Long-Lived, Charge-Shift States in Heterometallic, Porphyrin-Based Dendrimers Formed via Click Chemistry

Article abstract of DOI:10.1021/jp2012182

A series of multiporphyrin clusters has been synthesized and characterized in which there exists a logical gradient for either energy or electron transfer between the porphyrins. A central free-base porphyrin (FbP), for example, is equipped with periphera

Full text of DOI:10.1021/jp2012182

ARTICLE
pubs.acs.org/JPCA
Long-Lived, Charge-Shift States in Heterometallic, Porphyrin-Based
Dendrimers Formed via Click Chemistry
Loïc Le Pleux,† Yann Pellegrin,† Errol Blart,† Fabrice Odobel,*,† and Anthony Harriman*,‡
†Universitꢀe de Nantes, CNRS, Chimie et Interdisciplinaritꢀe: Synthꢁese, Analyse, Modꢀelisation (CEISAM), UMR CNRS No. 6230,
2 Rue de la Houssiniꢁere, BP 92208, 44322 Nantes Cedex 3, France
‡Molecular Photonics Laboratory, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU,
United Kingdom
S Supporting Information
b
ABSTRACT: A series of multiporphyrin clusters has been synthesized and
characterized in which there exists a logical gradient for either energy or
electron transfer between the porphyrins. A central free-base porphyrin (FbP),
for example, is equipped with peripheral zinc(II) porphyrins (ZnP) which act as
ancillary light harvesters and transfer excitation energy to the FbP under visible
light illumination. Additional energy-transfer steps occur at the triplet level, and
the series is expanded by including magnesium(II) porphyrins and/or tin(IV)
porphyrins as chromophores. Light-induced electron transfer is made possible
by incorporating a gold(III) porphyrin (AuPþ) into the array. Although
interesting by themselves, these clusters serve as control compounds by which
to understand the photophysical processes occurring within a three-stage
dendrimer comprising an AuPþ core, a second layer formed from four FbP
units, and an outer layer containing 12 ZnP residues. Here, illumination into a peripheral ZnP leads to highly efficient electronic
energy transfer to FbP, followed by charge transfer to the central AuPþ. Charge recombination within the resultant charge-shift state
is intercepted by secondary hole transfer to the ZnP, which occurs with a quantum yield of around 20%. The final charge-shift state
survives for some microseconds in fluid solution at room temperature.
’ INTRODUCTION
oxidative or reductive equivalents in close proximity.13 In this
way, it might become feasible to consider multielectron redox
reactions leading to fuel production or water oxidation.14 The
light-harvesting antenna present in photosynthetic reaction cen-
ters, which organize 9 or 18 (bacterio)chlorophylls into discrete
cyclic arrangements,15 is a convincing illustration of the validity of
these basic principles.16
Numerous multiporphyrin arrays have been prepared by
iterative Sonogashira cross-coupling reactions,17ꢀ20 by the Wil-
liamson nucleophilic reaction,21ꢀ24 or by way of nucleophilic
aromatic substitutions.25 Such systems rely on covalent bonding
to assemble the individual porphyrins into logical superstruc-
tures. Supramolecular-based dendrimers assembled by way of
strong coordinative bonds with zinc,26ꢀ28 phosphorus, and tin
porphyrins are also known.9,29ꢀ31 Furthermore, efficient elec-
tronic energy-32 and electron-transfer33 processes have been
described for several of these systems, and it should be stressed
that, in certain cases, unusually long-lived, charge-separated states
have been reported.34 In seeking to extend the field, it might be
noted that the synthesis of heteronuclear multiporphyrin dendri-
mers is a nontrivial task inasmuch as high-yielding reactions must
Multiporphyrin dendrimers are desirable target molecules for
use as large-scale photon collectors, or light-harvesting arrays, in
artificial photosynthesis. Indeed, the spherical branch symmetry
is well-suited for the realization of large absorption cross-sections
and for channeling accumulated photons or electronic charge
toward the center or periphery according to the nature of the
dendrimer. Changing the metal cation coordinated to the porphyrin
ring can generate a wide range of pigments, each displaying a
distinctive absorption spectrum, that combine to cover a broad
absorption window and at high absorptivity.1ꢀ4 Such properties
are key requisites for the design of efficacious solar concentra-
tors.5 Furthermore, the increasing density of pigment molecules
brought about with each incremental layer creates opportunities
for intercompartmental processes, such as excited-state annihila-
tion,6 energy- and/or electron-transfer reactions,7ꢀ10 and
quenching events.11 By arranging the pigments in preferred
sequences, additional possibilities might arise for the spatial
isolation of light-induced, charge-separated states, in turn leading
to long-lived redox species.12 A particularly attractive feature of
tetrapyrrolic-based dendrimers is the capability to combine both
options and thereby produce tailor-made optoelectronic devices
able to run under ambient light conditions. This could lead
to new molecular materials able to generate and store several
Received: February 7, 2011
Published: May 02, 2011
r
2011 American Chemical Society
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be used to limit defects and to reach higher generations. To this
end, the copper-catalyzed Huisgen cycloaddition reaction, com-
monly referred to as “click chemistry”, is a powerful tool for such
applications.35ꢀ38 This type of connection has been used success-
fully to link porphyrin substrates to various ancillary functions.39ꢀ50
Herein, we continue to apply click chemistry to assemble
functional porphyrin-based residues,40,51ꢀ53 with the specific
intention of preparing layered heterometallic dendrimers.
