Long-lived perpendicular conformation in the photoisomerization path of 1,1′-dimethylstilbene and 1,1′-diethylstilbene

► Photoisomerization of substituted stilbene. ► Ultrafast twist to perpendicular conformation. ► Perpendicular state is a common intermediate for trans-cis and cis-trans reaction. ► Long-lived (∼10 ps) perpendicular state.