2D transition metal carbides or nitrides, known as MXenes, are a new family of 2D materials with close to 30 members experimentally synthesized and dozens more theoretically investigated. Because of the abundant surface terminations, MXenes have been compounded with various materials by multi-interactions. In addition to the prevented aggregation and oxidation of MXene flakes, the MXene/polymer membranes exhibit outstanding mechanical, thermal, and electrical properties due to the synergistic effects. However, relatively little is currently known about the MXene/polymer membranes and a special review on the progress of the synthesis, properties, and applications of MXene/polymer membranes has not been reported to date. Herein, this Review starts with an introduction of the synthesis and properties of MXenes. Then the development of MXene/polymer membranes will be discussed, which aims to summarize various approaches of fabricating MXene/polymer membranes and their fascinating properties. The focus then turns to their exciting potential applications in various fields such as filtration, electromagnetic interference (EMI) shielding, energy storage devices, wearable electronics, etc. Finally, outlooks and perspectives for the future challenges and prospects of MXene/polymer membranes are provided.

Inspired by the formulation of quantum-electrodynamical time-dependent density functional theory (QED-TDDFT) by Rubio and co-workers [Flick et al., ACS Photonics 6, 2757-2778 (2019)], we propose an implementation that uses dimensionless amplitudes for describing the photonic contributions to QED-TDDFT electron–photon eigenstates. This leads to a Hermitian QED-TDDFT coupling matrix that is expected to facilitate the future development of analytic derivatives. Through a Gaussian atomic basis implementation of the QED-TDDFT method, we examined the effect of dipole self-energy, rotating-wave approximation, and the Tamm–Dancoff approximation on the QED-TDDFT eigenstates of model compounds (ethene, formaldehyde, and benzaldehyde) in an optical cavity. We highlight, in the strong coupling regime, the role of higher-energy and off-resonance excited states with large transition dipole moments in the direction of the photonic field, which are automatically accounted for in our QED-TDDFT calculations and might substantially affect the energies and compositions of polaritons associated with lower-energy electronic states.

Here, we design and synthesize an organic laser molecule, 2,7-diphenyl-9H-fluorene (LD-1), which has state-of-the-art integrated optoelectronic properties with a high mobility of 0.25 cm2 V–1 s–1, a high photoluminescence quantum yield of 60.3%, and superior deep-blue laser characteristics (low threshold of P th = 71 μJ cm–2 and P th = 53 μJ cm–2 and high quality factor (Q) of ∼3100 and ∼2700 at emission peaks of 390 and 410 nm, respectively). Organic light-emitting transistors based on LD-1 are for the first time demonstrated with obvious electroluminescent emission and gate tunable features. This work opens the door for a new class of organic semiconductor laser molecules and is critical for deep-blue optical and laser applications.

The photophysics of benzaldehyde are analyzed through the lens of TD-DFT adiabatic excited states and Boys or Edmiston−Ruedenberg localized diabatic states. We predict rate constants for two processes in excited benzaldehyde: (i) the intersystem crossing from S 1 → T 2 and (ii) the phosphorescence from T 1 → S 0 . We also study (iii) the conical intersection between T 2 and T 1 that is putatively responsible for an ultrafast internal conversion process, T 2 → T 1 . In agreement with Ohmori et al. (J. Phys. Chem. 1988, 92 (5), 1086−1093), our results suggest that the S 1 → T 2 intersystem crossing in benzaldehyde is rapid not only because of a large spin−orbit matrix element (i.e., El-Sayed's rule) but also because of a fortuitously small energy barrier. Furthermore, when studying the T 2 → T 1 internal conversion, we find that both Boys and Edmiston−Ruedenberg localization give remarkably stable and accurate diabatic states which will be useful for ongoing studies of dynamics near conical intersections. To our knowledge, this is the first example whereby localized diabatization techniques have been tested and have successfully recovered the topology of a conical intersection.

