Design and characterization of 4,5-bis(diarylamino)phthalic acid diesters as a new class of fluorophores exhibiting efficient blue emission in the solid state

Article abstract of DOI:10.1002/ejoc.201403272

We developed 4,5-bis(diarylamino)phthalic acid diesters as blue fluorophores exhibiting efficient photoluminescence in powder and in a thin film of pol(methyl methacrylate). Increasing the electron-donating nature of the substituents in the para positions

Full text of DOI:10.1002/ejoc.201403272

Job/Unit: O43272
/KAP1
Date: 03-12-14 18:07:33
Pages: 6
SHORT COMMUNICATION
DOI: 10.1002/ejoc.201403272
Design and Characterization of 4,5-Bis(diarylamino)phthalic Acid Diesters as a
New Class of Fluorophores Exhibiting Efficient Blue Emission in the Solid
State
Masaki Shimizu*[a] and Tomokazu Tamagawa[a]
Keywords: Materials science / Photochemistry / Charge transfer / Donor–acceptor systems / Fluorescence
We developed 4,5-bis(diarylamino)phthalic acid diesters as
blue fluorophores exhibiting efficient photoluminescence in
powder and in a thin film of pol(methyl methacrylate). In-
creasing the electron-donating nature of the substituents in
the para positions of the diarylamino moieties in the dimethyl
esters increased the redshift in the fluorescence spectrum.
Replacing the methoxycarbonyl groups with aryloxylcarb-
fluorescence spectrum. Thus, the color of the emission was
finely tuned by simple molecular modification. Single-crystal
X-ray analysis revealed a twisted conformation with loose
packing in the crystal. Density functional theory calculations
suggested that the excitation process was governed by intra-
molecular charge transfer from the diarylamino moieties to
the alkoxycarbonyl groups.
onyl groups also induced
a bathochromic shift in the
processes, such as internal conversion followed by vi-
brational relaxation and intersystem crossing. Fluorescence
efficiency in the solid state is also strongly related to intra-
molecular nonradiative decay as well as intermolecular
photophysical processes, such as the formation of excimers,
exciplexes, or physical dimers and the transfer of energy or
electrons, or both.[5] Accordingly, suppression of intermo-
lecular interactions that result in luminescence quenching is
key to the development of organic fluorophores that are
efficiently emissive in the solid state.
We recently reported that dimethyl 2,5-bis(diorgano-
amino)terephthalates 1 exhibited green to yellow fluores-
cence with good to high quantum yields in crystals and in
powder (Figure 1).[6] Terephthalates 1 are structurally char-
acterized by the presence of two donors (i.e., amino groups)
and two acceptors (i.e., methoxycarbonyl groups) and two-
fold ortho linkages of the donors and acceptors. The donor–
acceptor pairs induce intramolecular charge transfer that
results in a large Stokes shift. The ortho linkages force the
molecule to adopt a twisted conformation, which leads to
inhibition of dense packing in the solid state. Thus, the
structural characteristics of 1 minimize intermolecular en-
ergy migration from fluorophores in the excited state
through Förster and Dexter mechanisms.[7] On the basis of
these results, we envisioned the development of novel blue
fluorophores by changing the substitution patterns of the
two amino groups and the two alkoxycarbonyl groups,
while retaining the two sets of ortho linkages. We report
herein the structure, photophysical properties, and molecu-
lar orbital calculations of 4,5-bis(diarylamino)phthalic acid
diesters 2 as a new class of blue emitters exhibiting good
fluorescence efficiency in the solid state (Figure 1).
Introduction
Organic light-emitting diodes (OLEDs) are currently
attracting increased attention for applications in full-color
flat displays and lighting devices.[1] OLED emission is de-
rived from an organic emitter in a thin film or dispersed in
a host matrix. Hence, advances in OLEDs are highly de-
pendent on the development of organic chromophores that
radiate visible light in the solid state with high luminescence
efficiency.[2] Phosphorescent organometallic compounds
such as iridium complexes can in principle attain 100% in-
ternal quantum efficiency in OLEDs owing to the heavy-
metal effect of transition metals.[1d] Therefore, they serve as
very attractive emitters, especially for green and red
OLEDs. However, the lifetime and color purity of blue
phosphorescent OLEDs need improvement for practical ap-
plications as a result of the intrinsically wide band gap of
blue materials. Therefore, for continued development of
OLEDs, novel fluorophores are needed that efficiently emit
blue light in the solid state.[3]
Fluorescence is one of the possible de-excitation pro-
cesses of an excited molecule located at the lowest vi-
brational level of the singlet electronic state, that is, the S1
state.[4] The efficiency of fluorescence in solution depends
on the inhibition of other intramolecular photophysical
[a] Department of Biomolecular Engineering, Graduate School of
Science and Technology, Kyoto Institute of Technology,
1 Hashikami-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585,
Japan
E-mail: mshimizu@kit.ac.jp
http://www.cis.kit.ac.jp/~orgsynth/en/index.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403272.
