Effect of Binding Geometry on Charge Transfer in CdSe Nanocrystals Functionalized by N719 Dyes to Tune Energy Conversion Efficiency

Semiconductor quantum dots (QDs) functionalized by metal–organic dyes show great promise in photocatalytic and photovoltaic applications. However, the charge transfer direction and rateskey processes governing the efficiency of energy conversionare strongly affected by the QD–dye interactions, insights on which are challenging to obtain experimentally. We use density functional theory (DFT) and constrained DFT calculations to investigate a degree of sensitivity of the electronic level alignment and related QD–dye electronic couplings to binding conformations of N719 dye at the surface of the 1.5 nm CdSe QD. Our calculations reveal a lack of direct correlations between the strength of the QD–dye interaction in terms of their binding conformations and the donor–acceptor electronic couplings. While the QD–dye binding conformations are the most stable when the N719 dye is attached to the QD via two carboxylate groups, the strongest electronic coupling between the QD as an electron donor and the dye as an electron acceptor is observed in structures bonded via the isocyanate ligands. Such strong electronic couplings also are responsible for significant stabilization of the dye’s occupied orbitals deep inside in the valence band of the QD making the hole transfer from the photoexcited QD to the dye thermodynamically unfavorable in structures bound via isocyanates. Our results suggest that the most probable binding conformations are those occurring via two carboxylate linkers, which exhibit very weak electronic couplings contributing to the electron transfer from the photoexcited CdSe QD to the N719 dye but provide the most favorable conditions for the hole transfer. Overall, our computational work provides an insightful view about the surface chemistry of CdSe regarding the donor–acceptor interaction, energy level alignment, and charge transfer between CdSe and dye molecule, which can guide the rational design of QD-based materials for energy conversion applications

Heterometallic Potassium Rare-Earth-Metal Allyl and Hydrido Complexes Stabilized by a Dianionic (NNNN)-Type Macrocyclic Ancillary Ligand

The macrocyclic diamino diamine (1,7-Me₂TACD)­H₂ (1,7-Me₂TACD = 1,7-dimethyl-1,4,7,10-tetraazacyclododecane, 1,7-Me₂[12]­aneN₄), reacted under propylene elimination with [Ln­(η³-C₃H₅)₃(diox)] (Ln = Y, La) to give the mono­(allyl) complexes [(1,7-Me₂TACD)­Ln­(η³-C₃H₅)]₂ (Ln = Y (<b>1a</b>), La (<b>1b</b>)). A single-crystal X-ray diffraction study shows <b>1b</b> to be a centrosymmetric dimer with lanthanum atoms bridged by one of the two amido nitrogen atoms. Complexes <b>1a</b>,<b>b</b> were treated with 2 equiv of the potassium allyl KC₃H₅ to give the corresponding heterometallic allyl complexes [(1,7-Me₂TACD)­Ln­(η³-C₃H₅)₂K­(THF)]_<i>n</i> (Ln = Y (<b>2a</b>), La (<b>2b</b>)). A single-crystal X-ray diffraction study revealed that <b>2a</b>,<b>b</b> are polymeric in the solid state with allyl ligands bridging the metal centers in addition to the presence of μ₂-amido functions of the 1,7-Me₂TACD ligand. Hydrogenolysis of the yttrium compound <b>2a</b> with 1 bar of H₂ led to the formation of the heterometallic Y₄K₂ hydrido complex [(1,7-Me₂TACD)₂Y₂H₃K­(THF)₂]₂ (<b>3a</b>), which can also be synthesized from a 1:1 mixture of <b>1a</b> and KC₃H₅ with 1 bar of H₂. A single-crystal X-ray diffraction study of <b>3a</b> revealed a dimer of heterotrinuclear Y₂K trihydride aggregate. Treatment of <b>2b</b> with 1 bar of H₂ afforded the heptanuclear La₃K₄ heptahydrido complex [(1,7-Me₂TACD)₃La₃H₇K₄(THF)₇] (<b>3b</b>)

Heterometallic Potassium Rare-Earth-Metal Allyl and Hydrido Complexes Stabilized by a Dianionic (NNNN)-Type Macrocyclic Ancillary Ligand

