Solvatochromic shifts of
Abstract
In this work three molecules exhibiting dual sensing solvatochromic behaviors are examined in the context of solvation in binary solvent mixtures (BSMs). The compounds studied involve two functional groups with high responsiveness to solvent polarity namely pentacyanoferrate(II) (PC) and azo groups. Two of these compounds are [2]rotaxanes involving alpha- or beta- cyclodextrin (CyD) and the third is their CyD-free precursor. The dual solvatochromic behavior of these compounds is investigated in water/ethlylene glycol (EG) mixtures and their dual solvatochromic responses are assessed in terms of the intensity of solvatochromism and the extent of preferential solvation. To achieve this the linear solvation model by Kamlet, Abboud and Taft [J. Organomet. Chem. 1983, 48, 2877–2887] and the two-phase model of solvation by Bagchi and coworkers [J. Phys. Chem. 1991, 95, 3311–3314] are employed. The influence of the presence or lack of CyD (alpha- or beta-) on these dual solvatochromic sensors is analyzed.
Keywords
- solvatochromic dyes
- rotaxanes
- preferential solvation
- (non) specific solute-solvent effects
- azo dyes
1. Introduction
Nowadays, solvatochromic probes (SPs) are regularly utilized in various types of applications which require sensing of environmental/medium effects in either a qualitative or a quantitative manner [1, 2, 3, 4, 5, 6, 7, 8, 9]. Today there is a large variety of published solvatochromic dyes corresponding to different media, e.g. organic solvents [8, 10], ionic liquids [10, 11], solvent mixtures [10, 12, 13, 14, 15] or solvent comprising polarity modifiers [16]. What often appears to be challenging is the choice of a suitable solvatochromic probe for the description of a physicochemical problem encompassing solvent polarity effects. It has been observed that for the same solvent/cosolvent mixture, different solvatochromic dyes may provide different quantitative results [17, 18]. Indeed in many cases, different spectroscopic techniques applied on the same ternary system solvent/cosolvent/probe(solute) may provide different results. Therefore, probing solvent polarity effects and preferential solvation (PS) phenomena occurring in solvent mixtures of two or more solvents are considered as highly difficult tasks [17]. The complexity of those physicochemical problems is high and the interpretation of the solvent-solvent or solute-solvent effects sensed by SPs needs to be carefully undertaken. In this work, the authors examine the solvatochromic responses of two probing groups: the azo-group and the pentacyanoferrate(II) group of three molecules dissolved in binary solvent mixtures (BSMs) involving water and ethylene glycol (EG). From the three molecules employed two are [2]rotaxanes involving
2. Materials and methods
2.1 General information
All correlations (single or multi- parameter linear, polynomial regressions and contribution analyses were carried out using statistical software
All compounds involved in this work (
2.2 Examining the stability of compounds 1,2a,b in solution
All solvatochromic compounds used in this work are isolated as stable solid compounds of green-blue color. The measurements presented were conducted in fresh solutions of each compound in the desired H2O/EG mixtures (typically prepared 15 min prior to measurement). That time corresponds to the equilibration time (each sample was vigorously stirred after mixing). Directly after this period of time their electronic absorption spectra were recorded. It was observed that in all cases the solutions remained unmodified as concluded through check of the absorbances of the bands maxima which were found to be stable for at least 30 minutes after equilibration. This observation clearly indicates that all three compounds are stable in solution and therefore suitable for the current investigation.
2.3 Preferential solvation model
In this work a renowned PS model is employed in order to describe PS phenomena occurring in BSMs comprising solvatochromic solutes. The model was introduced by Bagchi and coworkers about thirty years ago and is also known as “the two-phase model of solvation” (TPMS) [32, 33, 34]. TPMS considers that solvent molecules in a BSM are distributed between a local phase and a bulk phase according to Eq. 1. The local phase lies in the vicinity of the solvation area.
