Electron-Transfer in Covalently Coupled Donor-Acceptor Complexes

Citation

Bachrach, Max (1996) Electron-Transfer in Covalently Coupled Donor-Acceptor Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/tzkw-0q94. https://resolver.caltech.edu/CaltechTHESIS:12152020-011716999

Abstract

A picosecond transient absorption experiment has been constructed and used for the study of intramolecular electron-transfer (ET) in three molecular systems. The first of these is a donor-bridge-acceptor (D(br)A) complex composed of a d⁸-d⁸ iridium core covalently coupled to two pyridinium acceptors with flexible phosphonite spacers. Varying the pyridiniums' substituents allows control of the driving-forces (ΔG°) of the photoinduced ET reactions from the core's singlet and triplet states. The rates of these reactions have been determined from steady-state and time-resolved emission experiments. Picosecond transient absorption has been used to measure the rates of subsequent charge-recombination, and determine their dependencies on ΔG°. The reorganization energy accompanying intramolecular electron-transfer within this system (λ ≈ 0.86 eV) has been determined by fitting the rates of photoinduced and thermal reactions to a single-mode quantum-mechanical ET model.

The rates of all the photoinduced and thermal ET reactions have been measured as a function of temperature (200° K - 280° K). These data show that the charge-recombination reactions, which lie within the Marcus inverted region (ΔG° > λ), have rates (k_BET) that are strongly dependent on temperature. Deuteration of the pyridinium acceptor has a temperature independent effect (k_H/k_D = 1.2) on k_BET, indicating that, although the inverted region reactions within these complexes exhibit behavior that is remarkably classical, quantum-mechanical effects do need to be considered. Both temperature and isotope effects suggest that the molecules' low internal reorganization energies (λ_in = 0.006 eV) account for their ET behavior.

Less comprehensive studies of two other electron-transfer systems are also reported. Unlike the iridium system, in which the donor-acceptor electronic coupling H_AB is less than 100 cm^-1, the couplings within these are over 2000 cm^-1. It has been found that despite the large amount of coupling between the metals of [(bpy)(tpy)Ru^IICNRu^II(NH₃)₅]²⁺ (bpy = 2,2'-bipyridine; tpy = 2,2',2"-terpyridine), our transient absorption experiment is able to detect formation of a charge-separated state following excitation into the Ru → immine charge-transfer band. It is proposed that this species can be described as [(bpy)(tpy•-)Ru^IICNRu^III(NH₃)₅]²⁺ and that unusually low coupling between the tpy ligand and the oxidized ruthenium leads to slow (1 x 10¹⁰ s^-1) charge-recombination.

The ground-state absorption features and excited-state decay kinetics of a series of ferrocene-based D(br)A complexes have also been measured. It is proposed that H_AB is much larger than 2000 cm^-1 for these complexes, and plays an important role in determining their nonlinear optical properties.

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