Investigations of the non-adiabatic photophysics of Cu(I)-phenanthroline complexes

Cu(I)-phenanthrolines are an important class of metal-organic molecules that exhibits much promise for solar energy harvesting and solar-driven catalysis applications. Although many experimental studies have been performed calling for high-level simulations to elucidate their photophysics, a complete picture is still missing. This is the goal of the present thesis. On the ultrafast (femtosecond) timescale we studied the non-adiabatic relaxation of a prototypical Cu(I)-phenanthroline, [Cu(dmp)2]+, by performing excited state simulations using two approaches: quantum dynamics and trajectory surface hopping. These simulations help to identify several mechanisms, internal conversion, pseudo Jahn-Teller distortion, intersystem crossing, occurring in the subpicosecond time scale. Surprisingly, we have found that intersystem crossing does not take place between the lowest singlet and triplet excited states, as previously proposed, but between the lowest singlet and higher triplet states. Moreover, we observed the initial stages (< 100 fs) of the solvent reorganization due to the electronic density changes in the excited state. This leads to an energy stabilization of the excited states that is associated with an increase of the non-radiative decay rate. The quantum dynamics simulations allowed us to provide indications for performing additional spectroscopy measurements by using the recently developed X-ray Free Electron Lasers (X-FELs). This technology can monitor both electronic and structural changes with an unprecedented time resolution of tens of femtoseconds and, therefore, is capable of revealing the aforementioned processes. In addition, we questioned the feasibility of such experiments and calculated the signal strengths for XAS and XES transient spectra. Finally, we analyzed the luminescence quenching, which has been observed for all Cu(I)-phenanthroline complexes when they are dissolved in strongly donating solvents. By performing Molecular Dynamics calculations we showed that, in contrast with the previously accepted model based on the formation of an exciplex (a species formed by two molecules, one in the excited state and one in the ground state), no stable exciplex is formed and that quenching is due to electrostatic solute-solvent interactions. In addition, we investigated how the geometry configuration can affect the luminescence lifetime in these molecules. We found a correlation between rigidity of the copper complex - inhibition of the pseudo Jahn-Teller distortion - and lifetime of the emission. The more the metal complex retains the ground state structure (large substituents), the longer its lifetime. This effect is attributed to a higher energy gap (excited state minus ground state energy) due to the reduction of relaxation. Our research reveals important insights into the relaxation mechanism and the complex interplay between geometry and electronic structure in Cu(I)-phenanthroline. These results can be exploited for guiding the synthesis of complexes with the desired physical properties.

A Theoretical Rationalisation of the Emission Properties of Prototypical Cu(I)-Phenanthroline Complexes

Gloria Capano, Thomas James Penfold, Ursula Röthlisberger, Ivano Tavernelli

The excited state properties of transition metal complexes have become a central focus of research owing to a wide range of possible applications that seek to exploit their luminescence properties. Herein, we use density functional theory (DFT), time-dependent DFT (TDDFT), classical and quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations to provide a full understanding on the role of the geometric and electronic structure, spin orbit coupling, singlet triplet gap and the solvent environment on the emission properties of nine prototypical copper(I)-phenanthroline complexes. Our calculations reveal clear trends in the electronic properties that are strongly correlated to the luminescence properties, allowing us to rationalize the role of specific structural modifications. The MD simulations show, in agreement with recent experimental observations, that the lifetime shortening of the excited triplet state in donor solvents (acetonitrile) is not due to the formation of an exciplex. Instead, the solute solvent interaction is transient and arises from solvent structures that are similar to the ones already present in the ground state. These results based on a subset of the prototypical mononuclear Cu(I) complexes shed general insight into these complexes that may be exploited for development of mononuclear Cu(I) complexes for applications as, for example, emitters in third generation OLEDs.

