Tetraphenylethylene-incorporated squaraine dyes: Structural and theoretical insights into the diverse emission behaviors in solution and solid state

Article abstract of DOI:10.1039/c9tc06406k

Endowing organic dyes with desired emissive behaviors in diverse states is of great importance for broad practical applications, but it remains a huge challenge. In this study, based on the incorporation of tetraphenylethylene (TPE) into squaraine dyes, we successfully synthesized 2TPE-SQ and TPE-SQ and both exhibited dual state emission with emission efficiencies (Φ_PL) up to 0.33 and 0.75, respectively. The intense emission in the crystalline state can be ascribed to highly restricted π-π stacking, while the different degrees of intramolecular phenyl motions in 2TPE-SQ and TPE-SQ result in weak and moderate emissions in solution. Combining density functional theory (DFT) calculations, local excited (LE)/twisted intramolecular charge transfer (TICT) and excited-state intramolecular proton transfer (ESIPT) mechanisms were proposed and accounted well for their very different emission wavelengths in solution and crystalline state, respectively. Besides, other emission behaviors such as aggregation-caused quenching (ACQ), crystalline-induced emission enhancement (CIEE) and thermo-/mechanochromism of these two derivatives were verified in detail, and all these results would bring a new insight into the investigation of organic luminogens in solution and solid state.

Full text of DOI:10.1039/c9tc06406k

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Nosang V. Myung, Yong-Ho Choa et al.
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ARTICLE
Tetraphenylethylene-incorporated Squaraine Dyes: Structural and
Theoretical Insight into the Diverse Emission Behaviors in Solution
and Solid State
Received 00th January 20xx,
Accepted 00th January 20xx
Qingqing Shao,a,b Kangli Liang,a,b Huan Ling,a Yigang Wang,a,b ZhiHong Yan,d Guomin Xia,a,c,d,
*
Hongming Wanga,c,*
DOI: 10.1039/x0xx00000x
Endowing organic dyes desired emissive behaviors in diverse state is of great importance for broad practical applications
but remains a huge challenge. In this contribution, based on the incorporation of tetraphenylethylene (TPE) into squaraine
dyes we successfully synthesized 2TPE-SQ and TPE-SQ, both emitting in dual-state with emission efficiency (ΦPL) up to 0.33
and 0.75, respectively. The intense emission in crystalline state can be ascribed to highly resisted π-π stacking, while their
different degrees of intramolecular phenyl motions in 2TPE-SQ and TPE-SQ resulted into weak and moderate emission in
solution. Combining density functional theory (DFT) calculations, local excited (LE)/twisted intramolecular charge transfer
(TICT) and excited-state intramolecular proton transfer (ESIPT) mechanism accounted well for their very different solution
and crystalline emission wavelength. Besides, other emission behaviors such as aggregation-caused quenching (ACQ),
crystalline-induced emission enhancement (CIEE) and thermo-/mechanochromism of these two derivatives were also
verified in detail, and all these results would bring a new insight into the investigation of organic luminogens in solution and
solid state.
Introduction
It is known that traditional organic luminophores are generally
in planar structures and highly emissive in solution but non-
fluorescent in the aggregated state due to active non-radiative
pathways.1,2 This phenomenon was referred to as (ACQ) by
Förster and Kasper and often hampers many fluorescence-
relative practical applications. In contrast to ACQ, the
aggregation-induced emission (AIE) effect was proposed by
Tang’s group3 in 2001 in the study of a series of twisted silole
derivatives, which was nonemissive in dilute solution but
became highly luminescent in aggregated formation. Since
then, a large number of AIE-active molecules were reported,
and largely increased the possibility of exploring organic
materials with high solid-state emission efficiency thereby
promoted diverse applications.4-16 Still and all, these
incompatible solution/aggregation fluorescence mean a large
gap between ACQ and AIE compounds, thus urges researchers
to take advantages of both two phenomena for developing the
materials with efficient dual state emission (DSE).17 These DSE-
active luminogens will definitely be beneficial for much
broader applications and molecular-level understanding the
relationship between the fluorescent properties and
monomeric/aggregated states.
