PCCP Blog

Chiral structures interact with circularly polarised light. Gennaro Pescitelli and co-workers have investigated the effects of the close contact between handed molecules in micro-crystals in their interaction with light. The well-rounded study explains—as promised–the specific case, but does it take us closer to a general structure-property relationship regarding the preferential absorption of one handedness of light? And is such as a correlation even possible?

In ”Intermolecular exciton coupling and vibronic effects in solid-state circular dichroism: a case study” by Gennaro Pescitelli, Daniele Padula and Fabrizio Santoro, the finer details of the interaction between circularly polarised light and chiral matter is discussed. In this specific case, homochiral microcrystals are investigated experimentally and theoretically. The results presented are impressive, as the theoretical model fits and explains the experimental observation, but why is this study so important? What is this chirality? And which answers are we looking for?

Chirality and the word chiral derive from the Greek word cheir or hand: chiral can be read as handed and chirality as handedness. The definition of a chiral object is that it cannot be superimposed on its mirror image, just as our hands. Chirality is hugely important as every biological building block that makes up our hands, and indeed the rest of us, is handed. It comes in a left-hand and a right-hand form.

As it is the entire biological machinery on earth is almost exclusively left-handed. How this came about, we do not know, but it is suggested that we, in our part of space, have an abundance of circularly polarised light emitters of a certain handedness, and that this has caused the handedness of our biology. Chirality and circularly polarised light are linked. Each can be described of having a specific handedness and one handedness of light interacts with one handedness of chiral matter. An abundance of right-handed light in our part of the universe may have induced the single handedness of our biology.

In order to figure out how the homo-chirality of the biosphere on earth arose, we need to understand how circularly polarised light and chiral material interacts; exactly what Gennaro Pescitelli and co-workers are investigating. Studies like this, where experiment and highly advanced theory is used to correlate observations and the chiral structure of the material, are needed if we are to understand how chirality in a molecular framework absorbs one specific handedness of light.

We understand how regular material (not handed) and normal light (linearly polarised) interact, and how to design materials that interact with light in a desired fashion. We cannot yet design materials that absorb a specific handedness of light. The paper  ”Intermolecular exciton coupling and vibronic effects in solid-state circular dichroism: a case study” by Gennaro Pescitelli, Daniele Padula and Fabrizio Santoro takes us one step closer, it was published in of Physical Chemistry Chemical Physics 2013, 15, 795-802.

by Dr Thomas Just Sørensen

Read this fascinating study today:

Intermolecular exciton coupling and vibronic effects in solid-state circular dichroism: a case study
Gennaro Pescitelli, Daniele Padula and Fabrizio Santoro
DOI: 10.1039/C2CP43660D

It is now possible to accurately predict the density, refractive index and molar polarisability of any imidazolium-based ionic liquid thanks to the recent work of Schröder et al.

Given the availability of at least one million simple ionic liquids, predicting which ionic liquid is best suited to a given application is a significant task. Molar polarisability is a key factor in describing solvation effects and, in principle, can be determined by various quantum-mechanical methods. However, these methods are time-consuming and can only be applied to a small subset of ionic liquids.

Schroder et al. used a Designed Regression Analysis to deconstruct the molar polarisability and molar volume into atomic contributions in this recent PCCP article. They used their approach to explore how the refractive indices of various imidazolium-based ionic liquids were influenced by the length of the alkyl chains.

Read the full PCCP article today:

Polarisabilities of alkylimidazolium ionic liquids
Christian Schröder, Katharina Bica, Maggel Deetlefs and Kenneth R. Seddon
DOI: 10.1039/C3CP43867H

We know much about the energetics in solutions and of species in solution. We can directly measure most parameters, when we measure energy. When it comes to the structure of solution or in solutions, we know next to nothing. Indirect measurements are our main source of information, as we cannot see the fluctuating solution structures. Miyuki Tanaka and co-workers have performed one of the most informative studies using indirect methods to probe solvent structures that I have come across.

