Spirofluorene-Bridged Through-Space Charge-Transfer Radicals with 1-Phenyl-1H-indole Donor

TL;DR

A new spirofluorene-bridged radical with a unique indole donor shows efficient light emission and stability for potential optoelectronic uses.

Contribution

A novel TSCT radical with a spirofluorene bridge and indole donor is designed and analyzed for optoelectronic performance.

Findings

PID-FR-TTM emits light at 609 nm with a 23.1% quantum yield and 90.1 ns lifetime.

The compound shows high thermal and electrochemical stability with decomposition at ≈340 °C.

Calculations confirm a TSCT-dominated excited state with SOMO–HOMO inversion.

Abstract

Organic luminescent radicals with through-space charge-transfer (TSCT) excited states are attractive for optoelectronic applications, yet donor-dependent structure–property relationships remain underexplored. Here we report a new spirofluorene-bridged TSCT radical, PID-FR-TTM, employing 1-phenyl-1H-indole (PID) as the donor. Single-crystal X-ray diffraction confirms a carbon-centered TTM radical and a less bulky, more planar five-membered N-heterocycle in the donor region. PID-FR-TTM shows TSCT-type absorption and an emission at 609 nm with a photoluminescence quantum yield (PLQY) of 23.1% and a 90.1 ns emission lifetime in cyclohexane. Calculations indicate a TSCT-dominated excited state and a pronounced singly occupied molecular orbital–highest occupied molecular orbital (SOMO–HOMO) inversion. Notably, PID-FR-TTM exhibits markedly improved stability, including high decomposition temperatures (≈340 °C), excellent electrochemical stability, and enhanced photostability. These results provide donor-structure insights for designing high-performance TSCT radical emitters.

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1. Introduction

Organic radical molecules exhibit distinct photophysical behaviors owing to their unpaired electron structure. Consequently, growing research interest has driven the development of diverse radical architectures [1], including heteroatom-centered radicals [2,3,4], diradicals [5,6,7,8], halogen-substituted radicals [9,10,11,12,13], and through-space charge-transfer (TSCT) radicals [14,15]. Corresponding materials have been extensively explored for a range of emerging applications, including organic light-emitting diode (OLED) [16,17,18,19], magnetoluminescence [20,21,22,23], circularly polarized luminescence [24,25,26], fluorescence imaging [27,28], and X-ray imaging [29,30,31]. Establishing clear structure–property relationships for radical systems remains a central challenge in luminescent radical research.

In our previous study, to elucidate the structure–property relationships of luminescent radicals with the TSCT excited state, we adopted a spirofluorene motif as a bridging unit and constructed two TSCT radicals, CZP-FR-TTM and TPA-FR-TTM (Figure 1a) [14]. Comprehensive structural and photophysical characterization established that the conformationally confined architecture, together with an increased donor–acceptor spatial separation, is markedly beneficial for suppressing nonradiative deactivation. Nevertheless, the donor motif in TSCT radicals has been less explored to date, and the influence of donor structural variations on radical performance remains insufficiently understood.

In this work, we designed and synthesized new TSCT radical molecules based on tris(2,4,6-trichlorophenyl)methyl radical (TTM), PID-FR-TTM, featuring a spirofluorene bridging unit and employing 1-phenyl-1H-indole (PID) as the donor (Figure 1a). The PID moiety forms a five-membered N-heterocycle within the spirocyclic framework, replacing the original six-membered N-fused ring. Such an N-fused five-membered ring is expected to reduce ring twisting and thereby relieve torsional strain within the spirocyclic framework, which may contribute to enhanced molecular stability. Through systematic structural and photophysical characterization and analysis, PID-FR-TTM exhibits a shorter emission wavelength, a respectable luminescence efficiency, and markedly improved thermal stability compared with previously reported TSCT radicals.

2. Results and Discussion

2.1. Synthesis and Structural Characterization

FRO-HTTM and PID were synthesized from commercially available reagents according to previously reported procedures [14,32]. The target radical PID-FR-TTM was prepared following the synthetic route (Figure 1b). Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, elemental analysis, infrared spectroscopy (IR), ^1^H NMR and ^13^C NMR spectroscopy were used to characterize the radical precursors and target radicals (Figures S1–S10). Electron paramagnetic resonance (EPR) spectroscopy measurements confirmed the presence of an unpaired electron at room temperature in dichloromethane solution (10^−3^ M) and powder state, affording a g-value of 2.0048 (Figure 2a) and 2.0049 (Figure S10).