2015

Capturing Non-local Effects in Kohn-Sham Density Functional Theory

Andrey Laktionov

Density Functional Theory (DFT) and its time-dependent extension (TDDFT) have become two of the most popular approaches for computer simulations of the electronic structure and response properties of quantum systems. A reasonable compromise between accuracy and computational cost allows to apply DFT to a wide range of systems from small molecules to biological complexes. Despite the in principle exact nature of DFT and TDDFT, practical calculations require the use of approximate DFT exchange-correlation (XC) functionals and TDDFT kernels and the accuracy of the obtained results is determined by the accuracy of the chosen XC description. These approximations inevitably introduce some limitations in the use of DFT-based methods.The inability of the local spin density approximation, generalized gradient approximations (GGA) and even some hybrid functionals to properly describe charge transfer (CT) excitations and predict intermolecular interaction energies of weakly bound complexes are two major drawbacks of current XC descriptions. This thesis is therefore devoted to the improvement of XC functionals for the special cases of weak interactions and charge-transfer excitations. Many original strategies have been suggested to cure DFT calculations from these failures. In particular, the Dispersion-Corrected Atom-Centered Potentials (DCACP) approach provides an accurate description of dispersion forces within generalized gradient approximations for the exchange-correlation functional. The DCACP method has been extensively used for the last seven years and has shown an excellent performance for a large class of applications. However, one of the drawbacks of the current implementation of DCACP is that the correct R-6 asymptotics of dispersion interaction is not reproduced. A first goal of this thesis was to enable DCACP to recover the R-6 asymptotic limit. To this end, we have designed a new 2-channel version of DCACP and carried out test calculations on both small molecules and large macromolecular complexes. The obtained results demonstrate the excellent performance and transferability of the DCACP approach. Moreover, for large macromolecular complexes, in which the binding energy is dominated by dispersion, [pi]-stacking, or hydrogen bonding, 2-channels DCACP were found to be the best method overall for correcting the popular BLYP functional. Our study clearly shows that account of R-6 asymptotics is crucial for the description of large complexes and that 2-channels DCACP are fully able to capture these effects. On the contrary, an account of R-6 asymptotics is of little importance for small molecules since the remaining errors of the underlying GGA functional are dominating. With the aim of deepening our understanding of the DCACP concept and the reasons for its excellent performance, we explored the properties of the two DCACP parameters and were able to establish some systematic trends. It turned out that variational tuning of the DCACP can be done in an analytical manner that enables the easy generation of DCACP potentials for the full periodic table. Furthermore, since DCACP have little but crucial impact on the electronic density, dispersion energies can be obtained from non-self-consistent electron densities. These empirical findings suggest that the high transferability of DCACP is due to their atom-centered form and the intrinsically weak nature of dispersion interactions. [...]

EPFL2014

Exploring the aqueous vertical ionization of organic molecules by molecular simulation and liquid microjet photoelectron spectroscopy

Jeremy Samuel Arey, Jennifer Guerard, Peter Rudolf Tentscher

To study the influence of aqueous solvent on the electronic energy levels of dissolved organic molecules, we conducted liquid microjet photoelectron spectroscopy (PES) measurements of the aqueous vertical ionization energies (VIEaq) of aniline (7.49 eV), veratrole alcohol (7.68 eV), and imidazole (8.51 eV). We also reanalyzed previously reported experimental PES data for phenol, phenolate, thymidine, and protonated imidazolium cation. We then simulated PE spectra by means of QM/MM molecular dynamics and EOM-IP-CCSD calculations with effective fragment potentials, used to describe the aqueous vertical ionization energies for six molecules, including aniline, phenol, veratrole alcohol, imidazole, methoxybenzene, and dimethylsulfide. Experimental and computational data enable us to decompose the VIEaq into elementary processes. For neutral compounds, the shift in VIE upon solvation, Delta VIEaq, was found to range from approximate to-0.5 to -0.91 eV. The Delta VIEaq was further explained in terms of the influence of deforming the gas phase solute into its solution phase conformation, the influence of solute hydrogen-bond donor and acceptor interactions with proximate solvent molecules, and the polarization of about 3000 outerlying solvent molecules. Among the neutral compounds, variability in Delta VIEaq appeared largely controlled by differences in solutesolvent hydrogen-bonding interactions. Detailed computational analysis of the flexible molecule veratrole alcohol reveals that the VIE is strongly dependent on molecular conformation in both gas and aqueous phases. Finally, aqueous reorganization energies of the oxidation half-cell ionization reaction were determined from experimental data or estimated from simulation for the six compounds aniline, phenol, phenolate, veratrole alcohol, dimethylsulfide, and methoxybenzene, revealing a surprising constancy of 2.06 to 2.35 eV.