J. Chem. SOC., Furaduy Trans. I , 1982, 78, 3411-3484 Photophysics of 1,l’-Binaphthyl and its Formation of a Complex with N-( 1 -propyl)-2,5-dimethylpyrrole BY XIU-JIN Luo,? GODFREY S. BEDDARD AND GEORGE PORTER Davy Faraday Laboratory, The Royal Institution, 2 1 Albemarle Street, London AND R. STEPHEN DAVIDSON* Department of Chemistry, The City University, Northampton Square, London EClV OHB Received I 1 th December, I98 1 Fluorescence lifetimes for 1,l ’-binaphthyl in cyclohexane, benzene, propan- 1-01 and acetonitrile have been measured and found to lie between 2.5 and 3.5 ns. The conformational changes which occur in the excited singlet state of 1 ,l’-binaphthyl are so rapid that they elude detection by sub-nanosecond fluorescence spectroscopy. 1,l ’-Binaphthyl forms an exciplex with N-( 1 -propyl)-2,5-dimethylpyrrole.This is weakly fluorescent in non-polar solvents. The kinetics of formation of the exciplex were examined. The radiative decay rate constant for the exciplex apppears to be relatively small. It is suggested that the pyrrole complexes with one of the naphthalene rings of the 1,l’-binaphthyl to give a complex which resembles that formed between the pyrrole and naphthalene. Interest has been shown in the photochemistry of biaryls, e.g. chlorobiphenyls,l 1 ,l’-binaphthy12v and 9,9’-bianthryl,** owing to the fact that in the ground state the two aryl rings are not co-planar, and this can affect both the photochemical and photophysical properties of these compounds. 2-Chlorobiphenyl dechlorinates far more readily than 4-chlorobiphenyl and this has been attributed to the loss of chlorine from the 2-substituted compound relieving steric strain which in turn reduces the overall free-energy change for the dechlorination process.’ The photophysics of 9,9’-bianthryl has been studied in detail.In the excited singlet state the interplanar angle between the two rings is reduced compared with the ground state. However, in polar solvents the excited singlet state of bianthryl undergoes intramolecular electron tran~fer.~ Early studies of the 1,l’-binaphthyl system indicated that the first excited singlet state of this compound is planar.2 However, this view has been chal- lenged and the suggestion made that the interplanar angle between the two naphthyl rings is reduced in the excited singlet state but is not reduced to zero.3 We have addressed ourselves to the question as to how fast the conformational changes in the first excited singlet state of 1,l’-binaphthyl occur and whether the relaxed species can form an exciplex with tertiary amines.The latter point is of interest in the light of the recent work by Yorozu et aL6 They found that the efficiency of quenching the fluorescence of optically active 1, I ’-binaphthyl in non-polar solvents by chiral amines is dependent upon the absolute configuration of the amine, indicating that in such solvents the exciplex formed between the aromatic hydrocarbon and the amine requires the partners to be correctly orientated with respect to each other and in close proximity. t Visiting research fellow, permanent address: Department of Chemistry, Jilin University, Changchun, People’s Republic of China. 34773478 PHOTOPHYSICS OF I J I - B I N A P H T H Y L RESULTS The fluorescence spectrum of 1 , 1 ’-binaphthyl shows some dependence upon solvent polarity (fig.1) but to nothing like the same degree as that of 9,9’-bianthr~l.~ Fluorescence lifetimes were measured by the technique of time-correlated single-photon counting having an instrumental response time of 280 ps. The results are shown in table 1 . In all cases the fluorescence decays were found to be exponential and showed I I 300 400 500 wavelength/nm FIG. 1 .-Fluorescence spectra of 1,l’-binaphthyl in various solvents (excitation wavelength 295 nm) : (---) propan-1-01, (--) benzene, (- - - -) acetonitrile, (-) cyclohexane.TABLE 1 .