Fluorescence excimers/exciplexes

D. Decay Characteristics of Molecular and Excimer (Exciplex) Fluorescence 178... 

In the absence of the reverse absorption the radiative transition probability fquantum yield of fluorescence qmC) and the decay constant l/r (C)= 2 [Pg.200]

Lifetimes t°(C are available from analyses of fluorescence decay curves as described in Section II.D where, to a good approximation, 1/t (C is given as the experimental parameter Ax describing terminal decay for a system exhibiting excimer (exciplex) fluorescence only. 

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... 

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. 

Rate constants are obtained from fluorescence decay analyses of the monomer decay profile I (t) and the excimer/exciplex profile Ig(t). These are fit to sums and differences of two exponential... 

Several studies on CD complexes with aromatic molecules using steady-state and nanosecond spectroscopy have been reported. These studies aimed to understand the photophysical and photochemical behavior of organic guests such as fluorescence and phosphorescence enhancement, excimer/exciplex formation, photocleavage, charge and proton transfer, energy hopping, and cis-trans photo-... 

A spectral shift due to interconversion between monomer and excimer/exciplex emission can be exploited to develop temperature sensors for the macroscopic environment. For example, a thermochromic sensor film was developed by immobilising perylene in a polystyrene film with iV-aUyl-iV-methylaniline (NA) [41]. In solution and in the absence of NA, perylene emits blue fluorescence — 475 nm). In thin films and in the presence of NA, an additional broad red emission band is observed (Aem = 551 nm), which is attributed to the perylene-NA exciplex. The fluorescence spectrum is temperature dependent on heating between 25 and 85 °C the relative intensity of the blue monomer emission increases at the expense of the exciplex emission band, indicating that at higher temperatures the monomer-exciplex equilibrium is shifted in favour of the monomer. A wavelength ratiometric approach based on the relative intensities of the two emission peaks as a function of temperature was used to calibrate the sensor film [41]. 

In this chapter, we have described the fundamental parameters that should be obtained when characterising an electronic, singlet or triplet, excited state and how to determine them experimentally including methodologies and required equipment. These characteristics include electronic energy, quantum yields, lifetimes and number and type of species in the excited state. Within this last context, i.e., when excited state reactions give rise to additional species in the excited state we have explored several excited state kinetic schemes, found to be present when excimers, exciplexes are formed and (intra and intermolecular) proton transfer occurs. This includes a complete formalism (with equations) for the steady-state and dynamic approaches for two and three-state systems, from where all the rate constants can be obtained. Additionally, we have explored additional recent developments in photophysics the competition between vibrational relaxation and photochemistry, and the non-discrete analysis (stretched-exponential) of fluorescence decays. 

In principle, an equation relating the emission intensity (of fluorescence, excimer or exciplex) to equilibrium constants and concentrations of G and C (and S for GSC2 complex formation) can be derived from dehnitions of the equilibrium constants and mass balances. The equilibrium constants can be obtained by numerical htting of the experimental data to the equation [12]. However, as the upper limit of fluorophore concentration for fluorescence measurement is low, the evaluation of the equilibrium constants involved in the formation of these complexes by using only fluorescence technique is often difficult. In most cases, the concentration dependences of other spectroscopic properties (e.g. absorption, circular dichroism, NMR) of the guest in the presence and in the absence of CD are used as complementary techniques. Detailed description of the methods of analysis can be found in Refs. [25-29]. 

The best evidence for a charge-transfer exciplex (hetero excimer) has been provided by Thomaz and Stevens.<148,149) They note a reduction in fluorescence yield of pyrene with increasing heavy-atom concentration and proposed the following set of reactions to explain their results ... 

A and D are the exciplex or excimer components, denotes the primarily excited species, k is the limiting photoassociation equilibrium constant, AHat ASa, and are the thermodynamic parameters for the exciplex-excimer, and p is the excited state dipole moment of the complex. Note that the large dipole moment for the exciplex indicates almost complete charge transfer in the excited state, (D+, A-). rfc and r, are the fluorescence lifetimes for the complex and the component. 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

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... 

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). 

