Multiple photoluminescence of spiro[acridine-fluorene]-based: O -carboranyl compounds with potential as a visual sensory material

Article abstract of DOI:10.1039/d0tc03801f

Two spiro[acridine-9,9′-fluorene]-based closo-o-carboranyl compounds, namely p-SAC and o-SAC, were prepared and fully characterized. p-SAC exhibited a weak high energy emission trace only in tetrahydrofuran (THF) at 298 K, while the photoluminescence (PL) spectra at 77 K exhibited intense emission in the low energy region. However, o-SAC exhibited an excellent dual-emissive pattern in THF at both 298 and 77 K. The electronic transition in each excited state (S1) was calculated, which confirmed that the high and low energy emission originated from locally excited (LE) states on the fluorene moieties and intramolecular charge-transfer (ICT) transitions corresponding to o-carboranes, respectively. All these characteristics indicated that ICT-based radiative decay was only favored in the rigid state, where structural fluctuations were restricted. Energy barriers were calculated based on relative energies at various dihedral angles around the o-carborane cages in p-SAC and o-SAC. The rotational motion of the o-carborane cage was less restricted in p-SAC when compared to o-SAC, resulting in suppression of the ICT-based emission when p-SAC was in solution. The PL experiments in the THF/water mixtures indicated that these features were caused by the aggregation-induced emission (AIE) effect. An acetonitrile solution containing relatively high concentrations of o-SAC (ca. 10-3 M) exhibited a dramatic emission color change from deep red to sky blue when the temperature was increased. The higher temperature caused a natural conversion from a colloidal state (slightly aggregated) to a clear solution. Consequently, the photophysical features of p-SAC and o-SAC demonstrated the application potential of π-aromatic conjugated o-carboranyl compounds as visual sensory materials. This journal is

Full text of DOI:10.1039/d0tc03801f

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JPolueransael odfoMnaotetraiadljsuCsht emmairsgtriynsC
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DOI: 10.1039/D0TC03801F
Journal of Materials Chemistry C
PAPER
Multiple Photoluminescence of Spiro[acridine-fluorene]-based o-
Carboranyl Compounds and Potential as a Visual Sensory Material
Min Sik Mun,‡a Chan Hee Ryu,‡a Hyunhee Soa, Mingi Kima, Ji Hye Leea, Hyonseok Hwanga, and Kang
Received 00th January 20xx,
Accepted 00th January 20xx
Mun Lee*a
DOI: 10.1039/x0xx00000x
Two spiro[acridine-9,9'-fluorene]-based closo-o-carboranyl compounds, namely p-SAC and o-SAC, were prepared and fully
characterized. p-SAC exhibited a weak high energy emission trace only in tetrahydrofuran (THF) at 298 K, while the
photoluminescence (PL) spectra at 77 K exhibited intense emission in the low energy region. However, o-SAC exhibited an
excellent dual-emissive pattern in THF at both 298 and 77 K. The electronic transition in each excited state (S1) was calculated,
and confirmed that the high and low energy emissions originated from locally excited (LE) states on the fluorene moieties
and intramolecular charge-transfer (ICT) transitions corresponding to the o-carboranes, respectively. All these
characteristics indicated that ICT-based radiative decay was only favored in the rigid state, where structural fluctuations
were restricted. Energy barriers were calculated based on relative energies at various dihedral angles around the o-
carborane cages in p-SAC and o-SAC. The rotational motion of the o-carborane cage was less restricted in p-SAC than o-SAC,
resulting in suppression of the ICT-based emission when p-SAC was in solution. The PL experiments in the THF/water
mixtures indicated that these features were caused by the aggregation-induced emission (AIE) effect. An acetonitrile
solution containing relatively high concentrations of o-SAC (ca. 10−3 M) exhibited a dramatic emission color change from
deep red to sky blue when the temperature was increased. The higher temperature caused a natural conversion from a
colloidal state (slightly aggregated) to a clear solution. Consequently, the photophysical features of p-SAC and o-SAC
demonstrated the application potential of π‒aromatic conjugated o-carboranyl compounds as visual sensory materials.
www.rsc.org/
report found that this ICT process in D‒A conjugated systems
can be controlled using strategic molecular geometry design.
Fox et al. reported that D‒A-type C-diazaboryl-o-carboranes
Introduction
Closo-carborane (C2B10H12) is a well-known icosahedral boron-
cluster component,1 3 and is considered a three-dimensional
(3D) variant of benzene. The high electronic and thermal
stability of closo-carborane has led to its extensive use as a 3D
analogue molecule in a wide variety of novel materials, such as
boron neutron capture therapy (BNCT) medications,4 7 novel
ligands of supramolecular building blocks,8 and promising
exhibited two different excited states depending on the
rotational motion of the diazaboryl derivatives.40 The twisted
intramolecular charge-transfer (TICT) states within o-
carboranes comprising various fluorescent donor groups lead to
‒
‒
multiple photoluminescence emission.47 56 Furthermore, our
‒
group recently reported that the structure of the molecular
scaffold itself, particularly the planarity of the π-aromatic
groups, played a dominant role in efficiently adjusting the
radiative decay of the locally excited (LE) states centered at the
π-aromatic groups and ICT transition corresponding to the o-
carborane moiety.44,45
‒14
optoelectronic materials.15,16 In particular, π-conjugated
aromatic group assembled closo-ortho-carborane (1,2-C2B10H12)
has exhibited unique photophysical properties due to strong
electron withdrawal via C-substitution and high polarizability of
‒
its σ-aromaticity.17 56 The electronic features of o-carborane
The effect of molecular structure on the intramolecular
electronic transitions of o-carborane D‒A conjugate systems
has been validated based on the aggregation-induced emission
(AIE) phenomenon. The radiative decay in the conjugates due
ICT transition is generally suppressed in the solution state at
ambient temperature due to numerous molecular motions
around the o-carborane cages (e.g. rotational motion of the o-
carborane cage and C‒C bond variation). However, the radiative
efficiency of the ICT emission is dramatically enhanced in an
aggregated solid state due to severe restriction of the molecular
lead to intramolecular charge-transfer (ICT) transitions between
‒
the π-conjugated aromatic moiety and o-carborane cage,21 46
resulting in the formation of an advantageous donor (D,
aromatic group)‒acceptor (A, o-carborane) system. A recent
a.Department of Chemistry, Institute for Molecular Science and Fusion Technology,
Kangwon National University, Chuncheon, Gangwon 24341, Republic of Korea. E-
mail: kangmunlee@kangwon.ac.kr
† Electronic Supplementary Information (ESI) available: 1H, 13C and 11B NMR spectra,
X-ray crystallographic data in CIF format (CCDC 2008760 for o-SAC), emission decay
curves, and computational details. For crystallographic data in CIF or other
electronic format. See DOI: 10.1039/b000000x/.
