Synthesis, electronic characterisation and significant second-order non-linear optical responses of meso-tetraphenylporphyrins and their Zn II complexes carrying a push or pull group in the β pyrrolic position

Article abstract of DOI:10.1002/ejic.200500292

This work describes the synthesis and characterisation by electronic absorption spectroscopy, cyclic voltammetry and by a solvatochromic investigation of meso-tetraphenylporphyrins and their Zn^II complexes substituted at the β pyrrolic position

Full text of DOI:10.1002/ejic.200500292

FULL PAPER
Synthesis, Electronic Characterisation and Significant Second-Order
Non-Linear Optical Responses of meso-Tetraphenylporphyrins and Their
ZnII Complexes Carrying a Push or Pull Group in the β Pyrrolic Position
Elisabetta Annoni,[a] Maddalena Pizzotti,*[a] Renato Ugo,[a] Silvio Quici,[b]
Tamara Morotti,[b] Maurizio Bruschi,[c] and Patrizia Mussini[d]
Keywords: Cyclic voltammetry / Nonlinear optics / Porphyrins / UV/Vis spectroscopy / Zinc
This work describes the synthesis and characterisation by
electronic absorption spectroscopy, cyclic voltammetry and
by a solvatochromic investigation of meso-tetraphenylpor-
phyrins and their ZnII complexes substituted at the β pyrrolic
incident wavelength of 1.907 µm. The porphyrin ring substi-
tuted at the β pyrrolic position by an electron-acceptor π-sys-
tem behaves as a significant donor group, comparable to a
ferrocenyl group or to a phthalocyanine. Unexpectedly, the
position by a pseudo-linear, π-delocalised organic linker car- porphyrin ring substituted at the β pyrrolic position with an
rying either an electron-withdrawing (pull) or electron-do-
nating (push) group. The second-order NLO response of
these push-pull porphyrinic chromophores has been investi-
gated by the EFISH technique working with a non-resonant
electron-donor π-system shows a larger and significant sec-
ond-order NLO response.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
additional advantages, such as good thermal and chemical
stability, acceptable solubility for the determination of di-
pole moments and their second-order molecular NLO re-
sponse and relatively easy synthetic pathways for the prepa-
ration of various architectures.[9] The highly polarisable π
ring, with strong πǞπ* transitions,[10] of push-pull metal
porphyrins asymmetrically substituted in the 5- and 15-po-
sitions, has been proposed as an excellent linker of the
pseudo-linear push-pull chromophore.[11] This latter point
was experimentally confirmed by Therien et al.,[6a,6b] Ng et
al.[6c] and recently also by some of us.[6d] However, to the
best of our knowledge, no-one, with the exception of two
rather preliminary communications,[12] has experimentally
investigated whether the π system of porphyrins or metall-
oporphyrins can act as a donor or acceptor group of a
pseudo-linear push-pull chromophore. Depending on the
number of π electrons, π orbitals, and on the heteroatom
electronegativities, heterocyclic rings can act as electron-
rich (donor) or electron-poor (acceptor) systems.[13] Pyrrole,
with six π electrons and five π orbitals, is an electron-rich π
system. Thus, a porphyrinic chromophore with four pyrrole
rings may be considered, at first glance, as a π-electron-
rich system in which electron-rich and electron-poor carbon
atoms may be identified.[14] In order to experimentally
prove some interesting theoretical suggestions,[14] we inves-
tigated the second-order NLO response of push-pull
pseudo-linear chromophores based on a 5,10,15,20-tet-
raphenylporphyrin, either as free base or as ZnII complex,
carrying a push or a pull group linked to the β pyrrolic
position through a π-delocalised spacer. One major prob-
Introduction
A wide variety of push-pull organic π-conjugated chro-
mophores have been extensively investigated, both theore-
tically and experimentally, for their significant second-order
optical nonlinearities.[1–3] When compared to organic chro-
mophores, pseudo-linear coordination and organometallic
chromophores may offer additional electronic features, act-
ing on the second-order non-linear optical (NLO) response,
such as intense charge-transfer transitions (ligand to metal
and vice versa) at relatively low energy that are easily tunea-
ble by changing the oxidation state of the metal, its coordi-
nation sphere and stereochemistry.[4] Attempts have been
made to design two-dimensional push-pull chromophores
based on π-delocalised macrocycles such as porphyrins and
metalloporphyrins,[5,6]
metallophthalocyanines[7a]
and
subphthalocyanines,[7b,7c] and metallotriazolohemiporphyr-
azines.[8] Porphyrins and their metal complexes offer some
[a] Dipartimento di Chimica Inorganica Metallorganica e Analit-
ica dell’Università di Milano, Unità di Ricerca dell’ INSTM,
Centro di eccellenza CIMAINA dell’Università di Milano, e
Istituto di Scienze e Tecnologie Molecolari del CNR (ISTM),
Via G.Venezian 21, 20133 Milano, Italy
Fax: +39-02-50314405
E-mail: maddalena.pizzotti@unimi.it
[b] Istituto di Scienze e Tecnologie Molecolari del CNR (ISTM),
Via Golgi 19, 20133 Milano, Italy
[c] Dipartimento di Biotecnologie e Bioscienze, Università di Mil-
ano Bicocca,
Piazza della Scienza 2, 20126 Milano, Italy
[d] Dipartimento di Chimica Fisica ed Elettrochimica dellЈUniver-
sità di Milano,
Via Golgi 19, 20133 Milano, Italy
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M. Pizzotti et al.
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lem with push-pull π-delocalised macrocycles is the correct Sonogashira coupling reaction of the ZnII complex of 2-
determination of their molecular second-order NLO re- bromo-5,10,15,20-tetraphenylporphyrin with 4-nitrophen-
sponse. The quadratic hyperpolarizability of asymmetric ylacetylene in refluxing THF/Et3N, using [PdCl2(PPh3)2]
porphyrins and their metal complexes has been measured and CuI as catalysts, followed by demetallation with
by HRS (Hyper-Rayleigh Scattering),[6a] Stark effect[6b] and CF3COOH in CH2Cl2 at room temperature to give porphy-
by EFISH (Electric Field Induced Second Harmonic) gen- rin 5 (Scheme 1).
eration techniques.[5,6c,6d] Depending on the incident wave-
length, these measurements can be affected by resonance
enhancements[6a,6d] and in the case of the HRS technique
also by a fluorescent contribution to the second harmon-
ic.[6a]
In order to avoid both these effects, the second order
NLO responses of compounds investigated in this work
were measured by the EFISH technique working at a non-
resonant incident wavelength of 1.907 µm. In addition, den-
sity functional theory (DFT)[15] calculations were carried
out in order to confirm experimental dipole moments and
to define the best structural geometries using the BP86
functional[16] and an all-electron valence triple-ξ basis set
with polarisation functions on all atoms.[17]
Results and Discussion
Synthesis of Porphyrins and Their ZnII Complexes
Scheme 1. i) 4-Nitrophenylacetylene, [PdCl2(PPh3)2] (cat), THF/
The asymmetric 5,10,15,20-tetraphenylporphyrins and
NEt3, reflux; ii) CF3COOH, CH2Cl2, room temp., 20 h.
their ZnII complexes investigated in this work are reported
in Figure 1.
Porphyrin 1 was synthesised by reaction of 2-bromo-
5,10,15,20-tetraphenylporphyrin, obtained by bromination
Porphyrins 7 and 11, which are already known,[21] were
of 5,10,15,20-tetraphenylporphyrin (TPP) with N-bromo- synthesised by a new and easier method based on the Wittig
succinimide, first with Ni(acac), then with CuCN[18] to af- condensation of 2-formyl-5,10,15,20-tetraphenylporphyrin
ford the NiII complex of 1, and finally by controlled deme- (13) with (4-nitrobenzyl)triphenylphosphonium bromide
tallation with concentrated sulfuric acid. Its ZnII complex and [4-(dibutylamino)benzyl]triphenylphosphonium chlo-
2 was obtained by reaction of 1 with Zn(OAc)2.[19] Porphy- ride respectively (Scheme 2).
rin 3 was obtained by reaction of the ZnII complex of TPP
Reactions were carried out in CH2Cl2 at room tempera-
with I2 and AgNO3 to afford 4,[20] followed by demetalla- ture in the presence of solid NaOH as base to afford, after
tion with concentrated HCl. Complex 6 was obtained by a purification by column chromatography, a mixture of (E)-
Figure 1. Asymmetrical meso-tetraphenylporphyrins and their ZnII complexes with β pyrrolic substitution by electron-withdrawing or
-donating groups.
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Scheme 2. i) POCl3, DMF, 1,2-dichloroethane, reflux, 24 h; ii) 96% H2SO4, CH3COONa 30% in H2O; iii) (4-nitrobenzyl)triphenylphos-
phonium bromide, NaOH, CH2Cl2, room temp., 8 h; iv) [4-(dibutylamino)benzyl]triphenylphosphonium chloride, NaOH, CH2Cl2, room
temp., 4 h; v) Zn(OAc)2·2H2O, CHCl3/MeOH, reflux.
Scheme 3. i) NaBH4, EtOH 96%, reflux, 4 h; ii) SOCl2, pyridine, Et2O; iii) PPh3, CHCl3, reflux; iv) 4-nitrocinnamaldehyde, DBU, room
temp.; v) Zn(OAc)2·2H2O, CHCl3/MeOH, reflux, 2 h.
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and (Z)-isomers. From this mixture the pure (E)-isomers 7
Again, treatment of the free porphyrin 9 with Zn(OAc)2
and 11 were obtained, in 58% and 69% yields respectively, in CHCl3/MeOH at reflux afforded the ZnII complex 10 in
by repeated crystallisation from CH2Cl2/MeOH. Porphyrin quantitative yield. All porphyrins and their ZnII complexes
13 was prepared in 70% yield by Vilsmeier formylation of were characterised by elemental analysis, H NMR spec-
1
the commercially available Ni complex of TPP (Scheme 2) troscopy and mass spectrometry
following a modification of the procedure reported in the
literature.[22] (4-Nitrobenzyl)triphenylphosphonium bro-
mide was easily obtained, in quantitative yield, by quat-
ernisation of triphenylphosphane with 4-nitrobenzylbro-
mide in refluxing toluene, while (4-dibutylamino)benzyltri-
Electronic Absorption, Emission Spectra and
Solvatochromic Behaviour
phenylphosphonium chloride was prepared from (4-dibu-
Metal complexation of tetraarylporphyrins increases the
tylamino)benzyl alcohol, obtained by reduction of (4-dibu- ring microsymmetry from D2h to D4h, with a parallel de-
tylamino)benzaldehyde with NaBH4, by adapting a pro- crease of the number of Q absorption bands from four to
cedure reported in the literature.[23] The ZnII complexes 8 two.[10,25,26] Although the introduction of a substituent in
and 12 were obtained in quantitative yield by treating por- the β pyrrolic position should produce a lowering of the
phyrins 7 and 11, respectively, with Zn(OAc)2 in CHCl3/ D4h microsymmetry, the usual decrease of the number of
MeOH.[19] Porphyrin 9, bearing a (1E,3E)-butadiene linker Q bands from four to two was reported when comparing
in the β position, was prepared by Wittig condensation of porphyrin 7 and its NiII complex.[21] For porphyrin 1 and
4-nitrocinnamaldehyde with the phosphonium salt 16 in the its CuII complex, a decrease of the number of Q bands from
presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as four to three was originally published,[18] but later a de-
base (Scheme 3).
crease from four to two was reported when comparing por-
The preparation of the phosphonium salt 16 was carried phyrins 1 and 3 with their CuII complexes.[25] When com-
out, starting from the aldehyde 13, according to the syn- paring porphyrins 1 and 3 with their ZnII complexes 2 and
thetic scheme reported in a preliminary communication by 4, we observed a decrease of the number of Q bands from
Bonfantini and Officer in 1993.[24] However the detailed ex- four to three, with complete disappearance of the Qα band
perimental procedure was never published, hence a careful above 600 nm but with a weak Qβ band at higher energy
description of the synthesis and characterisation of interme- still detectable (Table 1 and Figure 2).[27]
diate porphyrins 14, 15 and 16 is given in the Experimental
Section.
