Dynamics of Interfacial Charge Transfer to Formic Acid, Formaldehyde, and Methanol on the Surface of TiO₂ Nanoparticles and Its Role in Methane Production

Transient absorption and electron paramagnetic resonance (EPR) spectroscopies were used to study reactions of photogenerated electrons and holes on TiO₂ with methanol, formaldehyde, and formic acid (compounds that, together with methane, have been observed in the photocatalytic reduction of CO₂). The ultrafast dynamics of hole scavenging was found to be an order of magnitude faster on the surface of TiO₂ than in the corresponding homogeneous systems. Additionally, the equilibrium constant for the reaction of photogenerated electrons in TiO₂ with adsorbed CO₂ was estimated to be less than 3.2 M^–1, regardless of the presence of hole scavengers and product molecules. Formic acid serves as both the hole and the electron acceptor, yielding the protonated radical anions (OC^•OH), and formyl radicals, respectively. For methanol and formaldehyde only photooxidation, but no one-electron photoreduction, was observed by EPR spectroscopy; these molecules are either reduced in a two-electron process or act only as hole scavengers

Photon Upconversion Using Baird-Type (Anti)Aromatic Quinoidal Naphthalene Derivative as a Sensitizer

A naphtho-<i>p</i>-quinodimethane (QDM) exhibiting Baird’s 4<i>n</i> – π antiaromaticity was used as green photons-harvesting chromophore to sensitize perylene (Per) leading to upconverted blue photoluminescence. The solution phase QDM → Per triplet energy transfer (TET) could not be unraveled via the Stern–Volmer method, but transient absorption measurements revealed that the kinetics of T₁ → T<i>_n</i> for QDM (τ = 1.4 μs) was 1 order of magnitude reduced (τ = 0.17 μs) as a result of ³(Per)* formation. Furthermore, we demonstrated that incident light with power densities in the microwatt regime is sufficient to perform photon upconversion using the present set of molecular systems

Infiltrated Zinc Oxide in Poly(methyl methacrylate): An Atomic Cycle Growth Study

We have investigated the growth of zinc oxide in a polymer matrix by sequential infiltration synthesis (SiS). The atomic cycle-by-cycle self-terminating reaction growth investigation was done using photoluminescence (PL), Raman, and X-ray photoemission spectroscopy (XPS). Results show clear differences between Zn atom configurations at the initial stages of growth. Mono Zn atoms (O–Zn and O–Zn–O) exhibit pure UV emission with little evidence of deep level oxygen vacancy states (V_O). Dimer Zn atoms (O–Zn–O–Zn and O–Zn–O–Zn–O) show strong UV and visible PL emission from V_O states 20 times greater than that from the mono Zn atom configuration. After three precursor cycles, the PL emission intensity drops significantly exhibiting first evidence of crystal formation as observed with Raman spectroscopy via the presence of longitudinal optical phonons. We also report a first confirmation of energy transfer between polymer and ZnO where the polymer absorbs light at 241 nm and emits at 360 nm, which coincides with the ZnO UV emission peak. Our work shows that ZnO dimers are unique ZnO configurations with high PL intensity, unique O_1s oxidation states, and sub-10 ps absorption and decay, which are interesting properties for novel quantum material applications

η²‑SO₂ Linkage Photoisomer of an Osmium Coordination Complex

We report the discovery of an η²-SO₂ linkage photoisomer in the osmium pentaammine coordination complex, [Os­(NH₃)₅(SO₂)]­[Os­(NH₃)₅(HSO₃)]­Cl₄ (<b>1</b>). Its dark- and light-induced crystal structures are determined via synchrotron X-ray crystallography, at 100 K, where the photoinduced state is metastable in a single crystal that has been stimulated by 505 nm light for 2.5 h. The SO₂ photoisomer in the [Os­(NH₃)₅(SO₂)]²⁺ cation contrasts starkly with the photoinactivity of the HSO₃ ligand in its companion [Os­(NH₃)₅(HSO₃)]⁺ cation within the crystallographic asymmetric unit of this single crystal. Panchromatic optical absorption characteristics of this single crystal are revealed in both dark- and light-induced states, using concerted absorption spectroscopy and optical microscopy. Its absorption halves across most of its visible spectrum, upon exposure to 505 nm light. The SO₂ ligand seems to be responsible for this photoinduced bleaching effect, judging from a comparison of the dark- and light-induced crystal structures of <b>1</b>. The SO₂ photoisomerism is found to be thermally reversible, and so <b>1</b> presents a rare example of an osmium-based solid-state optical switch. Such switching in an osmium complex is significant because bottom-row transition metals stand to offer linkage photoisomerism with the greatest photoconversion levels and thermal stability. The demonstration of η²-SO₂ bonding in this complex also represents a fundamental contribution to osmium coordination chemistry

Photoexcited Carrier Dynamics of In₂S₃ Thin Films

Indium sulfide (In₂S₃) is a promising absorber base for substitutionally doped intermediate band photovoltaics (IBPV); however, the dynamics of charge carriers traversing the electronic density of states that determine the optical and electronic response of thin films under stimuli have yet to be explored. The kinetics of photophysical processes in In₂S₃ grown by oxygen-free atomic layer deposition are deduced from photoconductivity, photoluminescence (PL), and transient absorption spectroscopy. We develop a map of excited-state dynamics for polycrystalline thin films including a secondary conduction band ∼2.1 eV above the first, plus sulfur vacancy and indium interstitial defect levels resulting in long-lived (∼100 ns) transients. Band-edge recombination produces PL and stimulated emission, which both intensify and red-shift as deposition temperature and grain size increase. The effect of rapid conduction band electron relaxation (<30 ps) and deep defect levels on IBPV employing In₂S₃-based absorbers is finally considered

