Crystallization-induced emission enhancement of highly electron-deficient dicyanomethylene-bridged triarylboranes

Article abstract of DOI:10.1039/d1cc03311e

A highly electron-deficient dicyanomethylene-bridged triarylborane,^FMesB-TCN, was reported with a low-lying LUMO and crystallization-induced emission enhancement in its block-shape crystal. DFT calculations revealed lower re-organization energy

Full text of DOI:10.1039/d1cc03311e

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Crystallization-induced emission enhancement
of highly electron-deficient dicyanomethylene-
bridged triarylboranes†
Cite this: Chem. Commun., 2021,
57, 7926
Received 22nd June 2021,
Accepted 13th July 2021
c
Guanming Liao,ab Jia Zhang,a Xiaoyan Zheng, *a Xiaodi Jia,c Jialiang Xu,
a
Fenggui Zhao,a Nan Wang,
Kanglei Liu,a Pangkuan Chen *a and
a
DOI: 10.1039/d1cc03311e
Xiaodong Yin
*
rsc.li/chemcomm
A highly electron-deficient dicyanomethylene-bridged triarylborane, aggregation (FF = 0.18) and the crystal (FF = 0.38).17 Gu et al.
FMesB-TCN, was reported with a low-lying LUMO and crystallization- reported squaraines (SQs) which showed weak emission in solution
induced emission enhancement in its block-shape crystal. DFT calcu- while relatively strong in the crystal state. Furthermore, co-
lations revealed lower re-organization energy of the block crystal than crystallization between CIEE-SQ and chloroform induces dramatic
that of the weakly emissive acicular crystal. This work explored a novel emission enhancement, with about a 10-fold increase of the emis-
boron-containing skeleton with interesting optical properties.
sion efficiency and a slight blue shift.18 Inspired by these, triarylbor-
anes with tunable solid-state emission are envisioned to develop
Molecular aggregates with unique optical and electronic pro- novel CIEE systems. Herein, we report the synthesis of a series of
perties are of great importance for real-world applications.1–10 highly electron-deficient dicyanomethylene-bridged triarylboranes
Among them, organic crystalline materials have drawn consid- with a significant CIEE phenomenon observed in the block crystal
F
erable attention due to their potential applications in the area of MesB-TCN, which exhibits a much higher photo-luminescence
of organic field effect transistors and solid state lasers.11,12 The quantum yield (FF = 0.33) than that of the solution state and neat
emission enhancement observed in the aggregated state was coined film (FF = 0.01).
aggregation induced emission (AIE) by Tang and co-workers.13,14 As
The synthesis of FMesB-TCN and Mes*B-TCN is described
one of the branches of AIE, crystallization-induced emission in detail in the ESI.† 5,5-Dimethyl-5,12-dihydrobenzo[b]naphtho-
enhancement (CIEE) is a unique photophysical phenomenon in [2,3-e]stannine (TT) was prepared according to the reported proce-
which the compound emits weakly in the amorphous or solution dure by the Piers group.21 As shown in Fig. 1a, TT was treated with
state but becomes strongly emissive in crystalline states.15 Until BCl3 to obtain the chloroborane (Cl-BT), which was used for the
now, a few developments in CIEE based on organic compounds next step without purification beyond removal of the solvent and
F
have been achieved.16–20 Tang et al. reported a diphenylacetylene the Me2SnCl2 byproduct. The reaction of Cl-BT with Mes-Li and
F
derivative with fluorescence quantum efficiency changes from low Mes*-Li in toluene generated MesB-T and Mes*B-T with reason-
F
(FF = 0.09) to evidently enhanced (FF = 0.6) in the amorphous solid able yields, respectively. Oxidation of MesB-T and Mes*B-T with
and crystal state, respectively.16 Chujo et al. reported a novel CrO3 in refluxing acetic acid produced carbonyl-functionalized
anthracene-o-carborane dyad with both AIE and CIEE character with triarylboranes FMesB-TQ and Mes*B-TQ. Finally, the carbonyl
the quantum yield increasing from the solution in THF (FF = 0.02) to functionalities were transformed to dicyanomethylene groups
under Knoevenagel conditions to yield FMesB-TCN (55%) and
a Key Laboratory of Cluster Science, Ministry of Education of China,
Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials,
School of Chemistry and Chemical Engineering, Beijing Institute of Technology,
Beijing, 102488, P. R. China. E-mail: yinxd18@bit.edu.cn, pangkuan@bit.edu.cn,
xiaoyanzheng@bit.edu.cn
Mes*B-TCN (50%). These products exhibited sufficient stability
and could be purified by column chromatography on silica gel in
an ambient atmosphere without noticeable decomposition.
