Ultrafast Absorption Spectroscopy Research Articles

Ultrafast time-resolved absorption spectroscopy of geometric isomers of xanthophylls

This paper presents an ultrafast optical spectroscopic investigation of the excited state energies, lifetimes and spectra of specific geometric isomers of neoxanthin, violaxanthin, lutein, and zeaxanthin. All- trans- and 15,15′- cis-β-carotene were also examined. The spectroscopy was done on molecules purified by HPLC frozen immediately to inhibit isomerization. The spectra were taken at 77 K to maintain the configurations and to provide better spectral resolution than seen at room temperature. The kinetics reveal that for all of the molecules except neoxanthin, the S 1 state lifetime of the cis isomers is shorter than that of the all- trans isomers. The S 1 excited state energies of all the isomers were determined by recording S 1 → S 2 transient absorption spectra. The results obtained in this manner at cryogenic temperatures provide an unprecedented level of precision in the measurement of the S 1 energies of these xanthophylls, which are critical components in light-harvesting pigment-protein complexes of green plants.

Photophysics of an Intramolecular Hydrogen‐Evolving Ru–Pd Photocatalyst

Photoinduced electron-transfer processes within a precatalyst for intramolecular hydrogen evolution [(tbbpy)(2)Ru(tpphz)PdCl(2)](2+) (RuPd; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'c:3'',2'',-h:2''',3'''-j]phenazine) have been studied by resonance Raman and ultrafast time-resolved absorption spectroscopy. By comparing the photophysics of the [(tbbpy)(2)Ru(tpphz)](2+) subunit Ru with that of the supramolecular catalyst RuPd, the individual electron-transfer steps are assigned to kinetic components, and their dependence on solvent is discussed. The resonance Raman data reveal that the initial excitation of the molecular ensemble is spread over the terminal tbbpy and the tpphz ligands. The subsequent excited-state relaxation of both Ru and RuPd on the picosecond timescale involves formation of the phenazine-centered intraligand charge-transfer state, which in RuPd precedes formation of the Pd-reduced state. The photoreaction in the heterodinuclear supramolecular complex is completed on a subnanosecond timescale. Taken together, the data indicate that mechanistic investigations must focus on potential rate-determining steps other than electron transfer between the photoactive center and the Pd unit. Furthermore, structural variations should be directed towards increasing the directionality of electron transfer and the stability of the charge-separated states.

Stability and reaction dynamics of trifluorinated indolylfulgides

Quantum efficiencies and ultrafast dynamics of the ring-closure and ring-opening reaction of a trifluorinated dicyclopropyl indolylfulgide with improved photostability are investigated by stationary and ultrafast absorption spectroscopy. The ring-closure reaction occurs on the time scale of 200 fs and is found to be temperature independent ( T = 287–333 K). However, an activated behaviour is observed for the ring-opening reaction. A comparison with the corresponding non-substituted indolylfulgide shows that the dicyclopropyl group favours the open isomer via lower cyclisation and higher cycloreversion quantum efficiencies and faster dynamics of the ring-opening reaction.

Probing Ground‐state Hole Transfer Between Equivalent, Electrochemically Inaccessible States in Multiporphyrin Arrays Using Time‐resolved Optical Spectroscopy

A new strategy is described and implemented for determining the rates of hole-transfer between equivalent porphyrins in multiporphyrin architectures. The approach allows access to these rates between sites that are not the most easily oxidized components of the array. The specific architectures investigated with this new strategy are triads consisting of one zinc porphyrin (Zn) and two free base porphyrins (Fb). The triads employ a diphenylethyne linker (ZnFbFbU) and a phenylene linker (ZnFbFbPhi). The zinc porphyrin is selectively oxidized to produce Zn(+)FbFb, the free base porphyrins are excited to produce the excited-state mixture Zn(+)Fb*Fb and Zn(+)FbFb*, and the subsequent dynamics are monitored by ultrafast absorption spectroscopy. The system evolves by a combination of energy- and hole-transfer processes involving (adjacent and nonadjacent) zinc and free base porphyrin constituents that are complete within 100 ps of excitation; the rate constants of many of these processes are derived from prior studies of the oxidized forms of the benchmark dyads (ZnFbU and ZnFbPhi). One of the excited-state decay channels produces the metastable state ZnFbFb(+) that decays to a second metastable state ZnFb(+)Fb by the target hole-transfer process, followed by rapid hole transfer to produce the Zn(+)FbFb thermodynamic ground state of the system. The rate constant for hole transfer between the free base porphyrins in the oxidized ZnFbFb triads is found to be (0.5 ns)(-1) and (0.6 ns)(-1) across phenylene and diphenylethyne linkers, respectively. These rate constants are comparable to those recently measured, using a related but distinct strategy, for ground-state hole transfer between zinc porphyrins in oxidized ZnZnFb triads. The two complementary strategies provide unique approaches for probing hole transfer between equivalent sites in multiporphyrin arrays, with the choice of method being guided by the particular target process and the ease of synthesis of the necessary architectures.

