1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity

Article abstract of DOI:10.1021/jacs.1c01329

Curved π-conjugated molecules and open-shell structures have attracted much attention from the perspective of fundamental chemistry, as well as materials science. In this study, the chemistry of 1,3-diradicals (DRs) embedded in curved cycloparaphenylene (

Full text of DOI:10.1021/jacs.1c01329

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1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet
versus Triplet State and In-Plane Aromaticity
Yuki Miyazawa, Zhe Wang, Misaki Matsumoto, Sayaka Hatano, Ivana Antol,* Eiichi Kayahara,
Shigeru Yamago,* and Manabu Abe*
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ABSTRACT: Curved π-conjugated molecules and open-shell
structures have attracted much attention from the perspective of
fundamental chemistry, as well as materials science. In this study,
the chemistry of 1,3-diradicals (DRs) embedded in curved
cycloparaphenylene (CPPs) structures, DR-(n+3)CPPs (n = 0−
5), was investigated to understand the effects of the curvature and
system size on the spin−spin interactions and singlet versus triplet
state, as well as their unique characteristics such as in-plane
aromaticity. A triplet ground state was predicted for the larger 1,3-
diradicals, such as the seven- and eight-paraphenylene-unit-
containing diradicals DR-7CPP (n = 4) and DR-8CPP (n = 5),
by quantum chemical calculations. The smaller-sized diradicals
DR-(n+3)CPPs (n = 0−3) were found to possess singlet ground states. Thus, the ground-state spin multiplicity is controlled by the
size of the paraphenylene cycle. The size effect on the ground-state spin multiplicity was confirmed by the experimental generation of
DR-6CPP in the photochemical denitrogenation of its azo-containing precursor (AZ-6CPP). Intriguingly, a unique type of in-plane
aromaticity emerged in the smaller-sized singlet states such as S-DR-4CPP (n = 1), as proven by nucleus-independent chemical shift
calculations (NICS) and an analysis of the anisotropy of the induced current density (ACID), which demonstrate that
homoconjugation between the 1,3-diradical moiety arises because of the curved and distorted bonding system.
INTRODUCTION
■
In the past decade, open-shell molecules have attracted
considerable attention not only in the field of reactive
intermediates but also in materials science. For example,
radical-based light-emitting diodes have been developed using
isolatable triaryl methyl radicals,1−3 magnetically robust high-
spin molecules have been developed using new molecular
designs,4,5 and a new bonding system, π single bonding, has
emerged through research into localized diradicals.6−13
Furthermore, the isolation of singlet diradicaloids such as
Tchitchibabin derivatives has revealed their singlet fission
behavior and nonlinear optical character, and these are now
hot topics in π-conjugated materials.7,14−24 To date, we have
investigated the localized 1,3-diradicals DR (see Figure 1a),
which are key intermediates in bond homolysis.25−29 The
ground-state spin multiplicity is typically controlled by the
substituents at the C2 position, and the triplet ground state for
DR having X = H, CH3 can be switched to a singlet ground
state by the introduction of electron-donating and electron-
withdrawing substituents, for example, X = F, OR, SiR3 (Figure
1a).12,30
Figure 1. (a) Substituent effects on the ground-state spin multiplicity
of cyclopentane-1,3-diyl diradicals. (b) Effect of size on the quinoidal
character of cycloparaphenylenes. (c) This study.
Received: February 3, 2021
Published: April 26, 2021
In the late 2000s, pioneering studies in the synthesis of a
new series of π-conjugated molecules, cycloparaphenylenes
(CPPs), were reported; since then, these intriguing molecules
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have attracted much attention.31−33 The most striking feature
of these hoop-shaped molecules is the hypsochromic shift of
the HOMO−LUMO energy gap with an increasing number of
benzene rings in the CPP structures (Figure 1b).34−36 This
behavior is totally opposite to that of linear paraphenylenes.
This unique behavior is rationalized as being a result of the
quinoidal contribution of the small CPPs arising from the
diradical character induced by the curved structure of the
benzene rings.37 In the present study, we designed diradicals
DR-(n+3)CPPs to investigate the quinoidal character in the
hoop-shaped structures (Figure 1c). Among the precursors
AZ-(n+3)CPPs featuring curved paraphenylene units, the
novel azoalkane AZ-6CPP (n = 3) was synthesized to
investigate its electronic character and the corresponding
chemistry of the diradical DR-6CPP (n = 3). Furthermore,
quantum chemical calculations were carried out for all
diradicals DR-(n+3)CPP (n = 0−5) to understand the
experimental results for DR-6CPP and the effect of the ring
size on the diradical chemistry in the hoop-shaped structure. In
particular, we draw attention to the size effect on the ground-
state spin multiplicity (singlet (S) versus triplet (T)) and
interaction of the two spins with the curved π-electron system
(quinoidal structure).
a
Scheme 1. Synthesis of AZ-6CPP
RESULTS
■
Synthesis of AZ-6CPP. The synthetic route to AZ-6CPP
is shown in Scheme 1. Double lithium bromide exchange of
compound 1 and its addition to 4,4-dimethoxycyclohexane-
2,5-dien-1-one (2) gave the diketal intermediate 3. Subsequent
acid hydrolysis produced diketone 4.38 The double arylation of
4 using (4-bromophenyl)lithium produced a diol, followed by
dimethylation using NaH and MeI to afford dibromide 5 in
13% yield (four steps). The Yamamoto coupling of the
dibromide followed by the reductive aromatization31 of 6
afforded AZ-6CPP in approximately 1% total yield on the basis
of the starting material. Single crystals were obtained by the
gradual evaporation of a mixture of CHCl3 and MeOH, and
the strained structure was confirmed by an X-ray crystallo-
graphic analysis (Figure 2a). Two AZ-6CPP molecules were
found in a single crystal lattice, and in this structure, a molecule
a
Abbreviations: THF, tetrahydrofuran; cod, cyclooctadiene; DMF,
dimethylformamide; bpy, bipyridine; Naph, naphthalene.
