Monomer emission

The presence of a critical St content in ASt-x can also be seen in fluorescence spectra [29], This copolymer in aqueous solution shows an excimer emission peaking at 325 nra. As shown in Fig. 8, the intensity of the excimer emission increases, while the monomer emission decreases, with increasing St content. Eventually the excimer dominates the monomer emission at an St content of 72 mol%. The excimer emission becomes apparent at an St content of about 50 mol%, which agrees with the critical St content estimated by viscometry and NMR spectroscopy. The existence of the critical St content suggests the hydro-phobic self-aggregation to be a cooperative process. 

Fluorescence Measurement Fluorescence spectra were measured on a Spex Fluorolog 212 spectrofluorometer equipped with a 450 W xenon arc lamp and a Spex DM1B data acquisition station. Spectra were recorded in the front-face illumination mode using 343 nm as the excitation wavelength. Single scans were performed using a slit width of 1.0 mm. PDA fluorescence emission spectra were recorded from 360 to 600 nm, with the monomer and excimer fluorescence measured at 376.5 and 485 nm, respectively. Monomer and excimer peak heights were used in the calculation of the ratio of excimer to monomer emission intensities (Ie/Im). Excitation spectra were recorded from 300 nm to 360 nm and monitored at 376.5 and 500 nm for the monomer and excimer excitation, respectively. 

The energy change associated with the excimer emission is smaller than that for the monomer emission and so the excimer emission will occur at longer wavelengths than the monomer emission. 

Vibrational fine structure is absent from the excimer emission because the Franck-Condon transition is to the unstable dissociative state where the molecule dissociates before it is able to undergo a vibrational transition. In the case of the monomer emission, all electronic transitions are from the v = 0 vibrational level of M to the quantised vibrational levels of M, resulting in the appearance of vibrational fine structure. 

Fluorescent molecule (1) (Figure 6.5) acts as a host for Ca2+ ions. On addition of Ca2+ ions, the flexible polyether chain folds, leading to stacking of the anthracene fluorophores, and monomer emission is replaced by excimer emission. 

A first generation poly(amido amine) dendrimer has been functionalized with three calyx[4]arenes, each carrying a pyrene fluorophore (4) [30]. In acetonitrile solution the emission spectrum shows both the monomer and the excimer emission band, typical of the pyrene chromophore. Upon addition of Al3+ as perchlorate salt, a decrease in the excimer emission and a consequent revival of the monomer emission is observed. This can be interpreted as a change in the dendrimer structure and flexibility upon metal ion complexation that inhibits close proximity of pyrenyl units, thus decreasing the excimer formation probability. 1H NMR studies of dendrimer 4 revealed marked differences upon Al3+ addition only in the chemical shifts of the CH2 protons linked to the central amine group, demonstrating that the metal ion is coordinated by the dendrimer core. MALDI-TOF experiments gave evidence of a 1 1 complex. Similar results have been obtained for In3+, while other cations such as Ag+, Cd2+, and Zn2+ do not affect the luminescence properties of... 

B4.2.4). The dotted lines correspond to Eq. (B4.2.5). Insert Steady-state fluorescence spectra of corresponding solutions normalized to the monomer emission (reproduced with permission from Atik et al., 1979b ). 

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). 

The non-cyclic ethers E-2 (Figure 10.26), with two pyrenes linked at both ends of the chain, show strong intramolecular excimer formation. Addition of alkaline earth metal ions leads to an increase in monomer emission at the expense of the excimer band. The helical structure of the 1 1 complexes is supported by NMR spectra. Thanks to the pseudocyclic structure, the stability constants of the complexes with Ca2+, Sr2+ and Ba2+ in acetonitrile are quite high (106-107 for n — 5), but the selectivity is poor as a consequence of the flexibility of the oxyethylene chain. 

Although most boats are produced via the open mold spray-up process, environmental pressures are mounting due to the high styrene monomer emissions that are inherent in open molding techniques. Closed molded techniques are beginning to be commercially practiced, and it is anticipated that these processes will replace the open mold process. 

This model yields the following relation for the time-dependent monomer emission. 

Fig. 46 Pyrene monomer and excimer decay profiles in SDS micellar solutions [SDS] = 8.2 X 10 kmolm , [NaCl] = 10 kmolm , CMC = 1.5 x 10" kmolm", pyrene levels are indicated as the ratio of micellized SDS to added pyrene emission monitored at 383 nm for monomer and 480 nm for excimer. (A) Monomer emission for SDS/Py = 2160, (B) monomer emission for SDS/Py 108 (C) excimer emission for SDS/Py = 108...

The first two impregnation methods clearly dominate in North America and, probably, most of the world, whereas injection pultrusion is favored in several European countries where the permissible volatile levels in the workplace tend to be the lowest. Although injection pultrusion may be the simplest way of solving the monomer emission problem, it is clearly possible to reduce this disadvantage of the first two impregnation methods through proper ventilation. 

Functionalized polynorbornene derivatives, (V), prepared by Liaw et al. (4) using ring-opening metathesis polymerization exhibited strong carbazole fluorescence. Monomer emissions occurred in the near ultraviolet (UV) at approximately 380 nm and extended into the blue-violet region at 330 nm. 

Figure 6. Fluorescence quenching of pyrene by MV2+ in water, a) 1, b) 2. Monomer emission ( ), excimer emission ( ).

A broad, structureless fluorescence emission is observed for [2.2], [3.3], and [4.4] paracyclophane, but only structured monomer emission is seen in [4.5] and [6.6] paracyclophane. The fluorescence properties of the [2.3], [2.4], [3.4], [3.6], [4.6], [5.5], and [5.6] paracyclophanes have not been reported, although the latter three would be expected to yield only monomer emission. The UV absorption spectra of all of the above paracyclophanes have been reported, and all [m.n] phanes for which both m and n are 4 have absorption spectra that are identical to 1,4-bis (4 -ethylphenyl)butane, the open-chain analog. The UV absorption spectra of other paracyclophanes become increasingly red-shifted and broadened in the order [3.6], [3.4], [2.4], [3.3], [2.3], and [2.2] paracyclophane. 

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. 

The quantum yields for excimer emission e> for monomer emission in concentrated solution tfm and in dilute solution can be derived as follows from steady state concentrations of [A ] and [A ] ... 

The influence of the nonuniform character of the interior of zeolites on the photophysics of adsorbed guest molecules has been observed. Pyrene molecules included in zeolite faujasites show both monomer and excimer emission [232,233]. As in the case of silica surfaces, the excitation spectra of the emission corresponding to the monomer and the excimer differ (Figure 36), suggesting that there are at least two independent sites, each responsible for monomer emission and excimer emission. Time-resolved emission studies of pyrene included in Na + -X and Na + -Y (<0.1 molecule per cage) indicate... 

Figure 7.32 Kinetics of luminescence of pyrene following laser flash excitation. L, laser pulse profile M, monomer emission, E, excimer emission rise and decay. Horizontal axis, time in ns vertical axis, light intensity in arbitrary units. The three kinetic curves are normalized to a common maximum...

Excimer emission (460 nm) is observed from crystalline 16 at -186°C, but not at room temperature or in a methylcyclohexane glass (60). Photodimerization in the solid state results in a decrease in excimer fluorescence intensity and the appearance of monomer emission. Evidently, solid state photodimerization results in the isolation of monomer molecules among the dimers, as has been observed with anthracenes (65). The absorption spectrum of polycrystalline 16 is broader than the solution spectrum but the long-wavelength maximum (300 nm) is unchanged. 

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