Design, synthesis and electronic properties of push-pull-push type dye

Article abstract of DOI:10.1039/c5ra13416a

A Sonogashira cross-coupling protocol was employed for the construction of a push-pull-push type dye. An ethynyl π-spacer extends the effective π-conjugation length between push and pull units without altering the planarity of the electron donor/acceptor pair. The variation of the strength of the alkyne π-spacer electron push (or donor) units of these dyes has a strong effect on the shifting of both absorption and emission maxima and thereby on the Stokes shift. The dyes were solvatochromic and their solvatochromicity was highly dependent on the electron push unit. Strong red shifted emissions were likely to arise due to the internal charge transfer (ICT) from the electron push unit to the electron pull unit. Calculated energy values of HOMO → LUMO transitions are in good accordance with experimental observations. Alkyne conjugated electron push units (-CC-Ar; Ar = Ph, Ph-OMe, Ph-NMe₂) are more effective to increase the E_HOMO levels. Overall, experimental and theoretical results of the push-pull-push dyes indicate that they can be used as promising conjugated materials with predictable electronic properties for optoelectronic devices.

Full text of DOI:10.1039/c5ra13416a

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Design, synthesis and electronic properties of push-pull-push type dye
DOI: 10.1039/C5RA13416A
Rajen Kundu,*a Chandramouli Kulshreshtha *b
a Department of Chemistry, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
b Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic
of Korea
Graphical abstract

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DOI: 10.1039/C5RA13416A
Journal Name
ARTICLE
Design, synthesis and electronic properties of push-pull-push type
dye
Rajen Kundu,*a Chandramouli Kulshreshtha *b
Received 00th January 20xx,
Accepted 00th January 20xx
Sonogashira cross-coupling protocol was employed for the construction of a push-pull-push type dye. Ethynyl π-spacer
extends the effective π-conjugation length between push and pull units without altering the planarity of the electron
donor/acceptor pair. The variation of the strength of alkyne π-spacer electron push (or donor) units of these dyes has a
strong effect towards the shifting of both absorption and emission maxima and thereby on the Stokes shift. The dyes were
solvatochromic and their solvatochromicity was highly dependent on the electron push unit. Strong red shifted emissions
were likely to arise due to the internal charge transfer (ICT) from electron push unit to electron pull unit. Calculated energy
values of HOMO→LUMO transitions are in good accordance with experimental observations. Alkyne conjugated electron
push units (‒C≡C‒Ar; Ar = Ph, Ph-OMe, Ph-NMe2) are more effective to increase the EHOMO levels. Overall, experimental and
theoretical results of the push-pull-push dyes indicate that they can be used as a promising conjugated materials with
predictable electronic properties for optoelectronic devices.
DOI: 10.1039/x0xx00000x
www.rsc.org/
intramolecular charge transfer (ICT) from electron rich (push) to
electron deficient (pull) units, and thereby facilitates red shifted
Introduction
absorbance with high molar extinction coefficient. 42-51 The electron
deficient unit tunes the energy levels of frontier molecular orbitals
and at the same time the π-conjugated alkyne spacer connected
π-Conjugated donor-acceptor type molecules have been widely
used in material science as an efficient organic electronics material
1-5
for applications in organic light-emitting diode (OLED),
organic
6-13
17
field effect transistor (OFET),
organic photovoltaic device (OPV)
electron rich unit also involves to monitor the energy levels.
14-29 and so forth. Ethynyl (‒C≡C‒) and vinyl (‒HC=CH‒) π-conjugates
According to the donor capability of the donating unit, we can
monitor the energy level of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) and can
obtain different physical properties. 17, 52-59
are structurally very similar and could potentially reduce the band
30-35
gap energy.
However, vinyl π-conjugated systems show better
charge delocalization and smaller band gap energy over the ethynyl
36
π-conjugation.
Indeed, an ethynyl π-spacer is sterically less
hindered and more accommodating than a vinyl π-spacer. 37 It is also
noteworthy that the steric crowd between face-to-face groups of
vinyl and aromatic units result a non-planar conformation, which
38-39
reduces the charge delocalization efficiency.
Additionally, the
incorporation of an ethynyl π-spacer is a straightforward approach
and easier than a vinyl π-spacer in synthetic point of view. 40-41 Thus,
design and synthesis of novel ethynyl π-conjugated donor-acceptor
type molecules having high performance material properties remain
a promising area of optoelectronic research. The push-pull-push type
dye structure, where the electron donating units push the negative
charge density and electron accepting units pull the negative charge
density towards itself. Both the push and pull units are connected by
π-conjugated alkyne (‒C≡C‒) spacer unit which enables the
Fig. 1 Chemical structures of push-pull-push dye.
a.Department of Chemistry, Pohang University of Science and Technology, Pohang
790-784, Republic of Korea. Email: kundurajen@postech.ac.kr
b.Department of Chemical Engineering, Pohang University of Science and
Technology, Pohang 790-784, Republic of Korea. Email: mouli@postech.ac.kr
ꢀ Electronic Supplementary Informaꢁon (ESI) available: Normalized UV-visible and
fluorescence spectra, fluorescence images of the dyes, cyclic voltammograms,
cartesian coordinates, NMR spectra. See DOI: 10.1039/x0xx00000x
In our study, we demonstrate the incorporation of an alkyne π-
spacer between the donor (push) and acceptor (pull) units. The
alkyne π-spacer was introduced to extend the effective conjugation,
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and improve the photophysical properties.
