Temperature effect on the optical properties of two different triphenylamine-based organic dyes

Article abstract of DOI:10.1007/s11164-016-2431-0

In this paper, we present the absorption and photoluminescence (PL) spectra of conjugated, metal-free organic dyes, triphenylamine derivatives 3-(4-(bis(4-(5-(4-(hexyloxy)phenyl)thiophenyl)phenyl)amino)phenyl)-2-cyanoacrylic acid (OD3) and (E)-3-(5′-(4-(bis(4-(5-(4-hexyloxy)phenyl)thiophen-2-yl)phenyl)amino)phenyl)-2,2′-bithiophen-5-yl)-2-cyanoacrylic acid (OD3DT), with and without a π-conjugated bridge, respectively, in film at a temperature range from 13 to 400?K. We find that the intensities of absorption and PL of OD3 decrease gradually beyond 200?K, as well as the ratios of integral areas of absorption to PL spectra. However, the other compound, OD3DT, shows a very different behavior. An increase is discovered in the ratio of the absorption to PL integral area when the temperature is higher than 200?K. A π-conjugated bridge gives this compound a higher degree of symmetry, and therefore, a better alignment of the molecules in the film, which causes a stacking aggregation such that the temperature effect on this compound is different than OD3.

Full text of DOI:10.1007/s11164-016-2431-0

Res Chem Intermed
DOI 10.1007/s11164-016-2431-0
Temperature effect on the optical properties of two
different triphenylamine-based organic dyes
Che-Hung Hsu1 Chi-Tai Wu1 Shu-Chun Yang1
•
•
•
Yu-Cheng Chang2 Chih-Hong Huang2
•
•
Shu-Mei Chang2
Received: 14 November 2014 / Accepted: 15 February 2015
Ó Springer Science+Business Media Dordrecht 2016
Abstract In this paper, we present the absorption and photoluminescence (PL)
spectra of conjugated, metal-free organic dyes, triphenylamine derivatives 3-(4-(bis(4-
(5-(4-(hexyloxy)phenyl)thiophenyl)phenyl)amino)phenyl)-2-cyanoacrylic acid (OD3)
and (E)-3-(50-(4-(bis(4-(5-(4-hexyloxy)phenyl)thiophen-2-yl)phenyl)amino)phenyl)-
2,20-bithiophen-5-yl)-2-cyanoacrylic acid (OD3DT), with and without a p-conjugated
bridge, respectively, in film at a temperature range from 13 to 400 K. We find that the
intensities of absorptionand PL of OD3 decrease gradually beyond 200 K, as wellas the
ratios of integral areas of absorption to PL spectra. However, the other compound,
OD3DT, shows a very different behavior. An increase is discovered in the ratio of the
absorption to PL integral area when the temperature is higher than 200 K. A p-conju-
gated bridge gives this compound a higher degree of symmetry, and therefore, a better
alignment of the molecules in the film, which causesa stackingaggregation such thatthe
temperature effect on this compound is different than OD3.
Keywords Absorption Á Photoluminescence (PL) Á Dye-sensitized solar cells
(DSSC) Á Temperature effect
&
Shu-Chun Yang
scyang@mail.tku.edu.tw
Shu-Mei Chang
f10914@mail.ntut.edu.tw
1
2
Department of Physics, Tamkang University, Tamsui District, New Taipei City, Taiwan
Department of Molecular Science and Engineering, National Taipei University of Technology,
Taipei, Taiwan
123

