A systematic review on 1,8-naphthalimide derivatives as emissive materials in organic light-emitting diodes

Organic light-emitting devices (OLEDs) have garnered significant research attention owing to their immense application prospects in leading technologies for full-color flat panel displays and eco-friendly solid-state lighting. They demonstrate exceptional features such as mercury-free construction, wide viewing angle, superior color quality and captivating flexibility. The requirements of light-emitting organic materials pertaining to high stability, lifetime and luminescence quantum yield, combined with the fabrication of devices with high performance efficiency, are highly challenging. Rational molecular design of 1,8-naphthalimide (NI) derivatives can offer quite promising results in achieving standard-light-emitting materials with a wide range of colors for OLED applications. This review is mainly focused on the synthesis and usage of varyingly substituted NI frameworks as luminescent host, dopant, hole-blocking and electron-transporting materials for OLEDs that emit not only red, orange, green and blue colors, but also function as white emitters, which can really have an impact on reducing the energy consumption. The future prospects that could be explored to improve the research in the highly promising field of OLEDs are also discussed.


Introduction
An organic light-emitting diode (OLED) incorporates a thin film of an organic compound that can produce light as a response to an applied electric current, and this phenomenon is termed as electroluminescence (EL). OLEDs are bestowed with key beneficial characteristics such as bright solid-state emission with good luminescence efficiencies, realizable color tunability, fast response time, less energy consumption, facile fabrication to flat as well as large flexible displays with low cost and lightweight [1][2][3][4][5][6][7]. Hence, they find application in digital display devices including television screens, large flat panel computer displays, surface light sources and also in portable appliances such as personal digital assistants, mobile phones and handheld game consoles [8][9][10][11]. In terms of display applications, fluorescent OLEDs are more suitable compared to phosphorescent ones because of faster response rate and lower efficiency roll-off [4,12,13]. Ever since the innovative research on organic EL reported by Tang and VanSlyke and the pioneering report by Burroughes et al. on polymer EL, the field of OLEDs has fascinated several researchers and has subsequently led to various developments related to discovery of new organic light emitters and design of several devices using them [14][15][16][17][18][19][20].

Construction of an OLED
The fabrication of a typical single-layer OLED involves the introduction of an organic EL material layer amid anode and cathode, and all of them deposited on a substrate that supports the device [21]. However, multilayer OLEDs that incorporate two or more layers can be fabricated by inserting electron or hole-blocking layers (EBL or HBL) adjacent to the emissive layer (EML) to attain maximum recombination of carriers and thereby enhance the J Mater Sci (2022) 57:  device efficiency. Modern devices are mainly designed with a simple bilayer structure that comprises of a substrate, an anode, a conducting layer, an EML, and a cathode [22]. The selection of the anode material is based on certain fundamental qualities such as electrical conductivity, optical transparency, and chemical stability. Indium tin oxide (ITO) is the most frequently used anode owing to its high work function that facilitates the instillation of holes into the highest occupied molecular orbital (HOMO) of the organic layer. Gold and platinum are also rarely used as anodes. The conductive layer is generally poly (3,4-ethylenedioxythiophene) with polystyrene sulfonate (PEDOT:PSS) because its HOMO level lies between the HOMO of other frequently used polymers and the work function of ITO, thereby favoring decreased energy barriers for hole injection. Interestingly, the delocalization of p electrons in organic molecules with conjugated framework enables them to be electrically conducting because the frontier molecular orbitals (HOMO and lowest unoccupied molecular orbital-LUMO) energy levels of these organic semiconductors are comparable to the valence and conduction bands of the inorganic semiconductors. Thus, these organic materials can function both as conductive and EML of an OLED. The cathodic materials are usually barium (Ba), calcium (Ca) and magnesium (Mg), or their alloys, capped with a protective layer of aluminum (Al) or silver (Ag). These materials display low work functions that can support in electron injection into the LUMO level of the organic layer. The conducting or hole transport layer (HTL) transfers holes from the anode, whereas the EML or electron transport layer (ETL) transports electrons from the cathode. When a DC bias is applied to the electrodes, the recombination of these electrons and holes occurs in the organic layer, emitting light. The color of the light relies on the characteristics of the emissive material, whereas the brightness depends on the applied electrical current [23][24][25][26][27].

Organic materials for LED fabrication
The main technical features of an OLED include energy and current efficiency (CE), quantum efficiency (QE) of the emitter, operating and inclusion voltage, angle of view, brightness, contrast, life time and temperature range. Nevertheless, the main hurdle while using organic materials in LEDs for fullcolor display is their efficiency, lifetime, stability and inability to obtain pure red, green and blue (RGB) primary colors [28][29][30]. The vital strategy to improve the device efficiency is to balance the transport of charge carriers by introducing an ETL and a HTL to the diode architecture [31][32][33][34][35][36][37][38]. Though many organic compounds that can function as HTL are accessible, only very limited electron-deficient organic materials that unveil good electron-transporting features are known [39][40][41][42][43]. OLEDs based on tris-(8-hydroxyquinoline) aluminum (Alq 3 ) as the electron transporter material have been explored widely [32,[44][45][46][47]. Besides Alq 3 , different materials such as carbazole, oxadiazole, triazole and phenanthroline derivatives [48] are also reported as electron transporters as they own high electron affinity. Attention toward new organic materials started growing immensely since the first report published by Pope et al. on EL from a single crystal of anthracene [28,[49][50][51][52]. Consequently, immense experimental efforts were dedicated on designing newer functional materials for organic optoelectronics. Numerous p-conjugated molecules that display wide range of semiconducting and conducting behavior were developed as light-emitting and/or charge transport materials. The organic molecules that can potentially act as electron-transporting materials should possess sufficiently high electron affinity, greater than 3.0 eV to enable electron injection from the metallic cathode. In recent years, heterocyclic molecules especially those with strongly electron-withdrawing imide units with excellent intramolecular charge transfer (ICT) features have emerged as ETL materials with promising and potential use in the construction of OLEDs [40,[53][54][55].

