HKUST SPD - INSTITUTIONAL REPOSITORY

Title A Simple Approach to Achieve Organic Radicals with Unusual Solid-State Emission andPersistent Stability

Authors Zhao, Xueqian; Gong, Junyi; Alam, Parvej; Ma, Chao; Wang, Yanpei; Guo, Jing; Zeng,Zebin; He, Zikai; Sung, Herman; Williams, Ian Duncan; Wong, Kam Sing; Chen, Sijie; Lam,Wing Yip; Zhao, Zheng; Tang, Benzhong

Source CCS Chemistry, v. 3, 24 August 2021, p. 2518-2526

Version Published Version

DOI 10.31635/ccschem.021.202101192

Publisher Chinese Chemical Society Publishing

Copyright This article has been published in CCS Chemistry 2021; A Simple Approach to AchieveOrganic Radicals with Unusual Solid-State Emission and Persistent Stability is availableonline at https://doi.org/10.31635/ccschem.021.202101192

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A Simple Approach to AchieveOrganic Radicals with Unusual Solid-State Emission and Persistent StabilityXueqian Zhao1†, Junyi Gong1†, Parvej Alam1, Chao Ma2, Yanpei Wang3, Jing Guo3, Zebin Zeng3,Zikai He4, Herman H. Y. Sung1, Ian D. Williams1, Kam Sing Wong2, Sijie Chen5, Jacky W. Y. Lam1,Zheng Zhao6* & Ben Zhong Tang1,6,7,8,9*

1Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration

and Reconstruction, Institute for Advanced Study and Guangdong-Hong Kong-Macro Joint Laboratory of Optoelec-

tronic and Magnetic Functional Materials, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,

Hong Kong 999077, 2Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay,

Kowloon, Hong Kong 999077, 3State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry

and Chemical Engineering, Hunan University, Changsha, Hunan Province 410082, 4School of Science, Harbin Institute of

Technology, Shenzhen, HIT Campus of University Town, Shenzhen, Guangdong Province 518055, 5Ming Wai Lau Centre

for Reparative Medicine, Karolinska Institutet, Hong Kong 999077, 6School of Science and Engineering, Chinese

University of Hong Kong, Shenzhen, Guangdong Province 518172, 7HKUST-Shenzhen Research Institute, South Area

Hi-Tech Park, Nanshan, Shenzhen, Guangdong Province 518057, 8Center for Aggregation-Induced Emission, SCUT-

HKUST Joint Research Institute, State Key Laboratory of Luminescent Materials and Devices, South China University of

Technology, Guangzhou, Guangdong Province 510640, 9AIE Institute, Guangzhou Development District, Huangpu,

Guangzhou, Guangdong Province 510530

*Corresponding authors: zhaozheng@cuhk.edu.cn; tangbenz@ust.hk; tangbenz@cuhk.edu.cn; †X. Zhao and J. Gong

contributed equally to this work.

Cite this: CCS Chem. 2021, 3, 2518–2526

Stable organic radicals are promising materials for

information storage, molecular magnetism, electronic

devices, and biological probes. Many organic radicals

have been prepared, but most are non- or weakly

emissive and degrade easily upon photoexcitation. It

remains challenging to produce stable and efficient

luminescent radicals because of the absence of gen-

eral guidelines for their synthesis. Herein, we present

a photoactivation approach to generate a stable lu-

minescent radical from tris(4-chlorophenyl)phosph

ine (TCPP) with red emission in the crystal state.

The mechanistic study suggests that the molecular

symmetry breaking in the crystal causes changes of

molecular conformation, redox properties, andmolec-

ular packing that facilitates radical generation and

stabilization. This design strategy demonstrates a

straightforward approach to develop stable organic

luminescent radicals that will open new doors to pho-

toinduced luminescent radical materials.

Keywords: luminescent radicals, symmetry breaking,

tris(4-chlorophenyl)phosphine, anticounterfeiting,

photoinduced electron transfer

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Received: June 1, 2021 | Accepted: July 26, 2021

