Two in one: merging photoactivated chemotherapy and photodynamic therapy to fight cancer

The growing number of cancer cases requires the development of new approaches for treatment. A therapy that has attracted the special attention of scientists is photodynamic therapy (PDT) due to its spatial and temporal resolution. However, it is accepted that this treatment methodology has limited application in cases of low cellular oxygenation, which is typical of cancerous tissues. Therefore, a strategy to overcome this drawback has been to combine this therapy with photoactivated chemotherapy (PACT), which works independently of the presence of oxygen. In this perspective, we examine compounds that act as both PDT and PACT agents and summarize their photophysical and biological characteristics.

Visible and near-infrared light-induced photoclick reactions

Photoclick reactions combine the advantages offered by light-driven processes, that is, non-invasive and high spatiotemporal control, with classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photocrosslinking, protein labelling and bioimaging. Despite these advances, most photoclick reactions typically require near-ultraviolet (UV) and mid-UV light to proceed. UV light can trigger undesirable responses, including cellular apoptosis, and therefore, visible and near-infrared light-induced photoclick reaction systems are highly desirable. Shifting to a longer wavelength can also reduce degradation of the photoclick reagents and products. Several strategies have been used to induce a bathochromic shift in the wavelength of irradiation-initiating photoclick reactions. For instance, the extension of the conjugated π-system, triplet-triplet energy transfer, multi-photon excitation, upconversion technology, photocatalytic and photoinitiation approaches, and designs involving photocages have all been used to achieve this goal. Current design strategies, recent advances and the outlook for long wavelength-driven photoclick reactions are presented.

BODIPY-based photocages: rational design and their biomedical application

Photocages, also known as photoactivated protective groups (PPGs), have been utilized to achieve controlled release of target molecules in a non-invasive and spatiotemporal manner. In the past decade, BODIPY fluorophores, a well-established class of fluorescent dyes, have emerged as a novel type of photoactivated protective group capable of efficiently releasing cargo species upon irradiation. This is due to their exceptional properties, including high molar absorption coefficients, resistance to photochemical and thermal degradation, multiple modification sites, favorable uncaging quantum yields, and highly adjustable spectral properties. Compared to traditional photocages that mainly absorb UV light, BODIPY-based photocages that absorb visible/near-infrared (Vis/NIR) light offer advantages such as deeper tissue penetration and reduced bio-autofluorescence, making them highly suitable for various biomedical applications. Consequently, different types of photoactivated protective groups based on the BODIPY skeleton have been established. This highlight provides a comprehensive overview of the strategies employed to construct BODIPY photocages by substituting leaving groups at different positions within the BODIPY fluorophore, including the meso-methyl position, boron position, 2,6-position, and 3,5-position. Furthermore, the application of these BODIPY photocages in biomedical fields, such as fluorescence imaging and controlled release of active species, is discussed.

Crucial Roles of Leaving Group and Open-Shell Cation in Photoreaction of (Coumarin-4-yl)methyl Derivatives

Photoreactions of (coumarin-4-yl)methyl derivatives have been extensively studied in many fields of chemistry, including organic synthesis and photoinduced drug delivery systems. The identification of the reaction intermediates involved in the photoreactions is crucial not only for elucidating the reaction mechanism but also for the application of the photoreactions. In this study, the photoreactions of 7-diethylamino(coumarin-4-yl)methyl thioester 1a [-SC(O)CH3], thionoester 1b [-OC(S)CH3], and ester 1c [-OC(O)CH3] were investigated to clarify the intermediary species and their chemical behavior. While a radical pair [i.e., 7-diethylamino(coumarin-4-yl)methyl radical and CH3C(O)S•] plays an important role in the photoreactions of 1a and 1b, an ion pair [i.e., 7-diethylamino(coumarin-4-yl)methyl cation, and CH3CO2-] was the key in the photoreaction of 1c. 18O-isotope-labeling of 1c revealed a negligible recombination process within the ion pair. The unprecedented observation was rationalized by the open-shell character of the 7-diethylamino(coumarin-4-yl)methyl cation, whose formation was confirmed through product analysis and transient absorption spectroscopy.

