Monomer-dimer equilibrium of rhodamine B encapsulated in Si-Al and Si-Ti binary-oxide thin films

Si-Al and Si-Ti binary-oxide thin films including Rhodamine B (RB) have been prepared. They were dip-coated as a function of time after mixing of each sol-gel reaction system. The absorption and fluorescence spectra of the individual films have been observed. These spectra were analyzed in order to clarify the behavior of RB along with the change in the environment around the RB molecules, caused by the progress of the sol-gel reaction, in the fluid sol and the prepared thin films. Some amount of the RB dimers (H- and J-types) were formed in the Si-Al and Si-Ti binary-oxide films (Si : M = 75 : 25) prepared at the initial stage of the sol-gel reaction and aged under relative humidity of 60%. In the case of Si-Al binary-oxide films, the amount of the J-dimer decreased along with the reaction time at which the films were prepared, indicating that growing polymer networks of metal alkoxides around the RB molecules prevent the formation of the J-dimer. On the other hand, larger amounts of the H- and J-dimers were formed in the Si-Ti binary-oxide films prepared at longer reaction time of the solution. RB interacts more strongly with -TiOH compared with -AlOH. In the case of the Si-Ti binary-oxide films, with the progress of the sol-gel reaction, RB molecules in the prepared films easily cohere around the -TiOH and form the dimers because of increase in the amount of the -TiOH and contraction in the volume of the spaces where RB molecules exist.