Delayed fluorescence behaviors of aminopyridine oligomers: Azacalix[n](2,6)pyridines (n = 3 and 4) and their linear analog

Article abstract of DOI:10.1246/cl.131037

Aminopyridine oligomers exhibited long-lived emission at 77 K, which could be assigned to the delayed fluorescence. The macrocyclic structure predominantly dictated the emission behaviors. Their emission behaviors were elucidated from timedependent densit

Full text of DOI:10.1246/cl.131037

Received: November 5, 2013 | Accepted: December 10, 2013 | Web Released: December 14, 2013
CL-131037
Delayed Fluorescence Behaviors of Aminopyridine Oligomers:
Azacalix[n](2,6)pyridines (n = 3 and 4) and Their Linear Analog
Natsuko Uchida,1,4 Tohru Sato,2,3 Junpei Kuwabara,1,4 Yoshinobu Nishimura,1,5 and Takaki Kanbara*1,4
1Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), Graduate School of Pure and Applied Sciences,
University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573
2Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Katsura, Kyoto 615-8510
3Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8510
4Institute of Materials Science, Graduate School of Pure and Applied Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573
5Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573
(E-mail: kanbara@ims.tsukuba.ac.jp)
Aminopyridine oligomers exhibited long-lived emission at
N
N
N
N
77 K, which could be assigned to the delayed fluorescence.
The macrocyclic structure predominantly dictated the emission
behaviors. Their emission behaviors were elucidated from time-
dependent density functional theory calculations.
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
1
2
3
The luminescence mechanism of organic compounds can be
classified into fluorescence, phosphorescence, and delayed
fluorescence. Delayed fluorescence is an emission process that
proceeds via up-conversion from the first excited triplet state
(T1) to the first excited singlet state (S1) through reverse
intersystem crossing (RISC).1,2 It can be classified into two types
of mechanisms because of up-conversion through RISC: (i)
triplet-triplet annihilations and (ii) thermally activated delayed
fluorescence (TADF, also called E-type delayed fluorescence).
In 2009, the Adachi group realized the first application of
TADF materials for proposing the third-generation OLEDs
(organic light-emitting diodes), which are expected to show an
excellent electroluminescence efficiency with good stability in
high current density.1 Although the phenomena of delayed
fluorescence has been known from 1950s,3 TADF materials
draw renewed attention since their report.4 In the TADF
mechanism, a key factor is how to achieve a small energy gap
between the singlet and triplet states (¦EST) to enable an up-
conversion of the triplet exciton from a triplet to a singlet state. It
has been proposed that the molecular design for achieving a
small ¦EST includes a donor-acceptor framework to induce an
intramolecular charge-transfer transition from separate HOMO
to LUMO levels in a single molecule.5 However, this molecular
design for TADF compounds should involve an appropriate
selection of donor and acceptor units. The development of
another design for the delayed fluorescence compounds, in
addition to the conventional design, will expand the scope of
TADF materials.
Figure 1. Molecular structures of 1-3.
performed to elucidate their luminescence behaviors. The insight
would provide novel and valuable information for the molecular
design of delayed fluorescence compounds.
Azacalix[n](2,6)pyridine derivatives 1 (n = 3) and 3 (n = 4)
were synthesized by a method described in the literature.9 Linear
chain compound 2 was obtained in high yields by multistep
synthesis (Scheme S1).10 The UV-vis absorption spectra and
emission spectra of 1-3 in 2-methyltetrahydrofuran (Me-THF)
solutions at both room temperature and 77 K are shown in
Figure 2. The emission quantum yields (Φem) of 1, 2, and 3 at
room temperature were 27%, 45%, and 8%, respectively,
whereas the corresponding Φem values of 1-3 were increased
to 38%, 56%, and 67% at 77 K. The details of the emission
lifetimes (¸) of 1-3 are summarized in Table S1, and the
emission-decay curves are shown in Figure 3. At room temper-
ature, 1-3 showed a blue emission with a maximum emission
wavelength (-em) of around 420 nm. In a glass matrix at 77 K,
the emissions of 1-3 appeared at virtually the same wavelength
of 420 nm as those at room temperature, whereas 2 and 3 showed
new emissions of around 570 nm at 77 K. These 570-nm
emissions of 2 and 3 consist of one component, which was
confirmed from their emission-decay curves. The emission-
decay curves of 1-3 around 420 nm suggest that the emission
consists of two components with different lifetimes at both room
temperature and 77 K.11 It should be noted that in a glass matrix
at 77 K, the lifetimes for the emission of 1-3 were quite long; ¸
was in the several hundred ms range (Table S1 and Figure S1).
