Cubic nonlinear optical properties of new zinc tetraphenyl porphyrins peripherally functionalized with electron-rich Ru(II) alkynyl substituents

Article abstract of DOI:10.1016/j.tet.2012.09.108

We report the synthesis and characterization of new organometallic assemblies consisting of a central Zn(II) tetraphenylporphyrin (ZnTPP) core surrounded by four Ru(II) alkynyl complexes (trans-Ru(dppe)₂R) appended at the para phenyl positions

Full text of DOI:10.1016/j.tet.2012.09.108

Tetrahedron 68 (2012) 10351e10359
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Cubic nonlinear optical properties of new zinc tetraphenyl porphyrins
peripherally functionalized with electron-rich Ru(II) alkynyl substituents
,
Samuel Drouet a, Areej Merhi b, Dandan Yao b, Marie P. Cifuentes c, Mark G. Humphreyc
,
*
Malgorzata Wielgus d, Joanna Olesiak-Banska d, Katarzyna Matczyszyn d, Marek Samoc d
,
,e,
*
a
,
a,b,
*
*
ꢀ ꢀ
Frederic Paul , Christine O. Paul-Roth
a
ꢀ
Institut des Sciences Chimiques de Rennes, ISCR-UMR CNRS 6226, Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France
INSA-ISCR-UMR 6226, Universite Europeenne de Bretagne, INSA de Rennes, 35043 Rennes Cedex, France
b
ꢀ
ꢀ
c Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia
d Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, 50-370 Wroclaw, Poland
e Laser Physics Centre, Australian National University, Canberra ACT 0200, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2012
Received in revised form
24 September 2012
Accepted 25 September 2012
Available online 29 September 2012
We report the synthesis and characterization of new organometallic assemblies consisting of a central
Zn(II) tetraphenylporphyrin (ZnTPP) core surrounded by four Ru(II) alkynyl complexes (trans-Ru(dp-
pe)2R) appended at the para phenyl positions and acting as peripheral donor groups. The synthesis of the
tetra-chlorido derivative 2 (R¼Cl) is reported first, followed by that of three extended homologues (3eX)
obtained from this building block by substitution of the chlorido ligands by functionalized arylacetylenes
(R¼C^C(4-C6H4X); X¼NO2, H, OMe). Measurement of the nonlinear absorption properties of the pen-
tametallic derivatives 3eX reveals that these extended
metallic units behave as good two-photon absorbers in the visible and near-IR range.
Ó 2012 Elsevier Ltd. All rights reserved.
p-networks incorporating polarizable organo-
Keywords:
Porphyrin
Acetylide complex
Organometallics
Nonlinear optics
Pi-conjugation
1. Introduction
systems, these organometallic units permit great structural control,
as the diphosphines coordinating the metal centre, the oxidation
state of the metal, and the ligand trans to the alkynyl ligand can all
be varied at will. Moreover, due to the existence of kinetically stable
oxidized state(s) with distinct NLO properties at selected wave-
lengths, redox-switching of the NLO behaviour has been demon-
strated in several instances with these particular building blocks.8,9
Materials with cubic nonlinear optical (NLO) properties are re-
quired in emerging photonics-based technologies for various ap-
plications, including optical data storage, nanophotonics, and
biophotonics.1,2 The processing of optical signals such as ultrafast
switching or modulation of optical beams using materials with
sizeable third-order NLO responses is of significant interest.3
Amongst the wide range of molecular chromophores investigated
in the last 30 years, organometallic and coordination compounds
have emerged as very promising building blocks to access new
NLO-active materials.4,5 In particular, several examples of formally
alkynyl-d6 complexes featuring an equatorial ‘Ru(dppe)2’ core
[dppe¼1,2-bis(diphenylphosphino)ethane] have been explored.5,6
On another front, large metallated
p-compounds such as por-
phyrins or phthalocyanines were also identified very early as effi-
cient and robust cubic NLO-phores.10e12 For instance, Rao et al.
showed in 2000 that various metallated tetra-p-tolylporphyrins
(TTP) could exhibit quite high cubic optical nonlinearities at 532 or
600 nm.13 These properties were measured using nanosecond (and
to a lesser degree picosecond) pulses, the results being consistent
with contributions from mechanisms involving real excited states
rather than virtual states only, as is the case with nonresonant cubic
nonlinear responses, but rendering these molecules quite promis-
ing for optical limiting or all-optical switching purposes.2 More
recently, multi-porphyrin assemblies exhibiting enhanced cubic
nonlinear absorption properties were also developed by various
groups for related applications.14,15 Thus, depending on the overall
When such polarizable units are incorporated into extended
p
networks, a tremendous enhancement of the nonlinear optical
response was often observed.6,7 In the pursuit of optimized
* Corresponding authors. E-mail addresses: christine.paul@univ-rennes1.fr,
christine.paul@insa-rennes.fr (C.O. Paul-Roth).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.09.108

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S. Drouet et al. / Tetrahedron 68 (2012) 10351e10359
symmetry of a given porphyrin, on the nature of the central metal
ion, and on the peripheral substituents of the macrocyclic core,
widely different nonlinearities can result. The presence of a metal-
lic centre inside the porphyrin cavity usually proves to be crucial,
itself obtained after metallation27 of the corresponding tetra-
alkyne free base, affords access to the pentametallic complex 2
(Scheme 2) in an overall yield of 32%. Although the tetra-alkyne
Zn porphyrin 5 had been previously synthesized in fair yields
(29%),22 the present work (see Supplementary data) resulted in an
improved yield (34% over 3 steps) by using the BF3(OEt2) complex
as a Lewis-acid promoter to assist the synthesis of the free base
1.28 The tetra-vinylidene complex that forms upon reaction of the
Ru(II) precursor 4 with 5 by a 1,2-hydrogen shift was not char-
acterized,29 but was precipitated before being deprotonated in
situ by excess triethylamine to give 2. Completion of the reaction
forming 2 can be monitored by following the disappearance of the
1H NMR alkynyl proton (C^CH) of 5 at 3.37 ppm in CDCl3. This
new complex was characterized by the usual spectroscopies
(NMR, UVevis and IR), microanalysis and electrospray mass
spectrometry (ESI-MS).
