The Excimers and Exciplexes

The excited state dimer of a compound is called excimer. For example, a high concentrated solution of pyrene in toluene is irradiated with UV light, an excimer is formed. 

Exciplex is an excited complex formed by the reaction of an excited molecule of a compound with a quencher molecule. For example, when a solution of anthracene (A) in presence of diethylamine is irradiated with visible light, an exciplex is formed. Ground-state N,N-diethyl aniline acts as a quencher (0. 

---

In these solutions, K-(A -X)/(Ai-X2), F-K/(Ai-A2), A-(Ai X)/(X-Y), X-ki+k2+k3, Y-k4+k5, and Aj and A2 are given by Eqn. 16. For several well known (30) limiting cases, A3 and A2 are equivalent to and r2, the lifetimes of the pyrene singlet state and excited state complexes, respectively (see Eqns. 9-11). Activation parameters for pyrene excimer formation were calculated by two Independent methods. Since kx+k2 is known to be virtually temperature independent and k4 and ky are negligible(31), the ratios of fluorescent intensity maxima from the pyrene excimer and monomer maxima (Ig/Itl) the inverse of temperature yield the activation energy for pyrene excimer formation, E3. A similar experiment for the pyrene-CA system was not possible since its exciplex is not emissive. Activation parameters for the excimer and exciplex were also obtained from temperature and phase dependent pyrene fluorescent lifetime data. In the 1iquid-crystalline and isotropic phases of M, all pyrene decays were single exponential and the excimer decays could be expressed as the difference between two exponentials. 

The rare gas excimer lasers are based on bound-continuum transitions from an excited diatomic species to its dissociative ground state. The observed continuum emission is a superposition of the Franck-Condon factors from the vibrational levels of the upper state. Thus these molecular dissociation lasers display relatively broad fluorescence as a consequence of the steeply repulsive ground-state potential, and there is always a population inversion on such transitions. However, the net gain is significantly lower than that for a bound-bound transition because of the distribution of oscillator strength over the broad fluorescence band. Figure 1 illustrates schematic potential energy curves for such transitions in the excimer and exciplex lasers. 

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. 

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.

In addition to the excited-state chemical reactions and the radiative and radiationless relaxations, the energy of the excited state can also be degraded by bimolecular processes. One of such processes is the formation of exciplexes (A X) or excimers (A = X). In these processes, the energy is degraded when the excimer or exciplex decays via radiative or radiationless processes (Equation 6.80).38 10... 

Oxidized and reduced species can be produced when the excimer or exciplex is formed in a polar medium. The excited state-excited state annihilation reaction is another bimolecular process transforming the excited-state energy. An excited state of a higher energy, A in Equation 6.81, or charge separation (Equation 6.82) can be produced in the annihilation process. 

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.

---