Khushabu Thakur

ortho-Phenylenes: unusual conjugated oligomers with a surprisingly long effective conjugation length.

ortho-Phenylenes represent a fundamental but relatively unexplored class of conjugated molecular architecture. We have developed a robust synthetic approach to monodisperse o-phenylene oligomers which we have demonstrated by synthesizing a homologous series up to the dodecamer. The o-phenylenes exhibit complex conformational behavior but are biased toward a specific 2-fold-symmetric conformation which we believe corresponds to a stacked helix. Surprisingly, the series exhibits long-range delocalization, as measured by bathochromic shifts in UV/vis spectra. Although the overall magnitude of the shifts is modest (but comparable to some other classes of conjugated materials), the effective conjugation length of the series is approximately eight repeat units. The oligomers also exhibit an unusual hypsochromic shift in their fluorescence spectra with increasing length. The origin of these trends is discussed in the context of conformational analysis and DFT calculations of the frontier molecular orbitals for the series.

Sequential Oxidative Transformation of Tetraarylethylenes to 9,10-Diarylphenanthrenes and Dibenzo[g,p]chrysenes using DDQ as an Oxidant

Tetraarylethylenes can be sequentially transformed into 9,10-diarylphenanthrenes and dibenzo[g,p]chrysenes using 1 and 2 equiv of DDQ, respectively, in CH(2)Cl(2) containing methanesulfonic acid, in excellent yields. Efficient access to substituted dibenzochrysenes from tetraarylethylenes establishes the versatility of this procedure over the existing multistep syntheses of dibenzochrysenes. Moreover, the ready regeneration of DDQ from easily recovered reduced DDQ-H(2) continues to advance the use of DDQ/H(+) for the oxidative C-C bond forming reactions.

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.

Electrochemistry and Electrogenerated Chemiluminescence of π‑Stacked Poly(fluorenemethylene) Oligomers. Multiple, Interacting Electron Transfers

The electrochemistry, spectroscopy, and electrogenerated chemiluminescence (ECL) of a series of π-stacked poly(fluorenemethylene) oligomers (Fn, n = 1-6) were investigated. The pendant cofacially oriented fluorene moieties are essentially in contact with each other by Van der Waals interaction promoting electronic delocalization in these species. All six compounds give successive cyclic voltammetric one-electron (1e) oxidations in 1:1 acetonitrile/benzene (MeCN/Bz), and the multiple 1e transfer properties of all these compounds were confirmed by chronoamperometric experiments with an ultramicroelectrode and digital simulations. The potentials for oxidation of the successive 1e transfers can be explained in terms of electrostatic interactions among the fluorenes. The monomer (F1) shows one irreversible wave, while F2 shows two reversible 1e waves. F3 shows only two reversible 1e oxidation waves, which is consistent with the large energy to remove a third electron because of the greater electrostatic repulsion, so the third wave is shifted toward more positive potentials. Both F4 and F5 show three reversible 1e oxidation waves, while F6 shows four reversible 1e waves. The removal of the first electron from an oligomer becomes easier as n increases. The stability of the radical cations also increases with n. The removal of consecutive electrons from Fn can be correlated with the distance between fluorene moieties. No reduction peaks were observed except for some broad ones at ~-3.2 V vs SCE in THF, which is consitent with the wide highest occupied molecular orbital-lowest unoccupied molecular orbital gap in these compounds (absorbance at about 300 nm). No characteristic annihilation ECL signal was observed for these compounds in 1:1 MeCN/Bz mixed solvent. However, the ECL of F6 in the presence of the coreactant C(2)O(4)(2-) showed a long-wavelength ECL emission that was proposed to be electrolyzed byproduct from the radical cation.

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.

Synthesis of Doubly Annulated m-Terphenyl-Based Molecular Tweezers and Their Charge-Transfer Complexes with DDQ as a Guest

The synthesis of a doubly-annulated m-terphenyl-based tweezer platform has been developed, which affords ready incorporation of various pincer units from monobenzenoid to polybenzenoid electron donors. The complexation study with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as guest has been carried out, and the crystal structure of T-Py∩DDQ reveals the sandwich-type binding mode in the solid state.

From Wires to Cables: Attempted Synthesis of 1,3,5-Trifluorenylcyclohexane as a Platform for Molecular Cables.

Multiple molecular wires braided together in a single assembly, termed as molecular cable, are promising next-generation materials for effective long-range charge transport. As an example of the platform for constructing molecular cables, 1,3,5-trifluorenylcyclohexane (TFC) and its difluorenyl analogues (DFCs) were systematically investigated both experimentally (X-ray crystallography) and theoretically (DFT calculations). Although the syntheses of DFCs were successfully achieved, the synthesis of TFC, which involved a similar intramolecular Friedel-Crafts cyclization as the last step, was unsuccessful. An exhaustive study of the conformational landscape of cyclohexane ring of TFC and DFCs revealed that TFC is a moderately strained molecule (∼17 kcal/mol), and computational studies of the reaction profile show that this steric strain, present in the transition state, is responsible for the unusually high (∼5 years) reaction half-life. A successful synthesis of TFC will require that the steric strain is introduced in multiple steps, and such alternative strategies are being currently explored.