Photoluminescence of 1-phenyl,3-methyl pyrazoloquinoline derivatives

Paper presents the absorption and photoluminescence of 7-TFM, 6-F, 6-CN, 6-TBu and 6-COOEt derivatives of 1-phenyl-3-methyl-1H-Pyrazolo[3,4-b]quinoline (MPPQ). The measured spectra are compared with the results of the quantum chemical calculations performed by means of the semi-empirical methods (AM1 or PM3) that have been applied either to the equilibrium molecular conformations in vacuo (T=0 K) or combined with the MD simulations (T=300 K). The photoluminescent spectra of MPPQ dyes are highly solvatochromic. The emission bands broaden and shift to the red with the increasing of solvent polarity, indicating thus a substantial dipole moment of the excited states. According to the quantum chemical analysis the reason for the strong solvatochromism of MPPQ dyes is related with intramolecular charge transfer (ICT) state. Due to the large dipole moment in the twisted geometry the ICT state is believed to become the lowest excited state in a strongly polar environment. This would explain a considerable solvatochromic shift in the highly polar solvents observed for all MPPQ dyes in the experiment. Such hypothesis is supported by the semi-empirical quantum chemical evaluations.