UV-irradiation of self-assembled triphenylamines affords persistent and regenerable radicals

Article abstract of DOI:10.1039/c8sc04607g

UV-irradiation of assembled urea-tethered triphenylamine dimers results in the formation of persistent radicals, whereas radicals generated in solution are reactive and quickly degrade. In the solid-state, high quantities of radicals (approximately 1 in 1

Full text of DOI:10.1039/c8sc04607g

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UV-irradiation of self-assembled triphenylamines
affords persistent and regenerable radicals†
Cite this: DOI: 10.1039/c8sc04607g
All publication charges for this article Ammon J. Sindt, Baillie A. DeHaven, David F. McEachern, Jr,
have been paid for by the Royal Society
of Chemistry
D. M. M. Mevan Dissanayake, Mark D. Smith,
and Linda S. Shimizu
Aaron K. Vannucci
*
UV-irradiation of assembled urea-tethered triphenylamine dimers results in the formation of persistent
radicals, whereas radicals generated in solution are reactive and quickly degrade. In the solid-state, high
quantities of radicals (approximately 1 in 150 molecules) are formed with a half-life of one week with no
significant change in the single crystal X-ray diffraction. Remarkably, after decay, re-irradiation of the
solid sample regenerates the radicals to their original concentration. The photophysics upon radical
generation are also altered. Both the absorption and emission are significantly quenched without
external oxidation likely due to the delocalization of the radicals within the crystals. The factors that
influence radical stability and generation are correlated to the rigid supramolecular framework formed by
the urea tether of the triphenylamine dimer. Electrochemical evidence demonstrates that these
compounds can be oxidized in solution at 1.0 V vs. SCE to generate radical cations, whose EPR spectra
were compared with spectra of the solid-state photogenerated radicals. Additionally, these compounds
display changes in emission due to solvent effects from fluorescence to phosphorescence.
Understanding how solid-state assembly alters the photophysical properties of triphenylamines could
lead to further applications of these compounds for magnetic and conductive materials.
Received 16th October 2018
Accepted 9th January 2019
DOI: 10.1039/c8sc04607g
rsc.li/chemical-science
irradiation with UV light can regenerate the radicals in similar
quantities. Thus, solid-state assembly alters the photophysical
Introduction
Intentional design of functional supramolecular assemblies properties of TPAs and could prove helpful in the design of
requires precise control of intermolecular interactions as well as
an understanding of how complex structures modulate chem-
ical and physical properties to produce materials with emergent
1
qualities. This understanding is key for designing compounds
used to probe the enhancement or quenching of luminescence
2
of small molecules in the solid-state. Controlled assembly of
3
structures can also modulate conductivity and dichroism in
4
photoactive materials. Here, we synthesize urea tethered tri-
phenylamines (TPAs), and determine their photophysical
properties in solution and in crystalline assemblies. Upon UV-
irradiation, in both solution and the solid-state, these mate-
rials displayed radical formation with solid-state samples
proving to be quite stable (Fig. 1). Remarkably, solid-state
samples yield high quantities of persistent radicals with ꢀ1 in
150 molecules containing a radical. Moreover, aer decay, re-
Department of Chemistry and Biochemistry, University of South Carolina, Columbia,
South Carolina 29208, USA. E-mail: SHIMIZLS@mailbox.sc.edu
†
Electronic supplementary information (ESI) available: Experimental details;
synthesis and characterization of compounds; SC-XRD data; PXRD, absorbance
Fig. 1 (A) Self-assembly of a triphenylamine derivative affords
1
emission, DOSY NMR, EPR, H NMR, FTIR, CV, and electrolysis spectra; and
persistent radicals upon irradiation with UV light. (B) UV-irradiation
induces a noticeable change in color. (C) A significant radical signal is
observed which corresponds to 1 in ꢀ600 molecules displaying
a radical after 1 h of UV-irradiation and up to 1 in ꢀ150 after 8–11 h.
luminescent lifetime calculations and spectra. CCDC 1873066. For ESI and
crystallographic data in CIF or other electronic format see DOI:
1
0.1039/c8sc04607g
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conductive and magnetic materials that integrate TPA the solvent dependent photophysics of the triplet and singlet
components.
emissions of these molecules.
