(5,8)Picenophanedienes: Syntheses, Structural Analyses, Molecular Dynamics, and Reversible Intramolecular Structure Conversion

Article abstract of DOI:10.1021/jacs.0c08115

This study presents an important and efficient synthetic approach to 5,8-dibromo-2,11-di-tert-butylpicene (3), with multigram scale, which was then converted to a new series of picenophanes (6-10). The tub-shaped [2,2](5,8)picenophanediene 8 with two cis-ethylene linkers was explored using X-ray crystallography. The tub-to-tub inversion proceed through the successive bending of the linkers and the barrier for isopropyl-substituted derivative 10 was experimentally estimated to be 18.7 kcal/mol. Picenophanes with a large π-system and semi-rigid structure exhibited anomalous photophysical properties. The ethano-bridged picenophane shows the weak exciton delocalization while the cis-ethylene-bridged picenophane exhibits dual emission rendered by the weakly delocalized exciton and excimer. With the aid of the ultrafast time-resolved emission spectroscopy, the mechanism of the excimer formation is resolved, showing a unique behavior of two-state reversible reaction with fast structural deformation whose lifetime is around 20 ps at 298 K. This work demonstrates that the slight difference in the bridge of tub-shaped picenophanes renders distinct photophysical behavior, revealing the potential of harnessing inter-moiety reaction in the picenophane systems.

Full text of DOI:10.1021/jacs.0c08115

pubs.acs.org/JACS
Article
[2,2](5,8)Picenophanedienes: Syntheses, Structural Analyses,
Molecular Dynamics, and Reversible Intramolecular Structure
Conversion
Min-Chih Tang,§ Yu-Chen Wei,§ Yen-Chen Chu, Cai-Xin Jiang, Zhi-Xuan Huang, Chi-Chi Wu,
Tzu-Hsuan Chao, Pei-Hsun Hong, Mu-Jeng Cheng,* Pi-Tai Chou,* and Yao-Ting Wu*
Cite This: https://dx.doi.org/10.1021/jacs.0c08115
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: This study presents an important and efficient
synthetic approach to 5,8-dibromo-2,11-di-tert-butylpicene (3),
with multigram scale, which was then converted to a new series of
picenophanes (6−10). The tub-shaped [2,2](5,8)picenophanediene
8 with two cis-ethylene linkers was explored using X-ray
crystallography. The tub-to-tub inversion proceed through the
successive bending of the linkers and the barrier for isopropyl-
substituted derivative 10 was experimentally estimated to be 18.7
kcal/mol. Picenophanes with a large π-system and semi-rigid
structure exhibited anomalous photophysical properties. The
ethano-bridged picenophane shows the weak exciton delocalization
while the cis-ethylene-bridged picenophane exhibits dual emission
rendered by the weakly delocalized exciton and excimer. With the aid
of the ultrafast time-resolved emission spectroscopy, the mechanism of the excimer formation is resolved, showing a unique behavior
of two-state reversible reaction with fast structural deformation whose lifetime is around 20 ps at 298 K. This work demonstrates that
the slight difference in the bridge of tub-shaped picenophanes renders distinct photophysical behavior, revealing the potential of
harnessing inter-moiety reaction in the picenophane systems.
b′]dianthracenes (CODA), which are photofunctional materi-
als used as photoresponsive liquid crystals and light-melt
adhesive.3 π-Extended derivatives, cycloocta[1,2-b:5,6-b′]-
ditetracene (CODT) and cycloocta[1,2-b:5,6-b′]dipentacene
(CODP), exhibit characteristic excited-state photodynamics
for attaining intramolecular singlet fission.4 Moreover, TSAs
play major roles in supramolecular chemistry; for example,
dicorannulene-fused tetraphenylene is a buckycatcher used to
encapsulate a C60 molecule.5 Enlarging the central ring size of
DBCOT leads to the formation of cyclophanes,6 such as
[2,2](2,7)naphthalenophanediene (NPD),7 [2,2](3,6)-
phenanthro(2,7)naphthalenophanediene (PNPD),8 [2,2]-
(3,6)phenanthrophanediene (PPD),9 [2.2](2,8)-
dibenzothiophenophanediene (DBTPD),10 [2.2](3,10)benzo-
[ c ] p h e n a n t h r e n o p h a n e d i e n e ( B P P D ) , 1 1 a n d
[2.2](3,11)dibenzo[a,j]-anthracenophanediene (DBAPD).12
INTRODUCTION
■
Dibenzo[a,e]cyclooctatetraene (DBCOT) and tetraphenylene
are the simplest (double) tub-shaped arenes (TSAs, Figure 1)
and are crucial ligands1 and building blocks2 in organic
syntheses. Expanding π systems leads to the diversification of
compound properties, as exemplified by cycloocta[1,2-b:5,6-
Received: July 28, 2020
Figure 1. Ethylene-bridged cycloarenes and analogues.
https://dx.doi.org/10.1021/jacs.0c08115
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
© XXXX American Chemical Society
A

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Except DBTPD, other cyclophanes were used as precursors for
(attempted) syntheses of [8]carbon nanobelt and cycloarenes,
including [n]circulenes and Kekulene, and their structures and
properties have barely been explored. DBTPD with a quasi-
planar structure, accompanied by some distortion in the central
ring, is different from the aforementioned cyclophanes. PNPD8
and PPD9 were assigned to be tub-shaped (syn-conformers)
based on theoretical investigations or chemical shifts in the 1H
NMR spectra, whereas NPD,7c BPPD,11 and DBAPD12 were
proposed to adopt step-like anti-conformers. Nonetheless,
none of their structures were proved experimentally.
mide (NBS) and the subsequent base-mediated intramolecular
couplings of benzylic (dibromo)methine (ArCHBr2) and
bromomethene (ArCH2Br) of the thus generated hexabro-
mo-substituted intermediate.15 Debromination of 3 through
successive treatment with n-butyllithium and methanol
produced 2,11-di-tert-butylpicene (4).
