Enabling DPP derivatives to show multistate emission and developing the multifunctional materials by rational branching effect

Article abstract of DOI:10.1016/j.dyepig.2018.06.043

Conventional polycyclic organic molecules exhibit aggregation-caused quenching (ACQ) effect, and some organic molecules with twisted and propeller backbones could be made into aggregation-induced/enhanced emission (AIE/AEE). However, it was interesting but challenging to enable conjugated molecules derived from fused (hetero)cycles with high photo- and thermal-stability to show multistate emission, rather than only dual emission in solution and crystalline states. In the current work, we presented a rational strategy for realizing multistate emission of 1,4-diketo-pyrrolo [3,4-c]pyrrole (DPP) derivatives by employing imidazole as the additional acceptor bridge and constructing the branched DPP-centered molecule with four peripheral donors (triphenylamine). The resulting D?A?D compound (TIDPP) emitted yellow in solution with the fluorescence efficiency of ca. 70%, and its crystalline and amorphous states as well as aqueous suspensions also exhibited impressive fluorescence efficiencies of 40–50%, filling the gap between ACQ and AIE of DPP derivatives. Moreover, aggregation could greatly red-shift emission spectra and significantly enhance the two-photon absorption cross section and excitation fluorescence in the aqueous suspension. TIDPP solid was also found to exhibit the unique mechanochromic blue-shift luminescence and the commendable OFET performance, signifying the existence of aggregation-induced molecular planarization. This work demonstrated that the rational branching effect could manipulate the aggregation emission behaviors and develop the multifunctional materials.

Full text of DOI:10.1016/j.dyepig.2018.06.043

Accepted Manuscript
Enabling DPP derivatives to show multistate emission and developing the
multifunctional materials by rational branching effect
Kai Zhang, Zhenzhen Zhang, Xiaojing Fan, Liangliang Tang, Qi Ding, Kun Yang, Min
Zhai, Haichang Zhang, Shanfeng Xue, Wenjun Yang
PII:
S0143-7208(18)31180-X
10.1016/j.dyepig.2018.06.043
DYPI 6850
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 25 May 2018
Revised Date: 24 June 2018
Accepted Date: 25 June 2018
Please cite this article as: Zhang K, Zhang Z, Fan X, Tang L, Ding Q, Yang K, Zhai M, Zhang H, Xue
S, Yang W, Enabling DPP derivatives to show multistate emission and developing the multifunctional
materials by rational branching effect, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.06.043.
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ACCEPTED MANUSCRIPT
Graphical abstract
The branched DPP-centered derivatives exhibiting bright multistate emission with
strong aqueous 2PA and 2PEF, remarkable mechanohypsochromic luminescence, and
commendable OFET performance.

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Enabling DPP derivatives to show multistate emission and developing the
multifunctional materials by rational branching effect
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Kai Zhang,a Zhenzhen Zhang,a Xiaojing Fan,a Liangliang Tang,a Qi Ding,a Kun Yang,b
Min Zhai,a Haichang Zhang,a Shanfeng Xue, *a Wenjun Yang*a
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a. Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province
(QUST), School of Polymer Science & Engineering, Qingdao University of
Science & Technology, 53-Zhengzhou Road, Qingdao 266042, P. R. China.
E-mail: sfxue@ qust.edu.cn, ywjph2004@qust.edu.cn.
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b. Department of Materials Science and Engineering and the Shenzhen Key laboratory
for Printed Organic Electronics, South University of Science and Technology of
China (SUSTC), No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China.