O-nitrobenzyl Caged Molecule Enables Photo-controlled Release of Thiabendazole

Pesticides are essential in agricultural development. Controlled-release pesticides have attracted great attentions. Base on a principle of spatiotemporal selectivity, we extended the photoremovable protective group (PRPG) into agrochemical agents to achieve controllable release of active ingredients. Herein, we obtained NP-TBZ by covalently linking o-nitrobenzyl (NP) with thiabendazole (TBZ). Compound NP-TBZ can be controlled to release TBZ in dependent to light. The irradiated and unirradiated NP-TBZ showed significant differences on fungicidal activities both in vitro and in vivo. In addition, the irradiated NP-TBZ displayed similar antifungal activities to the directly-used TBZ, indicating a factual applicability in controllable release of TBZ. Furthermore, we explored the action mode and microcosmic variations by SEM analysis, and demonstrated that the irradiated NP-TBZ retained a same action mode with TBZ against mycelia growth.

Light in a Heartbeat: Bond Scission by a Single Photon above 800 nm

Photocages enable scientists to take full control over the activity of molecules using light as a biocompatible stimulus. Their emerging applications in photoactivated therapies call for efficient uncaging in the near-infrared (NIR) window, which represents a fundamental challenge. Here, we report synthetically accessible cyanine photocages that liberate alcohol, phenol, amine, and thiol payloads upon irradiation with NIR light up to 820 nm in aqueous media. The photocages display a unique chameleon-like behavior and operate via two distinct uncaging mechanisms: photooxidation and heterolytic bond cleavage. The latter process constitutes the first example of a direct bond scission by a single photon ever observed in cyanine dyes or at wavelengths exceeding 800 nm. Modulation of the beating rates of human cardiomyocytes that we achieved by light-actuated release of adrenergic agonist etilefrine at submicromolar concentrations and low NIR light doses (∼12 J cm-2) highlights the potential of these photocages in biology and medicine.

Near-infrared light controlled protein degradation by photo-caged lenalidomide and pomalidomide

Immunomodulatory drugs (e.g. thalidomide, lenalidomide and pomalidomide) have been proven highly successful in clinical treatment of multiple myeloma. However, systematic degradation of zinc finger transcriptional factors induced by these drugs could lead to severe systematic toxicity in patients. Previous reports of NVOC caged pomalidomide attempted to regulate its activity using UVA irradiation, but their application was limited by high cytotoxicity and low tissue penetration. Here, we reported red-shifted BODIPY caged lenalidomide and pomalidomide that enabled red-light controlled protein degradation with spatiotemporal precision.

Spin-Vibronic Coupling Controls the Intersystem Crossing of Iodine-Substituted BODIPY Triplet Chromophores

4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene (BODIPY) dyes are extensively used in various applications of their triplet states, ranging from photoredox catalysis, through triplet sensitization to photodynamic therapy. However, the rational design of BODIPY triplet chromophores by ab initio modelling is limited by their strong interactions of spin, electronic and vibrational dynamics. In particular, spin-vibronic coupling is often overlooked when estimating intersystem crossing (ISC) rates. In this study, a combined experimental and theoretical approach using spin-vibronic coupling to correctly describe ISC in BODIPY dyes was developed. For this purpose, seven π-extended BODIPY derivatives with iodine atoms in different positions were examined. It was found that the heavy-atom effect of iodine atoms is site specific, causing high triplet yields in only some positions. This site-specific ISC was explained by El-Sayed rules, so both the contribution and character of the molecular orbitals involved in the excitation must be considered when predicting the ISC rates. Overall, the rational design of BODIPY triplet chromophores requires using (i) the high-quality electronic structure theory, including both static and dynamical correlations; and (ii) the two-component wave function Hamiltonian, and rationalizing; and (iii) ISC based on the character of the molecular orbitals of heavy atoms involved in the excitation, expanding El-Sayed rules beyond their traditional applications.

Cation delocalization and photo-isomerization enhance the uncaging quantum yield of a photocleavable protecting group

Photocleavable protecting groups (PPGs) enable the light-induced, spatiotemporal control over the release of a payload of interest. Two fundamental challenges in the design of new, effective PPGs are increasing the quantum yield (QY) of photolysis and red-shifting the absorption spectrum. Here we describe the combination of two photochemical strategies for PPG optimization in one molecule, resulting in significant improvements in both these crucial parameters. Furthermore, we for the first time identify the process of photo-isomerization to strongly influence the QY of photolysis of a PPG and identify the cis-isomer as the superior PPG.