‒
structure.18,46,57 67 Further, the AIE of o-carborane conjugates
can also be tuned by changing the number and position of
substituents on the o-carborane moiety.66,67
‡ The first and second authors contributed equally to this work
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
Journal Name
The effects of structural features and molecular geometry on acid, saturated aqueous hydrochloric acid solution, magnesium
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the photophysical properties of o-carboranyl D‒A conjugated sulfate (MgSO4), sodium chloride (NaCl)D, OsoI:d10iu.1m039b/Dic0aTrCb0o3n8a0t1eF
systems have been previously reported. These studies provide (NaHCO3), 1.6
strong evidence that emissive characteristics can be controlled ethynylbenzene,
M
n-butyllithium in hexane (n-BuLi),
bis(triphenylphosphine)palladium(II)
via strategic molecular design. Specifically, a unique structural dichloride (Pd(PPh3)2Cl2), copper(I) iodide (CuI), diethyl sulfide
formation of appended π-aromatic groups on the o-carborane (Et2S), poly(methyl methacrylate) (PMMA)), and Alfa Aesar
shows promise. By extension, the previous findings (decaborane (B10H14) and basic alumina). Deuterated solvent
demonstrated the promise of these conjugate systems as novel (chloroform-d1 (CDCl3), and dichloromethane (CD2Cl2)) was
candidates for single-molecule visual sensing materials with a obtained from Cambridge Isotope Laboratories and was dried
prompt emissive response. Therefore, the structure of over activated molecular sieves (5 Å). Nuclear magnetic
molecular scaffolds appended to the o-carborane played a resonance (NMR) spectra were recorded on a Bruker Avance
significant role in embodying the o-carborane based sensory 400 spectrometer (400.13 MHz for 1H{11B}, 100.62 MHz for 13C,
materials, showing remarkable photophysical characteristics. In and 128.38 MHz for 11B{1H}) at ambient temperature. Chemical
line with this concept, we have a constant interest in shifts are given in ppm, and are referenced against external
spirobifluorene derivatives as one of the mother scaffolds for o- tetramethyl tin(IV) (Me4Si) (1H{11B} and 13C) and/or BF3·Et2O
carborane based materials possessing intriguing photophysical (11B{1H}). Elemental analyses, high resolution (HR)-mass
features, because these D‒A conjugated systems apparently spectroscopies, and dynamic light scattering (DLS)
exhibited both strong ICT-based emission and AIE measurements were performed on an EA3000 instrument
phenomenon.56
(EuroVector), JMS-700 (JEOL), and Mastersizer 3000 (Malvern
To further investigate such an intrinsic concept for the o- Instrument) at the Central Laboratory of Kangwon National
carboranyl-based D-A type conjugated systems, herein we University, respectively. 10-Phenyl-10H-spiro[acridine-9,9'-
explore whether the spirobifluorene-derivative-based o- fluorene] (SA) and both bromo-precursors (p-BrSA and o-BrSA
)
carboranyl conjugates can exhibit complex multiple were prepared following previously reported procedures.68 The
photoluminescence behavior, depending on their structural detailed synthesis procedures and spectroscopic data for SA, p-
properties, which can be useful in the design of a novel visual- BrSA and o-BrSA are given in the ESI†.
sensing material. The synthesis and photophysical properties of
General synthetic method for acetylene precursors
two 10-phenyl-10H-spiro[acridine-9,9'-fluorene]-based o-
carboranyl compounds, namely p-SAC and o-SAC (Scheme 1), The acetylene precursors (p-SAE and o-SAE) were synthesized
were evaluated. p-SAC and o-SAC were produced by using an adequate amount of starting materials. A cannula was
introducing o-carboranes to the para- or ortho- positions of used to add toluene and NEt3 (v/v, 2/1, 15 mL) to the mixture of
each fluorene moiety, respectively. This was the first report of bromo-precursors, CuI, and Pd(PPh3)2Cl2 at 25 °C. The reaction
spiro[acridine-9,9'-fluorene] possessing with o-carborane mixture was stirred for 10 min and ethynylbenzene (1.3 equiv.
groups. This study investigated the relationship between the per bromo-precursor) was added to the resulting dark brown
rotational motion of the o-carborane cages and the radiative slurry. The reaction mixture was refluxed at 110 °C for 24 h. The
decay efficiency of the ICT-based emissions. Further, an insight volatiles were removed via rotary evaporation to produce a
into the strategic directivity of using carboranyl conjugates as dark brown residue. The remaining solid was washed
visual sensory materials was provided. Detailed synthetic sequentially with MeOH and n-hexane, and purified by column
procedures, characterization, optical properties (with chromatography on silica (DCM/n-hexane = 1/8, v/v) to produce
theoretical calculations), and the application of the o- white solid acetylene precursors.
carboranyl conjugates to sensing materials have been described.
Data for p-SAE
p-BrSA (0.98 g, 2.0 mmol), CuI (34 mg, 0.18 mmol), PdCl2(PPh3)2
Experimental
(0.12 g, 0.18mmol), and ethynylbenzene (0.28 mL, 2.6 mmol)
1
afforded p-SAE as a white solid. Yield = 50 % (0.50 g). H NMR
General considerations
(CDCl3): δ 7.81 (dd, J = 7.8, 2.4 Hz, 2H), 7.72 (t, J = 7.6 Hz 2H),
All experiments were carried out under an inert N2 atmosphere
using standard Schlenk and glove box techniques. Anhydrous
solvents (toluene, THF, n-hexane, diethyl ether, methanol, and
trimethylamine (NEt3); Sigma-Aldrich) were dried by passing
each solvent through an activated alumina column. The solvents
were stored over activated molecular sieves (5 Å).
