Helical Oligonaphthodioxepins Showing Intense Circularly Polarized Luminescence (CPL) in Solution and in the Solid State

Article abstract of DOI:10.1002/chem.201702143

A series of oligonaphthodioxepins was synthesized, revealing a helically arranged octamer, (R,R,R,R,R,R,R)-3, which showed intense circularly polarized luminescence (CPL) both in solution and in the solid state. The fluorescence quantum yields (Φ_FL) in solution and in the solid state were 0.90 and 0.22, respectively, and the g_lum values in solution and in the solid state were +2.2×10^?3 and +7.0×10^?3, respectively. This is one of the highest solid-state CPL g_lum values yet reported. The high Φ_FL and g_lum values were due to the rigidity, as well as to the fact that (R,R,R,R,R,R,R)-3 was a non-planar molecule. Moreover, (R,R,R,R,R,R,R)-3 was highly stable both chemically and stereochemically.

Full text of DOI:10.1002/chem.201702143

A Journal of
Accepted Article
Title: Helical Oligonaphthodioxepins Showing Intense CPL in Solution
and in the Solid State
Authors: Kazuto Takaishi, Tadashi Ema, Takahiro Yamamoto, and
Sakiko Hinoide
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To be cited as: Chem. Eur. J. 10.1002/chem.201702143
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Helical Oligonaphthodioxepins Showing Intense CPL in Solution
and in the Solid State
Kazuto Takaishi*, Takahiro Yamamoto, Sakiko Hinoide, and Tadashi Ema*
|glum,solid| = 1.5  10–3; the Kuhn’s anisotropy factor glum is defined
Abstract: A series of oligonaphthodioxepins was synthesized,
as 2(IL – IR)/(IL + IR)).[8] Commercial applications would require
novel organic dyes that display much larger solid-state glum values.
revealing a helically arranged octamer, (R,R,R,R,R,R,R)-3, which
showed intense circularly polarized luminescence (CPL) both in
solution and in the solid state. The fluorescence quantum yields
(FL) in solution and in the solid state were 0.90 and 0.22,
respectively, and the glum values in solution and in the solid state
were +2.2  10–3 and +7.0  10–3, respectively. This is one of the
highest solid-state CPL glum values yet reported. The high FL and
glum values were due to the rigidity as well as to the fact that
(R,R,R,R,R,R,R)-3 was
a
non-planar molecule. Moreover,
(R,R,R,R,R,R,R)-3 was highly stable both chemically and
stereochemically.
Circularly polarized luminescence (CPL) is potentially useful in
displays,[1] memory,[2] and bioimaging[3] applications because it
produces three types of signals: left-handed CPL, right-handed
CPL, and non-CPL (+, –, and 0). Over the past decade, several
organic helical polymers and supramolecules with CPL activity
have been developed.[4] Small monomolecular CPL dyes such as
helicenes,[5] cyclophanes,[6] and binaphthyl compounds[7] have also
attracted attention for use in certain applications. Solid-state CPL is
preferred in many applications; however, almost all known
monomeric dyes display CPL only in solution. Exceptional cases
were reported by Imai and coworkers, who observed solid-state
CPL from BINOL-derived phosphoric acids (FL,solid = 0.26,
Figure 1. A typical binaphthyl and naphthodioxepin oligomers.
Dr. K. Takaishi, T. Yamamoto, S. Hinoide, Prof. Dr. T. Ema
Division of Applied Chemistry, Graduate School of Natural Science
and Technology, Okayama University
Tsushima, Okayama 700-8530 (Japan)
E-mail: takaishi@okayama-u.ac.jp (K. Takaishi)
ema@cc.okayama-u.ac.jp (T. Ema)
Figure 2. a) Selected stereoisomers 3. b) Top and side views of the
DFT-optimized structures of (B3LYP/6-31G(d,p) level). The
terminal benzyloxy groups were replaced with hydrogen atoms.
3
Supporting information for this article is given via a link at the end of
the document.
