Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “TRIPLETS”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 541 records · Page 30Linked to original sources
PubMed1997

Gly-Gly-containing triplets of low stability adjacent to a type III collagen epitope.

Collagens, in addition to their structural role in the extracellular matrix, possess a number of functional binding domains. In this study, the binding to collagen of a monoclonal antibody is used as a model to define the molecular features involved in triple-helix interactions with other proteins. Here we report the thermal stability of an overlapping set of triple-helical peptides that includes the epitope recognized by a monoclonal antibody to type III collagen. Although the sequences of these peptides are very closely related, by a translation of a single triplet along the collagen chain, substantial variations in the melting temperatures were observed. These variations in thermal stability could not be readily explained by differences in imino acid content, or in numbers of charged or hydrophobic residues. The results indicate that Gly-Gly-Y triplets, which are adjacent to the epitope, have a strong influence in reducing the thermal stability of triple-helical peptides. Further studies, which were carried out on a set of "host-guest" triple-helical peptides containing different Gly-Gly-Y guest triplets, confirm the destabilizing effect of such tripeptides. The presence of Gly-Gly-Y triplets may play an important role in specific functions of type III collagen by modulating the local triple-helical structure or dynamics.

Amino Acid Sequence↗
PubMed1997

Xanthophylls in light-harvesting complex II of higher plants: light harvesting and triplet quenching.

A spectral and functional assignment of the xanthophylls in monomeric and trimeric light-harvesting complex II of green plants has been obtained using HPLC analysis of the pigment composition, laser-flash induced triplet-minus-singlet, fluorescence excitation, and absorption spectra. It is shown that violaxanthin is not present in monomeric preparations, that it has most likely a red-most absorption maximum at 510 nm in the trimeric complex, and that it is involved in both light-harvesting and Chl-triplet quenching. Two xanthophylls (per monomer) have an absorption maximum at 494 nm. These play a major role in both singlet and triplet transfer. These two are most probably the two xanthophylls resolved in the crystal structure, tentatively assigned to lutein, that are close to several chlorophyll molecules [Kühlbrandt, W., Wang, N. D., & Fujiyoshi, Y. (1994) Nature 367, 614-621]. A last xanthophyll contribution, with an absorption maximum at 486 nm, does not seem to play a significant role in light-harvesting or in Chl-triplet quenching. On the basis of the assumption that the two structurally resolved xanthophylls are lutein, this 486 nm absorbing xanthophyll should be neoxanthin. The measurements demonstrate that violaxanthin is connected to at least one chlorophyll a with an absorption maximum near 670 nm, whereas the xanthophylls absorbing at 494 nm are connected to at least one chlorophyll a with a peak near 675 nm.

Chromatography, High Pressure Liquid↗
PubMed2002

Organometallic Pt(II) compounds. A complementary study of a triplet emitter based on optical high-resolution and optically detected magnetic resonance spectroscopy.

