Photoswitchable Fluorescent Assemblies Based on Hydrophilic BODIPY−Spiropyran Conjugates

Abstract

In search of strategies to design photoswitchable fluorescent probes and operate them in aqueous environments, we have envisioned the possibility of incorporating fluorescent, photochromic, and hydrophilic components within the same macromolecular construct. First, we synthesized a fluorophore−photochrome dyad, pairing a BODIPY fluorophore and a spiropyran photochrome in its molecular skeleton, and investigated the photochemical and photophysical properties of this compound in acetonitrile. Under these conditions, the photoinduced isomerization of the spiropyran causes a 56% decrease in the emission intensity of the BODIPY at the photostationary state. The photogenerated isomer has a lifetime of 2.7 × 102 s and reverts thermally to the original form, restoring the initial emission intensity. On the basis of these results, we copolymerized a similar BODIPY−spiropyran conjugate with a monomer bearing a pendant polyethylene glycol chain. The resulting polymer is soluble in aqueous environments, and its fluorescence can be modulated by operating the photochromic components with optical stimulation. Specifically, the emission intensity decreases by 40% at the photostationary state and reverts to the initial value after thermal reisomerization of the photochromic components. However, the lifetime of the photogenerated species in neutral buffer is significantly longer than that of the monomeric BODIPY−spiropyran in acetonitrile. The fluorescence of both monomeric and polymeric fluorophore−photochrome assemblies can be switched repeatedly between high and low values by alternating ultraviolet irradiation and storage in the dark. However, the fatigue resistance properties of both systems are relatively poor. In any case, our investigations demonstrate that our design is viable for the realization of hydrophilic and photoswitchable molecular assemblies. In principle, innovative fluorescent probes for biomedical applications can evolve from these studies, if methods to improve their fatigue resistance properties and optimize their reisomerization kinetics can be identified.