Tautomerism of4,4′-dihydroxy-1,1′-naphthaldazine studied byexperimental and theoretical methods
© Ahmedova et al; licensee Chemistry Central Ltd. 2013
Received: 25 October 2012
Accepted: 31 January 2013
Published: 11 February 2013
The title compound belongs to the class of bis-azomethine pigments. On thebasis of comparative studies on similar structures, insight into the complexexcited state dynamics of such compounds has been gained. It has been shown,for example, that only compounds that possess hydroxyl groups arefluorescent, and that the possibility for cis-trans isomerisationand/or bending motions of the central bis-azomethine fragment allows fordifferent non-radiative decay pathways.
The compound, 4,4'-dihydroxy-1,1'-naphthaldazine (1) was synthesized andcharacterized by means of spectroscopic and quantum chemical methods. Thetautomerism of 1 was studied in details by steady state UV-Vis spectroscopyand time resolved flash photolysis. The composite shape of the absorptionbands was computationally resolved into individual subbands. Thus, the molarfraction of each component and the corresponding tautomeric constants wereestimated from the temperature dependent spectra in ethanol.
According to the spectroscopic data the prevalent tautomer is the diol form,which is in agreement with the theoretical (HF and DFT) predictions. Theexperimental data show, however, that all three tautomers coexist insolution even at room temperature. Relevant theoretical results wereobtained after taking into account the solvent effect by the so-calledsupermolecule-PCM approach. The TD-DFT B3LYP/6-31 G** calculatedexcitation energies confirm the assignment of the individual bands obtainedfrom the derivative spectroscopy.
Keywords4,4′-dihydroxy-1,1′-naphthaldazine Flash photolysis Quantum chemical calculations Tautomerism Tautomeric constants
Synthesis and spectroscopic measurements
All solvents and reagents used were AR grade. Fluka silica gel/TLC-cards 60778with fluorescent indicator 254 nm were used for TLC chromatography andRf-value determination. The melting point was determined incapillary tube on MEL-TEMP 1102D-230 VAC (Dubuque, IA, USA) apparatus withoutcorrections.
The title compound, N,N’-bis-(4-hydroxy-naphthylmethylene)-hydrazine(1), was obtained from naphthaldehyde according to a standard procedure; i.e. toa solution of 1-hydroxy-4-naphthaldehyde (1 mmol) in EtOH (2 ml)hydrazine hydrate (0.5 mmol) was added and the mixture was stirred at roomtemperature for 4 h. No acid catalyst was used in an attempt to avoid theprotonation of the product. The residue formed was filtered off, washed withEtOH and then with Et2O, and dried in air to give 60% yield of 1 as ayellow powder; m. p. 235-236 °C (236°C); Rf - 0.15 (EtOAc:hexane 2:1); 1H NMR 7.03 (d,2H, J 8.0, CH-2 Ar), 7.58 (ddd, 2H, J 1.1, 6.8, 8.2, CH-7 Ar),7.70 (ddd, 2H, J 1.5, 6.8, 8.4, CH-8 Ar), 7. 97 (d, 2H, J 8.2,CH-3 Ar), 8.30 (dd, 2H, J 1.1, 8.2, CH-6 Ar), 9.25 (s, 2H,CH=N), 9.30 (d, 2H, J 8.4, CH-9 Ar), 10.96 (bs,OH) ppm; 13C NMR 108.5 (CH-2 Ar), 120.9(Cquat-4), 123.1 (CH-6 Ar), 125.2 (Cquat-10), 125.6 (CH-9 Ar), 125.8 (CH-7 Ar), 128.3(CH-8 Ar), 132.7 (Cquat-5), 133.4 (CH-3 Ar), 157.2 (Cquat-1), 162.0 (CH=N) ppm; COSY cross peaks 7.03/7.97,7.58/7.70, 7.58/8.30, 7.70/9.30; HSQC cross peaks 7.03/108.5, 7.58/125.8,7.70/128.3, 7.97/133.4, 8.30/123.1, 9.25/162.0, 9.30/125.6.
