Tuning the donors to control the lifetimes of charge-separated states in triazine-based donor-acceptor systems

Article abstract of DOI:10.1016/j.dyepig.2016.08.042

Six donor-acceptor systems with styrene based 9-phenyl carbazole, 4,4’-di(hydro, methyl, methoxy or octyloxy) triphenylamine and 4-methylphenyl indoline derivatives as donors, and s-triazine group as the acceptor were synthesized and characterized. The ch

Full text of DOI:10.1016/j.dyepig.2016.08.042

Accepted Manuscript
Tuning the donors to control the lifetimes of charge-separated states in triazine-based
donor-acceptor systems
Ting Lu, Haiya Sun, Nathan D. Colley, Chelsea N. Bridgmohan, Dongzhi Liu, Wei Li,
Wenping Hu, Xueqin Zhou, Tianyang Wang, Lichang Wang
PII:
S0143-7208(16)30494-6
10.1016/j.dyepig.2016.08.042
DYPI 5422
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 12 July 2016
Revised Date: 17 August 2016
Accepted Date: 18 August 2016
Please cite this article as: Lu T, Sun H, Colley ND, Bridgmohan CN, Liu D, Li W, Hu W, Zhou X, Wang
T, Wang L, Tuning the donors to control the lifetimes of charge-separated states in triazine-based donor-
acceptor systems, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.08.042.
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Tuning the donors to control the lifetimes of charge-separated
states in triazine-based donor-acceptor systems
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Ting Lu,a,b Haiya Sun,a,b Nathan D. Colley,c Chelsea N. Bridgmohan,c Dongzhi Liu,a,d
Wei Li,a,b,d Wenping Hu,b,e Xueqin Zhou,a,b,d Tianyang Wang,*,a,b,c and Lichang
Wang**,a,b,c
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a School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China
b Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China
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Department of Chemistry and Biochemistry and the Materials Technology Center, Southern
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Illinois University Carbondale, IL 62901, United States
d Tianjin Engineering Research Center of Functional Fine chemicals, Tianjin, 300072, China
e School of Science, Tianjin University, Tianjin, 300072,China
*Corresponding Author: Tianyang Wang, email: tianyangwang@tju.edu.cn;
**Lichang Wang, email: lwang@chem.siu.edu;
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Tuning the donors to control the lifetimes of charge-separated
states in triazine-based donor-acceptor systems
Abstract: Six donor-acceptor systems with styrene based 9-phenyl carbazole,
4,4’-di(hydro, methyl, methoxy or octyloxy) triphenylamine and 4-methylphenyl
indoline derivatives as donors, and s-triazine group as the acceptor were synthesized
and characterized. The charge-separated states of these donor-acceptor systems were
generated through the effective photoinduced electron transfers from excited donor
modules to acceptor modules and had lifetimes ranging from 57 ns to 215 ns. These
donor-acceptor systems are capable of fast charge separation, but have extremely slow
charge recombination. The back electron transfer processes of these donor-acceptor
systems occur in the inverted region of the Marcus curve. The driving forces of back
electron transfer and the reorganization energies of electron transfer decrease in the
order of 9-phenyl carbazole derivative, 4,4’-di(hydro, methyl, methoxy or octyloxy)
triphenylamine derivative and 4-methylphenyl indoline derivative. These two factors
work together to determine the different lifetimes of charge-separated states in these
donor-acceptor systems.
Keywords: Donor-acceptor system; Photoinduced electron transfer; Charge-separated
state; Lifetime; Driving force; Reorganization energy
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1. Introduction
In recent years, many researchers have expressed great interest in the simulation
of the long-lived charge-separated states of the reaction center in photosynthesis,
where the cascade formation of short-range photoinduced energy transfer and
multistep electron transfer occurs [1-7]. A variety of supramolecular systems were
developed as a result of the integration of various donors and acceptors to obtain long
distance charge-separated states [8-15]. However, a large amount of energy is lost
during the multistep electron transfer processes to reach the final charge-separated
state in both natural and artificial photosynthetic reaction centers [16,17]. Additionally,
the difficulty of synthesizing these multistep electron transfer molecules has
prevented the practical applications for construction of energy conversion systems
[17].
Alternatively, attempts have been made to design simple and small
Donor-Acceptor (D-A) systems in such a way that charge separation occurs rapidly
and quantitatively, while charge recombination is somehow impeded [18]. According
to the Marcus theory, the increase of the lifetime of charge-separated states was
accompanied with the decrease of the reorganization energy of electron transfer in the
Marcus inverted region. It is possible to obtain such simple D-A systems because the
reorganization energy of electron transfer is expected to reduce with decreasing D-A
distance [19]. Hence, a number of simple D-A systems were designed and synthesized
to obtain long-lived charge-separated states [18,20-24]. The relevant energy levels
determine the driving force of the electron transfer between the donor and acceptor
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[25-30], while the distance, spatial orientation, and flexibility of molecules
significantly influence the rate of photoinduced electron transfer and the efficiency of
creating charge-separated states [25,31-35]. However, the design principle between
donor and acceptor, which aimed at increasing the lifetimes of charge-separated states
in simple D-A systems, is unclear and should be studied further.
