9118
J. Phys. Chem. B 2000, 104, 9118-9125
Theoretical and Experimental Investigations of the Spectroscopic and Photophysical Properties of Fluorene-Phenylene and Fluorene-Thiophene Derivatives: Precursors of Light-Emitting Polymers Michel Belleteˆ te,† Serge Beaupre´ ,‡ Jimmy Bouchard,‡ Pierre Blondin,‡ Mario Leclerc,‡ and Gilles Durocher*,† De´ partement de Chimie, UniVersite´ de Montre´ al, Montre´ al, PQ, Canada H3C 3J7, and De´ partement de Chimie, Centre de Recherche en Sciences et Inge´ nierie des Macromole´ cules, UniVersite´ LaVal, Quebec City, PQ, Canada G1K 7P4 ReceiVed: April 7, 2000; In Final Form: July 13, 2000
The ground and excited states of covalently linked fluorene-based dimers were investigated by theoretical methods and by UV-vis and fluorescence spectroscopies. The optimized structures and the characterization of frontier molecular orbitals were obtained by HF/6-31G* ab initio calculations. All derivatives are nonplanar in their ground electronic states. The extent of nonplanarity depends on the nature of the aromatic ring bonded to the fluorene unit. All frontier orbitals involved both subunits of the dyads. The HOMO of each compound possesses an antibonding character between subunits, while the LUMO shows bonding character. The nature and the energy of the first 10 singlet-singlet electronic transitions have been obtained by ZINDO/S semiempirical calculations performed on the HF/6-31G* optimized geometries. All electronic transitions are of the ππ* type and involve both subunits of the molecules. For each derivative, excitation to the S1 state corresponds mainly to the promotion of an electron from the HOMO to the LUMO, and the S1 r S0 electronic transition is strongly favored and polarized along the long axis of the molecular frame. The energy of the first electronic transition of all derivatives follows the HOMO-LUMO energy gap computed from HF/631G* ab initio calculations. The absorption and fluorescence spectra of the fluorene derivatives have been recorded in cyclohexane. The first absorption band of each derivative can be assigned to the S1 r S0 electronic transition computed from ZINDO/S calculations. The overall shape of the absorption and fluorescence spectra suggests a smaller distribution of conformers in the S1 state than in the ground state. The fluorescence quantum yield and lifetime in cyclohexane have been obtained. In these systems, the photophysical properties are mainly governed by nonradiative processes.
1. Introduction Much effort has been undertaken recently to develop flexible and tunable light-emitting diodes (LEDS) from conjugated polymers.1-4 Until now, efficient and stable polymer blue-lightemitting LEDs are still undeveloped. Recently, our group has obtained and characterized blue-light-emitting materials from oligothiophenes incorporated into polyesters,5-7 but their low fluorescence quantum yields have put some limitations on their use in optical devices. Fluorene derivatives present an interesting alternative to blue-light-emitting materials. Indeed, fluorenes and oligofluorenes are well known as highly fluorescent compounds.8-10 These molecules contain a rigidly planar biphenyl structure in the fluorene monomer unit with facile functionalization at the C-9 position offering the prospect of controlling the polymer solubility and other physical properties. Moreover, the remote substitution at C-9 does not induce steric effects with adjacent aromatic rings.11 In this regard, various fully conjugated polyfluorenes have been studied for LED applications.12-15 However, the formation of excimers has been observed in these fully conjugated polymers, which limits their utilization in optical devices. * Corresponding author. † Universite ´ de Montre´al. ‡ Universite ´ Laval.