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Introduction

Stable luminescent radicals or doublet emitters are next-

generation luminophores in which the emission originates

from the radiative decay of the unusual lowest doublet

excited state (D1) to the doublet ground state (D0).1–3 Their

potential applications in, for example, organic light-emit-

ting diodes (OLEDs),4 light sources,5 and chemical sensors

are explored.6–8 Recently, it has been demonstrated that

radical emitters can easily achieve the upper limit (100%)

internal quantum efficiency of OLEDs due to their unusual

doublet nature.9,10 Thus, the potential of luminescent radi-

cals for practical applications have attracted considerable

attention in the last decade and have brought this

research field back to life. Thanks to the extensive efforts

of scientists, several luminescent radicals, such as tris-

2,4,6-trichlorophenylmethyl,11 perchlorotriphenylmethyl,12

and biphenylmethyl,7,13–15 have been successfully devel-

oped as luminous cores by exploiting their steric and

delocalization effects. In addition, strategies such as

chemical modification,16–18 physical doping,19 and even su-

pramolecular assembly20–23 have been utilized to improve

their stability and luminescence quantum yield (QY). In

spite of this gratifying progress, stable radical species

showing strong light emission are still relatively rare and

mainly limited to triarylmethyl radicals.8,24–27 On the other

hand, similar to many conventional luminophores, lumi-

nescent radicals exhibit low luminescence quantum effi-

ciency at high solution concentrations or in the solid state

due to the aggregation-caused quenching effect,28 and

such a problem must be solved because thin films or

aggregates of luminescent materials are widely utilized

in practical applications. However, the research on lumi-

nescent radicals is still in the preliminary stage. Besides,

most of the stable luminescent radicals are generated by

time-consuming chemical synthesis. As a result, the de-

velopment of new methods to generate luminescent radi-

cals is challenging but also exciting andmeaningful. In this

work, we demonstrate that the photoactivation of tris

(4-chlorophenyl)phosphine (TCPP) crystals can generate

stable radicals with unusual red emission at ambient con-

ditions. Figure 1a shows the schematic diagram of the

photoinduced radical generation process. X-ray crystallo-

graphic analysis combined with density functional theory

(DFT) calculations reveal that the symmetry breaking in

TCPP crystals upon UV irradiation results in charge sepa-

ration, which serves as a driving force for the generation of

stable solid-state emissive radicals. The TCPP radicals

exhibit persistent stability and show an emission half-life

of over oneweek evenwhen stored at ambient conditions.

Interestingly, the red radical emission is quenched only by

destroying the crystal packing or irradiating it with strong

visible light, suggesting that crystal packing acts as a

protective cage to stabilize the formed radicals. After

recrystallization, the ground sample can regenerate stable

radicals again by UV exposure. Taking advantage of the

reversible feature of radical generation, a photocontrol-

lable anticounterfeiting film was demonstrated. Although

there have been some achievements in generating stable

luminescent radicals by wet chemistry reactions, new

strategies for the design and synthesis of stable and effi-

cient radicals in the absence of any external additives are

also highly desirable. Thus, the current work may open up

a new avenue of achieving stable and photoresponsive

solid-state luminescent radicals.

Results and Discussion

TCPP sample was purchased from a commercial source

andpurified bymultiple column chromatography followed

by recrystallization three times in dichloromethane and

hexane mixture before use. The structure of TCPP was

confirmed by NMR spectroscopies and high-resolution

mass spectrometry (HRMS) as well as single-crystal X-ray

crystallography (Supporting Information Figures S1–S4).

After recrystallization, the final products appeared as col-

orless both in solution and the solid state and were almost

nonemissive (Supporting Information Figure S5). The

UV–vis spectrum of TCPP crystals exhibited an absorption

band centered at around 250 nm, which was consistent

with its colorless appearance (Figure 1b). However, upon

365-nm UV irradiation for a few seconds, the colorless

crystals gradually turned orange. Two new absorption

peaks that appeared at 496 and 520 nm were suggestive

of the generation of new species with narrow band gaps.

As most of the triphenylphosphine derivatives are elec-

tron-rich, they lose electrons readily during the oxidation

process to generate several oxidized products such as

radicals, oxides, or homocoupling products. These oxi-

dized products often show redder absorption than their

neutral counterparts.29–31 To verify the origin of the new

photogenerated species, high-performance liquid chro-

matography (HPLC) and 31P NMR analysis were carried

out. Results show that the signals at a retention time of

2.3 min and a chemical shift of 27.2 are in good agreement

with those of TCPP oxide (Supporting Information Figure

S6). However, amixture of TCPP oxide and pure TCPPwas

found to be colorless rather than orange. Thus, it is spec-

ulated that the orange color originates from TCPP radicals

because TCPP oxide can be generated from the radicals.

