Reversible excimer formation

Step 5 is the excimer formation step. Step 6, which is the reverse process involving the dissociation of the excimer into an excited and a ground state monomer, may need an energy of activation. 

M which is attributable to the formation of an excited dimer, termed an excimer, (32). Excimer fluorescence is characteristically broad, (cf. Fig. 6) and decay characteristics are complex, since excimer formation is reversible... 

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. 

Once again, time-resolved studies on copolymers have shown the existence of two kinetically distinct monomer species, an isolated chromophore capable only of excimer formation throu long-range interactions and a monomer capable of excimer formation which is also populated by reverse disrociation . An alternative explanation of observed behaviour is that two excimer species may exist which... 

Di(1-pyieny1)propane. Bauer et al. (32) observed an excimer-like emission in the fluorescence spectrum of 1Py(3)1Py on dry silica, which was attributed to intramolecular interactions in the ground state. The excimer emission disappeared on adsorption of 1-decanol to the surface. In contrast, strong intramolecular excimer fluorescence was found by Avnir et al. for 1Py(3)1Py adsorbed on a silica surface with up to a double-layer equivalent of 1-octanol (38), see Section 4.3.2. Excimer formation was also detected on reversed-phase Si-C g and on untreated silica, the ratio I /I depending on the amount of 1Py(3)1Py adsorbed (38), see Fig. 3. 

Pyrene in Reversed-Phase Silica Si-C g with Methanol/ Water. Excimer formation with Py was also used to study (21) later-... 

Excimer formation with pyrene in reversed-phase silica Si-C g in contact with methanol/water. Data from ref. (21). 

Intermolecular Excimer Formation with Pyrene on Sl-C g. The monomer as well as the excimer fluorescence decays of Py on reversed-phase Si-C g, were studied at low pyrene coverage (38), where, nevertheless, a superposition of static and dynamic excimer formation was found (Section 3.2.2). The excimer decay could only be fitted with three exponentials (38), see Fig. 11B. Excimer formation is predominantly dynamic, as shown by the excimer growing-in (45 ns). That ground-state dimers only play a minor role can be seen from the ratio of the negative-to-positive excimer amplitudes, having a value of -0.90, see Section 2.2. 

Decay parameters, from global analysis (61,62), for intermolecular excimer formation with pyrene on reversed-phase octadecylmethyl-silica Si-C. g. 

The evidence from photostationary measurements of fluorescence intensity can thus be interpreted in terms of reversible photodimerisation to form an excited dimer which falls apart when it fluoresces. As with static quenching, there is no permanent chemical change on dilution the original fluorescence spectrum is restored. The essential difference is that excimer formation occurs only when one of the molecules is excited in the ground state, the equilibrium A + A A—A is not observed. The equilibrium constant in the excited... 

Some molecules or ions can function as quenchers and inhibit luminescence. Quenching, caused by interactions between the luminescent probe and a quencher, may be reversible or irreversible. Excimer formation is a case of reversible quenching. Some luminescent probes react, in the excited state, with an identical molecule in the ground state and form an excimer ... 

Figure 9 shows the temperature dependence of the recovered kinetic rate coefficients for the formation (k bimolecular) and dissociation (k unimolecular) of pyrene excimers in supercritical CO2 at a reduced density of 1.17. Also, shown is the bimolecular rate coefficient expected based on a simple diffusion-controlled argument (11). The value for the theoretical rate constant was obtained through use of the Smoluchowski equation (26). As previously mentioned, the viscosities utilized in the equation were calculated using the Lucas and Reichenberg formulations (16). From these experiments we obtain two key results. First, the reverse rate, k, is very temperature sensitive and increases with temperature. Second, the forward rate, kDM, 1S diffusion controlled. Further discussion will be deferred until further experiments are performed nearer the critical point where we will investigate the rate parameters as a function of density. 

Bisnaphthalenes. Chandross and Dempster (16,110) showed that l,3-bls(a-naphtyl) propane [43], on excitation above -30 C, results in the formation of a complex which at this temperature can either fluoresce or collapse into product [44], Irradiation of [44] at 77 K resulted in the reverse reaction and excimer fluorescence, but no excimer phosphorescence of this cleaved dimer could be observed (111). This clearly indicated that on the way to product collapse an excited state complex is formed. 

No intramolecular product formation was observed for 1,2-bis (a-naphtyl)-ethane or l,4-bis(a-napthyl)-butane, while a g,6-llnked l,3-bis(naphtyl)-propane did show excimer fluorescence but no product formation. Excimer fluorescence quenching Is observed which according to the authors (16) could be due to formation of an unstable photoproduct that reverses very fast to the open form. 

Acridizinium salts also undergo [4+4] dimerization to yield (287) and aspects of the intramolecular interaction and reaction of the arene chromophores in compounds of type (288) with n=l-6 and 8 have been publishedf The quantum yields of the reversible intramolecular dimerization have been determined in several solvents and are reported to depend upon the value of n. Formation of the dimer arises from the intramolecular excimer and a common pericyclic transition state is assumed for the forward and back reactions. 

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