Persistent Room-Temperature Radicals from Anionic Naphthalimides: Spin Pairing and Supramolecular Chemistry

Article abstract of DOI:10.1002/chem.201902882

N-Substituted naphthalimides (NNIs) have been shown to exhibit highly efficient and persistent room-temperature phosphorescence from an NNI-localized triplet excited state, when the N-substitution is a sufficiently strong donor and mediates an intramolecu

Full text of DOI:10.1002/chem.201902882

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Accepted Article
Title: Persistent Room-Temperature Radicals from Anionic
Naphthalimides: Spin Pairing and Supramolecular Chemistry
Authors: Guoqing Zhang, Wenhuan Huang, and Biao Chen
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Persistent Room-Temperature Radicals from Anionic
Naphthalimides: Spin Pairing and Supramolecular Chemistry
Wenhuan Huang,[a] Biao Chen*[a] and Guoqing Zhang*[a]
Abstract: N-Substituted naphthalimides (NNIs) have been shown to
exhibit
highly
efficient
and
persistent
room-temperature
phosphorescence from an NNI-localized triplet excited state, when
the N-substitution is a sufficiently strong donor and mediates an
intramolecular charge-transfer (ICT) state upon photo-excitation.
Here we show that when the electron-donating ability of the N-
substitution is further increased in the presence of a carbanion or
phenoxide, spontaneous electron transfer (ET) occurs and results in
radical anions, verified with electron-paramagnetic resonance (EPR)
spectroscopy. However, the EPR-active anion is surprisingly
persistent and impervious to nucleophilic and radical reactions under
anionic conditions. The stability is thought to originate from an
intramolecular spin pairing between the N-donor and the NI acceptor
post ET, which is demonstrated in supramolecular chemistry.
-*
ICT
ET
h
h
4
1
2
3
Localized Singlet
Excited State
Localized Triplet
Excited State
Spin Pairing
Interactions
Fluorescence
Persistent RTP
Persistent Radical
Scheme 1. Chemical structures of NNI derivatives 1-4 and their corresponding
physical properties depending on the category of donor strength.
N-Substituted naphthalimides (NNIs) are a class of electron-
deficient aromatic compounds which are extensively exploited
as fluorescent sensors (1, Scheme 1)[1] Recently, we reported a
strategy to harvest the triplet excited state of NNIs by introducing
to the fact that substantial spin pairing interactions between the
anion donor and NI ring may reduce the energy of the open-shell
system. Although homodimeric spin-pairing interactions have
been routinely used in supramolecular chemistry by Stoddart et
al., [18] such interactions among heterodimers are rarely explored
until recently by Li et al.[19] Here we show that a polymer gel with
an anionic polymeric donor and an NI polymeric acceptor,
crosslinked by heterodimeric spin pairing interactions, can be
obtained.
a
mediating intramolecular charge-transfer state (e.g., 2,
replacing N-phenyl with N-methoxyphenyl) to bridge the large
energy gap between 1-* and 3-* of the NNI moiety.[2] As a
result, persistent room-temperature phosphorescence (RTP)
was obtained and used in low-background imaging. In light of
the previous study, one might wonder what happens when the
electron-donating ability is further increased, as in the case of
carbanion (3) or phenoxide (4) substituted NNI. A plausible
process is electron transfer (ET) when the highest occupied
molecular orbital (HOMO) of the anion is comparable to or
higher than the lowest unoccupied molecular orbital (LUMO) of
the NI ring.[3] It is well known that ET between a donor and
acceptor causes the formation of a pair of ionic radicals,[4] which
tend to be unstable and perish overtime.[5] If made persistent,
however, these organic radicals can have important applications
such as catalysis, sensing, quantum manipulation, light-emitting
and molecular recognition.[6-16] However, very limited classes of
organic radicals are known to exhibit stability against air,[17]
especially in the solution state. Here, we report the discovery
that N-substituted anionic NIs, such as 3 and 4, show typical
radical EPR signals, but are stable against air (up to three years)
and chemical reactions as long as their anionic form is
maintained, i.e., the organic/aqueous solution or solid is
deprotonating enough for the carbanion (3) or phenoxide (4) to
exist. Experimental methods including cyclic voltammetry (CV),
