Femtosecond to Microsecond Observation of Photochemical Pathways in Nitroaromatic Phototriggers Using Transient Absorption Spectroscopy

The synthetic accessibility and tolerance to structural modification of phototriggered compounds (PTs) based on the ortho- nitrobenzene (ONB) protecting group have encouraged a myriad of applications including optimization of biological activity, and supramolecular polymerization. Here, a combination of ultrafast transient absorption spectroscopy techniques is used to study the multistep photochemistry of two nitroaromatic phototriggers based on the ONB chromophore, O-(4,5-dimethoxy-2-nitrobenzyl)-l-serine (DMNB-Ser) and O-[(2-nitrophenyl)methyl]-l-tyrosine hydrochloride (NB-Tyr), in DMSO solutions on femtosecond to microsecond time scales following the absorption of UV light. From a common nitro-S1 excited state, the PTs can either undergo excited state intramolecular hydrogen transfer (ESIHT) to an aci-S1 isomer within the singlet state manifold, leading to direct S1 → S0 internal conversion through a conical intersection, or competitive intersystem crossing (ISC) to access the triplet state manifold on time scales of (1.93 ± 0.03) ps and (13.9 ± 1.2) ps for DMNB-Ser and NB-Tyr, respectively. Deprotonation of aci-T1 species to yield triplet anions is proposed to occur in both PTs, with an illustrative time constant of (9.4 ± 0.7) ns for DMNB-Ser. More than 75% of the photoexcited molecules return to their electronic ground states within 8 μs, either by direct S1 → S0 relaxation or anion reprotonation. Hence, upper limits to the quantum yields of photoproduct formation are estimated to be in the range of 13-25%. Mixed DMSO/H2O solvents show the influence of the environment on the observed photochemistry, for example, revealing two nitro-S1 lifetimes for DMNB-Ser in a 10:1 DMSO/H2O mixture of 1.95 ps and (10.1 ± 1.2) ps, which are attributed to different microsolvation environments.

Quadrupolar Dyes Based on Highly Polarized Coumarins

The fluorescence and other photophysical parameters of highly polarized, quadrupolar bis-coumarins possessing an electron-rich pyrrolo[3,2-b]pyrrole bridging unit are highly dependent on the linking position between both chromophores. Delocalization of the LUMO on the entire π-system results in intense emission and strong two-photon absorption.

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.

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.

Constructing Cross-Linked Nanofibrous Scaffold via Dual-Enzyme-Instructed Hierarchical Assembly

To explore the potential of step-by-step assembly in the fabrication of biological materials, we designed and synthesized two peptide-based molecules for enzyme-instructed hierarchical assembly. Upon the treatment of alkaline phosphatase, one molecule undergoes enzyme-instructed self-assembly forming uniformed nanofibers. The other one that can self-assemble into vesicles undergoes enzyme-induced transformation of self-assembly converting vesicles into irregular aggregates upon the treatment of carboxylesterase. Coadministration of two enzymes to a mixture of these two molecules in a stage-by-stage fashion leads to a physically knotted nanofibrous scaffold that is applicable as a nanostructured matrix for cell culture.

N2S2 pyridinophane-based fluorescent chemosensors for selective optical detection of Cd2+ in soils

A fluorescent sensor array for the quantitative determination of Cd²⁺ in soils based on two N₂S₂ pyridinophane chemosensors is presented.

Photodegradable coumarin-derived amphiphilic dendrons for DNA binding: Self-assembly and phototriggered disassembly in water and air-water interface

