Helical Molecular Orbitals to Induce Spin-Orbit Coupling in Oligoyne-Bridged Bifluorenes

Singlet-Triplet energy exchange is an area of active research due to its role in optoelectronic devices and photodynamic therapy. Large spin-orbit coupling (SOC) is difficult to achieve in simple hydrocarbon structures limiting the intersystem crossing (ISC) rates. A new approach to enhance the spin-orbit coupling via helical molecular orbitals is investigated in oligoyne-bridged bifluorenes. Transient absorption studies showed a singlet-to-triplet ISC rate of up to 6 ns^-1 resulting in 0.84 triplet yield. Density functional calculations revealed a direct relation between high ISC and large SOC values mediated by helical molecular orbitals. Calculations and spectroscopic data also suggested that El-Sayed forbidden ISC occurs as a direct transition between ¹ππ* and ³ππ*, which becomes allowed due to a symmetry-breaking interaction leading to mixing between orthogonal π-systems in the oligoyne fragment.