Conformational Planarization versus Singlet Fission: Distinct Excited-State Dynamics of Cyclooctatetraene-Fused Acene Dimers

Article abstract of DOI:10.1002/anie.201802185

A set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited-state dynamics in solution. While the anthracene dimer showed a fast V-shaped-to-planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems.

Full text of DOI:10.1002/anie.201802185

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Accepted Article
Title: Conformational Planarization vs Singlet Fission: Distinct Excited-
State Dynamics of Cyclooctatetraene-Fused Acene Dimers
Authors: Takuya Yamakado, Shota Takahashi, Kazuya Watanabe,
Yoshiyasu Matsumoto, Atsuhiro Osuka, and Shohei Saito
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802185
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Conformational Planarization vs Singlet Fission: Distinct Excited-
State Dynamics of Cyclooctatetraene-Fused Acene Dimers
Takuya Yamakado†, Shota Takahashi†, Kazuya Watanabe*, Yoshiyasu Matsumoto, Atsuhiro Osuka,
and Shohei Saito*
Abstract: A set of flapping acene dimers fused with an 8p
cyclooctatetraene (COT) ring showed distinct excited-state dynamics
in solution. While the anthracene dimer showed a fast V-shaped-to-
planar conformational change within 10 ps in the lowest excited
singlet state, reminding us of extended Baird aromaticity, the
tetracene dimer and the pentacene dimer underwent intramolecular
singlet fission (SF) in different manners: A fast and reversible SF
with a characteristic delayed fluorescence (FL), and a fast and
quantitative SF, respectively. Conformational flexibility of the fused
COT linkage plays an important role in these ultrafast dynamics,
demonstrating the utility of the flapping molecular series as a
versatile platform for designing photofunctional systems.
Flapping molecules (FLAP) bearing a fused 8p ring at the center
have been recently recognized as an emerging class of
photoresponsive systems. For example, the molecules in Figure
1, including FLAP0, were reported as key structures that show a
V-shaped-to-planar conformational change in the lowest singlet
excited state (S1), emitting fluorescence (FL) with large Stokes
shifts.[1] Along with this line, we have demonstrated the utilities
of FLAP as photofunctional materials such as molecular
viscosity probes,[2a] photoresponsive liquid crystals,[2b] and light-
melt adhesive.[2c] FLAP molecules have also attracted attentions
Figure 2. a)
A set of cyclooctatetraene(COT)-fused acene dimers. b)
from the viewpoint of 4np excited-state aromaticity, since this
planarization behavior is reminiscent of extended Baird’s
rule.[3,2b] However, the scope and limitations of the planarization
dynamics of FLAP have not been clarified, particularly for those
having potentially competing dynamics in the excited state.
Schematic energy diagram of the COT fused dimers. Two possible dynamics,
conformational planarization in S1 and singlet fission to a correlated triplet pair
(TT), are indicated.
Here we report on a set of acene dimers FLAP1–3 that
have an 8p cyclooctatetraene (COT) ring at the center. These
molecules display the conformational planarization or singlet
fission (SF) in the photo-excited state, depending on the acene
length (Figure 2). Since the first detection of SF was reported for
anthracene crystal,[4] various acene derivatives have been
explored to study SF. Recently, more interest has been focused
on SF, partly because it has a potential impact to enhance the
efficiency of organic photovoltaics.[5,6] In particular, the
intramolecular SF of covalently-linked dimers of tetracene,[7]
pentacene,[7g,8] and other chromophores[9] are widely explored
for the mechanistic study in solution. However, correlation
between the SF dynamics and the interchromophore
configuration in the dimers has remained elusive; previous
works have focused on dimer structures in which the
chromophores are either freely rotatable by a single bond[8b,c] or
fixed by a rigid spacer[7d,f,g,8k]. In this context, a structural motif
with possible flapping motion provides a unique opportunity to
Figure 1. a) Excited-state planarization of conformationally flexible molecules.
b) Reported 8p ring-fused molecules that show
a V-shaped-to-planar
conformational change in S1, emitting fluorescence with large Stokes shift.