In this article, we demonstrate the self-assembly and photoresponive behavior of a novel coumarin-based amphiphilic dendron in both aqueous solution and air-water interface. The dendritic structure, namely C-IG₁, was composed of a lipophilic cholesterol and hydrophilic poly(amido amine) (PAMAM) dendron, and the amphiphilic counterpart is interconnected by a photolabile coumarin carbonate ester, enabling the photoinduced degradation of the amphiphiles in protic solvents via S_N1-like mechanism. A Nile red solubilization fluorescence assay suggests a low critical aggregation concentration for the micelle formation of C-IG₁ in aqueous solutions (3.9 × 10^-5 M); the Langmuir analysis further indicates that C-IG₁ possesses significant compressibility in air-water interface, eventually forming homogeneous monolayers with a final molecular area (A₀) of 36 Ų. Notably, the micelles and Langmuir monolayer are quite stable until photo-triggered dissociation based on the photocleavage of C-IG₁ amphiphile activated by 365-nm incident light. Moreover, the transition in interfacial morphology of the Langmuir monolayer during the assembly and photodegradation processes also can be visually analyzed by incorporating Nile red probes with in situ monitoring through fluorescence microscopy. The thin film deposited on a glass substrate by the Langmuir-Blodgett technique also shows a photoresponsive behavior based on the change in the contact angles of a water droplet on the surface upon light stimulation. The binding affinity of C-IG₁ and cyclic DNA determined by the fluorescence quenching analysis of the coumarin reporter suggests a ground-state macromolecular complexation process occurring through polyvalent interactions between the pseudodendrimers and biomacromolecules. The ethidium bromide displacement assay further indicates thus dendriplex formation at low nitrogen-to-phosphorous value (N/P < 1) and confirms that the decomplexation accompanied by DNA release can be achieved through an active phototriggered route under spatiotemporal control.

A Click Cage: Organelle-Specific Uncaging of Lipid Messengers

Lipid messengers exert their function on short time scales at distinct subcellular locations, yet most experimental approaches for perturbing their levels trigger cell-wide concentration changes. Herein, we report on a coumarin-based photocaging group that can be modified with organelle-targeting moieties by click chemistry and thus enables photorelease of lipid messengers in distinct organelles. We show that caged arachidonic acid and sphingosine derivatives can be selectively delivered to mitochondria, the ER, lysosomes, and the plasma membrane. By comparing the cellular calcium transients induced by localized uncaging of arachidonic acid and sphingosine, we show that the precise intracellular localization of the released second messenger is crucial for the signaling outcome. Ultimately, we anticipate that this new class of caged compounds will greatly facilitate the study of cellular processes on the organelle level.

Photo-antimicrobial polymeric films releasing nitric oxide with fluorescence reporting under visible light

A novel molecular hybrid has been embedded in poly(lactic-co-glycolic acid) to give an antibacterial film photogenerating NO with a concomitant fluorescent reporter.

4-Trifluoromethyl-substituted coumarins with large Stokes shifts: synthesis, bioconjugates, and their use in super-resolution fluorescence microscopy

Bright and photostable fluorescent dyes with large Stokes shifts are widely used as sensors, molecular probes, and light-emitting markers in chemistry, life sciences, and optical microscopy. In this study, new 7-dialkylamino-4-trifluoromethylcoumarins have been designed for use in bioconjugation reactions and optical microscopy. Their synthesis was based on the Stille reaction of 3-chloro-4-trifluoromethylcoumarins and available (hetero)aryl- or (hetero)arylethenyltin derivatives. Alternatively, the acylation of 2-trifluoroacetyl-5-dialkylaminophenols with available (hetero)aryl- or (hetero)arylethenylacetic acids followed by intramolecular condensation afforded coumarins with 3-(hetero)aryl or 3-[2-(hetero)aryl]ethenyl groups. Hydrophilic properties were provided by the introduction of a sulfonic acid residue or by phosphorylation of a primary hydroxy group attached at C-4 of the 2,2,4-trimethyl-1,2-dihydroquinoline fragment fused to the coumarin fluorophore. For use in immunolabeling procedures, the dyes were decorated with an (activated) carboxy group. The positions of the absorption and emission maxima vary in the ranges 413-480 and 527-668 nm, respectively. The phosphorylated dye, 9,CH=CH-2-py,H, with the 1-(3-carboxypropyl)-4-hydroxymethyl-2,2-dimethyl-1,2-dihydroquinoline fragment fused to the coumarin fluorophore bearing the 3-[2-(2-pyridyl)ethenyl] residue (absorption and emission maxima at 472 and 623 nm, respectively) was used in super-resolution light microscopy with stimulated emission depletion and provided an optical resolution better than 70 nm with a low background signal. As a result of their large Stokes shifts, good fluorescence quantum yields, and adequate photostabilities, phosphorylated coumarins enable two-color imaging (using several excitation sources and a single depletion laser) to be combined with subdiffractional optical resolution.

