Donor-Acceptor molecule with TICT character: a new design strategy for organic photothermal material in solar energy

Organic photothermal materials, particular the donor-acceptor (D-A) type ones, have attracted increased attention because they could efficiently transform solar energy to heat in an environmentally friendly manner. So far, there are many strategies to improve the fluorescence of D-A type molecules, while how to quench the fluorescence via non-radiative decay process and thus maximize the solar-thermal conversion efficiency is rarely investigated. Herein, a new design strategy is proposed to overcome this bottleneck, that is, formatting a unique twisted intramolecular charge-transfer (TICT) excitation state for the D-A type molecule. To demonstrate this new design strategy, three D-A molecules TPA-BTO, TPA-BTO-Py+ and GTPA-BTO-Py+ are synthesized with the consideration of employing stronger donor/acceptor moieties. The photophysical measurements and theoretical calculations reveal that these molecules display strong tendency to form TICT state, and the stronger donor/ acceptor would significantly promote the TICT-based non-radiative decay process. Consequently, the target molecule GTPA-BTO-Py+ is totally non-fluorescent and could efficiently transform the absorbed solar energy to heat. Further combined with the wide absorption up to 1300 nm, GTPA-BTO-Py+ solid displays a high solarthermal conversion efficiency of 20.5%. Finally, the utility of GTPA-BTO-Py+ as highly efficient photothermal material in solar energy is highlighted by water evaporation and thermoelectric generation.