The excited-state proton transfer (ESPT) reaction is an important primary photochemical process because it is closely related to photophysical properties. Although ESPT research in aqueous solutions is predominant, alcoholic solvent-mediated ESPT studies are also significant in terms of photoacid-based reactions. Especially, the research for dihydroxynaphthalenes (DHNs) has been largely neglected due to the challenging data interpretation of two hydroxyl groups. A novel fluorescent dye, resveratrone, synthesized by light irradiation of resveratrol, which is famous for its antioxidant properties, can be regarded as a type of DHN, and it has distinctive optical properties, including high quantum yield, a large two-photon absorption coefficient, a large Stokes shift, and very high biocompatibility. In this study, we investigate the overall kinetics of the optical properties of resveratrone and find evidence for alcoholic solvent-mediated ESPT involvement in the radiative properties of resveratrone with a large Stokes shift. Our investigation provides an opportunity to revisit the overlooked photophysical properties of intriguing photoacid behavior and the large Stokes shift of the dihydroxynaphthalene dye.

Large Stokes shifts of fluorophores are significant in detection because they can reduce noise. The excited state proton transfer (ESPT) phenomenon is one of the closely related mechanisms leading this significant Stokes shift. Although ESPT research in aqueous solutions are predominant, alcoholic solvent-mediated ESPT studies are also significant in terms of photoacid-based reactions. Especially, research for 1,3-dihydroxynaphthalene (1,3-DHN) has been largely neglected due to challenging data interpretation of two hydroxyl groups. A novel fluorescent dye, resveratrone, synthesized by light irradiation of resveratrol which is famous for its antioxidant properties, can be regarded as a derivative of 1,3-DHN and it has distinctive optical properties including high quantum yield, large two photon absorption coefficient, large Stokes shift and very high biocompatibility. In this study, we investigate an overall kinetics of the optical properties of resveratrone and found the evidence for alcoholic solvent-mediated ESPT involvement in radiative properties of resveratrone with a large Stokes shift. Our investigation will provide an opportunity to revisit the overlooked photophysical property of intriguing photoacid behavior and large Stokes shift of dihydroxynaphthalene dye.

Given the immense challenge of excessive accumulation of carbon dioxide (CO2) in the earth's atmosphere, an extensive search is under way to convert atmospheric CO2 to compounds of more utility. With CO2 being thermodynamically extremely stable, activation of CO2 is the first and most important step toward its chemical conversion. Building upon our earlier model for the anionic activation of CO2 with azabenzene and inspired by the work of others on metal atom-CO2 complexes, we investigated the possibility of anionic activation of CO2 on small anionic metal clusters, which would have implications for catalytic conversion of CO2 on metal surfaces with atomic-scale structural irregularities. We carried out theoretical calculations using density functional theory to examine small anionic metal clusters of Cu, Ag, and Au to check whether they form a complex with CO2, with the sign of CO2 being chemically activated. We found that a class of anionic metal clusters Mn- with 1, 2, and 6 atoms consistently produced the activated complex (Mn-CO2)- for all three metals. There exists a strong interaction between the CO2 moiety and Mn- via a partially covalent M-C bond with a full delocalization of the electronic charge, as a result of electron transfer from the HOMO of Mn- to the LUMO of CO2 as in metal-CO2 π-backbonding. We examined the interaction of frontier orbitals from the viewpoints of the orbital geometry and orbital energetics and found that the above magic numbers are consistent with both aspects.

We report, for the first time, that the oxidation of bilayer graphene (BLG) can be reversibly and stacking-specifically controlled. The infrared (IR) absorption, IR nanoscopy, and Raman spectroscopy measurements on BLG consistently show reversible changes in the spectra and images upon exposure to O-2 and H-2 at elevated temperatures. We also obtain spectroscopic and theoretical evidence that stacking orders of graphene layers have a profound influence on the oxide structures: AB-BLG reacting with singlet and triplet oxygen results in endoperoxides (-C-O-O-C-), whereas AA'-BLG reacting with oxygen generates both the epoxides (singlet, -C-O-C-) and endoperoxides (triplet). We believe that our result provides deeper understanding on the layer-dependent catalytic activities of graphene, which is crucial for the design of high-performance graphene-based catalysts needed for various electrochemical, biological, and environmental applications.