-FLUORESCENCE LIFETIMES OF 1,l ’-BINAPHTHYL IN VARIOUS DEGASSED SOLVENTS solvent quantum yield lifetime/ns kF/ lo8 s-l cyclohexane 0.79 2.70 2.9 benzene 0.76 2.50 3.0 propan- 1-01 0.79 3.14 2.5 acet oni trile 0.67 3.22 2.0 no grow-in. This also proved to be the case when a highly viscous solvent such as ethylene glycol (> 26 cP) and liquid paraffin (20 cP) was used. In another experiment the fluorescence decay of 1,I’-binaphthyl in poly(methy1 methacrylate) was examined and again no grow-in was observed. Thus the conformational changes occurring in the excited singlet state of 1,l’-binaphthyl do so in less than 280 ps. Addition of N-( 1 -propyl)-2,5-dimethylpyrrole to cyclohexane solutions of 1,l I - binaphthyl quenched the fluorescence of the biaryl.The quenching was accompanied by a diminutive broadening of the binaphthyl fluorescence (fig. 2). The broadeningLUO, BEDDARD, PORTER A N D DAVIDSON 3479 wavelength/nm FIG. 2.-Fluorescence spectra of 1 , 1'-binaphthyl + pyrrole in cyclohexane (excitation wavelength 295 nm). Cl , 1 -BIN cPDMP (-1 1.406 x 0 (--) 1.403 x 10-4 7.579 x 10-3 (---) 1.399 x 2.572 x (- - - -) 1.329 x 4.598 x (--. .--) 1.381 x 6.114 x lo-' (. . . . . . . .) 1.379 x lop4 9.648 x lo-' is attributed to exciplex formation. When very high concentrations of the pyrrole are employed the emission owing to the exciplex becomes more discernible, but even under these conditions no discrete wavelength maximum for the exciplex is observable. A Stern-Volmer plot, based upon the results shown in fig.2, deviates from linearity being concave in shape. Unlike many systems exhibiting intermolecular exciplex f o r m a t i ~ n , ~ ~ , ~ increasing the solvent polarity did not give rise to a structureless fluorescence band, red-shifted from the hydrocarbon fluorescence, but instead little or no emission attributable to the exciplex could be observed (fig. 3). Only in benzene was some exciplex emission discernible. The pyrrole quenched the binaphthyl fluorescence in acetonitrile, the quenching process obeying simple Stern-Volmer kinetics. The rate constant for quenching was evaluated as being 3.4 x 1O1O dm3 mol-l s-l; i.e. at the diffusion-controlled limit (fig. 4). Quenching of the binaphthyl fluorescence by the pyrrole in various solvents was also examined by the technique of time-correlated single-photon counting in which all the light emitted to the red of 400 nm was collected.Since the quantum yield of exciplex fluorescence is3480 PHOTOPHYSICS OF ~,I’-BINAPHTHYL 01 1 1 300 400 xx) wavelength/nm FIG. 3.-Fluorescence spectra of 1, 1’-binaphthyl + pyrrole in various solvents (excitation wavelength 295 nm): (-) cyclohexane, (--) benzene, (---) propan-1-01, (- - -) acetonitrile. 2 2.00 0 + 1 .I* 1.00.’ / 20 [pyrrole] / 1 0-3 mol dmW3 FIG. 4.-Plot of Z,/Z against concentration of pyrrole for quenching of 1,l’-binaphthyl fluorescence in acetonitrile by the pyrrole. so small, it was considered that the percentage of counts owing to exciplex fluorescence will be minimal and would not affect analysis of the decay curves.In all cases the fluorescence decays were not mono-exponential, but they could be resolved into the sum of two exponentials. The fact that the decays are the sum of two exponentials substantiates the view that most of the emission emanates from the binaphthyl rather than the e~ciplex.‘~ The fluorescence decays were fitted to the equation Y(t) = F exp (- t/.rl) + (1 - F ) exp (- t / z JLUO, BEDDARD, PORTER A N D DAVIDSON 348 1 TABLE 2.-FLUORESCENCE LIFETIMES AND OTHER PARAMETERS OBTAINED FROM ANALYSIS OF THE DECAY CURVES FOR MIXTURES OF 1,l '-BINAPHTHYL AND N-( 1 -PROPYL)-2,5-DIMETHYLPYRROLE IN VARIOUS DEGASSED SOLVENTS concentration/mol dm-3 solvent 1 ,l'-binaphthyl pyrrole F a x 100 z,/ns z,/ns cyclohexane 3.77 x 10-4 3.64 x lo-, 0.083 10.96 1.67 benzene 1.73 x 10-4 3.30 x lop2 0.035 10.50 1.47 propan- 1-01 3.54 x 10-4 5.09 x lou2 0.028 8.61 1.78 acetonitrile i 3.30 x 10-4 3.97 x lo-, 0.008 5.65 1.16 a Fis the percentage of the decay component with the lifetime z, present at the onset of the decay.TABLE 3 .