Novel aluminum-BODIPY dyads: Intriguing dual-emission via photoinduced energy transfer

Article abstract of DOI:10.1039/c5dt05067g

Three novel BODIPY-based heterodinuclear complexes, [salen(3,5-^tBu)₂Al-(OC₆H₄-BODIPY)] (6), [salen(3,5-^tBu)₂Al-(OC₆F₂H₂-BODIPY)] (7), and [(mq)₂A

Full text of DOI:10.1039/c5dt05067g

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Novel aluminum–BODIPY dyads: intriguing
dual-emission via photoinduced energy transfer†
Cite this: DOI: 10.1039/c5dt05067g
Changho Sohn,a Jisu Jeong,b Ji Hye Lee,a Byung Hoon Choi,a Hyonseok Hwang,a
Gyun-Tack Bae,c Kang Mun Lee*a and Myung Hwan Park*c
Three novel BODIPY-based heterodinuclear complexes, [salen(3,5-tBu)2Al-(OC6H4-BODIPY)] (6), [salen-
(3,5-tBu)2Al-(OC6F2H2-BODIPY)] (7), and [(mq)2Al-(OC6H4-BODIPY)] (8) (salen = N,N’-bis(salicylidene)-
ethylenediamine, BODIPY = 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene, and mq = methyl-8-quino-
linolato) were prepared and characterized by multinuclear NMR spectroscopy. The specific structures of
6–8 were also determined by single crystal X-ray analysis. In particular, the salen-based heterodinuclear
complexes 6 and 7 exhibited higher thermal stability (Td5 = 309 and 306 °C, respectively) than that of the
closely related mononuclear aluminum or BODIPY compounds, except for 8. The UV/vis absorption and
PL spectra for 6 and 7 indicated a significant photoinduced energy transfer from the aluminum–salen
moiety to the BODIPY group in an intramolecular manner. Theoretical calculations revealed independent
transition states of the aluminum–salen moiety or the BODIPY group in the AlIII–BODIPY dyads, further
supporting these experimental results.
Received 31st December 2015,
Accepted 19th February 2016
DOI: 10.1039/c5dt05067g
www.rsc.org/dalton
Introduction
Tetra-coordinate BODIPY (boron dipyrromethane) compounds
have received considerable attention in a wide range of chemi-
cal applications, such as fluorescent probes for biological/
environmental fields,1 promising chromophores or fluoro-
phores in optoelectronic materials,2 chemical indicators for
other molecules,3 due to their outstanding photophysical pro-
perties. In order to enhance the inherent character of the
BODIPY group, a number of concepts have been developed in
conjunction with introduction of various functional groups
into the BODIPY moiety.4 In addition to tri-coordinate borane-
based BODIPY dyads (I in Chart 1)5 and triads (II),6 BODIPY-
based compounds containing various functional groups (III)7,8
were shown to expand their application. They exhibited intri-
guing spectroscopic and photophysical properties originating
from significant interactions such as partial energy transfer
between individual building units. This feature highly
Chart 1 Examples of BODIPY-based compounds.
depends on their molecular structures. For example, Thilagar and coworkers recently reported the triarylborane–BODIPY
dyad systems, demonstrating multiple emission bands as
chemical sensors for fluoride ions.9
In an effort to develop novel BODIPY-based dyads with high
stability, we were interested in the Al complexes based on the
Schiff base salen (N,N′-bis-(salicylidene)ethylenediamine) and
the quinolinate ligand as conjugates. Although the aluminum
salen complexes are well-known for various catalytic
systems,10–12 they have widely been investigated as excellent
luminophores in optoelectronic materials,13–15 owing to their
potential photophysical properties. Along with these studies, a
aDepartment of Chemistry, Institute for Molecular Science and Fusion Technology,
Kangwon National University, Chuncheon, Gangwon 24341, Republic of Korea.
