Femtosecond stimulated Raman study of excited-state evolution in bacteriorhodopsin

Femtosecond time-resolved stimulated Raman spectroscopy (FSRS) is used to examine the photoisomerization dynamics in the excited state of bacteriorhodopsin. Near-IR stimulated emission is observed in the FSRS probe window that decays with a 400-600-fs time constant. Additionally, dispersive vibrational lines appear at the locations of the ground-state vibrational frequencies and decay with a 260-fs time constant. The dispersive line shapes are caused by a nonlinear effect we term Raman initiated by nonlinear emission (RINE) that generates vibrational coherence on the ground-state surface. Theoretical expressions for the RINE line shapes are developed and used to fit the spectral and temporal evolution of the spectra. The rapid 260-fs decay of the RINE peak intensity, compared to the slower evolution of the stimulated emission, indicates that the excited-state population moves in approximately 260 fs to a region on the potential energy surface where the RINE signal is attenuated. This loss of RINE signal is best explained by structural evolution of the excited-state population along multiple low-frequency modes that carry the molecule out of the harmonic photochemically inactive Franck-Condon region and into the photochemically active geometry.     

The interplay of skeletal deformations and ultrafast excited-state intramolecular proton transfer: Experimental and theoretical investigation of 10-hydroxybenzo[h]quinoline

The excited-state intramolecular proton transfer in the aromatic polycycle 10-hydroxybenzo[h]quinoline is investigated by means of transient absorption experiments with 30 fs time resolution, classical dynamics and wavepacket dynamics. The experiments establish the ultrafast transfer after UV excitation and show signatures of coherent vibrational motion in the keto product. To elucidate details of the proton transfer mechanism, the classical dynamics is also performed for 2-(2′-hydroxyphenyl)benzothiazole and the results are compared. For both systems the proton transfer takes place on the ultrafast scale of 30–40 fs, with good agreement between the theoretical investigations and the measurements. The dynamics simulations show that for both molecules the proton is handed over by means of skeletal deformation of the molecule. Due to the more rigid structure of 10-hydroxybenzo[h]quinoline the hydrogen migration mode participates more actively than in 2-(2′-hydroxyphenyl)benzothiazole.     

Revealing Excited-State Trajectories on Potential Energy Surfaces with Atomic Resolution in Real Time

Photoexcited molecular trajectories on potential energy surfaces (PESs) prior to thermalization are intimately connected to the photochemical reaction outcome. The excited-state trajectories of a diplatinum complex featuring photo-activated metal–metal σ-bond formation and associated Pt−Pt stretching motions were detected in real time using femtosecond wide-angle X-ray solution scattering. The observed motions correspond well with coherent vibrational wavepacket motions detected by femtosecond optical transient absorption. Two key coordinates for intersystem crossing have been identified, the Pt−Pt bond length and the orientation of the ligands coordinated with the platinum centers, along which the excited-state trajectories can be projected onto the calculated PESs of the excited states. This investigation has gleaned novel insight into electronic transitions occurring on the time scales of vibrational motions measured in real time, revealing ultrafast nonadiabatic or non-equilibrium processes along excited-state trajectories involving multiple excited-state PESs.     

Femtosecond absorption study of photodissociation of diphenylcyclopropenone in solution: reaction dynamics and coherent nuclear motion

