Optical and EPR Detection of a Triplet Ground State Phenyl Nitrenium Ion.

Nitrenium ions are important reactive intermediates participating in the synthetic chemistry and biological processes. Little is known about triplet phenyl nitrenium ions regarding their reactivity, lifetimes, spectroscopic features, and electronic configurations, and no ground state triplet nitrenium ion has been directly detected. In this work, m-pyrrolidinyl-phenyl hydrazine hydrochloride (1) is synthesized as the photoprecursor to photochemically generate the corresponding m-pyrrolidinyl-phenyl nitrenium ion (2), which is computed to adopt a π, π* triplet ground state. A combination of femtosecond (fs) and nanosecond (ns) transient absorption (TA) spectroscopy, cryogenic continuous-wave electronic paramagnetic resonance (CW-EPR) spectroscopy, computational analysis, and photoproduct studies was performed to elucidate the photolysis pathway of 1 and offers the first direct experimental detection of a ground state triplet phenyl nitrenium ion. Upon photoexcitation, 1 forms S1, where bond heterolysis occurs and the NH3 leaving group is extruded in 1.8 ps, generating a vibrationally hot, spin-conserving closed-shell singlet phenyl nitrenium ion (12) that undergoes vibrational cooling in 19 ps. Subsequent intersystem crossing takes place in 0.5 ns, yielding the ground state triplet phenyl nitrenium ion (32), with a lifetime of 0.8 µs. Unlike electrophilic singlet phenyl nitrenium ions, which react rapidly with nucleophiles, this triplet phenyl nitrenium reacts through sequential H atom abstractions, resulting in the eventual formation of the reduced m-pyrrolidinyl-aniline as the predominant stable photoproduct. Supporting the triplet ground state, continuous irradiation of 1 in a glassy matrix at 80 K in an EPR spectrometer forms a paramagnetic triplet species, consistent with a triplet nitrenium ion.

Simple Air-Stable [3]Radialene Anion Radicals as Environmentally Switchable Catholytes in Water.

The hexacyano[3]radialene radical anion (1) is an attractive catholyte material for use in redox flow battery (RFB) applications. The substitution of cyano groups with ester moieties enhances solubility while maintaining redox reversibility and favorable redox potentials. Here we show that these ester-functionalized, hexasubstituted [3]radialene radical anions dimerize reversibly in water. The dimerization mode is dependent on the substitution pattern and can be switched in solution. Stimuli-responsive behavior is achieved by exploiting an unprecedented tristate switching mechanism, wherein the radical can be toggled between the free radical, a π-dimer, and a σ-dimer-each with dramatically different optical, magnetic, and redox properties-by changing the solvent environment, temperature, or salinity. The symmetric, triester-tricyano[3]radialene (3) forms a solvent-responsive, σ-dimer in water that converts to the radical anion with the addition of organic solvents or to a π-dimer in brine solutions. Diester-tetracyano[3]radialene (2) exists primarily as a π-dimer in aqueous solutions and a radical anion in organic solvents. The dimerization behavior of both 2 and 3 is temperature dependent in methanol solutions. Dimerization equilibrium has a direct impact on catholyte stability during galvanostatic charge-discharge cycling in static H-cells. Specifically, conditions that favor the free radical anion or π-dimer exhibit significantly enhanced cycling profiles.

Structure-Photoreactivity Studies of BODIPY Photocages: Limitations of the Activation Barrier for Optimizing Photoreactions.

BODIPY photocages are photoreactive chromophores that release covalently linked cargo upon absorption of visible light. Here, we used computations of the T1 photoheterolysis barrier to ascertain whether a computational approach could assist in a priori structure design by identifying new structures with higher quantum yields of photorelease. The electronic structure-photoreactivity relationships were elucidated for boron-substituted and core-functionalized 2-substituted BODIPY photocages as well as aryl substitutions at the meso-methyl position. Although there is a clear trend for the 2-substituted derivatives, with donor-substituted derivatives featuring both lower computed barriers and higher experimental quantum yields, no trend in the quantum yield with the computed activation barrier is found for the meso-methyl-substituted or boron-substituted derivatives. The lack of a correlation between the experimental quantum yield with the computed barrier in the latter two substitution cases is attributed to the substituents having larger effects on the rates of competing channels (internal conversion and competitive photoreactions) than on the rate of the photoheterolysis channel. Thus, although in some cases computed photoreaction barriers can aid in identifying structures with higher quantum yields, the ignored impacts of how changing the structure affects the rates of competing photophysical/photochemical channels limit the effectiveness of this single-parameter approach.

