Photophysical properties of closely-coupled, binuclear ruthenium(II) bis(2,2':6',2''-terpyridine) complexes

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) bis(2,2':6',2'-terpyridine) complexes via an alkynylene group are compared to those of the parent complex. The dimers exhibit red-shifted emission maxima and prolonged triplet lifetimes in deoxygenated solution. Triplet quantum yields are much less than unity and the dimers generate singlet molecular oxygen with low quantum efficiency. Temperature dependence emission studies indicate coupling to higher-energy triplet states while cyclic voltammetry shows that the metal centres are only very weakly coupled but that extensive electron delocalization occurs upon one-electron reduction. The radiative rate constants derived for these dimers are relatively low, because the lowest-energy metal-to-ligand, charge-transfer states possess increased triplet character. In contrast, the rate constants for nonradiative decay of the lowest-energy triplet states are kept low by extended electron delocalization over the polytopic ligand. The poor triplet yields are a consequence of partitioning at the second triplet level.     

Trends in metal-biradical exchange interaction for first-row M(II)(nitronyl nitroxide-semiquinone) complexes

We report molecular structures and temperature-dependent magnetic susceptibility data for several new metal complexes of heterospin triplet ground-state biradical ligands. The ligands are comprised of both nitronyl-nitroxide (NN) and semiquinone (SQ) spin carriers. Five compounds are five-coordinate M(II) complexes (M = Mn, Co, Ni, Cu, and Zn), and one is a six-coordinate Ni(II) complex. Five compounds were structurally characterized. During copper complex formation a reaction with methanol occurs to form a unique methoxy-substituted SQ ring. Variable-temperature magnetic susceptibility studies are consistent with strong intraligand (NN-SQ and NN-PhSQ) ferromagnetic exchange coupling. For the five-coordinate Mn, Co, and Ni complexes, the S = 1 ligand is antiferromagnetically coupled to the metal. For both the five-coordinate Cu complex and the six-coordinate Ni complex, the ligand is ferromagnetically coupled to the metal spins in accordance with orbital symmetry arguments. Despite the low molecular symmetries, the predicted trend in metal-ligand exchange interactions is supported by spin dimer analysis based on extended Hückel calculations. For (NN-SQ)NiTp(Cum,Me)() (Tp(Cum,Me)() = hydro-tris(3-cumenyl-5-methylpyrazolyl)borate), an antisymmetric exchange term was required for the best fit of the magnetic susceptibility data. Antisymmetric exchange was less important for the other complexes due to inherently smaller Deltag. Finally, it is shown that intraligand exchange coupling is of paramount importance in stabilizing high-spin states of mixed metal-biradical complexes.     

TRIPLET STATE STUDIES OF FLAVlNS BY ELECTRON PARAMAGNETIC RESONANCE—II

Abstract— The triplet states of proteins, bovine serum albumin, ovalbumin and d-amino acid oxidase, were observed by electron paramagnetic resonance at 77°K.
The triplet state of aromatic amino acids, tryptophan, tyrosine and phenylalanine was also detected.
The protein triplet originates from the tryptophan residues of these proteins.
It is suggested that an energy transfer takes place between tyrosine and tryptophan.     

How CO binds to hexacoordinated heme in neuroglobin protein

In the present work, density functional theory (DFT) has been used to investigate CO binding to the hexacoordinated heme in neuroglobin (Ngb) protein. Structural relaxation of the selected model system in the protein environment has been fully included by the alternative quantum and molecular mechanical optimizations. The polarized continuum model (PCM) was used to simulate interaction between the model system and the protein environment. The CO binding could take place in a concerted way and a barrier of 17.9 kcal mol(-1) was predicted on the concerted singlet pathway, which is not favorable in energy. The adiabatically sequential pathway requires an energy of 14.5 kcal mol(-1) for formation of the singlet intermediate. There exist two nonadiabatic sequential pathways for the CO binding, which involves the triplet and quintet states of intermediate. Both the singlet/triplet and singlet/quintet intersections play an important role in nonadiabatic sequential processes, which enhance the probability that the processes occur. The nonadiabatic processes that involve the triplet and quintet states of intermediate are the most probable pathways for the CO binding to the hexacoordinated heme in Ngb to form the product complex.     