Para substituted TPAs are prevalent examples of molecules
5
that exhibit persistent radicals. For example, Magic Blue, an
Results and discussion
antimony salt of tribromo TPA, is a commercial one-electron
6
oxidant employed for many chemical processes. The
The urea tethered triphenylamine 3 was synthesized in ve
steps from commercial 4-bromotriphenylamine using a Vils-
meier–Haack reaction with phosphoryl chloride to yield the
stability of substituted TPA radicals, has led to their use as
promising spin-containing units for organic polymer based
7
magnets. These organic magnets are designable offering
20
aldehyde, which was subsequently converted to the alcohol via
moldability and tunability. The oxidation of the TPA also alters
21
hydride reduction (Scheme 1). Aer bromination of the
8
its photophysics, leading to quenching of its uorescence.
22
alcohol, two TPA units were tethered through triazinanone
Typically, TPA compounds require para substitution on all the
under basic conditions. Deprotection of the urea afforded 3 as
a pale yellow powder. Colorless needles were regularly obtained
9
,10
phenyl rings to generate stable radical cations.
The extra
substitution helps to slow down degradation reactions such as
ꢁ1
by slow evaporation of ethyl acetate solutions (ꢀ20 mg mL
)
9
benzidine formation. Usually, chemical or electrochemical
and were used for all solid-state measurements. The crystals
were also subjected to X-ray diffraction analysis.
8
oxidation is required to generate the radicals. Even without
the oxidation to a radical, TPAs still nd many uses as two-
Triphenylamine 3 crystallized in the orthorhombic system in
the Pccn space group. The X-ray structure revealed the desired
compound with a linear trans–trans arrangement of the ureas
with the two TPA units outstretched on both sides of the
methylene urea tether in an anti-parallel manner. Crystallo-
graphically, the structure is disordered with two molecular
orientations present (Fig. S9†) with the major component pop-
ulation of 91%. The urea carbonyl, which is located on a crys-
1
1
12
photon absorbers, organic light emitting diode materials,
solvatouorochromatic intramolecular charge transfer (ICT)
1
3
molecules, and as aggregation induced emission (AIE)
1
4
compounds.
The Shimizu group utilizes the three-centered urea interac-
tion to drive assembly of linear and macrocyclic monomers into
15
tapes, rods, and columns. In the case of benzophenone con-
taining monomers, assembly inuences the photophysics and
2
tallographic C axis, is common to both components. The
16
affords surprisingly stable radicals upon UV-irradiation. For
comparison, unassembled structures in solution show no
radical formation upon UV-irradiation. Our hypothesis is that
supramolecular assembly signicantly enhances radical
stability. Here, we test if urea-tethered triphenylamines will be
affected in a similar manner.
individual molecules are organized into chains extending along
the crystallographic c-axis through characteristic three-centered
ꢂ
urea hydrogen bonds with a twisting angle of 51.6(1) . The
hydrogen-bonded urea groups (N(H)/O distances of 2.823(3)
˚
and 2.70(2) A, (Fig. 2A)) generate an X-shaped chain when
viewed down the c-axis (Fig. 2B). The twisting is likely caused by
the extra steric bulk of the TPA since similar dibenzylic systems
typically have straight urea chains according to a Cambridge
We synthesized and compared the structures and properties
of a methylene urea bridged 4-bromo TPA dimer (3) against 4-
bromo TPA (1) and a protected urea analog (2). The structure of
dimer 3 features one bromine on each TPA adduct to assist in
intersystem crossing (ISC) from the excited singlet to the triplet
state which can be aided by the heavy atom effect, and should
help promote radical generation from UV-irradiation. The heavy
23
Structural Database survey (CSD 5.39, September 28, 2018).
To examine how solvent and assembly affects the photo-
physics of the TPA compounds, the absorption and emission for
1, 2, and 3 were taken in six solvents and in the solid-state at
room temperature. The studies in dichloromethane, dimethyl
sulfoxide, ethyl acetate, ethanol, acetonitrile, and tetrahydro-
furan are summarized in Table 1 and S2.† In all the tested
solvents, the absorption spectra of 1–3 were nearly identical,
with a strong pp* transition at approximately 300 nm
17
atom effect increases ISC due to spin orbit coupling. Addi-
tionally, one para position on each TPA unit of 3 was le
intentionally unsubstituted. Typically, fully substituted TPA's
5
are required for radical stability. Here, we test if supramolec-
ular assembly can provide stability to unsubstituted TPA radi-
cals, which in turn, would allow for greater variability in TPA
structures with stable radical characteristics. Radical formation
was investigated by two methods: electrochemical oxidation
and UV-irradiation. Both of these methods can generate radical
8
cations in TPA compounds, with the former being well-known,
1
8
and the latter requiring a reducible agent in the molecule itself
19
or in molecules close-by (i.e. solvent). Our goal is to charac-
terize these systems by electron paramagnetic resonance (EPR)
spectroscopy to understand how solid-state organization inu-
ences their ability to generate stable radicals versus dissolution.