In Eq. 1 S1 and S2 symbolize the two mixed solvents in the bulk phase while
In Eq. 2
Finally, Eq. 3, provides
2.4 Applying the CNIBS/R-K equation
In order to determine isosolvation points (see below) pertaining to PS occurring in solutions of
2.5 Determining isosolvation points
For a BSM involving solvents
For the determination of
2.6 Quantifying the difference in the extent of PS
In order to quantify the difference of the extent of PS in aqueous EG through the two different types of transitions (MLCT(PCF) or
In Eq. 5
2.7 Single parameter regression analyses
To understand the role of various solvatochromic parameters expressing solvent polarity, single regression analyses were implemented (general Eq. (6)). SSP-LSERs
Where
2.8 KAT equation and contribution analysis
Moreover a multiparametric model was employed in order to assess the relative contribution of various solvatochromic parameters expressing solvent polarity, simultaneously. That model is the LSER introduced by Kamlet Abboud and Taft (KAT equation). This renowned LSER (Eq. 7) can provide information on the importance of dipolarity/polarizability, Hydrogen bond donor (HBD) acidity and Hydrogen bond acceptor (HBA) basicity of neat solvents or solvent mixtures.
Where
Through Eqs. 8–9 it is possible to determine the relative contribution (
and
where
1 | 2a | 2b | 1 | 2a | 2b | |
---|---|---|---|---|---|---|
0 | 73.12 | 74.26 | 74.26 | 51.46 | 49.02 | 50.71 |
0.051 | 71.48 | 73.88 | 73.50 | 49.06 | 48.67 | 48.85 |
0.097 | 72.38 | 73.50 | 73.88 | 48.44 | 47.16 | 47.18 |
0.139 | 70.77 | 72.38 | 72.57 | 47.26 | 46.15 | 47.18 |
0.178 | 69.91 | 71.48 | 71.48 | 46.23 | 45.36 | 46.67 |
0.212 | 69.73 | 71.48 | 71.48 | 46.34 | 44.50 | 46.50 |
0.245 | 69.73 | 71.48 | 71.84 | 45.29 | 44.10 | 46.50 |
0.392 | 69.73 | 70.95 | 71.66 | 45.78 | 43.74 | 44.85 |
0.492 | 69.73 | 70.77 | 71.12 | 45.06 | 42.94 | 43.66 |
0.659 | 69.73 | 70.77 | 70.77 | 43.95 | 41.85 | 42.83 |
0.854 | 69.73 | 70.77 | 70.77 | 42.94 | 41.08 | 41.94 |
1 | 69.73 | 70.42 | 70.42 | 41.60 | 40.35 | 40.94 |
3. Results and discussion
3.1 The bisensing solvatochromic compounds 1 and 2a,b
Recently Deligkiozi et al. [19] and subsequently Papadakis et al. [37] reported on the solvatochromic behavior of compounds
Interestingly, all three compounds also exhibit another transition which is significantly influenced by solvent polarity. The latter is attributed to the azobenzene group and corresponds to the forbidden
It is important to note here that the
Taken together, there is clear evidence that all three compounds are considered to be bisensing as they involve two functional groups (FC and azo groups) both responding to solvent polarity changes however at different extents (Table 1) as it will be thoroughly analyzed.
3.2 Resonance structures of compounds 1 and 2a,b
For a better understanding of the dual solvatochromic behavior of all compounds the analysis of their resonance structures is vital. while resonance structure
Finally, structure
The latter interaction of the azo group with its partly reduced neighboring viologen pyridin heterocycle obviously influences the
Frontier orbital representations of the tetraanion of dye
3.3 Solute-solvent interactions
It is also noteworthy that in cases
It is known that solvents with
3.4 Quantification of the solvatochromism of the FC and azo groups
A pertinent way to quantify, predict and rationalize solvent effects is the use of linear solvation energy relationships (LSERs). This approach has been employed in numerous research works focusing on solvent effect on a large variety of physicochemical properties [8, 10, 44, 45, 46]. The solvatochromism of PC complexes has been thoroughly investigated in this fashion as well [15, 16, 19, 31]. Particularly in case of PC complexes bearing pyridinic ligands (such as