E-mail: kangmunlee@kangwon.ac.kr
bDepartment of Chemistry, KAIST, Daejeon 34141, Republic of Korea
cDepartment of Chemistry Education, Chungbuk National University, Cheongju,
Chungbuk 28644, Republic of Korea. E-mail: mhpark98@chungbuk.ac.kr
†Electronic supplementary information (ESI) available: X-ray crystallographic
data of 6–8 in CIF format, NMR spectra, and computational details. CCDC
1400922–1400924. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5dt05067g
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heterodinuclear Al–salen/Ir complex recently reported was 1H NMR and 13C NMR spectra of all compounds showed the
shown that it could act as an emitter in organic light-emitting expected range corresponding to the aluminum moieties and
diodes (OLEDs).16 Nonetheless, to the best of our knowledge, the BODIPY. Additionally, the 11B NMR signals of 6–8 observed
Al(III)–BODIPY heterodinuclear complexes have never been at δ 0.27, 0.20, and 0.23 ppm, respectively, were consistent
investigated. Bearing this in mind, novel Al–BODIPY dyads with the presence of four-coordinate boron centers. The 11B
could be also considered promising candidates for opto- nucleus of the BODIPY group is coupled to the F atoms,
1
electronic applications, such as emitting materials in OLEDs.
leading to a sharp triplet (1JB–F = 28.2 Hz for 6 and 7; JB–F
=
In this regard, we report the Al–BODIPY dyad systems based 29.5 Hz for 8) similar to those found in previously reported
on two individual fluorescent luminophores, which are con- BODIPY derivatives.5–9 Furthermore, the solid-state structures
nected with a phenoxy spacer with the emergence of intriguing of 6–8 were determined by a single crystal X-ray diffraction
luminescent properties resulting from the efficient energy study (Fig. 1, Tables S1 and S2†). Single crystals suitable for
transfer between the two moieties. Herein, we describe the X-ray structural analysis were obtained by cooling a solution of
detailed synthesis, characterization, thermal stability, and saturated CH2Cl2/Et2O at −20 °C. The phenoxy group connects
photophysical properties of Al–BODIPY dyads (6–8) along with the aluminum centre with a BODIPY moiety indicating that
theoretical considerations.
Results and discussion
Synthesis and characterization
The synthetic procedures of mono-aluminum compounds (4a,
4b and 5), and heterodinuclear compounds 6–8, which were
linked between the aluminum and BODIPY groups, are shown
in Scheme 1. The aluminum compounds 4 and 5 were pro-
duced readily from the reaction of the corresponding alumi-
num precursor with phenol derivatives, and the aluminum–
BODIPY dyads 6–8 were prepared from the selective reaction of
the aluminum salen complex (2) with 1a or 1b, and the alumi-
num quinolinate complex (3) with 1a in toluene, respectively
in moderate yield (58% for 6, 42% for 7, and 39% for 8).
The formation of 4–8 was characterized by multinuclear
Fig. 1 X-ray crystal structures of 6–8 (50% thermal ellipsoids). H atoms
NMR spectroscopy (Fig. S1–S10†) and elemental analysis. The are omitted for clarity.
Scheme 1 Synthetic routes for mononuclear compounds (4a, 4b, and 5) and heterodinuclear compounds (6–8).
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the three complexes 6–8 are heterodinuclear complexes with
both a five-coordinate aluminum centre and a boron centre.
