Linked donor-acceptor

In the 1960s, Hassel8 studied the structures of many mixed crystals whose unit cells correspond to noncovalently linked donor-acceptor complexes, such as the... 

A. M. Oliver, D. C. Craig, M. N. Paddon-Row, J. Kroon, J. W. Verhoeven, Strong Effects of the Bridge Configuration on Photoinduced Chaige Separation in Rigidly Linked Donor-Acceptor Systems , Chem. Phys. Lett. 1988, 150, 366-373. 

Donor and acceptors can be covalently linked using a chemical spacer. Assume that we have the same D-A pair Eosin-Phenol Red. In this case we will have a mixture of two linked donor-acceptor species (Eosin-Phenol Red protonated and Eosin-Phenol Red unprotonated) characterized by the same distance distribution and different critical distances (ftoi = 28.3 A and Rm = 52.5 A) for FRET. A distribution of D- to -A distances will be present because the linker is typically flexible. The fractional intensity of the first species at time t = 0 is gi and that of the second species is (1 - 1). The fractional intensity at time t = 0 is equal to fractional concentration of each form, which can be in case of pH indicator (Phenol Red) calculated using Eq. (10.31). The donor fluorescence intensity decay of the mixture is described by the equation... 

Gust D, Moore TA (eds) (1989) Covalently linked donor-acceptor species. (Tetrahedron Symposia-in-Print, vol 45, No 15) Pergamon Press, New York)... 

Cyclization Reactions Involving Radical Cations and Radical Anions in Linked Donor-Acceptor Systems... 

The half-deprotected product was obtained in 65% yield. Of course, such a yield is insufficient from a synthetic point of view, the photovariant of the redox reaction is not simple instrumentally, and the duration of the reaction (5 h) is too long. Nevertheless, this approach is promising, and deserves attention and development. Thus, the photochemical method proved to be successful in the removal of protecting groups based on covalently linked donor-acceptor systems (Lee and Falvey 2000). 

In general, for the types of linked donor-acceptor systems to be discussed in this review, electron transfer is assumed to occur in the nonadiabatic regime. That is, the mixing between the electronic state of the donor and acceptor before electron transfer occurs and the corresponding state after electron transfer is weak (< kBT) [9], The actual electron transfer event is assumed to be fast compared to the time scale of nuclear motions. Marcus has proposed [11, 15] that the electron transfer rate constant ke, is given by Eq. 1. 

Although covalently linked donor-acceptor systems of small organic chromo-phores have been studied for some time in order to uncover the basic principles of electron and energy transfer, the first covalently linked cyclic tetrapyrroles were reported by Gouterman, Dolphin and coworkers in 1972 [27]. In 1976 the first dimeric chlorophyll-based models were reported. Structure la, based upon pyropheophorbide-a, was prepared by Boxer and Closs [28], whereas the pheo-phorbide-a derivative lb was reported by Wasielewski, Studier and Katz [29]. 

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]. 

Molecular electron transfer is the basis for many important natural and commercial processes. During the past decade photochemists have relied upon supramolecular arrays of molecules to facilitate their understanding of the chemical and physical basis for this fundamentally important process. It therefore seems appropriate that several chapters in this volume examine thermally and photo-chemically induced electron transfer in supramolecular assemblies consisting of inorganic molecular building blocks such as covalently linked donor-acceptor dyads, transition metal clusters, and nanocrystalline semiconductor particles. 

In many cases it will be seen, that the simple five-step scheme given above is not sufficient to describe more complex systems, i.e., donor-acceptor couples in solution or covalently linked donor acceptor couples. More sophisti-... 

The electroabsorption techniqne involves the measurement of the changes in absorption spectra that result when very large electric fields (typically tens of thousands of volts) are applied to glassy solntions at very low temperatures. It has been nsed primarily for covalently linked donor-acceptor complexes that are in the strongly conpled regime. ... 

Fig. 15. Example of a covalently linked donor-acceptor system featuring a photochromic control moiety 11) for the downregulation of photoinduced electron transfer under intense light conditions. Adapted from Ref. (133).

In the last part of this chapter, intramolecular charge transfer (ICT) in anthryl derivatives with linked donor-acceptor parts was discussed. Ultrafast spectroscopy has been applied both for structural characterization and for real-time probing of the ICT in this case. Microscopic solvation effects on the torsional motions and the ICT in the molecules have been examined by the use of their clusters with polar solvents. One of the most important problems which awaits for further studies is an ambiguous description of the electronic character of the charge-separated states in the systems. So far, high-level quantum-mechanical calculations have not been able to deal with such large molecular systems, but reliable evaluation of electronically... 

Some references of reviews besides the ones already cited are given [1,3, 5-9, 19, 23-25, 28, 31, 33]. Organometallic photochemistry [36] was excellently treated in [37] and may be compared with inorganic photochemistry to gain further inspiration [38-40]. A recent multiauthored book strongly overlaps with the subject matter of the present section, and should certainly be consulted [41]. Electron transfer reactions play a determinant role in many photocatalytic processes several recent reviews and books may be cited on this topic [42-44]. The photochemistry of the M-CO bond [45] and the theme of photocatalysis by transition metal complexes [46] have recently been reviewed. Covalently linked donor-acceptor systems for mimicry of photosynthetic energy transfer have been discussed in [47]. Several special issues of Coordination Chemistry Reviews have been devoted to the photochemistry and photophysics of coordination compounds [48-50], and a special issue to photochemistry [51]. Further developments in photochemistry were the subject of a special issue of Chemical Reviews [52]. Practical considerations useful for designing photochemical experiments may be found in [53]. 

Tetrahedron Symposia in Print No. 39, 1989, 45, special issue devoted to covalently linked donor-acceptor species for mimicry of photosynthetic electron and energy transfer. 

Fullerenes, especially Qo, have been used as electron acceptors in covalently-linked donor-acceptor dyads and triads (see Part IV on Artificial Photosynthesis). A critical comparison has been made between the rates of... 

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