Intersystem crossing (ISC), formally forbidden within nonrelativistic quantum theory, is the mechanism by which a molecule changes its spin state. It plays an important role in the excited state decay dynamics of many molecular systems and not just those containing heavy elements. In the simplest case, ISC is driven by direct spin–orbit coupling between two states of different multiplicities. This coupling is usually assumed to remain unchanged by vibrational motion. It is also often presumed that spin-allowed radiationless transitions, i.e. internal conversion, and the nonadiabatic coupling that drives them, can be considered separately from ISC and spin–orbit coupling owing to the vastly different time scales upon which these processes are assumed to occur. However, these assumptions are too restrictive. Indeed, the strong mixing brought about by the simultaneous presence of nonadiabatic and spin–orbit coupling means that often the spin, electronic, and vibrational dynamics cannot be described independe...

Spin-Vibronic Mechanism for Intersystem Crossing

Nonadiabatic mixed quantum–classical (NA-MQC) dynamics methods form a class of computational theoretical approaches in quantum chemistry tailored to investigate the time evolution of nonadiabatic phenomena in molecules and supramolecular assemblies. NA-MQC is characterized by a partition of the molecular system into two subsystems: one to be treated quantum mechanically (usually but not restricted to electrons) and another to be dealt with classically (nuclei). The two subsystems are connected through nonadiabatic couplings terms to enforce self-consistency. A local approximation underlies the classical subsystem, implying that direct dynamics can be simulated, without needing precomputed potential energy surfaces. The NA-MQC split allows reducing computational costs, enabling the treatment of realistic molecular systems in diverse fields. Starting from the three most well-established methods—mean-field Ehrenfest, trajectory surface hopping, and multiple spawning—this review focuses on the NA-MQC dynamics...

https://hal-amu.archives-ouvertes.fr/hal-01965458/document

Recent Advances and Perspectives on Nonadiabatic Mixed Quantum-Classical Dynamics.

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.

Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package

Proton-coupled electron transfer (PCET) is ubiquitous throughout chemistry and biology. This Perspective discusses recent advances and current challenges in the field of PCET, with an emphasis on the role of theory and computation. The fundamental theoretical concepts are summarized, and expressions for rate constants and kinetic isotope effects are provided. Computational methods for calculating reduction potentials and pKa’s for molecular electrocatalysts, as well as insights into linear correlations and non-innocent ligands, are also described. In addition, computational methods for simulating the nonadiabatic dynamics of photoexcited PCET are discussed. Representative applications to PCET in solution, proteins, electrochemistry, and photoinduced processes are presented, highlighting the interplay between theoretical and experimental studies. The current challenges and suggested future directions are outlined for each type of application, concluding with an overall view to the future.

Proton-Coupled Electron Transfer: Moving Together and Charging Forward

We present a current, up-to-date review of the surface hopping methodology for solving nonadiabatic problems, 25 years after Tully published the fewest switches surface hopping algorithm. After reviewing the original motivation for and failures of the algorithm, we give a detailed examination of modern advances, focusing on both theoretical and practical issues. We highlight how one can partially derive surface hopping from the Schrodinger equation in the adiabatic basis, how one can change basis within the surface hopping algorithm, and how one should understand and apply the notions of decoherence and wavepacket bifurcation. The question of time reversibility and detailed balance is also examined at length. Recent applications to photoexcited conjugated polymers are discussed briefly.

Understanding the Surface Hopping View of Electronic Transitions and Decoherence.

Two-coordinate Cu (I) complexes have attracted great interest recently because of the rich photophysical property in solid state, including the aggregation-induced thermal activated delayed fluorescence. Here, we summarize our theoretical investigations on the excited state structure and decay dynamics for the two-coordinate Cu (I) complexes in solution phase and solid state by the thermal vibration correlation function rate formalism we developed earlier coupled with time-dependent density-functional theory within polarizable continuum model and hybrid quantum and molecular mechanics. First, for the CAACCu (I)Cl complex, we found that the nature of the excited state undergoes a change from metal-to-ligand charge transfer (MLCT) in solution to hybrid halogen-to-ligand charge transfer and MLCT in solid state. The bending vibrations of the CCuCl and CuCN bonds are restricted in aggregates, reducing the non-radiative decay rate to cause strong solid-state fluorescence. Second, for CAACCu (I)Cz, we found that both intersystem crossing (ISC) and reverse intersystem crossing (rISC) are enhanced by 2–4 orders of magnitudes upon aggregation, leading to highly efficient thermally activated delayed fluorescence (TADF). The enhanced ISC and rISC rates can be attributed to the increase of the metal proportion in the frontier molecular orbitals, leading to an enhanced spin−orbit coupling between S1 and T1. The reaction barriers for ISC and rISC are much lower in solution than that in aggregate phase resulting in a decrease in energy gap ∆ E ST $$ \Delta {E}_{\mathrm{ST}} $$ and an increase in the relative reorganization energy through bending the angle ∠C − Cu − N for T1. Our theoretical studies provide a clear rationalization for the highly efficient solid-state luminescence character of two-coordinate Cu (I) complexes and may clarify the ongoing dispute on the understanding of the high TADF quantum efficiency. 

