Toward Benzobis(thiadiazole)-based Diradicaloids

Article abstract of DOI:10.1002/asia.201700732

We theoretically predicted that acetylene-bridged benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BBT) oligomers would show a quick increase of diradical character with the extension of chain length. To validate the hypothesis, six stable BBT-based diradicaloids were synthesized and fully characterized by X-ray crystallographic analysis and various spectroscopic measurements. Three of them showed prominent paramagnetic activity at elevated temperatures due to thermal population from the open-shell singlet ground state to triplet excited state. It was also found that substitution by electron-donating triphenylamine groups at the termini promoted the diradical character and reduced the singlet–triplet energy gap, and at the same time, resulted in intense near-infrared absorption.

Full text of DOI:10.1002/asia.201700732

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Toward Benzobis(thiadiazole)-based Diradicaloids
Authors: Jishan Wu, Yi Liu, Hoa Phan, Tun Seng Herng, Tullimilli Y.
Gopalakrishna, and Jun Ding
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To be cited as: Chem. Asian J. 10.1002/asia.201700732
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Chemistry - An Asian Journal
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COMMUNICATION
Toward Benzobis(thiadiazole)-based Diradicaloids
[a]
[a]
[b]
[a]
[b]
[a]
Yi Liu, Hoa Phan, Tun Seng Herng, Tullimilli Y. Gopalakrishna, Jun Ding, and Jishan Wu*
Abstract: We theoretically predicted that acetylene-bridged
benzo[1,2-c;4,5-c’]bis[1,2,5]thiadiazole (BBT) oligomers would show
a quick increase of diradical character with extension of chain length.
To validate the hypothesis, six stable BBT-based diradicaloids were
synthesized and fully characterized by X-ray crystallographic
analysis and various spectroscopic measurements. Three of them
showed prominent paramagnetic activity at elevated temperatures
due to thermal population from the open-shell singlet ground state to
triplet excited state. It was also found that substitution by electron-
donating triphenylamine groups at the termini promoted the diradical
character and reduced the singlet-triplet energy gap, and at the
same time, resulted in intense near-infrared absorption.
magnetic measurements difficult. Wudl et al first reported the
experimental observation of paramagnetic activity of BBT-based
donor-acceptor polymers and ascribed the magnetic property to
the intrinsic diradical character of BBT. To deep understand
the origins, oligomer approach is more desirable. The electron-
deficiency also implies that BBT-based compounds will have a
low-lying HOMO and thus are stable. Therefore, BBT-based
diradicaloids could be stable functional materials.
1
1
Recently, π-conjugated molecules with an open-shell singlet
diradical ground state have attracted a lot of attention due to
their unique electronic, optical and magnetic properties, and
1
promising applications in materials science. Most of the
currently investigated systems are based on pro-aromatic or
anti-aromatic π-conjugated polycyclic hydrocarbons (PHs), and
these studies have revealed that the number of aromatic sextet
1
1
1
8
6
4
2
0
4
2
0
yo
ES-T
1.0
0.8
0.6
0.4
2
3
ring, strain release and spatial distribution of frontier molecular
4
orbitals all played important roles in determining the diradical
character and singlet-triplet energy gap. By applying these
5
fundamental rules, even polyradicaloids can be synthesized.
However, a general problem for the PH-based diradicaloids and
polyradicaloids is their intrinsic high reactivity. Thus, chemists
also looked for heteroatom-containing open-shell singlet
0.2
0.0
6
7
diradicaloids. Indeed, oligothiophene,
thienoacene
and
1
2
3
4
5
6
8
n
porphyrinoid
based
diradicaloids
have demonstrated
reasonably good stability. Herein, we would like to exploit the
potential diradical character of another hetero-aromatic molecule,
benzo[1,2-c;4,5-c’]bis[1,2,5]thiadiazole (BBT) (Figure 1a).
Figure 1. (a) Resonance structures of BBT and acetylene-bridged BBT
oligomers BBT-n; (b) chain length dependence of the calculated diradical
character (y ) and singlet-triplet energy gap (ΔES-T) of BBT-n (n=1-6).