since it planarizes the
p-manifold, which is essential to achieve
high hyperpolarizabilities.11,12 Depending on its nature, the metal
ion can also provide an additional electronic contribution to the
NLO response. Accordingly, varying its nature is often a powerful
way to optimize the cubic NLO response of a porphyrin-based
chromophore tailored for a given application. In that respect,
metal centres with low-lying charge-transfer states16 or open-shell
structures have often been usefully used.11 In comparison, far fewer
investigations have focused on the role of the peripheral sub-
stituents. However, among the scant experimental data available on
metallated TPP derivatives, some investigations have suggested
that electron-releasing substituents at the para-positions of the
meso-aryl groups might afford higher cubic NLO activities.11,17
Some of us recently reported multistable alkynyl-d6complexes
exhibiting large and redox-switchable third-order (cubic) non-
linearities.18 Others have reported the synthesis and properties of
tetrafluorenyl porphyrin derivatives19 and evidenced the de-
termining role of the meso-substituents on the photophysical prop-
erties of these porphyrins.20 However, to the best of our knowledge,
only a few porphyrins featuring alkynyl metal complexes as pe-
ripheral substituents have been reported thus far,21 with very few
constructed around a TPP-core, such as in 1 (Scheme 1).22 Moreover,
none were studied from the perspective of their cubic NLO proper-
ties. We have consequently pursued the synthesis and third-order
NLO responses of a series of new ZnTPP derivatives functionalized
with electron-releasing alkynyl-d6 Ru(II) complexes at the para-lo-
cation of their meso-aryl groups.23 By using the controlled stepwise
reactivity of the cis-[RuCl2(dppe)2] and trans-Ru(C^CAr)Cl(dppe)2]
complexes,24,25 derivatives with organoruthenium termini possess-
ing differently functionalized terminal arylalkynyl ligands were tar-
geted, in order to evidence the effect of electron-releasing or
electron-attracting para substituents on their cubic NLO response.
In this contribution, we report the stepwise synthesis and charac-
terization of three new derivatives 3eX (NO2, H, OMe) via the cor-
responding tetra-chlorido precursor (2), followed by Z-scan
measurements of their cubic nonlinear absorption properties.
In its 1H NMR spectrum, 2 exhibits both the spectral signa-
tures of the porphyrin core (a characteristic singlet at ca. 9 ppm,
corresponding to the 8 equivalent
b-pyrrolic protons) and sig-
nals typical of the -alkynyl Ru(II) endgroups, such as the broad
s
singlet at ca. 3 ppm corresponding to the 32 equivalent CH2
protons of the ruthenium-coordinated dppe. Between 6.5 and
9 ppm are the signals of the various aromatic protons of the
porphyrin and of the dppe. The 31P NMR spectrum of 2 reveals
a sharp singlet corresponding to the 16 equivalent phosphorus
atoms from the 8 dppe ligands coordinated to the ruthenium
endgroups (Table 1); the chemical shift of this signal (ca.
50 ppm) is characteristic of monosubstituted
s-alkynyl ruthe-
nium complexes of this kind.30 Further evidence for the struc-
ture of this compound comes from the 13C NMR spectrum, in
which all carbon atoms of 2 can be observed and assigned by
polarization transfer studies, except for the weak quintuplet
corresponding to the Ca-alkyne carbon atoms, which escaped
from detection. The presence of the four alkynyl bonds in 2 is
nevertheless revealed by a broad singlet near 115 ppm, corre-
sponding to the equivalent Cb carbon atoms.25,31 The presence of
these alkynyl bonds is more clearly revealed by infrared, since 2
displays an intense absorption band at ca. 2050 cmꢀ1 (Table 1),
characteristic of the nC^C mode of these Ru(II)
s-alkynyl
complexes.25,30
Scheme 1. Selected TPP-based porphyrins substituted with alkynyl complexes.
2. Results and discussion
2.2. Synthesis and characterization of the tetra{ruthenium-
bis(alkynyl)} Zn(II) porphyrins
2.1. Synthesis and characterization of the tetra(ruthenium-
chloride) ZnTPP
Using the pentametallic porphyrin 2 as starting material, we
have isolated three new derivatives 3eX (X¼NO2, H, OMe) by li-
gand metathesis. This reaction was performed in the presence of
sodium hexafluorophosphate and with an excess of the
Reacting the known cis ruthenium(II) organometallic precursor
complex 426 with the known tetra-alkyne zinc(II) porphyrin 5,22

S. Drouet et al. / Tetrahedron 68 (2012) 10351e10359
10353
1/BF3(OEt2)
2/ p-Chloranil
CH2Cl2
O
TMS
H
NH
+
N
3/ K2CO3
MeOH / CH2Cl2 (1:3)
N
NH
N
45 %
H
Zn(CH3CO2)2.2H2O
CH2Cl2
71 %
1/ cis-RuCl2(dppe)2 (4)
NaPF6
2/ NEt3
N
N
Zn
CH2Cl2
73 %
N
N
5
Cl
[Ru]
Cl
[Ru]
N
N
Zn
N
N
[Ru]
Cl
2
[Ru]
Cl
Scheme 2. Synthesis of the peripherally tetraruthenated ZnTPP porphyrin 2.
