Photocatalytic Hydrogen Production
Using hydrogen as a form of green energy to replace traditional sources like fossil fuels is critical towards sustainably meeting the global energy demand. Our lab designs heteroleptic iridium (III) photocatalysts capable of producing hydrogen. These photosensitizers (PSs), when excited with light, promote an electron to a higher energy level where it can go through intersystem crossing from a singlet to a triplet state. This process allows for a longer lived excited state and creates an electron hole in which a sacrificial electron donor (ie. TEOA) can donate its electron to the PS. The PS can then give an electron to a water reduction catalyst (WRC), such as Pd nanoparticles, before relaxing down to the ground state. The water reduction catalyst can then reduce water into hydrogen.
With a system that contains a lot of moving parts, even the slightest change in the reaction conditions can have drastic effects on the amount of hydrogen produced. Altering the ligand identities on the PS can affect many of its properties such as the excited state lifetime, molar absorptivity, HOMO-LUMO gap, etc. Using a high-throughput, 108 well plate photoreactor, hundreds of different ligand identities can be easily screened to examine their activities. Other parameters such as the concentration of WRC, electron donor, or PS, or solvent identity can be easily explored.
Our team designs and builds maker-style instrumentation to automate and parallelize the exploration of solar fuels-related chemistries. The components of these novel photoreactors are fabricated using the 3D printing and laser cutting capabilities in our lab, but we also utilize CMU’s superb TechSpark facility to translate our computer aided design (CAD) plans to machine illumination platforms, camera mounts, vial holders etc. The instrumentation is mostly controlled by inexpensive single-board computers like Raspberry Pis or Arduinos that utilize custom-made software to orchestrate the measurement protocols. With the aid of these new setups we have been able to measure tens of thousands of photocatalytic reactions ranging from organic photoredox transformations to the photocatalytic generation of hydrogen from renewable biomass products such as alcohols or sugars. Data science and machine learning techniques in combination with DFT electronic structure calculations are used to interpret the unprecedented wealth of data measured by these setups. Future work will use the highly parallelized, automated infrastructure to find efficient and economically viable pathways to photolyse water into hydrogen and oxygen.
Circularly Polarized Luminescence
Circularly polarized luminescence can be observed from chiral molecules, but emission dissymmetry factors are typically small and limit the approach to produce materials with real world applications. In order to improve the degree of circularly polarized photoluminescence it is necessary to incorporate chromophores into large scale ensembles, preferably with the ability to form stable, long-range organized structures. One part of our work in this area uses luminophoric, furane-based polymers prepared by another research lab at CMU, the Noonan group, to investigate the degree of helical ordering with circularly polarized luminescence spectroscopy. The emission dissymmetry observed in these materials increases with larger degrees of polymerization. The chiroptical response is also highly dependent on the solvent environment and is unaffected by a polymer’s concentration, confirming that only intramolecular forces are involved. Other work on this project involves molecules synthesized by the Mayor group at the University of Basel, Switzerland, and chiral, perovskite-based luminophores designed by the Waldeck group at University of Pittsburgh. Some of our earlier work involved the use of enantiomerically pure hemi-cage ligands. The overall goal of the work is to create optoelectronic devices from these materials that could be used in sensors and 3D display applications.