Irradiation, fluorescent polymers

Fluorescence Analysis of Irradiated PET and PET-4,41-SD Yarns. As we noted above, the fluorescence emission at 460 nm in irradiated PET polymer has been attributed to the hydroxyterephthaloyl component (2). The fluorescence spectra of irradiated (100 hours) PET homopolymer yarns and PET-4,4 -SD copolymer yarns are identical and agree with that obtained by Day and Wiles for PET film (2). 

For this purpose, stabilization efficiency was defined as 1-As/Aq, where As and Aq represent the increase in absorbance in the blue spectral region (yellowing) in the presence and absence of stabilizer, respectively. The resulting stabilization efficiencies were found to decrease substantially over relatively short exposure times (ca. 40% decrease between 10 and 25 hrs irradiation). Difference absorption spectra obtained during accelerometer exposure exhibited a new absorption band at ca. 300 nm which overlapped strongly with polymer fluorescence (required for efficient RET quenching) and weakly with polymer absorption (screening).1... 

The importance of phenol formation by the proposed pathway was probed by irradiating l,3-diphenoxy-2-methyl-2-propanol (5) under the same conditions. Compared to 3, the rate of phenol formation was approximately 2 times slower. Since the 11-transfer step in Scheme II is not available to 5, the results provide support for the scheme as an important, but not sole, pathway for phenol formation. Irradiation of and 5 with an air purge resulted in faster rates of phenol formation (ca. 5-fold) relative to N2. These findings parallel the accelerated fluorescence intensity loss from polymer 1 films in air as compared to the results in vacuo (see Table I). 

Similarly, fluorescent silver clusters could be prepared in so called molecular hydrogels, formed by polyglycerol-b/oc -poly(acrylic acid) (PG-b-PAA), using a ratio COOH Ag of 2 1 with UV irradiation (365 nm). The emission band centered at 590 nm reached a maximum after 200 min of irradiation. The authors claim improved photostability of the clusters since they are still luminescent even after 9 h of irradiation, but it has to be mentioned that the irradiation source was weak, only 0.5 mW/cm2. They claim that it is the number of arms in the star polymer rather than the length of the arms (thus the density of COOH) that plays a crucial role in the formation of silver clusters [30]. 

Fig. 12 (a) Image of PMAA-protected fluorescent silver clusters prepared with increasing initial ratio Ag+ MAA from 0.5 1 to 12 1 and equal irradiation time, (b) Absorption spectra of the same samples as in (a), (c) Variation of absorption maxima of some of the samples in (a) with molar ratio. Black arrows indicate how the absorption band shifts to the blue with the addition of extra polymer to a fluorescent cluster solution explaining the transfer effect of silver clusters among PMAA chains [20]... 

The photochemistry of borazine delineated in detail in these pages stands in sharp contrast to that of benzene. The present data on borazine photochemistry shows that similarities between the two compounds are minimal. This is due in large part to the polar nature of the BN bond in borazine relative to the non-polar CC bond in benzene. Irradiation of benzene in the gas phase produces valence isomerization to fulvene and l,3-hexadien-5-ynes Fluorescence and phosphorescence have been observed from benzene In contrast, fluorescence or phosphorescence has not been found from borazine, despite numerous attempts to observe it. Product formation results from a borazine intermediate (produced photochemically) which reacts with another borazine molecule to form borazanaphthalene and a polymer. While benzene shows polymer formation, the benzyne intermediate is not known to be formed from photolysis of benzene, but rather from photolysis of substituted derivatives such as l,2-diiodobenzene ... 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

Morawetz et al. [105,106] were the first to use non-radiative energy transfer (NRET) fluorospectroscopy for exploring polymer-polymer miscibility. The basic principle is as follows. In a system containing two kinds of fluorescence chromophore, if the emission spectrum of one (donor D) overlaps the absorption spectrum of the other (acceptor A), a non-radiative energy transfer from the former to the latter may occur when the system is excited by irradiation that the former selectively absorbs. The efflciency of energy transfer (E) inversely proportional to Icj/Ia> where Id and la denote the emission intensities of D and A, respectively, depends on the average distance r between D and A according to the relationship ... 

The 8X12 library was simply irradiated with a hand-held UV lamp (365 nm) to discriminate easily fluorescent and nonfluorescent polymers and to visualize the corresponding emission color in solution. Then, with a spectrofluorimeter able to read 96-well plates for several excitation and emission wavelength combinations, the different excitation wavelengths were evaluated. By this procedure, new polymers showing green (39a,b, excitation at 460 nm, emission detection at 530 nm) or blue (39c-e, excitation at 360 nm, emission detection at 460 nm) emitting fluorescence were rapidly discovered (Fig. 5.13). 

Saeva mentions, in an early review [10] on LC materials and aspects of their photochemistry and photophysics, the irradiation of an unspecified stilbene containing polymer and changes in its physical properties. Creed et al. [25,61,62] have reported several observations of the photophysics and photochemistry of a rra/w-stilbene 4,4 -dicarboxylate containing MCLC polyester, 29, one of a series of such polymers with different spacers synthesized by Jackson, Morris, and coworkers [63]. This polymer is partly crystalline in the as cast state and has a N mesophase over a narrow temperature range (177-186°C). In solution, the structured UV-Vis absorption [62] and fluorescence [25,61] spectra are almost... 

Indeed, the usual fluorescence of the isolated aromatic amines (e.g., N,N-dimethylaniline, DMA) is quenched by excimer formation in compounds I and II. In the process of prolonged irradiation of I and II solutions the emission intensity increases gradually because of the loss of the C = C double bonds in the system due to the polymerization reaction. A polar environment favors the charge transfer and, therefore, the fluorescence quenching of the monomer is drastically decreased, whereas the polymer formation increases. 

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