To verify this hypothesis, electron paramagnetic reso-

nance (EPR) spectroscopy was carried out which indicat-

ed that the i-TCPP (irradiated TCPP) crystals exhibit a

strong radical signal with g value of 2.0035. On the other

hand, the pristine TCPP crystals were radical silent

(Figure 1c). We speculated that the crystal structure can

only protect the radicals generated inside the core but not

on the surface. Hence, only a trace amount of TCPP oxide

will be formed by the reaction of TCPP radicals with

ambient oxygen. Therefore, these results indicate that the

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photoirradiation of TCPP crystals can generate radicals

inside the crystal core. The three-dimensional image re-

construction of crystalline i-TCPP by confocal microscopy

confirmed the existence of luminescent radicals inside the

crystal core (Supporting Information Figure S7). Interest-

ingly, i-TCPP showed bright red emission at 620 nmwith a

QY of 4.6%, which was in great contrast with the none-

missive characteristic (QY < 0.1%) of TCPP in toluene

solution or in the crystal state (Figure 1d). This typical

behavior of i-TCPP is similar to that of luminogens with

aggregation-induced emission,32–34 which exhibit strong

emission in the aggregate or solid state but weak or no

emission in solution (Supporting Information Figure S8).

The photoluminescence (PL) emission lifetime of i-TCPP

crystals was measured to be 3.9 ns, suggestive of the

fluorescent nature of the light emission (Figure 1e). To

elucidate the origin of the red emission, a further detailed

investigation of the photophysical property was con-

ducted. Because impurity even in trace amountsmay exert

great influence on the luminescent property,35,36 the purity

of TCPP was confirmed by HPLC and elemental analysis

(EA). The difference between the obtained EA value and

the theoretical value is <0.3% which suggests the high

purity of the present molecule (Supporting Information

Figure S6a). Different batches of TCPP samples were also

purchased from different commercial sources and purified

through the same procedure. Despite the fact that they

exhibit varied luminescence properties, all the purified

samples showed identical optical properties with no or

weak emission, demonstrative of excellent data reproduc-

ibility and reliability (Supporting Information Figure S9). It

is now clear that pure TCPP crystals generate red-emitting

stable radicals readily under UV photoirradiation. As both

radicals and TCPP oxide are formed after photoirradiation

and the TCPP oxide is nonemissive (QY < 0.1%), the red

emission must be due to the radical formation. Notably,

the red emission of i-TCPP crystals along with its EPR

signal disappears upon grinding (Supporting Information

Figure S10). This observation supports the significance of

the crystalline state where the crystal lattice acts as a

protective cage to protect the TCPP radicals from water

and atmospheric oxygen.

To understand the optical properties and the genera-

tion mechanism of TCPP radicals, we analyzed TCPP’s

single-crystal structure and molecular stacking. Single-

crystal structure analysis (Figure 2a and Supporting

Figure 1 | (a) Schematic diagram of the photoinduced radical generation process. (b) UV–vis spectra, (c) EPR spectra,

and (d) PL spectra of TCPP crystals before and after UV light exposure for seconds at room temperature, λex = 365 nm.

The insets in (b) and (d) show the photographs of color and emission changes, QY = quantum yield. (e) Time-resolved

PL decay curves of i-TCPP measured at λmax = 620 nm at room temperature, λex = 400 nm.

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Information Table S1) indicates that TCPP adopts a D2h

symmetry rather than the commonly observed C3 sym-

metry for most of its analogs like triphenylphosphine

(Supporting Information Table S2). The dihedral angles

of the phenyl rings are not all identical. In particular, the

value (87.6°) of one pair (R2–R2′) was much larger than

the others (66.4° and 50.8°). Another structural charac-

teristic is that one phenyl ring (R1) is almost parallel with

the lone pair of the phosphorus atom. However, the

optimized molecular structure of TCPP in the gas phase

exhibited a C3 conformation, and the dihedral angles of

the phenyl rings were all equal. This indicates that the

symmetry breaking leads to the D2h conformation of

the TCPP molecule in the crystal state, which was first

noticed and described as an “abnormal” conformation

by Bin Shawkataly et al. in 1996.37 Although TCPP

adopts no donor–acceptor structure, DFT calculations

based on its D2h point group show distinct charge

separation with the highest occupied molecular orbital

(HOMO) located mainly on the phosphorous atom and

Figure 2 | (a) Molecular conformations of neutral TCPP obtained by DFT using BLYP/def2-SVP in the gas state (top)

and crystal state (bottom) with thermal ellipsoids at 50% probability. Hydrogen atoms are omitted for clarity.

(b) Electron cloud distributions of TCPP in the gas and crystal states calculated by TD-DFT M062X/def2-TZVP,

ORCA 4.1 program. (c) Crystal packing diagrams of TCPP. (d) Proposed mechanism of the photoinduced single-

electron transfer process.

