Forever young: polycyclic aromatic hydrocarbons as model cases for structural and optical studies

Laterally π-Extended Dithia[6]helicenes with Heptagons: Saddle-Helix Hybrid Molecules

A laterally π-extended dithia[6]helicene 1, representing an interesting saddle-helix hybrid molecule containing an unusual heptagon, has been synthesized by MoCl₅-mediated oxidative stitching of tetrakis(thienylphenyl)naphthalene precursor 2 involving reactive-site capping by chlorination and subsequent Pd-mediated dechlorination of tetrachlorinated intermediate 1-Cl₄. Highly distorted, wide helical structures of dithia[6]helicenes (1 and 1-Cl₄) were clarified by single-crystal X-ray diffraction analyses where heterochiral slipped π-π stacking was displayed in a one-dimensional fashion. Notably, theoretical studies on the thermodynamic behavior of 1 predicted an extraordinarily high isomerization barrier of 49.7 kcal·mol^-1, which enabled optical resolution and chiroptical measurements. Electronic structures of these huge helicenes were also examined by photophysical and electrochemical measurements.

Zirconacyclopentadiene-Annulated Polycyclic Aromatic Hydrocarbons

Syntheses of large polycyclic aromatic hydrocarbons (PAHs) and graphene nanostructures demand methods that are capable of selectively and efficiently fusing large numbers of aromatic rings, yet such methods remain scarce. Herein, we report a new approach that is based on the quantitative intramolecular reductive cyclization of an oligo(diyne) with a low-valent zirconocene reagent, which gives a PAH with one or more annulated zirconacyclopentadienes (ZrPAHs). The efficiency of this process is demonstrated by a high-yielding fivefold intramolecular coupling to form a helical ZrPAH with 16 fused rings (from a precursor with no fused rings). Several other PAH topologies are also reported. Protodemetalation of the ZrPAHs allowed full characterization (including by X-ray crystallography) of PAHs containing one or more appended dienes with the ortho-quinodimethane (o-QDM) structure, which are usually too reactive for isolation and are potentially valuable for the fusion of additional rings by Diels-Alder reactions.

Unlocking the electronic genome of halogenobenzenes

The drive to develop new organic materials for use in optoelectronic devices has created the need to understand the fundamental role functionalization plays concerning the electronic properties of conjugated molecules. Here density functional theory (DFT) is used to investigate how the HOMO-LUMO gaps of halogenobenzenes are affected as a function of substituent size, position, electronegativity, ionization potential, and polarizability. A detailed molecular orbital analysis is also provided. It is shown that the molecular static polarizability and ionization potential of the bound halogens are the primary physical descriptors governing the HOMO-LUMO gap within halogenobenzenes. Two secondary descriptors controlling the HOMO-LUMO gap in these materials are the aromaticity of the halogen substituted benzene rings (as monitored via the harmonic oscillator method of aromaticity index [HOMA]) and the reduced population of the halogen atomic orbitals in the frontier MOs (%X_HOMO or %X_LUMO). The molecular polarizability and aromaticity, as well as %X_HOMO and %X_LUMO, are shown to be a function of halogen electronegativity and size, as well as number and position on the ring. It is ultimately demonstrated that halogenobenzenes which are most polarizable and are either least aromatic and/or exhibit the smallest %X_LUMO (or largest %X_HOMO) values, have the smallest HOMO-LUMO gaps.

Corannulene-Helicene Hybrids: Chiral π-Systems Comprising Both Bowl and Helical Motifs

Two distinct structural elements that render π-systems nonplanar, i.e., geodesic curvature and helical motifs, have been combined into new polyarenes that contain both features. The resultant corannulene-[n]helicenes (n = 5, 6) show unique molecular dynamics in their enantiomerization processes, including inversion motions of both the bowl and the helix. Optical resolution of a corannulene-based skeletally chiral molecule was also achieved for the first time, and the influence of the bowl-motif annulation on the chiroptical properties was investigated.

A Shape-Persistent Cryptand for Capturing Polycyclic Aromatic Hydrocarbons

A shape-persistent cryptand 1, containing two face-to-face oriented electron-deficient 2,4,6-triphenyl-1,3,5-triazine units separated by approximately 7 Å, and bridged by two rigid 1,8-naphthyridine linkers and a pentaethylene oxide loop, is created for capturing polycyclic aromatic hydrocarbons. Cryptand 1 formed 1:1 complexes with PAH guest molecules, such as phenanthrene (6), anthracene (7), pyrene (8), triphenylene (9), and tetraphene (10). The single-crystal structure of complex 6⊂1 revealed that 6 was included in the cavity of 1 via face-to-face π···π stacking interactions. Soaking crystalline 1 in a toluene solution of anthracene resulted in anthracene from the toluene solution being picked up by the crystalline solid of 1.

Directed Metalation-Suzuki-Miyaura Cross-Coupling Strategies: Regioselective Synthesis of Hydroxylated 1-Methyl-phenanthrenes

A general, efficient, and regioselective synthesis of a series of hydroxylated 1-methylphenanthrenes 9 by a combined directed ortho metalation (DoM)-Suzuki-Miyaura cross-coupling-directed remote metalation (DreM) sequence is reported. Diversity to this methodology was achieved by a regioselective DoM rather than DreM reaction, affording more highly substituted phenanthrols ( Table 2 ). Application of the turbo-Grignard reagent (i-PrMgCl·LiCl) in the Ni-catalyzed Corriu-Kumada reaction gave efficient decarbamoylation ( Tables 3 and 4 ). Additional features are the TMS protecting group and halo-induced ipso-desilylation tactics applied to the regioselective synthesis of phenanthrenes ( Scheme 2 ).

Regiochemistry and substituent effects on pyrylogen and thiopyrylogen stability and electronic character

A physical organic analysis of a collection of 57 data sets generated by B3LYP/6-311+G(2d,p) geometry optimizations and TD-DFT calculations is presented. The analysis engenders recommendations for syntheses of new pyrylogen electron-transfer sensitizers that are potentially optimized for low energy absorbance and high extinction coefficients. It is argued that bathochromically shifted intense absorbance is especially useful to avoid competitive absorbance by electron-transfer substrates and for use at the low concentrations likely achievable in experimentally attractive nonpolar solvents.