From antenna to reaction center: Pathways of ultrafast energy and charge transfer in photosystem II.

The photosystem II core complex (PSII-CC) is the smallest subunit of the oxygenic photosynthetic apparatus that contains core antennas and a reaction center, which together allow for rapid energy transfer and charge separation, ultimately leading to efficient solar energy conversion. However, there is a lack of consensus on the interplay between the energy transfer and charge separation dynamics of the core complex. Here, we report the application of two-dimensional electronic-vibrational (2DEV) spectroscopy to the spinach PSII-CC at 77 K. The simultaneous temporal and spectral resolution afforded by 2DEV spectroscopy facilitates the separation and direct assignment of coexisting dynamical processes. Our results show that the dominant dynamics of the PSII-CC are distinct in different excitation energy regions. By separating the excitation regions, we are able to distinguish the intraprotein dynamics and interprotein energy transfer. Additionally, with the improved resolution, we are able to identify the key pigments involved in the pathways, allowing for a direct connection between dynamical and structural information. Specifically, we show that C505 in CP43 and the peripheral chlorophyll ChlzD1 in the reaction center are most likely responsible for energy transfer from CP43 to the reaction center.

Environment-sensitive fluorescence of COT-fused perylene bisimide based on symmetry-breaking charge separation.

Flexible and aromatic photofunctional system (FLAP) is composed of flapping rigid aromatic wings fused with a flexible 8π ring at the center such as cyclooctatetraene (COT). A series of FLAP have been actively studied for the interesting dynamic behaviors. Here, we synthesized a new flapping molecule bearing naphtho-perylenebisimide wings (NPBI-FLAP), in which two perylene units are arranged side by side. As a reference compound, we also prepared COT-fused NPBI (NPBI-COT) that contains only single perylene unit. In both compounds, inherent strong fluorescence of the NPBI moiety is almost quenched and the FL lifetime becomes much shortened in highly polar solvents (acetone and DMF). Through the analyses of environment-sensitive fluorescence, electrochemical reduction/oxidation, and femtosecond transient absorption, the fluorescence quenching behavior was attributed to rapid symmetry-breaking charge separation (SB-CS) for NPBI-FLAP and to intramolecular charge transfer (ICT) for NPBI-COT. Most of the excited species of these compounds decay with the bent geometry, which is in contrast with the excited-state planarization behavior of a previously reported COT-fused peryleneimides with the double-headed arrangement of the perylene moieties. These results indicate that changing the fusion manners between COT and other π skeletons offers new functional molecules with distinct dynamics.

Rapid-Scan Resonant Two-Dimensional Impulsive Stimulated Raman Spectroscopy of Excited States.

Photochemical reactions occur in the electronically excited state, which is effectively represented by a multidimensional potential energy surface (PES) with a vast degree of freedom of nuclear coordinates. The elucidation of the intricate shape of the PES constitutes an important topic in the field of photochemistry and has long been studied both experimentally and theoretically. Recently, fully time-domain resonant two-dimensional Raman spectroscopy has emerged as a potentially powerful tool to provide unique information about the coupling between vibrational manifolds in the excited state. However, the wide application of this technique has been significantly hampered by the technical difficulties associated with experimental implementation and remains challenging. Herein, we demonstrate time-domain resonant two-dimensional impulsive stimulated Raman spectroscopy (2D-ISRS) of excited states using sub-10 fs pulses based on the rapid scan of the time delay, which facilitates the efficient collection of time-domain vibrational signals with high sensitivity. As a proof-of-principle experiment, we performed 2D-ISRS of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) in solution. Through 2D Fourier transformation of the high-quality time-time oscillatory signal, we obtained a 2D frequency-frequency correlation map of excited-state TIPS-pentacene in the broad frequency window of 0-2000 cm-1. The data clearly resolve a number of cross peaks that signify the correlations among excited-state vibrational manifolds. The high capability of the rapid-scan-based 2D-ISRS spectrometer presented in this study enables the systematic investigation of various photochemical reaction systems, thereby further promoting the understanding and applications of this new multidimensional spectroscopy.

