Coherent Wave Packet Motion Research Articles

Spectral broadening and vibronic dynamics of the S2 state of canthaxanthin in the orange carotenoid protein

We have performed a series of broadband multidimensional electronic spectroscopy experiments to probe the electronic and vibrational dynamics of the canthaxanthin chromophore of the Orange Carotenoid Protein (OCP) from Synechocystis sp. PCC 6803 in its photoactivated red state, OCPR. Cross-peaks observed below the diagonal of the two-dimensional electronic spectrum indicate that absorption transitions prepare the bright S2 state of the ketocarotenoid canthaxanthin near to a sequence of conical intersections, allowing passage to the dark S1 state via the Sx intermediate in <50 fs. Rapid damping of excited-state coherent wavepacket motions suggests that the branching coordinates of the conical intersections include out-of-plane deformation and C=C stretching coordinates of the π-conjugated isoprenoid backbone. The unusual proximity of the Franck–Condon S2 state structure to the conical intersections with Sx and S1 suggests that the protein surroundings of canthaxanthin prepare it to function as an excitation energy trap in the OCPR–phycobilisome complex. Numerical simulations using the multimode Brownian oscillator model demonstrate that the ground-state absorption spectrum of OCPR overlaps with the fluorescence emission spectrum of allophycocyanin due to spectral broadening derived especially from the intramolecular motions of the canthaxanthin chromophore in its binding site.

Real-time capture of nuclear motions influencing photoinduced electron transfer.

Although vibronic coupling phenomena have been recognized in the excite state dynamics of transition metal complexes, its impact on photoinduced electron transfer (PET) remains largely unexplored. This study investigates coherent wavepacket (CWP) dynamics during PET processes in a covalently linked electron donor-acceptor complex featuring a cyclometalated Pt(ii) dimer as the donor and naphthalene diimide (NDI) as the acceptors. Upon photoexciting the Pt(ii) dimer electron donor, ultrafast broadband transient absorption spectroscopy revealed direct modulation of NDI radical anion formation through certain CWP motions and correlated temporal evolutions of the amplitudes for these CWPs with the NDI radical anion formation. These results provide clear evidence that the CWP motions are the vibronic coherences coupled to the PET reaction coordinates. Normal mode analysis identified that the CWP motions originate from vibrational modes associated with the dihedral angles and bond lengths between the planes of the cyclometalating ligand and the NDI, the key modes altering their π-interaction, consequently influencing PET dynamics. The findings highlight the pivotal role of vibrations in shaping the favorable trajectories for the efficient PET processes.

Origin of Vibronic Coherences During Carrier Cooling in Colloidal Quantum Dots.

Recent two-dimensional electronic spectroscopy experiments [Tilluck et al. J. Phys. Chem. Lett. 2021, 12 (39), 9677-9683] indicate the creation of coherent vibronic wavepackets in the first femtoseconds of hot carrier cooling in hexadecylamine-passivated CdSe quantum dots. Here we present a quantum chemical study of the origin of these coherences in a CdSe nanocrystal. We find that coherent wavepacket motions along vibrational coordinates with alkylamine character promote nonradiative relaxation through conical intersections between the exciton states of the inorganic core. Electronic excitations in the core are found to pass energy to the vibrations of the ligands via two distinct mechanisms: excitation of core phonon modes that are coupled to the ligand vibrations and direct excitation of ligand vibrations by delocalization of the exciton onto the ligands, both of which naturally arise within a photochemical framework based on many-electron potential energy surfaces. If these findings are demonstrated to be general, vibronic coherences may be leveraged to control photophysical outcomes in colloidal quantum dots.

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.

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

AbstractPhotoexcited 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.

Quantum pathways of carrier and coherent phonon excitation in bismuth

Ultrafast mid-infrared excitation of an electron-hole plasma breaks the crystal symmetry of bismuth transiently and opens alternative quantum pathways for the excitation of coherent lattice motions. Probing the transient crystal structure directly by femtosecond x-ray diffraction reveals oscillations of diffracted intensity at a frequency of 2.6 THz, which persist on a picosecond time scale. They reflect coherent wave packet motions along back-folded phonon coordinates in the crystal of reduced symmetry. Optically induced symmetry breaking thus allows for modifying phonon excitations.

