Tetrahedral Silicon-Centered Dibenzoylmethanatoboron Difluorides: Synthesis, Crystal Structure, and Photophysical Behavior in Solution and the Solid State

Article abstract of DOI:10.1002/cplu.201900732

Four tetrahedral silicon-centered derivatives of dibenzoylmethanatoboron difluoride (DBMBF₂) were synthesized and characterized. Their structural and optical features both in solution and the solid state were investigated by using X-ray crystal

Full text of DOI:10.1002/cplu.201900732

Accepted Article
Title: Tetrahedral Silicon-Centered Dibenzoylmethanatoboron
Difluorides: Synthesis, Crystal Structure and Photophysical
Behavior in Solution and Solid State
Authors: Yuriy Kononevich, Maxim M. Temnikov, Alexander A.
Korlyukov, Alexander D. Volodin, Pavel V. Dorovatovskii,
Viacheslav A. Sazhnikov, Andrey A. Safonov, Dmitriy S.
Ionov, Anatoly A. Ivanov, Nikolay M. Surin, Evgeniya A.
Svidchenko, and Aziz M. Muzafarov
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Tetrahedral Silicon-Centered Dibenzoylmethanatoboron
Difluorides: Synthesis, Crystal Structure and Photophysical
Behavior in Solution and Solid State
Yuriy N. Kononevich,*[a] Maxim N. Temnikov,[a] Alexander A. Korlyukov,[a][b] Alexander D. Volodin,[a]
Pavel V. Dorovatovskii,[c] Viacheslav A. Sazhnikov,[d][e] Andrey A. Safonov,[d] Dmitriy S. Ionov,[d] Anatoly
A. Ivanov,[d] Nikolay M. Surin,[f] Evgeniya A. Svidchenko,[f] and Aziz M. Muzafarov *[a][f]
building blocks for constructing various 3D porous metal-organic
Abstract: Four novel tetrahedral silicon-centered derivatives of
frameworks (MOFs),[1–3] dynamic covalent frameworks
dibenzoylmethanatoboron difluoride (DBMBF2) were synthesized
(DCFs)[4,5] and element-organic framework (EOF).[6] Silicon-
and characterized. Their structural and optical features both in
containing monomers with tetrahedral and triangular-pyramidal
solution and solid state were investigated using X-ray
shapes can be utilized for obtaining porous resins[7,8] and films.[9]
crystallography, steady and time-dependent spectroscopy, and DFT-
Tetrahedral scaffold-type silicon-containing molecules can be
based calculations. In dilute solutions, the molar absorption
used as rigid linkers for immobilization of various catalysts.[10–15]
coefficient increases with increasing the number of DBMBF2
Tripod-shaped oligo(p-phenylene)s joined together by a single
fragments in a molecule from 40500 to 175200 M-1cm-1, while, in
silicon atom can be utilized as anchors for the functionalization
contrast, the nonradiative rate constant of fluorescence decay
of different surfaces by various chromophores.[16–19] Dendrimer-
decreases from 0.49 to 0.34. In the solid state, absorption and
like structures based on different silanes[20,21] in particular
emission spectra depend on the degree of crystallinity and
organosilicon molecular antennas[22] have been obtained.
microcrystal size. Tris-DBMBF2 derivative forms fully overlapping
Tetrahedral silicon-centered cyano-functionalized silanes with
dimeric structures exhibiting excimer-like fluorescence, which is well
high thermal stability can be used as fluorescent sensors for
predicted by the quantum-chemical calculations. Mono-DBMBF2
diphenylamine detection[23] and flexible network based on
derivative exhibits fully reverse mechanofluorochromic behavior.
tetrahedral silicon-centered structures can be used for
fluorescence sensing of aromatic compounds.[24]
Triphenylsilylphenyl and triphenylsilylphenyl(ethynyl) bulky
substituents included into p-conjugated systems can suppress
π–π interaction due to their steric hindrance[25,26] and, thus, lead
Introduction
to an improvement of optoelectronic properties such
compounds.[27] The incorporation of silicon atoms into p-
conjugated systems can decrease LUMO energy levels
essentially due to a strong (Si-C)-p* hyperconjugation effect.
