Broadband spectral probing revealing ultrafast photochemical branching after ultraviolet excitation of the aqueous phenolate anion

Electron photodetachment from the aromatic anion phenolate excited into the π-π* singlet excited state (S(1)) in aqueous solution is studied with ultrafast transient absorption spectroscopy with a time resolution of better than 50 fs. Broad-band transient absorption spectra from 300 to 690 nm are recorded. The transient bands are assigned to the solvated electron, the phenoxyl radical, and the phenolate S(1) excited state, and confirmation of these assignments is achieved using both KNO(3) as electron quencher and time-resolved fluorescence to measure singlet excited state dynamics. The phenolate fluorescence lifetime is found to be short (～20 ps) in water, but the fast decay is only in part due to the electron ejection channel from S(1). Using global target analysis, two electron ejection channels are identified, and we propose that both vibrationally hot S(1) state and the relaxed S(1) state are direct precursors for the solvated electron. Therefore, electron ejection is found just to compete with picosecond time scale vibrational relaxation and electronic radiationless decay channels. This contrasts markedly with <100 fs electron detachment processes for inorganic anions.     

State preparation and excited electronic and vibrational behavior in hemes

The temporally overlapping, ultrafast electronic and vibrational dynamics of a model five-coordinate, high-spin heme in a nominally isotropic solvent environment has been studied for the first time with three complementary ultrafast techniques: transient absorption, time-resolved resonance Raman Stokes, and time-resolved resonance Raman anti-Stokes spectroscopies. Vibrational dynamics associated with an evolving ground-state species dominate the observations. Excitation into the blue side of the Soret band led to very rapid S2 --> S1 decay (sub-100 fs), followed by somewhat slower (800 fs) S1 --> S0 nonradiative decay. The initial vibrationally excited, non-Boltzmann S0 state was modeled as shifted to lower energy by 300 cm(-1) and broadened by 20%. On a approximately 10 ps time scale, the S0 state evolved into its room-temperature, thermal distribution S0 profile largely through VER. Anti-Stokes signals disappear very rapidly, indicating that the vibrational energy redistributes internally in about 1-3 ps from the initial accepting modes associated with S1 --> S0 internal conversion to the rest of the macrocycle. Comparisons of anti-Stokes mode intensities and lifetimes from TRARRS studies in which the initial excited state was prepared by ligand photolysis [Mizutani, T.; Kitagawa, T. Science 1997, 278, 443, and Chem. Rec. 2001, 1, 258] suggest that, while transient absorption studies appear to be relatively insensitive to initial preparation of the electronic excited state, the subsequent vibrational dynamics are not. Direct, time-resolved evaluation of vibrational lifetimes provides insight into fast internal conversion in hemes and the pathways of subsequent vibrational energy flow in the ground state. The overall similarity of the model heme electronic dynamics to those of biological systems may be a sign that the protein's influence upon the dynamics of the heme active site is rather subtle.     

Ultrafast S1 to S0 internal conversion dynamics for dimethylnitramine through a conical intersection

Electronically nonadiabatic processes such as ultrafast internal conversion (IC) from an upper electronic state (S(1)) to the ground electronic state (S(0)) though a conical intersection (CI), can play an essential role in the initial steps of the decomposition of energetic materials. Such nonradiative processes following electronic excitation can quench emission and store the excitation energy in the vibrational degrees of freedom of the ground electronic state. This excess vibrational energy in the ground electronic state can dissociate most of the chemical bonds of the molecule and can generate stable, small molecule products. The present study determines ultrafast IC dynamics of a model nitramine energetic material, dimethylnitramine (DMNA). Femtosecond (fs) pump-probe spectroscopy, for which a pump pulse at 271 nm and a probe pulse at 405.6 nm are used, is employed to elucidate the IC dynamics of this molecule from its S(1) excited state. A very short lifetime of the S(1) excited state (～50 ± 16 fs) is determined for DMNA. Complete active space self-consistent field (CASSCF) calculations show that an (S(1)/S(0))(CI) CI is responsible for this ultrafast decay from S(1) to S(0). This decay occurs through a reaction coordinate involving an out-of-plane bending mode of the DMNA NO(2) moiety. The 271 nm excitation of DMNA is not sufficient to dissociate the molecule on the S(1) potential energy surface (PES) through an adiabatic NO(2) elimination pathway.     