Squaraines (SQs), as an important organic dye, exhibit
sharp and intense absorption associated with a strong
fluorescent emission in solution from visible to near-infrared
region, which renders them favorable in advanced fields such
as optical sensors, solar cells and fluorescent histological
probes.18-21 Nevertheless, SQs are generally non-fluorescent in
solid state or aggregate assembly due to intermolecular π-π
interactions.22,23 This typical ACQ phenomenon has greatly
restricted their application in solid state and bioimaging. Based
on previous researches, it is reasonable that preventing
intermolecular close-packing could be a facile strategy to
access and even tune fluorescent SQs in solid state by
adjusting sterically hindered substituents on the conjugated
backbone.24-27 Moreover, Yin and colleagues have constructed
an intense emitting symmetric SQ with formation of J-type
nanostructures by introducing H-bonding in side carboxylic
acid groups.28 Very recently our group has also been
committed to the design and application of highly emissive
SQs in solid state and achieved a record-setting case with ΦPL
up to 0.81.29-31 Although state-of-the-art, all those elaborate
solid emissive SQs turned out to be weak or even non-
fluorescent in the dilute solution. This reversed situation
makes a challenge in the compatibility of their excellent
emission performance in solution and solid state, which would
greatly broaden the SQs practical applications.
a. Institute for Advanced Study, Nanchang University, Jiangxi, 330031, P.R. China
b. School of Materials Science, Nanchang University, Jiangxi, 330031, P.R. China
c. College of Chemistry, Nanchang University, Jiangxi, 330031, P.R. China
d. Jiangxi University of Traditional Chinese Medicine,Jiangxi, 330031, P.R. China
*
*
hongmingwang@ncu.edu.cn and gmxia_ncu@126.com
Electronic Supplementary Information (ESI) available: synthesis, characterization,
calculations and crystal data. See DOI:10.1039/x0xx00000x
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Scheme 1. Overall synthetic route to 2TPE-SQ and TPE-SQ. a) Pd(dppf)Cl2 / KOAc, dioxane, 100 ℃, 8h; b) Pd(dppf)Cl2 / CsF, dioxane / H2O, 90℃, 12h; c) Ethanol, reflux, 80℃, 8h.
Herein, DSE-active symmetric 2TPE-SQ and asymmetric
TPE-SQ (Scheme 1) were firstly designed and synthesized with
intersection of TPE group. Based on spectral analysis, 2TPE-SQ
turned out to be weak emissive in solution but bright green
emissive at 531 nm (ΦPL = 0.75) in crystalline state, which can
be mainly ascribed that substantial intramolecular phenyl
rotated in solution but highly resisted in solid state. As for TPE-
SQ, it emitted in both solution and crystalline state with the
highest ΦPL up to 0.33 at 564/588 nm and 0.61 at 627 nm,
respectively. Single crystal diffraction analysis revealed that
the efficient emission of TPE-SQ in solution was due to the
restricted intramolecular phenyl motions, and that in crystal
responded by a special packing mode dominated by multiple
C-HO and C-Hπ interactions. Combining theoretical DFT
calculations, the different emission mechanism as LE-TICT32-36
in solution and ESIPT37 in crystal was proposed and responded
to their different emission wavelength. Besides, other
emission behaviors such as ACQ, CIEE38 and thermo-
/mechanochromism39 of these derivatives were also verified in
detail, and all these results would bring some new insight into
the investigation of organic luminogens in solution and solid
state.
group with preset intramolecular H-bonds will enhance
intramolecular planarity thereby promote their rigidity in
solution; and (3) the potential ESIPT31,46,47 mechanism was
incubated to facilitate the remarkable red-shift and efficient
emission.
The overall synthetic route of desired 2TPE-SQ and TPE-SQ
was shown in Scheme 1. Both of them were isolated in great
crystalline state after reaction and exhibited bright-green and
intense red emission under 365 nm UV lamp, respectively. It is
worth mentioning that this condensation reaction can be
easily implemented with satisfied yields in the refluxing EtOH
solution, whereas a ester exchange reaction can be observed
in the refluxing BuOH/toluene under long-time azeotropic
conditions. The target intermediates and products were fully
characterized by 1H NMR, 13C NMR and mass spectra (Fig. S1-
S8). Additionally, rational assignments of the proton
resonances of 2TPE-SQ and TPE-SQ were carried out and
closely compared, which obviously implied that 2TPE-SQ and
TPE-SQ possessed similar steric configuration (see
Supplementary Information for details).
