smaller ground-state dipole moments when compared with
1a and 1b, their excited-state dipole moments are similar (1f)
or even larger (1c-e). The former is expected based on the sum of component bond moments, and the latter is consistent with the prediction of ZINDO calculations that suggest a larger charge-transfer character for the N-phenyl derivatives in the lowest excited state (Figure 2).
(33) (a) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood: Chichester, 1991, Chapter 2. (b) Parker, C. A. Photolumines-cence of Solutions; Elsevier: New York, 1968; pp 11-13.
(34) The Stokes shifts were determined in 10 solvents, including hexane, toluene, dichloromethane, diethyl ether, dibutyl ether, tetrahydrofuran, acetone, ethyl acetate, acetonitrile, and methanol,
(35) (a) Liptay, W. Z. Z. Naturforsch. 1965, 20a, 1441. (b) Lippert, E. Z. Elecktrochem. 1957, 61, 962-975. (c) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Chem. Soc. Jpn. 1956, 29, 465-470.
Figure 3. Fluorescence spectra of 1a-f in hexane.
Figure 4. Fluorescence spectra of 1c in (a) hexane, (b) toluene, (c) THF, (d) acetone, and (e) acetonitrile.
Table 4. Ground and Excited-State Dipole Moments for 1a-f
compd solvatochromic slope (cm-1)a µ
g(D)b µe(D)c 1a 6641d 2.0 10.1 1b 11254e 2.0 12.9 1c 8987 1.3 14.6 1d 9960 1.6 15.4 1e 9502 0.5 14.5 1f 7573 1.6 13.6
aCalculated based on eq 2.bCalculated by ZINDO algorithm.cValues
used for the radius of solvent cavity: 5 Å for 1a and 1b and 6 Å for 1c-1f.dFrom ref 5.eFrom ref 16.
∆νst) νabs- νfl) [2µe(µe- µg)/hca3]∆f
∆f ) ( - 1)/(2 + 1) - (n2- 1)/(2n2+ 1) (2)
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The temperature dependence of the fluorescence spectra of
1a-f was studied in hexane between 10 and 46°C. A significant reduction of the fluorescence intensity without changing the appearance of the spectra is observed for all aminostilbenes except 1e upon heating from 10 to 46 °C (Figure 5A). This suggests that there are activated nonradiative decay processes that compete with fluorescence decay for 1a-d and 1f. In the case of 1e, the fluorescence intensity is only slightly reduced and the spectra show a small blue shift (∆λ0,0) 2 nm) upon
heating from 10 to 46°C (Figure 5B). The noticeable spectral shift for 1e but not for the others with changing temperature indicates that the conformation of 1e is more dependent upon the temperature, presumably due to the bulkier triphenylamine group that becomes less planar at higher temperature. Accord-ingly, the small fluorescence intensity variations for 1e with changing temperature might be a consequence of temperature-induced conformational changes rather than the presence of weakly activated nonradiative decay processes.
Quantum Yields and Lifetimes. The fluorescence quantum
yields (Φf) for aminostilbenes 1c-f in hexane and acetonitrile
were measured (Table 5). Values ofΦffor 1c-e in hexane are
more than 1 order of magnitude larger than the values ofΦf∼
0.03 reported5,16for 1a and 1b. TheΦ
fvalues of 1c-e decrease
on going from hexane to acetonitrile, but the difference for 1e is relatively small. On the other hand, fluorescence enhancement is either small (hexane) or negligible (acetonitrile) for 1f vs 1a or 1b. Apparently, the geometry of the amino groups, defined by the θ and R values, is important in determining the fluorescence efficiency of the N-phenyl-substituted trans-4-aminostilbenes. TheΦfvalue of compounds 2 and 3 is more
sensitive to both amine substituents and solvent polarity than that of 1. However, the relative fluorescence efficiencies among compound series 2 and 3 are parallel to those observed for 1 with theΦfvalues in the order of e > c > f > a (Table 5).
Accordingly, the N-phenyl substituent effect on the fluorescence efficiency may be a general phenomenon for 4-amino-substituted stilbenes and 1,2-diarylethylenes.
