The influence of molecular structure and environment on the mechanism of photoinduced charge transfer in organic systems

A mechanistic understanding of long-distance charge transfer within solution-phase donor-bridge-acceptor molecules is both a challenge to our ability to describe electron transfer and necessary for the development of organic devices for photochemical energy conversion and molecular electronics. The charge transfer process is a combination of coherent tunneling and thermally-activated events, where the fraction of each is determined primarily by donor-acceptor orbital overlap, Coulombic effects, electrochemical potentials of redox centers, and interaction of the electron density with the environment.;In this work, we examine the dependence of the charge transfer mechanism on several of these intrinsic and environmental factors through transient absorption spectroscopy, optical magnetic resonance, and quantum dynamics simulations. We focus largely on the relative contributions of two mechanisms to charge transfer within organic donor-bridge-acceptor systems: superexchange, an indirect tunneling process, and thermally-activated hopping.;Measurement of the magnetic field effect (MFE) on charge recombination is an optical magnetic resonance technique used to find the magnitude of the magnetic superexchange interaction, 2J, between the two unpaired spins formed via photoinduced charge separation. We present an expression for 2J in terms of VDA, the electronic donor-acceptor coupling, which gives the probability for a superexchange process to occur. 2J has a sensitive dependence on molecular conformation. We completed temperature-dependent MFE experiments, in conjunction with rate measurements, on a donor-bridge acceptor arrays based on the 4-(N -piperidinyl)naphthalene-1,8-dicarboximide (6ANI) chromophore to determine the role of electronic coupling and molecular motion in through-bond and through-space superexchange charge transfer processes.;We also measured the charge transfer rate and 2J as a function of temperature and bridge length to investigate a conformationally-gated switch in mechanism from superexchange to thermally-activated hopping for the systems phenothiazine---p-phenylenen---perylene-3,4:9,10-bis(dicarboximide), n = 1--5.;Finally, we used density matrices coupled with a semigroup formalism in a simulation of a charge transferring system under the influence of semi-local and non-local dephasing to explore the best representation of the system-bath interaction.