Single crystals of the FMes- and Mes*-substituted triarylbor-
ane were obtained by slow evaporation of hexane/dichloro-
methane solutions. The structures of these compounds were
verified by X-ray crystallographic analyses (Fig. 1b and Fig. S1,
ESI†). Selected bond lengths and angles are listed in Table S3
(ESI†). The central boron atom of these molecules adopts a
trigonal planar geometry. It’s noteworthy that the dihedral
angles between the two fused benzene/naphthalene rings of
the dicyanomethylene-functionalized compounds are 28.761 for
b School of Chemistry and Chemical Engineering, Jiangxi Science and Technology
Normal University, Nanchang, Jiangxi, 330013, P. R. China
c School of Materials Science and Engineering, National Institute for Advanced
Materials, Nankai University, Tongyan Road 38, Tianjin, 300350, P. R. China
† Electronic supplementary information (ESI) available: Synthetic routes, data of
single crystal structures, characterization data, and DFT calculation data. CCDC
2010873, 2013631, 2013632, 2082134, 2081903, 2086880 and 2086881. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1cc03311e
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Fig. 1 (a) Synthesis routes of dicyanomethylene-functionalized triarylbor-
anes; and (b) X-ray structure plots of FMesB-TCN and Mes*B-TCN with
thermal ellipsoids drawn at 50% probability.
FMesB-TCN and 26.201 for Mes*B-TCN, revealing a non-planar
‘‘butterfly’’ structure due to the steric hindrance between the
dicyanomethylene moieties and the boratetracene skeleton.
The B–F distances of 2.510 and 2.645 Å for FMesB-TCN are
much shorter than the sum of the B and F van der Waals radii
of 3.39 Å,22 indicating the weak interaction between boron and
fluorine atoms.
Fig. 2 (a) UV-Vis spectra (1 Â 10À5 M) in THF and (b) cyclic voltammetry
data of carbonyl- and dicyanomethylene-functionalized compounds
in CH2Cl2 (FMesB-TQ and FMesB-TCN) and THF (Mes*B-TQ and
The photophysical properties of the carbonyl- and dicyano-
methylene-functionalized compounds were recorded in THF
(Fig. 2a). The absorption spectrum displayed a broad band,
which covered the region from 220 nm to 470 nm. A
slight redshift in the absorption of the dicyanomethylene-
functionalized derivatives compared to the carbonyl-bridged
compounds can be observed, confirming the influence of the
more electron-deficient group. The electrochemical properties
of the FMes- and Mes*-substituted triarylborane were examined
by cyclic voltammetry in CH2Cl2 and THF, respectively (Fig. 2b
and Fig. S7 and Table S6, ESI†). In the carbonyl-bridged
triarylboranes, the first reversible reduction waves were
observed at E1/2 = À1.51 (FMesB-TQ) and À1.81 V (Mes*B-TQ)
vs. Fc+/0. The dicyanomethylene derivatives FMesB-TCN and
Mes*B-TCN show relatively positive reduction potentials at ca.