Cy5 is one of the most widely used organic dyes with a photoswitching property. It can be reversibly photoconverted to the dark state through thiolation with primary thiols. Although photoswitching of Cy5 has been widely used in super-resolution nanoscopy, its thiolation mechanism remains unclear. We carried out time-dependent density functional theory calculations to investigate the excited state dynamics of Cy5 and observed its site-selective thiolation on both the ground and excited states. Scanning the excited state potential energy surfaces by rotating individual C-C bonds revealed structural similarity between the twisted form of Cy5 and the Cy5 subunit in the thiolated Cy5, which suggests that the dark state formation is strongly associated with the torsional motion on the excited state.

It is unmistakably paradoxical that the most vulnerable aspect of the photoactive organic-inorganic hybrid perovskite is its instability against light. Why and how perovskites break down under light irradiation and what happens at the atomistic level of these materials during the degradation process still remain unanswered. In this paper, we found the culprit and verified the mechanism for the irreversible degradation of hybrid perovskite materials from our experimental investigation and ab initio molecular dynamics (AIMD) simulation. We initially found that the electrostatic charges generated by light irradiation and trapped along the grain boundaries of the perovskite crystal result in oxygen-induced irreversible degradation in dry air. This result, together with our previous experimental finding on the same critical role of trapped charges in the perovskite degradation under moisture, suggests that the trapped charges are the main culprit in both the oxygen- and moisture-induced degradation of perovskite materials. Detailed roles of oxygen and water molecules were investigated using AIMD simulation by tracking the atomic motions in the outermost layers of the oxygen- or water-covered methylammonium lead triiodide (denoted MAPbI3 for CH3NH3PbI3) perovskite crystal with trapped charges. In the first few picoseconds of our simulation, trapped charges start disrupting the crystal structure, leading to a short-range interaction between oxygen or water molecules and the compositional ions of MAPbI3. We found that there exist different degradation pathways depending on both the polarity of the trapped charge and the kind of gas molecule. We also verified that a more structurally stable, multi-component perovskite material (with the composition of MA0.6FA0.4PbI2.9Br0.1) showed much stronger resistance against light-induced degradation than MAPbI3 even in 100%-oxygen ambience or humid air.

The breakdown process of CH3NH3PbI3 perovskite crystals by localized charges and its polarity-dependency have been revealed.

In our theoretical investigation for small Ag-Au alloy clusters, we found that pronounced polarization of their electrostatic charge occurs with some generic regularities. We propose a set of “rules” for the charge distribution, which may be extended to larger Ag-Au alloy clusters or even bulk solids: (1) atoms in lower-coordinated sites tend to be charged more negatively or less positively; (2) the charge of Au is affected by nearest neighbor atoms only, contrary to the case of Ag. The origins of these phenomenological rules may include site-dependent electronic polarization and the difference in s/p hybridization between Ag and Au.

Despite soaring performance of organic-inorganic hybrid perovskite materials in recent years, the mechanism of their decomposition at actual operation condition has been elusive. Herein, we elucidated the decrystallization process of CH3NH3PbI3 perovskite crystals via localized charges and identified polarity-dependent degradation pathway by carrying out time-evolution measurements for absorption spectra of perovskite films with underlying different charge transport layers and ab initio molecular dynamics calculations. It was found that the carrier polarity (hole-rich or electron-rich) inside the perovskite films played a critical role in the degradation rate, and polarity-dependent degradation pathway strongly depended on the combination of surrounding gaseous molecules. The hole-rich perovskite films degraded more rapidly in the existence of H2O than the electron-rich one, while the degradation trend became opposite in only-oxygen ambient. Strikingly, the hole-rich one was extremely weak to atmospheric air containing both H2O and O2, whereas the MAPbI3 film with excessive electrons rather stabilized in air. Ab initio molecular dynamics (AIMD) simulation was also done to find the detailed degradation pathway of MAPbI3 under atmosphere for different polarity of localized charge, which are in good agreement with experimental results. Furthermore, X-ray assisted spectroscopic measurements confirmed the production of Pb(OH)I as predicted from the simulation result.