To further elucidate the molecular structure, single crystals of PID-FR-TTM were successfully grown at room temperature by slow diffusion and solvent evaporation, using p-xylene as the good solvent and ethanol as the poor solvent. Single-crystal X-ray diffraction (SCXRD) confirmed the molecular structure of PID-FR-TTM (Figure 2b). The SCXRD results revealed that the carbon atom in the TTM moiety exhibits sp^2^ hybridization, providing direct structural evidence for the presence of a carbon-centered radical. Meanwhile, the donor unit PID in PID-FR-TTM exhibits a higher degree of planarity than that in CZP-FR-TTM (Figure 2c). The PID donor is also appreciably less bulky, further relieving steric hindrance on the TTM-radical side. Such geometric and steric modifications are expected to reduce the conformational energy, thereby contributing to enhanced stability.

2.2. Photophysical Properties

The UV–Vis absorption spectrum of PID-FR-TTM measured in cyclohexane solution shows well-defined absorption bands across the investigated wavelength range (Figure 3a). An intense absorption band peaking at approximately 375 nm is attributed to characteristic electronic transitions of the TTM radical, with a molar extinction coefficient (ε) of 2.03 × 10^4^ M^−1^ cm^−1^ (Figure S11). In addition, a weak absorption band analogous to that of CZP-FR-TTM is observed in the 500–600 nm region and is associated with through-space charge-transfer behavior. In the photoluminescence spectrum, PID-FR-TTM exhibits an emission maximum at 609 nm. Compared with CZP-FR-TTM, this blue-shifted emission is consistent with the weaker electron-donating character of the PID moiety, which increases the TSCT excited-state energy. Meanwhile, the full width at half maximum (FWHM) of PID-FR-TTM is 82.7 nm, comparable to that of CZP-FR-TTM (80.0 nm). Solvent-dependent photophysical measurements (Figure S12) show that the fluorescence spectrum of PID-FR-TTM exhibits a pronounced red shift with increasing solvent polarity, while the absorption spectrum remains essentially unchanged. This contrasting behavior is characteristic of an excited-state charge-transfer process and supports the presence of a TSCT state in PID-FR-TTM.

In cyclohexane solution, PID-FR-TTM exhibits a photoluminescence quantum yield (PLQY) of 23.1% and an emission lifetime of 90.1 ns (Figure 3b). Compared with bare TTM, the introduction of a through-space donor leads to a pronounced enhancement in PLQY [33], which is consistent with previous reports on TSCT radical systems [14]. However, the PLQY of PID-FR-TTM is noticeably lower than that of CZP-FR-TTM (Table S1). The relatively long emission lifetime further reflects the characteristic long-lived excited-state behavior of TSCT-emissive radicals and is attributed to suppressed nonradiative deactivation pathways and an intrinsically less allowed through-space charge-transfer transition, which together slow the excited-state decay. Based on Equations (1) and (2), the radiative decay rate constant (k_r_) of PID-FR-TTM in cyclohexane solution is determined to be 2.56 × 10^6^ s^−1^. Despite being less sterically bulky than the phenylcarbazole donor, the five-membered PID unit enables PID-FR-TTM to maintain a radiative decay rate comparable to that of CZP-FR-TTM (Table S1). In contrast, the reduced steric hindrance of the PID moiety alleviates conformational constraint on the TTM unit, thereby increasing the freedom of intramolecular motion. As a result, compared with CZP-FR-TTM, the TTM moiety undergoes more efficient vibrational relaxation in the excited state, leading to an increased non-radiative decay rate constant (k_nr_) of 8.53 × 10^6^ s^−1^, which is also further supported by the following theoretical calculations. In a 1 wt% PMMA-doped film, PID-FR-TTM exhibits a markedly reduced PLQY of 4.9%, accompanied by a slightly prolonged emission lifetime of 102.1 ns (Figure S14). The corresponding rate analysis indicates a pronounced decrease in the radiative decay rate from 8.53 × 10^6^ s^−1^ to 4.82 × 10^5^ s^−1^, suggesting that the rigid PMMA host is less favorable for the TSCT process in this system. Similarly, we found that the PLQY decreases markedly with increasing solvent polarity (Table S2). Given that TSCT emission is highly sensitive to the through-space donor-radical conformation, we propose that both the moderately polar microenvironment provided by the PMMA film and more polar solvent environments can shift the conformational distribution, thereby further weakening the electronic coupling and suppressing the radiative transition, ultimately leading to a pronounced reduction in PLQY.

