Axial and peripheral tetraarylethylene-modified subphthalocyanines with distinctive fluorescent performances

Article abstract of DOI:10.1016/j.tetlet.2021.153096

Special aromatic structure and unique geometric characteristics make subphthalocyanines possess distinctive electronic structures and physicochemical properties. In this paper, tetraarylethylenes with aggregation-induced emission were introduced to subphthalocyanine macrocycles at the axial direction and the periphery to improve the fluorescence emission properties. Results show that the modification at the two different positions of the subphthalocyanines has different effects on regulating the fluorescence performances. The subphthalocyanine modified axially by tetraphenylethylene shows outstanding fluorescence resonance energy transfer (FRET) phenomenon, and the modification on the periphery of subphthalocyanine is conducive to enhance the fluorescence intensity. These distinctive performances have the potential applications in fluorescence sensor and probe.

Full text of DOI:10.1016/j.tetlet.2021.153096

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Axial and peripheral tetraarylethylene-modified subphthalocyanines
with distinctive fluorescent performances
⇑
Wei Ding, Liying Yan, Fei Cao, Qianfu Luo
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science & Technology,
Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Special aromatic structure and unique geometric characteristics make subphthalocyanines possess dis-
tinctive electronic structures and physicochemical properties. In this paper, tetraarylethylenes with
aggregation-induced emission were introduced to subphthalocyanine macrocycles at the axial direction
and the periphery to improve the fluorescence emission properties. Results show that the modification at
the two different positions of the subphthalocyanines has different effects on regulating the fluorescence
performances. The subphthalocyanine modified axially by tetraphenylethylene shows outstanding fluo-
rescence resonance energy transfer (FRET) phenomenon, and the modification on the periphery of subph-
thalocyanine is conducive to enhance the fluorescence intensity. These distinctive performances have the
potential applications in fluorescence sensor and probe.
Received 20 January 2021
Revised 8 April 2021
Accepted 14 April 2021
Available online xxxx
Keywords:
Subphthalocyanine
AIE
Fluorescence
FRET
Ó 2021 Elsevier Ltd. All rights reserved.
Introduction
of subphthalocyanine according to the reported literature, which
makes them have the possibility of fluorescence resonance energy
Subphthalocyanine with
a
highly polarized
p
-conjugated
transfer (FRET) [25]. Recently, a considerable number of controlled
AIE materials containing AIE donors and tunable acceptors have
been reported, which could produce energy transfer reaction utiliz-
ing aggregation-induced emission [26–32]. Inspired by the AIE and
FRET mechanism, herein we try to introduce tetraphenylethylene
functional groups at the different positions of subphthalocyanine
to regulate its fluorescent performances in solid or aggregation
state, and the target compounds SubPc-1 and SubPc-2 have been
obtained by modifying at the axial and peripheral positions of
the subphthalocyanines. The structures of the new subphthalocya-
nines were characterized by 1H NMR, 13C NMR, and mass spec-
trometry, and the distinctive fluorescent properties were studied
in detail.
system and noncentrosymmetric cone-shaped structure has
become a promising building blocks for studying nonlinear optical
materials in recent years [1–3]. It has electron-rich feature and
strong absorption and emission in the visible light region, so it is
expected to be widely applied in the fields of organic semiconduc-
tors [4], solar cell [5], sensors and optical information storage
media [6,7]. At the same time, it has also been used as an excellent
building unit for asymmetric phthalocyanine synthesis and cat-
alytic degradation because of the characteristic of easy ring open-
ing and the twisted structure [8–19]. Moreover, some
subphthalocyanine derivatives are considered as promising fluo-
rescent probes, which emit strong near infrared fluorescence in
solution. However, with the increasing of the concentration in
solution, especially in the solid state and aggregation state, it often
shows aggregation-induced quenching (ACQ), which undoubtedly
limits its practical application. In order to overcome the ACQ effect,
it is an effective protocol to incorporate fluorescent functional
groups with aggregation-induced emission (AIE), such as tetraary-
lethylene, which is one of the outstanding representatives of
luminogenic materials with aggregation-induced emission
[20–24]. Moreover, there is significant overlap between the emis-
sion spectrum of tetraphenylethylene and the excitation spectrum
Results and discussion
Synthesis of axial tetraphenylethylene-substituted SubPc-1 and
peripheral substituted SubPc-2
The synthetic routes of the two target compounds were sum-
marized in scheme 1. Intermediate SubPc-0 and tetraphenylethy-
lene derivatives 1 and 2 were firstly prepared according to
literature procedures [33–35]. With them in hand, SubPc-1 was
then synthesized by the axial substitution reaction between the
hydroxyl-tetraphenylethylene 1 and subphthalocyanine SubPc-0
⇑
Corresponding author.
https://doi.org/10.1016/j.tetlet.2021.153096
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.
