Tuning the Optical Properties of Sulfonylaniline Derivatives: Degeneracy Breaking of Benzene Orbitals and Linkage through Nodal Planes

Article abstract of DOI:10.1002/cphc.201900135

The orbital degeneracy of benzene rings is resolved by an asymmetric push-pull system in 2,6-bis(methylsulfonyl)aniline (BMeSA), in which the highest occupied molecular orbital (HOMO) is located at the 4-position, while the lowest unoccupied molecular orbital (LUMO) is located at a different position and has a nodal plane through the carbon atoms at the 1- and 4-positions. Therefore, the π-extension of BMeSA at the 4-position reveals a strong overlap in the HOMO and a minimal overlap in the LUMO. Consequently, π-extended BMeSA derivatives exhibit longer absorbance and emission wavelengths in the order of the electron-donating abilities of their substituents at the 4-position, which is based on a decrease in an absolute HOMO-level-dependent HOMO-LUMO gap in accordance with the nodal arrangement. Positive fluorescent solvatochromism with polarity-dependent decrease in fluorescent intensity was also observed. The biaryls exhibited more planar geometries in the excited state than in the ground state. The charge transfer mechanism, which can be described as node-induced intramolecular charge transfer (NICT), differs from the planar intramolecular charge transfer (PICT) and twisted intramolecular charge transfer (TICT).

Full text of DOI:10.1002/cphc.201900135

Accepted Article
Title: Tuning the Optical Properties of Sulfonylaniline Derivatives:
Degeneracy Breaking of Benzene Orbitals and Linkage Through
Nodal Planes
Authors: Shoh Kudo, Nanami Hoshino, Teruo Beppu, and Hiroshi
Katagiri
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10.1002/cphc.201900135
ChemPhysChem
ARTICLE
Tuning the Optical Properties of Sulfonylaniline Derivatives:
Degeneracy Breaking of Benzene Orbitals and Linkage Through
Nodal Planes
Shoh Kudo, Nanami Hoshino, Teruo Beppu, and Hiroshi Katagiri*[a]
Abstract: The orbital degeneracy of benzene rings is resolved by an
asymmetric push-pull system in 2,6-bis(methylsulfonyl)aniline
(BMeSA), in which the highest occupied molecular orbital (HOMO) is
located at the 4-position, while the lowest unoccupied molecular
orbital (LUMO) is located at a different position and has a nodal plane
through the carbon atoms at the 1- and 4-positions. Therefore, the π-
extension of BMeSA at the 4-position reveals a strong overlap in the
HOMO and a minimal overlap in the LUMO. Consequently, π-
Figure 1. Representation of the two types of nodal planes on the two molecular
orbitals of benzene (π2 and π3) which have the same energy and are
extended BMeSA derivatives exhibit longer absorbance and emission
wavelengths in the order of the electron-donating abilities of their
substituents at the 4-position, which is based on a decrease in an
absolute HOMO-level-dependent HOMO-LUMO gap in accordance
with the nodal arrangement. Positive fluorescent solvatochromism
with polarity-dependent decrease in fluorescent intensity was also
observed. The biaryls exhibited more planar geometries in the excited
state than in the ground state. The charge transfer mechanism, which
can be described as node-induced intramolecular charge transfer
(NICT), differs from the planar intramolecular charge transfer (PICT)
and twisted intramolecular charge transfer (TICT).
degenerate.
The nodal plane, where the probability of finding an electron is
zero, is divided into two types: the nodal plane through bonds and
the nodal plane through atoms (Figure 1). The latter is particularly
important for π-conjugated molecules because the linkage
through the nodal plane enables the control of optical properties
via overlapping or non-overlapping of the frontier orbitals in π-
conjugation systems. Simple hydrocarbons such as pyrene,
fluorene, s-indacene, and azulene have potentially useful
architectures; their nodal planes, which cut through atoms with
free sites, are available for chemical modification. For example,
pyrene oligomers with a linkage through their nodal planes, which
serve as individual radical ion building blocks for organic magnetic
materials;[8] the control of charge recombination in dye-sensitized
solar cells using the orbital symmetries and nodal planes of
azulene, pyrene, and fluorene derivatives;[9] terazulenes, for
controlling the charge carrier polarity in organic transistors
through controlling the molecular orbital distribution;[10,11] and fine-
tuning the HOMO and LUMO energies of pyrene derivatives
through modifications at the free sites on the nodal plane.[12-15]
Several recent studies have focused on frontier orbital overlaps
and their nodal arrangements, which are important for promoting
large-scale electronic coupling in macromolecules and for self-
assembly.[16-19] Conversely, ordinary symmetrical aromatics such
as benzene have no nodal plane through their atoms, because
orbitals with the same energy are degenerate. Therefore, an
asymmetric arrangement of electron donors or acceptors is
necessary to disrupt the orbital degeneracy of benzene to
generate a nodal plane. In this study, based on our previous
concept of a push–pull system between amino and sulfonyl
moieties,[20,21] 2,6-bis(methylsulfonyl)aniline (BMeSA) exhibits an
asymmetric HOMO-LUMO distribution (Figure 2a), in which the
HOMO is located at the 1- and 4-positions, while the LUMO, which
is located at a different position, instead, generates a nodal plane
through the atoms at the 1- and 4-positions. BMeSA-Ar 1a-d
(Figure 2b), which are π-extended derivatives at the 4-position of
Introduction
π-Conjugated small molecules have been in the limelight of
organic optoelectronic materials research for years now, because
of their potential biosensing[1,2] and optoelectronic applications.[3-
6]
Various combinations of aromatic skeletons and substituent
effects can be adopted in the construction of π-conjugation
systems. However, a rational paradigm for their molecular design
is still lacking; therefore, new concepts need to be developed.
