Interplay of Electronic and Geometric Structure Tunes Organic Biradical Character in Bimetallic Tetrathiafulvalene Tetrathiolate Complexes

The
synthesis and design of organic biradicals with tunable singlet–triplet
gaps have become the subject of significant research interest, owing
to their possible photochemical applications and use in the development
of molecular switches and conductors. Recently, tetrathiafulvalene
tetrathiolate (TTFtt) has been demonstrated to exhibit such organic
biradical character in doubly ionized bimetallic complexes. In this
article we use high-level ab initio calculations
to interrogate the electronic structure of a series of TTFtt-bridged
metal complexes, resolving the factors governing their biradical character
and singlet–triplet gaps. We show that the degree of biradical
character correlates with a readily measured experimental predictor,
the central TTFtt C–C bond length, and that it may be described
by a one-parameter model, providing valuable insight for the future
rational design of TTFtt based biradical compounds and materials.