Nanosecond Laser Flash Photolysis Method Research Articles

Nature of charge separation and recombination processes within an organic dyad having short spacer

With the help of electrochemical, steady state and time resolved fluorescence (fluorescence lifetimes by using time correlated single photon counting technique) and nanosecond laser flash photolysis methods, the nature of charge separation along with the energy wasting charge recombination processes within a short-chained organic dyad 1-(4-Bromo-phenyl)-3-(2-methoxy-naphthalen-1-yl)-propenone (MNBA) has been revealed. In MNBA , the donor 2-methoxynaphthalene (2MNT) is connected with the acceptor p-bromoacetophenone (PBA) by an unsaturated olefinic bond. Though in the ground state elongated type structure (E-form) is observable from NMR spectra but on photoexcitation, another conformers possibly of the nature of folded type isomeric species (termed as Z-isomer) were also apparent from time resolved fluorescence measurements. However, preponderance of elongated form in the excited singlet state has been established from this time resolved measurements. NMR study on photoirradiated sample and theoretical predictions from computations using CIS method with Lanl2DZ basis set also indicate in favor of the propositions made on the formations of the two possible conformers and the stability of elongated isomeric species in the electronic excited state from the experimental results. The energy wasting charge recombination rate, k CR , determined from the transient absorption measurements by nanosecond laser flash photolysis (LFP) technique was found to be ( k CR , ∼1.9×10 5 s −1) significantly lower than the charge-separation rate, k cs ∼9.4×10 7 s −1, measured from the time resolved fluorescence. This observation demonstrates that MNBA may serve as an efficient candidate to construct artificial light energy conversion devices or components of molecular photovoltaic cells.

Generation and Characterization of 1,2-Diaryl-1,1,2,2-tetramethyldisilane Cation Radicals

Nanosecond laser flash photolysis methods were used to generate and spectrally characterize the cation radicals of 1,2-diaryl-1,1,2,2,-tetramethyldisilanes (Ar = p-X-Ph, X = H, CH(3), OCH(3)) in hexafluoroisopropanol (HFIP) at room temperature. The disilane cation radicals rapidly reacted with methanol, with bimolecular rate constants ranging from 0.63 to 2.1 x 10(8) M(-1) s(-1). The cation radicals were found to react with tert-butanol 4-5 times more slowly than methanol, consistent with a small steric effect for nucleophile-assisted fragmentation of the Si-Si bond. The standard potentials for oxidation of the disilanes in HFIP were determined by two different methods: first, by measuring equilibrium constants for electron exchange between the disilanes and the cation radical of hexaethylbenzene and, second, by combining electrochemical data from cyclic voltammetry with the lifetimes of the disilane cation radicals measured by laser flash photolysis in the same media. Agreement between the two methods was excellent (<or=3 mV). The oxidation of 1,2-di-p-methoxyphenyl-1,1,2,2,-tetramethyldisilane by slow scan cyclic voltammetry in acetonitrile was not found to be reversible, in contrast to prior literature reports. Possible explanations for the prior results are proposed.

Cis−TransPhotoisomerization of Fluorescent-Protein Chromophores

Photochromic variants of fluorescent proteins are opening the way to a number of opportunities for high-sensitivity regioselective studies in the cellular environment and may even lead to applications in information and communication technology. Yet, the detailed photophysical processes at the basis of photoswitching have not been fully clarified. In this paper, we used synthetic FP chromophores to clarify the photophysical processes associated with the photochromic behavior. In particular, we investigated the spectral modification of synthetic chromophore analogues of wild-type green fluorescent protein (GFP), Y66F GFP (BFPF), and Y66W GFP (CFP) upon irradiation in solutions of different polarities. We found that the cis-trans photoisomerization mechanism can be induced in all the chromophores. The structural assignments were carried out both by NMR measurements and DFT calculations. Remarkably, we determined for the first time the spectra of neutral trans isomers in different solvents. Finally, we calculated the photoconversion quantum yields by absorption measurements under continuous illumination at different times and by a nanosecond laser-flash photolysis method. Our results indicate that cis-trans photoisomerization is a general mechanism of FP chromophores whose efficiency is modulated by the detailed mutant-specific protein environment.

