Acceleration of Nonradiative Charge Recombination Reactions at Larger Distances in Kinked Donor-Bridge-Acceptor Molecules

Photoinduced electron transfer in donor-bridge-acceptor (D-B- A) molecular systems can occur via tunneling over long distances (rDA) of well over 10 angstrom. We commonly observe decreasing rates of electron transfer with increasing distances, a result of a decrease in the electronic coupling of the donor and acceptor moiety. In the study of D-B-A molecules with Ru(bpy)32+ as a bridge/core, Kuss-Petermann and Wenger observed the opposite trend (J. Am. Chem. Soc.2016, 138 , 1349); a maximum rate constant of electron transfer was observed at an intermediate electron transfer distance. Within the high-temperature limit of the classical Marcus equation, their observation was qualitatively explained by a sharp distance dependence of outer sphere (or solvent) reorganization energy, as predicted by Sutin and co-workers (J. Am. Chem. Soc.1984, 106 , 6858), and almost distance-independent electronic couplings. Here, we report another example of such an underexplored behavior with three kinked D-B-A systems of rDA similar to 10-19 angstrom, showing increasing rates of nonradiative charge recombination with increasing rDA. The three D-B-A systems are based on boron dipyrromethene and triphenylamine as electron acceptor and donor groups, respectively, with aryl bridges where the donor and acceptor moieties are connected at meso-positions. These D-B-A molecules exhibit radiative electron transfer reactions (or charge -transfer emission), which enables us to experimentally determine the solvent reorganization energy and the electronic couplings. The analysis of charge-transfer emission that explicitly considers electron-vibration coupling, in conjunction with the temperature -dependent analysis and computational method, revealed that the solvent reorganization energy indeed increases with distance, and at the same time, the electronic coupling decreases with distance expectedly. Therefore, under the right conditions for solvent reorganization energy and electronic coupling values, our results show that we can observe the acceleration of electron transfer reactions with increasing distance, even when we have the expected distance dependence of electronic coupling. This work indicates that the acceleration of electron transfer with increasing distance may be achieved with a fine-tuning of molecular design.