The photophysical properties of 2TPE-SQ and TPE-SQ in solution
Results and discussion
The absorption and emission spectra of 2TPE-SQ and TPE-SQ
were measured in dilute solution with different polarity, and
the results were summarized in Fig. 1 and Table S1. Both of
2TPE-SQ and TPE-SQ showed obvious dual absorption and
emission spectra in various solvents (Fig. 1a, b, d, e), which was
in a rather good mirror image relations and might be
associated to two different excited state. As shown, 2TPE-SQ
showed the maximum absorption peak (λab) at 448 nm and a
shoulder peak at 430 nm in tetrahydrofuran (THF, Fig. 1a). The
two peaks could undergo blue-shift upon increasing the
solvent polarity and merge into one broad band with short
wavelength centred at 425 nm in dimethylsulfoxide (DMSO).
Meanwhile, 2TPE-SQ presented the strongest cyan emission
color in toluene with a maximum peak (λem) around 473 nm
and a shoulder peak around 493 nm (Fig. 1b), but the emission
Molecular design and synthesis
Previously reported SQ derivatives were mainly based on the
activated arenes and electron-rich anhydrobases, whose
planar structures endow them excellent rigidity with
impressive emission in solution but quenched via
aggregation.40 As reported, the propeller-like TPE unit has
been widely used in the structural design and synthesis of
novel AIEgens and even DSEgens.41-45 So that under this
consideration, we attempted to synthesize symmetric 2TPE-
SQ and asymmetric TPE-SQ by intersecting two or one TPE
group on their conjugated skeletons. This design was expected
to afford the product SQs with novel properties: (1) the
twisted TPE was inserted to suppress closed intermolecular
molecular packing in the solid state; (2) the intersected aniline
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Fig. 1 a, d) Absorption and b, e) emission spectra of 2TPE-SQ (10 μM, λex = 420 nm) and TPE-SQ (10 μM, λex = 500 nm) in various solvents, respectively; and c, f) their emission changes
in THF solution with different fractions of water (fw), at room temperature. Insert, their corresponding fluorescent images in different solvents under a 365 nm UV lamp; from left to
right: Toluene, n-hexane, THF, CH2Cl2, DMSO.
efficiency (ΦPL) was only 0.07, which can be mainly responded
by the fact that too many free phenyl rotors were in the 2TPE-
SQ molecule and consumed large excited energy48. Upon
increasing the solvent polarity, the emission intensity of 2TPE-
SQ was gradually decreased except in toluene thus implying
there existed a charge transformation (CT) effect (Table S1).
The exception in toluene, however, may attribute to excellent
solvation effect between aromatic molecules of 2TPE-SQ and
toluene.17 As for TPE-SQ, it exhibited a red-shift dual
absorption and emission peak around 515/545 nm and
565/590 nm in varying solvents (Fig. 1d-e), suggesting its much
longer conjugation. Moreover, the emission intensity of TPE-
SQ would undergo similar decreasement in polar solvents as
compared to 2TPE-SQ, implying their extremely analogical
exited-state electronic structures in solution. It should be
notable that the ΦPL of TPE-SQ was up to 0.33 in toluene
(Table S1), indicating its much higher intramolecular rigidity
with decreased free phenyl rotors. As for these intramolecular
phenyl motions, this can be furtherly monitored by their
temperature-dependent luminescent characteristics in
toluene solution. As show in Fig. S9, 12.98 and 1.48 folds
increased of their emissive intensity can be observed in 77k as
compared to that at room temperature, confirming substantial
and less intramolecular phenyl motions in 2TPE-SQ and TPE-
SQ molecule in solution, respectively. Additionally, moderate
emission signals were always observed for those two
molecules even the temperature reached 373K, implying their
robust solution emission. Moreover, no ESIPT process
exponential function, with fluorescence lifetimes in the
nanosecond time scale (Table S1 and Fig. S10).
