Decoupling Fluorescence and Photochromism in Bifunctional Azo Derivatives for Bulk Emissive Structures

Bifunctional molecules that combine independent pushpull fluorophores and azo photochromes have been synthesized to create fluorescent structures upon light-induced migration in neat thin films. Their photochromic and emissive properties have been systematically investigated and interpreted in light of those of the corresponding model compounds. Fluorescence lifetimes and photoisomerization and fluorescence quantum yields have been determined in toluene solution. Kinetic analyses of the femtosecond transient absorption spectra reveal that the fluorophores evolve in a few picoseconds into a distorted intramolecular charge-transfer excited state, strongly stabilized in energy. Radiative relaxation to the ground state occurred competitively with the energy-transfer process to the azo moiety. Introduction of a 10 angstrom-long rigid and nonconjugated bridge between the photoactive units efficiently inhibits the energy transfer while it imparts enhanced free volume, which favors photoactivated molecular migration in the solid state.