Design of novel well-defined oligothiophenes bearing donor-acceptor groups (pyrene-porphyrin): Synthesis, characterization, optical properties and energy transfer

Article abstract of DOI:10.1016/j.molstruc.2019.01.078

Novel thiophene monomers containing porphyrin units attached via di(ethylene glycol) and tetra(ethylene glycol) spacers were synthesized: 3-methyl-4- (4- (1- (5-phenyl-10,15,20-triphenylporphyrin)) diethoxy) thiophene (M2PPh) and 3-methyl-4- (4- (1- (5-ph

Full text of DOI:10.1016/j.molstruc.2019.01.078

Journal of Molecular Structure 1183 (2019) 28e36
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Design of novel well-defined oligothiophenes bearing donor-acceptor
groups (pyrene-porphyrin): Synthesis, characterization, optical
properties and energy transfer
Efraín Rodríguez-Alba, Jesús Ortíz-Palacios, Mireille Vonlanthen,
Sandra M. Rojas-Montoya, Pasquale Porcu, Andrea Ruiu, Ernesto Rivera
*
Instituto de Investigaciones en Materiales, Universidad Nacional Aut oꢀ noma de M ꢀe xico, Ciudad Universitaria, C.P. 04510, DF, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 October 2018
Received in revised form
Novel thiophene monomers containing porphyrin units attached via di(ethylene glycol) and tetra(ethylene
glycol) spacers were synthesized: 3-methyl-4- (4- (1- (5-phenyl-10,15,20-triphenylporphyrin)) diethoxy)
thiophene (M2PPh) and 3-methyl-4- (4- (1- (5-phenyl-10,15,20-triphenylporphyrin)) tetraethoxy) thio-
phene (M4PPh). These monomers and other co-monomers with pyrene units 3-methyl-4-(diethoxy)
thiophene (M2) and 3-methyl-4-(tetraethoxy) thiophene (M4) previously reported by us, were linked to
thiophene and bithiophene via a Suzuki coupling reaction to give the corresponding terthiophenes and
23 January 2019
Accepted 24 January 2019
Available online 25 January 2019
000
0
quaterthiophenes: [3-diethoxypyrene, 3-diethoxyporphyrin, 4,4 dimethyl - 2,2: 5 2”- terthiophene
Keywords:
Oligothiophenes
Pyrene
Porphyrin
Fluorescence
Energy transfer
0
0
0
0
(
(
TT2PY-PPh)], [3-tetraethoxypyrene, 3-tetraethoxyporphyrin, 4, 4 dimethyl - 2,2:5 2”-terthiophene
0
0
000
0
0
00 00 000
TT4PY-PPh)], [3 -diethoxypyrene, 3 -diethoxyporphyrin, 4,4 methyl-2,2 :5 : 2 :5 ,2 -quaterthiophene
0
0
000
0
0
00 00 000
(QT2PY-PPh)], and [3 -tetraethoxypyrene, 3 -tetraethoxyporphyrin,4,4 methyl ꢀ2,2 :5 :2 :5 ,2 -qua-
terthiophene (QT4PY-PPh)]. The synthesized compounds were characterized by 1H NMR, C NMR and
MALDI-TOF mass spectrometry; their optical properties were studied by absorption and fluorescence
spectroscopy. The emission spectra of the oligothiophenes bearing pyrene-porphyrin units showed a
significant decrease in the pyrene monomer emission and the appearance of a new emission band at
13
650 nm, arising from the porphyrin, which revealed that a FRET phenomenon is occurring from the excited
pyrene to the porphyrin. The obtained FRET efficiencies were between 93 and 98% depending on the
oligomer structure.
©
2019 Elsevier B.V. All rights reserved.
1
. Introduction
allowed the development of novel organic semiconductors [7,8]
and the absorption bands of polythiophenes are red-shifted with
the increasing number of ethylenedioxy groups added to oligo-
thiophene skeleton [9]. Moreover, a greater stabilization was ob-
tained by increasing the oligothiophene chain length [10], the
presence of alkyl chains linked to the thiophene units improved the
solubility [11] and alkoxy groups contributed to enhance the con-
ductivity [12].
Pyrene is a highly fluorescent probe and is widely used in
polymer labeling to carry out research such as chain dynamics in
macromolecules [13], graphene exfoliation [14] and degradation of
environmental pollutants [15]. These applications are due to its
photophysical properties, particularly its ability to form excimers
Polythiophenes are versatile materials, which exhibit high
thermal stability and easily tunable optical and electrochemical
properties [1,2]. However, oligothiophenes, unlike polythiophenes,
present a well-defined structure and therefore they are more
appropriate for the elaboration of organic electronic devices such as
OFETs [3], OLEDs [4], electro-optic modulators and photovoltaic
cells [5]. Oligothiophenes are forming a novel class of molecular
fluorophores [6].
The properties of oligothiophenes have been modulated
through structural modifications. For instance, the studies of con-
ducting properties of thiophene tetramers and hexamers have
[16]. On the other hand, porphyrins play a very important role in
different fields such as photocatalysis [17] energy transfer [18], and
metal ions and molecules sensors [19,20]. Due to their photo-
physical properties [18] and easy structural modifications that
*
Corresponding author.
E-mail address: riverage@unam.mx (E. Rivera).
https://doi.org/10.1016/j.molstruc.2019.01.078
022-2860/© 2019 Elsevier B.V. All rights reserved.
0

E. Rodríguez-Alba et al. / Journal of Molecular Structure 1183 (2019) 28e36
29
permit to improve the system's properties, porphyrins have a sig-
nificant relevance in the design of materials for photovoltaic ap-
plications [21].
