The optical properties of compound 1 were studied by means of steady state absorption and emission spectroscopy and laser flash photolysis. Fluorescence measurements of ethanol solution of 1 showed that it is not fluorescent. Comparing with the fluorescent properties of the hydroxyl-group containing P.Y.101 and its derivatives, it can be concluded that the presence of intramolecular hydrogen bonding is operative for their strong fluorescence, while the competitive isomerisation via bond rotation might serve as a non-radiative decay pathway. This is in line with the observation of very strong fluorescence of P.Y.101 in solid state .
Absorption spectra of compound 1 were recorded in various solvents and the normalized spectra in absolute ethanol, diethyl ether and dioxane are presented in Figure 3. Bathochromic shift is observed going from diethyl ether to ethanol and DMSO showing maxima in the spectra at 381, 386 and 393 nm, respectively. Although that there is no significant difference in the shape of the absorption band (complex contour containing number of individual subbands, see Figure 3), we will emphasize on the change in relative intensities of the subbands, composing the overall contour, in three of the solvents. The observed shift in the position of the absorption maximum in these solvents is most probably caused by redistribution of the intensities of the individual subbands. In addition, the intensity of the shoulder at ca. 330 nm decreases when going from diethyl ether to ethanol. Complete list of the apparent maxima in ten different solvents and the estimated position of the subbands, obtained from derivative spectroscopy, is presented in Additional file 1: Table S1. Taking into account the possible tautomerism in this compound, the observed spectral changes can be attributed to solvent caused shift in the position of the tautomeric equilibrium .
Normalized spectra of compound 1 in absolute ethanol, diethyl ether and dioxane.
For further confirmation of the presence of tautomeric equilibrium in solution, temperature dependent absorption spectra were recorded in three different solvents - ethanol, DMSO and chloroform. The spectra in ethanol, recorded in the temperature range 20 – 60°C, are presented in Figure 4. Virtually the same temperature-dependent spectra were obtained in DMSO and chloroform. In all three solvents the spectral changes upon temperature elevation are characterized by a slight increase at ca. 330 nm and a decrease of the maximum at ca. 390 nm (see the difference spectra in the inset of Figure 4). Although the observed changes in the spectral shape and the related isosbestic points are not very well pronounced, they give the first indication that at least two species coexist in solution. Initially, the species absorbing at 386 and 330 nm were referred to as A and B, respectively, and the corresponding equilibrium constant, KT=[B]/[A], was estimated from the temperature dependent spectra in ethanol. Accordingly, the calculated values for ΔG are in the range of 2.26 to 1.53 kcal/mol (without accounting for the solvent contraction) and the corresponding values for ΔH and ΔS are estimated to 9.48 kcal/mol and - 24.01 cal/mol.K, respectively (Additional file 2: Table S2). Considering the complexity of the shape of the absorption band, we sought for additional experimental evidences for coexistence of tautomers in solution, as well as defining their types. Therefore, decomposition procedure and derivative spectroscopy were employed for the absorption spectra, and flashphotolysis experiments were performed.
Temperature dependence of the absorption spectra of 1 in absolute ethanol (1.7 × 10-5 M), recorded in the temperature range 20 – 60°C. Solid line - 20°C, dashed lines correspond to 30, 40, 50 and 60°C....
The different electronic structure of the tautomers, containing enol and keto fragment, determines quite different photochemical behavior. According to previous flash photolysis study of azonaphthol tautomeric compound 1-phenylazo-4-naphthol  upon absorption of a photon the E-form undergoes trans-cis isomerization (Scheme 1), whereas the keto tautomer does not show any detectable transient signal. In addition, the cis enol tautomer does not obey the direct cis-trans relaxation, being initially converted to the K-form and then to the corresponding enol form in order to restore the initial equilibrium tautomeric ratio. The process of relaxation of Ecis to K is governed by a rate constant k1 and the restoration of the tautomeric equilibrium is defined by kII = k2 + k3.
Possible tautomerization processes of compound 1 and the corresponding rate constants.
