Luminescent heteroleptic tris(dipyrrinato)indium(III) complexes

Article abstract of DOI:10.1021/ic500326u

To provide an improvement over the low fluorescence efficiencies often shown by homoleptic tris(dipyrrinato)indium(III) complexes, luminescent heteroleptic tris(dipyrrinato)indium(III) complexes bearing two types of dipyrrinato ligands are designed here by theoretical calculation and then synthesized. They possess frontier orbitals linked to suppression of the nonemissive charge-separated states; one shows a high fluorescence quantum yield (0.41) in toluene, which exceeds that of the corresponding BF₂ complex.

Full text of DOI:10.1021/ic500326u

Communication
pubs.acs.org/IC
Luminescent Heteroleptic Tris(dipyrrinato)indium(III) Complexes
Shinpei Kusaka, Ryota Sakamoto,* and Hiroshi Nishihara*
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan
S
* Supporting Information
Scheme 1. Synthesis of Tris(dipyrrinato)indium(III)
Complexes (LiHMDS = Lithium Hexamethyldisilazide)
ABSTRACT: To provide an improvement over the low
fluorescence efficiencies often shown by homoleptic
tris(dipyrrinato)indium(III) complexes, luminescent het-
eroleptic tris(dipyrrinato)indium(III) complexes bearing
two types of dipyrrinato ligands are designed here by
theoretical calculation and then synthesized. They possess
frontier orbitals linked to suppression of the nonemissive
charge-separated states; one shows a high fluorescence
quantum yield (0.41) in toluene, which exceeds that of the
corresponding BF2 complex.
etal complexes are becoming increasingly important in
M
photochemistry. A particular group of metal complexes,
photofunctional metal−organic frameworks (MOFs), is being
developed as, for example, luminescent materials,1 photo-
catalysts,2 and sensors.3 The spontaneous coordination of
dipyrrins with metal ions makes them useful in supramolecular
systems4 such as MOFs.5 Dipyrrin and its complexes are
promising photofunctional dyes because of their strong and
sharp absorption bands.6 Some dipyrrin complexes show notable
luminescence,7 particularly the BF2 complexes (4,4-difluoro-4-
bora-3a,4a-diaza-s-indacenes, or BODIPYs).8 However, metal
complexes of dipyrrins have remained relatively neglected
because of their low emission efficiencies. We recently developed
a heteroleptic bis(dipyrrinato)zinc(II) complex that showed a
unprecedentedly high fluorescence quantum yield.9 Improve-
ment was gained through suppression of the formation of
nonemissive charge-separated states between the two dipyrrinato
ligands that compete with the emissive 1π−π* state (Figures S1−
S3 in the Supporting Information, SI). Cohen and co-workers
reported the first homoleptic tris(dipyrrinato)metal complexes
with group 13 metals [gallium(III) and indium(III)]; however,
the complexes showed poor quantum efficiencies (e.g., 5 in
Scheme 1).10 Here we report the development of brightly
luminescent tris(dipyrrinato)indium(III) complexes by using the
same strategy that we employed to find the previous zinc(II)
complex. The result is the design and syntheses of heteroleptic
indium(III) complexes bearing two types of dipyrrinato ligand (6
and 7 in Scheme 1).
Figure 1. Frontier orbitals of heteroleptic complexes 6 and 7 estimated
by DFT calculations at the B3LYP/(LanL2DZ for indium and 6-31G*
for carbon, hydrogen, and nitrogen) level.
Theoretical calculations were conducted for complexes 6 and
7 (Figure 1) and 8 (Figure S4 in the SI). Frontier orbitals are
composed of π or π* orbitals on dipyrrinato ligands and contain
negligible contributions from the indium(III) center. No
remarkable energy differences were found among the frontier
orbitals composed of the same ligand (Table S1 in the SI). The
localization of both the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) on
ligand 3 was considered particularly important. The resulting
electronic structures of 6 and 7 were expected to suppress
photoinduced charge transfer between ligands 3 and 4 (Figure S5
in the SI). Only hydrogen was considered to be a suitable
substituent on the α position of the dipyrrinato ligand; larger
species would induce large steric repulsion that would
Received: February 11, 2014
© XXXX American Chemical Society
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dx.doi.org/10.1021/ic500326u | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
significantly destabilize the coordination sphere. Complexes
incorportating α-methylated dipyrrinato ligands, in fact,
dissociated quickly in solution (data not shown).
