PICOSECOND FLUORESCENCE KINETICS and ENERGY TRANSFER IN THE ANTENNA CHLOROPHYLLS OF GREEN ALGAE*

Single-photon timing measurements on flowing samples of Chlorella vulgaris and Chlamydomonas reinhardtii at low excitation intensities at room temperature indicate two main kinetic components of the fluorescence at open reaction centers (F₀) of photosystem II with lifetimes of approx. 130 and 500 ps and relative yields of about 30 and 70%. Closing the reaction centers progressively by preincubation of the algae with increasing concentrations of 3-(3′,4′-dichlorophenyl)-l,l-dimethylurea (DCMU) and hydroxylamine gave rise to a slow component with a lifetime increasing from 1.4 to 2.2 ns (F_max) The yield of the slow component increased to 65-68% of the total fluorescence yield in parallel to a decrease in the yield of the fast component to a value close to zero at the f_max-level. The 130 ps lifetime of the fast component remained unchanged. The middle component showed an increase of its lifetime from 500 to 1100 ps and of its yield by a factor of 1.5. Spacing of the ps laser pulses by 12 μs allowed us to resolve a new long-lived fluorescence component of very small amplitude which is ascribed to a small amount of chlorophyll not connected to functional antennae. The opposite dependence of the yield of the fast and the slow component on the state of the reaction centers at almost constant lifetimes is consistent with a mechanism of energy conversion in largely separately functioning photosystem II units. Yields and lifetimes of these two components are in agreement with the high quantum yield of photosynthesis. The lower lifetime limit of 1.4 ns of the slow component is assigned to the average transfer time of an excited state from a closed to a neighboring open reaction center and the increase in the lifetime to 2.2 ns is evidence for a limited energy transfer between photosystems II. Relative effects of changing the excitation wavelength from 630 to 652 nm on the relative fluorescence yields of the kinetic components were studied at the fluorescence wavelengths 682, 703 and 730 nm. Our data indicate that (i) the middle component has its fluorescence maximum at shorter wavelength than the fast component and (ii) that the antennae chlorophylls giving rise to the middle component are preferentially excited by 652 nm light. It is concluded that the middle component originates from the light-harvesting chlorophyll alb protein complexes and the major portion of the fast component from the chlorophyll a antennae of open photosystem II reaction centers.     

Ultrafast electrocyclic ring opening of 7-dehydrocholesterol in solution: the influence of solvent on excited state dynamics

Broadband UV-visible femtosecond transient absorption spectroscopy and steady-state integrated fluorescence were used to study the excited state dynamics of 7-dehydrocholesterol (provitamin D(3), DHC) in solution following excitation at 266 nm. The major results from these experiments are: (1) The excited state absorption spectrum is broad and structureless spanning the visible from 400 to 800 nm. (2) The state responsible for the excited state absorption is the initially excited state. Fluorescence from this state has a quantum yield of ～2.5 × 10(-4) in room temperature solution. (3) The decay of the excited state absorption is biexponential, with a fast component of ～0.4-0.65 ps and a slow component 1.0-1.8 ps depending on the solvent. The spectral profiles of the two components are similar, with the fast component redshifted with respect to the slow component. The relative amplitudes of the fast and slow components are influenced by the solvent. These data are discussed in the context of sequential and parallel models for the excited state internal conversion from the optically excited 1(1)B state. Although both models are possible, the more likely explanation is fast bifurcation between two excited state geometries leading to parallel decay channels. The relative yield of each conformation is dependent on details of the potential energy surface. Models for the temperature dependence of the excited state decay yield an intrinsic activation barrier of ～2 kJ/mol for internal conversion and ring opening. This model for the excited state behavior of DHC suggests new experiments to further understand the photochemistry and perhaps control the excited state pathways with optical pulse shaping.     