The Al–O and Al–N bond distances of 6–8 were observed as
ca. 1.8 Å and 2.0 Å, respectively, similar to those found in typical
pentacoordinate aluminum complexes.13,14 The distortion of
the coordination around an Al centre was generally determined
by geometric analysis such as the trigonality parameter τ (τ =
[α − β]/60, where α and β are the largest and next-largest inter-
ligand bond angles).17 The trigonality parameter τ for the per-
fectly square-pyramidal (sqp) geometry is 0 and τ for the
regularly trigonal-bipyramidal (tbp) geometry is 1. The τ values
for 6, 7, and 8 are 0.35, 0.39, and 0.82, respectively, indicating
that the geometries around the Al centre of 6 and 7 are sqp,
whereas 8 is more in favour of tbp. The meso-carbon atom of
BODIPY in 6–8 adopts a perfectly trigonal planar geometry, as
Table 1 Photophysical and thermal properties of mononuclear (1a, 1b,
4a, 4b, and 5) and heterodinuclear compounds (6–8)
λabs a/nm
b
Compound
(ε × 10−3 M−1 cm−1
)
λem a/nm
Φem
Td5 c/°C
1a
1b
4a
4b
5
6
7
380, 500 (1.30, 5.75)d
365, 507 (1.29, 5.19)
365 (0.81)
514
522
480
482
0.09
0.02
0.41
0.46
0.03
0.16
0.10
0.13
251
225
250
281
263
309
306
206
365 (0.85)
e
—
470
370, 500 (2.03, 5.46)
366, 503 (1.71, 5.02)
377, 500 (1.14, 4.65)
481, 514
477, 519
463, 513
8
a Measured using a 1.0 × 10−5 M CHCl3 solution. b Quinine sulfate as a
standard (0.5 M H2SO4, Φ = 0.55). c Determined by TGA at 5%
weight loss. d Molar absorption coefficient,
ε
(×10−3 M−1 cm−1).
e Not observed.
judged from the sum of the three C–Cmeso–C angles (∑(C–C–C)
=
360°, 6: 119.8°, 120.9°, and 119.3°, 7: 120.7°, 120.1°, and
119.2°, 8: 119.4°, 120.1°, and 120.5°, respectively). On the
other hand, the BODIPY-centred plane for 6–8 was signifi-
cantly distorted with respect to the phenoxy group linked to
the Al centre, resulting from the dihedral angles of 53.5°,
54.4°, and 41.4°, respectively. Thus, this feature might make it
difficult for achieving delocalization of electrons between the
bridged phenoxy group and the BODIPY moiety.
compared to that of each of the mono-aluminum complexes
(4a and 4b) or the BODIPY (1a and 1b) precursors, except for 8.
This feature is attributed to both the thermally robust salen–Al
bond and the significant bulkiness of 6 and 7.
Photophysical properties
The optical properties of mononuclear and heterodinuclear
compounds were investigated by UV/Vis absorption and photo-
luminescence (PL) measurements (Fig. 3 and Table 1) at room
temperature. The absorption spectra of 6 and 7 in CHCl3
exhibited two low-energy major absorption bands at
ca. 370 nm assignable to both the salen-centred π–π*
transitions13–15 and the BODIPY-centred S0 → S2 (π–π*) tran-
sitions, and at ca. 500 nm originating from the S0 → S1 (π–π*)
transitions in the BODIPY moiety (Fig. 3a).8 Despite a slight
change in the position of an absorption maxima below
300 nm, probably indicating minimal electronic communi-
cation between the two luminophores in 6 and 7,5a the absorp-
tion spectra of aluminum (4a and 4b) and BODIPY (1a and 1b)
mononuclear compounds mostly overlapped with those of the
corresponding heterodinuclear compounds (6 and 7). The dis-
torted arrangement of the two moieties is probably responsible
for this feature.8,18 Importantly, these independent absorption
features of the two luminophores in 6 and 7 provide a basis for
intramolecular energy transfer transitions.
Thermal properties
The thermal stabilities of 1a, 1b, and 4–8 were investigated by
thermogravimetric analysis (TGA) (Fig. 2 and Table 1). While
the mono-aluminum precursors 4a, 4b and 5 exhibited Td5
values of 250, 281, and 263 °C, respectively, the heterodi-
nuclear 6–8 showed Td5 values of 309, 306, and 206 °C, respecti-
vely, despite stepwise thermal degradation. It is expected that
the bond between the aluminum centre and phenoxy group
degraded at an early stage at a relatively low temperature. The
BODIPY precursors 1a and 1b also exhibited Td5 values of 251
and 225 °C, respectively, implying that the BODIPY moiety is
more thermally unstable than the aluminum moieties. Note
that 6 and 7 exhibit significantly enhanced thermal stability as
On the other hand, the low-energy absorption band (λabs
=
500 nm) of the heterodinuclear compound 8 is almost
identical to that of its closely related mononuclear BODIPY
compound (1a) due to no significant absorption of the
quinolinate-based aluminum compound 5 in the 350–500 nm
range. While the high-energy absorption bands of 6 and 7 were
slightly broadened by ligand-centered (LC) transitions ascrib-
able to the corresponding mono-aluminum and BODIPY com-
pounds, 8 showed a marginal absorption band in a region
ranging from 250 to 300 nm.