Journal Name
31, 60-61
In addition, the
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DOI: 10.1039/C5RA13416A
planer structure of alkyne facilitates efficient charge transport which
can improve the optoelectronic properties. We chose the readily
synthesizable thieno-[3,4-c]pyrrole-4,6-dione (TPD) as an electron
pull unit which is symmetric, rigidly fused planer structure having
62-72
strong electron withdrawing property.
This unit provides low
energy HOMO level, low band gap, and is convenient for the
introduction of variety of alkyl side chains into the pyrrole ring to
improve the solubility in organic solvents. Varieties of ethynyl donar
groups (Ar‒C≡C‒) are commercially available or easily synthesizable
according to the requirement. Sonogashira cross-coupling gives the
facile synthetic advantage of structural variation in the electron push
unit of the push-pull-push dye. Overall, the dyes showed tunable
electronic properties. Herein, we investigated the effect on variation
of the strength of alkyne π-spacer electron donor units of the push-
pull-push dye using a combination of experimental and theoretical
studies.
Results and discussion
Synthesis
The push-pull-push dyes were synthesized by cross coupling
reaction between the donor (push) and acceptor (pull) units and
are shown in Scheme 1. Sonogashira coupling 40-41 was used to
synthesize alkyne π-spacer conjugated dyes and Suzuki coupling
73 was used to attach the donor and acceptor units directly via
C(sp2)–C(sp2) coupling without alkyne π-spacer. The TPD
acceptor unit was synthesized from thiophene-3,4-dicarboxylic
acid. 62-64 Thus, the refluxing of thiophene-3,4-dicarboxylic acid
(
6
)
in acetic anhydride gave thiophene-3,4-dicarboxylic
anhydride ( ), which when treated with n-octylamine followed
by stirring with thionyl chloride afforded the TPD unit, 5-octyl-
4H-thieno[3,4-c]pyrrole-4,6(5H)-dione ( ). Compound was
7
Scheme 1 Synthesis of push-pull-push dye 1–5. Reagents and conditions: (a)
(CH3CO)2O, heat 140 oC, 12h; (b) C8H17NH2, toluene, reflux, 24h; (c) SOCl2, 50
oC, 8h; (d) N-bromosuccinimide (NBS), H2SO4, CF3COOH, 10h; (e) 2-
(tributylstannyl)thiophene, Pd2(dba)2, tri(O-tolyl)phosphine, toluene, reflux,
18h; (f) NBS, CHCl3, CH3COOH, 0 oC to rt, 30h; (g) Pd(PPh3)4, CuI, N,N-
diisopropylethylamine (DIPEA), DMF, 80 oC, 24h; (h) Pd(PPh3)2Cl2, K3PO4, DMF,
80 oC, 24h.
9
9
brominated to give dibromo product (10), which was further
reacted with 2-(tributylstannyl)thiophene to yield 5-octyl-1,3-
di(thiophen-2-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione
Bromination of 11 using N-bromosuccinimide and prolonged
reaction time resulted in symmetric dibromo compound 12
Finally, the dyes were synthesized via C(sp2)–C(sp3)
Sonogashira coupling between 12 and terminal alkynes 13 15
Whereas the dyes were synthesized via C(sp2)–C(sp2)
Suzuki coupling between 12 and boronic acids 16 17. The
(11).
.
1–3
–
.
Inconsistency behaviour in the absorption maxima was shown
in some of the intermediate polar solvents. Donating groups
have a strong effect on monitoring the absorptive properties of
4–5
–
intermediate and final products were characterized by NMR
and ESI MS (see ESIꢀ). The very pure and recrystallized form of
the dyes
push unit shows average absorption maxima at around 440 nm
and lies between 433 – 440 nm; whereas, dye containing 4-
1–3. The dye 1 containing phenyl group as the electron
the dyes (1–5) were used to study the photophysical and
3
optoelectronic properties.