1,8-Naphthalimide as an electrontransporting luminescent material
The two main requirements for an ETL material in an OLED is the presence of an electron-deficient p-system with good chemical and thermal stability. In recent years, optical, electrochemical and photoelectrical properties of electron-deficient 1,8-naphthalimide (NI) derivatives with planar rigid architecture [56] have gained increasing attention of researchers, who explore material features. NI derivatives have been utilized in different applications including laser active media, fluorescence switchers, fluorescent markers in biology, photoinduced electron transfer sensors, ion probes [57,58], etc. Attractive derivatives can be created from NI skeleton, which can display strong fluorescent emission on irradiation, and their rich photophysical properties enable them to function as prime molecular entities for use in optoelectronic devices [59,60]. The presence of electron-deficient center in NI derivatives generally permit them to achieve high electron affinity, which validates their usage as HBL in OLEDs [61,62]. These molecules can have low reduction potentials [48,63,64] and wide energy gaps [65][66][67], and hence are profoundly studied as electron-deficient n-type organic semiconductors [68,69]. Relatively greater electron affinity, superior charge transfer property, higher glass transition temperature, better film-forming capability, high fluorescence quantum yield (QY), desired chemical and photo-stability, and highly tunable fluorescence emission of NI derivatives demonstrate their possible use as emissive ETL to improve the lifetime and stability of not only small molecules, but also polymer-based OLEDs [29,68,[70][71][72][73][74][75][76][77][78][79].
Since the OLED working is based on the holeelectron recombination in the EML, the color emitted by a NI-based device depends primarily on the electronic features of the material [4,[80][81][82][83]. Moreover, the variations in the chemical, photophysical and excited-state optical behavior of NIs are extremely sensitive to the nature and position of the substituents in the aromatic system [66,[84][85][86]. Thus, NI derivatives with desired features can be synthesized by the reaction of suitable amines with appropriate 1,8-naphthalic anhydrides, which enable to achieve various functionalized derivatives of imide nitrogen and substituted NI skeleton. Extending the aromatic ring system by creating aromatic-or heteroaromaticfused derivatives can be another successful strategy to control the NI properties. The substitution using halo, nitro or amino groups mainly at C3 or C4 position of NI unit permits the introduction of further functional/pendant groups or allows extension of the existing substituent chain length. Moreover, the existence of electron-donating amino and alkoxy moieties at the 3, 4, 5 or 6-position in the aromatic rings of NI framework can not only enhance the fluorescence QY as they trigger a polar charge transfer excited state, but also can shift the absorption to visible region. Their emissions can be extensively and finely tweaked to produce blue to green or yellow fluorescence with a marked Stokes shift. Further, alteration in the nature of the ring or imide nitrogen substituent can direct the fluorescence emission further bathochromically toward red too. In some cases, these NI derivatives can be used as dopants to trap charges between the layers to facilitate luminescence at the host material [48].
NI derivatives are thus good electron-transporting emitters with high electron affinity of about 3.1 eV and electron mobilities as high as 0.16 cm 2 /(Vs) [87] that can be custom-modified specifically for an OLED to realize full-color emission having superior performance characteristics such as high fluorescence QY and good photo-stability [88]. Development of OLEDs that emit R/G/B (red/green/blue)-colored fluorescence with appropriate chromaticity and high efficiency are extremely imperative for full-color display applications [8,[89][90][91]. In this context, construction of large number of NI derivatives that have been investigated to achieve wide color-range is reported by various pioneers. Moreover, as white light-emitting OLEDs (WOLEDs) have created substantial performance advances to the general lighting [92][93][94][95], attributed to their significant environmental and energy saving benefits, few reports on NI grafted polymers for WOLED applications are also documented. The following sections illustrate the fabrication of OLEDs using different NI derivatives as ETL, in view of achieving optimum optical, electrochemical and thermal properties.

Red-green-blue (RGB) emitters
Organic frameworks that can offer full-color RGB displays are of high prospective demand. Maximum driving efficacy can be accomplished by means of self-emitting RGB pixels, and hence development of materials that can emit light in each of these primary colors is of crucial importance. NI derivatives have been established to be highly capable and color tunable emitters due to their plausible synthetic modification possibility at the C4 and the anhydride O positions. The attachment of diverse aliphatic systems and in particular aromatic skeletons can spread out the conjugation in these molecules to achieve desired features.