Triphenylamine and indoline could act as an effective electron donor and
potentially be widely used in organic photovoltaic materials [36-41]. Triazine (TRC)
was confirmed as an effective electron-accepting module in D-A systems, which
could improve the electron-injection and electron-transportation abilities of its
conjugated derivatives in previous studies by others [42-45] and our previous work
[46-49]. Therefore, we have designed and reported two D-A systems MTPA-TRC
[47-49] and YD-TRC [49], which used styrene based 4,4’-dimethyl-triphenylamine
(MTPA) and 4-methylphenyl indoline (YD) as the donors, respectively, and triazine
group (TRC) as the acceptor. Two lifetimes of charge-separated states were obtained
such that MTPA.+-TRC.- as 80 ns and YD.+-TRC.- as 215 ns. The important conclusion
indicated was that YD module was more effective than MTPA in forming long-lived
charge-separated states, which should be discussed and explained systematically in
more detail.
Therefore, more compounds based on D-TRC system should be designed and
characterized, and the relationship between the improvement of donor ability and the
lifetime of charge-separated states of the D-TRC system should be clarified and
defined in an intuitive way. To summarize the effect of tuning donors on lifetimes of
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charge-separated states, we synthesized four new compounds of D-TRC systems,
which used the replacement and modification of triphenylamine moiety to achieve
different donor ability in comparison with MTPA-TRC. On one hand, styrene based
non-substituted triphenylamine (TPA) and 9-phenyl carbazole (PCB) were alternatives
to MTPA and used to obtain TPA-TRC and PCB-TRC, respectively. On the other hand,
the dimethoxy groups and dioctyloxy groups were introduced to triphenylamine
moiety of TPA-TRC to construct the derivatives MeTPA-TRC and OeTPA-TRC,
respectively. Combining the results of these D-TRC systems with the reported results
of MTPA-TRC [48] and YD-TRC [49], this work will exhibit six D-TRC systems
which have the increased lifetimes of charge-separated states through the
enhancement of donor ability, and the reason for this conclusion will be discussed
systematically in this paper. Structures of key compounds characterized in this work
are provided in Scheme 1, while MTPA-TRC and YD-TRC were synthesized and
studied previously. PCB-TRC, TPA-TRC, MeTPA-TRC and OeTPA-TRC were first
reported here and their synthetic routes were shown in Scheme 1.
.....................................................Scheme 1....................................................................
Photophysical properties of these compounds were studied using both steady
state and transient UV-Vis and fluorescence measurements coupled with
electrochemical measurements and DFT calculations. The results of these
measurements and the corresponding analysis are provided in Section 3 following the
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description of methods in Section 2. Finally, the conclusions of this work are
summarized in Section 4.
2. Experimental and computational methods
2.1 Materials
3-(4-nitrostyryl)-9-phenyl-9H-carbazole (PCBn) [50], 4-(4-nitrostyryl)-triphen
ylamine (TPAn) [50], 4,4’-dimethyl-4”-(4-nitrostyryl)-triphenylamine (MTPAn) [5
0], 4,4’-dimethoxy-4”-(4-nitrostyryl)-triphenylamine (MeTPAn) [50], 4,4’-dioctylo
xy-4”-(4-nitrostyryl)-triphenylamine (OeTPAn) [50], 4-(4-methylphenyl)-7-(4-nitro
styryl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole (YDn) [50], 4,4’-dimethyl-4”-(4
-(4,6-dichloro-1,3,5-triazin-2-ylamino)styryl) triphenylamine (MTPA-TRC) [48] an
d 4-(4-methylphenyl)-7-(4-(4,6-dichloro-1,3,5-triazin-2-ylamino)styryl)-1,2,3,3a,4,8b
-hexahydrocyclopenta[b]indole (YD-TRC) [49] were prepared according to the li
terature. The synthetic pathways of 3-(4-(4,6-dichloro-1,3,5-triazin-2-ylamino)styr
yl)-9-phenyl-9H-carbazole (PCB-TRC), 4”-(4-(4,6-dichloro-1,3,5-triazin-2-ylamino)
styryl)triphenylamine (TPA-TRC), 4,4’-dimethoxy-4”-(4-(4,6-dichloro-1,3,5-triazin-
2-ylamino)styryl)-triphenylamine (MeTPA-TRC), 4,4’-dioctyloxy-4”-(4-(4,6-dichlor
o-1,3,5-triazin-2-ylamino)styryl)triphenylamine (OeTPA-TRC) were illustrated in
Scheme 1. All reagents and solvents were reagent grade and further purified b
y the standard methods when necessary. All synthetic procedures were carried
out under an atmosphere of dry nitrogen or dry argon unless otherwise indicate
d.
TPAa. TPAn (0.090 g, 0.23 mmol) was dissolved in absolute ethanol (40 mL) at
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room temperature. Then SnCl2•2H2O (0.23 g, 1.0 mmol) was added into the solution.
