Structural and photophysical studies of trans-AB2C-substituted porphyrin ligands and their zinc and copper complexes

Article abstract of DOI:10.1039/b713779f

Trans-AB₂C porphyrins with A = C₆H₄-COOR, C = C₆H₄-NX₂ and B = C₆H₅ (R = CH₃, H; X = O, H) have been synthesised by a rational high-yield procedure (1a-1d) and their zinc(ii) and copper(ii) complexes have been prepared (2a-2d, 3a-3d). 1a, 2a?THF and 3a display different distortions of the porphyrin core as shown by single crystal X-ray crystallography and NSD analyses. The Soret and Q bands of free-base and metalated porphyrins with mixed electron donating and withdrawing substituents (NH₂/COOR) are red-shifted as are the corresponding emission bands of free-base and zinc porphyrins. The electronic asymmetry revealed by spectrocopy is rationalised by DFT calculations. The Royal Society of Chemistry.

Full text of DOI:10.1039/b713779f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Structural and photophysical studies of trans-AB2C-substituted porphyrin
ligands and their zinc and copper complexes†‡
Katja Heinze* and Anja Reinhart
Received 7th September 2007, Accepted 31st October 2007
First published as an Advance Article on the web 9th November 2007
DOI: 10.1039/b713779f
trans-AB2C porphyrins with A = C6H4–COOR, C = C6H4–NX2 and B = C6H5 (R = CH3, H; X = O,
H) have been synthesised by a rational high-yield procedure (1a–1d) and their zinc(II) and copper(II)
complexes have been prepared (2a–2d, 3a–3d). 1a, 2a·THF and 3a display different distortions of the
porphyrin core as shown by single crystal X-ray crystallography and NSD analyses. The Soret and Q
bands of free-base and metalated porphyrins with mixed electron donating and withdrawing
substituents (NH2/COOR) are red-shifted as are the corresponding emission bands of free-base and
zinc porphyrins. The electronic asymmetry revealed by spectrocopy is rationalised by DFT calculations.
Introduction
meso-Substituted porphyrins are crucial building blocks in mul-
tiporphyrin arrays highly relevant for biomimetic and materials
chemistry.1–6 A variety of covalently linked porphyrin arrays has
been explored as biomimetic models of photosynthetic systems,
photonic materials, and functional molecular devices.1–6 Two
conceptually distinct approaches have been developed for the
stepwise synthesis of multiporphyrin arrays joined by covalent
linkers: (i) coupling of intact metalloporphyrin building blocks via
Pd-mediated C–C coupling, Glaser reactions or amide formation
and (ii) acid-catalysed condensation forming new porphyrins in
the process.4 Especially amide formation has been an attractive
synthetic route for coupling of redox and photoactive transition
metal complexes to form functional oligonuclear metal complexes.
Suitable compounds amenable to amide formation are depicted in
Scheme 1. Most of them are derived from a-amino acids and
these building blocks can be easily incorporated in peptide-like
structures (Scheme 1, I).7–19
However, communication or cooperative functions of metal
complex fragments e.g. electron transfer would be highly unlikely
in such peptide-like linear arrays (apart from steric interactions)
and have never been observed. Placing the transition metal
fragment between the functional groups instead of at the side
Scheme 1 Transition metal containing amino acids and peptides derived
chain of a-amino acids gives conceptually different building
thereof.
blocks (Scheme 1, II, III).20–25 Coupling key synthons II and
III by amide formation results in oligonuclear metal complexes
resulting in long-lived charge separated states has been achieved
with the transition metals positioned in the backbone of linear
using amide-linked ferrocene–porphyrin–fullerene arrays.26
arrays. Electronic communication is indeed observed in amide-
Thus free-base and metalated porphyrin amino acids, i.e. trans-
linked ferrocene–ferrocene and ferrocene–ruthenium conjugates
IV and V built from II and III.22–25 Photo-induced electron transfer
AB2C porphyrins§ with A bearing an amine and C bearing
a carboxylic acid functionality are attractive synthetic targets.
Such building blocks have potential applications as constituents
in light-harvesting arrays, in fluorescent sensors and in light-
induced charge separation processes. Access to a trans-AB2C
Department of Inorganic Chemistry, University of Heidelberg, Im Neuen-
heimer Feld 270, 69120, Heidelberg, Germany. E-mail: katja.heinze@
porphyrin with A = p-C6H4–NH2, B = mesityl and C = p-C6H4–
urz.uni-heidelberg.de; Fax: +49 6221 545707; Tel: +49 6221 8587
COOH has been recently reported by Moore and co-workers.27
† CCDC reference numbers 659519–659521. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b713779f
‡ Electronic supplementary information (ESI) available: Cartesian coor-
dinates of DFT optimised ZnTPP, 2a, 2b, 2c and 2d; energies of frontier
orbitals of ZnTPP, 2a, 2b, 2c and 2d; NSD analyses of 1a, 2a, 3a and
CuTPP; full NMR data tables. See DOI: 10.1039/b713779f
§ The ABCD nomenclature has been suggested by Lindsey28 and uses
capital letters to denote the arrangement and type of individual meso-
substituents.
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Desymmetrising the trans-A2B2 porphyrin 5,15-bis(4-carbo-
methoxyphenyl)-10,20-bis(2,4,6-trimethylphenyl)porphyrin (R1 =
R2 = COOCH3) gave the combination R1 = NH2/R2 = COOH
in four steps in 18% overall yield starting from the trans-A2B2
porphyrin.27 In order to thoroughly exploit the potential of such
building blocks a reliable rational high yield synthetic pathway to
“porphyrin amino acids” is desirable.
Results and discussion
A. Synthesis of porphyrins and complexes
Rational routes to trans-AB2C porphyrins have been developed by
Lindsey et al. for various combinations of A and C substituents,
especially alkyl, alkinyl and iodo substituted phenyl groups.28
However, the combination of R1 = NH2/R2 = COOH functional
groups requires modifications of the general rational route. Both
functional groups have to be “masked” or protected prior to
porphyrin formation and simple high yielding transformations
into the R1 = NH2/R2 = COOH combination are needed. For
his purpose a nitro group was chosen as a latent amine and the
acid functionality was protected as its methyl ester. The free-base
porphyrins and their ZnII and CuII complexes reported here are
depicted in Scheme 2.