Polyesters derived from fluorenes should give good control of the interchain interactions, and along these lines, we have recently reported the optical and photophysical properties of a terfluorene isolated and incorporated into a polyester.10 It was observed that the incorporation of the terfluorene into the polyester does not change the terfluorene’s conformation and spectral properties. The luminescence intensity of the polyester is slightly weaker than that of the isolated oligomer but remains quite intense, thus showing promising electrical and optical features for electro-optical applications. With these preliminary results, we decided to develop a new class of such aromatic polyesters with tunable emission colors (blue, green, and red). For this purpose, different aromatic molecules can be coupled to fluorene moieties to modulate the electronic structure of the resulting oligomers (dimers, trimers, etc.). Therefore, we report herein the spectroscopic and photophysical properties of five fluorene derivatives: 1-(9,9-dioctylfluoren-2-yl)phenylene (FP), 2-(9,9-dioctylfluoren-2-yl)thiophene (FT), 2-(9,9-dioctylfluoren2-yl)-3-methylthiophene (FMT), 2-(9,9-dioctylfluoren-2-yl)-3,4(ethylenedioxy)thiophene (FEDOT), and 9,9,9′,9′-tetraoctyl-2,2′bifluorene (FF). First, the ground-state conformation and the nature and energies of the frontier molecular orbitals are investigated using HF/6-31G* ab initio calculations. Next, the excited states are characterized, and the electronic transitions are computed by ZINDO/S calculations on the optimized
10.1021/jp001349b CCC: $19.00 © 2000 American Chemical Society Published on Web 09/07/2000
Fl-Phenylene and Fl-Thiophene Derivatives
Figure 1. Molecular structure of the fluorene derivatives.
geometries. Then the optical properties are reported and compared to theoretical results. Finally, the photophysical properties of the fluorene derivatives are given and discussed. All this information should lead to a rational design of tunable light-emitting materials. The molecules investigated are shown in Figure 1. 2. Experimental Section 2.1. Computational Methods. Ab initio calculations were performed on a Pentium III (450 Mz) personal computer with 128 MB of RAM using the Gaussian 98W program, version 5.2.16 The geometries were optimized at the HF level with the 6-31G* basis set. The Berny analytical gradient method was used for the optimizations. The requested HF convergence on the density matrix was 10-8, and the threshold values for the maximum force and the maximum displacement were 0.000 45 and 0.0018 au, respectively. The excited-state analyses were performed using the ZINDO/S method,17,18 implemented in version 5.01 of the Hyperchem program package. A configuration interaction scheme, including single excitations within an active space of 10 orbitals, i.e., all molecular orbitals between HOMO-4 and LUMO+4, was used in the calculations. The electron-repulsion integrals were evaluated using the Mataga-Nishimoto formula. The rotation angle θ defines the relative orientation of the dyad subunits. 2.2. Materials. 2-Bromofluorene, 1-bromobenzene, 2-bromothiophene, and 1-bromooctane were obtained from Aldrich Co. 3,4-Ethylenedioxythiophene was donated by Bayer Co. 2-Bromo-3-methylthiophene and tetrakis(triphenylphosphine) palladium(0) were synthesized following procedures reported in the literature.19,20 Cyclohexane was purchased from Aldrich Chemicals (>99%, anhydrous) and used as received. Prior to use, the solvent was checked for spurious emissions in the region of interest and found to be satisfactory. 2.3. Synthesis and Characterization. 2-Bromo-9,9-dioctylfluorene. To a solution of 2.50 g (10.1 mmol) of 2-bromofluorene in toluene (23 mL) was added 0.183 g (5.50 mmol) of tetrabutylammonium bromide as a transfer-phase catalyst. A
