Dissociation exists in s-triazine based donor-accepter organic systems by photo-induced electron transfer

Article abstract of DOI:10.1016/j.dyepig.2016.12.030

In the organic donor-acceptor system, the dissociation of the chloride anion in the s-triazine group as the acceptor was first investigated under irradiation. Introducing the electron-deficient chromophore as acceptor 2 to the donor-acceptor module allows the photo-induced electron transfer from the s-triazine module, making the dissociation less in the donor-acceptor1-acceptor2 architecture, which has been proven to be a good strategy to increase light-stability.

Full text of DOI:10.1016/j.dyepig.2016.12.030

Dyes and Pigments 139 (2017) 264e273
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Dissociation exists in s-triazine based donor-accepter organic systems
by photo-induced electron transfer
,
b
,
,
,
, b
Tianyang Wang a *, Haiya Sun a b, Ting Lu a b, Chelsea N. Bridgmohan c, Fengqing Li a
,
Dongzhi Liu a d, Wenping Hu b e, Wei Li a d, Xueqin Zhou a
**, Lichang Wang a
,
,
,
b,
,
b,
d,
, b, c, ***
a School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300354, China
b Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China
c Department of Chemistry and Biochemistry and the Materials Technology Center, Southern Illinois University, Carbondale, IL, 62901, United States
d Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin, 300354, China
e School of Science, Tianjin University, Tianjin, 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In the organic donor-acceptor system, the dissociation of the chloride anion in the s-triazine group as the
acceptor was first investigated under irradiation. Introducing the electron-deficient chromophore as
acceptor 2 to the donor-acceptor module allows the photo-induced electron transfer from the s-triazine
module, making the dissociation less in the donor-acceptor1-acceptor2 architecture, which has been
proven to be a good strategy to increase light-stability.
Received 17 October 2016
Received in revised form
1 December 2016
Accepted 7 December 2016
Available online 18 December 2016
© 2016 Elsevier Ltd. All rights reserved.
Keywords:
Donor-acceptor system
Donor-acceptor1-acceptor2 architecture
Dissociation of chloride anion
Photo-induced electron transfer
Light-stability
S-triazine
1. Introduction
electron transfer. It is recognized that D-A materials do in fact
improve the efficiencies of OSCs [7e12]. Moreover, some reports
Organic solar cells (OSCs) have been developed rapidly as an
inexpensive alternative to large-area Si-based solar modules, which
have the potential for low-cost and high-throughput production.
OSCs can be grouped into small organic molecule cells and polymer
cells, which are distinguished by the type of molecular solids
known as organic semiconductors, from which they are composed
[1e6]. In some cases, the separation of the excitons in OSCs could
also occur within the organic semiconductors that are used the
donor-acceptor (D-A) architecture to achieve photo-induced
have focused on the light-stability of D-A materials, which has been
proven to be the most important factor affecting the repeatability of
device efficiency [13,14]. This means that the effective photo-
induced electron transfer (PET) of the D-A architectured material
could improve the device efficiency of OSCs, but it may cause the
material to photo-degrade, thereby affecting the light-stability of
the device [15]. The light-stability of the OSC device determines its
application value. Therefore, it is also important to study the light-
stability of photovoltaic materials in the OSC devices and develop a
better strategy, other than the traditional D-A architecture method,
to promote PET and prevent photodegradation.
S-triazine as the electron-accepting module was the effective
linker to improve the electron-injection and electron-transport-
ation abilities of its conjugated derivatives [16,17]. In our previous
work [12,18e20], we reported a series of D-A1-A2 systems with the
triphenylamine (MTPA) or indoline (YD) derivative as a donor, s-
triazine derivative (TRC) as the first acceptor (A1), and anthraqui-
none derivative (AEAQ), or two perylene bisimide derivatives (PDI
and PBI) as the second acceptors (A2), denoted as D-TRC-A2
* Corresponding author. School of Chemical Engineering and Technology, Tianjin
University, Tianjin, 300354, China.
** Corresponding author. School of Chemical Engineering and Technology, Tianjin
University, Tianjin, 300354, China.
*** Corresponding author. Department of Chemistry and Biochemistry and the
Materials Technology Center, Southern Illinois University, Carbondale, IL, 62901,
United States.
E-mail addresses: tianyangwang@tju.edu.cn (T. Wang), zhouxueqin@tju.edu.cn
(X. Zhou), lwang@chem.siu.edu (L. Wang).
http://dx.doi.org/10.1016/j.dyepig.2016.12.030
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

T. Wang et al. / Dyes and Pigments 139 (2017) 264e273
265
systems. Different charge separation efficiency and lifetime of CS
2. Experimental and computational methods
states were achieved by changing the donor and second acceptor in
D-TRC-A2 systems. MTPA-TRC exhibits similarly low photoelectric
conversion efficiency (ƞ ¼ 0.03%) in the single-layer OSCs with
common organic semiconductors such as phthalocyane, poly(-
phenylenevinylene) and triarylamine derivatives [9,21e23] of
which the efficiency has been improved to greater than 6% and can
even exceed 10% by the combined application of other semi-
conductors and the proper design of solar cell structures [9,24e27].
Importantly, the single-layer OSC constructed using MTPA-TRC-
AEAQ (ƞ ¼ 0.89%) with long-lived CS states (650 ns) was found to
be significantly more efficient than that using MTPA-TRC. It in-
dicates that single-layer OSCs had been proven to be the highly
effective cell structure reflecting the effect of CS states with
different lifetimes of organic materials, and the far better photo-
voltaic characteristics of D-TRC-A2 than D-TRC suggests potential
applications in OSCs, if better structural designing of OSCs.
