利用奈秒時間解析近紅外/中紅外光譜闡明反應中間物在凝態光物理和光化學反應中的結構及動力學

Abstract

In this Thesis, the author presents unprecedentedly detailed studies on the structure and dynamics of ionic and neutral transient species that are of crucial importance in molecular photovoltaic devices, using nanosecond time-resolved near/mid-IR spectroscopy with the help of density functional theory (DFT) calculations and chemometrics techniques. The author has investigated (1) the back electron transfer (BET) dynamics in photoinduced intermolecular electron transfer reaction between pyrene (Py) and 1,4-dicyanobenzene (DCB) in acetonitrile and (2) the structure of the lowest excited triplet (T1) state of p-nitroaniline (PNA) in acetonitrile-d3. In the first work, the transient near/mid-IR spectra of Py radical dimer cation and DCB radical anion are observed in the nano- to microsecond (ns–μs) time regime after photoexcitation of Py. Global fitting analysis of the time-resolved IR data reveals a dual role of acetonitrile as solvent and “charge mediator” of the charge recombination between Py radical dimer cation and DCB radical anion in the BET reaction. This finding may have implications for dye-sensitized solar cells because acetonitrile is a commonly used solvent for redox couples in these types of devices. In the second work, the transient mid-IR spectra of PNA in the T1 state generated after photoexcitation of PNA and subsequent intersystem crossing are examined to characterize the structure of T1 PNA. Comparison of the experimental IR spectra with DFT calculated results on explicitly solvated PNA shows that T1 PNA has a partial quinoid structure, which sharply contrasts with the well-known zwitterionic charge-transfer structure of the lowest excited singlet state of PNA. The studies presented in this Thesis not only illustrate the applicability of the time-resolved near/mid-IR method to a wide variety of important photophysical and photochemical processes in the condensed phase, but they also provide otherwise hardly obtainable insights into the structure and dynamics of transient species (radicals and excited-state molecules) involved in charger transfer processes.

In this Thesis, the author presents unprecedentedly detailed studies on the structure and dynamics of ionic and neutral transient species that are of crucial importance in molecular photovoltaic devices, using nanosecond time-resolved near/mid-IR spectroscopy with the help of density functional theory (DFT) calculations and chemometrics techniques. The author has investigated (1) the back electron transfer (BET) dynamics in photoinduced intermolecular electron transfer reaction between pyrene (Py) and 1,4-dicyanobenzene (DCB) in acetonitrile and (2) the structure of the lowest excited triplet (T1) state of p-nitroaniline (PNA) in acetonitrile-d3. In the first work, the transient near/mid-IR spectra of Py radical dimer cation and DCB radical anion are observed in the nano- to microsecond (ns–μs) time regime after photoexcitation of Py. Global fitting analysis of the time-resolved IR data reveals a dual role of acetonitrile as solvent and “charge mediator” of the charge recombination between Py radical dimer cation and DCB radical anion in the BET reaction. This finding may have implications for dye-sensitized solar cells because acetonitrile is a commonly used solvent for redox couples in these types of devices. In the second work, the transient mid-IR spectra of PNA in the T1 state generated after photoexcitation of PNA and subsequent intersystem crossing are examined to characterize the structure of T1 PNA. Comparison of the experimental IR spectra with DFT calculated results on explicitly solvated PNA shows that T1 PNA has a partial quinoid structure, which sharply contrasts with the well-known zwitterionic charge-transfer structure of the lowest excited singlet state of PNA. The studies presented in this Thesis not only illustrate the applicability of the time-resolved near/mid-IR method to a wide variety of important photophysical and photochemical processes in the condensed phase, but they also provide otherwise hardly obtainable insights into the structure and dynamics of transient species (radicals and excited-state molecules) involved in charger transfer processes.