Simple Substitutions into Donor–Acceptor Radicals to Construct Highly Luminescent Radicals and Solvent-Responsive Ionic Radical Polymers

TL;DR

Researchers created highly luminescent radicals and solvent-responsive polymers by modifying donor-acceptor radical structures with simple substitutions.

Contribution

The study introduces new luminescent radical derivatives and solvent-responsive ionic radical polymers through simple structural modifications.

Findings

TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz showed high photoluminescence quantum efficiency (80% and 39%, respectively).

The polymers LT-Cz and LT-Dpa exhibited solid-state PLQE of 39% and 29%, respectively.

The polymers displayed solvent-responsive luminescence and could return to their original state after solvent evaporation.

Abstract

Constructing a donor–acceptor (D–A) architecture in luminescent radicals is an effective strategy for enhancing luminescent properties. However, further structural modification of the radical core through the simple substitutions in the framework of D–A radicals remains relatively underexplored. Herein, we synthesized two radical derivatives TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz through modification of the TTM unit of TTM-Cz and TTM-Dpa with imidazole and mesitylene groups. These radical derivatives exhibited high photoluminescence quantum efficiency (PLQE) (80% for TTM-Mes-Cz-Mz and 39% for TTM-Mes-Dpa-Mz) and photostability. The radical units were further covalently grafted onto the polymer chains to synthesize ionic radical polymers LT-Cz and LT-Dpa. LT-Cz and LT-Dpa showed PLQE of 39% and 29% in a solid state, respectively. Furthermore, the polymers exhibited solvent-responsive luminescence with dichloromethane and tetrahydrofuran. A significant redshift in emission wavelength and decrease in emission intensity were observed. The polymers could return to their initial state with solvent evaporation. This work advances the exploration of the role of simple substituent modifications in D–A radical systems, thereby enabling highly efficient luminescence in both small-molecule radicals and radical polymers.

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1. Introduction

Organic luminescent radicals with doublet-emission mechanisms can effectively overcome the breakthrough of 25% internal quantum efficiency, which represents a limitation in traditional closed-shell molecules. Due to the special open-shell electronic structure, organic radicals show higher reactivity with redox processes, limiting further application. However, chlorinated triphenylmethyl (trityl) radicals, such as perchlorotriphenylmethyl (PTM) and tris(2,4,6-trichlorophenyl)methyl (TTM), successfully achieve high kinetic and thermodynamic stability through steric protection provided by bulky chlorine atoms and spin delocalization over aromatic rings. More importantly, the excellent photoluminescence and electroluminescence properties [1,2,3] endow radicals with broad application prospects, such as their use in organic light-emitting diode [4,5,6], imaging [7,8,9,10,11], photothermal therapy [12,13,14], detection [15,16], and electron spinning [17]. With the introduction of an electron donor into the radical core through chemical bonding [18,19,20,21,22] and spatial interactions [23], the efficient donor–acceptor (D-A) structure can be constructed to improve luminescence efficiency and stability. Based on the D-A radical framework, researchers have further attempted to directly introduce donor or sterically hindered groups into the radical core to enhance luminescent properties [24,25,26]. Friend and Bronstein et al. achieved highly efficient luminescent radicals by further substituting the TTM unit in the D-A radical with a mesityl group [27]. Kuehne et al. achieved a PLQE as high as 87% by modifying the TTM unit with multiple 2,7-disubstituted carbazole groups [28]. We also enhanced the luminescence efficiency by directly grafting mesityl groups onto the TTM unit of [4-(N-Carbazolyl)-2,6-dichlorophenyl] bis(2,4,6-trichlorophenyl)methyl (TTM-Cz) [29]. These works demonstrate that introducing additional groups into the D-A radical is a viable and effective strategy for further enhancing luminescent performance. In this work, imidazole and mesitylene were introduced into the TTM unit of TTM-Cz [30] and TTM-Dpa [31] (parent D-A radicals). TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz demonstrated not only improved luminescence efficiency but also enhanced photostability, especially TTM-Mes-Cz-Mz. We covalently grafted small-molecule TTM derivatives into the polymer chains to gain novel ionic radical polymers LT-Cz and LT-Dpa with highly efficient solid-state luminescence (Figure 1). Furthermore, LT-Cz and LT-Dpa exhibited reversible solvent-responsive luminescence to dichloromethane (DCM) and tetrahydrofuran (THF).

2. Results and Discussion

2.1. Synthesis and Characterization

The synthesis procedures of TTM-Mes-Cz-Mz, TTM-Mes-Dpa-Mz, LT-Cz and LT-Dpa are detailed in the Supplementary Materials (Figures S1 and S2). The molecular structures and compositions of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz were characterized by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (Figure S3) and Fourier transform infrared spectra (FT-IR) (Figure S4). The presence of unpaired electrons was verified by electron paramagnetic resonance (EPR) spectroscopy (Figures S5 and S6). LT-Cz and LT-Dpa were characterized by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) (Figures S7 and S8) and solid-state ^13^C Nuclear Magnetic Resonance (NMR) (Figure S9).

