Why does thionating a carbonyl molecule make it a better electron accepter? Yi-Lin Wu Cardiff University, UK The past decade has witnessed a surge of biomedical and materials applications of thiocarbonyl molecules (R 2 C=S), such as in photodynamic theory, organic field-effect transistors, and rechargeable batteries. The success of these applications originates from the small optical gap in the visible region and the enhanced electron affinity compared to the carbonyl analogues (R 2 C=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 (2 p ) C /(2 p ) O interactions in C=O, the higher 3 p orbital of sulfur and its weaker overlap with carbon’s 2 p level result in a weaker antibonding interaction in π * C=S NBO, a prominent contributor to LUMO. Such an analysis also provides a semi-quantitative understanding of the effect of substituents on or in π-conjugation with the (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 andwould facilitatematerials development.
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