Electronically regulated thermally and light-gated electron transfer from anions to naphthalenediimides

Article abstract of DOI:10.1021/ja2055726

Anion-induced electron transfer (ET) to π-electron-deficient naphthalenediimides (NDIs) can be channeled through two distinct pathways by adjusting the Lewis basicity of the anion and the π-acidity of the NDI: (1) When the anion and NDI are a strong elect

Full text of DOI:10.1021/ja2055726

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pubs.acs.org/JACS
Electronically Regulated Thermally and Light-Gated Electron Transfer
from Anions to Naphthalenediimides
Samit Guha, Flynt S. Goodson, Sovan Roy, Lucas J. Corson, Curtis A. Gravenmier, and Sourav Saha*
Department of Chemistry and Biochemistry and Integrative NanoScience Institute, Florida State University,
95 Chieftan Way, Tallahassee, Florida 32306-4390, United States
S Supporting Information
b
Scheme 1. (a, b) Illustrations of (a) Thermal Anion f NDI
ABSTRACT: Anion-induced electron transfer (ET) to π-
electron-deficient naphthalenediimides (NDIs) can be chan-
neled through two distinct pathways by adjusting the Lewis
basicity of the anion and the π-acidity of the NDI: (1) When
the anion and NDI are a strong electron donor and acceptor,
respectively, positioning the HOMO of the anion above the
LUMO of the NDI, a thermal anion f NDI ET pathway is
turned ON. (2) When the HOMO of a weakly Lewis basic
anion falls below the LUMO of an NDI but still lies above its
HOMO, the thermal ET is turned OFF, but light can activate
ET Interactions from Strongly Lewis Basic Anions to Strongly
π-Acidic NDIs and (b) Anion f 1*NDI PET from Less Basic
Anions to NDIs That Show Weak or No Thermal ET; (c, d)
Energy Level Diagrams of (c) Thermal ET and (d) PET Events
1
an unprecedented anion f *NDI photoinduced ET path-
way from the anion's HOMO to the photogenerated 1*NDI's
SOMOÀ1. Both pathways generate NDI•À radical anions.
lthough anion-induced electron transfer (ET) is found in
A
nature (e.g., ascorbate reduces cytochromes b and c),1 anion-
mediated charge transfer (CT)2 and, especially, formal ET3 to
neutral π-acceptors analogous to π-donor/acceptor interactions4
are extremely rare. To display anion/π-acceptor5 and anion-induced
CT2 and ET3 interactions, the π-acceptors must possess strong
electron-accepting abilities, low LUMO (π*) levels, and large
positive quadrupole moments.
1,4,5,8-Naphthalenediimides (NDIs) are π-electron-deficient
colorless compounds (Scheme 1a) with easily tunable electronic
properties.6 Although π-donor/acceptor CT interactions of NDIs are
well-known,4 anion f NDI CT and ET interactions have remained
unexplored until recently. In 2010, Matile and co-workers2b reported
extremely weak CT interactions between polarizable anions
(BrÀ and IÀ) and π-acidic NDIs, while we3 discovered a unique
formal ET from strongly Lewis basic FÀ to an NDI that generated
an NDI•À radical anion. Herein we demonstrate for the first time
that by adjustment of the π-acidity (LUMO levels) of NDIs with
respect to the Lewis basicity (HOMO levels) of anions, anion f
NDI ET processes can be channeled through two distinct pathways
(Scheme 1), both of which generate NDI•À radical anions: (1)
When the HOMO of an anion is located above the LUMO of an
NDI, a thermal anion f NDI ET takes place from the anion's
HOMO to the NDI's LUMO (Scheme 1c). (2) When the HOMO
of an anion lies below the LUMO of an NDI receptor but above its
HOMO, the thermal anion f NDI ET is turned OFF, but light can
activate an unprecedented anion f 1*NDI photoinduced ET (PET)
pathway from the anion’s HOMO to the photogenerated 1*NDI's
SOMOÀ1 (Scheme 1d). Weak CT interactions take place
between strongly π-acidic NDIs and weakly Lewis basic but highly
polarizable anions. We also put forth two other possible
mechanisms for chromogenic anion/NDI interactions—nucleophi-
lic attack of the anion on the NDI to form covalent intermediates5d,7
and CH anion interactions8 with the NDI’s core protons—and
3 3 3
demonstrate how the experimental results not only rule out these
scenarios but also confirm the ET and PET phenomena.