THE ANAEROBIC PHOTOLYSIS OF LENS α-CRYSTALLIN: EVIDENCE FOR TRIPLET STATE MEDIATED PHOTODAMAGE

Anaerobic solutions of lens alpha-crystallin were subjected to near-UV (greater than 295 nm) irradiation, and the photoproducts were analyzed by fluorescence and room-temperature phosphorescence spectroscopy. The principal photoproduct was excited maximally at 340 nm, fluoresced maximally at 430 nm, and phosphoresced with an emission maximum at 510 nm. The phosphorescence intensity decay of this species was well fit by a sum of two exponentials with lifetimes of 9.2 ms (78%) and 61 ms (22%); this report is the first demonstration of a long-lived triplet state associated with a protein photolysis product. As reported previously, 3trp* is also long-lived in deoxygenated alpha-crystallin solution at room-temperature (Berger and Vanderkooi, 1989, Biochemistry 28, 5501-5508), hence both tryptophan and photoproduct triplet states are good candidates to mediate photodamage. Photolysis experiments in the presence of agents known to alter the tryptophan triplet yield provide evidence for the importance of triplet-state-mediated photodamage of lens crystallins in anaerobic solution. In 30 mM acrylamide where 3trp*, but not 1trp*, is efficiently quenched, anaerobic solutions exhibited marked resistance to protein photodamage, whereas the photoprotection in aerobic solution was minimal. In D2O, where photoionization is suppressed but triplet states are longer-lived, photodamage was accelerated in anaerobic solution but reduced in aerobic solutions. Finally, the anaerobic photodestruction rate was increased in 500 mM Cs+ solution where the triplet yield is increased by a heavy atom effect.     

Energetics of Photoinduced Electron Transfer in the Indole-O2 Cluster in Gas Phase: Possible Consequences for Photoexcited Tryptophan in Solution

Abstract— A direct process for an activationless electron transfer from photoexcited tryptophan to molecular oxygen is proposed. By photodetachment of mass-selected indole-O_2- clusters in gas phase a neutral indole₊ O_2- charge-separated exciplex state is found at 2.25 0.2 eV above the neutral ground state. By theory also, the existence of an excited charge separated state at 3.05 0.2 eV is postulated. In gas phase both charge-separated cluster states are energetically below the first singlet states ₁L_b and ₁L_a and the lower even below the first triplet state T₁ of indole. In gas-phase clusters these energetics imply a very efficient quenching of photoexcited indole by fast electron transfer to oxygen. We discuss a similar mechanism for tryptophan-O₂ in aqueous environment and find it without activation barrier and presumably extremely fast. In the collisional tryptophan_*-O₂ complex the efficiency and the time scale of the charge transfer process should be mostly solvent independent. In polar solvent a complete charge separation and free superoxide formation are expected. We correlate this model with previous fluorescence and phosphorescence quenching data of excited tryptophan by O₂ and propose electron transfer to be the relevant process.     

