Alkyl-substituted bis(4-((9H-fluoren-9-ylidene)methyl)phenyl)thiophenes: weakening of intermolecular interactions and additive-assisted crystallization

Article abstract of DOI:10.1039/d0ce01794a

Aggregation-induced emission (AIE) materials find their applications in organic optoelectronics, bio-imaging and sensors. In this work, we introduced alkyl substituents in AIE-active bis(4-((9H-fluoren-9-ylidene)methyl)phenyl)thiophene and elucidated thei

Full text of DOI:10.1039/d0ce01794a

Volume 23
Number 14
14 April 2021
Pages 2625–2774
CrystEngComm
rsc.li/crystengcomm
ISSN 1466-8033
PAPER
Maxim S. Kazantsev et al.
Alkyl-substituted bis(4-((9H-fluoren-9-ylidene)methyl)phenyl)
thiophenes: weakening of intermolecular interactions and
additive-assisted crystallization

CrystEngComm
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PAPER
Alkyl-substituted bis(4-((9H-fluoren-9-ylidene)
methyl)phenyl)thiophenes: weakening of
intermolecular interactions and additive-assisted
crystallization†
Cite this: CrystEngComm, 2021, 23,
2654
Alina A. Sonina,ab Christina S. Becker,a Anatoly D. Kuimov,a Inna K. Shundrina,a
a
Vladislav Yu. Komarovc and Maxim S. Kazantsev
*
Aggregation-induced emission (AIE) materials find their applications in organic optoelectronics, bio-
imaging and sensors. In this work, we introduced alkyl substituents in AIE-active bis(4-((9H-fluoren-9-
ylidene)methyl)phenyl)thiophene and elucidated their effect on the molecular and crystal structures,
crystallization and optical properties of materials. Ethyl-containing (C2-BFMPT) and octyl-containing (C8-
BFMPT) 2,5-bis(4-((2,7-dialkyl-9H-fluoren-9-ylidene)methyl)phenyl)thiophenes were synthesized in 4 steps.
The introduction of alkyl groups weakened intermolecular interactions, decreased the crystal quality,
melting point, and density, and increased the solubility of materials. Octyl-containing derivative was
demonstrated to generate two crystal forms obtained by the native (form I) and additive-assisted (form II)
crystallizations. The latter appeared to have a better crystal quality. C–H⋯π interactions and an extensive
positional disorder were revealed for both derivatives. In C8-BFMPT crystals (form II) only a half of
molecular backbones were well localized due to multiple intermolecular C–H⋯π interactions, whereas
another half demonstrated a high positional disorder. The AIE effect with a negligible photoluminescence
(PL) quantum yield (QY) in solution and a PL QY of 5% for C2-BFMPT and 2% for C8-BFMPT (form II)
crystals was demonstrated. The cooling of the C8-BFMPT form II resulted in 10-fold increase of PL QY.
The introduction of alkyl-substituents and additive-assisted crystallization are highlighted as powerful tools
for the control of crystal packing, morphology, polymorphism and the optical performance of AIE-
materials.
Received 10th December 2020,
Accepted 15th February 2021
DOI: 10.1039/d0ce01794a
rsc.li/crystengcomm
phase and highly efficient luminescence in the solid
(crystalline) state. AIE luminophores typically have a non-
1. Introduction
Organic highly-luminescent solids are practically demanding
materials for diverse applications, such as light-emitting
planar structure with an ability of intramolecular rotations
believed to be the main excited state relaxation channel in
solution. The crystalline environment locks rotations and
devices,1–6
organic
lasers,7,8
bio-imaging,9,10
anti-
counterfeiting and optical sensors.11–14 However, organic
fluorophores typically possess bright fluorescence in solution,
tremendously
increases
the
emission
from
these
molecules.18–21 Hence, the development of tools for
manipulation by the crystalline structure and morphology is
very important for the tuning of the optoelectronic properties
of AIE-active materials, and for the design of novel high
but
suffer
from
aggregation-caused
quenching.11,15
Aggregation-induced emission (AIE), being an opposite effect,
was reported by Prof. Tang and co-workers in early 2000s.16,17
AIE materials demonstrate a negligible emission in the liquid
performance
fluorophores.
The
introduction
of
substituents,22,23 polymorphism,24–26 external stimuli,27–29
various crystallization techniques30,31 and others are among
the most efficient strategies for the crystal packing and
microstructure control of conjugated materials.
