Structural changes of molecules accompanying ultrafast photophysical and photochemical processes in the excited state can be monitored by time-resolved vibrational probes with high spectral and temporal resolutions. Femtosecond stimulated Raman spectroscopy (FSRS) provides essential vibrational information for the structural changes of chromophores in the excited state in a wide dynamic range of vibrational frequencies. The spontaneous changes in the vibrational population and line-shape of time-resolved Raman spectra represent the changes in the electronic structure and molecular structures of molecules in the excited state. Many excited-state processes, including excited-state- intramolecular proton transfers and intramolecular charge transfers, have been investigated by FSRS. Besides the transient vibrational modes of solute molecules, the changes in the solvent vibrational modes (dimethyl sulfoxide) can be used for the indirect probe of the excited-state processes of the solute and the solvation dynamics of solute molecules. Ultrafast structural changes of solvent molecules in the solvation shells have been observed by the instantaneous changes in the stretching modes of dimethyl sulfoxide representing the hydrogen-bonded and isolated species.
Chemical reactions occurring in the heterogeneous media of cell membrane or protein complexes are distinct from the reactions in the solution phase in terms of the reaction kinetics and dependence on the local environments. Nanoscopic pools inside the reverse micelles are regarded as good model systems for heterogeneous reaction dynamics, where the size of micelles or the interfacial layer consisting of surfactant head groups can affect the reaction dynamics occurring on ultrafast time scales. Excited-state proton transfer dynamics of photoacids and intramolecular charge transfer dynamics with the twist of electron donor or acceptor group are investigated by femtosecond electronic spectroscopy of transient absorption and fluorescence upconversion. When confined in small reverse micelles with only a small number of solvent molecules, photoinduced chromophores often show distinct reaction dynamics from the bulk. Abnormally decreased solvent dynamics in small reverse micelles also result in the largely decreased solvent dependence from those observed in bulk phases.
The locally enlarged electric field around the metal nanoparticles is optimally used to enhance the fluorescence and Raman signals of molecules that exist close to the nanoparticles. Homogeneous silver nanoparticles of 60-200 nm in diameter can be obtained by the seeded-growth method, and the size-dependent dipole and quadrupole surface plasmons of the nanoparticles can be used to optimize the fluorescence enhancement of the fluorophores with significant spectral overlap between the fluorescence and the surface plasmon bands. Various noble metal nanosurfaces can also be used for the Raman enhancements of the analytes (surface-enhanced Raman scattering). Surface adsorption of small biological molecules and the dependence on the local environmental changes (e.g., pH or electric potential) in the major vibrational modes can be interpreted as the adsorption geometry changes of the analytes. The surface-enhanced Raman is considered as a highly sensitive probe for various biological and environmentally important molecules.