Specically, we are testing (1) if self-assembly can stabilize
radicals and (2) if UV-irradiation is a useful tool to generate TPA
radicals in reasonable quantities. Additionally, we will examine
Scheme 1 Synthetic scheme for 2 and 3.
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The emission spectra recorded in solution for 2 and 3
exhibited two main transitions either at approximately 370 nm
(Band 1) and/or 450 nm (Band 2). As seen in Fig. 3B and C, the
intensities of these bands varied widely on the basis of solvent
with 2 exhibiting more Band 1 character and 3 more Band 2.
Band 1 is generally considered the uorescence band for TPA
13
systems. To identify Band 2, further experiments were carried
out.
First, the emission of 1 was taken in different solvents to
probe if Band 2 was derived from an ICT process. Although ICT
26
typically requires a donor–p–acceptor system, the TPA units of
and 3 could rotate over to each other allowing the TPA units on
2
Fig. 2 Views from 3 in the solid-state. Disorder was omitted for clarity.
either side of the urea tether to act as both the donor and
acceptor in the ICT exchange without the need for a p-system
intermediate. For this case, the TPAs would have to adopt
acceptor characteristics since TPAs are typically only the donor
(
A) Packing is driven through zig-zagged chains of urea hydrogen
bonding. (B) Ureas adopt a twisting orientation creating an X-shape
looking down the c-axis.
27
in ICT systems. As seen in Fig. S15,† 1, which is only a single
TPA unit, also exhibited Band 2. This suggests that band 2 is not
due to an ICT process.
dominating the spectra with no other bands readily apparent
(Fig. 3A). This suggests that in solution the proximity of the two
Second, DOSY NMR studies were carried out on 3 to probe if
Band 2 originated from an AIE process. Since AIE has been
TPA units have little effect on the absorption properties. On
average, 2 and 3 were red-shied by 2 nm compared to 1. As
expected, the molar absorptivity for 2 and 3 were very similar
and twice that of 1 with values ranging from 4.70–5.53 ꢃ
28
known to create new emissive bands, ureas are known aggre-
29
14
gators, and AIE has been observed in TPA systems before it
seems reasonable that Band 2 could be derived from this
process. For DOSY NMR, aggregation is detected when the
observed hydrodynamic radius is signicantly higher than the
radius of the monomer. DOSY studies were conducted on
solutions of 3 in deuterated acetonitrile (1 mM and 100 mM).
This solution was chosen since it displayed the most signicant
Band 2 character of all the trials. As seen in Fig. S18 and S19,†
no aggregation was observed for 3 in acetonitrile since the
4
ꢁ1
ꢁ1
4
ꢁ1
ꢁ1
1
0 M cm and 2.12–2.68 ꢃ 10 M cm , respectively.
For solid-state samples, crystals of 3 were rst examined by
PXRD to probe if the bulk crystalline material was similar in
structure to the single crystal of 3. Fig. S11† compares the
experimentally observed PXRD pattern to the predicted powder
pattern simulated from the SC-XRD data. Seen here is an
excellent correlation, suggesting that the bulk material is single
phase and similar in structure to the solved crystal structure.
This indicates that the photophysical measurements of the bulk
material would be representative of the single crystals. Self-
assembly of 3 resulted in a red shi of the absorbance of
about 60 nm with slight broadening of the main peak (Fig. 3A).
A similar red shi has been reported before for other triphe-
˚
observed hydrodynamic radius of approximately 8 A for both
solutions is only slightly higher than that calculated from the
˚
crystal structure monomer of 3 (ꢀ6 A). The slightly higher
radius may be from solvation or a slight amount of dimeriza-
tion, but denitively no large-scale aggregation was observed.
Considering that more dilute solutions (10 mM) were used for
photophysical measurements and no aggregation was observed
24
nylamine derivatives on the basis of J-aggregates; however,
25
this is more common for planar dyes.
