Effect of the C(3)-Substituent in Verdazyl Radicals on their Profluorescent Behavior

Article abstract of DOI:10.2533/chimia.2016.172

Methods for the detection of reactive intermediates such as transient radicals are important in organic chemistry, polymer chemistry, biology or medicine. Along these lines we recently reported that 1,5-diphenyl-6- oxo verdazyl radicals can be used as flu

Full text of DOI:10.2533/chimia.2016.172

172 CHIMIA 2016, 70, No. 3
doi:10.2533/chimia.2016.172
Physical Organic chemistry
Chimia 70 (2016) 172–176 © Swiss Chemical Society
Effect of the C(3)-Substituent in Verdazyl
Radicals on their Profluorescent Behavior
Steffen Eusterwiemanna, David Matuscheka, Linda Stegemannb, Sina Klabundec,
Carsten Doerenkampc, Constantin G. Daniliuca, Nikos L. Doltsinis*d, Cristian A. Strassert*b,
Hellmut Eckert*ce, and Armido Studer*a
Abstract: Methods for the detection of reactive intermediates such as transient radicals are important in organic
chemistry, polymer chemistry, biology or medicine. Along these lines we recently reported that 1,5-diphenyl-6-
oxo verdazyl radicals can be used as fluorescent spin sensors. In situ generated C-centered radicals are efficiently
trapped by the verdazyls, which in turn undergo transformation from a paramagnetic non-fluorescent state to
a diamagnetic fluorescent state. Whereas the N-phenyl substituent in the spin probes is of high importance for
obtaining profluorescent behavior, the effect of the C(3)-substituent has not been investigated to date. We herein
present the synthesis and characterization of various 1,5-diphenyl-6-oxo-verdazyl radicals bearing differently
hybridized C-substituents at the C(3) position. Steady-state and time-resolved fluorescence spectroscopy in
solution and in the solid state along with time-dependent density functional theory (TDDFT) calculations reveal
that a C(3)-aryl substituent is crucial for obtaining fluorescence after spin trapping. In addition, it is shown that the
emission wavelength of the C(3)-aryl substituted verdazyl derivatives can be tuned by selective destabilization
of the HOMO and the LUMO.
Keywords: EPR · Profluorescence · Spin traps · TDDFT · Verdazyl radicals
species due to quenching of the excited are essential for obtaining profluorescent
state, whereas upon reaction with transient behavior. Diamagnetic spin trapping prod-
Introduction
radicals the fluorescence is restored in the ucts derived from N-alkyl verdazyl radi-
corresponding diamagnetic spin trapped cals are non-fluorescent.[6] However, the
products (Fig. 1). Since transient radicals effect of the C(3)-substituent in verdazyls
are common intermediates in organic syn- on their profluorescent properties has not
thesis, polymer, biological and environ- been investigated to date.
1,5-Diphenyl-6-oxo verdazyls are per-
sistent radicals,[1] which have been used as
building blocks for magnetic materials,[2]
as substrates for organic synthesis,[3] ESR
spin labels[4] and as mediators in living
radical polymerizations.[5] Recently, we
reported their profluorescent behavior,
allowing these paramagnetic species to
be used as spin sensors.[6] Fluorescence
is fully suppressed in these paramagnetic
mental chemistry, methods for the detec-
Hereinwereportthesynthesisandchar-
tion of these reactive intermediates are im- acterization of various C(3)-substituted
portant.[7] Prior to our report, this principle 1,5-diphenyl-6-oxo verdazyl radicals and
was applied to nitroxide radicals bearing address their profluorescent behavior by
a fluorophor covalently connected to the studying fluorescence after trapping with
nitroxyl moiety via a short tether.[8] Along C-radicals. Along these lines, we will in-
with their application as spin labels,[9] the vestigate the simple C(3)-H derivative
profluorescent behavior of nitroxides was and systems bearing differently hybrid-
also exploited to image cellular oxida- ized C-substituents (sp, sp² and sp³) at the
tive stress[10] and polypropylene degrada- C(3) position of the heterocycle. In addi-
tion.[11] In contrast, oxo-verdazyl radicals tion, we will show how the physicochemi-
do not require covalent attachment of a cal properties of such heterocycles can be
fluorophor, since these heterocycles in a tuned by introducing electron withdrawing
paramagnetic form show intrinsic fluores- and electron donating groups at the para-
cence properties. Our studies revealed that position of phenyl substituents at position
N-phenyl substituents in verdazyl radicals N(1), (N5) and C(3).
