J. Phys. Chem. 1984,88, 1163-1 168
1163
Rotamerism in (2-Anthry1)ethylenes. Evidence from Fluorescence Lifetimes and Quenching Studies' T. Wismontski-Knittei, P. K. Das,* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556
and E. Fischer Department of Structural Chemistry, Weizmann Institute of Science, Rehovot, Israel (Received: April 1 I , 1983; In Final Form: August 15, 1983)
The fluorescence decay of trans (2-anthry1)ethylenes containing phenyl, 2-naphthyl,and 2-thienyl groups as the second substituent is biexponential. The decay profiles in benzene are analyzable in terms of two lifetimes in the ranges 6-1 1 and 17-30 ns, respectively. The complex decay behavior as well as the observation of excitation wavelength dependence of fluorescence spectra, quantum yields, exciplex emission maxima, and fluorescence quenching constants (with oxygen, ethyl iodide (EtI), and N,N-dimethylaniline (DMA) as quenchers) is attributed to the existence of quasi-energetic rotamers in solutions of (2-anthry1)ethylenes. Exciplex lifetimes and transient absorption spectra of radical anions obtained by nanosecond laser flash photolysis in the presence of N,N-dimethylaniline are also presented.
Introduction Ground-state conformeric equilibria in solutions of several arylethylenes, and their manifestations in terms of heterogeneity in fluorescence and photochemistry of these systems, are now well recognized.2-8 Using trans- l-phenyl-2-(2-naphthyl)ethyleneas an example, the manner in which more than one conformer can arise by virtue of rotation of the 2-naphthyl group about the quasi-single bond at the 2-position is
P @ I
I
f@
n
Over the past several years, mounting evidence in support of the rotamerism in stilbene analogues containing a wide range of aryl moieties such as naphthalene, anthracene, phenanthrene, benzophenanthrene, pyrene, pyridine, and quinoline has been obtained from studies based on absorption-emission spectroscopy and photodynamical measurements. In an earlier paper3=from the laboratory of one of us, it has been demonstrated that 2-anthryl-substituted arylethylenes in (1) The work described here was supported by the Office of Basic Energy Sciences of the Department of Energy (T.W-K. and P.K.D.). This is Document No. NDRL-2459 from the Notre Dame Radiation Laboratory. (2) (a) Scheck, Yu. B.; Kovalenko, N. P.; Alfimov, M. V. J. Lumin. 1977, 15, 157-68. (b) Alfimov, M. V.; Scheck, Yu. B.; Kovalenko, N . P. Chem. Phys. Lett. 1976.43, 154-6. (c) Razumov, V. F.; Alfimov, M. V.; Shevchenko, G . A.; Kovalenko, N. P. Dokl. Phys. Chem. (Engl. Transl.) 1978,238, 135-7. (3) (a) Fischer, G.; Fischer, E. J . Phys. Chem. 1981, 85, 2611-3. (b) Fischer, E. Ibid. 1980, 84, 403-10. (c) Fischer, E. Bull. SOC.Chim. Belg. 1979, 88, 889-95. (d) Fischer, E. J. Photochem. 1981, 17, 331-40. (e) Goedicke, Ch.; Stegemeyer, H.; Fischer, G.; Fischer, E. Z . Phys. Chem. (Frankfurt um Main) 1976,101, 181-96. ( f ) Haas, E.; Fischer, G.; Fischer, E. J. Phys. Chem. 1978,82, 1638-43. (g) Fischer, E. J. Chem. Soc., Perkin Trans. 2 1981, 1264-6. (h) Fischer, G.; Fischer, E. Mol. Photochem. 1974, 6,463-72. (4) (a) Birks, J. B.; Bartocci, G.; Aloisi, G . G.; Dellonti, S.; Barigelletti, F. Chem. Phys. 1980, 51, 113-20. (5) Mazzucato, U. Pure Appl. Chem. 1982, 54, 1705-21. (6) (a) Ghiggino, K. P. J. Photochem. 1980, 12, 173-7. (b) Matthews, A. C.; Sakurovs, R.; Ghiggino, K. P. Ibid. 1982, 19, 235-44. (7) Muszkat, K. A.; Wismontski-Knittel, T. Chem. Phys. Lett. 1981,83, 87-90; J . Phys. Chem. 1981, 85, 3427-31. (8) Wismontski-Knittel, T.; Sofer, I.; Das, P. K. J. Phys. Chem. 1983, 87, 1745-52.
