Controlling the S1energy profile by tuning excited-state aromaticity

Article abstract of DOI:10.1021/jacs.0c05611

The shape of the lowest singlet excited-state (S1) energy profile is of primary importance in photochemistry and related materials science areas. Here we demonstrate a new approach for controlling the shape of the S1 energy profile which relies on tuning the level of excited-state aromaticity (ESA). In a series of fluorescent π-expanded oxepins, the energy decrease accompanying the bent-to-planar conformational change in S1 becomes less pronounced with lower ESA levels. Stabilization energies following from ESA were quantitatively estimated to be 10-20 kcal/mol using photophysical data. Very fast planarization dynamics in S1 was revealed by time-resolved fluorescence spectroscopy. The time constants were estimated to be shorter than 1 ps, regardless of molecular size and level of ESA, indicating barrierless S1 planarization within the oxepin series.

Full text of DOI:10.1021/jacs.0c05611

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Article
Controlling the S1 Energy Profile by Tuning Excited-State Aromaticity
Ryota Kotani, Li Liu, Pardeep Kumar, Hikaru Kuramochi, Tahei Tahara,
Pengpeng Liu, Atsuhiro Osuka, Peter B. Karadakov, and Shohei Saito
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05611 • Publication Date (Web): 10 Aug 2020
Downloaded from pubs.acs.org on August 10, 2020
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Controlling the S1 Energy Profile by Tuning Excited-State Aromatic-
ity
Ryota Kotani,† Li Liu,‡ Pardeep Kumar,‡,§ Hikaru Kuramochi,‡,§,ǁ Tahei Tahara,‡,§,* Pengpeng Liu,† At-
suhiro Osuka,† Peter B. Karadakov,^,* and Shohei Saito†, ǁ,*
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†Graduate School of Science, Kyoto University, Kitashirakawa Oiwake, Sakyo, Kyoto 606-8502, Japan
‡Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan
§Ultrafast Spectroscopy Research Team, RIKEN Center for Advanced Photonics (RAP), 2-1, Hirosawa, Wako 351-0198,
Japan
ǁPRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama, Japan
^Department of Chemistry, University of York, Heslington, York, YO10 5DD, U.K.
ABSTRACT: The shape of the lowest singlet excited state (S1) energy profile is of primary importance in photochem-
istry and related materials science areas. Here we demonstrate a new approach for controlling the shape of the S1 energy
profile which relies on tuning the level of excited-state aromaticity (ESA). In a series of fluorescent p-expanded oxepins,
the energy decrease accompanying the bent-to-planar conformational change in S1 becomes less pronounced with lower
ESA levels. Stabilization energies following from ESA were quantitatively estimated to be 10–20 kcal/mol using photo-
physical data. Very fast planarization dynamics in S1 was revealed by time-resolved fluorescence spectroscopy. The time
constants were estimated to be shorter than 1 ps, regardless of molecular size and level of ESA, indicating barrierless S1
planarization within the oxepin series.
INTRODUCTION
Very Fast
O
The shape of the S1 energy profile is important in the de-
sign of photofunctional molecules. For example, the
properties and functions of photochromic compounds,1
molecular rotors,2 and excited-state intramolecular proton
transfer (ESIPT) dyes3 are critically dependent on subtle
differences in the S1 energy profiles. Therefore, chemical
approaches to tuning the shape of the S1 energy profile
are invaluable for controlling the performance of photore-
sponsive materials,4 environment-sensitive fluorescent
probes,5 and in other applications. Conventionally, this
has been achieved by introducing electron-donating/with-
drawing substituents,6 changing aromatic rings,7 replac-
ing heteroatoms,8 providing internal steric constraints,9
etc. Here we propose a new conceptual approach for con-
trolling the shape of the S1 energy profile based on tuning
the level of excited-state aromaticity (ESA).
