Organic Chemistry Seminar

Electron donor-acceptor interactions form the basis for controlling electron transfer (ET) and transport at the molecular level.  They therefore form crucial, foundational elements in biological and artificial ET and photosynthesis, solar energy conversion and molecular electronics/spintronics (electron spin-based electronics).  Chief amongst the variables controlling ET/transport rates is the bridge-mediated donor-acceptor electronic coupling matrix element, H_DA – a type of resonance integral.

Photoexcitation of the donor (D) in a donor-bridge-acceptor molecule (D-B-A) yields a D*-B-A excited state which may undergo ET to form a charge-separated, spin singlet biradical D⁺-B-A^- state.  A convenient, straightforward way to demonstrate important, fundamental structure-property relationships for nonadiabatic ET rates is to utilize the correlation of H_DA with the readily-measured magnetic exchange coupling (J_DA) between spin ½ centers in a charge-separated biradical.

Covalently-linked D-B-A biradicals (where D = spin-½ 3-tert-butyl-orthosemiquinone, SQ and A = spin-½ nitronylnitroxide, NN, see Figure) serve as ground state analogues of PET-derived charge-separated states as well as models for understanding electronic structure contributions to single-molecule conductance.  Given our ability to design strong ferromagnetic bridge-mediated exchange coupling between SQ and NN, we can use this exchange coupling to control excited state wave function mixing that in turn modulates important excited state photophysical processes.

This talk will show how such biradicals can be used to demonstrate important structure property relationships in the strong-coupling regime and how this strong electronic coupling might be used to control excited state photophysical processes such as intersystem crossings.