The halochromism of the crown‐ether‐substituted betaine dyes 1 is very sensitive to the cation size of the added electrolytes. The strongest hypsochromic shift of the longest‐ wavelength UVIVIS absorption results for n = 0 with Na⊕, n = 1 with K⊕, and n = 2 with Cs⊕. Thus, the dyes 1 may be useful as indicators.
Synthesis, negative solvatochromism (Table 1, Figure l), and negative halochromism (Tables 2 -3, Figures 2 -3) of the new crown ether-substituted pyridinium N-phenolate betaine dyes 3 -6 are described. In contrast to other known chromoionophores, 3 -5 exhibit a cation-selective negative halochromism, which manifests itself as a hypsochromic shift of the longwavelength intramolecular charge-transfer (CT) absorption band of the betaine chromophore on addition of salts. As this The position of the longest-wavelength absorption band in the UV/Vis solution spectra of the pyridinium N-phenolate betaine dye 1 depends on the polarity of the solvent (solvatochromism), the temperature of the solution (thermochromism), the external pressure applied to the solution (piezochromism), the nature and concentration of added salts (halochromism), and possibly -in the case of chiral betaine dyes -on the configuration of enantiomerically pure, homochiral solvents (chiro-solvatochromism)~4~51. The unusually large negative solvatochromism of 1 has been used to introduce spectroscopically derived empirical parameters of solvent polarity, the so-called ET (30) It has been shown['.'71 that the salt-induced negative halo~hromism['~] of 1 and 2 stems mainly from an ion-pairing between the cation, delivered from the salt added, and the phenoxide part of the betaine dye. There is even a linear correlation between the halochromic band shifts and the charge-to-size ratio (effective charge) of the cations, at least in acetonitrile as solvent ['.'41. This cation-influenced negative halochr~rnism~'~~ should be enhanced and made more cation-selective by the introduction of crown ether rings into the phenolate moiety of 1. For this reason, we have synthesized und UV/Vis-spectroscopically studied the new chromoionophoric[''] betaine dyes 3 -6, which have several advantages over the chromoionophoric dyes known to CT absorption band lies within the visible spectral region, the cation-induced colour change can b e followed easily by eye, which suggests an application of these new chromoionophoric betaine dyes as cation indicators. The halochromism of 3-6 constitutes a new type of true halochromism, in contrast to the trivial halochromism first defined by Baeyer and Villiger in 1902. date[*". The crown ether-substituted betaine dyes 3 -6 possess cavities capable of binding suitable guest species such as alkaline metal cations by means of ion/dipole interactions -with concomitant colour changes in the visible spectral range as desirable result. Because the intensity of the longwavelength charge-transfer (CT) absorption band of 1 [**] (and of 2-5) is rather low ( E ca. 300-800 1 . mol-' . cm-'),
Sehr selektiv reagiert die Halochromie der Kronenether‐substituierten Betainfarbstoffe 1 auf die Kationengröße des zugesetzten Elektrolyten. Die stärkste hypsochrome Verschiebung der längstwelligen UV/VIS‐Absorption resultiert für n = 0 mit Na⊕, n = 1 mit K⊕ und n = 2 mit Cs⊕. Damit könnten die Farbstoffe 1 auch als Indikatoren in Frage kommen.
Key Words: Acetonitrile- Pyridinium N-phenoxide betaine dyes are solvatochromic, thermochromic, piezochromic, halochromic, and possibly chiro-solvatochromic. That is, the longest wavelength, intramolecular chargetransfer (CT) absorption band of their UV/Vis solution spectra depends on the polarity of the solvent, the temperature of the solution, the external pressure, the nature and concentration of added salts (electrolytes; ionophores), and possibly -in the case of homochiral betaine dyes -on diastereomorphic solute/solvent interactions with pairs of enantiomorphic solvent molecule^[^-^]. The exceptionally large negative solvatochromism of 1 has been used to define an empirical parameter of solvent polarity, the Ed30) values, which are known for numerous organic solvents and many binary solvent mixtures[3-51. The better water-soluble tris(methanesu1fon-y1)-substituted betaine dye 2 recently enabled us to determine ET(30) values of aqueous electrolyte solutions[61. The chance to determine empirically the polarity of aqueous and nonaqueous electrolyte solutions by means of the solvatochromic standard betaine dyes l and 2 has recently led to increased investigations by US[^-^] and other^[^-'^] of the halochromism of 1 and 2, caused by the addition of ionophores["I to solutions of 1 and 2 at room temperature.The study of the ionophore-induced halochromism of solvatochromic dyes other than 1 and 2 has also been intensified in recent years[19-24~ In this paper we report on the ionophore-induced halochromism of 1 in acetonitrile solution at 25 "C. Acetonitrile was chosen because of its good solvency for salts, its rather high relative permittivity (E, x 36), thus promoting the dissociation of ion pairs, and its non-hydrogen bond donor (non-HBD) character, thus avoiding specific solute/solvent interactions such as intermolecular hydrogen bonding.The addition of alkaline and alkaline earth iodides as well as magnesium perchlorate -up to a tenfold excess or up to a saturated salt solution -to a lop4 M solution of 1 in acetonitrile leads in all cases to a hypsochromic shift of its long-wavelength CT band by ca. 31 -213 nm (depending on the nature of the added ionophore; cf. Figure 1 and Table l), without substantially changing its shape. Only with tetra- decreases slowly after the addition of salt; therefore, the extinction coefficients E of the salt-containing betaine solutions are only approximate, measured relative to the absorption intensity of the salt-free betaine solution (cf. Table 1) mol/l; c(salt) 1 : 1 .
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