Bipolarons and polaron pairs in oligopyrrole dications

A series of oligopyrrole dications, n - Py 2 + , n = 1–12, 15, 18, 21, 24, 27 are studied with hybrid density functional theory at the B3PW91/6-31+G(d,p) level. Geometry optimization and electronic structure of oligopyrrole dications are revisited using spin-restricted and spin-unrestricted methods. It is found that the ground states of short oligopyrrole dications n - Py 2 + (n < 7) are spin-restricted singlet ground states. Thus, the charge carrier in these molecular dications has characteristics of a bipolaron. In contrast, medium size oligopyrrole dications ( 7 ⩽ n ⩽ 15 ) have a spin-unrestricted singlet ground state and display charge carriers that have a polaron pair character. With increasing oligomer length, the spin-unrestricted singlet and the triplet become degenerate such that for even longer oligopyrrole dications ( n - Py 2 + , n ⩾ 18 ) the ground state is a triplet and the charge carrier is predominantly a polaron pair.