Stable Diarylamine-Substituted Tris(2,4,6-trichlorophenyl)methyl Radicals: One-Step Synthesis, Near-Infrared Emission, and Redox Chemistry

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE5 Sep 2022Stable Diarylamine-Substituted Tris(2,4,6-trichlorophenyl)methyl Radicals: One-Step Synthesis, Near-Infrared Emission, and Redox Chemistry Chuan Yan, Dongyue An, Weinan Chen, Ning Zhang, Yanjun Qiao, Jing Fang, Xuefeng Lu, Gang Zhou and Yunqi Liu Chuan Yan Lab of Advanced Materials, State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200438 Google Scholar More articles by this author , Dongyue An Department of Materials Science, Fudan University, Shanghai 200438 Google Scholar More articles by this author , Weinan Chen Lab of Advanced Materials, State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200438 Google Scholar More articles by this author , Ning Zhang Department of Materials Science, Fudan University, Shanghai 200438 Google Scholar More articles by this author , Yanjun Qiao Department of Materials Science, Fudan University, Shanghai 200438 Google Scholar More articles by this author , Jing Fang Lab of Advanced Materials, State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200438 Google Scholar More articles by this author , Xuefeng Lu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Materials Science, Fudan University, Shanghai 200438 Google Scholar More articles by this author , Gang Zhou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Lab of Advanced Materials, State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200438 Google Scholar More articles by this author and Yunqi Liu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Materials Science, Fudan University, Shanghai 200438 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101513 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail A mild one-step radical-to-radical synthetic strategy has been developed to directly produce a new family of diarylamine-substituted tris(2,4,6-trichlorophenyl)methyl (TTM) radicals TTM-DPA, TTM-DBPA, and TTM-DFA. The obtained TTM radical derivatives are extremely stable during chromatography purification and long-term storage in the solid state. Upon simply introducing an electron-donating diphenylamine on the electron-withdrawing TTM radical, TTM-DPA exhibits a maximum photoluminescence (PL) wavelength at 705 nm in cyclohexane with a high PL quantum yield (PLQY) of 65%. With further extension of the conjugation of the diarylamine, the PL maximum bathochromically shifts to 748 nm for TTM-DBPA and 809 nm for TTM-DFA. Most importantly, upon the oxidation of the diarylamine-substituted TTM radicals by NO•SbF6, the unusual quinoidal structures of their iminium monocations accompanied by a slow valence tautomerization process and restricted rotation are identified by NMR spectroscopies and single-crystal X-ray analysis. This study will pave the way to the design novel high-performance organic radical emitters and facilitate the understanding of charge transfer and transport in TTM radical-related applications. Download figure Download PowerPoint Introduction Organic neutral radicals have attracted tremendous attention due to their extensive applications in electrical conductors, magnetic materials, spintronics, and information storage.1–7 In addition to their unique electronic and magnetic properties, their characteristic luminescence at room temperature has recently been exploited for organic neutral radicals, which opens the gate to chemical sensing,8–10 circularly polarized luminescence,11–13 and light-emitting diodes.14,15 The representative luminescent neutral radicals are triaryl methyl radicals which consist of three aryl groups surrounding the central methyl radicals and are mainly classified into three types of radical derivatives: bis(2,4,6-tirchlorophenyl)methyl (BTM),16–18 tris(2,4,6-trichlorophenyl)methyl (TTM),19–21 and perchlorotriphenylmethyl (PTM).22–25 However, the photoluminescence quantum yields (PLQYs) of the BTM series radicals are relavtively low26–28 while the PTM series radicals tend to be decomposed under light irradiation or during vacuum evaporation processing.29 Consequently, substantial effort has been devoted to the study of the TTM series radicals in the field of highly luminescent radicals. The PL properties of the TTM series radicals, such as the maximum PL wavelength and the PLQY, can be facilely tuned by simply introducing functional groups into one or more TTM arms. Among various functionalizations,30 carbazole-substituted TTM radical derivatives19–21,31–33 are regarded as one of the most fascinating luminescent radicals due to their