Perylenediimide - Metal ion dyads for photo-induced electron transfer

Article abstract of DOI:10.1039/b719243f

A novel perylene diimide incorporating a tetradentate ligand was synthesized and the photophysical properties of its Cu⁺, Cu ²⁺, and Fe³⁺ complexes were investigated by fluorescence experiments. The Royal Society of Chemistry.

Full text of DOI:10.1039/b719243f

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Perylenediimide—metal ion dyads for photo-induced electron transferw
Katrine Qvortrup,a Andrew D. Bond,b Anne Nielsen,b Christine J. McKenzie,b
a
a
˚
Kristine Kilsa* and Mogens Brøndsted Nielsen*
Received (in Cambridge, UK) 13th December 2007, Accepted 29th January 2008
First published as an Advance Article on the web 21st February 2008
DOI: 10.1039/b719243f
A novel perylene diimide incorporating a tetradentate ligand was
synthesized and the photophysical properties of its Cu+, Cu2+
dehyde 6 in dry THF at room temperature under an argon
,
atmosphere to afford the imidazolidine derivative 7 in quanti-
tative yield. This compound was then reduced with
NaBH3CN/CF3COOH, hereby opening the imidazolidine ring
to provide compound 8 in high yield. Refluxing this compound
overnight in MeOH with 10 molar equivalents of KOH
resulted in deacetylation to furnish the aniline derivative 9.
The strong polarity of this product rendered its purification
difficult, and it was condensed directly with N-(1-hexylhep-
tyl)perylene-3,4:9,10-tetracarboxylic 3,4-anhydride 9,10-imide
10, prepared according to Langhals and Jona,6 in imidazole at
70 1C to afford the ligand-functionalized PDI 117 in a yield of
80%. This product exhibits an excellent solubility in chlori-
nated solvents, such as dichloromethane.
and Fe3+ complexes were investigated by fluorescence experi-
ments.
Understanding photo-induced energy and electron transfers in
molecular donor–acceptor systems is crucial for the design of
artificial photosynthesis devices. Nature’s own photosynthetic
reaction centre is by far the most elegant system known and
illustrates how carefully oriented molecular arrays of photo-
and redox-active components can work in concert to convert
and store solar energy.1 One of us has shown that a dimange-
nese complex of the ligand N-methyl-N0-carboxymethyl-N,N0-
bis(2-pyridylmethyl)ethane-1,2-diamine 1 catalyzes the oxida-
tion of water to dioxygen using a chemical oxidant such as
tert-butylhydrogenperoxide.2 This complex is relevant as a
model system for the active site of the oxygen-evolving com-
plex of photosystem II.3 The challenge is to make the process
light-activated as in natural photosynthesis.
The Cu(II) complex of the ligand 8 and one chloride ligand
was precipitated as the hexafluorophosphate salt from a
MeOH solution of 8 and CuCl2. Electrospray ionization
(ESI) showed peaks at m/z = 487 and 451. The first one is
assigned to five-coordinated Cu(II) involving both the tetra-
dentate ligand 8 and one chloride ion [M+] and the second one
corresponds to [M ꢀ Cl ꢀ H]+. The stoichiometry was also
ascertained by elemental analysis. Similarly, the Fe(III) com-
plex of 8 incorporating two chloride ligands (according to ESI)
was precipitated as the hexafluorophosphate salt from a
MeOH solution of 8 and FeCl3. X-Ray crystallographic
analyses on complexes of the related tetradendate ligands 3
and 4 (Fig. 1) suggests that the coordination geometry of the
five-coordinated Cu(II) complex is likely to be distorted trigo-
nal bipyramidal and that of the hexa-coordinated Fe(III)
complex is likely to be distorted octahedral.w
We became interested in combining polydendate ligands
such as 1–4 with the strong light-harvestor perylene diimide 5
(PDI). PDIs possess a unique combination of chemical stabi-
lity, redox properties, excited state reactivity, luminescence
emission, and excited state lifetime.4 Here we present the
synthesis of the first such PDI functionalized with a polyden-
date ligand and the spectroscopic investigations of the photo-
induced electron transfer processes that occur upon coordina-
tion of metal ions.