The theory of ESA was proposed by Baird in 197210
and now the significant role of ESA in various photofunc-
tional systems has gained wide acceptance.11 Neverthe-
less, experimental evaluation of the quantitative energet-
ics of ESA has been very limited,12 and there are no re-
ports on tuning the level of ESA for adjusting the S1
downhill change from bent to planar conformations. To
demonstrate this concept, we focused on the excited-state
planarization behavior of dibenz[b,f]oxepin (1),13
(< 1 ps)
1
Aromatic Stabilization
in the Excited State
O
2
S1
O
Ar
Ar
O
E
3
Fluorescence
S0
Conformational Coordinate
Figure 1. Excited-state aromatization of p-expanded oxepins.
whose aromaticity in S1, as well as in T1, has been pre-
dicted theoretically.14 The fluorescence (FL) properties of
1 which exhibits a large Stokes shift provide a clear start-
ing point for discussing ESA energetics. Similar planari-
zation dynamics have recently been studied in a series of
flapping fluorophores bearing p-expanded cyclooctatet-
raene (COT),15 dihydrophenazine,16 phenothiazine17 and
dibenzoarsepin.18 However, the S1 energy profiles of
these fluorophores have double (or multiple) minima po-
tentials due to electronic configuration switching during
the structural planarization, which makes it difficult to
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discuss the effect of ESA.15a In addition, the more popular
Steady-State Fluorescence Spectra and the Environ-
1
2
3
4
5
6
7
8
COT system shows undetectable or faint FL when it has
a single minimum S1 energy profile (Figures S46–53).
Therefore, for the current study we selected an emissive
oxepin system and developed a new series of p-expanded
oxepins. Spectroscopic analyses and theoretical calcula-
tions demonstrate that the shape of the S1 energy profile
changes significantly with the level of ESA (Figure 1).
Very fast planarization dynamics in S1 was observed by
time-resolved FL spectroscopy, which suggests that these
oxepins exhibit barrierless S1 energy profiles. It should be
noted that most of the existing research on ESAis focused
on T1, which is much easier to access computationally in
comparison to S1 that we study here.
ment Dependence. UV/visible absorption and steady-
state FL spectra of 1–3 in CH2Cl2 are shown in Figure 2.
Upon p-expansion, oxepins 1–3 showed red-shifted ab-
sorption peaks (290 nm, 317 nm, and 327 nm, respec-
tively) with increased molar absorption coefficients. FL
of 1 was observed at 450, 483, and 516 nm with a charac-
teristic vibronic structure and a remarkably large Stokes
shift (12300 cm–1), most probably due to the bent-to-pla-
nar conformational change in S1.13 Importantly, we found
that the FL peak tops of 1–3 were not shifted despite p-
expansion (450 nm for 1, 449 nm for 2, and 450 nm for 3
in the shortest FL wavelength). This unshifted FL behav-
ior is unusual in the context of the well-known acene
chemistry, where both the absorption and FL spectra are
red-shifted with p-expansion.20 Therefore, we expect that
there are differences between the S1 energy profiles of 1–
3. It should be mentioned that, with p-expansion, the FL
quantum yields decreased, and the FL lifetimes became
shorter (Table 1). This trend is explained by the changes
in the nonradiative decay constants knr of the three com-
pounds considering the relatively small differences in the
radiative decay constants. Solvent effects in the oxepin
series were found to be small, supporting negligible con-
tributions of intramolecular charge transfer in the respec-
tive excited-state dynamics (Figures S5–7).
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RESULTS AND DISCUSSION
Synthesis and Structural Characterization. Three p-
expanded oxepins: 1, benzo[b]naphtho[2,3-f]oxepin (2)
and dinaphtho[2,3-b:2¢,3¢-f]oxepin (3) were synthesized
(Scheme 1). Precursors of bromoaryl phosphonium ylide
and tosyl-substituted aryl aldehyde were coupled by the
Wittig reaction, and thus two aryl rings were bridged with
a cis-olefin. After deprotection of the tosyl group, nucle-
ophilic aromatic substitution was employed to form the
oxepin ring. Each of these reactions was performed in a
similar way as a one-pot process, giving 1 in 79%, 2 in
57% and 3 in 46% yields based on the aldehyde precur-
sors. X-ray single-crystal structures of 1–3 were obtained
showing bent conformations. Bending angles as defined
in Figure 5a were observed to be 26.8–27.9° (Figures S1–
4), in agreement with those in DFT ground state (S0) op-
timized structures (vide infra). These angles are signifi-
cantly smaller than their counterparts in reported COT de-
rivatives (ca. 40°),15a,19 indicating that p-expanded ox-
epins feature straighter V-shaped structures. The nonaro-
matic character of the S0 bent structures was demon-
strated by aromaticity indices (Table S2).
