Design of novel luminescent porphyrins bearing donor-acceptor groups

Article abstract of DOI:10.1142/S1088424613501083

In this work, we report the synthesis and characterization of a novel series of porphyrins, some of them bearing donor-acceptor groups. meso-substituted free-base porphyrins:5-(4-amino-phenyl)-10,15,20- triphenylporphyrin (TPPNH₂) and 5-(4-acet

Full text of DOI:10.1142/S1088424613501083

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2014; 18: 209–220
DOI: 10.1142/S1088424613501083
Published at http://www.worldscinet.com/jpp/
Design of novel luminescent porphyrins bearing
donor–acceptor groups
a
a
b
Carolina Caicedo , Gerardo Zaragoza-Galán , Joaquim Crusats ,
b
a
a◊
Zoubir El-Hachemi , Ana Martínez and Ernesto Rivera*
a
Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México. Ciudad Universitaria,
C.P. 04510 D.F. México, Mexico
Department of Organic Chemistry and Institute of Cosmos Science, University of Barcelona, c. Martí i Franquès 1,
b
0
8028-Barcelona, Catalonia, Spain
Received 23 August 2013
Accepted 24 September 2013
ABSTRACT: In this work, we report the synthesis and characterization of a novel series of porphyrins,
some of them bearing donor-acceptor groups. meso-substituted free-base porphyrins:5-(4-amino-
phenyl)-10,15,20-triphenylporphyrin (TPPNH )and 5-(4-acetamidophenyl)-10,15,20-triphenylporphyrin
2
(
TPPNHAc), as well as their tribromo (Br TPPNH and Br TPPNHAc) and trimethylsilyl-substituted
3 2 3
homologs (TMS TPPNH and TMS TPPNHAc) were synthesized and characterized. The optical
3
2
3
properties of all compounds have been studied by absorption and fluorescence spectroscopy. Theoretical
calculations were performed in order to recognize the optimized geometry and the correlation with the
optical properties. On the other hand, an azobenzene containing porphyrin (TPPN PhC H ) was also
2
14 29
prepared to study the influence of the trans-cis photoisomerization on its optical properties. Almost all
porphyrin derivatives exhibited a Soret band at ca. l = 419–422 nm followed by four Q-bands in the range
between 500–700 nm. Besides, these porphyrins showed two emission bands located at ca. l = 654 nm
and 714 nm with different intensities, depending on the substituents. The relative quantum yields were
calculated for these compounds with respect to the most emissive species.
KEYWORDS: porphyrin, optical properties, fluorescence, electronic effects.
recycling and adapting of this important set of complexant
INTRODUCTION
agents to different processes [11, 12]. On the other hand,
porphyrins have been employed in the synthesis of
push–pull p-conjugated systems bearing donor–acceptor
groups and in the design of dendritic molecules able to
act as molecular antennae for photovoltaic applications
Porphyrins belong to a very important family
of fluorophores that have been widely studied in
macromolecular and materials science [1–4]. These
chromophores are highly delocalized p-systems which
are considered a unique category of ionic scavengers,
whose recognition properties come from the heteroatoms
present in their structure [5]. Due to their efficient light
absorption, porphyrins have been the subject of intense
research for the development of solar energy transfer
and electron transfer systems [6–10]. The incorporation
of porphyrins into polymers allows the easy handling,
[6–8]. The design of new porphyrin and multiporphyrinic
arrays is very attractive due to its application in catalysis
[
[
1], nonlinear optics (NLO) [2], two photon absorption
3], and molecular wires [4]. Several electro- and
photoactive units have been incorporated into porphyrins
in order to tune their electronic and photophysical
properties. The donor–acceptor character of porphyrins
can also be modified depending on their coordination
state and the photoactive units linked to them [13–18].
Thus, the preparation and properties of various porphyrin
derivatives linked to electro- and photoactive units
◊SPP full member in good standing
*
Correspondence to: Ernesto Rivera, email: riverage@unam.
mx, tel: +5255 5622-4733, fax: +5255 5616-1201
Copyright © 2014 World Scientific Publishing Company

2
10
C. CAICEDO ET AL.
(
fullerene C , [13] anthracene, [14, 15] pyrene, [16–18]
of molecular antennae and push–pull p-conjugated
systems, we synthesized and characterized soluble
precursor porphyrin derivatives able to act as donor or
acceptor groups. We performed theoretical calculations
in order to determine their optimal geometry and estimate
their photophysical properties [20]. Moreover, we studied
the optical properties of these compounds by absorption
and fluorescence spectroscopy and their spectra were
compared to those predicted by theoretical calculations.
The structures of the prepared porphyrin derivatives are
illustrated in Fig. 1.
6
0
functionalized porphyrins [19], etc) have been published
in the literature.
Free-base porphyrins usually exhibit an intense Soret
band at ca. l = 420 nm in their absorption spectra,
followed by four Q-bands which appear between l =
5
00 and 700 nm and are visible for all compounds.
However, when porphyrins are complexed with metallic
cations such as Zn(II), they became symmetrical so
that they show the typical Soret band followed only by
two Q-bands. Since porphyrins can exist in free-base or
metalated form, they can act either as donor or acceptor
groups, respectively. As our research focus on the design
On the other hand, azobenzene containing polymers
and azobenzene dyes have been considered as versatile
Fig. 1. Structures of the obtained porphyrins
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 210–220

DESIGN OF NOVEL LUMINESCENT PORPHYRINS BEARING DONOR–ACCEPTOR GROUPS
211
Fig. 1. (Continued)
materials due to the photoinduced motions which occur on
them when they are irradiated with linear polarized light
21]. Particularly, p-conjugated porphyrin-azobenzene
systems showed to be efficient light responsive materials
with potential applications in optical storage [22–27].