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Job/Unit: O43272
/KAP1
Date: 03-12-14 18:07:33
Pages: 6
M. Shimizu, T. Tamagawa
SHORT COMMUNICATION
Figure 1. Molecular structures of diaminoterephthalates 1 and di-
aminophthalates 2.
Results and Discussion
Diaminophthalates 2 were easily synthesized from com-
mercially available 4,5-dichlorophthalic acid through three
(for 2a–c) or five (for 2d–f) steps (see the Supporting Infor-
mation for details). The decomposition temperature (Td),
defined as the temperature at which 5% of the mass is lost,
ranged from 279 to 360 °C for all derivatives of 2, which
was indicative of their good thermal stability. Dimethyl
phthalate 2b crystallized from CH2Cl2/hexane solution in
the monoclinic space group P21/n.[8] The molecular and
crystal structures of 2b are shown in Figure 2. Both phenyl
groups of the amino moieties and methoxycarbonyl groups
largely or significantly deviated from the central benzene
Figure 2. (a) Molecular structure of 2b. Selected dihedral angles [°]:
C(2)–C(1)–N(1)–C(5) 55.57, C(6)–N(2)–C(2)–C(1) 59.85, C(3)–
C(4)–C(8)–O(2) 62.25, C(4)–C(3)–C(7)–O(1) 28.47. (b) Crystal
structure of 2b (viewed from the b axis). Space group: P21/n. Unit
cell dimensions:
a = 14.1377(8) Å, b = 9.1624(5) Å, c =
22.2157(12) Å, β = 104.6720(10)°.
plane, as expected (Figure 2, a). The dihedral angles of (PMMA) are shown in Table 1 and Figure 3 (b–d). Com-
C(2)–C(1)–N(1)–C(5), C(6)–N(2)–C(2)–C(1), C(3)–C(4)– parison of each absorption spectrum of 2 in toluene with
C(8)–O(2), and C(4)–C(3)–C(7)–O(1) were found to be its respective fluorescence spectrum in toluene showed that
55.57, 59.85, 62.25, and 28.47°, respectively. The corre- the spectral overlap was very small; this suggested that de-
sponding dihedral angles of C–C–N–C and C–C–C–O for activation of 2 from the excited state to the ground state
1a (R = Ph) were 44.62 and 48.47°, respectively (see Fig- through Förster-type energy migration was minimal (over-
ure S1, Supporting Information). Thus, the contortions of laid absorption and fluorescence spectra of 2b in toluene
the Ph2N moieties of 2b were both larger than those of 1a, are shown as an example in the Supporting Information).
whereas one CO2Me moiety was more twisted and the other The presence of electron-withdrawing (CF3) or electron-
was less twisted relative to those of 1a. There was no π–π donating (tBu) groups in the 4-positions of each phenyl
stacking in the crystal lattice, which minimized lumines- group of the amino moieties induced a blue- or redshift in
cence quenching, likely as a result of the twisted molecular the spectrum, respectively, relative to the spectrum of 2b,
conformation (Figure 2, b).
irrespective of the state of 2. Replacing the methoxycarb-
The absorption spectra of 2 in toluene are shown in Fig- onyl groups with aryloxycarbonyl groups also resulted in a
ure 3a as is the spectrum of 1a (R = Ph). The spectra of all bathochromic shift in the spectrum (compare that of 2b
derivatives of 2 were significantly blueshifted relative to the with those of 2d–f), presumably because the methoxy group
spectrum of 1a and all had small shoulders between 360 is more electron-donating than aryloxy groups. Among 2d–
and 380 nm, which were assigned to charge-transfer bands. f, a greater number of methyl groups on the aryloxy
The absorption edges of all derivatives of 2 ranged from moieties shortened the emission maximum, except for 2e
around 400 to 440 nm, much shorter than that of 1a dispersed in a PMMA film. Thus, the emission color could
(493 nm), which indicated that the highest occupied molec- be finely tuned by simple molecular modification. The sub-
ular orbital (HOMO)–lowest unoccupied molecular orbital stituent effects on the photophysical properties described
(LUMO) energy gaps of 2 were larger than that of 1a. Con- above were in good agreement with the anticipated intra-
sidering the molecular conformation disclosed by X-ray molecular charge transfer from the diarylamino moieties to
analysis, the blueshift may be attributed to the larger distor- the alkoxycarbonyl groups. Fluorescence quantum yields of
tion of the diarylamino moieties from the central benzene 2 in powder and doped PMMA film were generally good
ring. These results clearly demonstrate that the molecular and ranged from 0.39–0.58, except for 2a likely because of
design of 2 resulted in shorter effective conjugation lengths its twisted molecular conformation, which results in loose
than in 1. Indeed, 2 exhibited blue to bluish-green fluores- packing, and because of its electronic structure, which
cence in solution and in the solid state with good quantum exhibits a large Stokes shift. In some cases, the quantum
yields. The fluorescence data and spectra of 2 in toluene, yields in the solid states were higher than those in solution.