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2.3. Theoretical Calculations

Density functional theory (DFT) calculations at the UB3LYP/6-31G(d,p) level were performed to investigate the electronic structure of PID-FR-TTM. The calculated spin density distribution reveals predominant localization of the unpaired electron on the TTM moiety. The nearly orthogonal dihedral angle of approximately 90° between the fluorene unit and the PID moiety effectively suppresses π-conjugation, resulting in negligible spin density on the PID segment (Figure 4a).

Frontier molecular orbital (FMO) analysis (Figure 4b) shows that the lowest unoccupied molecular orbital (LUMO) is primarily distributed over the TTM and fluorene units, whereas the β-221 single-electron unoccupied molecular orbital (SUMO) is almost entirely confined to the TTM fragment. In contrast, both the α- and β-highest occupied molecular orbitals (HOMOs) are predominantly localized on the PID moiety. Meanwhile, the α-220 orbital corresponds to the singly occupied molecular orbital (SOMO), whose energy lies significantly lower than that of the α-221 (HOMO). This indicates that a pronounced SOMO-HOMO inversion (SHI) is also realized in the PID-FR-TTM molecule.

To gain deeper insight into the excited-state transition processes of the TSCT radical, time-dependent density functional theory (TD-DFT) calculations were performed on PID-FR-TTM. The results indicate that the transition from the ground state to the first excited state is primarily dominated by an electron transition from β-220 to β-221. Natural transition orbital (NTO) analysis reveals that this excitation involves electron transfer from the PID donor to the TTM moiety (Figure 4c). Electron–hole analysis further demonstrates that the excited-state transition predominantly occurs between the donor and radical fragments, with negligible contribution from the fluorene unit (Figure 4d). The overlap integral between the β-220 and β-221 orbitals is 0.07, which is significantly lower than the value of 1 corresponding to complete orbital overlap, indicating pronounced charge separation within PID-FR-TTM. These results clearly demonstrate that PID-FR-TTM exhibits TSCT characteristics.

To quantitatively evaluate the charge-transfer contribution, the interfragment charge-transfer (IFCT) analysis was performed by partitioning PID-FR-TTM into donor, fluorene, and radical fragments at IOp(9/40 = 4) precision. The results show that charge transfer from the donor to the TTM moiety accounts for 96.07% of the excitation, whereas the contribution from fluorene to TTM is only 1.70% (Table S3). Consequently, the excited state is overwhelmingly charge-transfer in nature, with a total CT contribution of 98.94% and only 1.06% local-excitation character, unambiguously confirming the dominant TSCT character of the excited-state process (Figure S15).

To gain deeper insight into the donor–radical interactions in TSCT radicals, weak interaction visualization analyses were performed on both PID-FR-TTM and CZP-FR-TTM, yielding the interaction maps shown in Figure S16. Pronounced weak interactions are observed between the donor and radical moieties in both systems. To further clarify the influence of these interactions on molecular conformation, comparative analyses of the optimized ground- and excited-state geometries were conducted. In CZP-FR-TTM, root-mean-square deviation (RMSD) analysis reveals a minimal structural deviation of 0.1796 Å between the optimized ground and excited states, indicating highly restricted intramolecular motion, which effectively suppresses vibrational relaxation. (Figure S17). However, this conformationally confined architecture also results in a higher conformational energy for CZP-FR-TTM. In contrast, the PID donor in PID-FR-TTM adopts a less bulky five-membered-ring configuration. While preserving favorable weak donor–radical interactions, this structural change partially relieves the restriction of intramolecular motion within both the PID unit and the TTM motif, leading to a more pronounced structural deviation (0.3703 Å). Although increased intramolecular motion can facilitate nonradiative deactivation, it can also reduce the overall conformational energy, which is conducive to improving the stability of the radical molecule.