Table 1
3eX are also observed, with the correct integrations, confirming
the presence of the tetraruthenated porphyrin core. In particular,
NMR is diagnostic of the high symmetry of the various compounds
isolated, indicating that the chloride metathesis occurs at all of the
Ru(II) centres of 2. The unique singlet observed in the 31P NMR
spectrum is now located at ca. 54 ppm (Table 1). This shift to lower
field relative to the same signal in 2 is diagnostic of the presence of
the bis-alkynyl Ru(II) moieties in 3eX.30 As is often observed for
bis-alkynyl Ru(II) complexes,23 both nC^C modes appear as a single
and broad absorption at ca. 2050 cmꢀ1 (Table 1). This characteristic
vibration is shifted to slightly lower energies (2042 cmꢀ1) for the
nitro compound 3eNO2 in comparison to the other 3eX com-
Selected spectral signatures for 2 and 3eX complexes
31P NMRb
a
Compounds
nC^C
2
2057
2042
2056
2056
51.0
54.7
55.2
55.1
3eNO2
3eH
3eOMe
a
IR pellet in KBr.
In CDCl3.
b
corresponding functional arylethyne, using well-established re-
action protocols developed for the trans-[Ru(C^CAr)Cl(dppe)2]
complexes (Scheme 3).26 The synthesis of these tetra-substituted
porphyrins featuring bis-alkynyl ruthenium endgroups was ach-
ieved in one pot, without attempt to isolate the intermediate vi-
nylidene species, triethylamine being added to the reaction mixture
together with the other reactants. The desired complexes 3eX were
obtained in good yields from 2 after ca. 72 h stirring at 40 ꢁC. These
new complexes were fully characterized by means of IR, UV and
NMR (as example for 3eX, for X¼OMe, see Supplementary data;
Fig. S2) spectroscopies, microanalyses, and CV. Attempts to detect
the molecular ions of these species by ESI-MS or MALDI-MS were
unsuccessful, but constituent fragments were observed in each
case;32 these complexes apparently undergo extensive fragmen-
tation in the mass spectrometer.
pounds,33 which exhibit this broad nC^C band at ca. 2056 cmꢀ1 31 As
.
seen previously with closely related bis-alkynyl Ru(II) com-
pounds,23 only one broad nC^C band is observed for 3eNO2 in spite
of the presence of two dissimilar alkynyl ligands at the Ru(II)
centre.29
2.3. Cyclic voltammetry
Cyclic voltammetry (CV) studies were then performed in
dichloromethane on 2 and on the 3eX derivatives (Table 4). For 2,
a reversible electrochemical process corresponding to the simul-
taneous oxidation of the four organoruthenium(II) termini is ob-
served at 0.48 V (Table 2). This value is not far removed from the
value of 0.44 V observed for the corresponding chlorido-
phenylalkynyl Ru(II) complex 6eH (Scheme 4).23 The compara-
tively higher oxidation potential indicates a slightly more difficult
While signals resembling those previously observed for 2 are
detected in the 1H and 13C NMR spectra, additional signals corre-
sponding to the presence of the terminal arylethynyl fragments in

10354
S. Drouet et al. / Tetrahedron 68 (2012) 10351e10359
X
Cl
Cl
X
[Ru]
[Ru]
[Ru]
[Ru]
X
(6 eq.)
NaPF6 / NEt3
N
Zn
N
N
Zn
N
N
N
N
N
CH2Cl2
85 % (X = NO2)
79 % (X = H)
[Ru]
[Ru]
[Ru]
[Ru]
78 % (X = OMe)
Cl
Cl
2
3-X
X
X
Scheme 3. Synthesis of the trans-substituted pentametallic porphyrins 3eX (X¼NO2, H, OMe).
undergoes two chemically reversible one-electron processes at ca.
0.92 V and 1.18 V versus SCE.34 In addition, the voltammogram of
the precursor porphyrin 5 (used to synthesize 2) shows two
pseudo-reversible oxidation waves at 0.87 V and 1.14 V versus SCE.
The present data indicate that para-substitution of the porphyrin
phenyl groups with ruthenium alkynyl moieties has only a weak
effect on the oxidation of the porphyrin core. This observation is
consistent with previous statements pertaining to substituent ef-
fects on the meso-aryl groups.34,35 In the present system, the well
known kinetic instability of Ru(III) alkynyl complexes in solu-
tion36,37 may decrease somewhat the chemical reversibility of the
porphyrin-centred oxidations at ambient temperatures.
Table 2
Oxidation potentials for complexes 2, 3eX and 5a,b
Compounds
ERꢁ uðIIIÞ=RuðIIÞðVÞ
EZꢁ nTPP=ZnTPPþ
ðVÞ
=ZnTPP2þ
2
0.49c
0.57c
0.46c
0.34c
d
0.87, 1.16
0.86, 1.21
0.88, 1.21
0.84d
3eNO2
3eH
3eOMe
5
0.87, 1.14
a
V versus SCE.
b
Conditions: CH2Cl2, 0.1 M [NBu4][PF6], scan rate 0.1 V sꢀ1
.
c
Fe(C^CPh)(dppe)(h
5-C5Me5) was used as an internal calibrant
EFꢁ eðIIIÞ=FeðIIÞ ¼ ꢀ0:15 V versus SCE).42
d
An irreversible oxidation occurs at 1.16 V.
