Ying-Zhong Ma

The photoreaction and functionalities of metastable state photoacids (mPAHs) have been examined almost exclusively through one-photon excitation so far. Here, we report two-photon excited fluorescence measurements on an aqueous solution of a highly water-soluble indazole mPAH. The readily observable fluorescence emission was found to not only follow the anticipated quadratic intensity dependence but also possess a sizeable two-photon absorption cross-section of ∼53 GM. The distinct time-resolved fluorescence decays and emission spectra observed upon one- and two-photon excitation offer indirect evidence for the activation of different isomers and their associated pathways in the respective photoreactions. Our findings pave a promising way of unlocking new functionalities and applications of these mPAHs that are inaccessible to one-photon excitation.

Generation of quantum states of light through off-resonance four-wave mixing in rubidium vapor is a straightforward and well-established technique. However, achieving a sufficiently high and consistent level of intensity difference squeezing (IDS) and intrinsic quantum correlations between photon pairs over an extended timescale, necessary for quantum-light-based nonlinear optical spectroscopy and microscopy, remains challenging and largely unexplored. Here, we report a simple stabilization method combining an active periodic laser frequency retuning with an automatic control algorithm based on quantitatively assessing the factors affecting the IDS level and squeezed light intensity. Validation of our method was performed by acquiring data over a 5-hour period, and the results demonstrate a remarkably stable squeezing level of -7.8 dB in combination with a > 4× reduction of the standard deviation of IDS level from 0.46 dB to 0.10 dB. The achieved stabilization further enables us to quantitatively assess the IDS reduction due to the scattering in a polystyrene bead suspension as a function of sample transmission. Our approach should enable a variety of applications requiring an extended squeezing stability over multiple hours, especially for those following biological processes and chemical reactions in real time.

It has been demonstrated recently that the use of specific quantum states of light for two-photon microscopy can lead to a substantial reduction of the intensity of light needed for the imaging of biological objects. This opens new ways to better understand the delicate mechanisms of the interactions of living organisms in the rhizosphere on an extended time scale without optical damage or alteration of their native behavior. Herein, we report a successful realization of entangled two-photon excited fluorescence microscopy of wheat plant roots and a set of biological samples, including purified iLOV protein, bacterial cells expressing iLOV, and combination of these under an extremely low excitation intensity. The entangled two-photon coupling strength of iLOV was modeled using electronic structure calculations which demonstrated that the FMN chromophore in the iLOV protein has an entangled two-photon absorption (ETPA) cross section that is large enough for imaging using weak quantum light for fluorescence excitation. Successful imaging of plant-associated Pantoea sp. YR343 cells expressing iLOV validates the feasibility of using iLOV as a molecular label for characterizing plant-microbe interactions based on entangled photon imaging. The results demonstrate the advantageous use of entangled photon imaging under noninvasive and nondestructive conditions.

Direct air capture (DAC) may reduce atmospheric CO₂ concentrations to preindustrial levels, yet the high energies and temperatures involved in current DAC technologies hinder large-scale deployment. In the case of aqueous-based CO₂ absorbents, a large energetic penalty is associated with heating and boiling off water, as required for thermally driven solvent regeneration. This could be avoided via photochemically driven pH swings involving photoacids or photobases, and harnessing abundant and renewable solar energy, though efficient solvent regeneration and recycling in a realistic multicycle DAC process remains challenging. Herein, we report a photochemically driven DAC process (photo-DAC) in which atmospheric CO₂ capture by an aqueous glycylglycine (GlyGly) solution is enabled through a pH swing by a pyridine-substituted diiminoguanidine (PyDIG) photobase. Upon irradiation with UV light, the PyDIG photobase undergoes photoisomerization from the E,E to the Z,Z isomer, corresponding to a pK_a increase of 2.8 units that activates GlyGly for DAC through deprotonation. After the GlyGly/PyDIG solvent is saturated with atmospheric carbon dioxide, leaving it in the dark under ambient conditions leads to the isomerization of PyDIG from the Z,Z back to the E,E isomer, which is accompanied by a pH drop and CO₂ release. To demonstrate the recyclability of the GlyGly/PyDIG solvent, we have completed six consecutive DAC cycles, with a measured average cyclic capacity in the range of 0.21-0.26 mol CO₂ per mol of GlyGly/PyDIG. These results open the prospect for energy-efficient DAC cycles completed entirely at ambient conditions, thereby avoiding the significant energy penalties associated with heating and boiling aqueous solvents.

