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1.
J Org Chem ; 2024 Jul 14.
Article in English | MEDLINE | ID: mdl-39004832

ABSTRACT

N-(4-Biphenylyl)-N-cyclopropyl nitrenium ion 5 and N-benzyl-N-cyclopropyl nitrenium ion (6) were generated through photolysis of their corresponding N-aminopyridinium ion photoprecursors. In the case of 5, stable products result from a combination of cyclopropyl ring expansion (N-biphenylazetium ion) and ethylene elimination (biphenylisonitrilium ion). When present in high concentrations, methanol can add to the cyclopropyl ring-forming N-3-methoxypropyl-N-biphenyl iminium ion. In contrast, the only detectable product from the N-benzyl-N-cyclopropyl nitrenium ion (6) is benzylisonitrile, resulting from the elimination of ethylene. Density functional theory (DFT) calculations predict the product distributions from the more stable biphenyl system 5 with reasonable accuracy. However, product distributions from the less stable benzyl system 6 are forecast with less accuracy.

2.
J Org Chem ; 88(14): 9870-9876, 2023 Jul 21.
Article in English | MEDLINE | ID: mdl-37418315

ABSTRACT

A visible light absorbing anthraquinone derivative 1-tosyloxy-2-methoxy-9,10-anthraquinone (QT) mediates both cationic and radical polymerizations depending on the intensity of visible light used. A previous study showed that this initiator generates para-toluenesulfonic acid through a stepwise, two-photon excitation mechanism. Thus, under high-intensity irradiation, QT generates acid in sufficient quantities to catalyze the cationic ring-opening polymerization of lactones. However, under low-intensity (lamp) conditions, the two-photon process is negligible, and QT photooxidizes DMSO, generating methyl radicals which initiate the RAFT polymerization of acrylates. This dual capability was utilized to switch between radical and cationic polymerizations to synthesize a copolymer using a one-pot procedure.

3.
Photochem Photobiol Sci ; 21(11): 1907-1914, 2022 Nov.
Article in English | MEDLINE | ID: mdl-35869409

ABSTRACT

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. It is shown that the geometric constraints provided by the bridging group (a) destabilize the triplet state of 1 relative to the singlet state (b) substantially increase the lifetime of 1 relative to Ph2N+ by inhibiting the unimolecular cyclization pathway possible in the latter and (c) modestly increases the barrier for nucleophilic addition to the ring carbons.


Subject(s)
Lasers , Photolysis , Cyclization , Density Functional Theory
4.
iScience ; 25(1): 103600, 2022 Jan 21.
Article in English | MEDLINE | ID: mdl-35005547

ABSTRACT

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). Using spectroscopic tools and kinetic modeling, we identify the reactive triplet state and a non-reactive reservoir triplet state. We present compelling evidence that the deactivation channel involves RISC from an excited triplet state to a highly vibrationally excited level of the electronic ground state. The work described here offers the enticing possibility of understanding, and ultimately controlling, the photochemistry and photophysics of a broad range of triplet processes.

5.
J Phys Chem A ; 125(24): 5227-5236, 2021 Jun 24.
Article in English | MEDLINE | ID: mdl-34129332

ABSTRACT

Two sulfonate ester derivatives of anthraquinone, 1-tosyloxy-2-methoxy-9,10-anthraquinone (1a) and 1-trifluoromethylsulfonoxy-2-methoxy-9,10-anthraquinone (1b), 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. Continuous wave diode laser photolysis also affects acid generation with a rate that depends quadratically on the light intensity. Density functional theory calculations, laser flash photolysis, and chemical trapping experiments support a mechanism, whereby an initially formed triplet state (T1) is excited to a higher triplet state which in turn undergoes homolysis of the RS(O2)-OAr bond. Secondary reactions of the initially formed sulfonyl radicals produce strong acids. It is demonstrated that high-intensity photolysis of either 1a or 1b can initiate cationic polymerization of ethyl vinyl ether.

6.
Photochem Photobiol ; 97(6): 1335-1342, 2021 Nov.
Article in English | MEDLINE | ID: mdl-34129686

ABSTRACT

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. This process leads to the formation of polymers with narrow molecular weight distribution, and the polymerization is able to be carried out in the presence of oxygen. A visible light absorbing substituted anthraquinone is synthesized, and nanosecond transient absorption spectroscopy is used to monitor the intermediates involved in the initiation mechanism. Photoproduct analysis indicates formation of methyl radicals as a result of DMSO oxidation. Furthermore, we show that the solvent outcompetes the chain transfer agent for interacting with the excited-state anthraquinone. These observations have a broad impact on photoinduced polymerizations performed in DMSO as many photocatalysts are strong oxidants in the excited state and are capable of reacting with the solvent. Therefore, the role of the solvent needs to be more carefully considered when proposing mechanisms for photoinduced polymerizations in DMSO.

7.
J Org Chem ; 85(14): 8844-8850, 2020 07 17.
Article in English | MEDLINE | ID: mdl-32589031

ABSTRACT

The behavior of the N,N-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. Mechanistic analysis indicates that its formation is through a separate pathway compared to that of the nitrenium ion, suggestive of a triplet mechanism.

