Dipole-mediated rectification of intramolecular photoinduced charge separation and charge recombination

Article abstract of DOI:10.1021/ja505618n

Controlling charge transfer at a molecular scale is critical for efficient light harvesting, energy conversion, and nanoelectronics. Dipole-polarization electrets, the electrostatic analogue of magnets, provide a means for "steering" electron transduction via the local electric fields generated by their permanent electric dipoles. Here, we describe the first demonstration of the utility of anthranilamides, moieties with ordered dipoles, for controlling intramolecular charge transfer. Donor- Acceptor dyads, each containing a single anthranilamide moiety, distinctly rectify both the forward photoinduced electron transfer and the subsequent charge recombination. Changes in the observed charge- Transfer kinetics as a function of media polarity were consistent with the anticipated effects of the anthranilamide molecular dipoles on the rectification. The regioselectivity of electron transfer and the molecular dynamics of the dyads further modulated the observed kinetics, particularly for charge recombination. These findings reveal the underlying complexity of dipole-induced effects on electron transfer and demonstrate unexplored paradigms for molecular rectifiers.

Full text of DOI:10.1021/ja505618n

Article
pubs.acs.org/JACS
Dipole-Mediated Rectification of Intramolecular Photoinduced
Charge Separation and Charge Recombination
Duoduo Bao,†,⊥ Srigokul Upadhyayula,†,‡,# Jillian M. Larsen,† Bing Xia,†,∇ Boriana Georgieva,†
Vicente Nunez,† Eli M. Espinoza,§ Joshua D. Hartman,§ Michelle Wurch,† Andy Chang,†
̃
Chung-Kuang Lin,† Jason Larkin,† Krystal Vasquez,§ Gregory J. O. Beran,*,§ and Valentine I. Vullev*,†,‡,§,∥
‡
§
∥
†Department of Bioengineering, Department of Biochemistry, Department of Chemistry, and Materials Science and Engineering
Program, University of California, Riverside, California 92521, United States
S
* Supporting Information
ABSTRACT: Controlling charge transfer at a molecular scale is
critical for efficient light harvesting, energy conversion, and
nanoelectronics. Dipole-polarization electrets, the electrostatic
analogue of magnets, provide a means for “steering” electron
transduction via the local electric fields generated by their
permanent electric dipoles. Here, we describe the first demon-
stration of the utility of anthranilamides, moieties with ordered
dipoles, for controlling intramolecular charge transfer. Donor−
acceptor dyads, each containing a single anthranilamide moiety,
distinctly rectify both the forward photoinduced electron transfer
and the subsequent charge recombination. Changes in the observed charge-transfer kinetics as a function of media polarity were
consistent with the anticipated effects of the anthranilamide molecular dipoles on the rectification. The regioselectivity of
electron transfer and the molecular dynamics of the dyads further modulated the observed kinetics, particularly for charge
recombination. These findings reveal the underlying complexity of dipole-induced effects on electron transfer and demonstrate
unexplored paradigms for molecular rectifiers.
With large intrinsic dipoles oriented along their axes, protein
helices are some of the best-known molecular electrets.12−15
Ordered amides and hydrogen-bonding networks in protein α-
helices and 310-helices produce intrinsic electric dipoles of
about five Debyes per residue.16−18 Polyproline helices, which
lack hydrogen bonding along their backbones, have intrinsic
dipoles that are smaller or oppositely oriented in comparison
with α-helices.18 These dipoles, oriented along the helix axis,
can rectify electron transfer19−27 and aid ion transport.12−14
Galoppini and Fox were first to report the dipole-induced
rectification of long-range CT using helices with a donor and an
acceptor attached to them.19,20,28 The electron transfer (ET)
toward the positive pole of the helix dipole was faster than the
ET toward the negative pole.19,20,28
INTRODUCTION
■
At a molecular level, charge-transfer rectification is a term
borrowed from electrical engineering to represent preferred
directionality of electron entrainment.1 Molecular rectifiers are
some of the principal building blocks for nanoscale
electronics.2−4 Accelerating forward charge transfer (CT) and
impeding charge recombination (CR) via charge-transfer
rectification are particularly important for energy-conversion
applications.5
In the context of electrical engineering and molecular
electronics, rectification is defined in terms of charge transport,
i.e., how the magnitude of the electric current depends on the
direction of the applied potential. However, numerous device
features, such as the interfaces with the molecules, can
dominate charge-transport rectification.6,7 Therefore, charge-
transfer rectification, representing the dependence of the CT
rates on the direction of intramolecular electron transfer, allows
for unequivocal examination of molecular features that govern
these processes.
