Excimer formation, studies

Many studies on side-chain modifications in PF were initially based on the idea of excimer formation, resulting in the green emission during LED operation or in solid-state PL on annealing PF films. This resulted in several proposed strategies for the design of fluorene side-chain homopolymers, where bulky substituents at position 9 of the fluorene moiety should sterically prevent (hinder) interchain interaction and thus improve the stability of blue emission. 

Similar results have been presented by Miller and co-workers, who capped the ends of oligo(dihexylfluorene)s and poly(dihexylfluorene)s with Frechet-type dendrons (Figure 7.17C) [73], Annealing experiments coupled with emission studies revealed that G-3 and G-4 dendrons were effective at preventing excimer formation, even when the poly(fluorene) spacer was 50-80 repeat units long. 

In order to study the molecular dynamics of the outer segments of a dendrimer, one pyrene moiety was selectively and covalently attached to one dendron of poly(aryl ester) dendrimers by Adams (in total three pyrene molecules per dendrimer) [24]. The fluorescence decay of pyrene in the THF solution of the labeled dendrimers provided details of the pyrene excimer formation, such as the excimer formation rate, the excimer decomposition rate constant and the equilibrium constant of the excimer formation. These parameters were utilized to evaluate the diffusional mobility of the dendrimer branches. 

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... 

In conclusion, the method of intramolecular excimer formation is rapid and convenient, but the above discussion has shown that great care is needed for a reliable interpretation of the experimental results. In some cases it has been demonstrated that the results in terms of equivalent microviscosity are consistent with those obtained by the fluorescence polarization method (described in Section 8.5), but this is not a general rule. Nevertheless, the relative changes in fluidity and local dynamics upon an external perturbation are less dependent on the probe, and useful applications to the study of temperature or pressure effects have been reported. 

Photophysical Studies of Molecular Mobility in Polymer Films and Bulk Polymers. 3. Dynamic Excimer Formation of Pyrene in Bulk PDMS, Macromolecules 23, 2217-2222. 

The phenomenon of fluorescence has been synonymous with ultraviolet (UV) and visible spectroscopy rather than near-infrared (near-IR) spectroscopy from the beginning of the subject. This fact is evidenced in definitive texts which also provide useful background information for this volume (see, e.g., Refs. 1-6). Consequently, our understanding of the many molecular phenomena which can be studied with fluorescence techniques, e.g., excimer formation, energy transfer, diffusion, and rotation, is based on measurements made in the UV/visible. Historically, this emphasis was undoubtedly due to the spectral response of the eye and the availability of suitable sources and detectors for the UV/visible in contrast to the lack of equivalent instrumentation for the IR. Nevertheless, there are a few notable exceptions to the prevalence of UV/visible techniques in fluorescence such as the near-IR study of chlorophyll(7) and singlet oxygen,<8) which have been ongoing for some years. 

The time-resolved spectroscopy is a sensitive tool to study the solute-solvent interactions. The technique has been used to characterize the solvating environment in the solvent. By measuring the time-dependent changes of the fluorescence signals in solvents, the solvation, rotation, photoisomerization, or excimer formation processes of a probe molecule can be examined. In conventional molecular solutions, many solute-solvent complexes. 

Self-Quenching and Excimer Formation. It is often desirable to use a high concentration of sensitizer to ensure that it will absorb all of the light, particularly if the substrate has any significant absorption at the wavelength of irradiation. Recent studies have shown that quantum yields determined under these conditions can be anomalously low if the sensitizer acts as a bimolecular quencher of its own triplets. 

Eximer Fluorescence. Since Forster and Kasper discovered concentration-dependent long-wavelength emission resulting from association of an electronically excited pyrene molecule with another ground state pyrene molecule,39 the phenomenon of excimer fluorescence has been studied extensively.40 The mechanism for excimer formation and emission can be represented by... 

Oyama et al. [23] utilized fluorescence spectroscopy to study how the molar masses of both PAA and PEO affect complexation. Actually, they used the exci-mer formation between the pyrene groups attached to the chain ends of PEO as a molecular probe. The molecular weights of PAA used were 1850, 4600 and 890,000, and those of PEO were 4800 and 9200. The latter are much lower than the PEO molecular weight in the work of Bednar et al. [19]. In order to monitor both intramolecular and infermolecular excimer formation, it was necessary to distinguish the two types of excimers clearly, so Oyama et al. used two kinds of solutions. One contained 99% of untagged PEO and only 1% PEO, where PEO refers to PEO whose chain ends are tagged by pyrene. In this solution, PEO is believed to behave Hke an individual PEO chain, which provides only intramolecular excimer. The other contained fully tagged PEO, which provides both intramolecular and intermolecular excimers. 

Frank et al. [29] studied the effect of hydrophobic interaction by comparing the fluorescent properties of PMAA/PEO and with those of PAA/PEO . Here PEO denotes pyrene end-labeled PEO. Figure 3 shows the intensity ratio le/Im of inframolecular excimer pyrene for PMAA/PEO (9200) and PAA/PEO (9200). It is seen that when added, PMAA more markedly reduces intramolecular excimer formation in PEO than does PAA. This difference is thought to be due to a stronger abihty of PMAA to combine PEO and the consequent suppression of intramolecular cychzation of PEO. 

The objective of this review is to characterize the excimer formation and energy migration processes in aryl vinyl polymers sufficiently well that the excimer probe may be used quantitatively to study polymer structure. One such area of application in which some measure of success has already been achieved is in the analysis of the thermodynamics of multicomponent systems and the kinetics of phase separation. In the future, it is likely that the technique will also prove fruitful in the study of structural order in liquid crystalline polymers. 

In all the above studies 4-9-37 41> the 1,3-diarylpropanes had a reduced monomer quantum yield due to substantial intramolecular excimer formation, but the monomer fluorescence spectrum was unchanged from that of the analogous monochromophoric... 

Despite the technical problems in the latter film study, we conclude that there is no intramolecular excimer formation in the compounds of Richards et al.143, and probably little intermolecular excimer formation in the pure films. The absence of an effect of solvent power 25) on the possible excimer fluorescence of the R = CH3 polymer may not be significant, since little change in the coil dimensions would be expected for the short ( 300 backbone atoms) polymers 143> which were studied. Additional work is needed on the fluorescence of such polymers having higher molecular weights, different aryl substituents (R = 2-naphthyl, for example), and fewer adventitious impurities. 

Excimer formation is studied in polyesters in which the repeating unit is represented by the formula ABm, where A isp-OOC—Ph-COO, B is CH2 and m = 2.6. In dichloroethane, dioxane... 

The prospensity for the formation of bends in the flexible spacer in polyesters containing naphthyl units is examined by the study of the fluorescence of a series of diesters. The dependence of the degree of intramolecular excimer formation on the length of the aliphatic spacer, under circumstances where the dynamics of rotational isomerism in the flexible spacer is suppressed, is evaluated by extrapolation of the measurements to infinite viscosity n. The extrapolated results exhibit an odd-even effect, with the more intense excimer emission being observed when the number of methylene groups is odd. The odd-even effect is rationalized by an RIS analysis of the diesters. 

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