Energy Transfer from Quantum Dots to Metal–Organic Frameworks for Enhanced Light Harvesting

Because of their efficient energy-transport properties, porphyrin-based metal–organic frameworks (MOFs) are attractive compounds for solar photochemistry applications. However, their absorption bands provide limited coverage in the visible spectral range for light-harvesting applications. We report here the functionalization of porphyrin-based MOFs with CdSe/ZnS core/shell quantum dots (QDs) for the enhancement of light harvesting via energy transfer from the QDs to the MOFs. The broad absorption band of the QDs in the visible region offers greater coverage of the solar spectrum by QD–MOF hybrid structures. We show through time-resolved emission studies that photoexcitation of the QDs is followed by energy transfer to the MOFs with efficiencies of more than 80%. This sensitization approach can result in a >50% increase in the number of photons harvested by a single monolayer MOF structure with a monolayer of QDs on the surface of the MOF

Photoinduced Electron Transfer in a BODIPY-<i>ortho</i>-Carborane Dyad Investigated by Time-Resolved Transient Absorption Spectroscopy

We report the results of photoinduced electron transfer (PET) in a novel dyad, in which a boron dipyrromethene (BODIPY) dye is covalently linked to <i>o</i>-carborane (<i>o</i>-Cb). In this dyad, BODIPY and <i>o</i>-Cb act as electron donor and acceptor, respectively. PET dynamics were investigated using a femtosecond time-resolved transient absorption spectroscopic method. The free energy dependence of PET in the S₁ and S₂ states was examined on the basis of Marcus theory. PET in the S₁ state occurs in the Marcus normal region. Rates are strongly influenced by the driving force (−Δ<i>G</i>), which is controlled by solvent polarity; thus, PET in the S₁ state is faster in polar solvents than in nonpolar ones. However, PET does not occur from the higher energy S₂ state despite large endothermic Δ<i>G</i> values, because deactivation via internal conversion is much faster than PET

A Detailed Evaluation for the Nonradiative Processes in Highly Phosphorescent Iridium(III) Complexes

To understand the intrinsic nature of nonradiative processes in heteroleptic cyclometalated Ir­(III) complexes, highly phosphorescent Ir³⁺ complexes containing 2-(3-sulfonylfluorophenyl)­pyridine (ppySO₂F) as the cyclometalated ligand were newly synthesized. Three ancillary ligands, acetylacetonate (acac), picolinate (pic), and <i>tetrakis</i>-pyrazolyl borate (bor), were employed for the preparation of the Ir­(III) complexes [Ir­(ppySO₂F)₂(acac)] (<b>Ir-acac</b>), [Ir­(ppySO₂F)₂(pic)] (<b>Ir-pic</b>), and [Ir­(ppySO₂F)₂(bor)] (<b>Ir-bor</b>). The molecular structures were characterized by X-ray crystallography. Blue phosphorescence maxima were observed at 458, 467, and 478 nm for <b>Ir-bor</b>, <b>Ir-pic</b>, and <b>Ir-acac</b>, respectively, at 77 K, and the corresponding emission quantum yields were determined to be 0.79, 0.80, and 0.98 in anaerobic CH₂Cl₂ at 300 K. Additionally, the phosphorescence decay times were measured to be 3.58, 1.94, and 1.44<i>μ</i>s for <b>Ir-bor</b>, <b>Ir-pic</b>, and <b>Ir-acac</b>, respectively. No temperature dependence was observed for the emission lifetimes in 298–338 K. These results indicate that there is no activation barrier to crossing to a nonradiative state like metal-centered (MC, d–d) state. The radiative rate constants (<i>k</i>_r) are within a narrow range of 3.0–5.5 × 10^–5 s^–1. However, the nonradiative rate constants (<i>k</i>_nr) are within a wide range of 14.2–0.52 × 10^–4 s^–1. The <i>k</i>_nr values showed exponetial correlation with the energy gap. We carried out <i>ab</i> <i>initio</i> calculations to evaluate the energy states and their corresponding orbitals. The nonemissive MC states lie at higher energies than the emissive metal-to-ligand charge transfer (MLCT) state, and hence, the MC states can be excluded from the nonradiative pathway