Rectification behavior results from asymmetry in CT
pathways. Local electric fields generated by molecular electric
dipoles provide a means for attaining such asymmetry.
Therefore, conjugates with codirectionally ordered electric
dipoles represent an important class of molecular rectifiers.
Similar to a molecular magnet,8,9 a molecular electret is a
conjugate containing polar groups with a codirectional
arrangement of their permanent electric dipole moments.10,11
Since these first reports,19,20,28 dipole effects on CT have
been studied almost exclusively in helical polypeptides.21−27,29
Self-assembled monolayers of polypeptide helices on gold
exhibited rectification consistent with the orientation of their
dipoles.21,22,26 A recent report on proline peptides with three
different redox residues showed that charges have a similar
effect on CT.23 Electrical junctions, comprising layers of
zwitterionic conjugates or of polypeptide α-helices, manifested
a pronounced dipole-induced current rectification.30−33
Received: June 9, 2014
Published: August 27, 2014
© 2014 American Chemical Society
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The electrets are dielectrics, which presents a fundamental
challenge in using them as charge-transfer media. Protein
helices are no exception: their large HOMO−LUMO gaps and
inaccessible reduction potentials render these biomolecular
electrets largely ineffectual for electronic applications (HOMO
= highest occupied molecular orbital, and LUMO = lowest
unoccupied molecular orbital). The peptide bonds, which are
aliphatic amides, have the narrowest HOMO−LUMO gaps
(exceeding 5 eV) along the protein backbones, and they are not
good electron donors or acceptors. Unless the CT pathways
involve redox residues or cofactors with sufficient electronic
coupling between them,34 proteins mediate ET via a super-
exchange mechanism, i.e., tunneling along virtual states, limiting
its efficiency to about 2 nm.35−37 The irreversible electro-
chemical oxidation of aliphatic amides has peak potentials
between 1.3 and 1.8 V versus SCE and leads to bond
cleavage.38 Hence, to prevent decomposition of these
biopolymers, the peptide bonds cannot be durable sites for
charges (holes, in particular), and tunneling accounts for the
experimentally observed features of CT through polypep-
tides.18,44
1a) allows for adjusting their electronic properties, enabling
efficient long-range electron or hole entrainment.11
In order for the anthranilamides to have utility as charge-
transfer electrets, they have to (1) rectify CT; (2) have wide
HOMO−LUMO gaps, typical for dielectrics; (3) possess no
permanent charges; and (4) accommodate charges on their
residues without initiating undesired decomposition processes.
That is, the residues have to manifest reversible oxidation or
reduction. The second requirement is important for preventing
the thermal generation of free charge carriers and semi-
conducting type electronic characteristics. Such mobile free
charges can readily redistribute and screen the local fields, thus
suppressing the dipole effects. Similar to polar solvents, free
counterions in the surrounding media can have the same
screening effect, implementing the reason for the third
requirement. While this requirement is not truly strict, it can
be viewed as a recommendation for preventing additional
complexity in the interpretation of the field effects on charge
transfer. Finally, the fourth requirement is especially important
for attaining long-range electron or hole hopping.
As a first step toward development of charge-transfer
molecular electrets, we focus here on a single anthranilamide
residue, 2-alkanamido-N-alkyl-5-(piperidin-N-yl)benzamide, Aa
(see Supporting Information). Aa exhibits reversible electro-
chemical oxidation. Its zero-to-zero energy, E00, is about 3 eV,
and its permanent dipole moment is about 6 D. Hence, this
residue is potentially a promising building block of an electret
dielectric that can mediate hole hopping. Most importantly, we
demonstrate here the Aa moiety rectifies the directionality of
forward and back electron transfer to and from a covalently
linked acceptor (Chart 1b,c). While most previous reports have
focused on the dipole effects on charge separation (CS),18−21
and a few have discussed charge recombination (CR),46 here
we examine the rectification of both, the forward and back CT.