Fc
Ph P PPh
N
N
N
2
2
N
N
N
N
Zn
Cl
Ru
C
C
X
Zn
Fc
Fc
N
Ph P
2
PPh
2
Fc
8
7
6-X
Fc
t
t
t
t
Bu
Bu
Bu
Bu
N
N
N
N
N
N
N
N
N
Zn
SiHex
3
Zn
SiMe
3
Hex Si
Bu
Zn
Fc
Fc
3
N
N
N
t
t
t
t
Bu
Bu
Bu
Bu
Fc
11
10
9
Scheme 4. Selected Ru(II) alkynyl complexes and porphyrins.
oxidation of the four Ru(II) centres in 2 than for the unique Ru(II)
centre in 6. The voltammogram of 2 shows two additional pseudo-
reversible processes at 0.87 V and near 1.16 V versus SCE, but de-
creased in intensity by approximatively 25%. These redox waves are
attributed to the stepwise one-electron oxidations of the metal-
lated porphyrin macrocycle. Indeed, ZnTPP (7; Scheme 4) usually
For 3eNO2 and 3eH compounds, similar voltammograms to 2
are obtained in the 0e1.3 V range, exhibiting three waves in a 4:1:1
ratio. The most intense process occurs at the lowest potential, again
corresponding to the simultaneous oxidation of the Ru(II) centres,
while the higher potential processes correspond to the first two
oxidations of the ZnTPP-core. For 3eOMe, an additional irreversible

S. Drouet et al. / Tetrahedron 68 (2012) 10351e10359
10355
Table 3
process masks the second porphyrin-based oxidation, while for
UVevis data for complexes 2, 3eX and 5
3eNO2, another strong irreversible wave is observed at ꢀ0.96 V,
which corresponds to the multi-electron reduction of the nitroaryl
groups.31,38,39
Compounds
l
in nm (
3 )
in 103 L molꢀ1 cmꢀ1
2
327 (93); 418 (364); 452 (sh, 97); 563 (31); 615 (39)
3eNO2
3eH
3eOMe
5
322 (77); 420 (248); 466 (sh, 104); 553 (sh, 42); 607 (30)
330 (94); 421 (249); 460(sh, 50); 563 (20); 612 (39)
321 (107); 422 (310); 460 (sh, 61); 562 (26); 611 (30)
302 (27); 424 (556); 552 (28); 594 (10)
In comparison to 2, the Ru(III/II) potentials of 3eH are slightly
lower (0.46 V vs 0.49 V), as expected from the more electron-
donating character of the arylalkynyl ligand relative to the chlor-
ido ligand.9 Within the 3eX series, classic substituent effects con-
trol the Ru-centred oxidation potentials.31,36 As expected, the
potential is lowered for the compound 3eOMe with four peripheral
electron-donating methoxy groups, while the opposite is found for
3eNO2 with strongly electron-withdrawing nitro groups (Table 2).
In contrast, the porphyrin-based oxidations appear only weakly
affected by modifications in the trans-ligand at the peripheral ru-
thenium centres (Table 2).
A comparison to other symmetrically functionalized porphyrins
bearing redox-active substituents on their periphery is also in-
structive. Thus, the Zn(II) porphyrin 8 (Scheme 4), which has four
ferrocenyl groups appended to its meso-positions, exhibits several
resolved Fc-based oxidations.40 In the present system, the obser-
vation of a single wave for the oxidation of the peripheral organ-
ometallic substituents in 2 or 3eX is indicative of a weaker
electronic interaction between them, as was observed for 9 with
the four ferrocenyl substituents on the para-positions of the meso-
phenyl groups.41 The observation of a chemically reversible and
simultaneous oxidation of the four metal-alkynyl substituents in 2
or 3eX is of interest in possible redox-switching of the NLO prop-
erties,8,9,18 since this oxidation will preserve the symmetry of the
compound, and thereby facilitate our analysis of the physics un-
derlying the switching phenomenon.
weakness of their shifts within the 3eX series is consistent with the
porphyrin core experiencing only a weak influence of the periph-
eral electron-rich Ru(II) endgroups. It is noteworthy that a larger
bathochromic shift (Dn¼486 cmꢀ1) of the Soret band relative to 5
had been observed in earlier work (Scheme 1), upon complexation
of platinum(II) to the terminal ethynyl groups,22 suggesting
a slightly different interaction between the porphyrin core and the
peripheral metal-alkynyl fragments in 1 relative to 2 or 3eX.