A similar decrease of the number of Q bands was ob-
served when comparing porphyrins 5 and 7 with their ZnII
Table 1. Visibile and fluorescence spectra in CHCl3.
Compound[a]
Soret B Half band width Soret B
Q bands
λa [nm] [logε]
Fluorescent emission[b]
λe [nm]
∆ν1/2 [cm–1 [c]
]
λa [nm] [logε]
2-Cyano-H2TPP[d] (1)
n.d.
420 [5.29][e]
517 [3.79], 552 [3.21]
593 [3.23], 648 [3.20]
518 [3.57], 555 [4.20]
593 [4.02]
527 [4.14], 560 [3.56]
605 [3.53], 665 [3.88]
522 [3.41], 555 [4.00]
598 [3.80]
525 [4.31], 562 [3.83]
601 [3.78], 657 [3.68]
521 [3.60], 557 [4.25]
594 [4.03]
525 [4.34], 569 [4.13]
602 [3.94], 659 [3.47]
526 [3.77], 558 [4.35]
597 [4.13]
659, 717
(659, 718)
607.5, 657
(606.5, 656.5)
716
(706)
661
(657)
677, 730
(671.5, 730.5)
621, 659.5
(614.5, 660)
671, 732.5
(672, 734)
617, 664
(616.5, 664.5)
668.5
(629, 678)
616.5, 666
(616.5, 667)
660, 724(sh)
(662, 728 sh)
650
2-Cyano-ZnTPP (2)
n.d.
427 [5.62][e]
427 [5.24]
425 [5.28]
430 [5.26]
431 [5.30]
431 [5.31]
432 [5.20]
425 [5.03]
430 [4.94]
422 [5.36]
422 [5.20]
2-Nitro-H2TPP (3)
2974 (1574)
1172 (1874)
2296 (1675)
1381 (1474)
1800 (2497)
3216 (2320)
3475 (2958)
2-Nitro-ZnTPP (4)
2-(4-Nitrophenyl)ethynyl-H2TPP (5)
2-(4-Nitrophenyl)ethynyl-ZnTPP (6)
(1E)-2-(4-Nitrophenyl)ethenyl-H2TPP (7)
(1E)-2-(4-Nitrophenyl)ethenyl-ZnTPP (8)
(1E,3E)-2-(4-Nitrophenyl)buta-1,3-dienyl-H2TPP (9)
523 [4.21], 572 [4.07]
601 [3.86], 665 [3.51]
559 [4.11]; 599 [3.98]
(1E,3E)-2-(4-Nitrophenyl)buta-1,3-dienyl-ZnTPP (10) 3341 (2741)
(1E)-2-[4-(Dibutylamino)phenyl]ethenyl-H2TPP (11)
(1E)-2-[4-(Dibutylamino)phenyl]ethenyl-ZnTPP (12)
1674 (1102)
1454 (1011)
521 [4.41], 579 [4.16]
603 [4.14], 656 [3.71]
556 [4.08], 601 [3.97]
(661)
[a] H2TPP = 5,10,15,20-tetraphenylporphyrin. [b] Values obtained by irradiation at the Soret wavelength and at the lower energy Qβ band
(in parentheses). [c] Values were obtained according to the equation ∆ν1/2 = f/4.33×10–9·ε (F. L. Pilar, Elementary Quantum Chem.,
McGraw-Hill Book Comp., 1968) where f is the oscillator strength and ε is the maximum extinction coefficient (values in parentheses are
graphically measured[28]). [d] Slightly different values of λa are given in ref.[21] [e] An absorption band at 406 nm (logε = 4.65) was also
detected for 2, while 1 showed only a shoulder of the B band at higher energy.
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Figure 2. (a) Electronic absorption spectrum of 2-cyano-ZnTPP (2) in CHCl3 (8.10×10–6 ). (b) Electronic absorption spectrum of 2-
nitro-ZnTPP (4) in CHCl3 (6.36×10–6 ).
complexes 6 and 8.[27] However, when comparing porphyrin tylamino group) and their NiII complexes.[21] Electronic
9 with its ZnII complex 10, with the electron-withdrawing transitions from the a1u and a2u HOMO to the eg LUMO
nitro group further away from the β pyrrolic carbon atom, orbitals in porphyrins and their metal complexes cause B
the number of Q bands decreases from four to two (Table 1 and Q absorption bands whose wavelengths and oscillator
and Figure 4). The above observations would suggest a sig- strengths are defined by the interaction configuration be-
nificant electronic asymmetry of the porphyrin ring when tween the a1u and a2u orbitals and therefore by their relative
the nitro group is linked to the β pyrrolic position directly energies.[10] These latter are reported to be affected by sub-
or through an arylethenyl or an arylethynyl bridge. This stitution at the β pyrrolic position; the effect is more pro-
suggestion is also supported by the asymmetry of the B nounced for the a1u orbital, which has a non-vanishing elec-
Soret band, with evidence for a shoulder at lower energy tron density on both the α and β pyrrolic carbon atoms.[25]
(see Figure 3 and Figure 4) and by its bandwidth increase For porphyrin 7 and its NiII complex, with the nitro group
(Table 1).[26]
not directly linked to the β pyrrolic position, the pertur-
However, when an electron-donating dibutylamino group bation of the energy of the a1u orbital has been suggested
is linked to the β pyrrolic position through an arylethenyl to be modest.[21] On the contrary, the decrease of Q bands
linker, the Soret B band remains quite symmetric and its from four to three upon complexation of 7 with ZnII, to-
bandwidth does not increase significantly (Table 1). Inter- gether with the significant asymmetry of the B band and
estingly, in the spectra of 1 and 2 we also observed an ab- parallel increased bandwidth, would suggest a significant
sorption band (logε = 4.5) close to the Soret B band but at perturbation of the porphyrin π orbitals. Due to the π delo-
higher energy (Figure 2 and Table 1) that has not been re- calised nature of the substituent at the β pyrrolic position,
ported in previous investigations.[25]
this effect could not be originated only by an inductive per-
The modulation of the π valence orbital levels of porphy- turbation of the energy levels of the a1u and a2u frontier
rins and metalloporphyrins by substitution at the β pyrrolic orbitals. As already suggested for porphyrin 7,[21] this per-
position was investigated by a combination of cyclic vol- turbation could be mainly due to the conjugation of the π
tammetry and electronic absorption spectroscopy.[25] A sim- orbitals of the linker with the π orbitals of the pyrrolic ring,
ilar approach was later applied when comparing porphyrins which may induce some charge-transfer character to por-
7 and 11 (the latter with a dimethylamino instead of a dibu- phyrin πǞπ* transitions that are no longer totally centred
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Figure 3. (a) Electronic absorption spectrum of 2-(4-nitrophenyl)ethynyl-ZnTPP (6) in CHCl3 (6.25×10–6 ). (b) Electronic absorption
spectrum of (1E)-2-(4-nitrophenyl)ethenyl-ZnTPP (8) in CHCl3 (4.52×10–6 ).
Figure 4. (a) Electronic absorption spectrum of (1E,3E)-2-(4-nitrophenyl)buta-1,3-dienyl-H2TPP (9) in CHCl3 (8.58×10–6 ). (b) Elec-
tronic absorption spectrum of (1E,3E)-2-(4-nitrophenyl)buta-1,3-dienyl-ZnTPP (10) in CHCl3 (2.19×10–5 ).
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meso-Tetraphenylporphyrin ZnII Complexes
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on the porphyrin ring,[28] as supported by the increased tron-withdrawing groups, while the red shift is much smaller
bandwidth of the Soret B band.[26] In agreement with this (about 20–400 cm–1) with electron-donating groups.[25] We
hypothesis, the available structural data indicate, at least for have found a relevant blue shift of about 1500 cm–1
the arylethenyl linker, that the porphyrin core and the styryl (Table 2) with respect to ZnTPP for complexes 4, 6, 8 and
group are essentially coplanar,[21,29] a structural feature 10 carrying a nitro group directly or indirectly bound to the
which should promote a facile interaction of the π orbitals β pyrrolic carbon atom. The centre of gravity appears to be
of the linker with the porphyrin π orbitals. Such a nearly quite independent of the different nature and length of the
planar arrangement was confirmed as the best geometry, by π linker (Table 2). On the contrary, the centre of gravity of
an ab initio DFT method, for the ZnII complexes 8 and 12 complex 12, with a π linker carrying an electron-donating
and the related porphyrins 7 and 11 (see below).
dibutylamino substituent, shows a completely different
The effects of substituents upon the relative energies of trend with a limited red shift of about 300 cm–1 (Table 2).
the frontier orbitals a1u and a2u of metal porphyrin com- The above observations suggest not only the presence of a
plexes may be evaluated, in a first instance, by the Shelnutt perturbation, but also a completely opposite trend when
analysis,[28] which correlates the energies of the B and Qα passing from an electron-withdrawing to an electron-donat-
transitions with the ratio of the square of their transition ing π delocalised substituent.[25] In conclusion, we have pro-
dipole moments. Shelnutt suggests that the closer in energy duced a significant and complementary body of evidence
the a1u and a2u orbitals are, the smaller the ratio of the for a significant electronic perturbation of the π porphyrin
square of the transition dipole moments of the Qα and B system by β pyrrolic substitution with a π delocalised linker
transitions, respectively, and the difference in energy EB
EQα
–
carrying a push or pull group. In order to confirm the role
of π conjugation, which should introduce some charge-
.