Since conversancy with the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) is
essential in understanding the optical properties of molecules,
controlling their energy levels and distributions will be of
significance in designing π-conjugated molecules in the future.[7]
[a]
S. Kudo, N. Hoshino, T. Beppu, Prof. Dr. H. Katagiri
Graduate School of Science and Engineering
Yamagata University
4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
E-mail: kgri7078@yz.yamagata-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/xxxxx. It contains NMR and HRMS spectra of novel
compounds, peak separation data, X-ray crystallographic data, and
DFT calculation details.
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10.1002/cphc.201900135
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BMeSA, exhibit HOMO-dependent tuning of the optical properties, Results and Discussion
a large dipole moment in the excited state, and positive
solvatochromism in fluorescence spectra with a large Stokes shift
based on asymmetric molecular orbital distribution and linkage
through the nodal plane.
Preparation of BMeSA and its π-extended derivatives,
BMeSA-Ar(1a-d).
BMeSA and its π-extended derivatives,
BMeSA-Ar(1a-d), were prepared through the synthetic route
shown in Scheme 1. BMeSA was synthesized in three steps from
2,6-difluoronitrobenzene, 2: a nucleophilic aromatic substitution of
the methanethiolate on
2
afforded sulfide, 3; catalytic
hydrogenation with palladium on carbon reduced the nitro group;
subsequent oxidation with mCPBA afforded good yields of
BMeSA. Notably, the bromination of BMeSA using N-
bromosuccinimide (NBS) proceeded selectively and afforded the
4-bromo derivative, 5, in quantitative yield. Subsequently,
Suzuki−Miyaura cross-coupling of 5 with the corresponding
boronic acid esters, 6a-d, afforded the desirable BMeSA-Ar (1a-
d) in moderate to good yields. The full structures of the final
BMeSA and BMeSA-Ar (1a-d) compounds were determined by
single-crystal X-ray diffraction. The details of the syntheses and
characterizations of the novel compounds are described in
Figures S1-8 in Supplementary Data.
Figure 2. (a) Schematic representation of 2,6-bis(methylsulfonyl)aniline
(BMeSA) with the asymmetric frontier orbitals and nodal plane through atoms.
(b) BMeSA-Ar: π-extended derivatives at the 4-position of BMeSA.
Scheme 1. Synthetic route to BMeSA and its π-extended derivatives, BMeSA-Ar.
Theoretical calculations on BMeSA and BMeSA-Ar(1a-d).
Density functional theory (DFT) and time-dependent density
functional theory (TDDFT) calculations were performed on
BMeSA and BMeSA-Ar (1a-d) using the polarizable continuum
model (PCM) to investigate the frontier molecular orbital
distributions, orbital energies, absorption spectra, and
fluorescent spectra, in dichloromethane. The geometry
optimizations were performed using the respective crystal
structures as the initial geometries, and the resulting
conformations for all molecules were in good agreement with
those of the crystal structures (except for 1d, because its
methylsulfonyl groups assumed syn conformation in single
crystals and anti conformation was adopted in the calculation,
which was the same for other molecules).
all the molecules, the LUMO was located in the BMeSA moiety
without orbital overlap; the HOMO of all the molecules were
distributed over the entire molecule with a large orbital overlap
between the 1- and 4-positions. Moreover, the HOMO energies
drastically increased in the order of BMeSA (–6.57 eV) < 1a (–
6.22 eV) < 1b (–5.60 eV) < 1c (–5.29 eV) < 1d (–5.10 eV) relative
to the electron-donating abilities of the aryl groups, whereas the
LUMO energies were nearly the same in these molecules.
Accordingly, the HOMO-LUMO energy gaps decreased in the
order of BMeSA > 1a > 1b > 1c > 1d. The interplanar angles of
the biphenyl groups of 1a-d are in the range of 143.0°-146.6°, all
of which are acceptable values for the orbital overlap between
two benzene rings. The results of these calculations suggest the
potential for controlling the HOMO-level-dependent energy gap
based on the strong orbital overlap in the HOMO and the minimal
orbital overlap in the LUMO.
As shown in Figure 3, the molecular orbital distribution of the
LUMO included the nodal plane through the atoms at the 1- and
4-positions of the BMeSA moiety in all molecules. Therefore, in
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10.1002/cphc.201900135
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Figure 3. Optimized geometry, frontier molecular orbitals, and orbital energies of BMeSA and BMeSA-Ar (1a-d) in the ground state. Dichloromethane was used as
the solvent for calculations (polarizable continuum model) at the B3LYP/6-31+G(d,p) level.
Table 1. Calculated absorption and fluorescence properties of BMeSA and BMeSA-Ar (1a-d) in dichloromethane.