C.-H. Hsu et al.
Introduction
In the development of modern industrialized society over the past decades, energy
demands and environmental pollution resulting in global warming have led to a
greater focus all over the world on renewable energy sources. Dye-sensitized solar
¨
cells (DSSCs) demonstrated by Grazel’s group have attracted considerable interest
for the ability to convert sunlight to electricity [1]. They are the most efficient [2]
and stable [3] excitonic photocells currently in existence, and have received wider
attention due to their high performances and low cost.
The main disadvantage in an organic solar cell is the low energy conversion
efficiency, poor stability, and intensity. Dye molecules play an important role in
DSSCs because the recombination of the electrons in the excited state with the
holes in ground state of the dye will emit light in the process of photoelectric
conversion and affect the battery efficiency. Although remarkable advances have
been made with the organic dyes as sensitizers for DSSCs [4–9], the goal
remains to optimize their chemical structures for further improvement on
performance.
Compared to Ru complexes, environmentally friendly metal-free organic dyes
are considered to be an alternative due to their high molar absorption coefficient,
low cost, and easy molecular tailoring and synthesis [10–15]. Organic sensitizers
used for efficient DSSC devices are generally composed of three units: a donor,
an acceptor (also as the anchor to the nanocrystalline TiO2 surface), and a p-
conjugated spacer connecting the donor and the acceptor. An ideal spacer should
allow effective photoinduced intramolecular charge transfer from the donor to
the acceptor. Meanwhile, intermolecular dye aggregation and recombination of
conduction-band electrons with tri-iodide present in the electrolyte have to be
suppressed.
The difference between the two dyes used in this experiment, OD3 and OD3DT,
is the p-conjugated bridge. In these samples, triphenylamine is the donor because
the N in triphenylamine has a lone pair of electrons, which could push the electron.
Bithiophene in OD3DT is the conductive p-conjugated bridge for a transport
channel of electrons, and the isocyanate group (–CN) with the carboxylic acid group
(–COOH) is the acceptor. High hole-mobility of polythiophene derivatives can
enhance the stability of the structure, avoiding the combination between water
molecules and dyes to reduce the damage of structure.
Based on the temperature variations across the surface of the earth, this
experiment will measure the absorption spectra of the dyes in film types at different
temperatures to study their ability to absorb light. It will also observe the
photoluminescence spectra at different temperatures in order to study the effects of
recombining the electron on the lowest unoccupied molecular orbital (LUMO) with
the hole on the highest occupied molecular orbital (HOMO). By comparing the
results of dyes both with and without the p-bridge, we can investigate the effect of
the p-bridge and the relationship between the molecular structures and their
photoelectric conversion efficiencies.
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Temperature effect on the optical properties of two…
Synthesis
The synthetic routes of the two triphenylamine dyes OD3 and OD3DT are shown in
Scheme 1. The synthesis of the dyes was carried out under classical procedures,
which involved the Suzuki coupling reaction [16], Vilsmeier reaction [17], and
Knoevenagel condensation reaction [18].
Synthesis of organic dye OD3
4-(Bis(4-iodophenyl)amino)benzaldehyde (1)
To begin, acetic acid (20 mL) and DI water (2 mL) were added to a mixture of
4-(diphenylamino)benzaldehyde (1.40 g, 5.12 mmol), potassium iodide (1.14 g,
6.88 mmol) and potassium iodate (1.10 g, 5.12 mmol). After the mixture was
Scheme 1 Molecular structures of OD3 and OD3DT
123

C.-H. Hsu et al.
heated at 85 °C for 3 h, the mixture was cooled to room temperature and filtered.
The crude products were recrystallized from ethanol to give a yellow solid of 2.04 g
(yield: 76.0 %; m.p.: 142.5–143.6 °C). 1H NMR (400 MHz, CDCl3): d 9.84 (1H, s),
7.70 (2H, d, J = 8.80 Hz), 7.61–7.63 (4H, m), 7.05 (2H, d, J = 8.40 Hz), 6.87–6.90
(4H, m).
4-(Bis(4-(5-(4-(hexyloxy)phenyl)thiophenyl)phenyl)amino)benzaldehyde (2)
A tetrahydrofuran (THF; 30 mL) solution of compound 1 (0.7 g, 1.33 mmol), 5-(4-
(hexyloxy) phenyl)-2-thiopheneboronic acid pinacol ester (1.13 g, 2.93 mmol),
Pd(PPh3)4 (0.12 g, 0.1 mmol) and 2 M aqueous solution of K2CO3 (10 mL) was
stirred at 80 °C for 24 h under nitrogen. After cooling, the solution was poured into
distilled water and extracted with dichloromethane, dried by anhydrous Na2SO4,
and evaporated under vacuum. The residue was purified by column chromatography
on silica gel using dichloromethane/hexane (2/1, v/v) as eluent. The product was a
yellow solid. Yield: 0.78 g (74 %). m.p.: 151–152 °C. 1H NMR (400 MHz, CDCl3):
d 0.90–0.93 (6H, t), 1.33–1.38 (8H, m), 1.46–1.48 (4H, m), 1.78–1.82 (4H, m),
3.97–4.00 (4H, t), 6.91–6.93 (4H, d), 7.13–7.24 (10H, m), 7.53–7.59 (8H, m),
7.72–7.74 (2H, d), 9.85 (1H, s).
3-(4-(Bis(4-(5-(4-(hexyloxy)phenyl)thiophenyl)phenyl)amino)phenyl)-2-
cyanoacrylic acid (OD3)
Compound 2 (0.25 g, 0.32 mmol) in 15 mL of dry THF was mixed with
cyanoacetic acid (0.27 g, 3.16 mmol) and piperidine (0.2 mL), and refluxed
overnight. After cooling, the solution was poured into distilled water and extracted
with dichloromethane, dried by anhydrous Na2SO4, and evaporated under vacuum.
The residue was purified by column chromatography on silica gel using
dichloromethane/MeOH (4/1, v/v) as eluent. The yield was an orange solid:
1
0.19 g (71 %). m.p.: 181–183 °C. H NMR (400 MHz, d6-DMSO): d 0.86–0.89
(6H, t), 1.30–1.42 (12H, m), 1.70–1.73 (4H, m), 3.97–4.01 (4H, t), 697–6.99 (4H,
d), 7.02–7.08 (2H, d), 7.13–7.19 (4H, d), 7.39–7.40 (2H, d), 7.45–7.46 (2H, d),
7.58–7.60 (4H, d), 7.62–7.68 (4H, d), 7.84–7.86 (2H, d), 7.91 (1H, s). FAB-mass:
calcd m/z: 856.34 found 857.
Synthesis of organic dye OD3DT
50-(4-Bis(4-iodophenyl)amino)phenyl)-2,20-bithiophene-5-carbaldehyde (1DT)
A mixture of 50-(4-(diphenylamino)phenyl)-2,20-bithiophene-5-carbaldehyde (0.30 g,
0.69 mmol), potassium iodide (0.15 g, 0.89 mmol), and potassium iodate (0.15 g,
0.69 mmol) was added to acetic acid (10 mL) and DI water (1 mL). After the mixture
was heated at 85 °C for 4 h, the mixture was poured into water and extracted with
DCM. The organic extracts were washed with brine and dried over anhydrous Na2SO4.
The solvent was removed by rotary evaporation, and the residue was purified by
column chromatography with hexane/DCM (1/3, v/v) eluent to provide 0.264 g (yield
123