Photoexcited Carrier Dynamics of Cu₂S Thin Films

Copper sulfide is a simple binary material with promising attributes for low-cost thin film photovoltaics. However, stable Cu₂S-based device efficiencies approaching 10% free from cadmium have yet to be realized. In this Letter, transient absorption spectroscopy is used to investigate the dynamics of the photoexcited state of isolated Cu₂S thin films prepared by atomic layer deposition or vapor-based cation exchange of ZnS. While a number of variables including film thickness, carrier concentration, surface oxidation, and grain boundary passivation were examined, grain structure alone was found to correlate with longer lifetimes. A map of excited state dynamics is deduced from the spectral evolution from 300 fs to 300 μs. Revealing the effects of grain morphology on the photophysical properties of Cu₂S is a crucial step toward reaching high efficiencies in operationally stable Cu₂S thin film photovoltaics

Self-Assembly of Highly Ordered Peptide Amphiphile Metalloporphyrin Arrays

Long fibers assembled from peptide amphiphiles capable of binding the metalloporphyrin zinc protoporphyrin IX ((PPIX)­Zn) have been synthesized. Rational peptide design was employed to generate a peptide, c16-AHL₃K₃-CO₂H, capable of forming a β-sheet structure that propagates into larger fibrous structures. A porphyrin-binding site, a single histidine, was engineered into the peptide sequence in order to bind (PPIX)Zn to provide photophysical functionality. The resulting system indicates control from the molecular level to the macromolecular level with a high order of porphyrin organization. UV/visible and circular dichroism spectroscopies were employed to detail molecular organization, whereas electron microscopy and atomic force microscopy aided in macromolecular characterization. Preliminary picosecond transient absorption data are also reported. Reduced hemin, (PPIX)­Fe^II, was also employed to highlight the material’s versatility and tunability

Size-Dependent Biexciton Quantum Yields and Carrier Dynamics of Quasi-Two-Dimensional Core/Shell Nanoplatelets

Quasi-two-dimensional nanoplatelets (NPLs) possess fundamentally different excitonic properties from zero-dimensional quantum dots. We study lateral size-dependent photon emission statistics and carrier dynamics of individual NPLs using second-order photon correlation (g⁽²⁾(τ)) spectroscopy and photoluminescence (PL) intensity-dependent lifetime analysis. Room-temperature radiative lifetimes of NPLs can be derived from maximum PL intensity periods in PL time traces. It first decreases with NPL lateral size and then stays constant, deviating from the electric dipole approximation. Analysis of the PL time traces further reveals that the single exciton quantum yield in NPLs decreases with NPL lateral size and increases with protecting shell thickness, indicating the importance of surface passivation on NPL emission quality. Second-order photon correlation (g⁽²⁾(τ)) studies of single NPLs show that the biexciton quantum yield is strongly dependent on the lateral size and single exciton quantum yield of the NPLs. In large NPLs with unity single exciton quantum yield, the corresponding biexciton quantum yield can reach unity. These findings reveal that by careful growth control and core–shell material engineering, NPLs can be of great potential for light amplification and integrated quantum photonic applications

A Naphtho‑<i>p</i>‑quinodimethane Exhibiting Baird’s (Anti)Aromaticity, Broken Symmetry, and Attractive Photoluminescence

We report a novel reductive desulfurization reaction involving π-acidic naphthalene diimides (NDI) <b>1</b> using thionating agents such as Lawesson’s reagent. Along with the expected thionated NDI derivatives <b>2</b>–<b>6</b>, new heterocyclic naphtho-<i>p</i>-quinodimethane compounds <b>7</b> depicting broken/reduced symmetry were successfully isolated and fully characterized. Empirical studies and theoretical modeling suggest that <b>7</b> was formed via a six-membered ring oxathiaphosphenine intermediate rather than the usual four-membered ring oxathiaphosphetane of <b>2</b>–<b>6</b>. Aside from the reduced symmetry in <b>7</b> as confirmed by single-crystal XRD analysis, we established that the ground state UV–vis absorption of <b>7</b> is red-shifted in comparison to the parent NDI <b>1</b>. This result was expected in the case of thionated polycyclic diimides. However, unusual low energy transitions originate from Baird 4nπ aromaticity of compounds <b>7</b> in lieu of the intrinsic Hückel (4n + 2)­π aromaticity as encountered in NDI <b>1</b>. Moreover, complementary theoretical modeling results also corroborate this change in aromaticity of <b>7</b>. Consequently, photophysical investigations show that, compared to parent NDI <b>1</b>, <b>7</b> can easily access and emit from its T₁ state with a phosphorescence ³(<b>7a</b>)* lifetime of τ_P = 395 μs at 77 K indicative of the formation of the corresponding “aromatic triplet” species according to the Baird’s rule of aromaticity

Enhancement of Local Piezoresponse in Polymer Ferroelectrics <i>via</i> Nanoscale Control of Microstructure

Polymer ferroelectrics are flexible and lightweight electromechanical materials that are widely studied due to their potential application as sensors, actuators, and energy harvesters. However, one of the biggest challenges is their low piezoelectric coefficient. Here, we report a mechanical annealing effect based on local pressure induced by a nanoscale tip that enhances the local piezoresponse. This process can control the nanoscale material properties over a microscale area at room temperature. We attribute this improvement to the formation and growth of β-phase extended chain crystals <i>via</i> sliding diffusion and crystal alignment along the scan axis under high mechanical stress. We believe that this technique can be useful for local enhancement of piezoresponse in ferroelectric polymer thin films