Synthesis, electrochemical and photophysical properties of covalently linked porphyrin-polyoxometalates

Article abstract of DOI:10.1039/c2dt31415k

Two covalently linked porphyrin-polyoxometalate hybrids have been prepared: an Anderson-type hexamolybdate [N(C₄H₉)₄] ₃[MnMo₆O₁₈{(OCH₂) ₃CNHCO(ZnTPP)}₂] with two pendant zinc(ii)- tetraphenylporphyrins, and a Dawson-type vanadotungstate [N(C₄H ₉)₄]₅H[P₂V₃W ₁₅O₅₉{(OCH₂)₃CNHCO(ZnTPP)}] with one porphyrin. Electrochemical studies show independent redox processes for the organic and inorganic parts at usual potentials. Photophysical studies reveal an electron transfer from the excited porphyrin to the Dawson polyoxometalate, but not to the Anderson polyoxometalate. Time resolved absorption spectroscopy allows the identification of the electron transfer pathways and the determination of the time constants.

Full text of DOI:10.1039/c2dt31415k

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Paper
Synthesis, electrochemical and photophysical properties of covalently linked
porphyrin-polyoxometalates
Clémence Allain, a,b,d Delphine Schaming, b,e Nikolaos Karakostas,b Marie Erard,b Jean-Paul
Gisselbrecht,c Sébastien Sorgues, b Isabelle Lampre b*, Laurent Ruhlmann b,c* and Bernold Hasenknopfa*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Two covalently linked porphyrinꢀpolyoxometalate hybrids have been prepared: an Andersonꢀtype
hexamolybdate [N(C4H9)4]3[MnMo6O18{(OCH2)3CNHCO(ZnTPP)}2] with two pendant zinc(II)ꢀ
tetraphenylporphyrins,
and
a
Dawsonꢀtype
vanadotungstate
[N(C4H9)4]5H[P2V3W15O59ꢀ
10 {(OCH2)3CNHCO(ZnTPP)}] with one porphyrin. Electrochemical studies show independent redox
processes for the organic and inorganic parts at usual potentials. Photophysical studies reveal an electron
transfer from the excited porphyrin to the Dawson polyoxometalate, but not to the Anderson
polyoxometalate. Time resolved absorption spectroscopy allows the identification of the electron transfer
pathways and the determination of the time constants.
porphyrinꢀPOM hybrids connected via a trisꢀalkoxo linker.
Andersonꢀtype POM {MnMo6O24} and Dawsonꢀtype POM
[P2V3W15O62]9– were investigated to study the influence of the
50 POM on the electrochemical and photophysical properties of
zinc(II)ꢀtetraphenylporphyrin.
15 Introduction
Polyoxometalates (POMs) are attractive molecular clusters
not only because of their rich structural chemistry,1 but also due
to their rich electronic and optical properties.2 In particular, they
are known for their ability to reversibly accept electrons without
20 significant structural changes.1 Porphyrins are also very
appealing and have photophysical properties that are widely used
in photoactive systems.3 Indeed, with appropriate metal ions, they
have been shown to act as electron donors in a variety of
supramolecular systems.4 Thus the combination of porphyrins
Results and Discussion
Synthesis
Porphyrin ZnTPPꢀTris (1, Scheme 1) was synthesized in three
55 steps with an overall 8.8% yield, which is satisfactory for
porphyrin derivatives. Detailed procedures are in the ESI. 5ꢀ(4ꢀ
carboxyphenyl)ꢀ10,15,20ꢀtriphenylporphyrin was prepared by the
Lindsey synthesis,17 and then metallated with zinc(II) acetate.
Amidation with trisꢀhydroxymethylmethane18 introduced the
60 necessary triol function. Functionalized Andersonꢀtype POM
[N(C4H9)4]3[MnMo6O18{(OCH2)3CNHCO(ZnTPP)}2] (2) was
prepared by the reaction of [N(C4H9)4]4[αꢀMo8O26] with
Mn(OAc)3 and ZnTPPꢀTris 1 in N,Nꢀdimethylacetamide (DMA)
for 60 h at 80ꢀ90°C, in adaptation of our published procedure.19
65 In this synthesis, the functionalized POM is formed in a selfꢀ
assembly type reaction from molybdate, MnIII ion and triol.