Scheme 3 Synthesis of free-base porphyrins 1a–1d.
R1 = NO2 and R2 = COOCH3 is formed by acid catalysed conden-
sation of the NO2-functionalised dipyrromethane-dicarbinol and
the ester functionalised dipyrromethane followed by oxidation
with DDQ (2,3-dicyano-5,6-dichloro-p-benzoquinone) in 20%
yield (Scheme 3). Transformation of 1a to 1d is achieved by
reduction of the nitro group to the amine followed by ester
hydrolysis or vice versa. The former pathway proved to be slightly
superior to the latter (see Experimental section). The nitro group of
1a is reduced to the amine by tin(II) chloride to give 1b in 80% yield.
Ester hydrolysis of 1a or 1b with potassium hydroxide proceeds
in 80–90% yield to give 1c and 1d, respectively. Thus, porphyrin
amino acid 1d can be prepared in two steps from porphyrin 1a in
64% isolated yield.
Metalation of free-base porphyrins 1a–1d with zinc(II)
or copper(II) was performed under standard conditions in
dichloromethane using zinc acetate dihydrate and copper acetate
hydrate, respectively (Scheme 4). The isolated yields of zinc and
copper complexes 2a–2d and 3a–3d are highest for metalation of 1a
and they drop from 1b to 1d. This is attributed to the propensities
of the NH2 and COOH moieties to cause streaking upon
chromatographic workup and not to any weaker coordinating
ability of 1b–1d as quantitative metalation is observed by UV/Vis
spectroscopic monitoring of the reaction.
Scheme 2 Free-base porphyrins, zinc and copper complexes.
A retrosynthetic analysis of trans-AB2C porphyrins reveals two
distinct approaches involving a BAB-dipyrromethane-dicarbinol
plus C-dipyrromethane or an ABC-dipyrromethane-dicarbinol
plus B-dipyrromethane. The former route is more efficient as only
one diacylation procedure is required. Thus pure dipyrromethanes
with identical benzoyl groups at the 1- and 9-positions are key pre-
cursors; impurities of monobenzoyl dipyrromethanes will result in
formation of trans-A2B2 porphyrins which are difficult to remove.
The successful dibenzoylation of 4-nitrophenyldipyrromethane
reported here is based on salts made in situ from phosphorous(V)
oxychloride and N-benzoyl morpholine (Scheme 3).
This modified Vilsmeier approach is compatible with the nitro
group, gives higher yields (72%, on a gram scale) and no
concomitant formation of monoacyldipyrromethanes as com-
pared with the Grignard route employed by Lindsey.28,29 Re-
duction of the dibenzoyl dipyrromethane to the corresponding
dipyrromethanedicarbinol is achieved with NaBH4 in methanolic
THF. The regioisomerically pure trans-AB2C porphyrin 1a with
Scheme 4 Metalation of free-base porphyrins 1a–1d.
B. Composition and structure of porphyrins and complexes
All compounds were characterised by IR spectroscopy, high-
resolution mass spectrometry, NMR spectroscopy (1H, 13C,
470 | Dalton Trans., 2008, 469–480
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◦
a
˚
2D-NMR for 1a–2d), EPR spectroscopy (for 3a–3d), cyclic
voltammetry as well as UV/Vis absorption and fluorescence
spectroscopy and thermal analyses. The single crystal structures
of 1a, 2a·THF and 3a were determined proving the desired trans-
AB2C substitution of porphyrin 1a (Fig. 1).
Table 1 Selected bend lengths (A) and angles ( ) and NSD analysis of
1a, 2a·THF, 3a and CuTPP
1a
2a·THF
3a
CuTPP
M–N1
M–N2
M–N3
M–N4
M–O5
O3–N5–C30–C31
—
—
—
—
—
2.062(4)
2.057(4)
2.071(4)
2.057(4)
2.137(4)
1.1(8)
2.001(4)
2.002(4)
1.995(4)
1.989(4)
1.981(7)
1.981(7)
1.981(7)
1.981(7)
—
—
10.9(3)
−12.6(9)
−20.0(9)
0.861
—
—
O1–C45–C24–C25 −13.3(3)
−0.5(10)
B2u (sad) b
−0.069
0.074
0.208
−0.182
−1.183
0.0
B1u (ruf )c
0.020
0.124
0.324
A2u (dom) d
Eg(x) (wav-x)e
Eg(y) (wav-y)e
A1u (pro)f
−0.004
−0.236
−0.051
0.055
0.072
−0.216
0.0
0.0
0.0
0.227
0.096
−0.007
0.020
0.339
−0.006
g
Doop
0.259
0.953
1.196
a Normal coordinate structural decomposition using NSD software from
ref. 32. b Saddling. c Ruffling. d Doming. e Waving. f Propellering. g Total
out-of-plane distortion.
Fig. 2 Results of the NSD analysis of 1a, 2a·THF and 3a.
role (Table 1). In contrast to CuTPP with four identical Cu–N
34
˚
distances of 1.981 A (due to crystallographic fourfold symmetry )
Fig. 1 Molecular structure of 1a, 2a·THF and 3a in the crystal.
3a is distorted with one shorter Cu–N distance (Table 1). A
comparison of the results of the NSD analyses (Fig. 2) reveals
that in this series the metal centres and co-ligands mainly influence
deformations of the macrocycle rather than the peripheral nitro
and ester substituents.
