A phenyl-removal strategy for accessing an efficient dual-state emitter in the red/NIR region guided by TDDFT calculations

Article abstract of DOI:10.1039/d0tc02596h

The development of molecules with efficient dual-state emission in the red/NIR region is significant for their broader application prospects in the biological field, but still remains a great challenge. Guided by in-depth TDDFT calculations, herein we fir

Full text of DOI:10.1039/d0tc02596h

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J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
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DOI: 10.1039/D0TC02596H
ARTICLE
A Phenyl-removed Strategy for Accessing Efficient Dual-state
Emitter at Red/NIR Region: Guided by TDDFT Calculations
a
b
b
b
a
a, b
Guomin Xia , Qingqing Shao , Kangli Liang , Yigang Wang , Lixia Jiang , Hongming Wang
*
Received 00th January 20xx,
Accepted 00th January 20xx
Developing molecules with efficient dual-state emission at red/NIR region is significant for their broader applications
prospect in biological area, but still remains a great challenge. Guided by in-depth TDDFT calculations, herein we firstly
demonstrate a phenyl-removed strategy in TPE-merged squaraine dyes for accessing intense red/NIR emission both in
solution and crystalline state, on the basis of full decoding the bidirectional TICT/ESIPT mechanism. With removing one
DOI: 10.1039/x0xx00000x
phenyl rotator in TPE segment, both less TICT formation and phenyl rotations facilitate the brightest red emission (λem
=
5
86/622 nm, ΦPL = 57.5%) of SQHTPE in solution, while the reserved hydrogen-bonding packing and ESIPT process guarantee
the intense red/NIR emission (λem = 670 nm, ΦPL = 51.2%) in its crystalline state. Moreover, efficient red bioimaging in
HUVECs has been achieved with naked SQHTPE, which provides a new vision for directly applying squaraine dyes in biological
applications.
Introduction
Most of the reported organic luminogens seems to be either
emissive in dilute solution but quenched in the aggregation or
emissive in the aggregation but quenched in dilute solution,
exhibit a short-wavelength emission in dual state due to their
decreased molecular conjugations and highly prevented
exciton interactions in twisted configurations. As such, a novel
strategy for accessing dual-state efficient luminogens at the
red/NIR region remains a huge challenge but highly desired,
1
,2
referred to as aggregation-caused quenching (ACQ) or
3
-6
aggregation-induced emission (AIE) effect. Yet despite all
that, organic materials with well-designed structures can be
highly emissive in both solution and solid state, which takes
advantage of both ACQ and AIE phenomenon thereby
2
1,22
especially in biologic area.
Squaraine dyes (SQs) have attracted much attention in
advanced fields due to its excellent photoelectrical properties,
7
-16
23-28
beneficial for broad practical applications.
015 Tang ’ s group accessed propeller-like triphenylamine
fluorophores with twisted triphenylethylene (HTPE, generally
For instance, in
but are still greatly limited by its typical ACQ behaviours.
2
Recently we have been devoted to the exploration of novel
SQs with solid-state emission and understanding of the
relationship among their molecular structures, self-assembled
1
7,18
regarded as AIE building block
) showing intense blue to
1
9
29-31
cyan emission in both solution and solid states. Very
recently, Zhu and co-workers developed planar diphenyl-
diacetylene structures with distorted cyanostilbenes and long
alkyl side chains featuring bright blue to green emission in
morphologies and photophysical properties.
From our
3
2
latest study on SQs with DSE behaviour, two interlaced
effects aroused us a great deal of confusion due to the
insertion of tetraphenylethylene (TPE) group into SQs: 1) on
the one hand the internal phenyl rotations in TPE segment was
regarded to facilitate the non-radiative decays, 2) while on the
other hand the suppressed twisted intramolecular charge
2
0
solution, amorphous and crystalline state.