1
of CHCl3 occupies the curved paraphenylene unit. In the H
NMR spectrum (vide infra), the two CH3 groups appeared at δ
−2.36 and −0.49 ppm relative to the signal of benzene (δ 7.16
ppm). The high-field resonance is a result of the aromatic ring
current induced by the benzene rings in the macrocyclic
structure. The dimethyl group was found to be trans-
configured relative to the cyclopentane ring. The bond lengths,
average torsion angles (θ = 17.2°), and bending of the benzene
rings (α = 17.7°) of the paraphenylene units of AZ-6CPP were
compared with those of p-sexiphenyl, the linear paraphenylene
C6H5−(C6H4)4−C6H5 (Figure 2b). Density functional theory
(DFT) calculations at the B3LYP/6-31G(d) level of theory
reproduced the molecular structure of AZ-6CPP well, although
the torsion angles between the two benzene rings were
computed to be slightly larger than those obtained in the X-ray
analysis. The average torsion angle of curved AZ-6CPP (α =
17.7°) was found to be approximately half of that of p-
sexiphenyl.
were observed in the UV−vis absorption spectra (Figure 3a).
The weak n−π* transition (ε ≈ 100 M−1 cm−1) of the azo
chromophore (−NN−) at approximately 360 nm seems to
be hidden behind the strong absorption band from the π-
conjugated system. Time-dependent (TD) DFT39 calculations
at the B3LYP/6-31G(d)40,41 level of theory revealed that the
absorption at around 400 nm can be attributed to the
HOMO−LUMO transition, S0 → S1 (λcalc = 431.7 nm, Figure
3b,g), having an oscillator strength (f) of 0.1748. The
corresponding HOMO−LUMO transition (S0 → S1) in the
six-membered CPP ([6]CPP) is reported to be symmetry
forbidden because the HOMO and LUMO conserve symmetry
(Laporte forbidden).42−44 A strong S0 → S5 absorption band,
which mainly corresponds to the HOMO-1−LUMO (0.44)
and HOMO−LUMO+2 (0.53) transitions, was computed with
f = 1.0250 at λcalc = 327.1 nm (Figure 3c,g), as found at
approximately 320 nm in the experimental absorption
spectrum (Figure 3a).
Absorption and Emission Spectroscopy Analysis of
AZ-6CPP. The absorption and emission spectra of AZ-6CPP
are shown in Figure 3. A broad absorption at approximately
400 nm (λmax = 404 nm, ε = 7916 M−1 cm−1) and a structured
band at about 330 nm (λmax = 331 nm, ε = 40509 M−1 cm−1)
A structured fluorescence emission was observed in the
spectra obtained under a nitrogen atmosphere; this emission,
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was accelerated by molecular oxygen. Recently, Suenobu and
Nakagawa reported46 similar S1 state quenching by molecular
oxygen for [9]-, [12]-, and [15]CPPs, thus generating triplet
excited states of the CPP molecules. The phosphorescence of
AZ-6CPP was not observed, even at 77 K, under a nitrogen
atmosphere, which is behavior similar to that of small [n]CPPs
(n = 5−7). The triplet energy was computed to be 55.6 kcal
mol−1 at the UB3LYP/6-31G(d) level of theory. Thus, the
energy difference with the S1 state, ΔEST ≈ 10 kcal mol−1, is
smaller than the singlet oxygen energy (1Δg = 22.5 kcal mol−1).
The generation of the T1 state of AZ-6CPP was expected in
47
3
the S1 state quenching by O2 (see Figure 4d).
Transient Absorption Spectroscopy Analysis of AZ-
6CPP. Sub-nanosecond and sub-microsecond transient
absorption (TA) measurements of AZ-6CPP (Abs355 = 1.0)
were conducted using randomly interleaved pulse train
(RIPT)48 and laser flash photolysis (LFP), respectively (Figure
4). Transient species were observed with absorption peaks at
approximately 540, 600, and 660 nm in the sub-nanosecond
TA analysis just after laser irradiation in argon-saturated
benzene at 298 K. These transient species did not originate
from the diradical generated by photochemical denitrogenation
but from the π-conjugated paraphenylene moiety, because
virtually no change in the UV−vis absorption spectra of the
sample was observed before and after the TA measurements
(Figures S56 and S57). This is reasonable because the molar
extinction coefficient (ε) at 355 nm of the π−π* excitation of
AZ-6CPP is 7000 times larger than that of the n−π* −N
N− chromophore of AZ-2Ph, ε = 83 M−1 cm−1, whose
excitation is necessary for denitrogenation to give the diradical
intermediate DR-2Ph having a triplet ground state. The
quantitative formation of the ring-closed compound CP-2Ph
via the photochemical denitrogenation of AZ-2Ph has been
reported with a high quantum yield of ΦN2 ≈ 1.0 (eq 1).49
Figure 2. (a) X-ray structure of AZ-6CPP (black, carbon; gray,
hydrogen; blue, nitrogen; green, chloride). (b) Experimentally and
computationally determined (at the B3LYP/6-31G(d) level of theory
in parentheses) torsion angles between the adjacent benzene-rings (θ
in deg), benzene bending angles (α in deg), and bond lengths (Å) of
the paraphenylene structures in AZ-6CPP and C6H5−(CH2)4−C6H5
(p-sexiphenyl).
at approximately 500 nm (λonset = 433 nm, λ = 475, 505 nm),
has a relatively high fluorescence quantum yield of 79%
(Figure 3d) and, using this, the S1 state energy was estimated
to be approximately 66 kcal mol−1. In contrast, the
fluorescence quantum yields of small [n]CPPs (n = 5−8) are
reported to be less than 10%. Thus, the embedded azo moiety
significantly changed the electronic transition and emission
character of the curved π-conjugated molecule. A similar
symmetry-breaking strategy to turn on HOMO−LUMO
absorption and emission in small CPPs was reported by
Jasti.45 The lifetime of the fluorescence at 500 nm was
determined to be τf = 5.0 ns under N2 at 295 K using the time-
correlated single-photon-counting (TCSPC) method (Figure
S76). Interestingly, the fluorescence quantum yield and
lifetime decreased and were shortened by about 20% in air
([O2] = 1.9 mM) to 4.2 ns and 62%, respectively (Figure 3 e
and Figure S62), suggesting that the nonfluorescence process
Two transient species with decay rate constants of 1.9 × 108
s−1 (τ = 5.2 ns, light green) and 1.5 × 105 s−1 (τ = 6.8 μs, light
blue) were observed in the sub-nanosecond TA spectroscopic
analysis of AZ-6CPP (Figure 4a,b). The slow decay rate
constant is consistent with that obtained in the sub-
microsecond TA analysis (τ = 6.5 μs, Figure 4b, light blue).