Unconstrained trans-stilbene derivatives that have fluores-cence as the dominant mode of excited-state decay (Φf> 0.5)
at room temperature has been limited to those with a few categories of substituent.1-7,36 One category consists of 4,4′
-disubstituted trans-stilbenes with strong electron-donating amino and methoxy groups.7,17,36,37 The highly fluorescent 4,4′
-diaminostilbene and its derivatives have long been employed
as optical brighteners.38A second category is formed by
trans-stilbenes with extended conjugation by aromaticπ-substituents such as 4-phenylstilbene (4-styrylbiphenyl),8 4-styrylstilbene
(1,4-bis(styryl)benzene),94,4′-diphenylstilbene,39 and
2-stryl-naphthalene (3, A ) H).32 The third category was recently
established for 3-amino-substituted trans-stilbenes and was attributed to “the m-amino effect”.4-6The N-phenyl derivatives 1c-e apparently represent a new category of strongly fluorescent trans-stilbenes. However, the “conjugated” nature of
N-phenyl-amino and N,N-diphenylN-phenyl-amino groups could be considered as “pseudo-π-substituents” belonging to the second category. In this context, it appears that resonance effects are more important than the electronic inductive effect of substituents in enhancing the fluorescence quantum yields of 4-monosubstituted trans-stilbenes.
Quantum yields for trans f cis photoisomerization (Φtc) of 1c-f in hexane are reported in Table 5. Values ofΦtcfor 1c-e
are smaller than the values of Φtc ∼ 0.5 reported for
trans-stilbene and 1a.5The Φ
tcvalue of 1f is larger than those of 1c-e but smaller than that of 1a, which is consistent with its
intermediateΦfvalue in hexane. Assuming that the decay of
the perpendicular p* state yields a 1:1 ratio of trans and cis isomers,3the sum of the fluorescence and isomerization quantum
yields (Φf+ 2Φtc) for 1c, 1d, and 1f in hexane is within the (36) Smit, K. J.; Ghiggino, K. P. Chem. Phys. Lett. 1985, 122, 369-374. (37) Zeglinski, D. M.; Waldeck, D. H. J. Phys. Chem. 1988, 92, 692-701. (38) Meier, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 1399-1420 and references
therein.
(39) Tan, X.; Gustafson, T. L. J. Phys. Chem. A 2000, 104, 4469-4474. Figure 5. Temperature dependence of the fluorescence spectra of 1c (A)
and 1e (B) in hexane recorded at an interval of 4°C between 10 and 46°C (top to bottom with increasing the temperature).
Table 5. Quantum Yields for Fluorescence (Φf) and
Photoisomerization (Φtc), Fluorescence Decay Times (τf), Rate Constants for Fluorescence Decay (kf) and Nonradiative Decay (knr), and Activation Energies (Ea) for Nonradiative Decay for 1-3
compd solvent Φf Φtc τf (ns) kf (108s-1) knr (108s-1) Ea (kcal/mol)f 1aa hexane 0.03b 0.49c ∼0.1c 3.0b 97b 3.6b acetonitrile 0.03 0.52 ∼0.1 3.0 97 1bd hexane 0.03 ∼0.1 3.0 97 3.6b acetonitrile 0.04 ∼0.1 4.0 96 1c hexane 0.51 0.31h 0.34 15 14 4.9 acetonitrile 0.30 0.39 7.7 18 1d hexane 0.64 0.18h 0.62 10 5.8 4.7 acetonitrile 0.31 1.86 1.7 3.7 1e hexane 0.57 0.14h 0.51 11 8.4 0.87 (1.9)g acetonitrile 0.53 1.37 3.9 3.4 1f hexane 0.11 0.43 ∼0.1 11 89 4.3 acetonitrile 0.04 ∼0.1 4.0 96 2ae cyclohexane 0.02 0.22 0.91 45 acetonitrile 0.03 0.18 1.7 54 2c hexane 0.37 0.29 13 22 acetonitrile 0.25 0.54 4.6 14 2e hexane 0.55 0.56 9.8 8.0 acetonitrile 0.39 1.52 2.6 4.0 2f hexane 0.08 ∼0.1 8.0 92 acetonitrile 0.05 ∼0.1 5.0 95 3ae cyclohexane 0.08 0.20 4.0 40 acetonitrile 0.05 0.23 2.2 41 3c hexane 0.46 0.34 14 16 acetonitrile 0.28 0.43 6.5 17 3e hexane 0.61 0.40 15 9.8 acetonitrile 0.42 1.09 3.9 5.3 3f hexane 0.20 acetonitrile 0.08 0.20 4 46
aFrom ref 5, unless otherwise noted.bFrom this work.cIn cyclohexane,
from ref 5.dFrom ref 16, unless otherwise noted.eFrom ref 4.fCalculated
based on eq 3 assuming kisc/kf∼ 0.gBased on Ifat 396 nm and the value in parentheses is obtained from kisc/kf) 0.42.hContaining 10% of THF by reason of solubility.
