Dibenzoarsepins: Planarization of 8π-Electron System in the Lowest Singlet Excited State

Article abstract of DOI:10.1002/anie.201904882

Dibenzo[b,f]arsepins possessing severely distorted cores compared to those of other heteropins were synthesized. These derivatives exhibited dual photoluminescence in the green-to-red region (500–700 nm) and the near-ultraviolet region (<380 nm), which could be attributed to the planarization of the arsepin core in the lowest singlet excited (S₁) state. The computational approach for the assessment of the aromatic indices revealed that the dibenzoarsepins studied show aromaticity (8π system) in the S₁ states in line with Baird's rule. The lone pair electrons of the arsenic atoms play a crucial role in the aromaticity in the S₁ states.

Full text of DOI:10.1002/anie.201904882

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Accepted Article
Title: Dibenzoarsepins: Planarization of 8π-Electron System in the
Lowest Singlet Excited State
Authors: Ikuo Kawashima, Hiroaki Imoto, Masatoshi Ishida, Hiroyuki
Furuta, Shunsuke Yamamoto, Masaya Mitsuishi, Susumu
Tanaka, Toshiki Fujii, and Kensuke Naka
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201904882
Angew. Chem. 10.1002/ange.201904882
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10.1002/anie.201904882
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Dibenzoarsepins: Planarization of 8-Electron System in the
Lowest Singlet Excited State
Ikuo Kawashima,[a] Hiroaki Imoto,[a] Masatoshi Ishida,[b] Hiroyuki Furuta,[b] Shunsuke Yamamoto,[c]
Masaya Mitsuishi,[c] Susumu Tanaka,[a] Toshiki Fujii, [a] and Kensuke Naka*[a]
Abstract: Dibenzo[b,f]arsepins, 3-Me and 3-Ph, possessing severely
distorted cores as compared to other heteropin families, were
synthesized. These derivatives exhibited dual photoluminescence in
the green-to-red region (500~700 nm) and the near-ultraviolet region
(<380 nm), which could be attributed to the planarization of the
arsepin cores in the lowest singlet excited (S1) states. The
computational approach for the assessment of the aromatic indices
revealed that 3-Me and 3-Ph show aromaticity (8π system) in the S1
states according to Baird’s rule. The lone pair electrons of the arsenic
atoms played a crucial role in the aromaticity in the S1 states.
systems, which are constructed by the overlap of the lone pair
electrons of their heteroatoms, should be of interest in terms of
structural transformation in the excited state.[10]
Heteropins have seven-membered cycloheptatriene cores
bridged by a heteroatom instead of an sp3 carbon atom, and their
conformations and electronic features, reflecting the nature of the
incorporated elements, have received much attention (Figure 1).[1-
Figure 1. Schematic representation of the conformation and electron system of
various heteropins. The corresponding aromaticity in the ground and excited
states is also summarized.
6]
Carbon group heteropins such as cycloheptatriene and silepin
have nonconjugated cores with a boat-like conformation, and due
to the low energy barrier, rapidly undergo boat-to-boat ring
inversion via a planar conformation.[1] The boron group heteropins
(e.g., borepin,[2] aluminepin,[3] and gallepin[4]) possess
isoelectronic structures with the tropylium ion and adopt the
planar conformation with a 6π-aromatic system at the heteropin
core in the S0 states.[2a,3,4b] In contrast, chalcogen-based
heteropins (e.g., oxepin and thiepin), which formally have an 8-
electron circuit, avoid the unstable antiaromatic structures by
adopting the distorted conformation in the S0 states. However,
upon photoexcitation, conformational planarization in the lowest
singlet excited state (S1) was predicted based on the
photochemical reactivity of the compounds.[5] Ottosson suggested
that this planarization is attributed to the Baird aromaticity rule.[7]
The concept of excited-state aromaticity according to Baird’s
rule is applicable to predict the photochemical properties of cyclic
4n and 4n+2 -electron systems in their excited states.[8] Among
them, the 8 annulenic cyclooctatetraene (COT) systems, which
are utilized as photoresponsive materials owing to their flapping
motions upon photoexcitation, have been widely studied in the
fields of structural and computational chemistry.[9] In-depth
investigations on seven-membered heteropins with 8-electron
Judging from the results of chalcogen-based heteropins, which
have 8-electron systems including the lone pair one, the