0022-3654/84/2088-1163%01.50/0
solution exhibit a spectacular dependence of fluorescence spectra on excitation wavelengths. The objective of the present work has been to show that the multiple fluorescence spectra of these systems are in turn associated with multiexponential decay times and, hence, establish the involvement of multiple conformeric fluorophores on a firmer basis. For additional support, the quenching of fluorescence under steady-state conditions has been investigated with special emphasis on excitation wavelength dependence of quenching constants and exciplex emission spectra. Finally, a nanosecond laser flash photolysis study has been carried out to obtain data concerning lifetimes of exciplexes and transient absorption spectra of radical anions produced by charge-transfer quenching of singlets of (2-anthry1)ethylenes. Three l-aryl-2-(2-anthryl)ethylenesin trans configurations bearing phenyl, 2-naphthyl, and 2-thienyl groups in the 1-position have been studied. The symbols that we have used for these systems are Ph-2A, 2N-2A, and 2T-2A, respectively. The structures, including those of possible quasi-energetic conformers in each case, are presented in Figure 1.
Experimental Section The sources of the (2-anthry1)ethylenes are given in an earlier paper.3a The sources and methods of purification of most of the solvents and reagents used are also described el~ewhere.~'J Bromobenzene (Fisher) and ethyl iodide (Aldrich) were distilled before use. 9,lO-Diphenylanthracene (Aldrich) was recrystallized from benzene. The absorption spectra were recorded in a Cary 219 spectrophotometer. Steady-state fluorescence measurements were performed in a photon-counting spectrofl~orimeter~ from SLM Corp. or a Perkin-Elmer MPF-44 i n ~ t r u m e n t Unless . ~ ~ otherwise not,&, the emission spectral data presented in this paper were obtained by using the former apparatus and are corrected for the detector response. The excitation was carried out at a right angle to the direction in which the emission was monitored. Except when a study of oxygen quenching was intended, the solutions were deaerated by purging with argon. For quantum yield (&) measurements, degassed solutions of 9,lO-diphenylanthracene in benzene optically matched with the solutions of (2-anthry1)ethylenes at the excitation wavelengths were used as the reference. The fluorescence quantum yield of 9,lO-diphenylanthracene in benzene was measured to be 0.81 with excitation at 370 nm by using quinine sulfate in 1.0 N H2S04as the reference (& = 0.55)1° (9) Chattopadhyay,S. K.; Das, P. K.; Hug, G. L. J. Am. Chem. Soc. 1982,
104. 4507-14.
(10) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229-35.
0 1984 American Chemical Societv
1164 The Journal of Physical Chemistry, Vol. 88, No. 6,1984
Wismontski-Knittel et al.
IO
5 0 400
500
400
500
WAVELENGTH, NM
Figure 2. Corrected fluorescence spectra of 2N-2A (A, B), 2T-2A (C, D), and Ph-2A (E, F), each at -1 X M, and exciplex emission spectra in deaerated benzene solutions with DMA as the quencher. For A, C, and E, A,, = 420 nm and for B, D, and F, A,, = 370 nm. For A-C,
Figure 1. Structures of trans (2-anthry1)ethylenes under study: (I) l-phenyl-2-(2-anthryl)ethylene (Ph-2A); (11) 1-(2-naphthyl)-2-(2anthry1)ethylene (2N-2A); (111) 1-(2-thienyl)-2-(2-anthryl)ethylene
(2T-2A). for it; this value is in reasonable agreement with 46s reported in the literaturelOJ’ and was assumed to be independent of excitation wavelengths at 330-420 nm. The fluorescence decay profiles were measured in a photoncounting setup from Photochemical Research Associates with a thyratron-operated, hydrogen-filled spark lamp as the pulsed excitation source. A Hamamatsu R955-P photomultiplier tube was used as the detector for fluorescence. Two Jobin-Yvon monochromators (set at 10-30-nm band-pass) allowed selection of appropriate spectral regions for excitation and emission, respectively. The laser flash photolysis experiments were conducted with a Molectron UV-400 nitrogen or a Quanta-Ray Nd-YAG (Model DCR-1 A) laser source for pulsed excitation. The description of the kinetic spectrophotometer for transient measurements is available in previous publication^^^^^ from the Radiation Laboratory. For 420-nm laser excitation, a methanolic solution of stilbene-420 (Exciton) pumped by the 355-nm output (third harmonic) from the Nd:YAG source was used in a dye laser system (Quanta-Ray Model PDL-1).