The porphyrin core of free-base porphyrin 1a is slightly distorted
with the waving mode wav-y contributing the largest part (Eg(y) =
˚
˚
0.227 A) to the total out-of-plane distortion Doop = 0.259 A
(Table 1).30–32 This deformation places the nitrogen atoms N1
Characteristic IR and NMR spectroscopic data of free-base
porphyrins 1a–1d and metalated porphyrins 2a–3d are compiled in
Tables 2–4. The IR spectra of the compounds display characteristic
peaks for the different substituents (Table 2). The NHpyrrol signal
˚
and N3 0.16 A above and below the mean plane thus minimising
the H1 · · · H4 contact within the macrocyclic ring. All meso-aryl
substituents are non-co-planar with the porphyrin core similar to
H2TPP in its triclinic modification.33 The nitro and ester groups
are rotated out of the respective aryl plane with torsion angles of
10.9◦ and −13.3◦, respectively (Table 1).
−1
=
of 1a–1c occurs at 3320 cm , the absorption for the m(C Oester
)
Table 2 Characteristic IR data of 1a–1d, 2a–2d and 3a–3d in CsI
In 2a·THF the zinc(II) ion is five-coordinated with a THF
−1
m(NHpyrrol)/cm−1 m(NH2)/cm−1 m(C O)/cm
m(NO2)/cm−1
molecule occupying the fifth position of a square pyramid (Fig. 1).
=
˚
The zinc ion is displaced above the mean plane by 0.31 A towards
1a 3322
1b 3320
—
1722
1724
1691
1691
1724
1723
1687
1707
1722
1720
1687
1691
1520, 1346
—
1520, 1347
—
1521, 1341
—
1517, 1344
—
1519, 1345
—
1520, 1350
—
the THF oxygen atom O5. The macrocycle is slightly distorted with
3468, 3375
—
n.o.
—
n.o.
—
n.o.
—
3471, 3382
—
˚
the saddling normal coordinate sad (B2u = 0.208 A) and the waving
1c
3320
˚
normal coordinate wav-x (Eg(x) = −0.216 A) describing most of
1d n.o.
˚
2a
2b
2c
2d
3a
3b
3c
3d
—
—
—
—
—
—
—
—
the total out-of-plane distortion Doop = 0.339 A. The NO2 and
COOCH3 substituents of 2a·THF are almost co-planar with their
aryl rings (Table 1). The macrocycle of the copper porphyrin 3a is
much more distorted, the total out-of-plane distortion Doop being
˚
0.953 A. The largest contribution arises from the saddling sad =
˚
˚
0.861 A and ruffling ruf = 0.324 A deformations (Table 1, Fig. 2).
n.o.
For CuTPP these normal coordinates also play the dominant
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vibration of 1a, 1b, 2a, 2b, 3a and 3b around 1722 cm−1, the signal
=
of the m(C Oacid) vibration of 1c, 1d, 2c, 2d, 3c and 3d around
1690 cm−1 and the symmetric and asymmetric NO stretching
vibrations of 1a, 1c, 2a, 2c, 3a and 3c are found at 1520 cm−1
and 1344 cm−1, respectively. Frequently, the signals for NH2
absorptions are absent which probably points to some aggregation
via hydrogen bonding or coordination to metal ions in the solid
state.
The characteristic NMR signals for pyrrol NH protons of free-
base porphyrins 1a–1d are observed at d = −2.7. As expected
these signals are absent for zinc porphyrins 2a–2d. The different
substituents induce characteristic chemical shifts of the protons
and carbon nuclei at ring 15 and especially at ring 5 (H152, H153,
H52 and H53; C152, C153, C154, C52, C53 and C54), irrespective of
zinc being present or absent ( Tables 3 and 4). The signals of the
amine protons of 1b, 1d, 2b and 2d are observed around d = 5 and
the proton resonances of the methyl groups of 1a, 1b, 2a and 2b
are found at d = 4.0. Due to the low solubility one-dimensional
13C NMR spectra were uninformative, however with the aid of 2D
NMR techniques (HSQC, HMBC, see Fig. 3 for assignment of
carbon signals) the residual proton signals have been assigned, as
have all 13C signals. The correlations and signal pattern observed
fully confirm the trans-AB2C substitution pattern and purity of
compounds 1a–1d and 2a–2d.
Fig. 3 HSQC (top) and HMBC spectrum (bottom) of 2a in [D8]-THF
(atom numbering scheme according to Scheme 3).
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Table 4 Characteristic 13C NMR data of 1a–1d and 2a–2d in [D]8-THF (atom numbering according to Scheme 3)
d(C52) (ppm)
d(C53) (ppm)
d(C54) (ppm)
d(C152) (ppm)
d(C153) (ppm)
d(C154) (ppm)
d(CH3) (ppm)
d(CO) (ppm)
1a 135.3 (s)
1b 136.1 (s)
1c 135.9 (s)
1d 136.3 (s)
2a 135.8 (s)
2b 136.2 (s)
122.5 (s)
112.9 (s)
122.3 (s)
113.0 (s)
122.0 (s)
112.7 (s)
122.1 (s)
119.1 (s)
148.9 (s)
149.7 (s)
148.5 (s)
143.4 (s)
148.6 (s)
148.7 (s)
148.6 (s)
150.9 (s)
135.3 (s)
134.9 (s)
135.3 (s)
134.9 (s)
135.2 (s)
135.2 (s)
135.0 (s)
134.9 (s)
128.5 (s)
128.1 (s)
128.6 (s)
128.5 (s)
127.9 (s)
128.0 (s)
128.1 (s)
128.0 (s)
130.8 (s)
130.4 (s)
131.6 (s)
131.3 (s)
130.3 (s)
130.0 (s)
131.2 (s)
130.7 (s)
52.2 (s)
51.9 (s)
—
167.0 (s)
167.3 (s)
167.6 (s)
169.6 (s)
167.0 (s)
167.3 (s)
168.9 (s)
167.4 (s)
—
52.2 (s)
52.2 (s)
—
2c
135.7 (s)
2d 135.5 (s)
—
C. EPR studies of copper porphyrins 3a–3d
The paramagnetic copper(II) porphyrins have been investigated by
X-band EPR spectroscopy in THF solution at room temperature
and in frozen THF solution at 130 K (Table 5). Under these
conditions aggregation appears to be almost negligible as resolved
hyperfine coupling to the copper nuclei and to nitrogen nuclei was
observed in all cases (Fig. 4 and 5). At room temperature the EPR
spectra of 3a–3d are very similar and almost indistinguishable
from that of CuTPP with this spectral resolution (Fig. 4). Typical
3
hyperfine couplings to 63/65Cu (I = /2) and to four nitrogen nuclei
(I = 1) are observed and estimated by EPR simulation using
XSophe35 giving giso = 2.097, Aiso(Cu) = 84 G and Aiso(N) = 15.4
G (Table 5).