The absorption spectrum of short-lived species was obtained
by subtracting the spectrum at 100 ns from that at 1 ns (Figure
4c, light green). The short-lived species with τ = 5.2 ns has an
absorption maximum at approximately 590 nm. The transient
species is assigned to the singlet excited state S1 because the
lifetime of 5.2 ns is consistent with that of the fluorescence
lifetime, τf = 5.0 ns, as shown in Figure 3f. After approximately
20 ns, another TA spectrum was obtained, and this contained
absorption peaks at 540 and 660 nm (Figure 4a, light blue)
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Figure 3. Absorption and emission analysis of AZ-6CPP in benzene: (a) UV−vis absorption spectrum; (b) computed S0 → S1 transition (λcalc
=
431.7 nm, f = 0.175) at the TD-B3LYP/6-31G(d) level of theory; (c) computed S0 → S5 transition (λcalc = 327.1 nm, f = 1.03) at the TD-B3LYP/
6-31G(d) level of theory; (d) emission spectrum for 400 nm excitation (Abs400 = 0.1) under an N2 atmosphere in benzene solution; (e) emission
spectrum for 400 nm excitation under air in benzene solution; (f) emission quantum yield (Φf) and lifetime (τf) in N2 and air; (g) energy diagram
of Kohn−Sham orbitals and electronic transitions computed at the TD-B3LYP/6-31G(d) level of theory.
Figure 4. Time-resolved transient absorption spectra during the photolysis of AZ-6CPP in benzene: (a) sub-nanosecond time-resolved transient
absorption spectra in benzene under an Ar atmosphere (355 nm, 80 μJ/pulse, 25 ps pulse width) at 295 K; (b) sub-microsecond time-resolved
transient absorption spectra in benzene under a N2 atmosphere (355 nm, 6 mJ/pulse, 4 ns pulse width) at 295 K; (c) transient absorption spectrum
of short-lived species (singlet excited state) with τ = 5.2 ns obtained by subtracting the spectrum at 100 ns from that at 1 ns; (d) time profile at 540
nm in the sub-nanosecond time-resolved spectroscopic analysis in Ar (black) and air (red).
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Figure 5. 1H NMR analysis (400 MHz) of the photolysis of AZ-6CPP (20 mM) by 355 nm laser (5.5 mJ/pulse) in degassed C6D6 solution: (a)
before irradiation; (b) after 2 h irradiation; (c) after 12.5 h irradiation; (d) after 45 h irradiation. Time profiles of the decomposition of azoalkanes
1
(e) AZ-2Ph and (f) AZ-6CPP as monitored by H NMR spectroscopy using a 355 nm laser (5.5 mJ).
that could be assigned to the triplet state (T1). Indeed, the
lifetime of the species detected at 540 nm was significantly
shortened to about 167 ns in air (Figure 4d, red). A careful
analysis of the triplet TA spectrum at 540 nm revealed that the
fluorescence quenching observed in the fluorescence measure-
ments in Figure 3e is due to an increase in the intersystem
crossing (ISC) from the S1 to the T1 state in the presence of
3O2 because the intensity of the triplet TA in air was
approximately 15% higher than that obtained in Ar (Figure
4d).
The denitrogenation quantum yield (ΦN2) of AZ-6CPP was
determined by comparing the decomposition rate with that of
AZ-2Ph, whose denitrogenation quantum yield is known to be
1.0.49 The decomposition of azoalkanes was monitored by H
1
NMR spectroscopy (Figure 5e,f). The function log([10I0 − 1]/
[10I − 1]), where I is the 1H NMR signal intensity of the CH3
groups in AZ-6CPP and AZ-2Ph (Figures S27 and S28),
increased linearly with irradiation time using a laser wavelength
of 355 nm. The photoreaction quantum yield of AZ-6CPP was
calculated from the ratio of the slopes of AZ-6CPP/AZ-2Ph =
εAZ‑6CPPΦN /εAZ‑2PhΦN , where εAZ‑6CPP (7 × 103 M−1 cm−1)
2
2
Product Analysis of the Photolysis of AZ-6CPP. As
found in the TA analysis, the quantum yield for the
denitrogenation of AZ-6CPP is too low to allow detection of
the diradical intermediate DR-6CPP directly using a time-
resolved spectroscopic analysis. To confirm the denitrogena-
tion of AZ-6CPP, the photoreaction of AZ-6CPP (20 mM)
using a 355 nm yttrium aluminum garnet (YAG) laser
and εAZ‑2Ph (83 M−1 cm−1) are the molar extinction coefficients
of AZ-6CPP and AZ-2Ph at 355 nm, respectively. From the
analysis, the denitrogenation quantum yield of AZ-6CPP was
determined to be 0.004 (0.4%). The low quantum yield is a
result of the 355 nm light being mainly absorbed by the curved
π-conjugated moiety, as suggested by the TA analysis (Figure
4).
1
(approximately 5.5 mJ/pulse) was monitored by H NMR
EPR Measurements. Although the quantum yield of the
photochemical denitrogenation of AZ-6CPP is very low, it
might be possible to observe DR-6CPP directly during the
photolysis of AZ-6CPP using an electron paramagnetic
resonance (EPR) spectroscopic analysis because of its high
sensitivity to paramagnetic species. Thus, low-temperature
photolysis of AZ-6CPP (11 mM) in degassed 2-methylte-
trahydrofuran (2MTHF) was conducted using a Hg lamp (λemi
> 250 nm) at 4 K (Figure 6). After 4 h, a typical signal
stemming from persistent triplet species was observed at 1650
G, and this was assigned to the ΔMS = 2 forbidden transition.
