Generation of Circularly Polarized Luminescence by Symmetry Breaking

Yoshitane Imai

doi:10.3390/sym12111786

Abstract

Circularly polarized luminescence (CPL) has attracted significant attention in the fields of chiral photonic science and optoelectronic materials science. In a CPL-emitting system, a chiral luminophore derived from chiral molecules is usually essential. In this review, three non-classical CPL (NC-CPL) systems that do not use enantiomerically pure molecules are reported: (i) supramolecular organic luminophores composed of achiral organic molecules that can emit CPL without the use of any chiral auxiliaries, (ii) achiral or racemic luminophores that can emit magnetic CPL (MCPL) by applying an external magnetic field of 1.6 T, and (iii) circular dichroism-silent organic luminophores that can emit CPL in the photoexcited state as a cryptochiral CPL system.

Highlights

The potential application of luminescent techniques to various systems, such as organic and organometallic electroluminescence devices and optoelectronic devices, has attracted considerable attention [1–6]
Unlike circular dichroism (CD), which indicates the chirality of the ground state, circularly polarized luminescence (CPL) spectroscopy elucidates conformational and structural information pertaining to optically active molecules in the photoexcited state
The conformation of two pyrene units changes to a chiral π-stacked spatial arrangement. These configuration transfers are responsible for the cryptochiral properties, meaning the silent CD and active CPL derived from the excimer pyrene moiety [53]

Summary

Introduction

The potential application of luminescent techniques to various systems, such as organic and organometallic electroluminescence devices and optoelectronic devices, has attracted considerable attention [1–6]. Chiral luminophores demonstrating CPL have attracted research attention, in the fields of chiral photonic science and optoelectronic materials science [7–18]. CPL generally requires chiral organic or organometallic luminophores. Chiral organic ligands coordinating with optically active metal ions induce chirality in the luminescent complex. In such a CPL-emitting system, a chiral organic molecule is indispensable. In practical applications of CPL, both right- and left-handed CPL are used, and their selective emission requires chiral organic or organometallic luminophores with opposite chirality. A cryptochiral CPL system based on CD-silent molecules in the photoexcited state is discussed

CD and CPL from Achiral and Racemic Molecules by Spontaneous Resolution

CPL from Cryptochiral Organic Luminophores

Findings

Conclusions