0
BBT is a well-known building block for the construction of
organic conductors, donor-acceptor type low band gap polymers,
and near-infrared (NIR) dyes due to their strong electron-
In this context, we first calculated the diradical characters (y
0
)
and the tetraradical characters (y ) of a series of acetylene-
1
9
bridged BBT oligomers (BBT-n) (Figure 1a). Our spin-
unrestricted calculations (UCAM-B3LYP/6-31G(d,p)) clearly
predict that with increase of repeat unit number of BBT (n), the
diradical character index (y ) quickly increases from 0.025 for
o
BBT-1, 0.30 for BBT-2, 0.63 for BBT-3, 0.82 for BBT-4, 0.92 for
BBT-5, to 0.96 for BBT-6 (Figure 1b and Supporting Information
SI)), which can be simply explained by the recovery of one
more aromatic thiadiazole sextet ring in each BBT unit in the
diradical form. However, the calculated tetraradical characters
withdrawing nature. However, another feature of this unique
molecule, that is, its diradical character, was not well
investigated. BBT can be drawn in a closed-shell form with a
hypervalent sulfur atom and an open-shell diradical form with
one additional aromatic thiadiazole ring (the pentagon shaded in
blue colour, Figure 1a). Therefore, BBT in principle has intrinsic
tendency to be diradicals. Such simple analysis was further
supported by theoretical calculations that BBT derivatives with
(
10
various substituents could show tunable diradical character.
(
y
1
) are negligible for all BBT oligomers (Table S1 in SI). At the
However, experimentally, these hypothesises were not well
validated because the target compounds usually have a small
diradical character and a large singlet-triplet energy gap, making
same time, the singlet-triplet energy gap (ΔES-T rapidly
)
decreases from 12.36 kcal/mol for BBT-1, 6.44 kcal/mol for
BBT-2, 3.35 kcal/mol for BBT-3, 2.24 kcal/mol for BBT-4, 1.65
kcal/mol for BBT-5, to 1.37 kcal/mol for BBT-6 (Figure 1b). This
interesting theoretical result inspired us to synthesize their
derivatives and analogues, and experimentally probe their
diradical character and magnetic activity. To make soluble
compounds, 3,5-di-tert-butylphenyl groups are attached to the
termini of BBT units, and the BBT monomer BBT-BP-1 and
dimer BBT-BP-2 were successfully synthesized (Scheme 1).
During the synthesis, the directly linked BBT dimer BBT-BP-3
was occasionally obtained. To investigate the effect of
substituent, electron-donating triphenylamine (TPA) terminated
[
a]
Y. Liu, Dr. H. Phan, Dr. T. Y. Gopalakrishna, Prof. J. Wu
Department of Chemistry, National University of Singapore
3
Science Drive 3, 117543, Singapore
E-mail: chmwuj@nus.edu.sg
[b]
Dr. T. S. Herng, Prof. J. Ding
Department of Materials Science and Engineering, National
University of Singapore, 119260, Singapore
Supporting information for this article is given via a link at the end of
the document.
1
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COMMUNICATION
1
2
analogues BBT-TPA-1 – BBT-TPA-3 were also synthesized
(Figure 2). Both BBT-BP-1 and BBT-BP-2 form a closely
stacked dimeric structure, with a face-to-face distance of about
3.634 Å and 3.341 Å, respectively. A number of [S•••N] short
(
Scheme 1). These molecules are supposed to be NIR dyes due
to intramolecular charge transfer.
contacts (2.977
neighboring BBT-BP-2 molecules lying in the same plane
Figure 2b). Interestingly, the backbone of BBT-BP-2 is bended
~ 3.221 Å) are observed between two
(
and approaches to each other in the stacked dimer (Figure 2b)
presumably due to the strong intermolecular spin-spin
interactions as well as the steric repulsion between the bulky
3,5-di-tert-butylphenyl substituents. The dihedral angle between
the two BBT planes is very small, indicating a good π-
conjugation between the two BBT units. BBT-TPA-3 adopts a
o
twisted structure with a large distortional angle of about 56 , and
there are neither close π-stacking between the molecules nor
regular [S•••N] close contacts (Figure 2c). Since the highest spin
density is expected to locate on the C4 and C8 sites of the BBT
unit (Figure 1a), bond length analysis of their neighboring C-C
bonds will be valuable for evaluating the relative contribution of
the diradical form. In BBT-BP-1, the bonds a and b are 1.413 Å
2
2
and 1.404 Å, shorter than a typical C(sp )-C(sp ) bond (~1.47 Å),
but much longer than a typical double bond (1.33-1.34 Å),
suggesting that the bonds a and b have a certain single bond
character and that the thiadiazole ring partially recovers its
aromaticity. In BBT-BP-2, the corresponding bonds, d and e
(
1.420 Å and 1.424 Å) are significantly longer than a and b,
indicating larger diradical contribution, which is further
a
supported by a shortened bond f (1.446 Å) compared to bond c
Scheme 1. Synthesis of two series of BBT derivatives: (a) Pd(PPh
3
)
)
2
Cl
Cl
2
,
,
(1.457 Å). The length of the central C(sp)-C(sp) triple bond
o
toluene, 80-100 C; (b) bis(tributylstannyl)acetylene (0.5 equiv), Pd(PPh
3
2
2
(
1.212 Å) is longer than that in the diphenylacetylene (1.205 Å),
o
toluene, 90-100 C.