The IR spectrum was recorded on a Bruker IFS-113 FTIR Spectrometer in KBr. TheNMR spectra were recorded on a Bruker Avance DRX 250 (for 1D) and Bruker AvanceII+ 600 (for 2D) spectrometers in DMSO-d6. The chemical shifts werequoted in ppm in δ-values against tetramethylsilane (TMS) as an internalstandard and the coupling constants were calculated in Hz. The assignment of thesignals in 1D NMR spectra is based on the observed cross peaks in 2D homo- andheteronuclear correlations COSY and HSQC, respectively. The UV-Vis spectralmeasurements were performed on a JASCO V-570 UV-Vis-NIR spectrophotometer,equipped with a Julabo ED5 thermostat (precision 1°C), in spectral gradesolvents. The obtained spectral curves were processed by a software foroverlapping bands decomposition [7, 8] and for derivative spectroscopy, developed by us . Laser flash photolysis experiments were performed using a setup thathas been described previously . Solutions were placed in quartz cells (4.5 cm long and1 cm wide) and excited by one of the following excitation sources: aLambda-Physik EMG 101 excimer laser operating at 308 nm (XeCl) with apulse energies of ca. 100 mJ and pulse widths of ca.30 ns. The photochemical stability of the samples was monitored.
Quantum chemical calculations
The quantum chemical calculations were performed with full geometry optimizationwithout any symmetry restrictions using the Gaussian 03 and Gaussian 09 programpackages . In order to evaluate the ground state properties of the studiedcompound, its potential energy surface was searched for stable conformers.Geometries of nineteen possible rotamers and tautomers were optimized by thesemiempirical AM1, ab initio Hartree-Fock and DFT methods. Vibrationalfrequencies were computed in order to verify that local energy minima wereattained. Selected structures were additionally optimized using the densityfunctional theory (DFT) and two hybrid B3LYP and M06-2X functionals [12–14] with two different basis sets, 6-31G** and def2TZVP . Vertical excitation energies were calculated employing ZINDO andtime-dependent DFT (TD-DFT) with the B3LYP/6-31G** at the equilibrium geometriesof the most stable conformers. The solvent effect was taken into considerationby the polarizable continuum model [16, 17] and IEF-PCM/B3LYP geometry optimization at the 6-31G** level inmethanol were carried out using the standard united-atom cavity model, asimplemented in G03 software. Solvent-solute interactions were modeled adding onemethanol molecule close to the enol OH group of compound 1 so thatintermolecular hydrogen bond, of type (solv)O….H-O, is formed. The modelswere optimized by B3LYP/6-31G** method, and the supermolecule-PCM approach wasemployed, doing IEF-PCM/B3LYP geometry optimization at the 6-31G** level inmethanol of the optimized solute-solvent complexes, similarly to someazonaphthols [18, 19].
Results and discussion
The optical properties of compound 1 were studied by means of steady stateabsorption and emission spectroscopy and laser flash photolysis. Fluorescencemeasurements of ethanol solution of 1 showed that it is not fluorescent.Comparing with the fluorescent properties of the hydroxyl-group containingP.Y.101 and its derivatives, it can be concluded that the presence ofintramolecular hydrogen bonding is operative for their strong fluorescence,while the competitive isomerisation via bond rotation might serve as anon-radiative decay pathway. This is in line with the observation of very strongfluorescence of P.Y.101 in solid state .
Based on the results from flash photolysis experiments it can be concluded thatthe species absorbing at 330 nm is the diketo tautomer (inFigure 2c) of compound 1 and coexists with themost stable diol tautomer (in Figure 2a). It mightbe expected, however, that the monoketo tautomer (in Figure 2b), being intermediate between (a) and (c), also exists in thesolution and this was actually suggested from the flash photolysis kineticcurves. In search of further confirmation to this supposition we performed adetailed assessment of the composite shape of the absorption spectrum observedin ethanol. A mathematical curve decomposition procedure for quantitativeanalyses of tautomeric equilibria, developed by some of us,  was applied for the temperature dependent spectra of compound 1 inethanol.