Ultrafast excited-state dynamics of adenine and monomethylated adenines in solution: implications for the nonradiative decay mechanism

The DNA base adenine and four monomethylated adenines were studied in solution at room temperature by femtosecond pump-probe spectroscopy. Transient absorption at visible probe wavelengths was used to directly observe relaxation of the lowest excited singlet state (S(1) state) populated by a UV pump pulse. In H(2)O, transient absorption signals from adenine decay biexponentially with lifetimes of 0.18 +/- 0.03 ps and 8.8 +/- 1.2 ps. In contrast, signals from monomethylated adenines decay monoexponentially. The S(1) lifetimes of 1-, 3-, and 9-methyladenine are similar to one another and are all below 300 fs, while 7-methyladenine has a significantly longer lifetime (tau = 4.23 +/- 0.13 ps). On this basis, the biexponential signal of adenine is assigned to an equilibrium mixture of the 7H- and 9H-amino tautomers. Excited-state absorption (ESA) by 9-methyladenine is 50% stronger than by 7-methyladenine. Assuming that ESA by the corresponding tautomers of adenine is unchanged, we estimate the population of 7H-adenine in H(2)O at room temperature to be 22 +/- 4% (estimated standard deviation). To understand how the environment affects nonradiative decay, we performed the first solvent-dependent study of nucleobase dynamics on the ultrafast time scale. In acetonitrile, both lowest energy tautomers of adenine are present in roughly similar proportions as in water. The lifetimes of the 9-substituted adenines depend somewhat more sensitively on the solvent than those of the 7-substituted adenines. Transient signals for adenine in H(2)O and D(2)O are identical. These solvent effects strongly suggest that excited-state tautomerization is not an important nonradiative decay pathway. Instead, the data are most consistent with electronic energy relaxation due to state crossings between the optically prepared (1)pipi* state and one or more (1)npi* states and the electronic ground state. The pattern of lifetimes measured for the monomethylated adenines suggests a special role for the (1)npi* state associated with the N7 electron lone pair.     

Femtosecond time-resolved transient absorption spectroscopy of xanthophylls

Xanthophylls are a major class of photosynthetic pigments that participate in an adaptation mechanism by which higher plants protect themselves from high light stress. In the present work, an ultrafast time-resolved spectroscopic investigation of all the major xanthophyll pigments from spinach has been performed. The molecules are zeaxanthin, lutein, violaxanthin, and neoxanthin. beta-Carotene was also studied. The experimental data reveal the inherent spectral properties and ultrafast dynamics including the S(1) state lifetimes of each of the pigments. In conjunction with quantum mechanical computations the results address the molecular features of xanthophylls that control the formation and decay of the S* state in solution. The findings provide compelling evidence that S* is an excited state with a conformational geometry twisted relative to the ground state. The data indicate that S* is formed via a branched pathway from higher excited singlet states and that its yield depends critically on the presence of beta-ionylidene rings in the polyene system of pi-electron conjugated double bonds. The data are expected to be beneficial to researchers employing ultrafast time-resolved spectroscopic methods to investigate the mechanisms of both energy transfer and nonphotochemical quenching in higher plant preparations.     

Direct observation of electronic relaxation dynamics in adenine via time-resolved photoelectron spectroscopy

We present femtosecond time-resolved photoelectron spectra of adenine in a molecular beam, recorded at pump wavelengths of 250, 267, and 277 nm. This leads to initial excitation of the bright S2(pipi*). Close to the band origin (277 nm), the lifetime is several picoseconds. Higher vibronic levels (267 and 250 nm excitation) show much shorter lifetimes of t < 50 fs, and we observe strong coupling between S2(pipi*) and S1(npi*). Rapid internal conversion (t < 50 fs) populates the lower lying S1(npi*) state which has a lifetime of 750 fs. At 267 nm, we found evidence for an additional channel which is consistent with the dissociative S3(pisigma*) state, previously proposed as an ultrafast relaxation pathway from S2(pipi*).     

