| Structure-property relationships are developed through Donor-Bridge-Acceptor (D-BA) biradical complexes as models of charge transfer excited states. By using a semiquinone (SQ) donor radical and a nitronylnitroxide (NN) acceptor radical, the magnetic exchange (JDA) is related to electronic coupling (HDA) via a valence bond configuration interaction (VBCI) model. Through auxochromic substituents, the torsional dependence of electronic coupling is investigated for para-phenylene bridged D-B-As. Then, by varying the number of bridge units from 0 to 2, the exponential decay in electronic coupling versus radical distance as well as the electronic coupling of one bridge unit to another is determined for oligo(para-phenylene), oligo(2,5-thiophene), and oligo(2,5-selenophene). This is followed by an analysis of cross-conjugated D-B-As where the electronic coupling of a meta-phenylene cross-conjugated bridge is found to involve a spin polarized pi-system to stabilize a singlet ground state. Applying concepts of the meta -phenyl system to cross-conjugated thiophene systems shows similar results with more ferromagnetic coupling. Finally, adding substituents ortho- to the SQ and para- to the NN in a meta-phenylene bridged cross-conjugated biradical complex provides single molecule models of quantum interference effect transistors. |