in powder, and in a thin film of poly(methyl methacrylate) The lower efficiency in solution is presumably ascribed to
2
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O43272
/KAP1
Date: 03-12-14 18:07:33
Pages: 6
Fluorophores Exhibiting Efficient Blue Emission in the Solid State
intramolecular free rotation of the diarylamino and alkoxy- Table 1. Fluorescent properties of 2.
carbonyl moieties. Good efficiency in the solid state indi-
catesstrongpotentialforuseasblue-emittinghostmaterialsand
dopants in OLEDs.
In toluene[a]
Powder[a]
In PMMA film[a,b]
λmax (nm) Φ[c]
λmax (nm) Φ[c]
λmax (nm) Φ[c]
2a
2b
2c
2d
2e
2f
432
463
495
488
482
480
0.34
0.43
0.42
0.34
0.47
0.49
428
458
476
496
495
461
0.33
0.58
0.39
0.50
0.44
0.46
420
451
472
468
474
460
0.20
0.49
0.45
0.47
0.49
0.41
[a] Excitation was effected by UV light at 320 nm. [b] Prepared by
spin coating. [c] Absolute quantum yield was determined by a cal-
ibrated integrating sphere system.
LUMO was mainly delocalized across the central benzene
ring and the two C(=O)O units. Time-dependent DFT cal-
culations assigned the lowest energy transition mainly to
the HOMO–LUMO transition. Hence, the small shoulders
between 360 and 380 nm in the absorption spectra of 2
originated from intramolecular charge transfer from the
amino moieties to the alkoxycarbonyl groups. The charge-
separated nature of the excited state was supported by the
redshift in the fluorescence spectra in more polar solvents
(see Figure S3, Supporting Information). The calculated
HOMO–LUMO energy gaps of 2 were much larger than
Figure 4. (a) HOMO and (b) LUMO drawings of 2b. (c) HOMO
and (d) LUMO drawings of 2d.
Figure 3. (a) Absorption spectra of 1a and 2 in toluene. Fluores-
cence spectra of 2 in (b) toluene, (c) powder, and (d) PMMA film.
Density functional theory (DFT) calculations were per-
formed at the B3LYP/cc-pVDZ//B3LYP/cc-pVDZ level by
using the Gaussian 09 package.[9] The HOMO and LUMO
diagrams of 2b and 2d are illustrated in Figure 4, and the
calculated energies and the energy gaps between the
HOMOs and LUMOs of 1a and 2 are depicted in Figure 5.
For both 2b and 2d, the HOMO developed over the central
Figure 5. Calculated HOMO and LUMO energies of 1a and 2; val-
ues between the HOMO and LUMO levels are the energy gaps
benzene ring and the two Ph2N moieties, whereas the between the HOMOs and LUMOs.
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3

Job/Unit: O43272
/KAP1
Date: 03-12-14 18:07:33
Pages: 6
M. Shimizu, T. Tamagawa
SHORT COMMUNICATION
2007, p. 295–412; c) D. F. Perepichka, I. F. Perepichka, H.
Meng, F. Wudl, Organic Light-Emitting Materials and Devices
(Eds.: Z. Li, H. Meng), Taylor & Francis, Boca Raton, 2007,
p. 45–293; d) A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G.
Jokisz, A. B. Holmes, Chem. Rev. 2009, 109, 897–1091; e) Z.