The main design principle behind this synthetic work has been
to create a large superstructure with the capability to harvest
photonic energy over most of the visible range and to utilize this
excitation energy to bring about efficacious charge separation. A
further requirement is that the resultant charge-separated state
should be relatively long-lived and have the promoted electron
localized at a single porphyrin residue. This latter feature is
necessary for the array to function as a light-harvesting unit for
organic solar cells or dye-injection photoelectrochemical devices.
To realize such units, we have adapted our earlier work54ꢀ56 with
electron affinic gold(III) porphyrins. Thus, the starting point for
assembly of the dendrimer is a gold(III) tetraarylporphyrin,
which will operate as the ultimate electron acceptor.57 The
complementary electron donor is a free-base porphyrin (FbP),
attached nearby to the gold(III) porphyrin (AuPþ) and forming
the first layer. To provide for spatial isolation of the charge-
separated state, a layer of zinc(II) porphyrins is added. This
strategy allows for the redox gradient that should drive hole
transfer from the oxidized free-base porphyrin to one of the
attached zinc(II) porphyrins. In addition, highly efficient elec-
tronic energy transfer should be expected58 from the zinc(II)
porphyrin (ZnP) to an adjacent free-base porphyrin, thereby
fulfilling the need to collect photons over a wide spectral window.
Energy transfer in such systems can involve both singlet and
triplet excited states59 such that the overall quantum efficiency
for the charge-separation event should be high. The limiting
efficiency, therefore, might be set by the effectiveness of the
subsequent hole-transfer step.
hydroxide in refluxing toluene as previously reported by Lindsey
and co-workers.64 The methylhydroxybutynyl protecting group
was preferred over the classical trimethylsilyl unit since it
provides easier separation of porphyrin byproduct via column
chromatography, because of the relatively high polarity of the
alcohol compared to the corresponding di-tert-butyl derivative.
The Huisgen cycloaddition reaction between the above porphyr-
in building blocks was conducted with copper(II) sulfate as
catalyst using ascorbic acid as reducing agent in dimethylforma-
mide. Reaction proceeds smoothly at 50 °C and gave high yields.
The resultant reaction mixture was best purified by size-exclusion
chromatography, where the triazole groups bind strongly to the
stationary phase (Scheme 2).
It was found that the magnesium porphyrin (MgP) in 10 and
D2 could be selectively demetalated with phosphoric acid in
tetrahydrofuran without removal of the zinc(II) cations from the
peripheral porphyrins. All of the new compounds were char-
1
acterized by H NMR spectroscopy and high-resolution mass
spectrometry, except dendrimers D2(Fb) and D2(Mg), which
failed to give a MALDI-TOF signal, most likely because of the
high molecular weights (>15 000 g/mol) of these compounds.
Electrochemistry. The dendrimers and relevant synthons
were studied by cyclic voltammetry and differential pulse vol-
tammetry in deoxygenated CH2Cl2 containing background
electrolyte to determine the half-wave potentials of each por-
phyrin in the array. The most easily oxidized porphyrin is MgP
(EOX = þ0.66 V vs SCE), closely followed by ZnP (EOX = þ0.74
V vs SCE), and finally by FbP (EOX = þ1.02 V vs SCE). These
derived values are in line with earlier work on simpler metallo-
porphyrins,65 thereby indicating that the triazole linker does not
perturb the electronic properties of the porphyrin. The AuPþ
unit cannot be oxidized within the available electrochemical
window imposed by the solvent. This potential gradient creates
the driving force by which to move a positive hole formed at the
FbP to a ZnP-forming part of the outer periphery in D2(Fb), but
this process is thermodynamically uphill for D2(Mg). All of the
electrochemical steps are quasi-reversible and highly reproduci-
ble. Similar values are observed in N,N-dimethylformamide
(DMF) solution.
’ RESULTS AND DISCUSSION
Synthesis. The molecular formula of the target dendrimer,
D2(Fb), is presented in Chart 1, together with structural
representations of some relevant control compounds. Synthesis
of these porphyrin-based dendrimers calls for three key building
blocks, 3, 6, and 7 (Schemes 1 and 2). The fully symmetrical
tetrakis-5,10,15,20-(4-(azidomethyl)phenyl)porphyrin, 2, was
prepared in 12% overall yield by condensation of the known
4-(azidomethyl)benzaldehyde, 1,60 with pyrrole as catalyzed by
BF3ꢀOEt2 in dichloromethane (Scheme 1). The corresponding
free-base porphyrin 2 was subsequently metalated with gold(III)
using standard conditions,61 namely, KAuCl4 in refluxing acetic
acid, to afford 3 in 40% yield.
Mixed aldehyde condensation of 1 with 4-((trimethylsilyl)-
ethynyl)benzaldehyde, 4,62 with pyrrole gave porphyrin 5 in 4%
yield. In spite of the low yield for this transformation, the one-
step synthesis and its facile purification by column chromatog-
raphy, due to the high polarity of the azido groups, enable
isolation of pure samples of porphyrin 5 in significant quantities.