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experimental error of 1.0, as is the case of 1a, which suggests that other channels of nonradiative decay are negligible. Although the sum is somewhat lower than unity (0.85) for 1e, other nonradiative decay pathways such as internal conversion6
should be of relatively minor importance.
The fluorescence lifetimes (τf) of the aminostilbenes in both
hexane and acetonitrile at room temperature are shown in Table 5. All decays can be well fit by single-exponential functions, although dual exponential fluorescence decay might be expected for 3 on the basis of the excitation wavelength-dependent fluorescence spectra.21A decay component comparable to or
much shorter than those reported in Table 5 for 3 would not be resolved by our lifetime apparatus, which has a resolution of ∼0.1 ns.40In accord with the larger fluorescence quantum yields,
the lifetimes of 1c-e are longer than those of 1a, 1b, and 1f. However, unlike theΦfvalues, theτfvalues of 1c-e are larger
in acetonitrile vs hexane. Consequently, the fluorescence rate constants (kf ) Φfτ-1) are smaller in acetonitrile (Table 5).
The fluorescence rate constants of 1c-f, when compared with those of 1a, are larger in hexane but similar in acetonitrile. The overall nonradiative deactivation (knr ) 1/τf - kf) was also
calculated. Values of knrare lower for 1c-e than 1a, 1b, and 1f, which might account for the longer fluorescence lifetimes
and the largerΦfvalues in the former cases.
Torsional Barriers. The temperature dependence of the
fluorescence spectra (Figure 4) allows estimation of the activa-tion energy (Ea) for nonradiative decay according to the
equation41,42
where Ifoand Ifare the limiting fluorescence intensity and the
fluorescence intensity measured at temperature T (K), kiscis the
rate constant for intersystem crossing, and A0is the Arrhenius
constant. When intersystem crossing is negligible (kisc/kf∼ 0)
in the excited singlet state decay, a linear Arrhenius plot of ln[(Ifo/If) - 1] against 1/T will give an Eavalue (slope ) -Ea/R)
for the nonradiative decay process. This plot requires several assumptions, including (1) constant solvent properties and kf values for aminostilbenes 1a-f over the temperature range of 10-46 °C, (2) a value of 1.0 for the limiting fluorescence quantum yield (Φfo) 1.0), and (3) Ifo) (Φfo/Φf)If(295 K).
The Arrhenius plots for 1a-f are shown in Figure 6, and the corresponding Eavalues are reported in Table 5. The Eavalue
of 3.6 kcal/mol provided by this method for 1a in hexane is reasonably consistent with the value of Ea ∼ 3.5 kcal/mol
previously determined based on the temperature-dependent lifetime data.5In addition, the E
avalue of 4.9 kcal/mol and knr
value of ∼1.4 × 109 for 1c in hexane corresponds to an
Arrhenius constant (A0) of∼6 × 1012, which agrees with the
literature values of A0 (1012-1014) for alkene isomerization.3
The temperature-induced fluorescence shift in 1e results in different intensity variations at different emission wavelengths (e.g., intensity decreased at 296 nm but increased at 308 nm) (Figure 5B). Thus, unlike those of 1a-d and 1f, the Eavalues
reported in Table 5 for 1e should not be given much weight.
