Ruchi Shukla

Oxidative C−C Bond Formation (Scholl Reaction) with DDQ as an Efficient and Easily Recyclable Oxidant

DDQ in the presence of an acid is known to oxidize a variety of aromatic donors to the corresponding cation radicals. Herein, we now demonstrate that the DDQ/H(+) system can be effectively utilized for the oxidative C-C bond formations or biaryl synthesis. The efficient preparation of a variety of polyaromatic hydrocarbons including graphitic hexa-peri-hexabenzocoronenes, ease of isolation of the clean products, and ready regeneration of DDQ from easily recovered reduced DDQ-H(2) advances the use of DDQ/H(+) for Scholl reactions.

Probing the arenium-ion (protontransfer) versus the cation-radical (electron transfer) mechanism of Scholl reaction using DDQ as oxidant.

DDQ/H(+) system readily oxidizes a variety of electron donors with oxidation potential as high as approximately 1.7 V to the corresponding cation radicals. A re-examination of the controversial arenium-ion versus cation-radical mechanisms for Scholl reaction using DDQ/H(+) together with commonly utilized FeCl(3) as oxidants led us to demonstrate that the reaction proceeds largely via a cation-radical mechanism. The critical experimental evidence in support of a cation-radical pathway for the Scholl reaction includes the following: (i) There is no reaction in Scholl precursors in a mixture of dichloromethane and various acids (10% v/v). (ii) The necessity to use powerful oxidants such as ferric chloride (FeCl(3)) or DDQ/H(+) for Scholl reactions is inconsistent with the arenium-ion mechanism in light of the fact that aromatization of the dihydro intermediates (formed via arenium-ion mechanism) can be easily accomplished with rather weak oxidants such as iodine or air. (iii) Various Scholl precursors with oxidation potentials <or=1.7 V vs SCE undergo ready oxidative C-C bond formation with DDQ/H(+) as oxidant, whereas Scholl precursors with oxidation potentials greater than >1.7 V vs SCE do not react. (iv) Finally, the feasibility of the dicationic intermediate, formed by loss of two electrons, has been demonstrated by its generation from a tetraphenylene derivative using DDQ/H(+) as an oxidant.

Synthesis, Optical, and Electronic Properties of Soluble Poly-p-phenylene Oligomers as Models for Molecular Wires

A homologous series of soluble poly-p-phenylenes containing up to eight phenylene moieties (PP(2)-PP(8)) with branched iso-alkyl (or bis-n-alkylmethyl) groups has been synthesized and the structure-property relationship with regards to various optical and electronic properties established. All electronic and optoelectronic properties of poly-p-phenylenes followed a 1/n relationship (where n is number of phenylene moieties) with the increasing number of phenylene moieties. The low-energy electronic transition of the PP(2)-PP(7) cation radicals, generated either by laser-flash photolysis or by chemical oxidation, varied as well according to the inverse (1/n) relationship. The observed inverse relationship of all measured electronic and optoelectronic properties against the increasing number of phenylene units in various PP(n)s, as well as X-ray crystallography of both neutral and a cation-radical salt of a representative tetra-p-phenylene oligomer allows us to demonstrate that the effective conjugation length in poly-p-phenylenes is, in part, controlled by the increasing number of interactions of ortho hydrogens which may prevent simultaneous planarization of the continuous arrays of a large number of phenylene moieties.

Cooperative interaction of hydronium ion with an ethereally fenced hexaarylbenzene-based receptor: an NMR and theoretical study.

Using (1)H and (13)C NMR and DFT calculations, the structure and interactions of the symmetric ethereally fenced hexaarylbenzene receptor 1 with hydronium ions were studied. Both 1 and its equimolecular complex 1.H(3)O(+) exhibit C(3v) symmetry. According to DFT, two similar optimal structures of the complex exist, the more stable one being 15.4 kJ/mol lower in energy. The equilibrium between 1 and 1.H(3)O(+) complexes is characterized by the stabilization constant K = 1.97 x 10(6) (i.e., the binding constant eta = 6.3) according to both proton and carbon NMR spectra. The exchange dynamics between 1 and the complex measured by the delay-varied CPMG sequence had to be corrected for the internal exchange processes in both 1 (conformation change) and the complex (vacillation between the two minima). After this correction, the correlation time of exchange was found to be 4.76 x 10(-5) s. Such relatively fast exchange can be explained only by it being mediated by the excess water molecules present in the system.

