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ABSTRACT: We introduce a variant of coumarin-based photoactivatable protecting groups and use it exemplarily for caging of a carboxylic acid, an amine, a phenol, and a carbonyl compound. The caged compounds are efficiently photolyzed at long-wavelength UV/vis irradiation. Compared to the corresponding (6-bromo-7-hydroxycoumarin-4-yl)methyl (Bhc) derivatives, the novel coumarin-type caged compounds are distinguished by (i) dramatically increased solubilities in aqueous buffers, (ii) lower pK(a) values of the C7 hydroxyl of the coumarin chromophore, thus permitting efficient photorelease at lower pH, and (iii) higher photolysis quantum yields in the case of photoprotected carbonyl compounds. The primary step of the photocleavages occurs with rate constants of about 10(9) s(-1).
The Journal of Organic Chemistry 03/2010; 75(9):2790-7. · 4.45 Impact Factor
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Journal of the American Chemical Society 06/2009; · 9.91 Impact Factor
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ABSTRACT: (Coumarin-4-yl)methyl esters (CM-A) are caged compounds that, upon excitation, release the masked biologically active acid HA and the highly fluorescent (coumarin-4-yl)methyl alcohol CM-OH very rapidly and in part with high efficiency. The results of photostationary and time-resolved investigations of 25 CM-A esters and corresponding CM-OH alcohols with varying substitution on the (coumarin-4-yl)methyl moiety and a wide variation in the structure of the acidic part have been analyzed. The initial step of the photoreaction is heterolytic ester cleavage leading to the singlet ion pair 1[CM+ A-] with rate constant k1. 1[CM+ A-] hydrolyzes to CM-OH and HA with rate constant k2 or recombines to ground-state CM-A with rate constant krec. 1[CM+ A-] is the key intermediate of the reaction. Stabilization of both CM+ by using electron-donating substituents and A- by increasing the acid strength leads to a strong enhancement of k1 and simultaneously to a diminution of krec. Therefore, stabilization of the ion pair has a two-fold positive effect on the photocleavage of (coumarin-4-yl)methyl esters: increasing the rate of the initial reaction step, which might require less than 30 ps, and increasing the efficiency of product formation.
The Journal of Physical Chemistry A 08/2007; 111(26):5768-74. · 2.95 Impact Factor
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Reinhard Schmidt
The Journal of Physical Chemistry A 07/2006; 110(24):7749. · 2.95 Impact Factor
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Reinhard Schmidt
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ABSTRACT: A large set of literature kinetic data on triplet (T(1)) sensitization of singlet oxygen by two series of biphenyl and naphthalene sensitizers in solvents of strongly different polarity has been analyzed. The rate constants and the efficiencies of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)). IC of (1,3)(T(1)(3)Sigma) encounter complexes is controlled by an energy gap law that is generally valid for the transfer of electronic energy to and from O(2). (1,3)(T(1)(3)Sigma) nCT complexes form in competition to IC (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) exciplexes if CT interactions between T(1) and O(2) are important. The rate constants of exciplex formation depend via a Marcus type parabolic model on the corresponding free energy change DeltaG(CT), which varies with sensitizer triplet energy, oxidation potential, and solvent polarity. O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)) are formed in the product ratio (1/6):(1/12):(3/4) in the CT deactivation channel. The balance between nCT and CT deactivation is described by the relative contribution p(CT) of CT induced deactivation calculated for a sensitizer of known triplet energy from its quenching rate constant. It is shown how the change of p(CT) influences the quenching rate constant and the efficiency of singlet oxygen formation in both series of sensitizers. p(CT) is sensitive to differences of solvent polarity and varies for the biphenyls and the naphthalenes as sigmoidal with DeltaG(CT). This quantitative model represents a realistic and general mechanism for the quenching of pipi triplet states by O(2), surpassing previous advanced models.