Ultrafast time-resolved absorption spectroscopy of geometric isomers of carotenoids

The structures of a number of stereoisomers of carotenoids have been revealed in three-dimensional X-ray crystallographic investigations of pigment–protein complexes from photosynthetic organisms. Despite these structural elucidations, the reason for the presence of stereoisomers in these systems is not well understood. An important unresolved issue is whether the natural selection of geometric isomers of carotenoids in photosynthetic pigment–protein complexes is determined by the structure of the protein binding site or by the need for the organism to accomplish a specific physiological task. The association of cis isomers of a carotenoid with reaction centers and trans isomers of the same carotenoid with light-harvesting pigment–protein complexes has led to the hypothesis that the stereoisomers play distinctly different physiological roles. A systematic investigation of the photophysics and photochemistry of purified, stable geometric isomers of carotenoids is needed to understand if a relationship between stereochemistry and biological function exists. In this work we present a comparative study of the spectroscopy and excited state dynamics of cis and trans isomers of three different open-chain carotenoids in solution. The molecules are neurosporene ( n = 9), spheroidene ( n = 10), and spirilloxanthin ( n = 13), where n is the number of conjugated π-electron double bonds. The spectroscopic experiments were carried out on geometric isomers of the carotenoids purified by high performance liquid chromatography (HPLC) and then frozen to 77 K to inhibit isomerization. The spectral data taken at 77 K provide a high resolution view of the spectroscopic differences between geometric isomers. The kinetic data reveal that the lifetime of the lowest excited singlet state of a cis-isomer is consistently shorter than that of its corresponding all- trans counterpart despite the fact that the excited state energy of the cis molecule is typically higher than that of the trans molecule. Quantum theoretical calculations on an n = 9 linear polyene were carried out to examine this process. The calculations indicate that the electronic coupling terms are significantly higher for the cis isomer, and when combined with the Franck–Condon factors, predict internal conversion rates roughly double those of the all- trans species. The electronic effects more than offset the decrease in coupling efficiencies associated with the higher system origin energies and explain the observed shorter cis isomer lifetimes.

Excited state processes of nitrobenzaldehydes probed by ultrafast fluorescence and absorption spectroscopy

We have investigated the isomers ortho-, meta-, and para-nitro-benzaldehyde (NBA) by means of ultrafast emission and absorption spectroscopy. The fluorescence dynamics of all three molecules are very similar and feature bi-phasic emission decays with time constants of <100 fs and ∼ 1 ps. The first process goes along with a strong loss of oscillator strength, which we assume is the result of a non-radiative π 1 π ∗ → n 1 π ∗ transition. These commonalities are contrasted by qualitatively different transient absorption signatures: For the photo-reactive isomer o-NBA, signatures of a ketene intermediate in the electronic ground state are observed. Its formation time matches the slower component of the emission decay. The ketene afterwards experiences pronounced cooling dynamics on the picosecond timescale followed by its decay in the nanosecond regime. For the non-reactive isomers m- and p-NBA, time constants with values similar to the ones derived from the emission experiment are found in the absorption experiment. In addition, at later delay times, a distinct spectral signature is observed for both isomers which decays within ∼ 1 ns and is assigned to a triplet state. This triplet decay is preceded by a process on the 10 ps timescale which we tentatively assign to a transition within the triplet manifold.

The Benzophenone S1(n,π*) → T1(n,π*) States Intersystem Crossing Reinvestigated by Ultrafast Absorption Spectroscopy and Multivariate Curve Resolution