Blue emitters
Blue emitters possess higher electron injection barrier. Hence the major challenge in developing an efficient blue-emitting material relies on the wide optical band gap required to accomplish relatively high-energy emission. NI that exhibit high electron affinity and electron-transporting or hole-blocking features perform as excellent materials suitable for balanced carrier injection in blue OLEDs [68]. The introduction of various electron-donating substituents to NI framework can readily tune their emission color to pure blue. Phosphorescent OLEDs were fabricated by Kolosov et al. by utilizing four violet-blue-emissive materials such as NI, N-phenyl-NI, N-2,6-dibromophenyl-NI (1), and bis-N,N-NI as EMLs (Fig. 1) [48]. The compound 1 prepared by treating 1,8naphthalic anhydride with 2,6-dibromo aniline showed good charge transport property as holeblocking and electron-conducting material, and also exhibited film-forming property with glossy appearance when doped with phosphorescent complexes. Two sets of OLEDs were designed: type 1 with 1 as a doped luminescent layer and undoped 1 as HBL; and type 2 device with doped CBP (1,4-N,N'-dicarbazolylbiphenyl) as luminescent layer and 1 as HBL. In both the device types, organometallic iridium (Ir) complexes 2, 3, and 4 that emit yellow, green and red colors, respectively, were used as dopants. The hole hopping between the dopant molecules of positive charge carriers in Ir dye-doped NI films enabled recombination of electrons and holes at the phosphorescent dye. OLED with 4 as dopant showed better performance with dopant emission having lowest energy excited state as the dominant relaxation pathway. The type 2 device with 2 showed voltage-dependent red emission at low bias (4-8 V) and green emission when the bias was increased.
Ulla et al. synthesized twelve molecules 5a-f and 6a-f by introducing different halogens, aldehyde and nitrile carrying phenoxy groups at the 4th position of NI unit to modify the electronic and optical features, including the fluorescence QY (Fig. 2a). NIs 5a-g were prepared from acenaphthene through a series of reactions-bromination, oxidation, imidation with ethanolamine, and final treatment with substituted phenols. On further acetylation, NIs 6a-f were obtained. These derivatives exhibited deep blue fluorescence in thin film state with excellent chromaticity and high stoke shifts [62,96]. The compounds were stable up to 260-280°C and displayed quite higher electron affinities ranging from 3.31 to 3.43 eV, compared to the frequently used ETL materials. Electrochemical band gaps were in 2.84-3.02 eV range and the low-lying frontier molecular orbital (FMO) energy levels endowed them with n-type and hole-blocking properties. Intensely blue emissive 5a that crystallized in triclinic space group P-1 was employed as an emissive ETL material in the un-optimized trilayer device, and subsequently as ETL alone in the bilayer device. The devices consisted of ITO as the transparent anode, (4,4'-bis[N-(1naphthyl)-N-phenyl-lamino]-biphenyl (a-NPD) with high HOMO level as HTL, bathocuproine (BCP) as HBL because it confines the reductant holes that did not recombine with the electrons in the emitting zone, Alq 3 as ETL, lithium fluoride (LiF) as the electron injection layer (EIL) and Al as cathode. The phosphorescent device with blue emitting 5a as ETL demonstrated an excellent external quantum efficiency (EQE) of 1.40% which was comparable to those with Alq 3 as the standard ETL and also without any ETL.
Ulla et al. further in 2017 synthesized two blueemissive chloro-phenoxy NI derivatives 5g and 6g, which possessed high electron affinity, wide band gaps (3.04 and 2.99 eV) and high stoke shifts for possible use as ETL in OLEDs ( Fig. 2a) [97]. They also displayed high molar extinction coefficient due to the charge transfer nature of long wavelength absorption band attributed to the S 0 ? S 1 transitions. HOMO and LUMO levels were quite low making these materials excellent electron-transporting and holeblocking emitters in OLEDs. The phosphorescent devices with NI as ETL exhibited superior performance compared to those without any ETL and similar performance to that of Alq 3 devices. 2,3,5,6-Tetrafluoro-7,7,8,8 tetracyanoquinodimethane (F 4-TCNQ) could efficiently inject the holes from the ITO anode to the HTL in the device.
Wang et al. synthesized eight NIs 7a-d and 8a-d by nucleophilic substitution at C4 position with either phenyloxy or tert-butyl group attached phenyloxy units and incorporation of N-(2-hydroxyethyl) or N-(2-acetoxyethyl) units at the carboximide sites ( Fig. 2) [68]. The compounds 7c and 8c with 2-(tertbutyl)phenyloxy group displayed blue fluorescence, good color purity and a QY of 0.29 in their solid films. However, the incorporation of an additional weakly electron-donating (tert-butyl) moiety exhibited greenish blue fluorescence and enhanced the photoluminescence QY to 0.42 owing to the D-p-A molecular architecture where D and A denote electron donor and acceptor, respectively. Besides, 8a, 8b and 8c showed higher QY than 7a, 7b and 7c due to end capping of acetylated hydroxyl group. Electrochemical and optical band gaps were found to be in the range of 2.95-3.01 eV. The compounds were suitable as blue doping materials or non-dopant blue emitters for ETL or HBL owing to high electron affinity values ranging from 3.24 to 3.29 eV and good electron injection abilities.
converted to the corresponding boronic acid pinacol ester by borylation with bis(pinacolato)diboron catalyzed by Pd(PPh 3 )Cl 2 /KOAc and then treated with 1,3,6,8-tetrabromopyrene to yield 9, 10 and 11 (Fig. 3). These molecules, which exhibited ICT were thermally stable up to 347-447°C with 5% weight loss. Solution processed devices were prepared with configuration: ITO|PEDOT:PSS-Nafion (Nf)|9/10/11:10% CBP|TmPyPb|LiF:Al. The device with CBP-doped 11 as emitter showed better performance with a maximum luminance of 3389 cdm -2 , a maximum EQE of 3.98%, luminance efficiency of 3.22 cdA -1 , and a turn-on voltage of 3.2 V. The better performance of 11 was due to the push-pull mechanism, suitable HOMO and LUMO levels and better charge mobility, which ensured a more balanced and improved recharge recombination behavior within the device.
The reported blue-emitting NI frameworks predominantly carry a substituted phenyloxy group at the C4 position and hydroxyethyl or acetoxyethyl substituents at the nitrogen atom of the electron-deficient NI skeleton. The current efficiencies (CEs) of devices with NI derivatives as electron transport materials are higher compared to the CEs of the devices with NI as EML indicating the charge transfer property of these compounds. Except for 5g and 6g (VI), EQE and maximum luminescence was better for devices with NI derivatives as ETL rather than EML with the turn-on voltage ranging from 3 to 9.4 V. Device 11 with pyrene-NI-based molecule exhibited maximum EQE of 3.98% among other reported blue emitters. Over the years, though the improvement in device efficiency was in research focus, the efficiency reported till date for blue OLED is less compared to other categories, and these NI derivatives have high energy gap leading to joule heating and defects in the device [99].

