Distance Dependence of Photoinduced Electron Transfer in Metalloporphyrin Dimers

Abstract

To study the dynamics and mechanism of intramolecular photoinduced electron transfer (PET) reactions, a series of (Zn II -Fe III ) meso-tetraarylmetalloporphyrin dimers were synthesized and the kinetics of their PET reactivity was measured. Molecular building blocks were prepared by selective nucleophilic aromatic substitution of a para fluorine on tetraarylporphyrins containing a single pentafluorophenyl group. This synthetic approach allows a wide variety of systematic modifications such as type and length of spacer, metal center, and redox-potential difference between donor and acceptor. The edge-to-edge distance between the two porphyrins varies from 14.4 to 27.3 Å. Into a symmetric dimer, with two identical porphyrins covalently linked by a rigid partly saturated bridge, one zinc(II) and one iron(III) can be inserted. From measurements of fluorescence lifetimes the rate constants for PET from the electronically excited state of the zinc porphyrin to the bis(imidazole)iron porphyrin cation were evaluated. The electron-transfer rate decreases by a factor of only 165 when the distance increases by 13 Å. This small decrease is indicative of a surprisingly weak attenuation of the electronic coupling with distance.

Enhanced Intersystem Crossing in Donor/Acceptor Systems Based on Zinc/Iron or Free-Base/Iron Porphyrins

“…It is also notable that the rate constants in our systems show essentially no solvent dependence. The total quenching rate constants observed for our ZnP/FeP systems are comparable to those previously reported for other rigid ZnP‐bridge‐FeP systems with similar donor–acceptor distances 18, 33, 35…”

Nonconjugated Hydrocarbons as Rigid‐Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry

“…To further understand the factors that control the dependence of ET rates on the distance between the electron-donor and the -acceptor, as eries of (Zn II -Fe III )5 ,10,15,20-tetraarylmetalloporphyrin dimers 189,with avariety of differentlinkers, including BCO, were synthesized and the kinetics of their PET reactivity were measured. [241] Through the use of fluorescence lifetimem easurements, the electronically excited states of the zinc porphyrin to the bis(imidazole)iron porphyrin cation could be determined. Results showedt hat when the distance was increased by 13 the rate of electron-transfer only decreased by af actor of 165 indicating as mall reduction of the electronic coupling with distance.…”

Condutância molecular e biomolecular

“…Dímeros de porfirinas com pontes contendo de um a três grupos benzeno tem sido estudados por McLendon 19 . Sistemas semelhantes em que o grupo benzeno central é substituído por um grupo saturado, como cicloexano ou piprazina, foram estudados por Portela e colaboradores 20 . Cálculos teóricos para o fator eletrônico nessas pontes foram realizados utilizando métodos de química quântica ab initio e semiempírico 21 .…”