Bridging-arylene effects on spectroscopic and photophysical properties of arylborane-dipyrrinato zinc(ii) complexes

Article abstract of DOI:10.1039/d0ra09029h

Novel bis(dipyrrinato)zinc(ii) derivatives having 4-[bis(2,4,6-trimethylphenyl)boryl]phenyl (ZnBph) or 4-[bis(2,4,6-trimethylphenyl)boryl]-2,3,5,6-tetramethylphenyl groups (ZnBdu) at the 5-position of the dipyrrinato ligands were designed and synthesized.

Full text of DOI:10.1039/d0ra09029h

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Bridging-arylene effects on spectroscopic and
photophysical properties of arylborane–
dipyrrinato zinc(II) complexes†
Cite this: RSC Adv., 2021, 11, 6259
Koyo Takaki, Eri Sakuda, *a Akitaka Ito,
a
b
a
Shinnosuke Horiuchi,
*
a
a
Yasuhiro Arikawa
and Keisuke Umakoshi
Novel bis(dipyrrinato)zinc(II) derivatives having 4-[bis(2,4,6-trimethylphenyl)boryl]phenyl (ZnBph) or 4-
bis(2,4,6-trimethylphenyl)boryl]-2,3,5,6-tetramethylphenyl groups (ZnBdu) at the 5-position of the
dipyrrinato ligands were designed and synthesized. In ZnBph with the smaller dipyrrinate–arylene and
arylene–dimesitylboryl dihedral angles, an intramolecular charge transfer arising from the presence of
the vacant p orbital on the boron atom participates in the pp* excited state in character in contrast to
the pure pp* excited state of ZnBdu. The synergistic pp*/ILCT excited state was weakly fluorescent, and
the fluorescence was enhanced upon binding of fluoride to the boron atom.
[
Received 23rd October 2020
Accepted 23rd January 2021
DOI: 10.1039/d0ra09029h
rsc.li/rsc-advances
complexes was explained by a disconnection of the CT systems in
Introduction
+
[Pt(Bdu-tpy)Cl] due to the perpendicularly-oriented durylene
Triarylborane derivatives exhibit a characteristic absorption/ (2,3,5,6-tetramethylphenylene) group. Thus, the triarylborane has
uorescence owing to an intramolecular charge transfer (CT) become one of the choices to control photophysical/

13–37
transition from the p orbital of aryl group to the vacant p orbital photochemical properties of a metal complex.
However, the
on the boron atom (p(aryl)–p(B) CT) and, therefore, various tri- number of the reports focusing on effects of a linker between the
1
–11
38–40
arylborane derivatives have been reported.
also able to tune the spectroscopic and photophysical properties
Triarylboranes are triarylborane and metal-complex moieties is still limited.
We recently reported synthesis, spectroscopic and photo-
of a metal complex by being introduced to the periphery of its physical properties of cyclometalated iridium(III) and platinum(II)
ligand(s). As a pioneering example reported by Kitamura and complexes with an arylborane-appended dipyrrinato ligand(s).
41
42
+
0
coworkers, a CHCl solution of [Pt(Bph-tpy)Cl] (Bph-tpy ¼ 4 -{4- These complexes showed intense visible absorption (molar
3
4
ꢀ1
ꢀ1
ꢀ3
0
0
00
[
bis(2,4,6-trimethylphenyl)boryl]phenyl}-2,2 :6 ,2 -terpyridine)
absorption coefficient 3 $ 10 M cm (M ¼ mol dm ) at ꢁ490
showed intense phosphorescence even at room temperature nm) and visible–near-IR phosphorescence ascribed to the syner-
(
(
emission quantum yield ¼ 0.011) whereas a metal-to-ligand CT gistic MLCT/pp*/p(aryl)–p(B) CT transition. Dipyrrinato metal
+
0
0
00
MLCT) excited state of a [Pt(tpy)Cl] (tpy ¼ 2,2 :6 ,2 -terpyridine) complexes exhibiting the CT-type excited states are characteristic
12
derivative is typically nonemissive in a solution phase. The at this stage since most of the excited states of dipyrrinato metal
+
enhanced emission from [Pt(Bph-tpy)Cl] originates in syner- complexes are ascribed to pp* transition in the dipyrrinato
43–52
gistic CT interactions between MLCT in the Pt(tpy)Cl moiety and ligand.
