Understanding the Fundamental Role of π/π, σ/σ, and σ/π Dispersion Interactions in Shaping Carbon‐Based Materials

Noncovalent interactions involving aromatic rings, such as π‐stacking and CH/π interactions, are central to many areas of modern chemistry. However, recent studies proved that aromaticity is not required for stacking interactions, since similar interaction energies were computed for several aromatic and aliphatic dimers. Herein, the nature and origin of π/π, σ/σ, and σ/π dispersion interactions has been investigated by using dispersion‐corrected density functional theory, energy decomposition analysis, and the recently developed noncovalent interaction (NCI) method. Our analysis shows that π/π and σ/σ stacking interactions are equally important for the benzene and cyclohexane dimers, explaining why both compounds have similar boiling points. Also, similar dispersion forces are found in the benzene???methane and cyclohexane???methane complexes. However, for systems larger than naphthalene, there are enhanced stacking interactions in the aromatic dimers adopting a parallel‐displaced configuration compared to the analogous saturated systems. Although dispersion plays a decisive role in stabilizing all the complexes, the origin of the π/π, σ/σ, and σ/π interactions is different. The NCI method reveals that the dispersion interactions between the hydrogen atoms are responsible for the surprisingly strong aliphatic interactions. Moreover, whereas σ/σ and σ/π interactions are local, the π/π stacking are inherently delocalized, which give rise to a non‐additive effect. These new types of dispersion interactions between saturated groups can be exploited in the rational design of novel carbon materials.     

Efficient Self‐Contained Photoacid Generator System Based on Photochromic Terarylenes

A series of highly sensitive neutral photoacid generators (PAGs) based on photochromic terarylenes was prepared. Like the example presented herein, these compounds show a subsequent thermal elimination of a Brønsted acid after a light‐triggered 6π‐electrocyclization, concomitant with the hexatriene aromatization. A novel type of molecular systems was developed, in which one thiazolyl moiety was replaced by a thienyl group. Depending on the solvents and on the nature of the acid source, the quantum yield (QY) for acid generation could reach up to 0.6. Comparative studies on the acid source clearly showed that aromatic leaving groups tend to extinguish the molecular system photoefficiency. A second type was also prepared, in which the nature of the hetero‐aromatic rings were identical to our previous example, but their sequence was modified. Therefore, a second level of improvement was achieved in nonpolar solvents, pushing the QY value up to 0.7. Finally, we demonstrated the mesylic acid‐releasing PAG as a photocatalyst in a chemically amplified positive resist system.     

A Multifunctional Photoswitch: 6π Electrocyclization versus ESIPT and Metalation

A terthiazole‐based molecular switch associating 6π electrocyclization, excited state intramolecular proton transfer (ESIPT), and strong metal binding capability was prepared. The photochemical and photophysical properties of this molecule and of the corresponding nickel and copper complexes were thoroughly investigated by steady‐state and ultrafast absorption spectroscopy and rationalized by DFT/TDDFT calculations. The switch behaves as a biphotochrome with time‐dependent photochemical outcome and displays efficient ESIPT‐based fluorescence photoswitching. Both photochemical reactions are suppressed by nickel or copper metalation, and the main factors contributing to the quenching of the electrocyclization are discussed.     

Nonsymmetrical Dithienylethenes Bearing Dithieno[3,2‐b:2′,3′‐d]thiophene Units with Photochromic Performance in the Crystalline Phase

Four novel nonsymmetrical photochromic diarylethene compounds containing dithieno[3,2‐b:2′,3′‐d]thiophene units were designed and synthesized to investigate their photochromic properties. All these molecules adopt a photoactive antiparallel conformation in single crystals, as revealed by X‐ray crystallographic analysis, and exhibit excellent photochromism in solution as well as in the crystalline phase.     

On the Reaction of Glycerol Dehydratase with But‐3‐ene‐1,2‐diol

Intriguing inactivation : Calculations suggest that the ability of relatively high‐energy radical intermediates to inactivate glycerol dehydratase (GDH) may reflect a general and hitherto unidentified inactivation mechanism in the reaction of coenzyme B₁₂‐dependent enzymes and 3‐unsaturated 1,2‐diols (see scheme; AdoCbl: adenosylcobalamin or coenzyme B₁₂).

     

Kinetic Stabilization and Reactivity of π Single‐Bonded Species: Effect of the Alkoxy Group on the Lifetime of Singlet 2,2‐Dialkoxy‐1,3‐diphenyloctahydropentalene‐1,3‐diyls

Kinetic stabilization and reactivity of π single‐bonded species have been investigated in detail by generating a series of singlet 2,2‐dialkoxy‐1,3‐diphenyloctahydropentalene‐1,3‐diyls ( DR s). The lifetime at 293 K in benzene was found to increase when the carbon chain length of the alkoxy groups was increased; 292 ns ( DRb ; OR=OR′=OCH₃) <880 ns ( DRc ; OR=OR′=OC₂H₅) <1899 ns ( DRd ; OR=OR′=OC₃H₇) ≈2292 ns ( DRe ; OR=OR′=OC₆H₁₃) ≈2146 ns ( DRf ; OR=OR′=OC₁₀H₂₁). DRh (OR=OC₃H₇, OR′=OCH₃; 935 ns) with the mixed‐acetal moiety is a longer‐lived species than another diastereomer DRg (OR=OCH₃, OR′=OC₃H₇; 516 ns). Activation parameters determined for the first‐order decay process reveal that the enthalpy factor plays a crucial role in determining the energy barrier of the ring‐closing reaction, that is, from the π‐bonding to the σ‐bonding compounds. Computational studies using density functional theory provided more insight into the structures of the singlet species with π single‐bonded character and the transition states for the ring‐closing reaction, thereby clarifying the role of the alkoxy group on the lifetime and the stereoselectivity of the ring‐closing reaction.     

