Photoprocesses in 2,2-diphenyl-5,6-benzo(2 H)chromene

Abstract

The photocolouration of 2,2-diphenyl-5,6-benzo(2 H)chromene (Chr), upon irradiation of the ring-closed form (Np), and the subsequent relaxation kinetics of one merocyanine isomer into Np were studied in several solvents at different temperatures. Deactivation of the excited Np form leads via the singlet manifold predominantly to the ring-opened cis– trans ( ct) isomer which converts thermally into Np. The activation energy, E ct→ Np ≈77 kJ mol −1 , and the pre-exponential factor, A≈10 12 s −1, are essentially independent of the solvent polarity. The trans– trans ( tt) isomer, which is suggested to be formed photochemically in a competing step, is thermally more stable. The triplet pathway of photocolouration can be induced by energy transfer using several sensitizers with a triplet energy of >200 kJ mol −1. The thermal relaxation time after either direct or naphthalene-sensitized colouration is the same. The mechanisms of ring opening and ring closure are discussed.