In this paper, we investigate the effects of intramolecular elec-
tron transfer on the light-stability of D-TRC-A2 systems, and report
the results and analyse the effects of second acceptors on the de-
gree of dissociation, which are relative to the electron transport
process. D-A system MTPA-TRC and D-TRC-A2 systems MTPA-TRC-
AEAQ and MTPA-TRC-PBI will be taken as the examples (Chart 1),
where experimental and computational results indicated that some
dissociations of the chloride anions exist in MTPA-TRC due to the
effective PET process from the excited MTPA to the TRC module.
However, the dissociation has been found to be less frequent in
MTPA-TRC-AEAQ and MTPA-TRC-PBI because of the sequential
electron transfers from excited MTPA to the TRC followed by the
AEAQ (or PBI) module, making MTPA-TRC-AEAQ and MTPA-TRC-PBI
more stable under irradiation.
2.1. Materials
Structures of the key compounds being 0synthesized and studied
in this work are provided in Chart 1. 4,4 -dimethyl-400-styryl-tri-
phenylamine (MTPA) [18], 4,40-dimethyl 400-(4-(4,6-dichloro-1,3,5-
triazin-2-ylamino)styryl)triphenylamine (MTPA-TRC) [18], 1-(2-
aminoethylamino)anthraquinone (AEAQ) [18], 1-((4,6-dichloro-
1,3,5-triazin-2-ylamino)ethylamino)-9,10-anthraquinone (AEAQt)
[18],
4,40-dimethyl-400-(4-(6-dichloro-1,3,5-triazin-2-ylamino-4-
ethylamino)styryl)triphenylamine (MTPA-TRC-EA) [18], 4,40-dime-
thyl-400-(4-(6-dichloro-1,3,5-triazin-2-ylamino-4-phenylamino)st-
yryl)triphenylamine (MTPA-TRC-BA) [20], D-A1-A2 systems 4,40-
dimethyl-400-(4-(4-chloro-6-(2-(9,10-dioxoanthracen-1-ylamino)
ethylamino)-1,3,5-triazin-2-ylamino)styryl)triphenylamine
(MTPA-TRC-AEAQ) [18], and 4,40-dimethyl-400-(4-(4-Chloro-6-(N-
(1-hexylheptyl)-N0-(4-amino)phenyl-1,7-di(4-tert-butylphenoxyl)
-perylene-3,4,9,10-tetracarboxylbisimide)-1,3,5-triazin-2-ylamino)
styryl)triphenylamine (MTPA-TRC-PBI) [20] were prepared ac-
cording to the literatures. cyanuric chloride (TRI) and melamine
(Mela) were purchased. The synthetic pathways of 2,4-dichloro-6-
anilino-1,3,5-triazine (TRC-Ph), 2-chloro-4,6-dianilino-1,3,5-triazi-
ne (TRC-diPh), 2,4,6-trianilino-1,3,5-triazine (TRC-triPh) and 4,40-
dimethyl-400-(4-(6-dichloro-1,3,5-triazin-2-ylamino-4-cyano)sty-
ryl)triphenylamine (MTPA-TRC-CN) were illustrated in Scheme 1.
All reagents and solvents were in reagent grade and further purified
by the standard methods if necessary. All synthetic procedures
were carried out under an atmosphere of dry nitrogen or dry argon
unless otherwise indicated.
Chart 1. The structure of key compounds.

266
T. Wang et al. / Dyes and Pigments 139 (2017) 264e273
Scheme 1. Synthesis scheme of TRC-Ph, TRC-diPh and TRC-triPh and MTPA-TRC-CN. a) N2, aniline, 0ꢀC, 10min; b) N2, aniline, 40ꢀC, 80min; c) N2, aniline, 90ꢀC, 5h. d) N2, sodium
cyanide, 10ꢀC, 3h.
2.1.1. Synthesis of TRC-Ph
60.62; H, 4.11; N, 23.48% (Calc. for C15H12ClN5: C, 60.51; H, 4.06; N,
23.52%).
Cyanuric chloride (TRI) (240 mg, 1.3 mmol) and aniline (0.19 mL,
2 mmol) were dispersed into 30 mL anhydrous THF in N2 atmo-
sphere. The solution was then stirred at 0 ꢀC for 10 min. Afterwards,
the solvent was removed by rotary evaporation and the residue was
purified by column chromatography on silica gel using petroleum
ether/ethyl acetate (1.5:1 v/v) as the eluent to yield a white solid
(275 mg, 87.4%). HRMS (ESI): m/z ¼ 241.0047 [MþH]þ (calcd for
2.1.3. Synthesis of TRC-triPh
Cyanuric chloride (TRI) (240 mg, 1.3 mmol) and aniline (0,28 mL,
3 mmol) were dispersed into 30 mL anhydrous THF in N2 atmo-
sphere. The solution was then stirred at 90 ꢀC for 5 h. Then the
solvent was removed by rotary evaporation and the residue was
purified by column chromatography on silica gel using petroleum
ether/ethyl acetate (3:1 v/v) as the eluent to yield a grayish solid
(175 mg, 37.6%). HRMS (ESI): m/z ¼ 355.1672 [MþH]þ (calcd for
C9H7Cl2Nþ4 : m/z ¼ 241.0048). 1H NMR (CDCl3, 500 Hz):
d 7.88 (s,1H),
7.53 (d, J ¼ 7.5 Hz, 2H), 7.40 (t, J ¼ 7.5, 15 Hz, 2H), 7.23 (t, J ¼ 7.5,
15 Hz, 1H). Elem. Anal.: Found: C, 44.75; H, 2.55; N, 23.31% (Calc. for
C9H6Cl2N4: C, 44.84; H, 2.51; N, 23.24%).