Energy transfer dynamics and the mechanism of biohybrid photosynthetic antenna complexes chemically linked with artificial chromophores.

A light-harvesting strategy is crucial for the utilisation of solar energy. In this study, we addressed the expanding light-harvesting (LH) wavelength of photosynthetic LH complex 2 (LH2, from Rhodoblastus acidophilus strain 10050) through covalent conjugation with extrinsic chromophores. To further understand the conjugation architecture and mechanism of excitation energy transfer (EET), we examined the effects of the linker length and spectral overlap integral between the emission and absorption spectra of the energy donor and acceptor pigments. In the former case, contrary to the intuition based on the Förster resonance energy transfer (FRET) theory, the observed energy transfer rate was similar regardless of the linker length, and the energy transfer efficiency increased with longer linkers. In the latter case, despite the energy transfer rate increases at higher spectral overlaps, it was quantitatively inconsistent with the FRET theory. The mechanism of EET beyond the FRET theory was discussed in terms of the higher-lying exciton state of B850, which mediates efficient EET despite the small spectral overlap. This systematic investigation provides insights for the development of efficient artificial photosynthetic systems.

Ultrafast energy transfer between self-assembled fluorophore and photosynthetic light-harvesting complex 2 (LH2) in lipid bilayer.

Photosynthetic light-harvesting (LH) systems consist of photosynthetic pigments, which are non-covalently self-assembled with protein scaffolds in many phototrophs and attain highly efficient excitation energy transfer via ultrafast dynamics. In this study, we constructed a biohybrid LH system composed of an LH complex (LH2) from Rhodoblastus acidophilus strain 10050 and a hydrophobic fluorophore ATTO647N (ATTO) as an extrinsic antenna in the lipid bilayer. Through the addition of ATTOs into a solution of LH2-reconstituted lipid vesicles, ATTOs were incorporated into the hydrophobic interior of the lipid bilayer to configure the non-covalently self-assembled biohybrid LH. Steady-state fluorescence spectroscopy clearly showed efficient energy transfer from ATTO to B850 bacteriochlorophylls in LH2. Femtosecond transient absorption spectroscopy revealed that the energy transfer took place in the time range of 3-13 ps, comparable to that of the covalently linked LH2-ATTO that we previously reported. In addition, the biohybrid LH system exhibited a much higher antenna effect than the LH2-ATTO system because of the higher loading level of ATTO in the membrane. These findings suggest that the facile self-assembled biohybrid LH system is a promising system for constructing LH for solar-energy conversion.

Vibrational Dephasing along the Reaction Coordinate of an Electron Transfer Reaction.

The role of molecular vibration in photoinduced electron transfer (ET) reactions has been extensively debated in recent years. In this study, we investigated vibrational wavepacket dynamics in a model ET system consisting of an organic dye molecule as an electron acceptor dissolved in various electron donating solvents. By using broad band pump-probe (BBPP) spectroscopy with visible laser pulses of sub-10 fs duration, coherent vibrational wavepackets of naphthacene dye with frequencies spanning 170-1600 cm-1 were observed in the time domain. The coherence properties of 11 vibrational modes were analyzed by an inverse Fourier filtering procedure, and we discovered that the dephasing times of some vibrational coherences are reduced with increasing ET rates. Density functional theory calculations indicated that the corresponding vibrational modes have a large Huang-Rhys factor between the reactant and the product states, supporting the hypothesis that the loss of phase coherence along certain vibrational modes elucidates that those vibrations are coupled to the reaction coordinate of an ET reaction.

Electron-Nuclear Dynamics Accompanying Proton-Coupled Electron Transfer.