Although our research group has reported the synthesis
of characterization of a large series of azobenzene based
materials (azo-dyes [28–31], azo-polymers [32–36] and
azo-dendrons [37, 38]) porphyrin-azobenzene molecules
haveattractedourinterestsincetheiropticalpropertiescan
be modified either electrochemically or photochemically
[39–41]. In addition, these compounds offer great
advantages, since they easily absorb light and exhibit
different oxidation states. The extension of the conjugated
porphyrins can be augmented by coupling them with
other aromatic units via molecular bridges consisting of
alkenes, alkynes, imines and other unsaturated functional
groups [42–47]. In previous experimental and theoretical
works, Anderson and co-workers demonstrated that the
azo group (N=N) showed to be the most efficient option
[
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 211–220

2
12
C. CAICEDO ET AL.
for this purpose. Porphyrin-azobenzene systems can be
prepared by coupling a porphyrinic diazonium salt in
the presence of an aryl compound bearing an electron–
donor group or by partial reduction of nitro, nitroso and
azoxyarene compounds. Another possible method is the
oxidation of two different arylamino compounds, one
of them bearing a porphyrin unit in the presence of an
11.6 mmol), and pyrrole (3.09 g, 46.2 mmol) in CH Cl
(1.2 L) was purged with nitrogen for 15 min, then 0.45 mL
2
2
of 2.5 M BF .OEt were added. The mixture was stirred
3
2
for 1.5 h at room temperature and p-chloranil (2.3 g,
9.35 mmol) was added. The mixture was stirred for 7 more
hours; afterwards the solvent was removed at reduced
pressure. The desired product was isolated and purified by
oxidizing agent such as MnO . The azoporphyrins show
column chromatography on Florisil CHCl –MeOH and
2
3
a strong interaction between the phenyl rings and the azo
bridge [47]. These molecules are promising candidates
for photonic applications since they also exhibit NLO of
third harmonic generation (THG).
subsequently by flash column chromatography on silica gel
using a mixture CHCl :MeOH (97:3) as eluent. TPPNHAc
3
(3a) was obtained as dark purple crystals (1.1 g, 1.64 mmol).
1
Yield 14.2%. H NMR (400 MHz, CDCl ): d , ppm -2.78
3
H
a
In this context, the interest for photochromic molecules
is mainly due to the electronic and geometrical changes
which take place on them. Many applications are possible
such as reversible photomodulation (photoswitching) and
catalysis. The catalytic activity of these compounds has
not been widely explored, however, there a few reports in
the recent literature [42–47]. For this reason, we decided
to synthesize an azo-porphyrin (TPPN PhC H ) in
(s, 2H, NH), 2.36 (s, 3H, COCH ), 7.48 (s, 1H, H ), 7.79–
3
7
6
7.72 (m, 9H, H ), 7.89 (d, J = 8.4 Hz, 2H, H ), 8.17 (d, J =
5
4
8.4 Hz, 2H, H ), 8.21 (dd, J = 7.4 Hz, J = 1.4 Hz, 6H, H ),
1
2
1,2,3
13
8.86–8.83 (m, 8H, H ) (see Fig. 1). C NMR (100 MHz,
7
11
CDCl ): d , ppm 145.9 (1C, C ), 142.2 (4C, C ), 141.8 (4C,
C ), 139.9 (1C, C ), 135.4 (6C, C ), 134.2 (9C, C ), 132.1
3
C
12
13
5
6
2
4
10
1
(3C, C ), 127.0 (2C, C ), 126.3 (8C, C ), 120.2 (1C, C ),
8
9
3
119.6 (1C, C ), 119.1 (3C, C ), 113.2 (2C, C ), 25.0 (1C,
2
14 29
order to study the influence of the porphyrin ring and
trans-cis photoisomerization on the optical properties;
the synthesis of the porphyrin derivatives mentioned
above is illustrated in Fig. 2 (vide infra).
NHCOCH ) (see Fig. 1). MALDI-TOF: m/z 672.2 (calcd.
3
+
for [M + H] 672.2, C H ON ). UV-vis (CHCl , 2.41 ×
4
6
33
-1
5
3
-6
-1
10 M): l , nm (e, M .cm ) 419 (433,000), 516 (17,300),
551 (8,750), 590 (6,300), 647 (4,200).
max
Synthesis of 5-(4-aminophenyl)10,15,20-trisphenyl-
porphyrin TPPNH (4a). TPPNHAc 3a (0.44 g, 0.65
2
EXPERIMENTAL
mmol) was dissolved in a mixture of EtOH (195 mL) and
concentrated HCl solution (130 mL) and heated to reflux
for 8 h. Then the reaction mixture was poured into water,
General procedures
and the organic layer was extracted with CHCl , washed
3
Unless specified, all reagents and solvents were
purchased from Aldrich and used as received. Dry
dichloromethane and toluene were freshly distilled over
calcium chloride. Moisture sensitive liquids and solutions
were transferred by syringes and introduced into reaction
vessels through rubber septa. All the reactions were
stirred with magnetic bars. The progress of the reactions
was monitored by thin-layer chromatography (TLC)
with an aqueous solution of NaHCO and dried with
3
anhydrous Na SO . Furthermore, the crude product was
2
4
isolated and purified by flash column chromatography on
silica gel using a mixture CHCl :MeOH (98:2) as eluent.
