Luminogens are molecules that emit light upon exposure to high-energy light, and fluorophores are one class of luminogens. Applications of fluorophores range from microviscosity sensors to light emitting diodes (LEDs), as well as biosensors, just to name a few. Many of these applications require the fluorophore to be in the aggregate or solid state. Some fluorophores become highly emissive in the aggregate state; these fluorophores are aggregation-induced emission (AIE) luminogens. Currently, very few quantum mechanical mechanisms have been proposed to describe the unique AIE behavior of luminogens. Boron difluorohydrazone (BODIHY) dyes are a new type of AIE fluorophore. The bright emission is from the S>1 excited state (“anomalous” emission) contrary to Kasha’s Rule. Thus, the mechanism Suppression of Kasha’s Rule (SOKR) was proposed to be responsible for the family of BODIHY dyes. We hypothesize that the SOKR mechanism can explain AIE as well as the anomalous emission of other fluorophores. New BODIHY derivatives (para-CO2H BODIHY, aluminum difluorohydrazone (ALDIHY), and paranitro ALDIHY) were predicted to be bright anomalous fluorophores through density functional theory (DFT) and time-dependent DFT (TDDFT) investigations. In addition, a series of anomalous fluorophores were investigated to determine if their photophysical properties could be explained by the SOKR mechanism (azulene, 1,6-diphenyl-1,3,5hexatriene, and zinc tetraphenylporphyrin). Finally, several triazolopyridinium and triazoloquinolinium dyes were computationally investigated by DFT and TDDFT calculations, and an accurate computational model for the large Stokes shifts of these dyes was developed. In conclusion, a better understanding of the photophysical properties through DFT and TDDFT modeling and spectroscopic investigation of hydrazone-based fluorophores has been achieved. In addition, the metal active sites and cofactors of metalloproteins were probed by optical spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and DFT modeling. In conjunction, these techniques can be used to elucidate the electronic structure responsible for the unique function of these metalloproteins. Specifically, a novel ironsulfur cluster of a metalloprotein that may be involved in endospore formation of Clostridium difficile, CotA, was characterized by magnetic circular dichroism (MCD) spectroscopy. We propose that CotA contains a high-spin [4Fe-4S] cluster and a Rieske [2Fe-2S] cluster. It appears that the multimerization of the protein is related to the cluster conversion at the interface of monomeric subunits where two [2Fe-2S] clusters combine to form the [4Fe-4S] cluster. In addition, a putative cobalamin acquisition protein from Phaeodactylum tricornutum, CBA1, was not expressed at sufficient concentrations in Escherichia coli for spectroscopic investigation. Finally, a new technique was developed using cobalt-59 NMR spectroscopy to better understand the nucleophilic character of cobalt tetrapyrroles, such as cobalamin (vitamin B12), as biological cofactors as well as synthetic catalysts. New insight into the electronic structure provides valuable information related to the mechanism of these metalloproteins.

Pour notre travail de thèse, nous avons établi une méthodologie, basée sur la DFT et sa variante dépendante du temps, qui permet l'évaluation efficace et la prédiction rapide des couleurs de différentes familles de colorants carbonylés dans différents environnements. Ainsi, nous sommes capables de fournir des valeurs théoriques très précises pour les énergies de transition des dérivés de l'anthraquinone, de la coumarine et de l'indigo ainsi que du thioindigo. Notre stratégie prend en compte aussi bien les effets de solvatation, dans le cadre d'une modélisation en continuum, que les effets du pH, sur la position de la bande d'absorption maximale du spectre UV/VIS. / The project of the Ph. D. work consists in the elaboration of a theoretical methodology able to evaluate and predict the color of organic dyes (see the List of recent publications for more details). Our methodology takes into account the solvent effects as well as the pH impact on the color of the dyes. The theoretical tools used are the Density Functional Theory (DFT, for ground-state optimization) and the Time Dependent TDDFT for excitation spectra calculations. The solvent effects are taken into account via the PCM (Polarizable continuum Model).