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 (RC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 (RCO). Although these observations seem to be contrary to the implication based on a simple electronegativity consideration (2.58 for sulfur and 3.44 for oxygen), a natural bond orbital (NBO) analysis gives a straightforward explanation for the LUMO-lowering effect of CO → CS substitution. In comparison to the valence (2p)/(2p) 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 NBO, a prominent contributor to the LUMO. Such an analysis also provides a semi-quantitative understanding of the electronic effect of substituents on or in π-conjugation with a (thio)carbonyl functionality. The intuitive concepts uncovered here offer a simple rule to predict the electronic properties of π-conjugated molecules that incorporate heavy heteroelements and would facilitate materials development.