Highly efficient room-temperature phosphorescent materials with a heavy-atom effect of bromine

Article abstract of DOI:10.1039/d0nj05713d

Room-temperature phosphorescent (RTP) materials with long luminescence lifetimes have stimulated considerable interest. However, pure organic molecules with persistent room-temperature phosphorescence are still rarely reported. In this work, we study two

Full text of DOI:10.1039/d0nj05713d

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Highly efficient room-temperature
phosphorescent materials with a
heavy-atom effect of bromine†
Cite this: New J. Chem., 2021,
45, 4930
Received 23rd November 2020,
Accepted 4th February 2021
Youfu Xia, * Yanqing Du, Qian Xiang and Mark G. Humphrey*
DOI: 10.1039/d0nj05713d
rsc.li/njc
Room-temperature phosphorescent (RTP) materials with long lumi- organic compounds can be multiplied by introducing halogen
nescence lifetimes have stimulated considerable interest. However, pure atoms (Br or I) that are favorable to enhance the intersystem
organic molecules with persistent room-temperature phosphorescence crossing process22 due to a pronounced heavy-atom effect.23,24
are still rarely reported. In this work, we study two molecules, DCzMPh
In the present work, carbazole and benzene are linked by
and TCzMPh, in which persistent room-temperature phosphorescence methylene, which increases distortion and activates crystal-
in the amorphous state is achieved via the heavy-atom effect of bromine induced phosphorescence.25 Such a combination of a heavy
atoms in their structure. Their final quantum yields are found to be as atom as a p-acceptor with a carbazole unit aims to improve the
high as 90 and 93%, respectively, at the phosphorescence lifetimes of phosphorescence efficiency of these compounds. The chemical
0.27 and 0.28 s, respectively. Moreover, both molecules are shown to be and structural analysis of both molecules was performed using
appropriate for security protection applications.
nuclear magnetic resonance (NMR) and single-crystal X-ray
diffraction (XRD) techniques. Their photophysical properties
Over the past few years, organic afterglow materials have attracted such as quantum yields and phosphorescence lifetimes were
a considerable amount of attention due to their unique pro- evaluated by ultraviolet-visible (UV/Vis) absorption spectroscopy
perties that find application in optoelectronic devices,1 data in a tetrahydrofuran (THF) solution and steady-state fluore-
security protection,2 molecular sensing,3 and mass spectrometric scence and phosphorescence spectrophotometry in the crystal-
analysis.4 Among these materials, the most effective are inorganics line state under ambient conditions (at atmospheric pressure
and organometallic room-temperature phosphorescence (RTP) and room temperature).
compounds.5–7 However, metal materials exhibit shortcomings
DCzMPh and TCzMPh compounds were prepared from trimethyl
such as instability, high cost, toxicity and poor flexibility. Further- bromide benzene 1 through a two-step process (Scheme 1). First,
more, high non-radiative deactivation rates along with weak inter- 2 and 3 were prepared via the routes described in the literature.26
system crossing (ISC) impede the efficient RTP in pure organic The substitution reactions of 2 and 3 with carbazole were
luminogens.8–10 Thus, designing organic (RTP) materials with performed under KOH conditions and DMSO was used as the
lower toxicity and longer emission lifetimes is one of the important solvent. DCzMPh and TCzMPh were recrystallized with dichlor-
tasks of researchers.
omethane and n-hexane until the colorless crystals were
In recent years, synthesis of organic materials with ultralong obtained. The final quantum yields of these molecules are found
phosphorescence has been presented by various strategies such
as host–guest design,11,12 H-aggregation,13,14 (co)crystallization,15,16
chelation,17,18 and polymerization.19,20 Meanwhile, achieving highly
efficient phosphorescence in pure organic materials under
ambient conditions is a challenging task because of a weak
spin–orbit coupling (SOC)21 between the singlet and triplet states.