This chapter describes the characteristics of the fluorescence emission of an excited molecule in solution. We do not consider here the photophysical processes involving interactions with other molecules (electron transfer, proton transfer, energy transfer, excimer or exciplex formation, etc.). These processes will be examined in Chapter 4. 

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. 

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. 

An example of exciplex formation in the solid state may be afforded by perylene doped crystals of pyrene which emit a green structureless fluorescence in addition to the blue and orange-red excimer bands of pyrene and perylene, respectively. Hochstrasser112 has shown that the energy of the emitting species is consistent with that of a charge transfer complex of pyrene and perylene molecules in a bimolecular unit of the pyrene lattice. 

A number of excimers, notably adjacent C-C and A-A, and exciplexes consisting of adjacent A-C and A-T, have been identified at 78 K [42]. Only A-T emits at room temperature [43], however, the emission being seen from poly(A T)-poly(A T). Significantly, the lifetime of this fluorescence at room temperature is 7 ns [44]. Of course, this is the radiative Hfetime, thus a lower limit on the possible lifetime of the excitation. Thus, despite the vigorous thermal motion, this excitation, which involves a distortion quite similar to that of a polaron, lasts at least nanoseconds. 

These lasers are also called—incorrectly— excimer lasers. It will be clear that they could be called exciplex lasers. The active material is a gas mixture which contains a halogen (F2 or Cl2 in most cases) and a rare gas such as Kr, Ar or Xe. These cannot form any stable compounds in their ground states, but excited state species do exist and can fluoresce. These excited state species e.g. KrF) are formed through the recombination of ions, for instance... 

The photodimerization of t-1 via a nonfluorescent singlet excimer is analogous to the behavior of anthracene (41,42). A possible explanation for the absence of excimer fluorescence is provided by the high limiting quantum yield for photodimeriza- tion (0.77 0.12) obtained from the intercept of a plot of versus [t-l] l according to eq. 9 (40). Excimer fluorescence is, in general, a slow process (< 1 x 10" s l), which evidently does not compete with cycloaddition in the exciplexes of t-1 or anthracene (42). 

Some properties of the t -dimethylhexadiene exciplex are summarized in Table 7. Its fluorescence maximum is at slightly shorter wavelength than that of the anthracene-dimethylhexadiene exciplex (435 nm) (51). While data on other unsubstituted arene-diene exciplexes are not available, t appears to be more reactive and to form more stable exciplexes with dienes than arenes of comparable electron affinity (101). The dipole moment of the - -t -dimethylhexadiene exciplex is estimated to be 7 D from the solvent dependence of its fluorescence maxima (36). This value is substantially lower than those for pure charge-transfer exciplexes (p > 15 D) and indicates that this exciplex is relatively nonpolar and might be better categorized as a hetero-excimer, than as an exciplex (83). That is, using the normal resonance description of an exciplex... 

As early as 1964, Corey had suggested the intermediacy of an oriented pi complex in the cycloaddition of enones (43) and soon thereafter Hammond and coworkers, on the basis of arene fluorescence quenching by dienes, suggested the possible involvement of a polar excited state complex with substantial charge transfer character (44). Since then the possibility of cycloadditions occurring through the intervention of exciplex or excimer intermediates per se or as precursors for radical ions pairs, eq. 12,... 

Steady-state fluorescence spectroscopy has also been used to study solvation processes in supercritical fluids. For example, Okada et al. (29) and Kajimoto and co-workers (30) studied intramolecular excited-state complexation (exciplex) and charge-transfer formation, respectively, in supercritical CHF3. In the latter studies, the observed spectral shift was more than expected based on the McRae theory (56,57), this was attributed to cluster formation. In other studies, Brennecke and Eckert (5,31,44,45) examined the fluorescence of pyrene in supercritical CO2, C2HSteady-state emission spectra were used to show density augmentation near the critical point. Additional studies investigated the formation of the pyrene excimer (i.e., the reaction of excited- and ground-state pyrene monomers to form the excited-state dimer). These authors concluded that the observance of the pyrene excimer in the supercritical fluid medium was a consequence of increased solute-solute interactions. 

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