3
TPPPNH (4a) was obtained as black purple crystals
2
1
(
0.41 g, 0.65 mmol). Yield 100%. H NMR (400 MHz,
CDCl ): d , ppm -2.76 (s, 2H, NH), 4.04 (s, 2H, NH ),
3
H
2
6
7
7
(
6
8
.07 (d, J = 8.4 Hz, 2H, H ), 7.80–7.72 (m, 9H, H ), 8.00
whenever possible. MnO was prepared according to the
5
2
d, J = 8.4 Hz, 2H, H ), 8.21 (dd, J = 6.2 Hz, J = 1.8 Hz,
1
2
method previously reported in the literature [49].
4
1
2
H, H ), 8.82 (s, 4H, H ), 8.83 (d, J = 4.2 Hz, 2H, H ),
1
H NMR spectra of the intermediates and final
3
13
.94 (d, J = 4.8 Hz, 2H, H ) (see Fig. 1). C NMR (100
MHz, CDCl ): d , ppm 146.2 (1C, C ), 142.5 (4C, C ),
42.4 (4C, C ), 135.9 (6C, C ), 134.7 (9C, C ), 132.6
3C, C ), 127.8 (2C, C ), 126.8 (8C, C ), 121.1 (1C, C ),
20.1 (1C, C ), 119.9 (3C, C ), 113.6 (2C, C ) (see Fig.
compounds in CDCl solution were recorded at room
7
11
3
3
C
temperature on a Bruker Avance 400 MHz spectrometer,
12
5
6
1
(
1
1
1
13
operating at 400 MHz and 100 MHz for H and C,
respectively.Themeltingpointsoftheobtainedporphyrins
could not be determined in a Fisher Instrument. DSC and
TGA experiments were performed and it was found that
these compounds start to decompose before melting,
beyond 400 °C. TGA and DSC (from 0 °C to 650 °C)
were carried out in a SDT Q600 de TA Instruments using
a heating rate of 10 °C/min.
2
4
10
1
8
9
3
+
). MALDI-TOF: m/z 630.2 (calcd. for [M + H] 630.3,
-6
C H N ). UV-vis (CHCl , 1.55 × 10 M): lmax, nm
4
4
31
-1
5
3
-1
(
5
e, M .cm ) 421 (449,646), 517 (20,154), 554 (11,075),
92 (6,503), 647 (5,795).
Synthesis of 5-(4-acetamidophenyl)10,15,20-tris(4-
trimethylsilyl)phenylporphyrin TMS TPPNHAc (3b).
3
Porphyrin TMS TPPNHAc was synthesized according
3
to the method previously described for the synthesis
of TPPNHAc. Yield 15%. Aldehyde 2b was prepared
according to the procedure described in the literature
Synthesis
Synthesis of 5-(4-acetamidophenyl)10,15,20-trisphe-
nylporphyrinTPPNHAc (3a). A solution of benzaldehyde
1
[49]. H NMR (400 MHz, CDCl ): d , ppm -2.77 (s, 2H,
3
H
(3.59 g, 33.9 mmol), 4-acetamidobenzaldehyde (1.89 g,
NH), 0.51 (s, 27H, Si(CH ) ), 2.35 (s, 3H, CH ), 7.49
3 3 3
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 212–220

DESIGN OF NOVEL LUMINESCENT PORPHYRINS BEARING DONOR–ACCEPTOR GROUPS
213
a
2
5
7
12
2
(
8
s, 1H, H ), 7.87 (d, 6H, H ), 7.89 (d, 2H, H ), 8.17 (d, J =
134.2 (8C, C ), 124.8 (3C, C ), 122.4 (2C, C ), 123.1
3
3
5
.4 Hz, 2H, H ), 8.20 (dd, J = 8.0 Hz, J = 2.4 Hz, 6H,
H ), 8.87–8.83 (m, 8H, H ) (see Fig. 1). C NMR (100
MHz, CDCl ): d , ppm 142.7 (4C, C ), 142.6 (4C, C ),
39.7 (1C, C ), 135.3 (4C, C ), 134.3 (8C, C ), 134.2 (8C,
C ), 131.8 (8C, C ), 120.4 (1C, C ), 120.1 (4C, C ), 118.1
4C, C ), 25.0 (1C, NHCOCH ), -0.59 (9C, (Si(CH ) ) )
(2C, C ), 115.9 (4C, C ) (see Fig. 1). MALDI-TOF:
m/z 863.61 (calcd. for [M + H] 863.99, C H N Br ).
UV-vis (CHCl , 2.67 × 10 M): l , nm (e, M .cm )
422 (478,864), 516 (30,174), 551 (19,475), 590 (14,093),
647 (12,658).
Synthesis of 5-(4-(p-tetradecyl)azobenzene)10,15,-
20-trisphenylporphyrin TPPN PhC H (5a). The
1
2
4
1
13
+
4
4
29
5
3
-1
6
8
-6
-1
3
9
C
3
max
4
2
1
3
7
1
1
5
(
(
3
3 3 3
see Fig. 1). MALDI-TOF: m/z 888.3 (calcd. for [M +
2
14 29
+
-6
H] 888.4, C H N OSi ). UV-vis (CHCl , 2.279 × 10
aminoporphyrin 4a (TPPNH ) (0.2 g, 0.32 mmol) and
4
4
31
5
3
-1
3
2
-
1
M): lmax, nm (e, M .cm ) 421 (35,4761), 517 (15,182),
4-tetradecylaniline (0.28 g, 0.96 mmol) were dissolved
in dry toluene (160 mL) and heated to reflux in a round-
bottom flask equipped with a Dean-Stark setup. Then,
5
54 (8,820), 592 (7,415), 648 (4,081).
