Origin of the conjugation break
4-Spirobifluorene derivatives - A new family of host materials for PhOLEDs

Unlike the 2-substituted SBF derivatives for which electronic conjugation can extend to the para-substituent of the SBF (see analogy with para-terphenyl figure 4), the 4-substituted derivatives belong to another class of SBFs for which conjugation is interrupted between the ortho-substituent of the SBF and the substituted fluorene (see analogy with ortho-terphenyl figure 4) mainly due to steric hindrance. This break in conjugation is described in figure 4 by comparing the XRD structure of crystals...