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At least 883 records · Page 49Linked to original sources
PubMed2003

Excited-state singlet manifold and oscillatory features of a nonatetraeniminium retinal chromophore model.

In this paper we use ab initio multireference Møller-Plesset second-order perturbation theory computations to map the first five singlet states (S(0), S(1), S(2), S(3), and S(4)) along the initial part of the photoisomerization coordinate for the isolated rhodopsin chromophore model 4-cis-gamma-methylnona-2,4,6,8-tetraeniminium cation. We show that this information not only provides an explanation for the spectral features associated to the chromophore in solution but also, subject to a tentative hypothesis on the effect of the protein cavity, may be employed to explain/assign the ultrafast near-IR excited-state absorption, stimulated emission, and transient excited-state absorption bands observed in rhodopsin proteins (e.g. rhodopsin and bacteriorhodopsin). We also show that the results of vibrational frequency computations reveal a general structure for the first (S(1)) excited-state energy surface of PSBs that is consistent with the existence of the coherent oscillatory motions observed both in solution and in bacteriorhodopsin.

Biomimetic Materials↗
PubMed2004

Laser flash photolysis of tert-butyl aroylperbenzoates: kinetics of the singlet and triplet states and the aroylphenyl radicals.

tert-Butyl aroylperbenzoates (1-4) were studied by laser flash photolysis (LFP). LFP (380 nm, pulse width approximately 350 fs) of 2 and 3 allowed direct observation of their singlet states, which showed broad absorption (lambda(max) approximately 625 nm; tau approximately 20 and approximately 7.9 ps, respectively). The triplet state of each (lambda(max) approximately 530-560 nm) rapidly dissociates by O-O cleavage as indicated by the short triplet lifetimes (e.g., triplet lifetime of 3 approximately 0.74 ns). The approximately 550 nm absorption obtained from the 355 nm LFP (pulse width approximately 7 ns) of 1, 2, and 4 has been assigned to the corresponding aroylphenyl radicals. Two representative radicals (4-benzoylphenyl 5 and 3-(4'-methylbenzoyl)phenyl 6) investigated in detail showed solvent-dependent lifetimes. Absolute bimolecular rate constants of reactions of these radicals with various quenchers including double-bond-containing monomers have been observed to range from 7.56 x 10(7) to 1.68 x 10(9) M(-1) s(-1) in CCl(4) at room temperature. A possible structure of the aroylphenyl radicals and the transition responsible for the 550 nm absorption are discussed.

Journal Article↗
PubMed2004

Singlet oxygen generation via two-photon excited FRET.

A new approach to two-photon excited photodynamic therapy has been developed. A dendritic array of eight donor chromophores capable of two-photon absorption (TPA) was covalently attached to a central porphyrin acceptor. Steady-state fluorescence measurements demonstrated that the donor chromophores transfer excited-state energy to the porphyrin with 97% efficiency. Two-photon excitation of the donor chromophores at 780 nm resulted in a dramatic increase in porphyrin fluorescence relative to a porphyrin model compound. Enhanced singlet oxygen luminescence was observed from oxygen-saturated solutions of the target compound under two-photon excitation conditions.

Journal Article↗
PubMed2003

Ultrafast internal conversion of excited cytosine via the lowest pipi electronic singlet state.

Computational evidence at the CASPT2 level supports that the lowest excited state pipi* contributes to the S1/S0 crossing responsible for the ultrafast decay of singlet excited cytosine. The computed radiative lifetime, 33 ns, is consistent with the experimentally derived value, 40 ns. The nOpi* state does not play a direct role in the rapid repopulation of the ground state; it is involved in a S2/S1 crossing. Alternative mechanisms through excited states pisigma* or nNpi* are not competitive in cytosine.

Cytosine↗
PubMed2005

Photochemistry of an azido-functionalized cryptand: controlling the reactivity of an extremely long-lived singlet aryl nitrene by complexation to alkali cations.