Synthesis of 5-(4-acetamidophenyl)10,15,20-tris(4-
trimethlsilyl) phenylporphyirin TMS TPPNH (4b).
freshly prepared MnO (1.4 g, 16 mmol) was added to
3
2
2
Porphyrin TMS TPPNH was prepared using a modified
the solution until all the aminoporphyrin was reacted.
The purple solution was filtered through a pad of
celite to remove the solid, and the residue was washed
with dichloromethane. The solution was concentrated
at reduced pressure and purified by flash column
3
2
method employed for the preparation of TPPNH . Yield
2
1
9
8%. H NMR (400 MHz, CDCl ): d , ppm -2.71 (s, 2H,
3
H
NH), 0.53 (s, 27H, Si(CH ) ), 3.98 (s, 2H, NH ), 7.05 (d,
3
3
2
5
2
J = 8.4 Hz, 2H, H ), 7.90 (d, J = 8.0 Hz, 6H, H ), 8.00
3
(
6
8
dd, J = 6.8 Hz, J = 1.8 Hz, 2H, H ), 8.22 (d, J = 8.0 Hz,
chromatography on silica gel using CHCl as eluent.
1
4
2
3
1
7
H, H ), 8.84 (s, 4H, H ), 8.85 (d, J = 4.0 Hz, 2H, H ),
TPPN PhC H was obtained as black purple crystals.
2
14 29
1
6
13
.92 (d, J = 4.8 Hz, 2H, H ) (see Fig. 1). C NMR (100
Yield 56%. H NMR (400 MHz, CDCl ): d , ppm -2.75
3
H
1
6
MHz, CDCl ): d , ppm 147.3 (1C, C ), 144.1 (4C, C ),
(s, 2H, NH), 0.88 (t, J = 4.8 Hz, J = 4.8 Hz, 3H, -CH ),
3
C
1
2
3
8
11
9
1
(
44.0 (1C, C ), 140.8 (3C, C ), 137.1 (6C, C ), 135.5
1.25 (m, 22H, –(CH ) ), 1.72 (q, J = 6.8 Hz, 2H, –CH -
2
11
2
1
0
4
7
6C, C ), 133.8 (1C, C ), 133.0 (8C, C ), 122.1 (3C,
(CH ) ), 2.75 (t, J = 8.0 Hz, 2H, –CH Ph), 7.42 (d, J =
10 7
2
11
2
1
2
2
3
5
C ), 121.4 (2C, C ), 121.2 (2C, C ), 114.8 (4C, C ), 0.62
8.0 Hz, 2H, H ), 7.79–7.73 (m, 10H, H ), 8.02 (d, J =
9
(
(
2
9C, [Si(CH ) ) ] (see Fig. 1). MALDI-TOF: m/z 847.4
8.0 Hz, 2H, H ), 8.22 (dd, J = 4.4 Hz, J = 1.6 Hz, 6H,
3
3 3
1
2
+
4
8
calcd. for [M + H] 846.4, C H N Si ). UV-vis (CHCl ,
H ), 8.29 (d, J = 8.4 Hz, 2H, H ), 8.37 (d, J = 8.4 Hz, 2H,
6 1 7
5
3
55
5
3
3
-
6
-1
-1
.02 × 10 M): l , nm (e, M .cm ) 422 (385,241), 516
H ), 8.84 (s, 4H, H ), 8.85 (d, J = 4.0 Hz, 9H, H ), 8.87
2 3
max
(
15,052), 551 (9,306), 590 (4,309), 647 (4,149).
(d, J = 4.8 Hz, 2H, H ), 8.91 (d, J = 4.4 Hz, 2H, H ) (see
13
Synthesis of 5-(4-acetamidophenyl)10,15,20-tris-
Fig. 1). C NMR (100 MHz, CDCl ): d , ppm 147.5
g k l j
3
C
(
4-tribromo)phenylporphyrin Br TPPNHAc (3c).
(8C, C ), 142.7 (1C, C ), 137.5 (1C, C ), 135.9 (8C, C ),
3
b
d
e
Br TPPNHAc was synthesized using the same method
134.0 (3C, C ), 130.7 (2C, C ), 129.2 (6C, C ), 128.1
3
f
c
a
m
employed for the preparation of TPPNHAc. Yield 14%.
(9C, C ), 124.5 (2C, C ), 123.1 (1C, C ), 122.4 (2C, C ),
1
h
n
o
H NMR (400 MHz, CDCl ): d , ppm -2.98 (s, 2H, NH),
115.5 (1C, C ), 114.4 (2C, C ), 103.7 (1C, C ), 102.4
3
H
a
i
p
q
r
2
2
2
8
1
1
.24 (s, 3H, CH ), 7.42 (s, 1H, H ), 7.77 (d, J = 2.4 Hz,
(3C, C ), 58.2 (1C, C ), 50.25 (1C, C ), 31.1 (10C, C ),
28.9 (1C, C ), 24.1 (1C, C ) (see Fig. 1). MALDI-TOF:
m/z 915.5 (calcd. for [M + H] 915.5, C H N ). UV-vis
(CHCl , 2.29 × 10 M): lmax, nm (e, M .cm ) 364
3
7
4
s
t
H, H ), 7.75 (d, J = 1.6 Hz, 4H, H ), 7.78 (d, J = 1.6 Hz,
3
2
5
+
H, H ), 7.94 (m, 6H, H ), 8.03 (d, J = 8.4 Hz, 2H, H ),
6
4
62
-1
6
6
1
-8
-1
.70 (m, 6H, H ), 8.76 (d, J = 4.4 Hz, 2H, H ) (see Fig.
