Alcoholic Solvent-Mediated Excited-State Proton Transfer Dynamics of a Novel Dihydroxynaphthalene Dye.

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.

Anionic Activation of CO2 via (Mn-CO2)- Complex on Magic-Numbered Anionic Coinage Metal Clusters Mn- (M = Cu, Ag, Au).

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.

Selective thiolation and photoswitching mechanism of Cy5 studied by time-dependent density functional theory.

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.

An atomistic mechanism for the degradation of perovskite solar cells by trapped charge.

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.

Shedding new light on an old molecule: quinophthalone displays uncommon N-to-O excited state intramolecular proton transfer (ESIPT) between photobases.

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.

Ab initio study on anomalous structures of anionic [(N-heterocycle)-CO2]- complexes.

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.

Live bio-imaging with fully bio-compatible organic fluorophores.

We synthesized a new organic fluorescent dye named resveratrone glucoside from the photoreaction of naturally-occurring phytoalexin compound resveratrol glucoside (resveratrol-3-ß-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.

Photoelectron spectroscopic and computational study of (M-CO2)(-) anions, M = Cu, Ag, Au.

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.