[*]
T. Yamakado, S. Takahashi, Prof. Dr. K. Watanabe, Prof. Dr. Y.
Matsumoto, Prof. Dr. A. Osuka, Prof. Dr. S. Saito
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: s_saito@kuchem.kyoto-u.ac.jp, kw@kuchem.kyoto-u.ac.jp
Prof. Dr. S. Saito
study SF dynamics as
a
function of
a
well-defined
interchromophore coordinate: the flapping angle. Therefore,
systematic investigation of excited-state dynamics of the COT-
fused anthracene, tetracene, and pentacene dimers FLAP1–3 is
expected to provide important insight in both chemistries of the
flapping fluorophores (FLAP) and the SF dynamics.
JST-PRESTO, FRONTIER
[†]
These authors contributed equally to this work.
CCDC 1822727, 1822728, and 1822729 contain the supplementary
crystallographic data for this paper. These data are provided free of
charge by The Cambridge Crystallographic Data Centre.
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FLAP1 was synthesized using Ni-catalyzed [2+2+2+2]
cycloaddition of naphthalene-based diynes.[2c,10] FLAP2 and
FLAP3 were synthesized in three steps from a tetraaldehyde of
dibenzo[a,e]COT in 14% and 9% yields, respectively
(Supporting Information, Section 2). To enhance the
of FLAP1 undergoes a fast conformational planarization without
SF. Absence of SF in FLAP1 is reasonable in consideration of
the relative energy levels of S1 and T1 of anthracene; namely,
E(S1) is much lower than 2E(T1) in anthracene.[5b]
photostability,
triisopropylsilyl(TIPS)ethynyl
groups
were
introduced at the tetracene and pentacene moieties,[11] although
the tetracene dimer FLAP2 still showed serious photosensitivity
particularly in the presence of oxygen (Supporting Information,
Section 4). Single-crystal X-ray structure analysis of FLAP1–3
demonstrated their V-shaped structures in the ground state
(Figure 3). Bent angles q of the COT ring are 41° in FLAP1 and
~45° in FLAP2 and FLAP3, while dihedral angles j between the
mean planes of two acene moieties are 109° in FLAP1 and 80–
84° in FLAP2 and FLAP3. The observed shallow V-shaped
conformation of FLAP1 is probably induced by packing force in
a tightly stacked columnar structure (Figure S7.1), similarly to
the previously reported FLAP0 derivatives.[1b,c] Their inversion
barriers between the V- and L–shaped conformations were
estimated to be lower than 10 kcal/mol (Table S9.1), suggesting
fast flapping behaviors of FLAP1–3 at room temperature.
Figure 4. a) UV-visible absorption spectrum of FLAP1 in CH2Cl2 (black) and
FL spectra of FLAP1 in a PMMA film (1.0 wt%) (blue) and in CH2Cl2 (green).
Excitation wavelength is 365 nm. b) Time-resolved FL spectra of FLAP1
excited at 400 nm in CH2Cl2.
Figure 3. X-ray crystal structures of a) FLAP1, b) FLAP2 and c) FLAP3. Bent
angles q of the COT ring and dihedral angles j between the mean planes of
the two acene moieties were summarized in the inset table.
FLAP1 showed a green FL (FF = 0.27) with a large Stokes
shift (5320 cm–1) in CH2Cl2, reflecting a V-shaped-to-planar
conformational relaxation in S1, as in the case of excited-state
dynamics of FLAP0.[1b] When FLAP1 was dispersed in a
poly(methyl methacrylate) (PMMA) film, a blue FL was observed
with a small Stokes shift because the conformation change in S1
was suppressed in the rigid PMMA matrix (Figure 4a). To track
the conformational planarization of FLAP1, time-resolved FL
measurement was performed using a CH2Cl2 solution (Figure
4b). Just after the excitation, a broad blue FL band at the
maximum of around 460 nm was observed, which was assigned
to an emission from V-shaped conformation since this FL was
similar to the steady-state blue FL of FLAP1 in a PMMA matrix.