While π-acidification of NDIs using electron-withdrawing
core substituents has been achieved,6b little is known about the
effects of imide N-substituents on the electronic and anion-recogni-
tion properties. Herein we show how electron-rich and -deficient
N-substituents impact the π-acidity of NDIs and regulate their
anion-recognition properties with remarkable precision. To
study the relationships between the π-acidity of NDIs and
the anion/NDI interactions, we installed electron-deficient (in
1À4) or electron-rich (in 6 and 7) groups on the imide N-centers
and compared the resulting NDIs with reference NDI 5
(Chart 1). These NDIs were prepared by bisimidization of
Received: June 16, 2011
Published: August 30, 2011
r
2011 American Chemical Society
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Table 1. NDI Redox Potentials (vs Ag/AgCl in MeCN), HOMO and LUMO Energies, and Thermal and Photoinduced Anion f
NDI ET in ODCB and MeCN
Chart 1. Molecular Structures of NDIs 1À7
naphthalene-1,4,5,8-tetracarboxydianhydride with the appro-
priate amines (Scheme S1 in the Supporting Information).6
NDIs 1À7 display nearly identical UV absorption spectra (λ =
340À390 nm) (Figures S1 and S2), indicating that the N-substi-
tuents do not affect the HOMOÀLUMO energy gaps. These
NDIs undergotwo reversible one-electron reductions (Figure S3),
forming NDI•À and NDI2À species.3,6,9 The electron-deficient N-
aryl substituents enhance the π-acidity of NDIs 1À4, as evidenced
by their smaller reduction potentials and lower LUMO levels
relative to NDI 5. The electron-rich N-substituents suppress the
π-acidity of NDIs 6 and 7, as they possess more negative reduction
potentials and higher LUMO levels. Electrochemical reduction of
NDIs to NDI•À radical anions produces highly featured absorp-
tion spectra with prominent peaks at ca. 475 (λmax), 605, 700, and
800 nm and establishes a clear isosbestic point at ca. 390 nm
(Figure 1a and Figure S4).3,9 The HOMO and LUMO energies of
NDIs 1À7 and their anion-recognition phenomena through ther-
Figure 1. (a, b) UV/Vis spectroscopic changes of NDI 1 as a result of
(a) electrochemical reduction (applied voltage Eap = À500 mV vs
Ag/AgCl, 0.1 M TBAPF6/ODCB) and (b) titration with FÀ (0À
10 equiv), demonstrating the formation of 1•À in both cases. (cÀf)
Anion-generated UV/Vis spectra of NDI•À radical anions of NDIs 1 (violet),
2 (indigo), 3 (cyan), 4 (green), 5 (gray), 6 (orange), and 7 (red) (each
30 μM in ODCB) in the presence of (c) FÀ (10 equiv), (d) AcOÀ (50
equiv), (e) H2PO4À (50 equiv), and (f) ClÀ (100 equiv). The spectra
show that while (c) FÀ and (d) AcOÀ generate the NDI•À radical anions
of 1À7, (e) H2PO4À does so only for 1 and 2 and (f) ClÀonly for NDI 1.
mal and photoinduced ET pathways are summarized in Table 1.
To understand the effects of the NDI’s π-acidity and the anion’s
electron-donating ability (as reflected by the anion’s Lewis basicity
and oxidation potential10) on the anion f NDI ET phenomena, we
surveyed the interactions between NDIs 1À7 and tetra-n-butyl-
ammonium (TBA+) salts of FÀ, AcOÀ, H2PO4À, ClÀ, BrÀ, IÀ, and
PF6À. To prevent the solvent from acting as an electron donor and to
determine the effect of the solvent on the anion/NDI interactions,
we conducted our studies in o-dichlorobenzene (ODCB) and
MeCN. The π-acidity of NDIs 1À7 follows the same trend in these
two solvents, but most NDIs have slightly stronger π-acidity in
MeCN, as shown by cyclic voltammetry (Figure S3). While the
absorption features of electrochemically generated NDI•À are
essentially the same in the two solvents, the λmax (ca. 475 nm)
and isosbestic point (ca. 390 nm) appear at slightly shorter
wavelengths (by ca. 8 nm) in MeCN than in ODCB (Figure S5).
