Role of structural flexibility in fluorescence and photochromism of the salicylideneaniline: the “aldehyde” ring rotation

Abstract

X-ray analysis, absorption and luminescent steady-state spectroscopy as well as quantum-chemical calculations have been used for comparative investigations of salicylideneaniline (i) an the more rigid phenylimine of 7-hydroxyindanine-1 (ii) molecules. In the latter the “aldehyde” ring A rotation is inhibited by inclusion of the saturated bridge fragment. Fluorescence with anomalous Stokes shift (ASS fluorescence) is typical for both (i) (only in glassy solvent) and (ii) (in liquid or glassy solvent). The ASS fluorescence occurs only as the result of excited-state intramolecular proton transfer (ESIPT). In liquid solvents the ESIPT in (i) may be followed by twisted intramolecular charge transfer (TICT) formation via adiabatic 90° ring A twist around the C ARC bond in the CN bond (Z)-isomer of the zwitterion NH structure. TICT state is one of the channels to quench the ASS fluorescence of (i) in the liquid solvent. The compound (ii) is found to have no photochromism unlike (i). The photochromism of the latter is connected not only with ESIPT but also with ring A 60–80° twist about the C ARC bond in the CN ( E)-isomer of the NH structure. It is proposed that the photocolored form originates from the concerted diabatic process (ring A twist and ( Z)-( E) isomerization) competing with ASS fluorescence and the formation of the “TICT-like” structure.