Circularly Polarized Luminescence of Isolated Small Organic Molecules

Inhaltsverzeichnis

1. Frontmatter

2. Chapter 1. Frontiers of Circularly Polarized Luminescence Chemistry of Isolated Small Organic Molecules

   Tadashi Mori

   Abstract

   Lately, circularly polarized luminescence (CPL), differential left- and right-polarized emission from chiral materials, has been attracted great attention, especially that of small organic molecules (SOMs). Despite the fact that the luminescence dissymmetry factor (g _lum) of SOMs, a measure of degree of chirality in CPL, is typically in a range of 10⁻³ to 10⁻⁵, considerably smaller than those based on materials with forbidden transition, the value of SOM-based CPL materials are progressively revised upwards. This trend is primarily due to the fact that a structural modification of SOMs is permanently rational and relatively easy, allowing a straightforward control of absorption and/or emission wavelengths as well as luminescence intensity. Biological compatibility and ease of device fabrication are additional advantages of SOM-based CPL materials. In this chapter, we outline the basics of CPL such as a definition and a quantification. Then, we briefly discuss about an information attained by the CPL measurement of SOMs. Finally, a perspective on the rapid progress of SOM-based CPL materials is provided.

3. Chapter 2. Circularly Polarized Luminescence of Axially Chiral Binaphthyl Fluorophores

   Yoshitane Imai

   Abstract

   Axially chiral fluorophores bearing a variety of functionalities have been successfully developed using chiral binaphthyl units. These axially chiral binaphthyl fluorophores emit circularly polarized luminescence (CPL) in their solution-dissolved states, organic polymer-film-dispersed states, and inorganic pellet-dispersed states. The CPL emitted from an axially chiral binaphthyl fluorophore is easily tuned by (1) adjusting the dihedral angle in the binaphthyl unit, (2) employing the neighboring group effect between binaphthyl units, and (3) controlling their external environments, without the need for the enantiomeric compound.

4. Chapter 3. Circularly Polarized Luminescence from Planar Chiral Compounds Based on [2.2]Paracyclophane

   Yasuhiro Morisaki

   Abstract

   In this chapter, recent development on molecules emitting intense circularly polarized luminescence (CPL) based on planar chiral [2.2]paracyclophane is described. Optical resolution routes of the planar chiral [2.2]paracyclophane compounds, optically active π-stacked molecules, and the CPL profiles are discussed. It is suggested that the optically active higher-ordered structures, such as V-, X-, triangle-, propeller-shaped structures, and so on, in the excited state are important for intense CPL with large dissymmetry factors (g _lum values).

5. Chapter 4. Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives

   Jeanne Crassous

   Abstract

   In this chapter, we discuss the circularly polarized luminescence (CPL) of helicene and helicenoid derivatives. The organic helicenic derivatives are classified according to the type of atom (heteroatom or carbon) incorporated within the helical backbone. Transition-metal complexes and chiroptical devices incorporating helicene derivatives and exhibiting CPL activity are also presented.

6. Chapter 5. Structural Control of Fluorescent Helicates for Improved Circularly Polarized Luminescence Properties

   Taku Hasobe

   Abstract

   This chapter focuses on the structural and photophysical control of fluorescent helicene derivatives and related homoleptic zinc(II) helicates for improved circularly polarized luminescence properties. Our synthetic strategy enables to significantly enhance the fluorescence quantum yield (Φ _FL) of helicene derivatives for observation of circularly polarized luminescence (CPL). Moreover, the zinc(II) helicate also demonstrated unusually strong chiroptical responses with absorption and luminescence dissymmetry factors such as |g _abs| = 0.20 at 615 nm and |g _lum| = 0.022 at 660 nm, respectively. The g _lum profiles significantly expanded up to ca. 850 nm.

7. Chapter 6. BODIPY Based Emitters of Circularly Polarized Luminescence

   Michael John Hall, Santiago de la Moya

   Abstract

   The boron dipyrromethenes (BODIPYs) are a ubiquitous class of fluorescent dyes which have found utility in a wide range of photonics applications. More recently the BODIPYs have been investigated for their use as small organic molecule (SOM) emitters of circularly polarized luminescence (CPL). Herein, we will discuss recent developments in the field of BODIPY-based CPL-SOMs. In particular, we will review the current design strategies for the induction of chirality in the otherwise planar BODIPY fluorophore, for both mono- and multi-fluorophore containing systems, and examine the impact of these designs on the resulting chiroptical properties, including CPL emission.