Although photoinduced proton-coupled electron transfer (PCET) plays an essential role in photosynthesis, a full understanding of the mechanism is still lacking due to the complex nonequilibrium dynamics arising from the strongly coupled electronic and nuclear degrees of freedom. Here we report the photoinduced PCET dynamics of a biomimetic model system investigated by means of transient IR and two-dimensional electronic-vibrational (2DEV) spectroscopies, IR spectroelectrochemistry (IRSEC), and calculations utilizing long-range-corrected hybrid density functionals. This collective experimental and theoretical effort provides a nuanced picture of the complicated dynamics and synergistic motions involved in photoinduced PCET. In particular, the evolution of the 2DEV line shape, which is highly sensitive to the mixing of vibronic states, is interpreted by accurate computational modeling of the charge separated state and is shown to represent a gradual change in electron density distribution associated with a dihedral twist that occurs on a 120 fs time scale.

Two-dimensional electronic-vibrational spectroscopy: Exploring the interplay of electrons and nuclei in excited state molecular dynamics.

Two-dimensional electronic-vibrational spectroscopy (2DEVS) is an emerging spectroscopic technique which exploits two different frequency ranges for the excitation (visible) and detection (infrared) axes of a 2D spectrum. In contrast to degenerate 2D techniques, such as 2D electronic or 2D infrared spectroscopy, the spectral features of a 2DEV spectrum report cross correlations between fluctuating electronic and vibrational energy gaps rather than autocorrelations as in the degenerate spectroscopies. The center line slope of the spectral features reports on this cross correlation function directly and can reveal specific electronic-vibrational couplings and rapid changes in the electronic structure, for example. The involvement of the two types of transition moments, visible and infrared, makes 2DEVS very sensitive to electronic and vibronic mixing. 2DEV spectra also feature improved spectral resolution, making the method valuable for unraveling the highly congested spectra of molecular complexes. The unique features of 2DEVS are illustrated in this paper with specific examples and their origin described at an intuitive level with references to formal derivations provided. Although early in its development and far from fully explored, 2DEVS has already proven to be a valuable addition to the tool box of ultrafast nonlinear optical spectroscopy and is of promising potential in future efforts to explore the intricate connection between electronic and vibrational nuclear degrees of freedom in energy and charge transport applications.

Sequential energy transfer driven by monoexponential dynamics in a biohybrid light-harvesting complex 2 (LH2).

Enhancing the light-harvesting potential of antenna components in a system of solar energy conversion is an important topic in the field of artificial photosynthesis. We constructed a biohybrid light-harvesting complex 2 (LH2) engineered from Rhodobacter sphaeroides IL106 strain. An artificial fluorophore Alexa Fluor 647 maleimide (A647) was attached to the LH2 bearing cysteine residue at the N-terminal region (LH2-NC) near B800 bacteriochlorophyll a (BChl) assembly. The A647-attached LH2-NC conjugate (LH2-NC-A647) preserved the integrity of the intrinsic chromophores, B800- and B850-BChls, and carotenoids. Femtosecond transient absorption spectroscopy revealed that the sequential energy transfer A647 → B800 → B850 occurs at time scale of 9-10 ps with monoexponential dynamics in micellar and lipid bilayer systems. A B800-removed conjugate (LH2-NC[B800(-)]-A647) exhibited a significant decrease in energy transfer efficiency in the micellar system; however, surprisingly, direct energy transfer from A647 to B850 was observed at a rate comparable to that for LH2-NC-A647. This result implies that the energy transfer pathway is modified after B800 removal. The results obtained suggested that a LH2 complex is a potential platform for construction of biohybrid light-harvesting materials with simple energy transfer dynamics through the site-selective attachment of the external antennae and the modifiable energy-funnelling pathway.

Non-condon Effect on Ultrafast Excited-State Intramolecular Proton Transfer.