Fast and slow fluorescence emissions from pyrimidine vapor

The pressure and excitation-energy dependence of the fluorescence quantum yield and lifetime of pyrimidine vapor has been investigated in the pressure range 10^?3-10 torr. The results indicate that in conformity to the intermediate case the fluorescence of the isolated pyrimidine molecule consists of fast and slow components with lifetimes of the order of 1 ns and 10 μs, respectively. The total fluorescence quantum yield amounts to as high as 0.045. The yield of the slow fluorescence component decreases significantly with increasing excitation energy; this observation is interpreted as being due mainly to the lengthening of the radiative lifetime of that component.     

Time-resolved fluorescence study of the single tryptophan in thiocyanate and azide derivatives of horseradish peroxidase: Implication for a p H-induced conformational change in the heme cavity

Detailed pH-dependent steady state and picosecond time-resolved tryptophan fluorescence studies on thiocyanate and azide complexes of horseradish peroxidase have been carried out. The fluorescence decay of the single tryptophan in these species was fitted to a discrete three exponential model. Maximum entropy method analysis also gave three distinct regions of lifetime distributions. The fast subnanosecond lifetime component was found to have > 97% amplitude contribution while other two longer lifetime components have small contributions. Small contributions from the nanosecond lifetime components possibly arise from apoprotein impurity or some small amount of disordered heme conformer of the protein. pH dependence of the fast picosecond lifetime components was found to show a systematic behavior which has been interpreted in the light of obligatory conformation change associated with activation of the enzyme at low pH.     

Multicoordinational excited state twisting of indan-1,3-dione derivatives

Excited state relaxation of indan-1,3-dione derivatives with different substituents attached to the phenyl ring and with the bridged amino group was investigated by means of the steady-state fluorescence and femtosecond time-resolved absorption pump–probe spectroscopy. Bridging of the amino group increases the fluorescence quantum yield and the excited state lifetime. Analysis of the results indicates that the phenyl ring twisting around a single central bond leads to the nonradiative state formation and to subsequent fast relaxation to the ground state. Double bond twisting takes place in molecules with the bridged amino group and causes a large Stokes shift and slightly slower excited state relaxation.     

Intra- and intermolecular electronic relaxation of the second excited singlet and the lowest excited triplet states of 1,3-dimethyl-4-thiouracil in solution

Intramolecular processes of deactivation of 1,3-dimethyl-4-thiouracil (DMTU) from the second excited singlet (S2) (pi, pi*) and the lowest excited triplet (T1) (pi, pi*) states have been studied using perfluoro-1,3-dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (tau(S2)) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of tau(S2) in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern-Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (tau0(T1)) and rate constants of self-quenching (k(sq)). The lifetimes, tau0(T1), of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group.     

New observations on the luminescence decay lifetime of Mn2+ in Zns: Mn2+ nanoparticles

A fast decay emission peaking at 645 nm with a decay lifetime within the experimental resolution of 0.14 micros is observed in ZnS:Mn2+ nanoparticles. This short-lived signal is also observed in pure ZnS and MgS: Eu3+ nanoparticles, which has nothing to do with Mn(2+)-doped ions but is from the deep trap states of the host materials. The short-lived component decreases in intensity relative to the Mn2+ emission at higher excitation powers, while it increases in intensity at low temperatures and shifts to longer wavelengths at longer time delays. Our observations demonstrated further that the emission of Mn2+ in ZnS: Mn2+ nanoparticles behaves basically the same as in bulk ZnS: Mn2+; the fast decay component is actually from the intrinsic and defect-related emission in sulfide compounds.     