-FLUORESCENCE LIFETIMES AND OTHER PARAMETERS OBTAINED FROM ANALYSIS OF THE DECAY CURVES FOR MIXTURES OF l,l'-BINAPHTHYL AND N-( l-PROPYL)-2,5-DIMETHYLPYRROLE IN DEGASSED CYCLOHEXANE SOLUTION concentration/mol dmP3 1, 1'-binaphthyl pyrrole F'x 100 zJns z,/ns 1.38 x 10-4 9.65 x lo-, 0.457 11.19 1.05 1.38 x 10-4 6.11 x 0.367 11.26 1.31 1.39 x 10-4 4.60 x lo-, 0.319 11.12 1.46 1.40 x 10-4 2.57 x lo-, 0.236 1 1.03 1.24 1.40 x 10-4 7.58 x 10-3 0.122 10.33 2.16 1.41 x 10-4 0 0 2.69b a See footnote to table 2; Mono-exponential decay.where F is a constant and z, and z, are lifetimes. The results are shown in table 2. A more detailed study was made of the quenching process in cyclohexane by examining the effect of varying the concentration of pyrrole upon the decay profiles. The results are shown in table 3. Kinetic analysis of the following reaction scheme enabled values for k,, k , and k , to be extracted from the lifetimes given in table 3:76 k3 kproduct BN + PY -JL BN* + PY . " ( B N PY) * + product k A 2 k , k d b6 BN+Py+hv BN + Py BN + Py+hv,,. BN + Py Here Bn is 1 ,l'-binaphthyl, Py is N-( 1 -propyl)-2,5-dimethylpyrrole, hex is the fluorescence from the exciplex,3482 PHOTOPHYSICS OF I,I’-BINAPHTHYL and 1 1 - x - = (k, + k2) (k, + k,) + k , k , [Py].71 7 2 k , is a composite rate constant containing the terms kproduct,kB and k6 k p = k5 + k6 + kprOduCt. From the fluorescence lifetime data in table 1, k , + k , = 3.7 x lo8 s-l. From the appropriate plots of z1 and z, (values in table 3) against concentration (fig. 5 and 6) the following rate constants were obtained : k, = 5.4 x lo9 dm3 mol-l s-l k4 = 6.5 x lo7 s-l k, = 8.45 x lo7 s - l . 1 1 5.0 10.0 [ pyrrolel / 1 0-2 mol dm-3 FIG. 5.-Plot of 1 /zl + 1 /z2 against pyrrole concentration for quenching of 1,l’-binaphthyl fluorescence in cyclohexane by the pyrrole. [pyrrole] mol dm-3 FIG. 6.-Plot of (1 /z,) x (1 /z2) against pyrrole concentration for quenching of 1, 1’-binaphthyl fluorescence in cyclohexane by the pyrrole.LUO, BEDDARD, PORTER A N D DAVIDSON 3483 DISCUSSION Although the absorption spectrum of 1,l'-binaphthyl shows some fine-structure, the fluorescence spectrum is virtually structureless (fig. 1).The lack of structure supports the view that the initially created excited state of binaphthyl undergoes a conform- ational change. This must be a particularly rapid process since it occurs even in viscous solvents within 280 ps. The lack of an appreciable solvent effect upon the fluorescence emission (fig. 2) suggests that little stabilisation of the excited state accrues from charge transfer. The conformational changes presumably lead to a species exhibiting greater delocalisation and it was anticipated that this species should form an exciplex with N-( 1 -propyl)-2,5-dimethylpyrrole which is considerably different to that formed between the pyrrole and naphthalene.8 In particular, it was expected that the exciplex formed from the binaphthyl should fluoresce to the red of the exciplex formed with naphthalene.This proved not to be the case. In cyclohexane solution, exciplex fluorescence produced by quenching of the binaphthyl fluorescence by the pyrrole is barely detectable, manifesting itself only as a broadening of the fluorescence band owing to the binaphthyl. The quantum yield of exciplex emission is extremely small. In polar solvents such as propan- 1-01 and acetonitrile the only detectable fluorescence is that owing to the unquenched binaphthyl. That exciplex formation is occurring in all the solvents is attested by the fact that the fluorescence decays show two distinct components (table 2).The occurrence of two decays is caused by (1) unquenched monomer fluorescence and (2) formation of excited monomer via break-up of the exciplex (having rate constant = k,). For acetonitrile solution, steady-state measure- ments show the quenching process to be diffusion controlled, which indicates that it is exothermic. Thus the lack of appreciable exciplex fluorescence in any of the solvents cannot be caused by inefficient exciplex formation. Indeed, in all solvents, the