In addition to the half-field signal at 1650 G, the allowed
transitions of ΔMS = 1 (2977 (z1), 3142 (xy1), 3560 (xy2),
and 3731 G (z2)) were observed at a resonance frequency of
9.40 GHz. From the z signals, the zero-field splitting (zfs)
parameters were determined to be |D/hc| = 0.035 cm−1 and |E/
hc| ≤ 0.001 cm−1 (Figure 6a).13 The triplet EPR spectrum was
consistent with the simulated spectrum by inputting zfs
spectroscopy at 295 K in degassed C6D6 solution in a sealed
NMR tube under vacuum conditions (Figure 5). During the
irradiation of AZ-6CPP, two new pairs of CH3 groups (a, 0.95
and 1.82 ppm; b, 0.58 and 1.52 ppm; Figure 5) were observed
in the photolysate with the concomitant decrease of two CH3
groups of AZ-6CPP (−2.36 and −0.49 ppm). After 45 h of
irradiation (Figure 5d), AZ-6CPP was almost completely
consumed under the photolysis conditions. The photoproducts
were purified by column chromatography on silica gel. Both of
these products were thermally labile and gradually decom-
posed under air conditions. Using quick column chromatog-
raphy, the separated samples were sealed under N2 and
analyzed by 1H NMR, 2D NMR, 2D-nuclear Overhauser effect
(NOESY), and mass spectroscopy, demonstrating that the
photoreaction products a and b are the trans-configured ring-
closed compound tCP-6CPP and a methyl-group-migrated
alkene product (MG-6CPP), respectively (Figures S29−S47).
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Figure 6. (a) EPR spectrum of the photolysis of AZ-6CPP (11 mM) using an Hg lamp (>250 nm) in 2MTHF matrix in a sealed quartz tube at 4 K
under vacuum conditions (resonance frequency 9.40 GHz). (b) EPR spectrum simulated using the zfs parameters |D/hc| = 0.035 cm−1 and |E/hc| =
0.00 cm−1. (c) Experimental and computed zfs parameters (D and E) for diradicals T-DR-2H, T-DR-2Ph, and T-DR-6CPP. (d) Temperature
dependence of the intensity of the EPR signal at 1650 G. (e) IT−T plot. (f) Least-squares fit for the Bleaney−Bowers analysis.
parameters of |D/hc| = 0.035 cm−1 and |E/hc| = 0 cm−1 (Figure
6b), and the average distance between the two dipoles was
determined to be 4.20 Å using the point dipole approx-
imation,50 which is longer than 3.76 Å in DR-2Ph (|D/hc| =
0.050 cm−1 and |E/hc| ≤ 0.001 cm−1)51,52 generated from AZ-
2Ph (eq 1), indicating that the diradical is more delocalized
over the benzene rings.51
To obtain more information about the triplet diradical
generated in the photolysis of AZ-6CPP, the spin densities of
diradicals T-DR-2Ph and T-DR-6CPP were computed at the
UB3LYP/6-31G(d) level of theory. The spin density at the
benzylic carbon of T-DR-2Ph was computed to be 0.753. The
corresponding value for T-DRexo-6CPP was calculated to be
0.706. A smaller value of 0.665 was found for the endo isomer,
T-DRendo-6CPP. To confirm the structural assignment of the
triplet diradical having a smaller D value (|D/hc| = 0.035 cm−1)
in comparison to that of DR-2Ph (|D/hc| = 0.050 cm−1), the
zfs parameters D and E were computed at the (RO)BP/EPR-
II//UB3LYP/6-31G(d)53,54 level of theory using ORCA
4.2.155,56 (Figure 6c). The zfs parameters of the parent triplet
diradical T-DR-2H (|D/hc| = 0.084 cm−1 and |E/hc| = 0.002
cm−1)57,58 were also calculated at the same level of theory to
confirm the reliability of the computation. As shown in Figure
6c, the experimentally obtained |D/hc| values in wavenumbers,
0.084 and 0.050 cm−1 for T-DR-2H and T-DR-2Ph,
respectively, were well reproduced by the computations: i.e.,
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0.088 and 0.050 cm−1, respectively. The D values for T-DRexo
-
6CPP and T-DRendo-6CPP were computed to be 0.041 and
0.034 cm−1, respectively. Thus, the triplet signals observed in
the photolysis of AZ-6CPP (Figure 6a) were assigned to T-
DRendo-6CPP. Although T-DRendo-6CPP was calculated to be
less stable than T-DRexo-6CPP by 7.9 kcal mol−1, the
assignment is reasonable because AZ-6CPP (Scheme 1) has
the same configuration as the endo isomer DRendo-6CPP,
featuring a trans configuration of the dimethylmethano moiety
with the cyclopentane ring.
The temperature dependence of the intensity of the triplet
signal at 1650 G was examined to determine the ground-state
spin multiplicity. The intensity measurements were conducted
at a microwave power of 20 mW, for which signal saturation
was not observed, even at 4 K (Figure S82). After the
generation of DR-6CPP at 4 K, the temperature was increased
by 1 to 15 K and then by 5 K until 30 K (Figure 6d).
Surprisingly, the triplet signal suddenly weakened at 40 K
(Figure 6d) and the signal intensity did not return to the
original intensity at 4 K. After regeneration of DR-6CPP at 6
K, the sample was heated to 30 K and then recooled to 6 K to
confirm the thermal stability of DR-6CPP at 30 K. The
intensity of 0.545 at 6 K before the sample was warmed was
nearly the same as 0.547 at 6 K after the sample was recooled
to 6 K (Figure S83), suggesting that DR-6CCP is persistent
below 30 K. The high reactivity, even at 40 K, is significantly
different from the persistent character of T-DR-2Ph at 77 K.59
In the IT−T plot below 30 K (Figure 6d), the IT value
gradually decreased with decreasing temperature, indicating
that the triplet state is thermally populated as an excited state.
