Characterization of the Oxidation Products of Styryl-Substituted Terthiophenes and Sexithiophenes Using Electronic Absorption Spectroscopy and Time-Dependent DFT

Computational investigations of click-derived 1,2,3-triazoles as keystone ligands for complexation with transition metals: a review

In recent years, metal complexes of organo 1,2,3-triazole click-derived ligands have attracted significant attention as catalysts in many chemical transformations and also as biological and pharmaceutical active agents. Regarding the important applications of these metal-organo 1,2,3-triazole-based complexes, in this review, we focused on the recently reported investigations of the structural, electronic, and spectroscopic aspects of the complexation process in transition metal complexes of 1,2,3-triazole-based click ligands. In line with this, the coordination properties of these triazole-based click ligands with transition metals were studied via several quantum chemistry calculations. Moreover, considering the complexation process, we have presented comparative discussions between the computational results and the available experimental data.

A Spectroscopic and Computational Study of the Neutral and Radical Cation Species of Conjugated Aryl-Substituted 2,5-Bis(2-thien-2-ylethenyl)thiophene-Based Oligomers

A spectroscopic and computational study of a series of 2,5-bis(2-thien-2-ylethenyl) thiophene-based oligomers with a para-R-arylethenyl substituent is reported. The primary aim of this investigation is to increase understanding of how charge moves through these molecules by comparing the neutral and oxidized structures for each molecule. To this end, the B3LYP/6-31G(d) computational method was used to calculate the geometry and vibrational spectra for all molecules considered and their oxidation products. For vibrational data, mean absolute deviations for frequencies between experimental and theoretical results ranging from 2 to 18 cm-1 were obtained. Experimental Raman spectroscopy, in conjunction with calculated bond length analyses, was used to gain an insight into the position and delocalization of the charged defect on the oxidized oligomers. The relative frequencies of different ethylene stretching modes served as a particularly useful probe in this regard. It was found that the ethenyl spacers do not impede pi-electron delocalization and, therefore, give rise to a longer conjugation length relative to the corresponding terthiophenes. Furthermore, the para-R-arylethenyl substituent was found to orientate the charged defect toward a specific region of the 2,5-bis(2-thien-2-ylethenyl)thiophene conjugation path.

Raman spectroscopy of short-lived terthiophene radical cations generated by photochemical and chemical oxidation

The Raman spectra of various terthiophene radical cations are investigated; namely those of unsubstituted terthiophene and two styryl-substituted terthiophenes. Transient pump-probe resonance Raman spectroscopy is used to measure the short-lived radical cation spectra of non-end-capped 2,2':5',2''-terthiophene (3T) and 3'-[(E)-2-(4-nitrophenyl)ethenyl]-2,2':5',2''-terthiophene (NO2-pe3T). For these two compounds, the radical cations are generated via either direct photogeneration or photochemically using the electron acceptor tetracyanoethylene. The radical cation of 5,5''-dimethyl-3'-[(E)-2-phenylethenyl]-2,2':5',2''-terthiophene (DM-pe3T) is stable for up to five minutes as a result of the two alpha end caps and continuous-wave resonance Raman spectroscopy and chemical oxidation is used to obtain the spectrum of this radical cation. The resonance Raman spectra of all three terthiophene radical cations are dominated by a group of very intense bands in the low-frequency region. These bands have been assigned, by density functional theory methods, to C-S stretching modes coupled to thiophene ring deformations. These modes are significantly less intense in the sigma-dimer of NO2-pe3T [i.e. the corresponding styryl sexithiophene (NO2-pe3T)2]. This observation is attributed to a smaller change in the C--S bond order in the sexithiophene compared to the analogous terthiophene. This bond order difference may be rationalised by consideration of the singly occupied molecular orbital and lowest unoccupied molecular orbital, which are involved in the electronic transition probed by the laser excitation wavelength.

Modulation of electronic properties in neutral and oxidized oligothiophenes substituted with conjugated polyaromatic hydrocarbons

The structural and electronic properties of neutral and oxidized terthiophenes substituted with polyaromatic systems have been investigated using a combination of both Raman and electronic absorption spectroscopy in conjunction with density functional theory calculations. Naphthylethenyl terthiophene exhibits structural and electronic properties, in both the neutral and oxidized species, that are dominated by the terthiophene backbone, in a manner similar to that previously reported for phenylethenylterthiophene. Anthracenylethenyl terthiophene, on the other hand, displays properties that are dominated by the anthracene group. Unlike both phenylethenyl and naphthylethenyl terthiophene, which have electronic absorption spectra dominated by transitions between molecular orbitals that are delocalized throughout the molecules, the absorption spectrum of anthracenylethenyl terthiophene consists of a simple addition of the absorption bands of the separate terthiophene and anthracenylethene chromophores. This is the result of a spatial partitioning of its molecular orbitals that effectively electronically decouples the anthracene and terthiophene moieties. Upon oxidation, the naphthylethenylterthiophene sigma-dimerizes to form sexithiophene charged species and spectral signatures of the sexithiophene backbone are evident in both the electronic absorption and resonance Raman spectra. In contrast, these signatures are absent in the corresponding spectra of the oxidized anthracenylethenylterthiophene, suggesting that the anthracene group is the primary site of the structural changes induced by oxidation.

