Recent advances in organic luminescent materials with narrowband emission

The International Telecommunication Union announced a new color gamut standard of broadcast service television (BT 2020) for ultra-high-definition TV in 2012. To satisfy the wide-color gamut standard of BT 2020, monochromatic red (R), green (G), and blue (B) emissions require a small full width at half-maximum, which is an important property for improving color purity. Although organic light-emitting diode (OLED) displays are currently one of the main types of display technologies, their broad emission via strong vibronic coupling between ground and excited states is a major hurdle to overcome in the development of next-generation wide-color gamut displays. Thus, the development of OLED emitters with narrowband R–G–B emissions is of great significance. In this review, the recent progress in the development of OLED materials with narrowband emission is summarized by grouping them into fluorescent, phosphorescent, and thermally activated delayed fluorescent emitters to reveal the correlation between molecular structures, optical properties, and device characteristics. We discuss rational molecular design strategies to achieve narrow photoluminescence and electroluminescence and the underlying mechanisms for controlling the emission bandwidth. Finally, the challenges in the realization of wide-color gamut OLED displays and the future prospects of such devices are discussed. Organic light-emitting diodes (OLEDs) with high color purity could be used in the next generation of high-definition televisions. The most widely used semiconductor, silicon, is an inorganic material but a wide range of organic alternatives are now emerging. These alternatives are especially in demand for light-emitting applications, where the performance of silicon is poor. Ji-Eun Jeong, Han Young Woo and colleagues from Korea University in Seoul, South Korea, reviewed recent progress in the development of OLEDs. An OLED tends to emit light over a relatively broad spectrum. This lack of color purity limits the device’s use in future ultra-high-definition TVs. The team presented an overview of the various molecular design strategies that have been used to reduce emission bandwidth and the physical mechanisms forming the basis of these strategies. With a growing demand for new emitters to realize ultra-high-definition displays, various types of organic emitters with narrow emission and high luminescent efficiency have been extensively studied. In this review, we summarized the recent developments of organic emitters (fluorescent, phosphorescent, and thermally activated delayed fluorescent) which show narrowband emission spectra with full-width half-maximum smaller than 50 nm.


Introduction
Since the first organic light-emitting diode (OLED) was successfully demonstrated by Tang and Slyke in 1987 1 , OLEDs have been extensively studied in both academia and industry, becoming a mainstream display technology in fullcolor televisions and smartphones. They have various advantages, such as a light weight, fast response time, wide viewing angle, facile chemical tunability of emitting molecules, low energy consumption, compatibility with flexible plastic substrates, and form factors for various types of displays 2 . Based on the light-emitting mechanisms, different types of OLEDs have been developed: fluorescence (1st generation)-based, phosphorescence (2nd generation)-based, and thermally activated delayed fluorescence (TADF, 3rd generation)-based OLEDs 3 . Recently, extensive research on the 4th generation of OLEDs is in progress to improve the device efficiency, lifetime, and color purity in particular. Despite the many advantages of OLEDs, their emission spectra often show broad bandwidths, which are extremely detrimental to achieving high color purity for future highend display electronics such as high-definition TV and ultra-high-definition TV (UHDTV).
The "CIE (International Commission on Illumination) 1931 color space" was first defined based on tristimulus values; then, the modified CIE 1976 was announced, and both CIE 1931 and 1976 became the most widely accepted standards to define emission colors in the field of displays 4 . The CIE coordinate visualizes the entire range of colors that can be obtained by mixing the three primary colors (red (R), green (G), and blue (B)) by varying the wavelength and emission intensity. In 2012, the International Telecommunication Union (ITU) announced a new color gamut standard for UHDTVs called the Broadcast Service Television 2020 (BT 2020) (Fig. 1) 5 . Compared to the previously reported BT 709, the color gamut became wider with CIE coordinates for the R, G, and B colors of (0.708, 0.292), (0.170, 0.797), and (0.131, 0.046), respectively. This change was made because of the growing demand for monochromatic R, G, and B colors to improve color purity (Fig. 1a) 6 .
Vibronic coupling in fluorescent and phosphorescent organic emitters between the singlet ground state (S 0 ) and the singlet (S 1 ), or triplet (T 1 ) excited states together with charge transfer (CT) interactions often induces broad peaks in both photoluminescence (PL) and electroluminescence (EL) spectra, showing a full width at halfmaximum (FWHM) of over 70 nm [7][8][9][10][11] . Organic emitters show significantly broader emission spectra than inorganic quantum dots (QDs) and perovskite-based emitters, limiting the potential for OLEDs with the high color purity proposed by BT 2020 12 . Thus, the development of OLED emitters with narrowband R, G, and B emissions with extremely small FWHM is of paramount importance. As seen in Fig. 1b, the color gamut becomes wider with decreasing FWHM values in the emission spectra, satisfying the color gamut standard of BT 2020 13,14 .
