Biaryl dopants

These results allowed the proposal, at the beginning of the 1980s, of a different molecular model for cholesteric induction 65,66 This model is sketched in Figure 7.15 in the case when both nematic host and chiral guest have a biaryl structure. Nematic molecules exist in chiral enantiomorphic conformations of opposite helicity in fast interconversion. The chiral dopant has a well-defined helicity (M in Figure 7.15) and stabilizes the homochiral conformation of the solvent In this way, the M chirality is transferred from the dopant to the near molecule of the solvent and from this to the next near one and so on. This... 

Figure 7.15 Chirality transfer from dopant to solvent. Chiral inducer with an (M)-helidty aligned with its biaryl axis parallel to biphenyl axis of solvent can have close contact only with molecules of solvent having same helicity Chirality is therefore transferred from dopant to near solvent molecule and from this to next near one and so on, via chiral conformations.

Several stereochemical applications of cholesteric induction have been described.52 55 Reliable results are obtained especially when the dopant has a high twisting power. One of the molecular fragments associated to a high value of twisting power is, as anticipated above, the biaryl unit therefore many experiments have been done on compounds containing this unit. 

Perylene (199) and its derivative (TBP, 200) have been widely used as blue dopant materials owing to their excellent color purity and stability. Efficient blue emitters with excellent CIE coordinates are found in biaryl compound 2,2 -bistriphenylenyl (BTP, 201) as shown in Scheme 3.62 [145]. A device of structure ITO/TPD/BTP/TPBI/Mg Ag emits bright blue emission with CIE (0.14, 0.11). A maximum brightness of 21,200 cd/m2 at 13.5 V with a maximum EQE of 4.2% (4.0 cd/A) and a power efficiency of 2.5 lm/W have been achieved. 

Based on the study of the Arl/doped systems, three distinct differences have been delineated between the ablative regime and the very low fluences regime. Specifically, differences are observed concerning (a) efficient formation of biaryl-type products, (b) efficiency of ArH-like photoproduct formation, and (c) kinetics of product formation. These results demonstrate that dopant reactivity is both qualitatively and quantitatively modified from that at low laser fluences. 

The twist induced by different molecular species can be qualitatively related to the molecular structure of the chiral species [40]. The twist induced in a nonchiral liquid crystal solvent by a chiral dopant also depends on the nature of the solvent, and it has been proposed that chiral dopants can preferentially promote chiral conformations of the solvent molecules [41]. This effect has also been observed in isotropic solutions, where an enhanced optical rotation in solutions of a chiral biaryl in a cyanobiph-enyl solvent was attributed [42] to an induction of chirality via preferential interactions... 

---