Direct Photon-by-Photon Analysis of Time-Resolved Pulsed Excitation Data using Bayesian Nonparametrics

Lifetimes of chemical species are typically estimated by either fitting time-correlated single-photon counting (TCSPC) histograms or phasor analysis from time-resolved photon arrivals. While both methods yield lifetimes in a computationally efficient manner, their performance is limited by choices made on the number of distinct chemical species contributing photons. However, the number of species is encoded in the photon arrival times collected for each illuminated spot and need not be set by hand a priori. Here, we propose a direct photon-by-photon analysis of data drawn from pulsed excitation experiments to infer, simultaneously and self-consistently, the number of species and their associated lifetimes from a few thousand photons. We do so by leveraging new mathematical tools within the Bayesian nonparametric. We benchmark our method for both simulated and experimental data for 1-4 species

Demonstration of a Light-Driven SO42- Transporter and Its Spectroscopic Characteristics.

In organisms, ion transporters play essential roles in the generation and dissipation of ion gradients across cell membranes. Microbial rhodopsins selectively transport cognate ions using solar energy, in which the substrate ions identified to date have been confined to monovalent ions such as H+, Na+, and Cl-. Here we report a novel rhodopsin from the cyanobacterium Synechocystis sp. PCC 7509, which inwardly transports a polyatomic divalent sulfate ion, SO42-, with changes of its spectroscopic properties in both unphotolyzed and photolyzed states. Upon illumination, cells expressing the novel rhodopsin, named Synechocystis halorhodopsin (SyHR), showed alkalization of the medium only in the presence of Cl- or SO42-. That alkalization signal was enhanced by addition of a protonophore, indicating an inward transport of Cl- and SO42- with a subsequent secondary inward H+ movement across the membrane. The anion binding to SyHR was suggested by absorption spectral shifts from 542 to 536 nm for Cl- and from 542 to 556 nm for SO42-, and the affinities of Cl- and SO42- were estimated as 0.112 and 5.81 mM, respectively. We then performed time-resolved spectroscopic measurements ranging from femtosecond to millisecond time domains to elucidate the structure and structural changes of SyHR during the photoreaction. Based on the results, we propose a photocycle model for SyHR in the absence or presence of substrate ions with the timing of their uptake and release. Thus, we demonstrate SyHR as the first light-driven polyatomic divalent anion (SO42-) transporter and report its spectroscopic characteristics

Ultrafast vibrational control of organohalide perovskite optoelectronic devices using vibrationally promoted electronic resonance

Vibrational control (VC) of photochemistry through the optical stimulation of structural dynamics is a nascent concept only recently demonstrated for model molecules in solution. Extending VC to state-of-the-art materials may lead to new applications and improved performance for optoelectronic devices. Metal halide perovskites are promising targets for VC due to their mechanical softness and the rich array of vibrational motions of both their inorganic and organic sublattices. Here, we demonstrate the ultrafast VC of FAPbBr3 perovskite solar cells via intramolecular vibrations of the formamidinium cation using spectroscopic techniques based on vibrationally promoted electronic resonance. The observed short (~300 fs) time window of VC highlights the fast dynamics of coupling between the cation and inorganic sublattice. First-principles modelling reveals that this coupling is mediated by hydrogen bonds that modulate both lead halide lattice and electronic states. Cation dynamics modulating this coupling may suppress non-radiative recombination in perovskites, leading to photovoltaics with reduced voltage losses

Tracking ultrafast chemical reactions at the aqueous interface with femtosecond time-resolved HD-VSFG spectroscopy

There have been numbers of reports suggesting that chemical reactions at the water interfaces are different from the reactions in the bulk phase. However, it is very difficult to directly investigate chemical reactions at the water interfaces because of lack of suitable experimental methods. Heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy is a powerful technique to study interfaces.1 Combined with the pump-probe method, HD-VSFG has been extended to time-resolved measurements, which opened a new door to investigate ultrafast dynamics at interfaces.2 HD-VSFG spectroscopy enables us to directly measure the spectrum of the second-order susceptibility (χ(2)) although conventional VSFG spectroscopy with homodyne detection can only provide the spectra of the absolute square of χ(2) (|χ(2)|2). This advantage of HD-VSFG becomes even more critical in the time-resolved measurements which detect the pump-induced change of the spectra. In fact, homodyne time-resolved VSFG can provide the pump-induced change of |χ(2)|2 (Δ|χ(2)|2) but it is very difficult to interpret it. In contrast, time-resolved HD-VSFG directly gives Δχ(2) spectra and, in particular, the imaginary part of Δχ(2) (ΔImχ(2)) can be directly compared to the time-resolved infrared and Raman spectra which correspond to ΔImχ(1) and ΔImχ(3) spectra, respectively. Fully utilizing this advantage of HD-VSFG, we developed UV-excited time-resolved HD-VSFG spectroscopy which enables tracking photochemical reactions and short-lived intermediates at aqueous interfaces.3 Very recently, we succeeded in tracking the photochemical reaction of phenol at the water interface.4 We observed several transients at the interface with femtosecond time resolution, and they were attributed to the reaction intermediates that also appear in the reaction in the solution phase. Surprisingly, however, it was found that dynamics at the interface is drastically accelerated, compared to the corresponding reaction in solution. We consider that this marked difference arises from the unique solvation structure around phenol at the interface,5 which significantly changes the relevant excited-state potential energy surface of phenol at the water interface. Largely different solvation environments at the interface is expected for all kinds of molecules, implying generality of the observation in our study, i.e., great difference in chemical reactions between the interface and the bulk.Published versio

Fifth-order impulsive stimulated Raman spectroscopy for visualizing vibrational coupling in reactive excited states

Chemical reactions proceed on the complex potential energy surface (PES), which consists of a vast degree of freedom of nuclear coordinates for polyatomic molecular systems. For unraveling (and manipulating) the reaction coordinate and molecular mechanisms that underlie the reaction, it is desirable to map out the PES, which has been a long-lasting central subject in both experimental and theoretical chemistries. In this quest, understanding of the vibrational coupling between normal mode coordinates is essential, since it actually characterizes the complex shape of the PES. Here, we report fifth-order time-domain Raman spectroscopy of a bacterial photoreceptor, photoactive yellow protein (PYP), with the aim to visualize vibrational coupling in its reactive excited state.Published versio