Zhao, J. W. Y. Lam, B. Z. Tang, J. Mater. Chem. 2012, 22,
23726–23740.
that of 1a, which is consistent with the trend in the absorp-
tion edges shown in Figure 3 (a) and the blueshifted
fluorescence spectra of 2. The observed electronic effects of
substituents in the para positions of each phenyl group of
the diarylamino moieties on the energy differences in the
HOMOs and LUMOs were consistent with the DFT calcu-
lations.
[3]
For recent examples of blue fluorophores, see: a) J.-Y. Hu, Y.-
J. Pu, F. Satoh, S. Kawata, H. Katagiri, H. Sasabe, J. Kido,
Adv. Funct. Mater. 2014, 24, 2064–2071; b) S. Wu, M. Aonuma,
Q. Zhang, S. Huang, T. Nakagawa, K. Kuwabara, C. Adachi,
J. Mater. Chem. C 2014, 2, 421–424; c) P. Y. Chou, H. H. Chou,
Y. H. Chen, T. H. Su, C. Y. Liao, H. W. Lin, W. C. Lin, H. Y.
Yen, I. C. Chen, C. H. Cheng, Chem. Commun. 2014, 50, 6869–
6871; d) J. Huang, N. Sun, P. Chen, R. Tang, Q. Li, D. Ma, Z.
Li, Chem. Commun. 2014, 50, 2136–2138; e) S. Y. Lee, T. Ya-
suda, Y. S. Yang, Q. Zhang, C. Adachi, Angew. Chem. Int. Ed.
2014, 53, 6402–6406; Angew. Chem. 2014, 126, 6520–6524; f) J.
Huang, N. Sun, Y. Dong, R. Tang, P. Lu, P. Cai, Q. Li, D. Ma,
J. Qin, Z. Li, Adv. Funct. Mater. 2013, 23, 2329–2337; g) H.
Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature
2012, 492, 234–238; h) Q. Zhang, J. Li, K. Shizu, S. Huang, S.
Hirata, H. Miyazaki, C. Adachi, J. Am. Chem. Soc. 2012, 134,
14706–14709; i) M. Shimizu, K. Mochida, T. Hiyama, J. Phys.
Chem. C 2011, 115, 11265–11274; j) M. Shimizu, K. Mochida,
Y. Asai, A. Yamatani, R. Kaki, T. Hiyama, N. Nagai, H. Yam-
agishi, H. Furutani, J. Mater. Chem. 2012, 22, 4337–4342; k)
H.-H. Chou, Y.-H. Chen, H.-P. Hsu, W.-H. Chang, Y.-H.
Chen, C.-H. Cheng, Adv. Mater. 2012, 24, 5867–5871; l) W. Li,
D. Liu, F. Shen, D. Ma, Z. Wang, T. Feng, Y. Xu, B. Yang, Y.
Ma, Adv. Funct. Mater. 2012, 22, 2797–2803; m) D. Yu, F.
Zhao, Z. Zhang, C. Han, H. Xu, J. Li, D. Ma, P. Yan, Chem.
Commun. 2012, 48, 6157–6159; n) I. Cho, S. H. Kim, J. H.
Kim, S. Park, S. Y. Park, J. Mater. Chem. 2012, 22, 123–129;
o) K. E. Linton, A. L. Fisher, C. Pearson, M. A. Fox, L.-O.
Palsson, M. R. Bryce, M. C. Petty, J. Mater. Chem. 2012, 22,
11816–11825; p) C. Liu, Y. Li, Y. Zhang, C. Yang, H. Wu, J.
Qin, Y. Cao, Chem. Eur. J. 2012, 18, 6928–6934; q) C. Liu, Y.
Gu, Q. Fu, N. Sun, C. Zhong, D. Ma, J. Qin, C. Yang, Chem.
Eur. J. 2012, 18, 13828–13835.
Conclusions
In conclusion, the molecular design of 4,5-bis(diaryl-
amino)phthalates, which involved construction of 1,2-
bis(donor)-4,5-bis(acceptor)-substituted benzenes, was ef-
fective for the development of blue fluorophores exhibiting
good luminescence efficiency in the solid state. The de-
signed blue emitters were easy to synthesize and were ther-
mally stable, which are attractive features for practical ap-
plications. The emission color could be finely tuned by
modifying the substituents in the para positions of each
phenyl group of the diarylamino groups as well as the
carbonaceous moieties of the alkoxycarbonyl groups. The
good luminescence efficiency in powder and in the poly-
(methyl methacrylate) host matrix indicates the strong
potential of these phthalates for use as blue-emissive host
materials and dopants in organic light-emitting diodes.
Further extension of this novel molecular design principle
to the development of non-blue-emissive materials and
application of the blue-emissive phthalates to organic light-
emitting diodes are in progress.