Thus, numerous luminescent materials based on tetrahedral
silicon-centered organic molecules due to their potential
application as well as organic light emitting devices (OLEDs)
were obtained and investigated.28–45
In recent years there has been an intense interest in the design
and synthesis of tetrahedral silicon-centered organic molecules
that can be used in numerous applications. These molecules
based on silane core have been proved to be fascinating
[a]
Dr. Yuriy N. Kononevich, Dr. Maxim N. Temnikov, Prof. Alexander A.
Korlyukov, Alexander D. Volodin, Prof. Aziz M. Muzafarov.
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian
Academy of Sciences
It is well known that boron difluoride complexes such as
DBMBF2, BODIPY and others possess valuable photophysical /
photochemical properties and are promising objects in the
development of new fluorescent materials for organic electronics
and photonics.[46–48] These compounds exhibit large extinction
coefficients and two-photon absorption cross section, show
intense fluorescence with high quantum yields both in solutions
and in the solid state, and also possess mechanofluorochromic
properties.[49–51] Such boron complexes can be utilized as
various fluorescent sensors.[52] for example for detecting of
BTEX.[53–61] It should be noted that such BTEX detection is
based on the formation of fluorescent exciplexes between
DBMBF2 and benzene derivatives. However, at relatively high
concentrations of DBMBF2 required for sensitive materials, a
competitive reaction of the formation of fluorescent excimers
between two adjacent DBMBF2 molecules can occur. It can be
expected that the use of silicon-centered derivatives of DBMBF2
with tetrahedral or triangular-pyramidal shapes can be used to
obtain sensitive materials with a reduced ability to form similar
dimeric associations of DBMBF2 in the ground or excited states.
Also note that derivatives of various silicon-cored compounds
119991 Moscow, Russian Federation
E-mail: kononevich.yuriy@gmail.com
[b]
[c]
[d]
Prof. Alexander A. Korlyukov
Pirogov Russian National Research Medical University
117997 Moscow, Russian Federation
Pavel V. Dorovatovskii
National Research Center “Kurchatov Institute”
123098 Moscow, Russian Federation
Dr. Viacheslav A. Sazhnikov, Dr. Andrey A. Safonov, Dmitriy S.
Ionov, Dr. Anatoly A. Ivanov
Photochemistry Center, FSRC “Crystallography and Photonics”,
Russian Academy of Sciences
1119421 Moscow, Russian Federation
[e]
[f]
Dr. Viacheslav A. Sazhnikov
Moscow Institute of Physics and Technology (State University)
141707 Dolgoprudny, Russian Federation
Dr. Nikolay M. Surin, Evgeniya A. Svidchenko, Prof. Aziz M.
Muzafarov
N.S. Enikolopov Institute of Synthetic Polymeric Materials, Russian
Academy of Sciences
117393 Moscow, Russian Federation
Supporting information for this article is given via a link at the end of
the document.
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typically exhibit higher absorption coefficients and fluorescence
quantum yields as compared to their parent molecules. It was
interesting to study how these effects which are of significant
value for fluorescent molecular sensors can be characteristic of
silicon-centered DBMBF2 derivatives. In addition, there is a great
deal of interest in understanding the fluorescence properties of
dimer-type structures in DBMBF2-based crystals.
Organosilicon derivatives of acetophenone 5 a-d were
obtained by removing the protecting group from 4 a-d in boiled
aqueous acetone in the presence of TsOH. After isolation 5 b-d
as colorless crystals and 5 a as a colorless oil were obtained.
Further, by the action with LiHMDS (solution in THF) on the
corresponding acetophenones 5 a-d, and then addition of
benzoylchloride lead to obtain of organosilicon 1,3-diketones 6
a-d, which were purified by column chromatography on silica
with toluene as an eluent. It should be noted that in the case of
Claisen condensation between corresponding acetophenones 5
a-d and ethylbenzoate in THF solution with NaH as a base no
positive results were obtained, since the cleavage of silicon-
carbon bond was observed.