Especially, two stereoisomers of bis(5-{4-[bis(2,4,6-tri-
p(aryl)–p(B) CT in the triarylborane unit. Interestingly, an anal- methylphenyl)boryl]phenyl}dipyrrinato)platinum(II), PtBph, pos-
+
0
ogous complex, [Pt(Bdu-tpy)Cl] (Bdu-tpy ¼ 4 -{4-[bis(2,4,6-tri- sessing the square-planar and distorted tetrahedral geometries
0
0
00
methylphenyl)boryl]-2,3,5,6-tetramethylphenyl}-2,2 :6 ,2 -terpyr-
show different absorption/emission spectra not only in the MLCT/
idine), did not exhibit such room-temperature phosphorescence pp*/p(aryl)–p(B) CT band but also in the ligand-centered (LC)
at all. The difference in the phosphorescence ability between the band. Both crystallographic data and theoretical calculations
suggest that a dihedral angle between the dipyrrinate and bridging
phenylene moieties in the square-planar isomer is larger than that
in the distorted tetrahedral one. The results remind us of the
a
Division of Chemistry and Materials Science, Graduate School of Engineering,
Nagasaki University, Bunkyo-machi 1-14, Nagasaki 852-8521, Japan. E-mail:
earlier Pt-tpy system and importance of further understanding of
Graduate School of Engineering, Kochi University of Technology, Miyanokuchi 185, the ligand itself in an arylborane-appended metal complex.
sakueri@nagasaki-u.ac.jp; kumks@nagasaki-u.ac.jp
b
Tosayamada, Kami, Kochi 782-8502, Japan
In order to extract the molecular/electronic effects of the
ligand itself in an arylborane–dipyrronato metal complex, we
chose dipyrrinato zinc(II) complexes. They exhibit intense
1
13
1
†
Electronic supplementary information (ESI) available: H and C{ H} NMR
spectra, crystallographic data of ZnBdu, detailed results of theoretical
1
calculations, uorescence decay proles and H NMR spectra in the absence
visible absorption (absorption maximum wavelength l
z
abs
and presence of TBAF. CCDC 2035525. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/d0ra09029h
4
ꢀ1
ꢀ1
470–500 nm; 3 z (5.0–10) ꢂ 10 M cm ) and uorescence
©
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1
0
1
13
1
11
1
19
1
ascribed to pure LC pp* excited states owing to the inert d
nature of the zinc(II) center.
of the H, C{ H}, B{ H} and F{ H} NMR spectra deter-
53–61
Furthermore, the spectroscopic mined in CDCl were given in ppm relative to tetramethylsilane
3
properties of a dipyrrinato zinc(II) complex are dependent on (0.00 ppm for H and C{ H}) as an internal standard, or boron
the substituent group at the 5-position of the dipyrrinato ligand. triuoride diethyl-ether complex (0.00 ppm for B{ H}) and
1
13
1
1
1
1
1
9
1
For example, Lindsey and coworkers revealed that bis[5-(2,4,6- hexauorobenzene (ꢀ164.9 ppm vs. CFCl
trimethylphenyl)dipyrrinato]zinc(II) (Znmes) showed much external standards. High-resolution fast-atom bombardment
higher uorescence quantum yield (F
¼ 0.36) than bis(5-phe- mass spectroscopies (HR-FAB-MS) were carried out on a JEOL
nyldipyrrinato)zinc(II) (Znph, F
3
for F{ H}) as
f
f
¼ 0.01) by effects of both uo- JMS-700N spectrometer. Elemental analyses (C, H, N) were
62
rescent and nonuorescent pathways. It has also been performed by a Perkin Elmer 2400II elemental analyzer. UV-vis
reported that the uorescence from Znmes is quenched in absorption spectra were recorded on a Jasco V-560 spectro-
a polar solvent by a thermal deactivation of the excited state via photometer. The corrected emission spectra were obtained by
58,63,64
a CT-type state.