in accordance with the cumulenic contribution in the diradical
form (Figure 1a). In BBT-TPA-3, the bond length pattern on BBT
is comparable with BBT-BP-1, but the bond between the BBT
units is very long (1.475 Å) due to large strain. FR-IR
measurement (Figure S1 in SI) show that the vibrational band of
the C-C triple bond is shifted to lower frequency from BBT-BP-2
(2153 cm ) to BBT-TPA-2 (2126 cm ), indicating a larger
diradical character for the latter.
No significant ESR signals were observed for compounds
Compound BBT-BP-1 was simply synthesized in 78% yield by
Pd-catalysed Stille coupling reaction between the commercially
available dibromo- BBT 1 and 2.2 equiv tributyl(3,5-di-tert-
butylphenyl)stannane 2 (Scheme 1). When the ratio of 2 to 1
was set to 6:5, the monobromo- BBT intermediate 3 was
isolated in 26% yield. Stille coupling reaction of 3 with 0.5 equiv
bis(tributylstannyl)acetylene afforded the acetylene-bridged BBT
dimer BBT-BP-2 in 19% yield. Prolonging the reaction time (24
-1
-1
h) resulted in the unexpected BBT dimer BBT-BP-3 in 10% yield. BBT-BP-1, BBT-BP-3, and BBT-TPA-1 at room temperature,
Since no reductive species is involved in the reaction, we
suppose that the formation of BBT-BP-3 is due to the
debromination of 2 followed by homo-coupling of the as-
generated radicals, which is reasonable considering of the
intrinsic radical character of BBT. Both compounds BBT-BP-2
and BBT-BP-3 are reasonably stable and can be purified by
silica gel column chromatography in air. However, attempted
synthesis of longer acetylene-bridged BBT oligomers suffered
from their inherent instability associated with their larger
diradical character (Scheme S1 in SI). The TPA-substituted BBT
analogues BBT-TPA-1 – BBT-TPA-3 were synthesized by using
suggesting that they have a small diradical character. Indeed,
DFT calculations predicted negligible diradical radical
character (y < 10%) and a large singlet-triplet gap for these
three molecules (Table 1 and Table S1 in SI). Broad ESR
signals for the solid sample were observed for BBT-TPA-2 and
a
0
e
BBT-TPA-3 with g value of 2.0038, 2.0039. Similar calculations
predict that BBT-TPA-2 and BBT-TPA-3 have a diradical
character of 35.5% and 11.4%, respectively. In both cases, the
spins are delocalized throughout the whole π-conjugated
skeleton (Figure 3c), which explains the broad feature of the
ESR spectrum. Superconducting quantum interference device
(SQUID) measurements were also performed on the freshly
prepared powder samples and it is found that both BBT-TPA-2
and BBT-TPA-3 have a singlet ground state which can be
thermally populated to the paramagnetic triplet excited state.
Fitting of the data by Bleaney–Bowers equation gave a singlet-
a
similar
strategy
starting
from
4-(N,N-di-(4-
methoxy)phenyl)aminophenyl-tributylstannane 4. By controlling
the ratio of 4 to 1, BBT-TPA-1 and the intermediate 5 were
obtained in 30% and 13% yield, respectively. Interestingly,
during the synthesis of 5, prolonging the reaction time from 12 h
to 24 h gave the homo-coupled product BBT-TPA-3 in 54% yield, triplet gap (ΔES-T) of -3.2 and -3.66 kcal/mol for BBT-TPA-2 and
implying larger radical character of compared to 3.