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.

Photo-Uncaging by C(sp3)-C(sp3) Bond Cleavage Restores ß-Lapachone Activity.

ß-Lapachone is an ortho-naphthoquinone natural product with significant antiproliferative activity but suffers from adverse systemic toxicity. The use of photoremovable protecting groups to covalently inactivate a substrate and then enable controllable release with light in a spatiotemporal manner is an attractive prodrug strategy to limit toxicity. However, visible light-activatable photocages are nearly exclusively enabled by linkages to nucleophilic functional sites such as alcohols, amines, thiols, phosphates, and sulfonates. Herein, we report covalent inactivation of the electrophilic quinone moiety of ß-lapachone via a C(sp3)-C(sp3) bond to a coumarin photocage. In contrast to ß-lapachone, the designed prodrug remained intact in human whole blood and did not induce methemoglobinemia in the dark. Under light activation, the C-C bond cleaves to release the active quinone, recovering its biological activity when evaluated against the enzyme NQO1 and human cancer cells. Investigations into this report of a C(sp3)-C(sp3) photoinduced bond cleavage suggest a nontraditional, radical-based mechanism of release beginning with an initial charge-transfer excited state. Additionally, caging and release of the isomeric para-quinone, α-lapachone, are demonstrated. As such, we describe a photocaging strategy for the pair of quinones and report a unique light-induced cleavage of a C-C bond. We envision that this photocage strategy can be extended to quinones beyond ß- and α-lapachone, thus expanding the chemical toolbox of photocaged compounds.

Steric Hindrance Favors σ Dimerization over π Dimerization for Julolidine Dicyanomethyl Radicals.

Metastable radicals exist in a steady-state equilibrium in solution with dimers, which can be either σ dimers or π dimers. Here, we show that steric hindrance at the para position causes julolidine-derived dicyanomethyl radicals to form σ dimers rather than π dimers, the opposite behavior as seen in other carbon-centered radicals, where steric hindrance typically favors pimerization. The change in dimerization mode can be attributed to weaker London dispersion forces and a decreased orbital overlap in the sterically hindered dicyanomethyl radical π dimers, while the bulky groups exert relatively little effect on the energy of the σ dimer.

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 pKa 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.

Mechanistic Insight into Phenol Dearomatization by Hypervalent Iodine: Direct Detection of a Phenoxenium Cation.

Phenol dearomatization is one of several oxidation reactions enabled by hypervalent iodine reagents. However, the presence of a proposed free phenoxenium intermediate in phenol dearomatization is a matter of debate in the literature. Here, we report the unambiguous detection of a free phenoxenium intermediate in the reaction of an electron-rich phenol, 2,4,6-trimethoxyphenol, and (diacetoxyiodo)benzene using UV-vis and resonance Raman spectroscopies. In contrast, we predominantly detect single electron oxidation products of less electron-rich phenols or alkoxy-substituted aromatics in their reaction with (diacetoxyiodo)benzene using UV-vis and electron paramagnetic resonance (EPR) spectroscopies. We conclude that the often-postulated free phenoxenium intermediate, while possible with highly stabilizing substituents, is unlikely to be a general mechanistic pathway in the reaction of typical phenols with hypervalent iodine reagents. The polar solvent 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) or the use of more strongly oxidizing hypervalent iodine reagents, such as [bis(trifluoroacetoxy)iodo]benzene (PIFA) or [hydroxy(tosyloxy)iodo]benzene (HTIB), can help reduce the formation of radical byproducts and favors the formation of phenoxenium intermediates.

Multiwavelength Control of Mixtures Using Visible Light-Absorbing Photocages.

Selective deprotection of functional groups using different wavelengths of light is attractive for materials synthesis as well as for achieving independent photocontrol over substrates in biological systems. Here, we show that mixtures of recently developed visible light-absorbing BODIPY-derived photoremovable protecting groups (PRPGs) and a coumarin-derived PRPG can undergo wavelength-selective activation, giving independent optical control over a mixture of photocaged substrates using biologically benign long-wavelength light.

Anti-Aromaticity Relief as an Approach to Stabilize Free Radicals.