Antiproliferative activity of meso-substituted BODIPY photocages: Effect of electrophiles vs singlet oxygen

A series of BODIPY compounds with a methylphenol substituent at the meso-position and halogen atoms on the BODIPY core, or OCH3 or OAc substituents at the phenolic moiety was synthesized. Their spectral and photophysical properties and the photochemical reactivity upon irradiation in CH3OH were investigated. The molecules with the phenolic substituent at the meso-position undergo more efficient photo-methanolysis at the boron atom, while the introduction of the OCH3 group at the phenolic moiety changes the reaction selectivity towards the cleavage at the meso-position. The introduction of the halogen atoms into the BODIPY increases the photo-cleavage reaction efficiency, as well as the ability of the molecules to sensitize oxygen and form reactive oxygen species (ROS). The efficiency of the ROS formation was measured in comparison with that of tetraphenylporphyrin. The antiproliferative effect of BODIPY molecules was investigated against three human cancer cell lines MCF-7 (breast carcinoma), H460 (lung carcinoma), HCT116 (colon carcinoma), and two non-cancer cell lines, HEK293T (embryonic kindey) and HaCaT (keratinocytes), with the cells kept in the dark or irradiated with visible light. For most of the compounds a modest or no antiproliferative activity was observed for cells in the dark. However, when cells were irradiated, a dramatic increase in cytotoxicity was observed (more than 100-fold), with IC50 values in the submicromolar concentration range. The enhancement of the cytotoxic effect was explained by the formation of ROS, which was studied for cells in vitro. However, for some BODIPY compounds, the effects due to the formation of electrophilic species (carbocations and quinone methides, which react with biomolecules) cannot be disregarded. Confocal fluorescence microscopy images of H460 cells and HEK293T show that the compounds enter the cells and are retained in the cytoplasm and membranes of the various organelles. When the cells treated with the compounds are irradiated, photo-processes lead to cell death by apoptosis. The study performed is important because it provides bases for the development of novel photo-therapeutics capable of exerting photo-cytotoxic effects in both oxygenated and hypoxic cells.

Far-red BODIPY-based oxime esters: photo-uncaging and drug delivery

Far-red BODIPY-based oxime esters for photo-uncaging were designed to release molecules of interest with carboxylic acids. The low power red LED light breaks the N-O oxime ester bond and frees the caged molecules. We studied the mechanism and kinetics of the uncaging procedure using a 1H NMR spectrometer. Moreover, the drug delivery strategy to release valproic acid (VPA) on demand was tested in vitro using this far-red BODIPY photo-uncaging strategy to induce apoptosis in tumor cells.

Giving the Green Light to Photochemical Uncaging of Large Biomolecules in High Vacuum

The isolation of biomolecules in a high vacuum enables experiments on fragile species in the absence of a perturbing environment. Since many molecular properties are influenced by local electric fields, here we seek to gain control over the number of charges on a biopolymer by photochemical uncaging. We present the design, modeling, and synthesis of photoactive molecular tags, their labeling to peptides and proteins as well as their photochemical validation in solution and in the gas phase. The tailored tags can be selectively cleaved off at a well-defined time and without the need for any external charge-transferring agents. The energy of a single or two green photons can already trigger the process, and it is soft enough to ensure the integrity of the released biomolecular cargo. We exploit differences in the cleavage pathways in solution and in vacuum and observe a surprising robustness in upscaling the approach from a model system to genuine proteins. The interaction wavelength of 532 nm is compatible with various biomolecular entities, such as oligonucleotides or oligosaccharides.

Record release of tetramethylguanidine using a green light activated photocage for rapid synthesis of soft materials

Photocages have enabled spatiotemporally governed organic materials synthesis with applications ranging from tissue engineering to soft robotics. However, the reliance on high energy UV light to drive an often inefficient uncaging process limits their utility. These hurdles are particularly evident for more reactive cargo, such as strong organobases, despite their attractive potential to catalyze a range of chemical transformations. Herein, two metal-free boron dipyrromethene (BODIPY) photocages bearing tetramethylguanidine (TMG) cargo are shown to induce rapid and efficient polymerizations upon exposure to a low intensity green LED. A suite of spectroscopic characterization tools were employed to identify the underlying uncaging and polymerization mechanisms, while also determining reaction quantum efficiencies. The results are directly compared to state-of-the-art TMG-bearing ortho-nitrobenzyl and coumainylmethyl photocages, finding that the present BODIPY derivatives enable step-growth polymerizations that are >10× faster than the next best performing photocage. As a final demonstration, the inherent multifunctionality of the present BODIPY platform in releasing radicals from one half of the molecule and TMG from the other is leveraged to prepare polymers with starkly disparate physical properties. The present findings are anticipated to enable new applications of photocages in both small-molecule photochemistry for medicine and advanced manufacturing of next generation soft materials.