Differences in the mechanism of photoisomerization of 1a-f are indicated by their Eavalues. The singlet-state mechanism
of photoisomerization for trans-stilbene is attributed to both the small rate constant for intersystem crossing (kisc) 3.9 × 107
s-1) and low barrier (∼3.5 kcal/mol) for singlet-state ethylene torsional relaxation.3Aminostilbenes 1a and 1b have values of Ea,Φtc,Φf, and knrall similar to trans-stilbene and thus have
been assumed to undergo photoisomerization mainly via the singlet excited state.1-3,5,16The singlet-state mechanism can also
account for the smaller values ofΦtcfor 1c, 1d, and 1f in hexane
on the basis of the larger Eavalues and the fact that intersystem
crossing is normally an unactivated process for stilbenes.1-3,41
Since intersystem crossing does not appear to compete with fluorescence and singlet torsion for aminostilbenes 1a-d and
1f,Φfshould attain a value of 1.0 for them at low temperatures.
This validates one of the assumptions required for estimating the Eavalues by the plots shown in Figure 6. On the other hand,
the value of Eafor 1e, if present, is too small to account for the
lowΦtcand highΦfvalues by the singlet-state mechanism. It
is thus concluded that 1e undergoes photoisomerization via the triplet-state pathway, presumably due to the presence of a high excited singlet state torsional barrier. Assuming that the twisted triplet (3p*) decays to yield a 1:1 ratio of trans and cis isomers,
quantum yields and rate constants for intersystem crossing can be estimated from the measured isomerization quantum yields and lifetimes (Φisc) 2Φtc, kisc) Φiscτf-1). The calculated value
of kiscfor 1e in hexane is 5.5× 108s-1, which lies between the
values for trans-stilbene (3.9 × 107s-1) and
trans-4-bromo-stilbene (5.0× 109s-1).3The larger value of k
iscfor 1e vs
trans-stilbene might be a consequence of the reduced S-T energy gap, since the energy of the 1t* state of 1e is∼14 kcal/mol
lower than that of trans-stilbene based on their 0,0 transition energies (Table 2). Although an Arrhenius plot of ln[(Ifo/If)
-1.42] against 1/T (eq 3) might be more appropriate for 1e than the one shown in Figure 6 due to the conclusion of a triplet-state photoisomerization mechanism, this does not provide new information. As mentioned above, the observed small temper-ature dependence of fluorescence spectra for 1e might be a consequence of weakly activated process of conformational changes, since the amine conformation defined by the param-etersθ and R strongly affects the fluorescence quantum yields and excited singlet state energies of N-phenyl substituted stilbenes, as is demonstrated by the dramatic difference inΦf
andλflfor 1c vs 1f. This argument is consistent with the fact of (40) The results of biexponential fitting might not be reliable except for the
case of 3e in acetonitrile, which providesτ1) 0.55 ns (11%) and τ2)
1.15 (89%).
(41) Saltiel, J.; Marinari, A.; Chang, D. W.-L.; Mitchener, J. C.; Megarity, E. D. J. Am. Chem. Soc. 1979, 101, 2982-2996.
(42) Sakurovs, R.; Ghiggino, K. P. Aust. J. Chem. 1981, 34, 1367-1372.
ln(Ifo/If- 1 - kisc/kf) ) ln(A0/kf) - Ea/RT (3)
Figure 6. Arrhenius plots of the fluorescence efficiencies for 1a-f.
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Φf+ 2Φtc< 1 for 1e, which suggests the presence of a minor
nonradiative decay pathway other than intersystem crossing. The photochemical behavior of aminostilbenes 1a-f is summarized by a simplified scheme shown in Figure 7.