UV/Vis titrations of NDIs 1À7 in ODCB with Lewis basic FÀ
display prominent NDI•À spectra (Figure 1c and Figure S4). As the
π-acidity of the NDI gradually diminishes in going from 1 to 7,
the extent of NDI•À formation decreases (Figure 1c). This trend
suggests that the thermal FÀ fNDI ET diminishes as the energy gap
between the anion’s HOMO and the NDI’s LUMO (ΔGE°T) gradu-
ally decreases (Scheme 1c,d) with the fading π-acidity. On the other
hand, as the Lewis basicity of the anions decreases (FÀ > AcOÀ >
H2PO4À > ClÀ),10 their electron-donating abilities and HOMO
levels diminish,2c weakening and eventually turning OFF the
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thermal ET (Scheme 1). As a result, in ODCB, less basic AcOÀ
generates weaker NDI•À signals from NDIs 1À7 (Figure 1d) than
those produced by FÀ, while H2PO4À generates very weak NDI•À
signals only with NDIs 1À4 (Figure 1e) and ClÀ triggers
extremely weak thermal ET only to NDI 1 (Figure 1f). The
extent of NDI•À formation via anion-induced thermal ET can
be quantified by comparing the molar absorptivities (εmax at
ca. 475 nm) of the anion-generated and electrochemically
generated NDI•À (Figure 1 and Figure S4). Anion/NDI inter-
actions between stronger electron donors and acceptors produce
more NDI•À radical anions and vice versa.
The anion-generated NDI•À absorption spectra and isosbestic
points are essentially identical to those of electrochemically
generated NDI•À radical anions in the absence of any electron-
donating anions (Figure 1a,b and Figure S4). These similarities
strongly suggest that anion f NDI ET events are indeed res-
ponsible for these spectroscopic changes and rule out the pos-
sibility of nucleophilic attack of anions on NDIs to form covalent
intermediates,5d,7 a scenario that would have produced UV/Vis
spectra and isosbestic points different from those displayed by
the electrochemically generated NDI•À radical anions.
Figure 2. (aÀc) UV/Vis spectra of NDI 1 (30 μM in MeCN, black traces)
and of 1 with large excesses (g50 equiv) of TBA+ salts of (a) AcOÀ, (b)
H2PO4À, and(c) ClÀ in the absence of light (red traces), showing extremely
weak NDI•À absorptions. (d) UV/Vis spectra of NDI 6 (30 μM in ODCB,
black trace) and of 6 with excess AcOÀ in the absence of light (red trace),
showing negligible NDI•À formation before irradiation. Irradiation of these
solutions with a W-lamp (aÀd) significantly enhanced the NDI•À signals
(blue traces), demonstrating the anion f 1*NDI PET.
Treatment of the anion-induced absorption changes at
475 nm (λmax of NDI•À) as a function of the NDI and anion
concentrations using the BenesiÀHildebrand method11 (Figure S6)
shows a good agreement for 1:1 interactions between NDI
1 and FÀ, AcOÀ, and H2PO4À with gradually decreasing affini-
ties (Ka = 1225, 70, and 40 MÀ1, respectively, in ODCB at 298 K).
Isothermal titration calorimetry (ITC) analyses (Figure S7) also
show 1:1 interactions between FÀ and NDIs 1À5 with gradually
extremely weak or absent. For instance, AcOÀ
f
1*NDI 6
1
1
(Figure 2d), H2PO4À f *NDI 1À5, and ClÀ f *NDI 1
(Figure S10) PET signals are observed in ODCB. Interestingly,
when a facile thermal anion f NDI ET process generates strong
NDI•À signals by directly populating the NDI’sLUMOintheground
state (Scheme 1c,d), irradiation of these samples does not improve
the NDI•À signal intensity, indicating that the anion f 1*NDI PET is
turned OFF. On the other hand, when the thermal ET interactions
are extremely weak or absent, the light-induced π f π* transition
in the NDI opens the door for a more facile PET from the anion’s
HOMO to the 1*NDI’s SOMOÀ1 level (Scheme 1d). Less basic
BrÀ, IÀ, andPF6À do not display any ET or PET to any of the NDIs
in either solvent (Figure S11). However, a large excess of highly
polarizable IÀproduces a broad, weak CT band (λCT = 505 nm)
with a concentrated solution (mM) of NDI 1 in ODCB but does
not produce any NDI•À (Figure S11a). Thus, by adjustment of the
NDI’s LUMO with respect to the anion’s HOMO, the entire
spectrum of ET, PET, and CT phenomena can be accessed. In
control experiments, irradiation of these NDIs in the absence of
anions did not produce any NDI•À radical anion (Figures S1 and S2),
ruling out the possibilities of intramolecular or solvent-mediated PET.