Luminescence of Ce3+ in Y2SiO5 nanocrystals: Role of crystal structure and crystal size

Here, we report the role of crystal structure and crystal size on the photoluminescence properties of Ce3+ ions in Y2SiO5 nanocrystals. The emission at 430 nm (5d1 --> 4f1) and lifetime of the excited state of Ce3+ ion doped Y2SiO5 nanocrystals are found to be sensitive to the crystal structure, crystal size, and dopant concentration. It is found that the overall lifetime tau of 0.5 mol % Ce doped Y2SiO5 nanocrystals are 8.78 and 3.45 ns for 1000 and 1100 degrees C heat-treated samples with the same crystal structure (X1-Y2SiO5 phase), respectively. However, a significant increase in the overall lifetime (35.21 ns) is observed for the 1300 degrees C annealed 0.5 mol % Ce doped Y2SiO5 sample having a different crystal structure (X2-Y2SiO5 phase). We found that the decay kinetic is biexponential. It is explained that the fast component arises due to sequential hole-electron capture on the luminescent ions and the slow component arises from isolated ions. Our analysis suggests that modifications of radiative and nonraditive relaxation mechanisms are due to local symmetry structure of the host lattice and crystal size, respectively.     

Ultrafast study of p-biphenylyldiazoethane. The chemistry of the diazo excited state and the relaxed carbene

Ultrafast photolysis of p-biphenylyldiazoethane (BDE) produces an excited state of the diazo compound in acetonitrile, cyclohexane, and methanol with lambdamax = 490 nm and lifetimes of less than 300 fs. The decay of the diazo excited state correlates with the growth of singlet carbene absorption at 360 nm. The optical yields of diazo excited states produced by photolysis of p-biphenylyldiazomethane (BDM) and BDE are the same; however, the optical yield of singlet p-biphenylylmethylcarbene (1BpCMe) is 30-40% less than that of p-biphenylylcarbene (1BpCH) in all three solvents. The results are explained by rearrangement in the excited state (RIES) of BDE to form p-vinylbiphenyl (VB) in parallel with extrusion of nitrogen to form 1BpCMe in reduced yield. This interpretation is consistent with product studies (ethanol-OD in cyclohexane) which indicate that there is an approximately 25% yield of VB that is formed by a mechanism that bypasses the relaxed singlet carbene. The decay of 1BpCMe is biexponential, and that of 1BpCH is monoexponential. This is attributed either to efficient relaxation of vibrationally excited 1BpCMe by 1,2 migration of hydrogen to form VB (minor) or to the increased number of low-frequency vibrational modes provided by the methyl group (major). A methyl group retards the rate of intersystem crossing (ISC), relative to a hydrogen atom, and ISC is more rapid in nonpolar solvents. Reaction of 1BpCMe with methanol is much faster than spin equilibration. Both the lifetime of 1BpCMe and 1BpCH are the same in cyclohexane and in cyclohexane-d12. This demonstrates that spin equilibration is faster than reaction of either carbene with the solvent. The lifetimes of 1BpCMe and 1BpCMe-d3 are the same in cyclohexane. This indicates that 1,2 hydrogen migration of 1BpCMe to form VB is slower than spin equilibration in cyclohexane. In acetonitrile, however, the lifetime of 1BpCMe-d3 is 1.5 times longer than that of 1BpCMe in the same solvent. Thus, in acetonitrile, where ISC is slow, the rate of 1,2 hydrogen shift of 1BpCMe is competitive with ISC. In cyclohexene, the lifetime of 1BpCH is shortened relative to that in cyclohexane. The lifetime of 1BpCMe is the same in cyclohexene and cyclohexane. The data indicate that spin relaxation is slow relative to reaction of 1BpCH with neat alkene but that spin relaxation is fast for 1BpCMe relative to reaction with neat cyclohexene.     