fluorescence decays contain a measurable amount of the longer-lived species, indicating that even in a polar solvent such as acetonitrile there is some feedback from the exciplex to give the excited monomer. The analysis of the kinetics for exciplex formation and decay in cyclohexane solution is informative.It shows that exciplex formation is favoured over exciplex ,dissociation to give excited monomer by a factor of ca. 100. The rate constant k, is a composite rate constant and contains contributions from kproduct, k , and k,. The rate constants k, and k, appear to be comparable, and this will account to some extent for the lack of emission from the exciplex. If k , (which could include intersystem crossing to give the triplet binaphthyl) is significantly larger than k,, this will also have a marked effect upon the quantum yield for exciplex formation. Radiative decay constants for exciplexes formed between styrenes and triethylamineg are all substantially less than 5 x lo7 s-l, which suggests that k, for the binaphthyl system may well be less than k, and k,.It is difficult to give an estimated value for k , since such factors as the tightness of the exciplex and the orientation of the partners, both of which are difficult to quantify, affect k,.l0 The lack of any exciplex emission in polar solvents such as acetonitrile is not so surprising, since electron transfer to give radical ions will also compete with the fluorescent decay processe~.~~9 Since the quenching of 1,l '-binaphthyl fluorescence by the pyrrole is a dynamic process, and therefore subject to diffusional control, the pyrrole must quench the conformationally relaxed excited singlet state of the binaphthyl. Surprisingly, what little exciplex emission that can be observed resembles that produced by the interaction of the excited singlet state of naphthalene with the pyrrole.With binaphthyl, the fluorescence maximum of the exciplex appears to be obscured by the fluorescence of the binaphthyl, which is red-shifted from naphthalene fluorescence. These fluorescent3484 PHOTOPHYSICS OF ~,~'-BINAPHTHYL properties of the exciplex indicate that the pyrrole is mainly associated with one of the naphthalene rings of the binaphthyl system, which suggests that there is not extensive delocalisation in the excited singlet state of 1 , 1'-binaphthyl. Furthermore, extensive delocalisation between the two rings should reduce the reduction potential of the binaphthyl, and consequently exciplexes formed with such a system should be red-shifted relative to the complex formed with naphthalene, since (from Ecomplex cc reduction potential Ecomplex = ID - E, +constant where ID is the ionisation potential of the donor and EA is the electron affinity of the a c ~ e p t o r ) .~ ~ ~ ~ Since this is not observed we conclude that in the conformationally relaxed excited singlet state of 1,l '-binaphthyl, which forms a complex with the pyrrole, there is not extensive electron delocalisation. EXPERIMENTAL N-( 1 -propyl)-2,5-dimethylpyrrole has been previously described.* 1 , 1'-Binaphthyl was syn- thesised from 1 -bromonoaphthalene according to the procedure described in the literature. l1 Fluorescence lifetimes were measured by the method previously described.8 Analysis of the fluorescence decay curves was carried out using an iterative non-linear least-squares program.12 X-J.L. acknowledges the financial support of the Ministry of High Education, People's Republic of China and Jilin University. We also thank the S.E.R.C. for financial support. N. J. Bunce, Y. Kumar, L. Ravanal and S. Safe, J. Chem. SOC., Perkin Trans. 2, 1978, 880. M. F. M. Post, J. Langelaar and J. D. W. Van Voorst, Chem. Phys. Lett., 1975,32,59; 1977,46,331; Chem. Phys., 1976, 14, 165. M. Irie, K. Yoshida and K. Hayashi, J. Phys. Chem., 1977, 81, 969; 1977, 81, 973. F. Schneider and E. Lippert, Ber. Bunsenges. Phys. Chem., 1968, 72, 1155; 1970, 74, 624. N. Nakashima, M. Murakawa and N. Mataga, Bull. 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