Strongly coupled zinc phthalocyanine-tin porphyrin dyad performing ultra-fast single step charge separation over a 34 A distance

Article abstract of DOI:10.1039/b711642j

A zinc(ii) phthalocyanine-tin(iv) porphyrin dyad with a strong electronic coupling was synthesized and upon light excitation shown to exhibit ultra-fast, long-range electron transfer in a single step. The Royal Society of Chemistry.

Full text of DOI:10.1039/b711642j

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Strongly coupled zinc phthalocyanine–tin porphyrin dyad performing
˚
ultra-fast single step charge separation over a 34 A distance{
Je´roˆme Fortage,a Erik Go¨ransson,b Errol Blart,a Hans-Christian Becker,*b Leif Hammarstro¨m*b and
Fabrice Odobel*a
Received (in Cambridge, UK) 30th July 2007, Accepted 8th October 2007
First published as an Advance Article on the web 17th October 2007
DOI: 10.1039/b711642j
A zinc(II) phthalocyanine–tin(IV) porphyrin dyad with a strong
electronic coupling was synthesized and upon light excitation
shown to exhibit ultra-fast, long-range electron transfer in a
single step.
The development of molecular systems that carry out rapid and
directional energy or electron transfer over a long distance is an
important research goal because these are key photofunctions
needed for photovoltaic, artificial photosynthesis and molecular
electronic applications. Although much attention has been paid to
the preparation and study of porphyrin1,2- or phthalocyanine3-
based molecular systems for energy or electron transfers, there are
few examples of porphyrin–phthalocyanine dyads that perform
electron transfer.4 Multichromophoric systems containing both
Scheme 1 Synthetic route to the dyads 6–8. Reagents and conditions: (a)
porphyrins and phthalocyanines are appealing because the distinct
piperidine, Pd2dba3–CHCl3, AsPh3, 40 uC, 15 h (43%); (b) Bu4NF, THF
absorption bands of these two pigments permit harvesting of a
(100%); (c) THF, Et3N, Pd2dba3–CHCl3, AsPh3, 60 uC, 15 h (33%); (d)
HCl, CH2Cl2 (91%); (e) pyridine, SnCl2–2H2O, 185 uC, 3 h (83%).
large part of the solar spectrum and enable selective excitations. In
most of the biomimetic porphyrin systems, the bridge is connected
to the porphyrin via a meso-phenyl group that limits the electronic
communication owing to poor frontier molecular orbitals overlap
between the connected elements.1,2,5,6 Alkynyl substituted phtha-
locyanines or porphyrins are attractive units for the design of
systems with a strong electronic coupling. This feature has been
previously used for the development of non-linear optical
chromophores,7,8 but it has rarely been tested for long-range
electron transfer.9 As part of our continuing interest in electro-
nically coupled systems to perform long-range photoinduced
energy or electron transfer,5,6 we report here the preparation and
the photophysical investigation of a strongly electronically coupled
tin porphyrin–zinc phthalocyanine dyad that performs ultra-fast
charge separation over a 34 s centre-to-centre distance.
The synthetic route for the preparation of the dyad 8 is shown in
Scheme 1. Zinc iodo phthalocyanine 1 was connected to the bridge
2 using the Heck ethynylation conditions.10 The triisopropyl
protecting group was removed by fluoride anion and the zinc
iodoporphyrin 5 was then appended to compound 4 by a second
cross-coupling reaction using the same conditions.
Zinc porphyrin in dyad 6 was selectively demetallated with
hydrochloric acid and the corresponding free base porphyrin 7 was
metallated by tin(II) chloride following the conditions reported by
Arnold and co-workers.11 It is noteworthy that during this
reaction the triple bond directly connected to the porphyrin was
reduced to a double bond to give 8. Besides, unlike most tin
porphyrins, the two chloro axial-ligands of 8 or 12 were not
replaced by hydroxo ligands, neither during the reflux in ethanol
solution of K2CO3, nor during the column chromatography on
silica gel.11,12 The symmetrical bis-porphyrin 12 was also prepared
to serve as reference compound for the electrochemical and
spectroscopic studies (Chart 1). Again, the metallation of the bis
free base porphyrin 10 with tin(II) chloride gave the tin(IV)
porphyrin dimer 12 in which the two triple bonds adjacent to the
porphyrin were also transformed into double bonds (see ESI{).
