Photophysical Pathways Research Articles

Heme Photolysis Occurs by Ultrafast Excited State Metal-to-Ring Charge Transfer

Ultrafast time-resolved resonance Raman spectra of carbonmonoxy hemoglobin (Hb), nitroxy Hb, and deoxy Hb are compared to determine excited state decay mechanisms for both ligated and unligated hemes. Transient absorption and Raman data provide evidence for a sequential photophysical relaxation pathway common to both ligated and unligated forms of Hb* (photolyzed heme), in which the excited state 1Q decays sequentially: Q 1 → H b I ∗ → H b II ∗ → Hb Hb ground state. Consistent with the observed kinetics, the lifetimes of these states are <50 fs, ≈300 fs, and ≈3 ps for 1Q, H b I ∗ and H b II ∗ , respectively. The transient absorption data support the hypothesis that the H b I ∗ state results from an ultrafast iron-to-porphyrin ring charge transfer process. The H b II ∗ state arises from porphyrin ring-to-iron back charge transfer to produce a porphyrin ground state configuration a nonequilibrium iron d-orbital population. Equatorial d– π* back-bonding of the heme iron to the porphyrin during the lifetime of the H b II ∗ state accounts for the time-resolved resonance Raman shifts on the ≈3 ps time scale. The proposed photophysical pathway suggests that iron-to-ring charge transfer is the key event in the mechanism of photolysis of diatomic ligands following a porphyrin ring π− π* transition.

Wavelength-Dependent Photochemistry of W(CO)4(en) (en = Ethylenediamine): Evidence for Distinct Chemical Reactivities from Singlet and Triplet Ligand Field Excited States

Quantum yields for ligand substitution of CO in W(CO)4(en) are strongly dependent on the excitation wavelength in the 313−476-nm region. The results illustrate that the ligand field (LF) excited states undergo photosubstitution via separate photophysical pathways and that reaction from the lowest lying LF triplet level occurs with a greatly reduced photoefficiency.

Role of thylakoid lipids in the structural flexibility of lamellar aggregates of the isolated light-harvesting chlorophyll a/b complex of photosystem II.

We studied the role of added thylakoid lipids in the light-induced reversible structural changes in isolated macroaggregates of the main light-harvesting chlorophyll a/b complex of photosystem II (LHCII). Loosely stacked lamellar macroaggregates were earlier shown to undergo light-induced reversible structural changes and changes in the photophysical pathways, which resembled those in thylakoid membranes exposed to excess light [Barzda, V., et al. (1996) Biochemistry 35, 8981-8985]. This structural flexibility of LHCII depends critically on the lipid content of the preparations [Simidjiev, I., et al. (1997) Anal. Biochem. 250, 169-175]. It is now reported that lamellar aggregates of LHCII are capable of incorporating substantial amounts of different thylakoid lipids. The long-range order of the chromophores is retained, while the ultrastructure of the lipid-protein macroaggregates can be modified significantly. Addition of thylakoid lipids to the preparations significantly enhances the ability of the LHCII macroaggregates to undergo light-induced structural changes. The lipid environment of the LHCII complexes therefore plays a significant role in determining the structural flexibility of the macroaggregates. As concerns the mechanism of these changes, it is proposed that the absorption of light and the dissipation of its energy in the macrodomains induces thermal fluctuations which bring about changes in the shape or in the stacking interactions of the membranes, this in turn affecting the long-range order of the embedded chromophores. In thylakoids, a similar mechanism is likely to explain the light-induced structural changes which are largely independent of the photochemical activity of the membranes.

