Amphiphilic Acetylacetone-Based Carbon Dots

On-going development of carbon dots (CDs) for different applications calls for search of novel methods for their synthesis and surface functionalization. For fabrication of light-emitting devices (LEDs), CDs should be soluble in non-polar solvents that are used for ink-printing of their functional layers, apart from the obvious requirement of bright luminescence. Herein, we introduce amphiphilic CDs synthesized from a mixture of benzoic acid and ethylenediamine in acetylacetone, which satisfy both above mentioned requirements. These CDs are quasi-spherical nanoparticles 20-50 nm in size, holding aliphatic, carbonyl, amide, imine, and carbamate groups at the surface which renders them amphiphilic and soluble in a variety of substances with relative polarity ranging from 0.002 to 1, such as toluene, chloroform, alcohols, and water. By variation of the molar ratio of benzoic acid and ethylenediamine, an optimal value for photoluminescence quantum yield of 36 % in non-polar solvents is achieved. Importantly, these CDs are easily mixable with a charge transport polymer – polyvinylcarbazole, a common component of organic LEDs. As a demonstration of use of developed amphiphilic CDs in LEDs, green emitting charge-injection devices are fabricated with a broad emission band centered at 515 nm, maximal luminance of 1716 cd/m 2 , and ССT of 5627 K.


Introduction
[3] Using different chemical precursors in the bottom-up synthesis of CDs makes it possible to functionalize their surface for different purposes starting from biomedical to energy/lighting applications. [4]For the latter direction, which include lightemitting devices (LEDs), solar cells, composite catalysts, etc., the energy level structure and existence of electron-withdrawing or -donating groups at the surface of CDs plays crucial role . [5- 8] n many cases however, the surface functionality of CDs is somewhat limited by the fact that traditionally their synthesis and further solubilization are performed in polar solvents.At the same time, the application of CDs in devices as a part of composite emissive/charge-transport layers demands their compatibility with a variety of polymers and other materials which are mostly soluble in non-polar solvents. [9]Despite this demand, studies devoted to development of CDs soluble in nonpolar organic solvents, and, even better, amphiphilic nanoparticles are still rare.
Currently, the non-polar based CDs have been produced by hot injection, [10] microwaveassisted [11,12] and solvothermal methods, [13][14][15] and by refluxing. [16]In the most of the studies where hydrophobic CDs were produced, combinations of aliphatic compounds (amines, aldehydes, acids) with hydrophobic molecules, including organic dyes, were used as precursors.For example, hydrophobic CDs were prepared by hot-injection method from melted Paraplast granules injected in toluene and stabilized with dodecanthiol. [10]In another study, [17] hydrophobic CDs were prepared from Rhodamine B and then functionalized with polyethylene glycol (PEG) for increasing their solubility in polar media.By refluxing dodecylamine in chlorobenzene, hydrophobic blue-emissive CDs with PL quantum yield (QY) reaching 10% were made; [16] these CDs could be easily mixed with polymers such as poly(methyl methacrylate) and polydimethylsiloxane.In another study, dodecylamine-based CDs were used as surfactants to stabilize multi-walled carbon nanotubes. [18]At the same time, the methods for fabrication of bright amphiphilic CDs are still in their infancy. [19]Amphiphilicity of CDs can be introduced via postsynthetic surface modification with long hydrophobic chain epoxides or acrylamides as was recently shown in the study by Chen et al. [20] Another example is the synthesis of CDs in acetone, which mixes well with both polar and non-polar solvents. [14] this work, we selected acetylacetone as a solvent for fabrication of amphiphilic CDs, made from benzoic acid (BA) and ethylenediamine (EDA) as precursors.The resulting CDs are characterized by amphiphilic properties, remarkable stability in a variety of solvent media, as well as chemical yield and reasonably high PL QY.Importantly, these CDs are easily mixable with a common charge transport polymer -polyvinylcarbazole (PVK), which allowed us to fabricate charge-injection green LEDs with a broad emission band and maximal luminance of 1716 cd/m 2 .