From the least-squares fit for the Bleaney−Bowers analysis (R2
= 0.9825, number of points 14, Figure 6e,f),60 the singlet
(ES)−triplet (ET) energy gap (ΔEST = 2J/kB = ES − ET; J/kB is
the exchange interaction) was determined to be −15.8 0.5
cal mol−1 (=−7.9 K, J/kB = 3.95 K), demonstrating the singlet
ground state of DRendo-6CPP (Figure 6e). Thus, the ground-
state spin multiplicity was switched by the curved macrocyclic
structure. The maximum in the I−T plot (Figure S84) was not
clearly observed in the temperature range of 4−30 K because
the exchange interaction value was J/kB = 3.95 K.
Figure 7. Structures of (a) DR-2Ph, (b) DRendo-6CPP, and (c)
DRendo-2Ph′ optimized at the UCAM-B3LYP/6-31G(d) level of
theory. Hydrogen atoms are omitted for clarity.
theory. Nearly planar structures were found in the cyclo-
pentane-1,3-diyl moiety for DR-2Ph (Figure 7a), and the C1−
C2−C3−C4 dihedral angles were computed to be approx-
imately 6.5° for both triplet and singlet states. The two planar
benzene rings have a coplanar orientation with the cyclo-
pentane-1,3-diyl moiety, as shown by the C5−C2−C6 angles,
which were calculated to be 167 and 157° for the singlet and
triplet states, respectively. The distances between C1 and C3
were found to be 240.4 and 240.2 pm in the singlet and triplet
states, respectively. Thus, nearly the same molecular structures
were found in the singlet and triplet states of DR-2Ph.
DISCUSSION
■
As found in the EPR experiments, the ground-state spin
multiplicity switched from triplet in DR-2Ph to singlet in
DRendo-6CPP, which has a curved π-conjugated structure,
although the singlet−triplet energy spacing is small. To gain
more insight into the macrocyclic effect, the singlet−triplet
energy spacings of DR-2Ph, DRendo-6CPP, and DRendo-2Ph′
were computed using DFT (CAM-B3LYP/6-31G(d))61 and
complete active space self-consistent field (CASSCF)62
calculations in Gaussian 16.63 The structure of DRendo-2Ph′
was obtained by replacing the middle four benzene rings with
two hydrogen atoms in DRendo-6CPP without optimizing the
curved structure of the two benzene rings at the radical sites
(Figure 7c). The open-shell singlet state was computed using
the broken-symmetry (BS)64 approach for the DFT calcu-
lations. The energy corrections were conducted using the
complete active space second-order perturbation theory
(CASPT2)65 for the CASSCF calculations. Thus, the effect
of the curvature on the open-shell singlet−triplet energy
spacing was appropriately computed.
In contrast to the planar and flat structures of DR-2Ph,
puckered and bent structures were found in DRendo-6CPP at
the same level of theory (Figure 7b). Puckered structures
having C1−C2−C3−C4 dihedral angles of 47.4 and 42.7°
were obtained for the singlet and triplet DRendo-6CPP,
respectively (Figure 7b). The C5−C2−C6 angles (bending
of the benzene rings) in the singlet and triplet states of DRendo
-
6CPP were computed to be 115 and 120°, respectively, which
are smaller than that of DR-2Ph (∼160°). In particular, the
atomic distance of 229.7 pm in S-DRendo-6CPP was
significantly shorter than that in the triplet state T-DRendo
-
6CPP (236.6 pm) and in the singlet and triplet states in DR-
2Ph (240.4 and 240.2 pm).
To understand the electronic structures of DR-2Ph and
DRendo-6CPP, the HOMO and LUMO occupation numbers in
their singlet states were compared (Figure 8). The occupation
numbers of S-DR-2Ph and S-DRendo-6CPP were determined at
the CASSCF(2,2)/6-31G(d) level of theory. As clearly shown
in the HOMO and LUMO images (Figure 8a), a parallel
alignment of the p orbitals was found in S-DR-2Ph. The
First, the molecular structures DR-2Ph and DRendo-6CPP
were optimized at the BS-UCAM-B3LYP/6-31G(d) level of
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Figure 8. HOMO and LUMO orbitals and their occupation numbers in (a) S-DR-2Ph and (b) S-DRendo-6CPP calculated at the CASSCF(2,2)/6-
31G(d) level of theory.
Table 1. Singlet−Triplet Energy Gap (ΔEST = ES − ET) in DR-2Ph, DRendo-6CPP, and DR-2Ph′
singlet−triplet energy gap/kcal mol−1 (ΔEST = ES − ET)
entry
diradical
DR-2Ph
DRendo-6CPP
DR-2Ph′
UCAM-B3LYP/6-31G(d)
CASSCF(14,14)/cc-pVDZ
CASPT2(14,14)/cc-pVDZ
1
2
3
+0.20
−1.77
−1.20
+0.37
nd
−3.84
+0.20
nd
−6.35
occupation numbers of electrons in the HOMO (ψA) and
LUMO (ψS) orbitals in S-DR-2Ph were computed to be 1.04
and 0.96, respectively, indicating a negligible bonding
interaction between the two radicals: that is, nearly pure
singlet diradical character in S-DR-2Ph. Thus, the triplet
ground state is reasonable for DR-2Ph. In contrast, a bent
conformation was found in S-DRendo-6CPP, which suggests a
bent-type (banana-like) bonding interaction between the two
radical sites (Figure 8b). Indeed, S-DRendo-6CPP possesses a
bonding combination of HOMO (ψS) and antibonding
LUMO (ψA), whose orbital order is, interestingly, opposite
in S-DR-2Ph (Figure 8a). The switch in the HOMO−LUMO
conversion in DR-2Ph is a result of through-bond
interactions.12,25,66−70 The occupation numbers in the bonding
(ψS) and antibonding (ψA) orbitals were found to be 1.47 and
0.53, respectively, and the bond order between the radical sites
was calculated to be 0.47 from the occupation numbers in the
bonding and antibonding orbitals.
singlet was calculated to be the ground state, having an energy
preference of 1.77 kcal mol−1, in DRendo-6CPP (entries 1 and
2). To confirm the energy gap obtained in the DFT
calculations, the DR-2Ph′ diradical having curved benzene
rings was used for the computation of the energy gap at the
CASPT2(14,14)/cc-pVDZ level of theory using OpenMol-
cas71 because the π-electrons in DR-6CPP are too large to
allow computation of the energy gap using the CASPT2
method. A singlet ground state with ΔEST = ES − ET = −3.84
and −6.35 kcal mol−1 was found for DRendo-2Ph′ at the
CASSCF(14,14)/cc-pVDZ and CASPT2(14,14)/cc-pVDZ
levels of theory, respectively (entry 3). At the same level of
theory, the triplet ground state was confirmed, having ΔEST
=
+0.20 kcal mol−1, for DR-2Ph (entry 1). Thus, the singlet
preference of DRendo-6CPP (Figure 6e) is rationalized to result
from the puckered structure of the diradical. The bent-bonding
interaction between the two radical sites is the key reason for
the singlet ground state in S-DRendo-6CPP (Figure 8b).
