Exciton-coupled Circular Dichroism Spectra Research Articles

Synthesis and Luminescence Properties of Self-Assembled Lanthanide Complexes with an EDTA-Type Chelating Ligand in Aqueous Ethanol Solution

Abstract A hexadentate chelating ligand based on ethylenediaminetetraacetic acid (EDTA) with four cholesteryl groups (L2) was designed and synthesized. L2 forms stable 1:1 complexes with lanthanide ions, and the resulting L2-lanthanide complexes form stable self-assemblies with an average particle size of ∼50 nm in aqueous ethanol solution (20 wt%). Under these conditions, the L2-lanthanide complexes exhibit amphiphilic properties and long-lived luminescence. Sodium 2-naphthoate was added as a guest anion to form a 1:2 complex (anion:Ln) with a lanthanide complex resulting in exciton-coupled circular dichroism spectra. 4-Alkyl benzoates were also used as guest anions, and the intensity of the sensitized luminescence largely depends in this case on the length of their alkyl chain.

Emergent chiroptical properties in supramolecular and plasmonic assemblies.

This tutorial provides a comprehensive description of the origin of chiroptical properties of supramolecular and plasmonic assemblies in the UV-visible region of the electromagnetic spectrum. The photophysical concepts essential for understanding chiroptical signatures are presented in the first section. Just as the oscillator strength (a positive quantity) is related to absorption, the rotational strength (either a positive or a negative quantity) defines the emergence of chiroptical signatures in molecular/plasmonic systems. In supramolecular systems, induced circular dichroism (ICD) originates through the off-resonance coupling of transition dipoles in chiral inclusion complexes, while exciton coupled circular dichroism (ECD) originates through the on-resonance exciton coupling of transition dipoles in chiral assemblies resulting in the formation of a bisignated CD signal. In bisignated ECD spectra, the sign of the couplet is determined not only by the handedness of chiral supramolecular assemblies, but also by the sign of the interaction energy between transition dipoles. Plasmonic chirality is briefly addressed in the last section, focusing on inherent chirality, induced chirality, and surface plasmon-coupled circular dichroism (SP-CD). The oscillator strength is of the order of 1 in molecular systems, while it becomes very large (104-105) in plasmonic systems due to the collective plasmonic excitations, resulting in intense CD signals, which can be exploited for the design of plasmonic metamaterial platforms for chiral sensing applications.

Probing Molecular Chirality Using Metallo-Bisporphyrin Hosts

Supramolecular chirogenesis is one of the most important interdisciplinary field to be looked into, because of its occurrences in many natural (DNA double helix, heme proteins, secondary α-helix structure of proteins etc.) and artificial systems. Although, chiroptical spectroscopy such as Circular Dichroism (CD) is inherently sensitive to chirality, it has not enjoyed tremendous use as a method for stereochemical determinations, mainly because the results of CD analysis are empirical. Exciton Coupled Circular Dichroism (ECCD) is a nonempirical spectroscopic method that is based on detecting the through-space exciton interaction between helically orientated independently conjugated chromophores. The relative orientation of the two chromophores in space results in a predicted sign of the couplet; i.e., a clockwise orientation of two interacting chromophores yields a positive couplet and vice versa. Therefore, the challenge lies in orienting two or more chromophoric receptor groups in a chiral fashion as a direct result of the binding of a chiral compound and extrapolating the chirality of the bound compound from the ECCD spectra. This presentation will showcase some of our recent research activity along the line. Representative referances: S. A. Ikbal, S. Brahma, S. P. Rath Chem. Commun. 2014, 50, 14037-14040.S. A. Ikbal, S. Brahma, S. P. Rath Chem. Commun. 2014, 50, ASAP. (DOI: 10.1039/c4cc07955h) S. Brahma, S. A. Ikbal, S P Rath Inorg. Chem. 2014, 53, 2381-2395.S. Brahma, S. A. Ikbal, S P Rath Inorg. Chem. 2014, 53, 49-62. S. Brahma, S. A. Ikbal, S. P. Rath Chem. Commun. 2012, 48, 4070-4072. Figure 1

Transition-density-fragment interaction combined with transfer integral approach for excitation-energy transfer via charge-transfer states

A transition-density-fragment interaction (TDFI) combined with a transfer integral (TI) method is proposed. The TDFI method was previously developed for describing electronic Coulomb interaction, which was applied to excitation-energy transfer (EET) [K. J. Fujimoto and S. Hayashi, J. Am. Chem. Soc. 131, 14152 (2009)] and exciton-coupled circular dichroism spectra [K. J. Fujimoto, J. Chem. Phys. 133, 124101 (2010)]. In the present study, the TDFI method is extended to the exchange interaction, and hence it is combined with the TI method for applying to the EET via charge-transfer (CT) states. In this scheme, the overlap correction is also taken into account. To check the TDFI-TI accuracy, several test calculations are performed to an ethylene dimer. As a result, the TDFI-TI method gives a much improved description of the electronic coupling, compared with the previous TDFI method. Based on the successful description of the electronic coupling, the decomposition analysis is also performed with the TDFI-TI method. The present analysis clearly shows a large contribution from the Coulomb interaction in most of the cases, and a significant influence of the CT states at the small separation. In addition, the exchange interaction is found to be small in this system. The present approach is useful for analyzing and understanding the mechanism of EET.

Chelation-Enhanced Circular Dichroism of Tripodal Bisporphyrin Ligands

A chiral tripodal ligand readily prepared from primary amines that contains two monosubstituted tetraphenylporphyrin (TPP) moieties is described. Complexation with Cu(II) in the presence of NH4SCN gives a fully metalated porphyrin and tripodal ligand complex. The free ligand shows a weak circular dichroism (CD) signal, but metalation gives a strong exciton-coupled CD spectrum. Reduction of the Cu(II) complex results in loss of CD signal.