Combined Raman and computational study of a novel series of macrocyclic π-conjugated diacetylene-bridged α-linked oligothiophenes

We present here a Raman study of a series of macrocyclic molecular materials containing various symmetrically butylated terthienyl or quinquethienyl segments connected through diacetylenic bridges, in the neutral state as solids. Comparison of the Raman features of the macrocycles with those of various linear diacetylenic-bridged oligothiophenes of short chain lengths suggests the coexistence in the macrocycles of different molecular conformations. In particular, Raman bands around 2180 cm(-1) (due to the diacetylenic bridges) and in the 1550-1450 cm(-1) region (due to the aromatic C=C stretchings) of the macrocycles are broader than those of the linear compounds, as a consequence of frequencies and intensity changes on cyclization, which qualify them as structure-sensitive marker bands. In addition, these Raman bands undergo frequency downshifts on going from -170 to +160degreesC, which is also consistent with conformational changes of low energy. This thermal study reveals, at the molecular level, that the compounds studied are stable between - 170 and + 160 degreesC, which is important for their practical use in technological applications. DFT and HF molecular geometries and vibrational calculations on suitable models indicate that the rotation of two oligothienyl segments around a central diacetylenic bridge induces almost negligible changes either in the skeletal bond distances or in the Raman scattering profile, thus not affecting the degree of pi conjugation of the macrocycle. This result is clearly attributable to the cylindrical symmetry of the diacetylenic spacer. On the other hand, calculations performed for different molecular arrangements of the thienyl units along a given oligothienyl brick predict higher spectral changes, in agreement with what is actually found. Nonetheless, the overall pi conjugation of the macrocycles seems to have very little dependence on the molecular conformation and to be primarily determined by the number of building blocks and the chain length of the oligothienyl segments.