A Reactive Molecular Dynamics Study of <i>n</i>‑Heptane Pyrolysis at High Temperature

<i>n</i>-Heptane is the most important straight chain paraffin in the fossil-fuel industry. In this work, pyrolysis of <i>n</i>-heptane at high temperature is investigated by a series of ReaxFF based reactive molecular dynamic simulations. The pyrolysis correlated intermediate reactions, important product/intermediate distributions, and corresponding kinetics behaviors are systematically analyzed at atomistic level. The results indicate that the entire pyrolysis process is radical-dominated. The unimolecular dissociation is the main pathway of <i>n</i>-heptane decomposition. Initiation of the decomposition is mainly through C–C bond fission. Central C–C bonds would dissociate prior to the terminal ones. Besides, the Rice–Kossiakoff theory is proved for the pyrolysis of <i>n</i>-heptane at the atomistic level. To give a better description of the pyrolysis behavior, some alkane related intermolecular reactions should be considered in the mechanism. The apparent activation energy extracted from the present simulations is 43.02–54.49 kcal/mol in the temperature range 2400–3000 K, which is reasonably consistent with the experimental results

Photophysical Properties of Self-Assembled Multinuclear Platinum Metallacycles with Different Conformational Geometries

In this work, spectroscopic techniques and quantum chemistry calculations were used to investigate the photophysical properties of various multinuclear platinum complexes with different conformational geometries. This suite of complexes includes a Pt–pyridyl square, a Pt–carboxylate triangle, and a mixed Pt–pyridyl–carboxylate rectangle, as well as two mononuclear Pt model complexes. Studying the individual molecular precursors in the context of larger assemblies is important to provide a complete understanding of the factors governing the observed photophysical properties of a given system. The absorption and emission bands of the parent linear dipyridyl donor (ligand 1) are largely preserved in the [4 + 4] square and the multicomponent [4 + 2 + 2] rectangle (<b>3</b> and <b>4</b>, respectively), with significant red shifts. The [3 + 3] Pt–carboxylate triangle containing <i>p</i>-phthalic acid is nonemissive. Phosphorescence and nanosecond transient spectroscopy on <b>3</b> and <b>4</b> reveal that the introduction of platinum atoms enhances spin–orbital coupling, thereby increasing the rate of intersystem crossing. This phenomenon is consistent with the low fluorescence quantum yields and short fluorescence lifetimes of <b>3</b> and <b>4</b>. Moreover, the electronic structures for the ground state and low-lying excited states of these compounds were studied using quantum chemistry calculations. The fluorescent states of the platinum complexes are local excited states of ligand-centered π–π* transition features, whereas the nonfluorescent states are intramolecular charge-transfer states. These low-lying intramolecular charge-transfer states are responsible for the nonemissive nature of small molecules <b>1</b> and <b>2</b> and triangle <b>5</b>. As the interactions between these components determine the properties of their corresponding assemblies, we establish novel excited-state decay mechanisms which dictate the observed spectra

A Revisit to the Orthogonal Bodipy Dimers: Experimental Evidence for the Symmetry Breaking Charge Transfer-Induced Intersystem Crossing

A series of Bodipy dimers with orthogonal conformation were prepared. The photophysical properties were studied with steady-state and time-resolved transient spectroscopies. We found the triplet-state quantum yield is highly dependent on the solvent polarity in the orthogonally linked symmetric Bodipy dimers, and the intersystem crossing (ISC) is efficient in solvents with moderate polarity. The photoinduced symmetry-breaking charge transfer (SBCT) in polar solvents was confirmed by femtosecond transient absorption spectroscopy, with the charge separation (CS) kinetics on the order of a few picoseconds and the charge recombination (CR) process occurring on the nanosecond time scale in dichloromethane. These observations are supported by the calculation of the charge separated state (CSS) energy levels, which are high in nonpolar solvents, and lower in polar solvents, thus the CR-induced ISC has the largest driven force in solvents with moderate polarity. These results clarify the mechanism of SOCT-ISC in the orthogonally symmetric Bodipy dimers. The acquired information, relating molecular structure and ISC property, will be useful for devising new strategies to induce ISC in heavy atom-free organic chromophores

Direct Observation of a Triplet-State Absorption-Emission Conversion in a Fullerene-Functionalized Pt(II) Metallacycle