The final stage in the synthesis of tetrahedral silicon-
centered derivatives of dibenzoylmethanatoboron difluoride 7 a-
d (Figure 1) was the reaction between 1,3-diketones 6 a-d and
boron trifluoride diethyl etherate in dichloromethane. In the case
of 7 a and 7 c after the completion of reaction, the solvent was
removed in vacuo and the residue was recrystallized from
toluene. In the case of 7 b and 7 d products were collected by
filtration and didn’t need any further purification. Boron difluoride
complexes 7 a-d were obtained as yellow substances. All
obtained compounds were characterized by 1H, 13C, 19F, 29Si
NMR, IR, MS, and elemental analysis (Supporting information).
Figure 1. Tetrahedral silicon-centered derivatives of diketonatoboron difluoride.
In this paper, we present a series of novel silicon-centered
dendritic diketonatoboron difluorides (Figure 1). The crystal
structure of synthesized compounds was determined by X-ray
diffraction analysis and their photophysical properties were
studied both in solution and in the solid state. In particular, it was
observed that (DBMBF2)3SiCH3 forms completely overlapped
excimer-like structures in the crystalline state, whereas
DBMBF2Si(CH3)3 exhibits reversible mechanofluorochromism.
Scheme 1. Synthesis of organosilicon derivatives of DBMBF2 7 a-d..
Results and Discussion
Synthesis of tetrahedral Si-centered diketonatoboron
difluorides. The organosilicon derivatives of diketonatoboron
difluorides 7 a-d were synthesized according to the route
outlined in Scheme 1. The starting compound in the synthesis of
7 a-d was 4-bromoacetophenone 1, the carbonyl group of which
was protected with a dioxolane group. The reaction was carried
X-ray analysis. Compounds 7 c and 7 d crystallized in
monoclinic cells, while 7 a and 7 b form triclinic ones. Analysis of
the crystal packing has shown that almost all phenyl rings and
boron-containing rings engaged in stacking interactions
(Supporting information). Phenyl cycles in 7 c participate in
stacking interaction with the same fragments of adjacent
molecules thus forming dimers (Figure 2, main text and Figures
S96-S98, Supporting information). The strongest stacking in this
crystal is formed by the rings C10C-C15C and C10D-C15D
(interplanar distance is equal to 3.322 (9) Å). Crystal 7 c is
colored yellow while all other crystals studied by X-ray diffraction
are light-yellow. The F1A atom in the 7 c crystal also forms a
hydrogen bond of medium strength with the molecule CHCl3
(donor-acceptor distance equal 3.086(17) Å) (Figure 2).
out by
a
known procedure, namely by boiling of 4-
bromoacetophenone 1 with ethylene glycol in toluene in the
presence of TsOH as a catalyst and with a Dean-Stark trap. The
product 2 was purified by a recrystallization from hexane.
In the next stage the compound 2 was treated with
equimolar amount of n-BuLi in THF at -80 °С and then with
corresponding chlorosilane. After the isolation and purification by
the recrystallization from hexane compounds
obtained with good yields.
4 a-d were
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The crystals 6 a and 7 a also has a layered structure. The
molecules in both cases are assembled by the -stacking of the
phenyl rings, but the dipole-dipole interaction between boron-
containing rings is presented only in 7 a packing (Figures S86
and S96, Supporting information). The boron ring in the 6 a
crystal interacts with other molecules only by weak hydrogen
bonds (C–H···O and C–H···).
CCDC 1983329 (6 a), 1907458 (6 b), 1907456 (6 d),
1907457 (7 a), 1907459 (7 b), 1907455 (7 c), 1907454 (7 d)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
Photophysical properties in solution. The photophysical
characteristics of the synthesized compounds were measured in
dichloromethane solutions and are summarized in Table 1.
Normalized electronic absorption and emission spectra of
organosilicon derivatives of DBMBF2
7 a-d obtained in
dichloromethane solution at room temperature are presented in
Figure 3.
Figure 3. Normalized electronic absorption (left) and emission (λex = 371 nm)
Figure 2. Molecular packing of 7 c in the crystal lattice. C (grey), O (red), Si
(blue), H (white), B (yellow), F (green), Cl (dark green).
(right) spectra of organosilicon derivatives of DBMBF2
7
a-d in
dichloromethane at room temperature. The dye concentration is 1 * 10-5 M for
all compounds.