OPTICALLY DETECTED MAGNETIC RESONANCE OF THE TRYPTOPHAN PHOSPHORESCENT STATE IN NATIVE PROTEINS*

Abstract— The phosphorescent triplet state of tryptophan has been studied by the method of optically detected magnetic resonance (ODMR) at pumped helium temperatures in zero magnetic field. Only one of the triplet sublevels is found to be significantly radiative; the other two decay radiationlessly. Although the phosphorescence and ODMR decay lifetimes are influenced by spin–lattice relaxation processes at T = 1.3°K, the lifetime of the radiative level can be estimated as approximately 2 s, whereas the lifetimes of the non–radiative levels are in excess of 10 s. Comparison of the ODMR signals and the phosphorescence spectra has been made for tryptophans in native proteins with the following results: the ODMR signals of the two types of tryptophan sites in horse liver alcohol dehydrogenase can be resolved due to a shift in the D and E values of the respective triplet states; binding of the substrate tri- N -acetylglucosamine to hen lysozyme leads to a considerable narrowing of the phosphorescence peaks and ODMR signals as well as to a shift in the E value of the triplet state.
The following tentative conclusions can be reached: the tryptophan triplet D and E values are measurably affected by the environment of the chromophore in the protein, as are the linewidths of the magnetic resonance transitions. The | E | value is reduced and the magnetic resonance linewidth is increased with increasing exposure of the tryptophan to hydroxylic solvent. Although a considerable part of the width of the magnetic resonance transition can be ascribed to a heterogeneity of environments in the sample, there appears to exist an intrinsic line–broadening process which at present is not understood.     

Mechanism of the photochemical ligand substitution reactions of fac-[Re(bpy)(CO)(3)(PR(3))](+) complexes and the properties of their triplet ligand-field excited states

We report herein the mechanism of the photochemical ligand substitution reactions of a series of fac-[Re(X(2)bpy)(CO)(3)(PR(3))](+) complexes (1) and the properties of their triplet ligand-field ((3)LF) excited states. The reason for the photostability of the rhenium complexes [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) was also investigated. Irradiation of an acetonitrile solution of 1 selectively gave the biscarbonyl complexes cis,trans-[Re(X(2)bpy)(CO)(2)(PR(3))(CH(3)CN)](+) (2). Isotope experiments clearly showed that the CO ligand trans to the PR(3) ligand was selectively substituted. The photochemical reactions proceeded via a dissociative mechanism from the (3)LF excited state. The thermodynamical data for the (3)LF excited states of complexes 1 and the corrective nonradiative decay rate constants for the triplet metal-to-ligand charge-transfer ((3)MLCT) states were obtained from temperature-dependence data for the emission lifetimes and for the quantum yields of the photochemical reactions and the emission. Comparison of 1 with [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) indicated that the (3)LF states of some 3- and 4-type complexes are probably accessible from the (3)MLCT state even at ambient temperature, but these complexes were stable to irradiation at 365 nm. The photostability of 3 and 4, in contrast to 1, can be explained by differences in the trans effects of the PR(3), py, and Cl(-) ligands.     

LIGHT-INDUCED CHANGE IN RHODOPSIN EMISSION: PHOSPHORESCENCE and FLUORESCENCE

Abstract— Phosphorescence measurements of rhodopsin in bovine rod disk membranes were made to study changes in protein conformation on bleaching by probing the environment of tryptophan and tyrosine residues of the protein. Bleaching decreased the tyrosine phosphorescence by about 25% and significantly affected the amplitude of triplet decay when rhodopsin was excited at 280 nm, where both tyrosine and tryptophan absorb. Computer analysis using one or two exponential model functions showed the presence of two components in the decay curve at 410 nm—one with a lifetime of 2.2 s, the other with a lifetime of 4.8 s>—which are typical of tyrosine and tryptophan respectively. When the rod outer segment sample was bleached, there was a significant decrease in the amplitude of the tyrosine component. However, the lifetime values of the two components did not change. Analyses of the fluorescence spectra of dark and bleached membranes at different excitation wavelengths and the phosphorescence change on bleaching suggest energy transfer between tyrosine and tryptophan singlet states, which may result from a conformational change of the opsin moiety on bleaching.     

Low-energy resonance states upon electron capture by five-membered heterocycles and cyclopentadiene

New resonance states were discovered for the negative molecular ions of thiophene and selenophene and a series of resonances was found for various heterocyclic compounds in the region 3.0–3.6 eV. The low-energy resonances at 1.65–2.3 eV are formed by a resonance mechanism of a form of the molecular ground state, while an electronically excited Feschbach resonance is responsible for the series of resonance states at 3.0–3.6 eV. The mother state for the latter resonance states is the first triplet state of these molecules. The first triplet state of selenophene is at 3.6±0.15 eV.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 925–927, April, 1990.     