This blue FL decayed with a time constant of 4.5 ± 0.1 ps, which
was followed by appearance of green FL with a clear vibronic
structure. This FL was virtually the same as the steady-state FL
of FLAP1 in CH2Cl2. In femtosecond time-resolved transient
absorption (TA) spectroscopy, the spectral change was
confirmed with a time constant of 4.1 ± 0.1 ps (Supporting
Information, Section 5). These data indicate that the S1 species
Figure 5. UV-vis absorption (black) and FL (red) spectra of a) FLAP2 and b)
FLAP3 in CH2Cl2. Excitation wavelengths are 515 nm for FLAP2 and 609 nm
for FLAP3. FL quantum yields (FF) are considered in the area of the FL
spectra. FL decay curve of FLAP2 is displayed as an inset of Figure 5a.
FLAP2 and FLAP3 showed red-shifted absorption bands at
554 nm and 662 nm, respectively, as compared with those of
bis(TIPS-ethynyl)-substituted tetracene (Tc) and pentacene (Pc)
(Figure 5 and Figure S4.1). Quite different from FLAP1, the
elongated acene dimers FLAP2 and FLAP3 showed weak FL
with a small Stokes shift of 160 cm–1 for both the molecules. FL
quantum yields were 0.14 for FLAP2 and < 0.01 for FLAP3,
being much smaller than that of Tc (FF = 0.86) and Pc (FF
=
0.21). These results indicate the following two features of the
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excited-state dynamics in FLAP2 and FLAP3: 1) The emission
mostly occurs with the V-shaped conformation; 2) The majority
of S1 is converted to another dark state. A possibility of a
dynamic conformational change into a non-emitting S1 state or a
conical intersection was excluded, because the FL quantum
yields of FLAP2 and FLAP3 were not enhanced even in a rigid
PMMA matrix (Table S4.1). From these results, the occurrences
of solution-phase SF are highly probable in FLAP2 and FLAP3.
ps. Note that the SF yield in FLAP3 was estimated to be FSF =
2.02 ± 0.03 by determining the molar extinction coefficient of
T1®Tn band by triplet sensitization (Supporting Information,
Section 6.3). This means nearly 100% of the photoexcited
species undergoes SF in FLAP3, which is consistent with its
practically zero FL quantum yield. (TT) likely possesses the V-
shaped conformation rather than the flattened one because the
absorption spectrum of (TT) (the TA spectrum at 20 ps in Figure
6b) reasonably agrees with those reported in other pentacene
dimers,[8] indicating absence of significant p-conjugation
between the two pentacene moieties. The lifetime of (TT) was
determined to be tTT = 5.3 ± 0.1 ns by TA measurement in
nanosecond range (Figure 7b). Comparison of the estimated tSF
and tTT of FLAP3 with those reported for other linked-pentacene
dimers may be useful for elucidating how through-bond coupling
in SF depends on the chemical structures of the linking moiety.
Both tSF and tTT of FLAP3 are lying between those reported for
a directly linked pentacene dimer (tSF = 0.76 ps and tTT = 0.45
ns) and a para-phenylene-linked pentacene dimer (tSF = 20 ps
and tTT = 16.5 ns) (Table S6.4).[8k] Therefore, a degree of the
electronic coupling mediated by the COT moiety is just between
those cases.
More importantly, a unique role of the flexible COT structure
in the SF dynamics is demonstrated. FLAP2 and FLAP3
possess C2v symmetry at their equilibrium geometries, whereas
the electronic coupling between S1 and (TT) vanishes with this
symmetry.[7d,14] Therefore, the observed efficient SF is likely to
be promoted by excitation of symmetry-breaking vibrational
modes originating from the conformational flexibility of the fused
COT linkage. The efficient SF in the ultrafast timescale realized
under the unfavorable symmetry for SF would provide new
insight into vital roles of symmetry and interchromophore
vibrations in SF.
Figure 6. Transient absorption spectra of a) FLAP2 excited at 525 nm and b)
FLAP3 excited at 600 nm in CH2Cl2.