The emitting triplet state of cyclometalated Pt(thpy)(CO)(Cl) monomers ((thpy)(-) = 2-(2'-thienylpyridinate), frequently also abbreviated as (2-thpy)(-)) is investigated at T = 1.2 K (typically) by use of the complementary methods of high-resolution optical spectroscopy and of optically detected magnetic resonance (ODMR) spectroscopy. Such a complimentary investigation is carried out for the first time for a Pt(II) compound. In solution, oligomer or short linear chain formation is also observed. However, the monomers can be investigated selectively, when they are dissolved in a relatively inert n-octane matrix (Shpol'skii matrix). This allows us to determine the energies of the T(1) triplet substates I, II, and III relative to the electronic ground state S(0)(0), the zero-field splittings (ZFSs) of T(1), and emission decay time constants (I/II <--> 0, 18012.5 cm(-1); III <--> 0, 18016.3 cm(-1); DeltaE(I,II) = 0.05437 cm(-1) (1.631 GHz), DeltaE(I,III) = 3.8 cm(-1) (114 GHz); tau(I) = 120 micros, tau(II) = 45 micros, tau(III) = 35 micros; spin-lattice relaxation time for the processes III --->I/II, tau(SLR) = 3.0 micros). The vibrational satellite structure observed in the emission of the T(1) state to the singlet ground state S(0) is also discussed. Moreover, it is possible to estimate the intersystem crossing time from the excited singlet state S(1) at 22952 cm(-1) to the triplet state T(1) to approximately 5 ps. The T(1) state is assigned as a thpy-ligand-centered (3)pipi* state with small metal-to-ligand charge-transfer (MLCT) admixtures. A comparison of Pt(thpy)(CO)(Cl) to a series of other organometallic Pt(II) compounds, such as heteroleptic Pt(ppy)(CO)(Cl) ((ppy)(-) = phenylpyridinate), Pt(dppy)(CO) ((dppy)(2-) = diphenylpyridinate), and Pt(i-biq)(CN)(2) (i-biq = 2,2'-bisisoquinoline) and homoleptic Pt(thpy)(2) and Pt(ppy)(2), is carried out. (The structures are shown in Figure 7.) Trends of photophysical properties are discussed. In particular, by chelation of two equal ligands the pattern of ZFS is strongly altered, resulting in a significant increase of the MLCT participation in the lowest triplet state of these organometallic compounds. This new observation represents an interesting further step concerning chemical tunability of photophysical properties.

Journal Article↗
PubMed2003

Lowest singlet and triplet potential energy surfaces of S2N2.

Forty four stationary points have been located on the lowest singlet and triplet potential energy surfaces of S(2)N(2). Ten minima and ten saddle points on the lowest singlet surface and eleven minima and thirteen saddle points on the lowest triplet surface were found. All saddle points were connected to minima or lower-order saddle points by following the intrinsic reaction coordinate. Renner-Teller effects in the linear isomers were studied by examining their bending curves. The S(2)N(2) polymerization mechanism was investigated by first locating the transition state corresponding to ring opening and then considering all species connected to it that are close in energy. The commonly accepted mechanism is problematic due to the number of species that would lead to dissociation to SN + SN. Other possible isomers that are consistent with the experimental evidence but do not connect to SN radicals in the dissociation limit were examined. A mechanism of polymerization to (SN)(x)() is proposed that involves excitation of the square planar singlet molecule to the triplet surface. The triplet species then undergoes a puckering, and polymerization occurs in a direction approximately perpendicular to the S(2)N(2) plane. Consideration of the predicted vibrational frequencies suggests the structure of the second isomer of S(2)N(2). This isomer has a trans-NSSN structure with a long SS bond. The energetics of trans-NSSN are consistent with the observed temperature effects in the dimerization of SN. Analysis of the bending curves of linear NSSN and NSNS indicates that trans-NSSN is the only isomer which has a small yet significant barrier to that dimerization.

Journal Article↗
PubMed2001

Switch-over in photochemical reaction mechanism from hydrogen abstraction to exciplex-induced quenching: interaction of triplet-excited versus singlet-excited acetone versus cumyloxyl radicals with amines.

