Entanglement entropy of electronic excitations

© 2016 Author(s). A new perspective into correlation effects in electronically excited states is provided through quantum information theory. The entanglement between the electron and hole quasiparticles is examined, and it is shown that the related entanglement entropy can be computed from the eigenvalue spectrum of the well-known natural transition orbital (NTO) decomposition. Non-vanishing entanglement is obtained whenever more than one NTO pair is involved, i.e., in the case of a multiconfigurational or collective excitation. An important implication is that in the case of entanglement it is not possible to gain a complete description of the state character from the orbitals alone, but more specific analysis methods are required to decode the mutual information between the electron and hole. Moreover, the newly introduced number of entangled states is an important property by itself giving information about excitonic structure. The utility of the formalism is illustrated in the cases of the excited states of two interacting ethylene molecules, the conjugated polymer para-phenylene vinylene, and the naphthalene molecule

Visualisation of electronic excited-state correlation in real space

A method for the visualisation of excited‐state electron correlation is introduced and shown to address two notorious problems in excited‐ state electronic structure theory, the analysis of excitonic correlation and the distinction between covalent and ionic wavefunction character. The method operates by representing the excited state in terms of electron and hole quasiparticles, fixing the hole on a fragment of the system and observing the resulting conditional electron density in real space. The application of this approach to oligothiophene, an exemplary conjugated polymer, illuminates excitonic correlation effects of its excited states in unprecedented clarity and detail. A study of naphthalene shows that the distinction between the ionic and covalent states of this molecule, which has so far only been achieved using elaborate valence‐bond theory protocols, arises naturally in terms of electron‐hole avoidance and enhanced overlap, respectively. More generally, the method is relevant for any excited state that cannot be described by a single electronic configuration

TheoDORE: a toolbox for a detailed and automated analysis of electronic excited state computations

The advent of ever more powerful excited-state electronic structure methods has lead to a tremendous increase in the predictive power of computation but it has also rendered the analysis of these computations more and more challenging and time-consuming. TheoDORE tackles this problem through providing tools for post-processing excited-state computations, which automate repetitive tasks and provide rigorous and reproducible descriptors. Interfaces are available for ten different quantum chemistry codes and a range of excited-state methods implemented therein. This article provides an overview of three popular functionalities within TheoDORE, a fragment-based analysis for assigning state character, the computation of exciton sizes for measuring charge transfer, and the natural transition orbitals used not only for visualisation but also for quantifying multiconfigurational character. Using the examples of an organic push-pull chromophore and a transition metal complex, it is shown how these tools can be used for a rigorous and automated assignment of excited-state character. In the case of a conjugated polymer, we venture beyond the limits of the traditional molecular orbital picture to uncover spatial correlation effects using electron-hole correlation plots and conditional densitie

Communication: Unambiguous comparison of many-electron wavefunctions through their overlaps

© 2016 Author(s). A simple and powerful method for comparing many-electron wavefunctions constructed at different levels of theory is presented. By using wavefunction overlaps, it is possible to analyze the effects of varying wavefunction models, molecular orbitals, and one-electron basis sets. The computation of wavefunction overlaps eliminates the inherent ambiguity connected to more rudimentary wavefunction analysis protocols, such as visualization of orbitals or comparing selected physical observables. Instead, wavefunction overlaps allow processing the many-electron wavefunctions in their full inherent complexity. The presented method is particularly effective for excited state calculations as it allows for automatic monitoring of changes in the ordering of the excited states. A numerical demonstration based on multireference computations of two test systems, the selenoacrolein molecule and an iridium complex, is presented

Uv absorption inmetal decorated boron nitride flakes: A theoretical analysis of excited states

© Informa UK Limited, trading as Taylor & Francis Group. The excited states of singlemetal atom(X=Co, Al and Cu) doped boron nitride flake (MBNF) B 15 N 14 H 14 -X and pristine boron nitride (B 15 N 15 H 14 ) are studied by time-dependent density functional theory. The immediate effect of metal doping is a red shift of the onset of absorption from about 220 nmfor pristine BNF to above 300 nm for all metal-doped variants with the biggest effect for MBNF-Co, which shows appreciable intensity even above 400 nm. These energy shifts are analysed by detailed wavefunction analysis protocols using visualisationmethods, such as the natural transition orbital analysis and electron-hole correlation plots, as well as quantitative analysis of the exciton size and electronhole populations. The analysis shows that the Co and Cu atoms provide strong contributions to the relevant states whereas the aluminium atom is only involved to a lesser extent

Exploring the excited state character of nitroarylcarbazole derivatives using wavefunction analysis

The solid state fluorescence behaviour of a family of 9-(4-Nitroaryl)carbazoles is investigated via calculations of their electronically excited states. Ground state optimisations show that generally, the most stable conformation has the nitroaryl moiety twisted relative to the carbazole. Vertical excitation calculations and subsequent wavefunction analysis [1] show that for molecules which are solid state emissive, the lowest lying singlet state has a major contribution from a charge transfer state from the carbazole donor to the nitroaryl acceptor. For molecules which are non-emissive, the lowest lying singlet state is found to be an nπ* state on the nitro group. Moreover, we demonstrate how the assignment of these states can be done via a fully automated procedure. We find that the energy of the charge transfer state decreases as the electron withdrawing power of the nitroaryl acceptor is increased. Calculations of excitations for molecules with orthogonal conformations suggest that the charge transfer state is dark, as indicated by a very small oscillator strength. The question, then, is: how can these molecules be solid state emissive if the lowest lying excited state is dark? A scan of the torsion angle shows that the oscillator strength increases significantly as the molecule becomes more planar, and the energy profile for this rotation is relatively flat. We therefore propose that provided that stacking in the solid state leads to small perturbations in the molecular geometry and the lowest lying singlet is a charge transfer state, the molecule will be solid state emissive. References 1. F. Plasser “Theodore 2.0.2: a package for theoretical density, orbital relaxation and exciton analysis”; available from http://theodore-qc.sourceforge.ne