Spectroscopic mapping of the gold complex oligomers (dimer, trimer, tetramer, and pentamer) by excited-state coherent nuclear wavepacket motion in aqueous solutions.

We investigate the excited-state dynamics of the [Au(CN)2-] oligomers following photo-initiated intermolecular Au-Au bond formation by carrying out femtosecond time-resolved absorption and emission measurements at various concentrations (0.080-0.6 mol dm-3) with different photoexcitation wavelengths (290-340 nm). The temporal profiles of the time-resolved absorption signals exhibit clear oscillations arising from the Au-Au stretch coherent wavepacket motion of the excited-state oligomers, which is initiated with the photo-induced Au-Au bond formation. The frequency of the observed oscillation is changed with the change of the concentration, excitation wavelength, and wavelength of the excited-state absorption monitored, reflecting the change in the size of the oligomers detected. Fourier transforms (FTs) of the oscillations provide 2D plots of the FT amplitude against the oscillation frequency versus the detected wavelengths. Because the FT amplitude exhibits a node at the peak wavelength of the absorption of the species that gives rise to the oscillation, the 2D plots enabled us to determine the peak wavelength of the excited-state absorption of the dimer, trimer, tetramer, and pentamer. We also performed femtosecond time-resolved absorption measurements for the 0.3 mol dm-3 solution with 260 nm photoexcitation, which is the condition employed in previous time-resolved X-ray studies (e.g., K. H. Kim et al. Nature, 2015, 518 (7539), 385-389). It was found that various excited-state oligomers, including the dimer, were simultaneously generated under this condition, although the analysis of the previous time-resolved X-ray studies was made by assuming that only the excited-state trimer was generated. The obtained results show that the excited-state dynamics of the trimer claimed based on the time-resolved X-ray data is questionable and that re-analysis and re-examining of its data are necessary.

Excitation energy transfer and vibronic coherence in intact phycobilisomes.

The phycobilisome is an oligomeric chromoprotein complex that serves as the principal mid-visible light-harvesting system in cyanobacteria. Here we report the observation of excitation-energy-transfer pathways involving delocalized optical excitations of the bilin (linear tetrapyrrole) chromophores in intact phycobilisomes isolated from Fremyella diplosiphon. By using broadband multidimensional electronic spectroscopy with 6.7-fs laser pulses, we are able to follow the progress of excitation energy from the phycoerythrin disks at the ends of the phycobilisome's rods to the C-phycocyanin disks along their length in <600 fs. Oscillation maps show that coherent wavepacket motions prominently involving the hydrogen out-of-plane vibrations of the bilins mediate non-adiabatic relaxation of a manifold of vibronic exciton states. However, the charge-transfer character of the bilins in the allophycocyanin-containing segments localizes the excitations in the core of the phycobilisome, yielding a kinetic bottleneck that enables photoregulatory mechanisms to operate efficiently on the >10-ps timescale.

Ultrafast branching in intersystem crossing dynamics revealed by coherent vibrational wavepacket motions in a bimetallic Pt(II) complex.