Inter-Vesicle Signal Transduction Using a Photo-Responsive Zinc Ionophore

Transmission of chemical information between cells and across lipid bilayer membranes is of profound significance in many biological processes. The design of synthetic signalling systems is a critical step towards preparing artificial cells with collective behaviour. Here, we report the first example of a synthetic inter-vesicle signalling system, in which diffusible chemical signals trigger transmembrane ion transport in a manner reminiscent of signalling pathways in biology. The system is derived from novel ortho-nitrobenzyl and BODIPY photo-caged ZnII transporters, in which cation transport is triggered by photo-decaging with UV or red light, respectively. This decaging reaction can be used to trigger the release of the cationophores from a small population of sender vesicles. This in turn triggers the transport of ions across the membrane of a larger population of receiver vesicles, but not across the sender vesicle membrane, leading to overall inter-vesicle signal transduction and amplification.

meso-Methyl BODIPY Photocages: Mechanisms, Photochemical Properties, and Applications

meso-methyl BODIPY photocages have recently emerged as an exciting new class of photoremovable protecting groups (PPGs) that release leaving groups upon absorption of visible to near-infrared light. In this Perspective, we summarize the development of these PPGs and highlight their critical photochemical properties and applications. We discuss the absorption properties of the BODIPY PPGs, structure-photoreactivity studies, insights into the photoreaction mechanism, the scope of functional groups that can be caged, the chemical synthesis of these structures, and how substituents can alter the water solubility of the PPG and direct the PPG into specific subcellular compartments. Applications that exploit the unique optical and photochemical properties of BODIPY PPGs are also discussed, from wavelength-selective photoactivation to biological studies to photoresponsive organic materials and photomedicine.

Stabilization of Carbocation Intermediate by Cucurbit[7]uril Enables High Photolysis Efficiency

A cucurbit[7]uril-based host-guest strategy is employed to enhance the efficiency of photolysis reactions that release caged molecules from photoremovable protecting groups. The photolysis of benzyl acetate follows a heterolytic bond cleavage mechanism, thereby leading to the formation of a contact ion pair as the key reactive intermediate. The Gibbs free energy of the contact ion pair is lowered by 3.06 kcal/mol through the stabilization of cucurbit[7]uril, as revealed by DFT calculations, which results in a 40-fold increase in the quantum yield of the photolysis reaction. This methodology is also applicable to the chloride leaving group and the diphenyl photoremovable protecting group. We anticipate that this research presents a novel strategy to improve reactions involving active cationics, thereby enriching the field of supramolecular catalysis.

Rapid hydrogel formation via tandem visible light photouncaging and bioorthogonal ligation

The formation of benign polymer scaffolds in water using green-light-reactive photocages is described. These efforts pave an avenue toward the fabrication of synthetic scaffolds that can facilitate the study of cellular events for disease diagnosis and treatment. First, a series of boron dipyrromethene (BODIPY) photocages with nitrogen-containing nucleophiles were examined to determine structure-reactivity relationships, which resulted in a >1,000× increase in uncaging yield. Subsequently, photoinduced hydrogel formation in 90 wt % water was accomplished via biorthogonal carbonyl condensation using hydrophilic polymer scaffolds separately containing BODIPY photocages and ortho-phthalaldehyde (OPA) moieties. Spatiotemporal control is demonstrated with light on/off experiments to modulate gel stiffness and masking to provide <100 μm features. Biocompatability of the method was shown through pre-/post-crosslinking cell viability studies. Short term, these studies are anticipated to guide translation to emergent additive manufacturing technology, which, longer term, will enable the development of 3D cell cultures for tissue engineering applications.

Spatiotemporal Control of Biology: Synthetic Photochemistry Toolbox with Far-Red and Near-Infrared Light

The complex network of naturally occurring biological pathways motivates the development of new synthetic molecules to perturb and/or detect these processes for fundamental research and clinical applications. In this context, photochemical tools have emerged as an approach to control the activity of drug or probe molecules at high temporal and spatial resolutions. Traditional photochemical tools, particularly photolabile protecting groups (photocages) and photoswitches, rely on high-energy UV light that is only applicable to cells or transparent model animals. More recently, such designs have evolved into the visible and near-infrared regions with deeper tissue penetration, enabling photocontrol to study biology in tissue and model animal contexts. This Review highlights recent developments in synthetic far-red and near-infrared photocages and photoswitches and their current and potential applications at the interface of chemistry and biology.

