Modulating the Photochemistry of Bipyridylic Compounds by Symmetric Substitutions

A quantum electronic study of the effect of substituents on (2,2′‐bipyridyl)‐3,3′‐diol and (2,2′‐bipyridyl)‐3,3′‐diamine is presented. A large difference in the photochemical behavior between the original and the substituted selected systems is expected. For the sake of simplicity, the study is restricted to the symmetrically bi‐substituted compounds: fluorine, the more electronegative atom and thus a strong σ‐acceptor but also a weak π‐donor group, and NO₂, a strong π‐acceptor substituent. Among the large set of compounds studied, two receive special attention: 5,5′‐dinitro‐(2,2′‐bipyridyl)‐3,3′‐diamine and 6,6′‐difluoro‐(2,2′‐bipyridyl)‐3,3′‐diol. While in the former case the nitro substitution transforms (2,2′‐bipyridyl)‐3,3′‐diamine, previously suggested to behave as a photomemory material, into a simple fluorescent species, the latter substitution turns (2,2′‐bipyridyl)‐3,3′‐diol into a fresh new candidate for a photomemory device.     

Photochemical Hydrogen Evolution at Metal Centers Probed with Hydrated Aluminium Cations,Al+(H2O)n,n=1–10

Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n=2 reveal that solvation shifts the intense 3s–3p excitations of Al⁺ into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S₁ to T₂ takes place, and photochemical hydrogen formation starts on the T₂ surface, which passes through a conical intersection, changing to T₁. On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n=8. Clusters with n=9 or 10 occur exclusively as HAlOH⁺(H₂O)_n-1 and are transparent in the investigated energy range. For n=4–8, a mixture of Al⁺(H₂O)_n and HAlOH⁺(H₂O)_n-1 is present in the experiment.     

A reduced-restricted-quasi-Newton–Raphson method for locating and optimizing energy crossing points between two potential energy surfaces

We present a method for the location and optimization of an intersection energy point between two potential energy surfaces. The procedure directly optimizes the excited state energy using a quasi-Newton–Raphson method coupled with a restricted step algorithm. A linear transformation is also used for the solution of the quasi-Newton–Raphson equations. The efficiency of the algorithm is analyzed and demonstrated in some examples. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 :992–1003, 1997     

Exploring Potential Energy Surfaces for Aggregation‐Induced Emission—From Solution to Crystal

Aggregation‐induced emission (AIE) is a phenomenon where non‐luminescent compounds in solution become strongly luminescent in aggregate and solid phase. It provides a fertile ground for luminescent applications that has rapidly developed in the last 15 years. In this review, we focus on the contributions of theory and computations to understanding the molecular mechanism behind it. Starting from initial models, such as restriction of intramolecular rotations (RIR), and the calculation of non‐radiative rates with Fermi's golden rule (FGR), we center on studies of the global excited‐state potential energy surfaces that have provided the basis for the restricted access to a conical intersection (RACI) model. In this model, which has been shown to apply for a diverse group of AIEgens, the lack of fluorescence in solution comes from radiationless decay at a CI in solution that is hindered in the aggregate state. We also highlight how intermolecular interactions modulate the photophysics in the aggregate phase, in terms of fluorescence quantum yield and emission color.     

Nonradiative Excited-State Decay via Conical Intersection in Graphene Nanostructures

Chemical groups are known to tune the luminescent efficiencies of graphene-related nanomaterials, but some species, including the epoxide group (−COC−), are suspected to act as emission-quenching sites. Herein, by performing nonadiabatic excited-state dynamics simulations, we reveal a fast (within 300 fs) nonradiative excited-state decay of a graphene epoxide nanostructure from the lowest excited singlet (S₁) state to the ground (S₀) state via a conical intersection (CI), at which the energy difference between the S₁ and S₀ states is approximately zero. This CI is induced after breaking one C−O bond at the −COC− moiety during excited-state structural relaxation. This study ascertains the role of epoxide groups in inducing the nonradiative recombination of the excited electron-hole, providing important insights into the CI-promoted nonradiative de-excitations and the luminescence tuning of relevant materials. In addition, it shows the feasibility of utilizing nonadiabatic excited-state dynamics simulations to investigate the photophysical processes of the excited states of graphene nanomaterials.     