For the initial photoinduced CS, the rates of ET along the Aa
dipole are up to 6.4 times faster than the ET rates against the
dipole. This degree of rectification is particularly impressive for
a single residue. For comparison, reported rates of ET along
and against the dipole in a polypeptide helix with 14 residues
differ 27-fold for optimal conditions, i.e., for a relatively
nonpolar media.19,20 The dependence on the media polarity
suggests that the Aa dipole governs the observed CT
rectification. The picosecond CS kinetics is further modulated
by molecular dynamics, while ET regioselectivity also plays an
important role, particularly for CR rectification. Overall, Aa can
be viewed as a molecular rectifier in which cumulative
contributions from molecular dynamics and regioselectivity
modulate the dipole-induced effects on charge transfer.
We have undertaken a bioinspired approach45 in designing
molecular electrets that possess the electronic advantages of
protein helices, that is, ordered amide and hydrogen bonds that
generate intrinsic dipoles of about 3 to 5 D per residue (Chart
1a).10,11,45 Unlike their biological counterparts, the bioinspired
a
Chart 1. Bioinspired Electrets Based on Anthranilamides
a
(a) Intrinsic electric dipole moment of anthranilamide electrets
originates from (1) the ordered orientation of the amide-bond dipoles
(solid arrows); and (2) the change in the polarization due to the shift
in the electron density from O to H upon hydrogen-bond formation
(hollow arrows). Unlike protein α-helices, the anthranilamide dipole is
oriented from the N- to the C-terminus. (b, c) Donor−acceptor dyads
with the acceptor linked to (b) the C-terminus and (c) the N-terminus
of the anthranilamide residue.
RESULTS AND DISCUSSION
■
Charge-Transfer Processes. To determine whether Aa
could rectify charge transfer, we need to compare its ability to
mediate charge transfer in which the electron moves toward its
C-terminus versus its N-terminus (Chart 1a). For this purpose,
we prepared dyads composed of Aa as an electron donor and 1-
alkylpyrene (Py) as an electron acceptor. Linking Py either to
the C- or the N-terminus of Aa yielded Aa-Py and Py-Aa,
respectively (Chart 1b,c). For mixtures of dichloromethane
(DCM) and acetonitrile (MeCN) as a solvent media, we
estimated that the ΔG(0) for the photoinduced electron transfer
from Aa to Py varied between −0.05 and −0.3 eV, that is,
between about 2- and 12-fold of the thermal energy, kBT (Table
1).
electrets based on anthranilamides are composed of covalently
linked aromatic moieties, forming extended π-conjugation
along their backbones (Chart 1a). Such a sequence of
electronically coupled aromatic residues is essential for attaining
long-range charge transfer via electron- or hole-hopping
mechanisms. Furthermore, altering the substituents on the
aromatic residues of the synthetic electrets (R1 and R2 in Chart
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Table 1. Redox and Charge-Transfer Properties of Aa (an Electron Donor) and Py (an Electron Acceptor) for Various Solvent
Media
a
b
c
d
d
e
f
f
(1/2)
Aa•+/Aa
(1/2)
(0)
(0)
solvent
TCE
v:v
ε
n
γ
μ/cP
E
V vs SCE
E
V vs SCE
E00/eV
ΔG /eV
ΔG /eV
mPy/mPy•‑
CS
CR
8.40
9.18
17.9
24.8
31.2
37.6
1.4923
1.4241
1.4020
1.3825
1.3565
1.3445
0.33
0.38
0.45
0.48
0.51
0.53
1.5
0.985
−2.26
3.02
3.03
3.05
3.02
3.04
3.03
−0.021
−0.060
−0.24
−0.25
−0.30
−3.0
−2.9
−2.8
−2.8
−2.7
−2.7
DCM
0.38
0.36
0.35
0.34
0.34
0.957
0.810
0.767
0.744
0.729
−2.24
−2.12
−2.08
−2.06
−2.05
DCM + MeCN
3:1
1:1
1:3
MeCN
−0.31
The dielectric constants were obtained from impedance-spectroscopy measurements.39,40 Solvent polarity, γ = n−2 − ε−1. Dynamic viscosity.39
a
b
c
d
The half-wave reduction potentials for the different solvents were extrapolated from cyclic voltammetry measurements employing samples with
different composition of the electrolyte solutions41,42 [Aa = 2-hexanamido-N-hexyl-5-(piperidin-N-yl)benzamide; mPy = 1-methylpyrene]. The
e
values for the zero-to-zero energy were extracted from the crossing points of the normalized absorption and emission spectra of Aa dissolved in the
f
corresponding solvent. The values of ΔG(0) for the photoinduced charge separation, CS, and for the back charge transfer leading to charge
recombination, CR, were estimated using the Rehm−Weller equation.41,43
From dielectric studies39 we determined that Aa has a dipole
moment of 6 2 D. Concurrently, we expect that a dyad with
Aa as a donor will manifest increased rates of photoinduced
charge separation when the electron moves in the same
direction as the Aa dipole, which points toward the C-
terminus.10,11 Hence, Aa-Py should manifest larger rates for
charge separation than Py-Aa.