The fluorescence and phosphorescence quenching of acetone by 13 aliphatic amines has been investigated. The bimolecular rate constants lie in the range of 10(8)-10(9) M(-1) s(-1) for singlet-excited acetone and 10(6)-10(8) M(-1) s(-1) for the triplet case. The rate data indicate that a direct hydrogen abstraction process dominates for triplet acetone, while a charge-transfer mechanism, namely, exciplex-induced quenching, becomes important for singlet-excited acetone. Pronounced stereoelectronic effects toward H abstraction, e.g., for 1,4-diazabicyclo[2.2.2]octane (DABCO), and significant steric hindrance effects, e.g., for N,N-diisopropyl-3-pentylamine, are observed. A negative activation energy (E(a) = -0.9 +/- 0.2 kcal mol(-1) for triethylamine and DABCO) and the absence of a significant solvent effect on the fluorescence quenching of acetone are indicative of the involvement of exciplexes. Full electron transfer can be ruled out on the basis of the low reduction potential of acetone, which was found to lie below -3.0 V versus SCE. The participation of H abstraction for triplet acetone is corroborated by the respective quenching rate constants, which resemble the reaction rate constants for cumyloxyl radicals. The latter were measured for all 13 amines and showed also a dependence on the electron donor properties of the amines. It is suggested that the H abstraction proceeds directly and not through an exciplex or ion pair. Further, abstraction from N-H bonds in addition to alpha C-H bonds has been corroborated as a significant pathway for excited acetone. Product studies and quantum yields for photoreduction of singlet- and triplet-excited acetone by triethylamine (8% for S(1) versus 24% for T(1)) are in line with the suggested mechanisms of quenching through an exciplex and photoreduction through direct H abstraction.

Journal Article↗
PubMed2001

Triplet excitation transfer through the walls of hemicarcerands: dependence of the electronic coupling on the size of the molecular cage.

Triplet excitation transfer from biacetyl trapped inside three hemicarcerands of different size (1, 2, and 3) to acceptors in the surrounding medium was investigated. The largest hemiracerand 1 employs four butyl linkers and the intermediate hemicarcerand 2 four o-xylyl linkers. The smallest hemicarcerand 3 contains only three methylene linkers. Both neat liquid triplet acceptors and acceptors dispersed in solvents were used. The primary objective of this work was to determine the dependence of the energy transfer rate on the size and the electronic structure of the molecular cages. There is a pronounced, more than 10-fold, increase of triplet energy transfer rates with decreasing size of the cage. The corresponding electronic coupling, /V/, increases approximately by a factor of approximately 3.5 from the largest hemicarceplex 1 to the smallest hemicarceplex 3. This increase of the electronic interaction is similar to that observed in covalently bound systems when the distance between the triplet donor and the acceptor is reduced by one carbon-carbon sigma-bond. The electronic structure of the hemicarcerand appears to be of secondary importance, at least when T(1) states of the donor and the acceptor are far from a resonance with the T(1) state of the cage. A very good agreement between the results obtained in neat acceptors and in solution was found, indicating that the association between the acceptors and the molecular cages is negligible, if at all present. An unexpectedly large interaction between the guest and the polarizable walls of the hemicarcerands manifested by emission red-shifts was observed in all cases. This suggests that the entrapment within the molecular cage gives rise to an environment considerably different from that of a single molecule in the gas phase. An interesting correlation between the magnitude of the phosphorescence spectral shift, Deltanu(0-0), and the guest-to-external acceptor electronic coupling, /V/, was found.

Journal Article↗
PubMed2002

The reaction of triplet flavin with indole. A study of the cascade of reactive intermediates using density functional theory and time resolved infrared spectroscopy.

As a model for riboflavin, lumiflavin was investigated using density functional theory methods (B3LYP/6-31G* and B3LYP/6-31+G**) with regard to the proposed cascade of intermediates formed after excitation to the triplet state, followed by electron-transfer, proton-transfer, and radical[bond]radical coupling reactions. The excited triplet state of the flavin is predicted to be 42 kcal/mol higher in energy than the singlet ground state, and the pi radical anion lies 45.1 kcal/mol lower in energy than the ground-state flavin and a free electron in the gas phase. The former value compares to a solution-phase triplet energy of 49.8 kcal/mol of riboflavin. For the radical anion, the thermodynamically favored position to accept a proton on the flavin ring system is at N(5). A natural population analysis also provided spin density information for the radicals and insight into the origin of the relative stabilities of the six different calculated hydroflavin radicals. The resulting 5H-LF* radical can then undergo radical[bond]radical coupling reactions, with the most thermodynamically stable adduct being formed at C(4'). Vibrational spectra were also calculated for the transient species. Experimental time-resolved infrared spectroscopic data obtained using riboflavin tetraacetate are in excellent agreement with the calculated spectra for the triplet flavin, the radical anion, and the most stable hydroflavin radical.