25 with POMs is expected to yield promising organicꢀinorganic
6
hybrids5,
capable of forming chargeꢀseparated states by
photoinduced electron transfer.7 Assemblies linking POMs and
porphyrins have been prepared as bulk materials8 or as thin
films,9 and used as catalysts10 or for electrocatalytic reductions.11
30 However, systems with stronger links and better control of the
stoichiometry and the relative orientation of the components
would be of interest for some applications such as
photocatalysis12 or NonꢀLinearꢀOptic materials.13
We prepared a series of coordination complexes between
35 POMs bearing pendant pyridyl groups and metalloporphyrins.14
Electronic communication between the polyoxometalate and the
porphyrin was established, but our study showed that a stronger
association between the two components of the complexes was
desirable. Covalent attachment of organic ligands to the POMs
40 appears as the way of choice.5 Harriman, Odobel and coll. have
reported their seminal work on single porphyrins and porphyrin
Because trivanadium substituted Dawsonꢀtype heteropolyanions
21
are known to be catalytically active,20,
we synthesized also
[N(C4H9)4]5H[P2V3W15O59{(OCH2)3CNHCO(ZnTPP)}]
3
by
70 reacting 1 with [N(C4H9)4]5H4 [P2V3W15O62] in DMA for 7 days
at 80ꢀ90°C, in adaptation of a procedure of Hill.20, 21 Here, the
POM structure is preformed and then grafted with the triol. Both
POMs were obtained in good yields (80%). They were isolated as
solids by precipitation with ether from the reaction mixtures and
75 thoroughly washed to obtain satisfactory chemical analysis and
clean NMR spectra. For the photophysical studies reported
below, the microcrystalline powders were redissolved in
assemblies connected to monolacunary α2ꢀ[P2W17O61]10– 15
Fukuzumi et al. presented recently the intramolecular
photoinduced electron transfer between a tin(IV) porphyrin and
45 directly coordinated [PW12O40]3–.16
.
We present here the preparation and characterization of
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acetonitrile and precipitated in ethyl acetate (repeated five times).
The mother liquor was very slightly coloured purple.
Electrochemical data of compounds 1ꢀ3, Andersonꢀtype POM
[MnMo6O18{(OCH2)3CNHCO(4ꢀC5H4N)}2]3– (4),14 and Dawsonꢀ
type POM [P2V3W15O62]9– (5)20 in organic solution (acetonitrile
35 or DMF) are gathered in Table
1 and typical cyclic
voltammograms (CV) are presented in Figure 1. All redox
potentials E, evaluated from cyclic voltammetry as (Eap + Ecp)/2,
are reported in V versus the standard saturated calomel electrode
(SCE). The electrochemical behavior of metalloporphyrins
40 involving nonelectroactive metals is well documented.26, , , , 30 In
the potential range of +1.5 to ꢀ2.5 V vs. Fc+/Fc, the oxidation and
the reduction of the
π ring proceed both via two reversible oneꢀ
electron steps, generating radical cations and dications or radical
anions and dianions, respectively.
45
Early electrochemical studies of the octaethylporphyrin (OEP)
and tetraphenylporphyrin (TPP) complexes – including their ZnII
complexes – utilized in large part cyclic voltammetry to measure
halfꢀwave potentials of each redox reaction. They demonstrated
that most of the porphyrins exhibited a constant potential
50 difference between the first and second macrocycleꢀcentered
oxidation, and between the first and second macrocycleꢀcentered
reduction.28 These porphyrins have also a similar HOMOꢀLUMO
gap of 2.25 ± 0.15 V, which corresponds to ꢁE = ꢁ|ERed1–EOx1|.31
This constant gap ꢁE was then used as diagnostic criteria to
55 distinguish macrocycleꢀcentered reactions from metalꢀcentered
ones.
Porphyrin-based redox processes
Porphyrin 1 presents the usual two oneꢀelectron reduction and
two oneꢀelectron oxidation steps (Figure 1). For compounds 2
60 and 3, similar signals are detected without a significant change of
Scheme 1: Synthesis of porphyrinꢀPOM hybrids 2 and 3.
the redox potentials (peaks labeled
ꢀ in Figure 1, Table 1). They
are therefore assigned to the porphyrin ligands on the POMs. The
potential difference between the first and second porphyrin
reduction is 0.41 V in 2 and 0.30 V in 3. The corresponding
65 values for porphyrin oxidation in 2 and 3 is 0.31 V. These values
are close to those typically observed for ring reductions and
oxidations of metalloporphyrins,28 and compare well to those in 1
(0.35 V and 0.31 V for reduction and oxidation, respectively).