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the phenyl ring that is parallel with the plane. On the

other hand, the lowest unoccupied molecular orbital

(LUMO) was located mainly on the two phenyl rings

vertical to the plane. No HOMO and LUMO separation of

TCPP was observed in the gas phase (Figure 2b). It is

worth noting that molecules with charge separation are

promising for generating radicals through the photoin-

duced electron transfer (PET) process. However, the

radicals generated are often transient, sensitive, and

quickly react with water and oxygen which lowers their

stability at ambient conditions. On the contrary, the

TCPP radicals show excellent stability at ambient con-

ditions, which may be correlated with their asymmetric

conformations and crystal structures. Further crystal

analysis shows that the TCPP molecules adopt an alter-

nate intermolecular donor–acceptor arrangement with

the closest distance between phenyl rings of about

3.29 Å, which is beneficial for the exciton separation

and stabilization through the electron hopping mecha-

nism (Figure 2c). In consideration of the considerable

stability of the radicals within i-TCPP crystals and the

molecular arrangement caused by this unique symme-

try breaking, we propose that the red emission of i-

TCPP crystals is due to TCPP radicals generated

through the PET process (Figure 2d).22,38–40

To confirm the rationality of our hypothesis, a theo-

retical calculation was carried out based on the

high-precision double-hybrid functional PWPB95 with

a def2-TZVP basis set. The free energy of the TCPP

dimer and the TCPP radicals (cations and anions) in the

ground state as well as that of the dimer in the excited

state was calculated (Figure 3a). Results show that both

radical cations and anions exhibit a much lower free

energy than the dimer in both ground state and excited

state, suggesting that the formation of radicals is

thermodynamically favorable. Interestingly, the opti-

mizedmolecular configuration of the TCPP radical anion

is almost identical to that of the TCPP crystal, and both

belong to a point group of D2h (Figure 3b and Supp

orting Information Table S3). It is worth mentioning that

despite the very crowded stacked structure, no interac-

tion, such as a CH···π interaction, exists in the packing

structure. Therefore, the molecular configuration of

TCPP in the crystal state is responsible for the formation

and stabilization of radicals.41,42 The spin distribution of

TCPP radicals in the crystal structure was also calculat-

ed (Figure 3c). For the TCPP radical cation, the unpaired

electron is mainly distributed on the phosphorus atom

and the benzene ring, parallel to the principal plane. For

the TCPP radical anion, the unpaired electron is distrib-

uted on benzene rings that are vertical to the plane.

Such distributions suggest that the radicals formed are

heterogenous instead of homogenous, which is consis-

tent with the complex hyperfine splitting pattern of the

EPR spectrum. The absorption and emission spectra of

TCPP radicals are also simulated by time-dependent

(TD)-DFT (Figures 3d and 3e). Interestingly, the super-

imposed absorption spectra of TCPP radical cation and

anion show two peaks at 457 and 482 nm, which corre-

late well with the experimental data and suggest the

reliability of the calculation results. Furthermore, in-

depth analysis indicates that the two absorption peaks

at 457 and 482 nm arise from the HOMO–9→singly

occupied molecular orbital (SOMO) transition of the

radical cation and the SOMO→LUMO+7 transition of the

radical anion, respectively (Figure 3d). The detailed

energy level diagrams andwave functions of the frontier

molecular orbitals of the TCPP radical anion and cation

are shown in Supporting Information Figure S11. The

emission spectra of the TCPP radical cation and anion

are also calculated using TD-DFT calculations based on

optimized structures in the first excited state byM062X/

def2-TZVP with two emission bands centered at around

549 and 667 nm and oscillator strengths of up to 0.021

(SOMO, LUMO+7) and 0.105 (HOMO-4, SOMO), respec-

tively (Supporting Information Table S4 and Figure S12).

Therefore, the red emission of the i-TCPP crystal is

mainly contributed by its radical cations, which is also

confirmed by the excitation spectra (Supporting

Information Figure S13).

The bright red emission of the i-TCPP crystal enables

it to serve as a potential optical crystalline material and

also a reporter to evaluate the stability of radicals

through a visualization method by measuring the emis-

sion change with time. As shown in Figure 4a, i-TCPP

crystals were placed at ambient conditions without any

protection, and their emission signals were measured by

a PL machine at different intervals of time. The radicals

and their red emission can survive for more than one

week in the crystal state at ambient conditions. Notably,

re-photoirradiation of the solid sample by UV light can

regenerate the radicals. This emission is an “on–off”

process that can be repeated many times and is techni-

cally feasible by alternate irradiation with UV and visible

light (Figure 4b).