1
This is clearly evidenced by the H NMR spectrum in which the
aLaboratoire de Synthe`se Organique, UMR 6513 CNRS & FR CNRS
2465, Universite´ de Nantes, Faculte´ des Sciences et des Techniques de
Nantes, BP 92208, 2 rue de la Houssinie`re, 44322 Nantes Cedex 03,
France. E-mail: hcb@fotomol.uu.se; Fabrice.Odobel@univ-nantes.fr
bChemical Physics, Department of Photochemistry and Molecular
Science, Uppsala University, Box 523, SE-751 20 Uppsala, Sweden.
E-mail: Leif@fotomol.uu.se
{ Electronic supplementary information (ESI) available: Synthetic proce-
dures for the preparation of 8 and 12; spectroelectrochemistry and
transient absorption spectra for 12 and 3; fluorescence spectra; experi-
mental details. See DOI: 10.1039/b711642j
Chart 1 Structures of the compounds used in this study.
Chem. Commun., 2007, 4629–4631 | 4629
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signals of the ethylenic protons came out as two doublets with a
16 Hz coupling constant and with chemical shifts that are in
agreement with those of porphyrinic systems linked at the meso
position with ethylenyl groups.6,13 The mass spectrometric analyses
of 8 and 12 also agreed with the reduced system.
The visible absorption spectra of dyad 8 can be well reproduced
by a linear combination of the spectra of references 3 and 12
(Fig. 1). This indicates that the electronic properties of the
porphyrin (SnP) and phthalocyanine (ZnPc) units are not
significantly perturbed by each other. On the other hand, the
presence of the OPE bridge reduces the symmetry of the SnP and
ZnPc units and leads to a conjugated p-system involving the
bridge. This results in red-shifted and broadened absorption
bands, as compared to compounds 13 and 14 (Fig. S1{), and a
split of the ZnPc Q-bands. Similar effects have been reported for
phthalocyanines or porphyrins connected to a p-conjugated unit
via a triple bond.5,9,14 In coordinating solvents such as PhCN or
pyridine, the spectra are well-resolved with narrow absorption
bands and their intensities follow the Beer–Lambert law, indicating
the absence of intermolecular interactions at low concentrations
(below 1026 M21).
Fig. 2 Transient absorption of dyad 8 in PhCN. The plot in (a)
corresponds to excitation at 680 nm and that in (c) to an excitation at
440 nm. The delay times are 1 ps (purple), 10 ps (blue), 50 ps (green), 200 ps
(orange) and 1000 ps (red). Plots (b) and (d) correspond to the kinetic
traces at 440 nm (purple), 690 nm (red) and 730 nm (green, multiplied by 3
for clarity) for the respective excitations.
showed a main lifetime component of less than 40 ps for both
1ZnPc and 1SnP, followed by slower components attributed to
impurities. The main lifetimes of 8 are significantly shorter than
The redox potentials of the dyad 8 in CH2Cl2 were determined
by square wave voltammetry.3,15,16 ZnPc is a good electron donor
since its half-wave ZnPc+/ZnPc-potential is found at 0.46 V vs.
SCE. The bridge is oxidized at 1.3 V but its reduction occurs
outside the solvent potential window (Ered , 21.6 V). SnP is a
moderately good electron acceptor with a half-wave SnP/SnP2-
potential located at 20.82 V vs. SCE in dyad 8. From these data
and the spectroscopically determined excited state energies, the
reaction driving force in PhCN was calculated as DGu = 20.66 eV
for photoinduced charge separation from 1ZnPc and DGu =
20.81 eV from 1SnP.17 The absorption spectra of the radical
cation of ZnPc and of the radical anion of SnP were recorded by
spectroelectrochemistry using the reference compounds 3 and 12.