The new irreversible wave for 3 observed at 0.81 V vs. Fc+/Fc
70 (Eox3, Table 1 and Figure 1) may be attributed to the oxidation of
an isoporphyrin,32, 33 which implies a nucleophilic attack of the
porphyrin dication ZnTPP2+ subunit. This agrees well with the
known redox behaviour of porphyrins in the presence of
nucleophiles34 and in particular with the well documented reports
75 of Hinman.32 We propose that the nucleophile is trace amount of
5
For compound 2 grafting of the organic ligand onto the
paramagnetic POM is evidenced in 1H NMR by a broad singlet at
δ
= 64 ppm for the CH2O groups linked to the POM, as well as a
strong line broadening for the porphyrin signals (see ESI) in
accordance with analogous structures.19,22 The ratio of integration
10 ligand / TBA cations is consistent with the grafting of two
1
porphyrins per POM unit. For compound 3, H, and 31P NMR
20, 21,23
spectra are consistent with earlier work
and indicate the
substitution of the three ꢀ2ꢀoxo ligands of the V3 cap in the parent
polyoxometalate by the three ꢀ2ꢀalkoxy groups of the triol 1. In
15 addition, 51V and 183W NMR spectra (see ESI, Figures S6 and S7)
show respectively one (δ = –538.3 ppm) and three peaks (δ =
–121.5, –150.1, –163.3 ppm), which is characteristic of a local
C3v symmetry.24 By comparison with literature data,25 the two
narrow 183W NMR signals at –121.5 and –163.3 ppm can be
20 assigned to the W3 cap and the adjacent W6 belt. The third signal
at –150.1 ppm is broadened as expected for the W6 belt linked to
the quadrupolar V atoms.
The IR spectra of 2 and 3 (see ESI) are also in agreement with
the proposed structures. They display the strong metalꢀoxo
25 vibrational bands between 700 and 1000 cm–1 for the inorganic
framework, and the bands for the organic ligand. Finally, the
electrospray mass spectra (see ESI) display the molecular anions
associated to varying numbers of TBA+ and H+ cations. All
signals of significant intensity are assigned, and all charge states
30 that fall within the spectral window are observed.
water present in the solution. The presence of reduction peak
ꢁ
on the CV curves of 3 (Figure 1) could be explained by reduction
of such an isoporphyrin. Similar reductions steps characterized
the redox chemistry of isoporphyrins.35 The oxidation peak
ꢁ
80 around –1.0 V only occurs after scanning beyond ꢀ2.5 V
(compare with the dotted CV in Figure 1). We attribute it to a
chemical reaction (protonation) of the dianion porphyrin which
becomes a strong base.
The experimental ꢁE values (HOMOꢀLUMO gap) of 1, 2 and
85 3 are in the range of 2.05 to 2.10 V, very close to the calculated
values of 2.25 ± 0.15 V, obtained by Zerner and Gouterman for
ZnTPP.31
Electrochemistry
2
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Table 1: Electrochemical data for compounds 1, 2, 3, 4 and 5 [a]
Compounds
Reduction
Oxidation
MVI/MV
VV/VIV MnIII/MnII
MnIII/IV
Ring
Ring
(W or Mo)
Ered1
–1.78
–1.76
Ered2
–2.13
–2.17
Eox1
0.29
0.29
Eox2
0.60
0.60
Eox3
1
2
Epc = –1.64
Epa = –0.71
–0.50b
–2.34
0.39
3
4
–1.76b
–2.06b
–2.24
–2.36
0.34b
0.65b,c
0.81b,c
–1.48b,c
–1.03b
0.44b
520
ꢀ0.31
ꢀ0.60
[a]: Data obtained from cyclic voltammetry (scan rate 100 mV/s) on glassy carbon electrode. Analyte concentration: 0.5 mM, with 0.1 M TBAPF6 as
electrolyte and DMF as a solvent unless otherwise noted. Ei, approximated by (Eap + Ecp)/2, are given in V vs. Fc+/Fc. [b]: in CH3CN, [c]: irreversible.
The electrochemical oxidations and reductions have also been
monitored by spectroelectrochemistry (see below) and these
results confirm these assignments.
are exchanged after these first two oxidations. This apparent
contradiction may be explained by a subsequent chemical
reaction in the case of electrolysis. At the electrode occurs the
25 first oxidation of the two metalloporphyrins as well the oxidation
of MnIII to MnIV. This in turn might react with itself by
disproportionation to form 0.5 equiv. of manganese (V) and 0.5
equiv. of the starting manganese (III) species. This species would
then be oxidized again at the electrode to manganese (IV). In the
30 end, such disproportionation would result in the overall transfer
of 2 electrons during bulk electrolysis and the formation of a
manganese (V) species.14
Moreover, the cathodic redox process MnIII/MnII is very slow
with the reduction peak at –1.64 V and the corresponding
35 reoxidation peak at ꢀ0.71 V (
the structurally
[MnMo6O18{(OCH2)3CNHCO(4ꢀC5H4N)}2] 4
ꢂ) (see also ESI, Figure S14). For
close
Anderson
POM
14
functionalized
by two pyridines instead of porphyrins, both redox couples
MnIII/MnIV and MnIII/MnII showed rapid, reversible electron
40 exchange (Table 1), but the couple MnIII/MnII was also
remarkably slowed down upon formation of a complex with
[Ru(CO)TPP].14 Thus, this process is most sensitive to the
ligands grafted onto the POM. The first reduction associated to
the MoVI/MoV couple is measured at ꢀ2.34 V (
45 potential observed for 4.14 A linear dependence of cathodic peak
currents of MnIII/MnII on the square root of the scan rates (v1/2
(Figure S15, ESI) indicates that this process is diffusionꢀ
controlled. Thus this wave is clearly not due to adsorption onto
the electrode but corresponds to the reversible exchange of the
50 electron which is slow in this case.
