Directional excitations in photosensitisers

Characterising excited states in transition metal complexes by looking at pictures of orbitals can be a tedious task. Even more, it is hard to eliminate personal in the process and produce quantitative results. In a study led by Pedro Sánchez-Murcia from the University of Vienna, we have taken a closer look at this problem in the case of various substituted complexes deriving from the archetype Ru(bpy)3 with the aim of quantifying how different substituents influence the localisation of the excited electron. The result is presented in the article “Orbital-free photophysical descriptors to predict directional excitations in metal-based photosensitizers,” which just appeared in Chemical Science.

The Columbus program system

Columbus is a collection of programs for high-level ab initio electronic structure computations. Through the use of multireference methods even highly challenging systems such as excited states and open-shell molecules are accessible. The availability of gradients and nonadiabatic coupling vectors allows for photodynamics simulations describing ultrafast internal conversion processes. The capabilities of Columbus have been showcased in a recent paper: The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry that just appeared in J. Chem. Phys. as part of a themed collection Electronic Structure Software.

A new release of the programm package, available to registered users, has been made available on the distribution page.

Paper: Rational design of chromophores for singlet fission

Singlet fission is a promising approach to photovoltaics where one pair of singlet charge carriers is converted intto two pairs of triplet charge carriers. Molecules undergoing singlet fission require a specific energetic condition: the first triplet state has to be half the energy of the first singlet state. In a study, lead by Max Pinheiro from Sao Paulo, Brazil, we investigated how this energetic condition can be achieved by substituting a tetracene molecule with B and N atoms in different positions. The resulting paper A systematic analysis of excitonic properties to seek optimal singlet fission: the BN-substitution patterns intetracene, which just appearred in J. Mat. Chem. C, shows the versatility of this approach in tuning the energies across a wide range. The results are rationalised by measuring unpaired electrons and compared to a more qualitative discussion in terms of bonding patterns and Clar sextets. We hope that the study will help by setting new accents in terms of design ideas for new chromophores.

For recent study presenting an alternative strategy of tuning energies for singlet fission, exploiting anti-aromaticity, see JACS 2019, 141, 13867.

Paper: Beyond the MO picture

The molecular orbital (MO) picture has become the standard tool used by chemists to understand molecular electronic structure and particularly excited states. In our newest paper Toward an understanding of electronic excitation energies beyond the molecular orbital picture, which just appeared as a Perspective in PCCP, we asked whether we can go beyond the MO picture in a systematic way in our understanding of excited states. This endeavour ended up more involved than initially expected but we hope that it presents some interesting physics in a memorable and sufficiently intuitive way.

Below, we exemplify the tools developed to study a donor-acceptor-donor system initially developed in JACS, 2014, 136, 18070 and revisited in PCCP, 2019, 21, 10580. The HOMO of this system is on the carbazole donor whereas the LUMO is on the anthraquinone acceptor. The lowest excited state of the system (in gas phase) is not reached via the HOMO-LUMO transition but it is a locally excited state on anthraquinone. Why?

Figure 1: Analysis of post-MO contributions of the first excited singlet state of a donor-acceptor-donor molecule.

The contributions affecting the energy of the first excited singlet state are shown in Figure 1, above. In the third row the natural transition orbitals (NTOs) are shown. These give rise to the density of the excitation hole (red) and the excited electron (blue), shown above. The Coulomb attraction between these two charge distributions now stabilises the excited state. This Coulomb attraction is represented by the electrostatic potentials (ESPs) induced by the charge distributions. The important observation is that these ESPs are non-uniform with stronger contributions in the centre. Thus, the locally excited state is stabilised and an energetic penalty is paid for charge transfer. At the bottom of Figure 1, the transition density is shown, which leads to a repulsive exchange interaction destabilising local singlet states. In the present case the Coulomb attraction dominates and the lowest state is fairly local in nature.

Paper: TheoDORE electronic structure analysis

The paper TheoDORE: A toolbox for a detailed and automated analysis of electronic excited state computations just appeared in the Journal of Chemical Physics in a special issue on Electronic Structure Software. Please, take a look if you are interested in speeding up the analysis of your excited-state computations. If you are already a user of TheoDORE, you can learn about the latest developments and obtain a compact overview of the underlying theory.

Paper: OpenMolcas – a powerful electronic structure code going open-source

You can find the new paper describing the OpenMolcas package in JCTC – OpenMolcas: From source code to insight. OpenMolcas represents the open-source release of the previously commercially distributed Molcas package. Use OpenMolcas to gain access to powerful multireference methods for free and to have full control if you need to modify the source.

My own contributions to Molcas are concerned with the implementation of the wavefunction analysis module &WFA , as described in JCTC 13, 5343 (2017), and the interface to Columbus. Let me know if you have any questions about these.

Paper: Details in the surface hopping method

Our paper “Strong Influence of Decoherence Corrections and Momentum Rescaling in Surface Hopping Dynamics of Transition Metal Complexes” was just accepted in JCTC. In this work we investigated the reliability of the surface hopping method in the case of a transition metal complex described using a linear vibronic coupling model. We found that various seemingly unimportant parameters can have a strong influence on the results.

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Paper: Two-photon absorption

Our new paper Effect of Symmetric and Asymmetric Substitution on the Optoelectronic Properties of 9,10-Dicyanoanthracene, written in collaboration with colleagues from Imperial College London, TU Vienna, University of Geneva, and the Polish Academy of Sciences just appeared in the new RSC journal Molecular Systems Design & Engineering. The paper illustrates design principles relevant for strong two-photon absorbers. The best two-photon absorption is obtained by using a symmetric D-A-D arrangement with sufficiently strong donors.

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Paper: Electronic structure methods for dynamics simulations

The challenge about running photodynamics simulations is that the computational cost is often so high that one might have to compromise in terms of the electronic structure method used. One is tempted to just check the vertical excitations at one geometry and run the dynamics if those look alright. How this can go wrong is investigated in the paper “The Influence of the Electronic Structure Method on Intersystem Crossing Dynamics. The Case of Thioformaldehyde” that just appeared in JCTC. Take a look if you are interested.

 

Copyright 2019 American Chemical Society.

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Paper: Delayed fluorescence

Small modifications can make a big difference. Swapping anthraquinone for a thiophene based acceptor in a donor-acceptor-donor system produces the desired red shift in the emission but limits its quantum efficiency. This conclusion was drawn from a recent paper lead by Stephanie Montanaro and Iain Wright at Loughborough: Red-shifted delayed fluorescence at the expense of photoluminescence quantum efficiency – an intramolecular charge-transfer molecule based on a benzodithiophene-4,8-dione acceptor which just a appeared in PCCP.

Computations reveal that the reason for the reduced emission quantum efficiency lies in the presence of low-lying locally excited states (4 triplets and one singlet) on the central unit.