Time-Resolved Electron Paramagnetic Resonance Study of Photoionization of Tyrosine Anion in Aqueous Solution

Abstract— Photoionization of the amino acid tyrosine in basic water was studied by time-resolved electron paramagnetic resonance (TREPR) at X-band (9.5 GHz). Photoionization of deprotonated tyrosine leads to a spin-polarized emissive/absorptive chemically induced dynamic electron polarization (CIDEP) spectrum produced by the radical pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction, J, is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of photoionization of deprotonated tyrosine is seen only upon addition of the electron acceptor 2-bro-mo-2-methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic acid). This channel is also found to be monophotonic.     

PHOTOLYSIS MECHANISM OF AQUEOUS TYROSINE UPON EXCITATION OF THE SECOND ABSORPTION BAND

Abstract— The formation mechanism of tyrosinyl radical was studied for aqueous solutions of tyrosine under irradiation at 235 nm which falls into the second absorption band. The work is based upon the analysis of the rate of bityrosine production for steady-state excitation at low intensity. The results indicate that monophotonic O-H bond cleavage of tyrosine, presumably involving the upper excited triplet state, is the initial photoprocess leading to the tyrosinyl radical when tyrosine is excited into the second absorption band.     

Achieving Adaptive Aromaticity in Cyclo[10]carbon by Screening Cyclo[n]carbon (n=8−24)

Discovery of species with adaptive aromaticity (being aromatic in both the lowest singlet and triplet states) is particularly challenging as cyclic species are generally aromatic either in the ground state or in the excited state only, according to Hückel's and Baird's rules. Inspired by the recent realization of cyclo[18]carbon, here we demonstrate that cyclo[10]carbon possesses adaptive aromaticity by screening cyclo[n]carbon (n=8?24), which is supported by nucleus‐independent chemical shift (NICS), anisotropy of the current‐induced density (ACID), π contribution of electron localization function (ELF_π) and electron density of delocalized bonds (EDDB) analyses. Further study reveals that the lowest triplet state of cyclo[10]carbon is formed by in‐plane ππ* excitation. Thus, the major contribution to the aromaticity from out‐of‐plane π molecular orbitals does not change significantly in the lowest singlet state. Our findings highlight a crucial role of out‐of‐plane π orbitals in maintaining aromaticity for both the lowest singlet and triplet states as well as the aromaticity dependence on the number of the carbon in cyclo[n]carbon.     

Photodissociation of N2O: triplet states and triplet channel

The role of triplet states in the UV photodissociation of N(2)O is investigated by means of quantum mechanical wave packet calculations. Global potential energy surfaces are calculated for the lowest two (3)A' and the lowest two (3)A' states at the multi-reference configuration interaction level of electronic structure theory using the augmented valence quadruple zeta atomic basis set. Because of extremely small transition dipole moments with the ground electronic state, excitation of the triplet states has only a marginal effect on the far red tail of the absorption cross section. The calculations do not show any hint of an increased absorption around 280 nm as claimed by early experimental studies. The peak observed in several electron energy loss spectra at 5.4 eV is unambiguously attributed to the lowest triplet state 1(3)A'. Excitation of the 2(1)A' state and subsequent transition to the repulsive branch of the 2(3)A' state at intermediate NN-O separations, promoted by spin-orbit coupling, is identified as the main pathway to the N(2)((1)Σ(g)(+))+O((3)P) triplet channel. The yield, determined in two-state wave packet calculations employing calculated spin-orbit matrix elements, is 0.002 as compared to 0.005 ± 0.002 measured by Nishida et al. [J. Phys. Chem. A 108, 2451 (2004)].     

Fluorescence of the antharacenes following Tn → T1 excitation studied by a double excitation method

The fluorescence from the lowest excited singlet state following excitation of the lowest triplet state was observed for anthracene, 9-methylanthracene, and 9-phenylanthracene in ethanol by a newly devised double excitation method which is essentially the combination of flash and laser photolysis. The quantum yield of intersystem crossing from the excited triplet state, T_n(n ? 2), to the lowest excited singlet state was markedly increased by methyl- and phenyl-substitution at the meso-position.     