Complexation of either Fe3+, Cu+, or Cu2+ (added as
FeCl3, CuCl or CuI and CuCl2 salts, respectively) by 11 in
CH2Cl2–MeOH (25 : 1) was observed by changes in the ligand
absorptions located in the UV-region (250–350 nm). The
absorption spectra correspond to a superposition of that of
The synthesis is shown in Scheme 1. First, N,N0-bis(2-
pyridylmethyl)-1,2-ethanediamine 2, prepared according to a
literature protocol,5 was condensed with 4-acetamidobenzal-
a Department of Chemistry, University of Copenhagen,
Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark. E-mail:
mbn@kiku.dk, kkj@nano.ku.dk; Fax: +45 3532 0212
b Department of Physics and Chemistry, University of Southern
Denmark, Campusvej 55, DK-5230 Odense M, Denmark
w Electronic supplementary information (ESI) available: Absorption
and fluorescence spectra and crystallographic data in CIF or other
electronic format (CCDC 671368 & 671369). See DOI: 10.1039/
b719243f
Scheme 1 Synthesis of perylene diimide incorporating a tetradentate
ligand.
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account of the low solubility of CuCl. However, extracting
CuCl into a solution of the ligand results in considerable
quenching of the PDI fluorescence. Dynamic quenching is
ruled out as addition of an excess of either Fe3+, Cu+, or
Cu2+ ions to a PDI devoid of the ligand group does not
change the fluorescence intensity.
According to simple geometry optimizations, the distance
between the PDI center and the metal ion is around 12 A. Only
Cu2+ complexes of the tetradendate ligand have an over-
lapping absorption band with the PDI emission band, but
with very low molar absorptivity; [Cu(II)Cl(3)]ClO4 exhibits an
absorption band in MeCN at lmax 702 nm with e = 154 Mꢀ1
cmꢀ1.w Energy transfer from the excited PDI (PDI*) to the
Cu2+ metal complex is thus possible, albeit of low probability.
For the other metals used, no energetic interaction is possible,
and fluorescence quenching in these cases cannot be explained
Fig. 1 X-Ray crystal structures of the cations (a) [Cu(II)Cl(3)]+ and
(b) [Fe(III)Cl2(4)]+. Counter anions are removed for clarity.
the metal complex of 8 alone and that of PDI, i.e., complexa-
tion does not alter the PDI absorptions (462, 489, and
527 nm), and, accordingly, the PDI unit can be selectively
excited at 489 nm.w
We note that the ability of Fe2+ and Fe3+ to quench the
fluorescence of more electron-rich aryloxy-substituted PDIs
has previously been observed via coordination of the metal
ions to ligand groups, such as terpyridines or amides.8 Elec-
tron-rich amine-substituted PDIs were also reported to under-
go photo-induced electron transfer processes efficiently.9 To
address the possibility of photo-induced energy or electron
transfer reactions between the PDI unit in 11 and coordinated
metal ion, the fluorescence behaviour was studied in dichlor-
omethane solution containing 10% methanol to dissolve the
metal salts. Complexation of Fe3+, Cu+, or Cu2+ ions by 11
has a strong quenching effect on the PDI fluorescence with no
discernible alteration in peak shape. Upon titration of a dilute
solution of 11 (10ꢀ5 M) with Fe3+ (Fig. 2) or Cu2+ ions, the
steady-state fluorescence follows the Stern–Volmer equation
for static quenching, I0/I = 1 + Ks[Q], where I and I0 are the
integrated emission intensities with and without quencher (Q)
present. Within the limited solubility of the metal ions, almost
full quenching (o10% intensity left) is obtained. The associa-
tion constants Ks for complexation of Fe3+ and Cu2+ were
determined to be 178 Mꢀ1 and 79 Mꢀ1, respectively, assuming
full quenching at infinite concentration of metal ions. Low
inherent fluorescence of [Fe(III)Cl2(11)]+ and [Cu(II)Cl(11)]+
would increase the resulting Ks by up to 20%, but also
introduce deviations from the static quenching equation. The
association constant for Cu+ could not be determined on
by either Forster or Dexter energy transfer.10 Instead, the
¨
quenching is most likely to occur by an electron transfer
process. In the case of Fe3+ and Cu2+, this electron transfer
should occur from PDI* to the metal ion, while instead Cu+ is
expected to reduce PDI*. Quenching originating from para-
magnetic (Fe3+ and Cu2+) and/or heavy atom species enhan-
cing the possibility of intersystem crossing11 seems unlikely:
firstly, significant quenching is also observed for the diamag-
netic Cu+ species; secondly, quenching is only observed as the
metal ion binds to the ligand.
