A series of 4,5,9,10-tetrahydropyrene-based tetraarylethenes: Synthesis, structures and solid-state emission behavior

Article abstract of DOI:10.1039/c8ra00057c

By controlling the number of 4,5,9,10-tetrahydropyrene segments around the tetraarylethene core, a series of 4,5,9,10-tetrahydropyrene-based tetraarylethenes were synthesized and structurally characterized. An aggregation-induced emission (AIE) study indicated that all the compounds are AIE active: they are weak emitters in good solvents but highly emissive in the condensed phase, and hence are potential solid-state emitters. Their optical properties, electrochemical properties and theoretical calculations were investigated, and the results prove that the π-conjugation degree of these compounds increases with the increasing number of 4,5,9,10-tetrahydropyrene units. However, the fluorescence quantum yield in the solid state doesn't increase with increasing π-conjugation. We studied the reason for this by analyzing the crystal structures of some compounds, and proposed that the close degree of molecular packing in the solid state may be responsible for it. Loose packing of tetraarylethenes in the solid state can restrict the rotation of the aromatic rings but cannot constrain other non-radiative pathways efficiently, such as vibration, which leads to the unpredictable emission of the compounds.

Full text of DOI:10.1039/c8ra00057c

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A series of 4,5,9,10-tetrahydropyrene-based
tetraarylethenes: synthesis, structures and solid-
Cite this: RSC Adv., 2018, 8, 15173
state emission behavior†
a
Zhao-Ming Zhang,ab Bao-Xi Miao,a Xin-Xue Tanga and Zhong-Hai Ni
*
By controlling the number of 4,5,9,10-tetrahydropyrene segments around the tetraarylethene core, a series
of 4,5,9,10-tetrahydropyrene-based tetraarylethenes were synthesized and structurally characterized. An
aggregation-induced emission (AIE) study indicated that all the compounds are AIE active: they are weak
emitters in good solvents but highly emissive in the condensed phase, and hence are potential solid-
state emitters. Their optical properties, electrochemical properties and theoretical calculations were
investigated, and the results prove that the p-conjugation degree of these compounds increases with
the increasing number of 4,5,9,10-tetrahydropyrene units. However, the fluorescence quantum yield in
the solid state doesn't increase with increasing p-conjugation. We studied the reason for this by
analyzing the crystal structures of some compounds, and proposed that the close degree of molecular
packing in the solid state may be responsible for it. Loose packing of tetraarylethenes in the solid state
can restrict the rotation of the aromatic rings but cannot constrain other non-radiative pathways
efficiently, such as vibration, which leads to the unpredictable emission of the compounds.
Received 3rd January 2018
Accepted 17th April 2018
DOI: 10.1039/c8ra00057c
rsc.li/rsc-advances
become strong emitters when the condensed phase are formed.17
In aggregates, intramolecular rotation which acts as a non-
1. Introduction
radiative pathway can be restricted effectively, meanwhile the
twisted structure of these compounds can avoid the formation of
p-stacking to a certain extent, thus achieving high solid-state
emission efficiency. As a representative member of AIE dyes,
tetraphenylethene (TPE) has attracted much research attention
because it enjoys the advantages such as efficient solid-state
emission, facile synthesis and easy functionalization.18–20 In
addition, by combing TPE and many kinds of polycyclic aromatic
hydrocarbons (PAHs), a large number of novel TPE derivatives
have been developed.21–27 It has been reported that the uores-
cence emission efficiency is affected by the p-conjugation length
of compounds.28 In the cases of TPE derivatives, the introduction
of PAHs can extend the p-conjugation of compounds and their
good performance in emission suggests that enlarging the p-
conjugation degree of AIE compounds is an effective strategy to
obtain solid-state dyes. Based on this fact, we speculate that in
solid state, intramolecular rotation of AIE compounds can be
effectively restricted and p–p interactions are completely absent,
whether the emission efficiency would increase with the
increasing p-conjugation?
The design and synthesis of organic materials with efficient
solid-state uorescence has attracted extensive attention
because the performance of organic photoelectric devices is
usually dependent on the solid-state properties of materials.1–4
And most luminogenic molecules, especially those with disc-
like shapes, normally exhibit quenched emission in the
condensed state due to the formation of intermolecular p-
stacking interactions.5–7 Several strategies have been developed
to solve this problem such as designing polymers with dendritic
structures and using antiaggregation reagents to modulate
aggregation.8–10 In these methods, the design ideas are
obstructing the formation of luminophoric aggregates.