Ultrafast fluorescence detection in tris(2,2'-bipyridine)ruthenium(II) complex in solution: relaxation dynamics involving higher excited states

The excited-state dynamics of a transition metal complex, tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)(3)](2+), has been investigated using femtosecond fluorescence upconversion spectroscopy. The relaxation dynamics in these molecules is of great importance in understanding the various ultrafast processes related to interfacial electron transfer, especially in semiconductor nanoparticles. Despite several experimental and theoretical efforts, direct observation of a Franck-Condon singlet excited state in this molecule was missing. In this study, emission from the Franck-Condon excited singlet state of [Ru(bpy)(3)](2+) has been observed for the first time, and its lifetime has been estimated to be 40 +/- 15 fs. Biexponential decays with a fast rise component observed at longer wavelengths indicated the existence of more than one emitting state in the system. From a detailed data analysis, it has been proposed that, on excitation at 410 nm, crossover from higher excited (1)(MLCT) states to the vibrationally hot triplet manifold occurs with an intersystem crossing time constant of 40 +/- 15 fs. Mixing of the higher levels in the triplet state with the singlet state due to strong spin-orbit coupling is proposed. This enhances the radiative rate constant, k(r), of the vibrationally hot states within the triplet manifold, facilitating the upconversion of the emitted photons. The vibrationally excited triplet, which is emissive, undergoes vibrational cooling with a decay time in the range of 0.56-1.3 ps and relaxes to the long-lived triplet state. The results on the relaxation dynamics of the higher excited states in [Ru(bpy)(3)](2+) are valuable in explaining the role of nonequilibrated higher excited sensitizer states of transition metal complexes in the electron injection and other ultrafast processes.     

A seemingly well understood light-induced peroxide decarboxylation reaction reinvestigated with femtosecond time resolution

The photoinduced (266 nm) ultrafast decarboxylation of the peroxyester tert-butyl 9-methylfluorene-9-percarboxylate (TBFC) in solution has been studied with femtosecond time resolution. While the photodissociation of TBFC occurs too fast to be resolved, the intermediate 9-methylfluorenylcarbonyloxy radical (MeFl-CO(2)) decarboxylates on a picosecond time scale. The latter process is monitored by pump-probe absorption spectroscopy at wavelengths between 400 and 883 nm. The measured transient absorbance signals reveal a dominant fast decay with a lifetime of a few picoseconds and, to a minor extent, a slow component with a lifetime of about 55 ps. Statistical modeling of MeFl-CO(2) decarboxylation employing molecular parameters calculated by density functional theory suggests that the fast component is associated with the decarboxylation of vibrationally hot radicals, whereas the 55 ps decay reflects the dissociation of thermally equilibrated MeFl-CO(2) at ambient temperature. The vast majority of MeFl-CO(2) radicals thus decarboxylate on a time scale about an order of magnitude faster than expected from the time constant of 55 ps reported by Falvey and Schuster for this reference reaction. This literature value turns out to refer to decarboxylation rate of MeFl-CO(2) at ambient temperature.     

Translational energy distribution of the excited hydrogen atom produced by controlled electron impact on HCl

The translational energy distribution of an atom can be calculated by differentiating the Doppler line shape of its emission line taken at a high optical resolution. The Balmer-β line of the excited hydrogen atom (n = 4) produced by electron impact on HCl has been measured at a high resolution (0.033Å) and at two angles (55° and 90°) with respect to the electron beam. The translation energy distribution depends on the electron energies and has almost two groups of components: ≈ 5 eV (fast) and ≈ eV (slow). Anisotropy is imporant for the slow component. The excitation function shows the corresponding structures. It is concluded that Rydberg states converging to the ²Π state of HCl⁺ produce the fast component and Rydberg states converging to the repulsive HCl⁺ states which cross the ²Σ⁺ state produce the slow component.     