2.2. Photophysical Properties

The photophysical properties of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz were investigated through the UV-Vis absorption and photoluminescence spectra in toluene solution (Figure 2a,b and Figure S10, Table 1). TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz exhibited pronounced absorption bands centered at 380 nm and 390 nm, respectively. These absorption features originated from the carbon-centered radical nature of the TTM derivatives. Additionally, weak absorption bands appeared at 605 nm for TTM-Mes-Cz-Mz and 646 nm for TTM-Mes-Dpa-Mz, which were attributed to the charge transfer from the donor (carbazole, diphenylamine) to the TTM acceptor. The emission band of TTM-Mes-Dpa-Mz (743 nm) exhibited a more significant bathochromic shift than TTM-Mes-Cz-Mz (668 nm) due to the stronger electron-donating ability of the diphenylamine unit. Further comparison of the absorption and emission spectra in different solvents revealed that absorptions remained largely unaffected, while the emission spectra exhibited a significant redshift with increasing solvent polarity (Figure S11). These observations indicated the presence of a CT excited state in TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz. The absolute PLQE values in toluene were measured with an integrating sphere. The PLQE of TTM-Mes-Cz-Mz (80%) and TTM-Mes-Dpa-Mz (39%) was significantly higher than that of TTM-Cz (68%) and TTM-Dpa (18%). The transient photoluminescence decay spectra of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz were also measured (Figure 2c,d). The measured lifetimes were 29.4 ns for TTM-Mes-Cz-Mz and 17.3 ns for TTM-Mes-Dpa-Mz. The radiative rate constant (k_r_) and non-radiative rate constant (k_nr_) were calculated to explain the luminescence efficiency. We found that upon attaching imidazole and mesitylene groups to the TTM unit, k_r_ showed little change, whereas a significant reduction was observed in k_nr_ relative to TTM-Cz and TTM-Dpa. The direct attachment of imidazole and mesitylene impeded the vibronic coupling of the TTM unit, resulting in the decrease in k_nr_.

2.3. Electrochemistry and Stability

The electrochemical properties of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz were measured via cyclic voltammetry (CV) (Figure S12). The reduction/oxidation potentials of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz were −0.98 V/+0.35 V and −1.4 V/+0.69 V, respectively. The energy levels of the α-SOMO and β-SUMO were calculated according to the electrochemical redox potentials (Table S1). The α-SOMO/β-SUMO levels in TTM-Mes-Cz-Mz (−5.15 eV/−3.82 eV) were higher than those in TTM-Cz (−5.35 eV/−3.82 eV). However, the changing trends of α-SOMO and β-SUMO levels in TTM-Mes-Dpa-Mz (−5.49 eV, −3.40 eV) were distinct in comparison with TTM-Dpa (−4.92 eV/−3.86 eV). The multiple cycles of CV scans demonstrated the excellent electrochemical stability of TTM-Mes-Cz-Mz. However, TTM-Mes-Dpa-Mz showed poor electrochemical stability, which might be associated with the diphenylamine unit during the redox process (Figure S13).

Radical photostability was further quantified by measuring photodegradation half-lives (t_1/2_) under continuous irradiation (375 nm) with a xenon lamp (150 W) (Figure 3a). The measured half-lives were 1.1 × 10^6^ s for TTM-Mes-Cz-Mz and 3.1 × 10^6^ s for TTM-Mes-Dpa-Mz. Compared with TTM-Cz and TTM-Dpa, TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz showed improvements in photostability, especially TTM-Mes-Cz-Mz. This result suggested that introducing appropriate substituents into the radical core could provide effective protection, thereby significantly improving the stability under prolonged light exposure. Thermogravimetric analysis (TGA) showed that the decomposition temperatures in air were 284 °C for TTM-Mes-Cz-Mz and 347 °C for TTM-Mes-Dpa-Mz. Moreover, the decomposition temperatures of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz in N_2_ were also similar (273 °C and 357 °C), indicating moderate thermal stability (Figure 3b).

