**Yoshitane Imai**

Department of Applied Chemistry, Faculty of Science and Engineering, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan; y-imai@apch.kindai.ac.jp

Received: 8 October 2020; Accepted: 23 October 2020; Published: 28 October 2020

**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.

**Keywords:** chiral; circularly polarized luminescence (CPL); magnetic circularly polarized luminescence (MCPL); spontaneous resolution

#### **1. 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]. Analogous to the chirality associated with molecules, there exists chirality of light, which is referred to as circularly polarized luminescence (CPL). Unlike circular dichroism (CD), which indicates the chirality of the ground state, CPL spectroscopy elucidates conformational and structural information pertaining to optically active molecules in the photoexcited state. Optically active luminescent materials may produce either clockwise or anti-clockwise CPL. Chiral luminophores demonstrating CPL have attracted research attention, particularly in the fields of chiral photonic science and optoelectronic materials science [7–18].

CPL generally requires chiral organic or organometallic luminophores. In 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 addition, 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. Chiral organic luminophores that can be prepared from achiral or racemic molecules are preferred as efficient and industrially useful chiral luminophores [19,20].

In this review, three types of non-classical CPL (NC-CPL) systems, including symmetry breaking CPL (SB-CPL) systems, are reported. The first is a spontaneous-resolution CPL system using achiral or racemic molecules. The second is a magnetic circularly polarized luminescence (MCPL) system based on achiral or racemic molecules under an external magnetic field. Finally, a cryptochiral CPL system based on CD-silent molecules in the photoexcited state is discussed.