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Abstract:
Conventional polycyclic organic molecules exhibit aggregation-caused quenching
(ACQ) effect, and some organic molecules with twisted and propeller backbones
could be made into aggregation-induced/enhanced emission (AIE/AEE). However, it
was interesting but challenging to enable conjugated molecules derived from fused
(hetero)cycles with high photo- and thermal-stability to show multistate emission,
rather than only dual emission in solution and crystalline states. In the current work,
we presented
a
rational strategy for realizing multistate emission of
1,4-diketo-pyrrolo[3,4-c]pyrrole (DPP) derivatives by employing imidazole as the
additional acceptor bridge and constructing the branched DPP-centered molecule with
four peripheral donors (triphenylamine). The resulting D‒A‒D compound (TIDPP)
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emitted yellow in solution with the fluorescence efficiency of ca. 70%, and its
crystalline and amorphous states as well as aqueous suspensions also exhibited
impressive fluorescence efficiencies of 40‒50%, filling the gap between ACQ and
AIE of DPP derivatives. Moreover, aggregation could greatly red-shift emission
spectra and significantly enhance the two-photon absorption cross section and
excitation fluorescence in the aqueous suspension. TIDPP solid was also found to
exhibit the unique mechanochromic blue-shift luminescence and the commendable
OFET performance, signifying the existence of aggregation-induced molecular
planarization. This work demonstrated that the rational branching effect could
manipulate the aggregation emission behaviors and develop the multifunctional
materials.
Introduction:
Design, synthesis and optoelectronic properties of new organic luminophores are
currently attracting much attention due to their academic significance and diverse
applications in organic photonics, electronics, optoelectronics, chemosensors and
bio-imaging, etc.[1-5] However, the early investigated fluorophores, such as cyanine,
fluorescein, rhodamine, and pyrene only emit brightly in dilute solutions, but the
weak and quenching emission was observed at high concentration and in the
aggregate states.[6-17] With the exploitation of promising applications of organic
luminophores in modern optical and optoelectronic fields, those exhibiting strong
fluorescence in solid states and aqueous suspensions have become one’s interest.[2,18-23]
Aggregation was the essential nature of conjugated organic dyes, and the key to
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conquering the thorny aggregation-caused-quenching (ACQ)[13-16] was how to
alleviate the intermolecular π-π stacking, especially for those molecules derived from
fused cycles and heterocycles. Thanks to the Tang’s discovery, many conjugated
organic molecules with twisted backbones and rotatable peripheral arylenes have been
found to be strong fluorescence upon aggregation although their solutions exhibited
weak or non fluorescence, which was named “aggregation-induced (enhanced)
emission (AIE/AEE)” and opened up an avenue for developing novel luminophores
with strong aggregate fluorescence.[2,18-30] However, excepting for the faint emission
in fluid solution, most AIE/AEE dyes were characterized by crystallization-enhanced
emission (CEE) and exhibited the decreased fluorescence efficiency in amorphous
aggregates.
The above facts signified that there was a large gap between ACQ and AIE dyes,
and filling the gap and achieving the highly efficient luminophores with multi-state
emission effect would definitely broaden the applications. Recently, there have been
some reports on solution and crystalline state dual emission taking the respective
advantages of ACQ and AIE dyes,[20,31-43] however, the organic dyes exhibiting
multi-state emission (MSE) not only in solution and crystalline state but also in
aqueous suspension and amorphous state were scarce.[18,23] Among various
chromophores, diketopyrrolopyrroles (DPPs) were a class of classical fused
heterocycles with excellent thermal and photo-stability and exhibited strong
fluorescence in dilute solution but typical ACQ effect.[44-49] To enable them to become
MSE and multifunctional materials, some attempts have recently been reported but
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the successful cases were limited (Chart 1).[47-52] For example, Tang and Sun et al.
ever laterally attached the most classical AIE-generator TPE to DPP core, however,
this popular strategy was only occasionally effective to DPPs (dyes 1‒3).[44] Dyes 5[45]
6[46], 7[47] and 10[48]were also ACQ but 4[47], 8[51] and 9[52] were AIE active. This meant
that conjugation bridge and end arylene could significantly influence the aggregation
behaviors of DPP-based derivatives. Dye 5 has been used as the highly effective NIR
photosensitizer for photodynamic/photo-thermal dual mode therapy by Dong and
co-workers. DPP dyes with AIE and large two-photon absorption (2PA) cross section
(δ) (such as dye 9) could be prepared as biocompatible nanoparticles for in vivo
two-photon fluorescence imaging. However, the existence of thermal and
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photo-unstable double bond in 2PA dyes was unfavorable, thus the fully aromatic 2PA
dyes with AIE and large δ were highly desirable. On the other hand, we recently
investigated mechanofluorochromic (MFC) properties of some DPP derivatives and
found that their MFC directions could be made red-shifted (dyes 6 and 7) and
blue-shifted (dye 10), demonstrating that DPP derivatives could be exploited as a new
and unique class of “smart materials” by manipulating conjugation bridge and end
arylene.