N,N-dimethylaniline group as electron push unit showed
absorption maxima at around 470 nm and lies between 465 –
485 nm (Fig. 2 a, e). The intermediate electron push unit 4-
methoxyphenyl shows absorption maxima at around 450 nm
and lies between 446 – 456 nm (Fig. 2 c). Thus, we observed ~10
Photophysical properties
First, we investigated the absorption photophysical properties
of the dyes (1–5) in different solvents of varying dielectric
constants. We observed 10 to 20 nm bathochromic shift of all
the dyes upon changing the solvent polarity from nonpolar
hexane to polar solvent dimethyl sulfoxide (DMSO). However,
in polar protic solvent MeOH, we observed hypsochromic shift,
and this is because of protic solvent–solute interaction between
the dye and the solvent.74-75
nm bathochromic shift from the dye
electron push unit phenyl (–Ph–H) to 4-methoxyphenyl (–Ph–
OMe) group and ~30 nm bathochromic shift from the dye to
, by changing phenyl (–Ph–H) to more efficient electron push
1 to 2, by changing the
1
3
unit 4-N,N-dimethylaniline (–Ph–NMe2) group. The dyes
4 and
5
absorb between 468 – 488 nm. However, the molar extinction
2 | J. Name., 2012, 00, 1-3
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0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.4
Hexane
Toluene
Dioxane
Ether
Hexane
Toluene
DOI: 10.1039/C5RA13416A
(b)
Hexane
Toluene
Hexane
Toluene
Dioxane
Ether
(a)
(4)
(4)
500
400
300
200
100
0
160
120
80
40
0
Dioxane
Ether
EtOAc
Dioxane
Ether
0.3
EtOAc
CHCl3
EtOAc
CHCl3
EtOAc
CHCl3
CHCl3
0.2
DiCl Ethane
DiCl Benzene
CH3CN
DMSO
DiCl Ethane
DiCl Benzene
CH3CN
DMSO
DiCl Ethane
DiCl Benzene
DiCl Ethane
DiCl Benzene
CH3CN
DMSO
CH3CN
0.1
DMSO
MeOH
MeOH
MeOH
MeOH
0.0
475
550
625
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775
300
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Wavelength (nm)
600
700
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Hexane
Toluene
Hexane
Toluene
Dioxane
Ether
Hexane
(a)
(b)
Hexane
Toluene
Dioxane
Ether
(5)
(5)
250
200
150
100
50
Toluene
Dioxane
Ether
Dioxane
Ether
EtOAc
EtOAc
CHCl3
EtOAc
CHCl3
EtOAc
CHCl3
CHCl3
DiCl Ethane
DiCl Benzene
CH3CN
DMSO
DiCl Ethane
DiCl Benzene
CH3CN
DMSO
DiCl Ethane
DiCl Benzene
CH3CN
DMSO
DiCl Ethane
DiCl Benzene
CH3CN
DMSO
MeOH
MeOH
MeOH
MeOH
0
475
0
450
300
400
500
600
550
625
700
775
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500
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Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Hexane
Toluene
Dioxane
Ether
Hexane
Toluene
Dioxane
Ether
160
120
80
40
0
0.4
0.3
0.2
0.1
0.0
Fig. 3 UV-visible (a, c) and fluorescence (b, d) spectra of the dyes 4 and 5
respectively in different solvents (10 µM, RT, λex = λUV max of each solvents).
Inset: Normalized UV- visible spectra of each dye.
EtOAc
CHCl3
EtOAc
CHCl3
DiCl Ethane
DiCl Benzene
CH3CN
DMSO
DiCl Ethane
DiCl Benzene
CH3CN
DMSO
MeOH
MeOH
dimethylaniline (–Ph–NMe2) group in dye 3 leads to a strong red
375
450
525
600
675
475
550
625
Wavelength (nm)
700
775
Wavelength (nm)
shift of about 75 nm (λmax,Hexane = 520 nm and λmax,EtOAc = 595
nm). The moderate electron donating group 4-methoxyphenyl
(–Ph–OMe) containing dye
2 showed 40 nm red shift from
Fig. 2 UV-visible (a, c, e) and fluorescence (b, d, f) spectra of the dyes 1, 2 and
3 respectively in different solvents (10 µM, RT, λex = λUV max of each solvents).
Inset: Normalized UV- visible spectra of each dye.
hexane (λmax,Hexane = 500 nm) to dichlorobenzene
(λmax,Dichlorobenzene = 540 nm) (see ESI†). As –Ph–NMe2 group is
the strongest electron donating group among the four, so best
charge delocalization through the alkyne π-spacer occurs in dye
coefficient of the dye
5
is more and almost doubled compared
provides
3
and it shows highest absorption and emission maxima and
to the dye 4 and this is because –NPh2 group in dye
5
also lowest band gap energy. Thus, from the above observations
it is clear that the electron push units of these push-pull-push
dyes have a strong effect towards the shifting of both
absorption and emission maxima and thereby on the Stokes
shift. The lack of alkyne π-spacer conjugation reduces the red
shift to 45 nm (λmax,Hexane = 533 nm and λmax,EtOAc = 578 nm) in
larger conjugated system than –NMe2 group provides in dye
(Fig. 3 a, c).
4
Next, we evaluated the fluorescence emission behaviour of
the dyes ( ) in different solvents moving from nonpolar
1–5
hexane to polar MeOH solvents (Fig. 2 b, d, f and Fig. 3 b, d). We
observed very interesting solvent dependent emission
properties upon changing the electron push unit. The emission
properties of the dyes were more effective on solvent polarity
than the absorption properties. All the dyes showed structured
band in low polar or medium polar solvents and structureless
broad emission band in high polar solvents. Interestingly, for all
the cases we observed strong red shifted fluorescence emission
with decrease in fluorescence intensity as the solvent polarity
changes from nonpolar to polar. These results clearly indicate
that the emission bands were arise due to the internal charge
transfer (ICT) from electron push unit to electron pull unit. The
large Stokes shift was observed with increasing solvent polarity
of all the dyes and it was also highly affected by the electron
push unit and the π-conjugation (Table 1, and see ESIꢀ). The
electron donor units used in this study follow the order –Ph–H
< –Ph–OMe < –Ph–NPh2 ≤ –Ph–NMe2. With increasing solvent
polarity a red shift of about 20 nm (λmax,Hexane = 487 nm and
dye
4
although it contains the strong electron donating 4-N,N-
containing the
dimethylaniline (–Ph–NMe2) group. Dye
5
electron push unit 4-N,N-diphenylaniline (–Ph–NPh2) group
showed 85 nm (λmax,Hexane = 530 nm and λmax,DMSO = 615 nm) red
shift although it does not contain the alkyne π-spacer
conjugation. The large conjugated system due to the –NPh2
group compensates the lack of alkyne π-spacer conjugation and
leads to a strong red shifted emission. Both the fluorescence
intensity and the quantum yield were decreased with increasing
solvent polarity for all the dyes (1–5). All the above observations
revealed that the dyes were solvatochromic and their
solvatochromicity was highly dependent on the electron push
unit and the π- conjugation. The photophysical properties of the
dyes (
1
–
5
) in CHCl3 solutions are summarized in Table 1.
