Parallel (face-to-face) versus perpendicular (edge-to-face) alignment of electron donors and acceptors in fullerence porphyrin dyads: The importance of orientation in electron transfer [2]

Full text of DOI:10.1021/ja004104l

9166
J. Am. Chem. Soc. 2001, 123, 9166-9167
Parallel (Face-to-Face) Versus Perpendicular
(Edge-to-Face) Alignment of Electron Donors and
Acceptors in Fullerene Porphyrin Dyads: The
Importance of Orientation in Electron Transfer
Dirk M. Guldi,*,† Chuping Luo,† Maurizio Prato,*,‡
Alessandro Troisi, Francesco Zerbetto,*, Michael Scheloske,§
Elke Dietel,§ Walter Bauer,§ and Andreas Hirsch*,§
Radiation Laboratory, UniVersity of Notre Dame
Notre Dame, Indiana 46556
Figure 1.
Dipartimento di Scienze Farmaceutiche
UniVersita` di Trieste, Trieste, Italy
Dipartimento di Chimica “G. Ciamician”
UniVersita` di Bologna, Bologna, Italy
Institut fu¨r Organische Chemie
UniVersita¨t Erlangen-Nu¨rnberg
Henkestrasse 42, 91054 Erlangen, Germany
benzaldehyde and pyrrole under Lindsey conditions7 and subse-
quent chromatographic separation from the constitutional isomer
5 (Scheme 1).
Irradiation of 2 with a short 532 nm laser pulse resulted in the
immediate Q-band bleaching of the porphyrin and the concomitant
formation of broad absorption characteristics, especially in the
region between 550 and 750 nm. These spectral features are
representative of the porphyrin singlet excited state. In contrast
ReceiVed NoVember 28, 2000
ReVised Manuscript ReceiVed March 23, 2001
1
to the slow intersystem crossing dynamics in ZnTPP 3, the *-
(π-π)ZnTPP absorption decays rapidly and monoexponentially
in dyad 2 (Table 1). The singlet lifetimes and the fluorescence
quantum yields in 2 are subject to a marked solvent dependence.
Particularly interesting are the strong solvent effects of dichloro-
methane. In analogy with previous work,9 such variations may
hint at an ET mechanism mediated by a solvent molecule situated
The role of the spatial arrangement of donor-acceptor couples
in electron transfer is a subject of primary interest to understand
the properties of fundamental systems such as those generating
the natural photosynthetic process1-3 and the one-dimensional
array of DNA base pairs aligned face-to-face to make π-π
stacks.4 The rates of electron transfer (ET) depend considerably
on the wave function mixing, which is gauged by the electronic
coupling |V| between the electron donor and the electron
acceptor.1-3 Despite the qualitative understanding reached in
recent years, the challenge to directly tune π-π interactions to
modify |V| awaits in depth exploration.
in the cavity between ZnP and C60.
•-
Formation of the charge-separated radical pair (i.e., C60
-
ZnTPP•+) was confirmed by transient absorption spectroscopy.
The one-electron reduced form of C60 displayed its fingerprint
absorption in the NIR (1060 nm), in good agreement with an
equatorial reference. The π-radical cation of ZnTPP was seen
with its strongest absorption in the visible (700 nm).
Here we report the ET rates due to two fundamentally different
orientations of a zinc tetraphenyl porphyrin (ZnTPP) and a
fullerene moiety. A full and comprehensive account will be given
of the effects of the face-to-face versus edge-to-face alignment
on the intramolecular ET rates. In particular, we are able to
determine that the magnitude of π-π interactions is the crucial
parameter that controls rates, efficiencies, and mechanisms of ET.
As model compounds for this investigation we chose the
previously reported dyad 1,5 where ZnTPP is attached in the
trans-2 position, and the new dyad 2, with an equatorial bridging
of ZnTPP.