To obtain more insights into the macrocyclic effect on the
reactivity of the diradicals, the ring-closing reaction to yield
CP-6CPP was computed, thus allowing comparison with the
corresponding reaction for DR-2Ph (Figure 9). First, the ring-
The singlet−triplet energy spacing in DR-2Ph and DRendo
-
6CPP was computed at the UCAM-B3LYP/6-31G(d) level of
theory (Table 1). In contrast to the triplet ground state of
diradicals DR-2Ph, ΔEST = ES − ET = +0.20 kcal mol−1, the
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Figure 9. Computations on the thermal reactivity of singlet diradicals S-DR-2Ph and S-DR-6CPP at the (U)CAM-B3LYP/6-31G(d) level of
theory.
closing reaction of S-DR-2Ph to CP-2Ph was computed at the
BS-(U)CAM-B3LYP/6-31G(d) level of theory. The kinetically
favored ring-closing reaction was found to give a cis-configured
compound, cCP-2Ph, with an energy barrier of 13.6 kcal mol−1
(Figure 9a). A slightly high energy barrier, 13.7 kcal mol−1, was
calculated for the reaction to the trans-configured compound,
tCP-2Ph. The repulsive interaction between Ph and CH2
groups in the transition state TSt increases the energy of
TSt for the formation of tCP-2Ph, whose observation is similar
to that of the previously reported kinetically favored formation
of a cis isomer.72 The persistent character of DR-2Ph at low
temperature (<100 K) is reasonable on the basis of the
relatively large energy barriers for the radical−radical coupling
reaction obtained by computation.
6CPP is proposed to be formed in the secondary photo-
reaction of CP-6CPP.
We wondered “why are the energy barriers from S-DRendo
-
6CPP to tCP-6CPP and from S-DRexo-6CPP to cCP-6CPP so
small?” As shown in Figure 8b, the puckered structure of S-
DRendo-6CPP, which already possesses a bent-bonding
character, is one of the reasons for the small energy barrier.
Similar molecular structures were found in S-DRexo-6CPP
(Figure S135). To gain more insight into the high reactivity of
DR-6CPP, the molecular strain energies were computed for
AZ-6CPP, S-DRendo-6CPP, T-DRendo-6CPP, S-DRexo-6CPP,
T-DRexo-6CPP, tCP-6CPP, and cCP-6CPP using eq 2 in
Table 2. The strain energies of AZ-6CPP and DR-6CPP were
determined to be 79−85 kcal mol−1 (entries 1−5), similar to
that of [7]CPP (84 kcal mol−1)73 calculated at the B3LYP/6-
31G(d) level of theory. The strain energy of DRendo-6CPP was
higher by ∼5 kcal mol−1 than that of the exo isomer (entries
2−5). Surprisingly, the strain energies of the ring-closed CP-
6CPP were found to be approximately 10 and 15 kcal mol−1
lower than those of the diradical intermediates S-DRexo-6CPP
and S-DRendo-6CPP, respectively (entries 6 and 7), although
the ring size of CP-6CPP is, in principle, smaller than that of
DR-6CPP by one carbon. The computational results
demonstrate that the ring-closing reaction releases molecular
strain, suggesting that the small energy barriers in the ring-
closing process in DR-6CPP were accelerated by the release of
molecular strain, as well as the bonding interaction in S-DR-
6CPP. A question quickly arises: “Why is the molecular strain
in CP-6CPP smaller than that in DR-6CPP in spite of the
smaller ring size in CP-6CPP in comparison to that in DR-
6CPP?” A careful analysis of the optimized structures revealed
that there is greater steric repulsion between the methyl group
and the adjacent benzene ring in S-DR-6CPP in comparison to
that in cCP-6CPP because the phenyl ring can twist in CP-
6CPP (see the optimized structures in Table 2).