) was reflected on the
The solvatochromicity of the dyes (
1–5
colour of the fluorescence images, which changed in different
solvents under a 254 nm wavelength transilluminator (see ESI†).
The influence of electron donating units on the shifting of
emission maxima of these push-pull-push dyes was also nicely
observed by the colour change in chloroform solvent under a
λmax,DMSO = 507 nm) for the dye
electron push unit) was observed. Replacement of the phenyl (–
Ph–H) group in dye with a strong electron donating 4-N,N-
1 (containing phenyl (–Ph–H) as
1
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(a)
(b)
Solvent: CHCl
3
Solvent: CHCl3
DOI: 10.1039/C5RA13416A
(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
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0.4
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1
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4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
500
600 700
Wavelength (nm)
800
375
450
Wavelength (nm)
525
600
450
525
600
Wavelength (nm)
675
750
400
500 600
Wavelength (nm)
700
800
Fig. 5 Normalized (a) UV-visible and (b) fluorescence spectra of the dyes
as film, spin coated on pre-cleaned quartz substrates using
chloroform solutions.
1–5
donating groups and the of alkyne π-spacer conjugation also
reflected on the photophysical properties in film state.
Fig. 4 Normalized (a) UV-visible and (b) fluorescence spectra the dyes 1–5 in
CHCl3 solvents (10 µM, RT, λex = λUV max). (c) Colours: colours in CHCl3 solvents
after irradiation of 1–5 in UV light (λ = 254 nm) under a UV-transilluminator.
Computational studies
Highest occupied molecular orbitals (HOMO) and lowest
unoccupied molecular orbitals (LUMO) of push-pull-push dye
(
76
1
–
5
) have been calculated using Gaussian 03 program package.
Table 1. Photophysical propertiesa of the dye 1–5
First the geometries were optimized using the B3LYP hybrid
λmax(Abs)
(nm)
εmax
M-1cm-1
λmax(Em)
(nm)
Stokes shift
(cm-1)
b
density functional and 3-21G* as the basis set. Next, the
optimized geometries were used for TD-SCF calculation using
the same B3LYP hybrid density functional and 3-21G* as the
basis set. For all the dyes, HOMO resides mainly on the electron
push units and/or alkyne π-spacer including thiophene units. In
contrast, LUMO resides mainly on the TPD moiety and
delocalized nearby two thiophene units attached with TPD
moiety. The electron density diagrams of HOMO and LUMO are
shown in Fig. 6. The calculated allowed transitions are listed in
Table 2. The lowest energy associated with strong absorption
band arises from S0→S1 transition which is attributed to the
electronic transition from HOMO→LUMO.
Dye
ΦF
1
2
3
4
5
440
453
485
477
482
501
529
581
566
572
2767
3171
3406
3296
3264
0.119
0.142
0.096
0.096
0.071
45400
37100
30600
18000
62500
a
b
Photophysical data in CHCl3 solvent. Quantum yield using
fluorescein in 0.1 N NaOH as a standard (Φf = 0.92), λex = 440, 453,
480 nm
254 nm wavelength transilluminator (Fig. 4). The colour varies
from cyan to green to yellow to orange region accordingly with
the electron donating capacity (–Ph–H < –Ph–OMe < –Ph–NPh2
≤ –Ph–NMe2) of the electron push unit and the extra π-
conjugation effect.
After solution state photophysical study, we investigated
the solid state photophysical properties of the dyes (1–5) as
film. To measure thin films, dyes were spin coated on pre-
cleaned quartz substrates using chloroform solutions. To ensure
the smooth films, spin coating time and speed were fixed at 60s
and 1000 rpm. It is noteworthy that both the solution and solid
state results follow similar trends. The UV-visible absorption
and fluorescence spectra of the dyes (
shown in Fig. 5. The dyes exhibit absorption peak centered
at 456, 456, 494, 480, and 485 nm respectively. Thus, all the
synthesized push-pull-push dyes ( ) absorbs visible light. The
fluorescence emission maxima of the dyes
610, 652, 623, and 638 nm respectively. Dye
1–5) in the film state are
1–5
1–5
1
–
5
centered at 583,
showed highest
3
absorption and emission maxima among all the dyes 1–5, which
can be attributed to increase in strength of the electron
donating group and π-conjugation. Thus, the effect of electron
Fig. 6 HOMOs (left) and LUMOs (right) of push-pull-push dye (1–5) (calculated
using Gaussian 03; Opt+Freq: B3LYP/3-21G* // TD-SCF: B3LYP/3-21G*).
