The Study of Two-photon-allowed Excited States of Octupolar Chromophoric Molecules by Resonance Hyper Raman and Surface-enhanced Resonance Hyper Raman Spectroscopy

The objective of this thesis is to study the two-photon-allowed excited states of octupolar chromophoric molecules by resonance hyper Raman (RHRS) and surface-enhanced resonance hyper Raman spectroscopy (SERHRS). These excited states were believed to dictate the nonlinear optical (NLO) properties of octupolar dyes.;Specifically, we have systematically studied two classes of octupolar molecules, organic dyes and transition metal complexes, respectively, by RHRS and/or SERHRS. The organic octupolar dyes were chosen because of their wide applications as optical imaging probes in biological sciences. The octupolar transition metal complexes were chosen because of their extensive use as photosensitizers in solar energy conversion. The ultimate goals of this thesis are to provide guidance for the design and synthesis of new NLO chromophoric molecules, and to pave the way for the development of NLO imaging techniques based on multi-photon excited hyper Raman scattering.;Chapter 1 gives a concise introduction and overview of Raman scattering and hyper Raman scattering under normal, resonance-enhanced, and surface-enhanced conditions. The basic and general properties of octupolar molecules are also outlined.;Chapters 2 to 4 report a systematic study of three organic octupolar dyes, crystal violet (CV), basic fuchsin (BF), and a triazine dye (T72), by both Raman and hyper Raman spectroscopy under normal, resonance-enhanced and surface-enhanced conditions. Raman excitation profiles (REPs) and hyper Raman excitation profiles (HREPs) were measured to establish the vibronic nature of the excited states. DFT and TD-DFT calculations were carried out to assign the normal modes of the molecules as well as the excited vibronic states involved. Two intensity calibration methods for SERHRS were developed, one of which uses pyridine as the external reference standard and another uses dissolved Na2MoO4 as the internal reference standard. Both methods were validated by testing on CV in the presence and absence of surface enhancement. The most important discoveries include (a) the revelation of two-photon-allowed excited states by HREPs, and (b) observation of multiplexed non-linear multiphoton scattering by single color excitation.;Chapters 5 to 7 report a systematic study of three octupolar transition metal complexes [M(bpy)3]2+ (bpy = 2,2'-bipyridine, M = Ru II, OsII and FeII) by Raman and hyper Raman spectroscopy under surface-enhanced resonance conditions. The main discovery is the observation of the two-photon-allowed but one-photon forbidden metal-to-ligand charge transfer (MLCT) excited state, revealed by HREPs, which could not be observed by conventional linear optical techniques. DFT and TD-DFT calculations were performed to assign the normal modes of vibration, as well as the excited states involved in the observed HREPs.;To the best of my knowledge, this thesis reports hitherto the most extensive study of both organic and inorganic octupolar molecules by two-photon excited hyper Raman spectroscopy under resonance and surface enhancement conditions. A brief summary and future perspective are given at the end of this thesis.