Thermochromism in Oligothiophenes: The Role of the Internal Rotation

On the origin of the solid-state thermochromism and thermal fatigue of polycyclic overcrowded enes

Aimed at unraveling the relative contribution of the folding, twisting and bending in the mechanism of the solid-state thermochromism of overcrowded polycyclic aromatic enes (PAEs), the structures of two typical heteromeric and homomeric representatives, 2-(thioxanthen-9-ylidene)indane-1,3-dione (1) and 9,9'-bi-9(10H)-anthracenylidene-10,10'-dione (bianthrone, 2), were studied by temperature-resolved single crystal X-ray diffraction (120-530 K) and solid-state UV-visible spectroscopy. Aside from negligibly small unfolding of the tricyclic moiety of 1, this first direct diffraction study of the high-temperature structures of solid PAEs did not unravel any significant and detectable changes in the time- and space-averaged intramolecular structures, thus, showing that the PAE-type thermochromism is not due to phase transitions or to major and permanent molecular distortions of a large portion of the material that would be caused by folding, twisting and/or bending. Instead, the experimental observations and theoretical modeling indicated that the color change is probably due to a dynamic process, where the absorption spectrum changes as a result of enhanced thermal oscillations of the two halves of the molecules around the central bridge. In addition to the reversible coloration, we also observed irreversible processes of thermal fatigue that afford stable chemical products that absorb in the visible region. We showed that the stable products are conductive and they act electrocatalytically toward oxidation of several biomarkers.

The Role of Linear Alkyl and Alkoxy Side Chains in the Modulation of the Structure and Electrical Properties of Bithiophene:a Theoretical Study

In the present work, we have studied from a theoretical perspective the geometry and electronic properties of 2,2′-bithiophene (BT) and its derivatives 3,4′-alkyl-2,2′-bithiophene (3,4′ABT) and 3,4′-alkoxy-2,2′-bithiophene (3,4′OABT). Properties such as planarity, bond lengths, electron density, highest occupied molecular orbital → lowest unoccupied molecular orbital (HOMO → LUMO) excitation energy and π-delocalization energy, which are related to the electrical conductivity, were calculated and analyzed as a function of both the nature and length of the substituent. The oxidation process was also studied for the single-polaronic state. The ionization potential and the intramolecular reorganization energy were calculated following the semiclassical Marcus treatment. As a conclusion, the introduction of alkoxy chains in 3,4′-positions improves the electrical properties with respect to the bithiophene molecule and the corresponding alkyl derivatives.

Twisting of Conjugated Oligomers and Polymers: Case Study of Oligo- and Polythiophene

Interring twisting (change in the dihedral angle between conjugated rings) of polythiophene was studied theoretically using periodic boundary conditions (PBC) at the B3LYP/6-31G(d) level. We find that the band gap of polymers is strongly dependent on the interring twist angle; yet twisting requires very little energy. A twist of 30 degrees increases the band gap by 0.75 eV in polythiophene, while requiring only 0.41 kcal mol(-1) per monomer unit. Such a small energetic value is of the order of crystal packing or van der Waals forces. These results are compared with calculations performed on model oligomers. Sexithiophene, its radical cations, and its dication are optimized at 0-180 degrees end-to-end twist angles (which correspond to 0-36 degrees interring dihedral angles) using the B3LYP/6-31G(d) method. The theoretical results suggest that the HOMO-LUMO gap, ionization potential, and charge distribution of oligomers are strongly dependent on twisting, whereas, similar to the case of polythiophene, twisting of neutral oligothiophenes costs very little energy. In the case of the radical cation, the lowest energy transition is shifted to a longer wavelength region on twisting, while the second-lowest energy transition is shifted to a shorter wavelength region. This implies that twisted, doped conducting polymers (modeled here by an oligomer radical cation), in contrast to planar, doped polymers, should be transparent within a certain optical window (in the far-visible region, at approximately 1.5 eV). This observation is explained on the basis of changes in the shape and overlap of the frontier molecular orbitals.