In this review, we summarize recent developments and important studies of organic light-emitting materials and devices with narrowband emission. We categorize these materials and devices based on their emission mechanism by grouping them into fluorescent, phosphorescent, and TADF emitters (Fig. 1c). Since there is currently no clear definition of "narrow emission", we mainly focused on recent reports of OLED materials with a FWHM smaller than 50 nm in their emission spectra (Fig. 1d). Rational molecular design strategies to achieve narrow PL and EL emission and the related electronic structure and light-emitting characteristics, including the resulting device properties, are discussed to understand the underlying mechanisms for controlling the emission bandwidth. Finally, we provide our perspective on the remaining challenges in this research area that must be overcome to develop the next generation of wide-color gamut OLED displays.
Fluorescent organic light-emitting materials π-Conjugated organic fluorophores suffer from limited color purity due to their broad emission spectra originating from the intrinsic vibronic coupling and structural relaxation of the S 1 state (Fig. 2a). To achieve narrow EL spectra with a small FWHM, color filters and optical microcavities have been considered in the fabrication of fluorescent OLEDs [15][16][17] . However, the development of efficient organic fluorescent emitters with narrow emission bandwidths for high color purity remains an important goal. As shown in Fig. 2b, the relative intensity (I 0-1 /I 0-0 ) of the 0-0 (between the ν = 0 vibrational levels of S 0 and S 1 ) and 0-1 (from ν = 0 of S 1 to ν = 1 of S 0 ) vibronic transitions is determined by the Huang-Rhys factor (S) 18 . When π-conjugated organic molecules have a locally excited (LE) state with a similar equilibrium geometry to the ground state (structural distortion, ΔQ 0), a large orbital overlap results in a dominant 0-0 vibronic transition, converging the S value to zero and producing a sharp single emission peak 19 . In contrast, an increase in ΔQ by the formation of the CT state can induce a strong contribution from 0 to n (n = 1, 2, 3…) vibronic transitions, resulting in a broader emission peak 9,18,20 . Thus, the design of rigid structures with a lack of CT character in the excited state is crucial to decrease the S value and develop organic fluorescent materials with narrowband emission.
In this chapter, we focus on the molecular design of narrowband emitting fluorophores and structure-optical property correlations by categorizing the molecules into (i) twisted structures with bulky substituents with suppressed intermolecular aggregation, (ii) rigid/fused aromatic molecules without CT character, (iii) 5,12-dihydroquinolino[2,3-b]acridine-7,14-dione (quinacridone) and (iv) 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives (Fig. 3). Much of the previous work concerning fluorescent singlet emitters with narrow emission studied blue-emitting materials and their EL devices. Some quinacridone and BODIPY-based fluorophores have demonstrated green and red emission spectra with remarkably small FWHM values. The narrow emission with a predominant 0-0 vibronic transition can be ascribed to the enhanced LE character and negligible CT interaction in the rigid and symmetrical molecular structures. The lightemitting properties of representative singlet emitters with narrowband emission and their device characteristics are summarized in Table 1.

Fused aromatic compounds with structural symmetry
Symmetrically structured fused aromatic compounds without ICT character have demonstrated narrow emission with similar molecular geometry in the S 0 and S 1 states. For example, flavanthrone is a well-known anthraquinone-type fluorophore with a rigid planar backbone, and several flavanthrone derivatives have been investigated in OLEDs where a dominant LE emission was observed with a high oscillator strength. Based on the flavanthrone core, Monkman et al. synthesized a series of 8,16-dialkoxybenzo[h]-benz [5,6] acridino [2,1,9,8-klmna] (Fig. 4b).
A series of rigid anthanthrene derivatives were synthesized by Neckers et al. with various phenyl substituents, including 4,10-diphenylanthanthrene (1-26) and p-tertbutylphenyl-4,6,10,12-tetraphenylanthanthrene (1-27) 37 . Compounds 1-26 showed a remarkably small FWHM of 14 nm with a λ PL of 442 nm and a PLQY of 33% in dichloromethane (DCM). Because anthanthrene in DCM had almost isoenergetic S 1 and T 2 states, nonradiative relaxation pathways via intersystem crossing (ISC) decreased the PL efficiency. However, in the solid state, the T 2 state had a slightly higher energy than the S 1 state, making the nonradiative ISC process far less likely to occur and yielding blueshifted emission with a higher PLQY compared to that in solution. OLED devices using    40 . In particular, the multiple resonance (MR) effect (discussed in detail in chapter 4) was achieved without any boron or ketone groups. The MR effect was generated by two nitrogen atoms enclosed in three aromatic rings with the LUMO distributed along the adjacent carbon atoms. The device employing tDIDCz showed a pure-violet EL emission at λ EL = 401 nm with a remarkably small FWHM (14 nm).