The Journal of Physical Chemistry A 06/2006; 110(18):5990-7. · 2.95 Impact Factor
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Reinhard Schmidt
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ABSTRACT: The quenching of excited triplet states of sufficient energy by O2 leads to O2(1sigma(g)+) and O2(1delta(g)) singlet oxygen and O2(3sigma(g)-) ground-state oxygen as well. The present work investigates the question whether in the absence of charge transfer (CT) interactions between triplet sensitizer and O2 the rate constants of formation of the three different O2 product states follow a generally valid energy gap law. For that purpose, lifetimes of the upper excited O2(1sigma(g)+) have been determined in a mixture of 7 vol % benzene in carbon tetrachloride, in chloroform, and in perdeuterated acetonitrile. They amount to 1.86, 1.40, and 0.58 ns, respectively. Furthermore, rate constants of O2(1sigma(g)+), O2(1delta(g)), and O2(3sigma(g)-) formation have been measured in these three solvents for five pi pi* triplet sensitizers with negligible CT interactions. The rate constants are independent of solvent polarity. After normalization for the multiplicity of the respective O2 product state, the rate constants follow a common dependence on the excess energies of the respective product channels. This empirical energy gap relation describes also quantitatively the rate constants of quenching of O2(1delta(g)) by 28 carotenoids. Therefore, it represents in the absence of CT interactions a generally valid energy gap law for the rate constants of electronic energy transfer to and from O2.
The Journal of Physical Chemistry A 04/2006; 110(8):2622-8. · 2.95 Impact Factor
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Volker Hagen,
Brigitte Dekowski,
Vasilica Nache, Reinhard Schmidt,
Daniel Geissler,
Dorothea Lorenz,
Jenny Eichhorst,
Sandro Keller,
Hiroshi Kaneko,
Klaus Benndorf,
Burkhard Wiesner
Angewandte Chemie International Edition 01/2006; 44(48):7887-91. · 13.45 Impact Factor
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ABSTRACT: Photolabile coumarinylmethyl esters of biomolecules (caged compounds) are new tools for studying spatial and time-dependent aspects of signal transduction in living cells. Herein we describe a fluoresence spectroscopic method for the determination of the rate constants of the photolysis steps of such caged compounds using (6.7-dimethoxycoumarin-4-yl)methyl diethyl phosphate (DMCM-DEP) and sodium (6,7-dimethoxycoumarin-4-yl)methyl sulfate (DMCM-S). DMCM-DEP and DMCM-S are caged compounds which photorelease a proton, the corresponding acid anion, and the strongly fluorescent alcohol DMCM-OH upon excitation. The results of stationary and time-resolved measurements of the photochemistry and the luminescence of both caged compounds indicate that DMCM-OH is produced already during the excitation pulse. The quantitative analysis of the data demonstrates that the first step of the reaction--heterolytic bond cleavage of the coumarinylmethyl ester leading to the ion pair of a DMCM cation and an acid anion--is very fast with a rate constant of k1 approximately 2 x 10(10) s(-1). Recombination of the ion pair occurs with a rate constant of k(rec) approximately 2.3 x 10(9) s(-1) and is about 10 times faster than the competing hydrolysis reaction of the DMCM cation yielding DMCM-OH and a proton. Thus, both caged compounds belong to the fastest phototriggers known.
The Journal of Physical Chemistry A 07/2005; 109(23):5000-4. · 2.95 Impact Factor
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Reinhard Schmidt
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ABSTRACT: A charge transfer (CT) channel and a non-CT deactivation channel, both leading to formation of O(2)((1)Sigma (g)(+)), O(2)((1) Delta(g)) and O(2)((3)Sigma(g)(-)), compete in the quenching of triplet states by O(2). Recent studies by our group demonstrated that these channels are described by rather simple and general quantitative relations. In the present paper we use the detailed kinetic data on the quenching by O(2) of pi pi* triplet sensitizers of three homologous aromatic series in CCl(4) to derive a parameter, which describes the balance between CT and non-CT deactivation. This quantity, p(CT), is the relative contribution of CT mediated deactivation and is easily calculated for a sensitizer of known triplet energy from its quenching rate constant. The parameter p(CT) quantitatively describes the balance between both deactivation channels without requiring any knowledge of oxidation potentials. It is shown how the variation of p(CT) influences the efficiencies and the rate constants of O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)) and O(2)((3)Sigma(g)(-)) formation in the quenching process.