Positronium formation in NaY-zeolites studied by lifetime, positron beam Doppler broadening and 3-gamma detection techniques

Results of positron annihilation measurements on NaY pressed powders and deposited thin films using slow positron beam and conventional fast positron techniques are presented. In lifetime experiments using an external ²²Na source an averaged long lifetime of 1.8 ns with a sum intensity of 27% was observed in pressed powders in the presence of air at room temperature (RT). In literature this lifetime is ascribed to positrons annihilating in water filled or β cages Habrowska, A.M., Popiel, E.S., 1987. Positron annihilation in zeolite 13X. J. Appl. Phys. 62, 2419. By means of isotopic exchange some of the Na was replaced by ²²Na. These powders showed a long lifetime component of 7–8 ns with an intensity increasing from 1 to 12% when heated under normal atmosphere from RT to 200°C. No significant increase of the shorter (1.5 ns) lifetime was observed, while its intensity dropped from 13.4 to 6.6%. Both effects are ascribed to the loss of water from cages only. The beam experiments revealed a high fraction of 3-gamma annihilations in the pressed powder and thin film samples, indicating the annihilation of o-Ps and thereby the existence of large open volumes.     

Ultrafast internal conversion in ethylene. I. The excited state lifetime

Using a combined theoretical and experimental approach, we investigate the non-adiabatic dynamics of the prototypical ethylene (C(2)H(4)) molecule upon π → π? excitation. In this first part of a two part series, we focus on the lifetime of the excited electronic state. The femtosecond time-resolved photoelectron spectrum (TRPES) of ethylene is simulated based on our recent molecular dynamics simulation using the ab initio multiple spawning method with multi-state second order perturbation theory [H. Tao, B. G. Levine, and T. J. Martinez, J. Phys. Chem. A 113, 13656 (2009)]. We find excellent agreement between the TRPES calculation and the photoion signal observed in a pump-probe experiment using femtosecond vacuum ultraviolet (hν = 7.7 eV) pulses for both pump and probe. These results explain the apparent discrepancy over the excited state lifetime between theory and experiment that has existed for ten years, with experiments [e.g., P. Farmanara, V. Stert, and W. Radloff, Chem. Phys. Lett. 288, 518 (1998) and K. Kosma, S. A. Trushin, W. Fuss, and W. E. Schmid, J. Phys. Chem. A 112, 7514 (2008)] reporting much shorter lifetimes than predicted by theory. Investigation of the TRPES indicates that the fast decay of the photoion yield originates from both energetic and electronic factors, with the energetic factor playing a larger role in shaping the signal.     

Aggregation-Induced Dual Phosphorescence from (o-Bromophenyl)-Bis(2,6-Dimethylphenyl)Borane at Room Temperature

Designing highly efficient purely organic phosphors at room temperature remains a challenge because of fast non-radiative processes and slow intersystem crossing (ISC) rates. The majority of them emit only single component phosphorescence. Herein, we have prepared 3 isomers (o, m, p-bromophenyl)-bis(2,6-dimethylphenyl)boranes. Among the 3 isomers ( o -, m - and p - BrTAB ) synthesized, the ortho-one is the only one which shows dual phosphorescence, with a short lifetime of 0.8 ms and a long lifetime of 234 ms in the crystalline state at room temperature. Based on theoretical calculations and crystal structure analysis of o - BrTAB , the short lifetime component is ascribed to the T₁^M state of the monomer which emits the higher energy phosphorescence. The long-lived, lower energy phosphorescence emission is attributed to the T₁^A state of an aggregate, with multiple intermolecular interactions existing in crystalline o - BrTAB inhibiting nonradiative decay and stabilizing the triplet states efficiently.     

Time-resolved fluorescence spectra using time-correlated photon counting: Picosecond fluorescence in aqueous solutions of Cr(III) complexes at room temperature

A new procedure for measuring time-resolved emission spectra has been implemented. This technique has subnanosecond time resolution combined with the sensitivity and dynamic range needed to cope with extremely weak luminescence. Using this method the emissions of Cr(NH₃)₂ (NCS)₄^? and Cr(NCS)₆^3- in aqueous solution at room temperature have each been analyzed into two components. The fast component has a broad spectrum and is assigned to prompt fluorescence with lifetime below 100 ps. The slow component is dominated by phosphorescence but may include some delayed fluorescence. The phosphorescence lifetime is 5.5 ± 0.5 ns in Cr(NH₃)₂ (NCS)₄^? and 1.65 ± 0.1 ns in Cr(NCS)₆^3-. Order of magnitude estimates have been derived for other photophysical parameters.     