In those
molecules, the delicate balance between intramolecular
rigidity and twisted conformations is regarded as the crucial
factor for accessing their highly efficient dual-state emission
3
3-37
transfer (TICT)
process on account of the big entire TPE
(
DSE), since the former is mentioned to restrict free
segment was regarded to facilitate radiative decays. These two
opposing results thus resulted in an ambiguous mechanism of
its laboratorial low emission efficiency (ΦPL) in dilute solution,
in spite of its desired high ΦPL through an exited-state
intramolecular motions in solution, while the latter is to resist
intermolecular π-π stacking in solid state. Upon this guidance,
it is very reasonable to speculate that those luminogens
3
8-41
intramolecular proton transfer (ESIPT)
channel in
crystalline state. As such, it is of great necessity and urgency to
further understand the two factors affecting emission in this
system. Guided by TDDFT calculations, we reported here a
phenyl-removed strategy in TPE-merged squaraine dyes for
accessing intense red/NIR emission in dual state. All the results
would be benefit for the design and controlling of efficient
DSE-active molecules in the near future.
a. Institute for Advanced Study and College of Chemistry， Nanchang University，
9
99, Xuefu Road, Nanchang, 330031, P.R. China.
b.
School of Materials Science and technology， Nanchang University， 999,
Xuefu Road, Nanchang, 330031, P.R. China
*
hongmingwang@ncu.edu.cn
Electronic Supplementary Information (ESI) available: synthesis, characterization,
calculations and crystal data. See DOI:10.1039/x0xx00000x
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PJ loe ua rsne a dl oo f nM o at t ea rdi aj ul ss Ct hm e am r gi si tnr ys C
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reversion energy barrier (ERTS2), which indicated a quickly
thermodynamic equilibrium with the minimum vi at us
V
b
ie
r
w
A
i
r
o
tic
n
le
s
O
thnl
ine
DOI: 10.1039/D0TC02596H
making SQHTPE possessed the strongest emission in TICT
states as well. In contrast, both low energy barrier (ETS2) and
relative energy (ES1P1) would cause easily an inverse TICT
process of much-anticipated SQTPE, which would greatly
increase vibrational relaxation thus resulting its low emission
in solution, even if taking no account of the internal phenyl
rotations in its TPE segment. In other words, it seems that the
solution ΦPL of SQHTPE would be much better than that of
SQTPE, together with its twisted HTPE segment for
suppressing π-π stacking and similar energy barriers of TS3 in
ESIPT channel (Fig. 1c, d), permitting SQHTPE a potential highly
efficient DSE-active molecule. Besides, the difference between
the emission band maxima in solution and solid state (define
as DSE shift) would be expected to become large from the
TDDFT calculations (Fig.S1).
These theoretical calculations encouraged us to
experimentally verify the phenyl-removed strategy in
brightening the emission and further helped us to thoroughly
evaluate the interlaced effects mentioned above. Therefore,
those target four analogues were elaborately synthesized, and
the overall synthetic routes of reaction intermediates and
desired derivatives were shown in detail in Scheme S1-5, with
1
13
fully characterized by H NMR,
C NMR and mass
spectroscopy (details in Supporting Information). After that,
their various emission behaviours and corresponding
mechanisms in solution and solid state were then discussed in
detail.
Fig. 1 a) Structures of SQTPE, SQHTPE, SQDP and SQP; b) Schematic illustrations of the
bidirectional TICT/ESIPT mechanism in different state; c) Reaction equation of ESIPT
process in these SQs through proton H transfers from N to O atom of central ring; d)
Computed relative energy of important stationary points on reaction potential energy
The photophysical properties of SQs in solution
-1
surface of SQTPE, SQHTPE, SQDP and SQP (unit: kcal mol ).
To examine whether the phenyl-removed strategy can
brighten solution emission for the designed molecules, the
photophysical properties of all compounds were firstly
investigated in common solvents with different polarity (Fig.
S4 and Table S2). It could be found that a rather good mirror
image relation between their dual absorption and emission
spectra in various solvents, which can be associated with their
LE and TICT states with Stokes shift around 15 and 80 nm,
respectively. Notably, the emission of SQTPE was red-shifted
with the ΦPL clearly decreased upon increasing the solvent
Results and discussion
Molecular design, TDDFT calculations and synthesis
To gain deep insights into this systems, we thus designed
SQTPE, SQHTPE, SQDP and SQP with gradually removed phenyl
rotator in the extended ethylene in TPE segment (Fig. 1a), and
next detailedly computed their photophysical and
4
2-44
photochemical mechanisms (Supporting Information).