Quinacridone derivatives
Various quinacridone derivatives based on a core structure consisting of five fused rings have proven to be green OLED emitters with great potential, showing intense fluorescence, high carrier mobility, and inherently narrow emission. However, the rigid and planar core structures readily form strong π-π stacking and intermolecular hydrogen bonding interactions in concentrated or aggregated conditions, which significantly quenches their fluorescence. Wang et al. studied the light-emitting characteristics of several quinacridone derivatives with various substituents to reduce intermolecular aggregation. For example, the quinacridone derivatives N,N-di(n-butyl) quinacridone  and N,N-di(n-butyl)-1,3,8,10-tetramethylquinacridone (1-31) showed narrow PL spectra in chloroform with a PLQY of over 90% at λ PL = 538 and 532 nm, respectively (FWHM < 30 nm) 41 . EL devices fabricated with these molecules showed only a slight increase in the FWHM (28 and 36 nm for 1-30 and 1-31, respectively). Fluorine substituents were subsequently introduced to yield N,N′-di(n-butyl)-2,9-difluoroquinacridone (C 4 -DFQA, 1-32) 42 . Similar to the non-fluorinated compounds 1-30, C 4 -DFQA showed a remarkably narrow (FWHM = 21 nm) and highly efficient green PL emission in THF at 535 nm with a PLQY of 97%. The introduction of strongly electron-withdrawing fluorine atoms decreased the HOMO and LUMO energy levels, resulting in efficient electron injection and transport to improve the OLED performance. As shown in Fig. 4c, the EL spectra of the ITO/NPB/C 4 -DFQA/Alq 3 /LiF/Al device exhibited a yellowish-green emission at 553 nm with a small FWHM of 30 nm and CIE coordinates of (0.42, 0.56). The authors also attempted to extend the conjugation of the core to tune the emission properties of quinacridone derivatives 43 . The indole-fused quinacridone structure 5,8,15,18-tetraoctyl-5,8,15,18-tetrahydroindolo[3,2-a]indole[3′,2′:5,6]quinacridone (IDQA, 1-33) in toluene showed a slightly redshifted PL maximum at 568 nm compared to that of quinacridone itself due to increased conjugation. In dilute solution, narrowband PL with an FWHM of 25 nm and a PLQY of 91% was measured, while a redshifted broadened PL spectrum with decreased PLQY was observed in concentrated solutions due to aggregation. The ITO/NPB/ Alq 3 :IDQA (1 wt%)/Alq 3 /LiF/Al EL device exhibited a narrow emission at λ EL = 588 nm with an FWHM of 45 nm.

Cyclometalated complex-based phosphorescent materials with narrow emission
Cyclometalated complexes containing Ir, Pt, Pd, Eu, and Tb in their core surrounded by π-conjugated ligands show intense phosphorescence even at room temperature 50,51 . Through efficient ISC from S 1 to T 1 , these phosphorescent emitters can achieve internal EL quantum efficiencies close to unity via strong spin-orbit coupling (SOC) between the emitting triplet state and high-lying singlet states, demonstrating their great potential as emitters in phosphorescent OLEDs (PhOLEDs) [52][53][54] . Figure 5a shows a simplified molecular orbital (MO) diagram for organo-transition metal compounds 55 . The electrons in most cyclometalated complexes are populated in the ligand-centered (LC) and metal-to-ligand charge transfer (MLCT) states upon excitation, yielding mainly four excited electronic states of the singlet and triplet MLCT ( 1 MLCT and 3 MLCT) and LC ( 1 LC and 3 LC) states. Subsequently, the low-lying 3 MLCT and 3 LC states can be occupied by excited carriers via internal conversion (IC) and ISC. In many phosphorescent cyclometalated complexes, the lowest excited T 1 state can be described as a combination (or intermixing) of 3 MLCT and 3 LC, forming the LUMO of a hybrid triplet state. MLCT-dominant phosphorescence is common in phosphorescent Ir and Ru coordination complexes and is characterized by broad, structureless emission with pronounced solvatochromism and rigidochromism originating from strong CT characteristics. The MLCT-dominant emission in various Ir complexes has a relatively short phosphorescence lifetime (τ p on the order of ns to μs) because of the efficient SOC with a singlet excited state such as 1 MLCT. By differentiating the central metal and organic ligands, cyclometalated complexes can also form various excited states, such as metal-centered (MC) and ligand-to-metal charge transfer (LMCT) states, as shown in Fig. 5b 56 . LMCT is the charge transfer from an MO with ligand-like character to that with metal-like character, which is an opposite process of MLCT from an MO with metal-like character to that with ligand-like character. Thus, LMCT reduces the metal center, while MLCT undergoes oxidation of the metal center. Unlike homoleptic phosphorescent complexes with identical ligand structures, heteroleptic complexes with incorporated ancillary ligands (L′) have additional transition states induced by the ligands, i.e., L′C, ML′CT, and ligand to ligand charge transfer (LL′CT) states, facilitating fine modulation of phosphorescence color tuning.