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Dinaphthodioxepins exhibit unique optical properties compared
to other 1,1’-binaphthyls.[9] We previously synthesized the (R)-
naphthodioxepin dimer 1, (R,S,R)-tetramer 2, and (R,S,R,S,R,S,R)-
octamer 3 (Figures 1 and 2).[10] The dioxepin ring structure leads to
a rigid and cisoid conformation that enabled partial conjugation
1
along the chiral axis involving the La transition[11]. Therefore, the
absorption and fluorescence maxima red-shifted as the degree of
oligomerization increased. The compounds also fluoresced both in
solution (FL,1,4-dioxane of up to 0.79) and in the solid state (FL,solid
of up to 0.29), unlike typical binaphthyl compounds. Naphthalene
oligomers without the dioxepin moiety exhibited neither solid-state
fluorescence nor red-shifted absorption properties because their
naphthalene rings were movable and orthogonal.[12]
In this study, we focused on a particular set of stereoisomers of 2
and 3. Seventy-two stereoisomers (36 enantiomeric pairs) of 3 exist,
and the optimized structures of four representative examples,
(R,R,R,R,R,R,R)-3, (R,R,R,S,R,R,R)-3, (R,S,R,R,R,S,R)-3, and
(R,S,R,S,R,S,R)-3, are shown in Figure 2.[13] Although the overall
structures of the stereoisomers 3 differed significantly from one
another, the dihedral angles between adjacent naphthalene rings
(C2–C1–C1’–C2’) were about 49°. The calculated potential energy
(an index of the molecular strain) did not differ significantly
among the stereoisomers: (R,R,R,R,R,R,R)-3 was the most stable
(e.g., E between (R,R,R,R,R,R,R)-3 and the most unstable
stereoisomer, (R,S,R,S,R,S,R)-3, was calculated to be 1.1 kcal mol–1
at the B3LYP/6-31G(d,p), 3.3 kcal mol–1 at the M062X/6-31G(d,p),
and 1.5 kcal mol–1 at the B97D/TZVP levels of theory). The axes
of (R,S,R,S,R,S,R)-3 were bent to form an arc due to an
accumulation of strain. The naphthalene rings of (R,S,R,S,R,S,R)-3
displayed an alternating arrangement similar to that observed in the
meso-form. Therefore, (R,S,R,S,R,S,R)-3 was not expected to
display significant chiroptical properties. On the other hand, the
naphthalene rings of (R,R,R,R,R,R,R)-3 took a left-handed helical
shape with linearly aligned chiral axes.[14] This helical
(R,R,R,R,R,R,R)-3 was expected to display excellent CPL
properties. Here, we report that (R,R,R,R,R,R,R)-3 showed intense
FL and CPL both in solution and in the solid state.
To construct a series of stereopure oligonaphthodioxepins, we
selected a bottom-up method using chiral BINOL as a starting
material (Scheme S1). The absolute configuration of the octamers
3 was determined using the exciton chirality method with two
TPP(Zn) chromophores.[15] The X-ray crystal structure of (R,R,R)-2,
shown in Figure 3, revealed a cisoid conformation (47–49°) similar
to the half structure of the DFT-optimized (R,R,R,R,R,R,R)-3. The
solubilities of the stereoisomers 3 showed more significant
differences. (R,R,R,R,R,R,R)-3 was highly soluble in CH2Cl2,
whereas (R,S,R,S,R,S,R)-3 was relatively insoluble. At 20 °C in
CH2Cl2, the solubility limits of (R,R,R,R,R,R,R)-3, (R,R,R,S,R,R,R)-
3, (R,S,R,R,R,S,R)-3, and (R,S,R,S,R,S,R)-3 were ≥200, 120, 90,
and 1 mg mL–1, respectively. The stacking interactions likely
increased in this order. The dipole moments may also have
significantly influenced solubility. Each dipole moment in the
helically arranged naphthalene rings was dispersive and cancelled
other dipole moments out, whereas those of the alternating
naphthalene ring were parallel and intensified. Indeed, TLC on
silica gel 60 (Merck, CHCl3/EtOAc = 50/1) indicated that the Rf
values were 0.63, 0.48, 0.35, and 0.05 for (R,R,R,R,R,R,R)-3,
(R,R,R,S,R,R,R)-3, (R,S,R,R,R,S,R)-3, and (R,S,R,S,R,S,R)-3,
respectively. VT NMR studies and DFT calculations indicated that
Figure 3. X-ray crystal structure of (R,R,R)-2 (the solvent molecules
are omitted for clarity): (left) ORTEP diagrams (thermal ellipsoids at
50% probability) and (right) a space-filling model.
the oligonaphthodioxepins were highly stable both chemically and
stereochemically (see the Supporting Information for details).
