Torsional Bias as a Strategy To Tune Single-triplet Gaps in Organic Diradicals

• Published in: Journal of physical chemistry. C 2018-06, Vol.122 (23), p.12148
• Main Authors: DeFrancisco, Justin R., Lopez-Espejo, Guzman, Zafra, Jose Luis, Yadav, Sangeeta, Messersmith, Reid E., Gomez-Garcia, Carlos J., Ottosson, Henrik, Casado, Juan, Tovar, John D.
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.8b01905

Quinoidal compounds with proaromatic structures possess differing degrees of diradical character, where the open-shell diradical resonance form has restored aromaticity throughout the compound. Methods to tune the diradical character of these compounds have traditionally focused on altering the length and the molecular composition of the pi-conjugated backbones. However, other molecular design strategies to tune the singlet-triplet gap of pi-conjugated quinoidal molecules have not been extensively explored. We previously reported a strikingly small energy gap between the quinoidal and diradical states of a quinoidal small molecule containing methano[10]annulene (TMTQ) that was dictated in large part by the unusual aromaticity of the central annulene ring. Here, we report on two alkylated derivatives of TMTQthat present substantially different torsional biases to the planarity of the TMTQ pi-system. Using a combination of electronic and vibrational spectroscopies, magnetic measurements, and quantum chemical calculations, we demonstrate here how a steric effect rather than pi-electron compositional molecular engineering can dramatically narrow the singlet-triplet gap of a quinoidal compound to as small as -0.52 kcal/mol, determined experimentally. This study offers important insight for the continued development of open-shell diradical molecules that need not rely exclusively on the design of synthesis of new and complex conjugated systems.