Electron-phonon coupling and vibrational properties of size-selected linear carbon chains by resonance Raman scattering

Universal vibrational anharmonicity in carbyne-like materials

Carbyne, an infinite linear chain of carbon atoms, is the truly one-dimensional allotrope of carbon. While ideal carbyne and its fundamental properties have remained elusive, carbyne-like materials such as carbyne chains confined inside carbon nanotubes are available for study. Here, we probe the longitudinal optical phonon (C mode) of confined carbyne chains by Raman spectroscopy up to the third overtone. We observe a strong vibrational anharmonicity that increases with decreasing C mode frequency, reaching up to 8% for the third overtone. Moreover, we find that the relation between vibrational anharmonicity and C mode frequency is universal to carbyne-like materials, including ideal carbyne. This establishes experimentally that carbyne and related materials have pronounced anharmonic potential landscapes which must be included in the theoretical description of their structure and properties.

Signature of electronically excited states in Raman spectra of azobenzene derivatives. Computational and experimental approaches

Raman spectroscopy can provide highly sensitive and detailed information about the structural fingerprint of molecules, enabling their identification. In this study, our aim is to understand the enhanced intensity observed in experimental Raman measurements. Five azobenzene derivatives were selected, each substituted with different functional groups, for both experimental and theoretical investigations. To reproduce the experimental trend, we employed various levels of theory using the QM-DFT approach. Theoretical results were compared to experimental data through both qualitative and quantitative analyses. A good correlation between theoretical and experimental results was achieved when considering electronic transitions to predict the theoretical Raman spectra and interpret the experimental data. Our theoretical results indicate that even dark (nπ*) transitions, which are forbidden and have an oscillator strength close to zero, can have a signature in the Raman spectra due to the resonance effect with incident energy. Additionally, the vibrational modes stimulated by the presence of ππ* bright states, being at the pre-resonance with the incident energy, was clearly separated from the vibrational frequencies of the dark states, which was evinced in the Raman fingerprint. Theoretical Raman spectra of azobenzene derivatives, substituted with push-pull moieties, revealed contributions from the charge transfer transitions (nπ*CT, ππ*CT) as well as back-donation of electron density, observed for the first time in an azobenzene derivative. Our protocol, proposing a quantitative and qualitative overlap between theoretical and experimental data, confirms the presence of combination modes between vibrational levels and electronically excited states.

Cinnamomum-Longepaniculatum-Leaves-Based Fe-N Doped Porous Carbon as an Effective Oxygen Reduction Catalyst

Developing low-cost, efficient, and scalable non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) remains a critical challenge in the field of energy conversion. Among various candidates, Fe-N-doped carbon materials have garnered attention as promising alternatives to commercial Pt/C catalysts for ORR. In this study, we report an Fe-N catalyst synthesized by incorporating iron phthalocyanine with Cinnamomum longepaniculatum waste leaves as the carbon source. This catalyst exhibited an excellent four-electron ORR activity and the half-wave potential (E1/2) reaches 0.875 V, which was superior to that of commercial Pt/C (E1/2 = 0.864 V). Additionally, the catalyst exhibits superior methanol tolerance and stability compared to commercial Pt/C. This approach, which utilizes biomass waste for the synthesis of electrocatalysts, not only provides an effective solution for reducing environmental waste but also addresses the issue of sluggish cathodic ORR kinetics in fuel cells, making it suitable for low-cost, large-scale industrial production.

Exploring the Growth Dynamics of Size-Selected Carbon Atomic Wires with In Situ UV Resonance Raman Spectroscopy

Short carbon atomic wires, the prototypes of the lacking carbon allotrope carbyne, represent the fundamental 1D system and the first stage in carbon nanostructure growth, which still exhibits many open points regarding their growth and stability. An in situ UV resonance Raman approach is introduced for real-time monitoring of the growth of carbon atomic wires during pulsed laser ablation in liquid without perturbing the synthesis environment. Single-chain species' growth dynamics are tracked, achieving size selectivity by exploiting the peculiar optoelectronic properties of carbon wires and the tunability of synchrotron radiation. Diverse solvents are systematically explored, finding size- and solvent-dependent production rates linked to the solvent's C/H ratio and carbonization tendency. Carbon atomic wires' growth dynamics reveal a complex interplay between formation and degradation, leading to an equilibrium. Water, lacking in carbon atoms and reduced polyynes solubility, yields fewer wires with rapid saturation. Organic solvents exhibit enhanced productivity and near-linear growth, attributed to additional carbon from solvent dissociation and low relative polarity. Exploring the dynamics of the saturation regime provides new insights into advancing carbon atomic wires synthesis via PLAL. Understanding carbon atomic wires' growth dynamics can contribute to optimizing PLAL processes for nanomaterial synthesis.

