Picosecond Coherent Vibrational Spectroscopy (CARS) of a DNA-Intercalating Ru Complex

Abstract

Vibrational spectra (1100−1800 cm-1) of the ground state and two excited electronic states, populated following the 5 ps, 440 nm excitation, of the [Ru(phen)2dppz]+2 (phen = 1,10-phenanthroline and dppz = dipyrido[3,2-a:2‘,3‘-c] phenazine) complex in aqueous solution are measured by coherent anti-Stokes Raman scattering (CARS). The band origin positions of vibrational features in the picosecond resonance CARS (PR/CARS) spectrum of ground state [Ru(phen)2dppz]+2 obtained with ω1 (probe) wavelengths of 491 and 530 nm are the same to within <1 cm-1 and correlate well with 488 nm resonance Raman data. The relative intensities of features in the 491 and 530 nm CARS spectra, however, are significantly different, reflecting the differences in Raman resonance enhancement associated with changes in excitation wavelengths. High signal-to-noise vibrational CARS spectra of samples containing [Ru(phen)2dppz]+2 and calf thymus DNA are also recorded using 491 and 530 nm ω1 excitation. Relative to CARS data from [Ru(phen)2dppz]2+ alone, small, but well-defined, shifts in band origin positions and large relative intensity changes are present. These changes are discussed in relation to possible structural changes in [Ru(phen)2dppz]2+ attributable to its interactions with DNA. Picosecond transient absorption (PTA) signals, recorded over the initial 4.0 ns interval after 440 nm, 5 ps excitation and using five probe wavelengths in the 500−560 nm range, confirm the presence of two photophysical intermediates that have previously been assigned as metal−ligand charge transfer (MLCT) excited states (i.e., MLCT(1) and MLCT(2)). Picosecond time-resolved CARS (PTR/CARS) spectra (ω1 = 530 nm) of [Ru(phen)2dppz]2+ in water, recorded at 23, 30, 100, 200, and 700 ps delays, provide five independent measurements of the MLCT(2) vibrational spectrum. Although the vibrational CARS features (positions and relative intensities) assigned to MLCT(2) remain the same throughout its 260 ps lifetime, the changes in their absolute intensities correlate closely with the PTA signals, thereby indicating that MLCT(2) has a well-defined and stable structure throughout and that little, if any, vibrational redistribution occurs during its 260 ps lifetime. The vibrational spectrum of another intermediate, not assignable to either ground state [Ru(phen)2dppz]2+ or its MLCT(2) state, is found in the 15 ps PTR/CARS data. Candidates for the assignment of this second intermediate, including MLCT(1), are discussed.