Synthesis of flexible dimeric meso-tetrakis-porphyrins

Article abstract of DOI:10.1016/j.tetlet.2007.03.047

Monofunctionalisation of meso-tetrakis-porphyrins through introduction of a carboxylic group in the meso position of the phenyl group confers the necessary characteristics to anchor them through stable amide bonds to functionalised supports or to molecules. In this Letter we describe the synthesis, characterisation and photophysical evaluation of such a functionalised flexible dimeric porphyrin, bis-(meso-tetrakis-5,10,15-triphenyl-20-(p-carboxyphenyl)-porphyrinyl)-1,6-hexanediamide.

Full text of DOI:10.1016/j.tetlet.2007.03.047

Tetrahedron Letters 48 (2007) 3145–3149
Synthesis of flexible dimeric meso-tetrakis-porphyrins
a
a
b
Abilio J. F. N. Sobral,a, Susana M. Melo, M. Luısa Ramos, Raquel Teixeira,
*
´
Susana M. Andradeb and Silvia M. B. Costab,*
aDepartamento de Quı´mica, Universidade de Coimbra, Rua Larga, 3004-535 Coimbra, Portugal
bCentro de Quı´mica Estrutural, Instituto Superior Te´cnico, UTL, 1049-001 Lisboa, Portugal
Received 12 February 2007; revised 5 March 2007; accepted 7 March 2007
Available online 12 March 2007
Abstract—Monofunctionalisation of meso-tetrakis-porphyrins through introduction of a carboxylic group in the meso position of
the phenyl group confers the necessary characteristics to anchor them through stable amide bonds to functionalised supports or to
molecules. In this Letter we describe the synthesis, characterisation and photophysical evaluation of such a functionalised flexible
dimeric porphyrin, bis-(meso-tetrakis-5,10,15-triphenyl-20-(p-carboxyphenyl)-porphyrinyl)-1,6-hexanediamide.
ꢀ 2007 Elsevier Ltd. All rights reserved.
The unique photophysical and photochemical properties
of porphyrins and related macrocycles make them an
extremely versatile class of compounds for a variety of
applications in medicine, catalysis, conversion of solar
energy and nanotechnology.1–7
angstroms, having saturated molecules, aromatics or
other highly conjugated motifs as spacers.8–13
In spite of this intense activity, synthesis of new bis-por-
phyrins is still an area of high potential, both in terms of
improving low yields generally obtained in the synthetic
process as well as discovering novel properties from new
assemblies of porphyrins and spacers, as in the case of
the long saturated spacers between meso-phenyl-porphy-
rins. Since most of the work has been done with conju-
gated spacers to favour the electronic interaction among
the two porphyrins, mostly with restricted rotation,
there is a lack in the evaluation of the interaction of
two porphyrins restricted in the same volume but with-
out direct conjugation between their electronic systems.
This has led us to plan and synthesise the title com-
pound, whose photophysical properties are also
presented.
Dimeric porphyrin arrays (bis-porphyrins) in particular
are important for supramolecular recognition and
energy or electron donor–acceptor systems. The bis-
porphyrins are the simplest elements in porphyrin based
nanostructures and consequently several methods have
been developed to achieve efficient and controlled syn-
thesis of porphyrin-spacer-porphyrin systems. Since
the nature of the linker has a strong influence on the
overall properties of the porphyrin dimers, affecting
both physical (solubility, aggregation behaviour or
photo response) and chemical properties (reactivity or
thermal stability) a large amount of work has been done
over the last four decades on the synthesis and evalua-
tion of the physical properties of a number of bis-por-
phyrins. The actual bis-porphyrin based design of
porphyrin-spacer-porphyrin systems, with organic ionic,
metal ionic or organic covalent linked spacers consti-
tutes a diverse set of molecular types, involving meso-
free porphyrins, meso-tetrakis-porphyrins and a combi-
nation of both, with spacers ranging from a few atoms
to complex spacers of lengths extending to dozens of
Optical absorption spectra of porphyrins were recorded
on a Jasco V-560 UV–vis absorption spectrophoto-
meter. Fluorescence measurements were made on a
Perkin–Elmer LS50B spectrofluorometer. The instru-
mental response at each wavelength was corrected by
means of a curve obtained using appropriate fluores-
cence standards. The sample holders of both instru-
ments were thermostated at 25.0 ꢁC and all
measurements were performed with [Porphyrin] ꢀ 4 lM
in acetonitrile using 10 mm path length quartz cells.
MM2 calculations were performed in a Pentium 4 work-
station at 3.2 GHz, via CambridgeSoft Chem3D Pro 7,
2001, USA, using a conjugated gradient algorithm for
*
Corresponding authors. Tel.: +351239854476 (A.J.F.N.S.); e-mail:
asobral@ci.uc.pt
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.03.047

