1
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Zeppuhar AN, Falvey DE. Lamp vs Laser: A Visible Light Photoinitiator That Promotes Radical Polymerization at Low Intensities and Cationic Polymerization at High Intensities. J Org Chem 2023. [PMID: 37418315 DOI: 10.1021/acs.joc.3c00636] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 07/09/2023]
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.
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Affiliation(s)
- Andrea N Zeppuhar
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Daniel E Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States
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2
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Liaros N, Gutierrez Razo SA, Thum MD, Ogden HM, Zeppuhar AN, Wolf S, Baldacchini T, Kelley MJ, Petersen JS, Falvey DE, Mullin AS, Fourkas JT. Elucidating complex triplet-state dynamics in the model system isopropylthioxanthone. iScience 2022; 25:103600. [PMID: 35005547 PMCID: PMC8717599 DOI: 10.1016/j.isci.2021.103600] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2021] [Revised: 11/10/2021] [Accepted: 12/03/2021] [Indexed: 11/19/2022] Open
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.
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Affiliation(s)
- Nikolaos Liaros
- Department of Chemistry & Biochemistry, University of Maryland, College Park, MD 20742, USA
| | | | - Matthew D. Thum
- Department of Chemistry & Biochemistry, University of Maryland, College Park, MD 20742, USA
| | - Hannah M. Ogden
- Department of Chemistry & Biochemistry, University of Maryland, College Park, MD 20742, USA
| | - Andrea N. Zeppuhar
- Department of Chemistry & Biochemistry, University of Maryland, College Park, MD 20742, USA
| | - Steven Wolf
- Department of Chemistry & Biochemistry, University of Maryland, College Park, MD 20742, USA
| | | | | | - John S. Petersen
- Department of Chemistry & Biochemistry, University of Maryland, College Park, MD 20742, USA
- imec, Kapeldreef 75, 3001 Leuven, Belgium
| | - Daniel E. Falvey
- Department of Chemistry & Biochemistry, University of Maryland, College Park, MD 20742, USA
| | - Amy S. Mullin
- Department of Chemistry & Biochemistry, University of Maryland, College Park, MD 20742, USA
| | - John T. Fourkas
- Department of Chemistry & Biochemistry, University of Maryland, College Park, MD 20742, USA
- Institute for Physical Science & Technology, University of Maryland, College Park, MD 20742, USA
- Maryland Quantum Materials Center, University of Maryland, College Park, MD 20742, USA
- Maryland NanoCenter, University of Maryland, College Park, MD 20742, USA
- Corresponding author
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3
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Zeppuhar AN, Wolf SM, Falvey DE. Photoacid Generators Activated through Sequential Two-Photon Excitation: 1-Sulfonatoxy-2-alkoxyanthraquinone Derivatives. J Phys Chem A 2021; 125:5227-5236. [PMID: 34129332 DOI: 10.1021/acs.jpca.1c01619] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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.
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Affiliation(s)
- Andrea N Zeppuhar
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Steven M Wolf
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Daniel E Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States
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4
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Thum MD, Hong D, Zeppuhar AN, Falvey DE. Visible-Light Photocatalytic Oxidation of DMSO for RAFT Polymerization †. Photochem Photobiol 2021; 97:1335-1342. [PMID: 34129686 DOI: 10.1111/php.13468] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2021] [Accepted: 06/13/2021] [Indexed: 11/28/2022]
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.
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Affiliation(s)
- Matthew D Thum
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD
| | - Donald Hong
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD
| | - Andrea N Zeppuhar
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD
| | - Daniel E Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD
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5
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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.
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Affiliation(s)
- Andrea N Zeppuhar
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 United States
| | - Daniel E Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 United States
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6
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Thum MD, Wolf S, Falvey DE. State-Dependent Photochemical and Photophysical Behavior of Dithiolate Ester and Trithiocarbonate Reversible Addition–Fragmentation Chain Transfer Polymerization Agents. J Phys Chem A 2020; 124:4211-4222. [DOI: 10.1021/acs.jpca.0c02678] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Affiliation(s)
- Matthew D. Thum
- University of Maryland, College Park, Maryland 20742, United States
| | - Steven Wolf
- University of Maryland, College Park, Maryland 20742, United States
| | - Daniel E. Falvey
- University of Maryland, College Park, Maryland 20742, United States
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7
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Zeppuhar AN, Hill-Byrne K, Falvey DE. Mechanism of the photorelease of alcohols from the 9-phenyl-9-tritylone protecting group. Photochem Photobiol Sci 2019; 18:1990-1995. [DOI: 10.1039/c9pp00183b] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
A mechanistic investigation of the 9-phenyl-9-tritylone photoremovable protecting group for alcohols revealed that the anion radical is the key intermediate required for clean deprotection.