Based on the above considerations, we have designed and synthesized a new DPP
derivative for not only filling the gap between ACQ and AIE but also developing the
optoelectronic properties by introducing the additional electron-withdrawing unit and
constructing the branched DPP-centered molecule with multiple D‒A‒D motif
(TIDPP, Scheme 1). Imidazole could not only enhance the acceptor ability of
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conjugation bridge but also provide the crowded branching points. Together with the
propeller conformation and the strong donating ability of tri-phenylamine, a branched
and twisted DPP-centered molecule with enhanced D-A interactions was obtained.
This structural motif and composition was expected to substantially rigidize
intramolecular motions in solution and prevent detrimental π-π interactions in
aggregate states. Thus, the red-shifted and multistate emission was anticipated,
including the promising multifunctional optoelectronic properties, rather than only the
dual emission in solution and crystalline state.
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2. Experimental section
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2.1. Materials
4,4'-(2,5-Bis(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo-[3,4-c]pyrrole-1,4-
diyl)dibenzaldehyde (M3) was from our previous work.[48] Toluene (PhMe) and
tetrahydrofuran (THF) over metallic sodium, and N,N-dimethylformamide (DMF) and
chloroform (CHCl3) over calcium hydride were distilled before use.
4-Bromobenzaldehyde, (4-(diphenylamino)phenyl)boronic acid, thiamine nitrate
(VB1), tetrakis(triphenylphosphine)-palladium (Pd(PPh3)4), potassium carbonate
(K2CO3), aniline, ammonium acetate (CH3COONH4), NH4NO3 and Cu(OAc)2·H2O
were obtained from Energy Chemical Ltd. Shanghai, China, and used without further
purification. Other solvents and chemicals were of analytical grade and obtained
commercially from available resources.
2.2. Sample preparation
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Stimulus-recovering experiments and sample preparation: Pristine solid was ground
on glass plate using a metal spatula at room temperature, and the annealing was
carried out in an oven at 225/270 oC for 30 min under a nitrogen atmosphere, and then
cooled to room temperature before the measurements. The fuming experiment was
conducted in a beaker containing a small amount of dichloromethane, and the sample
was put above the dichloromethane (DCM) level at room temperature. Meanwhile,
the fluorescence images were taken under 365 nm UV lamp and the spectra recorded
using Hitachi F-4600 spectrophotometer. THF/water dispersion of TIDPP (aqueous
dispersion) was prepared by slowly adding 1 mL of THF solution of TIDPP (1.0 ×
10–4 M) into a given amount of distilled water under ultrasound at room temperature.
2.3. Electrochemical Testing
Cyclic voltammetry (CV) was performed using a BAS 100W Bioanalytical
Systems, using a glass carbon disk (diameter = 3 mm) as the working electrode, a
platinum wire with a porous ceramic wick as the auxiliary electrode, and Ag/Ag+ as
the reference electrode standardized by the redox couple ferrocenium/ferrocene.
Anhydrous dichloromethane (CH2Cl2) containing 0.1 M tetrakis(n-butyl)-ammonium
hexafluorophosphate (NBu4PF6) as the supporting electrolyte was used as solvents
under a nitrogen atmosphere. All solutions were purged with a nitrogen stream for 10
min before measurements. The procedure was performed at room temperature, and a
nitrogen atmosphere was maintained over the solution during measurements. A scan
rate of 50 mV s‒1 was applied.
