Thibaud Etienne

Toward a Quantitative Assessment of Electronic Transitions’ Charge-Transfer Character

We hereby report studies devoted to a topological descriptor of photoinduced electronic charge density variation. Our novel index, symbolized as ϕS, consists in the detachment and attachment densities overlap, where the detachment density physically depicts the electron density removed from the ground state of a molecule during the transition while the attachment density consists in the rearranged density in the excited state. Our method provides a simple and efficient way to quantitatively evaluate how easy the charge-separation is made upon the chromophores light absorption. Furthermore, this model can be applied for instance to address a comment on new push-pull dyes charge-transfer ability in order to assess their potentiality as candidates for light absorption-based devices. Moreover, the ϕS assessment allows us to perform some methodological diagnostic tests concerning the use of long-range corrected exchange-correlation functional in a time-dependent density functional theory (TDDFT) framework. This paper relates the ϕS descriptors mathematical foundations from various perspectives (detachment/attachment densities or natural transition orbitals), together with its application to several types of chromophores. Connections and divergences with a formerly proposed index are finally evidenced.

A QM/MM Study of the Absorption Spectrum of Harmane in Water Solution and Interacting with DNA: The Crucial Role of Dynamic Effects

We present a time-dependent density functional theory computation of the absorption spectra of one β-carboline system: the harmane molecule in its neutral and cationic forms. The spectra are computed in aqueous solution. The interaction of cationic harmane with DNA is also studied. In particular, the use of hybrid quantum mechanics/molecular mechanics methods is discussed, together with its coupling to a molecular dynamics strategy to take into account dynamic effects of the environment and the vibrational degrees of freedom of the chromophore. Different levels of treatment of the environment are addressed starting from purely mechanical embedding to electrostatic and polarizable embedding. We show that a static description of the spectrum based on equilibrium geometry only is unable to give a correct agreement with experimental results, and dynamic effects need to be taken into account. The presence of two stable noncovalent interaction modes between harmane and DNA is also presented, as well as the associated absorption spectrum of harmane cation.

Transition matrices and orbitals from reduced density matrix theory

In this contribution, we report two different methodologies for characterizing the electronic structure reorganization occurring when a chromophore undergoes an electronic transition. For the first method, we start by setting the theoretical background necessary to the reinterpretation through simple tensor analysis of (i) the transition density matrix and (ii) the natural transition orbitals in the scope of reduced density matrix theory. This novel interpretation is made more clear thanks to a short compendium of the one-particle reduced density matrix theory in a Fock space. The formalism is further applied to two different classes of excited states calculation methods, both requiring a single-determinant reference, that express an excited state as a hole-particle mono-excited configurations expansion, to which particle-hole correlation is coupled (time-dependent Hartree-Fock/time-dependent density functional theory) or not (configuration interaction single/Tamm-Dancoff approximation). For the second methodology presented in this paper, we introduce a novel and complementary concept related to electronic transitions with the canonical transition density matrix and the canonical transition orbitals. Their expression actually reflects the electronic cloud polarisation in the orbital space with a decomposition based on the actual contribution of one-particle excitations from occupied canonical orbitals to virtual ones. This approach validates our novel interpretation of the transition density matrix elements in terms of the Euclidean norm of elementary transition vectors in a linear tensor space. A proper use of these new concepts leads to the conclusion that despite the different principles underlying their construction, they provide two equivalent excited states topological analyses. This connexion is evidenced through simple illustrations of (in)organic dyes electronic transitions analysis.

2,5-Dithienylpyrrole (DTP) as a donor component in DTP–π–A organic sensitizers: photophysical and photovoltaic properties

Organic dyes have been prepared to evaluate the ability of 2,5-dithienylpyrrole (DTP) to act as a donor substituent in D–π–A sensitizers for DSSCs. Using a styryl π-bridge the dyes were found to be excellent sunlight harvesters when adsorbed on TiO2 photoanodes with absorbances >3 in the 300–550 nm region. Calculations as well as transient absorption spectroscopy in both solution and on a TiO2 surface revealed that they were favourable for efficient injection and regeneration.

Fluorene-imidazole dyes excited states from first-principles calculations—Topological insights

We report the theoretical investigation of four organic dyes containing imidazole and fluorene moieties as donor and bridge in donor–bridge–acceptor molecular structures. Those target dyes were recently reported as potential agents for building metal-free light-to-electricity conversion devices [J. Org. Chem. (2014) 79, 3159]. Our contribution consists in the establishment of an appropriate computational protocol for obtaining the excited states of these dyes from a reliable level of theory. This benchmark was performed based on the possibilities offered by density functional theory and its time-dependent variant. The outcome of this screening allowed us to compute absorption properties of the target dyes, as well as the emission properties for one of them. Afterward, the electronic transitions computed from this reference method were characterized by a series of topological analysis tools, aimed for a qualitative and quantitative probing of the excited states nature. These tools rely on the formal depiction of the photogenerated hole and particle from density matrices or through the exploitation of the exciton wavefunction. Further linear algebraic operations based on these two types of objects lead to the elaboration of detachment/attachment density matrices and natural transition orbitals respectively, so that the outcome of these operations provides a qualitative depiction of the photoinduced electronic cloud polarization. Finally, quantitative insights were provided by the evaluation of quantum-mechanical metrics related to the charge transfer phenomenon caused by light absorption.

Vibronic properties of para-polyphenylene ethynylenes: TD-DFT insights

The first singlet excited states of a series of para-polyphenylene ethynylenes (PPEs) are investigated using time-dependent density functional theory (TD-DFT). Vibronic absorption spectra are calculated and show excellent agreement with the experiments, thus validating the adequacy of TD-DFT for such systems. The vibronic structure is assigned to the excitation of a few typical stretching and bending modes. The significant discrepancy between the simulated vertical-transition energies and the experimental absorption maxima in PPEs is underlined and explained. The evolution of the spectroscopic properties and of the electronic structure with the chain length is discussed and characterized.