Bidirectional electron transfer in molecular tetrads

Article abstract of DOI:10.1021/ja908865k

(Figure Presented) Two-color excitation of a molecular tetrad leads to directed electron transfer in opposite directions along the molecular axis according to which chromophore is illuminated and gives a 40000-fold disparity in the lifetimes of the resultant charge-separated states.

Full text of DOI:10.1021/ja908865k

Published on Web 12/11/2009
Bidirectional Electron Transfer in Molecular Tetrads
Andrew C. Benniston, Anthony Harriman,* and Peiyi Li
Molecular Photonics Laboratory, School of Chemistry, Bedson Building, Newcastle UniVersity,
Newcastle upon Tyne NE1 7RU, United Kingdom
Received October 17, 2009; E-mail: anthony.harriman@ncl.ac.uk
Numerous molecular-scale arrays have been built and examined
quantum yield and lifetime are greatly reduced relative to isolated
ZnP.9 Thus, the fluorescence lifetime is only 55 ( 5 ps, which can
be compared to the value of 2.4 ns for the reference ZnP compound.
Laser flash photolysis studies indicate that the ZnP π-radical cation
evolves during decay of the S1 state and that the ground-state system
is restored with a lifetime of 105 ( 8 ps (Figure 2). Consequently,
and on the basis of the many ZnP-BQ molecular dyads that have
been reported,6 attenuation of the S1 fluorescence can be attributed
to light-induced electron transfer from ZnP to BQ1 and is followed
by intramolecular charge recombination. The flash photolysis
records contain no indication that the nearby RuTpy unit enters
into the transient chemistry. Cyclic voltammetry in THF, used with
electrostatic corrections, shows that the driving force for light-
induced charge separation (CS) is ∼0.68 eV, while that for
subsequent charge recombination (CR) is ∼1.36 eV. The ZnP triplet
state is not populated during CR and under these conditions lies
above the charge-separated state by ∼0.22 eV. It is perhaps worth
stressing that the π-radical anion of BQ1 cannot be resolved by
optical means because of its relatively weak spectral signature,
which is hidden underneath the more intense features of ZnP.
as means by which to better understand the details of light-induced
electron transfer between well-defined cofactors.1 Such multicom-
ponent systems, covering dyads, triads, tetrads, pentads, etc., have
been highly successful as bioinspired mimics and as vehicles for
cascade effects leading to unusually long-lived, charge-separated
states.2 As a general rule, the temporal isolation of the charge-
separated state increases with increasing number of linear electron-
transfer steps, each propelled by a modest thermodynamic driving
force.3 Within the field, the notion of two-color systems has been
developed4 wherein a common electron-transfer process can be
driven by selective illumination of either of two different chro-
mophores. Related strategies have emerged for through-bond
electron exchange along linear arrays.5 In seeking to extend the
scope of such rational molecular architectures, we have developed
a new type of array in which the direction of electron transfer can
be alternated by selection of the illumination frequency. The
products are chemically identical but differ with respect to both
kinetic and magnetic properties.
Figure 1. Molecular formula of the target compound QPRQ.
The target compound, hereafter abbreviated as QPRQ, has a
benzo-1,4-quinone residue (BQ1) covalently attached to one of the
meso positions of a zinc(II) 5,10,15,20-tetraphenylporphyrin (ZnP)
(Figure 1). This is a tried and tested protocol that has formed the
basis of a multitude of artificial photosynthetic models.6 In the
present work, ZnP is attached via the opposite meso site to a
ruthenium(II) bis(2,2′:6′,2′′-terpyridine) complex (RuTpy), which
is also a popular module for constructing light-activated electron-
transfer systems.7 The opposite side of the RuTpy complex is linked
covalently to a second BQ moiety (BQ2) to complete the tetrad. A
key feature of this linear array is the ethynylnaphthalene unit (NAP)8
interposed between RuTpy and the appended BQ2 terminal. Indeed,
NAP introduces asymmetry into the metal complex and ensures
that charge transfer occurs selectively to that terpyridine ligand
under illumination. This is the most important part of the design
protocol, at least in terms of directed electron transfer.
Figure 2. Transient differential absorption spectra recorded at delay times
of 10, 30, 60, 80, 100, 150, 200, and 300 ps after excitation of QPRQ at
550 nm. Excitation forms the ZnP S1 state, which evolves into the ion pair.
The inset shows decay profiles for stimulated fluorescence (red) and
absorbance at 586 nm (blue).
On the other hand, illumination of QPRQ in THF at 490 nm,
where the RuTpy chromophore is the dominant (i.e., >95%)
absorber, results in rapid formation of the metal-to-ligand charge-
transfer (MLCT) triplet state localized on RuTpy (Figure 3).9 In
this species, the promoted electron is delocalized over the NAP-
substituted terpyridine ligand and therefore poised for transfer to
the adjacent BQ residue, BQ2. Indeed, the lifetime of the MLCT
triplet state is only 95 ( 12 ps, which is much shorter than the
value of 415 ns recorded for the relevant reference compound9
lacking both BQ and ZnP moieties. CR of the resultant ion pair
occurs via first-order kinetics with a lifetime of 440 ( 25 ps (Figure
In tetrahydrofuran (THF) at room temperature, QPRQ displays
the range of absorption transitions expected for ZnP and RuTpy
(see the Supporting Information). Thus, illumination at 550 nm
results in preferential (i.e., >90%) excitation into the first excited
singlet (S1) state resident on the ZnP unit. Weak fluorescence can
be observed that is characteristic of the S1 state, although its
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26 J. AM. CHEM. SOC. 2010, 132, 26–27
10.1021/ja908865k  2010 American Chemical Society

