J. Phys. Chem. B 2002, 106, 8959-8966
8959
Spectroscopic Study of Intermolecular Interactions in Various Oligofluorenes: Precursors of Light-Emitting Polymers Soujanya Tirapattur,† Michel Belleteˆ te,† Nicolas Drolet,‡,§ Jimmy Bouchard,‡ Maxime Ranger,‡ Mario Leclerc,‡ and Gilles Durocher*,† Laboratoire de photophysique mole´ culaire, De´ partement de Chimie, UniVersite´ de Montre´ al, C.P. 6128, Succ. Centre-Ville, Montre´ al, Que´ bec, H3C 3J7, Canada, Chaire de recherche du Canada en chimie des polyme` res photoactifs et e´ lectroactifs, Centre de recherche en sciences et inge´ nierie des macromole´ cules (CERSIM), UniVersite´ LaVal, Cite´ UniVersitaire, Que´ bec, G1K 7P4, Canada, and Institute of Microstructural Sciences, National Research Council of Canada M-50, Montreal road, Ottawa, Ontario, K1A 0R6, Canada ReceiVed: NoVember 29, 2001; In Final Form: April 16, 2002
A detailed analysis of the optical and photophysical properties of 2,7-bis(phenylene)-9,9-dioctylfluorene (PFP), 2,7-bis(biphenylene)-9,9-dioctylfluorene (BPFBP), 2,7-bis(2-thienyl)-9,9-dioctylfluorene (TFT), and 2,7-bis(2,2\-bithien-5-yl)-9,9-dioctylfluorene (BTFBT) in various environments are reported. The optical properties of the free molecules isolated in an alkane matrix are obtained and discussed in terms of the conformation adopted by each derivative in the electronic ground and first excited states. Also, conformational changes are responsible for the optical changes observed at high concentrations in an isopentane glass at 77 K. High quantum yields of all the oligofluorenes at 77 K indicate the absence of quenching effects such as excitonic or aggregation effects. The similar spectral and photophysical properties in matrix and glass environments are explained by the disorder introduced in oligofluorenes by long octyl chains at the C-9 position of the fluorene moiety. To study the effect of intermolecular interactions in the solid state, we recorded the spectra of thin films of these derivatives. The much red-shifted emission band in the solid state cannot be explained by conformational changes and has its origin in the π-stacking of conjugated oligomers in their relaxed S1 state. As an evidence to show the importance of the role played by octyl chains at the C-9 position of the fluorene moiety, we synthesized two new model compounds: one, without octyl chains at the C-9 position of the fluorene moiety, 2,7-bis(2-thienyl)fluorene (TFTWC) and another with more octyl chains, 1,4-bis(9,9dioctylfluoren-2-yl)phenyl (FPF). The spectral properties of these derivatives have been studied at room temperature and at 77 K. These systems serve as excellent examples to show the effect of intermolecular interactions on optical properties of oligofluorenes.
1. Introduction Conjugated polymers and oligomers have been investigated intensively in the past few decades because of their unusual electrical and optical properties, making them applicable to electronic and photonic devices, such as field-effect transistors and light-emitting diodes (LED).1,2 One critical issue in designing successful LED systems is the photoluminescence (PL) quantum efficiency. Impediments to high luminescence efficiency include the formation of quenching aggregates, the migration to nonradiative centers, and the twisting of molecular units. Various molecular design and polymer processing approaches have been suggested to overcome these barriers such as polymer blends,3 the introduction of bulky groups,4 cis inclusions,5 and block polymers.6 Polymeric derivatives of fluorene present an interesting approach to blue-light-emitting polymers.7-15 The facile functionalization at the C-9 position offers the prospect of controlling both the polymer solubility and the potential interchain interactions in films.16-22 These materials contain a rigidly planarized biphenyl structure in the fluorene monomer unit, where the remote substitution at C-9 produces less steric interaction in * To whom correspondence should be addressed. † Universite ´ de Montre´al. ‡ Universite ´ Laval. § Institute of Microstructural Sciences.