Electronic relaxation dynamics of water cluster anions

The electronic relaxation dynamics of water cluster anions, (H(2)O)(n)(-), have been studied with time-resolved photoelectron imaging. In this investigation, the excess electron was excited through the p<--s transition with an ultrafast laser pulse, with subsequent electronic evolution monitored by photodetachment. All excited-state lifetimes exhibit a significant isotope effect (tau(D)2(O)/tau(H)2(O) approximately 2). Additionally, marked dynamical differences are found for two classes of water cluster anions, isomers I and II, previously assigned as clusters with internally solvated and surface-bound electrons, respectively. Isomer I clusters with n > or = 25 decay exclusively by internal conversion, with relaxation times that extrapolate linearly with 1/n toward an internal conversion lifetime of 50 fs in bulk water. Smaller isomer I clusters (13 < or = n < or = 25) decay through a combination of excited-state autodetachment and internal conversion. The relaxation of isomer II clusters shows no significant size dependence over the range of n = 60-100, with autodetachment an important decay channel following excitation of these clusters. Photoelectron angular distributions (PADs) were measured for isomer I and isomer II clusters. The large differences in dynamical trends, relaxation mechanisms, and PADs between large isomer I and isomer II clusters are consistent with their assignment to very different electron binding motifs.     

Femtosecond time-resolved electronic sum-frequency generation spectroscopy: a new method to investigate ultrafast dynamics at liquid interfaces

We developed a new surface-selective time-resolved nonlinear spectroscopy, femtosecond time-resolved electronic sum-frequency generation (TR-ESFG) spectroscopy, to investigate ultrafast dynamics of molecules at liquid interfaces. Its advantage over conventional time-resolved second harmonic generation spectroscopy is that it can provide spectral information, which is realized by the multiplex detection of the transient electronic sum-frequency signal using a broadband white light continuum and a multichannel detector. We studied the photochemical dynamics of rhodamine 800 (R800) at the air/water interface with the TR-ESFG spectroscopy, and discussed the ultrafast dynamics of the molecule as thoroughly as we do for the bulk molecules with conventional transient absorption spectroscopy. We found that the relaxation dynamics of photoexcited R800 at the air/water interface exhibited three characteristic time constants of 0.32 ps, 6.4 ps, and 0.85 ns. The 0.32 ps time constant was ascribed to the lifetime of dimeric R800 in the lowest excited singlet (S(1)) state (S(1) dimer) that is directly generated by photoexcitation. The S(1) dimer dissociates to a monomer in the S(1) state (S(1) monomer) and a monomer in the ground state with this time constant. This lifetime of the S(1) dimer was ten times shorter than the corresponding lifetime in a bulk aqueous solution. The 6.4 ps and 0.85 ns components were ascribed to the decay of the S(1) monomer (as well as the recovery of the dimer in the ground state). For the 6.4 ps time constant, there is no corresponding component in the dynamics in bulk water, and it is ascribed to an interface-specific deactivation process. The 0.85 ns time constant was ascribed to the intrinsic lifetime of the S(1) monomer at the air/water interface, which is almost the same as the lifetime in bulk water. The present study clearly shows the feasibility and high potential of the TR-ESFG spectroscopy to investigate ultrafast dynamics at the interface.     

Femtosecond fluorescence dynamics of rotation-restricted azobenzenophanes: new evidence on the mechanism of trans --> cis photoisomerization of azobenzene

The ultrafast relaxation dynamics of two rotation-restricted (azobenzeno-2S-phane and azobenzeno-4S-phane) and one rotation-free (4,4'-dimethylazobenzene) azobenzene derivatives were investigated using femtosecond fluorescence up-conversion on both S(1)(n,pi) and S(2)(pi,pi) excitations. On S(2) excitation, pulse-limited kinetics with a decay coefficient of approximately 100 fs corresponding to ultrafast S(2) --> S(1) relaxation is found to be common for all molecules under investigation regardless of the molecular structure. This indicates that a direct rotational relaxation on the S(2) surface is unfavorable. On S(1) excitation, we observed biphasic fluorescence decay with a femtosecond component attributed to the decay of the Franck-Condon state prepared by excitation and a picosecond component attributed to the deactivation of the relaxed molecule on the S(1) surface. This picosecond component is slowed by at least a factor of 2 for the rotation-restricted 2S-bridged molecule compared to that of the rotation-free molecule; for the even stronger rotation-restricted azobenzeno-4S-phane, the decrease is by a factor of 10. These differences in deactivation suggest that the relaxed states and probably the trajectories for rotation-free and rotation-restricted molecules are different on the S(1) surface, which should be important for the quantum yield of photoisomerization.     