The emission spectra of 6 and 7 irradiated at 370 nm dis-
played dual emission patterns (λem = 481 and 514 nm for 6,
λem = 477 and 519 nm for 7), which correspond similarly to
those of the closely related mononuclear aluminum and
Fig. 2 TGA curves of mononuclear (1a, 1b, 4a, 4b, and 5) and heterodi-
nuclear complexes (6–8).
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Fig. 3 (a) UV/Vis absorption and (b) photoluminescence spectra in CHCl3 (5.0 × 10−5 M) for mononuclear (1a, 1b, 4a, 4b, and 5) and heterodinuclear
compounds (6–8) at room temperature.
BODIPY compounds (Fig. 3b and Table 1). The excitation the emissions at 470 nm and 520 nm for 6 or 7 in CHCl3
spectra for two emission maxima of 6–8 showed the maximum (5 × 10−5 M) as <4 and >6 ns, respectively, reveals that the
point around 370 nm, indicating that excitation at 370 nm can former is assignable to the Al-centre fluorescence and the
effectively reveal the dual emission for the aluminum and latter to the BODIPY-centre fluorescence (Fig. S19–S22†).
BODIPY moieties (Fig. S11–S13†). For this reason, we chose
On the other hand, the emission spectrum of 8 showed
370 nm as the excitation wavelength. A comparison of the emission maxima at 463 and 513 nm, which correspond
emission intensities for 6 and 7 with closely related mono- exactly to those of 1a and 5, respectively. This result indicates
nuclear aluminum (4a or 4b) and BODIPY compounds (1a or the absence of energy transfer between the two luminophores
1b) at the same concentration reveals that the intensity of the in 8, suggesting that each moiety behaves entirely indepen-
aluminum moiety decreased and that of BODIPY increased in dently within a heterodinuclear molecule. This feature is
both 6 and 7 (Fig. 3b). The observation that the emission further confirmed by the observation of the individual emis-
intensity of each moiety was only changed without any altera- sion bands without change originating from the corres-
tion in the emission maxima indicates the involvement of ponding mononuclear compounds from the solution of an
partial energy transfer from the Al-centred fluorophore to the equimolar mixture (λex = 370 nm, Fig. S16†).
BODIPY-centred chromophore. This feature resulted from the
Theoretical calculations and molecular orbital analyses
characteristic distorted nonplanar molecular structures of 6
and 7.
Furthermore, the observation of the obvious emission
bands attributable to the corresponding mononuclear com-
pounds from the solution of an equimolar mixture (λex
To gain insight into the electronic transitions and the elec-
tronic structures of 6–8, TD-DFT calculations on the ground
state (S0) were performed with the B3LYP functional and 6-31G(d)
basis sets (Fig. 4 and Table 2). The geometries of 6–8 for
the calculations were optimized from their X-ray structures. To
include the solvent effects of CHCl3, the conductor-like polar-
izable continuum model (CPCM) was used.20 The calculation
results for 6–8 show that the largest contribution ( f > 0.4) to
lower-energy absorptions below 400 nm is involved with the
transition from HOMO−2 to LUMO (λabs = 416 nm for 6 and
λabs = 420 nm for 7) or from HOMO−1 to LUMO (λabs = 416 nm
for 8, Table 2). While both the HOMO−2 (for 6 and 7) and the
HOMO−1 (for 8) are predominantly localized on the BODIPY
(>99%), the LUMOs (for 6–8) were distributed over the BODIPY
(ca. 89%) and the bridged phenyl group (ca. 10%) for all of the
=
370 nm, Fig. S14 and S15†) demonstrates such intramolecular
energy-transfer transition features in 6 and 7.8,18,19 Further
considerations of significant overlaps between the emission
bands of the aluminum–salen compounds (4a or 4b) and the
absorption bands of the BODIPY compounds (1a or 1b) also
confirm this feature (Fig. S17 and S18†). This result is also in
good agreement with the overlapping feature between the cor-
rected excitation spectra recorded for two emissions and the
absorption spectrum recorded over the entire spectral range
(Fig. S11 and S12†). This feature is clearly verified by the obser-
vation that measurement of the fluorescence decay lifetime of
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Fig. 4 Frontier molecular orbitals for 6–8 at their ground state (S0) optimized geometries and the lowest-energy electronic transition from TD-DFT
calculations (isovalue = 0.04).