The electrochemical reduction of SnP in 12 shows a spectrum
featuring a bleaching of the Soret and the Q-bands and the
appearance of a broad band between 650 nm and 850 nm
(Fig. S2{). Upon oxidation of ZnPc, there is a strong bleaching of
the Q-band at 700 nm and a modest one for the Soret band at
350 nm. New absorption bands build up between 470–600 nm and
1ZnPc
those observed for 12 (t
= 2.2 ns) and 3 (t
= 0.45 ns).
1SnP
To further investigate the fast deactivation processes occurring
in the dyad 8, femtosecond transient absorption spectroscopy
experiments were undertaken in PhCN. Upon excitation at 680 nm
1ZnPc is formed which rapidly decays to the charge separated
state. This can be clearly seen from the SnP ground state Soret
bleach that grows in with a 13 ps time constant, while most of the
phthalocyanine bleach remains (Fig. 2a). At the same timescale
there is a rise around 730 nm, due to the broad absorption band of
SnP2 and ZnPc+ in that range (Figs. S5–6{). The transient spectra
of both excited and radical states of porphyrins are often very
similar, and the same is true for phthalocyanines. Nevertheless the
observed spectrum ascribed to the charge separated state agrees
well with simulated spectra constructed from the spectroelec-
trochemistry of oxidized 3 and reduced 12 (Fig. S7{). The
spectrum is dominated by the bleach of both the SnP Soret band
and the ZnPc Q-band. These two signals disappear with the same
time constant (t = 85 ps), consistent with charge recombination
(Fig. 3).
between 750–900 nm (Fig. S3{). These characteristics are in
+
excellent agreement with the spectra of SP2 and ZnPc published
?
?
previously.15,16
Excitation in the SnP Soret band at 440 nm results in a very
short-lived S2-state (tIC , 1 ps in 12), which undergoes internal
conversion to the S1-state. The latter is rapidly quenched (t = 4 ps)
Selective excitation of the ZnPc Q-band was achieved by using
680 nm light, while 440 nm light selectively excited SnP in the Soret
band. The steady-state fluorescence spectrum of the dyad 8 in
PhCN showed a strong quenching of both 1ZnPc and 1SnP
emission relative to the corresponding reference compounds (3 and
12) (Fig. S4{). Time resolved emission measurements of the dyad 8
Fig. 1 Spectra of dyad 8 (unbroken line) along with those of 3 (dashed-
dotted line) and 12 (dashed line) recorded in PhCN. The dotted line is a
linear combination of the spectra of 3 and 12.
Fig. 3 Summary of the relevant states and transitions (in PhCN17).
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by a process that results in a bleach growing in around 690 nm,
corresponding to the ZnPc Q-band (Fig. 2c). This process is
ascribed to hole transfer from the 1SnP state, resulting in the same
charge separated state as in the previously described electron
transfer. The partial bleach recovery of the SnP Soret band is
consistent with a smaller net bleach of the SnP2-radical compared
to 1SnP (Fig. S2, S5{). Recombination to the ground state occurs
with the same time constant (85 ps) regardless of which
chromophore is initially excited, further strengthening the
conclusion that it is the same charge separated state.
Research for the ‘‘Action Concerte´e Incitative’’ (ACI) ‘‘Jeune
Chercheur’’ 4057 and the ANR program ‘‘PhotoCumElec’’.
Notes and references
1 M. R. Wasielewski, Chem. Rev., 1992, 92, 435–461.
2 V. Balzani, P. Piotrowiak, M. A. J. Rodgers, J. Mattay, D. Astruc,
H. B. Gray, J. Winkler, S. Fukuzumi, T. E. Mallouk, Y. Haas, A. P.
de Silva and I. Gould, in Electron Transfer in Chemistry, ed. V. Balzani,
Wiley-VCH, Weinheim, 2001, vol. 3.
