Solvent control of intramolecular proton transfer: is 4-hydroxy-3-(piperidin-1-ylmethyl)-1-naphthaldehyde a proton crane?

The solvent dependent excited state dynamics of 4-hydroxy-3-(piperidin-1-ylmethyl)-1-naphthaldehyde (compound 2), a candidate for a molecular switch based on intramolecular proton transfer, was investigated by ultrafast spectroscopy and quantum-chemical calculations. In acetonitrile a mixture of molecules in the enol and zwitterionic proton transfer (PT) form exists in the ground state. However, the zwitterion is the energetically favored one in the electronically excited state. Optical excitation of the enol form results in intramolecular proton transfer and formation of the PT form within 1.4 ps. In addition we observe the appearance of a long living species with a rate of 1/(330 ps) which returns to the original ground state on time scales beyond 2 ns and which is attributed to the triplet state. In toluene the enol form is the only observed ground state tautomer, but no light induced proton transfer occurs. Again the long living triplet state is formed, even with a faster rate of 1/(11 ps). In methanol hydrogen bonds between 2 and solvent molecules stabilize strongly the PT form in the ground as well as in the excited state. Also in this case no light induced intramolecular proton transfer was observed but the formation of a long living species was. However, its absorption spectrum is distinctly different from the triplet state seen in acetonitrile and methanol.

Toward Exploring Novel Organic Materials: MP4-DFT Properties of 4-Amino-3-Iminoindene

Tautomerism links with many applications and remains an attracting feature in exploring novel systems. In this regard, properties of indene-based HNCCCN segments have not received any considerable attention. In this computational organic chemistry study, first, to calculate the proton transfer energy barrier at a reasonable cost, the study identified an accurate forth order Møller–Plesset perturbation theory-density functional theory (MP4-DFT) protocol equivalent to the outstanding pioneering benchmark calculations. The calculations illustrate that the two tautomers of the 4-amino-3-iminoindene nucleus are separated by a considerable energy barrier while featuring different molecular orbital characteristics; frontier orbital distribution, λmax, and energies, which are known basic requirements in molecular switching and logic circuit applications. The N-H/BH₂ substitution was found to have significant influence on the electronic structure of the skeleton. Similarities in the two tautomers and the boron derivative to properties of known molecular materials have been found.

Comment on “Spectroscopic studies of keto–enol tautomeric equilibrium of azo dyes” by M. A. Rauf, S. Hisaindee and N. Saleh, RSC Adv., 2015, 5, 18097

The paper mentioned in the title contains inaccuracies and misleading interpretations that misrepresent basic knowledge about tautomerism in azo dyes. This Letter gives a better understanding of the basic principles of the tautomerisation process.