Efficiency of Functional Group Caging with Second-Generation Green- and Red-Light-Labile BODIPY Photoremovable Protecting Groups

BODIPY-based photocages release substrates by excitation with wavelengths in the visible to near-IR regions. The recent development of more efficient BODIPY photocages spurred us to evaluate the scope and efficiency of these second-generation boron-methylated green-light and red-light-absorbing BODIPY photocages. Here, we show that these more photosensitive photocages release amine, alcohol, phenol, phosphate, halides, and carboxylic acid derivatives with much higher quantum yields than first-generation BODIPY photocages and excellent chemical yields. Chemical yields are near-quantitative for the release of all functional groups except the photorelease of amines, which react with concomitantly photogenerated singlet oxygen. In these cases, high chemical yields for photoreleased amines are restored by irradiation under an inert atmosphere. The photorelease quantum yield has a weak relationship with the leaving group pK_a of the green-absorbing BODIPY photocages but little relationship with the red-absorbing derivatives, suggesting that factors other than leaving group quality impact the quantum yield. For the photorelease of alcohols, in all cases a carbonate linker (that loses CO₂ upon photorelease) significantly increases both the quantum yield and the chemical yield compared to those for direct photorelease via the ether.

Photouncaging of Carboxylic Acids from Cyanine Dyes with Near-Infrared Light

Near-infrared light (NIR; 650-900 nm) offers unparalleled advantages as a biocompatible stimulus. The development of photocages that operate in this region represents a fundamental challenge due to the low energy of the excitation light. Herein, we repurpose cyanine dyes into photocages that are available on a multigram scale in three steps and efficiently release carboxylic acids in aqueous media upon irradiation with NIR light up to 820 nm. The photouncaging process is examined using several techniques, providing evidence that it proceeds via photooxidative pathway. We demonstrate the practical utility in live HeLa cells by delivery and release of the carboxylic acid cargo, that was otherwise not uptaken by cells in its free form. In combination with modularity of the cyanine scaffold, the realization of these accessible photocages will fully unleash the potential of the emerging field of NIR-photoactivation and facilitate its widespread adoption outside the photochemistry community.

Design and Validation of the First Family of Photo-Activatable Ligands for Melatonin Receptors

Melatonin is a neurohormone released in a circadian manner with peak levels at night. Melatonin mediates its effects mainly through G protein-coupled MT₁ and MT₂ receptors. Drugs acting on melatonin receptors are indicated for circadian rhythm- and sleep-related disorders. Tools to study the activation of these receptors with high temporal resolution are lacking. Here, we synthesized a family of light-activatable caged compounds by attaching o-nitrobenzyl (o-NB) or coumarin photocleavable groups to melatonin indolic nitrogen. All caged compounds showed the expected decrease in binding affinity for MT₁ and MT₂. The o-NB derivative MCS-0382 showed the best uncaging and biological properties, with 250-fold increase in affinity and potency upon illumination. Generation of melatonin from MCS-0382 was further demonstrated by its ability to modulate the excitation of SCN neurons in rat brain slices. MCS-0382 is available to study melatonin effects in a temporally controlled manner in cellular and physiological settings.

Endogenous Proteins Modulation in Live Cells with Small Molecules and Light

The protein modulation by light illumination enables the biological role investigation in high spatiotemporal precision. Compared to genetic methods, the small molecules approach is uniquely suited for modulating endogenous proteins. The endogenous protein modulation in live cells with small molecules and light has recently advanced on three distinctive frontiers: i) the infrared-light-induced or localized decaging of small molecules by photolysis, ii) the visible-light-induced photocatalytic releasing of small molecules, and iii) the small-molecule-ligand-directed caging for photo-modulation of proteins. Together, these methods provide powerful chemical biology tool kits for spatiotemporal modulation of endogenous proteins with potential therapeutic applications. This Concept aims to inspire organic chemists and chemical biologists to delve into this burgeoning endogenous protein modulation field for new biological discoveries.

Computational Design, Synthesis, and Photochemistry of Cy7-PPG, an Efficient NIR-Activated Photolabile Protecting Group for Therapeutic Applications

Photolabile Protecting Groups (PPGs) are molecular tools used, for example, in photopharmacology for the activation of drugs with light, enabling spatiotemporal control over their potency. Yet, red-shifting of PPG activation wavelengths into the NIR range, which penetrates the deepest in tissue, has often yielded inefficient or insoluble molecules, hindering the use of PPGs in the clinic. To solve this problem, we report herein a novel concept in PPG design, by transforming clinically-applied NIR-dyes with suitable molecular orbital configurations into new NIR-PPGs using computational approaches. Using this method, we demonstrate how Cy7, a class of NIR dyes possessing ideal properties (NIR-absorption, high molecular absorptivity, excellent aqueous solubility) can be successfully converted into Cy7-PPG. We report a facile synthesis towards Cy7-PPG from accessible precursors and confirm its excellent properties as the most redshifted oxygen-independent NIR-PPG to date (λmax =746 nm).

