Phenyl Derivatives Modulate the Luminescent Properties and Stability of CzBTM-Type Radicals

The distinctive electron structures of luminescent radicals offer considerable potential for a diverse array of applications. Up to now, the luminescent properties of radicals have been modulated through the introduction of electron-donating substituents, predominantly derivatives of carbazole and polyaromatic amines with more and more complicated structures and redshifted luminescent spectra. Herein, four kinds of (N-carbazolyl)bis(2,4,6-tirchlorophenyl)-methyl (CzBTM) radicals, Ph2CzBTM, Mes2CzBTM, Ph2PyIDBTM, and Mes2PyIDBTM, were synthesized and characterized by introducing simple phenyl and 2,4,6-trimethylphenyl groups to CzBTM and PyIDBTM. These radicals exhibit rare blueshifted emission spectra compared to their parent radicals. Furthermore, modifications to CzBTM significantly enhanced the photoluminescence quantum yields (PLQYs), with a highest PLQY of 21% for Mes2CzBTM among CzBTM-type radicals. Additionally, the molecular structures, photophysical properties of molecular orbitals, and stability of the four radicals were systematically investigated. This study provides a novel strategy for tuning the luminescent color of radicals to shorter wavelengths and improving thermostability.

Not only the electronegativity but also the torsion angle of carbazole affects the change in the emission spectrum [31,32].Another successful approach to obtain blueshifted emission spectra is the introduction of a nitrogen atom into the carbazolyl moiety of CzBTM, which leads to a weakening of the electron-donating property of carbazolyl [33].Herein, a different strategy was carried out to change the performance of CzBTM-type radicals.Given that CzBTM is itself a donor-acceptor molecule, unlike the changes in the carbazole moiety from previous research, the modification on the radical side could also affect the luminescent properties.In this paper, weak electron-donating groups, phenyl and 2,4,6-trimethylphenyl, were introduced into the radical side of CzBTM and its analogue PyIDBTM (Figure 1).The molecular structures, photophysical properties, and stabilities of these derivatives were systematically investigated.The introduction of additional proper donors at the radical sides in the four derivatives could weaken the electron-accepting ability to the radical, resulting in blueshifts of the emission spectra.In addition, this molecular design strategy could also improve the photoluminescence quantum yield (PLQY) and thermostability of CzBTM-type radicals.

Synthesis and Structure Characterization
CzBTM and PyIDBTM were synthesized from commercially available reagents and the methods reported in the literature were followed [31,33].Four target radicals, Ph 2 CzBTM, Mes 2 CzBTM, Ph 2 PyIDBTM, and Mes 2 PyIDBTM, were synthesized utilizing the Suzuki-Miyaura coupling reaction as the final step in yields of 30%, 23%, 21%, and 15%, respectively.The synthetic details are outlined in the Supporting Information (Figure S1).The molecular structures of the four radicals were confirmed by MALDI-TOF mass spectra and Fourier transform infrared spectra (FT-IR) at first (Figures S2-S7).The existence of the unpaired electron, so as the radical nature, was confirmed by electron paramagnetic resonance (EPR) spectra at room temperature, with g values of 2.0037, 2.0038, 2.0021, and 2.0021 (Figure 2a-d).To further explore the molecular structures of these four radical molecules, attempts were made to gain single crystals.However, up to now, only ideal single crystals of the Ph 2 CzBTM and Mes 2 CzBTM radicals were obtained using the slow diffusion method with dichloromethane/ethanol mixed solution at room temperature.As for the Ph 2 PyIDBTM and Mes 2 PyIDBTM radicals, polycrystal samples were always obtained.Thus, only the structures of Ph 2 CzBTM and Mes 2 CzBTM were further determined through single-crystal X-ray diffraction measurements (Figure 2e,f).The solved structures reveal that the central methyl carbon atoms, C13 in both molecules, are sp 2 -hybridized and are coplanar with N1, C14, and C26 (or C29), which also confirmed the existence of an unpaired electron.The torsion angles of the dichlorophenyl rings and carbazole group to the radical center plane are approximately 45.As for the PL spectra (Figure 3b), Ph 2 CzBTM and Mes 2 CzBTM exhibit a red emission that peaked at 683 nm and 672 nm, with blueshifts of about 30 nm and 41 nm, respectively, compared to CzBTM (713 nm).Similar blueshifts could also be observed in the PyIDBTM series.The emission peak of Ph 2 PyIDBTM is at 653 nm, while that of Mes 2 PyIDBTM is at 630 nm, showing blueshifts of 11 nm and 34 nm, respectively, compared to PyIDBTM (664 nm).These results demonstrated that the molecular design strategy in this work is effective to realize blueshifts of the emission spectra of CzBTM-type radicals.And 630 nm for the emission peak is the shortest wavelength for the CzBTM derivatives reported.All the radicals show nearly unchanged absorption peaks in the solvents of different polarities, but the PL spectra undergo a significant redshift with increasing solvent polarity, which demonstrated the CT excited-state nature (Figures S10-S13).
The results show that, compared to CzBTM, the k r values of the four derivatives have been enhanced, while the k nr significantly decreased.The shorter emission wavelengths of all the derivatives, which means a larger energy gap as confirmed by following the theoretical calculations, serves to diminish the Frank-Condon factors, implying a decline in the internal conversion [34] and consequently a decrease in the k nr .Additionally, due to the larger steric hindrance in Ph 2 CzBTM and Mes 2 CzBTM, effective inhibition of the molecular rotation and vibration occurs, also leading to a significantly smaller k nr .Thus, Mes 2 CzBTM exhibits the highest PLQY.However, the k nr values of Ph 2 PyIDBTM and Mes 2 PyIDBTM do not decrease further compared to PyIDBTM, which may be affected by the LE-dominated excited state of PyIDBTM in low-polarity solvent [35].