the conjugated backbone itself. Polyfluorene derivatives also have extremely high luminescence efficiencies in solution, which are largely maintained in the polymer films. As a result, polyfluorenes have been intensively studied recently as blueemitting materials.7,9 However, chain aggregation tends to degrade the device performance. This physical degradation usually results in a red-shifted fluorescence with a reduced intensity because of the excitation migration and the relaxation through lower energy excimer traps.19 To depress chain aggregation and increase the fluorescence quantum efficiency, the introduction of disorder to the conjugated system seemed to be a good route. Recently, an attempt has been made to prevent aggregation by introducing carbazole units into polyfluorene polymer.23 On the other hand, Inbasekaran et al. have shown recently the improved device performance based on fluorene polymers by incorporating different aromatic units into the system.24 Moreover, Porzio et al. also reported the spectral properties of 2,7-diphenyl fluorene, with a cyclobutane moiety at the C-9 position.25 However, their work is still incomplete by lacking a detailed analysis of the spectra in solution and in the solid state. Furthermore, by increasing the repeat units in the molecules or by adding side chains on the fluorene rings, oligofluorenes permit color tuning. Along these lines, we recently published a detailed spectroscopic and conformational analysis of thiophene/ phenylene substituted fluorene co-oligomers and their respective
10.1021/jp0143544 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/14/2002
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SCHEME 1
polyesters.26,27 We present in this paper an extension of this study with the report of the spectroscopic features in solution at room temperature (RT) and at 77 K for various thiophene/ phenylene substituted fluorene co-oligomers with different ring arrangements trying to establish how the molecular size and the arrangement of the thiophene and phenylene rings influence the spectra. To this end, the absorption and fluorescence spectra were recorded both in the dilute regime in an alkane matrix (∼10-6 mol dm-3) and at higher concentrations in an isopentane glass (the solute concentration was varied from 10-5 mol dm-3 to 10-4 mol dm-3) according to whether intermolecular interactions play a role. The spectra of thin films were also measured and compared with those in solutions to elucidate the origin of the solid-state fluorescence. Scheme 1 shows chemical structures and abbreviated notations of the oligofluorenes investigated in this paper. Results show that the optical changes found for the isolated molecules in a 77 K matrix and in isopentane glass are associated with conformational changes. The spectral properties of oligomers in the solid state point to interactions between the molecules, which are controlled by octyl chains at C-9 position of the fluorene moiety. This is further corroborated by the spectral study of model compounds with and without side chains on the fluorene moiety. 2. Experimental Section 2.1. Materials. Tridecane (99+%), isopentane (99.5+%, HPLC grade), and chloroform were purchased from Aldrich Chemicals and used as received. Prior to use, all the solvents were checked for spurious emissions in the region of interest and found to be satisfactory. The synthesis and characterization of PFP, BPFBP, TFT, and BTFBT were already published.26,27 2.1.1. Synthesis of 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaboronal-2-yl) fluorene. In a 100-mL flask, 3,98 g (9,53 mmol) of 2,7-diiodofluorene,28 418 mg (0,57 mmol) of PdCl2(dppf),29 48 mL of freshly distilled dichloroethane (Aldrich), and 7,97 mL (57,17 mmol) of freshly distilled triethylamine (Aldrich) are mixed at room temperature under inert atmosphere. After complete dissolution, 4,15 mL (28,59 mmol) of pinacolborane (Aldrich) are added dropwise at room temperature to the solution until effervescence. The resulting mixture was stirred under inert atmosphere overnight at 80 °C.
The solution was put in 150 mL of CHCl3 and washed with two portions of HCl 10% (v/v). The organic phase was washed with two portions of brine and dried over magnesium sulfate. The solvent was removed under reduced pressure to afford 3 g of crude brown gummy material. The crude product was mixed in 100 mL of methanol and the particles are fractioned in a sonicated bath. The solution was filtrated with a Bu¨chner apparatus to give a brown solid. The pure product was obtained by sublimation at 210-220 °C and at 1 mmHg to provide 1,3 g (Y ) 32%) of the title product as white needles. 1H NMR (300 MHz, CDCl ) δ (ppm): 8.01 (s, 2H), 7.83 (s, 3 4H), 3.90 (s, 2H), 1.37 (s, 24H). 13C NMR (75 MHz, CDCl3) δ (ppm): 144.38, 143.13, 133.40, 131.40, 119.75, 83.80, 36.54, 24.92. 