The macrocyclic diamino diamine (1,7-Me₂TACD)­H₂ (1,7-Me₂TACD = 1,7-dimethyl-1,4,7,10-tetraazacyclododecane, 1,7-Me₂[12]­aneN₄), reacted under propylene elimination with [Ln­(η³-C₃H₅)₃(diox)] (Ln = Y, La) to give the mono­(allyl) complexes [(1,7-Me₂TACD)­Ln­(η³-C₃H₅)]₂ (Ln = Y (<b>1a</b>), La (<b>1b</b>)). A single-crystal X-ray diffraction study shows <b>1b</b> to be a centrosymmetric dimer with lanthanum atoms bridged by one of the two amido nitrogen atoms. Complexes <b>1a</b>,<b>b</b> were treated with 2 equiv of the potassium allyl KC₃H₅ to give the corresponding heterometallic allyl complexes [(1,7-Me₂TACD)­Ln­(η³-C₃H₅)₂K­(THF)]_<i>n</i> (Ln = Y (<b>2a</b>), La (<b>2b</b>)). A single-crystal X-ray diffraction study revealed that <b>2a</b>,<b>b</b> are polymeric in the solid state with allyl ligands bridging the metal centers in addition to the presence of μ₂-amido functions of the 1,7-Me₂TACD ligand. Hydrogenolysis of the yttrium compound <b>2a</b> with 1 bar of H₂ led to the formation of the heterometallic Y₄K₂ hydrido complex [(1,7-Me₂TACD)₂Y₂H₃K­(THF)₂]₂ (<b>3a</b>), which can also be synthesized from a 1:1 mixture of <b>1a</b> and KC₃H₅ with 1 bar of H₂. A single-crystal X-ray diffraction study of <b>3a</b> revealed a dimer of heterotrinuclear Y₂K trihydride aggregate. Treatment of <b>2b</b> with 1 bar of H₂ afforded the heptanuclear La₃K₄ heptahydrido complex [(1,7-Me₂TACD)₃La₃H₇K₄(THF)₇] (<b>3b</b>)

Structure and Capacitive Performance of Porous Carbons Derived from Terephthalic Acid–Zinc Complex via a Template Carbonization Process

High-performance porous carbons as supercapacitor electrode materials have been prepared by a simple but efficient template carbonization process, in which commercially available terephthalic acid–zinc complex is used as a carbon source. It reveals that the carbonization temperature plays a crucial role in determining the structure and capacitive performance of carbons. The <b>carbon-1000</b> sample has high surface area of 1138 m² g^–1 and large pore volume of 1.44 cm³ g^–1 as well as rationally hierarchical pore size distribution. In a three-electrode system, the <b>carbon-1000</b> sample possesses high specific capacitances of 266.0 F g^–1 at 0.5 A g^–1 and good cycling stability. In a two-electrode system, the operation temperature (25/50/80 °C) can greatly influence the electrochemical performance of the <b>carbon-1000</b> sample, especially with an extended voltage window (∼ 3 V). The temperature-dependent operation makes it possible for the application of supercapacitors under extreme conditions

Multifunctional Cationic Iridium(III) Complexes Bearing 2‑Aryloxazolo[4,5‑<i>f</i>][1,10]phenanthroline (N^N) Ligand: Synthesis, Crystal Structure, Photophysics, Mechanochromic/Vapochromic Effects, and Reverse Saturable Absorption

A series of 2-aryloxazolo­[4,5-<i>f</i>]­[1,10]­phenanthroline ligands (N^N ligands) and their cationic iridium­(III) complexes (<b>1</b>–<b>11</b>, aryl = 4-NO₂-phenyl (<b>1</b>), 4-Br-phenyl (<b>2</b>), Ph (<b>3</b>), 4-NPh₂-phenyl (<b>4</b>), 4-NH₂-phenyl (<b>5</b>), pyridin-4-yl (<b>6</b>), naphthalen-1-yl (<b>7</b>), naphthalen-2-yl (<b>8</b>), phenanthren-9-yl (<b>9</b>), anthracen-9-yl (<b>10</b>), and pyren-1-yl (<b>11</b>)) were synthesized and characterized. By introducing different electron-donating or electron-withdrawing substituents at the 4-position of the 2-phenyl ring (<b>1</b>–<b>5</b>), or different aromatic substituents with varied degrees of π-conjugation (<b>6</b>–<b>11</b>) on oxazolo­[4,5-<i>f</i>]­[1,10]­phenanthroline ligand, we aim to understand the effects of terminal substituents at the N^N ligands on the photophysics of cationic Ir­(III) complexes using both spectroscopic methods and quantum chemistry calculations. Complexes with the 4-R-phenyl substituents adopted an almost coplanar structure with the oxazolo­[4,5-<i>f</i>]­[1,10]­phenanthroline motif, while the polycyclic aryl substituents (except for naphthalen-2-yl) were twisted away from the oxazolo­[4,5-<i>f</i>]­[1,10]­phenanthroline motif. All complexes possessed strong absorption bands below 350 nm that emanated from the ligand-localized ¹π,π*/¹ILCT (intraligand charge transfer) transitions, mixed with ¹LLCT (ligand-to-ligand charge transfer)/¹MLCT (metal-to-ligand charge transfer) transitions. At the range of 350–570 nm, all complexes exhibited moderately strong ¹ILCT/¹LLCT/¹MLCT transitions at 350−450 nm, and broad but very weak ³LLCT/³MLCT absorption at 450−570 nm. Most of the complexes demonstrated moderate to strong room temperature phosphorescence both in solution and in the solid state. Among them, complex <b>7</b> also manifested a drastic mechanochromic and vapochromic luminescence effect. Except for complexes <b>1</b> and <b>4</b> that contain NO₂ or NPh₂ substituent at the phenyl ring, respectively, all other complexes exhibited moderate to strong triplet excited-state absorption in the spectral region of 440–750 nm. Moderate to very strong reverse saturable absorption (RSA) of these complexes appeared at 532 nm for 4.1 ns laser pulses. The RSA strength followed the trend of <b>7</b> > <b>11</b> > <b>9</b> > <b>3</b> > <b>2</b> ≈ <b>4</b> > <b>5</b> ≈ <b>10</b> ≈ <b>6</b> ≈ <b>8</b> > <b>1</b>. The photophysical studies revealed that the different 2-aryl substituents on the oxazole ring impacted the singlet and triplet excited-state characteristics dramatically, which in turn notably influenced the RSA of these complexes