These results indicate that structures of using a Jasco F-6500 spectrouorometer (excitation wavelength
aryl groups at 5-position of the dipyrronato ligands and the ¼ 365 nm). Fluorescence decay measurements were conducted
introduction of intramolecular CT interactions are key points to by using a Hamamatsu Photonics picosecond uorescence
control the spectroscopic/photophysical properties of a bis(di- lifetime measurement system C11200 equipped with pico-
pyrrinato)zinc(II) complex. The p(aryl)–p(B) CT of an arylborane second light pulser PLP-10 as a 405 nm excitation light source.
group is, therefore, one of possible candidates to tune the Emission quantum yields were determined by using a Hama-
points. We synthesized novel bis(dipyrrinato)zinc(II) derivatives matsu Photonic Absolute PL Quantum Yield Measurement
having 4-(dimesitylboryl)phenyl (ZnBph) or (dimesitylboryl) System C9920-02 (excitation wavelength ¼ 365 nm). For the
duryl group (mesityl ¼ 2,4,6-trimethylphenyl and duryl ¼ photophysical
measurements,
spectrophotometric-grade
2
,3,5,6-tetramethylphenyl) at 5-position of the dipyrrinato toluene was used as supplied.
ligands (ZnBdu), whose structures are shown in Chart 1. The
bridging phenylene and durylene moieties in the complexes
gave signicant differences especially in the uorescence and
Synthesis of (4-iodo-2,3,5,6-tetramethylphenyl)bis(2,4,6-
trimethylphenyl)borane (1)

uoride-binding properties. The characteristic photophysical
Synthesis was performed with minor changes in the reported
properties of the former complex (ZnBph) can be explained by
gained intraligand charge transfer (ILCT) character in the
excited state and discussed in terms of the molecular geome-
tries obtained by theoretical calculations.
27
procedures. An oven-dried Schlenk tube was evacuated and
lled subsequently with an argon gas. 1,4-Diiodo-2,3,5,6-

tetramethylbenzene (2.8 g, 7.2 mmol) and dry diethyl ether
ꢃ
(
20 mL) was added, then, cooled to ꢀ78 C in an acetone/dry-ice
bath. n-Butyllithium in n-hexane (1.6 M, 5.2 mL, 8.3 mmol) was
ꢃ
ꢃ
Experimental section
Chemicals
added to the reaction mixture at ꢀ78 C. Aer stirring at ꢀ78 C
for 1 h, the reaction mixture became a pale-yellow suspension.
Bis(2,4,6-trimethylphenyl)boron uoride (2.1 g, 7.9 mmol) dis-
solved in dry diethyl ether (20 mL) was added to the reaction
mixture, then, the mixture allowed to warm to room tempera-
ture and continuously stirred overnight. The reaction mixture
became a yellow suspension. Aer an addition of HCl(aq) (1 M,
All the reagents including organic solvents were commercially
available and used without further purication. 5-{4-[Bis(2,4,6-
trimethylphenyl)boryl]phenyl}dipyrrin was prepared according
41
to the literature methods. 4-[Bis(2,4,6-trimethylphenyl)boryl]-
,3,5,6-tetramethylbenzaldehyde (2) was synthesized similarly
2
2
2
0 mL), the mixture was extracted with diethyl ether (30 mL ꢂ
to the phenyl analogue. All the synthetic reactions and subse-
quent workup manipulations were carried out under air unless
otherwise noted.
). The combined organic extract was washed with water (50
mL), dried over anhydrous Na SO and concentrated under
2
4
reduced pressure. The crude product was washed with n-hexane
1
(
50 mL) to give pure 1 (2.0 g, 59%) as a colorless solid. H NMR
(400 MHz, CDCl ) d: 6.73 (4H, s, m-Ar-H of mesityl), 2.43 (6H, s,
,6-CH of duryl), 2.26 (6H, s, p-CH of mesityl), 2.09 (6H, s, 3,5-
CH of duryl), 1.94 ppm (12H, s, o-CH of mesityl).
