Helical molecular orbitals forming in oligoynes (chains of sp hybridized carbons) are intriguing for control of electron’s spin in simple organic molecules. Spin-Orbit coupling induced by mixing of orthogonal p electron orbitals leads to enhanced singlet-to-triplet intersystem crossing. However, the role of conformers for spin-orbit coupling is still unclear in oligoyne-bridged systems. Herein, we report a theoretical study of spin-orbit coupling and intersystem crossing rates as a function of axial torsion between bifluorene end-fragments bridged by various length oligoynes. Density functional theory computations revealed that conformers can exist at room temperature due to low torsional barrier. Highest calculated intersystem-crossing rates were found for torsional angles around 30° and agreed well with experimental values. Elongation of oligoyne bridge up to three triple bond fragments leads to steep increase of spin-orbit coupling, and thus, intersystem-crossing rates. Interestingly, at intermediate torsional angles oligoyne bridge allows to maintain high oscillator strength of the lowest singlet transition together with substantial spin-orbit coupling, which is a desirable property for photosensitizer and emitter molecules.
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