Recently, it has been proved that the construction of organic
compounds with aggregation-induced emission property is
a convenient but effective way to obtain outstanding lumi-
nogens.11–16 Different from the ordinary organic compounds
usually with excellent single-molecule emission in solvents, AIE
compounds are weakly emissive in good solvent but they
aSchool of Chemical Engineering and Technology, China University of Mining and
Technology, Xuzhou 221116, People's Republic of China. E-mail: nizhonghai@cumt.edu.cn
bGraduate School of Chemical Sciences and Engineering, Hokkaido University,
The best way to study this problem is to synthesize a series of
PAHs-substituted tetraarylethenes and analyze their emission
efficiency in solid state, in which the different p-conjugation
degree can be realized by controlling the number of PAHs
substituents. However, up to now, the reported PAHs-substituted
arylethenes are concentrated on the tetraarylethenes with one
and two PAHs moieties, but it is also necessary to obtain three
Sapporo 001-0021, Japan
† Electronic supplementary information (ESI) available: Crystal data and structure
renement parameters for compounds 1 and 4. CCDC 1548871 and 1548874. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8ra00057c
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and four PAHs-substituted tetraarylethenes in order to fully study diffractometer. The structures were solved by the direct method
the problem. Zhao et al. have successfully synthesized four and rened by full-matrix least squares on F2. Hydrogen atoms
naphthalene-substituted
naphthalen-2-yltriphenylethene
arylethene
to
compounds
from were added geometrically and rened using a riding model. CCDC
tetra(naphthalen-2-yl) numbers for compounds 1 and 4 are 1548871 and 1548874,
ethane.29 However, on account of the planar and bulk structure of respectively.
the naphthalene, these four compounds suffered from the
notorious p–p interactions, leading to a decrease tendency in
2.2 Synthesis procedure and structural characterization
emission efficiency with the increasing number of naphthalene
substitution. Because of the interference of the p–p interactions,
2.2.1 Synthesis of 2-benzoyl-4,5,9,10-tetrahydropyrene. To
these naphthalene-based compounds cannot be employed to a mixture of 4,5,9,10-tetrahydropyrene (1.03 g, 5 mmol),
study the proposed problem directly. Therefore, a special PAHs is aluminum chloride (1.60 g, 12 mmol) and carbon disulde (100
required urgently, based on which the constructed tetraar- mL), a solution of benzoyl chloride (0.73 g, 5.ꢁ1 mmol) in carbon
ylethenes can avoid the formation of p–p interactions in solid disulde (20 mL) was added dropwise at 0 C under nitrogen
ꢁ
state. In our previous work, we have reported that 4,5,9,10-tetra- atmosphere. Aer stirring for 30 min at 0 C, the mixture was
hydropyrene possesses both enlarged conjugation and certain warmed to room temperature and stirred overnight. The solu-
steric hindrance, and the introduction of it into tetraarylethene tion was poured into ice and extracted with dichloromethane.
system can expand the p-conjugation but prevent the formation The combined organic layers were washed with saturated brine
of p–p interactions.30,31 Therefore, in this work, based on solution and water, and dried over anhydrous magnesium
4,5,9,10-tetrahydropyrene, a series of 4,5,9,10-tetrahydropyrene- sulfate. Aer ltration and solvent evaporation, the residue was
based tetraarylethenes has been synthesized and their optical, puried by silica-gel column chromatography using hexane/
electrochemistry properties and theoretical calculations have dichloromethane as eluent. Pale yellow product was obtained
been fully investigated. Moreover, the effect of p-conjugation and in 68% yield. Mp: 117–119 ꢁC. IR (KBr, cmꢀ1): 3056, 2933, 2894,
molecule packing on solid-state emission has been discussed.
2831, 1654, 1595, 1326. 1H NMR (400 MHz, CDCl3), d (TMS,
ppm): 7.89–7.83 (m, 2H), 7.66–7.59 (m, 1H), 7.58–7.49 (m, 4H),
7.26–7.29 (m, 1H), 7.18–7.11 (d, 2H), 2.95 (s, 8H). 13C NMR (400
MHz, CDCl3), d (TMS, ppm): 196.64, 138.12, 136.19, 135.93,
135.28, 134.82, 132.12, 129.94, 129.85, 128.31, 128.23, 127.96,
2. Experimental
2.1 General
All reagents were purchased from commercial sources as 126.17, 28.15. GC/EI-QMS: m/z 310 [M]+.
analytically pure grade. Tetrahydrofuran (THF) was distilled 2.2.2 Synthesis of (4,5,9,10-tetrahydropyren-2-yl)
from sodium benzophenone ketyl under dry nitrogen immedi- triphenylethene (1). To a solution of diphenylmethane (0.45 g,
ately prior to use. Dichloromethane was dried and puried by 2.67 mmol) in dry tetrahydrofuran (10 mL) was added 0.9 mL of
distillation before use from CaH2. 4,5,9,10-Tetrahydropyrene a 2.5 M solution of n-butyllithium in hexane (2.22 mmol) at 0 ꢁC
was prepared by a method reported in the literature.32 The under an nitrogen atmosphere. The resulting orange-red solu-
synthetic procedure and the detailed characterization of 2,20- tion was stirred for 30 min at that temperature. To this solution
bis-(4,5,9,10-tetrahydropyrenyl)ketone and tetrakis(4,5,9,10- was added the 2-benzoyl-4,5,9,10-tetrahydropyrene (0.62 g, 2
tetrahydropyren-2-yl)ethene have been reported in ref. 30.