4 | J. Name., 2012, 00, 1-3
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Overall, the electron donor units of these push-pull-push dye
Table 2. Calculated lowest energy S0→S1 transition
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carefully balance both the EHOMO and ELUMO levels, which make them
promising optoelectronics material.
DOI: 10.1039/C5RA13416A
D
Major
Electronic Wavelength Oscillator
y
e
1
2
3
4
5
contributions
HOMO→LUMO
83.9%
transition
(nm)
strengths
S0→S1
S0→S1
S0→S1
S0→S1
S0→S1
485
505
546
510
528
1.8147
1.9778
2.0238
1.5026
1.6926
85.0%
86.9%
86.5%
88.8%
Conclusions
In summary, we have synthesized push-pull-push type dyes and
investigated the electronic properties using a combination of
both experimental and theoretical studies. Ethynyl π-spacer
improved the electronic properties of the dye by extending the
effective π-conjugation between electron donor/acceptor
units. All the dyes showed highly solvent dependent red shifted
emission and large Stokes shift. The solvatochromism of the
dyes significantly depends on the strength of the electron donor
unit. The red shifted emissions were assigned as internal charge
transfer (ICT) between electron donor/acceptor pair. The
influence of electron donating units on the shifting of emission
maxima of these push-pull-push dyes were reflected on the
Thus, all these low energy strong absorptions mostly (84% to
89%) originated from HOMO→LUMO correspond to π→π*
transitions which arise due to intramolecular charge transfer
(ICT) from electron push units to electron pull TPD unit through
the π-conjugation. The calculated energy values for S0→S1
transition of the dye 1–5 were in good agreement with the
experimental absorption energy values (Table 1–2).
colour change in chloroform solvent under
wavelength transilluminator. Strong electron donating 4-N,N-
dimethylaniline (–Ph–NMe2) group in dye leads to a strong red
a 254 nm
Cyclic voltammetry studies
To evaluate the energy parameters of the dye
1–5, cyclic
3
voltammetry (CV) measurements were carried out.
Ferrocene/ferrocenium (Fc/Fc+) redox couple was used as an
internal reference. The HOMO and LUMO energy levels were
shift of about 75 nm (λmax,Hexane = 520 nm and λmax,EtOAc = 595
nm). Calculated energy values of HOMO→LUMO transiꢀons are
in good agreement with experimental observations. The
electron push units (‒C≡C‒Ar; Ar = Ph, Ph-OMe, Ph-NMe2) are
more effective to increase the EHOMO levels. However, the ELUMO
levels are not significantly affected by the electron push unit.
Overall, findings indicate that these push-pull-push dyes can be
used as a promising conjugated materials with predictable
electronic properties for optoelectronic devices.
calculated from the onset of the first oxidation and reduction
onset
potentials using the equations EHOMO (eV) = - [Eox
onset
- E1/2
- E1/2 (Fc/Fc+) + 4.8],
(Fc/Fc+) + 4.8] and ELUMO (eV) = - [Ered
where E1/2 (Fc/Fc+) was the cell correction. It showed two peaks
at 0.389 and 0.485 V hence the E1/2 (Fc/Fc+) was equal to 0.437
V, which was used in equation to calculate the EHOMO and ELUMO
.
The energy parameters of the dye (1–5) are summarized in
Table 3 (see ESI† for the cyclic voltammogram). Thus, the
summarized EHOMO and ELUMO values show that the alkyne
conjugated electron push unit (‒C≡C‒Ar; Ar = Ph, Ph-OMe, Ph-
NMe2) is more effective to increase the EHOMO levels. However,
the ELUMO levels are not significantly affected by the electron
push unit. It is noteworthy that band gap energy decreases
upon introducing electron donor units into 11. The decrease in
Experimental section
General
All the reagents and solvents were purchased from commercial
suppliers and were used without further purification. The air-
and/or moisture-sensitive reactions were carried out under
nitrogen atmosphere. All the reactions were monitored with
analytical TLC silica gel 60 F254. The products were purified by
ELUMO and increase in EHOMO levels of the dyes (1‒5) upon
introducing electron donor units into 11 can be explained in
terms of inductive effect and resonance effect.17 TPD is a strong
electron withdrawing unit and shows strong inductive effect.
The conjugated electron donor units show resonance effect and
able to delocalize the charge density and or π-system with
neighbouring ethynyl π-spacer and or thiophene unit. Thus,
both the inductive effect and resonance effect increases the
effective delocalization which strongly affect the EHOMO and
1
silica gel (230-400 mesh size) column chromatography. H and
13C NMR spectra were recorded under 400 MHz and 100 MHz
Bruker Advance NMR spectrometer respectively using CDCl3 as
the solvent. All the UV–visible spectra of the compounds (10
µM) were measured in different solvents using a UV-Visible
spectrophotometer at room temp with a cell of 1 cm path
length. All the sample solutions prepared for UV–visible
experiments were further used for fluorescence experiments
immediately after UV–visible experiments. The fluorescence
spectra were obtained using a fluorescence spectrophotometer
at room temp using 1 cm path length cell. The fluorescence
quantum yields (Φf) were determined using fluorescein as a
reference with the known Φf (0.92) in 0.1 molar solution in
sodium hydroxide. The electrochemical cyclic voltammetry (CV)
was conducted on a PowerLab/AD instrument model system
with glassy carbon disk, Pt wire, and Ag/Ag+ electrode as the
working electrode, counter electrode, and reference electrode,
ELUMO levels to reduce the band gap energy of the dyes (
1‒5).