Ultrafast dynamics in thiophene investigated by femtosecond pump probe photoelectron spectroscopy and theory

A hybrid of a time-of-flight mass spectrometer and a time-of-flight "magnetic-bottle type" photoelectron (PE) spectrometer is used for fs pump-probe investigations of the excited state dynamics of thiophene. A resonant two-photon ionization spectrum of the onset of the excited states has been recorded with a tunable UV laser of 190 fs pulse width. With the pump laser set to the first intense transition we find by UV probe ionization first a small time shift of the maxima in the PE spectrum and then a fast decay to a low constant intensity level. The fitted time constants are 80+/-10 fs, and 25+/-10 fs, respectively. Theoretical calculations show that upon geometry relaxation the electronic state order changes and conical intersections between excited states exist. We use the vertical state order S1, S2, S3 to define the terms S1, S2, and S3 for the characterization of the electron configuration of these states. On the basis of our theoretical result we discuss the electronic state order in the UV spectra and identify in the photoelectron spectrum the origin of the first cation excited state D1. The fast excited state dynamics agrees best with a vibrational dynamics in the photo-excited S1 (80+/-10 fs) and an ultrafast decay via a conical intersection, presumably a ring opening to the S3 state (25+/-10 fs). The subsequently observed weak constant signal is taken as an indication, that in the gas phase the ring-closure to S0 is slower than 50 ps. An ultrafast equilibrium between S1 and S2 before ring opening is not supported by our data.     

Hidden Relaxation Channels in Aqueous Methylene Blue after Functionalization of Graphene Oxide Probed by Transient Absorption Spectroscopy

The mixture of graphene oxide (GO) and dye molecules may provide some new applications due to unique electronic, optical, and structural properties. Methylene blue (MB), a typical anionic dye, can attach on GO via π-π stacking and electrostatic interaction, and the molecule removal process on GO has been observed. However, it remains unclear about the ultrafast carrier dynamics and the internal energy transfer pathways of the system which is composed of GO and MB. We have employed ultrafast optical pump-probe spectroscopy to investigate the excited dynamics of the GO-MB system dispersed in water by exciting the samples at 400 nm pump pulse. The pristine MB and GO dynamics are also analyzed in t_andem for a direct comparison. Utilizing the global analysis to fit the measured signal via a sequential model, five lifetimes are acquired:(0.61±0.01) ps, (3.52±0.04) ps, (14.1±0.3) ps, (84±2) ps, and (3.66±0.08) ns. The ultrafast dynamics corresponding to these lifetimes was analyzed and the new relaxation processes were found in the GO-MB system, compared with the pristine MB. The results reveal that the functionalization of GO can alter the known decay pathways of MB via the energy transfer from GO to MB in system, the increased intermediate state, and the promoted energy transfer from triplet state MB to ground state oxygen molecules dissolved in aqueous sample.     

Time-resolved photoelectron imaging of large anionic methanol clusters: (methanol)n-(n approximately 145-535)

The dynamics of an excess electron in size-selected methanol clusters is studied via pump-probe spectroscopy with resolution of approximately 120 fs. Following excitation, the excess electron undergoes internal conversion back to the ground state with lifetimes of 260-175 fs in (CH3OH)n- (n=145-535) and 280-230 fs in (CD3OD)n- (n=210-390), decreasing with increasing cluster size. The clusters then undergo vibrational relaxation on the ground state on a time scale of 760+/-250 fs. The excited state lifetimes for (CH3OH)n- clusters extrapolate to a value of 157+/-25 fs in the limit of infinite cluster size.     