Next, how to effectively construct a cyclophane is equally
important. NPD,7 PNPD,8 BPPD,11 and DBAPD12 were
synthesized from the corresponding dithiacyclophanes using a
conventional route, which, regardless of chemical yields,
required many essential synthetic steps, including methylation,
oxidation, Stevens rearrangement, oxidation, and finally 1,2-
Hofmann elimination or pyrolysis. Normally, PPD was
achieved in 3−6 mg with small yields of 10−15% by
photocylization9a or nickel-catalyzed cyclization9b of halo-
substituted [2.2.2.2]paracyclophanetetraenes that were also
acquired inefficiently. The aforementioned synthetic ap-
proaches thus may not be suitable for picenophanes.
Alternatively, a new and efficient method was developed
herein where flexible acetyl groups were used as linkers for
connecting two picenyl moieties. Accordingly, palladium-
catalyzed Heck reaction of 3 with n-butyl vinyl ether was
conducted, which was followed by hydrolysis under acidic
conditions to obtain 5,8-diacetyl-2,11-di-tert-butylpicene (5).16
Picenophane 6 was produced through palladium-catalyzed α-
arylation17 of diketone 5 with an equal amount of 3. cis-
Ethylene linkers in 8 and ethano bridges in 9 were prepared by
reducing carbonyl groups of 6 and through the consecutive
dehydration and dehydroxylation of the resulting compound,
respectively. It needs to be emphasized that, with enough
dibromopicene 3 in hand, the title compound 8 was
accomplished on a semigram scale (415 mg). To study the
molecular dynamics of [2,2](5,8)picenophanediene experi-
mentally, isopropyl-substituted derivative 10 was prepared
from 5 and 5,8-dibromo-2-tert-butyl-11-isopropylpicene (11)
using the aforementioned synthetic method (see Scheme S1 in
the Supporting Information).
RESULTS AND DISCUSSION
■
Synthesis. In this study, a new series of picenophanes13
were synthesized, and their structures, molecular dynamics,
and photophysical properties were explored. Despite several
synthetic methods for preparations of picene14a,e and 5,8-
dihalopicene14e,f being reported, efficient and hence gram-scale
syntheses are still a great challenge.14g Picenophanes may also
suffer from low solubility in common organic solvents.
Introduction of additional alkyl chains should improve the
issue. Therefore, new synthetic approaches to di-tert-butyl-
substituted picene and several picenophanes were investigated
herein (Scheme 1). The Suzuki reaction of 4-bromo-2,3-
dimethylphenyl triflate (1) with 2,4,6-tris(5-tert-butyl-2-
methylphenyl)boroxin yielded 1,4-bis(5-tert-butyl-2-methyl-
phenyl)-2,3-dimethylbenzene (2), which was converted into
5,8-dibromo-2,11-di-tert-butylpicene (3) on a multigram scale
(2.32 g) through radical bromination with N-bromosuccini-
Scheme 1. Syntheses of Picenes and Picenophanes
Structural Analysis. Crystals of 6 and 8 were prepared
through the slow diffusion of methanol in dichloromethane
and chloroform solutions, respectively, at room temperature,
and their diffraction data were collected at 100 K. The notable
features of tub-shaped picenophane are mainly related to the
arrangement of two picenyl moieties, as verified by the
following structural parameters (see Figure 2a): (1) the
dihedral angle between the mean square planes (θ1) of the two
picenyl moieties; (2) degree of staggering, determined using
the torsion angle of a bridge (θ2); (3) opening (d1) and
shrinking (d2) of the tub, as estimated using the shortest
interplanar distances between any two peripheral carbon atoms
(C1, C2, C11, C12, C13, and C14) and inner carbon atoms
(C5, C6, C7, and C8), respectively; and 4) the average of the
distance (d3) between the internal hydrogen atoms (H6···H6′
and H7···H7′).
Compound 6 crystallized in the monoclinic P21/c space
group, with oxygen atoms and one tert-butyl group being
crystallographically disordered (Figure 2b). Moreover, 6 with
flexible linkers was expected to exhibit two conformers anti-6
and syn-6. However, only syn-6 was obtained in the solid state,
and the values of its structural parameters θ1, θ2, d1, d2, and d3
were determined to be 103.2°, 83.1°, 9.092(3) Å, 3.345(3) Å,
and 2.38 Å, respectively. According to the dihedral angle
between the mean square planes of the first and fifth six-
membered rings, one of the picenyl planes exhibited a higher
B
https://dx.doi.org/10.1021/jacs.0c08115
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
intermolecular interactions. As expected, the calculated
backbone of 10 is nearly identical to that of 8. PPD has a
tub-shaped structure similar to 8 and 10. BPPD adopts a step-
like anti-conformer, which is more stable than tub-shaped syn-
conformer by 4.0 kcal/mol (Figure S3 and S8). The calculated
structural parameters θ1, θ2, d1, d2, and d3 of syn-9 with C2
symmetry are 83.9°, 71.6°, 8.743 Å, 3.293 Å, and 2.122 Å,
respectively.
Molecular Dynamics. On the basis of DFT analyses, the
tub-to-tub inversion of 8 to 8′ (= 8) proceeded through the
successive bending of the linkers via the transition state 8-TS,
the intermediate 8-Int, in which one of the cis-ethylene linkers
was oriented upward and the other was oriented downward
from the backbone, followed by the transition state 8′-TS (= 8-
TS, Figure 3). The barrier of the tub-to-tub inversion (ΔG‡
)
inv
Figure 2. Structural parameters (a) and ORTEP drawings and
molecular packing of 6 (b and c) and 8 (d and e) with 50% thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
degree of bending than did the other (13.3° vs 3.6°). The only
syn-6 conformer and its bent picene fragment were presumably
caused by molecular packing. Each molecule 6 provided two
exo picenyl faces for intermolecular π···π surface overlapping
(Figure 2c) with numerous carbon−carbon nonbonded
distances of <3.434(3) Å (Figure S1 in the SI), which are
comparable to the sum of the van der Waals radii of two
carbon atoms (3.40 Å).18a Several CH···π hydrogen bonds
between the tert-butyl groups located in a cavity and concave
faces were observed, and the shortest hydrogen bond exhibited
a contact distance of 2.53 Å, which was measured as the
distance between the concerned hydrogen atom and the
centroid of the nearest six-membered ring.
