Structural and Spectroscopic Properties of Benzoylpyridine-Based Hydrazones.

Photochromic hydrazones are attracting the attention in the field of photochromic systems especially due to their P-type character. To understand the structural features and their correlation with the spectroscopic data, UV-Vis, vibrational and ellipsometry spectroscopic techniques are employed with the support of density functional theory (DFT) calculations to three hydrazone derivatives based on benzoylpyridine. Interestingly, analysis of the structure shows the presence of two distinct rotamers around the pyridine ring with different energy and the well-defined conjugation path that changes due to E to Z isomerization especially in the hydrazone -C=N-NH part of the skeleton. IR and Raman spectra are analyzed, showing a higher selectivity in the Z form; moreover, the comparison with the normal modes proves the effect of the reaction on the backbone structure. The experimental results are in good agreement with the theoretical predictions, especially in the case of the Raman spectrum. The molecular polarization also changes from E to Z forms as predicted by DFT calculations. Spectroscopic ellipsometry on thin films of TOPAS doped with 10 %wt of the dimethylamino hydrazone derivative is used to prove such change at the molecular level. A modulation of the refractive index is observed, and it is correlated with the concentration of the active moiety and the calculated electronic polarizabilities.

Photoswitching hydrazones based on benzoylpyridine.

The photoswitching properties of three readily accesible benzoylpyridine hydrazones were investigated. Interestingly, replacing classical stirring with ultrasound wave activation results in pure thermodynamically less stable E isomer crystallization at room temperature. The studied benzoylpyridine hydrazones exhibit both P- and T-photochromic behaviour, depending on the benzoyl substituent, and improved addressability compared to most of the previously published pyridyl based hydrazones and 2-pyridylcarboxaldehyde acylhydrazones. Low activation entropy and calculated transition state geometry favour the inversion mechanism of their thermal isomerization rather than tautomerization followed by rotation recently found for pyridyl-hydrazone ester or nitrile rotary switches. The association behaviour of the nitro derivative during its thermal E-to-Z isomerization in highly polar DMSO indicates an important role of intermolecular hydrogen bonding in the thermal kinetics of benzoylpyridine-based hydrazone photoswitches. Moreover, the addition of triethylamine significantly accelerates the rate of Z-isomer thermal isomerization from days to seconds and could thus pave the way to fast pyridyl hydrazone T-type photochromic compounds in polar solvents. This study could therefore contribute to general knowledge related to the photochromic behaviour of hydrazones as an important class of modern photoswitches.