mmol), and the reaction mixture was allowed to warm to room
1H and 13C NMR spectra were measured on a Bruker AV 400 temperature with stirring during a 6 h period. The reaction was
spectrometer in CDCl3 using tetramethylsilane (TMS; d ¼ 0) as quenched with addition of an aqueous solution and the organic
internal reference. Mass spectra were obtained on a JEOL JMS layer was extracted with dichloromethane and the combined
Q1000GC Mk II Quad GC/MS or a Bruker Ultraextreme MALDI organic layers were washed with saturated brine solution and
TOF/TOF mass spectrometer or an IonSpec HiResMALDI FT mass dried over anhydrous magnesium sulfate. The solvent was
spectrometer. IR spectra were recorded on KBr pellets using evaporated and the resulting crude alcohol (containing excess
a Nicolet 7199B FT/IR spectrophotometer in the region of 4000– diphenylmethane) was subjected to acid catalyzed dehydration
400 cmꢀ1. UV-Vis spectra were recorded on Shimadzu UV-3600 as follows.
with a UV-VIS-NIR spectrophotometer. Emission spectra were
The crude alcohol intermediate was dissolved in about
performed on a Hitachi uorimeter (F-4600). FF values of the 20 mL of toluene in a 100 mL Schlenk ask tted with a Dean–
amorphous lms were determined by FM-4P-TCSPC transient Stark trap. A catalytic amount of p-toluenesulfonic acid (0.076 g,
state uorescence spectrometer using an integrating sphere. 0.4 mmol) was added and the mixture was reuxed for 3 h and
Fluorescence lifetimes were examined on an Edinburgh FS5 cooled to room temperature. The toluene layer was washed with
spectrouorometer. Cyclic voltammetry experiments were per- 10% aqueous NaHCO3 solution and dried over anhydrous
formed with a CHI660E electrochemical work station using 0.1 M magnesium sulfate and evaporated to afford the crude
n-Bu4NPF6 in dichloromethane as supporting electrolyte. All compound 1. The crude product was puried by column chro-
measurements were carried out at room temperature with matography on silica gel using hexane/dichloromethane as
a conventional three-electrode conguration consisting of a plat- eluent. Compound 1 was obtained as a pale yellow solid in 82%
1
inum disk working electrode, a platinum auxiliary electrode and yield. H NMR (400 MHz, CDCl3), d (TMS, ppm): 7.25–7.04 (m,
a
calomel reference electrode. Single-crystal X-ray data of 18H), 6.76 (s, 2H), 2.88–2.79 (m, 4H), 2.74–2.64 (m, 4H). 13C
compounds 1 and 4 were collected on Bruker APEX II CCD NMR (400 MHz, CDCl3): 145.11, 143.96, 143.93, 143.51, 142.42,
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was obtained in 22% yield. 1H NMR (400 MHz, CDCl3), d (TMS,
ppm): 7.19–6.96 (m, 14H), 6.91–6.75 (d, 6H), 2.96–2.57 (m, 24H).
13C NMR (400 MHz, CDCl3): 142.37, 140.60, 135.33, 135.03,
134.38, 131.41, 130.76, 129.07, 128.91, 128.76, 128.66, 128.43,
127.75, 127.49, 126.77, 126.14, 125.87, 28.46, 28.15. MALDI-TOF
MS: m/z 716.30 [M]+.
3. Results and discussion
3.1 Synthesis
Up to now, there are two main methods to synthesize tetraar-
ylethenes. For asymmetric ethenes, Banerjee et al. proposed an
effective method which is based on the reaction of diphe-
nylmethyllithium and diaryl ketone.33 The other method is the
famous McMurry coupling in which the diaryl ketone can be
catalyzed by zinc powder and TiCl4 to afford the symmetric
ethenes.34,35 In the two methods, it is obvious that the inter-
mediate ketones play an important role in deciding the struc-
tures of the products. For the reported PAHs-substituted
ethenes, ketones were usually obtained by the Friedel–Cras
acylation reaction of PAHs and benzoyl chloride. Based on these
ketones, tetraarylethenes substituted by one and two PAH units
can be synthesized very efficiently, for example, compounds 1
and 2 in this work were obtained with the yields of 82% and
62% (Scheme 1), respectively. However, the preparation of
di(polycyclic aryl)ketones which are essential for the syntheses
of tetraarylethenes with three or four PAH segments is difficult
due to the limited synthetic methods. In our previous work, we
have developed a convenient method in which PAHs can
convert into their corresponding di(polycyclic aryl)ketones
directly by one step reaction.30 Based on the McMurry coupling
reaction of 2,20-bis-(4,5,9,10-tetrahydropyrenyl)ketone and 2-
benzoyl-4,5,9,10-tetrahydropyrene, tetraarylethene with three
4,5,9,10-tetrahydropyrene units can be obtained with relatively
low yield because compounds 2 and 5 were also produced at the
same time. Tetrakis(4,5,9,10-tetrahydropyrenyl)ethene can be
conveniently obtained by the McMurry coupling reaction of
pure 2,20-bis-(4,5,9,10-tetrahydropyrenyl)ketone, which was re-
ported in our previous work.30 In addition, a special molecule,
compound 3 was also synthesized in this paper by the reaction
of 2,20-bis-(4,5,9,10-tetrahydropyrenyl)ketone and diphenylme-
thyllithium. To the best of our knowledge, this is the rst tet-
raarylethene in which two PAH units are connected on one
carbon and the other ethene carbon are substituted by two
phenyl rings.