Since TPE is an archetype AIE luminogen, the emission
behaviors of those compounds in THF solution with different
fractions of water (fw, by volume) were then conducted (Fig. 1c,
f). When the fw increased from 0 to 80%, no nanoparticles were
observed and the emission intensity of 2TPE-SQ decreased
evidently with λem blue-shifted from 472 to 465 nm. As for this
unusual change, this can owe to that the protic H2O was able
to interact with 2TPE-SQ and TPE-SQ via multiple hydrogen-
bonding interactions, which may lead to a net stabilization of
its ground state and activation of nonradiation pathway of its
excited state for an abnormal blue-shift and the decrease of
emission intensity in the spectra.49 Upon fw over 90%, the
emission intensity continued to decrease accompanying with
λem red-shifted into the long wavelength at 525 nm, in which
tiny nanoparticles can be observed and the emission intensity
still decreased with fw reached 100%. Very similar emission
behaviors can be detected for TPE-SQ with increasing the fw
values, indicating
a typical ACQ effect in these TPE-
incorporated lumingens. The particle size distributions
revealed that the average particle size was 356.2 nm and 82.09
nm for 2TPE-SQ and TPE-SQ in mixture with fw = 90%,
respectively (details see Fig. S9). In viewing of intense green or
red emission can be observed in their crystals, the rapid
decrease of emission intensity can be mainly ascribed to the
formation of amorphous aggregates in the THF-water mixtures
with high fw, which quenched the fluorescence via rotations of
the phenyl groups on TPE units. This typical CIEE50,51 behavior
will be confirmed and discussed below in detail.
was involved evidenced by the small Stokes shift in various
solvents. The fluorescence decay profiles of both 2TPE-SQ and
TPE-SQ in solution were effectively fitted by applying a single
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Fig. 2 The molecular structures and packing mode in the TPE-SQ crystal. a) The molecular conformation and intramolecular hydrogen-bonding interactions. b) The dihedral angles
between vinyl bond and phenyl rotors. c) Intermolecular hydrogen-bonding interactions between adjacent TPE-SQ molecules. d) Intermolecular hydrogen-bonding and Ar-Hπ
interactions along a-axis. e) Intermolecular hydrogen-bonding between TPE-SQ and n-hexane solvent viewing from crystallographic a-axis.
Structural analysis and solid-state emission of 2TPE-SQ and TPE-
SQ
of the viny bond, was also observed within 2.81 Å (Fig. 2a). This
furtherly led to the locking of one phenyl rotor in the TPE-SQ
molecule. As compared to TPE molecule, the dihedral angles
between vinyl bond and phenyl rotors exhibited negligible
change for θ1 from 56.74° to 55.27° and θ2 from 45.66° to
45.00° in TPE-SQ molecule,52 while θ3 and θ4 increased from
46.18° to 63.92° and 48.43° to 54.12°, respectively (Fig. 2b).
This remarkable changes of dihedral θ3 and θ4 also directly
supported the enhanced rigidity of the whole TPE-SQ molecule.
As such, these limitations of its intramolecular phenyl rotation
would well respond to the fact that TPE-SQ exhibited efficient
emission in solution. Considering the structural symmetry, it
was reasonable that same intramolecular restriction also
existed in 2TPE-SQ, but extra free phenyl rotors would
consume large excited energy thus led to its extreme weak
emission efficiency in solution.
In order to further understanding emission behaviors in dilute
solution, a detailed analysis of their molecular conformation
was very necessary. Although it was difficult to obtain the
crystal structure of 2TPE-SQ, the suitable single crystal of TPE-
SQ was easy to obtain for X-ray crystallography, and all the
data were well summarized in Table S2. As shown in Fig. 2,
TPE-SQ crystal belonged to a monoclinic crystal system and
crystallized in the space group of P21/c with four TPE-SQ
molecules in one unit cell taking the parameters of a
=12.232(13), b = 9.804(10) and c = 32.921(4) Å. In the whole
TPE-SQ molecule, the SQ segment was nearly plane due to two
kinds of strong intramolecular C=OH hydrogen bonding (Fig.