Physical measurements and instrumentations
3
2
3
3
NMR spectra were recorded on a JEOL JNM-AL 400 Fourier-
transform NMR spectrometers (400 MHz). The chemical shis
3
3
Synthesis of 4-[bis(2,4,6-trimethylphenyl)boryl]-2,3,5,6-
tetramethylbenzaldehyde (2)
An oven-dried Schlenk tube was evacuated and lled subse-
quently with an argon gas. 1 (2.0 g, 4.9 mmol) and dry tetra-
ꢃ
hydrofuran (20 mL) was added, then, cooled to ꢀ78 C in an
acetone/dry-ice bath. n-Butyllithium in n-hexane (1.6 M, 4.4 mL,
ꢃ
7
.0 mmol) was added to the reaction mixture at ꢀ78 C. Aer
ꢃ
stirring at ꢀ78 C for 1 h, the reaction mixture became an
orange suspension. Dry N,N-dimethylformamide (2.0 mL) was
). added to the reaction mixture at ꢀ78 C. Aer stirring at ꢀ78 C
ꢃ
ꢃ
Chart 1 Chemical structures of ZnBph (R ¼ H) and ZnBdu (R ¼ CH
3
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+
for 1 h, the reaction mixture became a yellow solution. Then, FAB-MS (CH
2
Cl
2
) m/z: calculated for C66
H
65
B
2
N
4
Zn ([M +
+
HCl(aq) (1 M, 50 mL) was added and stirred for another 4 h. The H] ), 999.4687; found, 999.4687. Anal. calcd (%) for C H B -
6
6
64 2
reaction mixture was extracted with ethyl acetate (30 mL ꢂ 2). N Zn$CH OH: C, 77.96; H, 6.64; N, 5.43. Found: C, 77.61; H,
4
3
The combined organic extract was washed with water (50 mL), 6.63; N, 5.31.
dried over anhydrous MgSO and concentrated under reduced
4
pressure. The crude product was puried by recrystallization Synthesis of bis(5-{4-[bis(2,4,6-trimethylphenyl)boryl]-2,3,5,6-
from ethyl acetate to give pure 2 (1.3 g, 65%) as a pale-yellow tetramethylphenyl}dipyrrinato)zinc(II) (ZnBdu)
1
solid. H NMR (400 MHz, CDCl
3
) d: 10.7 (1H, s, CHO), 6.75
of duryl), 2.27
To a CH
2
Cl
2
solution (30 mL) of 3 (125 mg, 0.24 mmol),
$2H O (158 mg, 0.72
(
(
(
4H, s, m-Ar-H of mesityl), 2.31 (6H, s, 2,6-CH
6H, s, p-CH of mesityl), 2.02 (6H, s, 3,5-CH of duryl), 1.95 ppm
12H, s, o-CH of mesityl).
3
a CH OH solution (10 mL) of Zn(OAc)
3
2
2
3
3
mmol) was added and stirred at room temperature. Aer stir-
ring overnight, the solvent was evaporated under reduced
pressure. The crude product was puried by silica-gel column
chromatography, eluting with CH Cl . Recrystallization from
3
Synthesis of 5-{4-[bis(2,4,6-trimethylphenyl)boryl]-2,3,5,6-
tetramethylphenyl}dipyrrin (3)
2
2
CH Cl /methanol afforded pure ZnBdu (80.8 mg, 61%) as
2
2
1
a yellow-brown solid. H NMR (400 MHz, CDCl ) d: 7.50 (4H, s,
2
(501.4 mg, 1.22 mmol) was dissolved in neat pyrrole (25 mL)
and degassed by a nitrogen-gas bubbling for 30 min. Tri-
uoroacetic acid (20 mL, 0.23 mmol) was then added, and stirred
at room temperature for 10 min. The reaction mixture was
diluted with CH Cl (50 mL), washed with NaOH(aq) (1 M, 50
mL) and dried over anhydrous MgSO . Aer removing MgSO by
ltration and evaporation of CH Cl under reduced pressure,
3
1
,9-Ar-H of dipyrrinate), 6.79 (8H, d, J ¼ 6.8 Hz, m-Ar-H of
mesityl), 6.57 (4H, d, J ¼ 4.0 Hz, 2,8-Ar-H of dipyrrinate), 6.38

(
4H, d, J ¼ 4.2 Hz, 3,7-Ar-H of dipyrrinate), 2.30 (12H, s, p-CH
3
of
mesityl), 2.10 (12H, s, 2,6-CH
3
of durylene), 2.05 (12H, s, 3,5-CH
3
2
2
1
3
1
of durylene), 2.02 ppm (24H, s, o-CH
3
of o-mesityl). C{ H}
4
4
NMR (CDCl ) d: 150.3, 149.0, 144.6, 141.0, 140.8, 140.3, 139.4,

2
2
3
1
38.9, 134.9, 132.1, 131.3, 129.0, 128.8, 117.0, 29.7, 23.3, 22.9,
the remaining pyrrole was removed by vacuum distillation with
1
1
1
ꢃ
21.2, 19.9, 17.4 ppm. B{ H} NMR (CDCl ) d: 79.8 ppm. HR-
FAB-MS (CH
heating (40 C). Then, the product was added to a solution of p-
3
+
2
Cl
2
) m/z: calculated for C74
H
81
B
2
N
4
Zn ([M +
chloranil (500 mg, 2.04 mmol) in CH Cl (50 mL). The solution
2
2
+
H] ), 1111.5939; found, 1111.5938. Anal. calcd (%) for C74
Zn$CH OH: C, 78.71; H, 7.40; N, 4.90. Found: C, 78.89; H,
.35; N, 4.77.