Scheme 1 Synthetic routes to compounds 1–5.
141.16, 140.82, 135.33, 134.46, 131.46, 131.42, 131.27, 130.69,
128.97, 128.56, 128.40, 128.17, 127.86, 127.63, 127.59, 126.87,
126.34, 126.31, 125.90, 28.35, 28.12. MALDI-FT MS: m/z 460.2
[M]+.
2.2.3 Synthesis of dis(4,5,9,10-tetrahydropyren-2-yl)-1,2-
diphenylethene (2). To a solution of 2,20-bis-(4,5,9,10-tetrahy-
dropyrenyl)ketone (0.62 g, 2 mmol), zinc dust (0.26 g, 4 mmol)
in 80 mL dry THF was added dropwise titanium(IV) chloride (0.4
mL, 2 mmol) under nitrogen at ꢀ78 ꢁC. Aer stirring for 20 min,
the reaction mixture was warmed to room temperature and then
heated to reux for 12 h. The reaction mixture was cooled to
room temperature and poured into water. The organic layer was
extracted with dichloromethane and the combined organic
layers were washed with saturated brine solution and water, and
dried over anhydrous magnesium sulfate. Aer ltration and
solvent evaporation, the residue was puried by silica-gel
column chromatography using hexane/dichloromethane as
eluent. Green solid of compound 2 was obtained in 62% yield.
1H NMR (400 MHz, CDCl3), d (TMS, ppm): 7.23–7.02 (m, 16H),
6.80 (s, 4H), 2.88–2.79 (m, 4H), 2.74–2.64 (m, 4H). 13C NMR (400
MHz, CDCl3): 143.96, 142.57, 142.45, 140.98, 140.81, 135.30,
134.38, 131.50, 131.35, 130.70, 129.00, 128.85, 128.78, 128.71,
127.53, 126.79, 126.26, 125.86, 28.33, 28.09. MALDI-FT MS: m/z
588.3 [M]+.
2.2.4 Synthesis of dis(4,5,9,10-tetrahydropyren-2-yl)-2,2-
diphenylethene (3). The procedure was analogous to that
described for compound 1. Pale yellow of compound 3 was
obtained in 34% yield. 1H NMR (400 MHz, CDCl3), d (TMS,
ppm): 7.21–6.99 (m, 16H), 6.80 (s, 4H), 2.88–2.78 (m, 4H), 2.74–
2.64 (m, 4H). 13C NMR (400 MHz, CDCl3): 144.08, 143.48,
142.41, 141.30, 140.63, 135.30, 134.36, 131.30, 130.68, 129.00,
128.80, 128.53, 128.14, 127.52, 126.81, 126.19, 125.88, 28.34,
28.10. MALDI-FT MS: m/z 588.3 [M]+.
3.2 Optical properties
The aggregation-induced emissions of compounds 1–4 were
studied by monitoring the change in photoluminescence
intensity with various composition of the THF/water mixture,
which shows that all the ve compounds are AIE active (the
result of compound 5 see ref. 30). Taking compound 1 as an
example (Fig. 1), the emissions are really weak when the water
contents are lower than 80%, because the intramolecular rota-
tion deactivates the excited state energy via a nonradiative decay
pathway.17,36 Increasing the fraction of water to 80%, a peak
2.2.5 Synthesis
of
tris(4,5,9,10-tetrahydropyren-2-yl)
phenylethene (4). The procedure was analogous to that
described for compound 2. Yellow-green powder of compound 4
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Fig. 1 PL spectra of compounds 1 (a), 2 (b), 3 (c) and 4 (d) in THF/water mixtures with different water fraction (fw). Excitation wavelength: 360 nm.
located around 484 nm appears in the spectrum, and the the ve compounds can be obtained basically with the order of
emission intensity of this peak increase with the increasing 5 > 4 > 2 > 3 > 1. This order is consistent with that reected by
water fraction until the 99% water content reaches the strongest the optical band gaps which are derived from the onset of the
emission. As a poor solvent, the addition of water into the THF absorption spectra for the ve compounds (Table 1). The red
solution induce the aggregation of the molecules, and the shi of the UV spectra arising from the increasing number of
strong intermolecular interaction in aggregation can restrict the 4,5,9,10-tetrahydropyrene units indicates that molecules having
rotation of the aromatic rings, thus resulting in the strong more 4,5,9,10-tetrahydropyrene units would possess a much
emission. For the other compounds, same phenomenon can be stronger p-conjugation system. In addition, compared with
observed when water was added into the THF solution. But compound 3, compound 2 has a red-shied absorption, which
compounds 2–5 show a decrease of emission efficiency in high suggests that though both of the two compounds have two
water fraction, which has been well interpreted by many papers 4,5,9,10-tetrahydropyrene moiety, the structure of compound 3
and it is ascribed to the formation of larger particles.37
is not as effective as that of compound 2 to form a p-conjuga-
The UV-Vis spectra of the ve compounds in THF solution tion. As typical AIE compounds, these luminogens are weak
are shown in Fig. 2a. Absorption maxima of compounds 1, 2 and emitters in dilute phase, so their emission behaviors in solution
5 are observed at 334, 349 and 370 nm, respectively. Though were not investigated in detail. However, different from the
accurate values of absorption maxima for compounds 3 and 4 weak emission in solution, the solid lms of these compounds
are not so obvious, the tendency of the absorption maxima for exhibit intensive emissions. As shown in Fig. 2b, the
Fig. 2 (a) UV-Vis spectrums of compounds 1–5 in THF solutions. (b) Photoluminescence spectrums of compounds 1–5 in films.
