Electron transfer and intersystem crossing processes in new dyes based on 1H-pyrazolo[3,4-b]quinoxaline: Effect of temperature and solvent polarity on the CT fluorescence

Three dyes containing the 1H-pyrazolo[3,4-b]quinoxaline moiety have been synthesized and investigated as novel fluorophores exhibiting charge transfer fluorescence. The effect of changing temperature and solvent polarity on CT fluorescence of these compounds was investigated by steady-state and time-resolved fluorescence methods. Detailed investigation of the influence of temperature on charge transfer fluorescence band shape indicates the importance of the intramolecular vibrational mode of medium frequency in the radiative back electron transfer process.The second part of the paper deals with the intersystem crossing process leading to the population of the lowest triplet state of molecules. We found that population of the lowest molecular triplet state of 1H-pyrazolo[3,4-b]quinoxaline (I) in n-hexane proceeds via thermally activated process 1LE ⇒ Tn followed by the fast internal conversion Tn ⇒ T1. In polar solvents where the emitting singlet state has a CT character the reaction sequence 1CT ⇔ 3CT ⇒ T1 seems to be not operative in the triplet state population. One possible explanation bases on the fact that increasing polarity of the solvent reduces the energy gap between the 3CT and T1 and in consequence reduces the electron transfer process occurring in normal Marcus region. Another possible explanation bases on the supposition that the intersystem crossing occurs only from the locally excited singlet state(1LE). The emitting singlet state having a CT character possesses an admixture of the locally excited singlet state (π,π*) which contribution decreases when the solvent polarity increases due to rising up the energy gap between the states. A direct 1LE ⇒ T1 intersystem crossing is less probable in solvents not containing heavy atom and this fact seems to be responsible for the lack of phosphorescence of the investigated compounds at 77 K.