60 J. Phys. Chem. 1994,98, 60-67 Excited State Spectra and Dynamics of Phenyl-Substituted Butadienes Stacie E. Wallace-Williams,t Benjamin J. Schwartz,: Saren Merller,g Robert A. Goldbeck,t W. Atom Yee,'J M. Ashraf El-Bayoumi,li and David S. Kliger'vt Department zyxwvutsr of Chemistry and Biochemistry, University of California at Santa Cruz. Santra Cruz, California 95064, Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, and Department of Chemistry, Santa Clara University, Santa Clara, California 95053 Received: August IO, 1993' A combination of steady-state and dynamic spectral measurements are used to provide new insights into the nature of the excited-state processes of all-trans- 1,4-diphenyl- 1,3-butadiene and several analogs: 1,4-diphenyl- 1,3-~yclopentadiene, 1,1,4,4-tetraphenyIbutadiene, 1,2,3,4-tetraphenyl- 1,3 -cyclopentadiene, and E,E-diinda- nylidenylethane. Ground-state absorption, fluorescence, and nanosecond transient absorption measurements identify geometry changes upon excitation and provide information regarding the nature of the first excited singlet state. Femtosecond and picosecond transient absorption data indicate that phenyl torsional motion is not important to the excited-state dynamics and reveal alternative excited-state reaction pathways. The results demonstrate how molecular systems that are structually similar can exhibit different electronic properties and excited-state dynamics. Introduction Phenyl-substituted polyenes have often been used as model compounds in an attempt to understand mechanisms of polyene photoisomerization. Of this class of polyene, the substituted butadienes, especially all-trans-1 ,ediphenyl-l,3-butadiene (DPB), have received considerableattention.1-3 Because of its high molar absorptivity and relatively large fluorescencequantum yield, DPB has been studied by many researchers as a prototype for isomerizationprocesses in the chromophore of visual pigmentsM and has also served as a test for liquid-phase chemical reaction rate theories such as Kramers's theory.7.* However, despite this interest, there still remains ambiguity in the ordering of the first two electronic excited states of DPB in condensed phase because of the possibility of state mixing and interactions with the solvent environment.9 Given that the photoisomerization of these compounds is intimately connected to the electronic structure of the excited states, it is surprisingthat there have been few attempts to correlate photoisomerization dynamics with steady-state spectroscopic information. Adequate knowledge of the first excited state is imperative before these compounds can serve as useful models for understanding isomerization processes. DPB belongs to a class of molecules, all-trans a-w-diphen- ylpolyenes,whose excited states are characterized in terms of the A, or B, representations of the CZh symmetry group. These A and B representations can also adequately describe the excited states of a-w-diphenylpolyenes even when the actual symmetry of the polyene deviates somewhat from C2h. The electronicground states of all the diphenylpolyenes are described by the totally symmetric A, representation, whereas the symmetry of the first electronic excited state (SI) is dependent on the polyene chain length. TheSl statesof diphenylhexatriene (DPH) and thelonger chain polyenes have A, symmetry, while the SI state of stilbene (diphenylethylene) has B, symmetry. For DPB, the first two excited states of A, and B, symmetry are nearly degenerate, with * Author to whom correspondence should be addressed. t University of California at Santa Cruz. zyxwvutsrqpo t University of California at Berkeley. Present address: Department of University of California at Santa Cruz. Present address: Roskilde * Santa Clara University. University of California at Berkeley. zyxwvutsrq On leave from Faculty of Science, Abstract published in zyxwvutsrqp Aduance ACS Absrracrs, December 15, 1993. Chemistry, University of Texas, Austin, TX 78712. University Library, P.O. Box 258, D4-4000, Rjoskilde, Denmark. Alexandria University, Alexandria, Egypt. 0022-3654/94/2098-0060$04.50/0 the ordering dependent on the environment. In the gas phase, the 2A,state has been determined to lie below the 1 BU.10 However, interactions with the solvent can stabilize the lB, state so the state ordering can differ in different condensed-phase media.IlJ* Most solution-phase spectroscopic studies have identified the SI state of DPB as a B, state, and an excellent review of this work was recently published by Saltiel and Sun.4 The small Stokes shift and the good mirror image of the absorption and fluorescence bands indicate that the absorbing and emitting states are the same. Theoretical calculations and excited-state absorption measurements by Goldbeck et al.9 have identified the SI state as B,. In addition, the spectral properties of DPB are more similar to stilbene than to DPH and the longer polyenes, further supporting the B, assignment of the S1 state. However, two-photon fluorescence excitation studiesof DPB in cyclohexanesolutions~3J4 and one-photon absorption and fluorescence measurements of DPB in EPA glassI5 concluded that the 2A, state lies slightly below the lB, in condensed phase. It should be noted that Saltiel and Sun4 recently suggested that these studies are more consistent with a lowest B, excited-state assignment for DPB. Thus, consensus on the electronic nature of the SI state of DPB in condensed phase still has not been achieved. One significant attempt to correlate the ambiguous DPB SI spectral assignment and the excited-state dynamical processes of DPB in condensed phase has been the work of Rulliere et a1.16 In this study the authors used picosecond flash photolysis to examine the dynamics of the S, - SI excited-state absorption. To explain their observed dynamics, Rulliere et al. invoked a model of solvent-assisted level inversion modulated by large- amplitudephenyl torsional motion. While this model adequately explained the data obtained in their experiments, it predicts a dependence of the transient absorption spectrum on solvent polarizability that is not observed.17 Furthermore, isomerization studies of DPB derivatives with locked phenyl groups1 show a one-dimensionalexcited-state motion similar to DPB and do not support the hypothesis that phenyl motions are important to the excited-state dynamics in DPB. Clearly, the role of phenyl torsional motions in the excited-state ordering and dynamics for DPB in solution is not completely understood. In this paper, we have investigated the excited-state dynamics and photophysics of DPB and four structurally related phenyl- substituted butadienes: 1 ,Cdiphenyl- 1,3-cyclopentadiene(DPCP), E,E-diindanylidenylethane (stiff-SDPB), 1,1,4,4-tetraphenyl- Q 1994 American Chemical Society