Persistent room-temperature phosphorescence or high-contrast phosphorescent mechanochromism: polymorphism-dependent different emission characteristics from a single gold(i) complex

Article abstract of DOI:10.1039/d1dt00959a

Luminophores with persistent room-temperature phosphorescence (p-RTP) or effective phosphorescent mechanochromism features have significant potential applications in the field of optoelectronic materials. Until now, p-RTP and remarkable phosphorescent mechanochromism phenomena have been observed in some luminescent molecules with different molecular structures. However, separately realizing p-RTP and high-contrast phosphorescent mechanochromism in different polymorphs from a single luminophore is still a valuable and challenging topic. In this work, two polymorphs1Band1YGof a new gold(i) complex with blue and yellow-green luminescence, respectively, are reported. Interestingly,1Bexhibits high-contrast phosphorescent mechanochromic behavior, while1YGexhibits a persistent room-temperature phosphorescence effect. This is the first example of simultaneously obtaining double-purpose crystalline materials with a high-contrast phosphorescent mechanochromism or persistent room-temperature phosphorescence feature from a single luminophore.

Full text of DOI:10.1039/d1dt00959a

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Persistent room-temperature phosphorescence or
high-contrast phosphorescent mechanochromism:
polymorphism-dependent different emission
characteristics from a single gold(I) complex†
a,d
Cite this: DOI: 10.1039/d1dt00959a
Xiao-Yan Wang,‡a Ya Yin,‡b Jun Yin, a Zhao Chen
*
a,c and Sheng Hua Liu
*
Luminophores with persistent room-temperature phosphorescence (p-RTP) or effective phosphorescent
mechanochromism features have significant potential applications in the field of optoelectronic materials.
Until now, p-RTP and remarkable phosphorescent mechanochromism phenomena have been observed
in some luminescent molecules with different molecular structures. However, separately realizing p-RTP
and high-contrast phosphorescent mechanochromism in different polymorphs from a single lumino-
phore is still a valuable and challenging topic. In this work, two polymorphs 1B and 1YG of a new gold(I)
complex with blue and yellow-green luminescence, respectively, are reported. Interestingly, 1B exhibits
high-contrast phosphorescent mechanochromic behavior, while 1YG exhibits a persistent room-temp-
erature phosphorescence effect. This is the first example of simultaneously obtaining double-purpose
crystalline materials with a high-contrast phosphorescent mechanochromism or persistent room-temp-
erature phosphorescence feature from a single luminophore.
Received 23rd March 2021,
Accepted 26th April 2021
DOI: 10.1039/d1dt00959a
rsc.li/dalton
have triggered significant interest owing to their numerous
potential applications, including force sensing and optical
Introduction
Persistent room-temperature phosphorescence (p-RTP) recording, and in anti-counterfeiting systems.3 For high-per-
materials have gained increasing attention because of their formance mechanochromic materials, two factors are vital: (1)
unusual generation processes and promising practical appli- strong solid-state luminescence and (2) high-colour-contrast
cations, including biological imaging, chemical sensing, and before and after mechanical stress. At present, a range of
security protection.1 However, efficient phosphorescent emis- mechanochromic fluorophors, which involve fluorescent color
sion is commonly observed in inert gas atmospheres or at changes under mechanical stimulation, have been realized;4
ultra-low temperatures; achieving p-RTP in air and at room however, phosphorescent mechanochromic organic molecules
temperature is difficult because triplet excitons tend to relax are still scarce.5 In general, organometallic complexes are
nonradiatively via internal or external quenching.2 In recent more promising for obtaining phosphorescent mechanochro-
decades, mechanical-force-responsive luminescence materials mism in comparison with organic compounds. However, to
date, although some metal–organic complexes possessing
phosphorescent mechanochromic behavior have been
discovered,6,7 mechano-responsive metal complexes with long-
lived RTP features are rather rare.
aKey Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of
Chemistry, Central China Normal University, Wuhan 430079, China.
