Thermo- and photochromic dyes: indolino-benzospiropyrans. Part 1. UV–VIS spectroscopic studies of 1,3,3-spiro(2H-1-benzopyran-2,2′-indolines) and the open-chain merocyanine forms; solvatochromism and medium effects on spiro ring formation

Abstract

A kinetic and ultraviolet–visible study of a series of thermo- and photochromic dyes is reported. Ultraviolet irradiation of the indolino-benzospiropyran derivatives 1′–4′ leads to their transformation to the open-chain, colored merocyanine forms, which undergo thermal ring closure into the colorless spiro forms. The rate of this ring closure has been determined in different solvents. Plots of log (rate) vs. the solvent parameter ET are linear but are dependent on the nature of the substituent in the phenyl moiety, which yields information on the electronic character of the ground state (IIa↔IIb) and the transition state. The ring-closure process is characterized by a zwitterionic type transition state, TS3. The merocyanine dyes exhibit a significant solvatochromic effect, which unexpectedly shows sensitivity to substituents on the phenyl group. This observation finds explanation through semi-empirical molecular orbital (MOPAC) calculations. The MOPAC calculations also predict that the most stable configurational isomer of the merocyanines is the CTTC form. Key words: indolino-benzospiropyran–merocyanine interconversion, photochromic–solvatochromic dyes.