2.4. OFET Device Fabrication and Testing
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Bottom gate, bottom contact (BGBC) field effect transistors configuration was used
to fabricate devices. Highly doped Si (n doped) wafer with 300 nm thermal oxide
(Silicon Quest International) was used as the substrate, where the doped silicon layer
was used as back gate (G) and the oxide as the dielectric materials (Ci, capacitance of
10 nF cm‒2). After cleaning the substrate with Piranha (H2SO4/H2O2 = 3/1,v/v),
deionized water and acetone, oxygen plasma etching (10 min, PDC-001), the substrate
was immersed in OTS solution (5% in toluene) at room temperature overnight in an
Ar-filled glove box. Subsequently source (S) and drain (D) electrodes (gold, thickness:
50 nm) were deposited on the wafer through a shadow mask by electron-beam
evaporation. The distance between source and drain electrode is 50 µm and the
channel width is 1000 µm. 4 mg/mL TIDPP solution (in chloroform) was dropped on
the surface of the substrate through a syringe (with PTFE filter, 0.45 µm). The glass
Petri dish was then covered for evaporating the solvent slowly at room temperature
for 3 h. The device was then measured as "as prepared" device. The "as prepared"
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transistor device was then annealed at 200 C for 15 min on a hot plate in Ar-filled
glove box. Then these devices were measured again as "annealed" device. The field
effect mobility was calculated using ISD = (W/2L) µ Ci (VG–VT)2 based on the
saturation region of the output characteristic of the devices where L and W were the
channel length and width; Ci was the capacitance of gate oxide, VSD was voltage
between source and drain electrodes; ISD was the current between source and drain
electrodes; VG was the gate voltage.
2.5. Measurements
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1H and C NMR spectra were recorded on a Bruker AC500 spectrometer at 500
MHz and 125 MHz, respectively, when the compounds were dissolved in deuterated
chloroform (CDCl3) with tetramethylsilane (TMS) as the internal standard.
MALDI-TOF MS mass spectra were recorded using an AXIMA-CFRTM plus
instrument. UV-visible absorption spectra were recorded using a dual-beam grating
Hitachi U-4100 absorption spectrometer with a 1 cm quartz cell. Fluorescence
emission spectra were obtained by using Hitachi F-4600 spectrophoto-meter.
Differential scanning calorimetry (DSC) curves were determined on a Netzsch DSC
(204F1) instrument at a heating (or cooling) rate of 10 C min‒1. Thermogravimetric
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analysis (TGA) was performed on a Netzsch (209F1) thermogravimetric analyzer
under a nitrogen atmosphere (50 mL min‒1) at a heating rate of 10 C min‒1. Powder
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X-ray diffraction (PXRD) experiments were performed with X-ray diffractometry
(INCA Energy, Oxford Instruments) with operating at 4 kW. Solution fluorescence
quantum yields (ΦF) and solid-state fluorescence efficiencies were carried out with
FLS980 Spectrometer. The ground-state geometry was optimized at the level of
B3LYP/6-31G (d,p), and spatial distributions of the HOMO and LUMO of the
compound were obtained from the optimized ground state structure. Two-photon
absorption cross section (δ) was performed by using a femtosecond Ti:sapphire
oscillator (Avesta TiF-100M) with output laser pulses and average power of 100 mW
as the excitation source. The laser pulses have pulse duration of 80 fs and repetition
rate of 84.5 MHz in the wavelength range of 710-1000 nm. The pumping wavelengths
were determined by a monochromator-CCD system. Rhodamine B in methanol was
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used as a reference standard (r).[57] The two-photon absorption cross section (δ) of a
sample compound (s) can be calculated at each wavelength according to the equation:
δs = [IsΦrnr2cr]/[IrΦsns2cs]δr.[58,59] Where I is the integral area of the two-photon
excitation fluorescence, Φ is the fluorescence quantum yield (assuming that Φ is
unchanged under both one- and two-photon excitation), n is the refractive index, and c
is the number density of the molecules in solution. The solution concentrations for
both sample and reference are 1.0 × 10–5M.
2.6. Synthetic procedures and characterization
2.6.1. 1,2-Bis(4-bromophenyl)ethane-1,2-dione (M1).
A 250 mL of round-bottom flask was charged with VB1 (5 g, 20 mmol), deionized
water (5 mL) and ethanol (100 mL), and the mixture was cooled by an ice-salt bath.
When the temperature was below zero, a 10% NaOH aqueous solution was added
dropwise until the pH reached to 9. After that, 4-bromobenz-aldehyde (20 g, 108
mmol) was added, and the mixture was kept at 65 oC and stirred for 12 h. The solution
was cooled and the precipitate was filtered off, washed with water and ethyl alcohol.