O
O
2
1
0
1
2
Abs
O
FL
3
e
600
300
400
500
Wavelength (nm)
Figure 2. UV/visible absorption and FL spectra of 1–3 in
CH2Cl2. Excitation wavelength: 280 nm.
a
p
Scheme 1. Synthesis of -Expanded Oxepins
Table 1. Photophysical Constants of Oxepins 1–3
(X-Ray)
OTs Br
Br
(ii)
OTs
(i)
kr (107 s–1)c
1.5
knr (107 s–1)d
a
FF
tF (ns)b
10.7
14.9
15.8
6.2
+
Br
Ph3P
1
CHO
79% (total)
B
CH2Cl2
0.16
7.8
4.8
4.4
14
A
(1.2 eq.)
1
2
3
PMMA 0.28
1.9
OTs Br
OTs Br
OTs
(ii)
(i)
+
B
2
Crystal
CH2Cl2
0.31
0.09
2.0
CHO
57% (total)
C
(1.2 eq.)
1.5
Br
(i)
(ii)
PMMA 0.16
8.0
2.0
10
Br
Ph3P
C
+
3
Crystal
CH2Cl2
0.21
0.04
9.1
2.3
8.7
20
(1.2 eq.)
46% (total)
4.8
0.9
aConditions: (i) t-BuOK (1.4 eq), THF, 0 °C, 30 min then A
or C (1.0 eq), THF, 25 °C, 16 h. (ii) KOH (32 eq), EtOH/H2O,
reflux, 1 h, then K2CO3 (4.0 eq), NMP, 120 °C, 20 h. Ts: p-
Toluenesulfonyl. NMP: N-Methylpyrrolidone. In the X-ray
crystal structures of 1–3 on the right side, thermal ellipsoids
were set to the 50% probability level.
PMMA 0.09
Crystal 0.21
6.5
1.4
14
7.2
2.9
11
a FL quantum yield. b FL lifetime. c Radiative and d nonradi-
ative decay rate constants, calculated from FF and tF.
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We were surprised to observe that the steady-state
cases of COT-fused anthracene dimers.15a,15c Interestingly,
the FL spectrum for microcrystals of 1 was similar to that in
solution (Figure 3g). The excitation spectrum was red shifted
compared with that in solution (Figure S8), indicating the
presence of intermolecular interactions in the ground state.
However, the FL wavelengths are not largely shifted, and the
clear vibronic structure corresponding to the solution spec-
trum was observed for the microcrystals, which suggests that
the excited-state planarization still occurs in the crystal
packing. Whereas large structural changes in molecules are
usually thought to be prohibited in the crystalline phase, a
considerable number of exceptions have been reported re-
cently in photoresponsive crystals.21 To discuss the possibil-
ity of the structural planarization in crystals, we have ana-
lyzed the distribution of small voids in the crystal packing
structures of 1–3. As a result, small but significant void re-
gions were found near the “flapping wings” of 1–3 (benzene
and naphthalene moieties), which may allow the flapping
motions. (Figures 4 and S26–28). It is worth noting that the
FL spectrum for the microcrystals of 2 is not largely shifted
from the solution spectrum (Figure 3h), while the FL spec-
trum of 3 was significantly blue shifted in crystals (Figure
3i). These results suggest that the excited-state structural
change of the large-sized molecule would not provide com-
pletely planar geometry in the crystals, even considering the
presence of the small voids surrounding the molecule.
