Star-shaped oligomers with truxenone centre and triphenylene branches: mesomorphism, optical and electronic properties

Three star-shaped oligomers with an electron-deficient truxenone (TrO) as the central core and three electron-rich triphenylenes (TP) as the peripheral arms were prepared, in which the triphenylenes groups are, respectively, connected to the central truxenone core at 2, 7 and 12 positions through a highly flexible O(CH₂)_nO (n = 6,8,10) polymethylene chain. All these triphenylene-armed oligomers TrO[O(CH₂)_nOTP]₃ (n = 6,8,10) exhibit liquid crystalline over broad mesophase ranges including room temperature. Oligomer TrO[O(CH₂)₆OTP]₃ exhibit a rectangular columnar mesophase, while others with the linkage longer show a hexagonal columnar mesophase. This modulation of phase behaviours was demonstrated to be generated by steric perturbations introduced between the three decyloxy substituents in central truxenone core and the linkages with different lengths, which cause the disturbance of the intracolumnar structure and influence the degree of ordering in columnar phases. No new electronic absorption except for the usual bands of the individual truxenone or triphenylene part was observed in dichloromethane solution of TrO[O(CH₂)₆OTP]₃. However, two new emission peaks emerged and the intensity and position of the emission peaks were found to exhibit pronounced changes upon concentration variation, which can be attributed to the formation of intermolecular excimers.