The emitting T 1 state splits into three substates via zero-field splitting (ZFS) depending on the SOC 54 . The ZFS values correlate with the relative contribution of the 3 MLCT and 3 LC states to the LUMO of cyclometalated complexes. As shown in Fig. 5c, the octahedral Ir(4,6-dFppy) 2 (pic) complex exhibits a larger ZFS and a shorter emission decay time (ΔE(ZFS, Ir) = 67 cm −1 and τ(Ir) = 0.4 μs) compared to the square planar Pt(4,6-dFppy)(acac) complex (ΔE(ZFS, Pt) = 8 cm −1 and τ(Pt) = 3.6 μs) 57 . The emitting T 1 state in Pt(4,6-dFppy)(acac) is largely LC, showing a better resolvable emission compared to that of Ir(4,6-dFppy) 2 (pic). Large ΔE(ZFS) values over 50 cm −1 in quasi-octahedral structures indicate significant 3 MLCT character in the emitting T 1 state, showing favorable radiative emission from the triplet substates to the ground S 0 state via efficient SOC. The smaller ΔE(ZFS) from the emitting triplet state in Pt complexes originates from the weaker SOC between the lowest 3 LC ( 3 ππ*) and 1 MLCT ( 1 dπ*) states. In this case, indirect SOC mixing can occur if the 1 MLCT state interacts with the 3 MLCT state, which couples with 3 LC by a configuration interaction (CI) 54 . Compounds with ΔE(ZFS) smaller than 1 cm −1 show favorable 3 LC emitting states. Cyclometalated complexes, which have a T 1 state with dominant 3 MLCT character, show a short phosphorescence lifetime with high quantum yield; however, structureless broad emissions are commonly observed, originating from the strong CT character of these states. To induce 3 LC-dominant narrow phosphorescent emission, it is necessary to design phosphorescent molecules with a 3 LC state that is lower than the 3 MLCT by modifying the structure of the organometallic complex. Vibrational coupling to the ground state in the 3 LC-dominant emission can be effectively suppressed by employing rigid ligand frameworks without intra-or interligand CT interactions. In this chapter, we review cyclometalated complexes with narrow emission and discuss the correlation between the molecular structure and intermixing of the 3 LC and 3 MLCT states and the resulting light-emitting characteristics ( Table 2).

Ir-based phosphorescent cyclometalated complexes
Since the Thompson and Forrest group developed fac-tris (2-phenylpyridine)iridium (fac-Ir(ppy) 3 ) in the late 1990s 58 , heavy metal-based phosphorescent emitters have been extensively studied with the aim of exploiting their strong SOC and triplet excitons. Ir(III) complexes have shown good photo-and thermal stabilities, high quantum efficiencies, and short lifetimes. For example, fac-Ir(ppy) 3 is a representative green-emissive triplet emitter (λ PL = 519 nm) with a high PLQY of~100% at room temperature. However, it shows a broad emission spectrum due to its strong MLCT character at ambient temperature 59 . Therefore, cyclometalated Ir complexes with a variety of ligands have been synthesized to improve emission color purity by modulating the energy level and spatial geometry (Fig. 6). Bejoymohandas developed a series of Ir complexes (Ir1 (2-1), Ir2 (2-2), Ir3, and Ir4) by attaching electron-donating and electronwithdrawing substituents to the quinoline moiety in (benzo[b]thiophen-2-yl)quinoline cyclometalating ligands 60 . The HOMO and LUMO of Ir1-Ir4 are localized primarily in the cyclometalating ligands rather than the Ir center, resulting in an electronic transition with reduced MLCT character. As a result, the electron-donating methyl-substituted Ir1 and unsubstituted Ir2 showed narrow PL spectra at 655 nm in DCM with FWHM < 44 nm, while Ir3 and Ir4, which had strong electron-withdrawing trifluoromethyl and ethyl ester substituents, showed prominent CT character, resulting in a broad emission spectrum with a smaller emissive bandgap. The light-emitting properties of phosphorescent metal complexes can also be modulated by changing the ancillary ligand. Because of their higher triplet energy, nonchromophoric ancillary ligands do not influence the emission process directly, but different ligand field strengths can alter the SOC efficiency and structural distortion. Bejoymohandas developed new Ir complexes (Ir1-pic (2-3) and Ir2-pic (2-4)) by replacing the flexible thenoyltrifluoroacetylacetonate (tta) ancillary ligand of Ir1 and Ir2 (hereafter called Ir1-tta and Ir2-tta) with a rigid picolinate (pic) moiety 61 . As shown in Fig. 7a, the change in ancillary ligands had a negligible influence on the shape of the PL spectrum and FWHM (< 44 nm), but a slight blueshift in the PL spectra of the Ir1-pic and Ir2-pic was observed because of the stronger ligand field of the ancillary pic ligand compared to the tta moiety. The quantum efficiencies of Ir1-pic (49%) and Ir2-pic (37%) were significantly higher than their tta analogs (2-8%) because the rigid picoline ligand reduces the extent of the geometrical deformation of the excited state and thereby suppresses nonradiative relaxation. With the device architecture ITO/ PEDOT:PSS/PVK/CBP:TPBi:Ir1-pic (15%) or Ir2-pic (10%)/ TPBi/LiF/Al, EQEs of 5.03% and 3.41% were obtained for Ir1-pic and Ir2-pic-based devices with EL maxima at 657 and 664 nm, respectively.