3
1
3
6
). C NMR (100 MHz, CDCl ): d , ppm 145.1 (4C, C ),
(115,305), 420 (624,237), 516 (52,842), 554 (43,396),
591 (26,416), 647 (11,543).
3
C
8
9
4
44.0 (4C, C ), 141.1 (1C, C ), 138.3 (4C, C ), 136.7 (8C,
2
3
7
C ), 135.8 (8C, C ), 133.1 (8C, C ), 121.9 (1C, C-NHAc),
1
Absorption and fluorescence spectra of the porphyrinic
molecules were recorded in CH Cl (spectrophotometric
1
5
21.1 (4C, C ), 119.8 (4C, C ), 27.1 (1C, NHCOCH )
3
2
2
(
see Fig. 1). MALDI-TOF: m/z 965.41 (calcd. for [M +
grade), which was purchased from Aldrich. Prior to use,
this solvent was checked for spurious emission in the
region of interest and was found to be satisfactory. The
absorption spectra were recorded on a Varian Cary 1 Bio
UV/vis spectrophotometer (model 8452A) in solution for
the all compounds at the above indicated concentration,
using a 1 cm quartz cell. It has been verified that the
Beer–Lambert law applies for such concentration.
+
-8
H] 966.00, C H N OBr ). UV-vis (CHCl , 2.18 × 10
4
6
30
5
3
3
-
1
-1
M): lmax, nm (e, M .cm ) 422 (469,535), 518 (31,627),
5
52 (17,209), 588 (12,558), 644 (9,767).
Synthesis of 5-(4-acetamidophenyl)10,15,20-tris(4-
tribromo) phenylporphyrin Br TPPNH (4c). Br TP-
PNH was synthesized using the same method employed
for the preparation of TPPNH . Yield 95%. H NMR
3
2
3
2
1
2
(
2
400 MHz, CDCl ): d , ppm -2.95 (s, 2H, NH), 3.89 (s,
Fluorescence spectra corrected for emission detection
were carried out on a Fluorolog-3 spectrophotometer
with an FL3-11 special configuration, which is designed
to record simultaneously two emission scans (T-shape).
For all porphyrinic compounds emission spectra were
3
H
5
H, NH ), 6.90 (d, J = 6.0 Hz, 2H, H ), 7.73 (d, J = 8.0
4 3
2
Hz, 6H, H ), 7.82 (d, J = 8.0 Hz, 2H, H ), 7.91 (d, J = 6.0
2
1
Hz, 6H, H ), 8.67 (s, 6H, H ), 8.81 (d, J = 2.4 Hz, 2H,
6
13
H ) (see Fig. 1). C NMR (100 MHz, CDCl ): d , ppm
3
C
1
6
8
-7
1
(
50.1 (1C, C ), 146.2 (4C, C ), 145.0 (1C, C ), 142.7
3C, C ), 140.7 (6C, C ), 136.1 (6C, C ), 135.0 (1C, C ),
recorded in CH Cl solution (concentration 10 M)
2
2
1
1
9
10
4
exciting at l = 422 nm.
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 213–220

2
14
C. CAICEDO ET AL.
6
Density functional approximation as implemented in
Gaussian 09 [50] was used for all theoretical calculations.
Full geometry optimization without symmetry constrains
and frequency analyses were carried out for all the
stationary points using the three parameters hybrid
B3LYP functional [51–53]. In all cases, calculations were
done in gas phase with 6–31 g (d,p) basis sets [54, 55].
region, this porphyrin exhibits two signals at 8.81 (H )
1
and 8.67 (H ) ppm due to protons present in the pyrrolic
rings of the porphyrin, followed by four signals at 7.91
(H ), 7.82 (H ), 7.73 (H ) and 6.90 (H ) ppm arising from
the protons of the phenyl groups linked in the meso-
positions. In the up-field region, two more signals are
observed at 3.89 and -2.95 ppm, due to protons of the
2
3
4
5
terminal NH group and the inner protons NH of the
2
porphyrin ring, respectively.
RESULTS AND DISCUSSION
1
H NMR spectrum of TPPN PhC H (5a) is also
2
14 29
shown in the Supporting information. As we can see,
in the aromatic region this azo-compound exhibits
three signals at 8.91 (H ), 8.87 (H ) and 8.85 (H ) ppm
Synthesis of the porphyrin derivatives
3
2
1
The synthesis of the different substituted tetraphenyl-
corresponding to the b-pyrrolic protons, followed by a
porphyrins (X TPP) and the reference compound
7
1
6
3
series of peaks at 8.85 (H ) 8.84 (H ), 8.37 (H ), 8.29
(
tetraphenylporphyrin (TPP), reported in this work,
are shown in Fig. 2. These compounds were prepared
using the Lindsey’s method by reacting 3 eq of
benzaldehyde (2a) or the appropriate p-substituted
benzaldehyde (2b or 2c) with 4 eq of pyrrole and 1
eq of p-acetamidobenzaldehyde in the presence of
BF OEt , using dichloromethane as solvent. The solution
8
4
9
7
10
H ), 8.22 (H ), 8.02 (H ), 7.79–7.73 (H ), 7.42 (H )
ppm due to all protons present in the phenyl rings of the
molecule. In the aliphatic zone, we can observe a series
of signals at 2.75, 1.72, 1.25 and 0.88 ppm related to all
methylenes (CH ) and the methyl group (CH ) present
2
3
in the tetradecyl chain. Finally, an up-field shifted signal
appears at -2.75 ppm, which is attributed to the inner
protons of the amino groups (NH) of the porphyrin ring.