Electron Photodetachment from Iodide in Ionic Liquids through Charge-Transfer-to-Solvent Band Excitation

Solvation of iodide and electrons in an ionic liquid (N,N,N-trimethyl-n-propylammonium bis(trifluoromethanesulfonyl)imide; TMPA-TFSI) was studied through the absorption spectra of the charge-transfer-to-solvent (CTTS) state of iodide and of solvated electrons. The interaction between the TMPA cation and iodide was strong, whereas electrons were weakly solvated in TMPA-TFSI. We followed electron photodetachment from iodide to the ionic liquid and formation of the solvated electrons by observing absorption in the visible and near-infrared regions using a nanosecond laser flash photolysis method. The quantum yield of the photodetachment in TMPA-TFSI was estimated to be 0.34, which is much higher than that in a high-concentration aqueous salt solution previously reported. We also examined a reaction of the solvated electrons with the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Bmim-TFSI) as a solute in TMPA-TFSI. The reaction rate was determined to be 5.3 x 10(8) M(-1) s(-1). The electrons before full solvation (dry electrons) reacted with Bmim cations efficiently. These observations suggest that the electrons in TMPA-TFSI can move easily before solvation.

Three-component photo radical initiating system—the effect of 2-mercaptobenzothiazole as a co-initiator in polymer matrix

We studied the sensitization mechanism in a photo initiating system that consists of an aminostyryl sensitizing dye, 2-[ p-(diethylamino)styryl]naphto[1,2-d]thiazole (NAS), and a radical generator, 2,2′-bis(2-chlorophenyl)-4,4′,5,5′-tetraphenyl-1, 1′-bi-1 H-imidazole (HABI). In a photopolymer bearing NAS/HABI system, an improvement of the photosensitivity was observed by the addition of co-initiator, 2-mercaptobenzothiazole (MBT), to the initiating system. Photochemical and photophysical behavior of the three-component initiating system, NAS/HABI/MBT, in the PMMA film was studied by means of a nanosecond laser flash photolysis method employing a total reflection sapphire plate. It is considered that MBT can enhance to generate the triplet state of NAS, which is hardly observed by the direct excitation, and the mechanism of both singlet and triplet electron transfer from NAS to HABI is suggested.

Generation of Thiyl Radicals by the Photolysis of 5-Iodo-4-thiouridine

The photochemistry of 2',3',5'-tri-O-acetyl-5-iodo-4-thiouridine (3) in deoxygenated 1:1 CH(3)CN-H(2)O pH 5.8 (phosphate buffer) solution has been studied by means of steady-state and nanosecond laser flash photolysis methods. Under steady-state irradiation (lambda > or = 334 nm), the stable photoproducts were iodide ion, 2',3',5'-tri-O-acetyl-4-thiouridine (4), and two disulfides. The disulfides were the symmetrical bis-(2',3',5'-tri-O-acetyl-5-iodo-4-thiouridine) (5) and unsymmetrical 6, which contains both 4-thiouridine and 5-iodo-4-thiouridine residues. The formation of the dehalogenated photoproduct suggests that C(5)-I bond cleavage is a primary photochemical step. Attempts to scavenge the resulting C(5)-centered radical by suitable addends, bis-(N-alpha-acetyl)cystine-bis-N-ethylamide or benzene, were unsuccessful. Analysis of the photoproducts formed under these conditions showed that the S-atom is the reactive center. The photoproduct 4, obtained by irradiation of 3 in CD(3)CN-H(2)O, followed by reversed-phase HPLC isolation using nonlabeled eluents, did not contain deuterium. An analogous experiment performed in CH(3)CN-D(2)O gave deuterated product 4-d with 88% of the deuterium incorporated at C(5). Transient absorption observed upon laser excitation (lambda= 308 nm) of 3 was assigned to the 4-uridinylthiyl radical on the basis of the similarity of this spectrum with that obtained upon laser photolysis of the disulfide: bis-(2',3',5'-tri-O-acetyl-4-thiouridine) 14. On the basis of the results of steady-state and laser photolysis studies, a mechanism of the photochemical reaction of 3 is proposed. The key mechanistic step is a transformation of the C(5)-centered radical formed initially by C(5)-I bond cleavage into a long-lived S-centered radical via a 1,3-hydrogen shift. Theoretical calculations confirmed that the long-lived S-centered radical is the most stable radical derived from the 4-thiouracil residue.

The Carbonate Radical Is a Site-selective Oxidizing Agent of Guanine in Double-stranded Oligonucleotides

