Two-photon uncaging of bioactive compounds: Starter guide to an efficient IR light switch

Exploring the molecular design principles for efficient diarylethene photoacid and photohydride generators based on the photochemical reaction mechanism

Photoacid generators (PAGs) and photohydride generators (PHGs) are specific photolabile protecting groups that release acid and hydride, respectively. Over the past decade, great efforts have been devoted to developing novel PAGs and PHGs with advanced efficiency, among which, two of the promising candidates are diarylethene (DAE)-based PAGs and PHGs, which release acids/hydrides during photochromic electrocyclization. The release quantum yield for PAGs is acceptable, while that of PHGs is only 4.2% even after molecular structure modification. In this work, time-resolved transient absorption spectroscopies with femtosecond and nanosecond resolutions along with DFT/TD-DFT calculations were utilized to unravel the detailed photochemical reaction mechanisms of DAE-based PAGs (1o) and PHGs (2o), respectively. The results show that the different photochemical mechanisms are the key that leads to distinctive release quantum yields between 1o and 2o. The factors affecting the release quantum yield are discussed in detail, and several key design principles are proposed to facilitate future rational design of DAE-based PAGs and PHGs.

Triphenylamine Sensitized 8-Dimethylaminoquinoline: An Efficient Two-Photon Caging Group for Intracellular Delivery

Triphenylamine-sensitized 8-dimethylaminoquinoline (TAQ) probes showed fair two-photon absorption and fragmentation cross sections in releasing kainate and GABA ligands. The water-soluble PEG and TEG-analogs allowed cell internalization and efficient light-gated liberation of the rhodamine reporter under UV and two-photon (NIR) irradiation conditions.

Red-shifted two-photon-sensitive phenanthridine photocages: synthesis and characterisation

Herein we describe the rational design, synthesis and photophysical study of a novel class of phenanthridine-based, one- and two-photon sensitive, photoremovable protecting groups with absorption wavelengths extending beyond 400 nm. This design facilitated the development of scaffolds with enhanced uncaging quantum yield, paving the way for broader applications in controlled drug delivery and molecular manipulation.

Triplet-Triplet Annihilation Upconversion-Based Photolysis: Applications in Photopharmacology

The emerging field of photopharmacology is a promising chemobiological methodology for optical control of drug activities that could ultimately solve the off-target toxicity outside the disease location of many drugs for the treatment of a given pathology. The use of photolytic reactions looks very attractive for a light-activated drug release but requires to develop photolytic reactions sensitive to red or near-infrared light excitation for better tissue penetration. This review will present the concepts of triplet-triplet annihilation upconversion-based photolysis and their recent in vivo applications for light-induced drug delivery using photoactivatable nanoparticles.

Photorelease of Nitric Oxide (NO) in Mono- and Bimetallic Ruthenium Nitrosyl Complexes: A Photokinetic Investigation with a Two-Step Model

Two monometallic and three bimetallic ruthenium acetonitrile (RuMeCN) complexes are presented and fully characterized. All of them are built from the same skeleton [FTRu(bpy)(MeCN)]2+, in which FT is a fluorenyl-substituted terpyridine ligand and bpy is the 2,2'-bipyridine. The crystal structure of [FTRu(bpy)(MeCN)](PF6)2 is presented. A careful spectroscopic analysis allows establishing that these 5 RuMeCN complexes can be identified as the product of the photoreaction of 5 related RuNO complexes, investigated as efficient nitric oxide (NO) donors. Based on this set of complexes, the mechanism of the NO photorelease of the bimetallic complexes has been established through a complete investigation under irradiations performed at 365, 400, 455, and 490 nm wavelength. A two-step (A → B → C) kinetic model specially designed for this purpose provides a good description of the mechanism, with quantum yields of photorelease in the range 0.001-0.029, depending on the irradiation wavelength. In the first step of release, the quantum yields (ϕAB) are always found to be larger than those of the second step (ϕBC), at any irradiation wavelengths.

Development of a Two-Photon-Responsive Chromophore, 2-(p-Aminophenyl)-5,6-dimethoxy-1-(hydroxyinden-3-yl)methyl Derivative, as a Photoremovable Protecting Group

Photoremovable protecting groups (PPGs) are powerful tools that are widely used to investigate biological events in cells. An important requirement for PPGs is the efficient release of bioactive molecules by using visible to near-infrared light in the biological window (650-1350 nm). In this study, we report a new two-photon (2P)-responsive PPG, 2-(p-aminophenyl)-5,6-dimethoxy-1-(hydroxyinden-3-yl)methyl, with a donor-π-donor cyclic stilbene structure. The 2P cross section was approximately 40-50 GM at ∼700 nm. The quantum yield of the uncaging process of caged benzoate was greater than 0.7, demonstrating that the 2P uncaging efficiency was approximately 30 GM at around 700 nm. This newly developed 2P-responsive chromophore can be used in future biological experiments. The mechanism of the photo-uncaging reaction via the carbocation intermediate was elucidated using transient absorption spectroscopy and product analysis.

