PHOTOCHEMICAL RING-OPENING REACTION OF INDOLINOSPIROPYRANS STUDIED BY SUBPICOSECOND TRANSIENT ABSORPTION

The photochemical ring-opening reaction of spiropyrans BIPS (1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline]), its naphthopyran analogue naphtho-BIPS, and 6-nitro-BIPS in n-pentane to the corresponding merocyanines has been followed by transient absorption spectroscopy. A board absorption ca. 1 ps after UV excitation, covering the entire measurement range 380-680 nm, is assigned to transitions from the excited spiro compound. In the case of BIPS and naphtho-BIPS, the merocyanine absorption bands rise with time constants of 0.9 and 1.4 ps, respectively. The transient spectra are compared with spectra taken several microseconds after excitation and with the spectra obtained when the spiropyrans are irradiated at low temperature in an argon matrix. There is evidence that a distribution of isomers is already established 10 ps after UV excitation of the spiroform. The reaction paths leading from excited BIPS to ground-state merocyanine isomers are classified according to their steric requirements. The internal molecular temperature after fast internal conversion is estimated to be ca. 900 K. We suggest that, at this temperature, a primary merocyanine product in its electronic ground state may isomerize further on the picosecond time scale, resulting in the observed distribution of isomers.