Synthesis and properties of liquid‐ crystalline triblock copolymers by ATRP, having electron‐transporting thiadiazole unit

Abstract

ABA-type block copolymers composed of 2,5-diphenyl-1,3,4-thiadiazole (DPTD) oligoester and poly(methyl methacrylate) (PMMA) segments (M n = 16 200 and 23 000) were synthesized by atom-transfer radical polymerization and their liquid-crystalline (LC) and photoluminescence (PL) properties were examined. The structures of block copolymers were identified by Fourier transform infrared and 1 H NMR spectroscopies. Differential scanning calorimetry measurement, polarizing microscopy observation and wide-angle X-ray analysis revealed that the block copolymers form thermotropic LC phase (smectic C) independent of molecular weights of PMMA segments, but a model polymer (PMMA segments having the DPTD unit in the central part) has no LC melt. Solution and solid-state PL spectra indicated that all the block copolymers display blue emission arising from the DPTD unit. Their quantum yields are 17-21%, which increase with the PMMA chain lengths. The block copolymers have good aligned structures in the LC states, but their order parameter (S) values in sheared LC states were lower than those in the sheared LC compounds. The PL properties in the LC states were independent of the LC aligned structures. Cyclic voltammetry measurements showed that these block copolymers have deep HOMO levels compared with polymers composed of oxadiazole rings.

Electron‐transporting semi‐rigid polyester‐imides made up of a tetracarboxdiimide of p‐terphenyl analogue of 1,3,4‐oxadiazole: Preparation, and thermal, optical, and electrochemical properties

“…In the solution, PL spectra of the polymers 7 when excited at 332 nm, the peak maxima were observed at 386–389 nm with a bluish‐violet emission, the Stokes shifts being 54.5–57.0 nm, whose values were almost the same as our previously reported hyperbranched polymers26 composed of a phenylbenzothiadole moiety and higher than those of linear polyimides comprising DPOD and PI units 19. The quantum yields (ϕ PL ) of the polymers 7 measured in chloroform using coumarin 311 as a standard were 13–21% and comparable to those of block copolymers composed of an electron‐transporting 2,5‐diphenyl‐1,3,4‐thiadiazole (DPTD) unit 27. The UV–vis spectra of polymers 7 in films displayed similar broad absorption curves, having absorption maxima at 326.5–332.5 nm as those in solutions.…”

“…The observed E red values for polymers 7 were −1.42 to −1.56 V. The calculated values of HOMO (IP) levels of polymer films 7 were −6.58 to −6.72 eV and the LUMO (EA) level values were −3.24 to −3.38 eV. The E g OPT values were 3.31–3.34 eV and almost the same as those given in our previous article,27, 32 which were in rough agreement with the electrochemical band gaps ( E g EC : 3.08–3.20 eV), differences in the E ox and E red values. The HOMO values of polymers 7 were lower than those of polymers composed of the DPTD unit,32 DPOD‐containing polyethers,33 and PPV‐based polymers 31.…”

“…Table IV lists the barrier heights (Δ E e and Δ E h ) between the cathode (Al) or anode (ITO) and polyester‐imides 7 . Tables III and IV and Figure 7 suggest that the energy barriers between the HOMO level values and ITO anode (−4.8 eV) in the polymers 7 are larger than the DPTD‐27 and DPOD‐containing polymers33 and the differences in the barrier height (Δ E bh = Δ E e − Δ E h ) between Δ E e and Δ E h spread. The barriers of polymers 7c and 7d were higher than those of polymer 7a and 7b .…”

“…The barriers of polymers 7c and 7d were higher than those of polymer 7a and 7b . This suggests that the DDPOD unit lead the polymers 7 to increase in E e rather than E h and stabilized the HOMO level, but the energy barriers between LUMO level values and work function of Al cathode (−4.3 eV) were sufficiently small compared with the barriers in the DPTD‐27, 32 and DPOD‐containing polymers 33. From these data, it is suggested that the polyester‐imides (7) comprising the DDPOD moiety behave as electron‐transporting materials for organic EL devices.…”

Preparation and properties of novel thermotropic liquid crystalline hyperbranched polyesters composed of five‐membered heterocyclic mesogen by A2 + B3 approach

“…The quantum efficiencies ( Φ ) measured in the solutions using coumarin311 as a standard were 9.4–9.8% in the polymers (HBP1‐3) by the melt polycondendation and 9.1–13% in those (HBP4‐6) via the solution approach, which are the highest in the HBP5 polymer having the highest inherent viscosity and forming the most stable LC state. Against our expectation, the Φ values for HBP1‐6 (9.1–13.0%) are lower than those in random39 or block LC copolymers 40. This is probably due to that the highly‐branched structure in the HB polymers contributes to promotion of quenching.…”