S. Nagaraja Rao

Heterosupramolecular chemistry: programmed pseudorotaxane assembly at the surface of a nanocrystal.

Gold nanocrystals, stabilized by thiols covalently bound to a dibenzo[24]crown-8 moiety, have been programmed to recognize and selectively bind dibenzylammonium cations in solution. This results in a self-organization process at the surface of a nanocrystal with the assembly of a pseudorotaxane (see picture).

Hyperaromatic stabilization of arenium ions.

Benzene-cis- and trans-1,2-dihydrodiols undergo acid-catalyzed dehydration at remarkably different rates: k(cis)/k(trans) = 4500. This is explained by formation of a β-hydroxycarbocation intermediate in different initial conformations, one of which is stabilized by hyperconjugation amplified by an aromatic no-bond resonance structure (HOC(6)H(6)(+) ↔ HOC(6)H(5) H(+)). MP2 calculations and an unfavorable effect of benzoannelation on benzenium ion stability, implied by pK(R) measurements of -2.3, -8.0, and -11.9 for benzenium, 1-naphthalenium, and 9-phenanthrenium ions, respectively, support the explanation.

Heterosupramolecular Chemistry: Self‐Assembly of an Electron Donor (TiO2 Nanocrystallite)‐Acceptor (Viologen) Complex

A TiO2 nanocrystallite has been modified to recognise and selectively bind, by complementary hydrogen bonding, a uracil substrate incorporating a viologen moiety. Band-gap excitation of the self-assembled donor (TiO2 nanocrystallite)-acceptor (viologen) complex results in electron transfer. Some implications of these findings for the self-assembly of functional nanostructures containing both condensed phase and molecular components are considered.

Heterosupramolecular chemistry: toward the factory of the future

Abstract Summarised are the findings of recent studies in which a nanocrystal has been programmed to recognise and selectively bind in solution the following: a molecule, another nanocrystal, or a suitably patterned substrate. These studies form part of an ongoing effort to develop a systematic covalent and non-covalent chemistry of condensed phase and molecular components. Such a chemistry will under-pin emerging post-information technologies and point the way to the factory of the future.

Acid-catalysed aromatisation of benzene cis-1,2-dihydrodiols: a carbocation transition state poorly stablised by resonance

Acid-catalysed dehydration of 3-substituted benzene cis-1,2-dihydrodiols exhibits a Hammett plot with ρ=–8.2, consistent with reaction via a benzenonium ion-like intermediate; however, correlation of +M resonance substituents such as Me and MeO by σp rather than σ+ constants indicates a marked imbalance between resonance and inductive stabilisation of the transition state.

HETEROSUPRAMOLEKULARE CHEMIE : PROGRAMMIERTE PSEUDOROTAXAN-SELBSTORGANISATION AN EINER NANOKRISTALLOBERFLACHE

Gold-Nanokristalle, stabilisiert durch Thiole, die kovalent mit einer Kronenethereinheit verbunden sind, wurden auf das Erkennen und die selektive Bindung von Dibenzylammoniumionen in Losung programmiert. Das Ergebnis ist ein Selbstorganisationsprozes an der Nanokristalloberflache unter Bildung eines Pseudorotaxans (siehe Bild).

Heterosupramolecular chemistry: long-lived light-induced charge separation by vectorial electron flow in a heterotriad

A redox molecule (acceptor) is attached, using a surface chelate (spacer), to a semiconductor electrode (donor). Such donor-spacer-acceptor complexes, referred to as heterodyads, offer the prospect of testing important aspects of the theory of heterogeneous electron transfer (ET) at the semiconductor electrode-liquid electrolyte interface (SLI). Specifically, potentiostatically controlled ET from the conduction band of the semiconductor electrode to a redox species held at a fixed distance and orientation with respect to the SLI is possible. Extending the above approach, a modified SLI has been prepared at which potentiostatically controlled vectorial electron flow leading to long-lived charge trapping is possible. Specifically, a spacer-acceptor I-acceptor II complex is adsorbed at a semiconductor electrode to form a heterotriad. Application of a potential more negative than the potential of the conduction band at the SLI results in acceptor I mediated reduction of acceptor II. The reduced form of acceptor II is stabilized and long-lived charge trapping results. Efficient light induced charge separation by vectorial electron flow at the above modified SLI is also possible.

Heterosupramolecular chemistry and modulation of function in molecular devices

An organised heterosupramolecular assembly is prepared by attachment of linked molecular components to previously organised condensed phase nano-components. The above approach yields an intrinsic substrate through which the resultant molecular device may be modulated. Further, the modulation state can be inferred from the bulk properties of the intrinsic substrate. Specifically, we report results for a heterosupermolecule consisting of a TiO2 nanocrystallite attached to a linked viologen-quinone unit. The associated function is photo-induced vectorial electron flow. For the corresponding organised assembly we demonstrate the following: bandgap excitation of a semiconductor nanocrystallite results in light-induced vectorial electron flow; electrochemical modulation of the bulk properties of the intrinsic nanocrystalline semiconductor substrate modulates function; and the state of the intrinsic substrate provides information about the modulation state of the constituent units of the assembly.