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Affiliation(s)
- Andrea N. Zeppuhar
- Department of Chemistry and Biochemistry
- University of Maryland
- College Park
- USA
| | - Kevin Hill-Byrne
- Department of Chemistry and Biochemistry
- University of Maryland
- College Park
- USA
| | - Daniel E. Falvey
- Department of Chemistry and Biochemistry
- University of Maryland
- College Park
- USA
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8
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Falvey DE. Discrete Existence of Singlet Nitrenium Ions Revisited: Computational Studies of Non-Aryl Nitrenium Ions and Their Rearrangements. ACS Omega 2018; 3:10418-10432. [PMID: 31459169 PMCID: PMC6645317 DOI: 10.1021/acsomega.8b01038] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 05/17/2018] [Accepted: 08/08/2018] [Indexed: 06/10/2023]
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.
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Affiliation(s)
- Daniel E. Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States
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9
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Heymann RR, Thum MD, Hardee AL, Falvey DE. Visible light initiated release of calcium ions through photochemical electron transfer reactions. Photochem Photobiol Sci 2018; 16:1003-1008. [PMID: 28497836 DOI: 10.1039/c6pp00469e] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
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.
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Affiliation(s)
- Romina R Heymann
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA.
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10
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Thum MD, Falvey DE. Photoreleasable Protecting Groups Triggered by Sequential Two-Photon Absorption of Visible Light: Release of Carboxylic Acids from a Linked Anthraquinone-N-Alkylpicolinium Ester Molecule. J Phys Chem A 2018. [DOI: 10.1021/acs.jpca.8b00657] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Matthew D. Thum
- University of Maryland, College Park, Maryland 20742, United States
| | - Daniel E. Falvey
- University of Maryland, College Park, Maryland 20742, United States
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11
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Affiliation(s)
- Derek M. Denning
- Department of Chemistry
and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Daniel E. Falvey
- Department of Chemistry
and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
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12
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Denning DM, Pedowitz NJ, Thum MD, Falvey DE. Uncaging Alcohols Using UV or Visible Light Photoinduced Electron Transfer to 9-Phenyl-9-tritylone Ethers. Org Lett 2015; 17:5986-9. [DOI: 10.1021/acs.orglett.5b02924] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Affiliation(s)
- Derek M. Denning
- Department of Chemistry and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Nichole J. Pedowitz
- Department of Chemistry and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Matthew D. Thum
- Department of Chemistry and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Daniel E. Falvey
- Department of Chemistry and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
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13
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Affiliation(s)
- Derek M. Denning
- Department of Chemistry and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Matthew D. Thum
- Department of Chemistry and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Daniel E. Falvey
- Department of Chemistry and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
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14
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Kunsberg DJ, Kipping AH, Falvey DE. Visible Light Photorelease of Carboxylic Acids via Charge-Transfer Excitation of N-Methylpyridinium Iodide Esters. Org Lett 2015; 17:3454-7. [PMID: 26120927 DOI: 10.1021/acs.orglett.5b01490] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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.
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Affiliation(s)
- David J Kunsberg
- Department of Chemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Allison H Kipping
- Department of Chemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Daniel E Falvey
- Department of Chemistry, University of Maryland, College Park, Maryland 20742, United States
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15
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Falvey DE. Nitrenium ion analogues of nonclassical carbocations: cyclopropylnitrenium, allylnitrenium, and azetidenium ions and mechanisms for their interconversion. Org Lett 2015; 17:484-7. [PMID: 25611763 DOI: 10.1021/ol503488b] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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.
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Affiliation(s)
- Daniel E Falvey
- Department of Chemistry and Biochemistry, University of Maryland , College Park, Maryland 20742, United States
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16
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Affiliation(s)
- Derek M. Denning
- Department
of Chemistry and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
| | - Daniel E. Falvey
- Department
of Chemistry and
Biochemistry, University of Maryland, College Park, Maryland 20742, United States
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17
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Perrotta RR, Falvey DE. The 3,5-dinitroanilide anion: a singlet anilide anion with evidence for a thermally accessible triplet state. J PHYS ORG CHEM 2013. [DOI: 10.1002/poc.3118] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Affiliation(s)
- Raffaele R. Perrotta
- Department of Chemistry and Biochemistry; University of Maryland; College Park MD 20740-2012 USA
| | - Daniel E. Falvey
- Department of Chemistry and Biochemistry; University of Maryland; College Park MD 20740-2012 USA
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18
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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.