C O M M U N I C A T I O N S
3). From the cyclic voltammograms, the thermodynamic driving
forces for CS and CR are 0.13 and 1.80 eV, respectively. The
relatively slow rate of CR reflects the triplet character of the ion
pair, the large amount of energy to be dissipated, and the extended
separation distance. A control compound that has the precursor
hydroquinol functions protected by benzyl groups (see the Sup-
porting Information) does not show CS. Instead, triplet energy
transfer occurs to form the T1 state localized on ZnP,10 for which
the respective energy gap is 0.31 eV. This step is quite slow, with
an MLCT triplet lifetime of 450 ( 25 ps, because of the prior
localization of the promoted electron on the NAP-bearing terpy-
ridine ligand. Triplet energy transfer competes poorly with CS when
the BQ residue is in place.
triplet ion pair increases by ∼30% upon application of a strong
(i.e., 30-50 mT) magnetic field under conditions where the
corresponding singlet ion pair remains unaffected.
In conclusion, QPRQ displays bidirectional electron transfer
along the molecular axis, with the direction and flux being
determined by the wavelength of the incident light beam. Direct
excitation of the Soret band of ZnP at 420 nm causes electron
transfer in both directions as a result of the initial partition of the
ZnP S2 state into electron transfer to BQ1 and energy transfer to
RuTpy.
Figure 4. Transient differential absorption spectra recorded at delay times
of 0, 0.08, 1.3, 2.2, 4.0, 5.6, and 8.4 µs after excitation of QPRQ in THF
with a 4 ns laser pulse at 490 nm. The spectral changes relate to collapse
of the triplet ion pair to the ground state. The inset shows a kinetic trace
recorded at 460 nm.
Figure 3. Transient differential absorption spectra recorded at delay times
of 10, 40, 70, 120, 200, 300, 500, and 1,000 ps after excitation of QPRQ
in THF with a subpicosecond laser pulse at 490 nm. Decay of the RuTpy
T1 state is most evident at ∼800 nm, while the ZnP radical cation absorbs
strongly at 460 nm.
Acknowledgment. Financial support was provided by EPSRC
(EP/D032946/1) and Newcastle University.
Supporting Information Available: Brief experimental procedures,
structural proofs, and spectroscopic data for QPRQ. This material is
available free of charge via the Internet at http://pubs.acs.org.
Following excitation at 490 nm, the flash photolysis records
indicate that the ZnP π-radical cation11 is formed by way of hole
transfer (HT) to ZnP from the oxidized form of RuTpy (Figures 3
and 4). This step is apparent from the growth of an absorption
profile with prominent maxima at ∼460 and 660 nm, in agreement
with its assignment;11 the RuTpy triplet absorbs strongly in the
near-IR region, while the oxidized form of the metal complex can
be recognized by way of the shift in the bleaching of the MLCT
absorption band. There is a good driving force (∆G0 ) -0.48 eV)
for this process. On the basis of measured molar absorption
coefficients for the intermediate species involved in the overall
reaction, it appears that the ZnP π-radical cation is formed with a
quantum efficiency of ∼60%. Since the triplet ion pair is generated
with a quantum efficiency of >95%, it follows that HT competes
reasonably well with the primary CR event. The lifetime of the
secondary ion pair is extended to 4 ( 1 µs in THF at 295 K. This
lifetime increases to 23 ( 3 µs at 180 K.
Thus, we have shown that excitation into the ZnP unit results in
rapid electron transfer to BQ1 to form in quantitative yield a singlet
ion pair that lives for 105 ps. Conversely, excitation into the RuTpy
complex gives a 60% yield of a triplet ion pair for which the redox
products are more widely spaced and the electron is localized on
BQ2, not BQ1. There is a 40 000-fold prolongation of the lifetime
of the ion pair caused by a combination of spin factors, driving
forces, and separation distances. Furthermore, the lifetime of the
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