J. Phys. Chem. B, Vol. 104, No. 39, 2000 9119 freshly prepared solution of aqueous NaOH (50% w/w) (23 mL) was added at once to the solution. The mixture turned orange and became viscous, and 4.05 mL (27.4 mmol) of 1-bromooctane was added with a syringe. The mixture was warmed at 60 °C for 4 h. The whole mixture was diluted with 25 mL of ethyl acetate and washed several times with water. The organic layer was dried over magnesium sulfate, and the crude product was purified by column chromatrography using hexanes as the eluent (Rf ) 0.72) to provide 3.017 g of pale brown oil as the title product (yield, 63%). 13C NMR (75 MHz, CDCl3, ppm): δ 152.84, 150.19, 140.02, 139.91, 129.75, 127.33, 126.79, 126.01, 122.74, 120.87, 119.59, 55.24, 40.13, 31.66, 29.82, 29.06 (2C), 23.55, 22.47, 13.94. 1H NMR (300 MHz, CDCl3, ppm): δ 7.66 (m, 1H), 7.54 (m, 1H), 7.43 (m, 2H), 7.31 (m, 3H), 1.92 (m, 4H), 1.23-1.03 (m, 20H), 0.82 (t, 6H, J ) 6.6 Hz), 0.59 (m, 4H). 2-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene. To a solution of 2.671 g (5.7 mmol) of 2-bromo-9,9dioctylfluorene in THF (60 mL) at -78 °C was added dropwise 2.5 mL (6.3 mmol) of n-butyllithium (2.5 M in hexanes; Aldrich). The mixture was stirred at -78 °C, warmed to 0 °C for 15 min, and then cooled back to -78 °C; 1.74 mL (8.5 mmol) of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (Aldrich Co.) was added rapidly to the solution, and the resulting mixture was warmed to room temperature and stirred for 24 h. The mixture was poured into water and extracted with diethyl ether. The organic-layer extracts were washed with brine and dried over magnesium sulfate. The solvent was removed under reduced pressure, and the crude product was purified by column chromatography with 2% ethyl acetate/hexanes as the eluent (Rf ) 0.25) to give 1.97 g of a brownish oil as the title product (yield, 67%). 13C NMR (75 MHz, CDCl3, ppm): δ 151.18, 149.74, 144.01, 140.81, 133.59, 128.72, 127.34, 126.53, 122.80, 119.94, 118.82, 83.49, 54.94, 40.10, 31.65, 29.87, 29.06 (2C), 24.83, 23.55, 22.45, 13.94. 1H NMR (300 MHz, CDCl3, ppm): δ 7.80 (d, 1H, J ) 7.3 Hz), 7.72 (m, 3H), 7.32 (m, 3H), 1.97 (m, 4H), 1.38 (s, 12H), 1.25-1.09 (m, 20H), 0.81 (t, 6H, J ) 6.6 Hz), 0.59 (m, 4H). 2-Bromo-3,4-ethylenedioxythiophene. To a solution of 3,4ethylenedioxythiophene (Bayer, 2.00 g, 14.07 mmol) in DMF (20 mL) was added NBS (Aldrich, 1.25 g, 7.03 mmol). The reaction mixture was stirred for 1 h at room temperature in the absence of light. The resulting slurry was poured into water and then extracted with diethyl ether. The organic fractions were washed with brine and dried over magnesium sulfate. The solvent was removed under reduced pressure to afford 1.74 g of a mixture of 2-bromo-3,4-ethylenedioxythiophene and 3,4ethylenedioxythiophene in a 1:1 ratio. This monomer was used without further purification for the dimerization. Dimerization. The dimerization was performed by using carefully purified 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)-9,9-dioctylfluorene and the monobrominated aromatic compound (2 equiv) in the presence of 3 mol % (PPh3)4Pd(0) in an aqueous mixture of THF/2 M K2CO3. The solution was stirred and refluxed for 4 h under an inert atmosphere. The mixture was poured into water and extracted with diethyl ether. The organic extracts were washed with brine and dried over magnesium sulfate. The solvent was removed, and the residue was purified by column chromatography using hexanes as the eluent to afford the dimers in good yields (60-80%). 1-(9,9-Dioctylfluoren-2-yl)phenylene (FP). 13C NMR (75 MHz, CDCl3, ppm): δ 151.25, 150.86, 141.63, 140.64, 140.29, 139.91, 128.63, 127.07, 126.95, 126.89, 126.65, 125.81, 122.76, 121.39, 119.76, 119.59, 55.02, 40.28, 31.67, 29.92, 29.08 (2C),