Strategy for Engineering High Photolysis Efficiency of Photocleavable Protecting Groups through Cation Stabilization

Photolabile protecting groups (PPGs) enable the precise activation of molecular function with light in many research areas, such as photopharmacology, where remote spatiotemporal control over the release of a molecule is needed. The design and application of PPGs in recent years have particularly focused on the development of molecules with high molar absorptivity at long irradiation wavelengths. However, a crucial parameter, which is pivotal to the efficiency of uncaging and which has until now proven highly challenging to improve, is the photolysis quantum yield (QY). Here, we describe a novel and general approach to greatly increase the photolysis QY of heterolytic PPGs through stabilization of an intermediate chromophore cation. When applied to coumarin PPGs, our strategy resulted in systems possessing an up to a 35-fold increase in QY and a convenient fluorescent readout during their uncaging, all while requiring the same number of synthetic steps for their preparation as the usual coumarin systems. We demonstrate that the same QY engineering strategy applies to different photolysis payloads and even different classes of PPGs. Furthermore, analysis of the DFT-calculated energy barriers in the first singlet excited state reveals valuable insights into the important factors that determine photolysis efficiency. The strategy reported herein will enable the development of efficient PPGs tailored for many applications.

Transforming Dyes into Fluorophores: Exciton-Induced Emission with Chain-like Oligo-BODIPY Superstructures

Herein we present a systematic study demonstrating to which extent exciton formation can amplify fluorescence based on a series of ethylene-bridged oligo-BODIPYs. A set of non- and weakly fluorescent BODIPY motifs was selected and transformed into discrete, chain-like oligomers by linkage via a flexible ethylene tether. The prepared superstructures constitute excitonically active entities with non-conjugated, Coulomb-coupled oscillators. The non-radiative deactivation channels of Internal Conversion (IC), also combined with an upstream reductive Photoelectron Transfer (rPET) and Intersystem Crossing (ISC) were addressed at the monomeric state and the evolution of fluorescence and (non-)radiative decay rates studied along the oligomeric series. We demonstrate that a "masked" fluorescence can be fully reactivated irrespective of the imposed conformational rigidity. This work challenges the paradigm that a collective fluorescence enhancement is limited to sterically induced motional restrictions.

Controlling Antimicrobial Activity of Quinolones Using Visible/NIR Light-Activated BODIPY Photocages

Controlling the activity of a pharmaceutical agent using light offers improved selectivity, reduction of adverse effects, and decreased environmental build-up. These benefits are especially attractive for antibiotics. Herein, we report a series of photoreleasable quinolones, which can be activated using visible/NIR light (520–800 nm). We have used BODIPY photocages with strong absorption in the visible to protect two different quinolone-based compounds and deactivate their antimicrobial properties. This activity could be recovered upon green or red light irradiation. A comprehensive computational study provides new insight into the reaction mechanism, revealing the relevance of considering explicit solvent molecules. The triplet excited state is populated and the photodissociation is assisted by the solvent. The light-controlled activity of these compounds has been assessed on a quinolone-susceptible E. coli strain. Up to a 32-fold change in the antimicrobial activity was measured.

A Nucleophilic Deprotection of Carbamate Mediated by 2-Mercaptoethanol

Carbamates, typically used for the protection of amines, including Cbz, Alloc, and methyl carbamate, can be readily deprotected by treatment with 2-mercaptoethanol in the presence of potassium phosphate tribasic in N,N-dimethylacetamide at 75 °C. This nucleophilic deprotection protocol is superior to the standard hydrogenolysis or Lewis acid-mediated deprotection conditions for substrates bearing a functionality sensitive to these more traditional methods.

A long-lived fluorenyl cation: efficiency booster for uncaging and photobase properties

The photochemistry of fluorenols has been of special interest for many years. This is because both the fluorenol and the fluorenyl cation are antiaromatic in the ground state due to their 4n π-electrons according to the Hückel rule. The photolysis reaction of various fluorene derivatives takes place via a cation intermediate and is preferred due to its excited state aromaticity. Here we present an extremely long-lived fluorenyl cation and its effects on the uncaging of various leaving groups. We analyze the relationship between uncaging quantum yields of fluorene-based cages and the longevity of their fluorenyl cations with different spectroscopic methods in the steady state and on an ultrafast time scale and find that the uncaging quantum yield rises with the stability of the cation. In contrast to previous reports, the cation can be observed on a time scale of minutes, even in moderately protic solvents as methanol and ethanol. The stability of this cation depends on the dimethylamino-substituents on the fluorene scaffold and the properties of the solvent. In addition, with bis-dimethylamino fluorenol, a photobase is introduced that expands the small group of known photoinduced hydroxide emitters.