Temperature effect on the optical properties of two…
1
56 %) product 1DT as an orange solid, m.p.: 174–175 °C. H NMR (400 MHz,
CDCl3): d 9.85 (s, 1H), 7.66 (d, 1H), 7.56 (d, 1H), 7.47 (s, 2H), 7.3 (d, 4H), 7.13 (d,
2H), 7.09 (d, 2H), 6.86 (d, 4H).
50-(4-Bis(4-(5-(4-(hexyloxy)phenyl)thiophen-2-yl)phenyl)amino)phenyl)-2,
20-bithiophene-5-carbaldehyde (2DT)
A mixture of compound 1DT (0.192 g, 0.28 mmol) and tetrakis (triphenylphos-
phine) palladium (10 mg, 0.009 mmol) in THF (15 mL) was added to a THF
(5 mL) solution of 5-(4-(hexyloxy) phenyl)-2-thiopheneboronic acid pinacol ester
(0.27 g, 0.7 mmol) and potassium carbonate aqueous solution (2 M, 3 mL) at 60 °C
under an argon atmosphere. After the mixture was heated at 85 °C for 15 h, the
mixture was poured into water and extracted with DCM. The organic extracts were
washed with brine and dried over anhydrous Na2SO4. The solvent was removed by
rotary evaporation, and the residue was purified by column chromatography with
hexane/DCM (1/3, v/v) as eluent to provide 0.141 g (yield 53 %) of product 2DT as
a red solid. m.p.: 195.4–196.3 °C. 1H NMR (400 MHz, CDCl3): d 9.85 (s, 1H), 7.6
(d, 1H), 7.5–7.54 (m, 11H), 7.12–7.32 (m, 10H), 6.91 (d, 6H), 3.99 (t, 4H),
1.78–1.81 (m, 4H), 1.26–1.57 (m, 12H), 0.86–0.93 (m, 6H).
(E)-3-(50-(4-(bis(4-(5-(4-hexyloxy)phenyl)thiophen-2-yl)phenyl)amino)phenyl)-2,
20-bithiophen-5-yl)-2-cyanoacrylic acid (OD3DT)
THF (10 mL) was added to a mixture of compound 2DT (0.16 g, 0.168 mmol),
cyanoacetic acid (0.14 g, 1.68 mmol) and Piperidine(0.014 g, 0.168 mmol). After
the mixture was heated at 85 °C for 16 h, the mixture was poured into the PH = 2
phosphoric acid solution and extracted with DCM. The organic extracts were
washed with brine and dried over anhydrous Na2SO4. The solvent was removed by
rotary evaporation, and the residue was purified by column chromatography with
MeOH/DCM (1/6, v/v) as eluent to provide 0.106 g (yield 62 %) of OD3DT as a
1
deep red solid, m.p.: 243.4–244.2 °C. H NMR (400 MHz, CDCl3): d 8.12 (s, 1H),
7.35–7.63 (m, 12H), 6.94–7.11 (m, 16H), 3.96 (t, 4H), 1.68–1.71 (m, 4H), 1.3–1.41
(m, 12H), 0.87 (m, 6H).
Experimental
The compounds used in this work, OD3 and OD3DT, are two conjugated, metal-free
organic dyes, both triphenylamine derivatives, with and without a p-conjugated
bridge, respectively. The powder samples are first dissolved in acetone. The solution
is then drop-cast on a glass slide repeatedly to form a film with about 70 %
absorption at an absorption peak of 100 K. Absorption measurements are carried out
using TRIAX550 monochromator equipped with a CCD array detector and a
halogen lamp as the light source. The 457 nm line of an Ar? laser is used as the
excitation of the photoluminescence. The luminescence radiation is also analyzed
by TRIAX550 but with a photomultiplier detector.
123

