Vibrational energy transport and electron transfer in molecules investigated by nonlinear vibrational spectroscopy

This work is directed at application of ultrafast nonlinear infrared spectroscopy targeting a variety of atomic-scale characteristics for molecules, such as structural constraints, energy transport rates and pathways, and electron transfer efficiencies. Two major experimental approaches were used. One is the recently proposed dual-frequency relaxation-assisted two-dimensional infrared spectroscopy (RA 2DIR) method, which required further development and testing. Several molecular systems were investigated by RA 2DIR focusing on understanding of its applicability to decipher molecular structures. Dual-frequency RA 2DIR correlation spectra of two anomers of peracetylated D-glucopyranose are found to differ significantly, even in the regions where their linear absorption spectra are similar. RA 2DIR correlation spectra rely on the energy transport on a molecular scale, which can be very characteristic for a particular molecule. In the case of the two anomers, this energy transport patterns are found to be different, providing unique spectroscopic data specific to a particular anomer.;The off-diagonal anharmonicity for a pair of vibrational modes, designated as a shift of their combination level, Δ₁₂, is an experimentally accessible parameter, which can be linked to the molecular structure. To measure experimentally the absolute values of small off-diagonal anharmonicities (< 0.1 cm−1) is challenging as the cross-peak shape is not sensitive to Δ₁₂, while the absolute cross-peak measurements are difficult. A new approach relying on a series of relative RA 2DIR measurements is proposed to determine anharmonicities, which is based on sensitivity of high-frequency vibrational modes to temperature.;Understanding the energy transport dynamics in molecules is the key for deciphering structural information from the RA 2DIR spectra. Vibrational energy transport across coordination bonds was studied in a tetraethylammonium bis(maleonitrile dithiolate)iron(III)nitrosyl complex. It is shown that the energy transport via coordination bonds is almost as efficient as that via covalent bond, despite the weakness of the former. A correlation of the energy transport time with distance is found.;To push the boundaries of RA 2DIR spectroscopy, the energy transport was investigated in polyethylene glycol (PEG) oligomers of different length, end-labeled with azido and succinimide ester moieties. The through-bond energy transport time is found to be linearly dependent on the chain length of up to 60 Å, suggesting a ballistic energy transport regime in which the transport is described as a propagation of a vibrational wavepacket. The cross-peak amplitude at the maximum is reported to decay exponentially with the chain length. Also, the cross-peak amplitude at zero waiting time, determined by the end-to-end distance distribution, is found to decay with the chain length close to the predictions of the free flight chain model.;Electron transfer is one of the fundamental processes in chemistry; an ability to control its rate will certainly be useful in a variety of applications, ranging from molecular electronics to molecular biology. Modulation of electron transfer process via vibrational perturbation is investigated in a donor-bridge-acceptor complex featuring dimethylaniline and anthracene moieties as the electron donor and acceptor, and tethered by a guanosine-cytidine base pairing "bridge". Femtosecond three-pulse UV-IR-Vis pump-probe method was used in this study. The yield of the photoinduced charge separation reaction is found to decrease in the presence of vibrational excitation of high-frequency modes at the bridge. In contrast, the reaction becomes faster at higher temperatures.