Table 1 Measured photophysical properties for compounds 2 and 3 under different conditions
a
3 (ꢃ10 M ꢃ cmꢁ1
4
ꢁ1
b
c
Compound
Solvent
l
abs (nm)
)
l
ems (nm)
s<avg> (ns)
2
DCM
DMSO
EtOAc
EtOH
MeCN
THF
DCM
DMSO
EtOAc
EtOH
MeCN
THF
304
302
301
301
300
302
303
302
301
300
300
302
358
5.11
5.42
5.34
4.95
4.70
5.47
5.20
5.41
5.53
5.45
5.01
5.22
—
365, 435*
451
365
0.8
5.1
0.1
0.1
3.0
<0.1
2.2
4.1
0.1
0.1
2.8
<0.1
1.0
361
371, 451*
366*, 444
366, 449*
369, 452*
364*, 453
362*, 437
497
3
367*, 457
447
Solid
a
b
Peak position at largest absorption band. Peak positions at largest emission bands in nm (largest denoted with * if applicable, excited at labs.).
Average lifetime of largest emission peak.
c
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Fig. 3 Absorbance and emission spectra of 2 and 3 at room temperature. (A) Normalized absorbance of 2 and 3 in different solvents and 3 in the
solid-state. (B) Emission of 3 in different solvents and the solid-state. (C) Emission of 2 in different solvents. (D) Peak shift for 2 in the emission
spectra changing from ethyl acetate to acetonitrile.
in more concentrated samples, this suggests that AIE is not system (2 in EtOAc), the addition of acetonitrile slowly turned
responsible for Band 2.
on phosphorescence until the system was mostly phosphores-
With ICT and AIE ruled out for the occurrence of Band 2, cence clearly showing the solvent dependent nature of these
phosphorescence is suggested as the likely origin. Phospho- emissive bands.
rescence is common for structures containing TPAs that are
The luminescent lifetimes for 2 and 3 were found to be quite
30
employed in OLED materials. Additionally, the peak position short for TPA derivatives with uorescence and phosphorescent
in the emission spectra is in good agreement for where phos- lifetimes estimated to be around 0.1 ns and 3 ns, respectively
phorescence is typically observed in TPA compounds. The (Table 1, Fig. S20 and S21†). This may be due to competing non-
bromine substituent can increase spin-orbital coupling via the radiative decay processes introduced by the heavy atoms.
heavy atom effect, which gives access to the triplet state thus Typically, uorescent lifetimes for TPA containing compounds
1
7
34
enhancing phosphorescence. This effect can occur with either tend to hover around 2 ns, but heavy atom containing TPA
31
the heavy atom being directly connected into in the p-system,
or in close proximity. The former case could explain why all lifetimes (<0.1 ns). For the phosphorescent lifetimes, while the
three compounds exhibit Band 2, while the latter case could heavy atom effect does increase phosphorescence intensity it
derivatives with small p-systems have be seen to exhibit shorter
32
22
17
30
35
explain the intensity of this band (3 > 2 > 1). The emission can shorten the lifetimes as well.
spectra were also measured in the presence of a triplet quencher In the solid-state it was not indicatively clear if uorescence
triethylamine) and in an oxygen-saturated solution of or phosphorescence was occurring as both the Stoke's shi (89
dichloromethane. The emission was reduced in both cases nm) and lifetime (1.0 ns) were in-between the expected values
Fig. S31 and S32†), further suggesting that this band arises for uorescence (65 nm, 0.1 ns) and phosphorescence (150 nm,
from phosphorescence. 2.0 ns) determined from solution studies. Additionally,
(
(
For 1–3, increasing solvent polarity resulted in increased attempts at measuring accurate quantum yields for these
phosphorescence, except in the case of polar protic solvents compounds were unsuccessful due to radical generation and its
(
ethanol) which showed little to no phosphorescence (Fig. 3B subsequent effects on the photophysical properties.
and C). Solvent dependent phosphorescence has been observed The short observed lifetimes could be explained by the
were similar in energy. In this situation, formation of stable radicals or by other non-radiative pathways.