Transient absorption (TA) measurements were conducted
for FLAP2 and FLAP3 in CH2Cl2 (Figure 6). The TA of FLAP2 in
Figure 6a showed negative absorbance at 560 nm and 610 nm
due to the ground-state bleach and the stimulated emission,
respectively. The S1®Sn absorption band centered around at
433 nm decreased, which was followed by a rise of a new
absorption band at 534 nm that could be attributed to the
absorption of (TT). The occurrence of SF was confirmed by a
spectral decomposition analysis based on a triplet sensitization
experiment (Supporting Information, Section 6). Note that the
conversion from S1 to (TT) saturated at around 10 ps, indicating
that SF in FLAP2 reaches equilibrium with the inverse process,
triplet fusion, at this delay time. From the kinetic analysis
considering a partial photodegradation of FLAP2, fast and
efficient SF was indicated with the parameters of tSF ~ 3 ps and
FSF ~ 1.8 (Supporting Information, Sections 4 and 6). A FL
lifetime of 120 ns, observed as a main component (Figure 5a,
inset), is significantly longer than that of the tetracene monomer
Tc (tFL = 13 ns). Since the ratio of the slow-decay FL component
becomes lower as the temperature decreases (Figure S4.13),
the emission process can be assigned to a delayed FL of
FLAP2 in the thermal equilibrium between S1 and (TT). The
long-lived excited species causing the long FL lifetime was
rapidly quenched by air bubbling, which is consistent with the
involvement of (TT). Considering device applications, the long-
lived (TT) of FLAP2 is favorable for inducing the subsequent
separation of (TT) into two triplets.[12]
Figure 7. a) Population evolution of S1 and (TT) obtained by a global analysis
of the transient absorption spectra of FLAP3. See Section 6.4 in Supporting
Information for the detail, b) (TT) lifetime of FLAP3 in CH2Cl2. Transient
absorption of (TT) was monitored at 530 nm up to the time delay of 1.7 ns. c)
Overall SF dynamics of FLAP3.
Note that the time constants of conformational planarization
observed in FLAP1 and SF observed in FLAP3 are comparable,
about 4 ps in CH2Cl2 solution. To discuss the reason why
FLAP2 and FLAP3 did not show conformational planarization in
S1, we performed DFT and TD-DFT calculations on these acene
dimers at the PBE0/6-31+G(d) level of theory (Supporting
Information, Section 8). The TIPS groups of FLAP2 and FLAP3
FLAP3 exhibits clearer spectral changes indicating
quantitative SF. A global analysis[13] of TA spectra in Figure 6b
revealed that S1 with a broad positive band at 430–600 nm is
converted into (TT) with sharp absorption peaked at 530 nm.
The kinetic traces of the two species are plotted in Figure 7a,
which shows SF occurs with a time constant of tSF = 4.23 ± 0.02
were replaced with hydrogen atoms (FLAP2 and FLAP3 ) to
¢ ¢
reduce computational costs (Figure 8). To confirm the effect of
alkyne moieties, non-substituted tetracene dimer and pentacene
dimer, FLAP2¢¢ and FLAP3¢¢, were also calculated (Figure S8.1).
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DFT optimized structures of all the FLAP systems take V-
shaped conformations in S0, consistent with the X-ray crystal
structure analysis. S0 and S1 energy profiles were delineated by
constrained geometry optimization with fixed bent angle q in the
In summary, a set of flapping acene dimers that possess a
flexible COT core demonstrate distinct ultrafast dynamics. While
the COT-fused anthracene dimer shows a fast V-shaped-to-
planar conformational dynamics in S1 (t = 4 ps in CH2Cl2), the
tetracene dimer and the pentacene dimer exhibit intramolecular
SF in solution, retaining their V-shaped conformation. Although
the tetracene dimer shows serious oxygen sensitivity under
ambient light, fast and efficient SF (tSF ~ 3 ps, FSF ~ 1.8 in
CH2Cl2) has been confirmed with a delayed FL (tF ~ 120 ns).
