Solvatochromic study of three indoloquinoline derivatives: Effect of chloro group/s on the photophysics of thecompounds

Photophysical modulations of three indoloquinoline derivatives, viz., 5-Methyl-5H-indolo[3,2-c]quinoline (MIQ), 8-Chloro-5-methyl-5H-indolo[3,2-c]quinoline (CMIQ), and 2,8-Dichloro-5-methyl-5H-indolo[3,2-c]quinoline (DCMIQ) have been studied. These compounds belong to the cryptosanguinolentine family and are known to have potential anti-cancer abilities. In the present work we studied the effect of solvent polarity on introduction of one or two chloro (-Cl) group/s at chosen sites on the parent cryptosanguinolentine. The compounds exist mainly in two forms, neutral and zwitterionic, in equilibrium at ground state. Interestingly, all the three compounds exhibit hypsochromic shift in their absorption and fluorescence spectra with increase in relative permittivity of solvents in most of the cases. Blue-shift in fluoroscence might be due to lesser dipole moment of the species in the excited state. Time-resolved fluorescence spectroscopy clearly indicates the presence of two species in all the solvents in the excited state (except MIQ in water). The shift in the molecular polarity with chloro substitution/s seems to be responsible for the contributions of the two species in the excited state. The relative contributions are found to alter remarkably in a regular manner when MIQ is compared with CMIQ and DCMIQ. This behavior is supposed to be mainly due to dipole-dipole interaction between the solvent molecules and the fluorophores.