Absorption[a]
Fluorescence[b]
λmaxem [nm]
342
Oscillator
strength
λmaxabs [nm]
Oscillator strength
Contribution[c]
Contribution[c]
BMeSA
1a
306
244
332
283
383
314
416
337
441
356
0.151
0.113
0.097
0.666
0.038
0.66
H→L (0.69)
0.143
0.098
0.038
0.031
0.019
L→H (0.70)
H→L+1 (0.68)
H→L (0.70)
366
535
583
688
L→H (0.68)
L→H (0.68)
L→H (0.67)
L→H (0.68)
H→L+1 (0.70)
H→L (0.70)
1b
H→L+1 (0.70)
H→L (0.70)
1c
0.022
0.683
0.014
0.600
H→L+1 (0.70)
H→L (0.70)
1d
H→L+1 (0.68)
[a] Performed using the linear-response (LR) PCM method at the B3LYP/6-31+G(d,p) level. [b] Performed using the state-specific (SS) PCM method at the
ωB97XD/6-31+G(d,p) level. [c] H: HOMO, L: LUMO. The contribution of each pair of molecular orbitals is given in parentheses.
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As shown in Table 1, the wavelengths of the absorption and
fluorescence spectra simulated by TD-DFT calculations are in
the order of BMeSA < 1a < 1b < 1c < 1d, along with the HOMO-
LUMO energy gaps. The main absorption spectra of BMeSA and
BMeSA-Ar (1a-d) consisted of two excited states: the first lowest
energy corresponding to the HOMO-LUMO transition, and the
second lowest energy corresponding to the HOMO-LUMO+1
transition. In BMeSA-Ar (1a-d), the oscillator strengths of the
HOMO-LUMO transitions were markedly smaller than those of
the HOMO-LUMO+1 transitions. Theoretically, a small HOMO–
LUMO overlap leads to low oscillator strength. In fact, the
HOMO-LUMO transitions of all the BMeSA-Ar compounds
exhibited small orbitals overlaps in the order of 1d < 1c < 1b <
1a, owing to the electron-donating abilities of the aryl groups and
the linkages through the nodal planes. In contrast, the HOMO-
LUMO+1 transitions exhibited large oscillator strengths due to
the large orbitals overlappings of HOMO-LUMO+1 in the
BMeSA-Ar compounds. The calculated oscillator strengths
decreased with the electron-donating abilities of the aryl groups
in both absorbance and fluorescence. These results were in
agreement with the experimental measurements (see below).
HOMO-LUMO transition and the second excited state
corresponding to the HOMO-LUMO+1 transition. The
absorbance at the second excited state was, in fact, more
drastically red-shifted than that at the first excited state, as a
result of which these two peaks overlapped in 1b-d (Figure S9:
see the peak separations in Supporting Information). Basically,
importantly, the small shift in the HOMO-LUMO transition plays
a key role in the large Stokes shift of 188 nm in 1d. The
absorption extinction coefficients and fluorescent quantum yields
decreased with the electron-donating ability of the aryl group. As
described in the theoretical calculation part, small HOMO–LUMO
overlaps lead to small transition dipole moments on the part of
the first excited states in BMeSA-Ar (1a-d).
Absorbance, fluorescence, and electrochemical properties
of BMeSA and BMeSA-Ar (1a-d) in dichloromethane:
Demonstration of the decrease in the absolute HOMO-level-
dependent HOMO-LUMO gap, according to the nodal
arrangement. The experimentally observed absorption and
fluorescence spectra supported our initial theoretical calculations,
in that the longest-wavelength absorption and fluorescence
maxima red-shifted in the order of BMeSA < 1a < 1b < 1c < 1d
(Table 2 and Figure 4). The Hammett substituent constants, σ,
of the substituents on the aryl groups are –H (σ = 0.00) < –NH2
(σ = −0.66) < –NMe2 (σ = −0.83),[23] and the julolidyl group is
more electron-rich than –NMe2.[24] These red-shifts are, therefore,
an effect of the decreased HOMO-LUMO gap, along with the
increased HOMO energies owing to the donation of electrons by
the substituents. The absorption spectrum of all the molecules
showed two peaks: the first excited state corresponding to the
Table 2. Absorption and fluorescence properties of BMeSA and BMeSA-
Ar (1a-d) in dichloromethane.
λabs / nm
ε / M−1 cm−1
λem / nm[a]
381
Φ[b]
Figure 4. Absorption (a) and fluorescence (b) spectra of BMeSA and BMeSA-
Ar (1a-d) in dichloromethane.
BMeSA
1a
253, 324
279, 340
300, 360[c]
321, 378[c]
336, 401[c]
8211, 5527
0.43
0.12
0.07
0.07
0.03
22287, 4457
26285, 3298[c]
28106, 2801[c]
20042, 1904[c]
409
The cyclic voltammograms of BMeSA and BMeSA-Ar (1a-d) are
shown in Figure 5. The HOMO energy levels of these molecules
were estimated from their half-wave oxidation potentials; the
HOMO–LUMO gaps were calculated from the absorption edges;
and the LUMO levels were calculated by adding the HOMO–
LUMO gaps to the values of the HOMO levels (Table 3). The
HOMO levels were in the order of BMeSA > 1a > 1b > 1c > 1d,
along with the electron-donating abilities of donor units. In
contrast, the LUMO energies are nearly at the same levels.
These results genuinely represent the control of the HOMO--
level dependent energy gaps.
1b
478
1c
533
1d
589
[a] Excitation at 350 nm. [b] Determined relative to quinine sulfate in 0.1 M
H2SO4.[22] [c] Determined by peak separation.
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10.1002/cphc.201900135
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.