Theoretical Calculations
Density functional theoretical (DFT) calculations (B3LYP/6-31G (d,p)) were performed to understand the frontier molecular orbitals (MOs) of CzBTM-type radicals.The results show that the spin density distributions (Figure S15) of Mes 2 CzBTM and Mes 2 PyIDBTM are similar to those of CzBTM and PyIDBTM, while the spin density distributions of Ph 2 CzBTM and Ph 2 PyIDBTM are partially delocalized to the benzene rings.Through the calculation MOs (Figure 4a), the single occupied molecular orbital (α-SOMO) and the single unoccupied molecular orbital (β-SUMO) are mainly distributed on the central carbon atom and partially extend to the dichlorophenyl rings and carbazole moiety for Mes 2 CzBTM (160α and 160β), while those of Ph 2 CzBTM (184α and 184β) are distributed throughout the molecular skeleton.The highest occupied β molecular orbital (β-HOMO) of Mes 2 CzBTM (183β) and Ph 2 CzBTM (159β) is mainly distributed on the dichlorophenyl rings and carbazole moiety, which is different from CzBTM.The lowest unoccupied β molecular orbital (β-LUMO) is mainly distributed on the dichlorophenyl and outer phenyl groups for Ph 2 CzBTM (161β).However, the β-LUMO of Mes 2 CzBTM (183β) is only located on the dichlorophenyl groups, which is caused by the large dihedral angle between the mesityl and dichlorophenyl groups.The distribution of the frontier electronic orbitals of PyIDBTM, Ph 2 PyIDBTM, and Mes 2 PyIDBTM are shown in Figure 4b, showing a similar characterization as discussed above.The energy levels of the frontier MOs are summarized in Table S6.As can be seen in Figure 4, the energy levels of the α-SOMO and β-SUMO of Ph 2 CzBTM (−4.67 eV and −2.74 eV) and Mes 2 CzBTM (−4.71 eV and −2.67 eV) are higher than that of CzBTM (−4.99 eV and −3.01 eV).Similarly, the energy levels of Ph 2 PyIDBTM and Mes 2 PyIDBTM are also higher than that of PyIDBTM.From the above results, it can be understood that when the phenyl derivatives are introduced, the energy levels of the HOMO, α-SOMO, and β-SUMO obviously increase.However, for the donor itself, the carbazole or β-carboline do not change.Thus, the phenyl derivatives mainly caused the increase in the MO energy levels of the diphenyl methyl radical side, which is also equivalent to the weakening of the electron-accepting ability to the radical.As a result, the PL spectra show obviously blue shifts.S5) of Ph 2 CzBTM, Mes 2 CzBTM, Ph 2 PyIDBTM, and Mes 2 PyIDBTM originate from 159β→160β, 183β→184β, 159β→160β, and 183β→184β transitions, respectively.Furthermore, the transition energies of D 1 were calculated, with the CzBTM radicals and derivatives as 2.19 eV, 2.21 eV, and 2.27 eV (CzBTM < Ph 2 CzBTM < Mes 2 CzBTM) and the PyIDBTM radicals and derivatives as 2.27 eV, 2.28 eV, and 2.36 eV (PyIDBTM < Ph 2 PyIDBTM < Mes 2 PyIDBTM).This further confirms the blueshift observed in the PL spectra.