Crossover from Single-Step Tunneling to Multistep Hopping for Molecular Triplet Energy Transfer

Tracking Triplet-State Transfers In devices such as organic transistors and photovoltaic cells, energy flow from donor to acceptor sites can occur via electrons that have been excited into higher electronic levels, which create a triplet state with two unpaired spins. At short distances between donor and acceptor, the transfer occurs through direct tunneling, but at longer distances, the electron “hops” in a multistep process. Vura-Weis et al. (p. 1547) used femtosecond transient absorption spectroscopy to observe this crossover in transfer mechanism directly in a series of molecules with varying bridge lengths between the donor and acceptor. Ultrafast absorption spectroscopy reveals a change in mechanism when sites for electron transfer become more distant. Triplet energy transfer (TT), a key process in molecular and organic electronics, generally occurs by either strongly distance-dependent single-step tunneling or weakly distance-dependent multistep hopping. We have synthesized a series of π-stacked molecules consisting of a benzophenone donor, one to three fluorene bridges, and a naphthalene acceptor, and studied the rate of TT from benzophenone to naphthalene across the fluorene bridge using femtosecond transient absorption spectroscopy. We show that the dominant TT mechanism switches from tunneling to wire-like hopping between bridge lengths 1 and 2. The crossover observed for TT can be determined by direct observation of the bridge-occupied state.

Molecular actuator: redox-controlled clam-like motion in a bichromophoric electron donor.

The one-electron oxidation of tetramethoxydibenzobicyclo[4.4.1]undecane (4) prompts it to undergo a clam-like electromechanical actuation into a cofacially pi-stacked conformer as established by (i) electrochemical analysis, (ii) by the observation of the intense charge-resonance transition in the near IR region in its cation radical spectrum, and (iii) by X-ray crystallographic characterization of the isolated cation radical salt (4(+*) SbCl(6)(-)).

Charge Delocalization in Self-Assembled Mixed-Valence Aromatic Cation Radicals

The spontaneous assembly of aromatic cation radicals (D(+•)) with their neutral counterpart (D) affords dimer cation radicals (D(2)(+•)). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography.

Photochemical Elimination of Leaving Groups from Zwitterionic Intermediates Generated via Electrocyclic Ring Closure of α,β-Unsaturated Anilides

Methacrylanilides, ArN(CH3)COC(CH2LG)=CH2, with allylic leaving groups (LG(-) = BocAla, PhCO2(-), PhCH2CO2(-), PhO(-)) undergo photochemical electrocyclic ring closure to produce a zwitterionic intermediate. Further reaction of the intermediate results in expulsion of the leaving group to give an alpha-methylene lactam as the major product. In addition, a lactam product that retains the leaving group is formed via a 1,5-H shift in the intermediate. Elimination of the leaving group is generally preferred, even for LG(-) = PhO(-), although in benzene as the solvent the lactam retaining the phenolate group becomes the sole photoproduct. The electrocyclic ring closure occurs in the singlet excited-state for the para-COPh-substituted anilide derivative and is not quenched by 0.15 M piperylene or 0.01 M sodium 2-naphthalenesulfonate (2-NPS) as triplet quenchers. Comparable concentrations of 2-NPS strongly quench the transient absorption of the triplet excited state observed at 450-700 nm according to laser flash photolysis experiments. In aqueous media, quantum yields for total products are insensitive to leaving group ability, and Phi(tot)(para-CO2CH3) = 0.04-0.06 at 310 nm and Phi(tot)(para-COPh) = 0.08-0.1 at 365 nm, for which Phi(isc) = 0.15.

X-ray Structural Characterization of Charge Delocalization onto the Three Equivalent Benzenoid Rings in Hexamethoxytriptycene Cation Radical

Definitive X-ray crystallographic evidence is obtained for a single hole (or a polaron) to be uniformly distributed on the three equivalent 1,2-dimethoxybenzenoid (or veratrole) rings in the hexamethoxytriptycene cation radical. This conclusion is further supported by electrochemical analysis and by the observation of an intense near-IR transition in its electronic spectrum, as well as by comparison of the spectral and electrochemical characteristics with the model compounds containing one and two dimethoxybenzene rings.

Direct observation of electron-transfer-induced conformational transformation (molecular actuation) in a bichromophoric electron donor.

With the aid of laser-flash photolysis, the one-electron oxidation of conformationally mobile tetramethoxydibenzobicyclo[4.4.1]undecane (1), using photoexcited chloranil as an oxidant, allows us to show that extended 1(+•) undergoes a conformational transformation to π-stacked folded 1(+•) on a microsecond time scale (τ ≈ 1 μs), which is at least six times longer than that found for the conformationally locked model compound.