BBT-TPA-3, respectively. Compound BBT-BP-2 was
theoretically calculated to have a moderate diradical character
(y = 30.4%) which is slightly smaller than its counterpart BBT-
a
5
Compound BBT-TPA-2 were obtained in 26% yield by Stille
coupling between 5 and bis(tributylstannyl)acetylene.
0
Single crystals suitable for X-ray crystallographic analysis
were obtained for BBT-BP-1, BBT-BP-2 and BBT-TPA-3
TPA-2. However, the solid sample of BBT-BP-2 only showed
very weak ESR and SQUID signals at room temperature. Such
2
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COMMUNICATION
Figure 2. X-ray structures, molecular packing and selected bond lengths (in Å) for BBT-BP-1 (a), BBT-BP-2 (b) and BBT-TPA-3 (c). Hydrogen atoms are omitted
for clarity.
The UV-vis-NIR absorption spectra of all compounds were
recorded in DCM solution (Figure 4a and Table 1). Compound
BBT-BP-1 shows a broad absorption with λmax = 572 nm (εmax
=
-1
-1
1
4200 M cm ). The absorption spectrum of BBT-BP-2 exhibits
a similar band structure but is significantly red-shifted (λmax
=
1
0
8
6
4
2
0
BBT-BP-1
BBT-BP-2
BBT-BP-3
BBT-TPA-1
BBT-TPA-2
BBT-TPA-3
3200
3250
3300
3350
3260
3280
Field (G)
3300
3320
Field (G)

4
00
600
800
1000
1200
Wavelength (nm)
BBT-BP-1
BBT-BP-2
Figure 3. (a) ESR spectra of BBT-TPA-2 and BBT-TPA-3 in solid at room
temperature. (b) χ T-T curves in the SQUID measurements for BBT-TPA-2
BBT-BP-3
M
and BBT-TPA-3. (c) Calculated spin density distribution of the singlet
biradicals of BBT-TPA-2 and BBT-TPA-3.
BBT-TPA-1
BBT-TPA-2
phenomenon could be ascribed to strong intermolecular anti-
ferromagnetic coupling via π-π interaction and short [S•••N]
contacts as revealed by X-ray crystallographic analysis. Heating
the solid sample to higher temperatures (from 323 K to 410 K)
led to continuous increase of ESR signal and fitting the data by
Bleaney-Bows equation gave an estimated singlet-triplet gap of -
BBT-TPA-3
12.0 kcal/mol (Figure S2 in SI), which can be correlated to an
overall effect and is much larger than that in BBT-TPA-2. By
comparison this two series of compounds, it is clear that
extension of π-conjugation length or introduction of electron-
donating TPA groups both lead to increased diradical character
and decreased singlet-triplet gap.
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
+
Potential (V) vs Fc /Fc
Figure 4. (a) UV-vis-NIR absorption spectra of BBT-BP-1 – BBT-BP-3 and
BBT-TPA-1 – BBT-TPA-3 in DCM; (b) cyclic voltammograms of BBT-BP-1 –
BBT-BP-3 and BBT-TPA-1 – BBT-TPA-3 in DCM.
3
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COMMUNICATION
a
0
Table 1. Optical and electrochemical data, calculated diradical character (y ) and measured singlet-triplet gap (ΔES-T) of the BBT derivatives.
ε
(M
cm )
max
-1
ox1
1/2
ox2
red1
1/2
red2
1/2
red3
1/2
red4
1/2
EC
opt
λ
max
E
E
1/2
E
E
E
E
HOMO
(eV)
LUMO
(eV)
E
g
E
g
ΔES-T
(kcal/mol)
Comp.
y
0
(nm)
-1
(eV)
(eV)
(eV)
(eV)
(eV)
(eV)
(eV)
(eV)
BBT-BP-1
BBT-BP-2
BBT-BP-3
BBT-TPA-1
BBT-TPA-2
BBT-TPA-3
a
572
700
587
754
892
754
14200
26900
25700
22300
41700
42800
0.92
0.82
0.99
0.24
0.18
0.13
1.23
1.00
-
0.90
0.98
0.79
-1.24
-0.88
-1.09
-1.08
-0.94
-1.20
-
-
-
-5.70
-5.64
-5.74
-4.98
-4.92
-4.83
-3.57
-4.08
-3.86
-3.78
-3.95
-3.69
1.95
1.56
1.88
1.20
0.97
1.14
1.90
1.57
1.83
1.36
1.11
1.38
0.026
0.304
0.081
0.050
0.355
0.114
-
-1.15
-1.27
-1.26
-1.17
-1.39
-1.83
-
-1.96
-1.80
-2.06
-2.01
-
-2.19
-2.05
-2.25
onset
-12.0
-
-
-3.20
-3.66
onset
onset
onset
HOMO and LUMO levels were esimated according to the equations: HOMO=-(4.8+Eox
) eV and LUMO=-(4.8+Ered
) eV, where Eox
and Ered
are the
+
Ec
g
opt
g
onset potential (vs Fc /Fc) of the first oxidative and reductive redox wave, respectively. E
is electrochemical energy gap calculated from LUMO-HOMO. E
is
optical energy gap derived from lowest energy absorption onset in the electronic absorption spectra.
