Kunihiro Ichimura

Even though two isosbestic points are generated in UV-induced changes in UV-vis absorption spectra of films of a polymethacrylate with azobenzene side chains, the corresponding fourth-order derivative spectra disclose the presence of H-aggregate so that the dual process comprised of UV-induced disaggregation and E/Z photoisomerization of non-aggregated species are involved. Whereas two isosbestic points are observed in UV-induced changes in absorption spectra, there is no common crossing point in those in the fourth derivative spectra in the restricted wavelength region due to the formation of H-aggregation, verifying that the presence of isosbestic points is not always the evidence for the oneness of a reaction.

Higher order derivative spectra were applied at first to one-way ZEZ-to-EEE photoisomerisation of dimethyl ester (ZEZ-DPH1) of the titled compound in a methylcyclohexane solution. Many common crossing points emerged in UV-induced derivative-spectral changes to reveal the direct ZEZ-to-EEE photoisomerisation without the transient formation of an intermediate to suggest the bicycle-pedal mechanism. The solid-state photoisomerisation was subsequently monitored by tracing changes in the fourth-order derivatives of absorption spectra of a thin crystalline layer of ZEZ-DPH1 prepared by the drop-casting method, because the distortion of absorption spectra due to light scattering is cancelled. It was suggested that the solid-state photochemical event consists of three steps: fast ZEZ-to-EEE photoisomerisation, a subsequent slow ZEZ-to-EEE photoisomerisation and very slow disappearance of the EEE-isomer. Studies on powder XRD were also carried out for a drop-cast solid layer of ZEZ-DPH1 to disclose the coexistence of a crystal form other than the original one, and the former exhibited faster ZEZ-to-EEE photoisomerisation when compared with the original crystal form. The results revealed by XRD analysis are in line with those obtained by higher-order derivative spectra, confirming the solid-state one-way photoisomerisation to take place through the bicycle-pedal process.

The reproducibility of higher order derivative spectra of aqueous solutions of N-methyl-4-(p-formylstyryl)pyridinium salt (FSbQ) as a model compound were examined by the conversion of UV-Vis absorption spectra taken on six spectrophotometers into derivatives with aid of software for data analysis as a common tool for the differentiation and smoothing. The spectral shapes of crude fourth and eighth derivatives were not only quite noisy, but also considerably dependent on the spectrophotometers. But the Savizky-Golay smoothing under the same and appropriate conditions provided the satisfactorily identical derivative-spectra, confirming the reproducibility of derivative spectroscopy. In order to reveal the reliability of the quantitative analysis on the basis of the Beer’s law, absorbances (A) of aqueous solutions of FSbQ were plotted against dⁿA/dλⁿ (Dⁿ) values at sub-peaks of the fourth as well as eighth derivatives. Excellent linearity between A and Dⁿ was disclosed when A is smaller than ca. 1, leading to the confirmation that Dⁿ values are usable for kinetic studies on the photoisomerization of FSbQ as an example.

Photo-induced UV-Vis spectral changes of azobenzene (Az) in solution and aqueous crystalline dispersions were analysed by means predominantly of fourth order derivatives. Weak bands due to vibration level transitions (VLT) of E-isomers in solution were well-resolved in derivatives to estimate conversion of photoisomerisation by tracing extrema of a VLT sub-peak. The photo-induced derivative-spectral changes of Az crystals dispersed in aqueous PVA solutions generated many common crossing points to indicate that Az dissolves partially in a PVA solution to lead to photoisomerisation in a homogeneous phase and the reversible alteration of particle size. The solid-state photoisomerisation of Az was investigated by the preparation of PVA-free aqueous dispersions of powdery nanohybrids comprised of Az and silica nanoparticles. The fourth order derivatives of spectral changes revealed that the photochemical process involves three kinds of species including non-aggregated and aggregated E-isomers as well as Z-isomers.

The fourth-order derivative spectra reveal the individual generation of photodichroism of non-aggregated and aggregated species of a liquid-crystalline azo-polymer.

Hyper-branched polyester polyols (HBPs) comprised of 2,2-dimethyolpropionic acid (DMPA) were treated with cinnamoyl chloride to give the corresponding polycinnamates (DCi20, DCi30 and DCi40) in order to compare their spectroscopic and photochemical properties with those of poly(vinyl cinnamate) (PVCi) as a linear counterpart, as well as low-mass cinnamates. λ_max of the cinnamate residues of DCi20, DCi30 and DCi40 suffered from marked red-shifting when compared with those of PVCi and the model compounds in solution. Fourth as well as eighth-order derivative spectra revealed that DCi20, DCi30 and DCi40 give rise to a new sub-peak ascribable to a J-aggregate at ca. 320 nm, causing the red-shifting of λ_max in the absorption spectra. On the other hand, higher-order derivative spectra of PVCi consisted solely of the sub-peaks which are close to those of the low-mass compounds in dilute solution, indicating that PVCi does not form any aggregate at all. The new sub-peak due to the J-aggregate was more distinctly observed in the fourth-order derivative spectra for a film of DCi40, while a PVCi film shows again no sub-peak ascribable to aggregation. Photoirradiation of a DCi40 film with 313 nm light resulted in the rapid and thorough disappearance of the sub-peak due to the J-aggregate as a result of exclusive photodimerisation, when compared with that of the sub-peaks due to monomeric cinnamate, which displays both photoisomerisation and photodimerisation. It is assumed that dicinnamated DMPA residues attached densely to the branched side chains of HBPs form dendritic units that are prone to head–tail stacking to give the J-aggregate.