A nonconjugated radical polymer with stable red luminescence in solid state

1. Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction and Institute for Advanced Study, and Department of Chemical and Biological Engineering, and Department of Physics, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China. 2. Center for Computational Molecular Science and Technology, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA. 3. State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China. 4. Department of Chemistry, City University of Hong Kong, Hong Kong, China. 5. HKUST-Shenzhen Research Institute, No. 9 Yuexing 1st RD, Nanshan District, Shenzhen 518057, China. 6. Center for Aggregation-Induced Emission, State Key Laboratory of Luminescent Materials and Devices, SCUT-HKUST Joint Research Institute, South China University of Technology, Tianhe District, Guangzhou 510640, China. 7. AIE Institute, Guangzhou Development District, Huangpu, Guangzhou 510530, China 8. These authors contributed equally to this work: Zhaoyu Wang, Xinhui Zou, Yi Xie *email: ruquanye@cityu.edu.hk; tangbenz@ust.hk


Introduction
Synthetic organic chromophores are commonly featured with extended π-conjugation and closed-shell structure, but their synthesis could be tedious and challenging 9 . Most of the radicals with open-shell structure are not stable, and diligent efforts have been made to stabilize the unpaired electron by delicate structure design 10,11 . Yet they typically relax via a nonradiative decay pathway upon excitation, thereby showing non-luminescent property 5 . Since the first reported case in 2006 12 , luminescent radicals have been widely investigated from excited-states dynamics/mechanisms 13,14 to applications [15][16][17] . Nevertheless, luminescence from stable radicals remains a sporadic phenomenon and most of the structures are limited to triphenyl methyl radical derivatives and their analogues 7,[18][19][20][21] .
In nature, non-covalent interaction and self-assembly are playing a critical role in photophysical properties. 22,23 Modern photophysics suggests that in addition to the intrinsic energy states of chromophores, their luminescent properties could be affected or even reversed by the surrounding 24,25 . Here, we report that TEMPO, a non-luminescent radical 5 , could be transformed into a red-emissive radical after polymerization. The polymer is free from any conjugation and aromatic rings, yet the existence of narrow highest occupied molecular orbital to singly occupied molecular orbital (HOMO-SOMO) gap of nitroxide radical enables the emission in long wavelength. Experimental and theoretical data underscore the significance of intermolecular non-covalent interactions among TEMPO units. Our results disclose an unusual luminescence phenomenon and advances the development of luminescent radicals.

