Structural analysis of mixed α- and ß-amyrin samples.

Little is known about the structure and molecular arrangement of α- and ß-amyrin, a class of triterpenoids found within the cuticle of higher plants. Blends of both amyrin isomers with different ratios have been studied taking into consideration a combined methodology of density functional theory (DFT) calculations with experimental data from scanning electron microscopy, differential scanning calorimetry and Raman vibrational spectroscopy. Results indicate that trigonal trimeric aggregations of isomer mixtures are more stable, especially in the 1 : 2 (α : ß) ratio. A combination of Raman spectroscopy and DFT calculations has allowed to develop an equation to determine the amount of ß-amyrin in a mixed sample.

Structure determination of oleanolic and ursolic acids: a combined density functional theory/vibrational spectroscopy methodology.

Raw samples of oleanolic and ursolic acids, a class of terpenoid acids mainly found in the leaf and fruit cuticles of some plant species, can be defined as a blend of clusters of different conformers aggregated in dimers and tetramers by means of hydrogen bonds and stabilized by non-electrostatic interactions.

Microwave-assisted functionalization of carbon nanohorns with oligothiophene units with SERS activity.

Carbon nanohorns have been functionalized with oligothiophene units via the 1,3-dipolar cycloaddition reaction under microwave irradiation and solvent-free conditions. A dramatic Raman enhancement was found for one of the synthesized derivatives. Experimental and in silico studies helped to understand the enhancement, attributed to the modification of electromagnetic fields upon functionalization at the tip of the nanostructures.

Structure determination of amyrin isomers in cuticular waxes: a combined DFT/vibrational spectroscopy methodology.

We present a new methodology for the structural characterization of amyrins, a class of triterpenoids found within the fruit and leaf cuticles of higher plants. Two amyrin isomers (α and ß) have been studied taking into consideration a hydrophobic molecular scenario that mimics the cuticle matrix. DFT calculations have been employed in combination with experimental data from Raman vibrational spectroscopy and X-ray diffraction.

Functionalized branched EDOT-terthiophene copolymer films by electropolymerization and post-polymerization "click"-reactions.

The electrocopolymerization of 3,4-ethylenedioxythiophene (EDOT) with the branched thiophene building block 2,2':3',2″-terthiophene (3T) is presented as a versatile route to functional polymer films. Comparisons to blend systems of the respective homopolymers PEDOT and P3T by in situ spectroelectrochemistry and Raman spectroscopy prove the successful copolymer formation and the access to tailored redox properties and energy levels. The use of EDOT-N3 as co-monomer furthermore allows modifications of the films by polymer analogous reactions. Here, we exemplarily describe the post-functionalization with ionic moieties by 1,3-dipolar cycloaddition ("click"-chemistry) which allows to tune the surface polarity of the copolymer films from water contact angles of 140° down to 40°.

A combined MD/QM and experimental exploration of conformational richness in branched oligothiophenes.

Infrared (IR) absorption and vibrational Raman spectra of a family of branched oligothiophenes have been determined experimentally as well as theoretically. The molecular spectra have been compared to those of the linear analogues, with identification made of spectral features due to structural properties that are valued in organic solar cell applications. The theoretical spectra have been obtained through a newly developed method in which individual conformer spectra, calculated at the time-dependent DFT level in this work, are weighted by statistics extracted from classical molecular dynamics trajectories. The agreement with experiment for the resulting averaged spectra is at least as good as, and often better than, what is observed for Boltzmann-weighted spectra. As the weights are available before the costly step of spectrum calculation, the method has the additional advantage of enabling efficient approximations. For simulating the molecular dynamics of the studied α,ß-linked thiophenes and 2-methylthiophenes, high quality parameters have been derived for the CHARMM force field. Furthermore, the temperature dependence of the IR and Raman spectra has been investigated, both experimentally and theoretically.

Diradicals acting through diamagnetic phenylene vinylene bridges: Raman spectroscopy as a probe to characterize spin delocalization.