3 K. M. Kadish, K. M. Smith and R. Guilard, in The Porphyrin
Handbook, Elsevier, New York, 1999, vol. 16–19.
4 L. Li, S. Shen, Q. Yu, Q. Zhou and H. Xu, J. Chem. Soc., Chem.
Commun., 1991, 619–620; E. M. Maya, P. Vazquez and T. Torres,
Chem. Commun., 1997, 1175–1176; M. A. Miller, R. K. Lammi,
S. Prathapan, D. Holten and J. S. Lindsey, J. Org. Chem., 2000, 65,
6634–6649; J. M. Sutton and R. W. Boyle, Chem. Commun., 2001,
2014–2015; J. P. C. Tome´, A. M. V. M. Pereira, C. M. A. Alonso,
M. G. P. M. S. Neves, A. C. Tome´, A. M. S. Silva, J. A. S. Cavaleiro,
M. V. Mart´ınez-D´ıaz, T. Torres, G. M. A. Rahman, J. Ramey and
D. M. Guldi, Eur. J. Org. Chem., 2006, 257–267.
Energy transfer from the 1SnP to 1ZnPc is energetically possible,
but can be excluded since the subsequent charge separation from
1
1ZnPc is slower (tET = 13 ps) than the observed decay of SnP.
Thus a distinct intermediate state, with a 13 ps transient absorption
component, would have been seen. There is also a fraction of the
sample that is not quenched. The behaviour of this fraction is very
similar to that observed in references 3 and 12, starting with a
singlet signal that over time evolves into a triplet that subsequently
decays back to the ground state, on similar timescales (Fig. S4{).
With 440 nm excitation, sample degradation during measurements
made this component large, as seen by the significant porphyrin
bleach remaining at 1000 ps (Fig. 2c).
Electron transfer from 1ZnPc has to proceed through the direct
super-exchange mechanism,18 as the bridge is too difficult to
reduce for electron hopping to occur. Although similarly rapid
single step charge transfer has been reported for dyads containing
porphyrins or phthalocyanines with a direct ethynyl linkage,8,9
there is to our knowledge no system with similar rates over a
comparably long distance (34 s, centre-to-centre). Studies of long-
range electron transfer in similar systems, where the bridge binds
to the porphyrins via a meso-phenyl unit, report rates that are two
orders of magnitude slower than those reported here.19
5 C. Monnereau, J. Gomez, E. Blart, F. Odobel, S. Wallin, A. Fallberg
and L. Hammarstro¨m, Inorg. Chem., 2005, 44, 4806–4817.
6 F. Odobel, S. Suresh, E. Blart, Y. Nicolas, J.-P. Quintard, P. Janvier,
J.-Y. Le Questel, B. Illien, D. Rondeau, P. Richomme, T. Haupl,
S. Wallin and L. Hammarstro¨m, Chem.–Eur. J., 2002, 8, 3027–3046.
7 S. M. LeCours, H.-W. Guan, S. G. DiMagno, C. H. Wang and
M. J. Therien, J. Am. Chem. Soc., 1996, 118, 1497–1503; E. M. Garcia-
Frutos, S. M. O’Flaherty, E. M. Maya, G. De La Torre, W. Blau,
P. Vazquez and T. Torres, J. Mater. Chem., 2003, 13, 749–753.
8 N. P. Redmore, I. V. Rubtsov and M. J. Therien, J. Am. Chem. Soc.,
2003, 125, 8769–8778.
9 D. Gonzalez-Rodriguez, C. G. Claessens, T. Torres, S. Liu,
L. Echegoyen, N. Vila and S. Nonell, Chem.–Eur. J., 2005, 11,
´
3881–3893; A. J. Jime´nez, F. Spa¨nig, M. S. Rodr´ıguez-Morgade,
K. Ohkubo, S. Fukuzumi, D. M. Guldi and T. Torres, Org. Lett., 2007,
9, 2481–2484.