1
FL spectra of 1–3 showed no apparent dependence on the
viscosity of the DMSO/glycerol mixed solvents (Figure 3a–
c), whereas flapping systems studied previously have been
reported to exhibit remarkable viscosity responses.15c,16c The
apparent viscosity independence can be attributed to the very
fast planarization dynamics with a barrierless S1 energy pro-
file (vide infra). Ostensibly, the population of V-shaped spe-
cies does not accumulate even in viscous media and, there-
fore, emission from V-shaped species is not observed in the
steady-state FL spectra. More importantly, the large Stokes
shift of 1 was still observed even in a frozen medium of 2-
methyltetrahydrofuran (Figures 3d), most likely because the
small structural change from the relatively straight V-shaped
form to the planar form is not suppressed, as suggested in the
literature for 1.13 On the other hand, blue shifts in the shortest
FL wavelengths during the freezing process from –140 to –
192 °C became more explicit as the molecular size increases
(448 to 445 nm for 1, 446 to 438 nm for 2, and 449 to 437
nm for 3; Figures 3d–f). Since the excitation spectra were
not largely shifted during the freezing process (Figures S14,
S17, and S20), the Stokes shifts became smaller. This result
indicates that, in frozen media, the structural change in the
excited state is partially suppressed for the larger molecules
2 and 3. This tendency was also confirmed in a rigid polymer
(PMMA) matrix (Figures 3g–i), although the degree of the
structural suppression is still smaller than in the reported
2
3
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Viscosity
(cP)
3.4
DMSO/glycerol
DMSO/glycerol Viscosity
a
b
c
Viscosity
(cP)
3.4
5.5
9.2
DMSO/glycerol
(vol%)
(vol%)
90:10
(vol%)
90:10
(cP)
3.4
5.5
9.2
16
90:10
80:20
70:30
60:40
80:20
70:30
60:40
50:50
40:60
30:70
10:90
5.5
9.2
16
29
55
112
564
80:20
70:30
60:40
50:50
40:60
30:70
20:80
O
O
O
16
29
449–450
452–453
29
55
449–450
50:50
112
243
400
500
600
400
500
Wavelength (nm)
600
400
500
Wavelength (nm)
600
Wavelength (nm)
d
e
f
O
O
O
25 °C
25 °C
−140 °C
25 °C
−140 °C
448
445
437
446
438
−140 °C
(not completely frozen)
(not completely frozen)
(not completely frozen)
449
−192 °C
(completely frozen)
−192 °C
−192 °C
(completely frozen)
(completely frozen)
400
500
600
400
500
600
400
500
Wavelength (nm)
600
Wavelength (nm)
Wavelength (nm)
g
h
i
O
O
O
441
450
453
CH2Cl2
PMMA
Crystal
443
CH2Cl2
PMMA
Crystal
440
CH2Cl2
PMMA
Crystal
459
449
446
450
400
500
600
400
500
Wavelength (nm)
600
400
500
Wavelength (nm)
600
Wavelength (nm)
Figure 3. (a–c) FL spectra of 1–3 in DMSO/glycerol mixed solvents at 25 °C. FL spectrum in higher glycerol ratio is not shown
due to poor solubility. (d–f) Temperature-dependent FL spectra of 1–3 in 2-methyltetrahydrofuran (2-MeTHF). Note that the
melting point of 2-MeTHF is –136 °C. (g–i) FL spectra of 1–3 in CH2Cl2 (ca. 10–6 M, black), a PMMA matrix (0.01 wt%
dispersion, green) and microcrystals (as-prepared, red) at 25 °C. Excitation wavelength: 280 nm.
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a
significant disparity in the levels of S1 aromaticity in
1
2
3
4
5
6
7
8
these compounds which follow the order 1 > 2 > 3, where
stronger S1 aromaticity was indicated by higher
HOMHED and more negative NICS(1)zz values (Table 2).
Since the S1 electronic configurations of the planarized
geometries are dominated by the respective HOMO–
LUMO transitions (Figure 6), the CASSCF(2,2) method
was applied to the estimation of the S1 NICS values.22
(Moderate oscillator strengths for the S0–S1 transitions
are consistent with the fluorescent properties of 1–3.) As
a result, p-expansion of oxepins weakens the S1 aroma-
ticity, providing a flatter energy profile. Accordingly, the
lowest harmonic S1 frequencies, corresponding to the
flapping vibrational mode, were obtained as 69 cm–1 for
1, 46 cm–1 for 2, and 32 cm–1 for 3 at the TD B3LYP-
D3BJ/def2TZVP level.
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Side view
Top view
Crystal
S1 opt
Figure 4. (a) Representation of voids in the crystal pack-
ing structure of 1. Probe radius: 0.65 Å. (b) Superposi-
tions of the X-ray crystal structure (blue) and the S1 opti-
mized structure at the TD CAM-B3LYP/6-31+G(d) level
(red) for 1.
