The metal-to-ligand-charge-transfer (MLCT) excited state of Cu(I) diimine complexes is known to undergo structural reorganization, transforming from a pseudotetrahedral D(2d) symmetry in the ground state to a flattened D(2) symmetry in the MLCT state, which allows ligation with a solvent molecule, forming an exciplex intermediate. Therefore, the structural factors that influence the coordination geometry change and the solvent accessibility to the copper center in the MLCT state could be used to control the excited state properties. In this study, we investigated an extreme case of the steric hindrance caused by attaching bulky tert-butyl groups in bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I), [Cu(I)(dtbp)(2)](+). The two bulky tert-butyl groups on the dtbp ligand lock the MLCT state into the pseudotetrahedral coordination geometry and completely block the solvent access to the copper center in the MLCT state of [Cu(I)(dtbp)(2)](+). Using ultrafast transient absorption spectroscopy and time-resolved emission spectroscopy, we investigated the MLCT state property changes due to the steric hindrance and demonstrated that [Cu(I)(dtbp)(2)](+) exhibited a long-lived emission but no subpicosecond component that was previously assigned as the flattening of the pseudotetrahedral coordination geometry. This suggests the retention of its pseudotetrahedral D(2d) symmetry and the blockage of the solvent accessibility. We made a comparison between the excited state dynamics of [Cu(I)(dtbp)(2)](+) with its mono-tert-butyl counterpart, bis(2-tert-butyl-1,10-phenanthroline)copper(I) [Cu(I)(tbp)(2)](+). The subpicosecond component assigned to the flattening of the D(2d) coordination geometry in the MLCT excited state was again present in the latter because the absence of a tert-butyl on the phenanthroline allows flattening to the pseudotetrahedral coordination geometry. Unlike the [Cu(I)(dtbp)(2)](+), [Cu(I)(tbp)(2)](+) exhibited no detectable emission at room temperature in solution. These results provide new insights into the manipulation of various excited state properties in Cu diimine complexes by certain key structural factors, enabling optimization of these systems for solar energy conversion applications.
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