The reaction dynamics of excited-state intramolecular proton transfer (ESIPT) of 2,2'-dihydroxyazobenzene (2,2'-DHAB) was investigated by means of white-light supercontinuum femtosecond transient absorption spectroscopy. A coherent in-phase oscillation was observed in the entire wavelength range where stimulated emission of the photoproduct is dominant. This result indicates that the transition strength of the product state is dynamically modulated by a nuclear wavepacket motion (non-Condon effect). The observed vibration was assigned to the mode which modulates the distance between oxygen and hydrogen atoms. By integrating the result of time-dependent density functional theory calculation, the origin of the non-Condon effect was attributed to a dynamical change of configuration interaction between enol and keto characters along the vibrational coordinate, indicating that this vibration is strongly related to the reaction coordinate of ESIPT.

Vibrational decoherence induced by ultrafast intramolecular charge separation of an asymmetric bianthryl derivative.

Coherent wavepacket oscillation accompanying the ultrafast photoexcited intramolecular charge separation (CS) of 9,9'-bianthryl (BA) and 10-cyano-9,9'-bianthryl (CBA) in a room temperature ionic liquid, N,N-diethyl-N-methyl-N-(methoxyethyl)ammonium tetrafluoroborate (DemeBF4), was investigated by femtosecond time-resolved transient absorption spectroscopy. The frequency of the coherent oscillation observed for CBA in nonpolar n-hexane solution (Hex) was 377 cm-1, while this oscillation was undetectable in DemeBF4. For BA in DemeBF4, coherent oscillation with a frequency of 394 cm-1 was observed, which is similar to that for CBA in Hex. CS of CBA occurs in the ultrashort time range of ≤100 fs, while that of BA occurs in a few picosecond range [E. Takeuchi et al., J. Phys. Chem. C 120, 14502-14512 (2016)]. Hence, the oscillation of CBA in Hex and that of BA in DemeBF4 are assigned to the molecular vibration in the locally excited state, while this oscillation dephases instantaneously for CBA in DemeBF4 due to the ultrafast CS and no oscillation was generated in the CS state. This result suggests that the CS reaction is not mediated by a specific intramolecular vibration in the CS state but occurs incoherently through higher levels of multiple vibrational modes.

Ionization dynamics of a phenylenediamine derivative in solutions as revealed by femtosecond simultaneous and stepwise two-photon excitation.

Femtosecond transient absorption spectroscopy with off-resonant simultaneous and resonant stepwise two-photon excitation methods were applied to the direct observation of photoionization dynamics of a phenylenediamine derivative in n-hexane, ethanol and acetonitrile solutions. Upon the selective excitation of the solute via the off-resonant two-photon excitation to the energy level almost equivalent with the ionization potential in the gas phase, rapid appearance of the radical cation (within ca. 100-200 fs) was observed in polar and nonpolar solutions. On the other hand, in the case where the excited energy level from the ground state is 0.8 eV lower than the ionization potential in the gas phase, the radical cation appears only in polar solutions in sub-ps to ps time scales, indicating that the photoionization does not occur directly from the highly electronically excited state even in the polar solution. Comparison of the dynamics between ethanol and acetonitrile solutions strongly suggested that the solvation process of the precursor species leading to the ionization took a crucial role in the electron ejection process with lower energy in polar solutions.

Direct Observation of the Ultrafast Evolution of Open-Shell Biradical in Photochromic Radical Dimer.

Delocalized biradicals have been extensively studied because of fundamental interests to singlet biradicals and several potential applications such as to two-photon absorption materials. However, many of the biradical studies only focus on the static properties of the rigid molecular structures. It is expected that the biradical properties of the delocalized biradicals are sensitive to the subtle changes of the molecular structures and their local environments. Therefore, the studies of the dynamic properties of the system will give further insight into stable radical chemistry. In this study, we directly probe the ultrafast dynamics of the delocalized biradical of a photochromic radical dimer, pentaarylbiimidazole (PABI), by time-resolved visible and infrared spectroscopies and quantum chemical calculations with the extended multistate complete active space second-order perturbation theory (XMS-CASPT2). While the photogenerated transient species was considered to be a single species of the biradical, the present ultrafast spectroscopic study revealed the existence of two transient isomers differing in the contributions of biradical character. The origin of the two metastable isomers is most probably due to the substantial van der Waals interaction between the phenyl rings substituted at the imidazole rings. Unraveling the temporal evolution of the biradical contribution will stimulate to explore novel delocalized biradicals and to develop biradical-based photofunctional materials utilizing the dynamic properties.