The well-known benzophenone intersystem crossing from S(1)(n,pi*) to T(1)(n,pi*) states, for which direct transition is forbidden by El-Sayed rules, is reinvestigated by subpicosecond time-resolved absorption spectroscopy and effective data analysis for various excitation wavelengths and solvents. Multivariate curve resolution alternating least-squares analysis is used to perform bilinear decomposition of the time-resolved spectra into pure spectra of overlapping transient species and their associated time-dependent concentrations. The results suggest the implication of an intermediate (IS) in the relaxation process of the S(1) state. Therefore, a two step kinetic model, S(1) --> IS --> T(1), is successfully implemented as an additional constraint in the soft-modeling algorithm. Although this intermediate, which has a spectrum similar to the one of T(1)(n,pi*) state, could be artificially induced by vibrational relaxation, it is tentatively assigned to a hot T(1)(n,pi*) triplet state. Two characteristic times are reported for the transition S(1) --> IS and IS --> T(1), approximately 6.5 ps and approximately 10 ps respectively, without any influence of the solvent. Moreover, an excitation wavelength effect is discovered suggesting the participation of unrelaxed singlet states in the overall process. To go further discussing the spectroscopic relevancy of IS and to rationalize the expected involvement of the T(2)(pi,pi*) state, we also investigate 4-methoxybenzophenone. For this neighboring molecule, triplet energy level is tunable through solvent polarity and a clear correlation is established between the intermediate resolved by multivariate data analysis and the presence of a T(2)(pi,pi*) above the T(1)(n,pi*) triplet. It is therefore proposed that the benzophenone intermediate species is a T(1)(n,pi*) high vibrational level in interaction with T(2)(pi,pi*) state.

The Action Spectrum for Generation of the Primary Intermediate Revealed by Ultrafast Absorption Spectroscopy Studies of Pheomelanin¶

The observation that fair-skinned individuals are more susceptible to skin cancers is commonly explained by invoking an enhanced photoreactivity of the red melanin, pheomelanin compared with the black melanin, eumelanin. For the wavelength range from 500 to 1000 nm, pump-probe spectroscopic measurements reveal the photoexcitation of pheomelanin by UVA light that generates an immediate (<100 fs) transient absorption centered at 780 nm. Using a tunable femtosecond excitation source, the action spectrum between 300 and 390 nm for generation of the primary intermediate was measured. Similar action spectra are found for the sample with molecular weight (MW) between 1000 and 10 000 and the one with MW > 10 000 fractions of pheomelanin, indicating that the reactive chromophore has a low MW but is present and its photophysics is similar in the aggregated pigment. The shape of the action spectrum differs from the absorption spectrum of bulk melanin and mass-selected fractions but resembles reported absorption spectrum of benzothiazines, oxidation products of 5-S-cysteinyl-dopa, which are formed along the biosynthetic pathway of pheomelanin.

Nonlinear differential ultrafast laser absorption spectroscopy observation of charge carrier dynamicsof CdSxSe1−x nanocrystal doped glasses

Based on femtosecond transient absorption spectroscopy, the charge carrier dynamics ofCdSxSe1−x nanocrystal doped glasses was investigated by analyzing the nonlinear differential transientabsorption spectra. Time constants were extracted, which are different from the literatureresults using similar samples: they are pump laser intensity-dependent and decreasewith increasing pump laser intensity, whereas risetime constants are pump laserintensity-independent and high-order exciton recombination can be identified from thespectra. The threshold pump intensity was determined, which is lower than the literatureresult. Reasons for high-order recombination were analyzed, and broad size, statedistribution and high pump intensity are found as key factors. Auger recombination mayplay a key role in the multi-exciton recombination. Theoretical calculations andanalysis support this conclusion. A schematic model for electron transport and theformation of triexcitons in our samples are proposed to explain these results.

Large two-photon absorptivity of hemoglobin in the infrared range of 780–880nm

Porphyrin molecules have a highly conjugated cyclic structure and are theorized to have unusually large two-photon absorptivities (sigmaTPA), i.e., sigmaTPA approximately 10(2) GM. The authors tested this claim. Ultrafast two-photon absorption (TPA) spectroscopy was performed on solutions of hemoglobin, which contains a naturally occurring metaloporphyrin. They used a pump-probe technique to directly detect the change in transmission induced by TPA over the wavelength range of lambda0=780-880 nm. As controls, they measured the TPA of the dyes rhodamine 6G and B; their measurements both verify and extend previously reported values. In new results, hemoglobin was found to have a peak two-photon absorptivity of sigmaTPA approximately 150 GM at lambda0=825 nm, near a resonance of the Soret band. This value supports theoretical expectations. They also found a significant difference in the TPA of carboxyhemoglobin versus oxyhemoglobin, e.g., sigmaTPA=61 GM versus sigmaTPA=18 GM, respectively, at lambda0=850 nm, which shows that the ligand affects the electronic states involved in TPA.