C
21H19Nþ6 : m/z ¼ 355.1666). 1H NMR (CDCl3, 500 Hz):
d 7.58 (d,
J ¼ 8 Hz, 6H), 7.34 (t, J ¼ 8, 16 Hz, 6H), 7.17 (s, 3H), 7.11 (t, J ¼ 7.5,
15 Hz, 3H). Elem. Anal.: Found: C, 71.29; H, 5.20; N, 23.65% (Calc. for
2.1.2. Synthesis of TRC-diPh
C21H18N6: C, 71.17; H, 5.12; N, 23.71%).
Cyanuric chloride (TRI) (240 mg, 1.3 mmol) and aniline (0.19 mL,
2 mmol) were dispersed into 30 mL anhydrous THF in N2 atmo-
sphere. The solution was then stirred at 40 ꢀC for 80 min. The
solvent was then removed and the residue was purified by column
chromatography on silica gel using petroleum ether/ethyl acetate
(2:1 v/v) as the eluent. A white solid was obtained (245 mg, 62.2%).
HRMS (ESI): m/z ¼ 298.0858 [MþH]þ (calcd for C15H13ClN5þ: m/
2.1.4. Synthesis of MTPA-TRC-CN
Sodium cyanide (3.2 mg, 0.07 mmol) was added to the anhy-
drous DMSO (25 mL) solution of MTPA-TRC (70 mg, 0.13 mmol). The
mixture was stirred at 10 ꢀC for 3 h and then the solvent was
removed by rotorary evaporation. The residue was purified by
column chromatography on silica gel using n-hexane/ethyl acetate
(6:1 v/v) as the eluent and an orange solid was yielded (41 mg,
z ¼ 298.0859). 1H NMR (CDCl3, 500 Hz):
d 7.51e7.55 (m, 6H),
7.43e7.30 (m, 4H), 7.16 (t, J ¼ 7.5, 15 Hz, 2H). Elem. Anal.: Found: C,

T. Wang et al. / Dyes and Pigments 139 (2017) 264e273
267
(m, 12H), 2.33 (s, 6H). 13C NMR (CDCl3, 100.6 MHz),
d: 172.67, 171.43,
150.12, 147.27, 145.31, 136.51, 134.15, 132.59, 131.37, 129.74, 128.63,
127.05, 124.87, 123.78, 121.56, 115.26, 21.15. Elem. Anal.: Found: C,
72.74; H, 4.71; N, 16.01% (Calc. for C32H25ClN6: C, 72.65; H, 4.76; N,
15.89%).
2.2. Mass spectrometry
We obtained the ESI mass spectra using a Thermo Fisher mass
spectrometer, LCQ Deca XP MAX and the MALDI-TOF mass spectra
using a BrukerAutoflextof/tofIII mass spectrometer.
2.3. NMR spectrometry
1H NMR spectra and 13C NMR were collected on a VARIAN
INOVA 500 MHz spectrometer. The experiments were done at room
temperature.
2.4. Thermogravimetric analysis (TGA)
Thermogravimetric analysis was performed on a Rigaku Thermo
plus TG 8120 instrument. The thermal stability of the samples was
determined by measuring the weight loss while heating at a rate of
10 ꢀC minꢁ1 from room temperature to 500 ꢀC under the nitrogen
atmosphere.
2.5. Differential scanning calorimetry (DSC)
Differential scanning calorimetry at a heating rate of 10 ꢀC
minꢁ1 from 50 to 250 ꢀC was performed using a TA DSC Q 20 in-
strument under the nitrogen atmosphere.
2.6. EPR spectroscopy
The EPR measurements were carried out on a JEOL FA 200 in-
strument. The dye solution was introduced in a sample tube and
bubbled with argon for 30 min. The simulation of EPR spectra was
carried out using Bruker's WIN-EPR SimFonia Software Version
1.25. A USHI OPTICAL MODULEX spectrometer was used to assist in
the irradiation experiments and the light wavelength ranges from
350 to 410 nm.
Fig. 1. TGA (a) and DSC (b) curves of MTPA-TRC (black), MTPA-TRC-CN (olive), MTPA-
TRC-AEAQ (blue) and MTPA-TRC-PBI (orange). (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
60.0%). Mp > 300 ꢀC; HRMS (ESI): m/z ¼ 529.1905 [(MþH)þ] (calcd
for C32H26ClNþ6 : m/z ¼ 529.1902). 1H NMR (CDCl3, 500 Hz): 7.94 (s,
1H), 7.53 (dd, J ¼ 8.5, 8.5 Hz, 4H), 7.35 (d, J ¼ 8.5 Hz, 2H), 7.12e6.92
Scheme 2. The two dissociation pathways of compound MTPA-TRC.

268
T. Wang et al. / Dyes and Pigments 139 (2017) 264e273
2.7. Cyclic voltammetry
describe the effect of solvent, which was toluene in this work. The
self-consistent field (SCF) convergence was set to be 10ꢁ8 a.u. The
gradient and energy convergence was set to be 10ꢁ4 a.u. and 10ꢁ5
a.u., respectively. All the calculations were done using Gaussian09
package [32]. Frequency calculations were performed to confirm
the optimized geometry as a minimum. More detailed description
of the computational methods can also be found in the previous
work [33].