9120 J. Phys. Chem. B, Vol. 104, No. 39, 2000 23.66, 22.47, 13.95. 1H NMR (300 MHz, CDCl3, ppm): δ 7.71 (m, 4H), 7.56 (m, 2H), 7.45 (t, 2H, J ) 7.4 Hz), 7.32 (m, 4H), 1.97 (m, 4H), 1.19-1.04 (m, 20H), 0.80 (t, 6H, J ) 6.6 Hz), 0.67 (m, 4H). HRMS for C35H46: calcd, 466.35995; found, 466.35910. 2-(9,9-Dioctylfluoren-2-yl)thiophene (FT). 13C NMR (75 MHz, CDCl3, ppm): δ 151.38, 150.75, 145.12, 140.58, 140.51, 133,05, 127.88, 126.97, 126.68, 124.73, 124.30, 122.73, 122.68, 120.05, 119.89, 119.55, 55.02, 40.27, 31.66, 29.89, 29.08 (2C), 23.63, 22.47, 13.94. 1H NMR (300 MHz, CDCl3, ppm): δ 7.68 (m, 2H), 7.58 (m, 2H), 7.30 (m, 5H), 7.09 (m, 1H), 1.98 (m, 4H), 1.19-1.04 (m, 20H), 0.79 (t, 6H, J ) 6.6 Hz), 0.64 (m, 4H). HRMS for C33H44S1: calcd, 473.32419; found, 473.32230. 2-(9,9-Dioctylfluoren-2-yl)-3-methylthiophene (FMT). 13C NMR (75 MHz, CDCl3, ppm): δ 151.11, 140.60, 140.14, 138.61, 133.29, 132.85, 131.06, 127.72, 127.01, 126.72, 123.39, 122.99, 122.78, 119.64, 119.53, 55.02, 40.26, 31.69, 29.94, 29.12 (2C), 23.72, 22.50, 14.79, 13.97. 1H NMR (300 MHz, CDCl3, ppm): δ 7.70 (d, 2H, J ) 7.3 Hz), 7.43 (2H), 7.31 (3H), 7.19 (d, 1H, J ) 7.2), 6.94 (d, 1H, J ) 7.3 Hz), 2.37 (s, 3H), 1.97 (m, 4H), 1.06 (m, 20H), 0.80 (t, 6H, J ) 6.6 Hz), 0.67 (m, 4H). HRMS for C34H46S1: calcd, 486.33203; found, 486.33110. 2-(9,9-Dioctylfluoren-2-yl)-3,4-(ethylenedioxy)thiophene (FEDOT). 13C NMR (75 MHz, CDCl3, ppm): δ 151.03, 150.84, 142.24, 140.74, 139.61, 137.84, 131.83, 126.74, 126.60, 124.75, 122.73, 120.29, 119.61, 119.46, 118.30, 97.09, 64.70, 64.40, 54.99, 40.21, 31.68, 29.93, 29.10, 29.08, 23.64, 22.47, 13.93. 1H NMR (300 MHz, CDCl , ppm): δ 7.61 (m, 4H), 7.22 (m, 3 3H), 6.24 (s, 1H), 4.27 (m, 2H), 4.20 (m, 2H), 1.89 (m, 4H), 1.18-0.97 (m, 20H), 0.73 (t, 3H, J ) 6.6 Hz), 0.59 (m, 4H). HRMS for C35H46O2S1: calcd, 530.32184; found, 530.32030. 9,9,9′,9′-Tetraoctyl-2,2′-bifluorene (FF). 13C NMR (75 MHz, CDCl3, ppm): δ 151.33, 150.89, 140.67, 140.37, 140.18, 126.85, 125.90, 122.80, 121.28, 119.73, 119.58, 55.05, 40.25, 31.67, 29.92, 29.09 (2C), 23.70, 22.47, 13.95. 1H NMR (300 MHz, CDCl3, ppm): δ 7.75 (m, 4H), 7.63 (m, 4H), 7.35 (m, 6H), 2.02 (m, 8H), 1.25-1.07 (m, 20H), 0.80 (t, 12H, J ) 6.6 Hz), 0.70 (m, 8H). HRMS for C58H82: calcd, 466.35995; found, 466.35910. 2.4. Instrumentation. 1H and 13C NMR spectra were recorded on a Bruker AMX300 apparatus in deuterated chloroform solutions at 298 K. The absorption spectra were recorded on a Varian Cary 1 Bio UV-vis spectrophotometer at room temperarure using 1 cm quartz cells and a solute concentration of 2 × 10-6 M. It has been verified that the Beer-Lambert law is applicable in the region of the concentrations used. The fluorescence spectra corrected for the emission detection were recorded on a Spex Fluorolog-2 spectrophotometer with a F2T11 special configuration. Excitation and emission band-passes were 2.6 and 1.9 nm, respectively. Each solution was excited near the absorption wavelength maximum using a 1 cm path length quartz cell. Solute concentrations were about 2 × 10-6 M, yielding