Results Fluorescence Spectra, Quantum Yields, and Lifetimes. The absorption spectrum of each of the three (2-anthry1)ethylenes under study is characterized by a well-structured band system at 340-400 nm. The location and vibronic features of this band system are very similar to those of the lowest energy band system observed in the case of anthracene or 9,lO-diphenylanthracene. In addition, a prominent shoulder is noticeable at 400-430 nm in the absorption spectrum of each of the three (2-anthry1)ethylenes.3a A study of concentration dependence ( 10-4-10-5 M in benzene) shows that the tail absorption is not due to formation of aggregates. Recrystallization from methanol or purification by thin-layer chromatography on silica gel plates using heptane + diethyl ether mixtures as eluents does not effect any change (11) Stevens, B.; Algar, B. E. J . Phys. Chem. 1968, 72, 2582-7. (12) Das, P. K.; Small, R. D., Jr.; Scaiano, J. C. J. Am. Chem. SOC.1979, 101, 6965-70. Das, P. K.; Battacharyya, S . N. J . Phys. Chem. 1981, 85, 139 1-5.
[DMA] = 0.0, 13.0, 26.0, 52.0, and 184.0 mM for the spectra denoted by a-e, respectively. For D, [DMA] = 0.0, 5.0, 13.0, 26.0, and 184.0 mM for a-e, respectively. For E, [DMA] = 0.0, 13.0,31.5, and 158.0 mM for a-d, respectively. For F, [DMA] = 0.0, 2.6, 5.2, and 158.0 mM for a-d, respectively. The expanded exciplex emission spectra marked by e,e and d correspond to A,, = 370 nm and those marked by e’,e’ and d’ correspond to A,, = 420 nm. TABLE I: Emission Spectral Data of (2-Anthry1)ethylenes
solvent (temp) benzene (295 K) toluene (273 K) benzene (295 K) toluene (273 K)
fluorescence maxima? nm hex = 420 nm Ph-2A 412,437,465 421,450,480 413,436,462 426,452,482 2N-2A 421,444,475 430,455,485 415,442, 470 428,457, 488 hex = 370 nm
2T-2A benzene (295 K) toluene (273 K)
425,451,475 433,460,490 423,450,479 433,462,497 a The data in benzene were obtained by using the SLM spectrofluorimeter; those in toluene were recorded in the Perkin-Elmer instrument; neither set of spectral data was corrected for detector response. The two sets of emission peaks characterize the two assumed rotamers. in the relative intensity of the low-energy shoulder (400-430 nm) with respect to that of the band system at 340-400 nm. All of our photophysical measurements were confined to excitations in the spectral region 330-430 nm. In solutions, each of the three (2-anthry1)ethylenes exhibits strong fluorescence. In solvents such as methylcyclohexane, benzene, and toluene at room temperature the fluorescence spectra are modestly structured. In Figure 2, the spectra denoted by a’s illustrate this. The shape of the fluorescence spectra is found to vary with excitation wavelengths (Aex), particularly when A,, is varied along the long-wavelength edge of absorption (390-430 nm). The observed fluorescence spectral maximum (AF,max) becomes progressively red shifted as the excitation is carried out at longer and longer wavelengths. A compilation of fluorescence maxima in benzene at room temperature and in toluene at 273 K observed with A,, at 370 and 420 nm is presented in Table I. Essentially similar but better resolved spectral data in terms of two distinct sets of peaks have been obtained in toluene at lower temperatures (198-223 K).3a The energy separations between the corresponding peaks belonging to the two sets are in the range 700-900 cm-I. The fluorescence quantum yields (&) of the (2-anthry1)ethylenes, measured in benzene at room temperature, are 0.5-1 .O
The Journal of Physical Chemistry, Vol. 88, No. 6, 1984 1165
Rotamerism in (2-Anthry1)ethylenes TABLE 11: Fluorescence Quantum Yields (@F) for (2-Anthrv1)ethvlenesin Benzene at 296 K
0
336 377
412 412
352 366 379
420 419 418
Ph-2A 0.96 393 0.82 410 2N-2A 0.72 388 0.70 406 0.5 8 2T-2A 1.07 381 0.95 405 0.70 410
345 426 349 425 362 426 a Corrected for detector response; i 1 nm. *15%.
0
412 424
0.86 0.52
421 426
0.69 0.55
El
424 0.69 427 0.69 433 0.50 Estimated error,
(see Table 11). &=varies with excitation wavelengths particularly in the spectral region 390-430 nm. It decreases noticeably at longer A,,. Fluorescence decay profiles, measured by the time-correlated photon-counting technique, show the involvement of multiexponential processes. Resonably good, nonlinear least-squares fits13 (x2 -< 1.5) are obtained between experimental decay profiles and convoluted curves computed from lamp profiles using the biexponential fluorescence response function given by eq 1. Equation ZF(t) =
a1
exp(-t/71)
+ a 2 exp(-t/T2)
(1)
1 is applicable to a composite fluorescing system where two noninteracting fluorophores, with lifetimes +1 and T ~ respectively, , are excited by fractional light absorption at the exciting wavelength and contribute independently to the emission at the monitoring wavelength. While 7, and 7 2 are the experimental parameters that are of major interest, comparison of the preexponential coefficients, a , and a*,gives information on the relative contributions of the two fluorescing species under the conditions of an experiment. Specifically, cyt, expressed in eq 2, is determined4 by ai(Aex>Aem)
= (constant)LA(Xex)LF(Xem)ki
m
.