satisfactory PLQYs and stabilities. The simplest carbazole-functionalized TTM radical is TTM-Cz (Figure 1a) in which a carbazole unit is directly attached to one arm of the TTM radical. Interestingly, an intense red emission with a PL maximum of 628 nm31 can be realized due to the intramolecular charge transfer (ICT) interactions. To bathochromically shift the PL maximum, more carbazoles are incorporated to enhance their electron-donating ablitity. Upon the introduction of three carbazoles on one or three arms of the TTM radical, the PL maxima of TTM-3Cz and TTM-TCz (Figure 1a) have been bathochromically shifted to 654 and 689 nm,32,33 respectively. However, to further expand the emision into the near-infrared (NIR) region, a simple TTM radical system with high PLQY is still pursued. Figure 1 | Chemical structures of (a) typical emissive carbon radicals and (b) our diarylamine-substituted TTM neutral radicals TTM-DPA, TTM-DBPA, and TTM-DFA, and three possible resonance forms of their monocations. Download figure Download PowerPoint In general, TTM radical derivatives are synthesized in two key steps which include the synthesis of the hydrogen-protected precursor and the following dehydrogenation (Scheme 1a). In the first step for the synthesis of precursor HTTM-Ar, whichever TTM radical or its reduced precursor HTTM is used as the starting material, the reduction product is normally obtained, and thus a dehydrogenation reaction is necessary. Moreover, due to the low reactivity of chloride, harsh conditions are usually required for the C–C and C–N arylation reactions. For example, the introduction of carbazole has to be carried out at a high temperature, over 160 °C.15,31,34 This may limit the diversity of the TTM radicals. Therefore, expanding the scope of the TTM radicals under simplified preparation procedures remains a challenge. Scheme 1 | (a) Traditional two-step synthesis of TTM derivatives and (b) our one-step radical-to-radical synthesis of diarylamine-substituted TTM derivatives. Download figure Download PowerPoint TTM radicals exhibit ambipolar properties due to their unique unpaired electron structures. Electrons can be easily removed from or accepted by the TTM radicals in the electric field. Upon the removal of a electron, a TTM radical derivative may resonate among at least three different resonance forms (Figure 1b): a closed-shell monocation with the positive charge located at the methyl carbon in TTM, an open-shell diradical monocation with the charge at the nitrogen atom, and a closed-shell quinoidal monocation with the charge at the nitrogen atom. The resonance structures of a TTM cation have been previously speculated about with respect to various titrations monitored by spectrascopy techniques.32,35–38 However, to the best of our knowledge, no direct structure verification based on single-crystal analysis has been reported, probably due to the complication of growing the single crystal. In this context, diarylamines, which have been extensively exploited as electron-donating groups similar to carbazole, have been incorporated into TTM radicals. A mild one-step radical-to-radical synthetic methodology has been developed to directly produce the TTM radical derivatives, and the products are stable enough to be purified by routine silica gel column chromatography. Interestingly, by simply replacing the carbazole in TTM-Cz by diphenylamine (DPA), TTM-DPA (Figure 1b) displays a maximum PL wavelength at 705 nm in cyclohexane with a high PLQY of 65%. Upon replacing DPA by dibiphenylylamine (DBPA) and difluorenylamine (DFA), the PL maxima further bathochromically shift to 748 nm for TTM-DBPA and 809 nm for TTM-DFA (Figure 1b). Most importantly, after losing one electron, the resonant structures of their monocationic species are identified by single-crystal X-ray analysis, and variable temperature (VT) and two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY) NMR spectroscopies. Overall, the one-step radical-to-radical synthesis, the NIR emission with high PLQY, and the verified redox chemistry will extend the diversity of TTM radical derivatives with high PLQYs and facilitate the understanding of charge transfer and transport in TTM radical-based applications. Experimental Methods Materials and reagents All chemicals and reagents were purchased from commercial sources and used as received unless specified. Anhydrous tetrahydrofuran (THF) and toluene were distilled from sodium benzophenone ketyl. Dichloromethane (DCM) and chloroform were distilled from CaH2. All reactions and manipulations were carried out with the use of standard inert atmosphere and Schlenk techniques. Measurements