Table 5 EPR data of 3a–3d in THFa
3a
3b
3c
3d
CuTPP
giso
2.097
84.0
15.4
2.200
198.0
16.4
2.073
32.0
16.0
2.097
84.0
15.4
2.200
198.0
16.4
2.073
32.0
16.0
2.097
84.0
15.4
2.200
198.0
16.4
2.073
32.0
16.0
2.097
84.0
15.4
2.200
198.0
16.4
2.073
32.0
16.0
2.097
84.0
15.4
2.197
197.0
15.5
—
—
—
2.054
35.0
15.0
A
A
iso(Cu) a
iso(N)a
g1
b
A1(Cu)a,b
A1 (N)a,b
b
g2
A2(Cu)a,b
A2 (N)a,b
b
g3
2.024
32.0
16.0
2.024
32.0
16.0
2.024
32.0
16.0
2.024
32.0
16.0
A3(Cu)a,b
A3 (N)a,b
Fig. 5 X-Band EPR spectra of CuTPP and 3a in THF at 130 K (top);
high-field region (bottom).
a Hyperfine coupling constants in G. b Measured at 130 K.
In a frozen THF glass the EPR spectra of 3a–3d are again very
simular but different from that of CuTPP, especially in the high-
field region (Fig. 5). The high-field region in a THF matrix is well
resolved, while the low-field region is broadened (probably via
exchange interactions due to aggregation; note that in the crystal
˚
of 3a the shortest Cu · · · Cu contact amounts to 4.85 A).
In fact the EPR spectra of 3a–3d could not be simulated
satisfactorily assuming an axial spectrum as is possible for CuTPP
with fourfold symmetry [gꢀ = 2.197, g⊥ = 2.054, Aꢀ(Cu) = 197 G;
A⊥(Cu) = 35 G].36,37 For 3a an asymmetric coordination environ-
ment has been observed in the solid state (see above). With the
electronic and structural asymmetry of 3a–3d given simulations
of rhombic anisotropic EPR spectra were attempted. Satisfactory
agreement was obtained with the values given in Table 5. However,
such a large number of fitting parameters renders the accuracy of
the values less confident. Nevertheless, concluding electronic and
Fig. 4 X-Band EPR spectra of CuTPP and 3a in THF at 295 K and
simulated spectrum.
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Table 6 Absorption properties of 1a–3d in THFa
Soreta
Qy(1,0)a,b
Qy(0,0)b
Qx(1,0)b
Qx(0,0)b
A[Q(0,0)]/A[Q(1,0)]c
1a
418 (56.9)
419 (29.5)
418 (56.1)
419 (28.3)
416 (52.6)
419 (46.9)
424 (53.0)
426 (51.3)
424 (45.1)
425 (21.4)
423 (73.7)
423 (49.7)
415 (28.7)
417 (42.9)
416 (35.0)
415 (28.7)
415 (52.6)
414 (46.3)
514 (3.30) [1]
516 (1.28) [1]
514 (1.47) [1]
517 (1.64) [1]
513 (2.16) [1]
515 (1.87) [1]
557 (2.80) [1]
557 (3.44) [1]
557 (3.36) [1]
558 (0.84) [1]
556 (2.37) [1]
548 (2.09) [1]
540 (1.69) [1]
542 (2.86) [1]
540 (5.51) [1]
542 (1.69) [1]
540 (1.87) [1]
538 (1.92) [1]
548 [0.49]
554 [0.62]
550 [0.52]
554 [0.65]
547 [0.42]
549 [0.41]
598 [0.39]
598 [0.47]
598 [0.41]
599 [0.46]
595 [0.30]
586 [0.16]
576 [0.18]
582 [0.22]
575 [0.18]
580 [0.22]
583 [0.13]
574 [0.10]
590 [0.31]
592 [0.31]
592 [0.31]
595 [0.30]
592 [0.25]
591 [0.29]
—
—
—
—
—
—
—
—
—
—
—
—
647 [0.22]
651 [0.27]
648 [0.22]
651 [0.30]
649 [0.23]
650 [0.18]
—
—
—
—
—
—
—
—
—
—
—
—
0.54
0.68
0.57
0.73
0.50
0.46
0.39
0.47
0.41
0.46
0.30
0.16
0.18
0.22
0.18
0.22
0.13
0.10
1b
1c
1d
H2TPP
H2TPPd
2a
2b
2c
2d
ZnTPP
ZnTPPd
3a
3b
3c
3d
CuTPP
CuTPPe
a kmax in nm (e in 104 M−1 cm−1). b Intensities relative to Qy(1,0) = 1. c For free-base porphyrins the ratio of absorbances is A[Q(0,0)]/A[Q(1,0)] = {A[Qx(0,0)]
+ A[Qy(0,0)]}/{A[Qx(0,1)] + A[Qy(0,1)]}. d From ref. 38, measured in benzene. e From ref. 39, measured in CH2Cl2.
structural asymmetry from the anisotropic EPR spectra of 3a–3d
as compared to CuTPP is unaffected.
results in donor/acceptor substituted porphyrins with red shifted
absorption and emission bands (Fig. 6 and 7, Tables 6 and 7).
D. Absorption and emission studies
Absorption and emission spectra of free-base porphyrins 1a–1d
and zinc porphyrins 2a–2d as well as absorption spectra of copper
porphyrins 3a–3d were measured in THF at room temperature.