Please cite this article as: W. Ding, L. Yan, F. Cao et al., Axial and peripheral tetraarylethylene-modified subphthalocyanines with distinctive fluorescent
performances, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2021.153096

W. Ding, L. Yan, F. Cao et al.
Tetrahedron Letters xxx (xxxx) xxx
and its fluorescence performances caused by fluorescence reso-
nance energy transfer were further analyzed.
Fig. 1a shows a normalized comparison between the emission
spectrum of tetraphenylethylene 1 and the excitation spectrum
of subphthalocyanine precursor SubPc-0, from which we can find
there is obvious overlap. According to the basic principle of FRET,
in this case, the emission light generated by the excitation of the
tetraphenylethylene can be further used as an excitation light to
excite the fluorescence of the subphthalocyanine unit. It means
that if the two structural units are connected in a non-conjugated
way in a system and the distance is appropriate, the two units may
undergo this kind of energy transfer (Fig. 1b).
Because the absorptions of tetraphenylethylenes and subph-
thalocyanines are often strong at the B band but weak at Q band
around 480 nm (Fig. S1), 270 nm and 480 nm light are selected
as excitation light to observe and compare the different changes
in fluorescence between the unsubstituted subphthalocyanine
intermediate SubPc-0 and the target subphthalocyanine axially
substituted with tetraphenylethylene SubPc-1. Result shows that
substituted SubPc-1 performs remarkable fluorescence resonance
energy transfer as expected.
We first studied the fluorescence emission behavior of reference
compound, SubPc-0. Fig. 2 shows the changes in fluorescence
intensity of the unsubstituted subphthalocyanine precursor
SubPc-0 with different water content in THF solution. Under the
excitation of 480 nm light source, the THF solution of subphthalo-
cyanine SubPc-0 emitted significant fluorescence at around
610 nm, but the fluorescence intensity reduced slowly after adding
water-the poor solvent for subphthalocyanine. When the water
content was increased to 60%, the fluorescence was greatly
quenched. The fluorescence could hardly detect when the water
content was increased to 90%. It was a typical aggregation-induced
quenching (ACQ) phenomenon.
Scheme 1. The synthesis of the tetraarylethylene-modified subphthalocyanines.
in 1-chlorobenzene under high temperature conditions. SubPc-2
was obtained by the cyclization of tetraphenylethylene-dinitrile
derivative 2 using boron trichloride as a template in xylene solvent.
The synthesis and characterization of all the intermediates were
described in detail in the Supporting Information.
Then 270 nm was also chosen as excitation wavelength for
comparison. It was found that the fluorescence of the subphthalo-
cyanine SubPc-0 was very weak even in the 100% THF solvent. As
the water content increased, the fluorescence intensity of the sub-
phthalocyanine also reduced to almost 0 (Fig. S2).
Comparing the fluorescence emission of the unsubstituted sub-
phthalocyanine SubPc-0 under the excitation of these two differ-
ent light sources (480 nm and 270 nm), it can be found that in
the neat THF solution, the fluorescence emission intensity
ACQ and AIEE properties of the titled compounds
FRET (fluorescence resonance energy transfer) is a photophysi-
cal process in which the excited fluorescence donor transfers its
excitation energy to the acceptor through long-distance dipole–
dipole interaction. In order to apply the FRET mechanism to the
intelligent regulation of fluorescent molecules, we designed and
prepared the compound SubPc-1 by axially linking subphthalocya-
nine SubPc-0 with tetraphenylethylene 1 through an oxygen atom,
Fig. 1. (a) Overlap between the fluorescence spectrum of tetraphenylethylene 1 (ꢀꢀꢀ) in the aggregate state and the excitation spectrum of subphthalocyanine SubPc-0 (—)
in THF; (b) The schematic diagram of FRET process in SubPc-1.