Exhaustive bulk electrolysis of 2 is also carried out in the
reduction part. Controlledꢀpotential coulometry at –1.70 V
yielded 0.9 electron exchanged, which demonstrates that the first
reduction process involves only one electron. Electrolysis at
55 –1.88 V shows the transfer of 3.0 electrons. The two additional
electrons of the second wave correspond to the first reduction of
both ZnTPP macrocycles. The normal pulse voltammogram, the
linear sweep voltammogram and the differential pulse
voltammogram presented in Figure S17 (ESI) agree with this
ꢂ) close to the
5
Figure 1. Cyclic voltammograms of compounds 1 and 2 in DMF and of 3
in CH3CN, using 0.1 M TBAPF6 as electrolyte. v = 100 mVꢁs–1. (ꢀ) first
two oxidation and reduction peaks of 1. (ꢁ) peaks resulting from
chemical reactions after generation of the dianion or dication porphyrin
(see text). (ꢂ) peaks attributed to the reduction or oxidation of 2. (ꢀ)
)
10 reduction peaks related to 3. c = 0.5 mmolꢁL–1.
POM-based redox processes
The CV present furthermore the typical redox processes for the
POM frameworks. Considering the oxidation of the Anderson
15 POM 2, only the wave for MnIII/MnIV is well resolved at +0.39
V vs. Fc+/Fc (
ꢂ). Controlledꢀpotential coulometry at +0.47 V,
after this and the next anodic process (because the waves are too
close to be discriminated), results in the exchange of 4.1 electrons
per compound 2. The normal pulse voltammogram, the linear
20 sweep voltammogram and the differential pulse voltammogram
presented in Figure S17 (ESI), indicate that only three electrons
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observation. They show the sequence: a one electron wave
(MnIII/II couple of the POM subunit) followed by two twoꢀ
The steadyꢀstate absorption and fluorescence spectra recorded for
the ZnTPPꢀTris porphyrin 1 in DMF are identical to those
electron waves (first and second reductions of both porphyrins) 65 obtained for ZnTPP in the same solvent,38, 43 showing that the tris
and a oneꢀelectron wave (MoIV/V couple of the POM subunit).
Thinꢀlayer visible spectroscopy during the reduction of Anderson
POM 2 further confirms these assignments (see Figure S18, ESI).
At –1.70 V no change of the spectrum is observed. Only upon
reduction at –1.98 V, the intensity decrease of the Soret and Q
bands and the increase of two new bands at ca 460 nm and 715
10 nm are observed. These changes are typical for the generation of
porphyrin πꢀanion radicals.36 The reduced complex reverts
quantitatively to the initial compound 2 on the reverse potential
scan at –0.48 V. The change of the POM band (UV domain) has
not been examined because the used Pyrex cell cuts the
15 wavelength bellow 280 nm.37
substituent on one phenyl group has little influence on the
electronic transitions of the macrocycle. The electronic
absorption spectrum exhibits the expected Soret band at 426 nm
and the Q bands at 559 and 600 nm, associated to the S0 → S2
70 and S0 → S1 electronic transitions, respectively. The fluorescence
emission presents two bands at 610 and 665 nm.
5
The UVꢀvis absorption spectra (see ESI, Figure S8) of both
hybrid compounds
2 and 3 in DMF correspond to the
superpositions of the spectra of the precursors (compound 1 and
75 POM). Neither the position nor the intensity of the porphyrin
absorption bands are significantly influenced by the POM, and no
additional band is observed. This indicates the lack of strong
electronic interaction between the polyoxometalate and the
porphyrin in the ground state.
In the case of Dawson POM 3, the reduction processes at –0.50 V
and –1.48 V (ꢀ) correspond to the quasiꢀreversible reduction of
the VV, the first reduction WVI/WV being observed at –2.24 V.
Such quasiꢀreversible redox processes concerning VV were
20 already reported by Hill20 for [P2V3W15O62]9– and
[CH3C(CH2O)3P2V3W15O59]6–, even if the potentials are
somewhat shifted. These compounds have a high tendency to
stick to the electrode.