Synthesis of amphiphilic CDs
To synthesize amphiphilic CDs in acetylacetone, combination of BA and EDA was used (Figure 1a).This synthesis was inspired by 'classical' synthetic pathway towards bright CDs using citric acid and EDA, [21,22] whereas BA served as a hydrophobic component.Acetylacetone is a highly boiling solvent (140 °C) able to act as a reactive medium facilitating formation of amphiphilic groups at the CD's surface via keto-enol reaction.We envisage three possible mechanisms for the formation of amphiphilic CDs through interaction of acetylacetone with EDA only, BA only, and both precursors together, as demonstrated in Figures S1, S2 and S3, respectively.When EDA reacts with acetylacetone as its carbonyl derivative, bis(acetylacetone)ethylenediamine (Acacen) is formed. [23]Subsequently, Acacen in an excess of acetylacetone forms CDs enriched with imine groups at the surface (Figure S1).When BA reacts with acetylacetone, a denser CD core is formed, based mostly on acetylacetone covered with hydrophobic benzene rings (Figure S2).The reaction of acetylacetone with BA and EDA at the same time increases the hydrophobicity of the formed CDs and accelerates the formation of Acacen, so that esterification inside CDs and 1, 4-addition reactions are possible (Figure S3).We expect that the CDs formed by the latter reaction would possess a variety of functional groups at the surface which create an amphiphilic shell (Figure 1a) thus rendering those nanoparticles soluble in both polar and nonpolar media.Indeed, we have found that the produced CDs can be easily dispersed in a broad set of polar and nonpolar solvents such as water, methanol, ethanol, isopropanol, acetonitrile, dimethyl sulfoxide (DMSO), acetone, chloroform, toluene, and tetrachloromethane (TCM), as shown by the photographs in Figure 1b obtained under sunlight and UV excitation.In the following, we will elaborate in detail on morphology, surface chemistry, and optical properties of the produced amphiphilic CDs.In the forthcoming discussion, CD samples produced using EDA or BA as the only precursors are denoted as CDEDA and CDBA, respectively; for the CDs produced from the mixtures of BA and EDA in acetylacetone while changing the molar ratio of these two precursors, samples are designated as CDXX, where XX is the BA-to-EDA molar ratio.The details of the synthesis are provided in the Experimental section.