Table 3. Energy parameters of the dye
1–5
Dye
Eox (V) Ered (V) EHOMO (eV) ELUMO (eV) Eox-red (eV)
1
2
3
4
5
1.28
1.11
-0.754
-0.689
-0.76
-5.643
-5.473
-5.189
-5.219
-4.929
-3.609
-3.674
-3.603
-3.633
-3.51
2.034
1.799
1.586
1.586
1.419
0.826
0.856
0.566
-0.73
-0.853
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J. Name., 2013, 00, 1-3 | 5
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respectively
Journal Name
size) column chromatography using hexane-dichloromethane as
in
a
0.1
M
tetrabutylammonium
View Article Online
DOI: 10.1039/C5RA13416A
eluent to afford the white crystalline solid product, 4.72 g, 80% yield.
hexafluorophosphate (n-Bu4NPF6)-anhydrous acetonitrile
solution at a potential scan rate of 50 mV s−1. Material film was
drop cast onto the glassy carbon working electrode from a 2.0
mg mL-1 chloroform solution and dried. The electrochemical
onsets were determined at the position where the current
starts to differ from the baseline. The potential of Ag/AgCl
reference electrode was internally calibrated by using the
ferrocene/ferrocenium redox couple (Fc/Fc+). The HOMO and
LUMO energy levels were calculated from the onset of the first
1H NMR (400 MHz, CDCl3) δ 0.87 (3H, t, J = 6.4 Hz), 1.26 – 1.30 (10H,
m), 1.60 – 1.64 (2H, m), 3.58 (2H, t, J = 7.6 Hz); 13C NMR (100 MHz,
CDCl3) δ 14.1, 22.6, 26.8, 28.2, 29.1, 31.7, 38.7, 112.7, 134.8, 160.1;
HRMS calcd for C14H17Br2NO2S 420.9347, found 420.9338.
5-Octyl-1,3-di(thiophen-2-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-
dione (11). To a solution of 4.5 g (10.635 mmol) 1,3-dibromo-5-octyl-
4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (10) in toluene, 2-
(tributylstannyl)thiophene (10.317 g, 8.78 mL, 27.652 mmol),
tris(dibenzylideneacetone)dipalladium(0) (146.5 mg, 0.16 mmol, 1.5
mol%) and tri(O-tolyl)phosphine (194.1 mg, 0.638 mmol, 6 mol%)
were added under nitrogen atmosphere. The reaction mixture was
reflux for 18h. After completion of the reaction, the mixture was
extracted with dichloromethane. The organic layer was dried over
anhydrous magnesium sulfate followed by concentrated under
reduced pressure. The crude product was purified by silica gel (230-
400 mesh size) column chromatography using hexane-
dichloromethane as eluent to afford the yellow powdered product,
3.29 g, 72% yield. 1H NMR (400 MHz, CDCl3) δ 0.86 (3H, t, J = 6.8 Hz),
1.26 – 1.34 (10H, m), 1.63 – 1.71 (2H, m), 3.66 (2H, t, J = 7.2 Hz), 7.12
– 7.14 (2H, m), 7.44 (2H, d, J = 5.2 Hz), 8.01 (2H, d, J = 3.6 Hz); 13C
NMR (100 MHz, CDCl3) δ 14.2, 22.7, 27.1, 28.6, 29.3, 31.9, 38.7,
128.5, 128.7, 130.0, 132.5, 136.6, 162.7; HRMS calcd for C22H23NO2S3
429.0891, found 429.0885.
oxidation and reduction potentials using the equations EHOMO
onset
(eV) = - [Eoxonset - E1/2 (Fc/Fc+) + 4.8] and ELUMO (eV) = - [Ered
-
E1/2 (Fc/Fc+) + 4.8], where E1/2 (Fc/Fc+) was the cell correction.
Gaussian 03 program package was used for the computational
study. The geometries were optimized using the B3LYP hybrid
density functional and 3-21G* as the basis set. Next, the
optimized geometries were used for TD-SCF calculation using
the same B3LYP hybrid density functional and 3-21G* as the
basis set. For all the cases imaginary frequency values after opt-
freq (optimization followed by frequency) calculation were
zero.
Thieno[3,4-c]furan-1,3-dione (7). A solution of 4 g (23.242 mmol)
thiophene-3,4-dicarboxylic acid (6) in 150 mL acetic anhydride was
refluxed for 12h. After the completion of reaction, the solution was
evaporated under reduced pressure and the solid residue was
1
recrystallized from hot toluene, 3.402 g, 95% yield. H NMR (400
MHz, CDCl3) δ 8.10 (2H, s); 13C NMR (100 MHz, CDCl3) δ 129.5, 135.3,
156.5; HRMS calcd for C6H2O3S 153.9725, found 153.9711.
