Excimer fluorescence, compounds lifetimes

A similar but smaller intramolecular quenching effect was seen by Phillips and co-workers 44,4S) for 1-vinylnaphthalene copolymers incapable of excimer fluorescence. The monomer fluorescence lifetime of the 1-naphthyl group in the methyl methacrylate copolymer 44) was 20% less than the lifetime of 1-methylnaphthalene in the same solvent, tetrahydrofuran. However, no difference in lifetimes was observed between the 1-vinylnaphthalene/methyl acrylate copolymer 45) and 1-methylnaphthalene. To summarize, the nonradiative decay rate of excited singlet monomer in polymers, koM + k1M, may not be identical to that of a monochromophoric model compound, especially when the polymer contains quenching moieties and the solvent is fluid enough to allow rapid intramolecular quenching to occur. 

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. 

As noted earlier, the limiting lifetime of pyrene excimer fluorescence from concentrated solutions in PS and PMMA glasses was found to be the same as that of pyrene in cyclohexane solution. There have been no similar studies of naphthyl compounds in rigid glasses. Values of k and Q for the [2,6]-naphthalenophanes have not yet been determined for any solvent system. The bis(2-naphthyl) compounds have not been quantitatively characterized in rigid matrices, probably because excimer fluorescence is weak and difficult to detect under such conditions. Given such limited data, it can only be assumed that the values of QD and kD of 2-naphthyl excimers remain the same in rigid solution as in fluid solution. 

Excimer fluorescence from polychromophoric compounds in rigid systems, while easy to detect, is difficult to interpret. The transient response of the excimer can be empirically characterized by the limiting lifetime T p. In the absence of processes which convert D to M, this limiting lifetime is the reciprocal of k . We will examine xa]D for PS, P2VN, and other aromatic polymers to see if there is any difference between fluid and rigid solution at room temperature. 

Silica/i-Octanol. The monomer and excimer fluorescence decays of 1Py(3)1Py in the system silica/octanol were fitted with three exponentials (38), double-exponential fits giving unacceptable results. The decay times at 25°C (38) for the monomer (20, 43 and 146 ns), have values in the same range as those of the excimer (27, 51 and 106 ns). As was noted in studies with 1Py(3)1Py and related compounds in homogeneous solution (20,62), the monomer decay often contains a contribution from an impurity with a lifetime similar to that of e.g. 1-methylpyrene (x ), becoming more important with increasing fluorescence quenching. This then leads to the difference observed in the longest decay times of excimer and monomer. 

It is interesting to note that excimer bands of phenanthrene67 and of anthracene,124 which have defied detection in the prompt fluorescence spectra even at low temperatures, have been observed in the delayed emission spectra of these compounds at — 75°K. Presumably at the low temperatures necessary to observe these bands the high solvent viscosity completely suppresses photoassociation at the reduced concentration available, i.e., WM] 1 /r , whereas the reduced triplet-triplet annihilation rate constant mstationary concentration of the triplet state. 

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

Pyrene excimer formation still continues to be of interest and importance as a model compound for various types of study. Recent re-examinations of the kinetics have been referred to in the previous section. A non a priori analysis of experimentally determined fluorescence decay surfaces has been applied to the examination of intermolecular pyrene excimer formation O. The Kramers equation has been successfully applied to the formation of intermolecular excimer states of 1,3-di(l-pyrenyl) propane . Measured fluorescence lifetimes fit the predictions of the Kramer equation very well. The concentration dependence of transient effects in monomer-excimer kinetics of pyrene and methyl 4-(l-pyrenebutyrate) in toluene and cyclohexane have also been studied . Pyrene excimer formation in polypeptides carrying 2-pyrenyl groups in a-helices has been observed by means of circular polarized fluorescence" . Another probe study of pyrene excimer has been employed in the investigation of multicomponent recombination of germinate pairs and the effect on the form of Stern-Volmer plots ". 

Time resolved spectroscopy has developed assignments of intermediate species in radiation chemistry as revealed in the other sections. However, because solid polymers are less transparent, the works obtained so far seem to be limited mainly to polymer solution systems or liquid model-compounds. The lifetime of intermediates depends on LET the fluorescence lifetime of n-dodecane is shorter for higher LET radiation [83], which was studied as liquid model compounds for polyethylene. The observation is attributed to scavenging upon encountering of intermediates. Light emission from excimers of solid polystyrene has constant lifetime irrespective to LET [84], whereas polystyrene... 

The addition of -CD to an aqueous naphthalene solution caused the growth of its molecular fluorescence and the appearance of excimer emission [130]. By lowering the temperature of the solution, the excimer intensity grew at the expense of that of the monomer. The excimer emission was attributed to the association of 1 1 complexes to give 2 2 / -CD-naphthalene inclusion compounds. In air-saturated solutions, the three species have the following lifetimes 40 ns (free naphthalene), 48 ns (1 1 complex), and 68 ns (2 2 complex). The quenching rate constants derived from these lifetimes by the addition of I" were 6x 10 dm mol s , 3.9x 10 dm mol s , and 1.8 x 10 dm mol s , respectively, which confirmed the protection furnished by the cavity to the included molecules. 

The kinetics of excimer formation were also investigated for a Py2-PEO(9.6K) sample in water, different organic solvents, and their mixture with water [54]. The pyrene monomer and excimer decays could be fitted globally with three exponentials using free-floating pre-exponential factors. Unfortunately, the natural lifetime of the unquenched pyrene monomer was estimated from that of 1-methylpyrene which is not an ideal model compound due to its 30 ns shorter lifetime compared to that of the 1-pyrenemethoxy labels used for the Py2-PEO(9.6K) construct [70]. Thus, the kinetic parameters retrieved from this analysis were expected to be somewhat off. Also no final results were provided from the analysis of the fluorescence decays acquired with Py2-PEO(9.6K) solutions in water suggesting that the analysis did not yield reasonable parameters. 

Bazan and coworkers investigated the emission behavior of [2.2]paracyclophane-based compounds [48-55], They reported two types of emission mechanisms for [2.2]paracyclophane-based compounds, i.e., emission from the monomer state and emission from the phane state (excimer-like emission). The conjugation length of the stacked n-electron system, the extent of overlap, and the orientation between the stacked n-electron systems determine the mechanism. According to the photoluminescence spectra of 13 and 19 (Fig. 5) and their high d>pL (0.82 for 13 and 0.86 for 19), the emission of the [2.2]paracyclophane-based x-stacked polymer occurred from the monomer state. Fluorescence lifetime studies supported this hypothesis. Both fluorescence decay curves of 13 and 19 were a single exponential, and the fluorescence lifetime (r) of the polymer was 1.27 ns (j = 1.14), which was identical to the lifetime of 19 (r= 1.24 ns,1.00) [30]. 

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