Phenalenyl-based boron-fluorine complexes: Synthesis, crystal structures and solid-state fluorescence properties

Article abstract of DOI:10.1016/j.molstruc.2010.01.027

A novel series of phenalenyl-based boron-fluorine complex-type fluorophores, with a drastic fluorescence quenching in solution but high fluorescence emission in the solid-state, have been synthesized. The X-ray structure demonstrates that the bulky substituents result in steric hindrance and prevent the fluorophores forming short intermolecular interaction thereby enhancing the solid-state fluorescence emission.

Full text of DOI:10.1016/j.molstruc.2010.01.027

Journal of Molecular Structure 968 (2010) 85–88
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Phenalenyl-based boron–fluorine complexes: Synthesis, crystal structures and
solid-state fluorescence properties
*
Weibo Yan, Xiangjian Wan, Yongsheng Chen
State Key Laboratory for Functional Polymer Materials and Center for Nanoscale Science and Technology, Institute of Polymer Chemistry, College of Chemistry,
Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel series of phenalenyl-based boron–fluorine complex-type fluorophores, with a drastic fluores-
cence quenching in solution but high fluorescence emission in the solid-state, have been synthesized.
The X-ray structure demonstrates that the bulky substituents result in steric hindrance and prevent
the fluorophores forming short intermolecular interaction thereby enhancing the solid-state fluorescence
emission.
Received 10 October 2009
Received in revised form 12 January 2010
Accepted 12 January 2010
Available online 20 January 2010
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Phenalenyl-based
Boron–fluorine
Fluorophores
Solid-state
Fluorescence
1. Introduction
2. Experimental
Organoboron compounds with high efficiency fluorophores
have attracted considerable attention because of their excellent
photophysical properties and potential use in molecular sensors
[1], biomolecular probes [2], and in the construction of optoelec-
tronic devices such as organic light-emitting devices (OLEDs) [3].
However, most organoboron compounds, like other organic fluoro-
phores, undergo fluorescence quenching when aggregated due to
both intermolecular energy and electron transfer, which affects
their use in OLEDs [4].
On the other hand, solid-emissive fluorescent compounds have
attracted much attention for fundamental research of solid-state
photochemistry [5] and their possible applications in optoelectron-
ics [6]. For example, Tang and co-workers have reported a series of
molecules such as siloles, pyrans, fulvenes and butadienes which
show aggregation-induced emission (AIE) phenomenon [7]. And
several dyes have been proved to be excellent light-emitting mate-
rials for the device applications [8]. Park and co-workers have pre-
pared several new classes of fluorescent organic molecules which
assemble into stable nanoparticles (FONs), which show extremely
strong solid-fluorescence emission [9]. Yoshida et al. have synthe-
sized a class of heterocyclic quinol-type fluorophores with intense
solid-state fluorescence by introducing bulky substituents to the
original fluorophores [10].
2.1. General
All reagents are commercially available and purified by stan-
dard methods prior to use. Absorption spectra were observed with
a JASCO V-570 spectrophotometer and fluorescence spectra were
measured with a Fluora Max-3P spectrophotometer. Bruker SMART
1000 CCD automatic diffractometer was used for data collection at
113(2) K using graphite monochromated MoK
a-radiation
(k = 0.71073 Å). The fluorescence quantum yields (u) in solution
were determined by using quinine sulfate (u = 0.54, kex = 350 nm)
in 0.1 M H2SO4 as the standard. The solid-fluorescence quantum
yields (u) were determined by using a calibrated integration
sphere system (kex = 470 nm). The 1H NMR spectra were measured
on a Bruker AC-300 and a Bruker AC-400 using tetramethylsilane
(TMS) as internal standard. HRMS were recorded on a VG ZAB-HS
mass spectrometer with ESI resource.
2.2. Synthesis
2.2.1. Synthesis of 2,2-difluoro-3-hydro-2-bora-s-3-aza-1-
oxophenalene (2a)
To a degassed solution of 1a (0.195 g, 1 mmol) in 1,2-dichloro-
ethane was added BF3ÁEt2O (0.15 ml, 1.2 mmol), the resulting
mixture was stirred at ambient temperature for 2 h and then re-
fluxed for 12 h under an Ar atmosphere. After solvent was removed
under reduced pressure, the crude product was purified by column
* Corresponding author. Tel.: +86 22 23500693; fax: +86 22 23499992.
E-mail address: yschen99@nankai.edu.cn (Y. Chen).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.01.027