and characterizations The high-resolution mass spectroscopies (HRMS) were recorded on a Bruker McriOTOF11 Fourier transform ion-cyclotron resonance mass spectrometer (Bruker, Karlsruhe, Germany). Electron paramagnetic resonance (EPR) spectra of radicals were recorded with Bruker EMXnano (Bruker, Karlsruhe, Germany) at ambient temperature. Single-crystal X-ray diffraction data were collected on a Bruker D8 VENTURE MetalJet diffractometer (Bruker, Karlsruhe, Germany). UV–Vis–NIR absorption spectra were recorded on a PerkinElmer Lambda-750 spectrophotometer (PerkinElmer, Massachusetts, United States). PL spectra were measured on an Edinburgh fluorescence spectrometer (FLS1000, Edinburgh Instruments, Edinburgh, United Kingdom). Electrochemical measurements were performed using a CHI760 electrochemical analyzer (CH Instruments Inc., Shanghai, China) with a glass carbon disk as the working electrode, a platinum wire as the counter electrode, and an Ag/Ag+ electrode as the reference electrode at the rate of 100 mV s−1, while 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in DCM was the electrolyte. Redox couple ferrocenium/ferrocene was used as an internal standard. Thermal stability measurements were performed on a PerkinElmer Pyris 1 thermogravimetric analysis (TGA) analyzer (PerkinElmer, Massachusetts, United States). VT and 2D NOESY NMR spectra were measured on a Bruker AVANCE III HD spectrometer (500 MHz, Bruker, Karlsruhe, Germany). Computational Methods Density functional theory (DFT) calculations were conducted with the Gaussian 16 program39 using the B3LYP method and the 6-31G(d) basis set. Time-dependent DFT (TD-DFT) calculations were carried out using the same program with the m06 method. The geometries were optimized on their single-crystal structures using the default convergence criteria without any constraints. Results and Discussion Synthesis The traditional two-step synthetic route for TTM radical derivatives is depicted in Scheme 1a. First, via C–C or C–N arylation, a functional aryl group was attached on hydrogen-protected HTTM, and corresponding precursor HTTM-Ar was provided. Even if radical compound TTM was directly used as the starting material, the product was mostly reduced to hydrogen-protected precursor HTTM-Ar. A main product of HTTM-Ar with a trace amount of radical TTM-Ar was obtained. Consequently, intense NMR signals were recorded for the product of radical TTM initialized reaction. Next, extra dehydrogenation and oxidation reactions were thus required to oxidize HTTM-Ar into TTM radical derivative TTM-Ar. Taking reference TTM-Cz (Figure 1a) as an example, when radical TTM was used as the starting material, a condensation with carbazole was initially carried out with the presence of Cs2CO3 in N,N-dimethylformamide (DMF) at 160 °C.15,31 The product was reduced HTTM-Cz, which was further converted to TTM-Cz by dehydrogenation with t-BuOK and oxidation with chloranil. Therefore, to avoid the reduction during the arylation reaction and produce the TTM radical derivatives in one step, any possible weak reductant should be avoided. Herein, to prepare the diarylamine-substituted TTM radicals TTM-DPA, TTM-DBPA, and TTM-DFA, Buchwald–Hartwig coupling reactions40–43 were conducted, and the synthetic approach is depicted in Scheme 1b and Supporting Information Scheme S1. The reaction conditions, such as catalyst, ligand, base, solvent, and temperature, have been optimized, and the results are shown in Supporting Information Table S1. In only one step, using the optimal Buchwald–Hartwig coupling condition [Pd(OAc)2 as catalyst, P(t-Bu)3 as ligand, Cs2CO3 as base, toluene as solvent, 80 °C, 24 h], the radical compound TTM was successfully converted to the target diarylamine-substituted radicals TTM-DPA, TTM-DBPA, and TTM-DFA in 55%, 45%, and 39% yield, respectively. The main byproducts were bis- and tris-arylated TTM radical derivatives. Even the feed ratios of radical compound TTM were higher than the diarylamines. Nevertheless, the obtained diarylamine-substituted TTM radicals were stable enough and thus could be facilely purified by routine silica gel column chromatography. Moreover, upon increasing the feed ratio of diphenylamine and extending the reaction time, di- and trisubstituted products ( TTM-2DPA and TTM-3DPA, Supporting Information Schemes S2 and S3) were produced in 52% and 61% yields, respectively, which suggests that this approach is a general method for the synthesis of multifunctionalized TTM derivatives. It should be noted that the reaction temperature (80 °C) of C–N coupling is substantially milder compared to the temperature of 160 °C reported in