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

Mode Robustness in Raman Optical Activity

By reformulating Raman and ROA invariants we provide ground for the definition of robust modes in ROA spectroscopy. Introduction of two parameters defining robustness helps characterization and assignment of ROA bands. Application and use of robustness parameters to [<i>n</i>]­helicenes and oxirane/thiirane derivatives are presented

Chiral Peropyrene: Synthesis, Structure, and Properties

Herein we describe the synthesis, structure, and properties of chiral peropyrenes. Using <i>p</i>-terphenyl-2,2″,6,6″-tetrayne derivatives as precursors, chiral peropyrenes were formed after a 4-fold alkyne cyclization reaction promoted by triflic acid. Due to the repulsion of the two aryl substituents within the same bay region, the chiral peropyrene adopts a twisted backbone with an end-to-end twist angle of 28° that was unambiguously confirmed by X-ray crystallographic analysis. The chiral peropyrene products absorb and emit in the green region of the UV–visible spectrum. Circular dichroism spectroscopy shows strong Cotton effects (Δε = ±100 M^–1 cm^–1 at 300 nm). The Raman data shows the expected D-band along with a split G-band that is due to longitudinal and transversal G modes. This data corresponds well with the simulated Raman spectra of chiral peropyrenes. The chiral peropyrene products also display circularly polarized luminescence. The cyclization reaction mechanism and the enantiomeric composition of the peropyrene products are explained using DFT calculations. The inversion barrier for racemization was determined experimentally to be 29 kcal/mol and is supported by quantum mechanical calculations

Chiral Peropyrene: Synthesis, Structure, and Properties

Herein we describe the synthesis, structure, and properties of chiral peropyrenes. Using <i>p</i>-terphenyl-2,2″,6,6″-tetrayne derivatives as precursors, chiral peropyrenes were formed after a 4-fold alkyne cyclization reaction promoted by triflic acid. Due to the repulsion of the two aryl substituents within the same bay region, the chiral peropyrene adopts a twisted backbone with an end-to-end twist angle of 28° that was unambiguously confirmed by X-ray crystallographic analysis. The chiral peropyrene products absorb and emit in the green region of the UV–visible spectrum. Circular dichroism spectroscopy shows strong Cotton effects (Δε = ±100 M^–1 cm^–1 at 300 nm). The Raman data shows the expected D-band along with a split G-band that is due to longitudinal and transversal G modes. This data corresponds well with the simulated Raman spectra of chiral peropyrenes. The chiral peropyrene products also display circularly polarized luminescence. The cyclization reaction mechanism and the enantiomeric composition of the peropyrene products are explained using DFT calculations. The inversion barrier for racemization was determined experimentally to be 29 kcal/mol and is supported by quantum mechanical calculations

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

Persulfurated Coronene: A New Generation of “Sulflower”

We report the first synthesis of a persulfurated polycyclic aromatic hydrocarbon (PAH) as a next-generation “sulflower.” In this novel PAH, disulfide units establish an all-sulfur periphery around a coronene core. The structure, electronic properties, and redox behavior were investigated by microscopic, spectroscopic and electrochemical methods and supported by density functional theory. The sulfur-rich character of persulfurated coronene renders it a promising cathode material for lithium–sulfur batteries, displaying a high capacity of 520 mAh g^–1 after 120 cycles at 0.6 C with a high-capacity retention of 90%

Bottom-Up Synthesis of Heteroatom-Doped Chiral Graphene Nanoribbons

Bottom-up synthesis of graphene nanoribbons (GNRs) has significantly advanced during the past decade, providing various GNR structures with tunable properties. The synthesis of chiral GNRs, however, has been underexplored and only limited to (3,1)-GNRs. We report herein the surface-assisted synthesis of the first heteroatom-doped chiral (4,1)-GNRs from the rationally designed precursor 6,16-dibromo-9,10,19,20-tetraoxa-9a,19a-diboratetra­benzo­[<i>a</i>,<i>f</i>,<i>j</i>,<i>o</i>]­perylene. The structure of the chiral GNRs has been verified by scanning tunneling microscopy, noncontact atomic force microscopy, and Raman spectroscopy in combination with theoretical modeling. Due to the presence of oxygen–boron–oxygen (OBO) segments on the edges, lateral self-assembly of the GNRs has been observed, realizing well-aligned GNR arrays with different modes of homochiral and heterochiral inter-ribbon assemblies

Helical Sense-Responsive and Substituent-Sensitive Features in Vibrational and Electronic Circular Dichroism, in Circularly Polarized Luminescence, and in Raman Spectra of Some Simple Optically Active Hexahelicenes

Four different hexahelicenes, 5-aza-hexahelicene (<b>1</b>), hexahelicene (<b>2</b>), 2-methyl-hexahelicene (<b>3</b>), and 2-bromo-hexahelicene (<b>4</b>), were prepared and their enantiomers, which are stable at r.t., were separated. Vibrational circular dichroism (VCD) spectra were measured for compound <b>1</b>; for all the compounds, electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra were recorded. Each type of experimental spectrum was compared with the corresponding theoretical spectrum, determined via Density Functional Theory (DFT). Following the recent papers by Nakai et al., this comparison allowed to identify some features related to the helicity and some other features typical of the substituent groups on the helical backbone. The Raman spectrum of compound <b>1</b> is also examined from this point of view

Heteroatom-Doped Perihexacene from a Double Helicene Precursor: On-Surface Synthesis and Properties

We report on the surface-assisted synthesis and spectroscopic characterization of the hitherto longest periacene analogue with oxygen–boron–oxygen (OBO) segments along the zigzag edges, that is, a heteroatom-doped perihexacene <b>1</b>. Surface-catalyzed cyclodehydrogenation successfully transformed the double helicene precursor <b>2</b>, i.e., 12a,26a-dibora-12,13,26,27-tetraoxa-benzo­[1,2,3-<i>hi</i>:4,5,6-<i>h</i>′<i>i</i>′]­dihexacene, into the planar perihexacene analogue <b>1</b>, which was visualized by scanning tunneling microscopy and noncontact atomic force microscopy. X-ray photoelectron spectroscopy, Raman spectroscopy, together with theoretical modeling, on both precursor <b>2</b> and product <b>1</b>, provided further insights into the cyclodehydrogenation process. Moreover, the nonplanar precursor <b>2</b> underwent a conformational change upon adsorption on surfaces, and one-dimensional self-assembled superstructures were observed for both <b>2</b> and <b>1</b> due to the presence of OBO units along the zigzag edges