*Correspondence: Prof. N. L. Doltsinisd, Prof. Dr. H.
Eckertce, PD Dr. C. Strassertb, Prof. Dr. A. Studera
E-Mail: nikos.doltsinis@uni-muenster.de,
eckerth@uni-muenster.de, ca.s@uni-muenster.de,
studer@uni-muenster.de
aInstitute of Organic Chemistry, Department of Chemi-
stry and Pharmacy, Westfälische Wilhelms-Universität
Münster, Corrensstrasse 40, 48149 Münster, Germany
bCenter for Nanotechnology, Institute of Physics,
Westfälische Wilhelms-Universität Münster, Heisen-
bergstrasse 11, 48149 Münster, Germany.
E-mail: ca.s@uni-muenster.de.
Fig. 1. Profluorescent
verdazyl radicals.
O
O
R
N
N
N
N
N
N
N
N
cInstitut für Physikalische Chemie, Westfälische
Wilhelms-Universität Münster, Corrensstrasse 28/30,
48149 Münster, Germany
R
X
X
dInstitut für Festkörpertheorie and Center for Multi-
scale Theory & Computation, Westfälische Wilhelms-
Universität Münster, Wilhelm-Klemm-Strasse 10,
48149 Münster, Germany
X = H, C(sp), C(sp2), C(sp3)
eInstituto da Física em Sao Carlos, Universidade de
Sao Paulo, Avenida Trabalhador Saocarlense 400,
Sao Carlos, SP 13560, Brazil
Fluorescence off
Fluorescent ?

Physical Organic chemistry
CHIMIA 2016, 70, No. 3 173
The verdazyl radicals 4a–4m were ana-
lyzed in solution by UV/Vis and EPR spec- aryl ring like -NO2 (4m: 533 nm) or -F (4l:
stituents in the para-position of the C(3)-
Results and Discussion
troscopy and for radical 4i also by X-ray 559 nm) appear blue shifted compared to
Verdazyl radicals were prepared in two
steps via first condensing carbonohydra-
zide 1a or 1b with aldehydes 2 to afford
structure analysis (Fig. 2).
The UV/Vis spectra of these radi-
cals (see ESI, Fig. S7–S10) show a band, 581 nm) or -NMe (4k: 663 nm).
the red shifted absorptions of systems with
electron donating groups like -OMe (4j:
tetrazinanones 3a–3m (Table 1, for de-
2
tails, see ESI).[12] Oxidation with 1,4-ben-
zoquinone gave the corresponding verda-
zyl radicals 4a–4m in good to excellent
yields. Atom transfer coupling reaction[13]
of styryl bromide with the verdazyl radi-
cals provided the trapping products 5a–5l
in excellent yields. However, a nitro-sub-
stituted verdazyl radical 4m could not be
transformed to the desired product under
these conditions.
which depends strongly on the substituent.
The X-band EPR spectra (Fig. 3 and
Systems with H and alkyl substituents at Table 2) recorded on 1 mM solutions in
C(3) (4a: 481 nm; 4b: 497 nm; 4c: 495 degassed CH2Cl of all the radicals are
nm; 4d: 495 nm; 4e: 490 nm) show an ab- dominated by th2e magnetic hyperfine in-
sorption maximum from 481–497 nm. In teraction with the 14N nuclei (N-1,5 and
contrast, all other verdazyl radicals bear- N-2,4; quintet of quintets). As generally
ing a π-system at the C(3) position show found in the literature,[14] A(N-1,5) is sig-
bathochromic absorption behavior ranging nificantly smaller than A(N-2,4). In ad-
from 533–663 nm. In addition, the results dition, super-hyperfine coupling of the
illustrate that the absorption maxima of unpaired electron to the hydrogen of the
systems with electron withdrawing sub- different substituents via spin polarization
can be resolved in some cases: For com-
pounds 4a–4d, and 4g improved fits to
Table 1. Synthesis of verdazyl radicals 4 and trapping with C-centered radicals to 5.