The electrochemical properties were taken using a BAS 100 W
electrochemical analyzer with a three-electrode configuration and
a glassy carbon electrode as the working electrode, platinum as the
auxiliary electrode and Ag/AgNO3 electrode as the reference elec-
trode. The instrument was calibrated with a ferrocene/ferrocenium
redox couple as the external standard and a scan rate of 30 mV/s
was used. The dichloromethane containing 0.1 mol Lꢁ1 tetra-
butylammoniumhexafluorophosphate (TBAPF6) was utilized as
the medium for the cyclic voltammetric determination. The con-
2.9. Assembly of single-layer organic solar cells
centrations of compounds were 5 ꢂ 10ꢁ3 mol Lꢁ1
.
The single-layer organic solar cells were fabricated by placing
MTPA-TRC, MTPA-TRC-CN, MTPA-TRC-AEAQ and MTPA-TRC-PBI in
between FTO and Au electrodes. The FTO-coated glass substrate
was first cleaned with detergent, ultrasonicated in water, washed
with acetone and isopropyl alcohol, and then dried overnight in an
oven at 50 ꢀC under a vacuum of about 3 ꢂ 10ꢁ6 Torr. A film of
MTPA-TRC (MTPA-TRC-CN, MTPA-TRC-AEAQ or MTPA-TRC-PBI) was
prepared on the surface of FTO layer by spin coating from tetra-
hydrofuran solutions (10 mg/mL) and then a clean Au substrate of
2.8. Computational details
Geometry optimizations of all thesystemsshown in Chart 1 were
carried out using B3LYP (the three-parameter exchange functional
of Becke and correlation functional of Lee, Yang and Parr) [28e30]
with the 6-31G(d) basis set without any symmetry constraints. The
polarized continuum model (PCM) framework [31] was used to
Fig. 2. EPR spectra of MTPA-TRC (a), MTPA-TRC-CN (b), MTPA-TRC-AEAQ (c) and MTPA-TRC-PBI (d) in toluene recorded in nitrogen at 193 K. The black curves were determined
before irradiation and the red curves were obtained under irradiation with a high-pressure Hg lamp, of which the wavelength ranges in 350e410 nm. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)

T. Wang et al. / Dyes and Pigments 139 (2017) 264e273
269
cells was measured using solar simulator (Zolix) under AM 1.5 G
(100 mW/cm2), which was adjusted using a calibrated Si solar cell
(National Institute of Metrology), and recorded with a computer-
controlled digital source meter (Kethily 2400).
3. Results and discussion
3.1. Thermal properties
The thermal properties of MTPA-TRC, MTPA-TRC-CN, MTPA-TRC-
AEAQ, and MTPA-TRC-PBI were investigated by thermal gravimetric
analysis (TGA) and differential scanning calorimetry (DSC) under
the nitrogen atmosphere (Fig. 1). TGA curves (Fig. 1a) revealed the
onset decomposition temperatures (Td) of MTPA-TRC, MTPA-TRC-
CN, MTPA-TRC-AEAQ, and MTPA-TRC-PBI were 281 ꢀC, 266 ꢀC,
160 ꢀC, and 166 ꢀC, respectively, which indicated that these four
compounds had relatively high thermal-stability. Additionally, the
melting temperatures (Tm) of MTPA-TRC, MTPA-TRC-CN, MTPA-
TRC-AEAQ, and MTPA-TRC-PBI were measured as 159 ꢀC, 147 ꢀC,
135 ꢀC and 142 ꢀC, respectively, as determined by analyzing the DSC
thermograms (Fig. 1b). The introduction of the second acceptors
(AEAQ and PBI) clearly reduces the values of Td and Tm of MTPA-
TRC-AEAQ and MTPA-TRC-PBI in comparison to those belonging
to MTPA-TRC.
3.2. Dissociation of the chloride anion in D-A system MTPA-TRC
Fig. 3. MALDI-TOF mass spectra of MTPA-TRC (Mass: 537.1560 [18]) in dichloro-
methane solutions before (a) and after (b, c) exposure to the UV light (350e410 nm) for
6 h. The curve (c) was obtained with the solutions containing TEMPOL (Mass: 174.263)
and 1004.4 and 674.6 correspond to the molecular mass of a MTPA-TRC dimmer and a
TEMPOL derivative of MTPA-TRC respectively (Concentration: 5 ꢂ 10ꢁ5 mol Lꢁ1).
When the MTPA-TRC solutions were exposed to UV (visible)
light, the enhanced intramolecular charge transfer (ICT) within
MTPA from triphenylamine to phenylene moiety and PET from
phenylene moiety to TRC module [18,33] led to the formation of a
generous number of free electrons at TRC module, causing a
possible dissociation of the chloride anion of the MTPA-TRC module
to form the transient radical species MTPAþꢃ-TRꢃ (Scheme 2).
