Highly Efficient Multifunctional Luminescent Radicals

Open AccessCCS ChemistryRESEARCH ARTICLE1 Feb 2022Highly Efficient Multifunctional Luminescent Radicals Yihan Zhao, Alim Abdurahman, Yimeng Zhang, Ping Zheng, Ming Zhang and Feng Li Yihan Zhao State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Alim Abdurahman *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Yimeng Zhang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Ping Zheng State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author , Ming Zhang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author and Feng Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000737 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Stable organic luminescent radicals are a special class of compounds integrating optical, electrical, and magnetic properties. Luminescent radicals not only have potential applications in the field of the organic light-emitting diodes (OLEDs) but can also be applied in the fields of fluorescence sensing, bioimaging, and so forth. Nevertheless, due to the adverse effects of solvent polarity on the luminescent performance of radicals, no feasible approaches have been found in the literature toward fluorescent sensing. In this work, we report two luminescent radicals, 2αPyID-TTM and 2δPyID-TTM, whose emissions show high efficiency and less dependence on solvent polarity. Both radicals show remarkable protonation–deprotonation properties. Besides, 2αPyID-TTM exhibits significant fluorescence quenching and colorimetric response toward Fe3+ in aqueous solution. This suggests the possibility of a fluorometric/colorimetric dual-channel probe for Fe3+. Moreover, an optimized OLED using 2δPyID-TTM as an emissive dopant shows pure red emission and a maximum external quantum efficiency (EQE) of 10.6%. These results show promise for luminescent radicals as fluorescent probes and electroluminescent emitters. Download figure Download PowerPoint Introduction As a special category of free radicals, stable organic luminescent radicals integrating optical, electrical, and magnetic properties have broken the stereotypes of high activity and nonluminescence of traditional radicals and attracted significant attention.1–9 The emission of radicals comes from radiative decay of doublet excitons. Since there is no inhibition in the transition of doublet excitons, the upper limit of internal quantum efficiency (IQE) of organic light-emitting diodes (OLEDs) exploiting luminescent radicals as emitters has been theoretically increased to unit.10–16 In addition to OLED applications, luminescent radicals have also shown other potential applications, such as cation-responsive turn-on fluorescence,17 triplet sensitization,18 magnetoluminescence,19,20 circularly polarized luminescence,21,22 and photodynamic therapy.23 However, until now, there have been no reports regarding fluorescent sensing of luminescent radicals in an environment of mutual solubility with water. The main reason limiting these applications is that most luminescent radicals show low luminescent efficiency and even though few of them exhibit high photoluminescent quantum yield (PLQY), the high values are only observed in nonpolar solvents, whereas some fluorescent chemosensors must be dissolved in a mixture of polar solvents and water in actual applications. Fortunately, a rule for molecular design was proposed showing that non-alternant systems are required for highly efficient donor–acceptor (D–A)-type luminescent radicals. Meanwhile, the borrowed intensity from the higher-energy transitions can augment the oscillator strength of the charge-transfer (CT)-type emission and was verified.13 Since the excited states of radicals are hybridizations of local and CT states, the impact of solvent polarity on radical luminescence will be limited in terms of the intensity borrowing. Based on the above idea, herein, we obtained two luminescent radicals, 2αPyID-TTM and 2δPyID-TTM, as shown in Figure 1, in which the tris(2,4,6-trichlorophenyl)methyl (TTM) radical moiety connects with two α-carboline (αPyID) and two δ-carboline (δPyID) units, respectively. Both of them exhibit pure red emission with high PLQY in various solvents and less solvent polarity dependence. The high PLQY (up to 31%) in tetrahydrofuran solution, together with the excellent photostability, thermostability, and electrochemical stability make it feasible to apply 2αPyID-TTM and 2δPyID-TTM to fluorescent sensors and organic electroluminescence (EL). Figure 1 | Structures of 2αPyID-TTM and 2δPyID-TTM. Download figure Download PowerPoint Experimental Methods Precursors of TTM