33
1 n
before when S and T
different solvents stabilized either state in varying degrees Thus, we turned to X-band EPR spectroscopy to probe radical
resulting in different phosphorescent quantum yields for each formation within solution samples. First, a solution of 3
solvent, which may be the case here as well. To further inves- (ꢀ1 mM) was prepared in degassed DCM and was sealed under
tigate the solvent dependence, we examined if phosphoresce argon. While no EPR signal was observed pre UV, UV-irradiation
could be ‘turned on’ by the addition of a polar solvent to a non- (1 h) of the sample resulted in an EPR signal with a g-value of
polar system (Fig. 3D). Starting with a uorescent non-polar 2.005 (Fig. S22†). However, this radical was unstable and found
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to rapidly undergo degradation reactions. Fig. S26† compares amount of radicals increases steadily with irradiation time (1 to
1
the H NMR spectra of 3 in solution before and aer UV- 6 h) as seen in Fig. 4B part I. The plot of the double integration
irradiation. The post UV sample shows nearly a complete loss of the EPR signal versus time starts to plateau aer 7–11 h of UV-
of all of the parent resonances. Post-UV absorption and emis- irradiation with the crystals of 3 turning deep brown in color
sion spectra also followed this trend (Fig. S14 and S17†) clearly during the process (Fig. 1B). The concentration of radicals was
indicating that radicals of 3 generated in solution are not stable. approximated using a calibration with standard solutions of
This is not unexpected since electrochemical studies of control Magic Blue in DCM. Comparing the area of the EPR spectra of
36
1
indicate an unstable radical cation in solution and radical the solid-sample versus the Magic Blue calibration can give an
cations generated from TPAs with unsubstituted para positions approximate concentration of radicals generated in the solid-
9,10
are known to be unstable in solution.
state. Aer 11 hours of UV-exposure, 9.0 mg of 3 generated
Next, EPR spectra were recorded on crystals of 3 in order to the same amount of radicals as 100 mL of a 0.82 mM solution of
investigate how solid-state assembly inuences the formation Magic Blue, suggesting that 1 in 150 molecules of 3 have
of radicals. First, EPR spectroscopy was performed on a triply a radical (or 1 in 300 TPA units, Fig. S25†). Similar calculations
recrystallized sample of 3 (3.9 mg) which was UV-irradiated for for the 3.9 mg of the triply recrystallized sample were of
6
h. Fig. S23† shows the recorded EPR spectra, which displays a similar magnitude with ꢀ1 in 250 molecules exhibiting
a broad signal with an axial powder pattern shape. The observed a radical aer only 6 h of irradiation.
g-value is 2.006, which is in the range of TPA radical cations in
solution (2.002–2.005). Singly recrystallized samples of 3 (10 formation in crystalline samples (Fig. S27†), we investigated if
With no noticeable degradation of 3 occurring aer radical
5
mg) were also examined pre and post UV-irradiation (Fig. 1C). the radicals could be ‘regenerated’ aer decay with repeated UV
As expected, no signal was seen pre irradiation; however, aer 1 exposure. Typically, with chemical or electrochemical oxida-
hour of UV-irradiation a broad EPR signal identical to the triply tions of TPAs to their corresponding radical cations, loss of the
recrystallized sample was observed.
radical signal likely means the sample has degraded, and
The persistence of the photogenerated radicals was exam- samples must be resynthesized. Remarkably, once the signal of
ined using dark decay studies in which the recrystallized sample 3 decays to half signal, irradiation with UV-light restores the
was irradiated for one hour and then stored in the dark at room radical concentration back to its maximum value (Fig. 4B, Part
temperature under argon. The EPR spectrum was monitored II). The samples were re-irradiated for 6 hours at the start of
over a month to estimate its stability (Fig. 4A). EPR signals were weeks 2 and 3 to regenerate the signal. As seen in Fig. 4B part II,
doubly integrated to obtain the area, which was plotted versus the radicals decay at approximately the same rate over the three
time aer UV-irradiation (Fig. 4A, inset). A reliable radical signal week long cycles. Also notable is that similar quantities of
persists up to a month with a half-life of approximately one radicals are generated each time the crystals of 3 are UV-
week. Aer two months, when no radical signal was observable, irradiated, demonstrating the exceptional stability and repro-
1
we took a H NMR of the sample to see if the sample had ducible nature of the assembled structure versus in solution.