2.2. Apparatus
1H NMR and 13C NMR spectra of all compounds were recorded in
In a previous work, we have reported the preparation and
characterization of oligothiophenes containing two pyrene units. In
these oligomers, the intramolecular pyrene-pyrene interactions
depend significantly on the flexible spacer length (Fig. 1) as well as
on the oligomer backbone. Stronger intramolecular pyreneepyrene
interactions were observed for the oligomers that contained a
flexible spacer and a longer oligomer backbone [22]. We have re-
ported polythiophenes bearing pyrene attached via alkyl chains
that exhibit good thermal stability [23] and analyzed the effect of
the internal stacking in the optical and photophysical properties
CDCl solution, using a Bruker Avance 400 spectrometer, operating
3
1
13
at 400 MHz and 100 MHz for H and C, respectively. MALDI-TOF
mass spectra of the oligomers were recorded using dithranol as
matrix, on a Bruker Daltonics Flex Analysis.
Absorption spectra of the oligomers were scanned on a Unicam
UV300 spectrophotometer using quartz cells with a width of 1 cm
5
and CHCl
used as solvent.
3
(spectrometric grade, concentration 1 ꢁ 10ꢀ M) was
Steady state fluorescence spectra were recorded on a Fluorolog3
Horiba spectrofluorometer with Xenon lamp as light source. The
slit widths on the excitation and emission monochromators were
set to 1 and 1 nm, respectively. All spectra were obtained using
[
24e27]. We have also studied the energy transfer phenomenon
(
FRET) in dendritic molecules containing porphyrin and fullerene
ꢀ
6
units as acceptor, analyzing the effect of the donor-acceptor dis-
tance [28e30].
CHCl
pounds), with the usual right-angle configuration, exciting at
¼ 345 nm for the oligomers and at ¼ 415 nm for the porphyrin
monomers.
3
(spectrometric grade, concentration 1 ꢁ 10 M for all com-
The remarkable optical properties of the oligothiophenes are
arising from their conjugated backbone as well as from the inserted
side substituents [31]. In fact, the structure of the oligomers can be
modulated by modifying the conjugated backbone or by incorpo-
rating photoactive chromophores. These strategies allow control-
ling the optical and photophysical properties for the design of novel
materials [32e34].
l
l
2.3. Synthesis of monomers and oligomers (terthiophenes and
quaterthiophenes)
The oligothiophenes were synthesized according to the scheme
shown in Fig. 2. The spectroscopic characterization is described for
each compound and the assignation is referred to the supporting
This work describes the synthesis, characterization, optical
properties and energy transfer studies of a novel series of oligo-
thiophenes [terthiophenes (TT2PY-PPh, TT4PY-PPh) and quater-
thiophenes (QT2 PY-PPh, QT4 PY-PPh)], where TT means
Terthiophene, QT means Quaterthiophene, 2 and 4 indicate the
presence of di(ethylene glycol) and tetra(ethylene glycol) chains,
respectively, and PY-PPh means pyrene-porphyrin. These oligo-
thiophenes with pyrene and porphyrin units connected via
di(ethylene glycol) and tetra(ethylene glycol) spacers were pre-
pared to study the effect of the oligothiophene skeleton and of the
flexible spacer length on the optical properties and on intra-
molecular energy transfer. Monomers and oligomers were char-
information.
5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin
(PPh) was obtained using the methods previously reported by us
[28,35e37], and by Lindsey [38]. 3-Methyl-4-(4-(iodo) diethoxy)
thiophene (3) and 3-Methyl-4-(4-(iodo) tetraethoxy) thiophene (4)
were prepared following the procedure described in the literature
[22].
2.3.1. Synthesis of 3-Methyl-4- (4- (1- (5-phenyl-10,15,20-
triphenylporphyrin)) diethoxy) thiophene (M2PPh)
A solution of 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin
(PPh) (0.40 g, 0.63 mmol) and 3-methyl-4-(4-(iodo)diethoxy)thio-
phene (3) (0.23 g, 0.70 mmol) in acetone (50 mL) was reacted in the
1
13
acterized by H and C NMR spectroscopies and MALDI-TOF mass
spectrometry. The optical properties and energy transfer phe-
nomenon were studied by absorption and fluorescence
spectroscopies.
2 3
presence of K CO (0.61 g, 4.40 mmol), and 18-crown-6 ether as
catalyst. The reaction mixture was refluxed for 48 h; after this time,
it was cooled to room temperature and concentrated under
reduced pressure. The crude product was purified by flash column
chromatography in silica gel, using hexanes:dichloromethane 1:1
as eluent to remove the unreacted PPh, and then with dichloro-
methane. The pure product (M2PPh) was obtained as a purple solid
2
. Experimental work
2.1. Chemicals
(
310 mg, 0.38 mmol). Yield: 60%.
1
2
H NMR (CDCl
3
, 400 MHz, ppm): 8.96 (d, 2H, H , J ¼ 6 Hz), 8.95
All the reagents used in the synthesis were purchased from
1
6 4
(
s, 6H, H ), 8.31 (m, 6H, H ), 8.16 (d, 2H, H , J ¼ 9 Hz), 7.81 (m, 9H,
Sigma Aldrich and used without previous treatment. The solvents
used in the reactions were dried and purified by simple distillation
prior to be used.
3
5
a
b
H ), 7.23 (d, 2H, H , J ¼ 9 Hz), 6.90 (m, 1H, H ), 6.21 (d, 1H, H ,
J ¼ 4 Hz), 4.28 (t, 2H, PPh-O-CH , J ¼ 4 Hz), 4.17 (t, 2H, Thiop-O-CH
J ¼ 4.5 Hz), 4.01 (t, 2H, PPh -O-CH2-CH , J ¼ 4.5 Hz), 3.98 (t, 2H,
Thiop-O-CH - CH ). ꢀ2.62 (s, 2H, NH).