E-mail: chshliu@mail.ccnu.edu.cn
bHubei Key Laboratory of Catalysis and Materials Science, College of Chemistry and
Material Sciences, South-Central University for Nationalities, Wuhan 430074, PR
China
cJiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology
Normal University, Nanchang 330013, PR China. E-mail: chenzhao666@126.com
dState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, PR China
†Electronic supplementary information (ESI) available: Detailed experimental
section, NMR spectra, the details of the crystal data collection and characteriz-
ation data. CCDC 2008407 (1YG) and 2008408 (1B). For the ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/d1dt00959a
‡These authors contributed equally to this work.
Gold, as an important transition metal, is proven to be able
to display the maximum relativistic effect compared with its
neighbouring elements. Over the last two decades, gold(I)
chemistry has continued to receive growing attention, and a
variety of structurally novel gold(I) complexes with rich photo-
physical properties have been reported.8 Nevertheless, simul-
taneously achieving p-RTP and high-contrast phosphorescent
mechanochromism from a gold(I) complex still remains a
challenge.
Polymorphism is considered as an intriguing crystallization
phenomenon, in which a single substance can generate two or
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more crystalline phases in different or even the same crystalli-
zation environments, and these crystalline phases have dis-
tinct molecular arrangements in the crystal lattice.9 It is well
known that the physical and chemical characteristics of crys-
tals are closely associated with their crystal structures; thus,
different polymorphs resulting from the same substance are
likely to exhibit varying photophysical properties. Indeed,
obtaining multipurpose crystalline materials from a single
luminophore by taking advantage of structural polymorphism
is a significant but challenging research topic. Herein, a newly
reported gold(I) isocyanide complex 1 is demonstrated, which
generated two distinct polymorphic crystals 1B and 1YG
(Scheme 1) that emitted blue and yellow-green phosphor-
escence, respectively. Different luminescence of 1B and 1YG
resulted from their distinguishing molecular packings.
Importantly, polymorph 1B exhibited
a phosphorescent
Fig. 1 (a) Normalized steady-state (solid line) and delayed (dotted line)
emission spectra of polymorph 1B at room temperature. (b) Normalized
steady-state (solid line) and delayed (dotted line) emission spectra of
polymorph 1YG at room temperature. (c) Decay curve of 1B at the peak
emission wavelength of 510 nm at room temperature. (d) Decay curve of
1YG at the peak emission wavelength of 492 nm at room temperature.
mechanochromism phenomenon with a striking contrast in
luminescence colors, while polymorph 1YG exhibited a p-RTP
effect with a solid-state phosphorescence lifetime of up to
42.80 ms.
Results and discussion
Obviously, a red-shifted emission band centred at around
519 nm was observed. The luminescence lifetime of 1B at the
peak emission wavelength of 510 nm was 49.19 μs. The time-
gated luminescence spectrum and the microsecond-level
luminescence lifetime of 1B confirmed its typical room-temp-
erature phosphorescence (RTP) feature.
Gold(I) isocyanide complex 1 was prepared according to the
described synthesis procedures (Scheme S1†), and the struc-
ture of the synthesized complex was fully characterized via
NMR spectroscopy, mass spectrometry, and single crystal X-ray
diffraction. Notably, the crystallization of complex 1 by slowly
diffusing n-hexane vapor into its dichloromethane solution
produced two polymorphs 1B and 1YG concomitantly.
As shown in Fig. 1, the photoluminescence (PL) spectra of
the two types of polymorphs were investigated at room temp-
erature. Polymorph 1B showed multiple emission peaks in the
range of 400–500 nm. Under 365 nm UV light, 1B was blue
emissive with a luminescence quantum yield (Φ) of 17.3%.
Furthermore, its PL spectrum was also studied via gated
measurement with a delay time of 0.1 ms after light excitation.