80
H -
color changed from yellow to dark yellow-green. The solution
was stirred overnight and, then, ltered and evaporated to
B
7
2
N
4
3
remove resultant insolubles and CH
reaction mixture was processed by reprecipitation (CH
hexane) and silica-gel column chromatography, eluting with
CH Cl , to give the pure product (125 mg, 20%) as a pale yellow-
2
Cl
2
respectively. The
2
2
Cl /n-
X-ray crystal structure determinations
ꢃ
2
2
Diffraction data were collected at ꢀ180 C under a steam of cold
1
green solid. H NMR (400 MHz, CDCl ) d: 7.66 (2H, s, 1,9-Ar-H of
3
N gas on a Rigaku RA-Micro7 HFM instrument equipped with
2
dipyrrin), 6.78 (4H, s, m-Ar-H of mesityl), 6.39 (2H, dd, J ¼ 1.1,
a
Rigaku Saturn724+ CCD detector by using graphite-
4
.1 Hz, 2,8-Ar-H of dipyrrin), 6.36 (2H, dd, J ¼ 1.6, 3.8 Hz, 3,7-Ar-
monochromated Mo Ka radiation. The frame data were inte-
grated using a Rigaku CrystalClear program package, and the
data sets were corrected for absorption using a REQAB program.
The calculation was performed with a CrystalStructure soware
package except for renement, which was performed using
SHELXL Version 2018/3. The structures were solved by direct
methods and rened on F by the full-matrix least-squares
methods. Anisotropic renement was applied to all non-
hydrogen atoms with the exception of the crystal solvents. All
hydrogen atoms were put at calculated positions.
H of dipyrrin), 2.29 (6H, s, p-CH
of durylene), 2.01 (12H, d, J ¼ 7.9 Hz, o-CH
.94 ppm (6H, s, 2,6-CH of durylene). FAB-MS (CH
3
of mesityl), 2.05 (6H, s, 3,5-CH
of mesityl),
Cl ) m/z: 525
3
65
3
1
3
2
2
+
([M + H] ).
66
67
2
Synthesis of bis(5-{4-[bis(2,4,6-trimethylphenyl)boryl]phenyl}
dipyrrinato)zinc(II) (ZnBph)
To a CH
boryl]phenyl}dipyrrin (100 mg, 0.21 mmol), a CH
10 mL) of Zn(OAc) $2H O (138 mg, 0.63 mmol) was added and
stirred at room temperature. Aer stirring overnight, the solvent
2
Cl
2
solution (50 mL) of 5-{4-[bis(2,4,6-trimethylphenyl)
3
OH solution
(
2
2
Computational methods
was evaporated under reduced pressure. The crude product was Theoretical calculations for the complexes were conducted with
68
puried by silica-gel column chromatography, eluting with Gaussian 16W soware (Revision A.03). Optimizations of the
CH Cl . Recrystallization from CH Cl /methanol afforded pure ground-state geometries of the complexes were performed by
ZnBph (98.5 mg, 45%) as an orange solid. H NMR (400 MHz, using the B3LYP density functional theory (DFT).
