Zinc(II) Monomeric, Dimeric, and Trimeric Photosensitizers with Microsecond-Lived Intra-ligand Charge Transfer Excited States Investigated through Time-Resolved Optical and X-Ray Spectroscopy.

Zn photosensitizers relative to Cu complexes have received less attention due to their energetically higher metal-to-ligand charge transfer states. Three Zn complexes, namely a Monomer, a bimetallic helicate, and a trimetallic helicate, bearing phenanthroline ligands are hereby studied through time-resolved X-ray absorption (tr-XAS) and femto-microsecond optical transient absorption spectroscopy (OTA). The formation of intraligand singlet charge transfer (ILCT) excited states is achieved within femtoseconds, followed by intersystem crossing (ISC) in nanoseconds to generate microsecond-lived triplet (ILCT) states. Femtosecond OTA shows that the ILCT states in the Monomer, Dimer, and Trimer occur within 235 fs, 683 fs, and 730 fs, respectively, while nano-microsecond OTA and tr-XAS show their ILCT states to decay within 1.00 µs, 1.48 µs, and 1.51 µs. The ISC from the ILCT to the ILCT state for the Trimer is 42.8 ns compared to the Monomer and Dimer with ISC rates of less than 13 ns. These differences arise due to the stabilization by π-π and CH-π noncovalent interactions of the phenanthroline ligands. The dihedral and torsional angles indicate stronger ligand strains in the excited states of the Dimer and Trimer versus the Monomer. DFT calculations for the electrochemical oxidation potentials further highlight their capability in inducing photoredox processes.