Some details are shown in the EPR spectra: MTPA-TRC shows a
weak, broad EPR signal at a g value of 2 before irradiation (Fig. 2a),
whereas no EPR signal was determined with compound MTPA
(Chart 1). The g value agrees well with reported values of triaryl-
45 nm was covered onto this film with vacuum evaporation. The
fabricated solar cells were subsequently placed in a vacuum at-
mosphere of about 3 ꢂ 10ꢁ6 Torr for 3 h to remove the trace
amounts of the solvent. All these procedures were carried out un-
der an atmosphere of dry nitrogen. The active area of devices was
0.04 cm2 and the thickness of the film was about 100 nm.
amine radical cations [34] indicative of
p radical formation and
2.10. Photovoltaic characteristic determinations
The current density-Voltage (J-V) characteristic of obtained solar
0.00010
confirms the enhanced ICTcharacter in the HOMO-LUMO transition
of MTPA-TRC. Interestingly, the EPR signal intensity of MTPA-TRC
can be increased 4-fold upon exposure to UV light in the
(d)
(b)
(c)
(a)
0.00008
0.00006
0.00004
0.00002
0.00000
-0.00002
a
b
-1.5 -1.0 -0.5 0.0
-1.5 -1.0 -0.5 0.0
-1.5 -1.0 -0.5 0.0 -1.5 -1.0 -0.5 0.0
Potential / V Potential / V
Potential / V Potential / V
Fig. 4. Electrochemical curves of MTPA-TRC-EA (a), MTPA-TRC-BA (b), MTPA-TRC (c) and MTPA-TRC-CN (d) with potential from ꢁ1.7 V to 0.1 V in dichloromethane vs Ag/Agþ. The
concentration is 5 ꢂ 10ꢁ3 mol Lꢁ1. Dissociated ratio of chloride anions (4) in the anions of above compounds can be calculated using the equation 4 ¼ 1-b/a.

270
T. Wang et al. / Dyes and Pigments 139 (2017) 264e273
Fig. 5. Molecular orbitals of MTPA-TRC (a), MTPA-TRC- (b), MTPA-TRꢃ (c) and MTPAþ -TRꢃ (d) using B3LYP/6-31G(d) in dichloromethane.
ꢃ
350e410 nm range. Since the chloride anion is a common leaving
group in many organic reactions, we assumed this irradiation-
enhanced EPR signal was due to the formation of a transient
radical species MTPAþꢃ-TRꢃ, which was then confirmed by exposing
the experiment to UV light (350e410 nm) for 6 h.
The MS spectra of the resulting solutions indicated the presence
of dimers or derivatives of 4-hydroxy-2,2,6,6-tetramethylp-
iperidine-1-oxyl (TEMPOL) in the presence of TEMPOL (Fig. 3). In
order to confirm the presence of radical formation, we have per-
formed singlet state and triplet state calculations (Density Func-
tional Theory calculations using B3LYP/6-31G(d) level) of the
MTPAþꢃ-TRꢃ module. The triplet state of the MTPAþꢃ-TRꢃ structure
was 1.03 eV more stable than that of singlet, which clearly indicates
that radical formation is favourable.
The loss of the chloride anion in the MTPA-TRC module also can
be proven by cyclic voltammetry curve (Fig. 4), and the irrevers-
ibility of the reduction wave for the TRC module (ꢁ1.19 V vs Ag/
Agþ) is due to the dissociation of the chloride anion (Scheme 2).
This has been confirmed by the electrochemical determinations of
cyanuric chloride (TRI), 2,4-dichloro-6-anilino-1,3,5-triazine (TRC-
Ph), 2-chloro-4,6-dianilino-1,3,5-triazine (TRC-diPh), 2,4,6-triani-
lino-1,3,5-triazine (TRC-triPh) and melamine (Mela) (Supporting
Information, Fig. S1). The reduction waves are irreversible for the
former three compounds containing chloride, but reversible for the
latter two compounds for which all chlorides have been replaced by
aniline or amino groups.
Fig. 5. It is interesting to notice that when the chlorine anion is
removed from the MTPA-TRC- to form the MTPA-TRꢃ radical, the
HOMO and LUMO energy levels remain unchanged. However, when
we form a MTPAþꢃ-TRꢃ radical from MTPA-TRC, the HOMO and
LUMO levels were lowered significantly (ꢁ4.98 eV to ꢁ5.88 eV in
HOMO and ꢁ1.98 eV to ꢁ2.57 eV in LUMO). The calculated binding
energy (BE) reveals that ꢁ6.5 kcal/mol energy is released or
58.9 kcal/mol energy is needed (irradiated conditions are satisfied
in this paper) when the chlorine anion leaves from the MTPA-TRC-
module or MTPA-TRC module, respectively (Table 1). It is clearly
indicated that the formation of MTPA-TRꢃ or MTPAþꢃ-TRꢃ radicals
are favourable, which allows the chloride anion to depart from s-
triazine group. All of these experimental and calculated
results indicate that the chloride at s-triazine leaves easily as
additional electrons are supplied to TRC module, which results in
the formation of transient radical species MTPA-TRꢃ or MTPAþꢃ-TRꢃ
(Scheme 2).
Additionally, the computational results indicated that the sub-
stitution of one chlorine at TRC module by the electron-
Table 1
Calculatedþfree energy of MTPA-TRC, MTPA-TRC-, MTPA-
TRꢃ, MTPA -TRꢃ, Clꢁ using B3LYP/6-31G(d) level.
ꢃ
System
Energy/a.u.