2.4. Electrochemical Properties

Cyclic voltammetry (CV) measurements of PID-FR-TTM were performed using 0.1 M (Bu_4_N)PF_6_ in dichloromethane as the supporting electrolyte and the Fc^+^/Fc redox couple as the internal reference (Figure S18). Within the scanned potential window, PID-FR-TTM displayed two oxidation waves at 0.76 V and 0.98 V, together with a reduction wave at −1.18 V. The oxidation peak at 0.98 V and the reduction peak at −1.18 V can be assigned to the redox processes of the TTM radical, whereas the oxidation peak at 0.76 V is likely attributable to oxidation of the PID donor moiety. The SOMO and SUMO energy levels estimated from the CV data are in good agreement with the DFT-calculated values (Table S4). Accordingly, we further examined the oxidation behavior of the radical precursor, PID-FR-HTTM, and found an oxidation peak at approximately 0.88 V. These CV results show a redox pattern similar to that of CZP-FR-TTM and also provide experimental evidence for the distinctive SHI-type electronic structure in PID-FR-TTM [14].

2.5. Stability

The thermal stability of PID-FR-TTM was evaluated by thermogravimetric analysis (TGA) under both nitrogen and air atmospheres (Figure 5a). The temperature corresponding to 5% weight loss (T_d_) was determined to be 336.4 °C in air and 340 °C under nitrogen. In comparison, CZP-FR-TTM exhibits significantly lower decomposition temperatures of 274.6 °C under nitrogen and 256.6 °C in air, indicating that PID-FR-TTM possesses markedly superior thermal stability. This is consistent with our earlier analysis of the structural features and the increased intramolecular motion in this molecule. Furthermore, electrochemical stability was evaluated by repeated CV measurements. After 20 consecutive scans, the redox curves of PID-FR-TTM remained essentially unchanged, confirming excellent electrochemical stability (Figure 5b). The photostability of PID-FR-TTM was characterized by monitoring the temporal evolution of fluorescence intensity under continuous photoexcitation. The introduction of spatial interactions led to a significantly slower fluorescence decay, yielding a half-life of 1.82 × 10^4^ s (Figure S19), approximately one order of magnitude shorter than that of CzP-FR-TTM (Table S1). Overall, these results consistently point to the favorable stability of PID-FR-TTM.

3. Materials and Methods

All starting materials and reagents employed in this work were purchased from ERNEGI and Xilong Science Co., Ltd. (Shanghai, China) and were used directly without further purification. Infrared measurements were performed on a Bruker Tensor 27 spectrometer (Bruker, Berlin, Germany). For IR analysis, the radical sample and KBr were homogeneously blended at a weight ratio of 1:100, finely ground, and subsequently compressed into thin pellets. ^1^H NMR and ^13^C NMR spectra were recorded at ambient temperature using a Bruker AVANCE NEO 400 instrument (Bruker, Fällanden, Switzerland), with deuterated dichloromethane serving as the solvent. The elemental analysis data were recorded on an Elementar Vario micro cube spectrometer (Elementar Analysensysteme GmbH, Langenselbold, Germany). Mass spectrometric characterization was mainly carried out by MALDI–TOF, employing DCTB as the matrix. Electron paramagnetic resonance spectra were collected on a Bruker A320 spectrometer (Bruker, Germany), with measurements conducted at room temperature for both solid samples and dichloromethane solutions (0.001 M) (sweep width: 100 G, frequency: 9.852902 GHz). SCXRD data were obtained using a Bruker D8 Quest diffractometer (Bruker, Germany) at 100 K. Structural refinement and visualization were performed with the Olex2 [34] and Mercury programs [35]. UV–visible absorption spectra were measured on a Shimadzu UV-1900i spectrophotometer (Shimadzu, Kyoto, Japan) (excitation slit width: 3.0 nm; emission slit width: 3.0 nm; scan speed: 600 nm/min), while photoluminescence (PL) spectra were acquired using a Shimadzu RF-6000 fluorescence spectrometer (Shimadzu, Japan) (excitation wavelength: 375 nm). Time-resolved PL decay curves were recorded on an Edinburgh FLS1000 system (Edinburgh Instruments, Livingston, UK) (excitation wavelength: 375 nm), which was also used to determine absolute PLQY via an integrating sphere. DFT and TD-DFT calculations were conducted with the Gaussian 16 (C.02) [36] and GaussView 6.0 [37] packages. Analyses of electron–hole distributions [38], IFCT, and weak intermolecular interactions [39] were performed using the Multiwfn [40,41] and VMD [42] software suites. Thermogravimetric analyses were 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. CV experiments were performed using a CH Instruments CHI660E electrochemical workstation (CH Instruments, Shanghai, China) (scanning rate: 100 mV s^−1^; supported electrolyte: 0.1 M (Bu_4_N)PF_6_ in dichloromethane; working electrode: glassy carbon; auxiliary electrode: platinum; reference electrode: Ag/AgCl; ferrocene added as internal standard during measurements; measurements conducted in air at room temperature). Photostability evaluations were conducted under continuous xenon lamp illumination, with emission changes monitored using the Shimadzu RF-6000 spectrometer (Shimadzu, Japan) (excitation wavelength: 375 nm; incident light power: 150 W).