Reaction dynamics and coherent nuclear motions in the photodissociation of diphenylcyclopropenone (DPCP) were studied in solution by time-resolved absorption spectroscopy. Subpicosecond transient absorption spectra were measured in the visible region with excitation at the second absorption band of DPCP. The obtained spectra showed a new short-lived band around 480 nm immediately after photoexcitation, which is assignable to the initially populated S(2) state of DPCP before the dissociation. The dissociation takes place from this excited state (the precursor of the reaction) with a time constant of 0.2 ps, and the excited state of diphenylacetylene (DPA) is generated as the reaction product. The transient absorption after the dissociation decayed with a time constant of 8 ps that is very close to the S(2)-state lifetime of DPA, but the spectrum of this 8-ps component was different from the S(2) absorption observed with direct photoexcitation of DPA. We conclude that the dissociation of DPCP generates the S(2) state of DPA that probably has a cis-bent structure. At later delay times (>30 ps), the transient absorption signals are very similar to those obtained by direct photoexcitation of DPA. This confirmed that the electronic relaxation from the S(2) state of the product DPA occurs in a similar manner to that of DPA itself, i.e., the internal conversion to the S(1) state and subsequent intersystem crossing to the T(1) state. In order to examine the coherent nuclear dynamics in this dissociation reaction, we carried out time-resolved absorption measurements for the 480-nm band with 70 fs resolution. It was found that an underdamped oscillatory modulation with a 0.1-ps period is superposed on the decay of the precursor absorption. This indicates that DPCP exhibits a coherent nuclear motion having a approximately 330-cm(-1) frequency in the dissociative excited state. Based on a comparison with the measured and calculated Raman spectra of ground-state DPCP, we discuss the assignment of the "330-cm(-1) vibration" and attribute it to a vibration involving the displacement of the CO group as well as the deformation of the Ph-C[Double Bond]C-Ph skeleton. We consider that this motion is closely related to the reaction coordinate of the photodissociation of DPCP.     

Intermolecular vibrational coherence in bacteriochlorophyll a with clustered polar solvent molecules

We show that resonant impulsive excitation of the Qy absorption band of bacteriochlorophyll a (BChl) launches a rapidly damped (gamma < 200 fs) ground-state coherent wave-packet motion that arises from intermolecular modes with clustered solvent molecules. Femtosecond pump-probe, dynamic-absorption signals were obtained at room temperature with BChl solutions in pyridine, acetone, and 1-propanol. The vibrational coherence observed in the 0-800-fs regime is modeled in the time domain by two (or three, in the case of 1-propanol) modulation components with asymmetric, inhomogeneously broadened line shapes and frequencies in the 100-200-cm(-1) range. The mean frequency of the vibrational coherence exhibits at least a quadratic dependence on the dipole moment of the solvent molecules and a y-intercept in the 100-cm(-1) regime. This trend is modeled by an expression for the natural frequency of a "6-12" potential composed of attractive terms from van der Waals forces and a repulsive term from the exchange (Pauli exclusion) force. The model suggests that comparable contributions to the potential are provided by the dipole-dipole and London dispersion interactions. These results support the hypothesis that the low-frequency vibrational modes in the 100-cm(-1) regime that are coupled to the light-driven charge-separation reactions in the reaction center from purple bacteria are derived from intermolecular vibrational modes between the chromophores and the surrounding protein medium.     

Unraveling Vibrational Wavepacket Dynamics using Femtosecond Ion Yield Spectroscopy and Photoelectron Imaging

Time-resolved photoionization is a powerful experimental approach to unravel the excited state dynamics in isolated polyatomic molecules. Depending on species of the collected signals, different methods can be performed: time-resolved ion yield spectroscopy (TR-IYS) and time-resolved photoelectron imaging (TR-PEI). In this review, the essential concepts linking photoionization measurement with electronic structure are presented, together with several important breakthroughs in experimentally distinguishing the oscillating wavepacket motion between different geometries. We illustrate how femtosecond TR-IYS and TR-PEI are employed to visualize the evolution of a coherent vibrational wavepacket on the excited state surface.     

Broadband pump-probe spectroscopy with sub-10-fs resolution for probing ultrafast internal conversion and coherent phonons in carotenoids

We use pump-probe spectroscopy with broadband detection to study electronic energy relaxation and coherent vibrational dynamics in carotenoids. A fast optical multichannel analyzer combined with a non-collinear optical parametric amplifier allows simultaneous acquisition of the differential transmission dynamics on the 500–700 nm wavelength range with sub-10-fs temporal resolution. The broad spectral coverage enables on the one hand a detailed study of the ultrafast bright-to-dark state internal conversion process; on the other hand, the tracking of the motion of the vibrational wavepacket launched on the ground state multidimensional potential energy surface. We present results on all-trans β-carotene and on a long-chain polyene in solution. The developed experimental setup enables the straightforward acquisition and analysis of coherent vibrational dynamics, highlighting time–frequency domain features with extreme resolution.     

