Solvent structure and dynamics of ion pairs between a ruthenium photocatalyst and a countercation studied with infrared spectroscopy.

The strength of the ionic interaction between a counterion and photosensitizer can alter photocatalyst excited state stabilities and redox potentials, ultimately determining catalytic efficiency. Counterion identities are known to directly influence ionic interaction strength; however, recent literature indicates that the structure of the solvation shell around photosensitizer-counterion pairs can play a similarly important role in dictating photocatalytic properties. As such, the capacity to discriminate between different ionic solvation structures is imperative for understanding and optimizing the interplay of species in photocatalytic reaction environments. In this work, we probe ionic interactions within a model photocatalyst and counterion system: [Ru(4,4'-dicarboxy-2,2'-bipyridine)2(NCS)2]4-, a ruthenium dye known as N34-, and the Sc3+ cation of Sc(OTf)3, a Lewis acid. Using linear infrared (IR) spectroscopy, density functional theory (DFT) calculations, and two-dimensional infrared spectroscopy (2DIR), we look for signatures of contact ion pairs (CIPs) and solvent separated ion pairs (SSIPs) in two solvents of different dielectric strength. DFT calculations and linear IR spectroscopy indicate CIP formation between N34- and Sc3+ in a mixed solvent of D2O, acetonitrile, and trifluoroethanol. Linear spectral signatures of CIPs are not observed when D2O alone is used as the solvent. Spectral diffusion measurements from 2DIR enable comparison of solvation dynamics, revealing a significant slowing of solvent shell fluctuations for N34- and Sc3+ in D2O that is inconsistent with freely solvated ions, indicating SSIP formation. This study highlights the power of 2DIR for distinguishing between ion pair solvation structures and demonstrates its value for studying photocatalytic systems.