2.1.2. Synthesis of 2,7-Bis(2-thienyl)fluorene (TFTWC). Under inert atmosphere, 128 mg (0,79 mmol) of 2-bromothiophene (Aldrich), 200 mg (0,38 mmol) of 2,7-Bis(4,4,5,5tetramethyl-1,3,2-dioxaboronal-2-yl)fluorene, 16 mg (0,06 mmol) of PPh3 (Aldrich), and 3,4 mg (0,015 mmol) of Pd(OAc)2 (Aldrich) were dissolved in 1,6 mL of freshly distilled and degassed toluene. After complete dissolution, 1 mL of K2CO3(aq) 2 M (purged under argon atmosphere) was added to the solution. The mixture was heated at 110 °C with vigorous stirring for 24 h. The mixture was cooled to room temperature and washed with three portions of brine. The organic layer was dried over magnesium sulfate and the solvent was removed under reduced pressure to afford 238 mg of crude material. The crude product was purified by column chromatography (silica gel, 5% diethyl ether in hexanes as eluent, Rf: 0,30) to provide 18 mg of the title product as a white solid. 1H NMR (300 MHz, CDCl ) δ (ppm): 7.79 (m, 4H), 7.66 (d, 3 2H, J ) 7.8 Hz), 7.37 (d, 2H, J ) 3.2 Hz), 7.29 (d, 2H, J ) 5 Hz), 7.11 (t, 2H, J ) 4.1 Hz), 3.98 (s, 2H). 13C NMR (75 MHz, CDCl3) δ (ppm): 130.06, 128.31, 128.11, 125.05, 124.76, 123.07, 122.54, 121.13, 120.61, 120.31, 36.75. 2.1.3. Synthesis of 1,4-Bis(9,9-dioctylfluoren-2-yl)phenyl (FPF). Under inert atmosphere, 3 equiv of 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene,30 1 equiv 1,4-dibromobenzene (Aldrich), and 0.5-1% mol of Pd(0)(PPh3)48 were added to a mixture of THF/2M K2CO3(aq). The solution was stirred over 24 h at 80 °C. The mixture was poured in water
Spectroscopic Study of Intermolecular Interactions and extracted with chloroform. The organic extracts were washed with brine and dried over magnesium sulfate. The solvent was removed and the residue was purified by column chromatography (silica gel, hexanes as eluent) (Y ) 96%). 1H NMR (300 MHz, CDCl ) δ (ppm): 7.69 (m, 8H), 7.54 3 (m, 4H), 7.27 (m, 4H), 1.93 (m, 8H), 0.97 (m, 40H), 0.72 (t, 12H, J ) 6.8 Hz), 0.62 (m, 8H). 13C NMR (75 MHz, CDCl3) δ (ppm): 151.26, 150.83, 140.54, 140.30, 139.35, 127.71, 127.35 (2C), 126.85, 126.61, 125.61, 122.72, 121.25, 119.78, 119.56, 54.96, 40.22, 31.60, 29.86, 29.04 (2C), 23.61, 22.42, 13.89. 2.2. Instrumentation. Room-temperature absorption spectra were recorded on a Varian spectrometer, model Cary 1 Bio using 1-cm quartz rectangular cells. Fluorescence spectra corrected for the emission detection were recorded on a Spex Fluorolog-2 spectrophotometer. Each solution was excited near the absorption wavelength maximum. The fluorescence and excitation spectra were found independent of the excitation and emission wavelength, respectively, and the excitation spectra were identical to their respective absorption spectra. In rigid media at 77 K (tridecane matrix, isopentane glass), excitation and fluorescence spectra were recorded using the front face arrangement of the instrument to avoid any reabsorption or inner-filter effects. Measurements were taken in a quartz cylinder tube of 0.4 mm immersed in a dewar filled with liquid nitrogen. For thin films, the glass slide substrates (2 × 2 cm) have been washed with methanol and acetone and then dried at 60 °C and 30 mmHg in a vacuum oven for 12 h. The oligomers have been dissolved in chloroform (8 mg/mL) and then the solutions were filtered (1 µm). Thin films were prepared by the spin-coating method using few drops of the appropriate oligomer solution. The solutions were spin-coated at 1500 rpm for 50 s. Samples were dried at 40 °C and 30 mmHg in a vacuum oven for 2 h. Fluorescence lifetimes were measured on a multiplexed timecorrelated single-photon-counting fluorimeter (Edinburgh Instruments, model 299T). Details on the instrument have been published elsewhere.31 The instrument incorporates an all-metal coaxial hydrogen flashlamp. The reconvolution analysis was performed by fitting over all the fluorescence decay including the rising edge. The kinetic interpretation of the goodness-offit was assessed using plots of weighed residuals, reduced χ2 values, and Durbin-Watson (DW) parameters. Low-temperature measurements were taken in a quartz cylinder tube of 0.4 mm immersed in a dewar filled with liquid nitrogen. 3. Results and Discussion 3.1. Spectroscopic Analysis of Isolated Molecules at 298 and 77 K. A detailed spectroscopic and conformational analysis of oligofluorene derivatives depicted in Scheme 1 has been published recently.26,27 From these results, we obtained, in the ground state, dihedral angles of about 45° between phenylene and fluorene rings for PFP and BPFBP and slightly smaller dihedral angles of about 39° between thiophene and fluorene rings for TFT and BTFBT. The potential energy surfaces of the long oligomers (BPFBP and BTFBT) are expected to be complex and should possess many local minima. This should allow for a wider distribution of rotamers compared to those observed for shorter oligomers. The optimum conditions for the appearance of the quasi-linear spectra (Shpolskii effect) of linear aromatic molecules are obtained when the length of the long axes of the n-alkane solvent and the aromatic molecule are matched.32 The length of the molecules calculated from ground-state geometry optimized by
J. Phys. Chem. B, Vol. 106, No. 35, 2002 8961
Figure 1. Absorption (or) excitation and fluorescence spectra of (a) PFP, (b) TFT in tridecane at room temperature (solid), at 77 K (dot). The excitation and emission wavenumbers were near the maximum of the absorption (or excitation) and emission spectra, respectively. All intensities are arbitarily chosen to optimally fit the graph. Concentrations used were 2.4 × 10-6 and 2.0 × 10-6 mol dm-3 in tridecane for PFP and TFT, respectively.