-
1
-1
7
00 nm, εmax = 26900 M cm ) due to more extended π-
J. W. acknowledges financial support from the MOE Tier 3
programme (MOE2014-T3-1-004) and MOE Tier grant
(MOE2014-T2-1-080). We thank Dr. Tan Geok Kheng for her
help on X-ray crystallographic analysis.
2
conjugation. However, BBT-BP-3 just displays a slight red shift
(λmax = 587 nm, εmax = 25700 M cm ) due to its twisted structure.
-
1
-1
With the incorporation of electron-donating groups, the
absorption spectra of BBT-TPA-1, BBT-TPA-2 and BBT-TPA-3
are dramatically red shifted into the NIR region, with λmax
apearing at 754, 892 and 754 nm respectively, owing to
intramolecular charge transfer characcter. At the same time, the
intensity increases.
Cyclic voltammetry measurements were also performed for
the two series of BBT derivatives in anhydrous DCM and they all
showed amphoteric redox behavior (Figure 4b and Table 1). For
BBT-BP-1, two quasi-reversible oxidation waves and one
reversible reduction wave were observed. Passing to BBT-BP-2,
however, four reversible reduction waves were observed,
indicating strong electronic interactions between the BBT units.
The LUMO level is also dramtically decreased from -3.57 eV for
BBT-BP-1 to -4.08 eV for BBT-BP-2 while the HOMO is slightly
lifted up. For the twisted dimer BBT-BP-3, two reversible
reduction waves can be determined, implying a moderate
coupling between two BBT units, and the LUMO (-3.86 eV) lies
between those of BBT-BP-1 and BBT-BP-2. The TPA-
Keywords: diradicalod • benzobis(thiaziazole) • donor-acceptor
• near infrared dye • magnetic property
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substituted compounds BBT-TPA-1
– BBT-TPA-3 exhibit
significantly lowed first oxidation potentials correlated to the
oxidation of the TPA unit. Three (quasi)reversible reduction
waves are observed for BBT-TPA-1, and its LUMO level (-3.78
eV) is lower than that BBT-BP-1, presumably due to its larger
diradcial character. Compounds BBT-TPA-2 and BBT-TPA-3
show four and three reversible reduction waves, respectively,
and the LUMO level is slightly lifted up due to the electron-
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donating effect of TPA. Due to the intramolecular charge transfer,
EC
the electrochemical energy gaps (E
g
) of BBT-TPA-1 – BBT-
138, 13049.
TPA-3 are significantly smaller than the corresponding
[
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Chem. Int. Ed. 2007, 46, 9057.
compounds BBT-BP-1 – BBT-BP-3, in accordance with the
Opt
measured optical energy gaps (E
g
) (Table 1).
In summary, we have theoretically and experimentally
deomonstrated that BBT could be used as a useful building
block for construction of stable open-shell diradcialoids. It is
found that by increasing the π-conjugation length or introduction
of electron-donating groups to the termini, the diradical character
increases, leading to prominent paramagnetic acitivity and small
energy gap. The intensive NIR absorption of the TPA-substituted
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Chemistry - An Asian Journal
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[
2
COMMUNICATION
BBT-based diradicaloids! We
theoretically and experimentally
investigated the potential of using BBT
as a general building block for stable
open-shell singlet diradicaloids.
Extension of π-conjugation and
substitution by electron-donating
groups result in an increase of
diradical character and decrease of
singlet-triplet energy gap.
Y. Liu, H. Phan, T. S. Herng, T. Y.
Gopalakrishna, J. Ding, and J. Wu*
Page No. – Page No.
Toward Benzobis(thiadiazole)-based
Diradicaloids
5
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