The crude product (a mixture of benzoin and benzil) was recrystallized from ethanol
to give the white crystals (11 g, Yield: 55%). The white crystals (1.0 g, 2.7 mmol)
were added into a solution of NH4NO3 (0.27 g, 3.38 mmol) and Cu(OAc)2· H2O (62.8
mg, 0.34 mmol) in acetic acid (11.0 mL) and stirred and refluxed for 2 h. Upon
cooling to the room temperature, a precipitate was formed. Filtration followed by
washing with water and ethanol gave pure M1 as the yellow crystals (0.82 g, Yield:
84.5%). 1H NMR (500 MHz, CDCl3): δ ppm 7.83 (d, J = 8.4 Hz, 4H), 7.67 (d, J = 8.4
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Hz, 4H). Anal. Calcd. for C14H8Br2O2: C, 45.69; H, 2.19; Br, 43.42%. Found: C,
45.62; H, 2.21; Br, 43.47%. MALDI-TOF MS (mass m/z): Calcd. for C14H8Br2O2:
368.0201, found: 368.1002 [M+].
2.6.2. 1,2-Bis(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)ethane-1,2-dione (M2).
A mixture of M1 (0.50 g, 1.37 mmol), (4-(diphenylamino)-phenyl)boronic acid
(0.90 g, 3.142 mmol), K2CO3 (0.57 g, 4.1 mmol), THF (50 mL) and deionized water
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(10 mL) with Pd(PPh3)4 (64 mg, 55 µmol) acting as a catalyst was refluxed at 70 C
for 24 h under a nitrogen atmosphere. After the mixture was cooled down, 40 mL of
deionized water was added and extracted with dichloromethane. The organic phase
was dried over anhydrous MgSO4. After filtration and evaporation, the residue was
purified through silica gel column chromatography using CH2Cl2/petroleum ether (1:1,
v/v) as an eluent to give the product as an orange solid (0.75g, Yield: 78%). 1H NMR
(500 MHz, CDCl3) δ 8.03 (dd, J = 8.4, 2.5 Hz, 4H), 7.75–7.66 (m, 4H), 7.55–7.45 (m,
4H), 7.23–7.34 (m, 8H), 7.19–7.10 (m, 12H), 7.07 (dt, J = 8.5, 4.2 Hz, 4H). 13C NMR
(125 MHz, CDCl3) δ 194.13, 148.52, 147.21, 146.93, 132.41, 131.11, 130.56, 129.37,
128.00, 126.77, 124.90, 123.50, 122.91. Anal. Calcd. for C50H36N2O2: C, 86.18; H,
5.21; N, 4.02%. Found: C, 86.14; H, 5.19; N, 4.08%. MALDI-TOF MS (mass m/z):
Calcd. for C50H36N2O2: 696.2777, found:: 696.3321 [M+].
2.6.3. 3,6-Bis(4-(4,5-bis(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)-1-phenyl-1H-im-
idazol-2-yl)phenyl)-2,5-bis(2-ethylhexyl)pyrrolo-[3,4-c]pyrrole-1,4(2H,5H)-dione
(TIDPP).
Under nitrogen atmosphere, M3 (0.30 g, 0.53 mmol), M2 (0.74 g, 1.06 mmol),
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aniline (0.39 g, 4.24 mmol), ammonium acetate (0.20 g, 2.65 mmol) were added to
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CH3COOH (10 mL), and then stirred for 24 h at 140 C. After being cooled to the
room temperature, the mixture was poured onto water and extracted with
dichloromethane. The organic phase was dried over anhydrous MgSO4 and the
product was separated by silica column chromatography using CH2Cl2/ethyl acetate
(20:1, v/v). A red solid product was obtained (0.2g, yield: 27%). 1H NMR (500 MHz,
CDCl3) δ 7.71 (s, 9H), 7.53–7.45 (m, 17H), 7.34 (s, 7H), 7.25 (s, 6H), 7.22–7.07 (m,
32H), 7.03 (q, J = 7.5 Hz, 9H), 3.72 (s, 4H), 1.11 (d, J = 32.7 Hz, 18H), 0.80 (t, J =
7.2 Hz, 6H), 0.68 (s, 6H). 13C NMR (125 MHz, CDCl3) δ 162.70, 148.02, 147.64,
147.50, 146.95, 145.74, 139.84, 138.68, 138.59, 136.97, 134.82, 133.77, 132.82,
132.65, 131.59, 131.41, 129.23, 128.66, 128.46, 127.92, 127.83, 127.63, 127.45,
126.31, 124.71, 124.38, 124.30, 123.94, 123.82, 123.26, 122.99, 122.78, 110.07,
45.15, 38.30, 30.23, 28.19, 23.60, 22.84, 13.95, 10.35. Anal. Calcd. for C148H124N10O2:
C, 85.68; H, 6.02; N, 6.75%. Found: C, 85.62; H, 5.98; N, 6.83%. MALDI-TOF MS
(mass m/z): Calcd. for C14H8Br2O2: 2074.6340, found: 2074.7268 [M+].
