Excimer Formation and Decay

A series of papers from Cohen and co-workers165 has appeared in which absorption and emission from crystalline pyrene are reported. It is argued that excitation is localized at sites containing pairs of close-spaced molecules (Frenkel excitons). By taking into account anharmonicity (vibron-vibron interactions), it is possible to explain why, in ground-state-to-excimer absorption in pyrene, 

Avouris, J. Kordas, and M. A. El-Bayoumi, Chem. Phys. Letters, 1974, 26, 373. 

The modulation technique mentioned above has been used to identify triplet excimers in 1,2-benzanthracene and 1,2 3,4-dibenzanthracene at high solute concentrations167 and the differences between luminescence from naphthalene in fluid solution in the temperature range 353—173 and naphthalene in a rigid solution at 77 have been ascribed to phosphorescence from a triplet excimer.168 Excimer formation in solid poly-(2-vinylnaphthalene) and polystyrene is found to be dependent on the temperature at which the film is cast, and a statistical model based on the rotational isomeric state approximation has been used to formulate an expression for the fraction of excimer sites in the solid systems.168 Kinetic equations for dimer formation and decay, based on the statistical mechanics of ideal gases, have been obtained. These equations, derived from the N-atom von Neumann equation, take into account both bimolecular and termolecular equations.157 158 160 

The emission properties of some carbazole double molecules [l,n-bis(JV-carbazoyl)alkanes] have been examined and one compound in particular, 1,3-bis-(N-carbazoyl)propane (1,3-BCP), was found to be a useful model for poly-(/V-vinylcarbazole). Measurements of the temperature dependence of monomer and excimer decay constants have provided useful kinetic and thermodynamic information on this system. The binding energy for the intramolecular excimer of 1,3-BCP was shown to be 2.76 kcal mol-1, a rather low value. Measurements on other carbazole double molecules showed that Hirayama s n = 3 rule is obeyed and that the preferred geometry of the intramolecular excimer is sandwichlike.161 

The quenching of the photodimerization of coumarin by tetramethylethylene may be the result of quenching the excimer (precurser to cycloaddition).16  

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Fig, 33. Scheme proposed to explain excimer formation and decay in naphthyl methaaylates. (After Holden et al. 

Phillips, D., Roberts, A. J., Soutar, I. Transient decay studies of photophysical processes in aromatic polymers. V Temperature dependence of excimer formation and decay in copolymers of 1-vinyl naphthalene. As yet unpublished 1980... 

Theories dealing with the photophysics of excimer formation and decay involving the "isolated monomer" and "energy migration" concepts have been developed in order to explain the complex fluorescence decay curves observed for polymer systems (5.). Application of these fluorescence decay laws continues to be a topic of interest as will be demonstrated in chapters throughout this book. 

In our early work, it became clear that simple Birks kinetic schemes representing excimer formation and decay were inadequate for modelling fluorescence in excimer forming polymers [17-22]. The empirical observation that multiple (dual and triple) exponential terms could model successfully the decay of monomer and excimer fluorescence in poly(vinyl naphthalenes) led us to propose simple models which... 

Nagle et al. have reviewed the kinetic scheme that underlies excimer formation, as it applies to Pt complexes [19]. A plot of Id/Im (integrated emission intensities of excimer and monomer bands, respectively) versus concentration of the complex should be linear. The slope is determined by all the rate constants of formation and decay of both excimer and monomer, but is approximately related to the equilibrium constant for excimer formation. Typically for Pt complexes that do show excimer emission, the rate constants for formation and subsequent decay of the excimer are substantially larger than monomer decay. The decay of both monomer and excimer are thus governed by the rate of excimer formation, leading to nearly identical decay kinetics for the two species. 

Janssens H, Vanmarcke M, Desoppere E, Lenaerts J, Boucique R, Wieme W. (1987) A general consistent model for formation and decay of rare gas excimers in the 10 Vio+ mbar pressure range, with application to krypton. / Chem Phys 86 4925 934. 

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... 

In a prdiminary study on the time-resolution of fluorescence in pdy(l-vinyl naphthalaie) the kinetics were constrained to fit Scheme 1, yielding values of monomer decay times in methylene chloride sdutiMi ci 7.4 and 43.1 ns. Late-gated spectra indicated that reverse dissociaticm of the excimer occurred. With improvements in techniques, these studies have been greatly an lified recently. In particular, studies on copolymers have permitted more detailed analysis of the concentration dependence of excimer formation, and improved statistical analyses have permitted reEned modelling of the kinetics. We will discuss at some length one of these papers, and summarize rearlts on other sterns. 

At high temperatures, where excimer formation and dissociation are rapid, compared to the deactivation processes and kj >> kf+kg, an equilibrium is established, region, both monomer and excimer fluorescence will decay exponentially, with a common rate constant (38). 