To clarify the spectra of the long-lived emission components, the
time-dependent PL measurements for the long-lifetime compo-
nent were carried out at 77 K (Figure S3). The delayed
emissions of 1-3 at 420 nm appeared to have almost the same
spectra as the normal PL spectra at 77 K.
Herein, we report the first observation of delayed fluores-
cence behavior of macrocyclic and acyclic aminopyridine
oligomers 1-3, which depends on the size and topology of the
recurring units (Figure 1). Azacalix[n](2,6)pyridines are macro-
cyclic aminopyridine6 oligomers.7,8 Although the structural
features and reactivity of the azacalix[n](2,6)pyridines have
been investigated,7-9 the fundamental luminescence properties of
the compounds have not been well evaluated. Time-dependent
density functional theory (TD-DFT) calculations were also
Because delayed fluorescence proceeds through an up-
conversion from a triplet state to a singlet state, the emission
Chem. Lett. 2014, 43, 459–461 | doi:10.1246/cl.131037
© 2014 The Chemical Society of Japan | 459

(a) 3.9
S1 (3.833 eV)
3.8
3.7
T9 (3.732 eV)
T7 (3.636 eV)
T8 (3.661 eV)
3.6
3.5
3.4
3.3
3.2
T6 (3.494 eV)
T4 (3.405 eV)
T5 (3.412 eV)
T3 (3.380 eV)
T2 (3.178 eV)
T1 (3.134 eV)
3.1
(b)
HOMO−1
25
HOMO
LUMO
30
(c)
−1.0
26
27
24
28
29
23
−0.8
−0.6
−0.4
−0.2
0.0
22
Figure 2. UV-vis absorption spectra and photoluminescence
spectra of (a) 1 (at RT and 77 K both -ex = 320 nm), (b) 2 (at RT
and 77 K both -ex = 360 nm) and (c) 3 (at RT: -ex = 370 nm, at
77 K: -ex = 340 nm) in degassed Me-THF solution of 1.0 ©
10¹5 M.
0.2
0.4
0.6
0.8
Hückel−HOMO
Hückel−LUMO
21
20
19
1.0
13
14
15
16
17
18
Figure 4. (a) Energy diagram of 1. (b) Pictorial representa-
tions of the frontier MOs of 1. Calculated at the PBE0/6-
31G(d,p) level of theory. (c) Energy diagram of 1 within Hückel
approximation (The Coulomb integrals of nitrogen of amino
group and pyridine moiety and the resonance integrals were
taken from reference14).
delayed fluorescence and fast emissions (Table S1, ¸1 and ¸2 of
77 K) at the same wavelength as that of fluorescence.
For a large difference among 1, 2, and 3 at 77 K, 2 and 3
exhibit a new long-lived emission peak around 570 nm in
addition to the emission at 420 nm (Table S1 and Figure S3).
The results of time-dependent PL behaviors suggest that the
emissions at 570 nm indicate phosphorescence; the phospho-
rescence of an organic compound generally appears in a longer
wavelength region than the fluorescence spectrum at 77 K.2 The
Φem value also suggests the involvement of phosphorescence,
because the Φem values of 2 and 3 increase with a decrease in
temperature. Similar PL behavior was previously reported for
proflavine hydrochloride, which is a delayed fluorescence
compound.12 From the maximum emission wavelength of
phosphorescence and fluorescence, ¦EST of 2 and 3 were
observed to be 0.77 and 0.60 eV, respectively. In 1, it can be
assumed that ¦EST is nearly 0 eV because no phosphorescence
was observed. Even for the same recurring aminopyridine unit,
different conformations and numbers of ring sizes lead to
different ¦EST values and phosphorescence behaviors.
