Ultraviolet -visible

The ultraviolet-visible (UV-Vis) absorption spectrum of PPV typically consists of a well-defined low-energy maximum at approximately 3.3 eV, a second weaker feature at approximately 5 eV, and a strongly increasing absorption above 5 eV [75, 79] (Fig. 6). The optical absorption coefficient at the low-energy maximum is on the order of 2 x 10 cm [75]. 

PPV oligomers present optical absorption and luminescence spectra whose peaks are redshifted with increasing chain length (Fig. 7). The principal absorption peak position presents an approximately linear dependence with 1/m, where m is the number of carbon atoms in the shortest path between the ends of the conjugated chain [80]. The absorption and luminescence spectra of PPV oligomers in solution show a blueshift when compared to those in 

The absorption spectrum of PPV is also changed when prepared from freshly synthesized and undialyzed polysulfonium chloride, compared to that obtained from dialyzed prepolymer [34]. The water content in the prepolymer solution also plays an important role in determining the optical properties of the converted PPV [34]. The position of the absorption edge, which is directly dependent on the polymer bandgap, can be intentionally modified via substitution by various donor and acceptor groups onto the phenyl rings and/or the vinylene units of PPV [5, 41, 86-89]. These substitutions also produce modifications in the electroaffinity and ionization potential of the polymer and, for this reason. 

PPV and its derivatives are conventionally used in the form of thin films operating as active layers in LEDs, photodetectors, and other optoelectronic devices. An important characteristic of devices constructed with PPV is that they operate with the polymer layer submitted to high electric field strengths. To permit device operation at low voltages, on the order of a few volts, the polymer film thickness is reduced to around 10 run. The production of high-quality polymer thin films constitutes, for these reasons, an important step in the device construction process. 

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Van der Waals complexes can be observed spectroscopically by a variety of different teclmiques, including microwave, infrared and ultraviolet/visible spectroscopy. Their existence is perhaps the simplest and most direct demonstration that there are attractive forces between stable molecules. Indeed the spectroscopic properties of Van der Waals complexes provide one of the most detailed sources of infonnation available on intennolecular forces, especially in the region around the potential minimum. The measured rotational constants of Van der Waals complexes provide infonnation on intennolecular distances and orientations, and the frequencies of bending and stretching vibrations provide infonnation on how easily the complex can be distorted from its equilibrium confonnation. In favourable cases, the whole of the potential well can be mapped out from spectroscopic data. 

Yon can use a sin gle poin t calculation that determines energies for ground and excited states, using configuration interaction, to predict frequencies and intensities of an electron ic ultraviolet-visible spectrum. 

Ultraviolet visible (UV VIS) spectroscopy, which probes the electron distribution especially m molecules that have conjugated n electron systems Mass spectrometry (MS), which gives the molecular weight and formula both of the molecule itself and various structural units within it... 

Chromophore (Section 13 21) The structural unit of a mole cule principally responsible for absorption of radiation of a particular frequency a term usually applied to ultraviolet visible spectroscopy... 

Ultraviolet visible (UV VIS) spectroscopy (Section 13 21) An alytical method based on transitions between electronic en ergy states in molecules Useful in studying conjugated systems such as polyenes... 

In addition to total energy and gradient, HyperChem can use quantum mechanical methods to calculate several other properties. The properties include the dipole moment, total electron density, total spin density, electrostatic potential, heats of formation, orbital energy levels, vibrational normal modes and frequencies, infrared spectrum intensities, and ultraviolet-visible spectrum frequencies and intensities. The HyperChem log file includes energy, gradient, and dipole values, while HIN files store atomic charge values. 

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

TABLE 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy... 

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

The focus of this chapter is photon spectroscopy, using ultraviolet, visible, and infrared radiation. Because these techniques use a common set of optical devices for dispersing and focusing the radiation, they often are identified as optical spectroscopies. For convenience we will usually use the simpler term spectroscopy in place of photon spectroscopy or optical spectroscopy however, it should be understood that we are considering only a limited part of a much broader area of analytical methods. Before we examine specific spectroscopic methods, however, we first review the properties of electromagnetic radiation. 

Ultraviolet/visible absorption spectrum for bromothymol blue. 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

Figure 7.5 Ultraviolet-visible spectrum of dehydrohalogenated copolymers of styrene-l-chloro-1,3-butadiene. [Redrawn with permission from A. Winston and P. Wichacheewa, Macromolecules 6 200 (1973), copyright 1973 by the American Chemical Society.]...

Optical Properties. Teflon FEP fluorocarbon film transmits more ultraviolet, visible light, and infrared radiation than ordinary window glass. The refractive index of FEP film is 1.341—1.347 (74). 

Laser Photochemistry. Photochemical appHcations of lasers generally employ tunable lasers which can be tuned to a specific absorption resonance of an atom or molecule (see Photochemical technology). Examples include the tunable dye laser in the ultraviolet, visible, and near-infrared portions of the spectmm the titanium-doped sapphire, Tfsapphire, laser in the visible and near infrared optical parametric oscillators in the visible and infrared and Line-tunable carbon dioxide lasers, which can be tuned with a wavelength-selective element to any of a large number of closely spaced lines in the infrared near 10 ]lni. 

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