Electronic and steric effects on the photo-induced C → E ring-opening of structurally modified furylfulgides

Abstract

The ultrafast C → E ring-opening reactions of four selectively modified furylfulgides have been studied by means of ultrafast broadband transient absorption spectroscopy after femtosecond laser excitation at λ = 500 nm. A large difference in the dynamics was found in the case of benzannulation at the furyl moiety as an example for an electronic effect by extension of the conjugated π-electron system compared to furylfulgides carrying sterically different alkyl substituents at the central cyclohexadiene (CHD) ring. The measured very similar spectro-temporal absorption maps for the furylfulgides with a methyl or isopropyl group at the CHD ring or an intramolecular alkyl bridge from the CHD to the furyl moiety showed two distinctive excited-state absorptions with slightly different decay times. The first time constant (τ₁ = 0.39–0.57 ps) was assigned to the rapid departure of the excited wavepacket from the Franck–Condon region. The slightly longer second decay time of τ₂ = 0.66–0.92 ps, depending on the compound, was attributed to the electronic deactivation and ring-opening through a conical intersection to the S₀ state. In contrast, the benzannulation at the furyl moiety was found to lead to a bi-phasic excited-state decay with τ₂ = 4.7 ps and a much slower additional contribution of τ₃ = 17.4 ps, ≈25 times longer compared to the normal furylfulgides. The drastic change is attributed to a trapping of excited molecules in a local potential energy minimum en route to the conical intersection.