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

Donor−π–acceptor
complexes for solar energy
conversion are commonly composed of a chomophore donor and a semiconductor
nanoparticle acceptor separated by a π bridge. The electronic
coupling between the donor and acceptor is known to be large when
the π 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 S1 excited-state potential surfaces of O-Ph and O-Th along the dihedral coordinate of
donor–bridge coplanarization, τ. We find that O-Th accesses a nearly coplanar (τ = 8°) global minimum geometry
in S1 where significant intramolecular charge transfer
(ICT) character is developed. The S1 coplanar geometry
is populated in <10 ps and is stable for ca. 1 ns. Importantly, O-Ph is sterically hindered from rotation along τ and
therefore remains at its initial S1 equilibrium geometry
far from coplanarity (τ = 56°). 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 S1. 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.