Generation of phosphorescent triplet states via photoinduced electron transfer: energy and electron transfer dynamics in Pt porphyrin-Rhodamine B dyads

Control over generation and dynamics of excited electronic states is fundamental to their utilization in all areas of technology. We present the first example of multichromophoric systems in which emissive triplet states are generated via a pathway involving photoinduced electron transfer (ET), as opposed to local intrachromophoric processes. In model dyads, PtP-Ph(n)-pRhB(+) (1-3, n = 1-3), comprising platinum(II) meso-tetraarylporphyrin (PtP) and Rhodamine B piperazine derivative (pRhB(+)), linked by oligo-p-phenylene bridges (Ph(n)), upon selective excitation of pRhB(+) at a frequency below that of the lowest allowed transition of PtP, room-temperature T(1)→S(0) phosphorescence of PtP was observed. The pathway leading to the emissive PtP triplet state includes excitation of pRhB(+), ET with formation of the singlet radical pair, intersystem crossing within that pair, and subsequent radical recombination. Because of the close proximity of the triplet energy levels of PtP and pRhB(+), reversible triplet-triplet (TT) energy transfer between these states was observed in dyads 1 and 2. As a result, the phosphorescence of PtP was extended in time by the long decay of the pRhB(+) triplet. Observation of ET and TT in the same series of molecules enabled direct comparison of the distance attenuation factors β between these two closely related processes.     

A LASER FLASH PHOTOLYSIS AND PULSE RADIOLYSIS STUDY OF AMILORIDE IN AQUEOUS AND ALCOHOLIC SOLUTION

Abstract Laser flash photolysis and pulse radiolysis have been carried out on the diuretic drug amiloride. The primary photochemical processes in aqueous solution were found to be photoionisation and triplet formation. Photoionisation was found to occur by a biphotonic process for 265 nm excitation but by a monophotonic process for excitation at 353 nm. The spectral properties of the resulting cation radical of amiloride were determined by pulse radiolysis using one electron oxidation by the radical anion Br₂·^¯ Photoexcitation of amiloride in isopropanol did not result in photoionisation but instead a semireduced radical of amiloride was observed. The spectral properties of the semireduced radical of amiloride were determined using one electron reduction by the CO₂·^¯ radical.     

THE STEPWISE BIPHOTONIC PHOTOIONIZATION OF CHLORPROMAZINE AS SEEN BY LASER FLASH PHOTOLYSIS

It is generally accepted that both promazine (PZ) and chlorpromazine (CPZ) photionize monophotonically to their respective cation radicals and the corresponding hydrated electrons. It is also supposed that this photoinization has a role in the phototoxic effects of these drugs. However, using laser flash photolysis, we have observed that photoionization of CPZ during S1 excitation (lambda greater than 300 nm) is a stepwise biphotonic process. In the case of PZ our flash photolysis results are less clearcut, but are consistent with stepwise biphotonic photoionization for S1 excitation. We demonstrate, using computer simulation of the intramolecular kinetics, that the estimated triplet state lifetime of CPZ is sufficiently long (23 ns at room temperature) to account for the apparent monophotonic photoionization that has been observed by others at high light intensities and short pulse times. Our laser flash photolysis results also suggest that the photo-ionization mechanism of PZ and CPZ is wavelength-dependent. Both drugs exhibit apparent monophotonic photoionization when they are excited at 266 nm under conditions of laser pulse width and intensity similar to those at 355 nm. We suggest that photoionization is not an important mechanism in the observed phototoxic and photoallergic effects of PZ and CPZ in sunlight.     