In a recent paper in PCCP by Miyuki Tanaka, Tomoaki Yago and Masanobu Wakasa the microscopic structure of ionic liquid solvents is investigated. The diffusion of single particles is compared to the macroscopic measure of viscosity. They find that the ionic liquids are ‘more sticky’, when looking at the diffusion of single molecules, than more commonly used solvents; although the viscosities of the two solvents are identical.

In “Local structure of ionic liquids probed by self-quenching of thiobenzophenone” the movement of single microscopic particles in solution (molecular diffusion), are compared to measurement determining how difficult a macroscopic object moves in the same solution (the viscosity). A single particle is excited using light and gains energy. The energy can be released the by collision with another particle in the solution.  By following the evolution of the population of excited particles the molecular movements can be followed.

The result presented in this paper is another piece of the puzzle we have to assemble in order to understand the microstructure of solvents, and the conundrum of solvation. Solvents are everywhere, most ubiquitous is water. We have a limited understanding of the structure of pure solvents, and know even less about the structure of complex solutions. The collection of small molecules that constitute most liquids can have an astoundingly complex structure. A structure that we have to know and understand, if we are to comprehend the complex condensed phases such as the cells that make up most living things.

At the most advanced facility for structural investigation of matter, the SLAC national accelerator laboratory, an entire team of elite scientists has been assembled ‘just’ to elucidate the structure of water. The method applied by Tanaka and co-workers is a much simpler route to understanding molecular movements in solution. When a large library of different solvents has been investigated, we will be able to deduce effects of specific solvation. Following this, we may be able to explain exactly why the ionic liquids are ‘more sticky’ than traditional solvents.

“Local structure of ionic liquids probed by self-quenching of thiobenzophenone” by Miyuki Tanaka, Tomoaki Yago and Masanobu Wakasa was published in Physical Chemistry Chemical Physics (PCCP) at the beginning of 2013: M. Tanaka, T. Yago, M. Wasaka Phys. Chem. Chem. Phys.2013, 15, 787-794

By Dr Thomas Just Sørensen

In this insightful and well-rounded review, Gianfranco Pacchioni summarises the latest knowledge regarding the metal/oxide interface. Such knowledge has become crucial in the elucidation of reaction mechanisms and catalytic properties of metal-oxide catalysts.

Thanks to increasingly sophisticated techniques, it has become known that the original chemical and structural properties of a metal nanoparticle deposit can be significantly altered upon interaction with an oxide support. Particularly for catalysts containing metal nanoparticles smaller then about 1 nm, the phenomenon is particularly significant and somewhat complex.

For a long time, it was assumed that the oxide component of heterogeneous catalysts provided an “inert” support. Pacchioni takes us through the experimental and theoretical evidence that has shown that the support is in fact not as “innocent” as first thought, and focuses on the nature of the chemical bonds formed between metal atoms and clusters and oxide surfaces.

Read this PCCP Perspective in full today:

Electronic interactions and charge transfers of metal atoms and clusters on oxide surfaces
Gianfranco Pacchioni
DOI: 10.1039/C2CP43731G

US Scientists have successfully used second harmonic generation (SHG) measurements to probe the complicated plasmon resonances arising from inter-particle interactions within nanoparticle networks.

Kenneth Knappenberger Jr. and Manabendra Chandra from Florida State University performed systematic polarization-resolved single-particle SHG measurements on solid gold nanosphere dimers.  In addition, continuous polarization variation (CPV) experiments were used to obtain single particle non-linear optical data. They were able to demonstrate the superiority of their technique over measurements that rely exclusively on linearly polarized light to study structure-specific plasmonics.

A large SHG depolarization ratio was measured when the two nanoparticles forming a dimer were in close vicinity of each other. CPV spectra of single dimers revealed large inter-dimer variations, which can only be described by including magnetic-dipolar interactions.

Their work represents an important step towards a predictive understanding of the optical properties of nanostructured materials.

Read this HOT PCCP article today:

Nanoparticle surface electromagnetic fields studied by single-particle nonlinear optical spectroscopy
Manabendra Chandra and Kenneth L. Knappenberger
DOI: 10.1039/C2CP43271D, Paper