Sequential energy and electron transfer in aggregates of tetrakis[oligo(p-phenylene vinylene)] porphyrins and C60 in water

Article abstract of DOI:10.1021/ja054406t

Chiral aggregation of oligo(p-phenylene vinylene)-functionalized Zn and free-base porphyrins is observed in water. The formation of mixed assemblies containing both porphyrins results in sequential energy transfer from OPV via zinc porphyrin to free-base

Full text of DOI:10.1021/ja054406t

Published on Web 09/13/2005
Sequential Energy and Electron Transfer in Aggregates of
Tetrakis[oligo(p-phenylene vinylene)] Porphyrins and C60 in Water
Martin Wolffs, Freek J. M. Hoeben, Edwin H. A. Beckers, Albertus P. H. J. Schenning,* and
E. W. Meijer*
Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513,
5600 MB EindhoVen, The Netherlands
Received July 4, 2005; E-mail: a.p.h.j.schenning@tue.nl; e.w.meijer@tue.nl
Efficient cascade energy and electron transfer in natural photo-
synthetic systems depends highly on the molecular structure of
chlorophylls and the way in which they are mesoscopically ordered.1
Since interchromophoric order is crucial for efficient functioning
of organic electronic devices, understanding and mimicking pho-
tosynthetic processes may eventually lead to improved design
principles for such devices based on noncovalent interactions.
Energy2 and electron3 transfer processes in π-stacked architectures
have been reported;4 however, the use of sequential energy and
electron transfer steps has been far more elusive and has, up until
now, only been observed in, for example, elegantly designed
covalent molecules5 or supramolecular dimers.6 To the best of our
Figure 1. UV/vis and PL of 1 (10-5 M, T ) 20 °C) in water (solid line,
knowledge, we report here the first approach to cascade energy
and electron transfer in mixed π-conjugated assemblies in water,
based on our previous experience with water-soluble oligo(p-
phenylene vinylene)s (OPVs).7 For this purpose, we constructed
mixed assemblies consisting of (OPV4)4 porphyrins 1 and 2. When
these assemblies are formed in water, excitation of OPV4 should
result in energy transfer from OPV4 via Zn porphyrin to free-base
porphyrin. Moreover, incorporation of C60 into aggregated 1 or 2
results in redox-active assemblies yielding an additional electron
transfer step to C60. Compounds 1 and 2 have been synthesized
and fully characterized.8 Absorption spectra of 1 and 2 in chloroform
clearly show the distinctive Q-bands of the porphyrin,9 although
the Soret band is remarkably shifted and broadened, suggesting a
certain degree of ground state interaction of the separate chro-
mophores. Photoluminescence spectra of 1 and 2 in chloroform
show that upon predominant excitation of OPV4 (λex ) 346 nm)
the OPV emission is strongly quenched whereas almost exclusive
emission of the porphyrin is observed, indicating efficient intramo-
λex ) 344 nm) and chloroform (dashed line, λex ) 337 nm) and temperature-
dependent CD spectra of 1 in water (5 × 10-6 M, T ) 10-80 °C). Tapping
mode AFM image showing fibers of 1, dropcast from water (10-5 M) onto
a MICA surface (420 nm × 420 nm).
lecular energy transfer from the OPVs to the porphyrin (see the
Supporting Information for a comparison of 1 with reference
compounds).
In water, the absorption spectra of 1 and 2 display a red shoulder
at λ ) 475 nm, a decrease in extinction coefficient, and a
hypsochromic shift (4 nm) of the Soret band. Moreover, a clear
bathochromic shift in their emission maxima (25 nm) and a bisignate
Cotton effect in circular dichroism suggest chiral, face-to-face
aggregation of pure 1 and 2 in water (Figure 1).10
Temperature-dependent UV/vis and CD spectra of 1 and 2 in
water show a loss of optical activity at elevated temperatures (Figure
1), while the UV/vis spectra remain unchanged, indicating a
transition from chiral to achiral aggregation.8 AFM measurements
of 1 on mica show extended fibers of 3.5 nm in height that coagulate
into larger assemblies.8 If we assume an extended conjugated
system, this height corresponds to an angle of 64° between
cofacially stacked molecules and the MICA surface if two OPV
arms of each molecule interact with the surface.