Pt-and Pd-based phosphorescent cyclometalated complexes
The T 1 state of most octahedral Ir complexes is primarily determined by the 3 MLCT state, which induces broad PL emission with high PLQY because of the strong SOC between the triplet and singlet states. In contrast, square planar Pt and Pd complexes typically have narrowband emission, but longer phosphorescence lifetimes and lower PLQYs than those found in Ir complexes were observed because of weak SOC in the square planar complexes (Fig. 8). Li et al. modified the ligand structure of the PtON1 structure that had a bridging oxygen between the phenyl-pyrazole (ppz)-based cyclometalating ligand and the carbazolyl pyridine ancillary ligand by incorporating electron-donating substituents (methyl (PtON1-Me, 3-1), tert-butyl (PtON1-tBu, 3-2), and N,Ndimethylamino (PtON1-NMe 2 , 3-3)) at the 4-position of the pyridyl ring, resulting in narrow deep-blue emissions (FWHM 15-20 nm) at 445 nm with high PLQYs (80~95%) (Fig. 9) 65,66 . These results can be explained by the fact that the 1 MLCT/ 3 MLCT character in the original PtON1 was thermally accessible at room temperature, while the addition of an electron-donating group increased the energy levels of MLCT states, suppressing   70 . Although porphyrin is known to have a long-lived triplet state, PtOEP showed a decreased phosphorescence lifetime (91 μs) and 50% PLQY due to strong SOC. The Alq 3 :PtOEP-based PhOLED device has an emission spectrum similar to that of its porphyrin ligand and exhibits strong phosphorescence at 650 nm with a small FWHM of 19 nm, which is thought to originate from the dominant LC character of the emitting T 1 state. Thompson also reported another type of Pt-porphyrin complex; the two representative examples, PtOX (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) and PtDPP (3-16) had octaalkylporphyrin and arylporphyrin ligands, respectively 71 . PtOX showed a narrow emission peak at 648 nm with FWHM = 26 nm in polystyrene films, and PtDPP also displayed a narrow PL spectrum at λ PL = 630 nm (FWHM = 33 nm). The rotation of the phenyl group in PtDPP promoted nonradiative relaxation, making the PLQY of PtDPP (16%) lower than that of PtOX (44%). However, the spin-coated PtDPP:PS thin film showed an improved PLQY similar to that of PtOX because the rotation of the phenyl group was restricted. Narrow EL spectra of ITO/α-NPD/Alq 3    ring onto at least one of the benzo groups in tetrabenzoporpyrin 76 . Extension of the aromaticity in the porphyrin framework provided emission maxima in Pd complexes in the near infrared region (849-882 nm). In particular, cis-palladium(II)-meso-tetra-(4-fluorophenyl) dibenzodinaphthoporphyrin (Pd2NF, 3-23) exhibited PL at 868 nm with a small FWHM of 41 nm in toluene. Similar to those of Pt porphyrins, the optical properties of Pd porphyrins are determined by the degree of similarity between the ground and excited states of the porphyrin ligands.
In addition, industries have also tried to develop narrowband emitters; for example, deep red-emitting phosphorescent OLEDs with suppressed first vibronic transition in their EL spectra (molecular structure was not disclosed, λ E = 640 nm with a FWHM of 43 nm) were reported by Universal Display Co. and Beijing Summer Sprout Technology Co., Ltd. at the Society for Information Display (SID) conference in 2019 and 2020, respectively 77,78 .