Figure 3. Emission-decay curves of 1-3 in degassed Me-THF
solution of 1.0 © 10¹5 M (a) at RT of 1 (-ex = 380 nm,
-
em = 422 nm, black line), 2 (-ex = 380 nm, -em = 441 nm,
blue line), 3 (-ex = 360 nm, -em = 421 nm, red line) (b) at 77 K
glass matrix state of 1 (-ex = 373 nm, -em = 422 nm, black
line),
2 (-ex = 373 nm, -em = 441 nm, blue line) and 3
(-ex = 360 nm, -em = 421 nm, red line).
spectrum of the delayed fluorescence is generally similar to the
corresponding fluorescence spectrum.2 For the 1-3 compounds,
the long-lived emission spectra around 420 nm were almost
same as those of fluorescence spectra. In addition, it was
confirmed that the blue emission around 420 nm includes two
emission components with different emission lifetime ranges at
77 K. The long-lived emission at 420 nm at low temperatures
(Table S1, ¸3 of 77 K) of 1-3 can therefore be categorized into
Next, TD-DFT calculations for 1-3 were carried out using
Gaussian 09.13 The PBE0 functional with the 6-31G(d,p) basis
460 | Chem. Lett. 2014, 43, 459–461 | doi:10.1246/cl.131037
© 2014 The Chemical Society of Japan

set was employed; the PBE0 functional well reproduced the
experimental absorption wavelength of 1 (Figure S4 and
Table S2). The calculated ¦EST at the Franck-Condon states
of 2 and 3 were 0.72 and 0.65 eV, respectively, which are in
good agreement with the experimental ¦EST values of 2 and 3
(0.77 and 0.60 eV), as estimated from the fluorescence and
phosphorescence wavelengths discussed above. For 1, on the
other hand, the calculated ¦EST is 0.70 eV, while the exper-
imental ¦EST was almost 0 eV, because no phosphorescence was
observed.
References and Notes
1
A. Endo, M. Ogasawara, A. Takahashi, D. Yokoyama, Y.
Kato, C. Adachi, Adv. Mater. 2009, 21, 4802.
2
B. Valeur, M. N. Berberan-Santos, Molecular Fluorescence:
Principles and Applications, 2nd ed., Wiley-VCH, 2012.
doi:10.1002/9783527650002.
3
4
a) R. Williams, J. Chem. Phys. 1958, 28, 577. b) H. Sponer,
Y. Kanda, L. A. Blackwell, J. Chem. Phys. 1958, 29, 721. c)
C. A. Parker, C. G. Hatchard, Trans. Faraday Soc. 1961, 57,
1894. d) C. A. Parker, Nature 1963, 200, 331. e) C. A.
Parker, C. G. Hatchard, Trans. Faraday Soc. 1963, 59, 284.
f) C. A. Parker, Spectrochim. Acta 1963, 19, 989. g) R. G.
Kepler, J. C. Caris, P. Avakian, E. Abramson, Phys. Rev.
Lett. 1963, 10, 400.
a) A. Endo, K. Sato, K. Yoshimura, T. Kai, A. Kawada, H.
Miyazaki, C. Adachi, Appl. Phys. Lett. 2011, 98, 083302. b)
S. Y. Lee, T. Yasuda, H. Nomura, C. Adachi, Appl. Phys.
Lett. 2012, 101, 093306. c) H. Tanaka, K. Shizu, H.
Miyazaki, C. Adachi, Chem. Commun. 2012, 48, 11392. d)
Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki,
C. Adachi, J. Am. Chem. Soc. 2012, 134, 14706. e) F. A.
Salazar, A. Fedorov, M. N. Berberan-Santos, Chem. Phys.
Lett. 1997, 271, 361. f) M. N. Berberan-Santos, J. M. M.
Garcia, J. Am. Chem. Soc. 1996, 118, 9391. g) S. Park, O.-H.
Kwon, Y.-S. Lee, D.-J. Jang, S. Y. Park, J. Phys. Chem. A
2007, 111, 9649. h) R. Czerwieniec, J. Yu, H. Yersin, Inorg.
Chem. 2011, 50, 8293.
From the TD-DFT calculations, we propose a plausible
mechanism of the observed delayed fluorescence in 1-3.