The substitution at the β pyrrolic position in the ZnII transfer character to both the B and Q absorption bands,
porphyrin complexes investigated in this work produces a we carried out a solvatochromic investigation using a set of
substantial increase of both EB – EQα and the ratio of the solvents of increasing polarity, excluding those that show
square of the transition dipole moments of the Qα and B strong donor or hydrogen-bonding properties (see Experi-
transition, respectively, with respect to the parent complex mental Section), so that mainly the orientational effect of
ZnTPP (Table 2). Such an increase is more relevant when the solvent polarity (and not hydrogen bonding or strong
the π-delocalised substituent carries the strong donor dibu- basic ligation) dependency was studied. With this limitation
tylamino group. This evidence adds further support to a a poor solvatochromic behaviour of the B band of porphy-
significant perturbation of the energies of the a1u and a2u rin complexes was usually reported.[30,31] However, the sol-
orbitals, which shift further apart. However, the so-called vatochromic behaviour of the B band becomes significant
Shelnutt analysis[28] must be taken with some caution be- for a series of ZnII complexes of various isomers of H2TPP
cause different linkers may introduce different charge-trans- bearing two NO2 groups in different β pyrrolic positions.[32]
fer character to B or Qα transitions, or to both of them.
We have produced clear evidence for a significant solva-
In order to produce more evidence for a significant elec- tochromic behaviour of both B and Qβ (in porphyrins the
tronic perturbation, we also investigated the trends of a Q band is at higher energy) bands for porphyrins and their
parameter of the electronic absorption spectra of ZnII por- ZnII complexes investigated in this work when carrying a π
phyrin complexes that is independent of the extent of the delocalised linker (Table 3). With the exception of porphy-
configurational interaction of the a1u and a2u orbitals. This rin 9, the Soret B band is always shifted to higher energy
is the centre of gravity of the B and Qα bands, defined as (blue shift) with increasing polarity of the solvent when a
E = ½(EB + EQα), which is an indication of the relative nitro group is linked through a π spacer (Table 3).
perturbation of the energy levels of the a1u and a2u frontier
The strength of the charge-transfer character, deduced
orbitals.[25] A red shift of the centre of gravity with respect from the absolute value of ∆µeg (difference between excited
to CuTPP of about 350–700 cm–1 has been found for the and ground state dipole moments), which is negative for
substitution in the β pyrrolic position with a series of elec- blue-shifted bands, is relatively small for porphyrins 5, 7
Table 2. Some characteristics of the electronic spectra of some 2-substituted Zn(TPP) complexes in CHCl3.
Compound[a]
B band
Qα band
Center of gravity[b] EB – EQα
λmax [nm] [r2 10–34 [c]
]
λmax [nm] [r2 10–36 [c]
]
[cm–1]
[cm–1]
eg
eg
ZnTPP
2-Cyano-ZnTPP (2)
2-Nitro-ZnTPP (4)
2-(4-Nitrophenyl)ethynyl-ZnTPP (6)
(1E)-2-(4-Nitrophenyl)ethenyl-ZnTPP (8)
(1E,3E)-2-(4-Nitrophenyl)buta-1,3-dienyl-ZnTPP (10) 430 [1.15]
(1E)-2-[4-(Dibutylamino)phenyl]ethenyl-ZnTPP (12) 422 [0.895]
419 [1.22]
427 [n.d.]
425 [0.873]
431 [1.09]
432 [2.15]
584 [1.16]
593 [n.d.]
598 [5.73]
594 [3.67]
597 [4.96]
599 [3.95]
601 [5.37]
20494
21968
21956
21975
21859
21975
20167
5585.0
6555.8
6807.0
6366.8
6397.7
6561.3
7057.8
[a] H2TPP is 5,10,15,20-tetraphenylporphyrin. [b] The electronic centre of gravity is defined as (EB + EQα)/2 according to ref.[25] [c] The
transition dipole moment reg is related to the oscillator strength f by the following equation: r2 = 2.13×10–30 f/νeg, where νeg is the
eg
absorption in cm–1 according to ref.[49b]
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Table 3. Solvatochromic behaviour of the Soret B band and of the Qβ band at higher energy.
Soret B
Qβ
Compound[a]
∆µeg [D]
f
∆µeg [D]
f
ZnTPP
1.36
0.069
0.089
0.068
0.061
0.081
0.063
[b]
[b]
2-Cyano-ZnTPP (2)
–0.09
1.5
–0.6
2.32
–4.3
2-Nitro-H2TPP (3)
0.7
2.19
0.96
1.81
1.19
1.59
2.2
1.61
1.26
1.66
0.99
[b]
2-Nitro-ZnTPP (4)
2-(4-Nitrophenyl)ethynyl-H2TPP (5)
2-(4-Nitrophenyl)ethynyl-ZnTPP (6)
(1E)-2-(4-Nitrophenyl)ethenyl-H2TPP (7)
(1E)-2-(4-Nitrophenyl)ethenyl-ZnTPP (8)
(1E,3E)-2-(4-Nitrophenyl)buta-1,3-dienyl-H2TPP (9)
(1E,3E)-2-(4-Nitrophenyl)buta-1,3-dienyl-ZnTPP (10)
(1E)-2-[4-(Dibutylamino)phenyl]ethenyl-H2TPP (11)
(1E)-2-[4-(Dibutylamino)phenyl]ethenyl-ZnTPP (12)
–1.3
–5.7
–2.2
–8.6
7.1
–19.6
2.86
–16.8
2.6
0.082
[b]
[b]
9.3
–5.6
11.1
–19.2
0.085
0.062
0.22
0.060
[a] H2TPP is 5,10,15,20-tetraphenylporphyrin. [b] Not significantly solvatochromic.
and 9 but becomes more relevant for their ZnII complexes
In conclusion, our detailed investigation of the electronic
6 and 8, with an abrupt increase for complex 10 (Table 3). spectra and their solvatochromic behaviour has produced
The Qβ band of porphyrins 5, 7 and 9, which is only slightly strong evidence for a significant perturbation of the π por-
shifted to lower energy (red shift) with increasing polarity phyrin ring by β pyrrolic substitution with a π-delocalised
of the solvent, becomes blue shifted in the ZnII complexes linker carrying a donor or acceptor group. This pertur-
6 and 10 or remains unchanged in 8 (Table 3). As already bation involves the conjugation of both the π orbital sys-
reported,[32] the solvatochromic behaviour becomes insig- tems, as confirmed by the solvatochromic behaviour of both
nificant when the nitro group is directly linked to the β B and Qβ bands, but the significant excitation processes due
pyrrolic position, suggesting that the solvatochromism must to conjugation are different if the π-delocalised linker car-
be ascribed to a charge-transfer process originated by a sig- ries a push or a pull group.
nificant perturbation of the π frontier orbitals of the por-
phyrin ring through conjugation with π delocalised substit-
uents (Table 3). Porphyrin 11, with an arylethenyl linker
Voltammetric Investigation
carrying a dibutylamino group, and its ZnII complex 12
show an unexpectedly significant solvatochromism, blue
The anodic oxidation of porphyrins and their ZnII com-
shifted for 12 and red shifted for 11, of both B and Qβ plexes investigated in this work takes place by two reversible
bands. In this latter case it is difficult, however, to suggest one-electron steps.[34,35] For porphyrin 11 and its ZnII com-
that this solvatochromic behaviour may be due to a charge- plex 12, which carry a dibutylamino group, in addition to
transfer process involving conjugation of the π-delocalised the two main peaks we noticed a small oxidation prewave
linker and the porphyrin ring, since their B bands are sym- located before the second (for 11) and the first (for 12) main
metric with no increase of the bandwidth.[26] In this latter oxidation peak (Table 4). Porphyrins 5, 7 and 11, with a
case the perturbation of the π system of the porphyrin, different para substituent on the phenyl ring or a different
which influences the polarity of both the ground and ex- π system connecting the phenyl group to the porphyrin ring,
cited states, probably involves the strongly donor dibu- show the following oxidisability sequence for the first oxi-
tylamino group.
dation steps: 11 ϾϾ 7 Ͼ 5 (Table 4).
All porphyrins and their ZnII complexes investigated in
A significantly lower oxidation potential for the first oxi-
this work show a neat fluorescent emission band when irra- dation step has already been reported for a porphyrin struc-
diated either at the Soret B band or at the Qβ band. The turally related to 11 that carries a dimethylamino instead of
intensity of the emission for both porphyrins and their ZnII a dibutylamino group.[21] For this latter porphyrin the sec-
complexes is more relevant when the Soret B band is irradi- ond oxidation step was reported not to be reversible, while
ated. Porphyrin 3 and its ZnII complex 4, with the nitro we found that for 11 it is reversible and not at significantly
group directly bound to the β pyrrolic position, show, as lower oxidation potential with respect to porphyrins 5 and
already reported,[33] only one diffuse emission band 7 (Table 4). As a consequence, for porphyrin 11 it is better
(Table 1). The same behaviour is shown by the ZnII complex to take into account, when comparing its oxidisability with
12, with a dibutylamino group linked to the β pyrrolic posi- that of other porphyrins, the second oxidation step, which
tion through an arylethenyl linker. The wavelength of the seems to be centred, as for other porphyrins, on the HOMO
emission bands of both porphyrins and their ZnII com- energies of the porphyrin π core. In fact, as already sug-
plexes is affected by the length of the π-delocalised linker, gested,[21] the first step of oxidation of 11 could be centred
while it is not too affected by the substituents (Table 1). As off the HOMO energies of the porphyrin π core, probably
with the absorption B and Qβ bands, the emission bands at the dibutylamino functionality. The higher oxidisability
are also solvatochromic.
of 11 and the slightly lower oxidisability of 5 with respect
3864
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meso-Tetraphenylporphyrin ZnII Complexes
FULL PAPER
Table 4. Oxidation and reduction peak potentials Ep (V vs. SCE) for three porphyrins and their Zn complexes (glassy carbon electrode,
potential scan rate: 0.2 Vs–1).