1H NMR spectroscopy (CD3CN) confirms that while FÀ,
AcOÀ, and H2PO4À generate paramagnetic 1•À through ther-
mal ET, causing the NDI’s core HA signal to disappear
(Figure 3a and Figure S12), ClÀ does not do so in the absence
of light (Figure 3b). However, irradiation of the 1/ClÀ sample
quickly generates paramagnetic 1•À, as its HA signal disappears.
In contrast, less basic BrÀ never produces any NDI•À (Figure
S12). The fact that the NDI’s HA signal does not split or shift
from its original position before disappearing essentially rules
out the formation of a nonsymmetric covalent NDIÀanion
decreasing Ka values of 1230, 1080, 920, 710, and 530 MÀ1
,
respectively (298 K, ODCB). Thus, Ka is greater for interac-
tions between stronger Lewis basic anions and more π-acidic
NDIs. Electrospray ionization mass spectrometry (ESI-MS)
revealed several 1:1 NDI anion complexes (see below).
3
In a more polar solvent, MeCN, strongly basic FÀ generates
NDI•À absorption spectra of NDIs 1À7 via thermal ET (Figure S8),
indicating that the HOMO of FÀ is still located above the LUMOs of
all these NDIs. However, less basic AcOÀ and H2PO4 show
À
significantly weaker ET interactions with only NDIs 1 and 2
(Figure 2a,b) that produce much less NDI•À than in ODCB
(Figure 1d,e). All ClÀ f NDI thermal ET processes are essentially
turned OFF in MeCN, including that with the most π-acidic NDI,
1 (Figure 2c). The reducing abilities of less basic AcOÀ, H2PO4
,
À
and ClÀ are further depleted through CH anion interactions
3 3 3
with MeCN,8 which stabilize the HOMOs of these anions below
the LUMOs of the weakly π-acidic NDIs, shutting down the
thermal ET pathway (Scheme 1d).
We next explored whether a PET pathway12 from the HOMO
of an anion to the SOMOÀ1ofthe 1*NDI excited state (Scheme 1b,
d) could turn ON the NDI•À signal when the thermal anion f NDI
ET process is turned OFF. To investigate this hypothesis, we used a
W-lamp to irradiate NDI solutions containing anions that otherwise
showed negligible or no thermal ET interactions in the dark and
periodically recorded the UV/Vis spectra. Irradiation of a solution of
1 in MeCN in the presence of AcOÀ, H2PO4À, or ClÀ, which
showed negligible or no thermal ET, immediately turned ON the
anion f 1*NDI PET, generating much stronger NDI•À absorption
spectra that reached saturation within 10 min of irradiation
(Figure 2aÀc). Similar PET phenomena were displayed by NDIs
2À4 with AcOÀ and H2PO4À in MeCN (Figure S9).
intermediate via nucleophilic attack by anions5d,7 or a CH
3 3 3
anion interaction with the HA protons.8 The anion-generated
NDI•À radical anions are stable under dark, inert conditions.