Metalation of 5 with magnesium(II) was accomplished according
to the conditions developed by Lindsey and co-workers.63
Ethynylporphyrin 7 was prepared by condensation of 3,5-di-
tert-butylbenzaldehyde and 4-(3-methyl-3-hydroxybut-1-yn-1-yl)-
benzaldehyde with pyrrole followed by deprotection with sodium
For D2(Fb), the first reduction wave corresponds to addition
of an electron to AuPþ (ERED = ꢀ0.57 V vs SCE). Again, this
situation is fully consistent with earlier work,66 where it has been
concluded that the actual site of reduction is the metal cation and
not the porphyrin ring.67 At higher reductive potentials, the
FbP unit is reduced with a characteristic ERED of ꢀ0.89 V vs
SCE in CH2Cl2. Finally, the outermost ZnP units are reduced
(ERED = ꢀ1.35 V vs SCE). In examining this system it was noted
that whereas the ratio of the intensities of the ZnP oxidation wave
and the AuPþ reduction wave in dendrimer D1 is close to 4:1, the
same ratio for dendrimer D2(Fb) is much higher than expected.
This effect can be attributed to shielding by the ZnP (and FbP)
moieties that prevents close approach of AuPþ to the working
electrode. This phenomenon has been noted before in electro-
chemical studies with dendrimers68 and is reminiscent of the
situation known to occur in certain redox enzymes where the
protein matrix insulates the internal catalytic site and prevents fast
electron exchange with the electrode.69 Similar effects and half-
wave potentials were found for D2(Mg), with the MgP unit being
the most difficult species to reduce (ERED = ꢀ1.42 V vs SCE).
Absorption and Fluorescence Spectroscopy. The absorp-
tion spectrum recorded for the target dendrimer D2(Fb) in
DMFis shownasFigure 1andcontains arichvarietyofoverlapping
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Chart 1. Molecular Formulas and Generic Abbreviations for the Compounds Studied in this Worka
aS refers to synthon, D refers to dendrimer, and the numeral refers to the level of evolution.
transitions across the visible region. Assignment is made on the
basis of the various control compounds, supplemented with the
corresponding monomeric porphyrins. The Soret band appears
as a strong transition centered at 421 nm but cannot be resolved
into individual components. Across the Q-region, the absorption
spectrum has the appearance of a linear combination of the
individual porphyrin spectra, indicating little ground-state elec-
tronic interaction between the pigments. This property is con-
sistent with the saturated linkage used to connect the porphyrins.
The presence of AuPþ is evidenced by the intensification of the
absorbance at 550 nm and the presence of a shoulder at 515 nm,
which is the consequence of the overlap of its Q-bands, which
show peaks at 515 and 550 nm, with those of ZnP, which appear
respectively at 550 and 590 nm. The FbP unit shows absorption
bands at 591 and 648 nm, as well as Q-bands obscured by the
stronger transitions localized on ZnP. Thus, FbP can be illumi-
nated selectivity at 648 nm, but AuPþ does not possess a unique
absorption spectral feature and cannot be excited with any real
degree of selectivity. Because of the large number of ZnP
chromophores and their preferential absorption at 550 nm, this
pigment can be excited with around 85% selectivity. The absorp-
tion spectrum of the magnesium porphyrin (MgP) cannot be
distinguished from that of the ZnP such that spectra recorded for
S(Mg) or D2(Mg) (see insert to Figure 1) lack the diagnostic
information found for D2(Fb). Accordingly, selective excitation
of these two porphyrins is not possible.
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Scheme 1. Synthetic Route to the Key Porphyrin Building
Blocks 3 and 6 a
confirm the transient formation of the relevant charge-shift state
(see later). For the corresponding free-base porphyrin analogue,
D1(Fb), fluorescence from the peripheral porphyrin is also
quenched relative to the isolated reference compound, the
decrease being about 3.5-fold in DMF solution. Time-resolved
emission spectroscopy indicates that the S1 lifetime of the FbP
module is reduced to ca. 1.4 ns compared to 6.3 ns for the control.
Again, fluorescence quenching is attributed to a charge-shift
reaction for which the thermodynamic driving force is ꢀ0.32 eV.
The photophysical properties recorded for S(Mg) are difficult
to interpret because neither porphyrin can be excited with
meaningful selectivity, even allowing for the stoichiometry.
Indeed, the difference in absorption maxima for the Q-bands
of ZnP and MgP in DMF is less than 10 nm. There are marked
differences, however, in both fluorescence quantum yield (ΦF)
and excited-state lifetime (τS) recorded for the monomeric MgP
and ZnP reference compounds that can be used to follow the
course of reaction. Thus, because of the internal heavy-atom
effect,71 fluorescence from ZnP (ΦF = 0.033; τS = 2.2 ns) is
quenched relative to that from MgP (ΦF = 0.086; τS = 7.7 ns).
Excitation of S(Mg) in DMF at 510 nm gives a time-resolved
emission decay profile that is best fit to dual-exponential kinetics,
as might be expected, at all monitoring wavelengths. The two
lifetimes extracted by global fitting routines are 1.2 and 3.8 ns.
