Solvatochromism benzene

The variations of the simulated steady state solvatochromism, as a function of the polar solvent molarity, were found to be in good agreement with the experimental work of Krolicki et al. [4], both for absorption and fluorescence. The difference of Stokes-shifts between benzene and acetonitrile is 981 cm-1, compared to 1230 cm-1 obtained experimentally. These numbers are 870 cm-1 and 1910 cm-1, respectively, for methanol. 

In this work we presented the results of Molecular Dynamics simulations performed to study the solvatochromism and the dynamic stokes-shift of coumarin 153 in mixtures of solvents. We showed the ability of MD to reproduce available data of the time-dependent Stokes-shifts. Moreover, MD allowed us to interpret these dynamics in benzene-acetonitrile mixtures in terms of motions of benzene around the coumarin or rotation of acetonitrile. The role of benzene in the solvation process of Cl53 seems to be more important than usually assumed. 

In short, our S-MC/QM methodology uses structures generated by MC simulation to perform QM supermolecular calculations of the solute and all the solvent molecules up to a certain solvation shell. As the wave-function is properly anti-symmetrized over the entire system, CIS calculations include the dispersive interaction[35]. The solvation shells are obtained from the MC simulation using the radial distribution function. This has been used to treat solvatochromic shifts of several systems, such as benzene in CCI4, cyclohexane, water and liquid benzene[29, 37] formaldehyde in water(28, 38] pyrimidine in water and in CCl4(31] acetone in water[39] methyl-acetamide in water[40] etc. 

K. Coutinho et al., A Monte Carlo-quantum mechanics study of the solvatochromic shifts of the lowest transition of benzene. J. Chem. Phys. 112, 9874—9880 (2000)... 

Figure 7-8. The individual values of the solvatochromic shift of the first1B2U 17-77 transition of benzene in water obtained for each one of the 100 statistically uncorrelated configurations, composed of benzene surrounded by the first solvation shell of water...

Figure 7-9. The convergence of the average solvatochromic shift of the first 1 Ii2u tt-tt transition of benzene in water with respect to the number of uncorrelated configurations included in the quantum mechanical calculations. The solid line represents the final average value and the dotted lines the interval of the statistical error (see Eq. 7-4)...

In Table 7-1, we summarize the results obtained for the solvatochromic shift of the first B2u tt-tt transition of benzene in four different environments. As can be seen, the final calculated solvatochromic shifts ( >100 are in excellent agreement with the experimental results in all cases studied. For instance, in the case of liquid benzene 100 QM calculations were performed in uncorrelated supermolecular configuration selected in an interval of 800 successive configurations, each one composed of one benzene surrounded by 13 nearest benzene molecules. 

Table 7-1. Calculated solvatochromic shift of the first 1B21] tt-tt transition of benzene in different solvents. s is the correlation interval and (AE)ioo is the calculated average solvatochromic shift (in cm1) obtained with 100 uncorrelated configurations. The error in (AE)ioo is obtained from Eq. (7-4) and the experimental results are obtained from Bayliss and Hulme [66]...

As Table 7-1 shows, the final average result for the solvatochromic shift of the 1B2u tt-tt transition of homogeneous liquid benzene is calculated to be -306 11 cm-1, in excellent agreement with the experimental result of -332 30 cm-1 [66]. 

The primary standard betaine dye (44) is only sparingly soluble in water and less polar solvents it is insoluble in nonpolar solvents such as aliphatic hydrocarbons. In order to overcome the solubility problems in nonpolar solvents, the more lipophilic penta-t-butyl-substituted betaine dye (45) has additionally been used as a secondary reference probe [174]. The excellent Hnear correlation between the Ej values of the two dyes allows the calculation of t(30) values for solvents in which the solvatochromic indicator dye (44) is not soluble. Introduction of electron-withdrawing substituents e.g. Cl [323], F, CF3, C6F13 [324]) in the betaine molecule reduces the basicity of its phenolate moiety, which allows the direct determination of x(30) values for somewhat more acidic solvents. Moreover, the Hpophilic and fluorophilic penta(trifluoromethyl)-substituted betaine dye (46) is more soluble in nonpolar solvents e.g. hexafluoro-benzene) than the standard dye (44) [324]. Conversely, the solubility in aqueous media can be improved through replacement of some of the peripheral hydrophobic phenyl groups in (44) by more hydrophilic pyridyl groups, to yield the more water-soluble betaine dye (47) [325]. The Ej values of these new secondary standard betaine dyes correlate linearly with the x(30) values of (44), which allows the calculation of x(30) values for solvents in which only betaine dyes (45)-(47) are sufficiently stable and soluble for the UV/Vis spectroscopic measurements [324, 325]. 

A special property of these luminescent carbonyl diimine complexes is their solvatochromic behavior. The MLCT absorption bands exhibit a noticeable red shift when the polarity of the solvents decreases (negative solvatochromism). For example, the MLCT transition of [Re(CO)3(bpy)Cl] occurred at 370 and 400 nm in CH3CN and benzene respectively. Similar solvatochromism was also observed for the charge-transfer bands of other carbonyl complexes... 

Bosch, Roses and coworkers explored the use of solvatochromic probes in solvent mixtures involving various alcohols with water as well as with other alcohols, hexane and benzene in a series of papers106-110. The aniline derivatives they employed were 5 for the polarity/polarizability and the 8/5 probe pair for the EPD/HBA properties of the mixtures, in conjunction with other suitable non-aniline probes. In order to ensure that what is being... 

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. 

The long wavelength absorption band, due to the cross-conjugation, is very sensitive to the solvent polarity. For example, l,8-bis(dimethylamino)-4-nitronaphthalene (108) reveals a positive solvatochromism with Xmax values of 411, 444, 463 and 484 nm in n-hexane, benzene, methanol and DMSO, respectively80. On the other hand, 4-[2- 4, 5 -bis(dimethylamino)naphth-l -yl -vinyl]-l-methylpyridinium perchlorate (128) displays a negative solvatochromism, reminiscent in this respect of the known solvatochromic betaine 129 (Table 12). 

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