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

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 1 ππ* and 3 ππ*, which becomes allowed due to a symmetry-breaking interaction leading to mixing between orthogonal π-systems in the oligoyne fragment.

Sweet Spot of Intermolecular Coupling in Crystalline Rubrene: Intermolecular Separation to Minimize Singlet Fission and Retain Triplet–Triplet Annihilation

“…A sequential model was used to separate spectral features of two states and assign corresponding decay lifetimes. 35 , 36 …”

“…Global analysis of TA was performed by data analysis software “CarpetView” (Light Conversion). A sequential model was used to separate spectral features of two states and assign corresponding decay lifetimes. , …”

Helical versus linear Jahn–Teller distortions in allene and spiropentadiene radical cations

“…24,25 The experimental implications of this electrohelicity effect remain limited because both helicities are present simultaneously. 24–38 Consequently, this helical π-conjugation cancels out when the MOs are equally occupied in neutral closed-shell molecules.…”

Quantification of the Helicality of Helical Molecular Orbitals

“…Given the interest in helical MOs as a means to understand related properties and the range of molecular systems in which these MOs appear, − it is problematic that the assessment of helical MOs relies on visual inspection alone. It is thus altogether binary; if an MO is shaped like a coil, we may deem it to be helical.…”