A new strategy to stabilize free radicals electronically is described by conjugating formally antiaromatic substituents to the free radical. With an antiaromatic substituent, the radical acts as an electron sink to allow configuration mixing of a low-energy zwitterionic state that provides antiaromaticity relief to the substituent. A combination of X-ray crystallography, VT-EPR and VT-UV/Vis spectroscopy, as well as computational analysis, was used to investigate this phenomenon. We find that this strategy of antiaromaticity relief is successful at stabilizing radicals, but only if the antiaromatic substituent is constrained to be planar by synthetically imposed conformational restraints that enable state mixing. This work leads to the counterintuitive finding that increasing the antiaromaticity of the radical substituent leads to greater radical stability, providing proof of concept for a new stereoelectronic approach for stabilizing free radicals.

Spin Delocalization, Polarization, and London Dispersion Forces Govern the Formation of Diradical Pimers.

Some free radicals are stable enough to be isolated, but most are either unstable transient species or exist as metastable species in equilibrium with a dimeric form, usually a spin-paired sigma dimer or a pi dimer (pimer). To gain insight into the different modes of dimerization, we synthesized and evaluated a library of 15 aryl dicyanomethyl radicals in order to probe what structural and molecular parameters lead to σ- versus π-dimerization. We evaluated the divergent dimerization behavior by measuring the strength of each radical association by variable-temperature electron paramagnetic resonance spectroscopy, determining the mode of dimerization (σ- or π-dimer) by UV-vis spectroscopy and X-ray crystallography, and performing computational analysis. We evaluated three different hypotheses to explain the difference in the dimerization behavior: (1) that the dimerization behavior is dictated by radical spin densities; (2) that it is dictated by radical polarizability; (3) that it is dictated by London dispersion stabilization of the pimer. However, no single parameter model in itself was predictive. Two-parameter models incorporating either the computed degree of spin delocalization or the radical polarizability as well as computed estimates for the attractive London dispersion forces in the π-dimers lead to improved forecasts of σ- vs π-dimerization mode, and suggest that a balance of spin delocalization of the isolated radical as well as attractive forces between the stacked radicals, govern the formation of diradical pimers.

Efficient Far-Red/Near-IR Absorbing BODIPY Photocages by Blocking Unproductive Conical Intersections.

Photocages are light-sensitive chemical protecting groups that give investigators control over activation of biomolecules using targeted light irradiation. A compelling application of far-red/near-IR absorbing photocages is their potential for deep tissue activation of biomolecules and phototherapeutics. Toward this goal, we recently reported BODIPY photocages that absorb near-IR light. However, these photocages have reduced photorelease efficiencies compared to shorter-wavelength absorbing photocages, which has hindered their application. Because photochemistry is a zero-sum competition of rates, improvement of the quantum yield of a photoreaction can be achieved either by making the desired photoreaction more efficient or by hobbling competitive decay channels. This latter strategy of inhibiting unproductive decay channels was pursued to improve the release efficiency of long-wavelength absorbing BODIPY photocages by synthesizing structures that block access to unproductive singlet internal conversion conical intersections, which have recently been located for simple BODIPY structures from excited state dynamic simulations. This strategy led to the synthesis of new conformationally restrained boron-methylated BODIPY photocages that absorb light strongly around 700 nm. In the best case, a photocage was identified with an extinction coefficient of 124000 M-1 cm-1, a quantum yield of photorelease of 3.8%, and an overall quantum efficiency of 4650 M-1 cm-1 at 680 nm. This derivative has a quantum efficiency that is 50-fold higher than the best known BODIPY photocages absorbing >600 nm, validating the effectiveness of a strategy for designing efficient photoreactions by thwarting competitive excited state decay channels. Furthermore, 1,7-diaryl substitutions were found to improve the quantum yields of photorelease by excited state participation and blocking ion pair recombination by internal nucleophilic trapping. No cellular toxicity (trypan blue exclusion) was observed at 20 µM, and photoactivation was demonstrated in HeLa cells using red light.

Water-Soluble BODIPY Photocages with Tunable Cellular Localization.

Photoactivation of bioactive molecules allows manipulation of cellular processes with high spatiotemporal precision. The recent emergence of visible-light excitable photoprotecting groups has the potential to further expand the established utility of the photoactivation strategy in biological applications by offering higher tissue penetration, diminished phototoxicity, and compatibility with other light-dependent techniques. Nevertheless, a critical barrier to such applications remains the significant hydrophobicity of most visible-light excitable photocaging groups. Here, we find that applying the conventional 2,6-sulfonation to meso-methyl BODIPY photocages is incompatible with their photoreaction due to an increase in the excited state barrier for photorelease. We present a simple, remote sulfonation solution to BODIPY photocages that imparts water solubility and provides control over cellular permeability while retaining their favorable spectroscopic and photoreaction properties. Peripherally disulfonated BODIPY photocages are cell impermeable, making them useful for modulation of cell-surface receptors, while monosulfonated BODIPY retains the ability to cross the cellular membrane and can modulate intracellular targets. This new approach is generalizable for controlling BODIPY localization and was validated by sensitization of mammalian cells and neurons by visible-light photoactivation of signaling molecules.