Ultrafast Study of the Photodissociation and Recombination of Aqueous O3-

The photodissociation of the strongly solvated radical O3- (generating O2 and O-) has been studied in aqueous solution by femtosecond pump−probe spectroscopy. The near-UV absorption band of O3- exhibits a prompt bleach due to photolysis (390 nm excitation), which is followed by a partial recovery (3.5 ps) that is assigned to delayed geminate recombination. The transient spectrum of O3- shows no evidence of excess vibrational energy content, indicating that vibrational relaxation of O3- occurs on a faster time scale than the recombination process (3.5 ps). This overall picture is in strong contrast to the standard prototype for solution recombination reactions, i.e. the widely investigated photodissociation of I2 in nonpolar solvents. For I2, photodissociation and prompt recombination (∼0.3 ps) lead to hot I2 molecules that relax on the tens of picoseconds time scale. The extremely fast vibrational relaxation and the slow recombination of O3- in water apparently stem from strong solute−solvent interactions for both O3- and O-. The O3- behavior may in fact be a common situation for polar reactions in polar solvents. In these studies, O3- was generated via a known reaction by photolysis of an oxygenated basic solution of hydrogen peroxide. The quantum yield of cage escape for O3- photodissociation has been measured to be 0.50 ± 0.02, assuming that the long-lived bleach is due entirely to cage escape. The long-lived bleach may, however, also be due to other photophysical pathways that involve long-lived, low-lying O3- excited states. Both the apparent cage escape yield and recombination dynamics are identical within experimental error in H2O and D2O. The rotational reorientation time of O3- has been measured to be 2.3 ps.

Structural flexibility of chiral macroaggregates of light-harvesting chlorophyll a/b pigment-protein complexes. Light-induced reversible structural changes associated with energy dissipation.

In this paper, we show that stacked lamellar aggregates of the purified chlorophyll a/b light-harvesting antenna complexes (LHCII) and granal thylakoid membranes are capable of undergoing light-induced reversible changes in the chiral macroorganization of the chromophores as well as in the photophysical pathways. In granal thylakoids, the light-induced reversible structural changes, detected by circular dichroism (CD) measurements, are accompanied by reversible changes in the fluorescence yield that indicate an increased dissipation of the excitation energy. These changes become gradually more significant in excess light compared to nonsaturating light intensities, and can be eliminated by suspending the membranes in hypotonic, low-salt medium in which the chiral macroaggregates are absent. In lamellar aggregates of LHCII, the light-induced reversible changes of the main, nonexcitonic CD bands are also accompanied by reversible changes in the fluorescence yield. In small aggregates and trimers, no light-induced delta CD occurs, and the fluorescence changes are largely irreversible. It is proposed that the structural changes are induced by thermal effects due to the excess light energy absorbed by the pigments. Our data strongly suggest that the structure and function of the antenna system of chloroplasts can be regulated by the absorption of excess light energy with a mechanism independent of the operation of the photochemical apparatus.

Photophysical and photochemical behaviour of (( N,N-dimethylamino)methyl)styrenes

The photophysics and photochemistry of three different (( N,N-dimethylamino)methyl)styrenes, in which the methylene bridge is attached to different positions of the styrene moiety, have been investigated. Monomer fluorescence decay times are shorter than experimental time resolution (approximately 30 ps). Exciplex properties, e.g. fluorescence yields, are strongly dependent on the substitution pattern. In contrast with intermolecular exciplexes between corresponding methylstyrenes and triethylamine, the exciplex decay time increase with increasing solvent polarity. In addition to the intramolecular photophysical relaxation pathways, photochemical decomposition of the compounds was observed; a major product is the corresponding methylstyrene.

Photophysics of ruthenium(II)-diimine complexes in water-acetonitrile mixed solvents

The photophysical properties of Ru(bpy)[sub 3][sup 2+], Ru(bpz)[sub 3][sup 2+], and Ru(bpz)[sub 2](bpm)[sup 2+] (bpy = 2,2[prime]-bipyridine, bpz = 2,2[prime]-bipyrazine, bpm = 2,2[prime]-bipyrimidine) have been studied in neat and mixed CH[sub 3]CN-H[sub 2]O solutions. The temperature dependent lifetimes, room-temperature emission spectra, and quantum yields were determined for the complexes; the rate constants and activation parameters for the various photophysical pathways were calculated from the data. Shifts of the emission energy (E[sub cm]) as a function of solvent composition suggest that the excited states are preferentially solvated by water molecules. Although radiative decay is relatively insensitive to the change of solution medium, nonradiative decay (k[sub nr]) and the temperature-dependent thermal population of the metal centered d-d state, which account for more than 90% of the decay of the emitting MLCT state, are strongly dependent on solvent and temperature; the observed phenomena can be well understood on the basis that the energy level of the MLCT state, which possesses a static dipole moment, is sensitive to the polarity change of the solution medium. At lower temperatures, nonradiative decay is the dominant process, and the overall deactivation of the emitting state is faster in water-rich than in acetonitrile-rich solutions. 17 refs., 3 figs.,more » 1 tab.« less

Photochemistry of cyclodextrin host-guest complexes

Cyclodextrins, when used as molecular receptors, can induce significant modifications of the photophysical and photochemical deactivation pathways of aromatic molecules. On the basis of examples, taken from the authors' work, the factors determining the behaviour of a photoexcited guest molecule in a cyclodextrin cavity are discussed with particular attention to the role of cydodextrin as constraint to molecular degrees of freedom and as tool for controlling the evolution of reaction intermediates to final photoproducts.