Green emitters
Besides blue and red emissions, further manipulation of the electron-accepting NI framework has enabled construction of high-performance green and yellow EL materials with ICT characteristics as featured in this section. Novel triphenylamino NI derivatives 12a-c were synthesized via Suzuki cross-coupling using N-phenyl-NI and triphenylamine having distinct photophysical properties by Arunchai et al. (Fig. 4) [80]. The enhanced conjugation, the packing pattern and restricted molecular vibrations in thin film state displayed a blue-shifted emission compared to solution state. These molecules were thermally stable till 350°C with an electrochemical band gap in the range of 2.48-2.54 eV. A single-layer device with 12a exhibited low performance due to poor film-forming ability and electron migration barrier at the EML and LiF|Al electrode interface. BCP was incorporated to improve maximum luminance, and an EL device was fabricated with device structure-ITO|PEDOT:PSS|12a:CBP|BCP|LiF|Al, which exhibited a stable yellowish green fluorescence with CIE coordinates at (0.295, 0.600). The device displayed a maximum brightness of 10,404 cd/m 2 at an applied voltage of 19 V, turn-on voltage of 5.8 V, luminance efficiency of 3.77 cd/A, current density of 410 mA/m 2 , and EQE of 1.11% due to the efficient energy transfer from host BCP to 12a. The devices with 12b and 12c showed lower performance due to solubility issues during the spin-casting process.
Ding et al. synthesized nine amorphous NIs (13-15)a-c incorporating electron-transporting benzazole units through direct imidation of NI (Fig. 5a) [29]. The twisted molecular conformation of these molecules hindered their tendency to recrystallize, favoring stable amorphous phase, without altering the extent of conjugation as well as the emission color of the original chromophore. The QE of 13a-c with benzothiazole units was superior to 14a-c and 15a-c bearing benzoxazole and benzimidazole units, correspondingly. The electrochemical HOMO and LUMO levels were in -5.51 to -5.68 eV and -2.61 to -3.26 eV ranges, correspondingly. Green or yellowish green luminescence was observed with the device architecture ITO|NPB (75 nm)|EML (65 nm)|Mg:Ag|Ag (100 nm) with 13a, 14b and 15a as EML and ETL. Device with 15a showed better performance with a maximum brightness of 4500 cd/ m 2 (at 19 V) and a turn-on voltage of 10 V with CIE coordinates (0.43, 0.53) at 556 nm.
A NI derivative with fluorene was synthesized by treating 4-bromo naphthalic anhydride with 2-aminofluorene followed by reaction with dimethylamine (16) by Wang et al. to design a green EL device (Fig. 5b) [100]. The highly luminescent and environmental sensitive molecule 16 displayed excited-state charge transfer features with good electron affinity, and temperature-independent fluorescence. The derivative possessed an electrochemical band gap of 2.6 eV, and low-lying LUMO levels enabled its electron-transporting capability. The device with configuration ITO|TPD|16|Al exhibited yellowish green EL with a brightness of 3563 cd/m 2 at 19 V, EQE of 0.2% and luminous efficiency of 0.55 lm/W. Figure 5 Green-emitting NI derivatives with a electrontransporting benzazole [29] and b fluorene units [100] as dopants, and their device structures.
Bipolar light-emitting materials possess both electron affinitive and hole-transporting elements for improving the charge injection/transport ability [101]. NI owns good thermal stability due to the presence of imide linkage and high electron affinity owing to the existence of carbonyl moiety, thus favoring stable anion radical formation. Twisted bipolar NI derivatives with carbazole dyads (17a-f) and triads (18a-c) were synthesized through the imidation reaction of substituted naphthalic anhydride by Zhu et al. (Fig. 6) [101]. The prepared derivatives act as carrier-transporting fragments that could improve charge carrier balance and control the current flow to enhance the internal QE. Stable amorphous phase was favored in these molecules due to aliphatic side chain that can reduce the tendency of recrystallization. Neither the conjugation degree, nor the original emitting chromophore color changed, when the molecular assembly incorporated the hole-transporting moiety into the emitting element. The singlet-singlet energy transfers in these sterically hindered dyads and triads were possible owing to overlap between NI unit absorption and carbazole moiety emission. These thermally stable bipolar emitters with high glass transition temperatures produced stable anion and cation radicals for exciton recombination, and enhanced the stability and lifetime of the fabricated device. Single-layer EL device with these derivatives avoided the layer-to-layer exciton quenching and micro-cavity effect. Single and multiple layered devices having the configuration ITO|17a-f/18a-c (50-100 nm)|LiF (1 nm)|AlLi and ITO|CuPc (10 nm)|TPD (10 nm)|17a-f/18a-c (30 nm)|BePP 2 (45 nm)|LiF (1 nm)|AlLi, respectively were fabricated. The device with compound 18b displayed maximum brightness of 260 cd/m 2 at a driving voltage of 18 V. Bis[2-(2-hydroxyphenyl)-pyridine] beryllium (BePP2) served as the ETL, whereas phthalocyanine copper (CuPc) and TPD acted as the HTL to improve the device efficiency. Greenish yellow emission was purely from NI moiety, whereas the carbazole component did not contribute to the light emission, instead facilitated the electron-blocking and hole injection. Thus, these bipolar dyad emitters generated stable anion and cation radicals for the recombination of holes and electrons to produce excitons that undergo radiative decay in the NI moiety, resulting in its characteristic emission.
size of aryl-dioxin moiety influenced the emission characteristics of the derivatives. The FMO energy levels and the band gaps increased with the incorporation of a hexyloxy group and a second benzene ring in the dioxin moiety and showed a more pronounced effect with the aromatic ring extension. Among the two NIs 21 and 23 that were functionally tested as EL materials in device with ITO|PVK:TPD|21/23|Al architecture, better device performance with cyan-greenish emission was obtained with 21 as the EML because of the specific orientation of aliphatic chains toward the conjugated aromatic systems. A working OLED with NI 22 as EL could not be implemented as the long alkoxy group hindered the electron-hole recombination process.
The NI derivatives reported for green OLEDs usually have electron-deficient imine nitrogen and the 3rd or 4th position of NI backbone substituted with various moieties such as phenyl, long-chain aliphatic group, carbazole and fluorene. Most of these green-emitting NI derivatives were used as undoped emitters, except the single-layer device with 12 that exhibited poor efficiency. Hence, it was not only doped with CBP, but also BCP layer was added to improve the device efficiency. Maximum luminescence was observed when CBP was doped with 12 which is sixfold higher than other reported greenemitting NI derivatives for OLEDs.

Orange emitters
Efficient orange-emitting NI derivatives can be prepared either by shortening the p-conjugation length or by introducing non-planarity in the molecular conformation. Two divinylenes (24 and 25) that carried fluorene and phenylene units, together with a terminal electron-accepting NI moiety in their structural framework, were synthesized by Mikroyannidis et al. through Heck coupling reaction of 4-bromo-Ncyclohexylnaphthalimide with 9,9-dihexyl-2,7-divinylfluorene and 1,4-bis(dodecyloxy)-2,5-divinylbenzene, respectively (Fig. 8a) [103]. They exhibited greenish orange fluorescence with QY of 0.  2-naphthalene-1-yl-benzo[de]isoquinoline-1,3dione as electron-accepting core and arylamine substituents as both hole-transporting and electron-donating groups were prepared through N-arylation by Jung et al. (Fig. 8b) [104]. Electrochemical band gaps of both the molecules were * 2.42 eV, and thermal stabilities were found to be 368°C and 407°C for 26a and 26b, respectively. EL was observed at 574 and 588 nm with luminance efficiencies of 6.6 cd/A at 5.9 V and 5.9 cd/A at 6.3 V for 26a and 26b, correspondingly. These orange emitters were highly efficient because the asymmetric and bulky structure could control the intermolecular dipole-dipole interaction.
Zeng et al. synthesized two red-orange emitters 27 and 28 with rigid acridine unit as electron donor and NI moiety as acceptor that exhibited thermally activated delayed fluorescence (TADF) and possessed pre-twisted charge transfer state exhibiting EQE up to 30% [105] (Fig. 9a). The substitution at the acridine moiety can vary their electronic and photophysical properties. High QY, TADF and horizontally oriented emitting dipoles of these two emitters make them suitable EMLs in OLEDs. Horizontal dipole ratios of both the EMLs were higher than the host 9-(3-(9H-Carbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile (mCPCN) and was beneficial in optical outcoupling of OLEDs. Device was fabricated with configuration-ITO|MoO 3 |TAPC|mCP |mCPCN: x wt% 27 or 28|3TPYMB|LiF|Al. Both the emitters were doped with mCPCN (1.5 wt% of 27 and 6 wt% of 28) with an emission at 581-600 nm at a cut off voltage of 3 V with EQE of 21-29.2%.

