J . Phys. Chem. 1984, 88, 2803-2808 importance of their charge transfer character, the nonradiative processes are too complex to be explained on the basis of this parameter alone. In the limit of high ionic character, the energy gap law33appears to be applicable for the latter processes. This (33) (a) Robinson, G. W.; Frosch, R. P. J . Chem. Phys. 1963, 38, 1187-1203. (b) Englman, R.; Jortner, J. Mol. Phys. 1970,18, 145-64. (c) Henry, B. R.;Siebrand, W. In “Organic Molecular Photophysics”; Birks, J. B., Ed.; Wiley: New York, 1973; Vol. I, pp 153-237.
2803
is compatible with the explanation4given for the inverse correlation of internal conversion from ion pairs in polar solvents with the energy gap between them and the reactant partners. Registry No. 1, 95-47-6; 2, 108-38-3; 3, 106-42-3; 4, 108-67-8;5, 95-63-6;6, 488-23-3; 7, 527-53-7; 8, 95-93-2;9, 700-12-9; 10, 87-85-4; 11, 100-66-3;12, 91-16-7; 13, 151-10-0;14, 150-78-7;15,621-23-8;16, 634-36-6; 17, 135-77-3; 18, 578-58-5; 19, 100-84-5;20, 104-93-8;DCN, 3029-30-9.
A Laser Flash Photolysis Study of Triplets of trans-Naphthylethylenes. Relative Importance of Planar and Perpendicular Forms’ T. Wismontski-Knittelt and P. K. Das* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: October 24, 1983)
Triplets of five trans-1,2-diarylethylenesbearing phenyl, 1-naphthyl, and 2-naphthyl groups have been generated by laser flash photolysis under conditionsof direct excitation followed by assisted intersystem crossing (via heavy-atom, spin-exchange, energy-transfer, or charge-transfer interactions of singlets) as well as under triplet sensitization by biphenyl and fluorenone. Quantitative data concerning triplet-triplet absorption spectra A(,: = 39C-530 nm,,,e: = (8-30) X lo3 M-’ cm-’ in benzene), = 0.14-0.91 p s in benzene), and energy transfer quenching of several aromatic sensitizer triplets by the decay kinetics (q0 diarylethylenes are presented. On the basis of the kinetic data for the quenching of diarylethylene triplets by azulene and their decay lifetimes, analysis has been made for equilibria involving planar and perpendicular configurations of the triplets and intersystem crossing rates at the perpendicular geometry. Relatively large rate constants for olefin triplet quenching by oxygen and di-tert-butyl nitroxide (stable free radical) have been interpreted in terms of enhanced quenching of perpendicular forms via spin-exchange interactions.
Introduction Since the classic work* of triplet-sensitized photoisomerization of olefins by Hammond and co-workers and their proposition of the existence of a “phantom” triplet (twisted) at an energy below the spectroscopic triplet, an enormous amount of interest has been shown3 in the roles of twisted configurations in both singlet- and triplet-related photophysics and photochemistry of olefins. While a great deal of evidence in support of the distortion of geometries in the excited states has been gathered from steady-state photochemical studies, relatively recent time-resolved investigations4-’ of several olefin systems based on nanosecond laser flash photolysis and, in some cases, pulse radiolysis have led to the direct observaion and characterization of the short-lived triplets via their transient absorption. Examples of olefin triplets that have been examined by laser flash photolysis include styrene^,"^^ enones,sbstilbenes,“V6 trans- l-phenyl-2-(2-naphthyl)ethylene,5and 3,3-dimethyl- 1-(2naphthyl)- 1-butenes ( c i ~ / t r a n s ) . ~ It is of interest to see how the planar vs. twisted equilibrium in the triplet state and the decay behavior at the twisted geometry are affected when the phenyl groups of stilbene are substituted by polycyclic aromatic groups. Also, the fact that certain diarylethylenes are capable of existing in solutions as multiple, quasi-energetic r o t a m e r ~ ’ ~ -distinguishable ’~ in terms of fluorescence spectra/lifetimes and related photophysical behaviors leads to the question as to whether the distinctive nature of the rotamers is also maintained in the lowest triplet states. In this paper we present the results of a nanosecond laser flash photolysis study of the triplets of five trans-diarylethylenes bearing phenyl, 1-naphthyl, and 2-naphthyl groups at 1,2-positions. The triplets have been generated both by direct laser excitation (337.1 nm) followed by intersystem crossing (enhanced in the presence of suitable singlet quenchers) and under triplet energy transfer Present address: Department of Structural Chemistry, Weizmann Institute of Science, Rehovot, Israel.