PHOTOLYSIS MECHANISM OF AQUEOUS TYROSINE AND TYROSYL PEPTIDES*

Abstract— Laser flash photolysis at 265 nm has been employed for measuring the initial hydrated electron (e^-_aq) and p-alanylphenoxyl radical (Tyr) in aqueous Tyr, small Tyr peptides and R Nase A. The results indicate that monophotonic photolysis not involving the fluorescent or triplet states is the principal initial process. Equivalent yields of e and Tyr were found in all cases except Tyr, where the Tyr yield was 60% higher than e^-_aq attributed to splitting of the phenolic bond. Computer analysis of e^-_aq and Tyr decays for Tyr indicates the importance of electron-radical recombination in competition with electron scavenging and bimolecular radical-radical reactions. Evidence for intramolecular electron migration has been obtained in cystinyl-bis-Tyr.     

Mechanism of the photochemical substitution of 6-membered ring monoazaaromatic compounds by methanol in neutral and HCl acidified medium

6-Membered ring monoazaaromatic compounds such as pyridine, quinoline, 4-methylquinoline, isoquinoline and 9-phenylacridine are converted to the corresponding semiquinone radicals when irradiated in methanol acidified with HCl, ether or neutral methanol. In a neutral medium the hydrogen photoabstraction occurs from an nπ^* excited state by a monophotonic process, but in methanol acidified with HCl, the photoreaction corresponds to a biphotonic process. In this case the photoreaction involves an electron transfer from methanol to a protonated upper excited triplet state of the solute (Scheme 2, path 2).     

Involvement of excited triplet state in the photodissociation of cyclobutane

The potential energy curves (PECs) of the ground state and the low‐lying excited states for the photodissociation of cyclobutane have been calculated at the multi‐reference configuration interaction with singlet and doublet excitation (MRCISD) and the multi‐reference second order perturbation theory (MRPT2). Firstly, the PECs are constructed following a reaction path determined by semiclassical dynamics simulation, which suggests that the lowest triplet state of tetramethylene is involved in the photodissociation of cyclobutane. Then, the adiabatic PECs are calculated for the breaking processes of C1? C3 and C2? C4 bond respectively. The singlet‐triplet PECs' intersections have been found in the two breaking C? C bond processes. During the breaking process of the second C2? C4 bond, a local minimum has been found on the PEC of the lowest triplet state, which gives us some insight to reinterpret the experimental observed diradical intermediate as being trapped in its triplet state. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Laser-induced two-photon ionization of acridine in aqueous solution

The present work reports the formation of hydrated electrons upon excitation of a'queous solutions of acridine using the thud harmonic of a Nd-glass laser. The experimental results indicate that acridine is photoionized via a consecutive two-photon mechanism in which the intermediate states are both the fust excited singlet and the lowest triplet. However the triplet pathway is shown to be less efficient than the singlet pathway.     

The triplet state of cytosine and its derivatives: electron impact and quantum chemical study

The excitation of the lowest electronic states and vibrational excitation of cytosine (C) have been studied using electron energy loss spectroscopy (EELS, 0-100 eV) with angular analysis. The singlet states have been found to be in good agreement with UV-VIS absorption results on sublimed films, slightly blueshifted by about 0.1 eV. The EEL spectra recorded at residual energy below 2 eV show clear shoulders at energy losses of 3.50 and 4.25 eV (+/-0.1 eV). They are assigned to the lowest triplet electronic states of cytosine. Energies and molecular structures of the lowest-lying triplet state of C and its methylated and halogenated 5-X-C, 6-X-C, and 5-X, 6-X-C substituted derivatives (X=CH3, F, Cl, and Br) have been studied using quantum chemical calculations with both molecular orbital and density functional methods, in conjunction with the 6-311++G(d,p), 6-311++G(3df,2p), and aug-cc-pVTZ basis sets. The triplet-singlet energy gap obtained using coupled-cluster theory [CCSD(T)] and density functional theory (DFT) methods agrees well with those derived from EELS study. The first C's vertical triplet state is located at 3.6 eV, in good agreement with experiment. The weak band observed at 4.25 eV is tentatively assigned to the second C's vertical triplet excitation. For the substituted cytosines considered, the vertical triplet state is consistently centered at 3.0-3.2 eV above the corresponding singlet ground state but about 1.0 eV below the first excited singlet state. Geometrical relaxation involving out-of-plane distortions of hydrogen atoms leads to a stabilization of 0.6-1.0 eV in favor of the equilibrium triplet. The lowest-lying adiabatic triplet states are located at 2.3-3.0 eV. Halogen substitution at both C(5) and C(6) positions tends to reduce the triplet-singlet separations whereas methylation tends to enlarge it. The vibrational modes of triplet cytosine and the ionization energies of substituted derivatives were also evaluated.     