2a). One was 2.16 Å between Ar-H and carbonyl in four-
member ring, and another one was 1.96 Å between N-H and
adjacent methyl esters. This excellent conjugation was
regarded to facilitate its rigidity thus benefit to the limitations
of the intramolecular rotation. More importantly, another
moderate intramolecular hydrogen bonding, between
carbonyl in four-member ring and Ar-H at the zusammen side
Further analysis revealed that there exhibited no
intermolecular π-π stacking but multiple hydrogen bonding
interactions in the crystal of TPE-SQ. As shown in Fig. 2c, two
types of moderate hydrogen bonding interactions were
observed between adjacent TPE-SQ molecules. One was 2.50
4 | J. Name., 2012, 00, 1-3
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Fig. 3 The emission spectra of a) 2TPE-SQ and d) TPE-SQ assembiles in different condition at room temperature, the XRD patterns of b) 2TPE-SQ assemblies and e) TPE-SQ assemblies
and the simulated XRD patterns of corresponding crystals, PL microscopy and SEM images of c) 2TPE-SQ assemblies and f) TPE-SQ assemblies.
Å interaction in C=OH between the oxygen atom of methyl
ester and hydrogen atom of phenyl rotor, and another was
2.68 Å interaction in Ar-HO between the oxygen atom of
four-membered ring and hydrogen atom of phenyl rotor.
Meanwhile, along the a-axis the 2.69 Å interaction in Ar-HO
between the oxygen atom of four-membered ring and
hydrogen atom of phenyl rotor, accompanying a 3.08 Å
interaction in Ar-Hπ between phenyl rotors, were observe in
adjacent TPE-SQ molecules (Fig. 2d). It was noteworthy that
the incorporated n-hexane solvent played a key role in the
construction of the crystalline TPE-SQ. As shown in Fig. 2e, the
continuous 2.61 Å interaction in C=OH between the oxygen
atom of four-membered ring and hydrogen atom of n-hexane
was observed, which was in a “stair-like” type rising along the
b and c-axis. As such, this special packing mode dominated by
multiple intermolecular C-HO and C-Hπ interactions,
instate of π-π stacking, would well support the fact that TPE-
SQ exhibited efficient emission in crystalline state, so did that
in 2TPE-SQ crystals.
microrods were more than ten micrometers in width and one
hundred micrometers in length, whereas microplates were
about 13 µm in width and 20 µm in length as well as a thickness
around several micrometers (Fig. S11-12, Fig. 3c, f).
Furthermore, power X-ray diffraction (PXRD) patterns of their
pristine crystalline powders showed sharp and intense peaks,
indicating good microcrystalline structures (Fig. S13). The
simulated XRD patterns of TPE-SQ crystals turned out to be
coincided with that of crystalline assemblies (Fig. S13b),
suggesting the same molecular packing modes.
Thermogravimetric analysis (TGA) experiments revealed that
2TPE-SQ microrods were able to be unchanged until 324 ℃
(Fig. S14a), suggesting no solvent molecules were
incorporated in the crystals. While TPE-SQ microplates
gradually lost ≈ 5.94% of its weight before 74 ℃ (Fig. S14d),
and then stable until ≈ 260 ℃. The proportions of weight loss
coincided exactly with a molar ratio of 2:1 between TPE-SQ
and n-hexane molecules as observed in the crystal structures.
With these well-ordered assemblies, we next tested their
emission behaviors. As shown in Fig. 3, pristine 2TPE-SQ and
TPE-SQ assemblies emitted λem at 531 nm and 627 nm with the
ΦPL up to 0.75 and 0.61, respectively. These higher ΦPL can be
ascribed to their highly resisted intramolecular phenyl
rotations accompanying with their dominated hydrogen
bonding packing mode in crystals. It was worthy to noted that
the λmax both of 2TPE-SQ and TPE-SQ underwent red-shift
With those hydrogen bonding interactions, both 2TPE-SQ
and TPE-SQ held strong crystallization tendency. As shown,
well-defined 1D microrods and 3D microplates were easily
obtained when assembly of 2TPE-SQ and TPE-SQ molecules in
CH2Cl2/n-hexane mixtures (detail see supplementary
information). Photoluminescence (PL) microscopy and
scanning electron microscopy (SEM) images revealed that the
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Fig. 4 a) Calculated potential energy profiles (kcal mol −1 ) and wavelengths (nm) of vertical excitation and emission of TPE-SQ at the B3LYP/6-311G (d, p) and TD-B3LYP/6-311G (d,
p) levels. (red line: the proton transfer pathway; blue line: distortion of C1-N1 bond pathway) b) The image and energy level of HOMO and LUMO of all the transition of TPE-SQ
(energy unit: ev).
