Multi-spectroscopic study of electrochemically-formed oxide-derived gold electrodes.

Oxide-derived metals are produced by reducing an oxide precursor. These materials, including gold, have shown improved catalytic performance over many native metals. The origin of this improvement for gold is not yet understood. In this study, operando non-resonant sum frequency generation (SFG) and ex situ high-pressure X-ray photoelectron spectroscopy (HP-XPS) have been employed to investigate electrochemically-formed oxide-derived gold (OD-Au) from polycrystalline gold surfaces. A range of different oxidizing conditions were used to form OD-Au in acidic aqueous medium (H3PO4, pH = 1). Our electrochemical data after OD-Au is generated suggest that the surface is metallic gold, however SFG signal variations indicate the presence of subsurface gold oxide remnants between the metallic gold surface layer and bulk gold. The HP-XPS results suggest that this subsurface gold oxide could be in the form of Au2O3 or Au(OH)3. Furthermore, the SFG measurements show that with reducing electrochemical treatments the original gold metallic state can be restored, meaning the subsurface gold oxide is released. This work demonstrates that remnants of gold oxide persist beneath the topmost gold layer when the OD-Au is created, potentially facilitating the understanding of the improved catalytic properties of OD-Au.

Twisted Intramolecular Charge Transfer (TICT) Controlled by Dimerization: An Overlooked Piece of the TICT Puzzle.

Organic dyes have shown high efficiencies in solar cells, which is mainly attributed to the push-pull strategy present in such dyes upon attaching to the semiconductor surfaces. We deeply studied the fundamental photophysical properties of cyanoacrylic dyes, mostly the L1 dye, and found unique emission properties that depend on many factors such as the solvent polarity and the concentration of the dye and could present a complete emission picture about this family of dyes. The L1 dye shows an intramolecular charge transfer (ICT) emission state at low concentrations (approximately nanomolar scale) and shows a twisted intramolecular charge transfer (TICT) emission state in specific solvents upon increasing the concentration to the micromolar scale. Moreover, the associated emission lifetimes of the ICT and TICT states of the L1 dye depend on solvent basicity, highlighting the role of hydrogen bond formation on controlling such states. Density functional theory calculations are performed to gain insight into the photophysical properties of the dye and revealed that H-bonding between the carboxylic groups triggers the dimerization at low concentrations. Using femtosecond transient absorption, we assigned the rate of TICT formation to be in the range (160-650 fs)-1, depending on the size of the studied cyanoacrylic dye. Therefore, we add herein a new dimension for controlling the formation of the TICT state, in addition to the solvent polarity and acceptor strength parameters. These findings are not limited to the studied dyes, and we expect that numerous organic carboxylic acids dyes show similar properties.

Long-Lived Charge-Transfer State Induced by Spin-Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in a Compact Spiro Electron Donor/Acceptor Dyad.

We prepared conceptually novel, fully rigid, spiro compact electron donor (Rhodamine B, lactam form, RB)/acceptor (naphthalimide; NI) orthogonal dyad to attain the long-lived triplet charge-transfer (3 CT) state, based on the electron spin control using spin-orbit charge transfer intersystem crossing (SOCT-ISC). Transient absorption (TA) spectra indicate the first charge separation (CS) takes place within 2.5 ps, subsequent SOCT-ISC takes 8 ns to produce the 3 NI* state. Then the slow secondary CS (125 ns) gives the long-lived 3 CT state (0.94 µs in deaerated n-hexane) with high energy level (ca. 2.12 eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as 1 NI*→1 CT→3 NI*→3 CT. With time-resolved electron paramagnetic resonance (TREPR) spectra, an EEEAAA electron-spin polarization pattern was observed for the naphthalimide-localized triplet state. Our spiro compact dyad structure and the electron spin-control approach is different to previous methods for which invoking transition-metal coordination or chromophores with intrinsic ISC ability is mandatory.

Elucidation of the Intersystem Crossing Mechanism in a Helical BODIPY for Low-Dose Photodynamic Therapy.

Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). Herein, we report the co-existence of efficient ISC and long triplet excited lifetime in a heavy atom-free bodipy helicene molecule. Via theoretical computation and time-resolved EPR spectroscopy, we confirmed that the ISC of the bodipy results from its twisted molecular structure and reduced symmetry. The twisted bodipy shows intense long wavelength absorption (ϵ=1.76×105 m-1 cm-1 at 630 nm), satisfactory triplet quantum yield (ΦT =52 %), and long-lived triplet state (τT =492 µs), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical bodipy show efficient PDT-mediated antitumor immunity amplification with an ultra-low dose (0.25 µg kg-1 ), which is several hundred times lower than that of the existing PDT reagents.

