Elsevier

Dyes and Pigments

Volume 182, November 2020, 108628
Dyes and Pigments

Tautomerism in 8-(phenyldiazenyl)quinolin-5-ol: An attempt for pH activated rotary switch

https://doi.org/10.1016/j.dyepig.2020.108628Get rights and content

Highlights

  • Tautomerism of quinoline azo dye is studied by combination of theory and experiment.

  • Hydrazone form observed in solutions is stabilized by intramolecular hydrogen bond.

  • Upon deprotonation, slow rotation around the Cquin-N bond is observed.

  • Deprotonation is solvent assisted and facilitated by proton acceptor solvents.

  • The studied dye could be considered as rotary switch activated by addition of base.

Abstract

The tautomerism of 8-(phenyldiazenyl)quinolin-5-ol has been studied by a combination of theoretical (DFT calculations) and experimental (UV–Vis and NMR) methods. The detailed study of neutral molecules has shown that the hydrazone tautomeric form, stabilized by an intramolecular N–H⋯N hydrogen bond, is solely present in most of the solvents. In strong proton acceptor solvents, besides the dominant hydrazone form, the deprotonated form also appears. Solvent effects on the absorption maxima of the hydrazone form are interpreted by the linear solvation energy relationship concept, using Kamlet-Taft and Catalán models. Upon deprotonation, a substantial structural transformation is observed in the studied compound leading to a slow rotation around the Cquin-N bond. The process, as monitored by 1H NMR, is strongly solvent assisted and facilitated by proton acceptor solvents. Consequently, the investigated dye could be considered as a base-activated rotary switch.

Introduction

The tautomerism of azo dyes and especially – azonaphthols, is a well-known phenomenon, influencing their stability and colour [1,2]. Among them, the 4-(phenyldiazenyl)naphthalen-1-ol (1, Scheme 1) is perhaps one of the most studied tautomeric systems. Being discovered almost 140 years ago in a pure chemical manner [3,4], it represents, even now, a challenging system to study or to model with respect to structural and environmental effects on the position of the tautomeric equilibrium [1,2,[5], [6], [7], [8], [9], [10]]. Compound 1 always exists as an azo-hydrazone tautomeric mixture and the tautomeric ratio (quantitatively defined as a constant KT, KT = [H]/[A]) is very sensitive to the solvent environment and less - to the temperature [[11], [12], [13]]. Recently it has been used as a successful platform for the development of new systems, exploiting controlled proton transfer for signal conversion and molecular sensing [[14], [15], [16]]. This was achieved by the implementation of a mediator - host (nitrogen-containing unit, crown ethers or similar macrocyclic receptors), which transfers a signal from the external stimuli to the tautomeric backbone. These structural modifications come to answer the need for a single tautomeric form in the sensing/signaling systems, being able, under suitable stimuli, to switch fully to the other tautomer.

The stability of azo compounds and their tautomerism make them suitable targets for new switching systems as the efforts are now directed to dyes, existing as a single tautomer in the solution and being able fully to convert upon changing the external stimuli. In this context, hydrazone based compounds are suitable architectures for chemically activated configurational rotary switches [17,18]. Furthermore, quinoline based hydrazones are proven to be appropriate molecular switch prototypes induced by pH change [19].

Quinoline-based compounds are widely used in the synthesis of colorants, especially azo dyes. Among them, azo dyes obtained from hydroxyl-substituted quinolines have been widely studied [[20], [21], [22]]. These dyes are recognized as biologically active agents [23], while some of them are utilized as chemosensors for anion detection [24] and corrosion inhibitors [25]. Quinoline-based dyes, especially 8-hydroxy substituted quinolines, represent renowned structures in coordination chemistry due to their chelating capacity towards metal ions [26,27]. Such complexes possess luminescent properties that enable their application as promising materials for multilayer light-emitting devices (OLEDs) [28]. The azo-hydrazone tautomerism in azo dyes, derived from hydroxyl-substituted quinolines, is also discussed [[29], [30], [31], [32], [33], [34]].

