jo048282z_si_001.cif (22.48 kB)
Synthesis and Structural, Electronic, and Optical Properties of Oligo(thienylfuran)s in Comparison with Oligothiophenes and Oligofurans
dataset
posted on 2005-02-18, 00:00 authored by Yasuo Miyata, Tohru Nishinaga, Koichi KomatsuAlternate thiophene/furan oligomers having four and six heterocycles, i.e., oligo(thienylfuran) dimer
and trimer 2 (n = 4 and 6), were newly synthesized by repetitive Stille coupling reactions. The
structural, electronic, and optical properties of these oligomers were investigated by X-ray
crystallography (for n = 4), cyclic voltammetry (CV), UV−vis and fluorescence spectroscopy, and
DFT calculations, and the results were compared with those of corresponding oligothiophenes (1)
and oligofurans (3). The inter-ring torsional energy profiles calculated for bithiophene 1 (n = 2),
thienylfuran 2 (n = 2), and bifuran 3 (n = 2) at the B3LYP/6-31G(d) level indicated that the most
stable conformers of 2 (n = 2) and 3 (n = 2) are fully coplanar with transoid structure while that
of 1 (n = 2) is twisted with a dihedral angle of 158°. In accord with this, X-ray crystallographic
analysis of 2 (n = 4) revealed that the π-conjugated system is nearly planar with the inter-ring
CC−CC dihedral angles between the thiophene and furan rings of 173.6(7)°, −177.0(7)°, and
172.6(6)°. In the packing structure, these nearly planar molecules are arranged in a herringbone
pattern. The CV on a series of oligo(thienylfuran)s 2 showed irreversible oxidation peaks at +0.90,
+0.42, and +0.29 V vs Fc/Fc+ for n = 2, 4, and 6, which were 0.15−0.18 V lower than those for
corresponding oligothiophenes 1 and were closer to those for oligofurans 3. On the other hand, the
UV−vis spectra of 2 showed the longest wavelength absorption to be almost identical with those
of the corresponding 1, and more bathochromically shifted than those of the corresponding 3. The
results of CV and UV−vis measurements were supported by DFT calculations (B3LYP/
6-311+G(2d,p)//B3LYP/6-31G(d)). Thus, oligo(thienylfuran)s 2 have HOMOs which are higher than
those of oligothiophenes 1 and close to those of 3, and HOMO-LUMO gaps which are close to those
of 1 and smaller than those of 3. In fluorescence spectra, the quantum yield of 2 increased with
elongation of the π-system (n = 2 (3.5%), 4 (19%), 6 (24%)).