Effect of Two-Photon Excitation to 8-Azacoumarin Derivatives as Photolabile Protecting Groups

An improvement of the two-photon excitation was achieved using 8-azacoumarin-type caged compounds, which showed large values of the two-photon uncaging action cross-section (δu >0.1 Goeppert-Mayer (GM)). In particular, the 7-hydroxy-6-iodo-8-azacoumarin (8-aza-Ihc)-caged compound showed an excellent uncaging action cross-section value (δu = 1.28 GM). Therefore, 8-azacoumarin-type photolabile protecting groups (PPGs) can be used as two-photon excitation sources.

Three-Photon Infrared Stimulation of Endogenous Neuroreceptors in Vivo

To interrogate neural circuits and crack their codes, in vivo brain activity imaging must be combined with spatiotemporally precise stimulation in three dimensions using genetic or pharmacological specificity. This challenge requires deep penetration and focusing as provided by infrared light and multiphoton excitation, and has promoted two-photon photopharmacology and optogenetics. However, three-photon brain stimulation in vivo remains to be demonstrated. We report the regulation of neuronal activity in zebrafish larvae by three-photon excitation of a photoswitchable muscarinic agonist at 50 pM, a billion-fold lower concentration than used for uncaging, and with mid-infrared light of 1560 nm, the longest reported photoswitch wavelength. Robust, physiologically relevant photoresponses allow modulating brain activity in wild-type animals with spatiotemporal and pharmacological precision. Computational calculations predict that azobenzene-based ligands have high three-photon absorption cross-section and can be used directly with pulsed infrared light. The expansion of three-photon pharmacology will deeply impact basic neurobiology and neuromodulation phototherapies.

Near-infrared light-triggered prodrug photolysis by one-step energy transfer

Prodrug photolysis enables spatiotemporal control of drug release at the desired lesions. For photoactivated therapy, near-infrared (NIR) light is preferable due to its deep tissue penetration and low phototoxicity. However, most of the photocleavable groups cannot be directly activated by NIR light. Here, we report a upconversion-like process via only one step of energy transfer for NIR light-triggered prodrug photolysis. We utilize a photosensitizer (PS) that can be activated via singlet-triplet (S-T) absorption and achieve photolysis of boron-dipyrromethene (BODIPY)-based prodrugs via triplet-triplet energy transfer. Using the strategy, NIR light can achieve green light-responsive photolysis with a single-photon process. A wide range of drugs and bioactive molecules are designed and demonstrated to be released under low-irradiance NIR light (100 mW/cm2, 5 min) with high yields (up to 87%). Moreover, a micellar nanosystem encapsulating both PS and prodrug is developed to demonstrate the practicality of our strategy in normoxia aqueous environment for cancer therapy. This study may advance the development of photocleavable prodrugs and photoresponsive drug delivery systems for photo-activated therapy.

Riboflavin-Vancomycin Conjugate Enables Simultaneous Antibiotic Photo-Release and Photodynamic Killing against Resistant Gram-Positive Pathogens

Decades of antibiotic misuse have led to alarming levels of antimicrobial resistance, and the development of alternative diagnostic and therapeutic strategies to delineate and treat infections is a global priority. In particular, the nosocomial, multidrug-resistant "ESKAPE" pathogens such as Gram-positive methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus spp (VRE) urgently require alternative treatments. Here, we developed light-activated molecules based on the conjugation of the FDA-approved photosensitizer riboflavin to the Gram-positive specific ligand vancomycin to enable targeted antimicrobial photodynamic therapy. The riboflavin-vancomycin conjugate proved to be a potent and versatile antibacterial agent, enabling the rapid, light-mediated, killing of MRSA and VRE with no significant off-target effects. The attachment of riboflavin on vancomycin also led to an increase in antibiotic activity against S. aureus and VRE. Simultaneously, we evidenced for the first time that the flavin subunit undergoes an efficient photoinduced bond cleavage reaction to release vancomycin, thereby acting as a photoremovable protecting group with potential applications in drug delivery.

