A potential gradient along the layer-by-layer architecture for electron transfer rectification

Article abstract of DOI:10.1039/c5dt00788g

Electrochemical and photochemical measurements demonstrated that dendritic phenylazomethines, which can make complexes with SnCl₂ by a stepwise process, only permit an outbound electron transfer. The unique dendrimer effect allows efficient pro

Full text of DOI:10.1039/c5dt00788g

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A Potential Gradient along the LayerꢀbyꢀLayer
Architecture for Electron Transfer Rectification
Cite this: DOI: 10.1039/x0xx00000x
Takane Imaoka*, Hiroki Kobayashi, Makoto Katsurayama and Kimihisa Yamamoto*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Electrochemical
and
photochemical
measurements heteroatom-containing π-conjugating dendrimers as the potential
demonstrated that dendritic phenylazomethines, which can programming materials.16 By using dendritic phenylazomethine
make complexes with SnCl2 by a stepwise process, only backbones for the donor-acceptor linkage, the charge separation
permit an outbound electron transfer. The unique dendrimer efficiency and the lifetime was significantly increased.17,18 The sizes of
effect allows efficient production of photoꢀgenerated radical dendrimers are about 3-5 nm, suitable for the interfacial charge
ion pairs by suppressing their charge recombination. In rectification. However, the origin of such rectification property is yet
sharp contrast, the phenylenevinylene or benzylether ambiguous with respect to the relationship with the potential gradient
dendrimers, which lacks a heteroatom or πꢀconjugation, did and the topological factor.
not exhibit such nonꢀsymmetric characters.
We now report investigations employing various electron transfer (ET)
systems. One is the parallel system, in which the direction of charge
separation and potential gradient were identical. The other is anti-
parallel system, in which these directions are completely opposite. We
also compare a phenylenevinylene backbone as a non-cascade type
molecule. Previous study elucidated that this type of dendritic
structure does not have any electron density gradient together with
the significant dipole moment.17 Comparing these two backbones can
provide information about the effect of potential gradient.
A potential cascade principle in nature is now recognized as the
strategy for highly efficient artificial solar energy conversion.1,2
Looking at silicon-based or Grätzel-type photovoltaic devices, such
potential gradients (smooth cascades) autonomously emerge at p/n
junctions3 or Schottky barrier junctions.4 Thickness of this gradient
layer (depletion layer) is typically nanometer order; therefore,
backward electron tunneling leading to unfavorable carrier
recombination is effectively repressed.5 However, utilizing these
autonomous gradients with organic semiconducting materials is
difficult because the steady-state electron (or hole) carrier density is
very low. Another possible solution for the interfacial electron transfer
optimization is the fabrication of an artificial cascade layers. The
construction of self-assembling monolayers with large dipole moment
All dendrimers in this study were synthesized (Fig. 1). The
phenylazomethine dendrimers with a zinc tetraphenylporphyrin core
(
DPAG
reported.19,20 Phenylenevinylene dendrimers with a zinc porphyrin
core DPVGX-ZnTPP were synthesized from the corresponding
X
-ZnTPP
:
X
=
1~4) were synthesized as previously
(
)
dendrons and the core. The dendrons were prepared utilizing the
modified method from a previous report (see details in ESI).21 For the
quantitative evaluation, the core and the dendron units were
connected by the imine (C=N) bonds, which is identical to the
is one of the possible ways between two organic or organometallic
6-8
semiconducting interfaces.
In the case of electrode surface, redox
potential cascades fabricated on an electrode surface were also the
solutions.9-11 Alternatively, point charge can also provide such
a
phenylazomethine dendrimers (DPAG
resulted in the same redox potential of the ZnP/ZnP+ redox couple
between DPAG1-ZnTPP and DPVG1-ZnTPP Dendrimers with
a naphthalenediimide (DPAG -NDI = 1~4) were also synthesized
X-ZnTPP). This modification
cascade by the formation of imaginary induced dipoles on the
electrode surface.12 In any case, important thing is the sizes scale of
the interfacial gradient. Too thick cascade cannot block backward
electron tunneling (recombination). At least, potential gradient on a
several-nanometer scale is required.
.
X
: X
via the convergent approach from the corresponding
phenylazomethine dendrons and the core precursors with amine
groups (Details are shown in the Supplemental Information).
Dendritic architecture is one of the suitable molecular designs for
the management of electron or energy transfer and their
photochemical applications.13-15 We especially have studied
Benzylether dendrimers with
a zinc tetraphenylporphyrin core
(
BzEG
X
-ZnTPP
:
X
=
1~4) were prepared as previously
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reported.22 These novel and reported materials were characterized
using 1H, 13C NMR, MALDI-TOF-Mass and HPLC (SEC) measurements.
Fig. 1 Structures of dendrimers with a zinc porphyrin (photosensitizing electron
donor) or naphthalenediimine (electron acceptor) as the core. The molecular
CPK model represents DPAG4-ZnTPP
.