Photoreduction of Ru(bpy)(3)2+ by amines in aqueous solution. Kinetics characterization of a long-lived nonemitting excited state

The photochemistry of Ru(bpy)(3)+2 in the presence of amines was investigated in water by laser flash photolysis. N,N'-Dimethylaniline and p-phenylenediamine quench the luminescent metal to ligand charge transfer (MLCT) excited state of the complex by an electron transfer reaction that produces the semireduced form Ru(bpy)3+ in relatively high yields. On the other hand, triethylamine (TEA) and aniline do not quench the MLCT. Nevertheless, when laser flash irradiation at 532 nm is carried out in the presence of these amines, the formation of Ru(bpy)3+ is clearly detected by its transient absorption at 510 nm. These results are interpreted by an electron transfer reaction with the participation of a nonemitting excited state of the complex, formed independently of the MLCT from the Franck-Condon or the relaxed singlet excited state. The rate constants for the quenching of this state by TEA and aniline and the quantum yields for Ru(bpy)(3)+ were determined. The new state is formed in a very fast process and has a lifetime of ca 4 micros in water.     

Intermolecular electron transfer from naphthalene derivatives in the higher triplet excited states

Intermolecular electron transfer (ELT) from a series of naphthalene derivatives (NpD) in the higher triplet excited states (T(n)) to carbon tetrachloride (CCl(4)) in Ar-saturated acetonitrile was observed using the two-color two-laser flash photolysis method. The ELT efficiency depended on the driving force of ELT. Since the ELT from the T(n) state occurred competitively with the internal conversion (IC, T(n) --> T(1)) and the triplet energy transfer (ENT), the ELT became apparent only when sufficient free energy change of ELT was attained. On the other hand, ELT from the T(1) state was not observed, although ELT from the T(1) state with sufficiently long lifetime has a slightly exothermic driving force. The fast ELT from the T(n) state and lack of the reactivity of the T(1) state were explained well by the "sticky" dissociative electron-transfer model based on one-electron reductive attachment to CCl(4) leading to the C-Cl bond cleavage.     

Towards ruthenium(II) polypyridine complexes with prolonged and predetermined excited state lifetimes

The excited state lifetime of a Ru(bpy)3-motif is linearly related to the number of appended pyrenyl chromophores, but independent of connectivity; values for nine complexes range from 0.8 to 18.1 microseconds.     

Time-resolved spectroscopy of the excited singlet states of tirapazamine and desoxytirapazamine

Laser flash photolysis (LFP, 400 nm excitation) of the anti-cancer drug tirapazamine (TPZ) in acetonitrile produces the singlet excited-state S1 with lambda(max) = 544 nm. The lifetime of this state is 130 ps, in good agreement with the reported fluorescence lifetime. The excited state is reduced to the corresponding radical anion by KSCN or KI. The spectrum of the radical anion is in good agreement with previously reported pulse radiolysis studies and time-dependent density functional theory (TD-DFT) calculations. LFP of desoxytirapazamine (dTPZ) also produces the first excited singlet state, S1. The fluorescence quantum yield and lifetime (5.4 ns) of the dTPZ singlet excited state are both much greater than the corresponding values of TPZ. This is explained by DFT calculations that predict that cyclization of TPZ to form an oxaziridine is thermodynamically facile but that cyclization of dTPZ to form an oxadiaziridine is not. Thus, the S1 state of TPZ has a short lifetime and low fluorescence quantum yield due to ready cyclization whereas the cyclization of the S1 state of dTPZ is unimportant and does not limit either the fluorescence quantum yield or the fluorescence lifetime. This conclusion is confirmed by studies of dTPZ', an isomer of dTPZ containing the C=N-O moiety which has a low quantum yield and short fluorescence lifetime similar to that of TPZ.