Supporting Information (see footnote on the first page of this arti-
cle): General experimental methods, representative procedures for
synthesis, characterization data, NMR spectra of 2, summary of
X-ray crystallographic analysis of 2d, overlaid absorption and
fluorescence spectra of 2d in toluene, and procedures for measure-
ment and PMMA film preparation.
[4]
[5]
a) N. J. Turro, Modern Molecular Photochemistry, University
Science Books, Sausalito, USA, 1978; b) B. Valeur, Molecular
Fluorescence–Principles
and
Applications,
Wiley-VCH,
Weinheim, Germany, 2002.
a) M. Kasha, H. R. Rawls, M. A. El-Bayoumi, Pure Appl.
Chem. 1965, 11, 371–392; b) R. C. Powell, Z. G. Soos, J. Lumin.
1975, 11, 1–45; c) J. Cornil, D. Beljonne, J. P. Calbert, J. L.
Brédas, Adv. Mater. 2001, 13, 1053–1067; d) S. Varghese, S.
Das, J. Phys. Chem. Lett. 2011, 2, 863–873; e) J. Gierschner,
S. Y. Park, J. Mater. Chem. C 2013, 1, 5818–5832; f) C. Vijayak-
umar, V. K. Praveen, A. Ajayaghosh, Adv. Mater. 2009, 21,
2059–2063.
M. Shimizu, Y. Asai, Y. Takeda, A. Yamatani, T. Hiyama, Tet-
rahedron Lett. 2011, 52, 4084–4089.
M. Shimizu, Aggregation-Induced Emission: Fundamentals
(Eds.: A. Qin, B. Z. Tang), Wiley, Chichester, UK, 2013, p. 83–
104.
Acknowledgments
This work was financially supported by the Japan Science and
Technology Agency through a Grant-in-Aid for the Adaptable and
Seamless Technology Transfer Program through target-driven
R&D (no. AS2414069M). The authors thank Mr. Yuiga Asai and
Mr. Akinori Yamatani for their experimental assistance in the early
stage of this work.
[6]
[7]
[8]
[9]
CCDC-1026010 (for 2b) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B.
Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hase-
gawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M.
Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Starov-
erov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell,
[1] a) Z. H. Kafafi (Ed.), Organic Electroluminescence, Taylor &
Francis, Boca Raton, USA, 2005; b) K. Müllen, U. Scherf
(Eds.), Organic Light-Emitting Devices. Synthesis Properties
and Applications, Wiley-VCH, Weinheim, Germany, 2006; c) Z.
Li, H. Meng (Eds.), Organic Light-Emitting Materials and De-
vices, Taylor & Francis, Boca Raton, USA, 2007; d) H. Yersin
(Ed.), Highly Efficient OLEDs with Phosphorescent Materials,
Wiley-VCH, Weinheim, Germany, 2008; e) H. Sasabe, J. Kido,
J. Mater. Chem. C 2013, 1, 1699–1707.
[2] a) M. Shimizu, T. Hiyama, Chem. Asian J. 2010, 5, 1516–1531;
b) H. Meng, N. Herron, Organic Light-Emitting Materials and
Devices (Eds.: Z. Li, H. Meng), Taylor & Francis, Boca Raton,
4
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O43272
/KAP1
Date: 03-12-14 18:07:33
Pages: 6
Fluorophores Exhibiting Efficient Blue Emission in the Solid State
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M.
Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Ad-
amo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Mar-
tin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B.
Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09,
revision D.01, Gaussian, Inc., Wallingford, CT, 2009.
Received: October 28, 2014
Published Online: ᭿
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5

Job/Unit: O43272
/KAP1
Date: 03-12-14 18:07:33
Pages: 6
M. Shimizu, T. Tamagawa
SHORT COMMUNICATION
Fluorophores
Diorgano 4,5-bis(diarylamino)phthalates
exhibited blue fluorescence in solution and
in the solid state with good efficiency. The
emission spectra were finely shifted by
changing the substituents in the para pos-
itions of each phenyl group of the diaryl-
amino moieties as well as the carbonaceous
moieties of the alkoxycarbonyl groups.
M. Shimizu,* T. Tamagawa .............. 1–6
Design and Characterization of 4,5-Bis(di-
arylamino)phthalic Acid Diesters as a New
Class of Fluorophores Exhibiting Efficient
Blue Emission in the Solid State
Keywords: Materials science / Photochemis-
try / Charge transfer / Donor–acceptor
systems / Fluorescence
6
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0