Bis(dipyrrinato)zinc(II) Complexes: Emission in the Solid State

Article abstract of DOI:10.1021/acs.inorgchem.6b00431

This Communication reports the first observation of solid-state photoluminescence in bis(dipyrrinato)zinc(II) complexes with various substituents. The report discusses the effect of their substituents on their crystal structures and spectroscopic properties. Their meso-aryl groups are revealed to play important roles in the spectroscopic properties in the solid state.

Full text of DOI:10.1021/acs.inorgchem.6b00431

Communication
pubs.acs.org/IC
Bis(dipyrrinato)zinc(II) Complexes: Emission in the Solid State
†
,†,‡
†
†
†
Mizuho Tsuchiya, Ryota Sakamoto,* Masaki Shimada, Yoshinori Yamanoi, Yohei Hattori,
§
∥
,†
Kunihisa Sugimoto, Eiji Nishibori, and Hiroshi Nishihara*
†
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
‡
§
∥
Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Koto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
Division of Physics, Faculty of Pure and Applied Sciences, Tsukuba Research Center for Interdisciplinary Materials Science, and
Center for Integrated Research in Fundamental Science and Engineering, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki
05-8571, Japan
3
*
S Supporting Information
Scheme 1. Synthesis of Homoleptic Bis(dipyrrinato)zinc(II)
Complexes
a
ABSTRACT: This Communication reports the first
observation of solid-state photoluminescence in bis-
(
dipyrrinato)zinc(II) complexes with various substituents.
The report discusses the effect of their substituents on
their crystal structures and spectroscopic properties. Their
meso-aryl groups are revealed to play important roles in the
spectroscopic properties in the solid state.
ipyrrins are a class of π-conjugated organic molecules with
intense absorption in the visible region. In addition, their
1
D
pyrrolic nitrogen atoms provide coordinating ability, and thus
these compounds are often dyes and ligands. Of the known
dipyrrin−metal complexes, most are boron complexes (boron
dipyrromethenes, or BODIPYs), which are frequently used as
a
Ligand 2aH and its homoleptic complex 2a Zn are depicted as
2
2
,3
fluorescent dyes. Although dipyrrinato−metal complexes are
examples. The complexes investigated herein are 1a Zn, 1c Zn, 2a Zn,
2
2
2
generally nonemissive, exceptions include some
2b Zn, 2c Zn, 3b Zn, and 3c Zn.
2 2 2 2
1
,4−6
dipyrrinato−zinc(II) complexes.
Lindsey and colleagues
reported that a structurally rigid bis(dipyrrinato)zinc(II)
complex exhibits emission with a fluorescence quantum yield
properties of the five complexes were disclosed and discussed,
7
(
Φ ) of 0.36 in toluene. Later, we revealed that elaborate
8
F
together with those of 1a Zn and 1c Zn synthesized previously.
2
2
heteroleptic bis(dipyrrinato)zinc(II) complexes emit at Φ of up
to 0.76 in toluene, which are comparable to those of BODIPYs.
F
The complexes’ spectroscopic properties in solution are
summarized in Table 1, and their absorption spectra are shown
in Figure 1a. The regions of absorption maxima (488−511 nm),
8
,9
Antina and colleagues also discovered that double-helical
bis(dipyrrinato)zinc(II) complexes exhibit fluorescence with
5
5
−1
−1
molar extinction coefficients (1.2 × 10 −1.6 × 10 M cm ),
and emission wavelengths (510−558 nm) are typical of
bis(dipyrrinato)zinc(II) complexes and are attributable to the
1
0
Φ reaching 0.91 in cyclohexane. Thus, recent efforts have
F
realized luminescent bis(dipyrrinato)zinc(II) complexes in
solution. In sharp contrast, their luminescent property in the
1
8
π−π* transition of the dipyrrin core. For complexes 2a Zn−
2
11
solid state has hardly been studied. Here, we developed for the
first time a series of luminescent bis(dipyrrinato)zinc(II)
complexes in the solid state (Scheme 1), discussing the
relationship between their luminescent property and crystal
structures.
2
c Zn, there is an additional absorption band around 370 nm,
2
1
attributable to the π−π* transition of the anthracene subunit.
The constant absorption and emission wavelengths suggest that
the meso-aryl groups of the complexes have little effect on the
ground and photoexcited electronic states of the dipyrrin
subunit. However, the fluorescence quantum yields are strongly
affected by the meso-aryl groups; the conformation of the meso-
aryl group in the excited state greatly affects the accessibility of
The bulkiness of the meso-aryl group of dipyrrin ligands proved
to affect the emissive properties of dipyrrin complexes in
1
2
solution. In association with the previous knowledge, several
types of dipyrrin ligand bearing bulky aryl groups were prepared
here: meso-9-anthracenyl, and 4-tert-butylphenyl ligands 2aH,
13
nonradiative decay pathways. The 2.8−4.5 ns fluorescence
7
,9
lifetimes are comparable to those of reported structures.