Dynamics of Excitation Energy Transfer Between the Subunits of Photosystem II Dimer.

Energy transfer dynamics in monomer and dimer of the photosystem II core complex (PSII-CC) was investigated by means of femtosecond transient absorption (TA) spectroscopy. There is no profound difference between the TA dynamics of the monomer and the dimer in the weak excitation intensity condition (≤21 nJ). However, the fast recovery of the ground state bleach was pronounced at higher excitation intensities, and the excitation intensity dependence of the dimer was more significant than that of the monomer. This result indicates efficient exciton-exciton annihilation taking place in the dimer due to energy transfer between the two monomer units. The annihilation dynamics was reproduced by a simple model based on binomial theorem, which indicated that although PSII-CC dimer has two reaction centers, only one charge-separated state remained after annihilation.

Extension of Light-Harvesting Ability of Photosynthetic Light-Harvesting Complex 2 (LH2) through Ultrafast Energy Transfer from Covalently Attached Artificial Chromophores.

Introducing appropriate artificial components into natural biological systems could enrich the original functionality. To expand the available wavelength range of photosynthetic bacterial light-harvesting complex 2 (LH2 from Rhodopseudomonas acidophila 10050), artificial fluorescent dye (Alexa Fluor 647: A647) was covalently attached to N- and C-terminal Lys residues in LH2 α-polypeptides with a molar ratio of A647/LH2 ≃ 9/1. Fluorescence and transient absorption spectroscopies revealed that intracomplex energy transfer from A647 to intrinsic chromophores of LH2 (B850) occurs in a multiexponential manner, with time constants varying from 440 fs to 23 ps through direct and B800-mediated indirect pathways. Kinetic analyses suggested that B800 chromophores mediate faster energy transfer, and the mechanism was interpretable in terms of Förster theory. This study demonstrates that a simple attachment of external chromophores with a flexible linkage can enhance the light harvesting activity of LH2 without affecting inherent functions of energy transfer, and can achieve energy transfer in the subpicosecond range. Addition of external chromophores, thus, represents a useful methodology for construction of advanced hybrid light-harvesting systems that afford solar energy in the broad spectrum.

Room-Temperature Solution Fluorescence Excitation Correlation Spectroscopy.

Single-molecule fluorescence spectroscopy is a powerful tool for investigating the physical properties of individual molecules, yet elucidating the fast fluctuation dynamics of freely diffusing single molecules in solution at room temperature, where a variety of chemical and biological processes occur, remains challenging. In this study, we report on fluorescence excitation correlation spectroscopy of room-temperature solutions, which enables the study of spontaneous fluctuation of the excitation spectrum with microsecond time resolution. By employing Fourier transform spectroscopy with broadband femtosecond pulses and time-correlated single-photon counting, we achieved fluorescence excitation spectroscopy of a room-temperature solution at the single-molecule level. Building upon this single-molecule measurement, we obtained an excitation wavelength-resolved fluorescence autocorrelation function in the microsecond to millisecond range, demonstrating the potential of this method to elucidate fast, spontaneous, time-dependent changes of excitation spectra in statistically equilibrated systems. With further development, this method will allow the study of spectral exchange associated with transitions between sub-ensembles of solution-phase molecules with unprecedented time resolution.

Quasi-Reversible Photoinduced Displacement of Aromatic Ligands from Semiconductor Nanocrystals.