We next analyzed the optical properties of solutions of 1–3. The
optical spectra of the 1,4-dioxane solutions are shown in Figure 4,
and the luminescence properties are summarized in Table 1. The
absorption edges shifted to the red upon oligomerization, and the
stereoisomers displayed few differences (Figure 4b). TD-DFT
calculations of any of the octamers 3 indicated that both the
HOMO and LUMO were delocalized across the molecule with a
gap of 3.5 eV, without exception. Figure 4d plots the fluorescence
spectra. All compounds exhibited a high fluorescence quantum
yield (FL,1,4-dioxane), (R,R,R,R,R,R,R)-3 showed the highest FL
(FL,1,4-dioxane = 0.90) (Table 1). The stereoisomers displayed
similar FL spectra with somewhat different Stokes shifts. (R,R,R)-2
and (R,R,R,R,R,R,R)-3 provided smaller Stokes shifts than the other
stereoisomers, e.g., 1700 cm–1 for (R,R,R)-2, 3200 cm–1 for
(R,S,R)-2, 1100 cm–1 for (R,R,R,R,R,R,R)-3, and 2500 cm–1 for
(R,S,R,S,R,S,R)-3. In both the ground and excited states, (R,R,R)-2
and (R,R,R,R,R,R,R)-3 were more rigid than the other
stereoisomers, and the rigidity decreased the Stokes shift and
increased the fluorescence quantum yield. Although the optimized
ground and excited (S1) state structures of (R,R,R,R,R,R,R)-3 were
very similar to one another, the structure of the excited (S1) state of
(R,S,R,S,R,S,R)-3 was linear and did not assume an arc form
(Figure S4).[16] Excitation energy may be lost through the small
structural change.
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The CPL spectra are shown in Figure 4c. (R,R,R)-2 and
(R,R,R,R,R,R,R)-3 possessed significant (+)-CPL, and glum,1,4-dioxane
values were +1.6  10–3 for (R,R,R)-2 and +2.2  10–3 for
(R,R,R,R,R,R,R)-3. The signs and the g values of the CPL and the
CD related to the transition from the HOMO to LUMO (e.g., a gabs
value of (R,R,R,R,R,R,R)-3 was +1.5  10–3 at the first Cotton CD
band in Figure 4a) agreed well, as is often the case, indicating that
the structures in the ground and excited states were similar.
(R,R,R)-2 and (S,S,S)-2 provided clear mirror images. The spectral
shape and intensity of (R,R,R,R,R,R,R)-3 were not altered by the
solvents (1,4-dioxane, CH2Cl2, CH3CN, EtOAc, DMF, toluene)
and the temperature ranging 20–80 °C (Figures S6 and S7). That is,
the CPL properties were not affected by the environment.
Figure 4. a) CD, b) UV-Vis, c) CPL, and d) FL spectra of 1–3 in 1,4-
dioxane. Conditions: [1] = 1.0  10–5 M, [2] = 5.0  10–6 M, [3] = 2.5 
10–6 M, 20 °C, ex = 310 nm for 1, 330 nm for 2 and 3. The inset
presents a photograph of a picture of 2.5  10–6 M (R,R,R,R,R,R,R)-
3 solution under UV (365 nm) light.
Table 1: FL and CPL properties of 1–3.
1,4-dioxane sol.
solid state
[a]
[b]
compound
FL
glum
FL
glum
–1.1  10–3
+1.4  10–3
+2.1  10–4
+7.0  10–3
nd[c]
(R)-1
0.44
0.78
0.79
0.90
0.79
0.84
0.64
+6.8  10–4
+1.6  10–3
nd[c]
0.13
0.29
0.21
0.22
0.24
0.17
0.13
Figure 5. a) Solid-state CPL and b) FL Spectra of 1–3.