To study this behavior, we selectively excite the Aa moiety in
the donor−acceptor dyads. The UV absorption of Aa extends
to 400 nm, corresponding to the optical transitions to its lowest
singlet excited state (see Figure S14 in the Supporting
Information). This region of the spectrum is bathochromic
with respect to the alkylpyrene bands (Figure 1a),49,51−56 which
allows for selective excitation of the Aa moiety at about 390 to
400 nm. Upon photoexcitation of both dyads in DCM, broad
transient-absorption bands appear at about 410 and 710 nm,
which is indicative of the formation of the singlet-excited state
of the anthranilamide, 1Aa* (Figures 1c and 2a,b). A
picosecond growth of transient peaks at 500 and 580 nm
1
accompanies the decay of the Aa* absorption (Figure 2a,b).
We assign the 500 nm peak to the reduced pyrene,28,57 Py•−,
and the 580 nm peak to the radical cation of the
anthranilamide, Aa•+ (Figure 1b,c).
For each of the investigated samples the transient absorption
bands at 500 and 580 nm rise and decay simultaneously. The
growth of the bands at 500 and 580 nm, therefore, correspond
to the initial photoinduced CS, and the subsequent decay of
these radical-ion bands is due to CR (Figure 2d).
Figure 1. Absorption spectra of the donor−acceptor dyads, and of
their components and transients. (a) Steady-state spectra (30 μM in
DCM). (b) Spectra from electrochemical measurements recorded at
the cathodic and the anodic peak of cyclic voltammograms of Aa and
1-metylpyrene (mPy), respectively (100 mM tetrabutylammonium
hexafluorophosphate in MeCN). (c) Absorption spectra of Aa
transients recorded using pump−probe spectroscopy (λex = 350 nm,
5 μJ per pulse):47,48 singlet excited state, 1Aa*, recorded at 10 ps after
Studies employing two-dimensional nuclear magnetic reso-
nance (NMR) spectroscopy eliminated the possibility for
“through-space” donor−acceptor interactions (see Supporting
Information). Therefore, the observed electron-transfer pro-
cesses occurred via through-bond pathways mediated by the
methylene linkers. Analysis of the electron-transfer rates, using
the Marcus−Levich−Jortner formalism, indicated that CS and
CR were nonadiabatic processes, consistent with the lack of
detectable charge-transfer bands in the ground-state absorption
spectra (see Supporting Information).
3
the pulse for DCM solution; triplet excited state, Aa*, recorded 2 ns
after the pulse for solutions containing 1-bromobutane;40 radical
cation, Aa•+, recorded 2 ns after the pulse for aqueous samples of Aa
suspended with a surfactant (10 mM odium dodecyl sulfate) and an
electron-acceptor cationic fluorescence quencher, 10 mM CuSO4.49,50
stabilization of the CT states brings them closer to the ground
(0)
An increase in the media polarity causes an increase in both
the charge-separation and the charge-recombination rate
constants, kCS and kCR, respectively (Figure 3a,b).58 A polar
medium stabilizes the zwitterionic charge-transfer states, and,
state, making ΔGCR less negative (Table 1). This polarity-
(0)
induced decrease in the CR driving force, −ΔGCR
,
accompanies an increase in the observed kCR for both dyads
(Figure 3b). Hence, while CS follows the trends of the Marcus
normal region, CR occurs in a regime consistent with the
Marcus inverted region.59−61
(0)
hence, increases the CS driving force, −ΔGCS (Table 1). As
(0)
expected, this polarity-induced negative shift in ΔGCS
increases the observed kCS (Figure 3a). Conversely, the CR
Rectification Occurs for Both Charge Separation and
kinetics exhibited the opposite trends. The polarity-induced
Recombination. To quantify the Aa-induced rectification of
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Figure 2. Transient absorption spectra of the donor−acceptor dyads dissolved in DCM (λex = 395 nm, 5 μJ per pulse).47,48 (a, b) Transient
absorption spectra of Aa-Py and Py-Aa. (c, d) Transient absorption kinetics of Aa-Py, Py-Aa, and Aa, showing (c) the decays of the Aa singlet-exited
state; and (d) the rise and decay of the charge-transfer states, monitored at the absorption peak of the Py•− transient.