7.59 (t, J = 8.4 Hz, 3H) 7.50 (m, 4H), 7.44 (d, J = 8.2 Hz, 1H), 7.40
(td, J = 7.6, 2.2 Hz, 1H), 7.32 (m, 4H), 6.94 (td, J = 8.5, 1.6 Hz, 2H),
6.59 (td, J = 8.4, 2.0 Hz, 2H), 6.43 (dd, J = 7.8, 1.5 Hz, 2H), 6.37
(dd, J = 8.4, 0.9 Hz, 2H). 13C NMR (CDCl3): δ 157.29, 156.83,
141.33, 141.09, 139.65, 138.47, 131.62, 131.42, 131.30, 131.20,
129.30, 128.95, 128.59, 128.43, 128.27, 128.00, 127.81, 127.45,
125.98, 124.21, 123.38, 122.89, 120.67, 120.28, 119.98, 114.85,
90.16 (acetylene-C), 89.85 (acetylene-C), 56.85 (spiro-C). Anal.
Calcd for C39H25N: C, 92.28; H, 4.96; N, 2.76. Found: C, 91.96; H,
4.88; N, 2.54.
Spectrophotometric-grade
solvents
(toluene,
THF,
dichloromethane (DCM), acetone, acetonitrile (ACN), and
ethanol and n-hexane) were used as received from Alfa Aesar.
Commercial reagents were used as received from Sigma-Aldrich,
including 2-bromo-N,N-diphenylaniline, 9H-fluoren-9-one, 2-
bromo-9H-fluoren-9-one, 4-bromo-9H-fluoren-9-one, acetic
Data for o-SAE
2 | J. Name., 2012, 00, 1-3
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o-BrSA (1.1 g, 2.3 mmol), CuI (34 mg, 0.18 mmol), PdCl2(PPh3)2 (CD2Cl2): δ 160.11, 158.70, 141.06, 140.81, 140.61, 136.51,
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(0.12 g, 0.18mmol), and ethynylbenzene (0.32 mL, 3.0 mmol) 135.70, 131.18, 131.04, 130.88, 130.64, 1D30O.I2: 180,.1102399./4D30,T1C20398.1001F,
1
afforded o-SAE as a white solid. Yield = 38 % (0.43 g). H NMR 128.81, 128.61, 128.22, 127.69, 127.57, 127.29, 126.58, 126.52,
(CD2Cl2): δ 8.65 (d, J = 7.6 Hz, 1H), 7.71 (m, 4H), 7.60 (d, J = 8.0 125.72, 124.17, 120.41, 114.91, 90.18 (CB-C), 89.87 (CB-C),
Hz, 1H), 7.55 (dd, J = 8.2, 2.4 Hz, 1H), 7.50 (m, 2H), 7.45 (m, 4H), 55.89 (spiro-C). 11B{1H} NMR (CD2Cl2): δ −2.37 (br s, 3B), −3.38
7.41 (m, 2H), 7.33 (td, J = 7.7, 1.1 Hz, 1H), 7.27 (t, J = 7.6 Hz, 1H), (br s, 2B), −9.74 (br s, 2B), −11.03 (br s, 3B). Anal. Calcd for
6.92 (td, J = 8.6, 1.6 Hz, 2H), 6.55 (td, J = 7.6, 1.2 Hz, 2H), 6.37 (d, C39H35B10N: C, 74.85; H, 5.64; N, 2.24. Found: C, 74.75; H, 5.58;
J = 8.0 Hz, 2H), 6.5 (d, J = 7.8 Hz, 2H). 13C NMR (CD2Cl2): δ 157.29 , N, 2.08. HR-mass (ESI): m/z Calcd for C39H35B10N: 627.3700.
157.01 , 141.40 , 140.95 , 139.06 , 138.89 , 132.02 , 131.61 , Found: 627.3703.
131.17 , 131.12 , 128.89 , 128.71 , 128.66 , 128.58 , 127.97 ,
127.77 , 127.55 , 127.39 , 125.79 , 125.45 , 124.32 , 123.28 ,
Photophysical Measurements
122.85 , 120.48 , 116.58 , 114.88 , 93.75 (acetylene-C), 88.35 UV/Vis absorption and PL spectra of p-SAC and o-SAC were
(acetylene-C), 56.40 (spiro-C). Anal. Calcd for C39H25N: C, 92.28; acquired
using
V-530
(Jasco)
and
Fluoromax-4P
H, 4.96; N, 2.76. Found: C, 92.01; H, 4.89; N, 2.62.
spectrophotometers (HORIBA), respectively. The solution
UV/Vis absorption and PL measurements were performed at
298 K in degassed THF with a 1-cm quartz cuvette (5.0 × 10−5 M).
PL measurements were also carried out under various
General synthetic method for spiro[acridine-9,9'-fluorene] based
o-carborane compounds
An excess amount of diethyl sulfide (Et2S, 5 equiv. of acetylene conditions, including in different solvents (toluene, DCM, ACM,
moiety) was added to a toluene solution (10 mL) of decaborane and THF/distilled water mixtures (5.0 × 10−5 M)), in THF solution
(B10H14, 1.2 equiv. per acetylene moiety) and acetylene (5.0 × 10−5 M) at 77 K, and in the film state at 298 K. Thin films
precursors at 25 °C. The reaction mixture was heated to reflux of p-SAC and o-SAC in PMMA were obtained by spin-coating a
and stirred for 24 h. The solvent was removed under vacuum THF solution (1 mL) of PMMA (50 mg) containing either o-
and methanol (5 mL) was added. The resulting yellow solid was carborane compound (5 wt% versus PMMA) on a 10 × 10-mm
filtered and redissolved in toluene. The solution was purified by quartz plate (thickness
=
1
mm). The absolute
passing through a basic alumina column, and the solvent was photoluminescence quantum yields (PLQYs) in THF solution (5.0
removed in vacuo. The crude product was purified via × 10−5 M) and the film state, and CIE 1931 color coordinates of
recrystallization from n-hexane to produce the two o-carborane o-SAC in acetonitrile (7.0 × 10−4 M) were obtained at 298 K using
compounds as white solids.
