Highly solid-state emissive para-terphenyls laterally substituted with a diphenylamino group

Article abstract of DOI:10.1039/c1cc10502g

Intense solid-state emissions with good to excellent quantum yields were achieved through the introduction of bulky electron-donating diphenylamino groups at the side positions of an electron-accepting para-terphenyl framework.

Full text of DOI:10.1039/c1cc10502g

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 5518–5520
www.rsc.org/chemcomm
COMMUNICATION
Highly solid-state emissive para-terphenyls laterally substituted with a
diphenylamino groupw
Cui-Hua Zhao,* Yi-Hong Zhao, Hong Pan and Guang-Liang Fu
Received 25th January 2011, Accepted 22nd March 2011
DOI: 10.1039/c1cc10502g
Intense solid-state emissions with good to excellent quantum
yields were achieved through the introduction of bulky electron-
donating diphenylamino groups at the side positions of an
electron-accepting para-terphenyl framework.
similar manner, and thus we could provide another new
molecule design for organic emissive solids through the intro-
duction of lateral diarylamino groups into electron-accepting
p-conjugated systems (Fig. 1a). Although triarylamines have
been extensively utilized in various potential optoelectronic
and photonic applications, those containing lateral diarylamino
groups have been seldom investigated. So far as we know, only
one poly(1,4-phenylenevinylene) (PPV) p-conjugated system
bearing diphenylamino groups as side chains has been
reported.12 Herein, we designed a new kind of triarylamine,
in which diphenylamino groups were introduced at the side
positions of electron-accepting para-terphenyls 1–5 (Fig. 1b).
We found that these diphenylamino-substituted terphenyls
showed intriguing intense fluorescence, with good to excellent
quantum yields mainly in the solid-state.
To design and synthesize highly emissive organic materials
that can fluoresce, even in the solid-state, is a fundamental
topic for realizing a wide range of new optoelectronic and
photonic applications, such as organic light-emitting diodes
(OLEDs),1 organic light-emitting field-effect transistors (OLE-
FETs),2 organic solid-state lasers3 and organic fluorescent
sensors.4 In contrast to the great number of molecules that
are known to be highly emissive in solution, the examples of
highly emissive organic solids with a fluorescence quantum
yield close to unity are still quite limited, due to the severe
fluorescence quenching as a result of certain intermolecular
interactions.5 Therefore, the rational design of such emissive
organic solids is still a challenging issue. To achieve an intense
solid-state emission, several approaches have been adopted,
such as dendritic or bulky substituent protection,6 cross-dipole
stacking,7 aggregation-induced emission,8 J-aggregate formation9
and enhanced intramolecular charge transfer (CT) transition.10,11
Among them, we and others have recently focused our attention
on an interesting molecular design, in which the bulky electron-
accepting dimesitylboryl groups were introduced at the side
positions of an electron-donating p-conjugated framework.10
This effective molecular design is attributed to two factors of
the lateral dimesitylbroyl group. One is the steric bulkiness,
which can prevent an intermolecular interaction. The other is
the electron-accepting character, which induces an intramolecular
CT transition with a large Stokes shift and thus effectively
suppresses self-quenching in the condensed state. Based on this
design principle, we envisioned that the bulky electron-donating
groups, such as diarylamino groups, at the side positions of an
electron-accepting p-conjugated framework would act in a
The diphenylamino-substituted terphenyls were readily
synthesized in two steps, using 2,5-dibromoaniline as the
starting material. Thus, the first copper-catalyzed Ullman
coupling with iodobenzene afforded the key precursor
2,5-dibromo-N,N-diphenylaminobenzene in 48% yield. The
following Pd(0)-catalyzed Suzuki coupling with a variety of
para-substituted phenylboronic acids produced 1–5 in moderate
to good yields.
Single crystals of 2 suitable for X-ray analysis were obtained
by recrystallization from dichloromethane–hexane. The crystal
structure of 2 together with its packing structure is shown in
Fig. 2. Similar to other triarylamines, the benzene rings on the
lateral diphenylamino moiety are almost orthogonal to the
central benzene of the terphenyl framework. The dihedral
angle between benzene rings A and B is 81.21, which is similar
to that between A and C (80.91). Furthermore, the main chain
terphenyl framework exhibits a significantly twisted structure.
The terminal benzene ring D is twisted by a large dihedral
School of Chemistry and Chemical Engineering, Shandong University,
Shanda Nanlu 27, Jinan 250100, People’s Republic of China.