Cathodic peaks[a,b]
Anodic peaks[a,b]
2-(4-Nitrophenyl)ethynyl-H2TPP (5)
–1.53
–1.170 (–1.055)
–1.180 (–0.955)
–1.035
–1.015
–1.110
1.105
1.075
0.655
0.862
0.845
(0.390) 0.625
1.255
1.225
(0.935) 1.010
1.155
1.108
(1E)-2-(4-Nitrophenyl)ethenyl-H2TPP (7)
(1E)-2-[4-(Dibutylamino)phenyl]ethenyl-H2TPP (11)
2-(4-Nitrophenyl)ethynyl-ZnTPP (6)
–1.55
(1E)-2-(4-Nitrophenyl)ethenyl-ZnTPP (8)
(1E)-2-[4-(Dibutylamino)phenyl]ethenyl-ZnTPP (12)
–1.51
–1.68
–1.28 (–1.0)
0.790
[a] Reversibile monoelectronic peaks are italicised. [b] Prewaves are in parentheses.
to 7 (Table 4) can be interpreted as an increased stabilisa- a nitro group. Complexation of porphyrins to ZnII results
tion due to a significant perturbation, produced by the sub- in a significant anticipation, with respect to the parent por-
stituents carrying an electron-withdrawing nitro group, on phyrin, of both the first and second oxidation potentials,
the energy levels of the HOMO electrons, which stabilizes according to a relevant increase of the negative charge on
the π core of the porphyrin ring.
the porphyrin ring (Table 4). However, complexation to NiII
This effect is in agreement with a conjugation between of porphyrin 7 results in only a slight difference of the oxi-
the π system of the linker and the π orbitals of the porphy- dation potentials with respect to those of the parent por-
rin ring as proposed above. The slightly higher oxidisability phyrin (e.g. +0.03 V for the first oxidation peak, 0.00 V for
of 7 with respect to 5 can be interpreted in terms of the the second one),[21] in agreement with the suggestion of
higher efficiency of the triple bond of the linker in transmit- Kadish et al.[35] of a higher induction parameter of NiII
ting the perturbation of the electron-withdrawing nitro (1.8) with respect to ZnII (1.5), resulting in a negative
group to the porphyrin π core.
charge on the porphyrin ring that is much less significant
When a porphyrin ring is complexed to a metal like ZnII, for the NiII complexes. Finally, although the detection of
irrespective of the working medium and of the porphyrin three or four one-electron reduction waves is usually re-
structure, a fairly constant difference in the range from ported for porphyrins and their metal complexes when
–0.31 to –0.16 V between the first oxidation potential of working with an acceptably broad cathodic window,[37,38] in
the ZnII complex and that of its parent porphyrin has been the present work, due to the less broad window when work-
reported.[35,36] This range of negative values is shown by ing in CH2Cl2 as solvent, we observed only the first two
two of the three couples investigated in this work: –0.24 V reduction peaks and only the first one for 6 and 7. All the
for 6 vs. 5 and –0.23 V for 8 vs. 7 (Table 4). When compar- potentials but one (namely those of porphyrin 11) fall
ing 12 and 11, the difference turns out to be –0.27 V only within the expected range of 0.42 0.03 V of difference be-
if taking into account the oxidation prewave of the ZnII tween the first and the second reduction potential, namely
complex, while when comparing the first “regular” peak the 0.42 V for 5 (averaging between the first reduction peak and
difference (–0.03 V) appears to be anomalously low. How- its shoulder), 0.40 V for 8, 0.52 V for 11 and 0.40 V for 12.
ever, when taking into account the second oxidation peak, The first reduction step appears to be clearly monoelec-
as suggested above, the expected difference of –0.22 V is tronic and reversible only for ZnII complexes 6 and 8.
found. This is additional evidence that the first oxidation
As a conclusion, the voltammetric investigation has con-
peak in porphyrin 11 does not involve the HOMO electrons firmed a perturbation of the HOMO levels of the porphyrin
of the π porphyrin core. The difference ∆E(ox–red) between π core due to conjugation with the π orbitals of the linker
the first oxidation and the first reduction process of porphy- bound to the β pyrrolic position. This perturbation is more
rins and their ZnII complexes has been reported to be al- significant if the π linker carries a nitro group and if the
ways within 2.25 0.15 V (Kadish’s relationship), as ex- ZnII ion is complexed to the porphyrin ring. When the π
pected for a strictly ring-based process of oxidation and re- linker carries a donor dibutylamino group the perturbation
duction.[35] For porphyrins 5 and 7, the ∆E(ox–red) values of the π levels of the porphyrin ring is not only lower, but
obey Kadish’s relationship (2.16 V and 2.26 V respectively), the HOMO levels are centred on the dibutylamino group.
while again for porphyrin 11 this difference is only 1.69 V,
if we consider the first “regular” reduction peak. It becomes
2.05 V, as expected, when the second reduction peak is
Dipole Moments
taken into account. For ZnII complexes 6, 8 and 12 the
values of ∆E(ox–red) deviate from Kadish’s relationship
Although the experimental determination of the dipole
(1.88 V for 6, 1.96 V for 8 and 1.91 V for 12 when taking moment by the Guggenheim method[39] was not reproduc-
into account the first oxidation peak; the value becomes ible for the push-pull porphyrins and their metal complexes
2.07 V for 12 if we consider the second oxidation peak). It investigated previously, such that it was always calculated
thus appears that the π levels of the ZnII porphyrins investi- by semi-empirical methods,[5,6] we obtained quite reproduc-
gated in this work are quite affected by the perturbation ible experimental measurements, with the exception of por-
induced by the π system of the linker mainly when it carries phyrin 11 and its ZnII complex 12 (see Exp. Sect.). Porphy-
Eur. J. Inorg. Chem. 2005, 3857–3874
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M. Pizzotti et al.
FULL PAPER
rins 5, 7 and 9, which carry a linker with a nitro group, geometries and dipole moments were calculated theoretic-
show a dipole moment comparable to that of their ZnII ally by an ab initio approach based on the density func-
complexes 6, 8 and 10. The nature of the linker, either phen- tional theory (DFT)[15,16] using an extended basis set[17] (see
ylethynyl, phenylethenyl or even phenylbutan-1,3-dienyl, Experimental Section for details). The calculated optimised
does not induce a significant effect on the order of magni- geometries always support a quite planar arrangement of
tude of the experimental dipole moment (Table 5). The best the π porphyrin ring and the π system of the organic linker.
Table 5. Experimental and theoretical dipole moments, EFISH βλ in CHCl3 solution (1.907 µm incident wavelength) and β0.
Compound[a]
µexp (µtheor
[D]
)
β1.907
β0
µβ0
[b]
[c]
[10–30 esu]
[10–30 esu]
[10–48 esu]
2-Cyano-ZnTPP (2)
6.4 (5.43)
9.6[d]
7.30
5.17
10.2
22.7
11.3
19.7
22.4
32.6
32.6
57.9
97.5
46.7
23.3
65.2
138.5
80.2
196.0
157
202
231
308.6
503.1
2-Nitro-H2TPP (3)
4.5 (5.40)[e]
6.4 (5.42)
6.1 (8.55)[e]
7.1 (8.82)
6.8[d]
2-Nitro-ZnTPP (4)
13.4[d]
30.1[f]
20.4[f]
39.3[f]
29.7[f]
42.8[f]
43.1[f]
75.7[f][g]
2-(4-Nitrophenyl)ethynyl-H2TPP (5)
2-(4-Nitrophenyl)ethynyl-ZnTPP (6)
(1E)-2-(4-Nitrophenyl)ethenyl-H2TPP (7)
(1E)-2-(4-Nitrophenyl)ethenyl-ZnTPP (8)
(1E,3E)-2-(4-Nitrophenyl)buta-1,3-dienyl-H2TPP (9)
(1E,3E)-2-(4-Nitrophenyl)buta-1,3-dienyl-ZnTPP (10)
(1E)-2-[4-(Dibutylamino)phenyl]ethenyl-H2TPP (11)
(1E)-2-[4-(Dibutylamino)phenyl]ethenyl-ZnTPP (12)
[e]
6.6 (8.31)
7.0 (8.69)
6.2 (9.30)[e]
7.1 (9.46)
(5.33)[e][h]
(5.16)[h]
[f][g]
127.5
[a] H2TPP is 5,10,15,20-tetraphenylporphyrin. [b] Obtained from Equation (2) (see text) omitting the cubic contribution γ(–2ω; ω, ω, 0).
[c] Calculated from the two levels model assuming only the B band as the main charge-transfer process [Equation (2)]. [d] The error is
20–30%. [e] The theoretical value is the semi-sum of the theoretical dipole moments of two isomeric structures. [f] The error is 10–
15%. [g] Using theoretical dipole moment. [h] Theoretical value calculated by an ab initio approach.
1
Table 6. A comparison of the dipole moments and H NMR spectroscopic data of a series of (E)-arylethenyl structures.
[a] In CHCl3 or CDCl3 at 25 °C. [b] In benzene at 25 °C. [c] Calculated by an ab initio approach based on DFT Theory (see Exp. Sect.
and Table 5). [d] The stilbene substituent is linked to the β pyrrolic position of the porphyrin ring.
3866
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Eur. J. Inorg. Chem. 2005, 3857–3874

meso-Tetraphenylporphyrin ZnII Complexes
FULL PAPER
The calculated dipole moments of the two isomeric struc- ethenyl or an arylbutadienyl spacer with a nitro group in
tures of asymmetric porphyrins 3, 5, 7, 9 and 11 are quite the para position, the electronic contribution to γEFISH was
similar, thus the data reported in Table 5 are mean values. evaluated to be much smaller than the dipolar orientational
They are in good agreement with the experimental ones one.[41] In accordance, we neglected the electronic contri-
with the exception of the couple 9 and 10. For porphyrin bution during the evaluation of EFISH β1.907 from γEFISH
.
11 and its ZnII complex 12 the calculated dipole moments
Our push-pull porphyrinic chromophores are character-
are, as expected, rather low and with inversion of polarity ised by a dipole moment axis directed quite parallel to the
(Table 5). In this latter case, complexation to ZnII induces π linker axis, as confirmed by our DFT calculations on the
a slight lowering of the dipole moment with respect to the dipole moments and best geometries. It follows that a
free porphyrin, in agreement with an increased negative charge-transfer process involving the π linker, which may
charge on the porphyrin ring acting as acceptor. As ex- control the second-order NLO response, is probably located
pected, the opposite effect is calculated and observed exper- quite parallel to this axis, as in traditional two-dimensional,
imentally for the couples 5 and 6, 7 and 8, and 9 and 10 pseudo-linear, organometallic push-pull chromophores.[42]
(Table 5). The order of magnitude of the dipole moments If so, the component of βvec along the dipole moment axis,
of porphyrins 5, 7 and 9 and their ZnII complexes 6, 8 and that is EFISH β1.907 (see Experimental Section), should be
10, with a linker carrying a nitro group, supports a signifi- quite coincident with βvec, which is the vectorial part of the
cant electronic asymmetry.
quadratic hyperpolarizability tensor. EFISH β1.907 turns out
In conclusion, according to theoretical calculations the to be small for porphyrin 3 and for the ZnII complexes 2
substitution of the electron-withdrawing nitro group by the and 4, with the electron-withdrawing group linked directly
strongly electron-donating dibutylamino group introduces to the β pyrrolic position (Table 5). The order of magnitude
a lower electronic asymmetry with a completely opposite of the quadratic hyperpolarizabilities of 3 and 4 is in agree-
polarity. Therefore, we have further evidence that, already ment with that of a structurally related porphyrin and its
in the ground state, the conjugation of the π systems pro- ZnII complex but carrying completely fluorinated aromatic
duces a completely opposite electronic effect: the porphyrin rings in meso positions, although in this latter case the qua-
ring acts as an electron donor if the linker carries a nitro dratic hyperpolarizability was measured with the HRS tech-
group, while it acts as an electron acceptor if the linker car- nique (Hyper Raleigh Scattering) working with a resonant
ries a dibutylamino group. This ambivalent picture of the incident wavelength of 1.064 µm.[12b] In this latter case, on
ground-state electronic density of the porphyrins and their the basis of solvatochromic evidence the major contribution
ZnII complexes investigated in this work can be better mea- to the second-order NLO response was attributed to a
sured when comparing their dipole moments and 1H NMR charge-transfer process from the porphyrin ring (as donor)
chemical shifts with those of a series of (E)-stilbene struc- to the nitro group linked to the β pyrrolic carbon atom (as
tures (Table 6).
