Diarylethene derivatives are known to be photochromic, that is, upon irradiation they change their color because of ultrafast photocyclization. Their proposed use as nonvolatile data storage materials requires the cycloreversion process to be very slow in the dark. We apply density functional theory (DFT) methods to predict kinetics of thermal cycloreversion. The cycloreversion occurs through symmetry forbidden conrotatory electrocyclic mechanism with transition state of strong diradical character and requires the use of unrestricted broken-symmetry DFT formalism. Our results suggest that B3LYP and M05-2X are the best functionals to describe kinetics in these compounds (accurate to within 3À4 kcal/mol from the experimental values). The methods validated in this study show great promise as tools in rational design of the improved photochromic materials.

Significant ethylene epoxidation activity was observed over niobium (Nb) incorporated mesoporous silicate materials Nb-KIT-5, Nb-MCM-48, and Nb-TUD-1, with hydrogen peroxide (H 2 O 2) as oxidant and methanol (MeOH) as solvent under mild operating conditions (35 °C and 50 bars). No CO 2 as byproduct was detected at these conditions. The measured ethylene oxide (EO) productivity over Nb-TUD-1 materials (342-2539 g EO h-1 kg-1 Nb) spans a greater range than those observed with Nb-KIT-6 (234-794 g EO h-1 kg-1 Nb), Nb-KIT-5 (273-867 g EO h-1 kg-1 Nb) and Nb-MCM-48 (71-219 g EO h-1 kg-1 Nb) materials at similar operating conditions. However, significant H 2 O 2 decomposition and Nb leaching were observed in all cases. Computational studies employing minimal models of the catalytically active sites, suggest how the Brønsted acidity may lead to these detrimental pathways. Indeed, lowering the metal loading to significantly reduce the Brønsted acidity results in a dramatic increase in H 2 O 2 utilization towards EO formation (4304 g EO h-1 kg-1 Nb). The increased EO productivity either matches or surpasses that observed on the conventional Ag-based heterogeneous catalyst (with O 2 as oxidant) as well as a Re-based homogeneous catalyst (with H 2 O 2 as oxidant). These results are paving the way for further computational and experimental investigations aimed at the rational design of improved epoxidation catalysts that reduce H 2 O 2 decomposition and metal leaching to practically viable levels.

ABSTRACT:Certain organic compounds possess the ability to change color under the influence of light, called photochromism. This change is due to ultrafast chemical transition from open to closed ring isomers (photocyclization), which can be used for optical data storage and photoswitching applications. These applications require minimization of the irreversible photodegradation of the material, called photofatigue. This property is related to the chemical rate of byproduct formation. We use density functional theory methods to predict the mechanism and activation barriers to the byproduct formation for 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene in order to estimate its fatigue resistance. We also explain higher fatigue resistance for its methylated derivative. The methods used in this study may become a part of rational design strategy for the new photochromic materials.

We apply several exchange-correlation functionals in combination with time-dependent density functional theory to predict the maximum wavelengths in the absorption spectra for 29 diarylethene derivatives in both open and closed isomeric forms. Solvent effects and accurate molecular geometries are found to be important to obtain good agreement with experimental absorption wavelengths. In order to evaluate the quality of geometry optimization, we compare predicted bond length alternation parameters with experimental ones. We find the TD-M05/6-31G*/PCM//M05-2x/6-31G*/PCM theory level to give the best predictions for the structural and spectral parameters of the diarylethene derivatives. Applications of the photochromic diarylethene compounds as materials for optical switching and data storage based on their photocyclization properties are also discussed.

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