Again, THF was used in the measurements to reduce aggregation
via coordination of substitutents to the central metal ion M2+,
via hydrogen bonding between NH2/COOH or via p-stacking.
Indeed, THF is coordinated to Zn2+ in the solid state in 2a and
most likely also in solution. The photophysical data of all new
compounds and reference compounds are compiled in Tables 6
and 7.
As expected zinc complexes 2a–2d show a bathochromic and
copper complexes 3a–3d a hypsochromic shift of Soret bands rel-
ative to those of the free-base porphyrins 1a–1d in the absorption
spectra in THF (Table 6). Replacing the electron withdrawing
nitro group of 1a/1c by an electron donating NH2 group (1b/1d)
Fig. 6 Normalised [to Qy(1,0)] absorption spectra of 1a–1d in THF at
295 K.
Table 7 Fluorescence properties of 1a–2d in THF at 295 Ka
Q(0,0)a
Q(0,1)a
U
s1/2/ns
1a
652 (1.0)
658 (1.0)
652 (1.0)
660 (1.0)
650 (1.0)
652 (1.0)
613 (1.0)
612 (1.0)
612 (1.0)
610 (1.0)
602 (1.0)
585 (1.0)
719 (0.41)
720 (0.31)
719 (0.42)
723 (0.32)
718 (0.47)
718 (1.11)
654 (0.57)
658 (0.54)
655 (0.67)
658 (0.62)
654 (0.94)
647 (0.21)
0.14
0.16
0.14
0.16
0.13
0.11
0.11
0.06
0.11
0.08
0.06
0.033
9.9
8.9
10.0
9.0
10.0
13.6
1.7
1.5
1.7
1.6
1.8
1b
1c
1d
H2TPP
H2TPPb
2a
2b
2c
Fig. 7 Normalised [to Qy(0,0)] emission spectra of 1a–1d in THF at
295 K.
2d
ZnTPP
ZnTPPb
1.9
The absorbance ratio A[Q(0,0)]/A[Q(1,0)] = {A[Qx(0,0)] +
A[Qy(0,0)]}/{A[Qx(0,1)] + A[Qy(0,1)]} is used as a measure of
a kmax in nm (relative intensity). b From ref. 38, measured in benzene.
474 | Dalton Trans., 2008, 469–480
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the degeneracy of the configurations 1(a2u, eg) and 1(a1u, eg) in the
four-orbital model with a2u and a1u HOMO’s and eg LUMO’s.40,41
The closer the degeneracy of these configurations the weaker is the
Q(0,0) band while the intensity of the Q(1,0) band is unaffected.
Thusthemorepronouncedtheelectronicasymmetrythelarger will
be the absorbance ratio.41 A large electronic asymmetry manifests
itself in splitting of the eg level and a larger splitting of the a2u and
a1u levels. The experimental absorbance ratio is larger for the amine
substituted derivatives 1b/1d as compared to the nitro derivatives
1a/1c (Table 6).
Effects of electronic asymmetry are also present in metalated
porphyrins (Fig. 8) with the absorbance ratios A[Q(0,0)]/A[Q(1,0)]
of 2b/2d and 3b/3d being larger than those of 2a/2c and 3a/3c,
respectively (Table 6). Additionally, a red shift of Q bands for
3b/3d and of emission bands of 2b/2d is observed (Figs. 8 and 9,
Tables 6 and 7).
DFT calculations (B3LYP, LanL2DZ) indeed support the
electronic asymmetry in amino functionalised zinc porphyrins.
Exemplarily, the frontier orbitals of 2c and 2d are shown in
Fig. 10. Two electron withdrawing substituents in trans-AB2C-
substituted porphyrins (2a, 2c) only results in a slight splitting
of the Gouterman eg orbitals by 0.03 eV.40 However, an electron
withdrawing in combination with an electron donating substituent
also increases the energetic difference between the a1u and a2u
Gouterman orbitals of 2d by 0.09 eV relative to those of 2c.
The a2u orbital of 2d extends onto the amino substituted aryl
ring (although the aryl rings are not co-planar with the porphyrin
plane) while the lower energetic eg(1) orbital has large coefficients
on the carboxy substituted aryl ring (Fig. 10). Thus it can
be concluded that excitation of the donor/acceptor substituted
porphyrins includes some charge transfer character from the N-
terminal side to the C-terminal side in addition to the typical p–p*
excitation.
Fig. 8 Normalised [to Qy(1,0)] absorption spectra of 2a–2d (top) and
3a–3d (bottom) in THF at 295 K.
Fig. 10 Kohn–Sham frontier orbitals of 2c and 2d; the irreducible
representations are derived from the D4h point group of ZnTPP (isosurfaces
at 0.02 au).
Emission decay was found to be monoexponential in all cases
with the lifetimes given in Table 7. The electron withdraw-
ing/electron donating character of the substituents also influences
the quantum yields and excited state lifetimes of 1b/1d and 1a/1c
(Table 7). Interestingly, the quantum yields of amino substituted
porphyrins 1b/1d are slightly higher than those of 1a/1c while for
the amino substituted zinc complexes 2b/2d the quantum yields
drop significantly as compared to those of 2a/2c. This finding
could be due to intermolecular coordination of the NH2 groups to
the zinc ions in 2b and 2d. Indeed, 3,5-dimethylaniline coordinates
to 2a or 2c in CH2Cl2 solution resulting in a dramatic intensity
Fig. 9 Normalised [to Qy(0,0)] emission spectra of 2a–2d in THF at
295 K.
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reduction of the emission bands. Thus, it can be expected that in
THF solution the amine groups of 2b and 2d compete for the axial
coordination site of the zinc ions with the solvent.
ammonia or triethylamine to such a solution the corresponding
free-base porphyrin is obtained. On the other hand the copper
complexes are stable in the presence of concentrated hydrochloric
acid or trifluoroacetic acid.43
E. Stability studies
The thermal stability was probed by differential scanning
calorimetry (DSC). For the nitro derivatives in addition to
the endothermal melting process an exothermal feature around
400 ◦C is observed (Fig. 13). This is probably associated with the
decomposition of the nitro substituent.