Morphology and surface functionalities of CDs
The study of CD's morphology performed by combination of microscopy and spectroscopy methods has confirmed the assumption of their core-shell structure as shown by TEM images in  As can be seen from TEM image in Figure 2a, CDEDA consist of a carbonized core of ~4 nm in size, and a rather thick polymer shell (20-25 nm).CDBA consist of a carbonized core of up to 30 nm in size, whose interplanar distance of 0.34 nm (inset in Figure 2c) corresponds to the distance between sp 2 planes in graphite, [24] surrounded by thicker polymer shell (Figure 2c).The CD0.22 sample consists of carbonized core particles with a core of approx.5 nm and interplane distance of 0.25 nm, that can be ascribed to polyaromatic carbon domains, [25] that are surrounded by a polymer shell so that their overall size constitutes ~12.5±3.0 nm (Figure 2b) From the analysis of AFM images of CDEDA, CDBA, and CD0.22 shown in Figure S4a,b and Figure 2d, respectively, one can derive their height to be less than 20 nm, with a rather broad distribution (Figure S4c-e).The average height was determined as 6.0±3.2,12.0±8.0,and 10.4±4.0 for CDEDA, CD0.22, and CDBA, respectively.The sizes of the three CD samples estimated from TEM and AFM images are summarized in Figure 2e; one can see that they increases with the increase of BA amount.In the XRD pattern of CD0.22 shown in Figure 2f, wide peaks at app. 15, 23, and 42 2θ degree are observed and can be attributed to defective planar structures similar to those of graphitic carbon nitride. [26] should be noted that the broadening of XRD peaks in the diffraction pattern may be related both to the amorphous structure (more than 95% estimated amorphous content) and small size of crystallites (less than 2 nm).As shown in Figure 3a, amphiphilic CDs are composed of carbon, oxygen, and nitrogen (except for the case of CDBA where no nitrogen was present); the content of these three elements were determined as 67%, 77%, and 65% for carbon, 29%, 16%, and 32% for oxygen, and 1%, 4%, and 0% for nitrogen in CDEDA, CD0.22, and CDBA, respectively.The highest amount of carbon and nitrogen alongside with the lowest amount of oxygen was observed for CD0.22 synthesized from the mixture of BA and EDA in acetylacetone, which confirms that BA facilitates the reaction of EDA with acetylacetone as illustrated in Figure S2.From the high resolution XPS data shown in Figure S5, it is seen that CDBA are carbonized particles and their surface is rich with hydroxyl and carbonyl groups; for CDEDA, only a small amount of nitrogen atoms is incorporated into CD's structure as amide/imine groups and as pyridinic N, whereas for CDs produced from EDA and BA N-containing groups in the form of amide/imine and carbamate have been identified (Figure S5e,f).
Raman spectrum of CD0.22 (Figure 3b) has 5 distinct bands with peaks at 1245, 1315, 1415, 1570, and 1655 cm -1 , which are attributed to D*, D, D'', G, and D' bands typical for doped graphene and graphene oxide structures. [27]The D* band can be attributed to the sp 3 -hybridized carbon observed in polyene, [28] trans-polyacetylene chains, [29] and disordered graphite. [30]The D and G bands are breathing and stretching modes of sp 2 -hybridized carbon domains. [31]The D'' band originates from the amorphous carbon phase, [32] and the D' band corresponds to an intra-valley resonance with the G band and undergoes a splitting due to presence of impurities. [33]Thus, we infer that the CDs synthesized here are carbon polymorphs with N and O atoms incorporated into sp 2 -domains; they demonstrate a high degree of presence of amorphous/polymer phase (as manifested through the presence of the D'' and D' bands and a large width of the D and G bands) and a large amount of surface polymer chains such as trans-polyacetylene (manifested through the presence of the D* band).
From FTIR spectra of the CDs provided in Figure 3c, one can see that those spectra are different from the FTIR spectrum represented by a sum of the precursors (shown by a grey line).The Raman spectra in the range of 4000-2500 cm -1 of CDs formed from a mixture of BA and EDA have peaks at 3300, 3060, 29747, 2924, and 2886 cm -1 which can be attributed to the stretching vibrations of https://doi.org/10.26434/chemrxiv-2023-80dsgORCID: https://orcid.org/0000-0001-6841-6975Content not peer-reviewed by ChemRxiv.License: CC BY-NC-ND 4.0 N-H, Carom-H, asymmetric CH3, asymmetric CH2, and symmetric CH3, respectively.For the 2500-1300 cm -1 range, strong peaks at 1680 and 1610 cm -1 , and medium-intensity peaks at 1560, 1530, 1490, 1450, 1380, and 1360 cm -1 are observed.The former two peaks can be attributed to the stretching vibrations of -СО-СН₂-СО-bond in acetylacetone moieties and C=C groups; [34] the peak at 1490 cm -1 can originate from the arene stretching; and the peaks at 1300-1500 cm -1 can be attributed to the bonds of aromatic carbon with nitrogen and/or oxygen atoms.A gradual change in the peaks' intensities and their positions with increase of EDA and decrease of BA content in the precursor mixture is observed: the amount of aromatic phase decreases with increase of Ndoping, which manifests itself as the increase of intensity in the wavenumber regions attributed to -NH and C-N groups vibrations.From the above chemical composition analysis, it can be inferred that amphiphilic CDs are O,N-doped carbon particles with excess of aliphatic and aromatic groups at the surface, and at the same time they possess -СО-СН₂-СО-groups from acetylacetone, which should make them highly soluble in a variety of solvents.