Pump-shaped dump optimal control reveals the nuclear reaction pathway of isomerization of a photoexcited cyanine dye

Using optimal control as a spectroscopic tool we decipher the details of the molecular dynamics of the essential multidimensional excited-state photoisomerization - a fundamental chemical reaction of key importance in biology. Two distinct nuclear motions are identified in addition to the overall bond-twisting motion: Initially, the reaction is dominated by motion perpendicular to the torsion coordinate. At later times, a second optically active vibration drives the system along the reaction path to the bottom of the excited-state potential. The time scales of the wavepacket motion on a different part of the excited-state potential are detailed by pump-shaped dump optimal control. This technique offers new means to control a chemical reaction far from the Franck-Condon point of absorption and to map details of excited-state reaction pathways revealing unique insights into the underlying reaction mechanism.     

Tracking ultrafast excited-state bond-twisting motion in solution close to the Franck-Condon point

Applying optimal control to photoinduced trans-cis isomerization in condensed phase, the dynamics of bond-twisting motion of 1,1'-diethyl-4,4'-cyanine in methanol and propanol is revealed. The shape of the optimized pulse resulting from minimization of the photoisomer formation can be directly related to the initial excited-state dynamics in close proximity to the Franck-Condon point. The solvent viscosity-dependent ultrafast wavepacket motion is reflected in the prominent down-chirp of the optimized pulses and reveals a detailed picture of the control mechanism: The reduction of the isomer production is achieved by most efficient dumping of excited population back to the trans ground state. In the higher-viscosity solvent, propanol, wavelength-dependent oscillatory features are superimposed to the overall chirp structure pointing to the importance of excited-state vibrational coherences for the dumping process.     

Femtosecond dynamics of a non-steroidal anti-inflammatory drug (piroxicam) in solution: The involvement of twisting motion

In this contribution, we report on fast and ultrafast dynamics of a non-steroidal anti-inflammatory drug, piroxicam (PX), in methyl acetate (MAC) and triacetin (TAC), two solvents of different viscosities. The enol form of PX undergoes a femtosecond (shorter than 100 fs) electronically excited state intramolecular proton-transfer reaction to produce keto tautomers. These structures exhibit an internal twisting motion to generate keto rotamers in 2–5 ps, a time being longer in TAC. The transient absorption/emission spectrum is very broad indicating that the potential-energy surface at the electronically excited state is very flat, and reflecting the involvement of several coordinates along which the wavepacket of the fs-produced structures evolve.     

Coherent nuclear dynamics in ultrafast photoinduced structural change of bis(diimine)copper(I) complex

The photoinduced structural change of a prototype metal complex, [Cu(dmphen)(2)](+) (dmphen = 2,9-dimethyl-1,10-phenanthroline), was studied by ultrafast spectroscopy with time resolution as high as 30 fs. Time-resolved absorption measured with direct S(1) excitation clearly showed spectral changes attributable to the D(2d) (perpendicular) → D(2) (flattened) structural change occurring in the metal-to-ligand charge transfer singlet excited state ((1)MLCT) and the subsequent S(1) → T(1) intersystem crossing. It was confirmed that the two processes occur with time constants of ~0.8 ps (structural change) and ~10 ps (intersystem crossing), and their time scales are clearly well-separated. A distinct oscillation of the transient absorption signal was observed in the femtosecond region, which arises from the coherent nuclear motion of the perpendicular S(1) state that was directly generated by photoexcitation. This demonstrated that the perpendicular S(1) state has a well-defined vibrational structure and can vibrate within its subpicosecond lifetime. In other words, the S(1) state stays undistorted in a short period, and the coherent nuclear motion is maintained in this state. Time-dependent density functional theory (TDDFT) calculations gave consistent results, indicating a very flat feature and even a local minimum at the perpendicular structure on the S(1) potential energy surface. The vibrational assignments of the S(1) nuclear wavepacket motion were made on the basis of the TDDFT calculation. It was concluded that photoexcitation induces a(1) vibrations containing the Cu-ligand bond length change and a b(1) vibration attributed to the ligand-twisting motion that has the same symmetry as the flattening distortion. Ultrafast spectroscopy and complementary quantum chemical calculation provided an overall picture and new understanding of the photoinduced structural change of the prototypical metal complex.     