degraded similarly to the solution study. Remarkably, the NMRs
were identical to the initially synthesized materials indicating the properties of the crystals as a whole. First, we compared the
that 3 is photostable in its crystal form (Fig. S27†). photophysics of the crystals before and aer UV-irradiation (4
Next, we probed how the photogenerated radicals inuenced
Next, we estimated the maximum concentration of radicals h). Both the absorption and emission were signicantly
that could be generated through UV-irradiation by plotting the quenched upon radical formation (Fig. S13 and S16†). This was
area of the EPR signal versus time exposed to UV light. The quite striking considering the radical concentration was
Fig. 4 EPR data for 3 in the solid-state. (A) Dark decay after 1 hour of UV irradiation. Inset: the double integration of the dark decay spectra
plotted versus time post UV irradiation. (B) (I) The double integration of the EPR spectra over time of UV irradiation followed by (II) a regeneration/
decay study of the radicals. For part II, after the initial irradiation to the maximum radical concentration, the sample was irradiated for an
additional 6 hours at the start of weeks 2 and 3.
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relatively low compared to the bulk sample. Oxidation of TPAs
to radical cations is known to quench the photophysical prop-
8
erties, but typically it is quantitative in nature, at least in
solution. This indicates that the generated radical is strongly
delocalized to effect the whole system and behaves similarly to
a radical cation.
To further probe the nature of the radical, irradiated (4 h)
brown crystals of 3 were subjected to SC-XRD and IR spectros-
copy. No change was observed in the overall single crystal
structure; however, we noticed a minor change in the amount of
disorder in the crystal going from 91% major conformer to 95%.
This is likely correlated to the specic single crystal chosen for
SC-XRD and is probably unrelated to radical formation. We
expect that the TPA units are too bulky to self-correct during
crystal formation leading to an array of different conformer
percentages depending on the single crystal. For the IR studies,
pre and post UV spectra were found to be nearly identical with
no visible changes (Fig. S28†). The combination of SC-XRD and
IR suggest that either the radical is highly delocalized and/or is
not concentrated enough to be characterized by these methods.
Cyclic voltammetry (CV) was used to further characterize the
electronic structure of 3 in solution. Since the radicals in the
solid-state acted similarly to TPA radical cations, this method
could further characterize the types of radicals generated in this
system. The oxidation of 3 in DCM shows two pseudo-reversible Fig. 5 (A) CV for 1 mM 3 in a 0.1 M (n-Bu)
+
6ꢁ
4
N PF DCM solution, scan
ꢁ1
rate 100 mV s . SCE ¼ saturated calomel electrode. (B) Solution EPR
for 3 at 10 K after bulk electrolysis at first oxidative peak (red) along with
a solid-state EPR for 3 also at 10 K EPR after 3 hours of UV irradiation
oxidation waves at 1.0 V and 1.2 V vs. SCE (Fig. 5A). In
comparison, the oxidation of parent compound 1 leads to
a degradation that was immediately visible in the CV, which
suggests that 3 is more stable towards oxidation than the parent
compound. Controlled potential electrolysis performed on 3 at
36
(black). Inset: proposed structure of radical responsible for EPR signal.
+
1.2 V passed at total of 3.98 electrons per molecule (Fig. S30†).
temperature spectrum, consistent with population of lower
energy states. The peak at g-value ¼ 2.002 was consistent in the
electrolytic solution sample and the UV-irradiated crystals 3.
This suggests that crystalline 3 may form a similar radical
species to electrolytic sample. Additionally, this is in good
agreement with where TPA radical cations typically appear in
ꢁ
Thus, each oxidative wave in Fig. 5A is attributed to a 2e
oxidation of the symmetric compound 3. Each electron is ex-
pected to come from the TPA units of the molecule, which
would be consistent with previously reported results that have
only one TPA unit per molecule. For comparison, an electro-
chemical study on compound 2 showed that slower scan rates
36
5
EPR spectra. Thus, it is likely that the photogenerated radicals
ꢁ
1
(40 mV s ) were required to obtain pseudo-reversible oxidation
in assembled 3 are similar to radical triphenylamine cations
formed by electrolysis (at g-value ¼ 2.002), although it is not
clear what anion is being formed in the crystalline sample for
this process to occur. We are currently examining macrocyclic
derivatives and are planning high eld EPR studies that could
help probe this question. The second isotropic signal at g-value
waves also near 1.0 V and 1.2 V vs. SCE (Fig. S29†). This indicates
the rigidity of backbone of 2 has a clear impact on electron
transfer kinetics.
