Rearrangement, Elimination, and Ring-Opening Reactions of Cyclopropyl-Substituted Nitrenium Ions: A Computational and Experimental Investigation.

-(4-Biphenylyl)--cyclopropyl nitrenium ion and -benzyl--cyclopropyl nitrenium ion () were generated through photolysis of their corresponding -aminopyridinium ion photoprecursors. In the case of , stable products result from a combination of cyclopropyl ring expansion (-biphenylazetium ion) and ethylene elimination (biphenylisonitrilium ion). When present in high concentrations, methanol can add to the cyclopropyl ring-forming -3-methoxypropyl--biphenyl iminium ion.

Lamp vs Laser: A Visible Light Photoinitiator That Promotes Radical Polymerization at Low Intensities and Cationic Polymerization at High Intensities.

A visible light absorbing anthraquinone derivative 1-tosyloxy-2-methoxy-9,10-anthraquinone () mediates both cationic and radical polymerizations depending on the intensity of visible light used. A previous study showed that this initiator generates -toluenesulfonic acid through a stepwise, two-photon excitation mechanism. Thus, under high-intensity irradiation, generates acid in sufficient quantities to catalyze the cationic ring-opening polymerization of lactones.

Photochemical generation and characterization of the 5-endo-10,11-dihydrodibenzoazepine nitrenium ion.

A photochemical precursor to the 5-endo-10,11-dihydroazepine nitrenium ion (1) was synthesized and used to characterize the intermediate by laser flash photolysis and stable product analysis. In addition, DFT calculations were carried out on the nitrenium ion. These results were compared with earlier studies on the diphenylnitrenium ion in order to ascertain the effect of the 2 carbon bridge on the reactions and properties of 1.

Elucidating complex triplet-state dynamics in the model system isopropylthioxanthone.

We introduce techniques for probing the dynamics of triplet states. We employ these tools, along with conventional techniques, to develop a detailed understanding of a complex chemical system: a negative-tone, radical photoresist for multiphoton absorption polymerization in which isopropylthioxanthone (ITX) is the photoinitiator. This work reveals that the same color of light used for the 2-photon excitation of ITX, leading to population of the triplet manifold through intersystem crossing, also depletes this triplet population via linear absorption followed by reverse intersystem crossing (RISC).

Visible-Light Photocatalytic Oxidation of DMSO for RAFT Polymerization.

The solvent is an important, yet often forgotten part of a reaction mechanism. Many photochemical polymerizations are carried out using dimethyl sulfoxide (DMSO) as a way to promote the solubility of both the reactants and products, but its reactivity is rarely considered when initiation mechanisms are proposed. Herein, the oxidation of DMSO by an excited-state quinone is used to form initiating radicals resulting in the polymerization of methacrylate monomers, and the polymerization can be controlled with the addition of a chain transfer agent.

Photoacid Generators Activated through Sequential Two-Photon Excitation: 1-Sulfonatoxy-2-alkoxyanthraquinone Derivatives.

Two sulfonate ester derivatives of anthraquinone, 1-tosyloxy-2-methoxy-9,10-anthraquinone () and 1-trifluoromethylsulfonoxy-2-methoxy-9,10-anthraquinone (), were prepared and their ability to produce strong acids upon photoexcitation was examined. It is shown that these compounds generate acid with a yield that increases with light intensity when the applied photon dose is held constant. Additional experiments show that the rate of acid generation increases fourfold when visible light (532 nm) laser pulses are combined with ultraviolet (355 nm) compared with ultraviolet alone.

Generation of ,-Di(4-bromophenyl)nitrenium Ion under Acidic Conditions: Search for a Nitrenium Dication.

The behavior of the ,-di(4-bromophenyl)nitrenium ion under acidic aqueous conditions was examined via laser flash photolysis experiments. A long-lived species forms and can be assigned as the cation radical or the dication. This species is unreactive toward nucleophiles and reactive toward strong electron donors, consistent with a cation radical.