2.4. Theoretical Calculations

To gain deep insight into the electron density distribution and frontier molecular orbitals of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz, density functional theory (DFT) calculations (B3LYP/6-31G(d,p)) were performed (Figure 4). The optimized molecular structures are detailed (Figure S14, Tables S2 and S3). For TTM-Mes-Cz-Mz, the spin density distribution showed that the electron was largely localized on the TTM core and few electrons were distributed at the adjacent imidazole and carbazole units. Though TTM-Mes-Dpa-Mz also exhibited a relatively localized electron distribution at the TTM core, some electrons were also observed at the diphenylamine unit. The difference in spin density distribution at carbazole and diphenylamine units indicated a stronger D-A interaction between diphenylamine and the TTM core. The frontier orbitals revealed that the singly occupied molecular orbital (α-SOMO) of TTM-Mes-Cz-Mz was distributed at the TTM, carbazole, and imidazole units, while the singly unoccupied molecular orbital (β-SUMO) was concentrated on the TTM core. The α-SOMO and β-SUMO of TTM-Mes-Dpa-Mz also showed similar distribution. The α-SOMO/β-SUMO levels in TTM-Mes-Cz-Mz (−5.36 eV/−3.24 eV) and TTM-Mes-Dpa-Mz (−4.97 eV/−2.98 eV) were higher than those in TTM-Cz (−5.45 eV/−3.36 eV) and TTM-Dpa (−5.05 eV/−3.08 eV). The upward shift in frontier orbital energy levels arose from the electron-donating nature of imidazole and mesitylene. Time-dependent DFT (TD-DFT) calculations (B3LYP/6-31G(d,p)) showed that the doublet excited states (D_1_) of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz originated from the 204β (HOMO)→205β (SUMO) and 205β (HOMO)→206β (SUMO) transitions, respectively (Table S4).

2.5. Ionic Radical Polymers

By covalently grafting small-molecule radical TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz into the polymer chains, solid-state luminescent ionic radical polymers (LT-Cz, LT-Dpa) with solvent response were synthesized. The EPR spectra with g-factors at 2.0034 for LT-Cz and 2.0035 for LT-Dpa demonstrated the stable presence of radicals (Figure S6). SEM showed that the ionic radical polymers aggregated into microsphere particles in a range of about 50–100 um in diameter (Figure S7). Additionally, SEM-EDS analysis indicated that LT-Cz and LT-Dpa contained N, confirming that the small-molecule radical units were successfully bonded to the polymer chains (Figure S8). The polymers were further characterized by ^13^C NMR (Figure S9). TGA showed the higher thermal stability of ionic radical polymers LT-Cz and LT-Dpa compared to small-molecule radicals TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz (Figure S15).

The photophysical properties of LT-Cz and LT-Dpa were further summarized (Table S5). The solid-state photoluminescence spectra of LT-Cz and LT-Dpa showed that the emissions were at 689 nm and 740 nm, respectively (Figure 5). It was worth noting that LT-Dpa exhibited a 51 nm redshift in emission compared to LT-Cz. In addition, the solid-state PLQE reached 39% for LT-Cz and 29% for LT-Dpa. LT-Cz (29.4 ns) and LT-Dpa (20.4 ns) also exhibited lifetimes comparable to those of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz (Figure S16). The photostability of radical polymers was further quantified by measuring photodegradation under continuous irradiation (375 nm) with a xenon lamp (150 W); the emission intensity of LT-Cz decreased slowly and slightly, whereas that of LT-Dpa dropped sharply and significantly (Figure S17). The result clearly indicated that LT-Cz possessed markedly higher photostability than LT-Dpa. We further investigated the luminescence response of ionic radical polymers to different solvents (Figure S18). Only DCM and THF significantly affected the wavelength and intensity of emissions. When a small amount of DCM was dripped onto the radical polymer, the emission peak of LT-Cz redshifted from 689 nm to 765 nm. A redshift from 689 nm to 764 nm was also observed in LT-Cz with the addition of THF. LT-Dpa also exhibited a similar redshift from 740 nm to 767 nm (DCM) and from 740 nm to 768 nm (THF) (Figure 6). With the addition of DCM and THF, the solid-state luminescence intensity of the polymer also decreased significantly (Figure S19). The emission color of LT-Cz and LT-Dpa subsequently recovered to its initial state upon evaporation of the solvent (Figure 6c,d and Figure S20). The introduction and subsequent volatilization of DCM or THF in LT-Cz and LT-Dpa induced swelling and deswelling of the ionic radical polymers. This dynamic structural change altered the microenvironment of the radical units and modulated intramolecular/intermolecular interactions, ultimately resulting in high-contrast, reversible color-tunable emission [32].