Ultrafast stimulated emission and structural dynamics in nickel porphyrins

The excited-state structural dynamics of nickel(II)tetrakis(2,4,6-trimethylphenyl)porphyrin (NiTMP) and nickel(II)tetrakis(tridec-7-yl)porphyrin (NiSWTP) in a toluene solution were investigated via ultrafast transient optical absorption spectroscopy. An ultrashort stimulated emission between 620 and 670 nm from the S1 state was observed in both nickel porphyrins only when this state was directly generated via Q-band excitation, whereas such a stimulated emission was absent under B (Soret)-band excitation. Because the stimulated emission in the spectral region occurs only from the S1 state, this photoexcitation-wavelength-dependent behavior of Ni(II) porphyrins is attributed to a faster intersystem crossing from the S2 state than the internal conversion S2 --> S1. The dynamics of the excited-state pathways involving the (pi, pi*) and (d, d) states varies with the meso-substituted peripheral groups, which is attributed to the nickel porphyrin macrocycle distortion from a planar configuration. Evidence for intramolecular vibrational relaxation within 2 ps and vibrational cooling in 6-20 ps of a (d, d) excited state has been established for NiTMP and NiSWTP. Finally, the lifetimes of the vibrationally relaxed (d, d) state also depend on the nature of the peripheral groups, decreasing from 200 ps for NiTMP to 100 ps for the bulkier NiSWTP.     

Tracking of the nuclear wavepacket motion in cyanine photoisomerization by ultrafast pump-dump-probe spectroscopy

Understanding ultrafast reactions, which proceed on a time scale of nuclear motions, requires a quantitative characterization of the structural dynamics. To track such structural changes with time, we studied a nuclear wavepacket motion in photoisomerization of a prototype cyanine dye, 1,1'-diethyl-4,4'-cyanine, by ultrafast pump-dump-probe measurements in solution. The temporal evolution of wavepacket motion was examined by monitoring the efficiency of stimulated emission dumping, which was obtained from the recovery of a ground-state bleaching signal. The dump efficiency versus pump-dump delay exhibited a finite rise time, and it became longer (97 fs → 330 fs → 390 fs) as the dump pulse was tuned to longer wavelengths (690 nm → 950 nm → 1200 nm). This result demonstrates a continuous migration of the leading edge of the wavepacket on the excited-state potential from the Franck-Condon region toward the potential minimum. A slowly decaying feature of the dump efficiency indicated a considerable broadening of the wavepacket over a wide range of the potential, which results in the spread of a population distribution on the flat S(1) potential energy surface. The rapid migration as well as broadening of the wavepacket manifests a continuous nature of the structural dynamics and provides an intuitive visualization of this ultrafast reaction. We also discussed experimental strategies to evaluate reliable dump efficiencies separately from other ultrafast processes and showed a high capability and possibility of the pump-dump-probe method for spectroscopic investigation of unexplored potential regions such as conical intersections.     

Alignment, vibronic level splitting, and coherent coupling effects on the pump-probe polarization anisotropy

The pump-probe polarization anisotropy is computed for molecules with a nondegenerate ground state, two degenerate or nearly degenerate excited states with perpendicular transition dipoles, and no resonant excited-state absorption. Including finite pulse effects, the initial polarization anisotropy at zero pump-probe delay is predicted to be r(0) = 3/10 with coherent excitation. During pulse overlap, it is shown that the four-wave mixing classification of signal pathways as ground or excited state is not useful for pump-probe signals. Therefore, a reclassification useful for pump-probe experiments is proposed, and the coherent anisotropy is discussed in terms of a more general transition dipole and molecular axis alignment instead of experiment-dependent ground- versus excited-state pathways. Although coherent excitation enhances alignment of the transition dipole, the molecular axes are less aligned than for a single dipole transition, lowering the initial anisotropy. As the splitting between excited states increases beyond the laser bandwidth and absorption line width, the initial anisotropy increases from 3/10 to 4/10. Asymmetric vibrational coordinates that lift the degeneracy control the electronic energy gap and off-diagonal coupling between electronic states. These vibrations dephase coherence and equilibrate the populations of the (nearly) degenerate states, causing the anisotropy to decay (possibly with oscillations) to 1/10. Small amounts of asymmetric inhomogeneity (2 cm(-1)) cause rapid (130 fs) suppression of both vibrational and electronic anisotropy beats on the excited state, but not vibrational beats on the ground electronic state. Recent measurements of conical intersection dynamics in a silicon napthalocyanine revealed anisotropic quantum beats that had to be assigned to asymmetric vibrations on the ground electronic state only [Farrow, D. A.; J. Chem. Phys. 2008, 128, 144510]. Small environmental asymmetries likely explain the observed absence of excited-state asymmetric vibrations in those experiments.     

