Photogenerated cumulenic structure of adamantyl endcapped linear carbon chains: An experimental and computational investigation based on infrared spectroscopy

A DFT Study on the Excited Electronic States of Cyanopolyynes: Benchmarks and Applications

Highly unsaturated chain molecules are interesting due to their potential application as nanowires and occurrence in interstellar space. Here, we focus on predicting the electronic spectra of polyynic nitriles HC2m+1N (m = 0–13) and dinitriles NC2n+2N (n = 0–14). The results of time-dependent density functional theory (TD-DFT) calculations are compared with the available gas-phase and noble gas matrix experimental data. We assessed the performance of fifteen functionals and five basis sets for reproducing (i) vibrationless electronic excitation energies and (ii) vibrational frequencies in the singlet excited states. We found that the basis sets of at least triple-ζ quality were necessary to describe the long molecules with alternate single and triple bonds. Vibrational frequency scaling factors are similar for the ground and excited states. The benchmarked spectroscopic parameters were shown to be acceptably reproduced with adequately chosen functionals, in particular ωB97X, CAM-B3LYP, B3LYP, B971, and B972. Select functionals were applied to study the electronic excitation of molecules up to HC27N and C30N2. It is demonstrated that optical excitation leads to a shift from the polyyne- to a cumulene-like electronic structure.

Experimental and computational evaluation of the barrier to torsional rotation in a butadiyne-linked porphyrin dimer

The barrier to torsional rotation in a butadiyne-linked porphyrin dimer has been determined in solution using variable temperature UV-vis-NIR spectroscopy: ΔH = 5.27 ± 0.03 kJ mol(-1), ΔS = 10.69 ± 0.14 J K(-1) mol(-1). The value of ΔH agrees well with theoretical predictions. Quantum chemical calculations (DFT) were used to predict the torsion angle dependence of the absorption spectrum, and to calculate the vibronic fine structure of the S0 → S1 absorption for the planar dimer, showing that the absorption band of the planar conformer has a vibronic component overlapping with the 〈0|0〉 absorption of the perpendicular conformer. The torsion barrier in the porphyrin dimer is higher than that of 1,4-diphenylbutadiyne (calculated ΔH = 1.1 kJ mol(-1)). Crystallographic bond lengths and IR vibrational frequencies confirm that there is a greater contribution of the cumulenic resonance form in butadiyne-linked porphyrin dimers than in 1,4-diphenylbutadiyne. The DFT frontier orbitals of the twisted conformer of the porphyrin dimer are helical, when calculated in the absence of symmetry. The helical character of these orbitals disappears when D2d symmetry is enforced in the 90° twisted conformer. Helical representations of the frontier orbitals can be generated by linear combinations of the more localised orbitals from a symmetry-constrained calculation but they do not indicate π-conjugation. This work provides insights into the relationship between electronic structure and conformation in alkyne-linked conjugated oligomers.

Carbon-atom wires: 1-D systems with tunable properties

This review provides a discussion of the current state of research on linear carbon structures and related materials based on sp-hybridization of carbon atoms (polyynes and cumulenes). We show that such systems have widely tunable properties and thus represent an intriguing and mostly unexplored field for both fundamental and applied sciences. We discuss the rich interplay between the structural, vibrational, and electronic properties focusing on recent advances and the future perspectives of carbon-atom wires and novel hybrid sp-sp(2)-carbon architectures.

Raman spectroscopy as a tool to investigate the structure and electronic properties of carbon-atom wires

Graphene, nanotubes and other carbon nanostructures have shown potential as candidates for advanced technological applications due to the different coordination of carbon atoms and to the possibility of π-conjugation. In this context, atomic-scale wires comprised of sp-hybridized carbon atoms represent ideal 1D systems to potentially downscale devices to the atomic level. Carbon-atom wires (CAWs) can be arranged in two possible structures: a sequence of double bonds (cumulenes), resulting in a 1D metal, or an alternating sequence of single-triple bonds (polyynes), expected to show semiconducting properties. The electronic and optical properties of CAWs can be finely tuned by controlling the wire length (i.e., the number of carbon atoms) and the type of termination (e.g., atom, molecular group or nanostructure). Although linear, sp-hybridized carbon systems are still considered elusive and unstable materials, a number of nanostructures consisting of sp-carbon wires have been produced and characterized to date. In this short review, we present the main CAW synthesis techniques and stabilization strategies and we discuss the current status of the understanding of their structural, electronic and vibrational properties with particular attention to how these properties are related to one another. We focus on the use of vibrational spectroscopy to provide information on the structural and electronic properties of the system (e.g., determination of wire length). Moreover, by employing Raman spectroscopy and surface enhanced Raman scattering in combination with the support of first principles calculations, we show that a detailed understanding of the charge transfer between CAWs and metal nanoparticles may open the possibility to tune the electronic structure from alternating to equalized bonds.

Photophysics of threaded sp-carbon chains: the polyyne is a sink for singlet and triplet excitation

We have used single-crystal X-ray diffraction and time-resolved UV-NIR-IR absorption spectroscopy to gain insights into the structures and excited-state dynamics of a rotaxane consisting of a hexayne chain threaded through a phenanthroline macrocycle and a family of related compounds, including the rhenium(I) chlorocarbonyl complex of this rotaxane. The hexayne unit in the rhenium-rotaxane is severely nonlinear; it is bent into an arc with an angle of 155.6(1)° between the terminal C1 and C12 atoms and the centroid of the central C-C bond, with the most acute distortion at the point where the polyyne chain pushes against the Re(CO)3Cl unit. There are strong through-space excited-state interactions between the components of the rotaxanes. In the metal-free rotaxane, there is rapid singlet excitation energy transfer (EET) from the macrocycle to the hexayne (τ = 3.0 ps), whereas in the rhenium-rotaxane there is triplet EET, from the macrocycle complex (3)MLCT state to the hexayne (τ = 1.5 ns). This study revealed detailed information on the short-lived higher excited state of the hexayne (lifetime ∼1 ps) and on structural reorganization and cooling of hot polyyne chains, following internal conversion (over ∼5 ps). Comparison of the observed IR bands of the excited states of the hexayne with results from time-dependent density functional calculations (TD DFT) shows that these excited states have high cumulenic character (low bond length alternation) around the central region of the chain. These findings shed light on the complex interactions between the components of this supramolecular rotaxane and are important for the development of materials for the emerging molecular and nanoscale electronics.

Synthesis and properties of long [n]cumulenes (n ≥ 5)

This review highlights the synthesis and reactivity of longer [n]cumulenes (n ≥ 5) and summarizes their electronic and structural properties.

Vibrational characterization of dinaphthylpolyynes: a model system for the study of end-capped sp carbon chains

We perform a systematic investigation of the resonance and vibrational properties of naphthyl-terminated sp carbon chains (dinaphthylpolyynes) by combined multi-wavelength resonant Raman (MWRR) spectroscopy, ultraviolet-visible spectroscopy, and Fourier-transform infrared (FT-IR) spectroscopy, plus ab initio density functional theory (DFT) calculations. We show that the MWWR and FT-IR spectroscopies are particularly suited to identify chains of different lengths and different terminations, respectively. By DFT calculations, we further extend those findings to sp carbon chains end-capped by other organic structures. The present analysis shows that combined MWRR and FT-IR provide a powerful tool to draw a complete picture of chemically stabilized sp carbon chains.