In all compounds there are contacts H···F, but except for the
Table 1. Optical properties of organosilicon derivatives of DBMBF2 7 a-d and
above-described contact H3···F1A in 7 c, they are weak unsubstituted DBMBF2 in the solution.[a]
(corresponding interatomic distance more than 3.2 Å). Boron-
containing rings in the compounds 7 a-d are close to plane
conformation.
λabs
Kext
λem
Фf
0-0
τf
krf
(108
s)
knrf
(108
s)
Com-
(nm)
(M-1
(nm)
(cm-1
)
(ps)
pound
cm-1
)
7 a
7 b
371 (385)
372 (388)
373 (390)
374 (390)
365 (379)
40500
72900
101100
175200
39100
401 (421)
401 (422)
401 (423)
402 (423)
414 (398)
0.49
0.47
0.42
0.34
0.20
25400
25300
25300
25200
25700
1000
946
4.9
5
5.1
5.6
7.2
10
Among studied compounds it is easy to reveal the
compounds with very similar crystal packing. In other words,
crystal packing for pairs of compounds differing only by
presence of the BF2 fragment does not differ much. For example,
in 6 d and 7 d the same space group and packing motives were
revealed (Figures S90, S100, Supporting information). The
crystals consist of stacks along the b axis of unit cell, in which
the molecules are mainly connected by -stacking. Compounds
6 b and 7 b also have a similar packing. Both compounds are
packed with stacks that resemble arrows (Figures S88 for 6 b,
S94 for 7 b, Supporting information). In crystal 7 b all molecules
are orientated in the same direction, while the orientation of 6 b
molecules alternates layer by layer. The dimers of the 7 c are
linked together by -/dipole-dipole interaction and disposed
along c axis, so the interaction with molecules of another dimer
is possible (Figure 2). The cycles not participating in this
interaction are turned orthogonally to the plane ab. On the other
hand, the crystal packing of 6 a and 7 a is significantly different.
7 c
808
5.2
5.2
5.5
7 d
657
DBMBF2
470[b]
16
The optical properties were measured in dichloromethane (c = 1 * 10–5 M).
The wavelengths of the second absorption and fluorescence peak are given in
[a]
parentheses.
[b] Reference.[46]
The absorption spectra of compounds 7 a-d dissolved in
dichloromethane consist of almost identical structured strong
bands with two resolved peaks and shoulders: maxima at 371,
372, 373, 374 nm, second peaks at 385, 388, 390, 390 nm for
compounds 7 a, 7 b, 7 c and 7 d, respectively, and shoulders at
about 350 nm (Figure 3). The absorption maxima of compounds
7 a-d are red-shifted by 6-9 nm in comparison with unsubstituted
DBMBF2 and have a similar pattern of band distribution. Data
listed in Table 1 show that an increase in the number of DBMBF2
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fragments leads to nearly proportional increase of the molar
extinction coefficient that is in the range of 40500-175200 M-1cm-
and almost no effect on absorption spectra. Two resolved
peaks at 379 and 365 nm and a shoulder at about 350 nm of the
parent DBMBF2 are vibronic bands.[62] Obviously, similar peaks
for compounds 7 a-d in the region of 350-390 nm can be
attributed to vibronic transitions. It should be noted that the
intensity of the second band (at 385-390 nm) increases on going
from unsubstituted DBMBF2 and mono-DBMBF2 to tetra-
DBMBF2.