ab initio HF/6-31G* method are 17.4 Å for PFP and 16.4 Å for TFT. For this reason, tridecane (17.11 Å) has been chosen as the solvent to form a substitutional matrix for PFP and TFT, which should totally isolate these molecules. This method was used successfully to obtain the full vibronic resolution of the excitation and fluorescence spectra of quaterthiophenes at 4 K.33 Unfortunately, we could not do the matrix isolation studies for lengthier molecules, BPFBP (25.8 Å) and BTFBT (24.3 Å), because of the very poor solubility found for these compounds in very viscous alkane solvents. The absorbance (or excitation at 77 K) and fluorescence spectra of PFP and TFT in tridecane at room temperature and isolated in the solvent matrix at 77 K are shown in Figure 1. As seen from Figure 1a, the first absorption band of PFP is blue shifted and broad in comparison to the first absorption band of TFT (Figure 1b). These features of PFP are indicative of a wider distribution of conformers together with higher twisting angles in the ground state. The replacement of a phenyl group by a thiophene group is found to slightly destabilize the HOMO orbital and stabilize the LUMO orbital.15 This causes a decrease in the HOMO-LUMO energy gap of TFT compared to that of PFP. Consequently, a blue shift is observed in the absorption band of PFP. In contrast, fluorescence spectra of both the derivatives have better vibronic resolution and are sharper than their respective absorption spectra. This suggests that the molecules are more rigid in the first excited singlet state resulting in a narrower distribution of conformers. This is further
8962 J. Phys. Chem. B, Vol. 106, No. 35, 2002 supported by ab initio HF/6-31G* calculations, which favor planar geometries in the excited state for both PFP and TFT.34 It can also be seen from Figure 1 that the vibronic bands in the fluorescence spectra of PFP and TFT are better resolved on cooling to 77 K. However, the position of excitation and fluorescence bands does not change much for PFP on cooling to 77 K. Whereas for TFT, along with an increase in the vibronic resolution, there is a red shift of both excitation and fluorescence bands on going from RT to 77 K. It can also be observed from the figure that the mirror image relationship is better at 77 K for TFT than for PFP. We believe that this optical difference between TFT and PFP can be explained by the nature of the potential energy surface of 1-(9,9-diethylfluorene-2-yl)phenylene (FP) and 2-(9,9-diethylfluorene-2-yl)thiophene (FT) dimers.35 Indeed, our previous calculations on thiophene derivatives lead us to conclude that the length of the oligomer has almost no effect on the local rotational barriers between thiophene rings.36-38 Therefore, we extended our previous ab initio HF/ 6-31G* calculations on fluorene-based dimers to the systems shown in Scheme 1. For FP, the ground-state rotational barrier against planarity is 3.8 kcal/mol, which is much higher than that for FT (1.2 kcal/mol). Therefore, for PFP, molecules should remain in twisted forms at low temperatures. Moreover, the relatively large Stokes shift and the poor adherence to the mirror image relationship between absorption and fluorescence spectra at 77 K indicate a significant conformational change between ground and first excited singlet state for PFP. On the other hand, lower rotational barriers in the ground state for TFT make the bond rotation flexible at RT and favor planar conformations at 77 K. It can also be observed from Figure 1, a small red shift on cooling in the fluorescence spectrum of TFT, which indicates that the molecules are not completely planar at room temperature. One can also see that the fluorescence band of PFP in the matrix is slightly blue shifted compared to that at room temperature. This may be attributed to the nonuniformity of the matrix, which may trap more twisted conformers. The change of conformation adopted by TFT is responsible for the red shift observed in the excitation spectrum, relative to the room-temperature one, recorded in the tridecane matrix. The change of conformation in the ground state between roomtemperature solution and 77 K tridecane matrix probably occurs because of a better cohesion energy existing between planar molecules and the matrix. Now we will see how the spectral properties of oligofluorenes behave with increasing concentrations