Flavins↗
PubMed2002

The triplet-state lifetime of indole in aqueous and viscous environments: significance to the interpretation of room temperature phosphorescence in proteins.

The interpretation of room temperature phosphorescence studies of proteins requires an understanding of the mechanisms governing the tryptophan triplet-state lifetimes of residues fully exposed to solvent and those deeply buried in the hydrophobic core of proteins. Since solvents exposed tryptophans are expected to behave similarly to indole free in solution, it is important to have an accurate measure of the triplet state lifetime of indole in aqueous solution. Using photon counting techniques and low optical fluence (J/cm(2)), we observed the triplet-state lifetime of aqueous, deoxygenated indole and several indole derivatives to be approximately 40 micros, closely matching the previous reports by Bent and Hayon based on flash photolysis (12 micros; Bent, D. V.; Hayon, E. J. Am. Chem. Soc. 1975, 97, 2612-2619) but much shorter than the 1.2 ms lifetime observed more recently (Strambini, G. B.; Gonnelli, M. J. Am. Chem. Soc. 1995, 117, 7646-7651). However, we have now been able to reproduce the long lifetime reported by the latter workers for aqueous indole solutions and show that it likely arises from geminate recombination of the indole radical cation and solvated electron, a conclusion based on studies of the indole radical cation in water (Bent and Hayon, 1975). The evidence for this comes from a fast rise in the phosphorescence emission and measurements of a corresponding enhanced quantum yield in unbuffered solutions. This species can be readily quenched, and the corresponding fast rise disappears, leaving a monoexponential 40 micros decay, which we argue is the true indole triplet lifetime. The work is put in the context of room temperature phosphorescence studies of proteins.

Indoles↗
PubMed2002

A comparative photomechanistic study (spin trapping, EPR spectroscopy, transient kinetics, photoproducts) of nucleoside oxidation (dG and 8-oxodG) by triplet-excited acetophenones and by the radicals generated from alpha-oxy-substituted derivatives through Norrish-type I cleavage.

The photooxidation of 2'-deoxyguanosine (dG) and its derivative 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) by a series of acetophenones (AP-X) and benzophenone (BP) has been studied. The favorable absorption characteristics of the benzoyl chromophore enables time-resolved spectroscopy of the triplet ketones to assess their quenching kinetics by dG and 8-oxodG. Whereas the photolysis of acetophenone (AP), 2-acetoxyacetophenone (AP-OAc), and benzophenone (BP) does not produce radicals (group A ketones), the oxymethyl-substituted derivatives 2-hydroxyacetophenone (AP-OH) and 2-tert-butoxyacetophenone (AP-O(t)Bu) lead to carbon-centered radicals by alpha cleavage (group B ketones). For the latter ketones, this was confirmed by EPR studies with the spin trap 5,5-dimethylpyrroline N-oxide (DMPO) and by their triplet lifetimes that were shorter than those for the unsubstituted acetophenone. Both groups of ketones photooxidize dG and 8-oxodG; the oxidation products are spiroiminodihydantoin and guanidine-releasing products (GRP) in the case of dG and AP-OH also 8-oxodG. In the presence of O(2), the photooxidation by the group A ketones is efficient at high dG or 8-oxodG concentrations, whereas the group B ketones photooxidize dG and 8-oxodG also at low substrate concentrations. These results imply that peroxyl radicals are responsible for the photooxidation by the group B ketones, which are formed by alpha cleavage of the triplet ketone and subsequent O(2) trapping of the carbon-centered radicals. At higher dG concentrations, direct electron transfer from dG to the triplet ketone, as observed for the group A ketones, competes with the radical activity.