radical derivatives were synthesized from a carbon–nitrogen coupling reaction in a single step with a reaction yield of ∼20%. TTM radical derivatives were produced by precursors treated with potassium tert-butoxide and tetrachlorobenzoquinone at room temperature. The structures were characterized by 1H NMR spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS; Supporting Information Figure S1), and electron paramagnetic resonance (EPR; Supporting Information Figure S2) spectroscopy. Signals of the EPR spectrum confirmed the existence of unpaired electrons of radical molecules. Details of the synthesis methods, raw material handling, test apparatus model and parameter settings, and other structural characterization details are provided in the Supporting Information Scheme S1. Results and Discussion Photophysical properties Figure 2a (left column) shows the normalized absorption and PL spectra of 2αPyID-TTM and 2δPyID-TTM in various solvents. Both of them display two characteristic absorption bands. According to the results of time-dependent density functional theory (TD-DFT) calculations, the absorption band at 375 nm corresponds to a local transition from 207α [α-singly occupied molecular orbital (α-SOMO)] to 211α (LUMO+3) (LUMO = lowest unoccupied molecular orbital). The band at about 590 nm is attributed to the CT transition from 206β [β-highest occupied molecular orbital (β-HOMO)] to 207β (β-SOMO). As the solvent polarity increases, the shapes and positions of the absorption spectra remain almost unchanged. Nevertheless, the PL emission shows a slight red-shift and the vibration fine structure gradually weakens and disappears with increasing solvent polarity, which suggests a possible mixture of the local and CT states. Both the radicals reached a high PLQY up to above 90% in several solutions (see Supporting Information Table S1 for detail). Surprisingly, the PLQY of 2αPyID-TTM retains 79% and 31% even in dichloromethane and tetrahydrofuran, respectively. The fitting results of the Lippert–Mataga solvation model are shown in Figure 2a (right column). Estimated dipole moments of excited states of 2αPyID-TTM and 2δPyID-TTM were found to be 4.67 and 7.76 D in low-polarity solvents. In high-polarity solvents, the dipole moments of the excited states increase to 10.70 and 16.11 D. Additionally, in medium to high-polarity solvents, 2αPyID-TTM shows a lower CT characteristic compared with that of 2δPyID-TTM. The smaller dipole moment of 2αPyID-TTM is consistent with its lower dependence on solvent polarity. Figure 2 | (a) Normalized absorption and PL emission spectra of 2αPyID-TTM and 2δPyID-TTM in various solvents (10−5 M) (left) and fitted linear correlation of the Stokes shift as a function of solvent polarity (right). (b and c) Transient photoluminescence decay spectra of 2αPyID-TTM and 2δPyID-TTM in different solvents (10−5 M) at room temperature. Download figure Download PowerPoint To further analyze the effect of solvent polarity on the excited states of the radicals, we measured the transient PL decay of 2αPyID-TTM and 2δPyID-TTM in different solvents. As shown in Figures 2b and 2c, the fluorescent lifetimes (τ) of two radicals are present single-exponent decay. Both lifetimes decrease as the solvent polarity increases, whereas the tendency of 2αPyID-TTM is slower. 2αPyID-TTM always has a longer lifetime than that of 2δPyID-TTM in all test solvents. When the excited state shows characteristics of hybridizing local and CT states, the effect of solvent polarity weakens, and the emission can even survive in highly polar solvents. According to the values of τ and PLQY, we calculated the radiative (kr) and nonradiative (knr) transition rates in various solvents. From Supporting Information Table S2, we can see that kr is 2–12 times that of knr in various solvents (except for tetrahydrofuran and dichloromethane). The lesser effects of solvent polarity on the emission of 2αPyID-TTM enables it as a fluorescent probe. Electrochemical properties and theoretical calculations The redox properties of the two radicals were characterized by cyclic voltammetry (CV) with ferrocerium/ferrocene (Fc+/Fc) as a reference, TBAPF6 as an electrolyte, and dichloromethane as the solvent. Both the radicals display reversible redox processes (see Supporting Information Figure S3). The peak at a potential of about +0.5 V corresponds to the oxidation of the single electron on radical, and the one at about −1.0 V corresponds to the reduction. Energies of α-SOMO of 2αPyID-TTM and 2δPyID-TTM calculated using redox potential were found to be −5.26 and −5.38 eV, and the β-SOMO energies were −3.69 