Toward Higher Fatigue Resistance of Photochromic Polymer Layers Containing Novel Diarylethenes

For the use of diarylethenes (DAE) in optical microscopy with the aim of overcoming the Abbe limit, interdependent parameters must be coordinated. These include, in particular, the fatigue resistance X _A , the position of the isosbestic points (λ_isos > 400 nm), and the molar extinction coefficients of the closed form (ε_closed > 20,000 L/mol·cm). Thirteen photoswitchable DAEs were prepared and spectroscopically characterized (λ _isos , ε _closed , quantum yields φ), among those a first example of a pyridazine substituted derivative. The spectroscopic properties of the closed form of known and hitherto unknown DAEs can be predicted using the spectroscopic Hammett equation, since the excitation energy differences E _T,R - E _T,H/2.303·R·T correlate linearly with the Hammett parameters σ_p. Matrix effects of eight different polymers in the spin-coated absorbance modulation layer (AML) on the properties of model DAEs were investigated. A correlation was found between the glass transition temperature T _g and the degradation rate of the photochrome. We found that replacing PMMA with ZEONEX480R results in a 3-fold increase in photostability. DFT calculations of the frontier orbital profiles of the DAEs as well as the changes in the geometric requirements of the 6π-electrocyclic ring closure provide insights into structure-property relationships.