in isopentane at room temperature and at 77 K. 3.2. Spectroscopic Analysis of Oligofluorenes in an Isopentane Glass. In an attempt to study the effect of the intermolecular interactions on the optical properties of the fluorene derivatives, the optical spectra of PFP, BPFBP, TFT, and BTFBT in an isopentane glass and in the solid state (thin films) have been recorded. The absorption (or excitation) and fluorescence spectra of PFP and TFT in isopentane at room temperature and at 77 K are displayed in Figure 2 and that of BPFBP and BTFBT in Figure 3. The spectral properties are summarized in Table 1. In an attempt to create the aggregated forms, high concentrations of the solute (∼10-5 mol dm-3 to ∼10-4 mol dm-3) in isopentane at 77 K (glassy medium) have been used. The spectra at all concentrations are not shown in the figures for the sake of clarity. Isopentane instead of tridecane is employed because, at 77 K, this solvent forms a glass avoiding the possibility to trap free molecules in interstitial and substitutional lattices. For all the oligomers, there is no indication of appearance of any new band or extension of tail in the red-end region in the
Tirapattur et al.
Figure 2. Absorption (or) excitation and fluorescence spectra of (a) PFP, (b) TFT in isopentane at room temperature (solid), at 77 K (dot). The excitation and emission wavenumbers were near the maximum of the absorption (or excitation) and emission spectra, respectively. All intensities are arbitarily chosen to optimally fit the graph. Concentrations used were 3.0 × 10-5 and 3.6 × 10-5 mol dm-3 in isopentane for PFP and TFT, respectively.
absorption and fluorescence spectra with increase in concentration of solute. One can observe that the behavior of both absorption and fluorescence spectra in isopentane, at high concentrations, are very similar to that recorded in the alkane matrix. Moreover, the vibrational fine structure of the spectra in the tridecane matrix and in the isopentane glass are identical. Thus according to the above discussion, small conformational changes involving the dihedral angles between P-F and T-F rings should be mainly responsible for the optical changes observed in the excitation and fluorescence spectra in the glassy medium. The fluorescence emission is attributed to the free molecules both in dilute (alkane matrix) and in concentrated (isopentane) solutions, because the fluorescence excitation spectra correspond to the absorption spectra. This implies that no important intermolecular interactions take place in the ground or excited states. In our attempt to form the aggregates using the good and bad solvents, we found that the spectral behavior of all the oligofluorenes in a mixture of methanol and water solvent at RT and 77 K is not different from the spectra recorded in isopentane. It can be seen from Table 1 that absorption and fluorescence spectra for TFT and BTFBT are red shifted compared to that of corresponding phenylene substituted oligofluorenes. As discussed above, this is due to a decrease in the HOMO-LUMO energy gap for TFT and BTFBT. It can be observed from the table that with the increase in the length of the molecule from PFP to BPFBP and from TFT to BTFBT, the optical transitions are red shifted. This is attributed solely to the increase in the
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TABLE 1: Spectroscopic Parameters of Oligofluorenes in Isopentane at Room Temperature (RT), at 77 K, and in Solid State thin films νjabs (max)a RT FwhmAb compounds (cm-1) (cm-1) PFP BPFBP TFT BTFBT
30800, 33000 29600 28700, 27700 25600
5700 4600 3800 4400
νjflu (max)a,f RT (cm-1) 28400, 27200 26700, 25300 26600, 25100 23100, 21800
FwhmFc νjabs (max)a FwhmAb νjflu (max)a,f 77 K FwhmFc (cm-1) (cm-1 ) 77 K (cm-1) (cm-1 ) (cm-1) 3200 2800 2500 2400
30300, 31600 29800, 28500 28900, 27500 25200, 23600
5500 5500 3800 4000
28700, 27200 26100, 24700 26200, 24700 22400, 20100
2500 2700 2100 2100
νjabs (max)a (cm-1)
νjflu (max)a,f (cm-1)
29600, 31100 28100, 29800 27900, 26700 24600, 23200
25700, 24400 24500, 23300 22800, 24100 21800, 20500
a Wavenumbers taken at maximum of the band and shoulder. b Full width at half-maximum(fwhm ) of the absorption band. c Full width at A half-maximum(fwhmF) of the fluorescence band. f All the data given here were obtained by excitation at the wavelength of the respective absorption maximum.
