Four pure hydrocarbon molecules of 1,3,5-tris(9-phenyl-9H-fluren-9-yl)benzene (mTPFB), 4,4'-bis(9-phenyl-9H-fluoren-9-yl)biphenyl (pDPFBP), 1,4-bis(9-phenyl-9H-fluoren-9-yl)benzene (pDPFB) and 1,3-bis(9-phenyl-9H-fluoren-9-yl)benzene (mDPFB), with different connecting mode and core were prepared in one-pot reaction. Among them, mTPFB and pDPFDB were newly designed for the study of structure-properties relationship and developing good blue host for phosphorescent organic light-emitting devices (PhOLEDs). 10 The four materials show very similar photophysical properties in solution, with almost the same bandgap and triplet energy, as well as similar energy levels. However, their devices behave quite different with the best result from mTPFB. The conjugation blocked connection mode in the starburst mTPFB molecule with rigid and bulky substitutions results in high triplet energy in coupled with good and balanced electron and hole transport, as proved by internal reorganization energy calculation and mono charge device study, which makes it a good host for blue phosphor in PhOLED. The device shows external quantum efficiency of 15.7% and current efficiency of 15 30.6 cd/A, which is among the best of pure hydrocarbon host based devices. 65 fluorene 32 or spirofluorene 2, 33-34 based hosts for PhOLEDs have been reported, there are less evidences for bipolar transport in these materials. Furthermore, good pure hydrocarbon hosts of fluorene derivatives for blue PhOLED are rare 35 . Ye et al reported two fluorene derivatives of pure hydrocarbons, and employed 70