Afterwards, the suspension was refluxed at 90 °C for 8 h. After filtration, the solid
was washed with absolute ethanol. Then the organic phases were condensed by rotary
evaporation. The product was further purified by column chromatography on silica
gel using cyclohexane: dichloromethane (3:2 v:v) as the eluent to yield a pale yellow
+
solid (0.060 g, 72%). HRMS-ESI: Found: m/z=363.1855(M+H)+ (Calc. for C26H23N2 :
m/z=363.1856). 1H NMR (CDCl3, 500 MHz), δ: 7.76-7.62 (m, 2H), 7.61-7.29 (m, 4H),
7.29-7.19 (m, 6H), 7.19-6.76 (m, 8H), 6.44 (s, 2H).
Synthetic procedures of MeTPAa, OeTPAa and PCBa are the same as that of
TPAa except that TPAn was replaced by MeTPAn (0.10 g, 0.23 mmol), OeTPAn (0.15
g, 0.23 mmol) and PCBn (0.090 g, 0.23 mmol), respectively. The characterization data
of these compounds are as follows. .
MeTPAa. A pale green solid (0.065 g, yield: 67%) was obtained by column
+
chromatography. HRMS-ESI: Found: m/z=423.2065 (M+H)+ (Calc. for C28H27N2O2 :
m/z=423.2067). 1H NMR (CDCl3, 500 MHz), δ: 7.48 (dd, J=26.1, 19.7 Hz, 1H), 7.37
(d, J=8.3 Hz, 1H), 7.33-7.25 (m, 5H), 7.17 (d, J=8.8 Hz, 1H), 7.04 (t, J=16.8 Hz, 3H),
6.94 (dd, J=16.4, 7.2 Hz, 1H), 6.86 (dd, J=32.0, 7.8 Hz, 6H), 5.31 (s, 2H), 3.79 (d,
J=9.1 Hz, 6H).
OeTPAa. A pale green solid (0.085 g, yield: 60%) was obtained by column
+
chromatography. HRMS-ESI: Found: m/z=619.4261 (M+H)+ (Calc. for C42H55N2O2 :
m/z=619.4258). 1H NMR (CDCl3, 500 MHz), δ: 7.86-7.69 (m, 2H), 7.67-7.33 (m, 6H),
7.33-7.21 (m, 2H), 7.21-6.77 (m, 6H), 6.20 (dd, J=8.1, 1.3 Hz, 2H), 4.32 (t, J=6.7 Hz,
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2H), 4.06-3.83 (m, 4H), 2.11-0.56 (m, 30H).
PCBa. A yellow solid (0.060 g, yield: 73%) was obtained by column
chromatography. HRMS-ESI: Found: m/z=361.1697 (M+H)+ (Calc. for C26H21N2 :
+
m/z=361.1699). 1H NMR (CDCl3, 500 MHz), δ: 8.74-8.69 (m, 2H), 8.30 (s, 1H), 8.21
(dt, J=21.0, 10.5 Hz, 1H), 8.01 (dd, J=8.6, 1.4 Hz, 1H), 7.71-7.56 (m, 3H), 7.56-7.29
(m, 8H), 7.29-7.03 (m, 2H), 5.31 (s, 2H).
TPA-TRC. Cyanuric chloride (0.047 g, 0.26 mmol) was first dissolved in the
absolute tetrahydrofuran at 0 °C. Then TPAa (0.093 g, 0.26 mmol) was added into the
solution. Afterwards, the solution was warmed to and then kept stirring at room
temperature for 30 min. Then the solvent was removed by rotary evaporation and the
residue was purified by column chromatography on silica gel using petroleum ether :
dichloromethane (1:1 v:v) as the eluent to yield a yellow solid (0.11 g, 84%).
HRMS-ESI: Found: m/z=510.1250 (M+H)+ (Calc. for C29H22Cl2N5 : m/z=510.1247).
+
1H NMR (CDCl3, 500MHz), δ: 7.62-7.43 (m, 5H), 7.39 (d, J=8.6 Hz, 2H), 7.27 (dd,
J=9.8, 5.9 Hz, 4H), 7.12 (d, J=7.8 Hz, 4H), 7.09-7.01(m, 5H), 6.97 (d, J=16.3 Hz, 1H).
13C NMR (CDCl3, 100.6 MHz), δ:171.42, 163.92, 147.59, 147.50, 136.42, 135.42,
134.57, 131.14, 129.31, 128.62, 127.05, 124.59, 123.45, 123.15, 121.36. Anal.: Found:
C, 68.09; H, 4.03; N, 13.51% (Calc. for C29H21Cl2N5: C, 68.24; H, 4.15; N, 13.72%).
Synthetic procedures of MeTPA-TRC, OeTPA-TRC and PCB-TRC are the same
as TPA-TRC except that TPAa was replaced by MeTPAa (0.11 g, 0.26 mmol),
OeTPAa (0.16 g, 0.26 mmol) and PCBa (0.094 g, 0.26 mmol), respectively. The
characterization data of these compounds are as follows.