the literature,15,31,34 which suggests the applicability of this one-step radical-to-radical amination reaction. Neither proton nor carbon signal could be obtained in the NMR measurements for the neutral radicals TTM-DPA, TTM-DBPA, and TTM-DFA due to their paramagnetic characters which result in their silent NMR response. Therefore, HRMS was conducted, and the spectra exhibited single intense signals corresponding to the calculated masses of compounds, along with some weak signals assigned to the [M-Cl]+ and [M-2Cl]+ fragments ( Supporting Information Figures S1–S3). Moreover, EPR measurements were carried out for the three neutral radicals in DCM solutions (Figure 2) and in solid states ( Supporting Information Figure S4). As shown in Figure 2, all the three radicals in DCM solutions displayed strong signals at room temperature with g = 2.003. The g values for these three monoradicals are theoretically consistent with the conventional carbon-free electron g (2.0023),18 which further confirms the existence of the unpaired radical electron in the neutral radicals with an open-shell form. Figure 2 | EPR spectra of TTM-DPA, TTM-DBPA, and TTM-DFA in DCM solutions at 298 K. Download figure Download PowerPoint To verify the successful synthesis of the radicals and investigate the molecular structural features, single-crystal X-ray diffraction analysis was performed. The single crystals of TTM-DPA and TTM-DBPA were obtained by slow diffusion of water into their acetone solutions under natural ambient conditions.a The crystal structures are diplayed in Figure 3 and Supporting Information Figures S5–S7, and the crystallographic parameters are listed in Supporting Information Table S2. The central methyl carbon atom C0 in the TTM unit and its adjacent atoms C1, C2, and C3 in both TTM-DPA and TTM-DBPA (Figures 3a and 3b) were almost on the same plane. Moreover, the bond lengths of C0 and C1/C2/C3 in TTM-DPA and TTM-DBPA were in the range of 1.455–1.471 Å (Figures 3c and 3d and Supporting Information Tables S3 and S4), which were between the typical C–C bond (1.542 Å) and C=C bond (1.345 Å).44,45 Furthermore, the bond angles in TTM-DPA and TTM-DBPA ( Supporting Information Figures S8) are 121° and 121° for ∠C1C0C2, 121° and 120° for ∠C2C0C3, and 118° and 119° for ∠C1C0C3, respectively, which are close to 120°. All these results suggest that the central carbon atoms C0 in both TTM-DPA and TTM-DBPA are sp2 hybridized with open-shell structures. The harmonic oscillator model of aromaticity (HOMA) values46,47 of all individual rings were calculated to be larger than 0.9 on the basis of their crystallographic structures, which suggests that no quinoidal structures exist in either monoradical when all the benzene rings are linked by a single bond. On the other hand, the torsion angles ( Supporting Information Figure S8) were 52°, 39°, and 55°, respectively, for the chlorinated benzene rings A, B, and C in TTM-DPA with respect to the plane determined by atoms C0, C1, C2, and C3. Meanwhile, the corresponding torsion angles for TTM-DBPA were 44°, 47°, and 53°, respectively. Presumably the slight difference can be attributed to the distinct crystalline packing effects caused by the incorporated electron donor groups. These results indicate that the TTM radical moities in both TTM-DPA and TTM-DBPA is propeller-shaped and the carbon radicals are shielded and stabilized sterically by six chlorines. Furthermore, the distance of the two parallel benzene rings in the dimeric molecules increased from 3.39 Å for TTM-DPA to 5.45 Å for TTM-DBPA ( Supporting Information Figure S9), which indicates weak π–π stacking interactions in the diarylamine-substituted TTM radicals. Figure 3 | X-ray crystallographic structures, bond lengths (in Å), and calculated HOMA values of (a and c) TTM-DPA and (b and d) TTM-DBPA. The red numbers in the hexagon are the calculated HOMA values. Hydrogen atoms are omitted for clarity. Download figure Download PowerPoint Photophysical properties The UV–vis–NIR absorption and PL spectra of the diarylamine-substituted TTM radicals TTM-DPA, TTM-DBPA, and TTM-DFA were measured in cyclohexane solutions (ca. 10−5 M) and the corresponding photophysical data are summarized in Table 1. For comparison, the photophysical properties of TTM-Cz were also recorded. As shown in Figure 4a, all the TTM radicals demonstrated two distinct absorption bands. The intense absorption bands around 370 nm with moderate absorption bands around 450 nm originated from the characteristic π–π* transitions.48 The other absorption bands with the lowest energies were attributed to the ICT from the electron donor to the TTM core. The maximum