Electronic Spectra and Transition Moments of 6-(2’-Pyridiniumyl)phenanthridinium Photoactive DNA Intercalators

Journal article, 1997

The electronic transitions giving rise to the UV-visible absorption spectra of two pyridinium-phenanthridinium viologens, 6,7-dihydropyridol[2',1':3,4]pyrazinol[1,2-f]phenanthridinediium dication (1) and 7,8-dihydro-6H-pyrido[2',1':3,4]diazepino[1,2-f]phenanthridinediium dication (2), have been investigated with respect to energies, intensities, and transition moment directions. A combination of methods has been applied: UV-visible absorption, circular dichroism, magnetic circular dichroism, fluorescence anisotropy, Linear dichroism In stretched poly(vinyl alcohol) films, and semiempirical molecular orbital calculations. For both drugs, the lowest energy absorption band, occurring around 400 nm, results from two separate transitions. The corresponding electric transition dipole moments lie in the phenanthridine plane and are polarized, respectively, in the direction of the pyridine moiety (the lower energy transition) and parallel to the phenanthridine long axis (the higher energy transition). Up to four additional different pi --> pi* transitions account for a second band that peaks at 250 nm; they show different polarizations within the phenanthridine plane. The lowest energy transition of the whole spectrum of both drugs corresponds to the promotion of an electron from the HOMO to the LUMO, which are molecular orbitals mainly localized in the phenanthridine and pyridine rings, respectively, thereby implying a charge transfer, upon excitation, from the phenanthridine toward the pyridine ring. The experimental and theoretical results are discussed in relation to the spectroscopic, redox, and photochemical properties of these drugs.