Optical properties of amphiphilic CDs
Optical properties were monitored for amphiphilic CDs dissolved in three different solventswater, isopropanol, and chloroform (Figure 4 and Figure S6-S11).The peak positions for absorption, PL excitation and PL spectra of these samples are summarized in Table S1.The absorption spectra of all CDs show peaks at 220, 260, and 300-315 nm; upon addition of EDA, several other peaks emerge at 390, 465, and 490 nm (Figure 4a-c).While comparing the PLE-PL maps for CDBA with other samples (Figure S11c-e), the following observations can be made: CDBA in isopropanol has a PL band at 480 nm excited over a wide spectral range (240-400 nm), whereas the EDA-based CDs in isopropanol has a PL band at 350 nm efficiently excited at 240 and 290 nm and PL band at 460 nm efficiently excited at 240, 300, and 390 nm.The most intense emission among CDs dispersed in different solvents is observed for those dissolved in isopropanol, with a maximal PL QY of 36% for CD0.22 (Figure 4d).The least intense emission in general is observed for CDBA, meaning that optical centers formed under participation of EDA and BA result in CDs with a higher brightness.From the calculated radiative () and nonradiative (knr) relaxation rates of the CDs produced with the different BA/EDA molar ratios shown in Figure 4h, one can see that the radiative rates follow the trend of PL QY varying from 0.003 to 0.09 • 10 9 s -1 , whereas nonradiative rates fluctuate around the mean value of 0.2•10 9 s -1 .
Optical properties of CD0.22 were measured in different solvents with varying relative polarity given in brackets: tetrachloromethane (0.052), toluene (0.099), chloroform (0.259), acetone (0.355), dimethyl sulfoxide (0.444), acetonitrile (0.46), isopropanol (0.617), ethanol (0.654), methanol (0.762), and water (1.0), as summarized in Figure 5.With the increase of the relative polarity from 0.052 to 1.0, the absorption peaks slightly blue-shift from 402 to 385 nm (Figure 5a and 5b), while PL spectra blue-shift from 465 to 457 nm (Figure 5e and 5f).In comparison to reported hydrophobic CDs, [13,35] where the absorption and PL bands experienced much large spectral shifts of up to 160 nm, the observed changes in the absorption and PL peak positions are rather minor.Similar behavior was observed for amphiphilic CDs synthesized from (formylmethyl)triphenylphosphonium chloride in acetone, whose PL peak shifts from 595 and 620 nm by changing the solvent from water to chloroform. [14]The optical density of the amphiphilic CDs increases with the relative polarity of the solvent (Figure 5c), while PL QYs reach the maximal value of 36% in the solvent with the relative polarity of 0.617 (isopropanol) and are lower in both more polar and less polar solvents, going down to 3% only for CD0.22 in tetrachloromethane (Figure 5g).The change in PL QY can be explained by the change in either radiative or nonradiative decay rate depending on the type of the emission centers.Both radiative and nonradiative decay rates are affected by the interaction of the excited dipole with solvent molecules, however no single model can explain the diverse spectral properties displayed by fluorophores in various environment. [36]o further demonstrate the amphiphilicity of CDs, their optical spectra during several cycles of dissolution in water and chloroform were measured, and shown in Figure 5d and h.It is seen that both absorption and PL spectra are the same in water/chloroform after 1 st and 2 d dispersion cycle of CD0.22, in terms of the positions of the respective maxima, indicating that the produced CDs are indeed amphiphilic, as was shown in Figure 1b.

Energy level structure of CDs, and their use in CD-LEDs
https://doi.org/10.26434/chemrxiv-2023-80dsgORCID: https://orcid.org/0000-0001-6841-6975Content not peer-reviewed by ChemRxiv.License: CC BY-NC-ND 4.0 The amphiphilic property of the CDs opens up a possibility to fabricate composite materials in combination with non-polar conductive polymers such as polyvinyl carbazole (PVK), for both optoelectronic and photovoltaic devices.We studied the energy structure of CDEDA, CDBA, and CD0.22 by UPS, in order to derive their possible energy structure schemes as shown in Figure 6a for CD0.22 and Figure S12 for CDEDA and CDBA.The values of the energy level positions are also summarized in Table S2.For the CD0.22,their highest occupied molecular orbital (HOMO) lies at -7.2 eV, while the lowest unoccupied molecular orbital (LUMO) -at -4.4 eV.From the optical data discussed above, one can identify excited states of S1, S2, and S3 located at -4.0, -3.3, and -2.4 eV, which correspond to the absorption peaks Abs-1, Abs-2, and Abs-3 in Table S1; those levels are presented in Figure 6a   Considering that the highest PLQY of 36% was achieved for CD0.22, these particles were used for fabrication of CD-LEDs.A very common conductive polymer PVK was chosen as a polymer host for the CDs.CD0.22 were mixed with PVK in chlorobenzene with a final concentration of 25 wt%; the detailed procedure for the CD-LED fabrication is provided in Supporting Information.The multilayer structure and the band structure for the fabricated CD-LED is shown in Figure 6b.CD0.22 have lower HOMO level (-7.2 eV) as compared to PVK (-5.8 eV) but according to the CD0.22 energy scheme in Figure 6a, charge carriers in CD0.22 relax to the energy levels at -5.5 https://doi.org/10.26434/chemrxiv-2023-80dsgORCID: https://orcid.org/0000-0001-6841-6975Content not peer-reviewed by ChemRxiv.License: CC BY-NC-ND 4.0 and -5.8 eV (dotted lines in Figure 6a) which are very close to HOMO level of PVK.This helps to facilitate the charge carrier accumulation within CD-PVK layer.Figure S13 shows electroluminescence (EL) spectrum of the operating CD-LED in comparison with PL spectra of the same device excited at 405 nm from the ITO side and from the Al/LiF side.Both EL and PL bands of the device are redshifted as compared to the PL band of CD0.22 in isopropanol (shown by dashed line in Figure S13); EL spectrum is broader with increased intensity at 550-700 nm.The PL peak from the opposite side of CD-LED (with Al/LiF contact) is different from that observed from the ITO side: it is blue-shifted to 440 nm (Figure S13, violet line).We assume that the difference in PL spectra is mainly caused by the difference in the absorption of the layers that are prior to the CD-PVK layer.The Commission Internationale de L'Eclairage (CIE) coordinate of the EL spectrum is (0.332, 0.430) and the correlated color temperature (CCT) is 5627 K (Figure 6c).
The average brightness of fabricated CD-LEDs exceed 1000 cd/m 2 , and the maximal brightness reaches 1716 cd/m 2 .The latter value is comparable or better then those for reported CD-based LEDs listed in Table S3 for devices employing CDs with a PL QY near 40%.