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Affiliation(s)
- Derek M Denning
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States
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19
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Perrotta RR, Winter AH, Coldren WH, Falvey DE. 2-(3,5-Dinitrophenyl)-1,3-dithiane Carbanion: A Benzylic Anion with a Low Energy Triplet State. J Am Chem Soc 2011; 133:15553-8. [PMID: 21870822 DOI: 10.1021/ja204711a] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Raffaele R. Perrotta
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, United States
| | - Arthur H. Winter
- Department of Chemistry, Iowa State University, 2101d Hach Hall, Ames, Iowa 50011, United States
| | - William H. Coldren
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, United States
| | - Daniel E. Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, United States
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20
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Perrotta RR, Winter AH, Falvey DE. Photochemical Heterolysis of 3,5-Bis(dimethylamino)benzyl Alcohols and Esters: Generation of a Benzyl Cation with a Low-Energy Triplet State. Org Lett 2010; 13:212-5. [DOI: 10.1021/ol102606m] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
Affiliation(s)
- Raffaele R. Perrotta
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, United States
| | - Arthur H. Winter
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, United States
| | - Daniel E. Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, United States
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21
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Abstract
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., NH(2), NMe(2), OMe) or two moderate pi-donors (e.g., F, OH, Ar, vinyl) makes the triplet state the computed ground electronic state. In many cases, the singlet states for these beta pi-donor-substituted vinyl cations are prone to rearrangements, although such rearrangements can be inhibited through incorporation of the pi-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.
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Affiliation(s)
- Arthur H Winter
- Department of Chemistry & Biochemistry, University of Maryland, College Park, Maryland 20742-2021, USA
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22
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Borak JB, Falvey DE. Ketocoumarin dyes as electron mediators for visible light induced carboxylate photorelease. Photochem Photobiol Sci 2010; 9:854-60. [DOI: 10.1039/c0pp00072h] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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23
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Borak JB, Lee HY, Raghavan SR, Falvey DE. Application of PET deprotection for orthogonal photocontrol of aqueous solution viscosity. Chem Commun (Camb) 2010; 46:8983-5. [DOI: 10.1039/c0cc02203a] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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24
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Borak JB, Falvey DE. A new photolabile protecting group for release of carboxylic acids by visible-light-induced direct and mediated electron transfer. J Org Chem 2009; 74:3894-9. [PMID: 19361187 DOI: 10.1021/jo900182x] [Citation(s) in RCA: 64] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
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.
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Affiliation(s)
- J Brian Borak
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA
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25
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Sun K, Kumar R, Falvey DE, Raghavan SR. Photogelling Colloidal Dispersions Based on Light-Activated Assembly of Nanoparticles. J Am Chem Soc 2009; 131:7135-41. [DOI: 10.1021/ja9008584] [Citation(s) in RCA: 68] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Kunshan Sun
- Department of Chemical and Biomolecular Engineering and Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2111
| | - Rakesh Kumar
- Department of Chemical and Biomolecular Engineering and Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2111
| | - Daniel E. Falvey
- Department of Chemical and Biomolecular Engineering and Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2111
| | - Srinivasa R. Raghavan
- Department of Chemical and Biomolecular Engineering and Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2111
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26
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Affiliation(s)
- Daqing Gan
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
| | - Min Jia
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
| | - Pamela P. Vaughan
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
| | - Daniel E. Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
| | - Neil V. Blough
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
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27
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Affiliation(s)
- Rebecca C. Vieira
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
| | - Daniel E. Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
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28
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Affiliation(s)
- J. Brian Borak
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
| | - Susana López-Sola
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
| | - Daniel E. Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
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Winter AH, Gibson HH, Falvey DE. Carbazolyl nitrenium ion: electron configuration and antiaromaticity assessed by laser flash photolysis, trapping rate constants, product analysis, and computational studies. J Org Chem 2007; 72:8186-95. [PMID: 17892298 DOI: 10.1021/jo0708184] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Laser flash photolysis of 1-(carbazol-9-yl)-2,4,6-trimethylpyridinium tetrafluoroborate generates the carbazolyl nitrenium ion (tau = 333 ns, kobs = 3.0 x 106 M-1s-1) having absorption bands at 570 and 620 nm in CH3CN. The nitrenium ion is found to have reactivity comparable to structurally similar closed-shell diarylnitrenium ions, but spectroscopic evidence favors an open-shell singlet diradical assignment for the observed nitrenium ion. The carbazolyl nitrenium ion is also more reactive than diarylnitrenium ions as a likely result of antiaromatic character. Ab initio and hybrid DFT calculations were performed to address the degree of antiaromaticity in this and similar nitrenium ions through analysis of optimized geometries, nucleus independent chemical shifts, and isodesmic reactions.