a 3.2 inch integrating sphere (FM-sphere, HORIBA) equipped on
a Fluoromax-4P spectrophotometer (HORIBA). Fluorescence
decay lifetimes in THF solution (5.0 × 10−5 M) and in the film
Data for p-SAC
p-SAE (0.25 g, 0.50 mmol), B10H14 (73 mg, 0.60 mmol), and Et2S state were measured at 298 K using a time-correlated single-
(0.27 mL, 2.5 mmol) afforded p-SAC as a white solid. Yield = 38 % photon counting (TCSPC) spectrometer (FLS920–EDINBURGH
1
(0.12 g). H{11B decoupled} NMR (CDCl3): δ 7.80 (t, J = 7.4 Hz, instrument at the Central Laboratory of Kangwon National
2H), 7.67 (m, 3H), 7.60 (d, J = 8.0 Hz, 2H), 7.54 (m, 2H), 7.47 (d, University) equipped with a EPL-375ps pulsed semiconductor
J = 8.0 Hz, 1H), 7.35 (t, J = 7.5 Hz, 1H), 7.29 (d, J = 4.2 Hz, 3H), diode laser excitation source and a microchannel plate
7.18 (t, J = 8.0 Hz, 1H), 7.02 (m, 4H), 6.58 (t, J = 6.8 Hz, 2H), 6.46 photomultiplier tube (MCP-PMT, 200–850 nm) detector.
(d, J = 8.3 Hz, 2H), 6.16 (d, J = 8.8 Hz, 2H), 3.26 (br s, 2H, CB-BH),
X-ray crystallography
2.54 (br s, 5H, CB-BH), 2.35 (br s, 3H, CB-BH), 2.22 (br s, 1H, CB-
BH). 13C NMR (CDCl3): δ 156.55, 155.30, 141.90, 141.49, 140.93, Single o-SAC crystals were grown for X-ray diffraction from DCM
136.96, 131.29, 131.18, 130.68, 130.49, 130.34, 130.06, 129.42, and methanol mixtures. The single crystals were coated with
128.72, 128.70, 128.29, 127.87, 127.55, 126.81, 125.61, 124.27, paratone oil and mounted onto
a
glass capillary.
120.82, 120.66, 119.59, 114.86, 86.14 (CB-C), 85.54 (CB-C), Crystallographic measurements were performed using a Bruker
57.10 (spiro-C). 11B{1H decoupled} NMR (CDCl3): δ −3.19 (br s, D8QUEST CCD area detector diffractometer with graphite-
3B), −10.03 (br s, 3B), −11.38 (br s, 4B). Anal. Calcd for monochromated Mo-Kα radiation (λ = 0.71073 Å ). The structure
C39H35B10N: C, 74.85; H, 5.64; N, 2.24. Found: C, 74.77; H, 5.49; was assessed using direct methods, and all nonhydrogen atoms
N, 2.10. HR-mass (ESI): m/z Calcd for C39H35B10N: 627.3700. were subjected to anisotropic refinement by a full-matrix least-
Found: 627.3704.
squares method on F2 by using the SHELXTL/PC software
package. The X-ray crystallographic data of o-SAC was
presented in CIF format. CCDC‒2008760 contains the
Data for o-SAC
o-SAE (0.25 g, 0.50 mmol), B10H14 (73 mg, 0.60 mmol), and Et2S supplementary crystallographic data for this paper, and the
(0.27 mL, 2.5 mmol) afforded p-SAC as a white solid. Yield = 29 % data were provided free of charge by The Cambridge
1
(0.09 g). H{11B decoupled} NMR (CD2Cl2): δ 8.78 (d, J = 8.2 Hz, Crystallographic Data Centre. Hydrogen atoms were placed at
1H), 7.94 (d, J = 8.1 Hz, 1H), 7.69 (t, J = 7.3 Hz, 2H), 7.57 (t, J = their geometrically calculated positions and refined using a
8.0 Hz, 1H), 7.40 (t, J = 9.2 Hz, 5H), 7.26 (m, 4H), 7.10 (m, 3H), riding model on the corresponding carbon atoms with isotropic
6.85 (t, J = 7.8 Hz, 2H), 6.43 (t, J = 7.4 Hz, 2H), 6.27 (d, J = 8.1 Hz, thermal parameters. The detailed crystallographic data are
2H), 5.76 (d, J = 7.8 Hz, 2H), 3.78 (br s, 2H, CB-BH), 3.05 (br s, 2H, given in Tables S1† and S2†.
CB-BH), 2.62 (br s, 2H, CB-BH), 2.52 (br s, 4H, CB-BH). 13C NMR
This journal is © The Royal Society of Chemistry 20xx
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Computational calculation studies
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DOI: 10.1039/D0TC03801F
The geometries of p-SAC and o-SAC in their ground (S0) and first-
excited (S1) states in THF were optimized at the B3LYP/6-
31G(d,p)69 level of theory. Vertical excitation energies in the
optimized S0 and S1 state geometries were calculated using the
TD-DFT method70 at the same level of theory. Solvent effects
were evaluated using the self-consistent reaction field (SCRF)
based on the integral equation formalism of the polarizable
continuum model (IEFPCM) with THF as the solvent.71 The most
stable geometries were determined by constructing one-
dimensional potential energy surfaces (PESs) as a function of
dihedral angle (Ψ) by rotating the o-carborane cage vide infra
(Fig. 5) between ca. −180 and 180 at intervals of 30° to give a
Scheme 1 Synthetic routes for the formation of spiro[acridine-9,9'-fluorene]-based o-
carboranyl compounds (p-SAC and o-SAC). Reagents and conditions: (i) Ethynylbenzene,
CuI, PdCl2(PPh3)2, toluene/NEt3, 110 °C, 24 h. (ii) B10H14, Et2S, toluene, 110 °C, 24 h.
total of 13 initial conformations for each compound.
Conformations that exhibited physically impossible atomic
overlaps were excluded from further geometry optimization.