1,3-Bis(5-bromothiophen-2-yl)-5-octyl-4H-thieno[3,4-c]pyrrole-
4,6(5H)-dione (12). To a solution of 3.2 g (7.448 mmol) 5-octyl-1,3-
di(thiophen-2-yl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (11) in 1:1
mixture of CH3COOH – CHCl3 under ice bath, 3.314 g (18.62 mmol) N-
bromosuccinimide was added in eight portions. The reaction mixture
was allowed to come at RT and stirred for 30h. The solvent was
evaporated and the residue was purified by silica gel (230-400 mesh
size) column chromatography using hexane-dichloromethane as
5-Octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (9). To a solution of
3.35 g (21.739 mmol) thieno[3,4-c]furan-1,3-dione (7) in toluene, 7.2
mL n-octylamine was added and refluxed for 24h. The reaction
mixture was cooled down and the solvent was evaporated under
reduced pressure. The crude solid was dissolved in 200 mL thionyl
o
chloide (SOCl2) and stirred for 8h under 50 C. The volatile solvent
1
was removed and the crude product was purified by silica gel (230-
400 mesh size) column chromatography using hexane-
dichloromethane as eluent to afford the white solid product, 3.75 g,
65% yield. 1H NMR (400 MHz, CDCl3) δ 0.87 (3H, t, J = 5.2 Hz), 1.25 –
1.31 (10H, m), 1.61 – 1.67 (2H, m), 3.60 (2H, t, J = 7.2 Hz), 7.79 (2H,
s); 13C NMR (100 MHz, CDCl3) δ 14.2, 22.7, 27.0, 28.6, 29.2, 31.9, 38.6,
125.5, 136.8, 162.7; HRMS calcd for C14H19NO2S 265.1136, found
265.1125.
eluent to afford the yellow powdered product, 3.5 g, 80% yield. H
NMR (400 MHz, CDCl3) δ 0.86 (3H, t, J = 7.2 Hz), 1.26 – 1.31 (10H, m),
1.62 – 1.67 (2H, m), 3.61 (2H, t, J = 7.2 Hz), 7.05 (2H, d, J = 4.0 Hz),
7.63 (2H, d, J = 4.0 Hz); 13C NMR (100 MHz, CDCl3) δ 14.2, 22.8, 27.1,
28.6, 29.3, 31.9, 38.8, 116.9, 128.7, 129.9, 131.3, 133.9, 135.2, 162.4;
HRMS calcd for C22H21Br2NO2S3 584.9101, found 584.9113.
General procedure for the Sonogashira coupling: To a solution
of aryl halide 12 (1,3-bis(5-bromothiophen-2-yl)-5-octyl-4H-
thieno[3,4-c]pyrrole-4,6(5H)-dione) in 1:1 DMF–Et3N under
1,3-Dibromo-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (10).
To a solution of 3.7 g (13.946 mmol) 5-octyl-4H-thieno[3,4-c]pyrrole- nitrogen atmosphere, Pd(PPh3)4 (6 mol%), CuI (3 mol%) and
terminal alkyne (13 15, 2.6 eqv) were added. The reaction
–
4,6(5H)-dione (9) in a mixture of sulfuric acid (10 mL) and
trifluoroacetic acid (30 mL), 5.46 (30.681 mmol) N-
mixture was stirred for 24 h at 80 oC and monitored by TLC. After
the completion of the reaction, the solvent was evaporated and
the resulting residue was purified by silica gel (230-400 mesh
size) column chromatography using Hexane-Dichloromethane
as eluent to afford the product. The products were isolated with
very good yield and characterized by NMR, and mass
spectrometry.
g
bromosuccinimide was added in eight portions. The reaction mixture
was stirred at room temperature for 10h. The reddish brown solution
was diluted with water (120 mL) and the mixture was extracted with
dichloromethane. The organic layer was dried over anhydrous
magnesium sulfate followed by concentrated under reduced
pressure. The crude product was purified by silica gel (230-400 mesh
6 | J. Name., 2012, 00, 1-3
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ARTICLE
General procedure for the Suzuki coupling: To a solution of aryl 132.4, 132.9, 135.9, 150.5, 162.7; HRMS calcd for VCie4w2HAr4t1icNle3OOn2liSne3
DOI: 10.1039/C5RA13416A
halide
thieno[3,4-c]pyrrole-4,6(5H)-dione) in DMF under nitrogen
atmosphere, PdCl2(PPh3)2 (6 mol%), arylboronic acid (16 17
12
(1,3-bis(5-bromothiophen-2-yl)-5-octyl-4H- 715.2361, found 715.2350.
–
,
1,3-bis(5-(4-(dimethylamino)phenyl)thiophen-2-yl)-5-octyl-
2.5 eqv) and potassium phosphate (K3PO4, 6 eqv) were added. 4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (4). Using the general
The reaction mixture was stirred for 24 h at 80 oC and monitored procedure, to a solution of aryl halide 12 (100 mg, 0.170 mmol)
by TLC. After the completion of the reaction, the solvent was in DMF under nitrogen atmosphere, PdCl2(PPh3)2 (7.2 mg, 0.01
evaporated and the residue was washed with water and mmol, 6 mol%), 4-(N,N-dimethylamino)phenylboronic acid 16
extracted by dichloromethane. The crude product was purified (70.1 mg, 0.425 mmol) and K3PO4 (216.2 mg, 1.02 mmol) were
by silica gel (230-400 mesh size) column chromatography using added. The title compound 4 was isolated as red solid, 70.1 mg,
Hexane-Dichloromethane as eluent. The products were isolated 61 % yield. 1H NMR (400 MHz, CDCl3) δ 0.89 (3H, t, J = 6.6 Hz),
with very good yield and characterized by NMR, and mass 1.24 – 1.30 (10H, m), 1.66 – 171 (2H, m), 3.03 (12H, s), 3.68 (2H,
spectrometry.