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MeTPA-TRC. A yellow solid (0.12 g, yield: 82%) was obtained by column
chromatography. HRMS-ESI: Found: m/z=570.1460 (M+H)+ (Calc. for
+
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C31H26Cl2N5O2 : m/z=570.1458). H NMR (CDCl3, 500 MHz), δ: 7.57-7.46 (m, 2H),
7.33 (d, J=7.9 Hz, 2H), 7.46-7.08 (m, 4H), 7.08 (s, 1H), 7.06 (s, 2H), 7.10-6.90 (m,
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3H), 6.84 (d, J=8.7 Hz, 5H), 3.81 (s, 6H). C NMR (CDCl3, 100.6 MHz), δ: 171.24,
163.91, 158.41, 147.46, 137.28, 135.47, 134.58, 131.43, 129.26, 128.62, 127.03,
126.95, 124.38, 121.33, 113.67, 55.50. Anal.: Found: C, 65.13; H, 4.28; N, 12.03%
(Calc. for C31H25Cl2N5O2: C, 65.27; H, 4.42; N, 12.28%).
OeTPA-TRC. A yellow solid (0.16 g, yield: 78%) was obtained by column
chromatography. HRMS-ESI: Found: m/z=766.3651 (M+H)+ (Calc. for
+
1
C45H54Cl2N5O2 : m/z=766.3649). H NMR (CDCl3, 500 MHz), δ: 7.62-7.41 (m, 5H),
7.41-7.25 (m, 3H), 7.13-6.97 (m, 5H), 6.97-6.72 (m, 6H), 3.95 (dt, J=13.0, 6.5 Hz,
4H), 1.96-1.61 (m, 4H), 1.55-1.38 (m, 4H), 1.38-1.24 (m, 11H), 1.38-1.02 (m, 11H).
13C NMR (CDCl3, 100.6 MHz), δ: 171.26, 163.68, 158.63, 147.55, 136.26, 135.49,
134.56, 131.93, 129.09, 128.73, 127.25, 126.83, 126.72, 124.71, 121.42, 120.22,
115.32, 68.32, 31.95, 31.86, 29.73, 29.41, 29.28, 26.12, 22.69, 14.14. Anal.: Found: C,
70.32; H, 6.85; N, 8.97% (Calc. for C45H53Cl2N5O2: C, 70.48; H, 6.97; N, 9.13%).
PCB-TRC. A pale green solid (0.11 g, yield: 80%) was obtained by column
+
chromatography. HRMS-ESI: Found: m/z=508.1091 (M+H)+ (Calc. for C29H20Cl2N5 :
m/z=508.1090). 1H NMR (CDCl3, 500 MHz), δ: 8.28 (s, 1H), 8.18 (d, J=7.8 Hz, 1H),
13
7.68-7.37 (m, 10H), 7.35-7.24 (m, 5H), 7.15 (d, J=16.2 Hz, 1H), 5.30 (s, 1H). C
NMR (CDCl3, 100.6 MHz), δ: 171.40, 163.89, 141.38, 137.53, 137.02, 136.34,
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135.65, 132.42, 129.93, 129.40, 128.64, 127.60, 127.05, 126.99, 126.22, 123.82,
123.33, 122.34, 121.37, 121.18, 120.37, 118.62, 110.06. Anal.: Found: C, 68.43; H,
3.62; N, 13.59% (Calc. for C29H19Cl2N5: C, 68.51; H, 3.77; N, 13.78%).
2.2 Mass spectrometry. The ESI mass spectra were obtained on a Thermo Fisher
LCQ Deca XP MAX mass spectrometer. MALDI-TOF mass spectra were obtained on
a Bruker Autoflex tof/tofIII mass spectrometer.
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2.3 NMR spectrometry. H NMR spectra and C NMR spectra were obtained on a
VARIAN INOVA 500MHz spectrometer, and the testing temperature was set to 25 °C.
2.4 Elemental analysis. Elemental analysis for carbon, hydrogen, and nitrogen was
determined on a VARIO EL CUBE (ELEMENTAR, Germany) elemental analyzer.
2.5 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 min-1
from room temperature to 500 °C under the nitrogen atmosphere.
2.6 Differential scanning calorimetry (DSC). Differential scanning calorimetry at a
heating rate of 10 °C min-1 from 50 to 250 °C was performed using a TA DSC Q 20
instrument under the nitrogen atmosphere.
2.7 Cyclic voltammetry. The electrochemical properties were measured using a BAS
100W electrochemical analyzer utilizing the three-electrode configuration with a
glassy carbon electrode as the working electrode, Ag/AgNO3 electrode as the
reference electrode, and platinum as the auxiliary electrode. The analyzer was
calibrated using a ferrocene/ferrocenium redox couple as the external standard prior to
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the measurements. The scan rate was set to 30 mV/s. Dichloromethane containing 0.1
mol•L-1 tetra-butylammonium hexafluorophosphate (TBAPF6) was employed as the
medium for the cyclic voltammetric determination. The compound concentration was
5×10-4 mol•L-1.
2.8 Computational details. Geometry optimizations of the molecules were carried
out using the three-parameter exchange functional of Becke and correlation functional
of Lee, Yang, and Parr (B3LYP) [51-53] with the 6-31G(d, p) basis set in toluene
without any symmetry constraints. The polarized continuum model (PCM) framework
[54] was used to describe the solvent effect. The SCF convergence was 10-8 a.u, the
gradient and energy convergence was 10-4 a.u. and 10-5 a.u., respectively. All
calculations were performed using the Gaussian09 package [55]. In order to confirm
the optimized geometry as a global minimum, frequency calculations at the same level
of theory were performed. A detailed description of computational methods can also
be found in our previous work [48].