and −3.76 eV, respectively. It reveals that the substitution site of N in carboline had little effect on the energy level of the orbital 207β (β-SOMO). In contrast, the deeper 207α (α-SOMO) energy of 2δPyID-TTM compared with that of 2αPyID-TTM may arise from the different electron-donating abilities arising from the site of the nitrogen atom. Frontier molecular orbitals (MOs) of 2αPyID-TTM and 2δPyID-TTM set by DFT calculations (by using UB3LYP/6-31G (d,p)) are shown in Figures 3a and 3b. As shown, the electron clouds of the α-SOMO of the two radicals are distributed over the whole molecule. However, the 206β (β-HOMO) are mainly localized on the carboline and two benzene rings of the TTM. Based on TD-DFT calculations, it is found that the visible emission of the two radicals may be attributed to the transition from 207β to 206β, that is, the transition of D1 → D0. Following the electronic structure of the frontier orbits, it can be speculated that the emission from 2αPyID-TTM and 2δPyID-TTM possesses a certain CT property. This is consistent with the photophysical results. Figure 3 | Ground-state frontier orbitals of the (a) 2αPyID-TTM and (b) 2δPyID-TTM using DFT methods (UB3LYP/6-31G (d,p)). Download figure Download PowerPoint Radicals stability Stability is another important indicator of performance evaluation of multifunctional materials. To assess the feasibility of luminescent radicals for versatile applications, thermogravimetric analysis, redox stability, and photostability tests have been performed. It can be seen from Figure 4a that both radicals have high thermal decomposition temperatures. Compared with 2αPyID-TTM, 2δPyID-TTM has a higher thermal decomposition temperature. This may be related to the fact that the nitrogen atom is located in the δ position and more likely to form hydrogen bonds with others. Even after 20 cycles of CV scans on the radicals, no changes were observed. This indicates stability of the two radicals during the reversible redox process (Figure 4b). The photostabilities of 2αPyID-TTM and 2δPyID-TTM in chloroform solution were tested and compared with the TTM radicals. The solution was irradiated via a 355 nm pulsed laser with a power density of 17.8 kW cm−2 (pulse width: 8 ns, frequency: 10 Hz). The decay of the PL intensity (Figure 4c) suggests that 2αPyID-TTM and 2δPyID-TTM are more stable than TTM. Figure 4 | Stability testing of the 2αPyID-TTM and 2δPyID-TTM: (a) TGA curve of radicals under nitrogen flow. (b) Repeated CV measurements (20 cycles). (c) Time dependence of the emission intensity (I) for radicals in CHCl3 under 355 nm laser radiation. Download figure Download PowerPoint Proton responsive properties Next, we attempted to investigate the properties of 2αPyID-TTM and 2δPyID-TTM by protonation–deprotonation reaction on the nitrogen atoms of the carboline moieties. The performance of 2αPyID-TTM before and after protonation is shown here, and that of 2δPyID-TTM is summarized in the Supporting Information (see Supporting Information Figure S4). First, p-toluenesulfonic acid (TsOH) was added to a dichloromethane solution of 2αPyID-TTM (1 × 10−6 M), which causes a noticeable emission color change. Then the addition of triethylamine (Et3N) restored the PL emission to its original state (Figure 5a). The reversible response by UV–vis absorption and fluorescence spectra suggests the protonation–deprotonation reaction took place (Figures 5b–5e). Figure 5 | (a) Structure of 2αPyID-TTM and protonated 2αPyID-TTM ([NH-2αPyID-TTM]+) and photographs in CH2Cl2 under UV light. (b and d) Changes in absorption and emission spectra upon addition of TsOH (acid) to a CH2Cl2 solution of 2αPyID-TTM. (c and e) Recovery of the absorption and emission profile following the addition of Et3N (base) (the emission spectra were normalized). Download figure Download PowerPoint To investigate the detailed chemical processes behind the spectral changes and in view of the low solubility of TsOH in dichloromethane, we chose trifluoromethanesulfonic acid (TfOH) as the added acid in protonation to control the amount of acid more precisely. The absorption spectra of the two radicals when adding different equivalents of TfOH are shown in Supporting Information Figure S5. As an example, with the addition of different equivalents of TfOH, the absorption spectrum of 2αPyID-TTM solution (1 × 10−5 M) produced almost the same trend as that when TsOH was added, with a gradual enhancement of the absorption peak at 375 nm and suppression of the absorption peak at 425 and 585 nm. When 1 equiv of acid was added, a new absorption band at 309 nm appeared, which became