MTPA-TRC
MTPA-TRC-
MTPA-TRꢃ
ꢁ2390.5460
ꢁ2390.6168
ꢁ1930.2695
ꢁ1930.0945
ꢁ460.3576
We wanted to understand above phenomena in depth, so we
examined the contours of the molecular orbitals of MTPA-TRC,
MTPA-TRC-, MTPA-TRꢃ and MTPAþꢃ-TRꢃ, which are illustrated in
MTPAþ -TRꢃ
ꢃ
Clꢁ

T. Wang et al. / Dyes and Pigments 139 (2017) 264e273
271
Fig. 6. Molecular orbitals of MTPA-TRC-EA, MTPA-TRC-BA and MTPA-TRC-CN using B3LYP/6-31G(d) in dichloromethane.
withdrawing group cyano (-CN) could reduce the energy level of
lowest unoccupied molecular orbital (LUMO) of TRC module to
enhance the PET process from the MTPA to TRC module, but the
presence of an electron-donating group ethylamino/phenylamino
would weaken this PET process. (Fig. 6). The dissociation ratio of
chloride anions of MTPA-TRC- as 94.3%, the cyano derivative MTPA-
TRC--CN was 100%, and the ethylamino/phenylamino derivatives
MTPA-TRC--EA/MTPA-TRC--BA were 83.0% and 88.7% respectively
(Fig. 4). MTPA-TRC-CN exhibited more dissociation in comparison
with MTPA-TRC and MTPA-TRC-EA(or BA) in the testing of cyclic
voltammetry, moreover, the EPR signal intensity of MTPA-TRC-CN
(Fig. 2b) was increased by about 6 fold upon photoirradiation and
obviously higher than that of MTPA-TRC (Fig. 2a), indicating that
the electron donor ability of TRC module had been enhanced in the
MTPA-TRC-CN module. It is now necessary to introduce the
electron-deficient chromophore as the second acceptor (acceptor2)
to the MTPA-TRC module to receive the photo-induced electron
from the TRC module and achieve the consecutive PET from MTPA
to TRC and then to acceptor2. This means the dissociation of the
chloride anion in the MTPA-TRC-acceptor2 should be much less
than in the MTPA-TRC module, making the MTPA-TRC-acceptor2
system more stable than MTPA-TRC under irradiation.
3.3. Using D-A1-A2 architecture to reduce proportion of the
dissociation of chloride anion in MTPA-TRC-AEAQ and MTPA-TRC-
PBI
Some direct evidence has been presented to demonstrate the
significance of a D-A1-A2 architecture on light-stability. The EPR
spectrum of MTPA-TRC-AEAQ before irradiation revealed that the
enhanced ICT character within the MTPA module was retained by
showing a weak, broad signal at a g value of 2 (Fig. 2c), and the EPR
signal intensity did not increase upon photoirradiation, whereas no
Table 2
Photovoltaic performances of single layer organic photovoltaic cell FTO/MTPA-TRC-
AEAQ/Au under AM 1.5 G illumination over time.
Time/h
Voc/V
Jsc/mA/cm2
FF/%
ƞ/%
0
50
100
150
200
250
0.55
0.55
0.54
0.53
0.54
0.53
3.60
3.59
3.60
3.66
3.59
3.57
39.1
39.0
38.5
38.8
38.4
38.5
0.78
0.77
0.76
0.77
0.75
0.75
Fig. 7. The single photodegradation run of MTPA-TRC (square), MTPA-TRC (circle),
MTPA-TRC-AEAQ (triangle) and MTPA-TRC-PBI (star) in dichloromethane (Concentra-
tion: 5 ꢂ 10ꢁ6 mol Lꢁ1). O.D. ¼ optical density.

272
T. Wang et al. / Dyes and Pigments 139 (2017) 264e273
EPR signals were found in compounds AEAQ and AEAQt (Chart 1)
whether under photoirradiation or not. DFT calculations show that
the C-Cl bond distance in MTPA-TRC is 1.748 Å while that in MTPA-
TRC-AEAQ is 1.767 Å. Importantly, MTPA-TRC-AEAQ dimers or
TEMPOL derivatives of MTPA-TRC-AEAQ had not yet been found in
the MS spectrum of MTPA-TRC-AEAQ solution after exposure to the
UV light (350e410 nm) for 6 h in the presence of TEMPOL (SI,
Fig. S2). All of these results indicate that few C-Cl bonds have been
broken in MTPA-TRC-AEAQ module, and the substitution of chlo-
rine at TRC module of MTPA-TRC by the AEAQ module can enhance
the electron donor ability of TRC module under UV light so that
more MTPA-TRC-AEAQ molecules can undergo the consecutive PET
from MTPA to TRC, then to AEAQ [18]. Meanwhile, the other D-A1-
A2 system like MTPA-TRC-PBI [20] (Chart 1) also exhibited no sig-
nals of dimers or TEMPOL derivatives in the MS spectra and no
increment of the EPR signal intensity upon photoirradiation
(Fig. 2d) using the same testing conditions as MTPA-TRC-AEAQ,
confirming the advantage of the D-A1-A2 system on light-
stability. However, as shown in Fig. S3 of the SI, the reduction
waves of the TRC module were concealed by those of the AEAQ or
PBI module, the electrochemical results [20] can not illustrate the
effect of the introduction of the second acceptors (AEAQ and PBI) on
the dissociation on the TRC module.