Excited-State Modification of Phenylimidazole-Based Cyclometalated Ir(III) Complexes through Secondary Bulky Aryl Substitution and Inductive Modification Enhances the Blue Emission Efficiency in Phosphorescent OLEDs

To elucidate the key parameters governing the emission properties of phenylimidazole (pim)-based Ir(III) emitters, including their electronic structure and the bulky aryl substitution effect, a series of pim-based iridium(III) complexes (Ir(Rpim-X)3, Rpim-X = 1-R-2-(X-phenyl)-1H-imidazole) bearing secondary pendants of increasing bulkiness [R = methyl (Me), phenyl (Ph), terphenyl (TPh), or 4-isopropyl terphenyl (ITPh)] and three different primary pim ligands (X = F, F2, and CN) were designed and synthesized. Based on photophysical and electrochemical analyses, it was found that the excited state properties are highly dependent on the bulkiness of the secondary substituent and the inductive nature of the primary pim ligand. The incorporation of bulky TPh/ITPh substituents in the second coordination sphere significantly enhanced the emission efficiencies in the solid state (ΦPL = 72.1–84.9%) compared to those of the methyl- or phenyl-substituted Ir(III) complexes (ΦPL = 30.4% for Ir(Mepim)3 and 63.7% for Ir(Phpim)3). Further modification of the secondary aryl substituent (Ir(TPhpim)3 → Ir(ITPhpim)3) through the incorporation of an isopropyl group and F substitution on the primary pim ligand (Ir(TPh/ITPhpim)3 → Ir(TPh/ITPhpim-F/F2)3) resulted in a slight decrease in the LUMO and a significant decrease in the HOMO energy levels, respectively; these energy level adjustments consequently amplified emission blue shifts, thereby enabling efficient blue electroluminescence in phosphorescent organic light-emitting diodes. Theoretical calculations revealed that the excited-state properties of pim-based Ir(III) complexes can be modulated by the nature of the peripheral substituent and the presence of an EWG substituent. Among the fabricated blue-emitting TPh/ITPh-substituted Ir(III) complexes, Ir(ITPhpim-F)3, Ir(TPhpim-F2)3, and Ir(ITPhpim-F2)3 were tested as blue-emitting dopants for blue phosphorescent OLEDs owing to their high solid radiative quantum yields (ΦPL = 75.9–84.9%). The Ir(ITPhpim-F)3-doped multilayer device displayed the best performance with a maximum external quantum efficiency of 21.0%, a maximum current efficiency of 43.6 cd/A, and CIE coordinates of 0.18 and 0.31

Layer-by-Layer Fabrication of Oriented Porous Thin Films Based on Porphyrin-Containing Metal–Organic Frameworks

We report the synthesis and characterization of two thin films (<b>DA-MOF</b> and <b>L2-MOF</b>) of porphyrin-based MOFs on functionalized surfaces using a layer-by-layer (LbL) approach. Profilometry measurements confirm that the film thickness increases systematically with number of growth cycles. Polarization excitation and fluorescence measurements indicate that the porphyrin units are preferentially oriented, while X-ray reflectivity scans point to periodic ordering. Ellipsometry measurements show that the films are highly porous. Since there are currently few methods capable of yielding microporous MOFs containing accessible free-base porphyrins, it is noteworthy that the LbL growth permits direct MOF incorporation of unmetalated porphyrins. Long-range energy transfer is demonstrated for both MOF films. The findings offer useful insights for subsequent fabrication of MOF-based solar energy conversion devices

Investigation of Interface Characteristics and Physisorption Mechanism in Quantum Dots/TiO₂ Composite for Efficient and Sustainable Photoinduced Interfacial Electron Transfer