Anomalous Electronic Properties of Iodous Materials: Application to High-Spin Reactive Intermediates and Conjugated Polymers.

Manipulating frontier orbital energies of aromatic molecules with substituents is key to a variety of chemical and material applications. Here, we investigate a simple strategy for achieving high-energy in-plane orbitals for aromatics simply by positioning iodine atoms next to each other. The lone pair orbitals on the iodines mix to give a high-energy in-plane σ-antibonding orbital as the highest occupied molecular orbital (HOMO). We show that this effect can be used to manipulate orbital gaps, the symmetry of the highest occupied orbital, and the adopted electronic state for reactive intermediates. This electronic effect is not limited to reactive intermediates, and we demonstrate that this iodine buttressing strategy also can be used to achieve small HOMO-lowest unoccupied molecular orbital (HOMO-LUMO) gaps in organic electronic materials. Iodinated oligomers of several of the most popular conducting polymers are computed to have smaller HOMO-LUMO gaps than the unsubstituted materials. This iodine buttressing approach for generating high-energy in-plane HOMOs is anticipated to be highly general. While the unusual properties of fluorous materials are well established, at the other extreme on the periodic table novel properties of iodous materials may await discovery.

Direct Photorelease of Alcohols from Boron-Alkylated BODIPY Photocages.

BODIPY photocages allow the release of substrates using visible light irradiation. They have the drawback of requiring reasonably good leaving groups for photorelease. Photorelease of alcohols is often accomplished by attachment with carbonate linkages, which upon photorelease liberate CO2 and generate the alcohol. Here, we show that boron-alkylated BODIPY photocages are capable of directly photoreleasing both aliphatic alcohols and phenols upon irradiation via photocleavage of ether linkages. Direct photorelease of a hydroxycoumarin dye was demonstrated in living HeLa cells.

Solvent Effects on the Stability and Delocalization of Aryl Dicyanomethyl Radicals: The Captodative Effect Revisited.

The captodative effect postulates that radicals substituted with both electron donating and accepting groups enjoy a special enhanced stabilization, a model given theoretical support by simple MO and resonance arguments. A key prediction from theory is that captodative stabilization of radicals is larger in polar solvents than in nonpolar solvents or the gas phase, which can be viewed in the resonance model as solvent stabilization of charge-separated resonance forms. Yet, several experimental studies have failed to observe a solvent effect on radical stability, casting doubt on key aspects of the captodative effect. Here, we examine in detail the effect of solvent on the stability of structurally related captodative aryl dicyanomethyl radicals. An attractive feature of these radicals is that they exist as stable steady state populations of radicals in equilibrium with their dimers, allowing us to directly characterize from experiment their thermodynamic stabilities and spin delocalization in solvents of varying polarity. In contrast to the prior studies, we find that captodative radicals are indeed stabilized by polar solvents, as measured by a shift in the radical-dimer association constants by up to 100-fold toward the radical upon going from nonpolar toluene to more polar DMF. Moreover, in polar solvents, the spin is shifted onto the donor substituent and away from the benzylic carbon. Within the resonance model, these results can be explained by the increased contributions of the zwitterionic resonance structures to the overall hybrid. These results provide experimental support to a key prediction from theory that had previously been dismissed.

Effect of Structure on the Spin-Spin Interactions of Tethered Dicyanomethyl Diradicals.

Stable organic radicals with switchable spin states have attracted attention for a variety of applications, but a fundamental understanding of how radical structure effects the weak bonding interactions between organic radicals is limited. To evaluate the effect of chemical structure on the strength and nature of such spin interactions, a series of 14 tethered aryl dicyanomethyl diradicals were synthesized, and the structure and thermodynamic properties of the diradicals were investigated. These studies indicate that the nature of the dimer and the equilibrium thermodynamic parameters of the diradical-dimer equilibria are highly sensitive to the attachment point of the linker, the length of the linker, and the substituents on the radical itself. Values of the intramolecular Ka vary from as small as 5 to as high as 105 depending on these variables. An X-ray crystal structure for a linked ortho-substituted diradical shows that the diradical forms an intramolecular sigma dimer in the crystalline state with an elongated C-C bond (1.637 Å). Subtle changes to the radical structure influences the nature of the spin interactions, as fixing the dimethylamino substituent on the radical into a ring to make a julolidine-derived diradical leads to the weakest bonding interaction observed (Δ Gbonding = 1 kcal mol-1) and changes the spin-paired species from a sigma dimer to a diradical pimer. This work has implications for the design of stimuli-responsive materials that can reversibly switch between the dramatically different properties of closed-shell species and the unique properties of diradicals.