Photochemical and Photobiological Sciences 07/2005; 4(6):481-6. · 2.58 Impact Factor
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Angewandte Chemie International Edition 03/2005; 44(8):1195-8. · 13.45 Impact Factor
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ABSTRACT: Rate constants of photosensitized generation of O2(1Σg+), O2(1Δg), and O2(3Σg-) have been determined for a series of ππ* triplet sensitizers with strongly varying oxidation potential (Eox), triplet energy (ET), and molecular structure, in CCl4. We demonstrate that one common dependence on Eox and ET successfully describes these rate constants for the molecules studied here and also for all previously investigated ππ* sensitizers, independently of molecular structure or any other parameter. Photosensitized singlet oxygen generation during O2 quenching of ππ* triplet states can be generally described by a mechanism involving the successive formation of excited noncharge transfer (nCT) encounter complexes and partial charge transfer (pCT) exciplexes of singlet and triplet multiplicity 1,3(T13Σ), following interaction of O2(3Σg-) with the triplet excited sensitizer. Both 1,3(T13Σ) nCT and pCT complexes decay by internal conversion (ic) to yield O2(1Σg+), O2(1Δg), and O2(3Σg-) and the sensitizer ground state. ic is the rate-limiting step in the nCT channel, whereas exciplex formation is rate determining in the pCT channel. Rotation of the O2 molecule within the solvent cage of 1,3(T13Σ) nCT complexes is fast enough to allow for a completely established intersystem crossing (isc) equilibrium, whereas significant noncovalent binding interactions slow rotation and inhibit isc between 1(T13Σ) and 3(T13Σ) pCT complexes. Upon the basis of this mechanism, we propose a semiempirical relationship that can be generally used to estimate rate constants and efficiencies of photosensitized singlet oxygen generation during O2 quenching of ππ* triplet states in CCl4. The data set includes 127 rate constants for derivatives of naphthalene, biphenyl, fluorene, several ketones, fullerenes, porphyrins and metalloporphyrins, and other homocyclic and heterocyclic aromatics of variable molecular structure and size. It is suggested that the general relationship presented here can be used for the optimization of the singlet oxygen photosensitization ability of many molecules, including those used in biological and medical applications, such as the photodynamic therapy of cancer.
03/2003;
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ABSTRACT: Both excited singlet states, 1sigma(g)+ and 1delta(g), and the triplet ground state, 3sigma(g)-, of molecular oxygen are competitively formed during the quenching by O2 of triplet (T1) excited sensitizers of sufficient energy. The corresponding overall rate constants kT(1sigma), kT(1delta), and kT(3sigma) as well as the T1 state energies E(T) and the oxidation potentials E(ox), have been determined for a series of six fluorene derivatives. Graduated and in part strong charge transfer (CT) effects on kT(1sigma), kT(1delta), and kT(3sigma) are observed. These and literature data strongly indicate that quenching occurs in two different channels each capable of producing O2(1sigma(g)-), O2(1delta(g)), and O2(3sigma(g)-). One proceeds via internal conversion (IC) of excited 1,3(T1 x 3sigma) complexes with no CT character (nCT), which cannot be distinguished from encounter complexes, the other via IC of 1,3(T1 x 3sigma) exciplexes with partial CT character (pCT). The contributions of nCT and pCT deactivation channels to the overall formation of O2(1sigma(g)+), O2(1delta(g)). and O2(3sigma(g)-) depend on E(T) and E(ox). The rate constants of the nCT channel are controlled by the excess energies of the respective IC processes by an energy gap law. The rate constants of the pCT channel depend on the change of free energy deltaG(CET) for complete electron transfer from T1 excited sensitizer to O2. Equations are presented which show the functional form of the dependence of the oxygen quenching rate constants on E(T) and E(ox). Particular emphasis is laid on the question of whether these relations could generally be valid for pipi* triplet sensitizers.
Photochemical and Photobiological Sciences 05/2002; 1(4):263-9. · 2.58 Impact Factor
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ABSTRACT: Rate constants of formation of O2(1Σg+), O2(1Δg), and O2(3Σg-) in the quenching of triplet states T1 by O2 have been determined for a series of nine benzophenones (BPs) of strongly varying oxidation potential, Eox, but almost constant triplet-state energy ET. These data are analyzed considering data determined previously for T1(ππ*) sensitizers of very different ET and Eox. Much weaker charge transfer (CT) effects are observed for the T1(nπ*) BPs compared with those obtained with a series of structurally related T1(ππ*) biphenyls. The quenching proceeds for T1(nπ*) and T1(ππ*) sensitizers via two different channels, each capable of producing O2(1Σg+), O2(1Δg), and O2(3Σg-). One channel originates from excited 1,3(T1·3Σ) complexes with no CT character and the other from 1,3(T1·3Σ) exciplexes with partial CT character. Different energy gap relations determine the formation of O2(1Σg+), O2(1Δg), and O2(3Σg-) of T1(ππ*) and T1(nπ*) sensitizers in the nCT channel, whereby the excess energy (ΔE) dependence of the corresponding rate constants is much weaker for the T1(nπ*) ketones. In the pCT channel, the respective rate constants vary on a logarithmic scale linearly with the free energy change for complete electron transfer for both T1(ππ*) and T1(nπ*) sensitizers. This dependence too is much weaker for T1(nπ*) than for T1(ππ*) sensitizers. The comparison with CT induced quenching of O2(1Δg) by ground-state sensitizers reveals that the different electronic configurations leads to different sterical structures of 1,3(T1(nπ*)·3Σ) and 1,3(T1(ππ*)·3Σ) complexes. These differences strongly influence the complex deactivation and explain both the weaker ΔE dependence and the weaker CT effects in the quenching of T1(nπ*) by O2.
