Biradical vs singlet oxygen photogeneration in suprofen–cholesterol systems

• Fabrizio Palumbo,
• Francisco Bosca,
• Isabel M. Morera,
• Inmaculada Andreu and
• Miguel A. Miranda

Beilstein J. Org. Chem. 2016, 12, 1196–1202, doi:10.3762/bjoc.12.115

• ], whereas tiaprofenic acid (TPA) is a related drug that includes the 2-benzoylthiophene (BZT, Figure 1) chromophore and has a π,π* lowest triplet excited state [9][10]. Generally, the photochemical reactivity of the n,π* triplet state is higher than that of π,π* triplet state. It is also accepted that

Synthesis of 2,1-benzisoxazole-3(1H)-ones by base-mediated photochemical N–O bond-forming cyclization of 2-azidobenzoic acids

• Daria Yu. Dzhons and
• Andrei V. Budruev

Beilstein J. Org. Chem. 2016, 12, 874–881, doi:10.3762/bjoc.12.86

• through a 1,5-electrocyclization reaction [56][57]. Nonreacted singlet nitrenes A may undergo intersystem crossing (ISC) into the less reactive triplet state (Scheme 1, intermediate D). Although a multiplicity change is a spin-forbidden transition, it can be partially allowed in some cases. According to

• intersystem crossing for the initially formed singlet nitrenes (Scheme 1, intermediate A) to the triplet state (Scheme 1, intermediate D), thus resulting in decreased yields of the cyclization products 2c–e. In these cases, the formation of the triplet nitrene D should lead to increased yields of primary

Interactions between 4-thiothymidine and water-soluble cyclodextrins: Evidence for supramolecular structures in aqueous solutions

• Vito Rizzi,
• Sergio Matera,
• Paola Semeraro,
• Paola Fini and
• Pinalysa Cosma

Beilstein J. Org. Chem. 2016, 12, 549–563, doi:10.3762/bjoc.12.54

• several clinical diseases [12][21]. One of the most commonly used thiobase for PDT applications is S4TdR, whose triplet state is generated upon photoexcitation with a quantum yield of one [22][23][24][25][26]. Under these conditions the production of reactive oxygen species (ROS), well-known toxic agents

Surprisingly facile CO₂ insertion into cobalt alkoxide bonds: A theoretical investigation

• Willem K. Offermans,
• Claudia Bizzarri,
• Walter Leitner and
• Thomas E. Müller

Beilstein J. Org. Chem. 2015, 11, 1340–1351, doi:10.3762/bjoc.11.144

• -unrestricted calculations, a triplet state was considered and the minima of the corresponding spin-restricted calculations were used as initial structures. It turned out that the total energy of the geometrical optimised structures with triplet state was higher than the total energy of the spin-restricted

Photorelease of phosphates: Mild methods for protecting phosphate derivatives

• Sanjeewa N. Senadheera,
• Abraham L. Yousef and
• Richard S. Givens

Beilstein J. Org. Chem. 2014, 10, 2038–2054, doi:10.3762/bjoc.10.212

• benzyl (30b–d) moieties, and naphthylacetyl [37] reportedly lowered the singlet and triplet state energies only modestly [10][11] and were unsuitable because these chromophores remained intact and in competition for incident radiation. Photolysis of 1,4-HNA DEP (14a) in 1% aq MeOH released DEP with

• favored by aqueous-based solvents that serve both as a proton donor and an acceptor to the conjugate base generated from the chromophore’s triplet state heterolytic cleavage of the leaving group. The dual behavior of H2O is manifest in accepting a proton from the naphthol OH while simultaneously solvating

• -Favorskii rearrangement and release of DEP is initiated at the triplet state of 14 and proceeds according to the more extensively characterized rearrangement for pHP DEP. Due to a more favorable excitation wavelength of 350 nm for 1,4-HNA (vs 280 nm for pHP), it is anticipated that the 4-hydroxy-1

Photo, thermal and chemical degradation of riboflavin

• Muhammad Ali Sheraz,
• Sadia Hafeez Kazi,
• Sofia Ahmed,
• Zubair Anwar and
• Iqbal Ahmad