, J ¼ 4.5 Hz), 2.22 (s, 3H, CH
C NMR (CDCl , 100 MHz, ppm): 158.72 (C , thioph-O), 156.02 (C ,
Cphenyl-O), 142.37 (C , C-N), 135.71 (C , C-PPh, Cphenyl), 134.86
2
2
,
2
2
2
3
1
3
1
2
3
4
5
1
1
10
9
(
(
(
C , thiop-CH
C
C , C-thioph), (70.17, 70.08, 69.82, 67.78, O-CH
3
), 129.35 (C , CH-C-PPh), 127.88 (C , CH-S), 126.87
8
3
,12
6
), 120.24 (C7, O-C-CH, phenyl), 112.97 (C , Cmeso PPh), 96.79
), 13.00 (CH ).
2
3
2.3.2. Synthesis of 3-Methyl-4- (4- (1- (5-phenyl-10,15,20-
triphenylporphyrin)) tetraethoxy) thiophene (M4PPh).
A mixture of 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin
(PPh) (0.40 g, 0.63 mmol) and 3-methyl-4-(4-(iodo)diethoxy)thio-
phene (4) (0.28 g, 0.70 mmol) in acetone (50 mL) was reacted in the
presence of K CO (0.68 g, 4.90 mmol), and a catalytic amount of
18-crown-6 ether. The reaction mixture was heated to reflux for
2
3
Fig. 1. Chemical structure of oligothiophenes containing pyrene units [22].

30
E. Rodríguez-Alba et al. / Journal of Molecular Structure 1183 (2019) 28e36
Fig. 2. Synthesis of oligothiophenes: TT2PY-PPh, TT4 PY-PPh, QT2 PY-PPh, and QT4 PY-PPh.
1
1
10
4
8 h. After this time it was cooled to room temperature and
3
Cphenyl), 134.78 (C , thiop-CH ), 129.27 (C , CH-C-PPh), 127.80
8
,12
7
6
concentrated under reduced pressure. The crude product was pu-
rified by flash column chromatography in silica gel, using hex-
anes:dichloromethane 1:1 as eluent, to remove the unreacted PPh,
and then with dichloromethane. The pure product (M4PPh) was
obtained as a purple solid (355 mg, 0.39 mmol). Yield: 62%.
(C9, CH-S), 126.79 (C ), 120.14 (C , O-C-CH, phenyl), 112.95 (C ,
Cmeso PPh), 96.63 (C , C-thiop), 72.08, 71.07, 71.01, 70.86, 70.03,
69.85, 69.75, 69.79 (all O-CH ), 12.89 (CH ).
2 3
3
2.3.3. Synthesis of 2-bromo-4-methyl-3-(3-(1-(5-phenyl-10,15,20-
triphenylporphyrin)) diethoxy)thiophene (12)
This intermediate was synthesized from M2PPh, using a pro-
cedure that is similar to the one described by Castillo-Aguilera and
Bucevicius [39]. To a solution of N-bromosuccinimide (NBS) (80 mg,
0.012 mmol) in CHCl
10,15,20-triphenylporphyrin)) diethoxy) thiophene (M2PPh)
(100 mg, 0.012 mmol) and 70% HClO , (1 mol %) were added. The
1
2
H NMR (CDCl
3
, 400 MHz, ppm): 8.88 (d, 2H, H , J ¼ 6 Hz), 8.86
1
6
4
(
s, 6H, H ), 8.24 (m, 6H, H ), 8.12 (d, 2H, H , d ¼ 9 Hz), 7.77 (m, 9H,
3
5
a
b
H ), 7.27 (d, 2H, H , J ¼ 9 Hz), 6.82 (m, 1H, H ), 6.16 (d, 1H, H ,
J ¼ 4.4 Hz), 4.39 (t, 2H, P PPh -O-CH , J ¼ 4.8 Hz), 4.12 (t, 2H, Thiop-
O-CH ), 2.11 (s, 3H,
, J ¼ 4.9 Hz), 3.91e3.74 (m, 12H, rest of all O-CH
CH
). ꢀ2.74 (s, 2H, NH). C NMR (CDCl
thiop-O), 155.99 (C , Cphenyl-O), 142.31 (C , C-N), 135.67 (C ,
2
2
2
3
(10 mL), 3-methyl-4- (4- (1- (5-phenyl-
13
1
3
3
,100 MHz, ppm): 158.75 (C ,
2
4
5
4

E. Rodríguez-Alba et al. / Journal of Molecular Structure 1183 (2019) 28e36
31
reaction mixture was stirred for 1.5 h at room temperature; after-
wards K CO (ca. 20 mg) was added. The solids were filtered and
the residue was concentrated under vacuum to give 12 as a purple
2.3.6. Synthesis of terthiophene (TT4PY-PPh)
2
3
2,5-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophene
(10) (45.4 mg, 0.135 mmol), 2-bromo-4-Methyl-3-(3-(pyren-1-yl)
tetraethoxy)thiophene (9) (80.7 mg, 0.140 mmol) [34], 2-bromo-4-
methyl-3-(3-(1-(5-phenyl-10,15,20-triphenylporphyrin))tetrae-
thoxy)thiophene (13) (132.5 mg, 0.135 mmol), tetramethylammo-
solid (82 mg, 0.091 mmol). Yield: 75%.