The long-lived (TT) of the tetracene dimer is promising for the
subsequent separation into two triplets. The pentacene dimer
underwent fast and quantitative SF (tSF~ 4.2 ps, FSF ~ 2.0 in
range of 0°–45° at 5° intervals. The S1 energy profiles of FLAP2
¢
,
FLAP2¢¢, FLAP3 and FLAP3¢¢ showed monotonic increase as
¢
the structure was more planarized (Figure 8b–8c and Figure
S8.22), while the S1 energy profile of FLAP1 showed two
minima: a local minimum at a shallow V-shaped conformation (q
= 25.8°) and a global minimum at the planar conformation (q =
0°) (Figure 8a). The difference in these energy profiles is
discussed from molecular orbital perspective in the Supporting
Information, Section 8. As the conformation was more
planarized, the anthracene dimer FLAP1 showed an electronic
configurational change of S1 due to an enhanced participation of
8p COT in a planar geometry. On the other hand, the tetracene
CH2Cl2) with
a (TT) lifetime of 5.3 ns. Importantly, a
characteristic feature of the conformationally flexible COT
linkage was suggested for breaking the molecular symmetry to
activate SF, otherwise it is forbidden. The absence of the
conformational planarization dynamics in the tetracene and
pentacene dimers was supported by theoretical calculations in
S1, whose energy potential no longer gives a minimum at the
planar conformation because an electronic contribution of the 8p
COT ring is reduced as the acene moiety is elongated. Through
this study, unique roles of the fused COT linkage in ultrafast
dynamics of acene dimers were demonstrated, showing the
utility of the flapping structural motif as a versatile platform for
designing photofunctional systems. Using the conformational
flexibility, the orientation of two chromophores could be
dynamically controlled by external stimuli, such as mechanical
force[17] or hydrostatic pressure.[18] Stimuli-induced switching of
SF dynamics as well as dual fluorescence properties are
currently explored in our laboratory.
dimers FLAP2 and FLAP2¢¢, and the pentacene dimers FLAP3
¢ ¢
and FLAP3¢¢ showed no configuration switching of S1 during the
conformational planarization, because significant p conjugation
in their longer acene moieties ensures large contribution of
acene p orbitals in their S1 configurations irrespective of the
planarization. As a result, only FLAP1 showed the V-shaped-to-
planar conformational change in S1. It is worth noting that a
calculated S1 energy profile of the COT-fused naphthalene dimer
FLAP4 showed a steep energy decrease for the planarization
(Figure 8d), suggesting excited-state aromaticity of FLAP4 as it
has been predicted.[15] In case of FLAP1, however, in spite of
the observed spontaneous conformational planarization in S1,
excited-state aromaticity was not supported by our theoretical
calculations (Supporting Information, Section 9). This result
suggested that extended Baird’s rule (excited-state 4np
aromaticity in S1) cannot be necessarily applied to the molecules
that flatten in S1, although further theoretical study is expected
using multireference methods.[16]
Acknowledgements
This work was supported by JST, PRESTO Grant Numbers
JPMJPR12K5 and JPMJPR16P6 to S.S. and by JSPS,
KAKENHI Grant Numbers JP15H05482 and JP15H01083 to
S.S., and JP16H02249 to Y.M.
Conflict of interest
The authors declare no conflict of interest.
Keywords: Excited-state dynamics • Baird aromaticity • Singlet
fission • Cyclooctatetraene • Acene
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10.1002/anie.201802185
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Distinct excited-state dynamics were
Takuya Yamakado†, Shota Takahashi†,
Kazuya Watanabe*, Yoshiyasu
Matsumoto, Atsuhiro Osuka, and
Shohei Saito*
observed in
a set of COT-fused
flapping acene dimers (FLAP),
depending on the acene length.
While the anthracene dimer showed
a fast conformational planarization in
S1, the tetracene dimer and the
pentacene dimer underwent singlet
fission(SF), differently in a fast and
reversible manner and a fast and
quantitative manner, respectively,
providing new aspects of ultrafast
dynamics of the FLAP molecules.
Page No. – Page No.
Conformational Planarization vs
Singlet Fission: Distinct Excited-
State Dynamics of
Cyclooctatetraene-Fused Acene
Dimers
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