Excimer dissociation

The corrected yield ratio R for 1,2-benzanthracene in toluene at the same temperature increases with pressure up to 2500 atm but decreases at still higher pressures this is attributed to a transition between the limiting conditions described above as excimer dissociation is suppressed at higher pressures. The limiting slope at lower pressures is consistent with a value of — AV > 6 2 cm3 mole-1 for this molecule.22,23... 

It should be noted that excimer dissociation (process MD) is responsible for a nonexponential decay of molecular fluorescence which affords a criterion of photoassociation under conditions (A mdt 1) where neither self-... 

The increase in molecular fluorescence yield, and concomitant reduction in yield of excimer fluorescence with temperature at higher temperatures, illustrated in Figure 9, affords direct evidence for excimer dissociation, and under... 

In solution the over-all process of excimer dissociation includes escape of the dissociation products from the solvent cage, and the corresponding rate constant expressed as... 

Excimer dissociation (process MD) therefore reduces the Stern-Volmer selfquenching constant from its expectation value K = fcDMTp to... 

Just as photoassociation may be inferred from the observation of fluorescence self-quenching, so may the negative temperature coefficient of KM be adopted as a criterion of excimer dissociation in the absence of excimer fluorescence. The application of Eq. (31) to data obtained for the selfquenching of anthracene in the vapor phase leads to a value of — AHa 7.6... 

D + M - M-M + M Excimer dissociation to excited nearest-neighbor monomer k ... 

The bonding in the excimers is covalent in nature. In polar solvents the excimers dissociate forming redox changed radicals, the unpaired electron being localized on the phthalocyanine ring, e.g. 

It is concluded that below the transition temperature the monomer intensity increases with increasing temperature due to excimer dissociation to excited-state monomer. Above the transition temperature the excited-state equilibrium is apparently broken because the thermal energy is sufficient to activate excimer non-radiative decay by dissociation to ground-state monomer. Consequently, the monomer emission no longer increases with increasing temperature and the isobestic point disappears. Pyrene has been used as a fluorescent probe to monitor the conformational state of maleic acid with... 

Where describes the excimer dissociation to give excited monomer, r2 describes the quenched monomer at longer times and a describes the transient diffusion behaviour (J ) which is dependent on the rotational diffusion and collisional properties, though in a manner... 

The intrinsic weakness of benzene excimer emission derives from its extreme dipole forbiddeness (7). Its weakness relative to monomer emission under dilute solution conditions additionally results from the diffusion limit upon excimer formation rates, along with thermally induced excimer dissociation. These latter considerations are largely irrelevant to polymers, vide infra. 

It has been proposed that the monomer emission decay is predominantly exponential with an additional minor late exponential component attributed to excimer dissociation (10,11). The contribution of the late component is less than 1% of the population of... 

Fig. 1. Arrhenius plot for pyrene in n-decane of the rate constants of excimer formation (k ), excimer dissociation (k,), and the reciprocal excimer lifetime (1/t ). See Scheme (I).

The short propane chain of [lPy(3)lPy] does not allow direct interconversion from El to E2 or E2 to Ei without excimer dissociation, i.e., kj, and fc 3 are equal to... 

At low temperatures, if the rate of excimer dissociation is sufficiently low relative to alternative means of excimer state depopulation and provided it may be assumed that k and k are... 

If the temperature dependence of the rate coefficient for excimer dissociation, k, is sufficiently greater than that representative of the thermal dependence of k, it is to be expected that at sufficiently elevated temperatures k may be regarded as negligible relative to k and be expressed as ... 

Evidence for the failure of the assumptions inherent in the treatment discussed above to withstand empirical evaluation may be found both from steady state excitation and transient data. For example Nishijima [21] and David et al [22] have presented data (for poly 2- and 1-vinyl-naphthalenes, respectively) which indicate that does not increase in the manner expected in the high temperature region if excimer dissociation were dominant. More emphatically, Holden and Guillet [23] have tabulated individual rate parameters derived from transient decay data for poly(1-vinylnaphthalene) [23,24] and poly(1-naphthyImethacrylate)... 

Within experimental error, the monomer values were equal to those of the model compounds. Consequently it was proposed that this component arises not as a result of excimer dissociation but through the existence of an unquenched monomer species (M in scheme 2). It was supposed that such monomers exist in configuration too far removed from that of the excimer to be quenched within the excited state lifetime. 

The existence of three kinetically distinct species according to the triad distribution would be consistent with the necessity for triple exponential fitting to the data obtained in the monomer emission region of styrene copolymers [72]. However it has been claimed [73,75] that such a model is inconsistent with observations regarding excimer dissociation in time resolved spectroscopy [72], decay data for styrene... 

This interpretation is confirmed by means of the time correlated single photon counting technique at low temperatures, the decay of the locally excited state can be described by a two exponential function. Since the excimer dissociation to the locally excited state is only competitive at temperatures above -20 C, these two decay parameters are to be ascribed to two ground state conformations. From both lifetimes the ratio of their preexponentials and the lifetime of the unquenched chro-mophore all the rate constants present in the kinetic scheme and the ratio of the populations C7/C5 could be calculated. 

---