acceptor) along the dipole moment axis, as assumed
If compared to the benzene ring, the porphyrin ring, as above.[43] The very low second-order NLO response of
such or as a ZnII complex, behaves as a much better ac- chromophores 2–4 is in accordance with the assumption
ceptor and a slightly better donor on the basis of both di- that acceptor groups appended to an electron-rich centre,
pole moments and chemical shifts of the protons of the (E)- like the β position of the porphyrin ring, should not pro-
double bond when linked through the β pyrrolic position. duce large NLO responses.[13,14] The values of EFISH β1.907
It shows, however, lower donor properties than a benzene increase upon introduction of a π-delocalised linker be-
ring carrying a dimethylamino group in the para position. tween the β pyrrolic position of the porphyrin ring and the
Therefore, already in the ground state, a porphyrin ring sub- electron-withdrawing nitro group (Table 5). As in structur-
stituted in the β pyrrolic position with a push or pull aryl ally related pseudo-linear, organometallic, push-pull chro-
ethenyl system shows a relevant ambivalent role as donor mophores[42] and in structurally related push-pull phthalo-
or acceptor.
cyanines,[41] EFISH β1.907 increases slightly upon increasing
the length of the linker or by substitution of a triple bond
with an (E)-double bond. We have also shown that EFISH
β1.907 always decreases slightly on going from the free por-
phyrin to its ZnII complex, with the exception of the couple
Determination of the Quadratic Hyperpolarizability by the
EFISH Technique
The EFISH technique[40] allows the determination of the 9 and 10, which show comparable values (Table 5).
second-order NLO response, γEFISH, of a solute. In order
This limited effect of coordination to ZnII is in agreement
to avoid resonance enhancement, we worked with an off- with the assumption that the second-order NLO response
resonance incident wavelength of 1.907 µm (see Experimen- is controlled by a charge-transfer process, favoured by con-
tal Section).
jugation, from the occupied π levels of the pyrrolic ring,
In highly π conjugated two dimensional molecules, such acting as a push system, to the π* antibonding orbitals of
as porphyrins and phthalocyanines, the electronic contri- the linker. In fact, this latter process should be scarcely af-
bution to γEFISH in Equation (3) (see Experimental Section) fected by coordination of the porphyrin to ZnII, which
cannot, at first instance, be ignored.[5c,41] However, for mainly influences the energy of the a2u porphyrin orbital,
asymmetrically substituted phthalocyanines, structurally re- with nodes at the metal centre and little electron density on
lated to our porphyrinic chromophores, carrying an aryl- the exo pyrrole positions.[10,25]
Eur. J. Inorg. Chem. 2005, 3857–3874
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M. Pizzotti et al.
FULL PAPER
The positive value of the second-order NLO response of to Equation (1), assuming that the major charge-transfer
porphyrins with a nitro group linked to the β pyrrolic posi- process affecting the second-order NLO response is under
tion by a π linker (5, 7, 9 and 6, 8, 10) also supports the role the Soret B band.
of such a charge-transfer process, which is characterised by
the same polarity of the dipole moment.[42]
Interestingly, the values of EFISH β1.907 of our porphy-
rinic chromophores 5–10 are comparable to that of struc-
turally related chromophores based on the phthalocyanine
ring.[41]
The dominant role of the Soret B band is supported by
A larger and still positive second-order NLO response,
its significant charge-transfer character in all our porphy-
confirmed by a series of experimental determinations in two
rinic chromophores, as evidenced by the solvatochromic in-
different laboratories (see Experimental Section), was found
vestigation (Table 3) and by its high intensity. Any contri-
by substitution of the nitro group with the strongly electron
bution due to the Q bands, in particular the Qβ band, which
donating dibutylamino group as in porphyrin 11 and its
is not only slightly solvatochromic, but also of much lower
intensity, should be much less relevant.
ZnII complex 12. The second-order NLO response increases
by complexation to ZnII (Table 5). This response of both 11
Obviously, the application of the two-levels model to an
and 12 cannot be due to a charge-transfer process, as above,
extended two-dimensional π system, although already ap-
plied to porphyrins[6a,6b] and phthalocyanines,[41] is quite
debatable. Therefore, the β0 values calculated by this ap-
proach should be considered only for their order of magni-
tude.
from the occupied π orbitals of the pyrrolic ring to the π*
antibonding orbitals of the π linker, since this process
should produce a negative second-order NLO response as
its polarity is opposite to that of the calculated dipole mo-
ment.[42] It is known that in push-pull NLO molecules the
nature of the π conjugation can alter the electron donating
ability of push substituents.[13] Thus, we can tentatively pro-
pose that the high EFISH β1.907 values of both 11 and 12
are due to a relevant increase of the electronic effects of the
Conclusions
In this work we have produced, for the first time, clear
donor properties of the dibutylamino group induced by an evidence for a significant perturbation of the energy of the
extended π system involving the porphyrin ring, possessing π electron core of the porphyrin ring due to conjugation
π segments that are either electron rich or electron poor.[14] between the porphyrinic π system, when substituted at the
The conjugation process of the overall π system of the β pyrrolic position with a π delocalised linker carrying a
linker with the extended π system of the porphyrin ring nitro group, and the π system of the linker itself. The asym-
should stabilize the π* antibonding levels, thus producing a metric shape and increased bandwidth of the B band, the
shift of the nǞπ* transition, which is the main origin of Shelnutt analysis,[28] the trend of the centre of gravity of the
the second order NLO response, to much lower energy. In B and Qα bands and the sensitivity of the first two voltam-
accordance with this hypothesis, this latter transition, which metric oxidation steps, which involve the HOMO orbitals,
is usually located around 370–390 nm in para-dimethylami- with respect to the nature of the π linker, all support this
nostilbazoles,[42] is absent in the electronic spectra of both perturbation, where the porphyrin ring, because of the elec-
11 and 12, thus supporting a relevant red shift, probably tron-richness of the β position,[14] acts as an electron donor.
under the large B band or even under the Q bands at much This effect seems to be less relevant when the π linker car-
lower energy. In any case, our experimental observations are ries a dibutylamino group. In this latter case the HOMO
in agreement with the suggestion, based on a theoretical orbitals are centred on this group. Therefore, we have pro-
approach, of Marks, Ratner et al.[14] that the linkage to the duced clear evidence that, already in the ground state, the
electron-rich β pyrrolic position of a π system carrying a porphyrin ring shows an ambivalent character, acting also
strong electron donating dibutylamino group may be the as an acceptor, as supported, for instance, by the value and
origin of some depletion of electron density on the donor polarity of the dipole moments of porphyrin 11 and its ZnII
and thereby of the reduction of the ground-state polarisa- complex 12, both of which have a linker carrying the
tion with an increase of the effective strength of the donor strongly electron-donating dibutylamino group.
substituent and thereby of the second-order NLO re-
In the electron excitation process, the significant conjuga-
sponse.[14] The significant increase of EFISH β1.907 by coor- tion of the π levels of both the porphyrin ring and linker
dination of 11 to ZnII is in agreement with this hypothesis facilitates a charge transfer from the occupied π levels of
because complexation introduces a significant increase of the porphyrin π core, acting as a push group, to the anti-
the electron density on the porphyrinic ring, as confirmed bonding π* orbitals of the linker carrying the pull nitro
by our voltammetric investigation. The significant second- group, thus producing positive values of EFISH β1.907 of
order NLO response cannot be attributed to a aggregation the chromophores either as porphyrins (5, 7 and 9) or as
process, which was discarded by a careful spectroscopic in- their ZnII complexes (6, 8 and 10) (Table 5). Evidence for
vestigation carried out over a large range of concentrations. a charge-transfer process from the porphyrin ring is also
Finally, the static quadratic hyperpolarizability EFISH, provided by the significant solvatochromic behaviour of
β0, was calculated with the two-levels model,[44] according both B and Qβ bands and by the asymmetric shape and
3868
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meso-Tetraphenylporphyrin ZnII Complexes
FULL PAPER
increased bandwidth of the B band of chromophores 5–10.
Therefore, meso-tetraphenylporphyrins and their ZnII
In order to evaluate the unknown push properties of the complexes, carrying in the β pyrrolic position a π-delocal-
β position of the porphyrin ring we can compare, for in- ised linker with an electron-withdrawing nitro group, be-
stance, the quadratic hyperpolarizability EFISH β1.907 of have, in a push-pull chromophore, as a push group similar
the push-pull chromophore 8 with that of structurally re- to the ferrocenyl group.
lated push-pull chromophores having an organic aryl moi-
The ambivalent character of the porphyrin ring in the
ety or an organometallic system like ferrocene as push porphyrins and their ZnII complexes investigated in this
groups (Figure 5). The EFISH β1.907 of 8 is comparable to work is confirmed also by properties involving excited
that of the chromophore carrying a ferrocenyl push states, like the second-order NLO response. The lack of
group[45] but much smaller than the organic chromophore bandwidth increase of the B band, its symmetry and the
with para-dimethylamino as the push group.[46] A similar lower perturbation of the centre of gravity of the B and Qα
trend is observed for the porphyrinic chromophore having bands for both 11 and 12 do not produce evidence for an
a butadienyl linker like the ZnII complex 10, which shows a excitation process involving a significant charge-transfer
value of EFISH β1.907 comparable to that of the structurally from the π porphyrin core to the π linker, as occurs in the
related chromophore carrying a push ferrocenyl group,[45] series of chromophores 5–10. On the contrary, the signifi-
but much smaller than that of the organic chromophore cant positive EFISH β1.907 values of 11 and 12 would sug-
with the para-dimethylamino push group.[46] Not very dif- gest an effective screening of the ground-state polarisation
ferent is the trend with respect to the structurally related induced by the electron-rich β pyrrolic position of the por-
organic chromophore when the linker is acetylenic (Fig- phyrin ring, which behaves as a kind of acceptor and in-
ure 5).
creases, in the excitation process, the donor properties of
the dimethylamino push group.[14]
Experimental Section
All the commercially available solvents and chemicals were of rea-
gent-grade quality and, unless otherwise stated, were used without
further purification. Porphyrin 3,[20] its ZnII complex 4[20] and 4-
nitrophenylacetylene[47] were prepared according to literature meth-
ods. 1H NMR spectra were recorded on a Bruker AC-300 Spec-
trometer in CDCl3 as solvent. Electronic spectra were obtained in
CHCl3 with a Jasco V-530 Spectrometer. Emission spectra were
obtained in CHCl3 with a Jasco FP-777 spectrofluorimeter. FAB-
MS measurements were performed with an analytical VG 7070 EQ
instrument. Elemental analyses were carried out in the Analytical
Laboratories of the Department of Inorganic, Metallorganic and
Analytical Chemistry of Milan University. Dipole moments were
determined in CHCl3 solution with a WTW-DM01 dipolmeter (di-
electric constant) coupled with a Pulfrich Zeiss PR2 refractometer
(refractive index) according to the Guggenheim method.[39]
Cyclic Voltammetry: Cyclovoltammetric investigations (CV) were
carried out on carefully deaerated solutions with a station including
an AMEL 2049 potentiostat/galvanostat, an AMEL 568 function
generator and a Lynseis LY16100-II x/y recorder. The working
solutions were made up with HPLC-grade CH2Cl2 and tetrabu-
tylammonium perchlorate (TBAP; Ͼ99% Fluka) at concentrations
ranging from 3×10–4  to 7×10–4 . Data were obtained at dif-
ferent potential scan rates (ranging from 0.02 to 0.5 Vs–1) working
with two different electrodes: (a) an AMEL glassy carbon disk elec-
trode (GC, 2 mm diameter), and (b) a platinum wire electrode (Pt,
1 cm length, 0.05 cm diameter), using a platinum counter-electrode
and a saturated calomel electrode (SCE) as reference. The nearly
identical voltammograms indicate that the electrode materials have
no catalytic effect on the oxidation process.
