Formation of a Supramolecular Porphyrin-Spacer-Acceptor Ternary Complex and Intracomplex Electron Transfer

Abstract

A new ditopic spacer, N-propylisonicotinamidine, has been developed to assemble a ternary supramolecular complex, zinc-tetraphenylporphyrin (ZnTPP)-spacer-acceptor (3,4-dinitrobenzoic acid), via an axial coordination of the spacer to ZnTPP and a salt bridge interaction between the spacer and acceptor. Formation of this ternary complex was probed by 1 H HMR and UV-vis spectroscopy. The ternary complex is formed with an overall association constant of 1.9 × 10 8 M -2 in dichloromethane. The fluorescence is nearly completely quenched by the electron acceptor in the ternary complex as confirmed by the steady-state fluorescence measurements. The triplet excited state of ZnTPP is also quenched by the electron acceptor in the ternary complex. The ternary complex is also formed in acetonitrile, where fluorescence quenching is observed with an intracomplex quenching rate constant of 1.9 × 10 9 s -1 . Photoinduced intracomplex electron transfer is invoked as a likely mechanism of the fluorescence quenching and triplet bleaching.

Through‐Bond Excited Energy Transfer Mediated by an Amidinium–Carboxylate Salt Bridge in Zn–Porphyrin Free‐Base Porphyrin Dyads

“…These spectral changes are accounted for by the self-association of ZnPA-2 molecules through coordination of the amidine group of one molecule to the zinc ion in another. [20,21] Benzoic acid prevents this self-association of ZnPA-2 by forming the amidinium-carboxylate salt bridge with ZnPA-2. Thus, upon addition of benzoic acid to a solution of ZnPA-2, the spectrum reverted to the one for isolated ZnPA-2.…”

“…Keywords: energy transfer · porphyrinoids · salt bridges · supramolecular chemistry · through-bond interactions porphyrin connected through an amidinium-carboxylate salt bridge. [13][14][15][16][17][18][19][20][21][22][23] The same intermolecular bond was also used for the construction of an antenna-type (donor) 4 ·A C H T U N G T R E N N U N G (acceptor) complex. We found that the rates of excited energy-transfer processes in these supramolecules are much larger than expected from the Fçrster-type energy-transfer mechanism.…”

The Relation between Hydrogen Atom Transfer and Proton‐coupled Electron Transfer in Model Systems

“…An alternative approach for the study of PCET across amidinium-carboxylate interfaces relies on the formation of ternary complexes such as that shown for 16 [141]. A N-propylisonicotinamidine serves as a ditopic spacer between a Zn II tetraphenylporphyrin donor and 3,4-dinitrobenzoate acceptor.…”

“…As discussed previously, this approach can be problematic because the dynamics of fluorescence decay are not necessarily specific to charge transport. Furthermore, PCET can proceed from non-emitting states to which time resolved emission spectroscopy is blind [105,141]. Accordingly, there is great benefit offered by systems that allow the PCET kinetics to be determined directly from observed charge transport intermediates.…”

Supramolecular assemblies for electron transfer

“…Both the fluorescence and the triplet excited state of ZnP are nearly completely quenched by the electron acceptor in the ternary complex. Photoinduced intracomplex electron transfer is invoked as a likely mechanism of the fluorescence quenching and triplet bleaching [8].…”