À1.19 V and À1.33 V, respectively (Table 1). Then the LUMO
energy levels of these compounds can be estimated with the
equation of ELUMO = À(4.8 + Ered) to obtain the lowest LUMO at
À3.61 eV for FMesB-TCN among these compounds, demon-
strating the significant electron-deficient property of triarylbor-
Mes*B-TCN) with a scan rate of 100 mV sÀ1
.
conducted using the Gaussian 09 D.01 software package, and
optimized geometries and single point energies were obtained
at the B3PW91/6-311+G*//B3LYP/6-31G** level of theory. The
plots of selected frontier orbitals are illustrated in Fig. S9 (ESI†)
with the corresponding energy levels. The LUMOs of these
compounds mainly localized on the boron-doped six-member
ring and the dicyanomethylene group, and the LUMO energy
levels are in good agreement with the energy levels estimated
from CV data. TD-DFT calculations were also conducted at the
Table
1
Photophysical
and
electrochemical
data
of
the
dicyanomethylene-functionalized compounds
a
b
Compound
lonset (nm)
Eg (eV)
Ered (V)
LUMOc (eV)
Mes*B-TCN
FMesB-TCN
461
463
2.69
2.68
À1.33
À1.19
À3.47
À3.61
a
b
anes bearing a dicyanomethylene group. Density functional
Optical gaps were estimated with the equation Eg = 1240/lonset
.
Half-
F
theory (DFT) calculations of MesB-TCN and Mes*B-TCN were wave potentials of first reductive waves vs. Fc+/0
.
ELUMO = À(4.8 + Ered).
c
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PBE0/6-311+G* level of theory to get an in-depth understanding the reason for the highly emissive property of the powder
of the origins of the UV-vis absorption of the dicyanomethylene samples (see Fig. S2, ESI†). Further analysis of the molecular
functionalized compounds, and the theoretical data match well packing in the block single crystal of FMesB-TCN revealed a
with the experimental data (see Fig. S8 and S9 and Table S7, strong p–p interaction (3.296 Å, and 3.377 Å). On the contrary,
ESI,† for details).
the acicular crystal shows no significant p–p stacking interac-
The solutions of these two compounds are almost non- tions, as shown in Fig. 3b. It is unusual to see p–p stacking
emissive, but when we irradiate their solid powder with a UV enhance the photoluminescence property, since strong p–p
lamp, FMesB-TCN exhibited a significant photoluminescence interactions between luminogens usually cause emission
property with bright yellow-green fluorescence. This phenom- quenching.16,19 Several other intermolecular interactions can
enon cannot be observed in the neat film of FMesB-TCN, but also be observed in the block crystal, e.g. NÁ Á ÁH–C interactions
can be repeated in block-shape crystal samples, as shown in between cyano groups and naphthalene moieties (NÁ Á ÁH dis-
F
Fig. 3a. The block single crystal of MesB-TCN exhibited emis- tance of 2.5–2.7 Å), and FÁ Á ÁH–C interactions between trifluor-
sion at 537 nm which showed a ca. 30 nm red-shift in compar- omethyl groups and naphthalene moieties (FÁ Á ÁH distance of
ison with the emission in solution (lem = 508 nm) and the neat ca. 2.53 Å), as shown in Fig. S3a (ESI†). Instead, much fewer
film (lem = 505 nm). The photoluminescent quantum yield interactions can be observed in the acicular crystals, mainly
(PLQY) of the block single crystal of FMesB-TCN is ca. 33%, including C–HÁ Á Áp interactions and FÁ Á ÁH–C interactions, as
about 25 times higher than that in solution and the neat film shown in Fig. S3b (ESI†). More intermolecular interactions in
(Fig. 3c and Table S4, ESI†), demonstrating
crystallization-induced emission enhancement (CIEE) beha- skeleton, which is usually favorable for radiative decay.