The kinetic curves for solution of compound 1 in methanol, measured at 340 and at 400 nm, presented in Figure 5, show a typical keto and enol behavior, respectively. As can be seen from Figure 5 a, at 340 nm, there is a fast process (kI=3.18 ± 0.22 × 106 s-1) of accumulation of the keto tautomer, followed by a relatively slow process (kII=3.61 ± 0.13 × 104 s-1) of returning to the equilibrium state (Figure 5 a inset). The changes at 400 nm can be attributed to the initial decrease of the trans enol form, converted by the laser flash to cis form, followed by a rise - restoration of the equilibrium state through the keto form. The process is complicated and consists of fast and slow components with rate constants similar to those measured at 340 nm. This behavior can be attributed to the complexity of the tautomers in 1, where except the pure keto (c) and enol (a) tautomers, a mixed keto-enol form exists (b).
Time dependence of the transient absorption of 1 in methanol solution at 340 nm (a) and 400 nm (b).
Based on the results from flash photolysis experiments it can be concluded that the species absorbing at 330 nm is the diketo tautomer (in Figure 2c) of compound 1 and coexists with the most stable diol tautomer (in Figure 2a). It might be expected, however, that the monoketo tautomer (in Figure 2b), being intermediate between (a) and (c), also exists in the solution and this was actually suggested from the flash photolysis kinetic curves. In search of further confirmation to this supposition we performed a detailed assessment of the composite shape of the absorption spectrum observed in ethanol. A mathematical curve decomposition procedure for quantitative analyses of tautomeric equilibria, developed by some of us,  was applied for the temperature dependent spectra of compound 1 in ethanol.
The temperature dependent spectra in ethanol were decomposed into 7 components, which were further attributed to the three possible tautomers of compound 1. The latter was done by monitoring the change in integral area of each component with temperature increase. Additionally, second derivative spectra were used to verify the position of the components’ absorption maxima. Three of the components with absorption maxima at 410, 380 and 354 nm show gradual decrease of the calculated area with temperature increase and were attributed to the most abundant diol form (a). The sum of the integral areas of these three components is denoted as Iidiol. The other two components exhibit maxima at 392 and 324 nm and are attributed to the monoketo (b) and the diketo (c) forms, respectively, with the corresponding integral areas Iimonoketo and Iidiketo. The data were used to calculate the molar fractions of each component using equation (1) (for details see Additional file 3)
As can be seen from Table 1, the diol form (a) is the main component at 20°C. The quantity of the diketo form (c) exceeds the monoketo tautomer (b). With the temperature increase the quantity of diol form diminish, whereas those of the other two species rise gradually. At 60°C the quantity of the diketo form prevails that of the diol form, while the presence of the monoketo tautomer is ca. 10%. The current observation is in accordance with the temperature dependent tautomerism in 4-phenylazo-1-naphthol, which shows lowering the quantity of the enol form by increasing the temperature in ethanol, whereas the opposite trend is observed in the 2-phenylazo-1-naphthol, 1-phenylazo-2-naphthol and the corresponding Schiff bases. In the case of compound 1 the enol – keto tautomerism takes place in two steps a ↔ b ↔ c, which are referred to as process 1 and 2, respectively (in Table 1). From the calculated differences in the Gibbs free energies for these processes the enthalpy and the entropy were calculated, too. The obtained results are as follows: for process 1) ΔH1 = 5.66 ± 0.46 kcal/mol, ΔS1 = -13.76 ± 1.46 cal/mol.K; for process 2) ΔH2 = -4.17 ± 0.38 kcal/mol, ΔS2 = 9.11 ± 1.22 cal/mol.K.