C.-H. Hsu et al.
Results and discussion
The absorption spectra of two triphenylamine organic dyes in the temperature range
from 13 to 400 K at 1 bar are shown in Fig. 1. The main absorption peaks are at
around 428 and 424 nm for the dyes without (OD3) and with (OD3DT) the p-bridge
at room temperature (300 K), respectively. Red shifts in the absorption spectra are
observed in both OD3 and OD3DT when comparing to the dye without any
modification substituents on triphenylamine [19]. The absorption peaks mainly arise
from the intramolecular charge transfer (ICT) in donor–acceptor [20], which
produce intramolecular charge separation from a ground state to an excited state. It
was found that the triphenylamine coupled with the modified group at the end of the
chain could improve the overall absorption coefficient [19], therefore introducing a
red shift in the absorption peaks. Choi et al. [21] illustrated this type of plane
geometry conjugated bridge, noting that (1) extending the conjugated bridge
increases the shoot dye absorption coefficient, as well as (2) strengthens its
hydrophobicity, and increases light exposure and thermal stress stability. Common
organic dyes with extended conjugated thiophene units may increase the absorption
coefficient and show the red shift [22–24]. A red shift in the absorption spectrum
and a higher absorption coefficient could imply that both OD3 and OD3DT have a
more planar conjugated bridge than the dye without any modification substituents
on triphenylamine. Besides, a red shift is observed when comparing the offset of the
absorption spectra of OD3DT (*410 nm) to OD3 (*403 nm). This also indicates a
more planar structure of OD3DT than that of OD3.
The integral area of absorption spectra and the intensity of absorption peaks at
various temperatures (Figs. 2, 3, respectively) of both OD3 and OD3DT were
investigated. We found that in spite of the almost constant intensity of the
absorption peaks, the integral area of absorption spectra of OD3 decreased gradually
beyond 200 K. However, the other compound, OD3DT, showed a very different
behavior. In contrast to common organic compounds, the absorption intensity and
0.9
300K
200K
(a) OD3
(b) OD3DT
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
13K
100K
100K
200K
300K
400K
13K
400 500 600 700 800 900 400 500 600 700 800
Wavelength(nm)
Fig. 1 Absorption spectra of OD3 and OD3DT at various temperatures
123

Temperature effect on the optical properties of two…
280
240
200
160
OD3
OD3DT
120
80
0
100
200
300
400
Temperature(K)
Fig. 2 Total absorption of OD3 and OD3DT
0.9
0.8
0.7
0.6
0.5
OD3
OD3DT
0.4
0
100
200
300
400
Temperature (K)
Fig. 3 Intensity of absorption peak of OD3 and OD3DT
absorption integral area of OD3DT increased gradually with increasing temperature,
indicating the production of a p–p interaction between two adjacent phenyl rings.
We speculate that the p-conjugated bridge gives this compound a higher degree of
symmetry, and therefore, a better alignment of the molecules in the process of film-
making. This is due to the weight of the conjugated bridge in the –Ar– position in
Scheme 1, such that the planarity of the dye is increased and more easily stacked in
a certain way in a film. Therefore, stacking aggregation was shown to increase with
temperature. This stacking effect causes the difficulty in achieving the vibration of
the bond such that the temperature effect on this compound is different than in OD3.
A broadening effect in the absorption spectra of OD3DT as compared to OD3 was
also observed due to the stacked aggregation.
123