the initial photoinduced charge transfer, we compare the CS
rates exhibited by the two donor−acceptor dyads. A similar
comparison between the rates of the subsequent CR reveals the
effect of the orientation of Aa on the deactivation of the charge-
transfer state. Indeed, comparing a faster forward photoinduced
charge transfer (e.g., CS) with a slower back charge
entrainment (e.g., CR) has been occasionally employed for
illustrating molecular rectification of donor−acceptor and
donor−bridge−acceptor conjugates. Such a definition of
rectification, however, is misleading because CS and CR
represent different electronic transitions. While CS involves a
transition from a locally excited to a charge-transfer state, CR
represents the deactivation of the charge-transfer state to the
ground state or to comparatively low-lying triplet excited
states.50
indicate no rectification, while R = 1 would mean that the rate
of the electron transfer along the molecular permanent dipole is
an order of magnitude faster than the electron transfer against
the dipole. A negative value of R would correspond to electron
transfer being faster against the dipole than along it.
For each of the solvent mixtures of DCM and MeCN, kCS for
Aa-Py was larger than kCS for Py-Aa (Figure 3a), while kCR for
Aa-Py was smaller than kCR for Py-Aa (Figure 3b). As a result,
RCS and RCR assumed values between 0.1 and 0.8 (Figure 3c).
In other words, charge separation occurs more rapidly in the
direction of the dipole moment (i.e., faster in Aa-Py than in Py-
Aa), just as expected. Moreover, the positive RCR values indicate
that charge recombination behaves similarly, which means that
charge recombination is impeded after accelerated charge
separation in Aa-Py. While this behavior is desirable for energy-
conversion applications, it contradicts the Marcus transition-
state theory prediction of a negative RCR.
Varying solvent polarity provides further insights. Increasing
polarity decreases RCS (Figure 3c), which suggests that the
increased media polarity enhances electrostatic screening and
reduces the Aa dipole effect on charge separation. These
observations support the notion that the molecular dipoles
affect the electron-transfer kinetics by stabilizing or destabiliz-
ing the charge-transfer states.18 In Aa-Py, charge separation
moves the electron in the same direction as the intrinsic Aa
dipole. The charge-transfer state formed in this manner is
oriented against the ground-state dipole (i.e., the positive pole
of the Aa dipole points toward the radical anion). This
We define the charge-transfer rectification, R, separately for
CS and for CR as logarithmic ratios between the rate constants,
k, of electron transfer along and against the Aa dipole:
(kCS(Aa‐Py))
RCS = lg
(kCS(Py‐Aa))
(1a)
⎛
⎜
⎝
⎞
⎟
⎠
k
k
CR(Py‐Aa)
RCR = lg
(Aa‐Py)
(1b)
CR
Because in CS the electron moves toward Py and in CR
toward Aa, kCS(Aa-Py) is in the numerator of eq 1a and kCR(Py-
Aa) is in the numerator of eq 1b. A value of R = 0 would
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stabilization and destabilization of the charge-transfer states.
Specifically, RCR has a positive value, which increases slightly
with the increase in the solvent polarity (Figure 3c). The
(0)
ΔGCR values of about −3 eV (Table 1) place the CR
processes in the Marcus inverted region.59−61 Dipole-induced
(0)
stabilization of the charge-transfer state makes ΔGCR less
negative for Aa-Py, thereby increasing its CR rates. It would,
therefore, be expected that Aa•+-Py•− should undergo faster CR
than Py•−-Aa•+, resulting in negative values of RCR (eq 1b).