0
o
,
’
olo.ao
i . L
3’1.33
2b.67
(13) Bevington, P. R. “Data Reduction and Error Analysis for the Physical Sciences”; McGraw-Hill: New York, 1969
52.67
x10’
63.33
74.00
3,.j7
M) in deaerated Figure 3. Fluorescence decay of 2T-2A (3.5 X benzene solution at room temperature (Aex = 370 nm, , , ,A = 420 nm): (1) excitation lamp profile measured with a scattering suspension of BaS04 in water; (2) experimental fluorescencedecay profile (dotted line); (3) best-fit biexponential decay curve (solid line) computed by convolution of the lamp profile. The plot of percentage difference between the experimental and convoluted curves is shown at the top for each channel. TABLE 111: Results of Fluorescence Decay Measurements in Benzene at Room Temperature
(2)
(a) the fractional absorbance,AA (=A,/A,A = total absorbance) of the ith species at A,, (b) the fractional contribution, LF, to fluorescence at A,, (determined by the product of 4Fof the ith of the function representing its species and the value, at ,,A, intrinsic fluorescence shape), and (c) the rate constant, k,, of fluorescence decay (=~,-l). If it is possible to find an isoemissive wavelength wherefiF =fzF, measurement of decay profiles at this wavelength using varying Aex% would enable one to reconstruct the absorption spectra of the individual fluorescing components. Such a technique has in fact been used by Birks et al! in analyzing the composite fluorescence observed in the case of trans-lphenyl-2-(2-naphthyl)ethylene. A typical experiment for measurement of dual fluorescence lifetimes based on eq 1 is described in Figure 3. Data concerning T,, T ~and , a1/a2in benzene are presented in Table 111. Lifetimes were also measured in methylcyclohexane and bromobenzene at room temperature. In the former solvent, T~’S and T ~ ’ Swere found to be -1.5 and -3 times longer, respectively, than in benzene; in bromobenzene they were 1.5-2.0 times shorter. Nanosecond laser flash photolysis of (2-anthry1)ethylenes enabled us to observe clearly the dual decay behavior in emission profiles in a time domain of 50-400 ns. Using 337.1-nm for A,, we constructed time-resolved fluorescence spectra from experimental traces for fluorescence at various wavelengths (400-500 nm). Again, the maxima in the spectra corresponding to the long-lived component were found to be blue shifted by 5-10 nm relative to those corresponding to the short-lived component. Further studies under higher resolution using a SLM spectro-
42.30
CHANNEL u
Ph-2A
337 370 420
420 420 450
9.3 25.6 0.38 0.12 0.20 10.7 27.4 0.63 7.8 23.6 2.7 0.47 2N-2A 17.6 0.46 0.15 430 6.7 370 0.42 17.2 2.1 450 6.0 420 2T-2A 31.8 0.61 0.12 430 7.3 370 0.42 29.3 3.1 450 6.8 420 a Band-pass used: 10-20 nm. Average of two to three measurements; i- 20%. Calculated from the following relationship: p = (ollrl)/(ol,rl+ a2r2). fluorimeter with provisions for measuring time-resolved emission spectra are in progress. Quenching Studies. Exciplex Emission Maxima and Lifetimes. The results regarding fluorescence lifetimes presented in the previous section suggest clearly that for each of the (2anthry1)ethylenes two species predominantly contribute to the fluorescence. The lifetimes, T~ and 72, associated with these two species significantly differ from one another, T ~ / T ,being in the range 2-4. Thus, it is expected that a fluorescence quencher at a given concentration would preferentially suppress the emission from the longer-lived singlet allowing the emission spectrum of the short-lived singlet to be revealed in a prominent fashion. We have used oxygen, ethyl iodide (EtI), and N,N-dimethylaniline (DMA) as quenchers in the present study. Figure 4 shows how the emission spectrum of Ph-2A in toluene changes at increasing concentrations of oxygen in the solution. It is noted that the quenching effect of oxygen is the most pronounced at 412, 435, and 462 nm, suggesting that those wave-
1166 The Journal of Physical Chemistry, Vol. 88, No. 6,I984
Wismontski-Knittel et al. TABLE V: Data Concerning Quenching of Fluorescence of Trans (2-Anthry1)ethylenesby DMA and Et1 in Various Solvents
-I
z Y 6
solvent
fn V
methylcyclohexane
3
benzene
quencher (IoF/IgF - l)/[Q]",b Ph-2A DMA DMA
J IA
Et1
W
2
5a
acetonitrile
DMA
W
methylcyclohexane WAVELENGTH, NM
Figure 4. Uncorrected fluorescencespectra (A,
= 370 nm) of Ph-2A in toluene solution at room temperature: (a) argon flushed; (b) air saturated; (c) oxygen saturated. The emission spectra are normalized at 455
benzene
2N-2A DMA DMA Et1
nm. These were recorded in the Perkin-Elmer spectrofluorimeter.