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Table 1 Optical and electrochemical properties of compounds 1–5
HOMOc (eV)
Expt.
LUMO (eV)
Expt.
Egd (eV)
labs/nm
lem/nm
FF (%)
ox
onset
Compound
Solutiona
Filmb
Film
E
Calc.
Calc.
Expt.
Calc.
1
2
3
4
5
334
349
345
355
370
480
495
491
509
517
17.2%
62.6%
29.3%
52.4%
74.1%
1.16
1.03
1.04
0.93
0.82
ꢀ5.48
ꢀ5.35
ꢀ5.36
ꢀ5.25
ꢀ5.19
ꢀ5.122
ꢀ5.003
ꢀ5.005
ꢀ4.907
ꢀ4.823
ꢀ2.33
ꢀ2.41
ꢀ2.36
ꢀ2.46
ꢀ2.61
ꢀ1.299
ꢀ1.361
ꢀ1.356
ꢀ1.415
ꢀ1.435
3.15
2.94
3.00
2.79
2.58
3.823
3.642
3.649
3.492
3.388
a
1 ꢂ 10ꢀ5 M in THF solution. b Film drop-casted on quartz plate. c HOMO levels of compounds 1–4 were determined using the following equations:
HOMO ¼ ꢀe(Eonset ꢀ 0.48 V) ꢀ 4.8 eV, where the value 0.48 V is for FOC vs. SCE electrode. d Estimated from the onset of the absorption spectra:
1240/lonest
.
corresponding emission peaks of compounds 1–5 appear at exhibit a longer uorescence lifetimes. In addition, the uo-
480, 495, 491, 509 and 517 nm, respectively. It is noteworthy that rescence lifetimes of the compounds in THF solution were also
the order of the emission wavelength is in good agreement with measured, but no signal can be detected because of their AIE
that of the absorption maxima, which further reveals the p- properties.
conjugation rule of the ve compounds. Meanwhile, though
some conformation change is possible due to the intermolec-
ular interactions in aggregation, the emission of the lms also
indicates that the order of p-conjugation degree in lm is
similar with that in solution, which is very important to analyze
their solid state emission in the following section.
3.3 Electrochemical studies
The electrochemical properties of the ve compounds were
studied by cyclic voltammetry (CV) in anhydrous dichloro-
methane, and corresponding data has been summarized in
Table 1. The values of onset oxidation potentials (E
ox
onset
) of the
So far, it has been proved that all the ve compounds are AIE
active and thus their intramolecular rotation can be restricted
effectively in solid state. Moreover, the twisted structures of the
ve compounds and the special structure of their 4,5,9,10-tet-
rahydropyrene moiety can avoid the formation of p–p interac-
tions, which can be veried by the crystal packing structure
discussed in later section. Therefore, it is reasonable to specu-
late that the emission efficiency of the ve compounds in solid
state would be decided by their p-conjugation with the order of
5 > 4 > 2 > 3 > 1.28 However, the real uorescence quantum yields
of their solid lms which determined by using an integrating
sphere don't obey this order and their values are 17.2%, 62.6%,
29.3%, 52.4% and 74.1% for compounds 1–5, respectively. We
can nd that though the p-conjugation of compounds 2 is not
as good as compound 4, it has a higher value of uorescence
quantum yield in solid state. This problem is worthy to be
studied because it ishelpful to reveal the mechanism of the
solid-state emission and can provide reference for designing
new solid-state emitter.
