Realistic Modeling of Fluorescent Dye-Doped Silica Nanoparticles: A Step Toward the Understanding of their Enhanced Photophysical Properties.

The very high brightness and photo-stability of silica nanoparticles (NPs) encapsulating organic fluorophores have been demonstrated recently and have opened up new exciting opportunities for a number of biotechnological and information technological applications. However, a systematic theoretical study of fluorescent core-shell NPs remains a challenge, and as a result, the understanding of the fundamental interaction and microscopic dynamics of the dye/NPs assembly is still lacking. In the present work, different computational methods, as classical molecular dynamics simulations based on purposely tailored force-fields and TDDFT quantum mechanical calculations, are combined in an integrated strategy to elucidate the mechanisms behind the brightness enhancement of realistic models of rhodamine (TRITC) based C-dots (Cornell dots) for the first time. TD-B3LYP/MM calculations on the S1 excited state dynamics of the dye show that crossing between the low lying (bright) ππ* and (dark) nπ* states occurs both in solution and in silica NPs albeit in the latter case it is reduced by the caging and screening effect played by the silica matrix. Moreover, our calculations show that the negligible solvatochromic shift between free-TRITC in solution and TRITC-based C-dots observed experimentally is due to seizure and incorporation of water molecules during the synthetic process that mediate the dye-silica interaction.