Green Light-Activated Single-Component Organic Fluorescence-Based Nano-Drug Delivery System for Dual Uncaging of Anticancer Drugs

Developing green or red light-activated drug delivery systems (DDSs) for cancer treatment is highly desirable. Herein, we have reported a green light-responsive single component-based organic fluorescence nano-DDS by simply anchoring 2-hydroxy-6-naphthacyl (phototrigger) on both sides of the 1,5-diaminonaphthalene (DAN) chromophore. This green light (λ ≥ 500 nm)-activated DDS released two equivalents of the anticancer drug (valproic acid) in a spatio-temporally controlled manner. Our photoresponsive DDS [DAN-bis(HO-Naph-VPA)] exhibited interesting properties such as excited-state intramolecular proton transfer (ESIPT) accompanied with aggregation-induced emission (AIE) phenomena. AIE initiated the photorelease, and ESIPT enhanced the rate of the photorelease. Further, in vitro studies revealed that our green light-activated nano-DDS exhibited good cytocompatibility, excellent cellular internalization, and effective cancer cell killing ability.

Photoremovable Protecting Groups: Across the Light Spectrum to Near- Infrared Absorbing Photocages

We discuss the past decade of progress in the field of photoremovable protecting groups that allowed the development of photocages activatable by near-IR light and highlight the individual conceptual advancements that lead to general guidelines to design new such photoremovable protecting groups. We emphasize the importance of understanding the individual photochemical reaction mechanisms that was necessary to achieve this progress and provide an outlook of the subsequent steps to facilitate a swift translation of this research into clinical praxis. Since this issue of CHIMIA is dedicated to the late Prof. Thomas Bally, we decided to provide a personal perspective on the field to which he contributed himself. We tried to write this review with the general readership of CHIMIA in mind in a hope to pay a tribute to the extraordinary dedication and clarity with which Thomas Bally used to explain abstract chemical concepts to his students or colleagues. We are uncertain whether we matched such challenge but we believe that he would have liked such approach very much.

8-[4-(2-Hydroxypropane-2-yl)phenyl]-1,3,4,4,5,7-hexamethyl-4-boron-3a,4a-diaza-S-indacene

During recent years, the BODIPY core became a popular scaffold for designing photoremovable protecting groups (PPG). In this paper, we report the synthesis of a new molecule—8-[4-(2-hydroxypropane-2-yl)phenyl]-1,3,4,4,5,7-hexamethyl-4-boron-3a,4a-diaza-S-indacene—by the treatment of meso-(4-CO2Me-phenyl)-BODIPY with excess of MeMgI. The product was characterized by 1H, 13C NMR and HRMS. The combination of BODIPY core with tertiary benzilyc alcohol might be promising for utilizing this molecule as visible light removable PPG.

Wavelength-Selective Uncaging of Two Different Photoresponsive Groups on One Effector Molecule for Light-Controlled Activation and Deactivation

Photocleavable protecting groups (PPGs) play a pivotal role in numerous studies. They enable controlled release of small effector molecules to induce biochemical function. The number of PPGs attached to a variety of effector molecules has grown rapidly in recent years satisfying the high demand for new applications. However, until now molecules carrying PPGs have been designed to activate function only in a single direction, namely the release of the effector molecule. Herein, we present the new approach Two-PPGs-One-Molecule (TPOM) that exploits the orthogonal photolysis of two photoprotecting groups to first release the effector molecule and then to modify it to suppress its induced effect. The moiety resembling the tyrosyl side chain of the translation inhibitor puromycin was synthetically modified to the photosensitive ortho-nitrophenylalanine that cyclizes upon near UV-irradiation to an inactive puromycin cinnoline derivative. Additionally, the modified puromycin analog was protected by the thio-coumarylmethyl group as the second PPG. This TPOM strategy allows an initial wavelength-selective activation followed by a second light-induced deactivation. Both photolysis processes were spectroscopically studied in the UV/vis- and IR-region. In combination with quantum-chemical calculations and time-resolved NMR spectroscopy, the photoproducts of both activation and deactivation steps upon illumination were characterized. We further probed the translation inhibition effect of the new synthesized puromycin analog upon light activation/deactivation in a cell-free GFP translation assay. TPOM as a new method for precise triggering activation/deactivation of effector molecules represents a valuable addition for the control of biological processes with light.