We present a complete Raman spectroscopic study in two structurally well-defined diradical species of different lengths incorporating oligo p-phenylene vinylene bridges between two polychlorinated triphenylmethyl radical units, a disposition that allows sizeable conjugation between the two radicals through and with the bridge. The spectroscopic data are interpreted and supported by quantum chemical calculations. We focus the attention on the Raman frequency changes, interpretable in terms of: (i) bridge length (conjugation length); (ii) bridge conformational structure; and (iii) electronic coupling between the terminal radical units with the bridge and through the bridge, which could delineate through-bond spin polarization, or spin delocalization. These items are addressed by using the "oligomer approach" in conjunction with pressure and temperature dependent Raman spectroscopic data. In summary, we have attempted to translate the well-known strategy to study the electron (charge) structure of π-conjugated molecules by Raman spectroscopy to the case of electron (spin) interactions via the spin delocalization mechanism.

Zethrene biradicals: how pro-aromaticity is expressed in the ground electronic state and in the lowest energy singlet, triplet, and ionic states.

A analysis of the electronic and molecular structures of new molecular materials based on zethrene is presented with particular attention to those systems having a central benzo-quinoidal core able to generate Kekulé biradicals whose stability is provided by the aromaticity recovery in this central unit. These Kekulé biradicals display singlet ground electronic states thanks to double spin polarization and have low-energy lying triplet excited states also featured by the aromaticity gain. Pro-aromatization is also the driving force for the stabilization of the ionized species. Moreover, the low energy lying singlet excited states also display a profound biradical fingerprint allowing to singlet exciton fission. These properties are discussed in the context of the size of the zethrene core and of its substitution. The work encompasses all known long zethrenes and makes use of a variety of experimental techniques, such as Raman, UV-Vis-NIR absorption, transient absorption, in situ spectroelectrochemistry and quantum chemical calculations. This study reveals how the insertion of suitable molecular modules (i.e., quinoidal) opens the door to new intriguing molecular properties exploitable in organic electronics.

Interplay of α,α- versus α,ß-conjugation in the excited states and charged defects of branched oligothiophenes as models for dendrimeric materials.

This article investigates the excited and charged states of three branched oligothiophenes with methyl-thienyl side groups as models to promote 3D arrangements. A comparison with the properties of the parent systems, linear all-α,α-oligothiophenes, is proposed. A wide variety of spectroscopic methods (i.e., absorption, emission, triplet-triplet transient absorption, and spectroelectrochemistry) in combination with DFT calculations have been used for this purpose. Whereas the absorption spectra are slightly blueshifted upon branching, both the emission spectra and triplet-triplet absorption spectra are moderately redshifted; this indicates a larger contribution of the ß-linked thienyl groups in the delocalization of the S1 and T1 states rather than into the S0 state. The delocalization through the α,ß-conjugated path was found to be crucial for the stabilization of the trication species in the larger branched systems, whereas the linear sexithiophene homologue can only be stabilized up to the dication species.

Conformational control of the electronic properties of an α-ß terthiophene: lessons from a precursor towards dendritic hyperbranched oligo- and poly-thiophenes.

Herein we investigate the conformational and electronic properties of the 2,2';3',2"-terthiophene (B3T) unit as the building block of thiophene dendrimeric materials. By means of DFT ground electronic state dihedral energy profiles, we get insight in the flexibility of B3 T as the prominent feature promoting the 3D arrangement. The presence of diverse conformers is explored by Raman and (1)H NMR spectroscopies. A theoretical estimation of the Raman and (1)H NMR spectra over the most energetically accessible conformers is found to be crucial for the appropriate assignment of the major conformer population derived from the experimental spectra. We show that energy barriers for the interconversion of conformers also play a role. Finally, the impact on the optical spectra (absorption and emission) of the α-α and α-ß connections is studied and addressed by scanning the properties of the relevant low-lying excited states. These studies highlight the relevance of the architecture of the basic molecular unit to understand charge and exciton behavior in organic semiconductors, particularly for those useful in photovoltaics.