The electrochemical reversibility and electron number of each well-
defined peak were checked by classical tests,[48] including analysis
of the Ip vs. v1/2 characteristics, the Ep vs. logv characteristics, the
(Ep – Ep/2) vs. logv characteristics and the “stationary”, step-like
waves obtained by convolutive analysis of the original CV charac-
teristics.
Figure 5. Quadratic hyperpolarizability (β1.907) of some organic and
organometallic push-pull systems measured in CHCl3 by the EF-
ISH technique at an incident wavelength of 1.907 µm.
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Solvatochromic Investigation: The solvatochromic behaviour of the
Soret B and Qβ bands was investigated by using a set of polar, non-
protic and only weakly donor solvents (CH3CN, acetone, dichloro-
ethane, CH2Cl2, THF, ethyl acetate, CHCl3, toluene, CCl4, cyclo-
hexane, n-hexane). The values of ∆µeg, the difference between the
dipole moments of the excited and of the ground state, were ob-
tained from the Mc Rae equation (2)[49] evaluating the radius of
the cavity, a, occupied by the solute in the solvent empirically from
the molecular weight:
phenylbutadienyl structure. A loss of planarity of the π linker and
therefore a decrease of the π conjugation is usually the final result.
The final geometry of all the porphyrinic systems investigated,
either as the free-base porphyrin or its ZnII complex, shows a good
planarity of the π linker with the π system of the porphyrinic moi-
ety. The values of the dipole moments moduli and components
were finally computed using the Turbomole suite of programs[50] in
connection with the resolution of the identity (RI) approxi-
mation.[51]
Synthesis of Porphyrins and Their ZnII Complexes
2-Bromo-5,10,15,20-tetraphenylporphyrinatonickel(II):
2-Bromo-
5,10,15,20-tetraphenylporphyrin (205 mg, 0.296 mmol) was dis-
solved in slightly warmed 1,2-dichloroethane (30 mL) whilst stir-
ring, then Ni(acac)2 (84 mg, 0.326 mmol) was added. After 45 min-
utes the complete disappearance of the starting porphyrin was evi-
denced by TLC (n-hexane/toluene, 7:3) and by the change of the
colour of the reaction mixture from brown to red and finally to
orange. After removal of the solvent in vacuo, the crude product
was crystallised with CH2Cl2/pentane and washed with pentane to
afford the complex in quantitative yield.
g
where νa and νa are the frequencies of the absorption maximum
in a given solvent and in the gas phase, respectively, µ is the dipole
moment, a is the radius of the cavity (cm), and ε and n are the
solvent dielectric constant and refractive index, respectively. Since
in the determination of ∆µeg of both B and Qβ bands we were not
interested in their absolute value, but only in their order of magni-
tude and sign, the use of the molecular weight for a rather approxi-
mate assignment of the value of a (cavity radius) to rather flat
molecules, such those studied in this work, may be considered ac-
ceptable.
2-Cyano-5,10,15,20-tetraphenylporphyrin (1): This porphyrin was
prepared by demetallation of its NiII complex, synthesised accord-
ing to a literature procedure.[18] 2-Cyano-5,10,15,20-tetraphen-
ylporphyrinatonickel() (50 mg) was dissolved in 95% H2SO4
(2 mL) with magnetic stirring. The green solution was slowly added
to a saturated solution of (NH4)2CO3 and the dark brown suspen-
sion was filtered. The mother liquor was extracted with CH2Cl2
(2×60 mL), and the organic phase was washed with H2O, dried
with Na2SO4 and the solvents evaporated to dryness. The residue
was purified by column chromatography (silica gel, CH2Cl2/n-hex-
ane, 1:1) to afford 43 mg (94%) of pure 1 as a dark brown powder.
1H NMR (300 MHz, CDCl3): δ = –2.80 (br. s, 2 H, NH), 7.71–7.81
(m, 12 H, Hm,pPh), 8.16–8.19 (m, 8 H, HoPh), 8.74 (s, 2 H, Hpyrrolic),
8.90–8.98 (m, 4 H, Hpyrrolic), 9.36 (s, 1 H, Hpyrrolic) ppm. UV/Vis
(CHCl3): λmax (logε) = 420 (5.29), 517 (3.79), 552 (3.21), 605 (3.53),
593 (3.23), 648 (3.20) nm. C45H29N5 (639): calcd. C 84.48, H 4.57,
N 10.95; found C 84.40, H 4.73, N 10.94. MS-FAB(+): m/z = 640
[M + 1]+.
EFISH Measurements: EFISH measurements were performed in
our department in CHCl3 solutions, as a function of concentration,
working at an incident wavelength of 1.907 µm using a Q-switched
Nd:YAG laser with a 60- and 20-ns pulse duration, manufactured
by Atalaser. The 1.907 µm fundamental wavelength was obtained
by Raman shifting of the 1.064 µm emission of the Q-switched
Nd:YAG laser in a high pressure hydrogen cell (60 bar). A liquid
cell with thick windows in the wedge configuration was used to
obtain the Maker fringe pattern (harmonic intensity variation as a
function of liquid cell translation).[40] In the EFISH experiments
the incident beam was synchronised with a DC field applied in
order to break the centrosymmetry of the solution. The apparatus
for the EFISH measurements was a prototype made by SOPRA
(France).
From the concentration dependence (10–3–10–4 ) of the harmonic
signal with respect to that of the pure solvent, the NLO responses
EFISH βλ, were determined from the calculated γEFISH, according
to Equation (3):
2-Nitro-5,10,15,20-tetraphenylporphyrin (3): A solution of the ZnII
complex 4 (68.3 mg, 0.094 mmol) in 50 mL of CH2Cl2 was stirred
at room temperature with 1.5 mL of concentrated HCl for 5 min,
then the pH was raised to 7–8 by addition of NEt3. The brown
solution was transferred into a separating funnel and washed with
H2O, the organic phase was separated, dried with Na2SO4, evapo-
rated and the residue purified by column chromatography (silica
gel, toluene) to afford pure 3 (quantitative yield) as a violet powder.
1H NMR (300 MHz, CDCl3): δ = –2.58 (br. s, 2 H, NH), 7.71–7.83
(m, 12 H, Hm,pPh), 8.20–8.29 (m, 8 H, HoPh), 8.91–9.05 (m, 6 H,
where γEFISH is the sum of a cubic electronic contribution γ(–2ω;
ω, ω, 0) and of a quadratic orientational contribution µβλ (–2ω; ω,
ω)/5kT, where µ is the dipole moment and EFISH βλ is the projec-
tion along the dipole moment direction of the vectorial component
βvec of the tensorial quadratic hyperpolarizability, β, at the incident
wavelength λ.
Hpyrrolic), 9.07 (s, 1 H, Hpyrrolic) ppm. UV/Vis (CHCl3): λmax (logε)
= 427 (5.24), 527 (4.14), 560 (3.56), 605 (3.53), 665 (3.88) nm.
C44H29N5O2 (659): calcd. C 80.10, H 4.43, N 10.62; found C 80.40,
H 4.42, N 10.60. MS-FAB(+): m/z = 660 [M + 1]+.
Computational Methods: The optimised geometry of porphyrins as
two geometric isomers and of their ZnII complexes was determined
at an ab initio level using the DFT (Density Functional Theory)
approach[15] adopting the BP86 functional[16] and an all-electron
valence triple-ξ basis set with polarisation functions on all atoms
(TZVP).[17] The ab initio DFT approach was undertaken because
previous experiences[6d] have shown that semi-empirical Hamiltoni-
ans like MNDO or AM1 do not produce optimised geometries for
the extended π conjugation when the linker is based on a styryl or
2-[2-(4-Nitrophenyl)ethyn-1-yl]-5,10,15,20-tetraphenylporphyrin (5):
A solution of ZnII complex 6 (60 mg, 0.073 mmol) and trifluoro-
acetic acid (1 mL) in CH2Cl2 (25 mL) was stirred at room tempera-
ture for 12 h. The volatiles were removed in vacuo to afford a dark-
green residue which was dissolved in CH2Cl2 (100 mL) and care-
fully washed with a 5% aqueous solution of NaHCO3 (2×50 mL)
and with H2O (2×50 mL), dried with Na2SO4 and the solvents
evaporated to dryness. The residue was crystallised from CH2Cl2/
n-hexane to afford 52 mg (94%) of 5 as a red-brown powder. 1H
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meso-Tetraphenylporphyrin ZnII Complexes
FULL PAPER
NMR (300 MHz, CDCl3): δ = –2.65 (br. s, 2 H, NH), 7.49 C54H37N5O2 (787): calcd. C 82.32, H 4.73, N 8.89; found C 82.0,
(AAЈBBЈ, 2 H, HAr), 7.60–7.83 (m, 12 H, Hm,pPh), 8.15–8.22 (m, H 4.60, N 8.70. MS-FAB(+): m/z = 788 [M + 1]+.
10 H, 8HoPh, 2HAr), 8.78–8.91 (m, 6 H, Hpyrrolic), 9.13 (s, 1 H,
Synthesis of ZnII Complexes: ZnII complexes 2, 8, 10 and 12 were
Hpyrrolic) ppm. UV/Vis (CHCl3): λmax (logε) = 430 (5.26), 525 (4.31),
synthesised following the general procedure described in the litera-
562 (3.83), 601 (3.78), 657 (3.68) nm. C52H33N5O2 (759): calcd. C
ture.[19] In a typical preparation, the free porphyrin (100 mg) was
82.19, H 4.38, N 9.22; found C 81.75, H 4.40, N 8.96. MS-FAB(+):
dissolved in slightly warmed CHCl3 (50 mL) and a solution of
m/z = 760 [M + 1]+.
Zn(OAc)2·2H2O (free porphyrin/metal salt = 1:3) in MeOH (5 mL)
was then added. The mixture was heated at reflux for 2 h and the
progress of the reaction was monitored by TLC. After removal of
the solvent in vacuo, the crude product was purified by crystallisa-
tion or by washing with methanol.