Energy Profile and Evaluation of Aromaticity. To re-
veal the reasons for the anomalous unshifted FL behavior
in Figure 2, the S0 and S1 energy profiles were calculated
at the (TD)CAM-B3LYP/6-31+G(d) level, through con-
strained geometry optimizations at fixed values for the
bending angle of the oxepin ring (Figure 5). The bending
angle was defined as the dihedral angle between the O–
C3–C4 and the O–C1–C2–C3 planes. The S0 local mini-
mum geometries of 1–3 were found to be V-shaped with
bending angles of 25.7–26.9°; planar optimized geome-
tries were found to correspond to transition states of S0
flapping motion (Figures S23–25). The S0 inversion bar-
riers are very similar (4.2 for 1, 3.7 for 2, 3.6 for 3, in kcal
mol–1). To evaluate aromaticity in S0 and S1, further opti-
mizations were performed at the (TD)B3LYP-
D3BJ/def2TZVP level, followed by CASSCF(2,2)-
GIAO/6-311G(d) NICS calculations at the optimized ge-
ometries (Figures S44–45). The (TD)CAM-B3LYP/6-
31+G(d) and (TD)B3LYP-D3BJ/def2TZVP geometries
are in very good agreement, which suggests that further
improvements of the DFT approach are unlikely to intro-
duce significant changes. A singlet CASSCF(2,2) con-
struction can be used to approximate the S1 wavefunction
and calculate S1 NICS when S1 is dominated by the
HOMO–LUMO transition.22 The S0 planar transition
states of 1–3 are found to be strongly antiaromatic as in-
dicated by several calculated aromaticity indices:
HOMHED23, NICS(1)zz24, and ACID25 (Table 2, Figures
S36–41). In S1, single minimum energy profiles were ob-
tained, with planar optimal geometries of C2v and Cs sym-
metry, for 1 and 3, and 2, respectively (Figures S23–25).
According to the results of the S0 and S1 calculations, the
V-shaped S0 geometries of 1–3 are all planarized by pho-
toexcitation in S1. It is important to note that the extent of
energy stabilization by planarization is remarkably differ-
ent between 1–3 (Figure 5). The differences stem from the
Figure 5. Calculated S1 and S0 energy profiles of oxepins 1
(black), 2 (blue), and 3 (red) at the (TD)CAM-B3LYP/6-
31+G(d) level. Energies relative to those of the S1 and S0
optimized geometries were plotted vs the constrained bend-
ing angles.
Table 2. Aromaticity Indices of 1–3 in S0 (TS) and S1
HOMHED
(oxepin)a
HOMHED
(perimeters)a
NICS(1)zz
(ppm) a
S0
S1
S0
S1
S0
S1
1
2
3
0.676
0.685
0.686
0.835
0.814
0.802
0.849
0.875
0.890
0.926
0.932
0.942
22.3
19.9
18.3
–53.2
–51.5
–45.6
aCalculations were performed at planar transition state S0 geome-
tries and S1 local minimum geometries obtained at the (TD)
B3LYP-D3BJ/def2TZVP level. NICS(1)zz results come from
CASSCF(2,2)-GIAO/6-311G(d) calculations.
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must be different gaps in the S1 stabilization energies dur-
a
E (eV)
1
O
O
ing planarization (green arrows), that is, the differences
in the degree of S1 aromaticity. The relative energies of
the S0/S1 states at the bent/planar geometries obtained
from spectral data (Figure 2) are summarized in Figure 7b.
