Excimer intermolecular

Keywords Excimers Intermolecular interactions Light-harvesting Red-edge effects Resonance energy transfer Superquenching Wavelength-shifting... 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

Retna Raj C, Ramaraj R (1997) Cyclodextrin induced intermolecular excimer formation of thioflavin T. Chem Phys Lett 273(3 1) 285-290... 

A short excursion into the physics and spectroscopy of intermolecular interactions is intended to illustrate the effects of fluorescence spectra change on the transition of dye molecules from liquid solvents to solid environments, on the change of polarity and hydration in these environments, and on the formation of excited-state complexes (excimers and exciplexes). 

Methods based on intermolecular quenching or intermolecular excimer formation... 

The method using intermolecular excimer formation is based on the same principle because this process is also diffusion-controlled. Excimers should, of course, be formed during the monomer excited-state lifetime. In Section 4.4.1, it was shown that the ratio Ie/Im of the intensities of the excimer and monomer bands is proportional to ki provided that the transient term can be neglected. When the dissociation rate of the excimer is slow with respect to de-excitation, the relationship is... 

The serious drawback of the methods of evaluation of fluidity based on intermolecular quenching or excimer formation is that the translational diffusion can be perturbed in constrained media. It should be emphasized that, in the case of biological membranes, problems in the estimation of fluidity arise from the presence of proteins and possible additives (e.g. cholesterol). Nevertheless, excimer formation with pyrene or pyrene-labeled phospholipids can provide interesting in-... 

In contrast to intermolecular excimer formation, this process is not translational but requires close approach of the two moieties through internal rotations during the lifetime of the excited state. Information on fluidity is thus obtained without the difficulty of possible perturbation of the diffusion process by microheterogeneity of the medium, as mentioned above for intermolecular excimer formation. 

As in the case of intermolecular excimer formation, it should be recalled that difficulties may arise from the possible temperature dependence of the excimer lifetime, when effects of temperature on fluidity are investigated. It is then recommended that time-resolved fluorescence experiments are performed. The relevant equations established in Chapter 4 (Eqs 4.43-4.47) must be used after replacing ki[M] by k. ... 

The choice of method depends on the system to be investigated. The methods of intermolecular quenching and intermolecular excimer formation are not recommended for probing fluidity of microheterogeneous media because of possible perturbation of the translational diffusion process. The methods of intramolecular excimer formation and molecular rotors are convenient and rapid, but the time-resolved fluorescence polarization technique provides much more detailed information, including the order of an anisotropic medium. 

Vauhkonen M., Sassaroli M., Somerharju P. and Eisinger J. (1990) Dipyrenyl-phosphatidylcholines as Membrane Fluidity Probes. Relationship between Intramolecular and Intermolecular Excimer Formation Rates, Biophys. J. 57, 291-300. 

The effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

For the distyrylbenzene carbon-centered tetramer 46b, the fluorescence spectrum in the solid him differs from the spectra in solution or in a polymer matrix due to excimer formation [93]. A concentration of 5% in a polystyrene matrix is sufficient for a distinct broadening of the emission. For the higher homologue 46c, a fluorescence maximum of 472 nm was measured in freshly prepared films. If the film is thermally annealed, the spectrum shifts to 511 nm, probably due to intermolecular arrangement that favors excimer formation. 

Complex formation is important in photophysics. Two terms need to be described here first, an exciplex, which is an excited state complex formed between two different kinds of molecules, one that is excited and the other that is in its grown state second, an excimer, which is similar to exciplex except that the complex is formed between like molecules. Here, we will focus on excimer complexes that form between two like polymer chains or within the same polymer chain. Such complexes are often formed between two aromatic structures. Resonance interactions between aromatic structures, such as two phenyl rings in PS, give a weak intermolecular force formed from attractions between the pi-electrons of the two aromatic entities. Excimers involving such aromatic structures give strong fluorescence. 

Chandross and Ferguson64 find that the absorption spectra of dimers, produced65 by photolytic cleavage of photodimers of anthracene and mono-derivatives in a rigid methylcyclohexane glass at 77°K, are consistent with a symmetrical sandwich configuration these dimers also emit the characteristic excimer fluorescence. On the other hand, it is necessary to assume a 60° rotation of one component about the intermolecular axis of the 9,10-di-chloroanthracene dimer (as in the crystalline compound) to account for the observed resonance splittings of both absorption bands.64... 

Although excimer (exciplex) fluorescence is also exhibited by most dinucleotides,133 the observed phosphorescence from these systems, and from DNA, is characteristic of the lowest molecular triplet state. In the case of DNA at low temperatures this is identified132 as the triplet state of thymine which, in the absence of molecular intersystem crossing, must be populated by intermolecular energy transfer in the triplet manifold or by intersystem crossing from the XAT exciplex.134... 

