Demystifying the triplet state and the quenching mechanism of self-assembled fluorenoazomethines

The steady-state and time-resolved photophysical investigation of fluorene derivatives revealed their fluorescence is lowered upon incorporating an azomethine linkage. The heteroconjugated bond deactivates the singlet excited state by nonradiative modes of energy dissipation including internal conversion. Similarly, the azomethine bond quenches the triplet state, but it does not increase the yield of the intrinsically formed manifold by otherwise enhancing the amount of intersystem crossing. A series of aliphatic and partially conjugated azomethines were used to probe the quenching mechanism of triplet deactivation of fluorene derivatives. Although the measured rate constants (2 × 107–1 × 1010 M−1 s−1) for triplet quenching with various azomethines were lower than diffusion controlled processes, the endothermic energy transfer process is highly efficient when an azomethine bond is covalently bound to the triplet producing chromophore. Efficient intramolecular energy transfer is responsible for quenching the inherently formed triplet state within 10 ns leading to the absence of any detectable transients by laser flash photolysis. Steady-state photolysis at 254, 300, and 350 nm confirmed the azomethines are photostable and they do not photoisomerize.