Calculated molar fractions of the components present in ethanolsolution of 1 along with the corresponding equilibrium constants andthe Gibbs free energy differences
Quantum chemical calculations
Calculated relative energies (in kcal/mol) and dipole moments for themost stable conformers of the possible tautomeric forms of compound1 at the B3LYP/6-31 G** level (zero-point vibration energycorrection included)
Relative energies [kcal/mol]
Dipole moment [Debye]
Calculated vertical excitation energies (in nm) of the possibletautomers (a - c), their singly protonated forms* (a+, a+O), andbound methanol molecule to the corresponding tautomers (a_MeOH,b_MeOH) obtained by the ZINDO and TD-DFT –B3LYP/6-31 G**methods
368.83 nm (f=1.01)
354.18 nm (f=1.21)
393.30 nm (f=1.15)
379.22 nm (f=1.41)
346.46 nm (f=0.15)
344.28 nm (f=1.49)
319.99 nm (f=0.80)
257.12 nm (f=0.60)
528.25 nm (f=0.50)
454.06 nm (f=1.05)
373.62 nm (f=0.56)
575.49 nm (f=0.34)
404.56 nm (f=0.85)
349.21 nm (f=0.48)
373.95 nm (f=0.98)
355.95 nm (f=1.22)
397.34 nm (f=1.16)
381.71 nm (f=1.43)
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 thesupermolecule-PCM calculations
Relative energies [kcal/mol]
As can be seen from Table 4, formation ofintermolecular complex of (a) and (b) tautomers of compound 1 with a methanolmolecule do not cause appreciable change in the corresponding energy difference.It is the polarizable continuum model that leads to a lower energy differencebetween the tautomers. Taking into account the calculated dipole moments ofthese tautomers it is reasonable that the more polar (b) tautomer is betterstabilized in the polar methanol media. However, such effect cannot be expectedfor the relatively non-polar diketo tautomer (c). Indeed, the energy differencebetween (a) and (c) forms is lowered from 21.78 to 14.03 kcal/mol only, byinclusion the IEF-PCM in the DFT calculations. Nevertheless, this result doesnot agree quantitatively with the experimental data.
The tautomerism in4,4′-dihydroxy-1,1′-naphthaldazine (1) was studiedby time resolved and steady state absorption spectroscopy. Temperature dependentspectra and laser flash photolysis indicate that the three possible tautomers arepresent in solution. The absorption spectra were decomposed into individual subbandsin order to estimate the relative abundance of all species present in the solution,applying two- and three-component analysis. Reasonably, the quantitative dataobtained by the two- and by three-component approach are in close agreement.
It is frustrating to conclude that the calculated energy differences of the studiedtautomeric species agree only qualitatively with the experimental data. Inclusion ofthe solvent effect as polarizable continuum medium improves the resultssignificantly, but not enough considering the stability of the diketo tautomer (c).On the other hand, the optimized geometries and the vertical excitation energies arein accordance with the experiment. The complicated mechanism of the studiedtautomerism is possibly the reason for the poor agreement between the theoreticalmodels and the experimental data. As mentioned above, the tautomerism is accompaniedwith rotation about the C-C bonds. That is why the solvent molecules play crucialrole in the mechanism and the dynamics of the studied tautomeric processes.
The financial supports from Bulgarian National Science Fund (Projects TK-X-1716and UNA-17/2005) are gratefully acknowledged. We thank Dr P. Mueller (Universityof Basel) for the flash photolysis measurements. Publication of the paper isfinanced by the Bulgarian Ministry of Education, Youth and Science in the frameof contract № BG051PO001-3.3-05/0001 “Science and Business”,within the Operational Programme “Human Resources Development”.
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