Synergistic steric pairing effects of terfluorenes with ternary side groups on β-conformation transition: Experiments and computations

Rational molecular design has led to the achievement of β-conformations of terfluorenes that are usually difficult for oligoalkylfluorenes with less than 5 repeat units. Herein, under the guideline of synergistically molecular attractor-repulsor theory (SMART), we design four models of terfluorenes with the possibility of inducing β-conformation in the terfluorene 2,2′-(spiro[fluorene-9,9′-xanthene]-2,7-diyl) bis(9-(4-(octyloxy)phenyl)-fluoren-9-ol) (2O8-DPFOH-SFX) by manipulating the cooperative effects of steric hindrance, hydrogen bonding (HB) and van der Waals (vdW) forces after thermal annealing. For terfluorene 2O8-DPFOH-SFX, the two isomers, raceme and mesomer, with two side chains residing on the two sides and on the same side of the fluorene backbone, respectively, have different packing styles and hence different optical properties: only the raceme isomer could induce the formation of β-conformation. Due to the steric pairing effect of side chains on both sides and the cooperative interaction of the middle spiro[fluorene-9,9′-xanthene] (SFX) group, the racemes precisely interlace with each other and tend to adopt Δ-shaped stacking, which favors a planar π-conjugated structure for the raceme. This is rationalized by the larger content of planar conformations in the 2O8-DPFOH-SFX raceme, according to molecular dynamics (MD) simulations of the condensed phase. Thus, oligofluorenes with definite molecular structure can be considered as notable materials for gaining insight into the structure-property relationships for optical and electronics applications.