Fluorescence emission spectra poly

The broad emission and low-fluorescence quantum yield of PPS suggested a distribution of trapping sites in the Si skeleton, which were also considered responsible for the lower-than-expected conductivity. The far-IR spectrum of PPS suggested the existence of cyclohexasilane rings connected by linear chains.361,362 Subsequent investigations by Irie et al. on the electronic absorption spectra of radical ions of poly(alkylsilyne)s were taken to indicate the presence of various cyclic silicon species, in corroboration of this conclusion.363 The large Stokes shift and broadness of the fluorescence emission indicate a range of fluorophore structures, different from the chromophore structures. This is... 

Sato and Woody found a preference for binding left-handed conformers of ANS to the beta form of poly(Lys) and suggested that the dyes are bound to the polypeptide in regions of low polarity due to the increase in fluorescence quantum yield and the blue shift in the emission spectrum [261],... 

It is known that [3.3]paracyclophane, which has the almost highest transannular interaction of the less distorted benzenes (12), has the fluorescence emission at longer wavelength (356 nm) (18) than the excimer of 1,3-diphenylpropane (332 nm). The fluorescence spectrum of the cyclopolymer, poly(St-C3-St), recorded under the same conditions as for [3.3]paracyclophane is illustrated in Figure 1 (20). Both have the fluorescence at the same wavelength, and therefore the polymer is supported to contain [3.3]paracyclophane units as sequence units. The fluorescence emission at 312 nm is ascribed to the residual styryl groups. 

With a glassy solution of poly-1-vinylnaphthalene, the delayed emission spectrum has been shown to consist of an emission having a mean lifetime of approximately 80 ms at the normal fluorescence wavelength, in addition to the phosphorescence having a mean lifetime of about 2 s [159]. The delayed fluorescence did not appear in the spectrum of 1-ethylnaphthalene. With the polymer it was found to be inhibited by piperylene, a well-known triplet quencher. These results have been explained by mutual annihilation of two excited triplet states produced by the absorption of two photons by the same polymer molecule. They are considered as strong evidence for migration of the excited triplet state in poly-1-vinylnaphthalene. In polyacenaphthalene, however, which is chemically very similar to poly-1-vinylnaphthalene (see p. 409), no delayed fluorescence could be detected in the same experimental conditions [155]. 

It has been shown [155,171] that the dependence of excimer emission intensity on acceptor concentration obeys the Stern—Volmer equation whether M or D is the donor, whereas a second-order equation is obtained if both types of excited state simultaneously act as donor. It seems that in poly-1-vinylnaphthalene and polyacenaphthalene films at room temperature, energy transfer to benzophenone occurs from M, although normal fluorescence cannot be detected in the emission spectrum of the polymers in these conditions [155]. Decay time measurements have shown that the excimers in solid polyvinylcarbazole are traps rather than intermediates in the energy transfer process [148]. With polystyrene, however, it has been clearly demonstrated that energy transfer to tetraphenylbutadiene occurs from both excimer and isolated excited chromophore [171]. 

Fluorescence studies 14, 15) using pyrene, pyrene derivatives, and cationic probes in poly(methacrylic acid) have shown that a conformational transition from a closed compact coil to extended form induced by pH is a progressive process over several pH units (pH 4-6). The emission spectrum of 4 X 10 M R6G and 4 X 10 M RB excited at 480 nm in water is not dependent on pH. However, in aqueous solutions of PM A, the spectra are significantly dependent on pH (shown in Figure 6). At pH 4-5, the spectra are similar to the typical emission of RB at pH 2-3 and 6-7, the spectra in PMA display stronger emission at 550 nm and at pH 8, the spectra are identical to those in water. 

A methanol solution containing poly(3) exhibits abroad absorption band with A,max at 422 nm. In aqueous solution, the absorption maximum red-shifts to 432 nm and the absorption coefficient also decreases. The emission properties are greatly dependent upon the nature of the solvent. In a methanol solution, poly(3) exhibits a narrow emission band with A.max = 475 nm and a vibronic band at A,max = 502 nm. In aqueous solution, the emission band becomes broader and the fluorescence quantum yield (Ofi =0.016) is lower than that in methanol ( fi = 0.045). Interestingly, the emission maximum is at the same position as in methanol (475 nm) with a shoulder at 502 nm. Such behaviour is different from other PPE-based CPE, which exhibit large red-shifts of emission band in a poor solvent. The lack of a spectral shift for poly(3) is likely to be due to the fact that the aggregated state of the polymer has a much lower quantum yield, and therefore its contribution to the total emission spectrum is small. 

