Photoisomerization Dynamics Of Trans-Trans, Cis-Trans, And Cis-Cis Diphenylbutadiene From Broadband Transient Absorption Spectroscopy And Calculations

The photoisomerization path and dynamics of trans-trans (ttD), cis-trans (ctD), and cis-cis (ccD) 1,4-diphenyl-1,3-butadiene (DPB) in solution are studied with broadband transient absorption (TA) spectroscopy and quantum chemical calculations. For ttD in n-hexane, 2-photon-excited TA spectra indicate that the 2A(g) state is located above 1-photon allowed 1B(u) (S-1) by similar to 1000 cm(-1). Following S-0 -> S-1 optical excitation, the isomerization occurs via torsion about a butadiene double bond to perpendicular molecular configuration P. The P-state is detected in ccD with an excited-state absorption band at 390 nm. This P-band develops during S-1 -> P half-torsion with time of 0.15 ps, followed further by P -> S-0 half-torsion and simultaneous decay with 1.6 ps in acetonitrile and 5 ps in n-hexane. In addition, two oscillation cycles between P and S-1 population are observed before equilibration in n-hexane. For ctD, an indication of rising and decaying P is found in acetonitrile. The vast majority of ctD species photoisomerizes to ttD and not to ccD, in agreement a with calculated low torsional barrier about the cis double bond and high barrier about the trans double bond. Photoisomerization yield Y and time tau (i) depend drastically on the solvent polarity. Thus, in n-hexane, the isomerization ttD -> ctD has yield Y-tt,Y-ct = 0.1 and time tau (i) = 829 ps, while in acetonitrile, Y-tt,Y-ct = 0.4 and tau (i) = 27 ps. The 30-fold acceleration of the isomerization in acetonitrile clearly reflects a highly polar character of P, consistent with a dipole moment mu (P) > 9.6 D. The results for DPB are discussed in comparison to stilbene.