In order to have a deeper understanding of emission, the
uorescence lifetimes of the ve compounds were also
measured and the results are listed in Table 2. All these
compounds show two different lifetimes, which indicates that
there are two relaxation pathways in the process of their uo-
rescence decays. Therefore, it can be concluded that the emis-
sions of the compounds arise from two kinds of segments with
different p-conjugation lengths. For compounds 1, 2 and 5, the
pathway of s2 accounts for a larger proportion in the decay of
the excited state. On the contrary, the compounds 3 and 4
mainly decay through the s1 pathway. The weighted mean life-
times of the ve compounds are 2.71, 3.18, 2.00, 2.68 and 3.72
ns, respectively. Except for compound 1, there is a rule that the
compounds which have higher emission efficiency tend to
ve compounds are 1.16, 1.03, 1.04, 0.93, 0.82 V, respectively,
based on which the HOMO energy levels of these compounds
have been calculated with the values of ꢀ5.48, ꢀ5.35, ꢀ5.36,
ꢀ5.25 and ꢀ5.19 eV. Though these HOMO values are slight
lower than those obtained by theoretical calculation, they still
can reect the information of p-conjugation of the ve
compounds with the order of 5 > 4 > 2 > 3 > 1 which is identical
with the conclusion mentioned in the section of optical prop-
erties. The band gaps calculated based on the absorption edge
in the absorption spectra are somewhat higher than those
estimated by theoretical calculation, and their values are 3.15,
2.94, 3.00, 2.79 and 2.58 eV, respectively. The LUMO energy
levels of the ve compounds can be estimated by subtraction of
the optical band gap energies from the HOMO energy levels
with the values of ꢀ2.33, ꢀ2.41, ꢀ2.36, ꢀ2.46, and ꢀ2.61 eV,
respectively.
3.4 Theoretical calculation
The optical properties and electrochemical properties have
disclosed the p-conjugation regularity of the compounds, and it
is well known that the electronic structures can be employed to
analyze the properties from a theoretical level. Therefore,
Table 2 Fluorescence decay parameters of compounds 1–5
Compound
s1 (ns)
A1
s2 (ns)
A2
hsi (ns)
1
2
3
4
5
1.38
1.43
1.47
1.65
1.48
0.46
0.41
0.81
0.59
0.36
3.85
4.40
4.27
4.15
4.98
0.54
0.59
0.19
0.41
0.64
2.71
3.18
2.00
2.68
3.72
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density functional theory (DFT) calculations for the ve emission behaviors of compounds in solid state. In the crystal
compounds were carried out using a suite of Gaussian 09 structure of compound 1 (Fig. 4a), the centroid distance of two
program,38 and the nonlocal density functional of B3LYP with 6- phenyl rings in the 4,5,9,10-tetrahydropyrene moiety which
31G(d) basis sets was used for the calculation. Fig. 3 displays the belong to two adjacent molecules (the central two molecules in
˚
optimized molecular structures and orbital amplitude plots of the gure), is 5.48 A, which is much longer than the distance of
HOMO and LUMO levels for compounds 1–4. It can be seen typical p–p interactions. Except for these two phenyl rings,
from the orbital amplitude plots that the HOMO and LUMO of other aromatic rings are not likely to form p–p stacking.
ve compounds (the result of compound 5 see ref. 30) are Therefore, we can draw a conclusion that p–p interactions are
dominated by the orbitals from the aromatic rings as well as the successfully avoided in the crystal structure of compound 1.
central double carbon bond, indicating that the absorption and Besides, the shortest CH/centroid distances observed in the
˚
emission behaviors are associated with the p–p transitions and crystal are 3.64 and 3.89 A, which means that no strong C–H/p
exciton decays of the whole molecules. In particular, both of the hydrogen bonds form in the crystal structure.39,40 Fig. 4b is the
two phenyl rings in 4,5,9,10-tetrahydropyrene moiety contribute crystal structure of compound 4. The packing of the molecules
to the formation of the orbitals, which means that a 4,5,9,10- in the crystal structure are so loose that no potential p–p and C–
tetrahydropyrene can introduce larger p-conjugation than H/p interactions can be observed. However, it has been re-
a phenyl moiety, and this result can be used to explain why the ported that C–H/p hydrogen bonds can rigidify the molecular
conjugation of the compounds increase with the number of conformation and are benecial to the emission of
4,5,9,10-tetrahydropyrene moiety.
compounds.41 Considering the fact that compound 4 is AIE
active, the rotation of the aromatic rings in solid state should be
restricted by other weak intermolecular interactions such as van
der Waals forces. But this kind of interaction is not enough
strong to completely constrain the weak motion of the mole-
cules like vibration which also can consume the excited energy
in a non-radiative pathway. This may be the reason to explain
why the emission efficiency of compound 4 is lower than that of
compound 2.
Analyzing more crystal structures of TPE-based compounds,
further evidence can be found to reveal the relationship
between the degree of packing and their solid-state emission. In
contrast with TPE, compound 1 has a much expanded p-
3.5 Crystal structure
It is obvious that from the view of individual molecules, no
reasonable interpretation can be found out to explain why the
uorescence quantum yields of the compounds don't have the
same tendency with their p-conjugation. Fortunately, single
crystals of compounds 1 and 4 as well as the reported
compound 5 were grown from dichloromethane-methanol and
analyzed by X-ray diffraction crystallography (Table S1†). The
crystal structures of compounds can offer the information of
intermolecular interactions, which are crucial to study the
Fig. 3 Optimized molecular structures, and molecular orbital amplitude plots of HOMO and LUMO of compounds 1–4.
