Synthesis and Properties of Novel
Thiophene-Based Conjugated
Homologues:
9,9-Diphenylfluorene-Capped
Oligothiophenes
Ken-Tsung Wong,* Chung-Feng Wang, Chung Hsien Chou, Yuhlong Oliver Su,
Gene-Hsiang Lee, and Shie-Ming Peng
Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan [email protected]
Received September 30, 2002
ABSTRACT
A series of novel 9,9-diarylfluorene-capped oligothiophenes were synthesized by Suzuki coupling reactions in good yields. The color of the emissions can be controlled by varying the conjugation length of the oligothiophene core. The bulky and rigid terminal groups of the resulting oligomers are significantly beneficial for their high morphological and thermal stability. These new oligothiophenes exhibit intriguing reversible oxidation and reduction redox behavior.
Oligothiophenes with well-defined structures are of great interest due to their unique electronic properties. Oligoth-iophenes have been used in many applications as active materials, including, for example, field-effect transistors and light-emitting diodes.1The primary properties of crystalline oligothiophenes can be modified or improved upon by introducing pendant groups into the conjugated backbone.2 The strategy of introducing different terminal groups to cap oligothiophenes has been used to diversify the structure of a conjugated backbone, giving a new possibility of efficiently tuning the electronic and optical properties. For example, oligothiophenes terminated with phenyl or biphenyl groups3
exhibit interesting electroluminescent behavior.4 Oligoth-iophenes with terminal groups bearing diarylamino functional groups recently have been used as efficient emitters5 and have been reported to be potential hole-transporting materi-als.6The introduction of planar fluorene moieties as the end groups of bithiophene facilitates the π-π stacking of the crystals, resulting in the formation of new materials that exhibit extremely high field-effect mobility.7Other oligo-thiophene end-capping groups, including diarylboryl,8
(1) (a) Mu¨llen, K.; Wegner, G. Electronic Materials: The Oligomer
Approac; Wiley-VCH: Weinheim, Germany, 1998. (b) Fichu, D. Handbook of Oligo- and Polythiophenes; Wiley-VCH: Weinheim, Germany, 1999.
(2) (a) Adronov, A.; Malenfant, P. R. L.; Fre´chet, J. M. J. Chem. Mater. 2000, 12, 1463. (b) Wu, R.; Schumm, J. S.; Pearson, D. L.; Tour, J. M. J.
Org. Chem. 1996, 61, 6906.
(3) (a) Hotta, S.; Lee, S. A.; Tamaki, T. J. Heterocycl. Chem. 2000, 37, 25. (b) Hotta, S.; Lee, S. A.; Tamaki, T. J. Heterocycl. Chem. 2000, 37, 281. (c) Pelter, A.; Jenkins, I.; Jones, D. E. Tetrahedron 1997, 53, 10357.
(4) (a) Yanagi, H.; Morikawa, T.; Hotta, S. Appl. Phys. Lett. 2002, 81, 1512. (b) Yanagi, H.; Morikawa, T.; Hotta, S.; Yase, K. AdV. Mater. 2002,
13, 313.
(5) Noda, T.; Ogawa, H.; Noma, N.; Shirota, Y. J. Mater. Chem. 1999,
9, 2177.
(6) (a) Kisselev, R.; Thelakkat, M. Chem. Commun. 2002, 1530. (b) Wong, K.-T.; Hung, T. H.; Kao, S. C.; Chou, C. H.; Su, Y. O. Chem.
Commun. 2001, 1628. (c) Hartmann, H.; Gerstner, P.; Rohde, D. Org. Lett.
2001, 3, 1673. (d) Noda, T.; Ogawa, H.; Noma, N.; Shirota, Y. AdV. Mater. 1999, 9, 720. (e) Noda, T.; Imae, I.; Noma, N.; Shirota, Y. AdV. Mater. 1999, 9, 239.
(7) Meng, H.; Bao, Z.; Lovinger, A. J.; Wang, B.-C.; Mujsce, A. M. J.
Am. Chem. Soc. 2001, 123, 9214.
(8) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714.