2
2
2
2
1
69,70
The
and 6-31G(d,p) basis sets were used to treat the
4H, d, J ¼ 6.9 Hz, 3,5-Ar-H of phenylene), 7.54 (4H, s, 1,9-Ar-H geometrical structures of the zinc and all other atoms, respec-
of dipyrrinate), 6.86 (8H, s, m-Ar-H of mesityl), 6.68 (4H, d, J ¼ tively. Time-dependent DFT (TD-DFT) calculations were then
.8 Hz, 2,8-Ar-H of dipyrrinate), 6.38 (4H, d, J ¼ 3.8 Hz, 3,7-Ar-H performed to estimate the energies and oscillator strengths f of
of dipyrrinate), 2.33 (12H, s, p-CH of mesityl), 2.02 ppm (24H, s, electronic excitation transitions generating the 50 lowest-energy
71–73
74
CDCl
(
3
) d: 7.60 (4H, d, J ¼ 7.6 Hz, 2,6-Ar-H of phenylene), 7.55 LanL2DZ
3
3
1
3
1
o-CH of mesityl). C{ H} NMR (CDCl ) d: 149.9, 148.6, 146.4, singlet excited states. All of the calculations were carried out as
3
3
7
5
1
1
42.3, 141.8, 140.9, 140.3, 139.0, 134.8, 132.8, 130.4, 128.3, in toluene by using a polarizable continuum model (PCM).
1
1
1
3
17.2, 23.5, 21.3 ppm. B{ H} NMR (CDCl ) d: 75.7 ppm. HR- Optimized geometries, Kohn–Sham molecular orbitals and
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ꢀ
3
ꢀ
natural transition orbitals (isovalue ¼ 0.03 e A ) were visual- basis of the DFT calculations because single crystals of ZnBph
7
6
ized by GaussView 5.
suitable for X-ray structural analysis were not obtained in spite
of many efforts for recrystallization from various conditions.
Optimized geometries of ZnBph and ZnBdu are shown in
Fig. 1(b) and (c), respectively. The zinc centers in both
complexes take tetrahedral geometries, and zinc–nitrogen
Results and discussion
Synthesis and characterization
ꢀ
distances are in the range of 2.059–2.063 A. The dihedral angle
Novel dipyrrinato zinc(II) complexes ZnBph and ZnBdu were
successfully synthesized in moderate yields (45% for ZnBph,
between dipyrrinato ligating moiety and bridging phenylene
) in ZnBph (64.1 –64.3 ) is smaller than that in ZnBdu
89.0–89.2 ). Similar tendency was also observed for the tilt
ꢃ
ꢃ
group (q
(
1
61% for ZnBdu) by mixing Zn(OAc)
2 2
$2H O and relevant dipyrrin
ꢃ
in a dichloromethane/methanol mixture at room temperature.
1
13
1
angle between arylene group and the BC plane in the arylbor-
The complexes were identied by the H and C{ H} NMR
3
1
ꢃ
ꢃ
ꢃ
ꢃ
ane moiety (q ¼ 21.7 –25.6 and 57.1 –57.7 for ZnBph and
spectroscopies, HR-FAB-MS and elemental analysis. In the H
2
ZnBdu, respectively). On the basis of these angles, (cos q
1
ꢂ
NMR spectra of ZnBph and ZnBdu (see Fig. S3 and S4†), four
protons at 1- and 9-positions of dipyrrinate moiety were
observed as singlet at 7.50–7.54 ppm which was shied to
upeld upon the complexation. Two doublets ascribed to
protons at 2,8- and 3,7-positions were observed at 6.38–
cos q ) values as measures of the conjugation between the p
2
6
.68 ppm. The trends are similar to other bis(dipyrrinato)zinc(II)
54
complexes. Single crystals of ZnBdu suitable for X-ray crys-
tallographic analysis were obtained from CH Cl /n-pentane.
2
2
The X-ray structure of ZnBdu is shown in Fig. 1(a), whose
crystallographic data is summarized in Table S1.† The zinc
center takes a tetrahedral geometry with a dihedral angle for
ꢃ
two Zn–N
2
(dipyrrinato) planes being 90 . The distance between
ꢀ
the zinc center and each pyrrolic nitrogen is 1.983 A, which are
54
ꢀ
similar to those in Znph (1.973–1.988 A). The dihedral angle
between a dipyrrinato ligating and bridging durylene moieties
ꢃ
was 77.9 . The value is similar to those of the previous iridiu-
4
1
ꢃ
ꢃ
ꢃ
42
m(III) (71.1 ) and platinum(II) complexes (80.7 and 68.7 ).