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Affiliation(s)
- Arthur H Winter
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20740, USA
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30
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Winter AH, Falvey DE, Cramer CJ, Gherman BF. Benzylic Cations with Triplet Ground States: Computational Studies of Aryl Carbenium Ions, Silylenium Ions, Nitrenium Ions, and Oxenium Ions Substituted with Meta π Donors. J Am Chem Soc 2007; 129:10113-9. [PMID: 17655230 DOI: 10.1021/ja070143m] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Density functional theory (B3LYP/6-31G(d,p)) was used to predict the effect of meta substitution on aryl cationic (Ar-X+) species, including aryloxenium ions, arylsilylenium ions, arylnitrenium ions, and arylcarbenium ions. Multireference second-order perturbation theory (CASPT2) calculations were used to benchmark the quantitative accuracy of the DFT calculations for representative systems. Substituting the meta positions on these species with pi donors stabilizes a pi,pi* diradical state analogous to the well-known m-xylylene diradical. Notably, the 3,5-bis(N,N-dimethylamino)benzyl cation is predicted to have a triplet ground state by 1.9 kcal/mol by DFT and to have essentially degenerate singlet-triplet states at the CASPT2(10,9) level of theory. Adding electron-withdrawing CF3 groups to the exocyclic carbon of this meta-disubstituted benzyl cation further increases the predicted singlet-triplet gap in favor of the triplet. Other aryl cationic species substituted with strong pi electron-donating groups in the meta positions are predicted to have low-energy or ground-state triplet states. Systems analogous to the naphthaquinodimethane diradicals are also reported.
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Affiliation(s)
- Arthur H Winter
- Department of Chemistry & Biochemistry, University of Maryland, College Park Maryland 20742-2021, USA
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31
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Abstract
The reactive intermediates N,N-di(4-chlorophenyl)nitrenium ion and N,N-di(4-bromophenyl)nitrenium ion were generated through photolysis of the corresponding N-amino(2,4,6,-collidinium) ions. The behavior of these diarylnitrenium ions was characterized by laser flash photolysis, analysis of the stable photoproducts, and ab initio calculations with density functional theory. The latter predict these species to have singlet ground states. The halogenated diarylnitrenium ions are significantly longer lived than the unsubstituted diphenylnitrenium ion. Specifically, cyclization to form carbazole derivatives occurs negligibly, if at all, with the halogenated derivatives. They do, however, carry out most of the characteristic reactions of singlet arylnitrenium ions, including combining with nucleophiles on the aryl rings, adding to arenes, and accepting electrons from readily oxidized traps. Interestingly these species also abstract H atoms from 1,4-cyclohexadiene and various phenol derivatives. The implication of the latter process in relation to the computed singlet-triplet energy gaps of ca. -12.5 kcal/mol is discussed.
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Affiliation(s)
- Selina I Thomas
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA
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33
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Vieira RC, Falvey DE. Photoinduced Electron-Transfer Reactions in Two Room-Temperature Ionic Liquids: 1-Butyl-3-methylimidazolium Hexafluorophosphate and 1-Octyl-3-methylimidazolium Hexafluorophosphate. J Phys Chem B 2007; 111:5023-9. [PMID: 17474706 DOI: 10.1021/jp0630471] [Citation(s) in RCA: 68] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Photoinduced electron transfer in two room-temperature ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF(6)) and 1-octyl-3-methylimidazolium hexafluorophosphate (OMIM-PF(6)), has been investigated using steady-state fluorescence quenching of 9,10-dicyanoanthracene with a series of single electron donors. From these fluorescence quenching rates, reorganization energy (lambda) values and k(diff) values can be derived from a Rehm-Weller analysis. In many cases, these fluorescence quenching reactions occur at rates larger than what would be expected based on the Smoluchowski equation. In addition, lambda values of 10.1 kcal/mol and 16.3 kcal/mol for BMIM-PF(6) and OMIM-PF(6), respectively, have been determined.