Acetophenones↗
PubMed2003

Characterization of the triplet state of tris(8-hydroxyquinoline)aluminium(III) in benzene solution.

The lowest triplet state of tris(8-hydroxyquinoline)aluminium(III) (Alq3) has been prepared by pulse radiolysis/energy transfer from appropriate donors in benzene solutions and has an absorption maximum around 510 nm with a lifetime of about 50 mus. It is quenched by molecular oxygen, leading to singlet oxygen formation. From flash photolysis and singlet oxygen formation measurements, a quantum yield of triplet formation of 0.24 was determined for direct photolysis of the complex. A value of 2.10 +/- 0.10 eV was determined for the energy of the lowest triplet state by energy transfer studies and was confirmed by phosphorescence measurements on Alq3, either in the heavy atom solvent ethyl iodide or photosensitized by benzophenone in benzene. Dexter (exchange) energy transfer was observed from triplet Alq3 to platinum(II) octaethylporphyrin.

Journal Article↗
PubMed2005

Triplet-state formation along the ultrafast decay of excited singlet cytosine.

We address the possibility of populating the lowest triplet state of cytosine by an "intrinsic"mechanism, namely, intersystem crossing (ISC) along the ultrafast internal conversion pathway of the electronically excited singlet species. For this purpose, we present a discussion of the ISC process and triplet-state reactivity based on theoretical analysis of the spin-orbit strength and the potential energy surfaces for the relevant singlet and triplet states of cytosine. High-level ab initio computations show that ISC is possible in wide regions of the singlet manifold along the reaction coordinate that controls the ultrafast internal conversion to the ground state. Thus, the ISC mechanism documented here provides a possibility to access the triplet state, which has a key role in the photochemistry of the nucleic acid bases.

Cytosine↗
PubMed2004

Electrochemical detection of DNA triplet repeat expansion.

Hereditary neurodegenerative diseases are connected with the expansion of trinucleotide repetitive sequences in genomic DNA. Molecular diagnosis of these diseases is based on the determination of the triplet repeat length. Currently used methods involve PCR amplification followed by electrophoretic determination of the amplicon size. We propose a novel electrochemical technique based on hybridization of target DNA (tDNA) immobilized at magnetic beads with a reporter probe (RP) complementary to the triplet repeats (12 units per RP). The biotin-labeled RP is detected via an enzyme-linked electrochemical assay involving binding of streptavidin-alkaline phosphatase conjugate and transformation of electroinactive 1-naphthyl phosphate to electroactive 1-naphthol. Pyrimidine residues within sequences flanking the homopurine (GAA)n repeat in tDNA are premodified with osmium tetroxide, 2,2'-bipyridine (Os,bipy), introducing electroactive labels in tDNA. The length of the triplet expansion is calculated from the ratio of the intensities of electrochemical signals of hybridized RP/tDNA-Os,bipy. The normalized signal increases linearly with the repeat length between 0 and about 200 triplet units, allowing for discrimination between normal, premutated, and mutated alleles. Application of this method for the detection of the asymptomatic heterozygous carrier of expanded alleles is demonstrated.

2,2'-Dipyridyl↗
PubMed2004

Carbazole compounds as host materials for triplet emitters in organic light-emitting diodes: polymer hosts for high-efficiency light-emitting diodes.