A new absorption band appears near 330 nm upon functional-
ization of the pendant ethynyl substituents of 5 by organo-
ruthenium groups; this is a spectral region where dRu
/
pꢂC^C
transitions are often observed with Ru(II) alkynyl complexes.9
Given that such an absorption is also observed for 4, we tenta-
tively propose that it corresponds to a dRu
sition. Consistent with such an assignment, its energy across the
3eX series seems poorly influenced by a change in the terminal
arylalkynyl ligand. For these bis-alkynyl complexes, a second band
/
pꢂC^CTPP MLCT tran-
corresponding to a dRu
/
pꢂC^Cð4ꢀC H XÞ MLCT transition is also
6
4
observed in the same spectral region,9 except for 3eNO2, for which
it appears as a shoulder on the Soret band, at 466 nm, due to its
strong dRu
/
pꢂNO character. Such a transition is quite specific to
2
trans-[Ru(dppe)2] fragments with 4-nitroarylalkynyl ligands, and
2.4. UVevisible spectroscopy
was previously observed at 484 nm for 6eNO2.31
The UVevisible absorption spectra of compounds 2 or 3eX were
recorded at room temperature in dichloromethane and compared
to that of their tetra-ethynyl precursor 5 (Fig. 1). For 5, the ab-
sorption spectrum shows the characteristic spectrum of a metal-
lated porphyrin, with a Soret band at 425 nm and two Q bands at
552 and 594 nm.22
2.5. Nonlinear absorption measurements
Standard femtosecond Z-scan measurements were then carried
out on the various extended porphyrins 3eX in dichloromethane
(Fig. 2 (for X¼H) and see Supplementary data; Fig. S1 (for X¼OMe,
NO2); compound 2 proved insufficiently soluble for performing
such measurements). Measurements were performed in the spec-
tral range between 530 and 1600 nm, with simultaneous recording
of the open- and closed-aperture signals. It should be emphasized
that this region partly overlaps the one-photon absorptions of all
the compounds (the Q bands) and so two-photon transitions to
many states can be expected in this range, although it must be kept
in mind that different selection rules govern one-photon and two-
photon transitions. Due to the presence of various resonances, the
NLO properties in the investigated wavelength range are likely to
contain several contributions, including that of saturation of ab-
sorption (SA), so the NLO parameters need to be treated as ‘effec-
tive’ responses (as well as contributions from SA, the NLO
parameters may be laser pulse width- and pulse energy-
dependent). The Z-scan traces obtained from these studies were
used to calculate the effective values of the real and imaginary parts
400,000
2
300,000
3-OMe
200,000
3-NO2
100,000
3-H
5
0
280
380
480
wavelength (nm)
580
680
of the cubic hyperpolarizability
g of the chromophores. However,
Fig. 1. UVevisible absorption spectra of the zinc porphyrins 2 and 3eX (X¼NO2, H,
OMe) in CH2Cl2 at 25 ꢁC. For comparison, the compound 5 (with 3 /values divided by 3)
is also reported under the same conditions.
the real parts of were found to exhibit complex behaviour which,
g
together with inherently large errors in their determination (they
are calculated from differences between scans for a solution and
that for the solvent alone), has rendered it impossible to analyze
them at the present stage. We therefore focus here on the ab-
For 2 and the 3eX derivatives, a slight hypsochromic shift of the
Soret band is observed relative to 5 (Dn¼167e395 cmꢀ1), while the
two Q bands are bathochromically shifted (Table 3). No other ab-
sorption bands could be detected at longer wavelengths (until
2000 nm). Overall, the energies of the Soret and Q bands are only
weakly influenced by a change in the terminal X-substituent. The
sorptive nonlinearities, represented by the imaginary parts of
g,
which were determined and subsequently converted into two-
photon absorption cross-sections (s2eff) (Table 4). The data reveal
that all 3eX complexes behave as two-photon absorbers around
1100 and 710 nm. For the nitro- and methoxy-containing

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S. Drouet et al. / Tetrahedron 68 (2012) 10351e10359
Fig. 2. Nonlinear absorption measurements represented as effective two-photon absorption cross-sections overlayed with the one-photon absorption (OPA) spectrum (red curve)
and the same OPA spectrum plotted against twice the wavelength (green curve) for 3eH.
Table 4
correspond to the onset of OPA peaks at ca. 350 nm (Fig. 2), perhaps
suggestive of the involvement of a ‘dark’ MLCT state in this process.
Consistent with such a hypothesis, a more pronounced dependence
Comparison of extremal values of the effective two-photon absorption cross-
sections (s2eff) for given wavelengths between 600 and 1600 nm for 3eX complexes
Compound s2eff
l
s2eff
l
s2eff
l
of the s2eff values on the nature of the X-substituent is suggested by
the data (being mindful of the experimental error margins), the
strongest effective cross-section being obtained for the most
electron-withdrawing substituents. This is reminiscent of the fact
that related 4-nitroarylalkynyl Ru(II) complexes are often found to
present larger effective TPA cross-sections than their unsubstituted
couterparts.38,43 However, as mentioned above, the comparison
between effective TPA cross-sections determined for 3eX com-
pounds must be undertaken with care due to the occurrence of an
SA process in the same spectral range, as manifested by the presence
of negative values for the effective cross-section around 600 nm.13
Finally, comparison with the literature data reveals that related
porphyrins such as 1014 or 1115 (Scheme 4) possess very weak TPA
(<50 GM) above 700 nm, behaviour that was also predicted by in-
dependent DFT calculations conducted on Zn(II) porphyrins mod-
elling 11. These calculations also suggested that, for 11, a more
efficient TPA process may occur at higher energy, just below
600 nm, and would originate from a dark porphyrin-based excited
state with an energy just above that of the Soret band.44 With 3eX
derivatives, similar states could be at the origin of the TPA peak
detected near 700 nm. However, the effective cross-sections found
for these compounds are much larger than the theoretical pre-
dictions for the porphyrin modelling 11. Instead, as discussed above,
we believe that the state at the origin of this two-photon absorption
(first
max.)
[GM]
(first
max.) max.)
[nm] [GM]
(second
(second (first
(first
min.)
[nm]
max.)
[nm]
min.)
[GM]
a
3eNO2
3eH
6000ꢃ3000 770
1500ꢃ500 1000
1400ꢃ500 1300
1300ꢃ100 1000
ꢀ4500ꢃ200 620
ꢀ2800ꢃ600 595
ꢀ1400ꢃ400 620
4800ꢃ500
4200ꢃ500
710
710
3eOMea
a
An additional maximum is apparent at
l<530 nm with s2effꢄ6400 GM.
complexes, a third two-photon absorption (TPA) peak is found at
higher energies, near 530 nm. In addition, all 3eX complexes show
SA behaviour near 600 nm, in a spectral range, which roughly
corresponds to their second Q band.
The best TPA performances were obtained near 710 nm for
3eNO2 and 3eH, with effective cross-sections exceeding 4500 GM.