Eu-and Tb-based phosphorescent cyclometalated complexes
Owing to their sharp emission, trivalent lanthanide (Ln) metal complexes, including those with Eu 3+ and Tb 3+ ions, have attracted attention as red-and green-emissive phosphorescent materials (Fig. 10) 79 . The luminance of Eu and Tb complexes is mainly caused by f → f transitions in the metal and shows an extremely sharp emission (FWHM < 10 nm). Because the direct population of the excited emitting state in Ln 3+ is prohibited, the organic ligands in the Ln complex were first excited, and then the T 1 state was formed. Subsequently, Dexter-type electron exchange between the T 1 state of the ligand and the Ln ion induces the indirect population of the 4f level of Ln 3+ complexes, yielding sensitized luminescence [80][81][82] . In Ln metal complexes, ligands determine the sensitization phenomenon; therefore, the resulting PL intensity of the Ln ion is different depending on the structure of the ligand 83 . In Eu complexes, β-diketonate-type structures have been widely used as ligands to sensitize Eu 3+ ions, where control of the precise energy of the ligand T 1 energy level is important to ensure that it has a higher energy than the emissive level of Eu 3+ ( 5 D 0 ) and to optimize the sensitization. Martín-Gil et al. synthesized Eu(cbtfa) 3 (bath) (4-1), which has a halogenated β-diketonate-based ligand and a rigid bathophenanthroline ancillary ligand (bath) 84 . Upon excitation at 365 nm, the Eu(cbtfa) 3 (bath) powder, solution (in chloroform), and thin film showed almost identical PL spectra with a sharp red emission at 613 nm with a 5 nm FWHM. As shown in Fig. 11a, a strong electric dipole transition ( 5 D 0 -7 F 2 ) causes intense emission at 613 nm due to the highly polarizable ligand field around the Eu 3+ ion and has five satellite peaks corresponding to intraconfigurational f → f transitions. The EL spectra of the EL (ITO/PEDOT:PSS/ Eu(cbtfa) 3 (bath)/Ca/Al) device were similar to the PL spectra with a dominant emission at 613 nm. Because only one crystal field line is possible for a complex with D 3h symmetry 79 , emission from most Eu complexes occurs from a narrow 5 D 0 → 7 F 2 transition, regardless of the external environment or ligand structures. Sivakumar et al. synthesized Eu(TTA) 3 Phen-Fl-TPA (4-2), in which the tta ligand acts as an optical antenna with efficient energy transfer to Eu 3+ ions, exhibiting a sharp emission at 612 nm in both solution and film samples 85 . Borisov et al. reported a series of Eu complexes, Eu(HPhN) 3 phen (4-3), Eu(HPhN) 3 dpp (4-4), and Eu(HPhN) 3 DDXPO (4-5), containing 9-hydroxy-1H-phenalen-1-one (HPhN) as a sensitizing ligand and different ancillary ligands 86 . S 1 and T 1 excited states of the ancillary ligands with a higher energy than those of HPhN improved the sensitization process, and similar PL spectra at~611 nm were observed for all structures, showing~20% PLQY in polystyrene films. Similar results were reported by Vaidyanathan et al., who showed that all Eu-β-diketonate complexes had similar PL data with a sharp emission at 612 nm both in film and in chloroform solution, regardless of the ancillary ligand (4-6~4-10), indicating efficient energy transfer from the T 1 state of the β-diketonate ligand to the excited state of Eu 3+ (Fig. 11b) 87 .
