Biradicaloid with a Twist: Lowering the Singlet–Triplet Gap

Fusing two phenalenyl units such that a Kekule structure is obtained leads to biradicaloid' systems, which typically display low singlet-triplet (S-T) energy gaps. The value of the S-T gap in biradicaloids can be lowered by extending the conjugated system for an additional fused ring(s). Recently, we have described an alternative approach to decrease the S-T gap with the aid of through-space interactions that arise in helically twisted biradicaloids. Using [5]helicene-based biradicaloid cethrene' as an illustrative example, we have shown that the value of its S-T gap is substantially decreased when compared to that of its planar isomer heptazethrene, on account of the helical twist, inducing crucial intramolecular interactions.