1
2
H NMR (CDCl
3
, 400 MHz, ppm): 8.86 (d, 2H, H , J ¼ 6 Hz), 8.83
1
6
4
(
s, 6H, H ), 8.2 (m, 6H, H ), 8.13 (d, 2H, H , J ¼ 8.4 Hz), 7.75 (m, 9H,
3 5 a
H ), 7.31 (d, 2H, H , J ¼ 8.4 Hz), 6.85 (m, 1H, H ), 4.58 (t, 2H, PPh-O-
CH
PPh-O-CH
J ¼ 4.2 Hz), 2.24 (s, 3H, CH
2 3
nium bromide (ca.) and Na CO (7.1 mg, 0.067 mmol) (2 M) were
2
, J ¼ 4.6 Hz), 4.32 (t, 2H, Thiop-O-CH
2
, J ¼ 4.8 Hz), 4.12 (t, 2H,
dissolved in 30 mL of anhydrous toluene. The solution was purged
with nitrogen for 20 min; then bis(triphenylphosphine)palladiu-
m(II) dichloride (4.7 mg, 0.0067 mmol) was added and the reaction
mixture was heated with vigorous stirring at 80 C. The reaction
progress was followed by TLC. After 24 h, the mixture was cooled to
room temperature and extracted with dichloromethane. The ex-
2
-CH
2
,
J ¼ 4.8 Hz), 4.0 (t, 2H, Thioph-O-CH
2
-
CH
2
,
,
1
3
3
). ꢀ2.78 (s, 2H, NH). C NMR (CDCl
3
1
2
ꢂ
1
(
1
00 MHz, ppm): 160.02 (C , thiop-O), 157.71 (C , Cphenyl-O), 143.37
4
5
11
C , C-N), 137.27 (C , C-PPh, Cphenyl), 135.01 (C , thiop-CH
3
),
10
9
8,12
30.05 (C , CH-C-PPh), 128.58 (C , CH-S), 127.54 (9C, C ), 120.05
7
6
(
C , O-C-CH, phenyl), 114.07 (C , Cmeso PPh), (71.01, 70.36, 70.32,
2 4
tracts were washed with saturated brine and dried over Na SO .
6
9.10, O-CH
2
),12.50 (CH
3
).
The crude product was purified by column chromatography in silica
gel, employing chloroform:hexanes 80:20 as eluent to obtain
TT4PY-PPh (147 mg). Yield: 74%.
2.3.4. Synthesis of 2-bromo-4-methyl-3-(3-(1-(5-phenyl-10,15,20-
1
d
H NMR (CDCl
3
, 400 MHz, ppm): 8.85 (d, 2H, H , J ¼ 6.0 Hz), 8.83
triphenylporphyrin)) tetraethoxy)thiophene (13)
This intermediate was synthesized from M4PPh, using a pro-
cedure that is similar to the one described for the previous com-
d
a
(
s, 6H, H ), 8.60e7.29 (27H, pyrene and phenyl rings) 7.01 (d, 2H, H ,
b
J ¼ 4.1 Hz), 6.88 (m, 2H, H ), 4.5e3.66 (32H, O-CH
CH
), ꢀ2.79 (s, 2H, NH).
MS (MALDI-TOF): m/z calculated for C90
473.81; Found: 1473.63.
2
), 2.19 (s, 6H,
3
pound. To a suspension of NBS (49 mg, 0.028 mmol) in CHCl
-methyl-4- (4- (1- (5-phenyl-10,15,20-triphenylporphyrin)) tet-
raethoxy) thiophene (M4PPh) (252 mg, 0.028 mmol) and 70%
HClO , (1 mol %) were added. The reaction mixture was stirred
vigorously for 1.5 h at room temperature. Afterwards, K CO (ca.
0 mg) was added, the solids were filtered and the residue was
concentrated under reduced pressure to give 13 as a purple solid
3
10 mL,
þ
80
H N
4
O
10
S
3
[M] :
3
1
4
2.3.7. Synthesis of quaterthiophene (QT2PY-PPh).
2
3
This quatertiophene was obtained using a procedure similar to
2
0
the one described for the previous compounds. 5,5 -bis(4,4,5,5-
0
tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2 -bithiophene
(11)
(
222 mg, 0.22 mmol). Yield: 81%.
(
36.38 g, 0.0087 mmol), 2-Bromo-4-Methyl-3-(3-(pyren-1-yl)die-
toxy)thiophene (8) (42.4 mg, 0.0087 mmol), 2-Bromo-4-methyl-3-
3-(1- (5-phenyl-10,15,20-triphenylporphyrin)) diethoxy) thio-
phene (12) (114.1 mg, 0.12 mmol), tetramethylammonium bromide
ca.) and Na CO (4.61 mg, 0.043 mmol) (2 M) were dissolved in
0 mL of anhydrous toluene. The solution was purged with nitrogen
for 20 min; then bis(triphenylphosphine)palladium(II) dichloride
1
2
H NMR (CDCl
3
, 400 MHz, ppm): 8.88 (d, 2H, H , J ¼ 6 Hz), 8.86
1
6 4
(
s, 6H, H ), 8.24 (m, 6H, H ), 8.12 (d, 2H, H , d ¼ 8.9 Hz), 7.77 (m, 9H,
(
3
5
a
H ), 7.27 (d, 2H, H , J ¼ 9.6 Hz), 6.79 (m, 1H, H ), 4.39 (t, 2H, PPh-O-
CH , J ¼ 4.9 Hz), 3.91e3.74 (m,
, J ¼ 4.8 Hz), 4.12 (t, 2H, Thiop-O-CH
2H, rest of all O-CH ), 2.11 (s, 3H, CH
). ꢀ2.74 (s, 2H, NH). C NMR
CDCl , 75 MHz, ppm): 160.03 (C , thiop-O), 157.13(C , Cphenyl-O),
43.71 (C , C-N), 139.97 (C , Cphenyl), 135.98 (C , thiop-CH3),
2
2
(
3
2
3
13
1
(
1
1
2
3
1
2
3
4
5
11
8,12
(
0.30 mg, 0.0043 mmol) was added and the reaction mixture was
30.58 (C10, CH-C-PPh), 129.09 (C , CH-S), 127.59 (C ), 121.14 (C7,
9
ꢂ
heated with vigorous stirring at 80 C. The reaction progress was
monitored by TLC. After 24 h, the reaction was cooled to room
temperature and extracted with dichloromethane. The extracts
6
O-C-CH, phenyl), 114.09 (C , Cmeso PPh), 73.38, 73.17, 72.81, 72.19,
1.88, 71.23, 71.13, 70.88, (all O-CH ), 12.29 (CH ).
