Star-shaped hosts based on phenyls substituted carbazole: Effects of trifluoromethyl and tert-butyl moieties

Abstract

To investigate the effects of trifluoromethyl and tert-butyl moieties on the general optoelectronic properties of star-shaped hosts based on phenyls substituted carbazole, we used 3,6,9-tris (4-(tert-butyl)-phenyl)-9H-carbazole (M1) as a reference, and different numbers of electron-withdrawing 3,5-bis(trifluoromethyl) benzene were introduced into 3, 6 and 9 positions of carbazole to obtain compounds 9-(3,5-bis(trifluoromethyl)phenyl)-3,6-bis(4-(tert-butyl)phenyl)-9H-carbazole (M3), 3,6-bis(3,5-bis(trifluoromethyl)phenyl)-9-(4-(tert-butyl)phenyl)-9H-carbazole (M5) and 3,6,9-tris(3,5-bis(trifluoromethyl)phenyl)-9H-carbazole (M6). After the introduction of 3,5-bis (trifluoromethyl) benzene, the thermal temperature at 5 % mass loss of M3, M5 and M6 decreased to 344 °C, while the glass transition temperature and melting point of M3 and M5 showed a significant increase. However, M6 did not show obvious glass transition, but its melting point also showed a significant increase. M3 crystallized at 125 °C as evidenced by differential scanning calorimetry, and the linking model of M3 is helpful to crystalize. In addition, when compared with those of M1, the fluorescence emission peaks of M3 and M6 showed a blue-shift, while M5 showed a red-shift, all of which were in the deep blue emission range. The color purity of blue light for M1 in tetrahydrofuran solution is the highest, and M1 exhibit aggregation-induced emission characteristics. Compared to the photoluminescence quantum efficiency (PLQY) of compound M1 of 39 % in toluene, the PLQYs of M3 and M6 decreased to 34 % and 29 %, and the PLQY of M5 increased to 46 %. The function of the lowest unoccupied molecular orbital trap of M3 film under the external electrical field will increase, which not only offsets the increasing effect of the electrical field to the electron carrier and further decreases the carrier mobility when the electrical field increase, and the same situation is also feasible for the highest occupied molecular orbital traps in M5, M6 and M1. The organic blue light emitting diodes were prepared by using M1, M3, M5 and M6 as the hosts of bis [2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium, and the optimized device of M1 as the host showed the best performance with a maximum luminescence brightness of 2978 cd/m2, maximum current efficiency of 39.42 cd/A and maximum external quantum efficiency of 18.36 %.