Charge transfer reactions along a supramolecular redox gradient

Article abstract of DOI:10.1021/ja8063155

A rotaxane scaffold is used to align three photo/electroactive units along a supramolecular redox gradient leading to a cascade of through-space charge transfer reactions. Copyright

Full text of DOI:10.1021/ja8063155

Published on Web 10/21/2008
Charge Transfer Reactions along a Supramolecular Redox Gradient
Aurelio Mateo-Alonso,*,† Christian Ehli,‡ Dirk M. Guldi,*,‡ and Maurizio Prato*,†
Center of Excellence for Nanostructured Materials (CENMAT) and Italian InteruniVersity Consortium on Materials
Science and Technology (INSTM), Unit of Trieste, Dipartimento di Scienze Farmaceutiche, UniVersita` degli Studi di
Trieste, Piazzale Europa 1, 34127 Trieste, Italy, and Department of Chemistry and Pharmacy & Interdisciplinary
Center for Molecular Materials (ICMM), Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3,
D-91058 Erlangen, Germany
Received August 9, 2008; E-mail: amateo@units.it; guldi@chemie.uni-erlangen.de; prato@units.it
Scheme 1. Preparation of 3 and 4a
In recent years, the combination of synthetic organic chemistry,
supramolecular chemistry, and photophysics has largely improved
our understanding of the relation between molecular architecture
and photoinduced electron transfer efficiency. Such a multidisci-
plinary platform is fundamental for attaining novel and efficient
photovoltaic materials engineered at the molecular level. An elegant
strategy in the development of artificial reaction centers implies
the combination of fullerenes as electron accepting species to one
or several electron donors, typically metalloporphyrins and/or
ferrocene derivatives. A key factor is the structural organization of
the electron donating/accepting units, following an electrochemical
gradient, which promotes a unidirectional charge transfer cascade.
This approach has produced several photoactive systems,1-3 which
successfully (i) harvested visible light energy, (ii) converted it into
an electrochemical potential, and (iii) stored it for up to 380 mssan
efficiency similar to that of bacterial photosynthetic reaction
centers.4,5
Inspired by this encouraging results and by the recent ac-
complishments on donor-acceptor systems assembled by supramo-
lecular interactions,6-18 we have designed and prepared triad 3
(Scheme 1), in which three photo/electroactive units are organized
along a supramolecular redox gradient. Triad 3 is composed of a
fullerene (C60) electron-acceptor and two ferrocene (Fc) electron-
donors coupled by hydrogen bonds in a rotaxane fashion.16,17,19
Additionally, 3 presents a ruthenium carbonyl tetraphenylporphyrin
(Ru(CO)TPP) coordinated to a pending pyridyl group present
between the C60 and the Fc units. The alignment of the units along
a supramolecular electrochemical gradient promotes a unidirectional
cascade of two consecutive through-space charge transfer reactions
between the three units. This work demonstrates that the redox
gradient approach can be also applied to artificial reaction centers
with multiple units constructed solely by supramolecular interactions.
a (i) 5-Ferroceneacetoxyisophthaloyl chloride (12 equiv), p-xylylenedi-
amine (12 equiv), NEt3 (24 equiv), CHCl3 high dilution conditions, 17%;
(ii) Ru(CO)TPP (1 equiv) CH2Cl2.
characteristic transition around 880 nm, which transforms into the
triplet excited state (6.6 × 108 s-1) (not shown). An additional
feature of the C60 singlet excited-state is a fluorescence spectrum
in the 700-800 nm range with an overall quantum yield of 6.0 ×
10-4. In 2, a quenching of the C60-centered fluorescence is observed
(2.1 × 10-4), together with a fast deactivation of the C60 singlet
excited-state that leads to a radical ion pair state (C60•--Fc•+
)
formation, as illustrated by the fingerprint of the one-electron
reduced C60 at 1010 nm,22 for which a lifetime of 26 ns (3.8 × 107
s-1) was determined.
Triad 3 was prepared in two steps from thread 1. The macrocycle