Chart 1. (OPV4)4-H2 Porphyrin (1) and (OPV4)4-Zn Porphyrin
(2)
Mixed assemblies of 1 and 2 were studied in water, based on
ubiquitous reports using Zn and free-base porphyrin as energy donor
and acceptor, respectively.11 Aggregates made by premixing 1 and
2 in THF before injection into water8 showed a clear decrease in
the emission of 2 (λmax ) 623 nm, 5 × 10-7 M) and an increase in
the fluorescence of 1 (λmax ) 666 nm), indicating cascade energy
transfer from OPV4 via zinc porphyrin to free-base porphyrin
(Figure 2A). A control experiment performed by the addition of
aggregated 1 in water to aggregated 2 in water did not show any
significant change in emission of 2 (Figure 2B), indicating absence
of energy transfer between separate stacks of 1 and 2.
To incorporate redox activity, use was made of the specific,
attractive interactions of fullerenes with porphyrins.3b,12 UV/vis
spectra of mixed aggregates of C60 with either 1 or 2 in water (5 ×
10-7 M)8 exhibited a 5 nm bathochromic shift of λmax and a charge
9
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J. AM. CHEM. SOC. 2005, 127, 13484-13485
10.1021/ja054406t CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Since C60 has a higher affinity for free-base porphyrin than for
Zn porphyrin,12a we aimed at the formation of a mixed aggregate
consisting of 2 and 15 mol % of a 1:1 ratio of 1 and C60.8
Preliminary fluorescence data indicate cascade energy transfer from
OPV4 via zinc porphyrin to free-base porphyrin followed by
electron transfer from 1 to C60.8
In conclusion, we have shown the first examples of energy and
electron transfer in multichromophoric, π-conjugated assemblies
in water by construction of mixed stacks consisting of (OPV4)4-
Zn porphyrin, (OPV4)4-H2 porphyrin, and C60. Currently, we are
determining speeds for the different energy and electron transfer
steps.
Figure 2. PL spectra for the addition of (A) 0-36 mol % of 1 to 2 in
water, (B) 0-30 mol % of 1 in water to 2 in water (in both cases λex ) 346
nm, [2] ) 5 × 10-7 M, T ) 20 °C).
Acknowledgment. We would like to thank Dr. Xianwen Lou
for the matrix-assisted laser desorption time-of-flight (MALDI-TOF)
MS measurements. Pascal Jonkheijm is acknowledged for the AFM
measurement. The authors thank The Netherlands Organization for
Scientific Research (CW-NWO) for funding.
Supporting Information Available: Synthetic route to 1 and 2
and their full characterization. Optical comparison of 1 with reference
compounds (Figure S1). Solvent and temperature-dependent UV/vis
for 1 (Figure S2) and for 2 (Figure S3). AFM and height profile for 1
(Figure S4). UV/vis and quenching data for addition of C60 to,
respectively, 1 and 2 (Figures S5-S7). PIA spectra (Figure S8) and
energetic diagrams for 1 and 2 (Figure S9), including data for the Weller
equation (Table S1). PL data for a mixture of 1, 2, and C60 (Figure
S10). This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 3. Normalized PL spectra for mixtures containing 0-75 mol % of
C60 in (A) 1 and (B) 2 in water (λex ) 346 nm, 5 × 10-7 M, T ) 20 °C).
References
(1) Hu, X.; Schulten, K. Phys. Today 1997, 29-34.
(2) (a) Miyatake, T.; Tamiaki, H.; Holzwarth, A. R.; Schaffner, K. HelV. Chim.
Acta 1999, 82, 797-810. (b) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew.
Chem., Int. Ed. 2004, 43, 1229-1233. (c) Hoeben, F. J. M.; Herz, L. M.;
Daniel, C.; Jonkheijm, P.; Schenning, A. P. H. J.; Silva, C.; Meskers, S.
C. J.; Beljonne, D.; Phillips, R. T.; Friend, R. H.; Meijer, E. W. Angew.
Chem., Int. Ed. 2004, 43, 1976-1979.
(3) (a) Schenning, A. P. H. J.; v. Herrikhuyzen, J.; Jonkheijm, P.; Chen, Z.;
Wurthner, F.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 10252-10253.
