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 (S₁) by ∼1000 cm⁻¹. Following S₀ → S₁ 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 S₁ → P half-torsion with time of 0.15 ps, followed further by P → S₀ 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 S₁ 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 τ_i depend drastically on the solvent polarity. Thus, in n-hexane, the isomerization ttD → ctD has yield Y_tt,ct = 0.1 and time τ_i = 829 ps, while in acetonitrile, Y_tt,ct = 0.4 and τ_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 μ_P > 9.6 D. The results for DPB are discussed in comparison to stilbene.