Basicity bathochromic

Optical Spectra. The main (a) band in a variety of visual pigments exhibits absorption maxima in the range between 430 and 580 nm. It is this variability, as well as the basic bathochromic shift relative to a free PRSB in solution, which have provided the basis for most of the spectroscopic theories relevant to the structure of the chromophore and its environment in the binding site. Attempts to rationalize the shift in terms of charge-transfer complex formation between the (unprotonated) Schiff base and a protein functional group (200,210,212,228) have never... 

Such interactions are presumed to induce and control electron delocalization in the it system. As a commonly accepted principle in polyene spectroscopy (6,60,243), delocalization induces a basic bathochromic shift in the absorption maximum of the main (1B+) band. The magnitude of the shift in each pigment is controlled by specific electrostatic effects in the binding site. 

A bathochromic shift of about 5 nm results for the 320-nm band when a methyl substituent is introduced either in the 4- or 5-posiiion, The reverse is observed when the methyl is attached to nitrogen (56). Solvent effects on this 320-nm band suggest that in the first excited state A-4-thiazoline-2-thione is less basic than in the ground state (61). Ultraviolet spectra of a large series of A-4-thiazoline-2-thiones have been reported (60. 73). 

A 2-methylthio substituent decreases the basicity of thiazole pK = 2.52) by 0.6 pK unit (269). The usual bathochromic shift associated with this substituent in other heterocycles is also found for the thiazole ring (41 nm) (56). The ring protons of thiazole are shielded by this substituent the NMR spectrum of 2-methylthiothiazole is (internal TMS, solvent acetone) 3.32 (S-Me) 7.3 (C -H) 6.95 (Cj-H) (56, 270). Typical NMR spectra of 2-thioalkylthiazoles are given in Ref. 266. 

Electron-donating or -withdrawing properties of a substituent on the 4 and 5 positions have also been used in order to modulate the basicity in the hope to observe either hypsochromic or bathochromic shift (110). 

This bathochromic shift is typical of 77 —> tt transitions. The behavior of the water solution when acidified was attributed by Albert (175) absorption by the thiazolium cation, by analogy with pyridine. However, allowance is made for the very weak basicity of thiazole (pK = 2.52) compared with that of pyridine (pK = 5.2), Ellis and Griffiths (176) consider the differences between the spectrum of thiazole in water and in... 

A further strong bathochromic shift is observed as the basicity of the primary amines is increased by A/-alkylation, eg, malachite green [569-64-2] Cl Basic Green 4, =621 nm (5). 

Their physical properties closely resemble those of pterin, which has a basic pKt, of 2.20 and an acidic one of 7.86 associated with N-1 protonation and a hypsochromic shift of the long-wavelength absorption band in the UV spectrum, and N-3 deprotonation effecting a bathochromic shift respectively (Table 4). The xanthopterin (4) and isoxanthopterin types... 

Attempts have been made to deduce the structure of the predominant form of a potentially tautomeric compound from the shifts which occur in the ultraviolet spectrum of the compound in question on passing from neutral to basic or acidic solutions. The fact that no bathochromic shifts were observed for 2- and 4-hydroxy quinoline and 1-hydroxyisoquinoline under these conditions was taken as evidence that they existed in the oxo form [similar work on substituted quinol-4-ones led to no definite conclusions ]. A knowledge of the dissociation constants is essential to studies of this type, and the conclusions can, in any case, be only very tentative. A further dif-... 

The ultraviolet absorption spectra of carboline derivatives have been repeatedly recorded. Since the basic jpyr-N in the carbohnes and in 3,4-dihydro-jS-carbolines is part of a conjugated system, protonation affects the electronic absorption spectra significantly. It is unfortunate therefore that the spectra of the protonated, as well as those of the unprotonated, species have not been reported in all instances. Protonation leads to a bathochromic shift of 20-30 mp,. This is illustrated by the absorption of j3-carboline, 1-methyl-jS-carboline, 7-methoxy-l-methyl-jS-carbohne, and the salts of these compounds. 

Absorption and emission spectra of six 2-substituted imidazo[4,5-/]quinolines (R = H, Me, CH2Ph, Ph, 2-Py, R = H CH2Ph, R = Ph) were studied in various solvents. These studies revealed a solvent-independent, substituent-dependent character of the title compounds. They also exhibited bathochromic shifts in acidic and basic solutions. The phenyl group in the 2-position is in complete conjugation with the imidazoquinoline moiety. The fluorescence spectra of the compounds exhibited a solvent dependency, and, on changing to polar solvents, bathochromic shifts occur. Anomalous bathochromic shifts in water, acidic solution, and a new emission band in methanol are attributed to the protonated imidazoquinoline in the excited state. Basic solutions quench fluorescence (87IJC187). 