The hydrodynamic radii of two different dendrimers with a zinc
porphyrin core (DPAG -ZnTPP DPVG -ZnTPP) were identical. This
X
,
X
result suggests the similarity in their conformational structure
between these dendrimers reducing their back-foldings.23-25 In
addition, each dendrimer exhibited optical absorptions around 350 nm
attributed to the π-π* transition of the dendrons, together with the
Soret band (430 nm) and Q bands (600 nm) of the porphyrin core. The
Fig. 2 UV-vis absorption spectra of DPAGX-NDI (2.5 µM) on the addition of SnCl2
in benzene/acetonitrile (1/1) solvent. The isosbestic points (inset) were shifted
stepwise according to the equimolar amount of SnCl2. (a) X = 1, (b) X = 2, (c) X =
3, (d) X = 4. (e) Schematic representation of the stepwise complexation to the
dendrimer (X = 4).
dendron-based absorption exhibited
a red-shift in the UV-vis
absorption with the generation number increases up to 3 (Fig. S1 in
ESI). This result indicates an extension of the π-conjugation within the
three generations of monomers. The limited electron delocalization
on the dendrimers is important to retain their inherent dipole
moments due to the electron polarizations. Previous molecular orbital
(MO) calculations17 of each dendrimer model compound provided
results showed that the lowest unoccupied molecular orbital (LUMO)
of DPAG3-H is mainly distributed on the terminus monomer whereas
the highest occupied molecular orbital (HOMO) is on the core-side
one. The other unoccupied MOs of DPAG3-H above the LUMO
displayed potential steps. In contrast, there were no significant
These two types of dendrimers have different trends in their
electrochemical properties. In our previous study,19 the
phenylazomethine dendrimers with a zinc porphyrin core (DPAG
X-
ZnTPP) exhibited a cathodic-shift of the redox potentials
corresponding to the ZnP/ZnP+ redox couple with the generation
number increase.19 However, no significant shift was observed for the
open-shell dendrimers of which the cores (porphyrin) are not fully
encapsulated.27 These results suggest that the shift is not due to the
direct substitution effect based on the electronic induction through
the covalent bonds, but a result of the dendritic encapsulation.28 We
potential steps in DPVG3-H
.
The newly synthesized phenylazomethine dendrimer (DPAG
X
-
NDI) exhibited stepwise complexation of SnCl2 from the innermost
layer. Four independent isosbestic points in the UV-vis absorption
spectra (Fig. 2), which appeared in turn upon the stepwise additions,
were the evidence. Our previous studies26 have revealed that this
stepwise complexation is based on the dendron-specific property,
which does not affected by replacements of the core. This present
result also supports an idea that the potential gradient within the
also examined the trend in redox potentials of DPVG
the same experimental conditions. Interestingly, any shift due to the
encapsulation was not determined at all for DPVG -ZnTPP. Because
all the redox potentials of the core units in the phenylazomethine
dendrimers (DPAG -ZnTPP DPAG -NDI) exhibited a cathodic shift
X-ZnTPP under
X
X
,
X
due to the encapsulation, a unique electron transporting property of
the phenylazomethine dendrimer is expected. In other words, the
oxidation of the core units has become easier with the generation
number increases of the phenylazomethine shells probably due to
asymmetric feature of the ET through this type shell.
dendritic phenylazomethine structure is also available in DPAGX-NDI.
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5DT00788G
A
zinc tetraphenylporphyrin is known as
affording a radical ion pair in the presence of electron acceptor of DPAG
molecules.29 First, the singlet excited (1ZnP*) species was produced
The most significant feature in our study is that each attenuation
a
photosensitizer ZnP/ZnP+ redox couple observed in the electrochemical analysis
-ZnTPP
= 1 ~ 4).19
X
(X
upon photoexcitation at 532 nm. The 1ZnP* state was readily factor for the forward (kf) and backward (kb) ET was not equivalent.
converted (within 2~3 ns) to the triplet excited (3ZnP*) state through The trend in the less attenuated forward reaction (kf) was observed
an intersystem crossing. As preciously reported,18 intermolecular ETs with each electron acceptor. The observed rate constants of the
from the dendrimer to external acceptors such as 3,5-di-tert- forward (kf) and backward (kb) intermolecular reactions are much
buthylphenyl-naphtalenediimide (tBuPh-NDI) are starting from the slower than the diffusion limited rate constant calculated by
3ZnP* state. Radical ion pairs (RIPs) were produced in these systems Smoluchowski’s theory to be ks = 7.1 × 1010 mol–1 L s–1 29,30 which is also
,
as the result of this ET and ion pair dissociations. The RIPs then return applicable for the reactions of dendrimers.31 This fact strongly
to the ground state through intermolecular ET to the opposite suggests that the ET as the rate-determining step is not equivalent for
direction. All (forward and backward) ETs are exergonic for typical each direction of the ET.
electron acceptors.