However, since these processes partly overlap with the SA process
near 600 nm, the measured s2eff values result from a competition
between these two processes, a situation that likely leads to an
underestimation of the actual cross-section of the pure TPA process.
Also, the presence of ultrafast reverse saturable absorption (RSA)
processes that contributes to the effective TPA cross-sections near
710 nm found cannot be disregarded at this stage.13 However, these
appear unlikely due to the poor overlap with the second Q band.
This must also be considered when attempting any comparison
between the s2eff values near 710 nm for the different compounds.
Given the centrosymmetric nature of these molecules, selection
rules predict that excited states to which transitions are allowed for
one-photon processes should be forbidden for two-photon pro-
cesses, and vice-versa.12 Thus, examination of the OPA spectra is at
best indicative of the nature of the excited state involved in the TPA
process. The poor match between the OPA and TPA peaks suggests
that none of the observed TPA bands can be ascribed to a reduction
in symmetry due to the presence of conformers in solution. It is
especially interesting that the one-photon Soret band near 400 nm
does not have a two-photon analogue at ca. 800 nm; in contrast,
there seems to be some antiresonant behaviour in this region. The
small TPA peaks observed at ca. 1100 nm can be tentatively associ-
ated with excited states similar to those at the origin of the Q
bandsdtheir poor X-substituent dependency is consistent with
such an assignment. The main TPA peaks near 710 nm roughly
process corresponds to a dRu
slight substituent-dependency seen for the cross-sections.
/
pꢂC^C MLCT state, in line with the
3. Conclusions
We have synthesized and characterized four new centrosym-
metric architectures 2 and 3eX (X¼NO2, H, OMe) bearing four
peripheral ethynyl groups from the known ZnTPP complex 5. These
new compounds were characterized by the usual spectroscopies
and cyclic voltammetry. Both 2 and 3eX compounds exhibit
a chemically reversible oxidation at ca. 0.5 V versus SCE, which
corresponds to the simultaneous oxidation of all the Ru(II) centres.
Their electronic absorption spectra are dominated by a strong Soret
band in the visible range, with a strong charge-transfer band also
apparent as a shoulder in the case of 3eNO2. Preliminary studies of

S. Drouet et al. / Tetrahedron 68 (2012) 10351e10359
10357
the cubic NLO activity by Z-scan in the 500e1600 nm range for the
3eX derivatives revealed that all of these organometallic porphy-
rins behave as strong two-photon absorbers at ca. 700 and
1100 nm, with effective TPA cross-sections significantly larger than
those reported for related monomeric zinc porphyrin derivatives,
although the participation of RSA processes near 710 nm cannot be
definitively excluded yet. Their effective TPA characteristics exhibit
no obvious dependencies on the terminal X-substituent, except
perhaps for the TPA process at ca. 700 nm, for which slightly larger
effective cross-sections were obtained for X¼NO2. All compounds
possess substituent-dependent SA behaviour at ca. 600 nm, which
appears strongly favoured with electron-withdrawing X-sub-
stituents. Ongoing studies are directed at identifying the excited
states at the origin of these remarkable cubic NLO features, and at
assessing the potential of these redox-active compounds for
electro-switching their optical properties.
[NBu4][PF6] as supporting electrolyte. Potentials reported were
measured and are expressed relative to the saturated calomel
electrode (SCE). High-resolution mass spectra were recorded on
a ZabSpec TOF Micromass spectrometer in ESI positive mode in
CH2Cl2/MeOH (95:5) at the CRMPO (Univ. Rennes 1). The cis-
[RuCl2(dppe)2] complex 3 was prepared according to literature
methods.26 The synthetic protocol used to obtain the known por-
phyrin 522 is given in Supplementary data. The porphyrins were
precipitated from a chloroform/heptane mixture.
4.2. Synthesis
4.2.1. Zinc(II)-5,10,15,20-tetra{trans-(4-phenylethynyl)[ruthenium(II)
bis(bis(1,2-(diphenylphosphino)ethane)](chloride)} (2). NaPF6 (95 mg,
0.57 mmol) was added to a solution of 5 (100 mg, 0.13 mmol) and
cis-[RuCl2(dppe)2] (3; 551 mg, 0.57 mmol) in CH2Cl2 (100 mL). This
mixture was stirred at room temperature for 12 h and the solution
was filtered. The compound was then precipitated by addition of
Et2O. The resulting precipitate was dissolved in CH2Cl2 and NEt3 was
added (1 mL) and the solution was stirred for a 10 additional min-
utes. The solution was concentrated and hexane was added, and the
resulting precipitate was filtered and washed with MeOH (2ꢅ10 mL),
to afford 2 as a green product (424 mg, 0.094 mmol). Yield: 73%. 1H
4. Experimental section
4.1. General procedures
All reactions were performed under argon with magnetic stir-
ring. Solvents were distilled from appropriate drying agents prior to
use: CH2Cl2 from CaH2, CHCl3 from P2O5 and all other solvents were