86
W. Yan et al. / Journal of Molecular Structure 968 (2010) 85–88
chromatography (silica gel, petroleum ether/ethyl acetate 2:1, v/v)
to give 2a (0.206 g, 85%) as a light yellow powder. 1H NMR
(300 MHz, DMSO): d[ppm] = 7.26 (1H, d, J = 9.3 Hz), 7.50 (1H, d,
J = 9.3 Hz), 7.77 (1H, t, J = 7.5 Hz), 8.26 (1H, d, J = 7.5 Hz), 8.30
(1H, d, J = 9.3 Hz), 8.36 (1H, d, J = 7.5 Hz), 8.56 (1H, d, J = 9.3 Hz),
10.26 (1H, s). HRMS (ESI) calcd. for [C13H8ONBF2 + Na]+:
266.0559; found: 266.0564.
3. Results and discussion
3.1. Synthesis
In this paper, we have designed and synthesized a novel phenale-
nyl-based boron–fluorine complex-type fluorophores (2a–2e),
which are constructed to have a large rigid planar p-system with dif-
ferent sterical hindered substituents. The compounds 1a–1e were
synthesized using a methodology previously described [12]. The
fluorophores 2a–2e were synthesized in 80–90% yields by treatment
1a–1e with BF3 in o-xylene (2a, in ClCH2CH2Cl) (Scheme 1).
2.2.2. Synthesis of compounds 2b–2e
General procedure: To a degassed solution of 1 (1b–1e) (1 mmol)
in o-xylene was added BF3ÁEt2O (0.15 ml, 1.2 mmol), the resulting
mixture was stirred at ambient temperature for 2 h and then re-
fluxed at 140 °C for 12 h under an Ar atmosphere. After solvent
was removed under reduced pressure, the crude product was puri-
fied by column chromatography (silica gel, petroleum ether/ethyl
acetate 20:1, v/v) to give 2 (2b–2e) (yields vary between 80% and
90%).
3.2. Spectroscopic properties of 2a–2e
The visible absorption and fluorescence spectroscopic data of
2a–2e in solution and solid-state are summarized in Table 2. In
solution, the absorption maxima are at around 359–372 nm and
fluorescence maxima are at around 453–471 nm. Alkyl-substituted
2.2.2.1. Synthesis of 2,2-difluoro-3-n-propyl-2-bora-s-3-aza-1-oxoph-
enalene (2b). Yield 92%: 1H NMR (400 MHz, CDCl3): d[ppm] = 1.08
(3H, t, J = 7.4 Hz), 1.90 (2H, m), 3.82 (2H, t, J = 5.6) 7.23 (1H, d,
J = 9.6 Hz), 7.43 (1H, d, J = 9.2 Hz), 7.63 (1H, t, J = 7.6 Hz), 7.98
(1H, d, J = 9.6 Hz), 8.05 (1H, d, J = 9.6 Hz), 8.10 (1H, d, J = 8.0 Hz),
8.20 (1H, d, J = 9.2 Hz). HRMS (ESI) calcd. for [C16H14ONBF2 + Na]+:
308.1065; found: 308.1035.
Table 1
Experimental data for the X-ray diffraction studies of prepared compounds at
113(2) K.
Compounds
2b
2d
Numbers in CCDC
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
725287
C32H28B2F4N2O2
570.18
Monoclinic
P2(1)/c
18.541(4)
16.237(3)
8.8182(18)
90
100.28(3)
90
725288
C60H42B3F6N3O6
1047.40
Monoclinic
P2(1)/c
10.8705(6)
14.9755(7)
29.2018(15)
90
93.779(3)
90
4743.5(4)
4
2.2.2.2. Synthesis of 2,2-difluoro-3-phenyl-2-bora-s-3-aza-1-oxophe-
nalene (2c). Yield 91%: 1H NMR (300 MHz, CDCl3): d[ppm] = 6.88