Ultrafast excited state processes of transition metal complexes (TMCs) are governed by complicated interplays between electronic and nuclear dynamics, which demand a detailed understanding to achieve optimal functionalities of photoactive TMC-based materials for many applications. In this work, we investigated a cyclometalated platinum(II) dimer known to undergo a Pt-Pt bond contraction in the metal-metal-to-ligand-charge-transfer (MMLCT) excited state using femtosecond broadband transient absorption (fs-BBTA) spectroscopy in combination with geometry optimization and normal mode calculations. Using a sub-20 fs pump and broadband probe pulses in fs-BBTA spectroscopy, we were able to correlate the coherent vibrational wavepacket (CVWP) evolution with the stimulated emission (SE) dynamics of the 1MMLCT state. The results demonstrated that the 145 cm-1 CVWP motions with the damping times of ∼0.9 ps and ∼2 ps originate from coherent Pt-Pt stretching vibrations in the singlet and triplet MMLCT states, respectively. On the basis of excited state potential energy surface calculations in our previous work, we rationalized that the CVWP transfer from the Franck-Condon (FC) state to the 3MMLCT state was mediated by a triplet ligand-centered (3LC) intermediate state through two step intersystem crossing (ISC) on a time scale shorter than a period of the Pt-Pt stretching wavepacket motions. Moreover, it was found that the CVWP motion had 110 cm-1 frequency decays with the damping time of ∼0.2 ps, matching the time constant of 0.253 ps, corresponding to a redshift in the SE feature at early times. This observation indicates that the Pt-Pt bond contraction changes the stretching frequency from 110 to 145 cm-1 and stabilizes the 1MMLCT state relative to the 3LC state with a ∼0.2 ps time scale. Thus, the ultrafast ISC from the 1MMLCT to the 3LC states occurs before the Pt-Pt bond shortening. The findings herein provide insight into understanding the impact of Pt-Pt bond contraction on the ultrafast branching of the 1MMLCT population into the direct (1MMLCT → 3MMLCT) and indirect ISC pathways (1MMLCT → 3LC → 3MMLCT) in the Pt(II) dimer. These results revealed intricate excited state electronic and nuclear motions that could steer the reaction pathways with a level of detail that has not been achieved before.

Vibronic Excitons and Conical Intersections in Semiconductor Quantum Dots.

Surface defects and organic surface-capping ligands affect the photoluminescence properties of semiconductor quantum dots (QDs) by altering the rates of competing nonradiative relaxation processes. In this study, broadband two-dimensional electronic spectroscopy reveals that absorption of light by QDs prepares vibronic excitons, excited states derived from quantum coherent mixing of the core electronic and ligand vibrational states. Rapidly damped coherent wavepacket motions of the ligands are observed during hot-carrier cooling, with vibronic coherence transferred to the photoluminescent state. These findings suggest a many-electron, molecular theory for the electronic structure of QDs, which is supported by calculations of the structures of conical intersections between the exciton potential surfaces of a small ammonia-passivated model CdSe nanoparticle.

Ultrafast Excited-State Dynamics of Photoluminescent Pt(II) Dimers Probed by a Coherent Vibrational Wavepacket.

Intricate potential energy surfaces (PESs) of some transition metal complexes (TMCs) pose challenges in mapping out initial excited-state pathways that could influence photochemical outcomes. Ultrafast intersystem crossing (ISC) dynamics of four structurally related platinum(II) dimer complexes were examined by detecting their coherent vibrational wavepacket (CVWP) motions of Pt-Pt stretching mode in the metal-metal-to-ligand-charge-transfer excited states. Structurally dependent CVWP behaviors (frequency, dephasing time, and oscillation amplitudes) were captured by femtosecond transient absorption spectroscopy, analyzed by short-time Fourier transformation, and rationalized by quantum mechanical calculations, revealing dual ISC pathways. The results suggest that the ligands could fine-tune the PESs to influence the proximity of the conical intersections of the excited states with the Franck-Condon state and thus to control the branching ratio of the dual ISC pathways. This comparative study presents future opportunities in control excited-state trajectories of TMCs via ligand structures.

Time-Domain Observation of Surface-Enhanced Coherent Raman Scattering with 105-106 Enhancement.

Combining surface-enhanced Raman scattering (SERS) with the coherent nonlinear Raman technique is a promising route for achieving higher sensitivity and time-resolved SERS measurements, yet such attempts have just been started. Here, we report time-domain Raman measurements of trans-1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on gold nanoparticle assemblies (GNAs), which were carried out with impulsive stimulated Raman spectroscopy using sub-8 fs pulses. We observe coherent nuclear wavepacket motion of BPE on GNAs with drastic enhancement through the surface plasmon resonance, which provides information on the Raman-active vibrations in the time domain. Through Fourier transform of the measured time-domain Raman data, we obtained SERS spectra of BPE on GNAs with enhancement factors as high as 105-106. The present study not only demonstrates applicability of time-domain nonlinear Raman techniques in SERS, i.e., surface-enhanced impulsive stimulated Raman spectroscopy (SE-ISRS), but also provides a technical basis for femtosecond time-resolved SE-ISRS experiments to track ultrafast dynamics of the adsorbates.

Femtosecond Polarization Switching in the Crystal of a [CrCo] Dinuclear Complex.