Efficient Fluorescence Energy Transfer System between CdTe-Doped Silica Nanoparticles and Gold Nanoparticles for Turn-On Fluorescence Detection of Melamine

We here report an efficient and enhanced fluorescence energy transfer system between confined quantum dots (QDs) by entrapping CdTe into the mesoporous silica shell (CdTe@SiO₂) as donors and gold nanoparticles (AuNPs) as acceptors. At pH 6.50, the CdTe@SiO₂–AuNPs assemblies coalesce to form larger clusters due to charge neutralization, leading to the fluorescence quenching of CdTe@SiO₂ as a result of energy transfer. As compared with the energy transfer system between unconfined CdTe and AuNPs, the maximum fluorescence quenching efficiency of the proposed system is improved by about 27.0%, and the quenching constant, <i>K</i>_sv, is increased by about 2.4-fold. The enhanced quenching effect largely turns off the fluorescence of CdTe@SiO₂ and provides an optimal “off-state” for sensitive “turn-on” assay. In the present study, upon addition of melamine, the weak fluorescence system of CdTe@SiO₂–AuNPs is enhanced due to the strong interactions between the amino group of melamine and the gold nanoparticles via covalent bond, leading to the release of AuNPs from the surfaces of CdTe@SiO₂; thus, its fluorescence is restored. A “turn-on” fluorimetric method for the detection of melamine is proposed based on the restored fluorescence of the system. Under the optimal conditions, the fluorescence enhanced efficiency shows a linear function against the melamine concentrations ranging from 7.5 × 10^–9 to 3.5 × 10^–7 M (i.e., 1.0–44 ppb). The analytical sensitivity is improved by about 50%, and the detection limit is decreased by 5.0-fold, as compared with the analytical results using the CdTe–AuNPs system. Moreover, the proposed method was successfully applied to the determination of melamine in real samples with excellent recoveries in the range from 97.4 to 104.1%. Such a fluorescence energy transfer system between confined QDs and AuNPs may pave a new way for designing chemo/biosensing

Toward Broadband Reverse Saturable Absorption: Investigating the Impact of Cyclometalating Ligand π‑Conjugation on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium Complexes