Orange emitters with TADF properties
TADF can provide remarkably effective emission through singlet and triplet exciton harvesting. The small singlet-triplet energy gaps allow the thermal conversion of triplet excitons to singlet species in TADF emitting materials by reverse intersystem crossing (RISC) to achieve 100% theoretical internal Figure 8 Orange-emitting NI compounds with a divinylene fluorine and phenylene [103], b N-naphthyl substituentes [104].
QE of TADF-OLEDs. Recently, emitters with TADF have been reported as alternative for phosphorescent emitters for their utilization of both singlet and triplet excitons and their easily modified metal-free pure organic structures [106,107]. Though many blue, green and yellow TADF materials are reported, the development of the orange-red TADF emitters remains to be a big challenge.
Chen et al. developed two orange-red TADF active NI emitters (29 and 30) with benzofuran and benzothiophene fused to the acridine framework (Fig. 9b) [108]. These NI derivatives tend to possess plane and crooked form, where crooked form acridine leads to a lower twisting angle between donor and acceptor, thus exhibiting the TADF property.  Orange-emitting NIs with a acridine units [105] and b benzofuran and benzothiophene fused to the acridine framework [108] along with their device architecture. with 9,9-dimethyl-9,10-dihydroacridine (Fig. 10) [109]. These compounds possess higher thermal stability up to 405°C with excellent electrochemical properties, good TADF and circularly polarized luminescence properties. Devices were constructed by co-depositing CBP with 6 wt% of these emitters: ITO|HAT-CN|TAPC|TcTa|CBP|31a-b|TmPyPB| LiF|Al. The 31b-based device exhibited an orangered emission with a peak at 592 nm and high maximum EQE of 12.4%, and its maximum CE and PE were 28.5 cdA -1 and 28.8 lmW -1 , respectively.
The NI skeleton was appended with a variety of substituents to obtain orange emission, and in all the devices different hosts like Alq 3 , PVK and mCPCN were used. This category of emitters is considered as the linkage between green and red with mixed emission color and displayed a EQE of 5%, wherein orange-red TADF emitters showed maximum EQE of 20.3% with maximum luminescence of 2350 cdm -2 . D-A type and chiral molecules with orange-red TADF emission displayed improved EQE and maximum luminescence.

Red emitters
Red electrofluorescent materials are classified into molecules that (1) incorporate polycyclic aromatic hydrocarbon framework and (2) display ICT features with D-p-A structures. Only a few red emitters with high fluorescent QYs are reported for OLED fabrications till date [110,111]. Though few europium chelate complexes, pyran and porphyrin derivatives [112][113][114][115][116] are among those investigated as red-emitting materials, they suffer from various disadvantages such as (1) concentration/aggregation caused fluorescence quenching, (2) lack of good chromaticity, (3) lower EQE, (4) complicated synthetic routes, (5) inadequate overlap between the emission bands of the red dopant and the host matrix, (6) low doping levels and (7) high production cost [117][118][119][120]. Therefore, developing red emitters with improved color purity and high efficiency still remain a challenge. The rigid p-conjugated electron-withdrawing NI core skeleton can be manipulated to design longwavelength emitters by introducing electron donors at C4 position in order to achieve deep LUMO levels, bathochromically shifted emission wavelengths and improved fluorescence QY [56,121,122].
Azomethine derivatives of 4-amino-N-phenyl-NI, wherein the imine unit acts as a part of the conjugation linker joining the N-phenyl-NI and a phenyl unit, were found to exhibit wide ranges of emission colors. Gan et al. synthesized two series of amorphous Schiff bases 32a-e and 33a-b (Fig. 11) by condensing four hydrazino-NIs with suitable aldehydes [123]. ICT occurred in these derivatives between the electron providing amino substituent and the electron-accepting NI. The extended amino-conjugated system at the imide nitrogen and the increased electron-contributing ability of the moieties attached to NI backbone led to large bathochromic shift in the emission of the Schiff bases in their film form. These red-emitting materials did not exhibit any concentration causing quenching effects, and the fluorescence lifetimes of these emitters were dependent on the electron donor substituents connected through the C=N group. The OLED with 32d as the nondoping EML in the device ITO|CuPc (12 nm)|NPB (30 nm)|32d (45 nm)|sodium stearate (2 nm)|Al (100 nm) showed maximum luminance and current density values of 15.5 cd/m 2 and 2.9 mA/cm 2 , respectively, at an applied voltage of 22 V. CuPc served as HTL in the device to maintain the injection balance of electron and hole. The device performance was poor compared to that obtained with red-doping method.
Zhou et al. discovered that ICT featured NI 34 could function as a host material in orange-redemitting OLED through triplet-fusion mediated triplet harvesting (Fig. 12a) [124]. Molecule 34 displayed triplet-fusion delayed fluorescence (TFDF) with lesser exchange energy and lower lying pp* level than charge transfer state, thereby contributing to triplet harvesting via P-type delayed fluorescence, instead of thermally activated E-type one. The electrochemical molecular orbital levels were found to be -5.64 and -3.14 eV, correspondingly. The device configuration ITO|NPB (30 nm)|CBP (2 nm)|34:35 (6 wt%, 20 nm)|Bphen (40 nm)|Mg:Ag in which 4,4'-N,N'-dicarbazolylbiphenyl (CBP) and bathophenanthroline (Bphen) acted as HBL and ETL materials, respectively, showed maximum CE of 7.2 cd A -1 and brightness of 16,840 cd m -2 . The triplet-triplet annihilation (TTA) process dominated in devices with these NI derivatives functioning as both the doping matrix and non-doped emitter.
through imidation reaction of naphthalic anhydride with neutral red in quinoline. The HOMO and LUMO states were 5.4 and 3.2 eV, correspondingly. Highly efficient multilayer EL device was fabricated with 1% 39 as dopant having structure as ITO|NPB (60 nm)|Alq 3 :39 (30 nm)|Alq 3 (20 nm)|Mg-Ag (200 nm). The OLED presented maximum brightness of 18,400 cd/m 2 at 13 V exhibiting a unique constant CE of 5.2 cd/A at a wide current density range of 1-500 mA/cm 2 at different doping concentrations, with relatively narrow emission bands. The bright orange-red emitter 39 did not show any substantial drop in efficiency with increasing dopant concentration and/or current density and can function as a suitable material in passive matrix display which demands high excitation density. The emission wavelength can also be tuned to develop not only different color producing devices, but also whiteemitting OLED (WOLED).