Calculated molar fractions of the components present in ethanol solution of 1 along with the corresponding equilibrium constants and the Gibbs free energy differences
Additionally to the observed spectral changes upon increasing the temperature, another phenomenon was observed in chloroform solutions. Due to the poorer solubility in chloroform the solutions were sonicated for certain time. As a result, spectral changes were detected corresponding to protonation of compound 1. The observed protonation is produced by the hydrochloric acid that is formed during the chloroform sonication through radical intermediates. The mechanism of the sonodegradation of trihalomethanes is well studied  and applications of the protonation of some compounds as sonochemical dosimeters are suggested . In Figure 6 the spectra of compound 1 in chloroform are presented. The inset shows the temperature dependent spectra of a solution that was sonicated for 3 seconds. The observed temperature dependence is analogous to the one already described. However, additional sonication for 3 minutes at 50°C leads to the spectrum presented in Figure 6 with solid line. It can be clearly seen that a new intense band appears at 469 nm as a result from protonation of compound 1. Furthermore, the spectral changes upon decreasing the temperature down to 10°C were monitored in order to estimate the thermodynamic parameters of the observed equilibrium.
Effect of temperature and sonication on the absorption spectra of chloroform solution of compound 1 (1.7 × 10-5 M) – temperature range 50 – 10°C, sonication time - 3 minutes; solid line -...
In order to confirm these results, a series of spectra were recorded in methanol with increasing amount of added hydrochloric acid and are presented in Figure 7. The observed changes in the spectral shape are the same as the ones resulting from the sonication.
Absorption spectra of 1 in methanol (2.7 × 10-5 M) with addition of HCl. The solid line shows the spectrum without acid, dashed lines correspond to the spectra with increasing concentration of the acid added: 8 × 10...
Quantum chemical calculations
The nineteen optimized conformers of 1 stem from the three possible tautomeric forms shown in Figure 2; the diol form (a) the keto form (b) and the diketo form (c). Moreover, the rotation around the N-N bond, leading to cis and trans isomers, was also taken into account. The trans isomers were found to be the preferred ones for all three tautomers. Furthermore, the rotation about the C-C bonds bearing the naphthyl rings leads to formation of different endo and exo isomers. All tautomers shown in Figure 2, are in endo-endo form (both N-atoms’ electronic lone pairs are directed towards the neighboring naphthalene rings). The DFT (B3LYP/6-31 G**) calculated energies of the most stable conformers of the tautomers a – c are listed in Table 2 and the optimized structures of the possible conformers of the tautomers (a), (b) and (c) are depicted in Figure 8. It must be noted that the most stable isomer of compound 1 is the endo-endo rotamer of the diol tautomer, a-R1 in Figure 8, whereas the crystal structure of P.Y.101 shows that it exist in the exo-exo form of the diol tautomer (similarly to the a-R3 in Figure 8). Apparently, the specific intramolecular interactions are operative for stabilization of the corresponding rotamers; the presence of intramolecular H-bond (N….H-O) in P.Y.101 stabilizes the exo-exo form, while in compound 1 the endo-endo rotamer is stabilized by the N….H-C interaction. The N….H-C distance in the optimized structure of a-R1 is 2.213 Å, its geometry is completely planar and it is the isomer with the lowest energy. Rotation about each C-C bond in the structure of the diol tautomers leads to the other two conformers of the diol tautomer - the a-R2 and a-R3 structures, which are also planar as could be seen from Figure 8. On the other hand, upon tautomerization the endo-endo rotamer is no longer the most stable one of the monoketo tautomer (b) but the endo-exo form b-R2 (see Figure 8 and Table 2). These data show that tautomerism of compound 1 is accompanied with rotation about the C-C bonds. As could be expected from streric point of view, the b-R2 form has also planar structure whereas the other two rotamers of the monoketo tautomer are slightly twisted (b-R1 and b-R2’). On the contrary, the optimized structures of the diketo tautomer (c) show almost perpendicular orientation of the naphthyl rings and it is hard to distinguish the different rotamers. Therefore, only the energy of the most stable form is given in Table 2 and in Figure 8.
Optimized structures (B3LYP/6-31 G**) of the possible tautomers of compound 1 , diol (a), keto (b) with some of their isomers, and the diketo tautomer (c). The dihedral angles 1) and 3) are defined between the N- and quaternary C-atom, as shown...