The transient species characterized as RIPs (ZnP+
Such asymmetric character may also originate from the dendritic
tBuPh-NDI–) topology as the electron-transporting pathway.32 To confirm this, we
–
have identical absorption spectra based on the summation of the also examined the opposite ET system in which the direction of the
electrochemically generated ZnP+ (broad absorption peak at 690 nm) initial charge separation is inbound. The dendrimers with an electron-
and the acceptor– (the absorbance is much weaker). Similar spectral accepting core (DPAG
X-NDI: X = 1 ~ 4) were used with a non-dendritic
changes were also observed in other polar solvents (DMF, zinc tetraphenylporphyrin (ZnTPP) as the sensitizer and an electron
benzonitrile, benzene/acetonitrile). In any case, the generation donor. Upon the excitation of ZnTPP at 532 nm, similar transitions
number (
produced with each dendrimer. The trend that the lifetime increased was calculated from the decay curves. As the generation number
with the generation number was observed in the measurements of all increased, the rate constant of the forward ET kf significantly
the pairs using DPVG -ZnTPP and BzEG -ZnTPP as the electron decreased while the backward ET (kb) showed a relatively smaller
donors, and tBuPh-NDI
DPAG -NDI as the acceptors (see attenuation. Accordingly, the rate constants demonstrated
spectroscopic data in ESI). The quantum yield of 3ZnP* (ΦT) could be completely opposite trend to the pairs employing DPAG
-ZnTPP
experimentally calculated from the transient absorption spectrum, Further study using other dendrimer structure was carried out to
which was obtained without the electron acceptors. Similarly, we confirm whether this non-symmetric ET is a unique feature of the
could also calculate the yields of RIP (ΦRIP) from the transient spectra phenylazomethine architecture. We tested the benzylether (BzEG
obtained with the excess electron acceptors. Accordingly, the ZnTPP = 1 ~ 4) and phenylenevinylene (PPVGX-ZnTPP: X = 1 ~ 4)
quantum yield of the charge-separating step (ΦCS) could be estimated series. As usual, BzEG -ZnTPP and DPVG -ZnTPP also exhibited
as the division of ΦRIP by ΦT.18
two-step decays producing a transient RIP. Analysis of the decay
N = 0 ~ 4) significantly affects the lifetime of the FRIs involving two decays were found, and each rate constant (kf and kb)
(
)
X
X
,
X
a
X
.
X
-
:
X
X
X
The intermolecular ET rate constants could be calculated by the curve curves gave each rate constant (kf and kb) for the dendrimers with
fitting of the transient absorption decays corresponding to the two different generation numbers. BzEG4-ZnTPP exhibited an overall
transitions observed upon excitation. The forward ET from the trend that the ET is slower than those of the others. One possible
porphyrin core to the electron acceptor was observed as the decay of reason for the stronger attenuation is the difference in the π-
3ZnP*. As previously reported,18 the presence of tBuPh-NDI (3 mM) conjugation on these dendrimers. The partial conjugation of the
provided an initial monoexponential decay of the transient absorption benzylether structure may result in a weaker electronic coupling
(
DPAG4-ZnTPP) at 500 nm within a few µs with a residual absorption between the donor, bridge and acceptor. Again, DPVG
of the RIP. The apparent rate constant kobs) of the decay was exhibited a roughly similar lifetime of the RIPs to those of DPAG
proportional to the concentration of tBuPh-NDI (acceptor), ZnTPP because their attenuation factors are similar. Both the
suggesting that the quenching of 3ZnP* is
pseudo-first-order phenylenevinylene and benzylether dendrimers showed the
reaction. The plot between the concentration of acceptors and kobs symmetric character of the ET rates in contrast to the
gave the second-order constant kf as the slope. The same phenylazomethine series. These results suggest that the non-
X-ZnTPP
(
X-
a
(
)
treatments were successfully applied to the present pairs of the zinc symmetric ET was not due to the effects, which accompany the
porphyrins and the electron acceptors with different dendrimer common dendrimer architecture.
structures. The backward ET from the acceptor (radical anion) to the
Results of the bimolecular rate constants suggest that the ET
porphyrin core (radical cation) was purely observed as a second-order through the phenylazomethine dendritic shell has a non-symmetric
reaction, which displayed an inverse functional decay. This kinetics is property. We have also confirmed that the apparent redox potentials
due to the equivalent amount of each reactive species (radical anion of the core of the phenylazomethine dendrimers shifted in the
and cation). In this case, the second-order rate constant (kb) was negative direction. This observation supports a more facile outbound
calculated as the slope of the plot between the time and an inverse of ET than the opposite inbound transfer. This fact strongly suggests the
the RIP concentration (CRIP). Accordingly, each rate constant of the domination of
forward kf RIP formation) and backward ET kb
charge contribution by super-exchange ET is not excluded.33,34 Theoretical
recombination) was obtained. The calculated ratio of kb between
and 4 (kbG4 kbG1) was 16 for DPAGX-ZnTPP
tBuPh-NDI system, which
a electron hopping process although a partial
(
:
( :
X
= 1 representation of the non-symmetric ET is under consideration.
/
/
is almost equivalent to that of the interfacial ET rate constant for the
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Potential gradient along a phenylazomethine dendrimer structure can control favorable
electron transfer direction.