The carbonate radical anion (CO(3)) is believed to be an important intermediate oxidant derived from the oxidation of bicarbonate anions and nitrosoperoxocarboxylate anions (formed in the reaction of CO(2) with ONOO(-)) in cellular environments. Employing nanosecond laser flash photolysis methods, we show that the CO(3) anion can selectively oxidize guanines in the self-complementary oligonucleotide duplex d(AACGCGAATTCGCGTT) dissolved in air-equilibrated aqueous buffer solution (pH 7.5). In these time-resolved transient absorbance experiments, the CO(3) radicals are generated by one-electron oxidation of the bicarbonate anions (HCO(3)(-)) with sulfate radical anions (SO(4)) that, in turn, are derived from the photodissociation of persulfate anions (S(2)O(8)(2-)) initiated by 308-nm XeCl excimer laser pulse excitation. The kinetics of the CO(3) anion and neutral guanine radicals, G(-H)( small middle dot), arising from the rapid deprotonation of the guanine radical cation, are monitored via their transient absorption spectra (characteristic maxima at 600 and 315 nm, respectively) on time scales of microseconds to seconds. The bimolecular rate constant of oxidation of guanine in this oligonucleotide duplex by CO(3) is (1.9 +/- 0.2) x 10(7) m(-1) s(-1). The decay of the CO(3) anions and the formation of G(-H)( small middle dot) radicals are correlated with one another on the millisecond time scale, whereas the neutral guanine radicals decay on time scales of seconds. Alkali-labile guanine lesions are produced and are revealed by treatment of the irradiated oligonucleotides in hot piperidine solution. The DNA fragments thus formed are identified by a standard polyacrylamide gel electrophoresis assay, showing that strand cleavage occurs at the guanine sites only. The biological implications of these oxidative processes are discussed.

Photoinduced Charge Separation and Recombination in a Novel Methanofullerene−Triarylamine Dyad Molecule

Intramolecular charge separation (CS) and charge recombination (CR) processes of the dyad molecule linking C60 and N,N-di(6-tert-butylbiphenyl)benzenamine (BBA) were investigated by pico- and nanosecond laser flash photolysis methods. Furthermore, intermolecular electron transfer to a methanofullerene (C61H2) was also investigated; the electron-transfer rate between triplet excited C61H2 and BBA was somewhat slower than that of C60 due to less negative reduction potential of C61H2. As for the BBA−C60 dyad molecule, CS was successfully observed in polar solvents by excitation of C60 moiety with picosecond laser pulse, while only the intersystem crossing was observed in nonpolar solvent. The CS process in the Marcus “normal region” was confirmed. In moderately polar solvent, radical ion pairs recombined within a few nanoseconds to generate the ground state and the triplet excited state of the C60-moiety. On the other hand, in polar solvents, the radical ion pairs decayed by two-steps: Most part decayed within 1 ns, while the radical ions were also observed in the submicrosecond region for a ca. 10% yield of generated BBA•+-C60•-. An equilibrium between CS and triplet states was proposed for the mechanism of long-lived CS state.

Photoinduced Electron Transfer from Oligothiophenes/Polythiophene to Fullerenes (C60/C70) in Solution: Comprehensive Study by Nanosecond Laser Flash Photolysis Method

Photoinduced electron-transfer processes from oligothiophenes (nT)/polythiophene (poly-T) to fullerenes (C60/C70) have been studied by the nanosecond laser flash photolysis method, observing the transient absorption spectra in the visible and near-IR regions. When fullerene was selectively photoexcited in polar solvents, electron transfer from nT to the excited triplet state of fullerene was confirmed. The electron-transfer rate constants increased with the number of repeating unit (n) of nT. On the other hand, the efficiency of electron transfer showed a maximal value at n = 4; for n > 4, electron-transfer efficiency of nT decreased, indicating contribution of other processes such as energy transfer. By the photoexcitation of nT in polar solvent, both electron and energy transfer processes were observed for 4T and 6T. In the case of 3T, energy transfer occurred predominantly even in polar solvent. In nonpolar solvent, energy transfer was a predominant deactivation process. Electron-transfer efficiencies among these oligothiophenes and polythiophene were explained on the basis of free-energy changes for the electron transfers and triplet energy levels of nT.

Efficient photoinduced electron transfer between C60/C70 and zinc octaethylporphyrin studied by nanosecond laser photolysis method

Photoinduced electron-transfer processes between C 60 or C 70 and zinc octaethylporphyrin ( ZnOEP ) have been studied in polar solvents with the nanosecond laser flash photolysis method, observing the transient absorption spectra in the visible and near-IR regions. By the predominant excitation of ZnOEP with 532 nm laser light the transient absorption bands of 3 ZnOEP * decayed, accompanied by the appearance of the transient absorption bands of [Formula: see text] and [Formula: see text]. By the predominant excitation of C 60 and C 70 with 610 nm laser light the decays of [Formula: see text] and [Formula: see text] were observed, accompanied by the appearance of [Formula: see text] and [Formula: see text]. The electron transfer rate constants (k et ) and the quantum yields (Φ et ) of [Formula: see text] and [Formula: see text] formation via 3 ZnOEP * and [Formula: see text] or [Formula: see text] have been evaluated. These values increase with the solvent polarity; in polar benzonitrile these values are higher than for other porphyrins such as zinc tetraphenylporphyrin. The back electron transfer rate constants were evaluated from the decays of [Formula: see text] and [Formula: see text], which also show a solvent polarity dependence.