In general, the ET rates in the equatorial dyad 2 are ca. 10
times slower than those reported for the trans-2 dyad 1. In an
effort to correlate the different ET kinetics with the structure of
the dyad and to assess the location of possible solvent molecules,
we carried out molecular dynamics investigations.8 Six randomly
selected and superimposed conformers of each dyad (Figure S2)
support three critical aspects: (1) Due to the double linkage, the
variability of relative orientation of the two chromophores within
each dyad is very limited. In 1, the face-to-face alignment is
completely locked. In 2, only two types of face-to-edge orienta-
tions (Vertical and horizontal) were found. NOE measurements
(S3) further showed that the “horizontal” conformation has a very
low population (0.2%) relative to the preferred “vertical” con-
formation (99.8%). Both systems have therefore strong confor-
mational rigidity. (2) The face-to-face alignment in 1 with the
shortest interplanar distance (3 Å) leads to an appreciable π-π
stacking, whereas the edge-to-face alignment in 2 prohibits
effective π-π interactions (Figure 1). (3) 2 can accommodate
between the two moieties a solvent molecule such as toluene.
The red shift of the Soret-/Q-band transitions and the lower
extinction coefficients, relative to ZnTPP 3, are good fingerprints
of the presence of electronic coupling (Table S4). Unquestionably,
the interactions in the π-π stacked dyad 1 are significantly
stronger than those noted for 2.
The regioselective synthesis of 2 was accomplished via a tether
directed,6 2-fold cyclopropanation of C60 with bismalonate 3. The
latter was obtained from the statistical co-condensation of 4 with
* To whom correspondence should be addressed: Fax: (+01) 219 631
8068. E-mail: guldi.1@nd.edu, hirsch@organik.uni-erlangen.de
† University of Notre Dame.
‡ Universita´ di Trieste.
Universita´ di Bologna.
§ Universita¨t Erlangen-Nu¨rnberg.
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10.1021/ja004104l CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/23/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9167
Scheme 1. Synthesis of the Equatorial Dyad 2 and the Mixed Hexakisadduct 6a
a Conditions: (i) 2 equiv of Ph-CHO, 4 equiv of pyrrole, BF3*Et2O, room temperature; (ii) p-chloranile, reflux; (iii) ZnCl2, THF, reflux; (iv) C60, 2
equiv of iodine, DBU, toluene, room temperature; (v) DMA, toluene, room temperature; (vi) BrCH(COOEt)2, DBU, room temperature.
Table 1. Summary of the Photophysical Parameters of C60-ZnTPP Dyad 1 and Dyad 2
a
a
b
e
e
kET[s-1
dyad 1
]
kET[s-1
dyad 2
]
kET[s-1
dyad 2
]
-∆GBET° [eV]c -∆GET° [eV]d kBET[s-1
]
kBET[s-1
dyad 2
]
solvent
toluene
THF
Φf/(τ) ZnTPP
Φf dyad 2
dyad 2
dyad 2
dyad 1
0.04/3.47 ns
1.2 × 1011
f
f
0.04/3.22 ns 12.0 × 10-4 1.5 × 1011 9.8 × 109 6.7 × 109
1.39
0.61
2.6 × 109
3.8 × 105
5.4 × 105
4.9 × 106
dichloromethane 0.04/3.4 ns
4.4 × 10-4 1.3 × 1011g 2.6 × 1010 2.1 × 1010
1.5 × 1010g
dichlorobenzene 0.039/3.48 ns 9.5 × 10-4 1.4 × 1011 1.1 × 1010 8.9 × 109
1.33
1.21
1.18
0.67
0.79
0.82
benzonitrile
DMF
0.035/3.22 ns 5.6 × 10-4 1.3 × 1011 1.9 × 1010 1.8 × 1010
2.6 × 1010 9.1 × 105
0.035/3.29 ns 4.3 × 10-4
2.4 × 1010 2.9 × 1010
a kET ) [Φ(ZnTPP) - Φf(1,2)]/[τ(ZnTPP) Φf(1,2)] where 1,2 refer to the two dyads. b Determined from the *(π-π)ZnTPP lifetime. c Free
energy changes for BET of the C60•--ZnTPP•+ radical pair: -∆GBET° ) E1/2(D•+/D) - E1/2(A/A•-) + ∆GS; ∆GS ) (e2/(4πꢀ0))[(1/(2R+) + 1/(2R-)
- 1/RD-A)(1/ꢀS) - (1/(2R+) + 1/(2R-))(1/ꢀR)]; E1/2(D•+/D) ) 0.38 V versus Fc/Fc+; E1/2(A/A•-) ) -1.13 V vs Fc/Fc+; R+ ) 5 Å; R- ) 4.4 Å;
RD-A ) 15.1 Å. d Free energy changes for ET evolving from C60-1*ZnTPP: -∆GET° ) ∆E0-0 - (-∆GBET). e Determined from the radical pair
lifetime. f Energy transfer. g In dichloroethane.