The stability of the free-base porphyrins and metalated porphyrins
under acidic conditions was probed by UV/Vis spectroscopy.
Addition of concentrated hydrochloric acid to the free-base
porphyrins results in a reversible double protonation giving the
corresponding meso-tetraarylporphyrin diacids (H21)(Cl)2.
The porphyrin diacids display bathochromic shifts of the Soret
and Q bands and a reduction of the number of Q bands due
to the higher symmetry of the protonated macrocycle (Fig. 11).
The low-energy combination of the Gouterman transitions (Q
band) observed around 653 nm for (H21d)(Cl)2 (Fig. 11) has been
shown to possess significant chloride to porphyrin charge transfer
character.42
Fig. 13 DSC traces of nitro derivatives 1c, 2c and 3c.
F. Electrochemistry
As expected for free-base, Zn and Cu porphyrins up to four
reversible redox processes are observed in the voltage range
accessible under our conditions (Fig. 14). The reduction leads
to the p-anion radical while the two oxidations lead to the p-
cation radical and the porphyrin dication (Table 8), i.e. in all
cases the redox chemistry is ligand-centered. In general zinc
complexes are easier to oxidise and harder to reduce than free-
base porphyrins while the copper derivatives behave similary to the
free-base porphyrins (Table 8).37 For H2TPP, ZnTPP and CuTPP
Fig. 11 Acidification of 1d with conc. HClaq in H2O–THF (3 : 1).
Addition of aqueous ammonia restores the original porphyrin
spectra proving the reversibility of the acid/base equilibrium.
Protonation or deprotonation of NH2 and COOH groups, re-
spectively, has not been observed UV/Vis spectrophotometrically
during addition of acids or bases to 1d. However, under normal
conditions e.g. in THF solution it is expected that 1d exists as a
non-zwitterion [pKa PhCOOH 4.2; pKb PhNH2 9.4] which is also
supported by the chemical shift and integral of the signal of the
amine protons of 1d in the 1H NMR spectrum (see above).
Treatment of zinc porphyrins with dilute hydrochloric acid
results in immediate and irreversible demetalation to give the
doubly protonated porphyrins (Fig. 12). After addition of aqueous
Fig. 14 Cyclic voltammograms of 1a (top) and 1b (bottom) in
nBu4NPF6/CH2Cl2.
Fig. 12 Acidification of 2a with dilute HClaq in THF.
476 | Dalton Trans., 2008, 469–480
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Table 8 Redox potentials of 1a–3d in nBu4NPF6/CH2Cl2 at 295 K vs.
on a Biorad Excalibur FTS 3000 spectrometer using CsI disks.
UV/Vis spectra were recorded on a Perkin Elmer Lambda 19,
1 cm cells (Hellma, suprasil). EPR spectra were recorded with a
Bruker ELEXSYS E500 spectrometer (X-band), Xsophe, version
1.0.2b was used for simulation of the spectra. Mass spectra were
recorded on a JEOL JMS-700 spectrometer. Emission spectra were
recorded on a Varian Cary Eclipse Fluorescence Spectrometer,
slit widths 5 nm. Quantum yields were determined by comparing
the areas under the emission spectra on an energy scale [cm−1]
recorded for optically matched solutions of the samples and the
reference [U(H2TPP) = 0.12 in toluene]. Fluorescence lifetimes
were determined on a PicoQuant FluoTime 100 time-correlated
single-photon counting unit. Lifetimes were extracted from the
decay curve by an iterative reconvolution fitting routine with
nonlinear error minimisation using the FluoFit Software. Criteria
for the best fit were the values of v2. Cyclic voltammetry was
performed using a glassy carbon electrode, a platinum electrode
and a SCE electrode, 10−3 M in 0.1 M nBu4NPF6/CH2Cl2;
potentials are given relative to that of SCE.
SCE
E
1/2(red2)/V
E
1/2(red1)/V
E
1/2(ox1)/V
E1/2(ox2)/V
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
—
−1.13a
−1.34
−1.25b
−1.34
−1.21b
−1.43b
−1.50b
−1.50b
−1.30b
−1.44
−1.38b
−1.43
1.08
0.93b
1.08
1.00b
0.84
0.85b
1.07
0.85b
1.02
0.93b
1.04
0.93b
1.31
—
1.34
—
−1.67
−1.69b
1.15
1.17b
1.34
—
1.38b
—
1.36
—
a Two-electron process. b Irreversible or quasireversible; Ep/V given.
oxidation potentials of 0.94/1.29 V, 0.75/1.08 V and 0.92/1.30 V
are observed under our conditions, respectively.
However, the amine functionality renders oxidation processes
irreversible and multi-electron oxidations are observed which
are probably due to electropolymerisation (Fig. 14, bottom).44,45
The irreversible nature renders a direct comparison of oxidation
potentials of NO2 and NH2 substituted porphyrins meaningless.
Computational method
Density functional calculations were carried out with the
Gaussian03/DFT49 series of programs. The B3LYP formulation
of density functional theory was used employing the LanL2DZ
basis set.49 All optimised structures were characterised as minima
(Nimag = 0) by frequency analysis.
Conclusions
In this study a rational high-yield synthesis of trans-AB2C
porphyrins with A = C6H4–COOR, C = C6H4–NX2 and B =
C6H5 (R = CH3, H; X = O, H) has been reported together with
detailed structural, spectroscopic, photophysical, thermal and
electrochemical characterisations of free-base and metalated trans-
AB2C porphyrins (M = Zn, Cu). Mixed electron donating and
withdrawing substituents (NH2/COOR) significantly modulate
the porphyrin electronic states, while two electron withdrawing
groups (NO2/COOR) only marginally influence the electronic
situation of the porphyrin core both in the free-base and metalated
porphyrins.
