Why and how could an aliphatic bridge allow for a long-range photoinduced charge separation in Troger's bases derivatives

Troger's bases (TBs) have shown great potential to be used in different fields of science, such as biology, organic synthesis, photoelectronic applications, among others. As we have recently shown, a series of asymmetrically substituted Troger's base derivatives showed unexpected pull-push behavior. The aliphatic diazocine heterocycle which connect an electron donor and an electron acceptor moiety efficiently couples both electronic subsystems as if it were a typical p-conjugated linker. A thorough computational study was intended to shed light on the origin of the observed photophysical properties. A modified version of the CAM-B3LYP functional yielded an accurate prediction of the absorption and emission spectra of these species. In contrast, range-corrected and classical hybrid approaches showed too high and remarkably too low excited states energies, respectively. Unlike the typical pi-linked donor/acceptor systems, a threshold was found in the redox gap of the centers in order to obtain a full charge separation. The role of the aliphatic bridge was found to be related to the unusual topology of the frontier orbitals involved, to the tension and particular molecular shape of the Troger bicycle and to a contribution due to homoconjugation as well. Calculations on the actual Troger derivatives and specific models were able to quantify the magnitude of the different contributions that make possible the charge separation.

Automated summary

Troger's bases have shown great potential in various fields and a series of asymmetrically substituted derivatives displayed unexpected pull-push behavior. Computational study shed light on the photophysical properties and accurate prediction of absorption and emission spectra was obtained using a modified version of the CAM-B3LYP functional.