A small energy barrier, 1.87 kcal mol−1, was calculated for
the ring-closing reactions in the exo isomer of S-DR-6CPP, S-
DRexo-6CPP (black line, Figure 9b). In the reaction of endo
isomer S-DRendo-6CPP, a clear energy barrier was not found
during the scan calculation in the C1−C3 bond-formation
process to give the ring-closed compound tCP-6CPP (Figure
S88); thus, the energy barrier from S-DRendo-6CPP to tCP-
6CPP is should be very small (blue line, Figure 9b). The
observed high reactivity of DRendo-6CPP, even at 40 K, under
low-temperature matrix conditions is reasonable because of the
very small energy barrier. In the photolysis of AZ-6CPP,
another product, MG-6CPP, was formed (Figure 6). A
relatively large energy barrier of 41.9 kcal mol−1 was computed
for the methyl migration reaction in S-DRendo-6CPP (red line,
Figure 9b), which is much higher in energy than that of the
ring-closing reaction. The large energy barrier computed for
the migration reaction suggests that the formation of MG-
6CPP stems from an electronically excited state. Photolysis of
the matrix-isolated DR-2Ph has been reported to produce the
corresponding methyl migration product, MG-2Ph.59 The
photolysis of tCP-2Ph using 266 nm irradiation, indeed
produced MG-2Ph (Figures S85−S87). Thus, compound MG-
As shown by pioneering studies on the effects of size on the
electronic character of CPPs,33,35 the size-dependent change in
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singlet state was found for DR-3CPP (n = 0, entry 1), whereas
the triplet ground state was computed for DRendo-7CPP (n =
4, entry 5) and DRendo-8CPP (n = 5, entry 6). It should be
noted that the singlet−triplet energy gap drastically increased
Table 2. Molecular Strain Energies of AZ-6CPP, S-DRendo
6CPP, T-DRendo-6CPP, tCP-6CPP, and cCP-6CPP at the
(U)CAM-B3LYP/6-31G(d) Level of Theory
-
to 4.5 and 21.3 kcal mol−1 (i.e., singlet preference) for DRendo
-
4CPP and DRendo-3CPP, respectively (entries 1 and 2). The
singlet states of DRendo-3CPP and DRendo-4CPP are not
perfect open-shell molecules; rather, they are nearly closed-
shell molecules (quinoidal structures) because the occupation
numbers in HOMOs of S-DRendo-3CPP and S-DRendo-4CPP
were computed to be 1.91 and 1.64 at the CASSCF(2,2)//BS-
UCAM-B3LYP/6-31G(d) level of theory (entries 1 and 2),
demonstrating that the quinoidal structures S-DRendo-3CPP
and S-DRendo-4CPP are important. As the size of the rings
increases, there is a trend for the singlet state to increase in
diradical character, as judged by the occupation numbers of the
HOMOs and LUMOs. Finally, a triplet ground state was found
for DRendo-7CPP and DRendo-8CPP due to the small
difference in the occupation numbers in the HOMO (1.16
and 1.12) and LUMO (0.84 and 0.88) (Figure 8), having ΔEST
= +0.13 and +0.19 kcal mol−1 (entries 5 and 6). Similarly to
DR-2Ph (Figure 7), a nearly planar structure having C1−C2−
C3−C4 dihedral angles of 22 and 15° and the normal atom
distance of C1−C3 (240.4 and 241.1 pm) were found in S-
DRendo-7CPP and S-DRendo-8CPP, respectively.
The quinoidal structure of S-DRendo-CPP was evaluated
using the harmonic oscillator model of aromaticity
(HOMA),74 which was determined from the bond distances
computed in the curved paraphenylene moieties. As shown in
entries 1 and 2 of Table 3, the HOMA values of “ring 1” and
“ring 2” in the singlet state of DRendo-3CPP were found to be
significantly smaller than 1.0. As the ring size increased (entries
1−6), the HOMA values approached 1.0. In contrast to the
small HOMA values in the singlet states, the corresponding
values for the triplet states were found to be much larger than
those in the singlet states, even for DRendo-3CPP (entry 1).
Thus, the singlet state of DRendo-3CPP possesses a significant
bond-alternating quinoidal form. We realized that the
quinoidal structure of the singlet state of S-DRendo-(n
+3)CPPs would show in-plane aromaticity75−79 when the
homoconjugation80,81 of two radical sites exists in the
macrocyclic structures. The nucleus-independent chemical
shift values, NICS(0)zz and NICS(0)iso,82−84 at the center of
the ring for the singlet and triplet states were computed to
examine the in-plane aromaticity (Table 3). Interestingly, the
NICS values were prone to become negative with a decrease in
the ring size of the singlet state, indicating that the in-plane
aromaticity emerges in the small-sized S-DRendo-(n+3)CPPs.
The NICS value of S-DRendo-4CPP, NICS(0)zz (NICS(0)iso),
was highly negative, −19.2 (−11.1), although the NICS value
of the triplet state was found to be +10.4 (−1.0) (entry 2,
Figure 10a). The in-plane aromaticity of S-DRendo-4CPP was
clearly visualized using anisotropy of the induced current
density (ACID)85 plots (Figure 10b,c) and 2D-NICS plots
molecular structure and ground-state spin multiplicity is
particularly interesting in diradicals embedded in cyclo-
paraphenylenes: DRendo-(n+3)CPPs (n = 0−5). Thus, the
reaction energy (ΔrE in kcal mol−1) was computed for the
formation of tCP-(n+3)CPP from S-DRendo-(n+3)CPPs
(Table 3). Interestingly, ring-closed tCP-3CPP was not
found to be an equilibrium structure at the restricted
B3LYP/6-31G(d) level of theory, and optimization yielded
the ring-opened diradical S-DRendo-3CPP featuring the
quinoidal form (entry 1). Thus, the intramolecular cyclization
of S-DRendo-3CPP to tCP-3CPP is energetically disfavored,
although the strain energy (SE) of S-DRendo-3CPP was
computed to be high: SEDR = 91.3 kcal mol−1. The
exothermicity (ΔrE) of the intramolecular cyclization is
prone to increase with increasing size (n) of the paraphenylene
moiety (entries 2−6). The strain energies (SECP and SEDR) in
tCP-(n+3)CPP and S-DRendo-(n+3)CPP were found to
decrease with increasing macrocyclic ring size (entries 2−6).
The strain energy (SEDR) in S-DRendo-3CPP was smaller than
that in S-DRendo-4CPP (entries 1 and 2). The quinoidal form
of S-DRendo-3CPP would be the reason for this.
The effect of the ring size on the singlet−triplet energy gap
(ΔEST = ES − ET) computed at the (U)CAM-B3LYP/6-
31G(d) level of theory was also analyzed in DRendo-(n+3)CPP,
and the results are summarized in Table 3. Interestingly, the
energy gap was found to be significantly dependent on the ring
size. In particular, as the size of the rings increases, there is a
tendency for the triplet state to become more stable. For
example, a significant preference (by 21.3 kcal mol−1) for the
(Figures 10d,e). The smaller negative NICS value of S-DRendo
-
3CPP in comparison to that of S-DRendo-4CPP is rationalized
by the large bond alternation of the quinoidal structures, which
is reflected by the low degree of π-conjugation. As found in the
CASSCF calculations, the diradical character increases with an
increase in the ring size. Thus, the in-plane aromaticity
becomes low for the larger-sized S-DRendo (n+3)CPPs.