Solvent effect on the absorbance and fluorescence
spectroscopic properties of BMeSA and BMeSA-Ar (1a-d):
Demonstration of a node-induced intramolecular charge
transfer mechanism. The absorbance and fluorescence
spectra of BMeSA and BMeSA-Ar (1a-d) are shown in Figure 6,
and the fluorescence quantum yields are summarized in Table 4.
No solvent effects were observed in the absorption spectra of
BMeSA or BMeSA-Ar compounds. These molecules, therefore,
adopt a non-polarized structure in the ground state. Conversely,
typical solvatochromisms were observed in the fluorescence
spectra of 1b-d, which are categorized as positive
solvatochromisms corresponding to a redshift with increasing
solvent polarity. Therefore, these molecules adopt charge-
polarized structures in the excited state, which can be stabilized
by a polar solvent. Notably, no fluorescent solvatochromism was
observed in BMeSA and 1a, which have non-polarized structures
in their excited states, suggesting that the electron-donating
ability of the amino group is a key factor in the charge-polarized
structure and the fluorescent solvatochromism. Indeed, the
calculated dipole moments of BMeSA and 1a in their excited
states are small (1.3 and 4.9 D), whereas those of 1b-d are large
(18.3, 21.3, and 23.7 Debye) (Table 5).
Figure 5. Cyclic voltammograms for the compounds BMeSA and BMeSA-Ar
(1a-d) with 0.1 M solution of Bu4NPF6 in dichloromethane at room temperature
relative to the Fc/Fc+ couple.
Table 3. Electrochemical and photophysical data of BMeSA and BMeSA-
Ar (1a-d) in dichloromethane.
E1/2 / V[a]
EHOMO / eV[b]
ELUMO / eV[c]
EHOMO–LUMO
eV[d]
/
BMeSA
1a
+1.16
+0.85
+0.37
+0.24
+0.00
−5.96
−5.65
−5.17
−5.04
−4.80
−2.43
−2.35
−2.18
−2.19
−2.18
3.53
3.30
2.99
2.85
2.62
1b
1c
1d
[a] Versus Fc/Fc+. [b] Estimated from oxidation wave onset [HOMO = – (E1/2
+ 4.8 eV)]. [c] Calculated from the following equation: ELUMO = EHOMO–LUMO
+ EHOMO. [d] Estimated from absorption edge.
Figure 6. Absorption and fluorescence spectra of BMeSA and BMeSA-Ar (1a-
d) in various solvents. The excitation wavelength was 300 nm. Fluorescence
spectra of 1d were normalized at 420 nm due to the small quantum yields of
the intramolecular charge-transfer (CT) emission at longer wavelength.
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Table 4. Fluorescence emission maxima and quantum yields of BMeSA and BMeSA-Ar (1a-d) in various solvents, and in solid-state.
THF
CH2Cl2
CH3CN
DMSO
Solid-State
λem / nm[a]
384
Φ[b]
λem / nm[a]
381
Φ[b]
λem / nm[a]
384
Φ[b]
λem / nm[a]
388
Φ[b]
λem / nm[d]
387
Φ[e]
BMeSA
1a
0.55
0.18
0.07
0.07
0.03
0.43
0.12
0.07
0.07
0.03
0.38
0.12
0.02
0.01
0.59
0.20
0.01
< 0.01
0.12
0.12
0.08
0.13
0.01
407
411
409
409
414
1b
505
478
538
570
484
1c
526
533
583
600
494
[c]
[c]
[c]
[c]
1d
590
589
-
-
-
-
544
[a] Excitation at 300 nm. [b] Determined relative to quinine sulfate in 0.1 M H2SO4.[22] [c] Not determined due to weak fluorescence intensities. [d] Excitation at
350 nm. [e] Determined in an integrating sphere.
It has been reported and widely observed in the category of
push-pull type biphenyls that two factors are regarded as the
electronic-coupling elements between the donor and acceptor
subunits: (1) the torsion angle between aryl–aryl bonds, and (2)
the linear combination of atomic orbitals (LCAO) coefficients of
the 2pz orbitals on the acceptor’s LUMO and the donor’s HOMO
at the aryl–aryl bonds.[25] Interestingly, the excited-state
geometries on the biphenyl moieties in 1b-d were more planar
than the ground-state geometries (Table 5). The fluorescent
mechanism, therefore, appears to be based on PICT (planar
intramolecular charge transfer). However, the fluorescence
quantum yields markedly decreased with increasing solvent
polarity. Compound 1d exhibited dual fluorescence originating
from two different CT states, where the 420 nm and 600 nm
emission wavelengths were assignable to the BMeSA skeleton
and the whole 1d molecule, respectively.[26] These behaviors are
commonly observed in TICT (twisted intramolecular charge
transfer) mechanisms. The very weak molar extinction
coefficients of 1b-d, which correspond to the HOMO–LUMO
transitions, also suggest a twisted geometry and poor orbital
overlap. Generally, the contribution of the biaryl torsion effect to
the charge separation is greater than that of the LCAO
coefficients effect.[27] In BMeSA-Ar, however, the LCAO
coefficients on the acceptor’s LUMO were hindered by the nodal
plane. Therefore, the small LCAO coefficients on the acceptor’s
LUMO influence the charge separation in the excited states of
1b-d. This CT mechanism is different from that of both PICT and
TICT and can be described as “node-induced intramolecular
charge transfer.” The fluorescence wavelengths and quantum
yields in the solid-states were similar to those in less polar
solvents, such as THF and dichloromethane (Table 4). In
addition, the interplanar angles of the biphenyl groups in 1a-d
are in the range of 144.22°-167.58° are acceptable values for the
orbital overlap between two benzene rings (Table 5). Moreover,
no significant molecular stacking was observed in the crystal
packings, except for the crystal of BMeSA (Figure S11: see the
packing structures of BMeSA in Supporting Information). These
results indicate that this charge separation system is effective in
both the solution and solid-states without considering the
molecular motion and intermolecular interactions.
