Molecular aggregation and shape effects in the optical spectra of organic dye molecules in SiO2 and SiO2-PMMA matrices

The electronic spectra of four different types of organic colorants embedded into sol-gel SiO2 xerogel or into a hybrid SiO2-PMMA matrix were studied experimentally using optical absorption, photoluminescence excitation, emission techniques, atomic force microscopy and fluorescence microscopy. The results show a well defined set of energy levels for each type of dye. It turns out that the type of matrix and the degree of aggregation of the organic molecules have relatively small effects on the energy spectra, although they have a strong influence on the optical absorption of the material. A quantum mechanical description of the electronic spectra of these molecules was developed on the basis of free electron molecular orbitals approach. For that the molecule is considered as a two-dimensional potential well, using both periodic (Born-von Karman) boundary conditions to take into account aggregation effects, and the commonly used infinite wall conditions. The results obtained have a good agreement with experimental spectra without adjustable parameters. A new simple method to calculate the energy spectra of and electron in a two-dimensional potential well is proposed. For that, it has been considered the electron motion in a box with reflecting walls, giving authors a result identical to the classical model for rectangular-shaped molecules. This method also allows authors to treat the triangular-shaped molecules, the results obtained being in a reasonable agreement with experiment, also without any adjustable parameters. 23 refs.