A novel non-linear fluorescence (NLF) effect in polar dimethylaminobenzylidene 1,3-indandione (DMABI) crystals is presented and discussed. A single crystal or vacuum-evaporated layer of DMABI excited in the IR region at 1064 nm (by a Q-switched YAG laser) emits red light in the visible region of the spectrum at 600–740 nm depending on the DMABI crystalline modification. This anti-Stokes luminescence shows a quadratic dependence on the exciting light intensity which transforms to a linear dependence at very high excitation intensities. The observed NLF effect under IR excitation is determined by optical transitions into intermolecular charge transfer (CT) states. It has been suggested that the photoexcited singlet 1CT excitations may either interact in a quadratic singlet–singlet (1CT + 1CT) annihilation reaction or, more probably, form long-lived triplet 3CT excitons via a hyperfine interaction effect which may subsequently annihilate in a similar bimolecular fusion reaction (3CT + 3CT). In both cases the excited molecular singlet state S1 is formed and thus the NLF effect is due to singlet–singlet or triplet–triplet fusion of CT excitons. A detailed energy diagram and kinetic description of the model are presented. Quantum chemical calculations of the electronic structure of the DMABI molecule illustrate the dipolar nature of its ground and excited states.
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