Oxygen, excimer fluorescence

While the photodimerization of bis(l-naphthylmethyl) ether was acknowledged somewhat earlier 39), the photodimers were first characterized and the quantum yield of the dimerization determined by Todesco et al. U2). Both the syn- and anti-photodimers were formed in roughly equal amounts, and the quantum yield for formation of the anti-dimer was independent of solvent. However, the quantum yields for formation of the syn-dimer and for excimer fluorescence were found to vary with solvent such that their sum was independent of solvent. The fact that irradiation of l,3-bis(l-naphthyl)-1-propanol yields only the syn-photodimer 113> indicates that the conformational properties of oxygen are largely responsible for anii-dimerization in the ether compound. The possibility of photodimerization was unfortunately not considered in the fluorescence studies of protonated bis (1-naphthylmethyl) amine 115>, l,3-bis(4-methoxy-l-naphthyl) propane 116>, and meso-bis( 1 -(1 -naphthyl-ethyl) ether 13). 

What are the effects of a rigid polymer matrix and of oxygen on the excimer fluorescence lifetime and quantum yield ... 

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. 

Krueger and Weitz [1992] measured the diffusion coefficient of oxygen atoms in the ground 3P state in solid xenon by using the fluorescence of XeO excimers to monitor the 0(3P) concentration. The experimentally determined values are 5.4 x 10-18 and 2.0 x 10 17 cm2/s at 32 and 40 K, respectively. These diffusion coefficients are only about three orders of magnitude smaller than for hydrogen atoms ( 10 14cm2/s at 50 K), as... 

The PBA chemisorption film shows fluorescence at 376, 396 and 474 nm when excited at 365 nm, as shown in Fig. 10-20. The fluorescence spectra shape of the PBA film was almost the same as that in an ethanol solution (0.1 mmol dm PBA). Emissions of PBA monomers (376 and 396 nm) and excimers between PBA molecules (474 nm) were observed. Figure 10-20 shows that the fluorescence intensity of the film at 376, 396 and 474 nm depended on the oxygen concentration. Thus, the fluorescence of a PBA film is quenched by oxygen and this film is useful as an optical oxygen sensor. The intensity decreased with increase in oxygen concentration as shown in Fig. 10-21. This result indicates that the fluorescence of the PBA film was quenched by oxygen. 

Figure 10-22 shows a Stem-Volmer plot of the relationship between the fluorescence intensities of a PBA film at 474 nm, attributed to PBA excimer emission, and the oxygen concentration. At higher PBA concentrations, the emission of the PBA excimer is sensitized by energy transfer from the PBA monomer. 

Fig. 1.26 Fluorescence spectra of polystyrene in oxygen-free CH2CI2 solution (1 g L ). I Monomer emission, recorded at the end of a 10 ns flash (7exc = 257 nm). II Excimer emission, recorded 45 ns after the flash. Adapted from Beavan et al. [73] with permission from John Wiley 8l Sons, Inc.

Water soluble p-sulfonato calbc[w]arenes (w = 8, la-lb and n=6, 2a-2b) was employed as host to control the outcome of photodimerization and photoisomerization of 4-stilbazoles [17]. Novel macrocydic and medium-size stilbenophanes tethered by silyl chains were synthesized, and their photochemical and photophysical properties were examined (Figure 5.5) [18]. Direct irradiation of macrocydic stilbenophanes stereoselectively gave intramolecular photocydoadducts, and the efficiency increased with decreasing distance between the two stilbene units. The triplet-sensitized photoreaction of stilbenophanes caused cis-trans photoisomerization. Photoreactions of as-fixed stilbenophanes under an oxygen atmosphere selectively led to phenanthrenophanes. Fluorescence quantum yields increased with the introduction of silyl substituents, and hence those of silyl-tethered stilbenophanes were larger than that of unsubstituted fraws-stilbene. Intramolecular excimer emis-... 

For measurements at higher temperatures, MacCallum and Rudkin [27] developed a method for the determination of diffusion constants for oxygen in polystyrene and poly (methyl methacrylate). They studied the quenching of either the excimer emission of polystyrene or additive fluorescence in poly(methyl methacrylate) by oxygen in thin films after displacement of the nitrogen atmosphere over the sample by oxygen. 

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