Tunable Photochromism in the Robust Dithienylethene‐Containing Phospholes: Design, Synthesis, Characterization, Electrochemistry, Photophysics, and Photochromic Studies

Abstract

A series of dithienylethene-containing phosphole derivatives has been designed, synthesized and characterized. One of the compounds has been characterized by X-ray crystallography. Upon photoexcitation, the compounds exhibit drastic color changes, ascribed to the reversible photochromic behavior. Their photochromic, photophysical and electrochemical properties have been studied. They show photochromic reactivities with high photocyclization quantum yields. Their photophysical and photochromic properties are found to be facilely tuned in this system by substitution at the phosphole backbone, as well as variation on the extent of π-conjugation of the phosphole backbone. Some selected compounds have been demonstrated to exhibit photochromic properties in polymethylmethacrylate (PMMA) films.

Photochromic benzo[b]phosphole oxide with excellent thermal irreversibility and fatigue resistance in the thin film solid state via direct attachment of dithienyl units to the weakly aromatic heterocycle

“…The first oxidation of 1–6 is tentatively assigned as the oxidation of the dimethylthienyl moieties, with a mixing of the phosphole-centred oxidation due to the rather low sensitivity towards the P -substituents and the presence of electron-rich peripheral dimethylthienyl moieties. 18 Moreover, 1–6 showed reduction waves ranging from –0.91 to –1.99 V vs. SCE (Table S1†). The first reduction is tentatively assigned as the phosphole-centred reduction because of the strong dependence of the different substituents on the phosphole moiety.…”

“…The first reduction is tentatively assigned as the phosphole-centred reduction because of the strong dependence of the different substituents on the phosphole moiety. 18 The electronic absorption spectra of 1–8 in benzene show moderately intense absorption bands at about 300–480 nm (Table 1 and Fig. S1†), which are tentatively assigned as intraligand π–π* transitions with a possible involvement of intramolecular charge transfer transitions from the peripheral dimethylthienyl moieties to the central benzo[ b ]phosphole unit.…”

“…More importantly, this molecular design strategy involving the direct attachment of the bis-dimethylthienyl moieties to the weakly aromatic phosphole unit has been proven to greatly enhance the photochromic performance, in areas such as thermal irreversibility and fatigue resistance, as well as the photocyclization and photocycloreversion quantum yields in comparison to previously reported fused-heterocycle diarylethenes. 7–9,12,18 The present work provides important insights and guiding principles into the molecular design of photochromic diarylethene systems with excellent thermal irreversibility and fatigue resistance.…”

“…17 It has now been widely accepted that phosphole possesses less aromatic character due to the fact that the lone pair of electrons of phosphorus cannot be effectively delocalized into the π-system of the butadiene moiety, with the slight aromaticity of phosphole resulting from hyperconjugation between the endocyclic π-system and the exocyclic σ-bond. 15 With our continuous interest in designing various photochromic materials, 7–9,12,14,18 we envisioned that the direct attachment of two thienyl rings onto the phosphole ring would offer a strategy to optimize the photochromic performance of the system. Although phosphole-containing photochromic compounds have been reported previously, 18 the photochromic bis-thienyl units are not directly attached onto the ethene unit of the phosphole ring to take advantage of the low aromaticity of phosphole, and to the best of our knowledge, exploration of the direct attachment of the two thienyl units to the phosphole ring has never been reported.…”

“…15 With our continuous interest in designing various photochromic materials, 7–9,12,14,18 we envisioned that the direct attachment of two thienyl rings onto the phosphole ring would offer a strategy to optimize the photochromic performance of the system. Although phosphole-containing photochromic compounds have been reported previously, 18 the photochromic bis-thienyl units are not directly attached onto the ethene unit of the phosphole ring to take advantage of the low aromaticity of phosphole, and to the best of our knowledge, exploration of the direct attachment of the two thienyl units to the phosphole ring has never been reported. In addition, studies on the photochromic properties of thin films of fused-heterocycle diarylethenes under ambient conditions are still not as extensively explored as those of the perfluorocyclopentene analogues.…”