Table 5. Calculated biaryl torsion angles and dipole moments of BMeSA
and BMeSA-Ar (1a-d) in dichloromethane, and experimental data in the
single crystals.
Biaryl torsion angles / °
Dipole moments / Debye
GS[a]
ES[b]
Crystal[c]
-
GS[a]
ES[b]
BMeSA
1a
-
-
0.688
0.889
3.545
4.658
4.653
1.317
4.904
18.30
21.16
23.69
Origin of asymmetric molecular orbital distribution and
nodal planes on BMeSA. To generate a nodal plane through
the atoms on a benzene ring, it is necessary to resolve the orbital
degeneracy of the HOMO and LUMO, where there are two units
with the same energy level. As shown in Figure 7, the meta di-
substituted benzene, with two electron-withdrawing groups,
effectively generates the asymmetric molecular orbital
distribution and nodal plane through atoms with sites that are
available for modification. More importantly, the amino group of
BMeSA plays a role in separating the energy between the HOMO
and HOMO−1, and between the LUMO and LUMO+1. This
makes BMeSA a promising candidate for use in fine-tuning the
optical properties of organic molecules without the undesirable
143.02
144.53
146.43
146.60
153.22
164.05
164.65
164.68
146.28
152.64
144.22
167.58
1b
1c
1d
[a] Performed by ground state (GS) geometry optimization with equilibrium
solvation at the B3LYP/6-31+G(d,p) level. [b] Performed by the equilibrium
solvation state-specific calculation of the excited state (ES) at the
ωB97XD/6-31+G(d,p) level. [c] Average values.
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exchange of energy levels between the LUMO and LUMO+1. If
the differences in energies between the LUMO and LUMO+1
levels were small, the LUMO and LUMO+1 levels could be
changed by the introduction of a π-conjugation unit at the free 4-
position, due to the stabilization of the LUMO+1 level with a large
orbital overlap. Conversely, the introduction of an acceptor-
substituted Ar group, such as a 4-cyanophenyl or 4-formylphenyl
group, is different from the introduction of the π-conjugation units
and not acceptable for this system. DFT calculations suggested
that the acceptor units lowered the LUMO+1 level significantly,
leading to the change in the LUMO and LUMO+1 levels;
subsequently the LUMO were distributed over the entire
molecule (Figure S12: see the DFT calculations in Supporting
Information). Consequently, the decrease in the absolute HOMO
level-dependent HOMO-LUMO gap was not achieved in this
case.
shift. The mechanism of the node-induced intramolecular charge
transfer (NICT) is different from that of ordinary PICT and TICT.
The BMeSA skeleton is suitable and preferable for molecular
orbital tuning via resolution of the benzene ring degeneracy and
generation of a nodal plane. The chemical availability and
reactivity of brominated BMeSA derivative, 5, reveal its potential
as a valuable synthetic intermediate for π-extension at the 4-
position of BMeSA. Not surprisingly, a charge separation method
is important for the material design of OLED and OPV materials.
We believe that these findings will provide a rational approach to
constructing a wide variety of π-conjugated molecules for
photonic and optoelectronic applications.
.
Experimental Section
Synthesis and chemical characterization: All chemicals and solvents
were reagent grade, except for tetrahydrofuran (THF), which was distilled
before use. All reactions were performed in standard dry glassware under
an inert nitrogen (N2) atmosphere. Column chromatography was
performed using Kanto silica gel 60N, spherical neutral, 40 - 100 µm.
Thin-layer chromatography (TLC) was performed using Merck pre-coated
aluminum sheets covered with 0.20 mm silica gel with a UV-fluorescent
indicator (λ = 254 nm). Melting points were determined using a Sansyo
SMP-300 apparatus. Nuclear magnetic resonance spectra were obtained
using a JEOL JNM-ECX operated at 500 MHz for 1H or 125 MHz for 13C.
The samples for NMR were dissolved in deuterated chloroform containing
tetramethylsilane (TMS) as an internal reference; chemical shifts (δ) are
reported in parts per million. Elemental analyses were performed on a
Perkin Elmer 2400 Series II CHN/O elemental analyzer. IR spectroscopy
was conducted using a Jasco FT/IR-460 plus instrument with on KBr
(press). High-resolution field-deposition mass spectra (FD-HRMS) were
obtained using a JEOL JMS-T100GCV instrument employing anthracene
as an internal standard.
2,6-Bis(methylsulfanyl)nitrobenzene (3): Dimethyldisulfide (3.55 mL,
37.7 mmol) and sodium borohydride (1.43 g, 37.7 mmol) were added to
DMF (30 mL) at 50 °C under an N2 atmosphere. After 2 h of stirring, 2,6-
difluoronitrobenzene (1.33 mL, 12.6 mmol) was added to the solution at
0 °C. The resultant solution was stirred for 1 day at room temperature
under an N2 atmosphere. The reaction was then quenched with water.
