Donor-spacer-acceptor system

Photoactive Dyads. - Many new molecular dyads, comprising donor-spacer-acceptor systems, have been described in the recent literature. These systems are intended to reproduce the essential electron-transfer steps occurring in natural photosynthetic organisms by eliminating as many components as possible. The main events in the natural apparatus involve light-induced electron... 

Nishizawa. S. Kaneda, H. Uchida. T. Teramae. N. Anion sensing by a donor-spacer-acceptor system An intramo-leeular exciplex emission enhanced by hydrogen bond-mediated complexation. J. Chem. Soc., Perkin Trans. 2 1998. (11). 2325-2327. 

In a second paper, Sanders and coworkers presented a simple model mediated by pyridine ligation to zinc porphyrin. " In the donor-spacer-acceptor system, photoinduced ET occurs with a rate con.stant of 2.13 x I0 s for zinc mesoporphyrin II dimethyl ester 141 and 53.3 X lO s" for zinc tetraphenylporphyrin, while the recombination rate constants were 6.35 x 10 s and 3.81 X 10 s , respectively. Although the two porphyrins have similar redox potentials, the forward ET rates were unambiguously different. The authors speculated that the two photodonor porphyrins have different solvent reorganization energies. 

Further diverse systems have been developed and used in the investigation of intramolecular SET processes. These include styrene/amide-spacer/amine diads, 9-aminoacridine/polyether-spacer/benzoate ester diads, l-(4-cyano-phenyl)-4-(cyanomethylene)piperidine, 7i-donor/polyoxyethylene/Zn(II)por-phyrin/N,N -dimethyl-4,4 -bipyridinium systems, naphthalene/porphyrin/ quinone cyclophanes and their anthracene analogues, pyropheophytin-naphthoquinone diads, rigid donor/bridge/acceptor systems, anilide-substi-... 

For the attainment of marvelous electron transfer processes in the natural sequential potential fields, many noncovalentaly-bound donor-acceptor (DA) systems and covalently-bound DA systems " " have been previously reported. Most of them are artificial models of the photosynthesis comprising simple assemblies of the dyad (DA) or triad [donor-spacer-acceptor (DSA)] functional molecules with a chromophore such as a porphyrin. The quantum efficiency of such systems is lower (<25%) compared with the biological systems (=100%), and thus more efforts for constructing more efficient systems are necessary. Some of the covalen-taly-bound DA systems have been designed for the fabrication of molecule-scale devices based on a molecular electron-transport wire and/or highly ordered molecular arrays on the surface. " Most of such studies employed the DA nonconju-gated molecules. 

Electronic factors related to orbital overlap also appear to interfere significantly in the dynamics of concerted electron transfer/bond breaking reactions in donor-spacer-cleaving acceptor systems.94... 

The effects of spacer length on the fluorescence quantum yields of the benzofurazan compounds bearing a donor-acceptor system were reported <2002MI11>. 

The Forster resonance energy transfer can be used as a spectroscopic ruler in the range of 10-100 A. The distance between the donor and acceptor molecules should be constant during the donor lifetime, and greater than about 10 A in order to avoid the effect of short-range interactions. The validity of such a spectroscopic ruler has been confirmed by studies on model systems in which the donor and acceptor are separated by well-defined rigid spacers. Several precautions must be taken to ensure correct use of the spectroscopic ruler, which is based on the use of Eqs (9.1) to (9.3) ... 

A simple approach to understanding the factors which control the "conductivity of proteins towards electron tunneling is to develop "small molecule model systems to mimic intramolecular electron transfer in the protein systems. Appropriate models obviously require that the donor and acceptor be held at fixed distances and orientations which correspond to those in the protein-protein complexes. Models of this type have recently been obtained and investigated [103,104]. In these models the protein matrix is replaced by a simple synthetic spacer which separates two porphyrin molecules. By changing the chemical structure of the spacer, a series of molecules with different reaction distances and geometries has been synthesized. Typical examples of such molecules are presented in Fig. 21. 

The situation is different when donor and acceptor molecules are located at different interfaces, that are separated by a fatty acid monolayer of well defined thickness. There are no longer close pairs of donor and acceptor with a high probability of electron transfer as in the "contact" case. Consequently, no change in relative fluorescence intensity with increasing donor density is expected, contrary to the former case. Indeed, in systems with a spacer monolayer of... 

Cofacial bisporphyrin systems use rigid spacers to provide a unique placement of two chromophores (donor and acceptor) at a given distance,... 

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

This chapter provides a comprehensive overview of photoinduced ET in metal-organic dyads. The focus is on systems in which intramolecular ET occurs between a metal center and an organic donor (or acceptor) that are held in proximity by an organic spacer. Emphasis is placed on systems in which the thermodynamic driving force for ET is well defined. 

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