Similarly, various Tb complexes with different ligands also exhibit similar PL spectra arising from the 5 D 4 → 7 F 6 transition (Fig. 11c) 88 . Huang et al. reported a Tb complex, Tb(PMIP) 3 DPPOC (4-11), whose optical and electrical properties were comparable to those of Tb (PMIP) 3 (H 2 O) 2 (4-12) and Tb(PMIP) 3 (TPPO) 2 (4-13) (Fig. 11d) 89 . Fig. 11d also shows that similar emission spectra were observed for all Tb complexes with sharp Tb 3+ emission at 548 nm ( 5 D 4 → 7 F 5 ) with multiple satellite peaks. Since the energy level of the triplet state of DPPOC is closer to the 5 D 4 level of Tb, Tb(PMIP) 3 DP-POC showed a higher PLQY (16.7%) than the other Tb complexes. When the Tb(PMIP) 3  TADF-based light-emitting materials TADF, which was pioneered by Parker and Hatchard, is a phenomenon whereby triplet excitons are converted to singlet excitons by thermal activation and consequently undergo fluorescence 91 . Theoretically, 100% internal quantum efficiency (IQE) is possible via an efficient upconversion process called reverse intersystem crossing (RISC). Adachi's group successfully developed a new generation of TADF-based OLEDs 92 . The design of TADF emitters involves spatial separation of the electron-rich (donor) and electron-deficient (acceptor) moieties to separate the HOMO and LUMO distributions and create a small energy gap (ΔE ST ) between the S 1 and T 1 states (Fig. 12a) 93 . This design strategy has resulted in a significant improvement in EL efficiency, but it also enhances the structural relaxation in the excited states via ICT, resulting in broad CT emission (FWHM of 70-100 nm) with a large Stokes shift in TADF OLED devices (Fig. 12b) 94 . To achieve both high-performance and narrowband emission in TADF OLEDs, Hatakeyama et al. designed new TADF materials in which a rigid molecular framework with regular arrangements of boron and nitrogen atoms shows an MR effect; the HOMO is localized on the nitrogen atoms and at the meta-position with respect to the boron atom, whereas the LUMO is localized on the boron atom and at the orthoand para-positions (Fig. 12c) 94 . Thus, the boron and nitrogen atoms in the MR-TADF structures have the opposite resonance effect, and the HOMO and LUMO are separated onto different atoms without the need for electron-rich or electrondeficient substituents. The MR-TADF compounds shown in Fig. 13 undergo a limited amount of reorganization in the excited state and maintain their oscillating strength due to effective overlap of the electron and hole wavefunctions, ensuring narrow PL and EL spectra with high luminescence efficiency. In addition, the MR effect minimizes the bonding/antibonding characteristics of the HOMO and LUMO, and the resulting nonbonding MOs minimize the vibronic coupling and vibrational relaxation in the material, resulting in emission peaks with an extremely small FWHM 95 . In this chapter, various MR-TADF-and conventional D-A-type molecules are summarized, and strategies to achieve narrow emission are discussed (Table 3).
Ma et al. also reported a series of MR-TADF-based QAO derivatives of QA-PF (5-44), QA-PCN (5-45), QA-PMO , and QA-PCz (5-47) 114 . A design strategy to yield narrow emission was proposed by enhancing the low-frequency vibronic coupling strength while simultaneously reducing the high-frequency vibronic coupling strength of the commonly involved stretching modes. The fluorophenyl groups in QA-PF suppressed the highfrequency stretching vibrations coupled to the structural reorganization between S 0 and S 1 , resulting in a decrease in the overall reorganization energy. Based on this approach, all the QAO-based derivatives showed a narrower PL emission in toluene (FWHM = 23-29 nm) compared to that of QAO. The EL devices also showed narrow spectra (FWHM = 27-30 nm); however, the EQEs were still lower than those of the B/N core structures and suffered from severe efficiency roll-off originating from TTA and SPA. The close intermolecular packing was suggested as one reason for the EQE roll-off.

Summary and outlook
In this review, fluorescent, phosphorescent, and TADF emitters with narrowband emission were summarized, and the structure-property relationships were discussed. The development of narrow-emission OLED materials is of great importance to meet the new BT 2020 color standard for wide-gamut displays 118,119 . Recently, molecular design strategies and the underlying mechanisms for controlling the emission bandwidth have been studied extensively to achieve narrow PL and EL. π-Conjugated organic fluorophores normally suffer from limited color purity with broad emission owing to their intrinsic vibronic coupling between the S 0 and S 1 states as well as structural relaxation in the excited state. Twisted or rigid structures with fused aromatic backbones have shown narrow emissions with a dominant 0-0 transition caused by inhibition of CT interactions and intermolecular aggregation in the solid state. For example, a narrow EL spectrum (FWHM of 35 nm) was observed in TPA-PIM (1-9), which has suppressed vibrational splitting due to its fully twisted donor-acceptor structure, achieving the smallest CIE y of 0.046 ever reported at that time 31 . The rigid planar flavanthrone-based dyes (compounds 1-15~17) showed extremely small FWHM values of~22 nm at 535 nm in their EL spectra 36 . The quinacridone or BODIPY-based structures also exhibited a small FWHM ≤~50 nm in the green to red region.
The emitting T 1 state in cyclometalated phosphorescent emitters is often determined by intermixing the 3 MLCT and 3 LC states. Ir-based octahedral complexes have short phosphorescence lifetimes with high PLQYs due to strong SOC, but their MLCT-dominant emissions result in broad spectra. Several square planar Pt-and Pd-based structures have shown narrow phosphorescence, where the T 1 state is mainly determined by the 3 LC state. By modifying the ligand structures in the cyclometalated Pt/Pd complexes, the emission bandwidth can be further decreased, increasing the color purity. Li et al. modified the rigid tetradentate pyridyl ring of carbazolyl pyridine ancillary ligands to further suppress the vibrational coupling, resulting in improved phosphorescence quantum yield and achieving an FWHM of <20 nm with >80% PLQY (compounds 3-1~5). In particular, the PtON7-dtb (3-5)based EL device exhibited a high EQE of 24.8% with dominant emission at 451 nm (FWHM of 29 nm).
