The role of internal twisting in the photophysics of stilbazolium dyes

The synthesis of selectively bridged stilbazolium dyes related to DASPMI is described. The comparison of steady-state and time-resolved fluorescence s tudies as a function of temperature allows one to develop a kinetic model o n the basis of which the high photostability and absence of photoisomerizat ion for these dyes can be understood. There are two possible photochemical deactivation channels: non-radiative decay through single-bond twisting whi ch does not lead to a distinguishable photoisomer and through double bond t wisting leading to trans-cis photoisomerization. The latter process is more than one order of magnitude slower than single-bond twisting for these com pounds and is only observed in the compound where both single bonds are bri dged. Lifetime maxima observed at low temperature indicate a further early structural relaxation not connected with bond twisting but with bond length changes.