3. Results and discussion
3.1. Synthesis and characterization
The structure and synthetic route of TIDPP is shown in Scheme 1. Starting from
4-bromobenzaldehyde (S1), M1 could be facilely obtained according to the literature
method.
The
Suzuki
coupling
of
M1
and
commercial
(4-(diphenylamino)phenyl)-boronic acid (S2) afforded 1,2-diketo derivative M2. M3
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was from our previous work, which needed four-step synthesis in a good total yield.[48]
The condensation reaction of M2 and M3 assisted by aniline in acetic
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acid/ammonium acetate at 140 C produced the crude product TIDPP. Pure TIDPP
was obtained by silica-gel column chromatography in a yield of 27% and
characterized by NMR, MALDI-TOF MS and element analysis.
3.2. Solution and Solid-state photophysical properties and quantum chemical analysis
Figure 1 showed fluorescence photos of TIDPP in different solvents and solid states,
and the corresponding absorption and emission spectra were depicted in Figure 2.
TIDPP not only exhibited same absorption and emission spectra (peaks at ca. 580 nm)
(Figure 2a) and fluorescence colors (yellow) but also similar fluorescence quantum
yields (ca. 70%) in different solvents (Figure 1a). This implied the weak
solvatochromism, and the similar phenomena have been observed for other
DPP-based molecules with D‒A‒D motif by us and others, which might be ascribed
to their symmetrical and convergent frontier molecular orbital feature (Figure
3).[46,47,53] Interestingly, TIDPP emits bright red fluorescence in both solid powder
(639 nm) and spin-coated film (630 nm), and the fluorescence efficiency still
maintains up to 48.9% and 41.2%, respectively. The strong and multi-state emission
properties demonstrated that this structural motif could substantially rigidize the
intra-molecular motions in solution and prevent the intermolecular π-π interactions in
aggregate states, and thus the significantly red-shifted emissions in aggregate states
could signify that the aggregation has induced the molecular planarization.
The optimized molecular geometry indicates that all the aryl units adopt the more
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twisted conformations, moreover, the DPP core itself is also nonplanar and in a bent
configuration with the tilt angle of 7.1o (Figure 3). Our previous single crystal
analyses have indicated that DPP core could indeed exist in planar or bent
configurations to render the dyes different conformation and emission.[45,60] This
could explain the yellow emission of TIDPP in solutions since the twisted
conformation and the bent π-center configuration would lower molecular effective
conjugation degree. After aggregation, the intermolecular stacking interactions might
partly planarize the bent π-center and the twisted conformation to contribute the
red-shift emission, but the still twisted structure could prevent from the intermolecular
close π-π interactions to afford the effective aggregate emission.[24,26,44,52] This effect
was further evidenced by the formation of aqueous suspensions. When the molecules
were dispersed into the non-solvent water under sonication, the resulting suspension
emitted the bright and red-shifted fluorescence (Figure 2b). Moreover, the emission
spectra were extended to the near infrared (NIR) region, and significantly, the
fluorescence efficiency could still maintain more than half of the solutions. This
phenomenon was intriguing and valuable because water-dispersible donor‒acceptor
dyes with bright red/NIR fluorescence have been applied in bio-imaging, and
photodynamic and photothermal therapy.[47,52,61-63]
3.3. Solution and aqueous two-photon properties
Inspired by the multiple and enhanced D–A–D molecular structure and the
impressive fluorescence efficiency as well as the aggregation-induced red/NIR
emission, we investigated the 2PA and two-photon excitation fluorescence (2PEF)
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properties of TIDPP. The 2PA cross section (δ) was measured by 2PEF method using
femtosecond laser as excitation light, which could give reliable δ and avoid possible
complications of the excited-state absorption and the thermal lens effect.[64,65] To
confirm the occurrence of nonlinear absorption, fluorescence emission of TIDPP in
both THF solution and THF/water mixture (1/9) were recorded under the excitation of
860 nm laser with different input powers (50–150 mW). It was observed that the
fluorescence intensity was gradually increased with the input laser power (Figure 4a
and Figure S1), moreover, the obtained emission spectra were similar to its
one-photon excited ones (Figure 2), implying that the fluorescence emissions
occurred from same excited states, regardless of the excitation modes.
