Basic Chromophore

The question of the contribution of the benzene rings to the indigo chromogen led to a search for the basic chromophore. Consecutively removing parts of the mesomeric system leaves a cross-conjugated structure 7 that, as the basic chromophore, still exhibits the typical deep color and redox properties of indigo. 

A Pariser-Parr-Pople (PPP) calculation indicates that the long-wavelength absorption band is what would be expected for the trans-s-cis-s-cis configuration shown by crystal-structure analyses to occur in the indigo crystal [2], 

It was possible to test the various theoretical structures proposed for the basic chromophore 7 by synthesizing urindigo [4,4,4, 4 -tetramethyl(2,2 -bipyrrolidiny-lidene)-3,3 dione] [7], 

---

Several basic chromophore stmctures have been proposed for this purpose. Anthraquinone dyes appear to be predominant since they have a wider color range, excellent photostabiHty, good solubiHty in Hquid crystal media, and very high order parameters. Typical basic stmctures of the three primary colors are illustrated in Figure 11. Some examples are given in Table 10. The appropriate combination of three primary colors gives a black display. 

Tile basic chromophors of indigo and of some tetraazafulvalenes are very similar and therefore the frontier orbitals are comparable (90JPC949). PPP calculations on the 1,3,5,7-tetraazafulvalene 90 (R = Me) showed a good agreement with its UV/VIS spectrum. Tlie substitution of both methyl-sulfanyl groups in 90 by diethylamino/piperidino moieties led to a hypso-chromic shift of about 45 nm. 

The chemistry of these phthalides has much in common with that of the compounds reviewed in the next chapter, i.e., the feMco-triarylmethanes, and in fact, the parent dyes of the two classes share the same basic chromophoric system. However, the latter are true redox systems, rather than pH indicators, and consequently have a different range of technical applications. The situation is complicated further in that the triarylmethane cationic dyes can also bleach at high pH, giving a hydroxide addition... 

The basic chromophore in 4a,4b-dihydrophenanthrenes is a highly folded fully conjugated hexa-ene bridged by the central two atom unit of carbon atoms 4a and 4b (7 a). This bridge acts both as a poly alkyl substituent and also as a skeletal constraint. 

There has been little systematic study of the UV spectra of condensed 1,2,3-triazine derivatives, but even so the technique has occasionally proved to be of some value in the solution of certain structural problems (see p. 234 and references 90, 117, 118, 125, 147, 191). Different classes of triazine derivatives give rise, as expected, to spectra that are characteristic of the basic chromophores and, while substituent effects have been noted within individual groups of compoimds, confirmation of the structure of a given compound is sometimes possible on the basis of the UV spectrum. 

The diphenylmethyl and triphenylmethyl cations can be considered the basic chromophores of di- and triarylmethane dyes 11 —41. However, the electronic state of the donor-substituted di- and triarylmethine dyes can be described better by breaking down the chromophore of these dyes into a straight- or branched-chain polymethine subchromophore, respectively, and two ethylene units (from the formal cleavage of the benzene rings) [2], This model allows the similarities between the di- and triarylmethine dyes and the polymethine s to be recognized. 

Bond orders and charge densities in the indigo molecule have been calculated [7] and compared with the results of X-ray analysis [8], These studies confirm the structural formula 1 and answer questions about the basic chromophore of the dye (see Section 2.4). 

The structure of 2,2,5,5-tetramethyl-2,3,5,6-tetrahydrothieno[3,2-6]thiophene-3,6-dione (see equations 64, 73), the first compound containing the basic chromophoric system of the thioindigo dyes, was also determined with the aid of X-ray analysis (77CB1421). 

The third set of molecules that bring out the uniqueness of zeolites in the context of asymmetric induction is of 2,4-cyclohexadienones (6,6-dimethyl-2,4-cyclohexadienones 38, and 2,2-dimethyl-1,2-dihydronaphthalenones 39) [279,282,287,289,293-295]. The basic chromophore in these molecules is the conjugated dienone that undergoes oxa-di-rr-methane rearrangement to give a bicyclic product (Scheme 21). According to the accepted mechanism, the chirality is introduced into the system at the first step, yielding the diradical intermediate... 

The answer is a. (Murray, pp 505—626. Scriver, pp 4029-4240. Sack, pp 121-138. Wilson, pp 287-320.) In mammals, p-carotene is the precursor of retinal, which is the basic chromophore of all visual pigments. Isopen-tenyl pyrophosphate and dimethylallyl pyrophosphate are isoprenoid isomers formed from the repeated condensation of acetyl CoA units. By continued condensation in mammalian systems, cholesterol can be formed. In plant systems, carotenoids are formed. In addition to producing the color of tomatoes and carrots, carotenoids serve as the light-absorbing molecules of photosynthesis. Ketone bodies are derived from condensation of acetyl CoA units but not from isoprenoid units. Vitamin C (ascorbic acid), carnitine, and thiamine (vitamin BO are not derived from isoprenoid units. 

H Hirashima and M Sumimoto. Basic chromophore and lencochromophore in mechanical pnlps Possible repetition of photochemical rednction-oxidation-rednction. Tappi 77 146-154, 1994. 

Since most of these materials are described only in the patent literature unaccompanied by pertinent UV-visible spectral data, it was not possible to survey the effect of substituents on the absorption spectra of the basic chromophores in order to predict a priori what substituents (types and position) might generate sensitizers suitable for use in the mid-UV region. In light of this, we decided to calculate the theoretical electronic absorption spectra of 1 and 2 and to computationally assess the effect of a variety of substituents. We describe here the results of these studies and the use of the theoretical predictions to drive a synthetic program designed to produce viable mid-UV sensitizers. 

It has been demonstrated that, in accord with theoretical predictions, the presence of resonance attracting substituents in position 5 (either standard or hyperconjugative) of the basic chromophore 1 produces the desired red shift of the second electronic transition. The sulfonate group fulfills this function, in fact in the case of aryl derivatives such as 8, a bit too well for the strong red shift of the second transition in aryl sulfonates results in very little residual absorption in the mid-UV region particularly at 313 nm. 

Oxetan formation of the carbonyl group with a C=C bond which is an extension of the basic chromophore occurs in No examples of... 

From the viewpoint of basic chromophore structures, organic pigments of commercial significance today may be classified into four major groups, namely... 

In these oxidation reactions, the formation of a cyclic ester originates a lactone ring. Under mUd oxidation conditions, OH-lactones of chlorophylls are formed, and under more severe conditions, the purpurins 18. Although the basic chromophore structure is conserved, there are changes in the electron absorption spectrum and the polarity. 

The validity of the method has been demonstrated by comparing packing arrangements based on a powder analysis with structures obtained through independent single-crystal studies. - - ong the molecules studied was the commercially important triphendioxazine systems. 6,13-Dichlorotriphendioxazine (CjgFIgN202Cl2), as shown in Figure 5.19, is the basic chromophore unit of a number of commercially important dyestuffs. No full crystal structure information was initially available on... 

---