Controlled Heterocyclization/Cross‐Coupling Domino Reaction of β,γ‐Allendiols and α‐Allenic Esters: Method and Mechanistic Insight for the Preparation of Functionalized Buta‐1,3‐dienyl Dihydropyrans

Starting from β,γ‐allendiols and α‐allenic acetates, a chemo‐ and regiocontrolled palladium‐catalyzed methodology has provided access to enantiopure 3,6‐dihydropyrans that bear a buta‐1,3‐dienyl moiety. Thus, it has been demonstrated for the first time that the preparation of six‐membered heterocycles through cross‐coupling reactions of two different allenes can be accomplished. These heterocyclization/cross‐coupling reactions have been developed experimentally and their mechanisms have additionally been investigated by a computational study.     

Application of π‐Extended Ferrocene with Varied Anchoring Groups as Photosensitizers in TiO2‐Based Dye‐Sensitized Solar Cells (DSSCs)

Two new compounds, FcCHNC₆H₄COOH ( 1 ) and FcCHNCH₂CH₂OH ( 2 ) (Fc=C₅H₄FeC₅H₅), have been synthesized and characterized by elemental analyses, IR and ¹H NMR spectroscopy, and ESI‐MS. Attempt has been made to explain their quasi‐reversible redox behavior evidenced by cyclic voltammetry using density functional theory (DFT) calculations. Light‐harvesting properties of both the compounds and also the starting material, FcCHO ( 3 ), have been studied using these compounds as photosensitizers in TiO₂‐based dye‐sensitized solar cells having either a propylene carbonate‐based electrolyte or ionic liquid electrolyte, namely, 1‐propyl‐3‐methyl imidazolium iodide (PMII). Long‐term stability of the photocurrent output of the cell using compound 1 as photosensitizer has been monitored periodically over 1400 h.     

Stabilization of a Diphosphagermylene through pπ–pπ Interactions with a Trigonal‐Planar Phosphorus Center

N‐Heterocyclic carbenes and their heavier homologues are, in part, stabilized by delocalization of the N lone pairs into the vacant p‐orbital at carbon (or a heavier Group 14 element center). These interactions are usually absent in the corresponding P‐substituted species, owing to the large barrier to planarization of phosphorus. However, judicious selection of the substituents at phosphorus has enabled the synthesis of a diphosphagermylene, [(Dipp)₂P]₂Ge, in which one of the P centers is planar (Dipp=2,6‐diisopropylphenyl). The planar nature of this P center and the correspondingly short P? Ge distance suggest a significant degree of P? Ge multiple bond character that is due to delocalization of the phosphorus lone pair into the vacant p‐orbital at germanium. DFT calculations support this proposition and NBO and AIM analyses are consistent with a Ge? P bond order greater than unity.     

Competition Between π–π and CH/π Interactions: A Comparison of the Structural and Electronic Properties of Alkoxy‐Substituted 1,8‐Bis((propyloxyphenyl)ethynyl)naphthalenes

The structural and electronic consequences of π–π and C?H/π interactions in two alkoxy‐substituted 1,8‐bis‐ ((propyloxyphenyl)ethynyl)naphthalenes are explored by using X‐ray crystallography and electronic structure computations. The crystal structure of analogue 4 , bearing an alkoxy side chain in the 4‐position of each of the phenyl rings, adopts a π‐stacked geometry, whereas analogue 8 , bearing alkoxy groups at both the 2‐ and the 5‐positions of each ring, has a geometry in which the rings are splayed away from a π‐stacked arrangement. Symmetry‐adapted perturbation theory analysis was performed on the two analogues to evaluate the interactions between the phenylethynyl arms in each molecule in terms of electrostatic, steric, polarization, and London dispersion components. The computations support the expectation that the π‐stacked geometry of the alkoxyphenyl units in 4 is simply a consequence of maximizing π–π interactions. However, the splayed geometry of 8 results from a more subtle competition between different noncovalent interactions: this geometry provides a favorable anti‐alignment of C?O bond dipoles, and two C?H/π interactions in which hydrogen atoms of the alkyl side chains interact favorably with the π electrons of the other phenyl ring. These favorable interactions overcome competing π–π interactions to give rise to a geometry in which the phenylethynyl substituents are in an offset, unstacked arrangement.     

Vertically π‐Expanded Imidazo[1,2‐a]pyridine: The Missing Link of the Puzzle

The dehydrogenative coupling of imidazo[1,2‐a]pyridine derivative has been achieved for the first time. In cases in which the most‐electron‐rich position of the electron‐excessive heterocycle was blocked by a naphthalen‐1‐yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2‐de]naphtho[1,8‐ab]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion‐radical coupling reaction, which employed dry O₂ from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2‐a]pyridine in 63 % yield. Interestingly, whereas the fluorescence quantum yield (Φ_fl) of compound 3 , despite the freedom of rotation, was close to 50 %, the Φ_fl value of flat naphthalene‐imidazo[1,2‐a]pyridine was only 5 %. Detailed analysis of this compound by using DFT calculations and a low‐temperature Shpol′skii matrix revealed phosphorescence emission, thus indicating that efficient intersystem‐crossing from the lowest‐excited S₁ level to the triplet manifold was the competing process with fluorescence.