Heterogeneous Electron-Transfer Dynamics through Dipole-Bridge Groups

Abstract

Heterogeneous electron transfer (HET) between photoexcited molecules and colloidal TiO2 has been investigated for a set of Zn-porphyrin chromophores attached to the semiconductor via linkers that allow to change level alignment by 200 meV by reorientation of the dipole moment. These unique dye molecules have been studied by femtosecond transient absorption spectroscopy in solution and adsorbed on the TiO2 colloidal film in vacuum. In solution energy transfer from the excited chromophore to the dipole group has been identified as a slow relaxation pathway competing with S2-S1 internal conversion. On the film heterogeneous electron transfer occurred in 80 fs, much faster compared to all intramolecular pathways. Despite a difference of 200 meV in level alignment of the excited state with respect to the semiconductor conduction band, identical electron transfer times were measured for different linkers. The measurements are compared to a quantum-mechanical model that accounts for electronic-vibronic coupling and finite band width for the acceptor states. We conclude that HET occurs into a distribution of transition states that differs from regular surface states or bridge mediated states.

Dipole Effects on Electron Transfer are Enormous

“…Experimentally measured dependence of ET and electric currents on dipole orientation, however, often falls short of showing dramatic trends . The ET rates and the currents along vs. against the dipoles rarely differ by more than an order of magnitude, and occasionally, such differences are undetectable . This discrepancy suggests that the dipole effects on ET still remain not truly well understood, despite their importance.…”

Dipole-induced effects on charge transfer and charge transport. Why do molecular electrets matter?

“…Considerations about the morphology and the surface chemistry of the semiconductor, and the relatively small dipoles of push-pull conjugates, present challenges for obtaining the desired CT kinetic behavior. [148][149] D r a f t…”

“…The dipole orientation, however, has no noticeable effect on the electron-injection rates. The relatively small magnitude of the dipoles in the bridging conjugates is most likely the reason for the lack of observable effect on the CT kinetics 149. Placing organic polar molecules, in an ordered manner, between two electrodes provides a means for controlling the I-V properties of the formed junctions.…”

Bioinspired approach toward molecular electrets: synthetic proteome for materials

“…At metal-semiconductor interfaces, monolayers of organic molecules containing polar functional groups alter the rectification characteristics of such Schottky junctions [108][109][110]. Push-pull conjugates within CT organic structures on semiconductor surfaces introduce molecular dipoles that not only affect the electronic properties of the material but also modify the kinetics of interfacial ET [111]. Similarly, at a molecular level, dipole-generated fields shift the energy levels of the frontier orbitals of the electron donors and acceptors [12].…”