2-[(1E)-2-(4-Nitrophenyl)ethen-1-yl]-5,10,15,20-tetraphenylporphy-
rin (7): Solid NaOH (400 mg, 10 mmol) was added to a solution of
(4-nitrobenzyl)triphenylphosphonium bromide (596 mg,
1.25 mmol) and 2-formyl-5,10,15,20-tetraphenylporphyrin (13;
160 mg, 0.25 mmol) in 30 mL of 1,2-dichloroethane and the re-
sulting suspension was heated at reflux and stirred for 8 h. Then,
the reaction mixture was diluted with 150 mL of CH2Cl2, washed
with 100 mL of H2O and the organic phase was separated, dried
with Na2SO4 and the solvents evaporated. The residue was purified
by column chromatography (silica gel, CH2Cl2) to afford 160 mg
of product as a mixture 80:20 of (E)- and (Z)-isomers. This mixture
was purified by crystallisation from CH2Cl2/CH3OH to afford
2-Cyano-5,10,15,20-tetraphenylporphyrinatozinc(II) (2): The violet
powder was crystallised from CH2Cl2/n-pentane. Yield: 98%. 1H
NMR (300 MHz, CDCl3): δ = 7.60–7.80 (m, 12 H, Hm,pPh), 8.12–
8.20 (m, 8 H, HoPh), 8.87–8.93 (m, 6 H, Hpyrrolic), 9.46 (br. s, 1 H,
Hpyrrolic) ppm. UV/Vis (CHCl3): λmax (logε) = 427 (5.62), 518 (3.57),
555 (4.20), 593 (4.02) nm. C45H27N5Zn (702): calcd. C 76.87, H
3.87, N 9.96; found C 76.52, H 3.88, N 9.92. MS-FAB(+): m/z =
703 [M + 1]+.
1
110 mg (58%) of pure (E)-isomer 7. H NMR (300 MHz, CDCl3):
2-[(1E)-2-(4-Nitrophenyl)ethen-1-yl]-5,10,15,20-tetraphenylporphy-
rinatozinc(II) (8): The blue-violet powder was washed with n-hexane
and a small amount of MeOH in order to eliminate the excess of
δ = –2.60 (br. s, 2 H, NH), 7.11 (d, J = 16.0 Hz, 1 H, Hethenyl), 7.29
(d, J = 16.0 Hz, 1 H, Hethenyl), 7.30 (d, J = 8.6 Hz, 2 H, HAr), 7.74–
7.86 (m, 12 H, Hm,p), 8.14–8.24 (m, 10 H, 8HoPh, 2HAr), 8.70–8.82
(m, 6 H, Hpyrrolic), 9.01 (br. s, 1 H, Hpyrrolic) ppm. UV/Vis (CHCl3):
λmax (logε) = 431 (5.31), 525 (4.34), 569 (4.13), 602 (3.94), 659 (3.47)
nm. C52H35N5O2 (761): calcd. C 81.9, H 4.64, N 9.20; found C
81.6, H 4.62, N 9.16. MS-FAB(+): m/z = 762 [M + 1]+.
1
Zn(OAc)2. Yield: 90%. H NMR (300 MHz, CDCl3): δ = 7.18 (d,
J = 16.0 Hz, 1 H, Hethenyl), 7.24 (d, J = 16.0 Hz, 1 H, Hethenyl), 7.37
(d, J = 8.6 Hz, 2 H, HAr), 7.76- 7.85 (m, 12 H, Hm,pPh), 8.19–8.28
(m, 10 H, 8HoPh, 2HAr), 8.82–8.96 (m, 6 H, Hpyrrolic), 9.16 (br. s, 1
H, Hpyrrolic) ppm. UV/Vis (CHCl3): λmax (logε) = 432 (5.20), 526
(3.77), 558 (4.35), 597 (4.13) nm. C52H33N5O2Zn (824): calcd. C
75.70, H 4.04, N 8.49; found C 75.4, H 4.03, N 8.45. MS-FAB(+):
m/z = 825 [M + 1]+.
2-{(1E)-2-[4-(Dibutylamino)phenyl]ethen-1-yl}-5,10,15,20-tetraphen-
ylporphyrin (11): Solid NaOH (480 mg, 12 mmol) was added to a
solution of [4-(dibutylamino)benzyl]triphenylphosphonium bro-
mide (258 mg, 0.5 mmol) and 2-formyl-5,10,15,20-tetraphenylpor-
phyrin (13; 193 mg, 0,3 mmol) in 25 mL of CH2Cl2, and the re-
sulting suspension was stirred at room temperature for 4 h. Then,
the reaction mixture was purified by column chromatography (sil-
ica gel, CH2Cl2) to afford 205 mg of product as a 9:1 mixture of
(E)- and (Z)-isomers. This product was crystallised from CH2Cl2/
2-[(1E,3E)4-(4-Nitrophenyl)1,3-butadienyl]-5,10,15,20-tetraphenyl-
porphyrinatozinc(II) (10): The dark green powder was washed with
n-hexane and a small amount of MeOH in order to eliminate the
1
excess of Zn(OAc)2. Yield 90%. H NMR (300 MHz, CDCl3): δ =
6.40 (d, 1 H, Hbutyl), 6.75 (d, 2 H, Hbutyl), 7.14 (dd, 1 H, Hbutyl),
7.55 (d, 2 H, HAr), 7.78 (m, 12 H, Hm,pPh), 8.15 (d, 2 H, HAr), 8.24
1
n-pentane to give 175 mg (69%) of pure (E)-isomer 11. H NMR
(m, 8 H, HoPh), 8.88 (m, 6 H, Hpyrrolic), 9.07 (br. s, 1 H, Hpyrrolic
)
(300 MHz, CDCl3): δ = –2.60 (br. s, 2 H, NH), 1.01 (t, J = 7.20 Hz,
6 H, CH3 butyl), 1.20–1.70 (m, 4 H, 2CH2 butyl), 3.34 (t, J = 7.60 Hz,
4 H, NCH2), 6.60 (d, J = 8.80 Hz, 2 H, HAr), 6.72 (d, J = 16.0 Hz,
1 H, Hethenyl), 7.12 (d, J = 8.80 Hz, 2 H, HAr), 7.28 (d, J = 16.0 Hz,
1 H, Hethenyl), 7.65–7.88 (m, 12 H, Hm,pPh), 8.15–8.30 (m, 8 H,
ppm. UV/Vis (CH2Cl2): λmax (logε) = 430 (4.94), 559 (4.11), 599
(3.98) nm. C54H35N5O2Zn (850): calcd. C 76.20, H 4.14, N 8.23;
found C 76.20, H 4.05, N 8.01. MS-FAB(+): m/z = 851 [M + 1]+.
2-{(1E)-2-[4-(Dibutylamino)phenyl]ethen-1-yl}-5,10,15,20-tetraphen-
ylporphyrinatozinc(II) (12): The dark powder was washed with n-
hexane and a small amount of MeOH in order to eliminate the
HoPh), 8.65–8.85 (m, 6 H, Hpyrrolic), 8.96 (s, 1 H, Hpyrrolic) ppm.
UV/Vis (CHCl3): λmax (logε) = 422 (5.36), 521 (4.41), 579 (4.16),
603 (4.14), 660 (3.71) nm. C60H53N5 (843): calcd. C 85.37, H 6.33,
N 8.30; found C 85.0, H 6.35, N 8.26. MS-FAB(+): m/z = 844 [M
+ 1]+.
1
excess of Zn(OAc)2. Yield: 85%. H NMR [300 MHz, (CD3)2CO]:
δ = 1.02 (t, J = 7.2 Hz, 6 H, CH3), 1.45–1.51 (m, 4 H, CH2), 1.60–
1.73 (m, 4 H, CH2), 3.41 (t, J = 7.2 Hz, 4 H, CH2N), 6.69 (d, J =
8.8 Hz, 2 H, HAr), 6.82 (d, J = 16.0 Hz, 1 H, Hethenyl), 7.14 (d, J =
8.8 Hz, 2 H, HAr), 7.22 (d, J = 16.0 Hz, 1 H, Hethenyl), 7.72- 7.88
(m, 12 H, Hm,pPh), 8.15–8.30 (m, 8 H, HoPh), 8.70–8.90 (m, 6 H,
Hpyrrolic), 9.01 (s, 1 H, Hpyrrolic) ppm. UV/Vis (CHCl3): λmax (logε)
= 422 (5.20), 556 (4.08), 601 (3.97) nm. C60H51N5Zn (906): calcd.
C 79.47, H 5.63, N 7.73; found C 79.60, H 5.61, N 7.70. MS-
FAB(+): m/z = 907 [M + 1]+.
2-[(1E,3E)4-(4-Nitrophenyl)-1,3-butadienyl]-5,10,15,20-tetraphen-
ylporphyrin (9): 1,8-Diazabicyclo[5.4.0]-undec-7-ene (DBU;
509 mg, 3.34 mmol) was added to a solution of the phosphonium
salt of the tetraphenylporphyrin 16 (52 mg, 0.0562 mmol) and 4-
nitrocinnamaldehyde (12 mg, 0.0674 mmol) in CH2Cl2 (5 mL), and
the mixture was maintained under magnetic stirring and at room
temperature for one hour. The product was isolated by column
chromatography (silica gel; toluene/cyclohexane, 9:1) and further
purified by crystallisation from CH2Cl2/n-hexane to afford 38 mg
2-Nitro-5,10,15,20-tetraphenylporphyrinatozinc(II) (4): A solution of
AgNO3 (97.8 mg, 0.576 mmol) in 20 mL of CH3CN and a solution
of I2 (75 mg, 0.295 mmol) in 20 mL of CH2Cl2 were rapidly added
to a solution of 5,10,15,20-tetraphenylporphyrinatozinc()
1
(86%) of pure 9 as a purple solid. H NMR (300 MHz, CDCl3): δ
= –2.56 (s, 2 H, NH), 6.40 (d, 1 H, Hbutyl), 6.74 (d, 2 H, Hbutyl),
7.17 (m, 1 H, Hbutyl), 7.54 (d, 2 H, HAr), 7.78 (m, 12 H, Ho,pPh), (301 mg, 0.434 mmol) in 80 mL of CH2Cl2/CH3CN (1:1, v/v); the
7.79 (m, 6 H, Hpyrrolic), 8.15 (d, J = 6.98 Hz, 2 H, HAr), 8.23 (m, 8 resulting mixture was stirred at room temperature, under nitrogen,
H, HmPh), 8.95 (s, 1 H, Hpyrrolic) ppm. UV/Vis (CH2Cl2): λmax (logε) in the dark, for two hours. The reaction mixture was filtered, the
= 425 (5.03), 523 (4.21), 572 (4.07), 601 (3.86), 665 (3.51) nm. solvent was evaporated in vacuo and the residue was purified by
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column chromatography (silica gel, CH2Cl2) to afford 71.7 mg
(22%) of a violet powder. H NMR (300 MHz, CDCl3): δ = 7.67–
nm. C45H32N4O (644): calcd. C 83.83, H 5.00, N 8.69; found C
1
83.9, H 5.02, N 8.72. MS-FAB(+): m/z = 645 [M + 1]+.
7.95 (m, 12 H, Hm,pPh), 8.19–8.22 (m, 8 H, HoPh), 8.98–9.00 (m, 6
H, Hpyrrolic), 9.22 (s, 1 H, Hpyrrolic) ppm. UV/Vis (CHCl3): λmax
(logε) = 425 (5.28), 522 (3.41), 555 (4.00), 598 (3.80) nm.