The mixture was extracted with Et2O, washed with water and brine, and
dried over Na2SO4. After removal of the solvent, the residue was purified
by column chromatography on silica gel with hexane-ethyl acetate (3:1)
as the eluent to give 3 as a yellow solid (2.11 g, 78%).; m.p. 115.8 °C–
117.6 °C; 1H NMR (500 MHz, CDCl3): δ 7.41 (t, J = 8.0 Hz, 1H, phenyl),
7.18 (d, J = 8.0 Hz, 2H, phenyl), 2.48 (s, 6H, CH3); 13C NMR (125 MHz,
CDCl3): δ 147.66, 132.84, 120.33, 118.23, 17.79 ppm; IR (KBr): ν = 2981
(CH), 2924 (CH) cm−1; HRMS (FD+): m/z calcd for C8H9NO2S2 [M+]:
215.00747; found: 215.00759; Elemental analysis calcd (%) for
C8H9NO2S2: C 44.63, H 4.21, N 6.51, S 29.79; found: C 44.65, H 4.23, N
6.36, S 29.80.
Figure 7. Frontier molecular orbitals and orbital energies of benzene, 1,4-
bis(methylsulfonyl)benzene, and BMeSA calculated at B3LYP/6-31G(d) level
in a gas phase.
Conclusions
Tuning the optical properties of sulfonylaniline fluorophores was
demonstrated via asymmetric molecular orbital distribution and
linkage through nodal planes. Controlling the HOMO-dependent
energy gap was achieved by exploiting the strong orbital overlap
in the HOMO, and the minimal orbital overlap in the LUMO.
Furthermore, the nodal arrangement of the LUMO revealed a
large excited-state dipole moment and influence on charge
separation in the excited state, resulting in strong positive
solvatochromism in the fluorescence spectra with a large Stokes
2,6-Bis(methylsulfanyl)aniline (4): 10 wt% Pd/C (500 mg) was added
to a solution of 3 (1 g, 4.65 mmol) in THF (15 mL) and ethanol (15 mL)
under an N2 atmosphere. The reaction mixture was then continuously
shaken under an H2 atmosphere (balloon pressure) for 24 h. The black
suspension was filtered through celite and washed with CH2Cl2; the
solvent was then removed under reduced pressure to give 4 as a
1
colorless oil (800 mg, 93%).; H NMR (500 MHz, CDCl3): δ 7.29 (d, J =
8.0 Hz, 2H, phenyl), 6.67 (t, J = 8.0 Hz, 1H, phenyl), 4.93 (br, 2H, NH2),
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10.1002/cphc.201900135
ChemPhysChem
ARTICLE
2.37 (s, 6H, CH3) ppm; 13C NMR (125 MHz, CDCl3): δ 147.66, 135.88,
131.10, 124.30, 17.06 ppm; IR (NaCl): ν = 3439 (NH), 3340 (NH), 3056
(CH), 2985 (CH), 2918(CH) cm−1; HRMS (FD+): m/z calcd for C8H11NS2
[M+]: 185.00329; found: 185.00326; Elemental analysis calcd (%) for
C8H11NS2: C 51.85, H 5.98, N 7.56, S 34.60; found: C 52.00, H 6.29, N
7.53, S 34.57.
chromatography on silica gel with dichloromethane-ethyl acetate (1:1) as
the eluent to give 1b as a pale yellow solid (148 mg, 71%); m.p. 234.7
°C–237.2 °C; 1H NMR (500 MHz, CDCl3): δ 8.21 (s, 2H, phenyl), 7.36
(AA′XX′, 2H, phenyl), 6.75 (AA′XX′, 2H, phenyl), 6.53 (s, 2H, NH2), 3.80
(br, 2H, NH2), 3.13 (s, 6H, CH3) ppm; 13C NMR (125 MHz, CDCl3): δ
146.67, 143.30, 133.47, 131.06, 127.60, 127.44, 125.02, 115.64, 42.94
ppm; IR (KBr): ν = 3735 (NH), 3463 (NH) cm−1; HRMS (FD+): m/z calcd
for C14H16N2O4S2 [M+]: 340.05515; found: 340.05526; Elemental analysis
calcd (%) for C14H16N2O4S2: C 49.40, H 4.74, N 8.23, S 18.84; found: C
49.26, H 4.59, N 8.15, S 18.57.
2,6-Bis(methylsulfonyl)aniline (BMeSA): m-CPBA (65%) (4.81 g,
18.31 mmol) was added to a cooled solution (0 °C) of 4 (800 mg, 4.31
mmol) in CH2Cl2 (60 mL), and the resulting suspension was stirred for 24
h at 0 °C. The reaction was quenched with a saturated aqueous solution
of Na2SO3, diluted with 3M NaOH, and the aqueous layer was extracted
with CH2Cl2. The combined organic layers were washed with water and
brine. After drying over anhydrous Na2SO4, the solvent was removed
under reduced pressure to yield BMeSA as colorless crystals (770 mg,
72%).; m.p. 155.2 °C–162.9 °C; 1H NMR (500 MHz, CDCl3): δ 8.04 (d, J
= 8.0 Hz, 2H, phenyl), 6.95 (t, J = 8.0 Hz, 1H, phenyl), 6.62 (br, 2H, NH2),
3.11 (s, 6H, CH3) ppm; 13C NMR (125 MHz, CDCl3): δ 145.37, 136.34
124.33, 116.92, 42.77 ppm; IR (KBr): ν = 3465 (NH), 3362 (NH), 3005
(CH), 2925 (CH) cm−1; HRMS (FD+): m/z calcd for C8H11NO4S2 [M+]:
249.01295; found: 249.01264; Elemental analysis calcd (%) for
C8H11NO4S2: C 38.54, H 4.45, N 5.62, S 25.72; found: C 38.48, H 4.32,
N 5.46, S 25.78.