Materials and Methods
The detailed synthesis route of the target radicals can be found in Supporting Information 1.1.The raw materials and chemical reagents utilized in this investigation were procured from ERNEGI and Xilong Science Co., Ltd.(Shanghai, China), sans additional refinement.The infrared spectra were captured using a Bruker Tensor 27 spectrometer.The KBr and the radical were thoroughly mixed at a mass ratio of 100:1 and the mixture was ground well.Then, the mixture was pressed into a lamellar state for the IR measurements.The spectra of the NMR were obtained at room temperature on a Bruker AVANCE NEO 400.Deuterated chloroform was used as the solvent for both the 13 C NMR and 1 H NMR spectra.The mass spectra were primarily obtained through testing with two instruments: LCMS-IT-TOF and MALDI-TOF.DCTB was selected as the matrix for the MALDI-TOF testing.The EPR testing was conducted with a Bruker A320 spectrometer.The radical materials were tested at room temperature in both the solid state and solution (with dichloromethane as the solvent at a concentration of 0.1 M).The elemental analysis data were recorded on an Elementar Vario micro cube spectrometer.The single-crystal structure testing results were obtained using the Bruker D8 Quest instrument.The analysis of the single-crystal structures and their further analysis were conducted using the Olex2 software.The testing temperature for the single-crystal structures was maintained at 100 K.The UV-visible spectra were obtained using a Shimadzu UV-1900i UV-Vis spectrometer (Shimadzu Corporation, Kyoto, Japan).The photoluminescence spectra were acquired with a Shimadzu RF-6000 spectrometer (Shimadzu Corporation, Japan).The PL decays were captured employing an Edinburgh FLS1000 spectrometer (Edinburgh Instruments, Livingston, UK), and the absolute PLQYs were measured on the identical apparatus via the integrating sphere method.The DFT and TD-DFT calculations were executed using the Gaussian16 C.02 commercial software [36].The TGA test results were obtained using the TA INSTRUMENTS Q600 instrument (New Castle, DE, USA), with the tests conducted on the radicals in both air and nitrogen atmospheres, at a heating rate of 10 • C/min.The cyclic voltammetry measurements were carried out with a CH Instruments CHI660E electrochemical analyzer (Austin, TX, USA).The results of the photostability were obtained through testing under continuous xenon lamp irradiation, using the Shimadzu RF-6000 spectrometer.

Synthesis of Ph 2 CzBTM and Mes 2 CzBTM
The specific synthetic route is detailed in Figure S1

Conclusions
By introducing phenyl and 2,4,6-trimethylphenyl groups into the structures of CzBTM and PyIDBTM, we successfully synthesized four new CzBTM-type radicals.The PL spectra of the four derivatives exhibited rare blueshifts in luminescent radicals, especially with the introduction of the 2,4,6-trimethylphenyl group.An analysis of the energy levels of the frontier molecular orbitals from theoretical calculations and CV measurements demonstrated the reason for blueshifts.The introduction of phenyl derivatives mainly caused the increase in the MO energy levels of the radical side, equivalent to the weakening of the electron-donating ability from the carbazole or β-carboline.Additionally, the PLQYs and thermostability were also improved, particularly for Mes 2 CzBTM with a highest PLQY of 21% and a highest Td of 311 • C under nitrogen.The introduction of phenyl and 2,4,6-trimethylphenyl groups provides a new strategy for adjusting the luminescent color of CzBTM-type radicals to shorter wavelengths and improving the thermostability.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules29122900/s1, Figure S1 S2: The values of the characteristic torsion angles in the radical molecules in the crystal structures and the theoretical calculations; Table S3: The bond lengths of the radical molecules in the crystal structures and the theoretical calculations; Table S4: The parameters corresponding to the D 1 transition in the TD-DFT calculation results of the radicals; Table S5: Redox potential of free radicals and their corresponding orbital energy levels.