E-mail: chzhao@sdu.edu.cn; Fax: +86 531 88564464;
Tel: +86 531 88364464
w Electronic supplementary information (ESI) available: Experimental
procedures, spectral data for all new compounds, photophysical data,
theoretical calculations and crystallographic data of 2, CCDC 805836.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1cc10502g
Fig. 1 (a) The schematic representation and (b) the structures of
diphenylamino-substituted terphenyls 1–5.
c
5518 Chem. Commun., 2011, 47, 5518–5520
This journal is The Royal Society of Chemistry 2011

View Article Online
Fig. 4 Pictorial drawings of (a) the HOMO and (b) the LUMO of 2
calculated at the B3LYP/6-31G(d) level of theory by using its crystal
structure.
Fig. 2 The crystal structure of 2: (a) an ORETP drawing (50%
probability for the thermal ellipsoids) and (b) the packing structure.
Hydrogen atoms are omitted for clarity.
probably another factor that contributes to the large Stokes
shift of the present diphenylamino-substituted terphenyl 2.10b,11
It is most noteworthy that in spite of the moderately
intense fluorescence in solution, the diphenylamnio-substituted
terphenyl 2 exhibits a remarkably intense sky-blue fluorescence
(lem = 443 nm) with extremely high quantum yield (FF = 0.99)
for its spin-coated film, as determined by the calibrated inte-
grating sphere. A similar fluorescence property was also observed
for the powder of 2. In view of the absorption and emission
spectra, almost the same spectra as those in benzene solution
are maintained for the spin-coated film (see ESIw), suggesting
the formation of neither aggregates in the ground state nor an
excimer in the excited state.10d The present molecular design is
very effective in suppressing the intermolecular interaction,
which is presumably an important factor contributing to the
high solid-state fluorescence efficiency. On the other hand, a
large Stokes shift induced by the intramolecular CT transition
from the electron-donating lateral diphenylamino group to the
electron-accepting p-conjugated main chain framework may
be another important factor in suppressing fluorescence
quenching in the condensed state.10,11
angle (46.81), relative to the central ring A, and the dihedral
angle between benzene rings A and E is even larger (59.71) due
to the higher steric congestion between them. Apparently, the
significantly twisted conformation of the main chain framework
arises from the steric hindrance of the lateral diphenylamino
group. Presumably owing to the nonplanar structure of diphenyl-
amino moiety, as well as the main chain framework, the
molecules are far apart from each other. The distance between
the central A benzene planes of the two adjacent molecules is
7.33 A, which is supposedly large enough to prevent inter-
molecular electronic interactions.
The UV/vis absorption and emission spectra of the diphenyl-
amino-substituted terphenyl 2 are shown in Fig. 3. In cyclo-
hexane, in addition to an intense absorption band below
300 nm, 2 shows a relatively weaker absorption at 362 nm
(e = 2480 MÀ1 cmÀ1). In the fluorescence spectra, 2 exhibits
an emission at 432 nm with a moderate quantum yield
(FF = 0.33). One noticeable feature is the large Stokes shift
(4480 cmÀ1) for this terphenyl unit with such a limited
conjugation length, which may partially arises from significant
structure differences between the twisted structure in the
ground state and the presumably planar structure in the
excited state.10b,11 Single point calculations (B3LYP/6-31G(d)
level) using the geometry derived from the crystal structure
demonstrate that the HOMO is mainly localized over the
central diphenylaminophenylene unit and the LUMO is delo-
calized over the entire terphenyl framework (Fig. 4). More-
over, the time-dependent DFT calculations indicate that the
absorption at 362 nm is assignable to the intramolecular CT
transition from the HOMO to the LUMO. These calculated
results clearly suggest a polarized structure in the excited state,
which was further evidenced by a large solvatochromism in
fluorescence, from 432 nm in cyclohexane, to 453 nm in
benzene, 471 nm in THF and 486 nm in MeCN. The
significant intramolecular CT character in the excited state is
The intense solid-state fluorescence is a general characteristic
of the present molecular design. All of the other diphenylamino-
substituted terphenyl derivatives show an intense solid-state
emission with good to excellent quantum yields (FF
=
0.37–0.99) for the spin-coated films (Table 1). Since the
emission comes from the singlet excited state generated by
the intramolecular CT transition from the diphenylamino-
phenylene moiety to the terphenyl main chain framework,
the emission maxima can be finely tuned by choosing different
electron-withdrawing terminal groups. Thus, the introduction
Table 1 The photophysical properties of 1–5
c
l
abs/nma (e)b
lem [nm]
FF
1
2
3
4
5
Benzene
Filmd
Benzene
Filmd
Benzene
Filmd
Benzene
Filmd
353 (2860)
361
357 (2580)
363
391 (5250)
399
377 (2600)
389
421
419
453
443
483
473
479
485
495
515
0.14e
0.37f
0.26e
0.99f
0.31e
0.95f
0.26e
0.98f
0.21e
0.55f
Benzene
Filmd
390 (2800)
393
a
c
b
Only the longest absorption maxima are shown. MÀ1 cmÀ1
.