Unprecedented Ultralow Detection Limit of Amines using a Thiadiazole-Functionalized Zr(IV)-Based Metal-Organic Framework.

A luminescent Zr(IV)-based metal-organic framework (MOF), with the underlying fcu topology, encompassing a π-conjugated organic ligand with a thiadiazole functionality, exhibits an unprecedented low detection limit of 66 nM for amines in aqueous solution. Markedly, this ultralow detection is driven by hydrogen-bonding interactions between the linker and the hosted amines. This observation is supported by density functional theory (DFT) calculations, which clearly corroborate the suppression of the twisting motion of thiadiazole core in the presence of amine, reducing significantly the nonradiative recombination pathways and subsequently enhancing the emission intensity. Credibly, nicotine regarded as a harmful chemical and bearing an amine pending group is also detected with high sensitivity, positioning this MOF as a potential sensor for practical environmental applications. This finding serves also as a benchmark to understand the sensing mechanism in MOFs.

Pillar[5]arene-Stabilized Silver Nanoclusters: Extraordinary Stability and Luminescence Enhancement Induced by Host-Guest Interactions.

Herein, we report the synthesis of a new class of functional silver nanoclusters (AgNCs) capped with pillar[5]arene (P5)-based host ligands. These NCs are readily prepared through direct synthesis or ligand exchange synthesis and are stable at room temperature for over 4 months. The pillar[5]arene-stabilized NCs (Ag29 (LA-P5)12 (TPP)2 ) endorse reversible host-guest interactions with neutral alkylamines and cationic quaternary ammonium guests. This results in the formation of spherical assemblies with unparalleled changes in their optical properties including an astonishing circa 2000-fold luminescence enhancement. This is the highest luminescence enhancement ratio reported so far for such atomically precise NCs. Our synthetic protocol paves the way for the preparation of a new generation of metal nanoclusters protected by macrocyclic ligands with molecular recognition and selectivity toward specific guests.

Halogen Vacancies Enable Ligand-Assisted Self-Assembly of Perovskite Quantum Dots into Nanowires.

Interest has been growing in defects of halide perovskites in view of their intimate connection with key material optoelectronic properties. In perovskite quantum dots (PQDs), the influence of defects is even more apparent than in their bulk counterparts. By combining experiment and theory, we report herein a halide-vacancy-driven, ligand-directed self-assembly process of CsPbBr3 PQDs. With the assistance of oleic acid and didodecyldimethylammonium sulfide, surface-Br-vacancy-rich CsPbBr3 PQDs self-assemble into nanowires (NWs) that are 20-60 nm in width and several millimeters in length. The NWs exhibit a sharp photoluminescence profile (≈18 nm full-width at-half-maximum) that peaks at 525 nm. Our findings provide insight into the defect-correlated dynamics of PQDs and defect-assisted fabrication of perovskite materials and devices.

Bidentate Ligand-Passivated CsPbI3 Perovskite Nanocrystals for Stable Near-Unity Photoluminescence Quantum Yield and Efficient Red Light-Emitting Diodes.

Although halide perovskite nanocrystals (NCs) are promising materials for optoelectronic devices, they suffer severely from chemical and phase instabilities. Moreover, the common capping ligands like oleic acid and oleylamine that encapsulate the NCs will form an insulating layer, precluding their utility in optoelectronic devices. To overcome these limitations, we develop a postsynthesis passivation process for CsPbI3 NCs by using a bidentate ligand, namely 2,2'-iminodibenzoic acid. Our passivated NCs exhibit narrow red photoluminescence with exceptional quantum yield (close to unity) and substantially improved stability. The passivated NCs enabled us to realize red light-emitting diodes (LEDs) with 5.02% external quantum efficiency and 748 cd/m2 luminance, surpassing by far LEDs made from the nonpassivated NCs.

Intramolecular Energy and Electron Transfers in Bodipy Naphthalenediimide Triads.