pnictogen group is suitable for realizing Baird aromaticity due to
its lone pairs. For example, dibenzo[b,f]phosphepin has been
reported to adopt a boat-like bent conformation.[6] Among the
pnictogen group heteropins, arsepin, an arsenic analog of
heteropin, is beneficial for photodriven planarization because the
trivalent arsenic atom in the macrocycle has a lone pair electron
that is chemically inert in air, as opposed to the air-sensitive
phosphepin analogs.[11] However, despite the successful
synthesis of arsepins, their conformation, optical properties, and
aromatic character have not been elucidated in detail yet.[12] The
main obstacle to experimental studies on arsepins is the high
intrinsic toxicity and volatility of the precursor, arsenic
dichloride.[11d,13] Recently, we have developed simple synthetic
methodologies
using
cyclooligoarsines
such
as
pentamethylpentacycloarsine
(1-Me)
and
hexaphenylhexacycloarsine (1-Ph)[14] as shown in Scheme 1.[15]
When the As−As bonds of these cyclooligoarsines are cleaved,
reactive species such as radicals, electrophiles, and nucleophiles
are generated in situ.[16] Diiodoarsines (2-Me and 2-Ph) were then
prepared by the addition of iodine to the cyclooligoarsines, and
could be used for the construction of the cyclic arsepin core with
a dilithio intermediate.[16c]
[a]
I. Kawashima, Dr. H. Imoto, S. Tanaka, T. Fujii, Prof. Dr. K. Naka
Faculty of Molecular Chemistry and Engineering, Graduate School
of Science and Technology, Kyoto Institute of Technology
Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan)
E-mail: kenaka@kit.ac.jp
[b]
[c]
Dr. M. Ishida, Prof. Dr. H. Furuta
Department of Chemistry and Biochemistry, Graduate School of
Engineering and Center for Molecular System, Kyushu University
744 Moto-oka, Nishi-ku, Fukuoka 819-0395 (Japan)
Dr. S. Yamamoto, Prof. Dr. M. Mitsuishi
Institute of Multidisciplinary Research for Advanced Materials
(IMRAM), Tohoku University
2-1-1 Katahira, Aoba-ku, Sendai 980-8577 (Japan)
Scheme 1. Easy and safe As–C bond formation via in situ generation of
diiodoarsines from cyclooligoarsines.
Supporting information for this article is given via a link at the end of
the document.
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Herein, we report the synthesis of the new dibenzo[b,f]arsepin
derivatives (3-Me and 3-Ph) and their X-ray crystallographic
structures. The photophysical properties were examined in detail
to evaluate the conformational dynamics in their excited states.
Furthermore, computational calculations were conducted to
assess the aromatic character of these derivatives in the S1 states
using nucleus-independent chemical shift (NICS), harmonic
oscillator model of heterocyclic electron delocalization
(HOMHED) index, and anisotropy of the induced current density
(ACID) plot.
both 3-Me (θ1 = 34.0°, θ2 = 55.4°) and 3-Ph (θ1 = 32.2°, θ2 = 52.5°)
compared with other dibenzoheteropins (Table S4). The phenyl
ring in 3-Ph was oriented perpendicularly to the heterocyclic
double bond of the heteropin core, in contrast to the case of the
phosphepin derivatives.[6] Notably, bond alteration at the arsepin
cores was observed for 3-Me and 3-Ph. The specific C7−C8 bond
lengths were 1.33(2) Å (3-Me) and 1.330(4) Å (3-Ph), respectively,
which were identical to the typical carbon-carbon double bond.
The 1H-NMR spectra of 3-Me and 3-Ph supported the alkene
character; the signals observed at 7.07 ppm (3-Me) and 6.80 ppm
(3-Ph) could be attributed to the internal alkenes.
Compounds 3-Me and 3-Ph were synthesized as shown in
Scheme 2a. Cyclooligoarsines 1-Me and 1-Ph were mixed with
iodine in diethyl ether at 25 °C to prepare diiodoarsines 2-Me and
2-Ph, respectively, which were directly used for the subsequent
reaction. The resulting solution was reacted with the 2,2’-dilithio-
Z-stilbene intermediate generated from 2,2’-dibromo-Z-
stilbene[3,17] at 78 °C. The reaction mixture was then allowed to
warm to room temperature and stirred overnight to afford arsepins
3-Me and 3-Ph in 19% and 25% yields, respectively. The arsepins
were stable; no oxidation or decomposition was observed even
after storage in the solid state under ambient conditions for more
than one month. The lone pairs on the arsenic atom could be
utilized as a donor site for complexation; gold(I) metalation of 3-
Ph afforded adduct complex 4 in 62% yield (Scheme 2b). Single
crystals of 3-Me, 3-Ph, and 4 suitable for the analyses were
prepared by recrystallization from CH2Cl2 and methanol or
hexane by slow diffusion at 18 °C.
