Phenanthro[9,10-d ]imidazole-quinoline boron difluoride dyes with solid-state red fluorescence

Article abstract of DOI:10.1021/ol400605x

A new family of boron difluoride-rigidified dyes, phenanthro[9,10-d] imidazole-quinoline boron difluoride (PQBD), with solid-state fluorescence has been designed and synthesized. The novel series of PQBD are advantageous over the typical boron difluoride-rigidified dyes such as BODIPYs in terms of large Stokes shift and red fluorescence in the solid state.

Full text of DOI:10.1021/ol400605x

ORGANIC
LETTERS
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Vol. XX, No. XX
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Phenanthro[9,10‑d]imidazole-quinoline
Boron Difluoride Dyes with Solid-State
Red Fluorescence
Weiling Li, Weiying Lin,* Jiaoliang Wang, and Xiaoyu Guan
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry
and Chemical Engineering, Hunan University, Changsha, Hunan 410082, P. R. China
weiyinglin@hnu.edu.cn
Received March 6, 2013
ABSTRACT
A new family of boron difluoride-rigidified dyes, phenanthro[9,10-d]imidazole-quinoline boron difluoride (PQBD), with solid-state fluorescence has
been designed and synthesized. The novel series of PQBD are advantageous over the typical boron difluoride-rigidified dyes such as BODIPYs in
terms of large Stokes shift and red fluorescence in the solid state.
Efficient solid-state fluorescent dyes are required in the
fields of both fundamental research and optoelectronic ma-
terials. Thus, the development of solid-emissive fluorophores
has attracted great attention.1 A major hurdle faced in the
design of solid-emissive dyes is to overcome fluorescence
quenching in the solid state. Toward this end, several
strategies including aggregation-induced emission2 and in-
corporation of bulky groups3 have been employed. How-
ever, it is still very challenging to rationally design new
familiesof solid-emissivefluorophores, inparticular, based
on novel frameworks.
In this paper, we describe the design, synthesis, photo-
physical properties, and X-ray structural analysis of a
series of phenanthro[9,10-d]imidazole-quinoline boron di-
fluorides (PQBD, Scheme 1), a unique class of solid-state
fluorescent dyes with red emission.
The design strategy for the PQBD dyes is formulated on
the basis of the following considerations: (1) Selection of
the phenanthro[9,10-d]imidazole moiety. It is known that
phenanthro[9,10-d]imidazole dyes have large Stokes shifts
(up to 100 nm),4 which are favorable for solid-state emission
by minimizing reabsorption and self-quenching.5 In addition,
(1) (a) Brittenham, P. W.; Vittitow, M. P. Basic Research Needs for
Solid-State Lighting. Report of the Basic Energy Sciences Workshop on
Solid-State Lighting; May 22À24, 2006; Sandia Washington Program
Office: Washington, DC, 2006. (b) Birks, J. B. Photophysics of Aromatic
Molecules; Wiley: London, 1970. (c) Saragi, T. P. I.; Spehr, T.; Siebert, A.;
Fuhrmann-Lieker, T.; Salbeck, J. Chem. Rev. 2007, 107, 1011. (d) Figueira-
€
Duarte, T. M.; Mullen, K. Chem. Rev. 2011, 111, 7260.
(2) For some examples, see: (a) Yang, Y.; Su, X.; Carrollb, C. N.;
Aprahamian, I. Chem. Sci. 2012, 3, 610. (b) Zhang, Z.; Xu, B.; Su, J.;
Shen, L.; Xie, Y.; Tian, H. Angew. Chem. 2011, 123, 11858. (c) imizu, M.
Sh.; Takeda, Y.; Higashi, M.; Hiyama, T. Angew. Chem. 2009, 121, 3707.
(d) Wakamiya, A.; Mori, K.; Yamaguchi, S. Angew. Chem., Int. Ed.
2007, 46, 4273. (e) Kubota, Y.; Tanaka, S.; Funabiki, K.; Matsui, M.
Org. Lett. 2012, 14, 4682. (f) Hong, Y.; Lama, J. W. Y.; Tang, B. Z.
Chem. Commun. 2009, 4332. (g) Mutai, T.; Tomoda, H.; Ohkawa, T.;
Yabe, Y.; Araki, K. Angew. Chem., Int. Ed. 2008, 47, 9522.
(3) For some examples, see: (a) Qin, T.; Zhou, G.; Scheiber, H.;
€
Bauer, R. E.; Baumgarten, M.; Anson, C. E.; List, E. J. W.; Mullen, K.
