Picosecond time-resolved resonance Raman (picosecond-TR 3 ) spectroscopy has been used to conduct an extensive photophysical characterization of the "light-switch" complex [Ru(phen) 2 dppz] 2+ as a function of environment, in which studies have been carried out in aqueous and nonaqueous media and in DNA. The results are considered in relation to a previous report describing "environment-sensitive" lowest triplet MLCT states. Vibrational marker features and enhancement patterns were used to determine the rapid progression (<20 ps) between two triplet MLCT states in aqueous environment, followed by subnanosecond, nonradiative deactivation to the ground state. In nonaqueous environment, the long-lived, emissive triplet MLCT state is spectrally identified as the short-lived first triplet MLCT state observed in water, in agreement with the earlier proposed mechanism. The present data are shown to correlate well with previous nanosecond RR findings for the complex in each environment. Interestingly, a "precursor state" has been identified upon excitation in both nonaqueous solvent and in DNA, which precedes the triplet MLCT state, and the lifetime of which appears to be environment dependent. Observation of this state is discussed in relation to other recent femtosecond spectroscopic studies on this complex.

The photophysical properties of fac-[Re(CO)3(dppz)(py)]+ (1, where dppz = dipyrido[3,2-a: 2',3'-c]phenazine) in CH3CN have been investigated using a series of complementary techniques including visible and infrared transient absorption and resonance Raman spectroscopy on the picosecond and nanosecond timescales. The results confirm previous reports that the lowest-lying emissive state in 1 is a triplet intra-ligand (3IL) state localised on the dppz ligand and have provided detailed information on the dynamics of 1 upon photoexcitation, including the relative energies of the excited state species encountered and the electronic distribution within these. If the dppz ligand is viewed in terms of phenanthroline (phen) and phenazine (phz) moieties, the emissive state is probably more accurately described as a 3 pi-->pi *(phz) IL state. The picosecond studies have shown that this emissive state is formed, at least in part, within 30 ps of excitation from a precursor, which is possibly a 3 pi-->pi *(phen) IL state. On the nanosecond timescale, TRIR has been employed to elucidate further dynamics and reveal the presence of an energetically close-lying state in equilibrium with the emissive state. This has tentatively been assigned as being 3d pi(Re)-->pi *(phz) metal-to-ligand charge transfer (MLCT) in nature. A summary of the photophysics is proposed in the form of a Jablonski scheme. Time dependent density functional theory (TD-DFT) calculations support the relative ordering and suggested electronic character of the excited state species involved.

Time-resolved and steady-state luminescence and transient resonance Raman measurements have been carried out on the complex [Ru(phen) 2 dppz] 2+ (1) in the presence of single-stranded (ss) DNA that is either covalently attached to or mixed in 1:1 ratio with the complex. The well-known enhancement of luminescence (the "lightswitch" effect) exhibited by (1) when intercalated to double-stranded DNA is also observed in the presence of the single-stranded material, under conditions of covalent attachment or simple mixing. The evidence from both the luminescence and the transient Raman studies suggests that the enhancement need not necessarily reflect deep intercalation of the dppz ligand between the bases of the ss material.