8
,9
2bH, 2cH, 3bH, and 3cH. According to reported methods,
new zinc complexes 2a Zn, 2b Zn, 2c Zn, 3b Zn, and 3c Zn
Received: February 24, 2016
2
2
2
2
2
were prepared. Hereafter, the structures and luminescent
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b00431
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Table 1. Spectroscopic Properties of Homoleptic Bis(dipyrrinato)zinc(II) Complexes in Solution (Toluene) and in the Solid State
in toluene solution
in the solid state
λabs /nm 10−5ε /M−1cm−1 λexc /nm λem /nm
a
b
c
d
ΦF
e
τ /ns λabs /nm λexc /nm λem /nm
f
a
,
g
c
d
ΦF
e
τ (f ), τ (f ) /ns
1 1 2 2
f
h
h
h
h
h
h
1a2Zn
1c2Zn
2a2Zn
2b2Zn
2c2Zn
3b2Zn
3c2Zn
495
508
1.6
1.4
495
508
509
532
0.20
0.20
4.5
2.4
2.8
519
530
528
517
526
511
521
500
510
510
510
510
470
510
615
575
635
609
581
560
608
0.02
<0.01
0.03
0.64 (65%), 2.0 (35%)
0.43 (57%), 1.0 (43%)
h
h
h
h
498
493
511
488
506
1.39
1.45
1.29
1.39
1.21
484
480
496
470
490
513
519
558
510
538
0.090
<0.01
<0.01
<0.01
<0.01
<0.01
0.01
4.5
0.03
0.088 (71%), 1.1 (29%)
<0.01
a
b
c
d
e
Absorption maximum wavelength. Maximum molar absorptivity. Excitation wavelength. Fluorescence maximum wavelength. Fluorescence
f
g
h
quantum yield. Fluorescence lifetime excited at 470 nm. measured as drop-cast thin film. from ref 8.
Figure 1. Spectroscopic properties of complexes 1a Zn (orange dashed line), 1c Zn (light-blue dashed line), 2a Zn (red solid line), 2b Zn (green solid
2
2
2
2
line), 2c Zn (blue solid line), 3b Zn (light-green dotted line), and 3c Zn (purple dotted line): (a) absorption and (b) emission in toluene and (c)
2
2
2
emission in the solid state.
Figure 2. Crystal structures of (a) 1a Zn, (b) 2a Zn·3CH Cl , and (d) 3b Zn (thermal ellipsoids set at 50% probability). Hydrogen atoms and solvent
2
2
2
2
2
molecules are omitted for clarity. These three structures, whose fluorescence quantum yields are above 0.01, are shown as representative structures.
Color code: C, gray; N, purple; Zn, red.
Solid thin films of the complexes show absorption spectra
reminiscent of those in solution, with 15−30 nm red shifts and
broadened profiles compared with the complexes (Figure S1).
These differences can be ascribed to the stronger intermolecular
interactions in the solid. The seven complexes appear to be
ligands in different chemical atmospheres in the crystal phase
(vide infra). Given that the emission wavelengths, fluorescence
quantum yields, and lifetimes in the solid state are entirely
different from those in solution, the photophysics of the
dipyrrinato−zinc complexes must also be substantially different
in the solid state.
fluorescent in the solid state, and three of them had Φ values
F
above 0.01. The fluorescence properties in the solid state are
distinctly different from those in solution (Table 1 and Figure 1),
with each complex showing different emission wavelengths and
spectra dependent on their crystal packing structure. The
emission wavelengths are red-shifted compared with those in
The single-crystal X-ray structures of the complexes were also
studied (Figures S2−S8 and Tables S1−S7). Thermal ellipsoid
plots and packing structures for 1a Zn, 2a Zn·3CH Cl , and
2
2
2
2
3b Zn, which showed brighter solid-state luminescence than the
2
others, are shown in Figure 2. 1a Zn, a structure previously
2
8
solution, and the spectra are broadened. 1a Zn emits at 615 nm
reported by our group, has Zn−N distances of 1.976−1.984 Å.
2
with a Φ value of 0.02; the fluorescence shows a dual-
The dihedral angle between the mean plane of the dipyrrin
subunit and that of the mesitylene subunit is 84.4°. The two
dipyrrin subunits coordinating to the zinc center also show an
orthogonal conformationthe dihedral angle is 89.9°
showing that the zinc center adopts a tetrahedral coordination
sphere. Figure S9 shows a relative configuration between two
dipyrrinato ligands in proximity to each other. The two dipyrrins
has a head-to-tail orientation, with an offset distance of 3.199 Å.