Angew. Chem., Int. Ed. 2008, 47, 8292. (b) Fei, Z.; Kocher, N.; Mohrschladt,
C. J.; Ihmels, H.; Stalke, D. Angew. Chem., Int. Ed. 2003, 42, 783.
(c) Ozdemir, T.; Atilgan, S.; Kutuk, I.; Yildirim, L. T.; Tulek, A.;
Bayindir, M.; Akkaya, E. U. Org. Lett. 2009, 11, 2105. (d) Kubota,
Y.; Hara, H.; Tanaka, S.; Funabiki, K.; Matsui, M. Org. Lett. 2011, 13,
6544. (e) Li, D.; Zhang, H.; Wang, C.; Huang, S.; Guo, J.; Wang, Y.
J. Mater. Chem. 2012, 22, 4319. (f) Gao, F.; Liao, Q.; Xu, Z. Z.; Yue,
Y. H.; Wang, Q.; Zhang, H. L.; Fu, H. B. Angew. Chem., Int. Ed. 2010, 49,
732. (g) Lu, H.; Wang, Q.; Gai, L.; Li, Z.; Deng, Y.; Xiao, X.; Lai, G.; Shen,
Z. Chem.;Eur. J. 2012, 18, 7852. (h) Zhou, Y.; Xiao, Y.; Li, D.; Fu, M.;
Qian, X. J. Org. Chem. 2008, 73, 1571. (i) Kubota, Y.; Uehara, J.; Funabiki,
K.; Ebihara, M.; Matsui, M. Tetrahedron Lett. 2010, 51, 6195.
(4) Lin, W.; Long, L.; Yuan, L.; Cao, Z.; Chen, B.; Tan, W. Org. Lett.
2008, 10, 5577.
(5) (a) Wang, Z.; Lu, P.; Chen, S.; Gao, Z.; Shen, F.; Zhang, W.; Xu,
Y.; Kwok, H. S.; Ma, Y. J. Mater. Chem. 2011, 21, 5451. (b) Zhang, Y.;
Lai, S. L.; Tong, Q. X.; Chan, M. Y.; Ng, T. W.; Wen, Z. C.; Zhang,
G. Q.; Lee, S. T.; Kwonge, H. L.; Lee, C. S. J. Mater. Chem. 2011, 21,
8206.
r
10.1021/ol400605x
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the phenanthro[9,10-d]imidazole moiety possesses an aro-
matic NH, which can be taken advantage of to coordinate
with boron fluoride for enhancement of fluorescence
quantum yields.3d,6 Furthermore, the phenanthro[9,10-d]-
imidazole moiety can function as an effective electron
donor.7 (2) Selection of the quinoline unit. The aromatic
N atom of quinoline can be used as a ligand for the boron
complex. Furthermore, in PQBD, the quinoline moiety
bears a positive charge and thus can serve as an electron
acceptor. Hence, with phenanthro[9,10-d]imidazole and
quinoline moieties operating as electron donor and accep-
tor, respectively, PQBD may be considered as an effec-
tive donor (D)Àacceptor (A) system, which is desirable
for absorption and emission in the long wavelength. The
DÀA nature of PQBD is confirmed by density functional
theory (DFT) studies. As shown in Figure S1 (Supporting
Information), in PQBD, the π electrons of the HOMO
molecular orbital are mainly located on the 1H-phenanthro-
[9,10-d]imidazole moiety, whereas those of the LUMO
are mostly positioned on the quinoline unit. To the best
of our knowledge, boron difluoride-rigidified 1H-phenanthro-
[9,10-d]imidazol-2-yl dye has not been previously used in the
design of solid-state fluorescent dyes.