Ultrafast photophysics of the isolated indole molecule

The relaxation dynamics of the isolated indole molecule has been tracked by femtosecond time-resolved ionization. The excitation region explored (283-243 nm) covers three excited states: the two ππ* L(b) and L(a) states, and the dark πσ* state with dissociative character. In the low energy region (λ > 273 nm) the transients collected reflect the absorption of the long living L(b) state. The L(a) state is met 1000-1500 cm(-1) above the L(b) origin, giving rise to an ultrafast lifetime of 40 fs caused by the internal conversion to the lower L(b) minimum through a conical intersection. An additional ~400 fs component, found at excitation wavelengths shorter than 263 nm, is ascribed to dynamics along the πσ* state, which is likely populated through coupling to the photoexcited L(a) state. The study provides a general view of the indole photophysics, which is driven by the interplay between these three excited surfaces and the ground state.     

Distinguishing between relaxation pathways by combining dissociative ionization pump probe spectroscopy and ab initio calculations: a case study of cytosine

We present a general method for tracking molecular relaxation along different pathways from an excited state down to the ground state. We follow the excited state dynamics of cytosine pumped near the S(0)-S(1) resonance using ultrafast laser pulses in the deep ultraviolet and probed with strong field near infrared pulses which ionize and dissociate the molecules. The fragment ions are detected via time of flight mass spectroscopy as a function of pump probe delay and probe pulse intensity. Our measurements reveal that different molecular fragments show different timescales, indicating that there are multiple relaxation pathways down to the ground state. We interpret our measurements with the help of ab initio electronic structure calculations of both the neutral molecule and the molecular cation for different conformations en route to relaxation back down to the ground state. Our measurements and calculations show passage through two seams of conical intersections between ground and excited states and demonstrate the ability of dissociative ionization pump probe measurements in conjunction with ab initio electronic structure calculations to track molecular relaxation through multiple pathways.     

Semiclassical Liouville method for the simulation of electronic transitions: single ensemble formulation

In this paper, we describe a single ensemble implementation of the semiclassical Liouville method for simulating quantum processes using classical trajectories. In this approach, one ensemble of trajectories supports the evolution of all semiclassical density matrix elements, rather than employing a distinct ensemble for each. The ensemble evolves classically under a single reference Hamiltonian, which is chosen based on physical grounds; for electronic relaxation of an initially excited state, the initially populated upper surface Hamiltonian is the natural choice. Classical trajectories evolving on the reference potential then represent the time-dependent upper state population density and also the electronic coherence and the ground state density created by electronic transition. The error made in the classical motion of the trajectories for these latter distributions is compensated for by incorporating the difference between the correct and reference Liouville propagators into the calculation of the coefficients of the individual trajectories. This approach gives very accurate results for a number of model problems and cases describing ultrafast electronic relaxation dynamics.     

Photoinduced electron transfer between a carotenoid and TiO2 nanoparticle

The dynamics of photoinduced electron injection and recombination between all-trans-8'-apo-beta-caroten-8'-oic acid (ACOA) and a TiO(2) colloidal nanoparticle have been studied by means of transient absorption spectroscopy. We observed an ultrafast ( approximately 360 fs) electron injection from the initially excited S(2) state of ACOA into the TiO(2) conduction band with a quantum yield of approximately 40%. As a result, the ACOA(*)(+) radical cation was formed, as demonstrated by its intense absorption band centered at 840 nm. Because of the competing S(2)-S(1) internal conversion, approximately 60% of the S(2)-state population relaxes to the S(1) state. Although the S(1) state is thermodynamically favorable to donate electrons to the TiO(2), no evidence was found for electron injection from the ACOA S(1) state, most likely as a result of a complicated electronic nature of the S(1) state, which decays with a approximately 18 ps time constant to the ground state. The charge recombination between the injected electrons and the ACOA(*)(+) was found to be a highly nonexponential process extending from picoseconds to microseconds. Besides the usual pathway of charge recombination forming the ACOA ground state, about half of the ACOA(*)(+) recombines via the ACOA triplet state, which was monitored by its absorption band at 530 nm. This second channel of recombination proceeds on the nanosecond time scale, and the formed triplet state decays to the ground state with a lifetime of approximately 7.3 micros. By examination of the process of photoinduced electron transfer in a carotenoid-semiconductor system, the results provide an insight into the photophysical properties of carotenoids, as well as evidence that the interfacial electron injection occurs from the initially populated excited state prior to electronic and nuclear relaxation of the carotenoid molecule.     