absorption wavelength of TTM-DPA was located at 644 nm (ε = 1.01 × 104 M−1 cm−1). A bathochromic shift of 41 nm was observed as compared with the absorption maximum of TTM-Cz (λmax = 603 nm). This can be ascribed to the relatively stronger electron-donating ability of diphenylamine in comparison to carbazole,49 which results in enhanced ICT interactions in TTM-DPA. Upon the extension of the diphenylamine to dibiphenylylamine, the maximum absorption wavelength of TTM-DBPA bathochromically shifted to 664 nm (ε = 1.52 × 104 M−1 cm−1) due to the conjugation extension. When the dibiphenylylamine was replaced by difluorenylamine, the absorption maximum of TTM-DFA further shifted to 696 nm (ε = 1.74 × 104 M−1 cm−1) owing to the more planar structure of fluorene as compared with biphenyl. The absorption onsets of the diarylamine-substituted TTM radicals were located at 700, 730, and 780 nm for TTM-DPA, TTM-DBPA, and TTM-DFA, respectively. The corresponding optical gaps were determined to be 1.77 eV for TTM-DPA, 1.70 eV for TTM-DBPA, and 1.59 eV for TTM-DFA, respectively, which were distinctly lower than that for TTM-Cz (1.95 eV). Table 1 | Photophysical Properties of the TTM Radical Materials Radical λAbs (nm)a ε (104 M−1 cm−1)a λPL (nm)a ΦPL (%)b Stokes Shift (cm−1) E1/2ox (V)c E1/2red (V)c ESOMO (eV)c ESUMO (eV)c Eg (eV) TTM-Cz 603 0.60 633 50 786 0.57 −0.96 −5.29 −3.92 1.37 TTM-DPA 644 1.01 705 65 1344 0.20 −1.02 −4.92 −3.86 1.06 TTM-DBPA 664 1.52 748 28 1691 0.19 −1.03 −4.89 −3.85 1.04 TTM-DFA 696 1.74 809 5 2007 0.13 −1.03 −4.85 −3.85 1.00 aAbsorption (λAbs) and PL (λPL) maxima were measured in cyclohexane solutions (ca. 10−5 M). bMeasured in cyclohexane using rhodamine 6G as the standard.50,51 cThe SOMO and SUMO levels were determined from the first oxidation and reduction onset potentials which were calibrated with ferrocene. Figure 4 | (a) UV–vis–NIR absorption and (b) PL spectra of the TTM radical derivatives TTM-Cz, TTM-DPA, TTM-DBPA, and TTM-DFA in cyclohexane solutions (ca. 10−5 M). Download figure Download PowerPoint Figure 4b displays the PL spectra of the TTM radical derivatives in cyclohexane solutions (ca. 10−5 M). The maximum PL wavelengths of TTM-DPA, TTM-DBPA, and TTM-DFA were located in the NIR region at 705, 748, and 809 nm, respectively, which were in the same trend as the absorption maxima. In comparison to the maximum PL wavelength of TTM-Cz, significant bathochromic shifts of 72, 115, and 176 nm were observed for TTM-DPA, TTM-DBPA, and TTM-DFA, respectively, which can be ascribed to strengthened ICT interactions and extended effective conjugation lengths in the diarylamine-substituted TTM radicals. Moreover, the PLQYs of the TTM radical derivatives in dilute cyclohexane solutions were measured using rhodamine 6G as the standard.50,51 For comparison, the PLQY of reference TTM-Cz was measured to be 50%, which is similar to the previously reported value (53%).31 Under the same condition, TTM-DPA displayed an extremely high PLQY of 65%. The unusually large PLQY value of TTM-DPA with NIR emission suggests that TTM-DPA is an ideal candidate as an efficient NIR emitter for various applications. With the PL maximum bathochromically shifting, TTM-DBPA and TTM-DFA displayed gradually decreased PLQYs of 28% and 5%, respectively, which can be explained by the energy gap law.52 Moreover, to further investigate the ICT interactions in the diarylamine-substituted TTM radicals, the solvatochromic effects on the absorption and PL features were investigated. Taking TTM-DPA as an example, as illustrated in Supporting Information Figure S10a, the absorption spectrum of TTM-DPA was slightly solvent-dependent. Only a 7 nm shift was observed for the absorption maxima when solvents ranging from cyclohexane to DMF. However, unlike the rather weak solvatochromism in the absorption spectra, a remarkable solvatochromic PL was found for TTM-DPA. As depicted in Supporting Information Figure S10b, the cyclohexane solution of TTM-DPA displayed the maximum PL wavelength at 705 nm. With the solvent polarity increasing, the PL maximum of TTM-DPA gradually shifted bathochromically to 778 nm in toluene, 848 nm in DCM, and 861 nm in THF. Moreover, a notable decrease in the PL intensity was observed with an increase in solvent polarity. The PL of TTM-DPA in more polar solvents, such as acetone and DMF, could hardly be detected. The Stokes shift enhanced from 1344 cm−1 in cyclohexane and to 3794 cm−1 in THF. Such a significant solvatochromic effect is characteristic for an efficient charge transfer from the diphenylamine moiety to the TTM radical. The more significant solvatochromism in PL spectra as compared with