Organic fluorophores : nonlinear optical properties and environmental effects on photoluminescence

Abstract

This dissertation discusses the nonlinear optical properties and influence of molecular environment on the optical properties of several novel organic compounds developed by collaborators in chemistry. When z-scan technique is adopted to determine the nonlinear absorptive and refractive coefficient, femtosecond pulses with a 1 kHz repetition rate are more suitable as the incident source than the 76 MHz ones, as the latter would introduce a significant thermal effect. Performing the open aperture z-scan under several irradiances and estimating the two-photon absorption (2PA) coefficients from fitting would help to avoid the exaggeration of 2PA cross-sections due to other nonlinear effects such as excited state absorption, resulting in 2PA cross-sections estimated consistent with and comparable to other literature. The organic fluorophores investigated for nonlinear optical properties all have donor-π-acceptor structure motifs, whose lowest-energy 2PA bands are identical to those in one-photon absorption. The molecular 2PA cross-sections of these fluorophores are 50-250GM, increasing with the addition of donor/acceptor moieties, and influenced by alternation of donor/acceptor groups. For these fluorophores studied, the molecular environment, such as whether the fluorophores are dispersed or aggregated, what solvent is used, and the formation of crystals, can all influence optical properties, in aspects like absorption and emission spectra, fluorescence quantum efficiencies and lifetimes, as well as nonlinear optical properties. The aggregation enhancement in two-photon excited fluorescence can reach around two-orders of magnitudes. The 2PA cross-sections studied can be enhanced by the formation of aggregations, slightly influenced by solvent polarity, and reduced by the formation of a twisted intramolecular charge-transfer state for dipolar molecules with large charge separation. The changes in molecular conformations and molecular stacking modes in crystal polymorphs both influence the optical properties, via intra- and interchain conjugation as well as intra- and inter-molecular interactions.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology