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. Polar environments lead to a kinetically stronger, shorter, and more polar bond between the carbene and CO2. The current study shows that this solvent effect is general across a wide range of NHC complexes with CO2. Computational modeling at the DFT level shows that the lability of these bonds can be controlled by steric pressure due to substituents on the heteroatoms flanking the carbene center, as well as inductive electronic effects from substituents on the C4 and C5 positions. Moreover, a strong correlation between the gas-phase NHC–CO2 bond distance and the Gibbs free energy barrier for decarboxylation is demonstrated.

Photolysis of anthraquinone or flavin photosensitizers in the presence of calcium EDTA complexes results in decomposition of the EDTA complex, releasing free Ca2+. In the case of the flavin sensitizers, it is shown that millimolar concentrations of Ca2+ can be released using visible light (>440 nm) and with quantum yields as high as 0.31. The utility of this system is further demonstrated by in situ photogelation of an alginate solution.

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. 20 μs time scale.

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 1H NMR analysis. Photolysis reactions are carried out under mild, biphasic solvent conditions using a household LED lamp. Carboxylic acid release is reported in high yields, and the viability of this method for synthetic chemistry is demonstrated through a macroscale reaction.

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. Similarly to the carbon analogues, the singlet states of these species have several local minima with nonclassical bonding. Structures characterized include 3S, an N analogue of the bisected cyclopropylcarbinyl cation, 11S, an N analogue of the bicyclobutonium ion, and 6S, an unsymmetric 2-azidinylcarbinyl cation.

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.

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. These findings demonstrate that the decomposition process is a reversible decarboxylation forming the corresponding N-heterocyclic carbene (1,3-dimethylimidazolylidene, 3), followed by a rapid protonation of 3 by water or other protic species. The length and strength of the C–C bond between in the imidazolium ring and the carboxylate group (denoted CNHC–CO2) of 1 is remarkably dependent on the polarity of the solvent. Density functional theory (DFT) calculations predict a ca. 20 kcal/mol change in the barrier to decarboxylation in going from the gas phase to (SMD-simulated) water. Thus, addition of water has two effects on the stability of 1. At low concentrations, it provides a proton source for the trapping of the carbene 3 and accelerates decomposition. At higher concentrations, it increases the polarity of the medium. slowing the decarboxylation process and likewise the overall decomposition rate.

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. In the presence of nucleophiles, products such as 2-(2′-phenylacetoxy)acetic acid 5a and/or its ester derivatives are produced. Phenylacetic acid 6 is formed in some cases but under the conditions studied appears to be a minor product. The results are interpreted in terms of a general mechanism that features formation of an iso-tribromo intermediate 9 and/or a geminate radical-atom pair.

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. Experiments on this anion using chemical trapping, NMR, and the Evans method strongly suggest that this anion is either a triplet or a ground-state singlet with a very 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.

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.

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. The current study improves upon previous systems by using lower energy visible light absorbing mediators that induce substrate release over shorter irradiation periods (i.e. higher overall photolytic efficacies).

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.

A new aqueous-compatible photoinduced electron transfer based photolabile protecting group has been developed for the release of carboxylic acids. The reduction potential of this group is more positive than previous systems, thereby allowing the use of sensitizers with modest oxidation potentials. Release of several carboxylic acids has been demonstrated using tris(bipyridyl)ruthenium(II) as both a direct sensitizer and a mediator for electron transfer between a good donor and the protecting group.

One electron reduction of N-alkyl-4-picolinium (NAP) esters initiates C–O bond scission releasing a carboxylate anion. Previous experiments have demonstrated that this process can be initiated by photoinduced electron transfer from an electron-donating sensitizer. In the present study it is demonstrated that a comparable photorelease process can be initiated by photolysis of an electron acceptor (mediator), which in turn abstracts an electron from a ground state electron donor. The resulting mediator anion radicals donate an electron to the NAP ester, triggering release of the carboxylate anion. It is demonstrated that when benzophenone is used as a mediator, higher quantum yields for ester decomposition can be achieved compared with sensitizers that do direct photoinduced electron transfer.

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.