The synthesis, photophysics and reverse saturable absorption of a series of bis-cyclometalated Ir­(III) complexes Ir­(C^∧N)₂<b>L</b>·PF₆, where <b>L</b> = 3,8-bis­[9,9-di­(2-ethylhexyl)-9<i>H</i>-fluoren-2-yl]-1,10-phenanthroline and C^∧N = 2-phenylpyridine (ppy, <b>1</b>), 2-phenylquinoline (pqu, <b>2</b>), 1-phenylisoquinoline (piq, <b>3</b>), 2-phenylbenzo­[<i>g</i>]­quinoline (pbq, <b>4</b>), and 2,3-diphenylbenzo­[<i>g</i>]­quinoxaline (dpbq, <b>5</b>), are reported. By gradually increasing the π-conjugation along the pyridine or pyrazine ring of the C^∧N ligands, the energies of the lowest singlet (S₁) and triplet (T₁) excited states are significantly reduced, as reflected by the pronouncedly red-shifted charge transfer absorption bands at >450 nm and the emission band(s) in their UV–vis absorption and emission spectra, respectively. Additionally, our density functional theory (DFT) calculations confirm that the natures of the S₁ and T₁ states vary with the increased π-conjugation of the C^N ligands, with the S₁ state changing from the exclusive ¹LLCT (ligand-to-ligand charge transfer)/¹MLCT (metal-to-ligand charge transfer) transitions in <b>1</b>–<b>3</b> to the predominant ¹ILCT (intraligand charge transfer)/¹π,π*/¹MLCT/¹LLCT transitions in <b>4</b> and <b>5</b>, and with the T₁ state being altered from the predominant ligand <b>L</b> based ³ILCT or ³ILCT/³π,π* nature in <b>1</b> and <b>2</b>, respectively, to the C^∧N ligand-localized ³π,π*/³MLCT/³ILCT parentage in <b>3</b>–<b>5</b>. All complexes exhibit broad and positive transient absorption (TA) in the visible to the near-IR region (ca. 430–800 nm) upon nanosecond laser excitation at 355 nm. However, the TA spectral features and the triplet lifetimes vary dramatically from <b>1</b> to <b>5</b>, reflecting the different natures of the T₁ states when the degree of π-conjugation of the C^∧N ligands increases. Our nonlinear transmission experiments demonstrate moderate to strong reverse saturable absorption (RSA) for <b>1</b>–<b>5</b> for nanosecond laser pulses at 532 nm. The relative strength of the RSA follows the trend <b>1</b> > <b>3</b> > <b>2</b> > <b>4</b> > <b>5</b>. Our joined experimental and computational studies manifest that judicious choice of the C^∧N ligand with appropriate π-conjugation is an effective approach to obtain Ir­(III) complexes with desired photophysical properties for reverse saturable absorbers

Conditions for Directional Charge Transfer in CdSe Quantum Dots Functionalized by Ru(II) Polypyridine Complexes

Thermodynamic conditions governing the charge transfer direction in CdSe quantum dots (QD) functionalized by either Ru­(II)-trisbipyridine or black dye are studied using density functional theory (DFT) and time-dependent DFT (TDDFT). Compared to the energy offsets of the isolated QD and the dye, QD–dye interactions strongly stabilize dye orbitals with respect to the QD states, while the surface chemistry of the QD has a minor effect on the energy offsets. In all considered QD/dye composites, the dyes always introduce unoccupied states close to the edge of the conduction band and control the electron transfer. Negatively charged ligands and less polar solvents significantly destabilize the dye’s occupied orbitals shifting them toward the very edge of the valence band, thus, providing favorite conditions for the hole transfer. Overall, variations in the dye’s ligands and solvent polarity can progressively adjust the electronic structure of QD/dye composites to modify conditions for the directed charge transfer

A Multiscale Survival Process for Modeling Human Activity Patterns

<div><p>Human activity plays a central role in understanding large-scale social dynamics. It is well documented that individual activity pattern follows bursty dynamics characterized by heavy-tailed interevent time distributions. Here we study a large-scale online chatting dataset consisting of 5,549,570 users, finding that individual activity pattern varies with timescales whereas existing models only approximate empirical observations within a limited timescale. We propose a novel approach that models the intensity rate of an individual triggering an activity. We demonstrate that the model precisely captures corresponding human dynamics across multiple timescales over five orders of magnitudes. Our model also allows extracting the population heterogeneity of activity patterns, characterized by a set of individual-specific ingredients. Integrating our approach with social interactions leads to a wide range of implications.</p></div

Self-Adaptive Switch Enabling Complete Charge Separation in Molecular-Based Optoelectronic Conversion

Achieving high charge recombination probability has been the major challenge for the practical utilization of molecule-based solar harvesting. Molecular switches were introduced to stabilize the charge separation state in donor–acceptor systems, but it is difficult to seamlessly incorporate the ON/OFF switching actions into the optoelectronic conversion cycle. Here we present a self-adaptive system in which the donor and acceptor are bridged by a switchable moiety that enables a complete charge separation repeatedly. Calculations are presented for a platinum­(II) terpyridyl complex with an azobenzene bridge. The charge transfer induced by light extracts electrons from the azobenzene group, automatically triggering a <i>trans</i> → <i>cis</i> isomerization. The resulting conformation suppresses charge recombination. Energized charges are trapped in the acceptor, ready for charge collection by electrodes. The bridge then goes through inverse isomerization to restore the conjugation and conductance. This self-adaptive design provides a novel way to improve the performance of optoelectronic conversion and realize practical solar-harvesting applications in organic molecular systems