Time-resolved photoisomerization of 1,1′-di-tert-butylstilbene and 1,1′-dicyanostilbene

Highlights: • Photoisomerization of vinyl-substituted stilbenes (0, 1, 2, 3, 4). • Extinction spectra of 0, 1, 2, 3, 4. • Ultrafast (4) or slow (3) twist to perpendicular molecular conformation P. • Long-lived (∼100 ps) perpendicular conformation P. • Zwitterionic character of P, its dipole moment is of ∼3D. Photoisomerization of 1,1′-di-tert-butylstilbene (3) and 1,1′-dicyanostilbene (4) is monitored with stationary and broadband transient absorption spectroscopy. The electron affinity of the substituents correlates with the shift of the absorption band. The weak extinction of 3 complicates data interpretation, but comparison with earlier measured 1,1′-dimethylstilbene (1) and 1,1′-diethylstilbene (2) helps to assign transient spectra and relaxation paths. For 3 a long-lived perpendicular state P is observed with lifetime τ_P = 134 ps in acetonitrile. For 4 τ_P = 2.1 ps in acetonitrile and 27 ps in n-hexane, the difference indicating a substantial dipole moment (∼3D) of the P state.