presence of DIEA to provide the desired products PQBD
1, 2, 3, and 5, respectively. In addition, nitro compound
PQBD 3 or 5 was reduced by SnCl2 to afford amino
compound PQBD 4 or 6, respectively. Interestingly, unlike
PQBD 1, 3, 4, 5, and 6, PQBD 2 exists as a dimer as
confirmed by X-ray crystallography (vide infra). The
formation of the dimer product with a BÀOÀB center is
likely due to partial hydrolysis during the reaction.8
With the compounds PQBD 1À6 in hand, we examined
the absorption and emission spectra of these novel dyes in
dichloromethane. The absorption and emission profiles
are shown in Figure 1, and the photophysical data are
summarized in Table S1 (Supporting Information). The
major absorption peaks of PQBD 1À6 are at 476, 472, 518,
468, 530, and 482 nm, respectively. The time-dependent
density functional theory (TDDFT) studies show that the
maximal absorption band is at around 525 nm (S0fS1,
f = 0.3524) for PQBD 1, 539 nm (S0fS1, f = 0.1918) for
PQBD 3, 514 nm (S0fS1, f = 0.3491) for PQBD 4, 565
nm (S0fS1, f = 0.5364) for PQBD 5, and 533 nm (S0fS1,
f = 0.3320) for PQBD 6, respectively (Figure S3, Support-
ing Information). Thus, the trends in the calculated max-
imal absorption data are consistent with the experimental
ones. Notably, the maximum absorption of PQBD 3 and 5
is much more bathochromic than that of other com-
pounds, attributed to the strong electron-withdrawing
effect of the nitro group. In addition, PQBD 5 with a nitro
group at the para position exhibits a red-shift relative to
PQBD 3 with a nitro group at the meta position, high-
lighting the position effect of the substituent group. The
same phenomenon was noted when compared PQBD 6
with PQBD 4. Consistent with the trend observed in the
absorption, the emission of PQBD 3 and 5 display a
marked red-shift in comparison to other dyes. These
resultsareingoodagreement withthecalculatedmolecular
orbitals and energy levels of PQBD 1À6 (Figure S2,
Supporting Information). The Stokes shifts of PQBD
1À6 are up to 115 nm, which is significantly larger than
those (less than 15 nm) of typical boron difluoride-rigidi-
fied dyes such as BODIPYs.9 As shown in Figure S1
(Supporting Information), PQBD 1À6 display the efficient
electron transfer from the phenanthro[9,10-d]imidazole
unit to the quinoline moiety. This effective intramolecular
charge transfer process is consistent with the large Stokes
shifts of the new dyes.10
Scheme 1. Synthesis of Compounds PQBD 1À6
PQBD exhibit very bright luminescence in dichloro-
methane (Figure 2). The relative fluorescence quantum
yields of PQBD 1À6 calculated using rhodamine 6G as
a reference dye (ΦF = 0.95 in water) were estimated.
As shown in Table S1 (Supporting Information), the
fluorescence quantum yields of PQBD 3 and 4 are sig-
nificantly lower than those of other PQBD dyes. The
The key steps for the synthesis of new compounds
PQBD 1À6 are indicated in Scheme 1, and the full details
are given in the Supporting Information (Schemes S1À7).
Briefly, condensation of 1H-phenanthrene-9,10-dione
with substituted quinoline-2-carbaldehyde in the presence
of ammonium acetate gave the key intermediates 7, 8, 9,
(8) Belcher, W. J.; Hodgson, M. C.; Sumida, K.; Torvisco, A.;
Ruhlandt-Senge, K.; Ware, D. C.; Boyda, P. D. W.; Brothers, P. J.
Dalton Trans. 2008, 1602.
and 10, which were then treated with BF3 Et2O in the
3
(9) (a) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed.
2008, 47, 1184. (b) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891.
(10) (a) Wang, B.; Liao, H.; Yeh, H.; Wu, W.; Chen, C. J. Lumin.
2005, 113, 321. (b) Cornelissen-Gude, C.; Rettig, W. J. Phys. Chem. A
1999, 103, 4371.
(6) Kubota, Y.; Tsuzuki, T.; Funabiki, K.; Ebihara, M.; Matsui, M.
Org. Lett. 2010, 12, 4010.
(7) (a) Wang, J.; Lin, W.; Li, W. Chem.;Eur. J. 2012, 18, 13629. (b)
Ooyama, Y.; Nagano, S.; Yoshida, K. Tetrahedron 2009, 65, 1467.
B
Org. Lett., Vol. XX, No. XX, XXXX

with a calibrated integrating sphere system. The ab-
sorption and emission spectra of PQBD 1À6 in the solid
state are shown in Figure S6 (Supporting Information)
and Figure 3, respectively, and the photophysical data in
the solid state are collected in Table S2 (Supporting
Information). As anticipated, the new dyes in the solid
state display a red-shift in the emission profiles relative to
in dichloromethane. However, the emission wavelength of
PQBD 1À5 is much longer than that of PQBD 6, consis-
tent with the above observation that PQBD 1À5 and
PQBD 6 show red and yellow fluorescence in the solid
state, respectively. The Stokes shifts of PQBD 1À6 in the
solid state are very large. For instance, PQBD 4 bears a
Stokes shift of 187 nm. The large Stokes shifts of the new
family of dyes are consistent with the findings that they
exhibit intense emission in the solid. It is known that long
wavelength emission dyes tend to have quite low fluores-
cence quantum yields insolid. However, although PQBD 1
and 2 have long wavelength emission in the red region, the
fluorescence quantum yields of PQBD 1 and 2 are still very
high. In fact, the quantum yields of PQBD 1 and 2 are at
around 0.184 (Table S2, Supporting Information), which
represent the highest values of solid-state red fluorescence
boron difluoride-rigidified dyes. In particular, PQBD 1
shows a chromaticity value, CIE (0.63, 0.36), which is close
to the ideal red CIE (0.64, 0.34) (Figure S7, Supporting
Information), indicating that PQBD 1 with advantageous
characters of a high fluorescence quantum yield and al-
most ideal red feature is promising for applications in
optoelectronic materials.
