b S Supporting Information
' INTRODUCTIONPhosphorescent organic light-emitting devices (PHOLEDs) have attracted tremendous research interests owing to the spinÀ orbit coupling interactions of heavy metals, resulting in the harvest of both singlet and triplet excitons to give the approaching 100% internal quantum efficiency theoretically. 1À4 However, it is a prerequisite for high-performance PHOLEDs to blend the triplet emitters of heavy-metal complexes into host matrixes to reduce their concentration-quenching effect. Recently, considerable progresses have been achieved in small molecular host materials based blue PHOLEDs in terms of brightness, power efficiency, and durability. 5À8 However, small-molecule based devices require complex coevaporation techniques, high vacuum, and tedious and precise control process during device fabrication, thus greatly hindering the success of product development and commercialization. In contrast, solution process methods, such as spin-coating, inkjet printing, or screen printing, show more advantages of easier fabrication process and lower cost with phosphorescent polymer light-emitting diodes (PPLEDs). 9 In this context, the key issue is to develop the excellent polymer host materials for PPLEDs, especially for blue PPLEDs.Similar to small molecular host materials, high triplet energy (E T ) levels are crucial for suitable polymer host materials to effectively prevent reverse energy transfer from emitters to hosts and confine triplet excitons on the triplet emitters in PPLEDs. 10À12 In general, π-conjugated polymers usually have low E T levels, which are not suitable as host materials for blue and green phosphorescent heavy-metal complexes, 13,14 such as poly(2,7-fluorene)s and its derivatives which are the widely used π-conjugated polymer host materials possess low E T of 2.15À2.3 eV. 15,16 One effective method to increase the E T levels is to design the nonlinear π-conjugated polymers via the method, limiting the delocalization length of carriers and excitons. However, the obtained π-conjugated polymers, such as poly(3,6-carbazole)s (E T = 2.53À2.6 eV), 17,18 poly(3,6-fluorene)s (E T = 2.58 eV), 19 poly(3,6-silafluorene)s (E T = 2.55 eV), 20 and poly(m-phenylene) derivatives (E T = 2.64 eV) 21 still have too low E T levels to act as efficient host materials for typical blue electrophosphorescent emitters, such as bis[(4,6-difluorophenyl)pyridinato-N, C 2 (picolinato)iridium(III) (FIrpic) (E T = 2.65 eV). Other π-conjugation-interrupted technique also provides a useful strategy to improve the E T levels of polymer host materials through the introduction of sp 3 -hybrized carbon, silicon, or oxygen. 22 However, there are still rare polymer host materials for blue electrophosphorescent emitters up to date.With respect to π-conjugated polymers, π-stacked polymers as supramolecular semiconductors exhibit the unique carrier and exciton features in the applications of various polymer semiconducting devices. 23,24 The design of π-stacked polymers opens another way to develop the ...