ELECTRONIC-SPECTRA AND TRANSITION MOMENTS OF 6-(2'-PYRIDINIUMYL)PHENANTHRIDINIUM PHOTOACTIVE DNA INTERCALATORS

The electronic transitions giving rise to the UV-visible absorption sp ectra of two pyridinium-phenanthridinium viologens, idol[2',1':3,4]pyr azinol[1,2-f]phenanthridinediium dication (1) and rido[2',1':3,4]diaze pino[1,2-f]phenanthridinediium dication (2), have been investigated wi th respect to energies, intensities, and transition moment directions. A combination of methods has been applied: UV-visible absorption, cir cular dichroism, magnetic circular dichroism, fluorescence anisotropy, Linear dichroism In stretched poly(vinyl alcohol) films, and semiempi rical molecular orbital calculations. For both drugs, the lowest energ y 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 di rection of the pyridine moiety (the lower energy transition) and paral lel to the phenanthridine long axis (the higher energy transition). Up to four additional different pi --> pi transitions account for a sec ond band that peaks at 250 nm; they show different polarizations withi n the phenanthridine plane. The lowest energy transition of the whole spectrum of both drugs corresponds to the promotion of an electron fro m the HOMO to the LUMO, which are molecular orbitals mainly localized in the phenanthridine and pyridine rings, respectively, thereby implyi ng a charge transfer, upon excitation, from the phenanthridine toward the pyridine ring. The experimental and theoretical results are discus sed in relation to the spectroscopic, redox, and photochemical propert ies of these drugs.