The dihedral angle was fixed while the other geometrical
variables were fully relaxed for geometry optimization and
energy calculation of the resulting initial conformations using
the Gaussian 16 software program.72 The contribution (%) of a
group in a molecule to each molecular orbital was calculated
using the GaussSum 3.0 software program,73 while GaussView 6
was used to visualize the molecular properties.74
Results and discussion
Synthesis and characterization
The synthetic routes for two10-phenyl-10H-spiro[acridine-9,9'-
fluorene]-based compounds, namely p-SAC and o-SAC, with o-
carborane cages appended at the para- or ortho- positions of
each fluorene moiety, respectively, are given in Scheme 1. The
bromo-precursors (p-BrSA and o-BrSA
) were prepared
according to previously reported procedures.68 The mother-
scaffold structure, namely 10-phenyl-10H-spiro[acridine-9,9'-
fluorene] (SA), was synthesized,68 and its optical properties
were compared with p-SAC and o-SAC (see experimental
section for detailed schemes). Sonogashira couplings between
ethynylbenzene and either p-BrSA or o-BrSA led to the
formation of p-SAE and o-SAE, respectively, with moderate
yields (50 and 23%, respectively). p-SAC and o-SAC were
prepared via a cage forming reaction between decaborane
(B10H14) and either p-SAE or o-SAE in the presence of Et2S,
respectively (Scheme 1).75‒77
Fig. 1 X-ray crystal structure of o-SAC (50% thermal ellipsoids), where the solvents in the
unit cell and H atoms have been omitted for clarity. (a) Single molecule of o-SAC and (b)
distorted structure of o-SAC.
The precursors and o-carboranyl compounds were fully
characterized using multinuclear nuclear magnetic resonance
(NMR) spectroscopy, HR-mass spectroscopies (Figs. S1–S11 in
the ESI†), and elemental analysis. The 11B-decoupled 1H (1H{11B})
and 13C NMR spectra of p-SAC and o-SAC exhibited resonances
attributed to the aromatic groups, namely spiro[acridine-9,9'-
fluorene] moieties and terminal phenyl groups, as expected.
1
Several broad peaks between −2 and −12 ppm in the H{11B}
NMR spectra and two peaks around 90 to 80 ppm in the 13C
NMR spectra clearly indicated the presence of B‒H bonds (total
of 10 H atoms) and carbon atoms in the closo-o-carborane cages.
Further, the single peak at ca. 56 ppm in the 13C NMR spectra of
both compounds
Fig. 2 UV-vis absorption and PL spectra of (a) p-SAC (λex = 313 nm) and (b) o-SAC (λex
=
321 nm). Black: absorption spectra in THF solution (5.0  105 M); blue: PL spectra in THF
solution (5.0  105 M) at 298 K; green: PL spectra in THF solution (5.0  105 M) at 77 K;
and orange: PL spectra in a film (5 wt% doped with PMMA) at 298 K. Inset figures show
the emission color in each state under irradiation of a hand-held UV lamp (λex = 295 nm).
4 | J. Name., 2012, 00, 1-3
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View Article Online
DOI: 10.1039/D0TC03801F
Table 1 The photophysical properties of spiro[acridine-9,9'-fluorene]-based o-carboranyl compounds (p-SAC and o-SAC)
f
kre
/ 107 s−1
knr
λabsa/nm
d
λem /nm
ACNb
Φem
THFb
τobs /ns
λex
/nm
/ 108 s−1
(ε × 10−3
M−1 cm−1)
285 (30.0),
313 (23.0)
Tolb
386
THFb
388
77 Ka
469
filmc
428 (sh),
520
filmc
0.05
THFb
filmc
4.0
THFb
filmc
THFb
filmc
p-
SAC
g
313
321
389
<0.01
-
-
1.3
-
2.5
391
(sh),
461
393
(sh),
445
o-
SAC
278 (20.7),
321 (13.0)
440 (sh),
524
418
491
0.06
0.19
4.6
3.2
1.3
5.9
2.0
2.5
f
a 5.0 × 10−5 M in THF. b 5.0 × 10−5 M, observed at 298 K c Measured in the film state (5 wt% doped on PMMA). d Absolute PL quantum yield. e kr = Φem /τobs
knr = kr(1/Φem−1).
.
Fig. 3 Frontier molecular orbitals of p-SAC and o-SAC in their ground states (S0) and the first excited singlet states (S1), with relative energies from DFT calculations (isovalue = 0.04).
Transition energy (in nm) was calculated using the TD-B3LYP method with 6-31G(d) basis sets.
This observation suggested that the structural variation around
Table 2 Major low-energy electronic transitions for p-SAC and o-SAC in their
ground states (S0) and first excited singlet states (S1) calculated using the TD-B3LYP
method with 6-31G(d) basis setsa
the o-carborane moiety of p-SAC was much larger than in o-SAC
.
The structural variation behavior of o-SAC was evaluated based
on the molecular structure of o-SAC in the solid state, as
determined using X-ray crystallography (Fig. 1). The detailed
parameters, including selected bond length and angles, are
provided in Tables S1† and S2†. The structure of o-SAC
comprised a tetrahedral carbon center (average bond angle
around C15 = 109.37°, Fig. 1 and Table S2†) in the spirofluorene
moiety with an o-carborane cage. The nitrogen atom in the
acridine moiety adopted a perfect trigonal planar geometry, as
evidenced by the sum of the three C−N−C angles (ΣC−N−C = 359.9°
= 120.8°+ 120.0° + 119.1°), resulting in sp2-orbital hybridization.
These structural features suggested that the entire 10-phenyl-
state λcalc/ nm
fcalc
Assignmentb
336.44
S0
0.0639
0.3799
H → L (98.6%)
H−1 → L (94.9%)
p-SAC
o-SAC
305.21
475.68
S1
0.3534
0.2903
H → L (99.9%)
H−1 → L+1 (99.3%)
392.94
338.66
S0
0.1227
0.0895
H → L (98.3%)
H−1 → L (97.1%)
298.70
482.81
S1
0.1605
0.2143
H → L (99.9%)
H → L+1 (94.6%)
394.27
10H-spiro[acridine-9,9'-fluorene]
component
become
a
electronically delocalized. The fluorene group linked to the o-
carborane cage in o-SAC was significantly distorted, where the
angles between the two phenyl rings was 12.9° (Fig. 1b). These
structural features were attributed to severe steric hindrance
around the carborane cage in o-SAC, indicating that structural
variation centered at the cage was very restricted.
Singlet energies for the vertical transition calculated at the optimized S1
geometries. b H = HOMO; L = LUMO.
confirmed the presence of quaternary carbon atoms. All of the
1H-decoupled 11B NMR (11B{1H}) spectral peaks were integrated
to a total of 10 boron atoms. The peak spanning −2 to −4 ppm
in the p-SAC spectrum was broadly detected as a single peak
(−3.19 ppm, 3 B atoms), but was represented by two peaks in
the o-SAC spectrum (−2.37 and −3.38 ppm, 3 and 2 B atoms).