Plots
3.5 Single solvent polarity parameter involving LSERs
Single solvent polarity parameter involving LSERs (SSP-LSERs) were employed in order to investigate the importance of various solvent polarity parameters on
For all three compounds the two main solvatochromic functional groups are the PCF and azo groups which both behave as fairly good HBA-bases being prone to formation of hydrogen bonds between the –CN and –N=N– groups respectively and hydrogen atoms of protic solvents (like water and EG). Therefore, the nearly linear correlation observed between
Taken together, compound
Dimesionless BSM polarity parametersa | ||||
---|---|---|---|---|
0 | 1.000 | 1.20 | 1.17 | 0.14 |
0.051 | 0.971 | 1.19 | 1.01 | 0.44 |
0.097 | 0.948 | 1.18 | 0.96 | 0.49 |
0.139 | 0.930 | 1.17 | 0.92 | 0.52 |
0.178 | 0.916 | 1.16 | 0.87 | 0.53 |
0.212 | 0.904 | 1.15 | 0.87 | 0.54 |
0.245 | 0.895 | 1.14 | 0.87 | 0.55 |
0.392 | 0.861 | 1.08 | 0.88 | 0.53 |
0.492 | 0.844 | 1.04 | 0.88 | 0.55 |
0.659 | 0.821 | 0.98 | 0.87 | 0.60 |
0.854 | 0.806 | 0.93 | 0.88 | 0.55 |
1 | 0.790 | 0.92 | 0.90 | 0.52 |
3.6 Multiprameteric LSERs
An alternative way to compare and quantify the solvatochromism of the two solvatochromic chromophores for
Compound | %Pπ* | %P | %P | ||||||
---|---|---|---|---|---|---|---|---|---|
41.70 | 2.106 | 23.58 | 9.370 | 14.07 | 51.05 | 34.88 | 0.4687 | 0.8973 | |
36.73 | 6.425 | 24.15 | 11.64 | 34.70 | 39.18 | 26.12 | 0.2422 | 0.9762 | |
41.37 | 6.902 | 19.94 | 9.330 | 40.71 | 35.11 | 24.18 | 0.3461 | 0.9473 | |
5.724 | 17.50 | 20.44 | 4.753 | 51.27 | 26.63 | 22.10 | 0.6124 | 0.9630 | |
20.42 | 28.96 | 13.26 | 58.56 | 23.77 | 17.67 | 0.3999 | 0.9854 | ||
16.34 | 22.53 | 6.521 | 64.66 | 18.21 | 17.13 | 0.4214 | 0.9845 |
The behavior of compound
3.7 PS effects as sensed by the FC and azo groups
It is well established that when a polar compound is dissolved in a BSM consisted of two solvents of different polarity, the compound/solute gets solvated selectively by one of the two solvents [17, 18]. This effect is obviously associated with preferential solute-solvent interactions developed in the vicinity of the solute molecules. Due to this effect the region around the solute (the so-called cybotactic region) is characterized by a different solvent/cosolvent molar ratio when compared to the bulk part of the solution i.e. the regions away from the cybotactic region. This interesting phenomenon, is attenuated when the two solvents consisting the BSM are similar in terms of structure and polarity [18]. There are numerous published models allowing for the quantification of selective solvation phenomena applicable to various types of solutes and BSMs. These models are generally categorized in thermodynamic and spectroscopy-based models [18]. In the latter case a solvatochromic solute is often employed in order to probe preferential solvation phenomena in BSMs and using spectrally measured shifts as inputs one can obtain various types of information pertaining to preferential solvation as output e.g. the solvent and cosolvent molar ratios in the cybotactic region. Through various spectroscopy-based models thermodynamic properties can also be determined for instance the molar free energy of transfer of the solute from one solvent to its cosolvent [18]. In this work preferential solvation of compounds
By plotting the experimentally determined MLCT and
First of all, through the TPMS quantitative treatment the composition of the cybotactic region of solutions of
Compound 1 | Compound 2a | Compound 2b | ||||
---|---|---|---|---|---|---|