The amino acid 1d and its complexes are interesting building
blocks for linear directional porphyrin arrays and porphyrin
conjugates with for example ferrocene derivatives, ruthenium
oligopyridine complexes or organic chromophores. The free-base
porphyrin amino acid 1d and its copper complex 3d are highly
stable in acidic solution so that they are amenable to amide
coupling reactions, deprotection and resin cleavage procedures
etc. associated with peptide synthesis (in solution and solid phase
synthesis procedures22–25,46). The construction of linear directional
porphyrin containing arrays with potential impact on solar energy
conversion research and sensing applications is currently in
progress in our laboratory.
Crystal structure determinations
Intensity data were collected at 100(2) K with a STOE IPDS I
or a Bruker AXS Smart1000 CCD diffractometer using graphite-
˚
monochromated Mo-Ka radiation (k = 0.71073 A), and corrected
for absorption and other effects.50 Crystal data are compiled in
Table 9. The unit-cell parameters were calculated and refined from
the full data set. The structures were solved by direct methods and
refined by full-matrix least-squares based on F2. All non-hydrogen
atoms were given anisotropic displacement parameters, hydrogen
atoms were input in calculated positions, except when noted
otherwise. The calculations were performed using the programs
SHELXS-86,51,52 and SHELXL-97.53
In compound 1a the pyrrol hydrogen atoms H1 and H4 have
been located by a Fourier map and refined isotropically. Parts of
compound 2a·THF are disordered over three positions: the CH3
group is located at O2, O1 and O4 with occupancy factors of
0.5, 0.25 and 0.25; the coordinated THF molecule was modelled
over three positions with occupancy factors of 0.5, 0.25 and
0.25 and the aryl ring C33–C38 is disordered over two positions
with occupancy factors of 0.75 and 0.25. The disorder in the
different parts of the molecule arises from short contacts between
molecules which would arise if the disorder would be ignored
Experimental
General methods
˚
˚
(C46 · · · C46A 1.82 A with C46A; C46X · · · C37 1.89 A and
˚
˚
Unless otherwise noted, starting materials were obtained from
commercial suppliers and used without further purification. The
dipyrromethane precursors 4-nitrophenyldipyrromethane and 5-
(4-benzoic acidmethylester)dipyrromethane were synthesized ac-
cording to literature procedures.47,48 IR spectra were recorded
C46X · · · C38 2.14 A; C46Y · · · C51X 2.49 A and C46Y · · · C52X
˚
2.07 A). Additionally, one half lattice THF is disordered over two
positions with occupancy factors of 0.3 and 0.2. In compound
3a 0.25 lattice THF have been refined isotropically using mild
restraints.
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Table 9 Selected crystal data and refinement parameters for 1a, 2a·THF and 3a
1a
2a·0.5THF
3a·0.25THF
C47H29N5O4.25Cu
795.29
Triclinic
¯
P1
10.0786(8)
12.1933(10)
16.6574(13)
68.8270(10)
75.0110(10)
82.700(2)
1842.6(3)
2
Formula
C46H31N5O4
717.76
C52H35.15N5O5.5Zn
469.28
Formula weight
Crystal system
Space group
Triclinic
¯
P1
Monoclinic
I2/a
˚
a/A
6.5450(9)
12.8558(18)
23.214(3)
96.636(3)
91.808(3)
102.738(3)
1849.9(4)
2
12.8840(8)
31.4296(19)
21.9485(13)
90(0)
101.676(1)
90(0)
8703.9(9)
8
1.355
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
Volume/A
Z
Density (calc)/g cm−3
Absorption
1.289
0.084
1.433
0.648
0.621
coefficient/mm−1
F(000)
748
3684
818
Crystal size/mm
h range for data collection/◦
Index ranges
0.50 × 0.10 × 0.03
0.20 × 0.20 × 0.05
0.15 × 0.10 × 0.05
3.6 to 52.0
−11 ≤ h ≤ 12
−13 ≤ k ≤ 15
0 ≤ l ≤ 20
7266
7261
0.0
1.134
509
3.3 to 55.0
3.5 to 64.4
−8 ≤ h ≤ 8
−19 ≤ h ≤ 19
−47 ≤ k ≤ 47
−32 ≤ l ≤ 32
216499
14625
0.1374
−16 ≤ k ≤ 16
0 ≤ l ≤ 30
Reflections collected
Independent reflections
R(int)
44414
8483
0.0
1.035
503
Goodness-of-fit on F2
Parameters
1.078
678
Final R indices [I > 2r(I)]
R1 = 0.0540, wR2 = 0.1417
R1 = 0.0930, wR2 = 0.1576
0.392, 0.286
R1 = 0.0959, wR2 = 0.2344
R1 = 0.2113, wR2 = 0.3360
0.902, 1.538
R1 = 0.0640, wR2 = 0.1886
R1 = 0.1006, wR2 = 0.2065
1.391, 0.662
R indices (all data)
−3
˚
Largest diff. peak, hole/e A
3
Preparations
(d, JHH = 8.3, 2H, H2), 11.64 (s, 2H, NH). 13C NMR (CD2Cl2,
100 MHz): d = 44.9 (s, C5), 111.5 (s, C7), 121.0 (s, C8), 124.3 (s,
C2), 128.3 (s, C13), 129.7 (s, C12), 130.2 (s, C3), 131.8 (s), 132.1
(s, C14), 138.2 (s), 139.5 (s), 147.7 (s), 148.1 (s), 184.9 (s, C10).
MS (EI): m/z = 475 (100, M+), 370 (40, [M − C6H5CO]+), 353
(29, [M − C6H4NO2]+), 105 (60, [C6H5CO]+), 77 (23, [C6H5]+).
HR-MS (EI): m/z = 475.1534 (calcd for C29H21N3O4: 475.1532).
