Irreversible Bleaching of Donor–Acceptor Stenhouse Adducts on the Surfaces of Magnetite Nanoparticles

Donor–acceptor Stenhouse adducts (DASAs) have recently emerged as a new class of photoswitchable molecules. 2] These compounds feature a system of three conjugated C=C double bonds connecting an aliphatic amine “donor” and a 1,3-dicarbonyl “acceptor”, giving rise to an intense absorption in the visible (green) region. DASAs serve as an example of negative photochromic materials : exposing the triene isomer to visible light results in the formation of the closedring, colorless, zwitterionic isomer (Figure 1a). Green light, typically used to induce the decoloration reaction, is advantageous in that it enables the development of truly orthogonal photoswitching systems. In a relatively short time since the initial report on reversible photoswitching of DASAs, several creative applications have been reported. For example, Lee et al. showed that DASAfunctionalized polymers could be used as optical sensors for nerve gases, where the analytes bind to the amine function to form quaternary ammonium salts, thus inducing a drastic color change (bleaching) of the sensor. Importantly, these sensors could be reversibly activated by using light. In another study, Hopper and co-workers took advantage of the thermochromic properties of the zwitterionic form of the switch and fabricated sensors for mapping temperature localization with microscale precision. By replacing the aliphatic amines with aromatic ones, these researchers developed an improved family of switches, which could operate in a wide range of organic solvents. Depending on the exact substitution pattern on the nitrogen atom, these “second-generation” switches displayed absorption maxima across a wide (>100 nm) spectrum of visible light wavelengths (from the green up to the infrared region), thus paving the way towards further applications. Integrating inorganic materials with molecular switches, in particular, ones operated by visible light, represents an attractive strategy for fabricating novel functional materials. For example, diarylethenes have been used to remotely switch electrical conductivity; moreover, bistable rotaxanes and overcrowded alkenes have enabled the photocontrol of surface wettability. In addition, azobenzenes have played a key role in controlled drug delivery systems, whereas spiropyrans—in reversible information storage media. In contrast, the potential of DASAs immobilized onto the surfaces of inorTwo novel donor–acceptor Stenhouse adducts (DASAs) featuring the catechol moiety were synthesized and characterized. Both compounds bind strongly to the surfaces of magnetite nanoparticles. An adrenaline-derived DASA renders the particles insoluble in all common solvents, likely because of poor solvation of the zwitterionic isomer generated on the nanoparticle surfaces. Well-soluble nanoparticles were successfully obtained using dopamine-derived DASA equipped with a long alkyl chain. Upon its attachment to nanoparticles, this DASA undergoes an irreversible decoloration reaction owing to the formation of the zwitterionic form. The reaction follows firstorder kinetics and proceeds more rapidly on large nanoparticles. Interestingly, decoloration can be suppressed in the presence of free DASA molecules in solution or at high nanoparticle concentrations.