7 a
7 b
7 c
7 d
1
1.0
0.8
0.6
0.4
0.2
0.0
The fluorescence spectra of compounds
measured in dichloromethane solutions (c = 1  10-5 M) at room
temperature. Figure shows the normalized fluorescence
spectra of the parent DBMBF2 and synthesized compounds 7 a-
d, and Figure S107 (Supporting information) shows
7 a-d were
3
250
300
350
400
450
500
a
Wavelength (nm)
photograph of the fluorescence of these compounds in
dichloromethane. As can be seen in Figure 3 the emission
spectra consist of two bands at about 400 and 420 nm and a
shoulder at about 450 nm. Compounds 7 a-d exhibit higher
fluorescence quantum yield than parent DBMBF2 (0.2) that
drops on going from mono-chromophoric to tetra-chromophoric
derivatives (0.49 → 0.47 → 0.42 → 0.34). Figure S108
(Supporting information) demonstrates the fluorescence decay
curves obtained for the compound 7 a-d. The fluorescence
lifetime decreases slightly in this series. The observed changes
can be attributed to increasing the rate constants of non-
radiative processes, while radiative constants remain almost the
same and close to the value obtained for DBMBF2. The main
non-radiative decay pathway of DBMBF2 is the internal
conversion which is related to the torsion vibrations of phenyl
rings.[63,64] The addition of para-substituents significantly
decreases the non-radiative constant and the decrease is higher
for more massive substituents.[46] Thus, the decrease of the non-
radiative constant of 7 a is in a good agreement with this trend.
The increase of non-radiative constants observed for 7 b-d
appears to be attributable to changes in vibronic frequencies of
the single fluorophore environment due to proximity of other
fluorophores.
Figure 4. Normalized electronic absorption spectra of organosilicon
derivatives of DBMBF2 7 a-d in the solid state (grinded microcrystals with
average thickness < 0.15 μm).
The fluorescence behavior of compounds 7 a-d in the solid
state at room temperature was also investigated. For this
purpose, the fluorescence spectra of solid 7 a-d with different
degree of crystallinity were measured and studied (Figure 5,6,
Table 2). Figures 5 shows the normalized fluorescent spectra of
solid organosilicon derivatives of DBMBF2 7 a-d (grinded
microcrystals with average thickness < 0.15 μm, > 0.4 μm, and >
2 μm).
The obtained results also indicate that the degree of
conjugation throughout the molecules is relatively small. Thus,
the tetrahedral silicon core does not have significant effect on
the electronic properties of studied derivatives, which is
consistent with published data on similar compounds.[34]
Photophysical properties in the solid state. Absorption
and fluorescence spectra of 7 a-d were also investigated in the
solid state and obtained data are summarized in Table 2. The
absorption spectra of microcrystals 7 a-d were measured in a
thin layer on a quartz plate (10×25×1 mm) after grinding to an
average thickness < 0.15 μm (microcrystal sizes were evaluated
by fluorescent microscopy) (Figure 4). These spectra are very
similar to those obtained in the solution and consist of structured
strong bands with two resolved peaks and shoulders that are
red-shifted by 10-20 nm in comparison with spectra obtained in
solution.
Figure 5. Normalized fluorescence spectra (λex = 380 nm) of 7 a-d in the solid
state (grinded microcrystals with average thickness (< 0.15 μm; > 0.4 μm; > 2
μm). Spectra were measured using front-face configuration to minimize the
reabsorption.
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Table 4. Stokes shifts of components after Gaussian decomposition of
fluorescence spectra of organosilicon derivatives of DBMBF2 7 a-d in the
solid state (grinded microcrystals with average thickness < 0.15 μm).
Peak 1
Peak 2
(cm-1
Peak 3
(cm-1
Peak 4
Peak 5
(cm-1
)
)
)
(cm-1
)
(cm-1
)
Compound
7 a
7 b
7 c
7 d
478
478
478
478
4030
2620
2660
3940
5460
4730
4930
6030
-
-
6460
6640
7390
8220
8220
8790
As can be seen in Figure 5 spectra significantly depend on
the layer thickness. This can probably be explained by a
decrease in the crystallinity of samples and reabsorption.