Apparently, the large increase in fluorescence quantum yields for 1c,d vs 1a,b results not only from the modest increase in the singlet torsional barrier (∆Ea ) 1.1-1.3 kcal/mol), but
also from the increased radiative decay rate constants (Φf ) kf/(kf+ knr)). Likewise, the triplet-state photoisomerization for 1e results not only from the large barrier for singlet torsion,
but also from the increased rate constant for intersystem cross-ing, which is comparable with that for fluorescence (Table 5). A similar situation has recently been proposed for 4,4′ -diamino-stilbene and its tetramethyl derivative.7Photoisomerization via
the triplet state as a consequence of a large barrier for singlet torsion was reported for 3-aminostilbene and its disubstituted derivatives;5-7however, the comparable values of k
fand kiscin
these cases result from decreased rate constants for fluorescence instead of increased rate constants for intersystem crossing. Such an origin of triplet-state photoisomerization for aminostilbenes is apparently different from that for some nitro-, bromo-, or carbonyl-substituted trans-stilbenes, which have large rate constants for intersystem crossing (kisc ∼109-1010 s-1) as a
result of either the introduction of a low-lying n,π* state or an internal heavy atom effect.1-3
The increase of barrier height for singlet torsion in 1c-e suggests that the introduction of the N-phenyl group lowers the energy of the 1t* state more than that of 1p*.5 The relative
energies of1t* can be estimated from the position of their 0,0
transitions (Table 2). When compared with that of 1a, the 0,0 energies of 1c-f are lower by approximately 3, 5, 7, and 2 kcal/mol, respectively. Estimation of the energy of the1p* state
is not so straightforward. The nature of the1p* state for single
alkenes is not well defined but has been described as a resonance hybrid of biradical and charge-transfer configurations (eq 4).43
An electron-donating or electron-withdrawing substituent at the 4-position of stilbene would be expected to stabilize one of the two charge-transfer configurations and thus lower the energy of1p*. For trans-stilbene, the1t* and1p* states have been shown
to be approximately isoenergetic.44The similar quantum yields
of fluorescence and photoisomerization for trans-stilbene, 1a, and 1b,3,5,16 thus indicate that both amino and
N,N-dimethyl-amino substituents stabilize the1t* and 1p* states to a similar
extent. Accordingly, the N-methyl (or more likely N-alkyl in general) groups stabilize the1p* state as well as the1t* state,
whereas the N-phenyl groups stabilize the1t* state more than
the1p* state.
The larger stabilization of N-phenyl substituents to the1t*
vs1p* states of 4-aminostilbenes can be rationalized by a model
based on Lewis resonance structures, which was originally proposed to rationalize the large singlet torsional barrier (>7 kcal/mol) for 3-amino-substituted trans-stilbenes.5,6 As is
depicted below for 1c, the N-phenyl substituent provides an additional resonance structure for the planar1t* state (eq 5) but
not for the twisted1p* state (eq 6).
Likewise, there are two more resonance structures for the
1t* but not the1p* state in the case of 1e, which further lowers
the energy of1t* and further raises the singlet torsional barrier.
The lower 1t* state of 1d vs 1c is a consequence of the
introduction of the N-methyl group. Since an N-methyl group provides a similar extent of stabilization to both1t* and 1p*
states, it is as expected that the singlet torsional barrier for 1c and 1d should be similar (Table 5). The different effects of the
N-phenyl vs the N-methyl substituent on the1p* state are further
supported by comparison of 1b and 1c. The absorption maxima and 0,0 transition (1t*) energies of 1b and 1c are essentially
identical, but their singlet torsional barrier and fluorescence quantum yields in hexane are quite different (Table 5). Figure 8 depicts the potential energy for ethylene bond torsion in the lowest excited state for 1a-f in hexane.
Concluding Remarks
The strongly fluorescent nature of the N-phenyl and N,N-diphenyl derivatives of trans-4-aminostilbene is borne out by our detailed studies on the excited-state behavior of aminostil-benes 1c-f. The introduction of N-phenyl substituents extends the “conjugation” length of aminostilbenes and lowers the energy of the fluorescent1t* state, but it does not provide the
same amount of resonance stabilization to the twisted1p* state.
Consequently, the N-phenyl substituents stabilize the1t* state (43) Bonacˇic´-Koutecky´, V.; Ko¨hler, J.; Michl, J. Chem. Phys. Lett. 1984, 104,
440-443.
