Spectral and photophysical properties of some imidazo[1,2-a]purine derivatives related to acyclovir

Spectral and photophysical properties of 3,9-dihydro-3-[(2-hydroxyethoxy)methyl]-6-phenyl-9-oxo-imidazo[1,2-a]purine 7b (congener of acyclovir) and its three substituted derivatives 7a, c and 8a in organic solvents and in aqueous buffer solutions have been determined. Compound 7c has an ester group in the para position of the phenyl ring, compounds 7a and 8a are derivatives of 7b with a methyl group attached to imidazole N(5) or pyrimidine N(4) ring of the tricyclic skeleton, respectively. Experimental study has been supplemented with theoretical calculations of the singlet electronic transition energies and oscillator strengths by the time dependent density functional theory (TD-DFT). Combination of experimental and computational results have indicated that compounds 7b and c having mobile hydrogen exist in the ground state in a single N(5)H tautomeric form. The lowest excited S1 state of all compounds investigated has the π, π∗ configuration. The spectral and photophysical properties of 7a–c are similar. The fluorescence behaviour of the compounds depends on the solvent polarity and pH of the aqueous medium in the range of pH: 0–4 and 6–12. The pH dependence is a consequence of the protonation of the molecules (pKa=2.2 for 7a and pKa=2.0 for 7b and c) and their deprotonation in the ground state (pKa=8.0 and 8.2 for 7b and c, respectively). The absorption and fluorescence properties of the ionic forms have been determined. In contrast to 7a–c, the spectral and photophysical properties of 8a are not significantly influenced by the solvent polarity and they are independent on pH of aqueous solution within 4–12 range.