At 77 K, polymorph 1B showed multiple fine-structure
peaks in the range of 400–500 nm, and the delayed emission
spectrum of polymorph 1B was similar to its corresponding
steady-state emission spectrum (Fig. S1†). Furthermore, after
mechanical stimulation at room temperature, a new emission
peak appeared at 500 nm (Fig. 2a), and the phosphorescent
color of 1B changed from blue to green (Fig. 2e). The corres-
ponding Φ value of the obtained green luminescence was
13.2%. Upon fuming the ground sample with dichloro-
methane vapor for 30 s, the initial blue phosphorescence was
recovered. Therefore, 1B exhibited reversible high-contrast
phosphorescent mechanochromic behavior. In contrast to 1B,
the PL spectrum of the yellow-green-emitting polymorph 1YG
showed a broad emission band with two emission peaks at
492 nm and 534 nm and shoulder peaks in the range
400–480 nm (Fig. 1b); the time-gated emission spectrum with
a 0.1 ms delay for 1YG was similar to that without a time
delay. Moreover, for 1YG, both its steady-state and delayed
emission spectra at room temperature were similar to its
corresponding steady-state emission spectrum at 77 K.
Differing from the steady-state emission spectrum of 1YG at
77 K, the emission peaks in the range of 400–480 nm in the
delayed emission spectrum of 1YG at 77 K almost completely
disappeared (Fig. S2†). Surprisingly, the room temperature
phosphorescence lifetime of 1YG at the peak emission wave-
length of 492 nm was as high as 42.80 ms, which was about
three orders of magnitude longer than that of 1B at 510 nm.
Scheme 1 (a) Structure of the gold(I) isocyanide complex 1. (b) Single
crystal structure of polymorph 1B. (c) Single crystal structure of poly-
morph 1YG. The insets show the photoluminescence images under
365 nm UV irradiation of two polymorphs 1B and 1YG formed from
complex 1.
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Fig. 2 Normalized steady-state emission spectra of polymorphs 1B (a)
and 1YG (b) in different states (unground, ground, treated with CH2Cl2
vapor). The XRD patterns of polymorphs 1B (c) and 1YG (d) in various
states. The photographs of polymorphs 1B (e) and 1YG (f) in various
states taken under 365 nm UV irradiation.
After the removal of the photoexcitation source, a clear yellow-
green RTP lasting several seconds for 1YG could be directly
observed by the naked eye (see Video S1†), thus, demonstrating
the p-RTP characteristics. In addition, the polymorph 1YG also
displayed mechano-responsive phosphorescence involving an
inapparent color change from yellow-green (Φ = 19.7%) to
green (Φ = 16.5%) (Fig. 2b and f). From the powder X-ray diffr-
Fig. 3 (a) Transient emission spectra of polymorph 1YG. (b) Transient
PL decay image of polymorph 1YG; the color change from red to blue
indicated the decrease of emission intensity. (c) Schematic diagram for
mechanically regulated p-RTP of 1YG.
action (XRD) data (Fig. 2c and d), the two crystal forms of afterglow emission (see Video S2†). These results indicated
complex 1 exhibited distinctly different structures, and the that polymorph 1YG is a mechanically regulated p-RTP crystal-
observed phosphorescent mechanochromism phenomena of line material, and its “on–off” p-RTP effect could be effectively
1B and 1YG resulted from the morphological transition mediated via mechanical stimulation.
between the crystalline and amorphous states.