The photochemistry of lipoic acid: photoionization and observation of a triplet excited state of a disulfide.

Under short-wavelength UV irradiation, lipoic acid (LipSS) and its reduced form, dihydrolipoic acid (DHLA), undergo photoionization processes through a bi- or monophotonic pathway. After ionization, the LipSS radical cation (LipSS*+) and radical anion (LipSS*-) are generated. LipSS*- can be converted to equimolar amounts of LipSS and DHLA through second-order decay. Triplet acetone can be quenched by LipSS and DHLA with a rate close to the diffusion-controlled limit. The mechanism was further confirmed by continuous irradiation experiments. When LipSS is directly irradiated with UVA light, the first excited triplet state of LipSS is observed, with a lifetime tau=75 ns. Characteristic reactions include triplet energy transfer to oxygen and beta-carotene and addition to isoprene. The lifetime of triplet LipSS is also shortened by addition of water and methanol.     

Excited-state behavior and photoionization of 1,8-acridinedione dyes in micelles--comparison with NADH oxidation

The photophysics and photochemistry of 1,8-acridinedione dyes, which are analogues of reduced nicotinamide adenine dinucleotide (NADH), are studied in anionic and cationic micelles. Acridinedione dyes (ADDs) are solubilized in micelles at the micelle-water interface and are in equilibrium between the aqueous and micellar phase. The binding of the ADDs with micelles is attributed to hydrophobic interactions and the binding constants are determined with steady-state and time-resolved techniques. Nanosecond laser flash photolysis studies are carried out in aqueous, anionic, and cationic micellar solutions. The ADD undergoes photoionization in the excited state to give a solvated electron. The solvated electron reacts with the ADD to give an anion radical. In anionic micelles, the yield of the solvated electron increases because of the efficient separation of the cation radical and the electron. Cation radicals derived from the photooxidation of ADDs are involved in keto-enol tautomerization. Under acidic conditions, an enol radical cation of the acridinedione dye is formed from the keto form of the cation radical by intramolecular hydrogen atom transfer. In cationic micelles, due to electrostatic attraction, the electron cannot escape from the micelle and recombination of the cation radical and the electron results in the formation of a triplet state. For the first time, a solvated electron is observed in the laser flash photolysis of ADDs in anionic micelles. The photoionization of ADDs depends on the excitation wavelength and is biphotonic at 355 nm and monophotonic at 248 nm. From the results with this NADH model compound, the sequential electron-proton-electron transfer oxidation of NADH is confirmed and the nature of the intermediates involved in the oxidation is unraveled; these intermediates are found to depend on the pH value of the medium.     

Photoreactions of aromatic compounds—XXXV: Nucleophilic photosubstitution of methoxy substituted aromatic compounds. Monophotonic ionization of the triplet

Methoxy groups exert an activating and ortho/para directing influence in light induced nucleophilic substitution reactions (cyanation, hydroxylation, etc) of aromatic compounds in aqueous media. The first chemical step in these processes is monophotonic ionization of the aromatic compound in its lowest triplet state, followed by reaction of the radical cation with the nucleophile Quantum yields of photocyanation of 4-fluoro- and 4-chloroanisole indicate that in 99% (mole fraction) water virtually all triplets formed undergo electron ejection.This hypothesis is in agreement with the results of charge density calculations for the radical cations. It is directly supported by the similarity of the product composition of these photochemical substitutions with that of anodic substitutions, where the intermediacy of an aromatic cation is generally accepted. The presence of an oxidizing agent (oxygen, or persulphate) is required only when a hydrogen is replaced. The nucleophilic photosubstitution at electron rich aromatic systems in solvents as water can therefore be classified as an S_r⁺n¹(³Ar^*) process.     