80 The fluorescence emission spectra of ZnTPPꢀTris and the two
hybrids in DMF are shown in Figure 2. The emission of
Anderson 2 is identical to that of ZnTPPꢀtris, with the presence of
two bands peaking at 610 and 665 nm respectively. Whereas no
quenching is observed for 2, the fluorescence is strongly
85 quenched for 3. The emission intensity for Dawson 3 is reduced
by about 97% with respect to that of compound 1 (Figure 2). This
fluorescence quenching attests to a coupling between the excited
porphyrin and the POM cluster in hybrid 3. It is to note that,
although the fluorescence is always quenched for 3 compared to
90 1, the relative intensity and the position of the emission bands
depend on the excitation wavelength and that a band around 720
nm is also detected. While the excitation spectrum recorded at
610 nm is identical to the absorption spectrum of hybrid 3, the
excitation spectrum recorded at 720 nm presents the characteristic
95 features (Soret and four Q bands) of freeꢀbase porphyrins (see
ESI, Figure S9). These results reveal the presence of small traces
of free base porphyrin H2TPPꢀTris produced during the synthesis
of hybrid 3 and not entirely removed by purification.
Exhaustive bulk electrolysis of Dawson POM 3 at –0.68 V yields
25 1.0 electron transferred. Running the electrolysis of this
compound at –1.63 V yields 2.9 electrons. This demonstrates that
the first reduction process involves one electron while at the
potential of the second process, all the VV atoms seem to be
reduced to VIV. Electrolysis at the intermediate potential –1.18 V
30 yields 2.0 electrons transferred, suggesting that one process of
reduction of VV to VIV is masked but probably exists. Indeed, in
some scans, one illꢀdefined and broad additional wave can be
observed, which makes in total three processes for the three
vanadium reductions.
35 Electrolysis at –1.88 V shows the exchange of 4.1 electrons per
POM 3 in agreement with the reduction of the triꢀvanadium (V)
cap and the first reduction of the ZnTPP macrocycle.
Furthermore, the electrochemical reduction of compound 3 has
also been monitored by spectroelectrochemistry (Figure S19).
40 Study at –1.58 V (after the second reduction wave of vanadium
observed) shows a decrease of the band at ca 320 nm that is
POMꢀbased. This is accompanied by small variations of
porphyrin based Soret and Q bands. Thus, at this applied
potential, the POM is reduced which affects slightly the ZnTPP.
45 However, upon reduction at –1.83 V, a decrease in the intensity
of the Soret and Q bands accompanied by an increase of two new
bands at ca 460 nm and 715 nm, typical for the generation of
porphyrin πꢀanion radicals,36 are observed. The reduced complex
reverts quantitatively to the initial compound 3 on the reverse
50 potential scan at ꢀ0.38 V.
Spectroelectrochemical studies have also been performed for the
oxidations of POMs 2 and 3. For instance, thinꢀlayer visible
spectroscopy obtained during the oxidation of Anderson POM 2
indicates that electrolysis at +0.42 V produces the porphyrin πꢀ
55 cation radicals with the expected weakening of the Soret band
accompanied by the increase of new bands, one at ca 450 nm and
a broad band near 720ꢀ900 nm region (see Figure 18, ESI).37 The
spectral changes of 3 upon oxidation are similar to that of 2.
In conclusion, spectroscopic and electrochemical data are
60 consistent with our assignments of the redox processes in Table
1.
100
Figure 2: Emission fluorescence spectra of porphyrin 1 (black),
Andersonꢀtype POM 2 (blue) and Dawsonꢀtype POM 3 (red) in DMF
recorded for the same absorbance at the excitation wavelength 425 nm.
Fluorescence quenching in compound 3 could occur either from
105 energy or electron transfer process within the hybrid complex. In
order to make the difference, it is useful to estimate the redox
Photophysical properties
4
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structural flexibility in compound 3. Indeed, distinct conformers
potentials of the excited porphyrin. Given the electrochemical
data, the oxidation potential of the excited porphyrin, Eox*, can be 60 with different quenching efficiency have already been reported
approximated using the formula:
Eox* = Eox1 – Etr (1)
for Dawsonꢀtype POM covalently linked to porphyrin residues.15
5
where Eox1 is the first oxidation potential of the macrocycle and
Etr corresponds to the energy of the electronic transition. The
obtained values are Eox1* = –1.73 V and Eox2* = ꢀ2.57 V vs.
Fc+/Fc, for the porphyrin in the S1 or S2 excited singlet state,
respectively. Those values are well below the first reduction
10 potential of POM (Ered(VV/VIV) = –0.50 V vs. Fc+/Fc) leading to
negative ꢁG° for the electron transfer reactions from S1 and S2
(ꢁG1° = –1.13 and ꢁG2° = –1.97 eV, respectively). So, the
quenching in compound 3 is mainly attributed to electron
transfers from the excited states of the porphyrin to the POM.