Photoresponsive coumarin-tethered multifunctional magnetic nanoparticles for release of anticancer drug

Recently, photoresponsive nanoparticles have received significant attention because of their ability to provide spatial and temporal control over the drug release. In the present work, we report for the first time photoresponsive multifunctional magnetic nanoparticles (MNPs) fabricated using coumarin-based phototrigger and Fe/Si MNPs for controlled delivery of anticancer drug chlorambucil. Further, newly fabricated photoresponsive multifunctional MNPs were also explored for cell luminescence imaging. In vitro biological studies revealed that coumarin tethered Fe/Si MNPs of ~9 nm size efficiently delivered the anticancer drug chlorambucil into cancer cells and thereby improving the drug action to kill the cancer cells upon irradiation. Such multifunctional MNPs with strong fluorescence, good biocompatibility and efficient photocontrolled drug release ability will be of great benefit in the construction of light-activated multifunctional nano drug delivery systems.

7-(Dimethylamino)coumarin-3-carbaldehyde and its phenylsemicarbazone: TICT excited state modulation, fluorescent H-aggregates, and preferential solvation

The photophysical properties of 7-(dimethylamino)coumarin-3-carbaldehyde 3 and its phenylsemicarbazone 4 were investigated in solvents of various polarity and in differing solvent mixtures. The different fluorescent quantum yield (ΦF) behavior of 3 and 4 in highly polar solvents is discussed in terms of Twisted Intramolecular Charge-Tranfer (TICT) state formation and the specific solute-solvent interactions. Because of the weak intermolecular hydrogen bonding ability of both the radiative ICT and nonradiative TICT excited state of 3 and the linear steep decrease in ΦF from a medium to high polarity region, coumarin 3 could be a useful polarity probe for microenvironments containing hydrogen bonding groups. Compared to 3, coumarin 4 exhibits the highest ΦF values in highly polar solvents with strong hydrogen bond acceptor ability. The high quantum yield of fluorescence in DMSO, DMF, and alcohols qualifies coumarin 4 as a laser dye in the given medium, with kF higher than k(nr). Contrary to previous reports that many H-aggregates are nonfluorescent in nature, coumarin 3 forms highly fluorescent H-aggregates in MeOH and EtOH. On the basis of the restrictions of the Kasha-exciton theory model, we assume that the formation of fluorescent H-dimer aggregates of 3 is driven by π(+)-π(-) interactions. To the best of our knowledge, this is the first report on aggregation of coumarin dye in alcoholic solutions. In addition, restrictions in the fitting procedure relating to determination of the solvation number, n, using the Covington-Newman model of preferential solvation and also the solvent nonideality parameter, h', are discussed in this article.

Phosphorylated 3-heteroarylcoumarins and their use in fluorescence microscopy and nanoscopy

Photostable and bright fluorescent dyes with large Stokes shifts are widely used as markers in far-field optical microscopy, but the variety of useful dyes is limited. The present study introduces new 3-heteroaryl coumarins decorated with a primary phosphate group (OP(O)(OH)(2)) attached to C-4 in 2,2,4-trimethyl-1,2-dihydroquinoline fragment fused with the coumarin fluorophore. The general synthetic route is based on the Suzuki reaction of 3-bromocoumarines with hetarylboronic acids followed by oxidation of the methyl group at the C=C bond with SeO(2) (to an aldehyde), reduction with NaBH(4) (to an alcohol), and conversion into a primary phosphate. The 4 position in the coumarin system may be unsubstituted or bear a methyl group. Phosphorylated coumarins were found to have high fluorescence quantum yields in the free state and after conjugation with proteins (in aqueous buffers). In super-resolution light microscopy with stimulated emission depletion (STED), the new coumarin dyes provide an optical resolution of 40-60 nm with a low background signal. Due to their large Stokes shifts and high photostability, phosphorylated coumarins enable to combine multilabel imaging (using one detector and several excitation sources) with diffraction unlimited optical resolution.