Compound 8 with cis-ethylene linkers crystallized in the
orthorhombic Pbca space group (Figure 2d), and its cavity was
more rigid and smaller than that of 6, as evidenced by the
reduced values of θ1 (76°), θ2 (1.2°), d1 [7.321(4) Å], d2
[3.077(3) Å], and d3 (2.33 Å). Similar to 6, 8 crystallized with
one crystallographically disordered tert-butyl group and a bent
picenyl moiety of 17°. Molecules of 8 aggregated to form a
quasi-one-dimensional supramolecular chain through CH···π
interactions, where the concave π surface of the green molecule
(see Figure 2e) attracted two tert-butyl groups of two yellow
neighbors (one of each) to produce four hydrogen bonds with
distances of 2.62−2.75 Å (Figure S2). The exo-faces of the two
neighboring molecules (green and magenta) demonstrated
π···π interactions. Effective overlapping was obtained mainly
between the central biphenyls (second and fourth six-
membered rings) of picenyl moieties; the separation of the
mean square planes was 3.36 Å.
Figure 3. Energy profile for the ring-inversion process of 8. The
relative energies are reported in kcal/mol.
of 8 was estimated to be 20.3 kcal/mol. Several sterically
congested positions were observed in 8-TS, including one
short nonbonded H···H contact (1.683 Å) in the inner ring
and two pairs of peri hydrogen repulsions that distances
between alkenyl hydrogen atoms and their neighboring aryl
hydrogen atoms are 1.886 and 1.953 Å. The three values are
smaller than the sum of the van der Waals radii of the
hydrogen atoms (2.20 Å).18b Experimentally, we then
1
performed variable-temperature H NMR studies of 10 in
1,2-dichlorobenzene-d4. The results revealed the occurrence of
a slow tub-to-tub inversion at room temperature (Figure S5).
Two sharp doublets of diastereotopic methyl groups (1.33 and
1.30 ppm, Δv = 12.8 Hz) at 293 K were converted into a well-
resolved doublet (1.29 ppm) at 403 K. The coalescence
temperature for signal merging was 358 K; hence, the value of
ΔG‡ at the same temperature was approximately 18.7 kcal/
inv
mol, which agrees well with the calculated value (20.2 kcal/
mol, see Figure S6). Unlike 8 and 10, tub-to-tub inversions of
CODA and PPD do not suffer from peri hydrogen repulsions
and proceed via (quasi) planar transition states with barriers of
8.63b and 13.4 kcal/mol, respectively (Figure S7). We thus
conclude that 8 and 10 are more rigid than CODA and PPD.
Upon cooling of the solution of 9 in CD2Cl2 (Figure S9),
the signals of 1H NMR spectra progressively split into two sets
with a ratio of 1:1, which should be caused by C2-symmetric
syn-9, the most stable conformer (see below). The barrier
height (ΔG‡) for the dynamic process was experimentally
determined as 11.8−12.6 kcal/mol at 243 or 253 K, depending
on signals of different hydrogen atoms (Table S1). DFT
calculations conclude that syn-9, which adopts a twisted tub-
shaped structure, is more stable than anti-9 by 4.2 kcal/mol
(Figure 4). Therefore, signals of anti-9 should not be observed
The structures of 8, 9, 10, PPD and BPPD, and their
molecular dynamics were theoretically examined using density
functional theory19 (DFT, ωB97XD/6-311g**,20 see the
Supporting Information). The optimized geometry 8 obtained
theoretically was consistent with that obtained experimentally.
Picenyl moieties in the theoretically derived 8 were slightly
bent (approximately 8°), and their structural parameters (θ1 =
84°, θ2 = 0.0°, d1 = 7.903 Å, d2 = 3.095 Å, and d3 = 2.149 Å)
were slightly different from those in the solid state. These
distinct differences can be rationalized by perturbations of
C
https://dx.doi.org/10.1021/jacs.0c08115
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Table 1. Photophysical Properties of Picenophanes and
a
Their Fragments
Φ
EgP
(eV)
λabs (nm)
λem (nm)
(%)
b
b
b
4
8
9
330, 350 (broad)
386
363, 383
379, 400 , 424 , 449
6.5
2.7
13.3
3.54
3.14
3.24
b
b
b
386, 407 , 435 , 469, 496
b
b
b
b
385, 406 , 430 , 458
a
Measurements were conducted with samples (10 μM) in cyclo-
hexane. Emission spectra were recorded with 330 nm excitation. The
wavelengths of the absorption peak and the emission peak are
denoted by λabs and λem, respectively. ΔΦ and EgP are the quantum
b
yields and the optical HOMO-LUMO gaps, respectively. Vibronic
progression.
Figure 4. Energy profile for the ring-inversion process of 9. The
relative energies are reported in kcal/mol. Hydrogen atoms are
omitted for clarity.
in the NMR spectra on a long time scale at the studied
temperatures. anti-9 is the intermediate for the inversion of
syn-9 to syn-9′ (= syn-9). The conversion of syn-9 to anti-9
requires 12.3 kcal/mol, and the mechanism proceeded with a
twisted V-shaped transition state 9-TS through the successive
rotation of the enthano linkers. Notably, [2,2](2,7)-
naphthalenophane (NP),21 [2,2](3,6)phenanthro(2,7)-
naphthalenophane (PNP)8 and [2,2](3,6)phenanthrophane
(PP)22 have been prepared (Figure 5), but the most stable
conformer of only the first compound has been identified that
anti-NP is more stable than syn-NP by 11.0 kcal/mol.