12/2001;
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ABSTRACT: We have studied the charge-transfer-induced deactivation of nπ* excited triplet states of benzophenone derivatives by O2(3Σ), and the charge-transfer-induced deactivation of O2(1Δg) by ground-state benzophenone derivatives in CH2Cl2 and CCl4. The rate constants for both processes are described by Marcus electron-transfer theory, and are compared with the respective data for a series of biphenyl and naphthalene derivatives, the triplet states of which have ππ* configuration. The results demonstrate that deactivation of the locally excited nπ* triplets occurs by local charge-transfer and non-charge-transfer interactions of the oxygen molecule with the ketone carbonyl group. Relatively large intramolecular reorganization energies show that this quenching process involves large geometry changes in the benzophenone molecule, which are related to favorable Franck-Condon factors for the deactivation of ketone-oxygen complexes to the ground-state molecules. This leads to large rate constants in the triplet channel, which are responsible for the low efficiencies of O2(1Δg) formation observed with nπ* excited ketones. Compared with the deactivation of ππ* triplets, the non-charge-transfer process is largely enhanced, and charge-transfer interactions are less important. The deactivation of singlet oxygen by ground-state benzophenone derivatives proceeds via interactions of O2(1Δg) with the Ph rings.
Helvetica Chimica Acta 10/2001; 84(9):2493 - 2507. · 1.48 Impact Factor
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ABSTRACT: The charge-transfer induced quenching processes of the lowest excited triplet state (T1) of naphthalene derivatives by ground-state oxygen and of singlet oxygen O2(1Δg) by ground state naphthalene derivatives have been investigated in three solvents of different polarity. Both deactivation processes are described by one common Marcus type plot. The analysis of the data strongly indicates that exciplexes with the same degree of average partial charge transfer (pCT) and the same reorganization energy are formed in the rate-determining step of both quenching processes. The free energies of pCT complex formation are related to the corresponding free energy of complete electron transfer by ΔGCT = fΔGCET with a common corrective factor f, for T1 and O2(1Δg) deactivation. The reorganization energy increases from 34 kJ mol-1 in carbon tetrachloride to 92 kJ mol-1 in acetonitrile. The charge transfer character is shown to be significantly larger than 25% and to increase with increasing solvent polarity.
05/2001;
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ABSTRACT: The photochemistry of the (7-methoxycoumarin-4-yl)methyl (MCM) carboxylates 3a−d, the mesylate 4, and the phosphates 5a−e has been examined under near physiological conditions in acetonitrile or methanol/aqueous HEPES buffer solution, respectively. Analysis of photoproducts as well as measurements of photochemical quantum yields, fluorescence quantum yields, and lifetimes for the excited singlet state verified the similar photochemical and photophysical behavior of all the esters studied here. 4-(Hydroxymethyl)-7-methoxycoumarin (2) and the corresponding free acids were obtained as major products upon irradiation. The rates of deactivation of the excited MCM derivatives 3a−5e were found to be dependent on the leaving group ability of the anion concerned as well as on the solvent polarity. The polarity dependence and the exclusive formation of 18O-labeled 2 during irradiation of 5a in 18O-labeled water indicate that photocleavage of the excited singlet state of the MCM caged compounds 3a−5e proceeds via a photo SN1 mechanism (solvent-assisted photoheterolysis).
11/1999;
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Reinhard Schmidt
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ABSTRACT: This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of pi pi* excited triplet states by O2 proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O2. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet-state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge-transfer interactions similar to triplet-state quenching by O2. Finally, two recent developments in photosensitization of singlet oxygen are reviewed: Two-photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.
Photochemistry and Photobiology 82(5):1161-77. · 2.41 Impact Factor