Study of the Photochemical Properties and Conical Intersections of [2,2′‐Bipyridyl]‐3‐amine‐3′‐ol

The two isoelectronic bipyridyl derivatives [2,2′‐bipyridyl]‐3,3′‐diamine and [2,2′‐bipyridyl]‐3,3′‐diol are experimentally known to undergo very different excited‐state double‐proton‐transfer processes, which result in fluorescence quantum yields that differ by four orders of magnitude. In a previous study, these differences were explained from a theoretical point of view, because of topographical features in the potential energy surface and the presence of conical intersections (CIs). Here, we analyze the photochemical properties of a new molecule, [2,2′‐bipyridyl]‐3‐amine‐3′‐ol [BP(OH)(NH₂)], which is, in fact, a hybrid of the former two. Our density functional theory (DFT), time‐dependent DFT (TDDFT), and complete active space self‐consistent field (CASSCF) calculations indicate that the double‐proton‐transfer process in the ground and first singlet π→π* excited state in BP(OH)(NH₂) presents features that are between those of their “parents”. The presence of two CIs and the role they may play in the actual photochemistry of BP(OH)(NH₂) and other bipyridyl derivatives are also discussed.     

含BN旋转轴的光驱动分子马达的理论设计和机理研究

依据B=N键与C=C键的电子结构相似性,以Feringa型二苯乙烯型光驱动分子马达（CC-stilbene）为母体,设计了含极性旋转轴的模型马达BN-stilbene.CASPT2//CASSCF计算结果表明,优化所得的BN-stilbene分子的基态存在四个与CC-stilbene马达结构相似、相对能量一致的螺旋异构体;B=N极性共价双键对BN-stilbene的基态和激发态电子结构有显著影响.对BN-stilbene模型马达的工作机理研究表明,极性旋转轴的引入使得BN-stilbene中S₁/S₀-CI与激发态中间体构型更加相似且能量更低,同时可增加旋转的驱动力,起到改进分子马达光异构化过程的单向性的目的.     

The keto→enol photoisomerization of N‐salicilydenemethylfurylamine: Nonadiabatic ab initio dynamics simulation

The photoinduced isomerization of cis‐keto and trans‐keto isomers in N‐salicilydenemethylfurylamine has been studied using the surface‐hopping approach at the CASSCF level of theory. After the cis‐keto or trans‐keto isomer is excited to S₁ state, the molecule initially moves to a excited‐state local minimum. The torsional motion around relative bonds in the chain drives the molecule to approach a keto‐form conical intersection and then nonadiabatic transition occurs. According to our full‐dimensional dynamics simulations, the trans‐keto and enol photoproducts are responsible for the photochromic effect of cis‐keto isomer excited to S₁ state, while no enol isomer was obtained in the photoisomerization of trans keto on excitation. The cis keto to enol and cis keto to trans keto isomerizations are reversible photochemical reactions. It is confirmed that this aromatic Schiff base is a potential molecular switch. Furthermore, the torsion of C N bond occurs in the radiationless decay of trans‐keto isomer, while it is completely suppressed by an intramolecular hydrogen bonding interaction in the dynamics of cis‐keto form. Moreover, the excited‐state lifetime of cis keto is longer than that of trans‐keto form due to the O···H N hydrogen bond.     

基于转换控制定律的倒立摆控制器设计(英文)

本文研究倒立摆的控制器设计.运用控制转换定律和Lyapunov第二方法,有效解决了由钟的分离角和控制系统中小车的置换引起的强非线性和强耦合性难题.并证明该控制器对由摆的长度和质量以及小车的质量等参数决定的非线性摩擦系数的变化具有强鲁棒性.最后,仿真实验结果证明了该控制器设计的可行性.     

Stabilization via parametric excitation of multi-dof statically unstable systems

The problem of re-stabilization via parametric excitation of statically unstable linear Hamiltonian systems is addressed. An n-degree-of-freedom dynamical system is considered, at rest in a critical equilibrium position, possessing a pair of zero-eigenvalues and n − 1 pairs of distinct purely imaginary conjugate eigenvalues. The response of the system to a small static load, making the zero eigenvalues real and opposite, simultaneous to a harmonic parametric excitation of small amplitude, is studied by the Multiple Scale perturbation method, and the stability of the equilibrium position is investigated. Several cases of resonance between the excitation frequency and the natural non-zero frequencies are studied, calling for standard and non-standard applications of the method. It is found that the parametric excitation is able to re-stabilize the equilibrium for any value of the excitation frequencies, except for frequencies close to resonant values, provided a sufficiently large excitation amplitude is enforced. Results are compared with those provided by a purely numerical approach grounded on the Floquet theory.