The differences for light-stabilities among MTPA-TRC, MTPA-
TRC-CN, D-A1-A2 systems MTPA-TRC-AEAQ and MTPA-TRC-PBI
were also indicated by photodegradation in dichloromethane so-
lution under continuous UV light irradiation from 350 to 410 nm
(Fig. 7). The resulting decomposition half-lives were obtained by
monitoring the disappearance of the absorption band at maximum
wavelength of MTPA module over time (SI, Fig. S4). MTPA-TRC-CN
had the half-life of 593 min, and was the most unstable in above
four compounds. MTPA-TRC-AEAQ and MTPA-TRC-PBI were found
to be considerably more stable than MTPA-TRC with the half-lifes of
2161 and 2425 min, respectively, whereas the MTPA-TRC had a half-
life of 742 min. Additionally, we assembled the single-layer OSC
device using MTPA-TRC-AEAQ as the sensitizer, which has the
relatively high photoelectric conversion efficiency (SI, Fig. S5 and
Table S1), and then investigated the long-term stability of the de-
vice. The stability test of MTPA-TRC-AEAQ based single-layer OSC
was performed under sun-exposure for 250 h (Table 2). During the
stability test, The Jsc was stabilized at about 3.6 mA cmꢁ2, and the
free organic dyes in organic photovoltaics and other optoelec-
tronic device.
Acknowledgments
This work was supported by the National Nature Science
Foundation of China (Grant No. 21576195 and 21506151) and
Tianjin Science and Technology Support Program (No. 14TXGC-
CX00017). Lichang Wang acknowledges the support by the Tianjin
1000 talent program for her stay at Tianjin University.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.12.030.
References
[1] Günes S, Neugebauer H, Sariciftci NS. Conjugated polymer-based organic solar
cells. Chem Rev 2007;107(4):1324e38.
[2] Sharma GD, Siddiqui S, Nikiforou A, Zervaki GE, Georgakaki I, Ladomenou K,
et al. A mono (carboxy) porphyrin-triazine-(bodipy) 2 triad as a donor for bulk
heterojunction organic solar cells. J Mater Chem C 2015;3(24):6209e17.
[3] Lin Y, Dam HF, Andersen TR, Bundgaard E, Fu W, Chen H, et al. Ambient roll-
to-roll fabrication of flexible solar cells based on small molecules. J Mater
Chem C 2013;1(48):8007e10.
[4] Pensack RD, Asbury JB. Beyond the adiabatic limit: charge photogeneration in
organic photovoltaic materials. J Phys Chem Lett 2010;1(15):2255e63.
[5] Rolczynski BS, Szarko JM, Son HJ, Liang Y, Yu L, Chen LX. Ultrafast intra-
molecular exciton splitting dynamics in isolated low-band-gap polymers and
their implications in photovoltaic materials design.
J Am Chem Soc
2012;134(9):4142e52.
[6] Vandewal K, Albrecht S, Hoke ET, Graham KR, Widmer J, Douglas JD, et al.
Efficient charge generation by relaxed charge-transfer states at organic in-
terfaces. Nat Mater 2014;13(1):63e8.
[7] Bottari G, de la Torre G, Guldi DM, Torres T. Covalent and noncovalent
phthalocyanineꢁ carbon nanostructure systems: synthesis, photoinduced
electron transfer, and application to molecular photovoltaics. Chem Rev
2010;110(11):6768e816.
[8] Petrus ML, Morgenstern FS, Sadhanala A, Friend RH, Greenham NC,
Dingemans TJ. Device performance of small-molecule azomethine-based bulk
heterojunction solar cells. Chem Mater 2015;27(8):2990e7.
€
[9] Mishra A, Bauerle P. Small molecule organic semiconductors on the move:
promises for future solar energy technology. Angew Chem Int Ed 2012;51(9):
2020e67.
[10] Fukuzumi S, Ohkubo K. Long-lived photoinduced charge separation for solar
cell applications in supramolecular complexes of multi-metalloporphyrins
and fullerenes. Dalton Trans 2013;42(45):15846e58.
[11] Blanco GD, Hiltunen AJ, Lim GN, Chandra Bikram KC, Kaunisto K, Vuorinen T,
et al. Syntheses, charge separation and inverted bulk heterojunction solar cell
application of phenothiazine-fullerene dyads. ACS Appl Mater Interfaces
2016;8:8481e90.
[12] Lu T, Sun H, Colley ND, Bridgmohan CN, Liu D, Li W, et al. Tuning the donors to
control the lifetimes of charge separated states in triazine-based donor-
acceptor systems. Dyes Pigm 2017;136:404e15.
V
oc was kept at about 0.54 V, while the FF remained stable around
0.39. After 250 h of aging, the efficiency of the MTPA-TRC-AEAQ
device decreased by only 4% as compared with the initial effi-
ciency, meaning the performance of the MTPA-TRC-AEAQ device
was considered to be stable. These results clearly demonstrate the
light-stabilizing effect of D-TRC-A2 systems. However, in the mo-
lecular design of organic photovoltaic materials, the halogen groups
(except for fluorine) are normally prevented in the molecular
structure. Instead, the chlorine will be replaced with the cyano
group in D-TRC-A2 systems in our following work.
[13] Law C, Miseikis L, Dimitrov S, Shakya-Tuladhar P, Li X, Barnes PR, et al. Per-
formance and stability of lead perovskite/TiO2, polymer/PCBM, and dye
sensitized solar cells at light intensities up to 70 Suns. Adv Mater
2014;26(36):6268e73.
[14] Qu S, Qin C, Islam A, Wu Y, Zhu W, Hua J, et al. A novel DeA-p-A organic
sensitizer containing a diketopyrrolopyrrole unit with a branched alkyl chain
for highly efficient and stable dye-sensitized solar cells. Chem Comm
2012;48(55):6972e4.
