Progress in the Science of Functional Dyes

Polymethine dyes consist of heteroaromatic end and linkage groups. They are classified into cyanines, merocyanines, and oxonols. Their UV–Vis and near-infrared (NIR) absorption band are affected by the heteroaromatics and linkage groups. The change in the absorption band of polymethine dyes by aggregate formation has been explained by the molecular exciton coupling theory. Polymethine dyes have been used as data recording materials. Polymethine dyes can be used as sensitizers in dye-sensitized solar cells. Though usual dyes are solid, liquid trimethine dyes have been reported. The fluorescence intensity of the liquid trimethine dyes is drastically enhanced in liquid nitrogen.

Squaraine dyes which are composed of a cyclobutenedione core with aromatic or heterocyclic components show sharp and intense electronic absorption in the areas of visible to near-infrared regions and often fluorescence emission. These prominent optical properties arouse our interest in various fields of applications using the dye. In order to respond to the diverse demands of squaraine dyes in application fields, considerable effort have been made in the past decades to design and synthesize symmetrical and unsymmetrical squaraine dyes by means of classical condensation reaction of squaric acid moiety with electron-rich compounds. A novel approach using transition-metal catalyzed cross-coupling is developed to construct squaraine chromophores. This approach allows not only to attach desired functionalities on peripheral parts of chromophores but also to synthesize oligomeric and polymeric squaraine dyes. In addition to the molecular level of study, the supramolecular architectures have been constructed by non-covalent interaction between the dye molecules. This section gives an overview of the recent advances in syntheses and structures of squaraine dye with particular attention to the novel synthetic protocol.

It is well known that meso-tetraarylporphyrins are obtained readily by condensation reactions between pyrroles and aromatic aldehydes. The 2+2 type condensation of aryldipyrrylmethanes and the second aromatic aldehyde is useful for synthesizing porphyrins with mixed meso-aryl groups, and further transformations of the porphyrin periphery are performed conveniently by using organometallic methodology. Some meso-aryl substituted porphyrins are designed for supramolecular application and photosensitizing effect. Porphyrins substituted at pyrrole β-positions are obtainable by using Barton–Zard synthesis, and these compounds are further converted into benzoporphyrins that are structurally similar to phthalocyanines and play an important role in the development of a variety of photo-functional materials. Porphyrins more heavily π-extended than benzoporphyrin are focused in the last part of this chapter. Multiple porphyrin cores are directly bridged by π-units or aromatic substituents at the porphyrin periphery are forced to come into π-conjugation with the porphyrin π-system by Scholl reactions, leading to strong infrared absorption bands at longer wavelength beyond 1000 nm, very small HOMO–LUMO gap, and very high two-photon absorption efficiency.

Phthalocyanine (Pc) has been one of the most important industrial dyes exhibiting lasting blue or green color and used in various research fields including not only organic electronics but also biomedical field. The aim of this chapter is to overview the chemistry of Pc and its ring contracted analogue, subphthalocyanine (SubPc), with an emphasis on their syntheses and optical properties from a viewpoint of molecular symmetry–property relationships. A basic theoretical description of the absorption properties of Pc and SubPc, which is referred to as Gouterman's four orbital theory, is introduced. In addition, magnetic circular dichroism (MCD) spectroscopy, a powerful spectroscopic tool to analyze the absorption spectra of Pc, SubPc, and related analogues, is briefly described. Finally, recent advances in the synthesis of new Pc-based NIR chromophores will also be covered.

This chapter discusses organoboron complexes, especially the synthesis and correlation between molecular structure and optical properties. Organoboron complexes are among the most important fluorescent dyes. Boron complexation of the ligand (dye) contributes to the enhancement of the fluorescent properties of the dye through rigidization of the chromophore, which restricts non-radiative processes. Additionally, the optical properties of organoboron complexes, such as absorption and fluorescence maxima, and fluorescence quantum yields are strongly dependent on the ligand type (dye). Therefore, boron complexation of the ligand (dye) is an efficient strategy to synthesize novel fluorescent dyes. In fact, organoboron complexes have been actively developed and applied in various fields, including optoelectronics and biomedicine. Understanding synthetic methods and the correlation between molecular structure and optical properties help us in producing materials with the desired properties. This chapter discusses BODIPY dyes, the most famous fluorescent organoboron complexes, and then outlines the four-coordinate monoboron complexes possessing anionic bidentate ligands (N^N, N^O, O^O, and other types) and multinuclear boron complexes, with a special focus on the results reported by the author.

Molecular and crystal structures were reviewed on several dyes and coloured organic compounds with four or more polymorphic forms whose structural data are available from the online database. The difference in the molecular and crystal structures between polymorphs of the target compounds was briefly described. The result indicated that rigid dye chromophores, structural flexibility and weak intermolecular interactions such as hydrogen bonding and halogen interactions are important in terms of the occurrence of the polymorphs of organic dyes.