Organic-inorganic nanohybrids using semiconductor nanocrystals (NCs) coordinated with aromatic organic molecules have been widely studied in the fields of optoelectronic materials, such as solar cells, photocatalysis, and photon upconversion. In these materials, coordination bonds of ligand molecules are usually assumed to be stable during optical processes. However, this assumption is not always valid. In this study, we demonstrate that the coordination bonds between ligand molecules and NCs by carboxyl groups are displaced quasi-reversibly by light irradiation using zinc sulfide (ZnS) NCs coordinated with perylenebisimide (PBI) as a model system. Time-resolved spectroscopy over a wide range of time from tens-of femtosecond to second timescales and density functional theory calculations show that the photoinduced ligand displacement is driven by ultrafast hole transfer from PBI to ZnS NCs and that the dissociated radical anion of PBI survives over the second timescale. Photoinduced ligand displacements are important to be considered in various organic-inorganic nanohybrids and offer a possibility of NCs covered by nonphotoresponsive organic ligands for advanced photofunctional materials.

The initial charge separation step in oxygenic photosynthesis.

Photosystem II is crucial for life on Earth as it provides oxygen as a result of photoinduced electron transfer and water splitting reactions. The excited state dynamics of the photosystem II-reaction center (PSII-RC) has been a matter of vivid debate because the absorption spectra of the embedded chromophores significantly overlap and hence it is extremely difficult to distinguish transients. Here, we report the two-dimensional electronic-vibrational spectroscopic study of the PSII-RC. The simultaneous resolution along both the visible excitation and infrared detection axis is crucial in allowing for the character of the excitonic states and interplay between them to be clearly distinguished. In particular, this work demonstrates that the mixed exciton-charge transfer state, previously proposed to be responsible for the far-red light operation of photosynthesis, is characterized by the ChlD1+Phe radical pair and can be directly prepared upon photoexcitation. Further, we find that the initial electron acceptor in the PSII-RC is Phe, rather than PD1, regardless of excitation wavelength.

Concerted Electron-Nuclear Motion in Proton-Coupled Electron Transfer-Driven Grotthuss-Type Proton Translocation.

Photoinduced proton-coupled electron transfer and long-range two-proton transport via a Grotthuss-type mechanism are investigated in a biomimetic construct. The ultrafast, nonequilibrium dynamics are assessed via two-dimensional electronic vibrational spectroscopy, in concert with electrochemical and computational techniques. A low-frequency mode is identified experimentally and found to promote double proton and electron transfer, supported by recent theoretical simulations of a similar but abbreviated (non-photoactive) system. Excitation frequency peak evolution and center line slope dynamics show direct evidence of strongly coupled nuclear and electronic degrees of freedom, from which we can conclude that the double proton and electron transfer processes are concerted (up to an uncertainty of 24 fs). The nonequilibrium pathway from the photoexcited Franck-Condon region to the E2PT state is characterized by an ∼110 fs time scale. This study and the tools presented herein constitute a new window into hot charge transfer processes involving an electron and multiple protons.

Vibronic mixing enables ultrafast energy flow in light-harvesting complex II.

Since the discovery of quantum beats in the two-dimensional electronic spectra of photosynthetic pigment-protein complexes over a decade ago, the origin and mechanistic function of these beats in photosynthetic light-harvesting has been extensively debated. The current consensus is that these long-lived oscillatory features likely result from electronic-vibrational mixing, however, it remains uncertain if such mixing significantly influences energy transport. Here, we examine the interplay between the electronic and nuclear degrees of freedom (DoF) during the excitation energy transfer (EET) dynamics of light-harvesting complex II (LHCII) with two-dimensional electronic-vibrational spectroscopy. Particularly, we show the involvement of the nuclear DoF during EET through the participation of higher-lying vibronic chlorophyll states and assign observed oscillatory features to specific EET pathways, demonstrating a significant step in mapping evolution from energy to physical space. These frequencies correspond to known vibrational modes of chlorophyll, suggesting that electronic-vibrational mixing facilitates rapid EET over moderately size energy gaps.