Photoisomerization dynamics of trans–trans, cis–trans, and cis–cis diphenylbutadiene from broadband transient absorption spectroscopy and calculations Available to Purchase

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.