Figure 2. ORTEP drawings of (a) 3-Me, (b) 3-Ph, and (c) 4. Top views (top) and
side views (bottom) are shown (50% probability for thermal ellipsoids). Selected
bond lengths (Å) and angles are shown. Crystallographic data are provided in
Tables S1–S3.
The crystal structure of 4 indicated that the phenyl ring was
positioned above the arsepin core plane and that the tetrahedral
arsepin atom was coordinated to the gold(I) chloride unit in a
monodentate fashion. The sum of the C−As−C bond angles
(316.8(6)°) was larger than that for 3-Ph, resulting in enhanced
planarity of the arsepin core as the electronic repulsion between
the lone pair and As−C bonds was alleviated by the coordination.
Scheme 2. Synthesis of (a) arsepin derivatives 3-Me and 3-Ph, and (b) gold(I)
chloride complex (4).
Table 1. Photophysical data for 3-Me, 3-Ph, and 4.
CHCl3 solutiona
Solid
b
c
c
The X-ray crystallographic structures and data for 3-Me, 3-Ph,
and 4 are summarized in Figure 2 and Tables S1–S3.[18] The
seven-membered arsepin cores for 3-Me and 3-Ph adopted a
boat-like conformation. The sum of the inner angles of the seven-
membered arsepin cores for 3-Me (839.0(17)°) and 3-Ph
(846.2(6)°) were much smaller than that of the planar heptagon
model (900°). These angles were also significantly smaller than
those of the other dibenzoheteropins (856°–899°) as shown in
Table S4,[2a,3,4b,6b,19] indicating that 3-Me and 3-Ph had the most
distorted cores among the reported heteropins. The As−C bond
lengths were also significantly longer (1.93−1.96 Å) than those of
the other heteropins.[2a,3,4b,6b,19] The sum of the bond angles
(C1−As−C14, C1−As−C15, and C14−As−C15) around the
arsenic atoms was 295.4(5)° for 3-Me and 299.1(2)° for 3-Ph,
suggesting that the arsenic atoms had a trigonal pyramidal
geometry. The bent angles, θ1 and θ2, were remarkably large for
λabs
λem
τe
τf
λem
d
d
ФPL
ФPL
/nm
250
288
245
296
239
301
/nm
364
584
371
557
/ns
/ns
/nm
3-Me
3-Ph
4
NDg
NDg
0.38
1.00
0.12
0.39
2.30
NDg
424
0.03
0.06
488
383
375
< 0.01
< 0.01
aCHCl3 solution (1.0 × 10−4 M) in air at room temperature. bAbsorption
maxima. cEmission maxima (excited at the excitation maxima). dAbsolute
quantum yields. eFluorescence lifetime monitored feedback at 364 nm for
3-Me, 371 nm for 3-Ph, and 375 nm for 4. fFluorescence lifetime monitored
feedback at 584 nm for 3-Me and 557 nm for 3-Ph. gNot determined.
UV-vis absorption (Figure S7) and photoluminescence (PL)
spectra (Figure 3a-c) of 3-Me, 3-Ph, and 4 were measured in
CHCl3, and their photophysical data are summarized in Table 1.
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Intense bands around 250 nm and relatively broad bands around
300 nm were observed in the absorption spectra for both 3-Me
and 3-Ph. To gain insight into the vertical transition features of 3-
Me and 3-Ph, density functional theory (DFT) calculations with
(U)B3LYP functional and Grimme’s dispersion correction, B3LYP-
D3/6-31+G(d,p), were performed. The calculations well
reproduced the crystal structures, which revealed the distorted
tub-shaped arsepin cores of 3-Me and 3-Ph (vide infra, Figures
S17–S18, Table S6). Time-dependent (TD) DFT calculations for
3-Me and 3-Ph revealed that the characteristic transitions with the
lowest energy, S0S1 (323 nm and 336 nm, respectively),
assigned as HOMO–LUMO transitions had relatively smaller
oscillator strengths (f) of 0.0217 and 0.0205, respectively (Figure
S16, Table S5). The second intense bands could be visualized as
the contribution of the HOMO-1–LUMO transitions, for both the
compounds (Table S5).
measured from 145 K to 300 K, and the estimated Ea was 3.2
kJ/mol, meaning that the structural relaxation in the S1 state can
occur at room temperature.
