Fluorescence turn-on by photoligation - bright opportunities for soft matter materials

Abstract

Photochemical ligation has become an indispensable tool for applications that require spatially addressable functionalisation, both in biology and materials science. Interestingly, a number of photochemical ligations result in fluorescent products, enabling a self-reporting function that provides almost instantaneous visual feedback of the reaction's progress and efficiency. Perhaps no other chemical reaction system allows control in space and time to the same extent, while concomitantly providing inherent feedback with regard to reaction success and location. While photoactivable fluorescent properties have been widely used in biology for imaging purposes, the expansion of the array of photochemical reactions has further enabled its utility in soft matter materials. Herein, we concisely summarise the key developments of fluorogenic-forming photoligation systems and their emerging applications in both biology and materials science. We further summarise the current challenges and future opportunities of exploiting fluorescent self-reporting reactions in a wide array of chemical disciplines.

Fig. 1. Mechanism of tetrazole-based photocycloaddition via light-induced generation of the 1,3-nitrile imine intermediate, which can react with an alkene to form the cycloadduct. The 1,3-nitrile imine is susceptible to nucleophilic addition, which can be minimised by modification of the C-phenyl motif of the tetrazole (top left) using electron-deficient or ring-strained alkenes/alkynes (top right). The activation wavelength can be redshifted by modification of the N-phenyl motif by attachment of conjugated and electron-donating moieties (bottom).

Scheme 1. Tetrazole-containing macrocyclic structure for enhanced photoinduced 1,3-dipolar cycloaddition reactions with norbornene.

Fig. 2. Application of tetrazole-ene photoligation in the preparation of photoresponsive hydrogels. (A) Photo-crosslinking of 4-arm PEG containing tetrazole and methacrylate end-groups to form fluorescent hydrogels, or microgels by integrating with microfluidic devices. (B) Photo-induced intramolecular tetrazole-ene leads to disruption of a self-assembled structure and consequently degradation of the physically crosslinked hydrogels. (C) Fluorescent polymer networks, formed by tetrazole-based photo-crosslinking, can be degraded by aminolysis of the trithiocarbonate functionalities within the structure, leading to soluble fragments. Analysis of the fragments by fluorescence spectroscopy can provide quantitative information on the number of crosslinking junctions. (Reproduced with permission from ref. , , and .

Fig. 3. Application of tetrazole-ene photoligation in polymer synthesis and modification. (A) Photo-induced intermolecular or intramolecular crosslinking of polymer chains to form fluorescent microspheres (top) or single chain nanoparticles (bottom), respectively. (B) Step-growth photopolymerisation of AA and BB monomers, or an AB monomer containing a tetrazole and an electron-deficient alkene. The progress of photopolymerisation can be followed by the naked eye due to the fluorescence of the pyrazoline adducts. (C) Photo-patterning, via tetrazole-ene ligation, on cellulose-based substrates (top) or hydrogels (bottom left) to form fluorescent patterns, and attachment of bioactive component that enables cell attachment (bottom right). (Reproduced with permission from ref. , , , , and .

Fig. 4. Photoreactivity of sydnones. (A) Monoaryl sydnones can react with alkynes in a copper-catalysed reaction while diarylsydnone, readily prepared by C–H activation coupling, can undergo photo-cycloreversion to form a nitrile imine intermediate; the photoreactivity of diarylsydnones can be tuned by varying the substituent on the C-phenyl moiety. (B) Ring-strained diazobenzene structure in its trans-form has a high reactivity towards nitrile imine intermediate, in contrast to its cis-form, and thus the cycloaddition can be accelerated by visible light irradiation. Nevertheless the 1,3-dipole product is non-fluorogenic.

Fig. 5. Recently reported photochemical pericyclic reactions that form (pro)fluorogenic products and their applications. (A) [4 + 2] Cycloaddition of 9,10-phenanthrenequinone and vinyl ether forming a fluorescent phenanthrodioxine moiety (top left); the PQ-VE photocycloaddition can take place orthogonally to the tetrazole-ene photoligation, enabling λ-orthogonal labelling of hydrogel networks (right). (B) Photochemical reaction of o-methylbenzaldehydes and propiolate ester pendent groups for polymer crosslinking, enabling microparticles formation; addition of acid triggers aromatisation of the cyclic adduct, forming the fluorescent moiety. (C) [2 + 2] photocycloaddition of 1-trimethylammonium-6-pentafluorstyryl-pyrene chromophore forming fluorescent cyclobutane structures (Reproduced with permission from ref. .