Photochemistry of Nitrobenzenethiol. Selective Generation of the Thio Radical and Thione Triplet State as a Function of Solvent Polarity

Transient absorption spectra of p-nitrobenzenethiol (NBSH) have been measured by the nanosecond laser flash photolysis method. In nonpolar and less polar solvents, nitrobenzenethio radical (NBS•) was predominantly formed by the homolytic fission of the S−H bond of NBSH. In protic polar solvents, on the other hand, formation of the triplet state of deprotonated nitrobenzenethione [3(NBT-)*], which was confirmed by triplet quenching experiments with O2 and β-carotene, was observed. Quantum yield of intersystem crossing (ΦT) and the lowest triplet energy (ET1) of 3(NBT-)* were evaluated to be 0.36 and 58 kcal/mol, respectively, in ethanol. In aprotic polar solvents such as acetonitrile, both 3(NBT-)* and NBS• were produced in the same time. Electron transfer occurs from donors to 3(NBT-)* and to NBS•. For an electron acceptor, the electron-transfer reaction takes place from only 3(NBT-)* and not from NBS•. Thus, NBSH can be used as a selective generator of the thione triplet and thio radical by changing the properties, such as polarity and protic/aprotic character, of the solvents used.

Laser-flash photolysis study of dithiobis (tetrazole); reactivities of tetrazole-thio radical

The photo-cleavage of SS bond of 5,5′-dithiobis (1-phenyl-1H-tetrazole) has been studied by the nanosecond-laser flash photolysis method. The transient absorption band at ca. 430 nm was attributed to 1-phenyl-1H-tetrazole-5-thio radical forming by the SS bond fission. For the reaction with conjugated dienes, an addition reaction takes place forming the SC bond, suggesting that unpaired electron of the radical localizes mainly on the S-atom. From the decay rates of the radical, the addition reaction rate constant for 2-methyl-1,3-butadiene is evaluated to be 5.5 × 109 M−1 s−1 in THF at 23°C, which is as fast as diffusion controlled limit. The reactivity of the radical is ca. 100 times higher than that of the PhS˙. The reactivity of the thio radical to O2 was too low to evaluate, which is one of the characteristics of a S-centered radical. The rate constant for 1,4-cyclohexadienene (1.4 × 108 M−1 s−1) is larger than that of cyclohexene (2.8 × 107 M−1 s−1) suggesting the hydrogen abstraction is a main reaction. The MO calculations have been performed for these radicals to reveal the reason of the high reactivity of the radical. © 1996 John Wiley & Sons, Inc.

Study of photocycle and spectral properties of bacteriorhodopsin in Langmuir-Blodgett films

Langmuir-Blodgett (LB) films of bacteriorhodopsin (BR) in soya-phosphatidylcholine (PC) matrix were prepared on quartz plates and the photocycle and spectral properties of the films were studied. The transient absorption spectrum of BR LB film was measured by the nanosecond laser flash photolysis method in a wavelength range from 380 to 650 nm. The absorption maxima were at 540 and 415 nm for the L and M states respectively. In water the rate of decay of the L state was 2.3 × 10 4 s −1 and the rates of formation and decay of the M state were 2.3 × 10 4 s −1 and 3.0 × 10 2 s −1 respectively. The photocycle reaction was not observed in nitrogen-dried BR LB films, but when the films were put back in water, even after being stored for 8 months in dry conditions, the phenomenon could be observed. The energy transfer from trytophan to BR retinal takes place in both dry and wet LB films. The films were relatively stable to changing physical conditions as well as to laser excitation and no changes were observed following repeated photocycles. Molecular surface potentials of BR-soya-PC and soya-PC Langmuir films as a function of mean molecular area decreased towards zero with increasing surface pressure.

PHOTOPHYSICS AND IONIC PHOTODISSOCIATION OF POLYESTERS WITH PENDANT 1‐PYRENYL GROUPS IN SOLUTION

Abstract—Primary photoprocesses of polyesters with pendant pyrenyl groups in solution were studied by means of nanosecond laser flash photolysis method. The intrapolymer process of the Sl Sl annihilation was observed. Intense laser excitation brings about dense population of several fluorescent states in a polymer chain, and their mutual interaction leads to efficient deactivation. This process is confirmed by excitation intensity dependence of fluorescence spectra and fluorescence intensity. It was also confirmed that the T‐T absorption intensity decreases as the degree of polymerization increases. This low formation yield of the triplet state in the polyesters is ascribed mainly to the above Sl Sl interaction. In addition the polymer effect on the ionic photodissociation yield of the polyester‐electron acceptor was investigated. The yield decreases as the degree of polymerization increases. This effect is due to slow solvation processes, which may be due to low micropolarity and high microviscosily around the polymer chain.