Excimer and Exciplex Emission

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. 

Kalinowski J., Cocchi M., Virgili D., Fattoii V., and Gareth Williams J. A., Mixing of Excimer and Exciplex Emission A New Way to Improve White Light Emitting Organic Electrophosphorescent Diodes, Advanced Materials 19, 22, (November, 2007), 4000-4005... 

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 )... 

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.

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-... 

Exciplex and Excimer Formation. The formation of excited complexes (excimers and exciplexes) in quenching reactions of excited organic compounds is a well-known phenomenon (cf. 70,230, 231,249). Evidence for the intermediacy of exciplexes is most readily obtained when exciplex emission is observed. However, other techniques. Including flash absorption spectroscopy (250-252) and kinetic analysis of both steady-state and dynamic quenching rate data (253) have also been used to obtain evidence for the existence of these transients. 

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. 

In the case of exciplexes, where the aromatic chromophores are dissimilar, we might expect an increased red shift in the ground state absorption due to a greater charge transfer character to the transitions. It seems plausible then, that excimers and exciplexes account for the 350 nm excitation maximum for humic substance fluorescence, since they match the character of the fluorescence emission and may account for the observed excitation wavelengths. 

In some cases, long-wavelength emissions in electroluminescence occur that are not observed in the photoluminescence spectra. These long-wavelength emissions are postulated to arise from electromers and electroplexes, respectively. The mechanism of the latter type of emission is by phosphorescence or by direct radiative recombination of holes and electrons attached at two neighboring molecules [82]. The terms electromers and electroplexes are complete analogies to excimers and exciplexes. 

Since excimers and exciplexes give rise to distinct emission bands from monomeric fluorophores, the conformational changes of a given sensor bearing two identical... 

Excimer or exciplex emission is generally characterized by broad featureless emission red-shifted from the singlet emission of the molecule. Also, the emission wavelength and intensity change with solvent polarity. Emission from the monomer and the excimer are very often observed in the same spectrum. Although excimers and exciplexes can be formed via photo-excitation as observed in fluorescence spectroscopy, they are most likely to form in ECL due to the close proximity of the radical ions in the contact radical ion pair (122). 

All non-radiative electronic processes are isoenergetic transitions to another electronic state, which may or may not be the ground state. This other state may be localised on the same molecule, or on molecules produced by unimolecular chemical reaction from the excited-state or the process may involve interaction with states on another system which acts as a quencher of the excited-state, e.g. by energy transfer or bimolecular reaction. (The resultant electronic state(s) may themselves deactivate non-radiatively or radiatively, e.g. the phosphorescence from a triplet state formed from a higher singlet, emission from excimers and exciplexes formed from ground-state excited state interaction (see Sect 1.13.5.5), or emission from quencher states produced by energy transfer or chemical reaction.)... 

Excited states and, to a lesser extent, the ground state may be sensitive to changes in environmental parameters such as temperature, pressure, viscosity and polarity. This manifests itself in a number of quantifiable parameters, such as (i) spectral shifts (ii) change in the emission quanmm yield (iii) change in the nature of the emissive state (iv) change in non-radiative and radiative decay rates (v) variations in the intensity and resolution of vibronic fine structure and (vi) the emergence of excimer or exciplex emission. These response mechanisms can be used to design sensor platforms to monitor both microscopic and macroscopic environmental parameters. 

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. 

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