Well-known representatives of AIE-active chromophores
include silole-,16 tetraphenylethylene-,19 distyrylanthracene-20
and other derivatives.18,32–35 Compounds containing the
4-((9H-fluoren-9-ylidene)methyl)phenyl fragment36–43 have
recently attracted attention as model high-torsional freedom
a N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Lavrentieva 9,
630090, Novosibirsk, Russia. E-mail: maximkazantsev1988@gmail.com
b Novosibirsk State University, Pirogova 2, 630090, Novosibirsk, Russia
c A. V. Nikolaev Institute of Inorganic Chemistry, Lavrentieva 3, 630090,
Novosibirsk, Russia
† Electronic supplementary information (ESI) available: Spectra of synthesized
compounds, cyclic voltammetry, crystal data and X-ray analysis. CCDC 2047979,
2047980, 2059086 and 2059087. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/d0ce01794a
2654 | CrystEngComm, 2021, 23, 2654–2664
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“one-rotor” AIE systems for the study of the structure–
property relationships to reveal fundamental principles for
the molecular design of high-performance fluorophores. We
have previously reported the AIE behavior for bis(4-((9H-
fluoren-9-ylidene)methyl)phenyl)thiophene (BFMPT) with
irreversible thermo- and mechano-responsive fluorescence
switching triggered by conformational change.44 One of the
drawbacks of that system is relatively strong multiple
intermolecular C–H⋯π interactions, resulting in low
solubility and long crystallization time.44 The introduction of
substituents, particularly alkyl-groups, is an efficient way for
the tuning of the crystal packing, solubility, softness and
film-formation ability.45–47
In this work, we introduced alkyl substituents, viz., “short”
ethyl and “long” octyl groups, in the 2 and 7 positions of the
fluorene fragments of BFMPT and studied their influence on
the crystallization, structure, solubility and optical properties
of materials. Alkylated 2,5-bis(4-((2,7-dialkyl-9H-fluoren-9-
ylidene)methyl)phenyl)thiophene derivatives were synthesized
Thermogravimetry and differential scanning calorimetry
analyses were performed in helium atmosphere using a
NETZSCH STA 409 instrument; the heating rate was 10 °C
min−1. Cyclic voltammetry measurements in CH2Cl2 solution
were performed using
potentiostat (Elins, Russia) in combination with a three-
electrode cell (Gamry); 0.1 tetrabutylammonium
a computer-controlled P-8 nano
M
hexafluorophosphate was used as supporting electrolyte. The
Pt, Pt wire and Ag/AgCl were used as the working, counter
and reference electrodes, respectively. The measurements
were standardized by measuring the redox potential of the
Fc/Fc+ couple after each compound analysis. The HOMO and
LUMO energy levels were estimated using onset potentials
according to equations:
onset
EHOMO = −(E
+ 4.8)(eV)
+ 4.8)(eV)
ox
onset
Red
ELUMO = −(E
in
4 steps by acylation, reduction, cross-coupling, and
Knoevenagel reactions. The introduction of alkyl groups
resulted in a decrease of melting points and an increase of
the solubility due to the weakening of intermolecular
interactions. The crystal quality of the materials was also
reduced upon introduction of substituents, with the native
crystallization of the octyl-containing derivative in various
solvent systems leading to a polycrystalline sample. On the
contrary, the additive-assisted crystallization of C8-BFMPT
improved the crystal quality and gave its polymorphic phase
(form II). Both derivatives demonstrated an AIE effect with a
negligible photoluminescence (PL) quantum yield (QY) in
solution, and a PL QY of 5% for C2-BFMPT and 2% for C8-
BFMPT (form II) crystals.
Synthesis of the 2,7-diacyl-9H-fluorenes 1a, b. To a stirring
mixture of 1,2-dichloroethane (48 mL) and acyl chloride (48.0
mmol) in an oven-dried round bottom flask, AlCl3 (6.42 g,
48.0 mmol) was carefully added. A solution of fluorene (2.00
g, 12 mmol) in 1,2-dichloroethane (48 mL) was slowly added
to a mixture within 40 minutes. The reaction mixture was
stirred at room temperature for 40 minutes, and then was
refluxed for 23 h. The cooled solution was poured into a
mixture of ice and concentrated HCl (5 mL), and extracted
with CH2Cl2 (3 times). The combined extracts were washed
with water (3 times), dried over MgSO4 and evaporated. The
solid residue was washed on a glass filter with ethyl acetate,
dried under air and additionally purified by column
chromatography (silica gel/benzene).
For 1,1′-(9H-fluorene-2,7-diyl)diethanone (1a). Yield of white
solid – 2.0 g (67%). δH (CDCl3, 500.1 MHz) 8.12 (2H, s), 7.99
(2H, d, J = 8.0 Hz), 7.85 (2H, d, J = 8.0), 3.95 (2H, s), 2.63 (6H,
s). The 1H NMR spectrum corresponds to that described in
ref. 48.
2. Experimental
2.1 Synthesis and characterization
All reagents and solvents were purchased from commercial
sources (Sigma-Aldrich, Acros Organics), and used without
additional
2,5-Bis(tributylstannyl)thiophene
purification
unless
otherwise
was distilled
specified.
before
For 1,1′-(9H-fluorene-2,7-diyl)bis(octan-1-one) (1b). Yield of
white solid – 3.0 g (60%). M.p.: 138 °C (decomp.); IR (KBr):
2953, 2920, 2848, 1682, 1608, 1466, 1435, 1410, 1358, 1273,
1232, 1205, 1134, 1018, 964, 816, 771, 723, 704, 582, 430, 417
cm−1; δH (CDCl3, 400.1 MHz) 8.15 (2H, s), 8.01 (2H, d, J = 8.1
Hz), 7.86 (2H, d, J = 8.1), 3.99 (2H, s), 2.99 (4H, t, J = 7.4 Hz),
1.70–1.78 (4H, m), 1.31–1.41 (8H, m), 1.25–1.30 (8H, m), 0.87
(6H, t, J = 6.5 Hz); δC (CDCl3, 100.6 MHz) 200.5 (2 CO), 145.0,
144.6, 136.6, 127.7 (2 CH), 125.0 (2 CH), 120.8 (2 CH), 39.0 (2
CH2), 37.1 (CH2), 31.9 (2 CH2), 29.5 (2 CH2), 29.3 (2 CH2),
24.6 (2 CH2), 22.8 (2 CH2), 14.2 (2 CH3); HRMS found: [M]+
418.2864, C29H38O2 requires [M]+ 418.2866; Elem. an. Found:
C, 83.3; H, 9.1%; for C29H38O2 requires C, 83.2; H, 9.2%.
Synthesis of 2,7-diethyl-9H-fluorene (2a). 1,1′-(9H-fluorene-
2,7-diyl)diethanone (1a) (0.75 g, 3.0 mmol) was dissolved in
diethylene glycol (60 mL) under heating (95 °C) and stirring.
Then, hydrazine hydrate (0.96 mL, 18.0 mmol) was added;
coupling, b.p. 178 °C (1 Torr). 4-Bromobenzaldehyde was
dried by addition of anhydrous diethyl ether and subsequent
evaporation under reduced pressure, and the procedure was
repeated 3 times. The 1H NMR spectrum in CDCl3 of the
resulting white precipitate revealed no water. The reaction
mixtures were monitored by TLC using Macherey-Nagel pre-
coated TLC-sheets (Alugram Xtra SIL G/UV254). Macherey-
Nagel Kieselgel 60 was used for column chromatography.
