Daniela Nedeltcheva

Resolution of overlapping UV–Vis absorption bandsand quantitative analysis

Electronic transitions appear in the spectrum as individual bands, described by three basic parameters (position, intensity and width), which can be used for estimation of the fundamental transition characteristics as well as for quantitative analysis. The main problems concerning the mathematical resolution of overlapped individual bands in a complicated spectrum (estimation of the number of overlapping bands, the noise problem, artificial baseline and efficiency of the computing procedure) are discussed and a principal scheme and some solutions are suggested. The use of the resolution of overlapping bands technique for quantitative analysis is given step-by-step in its historical development on real spectral problems. Although the described methods for quantitative analysis are discussed only in terms of UV–Vis spectroscopy they can be easily used in all branches of molecular spectroscopy.

Exploiting Tautomerism for Switching and Signaling

Herein, we demonstrate a conceptual idea for a tautomeric switch based on implementation of a flexible piperidine unit in 4-(phenyldiazenyl)naphthalen-1-ol. The results show that a directed shift in the position of the tautomeric equilibrium can be achieved through protonation/deprotonation in a number of solvents. The developed molecular switch, in spite of the simple host–guest system, has shown acceptable complexation ability towards small alkaliand alkalineearth-metal ions and can be a promising basis for further development of effective molecular sensors through implementation of azacrown ethers. Organic molecular materials are increasingly recognized as suitable molecular-level elements (such as switching, signaling, and memory elements) for molecular devices, because the wide range of molecular characteristics can be combined with the versatility of synthetic chemistry to alter and optimize molecular structure in the direction of desired properties. Virtually every molecule changes its behavior when acted upon by external fields or other stimuli. True molecular switches undergo reversible structural changes, caused by a number of influences, which give a variety of possibilities for control. Several classes of photoresponsive molecular switches are already known; these operate through processes such as bond formation and bond breaking, cis– trans isomerization, and photoinduced electron transfer upon complexation. A conceptual scheme of a molecular switch based on molecular recognition is shown in Scheme 1. The host–guest system represents, for instance, a crown ether that can bind ions or a cyclodextrin that can bind other small molecules. It is bound to a signal converter. The complexation behavior is monitored by the state of the signal converter, and in turn its optical or electronic properties are determined by the complexation state of the host–guest system. The main requirement in the design of new molecular switches is to provide fast and clean interconversion between structurally different molecular states (on and off). Tautomerism could be a possibility, because change in the tautomeric state can be accomplished by a fast proton transfer reaction between two or more structures, each of them with clear and different molecular properties. Therefore, our aim herein is to show how tautomerism can be exploited for signal conversion. The conceptual idea of such a device is presented in Scheme 1. In this structure, a change in tautomeric state, labeled A and B, is linked to changes in the complexation abilities of the host–guest system by modulating the propensity of the system to hydrogen bond to the antenna. At the same time, engagement of this antenna causes a change in the tautomeric state. The sensitivity of the electronic ground and excited states of the tautomeric forms to environment stimuli (light, pH value, temperature, solvent) and to the presence of a variety of substituents or to hydrogen bonding can be exploited in the design of flexible tools for control. Obviously, such a device should be based on a tautomeric structure with easy proton exchange between the tautomers, which means that they must coexist in solution. At the same time, a main feature of systems of tautomers coexisting in solution is that the overall optical response is a mixture of the optical responses of the individual tautomers. Consequently, in the design of tautomeric switches, conditions for obtaining pure end tautomer in the corresponding off and on states must be provided. Herein we report the properties of two tautomeric switches, namely 3 and 4 (Scheme 2), based on 4-(phenyldiazenyl)phenol (1) and 4-(phenyldiazenyl)naphthalen-1-ol (2). The parent compound 2 is the first dye that was shown to tautomerize by Zincke and Bindewald in 1884. It has been the object of many spectral and theoretical studies because its tautomeric forms coexist in solution and the equilibrium Scheme 1. Molecular switch based on molecular recognition (left) and conceptual idea for a tautomerism-based molecular switch (right).

Tautomerism of 2-hydroxynaphthaldehyde Schiff bases

A UV–Vis spectroscopic study based on the recently developed chemometric approach for quantitative analysis of undefined mixtures is performed on a series of donor and acceptor substituted Schiff bases of 2-hydroxynaphthaldehydes. In CCl4 solution all compounds preferentially exist as the phenol tautomer independent of the nature of the respective substituent. With increasing polarity the tautomeric equilibrium is shifted towards the quinone form. In CHCl3 and, especially, ethanol a clear distinction between the effect of donors (stabilization of the quinone form) and acceptors (stabilization of the phenol tautomer) is evident. Ab initio calculations including solvent effects via the polarized continuum model of solvation as well as the supermolecule approach are used to rationalize the experimental findings.