A carbazole homopolymer and carbazole copolymers based on 9,9'-dialkyl-[3,3']-bicarbazolyl, 2,5-diphenyl-[1,3,4]-oxadiazole and 9,9-bis(4-[3,7-dimethyloctyloxy]phenyl)fluorene were synthesized and their electrical and photophysical properties were characterized with respect to their application as host in phosphorescent polymer light-emitting diodes. It is shown that the triplet energy of a polymer depends on the specific connections between its building blocks. Without changing the composition of the polymer, its triplet energy can be increased from 2.3 to 2.6 eV by changing the way in which the different building blocks are coupled together. For poly(9-vinylcarbazole) (PVK), a carbazole polymer often used as host for high-energy triplet emitters in polymer light-emitting diodes, a large hole-injection barrier of about 1 eV exists due to the low-lying HOMO level of PVK. For all carbazole polymers presented here, the HOMO levels are much closer to the Fermi level of a commonly used anode such as ITO and/or a commonly used hole-injection layer such as PEDOT:PSS. This makes high current densities and consequently high luminance levels possible at moderate applied voltages in polymer light-emitting diodes. A double-layer polymer light-emitting diode is constructed comprising a PEDOT:PSS layer as hole-injection layer and a carbazole-oxadiazole copolymer doped with a green triplet emitter as emissive layer that shows an efficacy of 23 cd/A independent of current density and light output.

Journal Article↗
PubMed2006

Triplet energy back transfer in conjugated polymers with pendant phosphorescent iridium complexes.

The nature of Dexter triplet energy transfer between bonded systems of a red phosphorescent iridium complex 13 and a conjugated polymer, polyfluorene, has been investigated in electrophosphorescent organic light-emitting diodes. Red-emitting phosphorescent iridium complexes based on the [Ir(btp)(2)(acac)] fragment (where btp is 2-(2'-benzo[b]thienyl)pyridinato and acac is acetylacetonate) have been attached either directly (spacerless) or through a -(CH(2))(8)- chain (octamethylene-tethered) at the 9-position of a 9-octylfluorene host. The resulting dibromo-functionalized spacerless (8) or octamethylene-tethered (12) fluorene monomers were chain extended by Suzuki polycondensations using the bis(boronate)-terminated fluorene macromonomers 16 in the presence of end-capping chlorobenzene solvent to produce the statistical spacerless (17) and octamethylene-tethered (18) copolymers containing an even dispersion of the pendant phosphorescent fragments. The spacerless monomer 12 adopts a face-to-face conformation with a separation of only 3.6 A between the iridium complex and fluorenyl group, as shown by X-ray analysis of a single crystal, and this facilitates intramolecular triplet energy transfer in the spacerless copolymers 17. The photo- and electroluminescence efficiencies of the octamethylene-tethered copolymers 18 are double those of the spacerless copolymers 17, and this is consistent with suppression of the back transfer of triplets from the red phosphorescent iridium complex to the polyfluorene backbone in 18. The incorporation of a -(CH(2))(8)- chain between the polymer host and phosphorescent guest is thus an important design principle for achieving higher efficiencies in those electrophosphorescent organic light-emitting diodes for which the triplet energy levels of the host and guest are similar.

Journal Article↗
PubMed2006

The triplet energy of thymine in DNA.

Norfloxacin (NFX) photosensitizes formation of thymine dimers (T<>T) in DNA, while its N(4') acetyl derivative (ANFX) does not. This is evident from the observation of single-strand breaks after enzymatic treatment with T4 endonuclease V and subsequent gel electrophoresis. The triplet energies of NFX and ANFX are estimated at 273 and 268 kJ/mol, respectively, on the basis of triplet-triplet energy transfer quenching by a set of biphenyl and naphthalene derivatives. Hence, the triplet energy of thymine in DNA (i.e., the value for a photosensitizer to produce T<>T) can be estimated at 270 kJ/mol.

DNA↗
PubMed2004

Reactivity of bovine whey proteins, peptides, and amino acids toward triplet riboflavin as studied by laser flash photolysis.