Combustion analyses were performed with a CHN-analyzer
(EURO EA). NMR spectra were recorded with Bruker AV 300,
Bruker AV 400 and Bruker AV 500 spectrometers. Mass
spectra were recorded with a Thermo Electron Corporation
DFS mass spectrometer. IR spectra were recorded in the
transmission mode with
spectrometer in potassium
a
Bruker Tensor 27 FT-IR
bromide pellets.
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the flask was equipped with a reflux condenser and the
mixture was stirred at 130 °C for 2 h. After cooling to 80 °C,
KOH powder (1.01 g, 18.0 mmol) was added. Then, the
reaction mixture was heated at 200 °C for 5 h (bath
temperature 230 3 °C). After cooling to room temperature,
the reaction mixture was poured into 100 mL of water. The
crude product was extracted with ethyl acetate (3 × 50 mL),
washed with water (3 × 70 mL), dried over sodium sulfate
and evaporated under reduced pressure. The product was
purified by column chromatography (silica gel/hexane) to
afford a white solid (0.66 g, 99%). M.p.: 123 °C; IR (KBr):
3028, 3005, 2960, 2928, 2870, 2856, 2721, 1890, 1880, 1641,
1612, 1581, 1471, 1419, 1398, 1371, 1321, 1306, 1213, 1055,
976, 862, 841, 820, 787, 721, 704, 581, 561, 505, 422 cm−1; δH
(CDCl3, 300.1 MHz) 7.73 (2H, d, J = 7.8 Hz), 7.43 (2H, s), 7.27
(2H, d, J = 7.8 Hz), 3.89 (2H, s), 2.80 (4H, q, J = 7.6 Hz), 1.38
(6H, t, J = 7.6 Hz); δC (CDCl3, 75.5 MHz) 143.6, 142.7, 139.6,
126.5 (2 CH), 124.6 (2 CH), 119.5 (2 CH), 36.8 (CH2), 29.2 (2
CH2), 16.1 (2 CH3); HRMS found: [M]+ 222.1407, C17H18
requires [M]+ 222.1403; Elem. an. Found: C, 91.4; H, 8.3%;
for C17H18 requires C, 91.8; H, 8.2%.
was filtered off, washed with water and a mixture of hexane
and MeOH (1 : 1), then dried under air. The product was
filtered through a short silica gel column using hot toluene
as the solvent. The filtrate was then evaporated, giving
dialdehyde 3 as a yellow powder (0.29 g, 66%). δH ((CD3)2CO,
500.1 MHz) 10.04 (2H, s), 7.97 (4H, d, J = 8.5 Hz), 7.94 (4H, d,
J = 8.5 Hz), 7.74 (1H, s); δC ((CD3)2CO, 125.0 MHz) 192.2 (2
CH), 144.5, 140.0, 136.8, 131.3 (4 CH), 127.9 (2 CH), 126.7 (4
CH); the NMR spectra corresponds to that described in ref.
49.
General synthesis of 2,5-bis(4-((2,7-dialkyl-9H-fluoren-9-
ylidene)methyl)phenyl)thiophenes C2-BFMPT, C8-BFMPT. To
an oven-dried two necked 50 ml round bottom flask
containing a magnetic stir-bar, dry DMF (25 ml) and the
corresponding fluorene derivative 2 (1.43 mmol) were added.
The flask was purged with Ar for 20 minutes, and then
t-BuONa (0.15 g, 1.56 mmol) was added and the flask was
closed by rubber septum. The mixture was vigorously stirred
for 10 minutes, and then 4,4′-(thiophene-2,5-diyl)
dibenzaldehyde (3) (0.2 g, 0.68 mmol) in DMF (10 mL) was
added dropwise. The reaction mixture was stirred at room
temperature for 1.5 h, and then poured into distilled water
(50 mL).
For 2,5-bis(4-((2,7-diethyl-9H-fluoren-9-ylidene)methyl)phenyl)
thiophene (C2-BFMPT). The resulting mixture was extracted
with ethyl acetate, washed with water and dried over MgSO4.
The residue was evaporated under reduced pressure, and
then purified by column chromatography on silica gel
(hexane/toluene = 1/1) to give a yellow powder (0.3 g, 60%).
M.p.: 230 °C; UV-vis λmax (THF) 402 (Irel, 100); IR (KBr): 3055,
3024, 2960, 2926, 2868, 2854, 1892, 1743, 1724, 1632, 1601,
1554, 1497, 1466, 1417, 1319, 1279, 1238, 1184, 1113, 1061,
970, 897, 870, 823, 800, 768, 746, 663, 571, 498, 480 cm−1; δH
(CDCl3, 500.1 MHz) 7.74 (4H, d, J = 8.3 Hz), 7.67 (4H, d, J =
8.3 Hz), 7.60–7.63 (6H, m), 7.58–7.60 (4H, m), 7.43 (2H, s),
7.14–7.19 (4H, m), 2.76 (4H, q, J = 7.7 Hz), 2.56 (4H, q, J = 7.7
Hz), 1.33 (6H, t, J = 7.7 Hz), 1.16 (6H, t, J = 7.7 Hz); δC (CDCl3,
125.6 MHz) 143.6, 143.0, 142.6, 140.0, 139.4, 137.2, 137.0,
136.9, 136.4, 133.9, 130.4 (4 CH), 128.6 (2 CH), 128.2 (2 CH),
126.0 (2 CH), 125.5 (4 CH), 124.6 (2 CH), 124.0 (2 CH), 119.8
(2 CH), 119.4 (2 CH), 119.3 (2 CH), 29.4 (2 CH2), 29.1 (2 CH2),
16.1 (2 CH3), 15.8 (2 CH3); HRMS found: [M]+ 700.3160,
C52H44S requires [M]+ 700.3158; Elem. an. Found: C, 88.7; H,
6.5; S, 4.3%; for C52H44S requires C, 89.1; H, 6.3; S, 4.6%.