7
2
3
2 4
were washed with saturated brine and dried over Na SO . The
2
.3.5. Synthesis of terthiophene (TT2PY-PPh)
This trimer was prepared using a procedure that is similar to the
crude product was purified by column chromatography with silica
gel, using chloroform:hexane 80:20 as eluent to give QT2PY-PPh
(91.12 mg). Yield: 76%.
one described by Duddukuri [40]. 2,5-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)thiophene (10) (30.9 mg, 0.009 mmol), 2-
Bromo-4-Methyl-3-(3-(pyren-1-yl)dietoxy)thiophene
1
d
H NMR (CDCl
3
, 400 MHz, ppm): 8.84 (d, 2H, H , J ¼ 6.2 Hz), 8.82
d
(8)
(s, 6H, H ), 8.48e7.31(27H, pyrene and phenyl rings), 7.05 (d, 2H,
a
b
c
(
44.4 mg, 0.092 mmol) [34], 2-Bromo-4-methyl-3-(3-(1-(5-phenyl-
H , J ¼ 4.2 Hz), 6.76 (d, 2H, H , J ¼ 4.1 Hz), 6.66 (m, 2H, H ) 4.53e3.98
10,15,20-triphenylporphyrin))diethoxy)thiophene (12) (138 mg,
(16H, O-CH
2
), 2.14 (s, 6H, CH
3
), ꢀ2.78 (s, 2H, NH).
þ
0
.150 mmol), tetramethylammonium bromide (ca.) and Na
2
CO
3
MS (MALDI-TOF): m/z calculated C86
Found: 1379.73.
H
66
N
4
O
6
S
4
[M] : 1379.73;
(
4.82 mg, 0.046 mmol) (2 M) were dissolved in 20 mL of anhydrous
toluene. The solution was purged with nitrogen for 20 min. Then,
bis(triphenylphosphine)palladium(II) dichloride (0.31 mg,
.005 mmol) was added and the reaction mixture was heated with
2.3.8. Synthesis of quaterthiophene (QT4PY-PPh).
0
0
0
5,5 -bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2 -
bithiophene (11) (54.36 mg, 0.13 mmol), 2-bromo-4-Methyl-3-(3-
(pyren-1-yl) tetraetoxy) thiophene (9) (92.5 mg, 0.162 mmol), 2-
bromo-4-methyl-3-(3-(1-(5-phenyl-10,15,20-triphenylporphyrin))
tetraethoxy)thiophene (13) (128.2 mg, 0.13 mmol), tetramethy-
ꢂ
vigorous stirring at 80 C. The reaction progress was followed by
TLC. After 24 h the reaction mixture was cooled to room tempera-
ture and extracted with dichloromethane. The extracts were
2 4
washed using saturated brine and dried over Na SO . The crude
product was purified by column chromatography in silica gel, using
chloroform:hexanes 90:10 as eluent to obtain (TT2PY-PPh)
2 3
lammonium bromide (ca.) and Na CO (6.88 mg, 0.065 mmol (2 M)
were dissolved in 30 mL of anhydrous toluene. The solution was
purged with nitrogen for 20 min, then bis(triphenylphosphine)
palladium(II) dichloride (4.55 mg, 0.0065 mmol) was added and the
(
85 mg). Yield: 72%.
1
d
H NMR (CDCl
3
, 400 MHz, ppm): 8.87 (d, 2H, H , J ¼ 6.0 Hz), 8.85
d
ꢂ
(
s, 6H, H ), 8.49e7.27 (27H, pyrene and phenyl rings), 6.85 (d, 2H,
reaction mixture was heated with stirring at 80 C. The reaction
a
b
H , J ¼ 4.0 Hz), 6.73 (m, 2H, H ) 4.49e4.03 (16H, O-CH
CH
), ꢀ2.76 (s, 2H, NH).
MS (MALDI-TOF): m/z calculated for
297.60; Found: 1296.25.
2
), 2.18 (s, 6H,
progress was followed by TLC. After 24 h, the mixture was cooled,
extracted with dichloromethane, washed with saturated brine and
3
þ
C
82
64
H N
4
O
6
S
3
[M] :
2 4
dried over Na SO . The crude product was purified by column
1
chromatography in silica gel, using a mixture chloroform:hexanes

32
E. Rodríguez-Alba et al. / Journal of Molecular Structure 1183 (2019) 28e36
8
0:20 as eluent to obtain QT4PY-PPh (137 mg). Yield: 68%.
triplet at 4.39 ppm corresponding to protons CH
porphyrin with J ¼ 4.8 Hz, a triplet at 4.12 ppm due to protons CH
close to the thiophene with J ¼ 4.9 Hz, a multiplet between 3.91 and
3.74 ppm due to all CH and a singlet at 2.11 ppm due to CH are also
2
close to the
1
d
H NMR (CDCl
3
, 400 MHz, ppm): 8.84 (d, 2H, H , J ¼ 6.2 Hz), 8.83
2
d
(
s, 6H, H ), 8.60e7.50 (27H, pyrene and phenyl rings) 7.03 (d, 2H,
a
b
H , J ¼ 4.0 Hz), 6.80 (d, 2H, H , J ¼ 4.2 Hz), 6.69 (m, 2H, Hc)
2
3
4
.45e3.64 (32H, O-CH2), 2.14 (s, 6H, CH
MS (MALDI-TOF): m/z calculated C94
Found: 1555.38.
