Excimer to monomer emission intensities

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. 

Although the theoretical study of EET in polymer systems has led to considerable recent advances, we choose in the second section of this paper to take a somewhat more pragmatic approach to the study of phase separation kinetics. To do so, we temporarily set aside any consideration of the details of any particular EET model and simply rely on a fundamental relationship between the ratio of the excimer to monomer emission intensities, and the... 

TABLE 4 Excimer to Monomer Emission Intensity Ratio for Dinaphthylpropanes in Water and in the Presence of 5 x 10 M CD ... 

The essential feature of the intramolecular cyclization of short polymer chains containing terminal pyrene groups is that the process is difiiision controlled at room temperature. As such, the cyclization process is expected to obey an Einstein relation with both the rate constant for cyclization and the ratio of excimer to monomer emission intensities proportional to... 

To explore this analogy, we will review previous work on the influence of stoichiometry as well as present new results on the effect of neutralization on macromolecular complex formation. In addition, we will review very recent results for adsorption on colloidal silica and present preliminary results for the adsorption on colloidal polystyrene. The excimer to monomer emission intensity ratio, the excitation spectra, and the lifetimes of the excimer and monomer are the observable experimental parameters. 

Figure 1. Intramolecular excimer to monomer emission intensity ratio as a function of the molar ratio of poly carboxylic acid) to PEG for Py-PEG-Py of weight-average molecular weights 4800 and 9200. All data are nomwlized by the Id/Im value for a 1% pyrene-tagged PEG solution with no polyacid added.

The experimental quantity easiest to observe in a photophysical experiment such as the ones described herein is l l uy the ratio of excimer to monomer emission intensities. Interpretation of the results is not completely straightforward, however. Winnik (28) has shown that IdUm for the end-labeled polymers in dilute solution is proportional to the rate constant for cyclization as such, it may be interpreted in terms of the influence of molecular weight and solvent quality on the configurational statistics of the chain. 

Early investigations were concerned primarily with considerations of the thermodynamic characteristics of the interaction through a study of the temperature and solvent dependence of the excimer to monomer emission intensity ratio. In addition, some kinetic information was... 

Having established the existence of the excimer emission of NDI based polyurethanes in solution, and realizing that the intramolecular excimer forming naphthyl carbamate groups are located on the backbone of the polymer, it becomes apparent that an excellent opportunity exists for chain conformational studies as a function of solvent. Figure 10 shows the steady-state fluorescence spjectra of NDI-650 in four solvents with distinctively different solvating power. In each case (curves a-d) both monomer and excimer emission are observed however, tlie ratios of excimer to monomer emission reflect conformational differences of the NDI-650 polymer in the solvent employed. The excimer to monomer intensity ratio... 

The lifetime of many aromatic fluorophores is of the order of 10 s, similar to the relaxation times for conformational transitions in paraffinic chains. Thus, the ratio of excimer and monomer" emission intensities can be used to estimate the rate constant for the hindered rotation required to form the excimer conformation in a molecule containing two aromatic residues. A detailed study based on this principle has been carried out in De Schryver s laboratory and is reported elsewhere in this volume. 

Following this same approach, Jin and coworkers have prepared a nearly identical pyrene-modified calixarene, 38, where the methoxy groups are replaced by ester functionalities [377], Na+ and K+ ions perturb the relative intensities of excimer and monomer emission in much the same manner as 37. Unlike 37, however, 38 is not sensitive to Li+ ion. 

Lariat crown ethers with two terminal pyrenyl sidearms connected to the same carbon 103 (/= 0, 1 m = 0-2 n = 0-2) or to two different carbon atoms 104 (m = 0-2 n 1,2) and 105 (m 0, 1) showed intramolecular excimer emission in the free state (Jt-Jt-stacking of the pyrene rings), whose intensity decreases with the increase of monomer emission intensity upon metal ion complexation <20020L2641, 2004JOC4403>. This response has been ascribed to the cooperative participation of one of the two sidearms in the complexation of the crown ring with the metal ion, which renders inoperative the Jt-Jt-stacking of aromatic rings. Most of these fluorophores show alkaline earth over alkali metal ion selectivities. 

Weller and Zachariasse thoroughly investigated exciplex formation and luminescence for donor acceptor systems in THF [18]. A particularly interesting result from their work came from an examination of the temperature dependence of radiative charge recombination between 9,10-dimethylanthracene anion (DMA") and TPTA+ in THF [19]. They found that both exciplex emission and fluorescence from DMA were observed in solution at low temperature (ca. —50°C). As the solution temperature is raised, the excimer emission decreases in relative intensity, and at room temperature the emission is nearly completely DMA fluorescence. The monomer-to-exciplex emission intensity ratio as a function of temperature follows Arrhenius kinetic behavior and yields an activation barrier that is nearly the same as the energy gap between the exciplex and the DMA states. Thus, their model consisted of reaction of the solvent-separated ions to form an intimate emissive ion pair which could dissociate to yield the singlet anthracene derivative. 

Noticeably, the presence of three linked and proximate fluorophores led to the formation of excimers upon irradiation of a metal free solution of 8 in MeOH/water (4 1, v/v). In particular, the strong excimer band at 485 nm (excimer-to-monomer intensity ratio = 17) does not vary over the 2-8 pH interval. At pH > 8 the emission is quenched according to a sigmoidal profile (see Figure 12). 

Figure 4.14 Time dependence of the ratio of the intensity of excimer to monomer fluorescence emission associated to the release of pyrene from pyrene-loaded micelles of poly(methacrylic acid)-poly(styrene)-poly(methacrylic acid). Reproduced from Reference 87 with permission of the American Chemical Society.

The experimental method to assess the phase behavior is to measure the ratio of excimer to monomer intensity, la/lm- Samples of the polymer blend are cast on sapphire disks at thicknesses of the order to 10-25 pm, and the fluorescence spectra are obtained. In a study of the excimer fluorescence of poly(2-vinyl naphthalene) (P2VN) in polystyrene, the monomer emission peak at 337 nm was compared with the excimer emission peak at 398 nm [334]. The ratio of the peaks versus polystyrene molecular weight is illustrated in Fig. 5.32 (a and b). Phase separation (at 0.3 wt% P2VN addition) was observed at PS Mn of 17,500. It was noted that the results show the onset of phase separation before any visual phase separation was observed. 

Numerous studies of the photophysics of aryl vinyl polymers have shown that the monomer emission intensity can be empirically fit to a triple exponential [15-17]. The immediate conclusion that may be drawn from this observation is that the Birks kinetic scheme [1], which was developed for a collision-induced intermolecular process between small molecules in solution, is inapplicable to intramolecular excimer formation in macromolecules. 

Yana et al. [27] prepared an a-helix peptide system 20 composed of 17 amino acids with two naphthalene units at the same side of fi-CT) in the peptide side chain (Fig. 8). This system was designed to signal the detection of a guest by an increase in the excimer emission intensity. 20 by itself exhibits strong monomer emission around 336 nm and relatively weak excimer emission around 390 nm. The excimer is formed between the two intramolecular naphthalene units outside the CD cavity. Upon addition of UDCA, the monomer emission intensity... 

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. 

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). 

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