Theoretical prediction and direct observation of the hot molecules of pyrazolotriazole azomethine dyes by steady state fluorescence

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Pyrazolotriazole (PT) azomethine dyes in the S1 electronic state were studied by emission spectroscopy and ab initio CI-singles and MCSCF methods. Electronically excited molecules are predicted to follow a barrierless relaxation trajectory involving a twisting of the arylamino fragment. Conformational searching yielded the only minimum on the CI-singles S1 potential energy surface (PES), with characteristic features being a twisted geometry and small S1–S0 gap (<0.3 eV). These features preclude its assignment to an experimentally observed fluorescent state (FS) characterized by the emission maximum at 550–600 nm. It is suggested that the FS is a non-Boltzmann population of vibrationally hot molecules with near-planar geometry in a relatively flat region of the S1 PES. The predicted nonclassical nature of the FS coincides with a remarkable observation of the unprecedented dependence of the fluorescence spectra on the excitation wavelength. In excellent agreement with computational models, a shift of the excitation towards longer wavelengths results in an emission band shift of comparable magnitude along with a significant decrease in a relative fluorescence quantum yield.