Aminoquinoxaline exists predominantly as such rather than in the tautomeric imino form. This is indicated by a comparison of the basic strength of the 2-amino compound (pKo 3.90) and those of its fixed methylated tautomers, 2-dimethylaminoquinoxaline pKa 3.72) and l-methylquinoxalin-2-oneimine (pfCn 8.70). The ultraviolet spectrum of the neutral molecule of 2-dimethylaminoquinoxaline shows the expected bathochromic shifts compared to that of 2-aminoquinoxaline these spectra differ from the ultraviolet spectrum of the neutral molecule of l-methylquinoxalin-2-oneimine (Fig. 1). The mono-cations (68) and (69) derived from 2-aminoquinoxaline and l-methylquinoxalin-2-oneimine have a similar chromophoric system and show almost identical ultraviolet absorption (Fig. 2). 

These values equal 2.0, 1.05, and 0.5, respectively, for PP, DPAcN, and PPA. It is possible that the contribution of excited states caused by n - it transitions accounts basically for a bathochromic luminescence of some PCSs and for a shift of the maxima in the luminescence spectra of polymers of this kind when proceeding from the solution to the solid phase. PCS solutions reveal concentration-quenching accom-... 

Basic strength, of polymethine dyes, 72 Bathochromic shift, of thiazolocyanines in relation with nitrogen atom in chain, 78... 

A similar effect can be observed in anthaquinones, mainly for the presence of an hydroxyl group. The ionization of hydroxyl groups under basic conditions also undergoes a bathochromic shift. Alizarin has two absorption bands in the vis region, simated at 567 and 609 nm carminic acid has a visible absorption maximum at around 500 nm and kermesic acid at 498 mn. 

The dramatic bathochromic shifts found for ligands such as 6 and various more elaborated probes in the presence of basic anions in organic solvents are mainly due to deprotonation effects [43,44]. 

Figure 4.10 shows the UV absorption spectra of a solution of procaine in 0.1 M HCl and O.IM NaOH. In procaine, the benzene chromophore has been extended by addition of a C = O group and under acidic conditions, as in Figure 4.10, the molecule has an absorption at 279 nm with an A (1%, 1 cm) value of 100. In addition to the extended chromophore, the molecule also contains an auxochrome in the form of an amino group, which under basic conditions has a lone pair of electrons that can interact with the chromophore producing a bathochromic shift. Under acidic conditions the amine group is protonated and does not function as an auxochrome but when the proton is removed from this group under basic conditions a bathochromic shift is produced and an absorption with A, max at 270 nm with an A (1%, 1 cm) value of 1000 appears. 

The chromophore of phenylephrine is not extended but its structure includes a phenolic hydroxyl group. The phenolic group functions as an auxochrome under both acidic and alkaline conditions. Under acidic conditions it has two lone pairs of electrons, which can interact with the benzene ring and under basic conditions it has three. Figure 4.11 shows the bathochromic and hyperchromic shift in the spectrum of phenylephrine, which occurs when 0.1 M NaOH is used as a solvent instead of 0.1 M HCl. Under acidic conditions the X max is at 273 and has an A (1 %, 1 cm) value of 110 and under alkaline conditions the X max is a 292 nm and has an A (1%, 1 cm) value of 182. 

The absorption and emission spectra of 10-CPT in water-methanol mixtures at room temperature (ca. 22 °C) in the pH range from neutral to basic in general exhibit a well-known naphthol-type behavior [4]. The absorption spectra in neutral water and methanol are nearly identical, the latter having a 3 nm bathochromic shift. The equilibrium between neutral (ROH, 380 nm) and Odeprotonated (RO, 420 nm) forms of 10-CPT is characterized by a pof 8.9 in a 68% mole H2O mixture (Fig. 1 right). The protonation of quinolinium nitrogen has a pKa of around 1.0, more than 3 pK units lower than the parent 6HQ. 

Azo dyes of this type are classed as donor-acceptor chromogens and basically a red (bathochromic) shift is produced by the introduction of electron withdrawing substituents in the diazonium component and by electron donating substituents in the coupler. An interesting anomaly is the large bathochromic shift produced by am-acetamido group in the coupler (79JCS(P1)1990>. 

The fluorescence of coumarin compounds has been well known since 1911, when it was observed that the absorption band of coumarin at 311 nm was bathochromically shifted by one of several auxochromic groups, such as hydroxyl or amino, in the 3- or 7-position. A comparison of the stilbene molecule (88) with 3-phenylcoumarin (93) indicates the similarity of structure, and basically similar substitution patterns in the latter molecule have produced similarly useful FBAs (69FRP1568007). 

Fig. 1 Basic structure (left side) and the absorption spectra (right side) of some photosensitizers. The absorption spectra of porphyrins (derived from porphins) consists of a Soret band (around 400 nm) and four Q-bands. Upon reduction of one or two double-bonds of the tetra-pyrrole structure or by expanding the number of 7i-electrons (by expanding the ring structure), the outermost Q-band becomes bathochromically shifted and the absorption coefficient increased as indicated on the figure. As described in the text, such chemical modifications are important for improving the therapeutic effect in deeper tissue layers...

---