Preparation Of Polymers Containing Azoindazole Side Chain Via RAFT Polymerization And Investigation On Their Optical Properties

In this paper, we report the observations of unusual fluorescence enission from light-driven and aggregates-induced polymers containing pendent azoindazole. The well-definded homopolymer poly6-{6-[(4-dimethylamino) phenylazo]-indazole}-hexyl methacrylate (PDHMA) and amiphiphilic diblock copolymer poly({6-[6-(4-dimethylamino)phenylazo]-indazole}-hexyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate) (PDHMAm-b-PDMAEMAn) were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization.(1) A novel azo monomer containing the indazole group was synthesized and characterized, which was named 6-{6-[(4-dimethylamino) phenylazo]-indazole}-hexyl methacrylate (DHMA). The DHMA homopolymer (PDHMA) was prepared by reversible addition fragmentation chain transfer (RAFT) polymerization in toluene solution using 2-cyanoprop-2-yl 1-dithionaphthalate (CPDN) as the RAFT agent and 2,2'-azobis(isobutyronitrile) (AIBN) as an initiator. Homopolymer was characterized by 1H NMR spectra and gel permeation chromatography (GPC), respectively. We found that most of the PDHMA chains were still"living". The nonfluorescent homopolymer in dichloromethane (DCM) undergo trans-to-cis photoisomerization upon UV irradiation accompanied by fluorescence enhancement. Moreover, the fluorescence properties of metalated azobenzene homopolymer and acidification were studied. The surface relief gratings and photoinduced birefringence for homopolymer at room temperature were investigated, too. The results indicade that light-driven homopolymer solution showed fluorescence enhancement upon cis isomerization. The reason could be inhibition of the photoinduced electron transfer (PET) mechanism. And fluorescence enhancement was observed for homopolymer CH₂Cl₂ solution in the present of Fe³⁺ due to the confindment of azo group hinder the photoisomerization. After irradiation, optical anisotropy was observed because of the homogeneously random alignment of the azobenzene chromophores. The maximum values of the photoinduced birefringence for PDHMA film reached about 0.113, and diffraction efficiency of about 2% was obtained.(2) Using hydrophobic PDHMA as the macro-RAFT agent, the soft hydrophilic 2-(dimethylamino) ethyl methacrylate (DMAEMA) as monomer was chosen to polymerize. The amphiphilic diblock copolymers poly({6-[6-(4-dimethylamino)phenylazo]-indazole}-hexyl methacrylate)-b-poly(2- (dimethylamino) ethyl methacrylate) (PDHMAm-b-PDMAEMAn) were prepared and characterized by 1H NMR spectra and gel permeation chromatography (GPC), too. Like homopolymer, the nonfluorescent copolymers in dichloromethane (DCM) also became fluorescent upon UV irradiation. Specially, the enhanced fluorescence from three amphiphilic diblock copolymers aggregates was observed upon addition of water in tetrahydrofuran (THF). The self-aggregation behavior of three copolymers was examined using transmission electron microscopy (TEM) and dynamic light scattering (DLS). The fluorescence emission was enhanced with addition the content of water in THF solution. The intensity is increased by almost 480 times in micrometer-scale aggregates, while 60 times in nano-aggregates. Aggregation-induced fluorescence was caused by the confinement of the azoindazole group, which leads to hinder the trans-to-cis isomerization, namely, the main reason was considered to be the restricted intramolecular rotation (RIR) of azo group.