Conclusions
We introduced the synthesis of amphiphilic CDs from a mixture of benzoic acid and ethylenediamine in acetylacetone, which are easily dispersible in a variety of solvents of different polarity, such as water, methanol, ethanol, isopropanol, acetonitrile, dimethyl sulfoxide, acetone, chloroform, toluene, and tetrachloromethane without any significant changes of the absorption and emission peak positions.Rather high PL QY of up to 36% makes these amphiphilic CDs suitable for application as active material for LEDs, where they can be used in combination with a common polymer PVK.The fabricated LEDs show the EL peak at 515 nm with a 200 nm width, maximal luminance of 1716 cd/m 2 , CIE coordinates of X=0.332, Y=0.430 and ССT of 5627 K. Apart from the demonstration of CDs as components of LEDs, amphiphilic CDs produced in this work can also be used as auxiliary components of perovskite-based solar cells work. [37]The developed synthesis of amphiphilic CDs opens up an opportunity to conveniently fabricate different optoelectronic and photovoltaic devices with improved characteristics.

Characterization
A Libra 200FE (Zeiss, Oberkochen, Germany) transmission electron microscope (TEM) and a Solver Pro-M (NT-MDT, Moscow, Russia) atomic-force microscope (AFM) were used to study size and shape of CDs.For AFM measurements, 10 μL solutions of CDs redispersed in 2methoxyethanol at a concentration of 18 mg/mL were spin-cast onto the mica substrates at 4000 rpm for 40 sec, and annealed at 130°C for 15 min.X-Ray photoelectron spectroscopy (XPS) measurements were performed on an Escalab 250Xi (Thermo Fisher Scientific, Waltham, MA, USA) photoelectron spectrometer with AlKα radiation (photon energy 1486.6 eV) in the constant pass energy mode at 100 eV for the survey XPS spectrum and at 50 eV for the core-level spectra of single elements, using an XPS spot size of 650 μm.X-ray diffraction (XRD) measurements were carried out on an X-ray powder diffractometer D2 Phaser (Brucker AXS GmbH, Germany), Cu-Kα, Ni filter monochromatization, LYNXEYE semiconductor linear detector (Bruker AXS) with an opening angle of 5 o , 2θ-θ scanning.All spectroscopic studies were performed under an ambient atmosphere.Raman spectra were measured on an inVia (Renishaw, Wotton-under-Edge, UK) microspectrometer equipped with a 20× objective (NA = 0.4) and a 514 nm laser source.Fourier-transform infrared (FTIR) spectra were recorded on a Tenzor II infrared spectrophotometer (Bruker, Billerica, MA, U.S.) in an attenuated total reflection mode.Absorption spectra were measured using a Shimadzu UV3600 spectrophotometer (Shimadzu, Kyoto, Japan).Photoluminescence (PL) spectra and PL excitation-emission (PLE-PL) maps were recorded on a Cary Eclipse instrument (Agilent, Santa Clara, CA, USA).Time-resolved PL measurements were performed on a confocal microscope MicroTime 100 (PicoQuant, Berlin, Germany) equipped with a 3× objective (NA = 0.1) and a 405 nm pulsed diode laser.Electroluminescence spectra of CD-LEDs were obtained using a CAS 120 spectrometer (Instrument Systems, Munich, Germany).I/V characteristics were measured using a Keithley 2401 measuring source (Keithley Instruments, Solon, Ohio, USA).PL spectra from CD-LED were taken by an LSM-710 (Zeiss, Oberkochen, Germany) laser confocal microscope with a 405 nm diode laser.