8.
J Phys Chem A ; 124(21): 4211-4222, 2020 May 28.
Article in English | MEDLINE | ID: mdl-32375484

ABSTRACT

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. Excitation of these agents into their S1 band results in negligible radical production, while excitation into the S2 band or higher results in the decomposition of dithioesters and trithiocarbonates, resulting in radical formation but with low quantum yields. Likewise, there is significant literature precedence for an electron transfer initiation mechanism, PET-RAFT. It is shown that the dithioesters and trithiocarbonates all show peak reduction potentials at ca. -1.0 V (vs SCE). However, transient absorption spectroscopy studies of the electron transfer from a mediator show that these reactions occur rapidly only when the mediator potential is more negative than -1.2 V (vs SCE).

9.
Photochem Photobiol Sci ; 18(8): 1990-1995, 2019 Aug 01.
Article in English | MEDLINE | ID: mdl-31243421

ABSTRACT

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.

10.
J Phys Chem A ; 122(12): 3204-3210, 2018 Mar 29.
Article in English | MEDLINE | ID: mdl-29518337

ABSTRACT

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. Through product analysis, laser flash photolysis, and steady-state UV-vis spectroscopy, it is demonstrated that carboxylate ion release is affected by sequential absorption of two photons. The initial photochemical step results in reduction of an anthraquinone chromophore to the corresponding hydroquinone. The latter either reacts with O2 to regenerate the starting material, or absorption of a second photon causes an electron transfer to the picolinium group triggering C-O bond scission and release of the carboxylate.

11.
ACS Omega ; 3(8): 10418-10432, 2018 Aug 31.
Article in English | MEDLINE | ID: mdl-31459169

ABSTRACT

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.) The substitution of the N+ center with alkyl, alkoxy, vinyl, acyl, and sulfonyl, among others, was evaluated. For these species, three properties are considered. (1) The stability of the nitrenium ions to unimolecular isomerizations such as 1,2 alkyl or H shifts; to the extent that the singlet states could be characterized as discrete minima on the potential energy surface (PES), (2) the effect of the substituents on singlet-triplet energy splitting as well as (3) the relative stabilities of the nitrenium ions as defined by N-hydration enthalpies (RR'N+ + H2O → RR'NOH2 +). Nearly all simple alkyl and di-alkyl nitrenium ion singlet states are predicted to rearrange without detectable barriers, largely through 1,2 H or alkyl shifts. Methyl and N,N-dimethylnitrenium ion singlet states could be characterized as formal minima on the PES. However, these species show small or insignificant barriers to isomerization. Disubstituted nitrenium ions that include an alkyl group and a conjugating substituent such as alkoxyl, vinyl, or phenyl show meaningful barriers to isomerization and are thus predicted to possess nontrivial lifetimes in solution. Alkyl groups substantially stabilize the singlet state relative to the situation in the parent nitrenium ion NH2 + to the point where the two states are nearly degenerate. Other groups that interact with the nitrenium ion center decrease the ΔE st in the order formoyl < vinyl < phenyl < alkoxy ∼ sulfonyl < cyclopropyl ∼ cyclobutyl. The latter two substituents interact strongly with the (singlet) nitrenium ion center through the formation of a nonclassical bonding reminiscent of the bisected cyclopropylcarbinyl ion case for carbocations. When singlet-state stability is evaluated in the context of N-hydration enthalpies, it is found that the ordering is acyl < vinyl < alkoxyl < phenyl < cyclopropyl and cyclobutyl.

12.
Photochem Photobiol Sci ; 16(6): 1003-1008, 2017 Jun 14.
Article in English | MEDLINE | ID: mdl-28497836

ABSTRACT

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.

13.
J Org Chem ; 82(3): 1552-1557, 2017 02 03.
Article in English | MEDLINE | ID: mdl-28067512

ABSTRACT

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.

14.
Org Lett ; 17(24): 5986-9, 2015 Dec 18.
Article in English | MEDLINE | ID: mdl-26605860

ABSTRACT

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.

15.
Org Lett ; 17(17): 4152-5, 2015 Sep 04.
Article in English | MEDLINE | ID: mdl-26273966

ABSTRACT

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.

16.
Org Lett ; 17(14): 3454-7, 2015 Jul 17.
Article in English | MEDLINE | ID: mdl-26120927

ABSTRACT

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. Carboxylic acid release is reported in high yields, and the viability of this method for synthetic chemistry is demonstrated through a macroscale reaction.

17.
Org Lett ; 17(3): 484-7, 2015 Feb 06.
Article in English | MEDLINE | ID: mdl-25611763

ABSTRACT

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.

18.
J Org Chem ; 79(10): 4293-9, 2014 May 16.
Article in English | MEDLINE | ID: mdl-24762208

ABSTRACT

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 C(NHC)-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.

19.
J Org Chem ; 78(5): 1934-9, 2013 Mar 01.
Article in English | MEDLINE | ID: mdl-23075383

ABSTRACT

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.

20.
J Am Chem Soc ; 133(39): 15553-8, 2011 Oct 05.
Article in English | MEDLINE | ID: mdl-21870822

ABSTRACT

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.

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