Winnik [49] used fluorescence measurements of transfer of the electronic excitation between donor-naphthalene and acceptor-pyrene chromophores attached to the same polymer chain for studies of thermoreversible phase separation of aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM). Dilute solutions of the doubly labelled polymer PNIPAM were heated from 277 K to 313 K, and the fluorescence emission intensity of pyrene (integrated spectrum) was measured when the system was excited with 290 nm, donor excitation, and when excited with 328 nm, acceptor excitation. Non radiative energy transfer between excited naphthalene and pyrene occurred in aqueous solution of the polymer. The increase in intensity of pyrene fluorescence when the solution was excited at 290 nm, shown in Figure 4.13, is due to a phase separation process at lower critical solution temperature (LCST). When the LCST was reached, the phase separation into polymer-rich and polymer-lean phases occurred. It was concluded that the collapse of the polymer chain leading to densification of polymer phase is followed by domination of intramolecular contributions to the energy transfer process. 

Fluorescence. We have also measured fluorescence produced by two-photon excitation for thick films of polysilane. For this experiment, the laser was a Spectra-Physics sub-picosecond dye laser system, focussed onto the polymer films to produce intensities of =440 MW/cm. Emission was focussed into a 0.5 m spectrometer and spectra were collected using an optical multichannel analyzer and analyzed on an IBM PC. For poly(di-n-hexylsilane), the two-photon induced emission is broadband (AXpwHM -10 nm at room temperature), with line center at =380 nm, as shown in figure 10. The emission spectrum is identical to that observed for this compound by UV excitation, and the average degree of fluorescence anisotropy (=0.2) produced at the two-photon resonance (579 nm) is quite similar to that oteerved for on-resonance UV excitations in polysilanes [26]. 

Measurements of the circularly polarized fluorescence (CPF) of this compound (PPA) as a function of temperature indicate the existence of two kinds of excimers one with a negative CPF dissymmetry with an emission maximum around 460 nm and predominant at higher temperatures, the other with a positive CPF dissymmetry emitting at wavelengths longer than 500 nm and mainly formed at lower temperatures (Egusa et al., 1985). In the emission spectrum of poly-N -(9-carbazolyl)-carbonyl-L-lysine, even four distinct species are observed besides the emission of locally excited carbazole. 

The heparin and poly-L-glutamate titrations show a markedly different behavior than do the DNA titrations. As polyanion is added, the fluorescence of the an-thrylpolyamine solution decreases until a well-defined minimum is reached. A new emission at 510 nm, which we assign to the anthracene excimer of 14, increases and decreases coincidently with the titrated fluorescence minimum. Likewise, the UV spectrum of 10 fiM 14 with added heparin shows hypochromism that occurs and disappears coincidently with the fluorescence minimum and a 2-nm red shift. We have proposed template-directed excimer formation as the physical basis for these observations. In the absence of heparin, fluorescence of the unassociated probe is observed. As heparin is added, the fluorescence decreases as a result of heparin-directed interaction between probe molecules. Additional heparin permits the fluorophore population to diffuse over the length of the poly anion, thus avoiding excimer formation and yielding a net CHEF. 

The third group ofpolychromophoric compounds to be discussed are homopolymers in which the pendant rings are separated from the backbone by one or more atoms. The polymers of allyl arenes, which lack only the n = 3 ring spacing of aryl vinyl polymers, have been studied very little. The fluorescence spectrum of poly(l-allyl-naphthalene) in dilute dichloromethane solution has been reported 28). Like 1-ethyl-naphthalene, the maximum intensity was seen at 337 nm, but a weak, broad shoulder was also recorded for the polymer at 410 nm. The fluorescence ratio Iu/IM for poly(l-allylnaphthalene) was only 1/100 th the value for P1VN 28). The excimeric nature of the 410 nm emission in the allyl-based polymer has not been confirmed, since neither the lifetime nor the excitation spectrum of this fluorescence band are known. 

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