Figure 6. Absorption (left) and emission (right, λex at 330 nm)
spectra of 4, 8 and 9 (1 × 10−5 M) in cyclohexane at 298 K.
Figure 5. Ethano-bridged cyclophanes NP, PNP, and PP.
Electrochemical Properties. The redox properties of 4, 8,
and 9 in dichloromethane were examined using cyclic
voltammetry, and their reduction signals were not observed
in this analytic window (Table S2 and Figure S10). Monomer
4 exhibited one irreversible oxidation wave with an onset
potential of 0.90 V (vs the ferrocene/ferrocenium couple),
indicating the low thermodynamic stability of the cationic
species. The cyclic voltammograms of cyclophanes 8 and 9 are
complex that their oxidation signals may contain multiple
oxidation processes, verified by differential pulse voltammetry
(DPV). The first oxidation potential of 9 (0.79 V) is lower
than that of 8 (0.86 V). Compound 6 in tetrahydrofuran
displayed one irreversible oxidation signal at 0.68 V, which
exceeds that of 8 by 0.07 V.
Photophysical Properties. Table 1 and Figure 6 present
the photophysical properties of the compounds. 4, 8, and 9 in
cyclohexane exhibited similar absorption spectra, presumably
because their frontier orbitals were mainly located at picenyl
moieties with unremarkable orbital interactions (Figure 7).
The π-orbital lobes of ethylene linkers in 8 were notable only
in LUMO where they exhibited antibonding states. The
HOMO−LUMO gaps (EgP) of 4, 8, and 9 were 3.54, 3.14, and
3.24 eV, respectively. On the other hand, the different vibronic
progressive feature in the emission spectra of picenophane 9
and monomer 4 reveals the exciton delocalization in 9. Based
Figure 7. Frontier orbitals of 4, 8, and syn-9.
on the previous report,23 the vibronic intensity ratio I0→0/I0→1
equals to N/λ2 relating to the Huang-Rhys factor λ and
delocalization number N. With this physical relation, the
Huang−Rhys factor of monomer 4 is evaluated as 1.03. The
slightly higher vibronic intensity ratio (1.36) in cyclophane 9
reflects that excitons delocalize at the picenyl moieties with N
≈ 1.36, indicating weak intramolecular interaction between
D
https://dx.doi.org/10.1021/jacs.0c08115
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
two picene moieties in 9. Such delocalization phenomena hold
at 313 K−253 K in methylcyclohexane (Figure S11).
As for the picenophane 8, the transition between the Frenkel
exciton state to the excimer state is captured (Figure 8 and
Importantly, picenophane 8 with cis-ethylene linkers
demonstrates anomalous dual emission. The first emission
band ranging from 380 to 420 nm (the F1 band) resembles the
emission profile of molecules 4 and 9. Clearly, such an
emission is attributed to the monomeric transition with slight
exciton delocalization dubbed as Frenkel exciton states. In
contrast, the broad emission band ranging from 450−650 nm
(the F2 band) is assigned as the excimer emission,24 which is
reminiscent of the emission of other diarylalkanes.25 This
assignment is firmly supported by the temperature-dependent
emission spectra (Figure S12), showing that the excimer
emission decreases with the lower temperature and disappears
at 77 K in methylcyclohexane. This result indicates that such
an emission band relates to the structural relaxation that is
assisted by the thermal energy, matching the phenomena of
excimer formation.
Comparing the dual emission and single emission bands for
picenophanes 8 and 9, respectively, the structure of the linker
between the two picenyl moieties plays a key role for the
excimer formation. A simple linker variation leads to much
distinction in photophysics is surprising, which may be caused
by different charge transfer coupling strength affected by the
rigidity of the linker. The excimer emission of 8 is highly
sensitive to the solvent polarity and viscosity (Figure S14).
Toluene with relatively low dielectric constant (ε = 2.2) and
low viscosity (η = 0.56 cP at 298 K) causes the highest
intensity ratio of excimer emission among the solvents studied
herein. Cyclohexane with low dielectric constant (ε = 2.0) and
relatively high viscosity (η = 0.86 cP at 298 K, cf. toluene) and
dichloromethane with high dielectric constant (ε = 8.9) and
low viscosity (η = 0.40 cP at 298 K) reduce the excimer
emission ratio. Based on the observations, we surmise that the
increase of the solvent viscosity hinders the structural
relaxation incorporating large-amplitude motion, resulting in
the less efficiency of the excimer formation. On the other hand,
the energy level of the inter-moiety charge transfer state and
the corresponding coupling strength may be affected by the
increase of solvent polarity, which interfere the excimer
formation. In other words, increasing either environmental
polarity or viscosity decreases the excimer intensity. Thus,
ratiometic emission for normal versus excimer can be
harnessed by the interplay between viscosity and polarity. In
contrast, the emission spectra of 9 are solvent-independent,
indicating that the flexible ethano-bridge give more degrees of
freedom for the structural variation, prohibiting the excimer
formation.
To elaborate the dynamics of the picenyl system, the time-
resolved emission spectroscopies combining the time-corre-
lated single photon counting (TCSPC) and the femtosecond
flourescence upconversion are applied with the time resolution
20 ps and 150 fs, respectively. 4 and 9 show single exponential
population decay with similar observed lifetimes (τobs) of 15 ns
(Figure S15). With the information on Φ and τobs, the radiative
decay rate constant (kr) of 4 and 9 can be evaluated. As a
result, kr of picenophane 9 is deduced to be 8.7 × 106 s−1,
which is around twice larger than that (4.3 × 106 s−1) of
monomer 4. This result verifies the superradiance in
picenophane 9, echoing with the exciton delocalization of 9
confirmed by the larger I0→0/I0→1 in the steady-state emission
spectra (vide supra).23
Figure 8. Time-resolved emission spectra of picenophane 8. (a)
Fluorescence up-conversion decay curves obtained in cyclohexane
with an excitation wavelength (λex) of 270 nm at 298 K. (b) TCSPC
measurements in cyclohexane with λex of 360 nm at 298 K.
a
Table 2. Fitting Lifetime of Picenophane 8 at 298 K
λmon (nm)
observed lifetime τobs (pre-exponential factor)
430
500
18.6 ps (0.65), 2241 ps (0.35)
18.6 ps (−0.28), 2153 ps (0.72)
a
λmon is the monitored emission wavelength.