15 In the case of compound 2, the fluorescence results give no
evidence of an electron transfer within the complex. From
electrochemical data, the electron transfer from the excited
porphyrin to the Anderson POM is thermodynamically feasible,
although less favourable. But, as the MnIII/MnII reduction appears
20 strongly slowed down in the hybrid, the electron transfer could be
avoided for kinetic reasons.
Figure 3: Fluorescence decay profiles of compounds 1 (black), 2 (blue)
and 3 (red) in DMF recorded at 610 nm (excitation wavelength 420 nm).
To gain a better understanding of the photophysical properties of 65 Nanosecond transient differential absorption spectra of the three
compounds 2 and 3, timeꢀresolved emission and both nanosecond
and femtosecond transient absorption studies were undertaken.
25 The experimental results detailed below yield ultimately the
schematic diagram for electron transfer in Figure 6.
compounds in deoxygenated DMF solution were recorded after
excitation with 3 ns laser pulses at 532 nm (see ESI Figure S11).
Porphyrin 1 shows the formation of a longꢀlived triplet state (τΤ =
1.1 ms), consistent with literature data for mesoꢀtetraphenylꢀ
The fluorescence decay profiles recorded at 610 nm for the three 70 porphyrin derivatives.39, 40, 41 The T1 triplet state is formed for the
compounds are depicted in Figure 3. The emission signal of
ZnTPPꢀTris follows a monoexponential decay with a lifetime of
30 1.9 ns, in agreement with data reported for related porphyrins in
DMF.38ꢀ, The emission profile of compound 2 is identical to that
hybrids too, but the lifetime and yield depend on the POM. For
the Anderson hybrid 2, the triplet lifetime is considerably
shortened (τT = 7.5 ꢀs) while the initial differential absorbance is
roughly 90% of that determined for ZnTPPꢀTris. For the Dawson
of the nonꢀcomplexed porphyrin indicating the absence of fast 75 compound 3, the triplet lifetime is slightly reduced (τT = 0.60 ms)
processes from the lowest singlet excited state in compound 2. In
contrast, the signal of compound 3 is satisfactorily fitted by a
35 fourꢀexponential decay with time constants: τf1 = 60 ps (76%),
but the initial differential absorbance corresponds to only 2% of
the value obtained for ZnTPPꢀTris in the same conditions (same
absorbance at the excitation wavelength, same laser pulse
energy). These results have to be regarded together with the
τ
f2 = 255 ps (22%), τf3 = 1.9 ns (2%), τf4 ≈ 8.4 ns (0.03%). It is to
note that the relative contributions to the decay depend on factors 80 fluorescence data. For compound 2, in the absence of
such as the fluorescence quantum yield of the species and
excitation and detection conditions. It is therefore not possible to
40 relate them to relative amounts of different species. The slowest
and fourth component with a very small contribution is attributed
fluorescence quenching, the triplet state originates from the
hybrid and its decay is accelerated because of heavy and
paramagnetic atom effects. For compound
3 with high
fluorescence quenching, we assume that the triplet signals, as
to the presence of slight traces of freeꢀbase porphyrin. The 85 well as the remaining steadyꢀstate fluorescence and the
obtained lifetime is also in agreement with reported values.38,39
Steadyꢀstate fluorescence and timeꢀresolved measurements
45 performed on 3 before and after additional purification (see ESI,
Figure S10) indicate that we could reduce the presence of this
τf3 = 1.9 ns fluorescence lifetime, are due to the presence of traces
of nonꢀcomplexed porphyrin.
Femtosecond transient absorption spectroscopy brings
information about fast photoinduced transformation. The excited
very small impurity, but not remove it entirely. The third 90 state properties of ZnTPPꢀTris are found similar to those reported
component has the lifetime of the free precursor porphyrin 1 (also
not detected by NMR spectroscopy) while the first two
50 components show that fast intramolecular quenching processes
take place. These first two lifetimes are related to either different
for ZnTPP.41,42 The picosecond differential spectrum of 1 in DMF
solution is characterised by a strong absorption band peaking at
456 nm between the Soretꢀband and Qꢀband bleachings and a
broad absorption band over 570ꢀ700 nm (see ESI Figure S12).
