Novel spiro-fluorenes from tandem radical addition for liquid crystalline monodisperse conjugated oligomers

Abstract

3′-Nonafluorobutylmethyl-4′-methyl-spiro[cyclopentyl-9,1′]fluorenes were successfully synthesized via tandem radical-addition reactions between 9,9-diallylfluorenes and perfluorobutyl iodide in the presence of a radical initiator followed by reduction under mild conditions. Single crystal analysis indicates that two substituents at 3,4-positions of cyclopentane are in a maleinoid form. Accordingly, four oligo(fluorene-co-bithiophene)s with the same molecular length of ∼10 nm (7 fluorene units and 12 thiophene units) containing one to three novel spiro-fluorene units were synthesized. The introduction of the spiro-fluorene units results in noticeable enhancement of both glass transition temperature (T_g) and clearing point temperature (T_c) of the oligomers, but has little effect on their photophysical properties. Exchanging three 9,9-dioctylfluorene units with 3′-nonafluorobutylmethyl-4′-methyl-spiro[cyclopentyl-9,1′] fluorene units results in a 37 °C enhancement of T_g and a 61 °C enhancement of T_c. All these results indicate that this new spiro-fluorene unit is an attractive building block for liquid crystalline conjugated polymers/oligomers with both high T_g and T_c.