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 (ε)-dependent excited-state relaxation behavior. In nonpolar toluene solvent, PZn–Pd(edtb)₂–PZn manifests an ultrafast S₁ → T₁ intersystem crossing time constant (τ_ISC ≈ 2 ps), a broad, high-oscillator strength T₁ → T_n transient absorption manifold (λ_max(T₁ → T_n) = 940 nm), and a near unity triplet-state formation quantum yield (Φ_T ≈ 1; τ_T = 2.2 μs). In contrast, in moderately polar solvents (e.g., dichloromethane (DCM) or THF), the S₁ → T₁ intersystem crossing quantum yield is significantly suppressed (Φ_T ≈ 0.2; τ_F ≈ 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 λ^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 S₁-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 S₁ state and providing an excited-state relaxation channel that bypasses T₁ 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.