2
The boron atoms in the arylborane groups take a planar sp -like
ꢀ
conguration with B–C bond lengths being 1.57–1.58 A and
ꢃ
C–B–C angles being ꢁ120 . Dihedral angle between the dur-
ꢃ
Fig. 2 UV-vis absorption (broken lines) and fluorescence spectra
solid lines) of ZnBph (red) and ZnBdu (blue) in toluene at 298 K. The
integrations of the fluorescence spectra in a wavenumber scale
correspond relatively to the fluorescence quantum yields of the
ylene and BC
3
plane was 54.1 . These structural characteristics
(
of ZnBdu in the crystalline phase are indicative of the conju-
gation between the dipyrrinate moiety and boron atom in the
ligand. The structures of the complexes were compared on the complexes. Inset: normalized fluorescence spectra.
Fig. 1 Perspective view of crystal structure for ZnBdu ((a) 50% probability ellipsoids) and DFT optimized geometries of ZnBph (b), ZnBdu (c) and
ꢀ
ZnBph$2F (d): carbon (gray), nitrogen (blue), boron (red), zinc (orange) and fluorine (green). Hydrogen atoms are omitted for clarity.
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Table 1 Spectroscopic and photophysical properties of the dipyrrinato zinc(II) complexes in toluene at 298 K
4
ꢀ1
ꢀ1
a
r
ꢀ1
a
ꢀ1
Complex
l
abs/nm (3/10 M cm
)
f
l /nm
F
f
f
s /ns
k
/s
knr /s
6
8
8
8
ZnBph
ZnBdu
334 (4.3), 487 (09.2)
333 (4.4), 486 (13.2)
383 (1.6), 481 (09.4)
485
322 (1.4), 482 (11.5)
487
517
504
503
500
500
501
501
0.01
0.34
0.02
0.006
1.3
2.7
0.2
0.09
7.7 ꢂ 10
1.3 ꢂ 10
7.6 ꢂ 10
2.4 ꢂ 10
ꢀ
8
9
ZnBph$2F
1 ꢂ 10
5 ꢂ 10
b
7
10
Znph
7 ꢂ 10
1 ꢂ 10
c
Znph
b
8
8
Znmes
Znmes
0.36
2.7
1.3 ꢂ 10
2.4 ꢂ 10
c
345, 485
a
b
c
Calculated by the equation, F
f
¼ k
r
/(k
r
+ knr) ¼ k
r
f
s .
2 2
Data in toluene compiled from ref. 62. Data in CH Cl compiled from ref. 54.
orbital of the dipyrrinate moiety and the vacant p orbital on the broad absorption bands at around 487 and 334 nm, respec-
boron atom via the bridging arylene moiety in ZnBph and tively. According to the DFT calculations (vide infra), the former
ZnBdu were calculated to be 40 and 1%, respectively. The band is assigned to the typical pp* transitions in a dipyrrinato
smaller dihedral angle in ZnBph than in ZnBdu strongly indi- ligand and the latter is ascribed to p(aryl)–p(B) CT transitions in
cates enhanced electronic interactions between the dipyrrinato an arylborane moiety, similarly to other arylborane–dipyrrinato
41,42
zinc(II) complex and the arylborane moieties, and therefore metal complexes.
The absorption maximum wavelengths of
larger contribution of the arylborane moieties to the ZnBph and ZnBdu (labs ¼ 487 and 486 nm, respectively) are
spectroscopic/photophysical properties of
expected.
a
complex are comparable with those of Znph and Znmes (labs ¼ 485 nm and
62
487 nm, respectively) without any arylborane substituents.
Interestingly, the molar absorption coefficients (3) at the
maximum wavelength (l ) of the low-energy band of the
abs
4
ꢀ1
ꢀ1
Absorption spectra
complexes (9.2 ꢂ 10
M
cm
0 M cm for ZnBdu) were larger than those of the aryl-
borane–dipyrrinato iridium(III) and platinum(II) complexes (3 ¼
for ZnBph and 1.32 ꢂ
5
ꢀ1
ꢀ1
1
Fig. 2 shows absorption spectra of ZnBph and ZnBdu in toluene
at 298 K, and the spectroscopic properties are summarized in
Table 1. The complexes exhibited intense/narrow and weak/
4
4
ꢀ1
ꢀ1
41,42
7
.4 ꢂ 10 and 2.2 ꢂ 10 M cm , respectively)
whose pp*
3 4
Fig. 3 NTOs for S and S states of the complexes in toluene. Hydrogen atoms are omitted for clarity.