Oxidation of anion-generated NDI•À with NOBF4 regenerates
the neutral NDIs, as the characteristic NMR signals return to
their full glory (Figure 3a,b and Figure S12). The oxidative
Photoinduced enhancement of anion-mediated NDI•À signals is
also observed in ODCB when the thermal ET interactions are
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strongly basic anion (FÀ), the more strongly π-acidic NDIs
become promiscuous to different anions. When the thermal
anion f NDI ET process is turned OFF because of energy
1
mismatch, light can turn ON the anion f *NDI PET pathway,
generating the NDI•À radical anions. Light- and electronically gated
anion-induced NDI•À formation could be exploited in anion
sensing, artificial photosynthesis, catalysis, and molecular electronics.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental section and ad-
Figure 3. 1H NMR spectra (CD3CN, 298 K) of NDI 1 [trace i in (a)
and (b)]; (a) 1 with 1 equiv of TBAF (trace ii) showing 1•À formation
via thermal ET, as indicated by the disappearance of the NDI’s core HA
signal; and (b) 1 with excess TBACl before (trace ii) and after (trace iii)
W-lamp irradiation, showing no 1•À formation through thermal ET but
1•À formation through PET, respectively. In both cases, NOBF4 oxidation
of 1•À regenerated neutral NDI 1 [trace iii in (a) and trace iv in (b)].
b
ditional experimental and computational results. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
saha@chem.fsu.edu
regeneration of the NDI further indicates formation of the
NDI•À in the first place and provides additional evidence against
NDIÀanion covalent bond formation.
’ ACKNOWLEDGMENT
This work was supported by startup funds from the Florida
State University Research Foundation and a FYAP Award.
19F NMR titration of NDI 2 with TBAF (Figure S13) shows a
broadening and gradual disappearance of the NDI’s F signals
(À118.80 and À118.96 ppm), indicating the formation of paramag-
netic 2•À. Conversely, titration of TBAF with NDI 2 shows a
broadening and gradual upfield shift of the FÀ signal (À117.2 ppm),
indicating possible shielding of FÀ by NDI 2 as a result of anion/
NDI complex formation. The TBAF signal disappears comple-
tely with 1 equiv of NDI, indicating F• formation via the thermal
FÀ fNDI ET process. EPR spectroscopy confirmed the formation
of paramagnetic NDI•À using FÀ by displaying excellent agree-
ment between the FÀ-generated and simulated NDI•À spectra
(Figure S14).3
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ESI-MS (Figure S15) reveals the complexes 1 FÀ (m/z1030.9),
À3
1 AcOÀ (1070.7), 1 H2PO4À (1108.7), 2 F (510.4), 2 AcOÀ
3
À3
3
3
(549.1), 2 H2PO4 (587.0), and 5 FÀ (437.1) as well as the
3
3
corresponding NDI•À radical anions. These data could be rationa-
lized by one of the following scenarios: (1) noncovalent anion/
π-acceptor interactions2,5 leading to anion f NDI ET events3
that generate NDI•À radical anions and oxidized anions (X•) or
(2) NDIÀanion covalent bond formation.5d,7 UV/Vis, NMR, and
EPR spectroscopies not only confirm the NDI•À formation via
anion f NDI ET events but also rule out NDIÀanion covalent
bond formation, as the anion-induced and electrochemically gen-
erated (anion-free) NDI•À radical anions display essentially iden-
tical spectroscopic signatures. The existence of NDI•À/X• radical
pairs (net charge = À1) could be surmised from the ESI-MS data.
However, in solutions X• species do not cause any discernible
perturbations of the spectroscopic signatures of the anion-generated
NDI•À radical anions, indicating the noncovalent nature of these
interactions before, during, and after the ET events. NOBF4 oxida-
tions of NDI•À radical anions regenerate neutral NDIs, but attempts
to capture the X• species with alkenes were inconclusive. It is plausi-
ble that the X• species may act as sacrificial agents13 that eventually
degrade by reacting with solvents, counterions, or themselves, thus
preventing the back-ET from NDI•À radical anions. B3LYP/6-31
+G** calculations showed that anions preferentially interact with the
electron-deficient imide rings of the NDIs (Figure S16).
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reducing ability (i.e., a more negative oxidation potential): FÀ > AcOÀ
>
H2PO4À > ClÀ > BrÀ > IÀ. Anion’s Lewis basicity is solvent-dependent.
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In conclusion, our studies clearly demonstrate how the inter-
play between the π-acidity of the NDI and the Lewis basicity
of the anion affects the anion/NDI CT and ET interactions. While
the more weakly π-acidic NDIs display better selectivity for a
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