UV-Vis absorption, luminescence, EPR, and NMR spectroscopy,
are used to seek the origin of their persistence. All results point
The synthesis of NNIs is rather straightforward via a one-step
reaction between an amine and 1,8-naphthalic anhydride.[2] The
most striking visual difference for 3 is the indigo color in the solid
state and purple in solutions while 1, 2 and the neutral form of 3
(3N) are colorless. While the absorption spectra for 1, 2, and 3N
in DMF are almost identical with a major peak at 335 nm, the
main absorption band for 3 is dramatically red-shifted to 588 nm,
using KOtBu as the deprotonating base due to its reasonable
solubility in organic solvent (Figure 1a and Table 1, the study of
4 and derivatives was conducted in aqueous solution and will be
presented separately) and the vibrational progressions (E =
1212 cm-1) of 3 indicate a localized -* transition instead of an
ICT state which is usually structureless. The control anion,
-
deprotonated acetophenone (AP ), however, was found to only
exhibit a weak, broad shoulder absorption at ~450 nm in the
visible region (Figure S1). Interestingly, the absorption of 3 is
also much red-shifted compared to that of an N-alky-substituted
NI radicals (~420 nm) according to a previous study,[20]
-
indicating the AP participation. Upon acidification with
CF3COOH, however, the absorption spectrum of 3 reverts back
to that of 3N; repeated deprotonation can restore the absorption
in the visible region again (Figure S2), indicating full reversibility
with pH control. Figure 1b shows the cyclic voltammogram of 1,
2, 3N and 3, where the first three neutral NNIs exhibit reversible
reduction at -1.164, -1.171, and -1.163 eV due to the formation
of an NNI radical anion; for 3 however, no cathodic or anodic
wave could be detected throughout the entire range workable
range (Figure S3). The calculated HOMO (~-6.9 eV) and LUMO
(~-3.5 eV) energies for 1, 2 and 3N are presented in Table S1,
while the values for 3 could not be determined.[21] However,
[a]
Mr. Wenhuan Huang, Dr. Biao Chen, and Prof. Guoqing Zhang
Hefei National Laboratory for Physical Sciences at the Microscale,
University of Science and Technology of China, Hefei, China
E-mail: biaochen@ustc.edu.cn; gzhang@ustc.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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-
1
since the measured oxidation potential for AP is -1.340 eV,
2c is the comparison of H-NMR spectra of 3 and 3N, it can be
clearly seen that in the presence of a deprotonating base (two
NMR peaks at 1.28 and 2.75 ppm corresponding to the t-butanol
–CH3 and –OH protons, respectively), the signals are
which translates to a HOMO energy level of -3.35 eV, it can thus
be inferred that ET from the AP unit to NI (LUMO ~ -3.5 eV) is a
-
thermodynamically favorable process, according to the Marcus
theory on ET.[22] The lack of reduction peak is understandable
given its anionic nature; the fact that a carbanion is resistant to
oxidation is quite surprising. Indeed, it was found that typical
radical or nucleophilic reactions by an -carbanion could not be
achieved by 3, e.g., for radical polymerization in the presence of
various monomers, radical coupling reactions, TEMPO
quenching, or Claisen condensitions (Table S2), possibly due to
the carbanion’s inability to give away electrons when spatially
confined to the NI acceptor.
indistinguishable
from
the
baseline,
consistent
with
paramagnetic molecules. Again, upon acidification with
CF3COOH, H-NMR spectrum characteristic of 3N re-appeared
1
(Figure S4). To eliminate possible ET from the base, we also
measured the mixture of 1 and KtOtBu under the same condition
and noted no EPR signal (Figure S5); UV-Vis titration may also
confirm that adding excess base does not cause spectroscopic
change (Figure S6, however, protonation of 3 with acetic acid
does cause a complete loss of EPR signal, and repeated
deprotonation by adding base can restore the paramagnetic
behaviors). The EPR signal of 3 was essentially constant
throughout a 90-day period in the presence of excess KOtBu
even at 10-3~10-4 M concentrations (Figure S7). In the solid
state, however, the sample may be indefinitely stored in a
capped vial since the EPR spectrum remains unchanged for
three years (Figure S8). It has to be noted that compared to well-
known organic radicals such as TEMPO, the signal of 3 is rather
weak since it usually takes sub-mM to obtain decent intensity.
