Friday, 10 March 2017

TDDFT for large conjugated systems

About five years ago, when we tried applying TDDFT to large conjugated systems, we noticed that it just did not work. This confused me for a while: What is so difficult about conjugated systems? Then I searched the literature, which showed that other people noticed the same problem quite a while ago (e.g. S. Grimme), that the phenomenon was interpreted in terms of exciton sizes, and that it was seen as "charge transfer in disguise." The thing that was still missing was a tangible way to analyze and talk about this problem. Therefore, we wanted to look at it from a somewhat different viewpoint. We took a set of conjugated polymers of varying sizes, performed TDDFT computations with different functionals, and analyzed the computations with our wavefunction analysis toolbox for TDDFT. The results are shown in our paper "Universal Exciton Size in Organic Polymers is Determined by Nonlocal Orbital Exchange in Time-Dependent Density Functional Theory" in JPCL.

The main quantity we are interested in is the exciton size, which corresponds to a dynamic charge transfer distance. The first striking observation is that the exciton size is largely independent of the molecular details but scales uniformly with the system size, as initially pointed out by Knupfer et al. The second point, important from a methodological point of view, is that huge variations between the functionals are observed. A bound exciton can only be formed if non-local exchange is included in the functional. The more non-local exchange is included, the stronger the observed binding.

Friday, 24 February 2017

Ultrafast Energy Transfer

There is another paper with some contributions from myself that just appeared: "Ultrafast Electronic Energy Transfer in an Orthogonal Molecular Dyad" in J. Phys. Chem. Lett. In this paper the question is discussed how it is possible to have energy transfer in a molecular dyad that occurs on the time scale of 100 fs. Clearly, no equilibrium Föster theory type approach is possible here but you need explicit nonadiabatic dynamics simulations, in this case using Newton-X.

The value of the dynamics simulations performed is not only to support the experimental measurements. It also gives new insight into the mechanism: The ultrafast energy transfer is mediated by a state with partial charge transfer character. Or in other words, the electron and hole are not transferred at the same time. As seen in the presented example trajectory: the electron goes first and pulls the hole behind itself.

Thursday, 16 February 2017

More Polyradicals

These days many people are interested in polyaromatic hydrocarbons because of their special electronic structure properties, such as reduction of the band gap, spin-polarization, and radical formation. The problem is that precisely these properties make computations on these systems very challenging. Previously, we have studied polyaromatic hydrocarbons with expensive correlated multireference methods. These methods do not only burn lots of computer time but they also require experts for their successful setup and interpretation. The idea of our newest work was to evaluate a very simple model based on Hückel theory and evaluate how this performs in comparison to high-level methods. The results are shown in the paper "Evaluation of the quasi correlated tight-binding (QCTB) model for describing polyradical character in polycyclic hydrocarbons" that just appeared in J. Chem. Phys.

Amazingly, the new method provides a semi-quantitative reproduction of the ab initio results in the cases we studied. Below, you can find a comparison the ab initio AQCC method with the QCTB model evaluated here. We are comparing polyacenes with isomeric phenacenes. It is well-known that the polyacenes become unstable with longer chain length, obtaining polyradical character, while the phenacenes remain stable. To evaluate this phenomenon, we compute an effectively number of unpaired electrons. Both methods, correctly predict that the unpaired electrons go up of the acenes and stay more or less constant for the phenacenes. But even more: there is a semi-quantiative agreement of the precise values.

The agreement between the two methods is quite amazing considering how much cheaper the QCTB method is. Because of this computational efficiency the QCTB method can even treat graphene nanosheets with thousands of atoms without significant computational cost. Below, the unpaired density for a "perforated" nanoribbon is shown.

Currently, the code is only available in a local Mathematica file. But I might add it as an addition to my Hückel program, at least a light version.

Thursday, 5 January 2017

Simulating Light-Induced Processes in DNA

We planned to write a short perspective of our experience of simulating UV excitations in DNA but ended up with a quite comprehensive article: "Challenges in Simulating Light-Induced Processes in DNA", which just appeared in the journal Molecules. The aim of this work was to illustrate the different tasks that are involved in the simulation of DNA and its components: (i) quantum chemistry, (ii) description of the excitation processes, (iii) nonadiabatic dynamics, (iv) comparison to experiment, and (v) analysis of the results. In all these cases significant challenges can occur, and a wide range of methods to tackle these have been developed by numerous researchers. For someone entering the field or even for active researchers, it is sometimes difficult to keep all this in mind. We hope that this new article will be provide a useful summary of the work that has been done.

Friday, 23 December 2016

Rational Design

One of the amazing things in chemistry is that people can actually predict the properties of molecules just by pushing around electron pairs on paper. In our newest article "Color Fine Tuning of Optical Materials Through Rational Design" that appeared in ChemPhysChem, we put this idea to the test. A set of oligothiophene derivatives are created by connecting different cap and linker groups, which are chosen according to chemical reasoning. These molecules are subsequently synthesized and characterized spectroscopically. And to check, we also perform TDDFT/M06-2X computations. The outcome: It all fits together. Chemical reasoning provides the correct qualitative trends. Computation predicts the outcome quantitatively.