In synthetic polymers, the interpretation is necessarily more difficult The form of Equation 4 and Equation 5 requires that the kinetics of formation and decay of complexes are modelled adequately by rate-constants and that they take place in a homogeneous medium. If, as in synthetic polymers, the population of excimer trap sites, may occur through energy migration or rotational diffusion, a rate-constant may not be an adequate representation of the process, some time-dependent parameter being required (see below.) Heterogeneity may also play an important role. Thus in earlier work the fluorescence decay of excimer-forming polymers was modelled adequately by a scheme based upon simple excimer kinetics to which had been added terms to account for the occurrence in co-polymers of monomer sites which, by their isolation, could not form excimers (4-10). For polymers which contain isotactic and syndiotactic sequences, or rather, are made up of meso and racemic triads (14), the kinetics may be similarly a superimposition of simple schemes appropriate for the different sequences. 

Time-resolved fluorescence spectroscopy and fluorescence anisotropy measurements have been applied to study (i) excimer formation and energy transfer in solutions of poly(acenaphthalene) (PACE) and poly(2-naphthyl methacrylate) (P2NMA) and (ii) the conformational dynamics of poly(methacrylic acid) (PMA) and poly (acrylic acid) as a function of solution pH. For PACE and P2NMA, analysis of projections in which the spectral, temporal and intensity information are simultaneously displayed have been used to re-examine kinetic models proposed to account for the complex fluorescence decay behaviour that is observed. Time-resolved fluorescence anisotropy measuranents of fluorescent probes incorporated in PMA have led to the proposal of a "connected cluster" model for the hypercoiled conformation of this polymer existing at low pH. 

The application of time-resolved fluorescence spectroscopy to studies of excimer formation and energy transfer in PACE and P2NMA provides an overview of the emitting species present and the dynamics of energy relaxation in these polymers. The results of fluorescence decay analyses suggest that kinetic models which have been proposed to explain monomer/excimer kinetics may require further refinanent. 

Monomer and excimer fluorescence decays of Py, 1Py(3)1Py and the alkylpyrenylsilanes PPS and PDS, adsorbed on silica surfaces have been reported in the literature (21, 31-43). However, whereas for inter- and intramolecular excimer formation in homogeneous solution the rate constants of excimer formation and dissociation could be determined from the fluorescence decays (11,15,23), a considerably more complex situation is encountered on the silica surfaces (c.f. Section 3.2.1). This is not surprising, as the multiple adsorption sites at the inhomogeneous surfaces ma)ce different pathways in the excimer formation process li)[Pg.61]

Excited singlet-state molecules may also participate in a number of bimolecular reactions such as energy transfer, exdted-state complex (excimer) formation, and emted-state proton transfer, all of which result in non-exponential fluorescence decays. A discussion of the kinetics of these processes is be3Tond the scope of this book, however. The interested reader may wish to consult (1). 

Fig. 6. Simplified arrangement of coplanar conjugated segments (HT dyads) illustrating the cofacial stacking which facilitates excimer formation and intermolecular nonradiative decay of the excited state [100]...

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. 

The first is an excitation of P, the second a radiative transition, the third excimer formation and dissociation, and the fourth a transition decay of the excimer. In every case, the kinetics is assumed to be first order, P(n) is a distribution probability from (12.1), 1 is the intensity of incident light, and ki, k E, and k[ are the rate constants of corresponding reactions. From the above processes, the following two rate equations are derived concerning MP P , and MP P 2 ... 

Fluorescence Rise and Decay Curves. Both monomer and excimer fluorescence decay curves of the unirradiated film are nonexponential and the excimer fluorescence shows a slow rise component. This behavior is quite similar to the result reported for the PMMA film doped with pyrene. (23) A delay in the excimer formation process was interpreted as the time taken for the two molecules in the ground state dimer to form the excimer geometry. Dynamic data of the ablated area observed at 375 no (monomer fluorescence) and 500 nm (exciner fluorescence) are shown in Figure 5. When the laser fluence increased, the monomer fluorescence decay became slower. The slow rise of the excimer fluorescence disappeared and the decay became faster. 

In order to study the molecular dynamics of the outer segments of a dendrimer, one pyrene moiety was selectively and covalently attached to one dendron of poly(aryl ester) dendrimers by Adams (in total three pyrene molecules per dendrimer) [24]. The fluorescence decay of pyrene in the THF solution of the labeled dendrimers provided details of the pyrene excimer formation, such as the excimer formation rate, the excimer decomposition rate constant and the equilibrium constant of the excimer formation. These parameters were utilized to evaluate the diffusional mobility of the dendrimer branches. 

The decay of monomer emission is thus a sum of two exponentials. In contrast, the time evolution of the excimer emission is a difference of two exponentials, the pre-exponential factors being of opposite signs. The time constants are the same in the expressions of iM(t) and iE(t) (/ , and fl2 are the eigenvalues of the system). The negative term in iE (t) represents the increase in intensity corresponding to excimer formation the fluorescence intensity indeed starts from zero because excimers do not absorb light and can only be formed from the monomer (Figure 4.8A). 

Excimer may relax (i) by emission of characteristic structureless band shifted to about 6000 cm-1 to the red of the normal fluorescence, (ii) dissociate nonradiatively into original molecules, (iii) form a photodimer. Those systems which give rise to photodimers may not decay by excimer emission. The binoing energy for excimer formation is provided by interaction between charge transfer (CT) state A+A- A-A and charge resonance state AA s A A. 

Excimer-forming complexes (GS = the ground state) Multiplicity of excimer Solvent Formation (K) and decay rate (k, in s ) constants Ref. 

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