Synthesis of CDs
BA was dissolved in 5 mL of acetylacetone under sonication, and different amounts of EDA were added to the mixture; the masses of BA and the volumes of EDA used to produce 7 different CD samples are provided in Table S4.The mixture was transferred into a 40 mL Teflon reactor and maintained at 190 °C for 8 h in an oven.After the solvothermal treatment, the CD solution was dialyzed through a membrane with a molecular weight cut-off of 7 kDa against deionized water for two days to remove any residual raw materials.After the dialysis, the CDs were redispersed in isopropanol and stored at 5 °C.

Figure 1 .
Figure 1.(a) Scheme of the synthesis of amphiphilic CDs from benzoic acid and ethylenediamine in acetylacetone.(b) Photographs of amphiphilic CDs in different solvents, taken under sunlight (left panel) and UV light (right panel).

Figure
Figure 2a-c, and outlined in the forthcoming discussion.

Figure 5 .
Figure 5. Optical properties of CD0.22 dispersed in a set of solvents with varying relative polarity: (a) absorption spectra, (b) long-wavelength absorption peak position and (c) optical density at its maximum depending on the relative polarity of the solvent; (e) PL spectra, (f) PL peak position excited at 400 nm and (g) PL QY as a function of the relative polarity of the solvent; (d) absorption and (h) normalized PL spectra excited at 400 nm recorded while changing the solvents from water to chloroform in 2 cycles (1 and 2).
as thin black lines.The radiative transitions of the two emissive centers corresponding to PL-1 and PL-2 in Table S1 are shown by colored arrows from S2 and S3 energy levels, respectively.One can see that CD0.22 demonstrate a large Stokes shift alongside with the fact that the lowest energy level, S1, is non-emissive.A very similar energy structure could be derived for CDEDA, as shown in Figure S12.In contrast, for the CDBA, HOMO and LUMO levels lie at -6.5 and -3.1 eV, respectively; and there are only two excited states, S1 and S2, with one radiative transition from S2.

Figure 6 .
Figure 6.(a) Energy level structure for the CD0.22.(b) Energy level diagram of the CD-LED: levels for PVK are within the cyan box, and those for the CD0.22 are within the empty box.(c) CIE coordinates for the green light emitted by the fabricated CD-LEDs; the inset shows a photograph of the operating LED.

Figure S7 .
Figure S7.Optical properties of CD0.15.(a) Absorption spectra, the inset shows zoom-in of the 450-550 nm spectral region; (b) peaks position, corresponding to the absorption bands Abs-1

Figure S9 .
Figure S9.Optical properties of CD1.(a) Absorption spectra, the inset shows zoom-in of the 450-550 nm spectral region; (b) peaks position, corresponding to the absorption bands Abs-1 (green

Figure S11 .
Figure S11.Optical properties of CDBA.(a) Absorption spectra, the inset shows zoom-in of the 450-550 nm spectral region; (b) peaks position, corresponding to the absorption bands Abs-2 (blue triangles) and Abs-3 (red circles) from Table S1, and to absorption peaks not contributing to emission: UV absorption band (black squares).(c-e) PLE-PL maps in water (c), isopropanol (d), and chloroform (e).

Figure S12 .
Figure S12.Suggested energy structure of CDEDA and CDBA.From the optical data, one can identify excited states shown as thin black lines of S1, S2, and S3 for CDEDA and S1 and S2 for CDBA, which correspond to the absorption peaks listed in TableS1.For CDEDA, the radiative transitions of the two emissive centers corresponding to PL-1 and PL-2 in TableS1are shown by colored arrows from the S2 and S3 energy levels, respectively.For CDBA, only one radiative transition is observed from the S2 excited level.

Figure
Figure S13.Electroluminescence (green solid line) of the CD-LED, PL excited at 400 nm of CD0.22 in isopropanol solution (dashed line), and PL excited at 405 nm of CD-LED from the ITO side (blue solid line) and from the Al/LiF side (violet solid line).