1,9-Bis(benzoyl)-5-(4-nitrophenyl)dipyrromethane. N-Benzoyl
morpholine (40 mmol, 7.65 g) and phosphorus(V) oxychloride
(80 mmol, 7.44 ml) were combined under an inert atmosphere
and kept at 65 ◦C for 3 h. After cooling to room temperature
the mixture was dissolved in dry 1,2-dichloroethane (60 ml) and
4-nitrophenyldipyrromethane48 (10 mmol, 2.65 g) was added in
one portion. The mixture was stirred at reflux for 2 h, quenched
with saturated aqueous NaOAc solution (60 ml) and heated at
reflux for 1 h. The mixture was cooled to room temperature
and the organic layer was separated. The aqueous layer was
extracted with CH2Cl2 and the combined organic phases were
washed with water (2×), dried with MgSO4 and purified by
chromatography (silica gel, CH2Cl2–ethyl acetate 10 : 1) to give
1,9-bis(benzoyl)-5-(4-nitrophenyl)dipyrromethane (3.42 g, 72%).
1H NMR (CD2Cl2, 400 MHz, see Scheme 5 for atom labels): d =
5.85 (s, 1H, H5), 5.99 (m, 2H, H7), 6.59 (m, 2H, H8), 7.41 (pt,
Synthesis of 1a. NaBH4 (30 mmol, 1.13 g) was added in
small portions to a stirred solution of 1,9-bis(benzoyl)-5-(4-
nitrophenyl)dipyrromethane (0.70 g, 1.5 mmol) in THF–methanol
(10 : 1, 20 ml) under nitrogen. A modest evolution of gas occurred.
After stirring for 1 h at room temperature saturated aqueous
NH4Cl and CH2Cl2 were added. The organic phase was separated,
washed with water (2×) and dried with MgSO4. Removal of
the solvent in vacuo afforded the dipyrromethane-dicarbinol as
a foamlike solid. To the dipyrromethane-dicarbinol (1.5 mmol,
assuming quantitative reduction) was added 5-(4-benzoic acid
methylester)dipyrromethane47 (0.51 g, 1.8 mmol) and acetonitrile
(100 ml). The mixture was stirred for 5 min to achieve complete
3
3JHH = 7.7 Hz, 4H, H13), 7.51 (t, JHH = 7.0 Hz, 2H, H14), 7.75
(d, 3JHH = 7.0 Hz, 4H, H12), 7.75 (d, 3JHH = 8.3 Hz, 2H, H3), 8.22
◦
dissolution. This solution was added to a cooled (0 C) solution
of DMF (1.0 ml), trifluoroacetic acid (1.4 ml) and acetonitrile
(220 ml) in small portions over 20 min. After 10 min 2,3-dicyano-
5,6-dichloro-p-benzoquinone (DDQ, 1.30 g, 5.7 mmol) was added
and the mixture was stirred for 1 h. Triethylamine (1 ml) was added
and the reaction mixture was filtered though a pad of alumina
and eluted with acetonitrile (300 ml). After removing the solvents
in vacuo purification by column chromatography (silica gel,
CH2Cl2) afforded a crystalline purple solid (0.24 g, 20%). HR-
MS (EI): m/z = 717.2383 (calcd 717.2376 for C46H31N5O4); mp
410 ◦C (decomp.).
Scheme 5
478 | Dalton Trans., 2008, 469–480
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Synthesis of 1b. Under nitrogen porphyrin 1a (0.33 g,
0.46 mmol) was suspended in 36% HCl (100 ml) and tin(II) chloride
(1.50 g, 6.6 mmol) was added. After stirring under reflux for 1 h
the solution was slowly neutralised with conc. aqueous ammonia
until the colour of the solution changed to purple-brown. The
aqueous layer was extracted with CH2Cl2 until the aqueous phase
was colourless. The organic phase was dried with MgSO4 and the
solvent was removed in vacuo giving 1b (0.25 g, 80%). HR-MS (EI):
m/z = 687.2658 (calcd 687.2634 for C46H33N5O2); mp 240 ◦C.
3a. HR-MS (EI): m/z = 778.1494 (calcd 778.1615 for
C46H29N5O4Cu); mp 245 ◦C, 395 ◦C (decomp.).
3b. HR-MS (FAB): m/z = 748.1746 (calcd 748.1873 for
C46H31N5O2Cu); mp 205 ◦C, 340 ◦C (decomp.).
3c. HR-MS (EI): m/z
=
764.1365 (calcd 764.1459 for
C45H27N5O4Cu); mp 355 ◦C, 390 ◦C (decomp.).
3d. HR-MS (FAB): m/z = 734.1632 (calcd 734.1717 for
C45H29N5O2Cu); mp 200 ◦C, 380 ◦C (decomp.).
Synthesis of 1c. A mixture of 1a (77 mg, 0.1 mmol) in THF–
ethanol (1 : 1, 50 ml) and KOH (0.51 g, 9 mmol) in water (5 ml) was
stirred under reflux for 3 d. The solvents were evaporated and the
residue suspended in water (50 ml). Acidification of the aqueous
suspension with concentrated hydrochloric acid and subsequent
filtration gave 1c as a purple powder (63 mg, 90%). HR-MS (EI):
m/z = 703.2208 (calcd 703.2220 for C45H29N5O4); mp 210 ◦C,
400 ◦C (decomp.).
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (Heisenberg
fellowship to K.H.) and the Graduate College 850 “Modeling of
Molecular Properties” (fellowship to A.R.) for financial support.
Notes and references
Synthesis of 1d from 1b. A mixture of 1b (69 mg, 0.1 mmol)
in THF–ethanol (1 : 1, 50 ml) and KOH (0.51 g, 9 mmol) in water
(5 ml) was stirred under reflux for 3 d. The solvents were evaporated
and the residue suspended in water (50 ml). Acidification of
the aqueous suspension with concentrated hydrochloric acid and
subsequent filtration gave 1d as a purple powder (54 mg, 80%).
HR-MS (EI): m/z = 673.2527 (calcd 673.2478 for C45H31N5O2);
mp 185 ◦C.
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until the colour of the solution changed to purple-brown. The
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was colourless. The organic phase was dried with MgSO4 and the
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2b. HR-MS (FAB): m/z = 749.1735 (calcd 749.1769 for
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