In this respect, the phosphorescence efficiency in metal-free
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
P. R. China. E-mail: xiayoufu2005@126.com, mark.humphrey@anu.edu.an
† Electronic supplementary information (ESI) available. CCDC 2022966 and
2022982. For ESI and crystallographic data in CIF or other electronic format
Scheme 1 Synthesis of DCzMPh and TCzMPh.
see DOI: 10.1039/d0nj05713d
4930 | New J. Chem., 2021, 45, 4930À4933
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to be as high as 90 and 93%, respectively. The thermal properties
of DCzMPh and TCzMPh were studied by thermogravimetric
analysis (TGA). DCzMPh and TCzMPh have good thermal stabi-
lity, the 5% weightlessness temperature (Td) is 363.3 1C and
394.0 1C, respectively (Fig. S2, ESI†). To characterize the frontier
molecular orbital distributions of DCzMPh and TCzMPh, density
functional theory (DFT) calculations were carried out on the
ground state by using density functional theory (DFT) with the
B3LYP/6-31G* basis set. As shown in Fig. S3 (ESI†), the LUMO is
clearly confined in the carbazole and benzene ring, respectively,
and the HOMO is also similarly distributed on the carbazole,
indicating a good overlap between the HOMO and LUMO.
Fig. 1 displays the UV/Vis absorption spectra of both molecules.
Except for a difference between the methyl and carbazole at the
adjacent positions, the structures of DCzMPh and TCzMPh are
close to each other, resulting in almost similar UV/Vis absorption
spectra of the two materials. Both compounds exhibit two pro-
nounced absorption peaks at 239 and 293 nm and signatures at
240 and 295 nm, respectively, with a small acromion, which are
the p–p* characteristic absorption peaks of the carbazole group
and the phenyl conjugated double bond. The absorption peaks at
327, 340, 329 and 342 nm possess low absorption intensities at
higher absorption wavelengths and refer to the n - p*absorption
from the solitary electron pairs to the empty orbitals on carbazole
and bromine atoms (Fig. 1).
Fig. 2 Steady-state fluorescence (black) and phosphorescence (red) spectra
of crystalline (a) DCzMPh and (b) TCzMPh under ambient conditions.
Fig. 2 displays the steady-state fluorescence and phosphore-
scence spectra of crystalline DCzMPh and TCzMPh under
ambient conditions. As shown in Fig. 2a, DCzMPh in a crystal-
line state exhibits a delayed luminescence in a wide wavelength
range (lmax = 431 nm) with an ultralong lifetime tp of up to
0.27 s and a quantum yield Fp of 6.58% (Fig. 4), along with a
fluorescence signal at lmax = 408 nm (lex = 356 nm). The large Stokes
shift between fluorescence and RTP emission of DCzMPh is around
yield Fp of 13.77% (Fig. 4), almost the same as the fluorescence
spectrum (lex = 400 nm).
The information available from UV-Vis absorption spectroscopy
data in Fig. 1 and 2 enabled one to assess the photophysical
parameters of DCzMPh and TCzMPh under ambient conditions.
Table 1 summarizes the fluorescence and phosphorescence char-
acteristics for both molecules, including quantum yields and
phosphorescence lifetimes. The quantum yield Fp of crystalline
DCzMPh was found to be 6.58% at an ultralong lifetime tp of up to
0.27 s. The appropriate values for TCzMPh in a crystalline state
were 13.77% and 0.28 s, respectively.
150 nm. Meanwhile, compared with a fluorescence emission (lmax
=
408 nm), the maximum emission redshift in a phosphorescence
mode (lmax = 431 nm) is 23 nm, which is testimony to the fact that
the afterglow is rather associated with phosphorescence than with
delayed fluorescence. For the crystalline TCzMPh (Fig. 2b) a delayed
luminescence is observed within a wide wavelength region (lmax
=
430 nm) with an ultralong lifetime tp of up to 0.28 s and a quantum
The afterglow decay curves of both samples are plotted in
Fig. 3 at various lifetimes. To elucidate the origin of ultralong
phosphorescence in both compounds, a single-crystal X-ray diffrac-
tion analysis was performed to thoroughly study their molecular
packing models and interaction. Fig. 4 shows the crystal packing
models of crystalline DCzMPh and TCzMPh under ambient
conditions. As seen in Fig. 4a, a DCzMPh molecule is presented
Table 1 Photophysical data of DCzMPh and TCzMPh at 298 K
lex
lFluo
lPhos
tF
tp
Compound [nm]a [nm]ab
[nm]ab
[ns]a [s]a FF(%)ac Fp(%)ac
DCzMPh 356
408
430
431
430
9.12 0.27 6.58 0.98
4.88 0.28 13.77 3.43
TCzMPh
400
a
b
In a crystalline state. Emission maximum upon excitation at the
c
Fig. 1 UV/Vis absorption spectra of DCzMPh (black) and TCzMPh (red) in THF. longest absorption maximum. Absolute quantum yield.