2.9 UV-Vis spectroscopy in solution. The absorption spectra were taken on a Thermo
Spectronic, Helios Gamma spectrometer. Quartz cells with a path length of 1 cm were
used to observe absorption in the UV region.
2.10 Fluorescence spectroscopy in solution. The fluorescence spectra were recorded
on a Varian CARY ECLIPSE fluorospectrophotometer.
2.11 Time-correlated single photon counting (TC-SPC). Excitation of samples was
done with picosecond diode lasers (Horiba JobinYvon Instruments) at 366 nm, and the
time resolution was ~ 150 ps. The laser pulse energy was ca. 15 pJ and attenuated
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(often more than an order of magnitude) to the desired count rate of ca. 1% or less of
the excitation frequency. A cooled (ca. -40 °C) Hamamatsu MCP- photomultiplier
R3809U 51 was used for detection of single photons, and the signal passed through a
discriminator (Ortec 9307) and into a TAC (Ortec 566, 100 ns range used). The
electrical trigger signal from the laser was also passed through a discriminator
(Tennelec TC454) and onto the TAC (Ortec 566). The TAC output was read by a
DAQ-1 MCA computer card using 1024 channels and collected with
HoribaJobinYvon Data Station 2.5. Measurements were made in reverse mode at 5
MHz and under magic angle polarization. A cutoff filter, GG400 (Excitation at 366
nm), was used to block stray excitation light. A dilute solution of Ludox was used to
record the instrument response function without any filter for solution measurements.
No monochromator was used; i.e., all wavelengths transmitted by the cut off filter
were collected. The sample concentrations were 5×10-6 mol•L-1, and the solutions
were bubbled with argon for 30 min before the measurements.
2.12 Nanosecond transient absorption spectroscopy. Nanosecond transient
absorption measurements were performed on a LP-920 laser flash photolysis setup
(Edinburgh). Excitation at 420 nm with a power of 2.0 mJ per pulse from a
computer-controlled Nd:YAG laser/OPO system from Opotek (Vibrant 355 II)
operating at 10 Hz was directed to the sample. The laser and analyzing light beams
passed perpendicularly through a 1 cm quartz cell. The complete time-resolved
spectra were obtained using a gated CCD camera (AndoriSTAR); the kinetic traces
were detected by a Tektronix TDS 3012B oscilloscope and a R928P photomultiplier
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and analyzed by Edinburgh analytical software (LP920). The samples used in the
flash photolysis experiments were bubbled with argon for 30 min before
measurements. The compound concentrations were 5×10-5 mol•L-1.
3. Results and discussion
We characterized and compared PCB-TRC, TPA-TRC, MTPA-TRC,
MeTPA-TRC, OeTPA-TRC and YD-TRC for the studies of photophysical processes.
We performed electrochemical and spectroscopic measurements and computational
calculations on these compounds. The results from these studies are discussed as
follows.
3.1 Thermal properties
The thermal properties of the six compounds were investigated by thermal
gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under the
nitrogen atmosphere (Figure 1). TGA curves (Figure 1a) revealed the onset
decomposition temperatures (Td) of PCB-TRC, TPA-TRC, MTPA-TRC,
MeTPA-TRC, OeTPA-TRC and YD-TRC were 294 °C, 283 °C, 281 °C, 236 °C,
222 °C, 249 °C, respectively, indicating that these six compounds had the relatively
high thermal-stability. Additionally, the glass transition temperatures (Tg) of six
compounds were obtained in the range of 96-99 °C by analyzing the DSC
thermograms (Figure 1b). The values of Td and Tg of YD-TRC are smaller than those
of PCB-TRC and TPA-TRC, and the introduction of octyloxy groups obviously
reduces the values of Td and Tg of OeTPA-TRC in comparison with those of
TPA-TRC.
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.....................................................Figure 1....................................................................
3.2 Electrochemical & Computational results
To reveal the orbital energy levels of these six compounds, the electrochemical
behaviors were investigated by cyclic voltammetry. The electrochemical
measurements are shown in Figure 2. The oxidation and reduction potentials as well
as the frontier orbital energies derived from these measurements are summarized in
Table 1.
................................................. ..Figure 2.......................................................................
.....................................................Table 1......................................................................
As shown in Figure 2, PCB-TRC, TPA-TRC, MTPA-TRC, MeTPA-TRC,
OeTPA-TRC and YD-TRC present the reversible oxidation waves, which could be
attributed to PCB, TPA, MTPA, MeTPA, OeTPA and YD modules, respectively, and
the irreversible reduction wave ascribed to the TRC module [48]. As shown in Table 1,
the charge transition first occurs in donor module from the highest occupied
molecular orbital (HOMO) to the lowest unoccupied molecular orbital plus one
(LUMO+1), and then the electron transfer from LUMO+1 to LUMO is possible in
these D-TRC systems. It can be concluded that all six molecules are typical D-A
systems with PCB, TPA, MTPA, MeTPA, OeTPA and YD as electron donors and the
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TRC module as electron acceptor. Meanwhile, the computational results (Figure 3)
show that the HOMO and LUMO+1 are located at the donor modules and TRC
module respectively. It should be noted that the LUMO and LUMO+1 energy levels
of these studied D-A systems are within the error bar and can have the same energy.