The fluorescence lifetime () of 3-Ph probed at 371 nm was
fitted by the sum of two exponential decays to be 0.30 ns (99%)
and 4.92 ns (1%) (Figure S8).[22] The exponential decay in the
blue wavelength region was rapid, in contrast, the decay
constants measured at longer wavelength (557 nm) with single
exponential fitting were higher ( = 2.30 ns). Time-resolved
fluorescence spectrum for 3-Ph in CHCl3 revealed that the weak
fluorescence band in the longer-wavelength region appeared
during the decay of the intense emission band around 400 nm
(Figure S12). These results suggested that the slow component
in emission was ascribed to the conformational relaxation from
the Franck-Condon state to the relaxed singlet state. Under
anaerobic conditions, no phosphorescence was detected in
CHCl3.[23] Additionally, no such conformational change occurred
in the excited state of 4 owing to the geometrical restriction at the
tetrahedral arsine atom, as inferred from the absence of longer-
wavelength emissions (Figure 3c) and the single exponential
fluorescence decay profile (Figure S8).
The planarization of the cores was demonstrated by the S1-
excited state calculations (Figures 4, S21).[24] The energy profiles
of the S0 and S1 species for 3-Me and 3-Ph indicated that the
decrease in the torsion angle between the mean planes of the two
benzene rings led to a monotonic decrease in the energies in the
S1 state. The distinct low energy emissions were predicted by
TDDFT calculations (em of 634 nm for 3-Me and 650 nm for 3-
Ph). The higher energy fluorescence around 360–370 nm was
thus attributed to the vertical transition, based on the Franck-
Condon principle, from the S1 to S0 states for 3-Me and 3-Ph,
while the fluorescence observed in the longer wavelength regions
could be derived from the transition geometry from the structurally
relaxed S1 to the S0 states. The calculated excited state dipole
moments (ES) were slightly decreased upon planarization for
both the compounds (Figures 4, S21), which suggested that the
relaxed conformers were partially favored in less polar solvents
such as n-hexane, as observed in Figures 3d and S13.
Figure 3. PL spectra (1.0×10−4 M, 298 K) of (a) 3-Me, (b) 3-Ph, and (c) 4 in
CHCl3 after air bubbling (black lines) and N2 bubbling (blue lines), and (d) 3-Ph
in various solvents.
Upon photoexcitation, arsepins 3-Me (Figure 3a) and 3-Ph
(Figure 3b) excited at 288 nm and 296 nm, respectively, showed
PL bands with large Stokes shifts of 7249 cm–1 and 6830 cm–1,
respectively. Concomitantly, broad bands were observed in the
green-to-red region (500−700 nm).[20] The intensities of the
longer-wavelength emissions were dependent on the solvent
used; high emission intensity was observed in less polar solvents
such as n-hexane (Figures 3d, S13) because the planar structure
was less polar than the bent structure in the S1 state, as discussed
below. When the arsepins were dispersed in the poly(methyl
methacrylate) matrix (Figure S10), the longer-wavelength
emission bands were diminished. Furthermore, we measured the
PL spectra of 3-Ph in various alkanes to investigate the effect of
solvent viscosity. The higher the viscosity, the lower the relative
intensity of the longer wavelength emission (Figure S14). These
results imply that the structural relaxation toward the planarization
was impeded by PMMA or viscous solvent matrix in the S1 state.
We estimate the activation energy (Ea) of the structural
relaxation from the Franck-Condon state to the relaxed singlet
state by the Stevens–Ban plot[21] of 3-Ph in 2-
methyltetrahydrofuran (Figure S15). The PL spectra of 3-Ph were
Figure 4. Energy diagram for S0 and S1 states of 3-Ph by changing the torsion
angle. Solid arrow stands for absorption process and dashed arrows for
emission. The calculated dipole moments, ES (excited state) and GS (ground
state), are provided.