This situation requires partial quenching of both excited states in
the cluster relative to their respective reference compound. For
direct excitation into the ZnP unit, quenching could involve both
electron abstraction (ΔG° = ꢀ0.09 eV) from the ground-state
MgP and electronic energy transfer. Electron transfer from the
excited-singlet state of MgP to a nearby ZnP (ΔG° = ꢀ0.05 eV)
is also possible following excitation into MgP, while excitation
energy transfer is thermodynamically uphill by a small amount.
Tentatively, we assign the shorter lifetime to the ZnP unit, but
this could not be proven by experiment. Such behavior is also
possible at the triplet level, but again the triplet energies are rather
close.58 This leads to a complex equilibrium mixture of triplet states
that interconvert on the sub-microsecond time scale.
Preferential excitation at 550 nm of the ZnP units present in
the target dendrimer D2(Fb) leads to weak fluorescence char-
acteristic of the ZnP S1 state (Figure 2). Time-resolved fluores-
cence decay profiles recorded at wavelengths where only the ZnP
unit emits indicate a lifetime of 160 ps, compared to 190 ps as
recorded for S(Fb) under the same conditions. Fluorescence
excitation spectra recorded for emission wavelengths where
fluorescence from the FbP dominates the profile confirm that
photons absorbed by ZnP are transferred in almost quantitative
yield to the FbP unit.
Thus, quenching is attributed to rapid electronic energy
transfer within the dendrimer, in accordance with the behavior
illustrated above for S(Fb). However, the S1 state localized on
the FbP unit is somewhat quenched in the dendrimer compared
to the synthon. Thus, the fluorescence lifetimes recorded for the
FbP units in D2(Fb) and S(Fb), respectively, are 1.3 and 6.1 ns in
DMF solution at ambient temperature (Figure 3). At the same
time, the fluorescence quantum yields measured following
selective excitation into the FbP unit at 610 nm confirmed that
the corresponding S1 state is quenched by a factor of ca. 5-fold in
the dendrimer compared to the synthon. Clearly, the difference
between the two compounds can be traced to the presence of the
central AuPþ fragment in D2(Fb). According to the electro-
chemical results, there is a substantial thermodynamic driving
(ΔG0 = ꢀ0.34 eV) for electron transfer from the S1 state resident
a Reagents and conditions: (a) Et2O.BF3, CH2Cl2, 12 h, r.t. then DDQ, r.t.,
1 h, (11%). (b) HAuCl4.3H2O, NaOAc, acetic acid, 120 °C, 2 h, (40%).
(c) Et2O.BF3, CH2Cl2, 12 h, r.t. then DDQ, r.t., 1 h, (4%). (d)
diisopropylethylamine, MgI2, CH2Cl2, 60 °C, 3 h, (66%).
The photophysical properties of S(Fb) have been reported
earlier52 and provide clear evidence for rapid energy transfer from
ZnP to the central FbP, at both singlet and triplet excited-state
levels. The rate constant for singlet energy transfer from ZnP to
FbP, as measured by time-resolved fluorescence spectroscopy, is
4.8 ꢁ 109 sꢀ1 and indicates that the probability for energy
transfer in this system exceeds 90%. The residual excited-singlet
state of ZnP produced upon excitation undergoes intersystem
crossing to form the corresponding triplet-excited state.59 This
latter species transfers excitation energy to the attached FbP with
a rate constant of 1.1 ꢁ 108 sꢀ1. In contrast, no spectroscopic
evidence could be obtained to indicate light-induced electron
transfer between the porphyrin units. Consequently, the excited-
state lifetimes of the FbP unit remain comparable to those
recorded for the corresponding monomeric porphyrin. Thus,
the singlet-excited-state lifetime (τS = 6.1 ns) is reduced slightly
relative to that of the monomeric reference compound(τS = 6.3ns)
but the corresponding triplet lifetime recorded for the FbP unit
present in S(Fb) (τT = 370 μs) is somewhat longer than that
recorded for the control compound (τT = 325 μs).
Energy transfer from ZnP to AuPþ in the corresponding
compound D1(Zn), where AuPþ replaces FbP, is thermodyna-
mically uphill and unlikely to take place. However, in DMF
solution fluorescence from the ZnP unit is quenched by a factor
of ca. 12-fold relative to the ZnP reference compound. Time-
resolved emission profiles recorded for ZnP fluorescence for D1
in DMF were monoexponential and allowed calculation of τS as
being 175 ps, compared to τS = 2.2 ns for the control. In this case,
quenching of the S1 state localized on the ZnP unit is attributed
to electron transfer to the AuPþ, as was reported for several
related molecular dyads comprising ZnP and AuPþ terminals.70
There is a significant thermodynamic driving force for this pro-
cess (ΔG0 = ꢀ0.79 eV) in D1, and laser flash photolysis studies
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Scheme 2. Click Chemistry Reactions for the Preparation of the Dendron D1(Zn) and the Two-Stage Dendrimersa
a Reagents and conditions: (a) CuSO4 5H2O, ascorbic acid, DMF, 50 °C, 4 days (75ꢀ99%). (b) K2CO3, MeOH, CH2Cl2, rt, 2 h (99%). (c) H3PO4,
3
THF, rt, 1 h (99%).
