SPECTROSCOPIC STUDIES OF DIPYRIDAMOLE DERIVATIVES IN HOMOGENEOUS SOLUTIONS - EFFECTS OF SOLUTION COMPOSITION ON THE ELECTRONIC ABSORPTION AND EMISSION

Electronic absorption and fluorescence spectra of three different dipyridamole (DIP) derivatives, RA 39, RA 14 and RA 25, were monitored in aqueous solution as a function of pH in the range 2-13. Extinction coefficients, quantum yields and fluorescence lifetimes were obtained in aqueous solutions at different pHs and in several other solvents. The absorption spectra are characterized by two broad bands centered at 405-415 nm and 290 nm for RA 39 and RA 14 and at 370 and 270 nm for RA 25. The first band used for fluorescence excitation has a value of epsilon congruent-to 5 x 10(3) M-1 cm-1 characteristic of a pi --> pi* transition. An increase in pH is accompanied by both fluorescence and absorption sharp changes around pH 6 giving a pK congruent-to 5.8 for RA 39 and RA 14, similar to the one observed for DIP. In the case of RA 25 two pKs are observed below 6.5, namely a pK1 congruent-to 2.25 and pK2 = 5.20. These changes are associated to protonation equilibria of nitrogens in the pi-conjugated system of the pyrimido-pyrimidine ring. At high pH a second transition is observed only in fluorescence with pK congruent-to 12.5 for RA 39 which is similar to DIP and pK congruent-to 12.1 for RA 25. For RA 14 this transition was not observed. Estimates of the pK* for the first singlet excited state showed that in comparison to the ground state pK congruent-to 5.8 or 5.2 (RA 25), the excited state pK* is more acid by almost a unit. In the case of RA 25 and the lowest pK of 2.25, an excited state reaction, possibly of proton transfer, seems to take place since emission and absorption data give opposite effects for pK*.