Photophysical properties of some donor–acceptor 1H-pyrazolo[3,4-b]quinolines: Radiative versus non-radiative electron transfer processes

The fluorescence behaviour of some new dyes based on the pyrazolo[3,4-b]quinoline (PQ) ring has been investigated. It was found that introduction of electron withdrawing and electron donating groups into pyrazolo[3,4-b]quinoline molecule causes an increase of the ground and excited state dipole moments but more important is introduction of the N,N-dimethylamine group directly into the pyrazolo[3,4-b]quinoline skeleton. Further introduction of the cyano groups in the peripheral phenyl rings brings about only minor changes in the transition dipole moments (absorption and fluorescence). Such compounds exhibit a CT fluorescence. The fluorescence of these compounds was investigated in terms of the radiative charge transfer transition to obtain the charge transfer parameters such as the energy gap between the charge transfer and the ground states, solvent reorganization energies and internal reorganization energy. The magnitude of these parameters was discussed in terms of a modern electron transfer theory and the parameters were used to calculate the rate constants for non-radiative CT transition.