State-Dependent Photochemical and Photophysical Behavior of Dithiolate Ester and Trithiocarbonate Reversible Addition-Fragmentation Chain Transfer Polymerization Agents.

The rise in popularity of photochemically initiated reversible addition-fragmentation chain transfer (RAFT) polymerization (photoRAFT) along with the broad spectrum of proposed, and possible, initiation mechanisms result in the need for careful characterization of the photophysical properties of some common RAFT agents. Direct irradiation of the RAFT agent as a means to generate radicals, also known as the photoiniferter mechanism, is one commonly proposed mechanism. The current study shows that dithioesters and trithiocarbonates have the lowest singlet and triplet excited-state energy levels that are close to, or lower than, the C-S bond dissociation energies.

Discrete Existence of Singlet Nitrenium Ions Revisited: Computational Studies of Non-Aryl Nitrenium Ions and Their Rearrangements.

Nitrenium ion species are examined using computational methods (DFT, MP2, coupled-cluster, and a composite method, CBS-APNO) with a particular emphasis on nonaromatic species (i.e., those lacking an aromatic or heteroaromatic ring in direct conjugation with the formal nitrenium ion center.

Mechanism of the photorelease of alcohols from the 9-phenyl-9-tritylone protecting group.

The 9-phenyl-9-tritylone photoremovable protecting group was examined using both photoproduct analysis and laser flash photolysis in order to determine the details of its mechanism of alcohol release. It is shown that formation of the tritylone anion radical is required for alcohol release. Attempts to trigger release via intramolecular photoinduced electron transfer were unsuccessful due to rapid back electron transfer reactions of the triplet diradical anion.

Photoreleasable Protecting Groups Triggered by Sequential Two-Photon Absorption of Visible Light: Release of Carboxylic Acids from a Linked Anthraquinone- N-Alkylpicolinium Ester Molecule.

A photoreleasable protecting group activated by sequential absorption of two visible photons is designed, synthesized, and tested. Specifically, an anthraquinone-based chromophore is covalently attached to an N-alkylpicolinium ester. Photolysis of this linked system results in the clean release of a corresponding carboxylic acid.

Visible light initiated release of calcium ions through photochemical electron transfer reactions.

Photolysis of anthraquinone or flavin photosensitizers in the presence of calcium EDTA complexes results in decomposition of the EDTA complex, releasing free Ca. In the case of the flavin sensitizers, it is shown that millimolar concentrations of Ca can be released using visible light (>440 nm) and with quantum yields as high as 0.31.

Substituent and Solvent Effects on the Stability of N-Heterocyclic Carbene Complexes with CO.

Carbon dioxide forms covalent complexes with N-heterocyclic carbenes. These complexes are of interest in catalysis as well as for their potential use in various carbon capture and storage strategies. A previous report showed that the stability of one such complex, N,N-dimethylimidazolium 2-carboxylate, was remarkably sensitive to solvent polarity.

Uncaging Alcohols Using UV or Visible Light Photoinduced Electron Transfer to 9-Phenyl-9-tritylone Ethers.

The clean and efficient photorelease of primary and secondary alcohols is reported from the deprotection of a new photoremovable protecting group, the 9-phenyltritylone (PTO) group. Deprotection is initiated by 350 nm excitation of the PTO chromophore in the presence of triethylamine or using 447 nm light in the presence of a visible light absorbing photocatalyst and triethylamine. Laser flash photolysis results are reported in support of a proposed deprotection mechanism for the release of alcohols on a ca.

Photochemical Reduction of CO2 Using 1,3-Dimethylimidazolylidene.

Product analysis along with fluorescence quenching and laser flash photolysis experiments demonstrate that it is possible to effect a net photochemical reduction of CO2 through photolysis of an excited state donor in the presence of 1,3-dimethylimidazolium-2-carboxylate.

Visible Light Photorelease of Carboxylic Acids via Charge-Transfer Excitation of N-Methylpyridinium Iodide Esters.

