Photophysics of Ru(phen)(2)(PHEHAT)(2+): A novel ''light switch'' for DNA and photo-oxidant for mononucleotides

The spectroelectrochemical properties of a novel light switch for DNA, Ru(phen)(2)(PHEHAT)(2+) (phen = 1,10-phenanthroline; PHEHAT = 1, 10-phenanthrolino[5,6-b]1,4,5,8,9,12-hexaazatriphenylene), are examined and compared to those of Ru(phen)(2)(DPPZ)(2+) (DPPZ = dipyrido[3,2-a;2',3'-c]phenazine) and Ru(phen)(2)(HAT)(2+) (HAT = 1,4,5,8,9,12-hexaazatriphenylene). The excited Ru(phen)(2)(PHEHAT)(2+) luminesces in organic solvents but not in water. It is shown that the orbitals involved in the absorption and luminescence spectroscopy are not the same as those in the electrochemistry. In aqueous solution, this complex luminesces upon intercalation of the PHEHAT ligand into the stacking of the DNA bases. Two modes of distribution of the complex on DNA can be evidenced from the titration curves of the complex with DNA. Laser flash photolysis experiments show that the excited state is able to abstract an electron from GMP (guanosine-5'-monophosphate) with a rather low efficiency, leading to the reduced complex and oxidized GMP. However, this process is not accompanied by the formation of photoproduct with GMP and cannot be detected with DNA on the time scale of the experiments.