Compounds 7 a-d exhibit the fluorescence quantum yield in the
range of 0.1-0.37 in the solid state. Decompositions of
fluorescence spectra of 7 a-d in the solid state (grinded
microcrystals with average thickness < 150 nm) into Gaussian
components are presented in Figure 6 and Tables 3,4. Note that
the nature of the relationships between the emission properties
and crystalline structures of organic luminescent compounds is a
controversial matter. Recently, the relationship between the
crystal-packing of various alkyl- and silyl-substituted
dibenzoylmethanatoboron difluoride derivatives were analyzed
in detail by Ikeda and co-workers.[65,66] The authors pointed out
that solid state luminescence of DBMBF2 derivatives depends on
packaging and particularly on the relative distance between
molecules which determines the relative position of chelate and
phenyl rings of two molecules in the crystal cell. Three types of
packaging were identified. The first packaging corresponds to
negligible overlap of phenyl rings with blue luminescence around
450 nm. The second packaging corresponds to overlap of
phenyl rings with yellow luminescence around 510 nm and the
last is corresponds to overlap of phenyl and chelate ring with
blue luminescence around 480 nm. In our case the observed
peaks in an amorphous state can be attributed to these
packaging. Peak 1 can be attributed to the noninteracting
molecules, peak 3 to overlap of phenyl rings, peak 2 to overlap
of phenyl and chelate rings. The peak 4 in this case can be
attributed to full overlap of the molecules in the face-to-face
pairs discussed below in the next section. The peak 5 seems to
arise due to non-gaussian form of the discussed luminescence.
Computational studies. The understanding of the
properties of excimers in molecular crystals is one of the most
interesting problems in solid state chemistry. As reported by
Tohnai et al.[67] the – stacking arrangement of aromatic
molecules (in particular, anthracene-based fluorophores) in the
crystalline state can lead to the formation of both excimers as
well as unusual excited oligomers. Although the concept of
fluorescent multimers of DBMBF2 was proposed and elaborated
in detail in the works mentioned above,[65,66] the authors noted
that there remains a possibility that the fluorescence emission
may be assigned to excimers. We decided to partially explore
this possibility. The crystal structures of tri-DBMBF2-substituted
silane 7 c with three DBMBF2 moieties involves DBMBF2
fragments arranged as stacked fully overlapping face-to-face
pairs (Figure 2). This structure is very close to the structure
Figure 6. Normalized absorption and fluorescence spectra of 7 a-d in solution
(λex = 371 nm) and in the solid state (λex = 380 nm) with deconvolution (grinded
microcrystals with average thickness < 150 nm). Spectra were measured
using front-face configuration to minimize the reabsorption.
Table 2. Optical properties of organosilicon derivatives of DBMBF2 7 a-d in
the solid state.[a]
Фf
λabs
λem
λem
λem
(nm)
(nm)
(nm)
(nm)
Compound
Crystals
< 0.15 nm
Crystals
< 0.15
nm
Crystals
> 0.4 nm
Crystals
> 2 μm
Crystals
> 2 μm[b]
7 a
7 b
7 c
7 d
381 (405)
376 (399)
401 (380)
401 (381)
485
514
548
541
487
513
548
541
496
522
553
543
0.37
0.14
0.29
0.10
[a]
For measurement of optical properties in the solid state grinded
microcrystals with average thickness: a) < 0.15 μm; b) > 0.4 μm; c) > 2 μm)
were used. Microcrystal sizes were evaluated by fluorescent microscopy. The
wavelengths of the second absorption and fluorescence peaks are given in
parentheses.
[b] Taking into account the reabsorption.
Table 3. Fluorescence maxima (nm) and contributions (%) of Gaussian
components after decomposition of fluorescence spectra of organosilicon
derivatives of DBMBF2 7 a-d in the solid state (grinded microcrystals with
average thickness < 0.15 μm).
Peak 1
(nm / %)
Peak 2
(nm / %)
Peak 3
(nm / %)
Peak 4
(nm / %)
Peak 5
Compound
(nm / %)
7 a
7 b
7 c
7 d
413 / 4
413 / 6
413 / 4
413 / 5
484 / 46
453 / 16
454 / 15
482 / 22
520 / 50
501 / 26
506 / 25
536 / 37
-
-
549 / 20
554 / 21
578 / 20
607 / 32
607 /35
629 / 16
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which is considered to be the main one when excimers are
formed in solutions. Excimer-like fluorescence of this crystal
indicates that the molecular pair form an excimer-like structure
at the excitation. To simulate this theoretically, we carried out
DBMBF2 derivatives in the highly concentrated solution and in
the amorphous state.[69]
Table 5. Optical properties of organosilicon derivatives of diketonatoboron
difluorides 7 a-d in solid state before and after grinding.