8. Chapter 7. Propeller Chirality: Circular Dichroism and Circularly Polarized Luminescence

   Tadashi Mori

   Abstract

   Hexaarylbenzenes (HABs) and related highly substituted aromatic molecules possess unique propeller-shaped structure. The propeller chirality derived therefrom turned out not to be a simple multiple combination of axial chirality. A domino-like corporation between the radial aromatic blades is critical to produce the highly enhanced chiroptical responses. More interestingly, such chirality can be either static or dynamic depending on the sterical hindrance between the aromatic blades. The latter systems could be easily controlled by subtle environmental factors such as temperature, solvent, as well as pressure, and hence feasibly manipulatable. The molecular systems with propeller chirality thus provide new design principle for advanced and superior chiroptical materials, particularly those emit efficient circularly polarized light at desired wavelength and, in addition, are modulable and are switched on-off at will.

9. Chapter 8. Photo-Switching of Circularly Polarized Luminescence

   Takuya Nakashima, Tsuyoshi Kawai

   Abstract

   Circularly polarized luminescence (CPL) emission from chiral molecular systems is readily switched by means of external stimuli such as temperature, solvent, chemicals, and light irradiation. Since CPL is one of the emission phenomena, it can be modulated in a similar manner to the emission switching. The ON-OFF switching of emission intensity of chiral molecular systems may simply lead to the modulation of CPL intensity. Apart from the modulation of emission intensity, the chiral structures including chiral arrangement of fluorophores or metal coordination geometries are switched by external stimuli, changing the dissymmetry factors, i.e., the quality of CPL. In this chapter, we review the design of chiroptical photo-switches based on photochromic molecules that modulate the CPL property in response to photo-irradiation in a dynamic manner.

10. Chapter 9. Circularly Polarized Luminescence of Chirally Arranged Achiral Organic Luminophores by Covalent and Supramolecular Methods

    Toshiaki Ikeda, Takeharu Haino

    Abstract

    Circularly polarized luminescence (CPL) produced by achiral organic luminophores is described. Achiral organic luminophores can exhibit CPL by the chiral arrangement of the achiral luminophores. Chiral arrangement of achiral luminophores can be constructed through a covalently linked chiral spacer like a binaphthyl moiety. A helical supramolecular assembly also provides chiral environment on an achiral luminophore. The helically stacked assemblies of achiral luminophores are excellent for realizing CPL of the achiral luminophore since the highly assembled structure in the helical assembly provides good CPL activity. The stimuli-responsivity of supramolecular systems provides stimuli-responsive CPL.

11. Chapter 10. Structural and Electronic Information Drawn from the Circularly Polarized Luminescence Spectra: Many Questions and Some Answers for Simple Organic Molecules, Polymers, and Molecular Aggregates

    Giovanna Longhi, Sergio Abbate

    Abstract

    In the last ten years, circularly polarized luminescence (CPL) has greatly advanced: lots of data have been collected and many compounds have been synthesized with the aim of enhancing this chiroptical response. We review here a few aspects with illustrative examples. After examination of the relation of circular dichroism (CD) and CPL signals, we investigate how CPL can be used to probe environment: aggregation phenomena or the presence of metal ions or pH variations. We also study the CPL of inherently dissymmetric chromophores and of metal complexes. We finally touch upon whether CPL originates from molecules or from aggregated inhomogeneous systems.