Excited state dynamics of common yellow dye quinophthalone (QPH) was probed by femtosecond transient absorption spectroscopy. Multi-exponential decay of the excited state and significant change of rate constants upon deuterium substitution indicate that uncommon nitrogen-to-oxygen excited state intramolecular proton transfer (ESIPT) occurs. By performing density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations, we found that adiabatic surface crossing between the S1 and S2 states takes place in the photoreaction. Unlike most cases of ESIPT, QPH does not exhibit tautomer emission, possibly due to internal conversion or back-proton transfer. The ESIPT of QPH presents a highly interesting case also because the moieties participating in ESIPT, quinoline and aromatic carbonyl, are both traditionally considered as photobases.

It is unmistakably paradoxical that the weakest point of the photoactive organic-inorganic hybrid perovskite is its instability against light. Why and how perovskites break down under light irradiation and what happens at the atomistic level during the degradation still remains unanswered. In this paper, we revealed the fundamental origin and mechanism for irreversible degradation of hybrid perovskite materials from our new experimental results and ab initio molecular dynamics (AIMD) simulations. We found that the photo-generated charges trapped along the grain boundaries of the perovskite crystal result in oxygen-induced irreversible degradation in air even in the absence of moisture. The present result, together with our previous experimental finding on the same critical role of trapped charges in the perovskite degradation under moisture, suggests that the trapped charges are the main culprit in both the oxygen- and moisture-induced degradation of perovskite materials. More detailed roles of oxygen and water molecules were investigated by tracking the atomic motions of the oxygen- or water-covered CH3NH3PbI3(MAPbI3) perovskite crystal surface with trapped charges via AIMD simulation. In the first few picoseconds of our simulation, trapped charges start disrupting the crystal structure, leading to a close-range interaction between oxygen or water molecules and the compositional ions of MAPbI3. We found that there are different degradation pathways depending on both the polarity of the trapped charge and the kind of gas molecule. Especially, the deprotonation of organic cations was theoretically predicted for the first time in the presence of trapped anionic charges and water molecules. We confirmed that a more structurally stable, multi-component perovskite material(MA0.6FA0.4PbI2.9Br0.1) exhibited a much longer lifespan than MAPbI3 under light irradiation even in 100 ambience.

Several unusual anionic complexes between carbon dioxide (CO2) and N-heterocycles (NHCs) possessing a significantly positive adiabatic electron affinity over 0.7 eV were studied by density functional theory calculations (UB3LYP/6-311++g(d,p)). Unlike all previously reported [NHC-CO2]- anions with a coplanar structure that ensures full delocalization of the negative charge through extended π-conjugation, this new class of anionic [NHC-CO2]- complexes has a strongly non-coplanar geometry and no π-bond character between CO2 and NHC. Despite the fundamental differences in chemical bonding between all prior cases and the new class of [NHC-CO2]- complexes, we found that the CO2 moiety in the latter still has a large negative charge (∼0.4 e) and a strongly bent geometry (O-C-O angle of ∼140°) just like in the former. This seemingly anomalous case was explained by a simple model based on the torsional steric effect and the electron affinities of the constituent moieties.

We synthesized a new organic fluorescent dye named resveratrone glucoside from the photoreaction of naturally-occurring phytoalexin compound resveratrol glucoside (resveratrol-3-β-mono-d-glucoside), which is abundant in various plants such as berries, herbs, nuts and grapes. Just like its predecessor molecule resveratrone that was previously discovered by our group, resveratrone glucoside possesses excellent optical properties including a high fluorescence quantum yield, a large Stokes' shift, and a large two-photon absorption cross section. In addition to these highly desirable properties, both fluorescent molecules can also be used as ideal bio-compatible organic fluorophores since they have remarkably low cytotoxicity, which we verified through our cell morphological study, trypan blue exclusion assay, Western blot analysis and fluorescence imaging of various live biological specimens. In particular, we note that resveratrone glucoside is much more soluble in aqueous solution because of its glycosidic side chain and therefore highly suitable for in vivo imaging. We demonstrated that resveratrone and resveratrone glucoside can be used in one- and two-photon fluorescence microscopic imaging of E. coli, yeast (S. cerevisiae), and mammalian cell lines including HeLa and MCF10A cells as well as to the live imaging and real-time tracking of the zebrafish embryo development. Both organic fluorophores can be readily obtained from a simple photoreaction of commercially available, inexpensive samples.