0 | 0 | 0 | 0 | 0 | 0 | 0 |
0.051 | 0.569 | 0.660 | 0.510 | 0.526 | 0.553 | 0.555 |
0.097 | 0.590 | 0.561 | 0.560 | 0.555 | 0.610 | 0.526 |
0.139 | 0.635 | 0.766 | 0.599 | 0.662 | 0.610 | 0.642 |
0.178 | 0.681 | 0.952 | 0.634 | 0.784 | 0.630 | 0.784 |
0.212 | 0.675 | 1.000 | 0.676 | 0.784 | 0.637 | 0.784 |
0.245 | 0.728 | 1.000 | 0.698 | 0.784 | 0.637 | 0.731 |
0.392 | 0.702 | 1.000 | 0.719 | 0.880 | 0.714 | 0.757 |
0.492 | 0.740 | 1.000 | 0.770 | 0.917 | 0.782 | 0.846 |
0.659 | 0.807 | 1.000 | 0.853 | 0.917 | 0.838 | 0.917 |
0.854 | 0.880 | 1.000 | 0.922 | 0.917 | 0.907 | 0.917 |
1 | 1 | 1 | 1 | 1 | 1 | 1 |
This becomes more obvious when plotting the
Compound 1 | Compound 2a | Compound 2b | |
---|---|---|---|
0.18 | 0.20 | 0.30 | |
0.055 | 0.15 | 0.13 | |
0.18 | 0.070 | 0.047 | |
0.12 | 0.053 | 0.17 |
4. Conclusions
A general conclusion of the present study is that two distinct functional groups acting as chromophores (specifically the FC and azo groups) can probe solvation effects in different ways however this is true only on a quantitative basis. Qualitatively, both functional groups probed a strong negative solvatochromic effect in all cases of molecules studied. It became apparent though that FC is more sensitive to the dipolarity/polarizability of the medium whereas the azo group is slightly more responsive to polarity changes associated to the Lewis acidity and HBD-acidity of the medium. This holds true for compound
Acknowledgments
The author would like to thank Dr. D. Matiadis (NCSR Demokritos, Athens, Greece) for fruitful discussions revolving around the solvatochromism of heterocyclic compounds. IKY (Greek State Scholarship Foundation) is gratefully acknowledged for financial support to R.P. during his PhD; a part of this work is connected to the work carried out then.
References
- 1.
Liu, H.; Xu, X.; Peng, H.; Chang, X.; Fu, X.; Li, Q.; Yin, S.; Blanchard G. J.; Fang, Y. New solvatochromic probes: Performance enhancement via regulation of excited state structures. Phys. Chem. Chem. Phys. ,2016 , , 25210-25220. DOI: 10.1039/C6CP04293G18 - 2.
Ali, R.; Lang, T.; Saleh, S. M.; Meier, R. J. Wolfbeis O. S. Optical sensing scheme for carbon dioxide using a Solvatochromic probe. Anal. Chem., 2011 ,83 , 2846-2851. DOI: 10.1021/ac200298j - 3.
Landis, R. F.; Yazdani, M.; Creran, B.; Yu, X.; Nandwana, V.; Cooke, G.; Rotello. V. M. Solvatochromic probes for detecting hydrogen-bond-donating solvents. Chem. Commun. ,2014 ,50 , 4579-4581. DOI: 10.1039/C4CC00805G - 4.
Florindo, C.; McIntosh, A. J. S.; Welton, T. ; Brancod, L. C.; Marrucho. I. M. A closer look into deep eutectic solvents: exploring intermolecular interactions using solvatochromic probes. Phys. Chem. Chem. Phys. ,2018 ,20 , 206-213. DOI: 10.1039/C7CP06471C - 5.
Liu, H,; Xu, X.; Shi, Z.; Liu, K.; Yu, L.; Fang Y. Solvatochromic probes displaying unprecedented organic liquids discriminating characteristics. Anal. Chem., 2016 ,88 , 10167-10175. DOI: 10.1021/acs.analchem.6b02721 - 6.
Li, Z.; Askim, J. R.; Suslick, K. S. The optoelectronic nose: Colorimetric and Fluorometric sensor arrays. Chem. Rev. 2019 ,119 , 231-292. DOI: 10.1021/acs.chemrev.8b00226 - 7.
Machado, V. G.; Stock, R. I.; Reichardt, C. Pyridinium N-Phenolate Betaine Dyes. Chem. Rev. 2014 ,114 , 10429-10475. DOI: 10.1021/cr5001157 - 8.
Reichardt, C. Solvatochromic dyes as solvent polarity indicators. Chem. Rev. 1994 ,94 , 2319-2358. DOI: 10.1021/cr00032a005 - 9.
Cabota, R.; Hunter, C. A. Molecular probes of solvation phenomena. Chem. Soc. Rev., 2012 ,41 , 3485-3492. DOI: 10.1039/C2CS15287H - 10.
Reichardt, C.; Welton. T. Solvents and solvent effects in organic chemistry. 4th Edn. Wiley-VCH, 2011, Weinheim - 11.
