Excited State Delocalization in DNA

One persistent topic in photobiology has been the question of how many bases are involved in the absorption of a photon of UV light, or in other words how delocalized the absorbing states are. There have been estimates ranging from completely localized monomeric states to delocalization over the whole helix. The question is challenging to study experimentally because the delocalization of the wavefunction is a quantum phenomenon without a direct experimentally observable counterpart. It is difficult to study from a computational point of view because of the extended system size, environmental interactions, and structural disorder. Computational studies using exciton models could not include structural disorder well. Explicit quantum chemistry studies had troubles because of smaller QM regions.

That is why we thought there was need for another more extended study. We used a QM region of eight nucleobases, which could be treated by TDDFT thanks to the GPU based TeraChem code, we did extended sampling by molecular dynamics using the Amber code (also GPU based), and we connected the two by a QM/MM scheme. In total we computed 6000 excited states. To analyze these systematically, we used the TheoDORE code. The results are shown in a new article "Electronic delocalization, charge transfer and hypochromism in the UV absorption spectrum of polyadenine unravelled by multiscale computations and quantitative wavefunction analysis" that just appeared in Chemical Science.

The main results of the study are:

• Photon absorption occurs predominantly through a collective excitation of two neighboring nucleobases.
• Full charge transfer (CT) states are only present at higher energies but states with non-neglible CT admixture account for about 50% of the spectral intensity.
• The experimentally observed hypochromism occurs through perturbed monomer states rather than excitonic or CT interactions.

Boron nitride nanoflakes

There is a new paper on metal doped boron nitride nanoflakes written by a colleague from India and myself. In this paper, we investigate what is the best way to represent the excited states in these systems using various quantitative and visual analysis tools. Check it out: "UV absorption in metal decorated boron nitride flakes: a theoretical analysis of excited states" in Mol. Phys. [free full text].

Local electron correlation

If you are interested in multireference methods that can be applied to large systems, then you can check out a new paper by us: "Local Electron Correlation Treatment in Extended Multireference Calculations: Effect of Acceptor-Donor Substituents on the Biradical Character of the Polyaromatic Hydrocarbon Heptazethrene" in JCTC. The paper reports a locally correlated implementation for the multireference configuration interaction method. The code is available within the COLUMBUS program package.
 

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.

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.