Unusual solvent polarity dependent excitation relaxation dynamics of a bis[p-ethynyldithiobenzoato]Pd-linked bis[(porphinato)zinc] complex

Abstract

We report the synthesis and excited-state dynamics of a bis[p-ethynyldithiobenzoato]Pd(II)-bridged bis.(porphinato) zinc(II)] complex (PZn-Pd.edtb)(2)-PZn) that exhibits unusual solvent dielectric (epsilon)-dependent excited-state relaxation behavior. In nonpolar toluene solvent, PZn-Pd(edtb)(2)-PZn manifests an ultrafast S-1 -> T-1 intersystem crossing time constant (tau(ISC) approximate to 2 ps), a broad, high-oscillator strength T-1 -> T-n transient absorption manifold (tau(max)(T-1 -> T-n) = 940 nm), and a near unity triplet-state formation quantum yield (FT approximate to 1; tau(T) = 2.2 mu s). In contrast, in moderately polar solvents (e. g., dichloromethane (DCM) or THF), the S-1 -> T-1 intersystem crossing quantum yield is significantly suppressed (Phi(T) approximate to 0.2; tau(F) approximate to 60 ps in DCM). Comparative femtosecond transient absorption studies in DCM and mixed DCM/toluene solvent systems reveal a new low-energy stimulated emission signal, the lambda(max)(em) of which is highly sensitive to solvent polarity. The lack of spectral signatures for radical species, and the emergence of intense stimulated emission indicate an additional low energy electronically excited-state (S*), populated via S-1-state relaxation, that also possesses substantial singlet character. As solvent polarity is progressively increased, the energy of S* progressively decreases, eventually becoming lower than the S-1 state and providing an excited-state relaxation channel that bypasses T-1 state formation. These data show that the nature of the PZn-Pd(edtb)(2)-PZn excited-state dynamics is strongly influenced by the solvent dielectric, and that this Pd(II)-based linker motif offers new opportunities to engineer excited-state spin distributions and lifetimes in strongly conjugated chromophore assemblies.