Analysis of branched oligophenylene by absorption and fluorescence spectra

Abstract

A branched oligophenylene has been synthesized based on 1,3,5-tri(4′-bromophenyl)benzene. Absorption and fluorescence spectra were studied and fluorescence quantum yields and lifetimes were measured for the compound in solution. It is demonstrated that the absorption spectrum is a superposition of p-quaterphenyl, p-terphenyl, and biphenyl chromophore absorption bands in a 1:2:1 ratio. The oligomer fluorescence spectrum is found to depend on the excitation wavelength. It is shown that the oligomer fluorescence is determined by two fluorochromic groups, namely fragments with branched p-terphenyl and p-quaterphenyl units. The main fluorescence maxima for these fluorochromic groups coincide with each other and lie in the vicinity of λ = 360 nm. A very weak fluorescence band found in the region 380-440 nm is excited by light with a wavelength lying beyond the oligomer self-absorption region. The reasons for a decrease in fluorescence quantum yields of branched models and the studied oligophenylene as compared with those of linear p-polyphenylene chromophores are discussed.Introduction. Branched oligomers have become very interesting in the last decade in connection with the search for novel materials with combinations of practically useful properties [1-3]. A promising area for application of branched oligomers containing luminophores is the development of new active additives to plastic scintillators [4], dye lasers [5,6], and light-emitting diodes in semiconducting photodiodes [7][8][9]. The preparation of branched oligomers and polymers with luminophores is a relatively new area. Comparatively few systems of this type have been prepared. Furthermore, their photophysical properties and their property-structure relationships are almost uninvestigated. One of the reasons for this situation is the difficulty of determining their molecular structures because the process for forming branched oligomers is random in nature [10,11].Herein a synthesized branched polymer (the Experimental section describes the synthetic method) is analyzed using spectrophotometry and fluorescence. According to the putative reaction, the produced macromolecule should have p-quaterphenyl (PQP) units at the branches and diphenyl (DP) and p-terphenyl (PTP) chromophores as the terminal groups. Issues with the determination of the ratio of these three chromophores in the produced macromolecule are examined. The units responsible for the fluorescence of the oligomer and the interactions between them are identified. These problems are resolved using a comparison of spectral data for the oligophenylene and the model low-molecularweight compounds PTP, PQP, and their branched analogs containing three PTP and PQP moieties.Experimental. The monomer 1,3,5-tri(4′-bromophenyl)benzene was prepared via the trimerization of 4-bromoacetophenone in benzene in the presence of triethylorthoformate and dry gaseous HCl by the standard method [12]. The oligophenylene was produced by Ni-catalyzed polycondensation of 1,3,5-tri(4′-bromophenyl)benzene by the literature me...

Fluorescent branched oligomers with polyphenyl and fluorenyl units

“…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 oligomers, the chromophores of which contain a conjugated chain of seven benzene rings, were prepared:…”

“…Herein we continue spectral investigations of fluorescent oligophenylenes [11]. Three branched oligomers containing different p-phenyl chromophores (m = 2, 3, 4) and two fluorenylphenylene oligomers, the chromophores of which contain a conjugated chain of seven benzene rings, were prepared:…”

“…Absorption and luminescent properties of branched oligophenylene OPh4-1 were studied in solutions and films [11]. It was found that two optical emission centers were responsible for the fluorescence of this compound.…”