Owing to their superior stability compared to those of conventional molecular dyes, as well as their high UV–visible absorption capacity, which can be tuned to cover the majority of the solar spectrum through size adjustment, quantum dot (QD)/TiO2 composites are being actively investigated as photosensitizing components for diverse solar energy conversion systems. However, the conversion efficiencies and durabilities of QD/TiO2-based solar cells and photocatalytic systems are still inferior to those of conventional systems that employ organic/inorganic components as photosensitizers. This is because of the poor adsorption of QDs onto the TiO2 surface, resulting in insufficient interfacial interactions between the two. The mechanism underlying QD adsorption on the TiO2 surface and its relationship to the photosensitization process remain unclear. In this study, we established that the surface characteristics of the TiO2 semiconductor and the QDs (i.e., surface defects of the metal oxide and the surface structure of the QD core) directly affect the QD adsorption capacity by TiO2 and the interfacial interactions between the QDs and TiO2, which relates to the photosensitization process from the photoexcited QDs to TiO2 (QD* → TiO2). The interfacial interaction between the QDs and TiO2 is maximized when the shape/thickness-modulated triangular QDs are composited with defect-rich anatase TiO2. Comprehensive investigations through photodynamic analyses and surface evaluation using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and photocatalysis experiments collectively validate that tuning the surface properties of QDs and modulating the TiO2 defect concentration can synergistically amplify the interfacial interaction between the QDs and TiO2. This augmentation markedly improved the efficiency of photoinduced electron transfer from the photoexcited QDs to TiO2, resulting in significantly increased photocatalytic activity of the QD/TiO2 composite. This study provides the first in-depth characterization of the physical adhesion of QDs dispersed on a heterogeneous metal-oxide surface. Furthermore, the prepared QD/TiO2 composite exhibits exceptional adsorption stability, resisting QD detachment from the TiO2 surface over a wide pH range (pH = 2–12) in aqueous media as well as in nonaqueous solvents during two months of immersion. These findings can aid the development of practical QD-sensitized solar energy conversion systems that require the long-term stability of the photosensitizing unit

Efficient Light Harvesting and Energy Transfer in a Red Phosphorescent Iridium Dendrimer

A series of red phosphorescent iridium dendrimers of the type [Ir­(btp)₂(pic-PC_<i>n</i>)] (<b>Ir-G</b>_{<b><i>n</i></b>}; <i>n</i> = 0, 1, 2, and 3) with two 2-(benzo­[<i>b</i>]­thiophen-2-yl)­pyridines (btp) and 3-hydroxypicolinate (pic) as the cyclometalating and ancillary ligands were prepared in good yields. Dendritic generation was grown at the 3 position of the pic ligand with 4-(9<i>H</i>-carbazolyl)­phenyl dendrons connected to 3,5-bis­(methyleneoxy)­benzyloxy branches (PC_<i>n</i>; <i>n</i> = 0, 2, 4, and 8). The harvesting photons on the PC_<i>n</i> dendrons followed by efficient energy transfer to the iridium center resulted in high red emissions at ∼600 nm by metal-to-ligand charge transfer. The intensity of the phosphorescence gradually increased with increasing dendrimer generation. Steady-state and time-resolved spectroscopy were used to investigate the energy-transfer mechanism. On the basis of the fluorescence quenching rate constants of the PC_<i>n</i> dendrons, the energy-transfer efficiencies for <b>Ir-G</b>_<b>1</b>, <b>Ir-G</b>_<b>2</b>, and <b>Ir-G</b>_<b>3</b> were 99, 98, and 96%, respectively. The energy-transfer efficiency for higher-generation dendrimers decreased slightly because of the longer distance between the PC dendrons and the core iridium­(III) complex, indicating that energy transfer in <b>Ir-G</b>_{<b><i>n</i></b>} is a Förster-type energy transfer. Finally, the light-harvesting efficiencies for <b>Ir-G</b>_<b>1</b>, <b>Ir-G</b>_<b>2</b>, and <b>Ir-G</b>_<b>3</b> were determined to be 162, 223, and 334%, respectively