The
calculated corresponding ground and exited potential energy
surface of four molecules in toluene as a function of C1-N1
rotations in TICT formation and ESIPT processes were showed
in Fig. 1b, Fig. S1-3 and Table S1. From the calculations, it was
easy to conclude that the dual emission of all SQs analogues
can be responded by their favourable local exited (LE)/TICT
process with the distortion of C1-N1 bond in S1 state occurred
in solution, while their red-shifted single emission followed the
ESIPT process from N1 to O1 atoms in crystalline state due to
TICT formation was strictly prohibited. It is interesting to note
that the energy barriers of TICT formations (ETS2) in those
analogues was in the order of SQHTPE > SQTPE > SQDP >> SQP,
strongly suggesting that the LE state should populate most
largely in SQHTPE than that in other SQ molecules, thereby
facilitating its strongest emission in LE state. Once reached
TICT state, the reversion of C1-N1 bond distortion for SQHTPE
was also the most difficult to take place because of the highest
polarity from hexane (22.3%), CH
(10.2%), furtherly proved its LE/TICT electronic characters.
While in toluene, SQTPE exhibited a brightest red emission
efficiency (ΦPL = 26.9%) at 579 nm together with a shoulder
peak at 616 nm (Table 1), which may resulted from excellent
2 2
Cl (11.8%) to DMSO
4
5
1
9
solvation effect between aromatic SQTPE and toluene. As for
protic CH OH, it was able to interact with SQTPE via multiple
3
hydrogen-bonding interactions, which may lead to a net
stabilization of its ground state and nonradiation of its excited
state for an abnormal blue-shift in the spectra and the lowest
4
6
PL
Φ . Very similar spectral phenomena were also observed in
SQHTPE, SQDP and SQP, suggesting their very analogical
ground/exited-state electronic structures in solution. Besides,
all the fluorescence decay profiles of those luminogens in
solution were well fitted by using a single exponential function
with two fluorescence lifetimes in the nanosecond time scale
(Table S2, and Fig. S5).
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PJ loe ua rsne a dl oo f nM o at t ea rdi aj ul ss Ct hm e am r gi si tnr ys C
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DOI: 10.1039/D0TC02596H
Fig. 2 a) Emissive photographs of SQTPE, SQHTPE, SQDP and SQP in solution (10 μM, from left to right: Toluene, Hexane, THF, CH
2 2 3
Cl , DMSO and CH OH) and in crystalline state
(under 365 nm UV illumination, scale bar: 50 μm); b) absorption/emission spectra of the luminogens in toluene; and c) in the crystalline state (λex = 500 nm).
As shown in Fig. 2b, SQHTPE exhibited obvious red-shifted
but stronger absorption profiles with maximum peak (λab) at
37/568 nm in toluene, and these changes were also recorded
in their emissions with (maximum peak) λem at 586/622 nm,
suggesting its bit larger conjugation than SQTPE in solution.
The emission intensity of SQHTPE, as expected, underwent a
great enhancement with ΦPL increased up to 57.5% in toluene
emission (λem = 558/593 nm) in toluene meant that it was in
obviously less conjugation than the others (Fig. 2b, Table 1),
and preferable TICT formation with lowest barrier energy (1.38
5
-1
kcal mol ) accompanying with lowest reversion energy barrier
-1
(4.40 kcal mol ) gave good explanation to its clearly lowest ΦPL
of 11.3% in toluene (Table 1). All the measured spectral data
are well consistent with that of theoretical calculation.
(
Table 1), directly verifying our theoretical prediction. As for
Structural analysis and solid-state emission of SQs
SQDP, it exhibited a very similar absorption (536/568 nm) and
emission spectra (582/621 nm) in toluene, suggesting a
comparative length conjugation as compared to that in
SQHTPE (Fig. 2b). The ΦPL of SQDP, notably, was also enhanced
to 42.7%, although its TICT formation hold a decreased energy
For the proton transfer channel, it showed that the energy
Table 1. Summary of optical properties of the luminogens (λ: nm, ΦPL: %a).
