Formation of excimers and exciplexes

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

The formation of excimers and exciplexes are diffusion-controlled processes. The photophysical effects are thus detected at relatively high concentrations of the species so that a sufficient number of collisions can occur during the excited-state lifetime. Temperature and viscosity are of course important parameters. 

The formation of excimers and exciplexes by reaction of radical ions generated electrochemically has been the subject of much research (Bard and... 

There is ample evidence that the formation of excimers and exciplexes in covalently linked, a, co-diarylalkanes is optimum for a chain length comprising three methylene groups, since this configuration allows a face-to-face sandwich-type structure to be achieved without imposing severe entropic penalties [58, 59], A two-carbon atom chain is rather too short for optimal exciplex formation, but it facilitates TB mediated ET from an aW-trans conformation, as shown in Figure 16b. 

An explanation of the observed properties may perhaps be found in the literature on photophysics of synthetic polymers ( ). A common process in the photophysics of synthetic polymers (in solution and in the solid phase) which contain aromatic substituents is the formation of excimers and exciplexes, achieved by the face to face alignment of aromatic ring structures, with a typical spacing of 4 S. The fluorescence of excimers is typically broadband, featureless and... 

In addition to the formation of singlet and triplet excited states, ion annihilation reactions can lead to the direct formation of excimers (excited dimers) and exciplexes (excited complexes). In most cases, the participating molecules must be able to align so that there is significant 7t-orbital overlap thus this occurs mostly among planar PAHs such as pyrene and perylene (118-120). Other reactions such as TTAprocess can also lead to the formation of excimers and/or exciplexes (121). The reactions associated with the formation of excimers and exciplexes are said to follow the E-route. The relevant reactions are summarized in Scheme 13.3. 

Fig. 4. Schematic (a) representation of excimer and exciplex formation in a dendrimer and (b) energy level diagram showing the three types of emissions that can result.

In emission (fluorescence) spectroscopy two classic examples of density-dependent processes are the phenomena of excimer and exciplex formation and decay, and the imprisonment (trapping) of resonance radiation in atomic (e.g., Hg, rare gases) gases. 

Rawashdeh-Omary, M.A., Omary, M.A., Patterson, H.H. and Fackler, J.P. Jr (2001) Excited-state interactions for [Au(CN)2 ]n and [Ag(CN)2 ]n oligomers in solution. Formation of luminescent gold-gold bonded excimers and exciplexes./oumol of the American Chemical Society, 123, 11237-11247. 

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). 

Rawashdeh-Omary, M. A. Omary, M. A. Patterson, H. H. Fackler, J. P. Excited-State Interactions for [AuJCNjjIn and [AgJCNjj"] Oligomers in Solution. Formation of Luminescent Gold-Gold Bonded Excimers and Exciplexes. J. Am. Chem. Soc. 2001, 123, 11237-11247. 

We will discuss briefly the reactive species such as an exciplex and radical ion species generated by the excitation of organic molecules in the electron-donor (D)-acceptor (A) system. An exciplex is produced usually in nonpolar solvents by an interaction of an electronically excited molecule D (or A ) with a ground-state molecule A (or D). It is often postulated as an important intermediate in the photocycloaddition between D and A. In the case of D = A, an excimer is formed as an excited reactive species to cause photodimerization. In some cases, a ter-molecular interaction of an exciplex with another D or A generates a triplex, which is also a reactive intermediate for photocycloaddition. The evidence for the formation of excimers, exciplexes, and triplexes are shown in the fluorescence quenching. Excimer and exciplex emission is, in some cases, observed and an emission of triplex rarely appears. 

Changes in the electronic and molecular structures after CT excitation can also result in chemical bond formation between the excited complex [AB[ and another molecule (Q) of the system, yielding an encounter complex AB-Q. The complex formed by interaction of an excited molecular entity with a ground state partner of the same structure is called an excimer, whereas an electronically excited dimer of definite stoichiometry, formed by interaction of an excited molecular entity with a ground state partner of a different structure, is named an exciplex [29], Both excimers and exciplexes have their own properties and can thus be regarded as new chemical species. Their generation is well documented by the concentration effect on the fluorescence of some solutes or by flash photolysis measurements [11,24],... 

Quenching Excimers and Exciplexes.—By measurements of decay times and fluorescence anisotropy of pyrene and the excimer in cellulose acetate films it has been found that the medium consists of spaces where small pyrene molecules have considerable freedom, Dissado and Walmsley have developed a complete theory of excimer formation and exciton-induced lattice distortion in crystals. Reference is made to data on 9-cyanoanthracene. The spectroscopy of chemically linked dimers of l,3-(l,l -dinaphthyl)propane in a... 

This chapter will focus on processes leading to the formation of localized excited states, excimers and exciplexes by annihilation of radical ions in solution. The article will not deal with either direct absorption or emission from intreimolecular charge-transfer excited states. In addition, the voluminous literature in this area prohibits any attempt to comprehensively cover published work in the space avail-... 

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