There has been rapid acceleration in synthesis and design of photocatalysts for use in renewable energy sources in recent years. Our focus has been on the design elements for creating and testing new photosensitizers which utilize earth-abundant coinage metals. These compounds have a linear structure of carbene-metal-amide which allows for efficient and tunable ligand-to-ligand charge transfer (LLCT) absorption energies and absorptivities. Linearity reduces the non-radiative decay rate which would be accelerated if Jahn-Teller distortions were allowed to take place, allowing for moderately long lifetimes (~ 1 µs) and near unity photoluminescent quantum yields (0.8 – 1.0). Different substitutions of the ligands allow for easy tunability from -1.4 V to -2.6 V for the oxidation of the amide. We employ radical anion and cation spectra measured recently by pulse radiolysis at Brookhaven National Lab, with our own transient absorption spectroscopy (TAS). Ultrafast triplet formation is observed on a tens of picosecond basis when (MAC)-Cu-DitBuCz, 1, is excited in deaerated toluene. Current work aims to determine the radical cation spectrum of the carbazole at short time scales (1 - 10 ns) by combining excited 1 with a sacrificial oxidative quencher, N-methylpthalimide. Similar work is carried out to determine the anion spectrum of MAC using a reductive quencher. TAS on 2, 3, and 4 is also performed. Combining these efforts, it should be possible to give a complete description of the efficiency and kinetics of charge transfer from the excited state species and suitability for nanoscale architectures for photocatalysis.

Molecular synthesis and design of novel photosensitizers with coinage metals using transient absorption spectroscopy

ACS Spring 2022

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