Modification of carrier dynamics in CdSe nanocrystals by excess Cd in deposition bath

Photoexcited carrier dynamics in thin CdSe nanocrystalline films prepared by chemical bath deposition are strongly dependent on the deposition parameters. In this paper, we show how an increase in concentration of CdSO 4 in deposition bath affects the photoexcited carrier dynamics in nanocrystals. We used ultrafast absorption and photoluminescence laser spectroscopy to compare the carrier dynamics in samples prepared with and without increased CdSO 4 concentration. We have found that in the Cd-rich samples the spectral dependences of both photoluminescence and absorption dynamics are considerably less pronounced and the dynamics are slower. The observed effects were explained by the suppression of surface mediated carrier relaxation due to the passivation of individual nanocrystals by cadmium hydroxide and/or by sulphatocomplexes of Cd 2+.

Electronic Energy Transfer in a Multiporphyrin‐Based Molecular Box

The molecular box 1 comprises of two zinc-porphyrin metallacycles connected by two free-base 4'-trans-dipyridylporphyrins, axially coordinated to the zinc centers. The photophysics of 1 were studied in chloroform by emission and ultrafast absorption spectroscopy. In the molecular box, fast singlet energy transfer (main component, tau=32 ps) is observed to occur from the zinc-porphyrin metallacycles to the free-base chromophores. From wavelength-dependent spectrofluorimetric data, the efficiency of the energy-transfer (ET) process is estimated as 0.5. The lower-than-unity value is tentatively attributed to the possibility of a competing electron-transfer quenching pathway. Molecular box 1 can be considered to be a simple, self-assembling, six-chromophore antenna system. It has an inner cavity, 11.4 Angstrom wide, that could be used, in principle, to host a variety of guest molecules and obtain higher-order assemblies.

Ammonia effect on surface-mediated carrier dynamics in CdSe nanocrystals

Properties of semiconductor nanocrystalline thin films are significantly influenced by quality of the nanocrystalline surface. We report on a strong effect of ammonia treatment on optical properties of CdSe nanocrystalline thin films prepared by chemical bath deposition. The treatment affects strongly photoexcited carrier dynamics, as measured by ultrafast absorption and photoluminescence laser spectroscopy, which reveals a crucial role of surface states in involved processes. The experimental data are interpreted in terms of ammonia passivation of the nanocrystal surface states.

Nonlinear ultrafast optical absorption and pump-probe spectroscopy in biased semiconductor superlattices

We present theoretical simulations of two important nonlinear optical phenomena in biased semiconductor superlattices: nonlinear ultrafast absorption and pump-probe spectroscopy. We find that for both these techniques, the excitonic Wannier-Stark ladder absorption peaks shift as the optical intensity is increased. We show that both of these measurements can be used to probe the ultrafast internal intraband electric field within the superlattice. We employ a nonperturbative formalism in an excitonic basis to model the dynamic response. This allows us to self-consistently include the self-generated internal intraband field that generates the spectral peak shifts.

On the unusual fluorescence properties of xanthone in water

Photo-excited xanthone is known to undergo ultrafast intersystem crossing (ISC) in the 1 ps time domain. Correspondingly, its fluorescence quantum yield in most solvents is very small ( approximately 10(-4)). Surprisingly, the quantum yield in water is 100 times larger, while ISC is still rapid ( approximately 1 ps), as seen by ultrafast pump probe absorption spectroscopy. Temperature dependent steady state and time resolved fluorescence experiments point to a delayed fluorescence mechanism, where the triplet (3)npi* state primarily accessed by ISC is nearly isoenergetic with the photo-excited (1)pipi* state. The delayed fluorescence of xanthone in water decays with a time constant of 700 ps, apparently by internal conversion between the (3)npi* state and the lowest lying triplet state (3)pipi*.

Strong Enhancement of the Two-Photon Absorption of Tetrakis(4-sulfonatophenyl)porphyrin Diacid in Water upon Aggregation

A strong enhancement of the two-photon absorption (TPA) cross section of tetrakis(4-sulfonatophenyl)porphyrin diacid (H(4)TPPS(2-)) at various wavelengths when a J-type aggregate is formed in water with respect to the one observed for the H(4)TPPS(2-) monomer in a mixture of water, dimethyl sulfoxide (DMSO), and urea is presented. The TPA properties are characterized by the open aperture Z-scan technique and the ultrafast two-photon absorption spectroscopy with white light continuum probe (TPA-WLCP) technique. The observed enhancement is discussed in terms of possible electronic cooperative effects in the aggregate.