The longest wavelength absorption peak was used to es-
timate the S0®S1 vertical transition energy at the S0 V-
shaped geometry, namely, the energy gap between
E(S0)//S0 and E(S1)//S0 (98.6 for 1, 90.5 for 2, and 87.4
for 3, all in kcal mol–1), while the shortest wavelength FL
peak was taken as the S1®S0 vertical transition energy at
the S1 planar geometry, namely, the energy gap between
E(S1)//S1 and E(S0)//S1 (63.5 for 1, 63.7 for 2, and 63.5
for 3, all in kcal mol–1). The S0 energy gap between
E(S0)//S0 and E(S0)//S1 (15.5 for 1, 14.4 for 2, and 13.5
for 3, all in kcal mol–1) was taken from the CAM-
B3LYP/6-31+G(d) calculations. As a result, the bent-to-
planar energy gaps in S1 (= E(S1)//S0 – E(S1)//S1) were
estimated to be 19.6 for 1, 12.4 for 2, and 10.4 for 3, all
in kcal mol–1. These values can be viewed as stabilization
energies gained by S1 aromaticity and are comparable to
those reported for a COT-based 8p system (21~22 kcal
mol–1), although the evaluation method is different.12
2
3
4
5
6
7
8
9
1
0
L+1
−1
−2
−6
−7
−8
−9
L
f = 0.05
≈
H → L (98%)
H
H−1
10
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q = 30°
q = 0°
30°
20°
10°
0°
Bending angle q
b
O
O
E (eV)
1
L+1
L
0
−1
−2
−6
−7
−8
−9
f = 0.10
H → L (94%)
H
≈
H−1
H−2
H−3
a
O
30°
20°
10°
0°
q = 30°
1
Bending angle q
O
Weaker S1 aromaticity
c
2
by p expansion
O
O
S1
O
E (eV)
3
E
Absorption
(red-shifted)
Fluorescence
(unshifted)
1
L+2
L+1
0
−1
−2
−6
−7
−8
−9
S0
Small difference
in the planarization barriers
L
f = 0.17
H → L (89%)
≈
Conformational coordinate
H
b
H−1
E (kcal mol−1
)
30°
20°
10°
0°
H−2
arbitrary scale
Stabilization energy
gained by S1 aromaticity
12.4
10.4
Bending angle q
E(S1)//S0
q = 30°
q = 0°
19.6
Figure 6. (a–c) Kohn-Sham molecular orbitals and their en-
ergies for constrained optimized geometries of 1–3 in S1 at
different bending angles, calculated at the TD CAM-
B3LYP/6-31+G(d) level. The two bending angles on the
benzene and naphthalene sides were fixed to be equal for 2.
Compositions (%) and oscillator strengths (f) are shown for
planar geometries. H and L stand for HOMO and LUMO,
respectively.
E(S1)//S1
98.6
90.5
Abs
63.5
63.7
63.5
FL
87.4
E(S0)//S1
15.5
14.4
13.5
E(S0)//S0
Bent
(S0 opt)
Planar
(S1 opt)
The shapes of the energy profiles of 1–3 is roughly
sketched in Figure 7a. The absorption wavelengths were
red-shifted by p-expansion (smaller in energy; purple ar-
rows), whereas unshifted fluorescence was observed (yel-
low arrows). Since the bent-to-planar energy barriers in
S0 show a small difference for 1–3 (black arrows), the
main factor responsible for the unshifted fluorescence
Figure 7. (a) Schematic energy diagrams for 1–3 showing
the energy balance for absorption, FL, planarization barriers
in S0, and excited state aromatization in S1. (b) Relative en-
ergy levels of the S0/S1 states at the bent/planar confor-
mations for 1 (black), 2 (blue), and 3 (red). The energies of
vertical transitions were obtained from experimental results.
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Figure 8. (a) Femtosecond time-resolved FL spectra of 1–3 in CH2Cl2 obtained after photoexcitation at 343 nm. The sharp
spectral feature at ~380 nm denoted by the asterisk is due to the Raman signal of the solvent. (b) Temporal profiles of the
time-resolved FL signals at 480 nm. (c) Early time windows of the temporal profiles of the time-resolved FL signals at 410
and 480 nm.