hand, the major transition of 8 in the range of 300–400 nm is
only assigned from HOMO to LUMO+1 (λabs = 384 nm, Fig. 4).
The HOMO is entirely localized on the quinolate ligand
(>99%). These findings suggest that the BODIPY has little
orbital contribution in the higher-energy region above 400 nm.
It is notable that the significant absorptions occur indepen-
dently of the salen- (6 and 7) or the quinolinate-centred (8)
π–π* transitions and the BODIPY-centred π–π* transitions
(6–8). Along with the experimental absorption spectra in
chloroform solution, these DFT calculation results further
confirm that the heterodinuclear complexes 6 and 7 only
exhibit partial intramolecular energy transfer from the salen
moiety to the BODIPY moiety.
Table 2 The major low-energy electronic transition for 6–8 from
TD-DFT calculationsa
λcalc/nm
fcalc
Assignment
6
7
8
416.05
392.99
360.37
419.90
388.53
363.09
416.26
384.33
0.4279
0.0328
0.0551
0.4338
0.0216
0.0690
0.4319
0.1114
HOMO−2 → LUMO (95%)
HOMO → LUMO+1 (98%)
HOMO−3 → LUMO+1 (95%)
HOMO−2 → LUMO (95%)
HOMO → LUMO+1 (98%)
HOMO−1 → LUMO+1 (97%)
HOMO−1 → LUMO (95%)
HOMO → LUMO+1 (92%)
a Singlet energies for the vertical transition calculated at the optimized
S0 geometries.
compounds (Tables S3, S5, and S7†). Although the transition
to the S1 ( f > 0.13, for 6) or S1 and S2 ( f > 0.02, for 7) state was
slightly observed in the lower-energy absorptions (Tables S3–
S6†), the Al moieties and bridged phenyl group very slightly
contribute to the major transitions below 400 nm.
However, most electronic transitions ( f > 0.02, Fig. 4 and
Table 2) to LUMO+1 (>99% for salen ligands (6 and 7) and qui-
nolate moiety (8)) in the range of 300–400 nm are limited to
the aluminum ligand moieties and the bridged phenyl groups.
The major contributions for 6 in this region are observed from
HOMO−3 to LUMO+1 (λabs = 360 nm) and from HOMO to
LUMO+1 (λabs = 393 nm). The HOMO−3 and HOMO corres-
pond to the delocalized electron density of the salen moiety
(52% and 93%) and bridged phenyl group (41% and 6%),
respectively. Similarly, the significant transitions of 7 in the
region are from HOMO−1 to LUMO+1 (λabs = 363 nm) and
from HOMO to LUMO+1 (λabs = 389 nm).
