Antiprotozoal agents as water soluble singlet oxygen photosensitizers: Imidazo[1,2-a]pyridine and 5,6,7,8-tetrahydro-imidazo[1,2-a]pyridine derivatives

In this study we report on the photophysical properties of some imidazo[1,2-a]pyridine and 5,6,7,8-tetrahydroimidazo[1,2-a]pyridine derivatives and their efficiency as singlet oxygen photosensitizers in aqueous media. In aqueous solution, the wavelengths of maximum absorption are in the range of 269-307 nm, while the wavelengths of maximum emission cover the range of 357-462 nm. It has been found that the fluorescence decay of these derivatives were fitted very well with mono-exponential function for one of the compounds and with bi- exponential function for the other compounds with χ2 of about 1.0±0.1 in all cases and show solvent isotope effect. It has also been found that excited singlet state relaxation is dominated by non-radiative relaxation pathways. Rate constants for oxygen quenching of the excited singlet states of these compounds are close to the diffusion rate constant in H2O and D2O. Singlet oxygen quantum yields photosensitized by imidazo[1,2-a]pyridine and 5,6,7,8-tetrahydroimidazo[1,2-a]pyridine derivatives were reported in D2O and found to be in the range 0.01-0.1 relative to phenalenone as a standard. The rate constants, kqΔ, for quenching of singlet oxygen by ground state sensitizers in D2O are also reported and found to be in the range (0.82-6.74)×109 M−1 s−1.