Beilstein J. Org. Chem. 2014, 10, 1999–2012, doi:10.3762/bjoc.10.208

• excited triplet state of RF [24][36][59] whereas the excited singlet state plays a role in the formation of LC and CDRF [14][24][35][42]. The excitation of the RF molecule on the absorption of light takes place very rapidly as the life spans of flavin excited singlet and triplet states are approximately 5

• the involvement of two pathways causing direct formation of LC (through excited singlet state) as well as through FMF (by excited triplet state) where the dominant role is played by the excited singlet state. Such formation of LC has also been reported by Song and Metzler [80] and Cairns and Metzler

• found to be much affected by an increase in buffer concentration which could be due to the involvement of the excited triplet state in the reaction. The same may also be hypothesized for LC as some of its fractions could be formed directly from FMF [21][23][25][26][30][35]. On the contrary, some buffers

On the mechanism of photocatalytic reactions with eosin Y

• Michal Majek,
• Fabiana Filace and
• Axel Jacobi von Wangelin

Beilstein J. Org. Chem. 2014, 10, 981–989, doi:10.3762/bjoc.10.97

• redox potential can be obtained indirectly via analysis of the thermodynamic cycle involving the energy of the triplet state eosin Y* (T1) (derived from fluorescence measurements) and the energy of the radical cation eosin Y+• (derived from cyclovoltammetric experiments, for more details see Supporting

Tailoring of organic dyes with oxidoreductive compounds to obtain photocyclic radical generator systems exhibiting photocatalytic behavior

• Christian Ley,
• Julien Christmann,
• Ahmad Ibrahim,
• Luciano H. Di Stefano and
• Xavier Allonas

Beilstein J. Org. Chem. 2014, 10, 936–947, doi:10.3762/bjoc.10.92

• rate constants of singlet and triplet state deactivation to the ground state, respectively; kISC the rate constant of the intersystem crossing from 1PS to 3PS; 1kq/A, 3kq/A, (1kq/D, 3kq/D) the bimolecular electron transfer rate constant of the singlet and triplet excited state of the PS by the acceptor

• . Intersystem crossing rate constant kISC was obtained from the triplet state quantum yield and singlet state lifetime according to [59]: which leads to: and to At this RB concentration, and in the absence of quencher, the triplet state lifetime was measured around 80 µs, leading to kdes3= 1.25·104 M−1s−1

Metal and metal-free photocatalysts: mechanistic approach and application as photoinitiators of photopolymerization

• Jacques Lalevée,
• Sofia Telitel,
• Pu Xiao,
• Marc Lepeltier,
• Frédéric Dumur,
• Fabrice Morlet-Savary,
• Didier Gigmes and
• Jean-Pierre Fouassier

Beilstein J. Org. Chem. 2014, 10, 863–876, doi:10.3762/bjoc.10.83

• panchromatic behavior in the 400–650 nm range. The PIC/Ph2I+/NVK system The photochemical properties of Ir(piq)2(tmd) A triplet-state lifetime of 1.1 μs has been determined for Ir(piq)2(tmd) by laser flash photolysis experiments. A relatively similar lifetime (1.3 μs) was previously measured for 3Ir(ppy)3 [68

• ]. Such long lifetimes for triplet states are important in photoredox catalysis to ensure an efficient quenching by the oxidation agents. Ir(piq)2(tmd) is characterized by an oxidation potential of 0.67 V vs SCE (Figure 7A); its triplet-state energy level (ET ~ 2.07 eV) was determined from absorption

• and C are the oxidation potential of Ir(piq)2(tmd), the reduction potential of Ph2I+, the excited triplet state energy of Ir(piq)2(tmd), and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents) [69]. This favorable 3PIC/Ph2I

Tuning the interactions between electron spins in fullerene-based triad systems

• Maria A. Lebedeva,
• Thomas W. Chamberlain,
• E. Stephen Davies,
• Bradley E. Thomas,
• Martin Schröder and
• Andrei N. Khlobystov

Beilstein J. Org. Chem. 2014, 10, 332–343, doi:10.3762/bjoc.10.31

• interactions in the electrochemically generated dianions of these triads, with one electron per fullerene cage were studied by EPR spectroscopy. The nature of electron–electron coupling observed at 77 K can be described as an equilibrium between doublet and triplet state biradicals which depends on the inter