2

W. Ding, L. Yan, F. Cao et al.
Tetrahedron Letters xxx (xxxx) xxx
Fig. 2. The fluorescence spectra of SubPc-0/ THF solution (5 ꢁ 10-6 M) with
different water contents (kex = 480 nm).
generated by excitation at 480 nm is significantly higher than that
generated by excitation at 270 nm. It shows that ultraviolet light
(270 nm) as the subphthalocyanine excitation light source is obvi-
ously not as efficient as the light source in the visible light region
(480 nm), and in comparison, it can even be ignored.
When axial tetraphenylethylene-substituted SubPc-1 was
selected to study the fluorescence emission under the two excita-
tion wavelengths, it showed totally different phenomenon from
aforementioned SubPc-0. In the THF solution, the fluorescence
intensity of the compound SubPc-1 is weaker than that of
SubPc-0 under the exciting light of 270 nm and 480 nm. It may
be because the axial substituent of SubPc-1 caused the molecular
structure looser and increased the chances to dissipate excited
state energy through intra-molecular rotation.
However, when adding water to the THF solution, the fluores-
cence emission of SubPc-1 (excited by the 270 nm light) continu-
ously enhanced with the increase of water content, as shown in
Fig. 3a. This behavior is contrary to that under the excitation of
480 nm light. It showed obviously fluorescence quenching upon
excitation with the light of 480 nm when adding water to the
THF solution of SubPc-1 (Fig. 3b).
Fig. 3. The fluorescence spectra of SubPc-1 / THF solution (5 ꢁ 10-6 M) with
different water content under 270 nm (a) and 480 nm (b) excitation.
In order to further investigate the effect of the substituent of
tetraphenylethylene at different positions, three tetraphenyl-
ethylenes were bonded to the peripheral of the subphthalocyanine
to obtain SubPc-2. Its fluorescent behavior was also examined and
compared with unsubstituted SubPc-0.
Results show that the fluorescence behavior of SubPc-2 is con-
sistent with that of SubPc-0 in the H2O-THF mixed solution. It also
shows obvious fluorescence quenching with the increase of water
content (Fig. 4). However, in the neat THF solution (5 ꢁ 10-6 M),
it is found that the peripherally substituted SubPc-2 has a signifi-
cant increase in fluorescence intensity compared with SubPc-0.
Through the comparative study of the ultraviolet absorption spec-
tra (Fig. S6), it is found that the absorption peak of the compound
SubPc-2 in the ultraviolet region has significantly enhanced
compared with that of the SubPc-0 and the overall red shift phe-
nomenon has occurred, which indicates that the tetraphenylethy-
lene units modified on the periphery of SubPc-2 have further
Judging from the fluorescence quenching behavior of unsubsti-
tuted SubPc-0 both at the excitation of 480 nm and 270 nm, even if
the axial substitution in SubPc-1 might increase the inter-molecu-
lar spacing and weaken the degree of aggregation, the fluorescence
quenching is unavoidable when excited by 480 nm light, as shown
in Fig. 3b. Therefore, the fluorescence enhancement of SubPc-1
under the excitation of 270 nm light should be attributed to the
FRET. It is to say that, with the excitation of 270 nm light, the axial
unconjugated tetraphenylethylene unit in SubPc-1 can emit fluo-
rescence at the range of 300–600 nm, which can be used as the
exciting light for subphthalocyanine unit (Fig. 1b). It is well known
that tetraphenylethylene shows prominent aggregation-induced
luminescence at around 300–600 nm [24,36–38]. With the adding
of water content and the formation of the aggregation state, the
fluorescence emitted by the tetraphenylethylene unit increases
significantly (Fig. S3), which means that the intensity of the effi-
cient exciting light around 480 nm for subphthalocyanine will
enhance accordingly. Therefore, compound SubPc-1 exhibits the
distinctive aggregation-induced emission enhancement (AIEE)
phenomenon through the action of fluorescence resonance energy
transfer (Fig. S4). The excitation spectra of two target compounds
with different water contents also confirmed the occurrence of
energy transfer (Fig. S5).
increased the degree of
p-p conjugation. The possible reason for
this fluorescence enhancement phenomenon may be due to the
increase in molecular conjugation, which increases the probability
of
p*?p transition between the lowest excited singlet state (S1)
and the ground state (S0). This experimental result shows that
the modification with tetraphenylethylene on the periphery of
the subphthalocyanine can enhance efficiently the fluorescence
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W. Ding, L. Yan, F. Cao et al.