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Table 3. Effect of the Ring Size on the Singlet−Triplet Energy Gap (ΔEST = ES − ET) in DRendo-[n+3]CPP (n = 0−5)
HOMA (benzene distortion (bent) angle
(α, deg))
NICS(0)
zz b
ring 1
ring 2
singlet triplet
(NICS(0)iso
)
DRendo
-
ΔrE = ECP − EDR/kcal
ΔEST = ES − ET/kcal
occupation no. in
a
entry
1
CPP
mol−1, SEDR/SECP
mol−1
HOMO/LUMO
singlet
0.465
triplet
singlet
−3.6
triplet
DRendo
-
−, 101.0/−
−21.29
−4.50
−1.90
−1.77
+0.13
+0.19
1.91/0.09
1.64/0.36
1.54/0.46
1.47/0.53
1.16/0.84
1.12/0.88
0.751
0.541 0.99
+11.9
(−0.8)
+10.4
(−1.0)
+5.2
(−2.3)
+3.6
(−2.2)
3CPP
(31.8)
0.876
(18.5)
0.942
(13.3)
0.951
(11.8)
0.968
(12.1)
0.963
(10.7)
(26.9)
(30.0)
0.971
(18.4)
0.993
(12.7)
0.995
(9.5)
0.995
(8.5)
0.995
(8.0)
(19.3)
0.990
(17.2)
0.994
(12.6)
0.995
(9.7)
0.995
(8.6)
0.995
(8.1)
(−5.5)
−19.2
(−11.1)
−2.9
(−5.2)
+0.1
(−3.5)
+2.4
(−2.1)
+2.1
(−1.7)
2
3
4
5
6
DRendo
-
−25.6, 103.0/95.6
−31.5, 92.3/79.0
−32.5, 82.6/68.3
−35.6, 78.2/60.8
−35.0, 70.9/54.1
0.888
4CPP
(18.7)
DRendo
-
0.930
5CPP
(14.2)
DRendo
-
0.940
6CPP
(12.3)
DRendo
-
0.966
+2.9
(−2.0)
7CPP
(12.3)
DRendo
-
0.963
+2.3
(−1.6)
8CPP
(10.7)
a
b
Occupation numbers in the HOMOs/LUMOs were determined at the CASSCF(2,2)//BS-UCAM-B3LYP/6-31G(d) level of theory. NICS
values at the ring centers were computed at the UB3LYP/6-31+G(d) level of theory.
Figure 10. (a) Molecular structure of S-DRendo-4CPP with a dummy atom at the ring center. (b) Top view and (c) side view of the ACID plot of
the ring current in S-DRendo-4CPP. 2D-NICS(0)zz plots for (d) S-DRendo-4CPP and (e) T-DRendo-4CPP.
compound tCP-6CPP. The intermediary diradical DRendo
-
CONCLUSION
■
6CPP was directly detected using EPR spectroscopic analysis
under low-temperature matrix isolation conditions. A singlet
ground state was revealed by EPR experiments, although the
parent diradical, DR-2Ph, has a triplet ground state. The
singlet ground state of DRendo-6CPP is rationalized as having a
puckered structure in the diradical unit, induced by the curved
structure of the paraphenylene moiety. A computational study
In this study, a novel azoalkane, AZ-6CPP, which is the
precursor of DR-6CPP, was synthesized. A relatively high
fluorescence quantum yield of 79% was observed for AZ-6CPP
because the embedded azo moiety turned on the HOMO−
LUMO absorption and emission in the small CPPs via
symmetry breaking. The photochemical denitrogenation of
AZ-6CPP was carried out to produce the ring-closed
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on the effects of size on the chemistry of DRendo-(n+3)CPP (n
= 0−5) demonstrated that (1) the ground-state spin
multiplicity is largely dependent on the ring size and the
singlet ground state was favored for (3−6)CPP derivatives, (2)
in-plane aromaticity emerged for small singlet states such as
DRendo-4CPP, which involves homoconjugation in the 1,3-
diradical moiety, and (3) S-DRendo-3CPP is proposed to
possess a closed-shell quinoidal structure due to the strongly
curved paraphenylene units. The singlet ground state of small-
sized diradicals DR-(n+3)CPP was experimentally proved by
the generation of DR-6CPP from AZ-6CPP.
ACKNOWLEDGMENTS
■
Mass spectrometry measurements were performed at the
Natural Science Center for Basic Research and Development
(N-BARD) of Hiroshima University. This work was supported
by JSPS KAKENHI Grant Numbers 17H03022 (M.A.),
20K21197 (M.A.), and 16H06352 (S.Y.), JST-CREST
(Grant Number JPMJCR18R4), and was also supported by
the International Collaborative Research Program of Institute
for Chemical Research, Kyoto University (grant 2020-43). We
thank UNISOKU Co., Ltd., for the use of the picosecond laser
system (picoTAS).
ASSOCIATED CONTENT
■
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AUTHOR INFORMATION
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Corresponding Authors
Ivana Antol − Laboratory for Physical Organic Chemistry,
Division of Organic Chemistry and Biochemistry, Ruđer
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Boskovic Institute, 10000 Zagreb, Croatia;
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Shigeru Yamago − Institute for Chemical Research, Kyoto
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Manabu Abe − Department of Chemistry, Graduate School of
Science, Hiroshima University, Hiroshima 739-8526, Japan;
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Authors
Yuki Miyazawa − Department of Chemistry, Graduate School
of Science, Hiroshima University, Hiroshima 739-8526,
Japan
Zhe Wang − Department of Chemistry, Graduate School of
Science, Hiroshima University, Hiroshima 739-8526, Japan;
orcid.org/0000-0002-9996-586X
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Singlet 1,3-Diradicals (Biradicals): From Putative Intermediates to
Persistent Species and Unusual Molecules with a π-Single Bonded
Character. Chem. Soc. Rev. 2012, 41 (10), 3808−3820.
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Hansmann, M. M.; Munz, D.; Guldi, D. M. Singlet Fission in
Carbene-Derived Diradicaloids. Angew. Chem., Int. Ed. 2020, 59 (20),
7906−7914.
Misaki Matsumoto − Department of Chemistry, Graduate
School of Science, Hiroshima University, Hiroshima 739-
8526, Japan
Sayaka Hatano − Department of Chemistry, Graduate School
of Science, Hiroshima University, Hiroshima 739-8526,
Japan
Eiichi Kayahara − Institute for Chemical Research, Kyoto
University, Uji, Kyoto 611-0011, Japan; orcid.org/0000-
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
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