Photoinduced ring-opening of a photochromic dihydroindolizine derivative monitored with femtosecond visible and infrared spectroscopy.

We present results of a femtosecond spectroscopy study of the ring-opening dynamics of the photochromic compound trimethyl-1'H-spiro[fluorene-9,1'-pyrrolo[1,2-b]pyridazines]-2',3',6'-tricarboxylate (also known as dihydroindolizine and abbreviated as DHI) in solvents of different polarities. We follow the ring-opening dynamics of photoexcited DHI by probing the transient response in the visible region between 450 and 700 nm, as well as in the fingerprint region between 1100 and 1800 cm(-1). We conclude that photoexcited DHI converts into the ring-opened betaine isomer while remaining in the electronic excited state. Subsequent electronic excited-state decay on a time scale of 40-80 ps results in regeneration of ground-state DHI (0.75-0.9 quantum yield) or betaine photoproduct, the exact value for DHI quantum yield recoveries and rates being solvent dependent.