Tetrahedron Letters xxx (xxxx) xxx
and fluorescence maximum emission wavelength of the compound
SubPc-1 had little difference from that of SubPc-0. However, the
fluorescence emission of SubPc-1 reduced, and the quantum yield
was only 0.6%. The reason might be that the spatial structure of the
subphthalocyanine after the axial substitution became looser, and
the degree of freedom of the molecule increased, so that the energy
of the excited state could be released through the movement of the
molecule to undergo a non-radiative transition back to the ground
state. When the periphery of SubPc-0 is modified with
tetraphenylethylenes, SubPc-2 has an obvious red shift in the max-
imum absorption and emission wavelengths due to the increase in
molecular conjugation and electron cloud density, and the fluores-
cence quantum yield reaches 13.7%, which shows significant
enhancement. These data indicate that the modification of AIE
units such as tetraphenylethene on the different position of subph-
thalocyanine is closely related to their photophysical properties.
Conclusions
Fig. 4. The fluorescence spectra of SubPc-2/THF solution (5 ꢁ 10-6 M) with different
water content under the exciting light of 480 nm.
In this paper, two novel subphthalocyanines, SubPc-1 and
SubPc-2, were constructed by modifying tetraarylethylenes at
the axial and peripheral positions, respectively. Their fluorescent
performances at different conditions were focused on. The experi-
mental results showed that the subphthalocyanine SubPc-1 with
the axial substituent of aggregation-induced emission (AIE) exhib-
ited typical aggregation-induced quenching (ACQ) phenomenon
when excited at 480 nm, but performed significant aggregation-
induced emission enhancement (AIEE) when the excitation wave-
length changed to the excitation wavelength of the tetraarylethy-
lene unit (270 nm), which was attributed to the fluorescence
resonance energy transfer. In addition, the peripherally modified
subphthalocyanine SubPc-2 did not exhibit any aggregation-
induced-emission properties, but the fluorescence intensity of
the whole molecule enhanced significantly in neat THF solution.
Such new subphthalocyanine molecules are expected to be used
in the fluorescence probe and fluorescence detection materials.
intensity of the subphthalocyanine molecule, but cannot overcome
the problem of fluorescence quenching in the aggregate state.
The fluorescence quantum yields of SubPc-1 and SubPc-2
In order to further quantitatively compare the fluorescence
changes before and after modification with tetraphenylethylenes
on the periphery and axial direction of the subphthalocyanines.
The ethanol solutions of SubPc-0, SubPc-1 and SubPc-2 were mea-
sured for fluorescence quantum yields with rhodamine 6G as a ref-
erence. The following Eq. (1) was used to calculate the fluorescence
quantum yield [39].
F ꢂ AStd ꢂ n2
£F ¼ £FðStdÞ
ð1Þ
2
FStd ꢂ A ꢂ nStd
Among them, £F(Std) is the standard fluorescence quantum
yield of the reference substance, and the reference substance here
is rhodamine 6G (£F = 0.94 in ethanol [40]). F and FStd represent
the integrated areas under the fluorescence emission curves of
the samples and the reference substance, respectively. A and AStd
are the corresponding absorbance of the sample and the reference
substance at their respective excitation wavelengths. n2 and
n2Std are the refractive index of the sample and the reference
substance in the solvent respectively. In order to simplify the
calculation, we adjusted the concentrations to make the absorption
curves of three target products and rhodamine 6G intersect at
about 488 nm, and used it as the excitation wavelength for the
measurement of fluorescence quantum yield.
The maximum absorption wavelength and maximum emission
wavelength of SubPc-0 in ethanol solution are 569 nm and
584 nm, respectively, and its fluorescence quantum yield is 2.2%,
as shown in Table 1. After it was modified with tetraphenylethy-
lene in the axial direction, the degree of molecular conjugation
did not change. Therefore, the maximum absorption wavelength
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work was supported by the Nature Science Foundation of
Shanghai (K100-2-13041). We are grateful to Prof. Yi Cheng from
East China University of Science & Technology for his valuable sup-
port on this project.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.153096.
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