Unique Trapped Dimer State of the Photogenerated Hole in Hybrid Orthorhombic CH₃NH₃PbI₃ Perovskite: Identification, Origin, and Implications

Revealing the innate character and transport of the photogenerated hole is essential to boost the high photovoltaic performance in the lead-based organohalide perovskite. However, knowledge at the atomic level is currently very limited. In this work, we systematically investigate the properties of the photogenerated hole in the orthorhombic CH₃NH₃PbI₃ using hybrid functional PBE0 calculations with spin–orbit coupling included. An unexpected trapping state of the hole, localized as I₂^– (I dimer), is uncovered, which was never reported in photovoltaic materials. It is shown that this localized configuration is energetically more favorable than that of the delocalized hole state by 191 meV and that it can highly promote the diffusion of the hole with an energy barrier as low as 131 meV. Furthermore, the origin of I dimer formation upon trapping of the hole is rationalized in terms of electronic and geometric effects, and a good linear correlation is found between the hole trapping capacity and the accompanying structural deformation in CH₃NH₃PbX₃ (X = Cl, Br, and I). It is demonstrated that good CH₃NH₃PbX₃ materials for the hole diffusion should have small structural deformation energy and weak hole trapping capacity, which may facilitate the rational screening of superior photovoltaic perovskites

Recovery of BoNT/B by PEG precipitation with various concentrations of PEG at various pH values.

<p>A, NaCl concentration: 0.1 M; B, NaCl concentration: 0.3 M. Recovery was defined as the ratio of the amount of BoNT/B measured by ELISA in PEG precipitation to that in the starting material.</p

SDS–PAGE analysis of the purified BoNT/B complex.

<p>1∼3, BoNT/B complex (diluted 1:2 with sample buffer containing DTT) after purification by hydrophobic interaction chromatography column; M, high molecular markers.</p

Further purification of toxins using hydrophobic interaction chromatography.

<p>Pooled fractions obtained from the Sephacryl™ S-100 size column (Fig. 4) were applied to a phenyl HP column. A, The BoNT/B complex was eluted at pH 5.8; B, the BoNT/B was purified at pH 8.0. The arrow shows the activity fraction.</p

SDS–PAGE analysis of the purified BoNT/B.

<p>1∼3, BoNT/B after purification by hydrophobic interaction chromatography column; M, markers.</p

Growth patterns of <i>C. botulinum</i> type B strain.

<p>It indicates the optical density of the cultures in three media.</p

Purification of the BoNT/B complex

a<p>The concentration of total BoNT/B complex was determined by a “sandwich” ELISA method using the purified BoNT/B as a standard. And the purified BoNT/B concentration was determined by BCA assay.</p>b<p>Minimum lethal dose (MLD) was defined as the minimum dose required to kill two mice as described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0039670#s4" target="_blank">Materials and Methods</a>.</p>c<p>The purification factor was estimated by the mouse toxigenicity test of BoNT/B complex.</p

Computational Exploration of Mechanism and Selectivities of (NHC)Nickel(II)hydride-Catalyzed Hydroalkenylations of Styrene with α‑Olefins

The [LNiH]⁺-catalyzed hydroalkenylation between styrene and α-olefins gives distinctive chemo- and regioselectivities with N-heterocyclic carbene (L = NHC) ligands: (a) the reaction with NHC ligands produces the branched tail-to-tail products, whereas the reaction with phosphine ligands (L = PR₃) favors the tail-to-head regio-isomers; (b) the reaction stops at heterodimerization with no further oligomerization even with excess α-olefin substrates; (c) typical side reactions with α-olefins, such as isomerization to internal olefins or polymerization, are either significantly diminished or eliminated. To understand the operating mechanism and origins of selectivities, density functional theory (DFT) calculations were performed, and several additional experiments were conducted. The olefin insertion step is found to determine both the regioselectivity and chemoselectivity, leading to the tail-to-tail heterohydroalkenylation product. With a small NHC ligand (1,3-dimethylimidazol-2-ylidene), the intrinsic electronic effects of ligand favor the tail-to-head regioisomer by about 1 kcal/mol in the olefin insertion step. With bulky NHC ligands (1,3-bis­(2,6-dimethylphenyl)­imidazol-2-ylidene or SIPr), the steric repulsions between the ligand and the substituent of the inserting alkene override the intrinsic electronic preference, making the tail-to-tail regioisomer favored (about 3 kcal/mol with both ligands). In the competition between homo- and heterodimerization, the insertion of the secondary styrene breaks its π-conjugation, making the insertion of styrene about 2 kcal/mol less favorable than that of alkyl-substituted alkenes. In addition, the interaction between nickel and phenyl group of styrene stabilizes the resting state and inhibits the side reactions with α-olefins, suggesting that styrene, or similar aryl olefins, is not only a substrate, but also an inhibitor for side reactions. This unique effect of styrene is verified by control experiments

Cosensitized Porphyrin System for High-Performance Solar Cells with TOF-SIMS Analysis

To date, development of organic sensitizers has been predominately focused on light harvesting, highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels, and the electron transferring process. In contrast, their adsorption mode as well as the dynamic loading behavior onto nanoporous TiO₂ is rarely considered. Herein, we have employed the time-of-flight secondary ion mass spectrometry (TOF-SIMS) to gain insight into the competitive dye adsorption mode and kinetics in the cosensitized porphyrin system. Using novel porphyrin dye <b>FW-1</b> and D–A−π–A featured dye <b>WS-5</b>, the different bond-breaking mode in TOF-SIMS and dynamic dye-loading amount during the coadsorption process are well-compared with two different anchoring groups, such as benzoic acid and cyanoacrylic acid. With the bombardment mode in TOF-SIMS spectra, we have speculated that the cyano group grafts onto nanoporous TiO₂ as tridentate binding for the common anchoring unit of cyanoacrylic acid and confirmed it through extensive first-principles density functional theory calculation by anchoring either the carboxyl or cyano group, which shows that the cyano group can efficiently participate in the adsorption of the <b>WS-5</b> molecule onto the TiO₂ nanocrystal. The grafting reinforcement interaction between the cyano group and TiO₂ in <b>WS-5</b> can well-explain the rapid adsorption characteristics. A strong coordinate bond between the lone pair of electrons on the nitrogen or oxygen atom and the Lewis acid sites of TiO₂ can increase electron injection efficiencies with respect to those from the bond between the benzoic acid group and the Brønsted acid sites of the TiO₂ surface. Upon optimization of the coadsorption process with dye <b>WS-5</b>, the photoelectric conversion efficiency based on porphyrin dye <b>FW-1</b> is increased from 6.14 to 9.72%. The study on the adsorption dynamics of organic sensitizers with TOF-SIMS analysis might provide a new venue for improvement of cosensitized solar cells

Dissociation constant (K_d) measurements by fluorescence anisotropy.

<p>A) βB-1; B) βB-20. Two nM of fluorescein labeled aptamers were titrated with increasing concentrations of β-BuTx. The anisotropy change (delta r) was the anisotropy of the initial fluorescein labeled aptamer subtracted from the anisotropy value at each β-BuTx concentration. Non-linear regression analysis revealed the K_d of βB-1 was 65.9±7.8 nM, and the K_d of βB-20 was 83.8±4.3 nM. Error bars represent standard deviations from three repeated experiments.</p