Additionally, due to the media screening of the dipole, the
increase in solvent polarity should result in a less negative RCR
(i.e., in a positive ΔRCR/Δγ slope where RCR is negative for
nonpolar solvents and approaches zero as the media polarity
increases).
The positive slope, ΔRCR/Δγ, and the positive values of RCR
indicate simultaneous contributions of polarity-dependent and
polarity-independent factors to the CR kinetics. While the
dipole effects depend on the polarity of the media, intra-
molecular factors, such as the donor−acceptor electronic
coupling, should be practically solvent independent, as our
theoretical studies suggest. For example, donor−acceptor
coupling in Py•−-Aa•+, which is larger than that in Aa•+-Py•−,
opposes the expected dipole effect. If such a difference between
the couplings is large enough, it can surpass the dipole effect
and make RCR positive even in the least polar solvent.
Therefore, as the solvent polarity increases, the dipole effect
decreases, making RCR more positive.
Alternatively, one might attribute the positive RCR values to
the triplet manifolds in the charge recombination pathways.
Upon the decay of the radical ions in a relatively nonpolar
medium, we observed the formation of a pyrene triplet state,
3Py*, as revealed by transient-absorption bands at 422, 490, and
523 nm that became apparent upon the CR (Figure 2a,b).62,63
Increasing the media polarity, however, decreased the relative
amplitude of the 3Py* transient absorption bands. Furthermore,
even though no triplet formation was observed in MeCN (see
Figure S4 in the Supporting Information), the value of RCR for
MeCN is still positive (γ = 0.53, Figure 3c). Therefore, charge
recombination pathways involving triplet states cannot account
for the positive shifts in RCR.
Conformational Dynamics is Particularly Important
for Charge Separation. Although the NMR studies results
suggest that the Aa-Py structures do not fold (see Supporting
Information), conformational dynamics do still appear to play a
role in the donor−acceptor coupling and charge-transfer
kinetics. Evidence for the impact of molecular dynamics on
the charge-transfer processes comes from the data fits to the
transient-absorption kinetics. Monoexponential functions could
not provide acceptable data fits for the rise of the radical-ion
transients when DCM and MeCN were used as solvents.
Hence, we extracted the CS rate constants from multi-
exponential data fits.64 Such heterogeneous kinetics is
characteristic for electron-transfer systems, in which flexible
linkers provide the coupling between the donor and the
acceptor.20,64 The multiexponential character of the CS kinetics
suggests that the initial photoinduced electron transfer from
1Aa* to Py involves more than one conformer. While the
rotation around the σ-bonds between Aa and Py does not bring
the two aromatic moieties in contact with each other, the
conformational dynamics of the locally excited states in the
dyads is essential for the observed trends of CS and RCS.
On the other hand, the charge-recombination behavior was
quite different. The radical-ion decay traces exhibit mono-
Figure 3. Dependence of charge-transfer kinetics and rectification on
the solvent polarity, γ = n−2 − ε−1. (a, b) Polarity dependence of the
rate constants of charge separation (CS) and charge recombination
(CR) measured for the two dyads, Aa-Py and Py-Aa (Chart 1b,c), for
mixtures of acetonitrile (MeCN) and dichloromethane (DCM) and
for 1,1,2,2-tetrachloroethane (TCE). The mixtures of DCM and
MeCN correspond to 0%, 25%, 50%, 75%, and 100% (v:v) of MeCN
(Table 1). (c) Dependence of the CS and CR rectification (as defined
by eq 1a and 1b) on the polarity of the solvent media.
orientation stabilizes the Aa•+-Py•− state, causing an increase in
the charge separation rate for the relatively small values of
−ΔGCS(0) (Table 1). For Py-Aa, on the other hand, the ground-
state dipole is oriented in the same direction as the charge-
transfer state, raising its energy, and decreasing the rate of CS.
Conversely, for CR we observed behavior that does not
completely agree with the notion of the dipole-induced
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exponential character for all solvents. This finding indicates that
the conformational relaxation dynamics of the CT states is
significantly faster than the measured CR rates. The CR
processes, therefore, likely originate from thermally relaxed
conformer populations. Electrostatic interactions of the Aa
dipole with the CT states, which are oppositely oriented for the
two dyads, would hypothetically steer the relaxation of Aa•+-
Py•− and Py•−-Aa•+ to different conformers with different
donor−acceptor coupling. A population of thermally relaxed
Aa•+-Py•− conformers with donor−acceptor coupling stronger
than the coupling in the most abundant Py•−-Aa•+ conformers
can potentially account for the positive values observed for RCR.
