Effects of Proaromaticity on Excited-State Lifetimes and Charge Separation in Near-Infrared Sensitizer Dyes in Solution and on TiO2

The influence of proaromaticity on the excited state dynamics of near-infrared (NIR)-absorbing sensitizer dyes is explored using a combination of ultrafast transient absorption spectroscopy (TAS) and computational chemistry. The addition of a proaromatic pi-bridge was found to stabilize the excited state and lead to lower excitation energies in solution and longer excited state lifetimes, contrary to what is expected by the energy gap law. When studied under standard device conditions on TiO2, it was found that the dye structure plays a significant role in determining excited-state dynamics. Computational chemistry results confirm the proaromatic nature of the dyes through both bond length analyses and nucleus-independent chemical shift (NICS) calculations. Through incorporation of excited-state aromaticity, a 10x increase in excited-state lifetime was observed for dyes with a near 0.5 V lower energy excited state.