0022-365418412088-2803$01.50/0
sensitization. In addition to quantitative spectral data of the triplets, results have been obtained for the kinetics of their decay, and their quenching by azulene, oxygen, and di-tert-butyl nitroxide. These in turn have been used to shed light on the roles of perpendicular forms in the fast intersystem crossing of the triplets to the ground state and their interaction with O2and the stable free radical, di-tert-butyl nitroxide. (1) The work described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2476 from the Notre Dame Radiation Laboratory. (2) Hammond, G. S.; Saltiel, J.; Lamola, A. A.; Turro, N. J.; Bradshaw, J. S.;Cowan, D. 0.; Counsell, R. C.; Vogt, V.; Dalton, C. J . Am. Chem. Soc. 1964,86, 3197-217. (3) For reviews see: (a) Saltiel, J.; Charlton, J. L. “Rearrangements in Ground and Excited States”;de Mayo, P., Ed.; Academic Press: New York, 1980 Vol. 3, p 25. (b) Saltiel, J.; D’Agostino, J.; Megarity, E. D.; Metts, L.; Neuberger, K. R.; Wrighton, M.; Zafiriou, 0. C. Org. Photochem. 1973,3,
1-1 13. (4) (a) Gorner, H.; Schulte-Frohlinde, D. J . Phys. Chem. 1981, 85, 1935-41. (b) Ibid. 1978, 82, 2653-9. (c) Ibid. 1979, 83, 3107-18. (5) Gorner, H.; Eaker, D. W.; Saltiel, J. J . A m . Chem. SOC.1981, 103, 7 164-9. ( 6 ) Sumitani, M.; Yoshihara, K.; Nagakura, S.Bull. Chem. SOC.Jpn. 1978,51, 2503-7. Sumitani, M.; Nagakura, S.; Yoshihara, K. Chem. Phys. Left. 1974, 29, 410-13. (7) (a) Bonneau, R. J. Photochem. 1979, 10,439-49. (b) Ibid.J. Am. Chem. SOC.1980, 102, 3816-22. (8) Caldwell, R. A.; Cao, C. V. J . Am. Chem. SOC.1982, 104, 6174-80. Caldwell, R. A,; Pac. C. Chem. Phys. Leu. 1979, 64, 303-6. Caldwell, R. A.; Cao, C. V. J . Am. Chem. SOC.1981, 103, 3594-5. (9) Arai, T.; Sakuragi, H.; Tokumaru, K.; Sakaguchi, Y . ;Nakamura, J.; Hayashi, H. Chem. Phys. Lett. 1983, 98,40-4. (10) (a) Fischer, E. J. Photochem. 1981, 17, 331-40, and references therein. J. Mol. Struct. 1982, 84, 219-26. (1 1) Birks, J. 8.; Bartocci, G.; Aloisi, G. G.; Dellonte, S.; Barigelletti, F. Chem. Phys. 1980, 51, 113-20. (12) Ghiggino, K. P. J . Photochem. 1980, 12, 173-7. Matthews, A. C.; Sakurovs, R.; Ghiggino, K. D. Ibid. 1982, 19, 235-44. (13) Sheck, Yu.B.; Kovalenko, N. P.; Alfimov, M. V. J. Lumin. 1977,15,
157-68.
0 1984 American Chemical Society
2804
Wismontski-Knittel and Das
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984
Q 00 I. Ph-IN
IC. Ph-2N
B
-
I
IU.IN- 2N
Ip.IN-IN
0
.
0
4
U
0.06
0.03
0.04
0.02
0.02
0.0I
000 -
300
380
460
540
620
Wavelength, n m
Figure 2. Transient absorption spectra observed under various conditions upon 337.1-nm laser flash photolysis of 1N-IN (-2 X lo4 M). Solvents and times for monitoring following laser flash are as follows: (A) benzene
X.2N-2N Figure 1. Structures and acronyms of trans-1,2-diarylethylenesunder study: (I) l-phenyl-2-(1-naphthy1)ethylene(Ph-IN); (11) l-phenyl-2(2-naphthy1)ethylene (Ph-2N); (111) l-(l-naphthyl)-2-(2-naphthyl)ethylene (1N-2N); (IV) 1,2-bis(l-naphthyl)ethylene (IN-IN); (V) 1,2-
bis(2-naphthy1)ethylene(2N-2N). The curved arrows indicate possible rotation of 2-naphthyl groups about quasi-single bonds giving rise to rotamers without imposing severe steric crowding. The structures and acronyms of the five naphthylethylenes are shown in Figure 1. It should be noted that many of the photophysical aspects 1-phenyl-2-(2-naphthyl)ethylene (Ph-2N) triplet have already been investigated by Gorner et ala5under tripletsensitized conditions; this olefin has been included in the present work primarily for comparison purposes. Experimental Section The naphthylethylenes were obtained as gifts from Prof. E. Fischer and were used as received. N,N-Dimethylaniline (Aldrich), ethyl iodide (Aldrich), and bromobenzene (Fisher) were distilled before use. Azulene (Aldrich), benzene (Aldrich, Gold Label), and di-tert-butylnitroxide (Eastman) were used without further purification. The sensitizers were of best grades available from Aldrich or Eastman and were recrystallized from benzene, toluene, ethanol, or their mixtures. The spectral measurements concerning absorption and fluoresence were carried out in a Cary 219 spectrophotometer and an SLM photon-counting spectrofluorimeter, respectively. The description of the latter apparatus is available e1~ewhere.l~For laser flash photolysis, a Molectron UV-400 nitrogen laser system (337.1 nm, 2-3 mJ, -8 ns) was mostly used as the excitation source. For some experiments, use was made of the outputs (425 nm, 2-4 mJ, -6 ns) from a dye laser system (Quanta-Ray PDL-1) containing methanolic solutions of Stilbene-420 (Exciton) and pumped by 355-nm laser pulses (third harmonic) from a Nd:YAG laser system (Quanta-Ray). The essential features of the kinetic spectrophotometer used for monitoring transient spectral absorption are given in previous publication^'^*^^ from this laboratory. All of the flash photolysis experiments were performed in Suprasil quartz cells of 2-3-mm pathlengths, with the laser beam intersecting the monitoring light beam at -20° in the photolysis cell. Unless otherwise specified, the solutions were deaerated by purging with argon. The details of the pulse radiolysis apparatus are given (14) Chattopadhay, S. K.; Das, P. K.; Hug, G. L. J. Am. Chem.SOC.1982, 104, 4507-14. (15) Das, P. K.; Encinas, M. V.; Small, Jr., R. D.; Scaiano, J. C. J. Am. Chem. SOC.1979, 101,6965-70 Das, P. K.; Bhattacharyya, S.N. J . Phys. Chem. 1981,85, 1391-5. Miedler, K.; Das, P. K. J . Am. Chem. SOC.1982,
104,7462-9.
+ 0.5 M EtI, 100 ns, (A’) benzene + 0.5 M EtI, 3.8 ps, (B) bromobenzene, 80 ns, (C) methylcyclohexane + 0.14 M DMA, 300 ns.
elsewhere.16 All measurements were carried out at room temperature (296 K). Results We have used mostly benzene and bromobenzene as solvents for laser flash photolysis of naphthylethylenes. In bromobenzene, understandably because of external heavy-atom effect on intersystem crossing, triplet formation upon direct excitation is relatively pronounced. On the other hand, in benzene (or methylcyclohexane) the observed yields of the lowest triplet formation are negligible (& I0.4 in benzene, see later). However, in these solvents, the triplets could be generated in relatively high yields via assisted intersystem crossing in the presence of appropriate singlet quenchers, namely, oxygen, ethyl iodide (EtI), di-tert-butyl nitroxide (DTBN), and N,N-dimethylaniline (DMA). Among these singlet perturbers, oxygen and DTBN are also efficient quenchers of naphthylethylene triplets (see later). Thus, the relatively high concentrations of oxygen and DTBN needed for sufficiently high yields of naphthylethylene triplets via quenching of the corresponding short-lived singlets concomitantly render the triplet lifetimes short and hence observable with difficulty. The enhanced triplet formation in the presence of DMA is a result of the decay of radiative exciplexes with lifetimes of 19-31 ns in methylcyclohexane and 40-60 ns in benzene. The use of DMA for exciplex-mediated formation of triplets is complicated by the facts that the lifetimes of the exciplex precursors are not short enough compared to the decay of some of the naphthylethylene triplets (see later) and that exciplex emission contributes strongly in the spectral region where the triplets absorb. For most of our work in benzene we have used Et1 (at 0.05-0.60 M) as the external heavy-atom perturber to induce intersystem crossing. With excitation at 340-360 nm the Stern-Volmer constants for the quenching of steady-state fluorescence of naphthylethylenes by Et1 in benzene are measured to be 1.4-4.8 M-l. Since the bimolecular rate constants for naphthylethylene triplet quenching by Et1 are relatively small (106-107M-’ s-l), it is convenient to generate and characterize the triplets by direct excitation in solutions in non-heavy-atom solvents with Et1 added to them at submolar concentrations. Besides direct excitation, sensitized energy transfer could be employed to generate the naphthylethylene triplets. None of the naphthylethylenes under study absorb significantly at wavelengths longer than 420 nm. Using 425-nm laser pulses for benzene (16) Patterson, L. K.; Lilie, J. IN. J . Radial. Phys. Chem. 1974,6, 129-41. Schuler, R. H.; Buzzard, G. K. Ibid. 1976, 8, 563-74.
Flash Photolysis Study of Triplets of trans-Naphthylethylenes
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2805 TABLE I: Spectral and Kinetic Data of Triplets of Naphthylethylenes in Benzene and Bromobenzene" %lax,c
E
I
I
I
1
0
.
I/
U !
-0.05 0.15
0 Q
0 0.10 5
0.00
I 1 I 4-0.05 400 500 600
naphthylethylene Ph-IN Ph-2N IN-IN 1N-2N 2N-2N
103 M-' cm-' 490 (495) 17 395 (400) 8 530 (535) 30 500 (505) 19 430 (435) 20 Xiax,b nm
TTO,~P S
@Td
0.04