Capability to control macroscopic molecular properties with external stimuli offers the possibility to exploit molecules as switching devices of various types. However, application of such molecular-level switching has often been limited by its speed and thus efficiency. Herein, we demonstrate ultrafast, photoinduced polarization switching in the crystal of a [CrCo] dinuclear complex by ultrafast pump-probe spectroscopy in the visible and mid-infrared regions. The photoinduced polarization switching was found to have a time constant of 280 fs, which makes the [CrCo] complex crystal the fastest polarization-switching material realized using the metastable state. Moreover, the pump-probe data in the visible region reveal the pronounced appearance of coherent nuclear wavepacket motion with a frequency as low as 22 cm-1 , which we attribute to a lattice vibrational mode. The pronounced non-Condon effect for its resonance Raman enhancement implies that this mode couples the relevant electronic states, thereby facilitating the ultrafast polarization switching.

Femtosecond Polarization Switching in the Crystal of a [CrCo] Dinuclear Complex

AbstractCapability to control macroscopic molecular properties with external stimuli offers the possibility to exploit molecules as switching devices of various types. However, application of such molecular‐level switching has often been limited by its speed and thus efficiency. Herein, we demonstrate ultrafast, photoinduced polarization switching in the crystal of a [CrCo] dinuclear complex by ultrafast pump–probe spectroscopy in the visible and mid‐infrared regions. The photoinduced polarization switching was found to have a time constant of 280 fs, which makes the [CrCo] complex crystal the fastest polarization‐switching material realized using the metastable state. Moreover, the pump–probe data in the visible region reveal the pronounced appearance of coherent nuclear wavepacket motion with a frequency as low as 22 cm−1, which we attribute to a lattice vibrational mode. The pronounced non‐Condon effect for its resonance Raman enhancement implies that this mode couples the relevant electronic states, thereby facilitating the ultrafast polarization switching.

Vibrational coherence in the composition-selected wavepacket of photoexcited pyrimidine.

Coherent wavepacket motion in photoexcited pyrimidine has been initiated and visualized in real time using femtosecond time-resolved ion-yield spectroscopy. A coherent superposition of at least four low-frequency Frank-Condon (FC) active modes is created in the first excited electronic state (S1), leading to a vibrational wavepacket. Its composition is manipulated experimentally by tuning the excitation wavelength in the range 309-313 nm to populate the selected vibrational levels. Interference among these vibrational levels is directly characterized by a clear quantum beat superimposed on a single-exponential decay. Fourier transform analysis of the wavelength-dependent transients shows modulation at different frequencies, providing a direct signature of multi-mode vibrational coherence resulting from the coherent excitation process. The sensitivity of the parent-ion transient to the vibrational wavepacket dynamics probably arises because different modes are connected by variable FC factors to the 3s and 3p Rydberg states.

Vibrational coherence in ultrafast electron transfer reaction observed by broadband transient absorption spectroscopy

Photoinduced electron transfer (ET) is one of the most important processes in light energy conversion systems. Marcus theory is one of the most famous frameworks on ET reaction in condensed phase. In this theory, reorganization of surrounding media is regarded as a main reaction coordinate. However, ultrafast ET beyond Marcus’ framework has been reported in various systems. To elucidate the role of molecular vibration in ultrafast ET, we have investigated the ET dynamics between a naphthacene dye and aniline derivatives by means of broadband transient absorption spectroscopy. Coherent wavepacket motions of naphthacene dye with frequencies of 300-1600 cm-1 were observed in time domain. The vibrational coherence of 310 cm-1 mode was reduced with increasing electron transfer rate, suggesting this vibration is coupled to electron transfer reaction.

