index - Laboratoire de Chimie et Physique Quantiques

Présentation du LCPQ

Le LCPQ (UMR 5626, Laboratoire de Chimie et Physique Quantique) est un laboratoire de recherche localisé sur le campus de l'Université Paul Sabatier de Toulouse. Il regroupe des chercheurs dont les activités couvrent plusieurs domaines de la Chimie Théorique -essentiellement quantique- et de la Physique Moléculaire Théorique.

Le LCPQ est membre de la Fédération de recherche FeRMI (Fédération de recherche Matière et Interactions - FR2051), anciennement IRSAMC (Institut de Recherche sur les Systèmes Atomiques et Moléculaires Complexes)..

Avant 2007 =>, voir le Laboratoire de Physique Quantique HAL-LPQ.

 

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Galvinoxyl, as one of the most extensively studied organic stable free radicals, exhibits a notable phase transition from a high-temperature (HT) phase with a ferromagnetic (FM) intermolecular interaction to a low-temperature (LT) phase with an antiferromagnetic (AFM) coupling at 85 K. Despite significant research efforts, the crystal structure of the AFM LT phase has remained elusive. This study successfully elucidates the crystal structure of the LT phase, which belongs to the P[1 with combining macron] space group. The crystal structure of the LT phase is found to consist of a distorted dimer, wherein the distortion arises from the formation of short intermolecular distances between anti-node carbons in the singly-occupied molecular orbital (SOMO). Starting from the structure of the LT phase, wave function calculations show that the AFM coupling 2J/kB varies significantly from −1069 K to −54 K due to a parallel shift of the molecular planes within the dimer.

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We performed several types of ab initio calculations, from Hartree-Fock to Complete-Active-Space second-order perturbation theory and Coupled Cluster, on compact clusters of stoichiometry XY, where X and Y are atoms belonging to the second row of the periodic table. More precisely, we considered the “cubic” structures of three isoelectronic groups, having a total of 48, 52, and 56-electrons, respectively. Notice that the highly symmetric cubic clusters of type X are characterized by an symmetry group, while the XY structures, with XY, have at most a symmetry. Binding energies and wave function analysis of these clusters have been performed, in order to investigate the nature, and the electron delocalization of these systems and establish a comparison between them. To this purpose, we also computed the Total-Position Spread tensor for each structure, a quantity which is related to the multi-reference nature of a system wave function.

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Recently, some of us reviewed and studied the photoionization dynamics of C<sub>60</sub> that are of great interest to the astrochemical community as four of the diffuse interstellar bands (DIBs) have been assigned to electronic transitions in the C<sub>60</sub>$^+$ cation. Our previous analysis of the threshold photoelectron spectrum (TPES) of C<sub>60</sub> [Hrodmarsson et al., Phys. Chem. Chem. Phys. 22, 13880–13892 (2020)] appeared to give indication of D<sub>3d</sub>$^+$ ground state symmetry, in contrast to theoretical predictions of D<sub>5d</sub>$^+$ symmetry. Here, we revisit our original measurements taking account of a previous theoretical spectrum presented in the work of Manini et al., Phys. Rev. Lett. 91(19), 196402 (2003), obtained within a vibronic model parametrized on density functional theory/local-density approximation electronic structure involving all h<sub>g</sub> Jahn–Teller active modes, which couple to the $^2$H<sub>u</sub> components of the ground state of the C<sub>60</sub>$^+$ cation. By reanalyzing our measured TPES of the ground state of the C<sub>60</sub> Buckminsterfullerene, we find a striking resemblance to the theoretical spectrum calculated in the work of Manini et al., Phys. Rev. Lett. 91(19), 196402 (2003), and we provide assignments for many of the hg modes. In order to obtain deeper insights into the temperature effects and possible anharmonicity effects, we provide complementary modeling of the photoelectron spectrum via classical molecular dynamics (MD) involving density functional based tight binding (DFTB) computations of the electronic structure for both C<sub>60</sub> and C<sub>60</sub>$^+$. The validity of the DFTB modeling is first checked vs the IR spectra of both species which are well established from IR spectroscopic studies. To aid the interpretation of our measured TPES and the comparisons to the ab initio spectrum we showcase the complementarity of utilizing MD calculations to predict the PES evolution at high temperatures expected in our experiment. The comparison with the theoretical spectrum presented in the work of Manini et al., Phys. Rev. Lett. 91(19), 196402 (2003), furthermore, provides further evidence for a DC<sub>5d</sub> symmetric ground state of the C<sub>60</sub>$^+$ cation in the gas phase, in complement to IR spectroscopy in frozen noble gas matrices. This not only allows us to assign the first adiabatic ionization transition and thus determine the ionization energy of C<sub>60</sub> with greater accuracy than has been achieved at 7.598 ± 0.005 eV, but we also assign the two lowest excited states ($^2E$<sub>1u</sub> and $^2E$<sub>2u</sub>) which are visible in our TPES. Finally, we discuss the energetics of additional DIBs that could be assigned to C<sub>60</sub>$^+$ in the future.

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Photocatalysis that uses the energy of light to promote chemical transformations by exploiting the reactivity of excited-state molecules is at the heart of a virtuous dynamic within the chemical community. Visible-light metal-based photosensitizers are most prominent in organic synthesis, thanks to their versatile ligand structure tunability allowing to adjust photocatalytic properties toward specific applications. Nevertheless, a large majority of these photocatalysts are cationic species whose counterion effects remain underestimated and overlooked. In this report, we show that modification of the X counterions constitutive of [Ru(bpy)<sub>3</sub>](X)<sub>2</sub> photocatalysts modulates their catalytic activities in intermolecular [2 + 2] cycloaddition reactions operating through triplet–triplet energy transfer (TTEnT). Particularly noteworthy is the dramatic impact observed in low-dielectric constant solvent over the excited-state quenching coefficient, which varies by two orders of magnitude depending on whether X is a large weakly bound (BAr$^F$<sub>4</sub>$^–$) or a tightly bound (TsO$^–$) anion. In addition, the counterion identity also greatly affects the photophysical properties of the cationic ruthenium complex, with [Ru(bpy)<sub>3</sub>](BAr$^F$<sub>4</sub>)<sub>2</sub> exhibiting the shortest 3MLCT excited-state lifetime, highest excited state energy, and highest photostability, enabling remarkably enhanced performance (up to &gt;1000 TON at a low 500 ppm catalyst loading) in TTEnT photocatalysis. These findings supported by density functional theory-based calculations demonstrate that counterions have a critical role in modulating cationic transition metal-based photocatalyst potency, a parameter that should be taken into consideration also when developing energy transfer-triggered processes.

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