Photolysis of the cryptand 1, bearing an intraannular azido substituent, results in a complex photochemistry. Low-temperature photolysis yields the triplet nitrene (3)2, which has been characterized by EPR spectroscopy. Small differences in ZFS parameters are detected between the uncomplexed nitrene-functionalized ligand (in EtOH: D' = 0.93 cm(-1)) and its sodium (NaBr@(3)2 in EtOH: D' = 0.88 cm(-1)) and potassium (KBr@(3)2 in MTHF: D' = 0.89 cm(-1)) complexes. If the photolysis of the free ligand is conducted at ambient temperature, a derivative of o-aminobenzaldehyde 4 is found to be the main product, which is formed by reaction of the o-iminoquinone methide 9 with water. The latter can be detected by UV/vis spectroscopy. Its lifetime is tau = 254 s in acetonitrile solution at ambient temperature. In the presence of diethylamine, the methyleneazepine derivative 5 is formed, which is indicative of didehydroazepine formation (7). Room-temperature photolysis of acetonitrile solutions of the sodium or potassium complexes also results in formation of the o-aminobenzaldehyde derivative. In the presence of diethylamine, however, no methyleneazepine 5 is found. Formation of the aniline derivative 8 instead points to free radical processes. Laser flash photolysis (LFP) of acetonitrile solutions of 1 leads to the detection of a short-lived (tau = 1.4 mus, lambda(max) = 445 nm plus weak absorption at lambda > 500 nm) intermediate A, which decays to transient B (tau = 8 ms, lambda(max) = 295 and ca. 350-400 nm). LFP of acetonitrile solutions of complexes NaBr@1 and KBr@1 gives similar transient spectra. In the presence of sodium and potassium cations, the lifetime of the short-lived transient A is reduced (Na(+): A', tau = 200 ns; K(+): A", tau = 160 ns). Transients A' and A" decay to long-lived transients B' + C' (B" + C"). Based on the results of our product studies, a comparison with the low-temperature results, and quantum mechanical calculations, the transients A, A', and A" are identified as singlet nitrenes (1)2, NaBr@(1)2, and KBr@(1)2, while the long-lived transients B, B', and B" are assigned to didehydroazepines 7, NaBr@7, and KBr@7. Transients C' and C" can be assigned to aminyl radicals NaBr@16 and KBr@16.

Journal Article↗
PubMed2004

4,5-dimethylthio-4'-[2-(9-anthryloxy)ethylthio]tetrathiafulvalene, a highly selective and sensitive chemiluminescence probe for singlet oxygen.

4,5-Dimethylthio-4'-[2-(9-anthryloxy)ethylthio]tetrathiafulvalene has been designed and synthesized as a highly selective and sensitive chemiluminescence (CL) probe for singlet oxygen ((1)O(2)). The design strategy for the probe is directed by the idea of photoinduced electron-transfer process and carried out through the incorporation of electron-rich tetrathiafulvalene unit into a reactive luminophore of anthracene specific for (1)O(2). Upon reaction with reactive oxygen species (ROS), such as hydrogen peroxide, hypochlorite, superoxide, hydroxyl radical, or (1)O(2), the probe exhibits both strong CL response to and high selectivity for (1)O(2) only, rather than the other ROS. This remarkable CL property permits (1)O(2) to be distinguished easily from the other ROS and makes the probe possible to be used widely for (1)O(2) detection in many chemical and biological systems and even in light water (H(2)O) environments. This applicability has been demonstrated by monitoring the (1)O(2) generation in a metal-catalyzed decomposition system of tert-butyl hydroperoxide. Moreover, the CL reaction mechanism of the present system is also discussed, clearly confirming that the introduction of electron-rich tetrathiafulvalene into the 9-position of anthracene can greatly activate its reactivity toward (1)O(2).

Journal Article↗
PubMed2004

Oligoacenes: theoretical prediction of open-shell singlet diradical ground states.

A series of oligoacenes from benzene to decacene were studied computationally with DFT and CASSCF methods. In contrast to the common view that acenes are closed-shell systems or may have a triplet ground state, these results offer the first theoretical predictions for the singlet ground state and diradical character for oligoacenes. The nature of the ground states of these molecules arises from the disjoint nature of the NBMOs that are singly occupied in the diradical.