For 2,5-bis(4-((2,7-dioctyl-9H-fluoren-9-ylidene)methyl)phenyl)
thiophene (C8-BFMPT). The resulting precipitate was filtered
off, washed successively with water and methanol, and dried
under air. The product was purified by column
chromatography on silica gel (hexane/toluene = 3/1) to give a
yellow powder (0.98 g, 66%). Mp 87 °C; UV-vis λmax (THF) 404
(Irel, 100); IR (KBr): 3024, 3003, 2953, 2922, 2850, 1632, 1601,
1497, 1466, 1419, 1348, 1279, 1113, 893, 868, 814, 800, 789,
744, 721, 669, 494, 471 cm−1; δH (CDCl3, 300.1 MHz) 7.74 (4H,
d, J = 8.0 Hz), 7.67 (4H, d, J = 8.0 Hz), 7.55–7.62 (10H, m),
7.42 (2H, s), 7.18 (2H, d, J = 7.7 Hz), 7.13 (2H, d, J = 7.7 Hz),
2.71 (4H, t, J = 7.5 Hz), 2.51 (4H, t, J = 7.5 Hz), 1.64–1.76 (4H,
Synthesis of 2,7-dioctyl-9H-fluorene (2b). Triethylene glycol
(40 mL) was heated to 110 °C, and 1,1′-(9H-fluorene-2,7-diyl)
bis(octan-1-one) (1b) (1.0 g, 2.4 mmol) was carefully added
under stirring in order to dissolve without conglutination.
Then, KOH powder (0.8 g, 14.3 mmol) followed by hydrazine
hydrate (0.76 mL, 14.3 mmol) were added. The reaction flask
was equipped with a reflux condenser and heated at 200 °C
for 3.5 h (bath temperature: 230
3 °C). After cooling to
ambient temperature, the reaction mixture was poured onto
50 mL of water. The crude product was then extracted with
ethyl acetate (3 × 30 mL), washed with water (2 × 50 mL),
dried over sodium sulfate and evaporated under reduced
pressure. The product was purified by column
chromatography (silica gel/hexane) to afford a white solid
(0.65 g, 70%). IR (KBr): 3037, 3007, 2956, 2922, 2850, 1468,
1417, 1389, 1377, 1138, 822, 785, 721, 704, 552, 424 cm−1; δH
(CDCl3, 500.1 MHz) 7.63 (2H, d, J = 7.7 Hz), 7.33 (2H, s), 7.15
(2H, d, J = 7.7 Hz), 3.82 (2H, s), 2.66 (4H, t, J = 7.8 Hz), 1.63–
1.68 (4H, m), 1.33–1.39 (8H, m), 1.25–1.33 (18H, m), 0.89 (6H,
t, J = 6.9 Hz); δC (CDCl3, 125.7 MHz) 143.5, 141.4, 139.6, 127.1
(2 CH), 125.2 (2 CH), 119.4 (2 CH), 36.9 (CH2), 36.3 (2 CH2),
32.1 (2 CH2), 32.0 (2 CH2), 29.7 (2 CH2), 29.5 (2 CH2), 29.4 (2
CH2), 22.8 (2 CH2), 14.2 (2 CH3); HRMS found: [M]+ 390.3279,
C29H42 requires [M]+ 390.3281; Elem. an. Found: C, 88.8; H,
11.0%; for C29H42 requires C, 89.2; H, 10.8%.
Synthesis of 4,4′-(thiophene-2,5-diyl)dibenzaldehyde (3). A
stirring mixture of 2,5-bis(tributylstannyl)thiophene (1.00 g,
1.5 mmol), 4-bromobenzaldehyde (1.15 g, 6.2 mmol) and
toluene (10.0 mL) was purged with argon for 20 minutes,
then Pd(PPh3)4 (0.17 g, 0.15 mmol) was added in one portion.
The flask was equipped with a reflux condenser and sealed.
The reaction mixture was heated at 80 °C for 2 days, and then
cooled to rt. Toluene was evaporated under reduced pressure;
the residue was diluted with water (10 mL) and hexane (10
mL), and left in a refrigerator for 1 night. The resulting solid
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m), 1.49–1.58 (4H, m), 1.24–1.38 (40H, m), 1.84–1.94 (12H,
m); δC (CDCl3, 75.5 MHz) 143.6, 141.7, 141.2, 139.9, 139.4,
137.2, 137.1, 136.8, 136.5, 133.9, 130.4 (4 CH), 129.1 (2 CH),
128.7 (2 CH), 125.9 (2 CH), 125.5 (4 CH), 124.6 (2 CH), 124.5
(2 CH), 120.3 (2 CH), 119.3 (2 CH), 119.2 (2 CH), 36.5 (2 CH2),
36.1 (2 CH2), 32.1 (4 CH2), 32.0 (2 CH2), 31.5 (2 CH2), 29.7 (4
CH2), 29.5 (2 CH2), 29.4 (4 CH2), 29.3 (2 CH2), 22.8 (4 CH2),
14.3 (4 CH3); HRMS found: [M]+ 1036.6903, C76H92S requires
[M]+ 1036.6914; Elem. an. Found: C, 88.4; H, 8.9; S, 3.1%; for
C76H92S requires C, 88.0; H, 8.9; S, 3.1%.
Montel mirrors) and PHOTON III CMOS detector was used to
collect diffraction intensities at 80 K. The samples were
cooled from 250 K at a rate of 3–6 K min−1. High-exposition
tests (200–300 seconds per degree) showed an abrupt
disappearance of observable diffraction intensities at d less
than 1.1–1.2 Å, pointing to the high static disorder. Data
reduction and structure refinement were done using the
same software. Attempts to refine the non-H atoms with
anisotropic atomic displacements gave irrelevant ellipsoids
with Uiso of ca. 0.15 Å2, which is 5 to 10 times higher than
could be expected for 80 K, and even higher than that in the
models obtained with 200 K data. The same restraints and
constraints as for the structures at 200 K except restraints
RIGU, which was replaced by SIMU for isotropic atomic
displacement, were used for the refinement. No apparent
local-ordered alternative positions were found, what indicates
that their number exceeds half a dozen. All atoms were
refined to be disordered over the two “uttermost” positions.
