New Perspectives to Trigger and Modulate Circularly Polarized Luminescence of Complex and Aggregated Systems: Energy Transfer, Photon Upconversion, Charge Transfer, and Organic Radical

Abstract

Conspectus Chiral functional materials with circularly polarized luminescence (CPL) have risen rapidly in recent years because of their fascinating characteristics and potential applications in various research fields. CPL refers to the differential spontaneous emission of left (L)- and right (R)-handed circularly polarized light upon photon or electron excitation. Generally, an outstanding CPL-active material needs to possess a high luminescence dissymmetry factor (g lum) (defined as 2­(I L – I R)/(I L + I R) where I is the emission intensity), which is between −2 and +2. Although the exciting development in CPL-active materials was achieved, the modulation of CPL signs is still a challenge. For small organic systems, a relatively small g lum value, one of the key parameters of CPL, limits their practical applications. Searching for efficient approaches for amplifying g lum is important. Therefore, over the past decades, besides optimizing the structure of small molecules, many other strategies to obtain efficient CPL-active materials have been developed. For instance, self-assembly has been well demonstrated as an effective approach to amplify the supramolecular chirality as well as the g lum values. On the other hand, chiral liquid crystals (CLCs), which are capable of selective reflection of left- and right-handed circularly polarized light, also to serve as a host matrix for endowing guest emitters with CPL activity and high g lum values. However, self-assembly focuses on modulating the conformation and spatial arrangement of chiral emitters. And the CPL of a luminophore-doped CLC matrix depends on the helix pitch and band gap positions. Lately, novel photophysical approaches to modulate CPL signs have gradually emerged. In this Account, we discuss the recent progress of excited-state-regulation involved CPL-active materials. The emergence, amplification, and inversion of CPL can be adjusted through regulation of the excited state of chiral emitters. For example, Förster resonance energy transfer (FRET) can amplify the g lum values of chiral energy acceptors in chiral supramolecular assemblies. By combining the concepts of photon upconversion and CPL, high-energy upconverted circularly polarized emission was achieved under excitation of low-energy light, accompanied by an amplified g lum. In addition, the organic systems with unpaired electrons, i.e., charge transfer (CT) system and open-shell π-radical, show favorable CPL properties, which can be flexibly tuned with an applied magnetic field. It should be noted that these photophysical process are associated with the excited state of chiral emitters. So far, while the main focus is on the regulation of the molecular and supramolecular nanostructures, direct regulation of the excited state of the chiral system will serve as a new platform to understand and regulate the CPL activity and will be helpful to develop smart chiroptical materials.

Quantifying the Overall Efficiency of Circularly Polarized Emitters

“…Finally, beyond the molecular structural optimization, new ways to directly regulate the excited state and therefore to increase the CPL activity ( g lum ) have been recently reported, [122] such as energy transfer amplified CPL, [123, 124] chiral radical CPL [125] and photon upconversion [126, 127] …”

500‐Fold Amplification of Small Molecule Circularly Polarised Luminescence through Circularly Polarised FRET

“…(M1E1 3 )U typically have an enhanced CP emission compared to the pure electric dipole coupling, and do not have the same distance‐dependence as V, such that chiral energy transfer does not only occur between nearest neighbours [31, 32] . The FRET process has been previously shown to generate reasonably weak CPL (| g PL | ≈3×10 −3 ) from achiral small molecule acceptors when incorporated into chiral supramolecular structures such as nanohelices [33–36] …”

“…[31,32] The FRET process has been previously shown to generate reasonably weak CPL (j g PL j % 3 10 À3 ) from achiral small molecule acceptors when incorporated into chiral supramolecular structures such as nanohelices. [33][34][35][36] Here, we describe the highly dissymmetric photoluminescence (j g PL j > 0.1) of an enantiopure chiral p-conjugated small molecule acceptor (oxa [7]superhelicene, hereafter oxa- [7]H) embedded within a range of ACP donors (PFO, F8BT, F8PFB, Figure 1 and Supporting Information, SI). The oxa- [7]H (10 wt %) induces a chiral phase within the ACPsanalogous to the phase formed in polymer/aza [6]H blends, which we have previously characterised.…”

Design, Synthesis and Applications of Chiral Metal-Organic Frameworks

“…崔勇课题组 [61] 3.4 圆偏振荧光 圆 偏 振 荧 光 (Circularly Polarized Luminescence, CPL)是指手性发光体系发射具有差异的左旋和右旋圆 偏振光的现象, 可反映手性发光体系的激发态结构信 息, 在 3D 信息显示、信息存储以及光学传感等领域具 有广阔的应用前景 [62] . [63] . 由于 具有芳香结构的手性有机小分子会通过分子间作用而 发生荧光猝灭, 合成具有高不对称因子和高效发光效率 的 CPL 材料是一个挑战性很高的工作 [64] .…”