Absorption Wavelength and Intensity

The requirements for the design of photochromic compounds are, among others, the adjustment of absorption wavelength and intensity, the attainment of thermal stability, clarification of reaction mechanism, and enhancement of quantum yield. The purpose of this chapter is to review recent computational studies in consideration of these requirements. 

Shinichiro Nakamura Yokohama Research Center, Mitsubishi Chemical Corporation, Yokohama-227, Japan. 

Organic Photochromic and Thermochromic Compounds, Vol. 2, edited by John C. Crano and Robert Guglielmetti. Kluwer Academic / Plenum Publishers, New York, 1999. 

Nonplanar molecules often show interesting properties. Their nonplanarity precludes the use of the re-electron approximation. The re-electron approximation is not adequate for the n-n transition and intramolecular hydrogen tending. The photochromic properties of many systems are, however, related to their nonplanarity as well as the electronic structure, including valence tr electrons. Recently, the ZESTDO method has shown its efficiency for these problems, to which the PPP method is not applicable. The capability of this method to reproduce experimental wavelength and intensity data of various organic dyes was tested extensively.8 

Most interesting photochromic molecules are large molecules. This often prevents calculations using the ab initio method from being extensively applied. However, recent theoretical progress is providing a fundamental understanding of the absorption properties of these molecules (e.g., see Ref. 21). 

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Solvatochromism Shift of UV/Vis absorption wavelength and intensity in the presence of solvents. A hypsochromic (blue) shift increases as solvent polarity inCTeases. The shift in the red direction is called bathochromic. 

If the objective is a quantitative determination by direct spectrophotometry, it suffices to read the absorbance at a given wavelength. By virtue of their simplicity, these methods are more precise than those involving chemical pretreatment. Therefore, if the absorption wavelength and intensity of the analyte are both adequate, pharmacopoeias and compilations of standard methods recommend using this approach. The choice between direct spectrophotometry and chemical derivatization is also influenced by the accuracy required in the result, the purity of the target analytes, and the interferences expected. If the sample allows for it, the solid, once powdered, can be extracted with water or another solvent (usually methanol or ethanol) at an appropriate pH, filtered, and diluted to the required concentration before measuring the absorbance. 

An auxochrome group is a saturated functional group presenting one or more lone pairs, it is attached to a chromophore and it is able to modify both its absorption wavelengths and intensity. 

We always recommend that structural changes occurring after the saponification procedure be verified. Since the hydroxyl groups have no influence on the chromophore, the wavelength of the maximum absorption, shape, and intensity of the ultraviolet-visible (UV-Vis) spectrum would be identical for unsaponified and saponified samples. 

The absorption in the dihydrodiazepinium monocations has been ascribed to a TT-TT transition (68M11 71M12). It occurs at a similar wavelength and intensity to the tt-tt transitions in open-chain m-2-iminoen-amines but not to those of pyrimidines, this being in accord with the lack of complete cyclic electronic interaction in the dihydrodiazepines (68MI1). 

Relationship of absorption positions and intensity to structures. While quantum mechanical calculations permit prediction of the correct number and approximate positions of absorption bands, they are imprecise. For this reason, electronic spectroscopy also relies upon a combination of empirical rules and atlases of spectra that can be used for comparison purposes.74 76 The following may help to orient the student. The position of an absorption band shifts bathochromically (to longer wavelength, lower energy) when the number of conjugated double bonds increases. Thus, butadiene absorbs at 46,100 cm 1 (217 nm) vs the 61,500 cm 1 of ethylene. As the number of double bonds increases further, the bathochro-mic shifts become progressively smaller (but remain more nearly constant in terms of wavelength than wave number). For lycopene (Fig. 23-10) with 11... 

With replacement of oxygen by sulfur, and then by selenium, visible absorption is weakened and shifted to longer wavelengths. On the other hand, in the near-ultraviolet region the selenium compound is intermediate between the oxygen and the sulfur isosteres, both with respect to wavelength and intensity of absorption. 

The performance of ablation lasers depends largely on two factors, namely the nominal laser wavelength and intensity, and whether the continuous or pulsed mode of irradiation is used. Studies on the first two factors have yielded controversial results that can be ascribed to differences in sample nature or grain size. Thus, ruby (694 nm), Nd YAG (1064, 532, 266 and, recently, 213 nm) and excimer (308, 248, 222, 193 and, recently, 157 nm) lasers have been used for laser ablation (LA). A comparison of the Nd YAG laser operating at 1064 nm and 266 nm revealed better absorption of UV wavelength by most types of samples and also decreased fractionation. The particle sizes produced during ablation were smaller with 266 than with 1064 nm, which was possibly the reason for the reduced... 

Optical absorption and emission studies on single crystals of Pt(tpy)2 [77], Pt(bhq)2 [113], and Pt2(ppy)4 [37] have been reported. For the Pt2(ppy)4 dimer, a study on the temperature (1.7-50 K) and magnetic field (H < 5T) dependence of the wavelength and intensity of the emission have allowed discussion of the spacing of the low energy levels derived from the Pt-Pt interaction [37]. 

The electronic spectra of dihydrodiazepinium salts are characterized by an intense absorption ( = 15,000-25,000) usually between 300 and 360 nm. The related bases absorb with somewhat reduced intensity at a slightly shorter wavelength. In the alkyl-substituted dications there is no appreciable absorption above 200 nm, but in the case of the 5,7-diphenyldihydrodiazepine there is a band at 290 nm (e = 23,500) the changes are due to loss of the conjugated dihydrodiazepinium system when it is protonated to form a dication. This absorption in the dihydrodiazepinium monocations has been ascribed to a —x transition.29 30 It occurs at a similar wavelength and intensity to the —ir transitions in open-chain cis-2-iminoenamines but not to those of pyrimidines, thus suggesting that there is no cyclic electronic interaction in the dihydrodiazepines.30... 

Yet another advance is development of semiconductor nanocrystals (or "quantum dots") with narrow absorption bands and intense fluorescence. The wavelength of absorption and fluorescence depends upon the size of the crystals. For example, CdSe crystals of diameter 2-5 nm coated with silica or with a surfactant fluoresce across the visible range. They have a variety of uses in biological staining. ... 

The factors that determine the wavelength and intensity of UV/visible absorption are complex, and we have already seen above how base stacking interactions can modify the absorbance of nucleic acids (the li iioclimiiiic cHcet). 

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