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group, which reduces to a large extent the changes in the indicated
intramolecular interactions. Since the C–HÁÁÁN interaction is char-
acterized by four values, this also suppresses the molecular
motions, protecting triplet excitons from the nonradiative transi-
tions. Each molecule in a crystalline TCzMPh is surrounded by the
two types of intermolecular interactions that are shown in Fig. 4e
(C–BrÁÁÁp, 3.906 Å). Simultaneously, a small overlap of the carbazyl
group with a formed pÁÁÁp stacking (3.393 Å) is found for a pair of
TCzMPh molecules in the crystalline state (Fig. 4f). A great deal of
intermolecular and intramolecular interactions results in a rela-
tively small rate of relaxation from T1 to S0, which explains the
ultralong lifetimes obtained for both molecules. The persistent
phosphorescence originates from the carbazoles involved in the
electronic interactions between the Br atoms and carbazole.27 In
particular, the H-aggregation formed in both crystalline com-
pounds can stabilize the triplet excitons for an ultralong lifetime
under ambient conditions.13
RTP materials with ultralong lifetimes may find applications
in anti-counterfeiting technology and encryption.28 For example,
Fig. 5 depicts a secure code pattern where DCzMPh serves as a ‘‘J’’
part and TCzMph is used as the ‘‘N’’ part (Daylight mode). Under
UV irradiation of 365 nm (UV light on mode), the pattern exhibits
white-light emission, which can act as the fluorescent security
protection mode. After turning off the UV excitation (UV light off
mode), it can be seen by the naked eye that ‘‘J’’ and ‘‘N’’ show
yellow RTP, thus making it difficult to distinguish each part.
In 2020, Yan’s group reported the synthesis of compound
(Ph4P)2Cd2Br6, which was synthesized difficultly, and with a low
overall yield, with the room temperature phosphorescence
quantum yield of 62.79% and lifetime of 37.85 ms.29 Our
synthesized materials DCzMPh and TCzMPh have lifetimes of
0.27 s and 0.28 s, respectively. Compared with the above
materials, we have achieved better phosphorescence lifetime
and higher yield materials.
Fig. 3 Afterglow decay curves vs. lifetimes of DCzMPh and TCzMPh
under ambient conditions.
by three types of intramolecular interactions, including C–HÁ Á ÁN
(2.469 and 2.557 Å). Strong C–BrÁ Á ÁN and C–BrÁ Á Áp interactions
effectively limit the molecular rotations and thus decrease the
nonradiative decay. Intramolecular Br bonds promote the spin–
orbit coupling (SOC) between the excited singlet and triplet
states. Each molecule in a crystalline DCzMPh is surrounded
by two types of intermolecular interactions (C–BrÁ Á Áp, 3.698 Å),
as shown in Fig. 4b. Moreover, a small overlap of the carbazyl
group with a formed pÁ Á Áp stacking (3.452 Å) is highlighted for a
DCzMPh molecule pair s in the crystal (Fig. 4c). In the crystalline
TCzMPh (Fig. 4d), there are three types of intramolecular interac-
tions, including C–HÁÁÁN (2.433, 2.484, 4.046 and 2.512 Å). Com-
pared with a DCzMPh structure, TCzMPh possesses a carbazole
In summary, we developed a concise and very simple chemical
approach to construct amorphous, heavy-atom containing pure
organic RTP materials, where the triplet exciton stabilization is
achieved through a combination of the bromine atom and
carbazole. For this, we produced DCzMPh and TCzMph molecules
with bromine as halogen atoms, in which carbazole and benzene
Fig. 5 Example of the use of DCzMPh and TCzMPh in data security
protection. Here, the ‘‘J’’ pattern is made of DCzMPh (powder) and the
‘‘N’’ pattern is obtained using TCzMPh (powder). The UV light excitation
wavelength is 365 nm.
Fig. 4 Crystal packing models of crystalline (a–c) DCzMPh and (d–f)
TCzMPh under ambient conditions.
4932 | New J. Chem., 2021, 45, 4930À4933
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Acknowledgements
Authorship agreed by all authors except for Mark G. Humphrey
who could not be contacted.
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