F
exponential decay with lifetimes of 0.64 ns (65%) and 2.0 ns
(
35%). Similarly, 2a Zn emits at a more red-shifted wavelength,
2
6
35 nm, with a Φ value of 0.03 and lifetimes of 0.43 ns (57%)
F
and 1.0 ns (43%). 3b Zn fluoresces at 560 nm, close to its
2
solution fluorescence wavelength (510 nm); its Φ value is 0.03,
F
and its lifetimes are 0.088 ns (71%) and 1.1 ns (29%). The series
of dual emission should stem from two or more dipyrrinato
B
DOI: 10.1021/acs.inorgchem.6b00431
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
In this scheme, the transition dipole moment of the S −S
spectroscopic properties and single-crystal X-ray structures. The
great differences between the emission properties of the
complexes in the solid state and in solution are ascribed to
their aggregation states. This is the first report to study the solid-
state emission of bis(dipyrrinato)zinc(II) complexes in depth. It
will contribute to the understanding, design, and further study of
bis(dipyrrinato)zinc(II) complexes.
0
1
transition of the dipyrrin ligand also possesses a head-to-tail
orientation, because it is propagated along the long axis of
1
2
dipyrrin. Therefore, the red-shifted solid-state emission in
13
1
a2Zn can be ascribed to exciton coupling. The single-crystal
structure of 2a Zn·3CH Cl shows characteristics similar to
2
2
2
those of 1a Zn: the Zn−N distances are 1.959−1.990 Å, the
2
dihedral angle between the anthracene and dipyrrin subunits is
8
4.3°, and the dihedral angle between the dipyrrin subunits
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00431.
■
coordinating to the zinc center is 88.9°. In addition to the
*
S
features shared with 1a Zn, intermolecular CH−π interaction
2
between the anthracene moieties (∼3.0 Å) and the dipyrrin
moieties (∼2.8 Å) is observed in 2a Zn·3CH Cl , and the
2
2
2
interaction contributes to the ordered alignment of the
Detailed synthetic and measurement procedures, UV−vis
absorption spectra of thin films, X-ray crystallographic
data, orientations of a pair of dipyrrinato ligands in the
crystal phase, and thermogravimetric analysis (PDF)
CIF file (CIF)
molecules. 2a Zn·3CH Cl also exhibits a dipyrrin−dipyrrin
2
2
2
configuration similar to that of 1a Zn (Figure S10): the two
2
dipyrrins has a head-to-tail orientation, with an offset distance of
3
.214 Å. This structural feature may account for the red-shifted
solid-state emission. However, the crystal and packing structures
of 3b Zn are totally different from those of 1a Zn and 2a Zn·
2
2
2
AUTHOR INFORMATION
Corresponding Authors
E-mail: sakamoto@chem.s.u-tokyo.ac.jp.
E-mail: nisihara@chem.s.u-tokyo.ac.jp.
Notes
■
3
CH Cl . Despite 3b Zn showing typical Zn−N distances of
2
2
2
1
.977−1.983 Å, its 60.1° dihedral angle between the 4-tert-
*
butylphenyl and dipyrrin subunits and its 78.9° angle between
the dipyrrin subunits are distorted relative to those of 1a Zn and
2
*
2
14
a2Zn, which can be attributed to a tighter packing structure.
Figure S11 displays a relative configuration between two
dipyrrinato ligands. The configuration is between the parallel
and head-to-tail ones, and an offset distance of 4.190 Å is much
further than those in 1a Zn and 2a Zn·3CH Cl . This may
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge Grants-in-Aid from the MEXT of
Japan (Grants 26620039, 26220801, 26708005, 15H00862,
16H00900, and 16H0095 and areas 2406, 2506, and 2509).
Single-crystal X-ray diffraction analysis for 2a Zn and 2c Zn was
2
2
2
2
explain the relatively small red shift of the solid-state
luminescence in 3b Zn.
2
Thermogravimetric analysis revealed 2a Zn to be thermally
2
2
2
stable below 380 °C (Figure S12). Therefore, a 2a Zn thin film
performed at the SPring-8 with the approval of JASRI (Grant
2015A1374).
2
was fabricated on a quartz substrate by vacuum deposition. The
resulting UV−vis absorption and emission spectra are given in
Figure 3. The absorption spectrum shows a maximum at 526 nm,
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DOI: 10.1021/acs.inorgchem.6b00431
Inorg. Chem. XXXX, XXX, XXX−XXX