a) I successfully established a straightforward approach to synthesize the highly strained 9,9’-bifluorenylidene cyclic dimer (CBF) from 9,9’-bifluorenylidene (BF) by unexpected intramolecular cyclization via typical Yamamoto coupling reaction conditions. Sequential Yamamoto coupling reactions potentially allowed us to obtain well-defined and highly twisted but conjugated 9,9'-BF ladder tetramer (LBF).
b) The photophysical and electrochemical properties of BF and CBF were systematically investigated by using UV-vis absorption and CV measurements. The BF and CBF were demonstrated to have ability to accept two electrons, probably favored by release of steric hindrance from proximal aromatic rings and development of a 14- electron system.
c) The thermal behavior and phase transition processes of BF and CBF was determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Characterization by DSC reveals no phase transitions between 25°C and 200°C for CBF, presumedly due to the existence of enantiomeric mixture namely (P, P) and (M, M), yielding CBF more amorphous than crystalline. In stark contrast, BF showed the broadening melting point at around 153°C in the second heating process, probably attributable to multi-conformations from BF. Surprisingly, in the cooling process, one sharp peak at 150°C in additional to one small peak at 154°C was observed, thus we infer that multiple phase transitions occur sequentially by alternating the conformation of BF.