ORGANIC
LETTERS
2002
Vol. 4, No. 25
4439-4442
10.1021/ol027001b CCC: $22.00 © 2002 American Chemical Society Published on Web 11/09/2002
Downloaded by NATIONAL TAIWAN UNIV on August 18, 2009
pyridyl,9diphenylphosphine,10and groups capable of charge
transfer,11 have also been documented. We report in this
communication a novel series of color-tunable amorphous emitters based on oligothiophene moieties with varying conjugation lengths as core chromophores. Bulky and rigid 9,9-diphenylfluorene was introduced as the terminal group to prevent intermolecular interactions and improve the morphological and thermal stability of the resulting oligo-mers.
The synthesis of 9,9-diphenylfluorene-capped oligoth-iophenes is illustrated in Scheme 1.
Treatment of 2-bromo-9,9-diphenylfluorene (1) with n-BuLi at -78°C, followed by quenching the lithiated complex with 2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane, afforded the boronic ester 2 in 82% yield. Pd-catalyzed Suzuki coupling reaction between 2 and dibromooligoth-iophen 3 (n ) 1-4) was carried out in the presence of a catalytic amount of PtBu
3 as a cocatalyst.12 The resulting
9,9-diphenylfluorene-capped oligothiophenes FTn (n ) 1-4) were isolated in good yields (Table 1) with satisfactory spectroscopic characterizations.
The single-crystal X-ray structure analysis of FT2 is shown in Figure 1. The bithiophene core of FT2 adopts a nearly
planar conformation with an interplanar twist angle of 1.0°, while the two sulfur atoms are in an unusual cis configura-tion. In contrast, the bulky fluorene moieties are in an anti configuration and are highly twisted from the planar bithiophene core with interplanar twist angles of -21.5 and 151.4°, respectively. The antiparallel arrangement between the two neighboring molecules is beneficial for better crystal packing. However, the bulkiness of the terminal 9,9-diphenylfluorene moieties plays an important role in prevent-ing intermolecular interactions. The closest plane-to-plane distance between the two neighboring planar bithiophene cores is calculated to be ca. 3.9 Å; thus, the intermolecular
π-π interactions in the crystal are negligible.
The electronic absorption behavior of the 9,9-diphe-nylfluorene-capped oligothiophenes FTn exhibits a perfect correlation to the conjugation length, i.e., the absorption maximum is significantly red-shifted as the conjugation length of the respective oligothiophene core increases (See Supporting Information for the spectra). The structureless broad absorption indicates that the central oligothiophenes are relatively flexible for the formation of different conform-ers in the ground state. All 9,9-diphenylfluorene-capped oligothiophenes FTn in this study are highly fluorescent. The quantum yields of the fluorescence in a dilute solution of (9) Hock, J.; Cargill Thompson, A. M. W.; McCleverty, J. A.; Ward,
M. D. J. Chem. Soc., Dalton Trans. 1996, 4257.
(10) Field, J. S.; Haines, R. J.; Lakoba, E. I.; Sosabowski, M. H. J. Chem.
Soc., Perkin Trans. 1 2001, 3352.
(11) (a) Apperloo, J. J.; Langeveld-Voss, B. M. W.; Knol, J.; Hummelen, J. C.; Janssen, R. A. J. AdV. Mater. 2000, 12, 908. (b) Wurthner, F.; Vollmer, M. S.; Effenberger, F.; Emele, P.; Meyer, D. U.; Port, H.; Wolf, H. C. J.
Am. Chem. Soc. 1995, 117, 8090.
Table 1. Chemical Yields and Physical Properties of 9,9-Diphenylfluorene-Capped Oligothiophenes FTn
FTn yield (%) λmax(nm)a; log PL λmax (nm)a Q (%)b E1/2ox (V)c E1/2red (V)d Tg (°C)e Td (°C)f FT1 75 374; 4.93 424, 447 52 1.19, 1.60 -1.87, -2.11 153 393 FT2 71 404; 4.80 465, 493 26 1.01, 1.36 -1.80, -2.02 153 483 FT3 82 425; 4.85 494, 528 35 0.86, 1.19 -1.76, -1.94 154 448 FT4 85 440; 4.66 515, 545 32 0.81, 1.09 -1.65, -1.79 153 449 aMeasured in CHCl
3; fluorescence was recorded by irradiating at the absorption maximum.bMeasured in ethyl acetate with Coumarin I as a standard.
cIn CH
2Cl2(0.1 M nBu4NPF6as a supporting electrolyte).dIn THF (glassy carbon electrode, 0.1 M nBu4NClO4as a supporting electrolyte).eBy DSC
analysis of the liquid nitrogen-quenched melt sample.fBy TGA analysis; the temperature was recorded corresponding to a 10% weight loss.
