Activation volumes associated with excited-state electron transfer across amidinium-carboxylate bridge

Abstract

Pressure was used to modulate interactions in an electron donor–acceptor system, composed of a zinc porphyrin (ZnP) and a fullerene (C₆₀), held together by an amidinium-carboxylate salt-bridge (ZnP-H⋯C₆₀). Two different trends evolved in steady-state absorption assays. Volume compression causes an absorbance intensification, and a solvatochromic-like red shift that stems from increased E-field density. Pressure-dependent femtosecond and nanosecond transient-absorption experiments were performed to investigate the activation volumes of the excited-state deactivation processes in ZnP-H⋯C₆₀. Solvent relaxation of S₁ is found to have a highly positive ΔV_k2^‡. The pressure-induced rate attenuation for this process is assumed to be linked to the solvent's viscosity increase. Intersystem crossing to the porphyrin-centered T₁ state is free of intrinsic and extrinsic reorganizations and, as such, the activation volume is close to zero. The same applies for the subsequent ground-state deactivation from T₁ to S₀. Charge-separation to afford (ZnP)˙⁺⋯H⋯(C₆₀)˙⁻ is linked to a volume compression towards the activated state with ΔV_k3^‡ = −5.7 ± 2.2 cm³ mol⁻¹. The charge-recombination undergoes, within the experimental margins of error, an equal volume expansion with ΔV_k4^‡ = +8.6 ± 0.7 cm³ mol⁻¹. This effect is linked to the generation and/or neutralization of charges, best described by the Jung equation for electrostrictive volume changes in dipolar zwitterionic entities. Importantly, volumetric contributions from a possible PT towards the activated state were not observed.