(b) Hasobe, T.; Kamat, P. V.; Troiani, V.; Solladie´, N.; Ahn, T. K.; Kim,
S. K.; Kim, D.; Kongkanand, A.; Kuwabata, S.; Fukuzumi, S. J. Phys.
Chem. B 2005, 109, 19-23.
Figure 4. Proposed molecular picture of a co-aggregate of 1 and C60.
Photoinduced absorption of 1 (dashed) and of a 1:1 ratio of 1 with C60
(solid) (λex ) 361 nm, 10-4 M, T ) 20 °C).
transfer band (1: λ ) 672 nm, 2: λ ) 625 nm), both of which are
known to be induced by porphyrin-fullerene interactions.13 Since
scattering effects were observed when the C60 incorporation
exceeded 50 mol %, these collective observations indicate formation
of a 1:1 co-aggregate of 1 or 2 with C60 in water. These complexes
yielded clear solutions that were stable in time. Furthermore,
photoluminescence spectra of mixtures (λex ) 346 nm, Figure 3)
show a dramatic quenching of porphyrin emission upon addition
of C60 to either 1 or 2, suggesting electron transfer to incorporated
C60. Near quantitative quenching of 1 and 2 luminescence is reached
at ∼20 mol % C60 incorporation.8
(4) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J.
Chem. ReV. 2005, 105, 1491-1546.
(5) (a) Kuciauskas, D.; Lidell, P. A.; Lin, S.; Johnson, T. E.; Weghorn, S. J.;
Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc.
1999, 121, 8604-8614. (b) Miller, M. A.; Lammi, R. K.; Prathapan, S.;
Holten, D.; Lindsey, J. S. J. Org. Chem. 2000, 65, 6634-6649.
(6) Rybtchinski, B.; Sinks, L. E.; Wasielewski. M. R. J. Am. Chem. Soc. 2004,
126, 12268-12269.
(7) (a) Jonkheijm, P.; Fransen, M.; Schenning, A. P. H. J.; Meijer, E. W. J.
Chem. Soc., Perkin Trans. 2 2001, 1280-1286. (b) Hoeben, F. J. M.;
Shklyarevskiy, I. O.; Pouderoijen, M. J.; Engelkamp, H.; Schenning, A.
P. H. J.; Christianen, P. C. M.; Maan, J. C.; Meijer, E. W. Submitted.
To directly prove intermolecular electron transfer to C60, pho-
toinduced absorption (PIA) was performed on a 1:1 mixed assembly
of 1 and C60 (Figure 4). The PIA spectrum shows a band at λ )
(8) See Supporting Information.
(9) Hashimoto, T.; Choe, Y.-K.; Nakano, H.; Hirao, K. J. Phys. Chem. A
1999, 103, 1894-1904.
(10) Schenning, A. P. H. J.; Hubert, D. H. W.; Feiters, M. C.; Nolte, R. J. M.
1070 nm that is characteristic for the C60 radical anion.14
A
Langmuir 1996, 12, 1572-1577.
bleaching band at λ ) 550 nm indicates that less porphyrins occupy
their ground state, suggesting that the porphyrin radical cation is
responsible for the observed absorption band at λ ) 750 nm.
However, on the basis of the Weller equation15 for these systems,
the difference in driving force for charge separation into either
OPV4 or porphyrin radical cation and C60 radical anion is negligible
at room temperature.8 Therefore, assignment of the band observed
at λ ) 750 nm is difficult. A 1:1 mixed assembly of 2 and C60 did
not show the λ ) 1070 nm band, which will be further investigated
on shorter time scales.8
(11) Chambron, J.-C.; Heitz, V.; Sauvage, J.-P. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New
York, 2000; Vol. 6.
(12) (a) Sun, D.; Tham, F. S.; Reed, C. A.; Chaker, L.; Boyd, P. D. W. J. Am.
Chem. Soc. 2002, 124, 6604-6612. (b) Boyd, P. D. W.; Reed, C. A. Acc.
Chem. Res. 2005, 38, 235-242.
(13) Tashiro, K.; Aida, T.; Zheng, J.-Y.; Kinbara, K.; Saigo, K.; Sakamoto,
S.; Yamaguchi, K. J. Am. Chem. Soc. 1999, 121, 9477-9478.
(14) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.;
Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607-2617.
(15) Weller, A. Z. Phys. Chem. 1982, 133, 93.
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