species or distinct conformers of compound 3. As there are traces 95 These spectral features correspond to the S1 excited state of the
of nonꢀcomplexed freeꢀbase porphyrin H2TPPꢀTris, one can argue
that traces of a DawsonꢀH2TPPꢀTris complex might be produced
55 during the synthesis. However, such tiny amounts should not
have any significant contribution. It rather seems highly probable
porphyrin. The transient absorption kinetics reveal that the
absorption band around 460 nm and the stimulated emission
around 660 nm appear with a time constant of 1.3 ps due to S2 →
S1 internal conversion after 395 nm excitation in the Soret band
that there exist distinct conformations that differ according to 100 (see ESI Figure S12). This time constant is in agreement with
their ability to undergo quenching (electron transfer) due to the
previous values reported for the S2 state depopulation of ZnTPP
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in DMF.43ꢀ45
The transient absorption spectra obtained for compound 3 in
DMF solution after excitation with 100 fs laser pulses at 395 nm
are shown in Figure 4. The spectrum at 1 ps with an absorption
band maximum located at 445 nm differs from the spectrum
recorded for ZnTPPꢀTris with a maximum at 456 nm. For the first
3 ps, a band around 442 nm rises, then drops revealing a band
around 460 nm. Subsequently, the band at 460 nm also decreases
and after 500 ps a spectrum with an absorption band maximum at
5
10 447 nm is observed. This spectrum differs notably from those
obtained for the S1 or T1 excited states of ZnTPPꢀTris revealing
the formation of a new longꢀlived state. The lifetime of this state
is too long to be determined within the time window of our setꢀup
(700 ps).
15
Figure 4: Transient absorption spectra obtained for compound 3 in DMF
solution after excitation with 100 fs laser pulse at 395 nm.
30
Figure 5: Transient absorption kinetics obtained for compound 3 in DMF
solution after excitation with 100 fs laser pulse at 395 nm. Solid lines are
threeꢀexponential fits.
The transient absorption kinetics analysis shows that all the
signals can be fitted by a constant associated with the final longꢀ
20 lived state and a threeꢀexponential function with the same time
constants, τ1 = 830 ± 90 fs, τ2 = 9.0 ± 1.3 ps and τ3 = 93.5 ± 3.5
ps (Figure 5). The absorption signal at 442 nm rises with the time
constant τ1 = 830 fs and then decay according to a biꢀexponential
law with time constants τ2 = 9 ps and τ3 = 93.5 ps. The kinetics
25 profiles obtained at 460 and 520 nm are fitted by a biꢀexponential
rise (τ1 = 830 fs and τ2 = 9 ps) and a slow exponential decay
(τ3 = 93.5 ps). The bleaching signal at 560 nm and the signal at
650 nm correspond to threeꢀexponential decays.
35 The formation of the S1 state by internal conversion (IC) from the
S2 state is characterized by the appearance of both intense
absorption at 460 nm and stimulated emission around 660 nm.
These signals grow with a time constant of 830 fs, faster than for
the free porphyrin 1. Moreover, a new band at 442 nm appears
40 with the same time constant. That suggests the presence of an
additionnal deactivation pathway for the S2 state in the complex.
We assign this deactivation channel to an electron transfer from
the excited porphyrin to the linked POM inducing the formation
of a charge transfer (CT) state which absorbs mainly around 440
45 nm. From the measured lifetimes (τ1 = 830 fs for compound 3
S
2
and τIC = 1.3 ps for ZnTPPꢀTris), the electron transfer rate, kET
is evaluated according to equation 2:
1
1
(2)
S2
kET
=
−
τ1 τIC
The kET value is found to be 4.3 × 1011 s–1, of the same order of
50 magnitude as the reported values for rate constants of electron
transfer from S2 in covalently linked ZnTPPꢀacceptor
molecules.42,44
S
2
The S1 state of the porphyrin in the complex, observed mainly at
460 nm follows a 93.5 ps decay, much faster than that measured
55 for the free porphyrin. That shows the existence of a new
deactivation process from the S1 state. We consider that, as for S2,
6
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the new deactivation pathway from S1 is an electron transfer
process leading to a longꢀlived CT state. It is to note that the time
transfers from the excited porphyrin unit to the Dawson POM
have been revealed. We will exploit this for the development of
constant of 93.5 ps determined by transient absorption does not 60 porphyrinꢀPOM hybrids with a photocatalytic activity.
match the fast lifetimes obtained by timeꢀresolved fluorescence
5
measurements, but rather their weighted average. That can be
explained if the two lifetimes are related to two conformers which
undergo electron transfer leading to similar final CT spectra.
From the measured lifetime of the S1 state in the complex
(τf1 = 60 ps) and in the free porphyrin (τS1 = 1.9 ns), the fastest
S
10
1
10 electron transfer rate from S1 is evaluated to be kET = 1.6 × 10
s–1. This value is in line with intramolecular electron transfer
rates reported for ZnTPP hybrids or dyads.39, 41, 42, 44, 48
The final CT state in which an electron has been transferred from
the excited singlet state of the zinc porphyrin to the covalently
15 linked POM is characterized by an absorption less intense and
shifted to the blue (447 nm) compared to the absorption of the S1
state. These spectral features are consistent with the formation of
the porphyrin πꢀradical cation49,50 and the reduced Dawson in
their ground state. Indeed, the absorption spectra of oxidised
20 ZnTPP porphyrin exhibit a sharp intense Soret band around 410
nm (ε ~2 × 105 mol–1L cm–1) with a shoulder around 450 nm
(ε ~3 × 104 mol–1L cm–1) and a broad and featureless band over
500ꢀ700 nm (ε ~1 × 104 mol–1L cm–1).51,52 Oneꢀelectron reduced
Dawson polyoxometalates also absorb, but in the UV domain (out
25 of our spectral window) and in the near infraꢀred region with a
low extinction coefficient (ε ~5000 mol–1L cm–1).53,54 The formed
CT state disappears due to reverse electron transfer on the
nanosecond time scale regenerating the groundꢀstate compound 3.