-1
barrier (2.64 kcal mol ) as compared to that in SQTPE (3.27
in toluene (10 μM)
in crystal
-1
Sample
kcal mol ). From all the theoretical and experimental data, we
can perhaps draw two main conclusions: 1) both less TICT
formation and phenyl rotations simultaneously facilitated the
radiative decays thus resulting in the brightest red emission
λ
ab
λ
em
Φ
PL
Sfb
19
λ
em
Φ
PL
Sfc
96
Dfd
77
SQTPE
SQHTPE
SQDP
532/560
537/568
536/568
508/544
579/616
586/622
582/621
558/593
26.9
57.5
42.7
11.3
656
670
682
675
73.1
51.2
40.5
8.1
18
14
14
102
114
131
84
(ΦPL = 57.5%) of SQHTPE in toluene, when removing one
100
117
phenyl rotator in TPE segment of SQTPE; and 2) by continuous
remove of another phenyl rotator, the obtained SQDP still
exhibited a brighter red emission (ΦPL = 42.7%) in toluene,
suggesting the further reduced phenyl rotations exerted
greater effect on its emission behaviour than its relatively
more TICT formation. While in SQP, the obvious blue-shifted
behaviours both in absorption (λab = 508/544 nm) and
SQP
a
b
c
Absolute ΦPL were determined using a Horiba FL-3018 Integrating Sphere;
The Stokes shift of LE state;
The Stokes shift of ESIPT state;
toluene
LE
d
DSE shift: Df = λ ec mr yst
-
λ
.
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Page 4 of 7
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DOI: 10.1039/D0TC02596H
Fig. 3 The dihedral angles between vinyl bond and phenyl rotors in a) SQTPE, b) SQHTPE, c) SQDP crystals. Intermolecular interactions including no π-π interactions with d) no free
phenyl rotors in SQTPE crystals, e) one free phenyl rotor in SQHTPE and f) weak π-π interactions in SQDP.
barriers of TS3 were almost equaled for these four molecules
Fig. 1d), which indicated that there was little effect on their
SQDP crystals, there also exhibited weak intermolecular π-π
interactions in 3.342 Å between central four-member ring and
distal phenyl group in adjacent SQDP molecule (Fig. 2f). This
mode permitted its crystalline assemblies showed an
obviously red-shifted λem centered at 682 nm with low ΦPL of
40.5%, in comparison with λem of 656 nm in SQTPE crystals
(Table 1). As for the low ΦPL of 8.1% in SQP crystals, this might
be ascribed to their limited conjugation and favourable
intermolecular π-π interactions in condensed phase. From
Table 1, it was found that the DSE shifts of SQTPE, SQHTPE,
SQDP and SQP were 77, 84, 100 and 117 nm, respectively, due
to their emissive mechanisms all changed from LE/TICT in
solution to ESIPT in crystalline state. To the best of our
knowledge, which represents the largest DSE shift among all
the DSE molecules (Table S5).
(
ESIPT process via remove of substituted phenyl rotator. In fact,
ESIPT process may mainly determine their single long
wavelength emission up to red/NIR region (Table 1), and their
intermolecular packing modes would greatly dominate their
emission efficiency in crystalline state for those four analogues.
As such, to deeply understand their structure-property
relationships, single crystals of SQTPE, SQHTPE and SQDP were
2 2
then obtained for X-ray crystallography in CH Cl /Hexane
mixtures (Table S3 and Fig. S6-8), so did their corresponding
well-ordered crystalline assemblies (Fig. S9-12, preparation
methods and characterizations in detail see Supporting
Information). As for the identical SQP segments in these
derivatives, two types of strong intramolecular C=OH
hydrogen bonding interactions would greatly facilitate their
intramolecular planarity and rigidity in solution, and two
methyl fork chains in the indoline moiety were very helpful for
preventing partial π-π stacking. Additionally, the triple
carbonyl groups in central four-membered ring and methyl
ester were particularly in favour of forming multiple
intermolecular hydrogen bonding interactions for potentially
generating hydrogen-bonding crystals. Thus, these inherent
structural characteristics were rewarding for the building of
these luminogens with potential DSE behaviour. Detailed
single crystals analyses of these three analogous have been
deciphered in the Supporting Information (Fig. S6-8).