Lithium–sulfur (Li–S) batteries are expected to overcome the limit of current energy storage devices by delivering high specific energy with low material cost. However, the potential of Li–S batteries has not yet been realized because of several technical barriers. Poor electrochemical performance is mainly attributed to the low electrical conductivity of the fully charged and discharged species, the irreversible loss of polysulfide anions and the decrease in the number of electrochemically active reaction sites during battery operation. Here, we report that the introduction of graphene quantum dots (GQDs) into the sulfur cathode dramatically enhanced sulfur/sulfide utilization, yielding high performance. In addition, the GQDs induced structural integrity of the sulfur–carbon electrode composite by oxygen-rich functional groups. This hierarchical architecture enabled fast charge transfer while minimizing the loss of lithium polysulfides, which is attributed to the physicochemical properties of GQDs. The mechanisms through which excellent cycling and rate performance are achieved were thoroughly studied by analyzing capacity versus voltage profiles. Furthermore, experimental observations and theoretical calculations further clarified the role played by GQDs by proving that C–S bonding occurs. Thus, the introduction of GQDs into Li–S batteries will provide an important breakthrough allowing their use as high-performance and low-cost batteries for next-generation energy storage systems. High-capacity cathodes with enhanced stability have been prepared by using graphene quantum dots (GQDs). Lithium–sulphur batteries are considered one of the most promising candidates for future battery systems. However, intermediate compounds readily dissolve into the electrolyte during battery operation, which results in a loss of active materials. A team led by Yung-Eun Sung and Byung Hee Hong of Seoul National University introduced a hierarchical structure with GQDs via wet chemical techniques. Oxygen-rich functional groups of GQDs gave rise to a tightly packed structure between carbon black and sulphur, enabling fast charge transfer. Moreover, experimental observations and theoretical analysis helped to clarify the role of GQDs. In particular, the introduction of GQDs significantly was found to improve cycling performance and rate capability through the formation of carbon–sulphur bonds. Graphene quantum dots (GQDs) decorated sulfur–carbon hierarchical structure serve as a high sulfur/sulfide utilization in Li–S battery. The oxygen rich functionalities of the GQDs induced structural integrity of the sulfur–carbon electrode composite. This hierarchical structure enables fast charge transfer, minimizes the loss of soluble polysulfides and completes reaction of sulfur/sulfide.

Fluorescence intensity and quantum yield of organic dyes often depend on their internal rotation in the excited electronic state. NIAD-4 is a novel amyloid-binding dye and a possible substitute for Thioflavin T in amyloid tagging. Its structure suggests that it should act like a molecular rotor whose photophysical properties are strongly governed by internal rotation, although little is known about the photophysics of this molecule to date. We investigated the molecular rotor property of NIAD-4 by measuring the viscosity dependence of fluorescence intensity against the molecular rotor model. The origin of torsion-dependent fluorescence switching of NIAD-4 is discussed.

In a combined photoelectron spectroscopic and computational study of (M-CO2)(-), M = Au, Ag, Cu, anionic complexes, we show that (Au-CO2)(-) forms both the chemisorbed and physisorbed isomers, AuCO2(-) and Au(-)(CO2), respectively; that (Ag-CO2)(-) forms only the physisorbed isomer, Ag(-)(CO2); and that (Cu-CO2)(-) forms only the chemisorbed isomer, CuCO2(-). The two chemisorbed complexes, AuCO2(-) and CuCO2(-), are covalently bound, formate-like anions, in which their CO2 moieties are significantly reduced. These two species are examples of electron-induced CO2 activation. The two physisorbed complexes, Au(-)(CO2) and Ag(-)(CO2), are electrostatically and thus weakly bound.