Bulk electrolysis on a ꢀ1 mM solution of 3 in DCM was
performed at the rst oxidation peak to generate a radical cation
in solution. The rst peak was chosen for the oxidation since it
likely affords an overall dication with each TPA unit being
oxidized once. Electrolysis was performed for ꢀ5 hours to afford
a bright yellow solution; however, once electrolysis was
completed the resulting sample was unstable at room temper-
ature and turned teal within 15 min. EPR analysis showed no
signal. Thus, the electrolysis was performed on a new sample
that was immediately immersed in liquid nitrogen for trans-
portation and the EPR recorded at 10 K. As seen in Fig. 5B, two
peaks for the electrolytic sample can be seen at g-values of 2.031
and 2.002.
¼
2.031 was exclusive to the electrolyte sample and is attributed
to degradation products.
Self-assembly of TPA urea dimers can stabilize the UV
generated organic radicals in stark contrast to their solution
counterparts. The concentration of the radicals is readily
controlled by irradiation time up to a maximum of 1 in ꢀ150
molecules. The presence of the radicals can be visualized simply
through their photophysical quenching behavior. Advantageous
to this system is that the radicals can be generated in the solid-
state without noticeable degradation to the starting materials
and display a half-life up to a week. Additionally, these radicals
can be regenerated upon re-irradiation without any loss in
radical concentration. A comprehensive study on how different
The UV-generated radicals of crystalline 3 were also recorded
at 10 K for comparison and showed no change in the line width
and a slight shi in g-value to 2.002 compared to its room
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halogen substituents on these TPA compounds inuences the
radical generation, stability, and concentrations may be
invaluable in revealing the factors that govern the photophysics
of these compounds.
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In summary, a TPA methylene urea-tethered dimer was
synthesized and readily afforded single crystals that organized
the TPA through urea hydrogen bonding interactions. This
solid-state assembly signicantly stabilizes UV-generated radi-
cals. Radicals formed in solution were unstable, as expected for
incomplete para substituted TPA systems. In the solid-state,
high quantities of radicals were formed, up to 1 in ꢀ150
molecules, which were persistent at room temperature with no
observable degradation or signicant changes in the single
crystal X-ray diffraction. Further, radicals generated within the
assembled framework have been shown to last up to a month
with a half-life around a week. Most remarkably, aer radical
decay, radicals can be regenerated to their original maximum
concentration with re-exposure to UV light. The photophysics of
these materials were signicantly quenched likely due to TPA
hole transport properties even with relatively low radical
concentration. Electrochemical evidence demonstrates that
these compounds can be oxidized in solution at 1.0 V vs. SCE to
generate radical cations, whose EPR spectra are similar to the
UV-generated radicals in the solid-state. This suggests that the
TPA radical cation is being formed in the solid-state and this
electron transfer is reversible and reforms the parent
compound over time. We are currently planning to carry out
high-eld EPR experiments as well as Dynamic Nuclear Polari-
zation Magic Angle Spinning solid-state C13-NMR to further
examine this process. Future work includes the synthesis of
additional halogenated on the TPA analogs to elucidate the
factors that govern radical formation, persistence, and quantity.
Understanding how assembly enhances the stability of radicals
would be exceedingly helpful in the end goal of making better
conductive and magnetic materials that incorporate TPA
scaffolds.
6
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2 J. Hu, X. Zhang, D. Zhang, X. Cao, T. Jiang, X. Zhang and
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3 Y. Kuramoto, T. Nakagiri, Y. Matsui, E. Ohta, T. Ogaki and
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4 M. Jiang, X. Gu, J. W. Y. Lam, Y. Zhang, R. T. K. Kwok,
K. S. Wong and B. Z. Tang, Chem. Sci., 2017, 8, 5440;
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5 L. S. Shimizu, S. R. Salpage and A. A. Korous, Acc. Chem. Res.,
Conflicts of interest
2014, 47, 2116.
1
6 M. F. Geer, M. D. Walla, K. M. Solntsev, C. A. Strassert and
L. S. Shimizu, J. Org. Chem., 2013, 78, 5568; B. A. DeHaven,
D. W. Goodlett, A. J. Sindt, N. Noll, M. De Vetta,
M. D. Smith, C. R. Martin, L. Gonz ´a lez and L. S. Shimizu,
J. Am. Chem. Soc., 2018, 140, 13064.
There are no conicts to declare.
Acknowledgements
This work was supported in part by the National Science
Foundation (CHE-1608874 and OIA-1655740), and a Fulbright
US Scholar award.
1
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18 A. Saha, M. Chen, M. Lederer, A. Kahnt, X. Lu and
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