Conditions:ex = 310 nm for 1, 330 nm for 2 and 3, 20 °C, nujol mull.
c) Photographs showing the solid-state FL of 1–3.
(R,R,R)-2
(R,S,R)-2
(R,R,R,R,R,R,R)-3
(R,R,R,S,R,R,R)-3
(R,S,R,R,R,S,R)-3
(R,S,R,S,R,S,R)-3
+2.2  10–3
+5.7  10–4
+3.8  10–4
+3.1  10–4
Finally, we measured the solid-state FL and CPL spectra of the
all-(R)-1–3 (Figure 5).[17] Compounds 1–3 fluoresced in the solid
(powder) state. The fluorescent (R)-1 was pale blue (FL,solid
=
nd[c]
0.13), (R,R,R)-2 was yellow green (FL,solid 0.29), and
=
(R,R,R,R,R,R,R)-3 was light blue (FL,solid = 0.22). The FL,solid
values were very high, although they were lower than the
corresponding values obtained in solution. The high values
obtained from solid-state fluorescence were due to the rigidity as
well as to the fact that the oligonaphthodioxepins, especially
helical (R,R,R,R,R,R,R)-3, were non-planar molecules. These
molecules avoided efficient stacking with the -planes of adjacent
molecules in the solid state. In fact, the em,solid values were very
nd[c]
Conditions: ex = 310 nm for 1, 330 nm for 2 and 3, 20 °C, solution;
[1] = 1.0  10–5 M, [2] = 5.0  10–6 M, [3] = 2.5  10–6 M. [a] The
absolute fluorescence quantum efficiencies of (R,R,R)-2 and
(R,R,R,S,R,R,R)-3 were measured as reference standards. [b]
Absolute fluorescence quantum efficiency. [c] Not detected.
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In
conclusion,
we
developed
helically
arranged
oligonaphthodioxepins with excellent chiroptical properties,
solubility, structural rigidity, and (stereo)chemical stability. It
should be noted that (R,R,R,R,R,R,R)-3 showed intense FL and
CPL both in solution and in the solid state (FL,1,4-dioxane = 0.90,
glum,1,4-dioxane = +2.2  10–3, FL,solid = 0.22, glum,solid = +7.0  10–3).
The values of glum,solid was one of the highest yet reported for
monomeric organic compounds to our knowledge. The -planes of
adjacent (R,R,R,R,R,R,R)-3 molecules were incapable of forming
efficiently stacked structures due to the non-planar molecular
conformation, which led to a high FL,solid value.
Acknowledgements
This work was supported by the JSPS KAKENHI Grant Number
JP17K05786, the Kyoto Technoscience Center, and the Okayama
Foundation for Science and Technology. We thank JASCO
Corporation for solid-state CPL measurements, Otsuka Electronics
Co., Ltd for solid-state FL measurements, and Dr. Hiromi Ota of
Okayama University for X-ray analyses.
[8] For monomeric dyes with solid-state CPL, see: a) T. Sato, N. Tajima,
H. Ueno, T. Harada, M. Fujiki, Y. Imai, Tetrahedron 2016, 72,
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Conflict of interest
The authors declare no conflict interest.
Keywords: Axial chirality • Chiroptical property • Circularly polarized
luminescence • Helical structure
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[17] a) Solid-state UV-Vis and CD spectra of (R,R,R,R,R,R,R)-3 are
shown in Figure S8. These spectra were similar to the solution-state
spectra; b) Powder XRD studies indicated that these materials
assumed a typical amorphous form (Figure S19).
[18] The possibility that the linear polarization distorted the CPL signal of
(R,R,R,R,R,R,R)-3 cannot be denied.
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Title
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COMMUNICATION
K.Takaishi,* T. Yamamoto, S. Hinoide,
T. Ema*
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Helical Oligonaphthodioxepins
Showing Intense CPL in Solution and
in the Solid State
Solid-state CPL dyes: A (R,R,R,R,R,R,R)-naphthodioxepin (dioxepin-fused
naphthalene) octamer was found to take a unique left-handed helical structure with
high rigidity, (stereo)chemical stability, and solubility. Interestingly, the octamer
emits strong circularly polarized luminescence (CPL) with high FL and glum values
both in solution and in the solid state.
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