Eilmes, A. Kubisiak, P. Explicit solvent modeling of Solvatochromic probes in ionic liquids: Implications of solvation Shell structure. J. Phys. Chem. B, 2015 ,119 , 113185-113197. DOI: 10.1021/acs.jpcb.5b07767 - 12.
Khajehpour, M.; Welch, C. M. Kleiner, K. A. Kauffman J. F. Separation of dielectric nonideality from preferential solvation in binary solvent systems: An experimental examination of the relationship between Solvatochromism and local solvent composition around a dipolar solute. J. Phys. Chem. A, 2001 ,105 , 5372-5379. DOI: 10.1021/jp010825a - 13.
Duereh, A.; Sato, Y.; Smith, R. L.; Inomata, H. Spectroscopic analysis of binary mixed-solvent-polyimide precursor systems with the preferential solvation model for determining solute-centric Kamlet–Taft Solvatochromic parameters. J. Phys. Chem. B, 2015 ,119 , 14738-14749. DOI: 10.1021/acs.jpcb.5b07751 - 14.
Papadakis, R. Preferential solvation of a highly medium responsive Pentacyanoferrate(II) complex in binary solvent mixtures: Understanding the role of dielectric enrichment and the specificity of solute−solvent interactions. J. Phys. Chem. B, 2016 ,120 , 9422-9433. DOI: 10.1021/acs.jpcb.6b05868 - 15.
Papadakis, R. Solute-centric versus indicator-centric solvent polarity parameters in binary solvent mixtures. Determining the contribution of local solvent basicity to the solvatochromism of a pentacyanoferrate(II) dye. J. Mol. Liq. 2017 ,241 , 211–221. DOI: 10.1016/j.molliq.2017.05.147 - 16.
Papadakis, R. The solvatochromic behavior and degree of ionicity of a synthesized pentacyano (N-substituted-4,40-bipyridinium) ferrate(II) complex in different media. Tuning the solvatochromic intensity in aqueous glucose solutions. Chem. Phys., 2014 ,430 , 29-39. DOI: 10.1016/j.chemphys.2013.12.008 - 17.
Ben-Naim, A. Theory of preferential solvation of nonelectrolytes. Cell Biophys. 1988 ,12 , 255-269. DOI: 10.1007/BF02918361 - 18.
Marcus, Y. Solvent mixtures: Properties and selective solvation, Marcel Dekker, Inc., 2002, New York - 19.
Deligkiozi, I; Voyiatzis, E.; Tsolomitis, A.; Papadakis, R. Synthesis and characterization of new azobenzene-containing bis pentacyanoferrate(II) stoppered push–pull [2]rotaxanes, with α- and β-cyclodextrin. Towards highly medium responsive dyes. Dyes Pigment., 2015 ,113 , 709-722. DOI: 10.1016/j.dyepig.2014.10.005 - 20.
Qu, D-H.; Ji, F-Y.; Wang, Q-C.; Tian H. A double INHIBIT logic gate employing configuration and fluorescence changes. Adv. Mater. 2006 ,18 , 2035-2038. DOI: 10.1002/adma.200600235 - 21.
Baroncini, M.; Gao, C.; Carboni, V.; Credi, A.; Previtera, E.; Semeraro, M.; Venturi, M.; Silvi, S. Light control of stoichiometry and motion in pseudorotaxanes comprising a cucurbit[7]uril wheel and an azobenzene-bipyridinium axle. Chem. Eur. J. 2014 ,20 , 10737-10744. DOI: 10.1002/chem.201402821 - 22.
Qu, D-H.; Wang, Q-C.; Tian, H. A half adder based on a photochemically driven [2] rotaxane. Angew. Chem. Int. Ed., 2005 ,44 , 5296-5299. DOI: 10.1002/anie.200501215 - 23.
Monk, P.M.S. The viologens: Physicochemical properties, synthesis and applications of the salts of 4,40-Bipyridine. John Wiley & Sons ltd; 1998, Chichester - 24.
Crano, J.C.; Guglielmetti, R.J. (Eds). Organic photochromic and thermochromic compounds. Main photochromic families, vol. 1. Kluwer Academic Publishers; 2002. New York. p. 341-67 - 25.
Deligkiozi, I.; Tsolomitis, A.; Papadakis, R. Photoconductive properties of a π-conjugated α-cyclodextrin containing [2]rotaxane and its corresponding molecular dumbbell. Phys. Chem. Chem. Phys., 2013 ,15 , 3497-3503. DOI: 10.1039/C3CP43794A - 26.