Journal Article↗
PubMed2005

Evidence for radical fragmentations from persistent singlet carbenes.

Upon warm up, persistent aminohydrazinocarbenes undergo beta-fragmentation reactions. Experimental as well as computational evidences are reported for the involvement of radical processes. This is the first example of a homolytic fragmentation for a persistent singlet carbene.

Journal Article↗
PubMed2005

Reversible light and air-driven lithography by singlet oxygen.

Monolayers and thin films of diphenylanthracene react in the presence of a singlet oxygen sensitizer by irradiation on air to the corresponding endoperoxides and reconvert to the starting compounds upon heating. This reaction has been applied to create 2D fluorescent pattern structures. Motifs which are written by this technique can be erased and replaced by new images.

Journal Article↗
PubMed2005

Synergism between molybdenum and lanthanum in the disproportionation of hydrogen peroxide into singlet oxygen.

The catalytic disproportionation of hydrogen peroxide into singlet molecular oxygen was studied using the combined action of lanthanum(III) and molybdenum(VI). A synergistic effect was observed between both metals, resulting in a strong acceleration of the H2O2 disproportionation. An optimum in the catalytic activity was found at La/Mo and La/NaOH molar ratios of 4/1 and 1/3, respectively.

Journal Article↗
PubMed2006

Singlet fission for dye-sensitized solar cells: can a suitable sensitizer be found?

We discuss possible improvements in the efficiency of dye-sensitized photovoltaic cells using dyes capable of singlet fission into two triplets, thus producing two electron-hole pairs from a single photon. It is pointed out that, in addition to derivatives of large alternant hydrocarbons, those of biradicals are also likely candidates for a favorable ordering of excited-state energy levels, E(T2), E(S1) > 2E(T1). A large number of potentially favorable structures has been examined by the semiempirical Pariser-Parr-Pople method and some also by the time-dependent density functional theory method. Several likely candidates have been identified for experimental examination.

Journal Article↗
PubMed2006

Solution phase isomerization of vibrationally excited singlet nitrenes to vibrationally excited 1,2-didehydroazepine.

Photolysis of phenyl and o-biphenylyl azide (at 270 nm) releases vibrationally excited singlet nitrene which isomerizes to the corresponding hot 1,2-didehydroazepine at a rate competitive with thermal relaxation. Using ultrafast vibrational spectroscopy we observe the formation of vibrationally excited 1,2-4,6-azacycloheptatetraene (1,2-didehydroazepine) in picoseconds following photolysis of phenyl azide in chloroform and o-biphenylyl azide in acetonitrile at ambient temperature.

Journal Article↗
PubMed2002

Calcium peroxide diperoxohydrate as a storable chemical generator of singlet oxygen for organic synthesis.

Calcium peroxide diperoxohydrate (CaO(2).2H(2)O(2)) is an environmentally friendly generator of singlet oxygen ((1)O(2), (1)Delta(g)) that can be used in organic synthesis as an alternative to the regular photochemical method. This compound produces (1)O(2) in various solvents and can be easily recovered by filtration for further regeneration. Both monitoring of (1)O(2) luminescence at 1270 nm and specific trapping have shown that CaO(2).2H(2)O(2) can be stored for several days at -80 degrees C and that the yield of (1)O(2) is equal to 25%. Oxidation of typical organic substrates in methanol or THF through [4 + 2] or [2 + 2] cycloaddition and ene reaction have been carried out on a preparative scale with total conversion and selectivity.

Journal Article↗
PubMed2001

A novel[2 + 3]cycloaddition reaction: singlet oxygen mediated formation of 1,3-dipole from iminodiacetic acid dimethyl ester and its addition to maleimides.

