Supramolecular organization and optical properties of BODIPY derivatives in Langmuir–Schaefer films
Four BODIPY (boron-dipyrrin) dyes with various condensed aromatic moieties (phenyl, naphthyl, anthryl and pyrenyl) as 8-substituents were chosen and their supramolecular organization and properties in Langmuir–Schaefer (LS) films with different number of layers were investigated. Spectral characteristics of the dyes in LS-films and solution and structure of the films were examined. BODIPY substituent structure influences the character of intermolecular interactions of the dyes in LS-films. Phenyl-substituted compound was found to form excimers with long wavelength fluorescence peak at 582 nm upon the layer number increase, yet only minor bathochromic shifts of the peak position were observed for films of other investigated compounds. Moreover, unlike in liquid solutions, dyes in LS-films were not found to exhibit solvatochromic properties. All of the compounds (except anthryl-substituted) were found to retain fluorescence in solid films. Transfer of films from a floating monolayer was stated from the further material analysis: measured average periodicity corresponds to a single-layer structure of BODIPY molecules located parallel to the surface with interplanar distance of d=0.3 nm. Obtained results broaden perspectives of BODIPY based thin film materials application, denoting versatile modification routes leading to increased fluorescence, tunable excimer formation equilibrium and crystallinity.