t, J = 7.2 Hz), 6.73 (4H, d, J = 9.0 Hz), 7.18 (2H, d, J = 3.9 Hz), 7.55
(4H, d, J = 9.0 Hz), 7.98 (2H, d, J = 4.2 Hz); 13C NMR (100 MHz,
1,3-bis(5-(phenylethynyl)thiophen-2-yl)-
5-octyl-4H- CDCl3) δ 14.1, 22.5, 27.0, 28.5, 29.1, 29.6, 31.7, 38.6, 39.7,
thieno[3,4-c]pyrrole-4,6(5H)-dione (1). Using the general 111.2, 112.2, 121.9, 125.3, 126.9, 127.4, 128.2, 131.0, 131.4,
procedure, to a solution of aryl halide 12 (100.0 mg, 0.170 134.6, 134.9, 136.6, 152.1, 162.4; HRMS calcd for C38H41N3O2S3
mmol) in 1:1 DMF–Et3N under nitrogen atmosphere, Pd(PPh3)4 667.2361, found 667.2346.
(11.8 mg, 0.01 mmol, 6 mol%), CuI (1.0 mg, 0.005 mmol, 3
mol%) and phenylacetylene, 13 (45.0 mg, 0.442 mmol) were 1,3-bis(5-(4-(diphenylamino)phenyl)thiophen-2-yl)-5-octyl-
added. The title compound 1 was isolated as red solid, 86.7 mg, 4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (5). Using the general
1
81% yield. H NMR (400 MHz, CDCl3) δ 0.88 (3H, t, J = 6.9 Hz), procedure, to a solution of aryl halide 12 (235 mg, 0.40 mmol)
1.27 – 1.33 (10 H, m), 1.66 – 1.68 (2H, m), 3.63 (2H, t, J = 7.5 Hz), in DMF under nitrogen atmosphere, PdCl2(PPh3)2 (16.9 mg,
7.18 (2H, d, J = 3.9 Hz), 7.34 – 7.36 (6H, m), 7.48 – 7.51 (4H, m), 0.024 mmol, 6 mol%), 4-(N,N-diphenylamino)phenylboronic
7.84 (2H, d, J = 3.9 Hz); 13C NMR (100 MHz, CDCl3) δ 14.2, 22.8, acid 17 (289 mg, 1.0 mmol) and K3PO4 (508 mg, 2.40 mmol)
27.1, 28.6, 29.3, 31.9, 38.8, 82.4, 96.5, 122.5, 126.7, 128.6, were added. The title compound 5 was isolated as red solid,
128.9, 129.1, 129.9, 131.6, 132.9, 133.4, 135.6, 162.4; HRMS 242.8 mg, 66 % yield. 1H NMR (400 MHz, CDCl3) δ 0.90 (3H, t, J
calcd for [C38H31NO2S3 + H]+ 630.1595, found 630.1571.
= 4.8 Hz), 1.29 – 1.35 (10H, m), 1.68 – 1.70 (2H, m), 3.66 (2H, t,
J = 7.3 Hz), 7.05 – 7.16 (16H, m), 7.21 (2H, d, J = 3.9 Hz), 7.28 –
7.33 (8H, m), 7.50 (4H, d, J = 8.7 Hz), 8.00 (2H, d, J = 4.2 Hz); 13
C
1,3-bis(5-((4-methoxyphenyl)ethynyl)thiophen-2-yl)-5-octyl-
4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (2). Using the general NMR (100 MHz, CDCl3) δ 14.3, 22.8, 27.2, 28.7, 29.4, 31.7, 31.9,
procedure, to a solution of aryl halide 12 (100 mg, 0.170 mmol) 38.7, 123.1, 123.3, 123.6, 125.0, 126.9, 127.1, 128.0, 129.5,
in 1:1 DMF–Et3N under nitrogen atmosphere, Pd(PPh3)4 (11.8 130.8, 131.3, 136.2, 147.3, 147.7, 148.2, 162.7; HRMS calcd for
mg, 0.01 mmol, 6 mol%), CuI (1.0 mg, 0.005 mmol, 3 mol%) and C58H49N3O2S3 915.2987, found 915.2970.
4-ethynylanisole 14 (58.4 mg, 0.442 mmol) were added. The
title compound 2 was isolated as red solid, 91.5 mg, 78% yield.
Acknowledgements
1H NMR (400 MHz, CDCl3) δ 0.85 (3H, t, J = 6.6 Hz), 1.23 – 1.30
The authors are thankful to Pohang University of Science and
(10H, m), 1.64 – 1.66 (2H, m), 3.63 (2H, t, J = 7.2 Hz), 3.81 (6H,
technology (POSTECH) for support.
s), 6.86 (4H, d, J = 9.0), 7.17 (2H, d, J = 3.9), 7.43 (4H, d, J = 8.7
Hz), 7.86 (2H, d, J = 3.9 Hz); 13C NMR (100 MHz, CDCl3) δ 14.2,
References
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128.1, 129.0, 130.0, 132.5, 133.0, 133.2, 135.8, 160.2, 162.6;
HRMS calcd for [C40H35NO4S3 + H]+ 690.1806, found 690.1821.
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