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DOI: 10.1039/D0TC03801F
Fig. 4 (a) UV−vis absorption and (b) PL spectra of o-SAC (5.0 × 10−5 M) in various organic solvents. The emission color in each state under irradiation of a hand-held UV lamp (λex
=
295 nm) are given in the insets. (c) Stokes shift as a function of empirical solvent-polarity parameter [ET(30)].
Table 3 Length (Å) of the C‒C bond in o-carborane, and the dihedral angles (Ψ, °) around
the o-carboranyl cage in p-SAC and o-SAC
Theoretical calculation of photophysical properties in solution
states
The photophysical properties of p-SAC and o-SAC were
investigated using ultraviolet-visible (UV-vis) absorption and
photoluminescence (PL) spectroscopy (Fig. 2 and Table 1).
Prominent absorption bands centered at 285 nm in the p-SAC
spectrum and 278 nm in the o-SAC spectrum were attributed to
vibronic structures undergoing spin-allowed π‒π* transitions
within the spiro[acridine-9,9'-fluorene] moiety. SA exhibited
similar strong absorption peaks in this region (Fig. S12†, λabs
=
p-SAC
o-SAC
Calc.a
S0 state
268 nm (ε = 11,500 M−1 cm−1), 297 nm (8,600 M−1 cm−1)). Low-
energy absorption between 310 and 340 nm with substantial
tailing to 350 nm was observed. The broad absorption features
were attributed to typical ICT transitions in the o-carborane
cages. Low-energy absorption bands were not observed in
thespectrum of SA (Fig. S12†). The UV-vis spectral properties
were highly consistent with the theoretical calculations (vide
infra).
Calc.a
Exp.b
S0 state
S1 state
2.39
S1 state
2.42
C‒C bond /
1.77
1.76
93.7
1.84
91.5
Å
Ψ /°
91.8
90.6
92.0
a
Calculated values based on the ground (S0) and first excited singlet state (S1)
optimized structures. b Experimental values based on the X-ray crystal structure.
The origin of the electronic transitions within p-SAC and o-SAC
were determined using time-dependent density functional
theory (TD-DFT) calculations. The ground (S0) and first-excited
(S1) states of the compounds were optimized at the B3LYP/6-
31G(d) level of theory (Fig. 3 and Table 2). The calculated
structure of o-SAC was based on its solid-state structure. An
integral equation formalism of the polarizable continuum
model (IEFPCM) was applied to account for the effect of the
tetrahydrofuran (THF) solvent.71 The S0 state calculations
indicated that the lowest-energy electronic transitions for p-
SAC and o-SAC were mainly associated with highest occupied
molecular orbital (HOMO) and transitions from the HOMO−1 to
the lowest unoccupied molecular orbital (LUMO) (Fig. 3 and
Table 2). The LUMO on the fluorene moieties was typically
occupied (63% in p-SAC and 64% in o-SAC, Tables S4† and S6†),
as well as a substantial proportion of the o-carborane units (29%
in p-SAC and 27% in o-SAC). However, the HOMO and HOMO−1
of both compounds were predominantly localized on the
triphenylamine (>96% in p-SAC and o-SAC, Tables S4† and S6†)
and fluorene groups (>77% in p-SAC and o-SAC), respectively.
Thus, the lowest-energy electronic transition (HOMO → LUMO)
of p-SAC and o-SAC was attributed to ICT transitions from the
triphenylamine to the fluorene and o-carborane units.
Furthermore, the intense absorption bands centered at ca. 280
nm were mainly attributed to fluorene centered π‒π* local
transitions.
The emission properties of both o-carboranyl compounds
were evaluated based on PL measurements under various
conditions (Fig. 2 and Table 1). The PL spectra of p-SAC in THF
at 298 K exhibited a faint emissive trace in the high-energy
region at 388 nm. SA also exhibited emission in this region (λem
= 387 nm, Fig. S12†), strongly suggesting that emission in this
region corresponded to the π‒π* transition on the
spiro[acridine-9,9'-fluorene] moiety, and was thus classified as
locally excited (LE) emission. The emission only shifted from 386
to 389 nm due to changes in solvent polarity when toluene was
replaced with acetonitrile (ACN), thereby further confirming the
occurrence of LE emission (Table 1 and Fig. S14†).
The PL spectrum of o-SAC in THF at 298 K exhibited additional
low-energy emission at 461 nm with an LE-based emission
shoulder pattern at 391 nm, resulting in blue colored emission
(inset in Fig. 2). Furthermore, PL experiments were performed
in solvents with different polarities, namely toluene,
dichloromethane (DCM), and ACN (Fig. 4). The spectra indicated
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the emissive pattern of o-SAC in THF at 77 K was characterized
View Article Online
as a 445 nm centered ICT-based emissDioOnI: 1w0.i1t0h39a/nD0eTCm0i3s8s0iv1eF
shoulder at 393 nm (Fig. 2b and Table 1), which was similar to
the observations at 298 K. It is well known that structural
variation around the o-carborane cages in solution at 298 K (e.g.
C–C bond variations or rotational motion of the cages) can
cause nonradiative decay of ICT transitions. Thus, ICT-based
emission corresponding to the o-carborane only occurred in the
rigid state.10,22,38,39,41,61 The optimized structures of p-SAC and
o-SAC based on their S0 and S1 states facilitated structural
fluctuations (Table 3). The bond lengths of the p-SAC and o-SAC
o-carborane cages in the S1 state were 2.39 and 2.42 Å,
respectively. This was much longer than in the S0 states (ca. 1.8
Å), as well as the experimentally measured value (1.76 Å) for o-
SAC based on the X-ray structure. Thus, the observed features
and previous considerations indicated that the structural
variation of o-SAC in solution at 298 K was much more
suppressed than p-SAC. The strain on the fluorene plane caused
by steric hindrance was represented in the X-ray structure of o-
SAC (Fig. 1), and further confirmed the restriction of structural
variation.
