Biradicaloid with a Twist: Lowering the Singlet–Triplet Gap

Fusing two phenalenyl units such that a Kekulé 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.