3
2
was stirred for 12 h at room temperature under argon
atmosphere. Later, the resulting precursor product was
oxidized with p-chloranil to give a mixture of products,
where the desired product was isolated and purified by
column chromatography (3a, 3b or 3c). Furthermore,
the obtained porphyrins bearing an acetamido group
were deprotected by heating to reflux with a mixture
HCl:ethanol 1:1.5 to give the desired TPP’s bearing a
terminal amino group (4a, 4b or 4c).
Absorption spectra of the porphyrin derivatives
The optical properties of the different porphyrins
(
TPPNHAc,TPPNH ,TMS TPPNHAc,TMS TPPNH ,
2 3 3 2
Br TPPNHAc, Br TPPNH and TPPN PhC H ) in
3
3
2
2
14 29
CH Cl solution were studied by absorption and fluores-
2
2
cence spectroscopy in the UV-vis range; the results are
In this manner, six different free-base tetraphenyl-
porphyrins were prepared: TPPNHAc (3a), TPPNH2
summarized in Table 1. Except for TPPN PhC H , the
2
14 29
absorption spectra of all the other obtained porphyrins are
practically identical with very slight shifts. The UV-vis
(
4a), TMS TPPNHAc (3b), TMS TPPNH (4b), Br -
3 3 2 3
TPPNHAc (3c) and Br TPPNH (4c). It is worth to
3
2
spectrum of Br TPPNHAc is shown in the Supporting
3
point out that we attempted to synthesize the trinitro-
information.
substituted tetraphenylporphyrin (NO ) TPPNHAc since
2
3
The UV-vis spectra of Br TPPNHAc showed the
3
NO is the best acceptor group. Unfortunately, this
2
typical characteristics of porphyrin derivatives, exhibiting
a Soret band at ca. l = 419–422 nm followed by four
Q-bands in the range between 500–700 nm. However,
we can see that in the absorption spectra of porphyrins
porphyrin resulted to be totally insoluble in common
organic solvents so that it was useless for this study.
On the other hand, TPPNH (4a) was reacted with
2
p-tetradecylaniline in the presence of MnO in toluene
to give the expected azo-compound bearing a porphyrin
2
bearing acceptor groups, such as Br TPPNHAc or
3
TMS TPPNHAc, the Soret band is slightly red-shifted
3
unit TPPN PhC H (5a).
2
14 29
by of 3–4 nm with respect to their non-substituted
homologue TPPNHAc. Similarly, discrete shifts in the
Q-bands are present. From these results, it is possible
to conclude that the presence of electron-withdrawing
groups and the presence of a donor group (amino
or amide) augment significantly the charge transfer
character of these compounds, increasing their dipole
moment values.
Characterization of the porphyrin derivatives
1
Theobtainedcompoundswerecharacterizedby HNMR
spectroscopy and the molecular weights were determined
by MALDITOF, thereby confirming the structure of the
obtained compounds. The spectroscopical data for all
porphyrin derivatives are given in the experimental section.
However, in order to clarify the main spectroscopical
features we explain in detail the characterization of
Br TPPNH (4c) and TPPN PhC H (5a).
In contrast, the absorption spectrum of the azo-dye
bearing a porphyrin unit (TPPN PhC H ) (Fig. 3)
2
14 29
exhibited an additional band at l = 364 nm, that was not
observed for the other porphyrin derivatives and comes
from the p–p* transition of the azobenzene moiety. For
non-substituted azobenzene this band usually appears at
3
2
2
14 29
1
The H NMR spectrum of Br TPPNH (4c) is
3
2
included in the Supporting information. In the aromatic
Copyright © 2014 World Scientific Publishing Company
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DESIGN OF NOVEL LUMINESCENT PORPHYRINS BEARING DONOR–ACCEPTOR GROUPS
215
Fig. 2. Synthesis of the porphyrin dyads
l = 350 nm and is more intense for the trans isomer [56].
band, and we perceived the appearance of an additional
absorption band at l = 448 nm, due to the n–p* transition,
which is more intense for the cis-isomer. This behavior
showed to be reversible when the sample was exposed
to natural light, which confirms that effectively these
When this azo-compound was irradiated with UV light,
2
with an intensity of 1.4 mW/cm at 254 nm for 3 min, the
trans-cis photoisomerization occurred so that the band at
3
64 nm (p–p*) decreased in intensity as well as the Soret
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 215–220

2
16
C. CAICEDO ET AL.
Table 1. Optical properties of the obtained porphyrins
a
Compound
Absorption, nm
Emission, nm
Relative quantum yield, F
TPPNHAc
419, 516, 551, 590, 647
421, 517, 554, 592, 648
422, 518, 552, 588, 644
421, 517, 554, 592, 647
421, 517, 554, 592, 647
422, 516, 551, 590, 647
364, 420, 516, 562, 588, 668
654, 714
654, 714
653, 714
654, 714
654, 714
655, 714
652, 714
0.677
0.706
0.243
0.85
1
TMS TPPNHAc
3
Br TPPNHAc
3
TPPNH2
TMS TPPNH2
3
Br TPPNH2
0.303
0.91
3
TPPN PhC H
2
14 29
a
Relative to F of TMS TPPNH . For the reference compound TTP F was found to be 27.1.
3
2
of an “azobenzene” where both the p–p* and n–p*
bands are well differentiated, exhibiting a slow trans-cis
photoisomerization rate.
Theoretical calculations were performed in order
to determine the optimized geometries and the dipole
moment values of these molecules (Fig. 4). The
theoretical results were compared with those obtained
experimentally and are summarized in Table 2.