The band at 442 nm, spectral signature of the photoinduced CT
30 state from S2, drops with a time constant of 9 ps whereas
concomitantly other signals rise or decay. At 30 ps, the measured
Figure 6: Schematic diagram showing the assumed deactivation processes
occuring in compound 3: internal conversion from S2 to S1
S
2
65 (kIC = 7.7 × 1011 s–1), electron transfer from the S2 state (kET
4.3 × 1011
=
s–1 electron transfer from the S1 state kET 1 = 1.6 × 1010 s–1), vibrational
S
),
(
relaxation in the charge transfer state (kVR ≈ 1.16 × 1011 s–1) radiative and
non radiative desexcitation processes from S1 to S0 and intersystem
crossing fron S1 to T1 (kS + kISC = 5.3 × 108 s–1), reverse electron transfer
70 (kRET < 1 × 109 s–1).
Acknowledgment. This work was supported by the CNRS, the
Université ParisꢀSud (Orsay, France), the Université Pierre et
75 Marie Curie (Paris, France), the Université of Strasbourg
(Strasbourg, France) and by the ANR agency, project n°
JC05_52437 (postꢀdoctoral fellowships to C.A. and N. K.). We
would like to thank Séverine Renaudineau and Joaquim Li for the
preparation of TBA5H4[P2V3W15O62], René Thouvenot for
80 recording the 51V and 183W NMR spectra, and Carlos Afonso and
JeanꢀClaude Tabet for help with the ESIꢀMS.
differential absorption spectrum corresponds mainly to
a
combination of the S1 state and the final CT state spectra, because
the substraction of the 500 ps spectrum from the 30 ps spectrum
35 results in a spectrum similar to that of the S1 state of the free
porphyrin (see ESI Figure S13). We assume that the electron
transfer from S2 leads to a vibrationally excited CT state, which
converts to the final CT state by vibrational energy relaxation
(VR). The measured relaxation time (9 ps) is in accordance with
40 the time scale of 1ꢀ40 ps reported for VR in ZnTPP πꢀcation
radical and porphyrins.50,52,55,56 From absorption and fluorescence
data, we have no evidence of recombination from unrelaxed CT
states to the S1 state of compound 3, as recently described for
porphyrinꢀacceptor dyad.45 Nevertheless, this pathway cannot be
45 completely excluded.
Figure 6 resumes the assumed deactivation processes occuring in
hybrid 3.
Conclusion
In conclusion, covalent porphyrinꢀPOMs have been synthesized
50 and characterized. They demonstrate how the fruitful
combination of multistep organic and inorganic synthesis yields
original functional molecules. Electrochemical studies reveal the
separate redox processes on the organic and inorganic parts, and
the lack of communication in the ground state. Photophysical
55 studies have shown an influence of the polyoxometalate on the
properties of the porphyrin, i. e. communication in the excited
state. Indeed, in the case of compound 3, photoinduced electron
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Table of Contents entry
Two polyoxometalates with the same zincꢀ
porphyrin photosensitizer are synthesized and
thoroughly characterized to reveal the possibility
of electron transfer.
5
10
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Notes and references
a UPMC Univ. Paris 06, Institut de Chimie Moléculaire, UMR CNRS
7071, 4 place Jussieu, 75005 Paris, France. E-mail:
5
bernold.hasenknopf@upmc.fr
b Université Paris-Sud 11, Laboratoire de Chimie Physique, UMR CNRS
8000, Bâtiment 349, 91405 Orsay cedex, France.
9
c Present address: Université de Strasbourg, Institut de Chimie,
Laboratoire d'Electrochimie et de Chimie Physique du Corps Solide,
10 UMR CNRS 7177, 4 rue Blaise Pascal, CS 90032, 67081 Strasbourg
cedex, France. E-mail: lruhlmann@unistra.fr
d Present address: ENS Cachan, PPSM, UMR CNRS 8531, 61, avenue du
Président Wilson, 94235 Cachan cedex, France.
e Present address: Université Paris Diderot - Sorbonne Paris Cité,
15 ITODYS, UMR CNRS 7086, Bât. Lavoisier, 15 rue Jean-Antoine de
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