Elucidating the Electronic Structure of TADF emitters

Thermally activated delayed fluoresence (TADF) is an exciting modern research area aimed at producing new OLED emitters. From a theoretical perspective TADF is particularly fascinating because it requires a detailed understanding of the different terms that contribute to the singlet and triplet excitation energies of the molecules studied. In a recent study led by Yihan Shao from the University of Oklahoma, we investigated a recently developed TADF emitter and showed how a combination of different wavefunction analysis tools provides deep insight into its excited-state properties. The paper just appeared in J. Phys. Chem. Lett.: Elucidating the Electronic Structure of a Delayed Fluorescence Emitter via Orbital Interactions, Excitation Energy Components, Charge-Transfer Numbers, and Vibrational Reorganization Energies.

Exciton-exciton annihilation

A new study led by C. Heshmatpour and J. Hauer from TU München studies exciton-exciton annihilation in a squaraine trimer. The experiment exploits 5th-order optical spectroscopy to study the evolution of the trimer after two-photon excitation into its bi-exciton state. Quantum chemistry computations performed by M. Menger, now located at Groningen, provide the required parameters to model the experimental signals within a Frenkel exciton model. The associated article Annihilation Dynamics of Molecular Excitons Measured at a Single Perturbative Excitation Energy just appeared in J. Phys. Chem. Lett.

Photoredox catalysis

A study led by O. Koleoso and M. C. Kimber at Loughborough University explored a new route of synthesising conjugated N-acyliminium compounds. The article entitled “A complementary approach to conjugated N-acyliminium formation through photoredox-catalyzed intermolecular radical addition to allenamides and allencarbamates” just appeared in a thematic issue on Advances on photoredox catalysis in the Beilstein Journal of Organic Chemistry.

Solution processed solar cells

A study lead by J. Lowe and A. Malkov from Loughborough University investigates how kesterite solar cells can be formed via a new cheap and non-toxic solvent system. The associated paper just appeared in J. Mat. Chem. C: Solution processed CZTS solar cells using amine–thiol systems: understanding the dissolution process and device fabrication.

Quantum chemical computations were used to aid in the assignment of the structures produced and characterised via infrared multiple photon dissociation spectroscopy. An interactive model showing the relevant molecular vibrations can be found here.

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: Nitrogen splitting

Splitting N2 is a challenging undertaking due to the strong triple bond holding the two nitrogen atoms together. Traditionally, this task is achieved via the Haber-Bosch process but recently interest has shifted to the possibilities of nitrogen splitting via homogeneous catalysis. Computations on the dinuclear transition metal complexes involved can be performed thanks to modern computational hardware and appropriate computer codes. However, the analysis of the excited states involved can become a significant challenge due to the large number of states and difficulties in assigning their character unambiguously. To tackle this problem, we have recently devised a strategy for a detailed analysis of the electronic wavefunctions of the states contributing to the reactive process. The associated paper, led by S. Rupp and V. Krewald from TU Darmstadt, just appeared in the European Journal of Inorganic Chemistry: Multi‐tier electronic structure analysis of Sita’s Mo and W complexes capable of thermal or photochemical N2 splitting.