3
H
), ꢀ2.78 (s, 2H, NH).
seen. Finally, a singlet at ꢀ2.74 ppm was assigned to the inner NH
þ
82
N
4
O
10
S
4
[M] : 1555.94;
protons of the porphyrin.
3
Since the obtained oligomers were fairly soluble in CDCl and
13
tend to self-aggregate, the C RMN spectra exhibited very low
intensity signals, even after long accumulation times. However, the
structures of the oligomers were confirmed by MALDI-TOF mass
spectrometry using dithranol as matrix. In each case, the obtained
molecular ion was in agreement with the calculated mass of the
oligomer (Fig. 3).
3
. Results and discussion
We carried out the synthesis and characterization of two novel
thiophene monomers bearing a porphyrin group (M2PPh and
M4PPh) and a new series of oligothiophenes: terthiophenes
(
TT2PY-PPh, TT4 PY-PPh) and quaterthiophenes (QT2 PY-PPh, QT4
PY-PPh) with pyrene-porphyrin as donor-acceptor pair. The syn-
thesis of the monomers and the oligomers is shown in Fig. 2.
Iodinated intermediates 3 and 4 were prepared from 3-bromo-4-
methyl-thiophene as previously reported by us [22]. The
porphyrin based monomers (M2PPh, M4PPh) were obtained by
3.1.1. Optical properties of the oligothiophenes
The optical properties of the synthesized thiophene monomers
and oligothiophenes bearing pyrene and porphyrin chromophores
were studied by absorption and emission spectroscopies in the
UVevis range; the obtained results are shown in Table 1.
The absorption spectra of compounds bearing diethylene glycol
spacers (M2PPh, TT2PY-PPh and QT2PY-PPh), and those contain-
ing tetraethylene glycol spacers (M4PPh, TT4PY-PPh and QT4PY-
PPh) are shown in Fig. 4a and Fig. 4b, respectively. For all the
reacting
these
intermediates
with
5-phenol-10,15,20-
triphenylporphyrin (PPh) through a Williamson etherification in
acetone. A bromination in position 2 of the thiophene ring was then
performed using NBS in CHCl and HClO to acidify the medium in
3 4
order to yield compounds 12 and 13, respectively.
compounds, a Soret band at
l
¼ 420 nm followed by four low-
The analogous monomers containing a pyrene unit as chromo-
phore as well as a brominated thiophene 8 and 9 were synthesized
according to a previously reported procedure [22]. Afterwards,
Suzuki coupling reactions were performed starting with the oxo-
borolane 10 or 11 and two monomers of the same spacer length,
one bearing a porphyrin unit (12 or 13) and the other one bearing a
intensity Q bands at
l
¼ 514, 549, 591 and 647 nm, which are
characteristic for the free base porphyrins were observed. The ab-
sorption spectra of the pyrene monomers mixture with the corre-
sponding porphyrin monomers (M2-M2PPh and M4-M4PPh) and
the absorption spectra of the oligomers are presenting four char-
acteristic absorption bands. The first one appears at 280 nm. It is a
pyrene unit (8 or 9), using Na
2
CO
3
and PdCl
2
(PPh
3
)
2
as catalyst to
0 3
well-defined band that corresponds to the S /S transition of the
yield terthiophenes (TT2PY-PPh, TT4PY-PPh) and quaterthio-
phenes (QT2PY-PPh, QT4PY-PPh), respectively.
pyrene unit and it overlaps with the absorption band of the thio-
phene monomers that is usually observed at 280 nm [37]. The
second band at 350 nm is broad and arises from the S
0
/S
2
tran-
*)
3.1. Characterization of the monomers and the oligothiophenes
sition of pyrene. Moreover, it overlaps with the S /S band (n-
0
1
p
of the oligothiophene backbone, since this band is expected to
appear at 385 nm. The obtained values can be compared to a series
of oligothiophenes containing alkyl and alkoxy groups that have
been previously reported in the literature [31]. The other two bands
are corresponding to the Soret and Q bands of the porphyrin
moiety. As we have previously reported, a large broadening in the
The structure of the monomers was confirmed by NMR spec-
troscopy. The 1H NMR spectrum of monomer M2PPh recorded in
3
CDCl is shown in Fig. S1 (Supporting information). First, a doublet
2
at 8.96 ppm corresponding to the aromatic protons H with
1
J ¼ 6 Hz, followed by a singlet at 8.95 ppm attributed to protons H
of the porphyrin are observed. Then a multiplet at 8.31 ppm
band at
l
¼ 350 nm for M2 and M4 monomers indicates the pres-
6
4
assigned to proton H , a doublet at 8.16 ppm corresponding to H
ence of intramolecular pyrene-tiophene interactions [22]. In the
porphyrin monomers no absorption band is appreciated at this
wavelength and in oligothiophene systems, where the porphyrin
unit is present, a considerable decrease in intensity of the absorp-
3
with J ¼ 9 Hz and a triplet at 7.81 ppm due to H , followed by a
5
doublet at 7.23 ppm assigned to proton H with J ¼ 9 Hz are
a
observed. A triplet at 6.9 ppm attributed to proton H , and a doublet
b
at 6.21 ppm due to proton H with J ¼ 4 Hz is also seen. Further-
tion band at
sorption of the macrocycle is more intense.
