Thermal and Mechanochemical Tuning of the Porphyrin Singlet-Triplet Gap for Selective Energy Transfer Processes: A Molecular Dynamics Approach.

Molecular dynamics simulations provide fundamental knowledge on the reaction mechanism of a given simulated molecular process. Nevertheless, other methodologies based on the "static" exploration of potential energy surfaces are usually employed to firmly provide the reaction coordinate directly related to the reaction mechanism, as is the case in intrinsic reaction coordinates for thermally activated reactions. Photoinduced processes in molecular systems can also be studied with these two strategies, as is the case in the triplet energy transfer process. Triplet energy transfer is a fundamental photophysical process in photochemistry and photobiology, being for instance involved in photodynamic therapy, when generating the highly reactive singlet oxygen species. Here, we study the triplet energy transfer process between porphyrin, a prototypical energy transfer donor, and different biologically relevant acceptors, including molecular oxygen, carotenoids, and rhodopsin. The results obtained by means of nanosecond time-scale molecular dynamics simulations are compared to the "static" determination of the reaction coordinate for such a thermal process, leading to the distortions determining an effective energy transfer. This knowledge was finally applied to propose porphyrin derivatives for producing the required structural modifications in order to tune their singlet-triplet energy gap, thus introducing a mechanochemical description of the mechanism.