Design, synthesis and photophysics of phosphole-fused dithienylethenes : from photochromism with excellent properties, visible light photoswitch, multi-responsive materials to supramolecular assembly

Abstract

A library of photochromic phosphole-fused dithienylethene-containing compounds has been newly designed, successfully synthesized and structurally characterized. Their electrochemical, photophysical and photochromic properties have been investigated. Upon photoirradiation, the benzo[b]phosphole derivatives were shown to display drastic color changes and reversible UV-vis absorption spectral changes. More interestingly, the novel photochromic benzo[b]phosphole oxides have been demonstrated to display photochromic properties with excellent fatigue resistance and thermal irreversibility in polymethylmethacrylate (PMMA) thin film under ambient conditions. The remarkable photochromic behavior could be achieved by rational molecular design, in which the weakly aromatic phosphole oxide was directly incorporated into the photo-responsive dithienylethene units. Photopatterning via photo-induced coloration and decoloration has been performed to demonstrate the repeatable and distinct transformation between the bistable states, making it a promising candidate with photoswitching properties for optoelectronic applications. On the other hand, another series of novel photochromic thieno[3,2-b]phosphole oxides has been shown to demonstrate photochromism without the need for the use of high-energy ultraviolet (UV) light irradiation while maintaining promising photochromic properties. Promising visible light-induced photochromic properties have been realized by the new molecular design, in which various π systems have been incorporated into the weakly aromatic phosphole backbone instead of the conventional modification of the peripheral diaryl units that usually leads to a drastic reduction of the photochromic quantum yields or even a loss of the photochromic behavior. Excellent fatigue resistance has been observed with no apparent loss of photochromic reactivity over ten photochromic cycles by alternate irradiation with violet (ca. 410 nm) and green (ca. 500 nm) light with high photochromic quantum yields (ϕO→C = 0.87 and ϕC→O = 0.44), rendering it a new promising candidate as visible light photoswitches for various potential applications.

Moreover, a series of novel benzo[b]phosphole alkynylgold(I) complexes has been demonstrated to display photochromic and mechanochromic properties upon applying the respective stimuli of light and mechanical force. Promising multi-responsive properties of the gold(I) complexes have been successfully achieved through the incorporation of the photochromic dithienylethene-containing benzo[b]phosphole into the triphenylamine-containing arylethynyl ligand that was susceptible to mechanical force-induced color changes via gold(I) complexation. With excellent thermal irreversibility and robust fatigue resistance of this series of gold(I) complexes, the benzo[b]phosphole alkynylgold(I) complexes showed repeatable photochromic and mechanochromic cycles without apparent loss of reactivity under ambient conditions, paving the way for further development of the underexplored photo-responsive gold(I) complexes and the multistate photo-controlled system.

In addition, a series of barbiturate-containing benzo[b]phosphole oxides has been demonstrated to show photochromic properties and co-assembly behavior with their ditopic receptor. Both the open form and the photogenerated closed form showed color changes with a bathochromic shift of the lowest-lying absorption maxima upon addition of the ditopic receptor. Apart from the multi-addressable states via photochromism and thermochromism, photo-modulated morphological transformation of the co-assembly in nanostructure from extensive network structures with smaller pores to adhering ring-like aggregates upon photoirradiation have also been demonstrated.