Table 2). Figure 8a demonstrates the corresponding rise-decay
rate with τobs of 18.6 ps, representing the observed transition
rate between Frenkel exciton and excimer states. In Figure 8b,
the similar population decay lifetimes (τobs ≈ 2.2 ns) for both
monomer and excimer emissions reveal the characteristic of
pseudo-equilibrium between these two excited states. Such
character indicates that the system here can be analogized as
the two-state reversible reaction followed by excited-state
dissipation (emission and non-radiarive decay), which has
been utilized to describe the dynamics of excited-state electron
transfer,26 proton-transfer27 and proton coupled electron
transfer.27,28 As a result, the equilibrium constant Keq and the
corresponding free energy difference ΔG can be derived by the
ratio of the pre-exponential between fast and slow decay (see
Figure 8), rendering Keq = 1.86, ΔG = −0.5 kcal/mol, which
also roughly accords with the emission intensities between the
Frenkel exciton state and excimer state. The relatively small
ΔG with the large Stokes shift of the excimer emission (∼13.6
kcal/mol) reflect that the redshift of the excimer emission
should result from the common solvent relexation of less than
few picoseconds.
We further performed tempeature-dependent dynamics of
excimer formation. As a reuslt, the slope of Arrhenius plot of
the excimer formation rate (Figure S16) deduces an acitvation
energy barrier (Ea) of 2.5 kcal/mol in methylcyclohexane. Such
relatively small Ea comparing to the ones of the other
diarylalkanes (3.4−4.2 kcal/mol) reveals that the cis-ethylene
linker may play a crucial role in the suppression of Ea.25b,29 The
results match well the shorter excimer formation lifetime of
picenophane 8 at 298 K (τ = 29 ps), which is much faster than
E
https://dx.doi.org/10.1021/jacs.0c08115
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
ones of the other diarylalkanes (τ = 10−100 ns).25b,29,30
According to the extracted frequency factor (1.13 ×1012 s−1),
such short excimer formation lifetime could correlate to the
low frequency vibronic motion of ∼40 cm−1. Based on the
results elaborated above, the mechanism of excimer formation
in picenophane 8 is summarized in Figure 9. Upon the
butyl groups. In the light of this information, accumulation of
metals on the concave π surface of picenophane is ongoing in
our laboratory, in order to explore metal-π and (potential)
metal−metal interactions.31
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c08115.
Complete ref 19, experimental procedures, character-
ization data, computational studies, and X-ray crystallog-
raphy data (PDF)
X-ray crystallographic data for 6 (CIF)
X-ray crystallographic data for 8 (CIF)
AUTHOR INFORMATION
■
Corresponding Authors
Yao-Ting Wu − Department of Chemistry, National Cheng
Kung University, 70101 Tainan, Taiwan; orcid.org/
0000-0001-7716-1855; Email: ytwuchem@
mail.ncku.edu.tw
Pi-Tai Chou − Department of Chemistry, National Taiwan
University, 10617 Taipei, Taiwan; orcid.org/0000-0002-
8925-7747; Email: chop@ntu.edu.tw
Figure 9. Excimer formation mechanism of picenophane 8.
Mu-Jeng Cheng − Department of Chemistry, National Cheng
Kung University, 70101 Tainan, Taiwan; orcid.org/
0000-0002-8121-0485; Email: mjcheng@mail.ncku.edu.tw
excitation of 8, the Frenkel exciton immediately delocalizes to
the two moieties. Subsequently, the population in the Frenkel
exciton state transfers to the excimer state with lifetime ∼20 ps
reversibly. The excited-state population then decays to the
ground state with a lifetime 2.2 ns.
Authors
Min-Chih Tang − Department of Chemistry, National Cheng
Kung University, 70101 Tainan, Taiwan
Yu-Chen Wei − Department of Chemistry, National Taiwan
University, 10617 Taipei, Taiwan
Yen-Chen Chu − Department of Chemistry, National Cheng
Kung University, 70101 Tainan, Taiwan
Cai-Xin Jiang − Department of Chemistry, National Cheng
Kung University, 70101 Tainan, Taiwan
Zhi-Xuan Huang − Department of Chemistry, National
Taiwan University, 10617 Taipei, Taiwan
Chi-Chi Wu − Department of Chemistry, National Taiwan
University, 10617 Taipei, Taiwan
Tzu-Hsuan Chao − Department of Chemistry, National
Cheng Kung University, 70101 Tainan, Taiwan
Pei-Hsun Hong − Department of Chemistry, National Cheng
Kung University, 70101 Tainan, Taiwan
CONCLUSION
■
In summary, the chemistry, X-ray crystallographic structures,
molecular dynamics, and photophysical properties of piceno-
phanes were investigated. Tub-shaped [2,2](5,8)-
picenophanediene with a large π-system and semirigid
structure exhibited anomalous photophysical properties and
photodynamics. Remarkably, we observed that slight difference
in the bridge of the picenophane system renders significant
change in the excited-state properties. Ethano-bridged
picenophane 9 shows weak exciton delocalization verified by
the vibronic progressive emission spectra and the superradiant
effect. In sharp contrast, cis-ethylene-bridged picenophane 8
exhibits dual emission rendered by weakly delocalized exciton
and excimer. The dynamics is elucidated by the time-resolved
emission spectroscopy showing that the transition between
Frenkel exciton and excimer states undergoes subtle structural
deformation reversibly with a time constant of 18.6 ps
(forward plus backward reactions) at 298 K, which is much
faster comparing to the excimer formation rates of most
diarylalkanes. This result together with the steady-state spectra
indicates that the cis-ethylene linker contributes to not only the
formation of excimer state in geometry but also the
acceleration of excimer formation rate. The molecular design
together with the comprehensive chemical analyses and insight
into the photophysical properties sheds light on the unique
excite-state behavior of tub-shaped picenophanes, providing a
simple system to harness the inter-moiety reaction. The
concave π surfaces of tub-shaped compounds 6 and 8 have the
capability to form intermolecular hydrogen bonds with tert-
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c08115
Author Contributions
§M.-C.T. and Y.-C.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Ministry of Science and
Technology of Taiwan (107-2113-M-006-013-MY3). The
Instrumentation Center at National Tsing Hua University
(X-ray crystallography), and the National Center for High-
performance Computing (computational resources) are
acknowledged.