Figure 1. Normalized absorption and emission spectra of PQBD
1À6 in dichloromethane.
fluorescence lifetimes (τ) of PQBD 1À6 in CH2Cl2 were
measured to be 4.89, 3.65, 2.67, 2.19, 2.18, and 2.96 ns,
respectively (Figure S5, Supporting Information). The
radiative (kf = Φf  τ) and nonradiative (knr = (1 -Φf)/τ)
rate constants were also calculated (Table S1, Supporting
Information). The relatively lower fluorescence quantum
yields of PQBD 3/5 when compared to PQBD 4/6 can be
attributed tothe fluorescence quenching nature ofthe nitro
group.
Generally, BODIPYs have no emission in the solid state
partly due to small Stokes shifts. By sharp contrast, our
new PQBD dyes have large Stokes shifts, which bodes well
for the solid-state emission. Indeed, PQBD 1À5 display
intense red emission in the solid state (Figure 2). By con-
trast, PQBD 6 shows solid-state yellow emission.
Figure 3. Fluorescence spectra of PQBD 1À6 in the solid state
(film). The excitation wavelength was 440, 470, 530, 520, 440,
and 470 nm for PQBD 1, PQBD 2, PQBD 3, PQBD 4, PQBD 5,
and PQBD 6, respectively.
Figure 2. Photographs of PQBD 1À6 in dichloromethane and in
the solid state.
To understand the spectroscopic properties of the new
dyes in the solid state, their X-ray structures may be
insightful; however, only single crystals of PQBD 1 and
PQBD 2 suitable for X-ray structure analysis were obtained
by the slow evaporation of CH2Cl2 or C2H5OH solutions
To gain the solid-state absorption and emission spectra,
the films were prepared by evaporation of 500 μM dyes in
dichloromethane onto a quartz plate. The absolute quantum
yields of the dyes in powder were measured by Horiba
Jobin Yvon-Edison Fluoromax-4 fluorescence spectrometer
Org. Lett., Vol. XX, No. XX, XXXX
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at ambient temperature. The crystal structures unambigu-
ously reveal the structural distinctions for these two dyes
(Figures S8 and S9, Supporting Information). PQBD 1
bears one FÀBÀF center, and the boron is coordinated in
a tetrahedral geometry by two nitrogen and two fluorine
˚
atoms. The bond lengths of BÀN are 1.641 and 1.616 A,
˚
and those of BÀF are 1.369 and 1.370 A. PQBD 1 is
essentially planar. The boron atom only deviates from the
plane of quinoline-phenanthro[9,10-d]imidazole at 0.88°,
which is consistent with DFT optimized structure of
PQBD 1 (Figure S1, Supporting Information). In contrast,
PQBD 2 is a dimer with two OÀBÀO centers. The boron is
coordinated in a tetrahedral geometry by two nitrogen and
two oxygen atoms. The bondlengths of BÀN are 1.550 and
˚
˚
1.641 A, and those of BÀO are 1.409 and 1.453 A. The
dihedral angle between one quinoline-phenanthro[9,10-d]-
imidazole plane with another one is 103.6°. Thus, unlike
planar PQBD 1, PQBD 2 is highly twisted and far away
from the coplanar framework. Despite that PQBD 1 is
planar, a careful examination of crystal lattice of PQBD 1
reveals that no πÀπ stacking interactions in a crystal lattice
for head-to-head overlapping along the b axis and vertical
overlapping along the c axis (Figure 4A,B,E, Figure S10,
Supporting Information), which account for strong fluor-
escence in the solid state. In addition, the highly twisted
structure of PQBD 2 inhibits πÀπ stacking, attributed to
high solid-state emission (Figure 4C,D,F, Figure S11,
Supporting Information).
In conclusion, a new family of boron difluoride-rigidi-
fied dyes, PQBD, with solid-state fluorescence has been
designed and synthesized. The new series of the PQBD
dyes are advantageous over typical boron difluoride-rigi-
dified dyes such as BODIPYs in terms of large Stokes shift
and red fluorescence in the solid state. We expect that the
novel PQBD dyes will find valuable applications in devel-
opment of fluorescent probes, organic luminescent displays,
and light sources.
Acknowledgment. This research was supported by NSFC
(20972044, 21172063), NCET (08-0175), and the Doctoral
Fund of Chinese Ministry of Education (20100161110008)).
Supporting Information Available. Experimental pro-
cedures, absorption and emission spectra, DFT calcula-
tions, lifetime decay profiles, crystallographic data, CIF,
and NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 4. A part of a crystal-packing pattern of PQBD 1 (A,
along the b axis; B, along the c axis) and PQBD 2 (C, along the a
axis; D, along the b axis) that shows no stacking of the aromatic
rings between the adjacent interlayered crystals. Packing dia-
grams view of PQBD 1 (E) and PQBD 2 (F).
The authors declare no competing financial interest.
D
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