FORMATION OF SEMIQUINONE RADICALS BY ULTRAVIOLET IRRADIATION OF PYRIMIDINES IN METHANOL

Abstract —Ultraviolet irradiation of pyrimidine and 4- and 5-methylpyrimidine in methanol at 113 K gave the corresponding semiquinone radicals in a monophotonic process. It is likely that this process involves an n * triplet state of the photoexcited pyrimidines.     

Photophysics and photochemistry of naphthoylnaphthvalene, and photoinduced valence isomerization of highly strained aromatic compounds (Or Quinones) yielding the valene-type isomers as well as related photochromism and photo-electro dualchromism

The lowest excited singlet state of naphthoylnaphthvalene (NNV) undergoes valence isomerization yielding ground-state naphthoylnaphthalene (NN) finally. Neither the lowest excited singlet nor triplet state of NN is formed upon excitation of NNV, and of particular interest in photoinduced NNV→NNV valence isomerization is the existence of an intermediate which is probably either a bond-cloven species or a valence isomer of NNV. The lowest excited triplet state of NNV populated in benzene deactivates to its ground state, but that populated in ethanol abstracts a hydrogen atom from the solvent molecule generating the NNV ketyl radical. Interestingly, this radical also undergoes rapid valence isomerization and recombination of two NN ketyl radicals thus formed yields naphthopinacol. Synthesis of poly-tert-butylpolyacenes, tri-tert-butylisobenzofuran and tri-tert-butylpolyacenequinones, and furthermore, their photoinduced valence isomerization yielding the valene-type isomers as well as related photochromism and photo-electro dualchromism are presented.     

FLASH PHOTOLYSIS OF N-ACETYL-L-TRYPTOPHANAMIDE: THE RELATIONSHIP BETWEEN RADICAL YIELDS AND FLUORESCENCE QUENCHING

Abstract— The flash photolysis of N-acetyl-L-tryptophanamide (NATA) in the presence of the fluorescence quenchers, imidazole, acrylamide and trichloroethanol, has been investigated. Imadazole and acrylamide induce a decrease in the NATA radical yield which correlates with their NATA fluorescence quenching action. These observations suggest that the fluorescent state is primarily responsible for the monophotonic photoionization processes. The acrylamide data also suggest that 40–65% of the NATA radicals arise from a long-lived state (τ˜μs) which must originate from the fluorescent state. Unlike imidazole and acrylamide, trichloroethanol enhances the radical yield by reaction with excited state precursors. Mechanisms for the quenching of fluorescence and the long-lived states are discussed.     

A possible contribution to the population of triplet states in electron impact measurements of total cross sections for triplet state excitation

It is proposed that low energy secondary electrons produced in medium energy electron impact experiments may play an important role in the excitation of triplet states even at low sample gas densities. A model calculation is carried out which shows that the population of the 4³S, S³P and 4³D triplet states in He from secondary electron excitation can be comparable to the population of these states by direct excitation at an incident electron energy of 1000 eV and sample gas pressures as low as 10^?3 torr. The model calculations show that the secondary electron mechanism becomes more important as the incident energy increases and that the produced populations are of a similar magnitude for the 3³P and 4³D states which in turn are about a factor of 4 larger than the population found for the 4³S state. The results indicate that the effect of secondary electron excitation in careful experimental measurements of electron impact total triplet state cross sections may have to be considered when incident electron excitation energies in the range of 1 keV or higher are used.