about 60 nm from solution state to crystalline state, which was
very different from most of the reported DSE-active
luminogens generally possessed a very similar λmax in blue to
green region both in solution and solid state.17,53 Those results
obviously suggested there would exist different emission
mechanism for different state of 2TPE-SQ and TPE-SQ
molecules. Considering their large Stokes shift (≈ 80 nm) and
configurational features, their intense emission in crystalline
state can be mainly ascribed to the well-known ESIPT
mechanism.
Considering that the n-hexane solvents were incorporated
in TPE-SQ crystals, we then studied their photophysical
properties under the external stimuli such as heating, grinding
and solvent fuming. As for TPE-SQ, when its pristine
assemblies were heated at 180 ℃ for 30 min, the TGA was
recorded and indicated that the solvent molecules were
mainly removed in the crystalline (Fig. S14f). Meanwhile, PXRD
patterns of the annealed powders revealed broad and
featured less reflection, suggesting the nearly amorphous
feature (Fig. S17b). The same results were also obtained when
thorough grinding the TPE-SQ assemblies over 30 min in the
mortar, confirmed by its TGA and PXRD experiments (Fig.
S14e, 18b). Both the annealed and ground assemblies showed
in a Petri dish at room temperature, the emission color and
PXRD turned out to be unchanged, suggesting that the
amorphous assemblies can not be recovered to the initial state
by treating with n-hexane solvent vapor (Fig. S17b, 18b). As
such, the thermo/mechanochromism of the TPE-SQ crystals
permitted it a promising candidate for stimuli-responsive54
emissive
organic
crystals.
Incidentally,
the
thermo/mechanochromic behaviors of 2TPE-SQ were also
examined and similar CIEE effect can be observed (Fig. S15, Fig.
S16). It was worth pointing out that the annealed 2TPE-SQ
assemblies exhibited slightly decreased intensity with ΦPL
changed from 0.75 to 0.58. This can be responded by the fact
that the intermolecular interaction can not be collapsed with
just annealling because no solvent was incorporated in
crystalline 2TPE-SQ, which was very different from that in TPE-
SQ crystals. As comparison with typical DSE-active
molecules,17,53 TPE-SQ exhibited obvious different emission
wavelength owing to distinct emission mechanism in solution
and solid state, and more importantly, its crystalline
assemblies manifested a intense red emission with λmax =627
nm. This red-emission characteristic is urgently needed for
enriching the design and tuning strategies of DSE-active
molecules, thereby benefiting to promote their biological
applications in the near future. All the photophysical data of
2TPE-SQ and TPE-SQ under different conditions were also
summarized in Table S1 and Fig. S10.
quickly decreased intensity at λem = 631 nm and λem = 622 nm
with ΦPL of 0.08 and 0.02, respectively (Fig. 3d, S16a, S17a).
Thus, we could conclude that removing n-hexane solvents
from the crystalline lattice induced a disordered state in the
molecular arrangements, which may facilitate the rotations of
the phenyl groups on TPE unit in TPE-SQ assemblies thus
quenching the emission, indicating a typical CIEE behavior.
This crystalline-amorphous transformation was also
responsible for the observed ACQ behavior of TPE-SQ in the
pure water. Moreover, we then tested the reaccess of the
solvent molecules in the annealed/ground assemblies. After
exposure of these assemblies to the n-hexane vapor for 30 min
Theoretical DFT calculations and LE-TICT/ESIPT mechanism
With the aim of gaining further insight into the optical
behaviors, the absorption and emission spectra and emissive
mechanisms for TPE-SQ in dual-state were calculated by time-
dependent density functional theory (TDDFT) method. All the
optimized geometries of TPE-SQ in the ground state (S0) and
in the excited state (S1) using B3LYP and TD-B3LYP calculations
were plotted in Fig. S19 and the calculated potential energy
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profiles of TPE-SQ photoreaction in S0 and S1 states were
presented in Fig. 4. In the ground state, a one-step proton
transfer mechanism was taken place via TS0 with an energy
barrier of 32.32 kcal mol-1 to produce PS0 that held an energy
of 10.73 kcal mol-1 above the reactant.
