Size-dependent modulation of intersystem crossing and singlet oxygen sensitization in porphyrin-silver cluster hybrid systems.

Density functional theory (DFT) and time-dependent DFT calculations including spin-orbit coupling effects were employed to systematically investigate the photophysical properties of a water-soluble porphyrin derivative, di(4-aminophenyl)-di(4-sulfophenyl)-diphenylporphyrin (TASP), and its hybrid complexes with silver clusters of varying size. Comparative analysis with the prototypical free-base porphyrin H2TPP reveals that molecular functionalization enhances visible-light absorption while maintaining triplet-state energetics favorable for Type-II oxygen sensitization. Hybridization of TASP with Agn (n =1, 2, 3, 4, and 13) clusters produces pronounced size-dependent changes in the excited-state landscape. For the smaller clusters, particularly Ag3, the porphyrin-centered π-π* character of the low-lying excitations is largely preserved, while moderate metal-ligand mixing and the heavy-atom effect lead to enhanced singlet-triplet spin-orbit coupling. In contrast, larger clusters such as Ag₁₃ shift the low-energy excited-state manifold toward cluster/interface-influenced states with very low excitation energies and weak oscillator strengths, together with triplet energies that are unfavorable for efficient Type-II 1O2 sensitization. Natural transition orbital analysis and excited-state energetics reveal a clear relationship between cluster size and the evolution of excited-state character. Quantitative spin-orbit coupling matrix element analysis for TASP and TASP-Ag3 further shows a one- to two-order-of-magnitude enhancement of singlet-triplet coupling upon Ag3 hybridization. These results provide mechanistic insight into how silver cluster size controls the balance between enhanced spin-orbit effects and retention of porphyrin-like photophysical functionality.