The redox behaviour of the [Cu(II)Cl(8)]+ and
[Fe(III)Cl2(8)]+ complexes (PF6ꢀ salts, vide supra) was studied
by cyclic voltammetry (CV) and differential pulse voltammetry
(DPV).12 The redox potential for [Cu(II)Cl(8)]+/[Cu(I)Cl(8)] is
ꢀ0.804 V vs. Fc+/Fc according to DPV, while the redox
potential for [Fe(III)Cl2(8)]+/[Fe(II)Cl2(8)] is ꢀ0.276 V vs.
Fc+/Fc. The CVs of both complexes reveal reversible electro-
chemistry (Fig. 3). It should be noted that the Fe(III)-complex
exhibits more complicated electrochemistry as evidenced by
additional shoulders in the CV. The PDI reference compound
12 is reversibly reduced at ꢀ1.132 V and ꢀ0.872 V vs. Fc+/Fc
and irreversibly oxidized at +0.485 V vs. Fc+/Fc. From the
measured potentials, it transpires that ground state electron
transfer is not possible in either case. However, upon excita-
tion of the PDI unit (E0 = 2.35 eV), the electron donor
strength is enhanced considerably; hence concomitant electron
transfer to the metal (Cu2+ or Fe3+)-ligand complex becomes
Fig. 2 Fluorescence behaviour upon titration of a dilute solution
(CH2Cl2 + 10% MeOH) of 11 (10ꢀ5 M) with Fe3+ ions (FeCl3). The
Fig.
[Fe(III)Cl2(8)]+ (green) (PF6 salts), and PDI 12 (red) in DMF with
Bu4NPF6 (0.2 M) as supporting electrolyte. Scan rate: 0.1 V sꢀ1
3
Cyclic voltammograms of
[Cu(II)Cl(8)]+
(blue),
ꢀ
arrow represents increasing concentration of Fe3+
.
.
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favourable with a driving force of more than 1 eV. Similarly,
the acceptor strength of the excited PDI is also enhanced, and
electron transfer from the Cu+ complex is now a favorable
process by more than 2 eV.
4 (a) D. Schlettwein, D. Wohrle, E. Karmann and U. Melville,
¨
Chem. Mater., 1994, 6, 3; (b) S. K. Lee, Y. Zu, A. Herrmann, Y.
Geerts, K. Mullen and A. J. Bard, J. Am. Chem. Soc., 1999, 121,
¨
3513.
5 (a) D. Schlettwein, D. Wohrle, E. Karmann and U. Melville, Chem.
¨
Mater., 1994, 6, 3; (b) S. K. Lee, Y. Zu, A. Herrmann, Y. Geerts, K.
In conclusion, by covalently attaching a tetradentate ligand
to PDI, metal ions can be brought into close proximity to the
Mullen and A. J. Bard, J. Am. Chem. Soc., 1999, 121, 3513.
¨
6 (a) H. Langhals and W. Jona, Eur. J. Org. Chem., 1998, 847.
7 Preparation of compound 11: Compound 8 (658 mg, 1.69 mmol)
was dissolved in abs. EtOH (5 mL), and KOH (289 mg, 5.15 mmol)
was added. The mixture was refluxed overnight, then cooled to rt,
and aqueous HCl (2 M) was added until pH B 7. The resulting
precipitate was removed by filtration, and the filtrate was concen-
trated in vacuo to provide the product 9 that was then ground
together with 10 (300 mg, 0.53 mmol) and imidazole (1.0 g) to a
homogenous powder. The mixture was heated at 70 1C, then
cooled to rt and dissolved in MeOH. Water was added, the
resulting precipitate was collected by centrifugation, washed with
water, and dried under vacuum. The product 11 was hereby
obtained as a dark red solid (383 mg, 80%). M.p. 115 1C (dec.).