Borondipyrromethene (BDP) naphthalenediimide (NDI) triads (BDP-NDI) and diiodo-BDP derivative (DiiodoBDP-NDI)) were synthesized to study the Förster resonance energy transfer (FRET) and its impact on the triplet state formation and dynamics. In these triads, diiodo-BDP and BDP are the energy donors and NDI is the energy acceptor. Nanosecond transient absorption spectra of triads indicated that triplet state is localized on NDI moiety, either by selective photoexcitation of the Diiodo-BDP or NDI unit. The intersystem crossing (ISC) is attributed to intramolecular heavy atom effect. The triplet state quantum yield was found to be 54% with a lifetime of 38 µs. However, no triplet state is observed for BDP-NDI system either by exciting BDP or NDI unit. Thus, we confirmed that charge recombination does not produce a triplet state. Interestingly, DiiodoBDP-NDI can be used as broadband excitable (500-620 nm) triplet photosensitizer, and high triplet-triplet annihilation (TTA) upconversion quantum yield of ΦUC = 2.8% was observed with 9,10-bis(phenylethynyl)-anthracene (BPEA) as a triplet acceptor/emitter.

Double Charged Surface Layers in Lead Halide Perovskite Crystals.

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface's optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

Time-Resolved IR Spectroscopy Reveals a Mechanism with TiO2 as a Reversible Electron Acceptor in a TiO2-Re Catalyst System for CO2 Photoreduction.

Attaching the phosphonated molecular catalyst [ReIBr(bpy)(CO)3]0 to the wide-bandgap semiconductor TiO2 strongly enhances the rate of visible-light-driven reduction of CO2 to CO in dimethylformamide with triethanolamine (TEOA) as sacrificial electron donor. Herein, we show by transient mid-IR spectroscopy that the mechanism of catalyst photoreduction is initiated by ultrafast electron injection into TiO2, followed by rapid (ps-ns) and sequential two-electron oxidation of TEOA that is coordinated to the Re center. The injected electrons can be stored in the conduction band of TiO2 on an ms-s time scale, and we propose that they lead to further reduction of the Re catalyst and completion of the catalytic cycle. Thus, the excited Re catalyst gives away one electron and would eventually get three electrons back. The function of an electron reservoir would represent a role for TiO2 in photocatalytic CO2 reduction that has previously not been considered. We propose that the increase in photocatalytic activity upon heterogenization of the catalyst to TiO2 is due to the slow charge recombination and the high oxidative power of the ReII species after electron injection as compared to the excited MLCT state of the unbound Re catalyst or when immobilized on ZrO2, which results in a more efficient reaction with TEOA.

A Fast and Scalable Approach for Synthesis of Hierarchical Porous Zeolitic Imidazolate Frameworks and One-Pot Encapsulation of Target Molecules.

A trimethylamine (TEA)-assisted synthesis approach that combines the preparation of hierarchical porous zeolitic imidazolate framework ZIF-8 nanoparticles and one-pot encapsulation of target molecules is presented. Two dye molecules, rhodamine B (RhB) and methylene blue (MB), and one protein (bovine serum albumin, BSA) were tested as the target molecules. The addition of TEA into the solution of zinc nitrate promoted the formation of ZnO nanocrystals, which rapidly transformed to ZIF-8 nanoparticles after the addition of the linker 2-methylimidazole (Hmim). Hierarchical porous dye@ZIF-8 nanoparticles with high crystallinity, large BET surface areas (1300-2500 m2/g), and large pore volumes (0.5-1.0 cm3/g) could be synthesized. The synthesis procedure was fast (down to 2 min) and scalable. The Hmim/Zn ratio could be greatly reduced (down to 2:1) compared to previously reported ones. The surface areas, and the mesopore size, structure, and density could be modified by changing the TEA or dye concentrations, or by postsynthetic treatment using reflux in methanol. This synthesis and one-pot encapsulation approach is simple and can be readily scaled up. The photophysical properties such as lifetime and photostability of the dyes could be tuned via encapsulation. The lifetimes of the encapsulated dyes were increased by 3-27-fold for RhB@ZIF-8 and by 20-fold for MB@ZIF-8, compared to those of the corresponding free dyes. The synthesis approach is general, which was successfully applied for encapsulation of protein BSA. It could also be extended for the synthesis of hierarchical porous cobalt-based ZIF (dye@ZIF-67).

Photoisomerization of the cyanoacrylic acid acceptor group--a potential problem for organic dyes in solar cells.

Organic solar cell dyes containing the most common anchoring group, cyanoacrylic acid, are shown to be photolabile and undergo photoisomerization. This may have significant consequences for dye-sensitized solar cells, as isomerisation competes with electron injection and leads to modifications of the dye and surface arrangement.

Ultrafast Events of Photoexcited Iron(III) Chloride for Activation of Benzylic C-H Bonds.