If molecular dynamics is the underlying cause for these CS
and CR trends, an increase in the media viscosity should affect
the observed charge-transfer kinetics.65,66 To examine such
viscosity dependence, we employed 1,1,2,2-tetrachloroethane
(TCE) as a solvent medium. TCE is about 4 times more
viscous than any of the mixtures of DCM and MeCN we used
(Table 1). Also, TCE is a non-hydrogen bonding solvent with a
polarity and polarizability close to those of DCM.
Figure 4. Natural transition orbitals75 for (a) the S1 and (b) S2 states
at the ground state geometry of Aa with truncated aliphatic chains.
The S1 state is dominated by a HOMO−LUMO transition, while the
S2 state is primarily a HOMO−LUMO + 1 transition. (c) Electron
spin density of the radical cation, Aa•+; green, excess spin up (i.e.,
radical cation); and purple, excess spin down.
Upon photoexcitation, both dyads mediated charge separa-
tion when dissolved in the viscous TCE solvent. Only for Aa-
Py, however, was the buildup of the Aa•+ and Py•− radical ions
sufficient to reliably evaluate the charge-transfer kinetics. In
contrast to the multiexponential kinetics of radical-ion
formation observed for DCM and MeCN, the charge-
separation kinetics in TCE exhibit a monoexponential
character. Furthermore, the measured kCS for Aa-Py in TCE
is about one-third the value expected from the polarity
dependence of kCS for the mixtures of DCM and MeCN
(Figure 3a). This finding is consistent with the hypothesis that
the increased solvent viscosity suppresses molecular motions,
density from the piperidinyl ring toward the C-terminus amide
bond. Subsequent geometric relaxation on the S1 excited state
surface alters the conformation of the piperidinyl ring, while the
rest of the structure remains nearly unchanged (see Figure S10a
in the Supporting Information). The geometric relaxation does
not significantly alter the character of the LUMO orbital that
becomes singly occupied in the S1 state (see Supporting
Information). Interestingly, the relaxed S1 structure is nearly
identical to that of the Aa radical cation (see Figure S10b in the
Supporting Information). As noted earlier, these results suggest
that the excitation and subsequent charge transfer do not
significantly affect the geometry in the amide bond region.
The second-lowest singlet excited state is predicted at 294
nm in DCM. Although the experiments here did not involve
this state, it provides an interesting contrast to the S1 state.
Whereas the S1 excitation steers electron density toward the C-
terminus amide, the S2 vertical excitation involves a HOMO to
LUMO + 1 transition that shifts the electron density toward the
N-terminus amide bond (Figure 4b). Relaxation on the S2
excited state surface once again primarily alters the piperidinyl
ring conformation, but here it adopts a geometry with the
piperidinyl ring perpendicular to the plane of the rest of the
molecule. This conformation is typical of twisted intramolecular
charge-transfer (TICT) states,71 and it differs significantly from
that of the relaxed S1 state.
Performing the same the calculations in polarizable
continuum MeCN or in the gas phase (no continuum solvent
model) has minimal impact on the predicted structures and the
character of the excited states; e.g., the vertical excitation
wavelengths change by less than 10 nm. While continuum-
solvent models cannot capture the sometimes important local
solute−solvent interactions, the polarizable continuum model
should adequately capture the bulk electrostatic and polar-
ization interactions that likely dominate the effects in these
aprotic solvents. Moreover, the predicted insensitivity to the
solvent is consistent with the absence of any experimentally
observed solvent effect on E00 for Aa (Table 1). The predicted
structures, excited-state characters, and excitation energies are
also robust with respect to the choice of density functional
(B3LYP vs PBE0)72 and the basis set (6-31G(d)73,74 vs 6-
1
making it harder to access the Aa*-Py conformers that enable
efficient photoinduced charge separation via improved donor−
acceptor electronic coupling. It follows then that the rates of
conformational sampling of the locally excited dyads and of
charge separation are comparable in media composed of DCM
and MeCN.