3146
A. J. F. N. Sobral et al. / Tetrahedron Letters 48 (2007) 3145–3149
energy minimisation in vacuum, with a final gradient of
0.05 kcal/(A mol).
These display common features, with both exhibiting a
single Soret (B) and four Q-bands. The similar spectra
and lack of splitting of the bands indicate that no distor-
tions occur in ring planarity. The lack of splitting of the
Soret band suggests that the electronic states of the di-
mer are not perturbed compared with that of the mono-
meric porphyrins.
˚
We isolated the mono functionalised porphyrin (1) in a
5% yield from the statistical mixture of porphyrins
resulting from the cross condensation of benzaldehyde
and 4-carboxy-benzaldehyde with 1H-pyrrole, in aer-
ated propionic acid and acetic anhydride solution at
130 ꢁC. After conversion of the carboxyl group to its
acyl chloride by SOCl2, the porphyrin was coupled with
hexane-1,6-diamine to give dimeric porphyrin (2)
(H4DTPP) or with n-butylamine to give porphyrin (3)
(TPP-CONHBu1), as shown in Scheme 1. All com-
pounds gave the expected spectroscopic and analytical
data for the suggested structures.
According to exciton coupling theory, the transition di-
pole moments for each moiety interact electrostatically
to create two exciton states, Wj and Wꢁ. The Wj state
corresponds to the case where both monomeric units
are excited in phase, while for the Wꢁ state, both mono-
mers are out of phase. The absorption energies of the
transitions depend on the angle between the transition
dipoles and the relative phase of their excitations. The
exciton is expected to be strong in directly meso-linked
arrays because of the short interconnection between
porphyrin moieties. However, in the present case the
interconnection between units is large enough to allow
The electronic absorption spectra were examined in ace-
tonitrile and compared with that of TPP-CONHBu1
under similar conditions. Figure 1 shows the absorption
spectra of H4DTPP (in bold) and of TPP-CONHBu1.
N
H
COOH
x
CHO
NH
N
N
H+
NH
N
N
x
x
(ii)
HN
(i)
HN
Column Flash
Chromatography;
Celite
aaaa
aaab
aabb
abab
abbb
bbbb
CHO
1st CH2Cl2 ;
x
2
nd EtOH/ CH2Cl2
(1:1)
(1)
a = H
b = COOH
COOH
(iii)
SOCl2
(v)
H2N
(iii)
(iv)
H2N
SOCl2
H
N
O
H
H
O
N
N
O
NH2
NH
N
N
HN
NH
N
N
NH
N
N
HN
HN
(2)
(3)
TPP-CONHBu1
H4DTPP
Scheme 1. Reagents and conditions: (i) Propionic acid, acetic anhydride, benzaldehyde, 4-carboxy-benzaldehyde, 1H-pyrrole; (1)/(2)/(3)
(0.0144 mol/7.0 · 10ꢁ3 mol/7.0 · 10ꢁ3 mol), 130 ꢁC, 3 h, 4%; (ii) column flash chromatography (first run CH2Cl2; second run EtOH/CH2Cl2
(1:1)); (iii) K2CO3 anhydrous, CH2Cl2, 32 ꢁC, SOCl2, 6 h; (iv) hexane-1,6-diamine, 32 ꢁC, 2 h, 10%; (v) 1-butylamine, 32 ꢁC, 1 h, 5%. (1) m/z (FAB
MS, NBA) 659.9 (100%, [M+H]+, C45H30N4O2 requires 658.7), 1H NMR 500 MHz (0.01 M in CDCl3): d 11.1 (br s, 1H, COOH), d 8.88 (m, 6H,
bHa), d 8.83 (m, 2H, bHb), 8.51 (dd, J 2.8 Hz, 2H, Subst. ArHo), 8.36 (dd, J 2.8 Hz, 2H, Subst. ArHm), d 8.24 (m, 6H, ArHm) d 7.80 (m, 9H, ArHo,p),
d ꢁ2.51 (m, 2H, NH). Inset: (a) MALDI-TOF mass spectra for (2), DHB matrix with calibration mixture Pepmix 1, M+Å at m/z 1396.96
(C96H72N10O2 requires 1396.58) and (b) ESI MS for (3) m/z (FAB, NBA) [M+H]+ at 714.6 (C49H39N5O requires 713.4).

