Synthesis, Electrochemistry, and Photophysics of Pd(II) Biladiene Complexes Bearing Varied Substituents at the sp³-Hybridized 10-Position

Abstract

A set of Pd(II) biladiene complexes bearing different combinations of methyl- and phenyl-substituents on the sp³-hybridized <i>meso</i>-carbon (the 10-position of the biladiene framework) was prepared and studied. In addition to a previously described Pd(II) biladiene complex bearing geminal dimethyl substituents a the 10-position (<b>Pd[DMBil]</b>), homologous Pd(II) biladienes bearing geminal methyl and phenyl substituents (<b>Pd[MPBil1]</b>) and geminal diphenyl groups(<b>Pd[DPBil1]</b>) were prepared and structurally characterized. Detailed electrochemical as well as steady-state and time-resolved spectroscopic experiments were undertaken to evaluate the influence of the substituents on the biladiene's tetrahedral <i>meso</i>-carbon. Although all three biladiene homologues are isostructural, <b>Pd[MPBil1]</b> and <b>Pd[DPBil1]</b> display more intense absorption profiles that shift slightly toward lower energies as geminal methyl groups are replaced by phenyl rings. All three biladiene homologues support a triplet photochemistry, and replacement of the geminal dimethyl substituents of <b>Pd[DMBil1]</b> (Φ_Δ = 54%) with phenyl groups improves the ability of <b>Pd[MPBil1]</b> (Φ_Δ = 76%) and <b>Pd[DPBil1]</b> (Φ_Δ = 66%) to sensitize ¹O₂. Analysis of the excited-state dynamics of the Pd(II) biladienes by transient absorption spectroscopy shows that each complex supports a long-lived triplet excited-state (i.e., τ > 15 μs for each homologue) but that the ISC quantum yields (Φ_T) varied as a function of biladiene substitution. The observed trend in ISC efficiency matches that for singlet oxygen sensitization quantum yields (Φ_Δ) across the biladiene series considered in this work. The results of this study provide new insights to guide future development of biladiene based agents for PDT and other photochemical applications.