In SQTPE and SQHTPE crystals, similar twisting TPE and
HTPE segments (Fig. 3a-b) helped to fully suppressed
intermolecular π-π interactions in the construction of
crystalline assemblies (Fig. 3d-e), and the free phenyl rotors
detected in SQHTPE crystals can be responded to its relatively
lower ΦPL of 51.2% at 670 nm than that of 73.1% at 656 nm in
SQTPE crystals without any free phenyl rotors (Fig. 3d-e). For
SQDP crystals, the dihedral angle between the phenyl group
and vinyl determined to be only 1.10° and 6.19° (Fig. 3c),
implying extended conjugated SQDP exhibited an exceedingly
planarized conformation in crystals on the whole. In spite of
multiple intermolecular hydrogen bonding interactions in the
Besides, these four amorphous assemblies, obtained by
grinding of their crystalline sample and confirmed by Powder
X-ray diffraction (PXRD, Fig. S10), exhibited rapidly decreased
intensity of their emission (Fig. S12), which can be mainly
ascribed to that their amorphous aggregates were in loosely
stacked state that cannot well restrict the intramolecular
rotations. These emission quenching behaviours from
crystalline-amorphous transformation were also responsible
for their observed ACQ from the tests in THF/water mixtures
(Fig. S13), suggesting a typical crystalline-induced emission
4
7-50
(CIE) effect.
Similarly, their fluorescence decay profiles in
amorphous or crystalline state were also well fitted by using a
single exponential function with one fluorescence lifetimes in
the nanosecond time scale (Fig. S12, and Table S4).
Cell bioimaging
As we all known, typical SQ derivatives normally need
cumbersome encapsulation to avoid nonfluorescent
aggregation and biological nucleophiles attacking in cell
5
1-55
bioimaging.
Here we established a really simple live-cell
imaging strategy by using “ naked ” SQHTPE regardless of
those two disadvantages. As shown in Fig. S14, SQHTPE was
quite stable in the presence of common biological results
indicated a very satisfactory biocompatibility of SQHTPE to the
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PJ loe ua rsne a dl oo f nM o at t ea rdi aj ul ss Ct hm e am r gi si tnr ys C
Conflicts of interest
There are no conflicts to declare.
View Article Online
DOI: 10.1039/D0TC02596H
Acknowledgements
We are grateful to the support of the National Natural Science
Foundation of China (21363014 and 21103082). This work was
also supported by the Natural Science Foundation of the
Jiangxi Province of China (Grant No. 20142BCB23003 and
2
0143ACB21021).
Notes and references
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SQHTPE (λex = 514 nm) followed by b) DAPI for another 30 min (λex = 405 nm); c) Bright
field and d) Merged imaging of a + b; Scale bar: 50 μm.
4
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-1
7. Y. Lei, Q. Liu, L. Dong, Z. Cai, J. Shi, J. Zhi, B. Tong, Y. Dong,
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solution was used for fluorescence cell imaging with HUVECs.
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bright red fluorescence from areas surrounding the DAPI-
stained nuclei (Fig. 4). For this, phagocytosis might be the
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Conclusions
In summary, we demonstrated the first time of a phenyl-
removed strategy in TPE-merged SQs for accessing efficient
emission both in solution and crystalline state, on the basis of
fully deciphering the bidirectional TICT/ESIPT mechanism in
these analogues by in-depth TDDFT. With removing one
phenyl rotator in TPE segment, both less TICT formation and
phenyl rotations allowed the brightest red emission (ΦPL = 57.5%
at 586/622 nm) of SQHTPE in solution. Meanwhile, the
reserved hydrogen-bonding packing mode and ESIPT process
in SQHTPE crystals responded to its intense red/NIR emission
2
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has been achieved with naked SQHTPE, promising these
molecules potential in biological applications. We believe this
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comprehensively considering the impact factors in DSE-active
molecules, but also practical for flourishing their constructions
in view of that there are many AIE-active luminogens with TPE
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Journal of Materials Chemistry C
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DOI: 10.1039/D0TC02596H
We report here a phenyl-removed strategy in TPE-merged squaraine dyes for accessing intense
red/NIR emission both in solution and crystalline state, guided by in-depth TDDFT calculations.