Proton Dynamics in the Excited States
Motivation:
Most redox reactions, especially those that are of interest for solar photoelectrochemical energy conversion, require transfer of several electrons and protons. Understanding the kinetics of such reactions, in particular the coupling of electron transfer to proton transfer, is at the frontiers of modern chemical dynamics. To control such processes, it is necessary to understand and engineer the pKa of molecules in the excited state, in various oxidation states, and in the presence of electric fields. Our work is motivated by this goal.
See-saw Effect in Excited State Proton Transfer
We have recently reported the kinetics of excited state proton transfer in a molecule with two transferrable protons. First, we found evidence that only one of the two protons transfer in the excited state. Second, we compared the results to the kinetics of a related singly protonated molecule. Unexpectedly, we observed that the rate of proton transfer for the singly protonated compound was faster. Theoretical collaboration with Tom Miller's group at Caltech revealed that the two hydrogen bonds in the doubly protonated molecule compete with each other reminiscent of a seesaw. This leads to a longer donor-acceptor distance for the doubly protonated compound.
Photobasicity: Thermodynamics, Kinetics, and Tuneability
We have established the thermodynamic origin, kinetics, and tunability of photobasicity, and have connected it to the known photoacidity phenomena. Inspired by our work on an intramolecular proton transfer model, we hypothesized that heterocyclic nitrogen containing aromatics should turn more basic in the excited state. Therefore, they can serve as molecules that turn light into “proton removal power”, at least for as long as the excited state lives. The origin, tunability, and the kinetics of this phenomenon were not well-understood prior to our work. We established that the excess charge density built on the heterocyclic nitrogen in the excited state is the main driver for this process. We showed that this excess charge density can be tuned by adjusting the electron-withdrawing strength of the substituents on the aromatic ring. We have used ultrafast transient absorption spectroscopy to measure the kinetics of proton capture from water, and have revealed the contribution of triplet states in photobasicity. (Link to our published work)
Protonic Photoconductivity
We have reported and explained protonic photoconductivity, a phenomenon at the boundary of electronic photoconduction and ionic conductivity. This process is the electrical manifestation of proton release by photoacids and to our knowledge was never reported before. We have not only demonstrated this experimentally, but have also explained that analogous to their semiconductor counterparts, this process is subject to recombination of carriers (conjugate base and proton). Here is a link to our publication.
