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

Phenyloxenium Ions: More Like Phenylnitrenium Ions than Isoelectronic Phenylnitrenes?

The geometries and energies of the electronic states of phenyloxenium ion 1 (Ph−O+) were computed at the multireference CASPT2/pVTZ level of theory. Despite being isoelectronic to phenylnitrene 4, the phenyloxenium ion 1 has remarkably different energetic orderings of its electronic states. The closed-shell singlet configuration (1A1) is the ground state of the phenyloxenium ion 1, with a computed adiabatic energy gap of 22.1 kcal/mol to the lowest-energy triplet state (3A2). Open-shell singlet configurations (1A2, 1B1, 1B2, 21A1) are significantly higher in energy (>30 kcal/mol) than the closed-shell singlet configuration. These values suggest a revision to the current assignments of the ultraviolet photoelectron spectroscopy bands for the phenoxy radical to generate the phenyloxenium ion 1. For para-substituted phenyloxenium ions, the adiabatic singlet−triplet energy gap (ΔEST) is found to have a positive linear free energy relationship with the Hammett-like σ+R/σ+ substituent parameters; for meta substituents, the relationship is nonlinear and negatively correlated. CASPT2 analyses of the excited states of p-aminophenyloxenium ion 5 and p-cyanophenyloxenium ion 10 indicate that the relative orderings of the electronic states remain largely unperturbed for these para substitutions. In contrast, meta-donor-substituted phenyloxenium ions have low-energy open-shell states (open-shell singlet, triplet) due to stabilization of a π,π* diradical state by the donor substituent. However, all of the other phenyloxenium ions and larger aryloxenium ions (naphthyl, anthryl) included in this study have closed-shell singlet ground states. Consequently, ground-state reactions of phenyloxenium ions are anticipated to be more closely related to closed-shell singlet arylnitrenium ions (Ar−NH+) than their isoelectronic arylnitrene (Ar−N) counterparts

Nucleophilic Addition to Singlet Diradicals: Heterosymmetric Diradicals

In the preceding paper, we examined the addition of nucleophiles to homosymmetric diradicals and showed that the reaction occurs with no symmetry restrictions or other electronic impediments. In this work, we examine the addition of nucleophiles to heterosymmetric diradicals, by using the addition of chloride to m-dehydrotoluene as a case study. Using CASPT2 and density functional theory calculations, we show that the addition of chloride to m-dehydrotoluene is predicted to be barrierless at the asymptotic limit if Cs symmetry is broken, and the reaction is allowed to proceed through a nonplanar geometry. A nonplanar cyclic allene acts as the transitioning structure between open-shell and closed-shell species for the addition of chloride, with a continuous and smooth changing of the wave function by the evolution of orbital and configuration interaction coefficients, such that there is no abrupt switch from diradical to closed-shell species along the reaction coordinate. The overall conclusion from our analysis is that both homosymmetric and heterosymmetric diradicals can undergo reaction with closed-shell reagents without a barrier, and one cannot rule out the direct addition of nucleophiles to diradicals when considering the reaction mechanism

Noncovalent Catch and Release of Carboxylates in Water

Association constants of a bis-(acetyl­guanidinium)­ferrocene dication to various (di)­carboxylates were determined through UV–vis titrations. Association constant values greater than 10⁴ M^–1 were determined for both phthalate and maleate carboxylates to the bis-(acetylguanidinium)­ferrocene salt in pure water. Density functional theory computations of the binding enthalpy of the rigid carboxylates for these complexes agree well with the experimentally determined association constants. Catch and release competitive binding experiments were done by NMR for the cation–carboxylate ion-pair complexes with cucurbit[7]­uril, and they show dissociation of the ion-pair complex upon addition of cucurbit[7]­uril and release of the free (di)­carboxylate

Vinyl Cations Substituted with β π-Donors Have Triplet Ground States

Computations at the CASPT2, CBS-QB3, and B3LYP levels of theory demonstrate that β-substitution of vinyl cations with π-donors switches the ground state of these ions from the familiar closed-shell singlet state to a carbene-like triplet state similar to the electronic state of triplet phenyl cations. Although the parent vinyl cation is a ground-state singlet species with a very large energy gap to the lowest energy triplet state, substituting the β hydrogens with just one strong π-donor (e.g., NH2, NMe2, OMe) or two moderate π-donors (e.g., F, OH, Ar, vinyl) makes the triplet state the computed ground electronic state. In many cases, the singlet states for these β π-donor-substituted vinyl cations are prone to rearrangements, although such rearrangements can be inhibited through incorporation of the π-donors into rings. For example, a vinyl cation based on 1,3-dimethyl-2-methylene imidazolidine (32) is predicted to show a substantial barrier to singlet state rearrangement as well as possess a triplet ground state with a large energy gap. In contrast to the singlet states, the stabilized triplet states appear to be well behaved and more immune to rearrangements. These triplet ions may exhibit substantially different properties and reaction chemistry than those seen for typical closed-shell vinyl cations

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

Heteroaryl Oxenium Ions Have Diverse and Unusual Low-Energy Electronic States

The electronic state orderings and energies of heteroaryl oxenium ions were computed using high-level CASPT2//CASSCF computations. We find that these ions have a number of diverse, low-energy configurations. Depending on the nature of the heteroaryl substituent, the lowest-energy configuration may be open-shell singlet, closed-shell singlet, or triplet, with further diversity found among the subtypes of these configurations. The 2- and 3-pyridinyl oxenium ions show small perturbations from the phenyl oxenium ion in electronic state orderings and energies, having closed-shell singlet ground states with significant gaps to an n,π* triplet state. In contrast, the 4-pyridinyl oxenium ion is computed to have a low-energy nitrenium ion-like triplet state. The pyrimidinyl oxenium ion is computed to have a near degeneracy between an open-shell singlet and triplet state, and the pyrizidinyl oxenium ion is computed to have a near-triple degeneracy between a closed-shell singlet state, an open-shell singlet state, and a triplet state. Therefore, the ground state of these latter heteroaryl oxenium ions cannot be predicted with certainty; in principle, reactivity from any of these states may be possible. These systems are of fundamental interest for probing the spin- and configuration-dependent reactivity of unusual electronic states for this important class of reactive intermediate

A Fine Line Separates Carbocations from Diradical Ions in Donor-Unconjugated Cations

Carbocations are traditionally thought to be closed-shell electrophiles featuring an empty orbital rich in p character. Here, density functional theory computations indicate that when strong π donors are not placed in direct conjugation with benzylic-type cations, alternative diradical configurations that resemble non-Kekulé diradicals are possible. For certain donor–acceptor frameworks, an open-shell singlet configuration is the computed ground state for the cation, whereas for coumarin and xanthenyl cations substituted with strong donors, a triplet diradical configuration is the computed ground state. Changing the substituent nature and attachment location substantially alters the energy gaps between the different electronic configurations and can manipulate the computed ground-state electronic configuration. There are few known examples of ground-state triplet carbocations, and, to our knowledge, no other examples of open-shell singlet carbocations. The open-shell singlet and triplet “carbocations” described here may have reactivity distinct from that of typical closed-shell singlet carbocations and, if appropriately stabilized, lead to organic materials with interesting electronic and magnetic properties