3.1. Synthesis of PID

Indole (1.932 g, 16.5 mmol), copper(I) iodide (0.285 g, 1.5 mmol), potassium phosphate (6.369 g, 30 mmol), and benzotriazole (0.357 g, 3 mmol) were added to a reaction flask. The flask atmosphere was replaced with nitrogen, followed by the addition of o-iodobromobenzene (2.1 mL) and dimethyl sulfoxide (DMSO, 60 mL). The reaction mixture was heated to 120 °C and stirred at 800 rpm for 30 h. After cooling to room temperature, the reaction was quenched with excess ultrapure water. The resulting precipitate was collected by filtration through a Büchner funnel. The upper portion of the filter cake was dissolved in ethyl acetate, and residual DMSO in the organic phase was thoroughly removed by washing with saturated brine. The crude product was obtained by vacuum evaporation and purified by column chromatography on silica gel (petroleum ether/dichloromethane = 4:1, v/v) to afford PID as a colorless liquid (3.0 g, 67% yield).

PID: MALDI-TOF (m/z): [M]^+^ calcd. for C_14_H_10_BrN, 271.000; found, 270.893. ^1^H NMR (400 MHz, Methylene Chloride-d2) δ 7.81 (d, J = 8.0 Hz, 1H), 7.70 (d, J = 6.6 Hz, 1H), 7.49 (d, J = 7.5 Hz, 2H), 7.40 (t, J = 9.1 Hz, 1H), 7.28 (d, J = 3.8 Hz, 1H), 7.19 (d, J = 6.0 Hz, 2H), 7.10 (d, J = 7.2 Hz, 1H), 6.72 (s, 1H). ^13^C NMR (101 MHz, Methylene Chloride-d2) δ 138.45, 136.84, 133.92, 129.88, 129.70, 128.83, 128.52, 128.48, 122.25, 121.93, 120.87, 120.26, 110.56, 103.00.

3.2. Synthesis of PID-FROH-HTTM

PID (0.63 g, 2.3 mmol) was added to a 50 mL reaction flask, sealed, and cooled to −78.5 °C. The flask was purged with nitrogen, after which anhydrous tetrahydrofuran (THF, 20 mL), pre-cooled to −78.5 °C, was introduced. n-Butyllithium solution (0.8 mL, 2.5 M in hexane) was added dropwise via a stainless-steel needle, and the mixture was stirred for 1 h. FRO-HTTM (1.07 g, 1.5 mmol) was then added, and the reaction was maintained for an additional 1 h. The mixture was allowed to warm to room temperature and stirred for 24 h. The reaction was quenched with methanol (10 mL), and the solvent was removed under reduced pressure. Purification by silica gel column chromatography (petroleum ether/dichloromethane = 1:1, v/v) afforded PID-FROH-HTTM (0.69 g, 34% yield).

PID-FROH-HTTM: MALDI-TOF (m/z): [M]^+^ calcd. for C_46_H_25_Cl_8_NO, 890.939; found, 890.651. As PID-FROH-HTTM is essentially insoluble in any organic solvent, it was not characterized by nuclear magnetic resonance spectroscopy.

3.3. Synthesis of PID-FR-HTTM

PID-FR(OH)-HTTM (0.69 g, 0.77 mmol) was placed in the reaction flask, and the atmosphere was replaced with nitrogen. Formic acid (30 mL) and hydrochloric acid (10 mL) were added, and the mixture was heated at 120 °C for 24 h. After cooling, the reaction was quenched with ultrapure water, and the pH was adjusted to basic using saturated sodium carbonate solution. The product was extracted with dichloromethane, and the organic phase was concentrated under reduced pressure. The crude product was purified by silica gel column chromatography (petroleum ether/dichloromethane = 1:1, v/v) to afford PID-FR-HTTM (0.16 g, 24% yield).