Papadakis, R.; Deligkiozi, I.; Giorgi, M.; Faure, B.; Tsolomitis, A. Supramolecular complexes involving non-symmetric viologen cations and hexacyanoferrate (II) anions. A spectroscopic, crystallographic and computational study. RSC Adv., 2016 ,6 , 575-585. DOI: 10.1039/C5RA16732A - 27.
Papadakis, R; Deligkiozi, I; Tsolomitis A. Synthesis and characterization of a group of new medium responsive non-symmetric viologens. Chromotropism and structural effects. Dyes Pigment., 2012 ,95 , 478-484. DOI: 10.1016/j.dyepig.2012.06.013 - 28.
Papadakis, R; Deligkiozi, I; Tsolomitis A. Spectroscopic investigation of the solvatochromic behavior of a new synthesized non symmetric viologen dye: Study of the solvent-solute interactions. Anal. Bioanal. Chem. 2010 ,397 , 2253-2259. DOI: 10.1007/s00216-010-3792-7 - 29.
Zhu, Y; Zhou, Y; Wang, Х. Photoresponsive behavior of two well-defined azo polymers with different electron-withdrawing groups on pushepull azo chromophores. Dyes Pigment., 2013 ,99 , 209-219. DOI: 10.1016/j.dyepig.2013.05.006 - 30.
Papadakis, R.; Tsolomitis, A. Study of the correlations of the MLCT Vis absorption maxima of 4-pentacyanoferrate- 4-arylsubstituted bispyridinium complexes with the Hammett substituent parameters and the solvent polarity parameters ETN and AN. J. Phys. Org. Chem. 2009 ,22 , 515-521. DOI: 10.1002/poc.1514 - 31.
Papadakis, R.; Tsolomitis, A. Solvatochromism and preferential solvation of 4-pentacyanoferrate 4-aryl substituted bipyridinium complexes in binary mixtures of hydroxylic and non-hydroxylic solvents. J. Solut. Chem., 2011 ,40 , 1108-1125. DOI: 10.1007/s10953-011-9697-z - 32.
Chatterjee, P.; Bagchi, S. Preferential solvation of a dipolar solute in mixed binary solvent: A study of UV-visible spectroscopy. J. Phys. Chem. 1991 ,95 , 3311-3314. DOI: 10.1021/j100161a064 - 33.
Banerjee, D.; Laha, A.K.; Bagchi, S. Preferential solvation in mixed binary solvent. J. Chem. Soc. Faraday Trans., 1995 ,91 , 631-636. DOI: 10.1039/FT9959100631 - 34.
Laha, A.K.; Das, P.K.; Bagchi, S. Study of preferential solvation in mixed binary solvent as a function of solvent composition and temperature by UV-vis spectroscopic method. J. Phys. Chem. A, 2002 ,106 , 3230-3234. DOI: 10.1021/jp0121116 - 35.
Redlich, O.; Kister, A.T.. Algebraic representation of thermodynamic properties and the classification of solutions. Ind. Eng. Chem. 1948 ,40 , 345-348. DOI: 10.1021/ie50458a036 - 36.
Krygowski, T.M.; Fawcett, W. R. Complementary Lewis acid-base description of solvent effects. I. Ion-ion and ion-dipole interactions. J. Am. Chem. Soc. ,1975 ,97 , 2143-2148. DOI: 10.1021/ja00841a026 - 37.
Papadakis, R.; Deligkiozi, I.; Nowak, E. K. Study of the preferential solvation effects in binary solvent mixtures with the use of intensely solvatochromic azobenzene involving [2] rotaxane solutes. J. Mol. Liq., 2019 ,274 , 715-723. DOI: 10.1016/j.molliq.2018.10.164 - 38.
Hofmann, K.; Brumm, S.; Mende, C.; Nagel, K.; Seifert, A.; Roth, I.; Schaarschmidt, D.; Lang, H.; Spange, S. Solvatochromism and acidochromism of azobenzene-functionalized poly(vinyl amines). New J. Chem., 2012 ,36 , 1655–1664. DOI: 10.1039/c2nj40313g - 39.