A. J. F. N. Sobral et al. / Tetrahedron Letters 48 (2007) 3145–3149
3147
Scheme 1 (continued)
0.06
0.04
0.02
0
sensed in the Q-band region are even less marked due to
the much smaller transition dipole moments compared
with those of B-bands.
1
0.8
0.6
0.4
0.2
The fluorescence emission spectrum of TPP-CONHBu1
in acetonitrile exhibits two vibronic emission bands cen-
tred at 650.5 and 717.5 nm (see Table 1) independent of
the excitation wavelength, Figure 2. In the case of
H4DTPP, however, besides two similar vibronics, a
totally new band is detected at higher energies (centred
at 543 nm) with an intensity (INT) that, in contrast to
the two longer wavelength emissions, depends on the
excitation wavelength, with INT(kexc = 375 nm) >
INT(kexc = 508 nm) > INT(kexc = 420 nm).
450
500
550
600
650
700
0
350
450
550
wavelength/nm
650
Figure 1. Normalised UV–vis absorption spectra of H4DTPP (black
line) and TPP-CONHBu1 (grey line) in acetonitrile. Inset: Expanded
absorption spectra in Q-band region.
The excitation spectra also show a dependence on the
emission wavelength. The spectrum obtained collecting
emission at the usual vibronic bands (650 or 712 nm)
is clearly superimposable on the absorption spectrum,
both in Soret and Q-band regions. However, when the
for a considerable angular distribution, such that exci-
ton coupling is weak (or nonexistent). The perturbations

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A. J. F. N. Sobral et al. / Tetrahedron Letters 48 (2007) 3145–3149
Table 1. Comparison of photophysical parameters of H4DTPP and TPP-CONHBu1 in acetonitrile
a
Absorption maxima (nm)
Emission maxima (nm)
/
f
B(Soret)
Qy(1,0)
Qy(0,0)
Qx(1,0)
Qx(1,0)
H4DTPP
TPP-CONHBu1
413.5(2.54)b
413.0(2.02)b
512.0
512.5
546.0
545.5
589.0
588.0
644.0
643.0
542.0
—
650.0
650.5
716.0
717.5
0.12
0.10
a kexc = 508 nm using for reference TPP in benzene with /f = 0.11.
b Molar extinction coefficient, e/105 Mꢁ1 cmꢁ1
.
1.2
1.2
a
b
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
x20
500
550
600
650
700
750
500
550
600
650
700
750
wavelength/nm
wavelength/nm
Figure 2. Normalised fluorescence spectra of H4DTPP (black line) and of TPP-CONHBu1 (grey line) in acetonitrile obtained upon excitation at (a)
375 nm and at (b) 508 nm (inset: amplification of fluorescence intensity in the high energy region).
emission is observed at shorter wavelengths (e.g.,
610 nm and 540 nm, in Fig. 3) a new band is detected
in the corresponding excitation spectra with a maximum
at 375 nm. In contrast, the intensity of the band com-
mon to absorption (centred at 413 nm) decreases, and
it is not detectable when emission is collected near the
new emission band maximum (540 nm). It is possible
that upon excitation, a different configuration of the
two porphyrin moieties is achieved, probably facilitated
by the polar aliphatic solvents.
1.2
1
This broad band at higher energies than the monomer
emission appears in many aprotic and protic polar ali-
phatic solvents and decreases with the temperature,
but is absent in the aromatic apolar toluene. Work is
in progress in order to clarify these features in the fluo-
rescence emission.
0.8
0.6
0.4
0.2
0
Preliminary molecular mechanic calculations in vacuum
of the open and closed forms (Scheme 2) show that the
‘closed form’ is 26 kcal/mol more stable than the ‘open
form’, due to favourable pꢂ ꢂ ꢂp stacking interactions.
Although this calculated vacuum value is too high to
correspond to solution conditions, and would dramati-
cally favour the stability of closed form, in contrast to
what is observed, this suggests a favourable interaction
between the linked macrocycles, which will be dimin-
ished by the solvent molecules and could even be totally
350
370
390
410
430
450
wavelength/nm
Figure 3. Normalised excitation spectra of H4DTPP in acetonitrile
collected at different emission wavelengths: 712 nm (black thin line);
610 nm (ꢃꢃꢃ); 540 nm (---) and (grey bold line for TPP-CONHBu1).
Scheme 2. Minimised conformational structures from molecular mechanical calculations (MM2) (a) ‘open form’ and (b) ‘closed form’.

A. J. F. N. Sobral et al. / Tetrahedron Letters 48 (2007) 3145–3149
3149
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Acknowledgements
Financial support from FCT (POCI/QUI/57387/2004),
´
and 3ꢁ Quadro Comunitario de Apoio is gratefully
acknowledged. S.M.A. thanks FCT for a BPD Grant
SFRH/BPD/24367/2005.
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