Our previously reported deborylation method9 allowed free
base dipyrrin 3 to be synthesized from BODIPY 1.11
Heteroleptic complexes 6 and 7 and reference homoleptic
complex 8 were synthesized by the treatment of InCl3 with the
corresponding equivalents of the lithium salts of dipyrrins 3 and
4. Neither silica nor alumina chromatography could effectively
isolate 6 and 7 because of decomposition, but the complexes
were isolated by gel permeation chromatography (GPC). Note
that the gallium(III) analogues of 6 and 7 could also be similarly
synthesized using GaCl3. Their formation was detected by mass
spectrometry, but their decomposition prevented their isolation,
even by GPC. The smaller atomic radius of gallium(III) (0.62
nm) in comparison with that of indium(III) (0.81 nm) may have
resulted in greater steric interference among the ligands.
Figure 3. Absorption (solid line) and emission (broken line) spectra for
complexes 6 (green), 7 (yellow), and 8 (red) in toluene. Excitation
wavelengths: 6, 459 nm; 7, 480 nm; 8, 530 nm.
Heteroleptic complexes 6 and 7 were identified by NMR
spectroscopy and elemental analysis. Their H NMR spectra
showed very different peak patterns, indicating no dissociation or
disproportionation of the dipyrrin ligands in solution (Figure S6
in the SI). The single-crystal X-ray diffraction of 7 demonstrated
its heteroleptic nature (Figure 2 and Table S2 in the SI). It
1
coupling among its three dipyrrinato ligands 3, such that the
single π−π* absorption band broadened and split into two
bands.
The emission spectra of complexes 6−8 each showed a single
peak at around 600 nm, whose wavelength position was
unaffected by which absorption band was photoexcited. This
indicates that the emission was derived exclusively from the
1π−π* excited state of the dipyrrinato ligand 3 and that
quantitative energy transfer occurred from the dipyrrinato ligand
4 to ligand 3. Among the three complexes, the highest quantum
yield was provided by 6 (0.41 in toluene); the efficiency
decreased with increasing number of dipyrrinato ligands 3 (0.34
for 7 and 0.28 for 8). The quantum yield of heteroleptic complex
6 was much greater than that previously reported for homoleptic
complex 5 (0.074 in hexane).10 Complex 6 surprisingly also
showed a greater fluorescence quantum yield in toluene than did
the corresponding BODIPY 2 (0.35). This counters the
preconception that BODIPYs emit more brightly than do
dipyrrinato metal complexes. The authors also describe solid-
state luminescence briefly. Complexes 7 and 8 exhibited red
emission (Figure 4), whereas complex 6 was hardly fluorescent.
Figure 2. Crystal structure of complex 7·toluene (thermal ellipsoid plot
at 50% probability). Color code: gray, carbon; blue, nitrogen; brown,
indium). Hydrogen atoms and toluene molecules are omitted for clarity.
R1 = 0.0427; wR2 = 0.1075.
showed a coordination sphere with a slightly distorted octahedral
geometry, and Δ and Λ isomers coexisted as a racemic mixture.
The In−N distances were 2.194(2)−2.242(3) Å, and the N−In−
N angles ranged from 83.78(10)° to 95.02(10)°; these values are
comparable to those of homoleptic complex 5.10
Figure 4. (a) Photographs of dichloromethane solutions of 6−8 under
ambient light (left) and 365 nm UV light (right). The absorbance of
each solution at 365 nm is normalized. (b) Photographs of solids of 6−8
under ambient light (top) and 365 nm UV light (bottom).
The optical properties of the series of indium(III) complexes
were disclosed in UV−vis absorption and emission spectroscopy
(Figure 3). Each heteroleptic complex showed two absorption
bands, corresponding to the 1π−π* transitions of the two types
of dipyrrinato ligands: π-extended dipyrrinato ligand 3 featured
the red-shifted 1π−π* band.
Complexes 6 and 7 show differently shaped peaks and
different absorption maxima because of exciton coupling.12
Heteroleptic complex 6 underwent exciton coupling between its
two dipyrirnato ligands 4, which resulted in the sub-band at 500
nm. Similarly, heteroleptic complex 7 showed an absorption
band (with its maximum at 564 nm) with a shoulder, which
resulted from exciton coupling between the two dipyrrinato
ligands 3. Homoleptic 8 underwent more significant excition
The absence of fluorescence in 6 might stem from intermolecular
interaction or bimolecular quenching, although the authors
cannot give a precise explanation because of the lack of crystal
structure. Solid-state fluorescence tends to be more sensitive to
crystal packing than to the molecular nature of the fluorophore
itself.