Density matrix method and ultrafast processes

When femto-second (fs) time-resolved experiments are used to study ultrafast processes, quantum beat phenomena are often observed. In this paper, to analyze the fs time-resolved spectra, we will present the density matrix method, a powerful theoretical technique, which describes the dynamics of population and coherence of the system. How to employ it to study the pump-probe experiments and fs ultrafast processes is described. The ππ*→nπ* transition of pyrazine is used as an example to demonstrate the application of the density matrix method. Recently, Suzuki’s group have employed the 22 fs time resolution laser to study the dynamics of the ππ* state of pyrazine. In this case, conical intersection is commonly believed to play an important role in this non-adiabatic process. How to treat the effect of conical intersection on non-adiabatic processes and fs time-resolved spectra is presented. Another important ultrafast process, vibrational relaxation, which takes place in sub-ps and ps range and has never been carefully studied, is treated in this paper. The vibrational relaxation in water dimer is chosen to demonstrate the calculation. It should be noted that the vibrational relaxation of (H2O)2 has not been experimentally studied but it can be accomplished by the pump-probe experiments.     

The effect of hydrogen bonding on the excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole: a TDDFT molecular dynamics study

The dynamics of the excited-state proton transfer (ESPT) in a cluster of 2-(2'-hydroxyphenyl)benzothiazole (HBT) and hydrogen-bonded water molecules was investigated by means of quantum chemical simulations. Two different enol ground-state structures of HBT interacting with the water cluster were chosen as initial structures for the excited-state dynamics: (i) an intramolecular hydrogen-bonded structure of HBT and (ii) a cluster where the intramolecular hydrogen bond in HBT is broken by intermolecular interactions with water molecules. On-the-fly dynamics simulations using time-dependent density functional theory show that after photoexcitation to the S(1) state the ESPT pathway leading to the keto form strongly depends on the initial ground state structure of the HBT-water cluster. In the intramolecular hydrogen-bonded structures direct excited-state proton transfer is observed within 18 fs, which is a factor two faster than proton transfer in HBT computed for the gas phase. Intermolecular bonded HBT complexes show a complex pattern of excited-state proton transfer involving several distinct mechanisms. In the main process the tautomerization proceeds via a triple proton transfer through the water network with an average proton transfer time of approximately 120 fs. Due to the lack of the stabilizing hydrogen bond, intermolecular hydrogen-bonded structures have a significant degree of interring twisting already in the ground state. During the excited state dynamics, the twist tends to quickly increase indicating that internal conversion to the electronic ground state should take place at the sub-picosecond scale.     

Laser pulse trains for controlling excited state dynamics of adenine in water

We investigate theoretically the control of the ultrafast excited state dynamics of adenine in water by laser pulse trains, with the aim to extend the excited state lifetime and to suppress nonradiative relaxation processes. For this purpose, we introduce the combination of our field-induced surface hopping method (FISH) with the quantum mechanical-molecular mechanical (QM/MM) technique for simulating the laser-driven dynamics in the condensed phase under explicit inclusion of the solvent environment. Moreover, we employ parametric pulse shaping in the frequency domain in order to design simplified laser pulse trains allowing to establish a direct link between the pulse parameters and the controlled dynamics. We construct pulse trains which achieve a high excitation efficiency and at the same time keep a high excited state population for a significantly extended time period compared to the uncontrolled dynamics. The control mechanism involves a sequential cycling of the population between the lowest and higher excited states, thereby utilizing the properties of the corresponding potential energy surfaces to avoid conical intersections and thus to suppress the nonradiative decay to the ground state. Our findings provide a means to increase the fluorescence yield of molecules with an intrinsically very short excited state lifetime, which can lead to novel applications of shaped laser fields in the context of biosensing.