Photoinduced Energy, Charge, and Spin Transfer in Organic Molecular Materials

Zachary E Dance

doi:https://doi.org/10.21985/N21X51

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Photoinduced Energy, Charge, and Spin Transfer in Organic Molecular Materials

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This thesis presents results and analysis on a series of donor-bridge-acceptor (D-B-A) charge transfer systems in which we use time-resolved spectroscopy and magnetic resonance techniques to study the relationship of molecular structure to energy, charge, and spin transfer dynamics. We find that the sequence of events following the initial charge separation within these systems indicates the considerable level of structurally and energetically dependent mechanistic complexity responsible for these processes in organic materials. We first examine a series of D-B-A molecules where a phenothiazine donor and a perylene-3,4:9,10-bis(dicarboximide) acceptor are linked by an oligo-p-phenylenen bridge, n = 1-5. Photoexcitation results in rapid electron transfer to produce 1(D+-B-A-). Above 150 K, when n = 2-5, the radical pair intersystem crossing mechanism produces spin-correlated radical ion pairs. Charge recombination in the radical pairs generates D-B-3*A, which exhibits a spin-polarized signal similar to that observed in photosynthetic reaction-center proteins and some biomimetic systems. At low temperatures and/or at shorter donor-acceptor distances, D-B-3*A is also formed from a competitive spin-orbit intersystem crossing (SO-ISC) mechanism. The second series also employs an oligo-p-phenylenen bridge, n = 1-4, to link a 3,5-dimethyl-4-(9-anthracenyl)julolidine donor to a naphthalene-1,8:4,5-bis(dicarboximide) acceptor. Similarly, charge separation produces a singlet radical pair state which undergoes intersystem crossing to the triplet radical pair state. Our results show directly that charge recombination of the radical pair initially produces a spin-polarized triplet state, D-B-3*A, that can only be produced by hole transfer involving the HOMOs of the D-B-A system. After the initial formation of D-B-3*A, triplet-triplet energy transfer occurs to produce 3*D-B-A. We also find that a SO-ISC mechanism becomes prevalent at shorter bridge lengths. Lastly, we examine the unusual SO-ISC mechanism that is operative in both of the D-B-A systems above. We use simple D-A systems linked by a single bond to compare how changes in a single degree of freedom affect photophysical behavior. Our studies show that the unusual SO-ISC mechanism depends on the degree of charge separation, the relative orientation of the orbitals involved in the charge transfer, and the magnitude of the electronic coupling between the donor and acceptor.

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