Excited at the longest absorption maxima. Spin-coated film
d
e
prepared from a chloroform solution. Calculated using quinine
f
sulfate as standard. Absolute quantum yield determined by
calibrated integrating sphere system.
a
Fig. 3 UV/Vis absorption and fluorescence spectra of 2.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5518–5520 5519

View Article Online
of dimesitylboryl groups (3) and cyano groups (4) at the
terminal positions shift the emission to greenish-blue at 473 nm
and bluish-green at 485 nm, respectively. The emission was
further red-shifted to yellowish-green at 515 nm through the
extension of the p-conjugation with formyl groups (5). It is
worth noting that compound 1, without the electron-
withdrawing groups at the terminal positions, emits purple-
blue fluorescence with a relatively low quantum yield (FF = 0.37)
in the solid-state, confirming that the intramolecular CT is an
important factor for the intense solid-state emission.10,11 As
for the fluorescence spectra, compounds 1–4 show very similar
spectra in the spin-coated films to those in benzene solution,
while the emission of 5 in the spin-coated film is more similar
to that in THF solution, suggesting that the environment
arising in the film is comparable to that in the dilute benzene
solution for 1–4 and to that in the more polar THF solution
for 5. It is noteworthy that the intense emissions were also
observed for the powder of these compounds, with quantum
yields in the range 0.38–0.99. While the fluorescence spectra of
the powder of 3 and 4 are very similar to those of the spin-
coated films, the emission spectra of 1 and 5 are greatly blue-
shifted when going from the spin-coated film to the powder
(see ESIw). This probably arises from the significant difference
in the molecular packing pattern between the film and the
powder for 1 and 5.13 Another noticeable characteristic for the
present diphenylamino terphenyls is that the quantum yields
of 1–5 in solution are much lower than those in the solid-state.
Such fluorescence behavior may be ascribed to the easily
exchangeable multiple conformations of the nonplanar structure
of the main chain, which facilitate the non-radiative decay of
the excited state. In the solid-state, the exchanges between
multiple conformations are greatly suppressed due to the
spatial congestion of the molecular stacking in the solid-state.8
In summary, a series of lateral diphenylamino-substituted
terphenyls have been designed and synthesized. Owing to the
influence of steric bulkiness and the electron-donating property
of the lateral diphenylamino group, these terphenyls exhibit a
twisted main chain structure and efficient intramolecular
charge transfer transition with a large Stokes shift, facilitating
the suppression of fluorescence quenching in the solid-state.
They display intense fluorescence with good to excellent
quantum yields mainly in the solid-state. We here disclosed
not only a new kind of emissive materials, but also a new
molecular design concept to attain organic emissive solids with
excellent quantum yields. Further studies on the applicability
of the present molecular design to other p-conjugated systems,
as well as the application of the obtained materials to organic
optoelectronics, are under way.
Notes and references
1 (a) H. Yersin, Highly Efficient OLEDs with Phosphorescent Materials,
Wiely-VCH, Weinheim, 2008; (b) K. Mullen and U. Scherf, Organic
Light-Emitting Devices: Synthesis Properties and Applications, Wiely-
VCH, Weinheim, 2006; (c) R. H. Friend, R. W. Gymer,
A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D.
C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund and
W. R. Salaneck, Nature, 1999, 397, 121.
2 (a) J. Zaumseil and H. Sirringhaus, Chem. Rev., 2007, 107, 1296;
(b) F. Cicoira and C. Santato, Adv. Funct. Mater., 2007, 17, 3421.
3 (a) I. D. W. Samuel and G. A. Turnbull, Chem. Rev., 2007,
107, 1272; (b) I. D. W. Samuel and G. A. Turnbull, Mater. Today,
2004, 7, 28; (c) U. Scherf, S. Riechel, U. Lemmer and R. F. Mahrt,
Curr. Opin. Solid State Mater. Sci., 2001, 5, 143;
(d) M. D. McGehee and A. J. Heeger, Adv. Mater., 2000,
12, 1655; (e) G. Kranzelbinder and G. Leising, Rep. Prog. Phys.,
2000, 63, 729; (f) N. Tessler, Adv. Mater., 1999, 11, 363;
(g) V. G. Kozlov and S. R. Forrest, Curr. Opin. Solid State Mater.
Sci., 1999, 4, 203; (h) A. Dodabalapur, E. A. Chandross,
M. Berggren and R. E. Slusher, Science, 1997, 277, 1787.
4 (a) S. Sreejith, K. P. Divya and A. Ajayaghosh, Chem. Commun.,
2008, 2903; (b) M. S. Meaney and V. L. McGuffin, Anal. Bioanal.