Contrasts in the 77 K emission spectra, structures, and dynamics of metal-to-metal and metal-to-ligand charge-transfer excited states

The 77 K emission spectrum of trans-[(ms-Me6[14]aneN4)Cr(CNRu(NH3)5)2]5+ has components characteristic of ligand field (LF) and metal-to-metal charge transfer (MMCT) excited states (ms-Me6[14]aneN4=5,12-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). The LF component of the emission is best resolved for irradiations at appreciably higher energies than the MMCT absorption band, while only the MMCT emission is observed for irradiations on the low-energy side of the MMCT absorption band. The LF emission component from this complex has vibronic structure that is very similar to that of the trans-[(ms-Me6[14]aneN4)Cr(CN)2]+ parent, but it is red-shifted by 560 cm-1 and the bandwidths are much larger. The red shift and the larger bandwidths of the ruthenated complex are attributed to configurational mixing between the LF and MMCT excited states, and the inferred mixing parameters are shown to be consistent with the known electron-transfer properties of the Ru(NH3)5 moieties. The MMCT excited-state lifetime is about 1 micros at 77 K and am(m)ine perdeuteration of this complex leads to an isotope effect of kNH/kND approximately 15-20. However, the contribution of the N-H stretching vibration to the emission sideband is too weak for a single vibrational mode model to be consistent with the observed lifetimes or the isotope effect. These features are very similar to those reported previously (J. Phys. Chem. A 2004, 108, 5041) for the MMCT emission of trans-[([14]aneN4)Cr{CNRu(NH3)5}2]5+ ([14]aneN4=1,4,8,11-tetraazacyclotetradecane), with the exception that the higher energy LF emission was not well resolved in the earlier work. The energies of the charge transfer absorption and emission maxima of both of these Cr(CN)Ru complexes are very similar to those of [Ru(NH3)4bpy]2+, but the latter has a 50-fold shorter 77 K excited-state lifetime, a 10-fold smaller NH/ND isotope effect, and a very different structure of its vibronic sidebands. Thus, the vibronic sidebands imply that the dominant excited-state distortions are in the metal-ligand vibrational modes for the Cr(CN)Ru complexes and in the bipyridine vibrational modes for the [Ru(NH3)4bpy]2+ complex. While an "equivalent" single vibrational mode model based on the frequencies and amplitudes of the dominant distortion modes is not consistent the observed lifetimes, such models do appear to be a good basis for qualitatively distinguishing different classes of excited-state dynamic behavior. A multimode, multichannel model may be necessary to adequately describe the excited-state dynamics of these simple electron-transfer systems.     

Excited-state singlet manifold and oscillatory features of a nonatetraeniminium retinal chromophore model

In this paper we use ab initio multireference M?ller-Plesset second-order perturbation theory computations to map the first five singlet states (S(0), S(1), S(2), S(3), and S(4)) along the initial part of the photoisomerization coordinate for the isolated rhodopsin chromophore model 4-cis-gamma-methylnona-2,4,6,8-tetraeniminium cation. We show that this information not only provides an explanation for the spectral features associated to the chromophore in solution but also, subject to a tentative hypothesis on the effect of the protein cavity, may be employed to explain/assign the ultrafast near-IR excited-state absorption, stimulated emission, and transient excited-state absorption bands observed in rhodopsin proteins (e.g. rhodopsin and bacteriorhodopsin). We also show that the results of vibrational frequency computations reveal a general structure for the first (S(1)) excited-state energy surface of PSBs that is consistent with the existence of the coherent oscillatory motions observed both in solution and in bacteriorhodopsin.     