• Information File 1). The half-field signal corresponding to the triplet state is not observed which is also consistent with previous reports for fulleride based triplets [30]. The presence of an intramolecular triplet would indicate that the distance between the two fulleropyrrolidine units is short enough to

Topochemical control of the photodimerization of aromatic compounds by γ-cyclodextrin thioethers in aqueous solution

• Hai Ming Wang and
• Gerhard Wenz

Beilstein J. Org. Chem. 2013, 9, 1858–1866, doi:10.3762/bjoc.9.217

• there are several explanations for the observed anti specificity: (a) ACE is tightly surrounded by the seven sulfur atoms of host 3, which may lead to an increased population of the triplet state and consequently to a heavy atom effect, which favors the formation of the anti dimer. Other CD derivatives

• reacts to form the syn-HH dimer, while the triplet state, with a longer lifetime, furnishes the anti-HH isomer [38][40]. Because most often mixtures of syn and anti isomers were obtained after irradiation of COU, any supramolecular control of the dimerization was highly desirable. Photodimerization of a

• atom effect of the S-atoms of CD derivatives 1–7 should facilitate intersystem crossing to the triplet state and therefore support the formation of the anti-HH dimer. Because this preference does not hold true in this case, especially for host 2, the multiplicity of the excited state seems not to be

Electron self-exchange activation parameters of diethyl sulfide and tetrahydrothiophene

• Martin Goez and
• Martin Vogtherr

Beilstein J. Org. Chem. 2013, 9, 1448–1454, doi:10.3762/bjoc.9.164

• triplet energy (222 kJ/mol [32]) is only lower by less than 90 kJ/mol. Its first excited singlet state has a lifetime of 2.9 ns [32], so reacts only marginally with the millimolar sulfide concentrations used in our experiments, but its triplet state (intrinsic intersystem-crossing efficiency, 0.48 [32

• protons; as stated above, the hyperfine coupling constant of Hα is also positive. For the radical pair, reverse electron transfer is only possible in its singlet state (ε = +1), because the triplet state of DES (281 kJ/mol [40]) lies even higher in energy than the initial excited singlet state of TPP

New core-pyrene π structure organophotocatalysts usable as highly efficient photoinitiators

• Sofia Telitel,
• Frédéric Dumur,
• Thomas Faury,
• Bernadette Graff,
• Mohamad-Ali Tehfe,
• Didier Gigmes,
• Jean-Pierre Fouassier and
• Jacques Lalevée

Beilstein J. Org. Chem. 2013, 9, 877–890, doi:10.3762/bjoc.9.101

• to radical polymerization. Laser flash photolysis Upon laser excitation of Py_3 at 355 nm, a triplet state T1 characterized by a maximum absorption at ~420 nm and a rather long lifetime is formed (t > 4 μs) (Figure S2 in Supporting Information File 1). This T1 state is similar to that of Py_1 [31

• ]. The short S1 lifetime (e.g., ~55 ns under N2 for Py_3) and the diffusion controlled interactions of S1 with Iod, PBr or MDEA (see above) prevent a significant production of T1 in Co_Py/Iod (PBr or MDEA). Therefore, a triplet-state pathway can be neglected in the investigated systems. Thermal processes

Spectroscopic characterization of photoaccumulated radical anions: a litmus test to evaluate the efficiency of photoinduced electron transfer (PET) processes

• Maurizio Fagnoni,
• Stefano Protti,
• Davide Ravelli and
• Angelo Albini

Beilstein J. Org. Chem. 2013, 9, 800–808, doi:10.3762/bjoc.9.91

• reduced and an oxidized product is formed. As an example, the largely positive reduction potential of aromatic nitriles [5][8][9] and cyanophthalimides [10] in the singlet excited state or of aromatic esters in the triplet state [5][11] makes PET a common occurrence upon irradiation of such substrates in

Photoionisation of the tropyl radical

• Kathrin H. Fischer,
• Patrick Hemberger,
• Andras Bodi and
• Ingo Fischer

Beilstein J. Org. Chem. 2013, 9, 681–688, doi:10.3762/bjoc.9.77

• eV photon energy, i.e., 200 meV higher, which may correspond to either vibrational excitation of the first electronic excited state or to the next electronic state. A noisy feature appears at 10.7 eV, and is followed by the highest intensity peak at 11.6 eV. The most stable triplet state has been