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Affiliation(s)
- Rebecca C Vieira
- Department of Chemistry and Biochemistry, University of Maryland at College Park, College Park, Maryland 20742, USA
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34
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Thomas SI, Falvey DE. Fast reactions of arylnitrenium ions with amino acids and proteins: a laser flash photolysis study. J PHYS ORG CHEM 2006. [DOI: 10.1002/poc.1031] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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35
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Sundararajan C, Falvey DE. Photorelease of carboxylic and amino acids from N-methyl-4-picolinium esters by mediated electron transfer. Photochem Photobiol Sci 2006; 5:116-21. [PMID: 16395436 DOI: 10.1039/b511269a] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
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.
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Affiliation(s)
- Chitra Sundararajan
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742-2021, USA.
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36
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Falvey DE. Dynamic Studies in Biology: Phototriggers, Photoswitches, and Caged Biomolecules Edited by Maurice Goeldner (Université L. Pasteur Strasbourg, France) and Richard Givens (University of Kansas, USA). Wiley-VCH GmbH & Co. KGaA: Weinheim. 2005. xxviii + 558 pp. $259.00. ISBN 3-527-30783-4. J Am Chem Soc 2005. [DOI: 10.1021/ja059783k] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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Abstract
Reactions of diphenylnitrenium ion were examined using laser flash photolysis (LFP), product analysis, and computational modeling using density functional theory (DFT). In the absence of trapping agents, diphenylnitrenium ion cyclizes to form carbazole. On the basis of laser flash photolysis experiments and DFT calculations it is argued that this process is a concerted cyclization/proton transfer that forms the H-4a tautomer of carbazole. Additional LFP experiments and product studies show that diphenylnitrenium ion reacts with electron-rich arenes (e.g., N,N-dimethylaniline, diphenylamine, and carbazole) through an initial one-electron transfer. The radical intermediates formed in this step then couple to form dimeric products. Secondary reactions between the diphenylnitrenium ion and these dimers results in the formation of oligomeric materials.
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Affiliation(s)
- Andrew C Kung
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742-2021, USA
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38
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Sundararajan C, Falvey DE. Photolytic Release of Carboxylic Acids Using Linked Donor−Acceptor Molecules: Direct versus Mediated Photoinduced Electron Transfer to N-Alkyl-4-picolinium Esters. Org Lett 2005; 7:2631-4. [PMID: 15957908 DOI: 10.1021/ol050744n] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
[reaction: see text] Efficient photorelease (Phi = 0.7) of carboxylic acids is achieved with a covalently linked mediator (benzophenone) protecting group (N-alkyl-4-picolinium ester) molecule. The mechanism involves initial photoreduction of the mediator, followed by rapid electron transfer to the protecting group.
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Affiliation(s)
- Chitra Sundararajan
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA
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39
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Sundararajan C, Falvey DE. Photorelease of Carboxylic Acids, Amino Acids, and Phosphates from N-Alkylpicolinium Esters Using Photosensitization by High Wavelength Laser Dyes. J Am Chem Soc 2005; 127:8000-1. [PMID: 15926809 DOI: 10.1021/ja050760f] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Visible light (>450 nm) is used to efficiently cleave carboxylic acids, amino acids, and phosphates from their N-methyl picolinium esters. Photolysis using pyrromethene dyes PM 546 and PM 597 and also coumarin 6 as photosensitizers effects release of carboxylic acids, N-protected amino acids, and phosphates in quantitative yields. The effective rate of photorelease by the dyes, Phiepsilon, was found to be as high as 4500 M-1 cm-1. The photorelease proceeds through photoinduced electron transfer from the dye sensitizers to the N-methyl picolinium group. Fluorescence quenching and laser flash photolysis experiments support the photoinduced electron-transfer mechanism.