The usage of rare-earth-metal catalysts in the synthesis of organic compounds is widespread in chemical industries but is limited owing to its environmental and economic costs. However, recent studies indicate that abundant-earth metals like iron(III) chloride can photocatalyze diverse organic transformations using blue-light LEDs. Still, the underlying mechanism behind such activity is debatable and controversial, especially in the absence of ultrafast spectroscopic results. To address this urgent challenge, we performed femtosecond time-resolved electronic absorption spectroscopy experiments of iron(III) chloride in selected organic solvents relevant to its photocatalytic applications. Our results show that the long-lived species [Fe(II) ← Cl•]* is primarily responsible for both oxidizing the organic substrate and reducing molecular oxygen through the diffusion process, leading to the final product and regenerating the photocatalyst rather than the most widely proposed free chloride radical (Cl•). Our study will guide the rational design of efficient earth-abundant photocatalysts.

Charge Transfer Process of a Solvated Hydrogen-Bonded Organic Network.

We show the first example of an organic linker (OL) terminated by carboxylic groups that can form a hydrogen-bonded network/polymer (HBN) in solution under controlled conditions in which the photogenerated charges can hop from a monomer OL to the hydrogen-bonded backbone of OLs, as probed by transient absorption (fs-TA). While fs-TA reveals a slow twisting process in the monomer form of the OL, the formation of a hydrogen-bonded network in solution suppresses such process and favors instead a charge transfer (CT) state along the low-lying hydrogen-bonded backbone. Theoretical calculations show that such solvated HBN in a specific polar solvent is stabilized due to the huge change of the dipole moment from monomer compared to the network, leading to a charge delocalization character due to the symmetry breaking. Our findings will open new avenues for implementing solvated hydrogen-bonded molecules in applications such as sensing and photocatalysis.

Ultrafast transient infrared spectroscopy for probing trapping states in hybrid perovskite films.

Studying the charge dynamics of perovskite materials is a crucial step to understand the outstanding performance of these materials in various fields. Herein, we utilize transient absorption in the mid-infrared region, where solely electron signatures in the conduction bands are monitored without external contributions from other dynamical species. Within the measured range of 4000 nm to 6000 nm (2500-1666 cm-1), the recombination and the trapping processes of the excited carriers could be easily monitored. Moreover, we reveal that within this spectral region the trapping process could be distinguished from recombination process, in which the iodide-based films show more tendencies to trap the excited electrons in comparison to the bromide-based derivatives. The trapping process was assigned due to the emission released in the mid-infrared region, while the traditional band-gap recombination process did not show such process. Various parameters have been tested such as film composition, excitation dependence and the probing wavelength. This study opens new frontiers for the transient mid-infrared absorption to assign the trapping process in perovskite films both qualitatively and quantitatively, along with the potential applications of perovskite films in the mid-IR region.

Highly Emissive Biological Bilirubin Molecules: Shedding New Light on the Phototherapy Scheme.

Bilirubin (BR) is the main end-product of the hemoglobin catabolism. For decades, its photophysics has been mainly discussed in terms of ultrafast deactivation of the excited state in solution, where, indeed, BR shows a very low green emission quantum yield (EQY), 0.03%, resulting from an efficient nonradiative isomerization process. Herein, we present, for the first time, unique and exceptional photophysical properties of solid-state BR, which amend by changing the type of crystal, from a closely packed α crystal to an amorphous loosely packed ß crystal. BR α crystals show a very bright red emission with an EQY of ca. 24%, whereas ß crystals present, in addition, a low green EQY of ca. 0.5%. By combining density functional theory (DFT) calculations and time-resolved emission spectroscopy, we trace back this dual emission to the presence of two types of BR molecules in the crystal: a "stiff" monomer, M1, distorted by particularly strong internal H-bonds and a "floppy" monomer, M2, having a structure close to that of BR in solution. We assign the red strong emission of BR crystals to M1 present in both the α and ß crystals, while the low green emission, only present in the amorphous (ß) crystal, is interpreted as M2 emission. Efficient energy-transfer processes from M2 to M1 in the closely packed α crystal are invoked to explain the absence of the green component in its emission spectrum. Interestingly, these unique photophysical properties of BR remain in polar solvents such as water. Based on these unprecedented findings, we propose a new model for the phototherapy scheme of BR inside the human body and highlight the usefulness of BR as a strong biological fluorescent probe.

Spin-Orbit Charge-Transfer Intersystem Crossing of Compact Naphthalenediimide-Carbazole Electron-Donor-Acceptor Triads.