PID-FR-HTTM: MALDI-TOF (m/z): [M]^+^ calcd. for C_46_H_23_Cl_8_N, 872.928; found, 872.692. ^1^H NMR (400 MHz, Methylene Chloride-d2) δ 7.95 (ddd, J = 7.7, 4.8, 1.1 Hz, 1H), 7.90 (dd, J = 7.6, 3.6 Hz, 1H), 7.70 (dd, J = 23.4, 8.3 Hz, 1H), 7.58–7.37 (m, 5H), 7.34–7.26 (m, 2H), 7.24–7.11 (m, 4H), 7.11–6.96 (m, 2H), 6.87 (t, J = 7.6 Hz, 1H), 6.78 (dd, J = 17.9, 7.7 Hz, 1H), 6.62 (dd, J = 22.4, 7.3 Hz, 1H), 6.52–6.45 (m, 1H), 6.44–6.39 (m, 1H), 6.26 (d, J = 1.9 Hz, 1H), 6.09 (d, J = 18.9 Hz, 1H). ^13^C NMR (101 MHz, Methylene Chloride-d2) δ 149.89, 149.56, 146.31, 146.11, 145.33, 142.63, 142.29, 141.30, 141.03, 141.00, 140.91, 140.07, 139.71, 139.20, 139.10, 138.17, 138.03, 137.95, 137.77, 137.49, 137.34, 137.05, 136.93, 136.13, 136.05, 135.32, 135.28, 134.34, 134.31, 134.24, 134.20, 133.44, 133.42, 133.38, 133.34, 132.77, 132.58, 131.08, 130.95, 130.34, 130.25, 130.19, 129.87, 129.86, 129.83, 129.80, 128.88, 128.84, 128.67, 128.48, 128.43, 128.29, 128.20, 128.17, 128.14, 124.02, 123.86, 123.81, 123.00, 122.97, 122.20, 121.40, 121.27, 120.69, 120.60, 120.22, 120.19, 120.13, 111.15, 111.11, 111.09, 110.85, 96.18, 95.97, 58.78, 58.65, 49.80, 49.74.

3.4. Synthesis of PID-FR-TTM

PID-FR-HTTM (0.16 g, 0.18 mmol) was added to a reaction flask, and the atmosphere was replaced with nitrogen. Under light-protected conditions, anhydrous THF (10 mL) and tetrabutylammonium hydroxide (50 wt% in water, 0.29 g, 0.5588 mmol) were added. After stirring for 5 h, tetrachlorobenzoquinone (0.1432 g, 0.5458 mmol) was introduced, and the reaction was continued for 1.5 h. The solvent was removed under reduced pressure to afford the crude product, which was purified by silica gel column chromatography (petroleum ether/dichloromethane = 4:1, v/v) to yield PID-FR-TTM as a red solid (0.0724 g, 46% yield).

PID-FR-TTM: MALDI-TOF (m/z): [M]^+^ calcd. for C_46_H_22_C_l8_N· 871.920; found, 871.700. Elem Anal. calcd. for C_46_H_22_Cl_8_N·(%): C, 63.34; H, 2.54; N, 1.61. Found (%) C, 63.46; H, 2.43; N, 1.58. IR (KBr) 2923(m), 2854(m), 1720(m), 1602(s), 1555(s), 1493(s), 1450(s), 1412(m), 1369(s), 1327(s), 1271(m), 1136(s), 1083(m), 1059(w), 858(s), 798(s), 746(s), 682(s).

4. Conclusions

A new spirofluorene-bridged TSCT radical, PID-FR-TTM, was designed and synthesized by introducing PID as the donor. Structural characterization verifies a carbon-centered TTM radical and shows that the PID donor adopts a less bulky, more planar five-membered N-heterocycle, relieving steric hindrance around the TTM unit and lowering the conformational energy. Photophysical measurements in cyclohexane confirm TSCT emission, a PLQY of 23.1%, and a long emission lifetime of 90.1 ns, while maintaining a radiative decay rate comparable to CZP-FR-TTM. The reduced steric confinement also increases intramolecular motion, leading to a higher nonradiative rate. Nevertheless, PID-FR-TTM achieves substantially improved thermal, electrochemical, and photostability. Theoretical calculations corroborate a TSCT-dominated excited state with pronounced charge separation and identify a clear SHI feature. Overall, this work expands donor-dependent structure–property understanding in TSCT radicals and provides actionable insights for improving stability in next-generation TSCT radical emitters.