Two-Dimensional Conjugated Metal-Organic Frameworks with a Ring-in-Ring Topology and High Electrical Conductance.

Electrically conducting two-dimensional (2D) metal-organic frameworks (MOFs) have garnered significant interest due to their remarkable structural tunability and outstanding electrical properties. However, the design and synthesis of high-performance materials face challenges due to the limited availability of specific ligands and pore structures. In this study, we have employed a novel highly branched D3h symmetrical planar conjugated ligand, dodechydroxylhexabenzotrinaphthylene (DHHBTN) to fabricate a series of 2D conductive MOFs, named M-DHHBTN (M=Co, Ni, and Cu). This new family of MOFs offers two distinct types of pores, elevating the structural complexity of 2D conductive MOFs to a more advanced level. The intricate tessellation patterns of the M-DHHBTN are elucidated through comprehensive analyses involving powder X-ray diffraction, theoretical simulations, and high-resolution transmission electron microscope. Optical-pump terahertz-probe spectroscopic measurements unveiled carrier mobility in DHHBTN-based 2D MOFs spanning from 0.69 to 3.10 cm2 V-1 s-1. Among M-DHHBTN famility, Cu-DHHBTN displayed high electrical conductivity reaching 0.21 S cm-1 at 298 K with thermal activation behavior. This work leverages the "branched conjugation" of the ligand to encode heteroporosity into highly conductive 2D MOFs, underscoring the significant potential of heterogeneous double-pore structures for future applications.

Rapid room-temperature phosphorescence chiral recognition of natural amino acids.

Chiral recognition of amino acids is very important in both chemical and life sciences. Although chiral recognition with luminescence has many advantages such as being inexpensive, it is usually slow and lacks generality as the recognition module relies on structural complementarity. Here, we show that one single molecular-solid sensor, L-phenylalanine derived benzamide, can manifest the structural difference between the natural, left-handed amino acid and its right-handed counterpart via the difference of room-temperature phosphorescence (RTP) irrespective of the specific chemical structure. To realize rapid and reliable sensing, the doped samples are obtained as nanocrystals from evaporation of the tetrahydrofuran solutions, which allows for efficient triplet-triplet energy transfer to the chiral analytes generated in situ from chiral amino acids. The results show that L-analytes induce strong RTP, whereas the unnatural D-analytes produce barely any afterglow. The method expands the scope of luminescence chiral sensing by lessening the requirement for specific molecular structures.

Orientation of Thiocyanate Ions Tuning the Electron-Phonon Interactions in Pseudohalide Perovskites.

Although the importance of electron-phonon interactions on the optoelectronic properties of perovskites has been well documented, the structural origin of electron-phonon interactions remains largely unexplored. In this study, using pseudohalide perovskites Cs2Pb(SCN)2I2(1-x)Br2x as a model, we have revealed how the orientation of SCN- anions tunes the electron-phonon interactions and the effective charge-carrier mobility by utilizing femtosecond sum frequency generation vibrational spectroscopy, supplemented by photoluminescence spectroscopy and femtosecond optical-pump terahertz-probe spectroscopy. The coupling between neighboring SCN- anions decreases as the Br content (x) increases but does not have a significant effect on the electron-phonon interactions. In contrast, the orientation angle of SCN- anions has a strong correlation with the electron-phonon interaction and effective charge-carrier mobility, that is, a more parallel orientation of SCN- anions leads to a higher electron-phonon interaction and lower effective charge-carrier mobility. This finding provides a molecule-level understanding of the inorganic lattice structure in tuning electron-phonon interactions and may offer valuable guidance for optimizing the optoelectronic properties of perovskites.

Stacking Arrangement and Orientation of Aromatic Cations Tune Bandgap and Charge Transport of 2D Organic-Inorganic Hybrid Perovskites.

Chemical modifications on aromatic spacers of 2D perovskites have been demonstrated to be an effective strategy to simultaneously improve optoelectronic properties and stability. However, its underlying mechanism is poorly understood. By using 2D phenyl-based perovskites ([C6 H5 (CH2 )m NH3 ]2 PbI4 ) as models, the authors have revealed how the chemical nature of aromatic cations tunes the bandgap and charge transport of 2D perovskites by utilizing sum-frequency generation vibrational spectroscopy to determine the stacking arrangement and orientation of aromatic cations. It is found that the antiparallel slip-stack arrangement of phenyl rings between adjacent layers induces an indirect band gap, resulting in anomalous carrier dynamics. Incorporation of the CH2 moiety causes stacking rearrangement of the phenyl ring and thus promotes an indirect to direct bandgap transition. In direct-bandgap perovskites, higher carrier mobility correlates with a larger orientation angle of the phenyl ring. Further optimizing the orientation angle by introducing a para-substituted element in a phenyl ring, higher carrier mobility is obtained. This work highlights the importance of leveraging stacking arrangement and orientation of the aromatic cations to tune the photophysical properties, which opens up an avenue for advancing high-performance 2D perovskites optoelectronics via molecular engineering.

Boosting Charge Transport in a 2D/3D Perovskite Heterostructure by Selecting an Ordered 2D Perovskite as the Passivator.

We demonstrate that an ordered 2D perovskite can significantly boost the photoelectric performance of 2D/3D perovskite heterostructures. Using selective fluorination of phenyl-ethyl ammonium (PEA) lead iodide to passivate 3D FA0.8 Cs0.2 PbI3 , we find that the 2D/3D perovskite heterostructures passivated by a higher ordered 2D perovskite have lower Urbach energy, yielding a remarkable increase in photoluminescence (PL) intensity, PL lifetime, charge-carrier mobilities (ϕµ), and carrier diffusion length (LD ) for a certain 2D perovskite content. High performance with an ultralong PL lifetime of ≈1.3 µs, high ϕµ of ≈18.56 cm2  V-1 s-1 , and long LD of ≈7.85 µm is achieved in the 2D/3D films when passivated by 16.67 % para-fluoro-PEA2 PbI4 . This carrier diffusion length is comparable to that of some perovskite single crystals (>5 µm). These findings provide key missing information on how the organic cations of 2D perovskites influence the performance of 2D/3D perovskite heterostructures.

Conformational Order of Alkyl Side Chain of Poly(3-alkylthiophene) Promotes Hole-Extraction Ability in Perovskite/Poly(3-alkylthiophene) Heterojunction.

Molecular structures of hole transport materials (HTMs) have significant impact on the optoelectronic properties of perovskite/HTM heterojunction. But the structure-property relationship in the heterojunction remains poorly understood. By using poly(3-alkylthiophene) (P3AT) as the HTM model, here we apply sum frequency generation vibrational spectroscopy to establish correlations among conformations of P3ATs, the hole extraction ability of P3ATs from the perovskite layer, and the charge mobility of P3ATs. It is revealed that with similar energy-level alignment, the conformational order of alkyl side chains in regioregular P3ATs can effectively regulate the hole extraction ability of P3ATs from perovskite layer by tuning reorganization energy. By contrast, the charge mobility of P3ATs strongly depends on the P3AT backbone's coplanarity. Our findings decouple the roles of the long-hidden conformational order of alkyl side chain and the polythiophene backbone's coplanarity on the performance of perovskite/HTM heterojunction, offering useful guidelines for boosting the performance of optoelectronic devices.