Sıdır, Y. G.; Sıdır, I; Taşal¸ E., E. Ermiş. Studies on the electronic absorption spectra of some monoazo derivatives. Spectrochim . Acta A2011 ,78 , 640–647. DOI: 10.1016/j.saa.2010.11.0 - 40.
Gasbarri, C.; Angelini, G. Polarizability over dipolarity for the spectroscopic behavior of azobenzenes in room-temperature ionic liquids and organic solvents. J. Mol. Liq., 2017 ,229 , 185–188. DOI: 10.1016/j.molliq.2016.12.033 - 41.
Qian, H-F.; Tao, T.; Feng, Y-N.; Wang, Y-G.; Huang, W. Crystal structures, solvatochromisms and DFT computations of three disperse azo dyes having the same azobenzene skeleton. J. Mol. Struct. 2016 ,1123 , 305-310. DOI: 10.1016/j.molstruc.2016.06.04 - 42.
Christoforou, D. Electronic effects of the azo group. PhD Thesis, University of Canterbury, New Zealand, 1981 - 43.
Mulski, M. J.; Connors, K.A. Solvent effects on chemical processes. 9. Energetic contributions to the complexation of 4-nitroaniline with α-cyclodextrin in water and in binary aqueous-organic solvents. 1995 ,4 , 271-278. DOI: 10.1080/10610279508028936 - 44.
Hickey, J.P.; Passlno-Reader D.R. Linear solvation energy relationships: “Rules of thumb” for estimation of variable values. Environ. Sci. Technol., 1991 ,25 , 1753-1760. DOI: 10.1021/es00022a012 - 45.
Endo, S; Goss, K-U. Applications of Polyparameter linear free energy relationships in environmental chemistry. Environ. Sci. Technol., 2014 ,48 , 12477−12491. DOI: 10.1021/es503369t - 46.
Williams, A. Free Energy Relationships in Organic and Bio-Organic Chemistry. Royal Society of Chemistry, Cambridge UK, 2003 - 47.
Kamlet, M.J.; Abboud, J.L.M.; Abraham, M.H.; Taft, R.W. Linear solvation energy relationships. 23. A comprehensive collection of the solvatochromic parameters, .Pi.*, .Alpha., and.Beta., and some methods for simplifying the generalized solvatochromic equation, J. Organomet. Chem. 1983 ,48, 2877–2887. DOI: doi.org/10.1021/jo00165a018 - 48.
The non-linear fit(s) in this figure correspond to polynomial fitting(s) and convey no physical meaning. They are simply used in order to visualize the sizable deviations from linearity - 49.
Ratts, K. W.; Howe, R. K.; Phillips, W. G. Formation of pyridinium ylides and condensation with aldehydes. J. Amer. Chem.Soc. 1969 ,91 , 6115-6121. DOI: 10.1021/ja01050a032 - 50.
Zoltewicz, J. A.; Smith, C. L.; Kauffman, G. M. Buffer catalysis and hydrogen-deuterium exchange of heteroaromatic carbon acids. Heterocycl. Chem. 1971 ,8 , 337-338. DOI: 10.1002/jhet.5570080236 - 51.
Elvidge, J.A.; Jones, J.R.; O'Brien, C.; Evans, E.A.; Sheppard, H.C. Base-Catalyzed Hydrogen Exchange. Adv. Heterocycl. Chem. 1974 ,16 , 1-31. DOI: 10.1016/S0065-2725(08)60458-4 - 52.
Marcus, Y. The use of chemical probes for the characterization of solvent mixtures. Part 2. Aqueous mixtures. J. Chem. Soc . Perkin Trans.1994 ,2 , 1751−1758. DOI: 10.1039/P2994000175 - 53.
Sindreu, R. J.; Moyá, M. L.; Sánchez Burgos, F.; González, A. G. Kamlet-Taft solvatochromic parameters of aqueous binary mixtures oftert-butyl alcohol and ethyleneglycol. J. Solut. Chem. 1996 ,25 , 289-293. DOI: 10.1007/BF00972526 - 54.
Deligkiozi, I.; Papadakis, R.; Tsolomitis, A. Synthesis, characterisation and photoswitchability of a new [2]rotaxane of α-cyclodextrin with a diazobenzene containing π-conjugated molecular dumbbell. Supramol. Chem. 2012 ,24 , 333-343. DOI: 10.1080/10610278.2012.660529