In summary, herein we demonstrated the VidewesAritgicnle Oanlninde
successful synthesis of DSE-active 2DTOPIE:-1S0Q.103a9n/Cd9TTCP06E4-S0Q6K,
featuring emission in both solution and crystalline state. From
spectra analysis, 2TPE-SQ turned out to be weak emissive
(highest ΦPL = 0.07) in solution but bright green emission at
531 nm with ΦPL = 0.75 in crystalline state, while TPE-SQ
emitted in solution and crystalline state with the highest ΦPL
up to 0.33 at 564 nm and 0.61 at 627 nm, respectively. Single
crystal diffraction analysis revealed that their emission in
solution can be ascribed to the restricted intramolecular
motions with enhanced structural rigidity and intramolecular
H-bonding interactions, while that in crystal was due to a
special packing mode dominated by multiple C-HO and C-
Hπ interactions. Based on theoretical DFT conclusions, the
different emission mechanism as LE-TICT in solution and ESIPT
in crystal was proposed and responded to their different
emission wavelength. Meanwhile, the self-assembly and
various emission behaviors such as ACQ, CIEE and
thermo/mechanochromism of the these compounds were
investigated in detail, and all these results would bring a novel
vision on the investigation of organic lumingens in diverse
state.
In the first excited state (S1), there were two possible
photoreaction pathways. One occurred directly between RS1
and PS1a via TS1a accompanying the distortion of C1-N1 bond
with an energy barrier of 5.27 kcal mol-1 and an endothermic
process of -2.22 kcal mol-1 (the blue line), suggested a typical
LE-TICT mechanism. The other that active H atom transferred
to O atom of central ring, occurring between RS1 and PS1 via
TS1 with only an energy barrier of 12.3 kcal mol-1 and a heat
release of 11.67 kcal mol-1 (the red line), suggested a typical
ESIPT mechanism. In fact, the distance between the active H
atom and O atom of ester group was much shorter than that
of O atom of central ring. For this, we initially believed that the
active proton would transfer to the O atom of ester group in
S1 state as well. However, when we optimized the structure
that the H atom connected with the O atom of the ester group,
the H atom spontaneously transferred to O atom of central
ring. Moreover, the scan of potential energy surface (PES) of
S1 state of TPE-SQ as a function of N-H bond length was done
(Fig. S20), which showed that the transfer of H atom from N to
O atom of ester group would increase the total energies both
in S0 and S1 states. This also indicated the ESPIT was difficult
to take place from active H atom to the O atom of the ester
group in S1 state. Therefore, we can conclude that the active
proton of N atom transferring to O atom of central ring was
more reasonable. Based on upon results, this dye molecule
that exhibited ESIPT existed predominantly as keto (RS0) form
in the ground state, however, by photoexcitation, they
underwent tautomerization into enol (PS1) form (keto∗→enol∗)
via an ultrafast and irreversible ESIPT process. The resulting
proton-transfer tautomer was substantially different in
structure and electronic configuration from its corresponding
ground state, thereby led to a large Stokes shifted emission
derived from enol*→enol.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thanks for professor Yan and Miss Ling assisted with data
analysis and characterization. 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 20143ACB21021).
Notes and references
Actually, because of low energy barrier, the distortion of
C1-N1 bond was easier to carry out than the proton transfer in
solution. While in the crystallline state, the distortion of C1-N1
bond was greatly prohibited because of the strong interaction
between molecules, triggering the proton transfer pathway.
Consequently, it was easy to draw the conclusion that the
excited proton transfer mechanism was favorable in solid
state, while the excited state C1-N1 bond distortion was easy
to occur in solution. In addition, the calculated emission bands
of PS1a and PS1 were centered at 576 and 610 nm,
respectively, which was in agreement with the experiment
data in dilute solution and crystalline state. At the same time,
as shown in Fig. 4b, all the transition of TPE-SQ was obviously
a predominant π to π*-type transition from the HOMO to the
LUMO.
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Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C9TC06406K
We report TPE-incorporated squaraines with dual-state emission and decode
their LE/TICT mechanism in solution and ESIPT mechanism in crystalline state