V-Shaped Tröger Oligothiophenes Boost Triplet Formation by CT mediation and Symmetry Breaking

Abstract

A new family of molecules obtained by coupling the Tröger’s base unit with dicyanovinylene-terminated oligothiophenes of different lengths has been synthesized and characterized by steady-state stationary and transient time-resolved spectroscopies. Quantum chemical calculations allow us to interpret and recognize the properties of the stationary excited states, as well as the time-dependent mechanisms of singlet-to-triplet coupling. The presence of the diazocine unit in the Tröger’s base derivatives is key to efficiently produce singlet-to-triplet intersystem crossing mediated by the role of the nitrogen atoms and of the almost orthogonal disposition of the two thiophene arms. Spin-orbit coupling mediated interstate ISC is activated by a symmetry breaking process in the first singlet excited state with a partial charge transfer character. This mechanism is characteristic of these molecular triads since the independent dicyano-vinylene oligothiophene branches do not display appreciable ISC. These results show how the Tröger’s base coupling of organic chromophores can be used to improve the ISC efficiency and tune their photophysics.