visible when 2 equiv of acid were added, and the absorption peaks at 425 and 585 nm nearly disappeared. As the acid equivalent was further increased, the changes at each absorption peak became saturated. The new absorption peak at 546 nm should correspond to the new transition from ground-state to the first excited state, which may be the reason for the change from red to yellow emission of the radical. We depicted the scatter plot ( Supporting Information Figure S5) of each absorption peak as a function of the amount of acid. It can be found that the inflection points appear at the acid equivalent of two, suggesting the protonation of the radical to be a stepwise diprotonation process. Through theoretical calculations, we found that the electronic-cloud distributions, energy levels, and dihedral angles between TTM and ligands were greatly changed after the two radicals were protonated (see Supporting Information Figure S6). Taking 2αPyID-TTM as an example, its energy gap between β-SOMO and β-HOMO increases from 2.78 to 3.17 eV after protonation (see Supporting Information Figure S7), resulting in a blue-shift of its emission peak, which is also consistent with the results of the TD-DFT calculation. The protonation–deprotonation properties of the two radicals endow them with good prospects in chemical sensing24 and fluorescent switching devices.25 Fluorescence sensing of 2αPyID-TTM to Fe 3+ Metal ions play an important role in human life. There is an urgent need for rapid and low-cost testing methods for a wide range of clinical, bioprocessing, and environmental applications.26–29 Among the various detection methods reported so far, the fluorescent sensor, based on organic luminescent molecules, is a remarkable one due to its high selectivity, high sensitivity, and fast response.30–33 As a new kind of luminescent material, the capability of radicals for fluorescent sensing is highly interesting. 2αPyID-TTM shows good PLQY in tetrahydrofuran, which can be mutually dissolved in water, and offers the possibility to detect metal ions. Figure 6a shows the images of 2αPyID-TTM in tetrahydrofuran after adding different metal ions in the day light and UV light. Only Fe3+ demonstrated apparent fluorescence quenching and color change. The fluorescent response testing of 2αPyID-TTM was performed (Figure 6b) and shown to agree well with the phenomenon in Figure 6a. Except for iron ions, the visual changes of fluorescence emission caused by other metal ions are almost negligible and do not interfere with the visualization results in the actual assay. The prominent response of Fe3+ using N as the chelating atom may be attributed to the better thermodynamic affinity and faster chelating rate of iron ions than the other transition-metal ions, as well as the stronger oxidizability.34 Hence, there may be chelating complexes formed between dissociative iron ions and 2αPyID-TTM. As further proof of the possible mechanism for sensing, we tested the absorption spectrum of the radical materials when coexisting with different contents of iron ions in tetrahydrofuran ( Supporting Information Figure S10). Upon addition of iron ions, the absorption band at 375 nm disappeared, a new one appeared at 340 nm, and increased by degrees. To investigate the response of 2αPyID-TTM to Fe3+ in detail, concentration-dependent titration was carried out. As shown in Figure 6c, the PL intensity gradually decreased with the addition of Fe3+. When the concentration of Fe3+ reached 9.0 × 10−4 M, the PL intensity of 2αPyID-TTM almost quenched. Moreover, in the inset of Figure 6c, we show that there is a linear relationship between the PL intensity and concentration of Fe3+. According to the limit of detection (LOD) calculation relationship,35 LOD = 3σ/m, the LOD of 2αPyID-TTM was calculated to be 9.5 × 10−6 M. Supporting Information Figure S8 shows the image of 2αPyID-TTM upon the addition of Fe3+ at different concentrations. As shown, even the naked eye can detect the color change of 2αPyID-TTM upon the addition of Fe3+ at a concentration level of 9.0 × 10−5 M. Supporting Information Figure S9 depicts the fluorescence lifetime curves of 2αPyID-TTM before and after the addition of Fe3+. It is noteworthy that the fluorescence lifetime of the radical remains almost unchanged after the addition of Fe3+, indicating static quenching of the fluorescence, that is, the formation of a ground-state complex between Fe3+ and radical.36 The Job’s plots ( Supporting Information Figure S11) and equilibrium constants ( Supporting Information Figure S12) for complexation are summarized in the Supporting Information. According