Tuning Effects for Some Cyclic Aromatic Carbenes Bearing Remote Amino Groups

Yamamoto and co-workers synthesized two cyclic aromatic carbenes with remote amino groups. Here we theoretically studied related compounds to explore tuning effects on the singlet–triplet splitting by variations of functional groups. For the Yamamoto compound, the lowest singlet state lies 15.7 kcal/mol below the lowest triplet. The singlet–triplet separation is reduced by ∼7 kcal/mol when the dimethylamino groups are replaced by H. In one set of carbenes, when X = O, we substitute S, Se, Te, SO, SeO, and TeO for X; the resulting Δ<i>E</i>(S–T) predictions are 9.9, 7.3, 3.9, 4.3, 2.3, and −0.1 kcal/mol, respectively. A different set of X fragments yields triplet electronic ground states with Δ<i>E</i>(S–T) values of −8.6 (X = BH), −6.8 (X = AlH), −7.2 (X = GaH), −7.5 (X = InH), and −7.0 kcal/mol (X = TlH). We also predicted Δ<i>E</i>(S–T) with N­(CH₃)₂ replaced by PH₂, AsH₂, SbH₂, BiH₂, BH₂, CH₃, OH, and F. Of all the molecules considered, that with N­(CH₃)₂ replaced with BH₂ and X = BH most favors the triplet state, lying 13.7 kcal/mol below the singlet. Finally, we have relocated the N­(CH₃)₂ and NH₂ groups from the (3, 6) positions to the (4, 5), (2, 7), and (1, 8) terminal ring positions, with very interesting results