Scheme 1
Figure 1. Crystal packing diagram of FT2.
4440 Org. Lett., Vol. 4, No. 25, 2002
Downloaded by NATIONAL TAIWAN UNIV on August 18, 2009
ethyl acetate range from 0.26 to 0.52 (Table 1). The photoluminescence (PL) features reveal typical characteristics of conjugated fluorene derivatives.13More importantly, the
emission maxima (Figure 2) of FTn can be controlled by
varying the number n (n ) 1-4) of the oligothiophene moieties of the conjugated backbone. The color of the fluorescence can be achieved from light blue to bright yellow (Figure 2, inset).
Cyclic voltammetry (CV) experiments were conducted on fluorene-capped oligothiophenes FTn at room temperature to probe their electrochemical properties. For FTn, two discrete reversible oxidation potentials were observed in CH2
-Cl2(0.1 M nBu4NPF6as a supporting electrolyte).
Unexpect-edly, two reversible reduction potentials were also detected in THF (glassy carbon electrode, 0.1 M nBu4NClO4 as a
supporting electrolyte) (Table 1). This finding indicates the possibility that an n-doping property is induced in the oligomers when 9,9-diphenylfluorenes are incorporated into the oligothiophenes as the terminal groups. Figure 3 shows a comparison of the cyclic voltammogram of FT2 and FT4. The oxidation potentials tend to decrease as the chain of the oliogothiophene core is lengthened, while FTn with shorter oligothiophene cores exhibit more negative reduction po-tentials. The oligomer with the longer conjugation length
shows a smaller potential difference between two redox processes. However, the pronounced potential differences between the two oxidations (410-280 mV) and two reduc-tions (240-140 mV) indicate that the monocationic and monoanionic species could efficiently delocalize the charge over the entire conjugated backbone. These results are consistent with the significant red shifts in the absorption bands. The increasing conjugation length is due to an increasing delocalization of theπ-electron system along the
backbone.
Figure 4 shows the spectral response of FT2 at various applied voltages. Between 0 and 1.00 V, the intensity of absorption at 405 nm of the neutral FT2 decreased, while new peaks grew at 609, 680, 1022, and 1202 nm. Upon increasing the applied voltage, the intensity of these new peaks was continuously enhanced with an isosbestic point at 460 nm (Figure 4a). These new forming visible bands (609 and 680 nm) and strong near-IR absorption bands (1022 and 1202 nm) were ascribed to the absorption of the first oxidation state. When the applied voltages were above 1.00 mV, the intensity of the long wavelength peaks started to diminish in intensity, while a new peak appeared at 351 nm with increasing intensity until the applied voltage reached 1.40 V. These spectral changes could be fully restored when the voltage was cycled between 0.0 and 1.40 V. The spectral changes at various applied voltages were in agreement with the CV experiments, in which the first and second oxidation potentials of FT2 were observed at 1.01 and 1.36 V, respectively. The spectroelectrochemistry indicated that the oxidation only involves a one-step one-electron redox process. In contrast, the spectral response of FT2 in the reduction region by using platinum gauze or reticulated vitreous carbon as a working electrode did not exhibit any isosbestic point(s) (Supporting Information). For the latter case, the intensity of a new broad absorption (580 nm) increased upon increasing the negative voltage from -0.55 to -2.12 V. This result suggests that the radical anion is relatively more reactive during the electrolysis as compared to its radical cation.