(44) Saltiel, J.; Waller, A. S.; Sears, D. F., Jr. J. Am. Chem. Soc. 1993, 115, 2453-2465.
Figure 7. Simplified scheme for the formation and decay of the fluorescent CT* (1t*) state of aminostilbenes 1a-f.
ψp*) c1ψbiradical (A•B•)+ c2ψCT (A+B-)+ c3ψCT (A-B+) (4)
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more than the1p* state, leading to an increase of the singlet
torsional barrier and thus a decrease of rate constants for photoisomerization. It is interesting to note that relatively small increases in the singlet torsional barrier (<1.5 kcal/mol) result in a large increase inΦfand decrease inΦtc. Since the amine
nitrogen lone pair mediates the interactions between the
N-phenyl and the stilbene groups, the geometry of the amine
moiety determines the extent of “conjugation” interactions. For previously reported 4-monosubstituted trans-stilbenes, only π-conjugated substituents such as phenyl8,39and styryl9groups
can significantly enhance the fluorescence quantum yields. Our results indicate that the N-phenylamino and N,N-diphenylamino groups behave as “nonconventional”π-conjugated substituents. Thus, we attribute the fluorescence enhancement in trans-4-aminostilbene upon N-phenyl substitutions to “the amino conjugation effect”. Indeed, the concept of the amino conjuga-tion effect has recently gained much attenconjuga-tion in materials chemistry from the comparison of the properties of arylamines with N-phenyl vs N-alkyl substitutions.10-14Since fluorescence
efficiency is an important issue for the development of organic light-emitting materials, the fluorescence enhancement of 1c-e vs 1b provides a new example showing the superior performance of N-phenyl vs N-alkyl substituents.
Insights into the amino conjugation effect on both photo-isomerization and fluorescence have been gained from our systematic investigation of the amine substituents a-f. The
1t* states of 1a-f are all calculated to be of relatively pure
HOMO f LUMO character. However, extending the conjuga-tion of the whole molecular system with N-phenyl substituents imposes charge redistribution, leading to decreased charge densities at the central ethylene portion in the HOMO and thus reduced antibonding character for the central ethylene bond in the excited singlet state. One line of evidence is nicely provided by the progressive changes on the vibronic structures of fluorescence spectra of 1a-f in hexane, which suggests a lesser extent of structural distortion for the strongly fluorescent
N-phenyl derivatives 1c-e in the excited state. Extending the
conjugation to the N-phenyl substituents also increases the charge-transfer character in the1t* state on going from 1a to 1c to 1e. Such differences in the1t* states of 1a-f should be
strongly associated with the efficiencies of both photoisomer-ization and fluorescence. It should be noted that the correlation between the relative fluorescence intensity ratio of the 0,0 to the 0,1 band and the excited-state structures of trans-stilbenes might be appropriate for a series of related substituents but not for those with different types or positions of substituents. In
addition, the decreased decay process of singlet torsion might not be completely compensated by enhanced fluorescence, because other nonradiative decay pathways can compete with fluorescence. A switch of photoisomerization pathway from the singlet-state mechanism to the triplet-state one on going from
1c and 1d to 1e clearly demonstrates this point. As a result of
intersystem crossing, the second N-phenyl substituent in 1e does not result in a larger fluorescence enhancement than the
N-methyl group in 1d.