Selective excitation of vibrations in a single molecule

The capability to excite, probe, and manipulate vibrational modes is essential for understanding and controlling chemical reactions at the molecular level. Recent advancements in tip-enhanced Raman spectroscopies have enabled the probing of vibrational fingerprints in a single molecule with Ångström-scale spatial resolution. However, achieving controllable excitation of specific vibrational modes in individual molecules remains challenging. Here, we demonstrate the selective excitation and probing of vibrational modes in single deprotonated phthalocyanine molecules utilizing resonance Raman spectroscopy in a scanning tunneling microscope. Selective excitation is achieved by finely tuning the excitation wavelength of the laser to be resonant with the vibronic transitions between the molecular ground electronic state and the vibrational levels in the excited electronic state, resulting in the state-selective enhancement of the resonance Raman signal. Our approach contributes to setting the stage for steering chemical transformations in molecules on surfaces by selective excitation of molecular vibrations.

Impact of Halogen Termination and Chain Length on π-Electron Conjugation and Vibrational Properties of Halogen-Terminated Polyynes

We explored the optoelectronic and vibrational properties of a new class of halogen-terminated carbon atomic wires in the form of polyynes using UV-vis, infrared absorption, Raman spectroscopy, X-ray single-crystal diffraction, and DFT calculations. These polyynes terminate on one side with a cyanophenyl group and on the other side, with a halogen atom X (X = Cl, Br, I). We focus on the effect of different halogen terminations and increasing lengths (i.e., 4, 6, and 8 sp-carbon atoms) on the π-electron conjugation and the electronic structure of these systems. The variation in the sp-carbon chain length is more effective in tuning these features than changing the halogen end group, which instead leads to a variety of solid-state architectures. Shifts between the vibrational frequencies of samples in crystalline powders and in solution reflect intermolecular interactions. In particular, the presence of head-to-tail dimers in the crystals is responsible for the modulation of the charge density associated with the π-electron system, and this phenomenon is particularly important when strong I··· N halogen bonds occur.

Size dependence of optical nonlinearity for H-capped carbon chains, H-(CC)n-H (n = 3-15): analysis of its nature and prediction for long chains

Based on a computational approach that can accurately describe their geometric structures and electronic spectra, we have theoretically studied the nonlinear optical (NLO) properties of H-capped carbon chains, H-(CC)n-H (n = 3-15), for the first time. Special attention was paid to the size dependence of the molecular (hyper)polarizability of these species through the nonlinear fitting of the data, which formed two power-law formulas of αiso(∞) = -0.206 + 0.264n1.498 and γ‖(∞) = -0.624 + 0.006n3.368 and was thoroughly discussed at the electronic structure level by in-depth wavefunction analyses. The fundamental gap (ΔE) between vertical ionization energy (VIE) and vertical electron affinity (VEA) is found to be related to the molecular (hyper)polarizability. The calculated (hyper)polarizability of the carbon chains H-(CC)n-H (n = 3-15) is more sensitive to the density functional theory (DFT) applied than to the basis set selected. The results are expected to provide theoretical guidance for the property prediction of arbitrarily long carbon chains not yet synthesized.

Disclosing Early Excited State Relaxation Events in Prototypical Linear Carbon Chains

One-dimensional (1D) linear nanostructures comprising sp-hybridized carbon atoms, as derivatives of the prototypical allotrope known as carbyne, are predicted to possess outstanding mechanical, thermal, and electronic properties. Despite recent advances in their synthesis, their chemical and physical properties are still poorly understood. Here, we investigate the photophysics of a prototypical polyyne (i.e., 1D chain with alternating single and triple carbon bonds) as the simplest model of finite carbon wire and as a prototype of sp-carbon-based chains. We perform transient absorption experiments with high temporal resolution (<30 fs) on monodispersed hydrogen-capped hexayne H─(C≡C)6─H synthesized by laser ablation in liquid. With the support of computational studies based on ground state density functional theory (DFT) and excited state time-dependent (TD)-DFT calculations, we provide a comprehensive description of the excited state relaxation processes at early times following photoexcitation. We show that the internal conversion from a bright high-energy singlet excited state to a low-lying singlet dark state is ultrafast and takes place with a 200 fs time constant, followed by thermalization on the picosecond time scale and decay of the low-energy singlet state with hundreds of picoseconds time constant. We also show that the time scale of these processes does not depend on the end groups capping the sp-carbon chain. The understanding of the primary photoinduced events in polyynes is of key importance both for fundamental knowledge and for potential optoelectronic and light-harvesting applications of low-dimensional nanostructured carbon-based materials.