According to Fig. 4, in all the compounds the central
porphyrin ring is planar with the phenyl groups partially
twisted in order to diminish steric effects. The dihedral
angles were calculated for all porphyrins and are shown
in Table 3. As Table 2 reports, the theoretical values of
the Soret band are in agreement with the experimental
results, with a range of error between 1.2 and 3.58%.
Although the different porphyrins exhibited almost
the same absorption values in their UV-vis spectra, the
calculated dipole moment values show that Br TPPNH
Fig. 3. Absorption spectra of TPPN PhC H , before and after
irradiation with UV light
2
14 29
3
2
exhibits the highest value (m = 5.18 D), indicating that
spectral changes are due to trans-cis photoisomerization
and not to other phenomena (Fig. 3).
this compound is the best push–pull system of the series,
where NH and Br act as donor and acceptor groups,
2
Rau classified azobenzenes into three main
groups based on their photochemical behavior [49].
Unsubstituted photochromic azobenzene makes up the
first group, known as “azobenzenes.” The thermally
stable trans isomer exhibits a strong p–p* transition at
respectively.
Regarding the azobenzene-porphyrin molecule,
TPPN PhC H , this compound exhibits a low dipole
2
14 29
moment value because of the absence of donor–acceptor
groups. In the optimized geometry of this compound, the
phenyl groups of the azobenzene unit and the porphyrin
ring are almost aligned at the same plane, which allows
the electronic delocalization. The trans isomer exhibits a
very low dipole moment value (m = 0.53 D), whereas the
cis isomer shows a higher value (m = 3.60 D) as it could
be expected.
3
50 nm and a weak n–p* transition at 440 nm, whereas
the cis isomer undergoes similar transitions but with a
more intense n–p* band. In addition, “azobenzenes”
have a relatively poor p–p* and n–p* overlap. The
second group, known as “aminoazobenzenes” typically
includes azobenzenes that are substituted by an electron-
donor group and are characterized by the overlapping of
the p–p* and n–p* bands. Finally, azobenzenes bearing
both electron-donor and electron-acceptor groups belong
to the third category, named “pseudostilbenes,” where
the p–p* and n–p* bands are practically superimposed
and inverted on the energy scale with respect to the
Fluorescence spectra of the porphyrin derivatives
The fluorescence spectra of the obtained porphyrins
are shown in Fig. 5 and the results are presented in
Table 2. In general, the emission spectra of free base
porphyrins and Zn-metalated porphyrins have two
emission regions: the first one situated between 400
and 500 nm and the second one located between
“
azobenzenes” bands.
AccordingtotheabsorptionspectraofTPPN PhC H
2
14 29
before and after irradiation with UV light, we can observe
that our azo-compound shows the typical behavior
Copyright © 2014 World Scientific Publishing Company
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DESIGN OF NOVEL LUMINESCENT PORPHYRINS BEARING DONOR–ACCEPTOR GROUPS
217
Fig. 4. Optimized geometries calculated for the obtained porphyrin dyads
Table 2. Theoretical and experimental absorption spectra of the obtained porphyrins
Compound
Theoretical Soret l, nm
Experimental Soret l, nm
% Error
Dipolemoment m (D)
TPPNH2
434
436
440
409
411
413
425
419
422
422
419
421
422
420
3.58
3.32
4.27
2.4
2.80
(
TMS) TPPNH2
2.49
3
Br TPPNH2
5.18
3
TPPNHAc
3.68
(
TMS) TPPNHAc
2.4
3.94
1.18
3
Br TPPNHAc
2.1
3
TPPN PhC H
1.2
0.53 (trans) 3.60 (cis)
2
14 29
5
50 and 800 nm [11]. Herein, we can see that all
The TMS-substituted porphyrin (TMS TPPNHAc F =
3
porphyrin derivatives showed two emission bands
located at l = 654 nm and 714 nm with different
intensities, depending on the present substituents.
It is worth to point out that a slight blue shift was
observed in the second emission band. The relative
quantum yields (F) of the different porphyrins
were calculated with respect to the most emissive
compound, in this case TMS TPPNH (F = 1).
0.706) resulted to be good emissive compounds as well
as the azobenzene derivative containing a porphyrin
moiety, which exhibited a F = 0.91. Non-substituted
porphyrins such as TPPNHAc and TPPNH showed
2
F values of 0.677 and 0.85, respectively. Finally, the
brominated porphyrin derivatives Br TPPNHAc and
3
Br TPPNH exhibited the lowest emission intensity
3
2
with F values about 0.243 and 0.303, respectively.
3
2
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 217–220

2
18
C. CAICEDO ET AL.
Fig. 6. Electronic effect on relative quantum yields
Fig. 5. Fluorescence spectra of the obtained porphyrin dyads
in CH Cl solution
2
2
This can be due to an intramolecular energy transfer
from the porphyrin to the halogen groups, which
quenches the fluorescence as other heavy atoms do.
On the other hand, these molecules exhibit also an
elevated CT character due to the presence of donor and
acceptor groups.
