Tunable Lifetimes of Intramolecular Charge Separated States in Molecular Donor-Acceptor Dyads

We report ultrafast transient UV-vis absorption and electrochemical spectroscopies on the photoinduced charge separation dynamics in a recently synthesized family of metal-free donor-acceptor systems, where two redox-active molecules are fused into a compact and planar structure upon annulation of a tetrathiafulvalene and a benzothiadiazole as electron donor and acceptor, respectively. We found extraordinary tunability of the lifetime of the photoinduced charge separation by more than 2 orders of magnitude (from 6 to 900 ps) upon small changes of the peripheral residues on the acceptor and the polarity of the environment. Contrary to expectations, the lifetime of the charge separation state decreases in more polar environments and with more electronegative acceptors. This study proves that such fused donoracceptor systems give rise to a new class of electronically versatile materials whose physicochemical properties can be tuned via a targeted substitution or a suitable choice of local electrostatics.