the experimental spectra can be obtained
1
by considering the effect of the H nuclei
associated with the N-phenyl substituents
in ortho-, meta- and para-position (4:4:2
ratio). For the other compounds, hyperfine
couplings with these aromatic protons were
also considered, however, generally found
to be near or below 0.5 G. For compound
O
O
O
1
1
R
R
O
Ar
Ar
Ar
Ar
Ar
Ar
N
N
N
N
N
N
a)
b)
c)
N
N
H
N
N
H
N
N
N
N
NH2 N 2
H
2
2
2
R
R
Ph R
1
3
1
1
a (R =H)
4
5
1
b (R =OMe)
4a the hyperfine coupling constant of the
1H nuclei attached to C(3) is too small to
entry R1
R2
H
3 (yield)
3a (64%)
3b (62%)
3c (61%)
3d (73%)
3e (86%)
4 (yield)
4a (98%)
4b (89%)
4c (64%)
4d (91%)
4e (55%)
5 (yield)
5a (94%)
be detected. The modest values of these
1
H
H
H
H
H
H
H
proton super-hyperfine coupling constants
(with the exception of compounds 4b and
4c), show that the radical density is con-
centrated on the constituent atoms of the
verdazyl ring.
2
Me
5b (96%)
5c (99%)
5d (94%)
5e (96%)
3
Et
4
iso-Pr
tert-Bu
5
In comparison to the colored verdazyl
radicals the diamagnetic trapping products
6
3f (68%)
4f (98%)
5f (88%)
≡-TMS
5 are all colorless solids, which were in
7
CH=C(CH3)2
3g (50%)
3h (67%)
3i (68%)
3j (63%)
3k (30%)
3l (42%)
3m (76%)
4g (85%)
4h (89%)
4i (88%)
4j (99%)
4k (94%)
4l (89%)
4m (45%)
5g (96%)
5h (91%)
5i (93%)
5j (89%)
5k (85%)
5l (97%)
-
some cases characterized in the solid state
by single-crystal X-ray crystallography
(Fig. 4 and Fig. 5). Single crystals of 5b,
5e, 5i and 5l were obtained by slow evapo-
ration of a solution of the sample in EtOAc
or CHCl3. In contrast to the almost planar
heteroarene in the X-ray crystal structure
of verdazyl radical 4i (CNNC dihedral
8
OMe p-OMe-C6H4
9
OMe Ph
10
11
12
13
H
H
H
H
p-OMe-C6H4
p-NMe2-C6H4
p-F-C6H4
p-NO2-C6H4
angle = –1.5(1)°), the hetero core of 5i
a) R2-CHO (2) (1.0 equiv.), MeOH, reflux; b) 1,4-benzoquinone (1.7 equiv.), CH2Cl2, sealed tube,
60 °C, 2–3 h; c) 1-bromoethylbenzene (1.2–1.5 equiv.), Cu (1.2–1.5 equiv.), Cu(OTf)2 (3–5 mol%),
4,4'-di-tert-butyl-2,2'-dipyridyl (6–10 mol%), PhH, 80 °C, 3–6 h.
appears distorted with a dihedral angle of
–25.7(3)°, which was also found for oth-
er derivatives of the trapping products[6]
(CNNC dihedral angles for 5b: 31.0(4)°,
5e: 33.9(3)°, 5l: 34.8(2)°) (for more details
and X-ray crystal structures of 5e and 5l,
see ESI, Fig. S3 – S6).
The fluorescence properties of the trap-
ping products 5 were analyzed by UV/Vis,
steady-state and time-resolved fluores-
Fig. 2. X-ray crystal
structure analysis of
verdazyl radical 4i
(thermal ellipsoids
are shown with 30%
probability).
cence spectroscopy in solution and in the
solid state (Table 3 and Fig. 6, for more
details, see ESI, Fig. S11 – S61).