Tautomeric transformations of piroxicam in solution: a combined experimental and theoretical study

Piroxicam tautomerism was studied in solution by using UV-Vis spectroscopy, NMR measurements and advanced chemometrics. It has been found that in ethanol and DMSO the enol-amide tautomer is present mainly as a sandwich type dimer. The addition of water leads to distortion of the aggregate and to gradual shift of the equilibrium towards the zwitterionic tautomer. Quantitative data for the aggregation and the tautomeric equilibria are presented. Quantum chemical calculations (M06-2X/TZVP) have been used to explain stability of the tautomers as a function of the solvent and concentration.

Gas-phase tautomerism in hydroxy azo dyes – from 4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-ol†

The tautomeric constants of a series of azo dyes were estimated in the gas phase by using electron ionization mass spectrometry. It was shown that the relative amount of the keto tautomer increases from 4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-ol, thus confirming the quantum-chemical predictions. The existence of the enol tautomer of 4-phenylazo-anthracen-1-ol is shown for the first time by mass spectrometry in the gas phase. This finding is supported by flash photolysis measurements in solution.

Controlled tautomerism – switching caused by an “underground” anionic effect

In a previous communication, we demonstrated a conceptual idea for a tautomeric switching system based on implementation of a flexible piperidine unit in 4-(phenyldiazenyl)naphthalen-1-ol (1). The results showed that a directed shift in the position of the tautomeric equilibrium can be achieved through protonation/deprotonation in a number of solvents. However, the effect of the counter ion in the process of protonation was never considered. The crystallographic analysis of protonated cyano and nitro derivatives of 4-(phenyldiazenyl)-2-(piperidin-1-ylmethyl)naphthalen-1-ol have shown an interesting and unexpected feature: the counter ion is captured in the process of protonation and the shift in the position of the tautomeric equilibrium is achieved through a bridged complex formation. To the best of our knowledge this is a rare example when controlled shift in the position of tautomeric equilibrium is achieved through anion complexation. The results from the solid state analyses are confirmed by NMR spectroscopy in solution and by quantum-chemical calculations.

Gas‐phase tautomerism in 1‐phenylazonaphthalene‐4‐ol: verification of the responses of individual tautomers

Gas-phase tautomerism in 1-phenylazonaphthalene-4-ol (1) was studied by using electron ionization (EI) mass spectrometry on the basis of the fragmentations of the model enol and keto tautomers, where the movable proton is replaced by a methyl group. These fixed tautomers were obtained as an easy separable mixture by simple methylation of the cheap and easily accessible diazene (1). It was found that their EI mass spectral fragmentations are in full congruence with the already published theoretical predictions. The relative energies required for bond cleavage in 1 and its fixed tautomers were estimated by stepwise increasing of the electron energy of the ion source of the mass spectrometer. A simple equation for the approximate estimation of the molar fractions of the individual tautomers was suggested. It was shown that the enol form is dominant in the temperature range of 200-300 degrees C.

10-hydroxybenzo[h]quinoline: Switching between single and double-well proton transfer through structural modifications

Proton transfer in 10-hydroxybenzo[h]quinoline (HBQ) and structurally modified compounds was investigated experimentally (steady state UV-Vis absorption and emission spectroscopy, NMR and advanced chemometric techniques) and theoretically (DFT and TD-DFT M06-2X/TZVP calculations) in the ground and excited singlet state. We observed that the incorporation of electron acceptor substituents on position 7 of the HBQ backbone led to appearance of a keto tautomer in ground state and changes in the excited state potential energy surface. Both processes were strongly solvent dependent. In the ground state the equilibrium could be driven from the enol to the keto form by change of solvent. The theoretical calculations explain the substitution-determined transition from a single- to a double-well proton transfer mechanism.

Gas-phase study of molecular switches based on tautomeric proton transfer

The keto–enol tautomeric equilibrium in two newly developed molecular switches was studied by using various mass spectral techniques. In these two compounds, namely 4-(phenyldiazenyl)-2-(piperidin-1-ylmethyl)naphthalen-1-ol and 2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4-(phenyldiazenyl)naphthalen-1-ol, the switching on/off states are achieved by a controlled shift of the tautomeric equilibrium. In the first compound, electron impact–mass spectrometry confirms that the unprotonated dye exists as an enol tautomer, while the electrospray ionization tandem mass spectrometry (ESI-MS/MS) experiment proves the clear shift to the keto tautomeric form under switching with acid addition. In the second compound, the addition of the alkali metal ions causes transition of the tautomeric equilibrium from the pure enol to the pure keto form. The ESI-MS study demonstrated better sensitivity towards lithium ions.