N4′,N4′-Dimethyl-3,5-bis(methylsulfonyl)-[1,1′-biphenyl]-4,4′-diamine
(1c): A mixture of Pd(PPh3)4 (77 mg, 0.0663 mmol), 5 (272 mg, 0.829
mmol),
N,N-dimethyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)aniline (6c, 164 mg, 0.995 mmol), and K2CO3 (573 mg, 4.14 mmol) in
toluene-ethanol-water (22 mL, 14 mL, 3 mL) mixture was stirred at 60 °C
for 24 h under an N2 atmosphere. The reaction was then quenched with
water. The mixture was extracted with CH2Cl2, washed with water and
brine, and dried over Na2SO4. After removal of the solvent, the residue
was purified by column chromatography on silica gel with hexane-ethyl
acetate (1:1) as the eluent to give 1c as a yellow solid (203 mg, 67%).;
m.p. 267.1 °C–271.2 °C; 1H NMR (500 MHz, CDCl3): δ 8.23 (s, 2H,
phenyl), 7.45 (AA′XX′, 2H, phenyl), 6.79 (AA′XX′, 2H, phenyl), 6.50 (s, 2H,
NH2), 3.13 (s, 6H, CH3), 3.01 (s, 6H, CH3) ppm; 13C NMR (125 MHz,
CDCl3): δ 150.31, 142.89, 133.11, 131.08, 126.95, 124.94, 124.93,
4-Bromo-2,6-bis(methylsulfonyl)aniline (5): NBS (660 mg, 3.71 mmol)
was added to a solution of BMeSA (770 mg, 3.09 mol) in DMF (30 mL),
and the resulting solution was stirred for 24 h at room temperature. The
mixture was poured into water, and the resultant precipitate was filtered.
The cake was washed with water and dried in vacuo to give 5 as a white
solid (1.00 g, 99%).; m.p. 204.7 °C–205.1 °C; 1H NMR (500 MHz, CDCl3):
δ 8.13 (s, 2H, phenyl), 6.59 (br, 2H, NH2), 3.11 (s, 6H, CH3) ppm; 13C
NMR (125 MHz, CDCl3): δ 144.31, 138.72, 126.19, 108.27, 43.01 ppm;
IR (KBr): ν = 3472 (NH), 3376 (NH) cm−1; HRMS (FD+): m/z calcd for
C8H10NO4S2Br [M+]: 326.92346; found: 326.92348; Elemental analysis
calcd (%) for C8H10NO4S2Br: C 29.28, H 3.07, N 4.27, S 19.54; found: C
29.27, H 3.01, N 4.24, S 19.64.
112.72, 42.80, 40.43 ppm; IR (KBr): ν = 3473 (NH), 3369 (NH) cm−1
;
HRMS (FD+): m/z calcd for C16H20N2O4S2 [M+]: 368.08645; found:
368.08638; Elemental analysis calcd (%) for C16H20N2O4S2: C 52.15, H
5.47, N 7.60, S 17.40; found: C 52.04, H 5.34, N 7.50, S 17.63.
2,6-Bis(methylsulfonyl)-4-(2,3,6,7-tetrahydro-1H,5H-
benzo[ij]quinolizin-9-yl)aniline (1d): A mixture of Pd(PPh3)4 (106 mg,
0.0914 mmol), 5 (250 mg, 0.762 mmol), 9-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizine
(6d,
366 mg, 1.22 mmol), and K2CO3 (526 mg, 3.81 mmol) in a toluene-
ethanol-water (20 mL, 13 mL, 3 mL) mixture was stirred at 60 °C for 24 h
under an N2 atmosphere. The reaction was then quenched with water.
The mixture was extracted with toluene, washed with water and brine,
and dried over Na2SO4. After removal of the solvent, the residue was
purified by column chromatography on silica gel with dichloromethane as
the eluent to give 1d as a yellow solid (143 mg, 44%).; m.p. 153.7 °C–
3,5-Bis(methylsulfonyl)-[1.1′-biphenyl]-4-amine (1a):
A mixture of
Pd(PPh3)4 (28 mg, 0.0244 mmol), 5 (100 mg, 0.305 mmol), 4,4,5,5-
tetramethyl-2-phenyl-1,3,2-dioxaborolane (6a, 75 µL, 0.366 mmol), and
K2CO3 (211 mg, 1.52 mmol) in a toluene-ethanol-water (15 mL, 10 mL, 2
mL) mixture was stirred at 60 °C for 24 h under an N2 atmosphere. The
reaction was then quenched with water. The mixture was extracted with
CH2Cl2, washed with water and brine, and dried over Na2SO4. After
removal of the solvent, the residue was purified by column
chromatography on silica gel with dichloromethane-ethyl acetate (10:1)
as the eluent to give 1a as colorless solid (79 mg, 80%).; m.p. 219.3 °C–
220.5 °C; 1H NMR (500 MHz, CDCl3): δ 8.31 (s, 2H, phenyl), 7.56
(AA′MM′X, 2H, phenyl), 7.46 (AA′MM′X, 2H, phenyl), 7.38 (AA′MM′X, 1H,
phenyl), 6.64 (s, 2H, NH2), 3.14 (s, 6H, CH3) ppm; 13C NMR (125 MHz,
CDCl3): δ 144.26, 137.51, 134.46, 130.78, 129.34, 128.22, 126.48,
125.12, 43.02 ppm; IR (KBr): ν = 3472 (NH), 3372 (NH) cm−1; HRMS
(FD+): m/z calcd for C14H15NO4S2 [M+]: 325.04425; found: 325.04492;
Elemental analysis calcd (%) for C14H15NO4S2: C 51.67, H 4.65, N 4.30,
S 19.71; found: C 51.71, H 4.89, N 4.23, S 19.68.
