Excited-State Planarization in Donor-Bridge Dye Sensitizers: Phenylene versus Thiophene Bridges

Donor-pi-acceptor complexes for solar energy conversion are commonly composed of a chomophore donor and a semiconductor nanoparticle acceptor separated by a pi bridge. The electronic coupling between the donor and acceptor is known to be large when the pi systems of the donor and bridge are coplanar. However, the accessibility of highly coplanar geometries in the excited state is not well understood. In this work, we clarify the relationship between the bridge structure and excited-state donor-bridge coplanarization by comparing rhodamine sensitizers with either phenylene (O-Ph) or thiophene (O-Th) bridge units. Using a variety of optical spectroscopic and computational techniques, we model the S-1 excited-state potential surfaces of O-Ph and O-Th along the dihedral coordinate of donor-bridge coplanarization, tau. We find that O-Th accesses a nearly coplanar (tau = 8 degrees) global minimum geometry in S-1 where significant intramolecular charge transfer (ICT) character is developed. The S-1 coplanar geometry is populated in <10 ps and is stable for ca. 1 ns. Importantly, O-Ph is sterically hindered from rotation along tau and therefore remains at its initial S-1 equilibrium geometry far from coplanarity (tau = 56 degrees). Our results demonstrate that donor-bridge dye sensitizers utilizing thiophene bridges should facilitate strong donor-acceptor coupling by an ultrafast and stabilizing coplanarization mechanism in S-1. The coplanarization will result in strong donor-acceptor coupling, potentially increasing the electron transfer efficiency. These findings provide further explanation for the success of thiophene as a bridge unit and can be used to guide the informed design of new molecular sensitizers.