An interesting triplet excited-state absorption-emission conversion of a fullerene-functionalized Pt­(II) metallacycle (<b>C</b> _<b>60</b> <b>–Pt</b>) caused by a concentration effect was directly observed by nanosecond transient absorption (ns TA) spectroscopy. In dilute solution, the triplet excited-state absorption (TESA) band was observed at about 750 nm with a lifetime of ca. 10.7 μs. However, with increasing the concentration, the absorption band converted to a triplet excited-state emission (TESE) band with a longer lifetime of ca. 15.4 μs. Femtosecond transient absorption experiments and quantum chemistry calculations were performed to reveal the excited-state decay pathways of <b>C</b> _<b>60</b> <b>–Pt</b> in concentrated solution. This conversion was ascribed to the formation of a triplet excimer, which forms at localized ³C₆₀ ^* states. This work demonstrates that radiative excimers with longer-lived triplet excited states can exist in concentrated solution, and this finding will provide useful information for applications of fullerene complexes, especially as photosensitizers

Constructing Sensitive and Fast Lead-Free Single-Crystalline Perovskite Photodetectors

We developed a high-performance photodetector based on (CH₃NH₃)₃Sb₂I₉ (MA₃Sb₂I₉) microsingle crystals (MSCs). The MA₃Sb₂I₉ single crystals exhibit a low-trap state density of ∼10¹⁰ cm^–3 and a long carrier diffusion length reaching 3.0 μm, suggesting its great potential for optoelectronic applications. However, the centimeter single crystal (CSC)-based photodetector exhibits low responsivity (10^–6 A/W under 1 sun illumination) due to low charge-carrier collection efficiency. By constructing the MSC photodetector with efficient charge-carrier collection, the responsivity can be improved by three orders of magnitude (under 1 sun illumination) and reach 40 A/W with monochromatic light (460 nm). Furthermore, the MSC photodetectors exhibit fast response speed of <1 ms, resulting in a high gain of 108 and a gain-bandwidth product of 10⁵ Hz. These numbers are comparable to the lead-perovskite single-crystal-based photodetectors

Intraligand Charge Transfer Sensitization on Self-Assembled Europium Tetrahedral Cage Leads to Dual-Selective Luminescent Sensing toward Anion and Cation

Luminescent supramolecular lanthanide edifices have many potential applications in biology, environments, and materials science. However, it is still a big challenge to improve the luminescent performance of multinuclear lanthanide assemblies in contrast to their mononuclear counterparts. Herein, we demonstrate that combination of intraligand charge transfer (ILCT) sensitization and coordination-driven self-assembly gives birth to bright Eu^III tetrahedral cages with a record emission quantum yield of 23.1%. The ILCT sensitization mechanism has been unambiguously confirmed by both time-dependent density functional theory calculation and femtosecond transient absorption studies. Meanwhile, dual-responsive sensing toward both anions and cations has been demonstrated making use of the ILCT transition on the ligand. Without introduction of additional recognition units, high sensitivity and selectivity are revealed for the cage in both turn-off luminescent sensing toward I^– and turn-on sensing toward Cu²⁺. This study offers important design principles for the future development of luminescent lanthanide molecular materials

Experimental and Theoretical Study on the Photophysical Properties of 90° and 60° Bimetallic Platinum Complexes

The 90° and 60° bimetallic platinum complexes with special structures are widely used in coordination-driven self-assembled metallosupramolecular architectures, and these complexes are the key components of triangular, rectangular, and polygonal metallacycle and metallocage supramolecules. Therefore, spectroscopic techniques and quantum chemistry calculations were employed in this article to investigate the photophysical properties of these bimetallic platinum complexes. Compared with spectra for the ligands, the absorption spectra of these Pt complexes are red-shifted, and the fluorescence spectra become wider and are also red-shifted. Moreover, the reasons for the low fluorescence quantum yields and short fluorescence lifetimes of these compounds were investigated using quantum chemistry calculations. We demonstrate that the fluorescent states of the bimetallic platinum complexes can be considered as local excited states, and that they possess a ligand-centered π–π* transition feature. Meanwhile, the platinum metals act as perturbation for these transitions, whereas the nonfluorescent states are classified as intramolecular charge-transfer states. Furthermore, a new fluorescence modulation mechanism is developed to explain the different emission processes of these complexes with different ligands