Development of the 8-aza-3-bromo-7-hydroxycoumarin-4-ylmethyl group as a new entry of photolabile protecting groups

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Synthesis of hydrophilic caged DAG-lactones for chemical biology applications

The 6-bromo-7-hydroxy-coumarin-4-ylmethyl (Bhc) group has been used widely in cage chemistry because of its high molar absorptivity and photolytic efficiency. One of the drawbacks of coumarins however is their low aqueous solubility. Aqueous solubility is important in the behavior of caged compounds because hydrophobic caged compounds might be aggregated in physiological conditions and consequently the photocleavage would be impaired. The 8-azacoumarin-4-ylmethyl derivatives with bromine (8-aza-Bhc) or iodine (8-aza-Ihc), which were previously developed in this laboratory, have aqueous solubilities that are higher than those of related coumarins. Here, to improve the hydrophilicity and management of caged diacylglycerol lactones (DAG-lactones), 8-aza-Bhc and 8-aza-Ihc were introduced into the DAG-lactone structure. The synthesized caged compounds showed high hydrophilicity compared with the parent Bhc-caged DAG-lactone, and the 8-aza-Ihc-caged DAG-lactone (2) showed excellent photolytic efficiency, which allows rapid release of the DAG-lactone (1) by brief photoirradiation. The 8-aza-7-hydroxy-6-iodo-coumarin-4-ylmethyl group might be useful for caging of bioactive compounds, especially hydrophobic compounds.

Coumarin-Caged Compounds of 1-Naphthaleneacetic Acid as Light-Responsive Controlled-Release Plant Root Stimulators

Six coumarin-caged compounds of 1-naphthaleneacetic acid (NAA) comprising different substituents on the coumarin moiety were synthesized and evaluated for their photophysical and chemical properties as light-responsive controlled-release plant root stimulators. The ¹H NMR and HPLC techniques were used to verify the release of NAA from the caged compounds. After irradiation at 365 nm, the caged compounds exhibited the fastest release rate at t_1/2 of 6.7 days and the slowest release rate at t_1/2 of 73.7 days. Caged compounds at high concentrations (10^-5 and 10^-6 M) significantly stimulate secondary root germination while free NAA at the same level is toxic and leads to inhibition of secondary root germination. The cytotoxicity of the caged compounds against fibroblasts and vero cells were evaluated, and the results suggested that, at 10^-5-10^-6 M, caged compounds exhibited no significant cytotoxicity to the cells. Thus, the caged compounds of NAA in this study could be of great benefit as efficient agrochemicals.

Combining Top-Down and Bottom-Up with Photodegradable Layer-by-Layer Films

Layer-by-layer (LbL) self-assembly of polymer coatings is a bottom-up fabrication technique with broad applicability across a wide range of materials and applications that require control over interfacial properties. While most LbL coatings are chemically uniform in directions both tangent and perpendicular to their substrate, control over the properties of surface coatings as a function of space can enhance their function. To contribute to this rapidly advancing field, our group has focused on the top-down spatiotemporal control possible with photochemically reactive LbL coatings, harnessed through charge-shifting polyelectrolytes enabled by photocleavable ester pendants. The photolysis of the photocleavable esters degrades LbL films containing these polyelectrolytes. The chemical structures of the photocleavable groups dictate the wavelengths responsible for disrupting these coatings, ranging from ultraviolet to near-infrared in our work. In addition, spatially segregating reactive groups into "compartments" within LbL films has enabled us to fabricate reactive free-standing polymer films and multiheight photopatterned coatings. Overall, by combining bottom-up and top-down approaches, photoreactive LbL films enable precise control over the interfacial properties of polymer and composite coatings.

[Novel Visible Light Photoactivatable Caged Neurotransmitters Based on a N-Methyl Quinolinium Chromophore]

The development of novel photolabile protecting groups with practical levels of photolytic efficiency and hydrophilicity can provide smart photochemical tools, such as caged compounds. One of the long-standing problems of most reported photolabile protecting groups is the requirement for one-photon activation, of ultraviolet light (250-400 nm), that is harmful to living cells and has low tissue penetration power. An attractive approach to overcome this would be the use of longer-wavelength light for one-photon activation; advantages would include both lower phototoxicity and higher tissue penetration power than UV irradiation. As part of our research aimed at developing new photochemical tools, we have developed the N-methyl-7-hydroxyquinolinium (N-Me-7-HQm) caging chromophore as a novel photocage, sensitive to visible light. A key to the success of the development of the N-Me-7-HQm photocage was simple N-methylation of the 7-hydroxyquinoline chromophore. This modification allows access to visible light absorbance, facile photoactivation by blue-LED light (458 nm) with high photolytic efficiency, excellent water solubility, and high resistance to spontaneous hydrolysis. The success of the late stage upgrading of a chromophore in the synthetic sequence suggests that further functionalization of the caging chromophore will be possible, and should aid in the rapid generation of structurally diverse libraries of visible light-sensitive photocages.

UV and NIR-Responsive Layer-by-Layer Films Containing 6-Bromo-7-hydroxycoumarin Photolabile Groups

This paper describes polyelectrolyte multilayer films prepared by the layer-by-layer (LbL) technique capable of undergoing dissolution upon exposure to either ultraviolet or near-infrared light. Film dissolution is driven by photochemical deprotection of a random methacrylic copolymer with two types of side chains: (i) 6-bromo-7-hydroxycoumarinyl esters, photocleavable groups that are known to have substantial two-photon photolysis cross sections, and (ii) cationic residues from the commercially available monomer N,N-dimethylaminoethyl methacrylate (DMAEMA). In addition, the dependence of stability of both unirradiated and irradiated films on pH provides experimental evidence for the necessity of disrupting both ion-pairing and hydrophobic interactions between polyelectrolytes to realize film dissolution. This work therefore provides both new fundamental insight regarding photolabile LbL films and expands their applied capabilities to nonlinear photochemical processes.

Utilization of the Heavy Atom Effect for the Development of a Photosensitive 8-Azacoumarin-Type Photolabile Protecting Group

A remarkable improvement of the photochemical properties of coumarin-type photolabile protecting groups was achieved by iodine substitution. The newly identified 7-hydroxy-6-iodo-8-azacoumarin (8-aza-Ihc)-caged acetate showed excellent photolytic efficiency, significantly higher than that of the corresponding bromine-containing coumarin- and azacoumarin-type caging groups. The results provide a solid approach to improving the photosensitivity of photolabile protecting groups.