The reaction between the triplet excited state of riboflavin and amino acids, peptides, and bovine whey proteins was investigated in aqueous solution in the pH range from 4 to 9 at 24 degrees C using nanosecond laser flash photolysis. Only tyrosine and tryptophan (and their peptides) were found to compete with oxygen in quenching the triplet state of riboflavin in aqueous solution, with second-order rate constants close to the diffusion limit, 1.75 x 10(9) and 1.40 x 10(9) L mol(-1) s(-1) for tyrosine and tryptophan, respectively, with beta-lactoglobulin and bovine serum albumin having comparable rate constants of 3.62 x 10(8) and 2.25 x 10(8) L mol(-1) s(-1), respectively. Tyrosine, tryptophan, and their peptides react with the photoexcited triplet state of riboflavin by electron transfer from the tyrosine and tryptophan moieties followed by a fast protonation of the resulting riboflavin anion rather than by direct H-atom abstraction, which could be monitored by time-resolved transient absorption spectroscopy as a decay of triplet riboflavin followed by a rise in riboflavin anion radical absorption. For cysteine- and thiol-containing peptides, second-order rate constants depend strongly on pH, for cysteine corresponding to pKaRSH = 8.35. H-atom abstraction seems to operate at low pH, which with rising pH gradually is replaced by electron transfer from the thiol anion. From the pH dependence of the second-order rate constant, the respective values for the H-atom abstraction (k = 1.64 x 10(6) L mol(-1) s(-1)) and for the electron transfer (k = 1.20 x 10(9) L mol(-1) s(-1)) were determined.

Amino Acids↗
PubMed2005

(Sensitized) photolysis of diazonium salts as a mild general method for the generation of aryl cations. Chemoselectivity of the singlet and triplet 4-substituted phenyl cations.

The photolysis of a series of 4-X-benzenediazonium tetrafluoroborates is studied in MeCN. Loss of nitrogen occurs from the singlet excited state with X=H, t-Bu, and NMe2 and leads to the singlet aryl cation. This adds to the solvent yielding the corresponding acetanilides. With other substituents, ISC competes with (X=Br, CN) or overcomes (X=COMe, NO2) fragmentation and the aryl cation is formed in part or completely in the triplet state. In neat MeCN, this either abstracts hydrogen from the solvent (in most cases inefficiently) or undergoes intersystem crossing to the more stable singlet that reacts as above. In the presence of pi nucleophiles (allyltrimethylsilane or benzene), the triplet aryl cation is efficiently trapped giving substituted allylbenzenes and biphenyls, respectively. By triplet sensitization by xanthone, the triplet cation and the products from it are obtained from the whole series considered. The direct or sensitized photodecomposition of diazonium fluoroborates, substituted with both electron-donating and -withdrawing substituents, in the presence of alkenes and arenes offers an access to an alternative arylation procedure.

Journal Article↗
PubMed2004

Triplet diphenylcarbenes protected by (trimethylsilyl)ethynyl groups.

(2,4,6-Tribromophenyl)(4-tert-butyl-2,6-dimethylphenyl)diazomethane (1a) was shown to be stable enough to survive Sonogashira coupling reaction conditions at an elevated temperature and gave not only a para-monosubstituted product, (4-trimethylsilylethynyl-2,6-dibromophenyl)(4-tert-butyl-2,6-dimethylphenyl)diazomethane (1b), but also a disubstituted one, [2,4-bis(trimethylsilylethynyl)-6-bromophenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethane (1c), and a trisubstituted product, [2,4,6-tris(trimethylsilylethynyl)phenyl](4-tert-butyl-2,6-dimethylphenyl)diazomethane (1d). Triplet diphenylcarbenes (DPCs) generated by photolysis of those ethynylated diphenyldiazomethanes were characterized by ESR and UV-vis spectroscopies at low temperature and laser flash photolysis techniques in solution at room temperature. Although ESR data indicated that ethynyl groups at the ortho positions are likely to stabilize triplet DPCs both sterically and electronically more effectively than o-bromine groups, kinetic studies suggested that the stability of triplet DPCs is not increased by o-ethynyl groups, as opposed to o-bromine groups. It is likely that triplet DPCs decay by interacting with the o-ethynyl groups.

Journal Article↗