F
https://dx.doi.org/10.1021/jacs.0c08115
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(b) Povie, G.; Segawa, Y.; Nishihara, T.; Miyauchi, Y.; Itami, K.
Synthesis and Size-Dependent Properties of [12], [16], and [24]
Carbon Nanobelts. J. Am. Chem. Soc. 2018, 140, 10054. (c) Liljefors,
L.; Wennerstrom, O. Molecular mechanics calculations on the
structures and conformational properties of [8]circulene and some
related phane compounds. Tetrahedron 1977, 33, 2999.
REFERENCES
■
(1) (a) Wender, P. A.; Lesser, A. B.; Sirois, L. E. Rhodium
Dinaphthocyclooctatetraene Complexes: Synthesis, Characterization
and Catalytic Activity in [5+2] Cycloadditions. Angew. Chem., Int. Ed.
2012, 51, 2736. (b) Singh, A.; Sharp, P. R. Platinum(II) and
Palladium(II) Dibenzo[a,e]cyclooctatetraene (DBCOT) Oxo and
Halide Complexes: Comparison to 1,5-COD Analogues. Organo-
metallics 2006, 25, 678. (c) Hiroi, Y.; Komine, N.; Komiya, S.; Hirano,
M. Regio- and Enantioselective Linear Cross-Dimerizations between
Conjugated Dienes and Acrylates Catalyzed by New Ru(0)
Complexes. Organometallics 2014, 33, 6604. (d) Dzik, W. I.; Fuente
Arruga, L.; Siegler, M. A.; Spek, A. L.; Reek, J. N. H.; de Bruin, B.
Iridium Olefin Complexes Bearing Dialkylamino/amido PNP Pincer
Ligands: Synthesis, Reactivity, and Solution Dynamics. Organo-
metallics 2011, 30, 1902.
(2) For selected examples, see: (a) Man, Y.-M.; Mak, T. C. W.;
Wong, H. N. C. Synthesis of Benzo-Fused Tetraphenylenes and
Crystal Structure of a 4:1 Clathrate Inclusion Compound of
Dibenzo[b,h]tetraphenylene with p-Xylene. J. Org. Chem. 1990, 55,
3214. (b) Pun, S. H.; Wang, Y.; Chu, M.; Chan, C. K.; Li, Y.; Liu, Z.;
Miao, Q. Synthesis, Structures, and Properties of Heptabenzo[7]-
circulene and Octabenzo[8]circulene. J. Am. Chem. Soc. 2019, 141,
9680. (c) Feng, C.-N.; Kuo, M.-Y.; Wu, Y.-T. Synthesis, Structural
Analysis, and Properties of [8]Circulenes. Angew. Chem., Int. Ed.
2013, 52, 7791. (d) Miller, R. W.; Duncan, A. K.; Schneebeli, S. T.;
Gray, D. L.; Whalley, A. C. Synthesis and Structural Data of
Tetrabenzo[8]circulene. Chem. - Eur. J. 2014, 20, 3705.
(3) (a) Yuan, C.; Saito, S.; Camacho, C.; Irle, S.; Hisaki, I.;
Yamaguchi, S. A π-Conjugated System with Flexibility and Rigidity
That Shows Environment-Dependent RGB Luminescence. J. Am.
Chem. Soc. 2013, 135, 8842. (b) Yamakado, T.; Otsubo, K.; Osuka,
A.; Saito, S. Compression of a Flapping Mechanophore Accompanied
by Thermal Void Collapse in a Crystalline Phase. J. Am. Chem. Soc.
2018, 140, 6245. (c) Saito, S.; Nobusue, S.; Tsuzaka, E.; Yuan, C.;
Mori, C.; Hara, M.; Seki, T.; Camacho, C.; Irle, S.; Yamaguchi, S.
Light-melt adhesive based on dynamic carbon frameworks in a
columnar liquid-crystal phase. Nat. Commun. 2016, 7, 12094.
(4) Yamakado, T.; Takahashi, S.; Watanabe, K.; Matsumoto, Y.;
Osuka, A.; Saito, S. Conformational Planarization versus Singlet
Fission: Distinct Excited-State Dynamics of Cyclooctatetraene-Fused
Acene Dimers. Angew. Chem., Int. Ed. 2018, 57, 5438.
́
(10) Qiao, Y.; Wei, Z.; Risko, C.; Li, H.; Bredas, J.-L.; Xu, W.; Zhu,
D. Synthesis, experimental and theoretical characterization, and field-
effect transistor properties of a new class of dibenzothiophene
derivatives: From linear to cyclic architectures. J. Mater. Chem. 2012,
22, 1313.
(11) Staab, H. A.; Diederich, F.; Caplar, V. Cycloarenes, a New Class
of Aromatic Compounds, III. Studies towards the Synthesis of
Cyclo[d.e.d.e.e.d.e.d.e.e]decakisbenzene. Liebigs Ann. Chem. 1983,
1983, 2262.
(12) (a) Staab, H. A.; Diederich, F. Cycloarenes, a New Class of
Aromatic Compounds, I. Synthesis of Kekulene. Chem. Ber. 1983,
116, 3487. (b) Diederich, F.; Staab, H. A. Benzenoid versus
Annulenoid Aromaticity: Synthesis and Properties of Kekulene.