The absorption spectra (Fig. 6) show
a band around 260 nm for all trapping
products, except for 5k, which appears 20
nm red-shifted. Upon substitution with an
alkenyl, alkynyl or phenyl group (5f–5l,
5n), a new absorption band around 330
nm appears, due to electronic delocaliza-

174 CHIMIA 2016, 70, No. 3
Physical Organic chemistry
4a
4b
4c
4h
4i
4j
4d
4e
4k
4l
Fig. 5. X-ray crystal structure analysis of aryl
substituted trapping product 5i (thermal ellip-
soids are shown with 50% probability).
4f
4m
4g
negligible oscillator strength explains why
no emission could be detected for trapping
products that do not bear a third aryl sub-
2.02
2.01
2.00
1.99
2.02
2.01
2.00
1.99
g-value
g-value
stituent (see also Table 3). As an example,
Fig. 3. Experimental (black) and fitted (red) EPR spectra of verdazyl radicals 4a–4m.
the oscillator strength is five times higher
for 5i than for 5e, explaining its compara-
Table 2. EPR parameters (hyperfine coupling constants (in G)) and g-values for verdazyl radicals
4a–4m. Simulations include the effects of the modulation amplitudes and an additional Lorentzian
line broadening parameter of 0.1 G.
bly high quantum yield. The low intensity
of 5b and 5e can be understood in terms of
the frontier orbitals (HOMO and LUMO)
participating in the electronic transition.
The HOMO mainly involves the hetero-
cycle and two aromatic moieties, whereas
A
A
A
A
A
A
g-value
(N-2,4)
(N-1,5) (H-ortho) (H-meta) (H-para) (H-C(3))
the LUMO has significant delocalization
on the third aromatic moiety (Fig. 7) that
confers a push-pull character to the excited
4a
4b
4c
4d
4e
4f
6.467
6.451
6.546
6.533
6.583
6.508
6.528
6.350
6.254
6.513
6.552
6.438
6.274
4.575
4.439
4.526
4.436
4.419
4.684
4.434
4.665
4.784
4.525
4.439
4.572
4.720
0.7
2.55
1.69
1.13
0.7
0.6
2.40
0.7
<0.5
0.8
<0.5
2.00394
2.00452
2.00394
2.00395
2.00395
2.00396
2.00388
2.00395
2.00389
2.00391
2.00395
2.00392
2.00391
0.6
state. The TMS-acetylene substituted de-
rivative 5f is an exception, as it does not
have a phenyl substituent but still shows
room temperature fluorescence, owing to
the electron-rich TMS group, which intro-
duces a slight LUMO delocalization. To
corroborate this view, we investigated the
effect of the silicon on the photophysical
0.5
<0.5
<0.5
<0.5
<0.5
0.5
0.7
0.6
4g
4h
4i
0.9
0.8
0.5
<0.5
<0.5
0.6
properties and treated 5f with KF to give
the TMS-deprotected acetylene 5n in 70%
yield (Scheme 1).
<0.5
0.6
4j
<0.5
<0.5
<0.5
Not surprisingly, no emission was de-
4k
4l
0.6
0.5
tectable at room temperature for 5n. The
0.6
0.6
predicted emission wavelengths for the
aryl-substituted derivatives 5i, 5h, and 5j
nicely coincide with the measured wave-
lengths in frozen matrices at 77 K. We ob-
4m
tion. Emission spectra, photoluminescence serve an emission red-shift upon HOMO
quantum yields and fluorescence lifetimes destabilization by the methoxy groups (5h,
(Table 3 and Fig. 6) demonstrate the signif-
5i), and a blue-shift upon LUMO destabi-
icance of the aromatic substituent for room lization (5h, 5j, 5k). The fluorine atom, on
temperature fluorescence. Fluorescence the other hand, stabilizes the LUMO caus-
wavelengths were additionally calculat- ing a bathochromic shift (5l). At 77 K, the
ed for selected species in vacuum using solvent molecules cannot rearrange their
time-dependent density functional theory orientation to accommodate to the polar-
(TDDFT). The predicted values for the 0-0
transition are shown in Table 4 together
room temperature, however, the measured
with the oscillator strengths for the verti- emission wavelengths show a slightly dif-
cal fluorescent de-excitation. The emission
ferent trend, as the solvent dipoles can fit
ity changes upon optical excitation. At
spectra of species 5b and 5e are predicted the changing polarity of the molecules.
to be considerably blue-shifted as com- For instance, 5i and 5j are red-shifted as
pared to 5f, 5h, 5i, and 5j. However, their compared to 77 K, while 5h appears blue-
Fig. 4. X-ray crystal structure analysis of meth-
yl-substituted trapping product 5b (thermal el-
lipsoids are shown with 30% probability).