• observed before at 9.63 eV [16], and is calculated to lie 3.1 eV above the ground state [9], i.e., at 9.33 eV using the newly determined adiabatic ionisation energy of 6.23 eV. In the same work, Lee and Wright predict the next triplet state at 3.9 eV excitation energy, i.e., at 10.13 eV photon energy. We

• vertical excitation energies. The shoulder at 9.85 eV may thus either be due to the second triplet state, or to a vibrational fundamental of the excited state. For example a C–C stretching mode may be responsible for this peak, 1600 cm–1 further to the blue with respect to the first one. The electronic

Complete σ* intramolecular aromatic hydroxylation mechanism through O₂ activation by a Schiff base macrocyclic dicopper(I) complex

• Albert Poater and
• Miquel Solà

Beilstein J. Org. Chem. 2013, 9, 585–593, doi:10.3762/bjoc.9.63

• kcal·mol−1, evolving to the μ-η1:η2-peroxo isomer c with an energetic stabilization of 12.9 kcal·mol−1 with respect to the preceding complex b. Furthermore, this step also requires change to a biradical singlet ground state, although the triplet state is only 1 kcal·mol−1 higher as a result of the long

Synthesis and photooxidation of styrene copolymer bearing camphorquinone pendant groups

• Branislav Husár,
• Norbert Moszner and
• Ivan Lukáč

Beilstein J. Org. Chem. 2012, 8, 337–343, doi:10.3762/bjoc.8.37

• absence of oxygen [10][11][12][13][14][15][16] and in the presence of oxygen [10][13][17][18][19][20] has been studied extensively. In an inert atmosphere, the excited n→π* triplet state of the carbonyl group of CQ abstracts an H-atom from a hydrogen donor. The two primarily formed radicals undergo

• subsequent reactions leading to photoproducts [21]. The rate-determining step in photoinitiation by CQ/amine is hydrogen transfer by the excited n→π* triplet state of the carbonyl group of CQ from the alkylamino group [8][9]. The photochemistry of the low molecular CQ in the polystyrene (PS) film was the

• CQ photooxidation in PS are summarized in Scheme 3 [21]. The addition of molecular oxygen to the excited n→π* triplet state of ketones and 1,2-diketones to form 1,4-biradicals is a generally accepted mechanism, which has been theoretically treated and reviewed [37]. The oxygen atom released during

Fine-tuning alkyne cycloadditions: Insights into photochemistry responsible for the double-strand DNA cleavage via structural perturbations in diaryl alkyne conjugates

• Wang-Yong Yang,
• Samantha A. Marrone,
• Nalisha Minors,
• Diego A. R. Zorio and
• Igor V. Alabugin

Beilstein J. Org. Chem. 2011, 7, 813–823, doi:10.3762/bjoc.7.93

• ]. The mechanism of triplet photocycloaddition involves a sequence of radical closures initiated by the formation of a triplet 1,4-diradical via the reaction of 1,4-CHD and the alkyne π,π*-triplet state. Although several plausible mechanistic pathways converge at the same homoquadricyclane product in

• of the reaction mixture. This observation suggests that the ISC to the triplet state with m-acetamidyl acetylene 4 is more efficient than with p-acetamidyl acetylene 3, the lifetime of the triplet of 4 is longer than that of 3, or the triple state of 4 is more electrophilic than the triplet state of

• Scheme 1, the maximum quantum yield of 0.50 along with the DFT activation barriers at the triplet hypersurface suggest that 5-exo-trig attack of electrophilic vinyl radical at the remaining 1,4-CHD double bond is the most likely subsequent step. Because this photocycloaddition occurs from the triplet

Photoinduced electron-transfer chemistry of the bielectrophoric N-phthaloyl derivatives of the amino acids tyrosine, histidine and tryptophan

• Axel G. Griesbeck,
• Jörg Neudörfl and
• Alan de Kiff

Beilstein J. Org. Chem. 2011, 7, 518–524, doi:10.3762/bjoc.7.60

• and ФISC < 0.01 for N-arylphthalimides [1]. If necessary, the population of the triplet state is also possible by sensitization, e.g., with triplet sensitizers such as acetone or benzophenone. With a triplet energy ET of 293–300 kJ mol−1 and a ground-state reduction potential E0 of −1.85 V vs Fc/Fc

