Primary photo-events in a metastable photomerocyanine of spirooxazines

Abstract

We report on the ultrafast excited-state relaxation dynamics of the metastable photo-merocyanine (open-form) isomer of spiro-phenantroxazine, measured by pump and probe spectroscopy with sub-40-fs temporal resolution. We found that the photo-induced yield for ring-closure is negligible, and that the excited-state lifetime is only on the order of 300 fs. Relaxation leads to the non-adiabatic formation of a hot ground state (HGS). In this state, a coherent oscillation with 45 cm−1 frequency is present, showing strong anharmonicity. We attribute it to the motion (torsion/bending) of the molecular backbone attempting geometric relaxation to the close form. The strength of the coherent oscillation and the subsequent spectral relaxation in the HGS, together with the ultrashort lifetime, points to a crossing through a conical intersection (CI). We conclude that excited-states on the merocyanine form pass through a CI that is different from the one that would lead to ring-closure. We discuss design rules for the spiro-oxazine class, allowing for bidirectional switching avoiding this parasitic CI.