The degree of structural fluctuation around the o-carborane
cages of both carboranyl compounds was further investigated,
as well as the relationship between the radiative mechanism for
ICT-based emissions and the structural motion of the o-
carboranyl units. The energy of p-SAC and o-SAC in the S0 state
was calculated as a function of dihedral angle (Ψ) of the o-
carboranyl cage (Table 3). Calculations were based on the B3LYP
functional and the 6-31G(d) basis set. The thermodynamic
stability of each compound was represented as the relative
energy (ΔE/kcal·mol−1) in reference to the S0-optimized
structure of p-SAC and o-SAC in THF (i.e. the thermal energy of
the optimized structures were considered 0 kcal·mol−1). The
relative energy was calculated at each Ψ value from −180° to
180° at intervals of 30° (Fig. 5). The Ψ values of S0-optimized p-
SAC and o-SAC were 91.8° and 91.5°, respectively, which closely
reflected the experimental value for o-SAC based on the X-ray
structure (93.7°). Further, the values were similar to the S1 state
Ψ values, namely 90.6° for p-SAC and 92.0° for o-SAC. Thus, the
structural formation of both compounds was almost fixed
during radiative excitation and relaxation. Importantly, the
relative energy barrier for p-SAC did not exceed 2.6 kcal·mol−1
throughout the entire Ψ range (Fig. 5). However, the low energy
Fig. 5 Relative energy diagrams (ΔE/kcal·mol−1) for p-SAC and o-SAC, with relative energy
as a function of dihedral angle (Ψ) in each ground (S0) state. The structures of Ψ = −30°
and 0° in o-SAC were not possible due to intramolecular steric hindrance.
that low-energy emission was associated with the ICT transition.
The PL spectra of o-SAC exhibited solvent-dependent emission
bands that red-shifted dramatically with increasing solvent
polarity from the violet region in toluene (λem = 418 nm, inset
figure in Fig. 4b) to bluish-green in ACN (491 nm), as is
characteristic of the positive solvatochromic effect. The
emissive behavior of o-SAC was indicative that the excited
states had a significantly polar character, as confirmed by the
ICT transitions. Furthermore, solvatochromism led to a straight
positive slope in the plot of Stokes shifts (from 7217 cm−1 in
toluene to 10731 cm−1) against the empirical solvent polarity
parameter [ET(30)] (Fig. 4c).78 The large Stokes shifts confirmed
the ICT characteristics, while the computational results of the S1
state in both o-carboranyl compounds supported these origins
of each electronic transition.
The computational results for the S1 states of p-SAC and o-SAC
indicated that the high-energy emissions were dominated by
HOMO−1 (for p-SAC) or HOMO (for o-SAC) → LUMO+1
transitions (Fig. 3 and Table 2), respectively. The HOMO−1 of p-
SAC, HOMO of o-SAC, and LUMO+1 of both compounds were
significantly localized on the spiro[acridine-9,9'-fluorene]
moiety. The localization was particularly high on the fluorene
units (ca. 74–88%, Tables S4† and S6†), strongly suggesting that
the experimentally observed LEs were attributed to π−π*
transitions on the fluorene moieties. In addition, the low-energy
emissions of p-SAC and o-SAC were mainly associated with
HOMO → LUMO transitions (Fig. 3 and Table 2). The LUMOs of
both compounds were mainly occupied within the o-carborane
cages (ca. 73%, Tables S4† and S6†) while, the HOMOs were
predominantly localized on the fluorene units (63% in p-SAC
and 77% for o-SAC). These computational results further
confirmed that the low-energy emissions originated from ICT
processes between the o-carborane cages and fluorene groups
of the spiro[acridine-9,9'-fluorene] moieties.
region of o-SAC was mainly observed between Ψ = 90° and −90°
(Fig. 5) and the energy gap between the minimum and
maximum points was ca. 18 kcal·mol−1 (18.1 kcal·mol−1 at Ψ =
30°). The structures of o-SAC at Ψ = −30° and 0° were not
optimized due to intramolecular steric hindrance limitations.
The thermal energy value of 0.59 kcal·mol−1 was calculated as
kBT (kB = Boltzmann constant, 1.98 cal·K−1·mol−1) at 298 K, and
was compared to the relative energy values. The o-carborane
cages in p-SAC freely rotated in the solution state at ambient
temperature, but the S0 and S1 optimized structures (Ψ
≈ 90°)
of o-SAC were fixed in all states. Consequently, the calculation
and experimental results confirmed that the structural rigidity
of o-SAC led to efficient radiative decay, which affected the ICT
transitions associated with the o-carborane moieties.
The ICT-based low-energy emission of p-SAC in THF was not
detected in the PL spectra at 298 K, but strong emission
observed at 77 K around 469 nm (Fig. 2a and Table 1). Further,
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Page 8 of 11
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Journal Name
Photophysical properties in solid states based on aggregation
induced emission phenomenon
Applications of o-SAC as an emissive thermal indicator
View Article Online
DOI: 10.1039/D0TC03801F
The significantly different emissive colors of o-SAC when in
The PL spectra of p-SAC and o-SAC in the film state (5 wt% solution (ICT based emission) and in a solid film (AIE
doped on poly(methyl methacrylate) (PMMA)) exhibited phenomenon) indicated that the compound has potential as a
unusual emissive patterns. Both compounds were characterized thermal indicator,79 where the emission color differs according
by a broad emissive band in the low energy region (500‒600 nm) to temperature. The solubility of o-SAC was relatively low at
and an ICT based emission shoulder centered at ca. 430 nm (Fig. high concentrations (ca.10−3 M) in ACN. Its PL spectrum and the
2 and Table 1). Thus, strong reddish-orange emission was Commission Internationale de l'Eclairage 1931 (CIE 1931) color
observed in the rigid state (e.g. solid and solution at 77 K) coordinates in ACN (7.0 × 10−4 M) between 20 °C and 60 °C were
(emission in film state, inset in Fig. 2). The origin of the low- investigated. Intense red-emission centered at ca. 630 nm was
energy emission was investigated based on further PL observed at 20 °C due to AIE (Fig. 7a). The emissive character
measurements of p-SAC and o-SAC in a THF−water mixture (5.0 gradually decreased, while the ICT-based emission at ca. 490

10–5 M). The low energy emissions around 600 nm were nm increased with increasing temperature until 60 °C. These
drastically enhanced in an increased water fraction (fw). observations indicated that the phase of o-SAC changed from
Consequently, the maximized aggregation states in THF/water the colloidal state to the solution state. To elucidate the phase
(fw ≈ 98% for p-SAC and 90% for o-SAC) were associated with of o-SAC in ACN at 20 °C, dynamic light scattering (DLS)
intense emission patterns similar to those observed in the film measurements was performed (Fig. S17†). The result reveals
state. These observations were characteristic of AIE that the o-SAC particle has an average diameter of 2.0 μm in
phenomena. The strong AIE effect of both o-carboranyl ACN at 20 °C (total range = 0.57‒4.1 μm), indicating the colloidal
compounds tripled the quantum yield (Φem) in the film state suspension state. This temperature-dependent change in
compared to in solution at 298 K, where p- the Φem value of SAC emissive features led to a dramatic change in emissive color
increased from < 1% in THF to 5% in the film and o-SAC from red at 20 °C, to orange at 35 °C, to yellowish-green at 50 °C,
increased from 6 to 19% (Table 1).