on the FbP residue to a nearby AuPþ unit. We will return to this
point in the next section.
derived by global analysis, are 150 ps and 1.1 ns, with the latter
being the more intense at all monitoring wavelengths. These
values are to be compared to those derived for the ZnP(τS =1.2 ns)
and MgP units (τS = 3.8 ns) present in S(Mg). It is difficult to
arrive at a logical interpretation as to why the fluorescence
lifetime for the ZnP residue should shorten in the dendrimer,
since it is remote from the central AuPþ, but the MgP is able to
undergo light-induced electron transfer (ΔG° = ꢀ0.83 eV) to the
AuPþ unit. Consequently, we assign the longer lifetime to ZnP
and the shorter lifetime to MgP. Clearly, this situation requires
that the S1 state localized on the MgP unit in D2(Mg) is heavily
involved in electron transfer to the central porphyrin. A similar
For dendrimer D2(Mg) in DMF solution, excitation at
560 nm gives rise to rather weak emission that appears consistent
with that expected from the ZnP unit, with little contamination
from the MgP unit. It must be stressed, however, that the two
spectral profiles are quite similar, and consequently it is difficult
to be quantitative on this point. More informative are the time-
resolved emission profiles recorded after statistical excitation into
the ZnP unit. Like the situation found for S(Mg), fluorescence
decay curves recorded for D2(Mg) are dual-exponential across
the entire wavelength range. The averaged emission lifetimes,
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Figure 1. Absorption spectrum recorded for D2(Fb) in DMF. The
Soret region is shown after dilution of the solution. The insert shows the
corresponding Q-band region as recorded for D2(Mg) in DMF.
Figure 3. Time-resolved fluorescence profiles recorded for D2(Fb) in
DMF solution at room temperature. The instrumental response func-
tion is shown as a gray curve superimposed onto the experimental data
points: (a) fluorescence from the ZnP unit and (b) fluorescence from
the FbP unit.
Table 1. Summary of Main Fluorescence Quenching Events
Deduced from Time-Resolved Emission Spectroscopy for the
Various Clusters in DMF Solution
compd
process donor acceptor
ꢀΔG°a/eV k b/(108 sꢀ1
)
S(Fb)
EET
ElT
ElT
ElT
ElT
EET
ElT
ElT
ElT
ZnP*
MgP
MgP*
ZnP*
FbP*
ZnP*
FbP*
MgP
MgP*
FbP
0.19
0.09
0.05
0.79
0.32
0.19
0.32
0.09
0.83
48
3.8
2.3
53
5.6
58
S(Mg)
ZnP*
ZnP
AuPþ
AuPþ
FbP
AuPþ
ZnP*
AuPþ
Figure 2. Fluorescence spectrum recorded for D2(Fb) in DMF at room
temperature. The excitation wavelength was 550 nm, and the region
corresponding to emission from the ZnP unit is shown as an expansion.
D1(Zn)
D1(Fb)
D2(Fb)
6.1
3.8
situation arises for the triplet manifold since laser flash photolysis
studies made with 25 ns temporal resolution failed to detect any
transient species. In contrast, the triplet-state lifetime (τT) recor-
ded for the monomeric MgP reference compound in deoxyge-
nated DMF is 290 μs. A similar triplet lifetime was found for
S(Mg) (τT = 265 μs), although this cannot be attributed solely to
MgP since the triplet states localized on MgP and ZnP are almost
isoenergetic. Nonetheless, the triplet-state resident on MgP must
be quenched in the dendrimer relative to that for the synthon in
deoxygenated DMF at ambient temperature. This effect can be
ascribed to electron transfer to the AuPþ.
We now summarize the various processes occurring in the
multicomponent arrays by way of Table 1. The two key fluores-
cence quenching events relate to electronic energy transfer
(EET) and light-induced electron transfer (ElT) within the
cluster while the accompanying thermodynamic parameters refer
to the energy gap between relevant electronic levels (ΔESS) or
the change in Gibbs free energy (ΔG°) associated with electron
transfer. The rate constants (kEET or kElT) are derived from time-
resolved emission spectroscopy by comparing fluorescence life-
times for a particular chromophore in the cluster with the isolated
control compound in DMF, using the same excitation wave-
length. In general, the electron-transfer rates reflect the driving
force for that process but give no useful information about the
D2(Mg)
65
a For EET substitute ΔESS. kEET for energy transfer and kElT for
b
electron.
underlying mechanism. The time scales for electron transfer
within the various clusters seem to be on the order of some
hundreds of picoseconds which means that competing processes
need to be fast.
Transient Absorption Spectroscopy. Starting first with D1-
(Zn) in deoxygenated DMF, laser flash photolysis studies were
carried out with excitation at 532 nm using a 30 ps laser pulse.
Immediately after excitation, the transient absorption records
show the presence of the S1 state associated with the ZnP unit.