TADF molecules can achieve 100% IQE through efficient upconversion via the RISC process; however, these emitters are often designed by spatial separation of the HOMO and LUMO distributions to decrease the ΔE ST between the S 1 and T 1 states. This strategy enhances the structural relaxation in the excited states via ICT, resulting in a broad CT emission with FWHM values of 70-100 nm and large Stokes shifts. Recently, new MRbased TADF molecules with rigid skeletons and regular arrangements of boron and nitrogen atoms have been investigated extensively by Hatakeyama et al. Owing to the MR effects between the electron-accepting boron and electron-donating nitrogen atoms, the strategic separation of the HOMO and LUMO localized around the individual atoms could achieve not only high EQE but also narrow emission bandwidth. For example, MR-TADF OLEDs based on ν-DABNA (5-14) demonstrated extremely narrow emission with an FWHM of 18 nm and a remarkably high EQE of 31.4% 95 .
Although many promising results for the development of singlet and triplet emitters with narrow emission have been reported, several challenges remain. To further decrease the emission bandwidth, the 0-0 vibronic transition should be intensified with higher 0-n (n = 1, 2, 3…) vibronic transitions suppressed. Thus far, it is not clearly understood how the 0-n vibronic transitions can be efficiently suppressed or controlled by the molecular design of fluorophores. The vibrational normal modes coupled with the electronic S 1 to S 0 transition need to be studied to understand how to control their Huang-Rhys factors and minimize the 0-n vibronic transition. The number of green-and red-emitting singlet fluorescent emitters is very limited, with most of the reported narrowband fluorophores being blue-emitting materials. Molecules are often designed to exploit the ICT interaction to extend the effective conjugation with a reduced bandgap; however, the CT character induces undesirable broad emission. Given these limitations, the development of efficient green-and red-emitting fluorescent materials with small FWHM values is of paramount importance. Hyperfluorescent OLEDs made by combining TADF sensitizers and narrow-emitting fluorescent dopants can be an effective strategy to achieve both high EQE and color purity. In the case of phosphorescent triplet emitters, the structural optimization of square planar organometal complexes is still needed. For instance, rigid tetradentate ligands without intraligand or interligand CT interactions can reduce vibronic coupling with decreased emission bandwidth. Similar to the case of fluorescent materials, the investigation of the vibrational normal modes coupled with the LC S 1 to S 0 transition to decrease their Huang-Rhys factors may suggest an affordable solution to intensify the 0-0 transition with reduced 0-n vibronic peaks. The compound library of phosphorescent emitters showing narrow emission and high PLQY needs to be further expanded. As shown in Fig. 1d, blue-and green-MR-TADF emitters have been widely studied to achieve narrow emission and high EQE. However, the development of red-emitting MR-TADF molecules is still far behind, and extending the conjugation of these molecules without broadening the emission bandwidth remains challenging. In addition, the efficiency roll-off in MR-TADF OLEDs that occurs at a high current density remains a significant barrier to the industrialization of MR-TADF OLEDs. To overcome the EQE and its roll-off that originates from TTA, SPA, aggregation quenching, etc., the optimization of emitter structures and the optimization of the device architecture should be considered together. For instance, Adachi recently reported an MR-TADF OLED employing ν-DABNA as an emitter by combining a TADF sensitizer, showing a maximum EQE of 41% with suppressed efficiency roll-off 120 . Monkman also adopted ν-DABNA as a hyperfluorescent emitter, suppressing efficiency roll-off by decreasing the excimer quenching with optimization of its doping ratio 121 . In addition, the enhancement of outcoupling efficiency by controlling the molecular orientation of MR-TADF emitters needs to be studied to further improve the EQE and roll-off. As discussed, the underlying mechanism needs to be further clarified, and fine optimization of the host, TADF dopants, and charge injection/transport layers is necessary to improve the charge balance and efficiency roll-off. In particular, special attention should be given to the design of narrowband green emitters because their CIE coordinates need to be further optimized to a further degree than those of blue and red emitters to satisfy the BT 2020 standards. Despite the current challenges, narrowband emissive OLEDs have great potential to become an efficient and extensively adopted display technology in the next generation of highresolution and wide-color gamut displays if both the resolution and color chromaticity are considered.