Bichromophoric molecules

The synthetic applications 440) and mechanistic aspects 4411 of intermolecular photocycloaddition reactions of arenes to olefins have been reviewed recently. Intramolecular cycloadditions442a,b) have been studied in the context of the photochemical behaviour of bichromophoric molecules, as to investigate interchromophoric interactions in polyfunctional molecules. Three types of addition products can be formed in the photocycloaddition of benzene to an alkene (4.37)441. ... 

De Schryver F., Collart P., Vandendriessche J., Goedeweeck R., Swinnen A. and Van der Auweraer M. (1987) Intramolecular Excimer Formation in Bichromophoric Molecules Linked by a Short Flexible Chain, Acc. Chem. Res. 20, 159-166. 

Valeur B. (1989) Intramolecular Excitation Energy Transfer in Bichromophoric Molecules, in Jameson D. and Reinhart G. D. (Eds), Fluorescent Biomolecules,... 

B. Valeur, J. Pouget, and J. Bourson, Photoinduced electronic energy transfer in a supramolecular complex between a coumarin bichromophoric molecule and lead(II), 7. Lumin. 52, 345-347 (1992). 

Other examples of determination of the absolute configuration of bichromophoric molecules having two or more rotamers in the equilibrium include naphthalene/A -vinyl carbamate in a 1,2,3,4-tetrahydropyridine derivative123, naphthalenc/purine in deoxyadenosine adducts124-126, phenanthrene/purine in deoxyadenosine and deoxyguanosine adducts127 and pyrene/purine in adenosine adducts128. 

The occurrence of TICT states was regarded for a long time as a very special property of a few unusual molecules. It seems now to be a much more general property common to many bichromophoric molecules. 

Figure 4.12 Examples of bichromophoric molecules used for the study of intramolecular electron transfer, (a) Dimethoxynaphthalene electron donors dicyanoethylene electron acceptor with rigid spacers (b) porphyrin donor and quinone acceptors separated by flexible spacers...

Typical intramolecular (2 + 2) photocycloaddition of electron-rich alkenes to the 1-cyanonaphthalene ring via an exciplex has been reported by McCullough and Gilbert, independently. McCullough reported intramolecular photocycloaddition of bichromophoric molecules 255,258, and 259, in which the chromophores are linked by a three-atom ether chain [286,287] (Scheme 71). The fluorescence of the 1-cyanonaphthalene chromophore of 255,258, and 259 is efficiently quenched by alkenes and the typical intramolecular exciplex emissions are observed. Photocycloaddition of these 1-cyanonaphthalenes in benzene occurs at 1,2-, 3,4-, or 5,6-positions of the naphthalene ring, respectively. 

Intramolecular Photocycloaddition of N—P4—A. Although the photocycloaddition of anthracene [85-100] and that of naphthalene [65-67,80-82] have been extensively studied, until recently relatively little has been reported on the cross-photocycloaddition between an anthracene and a naphthalene moiety [101-110], The main reason for this is the large difference in the quantum yields between the photocycloaddition of anthracene and the cross-photocycloaddition of anthracene and naphthalene. Thus, bichromophoric molecules with anthryl as one chromophore and naphthyl as the other generally undergo intermolecular an-thryl-anthryl cycloaddition rather than intramolecular cross-cycloaddition when irradiated. 

On the other hand, azobenzene can quench the fluorescence of other moleetdes. This has been investigated in molecules containing both a fluorescing and an azobenzene unit. It was found that the E-form is about 3 times, or even up to 13 times, as effective a quencher as the Z-form. The influence of the environment on such bichromophoric molecules was studied by Eisenbach et al. ... 

The lack of energy transfer in 24 is in marked contrast to the results for a variety of other bichromophoric molecules where singlet energy transfer occurs over many tens of angstroms via the Forster dipole-dipole mechanism. Since the effidency of Forster energy transfer depends upon the fluorescence quantum yield of the donor, we postulated that the lack of energy transfer in 24 was due to the very low fluorescence quantum yield of the carotenoid, and further concluded that energy transfer from carotenoid polyenes to chlorophyll in photosynthetic reaction centers could therefore not occur by the dipole-dipole mechanism [72]. 

The selection of systems reviewed clearly Illustrates the synthetic potential of bichromophoric molecules. It further emphasizes the role played by excitation delocalization and its impact on the choice of deactivation pathways available for the molecule. Systematic studies give an Insight into the dynamics of the linking chain, the importance of ground state interaction, and the role played by excited state complexes. 

In order to mimic electronic energy transfer from the light-harvesting complex to the special pair of the reaction centre complex, a variety of bichromophoric molecules have been synthesized in which terminal donor and acceptor units are separated by a transparent spacer group. Provided the geometry of the dyad is known, especially the separation distance and mutual orientation, it becomes possible to consider the rates of intramolecular energy transfer in terms of... 

Nakabayashi, T., Wu, B., Morikawa, T., Iimori, T., Rubin, M.B., Speiser, S. and Ohta, N. (2006) External electric field effects on absorption and fluorescence of anfhracene-(CH2)n-naphthalene bichromophoric molecules doped in a polymer film. /. Photochem. Pholobiol. A, 178, 236-241. 

Intramolecular electronic energy transfer in bichromophoric molecules consisting of two coumarins linked by a variable number... 

Frequency domain fluorometry has been used to study end to end diffusion of flexible bichromophoric molecules by intramolecular energy transfer o - . ... 

Pyrene photophysics has produced the usual, and now to be expected, crop of papers on diverse topics. Aggregation in concentrated solutions has been evidenced by the Shpol ski effect and two photon excitation spectra have yielded new electronic state assignments. The maximum entropy method, mentioned earlier, for the determination of fluorescence lifetimes shows that in dipyrenylpropane the data for luminescence decay do not fit a simple 3-state model.This study adds more information upon a system hitherto open to much dispute. It is unlikely that the problem will be considered as solved. The presence of a ground state dimer of pyrene moieties has been shown by NMR in the bichromophoric molecules of (pyrenylcarboxyl) alkanesand also with racemic and meso dipyrenyl alkanes. Strong circular polarization in excimer emission has been detected from pairs of pyrene groups linked to a polypeptide chain. ... 

Wegewijs, B. Hermant, R.M. Verhoeven, J.W. Kunst, A.G.M. Rettschnick, R.P.H. Determination of the barrier to intramolecular exciplex formation in a jet-cooled, bichromophoric molecule. Chem. Phys. Letters 1987,140, 587-590. 

Fig. 1 The bichromophoric molecules used for studies of intramolecular electronic energy transfer.

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