The obtained low-temperature structure models confirmed
the principal consistency of the localization of aromatic
fragments with the ones at 200 K. The latter were simpler
and were used for the crystal structure analysis (section 3.2).
The low-temperature experiments also confirmed the
principal static nature of the disorder, revealed the expansion
of alternative positions of molecules upon the decrease of
temperature, and allowed us to estimate the difference in the
conformations (see Fig. S25†). Additional experimental
details and parameters characterizing the data collection and
refinement, as well as crystal data, are presented in ESI.†
The crystal structures were analyzed for intermolecular
interactions using PLATON54 and Mercury.55 Crystallographic
data for the structures have been deposited at the Cambridge
Crystallographic Data Centre as ESI† publication CCDC no.
2047979–80 (200 K) and 2059086–7 (80 K).
2.2 Crystal growth and analysis
C2-BFMPT was crystallized by slow diffusion of methanol
vapors into its CH2Cl2 solution with a concentration of 0.6 g
l−1. In the case of neat C8-BFMPT, various solution methods
such as solvent diffusion and slow solvent evaporation were
used (CH2Cl2, toluene, benzene, dichlorobenzene, ethyl
acetate, THF, acetone, acetonitrile as solvents and hexane,
i-PrOH, MeOH as antisolvents); typically, the polycrystalline
sample was obtained. To improve the crystal quality of the
material, it was crystallized from
a toluene solution
containing 15 mol% of anthracene with respect to C8-BFMPT.
The crystals were examined using an optical microscope
(POLAM R-312, “Lomo”, Russia) in transmitted light and
under blue laser irradiation (405 nm).
X-ray diffraction. The X-ray diffraction experiments at 200
K
were performed using
a
Bruker KAPPA APEX II
diffractometer with graphite monochromated Mo Kα
radiation. Integration and scaling of the intensity data were
accomplished with SAINT.50 Absorption corrections were
applied using SADABS.51 The structures were solved by direct
methods with SHELXT.52 Refinement was carried out by the
full-matrix least-squares technique with SHELXL52 using
Olex2 software.53 The structure of C2-BFMPT was refined as a
merahedral twin (twin axis (001), twin component ratio 0.38).
All non-hydrogen atoms in both structures were refined
anisotropically. The restraints (SADI for 1,2-, 1,3- and some
1,4-distances and FLAT for the aromatic fragments, DFIX/
DANG for alkyl 1,2-, 1,3-distances), and atomic displacement
restraints (RIGU) and constraints (EADP for atoms close
alternative positions) were used for the refinement. Hydrogen
atoms were calculated at idealized positions according to the
riding model, with Uiso(H) = 1.5Ueq(C) for the alkyl H atoms
and 1.2Ueq(C) otherwise. Molecules of C2-BFMPT have
disordered ethyl groups over two positions with an
approximate site occupancy of 1 : 1 and 1 : 2. C8-BFMPT has a
disordered phenylene with a site occupancy of 1 : 2, and octyl
groups with site occupancy of 1 : 1 and 1 : 2. Elongated atomic
displacement ellipsoids with a somewhat exaggerated Uiso
pointed on the static or dynamic disorder in the crystal
structures. Low data-to-parameters ratio did not allow us to
refine the structure using more detailed models; additional
low-temperature (80 K) experiments were carried out.
Powder X-ray diffraction (PXRD) patterns were collected
using Stoe Stadi-MP diffractometer (Cu Kα1 radiation, curved
Ge monochromator, transmission/Debye–Scherrer mode,
MYTHEN 1 K detector).
2.3 Optical spectroscopy
UV/vis spectra were recorded in a diluted (10−5 M) THF
solution
with
a
Varian
Cary
5000
UV-vis-NIR
spectrophotometer in 1 × 1 cm quartz cuvettes. The PL
spectra were recorded with a Varian Cary Eclipse fluorescence
spectrophotometer in 1 cm quartz cuvettes. The PL QY in
THF solution was measured according to the standard
procedure. A solution of fluorescein in 0.1 M NaOH (PL QY =
0.95) was used as a reference standard.56 The PL spectra in
binary THF/water mixtures were recorded in 1 cm quartz
cuvettes with a constant dye concentration of about 10−5 M.
The fraction of water/THF was varied from 0 to 90%, and the
excitation wavelength was 365 nm. The PL QY of the single
crystals was measured using a 3.3 inch-diameter integrating
sphere (Newport 819C-SL-3.3) coupled with UV-vis
spectrometer (QE Pro, Ocean Optics). The excitation was
The next generation Bruker D8 Venture instrument with
IμS 3.0 Cu Kα radiation (Incoatech, multilayer focussing
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Scheme 1 A synthetic route to 2,5-bis(4-((2,7-dialkyl-9H-fluoren-9-ylidene)methyl)phenyl)thiophenes C2-BFMPT and C8-BFMPT.
performed with a diode laser emitting at 405 nm (Laserglow).
The detailed experimental procedure is described in ref. 57.
The cooling of the crystalline samples was performed by a
liquid nitrogen bath with subsequent measurement of the PL
signal in an integrating sphere.
atmosphere. Both compounds demonstrate a high thermal
stability with a temperature of 10% weight loss of 400 °C.
The melting points are significantly lower, as compared to
that of the parent BFMPT compound44 (326 °C) being 226.3
°C for C2-BFMPT and 83.9 °C for C8-BFMPT. The solubility of
the material increases upon alkylation: in contrast to BFMPT
having a solubility in toluene of 2.5 10−4 M,44 the solubility of
C2-BFMPT in toluene is 1.3 10−2 M and the lower estimate of
the solubility of C8-BFMPT in toluene is 0.2 M.