Figure 2d shows the results from temperature variable EPR in
solid-state DMF (m.p. = 212 K), which do not exhibit any fine
structures. Typically, for an intermolecular spin-pairing process,
lower temperatures should favor diamagnetic singlet
configurations when donor-acceptor distance is reduced. In this
case, however, the distance should be constant and this rule
may not apply, and the signal increase may be related to
reduced quenching at low temperatures. Based on the above
spectroscopic results, it is reasonable to believe that the ground
state of 3 is a mix of a singlet state and a triplet one, which
confer stability and magnetism, respectively (Figure 2e).
Following the ET process, the two electrons are still spatially
confined to the NNI molecule and are expected to form strong
intramolecular spin-pairing interactions which can mitigate the
unpaired character, but also reduce their energy.
Figure 1. a) Normalized UV-Vis absorption spectra of NNIs 1, 2, 3 and 3N (5
10-4 M), the neutral form of 3; b) cyclic voltammograms of these NNIs,
(conditions: 5 × 10-5 M in DMF; reference electrode, Ag/AgCl; working and
auxiliary electrodes, Pt with Bu4NPF6 ;0.1 M; 298 K; scan rate: 200 mV/s, for
1, 2, 3 and 3N), the CV patterns showing the reversible formation of one single
ionic species that correspond to the reduction of the NNIs; for 3, no reduction
or oxidation could be observed.
Table 1. Luminescence properties of 1, 2 ,3N and 3 in DMF at room
temperature in air.
[a]
compounds
λabs (nm)
F[b] (nm)

[c] (ns)

[d] (%)
1
2
335
335
335
585
434
435
435
682
1.52
1.34
2.99
7.42
23.3
8.5
8.4
0.1
3N
3
[a]Maximum absorption wavelength in DMF; [b]Fluorescence emission
maximum from steady-state spectra (λex = 365 nm for 1, 2, 3N; λex = 548 nm
for 3); [c]Fluorescence lifetime fitted from a single-exponential decay (λex = 374
nm for 1, 2, 3N; λex = 455 nm for 3; nanoLED); [d] Absolute quantum yield.
In DMF solution, 1 strongly fluoresces in the blue region (em
=
434 nm) while 2 is more weakly fluorescent (em = 435 nm) due
to the quenching of a dark ICT state (Figure 2a and Table 1).[2]
The luminescence spectrum of 3 fluoresces in the red region
with an emission maximum at 682 nm and a calculated intrinsic
lifetime (/) of 7.42 s at room temperature, indicative of a
“luminescence” state that has both singlet and triplet
characteristics.[23] This is in huge contrast to 3N, which has
essentially the same fluorescence profile as 2. The DMF
solutions were then subjected to EPR measurement (Figure 2b);
only 3 has signals observable at room temperature even in air.
The splitting patterns indicate that the unpaired electrons are in
close vicinity of one nitrogen atom and two hydrogen atoms. We
suspect that the –CH2 group is in reasonable spatial proximity to
the imide N atom to co-influence the unpaired electrons. Figure
Figure 2. a) Steady-state photoluminescence emission spectra of NNIs 1, 2,
3N, and 3 in DMF at room temperature with inset showing the single-
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exponential lifetime decay of 3; b) liquid-phase EPR spectrum of 3 in DMF
solution (8.910-3 M) at 273 K; c) 1H-NMR spectra of 3 (above) and 3N (below)
showing that the anionic form is completely NMR-silent; d) solid-phase EPR
spectra of 3 in DMF solution (8.910-3 M) at 212 K, 180 K, and 140 K; e)
schematic illustration of spin-pairing as a result of intramolecular ET from the
To demonstrate that the hetero spin-pairing interactions
among the AP and the NI ring can be harvested as a driving
-
force for supramolecular chemistry,[19] we designed a polymer
system shown in Figure 4a, where two linear acrylate polymers
bear acetophenone and NNI molecules as the side chains,
respectively. When an individual polymer was dissolved in DMF
in the presence of KOtBu, no conspicuous color or viscosity
change could be noted. The electrochemical oxidation or
reduction of an individual polymer or mixed one did not yield any
observable change as well. Again, the mixed polymers solution
was also colorless without adding KOtBu. However, a dark gel
immediately formed when the two polymers were mixed in the
presence of a deprotonating base (e.g., KOtBu, NaH, LDA, etc.,
Figure 4b) and strong EPR signal could again be recorded. The
variation in EPR pattern between the gel and 3 in DMF could be
due to different solvation environments or concentration effects
as shown in Figure 4c. We measured the storage modulus (G’)
and loss modulus (G’’) of the polymeric gel as a function of
angular frequency (ω) (Figure 4d), the frequency sweep plots
exhibit an invariance of G’ and G’’ values over a wide range of ω,
and moreover, the values of G’ are always higher than those of
G’’, which are characteristics of gels. It has to be noted that the
gel starts to liquefy at elevated temperatures and gelates again
at lower ones, which indicates that no covalent bonds are
formed in the process.