We present a complete derivation of derivative couplings between excited states in the framework of adiabatic time-dependent density functional response theory. Explicit working equations are given and the resulting derivative couplings are compared with derivative couplings from a pseudo-wavefunction ansatz. For degenerate excited states, i.e., close to a conical intersection (CI), the two approaches are identical apart from an antisymmetric overlap term. However, if the difference between two excitation energies equals another excitation energy, the couplings from response theory exhibit an unphysical divergence. This spurious behavior is a result of the adiabatic or static kernel approximation of time-dependent density functional theory leading to an incorrect analytical structure of the quadratic response function. Numerical examples for couplings close to a CI and for well-separated electronic states are given.

Electronically photoexcited dynamics are complicated because there are so many different relaxation pathways: fluorescence, phosphorescence, radiationless decay, electon transfer, etc. In practice, to model photoexcited systems is a very difficult enterprise, requiring accurate and very efficient tools in both electronic structure theory and nonadiabatic chemical dynamics. Moreover, these theoretical tools are not traditional tools. On the one hand, the electronic structure tools involve couplings between electonic states (rather than typical single state energies and gradients). On the other hand, the dynamics tools involve propagating nuclei on multiple potential energy surfaces (rather than the usual ground state dynamics). In this Account, we review recent developments in electronic structure theory as directly applicable for modeling photoexcited systems. In particular, we focus on how one may evaluate the couplings between two different electronic states. These couplings come in two flavors. If we order states energetically, the resulting adiabatic states are coupled via derivative couplings. Derivative couplings capture how electronic wave functions change as a function of nuclear geometry and can usually be calculated with straightforward tools from analytic gradient theory. One nuance arises, however, in the context of time-dependent density functional theory (TD-DFT): how do we evaluate derivative couplings between TD-DFT excited states (which are tricky, because no wave function is available)? This conundrum was recently solved, and we review the solution below. We also discuss the solution to a second, pesky problem of origin dependence, whereby the derivative couplings do not (strictly) satisfy translation variance, which can lead to a lack of momentum conservation. Apart from adiabatic states, if we order states according to their electronic character, the resulting diabatic states are coupled via electronic or diabatic couplings. The couplings between diabatic states |ΞA⟩ and |ΞB⟩ are just the simple matrix elements, ⟨ΞA|H|ΞB⟩. A difficulty arises, however, because constructing exactly diabatic states is formally impossible and constructing quasi-diabatic states is not unique. To that end, we review recent advances in localized diabatization, which is one approach for generating adiabatic-to-diabatic (ATD) transformations. We also highlight outstanding questions in the arena of diabatization, especially how to generate multiple globally stable diabatic surfaces.

Electrically pumped organic lasing is one of the most challenging issues in organic optoelectronics. We present a systematic theoretical investigation to screen out electrical pumping lasing molecules over a wide range of organic materials. With the electronic structure information obtained from time-dependent density functional theory, we calculate multiple photophysical parameters of a set of optical pumping organic laser molecules in our self-developed molecular material property prediction package (MOMAP) to judge whether the electrically pumped lasing conditions can be satisfied, namely, to avoid reabsorption from excitons and/or polarons, and the accumulation of triplet excitons. In addition, a large oscillator strength of S1 and weak intermolecular π–π interaction are preferred. With these criteria, we are able to conclude that BP3T, BSBCz, and CzPVSBF compounds are promising candidates for electrically pumped lasing, and the proposed computational strategy could serve as a general protocol for molecular design of organic lasing materials.

Accurate theoretical description of the electronic structure of boron dipyrromethene (BODIPY) molecules has been a challenge, let alone the prediction of fluorescence quantum efficiency. In this Letter, we show that the electronic structures of BODIPYs can be accurately evaluated via the spin-flip time-dependent density functional theory with the B3LYP functional. With the resulting electronic structures, the experimental spectral line shapes of representative BODIPYs are successfully reproduced by our previously developed thermal vibration correlation function method. Most importantly, a two-channel scheme is proposed to describe the internal conversion of S 1 to S 0 in BODIPYs: channel I via direct vibrational relaxation within the harmonic region and channel II via a distorted S 0 /S 1 minimum energy crossing point well away from the harmonic region. The fluorescence quantum yields are accurately predicted within this two-channel scheme, which can therefore serve as a generalized method for predicting the photophysical parameters of organic fluorescent compounds.