3. Results and discussion
3.1 Synthesis and general properties
Scheme 1 demonstrates the synthetic approach for the ethyl
(C2-BFMPT) and octyl (C8-BFMPT) substituted BFMPT. Target
compounds were synthesized in 4 steps, including acylation,
reduction, cross-coupling reaction and Knoevenagel
condensation. 2,7-Dialkyl-9H-fluorenes 2a, b were synthesized
via acylation of fluorene, followed by the reduction of the
carbonyl group. The Friedel–Crafts acetylation of fluorene
was previously reported to proceed preferably into positions 2
and 7, and less favorable in 4 and 5.48 The same reaction
conditions were successfully applied to obtain 1,1′-(9H-
fluorene-2,7-diyl)bis(octan-1-one) (1b) with 60% yield. The
reduction procedure is precisely described in the
Experimental section. It is noteworthy that the temperature
control at this step throughout the process is very important
to avoid a formation of byproducts. To reach the maximum
yield, the best conditions in both cases were found (see
Experimental details). Target compounds C2-BFMPT and C8-
BFMPT were obtained via condensation of 4,4′-(thiophene-
2,5-diyl)dibenzaldehyde (3) with two equivalents of the
corresponding substituted fluorene using t-BuONa as a base
(Scheme 1).
Therefore, the introduction of the alkyl substituents
significantly weakens the intermolecular interactions in
crystals of the alkyl derivatives of BFMPT, which should
dramatically impact the crystallinity, structure and optical
properties of the materials.
3.2 Crystal growth and analysis
The crystals of C2-BFMPT grown by solvent-antisolvent
method typically have a needle-like habit with a green
photoluminescence
under
blue
laser
irradiation
(Fig. 2a and b). C8-BFMPT was attempted to be crystallized
from solution by solvent/antisolvent crystallization and
solvent evaporation, but none of the used solvent systems
(CH2Cl2, toluene, benzene, dichlorobenzene, ethyl acetate,
THF, acetone, acetonitrile as solvents; hexane, i-PrOH, MeOH
as antisolvents) provided the crystals of sufficient quality for
The obtained compounds were characterized by 1H and
13C NMR, HRMS, and FT-IR spectroscopies (see ESI,† Fig. S1–
S24)
and
elemental
analysis.
Cyclic
voltammetry
measurements (Fig. S23†) revealed reversible first oxidation
and reduction waves for both compounds. The energies of
the frontier molecular orbitals estimated from the onset
oxidation and reduction potentials are almost identical:
EHOMO = −5.5 eV; ELUMO = −2.78 eV, indicating a negligible
effect of the substituents on the electronic properties of the
studied
compounds.
Fig.
1
demonstrates
the
Fig.
1 Thermogravimetric (dashed lines) and differential scanning
thermogravimetric and differential scanning calorimetry
analyses of the as-synthesized BFMPTs in helium
calorimetry (solid lines) analyses of the C2-BFMPT (red) and C8-BFMPT
neat samples (blue) in He atmosphere.
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allowed us to obtain a new polymorphic form of C8-BFMPT
(form II) with a higher melting point of 89.4 °C. Since we did
not observe any additional peaks in the DSC thermograms
(Fig. 3) and PXRD patterns (Fig. S27†), both form I and form
II were concluded to have sufficient phase purity. After the
melting of form II, the obtained sample crystallizes upon
cooling into form I (Fig. 3, blue), corresponding to the as-
synthesized powder. According to TGA analysis, the
crystallized Forms did not demonstrate a weight loss up to
the melting point, what excludes the formation of solvates.
Since the melting point of form II is higher than that for
form I, we concluded that the crystallization with anthracene
resulted in stronger intermolecular interactions in form II
crystals of C8-BFMPT than that in the neat sample (form I).64
To gain further insight into the structure and properties
of the alkyl-derivatives, X-ray diffraction experiments (both at
200 K and 80 K, see Experimental details) were performed.
C2-BFMPT crystallizes in the chiral monoclinic space group
(P21), with two molecules in the asymmetric unit designated
as molecule A and molecule B. The asymmetric unit of the
structure with corresponding labeling scheme is shown in
Fig. 4a. The ethyl groups exhibit extensive positional disorder
because they do not form any C–H⋯π interactions (Fig. 4).
The conformation of the molecules is described by four
torsion angles: C2–C1–C24′–C25′ (φ1A/B), C26′–C21′–C5′–C4′ (φA2/B),
C3′–C2′–C11′–C16′ (φA3/B), and C15′–C14′–C1–C2 (φ4A/B) (Fig. 4a).
The molecular conformations of C2-BFMPT are non-planar
because of the intramolecular sterical hindrance between
fluorene and the phenylene fragments,44 coupled with the
multiple intermolecular C–H⋯π interactions of the phenylene/
thiophene backbone and fluorene groups. All noncovalent
interactions are formed along axis b, corresponding to the
direction of the crystal growth. Molecules A form stacks by C–
Fig. 2 Optical images of: C2-BFMPT crystal in transmitted light (a) and
under blue laser irradiation 405 nm (b); C8-BFMPT crystal grown in the
presence of anthracene in transmitted light (c) and under blue laser
irradiation 405 nm (d).
X-ray study. The polycrystalline powders (Fig. S24†) were
obtained in numerous native crystallization experiments. This
is assigned to the poor crystal quality due to the disordered
octyl groups, inducing very weak intermolecular interactions
and loose packing of the C8-BFMPT molecules in crystals.
To improve the crystal quality of C8-BFMPT, we applied a
method of crystallization in the presence of an additive.
