Poly fluorescence excitation spectra

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. 

Figure 8.15 (a) Steady state anisotropy of fluorescence excitation of 1 recorded in poly(vinyl butyral) film as a function of temperature. The spectra were taken at 293, 250, 215, 165, 125, 85, 65, 45, and 7.5 K. (b) The excitation spectrum at 85 K monitored at 15900cm . ... 

Polymer Luminescence Spectra. Figure 1 shows typical fluorescence and phosphorescence excitation and emission spectra obtained from commercial polypropylene film (or powder). Poly(4-methylpent-l-ene) exhibits similar spectra to those of polypropylene. The excitation spectrum for the fluorescence has two distinct maxima at 230 and 285 nm while that of the phosphorescence has only one distinct maximum at 270 nm with rather weak and diffuse structure above 300 nm. It is clear from these results that the fluorescent and phosphorescent chromophoric species cannot be the same. This, of course, does not rule out the fact that both may arise from carbonyl emitting species, as will be shown later, since these chromophoric groups when linked to ethylenic unsaturation can have quite distinct absorption (14) and emission spectra (15,16,17). 

Fig. 11 Illustration of the excited state relaxation derived from experimental results obtained for poly(dA).poly(dT) by steady-state absorption and fluorescence spectroscopy, fluorescence upconversion and based on the modeling of the Franck-Condon excited states of (dA)io(dT)io. In red (full line) experimental absorption spectrum yellow circles arranged at thirty steps represent the eigenstates, each circle being associated with a different helix conformation and chromophore vibrations.

In order to proceed it is now necessary to consider the nature of the lowest excited state of these polymers. One description which appears to be particularly appropriate to these materials is that given by the molecular exciton theory (37,38). This of course is suggested by the nature of the fluorescence spectrum itself and in addition this approach has proven to be quite successful in the Interpretation of the electronic states of the alkanes, the structural analogs of the poly(organosllylenes) ( 3, 6). The basic assumption... 

We employ method B to study effects of this type. In this mode, our apparatus yields relative high-resolution fluorescence spectra at different time windows after excitation of the sample by the 355 nm pulse. The spectra are acquired by the upconversion method. The upconverted fluorescence spectrum is recorded simultaneously at all monitored wavelengths by an optical multichannel analyzer. It is constructed from a poly-chromator (HR320 Instruments SA) and an intensified silicon photodiode array detector (Princeton Applied Research Model 1412). The detector is interfaced to our Cromemco computer. 

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]. 

Fullerene-styrene copolymers have been prepared in radical initiated and thermal polymerization reactions [148-151]. In radical copolymerizations of Cgg and styrene, copolymers with Cgg contents up to 50% (wt/wt) can be obtained [150]. Electronic absorption spectra of the copolymers are very different from that of monomeric C o (Fig. 36). The absorptivities per unit weight concentration of the copolymers j increase with increasing C q contents in the copolymers in a nearly linear relationship (Fig. 37). Fluorescence spectra of the Cgg-styrene copolymers, blue-shifted from the spectrum of monomeric Cgo, are dependent on excitation wavelengths in a systematic fashion [149]. Interestingly, the observed absorption and fluorescence spectral profiles of CgQ-styrene and Cyg-styrene copolymers are very similar, even though the spectra of monomeric CgQ and C70 are very different. The absorption and fluorescence spectra of the fullerene-styrene copolymers are also similar to those of the pendant Cgg-poly-styrene polymer (19) prepared in a Friedel-Crafts type reaction [150,156]. 

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. 

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. 

Figure 9.1 (a) UV-vis electronic spectrum (cyclohexane) and (b) fluorescence spectrum (cyclohe)ane, excitation wavelength =300 nm) of poly(phenylcarbyne) obtained by this methodology. 

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]. 

Figure 2. Fluorescence spectrum of a colloidal particle composed of poly(viny] acetate) [PVAc] and poly(2 ethylhexyl methacrylate) labelled with phenanthrene in the PVAc phase, curve a. Curve b is the fluorescence spectrum of an identical unlabelled particle. Curve c is obtained by subtracting b from a. The excitation wavelength was 290 nm.

---