This species possesses a reasonably characteristic spectral signa-
ture72 with strong absorption at around 450 nm and distinctive
bleaching signals at longer wavelengths. The S1 state decays via
first-order kinetics with a lifetime of 140 ps, derived from global
analysis, leading to increased absorptivity across most of the
spectral window and the evolution of a new spectral profile
(Figure 4). On the basis of earlier work,70 this latter transient
species can be identified as being the charge-shift state formed as
a result of electron transfer from the ZnP S1 state to the nearby
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Figure 5. Transient absorption spectral records showing the reaction
profile after selective excitation into the ZnP unit present in D1(Fb) in
DMF solution at room temperature. Delay times are as follows: 0.05,
0.20, 0.60, 1.0, 2.0, 4.0, and 8.0 ns. The insert shows a typical kinetic trace
recorded at 518 nm.
Figure 4. Transient absorption spectral records showing the reaction
profile after selective excitation into the ZnP unit present in D1(Zn) in
DMF solution at room temperature. Delay times are as follows: 30, 60,
100, 200, 300, 400, 600, 800 and 1000 ps. The insert shows a typical
kinetic trace recorded at 460 nm.
made at 598 nm where only the FbP absorbs. The transient signal
decays with a lifetime of 1.1 ns to form the triplet-excited state73
of the FbP unit in low yield (Figure 5). This latter species is most
likely formed by way of intersystem crossing from the S1 state. It
decays with a lifetime of 190 μs, which is not too dissimilar from
that measured for the control compound (τT = 325 μs) under
identical conditions. Thus, the triplet state does not enter into
electron-transfer reactions with the nearby AuPþ, most likely
because of unfavorable thermodynamics (ΔG° = þ0.12 eV).
Quenching of the S1 state is attributed to the charge-shift reac-
tion, as outlined above on the basis of the time-resolved emission
spectroscopy. Since the charge-shift state is not seen clearly in the
flash photolysis records, we surmise that charge recombination
occurs on a time scale comparable to that of the forward charge-
shift reaction. The transient absorption spectral records were
closely interrogated at 493 nm, which represents an isosbestic
point for the FbP excited singlet and triplet states but where the
charge-shift state is expected to show significant absorption.
Simulation of the temporal profile collected at this wavelength,
on the basis that it evolves with a lifetime of 1.1 ns, suggests the
charge-shift state has a lifetime less than 0.7 ns. This can be
compared with that derived for the species formed for D1(Zn)
under related conditions. Although the charge-shift state asso-
ciated with D1(Fb) has to dissipate more vibrational energy
during the reverse step (ΔGCSR = ꢀ1.59 eV) relative to D1(Zn)
where ΔGCSR = ꢀ1.31 eV, it is able to undergo intersystem
crossing to the local triplet-state resident on FbP. This latter
process might serve to offset the full effects74 of the Marcus inver-
ted region.75
Laser flash photolysis studies made with S(Fb) in deoxyge-
nated DMF confirmed the presence of the S1 state localized
on ZnP at the end of the laser pulse72 Again, this S1 state
decayed much faster than that of the isolated reference compound
(τS = 2.2 ns), with the derived lifetime of 180 ps being in very
good agreement with that determined by time-resolved fluores-
cence spectroscopy (τS = 190 ps). Electron transfer is unlikely in
this system because of unfavorable thermodynamics, but energy
transfer is a strong possibility and has been introduced already to
account for the fluorescence spectral results. This situation was
confirmed by transient absorption spectroscopy, as indicated in
Figure 6 for S(Fb). Thus, decay of the ZnP S1 state is matched by
Scheme 3. Representation of the Events That Follow from
Selective Excitation of the ZnP Unit Present in D1(Zn)
AuPþ. The thermodynamic driving force for this process,
calculated on the basis of the cyclic voltammetry results, is
estimated to be ca. ꢀ0.8 eV. It is notable that the rate constant
for the forward charge-shift step (kCSR = 6.7 ꢁ 109 sꢀ1) derived
from the transient absorption studies is in fair agreement
with that obtained from time-resolved emission spectroscopy
(kCSR = 5.3 ꢁ 109 sꢀ1). Global analysis applied to the transient
absorption spectral records indicates that the charge-shift state
decays with a lifetime of ca. 290 ps to restore the ground state,
without intermediate population of a triplet species. Indeed, the
small amount of ZnP triplet state formed in competition to
electron transfer can be seen after decay of the charge-shift state.
By necessity, the T1 state is formed for those conformations that
do not favor electron transfer at the S1 level. The lifetime of the
T1 state for D1(Zn) is decreased to only 25 ns, compared to 280
μs found for the isolated control compound. Quenching is
attributed to intramolecular electron transfer to form the triplet
charge-shift state, for which the driving force is ca. ꢀ0.28 eV. In
this case, decay of the charge-shift state occurs faster than the
forward reaction, probably because of an accompanying change
in conformation to favor electron transfer. The overall behavior
deduced for D1(Zn) is illustrated by way of Scheme 3.