(12) Representative Procedure for Synthesis of FT2. 9,9-Diphenylfluo-rene-2-pinacol boronate (2) (930 mg, 2.1 mmol), 5,5′-dibromo-2,2′ -bithiophene (3, n ) 2) (324 mg, 1.0 mmol), Pd(PPh3)4(23 mg, 0.02 mmol),
2 M K3PO4(2 mL), 1,4-dioxane (20 mL), and tri-tert-butylphosphine (0.05
M in toluene, 1.2 mL, 0.06 mmol) were stirred and refluxed under argon for 2-3 days. The cooled reaction mixture was quenched with saturated sodium bicarbonate and extracted with chloroform. The combined organic extracts were dried over MgSO4and concentrated by rotary evaporation.
The crude product after washing with hot hexane was recrystallized from CHCl3/hexanes to afford FT2 (567 mg, 71%) as a yellow solid (see the
Supporting Information for spectroscopic characterization).
(13) (a) Katsis, D.; Geng, Y. H.; Ou, J. J.; Culligan, S. W.; Trajkovska, A.; Chen, S. H.; Rothberg, L. J. Chem. Mater. 2002, 14, 1332. (b) Belleteˆte, M.; Ranger, M.; Beaupre´, S.; Leclerc, M.; Durocher, G. Chem. Phys. Lett.
2000, 316, 101.(c) Weinfurtner, K.-H.; Weissortel, F.; Harmgarth, G.;
Salbeck, J. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3476, 40.
Figure 2. Photoluminescent spectra of FTn (n ) 1-4). Inset shows
the color of emission in CHCl3, from top to bottom, FT1, FT2,
FT3, FT4.
Figure 3. Comparison of the cyclic voltammogram of FT2 and FT4.
Org. Lett., Vol. 4, No. 25, 2002 4441
Downloaded by NATIONAL TAIWAN UNIV on August 18, 2009
The introduction of bulky 9,9-diphenylfluorene into the oligothiophene cores as the end-capping group was signifi-cantly beneficial in improving the morphological stability and the resistance to thermal decomposition. The morpho-logical stability and the thermal property of 9,9-diphenylfluo-rene-capped oligothiophenes FTn were investigated by DSC and TGA analysis, respectively.
It is noteworthy that all fluorene-capped oligothiophenes exhibited a distinct glass transition temperature (Tg) of ca.
153°C (Table 1), irrespective of the conjugation length of the oligothiophene cores. The independence of Tg of the
molecular weight of the homologues is different from other results reported in the literature.7,14 However, the result
indicates that the bulkiness of the terminal groups could effectively preserve molecular morphological stability.
FT1-FT4 exhibited high thermal stability: no weight loss was
detected below 380°C. The temperature corresponding to a 10% weight loss upon heating under nitrogen ranged from 393-483°C (Table 1). FT2 displayed the highest decom-position temperature (up to 483°C) compared with that of the other homologues.
In summary, we have synthesized a new series of 9,9-diphenylfluorene-capped oligothiophenes by using a Suzuki coupling reaction. The introduction of bulky and rigid 9,9-diphenylfluorene as the terminal group of oligothiophenes had significant effects on improving morphological and thermal stability. The fluorescent color of the resulting new materials could be readily controlled by varying the conjuga-tion length of the oligothiophene core. These findings to-gether with the bipolar property make these fluorene-capped homologues attractive for future application in OLEDs.
Acknowledgment. This work was financially supported
by the National Science Council and Ministry of Education Taiwan. The authors thank Mr. S. C. Kao for DSC and TGA analyses.
Supporting Information Available: Detailed
experi-mental procedure, spectroscopic characterization of new compounds, comparison of the absorption spectra of FTn (n ) 1-4), and an X-ray structure of FT2 (CIF file). This material is available free of charge via the Internet at http://pubs.acs.org.
OL027001B
(14) Naito, K.; Miura, A. J. Phys. Chem. 1993, 97, 6240.
Figure 4. Spectral response of FT2 at various applied voltages in CH2Cl2 (0.1 M nBu4NPF6as a supporting electrolyte) at (a) the first oxidation state and (b) the second oxidation state.
4442 Org. Lett., Vol. 4, No. 25, 2002
Downloaded by NATIONAL TAIWAN UNIV on August 18, 2009
