UNUSUAL PHOTOPHYSICS OF A RHENIUM(I) DIPYRIDOPHENAZINE COMPLEX IN HOMOGENOUS SOLUTION AND BOUND TO DNA

The photophysics of [fac-(dppz)Re-I(CO)(3)(4-MePy)][Cl] (1, where dppz = dipyrido[3,2-a:2',3'-c]phenazine and 4-MePy = 4-methylpyridine) have been examined in organic solvents and in aqueous solution with and without calf thymus DNA. Emission and nanosecond to microsecond transient absorption studies of 1 in organic solvents such as MeOH, MeCN, and CH2Cl2 indicate that the lowest excited state of the complex is a dppz-based intraligand triplet state ((3)IL(dppz)). The (3)IL(dppz) state is characterized by weak room temperature phosphorescence and a strong T-0 --> T-n absorption band with lambda(max) = 470 nm. Absorption and emission studies of 1 in aqueous solution with added DNA indicate that the complex binds to the biopolymer, persumably by intercalation of the dppz ligand. Complex 1 is non-luminescent in aqueous solution; however, (3)IL(dppz) phosphorescence is observed from the DNA-bound form of the complex in aqueous solution. The (3)IL(dppz) assignment for the emission from the DNA-bound complex is supported by the observation of the characteristic T-0 --> T-n transient absorption feature at 470 nm in the absorption-difference spectrum of the DNA-bound complex.