Similar experiments made with compound D1(Fb) in deox-
ygenated DMF confirmed the presence of the S1 state localized
on FbP at the end of the laser pulse.72,73 Here, excitation was
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Scheme 4. Representation of the Events That Follow from
Selective Excitation of the ZnP Unit Present in S(Fb)
emission lifetime is decreased to 175 ps. On the basis of the work
described above, fluorescence quenching can be attributed to
electronic energy transfer to an FbP fragment within the dendri-
mer. Although EET is highly efficient in this system, fluorescence
from the FbP is also quenched; in this case, quenching is due to
light-induced electron transfer to a nearby AuPþ. Indeed, the
emission lifetime for the FbP unit in D2(Fb) is reduced to 0.85 ns
compared to 6.1 ns found for S(Fb). In this case, the time-
resolved fluorescence decay profile does not fit particularly well
to a single-exponential step and is better described in terms of
dual-exponential kinetics. This behavior most likely is a reflection
of hindered mobility of the FbP unit in the second-level
dendrimer. Now, the best fit to the decay curves has lifetimes
of 0.60 ns (70%) and 1.05 ns (30%), which are taken to represent
two different families of conformers. Apart from this effect, the
general situation found for D2(Fb) closely resembles that
described for the lower level dendrimer D1(Fb). A marked
difference, however, is that there is a pronounced fall in the yield
of the excited-triplet state of FbP in the larger dendrimer as seen
on relatively slow time scales. There is, in addition, a further
transient species that cannot be resolved for D1(Fb) under the
same conditions, and this species possesses the characteristic
spectral features of the charge-shift state.38
Figure 6. Transient absorption spectral records showing the reaction
profile after selective excitation into the ZnP unit present in S(Fb) in
DMF solution at room temperature. Delay times are as follows: 0.05,
0.10, 0.20, 0.30, 0.50, 0.75, 3.0, 6.0, and 10.0 ns. The insert shows typical
kinetic traces recorded at 508 and 582 nm.
Figure 7. Decay trace recorded at 508 nm for the ZnP triplet excited
state of S(Fb) in DMF. Note that the residual signal is due to the triplet
state of the FbP unit.
Detailed analysis of the transient absorption spectral records
obtained for D2(Fb) in DMF indicates that the charge-shift state
evolves on sub-nanosecond time scales but does not decay within
a 10 ns temporal window. On much longer time scales (Figure 8),
it is seen that the charge-shift state coexists with the triplet-
excited state of the FbP unit and survives for some tens of
microseconds. Indeed, global analysis protocols indicate that the
charge-shift state possesses a lifetime of ca. 60 μs in DMF at room
temperature. It is most likely that this charge-shift state forms by
way of hole transfer from the FbP π-radical cation, this being part
of the primarycharge-shiftstate, to a nearby ZnP. There is a modest
thermodynamic driving force for this process (ΔG° = ꢀ0.28 eV).
The transient absorption spectra are fully consistent with this
hypothesis in that the profile derived for the long-lived charge-shift
state clearly bears the characteristic signature of the ZnP π-radical
cation (see Supporting Information). Thus, transient differential
absorption spectra were compiled for the charge-shift states formed
between ZnP/AuPþ and FbP/AuPþ on the basis of the individual
spectra obtained by spectroelectrochemistry. These spectra are
similar but differ in the region where the Q-bands of the porphyrins
are bleached. The net result is that the charge-shift spectrum
involving the FbP shows pronounced bleaching at ca. 660 nm
whereas that formed from the ZnP has its principal bleaching region
centered at ca. 560 nm. The charge-shift state derived for D2(Fb)
after subtraction of the contribution due to the FbP triplet-excited
formation of the S1 state associated with the FbP unit.73 This
effect is most noticeable by the fast decay process occurring at
508 nm and the concomitant growth of a signal at 582 nm
(Figure 6). The efficiency of electronic energy transfer in S(Fb)
is around 92%, with the residual S1 state primarily undergoing
intersystem crossing to the triplet state. The resultant metastable
species also transfers excitation energy to the FbP unit,27 as
indicated by the laser flash photolysis records. In this case, kinetic
measurements were made at wavelengths (i.e., 493, 510, 567, and
588 nm) corresponding to isosbestic points between the tran-
sient absorption signals recorded for the excited-singlet and
triplet states localized on the FbP unit. The transient decay seen
under these conditions is due entirely to the ZnP triplet state,
which decays with an averaged lifetime of 9 ns (Figure 7). The
overall behavior is illustrated by way of Scheme 4. Laser flash
photolysis studies carried out with S(Mg) were inconclusive
other than to confirm the shortened S1 lifetimes and the presence
of low concentrations of triplet species that decayed on the
microsecond time scale.
Attention now turns to the target dendrimer G2(Fb) in DMF
solution. The dominant chromophore is the ZnP unit, which can
be illuminated with modest selectivity at 560 nm. Under such
conditions, weak fluorescence is observed in the region expected
for ZnP, but the quantum yield is reduced to only 0.003 while the
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The Journal of Physical Chemistry A
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conclusion that the secondary charge-shift state is formed with
a quantum yield of ca. 20%. The overall behavior is summarized
in Scheme 5.
The long-lived charge-shift state is not observed for D2(Mg),
where any electron-transfer chemistry is complete within a few
nanoseconds of excitation. Here, it is not possible to selectively
excite either ZnP or MgP units; in principle the AuPþ unit could
be selectively illuminated at 525 nm, but there is too much
competitive absorption by the ZnP, this being in large excess, to
make such studies viable. The photophysical properties differ
from those mentioned for S(Mg) and are difficult to interpret in
detail because of competitive light absorption and multiple quench-
ing pathways. In this case, the system does not facilitate hole
transfer to spatially isolate the redox pair and serves to emphasize
the importance of setting up a suitable redox gradient within the
dendrimer.