Red emitters with AIE and TADF properties
Aggregation-induced emission (AIE) can augment solid-phase luminescence intensity by suppressing exciton annihilation and aggregation-induced luminescence quenching. Certain molecules (AIEgens) can aggregate to show significantly enhanced emission in solid state compared to its individual molecular form. Their molecular motions are arrested in the aggregate form due to unique structures and stacking modes. The use of AIEgens can overcome the issues of aggregation-induced quenching, and enable concentration unaffected doping or even non-doped EMLs for OLEDs [127][128][129][130]. Generally, the fluorophores can use only 25% singlet excitons in an OLED. Though small singlet-triplet energy gaps and high photoluminescence QY are essential features for TADF emitters, high-performance red TADF materials are difficult to achieve. This is because the electron and hole should be decoupled on the spatially separated frontier orbitals to achieve the small energy gap, which would possibly reduce the ICT resulting in low QY. The following examples demonstrate the design and synthesis of AIE-TADF NI derivatives as red emitters for OLEDs.
Recently, two red emitters 40 and 41 have been developed by Wu et al. with strongly electron-accepting NI component and electron-donating nonpolar groups such as 10-H-phenothiazine or 9,9dimethyl-9,10-dihydroacridine connected through 2,6-dimethylphenyl as a p-linker (Fig. 14a) [131]. The molecules with D-p-A architecture were synthesized Figure 13 Red-emitting NI compounds and their devices with a D-p-A structure possessing DMAP or TMJ as electron donors [117] and b ICT active moiety as dopant [126].
via Buchwald-Hartwig C-N coupling utilizing a bromide intermediate and exhibited unique AIE and TADF characteristics. The twisted confirmation due to the p-bridge was responsible for restriction in the intermolecular rotation (RIR) leading to AIE property enabling high solid-state red emission, with an internal QY of 55% and 39% for 40 and 41, respectively. Moreover, their twisted structures led to efficient HOMO and LUMO separation and resulted in small energy gaps between lowest singlet (S1) and triplet (T1) excited states. Highly effective RISC process was realized to convert triplet to singlet excitons for robust TADF emission in these molecules. They exhibited good thermal stability for wide range of  [56]. The target molecules were obtained by Suzuki coupling, Ullmann reaction or Buchwald-Hartwig reaction between the boric acid, its ester or secondary amine derivatives with the bromide intermediate (Fig. 14b). The incorporation of N-([1,1'-biphenyl]-4-yl)-9,9-dimethyl-9H-fluoren-2-amine and 2,6-diphenyl-4-toluene in 42 introduced large steric-hindrance, improved the QY and inhibited p-p stacking and strong molecular associations in the aggregate form, resulting in AIE, whereas in 43, the presence of the p-bridge increased the fluorescence rate constant and QY. Both the molecules were thermally stable up to 440°C. Non-doped devices with 42 and 43 exhibited lower performance compared to CBP-hosted devices, attributed to the low photoluminescence QY and triplet exciton utilization efficiency. As dilution of emitters in the host matrix is possible in the host-guest system, efficiency of the device can further be enhanced by an efficient Fö rster energy transfer. Based on this concept, a multilayer OLED was prepared with the structure: ITO|HAT-CN (15 nm)|TAPC (40 nm)|TcTa (5 nm)|x wt%-42 or 43:CBP (20 nm)|TmPyPB (40 nm)|LiF (1 nm)|Al (100 nm). Doped OLED with CBP as host and 5 wt % of 42 and 43 as dopants could produce a CE of 14.7 and 28.0 cd A -1 as well as EQE of 4.81 and 7.59%, correspondingly. 2,3,6,7,10,11-Hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN) and LiF served as hole injection layer (HIL) and EIL, respectively. 1,1-Bis(4-di-p-tolylaminophenyl)cyclohexane (TAPC) and (3,3'-[5'-[3-(3-pyridinyl)phenyl][1,1':3',1''-terphenyl]-3,3''-diyl]bispyridine (TmPyPB) were used as the HTL and ETL, respectively. However, peak efficiencies decreased with increase in dopant concentration. A thin TcTa layer was introduced at the EML/HTL interface in order to lessen the hole injection barrier between CBP-hosted EML and TAPC. Sequential charge trapping occurred due to the larger band gap of CBP compared to that of 42 and 43 as dopants. Moreover, Figure 15 Synthesis and device architecture of TADF red emitters with a strong electron-deficient benzoisoquinoline-1,3-dione [132] and b phenoxazine or phenothiazine donor moieties [133]. TmPyPB having a low HOMO level was used to prevent hole leakage and permit a better confinement of excitons and charge carriers within the EML. Nondoped EML displayed orange and red emission with maximum EQE of 1.53 and 1.39%, CE of 2.44 and 3.71 cd A -1 and PE of 1.85 and 2.39 lm W -1 for 42 and 43, respectively. The CIE coordinates of redemitting CBP-free device with derivative 42 was very close to National Television Standards Committee (NTSC) standard red. The doped devices based on 42 and 43 displayed hypsochromic shift and narrow EL spectra compared to host-free device, with CIE color ranging from orange to red and green to orange, respectively, with increasing dopant concentration.
Two red-emitting donor-acceptor TADF emitters (44a-b) with strong electron-deficient benzoisoquinoline-1,3-dione were prepared by Yun et al. by treating bromo NI compounds with aniline and subsequent reaction with 9,9-dimethyl-9,10-dihydro acridine (Fig. 15a) [132]. Both the molecules exhibited charge transfer property and emitted in red region around 578 and 601 nm. Multilayer devices were prepared by using these emitters as dopants with configuration: ITO|PEDOT:PSS|TAPC|mCP|CBP: TPBi:44a-b (25 nm, 5% doping)|TSPO1|TPBi| LiF|Al. EL peak for these devices was around 600 nm with color coordinates of the 44a and 44b as (0.56, 0.44) and (0.54, 0.46), respectively. Recently Wang et al. have reported TADF active red emitters (45a-b) by attaching an electron-deficient NI acceptor moiety with a phenoxazine or phenothiazine donor moiety (Fig. 15b) [133]. The two emitters exhibited distinct TADF characteristics with small energy gaps between the lowest singlet and triplet excited states, which originated from the well-separated HOMO and LUMO levels. These emitters are thermally stable up to 310°C and exhibited solid-state emission at 588 and 600 nm for 45a-b. Multilayer devices were fabricated with the design: ITO|MoO 3 | TAPC|mCP|o-mCPBI: 45a-b (10 wt%) Bphen|Li-F|Al. Both the devices exhibited relatively small efficiency roll-offs. The maximum power efficiency values of 14.8 and 9.8 lmW -1 and EQE values reducing to 9.4% and 6.0% at a luminance of 1000 Cdm -2 were realized for devices with 45a and 45b, respectively.
NI moiety on its own does not display any red emission in the absence of extended conjugation. Hence, incorporation of bulkier aromatic substitutions electron-releasing moieties at the C4 site of NI core can not only alter the emissions from blue to red, but also augment the fluorescence QE. Thus, designing molecules with D-linker-A skeleton that combine both TADF and AIE features can greatly improve the efficiency of OLEDs. All the EMLs were doped with CBP and Alq 3 . Though the development of these types of devices are in progress, an EQE of 13.6% was achieved till date. Devices with CBP have shown better performance than devices with Alq 3 . Highest CE was observed in red emitters in comparison with other color emitters of NI.