.....................................................Figure 3....................................................................
As shown in Table 1, even though there is little difference of LUMO level among
these compounds, the HOMO level of PCB-TRC is lower than that of TPA-TRC by
0.15 eV. The HOMO levels of MTPA-TRC, MeTPA-TRC and OeTPA-TRC
introduced by methyl groups, methoxy groups and octoxy groups increased by 0.03
eV, 0.19 eV and 0.24 eV, respectively. In comparison with the triphenylamine
derivatives, YD-TRC has the highest HOMO level in these D-A systems. This
demonstrates that donor modules with stronger electron donating abilities (PCB <
TPA < MTPA < MeTPA < OeTPA < YD) have the stronger promoted effects on the
HOMO levels, but have little effects on LUMO levels in the D-A systems.
3.3 Absorption and fluorescence properties
The UV-vis absorption spectra of PCB-TRC, TPA-TRC, MTPA-TRC,
MeTPA-TRC, OeTPA-TRC and YD-TRC in toluene are shown in Figure 4. All six
compounds show two obvious absorption bands from 200 nm to 600 nm in toluene.
The bands appeared at 300 nm are due to the π-π* transition of 9-phenyl carbazole,
triphenylamine and 4-methylphenyl indoline. The maximum absorption bands are
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ascribed to the intramolecular charge transfer (ICT) transitions. Once the methyl
groups, methoxy groups and octoxy groups are introduced to triphenylamine, the
absorption maxima of MTPA-TRC, MeTPA-TRC and OeTPA-TRC become 393 nm,
396 nm and 397 nm, which are red-shifted 7nm, 10nm and 11nm from 386 nm of
TPA-TRC, respectively. Both PCB-TRC and YD-TRC have lower absorption
intensities than TPA-TRC. The absorption maxima of PCB-TRC and YD-TRC are at
359 nm and 402 nm, which are blue-shifted 27 nm and red-shifted 16 nm, respectively,
in comparison with that of TPA-TRC. The absorption maxima of these D-A systems
in order of decreasing wavelength is YD-TRC > OeTPA-TRC > MeTPA-TRC >
MTPA-TRC > TPA-TRC > PCB-TRC which is in agreement with the degree of
enhanced ICT character of donor modules in the D-A systems (YD > OeTPA >
MeTPA > MTPA > TPA > PCB).
.....................................................Figure 4....................................................................
To explore the differences of photophysical processes from the excited donor
(PCB, TPA, MTPA, MeTPA, OeTPA and YD) module to acceptor (TRC) module, the
fluorescence spectra at the excitation wavelength of 380 nm were exhibited in toluene
(Figure 5). The emission maxima are in a series of decreasing wavelength of
OeTPA-TRC (470 nm), MeTPA-TRC (468 nm), MTPA-TRC (444 nm), TPA-TRC
(437 nm) and PCB-TRC (424 nm). However, the emission maxima of YD-TRC (452
nm) is significantly blue-shifted compared with that of OeTPA-TRC, which
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corresponds to a much smaller Stokes shift of YD-TRC. It implies that the structure
reorganization of YD-TRC upon photoexcitation is the smallest among these D-A
systems.
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.....................................................Figure 5....................................................................
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Meanwhile, the fluorescence quantum yields (Ф) were obtained as 0.005, 0.054,
0.055, 0.108, 0.132 and 0.018 for PCB-TRC, TPA-TRC, MTPA-TRC, MeTPA-TRC,
OeTPA-TRC and YD-TRC molecules, respectively. Taking reported compound
MTPA-TRC for instance, its fluorescence intensity is substantially quenched in
comparison with that of the compound MTPA [56] (Ф=0.690). The fluorescence of
MTPA-TRC follows a bi-exponential decay equation from the emission fluorescence
decay experiments: the fast component (0.29 ns) of the fluorescence is assigned to the
electron transfer from MTPA to the TRC module, which results in a charge-separated
state of MTPA.+-TRC.-, and the slow component (1.73 ns), which is attributed to the
solvation relaxation of MTPA singlet. We also carried out the emission fluorescence
decay experiments to investigate the fluorescence of the other five compounds (Figure
6). It is clear that the fluorescence from PCB, TPA, MeTPA, OeTPA and YD modules
follow the bi-exponential decay process. The emission lifetimes and proportions are
provided in Table 2, illustrating that the photoinduced electron transfer, which is
attributed to the short lifetime should lead to the low fluorescence yield of these D-A
systems.
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.....................................................Figure 6....................................................................
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.....................................................Table 2......................................................................