15178 | RSC Adv., 2018, 8, 15173–15180
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Fig. 4 Packing structures of compound 1 (a), compound 4 (b), and compound 5 (c).
conjugation system, whereas its solid-state uorescence cannot completely constrain other weakly non-radiative
quantum yield (17.2%) is much lower than that of TPE whose pathway such as vibration. Therefore, not only the properties
value is 49.2%.29 From the view of packing, TPE has a much of individual molecules but also their aggregates can affect the
tighter stack in its crystal structure in which multiple C–H/p emission of compounds in solid state, and these two aspects
hydrogen bonds with short distances such as 3.047, 2.859 and need to be considered systematically when design new solid-
15
˚
2.826 A can be observed. It is noteworthy that similar with state emitters.
TPE, molecules of compound 5 in crystal state are also tightly
stacked and numerous C–H/p interactions with distance of
Conflicts of interest
˚
3.20 and 3.34 A are formed (Fig. 4c). Consequently, compound 5
also has good performance in its solid-state emission with
74.07% uorescence quantum yield. Among these compounds,
why TPE and compound 5 have a much tighter packing than
others? Chemical structure of compounds is their intrinsic
characteristic and is the decided factor for the packing. In TPE
and compound 5, the ethenes are substituted by four same
aromatic rings and thus they have a highly symmetrical struc-
ture, which should be more likely to form tightly packing. As for
the higher emission efficiency of compound 2, though the
crystal of compound 2 was failed to be obtained, it can be
speculated that in solid state of compound 2, a much tighter
packing may be formed compared with compound 4 because of
its relatively symmetrical structure.
There are no conicts to declare.
Acknowledgements
This work was supported by the Fundamental Research Funds
for the Central Universities (2015XKZD08) and the Priority
Academic Program Development of Jiangsu Higher Education
Institutions.
References
1 G. M. Farinola and R. Ragni, Chem. Soc. Rev., 2011, 40, 3467.
2 J. Roncali, P. Leriche and A. Cravino, Adv. Mater., 2007, 19,
2045.
3 V. S. Padalkar and S. Seki, Chem. Soc. Rev., 2016, 45, 169.
4 Z. Q. Xie, B. Yang, F. Li, G. Cheng, L. L. Liu, G. D. Yang, H. Xu,
L. Ye, M. Hanif, S. Y. Liu, D. G. Ma and Y. G. Ma, J. Am. Chem.
Soc., 2005, 127, 14152.
5 A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz and
A. B. Holmes, Chem. Rev., 2009, 109, 897.
6 J. Z. Liu, J. W. Y. Lam and B. Z. Tang, Chem. Rev., 2009, 109,
5799.
7 T. P. I. Saragi, T. Spehr, A. Siebert, T. Fuhrmann-Lieker and
J. Salbeck, Chem. Rev., 2007, 107, 1011.
4. Conclusions
In conclusion, we have successfully synthesized a series of tet-
raarylethenes based on 4,5,9,10-tetrahydropyrene. These
compounds are weakly uorescent in solutions, but become
strong emitters when the aggregations were formed, revealing
distinct aggregation-induced emission characteristics. From
the view of p-conjugation, corresponding characterizations
including absorption and emission spectra, electrochemistry,
and theoretical calculation support that the increase of 4,5,9,10-
tetrahydropyrene moieties in compounds lead to the improve-
ment of the p-conjugation with the order of 5 > 4 > 2 > 3 > 1.
However, the uorescence quantum yield of the compounds in
lms not t the same order with the degree p-conjugation, and
the value of compound 4 is not as high as that for compound 2.
8 J. Wang, Y. F. Zhao, C. D. Dou, H. Sun, P. Xu, K. Q. Ye,
J. Y. Zhang, S. M. Jiang, F. Li and Y. Wang, J. Phys. Chem.
B, 2007, 111, 5082.
´
9 S. Hecht and J. M. J. Frechet, Angew. Chem., Int. Ed., 2001, 40,
74.
A reasonable interpretation can be found in the crystal structure 10 B. T. Nguyen, J. E. Gautrot, C. Ji, P. L. Brunner, M. T. Nguyen
of compound 4. Compared with other compounds, its crystal and X. X. Zhu, Langmuir, 2006, 22, 4799.
structure has a much looser packing. We speculated that the 11 J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam and
loose packing can restrict the rotation of the aromatic rings but
B. Z. Tang, Chem. Rev., 2015, 115, 11718.
This journal is © The Royal Society of Chemistry 2018
RSC Adv., 2018, 8, 15173–15180 | 15179

View Article Online
RSC Advances
Paper
12 H. Wang, E. G. Zhao, J. W. Y. Lam and B. Z. Tang, Mater. 26 Y. Liu, X. Ye, G. F. Liu, Y. Lv, X. Y. Zhang, S. M. Chen,
Today, 2015, 18, 365.
J. W. Y. Lam, H. S. Kwork, X. T. Tao and B. Z. Tang, J.
Mater. Chem. C, 2014, 2, 1004.
27 T. F. Zhang, R. Zhang, Y. Zhao and Z. H. Ni, Dyes Pigm., 2018,
148, 276.
