Why does thionating a carbonyl molecule make it a better electron acceptor?

The past decade has witnessed a surge of biomedical and materials applications of thiocarbonyl molecules (R2C= S), such as in photodynamic therapy, organic field-effect transistors, and rechargeable batteries. The success of these applications originates from thiocarbonyl's small optical gap in the visible region and the enhanced electron affinity compared to the carbonyl analogues (R2C= O -> C= S substitution. In comparison to the valence (2p)(C)/(2p)(O) interactions in C= O, the higher 3p orbital of sulfur and its weaker overlap with the 2p level of carbon result in a weaker antibonding interaction in pi(C-S)* NBO, a prominentcontributor to the LUMO. Such an analysis also provides a semi-quantitative understanding of theelectronic effect of substituents on or inp-conjugation with a (thio)carbonyl functionality. The intuitiveconcepts uncovered here offer a simple rule to predict the electronic properties ofp-conjugatedmolecules that incorporate heavy heteroelements and would facilitate materials development.