Excimer formation has been shown to be a diffusion controlled process 41-43 in which a sandwich or face-on configuration of the two interacting molecules is required.44-48 It has been deduced that intermolecular distance in the excimer state is smaller than for the same configuration with both molecules in their electronic ground states.44-48 Apart from pyrene, excimer-like emission has been observed from a wide range of aromatic compounds including many alkyl derivatives of such hydrocarbons43-47 and vinyl polymers.48-80... 

Oyama et al. [23] utilized fluorescence spectroscopy to study how the molar masses of both PAA and PEO affect complexation. Actually, they used the exci-mer formation between the pyrene groups attached to the chain ends of PEO as a molecular probe. The molecular weights of PAA used were 1850, 4600 and 890,000, and those of PEO were 4800 and 9200. The latter are much lower than the PEO molecular weight in the work of Bednar et al. [19]. In order to monitor both intramolecular and infermolecular excimer formation, it was necessary to distinguish the two types of excimers clearly, so Oyama et al. used two kinds of solutions. One contained 99% of untagged PEO and only 1% PEO, where PEO refers to PEO whose chain ends are tagged by pyrene. In this solution, PEO is believed to behave Hke an individual PEO chain, which provides only intramolecular excimer. The other contained fully tagged PEO, which provides both intramolecular and intermolecular excimers. 

The volume change AV associated with intermolecular excimer formation has been determined for naphthalene and various alkyl derivatives through the application of pressure 74). For naphthalene and the two methylnaphthalene isomers, the value of AV = — 16cm3/mole was measured at room temperature. Assuming the sandwich structure for the excimer, and taking the projected area of the naphthalene molecule... 

There is a substantial entropy decrease AS associated with intermolecular excimer formation, as given in the tabulation by Birks 71). For all solvents (except 95% ethanol 75), the value AS ss —20 cal/mole-K was observed for naphthalene and its derivatives. For comparison, the entropy of fusion of unsubstituted aromatic hydrocarbons such as naphthalene falls in the range of —8 to —15 e.u. The large loss of entropy in the intermolecular excimer formation process indicates a very constrained symmetric structure. 

The rate constant kTD for fluorescence of the pyrene intermolecular solution excimer has been found to follow the relation kFD = n2(kFD)n=I, where n is the the refractive index of the solvent69 . The values of kTO for the 1-methylnaphthalene excimer in ethanol at various temperatures are also consistent with the above relation 76). The fact that (kFD)n=I is independent of solvent and temperature indicates that the excimer has a specific structure, according to Birks 69,71). Experimentally, it was observed much earlier that kFM = n2(kFM)n=i for the polycyclic aromatic hydrocarbons, and that k /kp is independent of solvent and temperature. Table 5 shows that agreement between independent investigators of the excimers of naphthalene compounds is not always good, as in the case of 1-methylnaphthalene. 

In summary, all available evidence suggests that the intermolecular excimers of naphthalene compounds have a sandwich structure in which the ring planes are parallel and the molecular axes are aligned. While the intermolecular excimer appears to adopt the eclipsed sandwich structure in solution, there may be differences in the structure of excimers constrained by hydrocarbon links or by rigid matrices. These constrained excimers will be considered next. 

Excimer fluorescence has been observed in a variety of systems in which intermolecular diffusion does not play a role in excimer formation. Five such systems involving the naphthyl chromophore will be discussed (1) Crystals of aromatic hydrocarbons ... 

Phane compounds, i.e. two chromophores held face-to-face by at least two hydrocarbon links (4) Sandwich dimers, which are chromophore pairs produced by photolysis of photodimers in rigid matrices and (5) Bichromophoric compounds having a single saturated hydrocarbon linkage, which form intramolecular excimers as allowed by the rotational isomers of the linkage. In each case, we will utilize the intermolecular excimer formed in solution as the standard against which the properties of constrained excimers will be measured. 

Despite the technical problems in the latter film study, we conclude that there is no intramolecular excimer formation in the compounds of Richards et al.143, and probably little intermolecular excimer formation in the pure films. The absence of an effect of solvent power 25) on the possible excimer fluorescence of the R = CH3 polymer may not be significant, since little change in the coil dimensions would be expected for the short ( 300 backbone atoms) polymers 143> which were studied. Additional work is needed on the fluorescence of such polymers having higher molecular weights, different aryl substituents (R = 2-naphthyl, for example), and fewer adventitious impurities. 

The pure-film spectra of the alternating copolymers have been reported only for P(S-fl/t-MMA) and P(2VN-a//-MMA)5S>. For the former polymer, a fluorescence band similar to toluene was observed, distinguished only by a slight broadening at X > 290 nm. The P(2VN-a//-MMA) film spectrum was described as having a maximum at 365 nm, with appreciable fluorescence intensity at 425 nm. This emission, which could not be positively identified as excimer fluorescence, was attributed to an impurity by Fox et al.55). If the results for P(S-a//-MMA) are representative of all the copolymer film spectra, they indicate that very few intermolecular EFS are formed. 

In both intra- and intermolecular excimers, the aromatic rings are placed in a fully-eclipsed, parallel sandwich arrangement. This is true only for PS and P2VN exceptions have been noted for PVK and P1VN. 

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