The 8-hydroxyquinoline derivatives need special attention, due to the possible involvement of the 8-OH group in an intramolecular hydrogen bond with the nitrogen atom of the quinoline ring as well as intermolecular hydrogen bonds with the N atom of an adjacent quinoline molecule [35], which influences the tautomeric state of the system. Heterocyclic and carbocyclic dyes based on this heterocycle exist as azo tautomers in the solid state, while in N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO) these dyes exhibit azo-common anion equilibrium [29,31]. On the other hand, some authors have observed azo-hydrazone tautomerism of a particular 8-hydroxy substituted azoquinoline dyes in some dipolar aprotic and basic solvents, while in nonpolar and proton-donor solvents dyes adopt an azo form [30,33]. Structural investigations of 4-hydroxy-2-quinolone (2-hydroxyquinoline-4-one) based azo dyes in the solutions show that they appear as two hydrazone-keto forms that form strong intramolecular hydrogen bonds with the corresponding keto group [17,32].

While 2- and 8-hydroxyquinolines have been frequently used in the synthesis of various azo dyes, 5-hydroxyquinoline, to our best knowledge, has not been used. Although the synthesis of 8-(phenyldiazenyl)quinolin-5-ol (2) was reported using a different synthetic pathway [36], its tautomerism has never been studied. It gives interesting possibilities for additional stabilization due to intramolecular hydrogen bonding and shift of the equilibrium upon the change of pH (Scheme 2). Therefore, in the current paper, we report a new synthetic pathway and in-depth investigation of the tautomerism of 2. To our best knowledge, this is the first study of the tautomerism and spectral properties of 8-(phenyldiazenyl)quinolin-5-ol by using a combined experimental and theoretical approach. For quantitative elucidation of the solvent's effects on the position of absorption spectra, the concept of linear solvation energy relationships (LSER) is used. Furthermore, the possibility of using 2 as a pH stimulated rotary switch is investigated.

Section snippets

Materials and measurements

All reagents were purchased from Fluka, Aldrich and Acros Organics and were used without purification. Solvents used for UV–Vis analysis were of spectrophotometric grade. The melting point was obtained on the melting point system Stuart SMP30. FT-IR (Fourier transform infrared spectroscopy) spectrum of 2 was obtained on a Nicolet™ iS™ 10 FT-IR Spectrometer (Thermo Fisher SCIENTIFIC) spectrometer with Smart iTR™ Attenuated Total Reflectance (ATR) Sampling accessories in the range 500–4000 cm−1

Spectral characterization

Possible neutral and ionized forms of compound 2 are presented in Scheme 2. ATR FTIR spectrum of the dye 2 suggests that the dye adopts the azo form in the solid state (A form). Characteristic vibrations originating from the quinoline ring are observed at 1628 and 1578 cm−1 and are ascribed to stretching vibrations of Cdouble bondN imino and Cdouble bondC groups, respectively [52]. The band located at 1333 cm−1 is attributed to the in-plane-bending of OH group, while the broad signal at 3252 cm−1 corresponds to the

Conclusions

Tautomerism and spectral behavior of 8-(phenyldiazenyl)quinolin-5-ol are investigated in depth by combining experimental and theoretical approaches. The study involves a neutral molecule as well as its ionized forms. ATR FTIR spectrum of the dye suggests the existence of an azo form in the solid state. The spectral behavior of the investigated dye in solutions substantially differs from the naphthalene analog, 4-(phenyldiazenyl)naphthalen-1-ol and implies that the dye under investigation solely

CRediT authorship contribution statement

Jelena Lađarević: Investigation, Writing - original draft, Writing - review & editing. Dušan Mijin: Investigation, Validation, Writing - original draft, Writing - review & editing. Liudmil Antonov: Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Authors would like to acknowledge the financial support from the Ministry of Education, Science and Technological Development of the Republic of Serbia under Contract No.451-03-68/2020-14/200135 (J.L., D.M.) and from Bulgarian National Science Fund under the project MolRobot DN09/10 (L.A.).

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