the quantum-chemical calculation of
a pair of DBMBF2
fragments in the excited state. The fragment involved the Si
atom of the initial silane terminated by hydrogens. For excited
state structures and emission spectra we used combined
procedure proposed earlier for exciplexes of DBMBF2 with
methylbenzenes.[68]
Geometry optimization in the excited state was performed
using the CIS-D method including the Grimme’s dispersion
correction with the double-zeta def2-SVP basis set. Positions of
fluorescence bands were calculated as electronic transition
energies at the geometries optimized for the excited state by the
TDDFT method with the double-hybrid functional mPW2PLYP
and the triple-zeta def2-TZVP basis set. To simulate geometrical
constraints imposed by the crystal structure, positions of the Si
atoms and terminating hydrogens were fixed in the optimization
procedure.
λem before
grinding
(nm)
λem after
grinding
(nm)
Shift of emission
maximum
(nm)
Compound
7 a
7 b
7 c
7 d
486
474
571
473
490
507
512
491
+ 4
+ 31
- 59
+ 18
The distance between the nearly parallel fragments in the
crystal structure is 3.30–3.38 A. According to calculations, in the
excimer-like moiety, the distance between the planes of
diketonate cycles decreased to 2.58–2.70 A. The calculated
position of the fluorescence band (electronic transition energy at
the geometry optimized for the excited state) was about 570 nm
(2.18 eV). This is in the excellent agreement with the position of
excimer-like band in the spectra of 7 c. The calculations clearly
demonstrate that the observed redshift of the excimer emission
is due to the large geometrical change upon the electronic
transition (Figures S119, Supporting information).
As shown in Figures S118 (Supporting information) all the
crystal-packing structures of silicon-centered derivatives of
DBMBF2 are characterized by overlap of the π-orbitals of the
phenyl rings and central boron-containing rings of two adjacent
molecules. Several modes of molecular overlap observed for the
crystalline state of compounds 7 a-d require more thorough
theoretical consideration. We believe that our approach provides
a reliable tool for detailed understanding of optical properties of
such structures.
Figure 7. Normalized emission spectra (λex = 365 nm) of crystals 7 a-d before
and after grinding.
For compounds
7
a-d, their mechanofluorochromic
Mechanofluorochromic properties. As mentioned above,
all synthesized compounds 7 a-d in the solid state exhibit
properties were also studied. For this purpose, crystals 7 a-d
were grinded and their fluorescent properties were
reinvestigated. As can be seen from the fluorescence spectra
presented in Figure 7, the fluorescence maxima of compounds 7
a,b,d after grinding are red-shifted by 4, 31 and 18 nm,
respectively (Table 5). This shift can be explained by the
appearance of a package where only phenyl rings overlap.[66] In
case of 7 c, the fluorescence maximum shifts in the blue region
by 59 nm. The difference in the direction of maximum shifts
between 7 a,b,d and 7 c can be explained by the assumption
that mechanical stress and the transition into an amorphous
state lead to the appearance of new packages with varying
degrees of overlap. After grinding, the DBMBF2 fragments in 7
a,b,d have a greater freedom of rotation for the formation of
fluorescent excimers. In contrast, 7 с exhibits a blue-shifted
fluorescence after grinding, which may be connected with the
destruction of the ground state structures, which are
fluorescent properties. As-synthesized crystals
greenish blue (λem = 486 nm), 7 b show light blue (λem = 474 nm),
7 c show deep yellow (λem = 571 nm) and 7 d show blue (λem
7 a show
=
473 nm) fluorescence at the excitation by light with 365 nm
wavelength (Figures 7,8). According to X-ray data, the
fluorescence of 7 a,b,d is in a blue region due to the weaker --
interaction between chromophores in the crystal. In contrast, 7 c
has yellow fluorescence. As discussed above, all DBMBF2
fragments in 7 c have strong stacking between all chromophores
in the crystal. After excitation, each fragment can form a highly
fluorescent excimer with the nearest fragment located in the
most favorable manner. As mentioned above, such excimers
can fluoresce in the yellow region of the spectrum. The process
of DBMBF2 excimer formation at the excitation is well known for
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characterized by the highest degree of overlap. These
assumptions correlate well with obtained XRD) patterns (Figure
S101, Supporting Information). (Figure S101, Supporting
information).
