Photophysical properties of derivatives of 1H-pyrazolo[3,4-b]quinoline and 1H-pyrazolo[3,4-b]quinoxaline

The fluorescence behaviour of 1,3-diphenyl-1H-pyrazolo[3,4-b]quinoline (PQ1), 1,3-diphenyl-1H-pyrazolo[3,4-b]quinoxaline (PQX1) and 3-methyl-1-phenyl-1H-pyrazolo[3,4-b]quinoxaline (PQX2) has been compared. It has been established that these dyes form exciplexes with N,N'-dimethylaniline (DMA) and p-cyano N,N'-dimethylaniline (DMABN) in weakly polar dibutyl ether. The reduction potential of these compounds has been measured by cyclic voltammetry. The different behaviour with respect to intersystem crossing process in pyrazoloquinoline and pyrazoloquinoxalines has been explained in terms of larger energy gap between the first excited singlet state and the higher triplet state having a different symmetry in the former system. This is supported by quantum chemical calculations and it explains the lack of the triplet-triplet absorption bands in the flash photolysis experiments in pyrazoloquinoline system in any solvent.