TABLE 2: Photophysical Parameters of Oligofluorenes at 298 and 77 K Investigated in Isopentane compounds
φfa RT
φfa 77 K
τb RT (ns)
τb 77K (ns)
PFP BPFBP TFT BTFBT
0.71 0.78 0.84 0.42
0.75 0.52 0.60 0.46
0.88 0.72 0.96 0.59
0.94 0.82 0.99 1.03
a
Figure 3. Absorption (or) excitation and fluorescence spectra of (a) BFPBP, (b) BTFBT in isopentane at room temperature (solid), at 77 K (dot). The excitation and emisson wavenumbers were near the maximum of the absorption (or excitation) and emission spectra, respectively. All intensities are arbitarily chosen to optimally fit the graph. Concentrations used were 1.5 × 10-5 and 1.2 × 10-5 mol dm-3 in isopentane for BPFBP and BTFBT, respectively.
conjugation length of the molecules. Indeed, our previous calculations on these molecules have shown that the increase in the conjugation length does not improve the molecular planarity of thiophene/phenylene-based fluorene oligomers.26,27 Accordingly, the absorption and fluorescence bandwidths of TFT, BTFBT and PFP, BPFBP are similar. Now it is clear that the excitonic effects, which were observed earlier for unsubstituted oligothiophenes, are absent for oligofluorenes.39,40 The important question still to be answered is why there is no change in the spectral behavior of oligofluorenes with increasing concentrations. Another possibility is that the aggregation bands are hidden under monomer emissions or are not spectrally resolved from the main emission band. For this reason, the quantum yields and fluorescence lifetimes of all oligofluorenes have been recorded at room temperature and at 77 K in isopentane to check for any anomalous behavior.
Fluorescence quantum yields. b Fluorescence lifetimes.
3.3. Photophysical Analysis of Oligofluorenes at RT and 77 K. The photophysical parameters of the oligofluorene derivatives investigated in isopentane at RT and 77 K are listed in Table 2. Fluorescence quantum yields at 77 K are obtained using the room-temperature quantum yield of each oligofluorene derivative as a standard. The change of the refractive index of isopentane from 298 to 77 K has been neglected. The fluorescence quantum yields obtained at 77 K are less accurate than those measured at 298 K because the concentrations used to form aggregates are relatively high and also because absorbances measured at 77 K are less precise. The relative error in quantum yield measurements at RT is (5% and (10% at 77 K. We did not attempt to calculate the fluorescence quantum yields of thin films, obviously because of discrepancies involved in the measurement of quantum yields in the solid-state like selfabsorption of emission and total internal reflection of the emission because of high refractive indices in the thin films. Usually, the aggregation or excimer formation will drastically quench the fluorescence, as observed recently for oligothiophenes. Fluorescence quantum yields of oligofluorenes investigated in solution at RT are significantly high in the range 0.40-0.80. The remarkable feature of these derivatives is that the high quantum yield values are more or less retained at 77 K. This supports our earlier conclusion that there is no significant aggregation formation for all the oligomers at 77 K. Moreover, the fluorescence emissions of the oligomers decay with a single exponential, whose lifetime is a good measure of the extension of the π-conjugation length. Also, fluorescence lifetimes of all the derivatives does not change much at 77 K except for the slight increase in lifetimes. One can also see that the fluorescence quantum yield of BTFBT is much lower than that of TFT. Whereas for the couple PFP:BPFBP, the values are almost same. For oligothiophenes, it is well established that the main deactivation pathway of the S1 state involves an intersystem crossing process because of the presence of heavy sulfur atoms.41,42 Accordingly, the intersystem crossing probability of BTFBT, which possesses two additional sulfur atoms, should increase and consequently fluorescence quantum yield is reduced. High quantum yields at low temperatures for these derivatives is an encouraging factor to use these further for LED applications. However, it is important to study their spectral behavior in the solid state.
8964 J. Phys. Chem. B, Vol. 106, No. 35, 2002
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Figure 4. Absorption (or) excitation and fluorescence spectra of (a) PFP, (b) TFT in solid state (solid), in isopentane at 77 K (dot). The excitation and emission wavenumbers were near the maximum of the absorption (or excitation) and emission spectra, respectively. All intensities are arbitarily chosen to optimally fit the graph. Concentrations used were 3.0 × 10-5 and 3.6 × 10-5 mol dm-3 in isopentane for PFP and TFT, respectively.
