Competition between annihilation and trapping leads to strongly reduced yields of photochemistry under ps-flash excitation

Photoelectrochemical Two-Dimensional Electronic Spectroscopy (PEC2DES) of Photosystem I: Charge Separation Dynamics Hidden in a Multichromophoric Landscape

We present a nonlinear spectroelectrochemical technique to investigate photosynthetic protein complexes. The PEC2DES setup combines photoelectrochemical detection (PEC) that selectively probes the protein photogenerated charges output with two-dimensional electronic spectroscopy (2DES) excitation that spreads the nonlinear optical response of the system in an excitation-detection map. PEC allows us to distinguish the contribution of charge separation (CS) from other de-excitation pathways, whereas 2DES allows us to disentangle congested spectral bands and evaluate the exciton dynamics (decays and coherences) of the photosystem complex. We have developed in operando phase-modulated 2DES by measuring the photoelectrochemical reaction rate in a biohybrid electrode functionalized with a plant photosystem complex I-light harvesting complex I (PSI-LHCI) layer. Optimizing the photoelectrochemical current signal yields reliable linear spectra unequivocally associated with PSI-LHCI. The 2DES signal is validated by nonlinear features like the characteristic vibrational coherence at 750 cm-1. However, no energy transfer dynamics is observed within the 450 fs experimental window. These intriguing results are discussed in the context of incoherent mixing resulting in reduced nonlinear contrast for multichromophoric complexes, such as the 160 chlorophyll PSI. The presented PEC2DES method identifies generated charges unlike purely optical 2DES and opens the way to probe the CS channel in multichromophoric complexes.

Singlet-singlet annihilation kinetics in aggregates and trimers of LHCII

Singlet-singlet annihilation experiments have been performed on trimeric and aggregated light-harvesting complex II (LHCII) using picosecond spectroscopy to study spatial equilibration times in LHCII preparations, complementing the large amount of data on spectral equilibration available in literature. The annihilation kinetics for trimers can well be described by a statistical approach, and an annihilation rate of (24 ps)(-1) is obtained. In contrast, the annihilation kinetics for aggregates can well be described by a kinetic approach over many hundreds of picoseconds, and it is shown that there is no clear distinction between inter- and intratrimer transfer of excitation energy. With this approach, an annihilation rate of (16 ps)(-1) is obtained after normalization of the annihilation rate per trimer. It is shown that the spatial equilibration in trimeric LHCII between chlorophyll a molecules occurs on a time scale that is an order of magnitude longer than in Photosystem I-core, after correcting for the different number of chlorophyll a molecules in both systems. The slow transfer in LHCII is possibly an important factor in determining excitation trapping in Photosystem II, because it contributes significantly to the overall trapping time.

Theories for kinetics and yields of fluorescence and photochemistry: how, if at all, can different models of antenna organization be distinguished experimentally?

The models most commonly used to describe the antenna organization of the photosynthetic membrane are the connected units model and the domain model. The theoretical descriptions of the exciton dynamics according to these models are reviewed with emphasis on a common nomenclature. Based on this nomenclature we compare for the two models the kinetics and yields of photochemistry and fluorescence under non-annihilation and annihilation conditions both under continuous light and under flash excitation. The general case is considered, that all initially open reaction centers become gradually closed and that exciton transfer between photosynthetic units (PSUs) is possible. Then, calculated kinetics and yields depend on the model assumptions made to account for the exciton transfer between PSUs. Here we extend the connected units model to flash excitation including exciton-exciton annihilation, and present a new simple mathematical formalism of the domain model under continuous light and flash excitation without annihilation. Product and fluorescence yields predicted by the connected units model for different degrees of connectivity are compared with those predicted by the domain model using the same sets of rate constants. From these calculations we conclude that it is hardly possible to distinguish experimentally between different models by any current method. If at all, classical fluorescence induction measurements are more suited for assessing the excitonic connectivity between PSUs than ps experiments.