PhD in chemistry (M/F) – Chiral Properties of Achiral Molecules on Chiral Surfaces

The recruited PhD student will be involved in the second work-package of the ChiExCo program, which aims to develop a reliable computational protocol to predict, for organic chromophores, both chirality quantifying factors (gabs and glum) resulting from excitonic couplings. This will require the student to adequately select the computational approaches needed for all computational steps. More details on the three envisaged steps (ca. 1/PhD year) are listed below.

The first task will be to define a TD-DFT protocol for obtaining accurate gabs and glum values for nearly-isolated (i.e. solvated in apolar media) intrinsically chiral dyes. In a first phase, the PhD student will perform vertical calculations of chiroptical properties of a series of experimentally-relevant chromophores using several TD-DFT schemes and compare the responses to those obtained with reference wavefunction approaches. A few adequate functionals will be selected, with a special focus on long-range effects, essential for the following. In a second phase, the PhD student will use experimentally available values for chiroptical spectra and responses (gabs and glum) and model them accounting for vibronic effects inspired by literature. Various vibronic schemes will be assessed using not only band shapes but also absolute response values as metrics.
 

Since the experimental response to be modelled originates from a collective chiral response of intrinsically achiral molecules, likely due to an excitonic effect, the PhD student will design an approach to tackle this scenario. In this context, two types of dimers should likely be considered: homodimers of chiral molecules so as to ensure perfectly degenerate excited states and heterodimers composed of one chiral molecule and one achiral dye yet presenting similar excitation energies. Various relative geometries corresponding to, e.g., H and J aggregates will be modelled. For all cases, reference calculations will be achieved using the model resulting from the first task. It is likely that using heterodimers will offer first insights into chiral induction: can the response of such a dimer exceed that of a single chiral molecule? In the second stage, the PhD candidate will build an excitonic coupling model suited for the larger (real) systems. The performances of TD-DFT, sTD-DFT, and TD-DFTB will be compared so as to evaluate the performances of the two latter.
 

In this final step, the PhD student will use realistic systems coming from partners of the project to evaluate chiral induction by the surface for a cluster of achiral dyes. Again, two steps will be considered, each requiring embedding strategies. First, electrostatic embedding will be used for the absorption since such techniques are available, allowing effective comparisons between an actual dimer and excitonic models. Second, for emission (glum), the PhD student will next design an approach allowing self-consistently obtaining the environmental charges for the considered excited state. In both cases, the environmental parameters will be provided by the Partner in Lyon.

This PhD thesis is part of the ChirExCo project which aims to develop a multiscale modelling procedure to understand the origin of the chirality induction from chiral surface (e.g., helical silica) to achiral adsorbed molecules. The induction of chirality in achiral chromophores via adsorption on helical silica has been demonstrated experimentally by several groups and applied, for instance, in circularly polarized light (CPL) emission. It has also been observed that the efficiency of this induction is related to the packing density of the adsorbed molecules. While  the phenomenon is well documented, the underlying mechanism of this chirality induction is still unknown. The proposed PhD thesis will take place in the framework of this project involving theoretical chemistry teams in Lyon, Marseille, Rennes and Nantes.

• A recent Master’s degree (or equivalent) in Chemical Physics, Physical Chemistry, Theoretical Chemistry, Theoretical Physics, or a related discipline.
• Experience with recognized computational chemistry software (e.g., Gaussian, Dalton, TurboMole) is highly desirable.
• Experience in Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) are desirable assets.
• Strong analytical and problem-solving skills, with a proactive and motivated mindset.
• Ability to work both independently and collaboratively within a research team.
• Excellent communication skills, both written and verbal.