Photochromism of 2<i>H</i>-Naphtho[1,2-b]pyrans:  A Spectroscopic Investigation

Abstract

The photophysical and photochromic properties of 2,2-diphenyl-2H-naphtho[1,2-b]pyran and substituted derivatives were investigated by steady state and time-resolved optical absorption and emission spectroscopy in solution at room temperature and in a frozen matrix at 77 K. Fluorescence quantum yields, fluorescence lifetimes, and singlet energies depend strongly on the substitution patterns. Photoexcitation of the naphthopyrans ( A ) leads to efficient ring opening to produce the merocyanines ( B ) and ( C ). The optical absorption of the merocyanines can be tuned by the substituents and can cover most of the visible spectrum (400−700 nm). The decoloration kinetics of the open forms (merocyanines) B and C to produce naphthopyrans ( A ) depends strongly on the substituents. The ring closure rate constants ( k B → A ) range from 0.0009 to 0.04 s -1 .