White emitters
White organic and white polymeric light-emitting diodes (WOLEDs and WPLEDs) have been lately viewed as one among the emerging technologies [134] because of their impending usage in full-color light-emitting diodes, low-cost backlight for liquidcrystal displays, and several other lighting sources. WOLEDs have garnered special attention due to associated key properties such as energy saving ability, lightweight, flexibility and optimal heat dissipation, thereby replacing the conventional white light sources using large area panels. Typically, in WPLEDSs, white emission is achieved either by using polymer-blended systems comprising of red, green and blue light-emissive polymers, or by doping a small quantity of orange/red-emissive polymer into a blue-emissive polymer. Based on the white light generation, by varying the content of orange/redemitting NI conjugated to blue-emitting polymers and the substantial alterations in the current-voltage responses of the devices centered on these materials, few white-emitting polymers incorporating NI unit were studied as illustrated below.
Tu et al. developed a white light-emitting polymer 46 by incorporating 0.05-8 mol % of orange-emitting NI chromophore into a blue light-emitting polyfluorene system using Yamamoto polycondensation reaction with catalytic amount of Ni(0) (Fig. 16a) [134]. The NI quantity in the polymer was responsible for the tuned emission. A WPLED with device structure ITO|PEDOT (40 nm)|46 (80 nm)|Ca (10 nm)|Al (100 nm) presented CE and PE of 5.3 cd/ A and 2.8 Lm/W at 6 V, respectively, with CIE coordinates at (0.25,0.35) and maximum brightness of 11,100 cd/m 2 . The polymer-blended device presented a very stable white light emission in terms of both color and efficiency at different brightness and J Mater Sci (2022) 57:105-139 driving voltage. The chemical doping was advantageous over physical doping because the blended systems displayed lesser CE. The emission from dopant molecules increased with rising driving voltage, which indicated the dependency of the CIE coordinates on the driving voltage.
Later, polymers 47a-e with blue-emisssive poly(fluorine-alt-phenylene) that contained comonomers carrying red-emissive NI pendant units with varying concentration (0, 0.0005, 0.005, 0.02 and 0.08 wt%) were developed by Coya et al. as luminescent materials for WOLEDs (Fig. 16b) [135]. Devices configured as ITO|PEDOT:PSS|47a-e|Ba|Al were emissive at low driving current range of 47-73 lA. EL emission was dominated by the NI contribution, which exhibited a redshift with increased concentration of the chromophore. Pure white light emission was observed from the copolymer bearing 0.0005% chromophoric NI groups of 47b with luminous efficiency of 9.42 Cd/A at 50 lAs from almost pure white color with CIE coordinates (0.26, 0.30) for low currents to stable cool white (0.21, 0.23) for different polymers. Increase in NI content (47d-e) resulted in stable green to orange fluorescence with efficiency of 6.7 Cd/A.
The emission color was dependent on the concentration of NI in the copolymer. Higher concentration of these derivatives showed green to orange emission with decreased CE. Small molecules used in WOLED with triplet excitons in the radiative recombination process perform better as they involve spin-orbital coupling allowing radiative decay and are also lowcost material [136]. Using a single and multilayer TADF emissive layer, very low turn-on voltages (V), Figure 16 White-emitting NI blended polymers and their device architectures based on combination emission strategy [134,135].
high PE (lm/W), CE (cd/A), and even a maximum of 28% EQE with a CIE coordinate close to standard (0.33, 0.33) can be achieved [93].