Engendering Long-Term Air and Light Stability of a TiO₂‑Supported Porphyrinic Dye via Atomic Layer Deposition

Organic and porphyrin-based chromophores are prevalent in liquid-junction photovoltaic and photocatalytic solar-cell chemistry; however, their long-term air and light instability may limit their practicality in real world technologies. Here, we describe the protection of a zinc porphyrin dye, adsorbed on nanoparticulate TiO₂, from air and light degradation by a protective coating of alumina grown with a previously developed post-treatment atomic layer deposition (ALD) technique. The protective Al₂O₃ ALD layer is deposited using dimethylaluminum isopropoxide as an Al source; in contrast to the ubiquitous ALD precursor trimethylaluminum, dimethylaluminum isopropoxide does not degrade the zinc porphyrin dye, as confirmed by UV–vis measurements. The growth of this protective ALD layer around the dye can be monitored by an in-reactor quartz crystal microbalance (QCM). Furthermore, greater than 80% of porphyrin light absorption is retained over ∼1 month of exposure to air and light when the protective coating is present, whereas almost complete loss of porphyrin absorption is observed in less than 2 days in the absence of the ALD protective layer. Applying the Al₂O₃ post-treatment technique to the TiO₂-adsorbed dye allows the dye to remain in electronic contact with both the semiconductor surface and a surrounding electrolyte solution, the combination of which makes this technique promising for numerous other electrochemical photovoltaic and photocatalytic applications, especially those involving the dye-sensitized evolution of oxygen

Electronic Alteration on Oligothiophenes by <i>o</i>‑Carborane: Electron Acceptor Character of <i>o</i>‑Carborane in Oligothiophene Frameworks with Dicyano-Vinyl End-On Group

We studied electronic change in oligothiophenes by employing <i>o</i>-carborane into a molecular array in which one or both end(s) were substituted by electron-withdrawing dicyano-vinyl group(s). Depending on mono- or bis-substitution at the <i>o</i>-carborane, a series of linear A₁-D-A₂ (<b>1a</b>–<b>1c</b>) or V-shaped A₁-D-A₂-D-A₁ <b>(2a</b>–<b>2c</b>) oligothiophene chain structures of variable length were prepared; A₁, D, and A₂, represent dicyano-vinyl, oligothiophenyl, and <i>o</i>-carboranyl groups, respectively. Among this series, <b>2a</b> shows strong electron-acceptor capability of <i>o</i>-carborane comparable to that of the dicyano-vinyl substituent, which can be elaborated by a conformational effect driven by cage σ*−π* interaction. As a result, electronic communications between <i>o</i>-carborane and dicyano-vinyl groups are successfully achieved in <b>2a</b>

Electronic Alteration on Oligothiophenes by <i>o</i>‑Carborane: Electron Acceptor Character of <i>o</i>‑Carborane in Oligothiophene Frameworks with Dicyano-Vinyl End-On Group

We studied electronic change in oligothiophenes by employing <i>o</i>-carborane into a molecular array in which one or both end(s) were substituted by electron-withdrawing dicyano-vinyl group(s). Depending on mono- or bis-substitution at the <i>o</i>-carborane, a series of linear A₁-D-A₂ (<b>1a</b>–<b>1c</b>) or V-shaped A₁-D-A₂-D-A₁ <b>(2a</b>–<b>2c</b>) oligothiophene chain structures of variable length were prepared; A₁, D, and A₂, represent dicyano-vinyl, oligothiophenyl, and <i>o</i>-carboranyl groups, respectively. Among this series, <b>2a</b> shows strong electron-acceptor capability of <i>o</i>-carborane comparable to that of the dicyano-vinyl substituent, which can be elaborated by a conformational effect driven by cage σ*−π* interaction. As a result, electronic communications between <i>o</i>-carborane and dicyano-vinyl groups are successfully achieved in <b>2a</b>