Results and discussion
The non-conjugated radical polymer, poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PGTEMPO), was synthesized via the ring-opening polymerization of stable radical monomers initiated by potassium tert-butoxide ( Figure 1a). The TEMPO derivative, 4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (GTEMPO), was used as the monomer as it is stable at a wide range of temperatures and easy to crystalize. PGTEMPO is orange and it has a numberaverage molecular weight of 4.9 kg mol -1 and a narrow molecular weight distribution (Đ = 1.32). It is readily soluble in common organic solvents, such as tetrahydrofuran (THF), chloroform, dichloromethane, and dimethylsulfoxide. Electron paramagnetic resonance (EPR) spectroscopy and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy confirm the existence of stable radical and nitroxide function group respectively ( Figure S3 and Figure S4). Besides, the thermal property of PGTEMPO was characterized by thermogravimetric analysis (TGA), presenting a degradation temperature (Td) of ~246 °C ( Figure S5).  To confirmed that the luminescence is associated with the radical on TEMPO, we first designed an experiment to chemically quench the radical site on PGTEMPO with acid (vitamin C, VC) 27 and observed the subsequent luminescence change. After reacting with VC, the nitroxide group (N-O) was reduced into hydroxylamine group (N-OH), generating poly(4-glycidyloxy-2,2,6,6tetramethylpiperidine-1-hydroxyl) (PGTEMPOH) (Figure 2a). The suppression of EPR signal ( Figure 2b) and the emergence of NMR spectrum of PGTEMPOH ( Figure S9) suggested the successful quenching of radicals. The orange color of PGTEMPO solution also faded to colorless after reduction into PGTEMPOH ( Figure 2c). As we expected, the red-emissive peak of PGTEMPO was significantly weakened along with the quenching of radicals (Figure 2d, top). The response to VC is also very sensitive and rapid ( Figure S10). On the other hand, the quenching of radical generates PGTEMPOH, which is a classic clusteroluminogen 28 . Previous study suggests that the clustering effect from the inter/intramolecular hydrogen bond interactions could trigger the emission. 29 As expected, we observed a blue emission from PGTEMPOH under excitation of 360 nm, which is absent from PGTEMPO (Figure 2d, bottom).
The luminescence quenching experiment proves that radical is playing a crucial role in the unusual red-luminescence property of PGTEMPO.
To understand the origin of luminescence from TEMPO units, we studied the dependence of luminescence on polymer packing. A cycle of temperature-dependent PL was performed between -20 and 50 o C, and compared to the differential scanning calorimetry (DSC) result.
The PGTEMPO presents a glass transition temperature (Tg) of 17.40 °C. In general, the luminescence intensity decreases as the temperature increases, which is because of the favourable non-radiative decay at higher temperature 30 . Surprisingly, there is a significant drop of PL intensity between 7 to 17 o C, where the polymer undergoes a glassy transition. It is hypothesized that the glassy transition breaks the rigidity of polymer, which decreases the intensity 31 . Besides, the real-time monitoring of the luminescence of PGTEMPO at 80°C under N2 was performed to probe the dynamic structural evolution of PGTEMPO. As high temperature will boost the non-radiative decay, the maximum PL intensity decreased rapidly within the first 20 min. Serendipitously, the PL intensity rebounded afterwards. Previous Monte Carlo simulations suggested that the annealing will form a continuous percolation network among TEMPO units for charge transport 32 . Therefore, it is plausible that the increasing population of through-space interaction among the TEMPO units stimulated by the annealing process account for the escalating PL intensity. This is also supported by the observation of a red shift from 635 to 647 nm during the real-time annealing (Figure 3c).
To further understand the luminescence mechanism of PGTEMPO, we combined the structural information and calculations. We first investigate the properties of the monomer, GTEMPO.
The X-ray diffraction analysis revealed that the GTEMPO powder is orderly packed ( Figure   S11). The single crystal structure of GTEMPO was obtained as shown in Figure S12. In the side view, the nitroxide groups are sterically hindered by the surrounding methyl groups. The nearest distance between two nitroxide groups is 5.817 Å. If we term the nitroxide site as the head of the molecule, from the top view, one can observe that GTEMPO adopts a head-to-tail model, and the distance between adjacent nitroxide groups is 6.140 Å. Then we use timedependent density functional theory (TDDFT) calculations with a B3LYP functional and def2-TZVP basis set via Q-Chem, to reveal the orbital states. The ground state of TEMPO is a doublet (D0) due to the existence of an unpaired electron. As depicted in Figure S13  In comparison, for PGTEMPO, the backbone of polymer readily breaks the orderly packed conformation, as shown by the powder X-ray diffraction (XRD) results that the fresh PGTEMPO sample lose the fine peaks and became completely amorphous after annealing at 80 °C ( Figure S14).
To understand the energy state of PGTEMPO, we first simulate the structure by aligning the TEMPO dimer to form a parallelogram. 32,33 We further added extra TEMPO units near the parallelogram to form trimer and tetramer clusters to model the effect of multi-unit clusters.
For convenience, we defined the y-axis of the geometry as the line running through the atoms N1 and C4, and the x-axis as the line running through atoms C3 and C5 ( Figure S16a). We used the displacement between the TEMPO unit on x-axis (Δx) and y-axis (Δy) to specify the configuration of such TEMPO dimers ( Figure S16b). Trimers and tetramers are constructed by placing the extra TEMPO unit above and below the nitroxide radical parallelogram plane in the dimer ( Figure S16c). For all dimers in this section, we choose the value of Δx to be 1.5 Å and Δy to be between 5.5 Å and 6.0 Å, so that the two TEMPO units can approach each other to form the parallelogram between nitroxide radicals, while avoiding direct collision between the radicals and methyl groups on C2 and C6. The ground state frontier orbitals of TEMPO dimer, trimer, and tetramer were shown in Figure S17, Figure S18, and Figure S19, respectively.
The excitation energy of cluster at various distance was calculated and plotted as shown in

Conclusions
We have synthesized a nonconjugated radical polymer showing luminescence in the solid state.
To our knowledge, this is the first report that stable radical without any conjugated or aromatic structures can emit light. Luminescence quenching experiment confirms the key role of nitroxide radicals. Combining the structural information and calculation, we propose that the intermolecular interactions of TEMPO clusters account for the long-wavelength emission.
Using the redox characteristic of TEMPO, a biological application of intracellular VC visualization was demonstrated. This work expands the family of luminescent radicals. We envision that further experimental and theoretical researches on this unconventional luminescence phenomenon will provide insights into principles governing radical luminescence and find applications in broad fields.

Synthesis of GTEMPO
The monomer, GTEMPO, was synthesized as reported 36 . Briefly, TEMPO-OH was purified by recrystallization before use. Sodium hydroxide (NaOH) ( as the eluent. The oily product obtained was freeze-dried for 1 day to yield the monomer, GTEMPO, as a red crystalline solid.

Synthesis of PGTEMPO
The polymerization of the monomer was achieved using a procedure optimized from the literature 37 . GTEMPO was further dried under reduced pressure for one day before use and stored in glove box. Inside a glove box, a mixture of GTEMPO (300 mg, 1.31 mmol) and potassium tert-butoxide (12 mg) were added into a 10 mL Schlenk tube with a stirring bar, which was already dried in a hot oven overnight. Then, the tube was sealed with a rubber stopper. The reaction mixture was heated at 80 °C for 2 hours without solvent and then was injected with 2 mL anhydrous THF for another 8 hours. The mixture was vortexed after addition of solvent to make sure the mixture was well dissolved in THF. After cooling down to room temperature, NaCl aqueous solution was added to mixture, followed by extraction with chloroform for three times. The organic solvent was removed under reduced pressure and the crude polymer dissolved with a small volume of THF. The crude polymer solution was passed through a simple column filled with neutral Al2O3 powder and precipitated in hexane. The precipitates were collected by ultracentrifugation (7000 rpm for 3 min). This protocol was repeated for three times to remove excess unreacted monomer. The polymer was dried overnight in a vacuum oven at room temperature to obtain an orange solid. Mn = 4,900; Mw = 6,500; Mw/Mn = 1.32 (GPC, polystyrene calibration).

Data Availability
All experimental data are available in the main text or the supplementary materials.