HPLC grade. Commercially available reagents were used without
further purification unless otherwise stated. All reactions were
monitored by thin-layer chromatography (TLC) with Merck pre-
coated aluminium foil sheets (Silica gel 60 with fluorescent in-
dicator UV254). Compounds were visualized with ultraviolet light at
254 and 365 nm. Column chromatography was carried out using
silica gel from Merck (0.063e0.200 mm). 1H NMR, 31P and 13C NMR
in CDCl3 were recorded using Bruker 200 DPX, 300 DPX and 500
DPX spectrometers. The chemical shifts are referenced to internal
TMS. The assignments follow the numbering scheme shown in
Scheme 5 and were obtained from 2D NMR experiments: COSY
(Correlation Spectroscopy), HMBC (Heteronuclear Multiple Bond
Correlation) and HMQC (Heteronuclear Multiple Quantum Co-
herence). IR spectra were recorded on a Bruker IFS 28 spectrometer
using KBr pellets. UVeviseNIR spectra were recorded on an UVI-
KON XL spectrometer from Biotek instruments. Cyclic voltammetry
studies were performed in CH2Cl2 with a concentration of 0.1 M
NMR (500 MHz, CDCl3, d in ppm): 9.22 (s, 8H, Hb-pyrrolic), 8.05 (d, 8H,
3JH,H¼8.2 Hz, Hb), 7.79 (m, 32H, Hpara/Ar/dppe), 7.40e7.03 (m, 128H,
3
H
orthoemeta/Ar/dppe), 7.13 (d, 8H, JH,H¼8.2 Hz, Hc), 2.85 (m, 32H, CH2/
dppe). 13C NMR (125 MHz, CDCl3,
d in ppm): 151.1 (s, Ca-pyrrolic), 138.0
(s, CAr[Ca]), 137.6 and 136.3 (m, Cipso/dppe), 135.3 and 135.2 (s, CHAr/
dppe), 134.9 (s, CHAr[Cb]), 132.6 (s, Cb-pyrrolic), 130.2 (s, CAr[Cd]), 129.7
and 129.5 (s, CHAr/dppe), 128.9 (s, CHAr[Cc]), 128.1 and 127.7 (s, CHAr/
dppe), 122.6 (s, Cmeso), 114.8 (s, RuC^C[Cf]), 31.7 (m, CH2/dppe);
1RuC^C[Cf] not observed, possibly overlapped. 31P NMR (81 MHz,
CDCl3,
d in ppm): 51.0 (s, 16P, (dppe)2Ru). Anal. Calcd for
C260H216Cl4N4P16Ru4Zn$CHCl3: C, 67.81; H, 4.73; N, 1.21. Found: C,
67.84; H, 4.77; N 1.32. FTIR (n
, KBr, cmꢀ1): 2057 (vs, RuC^C). HRMS-
ESI (m/z): calcd for [C260H216Cl4N4P16Ru4Zn]2þ: 2251.8556, found:
2251.8359 (Scheme 5).
4.2.2. Zinc(II)-5,10,15,20-tetra{trans-(4-phenylethynyl)[ruthenium(II)
bis(bis(1,2-diphenylphosphino)ethane)](4-nitrophenylethynyl)}
(3eNO2). NaPF6 (18 mg, 0.11 mmol) and NEt3 (0.5 mL) were added
Scheme 5. Atomic numbering scheme for the labelling of the NMR spectra of 2 and 3eX.

10358
S. Drouet et al. / Tetrahedron 68 (2012) 10351e10359
to
a
solution of
2
(70 mg, 0.015 mmol) and 1-ethynyl-4-
Calcd for C296H244N4O4P16Ru4Zn$7CHCl3: C, 63.60; H, 4.42; N, 0.98.
nitrobenzene (10 mg, 0.060 mmol) in CH2Cl2 (10 mL). The mix-
ture was then heated at reflux for 72 h and the reaction mixture
was filtered. The compound was subsequently precipitated by
addition of hexane to the filtrate and washed with MeOH
(2ꢅ10 mL) to afford 3eNO2 as a green product (63 mg). Yield: 85%.
Found: C, 63.98; H, 4.64; N 1.31. FTIR (n
, KBr, cmꢀ1): 2056 (RuC^C).
4.3. Z-scan measurements
Third-order nonlinear optical properties were investigated
with an amplified femtosecond laser system using a Clark-MXR
CPA-2001 Ti-sapphire regenerative amplifier to pump a Light
Conversion TOPAS optical parametric amplifier. Experiments
were performed in a wide range of wavelengths using different
modes of the OPA output and employing polarizing optics,
spatial filtering and colour glass filters to reject unwanted
wavelengths. The pulse duration was approximately 150 fs and
the repetition rate was 250 Hz. The pulse energy was adjusted to
keep the nonlinear phase shifts that were obtained from the
samples in the range of roughly 0.3e1.5 rad, which typically
corresponded to light intensities of the order of 100 GW/cm.2
Solutions of the compounds in dichloromethane of ca. 0.5 w/w
% concentration were placed in 1 mm stoppered Starna glass
cells. An identical cell was used for measurements of Z-scans on
pure solvent. All measurements were calibrated by referencing
to signals obtained from a 3 mm thick fused silica plate, and the
NLO properties of the solute were determined as described
previously.38,45
1H NMR (500 MHz, CDCl3,
d
in ppm): 9.23 (s, 8H, Hb-pyrrolic), 8.13 (d,
3
3
8H, JHH¼7.6 Hz, Hb), 8.03 (d, 8H, JH,H¼8.5 Hz, Hk), 7.89 (m, 32H,
H
para/Ar/dppe), 7.42 (m, 32H, HAr/dppe), 7.40e6.90 (m, 96H, HAr/dppe),
7.31 (d, 8H, 3JH,H¼8.1 Hz, Hc), 6.62 (d, 8H, 3JH,H¼8.4 Hz, Hj), 2.80 (m,
32H, CH2/dppe). 13C NMR (125 MHz, CDCl3,
d in ppm): 150.9 (s, Ca
-
pyrrolic), 143.1 (s, CAr[Cl]), 138.4 (s, CAr[Ca]), 138.1 (s, CAr[Ci]), 137.5,
137.1 (m, Cipso/dppe), 135.0 and 134.4 (s, CHAr/dppe), 134.8 (s, CAr[Ci]),
132.6 (s, Cb-pyrrolic), 130.4 (s, CHAr[Cj]), 129.9 (s, CAr[Cd]), 129.6 and
129.5 (s, CHAr/dppe), 128.5 (s, CHAr[Cc]), 127.9 and 127.7 (s, CHAr/
dppe), 123.9 (s, CHAr[Ck]), 122.3 (s, Cmeso), 119.4 and 119.0 (s, RuC^C
[Ce/h]), 32.0 (m, CH2/dppe); 2RuC^C[Cf/g] not observed, possibly
overlapped. 31P NMR (81 MHz, CDCl3,
(dppe)2Ru). Anal. Calcd for C292H232N8O8P16Ru4Zn$2CHCl3: C,
d in ppm): 54.7 (s, 16P,
68.10; H, 4.55; N, 2.16. Found: C, 68.47; H, 4.56; N 2.25. FTIR (n, KBr,
cmꢀ1): 2042 (RuC^C).