that in absorption spectra is due to the more polarized excited states.53,54 When the electrons were excited, the dipole was enhanced. Therefore, a more polar solvent was able to stabilize such a polarized excited state by the reorientation of the solvent molecules to accommodate the increased dipole, lowering the energy of the system and thereby leading to the more distinct bathochromic shift in the PL spectra. The solid states of the TTM radical derivatives were not photoluminescent due to the aggregation-caused quenching. Therefore, the thin films of the TTM radical derivatives were prepared by dispersing them in polymethyl methacrylate (PMMA) on quartz substrates. In contrast to the absorption spectra in cyclohexane solutions, the films displayed maximum absorption wavelengths at 605, 648, 672, and 701 nm for TTM-Cz, TTM-DPA, TTM-DBPA, and TTM-DFA ( Supporting Information Figure S11a). Bathochromic shifts of 2, 4, 8, and 5 nm were observed in comparison to those in cyclohexane solutions, which were attributed to the weak π–π stacking interactions among the radial molecules in solid states. Under the same conditions, the films of TTM-Cz, TTM-DPA, TTM-DBPA, and TTM-DFA, exhibited PL maxima at 694, 773, 826, and 858 nm ( Supporting Information Figure S11b) with PLQYs of 7%, 6%, 1%, and 1%, respectively. Distinct bathochromic shifts and decreased PLQYs were observed in comparison to their cyclohexane solutions, which is probably owing to the polarization effect of fluorophores caused by the polar PMMA matrix. Stability properties Stability is an important issue for the photoelectromagnetic applications of neutral radicals. Therefore, the stability properties of the diarylamine-substituted TTM radicals were firstly investigated by TGA in nitrogen atmosphere. For comparison, the stability properties of TTM-Cz were measured under the same conditions. As shown in Figure 5a, the decomposition temperatures (Td) for the diarylamine-substituted TTM radicals range from 313 to 338 °C, which are close to TTM-Cz (340 °C) and can be attributed to the decomposition of the aryl groups. The high Td values over 300 °C suggest their excellent thermal stability. Figure 5 | (a) TGA curves of the TTM radical derivatives in nitrogen atmospheres, (b) absorption intensity changes at λmax, and (c) photographs of TTM-Cz, TTM-DPA, TTM-DBPA, and TTM-DFA in dilute cyclohexane solutions exposed in ambient environment. Download figure Download PowerPoint The diarylamine-substituted TTM radicals exhibited excellent chemical stability in solid states for several months. No obvious changes were observed in the absorption and PL spectra for the as-prepared TTM radical derivatives after long-term storage in the solid state. In general, the dispersion in solution may accelerate the chemical reaction and degradation. Therefore, the stability of the solution of the TTM radical derivatives was investigated by evaluating the degradation of their dilute solutions upon continuous irradiation by the natural light. Supporting Information Figure S12 displays the absorption spectra of the TTM radical derivatives at different times. The absorbance of the four TTM radicals at corresponding maximum absorption wavelengths in cyclohexane solutions are shown in Figure 5b. We found that the absorbance of TTM-Cz degenerated rapidly and that the red color of the solution completely disappeared after irradiation for 24 h (Figure 5c). At the same time, no obvious change in the solution color was observed for the three diarylamine-substituted TTM radicals. As shown in Figure 5b, the half-life time (t1/2) of TTM-DPA (167 h), TTM-DBPA (500 h), and TTM-DFA (806 h) were 15, 45, and 73 times longer than that for TTM-Cz (11 h), respectively. This indicates that the solution stability of TTM-Cz can be dramatically improved by replacing the carbazole unit with diarylamine groups. Moreover, the absorbance decays gradually and slows down when the substituent groups switch from diphenylamine to dibiphenylylamine, and further to difluorenylamine. These results suggest that a more conjugated system can better delocalize the TTM radical charge to suppress the potential photocyclisation for the TTM core and thus improve the radical stability.33 Electrochemical properties The electrochemical behavior of the diarylamine-substituted TTM radicals TTM-DPA, TTM-DBPA, and TTM-DFA as well as reference TTM-Cz were investigated by cyclic voltammetry (CV). It was found that the diarylamine-substituted TTM radicals exhibited two reversible redox couples in the cathodic and anodic regions (Figure 6), which can be assigned to the oxidation of the diarylamine groups and reduction