Electrostatically Tuning the Photodissociation of the Irgacure 2959 Photoinitiator in the Gas Phase by Cation Binding

The low-lying electronic states of Irgacure 2959, a Norrish-type I photoinitiator, complexed with a single metal cation are investigated in the gas phase by photodissociation action spectroscopy. Analysis of the band shifts using quantum chemical calculations (TD-DFT and SCS-CC2) reveals the underlying influence of the charge on the key electronic energy levels. Since the cations (H+, Li+, Na+, K+, Zn2+, Ca2+, and Mg2+) bind at varying distances, the magnitude of the electric field at the center of the chromophore due to the cation is altered, and this shifts the electronic states by different amounts. Photodissociation action spectra of cation-Irg complexes show that absorption transitions to the first (1)pi pi* state are red-shifted with a magnitude proportional to the electric field strength (with red shifts >1 eV), and in most cases, the cation is essentially acting as a point charge. Calculations show that a neighboring (3)n pi* state, a key state for the alpha-cleavage pathway, is destabilized (blue-shifted) by the orientated electric field. As such, if the (1)pi pi*-(3)n pi* energy gap is reduced, increased intersystem crossing rates are expected, resulting in higher yields of the desired radical photoproducts, and this is controlled by the orientated electric field arising from the cation.

Automated summary

In this study, the low-lying electronic states of the Norrish-type I photoinitiator Irgacure 2959 complexed with a single metal cation were investigated using photodissociation action spectroscopy in the gas phase. The charge of the cations was found to influence the key electronic energy levels, leading to shifts in the electronic states. The oriented electric field generated by the cation affects the energy gap between different electronic states, ultimately impacting the yield of radical photoproducts.