Enhanced Electron‐Transfer Properties of Cofacial Porphyrin Dimers through π–π Interactions

Abstract

pi-pi assisted: Photoinduced electron transfer from cofacial porphyrin dimers to electron acceptors is prominently accelerated, whereas the back electron transfer is decelerated, relative to the corresponding porphyrin monomer (see figure).The radical cation of zinc tetrapentylporphyrin is dimerized with an excess of the neutral counterpart to form the dimer radical cation in which the unpaired electron is delocalized over both porphyrin rings. The dimeric radical cation exhibits an NIR absorption spectrum characteristic of weak pi-bond formation between the porphyrin rings. When cofacial porphyrin dimers, linked by different spacers, are oxidized such pi-bond formation between the porphyrin rings is also recognized in cyclic voltammetry, and Vis/NIR and ESR spectroscopic measurements. The dynamics of photoinduced electron transfer from the triplet excited states of cofacial porphyrin dimers to a series of electron acceptors were investigated by using laser flash photolysis measurements and compared with the porphyrin monomer. The rates of photoinduced electron-transfer reactions of cofacial porphyrin dimers are prominently accelerated compared with the reference monomer. The driving-force dependence of the rate constants of photoinduced electron-transfer reactions was analyzed in light of the Marcus theory of electron transfer to afford the reorganization energies of electron transfer (lambda). The lambda values of cofacial porphyrin dimers are significantly smaller than those of the porphyrin monomer when compared at the same driving force of the photoinduced electron transfer. The lambda values increase linearly with an increase in the driving force of the photoinduced electron transfer. This is accompanied by an increase in the distance between electron donor and acceptor molecules, where the electron transfer occurs. The enhanced electron-transfer properties of cofacial porphyrin dimers, in relation with the important role of the special pair in the photosynthetic reaction center, result from the smaller reorganization energy (lambda) together with the larger driving force of the photoinduced electron transfer due to the pi-electron delocalization in the dimer radical cations.

Tryptophan-mediated charge-resonance stabilization in the bis -Fe(IV) redox state of MauG

“…4B) (4,(28)(29)(30)(31)(32)(33). We term the former case with an odd number of spin/charge as type I CR and the latter case with an even number as type II CR.…”

“…Notably, both types of CR-transition phenomena may be observed from the same system and further distinguished by the relative positions of their CR bands (4,(31)(32)(33). Despite the inherent energy penalty caused by electrostatic repulsion in the doubly charged complex, the ΔE CR value of type II CR usually is larger than that of type I CR from the same system, which causes a blue-shift of the CR band of the (Π +• ) 2 complex compared with the (Π) 2 +• complex (4,(31)(32)(33). In principle, both types of CR complexes bear a close relationship to the conventional charge-transfer complexes derived from closed-shell diamagnetic precursors consisting of electron-rich donors and electron-poor acceptors.…”

Flexible Amide-Bonded Zn(II) Porphyrin Dimer: Electronic Structure Investigation and Its Induced Chirality

“…On the other hand, no spectral changes observed during the oxidation of Zn II Por dimer 1 in low polar solvent o-DCB. The effect of the counter anion of the supporting electrolyte on the reductive spectroelectrochemical behavior of 1 has also been examined (Figures 5,6), and was found to have a significant effect on the potentials and reversibility of the electroreductions in both DMF and o-DCB.…”

“…[4] From this point of view, there has been considerable research interests in the synthesis and properties of cofacial porphyrin dimers, [5] since these compounds provide a possible pathway for mimicking the electron transfer properties of the bacteriochlorophyll dimer in photosystem II, which is often referred as the "special pair". [6] On the other hand, supramolecular chirality is a growing multidisciplinary field of modern research and attracts much strong attention from the scientific community because of its vital importance for various natural processes and for its attractive possibilities for new smart technologies. [7] Of the vast number of host-guest and self-associated systems, supramolecular assemblies based on porphyrin chromophores are of particular interest for in depth investigation and potential application as a consequence of them having specific and well-suited physicochemical and spectroscopic properties.…”

Spectroscopic investigations and theoretical calculations of DABCO induced xanthene bridged self-assembled zinc(II) porphyrin dimer

“…In recent years, there has been considerable interest in the synthesis and properties of cofacial porphyrin dimers [1-4], since these compounds provide a possible pathway for mimicking the electron transfer properties of the bacteriochlorophyll dimer in photosystem II, which is often referred as the "special pair" [5]. Self-assembly of Zn(II)-porphyrins is particularly important in this regard, since axial ligation of the metal-center from above and below the porphyrin ring occurs readily in the presence of a nitrogencontaining Lewis base guest molecule such as pyridine, 4,4-bipyridine, piperazine, or 1,4-diazabicyclo[2.2.2] octane (DABCO) [6].…”