Electronic effects on fluorescence emission
of porphyrins
Structure-properties correlation is an important factor
in the design of new photoactive materials since we try
to improve their electronic properties by the appropiate
structural design. Relative quantum yields (RQY, named
here F) of synthesized porphyrins were determined with
respect to the porphyrin exhibiting the highest absolute
fluorescence emission (TMS TPPNH ; F = 1). On this
basis, F were assigned for the rest of porphyrins. It is
worth to note that brominated porphyrin emission is
three times lower than that of its silylated analogs. It is
well-known that fluorescence emission is very sensitive
to different parameters that can affect emission of
chromophore. For instance, heavy atoms tend to quench
or reduce the quantum yields of certain chromophores
due to an efficient spin-orbit coupling, [57] or well, some
conformational changes in the cromophore undergo
non-radiatively deactivate fluorescence. [58] Herein, we
observed that electronic effects strongly affect emission
in our porphyrins. Fluorescence emission of synthesized
compounds correlates with the electronic nature of
para-substituted rings located at the meso position. This
behavior can be noticed when the Hammet parameter
Fig. 7. Dihedral angle determination of the TPP derivatives
3
2
Table 3. Dihedral angles of C3–C4 and
C1–C2 bonds
Sample
Dihedral angle
TPPNH
65.77°
65.08°
65.37°
65.43°
63.81°
65.27°
64.27°
2
Br TPPNH
3
2
TMS TPPNH
3
2
TPPNHAc
Br TPPNHAc
3
TMS TPPNHAc
3
TPP
[
59] is contrasted with F. As the electron-withdrawing
C1–C2 bonds (Table 3). Dihedral angles of C3–C4
and C1–C2 as depicted in Fig. 7 are not so different in
these porphyrins. As a matter of fact, the twisting in
the aromatic ring is almost equal when we compared
a hydrogen-substituted ring with a trimethylsilyl-
substituted one. This suggests that substitution does
group increases, the quantum yield decreases (Fig. 6).
Since fluorescence emission can also be affected by
conformational changes in porphyrinic macrocycle;
steric effects of para-substituted groups were investigated
by determining the dihedral angle between C3–C4 and
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 218–220

DESIGN OF NOVEL LUMINESCENT PORPHYRINS BEARING DONOR–ACCEPTOR GROUPS
219
not really affect the pophyrin ring conformation and
3. Pawlicki M, Collins HA, Denning RG andAnderson
HL. Angew. Chem., Int. Ed. 2009; 48: 3244.
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consequently, fluorescence deactivation is more affected
by the electrowithdrawing or electrodonating character
of the substitutuents present on it.
2
007; 11: 299.
6
. Petit L, Quartarolo A, Adamo C and Russo N.
J. Phys. Chem. B. 2006; 110: 2398.
CONCLUSION
Six different free-base substituted tetraphenylpor-
phyrins were successfully synthesized and characte-
rized: TPPNHAc, TPPNH , TMS TPPNHAc, TMS -
7. Wasielewski MR. Chem. Rev. 1992; 92: 435.
8. Zaragoza-Galán G, Fowler M, Duhamel J, Rein R,
Solladié N and Rivera E. Langmuir 2012; 28: 11195.
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Hascoat P, Diers JR, Seth J, Lammi RK, Bocian
DF, Holten D and Lindsey JS. Chem. Mater. 2001;
13: 1023.
2
3
3
TPPNH , Br TPPNHAc, and Br TPPNH . These
2
3
3
2
compounds exhibited a Soret band at ca. l = 419–422 nm
followed by four Q-bands in the range between 500–
7
00 nm. These porphyrins showed two emission bands
at l = 654 nm and 714 nm with different intensities,
depending on the substituents. Theoretical values of the
spectra are in good agreement with the experimental
results. Theoretical dipole moments allow us to conclude
that the presence of electron-withdrawing groups and the
presence of a donor group (amino or amide) augment
significantly the charge transfer character of these
compounds, increasing their dipole moment values.
The relative quantum yields were calculated for
these compounds with respect to the most emissive
specie TMS TPPNH , showing values from F = 0.243
10. Liddell PA, Kodis G, Moore AL, Moore TA and
Gust D. J. Am. Chem. Soc. 2002; 124: 7668.
11. Aguilar-Ortiz E, Zaragoza-Galán G, Rein R,
Solladié N, Aguilar-Martinez M, Macías-Ruvalcaba
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J. Porphyrins Phthalocyanines 2008; 12: 1232.
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Hackbarth S and Roder B. Chem.-Eur. J. 1999; 5:
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3
2
14. Brettar J, Gisselbrecht JP, Gross M and Solladié N.
Chem. Commun. 2001; 733.
to 1. On the other hand, an azobenzene containing
porphyrin TPPN PhC H was also synthesized and
15. Flamigni L, Talarico AM, Ventura B, Rein R and
Solladié N. Chem. Eur. J. 2006; 12: 701.
2
14 29
characterized. The absorption spectra of this porphyrin
were recorded before and after irradiation with UV light
and it was observed that this azo-compound shows the
typical behavior of an “azobenzene” where both the
p–p* and n–p* bands are well differentiated, thereby
exhibiting a slow trans-cis photoisomerization rate.
It was demostrated that the optical properties such as
fluorescence intensity is more affected by electronic
effects than by steric hindrance.
16. Knör G. Inorg. Chem. Comm. 2001; 4: 160.
17. Sheng N, Sun J, Bian Y, Jiang J and Xu D.
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Acknowledgements
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We thank Gerardo Cedillo, Damaris Cabrero and
Miguel Canseco for their technical assistance in the
characterization of the compounds. We are also grateful
to PAPIIT (Project IN-100513) and CONACYT (Project
2
2
2
1
3. Bossi ML, Murgida DH and Aramendía PF. J. Phys.
Chem. B. 2006; 110: 13804.
4. Yagai S, Iwashima T, Kishikawa K, Nakahara S,
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Supporting information
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Supplementary material is available free of charge
via the Internet at http://www.worldscinet.com/jpp/jpp.
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