-
AP unit to the NI unit, which leads to reduced energy and loss of chemical
reactivity in nucleophilic reactions such as the Claisen condensation.
To expand the generality of the anionic system, we also
synthesized a series of six NNIs with N-phenoxide donors
(Figure 3a and 3b) which are soluble in both DMF and water. As
the figure indicates, in aqueous solutions, NNIs 4-6 exhibit EPR
signals while 7-9 do not (with 9 showing a slightly distinguishable
pattern). Structurally speaking, these two groups do not have
dramatic differences. However, their aqueous solutions prepared
from reacting with excess KOH did show some visually
difference: the first group exhibits a brownish yellow color while
the second group is essentially colorless at optically dilute
concentrations (Figure S9); as the concentration increases, the
first group starts to turn very dark while the second group
remains colorless (7 and 8) or turns brownish yellow (9). In
Figure 3c, the UV-Vis spectra do seem to show that the
absorption of the first group NNIs slightly extends beyond 390
nm. As coloration is an indication for possible charge or electron
transfer, it is possible that 4-6 has better HOMO-LUMO energy
match than that of 7-9 in water. Again, as long as the pH was
deprotonating enough for the N-phenols, the EPR signal was
always detectable over a period of 90 days despite that higher
concentrations are required vs. detection in DMF. It is possible
that the energy of the phenoxide is substantially reduced in high-
polarity solvents, so that ET could only happen during a
fluctuation in which water molecules adopt a less efficient
arrangement for solvation. Nonetheless, the polarity-dependent
EPR intensity can be very useful if there NNIs are to be used as
molecular probes in polarity or hydrophobicity sensing.
In conclusion, we discovered that spontaneous ET in anionic
NNI derivatives can result in persistent radicals at room
temperature in organic and aqueous solutions and ascribe the
phenomenon to spin-pairing interactions which confer both
stability and magnetism. The interactions are explored in
supramolecular chemistry to prepare a polymeric gel that is
EPR-active and may be useful in applications such as
chemochromic magnetic materials.
Figure 4. a) Synthetic procedures of polyacetophenone (PAP) and
polynaphthalimide (PNI); b) pictures showing the mixture of PAP and PNI at
1:1 ratio (0.1mM) in DMF 0 min (thick liquid, left) and 5 min (gelation, right)
after the KOtBu base is added at room temperature in air; c) EPR spectrum of
the PAP/PNI polymeric gel at room temperature in air from b); d) Oscillation
frequency dependency of the storage modulus G’ and loss modulus G’’.
Figure 3. a)Chemical structures of anionic NNIs that are EPR-active and b)
EPR-inactive in aqueous solutions; c) UV-Vis absorption spectra of NNIs 4-9
at 110-5 M; inset showing varying colors of the NNI aqueous solutions at
110-3 M; d)-f) liquid-phase EPR spectra of 4-6 in aqueous solutions (810-3
M) at 298 K.
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Acknowledgements
We thank the National Key R&D Program of China
(2017YFA0303500 to G. Z.) for financial support of this work.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Supramolecular Chemistry • Persistent Room-
Temperature Radicals • Naphthalimides • Spin Pairing • Polymer
Gel
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What makes an anion stable and
exhibit radical characters? When it is
spatially confined to an electron
deficient aromatic molecule, a
carbanion or phenoxide becomes a
persistent radical-like species at room
temperature, presumably due to spin-
pairing interactions. Interesting
supramolecular chemistry may thus
be constructed.
Wenhuan Huang, Biao Chen*, and
Guoqing Zhang*
Page No. – Page No.
Persistent Room-Temperature
Radicals from Anionic
Naphthalimides: Spin Pairing and
Supramolecular Chemistry
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