Femtosecond Transient Absorption Spectroscopy Of Complex Molecular Systems And Photocatalytic Nanosystems

This dissertation reports on a systematic study of ultrafast dynamics and involved mechanisms in several complex molecular systems and photocatalytic nanosystems by means of femtosecond time-resolved transient absorption spectroscopy, towards providing fundamental guidance for relevant applications.This dissertation includes two parts:(1) Ultrafast spectroscopy and dynamics studies on several complex molecular systems:(a) Transient spectroscopy and dynamics of endohedral metallofullerenes (EMFs) and their derivatives; (b) Transient spectroscopy and dynamics of lanthanide coordination complexes. (2) Ultrafast spectroscopy and dynamics studies on several photocatalytic nanosystems:(a) Ultrafast carrier dynamics of a metal-organic framework (MOF) material PCN-222; (b) Ultrafast carrier dynamics of a hybrid material that integrates an inorganic semiconductor with a MOF.(1) Ultrafast dynamics of molecular systems(a) Endohedral metallofullerenes (EMFs) and their derivativesEndohedral metallofullerenes (EMFs) have become an important class of molecular materials for optoelectronic applications. The performance of EMFs is known to be dependent on their symmetries and characters of the substituents, but the underlying electron dynamics remain unclear. Here ultrafast transient absorption (TA) spectroscopy is used to examine the photoexcited electron dynamics of three isomers of Sc2C2@Cg2 with different cage symmetries (i.e., Cs, C2v, and C3v) and two representative Sc2C2@C3v-C82 derivatives. It is found that the opening of the triplet channels is highly correlated to the fullerene cage symmetry as well as the electronic character of the substituents. This systematic study could provide valuable guidance for potential applications of endohedral metallofullerene molecular systems in visible-light solar energy harvesting.(b) Lanthanide coordination complexesLanthanide coordination complexes with organic ligands are widely used as photoluminescence materials. Here we examine the ultrafast dynamics of lanthanide coordination complexes Er(TTA)3(TPPO)2. It is found that the reverse intersystem crossing (RISC) process occurs under certain conditions and its rate can be measured by analyzing the spectral lineshape and its variation.(2) Ultrafast carrier dynamics of photocatalytic nanosystems(a) A metal-organic framework material PCN-222It is highly desirable to convert CO2 to valuable fuels or chemicals by means of solar energy. We demonstrate that a metal-organic framework (MOF), PCN-222, can selectively capture and further photoreduce CO2 with high efficiency under visible-light irradiation. Mechanistic information gleaned from ultrafast transient absorption spectroscopy (combined with time-resolved photoluminescence spectroscopy) has elucidated the relationship between the photocatalytic activity and the electron-hole separation efficiency. It is found that the presence of a deep electron trap state in PCN-222 effectively inhibits the detrimental radiative electron-hole recombination. This work provides important insights into the design of MOF-based materials for CO2 capture and photoreduction.(b)A hybrid material that integrates an inorganic semiconductor TiO2 with a MOFWe demonstrate by using ultrafast transient absorption spectroscopy that the photogenerated electrons can be effectively transferred from the semiconductor to the MOF in the integrated nanosystem of an inorganic semiconductor TiO2 with MOF Cu3(BTC)2@TiO2. Such an effective charge transfer substantially suppresses electron-hole recombination, thereby improving the photocatalytic performance of the hybrid nanosystem.