Angew. Chem., Int. Ed. Engl. 1978, 17, 372.
(13) Scifinder key words search (picenophane and cyclophane
refined by picene) on July 1st, 2020 revealed that this class of
compounds did not exist in the database.
(14) (a) McAtee, C. C.; Riehl, P. S.; Schindler, C. S. Polycyclic
Aromatic Hydrocarbons via Iron(III)-Catalyzed Carbonyl-Olefin
Metathesis. J. Am. Chem. Soc. 2017, 139, 2960. (b) Xia, Y.; Liu, Z.;
Xiao, Q.; Qu, P.; Ge, R.; Zhang, Y.; Wang, J. Rhodium(II)-Catalyzed
Cyclization of Bis(N-tosylhydrazone)s: An Efficient Approach
towards Polycyclic Aromatic Compounds. Angew. Chem., Int. Ed.
2012, 51, 5714. (c) Chang, N.-H.; Chen, X.-C.; Nonobe, H.; Okuda,
Y.; Mori, H.; Nakajima, K.; Nishihara, Y. Synthesis of Substituted
Picenes through Pd-Catalyzed Cross-Coupling Reaction/Annulation
Sequences and Their Physicochemical Properties. Org. Lett. 2013, 15,
3558. (d) Protti, S.; Artioli, G. A.; Capitani, F.; Marini, C.; Dore, P.;
Postorino, P.; Malavasi, L.; Fagnoni, M. Preparation of (substituted)
picenes via solar light-induced Mallory photocyclization. RSC Adv.
2015, 5, 27470. (e) Okamoto, H.; Yamaji, M.; Gohda, S.; Kubozono,
Y.; Komura, N.; Sato, K.; Sugino, H.; Satake, K. Facile Synthesis of
Picene from 1,2-Di(1-naphthyl)ethane by 9-Fluorenone-Sensitized
Photolysis. Org. Lett. 2011, 13, 2758. (f) Nakae, T.; Ohnishi, R.;
Kitahata, Y.; Soukawa, T.; Sato, H.; Mori, S.; Okujima, T.; Uno, H.;
Sakaguchi, H. Effective synthesis of diiodinated picene and dibenzo-
[a,h]anthracene by AuCl-catalyzed double cyclization. Tetrahedron
Lett. 2012, 53, 1617. (g) Carrera, M.; de la Viuda, M.; Guijarro, A.
3,3′,5,5′-Tetra-tert-butyl-4,4′-diphenoquinone (DPQ)-Air as a New
Organic Photocatalytic System: Use in the Oxidative Photocyclization
of Stilbenes to Phenacenes. Synlett 2016, 27, 2783.
(15) (a) Marcinow, Z.; Grove, D. I.; Rabideau, P. W. Synthesis of a
New C32H12 Bowl-Shaped Aromatic Hydrocarbon: Acenaphtho-
[3,2,1,8-ijklm]diindeno[4,3,2,1-cdef:1‘,2‘,3‘,4‘-pqra]triphenylene. J.
Org. Chem. 2002, 67, 3537. (b) Goretta, S.; Tasciotti, C.; Mathieu,
S.; Smet, M.; Maes, W.; Chabre, Y. M.; Dehaen, W.; Giasson, R.;
Raimundo, J.-M.; Henry, C. R.; Barth, C.; Gingras, M. Expeditive
Syntheses of Functionalized Pentahelicenes and NC-AFM on
Ag(001). Org. Lett. 2009, 11, 3846.
(16) Vallin, K. S. A.; Larhed, M.; Hallberg, A. Aqueous DMF-
Potassium Carbonate as a Substitute for Thallium and Silver Additives
in the Palladium-Catalyzed Conversion of Aryl Bromides to Acetyl
Arenes. J. Org. Chem. 2001, 66, 4340.
(5) Sygula, A.; Fronczek, F. R.; Sygula, R.; Rabideau, P. W.;
Olmstead, M. M. A Double Concave Hydrocarbon Buckycatcher. J.
Am. Chem. Soc. 2007, 129, 3842.
(6) For reviews on cyclophanes, see: (a) Hassan, Z.; Spuling, E.;
̈
Knoll, D. M.; Brase, S. Regioselective Functionalization of [2.2]-
Paracyclophanes: Recent Synthetic Progress and Perspectives. Angew.
Chem., Int. Ed. 2020, 59, 2156. (b) Liu, Z.; Nalluri, S. K. M.; Stoddart,
J. F. Surveying macrocyclic chemistry: from flexible crown ethers to
rigid cyclophanes. Chem. Soc. Rev. 2017, 46, 2459. (c) Ghasemabadi,
P. G.; Yao, T.; Bodwell, G. J. Cyclophanes containing large polycyclic
aromatic hydrocarbons. Chem. Soc. Rev. 2015, 44, 6494. (d) Modern
Cyclophane Chemistry; Gleiter, R.; Hopf, H., Eds.; Wiley-VCH:
Weinheim, 2004.
(7) (a) Davy, J. R.; Reiss, J. A. Synthesis of [2,2](2,7)-
naphthalenophane-1,11-diene. J. Chem. Soc., Chem. Commun. 1973,
806. (b) Davy, J. R.; Reiss, J. A. Cyclophanes. IV. Synthetic
approaches to cyclophane dienes. The preparation of [2,2](2,7)-
Naphthalenophane-1,11-diene. Aust. J. Chem. 1976, 29, 163.
(c) Rioseras-garcia, M. J.; Hernando-huelmo, J. M. Theoretical
conformational study of the stabilities and geometries of [2,2](2,7)-
naphthalenophane-1,11-diene and related compounds. Int. J. Quantum
Chem. 1994, 52, 651.
(8) Jessup, P. J.; Reiss, J. A. Cyclophanes. VIII. An attempted
preparation of [7]circulene from [2,2](3,6)Phenanthro(2,7)-
naphthalenophane-1,11-diene. Aust. J. Chem. 1977, 30, 851.
(9) (a) Thulin, B.; Wennerstrom, O.; Lindstrom, B.; Schaumburg,
K.; Vialle, J.; Anthonsen, T. Synthesis of [2.2](3,6)-
Phenanthrenophanediene. Acta Chem. Scand. 1976, 30b, 369.
(17) Churruca, F.; SanMartin, R.; Carril, M.; Tellitu, I.; Domínguez,
E. Towards a facile synthesis of triarylethanones: palladium-catalyzed
arylation of ketone enolates under homogeneous and heterogeneous
conditions. Tetrahedron 2004, 60, 2393.
(18) (a) Pauling, L. The Nature of the Chemical Bond, 3rd ed.;
Cornell University Press: Ithaca, 1960. (b) Rowland, R. S.; Taylor, R.
Intermolecular Nonbonded Contact Distances in Organic Crystal
Structures: Comparison with Distances Expected from van der Waals
Radii. J. Phys. Chem. 1996, 100, 7384.
G
https://dx.doi.org/10.1021/jacs.0c08115
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(19) Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT,
2016.
(20) Chai, J.-D.; Head-Gordon, M. Systematic optimization of long-
range corrected hybrid density functionals. J. Chem. Phys. 2008, 128,
084106.
(21) Ohtsuka, K.; Cai, G.; Fujita, K.; Miyoshi, H.; Tobe, Y. Synthesis
and structures of [2n](2,7)naphthalenophanes (n = 2-4). Can. J.
Chem. 2017, 95, 445.
(22) Thulin, B.; Wennerstrom, O. Synthesis and Photocyclization of
[2.0.2.0]-, [2.2.2.0]- and [2.2.2.2]Cyclophanediene. Acta Chem. Scand.
1983, 37b, 297.
(23) Hestand, N. J.; Spano, F. C. Expanded Theory of H- and J-
Molecular Aggregates: The Effects of Vibronic Coupling and
Intermolecular Charge Transfer. Chem. Rev. 2018, 118, 7069.
(24) Tube-shaped [2.2](3,3′)biphenylo(3,6)phenanthrene does not
form excimer. For details, see: Nakamura, Y.; Tsuihiji, T.; Mita, T.;
Minowa, T.; Tobita, S.; Shizuka, H.; Nishimura, J. Synthesis,
Structure, and Electronic Properties of syn-[2.2]Phenanthrenophanes:
First Observation of Their Excimer Fluorescence at High Temper-
ature. J. Am. Chem. Soc. 1996, 118, 1006.
(25) (a) Chandross, E. A.; Dempster, C. J. Intramolecular excimer
formation and fluorescence quenching in dinaphthylalkanes. J. Am.
Chem. Soc. 1970, 92, 3586. (b) De Schryver, F. C.; Collart, P.;
Vandendriessche, J.; Goedeweeck, R.; Swinnen, A. M.; Van der
Auweraer, M. Intramolecular excimer formation in bichromophoric
molecules linked by a short flexible chain. Acc. Chem. Res. 1987, 20,
159. (c) Hirayama, F. Intramolecular Excimer Formation. I. Diphenyl
and Triphenyl Alkanes. J. Chem. Phys. 1965, 42, 3163.
(26) Chen, K.-Y.; Hsieh, C.-C.; Cheng, Y.-M.; Lai, C.-H.; Chou, P.-
T.; Chow, T. J. Tuning Excited-State Electron Transfer from an
Adiabatic to Nonadiabatic Type in Donor-Bridge-Acceptor Systems
and the Associated Energy-Transfer Process. J. Phys. Chem. A 2006,
110, 12136.
(27) (a) Tang, K. C.; Chang, M. J.; Lin, T. Y.; Pan, H. A.; Fang, T.
C.; Chen, K. Y.; Hung, W. Y.; Hsu, Y. H.; Chou, P.-T. Fine Tuning
the Energetics of Excited-State Intramolecular Proton Transfer
(ESIPT): White Light Generation in A Single ESIPT System. J.
Am. Chem. Soc. 2011, 133, 17738. (b) Liu, Z.-Y.; Hu, J.-W.; Chen, C.-
L.; Chen, Y.-A.; Chen, K.-Y.; Chou, P.-T. Correlation among
Hydrogen Bond, Excited-State Intramolecular Proton-Transfer
Kinetics and Thermodynamics for -OH Type Proton-Donor
Molecules. J. Phys. Chem. C 2018, 122, 21833.
(28) Cheng, C.-C.; Chang, C.-P.; Yu, W.-S.; Hung, F.-T.; Liu, Y.-I.;
Wu, G.-R.; Chou, P.-T. Comprehensive Studies on Dual Excitation
Behavior of Double Proton versus Charge Transfer in 4-(N-
Substituted amino)-1H-pyrrolo[2,3-b]pyridines. J. Phys. Chem. A
2003, 107, 1459.
(29) (a) Zachariasse, K. A.; Maca̧ nita, A. L.; Kuhnle, W. Chain
̈
Length Dependence of Intramolecular Excimer Formation with 1,n-
Bis(1-pyrenylcarboxy)alkanes for n = 1-16, 22, and 32. J. Phys. Chem.
B 1999, 103, 9356. (b) Goldenberg, M.; Emert, J.; Morawetz, H.
Intramolecular excimer study of rates of conformational transitions.
Dependence on molecular structure and the viscosity of the medium.
J. Am. Chem. Soc. 1978, 100, 7171.
(30) Snare, M. J.; Thistlethwaite, P. J.; Ghiggino, K. P. Nonlinear
fluorescence quenching and the origin of positive curvature in Stern-
Volmer plots. J. Am. Chem. Soc. 1983, 105, 3328.
(31) A preliminary study indicated that picenophane 8 should be a
suitable ligand for metal cations. Treatment of a solution of 8 in
chloroform with silver trifluoromethanesulfonate yielded [8•Ag]+
complex, which has been verified by mass spectra. Identification of
the accurate coordination position is in progress.
H
https://dx.doi.org/10.1021/jacs.0c08115
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX