Excimer emission polymers

Polybenzobisthiazoles 608 and polybenzobisoxazole 609 have been used as efficient electron transport materials in PLEDs [71] (Chart 2.143). Although these polymers show poor fluorescence quantum yields in thin films likely due to excimer formation [700], double-layer devices ITO/PEDOT/polymer/ETL/Al with PPV or MEH-PPV as emissive polymers and... 

In a film, however, molecular mobility is severely limited, so that excimer fluorescence must arise mainly from pairs or groups of pyrene molecules that were approximately in the excimer configuration when the film was cast. Thus, the intensity of the excimer emission is also an indication of the local concentration of pyrene in the cast film. If the pyrene aggregates, we expect that the excimer fluorescence would increase with aggregation. This system can be used to look at the aggregation of very low concentrations of a small molecule dye in a polymer film, and potentially detect molecular aggregation before it would be observable by other tech-... 

A fourfold decrease in the IDIIM ratio was observed for the 5.3% peracetylated pyrenylmethyl polyethylenimine derivative in glycerol compared to methanol. The higher viscosity of the glycerol limits the mobility of the attached pyrene group necessary to form excimer, decreases the association rate, and hence lowers ID/IM. These samples at 77°C showed essentially no excimer emission. Clearly, diffusion of the pyrene moieties attached to the polymer side chains is necessary for excimer formation. 

Farid and co-workers88 have investigated the effect of a glassy polymer host on the spectral position of the excimer emission peak produced by high concentrations of the compound methyl 4-(l-pyrenyl)-butyrate. The excimer peak position in a glassy polymer host was compared to the peak position in fluid solution for the following polymer hosts (and solvents) PS(toluene), PMMA(methyl isobutyrate), and poly-(vinyl benzoate) (methyl benzoate). The excimer emission peak of the pyrene compound in all three solvents occurred at about 20,800 cm-1, but the emission peak in all three polymer hosts was blue-shifted about 1900 cm-1 relative to the solution value. This is in contrast to the behavior of unsubstituted pyrene in PMMA 82) and PS 83), whose excimer peak does not shift from the solution value. 

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... 

In the second category, polymers with the repeat unit [CH2— CRR —(CH2)m CRR —CH2], where R = phenyl, m = 3-6 or 10, and R = hydrogen or methyl, were synthesized by Richards et al.143). The fluorescence of the R = H compounds 144) and the R = CH3 compounds 2S 145) were studied in fluid and rigid solution and in pure films. Although no spectra were given for the R = H compounds, these were stated 144) to have no excimer emission at 330 nm in fluid solution nor in pure films. A similar report was made25) for the R = CH3 compounds in 2-methyltetrahydrofiiran solution at room temperature, and in such solutions to which methanol had been added to the point of opalescence. These results were confirmed for the R = CH3 compounds in solution, and the spectra of pure films did not show significant amounts of excimer fluorescence at 330 nm t4S). However, an extraneous emission at 310 nm in the film spectra made quantitative measurement of the 330 nm excimer band impossible. 

Few conclusions can be drawn for room-temperature films of copolymers or polymers. David et al.187) measured p g 0.02 for copolymers containing as little as 35% of the aromatic monomer. In contrast, MacCallum et al.188>189) found that p 0.2 for the excimer emission of PS and P(a-methyl S). The latter may be consistent with limited energy migration in pure PS, because the fraction of EFS chromophores in pure PS has recently been estimated to be about 40 %148). 

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. 

Ad-C3-Ad and poly-VAd exhibited the monomeric fluorescence at about 320 nm in ethanol. This finding suggests that the excimer was not formed for both dimeric and polymeric compounds in ethanol solution. Furthermore, the excimer could not be obtained either in the case of dimers or polymers of 6-methylaminopurine derivatives, though excimer emissions were observed in water-ethylene glycol. The absence of the excimer may be explained by the fact that the stacked forms of the nucleic acid bases are unstable in ethanol solution where the bases appear to be solvated with ethanol molecules23. ... 

Wavelengths of maximum excimer emission (nm) exhibited by [12] in polymers and fluid medium"... 

Studies continue on the effects of a polymer host matrix on the excited-state properties of guest molecules. For example, the lifetime of excited singlet-state species may be greatly prolonged through restrictions of molecular motions (Gusten and Meisner, inter alia). Accurate information on the blend miscibility of polymers is provided by studies of excimer emission (Mikes et al.), and George et al. claim that the service life of many polymers can be predicted from their luminescence properties see also Martin. 

The series of copolymers of 1-vinyl naphthalene with methyl methacrylate used in a recent study is shown in Table 3 ete the fonctions are the mole fraction of naphthalene chromophores in the copolymer, fn the fraction of linkages between naphthalene species in the polymer chain, f n, and the mean sequence length of aromatic species, 1 . Fluorescence decays in the monomer and excimer emission... 

The principal difference between considerations of fluotescenre behaviour of acenaphthylene polymers VI and those of the photophysical characteristics of vinylaro-matic polymers is that whilst the latter sterns may form excimers throu interactions between nearest-neighbour chromophores on the polymer chain, steric restrmnts preclude such a mechanism in polymers of acenaphthylene It has been suggested that the dominant mechanism for excimer formation in acenaphthylene derived systems involves interactions between next to nearest nei bours. Such a proposal has been validated for copolymers of acenaphthylene with methyl methacrylate and methyl acrylate and is reinforced by the observation of excimer emission in alternating copolymers incorporating acenaphthylene chromophores in steady state excitation. 

Excimer emission from polymers continues to attract widespread interest particularly with regard to segmental motion in polymers. Intramolecular excimer formation of me30-2,4-di( N-carbazoyl)pentane dissolved in poly(propylene oxide) is shown to be controlled by... 

Recent studies in our laboratory were aimed at defining more closely the conditions governing Intramolecular excimer formation in dilute polymer solutions (15). An alternating copolymer of styrene with maleic anhydride or methylmethacrylate showed no excimer emission, confirming that interactions of other than neighboring phenyl residues made no significant contribution to... 

Intramolecular excimer emission, the polarization of fluorescence, nonradiative energy transfer and the use of medium-sensitive fluoropliores has been used to study the conformational mobility of polymers in dilute solution, the interpenetration of drain molecules, the association of polymers with each other or with small species and the cooperative transition of certain poly-carboxylic acids from a compact to an expanded state. 

It was found that excimer emission was reduced only by 27Z when the dibenzylacetamide was built into the center of a long polyoxyethylene chain as compared to the low molecular weight analog in which R were methyl groups.This proves that hindered rotation In long polymer chains takes place at similar rates as in small molecules. 

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