As TIDPP has no linear absorption after 700 nm (Figure 2a), the appearance of
fluorescence emission upon long-wavelength excitation indicated the existence of
non-linear absorption. Furthermore, the plot of logarithmic output fluorescence
intensity versus logarithmic input laser power showed the near square dependence
with exponent 1.84 and 1.91 for THF solution and aqueous suspension, respectively
(inset of Figure 4a, Figure S1), which clearly indicated a two-photon excitation
(absorption) process.
The δ was determined under 90 mW of input laser power (which fell within the
power range of two-photon excitation, Figure 4a) as a function of excitation
wavelength (760–1000 nm). Figure 4b depicts the two-photon excitation spectra of
TIDPP and indicates that the 2PA bands are distributed throughout the measurement
wavelength range. Importantly, the δ values are significantly enhanced by aggregation,
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and the maximal δ values are only 320 GM in THF solution but up to 1960 GM in the
aqueous suspension. It is known that the planarization would increase molecular
effective conjugation and D-A interaction strength, which is extremely favorable for
large δ. In other words, the aggregation-enhanced δ could provide a reasonable
evidence for the aggregation-induced molecular planarization. Overall, TIDPP is the
2PA dye that simultaneously possesses large δ and strong red/NIR 2PEF in aqueous
media. Together with the fully aromatic structure and the tunable two-photon
excitation wavelength, TIDPP should be a more promising two-photon dye applicable
in biological science and technology.
3.4. Solid-state stimuli-responsive behaviors and mechanism analysis
Based
on
the
significantly
aggregation-red-shifted
emission
and
aggregation-enhanced δ, it could be deduced that TIDPP might adopt the less bent
configuration of DPP core and the more planar molecular conformation in the pristine
crystalline and aqueous aggregate states. If so, the transformation from crystalline to
amorphous states should afford a hypsochromic shift luminescence. At present, it was
no longer difficult to find an organic mechanobathochromic shift luminophor,
however, the reports on mechanohypsochromic shift are limited.[48,60,66-71] Thus, we
mechanically ground the pristine solid powder and found that the emission peaks were
blue-shifted from 639 nm to 620 nmv (Figure 5). When the ground solid was
solvent-fumed at room temperature, a deep-red-emitting state (667 nm) appeared. The
reversible fluorescence change could readily happen upon grinding and
o
solvent-fuming cycle. However, annealing the ground state below 200 C hardly
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changed the colors, and only the temperature was elevated to 270 oC in N2, a deep-red
emission state (672 nm) was observed. Nevertheless, a new mechanohypsochromic
shift luminophor was obtained, and the mechanohypsochromic shift luminescence
also might due to the existence of aggregation-induced molecular planarization.
To further understand the fluorescence emission behaviors and the spectral changes
of TIDPP in different solid states, X-ray diffraction (XRD) and differential scanning
calorimetry (DSC) analyses were conducted and depicted in Figure 6. On the whole,
the pristine and ground solids were in the crystalline and amorphous states,
respectively, and both the annealed and fumed solids were in the crystalline states.