Although less information is available for the stilbenoid systems 2 and 3, the discussion for 1 might also apply to these two cases on the basis of their similar and parallel spectroscopic properties. We are currently investigating the N-phenyl sub-stituent effect on the excited-state behavior of several other stilbenoid systems. Our preliminary results indicate that fluo-rescence enhancement by N-phenyl substitutions is a general phenomenon for trans-4-aminostilbenoid systems. Such a strongly fluorescent, conformation-dependent nature of N-phenyl substituent effect might be useful for the design of new stilbene-based fluorescent probes45and light-emitting materials.19,28a,46 Experimental Section
Methods.1H NMR and13C NMR spectra were recorded in CDCl 3
solution using a Bruker DRX-200 spectrometer with TMS as internal standard. Infrared spectra were recorded on a Bruker VECTOR 22 spectrometer. Elemental analyses and mass spectra were determined by the Instrumentation Center of National Cheng-Kung University or by that of National Taiwan University. UV spectra were measured on a Jasco V-530 double beam spectrophotometer. Fluorescence spectra were recorded on a PTI QuantaMaster C-60 spectrofluorometer at room temperature. A cell holder that allows for internal circulation was connected to a circulating bath with temperature variation of
(0.05°C and used to obtain fluorescence spectra in the 10-46 °C
temperature range. Anthracene (Φf) 0.27 in hexane)47was used as
standard for the fluorescence quantum yield determinations (λex)
338 nm) with solvent refractive index correction. The optical density of all solutions was about 0.1 at the wavelength of excitation. All fluo-rescence spectra are uncorrected and an error of (10% is estimated for the fluorescence quantum yields. Fluorescence decays were measured at room temperature by means of an Edinburgh photon counting apparatus (OB900-14A) with a gated hydrogen arc lamp using a scatter solution to profile the instrument response function. The goodness of
nonlinear least-squares fit was judged by the reducedχ2value (<1.3
in all cases), the randomness of the residuals, and the autocorrelation function. Quantum yields of photoisomerization were measured on
optically dense degassed solutions (∼10-3M) at 313 nm using a 75 W
Xe arc lamp and monochromator. trans-Stilbene was used as a reference
standard (Φtc) 0.50 in hexane).48The extent of photoisomerization
(<10%) was determined using HPLC analysis (HP1100, Whatman Partisil M9, 10/25, solvent gradient starting with hexane and ending at hexane:dichloromethane ) 1:2). The reproducibility error was <15% of the average. MOPAC-AM1 and INDO/S-CIS-SCF (ZINDO) calculations were performed on a personal computer using the algorithms supplied by the package of Quantum CAChe Release 3.2,
(45) For examples of stilbene-based probes, see: (a) Lednev, I. K.; Hester, R. E.; Moore, J. N. J. Am. Chem. Soc. 1997, 119, 3456-3461. (b) Le´tard, J. F.; Lapouyade, R.; Rettig, W. Pure Appl. Chem. 1993, 65, 1705-1712. (c) Lo¨hr, H.-G.; Vo¨gtle, F. Acc. Chem. Res. 1985, 18, 65-72. (46) Examples of stilbene-based light-emitting dendrimers: (a) Dı´ez-Barra, E.;
Garcı´a-Martı´nez, J. C.; Ria´nsares del Rey, S. M.; Rodrı´guez-Lo´pez, J.; Sa´nchez-Verdu´, P.; Tejeda, J. J. Org. Chem. 2001, 66, 5664-5670. (b) Segura, J. L.; Go´mez, R.; Martı´n, N.; Guldi, D. M. Org. Lett. 2001, 3, 2645-2648. (c) Pillow, J. N. G.; Halim, M.; Lupton, J. M.; Burn, P. L.; Samuel, I. D. W. Macromolecules, 1999, 32, 5985-5993.
(47) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970.
(48) Malkin, S.; Fischer, E. J. Phys. Chem. 1964, 68, 1153-1163. Figure 8. Potential energy diagram for ethylene bond torsion in the lowest
excited state for 1a-f in hexane. Energies are relative to1t* for 1a.
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a product of Fujitsu Limited. The synthesis and characterization of compounds 1-3 are provided in Supporting Information.
Acknowledgment. Financial support for this research was provided by the National Science Council of Taiwan, ROC. We greatly appreciate Professor K. C. Hwang (NTHU) and Miss L. A. Dai for obtaining the lifetime data, Professor W.-R. Lee (NCU) for using his HPLC apparatus, Professor F. D. Lewis (Northwestern University) for helpful discussions, and the reviewers for helpful comments.
Supporting Information Available: Detailed synthetic pro-cedures and product characterization data, ZINDO calculated frontier orbitals for 1c and 1e, Lippert-Mataga plots according to eq 2 for 1c-f, fluorescence spectra of 1-3 in hexane recorded with different excitation wavelengths, and fluorescence spectra of compound series 2 and 3 in hexane (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
JA016416+
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