1
161.2 °C (decomp); H NMR (500 MHz, CDCl3): δ 8.18 (s, 2H, phenyl),
7.00 (s, 2H, phenyl), 6.47 (s, 2H, NH2), 3.19 (t, J = 6.0 Hz, 4H, CH2), 3.12
(s, 6H, CH3), 2.80 (t, J = 6.5 Hz, 4H, CH2), 1.99 (dd, J = 6.0, 6.5 Hz, 4H,
CH2) ppm; 13C NMR (125 MHz, CDCl3): δ 142.91, 142.61, 132.83, 131.42,
124.82, 124.62, 123.92, 122.00, 49.90, 42.82, 27.73, 21.89 ppm; IR
(KBr): ν =3643 (NH), 3360 (NH), 2927 (CH) cm−1; HRMS (FD+): m/z calcd
for C20H24N2O4S2 [M+]: 420.11775; found: 420.11789; Elemental analysis
calcd (%) for C20H24N2O4S2: C 57.12, H 5.75, N 6.66, S 15.25; found: C
56.85, H 5.75, N 6.29, S 15.16.
Optical
spectroscopy
and
electrochemical
studies:
All
measurements were performed using Kanto HPLC grade solvents.
UV/Vis spectra of solutions were obtained using a Hitachi U-2810
spectrophotometer (concentration: 10−4 M). Fluorescence spectra were
obtained using a Shimadzu FR-6000 instrument (concentration: 10−5 M).
Cyclic voltammograms (CVs) were recorded on an ALS electrochemical
3,5-Bis(methylsulfonyl)-[1,1′-biphenyl]-4,4′-diamine (1b): A mixture of
Pd(PPh3)4 (56 mg, 0.0487 mmol), 5 (200 mg, 0.609 mmol), 4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (6b, 160 mg, 0.731 mmol),
and K2CO3 (421 mg, 3.05 mmol) in toluene-ethanol-water (16 mL, 10 mL,
2 mL) mixture was stirred at 60 °C for 24 h under an N2 atmosphere. The
reaction was then quenched with water. The mixture was extracted with
CH2Cl2, washed with water and brine, and dried over Na2SO4. After
removal of the solvent, the residue was purified by column
analyzer
CH
Model
612E
in
dichloromethane
containing
tetrabutylammonium hexafluorophosphate (Bu4NPF6, 0.1 M) as the
supporting electrolyte at a scan rate of 100 mV/s. Counter and working
electrodes were made of Pt, and the reference electrode was Ag/Ag+. All
potentials were calibrated versus a ferrocene standard.
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10.1002/cphc.201900135
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ARTICLE
Theoretical calculations of BMeSA derivatives: Ground-state
geometry optimizations were performed at the B3LYP/6-31+G(d,p) level;
Excited-state geometry optimizations were performed at the ωB97XD/6-
31+G(d,p) level. The solvent models were introduced using the
polarizable continuum method (PCM). Absorption properties were
calculated using the linear-response (LR)[28,29] model at the B3LYP/6-
31+G(d,p) level; emission property calculations were performed using the
State-Specific (SS)[30] model at the ωB97XD/6-31+G(d,p) level. Ground-
state dipole moments were determined by ground state geometry
optimization with equilibrium solvation; excited-state dipole moments
were determined by equilibrium solvation state-specific calculation of the
excited state. Other proven functionals such as PBE0 and M06-2X were
employed in the ground-state calculations; PBE0 and CAM-B3LYP were
employed in the excited-state calculation (Table S2 and S3: see the DFT
calculations in the Supporting Information).
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This study was supported by the Naito Foundation (No. 287 to
H.K.), a Grant-in-Aid for Scientific Research (C) (15K05468 to
H.K.) from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), and the YU-COE (M) program from
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10.1002/cphc.201900135
ChemPhysChem
ARTICLE
Entry for the Table of Contents
ARTICLE
Degeneracy breaking: The orbital
degeneracy of benzene rings is
resolved by biased push-pull system
and asymmetric molecular orbital
distribution. A linkage through the
nodal plane provides HOMO-level-
dependent tuning of the optical
properties, a large dipole moment in
the exited state, and positive
solvatochromism in fluorescence with
a large Stokes shift produced by a
node-induced intramolecular charge
transfer mechanism.
Shoh Kudo, Nanami Hoshino, Teruo
Beppu, and Hiroshi Katagiri*
Page No. – Page No.
Tuning the Optical Properties of
Sulfonylaniline Derivatives:
Degeneracy Breaking of Benzene
Orbitals and Linkage Through Nodal
Planes
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