l
¼ 350 nm is perceived, due to the fact that the ab-
more, a triplet at 4.28 ppm corresponding to protons CH
2
close to
the porphyrin J ¼ 4 Hz as well as a triplet at 4.17 ppm due to protons
CH
close to the thiophene J ¼ 4.5 Hz are appreciated, followed by a
triplet at 4.01 ppm attributed to protons (PPh-O-CH -CH ),
J ¼ 4.5 Hz, and a triplet at 3.98 ppm assigned to protons (thiophene-
O-CH -CH appears in
The fluorescence spectra of monomers M2PPh, M4PPh and
oligothiophenes TT2PY-PPh, TT4PY-PPh, QT2PY-PPh and QT4PY-
PPh are shown in Fig. 5. The fluorescence spectra of M2PPh and
M4PPh were acquired at room temperature exciting at 415 nm.
Both spectra exhibit two bands at 650 nm and 718 nm, corre-
sponding to the S / S transition from the Q (0, 0) and Q (1, 0),
1 0
respectively. These bands are characteristic of the emission of tet-
raphenylporphyrin [37,41].
2
2
2
2
2
), J ¼ 4.5 Hz. A singlet at 2.22 ppm due to CH
3
the aliphatic region of the spectrum. Finally, a singlet at ꢀ2.62 ppm
was assigned to the inner NH protons of the porphyrin.
1
The H NMR spectrum of monomer M4PPh in CDCl
3
is shown in
Fig. S2 (Supporting information). In the aromatic region, a doublet
The fluorescence spectra of oligothiophenes TT2PY-PPh, TT4PY-
PPh, QT2PY-PPh and QT4PY-PPh (Fig. 5) were recorded at room
temperature exciting at 345 nm. In the spectra, a pyrene emission
2
at 8.88 ppm due to the aromatic protons H with J ¼ 6 Hz, followed
1
by a singlet at 8.86 ppm attributed to protons H , a multiplet at
6
8
.24 ppm assigned to proton H and a doublet at 8.12 ppm corre-
band is observed between
transition of pyrene in the non-associated state “monomer emis-
sion” [9]. In addition, a broad emission band of low intensity is
l
¼ 380e430 nm, due to the S
1 0
/ S
4
sponding to H with J ¼ 9 Hz are seen. Moreover, a triplet at
3
7
.77 ppm due to H , followed by a doublet at 7.27 ppm assigned to
5
proton H with J ¼ 9 Hz, a triplet at 6.82 ppm attributed to proton
present at
l
¼ 470e550 nm and is attributed to the emission of the
a
b
H
and a doublet 6.16 ppm assigned to H with J ¼ 4.4 Hz are
oligothiophene backbone. Similar values of fluorescence wave-
observed. Furthermore, in the aliphatic region of the spectra, a
lengths have been reported for other oligotiophene series

E. Rodríguez-Alba et al. / Journal of Molecular Structure 1183 (2019) 28e36
33
Fig. 3. Mass Spectra of terthiophenes: a)TT2PY-PPh, b)TT4PY-PPh and. quaterthiophenes: c) QT2PY-PPh, d) QT4PY-PPh.
Table 1
Optical properties of the monomers (M2PPh, M4PPh) and of the oligothiophenes (TT2PY-PPh, TT4PY-PPh, QT2PY-PPh, QT4PY-PPh).
ꢀ
Compound
ε (M 1. cmꢀ1) at
l
¼ 420 nm
Absorption
420a
l
(nm)
Cut off (nm)
Emission
l
(nm)
Cut off (nm)
M2PPh
TT2PY-PPh
485,882
546,666
670
670
650,718
380e430
760
760
b
c
350
d
c
650e750
QT2PY-PPh
540,000
350b
670
380e430
760
6
50e750d
b
Mixture M2-M2PPh
M4PPh
TT4PY-PPh
e
350
420
350
670
670
670
e
760
760
760
a
478,750
505,384
650e718
380e430
b
c
d
b
650e750
QT4PY-PPh
510,000
350b
350b
670
670
380e430
760
760
6
50e750d
Mixture M4-M4PPh
e
e
a
Absorption band of the porphyrin.
Absorption band of the pyrene.
Emission band of the pyrene.
Emission band of the porphyrin.
b
c
d
containing alkyl and alkoxy substituents [31]. Finally, a band at
pyrene interactions were present. In contrast, the spectra of the
oligothiophenes containing donor-acceptor pair (pyrene-
porphyrin) are showing a drastic fluorescence quenching. Only a
monomer emission band at
¼ 380e430 nm is observed, followed
by an emission band at
¼ 470e550 nm, coming from the oligo-
thiophene segment. Moreover, an additional emission band at
l
¼ 650 nm, due to the emission of the porphyrin unit is observed.
a
This last emission band reveals an energy transfer process from the
excited pyrene to the ground state porphyrin, since by exciting at
l
l
exc ¼ 345 nm the absorption occurs mainly through the pyrene
group.
The emission spectra of the oligothiophenes containing two
l
l
¼ 650 nm arising from the porphyrin unit after the FRET phe-
pyrene units [22] can be compared with the emission spectra of the
oligothiophenes bearing the donor-acceptor pair (pyrene-
porphyrin). In the spectra of the oligomers with two pyrene units, a
nomenon is observed.