Calculated relative energies (in kcal/mol) and dipole moments for the most stable conformers of the possible tautomeric forms of compound 1 at the B3LYP/6-31 G** level (zero-point vibration energy correction included)
The optimized structures explain well the obtained dipole moments (Table 2) and the calculated vertical excitation energies (Table 3). While in tautomers (a) and (b), the relative planarity permits the conjugation between the electron donor and acceptor parts of the molecule, the structure of tautomer (c) clearly shows that such conjugation is not possible. Correspondingly, the TD-DFT calculated excitation energy for (c) is ca. 346 nm, while those for tautomers (a) and (b) are 367 and 393 nm, respectively. We shall emphasize that this results agree well with the assignment of the experimentally observed spectra based on the decomposition procedure and the derivative spectroscopy (vide supra). Unlike the calculated spectral properties, the theoretically predicted energies and relative stabilities in gas phase agree only qualitatively with the experimental observations. Therefore, we performed additional calculations using a fitted hybrid meta-GGA functional with 54% HF exchange - M06-2X, specially developed to describe main-group thermochemistry and non-covalent interactions, in combination with a large basis set, def2TZVP. The latter basis set was used also in calculation with the B3LYP functional and Hartree-Fock method. The ab initio HF calculations were perfomed for the sake of comparison, since it has been shown that the DFT methods do not predict correctly the relative energies of substituted azonaphthols . All data from our calculations are summarized in tables given in Additional file 4. The main conclusions from the isolated-molecule gas-phase calculations are that both hybrid functionals with both basis sets give virtually the same geometrical and energetical picture, predicting too large energy differences between the different tautomers of compound 1 (up to 20 kcal/mol). Such a disagreement with the experiment could point out that the role of the media is not negligible and the specific interactions between the solute and the solvent molecules most possibly contribute by a large extent to the stabilization of the various tautomeric forms.
Calculated vertical excitation energies (in nm) of the possible tautomers (a - c), their singly protonated forms* (a+, a+O), and bound methanol molecule to the corresponding tautomers (a_MeOH, b_MeOH) obtained by the ZINDO and TD-DFT –B3LYP/6-31 G** methods...
In order to account for the interaction between the solvent molecules and the solute, different types of adducts of compound 1 with a methanol molecule were modeled and optimized. Amongst them the most stable ones, for the corresponding tautomeric form, were those in which the methanol molecule is attached by intermolecular H-bonding to the OH-group of the compound 1. The energy difference between the diol (a) and the monoketo (b) forms, with attached methanol molecule, is 6.09 kcal/mol as obtained from the B3LYP/6-31 G** calculations. Furthermore, we reoptimized these solute-solvent complexes in methanol media using IEF-PCM calculation. The result from this supermolecule-PCM calculation lead to decrease of the energy difference between the diol (a) and the monoketo (b) forms down to 2.46 kcal/mol. The obtained relative energies from the PCM calculation of individual molecules and the supermolecule-PCM, including a methanol molecule, are compared in Table 4.
B3LYP/6-31 G** calculated energy differences (in kcal/mol) between the diol form a-R1 and the monoketo form b-R3 of compound 1, compared with the IEF-PCM (solvent methanol) and the supermolecule-PCM calculations
As can be seen from Table 4, formation of intermolecular complex of (a) and (b) tautomers of compound 1 with a methanol molecule do not cause appreciable change in the corresponding energy difference. It is the polarizable continuum model that leads to a lower energy difference between the tautomers. Taking into account the calculated dipole moments of these tautomers it is reasonable that the more polar (b) tautomer is better stabilized in the polar methanol media. However, such effect cannot be expected for the relatively non-polar diketo tautomer (c). Indeed, the energy difference between (a) and (c) forms is lowered from 21.78 to 14.03 kcal/mol only, by inclusion the IEF-PCM in the DFT calculations. Nevertheless, this result does not agree quantitatively with the experimental data.
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