Triplet Energy Transfers in Well-Defined Host–Guest Porphyrin–Carboxylate/Cluster Assemblies

The dyes (5-(4-carboxylphenyl)-10,15,20-tritolylporphyrinato)­zinc­(II) (<b>MCP</b>) and (5,15-bis­(4-carboxylphenyl)-15,20-ditolylporphyrinato)­zinc­(II) (<b>DCP</b>), as their sodium salts, were used to form assemblies with the unsaturated cluster Pd₃(dppm)₃(CO)²⁺ (<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>, dppm = (Ph₂P)₂CH₂) via ionic CO₂^–···Pd₃²⁺ interactions. The photophysical properties in their triplet states were studied. The position of the T₁ state of <b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b> (∼8190 cm^–1) has been proposed using DFT computations and was corroborated by the presence of a T_<i>n</i> → S₀ delayed emission at 680–700 nm arising from a T₁–T₁ annihilation process at 77 K. The static quenching of the near-IR phosphorescence of the dyes at 785 nm (T₁ → S₀) was observed. Thermodynamically poor reductive and oxidative driving forces render the photoinduced electron transfer quenching process either inoperative or very slow in the T₁ states. Instead, slow to medium T₁–T₁ energy transfer (³dye*···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b> → dye···³<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>*) operates through a Förster mechanism exclusively with <i>k</i>_ET values of ∼1 × 10⁵ s^–1 on the basis of transient absorption measurements at 298 K

Electron-Transfer Kinetics within Supramolecular Assemblies of Donor Tetrapyrrolytic Dyes and an Acceptor Palladium Cluster

9,18,27,36-Tetrakis­[<i>meso</i>-(4-carboxyphenyl)]­tetrabenzoporphyrinatozinc­(II) (TCPBP, as a sodium salt) was prepared in order to compare its photoinduced electron-transfer behavior toward unsaturated cluster Pd₃(dppm)₃(CO)²⁺ ([Pd₃²⁺]; dppm = Ph₂PCH₂PPh₂ as a PF₆^– salt) with that of 5,10,15,20-tetrakis­[<i>meso</i>-(4-carboxyphenyl)]­porphyrinatozinc­(II) (TCPP) in nonluminescent assemblies of the type dye···[Pd₃²⁺]_<i>x</i> (<i>x</i> = 0–4; dye = TCPP and TCPBP) using femtosecond transient absorption spectroscopy. Binding constants extracted from UV–vis titration methods are the same as those extracted from fluorescence quenching measurements (static model), and both indicate that the TCPBP···[Pd₃²⁺]_<i>x</i> assemblies (<i>K</i>₁₄ = 36000 M^–1) are slightly more stable than those for TCPP···[Pd₃²⁺]_{<b><i>x</i></b>} (<i>K</i>₁₄ = 27000 M^–1). Density functional theory computations (B3LYP) corroborate this finding because the average ionic Pd···O distance is shorter in the TCPBP···[Pd₃²⁺] assembly compared to that for TCPP···[Pd₃²⁺]. Despite the difference in the binding constants and excited-state driving forces for the photoinduced electron transfer in dye*···[Pd₃²⁺] → dye^•+···[Pd₃^•+], the time scale for this process is ultrafast in both cases (<85 fs). The time scales for the back electron transfers (dye^•+···[Pd₃^•+] → dye···[Pd₃²⁺]) occurring in the various observed species (dye···[Pd₃²⁺]_<i>x</i>; <i>x</i> = 0–4) are the same for both series of assemblies. It is concluded that the structural modification on going from porphyrin to tetrabenzoporphyrin does not greatly affect the kinetic behavior in these processes

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

The sodium 9,18,27,36-tetra-(4-carboxyphenyl­ethynyl)­tetrabenzo­porphyrinatozinc­(II) (<b>TCPEBP</b>) and sodium 5,10,15,20-tetra-(4-carboxy­phenyl­ethynyl)­porphyrinatozinc­(II) (<b>TCPEP</b>, 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⁺ (<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>, dppm = Ph₂PCH₂PPh₂ as a PF₆^– salt). Nonemissive dye···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i></b>} assemblies (<i>x</i> = 1–4) are formed in methanol with <i>K</i>_1<i>x</i> (binding constants) values of 83 200 (<b>TCPEBP</b>) and 70 400 M^–1 (<b>TCPEP</b>; 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 <b>TCPEBP</b> dye. This conclusion is corroborated by electrochemical data, which indicate a lower oxidation potential of the <b>TCPEBP</b> dye (+0.46 V) vs <b>TCPEP</b> (+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*···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i></b>} <b> → </b> dye^<b>+•</b>···<b>[Pd</b>_<b>3</b>^<b>+•</b><b>]­[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i>–1</b>} are estimated for <b>TCPEBP</b> (+1.22 V vs SCE) and <b>TCPEP</b> (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^<b>+•</b>···<b>[Pd</b>_<b>3</b>^<b>+•</b><b>]­[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i>–1</b>} <b> → </b> dye···<b>[Pd</b>_<b>3</b>^<b>2+</b><b>]</b>_{<b><i>x</i></b>}) occur well within 1 ps (respectively 650 and 170 fs for <b>TCPEBP</b> and <b>TCPEP</b>). Arguments are provided that the reorganization energy governs this difference