Following coherent multichannel nuclear wave packets in pump-probe studies of O2 with ultrashort laser pulses

We reexamine the recent pump-probe experiment with ${\mathrm{O}}_{2}$ using short intense infrared laser pulses theoretically. Using parameters that closely mimic the experimental conditions and taking into account the angle-dependent population redistribution due to resonant coupling between the relevant states, we show that the observed kinetic energy release spectra, including the energy-dependent structure and the quantum beat frequencies, can be accurately reproduced. Our results reveal additional important channels that were missed earlier. In particular, the strong contributions from ${\mathrm{O}}_{2}{}^{+}\phantom{\rule{4pt}{0ex}}a{\phantom{\rule{0.16em}{0ex}}}^{4}{\mathrm{\ensuremath{\Pi}}}_{u}$ and $b{\phantom{\rule{0.16em}{0ex}}}^{4}{\mathrm{\ensuremath{\Sigma}}}_{g}^{\ensuremath{-}}$ states lead to the possibility of observing the interchannel beating. We further demonstrate that, by varying the laser parameters, the coherent nuclear wave-packet motions on different potential energy surfaces (PESs) can be probed and the properties of the PES can be examined. Future experiments with different wavelength lasers are proposed for better probing and controlling nuclear dynamics on different PESs.

Solvent dependent trans → cis photoisomerization of N, N′-diacetylindigo studied by femtosecond time-resolved transient absorption spectroscopy

Solvent dependence of trans→cis photoisomerization of N,N′-diacetylindigo (DAI) was investigated by femtosecond transient absorption (TA) spectroscopy. A strong and sharp singlet excited state absorption band centered at 595nm was observed in the TA spectrum and the differential absorbance (ΔAbs) and the wavelength at the maximum of the band were periodically modulated by coherent vibrational wavepacket motions. However, even in the “reactive solvent”, ethyl acetate (EtAc), where the isomerization takes place, the formation of cis-DAI was unobservable in our experimental time window of 2ns which means that isomerization is occurring in a longer time scale. Preceding studies suggest that intermolecular hydrogen-bond (H-bond) with the solvent quenches the excited state and blocks the isomerization. Interestingly, we have observed that dimethylformamide (DMF) blocked the isomerization more efficiently than methanol. The excited state lifetime was shorter in DMF compared to EtAc solution suggesting that weak H-bonding with the formyl group of DMF is sufficient to prevent the slow trans→cis photoisomerization of DAI.

The Primary Photochemistry of Vision Occurs at the Molecular Speed Limit.

Ultrafast photochemical reactions are initiated by vibronic transitions from the reactant ground state to the excited potential energy surface, directly populating excited-state vibrational modes. The primary photochemical reaction of vision, the isomerization of retinal in the protein rhodopsin, is known to be a vibrationally coherent reaction, but the Franck-Condon factors responsible for initiating the process have been difficult to resolve with conventional time-resolved spectroscopies. Here we employ experimental and theoretical 2D photon echo spectroscopy to directly resolve for the first time the Franck-Condon factors that initiate isomerization on the excited potential energy surface and track the reaction dynamics. The spectral dynamics reveal vibrationally coherent isomerization occurring on the fastest possible time scale, that of a single period of the local torsional reaction coordinate. We successfully model this process as coherent wavepacket motion through a conical intersection on a ∼30 fs time scale, confirming the reaction coordinate as a local torsional coordinate with a frequency of ∼570 cm-1. As a result of spectral features being spread out along two frequency coordinates, we unambiguously assign reactant and product states following passage through the conical intersection, which reveal the key vibronic transitions that initiate the vibrationally coherent photochemistry of vision.

Vibrational Spectrum of an Excited State and Huang-Rhys Factors by Coherent Wave Packets in Time-Resolved Fluorescence Spectroscopy.

Coherent nuclear wave packet motions in an electronic excited state of a molecule are measured directly by time-resolved spontaneous fluorescence spectroscopy with an unprecedented time resolution by using two-photon absorption excitation and fluorescence upconversion by noncollinear sum frequency generation. With an estimated time resolution of approximately 25 fs, wave packet motions of vibrational modes up to 1600 cm-1 are recorded for coumarin 153 in ethanol. Two-color transient absorption at 13 fs time resolution are measured to confirm the result. Vibrational displacements between the ground and excited states and Huang-Rhys factors (HRFs) are calculated by quantum mechanical methods and are compared with the experimental results. HRFs calculated by density functional theory (DFT) and time-dependent DFT reproduce the experiment adequately. This fluorescence-based method provides a unique and direct way to obtain the vibrational spectrum of a molecule in an electronic excited state and the HRFs, as well as the dynamics of excited states, and it might provide information on the structure of an excited state through the HRFs.