Photochromic compounds can be divided into two types, T- and P-types. T-type photochromic compounds include azobenzene, spiropyran, hexaarylbiimidazole, spirooxazine, naphthopyran, and the donor-acceptor Stenhouse adduct, as introduced here. In P-type photochromic compounds, there are furylfulgide and diarylethene. Diarylethene derivatives have the most excellent photochromic compounds, such as thermal stability of both isomers, high fatigue-resistance, high response, high sensitivity, high coloration quantum yield, and high reactivity even in the crystalline phase. In this chapter, the photochromic properties of these compounds have been focused.

The two-photon excitation technique has several advantages such as reduced autofluorescence and deeper penetration in tissues, less photodamage and photobleaching, and higher three-dimensional resolution, which cannot be achieved by linear one-photon excitation. Because of these advantages, two-photon absorption (TPA) of organic molecules has been developed in biological and materials science fields. In particular, fluorescent TPA dyes have attracted the attention of chemists as well as biologists because of their application in biological imaging. At present, various types of fluorescent TPA dyes that have large TPA cross sections as well as good fluorescence quantum yields are accessible. However, the fluorescent color has mostly been restricted to the shorter wavelength region of blue and green light, which competes with the autofluorescence from intrinsic biomolecules. To achieve efficient two-photon imaging, the longer wavelength emission of red and near-IR light is required for two-photon excitation because this light lies in the biological optical window. In this chapter, we focus first on fluorescent TPA dyes that have red and near-IR light-emitting properties. Then, we summarize the recent advances made in the development of these dyes for applications in biological systems. In the last section, the unique features of two-photon absorption in aggregate systems are summarized.

Luminescent organic dyes are known to be an essential building block for developing modern organic optoelectronic devices. In these devices, organic dyes are normally used as a film. However, most of the organic dyes suffer from the quenching effect induced in the condensed state, named as concentration quenching or aggregation-caused quenching (ACQ). Therefore, so far, various strategies have been proposed for overcoming ACQ. In this chapter, we survey conventional strategies for suppressing ACQ and obtaining intense solid-state emission mainly regarding boron-containing materials which are known to be a platform for designing luminescent dyes. A series of typical examples to exhibit solid-state emission are presented in each part. Initially, luminescent organic– inorganic hybrids are illustrated. ACQ is often induced through non-specific intermolecular interactions in the condensed state. In order to realize the solution-like situation where each dye molecule is isolated from each other, the transparent hybrid matrices were useful as a scaffold. Recent progresses on the development of conjugated polymer-based hybrids by employing hybrid molecules as a building block is also demonstrated. The similar strategy for isolating dye molecules is accomplished by introducing steric substituents and structures near the chromophore moiety. Particularly, this strategy is conventionally applied for obtaining luminescent polymer films. Moreover, by assembling heterogeneous types of dyes at the cardo structure in the polymer main chains, the multiple emission bands originating from each dye were able to be simultaneously observed from the film samples. These results on simultaneous multiple emission bands are explained. Next topics are regarding aggregation-induced emission (AIE)-active molecules. By suppressing molecular motions at the steric moiety, intense emission can be observed in the aggregation state. Furthermore, it was shown that emission color from some of the AIE-active molecules was sensitive to molecular distributions in the solid state. Based on these characteristics, stimuli-responsive luminochromism was obtained. Basic design strategies and recent applications are described.

In this chapter, chiroptical properties, especially circularly polarized luminescence (CPL) properties of optically active molecules based on planar chiral [2.2]paracyclophane are mainly introduced. In addition, practical optical resolution methods of disubstituted and tetrasubstituted [2.2]paracyclophane molecules are also focused on. The enantiopure [2.2]paracyclophane compounds have been used as chiral building blocks to synthesize the optically active molecules by means of optical resolution. The [2.2]paracyclophane-based molecules are π-stacked molecules, which construct optically active second-ordered structures, such as V-, X-, triangle-shaped, and one-handed double helical structures, due to the orientation of stacked π-electron systems. Photoexcitation allows them to emit bright CPL with good photoluminescence (PL) quantum efficiencies and large dissymmetry factors (g_lum values). Thus, planar chiral [2.2]paracyclophane is the ideal scaffold to achieve excellent CPL properties.

Chemosensors are molecules capable of monitoring changes in the concentration, structure, or location of chemical species based on a detectable physical signal and can therefore be used in quantitative analysis or for the monitoring and/or visualization of targeted analytes (ions, biomolecules, organelles, etc.). Besides the analyte itself, chemosensors can be used indirectly to observe chemical reactions, biological events, or specific phases in materials where the analyte appears (e.g., the concentration of reactive oxygen species in mitochondria is related to apoptosis). As functional dyes show inherent optical properties such as photon absorption/emission at distinct wavelengths, they are potential optical signal indicators in chemosensors. In fact, in the more than 150 years since F. Goppelsrönder invented a morin-based chemosensor to detect aluminum anion using fluorescence signal (Wu et al. in Chem Soc Rev 46:7105–7123, 2017), a number of such “fluorescent chemosensors” have been developed. Some of them are widely used in practical scientific fields such as biology, physiology, pharmacology, food chemistry, and environmental chemistry, as well as in industrial and military/defense fields. This chapter describes the representative principles and molecular designs of fluorescent chemosensors, and several historically important progresses are introduced. The reason to limit the discussion to fluorescence is that fluorescence-based techniques generally exhibit superior sensitivity to absorption-based ones, and therefore many dyes have been reported for the development of fluorescent chemosensors.

