many different heteroatom organic blocks, such as carbazole, [ 10 ] pyridine, [ 11 ] oxadiazole, [ 12 ] acridine, [ 13 ] phosphine oxide, [ 14 ] etc., are introduced to meet these requirements, because the frontier orbitals of host materials can be delicately tuned with different groups. [ 15 ] These differences in heteroatom compounds give us more options in material's design and, however, bring some unknown risks in chemistry. It is primarily resulted from the fact that organic functional groups consist of heteroatoms always serve as the activated sites by themselves or activate adjacent positions. For example, Qiao and co-workers studied the molecular stability under device operation and found that C-C bond is signifi cantly stronger than C-S, C-P, or C-N bonds. [ 16 ] Thus, there is still a quest for host materials constituted by pure hydrocarbon (PHC) backbone in academia. But in practical, these heteroatom-free hosts still lag far behind the development of heteroatom hosts. Take the sky-blue emitter FIrpic as guest for example, the best results based on PHC hosts were just around 10% in EQE, [ 17 ] while the heteroatom hosts can achieve >20% EQE, approaching the ≈100% IQE in theory. [10][11][12][13][14] There could be big room for improvement of PHC hosts; we would therefore go forward to explore which is the effective way to arrange the hydrocarbon blocks for high efficient hosts.In this communication, two new spirofl uorene-based PHC materials, SF33 and SF34, are reported by combining two 9,9′-spirofl uorene blocks in different linking ways. SF33 adopts symmetrical confi guration with meta-meta linkage, while SF34 has unsymmetrical confi guration with meta-ortho linkage. Most of previously researches about PHC hosts adopt symmetrical or repeated units as SF33, as it can increase the molecular weight for the balance between suffi cient glass-transition temperature and suitable volatility. [ 17d ] But by breaking the symmetry of SF33 in this case, SF34 presented similar thermal property but quite different electrical and optical properties. In thermogravimetric analysis (TGA) and differential scanning calorimetry measurements ( Figure S1 and S2, Supporting Information), SF33 and SF34 exhibited good thermal stability. The decomposition temperatures ( T d ), which correspond to 5% weight loss upon heating during TGA, were measured to be 433 °C and 407 °C for SF33 and SF34, respectively. In addition, SF33 and SF34 also have a high glass-transition temperature ( T g ) of 193 °C and 177 °C, respectively.The signifi cant difference between SF33 and SF34 is in the spectra measurements. The UV-vis absorption and photoluminescence (PL) spectra of SF33 and SF34 in dilute hexane solution (1 × 10 −6 mol L −1 ) were measured to investigate their photophysical properties ( Figure 1 ). SF33 and SF34 present Organic light-emitting diodes (OLEDs) have been widely touted as next-generation displays and solid-state lighting technologies because of their superior characteristics. [ 1 ] However, effi ciency and lifetime...