acetonitrile
TABLE IV: Quenchingof Fluorescence of (2-Anthry1)ethylenes by Oxygen in Toluene at Room Temperature quencher A, nm I,F/IF a
methylcyclohexane
DMA 2T-2A DMA
'f
air
air
oxygen
air
2.38 (413) 1.85 1.45 (426)
2N-2A 3 70 415 422 370 415 422
1.82 (415) 1.45 1.39 (428) 5.26 (415) 3.33 3.45 (428)
2T-2A 3 70
415
oxygen
bepzene
Ph-2A 380 405 425
430 370 415 430
2.56 (423) 1.82 1.45 (433) 9.09 (423) 4.35 3.70 (433)
a IoFand I F denote the relative emission intensities in argonand air- or 0,-flushed solutions, respectively. For excitations at 370-380 and 422-430 nm, the ratios of intensities correspond to those observed at the respective fluorescence maxima, given (in nanometers) in the parentheses. For excitations at 405-415 nm, the ratios of the areas under the observed fluorescence spectra (uncorrected) are given. All data were obtained by using the Perkin-Elmer spectrofluorimeter.
lengths correspond to the maxima of fluorescence from the longer-lived singlet. Table IV summarizes the fluorescence quenching by air and by oxygen of the three (2-anthry1)ethylenes in toluene. These data indicate a definite dependence of the extent of oxygen quenching upon lex. Clearly, the fluorescing species that absorbs preferentially at short wavelengths (370-380 nm) are affected by oxygen much more than the one that absorbs at longer wavelengths (422-430 nm), indiating that its T~ is longer. The Stern-Volmer plots for the quenching of fluorescence originating from a system containing multiple fluorophores are expected to bend in a sublinear fashion.s However, as it has been shown in a previous studys concerning naphthylethylenes, the curvilinearity of the plots may be too small to be detected within the usual precision of fluorimetry if a small range of quencher concentrations is used. Use of quencher concentrations over a wide range brings in complicating factors such as changing the nature of the solvent and the activity coefficient of the quencher, screening of the exciting light by the quencher, etc. Table V summarizes the effects of DMA and Et1 as quenchers for fluorescence of (2-anthryl)ethylenes, as expressed in terms of quenching constants obtained from measurements at a single
DMA
Et1 acetonitrile
DMA
65.4 ( 3 8 0 , 4 1 4 ) 30.8 ( 4 2 5 , 4 4 3 ) 138.5 ( 3 8 0 , 4 1 3 ) 55.8 ( 4 2 5 , 4 5 0 ) 1.8 ( 3 7 0 , 4 1 3 ) 1.5 ( 4 1 0 , 4 5 0 ) 434.6 ( 3 7 0 , 4 3 8 ) 192.3 (420, 446) 31.9 (370, 419) 18.8 ( 4 2 0 , 4 5 0 ) 65.4 (370, 421) 53.8 (420, 430) 3.6 (370, 420) 2.1 (420, 430) 242.3 ( 3 7 0 , 4 4 5 ) 170.4 (420, 448) 22.3 ( 3 7 0 , 4 2 5 ) 18.5 (420, 427) 53.8 ( 3 7 0 , 4 2 5 ) 37.7 (420, 427) 0.6 (370, 425) 0.6 (420, 433) 259.2 ( 3 7 0 , 4 5 0 ) 173.1 (420, 452)
[Q] = 0.026 M for DMA and 0.33 M for EtI. in nanometers, are shown in parentheses.