Conversely, the viscosity of the media does not appear to
affect the observed CR for Aa-Py. The measured value of kCR
for TCE follows the polarity-dependence trend that we
recorded for the mixtures of DCM and MeCN (Figure 3b);
i.e., slowing down the molecular dynamics of the CT state of
the dyad does not have a major effect on the charge
recombination.
Regioselectivity Also Contributes to the Charge-
Transfer Rectification. Density functional theory (DFT)
and time-dependent DFT (TDDFT) calculations for Aa at the
B3LYP/6-311+G(d,p)67−69 level provide insight into the
regioselectivity of CT and how it impacts the charge-transfer
rectification. TDDFT calculations for the 25 lowest states in a
DCM or MeCN polarizable continuum solvent model predict
absorption spectra in good agreement with the experimental
one (see Figure S14 in the Supporting Information), providing
some validation for the model. The errors relative to the
experimental spectrum lie well within the few tenths of an eV
accuracy (which maps to a few tens of nanometers here)
typically expected for valence excited states in TDDFT.70
The lowest singlet excited state S1 is predicted to occur at
353 nm in DCM. This excitation, which corresponds to a
HOMO−LUMO transition, is the state accessed experimentally
with 400 nm radiation. As shown in the natural transition
orbitals (Figure 4a), the vertical excitation shifts electron
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Article
311+G(d,p)), providing further confidence in the predictions
(see Supporting Information for details).
techniques used; electron-transfer analysis; and details for the
computational work. This material is available free of charge via
the Internet at http://pubs.acs.org.
These computational results suggest that regioselectivity
contributes to the observed charge rectification. Photo-
excitation to the S1 state shifts electron density toward the C-
terminus amide in the Aa donor, which improves the electronic
coupling to the Py acceptor when the Py moiety is linked to the
C-terminus (as in Aa-Py). This leads to faster charge separation
rates for Aa-Py than for Py-Aa.
Conversely, these calculations predict that the Aa structures
and excited state character depend minimally on the solvent
polarity, which contrasts the pronounced solvent effect on RCS
observed experimentally (Figure 3c). Thus, the charge
separation rectification depends more strongly on the dipole
effect than on the regioselective electronic coupling that is
dictated by the positioning of the linker relative to the
piperidinyl ring.
Indeed, previous studies have shown that charge-transfer
regioselectivity associated with the position of donor−acceptor
linkers is more important for charge recombination than for the
initial photoinduced charge separation.76 Charge recombination
will involve the singly occupied molecular orbital (SOMO) on
the Aa radical cation, which is very similar to the Aa ground
state HOMO and which extends over the N-terminus amide
(Figure 4c). Hence, charge-recombination kinetics should be
faster for Py-Aa than for Aa-Py, as was observed experimentally
(Figures 2 and 3), leading to a positive value of RCR. However,
the lack of solvent dependence for the Aa radical cation SOMO
cannot account for the positive ΔRCR/Δγ slope observed in the
experiments. Hence, while regioselectivity can help explain the
positive sign of RCR, the polarity dependence of RCR likely arises
from dipole effects on charge recombination.
In summary, the observed rectification of charge separation
can largely be attributed to the electric dipole effect, which
depends on media polarity. Regioselectivity plays a smaller role
in charge separation. Conversely, regioselectivity appears to be
more important for the rectification of charge recombination,
though the electric dipole contributions also play a role.
AUTHOR INFORMATION
■
Corresponding Authors
gregory.beran@ucr.edu
vullev@ucr.edu
Present Addresses
⊥PrimusLabs, Santa Maria, CA 93455, United States.
#Department of Cell Biology, Harvard Medical School, Boston,
MA 02115, United States, and Program in Cellular and
Molecular Biology, Boston Children’s Hospital, Boston, MA
02115, United States.
∇GlaxoSmithKline, MDR-Boston, Waltham, MA 02451, United
States.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding for this work was from the National Science
Foundation (CBET 0935995, CBET 0923408, and DBI
0731660, as well as GRFP 2011081805 for V.N. and IGERT
DGE 0903667 for J.M.L), and from the City of Riverside, CA,
as well as from NSF grants CHE-1112568 and CHE-1362465,
and supercomputer time from XSEDE (TG-CHE110064) for
the theoretical studies (G.J.O.B. and J.D.H.), is gratefully
acknowledged.
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