with the two Fc electron donors was clipped following a modification
of Leigh’s protocol,16 yielding rotaxane 2 (Supporting Information,
Figure S1-S4). Addition of Ru(CO)TPP to 2, resulted in red shifts of
the Soret- (2 nm) and Q-bands (3 nm) in absorption measurements,
and in upfield shifts of the pyridyl signals A (7.1 ppm) and B (2.2
ppm) (see lettering in Scheme 1) in NMR measurements, which is
consistent with previous observations,15,18,20,21 providing conclusive
evidence for the formation of triad 3.
Similar experiments were carried out with 1 coordinated to
Ru(CO)TPP in dichloromethane that yield the reference dyad 4
(Figure S6). Femtosecond excitation (420 nm) of 4 generates the
fingerprint absorption of the Ru(CO)TPP radical cation (i.e.,
400-800 nm with maxima at 470, 510, 565, 585, 645, and 715
nm), in good agreement with the spectrum of the Ru(CO)TPP
radical cation generated electrochemically (maxima at 460, 565,
580, 640, and 715 nm and shoulders at around 510 and 560 nm)
(Figure S7).23 At the same time, the C60 radical-anion develops
(1.4 × 1010 s-1) predominantly in the near-infrared part with a
maximum at 1010 nm, providing evidence for the formation of
C60•--Ru(CO)TPP•+. Such radical ion pair state decays after 2200
ps to regenerate the singlet ground state.
The photophysical behavior of the different components and their
combination was investigated in detail by time-resolved absorption
measurements in dichloromethane. Photoexcitation of 1 at 387 nm
reveals the formation of the C60 singlet excited-state with the
When Ru(CO)TPP was coordinated to 2 in dichloromethane to
afford triad 3, a trend was observed that differs from that seen for
† Universita` degli Studi di Trieste.
‡ Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg.
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14938 J. AM. CHEM. SOC. 2008, 130, 14938–14939
10.1021/ja8063155 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
polyads with donor-acceptor geometries that could not be otherwise
accessed by covalent chemistry.
Acknowledgment. This work was carried out with partial
support from the University of Trieste, INSTM, MIUR (PRIN 2006,
prot. 200634372 and Firb, prot. RBNE033KMA), the EU (RTN
networks “WONDERFULL” and “EMMMA”), the Deutsche For-
schungsgemeinshaft (SFB 583), FCI, and the Office of Basic Energy
Science of the U.S. Department of Energy. This paper is dedicated
to Prof. Dan Meyerstein on the occasion of his 70th birthday.
Supporting Information Available: Characterization of 2 and 3.
Differential absorption spectra and time absorption profile of 4.
Spectroelectrochemistry of methyl 5-ferroceneacetoxyisophthalate and
Ru(CO)TPP. This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 1. Differential absorption spectra (visible and near-infrared) obtained
upon femtosecond flash photolysis (420 nm-150 nJ) of 3 in CH2Cl2 with
several time delays between 0 and 167 ps at room temperature. Arrow
illustrates the time evolution. (Insert) Time-absorption profile at 480 nm
(black) and 532 nm (gray), reflecting the charge separation and charge shift
dynamics.
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•-
kinetically. As a matter of fact, the lifetime of the C60
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•-
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In conclusion, we have prepared a triad by a combination of
hydrogen bonding and metal coordination that were employed to
organize the different units along a supramolecular redox gradient.
The excitation of the central Ru(CO)TPP unit results in the transfer
of an electron from Ru(CO)TPP to C60, followed by a charge shift
to yield C60•--Fc•+, as demonstrated by a systematic study based
on time-resolved absorption measurements. This work illustrates
that the redox gradient approach can also be applied to the
construction of supramolecular donor-acceptor systems, opening
the door to the preparation and investigation of supramolecular
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