Figure 4a shows the energy diagram of 1, whereas Figures S5a
and S5b show the energy diagrams of 2 and 3, respectively. It
should be noted that 1-3 have quasi-degenerate excited triplet
states Tn (where n = 7, 8, and 9 for 1; n = 9, 10, 11, and 12 for
2; and n = 9 and 10 for 3) near the lowest excited singlet state
S1. The energy gap between S1 and the quasi-degenerate Tn
states of 1-3 close to S1 is less than 240 meV; thus, the energy
barriers can be overcome by thermal excitation via RISC. In
contrast, the energy gaps between S1 and T1 of 1, 2, and 3 are too
large to be overcome through thermal excitation. Therefore,
delayed fluorescence via the T1 states seems to be almost
impossible. These results suggest that the observed delayed
fluorescence originates from the higher quasi-degenerate triplet
states Tn. According to the energy diagram of 1 within the
Hückel approximation, these quasi-degenerate states of 1-3 are
due to the nonbonding orbitals formed by the meta-linkage of
the pyridine ring through the amino groups (Figure 4c);15 these
Tn states close to the S1 states are excitations between the
orbitals localized on the tolyl and pyridine moieties (see the
MOs of Figures 4b and 4c). The T1 state of 1 is an excitation
from the HOMO mainly localized on the remaining pyridine
moieties to the LUMO. Therefore, the overlap density between
T1 and T9 is small because the region where the HOMO is
localized differs from the region where the next HOMO
(HOMO¹1) is localized (Figure 4b). The difference in the
phosphorescence behavior of 1-3 indicates that the orbital
localization of the quasi-degenerate excited triplet states might
be somewhat weak in 2 and 3.16
5
6
H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi,
Nature 2012, 492, 234.
a) T. W. Bell, A. B. Khasanov, M. G. B. Drew, J. Am. Chem.
Soc. 2002, 124, 14092. b) J.-D. Cheon, T. Mutai, K. Araki,
Org. Biomol. Chem. 2007, 5, 2762.
7
a) Y. Miyazaki, T. Kanbara, T. Yamamoto, Tetrahedron Lett.
2002, 43, 7945. b) T. Kanbara, Y. Suzuki, T. Yamamoto, Eur.
J. Org. Chem. 2006, 3314. c) N. Uchida, A. Taketoshi,
J. Kuwabara, T. Yamamoto, Y. Inoue, Y. Watanabe, T.
Kanbara, Org. Lett. 2010, 12, 5242. d) N. Uchida, J.
Kuwabara, A. Taketoshi, T. Kanbara, J. Org. Chem. 2012,
77, 10631.
8
9
a) E.-X. Zhang, D.-X. Wang, Q.-Y. Zheng, M.-X. Wang,
Org. Lett. 2008, 10, 2565. b) H.-Y. Gong, Q.-Y. Zheng,
X.-H. Zhang, D.-X. Wang, M.-X. Wang, Org. Lett. 2006, 8,
4895.
Y. Suzuki, T. Yanagi, T. Kanbara, T. Yamamoto, Synlett
2005, 263.
In conclusion, we observed the delayed fluorescence behav-
ior in 1-3. The TD-DFT calculations indicate that each of them
has a small energy gap, ¦EST, between the fluorescence state S1
and the higher excited quasi-degenerate triplet state Tn than the
T1 state. The macrocyclic structure of 1 predominantly dictated
the photoluminescence properties. To clarify this peculiarity in
1, further theoretical calculations for the excited states are
required. We will discuss the results in another report in the near
future.
10 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
11 Similar case was reported: C.-S. Choi, K.-S. Jeon, K.-H. Lee,
J. Photosci. 2004, 11, 71.
The authors kindly acknowledge the emission lifetime
measurements provided by HORIBA, Ltd. We appreciate the
help received from S. Mimura, Dr. Y. Nakata, and S. Akaji of
HORIBA, Ltd., with the emission lifetime measurements. Prof.
Dr. T. Nabeshima and Dr. M. Yamamura are grateful for the
support provided during quantum yield measurements and
long-term photoluminescence spectra measurements. Numerical
calculations were performed partly at the Supercomputer
Laboratory of Kyoto University and the Research Center for
Computational Science, Okazaki, Japan.
12 C. A. Parker, C. G. Hatchard, J. Phys. Chem. 1962, 66, 2506.
13 M. J. Frisch, et al., Gaussian 09 (Revision D.01), see
Supporting Information.
14 A. Streitwieser, Molecular Orbital Theory for Organic
Chemists, John Wiley & Sons, 1961.
15 E. Heilbronner, H. Bock, The HMO Model and Its
Application: Basis and Manipulation, John Wiley & Sons,
1976, Vol. 1.
16 T. Sato, M. Uejima, N. Iwahara, N. Haruta, K. Shizu, K.
Tanaka, J. Phys.: Conf. Ser. 2013, 428, 012010.
Chem. Lett. 2014, 43, 459–461 | doi:10.1246/cl.131037
© 2014 The Chemical Society of Japan | 461