Ultrafast light-induced charge pair formation dynamics in poly[3-(2′-methoxy-5′octylphenyl)thiophene

Charge pair photogeneration was investigated by ultrafast absorption spectroscopy for different excitation photon energies in poly[3-(${2}^{\ensuremath{'}}$-methoxy-${5}^{\ensuremath{'}}$octylphenyl)thiophene] $(\mathrm{POMeOPT})$ film with and without an external electric field. Electric field-assisted charge pair photogeneration in $\mathrm{POMeOPT}$ occurs from vibrationally relaxed singlet excitons during their entire lifetime and charge pair formation takes place in this manner even in the absence of an external electric field. From our data there are no indications of hot exciton dissociation to charge pairs even when a large amount of excess energy is supplied to the excitons. To explain these observations we present a model with energy transfer to low-barrier dissociation sites as a key feature.

Dynamic Regulation of the Inducible Nitric-oxide Synthase by NO: COMPARISON WITH THE ENDOTHELIAL ISOFORM

We studied by ultrafast time-resolved absorption spectroscopy the geminate recombination of NO to the oxygenase domain of the inducible NO synthase, iNOSoxy, and to mutated proteins at position Trp-457. This tryptophan interacts with the tetrahydrobiopterin cofactor BH4, and W457A/F mutations largely reduced the catalytic formation of NO. BH4 decreases the rate of NO rebinding to the ferric iNOSoxy compared with that measured in its absence. The pterin has a larger effect on W457A/F than on the WT protein by increasing NO release from the protein. Therefore, BH4 raises the energy barrier for NO recombination to the mutated proteins in contrast with our observations on eNOS (Slama-Schwok, A., Négrerie, M., Berka, V., Lambry, J.-C., Tsai, A.-L., Vos, M., and Martin, J.-L. (2002) J. Biol. Chem. 277, 7581-7586). Thus, we show a differential effect of BH4 on NO release from eNOS and iNOS. Compared with the position of this residue in the BH4-repleted enzyme, simulations of the NO dissociation dynamics point out at a swing of Trp-457 toward the missing pterin in the absence of BH4. NO geminate-rebinding data show a more efficient NO release from eNOS than from iNOS once NO is formed. Consistently, NO produced by iNOS is regulated by its ferric nitrosyl complex in contrast with eNOS. We show that the small enhancement of the NO geminate recombination rate in W457A/F compared with that in the WT enzyme cannot explain the decrease of NO yield because of the mutation; the major effect of the mutation thus arises from an uncoupled catalysis (Wang, Z. Q., Wei, C. C., Ghosh, S., Meade, A. L., Hemann, C., Hille, R., and Stuehr, D. J. (2001) Biochemistry 40, 12819-12825).

Calcium Photorelease from a Symmetrical Donor−Acceptor−Donor Bis-crown-fluoroionophore Evidenced by Ultrafast Absorption Spectroscopy

The excited-state properties of a calcium dicomplex (AM2) of 2,5-[bis-[4N-(aza-15-crown-5)phenyl]methylene]cyclopentanone (A) in acetonitrile are investigated by ultrafast absorption spectroscopy. Spectroscopic evidence of an ultrafast cation decoordination following subpicosecond excitation of the chromophore is given. A global analysis of the time-resolved data is performed, and a kinetic scheme is tentatively proposed, which quantitatively describes the dynamics of the deactivation pathway of the AM2 dicomplex. A first step would involve the disruption of one of the two nitrogen−calcium bonds at a rate constant of 4 × 1011 s-1, followed by the diffusion of the cation out of the crown at a rate of 5.8 × 1010 s-1. The resulting AM complex would then relax both to its ground state (5.4 × 108 s-1) and to its triplet state (2.8 × 108 s-1).

The action spectrum for generation of the primary intermediate revealed by ultrafast absorption spectroscopy studies of pheomelanin.

The observation that fair-skinned individuals are more susceptible to skin cancers is commonly explained by invoking an enhanced photoreactivity of the red melanin, pheomelanin compared with the black melanin, eumelanin. For the wavelength range from 500 to 1000 nm, pump-probe spectroscopic measurements reveal the photoexcitation of pheomelanin by UVA light that generates an immediate (< 100 fs) transient absorption centered at 780 nm. Using a tunable femtosecond excitation source, the action spectrum between 300 and 390 nm for generation of the primary intermediate was measured. Similar action spectra are found for the sample with molecular weight (MW) between 1000 and 10 000 and the one with MW > 10 000 fractions of pheomelanin, indicating that the reactive chromophore has a low MW but is present and its photophysics is similar in the aggregated pigment. The shape of the action spectrum differs from the absorption spectrum of bulk melanin and mass-selected fractions but resembles reported absorption spectrum of benzothiazines, oxidation products of 5-S-cysteinyl-dopa, which are formed along the biosynthetic pathway of pheomelanin.