Ultrafast Electron Transfers in Organometallic Supramolecular Assemblies Built with a NIR-Fluorescent Tetrabenzoporphyrine Dye and the Unsaturated Cluster Pd₃(dppm)₃(CO)²⁺

Abstract

The sodium 9,18,27,36-tetra-(4-carboxyphenyl­ethynyl)­tetrabenzo­porphyrinatozinc­(II) (TCPEBP) and sodium 5,10,15,20-tetra-(4-carboxy­phenyl­ethynyl)­porphyrinatozinc­(II) (TCPEP, for comparison purposes) salts were prepared to investigate the ionic driven host–guest assemblies made with the unsaturated redox-active cluster Pd3(dppm)3(CO)2+ ([Pd 3 2+ ], dppm = Ph2PCH2PPh2 as a PF6 – salt). Nonemissive dye···[Pd 3 2+ ] x assemblies (x = 1–4) are formed in methanol with K 1x (binding constants) values of 83 200 (TCPEBP) and 70 400 M–1 (TCPEP; average values extracted from graphical methods (Benesi–Hildebrand, Scott, and Scatchard), matching those obtained from fluorescence quenching experiments (static model)). These values are consistent with the more electron rich TCPEBP dye. This conclusion is corroborated by electrochemical data, which indicate a lower oxidation potential of the TCPEBP dye (+0.46 V) vs TCPEP (+0.70 V vs SCE) and by shorter calculated average Pd···O distances (DFT (B3LYP): 3.259 vs 3.438 Å, respectively). Using the position of the 0–0 component of the Q-bands and the electrochemical data, the excited-state driving forces for dye*···[Pd 3 2+ ] x → dye +• ···[Pd 3 +• ]­[Pd 3 2+ ] x–1 are estimated for TCPEBP (+1.22 V vs SCE) and TCPEP (1.08 V vs SCE). The time scale for this process occurs within the laser pulse (fwhm <75–110 fs) during the measurements of the femtosecond transient absorption spectra. Conversely, the back electron transfers (dye +• ···[Pd 3 +• ]­[Pd 3 2+ ] x–1 → dye···[Pd 3 2+ ] x ) occur well within 1 ps (respectively 650 and 170 fs for TCPEBP and TCPEP). Arguments are provided that the reorganization energy governs this difference.