Combined Raman and Computational Study of a Novel Series of Macrocyclic π-Conjugated Diacetylene-Bridged α-Linked Oligothiophenes

Abstract

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 diacetylenicbridged 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 CdC 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 +160°C, 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°C, 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 π 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 π 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.

Polymorphism in intercalated poly(vinylidene fluoride)/clay nanocomposites

“…The well‐accepted effective conjugation coordinate (ECC) model developed by Zerbi and co‐workers accounts for the observation of a limited number of overwhelmingly strong Raman scatterings, even for systems with complex chemical structures 35f. 36e This model postulates the existence of a unique collective CC/CC stretching mode strongly involved in the electron–phonon coupling mechanism which characterizes these one‐dimensional π‐conjugated chains 39.…”

Application of Raman spectroscopy and quantum chemistry for featuring the structure of positively charged species in macrocyclic π‐conjugated diacetylene‐bridged oligothiophenes

“…10,11 With the main goal of obtaining relevant structural and molecular information, we have previously studied the Raman spectra of these macrocyclic -conjugated molecular materials, 12 having found that these compounds show unusual features directly related to the degree of -electron delocalization in neutral state. 13 -21 The correlation between the experimental Raman wavenumber and intensity data from one macrocycle to another has allowed, in recent work, the estimation of their effective conjugational properties.…”

“…Furthermore, the comparison of the Raman data for the macrocycles and those obtained for a homologous series of openchain oligothiophenes intercalated by a central diacetylene spacer, aided by some Raman vibrational calculations on model systems, has revealed the occurrence of a broadening and/or splitting of the main scatterings upon cyclization, seemingly due to the coexistence of oligothienyl sequences with different conformational arrangements in the same macrocycle. 12 Accordingly, effective -conjugation seems to be scarcely dependent on, infact almost independent of, the molecular conformation of the neutral macrocycle. 12 Nowadays, the completion of this research requires the study of the spectroscopic features of the oxidized forms since many of the applications of these macrocyclic systems are based on the interplay between neutral and stable oxidized species.…”

“…12 Accordingly, effective -conjugation seems to be scarcely dependent on, infact almost independent of, the molecular conformation of the neutral macrocycle. 12 Nowadays, the completion of this research requires the study of the spectroscopic features of the oxidized forms since many of the applications of these macrocyclic systems are based on the interplay between neutral and stable oxidized species. On the other hand, it is difficult to assess detailed structural information on the oxidized species in terms of bond lengths and angles by means of experimental techniques only.…”

Fourier Transform Raman and DFT Study of Three Annulated Oligothiophenes with Different Molecular Shapes

“…www.chemphyschem.org stitution patterns), [16] in most previous articles the systems under study were different homologous series of a-linked oligoheterocycles (i.e. either with linear or cyclic arrangements of the successive p-conjugated rings), but not annulated systems such as acenes or heteroacenes, for which the currently available Raman scattering information is quite scarce in spite of their readily growing relevance in the field of molecular electronics.…”