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Affiliation(s)
- Chitra Sundararajan
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA
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40
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Abstract
N-Methyl-N-1-naphthylnitrenium ion (2) was generated through photolysis of 1-(N-methyl-N-(1-naphthyl)amino)-2,4,6-trimethylpyridinium tetrafluoroborate (1). Laser flash photolysis (LFP) with time-resolved UV-vis (TRUV) detection as well as photoproduct analysis verified that the expected nitrenium ion was formed cleanly and rapidly following photolysis. Consistent with an earlier study, which used competitive trapping methods (Novak, M. et al. J. Org. Chem. 1999, 64, 6023-6031), it is found that 2 reacts rapidly with a variety of nucleophiles. The high reactivity of 2 relative to other arylnitrenium ions is discussed in terms of steric and electronic effects.
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Affiliation(s)
- Andrew C Kung
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742-2021, USA
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41
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Abstract
[reaction: see text] Laser flash photolysis experiments, chemical trapping studies, and time-dependent density functional theory calculations demonstrate that photolysis of protonated 1,1-diarylhydrazines generates N,N-diarylnitrenium ions.
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Affiliation(s)
- Arthur H Winter
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA
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42
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Sundararajan C, Falvey DE. C-O bond fragmentation of 4-picolyl- and N-methyl-4-picolinium esters triggered by photochemical electron transfer. J Org Chem 2004; 69:5547-54. [PMID: 15307722 DOI: 10.1021/jo049501j] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Photochemical reduction of several 4-picolyl- and N-methyl-4-picolinium esters was examined using product analysis, laser flash photolysis, and fluorescence quenching. It is demonstrated that the radical (anions) formed in these reactions readily fragment to yield a carboxylic acid and a 4-pyridylmethyl radical intermediate. The high chemical and quantum yields observed for these photoreactions suggests that these esters can be used as photolabile protecting groups.
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Affiliation(s)
- Chitra Sundararajan
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, USA
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43
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Winter AH, Falvey DE, Cramer CJ. Effect of meta Electron-Donating Groups on the Electronic Structure of Substituted Phenyl Nitrenium Ions. J Am Chem Soc 2004; 126:9661-8. [PMID: 15291569 DOI: 10.1021/ja047677x] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Density functional theory (UB3LYP/6-31G(d,p)) was used to determine substituent effects on the singlet-triplet-state energy gap for 21 meta-substituted phenylnitrenium ions. It was found that strongly electron-donating substituents stabilize the triplet state relative to the singlet state. With sufficiently strong meta electron donors (e.g., m,m'-diaminophenylnitrenium ion) the triplet is predicted to be the ground state. Analysis of equilibrium geometries, Kohn-Sham orbital distributions, and Mulliken spin densities for the triplet states of this series of nitrenium ions leads to the conclusion that there are two spatially distinct types of low-energy triplet states. Simple arylnitrenium ions such as phenylnitrenium ions as well as those having electron-withdrawing or weakly donating meta substituents have lowest-energy triplet states that are n,pi in nature. That is, one singly occupied molecular orbital is orthogonal to the plane of the phenyl ring and one is coplanar. These n,pi triplets are generally characterized by large ArNH bond angles (ca. 130-132 degrees ) and an NH bond that is perpendicular to the plane of the phenyl ring. In contrast, meta donor arylnitrenium ions have a lowest-energy triplet state best described as pi,pi. That is, both singly occupied molecular orbitals are orthogonal to the aromatic ring. Such pi,pi states are characterized by NH bonds that are coplanar with the phenyl ring and have ArNH bond angles that are more acute (ca. 110-111 degrees ). These triplet nitrenium ions have electronic structures analogous to those of meta-benzoquinodimethane derivatives.
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Affiliation(s)
- Arthur H Winter
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, USA
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Abstract
Photoremovable protecting groups (also known as photolabile protecting groups, phototriggers, or caged molecules) are functional groups that are attached to a molecule in such a way as to render the latter inactive. Exposure to light releases the protecting group, restoring functionality to the molecule. The use of photoremovable protecting groups (PRPGs) allows for precise spatial and temporal control of chemical reactions. Such groups have found use in many diverse applications, ranging from time resolved studies of physiological processes, to fabrication of spatially resolved combinatorial libraries of DNA. Recent research efforts have focused on designing protecting groups that are removed through photoinduced electron transfer (PET), rather than by direct photolysis. The PET strategy allows the light absorption step to be decoupled from the bond breaking step, thus permitting more control over the wavelengths of light used in the release process. The application of these types of protecting groups to the photochemical release of amines, alcohols, ketones, and carboxylic acids is described.