Ultrafast Dynamics. To obtain deeper insights into the
planarization dynamics and the S1 potential energy land-
scape of 1–3, we investigated their excited-state dynam-
ics by femtosecond time-resolved FL spectroscopy. Fig-
ure 8a shows the time-resolved FL spectra of 1–3 in
CH2Cl2 upon 343-nm photoexcitation. In all cases of 1–3,
the spectral and temporal behaviors were found to be
quite similar. Immediately after photoexcitation (i.e., at 0
ps), a broad FL spectrum with peaks around 410 nm and
445 nm was observed for each of 1–3. This early time
spectrum looks very different from the corresponding
steady-state FL spectrum and it is attributed to the FL
from the V-shaped conformation. Subsequently, the time-
resolved FL spectrum showed rapid spectral change to-
wards 1 ps, giving rise to a spectral shape almost identical
to the steady-state FL spectrum of each sample. Such
spectral changes are clearly observable in the time-re-
solved FL but not in the time-resolved absorption (Figure
S54). After 1 ps, the entire FL band decayed slowly on the
ns time scale (Figure 8b), in agreement with the FL life-
times estimated separately (see Table 1).
Our time-resolved FL data clearly show that the
V-shaped-to-planar conformational change in 1–3 occurs
on the ultrafast time scale. Remarkably, the FL signal of
the V-shaped conformation is observed only around the
time origin and the FL signal due to the planar confor-
mation (e.g., at 480 nm) rises almost instantaneously as
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shown in Figure 8c. These results indicate that the V-
AUTHOR INFORMATION
1
2
3
4
5
6
7
8
shaped-to-planar conformational change in 1–3 is com-
pleted within the instrumental response of the setup (~330
fs). This planarization time scale is much shorter than
those of other flapping molecules in low viscous me-
dia,15a-b,16c-d,17 and suggests that the planarization pro-
cesses of 1–3 are all downhill and barrierless, and proceed
continuously on the S1 potential energy surface.
Corresponding Authors
tahei@riken.jp
peter.karadakov@york.ac.uk
s_saito@kuchem.kyoto-u.ac.jp
Funding Sources
This work was supported by JST, PRESTO, Grant Numbers
JPMJPR16P6 to S.S. and JPMJPR17P4 to H.K., by JSPS,
KAKENHI, Grant Numbers JP18H01952 and JP17H05258
to S.S., by JSPS, Research Fellowship for Young Scientists
Grant Number JP18J22477 to R. K.
According to the theoretical calculations shown
in Figures 5 and 7, the S1 state of oxepins has a steeper
downhill profile as the molecule becomes smaller. In this
case, one would expect faster decay of the V-shaped con-
formation for the smaller oxepin. Interestingly, the signal
around 410 nm looks less pronounced as the molecule be-
comes smaller. (This is the reason why the temporal evo-
lution of the time-resolved spectrum looks slightly differ-
ent for 1–3 in Figure 8a.) Although the temporal behav-
iors are not fully resolved with our time resolution, this
result suggests that the lifetime of the V-shaped confor-
mation becomes shorter as the molecule becomes smaller.
In other words, it is expected that the planarization time
scale is governed by the steepness of the S1 energy slopes
along the planarization coordinate, as the theoretical cal-
culation suggested.
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ACKNOWLEDGMENT
We thank Prof. Dr. Rainer Herges and Fynn Röhricht (Kiel
University) for providing us the program of the ACID calcu-
lations, and Dr. Kazuya Otsubo (Kyoto University) for ad-
vice on the representation of void in the crystal packing.
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CONCLUSION
In conclusion, we have demonstrated that tuning the level
of excited-state aromaticity provides a convenient way of
controlling the S1 energy profiles of photoresponsive
molecules. Aromatic stabilization energies were experi-
mentally compared for a series of oxepins, in which fur-
ther p-expansion was shown to lead to weaker S1 aroma-
ticity and produce a flatter S1 downhill planarization en-
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with the degree of geometrical and magnetic measures of
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for 1–3) were observed in all oxepins we studied, despite
the differences in molecular size and level of ESA.
Changing the degree of the S1 aromatic stabilization en-
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soft materials and supramolecules, because the S1 aroma-
tization can serve as a driving force for the ultrafast per-
turbation of their packing structures.15b
ASSOCIATED CONTENT
The Supporting Information is available free of charge on
the ACS Publications website. Crystallographic data are
also available free of charge from The Cambridge Crystal-
lographic Data Centre, codes 1997512 for 1, 1997513 for 2
and 1997513 for 3.
Synthetic protocols, characterization data, photophysical
spectra, theoretical calculation (PDF)
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Table of Contents (TOC)
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44
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46
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50
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53
54
55
56
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10
ACS Paragon Plus Environment