Ultrafast pump-probe study of excited-state charge-transfer dynamics in umecyanin from horseradish root

We have applied femtosecond pump-probe spectroscopy to investigate the excited-state dynamics of umecyanin from horseradish roots, by exciting its 600-nm ligand-to-metal charge-transfer band with a 15-fs pulse and probing over a broad range in the visible region. The decay of the pump-induced ground-state bleaching is modulated by clearly visible oscillations and occurs exponentially with a time constant depending on the observed spectral component of the transmission difference signal, ranging from 270 fs up to 700 fs. The slower decaying process characterizes the spectral component corresponding to the metal-to-ligand charge-transfer transition. The excited-state decay rate is significantly lower than in other blue copper proteins, probably because of the larger energy gap between ligand- and metal-based orbitals in umecyanin. Wavelength dependence of the recovery times could be due to either the excitation of several transitions or the occurrence of intramolecular vibrational relaxation within the excited state. We also find evidence of a hot ground-state absorption, at 700 nm, persisting for several picoseconds. The vibrational coherence induced by the ultrashort pump pulse allows vibrational activity to be observed, mainly in the ground state, as expected in a system with fast excited-state decay. However, we find evidence of a rapidly damped oscillation, which we assign to the excited state. Finally, the Fourier transform of the oscillatory component of the signal presents additional bands in the low-frequency region which are assigned to collective motions of the protein.     

Time resolved spectroscopy on Pigment Yellow 101 in solid state

The photochemistry of the fluorescent Pigment Yellow 101 (P.Y.101) in the crystalline phase is investigated combining time resolved vibrational and electronic spectroscopy. The experiments reveal a complex reaction dynamics spanning several orders of magnitude in time and including excited state intramolecular proton transfer (ESIPT) from the initial trans-diol conformer. Following photoexcitation and the subsequent wavepacket motion out of the Franck–Condon region two tautomers, an excited trans-diol and trans-keto state are formed. The cis–trans isomerization of the keto form, which was experimentally observed and theoretically confirmed in DFT calculations in studies on P.Y.101 in solution is obstructed in the solid state, consequently the formation of the cis conformer is not directly observed. In addition, a long lived photoproduct with red shifted vibrational frequencies is identified. The life time of this intermediate is determined to be 50 μs, although an unambiguous assignment to a specific molecular configuration cannot be given at present.     

Vibrational analysis of excited and ground electronic states of all-trans retinal protonated Schiff-bases

We report on vibrational coherence dynamics in excited and ground electronic states of all-trans retinal protonated Schiff-bases (RPSB), investigated by time-resolved Degenerate Four-Wave-Mixing (DFWM). The results show that wave packet dynamics in the excited state of RPSB consist of only low-frequency (<800 cm(-1)) modes. Such low-frequency wave packet motion is observed over a broad range of detection wavelengths ranging from excited state absorption (～500 nm) to stimulated emission (>600 nm). Our results indicate that low-frequency coherences in the excited state are not activated directly by laser excitation but rather by internal vibrational energy redistribution. This is supported by the observation that similar coherence dynamics are not observed in the electronic ground state. Challenging previous experimental results, we show that the formation of low-frequency coherence dynamics in RPSB does not require significant excess vibrational energy deposition in the excited state vibrational manifolds. Concerning ground state wave packet dynamics, we observe a set of high-frequency (>800 cm(-1)) modes, reflecting mainly single and double bond stretching motion in the retinal polyene-chain. Dephasing of these high-frequency coherences is mode-dependent and partially differs from analogous vibrational dephasing of the all-trans retinal chromophore in a protein environment (bacteriorhodopsin).     

Quantum control spectroscopy of vibrational modes: Comparison of control scenarios for ground and excited states in β-carotene

Quantum control spectroscopy (QCS) is used as a tool to study, address selectively and enhance vibrational wavepacket motion in large solvated molecules. By contrasting the application of Fourier-limited and phase-modulated excitation on different electronic states, the interplay between the controllability of vibrational coherence and electronic resonance is revealed. We contrast control on electronic ground and excited state by introducing an additional pump beam prior to a DFWM-sequence (Pump-DFWM). Via phase modulation of this initial pump pulse, coherent control is extended to structural evolution on the vibrationally hot ground state (hot-S₀) and lowest lying excited state (S₁) of β-carotene. In an open loop setup, the control scenarios for these different electronic states are compared in their effectiveness and mechanism.