Computational studies on the excited state decay rates in aggregates of two‐coordinate
 Cu (I)
 complexes:
 Thermally Activated Delayed Fluorescence
 and
 Aggregation Induced Emis

Recently, the development of machine learning (ML) potentials has made it possible to perform large-scale and long-time molecular simulations with the accuracy of quantum mechanical (QM) models. However, for different levels of QM methods, such as density functional theory (DFT) at the meta-GGA level and/or with exact exchange, quantum Monte Carlo, etc., generating a sufficient amount of data for training an ML potential has remained computationally challenging due to their high cost. In this work, we demonstrate that this issue can be largely alleviated with Deep Kohn-Sham (DeePKS), an ML-based DFT model. DeePKS employs a computationally efficient neural network-based functional model to construct a correction term added upon a cheap DFT model. Upon training, DeePKS offers closely matched energies and forces compared with high-level QM method, but the number of training data required is orders of magnitude less than that required for training a reliable ML potential. As such, DeePKS can serve as a bridge between expensive QM models and ML potentials: one can generate a decent amount of high-accuracy QM data to train a DeePKS model and then use the DeePKS model to label a much larger amount of configurations to train an ML potential. This scheme for periodic systems is implemented in a DFT package ABACUS, which is open source and ready for use in various applications.

DeePKS+ABACUS as a Bridge between Expensive Quantum Mechanical Models and Machine Learning Potentials

Multifunctional Fe-doped carbon dots and metal-organic frameworks nanoreactor for cascade degradation and detection of organophosphorus pesticides

Integration of luminescence and carrier transport is of prime interest for optoelectronics, but full with challenges in organic semiconductors. Previously, it has been demonstrated that intermolecular charge transfer (ICT) in the herringbone H‐aggregate can lend oscillator strength to the lowest‐lying dark state if electron transfer (te) and hole transfer (th) integrals are in phase (same sign) and greater than 2J$\sqrt 2 J$ , where J is exciton coupling. In this work, both herringbone and π–π stackings are considered and a universal descriptor I =2teth/[(|te|+|th|)·|J|]$I\;{\bf = }2{t_{\rm{e}}}{t_{\rm{h}}}/\left[ {(|{t_{\rm{e}}}|{\bf + }|{t_{\rm{h}}}|)\cdot|J|} \right]$ is proposed. It is found that for π–π stacking with I > 1 and for herringbone with I>2$I{\bf > }\sqrt 2 $ , respectively, the lowest dark state could become bright through mixing ICT component. Accordingly, it is found that eclipsed packing and use of molecules with acene‐like frontier molecular orbital profiles favor larger I value than the other packing patterns. Finally, with the help of the proposed descriptor, 13 high‐mobility emissive unit cores from 32 fused ring molecules are effectively screened out.

Influence of Intermolecular Packing on Light Emitting Efficiency and Carrier‐Mobility of Organic Semiconductors: Theoretical Descriptor for Molecular Design

We computationally investigated the molecular aggregation effects on the excited state deactivation processes by considering both the direct vibrational relaxation and the S0/S1 surface crossing, that is, the minimum energy conical intersection (MECI). Taking classical AIEgens bis(piperidyl)anthracenes (BPAs) isomers and the substituted silole derivatives as examples, we show that the deformation of MECI always occurs at the atom with greater hole/electron overlap. Besides, the energetic and structural changes of MECI caused by substituent has been investigated. We find that effective substituent such as the addition of the electron‐donating groups, which can polarize the distribution of hole/electron density of molecules, will lead to the pyramidalization deformation of MECI occurring at the substituent position and simultaneously reduce the required energy to reach MECI. And MECI is sterically restricted by the surrounding molecules in solid phase, which remarkably hinders the non‐radiative decay through surface crossing. Through quantitative computational assessments of the fluorescence quantum efficiency for both solution and solid phases, we elucidate the role of MECI and its dependence on the substitutions through pyramidalization deformation, which give rise to the aggregation‐induced emission (AIE) phenomenon for 9,10‐BPA, to aggregation‐enhance emission (AEE) behavior for 1,4‐BPA, and to conventional aggregation‐caused quenching (ACQ) for 1,5‐BPA. We further verify such mechanism for siloles, for which we found that the substitutions do not change the AIE behavior. Our findings render a general molecular design approach to manipulating the aggregation effect for optical emission.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/agt2.291

Qi Ou

Papers

Computational studies on the excited state decay rates in aggregates of two‐coordinate Cu (I) complexes: Thermally Activated Delayed Fluorescence and Aggregation Induced Emis

DeePKS+ABACUS as a Bridge between Expensive Quantum Mechanical Models and Machine Learning Potentials

Multifunctional Fe-doped carbon dots and metal-organic frameworks nanoreactor for cascade degradation and detection of organophosphorus pesticides

Influence of Intermolecular Packing on Light Emitting Efficiency and Carrier‐Mobility of Organic Semiconductors: Theoretical Descriptor for Molecular Design

Substituent‐controlled aggregate luminescence: Computational unraveling of S1/S0 surface crossing