To obtain deeper insight into the effect of intermolecular
To further understand the p-RTP phenomenon of poly- interactions on the emission properties of 1B and 1YG, the
morph 1YG, the time-resolved emission spectra (TRES) of 1YG crystal packings of the two polymorphs 1B and 1YG were sys-
were investigated. From Fig. 3a, it can be seen that the emis- tematically analyzed. As illustrated in Fig. 4, the shortest
sion peaks in the range 400–450 nm disappeared on stopping Au⋯Au distance between adjacent molecules was 4.745 Å (1B)
photoexcitation. The maximum emission peak was located at and 4.653 Å (1YG), respectively, indicating no significant inter-
492 nm with a shoulder peak at 526 nm, which could be molecular aurophilic interactions.10 Dipole–dipole interactions
assigned to p-RTP emission. Two weak emission peaks were were observed between the F atom of pentafluorophenyl and
still present even after about 600 ms (Fig. 3b), which was con- the C atom of the isocyano group in 1YG (Fig. 4d, d = 3.099 Å)
sistent with the fact that the yellow-green phosphorescence or 1B (Fig. 4b, d = 3.174 Å), which promoted intersystem cross-
could still be seen with the naked eye after removal of the exci- ing and induced RTP generation.11
tation light (Fig. 3c). Interestingly, upon mechanical grinding,
Multiple hydrogen-bonding interactions were found in the
the emission peaks for 1YG at short wavelengths (400–480 nm) 1B and 1YG crystals (Fig. 5), and these intermolecular forces
disappeared, and the phosphorescent peaks at long wave- efficiently impeded their molecular motions and facilitated
lengths (480–580 nm) remained basically unchanged (Fig. 2b). the molecular packings of 1B and 1YG. For 1B, each molecule
In contrast, the luminescence lifetime of 1YG at 507 nm was connected to the surrounding molecules through six C–
sharply decreased to 4.97 μs after grinding (Fig. S6a†). After H⋯F interactions and two C–H⋯O interactions (Fig. 5a, dH⋯F
removing the 365 nm UV lamp, the ground 1YG did not show = 2.563 Å, 2.606 Å, 2.622 Å; Fig. 5b, dH⋯O = 2.514 Å), forming a
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of the intersystem crossing process and the formation of a
more stable excited triplet state.
Conclusions
In summary, we successfully prepared two types of crystalline
materials 1B and 1YG exhibiting blue and yellow-green phos-
phorescence respectively from
a single gold(I)-containing
complex by taking advantage of structural polymorphism.
Furthermore, polymorph 1B exhibited phosphorescent
mechanochromic behavior involving a high-contrast color
change from blue to green, while polymorph 1YG exhibited a
p-RTP effect. Furthermore, an “on–off–on” effect of p-RTP
could be effectively achieved by successive treatment via
mechanical grinding and solvent fuming. Compared with 1B,
the more abundant intermolecular interactions and more rigid
conformations of 1YG played a key role in promoting intersys-
tem crossing and suppressing nonradiative decay and thus
promoting the occurrence of the notable p-RTP phenomenon.
To the best of our knowledge, this is the first report describing
two types of crystalline materials with high-contrast phosphor-
escent mechanochromism or p-RTP behavior from a single
luminogen. The structural polymorphism strategy demon-
strated here may open up a new avenue for realizing p-RTP and
high-contrast mechanochromism.
Fig. 4 (a) The shortest Au⋯Au distance and C–H⋯π interactions in
polymorph 1B. (b) Dipole–dipole interaction in polymorph 1B. (c) The
shortest Au⋯Au distance and C–H⋯π interactions in polymorph 1YG. (d)
Dipole–dipole interaction in polymorph 1YG.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Fig. 5 (a) Intermolecular C–H⋯F interactions in polymorph 1B. (b)
Intermolecular C–H⋯O interactions in polymorph 1B. (c) Intermolecular
C–H⋯F interactions in polymorph 1YG. (d) Intermolecular C–H⋯O
interactions in polymorph 1YG.
The authors acknowledge the financial support from the
National Natural Science Foundation of China (21472059,
22061018, 21702079 and 21772054), the 111 Project (B17019),
and the Huabo Project of CCNU.
Notes and references
loose 3D molecular arrangement. Under the stimulation of a
mechanical force, the loose packing motif was broken, and the
ordered crystalline state was converted into a chaotic amor-
phous state. As a result, 1B exhibited a high-contrast phos-
phorescent mechanochromic behavior. As for 1YG, stronger
hydrogen-bonding interactions (Fig. 5c, dH⋯F = 2.540 Å,
2.543 Å, 2.546 Å; Fig. 5d, dH⋯O = 2.424 Å), C–H⋯π interactions
(Fig. 4c, d = 2.707 Å) and dipole–dipole interactions (Fig. 4d, d
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stabilized the triplet excitons. In comparison with 1B, the
long-lived RTP effect of 1YG is possibly due to the facilitation
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