28 Y. Yamaguchi, Y. Matsubara, T. Ochi, T. Wakamiya and
Z.-I. Yoshida, J. Am. Chem. Soc., 2008, 130, 13867.
29 J. Zhou, Z. F. Chang, Y. B. Jiang, B. R. He, M. Du, P. Lu,
Y. N. Hong, H. S. Kwork, A. J. Qin, H. Y. Qiu, Z. J. Zhao
and B. Z. Tang, Chem. Commun., 2013, 49, 2491.
13 J. Mei, Y. N. Hong, J. W. Y. Lam, A. J. Qin, Y. H. Tang and
B. Z. Tang, Adv. Mater., 2014, 26, 5429.
14 R. R. Hu, N. L. C. Leung and B. Z. Tang, Chem. Soc. Rev.,
2014, 43, 4494.
15 Y. N. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev.,
2011, 40, 5361.
16 Y. N. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun.,
2009, 4332.
17 J. D. Luo, Z. L. Xie, J. W. Y. Lam, L. Cheng, H. Y. Chen, 30 Z. M. Zhang, Y. Zhao, R. Zhang, L. F. Zhang, W. Q. Cheng
C. F. Qiu, H. S. Kwok, X. W. Zhan, Y. Q. Liu, D. B. Zhu and
B. Z. Tang, Chem. Commun., 2001, 1740.
and Z. H. Ni, Dyes Pigm., 2015, 118, 95.
31 Z. M. Zhang, F. F. Han, R. Zhang, N. Li and Z. H. Ni,
Tetrahedron Lett., 2016, 57, 1917.
¨
18 Y. N. Hong, M. Haubler, J. W. Y. Lam, Z. Li, K. K. Sin,
Y. Q. Dong, H. Tong, J. Z. Liu, A. J. Qin, R. Renneberg and 32 S. Suzuki, T. Takeda, M. Kuratsu, M. Kozaki, K. Sato,
B. Z. Tang, Chem.–Eur. J., 2008, 14, 6428. D. Shiomi, T. Takui and K. Okada, Org. Lett., 2009, 11, 2816.
19 Y. Liu, Y. H. Tang, N. N. Barashkov, I. S. Irgibaeva, 33 M. Banerjee, S. J. Emond, S. V. Lindeman and R. Rathore, J.
J. W. Y. Lam, R. R. Hu, D. Birimzhanova, Y. Yu and
Org. Chem., 2007, 72, 8054.
B. Z. Tang, J. Am. Chem. Soc., 2010, 132, 13951.
34 J. E. McMurry, Chem. Rev., 1989, 89, 1513.
20 H. P. Shi, S. J. Wang, L. Fang and B. Z. Tang, Tetrahedron 35 J. E. McMurry and M. P. Felming, J. Am. Chem. Soc., 1974, 96,
Lett., 2016, 57, 4428. 4708.
21 Z. J. Zhao, S. M. Chen, J. W. Y. Lam, Z. M. Wang, P. Lu, 36 N. W. Tseng, J. Liu, J. C. Y. Ng, J. W. Y. Lam, H. H. Y. Sung,
F. Mahtab, H. H. Y. Sung, L. D. Williams, Y. G. Ma, I. D. Williams and B. Z. Tang, Chem. Sci., 2012, 3, 493.
H. S. Kwork and B. Z. Tang, J. Mater. Chem., 2011, 21, 7210. 37 X. Y. Shen, Y. J. Wang, E. G. Zhao, W. Z. Yuan, Y. Liu, P. Lu,
22 N. Xie, Y. Liu, R. R. Hu, N. L. C. Leung, M. Arseneault and
A. J. Qin, Y. G. Ma, J. Z. Sun and B. Z. Tang, J. Phys. Chem. C,
2013, 117, 7334.
B. Z. Tang, Isr. J. Chem., 2014, 54, 958.
23 Y. Liu, Y. Lv, X. Y. Zhang, S. M. Chen, J. W. Y. Lam, P. Lu, 38 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
R. T. K. Kwork, H. S. Kwork, X. T. Tao and B. Z. Tang,
Chem.–Asian J., 2012, 7, 2424.
M. A. Robb and J. R. Cheeseman, et al., Gaussian 09,
revision A.02, Gaussian, Inc., Wallingford CT, 2009.
¨
24 D. Jana, S. Boxi and B. K. Ghorai, Dyes Pigm., 2013, 99, 740. 39 M. Brandl, M. S. Weiss, A. Jabs, J. Suhnel and R. Hilgenfeld,
25 W. Z. Wang, T. T. Lin, M. Wang, T. X. Liu, L. L. Ren, D. Chen
J. Mol. Biol., 2001, 307, 357.
and S. Huang, J. Phys. Chem. B, 2010, 114, 5983.
40 M. Goel and M. Jayakannan, Chem.–Eur. J., 2012, 18, 2867.
41 E. Gagnon, T. Maris, P. M. Arseneault, K. E. Maly and
J. D. Wuest, Cryst. Growth Des., 2009, 10, 648.
15180 | RSC Adv., 2018, 8, 15173–15180
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