10 R. W. Wagner, T. E. Johnson, F. Li and J. S. Lindsey, J. Org. Chem.,
1995, 60, 5266–5273.
11 D. P. Arnold and J. Blok, Coord. Chem. Rev., 2004, 248, 299–319.
12 M. J. Crossley, P. Thordarson and R. A. S. Wu, J. Chem. Soc., Perkin
Trans. 1, 2001, 2294–2302.
13 F. Odobel, F. Suzenet, E. Blart and J.-P. Quintard, Org. Lett., 2000, 1,
131–133.
14 R. Kumble, S. Palese, V. S. Y. Lin, M. J. Therien and R. M.
Hochstrasser, J. Am. Chem. Soc., 1998, 120, 11489–11498; R. Shediac,
M. H. B. Gray, H. T. Uyeda, R. C. Johnson, J. T. Hupp, P. J. Angiolillo
and M. J. Therien, J. Am. Chem. Soc., 2000, 122, 7017–7033.
15 Phthalocyanines: Properties and Applications, ed. C. C. Leznoff and
A. B. P. Lever, Wiley-VCH, Weinheim, 1989.
Starting from 1SnP, a hole-hopping mechanism via a transiently
oxidized bridge is energetically feasible (Fig. 3). Thus, it is
conceivable that the very rapid charge separation (tHT = 4 ps) is
due to a hopping mechanism, while the subsequent charge
recombination can only occur via a long-range super-exchange
mechanism and is therefore 20-fold slower. This is an interesting
design principle for achieving rapid, long-range charge separation
via transient states without losing energy in the intermediate steps,
as is done in a conventional multi-step, exergonic charge
separation approach demonstrated in triads and tetrads etc.1,20
The close similarity in transient spectra and lifetime of the
reference 12 to a simple SnP unit shows that the B+–SnP2 state lies
16 K. M. Kadish, Q. Y. Y. Xu, G. B. Maiya, J. M. Barbe and R. Guilard,
J. Chem. Soc., Dalton Trans., 1989, 1531–1536.
17 Using the conventional assumption of DGu = 2e(EuDon 2 EuAcc) 2 E00.
21
For reactions in PhCN the correction term (e2/4pe0r)[(ePhCN
)
2 (eCH Cl )
21] was added. 3ZnPc energy from Gonzalez-Rodriguez
et al., ref. 9. Comparing SnP to other porphyrins with similar singlet
2
2
1
higher in energy than the SnP state, as suggested in Fig. 3. The
3
energies, SnP should lie between 1.4–1.5 eV (K. Kalyanasundram, in
predicted activation energy for hopping and super-exchange are
both very small so that temperature-dependent measurements
would not be able to safely distinguish these mechanisms.21 Instead
further studies with new compounds, modifying the relative energy
of the bridge state, are required to explore this interesting
possibility.
Photochemistry of Polypyridine and Porphyrin Complexes, Academic
Press, London, 1992).
18 W. B. Davis, W. A. Svec, M. A. Ratner and M. R. Wasielewski, Nature,
1998, 396, 60–63; H. McConnel, J. Chem. Phys., 1961, 35, 508–515.
19 K. Pettersson, J. Wiberg, T. Ljungdahl, J. Ma˚rtensson and B. Albinsson,
J. Phys. Chem. A, 2006, 110, 319–326.
20 D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2001, 34,
40–48; H. Imahori, Y. Mori and Y. Matano, J. Photochem. Photobiol.,
C, 2003, 4, 51–83.
21 Energy variation by changing solvent polarity was precluded because of
solubility and aggregation.
This work was supported by The Swedish Foundation for
Strategic Research, The K & A Wallenberg Foundation, The
Swedish Energy Agency, COST D35, and the French Ministry of
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Chem. Commun., 2007, 4629–4631 | 4631