Iodide contrast sensitization to direct irradiation of charge transfer salts incurs carboxylic acid release via visible light absorption. The photochemical reduction of N-methyl-4-pyridinium iodide esters to release carboxylic acids is examined using (1)H NMR analysis. Photolysis reactions are carried out under mild, biphasic solvent conditions using a household LED lamp.

Nitrenium ion analogues of nonclassical carbocations: cyclopropylnitrenium, allylnitrenium, and azetidenium ions and mechanisms for their interconversion.

Cyclopropylnitrenium 3S, allylnitreium 6S, and azetidenium (i.e., the nitrogen analogue of cyclobutylcarbenium) ions were examined using density functional theory and a complete basis set method.

Solvent-dependent decarboxylation of 1,3-dimethylimdazolium-2-carboxylate.

1,3-Dimethylimidazolium-2-carboxylate 1 is stable in both pure water and pure acetonitrile. However, in mixtures of the two solvents, this compound suffers a rapid decarboxylation/protonaton reaction, forming 1,3-dimethylimidazolium cation 2. A series of kinetic and mechanistic experiments, along with DFT calculations, were carried out to understand the mechanism of this process and to elucidate the role of solvation on the stability of 1.

Solvent-dependent photochemistry of 2,2,2-tribromoethyl-(2'-phenylacetate).

Photolysis (254 nm) of the title compound 1 produces a variety of stable products, which vary significantly with the nature of the solvent. Solvents that serve as efficient H atom donors (methanol, ethanol, isopropyl alcohol) favor products arising from a net reduction of one or more of the C-Br bonds. These include 2,2-dibromoethyl-(2'-phenylacetate) 2 and 2-bromoethyl-(2'-phenylacetate) 3.

2-(3,5-Dinitrophenyl)-1,3-dithiane carbanion: a benzylic anion with a low energy triplet state.

Calculations at the DFT level predict that benzyl anions with strong π-electron-withdrawing groups in the meta position(s) have low energy diradical or triplet electronic states. Specifically, the 2-(3,5-dinitrophenyl)-1,3-dithiane carbanion is predicted to have nearly degenerate singlet and triplet states at the (U)B3LYP level as a free anion. Its lithium ion pair is predicted to be a ground-state triplet with a substantial (26 kcal/mol) singlet-triplet energy gap.

Photochemical heterolysis of 3,5-bis(dimethylamino)benzyl alcohols and esters: generation of a benzyl cation with a low-energy triplet state.

An earlier computational study (CASPT2/pVDZ) by Winter et al. predicts the 3,5-bis(dimethylamino)benzyl cation to have nearly degenerate singlet and triplet states. Through product studies it is demonstrated that photolysis of 3,5-bis(dimethylamino)benzyl alcohol and its corresponding acetate and phenylacetate esters in alcoholic solvents produces a solvent incorporated adduct, 3,5-bis(dimethylamino)benzyl ethers, and 3,5-bis(dimethylamino)toluene.

Application of PET deprotection for orthogonal photocontrol of aqueous solution viscosity.

Photorelease and photoisomerization of trans-cinnamic acid in aqueous CTAB solutions induces a bulk solution viscosity increase and decrease, respectively, triggered by orthogonal irradiation wavelengths.

Ketocoumarin dyes as electron mediators for visible light induced carboxylate photorelease.

Photoinduced mediated electron transfer to N-alkylpicolinium(NAP)-based esters has previously been shown to induce release of carboxylate anions. In this mechanism, electrons are shuttled between a good electron donor through a sensitizer/mediator to the NAP group which subsequently induces C-O bond cleavage. Previous studies have used UV-absorbing mediators to initiate substrate release with high photorelease efficiencies, but require long irradiation periods for quantitative deprotection.

Vinyl cations substituted with beta pi-donors have triplet ground states.

Computations at the CASPT2, CBS-QB3, and B3LYP levels of theory demonstrate that beta-substitution of vinyl cations with pi-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 beta hydrogens with just one strong pi-donor (e.g.

Photogelling colloidal dispersions based on light-activated assembly of nanoparticles.

Photorheological (PR) fluids, i.e., fluids whose rheology can be tuned by light, have been a recent focus for our laboratory.