However, the careful inspection on XRD patterns indicates that the pristine solid has
the characteristics of crystalline and amorphous co-existence, which is further
confirmed by DSC analysis that indicates a clear cold-crystallization transition around
o
o
256 C before isotropic melt transition (303 C) upon heating process. This could
explain why the emission spectrum of the pristine solid was situated between those of
the ground and annealed solids. The ground solid exhibits an additional exothermic
o
o
transition (cold-crystallization) around 217 C, besides that at 256 C. The transition
o
o
at 217 C disappeared but the transition at 256 C remained upon annealing the
o
ground sample at 225 C, which afforded a red-emitting state (635 nm) similar to the
pristine state. Only the temperature was elevated to 270 oC in N2, the transition at 256
oC disappeared, affording a deep-red emission state (672 nm). In contrast, the
solvent-fuming could readily eliminate both cold-crystallization transitions to afford
more ordered crystals with redder fluorescence. Overall, the mechanofluorochromic
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behavior was ascribed to the phase transformation between crystalline and amorphous
states, and manipulating the DPP-centered derivatives could provide a family of
mechanohypsochromic shift luminophores.
3.5. Electrochemical behavior and OFET properties
Since the existence of DPP unit, multiply enhanced D-A structure and
aggregation-induced molecular planarization, TIDPP might be a promising electronic
material. Cyclic voltammetry (CV) was performed using a BAS 100W Bioanalytical
Systems and the calculated HOMO and LUMO levels of TIDPP were -5.47 and -3.66
eV, respectively (Figure S3). The charge transport performance of TIDPP was
evaluated by OFET devices with the bottom-gate, bottom-contact configuration on the
n-type silicon wafer with a layer of 300 nm SiO2 as the dielectric material. All the
devices were fabricated by drop-casting the TDPP solution onto the OTS-treated
wafer with pre-patterned gold source and drain electrodes and measured under
vacuum condition. Drop-casting technique instead of spin-coating was chosen to
allow the solvent slow evaporation, which was favorable for the formation of more
ordered (crystalline) thin film and the charge transport between the neighboring
molecules. Figure 7 shows the transfer and out-put characteristics of the transistors,
and the corresponding data of device performance are summarized in Table S1.
TIDPP exhibited p-type behavior with the commendable hole mobility (µh) of 0.168
cm2 V‒1 s‒1 and a threshold voltage of 3.1 V.[72,73] The on-to-off current ratio was
found to be ca. 106. Moreover, the OFET performance was almost unchanged (µh =
0.171 cm2 V‒1 s‒1, Figure S2 and Table S1) after the thermal annealing of thin film at
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200 C, which could be ascribed the excellent thermal and morphological stability of
o
TIDPP solid before 200 C (Figure 6). Therefore, TIDPP might also be a promising
p-type material for electronics application.
4. Conclusions
In summary, a new red/NIR-emitting DPP-based derivative (TIDPP) with imidazol
ring as the acceptor conjugation bridge has been designed and synthesized. TIDPP
exhibited multistate emission and could retain strong fluorescence even in aqueous
suspensions. Moreover, large δ and strong red/NIR 2PEF upon aqueous aggregation
were simultaneously achieved, signifying its promising potential in two-photon
biological science and technology such as photodynamic and photothermal therapy as
well as bio-imaging. Furthermore, unlike the present most mechanofluorochromic
materials, TIDPP exhibited the rare mechanohypsochromic luminescence,
emphasizing the role of π-center and conjugation bridge in determining the
stimuli-responsive behaviour. It was quite possible to prepare new and multifunctional
DPP derivatives following the designed and synthetic strategy described in this work.
Thus, the types of stimuli responsive materials were enriched and the types and
optical properties of DPP derivatives were further expanded.
Acknowledgment
Financial support for this research was provided by the National Natural Science
Foundation of China (No. 51673105, 51573081and 51473084), Natural Science
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Foundation of Shandong Province (No. ZR2018MB024, ZR2016JL016), the Key
Resarch and Development Program of Shandong Province of China (No.
2018GGX102022) and the Natural Science Foundation of Qingdao City of China (No.
16-5-1-89-jch). We thank Open Project of the State Key Laboratory of
Supramolecular Structure and Materials of Jilin University (SKLSSM-201828).
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