Fluorescence intramolecular energy transfer

Hossain, M. A., Mihara, H. and Ueno, A. (2003). Novel peptides bearing pyrene and coumarin units with or without beta-cyclodextrin in their side chains exhibit intramolecular fluorescence resonance energy transfer. J. Am. Chem. Soc. 125, 11178-11179. 

Co-free PAE). In PAE-CoCpl, the fluorescence quantum yield is only 18% of that observed for Co-free PAE, even though the quencher substitutes less than 0.1% of the aryleneethynylene units. The fluorescence in solution disappeared in PAE-CoCp4, where every fifth unit is a cyclobutadiene complex. The mechanism by which this quenching occurs is via the cobalt-centered MLCT states [82,83], conferred onto the polymer by the presence of cyclobutadiene complexes. Even in the solid state the polymers PAE-CoCpl-2 are nonemissive. It was therefore shown that incorporation of CpCo-stabilized cyclobutadiene complexes into PPEs even in small amounts leads to an efficient quenching of fluorescence in solution and in the solid state. Quenching occurs by inter- and intramolecular energy transfer [84]. 

Most interesting applications of intramolecular energy transfer between nonconjugated chromophores are found in the conformational studies of biomolecules like nucleic acids and proteins. The experiments on rotational depolarization of emission from intrinsic fluorescent groups on externally attached fluorescent probes, have resulted in a vast store of knowledge which has helped to enrich the subject of photobiology. 

Salicylates.—In the course of his intramolecular energy transfer studies (p. 130) on europium chelates Weissman 630 prepared the 3-nitro and 5-nitro salicylate complexes. He was one of the first to demonstrate the phenomena of intramolecular energy transfer (IMET) from the coordinated ligands to the central rare earth ions giving characteristic fluorescence of the metal ions. However, no analytical data on these compounds are available. 

In most of the europium complexes the fluorescence is due to the last effect. The intramolecular energy transfer within europium and terbium complexes was first observed by Weissman [630] in 1942, and subsequently investigated by many other workers. 

The efficiency of intramolecular energy transfer has been studied in certain molecules which consist of two independent systems of conjugated double bonds separated by up to three saturated carbon atoms.4882,94 Only the lowest states of any multiplicity were found to emit. Thus although the absorption spectrum of such a species consists of a superposition of characteristic absorption due to the separate conjugated groups, fluorescence and phosphorescence from only one group are observed even when the other group is excited. Further studies... 

We have seen earlier that Eu3+ possess two resonance levels, 5Z>o and 5 >i, from which fluorescence transitions to the J manifold of 1F takes place. The Do - 1Fq transition is strictly forbidden for regular octahedral symmetry but is observed in some complexes due to the lack of centro-symmetry. The intensity of the fluorescence transition is not directly dependent only on the amount of T 4f energy transfer, but mainly on the transition probabilities from a particular resonance level to the various J manifolds. However, the transition probabilities are sensitive functions depending on the ligand. A schematic representation of the intramolecular energy transfer in a Eu3+ complex is given in Fig. 23. 

Fluorescence resonance energy transfer (FRET) is a technique that has been used to measure distances between pairs of proximal fluorochromes. A suitable pair consists of a donor fluorochrome, which has an emission spectrum that significantly overlaps with the absorption spectrum of an acceptor fluorochrome (2). With the availability of monoclonal antibodies to many cell-surface determinants, intramolecular distances between nearby epitopes and intermolecular distances between adjacent cell-surface macromolecules can be investigated to analyze molecular interactions influencing important cellular events. Such monoclonal antibodies can be conjugated to fluorescein-isothiocyanate (FITC) as the donor, and either tetramethyl-rhodamine-isothiocyanate (TRITC) or phycoerythrin (PE) as the acceptor. 

Fig. 15.3 (a) Intramolecular energy transfer luminescence of Eu(III) complexed with a Wang-decapeptide-Lariat ether library member (b) metal ion allosteric interactions with the peptide chain induces a molecular reorganization followed by europium fluorescence enhancement, (from Rivero et al.,34 with permission. Copyright 2006. Bentham Science Publishers)... 

Intramolecular excimer emission, the polarization of fluorescence, nonradiative energy transfer and the use of medium-sensitive fluoropliores has been used to study the conformational mobility of polymers in dilute solution, the interpenetration of drain molecules, the association of polymers with each other or with small species and the cooperative transition of certain poly-carboxylic acids from a compact to an expanded state. 

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