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

The encapsulation of organic dye molecules insilica-based nanostructures leads to composite materials withnovel optical properties and a wide range of applications in thefield of nanotechnology. The design of new dye-doped silicabaseddevices requires a deep understanding of how host−guest interactions affect the optical properties and stability ofthe dye/silica assembly. In this work, density functional theory(DFT) and time-dependent DFT (TD-DFT) calculations havebeen employed to investigate the effect of the host−guestinteractions on the structural, optical, and electronic propertiesof two 7-aminocoumarin dyes (labeled C339 and C340) incorporated into MCM-41. Our calculations show that the interactionof the carbonyl groups of the coumarin molecules with the silanol groups on the silica surface is responsible for the dyestabilization and is strengthened upon photoexcitation. As a result, the computed absorption and emission spectra of theincorporated dyes are red-shifted compared to those in toluene, in perfect agreement with experimental measurements. Thecomputed 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 electronictransition.