ROLE OF STRUCTURAL FLEXIBILITY IN FLUORESCENCE AND PHOTOCHROMISM OF THE SALICYLIDENEANILINE - THE ALDEHYDE RING ROTATION

X-ray analysis, absorption and luminescent steady-state spectroscopy a s well as quantum-chemical calculations have been used for comparative investigations of salicylideneaniline (i) an the more rigid phenylimi ne of 7-hydroxyindanine-1 (ii) molecules. In the latter the ''aldehyde '' ring A rotation is inhibited by inclusion of the saturated bridge f ragment. Fluorescence with anomalous Stokes shift (ASS fluorescence) i s typical for both (i) (only in glassy solvent) and (ii) (in liquid or glassy solvent). The ASS fluorescence occurs only as the result of ex cited-state intramolecular proton transfer (ESIPT). In liquid solvents the ESIPT in (i) may be followed by twisted intramolecular charge tra nsfer (TICT) formation via adiabatic 90 degrees ring A twist around th e C-AR=C bond in the C=N bond (Z)-isomer of the zwitterion NH structur e. The TICT state is one of the channels to quench the ASS fluorescenc e 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 degrees twist about th e 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 pro cess (ring A twist and (Z)-(E) isomerization) competing with ASS fluor escence and the formation of the ''TICT-like'' structure.