1
The different donor-acceptor orientations (i.e., face-to-face
versus face-to-edge) have a strong impact on the back electron
transfer (BET) dynamics (Table 1). Previously, we reported that
the lifetime of the C60•--ZnTPP•+ radical pair in the trans-2 dyad
1 is of the order of a few hundred picoseconds (and that BET is
facilitated by the strong interaction between the fullerene and
porphyrin π-systems).5 Analysis of the decay kinetics of the
radical pair of 2 gives a lifetime of the order of microseconds.
Modification of the relatiVe alignment between acceptor (C60) and
donor (ZnTPP), and its alteration of the π-π interactions,
changes the lifetime of the charge-separated state by 4 orders of
magnitude. The dependence of kET versus free energy changes
indicates stabilizing effects for the charge-separated state at higher
-∆G° values (i.e., in solvents of lower polarity) in both systems.
These are clear attributes describing the inverted region of the
classical Marcus parabola, the highly exergonic region, where
kET start to decrease with increasing exothermicity.10
solvation is therefore more efficient in the former than in the latter.
In any event, the calculated |V| prove the remarkably different
aspects that the same process of ET takes in the two dyads which
differ only by the relative orientation of their moieties. The
simulations also show that in 1 the largest BET probability is
calculated from the 1,1′ positions of the pyrrole ring closest to
C60, whereas in 2 the electron “jumps” from the 2,2′ sites. Super-
exchange transfer through a solvent molecule (toluene or CH2-
Cl2) was also considered for 2. The largest |V| value was found
for the transfer of an electron from the porphyrin HOMO to the
toluene HOMO with simultaneous transfer from the toluene
HOMO to the C60 LUMO (13 cm-1). This small |V| value makes
us conclude that the solvent’s main role is not to act as a bridge
as in Zimmt’s systems.13 It should, however, be remarked that
apart from super-exchange and modification of λ, the solvent can
assist the transfer changing directly the value of |V|. This
investigation is now in progress and will be reported in the future.
Semiempirical molecular orbital calculations11 in a scheme
similar to what was used previously for electron transfer12 were
used to obtain |V| for 1 and 2. At the geometries obtained by the
MD simulations, their ratio was ∼7 (415 cm-1 vs. 59 cm-1). In
terms of rate constants, if the ET in the two systems has a similar
reorganization energy, λ, the corresponding ratio is 49. Notice
that as suggested by one referee, it is likely that λ(2) g λ(1)
because the charges are more separated in 2 than in 1 and
Acknowledgment. This work was supported by the Stiftung Volk-
swagenwerk, the European Community (HPRNT-CT-1999-00011 pro-
gram FUNCARS), the Office of Basic Energy Sciences of the U.S.
Department of Energy, and CNR (legge 95/95). This is contribution
NDRL-4311 of the Radiation Laboratory.
Supporting Information Available: Details regarding spectroscopic
characterization and conformational studies of dyad 2, along with
absorption data and calculation of electronic coupling (V) (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
(10) Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111.
(11) Zerner, M. C. Semi Empirical Molecular Orbital Methods. In ReViews
of Computational Chemistry; Lipkowitz, K. B., Boyd, D. B., Eds.; VCH
Publishing: New York, 1991; Vol. 2, p 313.
(12) Orlandi, G.; Troisi, A.; Zerbetto, F. J. Am. Chem. Soc. 1999, 121,
5392.
JA004104L
(13) Read, I.; Napper, A.; Kaplan, R.; Zimmt, M. B.; Waldeck, D. H. J.
Am. Chem. Soc. 1999, 121, 10976.