The Effect of Torsional Motion on Multiexciton Formation through Intramolecular Singlet Fission in Ferrocene-Bridged Pentacene Dimers

A series of ferrocene(Fc)-bridged pentacene(Pc)-dimers [Fc-Ph(2,n)-(Pc)2: n=number of phenylene spacers] were synthesized to examine the tortional motion effect of Fc-terminated phenylene linkers on strongly coupled quintet multiexciton (5TT) formation through intramolecular singlet fission (ISF). Fc-Ph(2,4)-(Pc)2 has a relatively small electronic coupling and large conformational flexibility according to spectroscopic and theoretical analyses. Fc-Ph(2,4)-(Pc)2 exhibits a high-yield 5TT together with quantitative singlet TT (1TT) generation through ISF. This demonstrates a much more efficient ISF than those of other less flexible Pc dimers. The activation entropy in 1TT spin conversion of Fc-Ph(2,4)-(Pc)2 is larger than those of the other systems due to the larger conformational flexibility associated with the torsional motion of the linkers. The torsional motion of linkers in 1TT is attributable to weakened metal-ligand bonding in the Fc due to hybridization of the hole level of Pc to Fc in 1TT unpaired orbitals. Ferrocene(Fc)-terminated phenylene-bridged pentacene (Pc) dimers were synthesized to examine the effect of torsional motion on quintet multiexciton (5TT) formation through intramolecular singlet fission. The optimized dimer exhibits an efficient 5TT yield together with quantitative singlet triplet pair (1TT) generation. The torsional motion in 1TT is ascribed to weakened metal-ligand bonding in the Fc unit.image