This Review discusses how molecular organization at functionalized interfaces governs CO ₂ transport and capture, and how reversible photoacids and photobases enable light-driven CO ₂ binding and release.

The photoreaction of a metastable‐state photoacid (mPAH) generally involves multiple isomers with various connected pathways of photoinduced structural changes during a single reaction cycle. However, only a limited number of isomers have been identified experimentally so far owing to the inherent complexity in combination with the presence of various competing electronic and vibrational processes, as well as the constantly varying interactions between mPAH isomers and solvent molecules. Here, an optical spectroscopic study on a benzimidazole‐based mPAH, a novel photoacid using benzimidazole as the structural moiety with the active proton, is reported. Through measurements of linear absorption and steady‐state fluorescence in neat solvents and binary mixtures, a pronounced effect of neat water and its binary mixture with glycerol is discovered on the photoreaction of this benzimidazole‐mPAH, manifested by the remarkably distinct spectral responses to irradiation from that observed for an organic solution under identical conditions. Measurements of time‐ and frequency‐resolved fluorescence emission further enable to access transient isomers and the associated spectral characteristics from other competing electronic excited‐state relaxation processes. Spectral deconvolution analysis and time‐dependent density functional theory calculations are applied to separate distinct spectral components and access their potential origin.

Photoswitchable molecules involving large-scale structural changes such as E/Z photoisomerization offer remarkable opportunities for light-stimulated catch-and-release chemical separations. While the feasibility of this photochemically driven mechanism has been demonstrated in pioneering studies, the electronic excited-state relaxation processes and its concomitant structural changes of such a functional photoswitcher remains largely unexplored. Here, we investigate an exceptional photoswitchable molecule, 2-pyridyl-diiminoguanidinium (2PyDIG), which exhibits strong and selective anion binding, along with an extraordinary capability for light-induced release of a guest ion. Through time-resolved fluorescence measurements, multireference and time-dependent density functional theory calculations we reveal the dynamics underlying electronic excited state relaxation and photoisomerization central to photoswitching. A very rapid and dominant decay component was found that is consistent with radiationless de-excitation from S₁ to S₀ through conical intersections. This process competes effectively with the slower photoisomerization process taking place in 94 ps. We further identified the underlying causes through theoretical calculations and potential routes towards improved photoisomerization efficiencies.

Direct air capture (DAC) is considered an important element in our efforts to reduce atmospheric CO2, yet the high energies and temperatures involved in current DAC technologies make their large-scale deployment largely impractical and economically unfeasible. In the case of aqueous-based CO2 absorbents, which have been identified as the basis for one of the most promising and scalable DAC approaches, a large energetic penalty is associated with heating and boiling off water, as required for solvent regeneration in a conventional thermal-swing DAC process. This could be avoided by employing alternative solvent regeneration methods based on electrochemical or photochemical pH swings. Photochemical approaches involving photoacids or photobases are particularly promising, as they offer the prospect for using abundant and renewable solar energy, though efficient solvent regeneration and recycling in a realistic multi-cycle DAC process remains challenging. Here, we report a photochemically-driven DAC process (photo-DAC) in which atmospheric CO2 capture by an aqueous oligopeptide, i.e., glycylglycine (GlyGly), is enabled through a pH swing by a guanidine photobase, i.e., pyridine-substituted diiminoguanidine (PyDIG). Upon irradiation with UV light, the PyDIG photobase undergoes photoisomerization from the E,E to the Z,Z isomer, corresponding to a pKa increase of 2.8 units that activates GlyGly for DAC through deprotonation. After the GlyGly/PyDIG solvent is saturated with atmospheric carbon dioxide, leaving it in the dark under ambient conditions leads to isomerization of PyDIG from the Z,Z back to the E,E isomer, which is accompanied by a pH drop and CO2 release. To demonstrate the recyclability of the GlyGly/PyDIG solvent, we have completed three consecutive DAC cycles, with a measured average cyclic capacity in the range of 0.21–0.26 mols CO2 per mol of GlyGly/PyDIG, and minimal loss in efficiency from one cycle to another. These results open the prospect for energy-efficient DAC cycles completed entirely at ambient conditions, thereby avoiding the significant energy penalties associated with heating and boiling aqueous solvents.