Bioorthogonally Assisted Phototherapy: Recent Advances and Prospects

Photoresponsive materials offer excellent spatiotemporal control over biological processes and the emerging phototherapeutic methods are expected to have significant effects on targeted cancer therapies. Recent examples show that combination of photoactivatable approaches with bioorthogonal chemistry enhances the precision of targeted phototherapies and profound implications are foreseen particularly in the treatment of disperse/diffuse tumors. The extra level of on-target selectivity and improved spatial/temporal control considerably intensified related bioorthogonally assisted phototherapy research. The anticipated growth of further developments in the field justifies the timeliness of a brief summary of the state of the art.

Phototriggered structures: Latest advances in biomedical applications

Non-invasive control of the drug molecules accessibility is a key issue in improving diagnostic and therapeutic procedures. Some studies have explored the spatiotemporal control by light as a peripheral stimulus. Phototriggered drug delivery systems (PTDDSs) have received interest in the past decade among biological researchers due to their capability the control drug release. To this end, a wide range of phototrigger molecular structures participated in the DDSs to serve additional efficiency and a high-conversion release of active fragments under light irradiation. Up to now, several categories of PTDDSs have been extended to upgrade the performance of controlled delivery of therapeutic agents based on well-known phototrigger molecular structures like o-nitrobenzyl, coumarinyl, anthracenyl, quinolinyl, o-hydroxycinnamate and hydroxyphenacyl, where either of one endows an exclusive feature and distinct mechanistic approach. This review conveys the design, photochemical properties and essential mechanism of the most important phototriggered structures for the release of single and dual (similar or different) active molecules that have the ability to quickly reason of the large variety of dynamic biological phenomena for biomedical applications like photo-regulated drug release, synergistic outcomes, real-time monitoring, and biocompatibility potential.

Multimeric RGD-Based Strategies for Selective Drug Delivery to Tumor Tissues

RGD peptides have received a lot of attention over the two last decades, in particular to improve tumor therapy through the targeting of the αVβ3 integrin receptor. This review focuses on the molecular design of multimeric RGD compounds, as well as the design of suitable linkers for drug delivery. Many examples of RGD-drug conjugates have been developed, and we show the importance of RGD constructs to enhance binding affinity to tumor cells, as well as their drug uptake. Further, we also highlight the use of RGD peptides as theranostic systems, promising tools offering dual modality, such as tumor diagnosis and therapy. In conclusion, we address the challenging issues, as well as ongoing and future development, in comparison with large molecules, such as monoclonal antibodies.

Sensitized 1-Acyl-7-nitroindolines with Enhanced Two-Photon Cross Sections for Release of Neurotransmitters

Precise photochemical control, using two-photon excitation (2PE), of the timing and location of activation of glutamate is useful for studying the molecular and cellular physiology of the brain. Antenna-based light harvesting strategies represent a general method to increase the sensitivity to 2PE of otherwise insensitive photoremovable protecting groups (PPGs). This was applied to the most commonly used form of "caged" glutamate, MNI-Glu. Computational investigation showed that a four- or six-carbon linker attached between the 4-position of thioxanthone (THX) and the 4-position of the 5-methyl derivative of MNI-Glu (MMNI-Glu) would position the antenna and PPG close to one another to enable Dexter energy transfer. Nine THX-MMNI-Glu conjugates were prepared and their photochemical properties determined. Installation of the THX antenna resulted in a red shift of the absorption (λ_max = 385-405 nm) along with increased quantum yield compared to the parent compound MNI-Glu (λ_max = 347 nm). The THX-MMNI-Glu conjugate with a four-carbon linker and attachment to the 4-position of THX underwent photolysis via 1PE at 405 and 430 nm and via 2PE at 770 and 860 nm, yielding glutamate. The two-photon uncaging action cross section (δ_u) was 0.11 and 0.29 GM at 770 and 860, respectively, which was greater than for MNI-Glu (0.06 and 0.072 GM at 720 and 770 nm, respectively). The THX sensitizer harvested the light via 2PE and transferred its resulting triplet energy to MMNI-Glu. Release of glutamate through 2PE at 860 nm from the compound (100 μM) activated iGluSnFR, a genetically encoded, fluorescent glutamate sensor, on the surface of cells in culture, portending its usefulness in studies of neurophysiology in acute brain slice.

Two-Photon Absorption: An Open Door to the NIR-II Biological Window?

The way in which photons travel through biological tissues and subsequently become scattered or absorbed is a key limitation for traditional optical medical imaging techniques using visible light. In contrast, near-infrared wavelengths, in particular those above 1000 nm, penetrate deeper in tissues and undergo less scattering and cause less photo-damage, which describes the so-called "second biological transparency window". Unfortunately, current dyes and imaging probes have severely limited absorption profiles at such long wavelengths, and molecular engineering of novel NIR-II dyes can be a tedious and unpredictable process, which limits access to this optical window and impedes further developments. Two-photon (2P) absorption not only provides convenient access to this window by doubling the absorption wavelength of dyes, but also increases the possible resolution. This review aims to provide an update on the available 2P instrumentation and 2P luminescent materials available for optical imaging in the NIR-II window.