Exploring the excited state character of nitroarylcarbazole derivatives using wavefunction analysis

The solid state fluorescence behaviour of a family of 9-(4-Nitroaryl)carbazoles is investigated via calculations of their electronically excited states. Ground state optimisations show that for many of the molecules, the most stable conformation has the nitroaryl moiety twisted relative to the carbazole. Vertical excitation calculations and subsequent wavefunction analysis [1] show that for molecules which are solid state emissive, the lowest lying singlet state has a major contribution from a charge transfer state from the carbazole donor to the nitroaryl acceptor. For molecules which are non-emissive, the lowest lying singlet state is found to be an n – π* state. We find that the energy of the charge transfer state decreases as the electron withdrawing power of the nitroaryl acceptor is increased. Calculations of excitations for molecules with orthogonal conformations suggest that the charge transfer state is dark, as indicated by a very small oscillator strength. The question, then, is: how can these molecules be solid state emissive if the lowest lying excited state is dark? A scan of the torsion angle shows that the oscillator strength increases significantly as the molecule becomes more planar, and the energy profile for this rotation is relatively flat. We therefore propose that provided that stacking in the solid state leads to small perturbations in the molecular geometry and the lowest lying singlet is a charge transfer state, the molecule will be solid state emissive. [1] F. Plasser “Theodore 2.0.2: a package for theoretical density, orbital relaxation and exciton analysis”; available from http://theodore-qc.sourceforge.ne

Evaluation of the quasi correlated tight-binding (QCTB) model for describing polyradical character in polycyclic hydrocarbons

We present a verification and significant algorithmic improvement of the quasi-correlation tightbinding (QCTB) scheme (a H¨uckel-Hubbard-type model mimicking electron correlation) for describing effectively unpaired electrons in the spirit of Head-Gordon’s approach [M. Head-Gordon, Chem. Phys. Lett. 380, 488 (2003)]. For comparison purposes, results based on the high-level ab initio multireference averaged quadratic coupled cluster method previously computed in our works are invoked. In doing so, typical polyaromatic hydrocarbons (polyacenes, periacenes, zethrenes, and the Clar goblet) are studied. The evaluation shows that the QCTB H¨uckel-like scheme extended for electron correlation effects provides a qualitatively and in several cases also quantitatively good picture of the unpairing electrons in formally closed-shell electronic systems. Additionally, fairly large nanographene systems of triangulene structure (C426) and a perforated nanoribbon (C8860) have been treated at QCTB level. Two analytical model problems in the framework of QCTB prove the ability of this approximation to give a correct description of natural orbital occupancy spectra. For the studied QCTB scheme, an efficient algorithm is elaborated, and large-scale calculations of radical characteristics for nanographene networks with thousands of carbon atoms are possible

Surface hopping within an exciton picture. An electrostatic embedding scheme

We report the development and the implementation of an exciton approach that allows ab initio nonadiabatic dynamics simulations of electronic excitation energy transfer in multichromophoric systems. For the dynamics, a trajectory-based strategy is used within the surface hopping formulation. The approach features a consistent hybrid formulation that allows the construction of potential energy surfaces and gradients by combining quantum mechanics and molecular mechanics within an electrostatic embedding scheme. As an application, the study of a molecular dyad consisting of a covalently bound BODIPY moiety and a tetrathiophene group is presented using time-dependent density functional theory (TDDFT). The results obtained with the exciton model are compared to previously performed full TDDFT dynamics of the same system. Our results show excellent agreement with the full TDDFT results, indicating that the couplings that lead to excitation energy transfer (EET) are dominated by Coulomb interaction terms and that charge-transfer states are not necessary to properly describe the nonadiabatic dynamics of the system. The exciton model also reveals ultrafast coherent oscillations of the excitation between the two units in the dyad, which occur during the first 50 fs

Orbital-free photophysical descriptors to predict directional excitations in metal-based photosensitizers

The development of dye-sensitized solar cells, metalloenzyme photocatalysis or biological labeling heavily relies on the design of metalbased photosensitizes with directional excitations. Directionality is most often predicted characterizing manually excitations via canonical frontier orbitals. Although widespread, this traditional approach is, at the very least, cumbersome and subject to personal bias, as well as limited in many cases. Here, we demonstrate how two orbital-free photophysical descriptors allow an easy and straightforward quantification of the degree of directionality in electron excitations using chemical fragments. As proof of concept we scrutinize the effect of 22 chemical modifications on the archetype [Ru(bpy)3] 2+ with a new descriptor coined “substituent-induced exciton localization” (SIEL), together with the concept of “excited-electron delocalization length” (EEDLn). Applied to quantum ensembles of initially excited singlet and the relaxed triplet metal-to-ligand charge-transfer states, the SIEL descriptor allows quantifying how much and whereto the exciton is promoted, as well as anticipating the effect of single modifications, e.g. on C-4 atoms of bpy units of [Ru(bpy)3] 2+. The general applicability of SIEL and EDDLn is further established by rationalizing experimental trends through quantification of the directionality of the photoexcitation. We thus demonstrate that SIEL and EEDL descriptors can be synergistically employed to design improved photosensitizers with highly directional and localized electron-transfer transitions.</p