Thermal-resistant semiconductive oligomers via bis(phenyl)fluorene-functionalized phenoxy-imine frameworks

Abstract

In this study, two oligo(phenoxy-imine)s containing a bis(phenyl)fluorene unit were synthesized. Initially, two Schiff base monomers (AF-HBA and AF-SA) were synthesized through the one–pot condensation reaction of 9,9-bis(4-aminophenyl)fluorene (AF) with 4-hydroxybenzaldehyde (HBA) and 5-bromosalicylaldehyde (SA), respectively. Subsequently, these Schiff base monomers were oligomerized to Oligo(AF-HBA) and Oligo(AF-SA) using ammonium persulphate (APS) as an oxidant. The structures of the synthesized compounds were confirmed through FT-IR, ¹H NMR, ¹³C NMR, and UV–Vis spectroscopic analyses. Additional characterization techniques included optical measurements, electrochemical assessments, fluorescence (PL) and TG analyses. The AF-SA Schiff base exhibited blue photoluminescence in DMF, with a maximum emission intensity of 949 a.u. at the optimal excitation wavelength of 460 nm, and a quantum yield (QY) value of 18 % corresponding to the emission at 477  nm. The number-average molecular weights (M_w) and polydispersity indexes (PDI) of the oligomers were determined to be approximately 3750 Da with a PDI of 1.103 for Oligo(AF-HBA), while those for Oligo(AF-SA) were found to be 3900 Da with a PDI of 1.069 using GPC instrumentation. The low optical and electrochemical band gaps exhibited by the synthesized oligomers suggested their potential as semiconductive materials. The application of TGA and DSC analyses revealed that both Oligo(AF-HBA) and Oligo(AF-SA) exhibited thermal stability up to 234 °C and 294 °C respectively. Furthermore, their char amounts at 1000 °C were calculated to be relatively high at approximately 50.14 % for Oligo(AF-HBA) and 50.57 % for Oligo(AF-SA). These thermal properties, in conjunction with the obtained Limiting Oxygen Index (LOI) values, suggest that the synthesized oligomers hold potential for application in flame-retardant materials.