Excimers and exciplexes

Since in supramolecular systems electron-and energy transfer processes are no longer limited by diffusion, they take place by first order kinetics and in suitably designed supramolecular systems they can involve even very short lived excited states. The reactions described by (2.13-2.15) correspond to the key step (first order rate constant kg. Fig. 2.1) occurring in the analogous bimolecular reactions (2.1-2.3) taking place in the encounters formed by diffusion. The parameters affecting the rates of such unimolecular reactions will be discussed in Sects. 2.3 and 2.5. 

In most cases, the interaction between excited and ground state components in a supramolecular system, and even more so in an encounter, is weak. When the interaction is strong, new chemical species, which are called excimers (from excited dimers) or exciplexes (from excited complexes), depending on whether the two interacting units have the same or different chemical nature. The scheme shown in Fig. 2.3 refers to a supramolecular system, but it holds true also for species in an encounter complex. It is important to notice that excimer and exciplex formation are reversible processes and that both excimers and exciplexes sometimes can give luminescence. Compared with the monomer emission, the 

Excimers are usually obtained when an excited state of an aromatic molecule interacts with the ground state of a molecule of the same type. For example, between excited and ground state of anthracene units. Exciplexes are obtained when an electron donor (acceptor) excited state interacts with an electron acceptor (donor) ground state molecule, for example, between excited states of aromatic molecules (electron acceptors) and amines (electron donors). 

An excimer is a dimer that is formed between an excited state and a ground state molecule  

An exciplex is a new excited state complex that is formed between a molecule of the excited state M and a molecule of the quencher Q  

Excimer formation is only to be expected when the concentration of the complex is high, and its excited state lifetime is long. Both excimers and exciplexes are new electronically excited species that have their own individual electronic and geometrical structures, vibrational energy levels, and excited state reactivities. Both excimers and exciplexes are also expected to have their own characteristic fluorescent and phosphorescent properties, which will be different from those of the 

Because of the shorter excited state lifetimes of metal complexes as compared to organic molecules, the observation of excimers and exciplexes is much less common for complexes. Nevertheless, a number of cases exist where a spectral shift is observed upon the addition of a quencher that can bind with the excited state of the metal complex. 

Measurements of excimer or exciplex fluorescence can be used to determine whether a pair of macromolecules or two regions of a macromolecule are able to come in close contact during the lifetime of the excited state. Derivatives of pyrene that can be attached to various functional groups in proteins, lipids or polysaccharides lend themselves well to such studies [57-61]. In one application, the A-terminus of the EcoRl restriction endonuclease was labeled with N-(l-pyrenyl)iodoacetamide [62]. A broad excimer emission band at 480 nm indicated that the A-termini of two molecules come into close proximity when the protein dimerizes. The A-termini are essential for enzymatic activity but are too disordered to be seen in a crystal structure of the protein. 

Consider a molecule that has an absorption band at 700 nm with a dipole strength of 25 debye. Suppose two such molecules form a co-planar complex with a center-to-center distance of 8 A as sketched below. The arrows represent intramolecular vectors that are aligned with the transition dipoles. 

Calculate the dipole-dipole interaction energy Hab) in the point-dipole approximation for a medium with index of refraction = 1.2. Specify the sign and units of your answer. Save your result for use in Chap. 10. 

What are the excitation energies for the dimers two excited states  

Calculate the dipole strengths of the two exciton absorption bands of the dimer. (Assume that each monomeric molecule has only one excited state.) 

Time-resolved measurements of pyrene fluorescence show that only the structured monomer emission is observed immediately after excitation, because the pyrene molecules are not associated in the ground state. The broad emission then grows in as the excimers are formed by diffusional encounter and the equilibrium between monomers and excimers is reached. 

AexcG = AexcH—TAexcS 0 requires AexcH —30 kJ mol 1 (T = 298 K) Equation 2.49 Thermochemistry of excimer formation 

The S0 Si transition of pyrene is fairly weak, log( r/M 1 cm ) 2 (see Section 4.7), so that only a small exciton stabilization would be predicted by the simple treatment outlined above. In this case, higher excited states and also charge transfer states of the type 

Quenching is a radiationless process involving two molecules. There are also radiative processes involving two molecules. These are necessarily cooperative processes involving non-covalent molecular complexes. If two molecules act cooperatively to 6sor6 a photon, an absorption complex is involved. If two molecules act together to emit a photon, an exciplex (electronically excited complex) is involved. The particular case of an exciplex in which the two molecules are the same is termed an excimer (electronically excited dimer). 

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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. 

Excimers and exciplexes are formed in the excited states. Excimers are complexes of excited 1M and unexcited 1M molecules in the excited state ... 

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 )... 

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). 

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. 

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. 

Jenekhe SA, Osaheni JA (1994) Excimers and exciplexes of conjugated polymers. Science 265 765... 

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. 

Figure 11.3 The excited state of a chromophore such as pyrene can form a complex with a ground state molecule. If the result is an excited state dimer the complex is known as an excimer, while if the excited complex is formed between two different molecule it is termed and exciplex. Excimers and exciplexes emit at lower energy than the corresponding monomers.

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],... 

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

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.

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