(1H, d, J = 9.6 Hz), 7.42–7.57 (6H, m), 7.68 (1H, t, J = 7.5), 7.94
(1H, d, J = 9.6 Hz), 8.02 (1H, d, J = 7.2 Hz), 8.18 (1H, d, J = 8.1 Hz),
8.32 (1H, d, J = 9.0 Hz). HRMS (ESI) calcd. for [C19H12ONBF2 + Na]+:
342.0872; found: 342.0879.
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
2612.1(9)
4
1.450
Z
Dcalcd. (mg/m3)
Crystal size (mm)
1.476
0.18 Â 0.14 Â 0.02 0.18 Â 0.16 Â 0.14
2.2.2.3. Synthesis of 2,2-difluoro-3-(4-methoxyphenyl)-2-bora-s-3-
aza-1-oxophenalene (2d). Yield 83%: 1H NMR (400 MHz, CDCl3):
d[ppm] = 3.88 (3H, s), 6.92 (1H, d, J = 9.6 Hz), 7.04 (2H, d,
J = 8.8 Hz), 7.35 (2H, d, J = 8.8 Hz), 7.51 (1H, d, J = 8.8 Hz), 7.67
(1H, t, J = 8.0 Hz), 7.93 (1H, d, J = 9.6 Hz), 8.01 (1H, d, J = 7.2 Hz),
8.16 (1H, d, J = 8.0), 8.30 (1H, d, J = 9.6). HRMS (ESI) calcd. for
[C20H14O2NBF2 + Na]+: 372.0983; found: 372.0982.
l
(mmÀ1
)
0.109
1184
0.110
2160
F(0 0 0)
h Range for data collection (°)
Index ranges
3.04–25.02
À19 6 h 6 22,
À19 6 k 6 19,
À10 6 l 6 9
99.8
1.40–27.89
À14 6 h 6 14,
À19 6 k 6 18,
À38 6 l 6 32
99.6
Completeness (%)
Reflections collected/unique
Rint
17,602/4605
0.1760
34,194/11,296
0.0640
Data/restraints/parameters
4605/0/381
1.125
11,296/0/706
1.048
Goodness-of-fit (GOF) on F2
2.2.2.4. Synthesis of 2,2-difluoro-3-(4-cyanophenyl)-2-bora-s-3-aza-
1-oxophenalene (2e). Yield 80%: 1H NMR (300 MHz, CDCl3):
d[ppm] = 6.85 (1H, d, J = 9.3 Hz), 7.55 (1H, d, J = 9.0), 7.60 (2H, d,
J = 8.4), 7.74 (1H, t, J = 7.5 Hz), 7.85 (2H, d, J = 8.1 Hz), 8.04 (1H, d,
J = 9.3 Hz), 8.10 (1H, d, J = 7.2 Hz), 8.24 (1H, d, J = 8.1 Hz), 8.38
(1H, d, J = 9.0 Hz). HRMS (ESI) calcd. for [C20H11ON2BF2 + Na]+:
367.0825; found:367.0832.
Final R indices [I > 2
r
(I)]
R1 = 0.0958,
wR2 = 0.2822
R1 = 0.1809,
wR2 = 0.3207
R1 = 0.0638,
wR2 = 0.1441
R1 = 0.0984,
wR2 = 0.1660
R indices (all data)
Largest difference peak and hole
0.380 and À0.376 0.298 and À0.384
(e AÀ3
)
2.3. X-ray crystallography
BF3Et2O
The diffraction-quality single crystals of 2b and 2d were
mounted on glass fibers in random orientation using epoxy-glue.
The structures were solved by direct methods, and all non-hydro-
gen atoms were subjected to anisotropic, full-matrix least squares
refinement on F2 using the SHELXTL package [11]. More details on
data collection and structure calculation are summarized in Table
1. Crystallographic data for all structures reported here have been
deposited with the Cambridge Crystallographic Data Centre; CCDC
numbers are given in Table 1.
O
F
N
F
O HN
B
R
R
1a: R = H
2a: R = H
1b: R = n-C3H7
1c: R = C6H5
2b: R = n-C3H7
2c: R = C6H5
1d: R = C6H5OMe-p
1e: R = C6H5CN-p
2d: R = C6H5OMe-p
2e: R = C6H5CN-p
Scheme 1. Synthetic route of 2a–2e.

