Photoprocesses in LB films containing non-isomerizing diphenyldiacetylene chromophores

Monolayers of polyelectrolyte complexes of non-isomerizable diphenyldiacetylene chromophores can be transferred to solid substrates to give highly ordered Langmuir–Blodgett (LB) films. The LB films have a well-defined structure with densely packed chromophores. The dense packing causes strong π–π interaction of the chromophores resulting in strong changes of the UV/vis spectra of the monolayers and LB films compared to solutions. The LB films have a significant optical anisotropy indicating a preferential orientation of the chromophores parallel to the dipping direction. The anisotropy is caused by the flow of material during the LB transfer. Upon polarized irradiation the optical anisotropy is changed, an effect that has been rationalized in previous reports as being due a photoreorientation of the chromophores. However, the irradiation also causes changes in the spectrum of the LB films, which indicate reduced π–π interactions. Furthermore, UV/Vis spectroscopy under grazing incidence reflection has indicated that an out-of-plane reorientation can be excluded as a reason for the changes in the spectra of the LB films. Thus, the changes in the optical anisotropy of the LB films upon polarized irradiation are not caused by a photoreorientation of the chromophores, but by a angle dependent excitation and subsequent photoreaction. This leads to changes in the packing of the chromophores and to an angle dependent increase in absorbance in the π-band region of the UV/vis spectra.