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.

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.

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.

A functional linkage between light-harvesting and photocatalytic components is a pivotal issue for using solar energy in chemical conversions; however, this concept is far from being practically realized. Here, we constructed a system that integrates an artificially extended photosynthetic light-harvesting complex 2 (LH2) and a light-harvesting 1-reaction center (RC) core complex (LH1-RC). A biohybrid LH2, whose light-harvesting ability was extended by a covalently attached hydrophobic fluorophore ATTO647N (LH2-ATTO) to cover the absorption gap of LH2, was assembled with LH1-RC in a lipid bilayer. Femtosecond transient absorption spectroscopy revealed that upon an excitation of the ATTO, sequential excitation energy transfer (EET), ATTO → LH2 → LH1 → RC, was followed by a subsequent electron transfer in the RC with rates comparable to those in a native system. As a proof of concept, the functional linkage between light-harvesting and photocatalytic reactions was quantitatively demonstrated through the enhancement in the charge separation yield and photocurrent generation activity of LH2-ATTO/LH1-RC assembled on an electrode.

We demonstrated one-colour control of activation, excitation and deactivation of a fluorescent diarylethene derivative by using a 532 nm CW laser at a weak output power. This one-colour control method was applied to single-molecule tracking in polymer films over a total duration of a few hours at room temperature and PALM for the nanostructures of polymer systems.

Solvent polarity dependence of photochromic reactions such as cyclization and cycloreversion of a photochromic diarylethene derivative, 1,2-bis­(2-methyl-3-benzothienyl)­perfluorocyclopentene, was investigated by steady-state spectroscopic and femtosecond laser photolysis methods. For the cyclization reaction, it was revealed that the quantum yield decreased with an increase in solvent polarities, mainly due to the decrease in the fraction of the conformer with C 2 symmetry favorable for the cyclization. This result indicated that the branching ratio for the cyclization and the deactivation to the open-ring isomer at the conical intersection was almost independent of the solvent polarity. On the other hand, it was found for the cycloreversion process that the closed-ring isomer in the S1 state rapidly deactivated into the ground state in competition with the activated process leading to the conical intersection providing a pathway toward both open- and closed-ring minima in the ground state. The cycloreversion reaction quantum yield also decreasing with an increase in the solvent polarity was attributed to larger increase of the direct deactivation into the ground state from the excited state minimum of the closed-ring isomer.

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.

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.