Chromophores, benzene derivatives

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...

Many chromophores are suitable for use in the exciton chirality method. One of the features required for such a chromophore is its planarity or near-planarity. Nonplanar (inherently dissymmetric) chromophores would contribute to the CD spectra by other mechanisms. The other limiting factor is the position of the transition in the spectral region studied. For example, the 1B transition in the alkyl-substituted benzene chromophore appears near the short-wavelength recording limit around 200 nm, making its use in the exciton chirality method less attractive. Furthermore, the direction of polarization of the lB transition in alkyl-substituted benzene derivatives is not readily determined. In such cases calculation of the rotatory strength is more reliable than qualitative analysis. 

More numerous, however, are aromatic compounds with an effectively planar n system, the optical activity of which comes from chiral perturbations. Benzene derivatives such as phenylalanine (3) are particularly important. The Lb band of the benzene chromophore (A = 260 nm) is neither magnetically nor electrically allowed. The symmetry selection rule may be broken by vibronic interactions or due to substituents. The interpretation of the observed rotatory strength is not easy, but empirical sector rules have been proposed. (Cf. Charney, 1979 Pickard and Smith, 1990.)... 

Benzoid Chromophobe. The spectrum of the aromatic parent, benzene (Figure 2), displays considerable fine structure, a property which is not shared to the same extent with many of its derivatives. The three-bonded spectrum (248, 254, and 260 /x) of benzene will be considered as one chromophore. Benzene absorbs at 184 fi (Om 60,000), 203.5 fi (Om 7400), and 254 fi (Om 204) in hexane (4). These maxima are considered as the 7T 7T bands of the benzene chromophore. Increasing alkyl substitution causes a bathochromic shift of the 254-/i band, an eflFect which reaches its maximum at tetrasubstitution. New intense bands appear in the spectrum of benzoid compounds upon introduction of a substituent... 

Broadly speaking, the cavity sizes of a-, f -, and y-CD are appropriate for binding simple derivatives of benzene, naphthalene, and anthracene, respectively (Sanemasa and Akamine, 1987 Fujiki et al., 1988 Sanemasa et al., 1989). Many studies of the inclusion of aromatics, particularly of dyes and other molecules with strong chromophores, have been reported, and these have been useful in delineating the main features of CD binding (Bender and Komiyama, 1978 Saenger, 1980 Szejtli, 1982 Atwood et al., 1984 Stoddart and Zarzycki, 1988). In contrast, the affinity of small to medium aliphatic molecules for CDs have been less well studied, most... 

In the last 15 years, Wagner and his co-workers have developed the intramolecular ortho photocycloaddition of alkenes to the benzene ring of the acetophenone chromophore. In these photoreactions, the ring opening products of ortho photocycloadducts, 1,3,5-cyclooctatriene derivatives (220), and their secondary photocycloadducts, tricyclic compounds (221), were often produced [271,272] (Scheme 63). The triplet exciplexes generated from ir, ir -triplet states of acetophenone derivatives were postulated as the reactive intermediates [273],... 

An extended study of the optical behavior of this class of products has been performed in connection with investigations on the conformations of lycorine (1) and related compounds. An empirical rule, similar to the octant rule, which allows the prediction of both the sign and magnitude of the Cotton effect at 290 nm from the steric orientation of the atoms about the aromatic chromophore, has been deduced from considerations on the ORD and CD curves of lycorine (1) and derivatives. When the benzene ring is viewed along the — z —+ z axis (the coordinates are fixed on the aromatic ring as depicted in Fig. 1), four back octants are defined which contribute, with the signs indicated, to the Cotton effect. 

Macralstonine, C44H54N4O5, crystallizes as colorless, rectangular rods, mp 293° (dec.), [a]D +27.5°, and contains one methoxyl, at least two C-methyl, and three, or possibly four, methylimino groups (33, 36). Its IR-spectrum discloses the presence of an imino or hydroxyl group, an alkoxyacrylic ester chromophore (peaks at 1652 and 1628 cm-1), and an o-disubstituted benzene nucleus. The formation of a derivative with dinitrophenylhydrazine indicates the presence of a potential carbonyl function. Macralstonine has also been reported to possess a marked hypotensive activity (36). 

Table I shows the effect of various systems such as micelles, swollen micelles (achieved by adding hexanol to CTAB), microemulsion systems, vesicles formed from a double-chain CTAB surfactant, and reversed micelles with water cores formed with benzyl dimethylcetylammonium bromide in benzene. Hie active chromophore exists either as pyrene, pyrene sulfonic acid or pyrene tetrasulfonlc acid. Essentially the concept here is that the polar derivatives of pyrene will always locate pyrene at the surface of the micelle as these anionic species of pyrene complex with the positively charged surface. Dimethylaniline is used as an electron donor in each case, it can be seen that for pyrene, a continual decrease in the yield of the pyrene anion (ion yield of unity in the micelle) is observed on going from micelle to swollen micelle, to microemulsion, and no yield of ions is observed in a reversed micelle system. With pyrene tetrasulfonic acid the yield of ions over the different systems is fairly constant, even across to the reverse micellar system. However, the lifetime of the ions is extremely short in the reversed micellar system. An explanation for such behavior can be given as follows as we transverse across the...

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