“Tailor-made” auxiliaries58 were previously demonstrated to
direct the crystallization of amino-acids59–62 and nitro-
aromatics.63 However, to the best of our knowledge, this
approach has never been used for AIE-active and fluorescent
materials. As an additive, we chose a conjugated small
molecule – anthracene, which was intentionally added to the
crystallization solution of C8-BFMPT. The concentration of
anthracene was about 15 mol% with respect to C8-BFMPT. In
addition to the anthracene crystals, the thin needle-like
crystals of C8-BFMPT were reproducibly obtained in these
experiments (Fig. 2c and d). It should be noted that we did
not noticed co-crystals, which may be formed from
anthracene and C8-BFMPT as follows from the single crystal
and powder X-ray diffraction study (vide infra). This is most
probably due to the large structural distinction of anthracene
and the C8-BFMPT molecules. Nevertheless,
a small-
molecular additive significantly improves the crystal quality
of the C8-BFMPT material (cf. Fig. 2 and S24†). Moreover,
according to DSC analysis (Fig. 3) and powder X-ray
diffractions (Fig. S27†), the crystallization with the additive
H⋯π interactions between the phenyl and fluorene groups (C13′
–
H⋯π2, C13(Cg 8)–H⋯π4 and C15′–H⋯π8) (Fig. 4b and Table S2†).
Molecules B form stacks by π-stacking between the fluorene
groups (π7–π6), and C–H⋯π interaction between the phenyl ring
and fluorene group (C25′–H⋯π4; Fig. 4c and Table S2†). So, the
fluorene groups of molecules B form pi-stacking, whereas the
fluorene groups of molecules A only participate in the C–H⋯π
interactions. These stacks are bonded by C–H⋯π interactions
between molecules A and B (C14(Cg 5B)–H⋯π1(A), C25′(A)–H⋯π8(B)
,
C26′(A)–H⋯π7(B), C4′(A)–H⋯π9B; Fig. 4d and Table S2†). The
powder X-ray diffraction study demonstrated that the as-
synthesized powder of C2-BFMPT corresponds to its single
crystalline structure (Fig. S26†).
The neat crystalized C8-BFMPT sample (form I) was not
suitable for the single crystal X-ray study. The powder
X-ray diffraction pattern of form
I
(Fig. S27†)
demonstrated broadened peaks and low-resolution, which
makes it unsuitable for structure solution from powder
data. However, we managed to solve the structure of C8-
BFMPT form II obtained by the additive-assisted
crystallization. Form II crystallized in the centrosymmetric
monoclinic space group (P21/c) with one molecule in the
asymmetric unit; the corresponding labeling scheme is
Fig. 3 Thermal gravimetric (dashed lines) and differential scanning
calorimetry (solid lines) analyses of C8-BFMPT: form I (red) and form II
(dark), and the same sample after melting and cooling to room
temperature (blue) in He atmosphere.
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Fig. 4 Molecular structure of C2-BFMPT with the disordered ethyl groups, dihedral angles (ϕ1, ϕ2, ϕ3, ϕ4), atom and cycle numbering involved in
the noncovalent interactions drawn at 50% probability atomic displacement ellipsoids (a); fragments of the crystal structure C2-BFMPT, and the
intermolecular interactions with molecules A (b) and molecules B (c), and between A and B (d). Dashed blue lines represent the π-stacking and C–
H⋯π interactions. The arrows indicate the orientation of the crystallographic axes.
Fig. 5 Molecular structure of C8-BFMPT with disordered groups (translucent) drawn with fixed atomic radii for clarity, dihedral angles (ϕ1, ϕ2, ϕ3,
ϕ4 and translucent ϕ3, ϕ4 for disordered part), atom numbering and cycle numbering involved in the noncovalent interactions (a); fragment of the
crystal structure C8-BFMPT (cut for clarity) demonstrating the intermolecular interactions between the well-localized fluorene and octyl groups
(b). Dashed blue lines represent the C–H⋯π interactions. The arrows indicate the orientation of the crystallographic axes.
shown in Fig. 5a. The conformation of C8-BFMPT is also
non-planar due to the steric hindrance of the fluorene and
phenyl groups, which is confirmed by the disorder of the
phenylene and fluorene groups (Fig. 5a). Remarkably, the
coordinate sites of one octyl group and half of the molecular
backbones are well localized, and do not demonstrate
positional disorder due to the formation of C–H⋯π
interactions between the fluorene and octyl groups (C28–
H28b⋯π9, C2A–H2AA⋯π7; Fig. 5b and Table S2†). This half of
the molecules are additionally stabilized by the interaction
between the phenyl ring and fluorene fragment (C25′–H⋯π7;
Table S2†). Molecules form stacks along axis a,
corresponding to the direction of crystal growth. Another
three octyl groups and one phenylene group exhibited
extensive positional disorder, and do not participate in
numerous C–H⋯π interactions as the former (Fig. 5a). Half
of the octyl groups are curved due to the close crystal
packing. Their extensive positional disorder in connection
with the lack of short contacts are in accordance with the low
crystal density (1.099 g cm−3), as compared with that for C2-
BFMPT (1.214
g
cm−3) and parent BFMPT compound
(1.285).44 So, the increasing of the length of the alkyl
substituent leads to the loose crystal packing and decrease of
the crystal density.
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3.3 Optical properties
in Fig. 6 and Table 1, respectively. Both C2-BFMPT and C8-
BFMPT exhibit very weak emission with unstructured PL
spectra in solution, which might be assigned to the high
torsional freedom and fast relaxation of the excited state, as
demonstrated for the parent BFMPT compound.44 The
Absorption and photoluminescence (PL) spectra for C2-
BFMPT and C8-BFMPT were recorded in THF solution. The
corresponding curves and photophysical data are presented
Fig. 6 Optical spectra for C2-BFMPT (left) and C8-BFMPT (right): a) and b) absorption (black) and PL (red) in THF solution (10−5 M), c) and d) PL
spectra of the binary mixtures of THF and water with different volume water contents (0–90%) at constant dye concentration (10−5 M); the insets
demonstrate the dependence of the ratio of the spectral intensity (I) to the initial spectral intensity (I0) on the water fraction; the excitation
wavelength was 365 nm, e) and f) PL spectra of the crystals at room temperature (blue) and liquid nitrogen (red) conditions; the spectrum of the
neat crystallized C8-BFMPT sample form I (olive).