C.-H. Hsu et al.
The photoluminescence (PL) emission spectra due to singlet exciton transitions
of OD3 and OD3DT at 1 bar at the same temperature range are shown in Fig. 4. The
peak position of the PL of OD3 is almost the same, as well as that of OD3DT at low
temperature, but when the temperature is higher than 200 K, a slightly blue shift in
the spectrum of OD3DT was observed (Fig. 5). This could be attributed to a higher
carrier density [25, 26] making the number of free electrons increase, such that
carriers have sufficient kinetic energy to create a higher energy level, resulting in an
increase in the p–p* transition energies. This is characteristic of donor–acceptor pair
recombination [27, 28]. At low temperature, only a small fraction of the donor and
acceptor levels trap carriers. This leads to recombination from distant pairs only. At
higher temperatures, more impurity levels trap carriers, which leads to contribution
(b) OD3DT
(a) OD3
100K
200K
200K
100K
13K
300K
300K
400K
400K
500
600
700
800
500
600
700
800
Wavelength(nm)
Fig. 4 Photoluminescence spectra at various temperatures
616
614
612
610
608
606
604
602
OD3DT
600
0
100
200
300
400
Temperature (K)
Fig. 5 Peak position of PL of OD3DT at various temperatures
123

Temperature effect on the optical properties of two…
from closer pairs as well. This probability of radiative transition increases as the
separation between the pairs decreases when the temperature rises. Therefore, we
observe a shift of the emission band peak wavelength to shorter wavelengths as the
temperature increases.
Except for the blue shift, the intensity of the PL spectrum of OD3 decreased with
increases in temperature, the same as is seen in common organic dyes. However, the
intensity of the PL of OD3DT first increased with the temperature rise up to 200 K,
and then decreased at a higher temperature range (Fig. 4). The p-bridge gives
OD3DT the planar configuration, therefore reducing the distance between adjacent
layers of molecules. At higher temperatures, the electron has greater kinetic energy
and could jump between two neighboring chains if the distance between two chains
is short enough. The recombination of electron–hole pairs would be more difficult if
this happened. Thus, the PL would decrease.
In order to clarify the particular behavior of the PL of OD3DT, the ratios of
integral areas of absorption and photoluminescence spectra of two compounds at
various temperatures are shown in Fig. 6. In spite of the fluctuation of the data
(a) OD3
3
2
1
3
(b) OD3DT
2
1
0
100
200
300
400
Temperature (K)
Fig. 6 Temperature dependence of the ratios of integral absorption and photoluminescence spectra
123

C.-H. Hsu et al.
points due to the higher kinetic energy of carriers at a higher temperature range, we
were still able to observe a different trend in these two samples. This ratio of OD3
decreased slightly with increasing temperature. However, an increase in this ratio
when the temperature is higher than 200 K was discovered in OD3DT. The
modification substituents of the dye make the electron cloud distribution of the
HOMO and LUMO wider. When the temperature rises, electron transition is
increased. A p-bridge in OD3DT allows an electron to transfer along the chain and
therefore could enlarge the distance between an electron and a hole, which could
reduce the recombination of electrons and holes. From our absorption measure-
ments, the absorption ability of OD3DT above 200 K is found to increase with
rising temperature. Thus, more electrons could stay in the excited state at higher
temperatures. It has been reported that DSSCs using dyes with planar bridging units
have higher efficiency [29]. Thus, the existence of a p-bridge would make OD3DT
molecules more planar in a film, which could reduce the distance between two
adjacent chains and enhance the interchain interaction. Therefore, the electrons
could jump between two neighboring chains and could have higher charge-transport
efficiency.
Conclusion
The absorption spectra of organic dyes, OD3 and OD3DT, on films at various
temperatures showed very different trends. A red shift in the absorption offset of
OD3DT indicates a more planar configuration. The absorption peak intensity of
OD3DT increases gradually with temperature, as does the ratio of the integral area
of absorption and photoluminescence of OD3DT when the temperature is higher
than 200 K. The p-conjugated bridge does not increase the carrier mobility in this
dye as expected, but does affect its optical properties by increasing the symmetry of
the chemical structure.
Acknowledgments The authors are grateful to the Ministry of Science and Technology and to Tamkang
University, Taiwan, R.O.C. for the financial support.
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