Photophysics of 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine: emission from two states

Steady state photophysics of 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ) has been studied in different pure and mixed solvents. The fluorometric behaviour of AODIQ is modified dramatically with a change in the solvent polarity. The fluorescence quantum yield, when plotted against the solvent polarity parameter (ET(30)), passes through a maximum. Plot of the fluorescence parameters in a graded series of dioxane–water mixture against ET(30) shows segmented linearity with two independent slopes; reflecting a lower sensitivity in the less polar region and a much higher sensitivity in higher polarity range. The observations lead to the proposition that the fluorescence of AODIQ originates from two different states, viz., a locally excited (LE) and a charge transfer (CT) one. Studies of the phosphorescence and heavy atom quenching of the fluorescence reveal that the CT state lies above the lowest triplet state even in a very polar environment.