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

Abstract

<p>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.</p>