A FLUORESCENCE STUDY OF THE MISCIBILITY OF OMEGA-ARYLALKANOIC ACIDS IN LANGMUIR-BLODGETT-FILMS

On increasing the amount of myristic acid in mixed multilayers of 7-(2 -anthryl)-heptanoic acid (2A7) and myristic acid the emission of shall ow traps emitting at 420 nm is favoured compared with that of deeper t raps emitting at 455 nm. This change in the emission spectrum is accom panied by a change in the excitation spectrum from a spectrum resembli ng the absorption spectrum of an anthracene single crystal to a spectr um resembling the absorption spectrum of a dilute solution of 2A7. To obtain more efficient mixing, myristic acid was replaced by 8-(2-napht hyl)-octanoic acid (2N8). On excitation at 340 nm, where light absorpt ion is mainly due to the anthracene chromophores, the emission of mixe d multilayers of 2A7 and 2N8 with a mixing ratio of 1:2 is mainly due to the shallow traps emitting at 420 nm. However, on excitation at 290 nm, where light absorption is mainly due to 2N8, the emission spectru m resembles that of a dilute solution of 2A7 at mixing ratios between 1:2 and 1:200. These spectral data suggest that in addition to isolate d molecules of 2A7 disolved in 2N8, there exists in mixed Langmuir-Blo dgett films of 2A7 and 2N8 or myristic acid also a second phase contai ning aggregated 2A7 molecules. While excitation at 290 nm will favour the population of the excited state of isolated molecules of 2A7 by en ergy transfer from 2N8, excitation at 340 nm will favour the excitatio n of aggregated 2A7 molecules.