1H-NMR (CDCl3, 300 MHz): d = 0.80–0.843 (m, 4H), 1.22–1.86
(m, 12H), 1.80–1.88 (m, 4H), 2.18–2.35 (m, 4H), 2.82–2.85 (m, 2H),
2.92–3.00 (m, 2H), 3.72 (s, 2H), 3.76 (s, 2H), 4.00 (s, 2H), 5.10–5.20
(m, 1H), 7.03–8.58 ppm (m, 20H). HR-MS (FAB+): m/z =
903.4587 [M + H]+; C58H59N6O4 requires: 903.4598.
PDI unit. While complexation of metal ions (Cu+, Cu2+
,
Fe3+) does not perturb the PDI absorption, the fluorescence
of compound 11 is partly quenched. The exact mechanism will
be the subject to detailed time-resolved spectroscopic studies
in future work, but is most likely owing to photo-induced
electron transfer. This is promising in view of the ultimate goal
to explore derivatives of 11 for light-induced water splitting.
We are currently aiming at the related compound containing a
carboxylate arm as in compound 1 in order to prepare the
dimanganese complex analogous to that reported to catalyse
water oxidation.2 In addition, preliminary experiments reveal
that the Cu2+ complex of the ligand 8 can be employed for
oxidative homo-coupling of phenylacetylene in the presence of
O2 (Glaser coupling), requiring, however, NaH as base for
deprotonation of the alkyne. The possibility for light-driven
alkyne dimerization, under oxygen-free conditions, using the
PDI-functionalized Cu2+ complex is another interesting
aspect to be pursued in the future.
8 (a) R. Dobrawa and F. Wurthner, Chem. Commun., 2002, 1878; (b) Y.
¨
Li, N. Wang, H. Gan, H. Liu, H. Li, Y. Li, X. He, C. Huang, S. Cui,
S. Wang and D. Zhu, J. Org. Chem., 2005, 70, 9686; (c) R. Dobrowa,
M. Lysetska, P. Ballester, M. Grune and F. Wurthner, Macromole-
¨
¨
cules, 2005, 38, 1315; (d) Y. Li, H. Zheng, Y. Li, S. Wang, Z. Wu, P.
Liu, Z. Gao, H. Liu and D. Zhu, J. Org. Chem., 2007, 72, 2878.
9 (a) B. Rybtchinski, L. E. Sinks and M. R. Wasielewski, J. Am.
Chem. Soc., 2004, 126, 12268; (b) Y. Shibano, T. Umeyama, Y.
Matano, N. V. Tkachenko, H. Lemmetyinen, Y. Araki, O. Ito and
H. Imahori, J. Phys. Chem. C, 2007, 111, 6133.
The Danish Research Agency (grants #2104-06-0060 and
#2111-04-0018), the Carlsberg Foundation, and University of
Copenhagen are acknowledged for financial support.
10 N. J. Turro, Modern Molecular Photochemistry, University Science
Books, Sausalito, California, USA, 1991.
11 P. Suppan, Chemistry and Light, Royal Society of Chemistry,
Cambridge, UK, 1994.
12 Cyclic and differential pulse voltammetries were measured using a
glassy carbon working electrode and a Pt wire counter electrode.
All potentials are expressed relative to that of Fc+/Fc (0.31 V vs.
SCE; A. J. Bard and L. R. Faulkner, Electrochemical Methods,
Fundamentals and Applications, John Wiley & Sons, Inc., Cam-
bridge, UK, 2nd edn, 2001.
Notes and references
1 Electron transfer in Chemistry, Vol. 3: Biological and Artificial
Supramolecular Systems, ed. V. Balzani, Wiley-VCH, Weinheim,
2001.
2 A. Poulsen, A. Rompel and C. J. McKenzie, Angew. Chem., Int.
Ed., 2005, 44, 6916.
3 S. Mukhopadhyay, S. K. Mandal, S. Bhaduri and W. H. Arm-
strong, Chem. Rev., 2004, 104, 3981.
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This journal is The Royal Society of Chemistry 2008
1988 | Chem. Commun., 2008, 1986–1988