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Interestingly, different donor modules affect the emission lifetimes and
proportions of the fluorescence decay of D-A systems differently. As shown in Table
2, the lifetimes of fluorescence for PCB-TRC and YD-TRC are obviously larger than
those of TPA-TRC, MTPA-TRC, MeTPA-TRC and OeTPA-TRC. The proportions of
the fluorescence decay related to photoinduced electron transfer are different in these
six compounds. The proportion of photoinduced electron transfer is given in
decreasing value in these D-A systems (66.8% for YD-TRC > 64.3% for
OeTPA-TRC > 45.9% for MeTPA-TRC > 39.7% for MTPA-TRC > 33.9% for
TPA-TRC > 12.3% for PCB-TRC). This result reveals that using the YD module as
the donor in these D-A systems should be more effective than other donor modules to
form the long-lived charge-separated states.
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Therefore, to determine the lifetimes of charge-separated states for compounds
PCB-TRC, TPA-TRC, MTPA-TRC, MeTPA-TRC, OeTPA-TRC, and YD-TRC,
nanosecond transient absorption measurements have been performed. As we reported
previously, there is no signal for the triplet absorption by compounds MTPA and YD,
and the positive transient absorptions of MTPA-TRC and YD-TRC in the range
400-700 nm were attributed to the MTPA.+ and YD.+ absorptions [48,49]. The
absorptions of PCB.+, TPA.+, MeTPA.+ and OeTPA.+ should be similar with that of
MTPA.+. The lifetimes of charge-separated states of MTPA.+-TRC.- and YD.+-TRC.-
were reported as 80ns [48] and 215 ns [49], respectively. Meanwhile, the lifetimes of
charge-separated states PCB.+-TRC.-, TPA.+-TRC.-, MeTPA.+-TRC.-, OeTPA.+-TRC.-
were estimated as 57 ns, 75 ns, 115 ns and 131 ns, respectively, and have been
exhibited in Figure 7 (a)-(d). It is obvious that the lifetimes of six compounds are in
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the increasing order shown as PCB-TRC < TPA-TRC < MTPA-TRC < MeTPA-TRC
< OeTPA-TRC < YD-TRC, which is also in agreement with the degree of enhanced
ICT character of donor modules in the D-A systems (PCB < TPA < MTPA < MeTPA
< OeTPA < YD).
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.....................................................Figure 7....................................................................
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3.4 Decay pathways of charge-separated states in PCB-TRC, TPA-TRC,
MTPA-TRC, MeTPA-TRC, OeTPA-TRC and YD-TRC
Here we derive and compare the decay pathways and charge-separated states
lifetimes of PCB-TRC, TPA-TRC, MTPA-TRC, MeTPA-TRC, OeTPA-TRC and
YD-TRC. It is beneficial to comprehend the principle of tuning the donors to control
the lifetimes of charge-separated states in these triazine-based D-A systems. It also
exhibits the photoinduced electron transfer from LUMO of donor modules (LUMO+1)
to the LUMO of TRC module, which then generates the respective charge-separated
states. The energy diagram and photophysical processes of six compounds were
therefore constructed and exhibited in Figure 8.
....................................................Figure 8....................................................................
In PCB-TRC, about 12.3% of excited singlet PCB decay to the photoinduced
electron transfer pathway to form the charge-separated state, which is less than that of
TPA-TRC as about 33.9%. For MTPA-TRC, MeTPA-TRC, OeTPA-TRC and
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YD-TRC about 39.7%, 45.9%, 64.3% and 66.8% of donor singlet excited states
convert into the charge-separated states through the photoinduced electron transfer
pathway, respectively. The value in YD-TRC is significantly larger than those in other
D-A systems. This demonstrates that the tuning of the donors to control photoinduced
electron transfer was reasonable and effective, which will change the quantum yield
of long-lived charge-separated states in D-A systems.
The Marcus theory of electron transfer offers a valuable guide to controlling and
optimizing the efficiency of charge separation versus charge recombination.
According to the Marcus theory of electron transfer [57], the rate constants of both
forward electron transfer and back electron transfer are given by eq (1), where V is the
electronic coupling matrix element, KB is the Boltzman constant, h is the Planck
constant, and T is the absolute temperature, ꢀGET is the free energy change of electron
transfer, and λ is the reorganization energy of electron transfer.
ꢅ
ꢋ
(
)
ꢃꢄ
ꢇ
ꢓꢔ ꢗꢈ
ꢕꢖ
ꢍ
ꢌ
ꢌ
ꢀꢁꢂ = (ꢆ ꢈꢉ ꢂ
)
ꢎ ꢏꢐ ꢑ ꢒ−
ꢇꢘ
(1)
ꢃꢈꢉ ꢂ
ꢊ
ꢊ
ꢀGET is the free energy change of electron transfer, and λ is the reorganization
energy of electron transfer.
The most significant concept of the Marcus theory is that the electron transfer
rate constant is expected to decrease as the driving force of electron transfer (-ꢀGET)
increases in the region where the driving force is larger than the reorganization energy
of electron transfer (-ꢀGET > λ) [17]. This region (-ꢀGET > λ) is referred to as the
Marcus inverted region [58-61]. The reorganization energy of electron transfer (λ) is
the energy required to structurally reorganize the donor, acceptor and their solvation
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spheres upon electron transfer [17]. λ is one of the most important terms, which has a
strong impact on the lifetime and quantum yield. The small reorganization energy of
electron transfer reactions and large driving force of back electron transfer are
required to form long-lived charge-separated states [62-65].