Sensitized photolysis of iminodiacetic acid methyl ester and maleimides follows a [2 + 3] cycloaddition pathway yielding pyrrolidine derivatives. This is similar to the photochemical reaction between C(60) and amines. A series of pyrrolidine derivatives are prepared by the method including multipyrrolidines from bis- and tris-maleimide starting materials. The yields range from 13% to 85%. The reaction is highly stereoselective. All the isolated products have the 1,3-dimethoxycarbonyl groups in the cis configuration. Various sensitizers may be used with slightly different yields. A plausible mechanism is proposed that involves the singlet oxygen abstraction of two alpha hydrogen atoms from the iminodiacetate and formation of a 1,3-dipole with a structure similar to the classical thermally generated 1,3-dipole.

Journal Article↗
PubMed2003

Reductive PET cycloreversion of oxetanes: singlet multiplicity, regioselectivity, and detection of olefin radical anion.

Cycloreversion of 2-(p-cyanophenyl)-4-methyl-3-phenyloxetane (1) is achieved using 1-methoxynaphthalene (2) as electron-transfer photosensitizer. The experimental results are consistent with the reaction taking place from the singlet excited state of the sensitizer. Ring splitting of the radical anion 1*- occurs with cleavage of O-C2 and C3-C4 bonds, leading to products (acetaldehyde and p-cyanostilbene) different from the reagents used in the Paterno-Büchi synthesis of 1. The olefin radical anion involved in the electron-transfer process has been detected by means of laser flash photolysis.

Journal Article↗
PubMed2004

Enecarbamates as selective substrates in oxidations: chiral-auxiliary-controlled mode selectivity and diastereoselectivity in the [2+2] cycloaddition and ene reaction of singlet oxygen and in the epoxidation by DMD and mCPBA.

The stereochemical course of the oxidation of chiral oxazolidinone-substituted enecarbamates has been studied for singlet oxygen ((1)O(2)), dimethyldioxirane (DMD), and m-chloroperbenzoic acid (mCPBA) by examining of the special structural and stereoelectronic features of the enecarbamates. Valuable mechanistic insight into these selective oxidations is gained. Whereas the R(1) substituent on the chiral auxiliary is responsible for the steric shielding of the double bond and determines the sense of the pi-facial diastereoselectivity, structural characteristic such as the Z/E configuration and the nature of the R(2) group on the double bond are responsible for the extent of the diastereoselectivity. Stereoelectronic steering by the vinylic nitrogen functionality controls the mode selectivity (ene reaction vs [2+2] cycloaddition) in the case of (1)O(2).

Journal Article↗
PubMed1996

Ring Strain Effects on the Interconversion of Intermediates in the Reaction of Organic Sulfides with Singlet Oxygen.

Ab initio methods are used to investigate ring strain effects on sulfide-singlet oxygen reaction intermediates. The optimized persulfoxide and thiadioxirane structures derived from 3-, 4-, and 5-membered ring sulfides showed minor albeit systematic changes in geometry. These persulfoxides and thiadioxiranes are best described as distorted tetrahedral and trigonal bipyramidal in nature, respectively. We find that the persulfoxy sulfur becomes less sulfonium-ion-like in character with decreasing ring size. In addition, the persulfoxide and the thiadioxirane are nearly isoenergetic in all cases and their interconversion barriers are nearly identical. We speculate that the anticipated ring strain effect in the persulfoxide is compensated for by a weaker sulfur-oxygen interaction and the corresponding relaxation of the need to attain the energetically preferred tetrahedral geometry.

Journal Article↗
PubMed1996

Unusual Facial Selectivity in the Cycloaddition of Singlet Oxygen to a Simple Cyclic Diene(1).

Syntheses of 5-isopropyl-1,3-cyclohexadiene and syn-5-isopropyl-2,3-dioxabicyclo[2.2.2]octane, by routes that would allow completely diastereoselective introduction of deuterium labels, are described. The reaction of the isopropyl cyclohexadiene with singlet oxygen is shown to give an endoperoxide that is derived by preferential attack on the more sterically hindered face of the diene. A possible mechanistic explanation of this result is that the attack from the less hindered face leads to "ene" reaction rather than endoperoxide formation. However, this mechanism would require that the "ene" reaction and cycloaddition proceed via a common intermediate-presumably a perepoxide.

Journal Article↗