Interestingly, a TCPP-loaded filter paper shows pres-

sure responsiveness upon UV irradiation which makes it

possible to utilize as a smart responsive material for

applications such as photopatterning or anticounterfeit-

ing (Figures 5a and 5b). By immersing a filter paper in a

dichloromethane solution of TCPP, a TCPP-loaded filter

paper was prepared. By stamping the freshly soaked

paper with a seal of a Chinese character “唐” of Tang and

then irradiating the paper with UV light, the patterned

area exhibited a red emission while the rest of the paper

only showed autofluorescence. The red emission could

be erased upon exposure to a green lamp (520 nm).

Since the radicals are only stable in the crystal state, we

speculate that exerting pressure can induce the forma-

tion of microcrystals in the filter paper.

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Figure 4 | Plots of relative PL intensity at 620 nm under (a) ambient condition and (b) under UV (365 nm) or visible

light (520 nm) irradiation.

Figure 3 | (a) Schematic diagram showing the free energy profile of the process obtained byDFT using PWPB95/def2-

TZVP. (b)Molecular structures of TCPP radical cation and anion in the gas phase obtained byDFT using the BLYP/def2-

SVP. (c) The spin density distribution of TCPP radical anion and cation with single point (SP) calculation based on

structures in the gas phasebyDFTusing theM062X/def2-TZVP. (d–e)Absorption and emission spectra of TCPP radical

anion and cation simulated on TD-DFT calculation using PBE0/def2-TZVP with structures in the gas phase.

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Conclusion

The photoinduced generation of radicals in TCPP crys-

tals with bright red emission has been reported. A

mechanistic study reveals that radicals are generated

within the TCPP crystals, and the red emission is mainly

ascribed to the radical cation. Theoretical calculation

integrated with crystal structure analysis indicates that

the unique symmetry breaking results in molecular con-

formation and photoredox property change of TCPP

crystals, facilitating the generation of radicals through

PET. The TCPP crystal lattice protects the radicals very

well from quenching by water and oxygen to endow the

TCPP radicals with considerable stability at ambient

conditions. The present results not only demonstrate

the possibility of in-situ generation of stable radicals

with the assistance of light irradiation but also open a

new window to producing photoresponsive stable lumi-

nescent radicals. Further potential applications of this

material are being explored.

Footnote

CCDC 2080878 contains the supplementary crystallo-

graphic data for TCPP.37 These data can be obtained free

of charge from The Cambridge Crystallographic Data

Centre (see www.ccdc.cam.ac.uk).

Supporting Information

Supporting Information is available and includes materi-

als and methods, characterizations, DFT calculations,

crystallographic data, photophysical data, and imaging

data.

Conflicts of Interest

There are no conflicts to declare.

Funding Information

This projectwas financially supported by theNational Nat-

ural Science Foundation of China (grant no. 21788102), the

NaturalScienceFoundationofGuangdongProvince(grant

nos. 2019B121205002 and 2019B030301003), the Re-

search Grants Council of Hong Kong (grant nos. 1630

5618, 16305518, C6014-20W, C6009-17G, and AoE/P-02/

12), the National Key Research and Development Program

(grant no. 2018YFE0190200), the Innovation andTechnol-

ogy Commission (grant no. ITC-CNERC14SC01), and the

Science and Technology Plan of Shenzhen (grant nos.

JCYJ20180306174910791, JCYJ20170818113530705, JCY

J20170818113538482, and JCYJ20160229205601482).

Preprint Acknowledgment

Research presented in this article was posted on a pre-

print server prior to publication in CCS Chemistry. The

corresponding preprint article can be found here: DOI:

10.26434/chemrxiv.12479321.v1

Acknowledgments

We gratefully acknowledge the contributions of Lili Du

and PHILLIPS, David Lee at the University of Hong Kong

(HKU), Zhijiao Tang at Shenzhen University, Congwu Ge

and Xike Gao at the Shanghai Institute of Organic Chem-

istry (SIOC) Chinese Academy of Sciences (CAS), Qiang

Wei at the Ningbo Institute of Industrial Technology

(CNITECH), Chinese Academy of Sciences (CAS), Xing

Feng at the Guangdong University of Technology, and

Ruoyao Zhang at the Hong Kong University of Science

and Technology for the experimental measurements.

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