C44H27N5O2Zn (722): calcd. C 73.08, H 3.76, N 9.69; found C
73.20, H 3.89, N 9.80. MS-FAB(+): m/z = 723 [M + 1]+.
2-(Chloromethyl)-5,10,15,20-tetraphenylporphyrin (15): A solution
of thionyl chloride (0.13 g, 1.36 mmol) in 5 mL of Et2O was slowly
added, keeping the temperature in the range 0–5 °C, to a stirred
solution of porphyrin 14 (87.5 mg, 0.136 mmol) and pyridine
(0.13 g, 1.63 mmol) in 30 mL of Et2O. Once the addition was com-
plete, the temperature was raised to room temperature and the reac-
tion mixture was stirred for 30 min. The mixture was then transfer-
red into a separating funnel, diluted with 100 mL of CH2Cl2 and
washed with water (70 mL) and aqueous 5% NaHCO3 (60 mL).
The organic phase was dried with Na2SO4 and the solvents evapo-
rated in vacuo to afford 95 mg of residue, which was purified by
crystallisation from CH2Cl2/n-pentane to give 85 mg (95%) of a
2-[2-(4-Nitrophenyl)ethyn-1-yl]-5,10,15,20-tetraphenylporphyrinato-
zinc(II) (6): A mixture of 2-bromo-5,10,15,20-tetraphenylporphy-
rinatozinc() (150 mg, 0.182 mmol), [PdCl2(PPh3)2] (19 mg,
0.027 mmol), CuI (15.4 mg, 0.0081 mmol), 4-nitrophenylacetylene
(61.5 mg, 0.42 mmol) and triethylamine (3 mL) in 40 mL of THF
was heated at reflux under nitrogen, in the dark, for 5 h. The sol-
vent was evaporated in vacuo and the residue was purified by col-
umn chromatography (silica-gel, toluene/cyclohexane, 1:1) to afford
63 mg (42 %) of pure 6 as a dark-violet powder. 1H NMR
(300 MHz, CDCl3): δ = 7.50 (d, J = 8.79 Hz, 2 H, HAr), 7.61–7.83
(m, 12 H, Hm,pPh), 8.17–8.22 (m, 10 H, 8HoPh, 2HAr), 8.65–8.93 (m,
6 H, Hpyrrolic), 9.27 (br. s, 1 H, Hpyrrolic) ppm. UV/Vis (CHCl3):
λmax (logε) = 431 (5.30), 521 (3.60), 557 (4.25), 594 (4.03) nm.
C52H31N5O2Zn (822): calcd. C 75.87, H 3.80, N 8.51; found C 76.2,
H 3.89, N 8.35. MS-FAB(+): m/z = 823 [M + 1]+.
1
purple solid. H NMR (300 MHz, CDCl3): δ = –2.79 (br. s, 2 H,
NH), 4.79 (s, 2 H, CH2Cl), 7.64- 7.75 (m, 12 H, Hm,pPh), 8.10–8.25
(m, 8 H, HoPh), 8.65 (d, J = 4.87 Hz, 2 H, Hpyrrolic), 8.78–8.87 (m,
4 H, Hpyrrolic), 8.96 (s, 1 H, Hpyrrolic) ppm. UV/Vis (CHCl3): λmax
(logε) = 420 (5.54), 516 (4.26), 550 (3.84), 592 (3.73), 650 (3.81)
nm. C45H31ClN4 (662): calcd. C 81.50, H 4.71, N 8.45; found C
81.15, H 4.78, N 8.49. MS-FAB(+): m/z = 663 [M + 1]+, 628 [M –
Cl]+.
[2-(5,10,15,20-Tetraphenylporphyrin-2-yl)methylene]triphenylphos-
phonium Chloride (16): A solution of porphyrin 15 (75.1 mg,
0.113 mmol) and triphenylphosphane (240 mg, 0.9 mmol) in 50 mL
of CHCl3 was refluxed under magnetic stirring for 16 h. The sol-
vent was then evaporated in vacuo, the residue was washed with n-
hexane in order to remove the excess of PPh3, and the purple solid
residue was dried to afford 84 mg (80%) of pure 16. 1H NMR
(300 MHz, CDCl3): δ = –2.78 (br. s, 2 H, NH), 5.31 (d, J = 15 Hz,
2 H, CH2P), 7.10- 7.20 (m, 6 H, 3 Hm,pPh + 3 HpAr–P), 7.22–7.35
(m, 5 H, 3 Ho Ar–P + 2 HoPh), 7.41 (d, J = 7.2 Hz, 2 H, 2 HoPh),
7.50–7.95 (m, 18 H, 3 HoAr–P + 6 HmAr–P + 9 Hm,pPh), 8.10–8.20
(m, 4 H, 4HoPh), 8.34 (d, J = 3.6 Hz, 1 Hpyrrolic), 8.45 (d, J = 4.9 Hz,
1 Hpyrrolic), 8.70–8.90 (m, 5 Hpyrrolic.) ppm. UV/Vis (CH2Cl2): λmax
(logε) = 423 (5.60), 519 (4.26), 556 (3.72), 595 (3.69), 652 (3.70)
nm. C63H46ClN4P (924): calcd. C 81.76, H 5.01, N 6.05; found C
81.50, H 5.03, N 6.10. MS-FAB(+): m/z = 889 [M – Cl]+, 627 [M –
Cl – PPh3]+.
2-Formyl-5,10,15,20-tetraphenylporphyrin (13): A solution of di-
methylformamide (2 mL) and phosphoryl chloride (2 ml) in 10 mL
of 1,2-dichloroethane was left at room temperature and stirred
magnetically for 30 min. After this time, a solution of tetrakis-
5,10,15,20-tetraphenylporphyrinatonickel() (300 mg, 0.449 mmol)
in 100 mL of 1,2-dichloroethane was added and the reaction mix-
ture was refluxed for 24 h. The dark-green solution was then evapo-
rated to dryness and the residue was treated, while cooling with an
ice bath, with 10 mL of 96% H2SO4 to afford a green viscous solu-
tion, which was diluted with a saturated aqueous solution of
CH3COONa until the pH was basic. The product was extracted
with CH2Cl2 (3 × 150 mL), the organic phase was washed with
aqueous 5% NaOH (100 mL) and then with H2O (150 mL) and
the solvents evaporated in vacuo to afford 320 mg of crude product.
Purification by column chromatography (silica gel, CH2Cl2) af-
forded 220 mg (70 %) of pure 13 as a purple solid. 1H NMR
(300 MHz, CDCl3): δ = –2.60 (br. s, 2 H, NH), 7.64–7.84 (m, 12
H, Hm,pPh), 7.95–8.05 (m, 2 H, HoPh), 8.12–8.28 (m, 6 H, HoPh),
8.65–8.81 (m, 2 H, Hpyrrolic), 8.85–8.87 (m, 4 H, Hpyrrolic), 9.23 (br.
s, 1 H, Hpyrrolic), 9.42 (s, 1 H, CHO) ppm. UV/Vis (CHCl3): λmax
(logε) = 432 (5.47), 526 (4.21), 568 (3.80), 605 (3.70), 664 (3.83)
nm. C45H30N4O (642): calcd. C 84.09, H 4.70, N 8.72; found C
84.35, H 4.83, N 8.79. MS-FAB(+): m/z = 643 [M + 1]+.
4-(Dibutylamino)benzyl Alcohol: A sample of 4-(dibutylamino)-
benzaldehyde (2.33 g, 10 mmol) was dissolved in 100 mL of 96%
EtOH. Then, NaBH4 (760 mg, 20 mmol) was added and the sus-
pension was stirred at reflux for 2 h. The reaction mixture was
evaporated in vacuo, the residue was taken up into a separating
funnel with 60 mL of H2O and 100 mL of Et2O and the organic
phase was separated. The aqueous phase was extracted with Et2O
(2×50 mL). The organic phase was dried with MgSO4 and the sol-
vents evaporated to dryness to afford 2.28 g (97%) of pure product
as a thick, colourless oil. 1H NMR (200 MHz, CDCl3): δ = 0.95 (t,
J = 7.2 Hz, 6 H, CH3), 1.25–1.45 (m, 5 H, CH2 + OH), 1.45–1.65
(m, 4 H, CH2), 3.26 (t, J = 7.2 Hz, 4 H, CH2N), 4.54 (d, J = 5.7 Hz,
2 H, CH2O), 6.62 (d, J = 8.8 Hz, 2 H, AAЈBBЈ), 7.20 (d, J = 8.8 Hz,
2 H, AAЈBBЈ) ppm.
2-Hydroxymethyl-5,10,15,20-tetraphenylporphyrin (14): A sample of
2-formyl-5,10,15,20-tetraphenylporphyrin (13; 375 mg,
0,583 mmol) was dissolved in 50 mL of refluxing dry tetra-
hydrofuran, then NaBH4 (609 mg, 16.1 mmol) was added and the
suspension was stirred for 15 min. The reaction mixture was cooled
to room temperature, 100 mL of H2O was added in order to elimin-
ate the excess of NaBH4, then the reaction mixture was transferred
into a separating funnel and extracted with 100 mL of CHCl3. The
organic phase was separated, dried with Na2SO4 and evaporated
under reduced pressure. The residue was purified by crystallisation
with CH2Cl2/n-hexane to afford 335 mg (89%) of pure 14 as a pur-
ple solid. 1H NMR (300 MHz, CDCl3): δ = –2.75 (br. s, 2 H, NH),
1.95 (t, 1 H, exchange with D2O), 4.88 (d, J = 5.06 Hz, 2 H, CH2O),
[4-(Dibutylamino)benzyl]triphenylphosphonium Chloride: A suspen-
sion of 4-(dibutylamino)benzyl alcohol (1.18 g, 5 mmol) in 20 mL
of CH3CN, cooled to 0 °C, was saturated with gaseous HCl until
a light yellow solution had formed. This solution was heated at
reflux and stirred for 40 h. The reaction mixture was evaporated in
7.64–7.80 (m, 12 H, Hm,pPh), 8.08–8.11 (m, 2 H, HoPh), 8.17–8.22 vacuo, the residue was taken up in a separating funnel with 120 mL
(m, 6 H, HoPh), 8.50–8.61 (m, 2 H, Hpyrrolic), 8.75 and 8.87 (m, 4 of CH2Cl2, washed with aqueous 10% Na2CO3 (2×30 mL) and
H, Hpyrrolic), 8.94 (s, 1 H, Hpyrrolic) ppm. UV/Vis (CHCl3): λmax 50 mL of brine. The organic phase was dried with MgSO4 and the
(logε) = 419 (5.71), 515 (4.27), 549 (3.77), 589 (3.74), 645 (3.50)
solvents evaporated to dryness to give a white solid residue which
3872
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 3857–3874

meso-Tetraphenylporphyrin ZnII Complexes
FULL PAPER
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Received: April 6, 2005
Published Online: August 4, 2005
3874
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 3857–3874