Ab initio study of hydrogen bonding interaction and photoinduced electron transfer between 4‐nitroquinoline‐1‐oxide and tryptophan

In the present article, the electronic structure of 4‐nitroquinoline‐1‐oxide (4NQO) has been theoretically investigated at the level of HFSCF/6‐31G**. Conformation of monomeric 4NQO has been found to be the most stable when the nitro group twists from the quinoline plane by about 24°. The absorption spectra of the twisted 4NQO have been calculated by using complete active space self‐consistent field method. The hydrogen bonded complexes of 4NQO and tryptophan have been intensively studied. The absorption spectra of the most stable complex have been calculated to investigate the photoinduced electron transfer between 4NQO and tryptophan. The first and second excited singlet states are found to be charge‐separated. Population analysis shows that the low‐lying triplet states are charge‐separated as well, except for the lowest one that presents the characters of a locally excited state. A solvent effect on transition energies has been considered based on the Ooshika–Lippert–Martaga equation. Theoretical study shows that the optical excitation can lead to charge separation in the 4NQO–tryptophan complex. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Full six-dimensional nonadiabatic quantum dynamics calculation for the energy pooling reaction O(2)(a (1)Delta)+O(2)(a (1)Delta)-->O(2)(b (1)Sigma)+O(2)(X (3)Sigma)

Six new potential energy surfaces of four singlet states and two triplet states for the title oxygen molecule reaction along with the spin-orbit coupling among them have been constructed from the complete active space second-order perturbation theory with a 6-311+G(d) basis. Accurate integral cross sections are calculated with a full six-dimensional nonadiabatic time-dependent quantum wave packet method. The thermal rate constant based on the integral cross sections agrees well with the result of the experimental measurements, and the intersystem crossing effects are also discussed in this electronic energy-transfer process.     

Rhenium(I) tricarbonyl polypyridine complexes showing strong absorption of visible light and long-lived triplet excited states as a triplet photosensitizer for triplet-triplet annihilation upconversion

The preparation of rhenium(I) tricarbonyl polypyridine complexes that show a strong absorption of visible light and long-lived triplet excited state and the application of these complexes as triplet photosensitizers for triplet-triplet annihilation (TTA) based upconversion are reported. Imidazole-fused phenanthroline was used as the N^N coordination ligand, on which different aryl groups were attached (Phenyl, Re-0; Coumarin, Re-1 and naphthyl, Re-2). Re-1 shows strong absorption of visible light (ε = 60,800 M(-1) cm(-1) at 473 nm). Both Re-1 and Re-2 show long-lived T(1) states (lifetime, τ(T), is up to 86.0 μs and 64.0 μs, respectively). These properties are in contrast to the weak absorption of visible light and short-lived triplet excited states of the normal rhenium(I) tricarbonyl polypyridine complexes, such as Re-0 (ε = 5100 M(-1) cm(-1) at 439 nm, τ(T) = 2.2 μs). The photophysical properties of the complexes were fully studied with steady state and time-resolved absorption and emission spectroscopes, as well as DFT calculations. The intra-ligand triplet excited state is proposed to be responsible for the exceptionally long-lived T(1) states of Re-1 and Re-2. The Re(I) complexes were used as triplet photosensitizers for TTA based upconversion and an upconversion quantum yield up to 17.0% was observed.     

Ground- and excited-state properties of the mixed-valence complex [(NH3)5Ru(III)NCRu(II)(CN)5]-

A computational study of the ground- and excited-state properties of the mixed-valence complex [(NH 3) 5Ru (III)NCRu (II)(CN) 5] (-) is presented. Employing DFT and TDDFT calculations for the complex in the gas phase and in aqueous solution, we investigate the vibrational and electronic structure of the complex in the electronic ground state as well as the character of the electronically excited states. The relevance of the various excited states for the intervalence metal-metal charge-transfer process in the complex is analyzed based on the change of charge density, spin density, and dipole moment upon photoexcitation as well as by a Mulliken-Hush analysis. Furthermore, those intramolecular modes, which are important for the charge-transfer process, are identified and characterized.     

