Role of Host-Guest Interactions in Tuning the Optical Properties of Coumarin Derivatives Incorporated in MCM-41: A TD-DFT Investigation

The encapsulation of organic dye molecules in silica-based nanostructures leads to composite materials with novel optical properties and a wide range of applications in the field of nanotechnology. The design of new dye-doped silica-based devices requires a deep understanding of how host-guest interactions affect the optical properties and stability of the dye/silica assembly. In this work, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been employed to investigate the effect of the host-guest interactions on the structural, optical, and electronic properties of two 7-aminocoumarin dyes (labeled C339 and C340) incorporated into MCM-41. Our calculations show that the interaction of the carbonyl groups of the coumarin molecules with the silanol groups on the silica surface is responsible for the dye stabilization and is strengthened upon photoexcitation. As a result, the computed absorption and emission spectra of the incorporated dyes are red-shifted compared to those in toluene, in perfect agreement with experimental measurements. The computed electronic spectra reproduce well both the solvatochromic and Stokes shifts of the dye molecule in toluene and MCM-41, and also, the band-shape can be reconstructed by simply including the vibrational fine structure associated to the electronic transition.