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Given that the arsepins adopt a quasi-planar conformation in
the excited states after the structural relaxation, the arsepin cores
possessing 8π-conjugated systems, including the lone pair
electrons on the arsenic atoms, would exhibit reversal of
aromaticity in the S1 states according to Baird’s rule. To evaluate
the distinct aromaticity of 3-Me and 3-Ph in the S1 states, we
performed calculations with the B3LYP-D3 functional (Figures
S24–S27). As mentioned, both 3-Me and 3-Ph adopted bent
geometries in the S0 states, and thus showed non-aromatic
character, as inferred from the ACID plots, revealing the
disconnected electron conjugation between the arsine and the
adjacent carbons on the arsepin core (Figures S24–S25). In
addition, the HOMHED values[25] of the arsepin cores were small
(0.37 and 0.28 for 3-Me and 3-Ph, respectively) due to the
significant bond length alteration of the 8π-conjugated
systems.[26] The NICS(1) values that were nearly zero at the
arsepin cores also supported this conclusion (Table 2).
In turn, the arsepin cores of 3-Me and 3-Ph adopted a
coplanar structure in the S1 states (vide supra), and the C1/C14–
As bond lengths were estimated to be significantly shorter than
those of the ground-state species (Figures S20–S21, Table
S7).[27] In addition, the C7–C8 bonds were longer in the S1 states.
Therefore, the HOMHED values of the flattened arsepin cores
were estimated to be 0.89 for 3-Me and 0.90 for 3-Ph (Table 2),
which could be attributed to the effective -conjugation in the
arsepin cores in the S1 excited states. This indicated the reversal
of aromaticity based on Baird’s rule. Moreover, the NICS plot
scanned along the molecular z-axis showed moderate aromatic
character for the arsepin cores as evident from the negative
NICS(1) values above the center of the rings (e.g., 6.7 ppm and
9.3 ppm, respectively), although the gauge independent atomic
orbital (GIAO) method was applied to the virtually constructed
excited state structures in this calculation. The ACID plots
represented clockwise ring currents on the arsepin rings as well
as the adjacent benzene units for both 3-Me and 3-Ph (Figures
S24–S25). The arsenic lone pair could be hybridized with the
neighboring -orbitals of the benzene rings that realized the global
conjugation. Using these coplanar geometries, we also assessed
the aromaticity in the S0 state with the NICS-scan plots (Figures
S26–S27). The flattened arsepins 3-Me and 3-Ph showed distinct
antiaromatic character (+7.6 and +11.7 ppm, respectively) at the
global center of the arsepin rings, indicating the reversal of Hückel
aromaticity.
and HOMHED values, 3-Me and 3-Ph have distinct aromaticity in
the S1 states according to Baird’s rule, in contrast to the
nonaromatic nature in the S0 states. Moreover, the lone pairs of
the arsenic atoms played a pivotal role in the construction of the
8π-electron heteropin circuits. These novel class of arsepin
molecules exhibiting drastic photodriven conformational change
are not only promising candidates for stimuli-responsive materials
but also allow for a deeper understanding of Baird aromaticity.
Acknowledgments
The work in Tohoku was supported by the Research Program of
"Dynamic Alliance for Open Innovation Bridging Human,
Environment and Materials" in "Network Joint Research Center
for Materials and Devices". The work in KIT was supported by
JSPS KAKENHI, grant Number JP17H04577 (Coordination
Asymmetry), to HI. The work in Kyushu was supported by JSPS
KAKENHI, grant numbers JP19H04586 and JP19K05439, to MI.
Keywords: arsepin, lone pair, Baird’s rule, excited-state
aromaticity, aromatic indices.
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S0
S1
S0
S1
3-Me
3-Ph
[5]
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2.6
6.7
0.37
0.89
1.2
9.3
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NICS(1)/ ppm
HOMHED
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geometries in the S1 state were investigated by experimental
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[27] To verify the substitution effect of the heteroatom on the aromaticity of
the 8 heteropin cores, the same calculations were conducted for the
dibenzo[b,f]azepin and phosphepin derivatives. For details, see SI
(Figures S28−S29).
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10.1002/anie.201904882
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
I. Kawashima, H. Imoto, M. Ishida, H.
Furuta, S. Yamamoto, M. Mitsuishi, S.
Tanaka, T. Fujii, K. Naka*
Page No. – Page No.
Dibenzoarsepins: Planarization of 8
system in the Lowest Singlet Excited
State
Dibenzoarsepins possessing formally 8-electron systems showed highly bent
structures in the ground states, while quasi-planar structures were favored in the
singlet excited states. The drastic conformational change upon photo-excitation was
studied based on Baird’s rule.
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