12. Chapter 11. Circularly Polarized Luminescence from Gelator Molecules: From Isolated Molecules to Assemblies

    Tonghan Zhao, Pengfei Duan, Minghua Liu

    Abstract

    Currently, molecular gels have become one kind of the fascinating candidates for fabricating chiroptical materials with circularly polarized luminescence (CPL) properties, due to the tunable and modifiable structure, simple yet facile synthesis, controlled and reversible assembly, and so on. Since self-assembly approach has been regarded as an efficient way for amplifying the chirality, supramolecular gelation provides a remarkable method for fabricating CPL materials with high dissymmetry factor (g _lum). Various gel systems, including chiral-, achiral-, organic-inorganic hybrid systems can be endowed with CPL activities through supramolecular gelatinization, possessing excellent luminescence circular polarization. This chapter summarizes and reviews the present status and progress of supramolecular gel systems with CPL activity.

13. Chapter 12. Circularly Polarized Luminescence from Intramolecular Excimers

    Francesco Zinna, Elodie Brun, Alexandre Homberg, Jérôme Lacour

    Abstract

    In this chapter, examples of circularly polarized luminescence (CPL) stemming from intramolecularly formed excimers will be reviewed. Emission from excimers has peculiar photophysical properties with respect to fluorescence of regular monomers. In addition, if the fluorophoric couple forming the excimer in the excited state is mounted on a chiral scaffold, a strong CPL can be usually observed. Examples of chiral scaffolds include oligopeptides, macrocycles, binaphthyl, and diaminocyclohexane derivatives. CPL from excimers has mainly been observed from pyrenes but other molecules are also able to give rise to such phenomenon, e.g., perylenes and 1,8-naphthalene monoimide. Excimer CPL can provide important information about the conformation of a molecule in the excited state and how it evolves depending on the environment (e.g., solvent and temperature) or external stimuli (e.g., light irradiation and cation addition). Moreover, thanks to the peculiar photophysical nature of excimers, the degree of circular polarization associated with excimer emission is usually much larger than the one associated with the absorption (electronic circular dichroism, ECD) for the same molecule. This allows to study chiroptical emission properties of molecules which are ECD-silent (ground state cryptochirality). As a whole, excimer CPL is an interesting and useful strategy to develop organic molecular systems endowed with bright and highly polarized luminescence.

14. Chapter 13. Design of Circularly Polarized Thermally Activated Delayed Fluorescence Emitters

    Gregory Pieters, Lucas Frederic

    Abstract

    This chapter focuses on the molecular designs of Small Organic Molecules (SOM) merging Circularly Polarized Luminescence (CPL) and Thermally Activated Delayed Fluorescence (TADF) properties. In Introduction, the benefits associated with the combination of these properties into SOM for their application as emitting materials to construct Circularly Polarized Organic Light Emitting Diodes (CPOLED) are presented. Next, the different molecular designs leading to CPTADF SOM are described depending on the nature of the chirality of these molecules (point, axial, or planar chirality). The synthesis, photophysical, and chiroptical properties of these molecules and the performance of related CPOLED devices are discussed.

15. Chapter 14. Principles and Applications of Circularly Polarized Luminescence Spectrophotometer

    Satoko Suzuki

    Abstract

    Materials that exhibit circularly polarized light characteristics are finding an increasing range of applications, such as liquid crystal display backlights, three-dimensional displays, holographic displays, plant growth control illumination, security systems for optical communications, and printing. In this chapter, we will describe the basic principles of circularly polarized luminescence (CPL) spectroscopy, the instruments used to measure CPL signals, including optical systems and signal acquisition methods in commercially available CPL instruments, and also methods for calibrating such equipment. Example measurements are presented for camphor and camphorquinone, which are long-established CPL samples, in addition to lanthanoid complexes that are promising CPL luminescent materials, biopolymers that are highly sensitive to circularly polarized light, and solid samples that have been attracting attention in recent years.

16. Chapter 15. Transient Circular Dichroism Approach to Chirality Detection in Dark Photo-Excited States

    Yasuyuki Araki

    Abstract

    A transient circular dichroism (CD)-based approach, called time-resolved circular dichroism (TRCD) spectroscopy, serves as an alternative to circularly polarized luminescence (CPL) spectroscopy for the investigation of excited-state chirality. The experimental apparatus for TRCD measurements was designed and is presented here. The Jones calculus approach to assessing experimental observables is also reported. Finally, to demonstrate the feasibility of TRCD, the CD spectrum of the photo-excited state of [6]carbohelicene is obtained.