Push-pull chromophores aggregation in SiO2 sol-gel films doped with silver nanoparticles

Sol-gel films used for second-order non-linear optical applications often contain some organic molecules working as spacers between chromophores with large permanent dipolar moments. The spacers improve the optical quality of the films because they avoid the chromophores aggregation. We propose the use of silver nanoparticles as good spacers, instead of organic molecules. In this work the effect of silver nanoparticles on the arrangement of push-pull chromophores inside SiO2 sol-gel films is investigated by UV-vis optical absorption spectroscopy. Key variables for a good performance of spacers are: their polarizability, their concentration with respect to the concentration of the chromophores, as well as the temperature of the system. Disperse Red 1 chromophores, well known push-pull chromophores, arrange themselves forming H-aggregates when they are inside films in high enough concentrations. Those aggregates are clearly detected by UV-vis optical absorption spectroscopy, where they show an absorbance peak close to 404 nm. That peak disappears as the temperature of the films and/or the concentration of the spacers and/or the polarizability of the spacers increase. A possible electrostatic shielding between the chromophores, created by the spacers, makes us to think about the possibility to use metallic nanoparticles as a new kind of molecular spacer.