A,,
and ,,A
quencher concentration. In most of the cases studied, SternVolmer plots were drawn by using quenching data at varying concentrations of DMA and Et1 over the ranges 0-0.04 and 0-0.4 M, respectively, and were found to be practically linear. The data in Tables IV and V clearly establish the dependence of quenching constants on both A,, and, , ,A, a manifestation of the involvement of multiple fluorescing species possessing distinct absorption and emission spectra. On the basis of the assumption that the quenching data obtained by using short Aexk (370-380 nm) and Amon's(414-425 nm) are related essentially with the fluorescing components with longer lifetimes (Q's), the bimolecular rate constants (kqF)for quenching by DMA and Et1 in benzene are calculated to be in the ranges (1.7-5.1) X lo9 and (0.18-2.0) X los M-' s-'. Since T Z values in methylcyclohexane are found to be 2-3 times longer than those in benzene, the comparison of quenching data for DMA in benzene and methylcyclohexane suggests a (4-6)-fold decrease in kqFon going from the former to the latter solvent. On the other hand, a (3-5)-fold increase is noted in the fluorescence quenching constants for DMA on changing solvent from benzene to acetonitrile (Table V); this appears to be primarily a reflection of an increase in kpFin the polar solvent, as expected from the charge-transfer nature of the interactions in the quenching process: In nonpolar solvents, namely, benzene and methylcyclohexane, the quenching of the singlets of (2-anthry1)ethylenes by DMA is accompanied by the formation of radiative exciplexes. The exciplex emission spectra, as they develop at increasing concentrations of DMA in benzene, are presented in Figure 2. Again, changing excitation from short to long wavelengths, e.g., from 370 to 420 nm, results in the shifting of the exciplex emission maxima (A,,,,) to the red (by 10-13 nm) (see Figure 2). The spectral data for exciplex emissions are summarized in Table VI. Note that in benzene are red shifted by 30-40 nm relative to those in methylcyclohexane. The exciplex lifetimes (7,) are long enough to be measurable by nanosecond laser flash photolysis using 678-11s laser pulses for excitation. The lifetime data with DMA as the quencher in methylcyclohexane and benzene, obtained by 337.1- and 420-nm excitation, are given in Table VI. Practically no variation in T,IS
The Journal of Physical Chemistry, Vol. 88, No. 6,1984 1167
Rotamerism in (2-Anthry1)ethylenes TABLE VI: Exciplex Emission Spectral Data for (2-hnthry1)ethyleneswith DMA as the Quencher he,maxF nm hex=
solvent
370nm
methylcyclohexane benzene
487 520
methylcyclohexane benzene
482 522
he,=
420nm
.re,b ns hex =
hex= 337.1 nm
420nm
Ph-2A
2N-2A
497 532
60
29
28 61
492 535
31 54
29 51
2T-2A
methylcyclohexane
23 480 493 34 benzene 5 20 532 50 43 a Observed at [DMA] = 0.16-0.18 M ; i 2 nm. Observed at [DMA] = 0.20 M ; A,,= 450-580 nm; +lo%.
0.10
0.05
was observed upon changing the monitoring wavelength throughout the exciplex emission spectra. At the same DMA concentration (0.20 M), the exciplex lifetimes in methylcyclohexane are about half of those in benzene. With Ph-2A and 2N-2A, the exciplex lifetimes with 337.1 and 420 nm as lea’sare nearly identical. For 2T-2A, 7 , at the longer A,, is shorter. Although no attempt has been made to analyze the exciplex decay in this case in terms of a biexponential process, the data representing “average” lifetimes qualitatively signify that the exciplex from the component with a smaller T~ is also slightly shorter-lived. Transient Spectra of Radical Anionr. The quenching of singlets of (2-anthry1)ethylenes by Et1 and DMA in benzene and methylcyclohexane results in enhanced formation of triplets. The following paper14 in this issue describes the photophysical properties of the triplets. In acetonitrile, quenching by DMA leads primarily to the formation of radical ions. The transient spectra obtained by 337.1-nm laser photolysis are shown in Figure 5. The absorption spectra of the radical anions are all characterized by maxima at 445-450 nm. That the observed spectra are primarily due to radical anions (rather than triplets) is established by the lack of similarity with the T-T absorption spectral4 (particularly in the 600-650-nm region) and relatively high rate constants (-1.0 X 1O’O M-’ s-l) for their quenching by oxygen. Rate constants for the quenching of (2-anthry1)ethylene triplets by oxygen in acetonitrile are much smaller ((4.5-5.0) X lo9 M-I s-l ).
Discussion The results from both lifetime measurements and quenching studies amply establish the heterogeneous character of the photophysical properties of the (2-anthry1)ethylenes. The most plausible manner in which this heterogeneous behavior may be explained is that more than one conformer arising from rotation about the quasi-single bonds, as shown in Figure 1, exist. The goodness of fit of the fluorescence decay profiles with biexponential decay functions convoluted from eq 1 suggests that predominantly two fluorescing conformers contribute in the case of each of the (2-anthryl)ethylenes, although for 2N-2A and 2T-2A four structurally distinct conformers are possible without imposing any steric hindrance (Figure 1). It is possible that some of these conformers are very similar in terms of photophysical properties or have contributions too small to be revealed under low-resolution conditions. As a first approximation, an understanding of the spectral and photophysical behaviors of the (2-anthry1)ethylenes can be obtained on the basis of the existence of two rotamers (designated by 1 and 2) in each case. An intercomparison of the properties of the pair of conformers existing in the solution of a particular (2-anthry1)ethylene is of ~ interest. Conformer 1, characterized by a shorter lifetime ( T = 6-1 1 ns), absorbs and emits at longer wavelengths. Also, the exciplex (with DMA) from this conformer has its emission ~~
~
~~
(14) Wismontski-Knittel, T.; Das, P. K. J. Phys. Chern., following paper in this issue.