Figure 5. Absorption (or) excitation and fluorescence spectra of (a) BPFBP, (b) BTFBT in solid state (solid), in isopentane at 77 K (dot). The excitation and emission wavenumbers were near the maximum of the absorption (or excitation) and emission spectra, respectively. All intensities are arbitarily chosen to optimally fit the graph. Concentrations used were 1.5 × 10-5 and 1.2 × 10-5 mol dm-3 in isopentane for BPFBP and BTFBT, respectively.
3.4. Spectroscopic Analysis of Thin Films of Oligofluorenes. Since the spectral behavior in the solid state should reveal the extent of intermolecular interactions, we recorded the absorption and fluorescence spectra of thin films of all the derivatives. Figures 4 and 5 show the absorption and fluorescence spectra of PFP, TFT and BPFBP, BTFBT, respectively. For the sake of comparison, the respective excitation and fluorescence spectra in isopentane recorded at 77 K are also illustrated. From these figures, it is clear that absorption spectra of all the thin films are not much broader but are less structured and red shifted when compared to the corresponding excitation spectra in isopentane at 77 K. This might be explained by the fact that the aggregation process does not really occur in this environment. However, even if there are no or few interactions between these electronic systems in the ground states, it might be possible that various interaction processes arise in the excited electronic states. This is because the electronic affinity of a molecule is large and its ionization potential is smaller in the electronically excited state than in the ground state. One can see that the fluorescence spectrum of all the oligomers in the solid state presents a large red shift, is structured, and is independent of the excitation wavelength. Such a large red shift observed for all the oligomers in the solid state is not due to conformational changes.43,44 From ab initio HF/6-31G* calculations on PFP and TFT, we obtained planar conformers as minimum energy geometries in the excited state. The electronic interactions involved between the oligomer molecules induced by a good
π-stacking process in the solid state should be responsible for huge red shift in the emission band.39,40 The well-separated vibronic bands of the fluorescence spectra rule out the possibility of excimer formation, spectra of which are usually broad and featureless. Moreover, we did not notice the appearance of any new absorption band or the extension of absorption band at the red region with increasing concentrations. At this stage, we infer the red-shifted fluorescence spectra of thin film to packing effects that are more prominent in the solid state than in a glassy medium giving rise to a π-stacking process. We believe that the different behavior between oligothiophenes and oligofluorenes in thin films or high concentrated solutions might be due to the intermolecular separation.39,40 Indeed, the presence of octyl chains at the C-9 position of the fluorene rings should increase the intermolecular distances in the oligofluorenes, which should weaken the ground-state electronic interactions between molecules, interactions that are mainly responsible for H-type excitonic effect or aggregate formation. To shed more light on this aspect, the spectral behavior of model compounds without or with more octyl chains at the C-9 position should provide convincing evidence about the effect of the side chains on intermolecular distances between oligomer molecules. Along these lines, we synthesized and studied the spectral properties of two model compounds: one based on thiophene-based oligomer, TFTWC, without octyl chains at the C-9 position of the fluorene moiety and a phenylene-based oligomer, FPF molecule, with more octyl chains. The absorption or excitation and the fluorescence spectra
Spectroscopic Study of Intermolecular Interactions
J. Phys. Chem. B, Vol. 106, No. 35, 2002 8965 FPF in isopentane at 77 K and in solid state, we notice a red shift of about 1300 cm-1 for FPF, which is much lower than what we observed for PFP (3000 cm-1). Also, the fluorescence quantum yield of FPF is similar at RT (0.80) and at 77 K (0.75). This behavior is due to the presence of more side chains in FPF, which increases the intermolecular distances and thereby prevents the π-stacking of oligomer molecules in the solid state. Similar studies on polyethynylenes shows that the incorporation of bulky groups prevent π-stacking of the polymer backbone in the solid state.45 4. Concluding Remarks
Figure 6. Absorption (or) excitation and fluorescence spectra of (a) TFTWC in isopentane at room temperature (solid), in isopentane at 77 K (dot) and (b) FPF in isopenante at 77 K (dot) and in solid state (solid). The excitation and emission wavenumbers were near the maximum of the absorption (or excitation) and emission spectra, respectively. All intensities are arbitarily chosen to optimally fit the graph. Concentrations used were 2.4 × 10-5 and 2.2 × 10-5 mol dm-3 in isopentane for TFTWC and FPF, respectively.