• electronic interaction between the donor indole and the acceptor phthalimide (Figure 2). Such interactions also in solution phase would facilitate decay processes from the excited singlet or triplet state. This concept has also been described by Gawronski et al. from exciton CD Cotton effects that have been

Effects of anion complexation on the photoreactivity of bisureido- and bisthioureido-substituted dibenzobarrelene derivatives

• Heiko Ihmels and
• Jia Luo

Beilstein J. Org. Chem. 2011, 7, 278–289, doi:10.3762/bjoc.7.37

• benzophenone, a triplet-state di-π-methane rearrangement is induced. Thus, in the initial reaction step connection between one vinyl and one benzo carbon atom takes place, i.e., a so called vinyl–benzo bridging, that leads to the intermediate biradical BR1a [29]. Subsequent rearomatization with the formation

• nm) in the gas phase, and the latter compound is able to sensitize a triplet-state E/Z-isomerization of alkenes [48]. On the other hand, the thioureido-substituted dibenzobarrelene derivative 1i does not undergo a DPM rearrangement upon direct irradiation, despite the potentially sensitizing 3,5-bis

• association with anions. Specifically, the DPM rearrangement of the thioureido derivative 1i to give the dibenzosemibullvalene 2i can only be performed successfully when the self-quenching of the triplet state is suppressed by complex formation. At the same time it was shown in preliminary experiments that

Photoinduced homolytic C–H activation in N-(4-homoadamantyl)phthalimide

• Nikola Cindro,
• Margareta Horvat,
• Kata Mlinarić-Majerski,
• Axel G. Griesbeck and
• Nikola Basarić

Beilstein J. Org. Chem. 2011, 7, 270–277, doi:10.3762/bjoc.7.36

• (Scheme 1) [51]. The mechanism of the photoinduced domino reaction was investigated and it was found that it probably takes place from a higher excited triplet state or the singlet state [52]. Herein, we report the synthesis and photochemistry of a phthalimide derivative of homoadamantane 5. The research

• unreacted 5 was recovered. This finding is in accordance with acetone acting as a triplet sensitizer and the anticipated triplet state reactivity of the phthalimide in the H-abstraction reactions. Furthermore, the addition of H2O as a protic solvent also increased the reactivity of the phthalimide, based on

• notation of atoms see the Experimental), which precluded unambiguous assignment of the stereochemistry to the isomers 7. According to the above product study, a mechanism for the photochemical transformation of 5 can be proposed. On excitation (direct or sensitized) the triplet state of 5 is populated and

Synthesis of bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine oxide – a tailor-made photoinitiator for dental adhesives

• Norbert Moszner,
• Iris Lamparth,
• Jörg Angermann,
• Urs Karl Fischer,
• Frank Zeuner,
• Thorsten Bock,
• Robert Liska and
• Volker Rheinberger

Beilstein J. Org. Chem. 2010, 6, No. 26, doi:10.3762/bjoc.6.26

• between ~360 and 400 nm can be generally assigned to symmetry forbidden n-π* transitions, which are responsible for α-cleavage and formation of free radicals [17]. After excitation with light in the near UV–vis, the excited triplet state undergoes cleavage of the carbon-phosphorus bond, thereby producing

Reversible intramolecular photocycloaddition of a bis(9-anthrylbutadienyl)paracyclophane – an inverse photochromic system. (Photoactive cyclophanes 5)

• Henning Hopf,
• Christian Beck,
• Jean-Pierre Desvergne,
• Henri Bouas-Laurent,
• Peter G. Jones and
• Ludger Ernst

Beilstein J. Org. Chem. 2009, 5, No. 20, doi:10.3762/bjoc.5.20

• observed to occur under geometrical constraints [11][12][24] through a triplet state process [25]. [4+2]-Cycloadditions can take place in a hot ground state [26]. This result is reminiscent of the [4+2]-photocycloaddition between two naphthalene substrates for the anti-[2.2](1,4)-naphthalenophane leading