Family of BODIPY Photocages Cleaved by Single Photons of Visible/Near-Infrared Light.

Photocages are light-sensitive chemical protecting groups that provide external control over when, where, and how much of a biological substrate is activated in cells using targeted light irradiation. Regrettably, most popular photocages (e.g., o-nitrobenzyl groups) absorb cell-damaging ultraviolet wavelengths. A challenge with achieving longer wavelength bond-breaking photochemistry is that long-wavelength-absorbing chromophores have shorter excited-state lifetimes and diminished excited-state energies. However, here we report the synthesis of a family of BODIPY-derived photocages with tunable absorptions across the visible/near-infrared that release chemical cargo under irradiation. Derivatives with appended styryl groups feature absorptions above 700 nm, yielding photocages cleaved with the highest known wavelengths of light via a direct single-photon-release mechanism. Photorelease with red light is demonstrated in living HeLa cells, Drosophila S2 cells, and bovine GM07373 cells upon ∼5 min irradiation. No cytotoxicity is observed at 20 µM photocage concentration using the trypan blue exclusion assay. Improved B-alkylated derivatives feature improved quantum efficiencies of photorelease ∼20-fold larger, on par with the popular o-nitrobenzyl photocages (ÎµΦ = 50-100 M-1 cm-1), but absorbing red/near-IR light in the biological window instead of UV light.

Dynamics of Oxygen-Independent Photocleavage of Blebbistatin as a One-Photon Blue or Two-Photon Near-Infrared Light-Gated Hydroxyl Radical Photocage.

Development of versatile, chemically tunable photocages for photoactivated chemotherapy (PACT) represents an excellent opportunity to address the technical drawbacks of conventional photodynamic therapy (PDT) whose oxygen-dependent nature renders it inadequate in certain therapy contexts such as hypoxic tumors. As an alternative to PDT, oxygen free mechanisms to generate cytotoxic reactive oxygen species (ROS) by visible light cleavable photocages are in demand. Here, we report the detailed mechanisms by which the small molecule blebbistatin acts as a one-photon blue light-gated or two-photon near-infrared light-gated photocage to directly release a hydroxyl radical (•OH) in the absence of oxygen. By using femtosecond transient absorption spectroscopy and chemoselective ROS fluorescent probes, we analyze the dynamics and fate of blebbistatin during photolysis under blue light. Water-dependent photochemistry reveals a critical process of water-assisted protonation and excited state intramolecular proton transfer (ESIPT) that drives the formation of short-lived intermediates, which surprisingly culminates in the release of •OH but not superoxide or singlet oxygen from blebbistatin. CASPT2//CASSCF calculations confirm that hydrogen bonding between water and blebbistatin underpins this process. We further determine that blue light enables blebbistatin to induce mitochondria-dependent apoptosis, an attribute conducive to PACT development. Our work demonstrates blebbistatin as a controllable photocage for •OH generation and provides insight into the potential development of novel PACT agents.

Nucleophilic Addition to Singlet Diradicals: Homosymmetric Diradicals.

Experiments have demonstrated that nucleophiles can attack singlet diradicals to generate bonded, closed-shell addition products. Here, we present a molecular orbital analysis for this reaction, focusing on the addition of nucleophiles to homosymmetric diradicals. We show that beginning with the Salem-Rowland molecular orbital description of homosymmetric diradicals, a continuous progression from open-shell diradical to closed-shell addition product occurs during the reaction via a gradual evolution of orbital and configuration interaction coefficients. This theoretical framework is supported by high-level multireference computations (CASPT2, EOM-SF-CCSD(dT)) using the addition of chloride to p-benzyne to generate a p-chlorophenyl anion as a case study. When using levels of theory that include dynamic correlation, the reaction is predicted to be barrierless. No abrupt switch from diradical to closed-shell species happens during the mechanism, but rather a gradual decrease in diradical character occurs as the nucleophile approaches the radical center before ultimately transforming into the closed-shell anion. The overarching conclusion from this work is that there are no electronic impediments of any kind, deriving from orbital symmetry or from any other source, that exist for the addition of nucleophiles to homosymmetric singlet diradicals.