4.2.3. Zinc(II)-5,10,15,20-tetra{trans-(4-phenylethynyl)[ruthenium(II)
bis(bis(1,2-diphenylphosphino)ethane)](phenylethynyl)} (3eH). NaPF6
(35 mg, 0.21 mmol) and NEt3 (0.5 mL) were added to a solution of 2
(60 mg, 0.013 mmol) and phenylacetylene (9 mL, 0.08 mmol) in
Acknowledgements
CH2Cl2 (10 mL). The mixture was heated at reflux for 72 h and this
reaction mixture was filtered. The compound was precipitated by
addition of hexane to the filtrate and washed with MeOH (2ꢅ10 mL)
to afford 3eH as a green product (50 mg). Yield: 79%. 1H NMR
ꢀ
ꢀ
The ‘Universite Europeenne de Bretagne’ (UEB), FEDER, and RTR
BRESMAT are acknowledged for an EPT grant, and the CNRS (PICS
program Nꢁ 5676) is acknowledged for financial support. S.D.
ꢁ
(500 MHz, CDCl3,
d in ppm): 9.22 (s, 8H, Hb-pyrrolic), 8.12 (d, 8H,
thanks the ‘Ministere National de la Recherche et de la Technologie’
3JHH¼8.1 Hz, Hb), 7.84 (m, 8H, HAr/dppe), 7.75 (m, 32H, Hpara/Ar/dppe),
7.62 (m, 32H, HAr/dppe), 7.55e6.90 (m, 88H, HAr/dppe), 7.32 (t, 4H,
(MNERT), A.M. thanks the ‘Region Bretagne’ and D.Y. thanks the
China Scholarship Council (CSC) for their Ph.D. grants. M.S., K.M.,
J.O.-B. and M.W. were supported by the Foundation for Polish Sci-
ence Welcome grant and by a statutory activity subsidy from the
Polish Ministry of Science and Higher Education for the Faculty of
Chemistry of WUT. M.G.H. and M.P.C. thank the Australian Research
Council for support, an ARC Australian Professorial Fellowship
(M.G.H.) and an ARC Australian Research Fellowship (M.P.C.). The
authors are grateful to M.S. Jennaway (RSC, ANU) for technical
support and S. Sinbandhit (CRMPO) for technical assistance and
helpful discussions.
3JH,H¼7.3 Hz, Hl), 7.19 (d, 8H, JH,H¼7.3 Hz, Hc), 7.17 (d, 8H,
3
3JH,H¼7.2 Hz, Hk), 6.82 (d, 8H, JH,H¼7.1 Hz, Hj), 2.80 (m, 32H, CH2/
3
dppe). 13C NMR (125 MHz, CDCl3,
d in ppm): 151.1 (s, Ca-pyrrolic), 137.9
(m, Cipso/dppe), 135.2 and 135.1 (s, CHAr/dppeþCHAr[Cb]), 132.6 (s, Cb
-
pyrrolic), 131.4 (s, CAr[Ca]), 130.7 (s, CHAr[Cj]), 130.4 (CAr[Cd]), 129.5 and
129.3 (s, CHAr/dppeþCHAr[Cc]), 128.9 (s, CAr[Ci]), 128.1 (s, CHAr[Ck]),
127.9 and 127.8 (s, CHAr/dppe),123.6 (Cl, CHAr[Cl]),122.6 (s, Cmeso),117.6
and 117.5 (s, RuC^C[Ce/h]), 32.3 (m, CH2/dppe); 2RuC^C[Cf/g] not
observed, possibly overlapped. 31P NMR (81 MHz, CDCl3,
d
in ppm):
55.2 (s, 16P, (dppe)2Ru). Anal. Calcd for C292H236N4P16Ru4Zn$2CHCl3:
C, 70.55; H, 4.79; N,1.12. Found: C, 70.83; H, 5.04; N 1.33. FTIR (n, KBr,
Supplementary data
cmꢀ1): 2056 (RuC^C).
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.09.108.
4.2.4. Zinc(II)-5,10,15,20-tetra{trans-(4-phenylethynyl)[ruthenium(II)
bis(bis(1,2-diphenylphosphino)ethane)](4-methoxyphenylethynyl)}
(3eOMe). NaPF6 (24 mg, 0.13 mmol) and NEt3 (0.5 mL) were added
to a solution of 2 (70 mg, 0.015 mmol) and 1-ethynyl-4-methox-
ybenzene (12 mg, 0.094 mmol) in CH2Cl2 (10 mL). The mixture was
heated at reflux for 72 h and the reaction mixture was filtered. The
compound was then precipitated by addition of hexane and washed
with MeOH (2ꢅ10 mL) to afford 3eOMe as a green product (60 mg).
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