vior. Moreover, an acicular shape crystal of FMesB-TCN can
DFT calculations were conducted to study the electronic
a typical the block crystal mean that it has a more rigid molecular
be obtained via fast evaporation from its CH2Cl2 solution. structures of the excited states of FMesB-TCN in the solid
Interestingly, the acicular shape crystal exhibited a different state (see the ESI† for details). Basically, the fluorescence of
photo-luminescence property in comparison with the block FMesB-TCN in the solid state can be assigned to the LUMO -
shape crystal with much weaker and blue-shifted fluorescence HOMO transition, and the electronic density contours of the
(lem = 504 nm, PLQY = 1.3%). Powder X-ray diffraction (PXRD) HOMO and LUMO of the block and acicular crystals are plotted
measurements were performed on the highly emissive powder in Fig. S11 (ESI†). The corresponding electronic bandgaps
sample, which match well with the XRD peaks of the block of the block crystal and acicular crystal are 3.3171 eV and
crystal but are different to those of the acicular crystal. It 3.3260 eV, respectively, explaining the bathochromic emission
demonstrated that the molecular packing mode in the powder of the block crystal compared with the acicular crystal. The
samples is similar to that in the block crystals, which could be fluorescence quantum yield (FF) is FF E kr/(kr + kic). kr can
Fig. 3 (a) Fluorescence photos of the block crystal and acicular crystal of FMesB-TCN; (b) molecular packing structures of the FMesB-TCN block crystal
and acicular crystal; (c) PL spectra of FMesB-TCN in different states with lex = 400 nm; (d) PL spectra of the FMesB-TCN block single crystal at different
temperatures with lex = 360 nm, inset: fluorescence photos of the FMesB-TCN block single crystal at different temperatures; and (e) recycles of the PL
intensity of FMesB-TCN (block crystal) at 540 nm as the temperature changed.
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be estimated by simple spontaneous emission relationship that of the acicular crystal, which matches well with the phenom-
E B fDE2vert, where f is the dimensionless oscillator strength enon of the highly emissive property of the block shape crystal.
and DEvert is the energy difference in units of cmÀ1 between the This work uncovered the structural and functional diversity of
S1 and S0 states at the optimized S1 geometry. As shown in boron-containing molecules, while providing a new strategy for
Table S8 (ESI†), f and DEvert for the block and acicular crystal designing novel functional materials.
are similar to each other. Therefore, the kr values of the block
This work was funded by the National Natural Science Foun-
and acicular crystals are similar to each other. The reorganiza- dation of China (21772012, 21803007), and Beijing Institute of
tion energy from S0 to S1 and vice versa can be obtained by the Technology Research Fund Program for Young Scholar. The
adiabatic potential (AP) energy surface method. We find that authors acknowledge the Analysis and Testing Center of Beijing
the total reorganization energy of the block crystal (430 meV) is Institute of Technology for characterizations. G. M. L. also thanks
smaller than that of the acicular crystal (450 meV), see Table S9 the Project of the Science Funds of Jiangxi Education Office
(ESI†). As known from previous work, the decrease of the (GJJ180629) and the Project of Jiangxi Science and Technology
reorganization energy can sharply retard the electron–vibration Normal University (2016XJZD009) for financial support.
coupling caused nonradiative decay rate constant;23–27 thus, kic
of the block crystal is smaller than that of the acicular phase.
Therefore, the block crystal has a higher fluorescence quantum
yield than the acicular crystal. Besides, the photo-luminescent
Conflicts of interest
There are no conflicts to declare.
properties of Mes*B-TCN in the block crystal, neat film and
CH2Cl2 solution were also studied, and all these samples are
weakly emissive, indicating that Mes*B-TCN exhibits no CIEE
behavior (see Fig. S6 and Table S5, ESI†).
Notes and references
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retaining good stability. Interestingly, one of these compounds,
FMesB-TCN, exhibits
a
crystallization-induced emission
enhancement property, with a much higher PLQY (ca. 33%)
of the block shape crystal than that of the solution, the
neat film, and an acicular crystal from the same compound
(ca. 1.3%). Single crystal X-ray diffraction indicated a much tighter
packing mode in the block crystal with strong p–p interactions,
which was not observed in the acicular crystal. DFT calculations
revealed a smaller reorganization energy of the block crystal than
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