Time-resolved absorption studies on the photochromic process of 2H-benzopyrans in the picosecond to submillisecond time domain

Picosecond to submillisecond photochromic reactions of 2,4-diphenyl-2H-benz opyran and 2,2,4-triphenyl2H-benzopyran have been investigated by time-reso lved absorption spectroscopy. The C-O bond cleavage of the benzopyrans (clo sed forms) occurs via the first excited singlet state within 2 ps to produc e vibrationally excited open forms in the ground electronic state. In the s ubnanosecond to submillisecond time domain, several decay components with a lmost the same spectral profiles are observed. These components are assigne d to respective stereoisomers with respect to two double bonds and one sing le bond of the open enone forms. From the pump-laser power dependencies of the yields of the open forms, it is suggested that the photocleavage gives at first only the open forms revertible to the closed form by a single-bond rotation, and that the photoexcitation of the first generated open forms g ives rise to other open forms which need a double-bond rotation for reversi on to the closed form. The photochromic reactions of a series of 2H-benzopy rans bearing substituents on the pyran ring have also been studied using na nosecond time-resolved absorption spectroscopy. The size of a substituent i n the 4-position fairly affects the rate constants of the thermal reversion of the open form to the closed form.