Red crystals of N,N′-bis(3-methoxysalicylidene)-1,5-diiminonaphthalene were obtained after Schiff base condensation in ethanol. Recrystallization from acetone afforded yellow crystals, a process which is reversible and reproducible. Single crystal X-ray diffraction evidences two polymorphs differing in their space group and dihedral angle between aromatic moieties. DFT periodic calculations further confirmed the existence of two minima on the potential energy surface corresponding to red and yellow crystals. The red polymorph irreversibly (monotropically) transforms at 165–190 °C into the yellow one with a 3% increase of the unit cell volume, as shown by X-ray powder diffraction and periodic DFT calculations. Both polymorphs are thermochromic but only the red one displays photochromism upon irradiation at λ = 365 nm, which is reversible and exhibits a relatively slow thermal relaxation. A temperature induced cis/trans-keto equilibrium is for the first time identified for an N-salicylidene aniline derivative. Static DFT molecular and periodic calculations as well as ab initio Born–Oppenheimer dynamics simulations were performed to characterize the stability of both polymorphs and to determine the relative populations of the enol/cis-keto/trans-keto isomers at various temperatures.

Analysis of the hydrogen bond network in aprotic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide has been performed based on structures obtained from ab initio or classical molecular dynamics simulations. Statistics of different donor and acceptor atoms and the amount of chelating or bifurcated bonds has been presented. Most of hydrogen bonds in EMIM–TFSI are formed with oxygen atom as hydrogen acceptors; and the most probable bifurcated bonds are those with a mixed pair of oxygen and nitrogen acceptors. Spectral graph analysis has shown that the cations may form hydrogen bonds to up to five different anions and the connectivity of the whole hydrogen bond network is supported mainly by H-O bonds. In the structures of the liquid simulated via force field-based dynamics the number of hydrogen bond is smaller and fluorine atoms are the most favored hydrogen acceptors. One-dimensional potential energy profiles for hydrogen atom displacements and corresponding vibrational frequencies have been calculated for selected C-H bonds. Individual C-H stretching frequencies vary by 200-300 cm-1 indicating differences in local environment of hydrogen atoms forming C-H···O hydrogen bonds.

While tetrazole (TZ) has much lower basicity than imidazole and may not be fully protonated in the presence of phosphoric acid (PA), DFT calculations suggest that the basicity of TZ groups can be increased by the introduction of a 2,6-dioxy-phenyl-group in position 5 of TZ. This structure allows hydrogen bonds between TZ protons and ether oxygen atoms, and thereby establishes a resonance stabilised, co-planar structure for tetrazolium ions. Molecular electrostatic potential (MEP) calculations also indicate that tetrazolium ions possess two sites for proton hopping. This makes such materials interesting for use in a high temperature fuel cell (HT PEMFC). Based on these findings, two polymers incorporating the proposed TZ groups were synthesised, formed into membranes, doped with PA and tested for fuel cell relevant properties. At room temperature, TZ-PEEN and commercial meta-PBI showed an equilibrium uptake of 0.5 and 4.7 mol PA per mol heterocycle, respectively, indicating that PBI has higher affinity for PA than TZ-PEEN. The highest achieved PA uptake was ca. 110 wt%, resulting in a proton conductivity of 25 mS cm⁻¹ at 160 °C with a low activation energy of about 35 kJ mol⁻¹. In a first HT PEMFC test at 160 °C, a peak power density of 287 mW cm⁻² was achieved.