Studies on the Bridge Dependence of Bis(triarylamine) Diradical Dications: Long-Range π-Conjugation and π-π Coupling Systems

Three bis(triarylamine) dications bridged with 2,2'-bithienyl group (1(2+), biphenyl group (2(2+)) and [2,2]-paracydophane (3(2+)) have been successfully isolated. The electronic structures of 1(2+) - 3(2+) show bridge dependence. Magnetic studies and DFT calculations show that dications 1(2+) - 3(2+) possess an open-shell singlet ground state with a thermally excited triplet state. Dication 3(2+) has rather small singlet triplet energy gap and could basically be regarded as a nearly pure diradical. Diradical dications 1(2+) - 3(2+) represent rare examples of diradicaloids with intramolecular long-range pi-conjugation or pi-pi coupling interaction.