Liquid Crystalline Solvents as Mechanistic Probes. VII. Investigation of a Photochromic Merocyanine to Indolinospiropyran Isomerization in the Isotropic and Smectic Phases of n-Butyl Stearate and in Polybutadiene Oligomers

Abstract

Abstract The unimolecular isomerization of a merocyanine (t - 2a) to an indolinospiropyran, 1', 3', 3'-trimethyl-6-nitrospiro [2H-benzopyran-2,2'-indoline] (1a), has been followed in the isotropic and smectic liquid crystalline phases of n-butyl stearate (BS) and in three polybutadiene oligomers (PB). The rate constants, k−t, for isomerization in PB demonstrate that viscosity is not a factor as long as the solvent chains are mobile. The k−t for all three PB fractions (η 13.1-506 cP) at 25°C range only from 1.0-3.6 min−1. Comparison of the activation parameters in the two phases of BS (Ea = 17.4 kcal mole−1 and ΔS‡ = -9.3 e.u. in the isotropic phase; Ea = 38.2 kcal mole−1 and ΔS‡ ≈ +55 e.u. in the smectic phase) with each other and with other solvents reveals that polarity considerations, although important, are incapable of explaining the data completely. The effect of solvent order on the activation parameters must be included.