Figure 8. Transient absorption spectral records showing the reaction
profile after selective excitation into the ZnP unit present in D2(Fb) in
deoxygenated DMF solution at room temperature. Delay times are as
follows: 0.01, 10, 20, 40, 60, 100, 150, and 250 μs. The insert shows a
typical kinetic trace recorded at 503 nm, which is an isosbestic point for
the triplet state of the FbP unit.
’ CONCLUSIONS
The new porphyrin-based building blocks described herein
provide straightforward access to the first series of heterometallic,
multiporphyrin dendrimers by making use of the so-called click
chemistry. The approach is exemplified by the synthesis of a
three-layer dendrimer comprising 17 porphyrin modules ar-
ranged in a rational sequence. A key strength of the synthetic
protocol relates to the mild deprotection of the TMS group in
synthon 6 followed by the copper-catalyzed Huisgen cycloaddi-
tion with the azido porphyrin, which is almost quantitative. A
second advantage of this strategy concerns the opportunity to
modify the sequence of porphyrin-based modules within the
assembly, which is an important feature by which to control the
directionality of energy or electron transfer. In this study, we have
reported on the first and second dendrimer generations, but it
seems reasonable to anticipate that the iterative reactions of
dendron 10 with any metal porphyrin 5 would provide access to
higher generations. Dendrimers made to date are readily soluble
in DMF and show no signs of aggregation of individual porphyr-
ins. According to the photophysical measurements, these den-
drimers demonstrate energy- and electron-transfer reactions
with relatively high yield that are of potential interest for solar
energy conversion.
Scheme 5. Representation of the Events That Follow from
Selective Excitation of the ZnP Unit Present in D2(Fb)
state agrees well with the latter spectrum but not the former (see
Supporting Information).
Within experimental limits, decay of the charge-shift state
follows first-order kinetics and is independent of the concentra-
tion of D2(Fb) and of laser intensity. The quantum yield, being
much less than unity, can be estimated as follows: The S1 state
associated with the FbP fragment is populated with 93% prob-
ability by way of EET from the ZnP chromophore. Light-induced
charge shift with the central AuPþ unit accounts for some 85% of
this S1 state, the remainder decaying by way of fluorescence and
intersystem crossing to the T1 state. For the isolated FbP control
compound, the T1 state is formed with a quantum yield of 0.70
and we assume that the same relative probability holds for the
dendrimer. There is a further crop of the T1 state because of
tripletꢀtriplet energy transfer from the ZnP. Thus, the overall
quantum yield for formation of the FbP T1 state in the dendrimer
is ca. 15%. The molar absorption coefficient of this species is
6500 Mꢀ1 cmꢀ1 at 780 nm. On the basis of spectro-electrochemical
measurements made with D1(Zn), the molar absorption coeffi-
cients of the ZnP π-radical cation and the AuP neutral radical
respectively are 7100 and 5800 Mꢀ1 cmꢀ1 at 493 nm, where the
FbP triplet does not contribute to the transient signal. From the
relative absorbance values for the two species we reach the
The best performance is provided by D2(Fb) where highly
efficient EET is followed by successive electron-transfer steps
that lead to long-lived, charge-separated species in modest yield.
The limitation of this system is set by the relatively slow charge-
shift reaction in which the FbP donates an electron to the nearby
AuPþ fragment. Subsequent hole transfer to a peripheral ZnP
unit struggles to compete with primary charge recombination,
although the resultant secondary charge-shift state survives for
ca. 60 μs in DMF at room temperature. It is difficult to see how to
increase the overall efficiency of this basic design since the system
represents a critical balance of competing events, each of which is
subjected to thermodynamic control. Perhaps a different approach
is warranted. Thus, the synthetic strategy is easily modified to
enable the replacement of the interior FbP layer with one formed
from tin(IV) porphyrins (SnP). Illumination of the outermost
ZnP modules should lead to light-induced electron transfer to
one of the SnP units.76 The resultant SnP π-radical anion77 is
able to reduce the central AuPþ, leading to the same secondary
charge-shift state as characterized for D2(Fb). Preliminary
investigations made with S(Sn), where the corresponding SnIVP
replaces the FbP in S(Fb), confirm that the first step occurs with
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high efficiency, although it is not possible to selectively illuminate
ZnP. There is now a modest driving force for reduction of AuPþ,
which might be competitive with charge recombination. An
advantage of this sequence is that the ZnP triplet state is also
capable of reducing SnP so few photons are lost. Likewise, any
photons absorbed directly by SnP will be used to drive the same
electron-transfer reaction.
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’ ASSOCIATED CONTENT
S
Supporting Information. Text giving experimental de-
b
tails for the synthesis of all new compounds and for making the
photophysical measurements and figures showing comparative
differential spectra for the two charge-shift states associated with
D2(Fb). This material is available free of charge via the Internet
at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*Fax: 0044 191 222 8660. E-mail: anthony.harriman@ncl.ac.uk.
’ ACKNOWLEDGMENT
We thank Newcastle University and the Agence Nationale de
la Recherche ANR (“PhotoCumElec” program) for financial
support of this work.
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