A Novel Photosensitizer for Lipid Droplet-Location Photodynamic Therapy

Lipid droplets (LDs), an extremely important cellular organelle, are responsible for the storage of neutral lipids in multiple biological processes, which could be a potential target site for photodynamic therapy (PDT) of cancer. Herein, a lipid droplet–targeted photosensitizer (BODSeI) is developed, allowing for fluorescence imaging–guided PDT. Owing to the location of lipid droplets, BODSeI demonstrates enhanced PDT efficiency with an extremely low IC50 value (around 125 nM). Besides, BODSeI shows good biocompatibility and high photostability. Therefore, BODSeI is promising for droplet-location PDT, which may trigger wide interest for exploring the pathway of lipid droplet–location PDT.

Optical control of targeted protein degradation

Molecular glues and proteolysis targeting chimeras (PROTACs) have emerged as small-molecule tools that selectively induce the degradation of a chosen protein and have shown therapeutic promise. Recently, several approaches employing light as an additional stimulus to control induced protein degradation have been reported. Here, we analyze the principles guiding the design of such systems, provide a survey of the literature published to date, and discuss opportunities for further development. Light-responsive degraders enable the precise temporal and spatial control of protein levels, making them useful research tools but also potential candidates for human precision medicine.

Interrogating biological systems using visible-light-powered catalysis

Light-powered catalysis has found broad utility as a chemical transformation strategy, with widespread impact on energy, environment, drug discovery and human health. A noteworthy application impacting human health is light-induced sensitization of cofactors for photodynamic therapy in cancer treatment. The clinical adoption of this photosensitization approach has inspired the search for other photochemical methods, such as photoredox catalysis, to influence biological discovery. Over the past decade, light-mediated catalysis has enabled the discovery of valuable synthetic transformations, propelling it to become a highly utilized chemical synthesis strategy. The reaction components required to achieve a photoredox reaction are identical to photosensitization (catalyst, light source and substrate), making it ideally suited for probing biological environments. In this Review, we discuss the therapeutic application of photosensitization and advancements made in developing next-generation catalysts. We then highlight emerging uses of photoredox catalytic methods for protein bioconjugation and probing complex cellular environments in living cells.

Boron dipyrromethene (BODIPY) in polymer chemistry

The present review provides both a summary and outlook on the exciting field of BODIPYs in polymer chemistry.

Taking phototherapeutics from concept to clinical launch

More than four decades have passed since the first example of a light-activated (caged) compound was described. In the intervening years, a large number of light-responsive derivatives have been reported, several of which have found utility under a variety of in vitro conditions using cells and tissues. Light-triggered bioactivity furnishes spatial and temporal control, and offers the possibility of precision dosing and orthogonal communication with different biomolecules. These inherent attributes of light have been advocated as advantageous for the delivery and/or activation of drugs at diseased sites for a variety of indications. However, the tissue penetrance of light is profoundly wavelength-dependent. Only recently have phototherapeutics that are photoresponsive in the optical window of tissue (600-900 nm) been described. This Review highlights these recent discoveries, along with their limitations and clinical opportunities. In addition, we describe preliminary in vivo studies of prospective phototherapeutics, with an emphasis on the path that remains to be navigated in order to translate light-activated drugs into clinically useful therapeutics. Finally, the unique attributes of phototherapeutics is highlighted by discussing several potential disease applications.

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.

Photoresponsive molecular tools for emerging applications of light in medicine

Light-based therapeutic and imaging modalities, which emerge in clinical applications, rely on molecular tools, such as photocleavable protecting groups and photoswitches that respond to photonic stimulus and translate it into a biological effect. However, optimisation of their key parameters (activation wavelength, band separation, fatigue resistance and half-life) is necessary to enable application in the medical field. In this perspective, we describe the applications scenarios that can be envisioned in clinical practice and then we use those scenarios to explain the necessary properties that the photoresponsive tools used to control biological function should possess, highlighted by examples from medical imaging, drug delivery and photopharmacology. We then present how the (photo)chemical parameters are currently being optimized and an outlook is given on pharmacological aspects (toxicity, solubility, and stability) of light-responsive molecules. With these interdisciplinary insights, we aim to inspire the future directions for the development of photocontrolled tools that will empower clinical applications of light.