Compact electron donor-acceptor triads based on carbazole (Cz) and naphthalenediimide (NDI) were prepared to study the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). By variation of the molecular conformation and electron-donating ability of the carbazole moieties, the electronic coupling between the two units was tuned, and as a result charge-transfer (CT) absorption bands with different magnitudes were observed (ε = 4000-18 000 M-1 cm-1). Interestingly, the triads with NDI attached at the 3-C position or with a phenyl spacer at the N position of the Cz moiety, thermally activated delayed fluorescence (TADF) was observed. Femtosecond transient absorption (fs-TA) spectroscopy indicated fast electron transfer (0.8-1.5 ps) from the Cz to NDI unit, followed by population of the triplet state (150-600 ps). Long-lived triplet states (up to τT = 45-50 µs) were observed for the triads. The solvent-polarity-dependent singlet-oxygen quantum yield (ΦΔ) is 0-26%. Time-resolved electron paramagnetic resonance (TREPR) spectral study of TADF molecules indicated the presence of the 3CT state for NDI-Cz-Ph (zero-field-splitting parameter D = 21 G) and an 3LE state for NDI-Ph-Cz (D = 586 G). The triads were used as triplet photosensitizers in triplet-triplet annihilation upconversion by excitation into the CT absorption band; the upconversion quantum yield was ΦUC = 8.2%, and there was a large anti-Stokes shift of 0.55 eV. Spatially confined photoexcitation is achieved with the upconversion using focusing laser beam excitation, and not the normally used collimated laser beam, i.e., the upconversion was only observed at the focal point of the laser beam. Photo-driven intermolecular electron transfer was demonstrated with reversible formation of the NDI-• radical anion in the presence of the sacrificial electron donor triethanolamine.

Using a first-derivative spectrophotometric method for simultaneous determination of Fe(II) and Ni(II) in mineral vitamins by interaction with 2,4-diamino-5-(2-hydroxy-5-nitrophenylazo)benzenesulfonic acid sodium salt in the presence of Tween 20.

A highly selective and sensitive derivative spectrophotometric method has been developed for the determination of Fe(II) and Ni(II) in different mineral vitamins. The method is based on the formation of binary complexes of Fe(II) and Ni(II) with 2,4-diamino-5-(2-hydroxy-5-nitrophenylazo) benzenesulfonic acid sodium salt (Mordant Brown 33) at pH 5.6 with Tween 20. At lambda(max) 512 and 493 nm, the molar absorbtivity was 2.09 x 10(4) and 0.58 x 10(4) L/mol cm for Fe(II) and Ni(II), respectively. Fe(II) can be determined in the range 0.55-2.79 microg/mL in the presence of 2.92 microg/mL Ni, and Ni(II) can be determined in the range 1.17-3.5 microg/mL in the presence of 2.79 microg/mL Fe(II) in the presence of Tween 20 (4%). The detection limits were 9.0 and 15.0 ng/mL for Fe(II) and Ni(II), respectively. The effect of foreign ions was elucidated. The RSD values were, in all instances, less than 1.3%. The proposed method was successfully applied for the simultaneous determination of Fe(II) and Ni(II) in different mineral vitamins.

Critical Role of Protons for Emission Quenching of Indoline Dyes in Solution and on Semiconductor Surfaces.

By combining time-correlated single photon counting (TCSPC) measurements, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations, we herein investigate the role of protons, in solutions and on semiconductor surfaces, for the emission quenching of indoline dyes. We show that the rhodanine acceptor moieties, and in particular the carbonyl oxygens, undergo protonation, leading to nonradiative excited-state deactivation. The presence of the carboxylic acid anchoring group, close to the rhodanine moiety, further facilitates the emission quenching, by establishing stable H-bond complexes with carboxylic acid quenchers, with high association constants, in both ground and excited states. This complexation favors the proton transfer process, at a low quencher concentration, in two ways: bringing close to the rhodanine unit the quencher and assisting the proton release from the acid by a partial-concerted proton donation from the close-by carboxylic group to the deprotonated acid. Esterification of the carboxylic group, indeed, inhibits the ground-state complex formation with carboxylic acids and thus the quenching at a low quencher concentration. However, the rhodanine moiety in the ester form can still be the source of emission quenching through dynamic quenching mechanism with higher concentrations of protic solvents or carboxylic acids. Investigating this quenching process on mesoporous ZrO2, for solar cell applications, also reveals the sensitivity of the adsorbed excited rhodanine dyes toward adsorbed protons on surfaces. This has been confirmed by using an organic base to remove surface protons and utilizing cynao-acrylic dye as a reference dye. Our study highlights the impact of selecting such acceptor group in the structural design of organic dyes for solar cell applications and the overlooked role of protons to quench the excited state for such chemical structures.