Physical Organic chemistry
CHIMIA 2016, 70, No. 3 175
Table 3. Photophysical data of trapping products 5a–5l and 5n.
a
a
a
λabs
[nm]
λexc
[nm]
λem
[nm]
τ
Φ
[%]
λexc
[nm]
λem
[nm]
τ
λexc
[nm]
λem
[nm]
τ
Φ
[%]
[ns]
[ns]
[ns]
room temperature
77 K
415
426
378
378
433
442
423
423
438
414
406
452
444
solid state
5a
5b
5c
5d
5e
5f
257
255
256
256
258
259
258
264
255
268
341
259
255
–
–
–
–
–
–
–
–
–
–
–
–
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
0.9
300
300
270
300
273
348
277
277
285
336
270
275
269
-
–
–
–
–
–
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
<0.5
2.6
3.825
4.796
0.180
33.237
29.067
9.574
16.224
22.395
12.595
3.550
17.485
17.427
–
–
–
–
–
–
–
–
–
–
–
–
–
278
–
473
–
0.104
–
–
–
.
5g
5h
5i
–
–
–
269
250
277
261
341
–
377
486
442
404
420
–
2.110
0.590
0.395
0.091
0.820
–
294
300
324
285
295
–
441
434
409
423
415
–
2.0523
2.448
1.984
0.097
2.488
–
1.4
13.7
13.0
1.1
5j
<0.5
<0.5
<0.5
<0.5
5k
5l
12.9
<0.5
5n
aIntensity weighted average lifetime
Fig. 6. Absorption
(top) and emission
(bottom) spectra for
the trapping products
at room temperature
in acetonitrile
Table 4. Calculated parameters in vacuum
obtained from TDDFT/PBE0/6-31G*
calculations: λem is the 0-0 emission
wavelength in nm and f is the oscillator
strength for the vertical transition from the S1
minimum.
(10–5 M).
f
λem
360
346
436
428
447
396
5b
5e
5f
0.0084
0.0106
0.0232
0.0656
0.0556
0.0710
5h
5i
5j
solvents. 5i and 5j on the other hand are
less symmetric and exhibit a more polar
excited state compared with the ground
state (more push-pull character), thus lead-
ing to red-shifted emission in solution.
In addition to the static TDDFT
cal-
culations, we have also performed excited
state Car-Parrinello molecular dynamics
simulations for species 5a, 5b, and 5f to
investigate their stability. Although the
simulations were carried out at an elevated
temperature to account for the energy re-
lease during geometric relaxation from the
Franck-Condon region to the S minimum,
there are clear indications that1 these mol-
ecules are prone to fragmentation due to
C–N homolysis and formation of a styryl
radical. This is experimentally corrobo-
rated by the study of compound 5e, which
derivative 5h probably has a ground state slowly decomposes to verdazyl radical 4e
with higher polarity than its excited state, in the presence of light.
shifted, thus falling between 5i and 5j in
fluid solution. This can be attributed to the
leading to a blue-shifted emission in polar
fact that the threefold methoxy-substituted

176 CHIMIA 2016, 70, No. 3
Physical Organic chemistry
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deprotected acety-
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O
O
Ph
Ph
Ph
Ph
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In summary, we presented the synthesis
and characterization of various substituted
1,5-diphenyl-6-oxo verdazyl radicals and
their use as potential fluorescent spin sen-
sors. As pointed out, the C(3)-substituent
clearly is substantial for the profluores-
cent behavior. Spectroscopic measure-
ments demonstrate the significance of the
aryl substituent at the C(3)-position for
room temperature fluorescence, as it en-
hances the delocalization of the LUMO.
Interestingly, the emission wavelength can
be tuned by selective destabilization of the
HOMO and the LUMO.
Acknowledgements
Financial support for this project was pro-
vided by the Deutsche Forschungsgemeinschaft
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