The driving force (-ꢀGBET in eV) for back electron transfer process from TRC
radical anion to donor radical cation were calculated by eq (2), where ꢏ stands for
the elementary charge, ꢙꢚꢜꢛ(D.+/D) is the first one-electron oxidation potential of the
donor module, while ꢙꢝꢜꢁꢞ(A.-/A) refers to the first one-electron reduction potential of
the acceptor moiety [57].
-ꢀGBET = e [ꢙꢚꢜꢛ(D.+/D)-ꢙꢝꢜꢁꢞ(A.-/A)]
(2)
Furthermore, the forward electron transfer driving force (-ꢀGET in eV) is
determined by eq (3), ꢀE0-0 is the energy of the 0-0 transition energy gap between the
lowest excited state and the ground state [57]. The driving forces of forward electron
transfer (-ꢀGET) and back electron transfer (-ꢀGBET), the rate constants of forward
electron transfer (kET) and back electron transfer (kBET) were calculated and shown in
Table 3.
-ꢀGET= ꢀE0-0 +ꢀGBET
(3)
From Table 3 we can see photoinduced forward electron transfer processes in
these D-A systems are extremely rapid, and back electron transfer processes are up to
three orders of magnitude slower. We investigated the electron transfer processes of
these D-A systems at a specific temperature (298 K) and also estimated the values of λ
and V according to eq (1) (Table 3) and drew the driving forces dependence of
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intramolecular electron transfer rate constants of these D-A systems (Figure 9).
.....................................................Table 3......................................................................
....................................................Figure 9.....................................................................
It elucidates that back electron transfer processes in these D-TRC systems occur
in the inverted region of the Marcus curve, which plays a key role in the differences of
charge-separated state lifetimes for these D-A systems. The forward electron transfer
processes of PCB-TRC, TPA-TRC, MTPA-TRC are situated in the normal region of
the Marcus curve, and the forward electron transfer processes of MeTPA-TRC and
OeTPA-TRC are located in the beginning of the inverted region. For YD-TRC, the
forward electron transfer process is within the deeply inverted region of the Marcus
curve. As illustrated in table 3 and Figure 9, the driving forces of the back electron
transfer decrease gradually in the order of PCB-TRC > TPA-TRC > MTPA-TRC >
MeTPA-TRC > OeTPA-TRC > YD-TRC. The reorganization energies also decrease
in the same order, and are used to determine the rate constants of back electron
transfer in these D-A systems. These two factors work together to determine the
differences of lifetimes of charge-separated states of PCB-TRC < TPA-TRC <
MTPA-TRC < MeTPA-TRC < OeTPA-TRC < YD-TRC. It should be noted that the
electronic coupling matrix element of YD-TRC is clearly larger than that of other D-A
systems, which demonstrates that the electronic coupling of electron transfer
processes of YD-TRC is enhanced remarkably.
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Above all, there are two reasons that account for differences in the lifetimes of
the charge-separated states among these D-TRC systems: one is the driving force of
back electron transfer and the other is the reorganization energy of electron transfer.
These two factors work together to determine the different lifetimes of
charge-separated states in these D-A systems.
4. Conclusions
In this paper, we designed and synthesized four new D-A systems PCB-TRC,
TPA-TRC, MeTPA-TRC, OeTPA-TRC, together with some results of two reported
D-TRC systems, MTPA-TRC and YD-TRC, to compare their photophysical processes
using steady-state and transient absorption and fluorescence spectra as well as the
electrochemical measurements and computational calculations. Meanwhile, the
thermal properties of six compounds had been studied by TGA and DSC. In the above
D-TRC systems, charge-separated states, PCB.+-TRC.-, TPA.+-TRC.-, MTPA.+-TRC.-,
MeTPA.+-TRC.-, OeTPA.+-TRC.- and YD.+-TRC.-, were generated through the effective
photoinduced electron transfer with the lifetimes of 57 ns, 75 ns, 80 ns, 115 ns, 131 ns
and 215 ns, respectively.
In accordance with the Marcus theory of electron transfer, these simple D-TRC
systems are capable of fast charge separation, but have extremely slow charge
recombination, and the back electron transfer processes of these D-TRC systems
occur in the inverted region of the Marcus curve. The back electron transfer driving
forces decrease in the order of PCB-TRC > TPA-TRC > MTPA-TRC > MeTPA-TRC >
OeTPA-TRC > YD-TRC. The reorganization energies decrease in the same order as
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the back electron transfer driving forces, and are used to determine the rate constants
of back electron transfer in these D-A systems. The electron transfer driving forces
and reorganization energies of electron transfer work together to determine the
different lifetimes of charge-separated states in these simple D-TRC systems.
This work indicates that tuning donor ability to control the lifetime of
charge-separated state in D-A system was reasonable and achievable while providing
an important theoretical basis for the design of organic optoelectronic materials.
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. 14TXGCCX001). Lichang Wang acknowledges the support by the
Tianjin 1000 talent program for her stay at Tianjin University, and Tianyang Wang
acknowledges the support by the Tianjin excellent postdoctoral international training
program.
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