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...[
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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 (D
2A
2 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 (AD
3 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.
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