Surface-enhanced resonance hyper-Raman scattering (SERHRS) is a multiphoton nonlinear optical process boosted by both molecular resonance and localized surface plasmons of nanostructures. By taking the advantages of plasmonic enhancement effect and metallic adsorption induced symmetry-lowering effect, two-photon allowed but one-photon forbidden excited states of quadrupolar and octupolar chromophores can be characterized by the wavelength-scanned SERHRS. Three types of quadrupolar chromophores, including one each of organic, organometallic and biomimetic molecules, and one octupolar organic chromophore are investigated in this dissertation. The multipolar chromophores are chosen for their important applications in biological imaging, solar energy conversion and photodynamic the...[ Read more ]

Surface-enhanced resonance hyper-Raman scattering (SERHRS) is a multiphoton nonlinear optical process boosted by both molecular resonance and localized surface plasmons of nanostructures. By taking the advantages of plasmonic enhancement effect and metallic adsorption induced symmetry-lowering effect, two-photon allowed but one-photon forbidden excited states of quadrupolar and octupolar chromophores can be characterized by the wavelength-scanned SERHRS. Three types of quadrupolar chromophores, including one each of organic, organometallic and biomimetic molecules, and one octupolar organic chromophore are investigated in this dissertation. The multipolar chromophores are chosen for their important applications in biological imaging, solar energy conversion and photodynamic therapy among others.

Chapter 1 gives a brief overview of nonlinear optical processes and SERHRS.

Chapter 2 reports a systematic study of an organic quadrupolar (D-π-A-π-D type) chromophore with a structural inversion center (Squarylium Dye III or SQ-III) by two-photon excited (2PE-) and three-photon exited (3PE-) SERHRS. The location and vibronic nature of the 2P-allowed but 1P-forbidden donor-to-acceptor charge transfer (DACT) excited states are characterized by the wavelength-scanned 2PE-SERHRS. Two possible 3P-resonant enhancement mechanisms, direct resonance involving one excited state and double resonance involving two excited states, are proposed to account for the observed 3PE-SERHRS excitation profiles.

Chapter 3 focuses on the study of a quadrupolar organometallic coordination complex (A-D-A type), Ruthenium (II) bis(terpyridine) cation ([Ru(tpy)₂]²⁺) without a structural inversion center. This chromophore is characterized by its metal-to-ligand charger transfer (MLCT) excited states whose location and vibronic nature in terms of Herzberg-Teller coupling via totally symmetric “a1” and nontotally symmetric “e” vibrational modes are revealed by the 2PE-SERHRS with the aid of DFT/TD-DFT calculation.

Chapter 4 systematically investigated the linear and nonlinear optical properties of a dipolar (D-π-A type) and a quadrupolar (D₂A₂ type) Zn (II) metalloporphyrin derivatives by wavelength-scanned RRS, SERRS, RHRS and SERHRS. Both one- and two-photon allowed DACT excited states and their mixing with π-π* excited state of porphyrin macrocycle are elucidated by the selective enhancement of vibration modes with different symmetries.

Chapter 5 reports for the first time a systematic investigation of an octupolar (AD₃ type) chromophore crystal violet (CV) adsorbed on gold nanoparticles by 2PE- and 3PE-SERHRS. The two-photon allowed but one-photon forbidden DACT excited state and plasmons-molecule coupling are characterized by the wavelength-scanned SERHRS. The resonance mechanisms in 3PE-SERHRS are proposed, including direct resonance involving one excited state and double resonance involving two excited states. Moreover, SERHRS on gold nanoparticles demonstrate more promising applications in both chemical and biological systems due to their higher biocompatibility, stability and geometry controllability.

In addition, the enhancement factors of SERHRS on silver and gold aggregations are also estimated, which can be from the combined contributions of plasmonic enhancement and molecular resonance enhancement. By choosing the nonresonant 4-MBA as the external standard, the double resonance effect in 3PE-SERHRS of CV is further confirmed.

This dissertation systematically investigated the 2PE- and 3PE-SERHRS on aggregated Ag and Au nanoparticles, respectively. The obtained results demonstrate a unique approach to understand the electronic structures and multiphoton enhancement mechanisms in multipolar molecules. We hope that our work can provide guidance for the design and synthesis of new nonlinear optical materials and pave the way for the development of a new nonlinear optical technique for analysis and imaging based on multi-photon excited hyper-Raman scattering with more spectral information and higher spatial resolution.