Ultraviolet resonance Raman spectroscopy of folded and unfolded states of an integral membrane protein

The vibrational structure of native anchoring tryptophan (Trp) and tyrosine residues in an integral membrane protein, bacterial outer membrane protein A (OmpA), have been investigated using UV resonance Raman (UVRR) spectroscopy for the first time. Spectra of native OmpA, a single-Trp mutant, and a Trp-less mutant were recorded in folded and unfolded states, and reveal significant changes in tryptophan structure and local environment. Salient alterations upon folding include loss of hydrogen-bonding character of indole N1H, evidenced by a shift in W17 frequency from 874 and 878 cm(-1), and growth in hydrophobicity of the local tryptophan environment, supported by increase in the ratio I1361/I1340. In addition to these site-specific changes in a single tryptophan residue, modification of the vibrational structure of the remaining native tryptophan and tyrosine amino acids is also evident. Finally, the UVRR data presented here indicate that the structures of OmpA folded in vesicle and folded in detergent may differ, and provide important foundations for ongoing studies of membrane protein folding.     

Singlet excited-state dynamics of nitropolycyclic aromatic hydrocarbons: direct measurements by femtosecond fluorescence up-conversion

Understanding the dynamics of the electronically excited states of nitrated polycyclic aromatic hydrocarbons (NPAHs) is of great importance since photochemical reactions determine the atmospheric stability of these toxic pollutants. From previous studies, it is known that electronically excited NPAHs evolve through two parallel pathways: The formation of the first triplet state and the dissociation of nitrogen (II) oxide. In this contribution, we present the first time-resolved emission measurements of the singlet excited states which are the precursors in the aforementioned photoprocesses. We analyzed 1-nitronaphthalene, 9-nitroanthracene, 1-nitropyrene, 6-nitrochrysene, and 3-nitrofluoranthene in solution samples. Although these compounds are considered nonfluorescent, with the frequency up-conversion method it was possible to detect the emission from the S1 states despite their femtosecond and picosecond lifetimes. Except for 1-nitronapthalene, where a single exponential is observed, for the rest of the compounds, the emission shows double-exponential decays indicating ultrafast structural changes in the excited states. From anisotropy measurements, we conclude that no significant internal conversion occurs in the singlet manifold after excitation in the first absorption band. In accord with El-Sayed rules and with previous calculations, the highly efficient intersystem crossing implied by the large triplet yields and the ultrafast S1 decays is accounted by the pi-pi* nature of the S1 and T1 states together with the existence of higher triplet configurations which act as receiver states. Our measurements show that NPAHs have the largest intersystem crossing rates observed to date in an organic molecule.     

Effect of the parent ligand on the photophysical properties of closely-coupled, binuclear ruthenium(II) tris(2,2'-bipyridine) complexes

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) tris(2,2'-bipyridine) complexes via an alkynylene group differ significantly from those of the relevant mononuclear complex. In particular, the energy of the first triplet excited state is lowered relative to the parent complex, because of the presence of the alkynylene substituent, while the triplet lifetime is prolonged, in part, because of extended electron delocalization. We now report that the triplet lifetime is also affected by the nature of the spectator 2,2'-bipyridyl ligands. Thus, replacing the parent 2,2'-bipyridine ligands with the corresponding 4,4'-dinitro-substituted ligands serves to decrease the luminescence yield and lifetime. With the corresponding carboxylate ester, the luminescence yield and lifetime are increased. Perdeuteration of the parent 2,2'-bipyridine ligands also leads to a modest increase in the luminescence yield. Such observations are indicative of electronic coupling between the various metal-to-ligand, charge-transfer excited triplet states. Temperature dependence studies confirm that these excited states are closely spaced and thermally accessible at ambient temperature. For some of the binuclear complexes, the quantum yield for formation of the lowest-energy triplet state is significantly less than unity.