From 2,5-Diformyl-1,4-dihydropyrrolo[3,2-b]pyrroles to Quadrupolar, Centrosymmetric Two-Photon-Absorbing A-D-A Dyes

An original approach has been developed for the insertion of formyl substituents at positions 2 and 5 of 1,4-dihydropyrrolo[3,2-b]pyrroles by conversion of thiazol-2-yl substituents. The synthetic utility of these formyl groups was investigated, and a series of centrosymmetric A-π-D-π-A frameworks were constructed. The two-photon absorption of the quadrupolar pyrrolo[3,2-b]pyrrole possessing two dicyanovinylidene flanking groups is attributed to an S0 → (S1) → S4 transition which has a large TPA cross-section (1300 GM) for a molecule of this size.

Stealth Luminescent Organic Nanoparticles Made from Quadrupolar Dyes for Two-Photon Bioimaging: Effect of End-Groups and Core

Molecular-based Fluorescent Organic Nanoparticles (FONs) are versatile light-emitting nano-tools whose properties can be rationally addressed by bottom-up molecular engineering. A challenging property to gain control over is the interaction of the FONs’ surface with biological systems. Indeed, most types of nanoparticles tend to interact with biological membranes. To address this limitation, we recently reported on two-photon (2P) absorbing, red to near infrared (NIR) emitting quadrupolar extended dyes built from a benzothiadiazole core and diphenylamino endgroups that yield spontaneously stealth FONs. In this paper, we expand our understanding of the structure-property relationship between the dye structure and the FONs 2P absorption response, fluorescence and stealthiness by characterizing a dye-related series of FONs. We observe that increasing the strength of the donor end-groups or of the core acceptor in the quadrupolar (D-π-A-π-D) dye structure allows for the tuning of optical properties, notably red-shifting both the emission (from red to NIR) and 2P absorption spectra while inducing a decrease in their fluorescence quantum yield. Thanks to their strong 1P and 2P absorption, all FONs whose median size varies between 11 and 28 nm exhibit giant 1P (106 M−1.cm−1) and 2P (104 GM) brightness values. Interestingly, all FONs were found to be non-toxic, exhibit stealth behaviour, and show vanishing non-specific interactions with cell membranes. We postulate that the strong hydrophobic character and the rigidity of the FONs building blocks are crucial to controlling the stealth nano-bio interface.

Efficient Visible/NIR Light-driven Uncaging of Hydroxylated Thiazole Orange-based Caged Compounds in Aqueous Media

In photoactivation strategies with bioactive molecules, one-photon visible or two-photon near-infrared light-sensitive caged compounds are desirable tools for biological applications because they offer reduced phototoxicity and deep tissue penetration. However, visible-light-sensitive photoremovable protecting groups (PPGs) reported so far have displayed high hydrophobicity and low uncaging cross sections (εΦ < 50) in aqueous media, which can obstruct the control of bioactivity with high spatial and temporal precision. In this study, we developed hydroxylated thiazole orange (HTO) derivatives as visible-light-sensitive PPGs with high uncaging cross sections (εΦ ≈ 370) in aqueous solution. In addition, 2PE photolysis reactions of HTO-caged glutamate were achieved using a NIR laser (940 nm). Moreover, HTO-caged glutamate can activate N-methyl-d-aspartic acid receptors in Xenopus oocytes and mammalian cells with green-light illumination, thus allowing optical control of biological functions.

A hydroxylated thiazole orange (HTO)-caged glutamate efficiently releases a glutamate for temporal activation of ion channels under visible-to-NIR light in aqueous media.

Two-Photon Sensitive Coumarinyl Photoremovable Protecting Groups with Rigid Electron-Rich Cycles Obtained by Domino Reactions Initiated by a 5-exo-Dig Cyclocarbopalladation

We herein report the design, synthesis, and photophysical characterization of extended and rigid coumarinyl derivatives showing large two-photon sensitivities (δ_aΦ_u ≤ 125 GM) at 740 and 800 nm. To efficiently synthesize these complex photoremovable protecting groups (PPGs), we used step-economic domino reactions. Moreover, those new coumarinyl PPGs display unique bathochromic shifts (≤100 nm) of the uncaging subproducts as a result of the formation of a more conjugated fulvene moiety.