400
500
600
700
WAVELENGTH,NM
Figure 5. Transient absorption spectra of radical anions of (2-anthry1)ethylenes (plus radical cation of DMA) observed by 337.1-nm laser excitation of their solutions ((1-5) X M) in deaerated acetonitrile containing 0.20 DMA: (A) Ph-2A; (B) 2N-2A; and (C) 2T-2A. Each spectrum corresponds to -50 ns following the laser flash. maximum located at a longer wavelength. From the trend in the excitation wavelength dependence of apparent fluorescence quantum yields (Table 11), it can be concluded that the fluorescence yield of conformer 1 is smaller and, as observed in the cases of 2T-2A and Ph-2A, may be as small as half of the yield of its counterpart. The implication of this qualitative observation is that, although the observed lifetimes (7:s) for a pair of conformers differ by factors of 2-4, the corresponding intrinsic radiative lifetimes, given by ~i/+F,j, do not seem to differ from one another to that extent. In fact, they may even be identical, because the spectral resolution is poor, and the observed +F)s do not represent the two pure modifications characterized by T~ and 72.
The fractional contributions of conformers 1 and 2 to the observed fluorescence may be suitable expressed in terms of @, given by eq 3. Using the data concerning a I / qand 7:s obtained from (3) fluorescence decay measurements, @ has been calculated (see column 6 of Table 111). As expected, the fractional contribution are shifted of conformer 1, given by fi, increases as A,, and A, to longer wavelengths. From the point of view of the fact that +F,l < +F,2, the fractional contribution of conformer 1 in terms of absorption alone (at A), is larger than that expressed by @. Fluorescence observed by excitation at 370-380 nm and monitoring at 420-430 nm arises primarily from the conformer with long lifetime (72). The quenching data with oxygen as the quencher for the three anthrylethylenes in toluene (Table IV) obtained with excitation/monitoring in these spectral regions reflects faithfully the trend observed in T ~ ’ Sof these systems in benzene (Table 111), that is, 2T-2A > Ph-2A > 2N-2A. This suggests that the bimolecular rate constants (k:) for fluorescence quenching of the anthrylethylenes are all very high (-2 X 1Olo M-’ s-l) as well as very similar to one another. On the other hand, the corresponding quenching data with DMA as the quencher in benzene (Table V) indicate a systematic decrease in kqFas the second substituent in (2-anthry1)ethylenes is changed in the following order: Ph, 2N, and 2T. A parallel change in the excited-state reduction potential (oxidizing character) is implied and this trend may arise in part from the corresponding parallel decrease in singlet energies as evident from the 0,O bands of fluorescence spectra (Table I). A comparison of fluorescence lifetimes of (2-anthry1)ethylenes with those of related stilbene analogues reveals an interesting trend.
J . Phys. Chem. 1984,88, 1168-1 173
1168 >-
t v)
Fluorescence maxima, nm 0 : 420 447 476
A : 4 0 9 433 459
400
450
500
10 9 6 3495
550
WAVELENGTH, N M
Figure 6. Fluorescence spectra, A and B, of component rotamers with lifetimes 34.95 and 10.96 ns, respectively, observed from a solution of Ph-2A in toluene (Ae,, = 370 nm) by the PRS method of SLM.
There is, in general, a progressive decrease in the radiative rate , the aromatic subconstant, kF (calculated by kF = ~ F / T F ) as stituents in arylethylenes become more and more extended (polycyclic). Thus, while stilbene is associated with a very large kF value (- lo9 s-l in hydrocarbon solvents),l5,l6kF for l-phenyl2-(l-naphthyl)ethylene in n-hexane5 is 3.4 X lo* s-l. 1(15) Good, H. P.; Wild, U. P.; Haas, E.; Fischer, E.; Resewitz, E.-P.; Lippert, E. Ber. Bunsenges. Phys. Chem. 1982, 86, 126-9. (16) Birch, D. J. S.; Birks, J. B. Chem. Phys. Lett. 1976, 38, 432-6.