of TFTWC in isopentane at room temperature and at 77 K are depicted in Figure 6a. The excitation spectrum of TFTWC in isopentane at 77 K is red shifted and is more structured when compared to absorption spectrum at room temperature. This is in contrast to unsubstituted oligothiophenes, where splitting of excitation spectrum because of excitonic effects is observed.39,40 However, the much red-shifted emission band, which we could observe for other derivatives only in the solid state can be seen for TFTWC in the isopentane itself. These results give ample evidence for the role played by octyl chains in increasing the intermolecular distances. In contrast to similar values of fluorescence quantum yield for TFT at RT and 77 K (see Table 2), the fluorescence quantum yield of TFTWC is drastically decreased from 0.88 measured at RT to 0.19 at 77 K. This quenching behavior for TFTWC, at 77 K, can be interpreted in terms of better π-π stacking of the molecules because of the absence of octyl chains in the C-9 position. The molecule FPF is interesting for two reasons: (1) it is similar to PFP and (2) on account of two fluorene rings in each FPF molecule, it has more octyl chains than PFP. The absorption (or excitation at 77 K) and fluorescence spectra of FPF in isopentane at 77 K and in the solid state are shown in Figure 6 b. As we can see from the figure, on comparison to PFP, the red-shifted absorption and fluorescence spectra of FPF at room temperature is attributed to a longer conjugation length in FPF. From the above discussion, intermolecular interactions should be reduced in FPF. On comparison of respective fluorescence spectra of PFP and
In an attempt to study the effect of intermolecular interactions in polyfluorenes, we investigated in detail the spectral and photophysical properties behavior of oligofluorenes in different environments. The similar spectral properties of these derivatives isolated in an alkane matrix and at high concentrations in an isopentane glass are attributed to conformational changes adopted by each molecule. For all the oligomers, the fluorescence spectra recorded in the solid state exhibit a huge red shift when compared to the fluorescence spectra at 77 K. This is explained by intermolecular interactions induced by π-stacking of oligomer molecules in the solid state. As model compounds to show the importance of the octyl chains in dictating the spectral properties, we synthesized and studied TFTWC, having no octyl chains at the C-9 position of the fluorene moiety, and FPF, having more octyl chains compared to PFP. From the results on these model compounds, we have shown how effectively the emitting spectral region of oligomers can be tuned by altering the side chains attached to the fluorene moiety. This work leads us further in designing new promising systems for LED applications. Indeed, recent work on polyfluorene copolymers with phenylene and thiophene rings substituted with bulky side chains have no aggregation band in the solid state and have drawn attention as efficient blue- and green-emitting polymers, respectively.7(b,c) Acknowledgment. This work was supported by strategic and research grant from NSERC. M.R. is grateful to NSERC for a fellowship. References and Notes (1) Conjugated polymers and related materials; Salaneck, W. R., Lundstrom, I., Ranby, B., Eds.; Oxford University Press: Oxford, 1993; pp 65-169. (b) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605 and the references cited. (2) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Logdlund, M.; Salaneck, W. R. Nature (London) 1999, 397, 121. (3) Hu, B.; Yang, Z.; Karasz, F. E. J. Appl. Phys. 1994, 76, 2419. (4) Blatchford, J. W.; Jessen, S. W.; Lin, L. B.; Gustafson, T. L.; Fu, D. F.; Wang, H. L.; Swager, T. M.; MacDiarmid, A. G.; Epstein, A. J. Phys. ReV. B 1996, 54, 5180. (5) Son, S.; Dodabalapur, A.; Lovinger, A. J.; Galvin, M. E. Science 1995, 269, 376. (6) Yang, Z.; Sokolik, I.; Karasz, F. E. Macromolecules 1993, 26, 1188. (7) (a) Miller, R. D.; Scott, J. C.; Kreyenschmidt, M.; Karg, S.; Klaerner, G.; Chen, W.; Lee, V. Y.; Ashenhurst, J.; Kwak, J. Macromolecules 1988, 21, 1099. (b) Wang-Lin, Y.; Jian, P.; Yong, C.; Wei, H.; Heeger, A. J. Chem. Commun. 1999, 1837. (c) Jian, P.; Wang-Lin, Y.; Wei, H.; Heeger, A. J. Chem. Commun. 2000, 1631. (8) Ranger, M.; Rondeau, M.; Leclerc, M. Macromolecules 1997, 30, 7686. (9) Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996, 118, 7416. (10) Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. AdV. Mater. 1997, 9, 798. (11) Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J. Appl. Phys. 1991, L1941. (12) Nijegorodov, N. I.; Downey, W. S. J. Phys. Chem. 1994, 98, 5639.
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