2. 2 .
Photophysical Properties The ultraviolet-visible (UV-Vis) absorption spectra and photoluminescence (PL) spectra of Ph 2 CzBTM, Mes 2 CzBTM, Ph 2 PyIDBTM, and Mes 2 PyIDBTM were measured in cyclohexane solvent (Figure 3a,b).At the same time, the photophysical properties of CzBTM and PyIDBTM were also measured for comparison.Similar to CzBTM and PyIDBTM, Ph 2 CzBTM, Mes 2 CzBTM, Ph 2 PyIDBTM, and Mes 2 PyIDBTM also display three absorption bands, including a strong absorption band at 280 nm caused by the carbazole moiety or β-carboline, a medium absorption band around 400 nm attributed to the characteristic absorption of carbon-centered radicals, and a weak absorption band beyond 500 nm primarily attributed to the intramolecular charge-transfer (CT) state.However, in comparison to Mes 2 CzBTM and Mes 2 PyIDBTM, there is a significant redshift in the medium absorption band of Ph 2 CzBTM and Ph 2 PyIDBTM, which is attributed to the difference in their transition energy caused by the difference in their molecular structures.Compared to the substituents in Mes 2 CzBTM and Mes 2 PyIDBTM, the phenyls in Ph 2 CzBTM and Ph 2 PyIDBTM formed larger conjugated structures due to the smaller dihedral angles, resulting in changes in the characteristic absorption of the carbon-centered radicals, which is also confirmed by the subsequent theoretical calculations.

Figure 4 .
Figure 4. Frontier molecular orbitals calculated by DFT calculations.(a) CzBTM, Ph 2 CzBTM, and Mes 2 CzBTM; (b) PyIDBTM, Ph 2 PyIDBTM, and Mes 2 PyIDBTM.(Red and dark green distributions on molecule represent electron cloud distributions; green, blue, gray and white balls represent Cl, N, C and H atoms respectively.)Additionally, time-dependent DFT (TD-DFT) calculations (B3LYP/6-31G (d,p)) were carried out to further investigate the excited states.The results show that the doublet excited states (D 1 ) (Table S5) of Ph 2 CzBTM, Mes 2 CzBTM, Ph 2 PyIDBTM, and Mes 2 PyIDBTM originate from 159β→160β, 183β→184β, 159β→160β, and 183β→184β transitions, respectively.Furthermore, the transition energies of D 1 were calculated, with the CzBTM radicals and derivatives as 2.19 eV, 2.21 eV, and 2.27 eV (CzBTM < Ph 2 CzBTM < Mes 2 CzBTM) and the PyIDBTM radicals and derivatives as 2.27 eV, 2.28 eV, and 2.36 eV (PyIDBTM The decomposition temperatures (Td, corresponding to 5% weight loss) for the four radicals are 254 • C, 301 • C, 284 • C, and 303 • C, respectively, in an ambient atmosphere and 273 • C, 311 • C, 291 • C, and 301 • C, respectively, in a nitrogen atmosphere, which are obviously higher than those of CzBTM (230 • C) and PyIDBTM (241 • C).In particular, the introduction of 2,4,6-trimethylphenyl significantly increased the decomposition temperature of the radicals, effectively enhancing their thermostability.Two main factors contribute to this result: firstly, the substituents are introduced into the more reactive chlorine sites of the CzBTM and PyIDBTM radicals, reducing the reactivity; secondly, the introduction of phenyl derivatives increases the spatial steric hindrance of the molecules, reducing the molecular thermal vibrations and thereby improving the thermal stability of the radical molecules.

Figure 5 .
Figure 5.The TGA curves of four radicals with a heating rate of 10 • C/min under (a) air and (b) nitrogen.(The horizontal dashed black line represents the level of 5% mass loss.)Photostability is another important property for luminescent radicals.The fluorescence intensity decays of the Ph 2 CzBTM, Mes 2 CzBTM, Ph 2 PyIDBTM, and Mes 2 PyIDBTM