7 a
7 b
7 c
7 d
7 a
7 b
7 c
7 d
Figure 8. Fluorescence microscope images of crystals 7 a-d before (left) and
after grinding (right). Size of the images is 700 х 1000 m.
The ability of compounds 7 a-d to relaxation in the initial
state after grinding was investigated and kinetics of this process
was studied (Figure 9). It was found that only 7 a exhibits
Figure 9. Solid-state fluorescence spectra during time (left) and kinetics of
solid-state fluorescence intensity (right) of crystals 7 a-d after grinding.
external
stimuli-responsive
self-reversible
solid-state
fluorescence. Figure 9 shows solid state fluorescence spectra of
7 a for various periods of time after grinding. As can be seen in
Figure 9, immediately after grinding a long-wavelength shoulder
around 560 nm is observed in the spectrum. The shoulder slowly
disappears within 100 minutes and the solid-state fluorescence
of 7 a completely restored in a few hours at room temperature.
Figure 10 shows the fluorescence microscopy images of crystals
7 a for 5 min after mechanical grinding at room temperature.
Figure 9 demonstrates changes in solid state fluorescence
spectrum observed after grinding of 7 b. The main changes in
the shape of the spectral band for 7 b are observed immediately
Figure 10. Fluorescence microscopy images of crystals 7 a for 5 min after
mechanical grinding at room temperature.
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after grinding. Further observations during 4000 minutes reveal
only decrease of integral intensity without significant changes in
the line shape. The kinetics of solid-state fluorescence intensity
of 7 b at 485 and 600 nm presented in Figure 9 show almost
This study was supported by the Russian Science Foundation
(grant no. 18-73-10152). The NMR, IR spectroscopic and X-ray
analysis data were obtained using the equipment of Center for
molecule composition studies of INEOS RAS. The analysis of
fluorescent properties was made using the equipment of the
Collaborative Access Center “Сenter for Polymer Research” of
ISPM RAS.
exponential decay. In the case of
7 c the solid-state
fluorescence spectrum shifts toward shorter wavelengths, in
contrast to other compounds and observed changes can be
divided into two stages. At first, a gradually blue shift and an
increase of intensity during 1000 minutes are observed. This
step corresponds to an increase in fluorescent intensity at 485
nm. After this, there is only a decrease in fluorescence intensity
without any spectral shape and position changes. As shown in
Figure 9, a blue shift of fluorescence maximum is observed for 7
d after grinding. The spectrum becomes similar to 7 a. Upon
further observation the shoulder peak at 560 nm slowly
decreases during 700 minutes. This process corresponds to a
plateau in the kinetic curve for fluorescence intensity at 485 nm.
After this, the fluorescence intensity gradually decreases without
any changes in the spectral shape. The main features that were
observed in the solid-state fluorescence after grinding for all
compounds 7 a-d are maximum at about 490 nm and 560 nm.
Keywords: crystal engineering • diketonatoboron difluorides •
luminescence
compounds
•
mechanofluorochromism
•
organosilicon
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Entry for the Table of Contents
FULL PAPER
Yuriy N. Kononevich,* Maxim N.
Temnikov, Alexander A. Korlyukov,
Alexander D. Volodin, Pavel V.
Dorovatovskii, Viacheslav A. Sazhnikov,
Andrey A. Safonov, Dmitriy S. Ionov,
Anatoly A. Ivanov, Nikolay M. Surin,
Evgeniya A. Svidchenko, and Aziz M.
Muzafarov *
Four novel tetrahedral silicon-centered derivatives of dibenzoylmethanatoboron
difluoride (DBMBF2) were synthesized and characterized. Tris-DBMBF2 derivative
forms fully overlapping dimeric structures exhibiting excimer-like fluorescence.
Mono-DBMBF2 derivative exhibits fully reverse mechanofluorochromic behavior.
Page No. – Page No.
Tetrahedral Silicon-Centered
Dibenzoylmethanatoboron
Difluorides: Synthesis, Crystal
Structure and Photophysical Behavior
in Solution and Solid State
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