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Table 1 Photophysical properties of C2-BFMPT and C8-BFMPT in solution and crystalline samples
λabs (nm)
λem (nm)
Stokes shift (eV)
PL QY (%)
C2-BFMPT in THF
C8-BFMPT in THF
C2-BFMPT crystal
402
404
—
542
528
562
0.80
0.72
—
<0.1
<0.1
5
C8-BFMPT crystal. Form II
—
562
—
2
C2-BFMPT crystal in liquid nitrogen (∼77 K)
—
—
547, 562
553, 578
—
—
5
19
C8-BFMPT crystal form II in liquid nitrogen (∼77 K)
λabs and λem – absorption and PL maxima.
relatively high Stokes shift for C2-BFMPT and C8-BFMPT (0.8
eV and 0.72, respectively) also indicates the substantial
reorganization of the molecular geometry in the excited state.
To investigate the AIE behavior of the obtained
compounds, the PL spectra of the binary mixtures of THF
and water with different volume water fractions were
investigated (Fig. 6c and d). Both studied molecules have a
clear AIE behavior, and the PL intensity increased about 32
and 11 times upon the addition of 90% of water for C2-
BFMPT and C8-BFMPT, respectively (Fig. 6c and d). The
higher AIE activity of C2-BFMPT is assigned to the stronger
intermolecular interactions in its crystals, which corresponds
to the lower solubility and higher melting point of the
former.
BFMPT form II upon cooling demonstrated a structured
PL spectrum, and the PL QY sharply increased up to 19%
(Fig. 6 and Table 1). The same procedure for form I did
not result in the improvement of the PL. This is assigned
to the different crystal packing, defects, intermolecular
interactions and rigidity of the molecular environment in
the studied polymorphs. Therefore, the structural ordering
achieved by the crystallization with the additive not only
improved the crystal quality and enhanced the
intermolecular interactions, but also improved the PL
performance of the C8-BFMPT crystals and their response
to the cooling.
Conclusions
To further reveal the optical properties of the studied
compounds, we measured the PL of the C2-BFMPT and
C8-BFMPT crystals. C2-BFMPT demonstrated a red-shifted
PL maximum, as compared to that in solution and a PL
quantum yield (PL QY) of 5%. We were unable to
measure any meaningful PL signal from the neat-
crystallized C8-BFMPT form I sample, which is assigned to
the high structural disorder, defects within the sample
and the scattering effect (Fig. 6f, olive). On the other
hand, the crystals of form II grown by additive method
demonstrated a PL QY of 2% and red-shifted PL maxima,
as compared to that in solution. The PL efficiency of the
alkyl-substituted BFMPTs is higher than that in solution
(Table 1), but significantly lower than that of the
unsubstituted BFMPT.44 Therefore, the weakening of the
intermolecular interactions in the alkyl-substituted BFMPTs
deteriorates the PL efficiency in the studied crystalline
materials. The longer the alkyl substituent is, the less
ordered material and the lower PL QY obtained. However,
by the additive-assisted crystallization, we improved the
In summary, we studied the effect of the alkyl-substituents
and additive-assisted crystallization on the structure and
optical properties of the AIE-active bis(4-((9H-fluoren-9-
ylidene)methyl)phenyl)thiophenes. The alkylated 2,5-bis(4-
((2,7-dialkyl-9H-fluoren-9-ylidene)methyl)phenyl)thiophenes
were synthesized in 4 steps by acylation, reduction, cross-
coupling, and Knoevenagel condensation reactions. The
introduction of the alkyl groups resulted in the weakening of
intermolecular interactions and decreasing the crystal
density, melting points. The solubility of the materials also
increased upon the introduction of the alkyl groups. To
improve the crystal quality of the octyl-containing derivative,
we applied an additive-assisted crystallization strategy that
resulted in the polymorphism of C8-BFMPT evidenced from
the thermal analysis and X-ray diffraction data. C–H⋯π
intermolecular interactions with an extensive disorder were
revealed for both derivatives, with the crystal density being
reduced upon the elongation of the alkyl groups. Only half of
the C8-BFMPT molecular backbones were demonstrated to
participate in the intermolecular C–H⋯π interactions. Both
derivatives demonstrated an AIE effect with a negligible
photoluminescence (PL) quantum yield (QY) in solution, and
a PL QY of 5% for C2-BFMPT and 2% for C8-BFMPT form II.
In contrast to the neat crystallized form I, cooling of the C8-
BFMPT form II resulted in a 10-fold increase of the PL QY.
The introduction of alkyl groups and the additive-assisted
crystallization are highlighted as powerful tools for
manipulation by the crystal structure, morphology and
optical properties of the materials.
crystal
quality
and
enhanced
the
intermolecular
interactions in C8-BFMPT form II, resulting in the
increase of PL QY (no emission for form I vs. measurable
signal for form II). To further reveal the PL behavior of
the studied compounds and the effect of the rigidification
of environment on the photophysical properties, we have
studied the PL of crystals under cooling by liquid
nitrogen. C2-BFMPT did not show any significant
influence of cooling on PL QY, although the small blue
shift of PL was observed. However, the PL spectra of C8-
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Conflicts of interest
21 P. Srujana, P. Sudhakar and T. P. Radhakrishnan, J. Mater.
Chem. C, 2018, 6, 9314–9329.
22 J. Gierschner and S. Y. Park, J. Mater. Chem. C, 2013, 1,
5818–5832.
There are no conflicts to declare.
Acknowledgements
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Goto, W. Lee, Y. Kim, J.-B. Arlin, J. Karpinska, A. R. Kennedy,
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This work was supported by the State Assignment of Ministry
of Science and Higher Education of Russian Federation,
project #0238-2019-0004. The authors thank the XRD Facility
of NIIC SB RAS for the data collection. Powder diffraction
experiments were carried out at the Research and Education
Centre of the Novosibirsk State University REC-008. The
assistance of Dr. Evgeny Losev is gratefully acknowledged.
The authors acknowledge the Multi-Access Chemical Service
Center SB RAS for the spectral and analytical measurements.
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