Fluorescence structure effects

J.-S. Yang and T.M. Swager, Fluorescent porous polymer films as TNT chemosensors electronic and structural effects, J. Am. Chem. Soc., 120 11864-11873, 1998. 

In Section 3.4, structural effects were often discussed in conjunction with the nature of the solvent. As emphasized in the introduction to this book, the fluorescence emitted by most molecules is indeed extremely sensitive to their microenvironment (see Figure 1.3), which explains the extensive use of fluorescent probes. The effects of solvent polarity, viscosity and acidity deserves much attention because these effects are the basis of fluorescence probing of these microenvironmental characteristics and so, later chapters of this book are devoted to these aspects. The effects of polarity and viscosity on fluorescence characteristics in fluid media and the relevant applications are presented in Chapters 7 and 8, respectively. The effect of acidity is discussed in Sections 4.5 and 10.2. This section is thus mainly devoted to rigid matrices or very viscous media, and gases. 

R. W. Cowgill, Fluorescence and protein structure X. Reappraisal of solvent and structural effects, Biochim. Biophys. Acta 133, 6-18 (1967). 

Besides the impressive difference in the chemiexcitation efficiency, also the fluorescence yield of the meta-pattemed emitter m-17 is by more than an order of magnitude ( ) higher than that of the para regioisomer p-17 . Evidently, crossed-conjugated emitters are advantageous for the design of efficient intramolecular CIEEL systems. In Sections V.A-V.C we shall consider additional internal (substrate structural effects) and external (medium influence) factors, which play an essential role in the development of efficient dioxetane-based analytical probes. 

For pro tic solvents with larger dielectric constants and stronger basicity, the La and 1Lb states are inverted and relaxation from xLb to xLa takes places but there is no proton transfer to the solvent. The fluorescence is then due to the 1LB state with a small Stokes shift. The intermediate sized water clusters (n = 10-20) belong in this category. The clusters with methanol for any size n < 10 (due to a weak basicity or a small dielectric constant) follow this mechanism. From the evaluated proton affinities (see Figure 4-16), it can be seen that for n k 10 molecules of methanol (PA 243 kcal mol-1 which corresponds to the limit for proton transfer evidence in 1-naphthol complexes with piperidine or ammonia), a proton transfer should be observed. The absence of such a transfer can be related to a cluster structure effect. 

In summary, we have tried to overview the state of the field of nanoaperture enhanced fluorescence. While much is known currently about the photophysics of an isolated aperture, it is clear that much more work needs to be performed in order to understand, and maximize, fluorescence enhancement effects from arrangements of apertures and structured apertures. In particular, the relative contributicms of radiative and non-radiative processes have (Hily beoi studied in a couple of experiments, and limited computaticHial work has been performed. In addition, the role of emission directicHiality is not clear at present and requires further study. 

Structural Effects. Excitation and emission spectra of the PMDA based polyimide films obtained from Dow were measured. The fluorescence intensity decreases in the order PMDA-ODA > PMDA-MDA > PMDA-IPDA, as the ether linkage is replaced with more bulky, less flexible linkages. X-ray diffraction data, provided by Dow, also show that this is the order of increasing intermolecular distance, d. The wavelength of the excitation maximum also shifts to higher energy with reduced flexibility and increased bulkiness of this linkage. These results are tabulated in Table III below. 

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