Ab initio investigation of potential-energy surfaces involved in the photophysics of benzene and pyrazine

Potential-energy surfaces of the lowest singlet and triplet excited states of benzene and pyrazine have been calculated using complete-active-space self-consistent-field and multireference configuration interaction (MRCI) techniques. We have focused our attention on the saddle points and surface intersections associated with the reaction path to a biradical form called prefulvene. The barrier heights separating the prefulvenic minimum from the minimum of the planar aromatic form on the ππ* excited singlet surface and on the ground-state surface have been estimated by large-scale MRCI calculations. The conical intersection of the lowest ππ* excited singlet surface with the S0 surface has been mapped out in two dimensions, the reaction coordinate to prefulvene and the coordinate of maximum coupling perpendicular to it. The relevance of these ab initio potential-energy data for the interpretation of photophysical relaxation pathways in benzene and pyrazine (‘‘channel-three’’ effect) is discussed.

Kinetic spectroscopy of pyrazolotriazole azomethine dyes

The photophysical properties of pyrazolotriazole azomethine dyes have been investigated using both picosecond and nanosecond flash photolysis. On nanosecond timescales, prompt formation of a photoisomer is observed, the rate of decay of which shows a solvent dependence. In the presence of a triplet sensitiser a triplet pathway to the photoisomer has been established, the yield of isomer from the triplet state being considerably higher than from direct excitation. On picosecond timescales, two transients are observed; the first has a very short, solvent-independent lifetime, while the second has a longer solvent-dependent lifetime. These two transients are assigned as states formed during the relaxation of the molecules along the first excited singlet and the ground-state potential-energy surfaces, respectively. Similar kinetic behaviour is observed in a high concentration, high viscosity environment designed to mimic the photographic emulsion, indicating that the photophysical relaxation pathways are still very rapid even in this type of environment.

Photophysical pathways in metal complexes

In this article, the author focuses on the photophysical relaxation pathway for decay of excited states, pointing out several important differences between inorganic and organic systems. Applicatio...

Emission properties of a polymer and model containing the 4-alkoxycarbonyl-4′,4′'-dibromotriphenylamine unit

The photophysical deactivation pathways for a polymer and model containing the 4-alkoxycarbonyl-4′,4′'-dibromotriphenylamine chromophore were investigated. Optical absorption, fluorescence emission, phosphorescence, and transient absorption spectra were obtained in frozen glass, and in non-polar (toluene) and in polar (DMF and acetonitrile) solvents. The possibility of twisted internal charge-transfer excited states and the influence of polymer chain effects on the photophysical properties of the model and polymer were examined.

Microemulsions, micelles, and vesicles as media for membrane mimetic photochemistry

Microemulsions, micelles, and vesicles are compared as media for membrane mimetic photochemistry. These systems solubilize, concentrate, compartmentalize, organize, and localize reactants; maintain proton and/or reactant gradients; alter quantum efficiencies; lower ionization potentials; change oxidation and reduction properties; change dissociation constants; affect vectorial electron displacements; alter photophysical pathways and rates; alter chemical pathways and rates; stabilize reactants, intermediates, and products; and separate products (charges). Formation of structures of microemulsions, micelles, and vesicles as well as substrate solubilization therein are summarized. Attention is focused on the utilization of microemulsions as reaction media. 72 references.

The effects of temperature and pressure on the lifetime of benzophenone emission

The variation in the lifetime of flash-excited gaseous benzophenone with pressure and temperature indicates that (1) self-quenching is a relatively inefficient process for the long-lived emission, k sq = 9 × 10 5 M −1 s −1 (estimated from solution data) at 25°C and 1.2 × 10 7 M −1 s −1 at 170°C and (2) the lifetime decreases with increasing temperature as a result of photochemical and photophysical decay pathways which have significant activation energies. The importance of diffusion to the walls on lifetime measurements is discussed.