Experimental
General considerations
All operations were performed under an inert nitrogen atmos-
phere using standard Schlenk and glove box techniques. Anhy-
drous grade solvents (Aldrich) were dried by passing them
through an activated alumina column and stored over acti-
vated molecular sieves (5 Å). Spectrophotometric grade CHCl3
(Aldrich) was used as received. Commercial reagents were used
without any further purification after purchasing from Aldrich
(salicylaldehyde derivatives, ethylenediamine, 8-hydroxyquin-
aldine, trimethylaluminum (2.0 M in toluene), phenol deriva-
tives, benzaldehyde derivatives, hexamethylenetetramine,
pyrrole, trifluoroacetic acid, DDQ (2,3-dichloro-5,6-dicyano-1,4-
benzoquinone), triethylamine, BF3·OEt2). Compounds 1a,21
2,22 and 4a 14 were analogously prepared according to the
reported procedures. Deuterated solvents from Cambridge
Isotope Laboratories were used after drying over activated
molecular sieves (5 Å). NMR spectra were recorded on a Bruker
The delocalized electron density of HOMO−1 and HOMO
for 7 is distributed over the salen moieties (92% and 96%) and
bridged phenyl group (8% and 3%), respectively. On the other
1
Avance 400 spectrometer (400.13 MHz for H, 100.62 MHz for
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13C, and 128.38 MHz for 11B) at ambient temperature. Chemi- cooling to ambient temperature, the solvent was removed
cal shifts are given in ppm, and are referenced against external under reduced pressure and washed with pentane to afford a
1
Me4Si (1H, 13C), and BF3·OEt2 (11B). Elemental analyses were yellow solid 4b (0.84 g, 87%). H NMR (CDCl3): δ 8.39 (s, 2H),
performed on an EA3000 (Eurovector) in the Central Labora- 7.49 (d, 2H), 7.01 (d, 2H), 6.49 (m, 2H), 6.32 (m, 1H), 4.19 (m,
tory of Kangwon National University. UV/Vis absorption and 2H), 3.73 (m, 2H), 1.44 (s, 18H), 1.29 (s, 18H). 13C NMR
PL spectra were recorded on a Jasco V-530 and a Spex Fluorog-3 (CDCl3): δ 170.16, 163.31, 156.87, 154.46, 140.95, 138.26,
Luminescence spectrophotometer, respectively. Fluorescence 137.44, 130.87, 127.05, 118.21, 115.00, 110.65, 55.08, 35.52,
decay lifetimes were measured using a time-correlated single- 33.97, 31.47, 29.61.
photon counting (TCSPC) spectrometer (FLS920-EDINBURGH
Synthesis of 5. To a solution of 3 (0.54 g, 1.5 mmol) in
Instruments in the Central Laboratory of Kangwon National toluene (20 mL) was slowly added phenol (0.14 g, 1.5 mmol) in
University) equipped with a EPL-375ps pulsed semiconductor toluene (20 mL). After stirring for 20 min, the reaction mixture
diode laser as an excitation source and a microchannel plate was then heated at 110 °C for 4 h. After cooling to ambient
photomultiplier tube (MCP-PMT, 200–850 nm) as a detector at temperature, the solvent was removed under reduced pressure.
298 K. Thermogravimetric analysis (TGA) was performed The residue was washed with n-hexane and recrystallization
under a N2 atmosphere using a TA Instrument Q500 at a from a mixture of CH2Cl2/n-hexane afforded 5 as a white solid
heating rate of 20 °C min−1 from 50 °C to 800 °C.
(0.50 g, 76%). 1H NMR (CDCl3): δ 8.19 (d, 2H), 7.45 (t, 2H),
Synthesis of 1b. A 3,5-difluoro-4-hydroxybenzaldehyde 7.33 (d, 2H), 7.19 (d, 2H), 7.09 (d, 2H), 6.83 (t, 2H), 6.51 (t, 2H),
(1.58 g, 10 mmol) was dissolved in pyrrole (17.4 mL, 0.25 mol). 6.35 (d, 2H), 2.98 (s, 6H). 13C NMR (CDCl3): δ 159.50, 157.34,
After stirring for 30 min at ambient temperature, the solution 156.10, 139.47, 139.19, 129.03, 128.93, 126.72, 124.08, 119.05,
was cooled to 0 °C. To this solution was slowly added trifluoro- 117.32, 114.20, 112.84, 22.77. Anal. Calcd for C26H21AlN2O3: C,
acetic acid (0.08 mL, 1 mmol) via a syringe and the reaction 71.55; H, 4.85; N, 6.42. Found: C, 72.14; H, 4.92; N 6.60.
