Excimer lifetime

As in the case of intermolecular excimer formation, it should be recalled that difficulties may arise from the possible temperature dependence of the excimer lifetime, when effects of temperature on fluidity are investigated. It is then recommended that time-resolved fluorescence experiments are performed. The relevant equations established in Chapter 4 (Eqs 4.43-4.47) must be used after replacing ki[M] by k. ... 

Farid 88) did not report on the excimer lifetime of the pyrene compound in the systems that were studied. Nevertheless, they proposed that the blue shift of the excimer emission peak in glassy polymers relative to solution was due to improper orientation of the excimer components in the polymer matrix 88). This proposal is supported by the observation 88) that the blue shift of the excimer peak for the pyrene... 

The inter-ring separation in [4.4] paracyclophane has been calculated to be 3.73 A, assuming normal bond angles and planar benzene rings. At this distance, there is no ground-state overlap, and the UV absorbance does not extend past 280 nm. Nevertheless, the peak of the excimer fluorescence intensity of [4.4] paracyclophane is red-shifted 1900 cm"1 relative to the peak of the solution excimer of toluene at 31,300 cm-1. Neither the excimer lifetime nor the excimer fluorescence response function have been reported for any of the exrimer-forming paracyclophanes, so little is known about the kinetics of excimer formation in these compounds. 

The final expression is in the form of the Stern-Volmer equation, where tj, is the monomer lifetime in dilute solution and te is assumed to be excimer lifetime in infinitely dilute solution. 

Ikeda et al. [54] studied the excimer-like fluorescence of phenylenediacry-late chromophores in MCLC polyesters of type 23. These results are discussed in Section III.B of this chapter. A correlation was noted between the excimer fluorescence lifetime and the AS values for mesophase formation. The less-ordered, more mobile mesophases had shorter excimer lifetimes presumably because of more facile nonradiative decay. Kurihara, Ikeda, and Tazuke [129,130] also studied the emission properties of SCLC polymers of type 64, containing the... 

Deep UV Resist Reactions of CMS. The laser photolysis studies on CMS and polystyrene solutions in cyclohexane were carried out at 248 nm using a KrF excimer laser. The intensity of monomer and excimer fluorescence of CMS becomes weaker with increasing chloromethylation ratio, but the lifetime of the excimer is essentially independent of the chloromethylation ratio being almost the same as the excimer lifetime of polystyrene (20 ns). The lifetime of the monomer fluorescence has not been investigated by nanosecond laser photolysis because of the short lifetime (about 1 ns) (20,21) of the singlet. 

In [1], and 0 are the fluorescence quantum yields of the excimer and the monomer, respectively. The rate constants k and k, depend on solvent viscosity and hence on temperature (10-15). In contrast, the ratio of the radiative rate constants k /k as well as the excimer lifetime do not strongly depend on temperature. This is shown in Fig. 1, where k, k, and the reciprocal excimer life-... 

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).

Singer (33), analyzed the fluorescence decays of Py on dry silica in a manner similar to that employed for excimer formation in micelles containing a varying number of Py molecules. In analogy, the Py molecules on the silica were taken to have distributions of neighbours within the domain accessible by surface diffusion. This treatment of the decay data leads to exceptionally low values for the excimer lifetimes (25 ns), c.f. Section 4.2.2. 

Dynamic Excimer Formation of Pyrene on Decanol-Covered Silica. The temperature dependence of the monomer and excimer fluorescence decays of Py adsorbed on silica gel with a monolayer coverage of 1-decanol has been studied by de Mayo et al. (39). The literature data for the two times T2 and x. of the double-exponent ial excimer decays are depicted in Fig. 9A. It is seen that the shorter decay time X2 approaches a value of about 60 ns upon lower ing the temperature, a value similar to the excimer lifetime of Py in methylcyclohexane (60), Fig. 9B. From the observation that the... 

Py(2)]) is the total Py concentration. For the excimer, two decay times (t and T2> are equal to those of the monomer and the fluorescence grows in (t ) with the excimer lifetime see Sect-... 

The appearance of at least two fluorescence lifetimes for cations 1 and 2 indicates that more than one excimer is present in the reversed micelle solution (Table 4). The percent contribution of the lifetime components change with . This finding is consistent with the notion that the size of the water pool affects the photochemistry of the stiibazoiium cation. At this point we do not have enough information to determine whether or not the different excimer lifetimes represent different states which lead to the production of different photodimers. 

In the analogous complex-formation study with polar PAA, the excimer excitation frequency was substantially shifted upon complexation it was not, however, accompanied by the decrease in excimer lifetime found in the adsorption work. Two representative results are included in Table I corresponding to [PAA]/[Py-PEG-Py(4800)] molar ratios of 1 and 2. This difference indicates that perhaps the excimer is not as strained or destabilized by complexation as it is by adsorption onto solid particles. Therefore, the complexation may result in enough environmental change to shift the excimer excitation spectrum, but the excimers are not strained by complexation nearly as much as by adsorption onto a solid surface. 

Bright and coworkers investigated pyrene-excimer formation in supercritical fluids from a somewhat different angle using not only steady-state but also time-resolved fluorescence techniques (47,167). They measured fluorescence lifetimes of the pyrene monomer and excimer at a pyrene concentration of 100 p,M in supercritical ethane, CO2, and fluoroform at reduced densities higher than 0.8. Since the kinetics for pyrene-excimer formation was found to be diffusion controlled in ethane and CO2 and less than diffusion controlled in fluoroform, they concluded that there was no evidence for enhanced pyrene-pyrene interactions in supercritical fluids. The less efficient excimer formation in fluoroform was discussed in terms of the influence of solute-solvent clustering on excimer lifetime and stability. Experimentally, their fluorescence measurements were influenced by extreme inner-filter (self-absorption) effects due to the high pyrene concentration in the supercritical fluid solutions (35). 

T and T represent the excited monomer and excimer lifetimes,respectively. 

Fig. 4 gives evidence of the temperature dependence of the excimer lifetime of diphant even in the range of existence of the isoemissive point. But contrary to T, x Plotted versus temperature exhibits little scatter within the range of polymers studied. The similar values obtained in solution (5) show that X is unaffected by the rigidity of the matrix and is, indeed, and intrinsic characteristic of the fluorescence probe. 

Fig. 4 - Temperature evolution of the excimer lifetime of diphant in polybutadiene, polyisoprene, and their respective copolymers with styrene.

The fluorescent probe methods were used to determine miCToviscosity (T ,) of potassium A-acylalaninates and potassium A-acylvalinates micelles. The results obtained show that 11 on the micellar surface is larger in A-acylalaninates than in 7V-acylvalinates, whereas in the micellar core remains same in the micelles of the two surfactant series. The values of 11 of SDS, lithium dodecyl sulfate (LDS), CTACl, and CjjEg, determined at 15°C by monomer/excimer intensity ratio and excimer lifetime of dipyrenylpropane dissolved in micelles, are 19, 19, 39, and 57 cP, respectively. The fluorescent probe technique was used to determine r m values of SDS, CTABr, and CTACl micelles at different applied pressure. The derived values of SDS, CTABr, and CTACl micelles at 25°C and atmospheric pressure are 12,47, and 27 cP, respectively. The addition... 

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