ABSTRACT
The oxygen evolution reaction, which involves high overpotential and slow charge-transport kinetics, plays a critical role in determining the efficiency of solar-driven water splitting. The chiral-induced spin selectivity phenomenon has been utilized to reduce by-product production and hinder charge recombination. To fully exploit the spin polarization effect, we herein propose a dual spin-controlled perovskite photoelectrode. The three-dimensional (3D) perovskite serves as a light absorber while the two-dimensional (2D) chiral perovskite functions as a spin polarizer to align the spin states of charge carriers. Compared to other investigated chiral organic cations, R-/S-naphthyl ethylamine enable strong spin-orbital coupling due to strengthened π-π stacking interactions. The resulting naphthyl ethylamine-based chiral 2D/3D perovskite photoelectrodes achieved a high spin polarizability of 75%. Moreover, spin relaxation was prevented by employing a chiral spin-selective L-NiFeOOH catalyst, which enables the secondary spin alignment to promote the generation of triplet oxygen. This dual spin-controlled 2D/3D perovskite photoanode achieves a 13.17% of applied-bias photon-to-current efficiency. Here, after connecting the perovskite photocathode with L-NiFeOOH/S-naphthyl ethylamine 2D/3D photoanode in series, the resulting co-planar water-splitting device exhibited a solar-to-hydrogen efficiency of 12.55%.
ABSTRACT
Anisotropic circular dichroism (CD) refers to the CD of oriented molecules, which varies with the direction of light propagation toward the molecules. Thus, anisotropic CD spectroscopy has been used to investigate the orientations of molecules in anisotropic media such as liquid crystals and thin films. However, it is unclear whether anisotropic CD results from isolated chromophores or their intermolecular interactions with other atoms or molecules that form anisotropically aligned structures. Herein, anisotropic CD of isolated chiral molecules was observed for the first time. The resonant two-photon ionization CD spectra of jet-cooled pseudoephedrine and styrene oxide indicated a difference between the CD values of the P/R and Q branches of the origin bands of the S0-S1 transition. This difference may have resulted from the anisotropic CD phenomena of these molecules, which are oriented via photoselection. Although jet-cooled molecules may have nearly random orientations, those excited to the P/R or Q branch become oriented because the transition probability to these branches depends on the molecular orientation relative to the direction of light propagation. These results demonstrate that the CD spectra of cold, isolated molecules, such as those in an interstellar medium, may exhibit anisotropic CD values.
ABSTRACT
A wireless solar water splitting device provides a means to achieve an inexpensive and highly distributed solar-to-fuel system owing to its portability, flexible scale, and simple design. Here, a highly efficient hydrogen-generating artificial leaf is introduced, which is a wireless configuration for converting solar energy into chemical energy, by integrating a hybrid perovskite (PSK) as the light absorber with catalysts for electrochemical reaction. First, a single integrated photoelectrochemical photocathode, and a spatially decoupled hydrogen evolution reaction catalyst, are fabricated. A decoupled geometry is adopted to enable the physical protection of the PSK layer from the electrolyte, thus allowing excellent stability for over 85 h. Additionally, an efficient dual photovoltaic module photocathode is fabricated to produce sufficient photovoltage to drive water splitting reactions, as well as a high photocurrent to achieve the applied-bias photoconversion efficiency (13.5%). To investigate the overall water splitting performance, a NiFe-OH catalyst is employed, and the device with a wired configuration achieves a photocurrent density of 9.35 mA cm-2 , corresponding to a solar to hydrogen (STH) efficiency of 11.5%. The device with a fully integrated wireless artificial leaf configuration exhibited a similar STH efficiency of over 11%, demonstrating the effectiveness of this cell design.
ABSTRACT
Circular dichroism (CD) spectroscopy of jet-cooled molecules provides conformation-specific CD spectra. However, its widespread utilization has been limited by the weak CD effects and the low density of gas-phase molecules. Here, we developed a dual-beam method to improve the sensitivity and accuracy of gas-phase CD measurements. Circularly and linearly polarized pulses were generated from a single laser pulse and used to irradiate a single molecular-beam pulse to produce two ion peaks. The ion peaks induced by linearly polarized pulses were subtracted from those induced by circularly polarized pulses to correct the CD values for the pulse-to-pulse fluctuations in laser power and gas density. The resonant two-photon ionization CD spectrum of (1R,2R)-(-)-pseudoephedrine revealed that the standard deviations of CD values measured using the dual-beam method were three times lower than those measured using a single-beam method. The dual-beam method provides an effective, accurate, and easy-to-use tool to obtain gas-phase CD spectra.