The determination of the relative quantum yields was per-
formed using as standards tetraphenylporphyrin TPP (
toluene for monomers (M2PPh, M4PPh) and pyrene (F
cyclohexane [16] for oligothiophenes (TT2PY-PPh, QT2PY-PPh,
F
¼ 0.11) in
“
monomer” emission band at
l
¼ 379e450 nm, followed by a very
¼ 0.32) in
intense excimer emission at 570 nm, due to intramolecular pyrene-

34
E. Rodríguez-Alba et al. / Journal of Molecular Structure 1183 (2019) 28e36
Fig. 5. Emission spectra of the monomers and oligothiophenes: a) M2PPh, TT2PY-PPh,
3
QT2PY-PPh and b) M4PPh, TT4PY-PPh, QT4PY-PPh in CHCl solution.
Fig. 4. Absorption spectra of: a) M2PPh, TT2PY-PPh, QT2PY-PPh and b) M4PPh,
(spectrometric grade, concentration 1 ꢁ 10ꢀ M).
6
TT4PY-PPh, QT4PY-PPh in CHCl
3
Table 2
Quantum yields and energy transfer efficiency (EFRET%).
TT4PY-PPh, QT4PY-PPh) following reported procedures [42,43].
The results obtained for the oligothiophenes with the pyrene-
porphyrin pair were compared to those previously reported for
the oligothiophenes bearing two pyrene units [22] (Table 2).
The monomers (M2PPh, M4PPh) and the porphyrinic com-
pound used as reference TTP exhibit quantum yields with values of
Compound
Relative Quantum Yield
Py) (
exc ¼ 345 nm)
E
FRET % Energy
(
F
l
Transfer
Pyrene
TPP
M2PPh
M4PPh
TT2
QT2
TT4
QT4
TT2PY-PPh
QT2PY-PPh
TT4PY-PPh
QT4PY-PPh
0.32
0.11
0.18
0.21
0.68
0.74
0.69
0.68
0.035
0.022
0.069
0.046
e
e
e
e
F
¼ 0.18, 0.21 and 0.11 respectively, showing that the luminescence
of porphyrin is lower than that of pyrene (
¼ 0.32). On the other
hand, oligothiophenes bearing only pyrene groups (TT2, QT2, TT4,
QT4) exhibit higher quantum yields than pyrene (
¼ 0.32), with
e
e
F
e
e
F
96.5%
97.8%
93.1%
95.4%
values between 0.68 and 0.74, due to the presence of two pyrene
units, which favors the formation of excimers thereby increasing
considerably the luminescence. In contrast, oligothiophenes
bearing the pyrene-porphyrin pair have lower quantum yields than
those of the oligomers bearing only pyrene groups since an energy
transfer phenomenon takes place from pyrene to porphyrin, giving
quantum yield values between 0.022 and 0.069. These values reveal
an almost total fluorescence quenching in these oligothiophenes.
Based on this fluorescence quenching the energy transfer effi-
ciencies (EFRET) were calculated, giving EFRET values of 96.5, 97.8,
QT4PY-PPh, respectively.
. Conclusions
4
A new series of oligothiophenes bearing pyrene-porphyrin units
attached via well defined oligoethylene glycol spacers were
9
3.1 and 95.4% for TT2PY-PPh, QT2PY-PPh, TT4PY-PPh and

E. Rodríguez-Alba et al. / Journal of Molecular Structure 1183 (2019) 28e36
Nanoscale 5 (2013) 9073e9080.
35
synthesized and characterized. The optical and photophysical
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15] Marc R. Elie, Christian A. Clausen, Cherie L. Geiger, Reduction of benzo[a]
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these compounds were studied. A drastic fluorescence quenching
was observed since only a “monomer” emission band was
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lutions and in other organized media, Chem. Rev. 93 (1993) 587e614.
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¼ 470e550 nm due to oligothiophene backbone. Finally, an
emission band at
l
¼ 380e430 nm, followed by an emission band at
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17] M. Neam t¸ u, C. N ꢁa dejde, V.D. Hodoroaba, R.J. Schneider, U. Panne, Singlet ox-
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after FRET was also observed. This phenomenon was confirmed by
the low quantum yield values obtained for these compounds.
The energy transfer efficiency was calculated for these oligo-
thiophenes, giving values close to 100%. The obtained values for
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E
FRET were 96.5, 97.8, 93.1 and 95.4% for TT2PY-PPh, QT2PY-PPh,
TT4PY-PPh and QT4PY-PPh, respectively. The oligomer that
exhibited the highest energy transfer efficiency was the one that
have the shortest flexible spacer and longest oligothiophene
backbone (QT2PY-PPh).
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21] B. Pan, Y. Zhu, D. Ye, J. Zheng, Improved conversion efficiency in dye-
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Acknowledgements
b E. Rivera, M. Bellet e^ te, X.X. Zhu, G. Durocher, R. Giasson, Novel poly-
acetylenes containing pendant 1-pyrenyl groups: synthesis, characterization,
and thermal and optical properties, Polymer 43 (2002) 5059e5068.
ꢀ
We thank Miguel A. Canseco Martínez, María de los Angeles
Pe n~ a Gonz aꢀ lez, Elizabeth Huerta Salazar, Gerardo Cedillo and
Er eꢀ ndira García Ríos for their technical assistance in the charac-
terization of the monomers and oligomers (FTIR, NMR and MAL-
DITOF). We are also grateful to PAPIIT-DGAPA (Project IN-100316)
and CONACyT (Projects 253155 and 279380) for financial support.
[
23] E.G. Morales-Espinoza, E. Aguilar-Ortíz, A. V aꢀ zquez-Arce, H. V aꢀ zquez-Torres,
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at
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