Tuning the Quinoid versus Biradicaloid Character of Thiophene-Based Heteroquaterphenoquinones by Means of Functional Groups

A series of quinoidal bithiophenes (QBTs) with controlled variations in steric hindrance and electron activity of the substituents has been synthesized. Evidence of their quinoidal versus biradicaloid ground-state electronic character has been experimentally detected and coherently identified as fingerprints by spectroscopic methods such as NMR, UV–vis, multiwavelength Raman. From this analysis, alkoxy groups have been shown to strongly affect the electronic structure and the ground-state energy and stability of QBTs. Quantum-chemical calculations correctly predict the experimental spectroscopic response, even while changing the alkyl on phenone from a tertiary carbon atom to secondary to primary toward an unsubstituted phenone, further confirming the validity of the approach proposed. A control of the electronic structure accompanied by negligible variations of the optical gap of the molecules has thus been demonstrated, extending the potential use of quinoidal species in fields ranging from photon harvesting to magnetic applications

Ab Initio Calculation of the Crystalline Structure and IR Spectrum of Polymers: Nylon 6 Polymorphs

State-of-the-art computational methods in solid-state chemistry were applied to predict the structural and spectroscopic properties of the α and γ crystalline polymorphs of nylon 6. Density functional theory calculations augmented with an empirical dispersion correction (DFT-D) were used for the optimization of the two different crystal structures and of the isolated chains, characterized by a different regular conformation and described as one-dimensional infinite chains. The structural parameters of both crystalline polymorphs were correctly predicted, and new insight into the interplay of conformational effects, hydrogen bonding, and van der Waals interactions in affecting the properties of the crystal structures of polyamides was obtained. The calculated infrared spectra were compared to experimental data; based on computed vibrational eigenvectors, assignment of the infrared absorptions of the two nylon 6 polymorphs was carried out and critically analyzed in light of previous investigations. On the basis of a comparison of the computed and experimental IR spectra, a set of marker bands was identified and proposed as a tool for detecting and quantifying the presence of a given polymorph in a real sample: several marker bands employed in the past were confirmed, whereas some of the previous assignments are criticized. In addition, some new marker bands are proposed. The results obtained demonstrate that accurate computational techniques are now affordable for polymers characterization, opening the way to several applications of ab initio modeling to the study of many families of polymeric materials

Molecular Level Investigation of the Film Structure of a High Electron Mobility Copolymer via Vibrational Spectroscopy

Vibrational spectroscopy is adopted to investigate the film structure of poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)} (P­(NDI2OD-T2)) at the molecular level. Both Raman and IR spectra are measured for P­(NDI2OD-T2) solutions and films. A good match with density functional theory (DFT) calculations at the B3LYP/6-311G** level is obtained, so that the main spectral features could be assigned. No significant spectral shifts are recorded when passing from very diluted solutions to the solid state, while clear variations in the relative intensity of specific spectral markers are observed. The comparison of the spectral patterns shown by IR spectra recorded with reflection–absorption IR spectroscopy (RAIRS) and in normal transmission experiments allows to derive a structural model of the polymer. In as-cast films, or in films subjected to mild thermal treatments, below the melting point, the backbone of the polymer chains lies preferentially in the substrate plane, with the T2 units lying flat parallel to the substrate and the NDI2OD unit featuring a dihedral angle θ with the T2 unit (θ ≈ 38°). This structure and polymer orientation is consistent with reported good bulk electron mobility in vertical diodes structures and high field-effect mobility in lateral field-effect transistors. Furthermore, we observe that upon a melt-annealing treatment, a clear modification of the RAIRS spectrum occurs suggesting either a loss of the preferential orientational order of the film or a flip of some domains featuring the polymer segments tilted out of the substrate

Structural Characterization of Highly Oriented Naphthalene-Diimide-Bithiophene Copolymer Films via Vibrational Spectroscopy

Epitaxially grown highly oriented crystalline films, named form I and form II, and spin-coated films of poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)}, P­(NDI2OD-T2), have been investigated through infrared vibrational spectroscopy techniques (infrared absorption in double transmission at normal incidence (IRA-TR) and reflection absorption infrared spectroscopy at grazing angle incidence (RAIRS)) to get access to polymer chain orientation and structure. An analytic model to correlate the experimental intensities of the IR bands with structural parameters has been developed and applied for the three film morphologies. While spin-coated and form I films show P­(NDI2OD-T2) chains lying parallel to the substrate in the face-on arrangement, form II films feature a structure with chains tilted out from the surface. The combined experimental and theoretical methodology gives insights into the local molecular orientations of naphthalene diimide (NDI2OD) and bithiophene (T2) counits. This approach can be easily extended to a variety of organic polymer semiconductors, allowing one to directly correlate molecular structure to properties such as charge transport, which is of fundamental relevance for developing quantitative models for applications in organic electronics and photovoltaics

π‑Conjugation and End Group Effects in Long Cumulenes: Raman Spectroscopy and DFT Calculations

We have investigated the structure and spectroscopic properties of cumulenic carbon chains, focusing on the peculiar π-conjugation properties and end-group effects that influence their behavior. With support from Density Functional Theory (DFT) calculations, we have analyzed the IR and Raman spectra of cumulenes characterized by different end-capping groups and we have related them to the bond length alternation (BLA) pattern and local spectroscopic parameters associated with the CC bonds along the sp-carbon chain. For cumulenes we observe a breakdown of the correlation existing in polyynes among frequencies, Raman intensities of the R line (longitudinal CC stretching modes), and BLA. While the low R line frequency and equalized CC bonds would indicate the “metallic” character of cumulenic species, we obtain an unusually strong Raman intensity, which is typical of bond-alternated (semiconductive) structures. DFT calculations reveal that this is a consequence of π-electron conjugation, which markedly extends from the sp-carbon chain to the aryl rings belonging to the end groups. These findings suggest the existence of a strong electronic, vibrational and structural coupling between sp-carbon chains and sp²-carbon species, which could play a key role in nanostructured sp/sp²-hybrid carbon materials (e.g., linear carbon chains coupled to graphene domains). Within this context, Raman spectroscopy is a valuable tool for the detailed characterization of the molecular properties of this kind of materials

Adding Four Extra K‑Regions to Hexa-<i>peri</i>-hexabenzocoronene

A multistep synthesis of hexa-<i>peri</i>-hexabenzo­coronene (HBC) with four additional K-regions was developed through a precursor based on two benzotetraphene units bridged with <i>p</i>-phenylene, featuring preinstalled zigzag moieties. Characterization by laser desorption/ionization time-of-flight mass spectrometry, Raman and IR spectroscopy, and scanning tunneling microscopy unambiguously validated the successful formation of this novel zigzag edge-rich HBC derivative. STM imaging of its monolayers revealed large-area, defect-free adlayers. The optical properties of the modified HBC were investigated by UV/visible absorption spectroscopy