Our previously synthesized heteroleptic bis(dipyrrinato)zinc-
(II) complexes recovered their fluorescence quantum yields by
suppressing nonemissive charge-separated states competing with
emissive singlet π−π* excited states (Figures S1−S3 in the SI).9
A similar mechanism is proposed here (Figure S5 in the SI).
Heteroleptic complex 6 with one emissive dipyrrinato ligand 3
avoided nonemissive charge-separated states. In contrast,
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Communication
Table 1. Optical Properties of Dipyrrin Complexes
a
b
c
d
d
10−4ε (M−1 cm−1
6.11
)
λabs (nm)
λem (nm)
ϕF (in toluene)
ϕF (in CH2Cl2)
τ
(ns, in toluene)
τ
(ns, in CH2Cl2)
1.52
2
5
6
7
8
604
496
650
522
592
596
600
0.35
0.074
0.41
0.34
0.18
2.53
1.93
2.44
2.53
2.49
e
6.01
11.8, 6.73
459, 570
0.053
0.021
0.015
1.29
1.42
1.48
f
12.4, 9.24 (sh ), 11.3
478, 542 (sh), 564
15.6, 14.0
528, 572
0.28
a
b
c
d
Absorption wavelength. Emission wavelength. Fluorescence quantum yields. Mean fluorescence lifetimes. For details, please refer to the SI.
e
f
From ref 10, where all data are in a hexane solution. Shoulder.
complexes 7 and 8 underwent charge separation among their
multiple dipyrrinato ligands 3, with greater charge separation
shown by 8, reflecting its greater number of dipyrrinato ligands 3.
Optical data in dichloromethane were also acquired. The
fluorescence quantum yields of 6−8 were respectively 13%,
6.2%, and 5.4% of the values recorded in toluene. The smaller
drop shown by 6 may reflect its effective suppression of charge-
separated states, which is more effective in polar solvents such as
dichloromethane. Fluorescence lifetime studies were also
conducted to assess the photophysical properties of the
complexes. The emission lifetimes of the complexes in toluene
ranged from 2.4 to 2.5 ns (Table 1), consistent with the singlet
nature of their luminescence. Each decay curve was fitted using
one main and two minor exponential decays (Figures S7−S9 and
Tables S3−S5 in the SI). The fluorescence lifetimes decreased to
1.3−1.5 ns in dichloromethane, reflecting fluorescence quench-
ing. In this medium, the fastest decay component gained
population (Figures S10−S12 and Tables S6−S8 in the SI).
These findings may also be associated with the existence of the
quenching charge-separated state.13
In conclusion, we synthesized the first heteroleptic tris-
(dipyrrinato)indium(III) complexes using a combination of
plain (4) and π-extended (3) dipyrrin ligands. Heteroleptic
complexes 6 and 7 exhibited higher fluorescence quantum yields
than homoleptic complexes 5 and 8, and the finding was
attributed to suppression of the nonemissive charge separated
states. This work establishes tris(dipyrrinato)indium(III)
complexes as potentially useful in, for example, optically active
supramolecular and MOF systems.
Emgineering, The Kao Foundation for Arts and Sciences, The
Asahi Glass Foundation, The Noguchi Institute, and The
Tokuyama Science Foundation for financial support. We thank
Dr. Tetsuro Kusamoto (The University of Tokyo) for measure-
ment of the electrospray ionization spectrometry.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: sakamoto@chem.s.u-tokyo.ac.jp.
*E-mail: nisihara@chem.s.u-tokyo.ac.jp.
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ACKNOWLEDGMENTS
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The authors acknowledge Grants-in-Aid from MEXT of Japan
(Grants 24750054, 21108002, and 25107510; area 2107
[Coordination Programming] and area 2406 [AnApple]) and a
JSPS fellowship for young scientists. R.S. is grateful to The
Ogasawara Foundation for the Promotion of Science &
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dx.doi.org/10.1021/ic500326u | Inorg. Chem. XXXX, XXX, XXX−XXX