ABSTRACT
The induced circular dichroism (ICD) of phenol complexed with (R)-(-)-2-butanol [PhOH-(-)BOH] in a supersonic jet is investigated using resonant two-photon ionization circular dichroism (R2PICD) spectroscopy. The R2PICD spectrum of PhOH-(-)BOH exhibits nonzero ICD bands near the absorption region of bare PhOH, where (-)BOH is transparent. Two different conformers containing a single hydrogen bond between PhOH and (-)BOH are identified using ultraviolet-ultraviolet hole-burning and infrared ion-dip spectroscopy combined with quantum theoretical calculations. The ICD values of the two conformers are similar to each other. To understand these similar ICD effects of the conformers, the geometrical asymmetry of the PhOH moiety bound to (-)BOH and the coupling strength of the electric transition dipole moments between PhOH and (-)BOH are estimated. Comparing the ICD values of PhOH-(-)BOH with those of PhOH-(-)-l-methyl lactate in the previous report [ Hong , A. ; J. Phys. Chem. Lett. 2018 , 9 , 476 -480 ], we investigate the physical properties that may govern the differences of the ICD values between the two complexes.
ABSTRACT
We obtained resonant two-photon ionization circular dichroism (R2PICD) spectra of jet-cooled phenylalanine (Phe) and its hydrated clusters (Phe(H2O)n, n = 1-2) near the origin band of the S0-S1 transition. The R2PICD spectra of Phe exhibit well-resolved CD bands of six different conformers present in the jet, which vary in sign and magnitude depending on their conformations. We revised the previous structural assignments of the Phe conformers based on the comparison between the experimental and theoretical CD signs, infrared spectra, and rotational band contours. The R2PICD spectra of Phe(H2O)n reveal that hydration with one or two water molecule(s) does not affect the CD signs of Phe conformers but significantly increases their CD magnitudes. Furthermore, conformational selection by solvation alters relative populations of Phe conformers, leading to a sign inversion in the CD spectra of Phe(H2O)n compared with that of Phe monomer.
ABSTRACT
The resonant two-photon ionization circular dichroism (R2PICD) spectrum represents the cumulative circular dichroism (CD) of one-photon excitation and the subsequent one-photon ionization, whereas the fluorescence-detected circular dichroism (FDCD) spectra exhibit only the CD of one-photon excitation, similar to conventional CD spectra. We obtained the FDCD spectra of jet-cooled ephedrine (EPD) near the origin band of the S0-S1 transition to measure the CD of one-photon absorption and thus the CD of the ionization process in R2PI in comparison with the R2PICD spectra. The CD effects of the ionization following excitation of the A (0-0) and C (930 cm(-1)) bands in the spectrum are small, whereas those of the B band (530 cm(-1)) are anomalously large, leading to opposite CD signs for the FDCD and R2PICD spectra. Based on the intermediate state-selective fragmentation patterns in the R2PI spectra, this large CD effect is attributed to the state-selective isomerization that occurs after excitation of the B band. By comparing the experimental and theoretical spectra, we determined that the B band corresponds to an asymmetric ring distortion mode that involves torsional motions of the side chain, which may facilitate the isomerization process. This study demonstrates that FDCD spectroscopy combined with R2PICD spectroscopy provides a powerful tool to measure the CD effects of the excitation and ionization processes separately in R2PI and thus probe the structural changes that occur during the ionization process following excitation to an intermediate state.
ABSTRACT
The CD spectroscopy of a chiral compound in solution yields an average CD value derived from all of the conformations of a chiral molecule. By contrast, CD spectroscopy of cold chiral molecules in the gas phase distinguishes specific conformers of a chiral molecule, but the weak CD effect has limited the practical application of this technique. Reported herein is the first resonant two-photon ionization CD spectra of ephedrines in a supersonic jet using circularly polarized laser pulses, which were generated by synchronizing the oscillation of the photoelastic modulator with the laser firing. The spectra exhibited well-resolved CD bands which were specific for the conformations and vibrational modes of each enantiomer. The CD signs and magnitudes of the jet-cooled chiral molecules were very sensitive to their conformations and thus offered crucial information for determining the three-dimensional structures of chiral species, as conducted in combination with quantum chemical calculations.
