Abstract
Here we report results of our study of chain conformation, molecular packing, and corresponding effects on photo-excitation and luminescence behavior of poly(9,9-di-n-oxtyl-2,7- fluorene) (PFO) and poly(9,9-di-n-hexyl-2,7-fluorene) (PFH) using quantum mechanical (QM, Gaussian 03) and molecular mechanics (MM, Accelrys Cerius2) calculations. We demonstrate that the single-chain excitation energy can be slightly different upon pi-flipping of a fluorene monomer; this signifies difference between the two most likely (i.e., UDUD vs. DDUU) candidates for backbone conformation in the crystalline state. Excluding the DDUU conformation based on monomer length from MM, its tendency to bend from QM calculation, and symmetry requirement from crystallographic argument, we adopt UDUD conformation with alternating fluorene torsion angles of +_140 as the single chain conformation and proceed with MM simulation of chain packing in the α phase of PFO and PFH. Results agree well with experimental zone patterns from selected-area electron diffraction and indicate that the n-hexyl side-chains of PFH tend to take bi-radial I conformation whereas the octyl side-chains of PFO tend to take bi-radial II conformation. Due to differences in the preferred conformation, side-chains in PFH show stronger tendency to embrace a neighboring backbone than those in PFO.