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Affiliation(s)
- Daniel E Falvey
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
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45
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Abstract
Plants containing aloin A, aloe emodin, and structurally related anthraquinones have long been used as traditional medicines and in the formulation of retail products such as laxatives, dietary supplements, and cosmetics. Since a recent study indicated that topically applied aloe emodin increases the sensitivity of skin to UV light, we examined the events following photoexcitation of aloin A and aloe emodin. We determined that incubation of human skin fibroblasts with 20 microM aloe emodin for 18 h followed by irradiation with UV or visible light resulted in significant photocytotoxicity. This photocytotoxicity was accompanied by oxidative damage in both cellular DNA and RNA. In contrast, no photocytotoxicity was observed following incubation with up to 500 microM aloin A and irradiation with UVA light. In an attempt to explain the different photobiological properties of aloin A and aloe emodin, laser flash photolysis experiments were performed. We determined that the triplet state of aloe emodin was readily formed following photoexcitation. However, no transient intermediates were formed following photoexcitation of aloin A. Therefore, generation of reactive oxygen species and oxidative damage after irradiation of aloin A is unlikely. Although aloin A was not directly photocytotoxic, we found that human skin fibroblasts can metabolize aloin A to aloe emodin.
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Affiliation(s)
- Wayne G Wamer
- Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, College Park, MD 20740, USA.
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46
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Kung AC, Chiapperino D, Falvey DE. Photochemically generated arylnitrenium ions: substituent effects on reactivity studied by laser flash photolysisThis paper is dedicated to Professor Fred Lewis on the event of his 60th birthday. Photochem Photobiol Sci 2003; 2:1205-8. [PMID: 14690235 DOI: 10.1039/b307994e] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Laser flash photolysis is used to examine reaction rates of four arylnitrenium ions: N-methyl-N-(4-chlorophenyl)-, -(4-biphenylyl)-, -(4-methylphenyl)-, and -(4-methoxyphenyl)nitrenium ion. These intermediates are generated from photolysis of appropriately substituted N-aminopyridinium ions. Kinetic analysis of trapping rates shows that the nitrenium ions react with weak nucleophiles in the order 4-Cl approximately 4-Me > 4-Ph > 4-OMe. Generally, amines react with the arylnitrenium ions at or near the diffusion-limited rate. Only in cases where the least reactive nitrenium ion (4-OMe) reacts with sterically hindered amines is any deviation from this generalization observed. The addition of a 2-acetyl substituent to these structures produces an interesting effect. In the case of the 4-Cl and 4-Ph analogs, the electron-withdrawing acetyl group decreases the rate of nucleophilic addition, yet with the 4-OMe and 4-Me compounds, the acetyl group increases the rate of addition. These results are discussed in terms of competing electronic and steric effects.
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Affiliation(s)
- Andrew C Kung
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742-2021, USA
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47
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Abstract
Oxetane adducts of 1,3-dimethyluracil and 1,N4,N4-trimethylcytosine were prepared and their behavior under photoinduced electron transfer was examined by fluorescence quenching, laser flash photolysis and product analysis. The excited state electron donor, N,N,N',N'-tetramethylbenzidine, was shown to photosensitize a net cycloreversion of these oxetanes to give the pyrimidine derivative and benzophenone. It is demonstrated that this reaction occurs via the anion radical of the oxetane and that the latter cleaves very rapidly (>10(7) s(-1)).
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Affiliation(s)
- Arul Joseph
- Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
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48
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Robbins RJ, Yang LLN, Anderson GB, Falvey DE. Photogenerated Arylnitrenium Ions: Reactions of N-tert-Butyl(4-substituted 2-acetylphenyl)nitrenium Ions with Alcohols and Water Studied by Laser Flash Photolysis. J Am Chem Soc 2002. [DOI: 10.1021/ja00129a019] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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49
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Shields CJ, Falvey DE, Schuster GB, Buchardt O, Nielsen PE. Competitive singlet-singlet energy transfer and electron transfer activation of aryl azides: application to photo-cross-linking experiments. J Org Chem 2002. [DOI: 10.1021/jo00250a017] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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50
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Falvey DE, Schuster GB. Picosecond time scale dynamics of perester photodecomposition: evidence for an acyloxy radical intermediate in the photolysis of tert-butyl 9-methylfluorene-9-percarboxylate. J Am Chem Soc 2002. [DOI: 10.1021/ja00283a053] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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