Chem., 2008, 391, 2557; (c) S. W. Thomas III, G. D. Joly and
T. M. Swager, Chem. Rev., 2007, 107, 1339; (d) L. Basabe-
Desmonts, D. N. Reinhoudt and M. Crego-Calama, Chem. Soc.
Rev., 2007, 36, 993; (e) T. J. Dale and J. Jr. Rebek, J. Am. Chem.
Soc., 2006, 128, 4500; (f) O. S. Wolfbeis, J. Mater. Chem., 2005,
15, 2657; (g) J. F. Callan, A. P. de Silva and D. C. Magri,
Tetrahedron, 2005, 61, 8551; (h) S.-W. Zhang and T. M. Swager,
J. Am. Chem. Soc., 2003, 125, 3420; (i) R. Martınez-Manez and
F. Sancenon, Chem. Rev., 2003, 103, 4419.
5 For reviews of emissive organic solids, see M. Shimizu and
T. Hiyama, Chem.–Asian J., 2010, 5, 1516.
6 (a) J. Wang, Y. Zhao, C. Dou, H. Sun, P. Xu, K. Ye, J. Zhang,
S. Jiang, F. Li and Y. Wang, J. Phys. Chem. B, 2007, 111, 5082;
(b) T. Sanji, T. Kanzawa and M. Tanaka, J. Organomet. Chem.,
2007, 692, 5053; (c) H. Langhals, O. Krotz, K. Polborn and
P. Mayer, Angew. Chem., Int. Ed., 2005, 44, 2427; (d) T. Sato,
D.-L. Jiang and T. Aida, J. Am. Chem. Soc., 1999, 121, 10658.
7 Z. Xie, B. Yang, F. li, G. Cheng, L. Liu, G. Yang, H. Xu, L. Ye. M.
Hanif, S. Liu, D. Ma and Y. Ma, J. Am. Chem. Soc., 2005, 127, 14152.
8 (a) Y. Dong, J. W. Y. Lam, A. Qin, J. Sun, J. Liu, Z. Li, J. Sun, H.
H. Y. Sung, I. D. Williams, H. S. Kwok and B. Z. Tang, Chem.
Commun., 2007, 3255; (b) Z. Ning, Z. Chen, Q. Zhang, Y. Yan,
S. Qian, Y. Cao and H. Tian, Adv. Funct. Mater., 2007, 17, 3799;
(c) S. Kim, Q. Zheng, G. S. He, D. J. Bharali, H. E. Pudavar,
A. Baev and P. N. Prasad, Adv. Funct. Mater., 2006, 16, 2317;
(d) H. Tong, Y. Hong, Y. Dong, M. Haußler, J. W. Y. Lam, Z. Li,
Z. Guo, Z. Guo and B. Z. Tang, Chem. Commun., 2006, 3705;
(e) K. Itami, Y. Ohashi and J. Yoshida, J. Org. Chem., 2005,
70, 2778; (f) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen,
C. Qiu, H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang,
Chem. Commun., 2001, 1740.
9 T. E. Kaiser, H. Wang, V. Stepanenko and F. Wurthner, Angew.
Chem., Int. Ed., 2007, 46, 5541.
10 (a) C.-H. Zhao, A. Wakamiya, Y. Inukai and S. Yamaguchi,
J. Am. Chem. Soc., 2006, 128, 15934; (b) A. Wakamiya, K. Mori
and S. Yamaguchi, Angew. Chem., Int. Ed., 2007, 46, 4273;
(c) C.-H. Zhao, A. Wakamiya and S. Yamaguchi, Macromolecules,
2007, 40, 3898; (d) C.-H. Zhao, E. Sakuda, A. Wakamiya and
S. Yamaguchi, Chem.–Eur. J., 2009, 15, 10603.
We acknowledge the financial support from the National
Nature Science Foundation of China (Grant Nos. 21072117,
20802041), Nature Science Foundation of Shandong Province
(No Q2008B02), and Science Foundation of Ministry of
Education of China (No 200804221009).
11 M. Shimizu, Y. Takeda, M. Higashi and T. Hiyama, Angew.
Chem., Int. Ed., 2009, 48, 3653.
12 Y. Takahshi, T. Kurata, H. Nishide, C.-H. Lee, E.-D. Do and
J. Jin, Sci. Technol. Adv. Mater., 2006, 7, 475.
13 J. Chen, B. Xu, K. Yang, Y. Cao, H. H. Y. Sung, I. D. Williams
and B. Z. Tang, J. Phys. Chem. B, 2005, 109, 17086.
c
5520 Chem. Commun., 2011, 47, 5518–5520
This journal is The Royal Society of Chemistry 2011