Phenyl-2-(2-naphthyl)ethylene is characterized by the presence of two rotamers in solutions with estimated kF value^,^ 1.6 X 10’ and 0.2 X lo8 s-l (in n-hexane). Our lifetime and quantum yield data for Ph-2A lead to approximate estimates for kF values for the longer-lived conformer as 0.4 X lo8 s-’ in benzene and 0.16 X 10’ s-l in methylcyclohexane. As proposed for l-phenyl-2(2-naphthy1)ethylene by Aloisi et a1.,17 the decreasing trend in the radiative rate constant is probably indicative of involvement of low-lying weakly allowed states such as the dipole-forbidden lA; state of the ethylenic moiety or state(s) of low oscillator strengths related with the aryl groups. Note Added in Proof. Recently, SLM’s Customers Application Lab analyzed the composite fluorescence from a solution of Ph-2A in toluene by applying the technique called phase resolution of spectra (PRS). The results, shown in Figure 6, are in good agreement with those obtained in the present study. We thank SLM for the PRS analysis. Acknowledgment. T.W-K. and P.K.D. are grateful to Drs. L.
K. Patterson and G. L. Hug for help and guidance in the use of the photon-counting equipment for nanosecond lifetime measurements and of computer programs for deconvolution of twoexponential decay profiles. We thank Prof. A. E. Siegrist, Basel, for generous gifts of (2-anthry1)ethylenes. Registry No. Ph-2A, 78405-79-5; 2N-2A, 31 136-37-5; 2T-2A, 78405-84-2; oxygen, 7782-44-7;EtI, 75-03-6;DMA, 121-69-7; Ph-2A radical anion, 88824-48-0; 2N-2A radical anion, 88824-49-1; 2”-2A radical anion, 88824-50-4. (17) Aloisi, G. G.; Mazzucato, U.; Birks, J. B.; Minuti, L. J . Am. Chem. SOC.1977, 99, 6340-7.
Laser Flash Photolysis Study of (2-Anthry1)ethylenes. Triplet-Related Photophyslcal Behaviors’
T. Wismontski-Knittel and P. K. Das* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: April 11, 1983; In Final Form: August 15, 1983)
The triplet-state photophysical behaviors of three trans-l,2-diarylethyleneswith a phenyl, 2-naphthyl, or 2-thienyl group as one substituent and a 2-anthryl as the other have been studied by laser flash photolysis and pulse radiolysis. The triplets have been generated by energy transfer under sensitized conditions, direct intersystem crossing (in bromobenzene), and assisted intersystem crossing in the presence of such singlet quenchers as oxygen, ethyl iodide (EtI), di-tert-butyl nitroxide (DTBN), and N,N-dimethylaniline (DMA). The absorption spectra of the triplets are sharp, intense, and well resolved and display two major band systems at 400-500 and 550-650 nm, respectively. The observed triplet lifetimes ( T T ) are in the range 15-120 M S . On the basis of the data concerning triplet-triplet (T-T) spectra, TT’s, triplet quenching by azulene and di-tert-butyl nitroxide, and Stern-Volmer behavior for triplet formation via exciplexes with N,N-dimethylaniline, the triplets are assignable to predominantly one conformeric species, or multiple species with indistinguishablespectral and kinetic behaviors, with the excitation energy in either case being localized primarily on the anthracene moiety. In the case of 1-(2-thienyl)-2-(2anthryl)ethylene,triplet-related formation and decay processes with associated lifetimes 100-1 20 ns are observed at the initial stage following the laser excitation; these are tentatively attributed to intramolecular relaxation in the initially formed triplet conformers in the form of rotation of aryl groups about quasi-single bonds.
Introduction Extensive photochemical and photophysical research on stilbenes in the past two decades has established that perpendicular (twisted) configuration^^^^ play important roles in both singlet- and trip(1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2460 from the Notre Dame Radiation Laboratory. (2) Saltiel, J.; DAgostino, J.; Megarity, E. D.; Metts, L.; Neuberger, K. R.; Wrighton, M.; Zafiriou, 0. C. Org. Photochem. 1973, 3, 1-113 and references therein.
0022-3654/84/2088-1168$01.50/0
let-mediated photoisomerization (cis-trans) processes of these systems. An obvious extension of this work would be to study photodynamical behaviors of stilbene analogues in which the phenyl groups are replaced by more extended aromatic systems. On going from stilbenes to arylethylenes bearing polycyclic aromatic groups, one expects major changes4” to occur in the order (3) Gorner, H.; Schulte-Frohlinde, D. J . Phys. Chem. 1981,85, 1835-41 and references therein. (4) Hammond, G. S.;Shim, S.C.; Van, S.P. Mol. Photochem. 1969, 1 , 89-106.
0 1984 American Chemical Society