W. Yan et al. / Journal of Molecular Structure 968 (2010) 85–88
87
Table 2
2b and the parent compound 2a show strong fluorescence emis-
sion, while the aryl-substituted 2c–2e exhibit very weak fluores-
cence emission in solution in comparison with 2a–2b. It is
believed that the active intramolecular rotations in compound
2c–2e of the peripheral phenyl ring around the axes of the single
bonds linked to the central ring effectively annihilate the excitons,
thus making the molecular fluorescence weakly emissive [7a]
(Fig. 1).
In the solid-state, compounds 2a–2e all show red-shift of
absorption compared with that in solution with compounds
2c–2e exhibiting a greater red-shift (45–60 nm). Surprisedly, in
the solid-state, the fluorescence of 2a–2b is almost completely
quenched while 2c–2e show much stronger fluorescence emission
when excited at their maximum absorption, as indicated by the
quantum yields listed in Table 2. The solid-fluorescence quenching
of compounds 2a and 2b may be due to the following: (1) smaller
substituent groups on the nitrogen atom cannot prevent close
packing of the molecules in the solid-state as discussed in the sin-
gle-crystal structure analysis below; (2) the existence of hydrogen
on nitrogen in 2a could induce intermolecular hydrogen bonding.
On the other hand, for compound 2c–2e, the large aryl groups frus-
Spectroscopic properties of 2a–2e in THF and in the solid-state.
Compounds In THF
In the solid-state
kemxax=nm
kambasx=nm
kfml ax=nm
kfml ax=nm
u
u
2a
2b
2c
2d
2e
359(15,300) 456
365(28,500) 471
366(21,000) 454
366(19,700) 453
372(26100) 453
0.344 467
0.399 467
0.006 499
0.007 510
0.003 500
506
508
538
550
539
<<0.01
<0.01
0.04
0.08
0.03
trate intermolecular ordered
p-stacking between the phenalenyl
Fig. 1. Top: compounds 2a–2e dissolved in THF (1 Â 10À5 mol LÀ1) and bottom: in
solid-state under 365 nm UV light.
Fig. 3. Crystal packing of 2d: (a) a stereo view of the molecular packing structure
and (b) side view of face-to-face overlap between the fluorophores. The
interplanar distance between the rings is ca. 3.413, 3.473 and 3.476 Å.
Fig. 2. Crystal packing of 2b: (a) a stereo view of the molecular packing structure
and (b) a side view of a face-to-face overlap between the fluorophores.
a
a

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W. Yan et al. / Journal of Molecular Structure 968 (2010) 85–88
rings in the molecules and thus give efficient solid-fluorescence
emission [7a]. Also, compound 2d with electron-donating effect
of the peripheral phenyl ring has stronger solid-state fluorescence
emission with red-shift compared with that of compound 2c, while
2e with electron-drawing effect shows no obvious change com-
pared with 2c (Table 2 and Fig. 1). The possible reasons are yet
to be understood.
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The authors gratefully acknowledge the financial support from
the NSFC (Nos. 50933003, 50902073, 50903044, 20774047), MOST
(No. 2006CB932702), MOE (No. 708020) of China and NSF of Tian-
jin City (No. 07JCYBJC03000).
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