4. Conclusions
[15] Wood S. Natures of optical absorption transitions and excitation energy
dependent photostability of diketopyrrolopyrrole (DPP)-based photovoltaic
copolymers. Energy Environ Sci 2015;8(11):3222e32.
Experimental and computational results illustrated that sub-
stantial dissociations of chloride anions exist in the TRC module of
the D-A system MTPA-TRC under irradiation, which is attributed to
the effective PET process from MTPA to TRC module, but signifi-
cantly less dissociation took place in the D-A1-A2 systems MTPA-
TRC-AEAQ and MTPA-TRC-PBI. This is because introducing the
electron-deficient chromophore AEAQ (or PBI) as acceptor2 to the
MTPA-TRC module promotes the photo-induced electron transfer
from TRC to AEAQ (or PBI), thus preventing the dissociation of
chloride anion. The outstanding light-stability of the D-A1-A2 ar-
chitecture may serve as an important design reference for metal-
ꢀ
[16] Lazarides T, Charalambidis G, Vuillamy A, Reglier M, Klontzas E, Froudakis G,
et al. Promising fast energy transfer system via an easy synthesis: bodi-
pyeporphyrin dyads connected via a cyanuric chloride bridge, their synthesis,
and electrochemical and photophysical investigations. Inorg Chem
2011;50(18):8926e36.
[17] Zhou H, Zheng Z, Xu G, Yu Z, Yang X, Cheng L, et al. 1, 3, 5-Triazine-cored
derivatives dyes containing triphenylamine based two-photon absorption:
synthesis, optical characterization and bioimaging. Dyes Pigm 2012;94(3):
570e82.
[18] Wang T, Weerasinghe KC, Liu D, Li W, Yan X, Zhou X, et al. Ambipolar organic
semiconductors with cascades of energy levels for generating long-lived
charge separated states: a donoreacceptor1eacceptor2 architectural triaryl-
amine dye. J Mater Chem C 2014;2(28):5466e70.

T. Wang et al. / Dyes and Pigments 139 (2017) 264e273
273
[19] Wang T, Zhao C, Zhang L, Lu T, Sun H, Weerasinghe KC, et al. Enhancing
photoinduced charge separation through donor moiety in donor-accepter
organic semiconductors. J Phys Chem C 2016;120(44):25263e75.
[20] Wang T, Weerasinghe KC, Sun H, Hu X, Lu T, Liu D, et al. Effect of triplet state
on the lifetime of charge separation in ambipolar D-A1-A2 organic semi-
conductors. J Phys Chem C 2016;120(21):11338e49.
solar cell with 10.6% power conversion efficiency. Nat Commun 2013;4:1446.
[27] Zhou H, Yang L, Stuart AC, Price SC, Liu S, You W. Development of fluorinated
benzothiadiazole as a structural unit for a polymer solar cell of 7% efficiency.
Angew Chem Int Ed 2011;123(13):3051e4.
[28] Becke AD. Density-functional exchange-energy approximation with correct
asymptotic behavior. Phys Rev A 1988;38(6):3098.
[21] Antohe S. Electrical and photoelectrical properties of the single-, and multi-
[29] Becke AD. Density-functional thermochemistry. III. The role of exact ex-
change. J Chem Phys 1993;98(7):5648e52.
layer organic photovoltaic cells.
J Optoelectron Adv Mater 2000;2(5):
498e514.
[30] Lee C, Yang W, Parr RG. Development of the Colle-Salvetti correlation-energy
formula into a functional of the electron density. Phys Rev B 1988;37(2):785.
[31] Cances E, Mennucci B, Tomasi J. A new integral equation formalism for the
polarizable continuum model: theoretical background and applications to
isotropic and anisotropic dielectrics. J Chem Phys 1997;107(8):3032e41.
[32] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al.
Gaussian 09, revision D. 01. Wallingford, CT, USA: Gaussian, Inc; 2013.
[33] Walkup LL, Weerasinghe KC, Tao M, Zhou X, Zhang M, Liu D, et al. Importance
of dynamics in electron excitation and transfer of organic dyes. J Phys Chem C
2010;114(45):19521e8.
[22] Ghosh AK, Morel DL, Feng T, Shaw RF, Rowe Jr CA. Photovoltaic and rectifi-
cation properties of Al/Mg phthalocyanine/Ag Schottky-barrier cells. J Appl
Phys 1974;45(1):230e6.
[23] Karg S, Riess W, Dyakonov V, Schwoerer M. Electrical and optical character-
ization of poly (phenylene-vinylene) light emitting diodes. Synth Met
1993;54(1e3):427e33.
[24] Cnops K, Rand BP, Cheyns D, Verreet B, Empl MA, Heremans P. 8.4% efficient
fullerene-free organic solar cells exploiting long-range exciton energy trans-
fer. Nat Commun 2014:5.
[25] Saeki A, Tsuji M, Yoshikawa S, Gopal A, Seki S. Boosting photovoltaic perfor-
mance of a benzobisthiazole based copolymer: a device approach using a zinc
oxide electron transport layer. J Mater Chem A 2014;2(17):6075e80.
[26] You J, Dou L, Yoshimura K, Kato T, Ohya K, Moriarty T, et al. A polymer tandem
[34] Rao KP, Kusamoto T, Toshimitsu F, Inayoshi K, Kume S, Sakamoto R, et al.
Double protonation of 1, 5-bis (triarylaminoethynyl) anthraquinone to form a
paramagnetic pentacyclic dipyrylium salt. J Am Chem Soc 2010;132(35):
12472e9.