Helical Molecular Orbitals to Induce Spin–Orbit Coupling in Oligoyne-Bridged Bifluorenes
Paulius Baronas, Regimantas Komskis, Eglė Tankelevičiūtė, Povilas Adomėnas, Ona Adomėnienė, Saulius Juršėnas
Abstract
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.