Photophysical, electrochemical, and crystallographic investigation of conjugated fluoreno azomethines and their precursors

The photophysical investigation of amino- and aldehyde-substituted fluorenes revealed that these compounds are not only highly fluorescent, but dissipation of their singlet excited energy occurs by a combination of nonradiative means involving intersystem crossing (ISC) and internal conversion (IC). Quantification of the triplet state formed by ISC was possible by laser-flash photolysis (LFP). The efficiency by which this manifold was populated varied between 10 and 40% depending on the fluorene substitution. Condensation of these aldehyde and amine precursors yielded conjugated thiopheno azomethines with robust covalent bonds. Fluorescence of the azomethine fluorene derivatives was reduced relative to their precursors while the degree of IC remained unchanged. Deactivation of the singlet excited state occurred predominately by ISC and the resulting triplet state was rapidly and efficiently quenched by energy transfer by the azomethine linkage. Cyclic voltammetry of the fluoreno azomethines showed both oxidation and reduction processes, and the measured redox potentials and the band-gaps are lower than a bisfluorene analogue. The fluoreno azomethine LUMO energy levels are sufficiently low, making them compatible with common cathodes, therefore eliminating the use of an electron-injection layer.