Fluorescent branched oligomers with polyphenyl and fluorenyl units

Abstract

Absorption spectra and fluorescence parameters (spectra, quantum yields, and lifetimes) were obtained for a number of branched oligophenylenes (OPh) with long-wavelength chromophores such as p-quaterphenyl (OPh4-2), p-pentaphenyl (OPh5), and p-hexaphenyl (OPh6) and for oligofluorenylphenylenes (OFl) in solutions and films. It is demonstrated that the absorption spectrum of OPh4-2 is a superposition of p-quaterphenyl, p-terphenyl, and diphenyl absorption bands taken in a ~3:2:2 ratio. The obtained OPh5 and OPh6 absorption and fluorescence spectra are shown to be determined mainly by the longest-wavelength chromophores, p-pentaphenyl and p-hexaphenyl, respectively. It is demonstrated that the obtained compounds contain traces of impurities with fluorescence at longer wavelengths than that of the base material. The branched oligomers exhibit high fluorescence quantum yields in solutions, those for OPh5 and OPh6 reaching 1. Transparent fluorescent films were produced from all of the synthesized oligomers. Keywords: branched oligophenylenes and oligofluorenylphenylenes, UV absorption, fluorescence, quantum yield, fluorescence lifetime. Introduction. Advances in the chemistry of branched oligomers during the last two decades have made it possible to fabricate luminescent compounds with new molecular structures. Branched oligomers incorporating phenylenevinylene [1], phenyleneacetylene [2], carbazole [3], fluorene [4], thiophene [5], and several other chromophores were synthesized. Homogeneous films based on branched oligomers were used to fabricate new semi-conducting LEDs [6,7]. This, in turn, prompted studies on the preparation of new branched fluorescent oligomers. Let us describe in more detail branched oligophenylenes. One of the first studies [8] dedicated to branched oligophenylenes containing the diphenyl fragment as the chromophore was focused on the determination of the molecular weight distribution, thermophysical properties, and the potential for chemical modification. However, the spectral properties of the synthesized oligomers were not discussed. Later, branched oligoarylenes with phenyl, fluorenyl, carbazole, and thiophene units were prepared [9,10]. The absorption and fluorescence properties were reported in these studies but the relationship of these properties to the molecular structures of the prepared oligomers was not examined. Recently we prepared the branched oligophenylene OPh4-1 and analyzed its structure and spectral and luminescent properties [11]. It was shown that the absorption spectrum was composed of bands from the chromophores p-quaterphenyl (PQP), p-terphenyl (PTP), and diphenyl (DP) (in ~1:2:1 ratio). It was found that two optical centers were responsible for the fluorescence of OPh4-1. An anomalous fluorescent unit, p-hexaphenyl, was observed at a negligibly low concentration in the oligomer.Herein we continue spectral investigations of fluorescent oligophenylenes [11]. Three branched oligomers containing different p-phenyl chromophores (m = 2, 3, 4) and two fluorenylphenylene oli...