mixture was allowed to cool slowly to room temperature. After
Synthesis of 6. To a solution of 2 (0.80 g, 1.5 mmol) in
stirring for 2 h, a 0.1 M aqueous solution of NaOH was added toluene (30 mL) was slowly added 1a (0.43 g, 1.5 mmol) in
and the resulting solution was extracted with ethyl acetate. The toluene (20 mL). After the solution was heated at reflux for 4 h,
combined organic portions were dried over MgSO4 and the it was cooled to ambient temperature, and then the solvent
solvent was removed under reduced pressure. The residue was was removed under reduced pressure. The residue was washed
redissolved in THF (150 mL) and 2,3-dichloro-5,6-dicyano-1,4- with diethyl ether (50 mL) and recrystallization from a mixture
benzoquinone (2.27 g, 10 mmol) was added. After stirring for of CH2Cl2/n-hexane afforded 6 as an orange solid (0.70 g,
12 h, triethylamine (13.9 mL, 0.10 mol) and BF3·OEt2 58%). Single crystals suitable for X-ray diffraction study were
1
(18.4 mL, 0.15 mol) were added and stirred for 2 h. After the grown from the CH2Cl2/Et2O solution of 6. H NMR (CDCl3): δ
reaction was quenched with a 1 M aqueous solution of NaOH 8.44 (s, 2H), 7.80 (s, 2H), 7.56 (d, 2H), 7.14 (d, 2H), 7.07 (d,
(100 mL), the organic layer was separated, and the aqueous 2H), 6.84 (d, 2H), 6.46 (m, 4H), 4.13 (m, 2H), 3.78 (m, 2H), 1.46
layer was extracted with CH2Cl2 (50 mL). The combined (s, 18H), 1.31 (s, 18H). 13C NMR (CDCl3): δ 170.39, 165.02,
organic portions were dried over MgSO4 and concentrated 163.31, 141.80, 141.06, 138.82, 134.57, 132.68, 131.33, 131.16,
under reduced pressure. Purification by column chromato- 127.16, 123.19, 120.29, 118.32, 117.48, 55.11, 35.60, 34.05,
1
graphy (eluent: CH2Cl2/n-hexane, 2 : 1 v/v) afforded 1b as a 31.36, 29.66. 11B NMR (CDCl3): δ 0.27 (t, JB–F = 28.2 Hz). Anal.
brown solid (0.93 g, 29%). 1H NMR (CDCl3): δ 7.94 (s, 2H), 7.18 Calcd for C47H56AlBF2N4O3: C, 70.50; H, 7.05; N, 7.00. Found:
(m, 2H), 6.94 (d, 2H), 6.56 (d, 2H); 13C NMR (CDCl3): δ 152.61, C 70.39; H, 7.11; N, 6.96.
150.16, 144.46, 135.40, 134.49, 131.20, 124.96, 119.00, 114.28.
Anal. Calcd for C15H9BF4N2O: C, 56.29; H, 2.83; N, 8.75. analogous to the synthesis of 6 using 1b. A brown solid was
Found: C, 56.13; H, 2.66; N, 8.73. obtained (0.53 g, 42%). Single crystals suitable for an X-ray
Synthesis of 7. This compound was prepared in a manner
Synthesis of 3. 8-Hydroxyquinaldine (0.96 g, 6 mmol) was diffraction study were grown from the CH2Cl2/Et2O solution of
1
dissolved in toluene (50 mL) and cooled at −78 °C. Trimethyl- 7. H NMR (CDCl3): δ 8.46 (s, 2H), 7.84 (s, 2H), 7.52 (d, 2H),
aluminum (2.0 M in toluene, 1.5 mL, 3 mmol) was slowly 7.06 (d, 2H), 6.81 (dd, 2H), 6.78 (d, 2H), 6.47 (dd, 2H), 4.24 (m,
added via a syringe to the 8-hydroxyquinaldine solution. The 2H), 3.81 (m, 2H), 1.42 (s, 18H), 1.29 (s, 2H). 13C NMR (CDCl3):
reaction mixture was then slowly heated at 110 °C for 1 h. After δ 170.53, 163.14, 156.49, 154.06, 146.37, 143.24, 141.36,
cooling to room temperature, all volatiles were removed at 140.91, 138.70, 134.46, 131.15, 127.20, 120.81, 118.23, 114.02,
reduced pressure and washed with toluene and n-hexane to 55.22, 35.55, 34.02, 31.35, 29.62. 11B NMR (CDCl3): δ 0.20 (t,
afford an ivory solid residue 3 (0.90 g, 84%). It was used in situ 1JB–F = 28.2 Hz). Anal. Calcd for C47H54AlBF4N4O3: C, 67.46; H,
for the next step without any purification. 1H NMR (CDCl3, 6.50; N, 6.70. Found: C, 67.19; H, 6.76; N, 6.82.