The emission decay lifetimes (τobs) of both compounds lasted
several nanoseconds, where p-SAC was 4.0 ns in film and 4.6 ns
in THF, and o-SAC was 3.2 ns in film. Thus, the emission was
attributed to fluorescence (Table 1 and Figs. S15† and S16†).
The fluorescence lifetime and quantum yield of p-SAC and o-
SAC were used to calculate the radiative (kr) and nonradiative
(knr) decay rate constants (Table 1). The film state kr value for o-
SAC (5.9 × 107 s−1) was about 4.5 times larger than p-SAC (1.3 ×
107 s−1), but the knr value of both compounds was very similar
(2.5 × 108 s−1). These results confirmed the structural instability
of p-SAC. Specifically, the structure could not be fixed around
the S0- and S1-optimized structures, which interrupted the
radiative decay mechanism caused by the ICT transition in o-
carborane and molecular aggregation. The kr value of o-SAC was
much lower in THF (1.3 × 107 s−1) than in the film state, but the
knr values in the two states did not differ much (2.0 and 2.5 ×
107 s−1 in THF and film, respectively). Further, this structural
fluctuation clearly reduced the efficiency of the radiative
mechanism.
Fig. 7 (a) PL spectra of o-SAC in ACN (7.0  10–4 M, λex = 330 nm) according to changes in
temperature between 20 °C and 60 °C. The upper inset shows the emission color of o-
SAC in ACN (7.0  10–4 M) viewed under a hand-held UV lamp (λex = 365 nm). (b) CIE 1931
color coordinates of the PL spectra in consecutive heating and cooling cycling.
Fig. 6 PL spectra of (a) p-SAC and (b) o-SAC in THF/distilled water mixtures (5.0  10–5 M,
λex = 330 nm). The emission color in each state under irradiation by a hand-held UV lamp
(λex = 365 nm) are given in the insets.
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and to sky blue at 60 °C under a hand-held UV lamp (λex = 365
nm, inset in Fig. 7a). The change in luminescent response was
quantified as a function of temperature, and the CIE 1931 color
coordinates of o-SAC in ACN (7.0 × 10−4 M) were examined (Fig.
7b). The color coordinates demonstrated the emissive color
change from red (CIE x, y = ca. 0.69, 0.27 at 20 °C) to sky blue
(CIE x, y = ca. 0.06, 0.54 at 60 °C) via orange (CIE x, y = ca. 0.51,
0.44 at 35 °C) and bluish-green (CIE x, y = ca. 0.26, 0.55 at 50 °C)
as temperature increased. The respective color coordinates at
each temperature were maintained throughout consecutive
heating and cooling cycling. The temperature dependent
emissive behavior of o-SAC provided strong evidence of the
optimal condition of o-SAC required to achieve each emissive
color at specific temperatures. These changes were dependent
on the phase transition between the colloid and solution states
as the temperature increased. Consequently, o-SAC is a
Notes and references
View Article Online
DOI: 10.1039/D0TC03801F
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Conclusions
S. P. Fisher, A. W. Tomich, S. O. Lovera, J. F. Kleinsasser, J. Guo,
Two
spiro[acridine-9,9'-fluorene]
based
were prepared and
o-carboranyl
M. J. Asay, H. M. Nelson and V. Lavallo, Chem. Rev., 2019, 119
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,
compounds (p-SAC and o-SAC
characterized. The structural variation of o-SAC was more
restricted than p-SAC. This was attributed to the severely
)
K. -R. Wee, Y. -J. Cho, S. Jeong, S. Kwon, J. -D. Lee, I.-H. Suh
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distorted fluorene moiety in the solid-state structure of o-SAC
,
which was caused by intramolecular steric repulsion. The
calculated energy barriers (based on rotational motion around
the o-caborane cage) further contributed to the restriction.
Consequently, the ICT-based radiative decay efficiency of o-SAC
in solution state at 298 K was enhanced. Interestingly, both
compounds demonstrated AIE characteristics, resulting in
intense red emission in the solid state. However, only o-SAC
exhibited a drastic change in emissive color between the
solution and aggregated states. Furthermore, a relatively high-
concentrated ACN solution of o-SAC exhibited distinct changes
in emissive color due to the transition between a clear solution
and colloid caused by changing the temperature. The
experimental and theoretical findings strongly suggest that the
π‒aromatic group appended to the o-carboranyl conjugate
system in both o-carboranyl compounds can be used in visual
sensory materials for detecting temperature.
A. González-Campo, E. J. Juárez-Pérez, C. Viñas, B. Boury, R.
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of Korea (NRF) grant (NRF-2020R1A2C1006400 and NRF-
2016M3A7B4909246 for K. M. Lee and 2020R1I1A1A01073381
for J. H. Lee) funded by the Ministry of Science and ICT and the
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JPolueransael odfoMnaotetraiadljsuCsht emmairsgtriynsC
View Article Online
DOI: 10.1039/D0TC03801F
Journal of Materials Chemistry C
PAPER
Graphical and Textual Abstract for TOC
Two spiro[acridine-9,9'-fluorene]-based closo-o-carboranyl
compounds were prepared and they demonstrated the
application potential of π‒aromatic conjugated o-carboranyl
compounds as visual sensory materials.
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