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

A set of Pd(II) biladiene complexes bearing different combinations of methyl- and phenyl-substituents on the sp-hybridized -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 (), homologous Pd(II) biladienes bearing geminal methyl and phenyl substituents () and geminal diphenyl groups() 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 -carbon. Although all three biladiene homologues are isostructural, and 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 (Φ = 54%) with phenyl groups improves the ability of (Φ = 76%) and (Φ = 66%) to sensitize O. 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) varied as a function of biladiene substitution. The observed trend in ISC efficiency matches that for singlet oxygen sensitization quantum yields (Φ) 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.