In material science, organic compound-based white-light emission (WLE) systems that emit in the entire visible region have received much attention due to low-cost, lightweight, easy fabrication of thin film, and so on. Synthesis of single molecules with WLE function has been still a challenge because of constraints imposed by Kasha’s rule, whereas it is easier to fabricate multicomponent-based emissive systems. In the latter case, supramolecular organization of the related components is promising, where well-tailored intermolecular interactions between them would tune their interplay on the physical properties to give WLE. Toward this end, the use of Förester resonance energy transfer (FRET) is benefitable for the interplay between the emissive components. Significant overlap between the emission spectrum of donor dye and absorption band of adjacent acceptor one is indispensable. This chapter describes supramolecular ensembles capable of tuning the primary colors, i.e., red, green, and blue, or two complementary colors, e.g., cyan and orange, with dynamic and reversible non-covalent bonds involving hydrogen bonds, metal coordination, electrostatic interactions, hydrophobic interactions, and π–π stacking interactions. Emissive systems other than FRET behavior are also introduced in this chapter.

Photosensitizing dyes with the ability to produce singlet oxygen (¹O₂) under light irradiation can be used for photodynamic therapy (PDT), a treatment of early-stage cancer with less stress on bodies, where the photogenerated ¹O₂ destroys cancer cells. For PDT use, photosensitizing dyes require several capabilities such as strong photoabsorption in the phototherapeutic window (650–900 nm), high ¹O₂ generation quantum yield (Φ_Δ), good water solubility, and low toxicity without light irradiation. Therefore, the effects of chemical structures and substituents of photosensitizing dyes on these properties have been investigated, which enables to create new photosensitizing dyes with excellent performance for PDT. This chapter overviews recent studies and developments in photosensitizing dyes with a focus on porphyrin, phthalocyanine, boron-dipyrromethene (BODIPY), xanthene, phenothiazinium, heteropolycycle, pyrylium, azinium, squalin, and transition metal (Ru, Ir, Pt) complex skeletons.

Organic solar (photovoltaic) cell is one of the most promising new renewable photovoltaic cells. In particular, dye-sensitized solar cells (DSSCs) based on dye photosensitizers adsorbed on photoelectrodes (oxide semiconductor electrodes such as TiO₂, ZnO, and NiO) have received considerable attention from the viewpoint of the fascinating construction and operational principles, decorative natures, low cost of fabrication, and high power conversion efficiency. For improvement of the photovoltaic performances of DSSCs, it is essential to create new efficient dye photosensitizers. Therefore, a variety of ruthenium (Ru) complex dyes and organic dyes that possess excellent light-harvesting properties over the wide spectral region of sunlight and enhanced electron communication between the dye and photoelectrode and between the dye and electrolyte have been designed and developed. To date, the solar energy-to-electricity conversion yield (η) of up to 13% has been achieved in DSSCs.

Recently, a wide variety of π-conjugated polymers have been developed for the use as the active layer in organic photovoltaics (OPVs). This chapter will summarize the recent development of π-conjugated polymers with donor–acceptor motifs, specifically, based on naphthalene-based nitrogen-containing heteroaromatics as the acceptor unit. With the strong electron-deficient nature as well as the large π-conjugation system, incorporation of these heteroaromatics in the backbone allows us to create π-conjugated polymers having wide absorption range, i.e., narrow bandgap, deep HOMO energy levels, and high crystallinity, all of which are important for improving the efficiency of OPV cells. We describe the syntheses, properties, structural order in the thin films, and OPV performances of the π-conjugated polymers based on naphthobisthiadiazole and its analogues.

Nowadays, an organic light-emitting diode (OLED), consisting of nano-order luminescent and semiconductive organic thin films, attracts considerable interest from the viewpoint of application to flat panel displays and illumination devices. Among the constituent materials of OLEDs, emitting materials play an important role in determination of the device performance such as luminous efficiencies and chromaticity coordinates of electroluminescence. Here, luminescent dyes and related compounds used as emitters in OLEDs are reviewed in accordance with the mechanism of the exciton generation. First, fluorescent emitters based on organic dyes and π-conjugated polymers are focused on, and then the topic moves to phosphorescent organometallic emitters affording high device efficiencies. In the last part, organic TADF emitters are reviewed, which can realize the 100% internal quantum efficiency of electroluminescence in theory without the employment of precious metal-containing organometallic structures.