3. Materials and Methods

All raw materials and chemical reagents used for the synthesis of small-molecule radicals in this study were purchased from Energy Chemical and Xilong Scientific Co., Ltd. (Shanghai, China), and were used without further purification. Polymer L was chloromethyl polystyrene resin cross-linked with 1% divinylbenzene (200–400 mesh, loading capacity: 0.8–1.3 mmol/g) (TCI). IR spectra were recorded on Thermo Nicolet IS5 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Samples for IR analysis were prepared by thoroughly grinding the radical compound with KBr at a mass ratio of 1:100, followed by compression into thin pellets. Solid-state ^13^C NMR spectroscopy was performed using Bruker Avance NEO 600M NMR Spectroscopy (Bruker, Karlsruhe, Germany). Mass spectrometry data were primarily obtained using LCMS-IT-TOF (Waters, Milford, MA, USA) and MALDI-TOF (Bruker, Karlsruhe, Germany) instruments. For MALDI-TOF analysis, DCTB was employed as the matrix. EPR measurements were conducted on a Bruker A320 spectrometer (Bruker, Karlsruhe, Germany) at room temperature, with samples tested in both the solid state and dichloromethane solution (concentration: 0.1 M). SEM-EDS was performed using Hitachi SU8010 (Hitachi High-Technologies Corporation, Tokyo, Japan) and Zeiss 300 instruments (Carl Zeiss AG, Oberkochen, Germany). UV-Vis absorption spectra were collected on a Shimadzu UV-1900i spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Steady-state photoluminescence (PL) spectra were measured with a Shimadzu RF-6000 spectrofluorometer (Shimadzu Corporation, Kyoto, Japan). The excitation wavelengths of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz were 380 nm and 390 nm, respectively; excitation bandwidth: 3 nm; emission bandwidth: 3 nm; scan speed: 600 nm/min. Time-resolved PL decay profiles were obtained using an Edinburgh FLS1000 spectrophotometer (Edinburgh Instruments, Livingston, UK) without a long-pass filter. Absolute photoluminescence quantum efficiency (PLQE) was determined on the same instrument via an integrating sphere attachment (the excitation wavelengths of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz were 380 nm and 390 nm, with a slit width of 1 nm; the excitation wavelength of LT-Cz and LT-Dpa is 375 nm, with a slit width of 1 nm). DFT and TD-DFT calculations were performed using the Gaussian 16 C.02 software package [33]. TGA was carried out on a TA Instruments Q600 analyzer (TA Instruments, New Castle, DE, USA) under both air and nitrogen atmospheres at a heating rate of 10 °C min^−1^. CV measurements were conducted with a CH Instruments CHI660E electrochemical workstation (scan rate: 300 mVs^−1^; supporting electrolyte: 0.1 M (Bu_4_N)PF_6_; working, counter and reference electrodes: glassy graphite, platinum, and Ag/AgCl; ferrocene was added as an internal standard). Photostability assessments were performed under continuous xenon lamp irradiation, with spectral monitoring carried out using a Shimadzu RF-6000 spectrofluorometer (Shimadzu Corporation, Kyoto, Japan).

Synthesis of TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz

TTM-Mes-Cz-Mz: MALDI-TOF (m/z): [M]^+^ calcd. for C_43_H_28_Cl_6_N_3_, 798.04; found, 798.12. Elem.Anal.: C, 64.61; H, 3.53; Cl, 26.61; N, 5.26; IR (KBr) 2919 (w), 2848 (s), 1573 (m), 1490 (w), 1452 (w), 1373 (w), 1332 (m), 1303(s), 1224 (s), 1187 (m), 1149 (w), 1085 (w), 1054 (w), 997 (w), 921 (w), 850 (w), 802 (m), 748 (m), 719 (m), 653 (w), 586 (w), 518 (w).

TTM-Mes-Dpa-Mz: MALDI-TOF (m/z): [M]^+^ calcd. for C_43_H_30_Cl_6_N_3_, 802.05; found, 802.13. Elem.Anal.: C, 64.44; H, 3.77; Cl, 26.54; N, 5.24; IR (KBr) 2919 (w), 2848 (s), 1689 (m), 1589 (w), 1533 (w), 1490 (w), 1450 (m), 1373(s), 1328 (s), 1288 (m), 1180 (w), 1078 (w), 985 (w), 856 (w), 827 (w), 796 (w), 754 (m), 696 (m), 642 (m), 588 (w), 511 (w).

4. Conclusions

In summary, we directly introduced two distinct simple substituents (imidazole and mesitylene) into the TTM radical units of TTM-Cz and TTM-Dpa. This modification effectively enhanced the luminescence efficiency and photostability of the TTM derivatives TTM-Mes-Cz-Mz and TTM-Mes-Dpa-Mz. We further covalently incorporated these small-molecule radical derivatives into polymer chains and successfully synthesized two novel ionic radical polymers LT-Cz and LT-Dpa. These polymers not only achieved highly efficient solid-state luminescence (39% for LT-Cz and 29% for LT-Dpa) but also exhibited reversible luminescent responses to DCM and THF. Upon droplet addition of DCM or THF, the polymers exhibited significant redshift in emission accompanied by a pronounced decrease in emission intensity. After solvent evaporation, the polymers recovered to their initial state. This work demonstrates that simple substitution into donor–acceptor radicals could construct small-molecule radicals and radical polymers with highly efficient luminescence.