Parallel ultrafast E–C ring closure and E–Z isomerisation in a photochromic furylfulgide studied by femtosecond time-resolved spectroscopy

The photo-induced parallel E→C ring closure and E→Z isomerisation reactions of the (E) isomer of the photochromic furylfulgide 2-[1-(2,5-dimethyl-3-furyl)-ethylidene]-3-isopropylidene succinic anhydride (1) in n-hexane have been studied using femtosecond time-resolved spectroscopy. Broadband transient absorption data after femtosecond laser excitation at λpump = 335 nm provide time constants of 100 fs and 250 fs that belong to the formation of the (C) and the (Z) isomers, respectively, to yield a (C):(Z) product ratio of about 2:1. The results are consistent with a conformer-specific photoreaction of the (E)-isomer of 1, where one conformer (α) undergoes predominantly E→C ring closure and the other (β) E→Z isomerisation, or alternatively with an ultrafast branching of the excited wavepacket of the α-conformer within Δt < 250 fs after the pump pulse. The observed isomerisation times suggest that the ensuing transformations proceed via distinctive conical intersections between the respective potential energy hypersurfaces. Oscillations of the transient absorption with frequencies of ≈64 and 114 cm−1 are found and interpreted as excited-state vibrations induced during the E→Z isomerisation reaction. Slower spectral dynamics at delay times up to ≈10 ps reflect the cooling of the vibrationally hot reactant and product molecules after their return to their electronic ground states. Time-dependent DFT calculations were performed to shed light on the reaction coordinates involved. The emerging picture for the dynamics of 1 obtained in this work is of interest in the broader context for our understanding of conformer-specific photochemistry and competing ultrafast reactions in polyatomic molecules.