From blue fluorescence to red fluorescence: Solid-state oxidative coupling polymerization of fluorene and anthracene or naphthalene

Through the solid-state oxidative coupling polymerization, a series of fluorene-co-anthracene copolymer (PFA) and fluorene-co-naphthalene copolymers (PFN) were synthesized using FeCl3 as the oxidant. The structures of the copolymers were characterized by FT-IR and 1H NMR spectra, showing the random structures composed of fluorene units and 9,10-anthracene units or 1,4-linked naphthalene units. The increasing fluorene units in the backbones resulted in the increasing crystallization of copolymers by XRD patterns. All the copolymers exhibited good thermal stability. The optical properties could be adjusted by using different fluorene and anthracene or naphthalene ratios. Moreover, the polymer concentrations that affected the photoluminescence emission ranged from blue light to red light. The concentration-induced discoloration property also could be observed from the copolymers on the filter paper of TLC plates. This is attributed to the change in the effective conjugation length of a single polymer and the formation of polymer aggregates and excimers. These unique properties endow the PFA and PFN with potential applications in optoelectronics. Especially, these polymers exhibit enhanced emissions and on/off fluorescent switching which senses THF vapor efficiently.