to the Job’s plots, the curve inflects at one-half of the horizontal coordinate, which indicates a complexation ratio of 1:1 between Fe3+ and radical. To the best of our knowledge, this is the first report on the application of luminescent radicals in the field of fluorescent sensing for water-soluble ions, which extends the range of practical applications of luminescent radicals in fluorescence sensing. Figure 6 | (a) Images of 2αPyID-TTM in tetrahydrofuran after adding various metal ions in the day light (up) and UV light (down). (b) The fluorescence response of 2αPyID-TTM (1.0 × 10−6 M tetrahydrofuran) in the presence of Zn(OAc)2, PbCl2, KOAc, NaOAc, CaCl2, MgCl2, HgCl2, CrCl3, CdCl2, FeCl3, Cu(OAc)2, Co(OAc)2, Mn(OAc)2, and Ni(OAc)2 at a concentration of 3.0 × 10−4 M under the excitation wavelength at 375 nm (a–n). (c) The PL spectra of 2αPyID-TTM upon the addition of Fe3+, inset: the linear fit of the 2αPyID-TTM PL intensity and concentration of Fe3+. Download figure Download PowerPoint EL properties To further evaluate the prospects of radicals for display technology applications, OLED devices were fabricated by vacuum thermal evaporation onto a clean glass substrate coated with indium tin oxide. 2δPyID-TTM doped in TPBi was used as the emitter. The optimized device structure is as follows (Figure 7a, structures of TAPC, TPBi, and PO-T2T are shown in Supporting Information Figure S13): ITO/MoO3 (3 nm)/TAPC (40 nm)/5% 2δPyID-TTM in TPBi (30 nm)/PO-T2T (60 nm)/LiF (0.8 nm)/Al (100 nm) (ITO, indium tin oxide; TAPC, 1,1-bis[(di-4-tolylamino) phenyl]cyclohexane; TPBi, 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-pheny l-1-H-benzimidazole); PO-T2T, 2,4,6-tris[3-(diphenylphosphoryl)phenyl]-1,3,5-triazine). TAPC and PO-T2T were used as hole- and electron-transporting materials, respectively. As can be seen from Figure 7b, the devices exhibit pure red emission around 660 nm with Commission Internationale de L’Eclairage (CIE) coordinates of [0.66, 0.30], which agrees with the photoluminescence spectrum of 2δPyID-TTM. When the voltage was varied from 5.5 to 12 V, the EL spectrum showed good stability, almost barely changing. The current density–voltage–luminance (J–V–L) characteristics are shown in Figure 7c, which reveal that the turn-on voltage is at about 5 V. The relationship between external quantum efficiency (EQE) and the voltage (Figure 7d) shows that the EQE of the device reaches its maximum, 10.6%, at a voltage of 6 V. Figure 7 | (a) Energy level diagram of the 2δPyID-TTM-based OLED. (b) EL spectra of OLEDs at different driving voltages. (c) J–V–L characteristics of the device. (d) EQE versus voltage. Download figure Download PowerPoint Conclusion Two double-substituted TTM radical derivatives with α- and δ-carboline as substituents were obtained. The introduction of nonalternant groups greatly ameliorates the emission efficiency of TTM radicals with PLQY over 90% in solution with low to medium polarity, still remaining 31% in tetrahydrofuran. Both radicals exhibit remarkable photostability, thermal stability, and protonation–deprotonation properties. Moreover, 2αPyID-TTM can be used as a fluorometric/colorimetric dual-channel probe to Fe3+ in aqueous solution with high selectivity and sensitivity. In addition, the OLED device using 2δPyID-TTM doped with TPBi as an emission layer showed pure red emission with a maximum EQE of 10.6%. Our results demonstrate that reasonable molecular design can greatly improve the luminescent properties of radicals and show the promising application of the two radicals as fluorescent probes and electroluminescent emitters. Supporting Information Supporting Information is available and includes materials synthesis and characterization, EPR spectra, detailed photophysical data in different solvents, Lippert–Mataga equation and related parameters, electrochemical properties, proton responsive properties, ground-state frontier orbitals and the molecular structures of the radicals before and after protonation, data of DFT and TD-DFT calculations, the images, absorption spectra and Job’s plots of 2αPyID-TTM upon the addition of Fe3+, and the equilibrium constants for protonation and complexation. Conflict of Interest There is no conflict of interest to report. Funding Information This research was made possible by a generous grant from the National Natural Science Foundation of China (grant nos. 51925303, 21875083, and 91833304), the China Postdoctoral Science Foundation (grant nos. 2020TQ0117 and 2020M681033), and the program “JLUSTIRT” (grant no. 2019TD-33). References 1. 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