Theoretical studies
Theoretical studies play a significant role in developing materials with desired features for preferred applications. Several investigations have established the relationship between theory and experiment that can provide useful insights into the behavioral aspects of molecules designed for distinctive applications [137,138]. Theoretical probing bestows exhaustive understanding of the optical, electronic, and charge-transporting characteristics of a designed molecule, and wisely tune the substituents attached to achieve a final OLED material with the most desired properties.
D-A molecular arrangement of NI derivatives 48af with fluorene substituents was investigated by Chai et al. to understand their optical, electronic and charge transfer features for OLED application (Fig. 17a) [139]. The electronic and optical characteristics were influenced by the substituent units in fluorene skeleton. All molecules demonstrated potential electron and hole transport properties, while derivatives incorporating dibenzothiophene fragment could function only as a hole transport material. D-p-A bipolar molecules 49aj with triphenylamine components as blue lightemitting donors, NI unit as acceptors, and diverse pconjugated spacers with vertical absorption and emission transitions were designed by Jin et al. as presented in Fig. 17b [140]. Varying the electron-donating p-conjugated spacers of 49a-j is an extremely favorable approach to develop hole transport luminescent materials for OLED. Sun et al. designed a series of N-butyl-1,8-naphthalimide derivatives 50a-i as luminophores for OLEDs (Fig. 17c) [141]. Among these molecules, 50 g and 50i with benzo[c]thiophene and benzo[d]thieno [3,2-b]thiophene units exhibited hole-transporting features. The FMOs analysis of all these molecules 48-50 revealed that the absorption and emission process were ICT-characterized. The calculations suggested that the substituent groups in the molecular structure affect the optical and Figure 17 Molecular structures of NI derivatives identified as a, b, c potential light-emitting materials [139][140][141] d, e as hole or electron transport materials [142,143] through theoretical investigations.  -87  12  ---17b  539  -90  11  ---17c  540  -120  11  ---17d  535  -110  11  ---17f  532  -98  10  ---18a  539  -238  7  ---18b  538  -260  8  ---18c  534  -224  8 --- electronic properties and these molecules are promising candidates for OLED application. Optoelectronic properties like charge transfer and luminescence of a series of NI materials that carry electron-donating and aromatic groups 51a-i were investigated by Jin et al. (Fig. 17d) [142]. The optical and electronic properties varied with substitution at 4th position of NI derivatives. However, ICT property of substituents was not significant and stability of the molecules did not change with the substitution. Later in 2015 same research group studied starshaped D-p-A small molecules (52-55)a-d with NI structures for OLED application (Fig. 17e) [143], and using the FMO and local density of states analysis, the vertical electronic transitions of absorption and emission were characterized as ICT. The end groups and spacers affected the optical properties. These star-shaped compounds had lower band gap, hence extended the absorption spectra to longer wavelengths and especially 54a could be used as hole or electron transport material.

Summary
Highly intense luminescence and excellent carrier mobility that arises from planar molecular geometry and extended p-conjugated systems are the most desired features of OLED materials. In this context, the possibility of grafting a large variety of auxochromic groups into the NI skeleton, and properties of the surrounding medium that can enable finetuning of the emission colors for OLEDs are illustrated in this review. The device performances with NIs having various structural frameworks that emit R/G/B and white colors, respectively, are summarized in the Table 1. The luminescence intensity is related to the extent of conjugation allowed within the molecule, which escalates based on the interactions between the donor and acceptor elements of the molecule. Hence by exploiting the scope of introduction of different substituents, a range of NI derivatives have been prepared and utilized in both small molecule blue, red, orange, green-emitting OLEDs and polymer-based WOLEDs. Typically, proper selection of donor as well as acceptor moieties enable to achieve suitable separation of the HOMO and LUMO energy levels in the D-A-type molecular structure for OLED fabrication. Moreover, emission from ICT states contribute immensely to EL of these devices. Search for newer and efficient charge-transporting functional materials for OLED applications holds great importance.

Future prospects
Organic solid-state lighting is now progressing toward technical lighting and general illumination from decorative applications. This evolving transition demands device architectures with higher efficiency, better color purity, increased lifetime, and reduced production costs. Few important aspects that need technical advancements are highlighted below.
Development of blue emitters is one of the challenging research areas for material chemists as generally they have lesser thermal stability, luminescent efficiency and lifetime, as well as narrow emission wavelength for color purity compared to green and red emitters [144,145]. Besides, low electron affinities and wide energy band gap contribute to higher barrier for electron injection, resulting in poorer device performance. Though efficient blue-emitting p-type materials have been constructed and this review portrays investigations on n-type NI derivatives as blue OLED materials, more studies on stable and efficient n-type blue materials are essential for commercialization of blue OLEDs. While being used as red dopants in commercial applications, high-performance n-type red emitters are much sought after due to compromised color purity, chemical stability and device efficiency. NI derivatives with higher fluorescence QE, narrower FWHM (full width at half maximum) and far-infrared emission are extremely required. Though design of triplet red emitters with extended lifetime have been achieved, their efficiency decreases with increase in brightness [146], which challenges their use in passive dot-matrix displays. Red-emitting compounds possess low energy gap between HOMO and LUMO levels which leads to high non-radiative transition rate of exciton in some cases and finally results in the quenching of emission. TADF materials can solve this problem to some extent, and though some of red TADF materials are reported, further investigation can be oriented in this direction. In addition, except few reports with devices of around 20% EQE, most red TADF materials featured only 5-15% EQE range and lag behind in displaying high performance in OLED applications [121]. Besides, design of highly efficient TADF-AIEgens which can facilitate high concentration doping combined with red emission beyond 600 nm are of high demand as they offer efficient and enhanced solid-state luminescence. The molecular design of red-emitting TADF featured AIEgens with both strong ICT state and twisted molecular conformation are also of extreme emphasis.
Even though it gives the impression that the triplet energy level of NI with maximum phosphorescence near 540 nm [147] may hinder their possible use only as a host to red and orange dopants, the prospects of exploiting these molecules for efficient electro-phosphorescence from blue or green devices can be further investigated. Moreover, an important factor for degradation of OLED is its operational current.
Reducing the operational current and retaining luminescence is a task which also demand further attention [148][149][150]. Innovations in WOLED architectures that can allow color tuning from cool to warm white or even full-color tunability are highly desired. Host materials for green technology devices with adjustable brightness levels that become even more efficient when dimmed are also looked for. Though the main focus in current WPLED technology is to reduce manufacturing costs and to improve mass-production processes, WPLED efficiency with very low Cd/A values is still a major concern [151,152]. Accordingly, there exists a substantial necessity for novel organic semiconductors that uniquely combine enhanced processability, stability and performance with efficient charge transport properties and light emission. Thin, flexible and transparent devices of almost any shape, which can emit from both sides, can enable a completely new experience that is both attractive and highly efficient.
Thus, in spite of extensive research that have been conducted toward refining the stability, color tunability and efficiency of OLEDs either through molecular engineering or modifying device configuration [134,136], design and synthesis of new materials for desired colors, maximization of the EQE, and modes of addressing fabrication of full-color display devices with optimized resolution continue to remain a challenge. We anticipate that this comprehensive report would benefit the researchers who are actively engaged or planning to enter the arena on OLED materials search and could kindle further research in this area.

Funding
Open access funding provided by Manipal Academy of Higher Education, Manipal.

Declarations
Conflict of interest The authors declare that they have no conflict of interest.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licen ses/by/4.0/.