Torsional effects on excitation energies of thiophene derivatives induced by β-substituents: Comparison between time-dependent density functional theory and approximated coupled cluster approaches

The influence of methyl or phenyl substitution in beta-position of dioxygenated terthiophene and diphenylthiophene on the optical properties is investigated by first-principles calculations. We compare the approximated singles and doubles coupled cluster (CC2) approach with time-dependent density functional theory methods. CC2 reproduces experimental excitation energies with an accuracy of 0.1 eV. We find that the different substituents modify the inter-ring torsional angle which in turn strongly influences the excitation energies. The steric contribution to the excitation energies have been separated from the total substituent effects.

Tuning from π,π* to Charge-Transfer Excited States in Styryl-Substituted Terthiophenes: An Ultrafast and Steady-State Emission Study

The steady-state and transient emission properties of unsubstituted terthiophene and a series of 3'-[E-2-(4-R-phenyl)ethenyl]-2,2':5',2' '-terthiophenes (where R = H, MeO, NH(2), CN, NMe(2), NO(2)) have been examined. The R = NO(2) compound is nonemissive at room temperature in all solvents but cyclohexane. All of the other compounds show measurable steady-state emission in a variety of solvents. The behavior of these spectra may be split into two groups. The first group, those substituted compounds with R = CN, NH(2) and NMe(2), show solvatochromic behavior, where their Lippert-Mataga plots suggest changes in dipole upon photoexcitation ranging from 12.5 to 16.0 D. For the second group, where R = H and MeO (and unsubstituted terthiophene as well), the Lippert-Mataga plots indicate dipole moment changes ranging from 0 to 7.9 D. The difference in behavior between the two groups of emissive compounds can be attributed to a charge-transfer character of the emitting state in the first group. This conclusion is supported by density functional theory calculations, which show that the frontier MOs in the group one compounds are spatially separated whereas those of group two have frontier MOs that are delocalized over both the styryl and terthiophene moieties. Picosecond time-resolved fluorescence spectroscopy reveals that unsubstituted terthiophene has the shortest emission lifetime of 140 ps in acetonitrile. For the styryl substituted terthiophenes, the lifetimes are much longer and range from 320 to 670 ps for R = CN and NMe(2) respectively, a result that can be explained in terms of a smaller rate of intersystem crossing in these compounds.

The effect of oxidation on the structure of styryl-substituted sexithiophenes: A resonance Raman spectroscopy and density functional theory study

The structures and vibrational properties of a series of styryl-substituted sexithiophenes and their charged species have been examined using resonance Raman spectroscopy in conjunction with density functional theory calculations. The calculated geometries of the radical cations and dications indicate that the quinoidal charged defects are more strongly localized in the center of the thiophene backbone than is observed in other sexithiophenes. This defect confinement, induced by the positions of the styryl substituents, is particularly evident in the dication species. However, the defect confinement weakens when alkoxy groups are added onto the phenyl rings by causing the extension of the charged defect into the styryl groups. The Raman spectra of the neutral styryl sexithiophenes are dominated by intense thiophene symmetrical stretching modes in both the measured and predicted spectra. Oxidation generates radical cations and dications, both of which can be observed in the solution state resonance Raman spectra. Unlike other sexithiophenes, which generally show a downshift of the intense thiophene stretching mode from the radical cation to the dication, a small upshift is observed for the styryl-substituted sexithiophenes. The theoretical spectra predict an insignificant change during this transition and the eigenvector for this mode reveals that it is localized over the same area occupied by the confined defect. In contrast, the solid state resonance Raman spectra of electrochemically oxidized films reveal evidence of solely radical cations and there is an appreciable downshift of the intense thiophene stretching mode compared with the corresponding mode in the solution spectra. This implies that the increase in the effective conjugation length from the solution to the solid state is greater for the radical cations than for the neutral species. It therefore appears that the radical cations form pi stacks in the solid film and the resulting intermolecular interactions effectively allow a further extension of the electron delocalization.

Towards functionalized poly(terthiophenes): regioselective synthesis of oligoether-substituted bis(styryl)sexithiophenes

A variety of new bis(oligo(oxyethylene)styryl)sexithiophenes have been prepared by chemical oxidation of ether-substituted styrylterthiophenes with FeCl3. In all cases dimers are formed in high yields, rather than the expected polymers. In addition, although three different regioisomers can potentially be formed from such an oxidation, the isolable products are shown to consist of only the head-to-head regioisomer. Theoretical calculations on alkoxystyrylterthiophenes show that this can be understood in terms of an uneven electron spin density distribution at the two alpha-positions available for polymerization. Electron density calculations on the resulting head-to-head alkoxystyrylsexithiophenes show that the spin density is concentrated in the core of the molecule rather than at the alpha-positions, a result that explains the absence of significant amounts of higher oligomers.