Stefan Knippenberg

The research, which I like to reflect to, aims at both applications and method development. It is very much in line with the scope of our division of theoretical chemistry and biology, where experimental-theoretical collaboration always is at focus. According to the Swedish system, I am 'Docent i teoretisk fotokemi och fotobiologi' (March 2017).

Although they are not restricted to these topics and despite my broad interest, my activities can be divided into the following main areas:

1. Non-linear optics & excited state properties

The Algebraic Diagrammatic Construction (ADC) scheme has been proven to be very robust and reliable to investigate excited states.

In collaboration with the group of Prof. Dreuw (Heidelberg) and of Prof. A. Trofimov (Irkutsk), an earlier proposed approach to molecular response functions based on the intermediate state representation (ISR) of the polarization propagator and the ADC approximations has been for the first time employed for calculations of nonlinear response properties. Employing furtheron a pragmatic sum-over-states expression and together with Prof. Brédas and his collaborators (Atlanta), this approach has been used to investigate and assess the real and imaginary parts of the second hyperpolarizability of streptocyanine molecules, which are important species in all-optical switching.

For more information, we like to refer to Mol. Phys. 108 (2010), 2801; Chem. Phys. Chem. 12 (2011), 3180; J. Comp. Chem. 33 (2012), 1797; J. Chem. Phys. 136 (2012), 064107 and J. Chem. Theory Comput. 12 (2016), 5465.

2. Multiscale modeling

The research of the Division has in the past taken steps in the direction of effective low scaling modelling, and nowadays also general multiscale/multiphysics modelling. An emphasis is now put on hybrid density functional theory/molecular mechanics (DFT/MM) approaches, where a quantum mechanical (QM) region is treated at the DFT level and the surroundings at the molecular mechanics (MM) level. The full interaction between the layers is accounted for in the property calculations.

The research in my group, which is centered around optical properties and excited states, involves also the investigation of fluorescent probes in bilipid membranes. An example is the fluorescent marker Laurdan and its new derivative, C-Laurdan, which have been

investigated by means of theoretical calculations in a DOPC lipid bilayer membrane at room temperature and whose results have been compared with those of fluorescence experiments. Experimentally, C-Laurdan is known to have a higher sensitivity to the membrane polarity at the lipid headgroup region and has higher water solubility. Results from Molecular Dynamics (MD) simulations show that C-Laurdan is oriented with the carboxyl group toward the head of the membrane, with an angle of 50° between the molecular backbone and the normal to the bilayer, in contrast to the orientation of the Laurdan headgroup whose carbonyl group is oriented toward the polar regions of the membrane and which describes an angle of ca. 70−80° with the membrane normal. This contrast in orientation reflects the differences in transition dipole moment between the two probes and, in turn, the optical properties. QM/MM results of the probes show little differences for one- (OPA) and two-photon absorption (TPA) spectra, while the second harmonic generation (SHG) beta component is twice as large in Laurdan with respect to C-Laurdan probe. The fluorescence anisotropy decay analysis of the first excited state confirms that Laurdan has more rotational freedom in the DOPC membrane, while C-Laurdan experiences a higher hindrance, making it a better probe for lipid membrane phase recognition.

For more information, we like to refer to Langmuir 32 (2016), 3495; J. Chem. Theory Comput. 12 (2016), 6169; J. Am. Chem. Soc. 139 (2017), 4418; Langmuir (2018), DOI: 10.1021/acs.langmuir.8b01164

3. Photochemistry: photochromism and excitonic coupling

One of the examples of our work in photochemistry and excited state dynamics, in which an interesting interaction with experimental oriented peers can be set up, is the study upon photoenolization and photochromism or the change of color upon irradiation. It is a general property of quinoline derivatives, yet subtle differences in the geometric structure influence its occurrence.

To investigate this relation, the mechanism of photoenolization of the photochromic compounds 3-benzoyl-2-benzyl-1-methyl-1H-quinoline-4-one (1) and 3-benzoyl-1,2-dibenzyl-1H-1,8-naphtyridin-4-one (2) as well as of the structurally closely related but nonphotochromic 3-benzoyl-1-benzyl-2-methyl-1H-1,8-naphtyridin-4-one (3) has been investigated theoretically using state-of-the-art quantum chemical methods. Focusing on the difference between 2 and 3 and stressing the absence of a phenyl group in the latter, the excited state potential energy surfaces along the photoenolization coordinate have been calculated for both. While the initial proton transfer initializing photoenolization is feasible when the phenyl group is present in 1 and 2, it is suppressed in 3.

Another work in photochemistry focusses on the effects of excitonic coupling upon the absorption spectrum of a BisBODIPY molecule. Recently, the latter one is investigated using high-level quantum chemical methodology and the results are compared with experimental data. The S1 and S2 excited states are examined in detail to illuminate and to understand the electronic coupling between them. With the help of model systems in which the distance between the BODIPY monomers is increased or in which the dihedral angle between the subunits is changed, the electronic coupling is quantified, and its influence on energetics and oscillator strengths is highlighted. For the explanation of the experimental spectrum, orbital interaction effects are found to be important. Because of the large experimental Stokes shift of BisBODIPY, the nature of the emissive state is investigated and found to remain C2 symmetric as the ground state, and no localization of the excitation on one BODIPY subunit occurs. The excitonic coupling is in BisBODIPY still larger than the geometry relaxation energy, which explains the absence of a pseudo-Jahn-Teller effect.

For more information, we like to refer to Phys. Chem. Chem. Phys. 12 (2010), 8190-8200; Vib. Spectrosc. 56 (2010), 13; J. Phys. Chem. A 116 (2012), 12321; J. Phys. Chem. A. 117 (2013), 2782; J. Phys. Chem. A 119 (2015), 1323.

4. Ionization

Since my Ph.D. time, I am also interested in ionization phenomena and simulations of ultraviolet photoemission spectra or electron impact experiments like electron momentum spectroscopy (EMS). Discarding the details, it is closely related to the calculation of excitations of the cation in the geometry of the neutral.

An interesting molecule is the cage compound norbornane: its EMS spectrum exhibits a large band at 25 eV, which is missing in UPS spectra as well as in simulations by means of the algebraic diagrammatic construction scheme (ADC). A detailed analysis tentatively describes this issue in terms of ultrafast nuclear dynamical effects and autoionization processes.

More information can be found in Spectrochim. Acta A 88 (2012), 102; Chem. Phys. Lett. 584 (2013), 24 and references therein.