400.13 MHz, ppm): δ = 8.16 (d, 2H), 7.39 (m, 4H), 7.11 (d, 2H),
7.00 (d, 2H), 3.06 (s, 6H), −0.65 (s, 3H).
Synthesis of 8. This compound was prepared in a manner
analogous to the synthesis of 6 using 3 with 1a. A brown solid
Synthesis of 4b. To a solution of 2 (0.80 g, 1.5 mmol) in was obtained (0.37 g, 39%). Single crystals suitable for an X-ray
toluene (20 mL) was slowly added 2,6-difluorophenol (0.20 g, diffraction study were grown from the CH2Cl2/Et2O solution of
1.5 mmol) in toluene (20 mL). After stirring for 20 min, the 8. 1H NMR (CDCl3): δ 8.24 (d, 2H), 7.81 (s, 2H), 7.48 (t, 2H),
reaction mixture was then heated at 110 °C for 4 h. After 7.39 (d, 2H), 7.23 (d, 2H), 7.11 (d, 2H), 6.78 (d, 2H), 6.45 (m,
Dalton Trans.
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4H), 3.00 (s, 6H). 13C NMR (CDCl3): δ 163.51, 157.34, 155.84, nuclear compound, except for 8. UV-Vis absorption and emis-
148.59, 141.22, 139.48, 134.52, 132.78, 131.07, 129.26, 126.75, sion spectra of
6
and
7 demonstrated obvious partial
124.11, 123.42, 119.30, 117.62, 114.50, 113.11, 53.41, 22.87. 11
B
intramolecular energy transfer from the salen moiety to
1
NMR (CDCl3): δ 0.23 (t, JB–F = 29.5 Hz). Anal. Calcd for BODIPY. Theoretical calculations further supported the
C36H28AlBCl2F2N4O3: C, 60.79; H, 3.97; N, 7.88. Found: C, experimental features. To the best of our knowledge, these
60.90; H, 3.86; N, 7.82.
dyad systems are the first example of BODIPY-based com-
pounds containing various functional groups. For these
reasons, these heterodinuclear complexes can constitute a
novel class of luminophores. Relevant studies on modifying
the given system to make novel BODIPY-based compounds are
in progress.
X-ray crystallography
Single crystals of 6–8 were coated with Paratone oil and
mounted onto a glass capillary. The crystallographic measure-
ments were performed on a Bruker SMART Apex II CCD area
detector diffractometer with graphite-monochromated Mo-Kα
radiation (λ = 0.71073 Å). The structure was solved by direct
methods and all non-hydrogen atoms were subjected to aniso-
tropic refinement by the full-matrix least-squares method on
F2 by using the SHELXTL/PC package, resulting in the X-ray
crystallographic data of 6–8 in CIF format (CCDC
1400922–1400924). Hydrogen atoms were placed at their geo-
metrically calculated positions and refined riding on the
corresponding carbon atoms with isotropic thermal para-
meters. The detailed crystallographic data are given in Tables
S1 and S2.†
Acknowledgements
This work was supported by the Basic Science Research
Program (NRF-2013R1A1A2061792 for M. H. Park and
NRF-2015R1D1A1A01057396 for K. M. Lee) through the
National Research Foundation of Korea (NRF) funded by the
Ministry of Education. K. M. Lee thanks the support from the
2015 Research Grant from Kangwon National University.
UV/Vis absorption and photoluminescence (PL)
measurements
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Conclusions
We have synthesized and characterized Al(III)–BODIPY dyads
(6–8) from the selective coupling of the mono-aluminum com-
plexes (4a, 4b and 5) and BODIPY compounds (1a and 1b). The
structures of the novel three heterodinuclear complexes (6–8)
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