Hypsochromic solvatochromism

Dimroth et al. introduced 8 as a solvatochromic probe of solvent polarity having absorption in the visible region it shows the largest solvatochromic shift of any substance yet reported. Ey (30) is calculated with Eq. (8-76), like Z. (The peculiar symbolism arose because compound 8 happened to be No. 30 on the list of substances studied by Dimroth et al.) The shift is hypsochromic as solvent polarity is increased. Table 8-16 gives some Ey (30) values. - (30) is linearly... 

Compounds are called solvatochromic when the location of their absorption (and emission) spectra depend on solvent polarity. A bathochromic (red) shift and a hypsochromic (blue) shift with increasing solvent polarity pertain to positive and negative solvatochromism, respectively. Such shifts of appropriate solvatochromic compounds in solvents of various polarity can be used to construct an empirical polarity scale (Reichardt, 1988 Buncel and Rajagopal, 1990). 

To estimate how many dye molecules fit into the dendritic micelles, UV-titra-tion experiments have been employed. In comparison with the spectra of a pure pinacyanol chloride solution in water, the peaks of the absorption maxima of the dye in the presence of the dendrimer are shifted bathochromically due to solvatochromic effects, which indicates the incorporation of the dye within the branches of the dendrimer. At dye-to-dendrimer molar ratios higher than 4 1, in addition to the bathochromic shifts, hypsochromically shifted peaks start to appear, indicating that the dendrimer is not incorporating further dyes. We interpret this as an incorporation of up to four dyes within the branches of the dendrimer. This observation correlates with the calculated available space within the dendrimer, obtained from the molecular simulations. Further studies of the interactions of the dyes within the dendritic micelle are in progress. 

A hypsochromic shift in the emission band would be observed if the amount of excited-state distortion were lower in a rigid matrix than in fluid solution. Such an interpretation has been recently suggested for Cu4l4py4 and related copper clusters, and this constitutes an unusual case of molecules that exhibit rigidochromism but not solvatochromism in its emission from a cluster-centered excited state [115]. It is likely that both excited-state destabilization and distortion arise during the polymerization processes in the tungsten and rhenium organometallic systems. 

A series of yellow to greenish-blue aziridinyl azo dyes and their azo precursors containing a thienyl coupling moiety (i.e., 263), which have been prepared from 2-aminothiophenes, are relatively bathochromic. From the viewpoint of solvatochromism, a clear contrast existed between A ax values in different solvents thus, a positive solvatochromism was observed in aprotic solvents, whereas a hypsochromic shift was brought about in polar protic solvents <1999DP(40)99>. 

A particularly interesting solvatochromic merocyanine dye is l-methyl-4-[(4-oxocyclohexadienylidene)ethylidene]-l,4-dihydropyridine also called Brooker s merocyanine [48]. First it exhibits a bathochromic and then a hypsochromic shift of the long-wavelength n n absorption band as the solvent polarity inereases [309] cf. also entry 14 in Table 6-1. 

The visible n n absorption band is shifted hypsochromically by A1 = 53 nm on changing the medium from n-hexane to water. Based on the negative solvatochrom-ism of this aminyloxide radical, a spectroscopic solvent polarity scale, called the scale of Eewis acidity has been proposed [336]. Because of its pronounced negative sol-vatochromism, 4-nitropyridine-l-oxide has been recommended as an empirical indicator of the HBD acidity of solvents [330] cf. Section 7.4. The solvatochromic range of this A-oxide, measured in 48 different solvents, amounts to Av = +2840 cm (A1 = —31 nm) for the solvent change n-hexane water. 

Tamura and Imoto [74] and Hendrickson, Drickamer et al. [182] observed pressure effects on the solution spectra of (44). In all solvents used they found a hyp-sochromic shift of the longest-wavelength absorption band with increasing pressure. The observed hypsochromic shift of a betaine solution in methanol amounts to Ad = — 15 nm (Av = +600 cm AEj = +1-7 kcal/mol) on raising the pressure from 1 to 10 kbar [182]. On the supposition that this piezo-solvatochromism results from better solute/ solvent interactions with increasing pressure, it can be stated that, the higher the pressure, the more polar the solvent, and the higher the t(30) value. 

The addition of electrolytes (ionophores) to solutions of (44) causes hypsochromic shifts of its solvatochromic absorption band [197, 209-213]. This phenomenon can be designated as halo-solvatochromism ) For example, the addition of KI, Nal,... 

Another UV/Vis spectroscopic method for the determination of water in organic solvents involves the use of solvatochromic dyes (such as the pyridinium A-phenolate betaine dye (44) m Chapter 7.4), and is based on the observation that water has a very high polarity compared with most organic solvents [142-145]. Even small amounts of water cause a strong hypsochromic band shift of the dissolved solvatochromic dye, which can be related to the water content by a cahbration curve. A typical detection hmit of this method is of the order of 1 mg water in 1 mL solvent for routine spectrophotometers [142], An analogous solvatochromic method has been developed for the determination of aqueous ethanol mixtures [146],... 

Molecular EDA complexes as well as charge-transfer ion pairs show (negative) solvatochromism [128], i.e. the charge-transfer absorption maxima (2cx) undergo hypsochromic shifts with increasing solvent polarity. The solvatochromic effect is readily explained on the basis of the Marcus correlation for charge-transfer energies in solution [129], (Eq. 9) ... 

Wetzler and coworkers123 employed 4-aminophthalimide (63) and 4-amino-lV-methyl-phthalimide (64) as solvatochromic (and thermochromic) fluorescent probes in solvent mixtures. A bathochromic shift of the emission spectra was found in mixtures of toluene with ethanol and with acetonitrile123 when the more polar solvent was added to toluene, but raising the temperature causes a relative hypsochromic effect. Mixtures of benzene and acetonitrile were studied by Nevecna and coworkers124 for their polarity by means of the probes 46 and 47 and with respect to the correlation of this with the rate constants of the reaction of triethylamine with ethyl iodide. The fluorescence of the ammonium salt of 4-(l-naphthylsulfonate)aniline (84) in dioxane and water mixtures was studied by Hiittenhain and Balzer125. 

In the case of tire negative solvatochromism (blue or hypsochromic shift), the ground state is better stabilized by the polar solvent than the CT excited state. 

Polar compounds of a heterocyclic (6 5 6) system display a negative solvatochromism, the hypsochromic shift of the absorption maximum in solvents of increased polarity <80UKZ835, 92ZOB1903>. This effect is especially large for betaine (139) <85JOC4855>. 

The term "solvatochromism describes a significant change in position or intensity of an absorption or emission band of a compound in solution, when the polarity of the medium is changed. A bathochromic (or red) shift of the absorption band with increasing solvent polarity is known as positive solvatochromism The corresponding hypsochromic (or blue) shift is termed negative solvatochromism. 

Compound 2 (/ i = i 2= H) shows an opposite behavior. Its zwitterionic ground state has a calculated dipole moment of 22.6 D, which is reduced in its quinoid, excited state to a value of 13.7 D. An increase in solvent polarity destabilizes the charge-transfer transition of this compound. shifting the value of its longest wavelength band hypsochromically, from 620 nm in nonpolar CHCI3 to 442 nni in water (negative solvatochromism) (Fig. 3). 

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. 

---