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states of the complexes. Both S
3
and S
4
states of ZnBph originate
in electronic transitions from the dipyrrinate moiety to the
whole ligand, gaining the p(aryl)–p(B) CT character, whereas
those of ZnBdu are assignable to the pure pp* transitions in a 5-
duryldipyrrinate moiety.
Chart 2 Structural change of an arylborane compound upon the
binding of fluoride.
Fluorescence spectra and photophysical properties
transitions synergistically interact with MLCT and/or p(aryl)– As shown in Fig. 2, the uorescence from ZnBph (F ¼ 0.01 in
f
p(B) CT ones. Such differences in the 3 values indicates that an toluene at 298 K) was signicantly weak compared with that
introduction of the charge-transfer character to the pp* tran- from ZnBdu (F ¼ 0.34). Furthermore, the uorescence spec-
f
sition in a dipyrrinato–metal complex decreases the relevant trum of ZnBph (maximum wavelength lf ¼ 517 nm) was
oscillator strength. Thus, smaller 3 value of ZnBph suggests the broadened and shied to lower-energy compared with those of
62
62
existence of the p interactions throughout the ligand presum- ZnBdu (504 nm), Znph (501 nm) and Znmes (500 nm) see
ably owing to the smaller dihedral angles between dipyrrinate– Table 1. The spectroscopic data of ZnBph indicate a partial
phenylene and phenylene–dimesitylboryl moieties. These contribution of a CT character arising from the low-energy p(B)
discussions were theoretically supported by TD-DFT calcula- to the uorescent excited state and energetic stabilization of the
tions as summarized in Tables S2–S5.† For both complexes, excited state by the solvation. In practice, the radiative rate
6
ꢀ1
8
intense absorption bands at ꢁ487 nm appeared as electronic constant (k ) of ZnBph (7.7 ꢂ 10 s , see Table 1) was 17-times
r
ꢀ1
transitions generating third (S ) and forth excited states (S ). smaller than that of ZnBdu (1.3 ꢂ 10 s ) owing to a decreased
3
4
Fig. 3 shows natural transition orbitals (NTOs) for the S and S
wavefunction overlap between the excited and ground states.
3
4
Fig. 4 UV-vis absorption and fluorescence spectral changes upon an addition of TBAF (0–4.0 equiv.) in toluene: ZnBph ((a and b) red to green)
and ZnBdu ((c and d) blue to yellow).
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Furthermore, the CT character in the excited state accelerated atom in ZnBph increased electron density around the boron
thermal deactivation to the ground state as the nonradiative atom.
8
ꢀ1
ꢀ
decay rate constant (k ) of ZnBph (7.6 ꢂ 10 s ) was three-
The structure of ZnBph$2F was theoretically investigated by
). It the DFT calculation and the optimized geometry is shown in
nr
8
ꢀ1
times larger than that of ZnBdu (k ¼ 2.4 ꢂ 10
s
nr
should be noted that the k
r
and knr values of ZnBdu are Fig. 1(d). Each boron atom possesses a tetrahedral geometry,
8
ꢀ1
comparable to those of Znmes (k
r
¼ 1.3 ꢂ 10 s and knr ¼ 2.4 and the dihedral angles between a dipyrrinato ligating moiety
8
ꢀ1 54
ꢃ
ꢂ 10 s ). Thus, the strong uorescence from ZnBdu would and bridging phenylene group (q
1
) were reduced to be 56.8 –
ꢃ
originate in the pure pp* excited state and suppressed non- 58.7 . The structural and electronic changes increased the
radiative decay processes owing to the presence of the bulky transition energy of the lowest-energy absorption band through
durylene bridging units. On the other hand, the k and k values efficient electron donation from the uorinated arylborane
r
nr
of ZnBph are signicantly smaller than those of Znph (k ¼ 7 ꢂ group to the dipyrrinato moiety, leading to the decrease of
r
7
ꢀ1
10 ꢀ1 54
1
0 s and k ¼ 1 ꢂ 10
s ), suggesting the existence of CT electron density on the dipyrrinato moiety and therefore the
nr
interactions in the excited state. As results, the uorescence downeld-shi of proton signals in the pyrrole rings (see
from ZnBph was well characterized by the pp*/ILCT excited Fig. S15† for HOMO and HOMOꢀ1). The uorescence from
state, and the participation of the ILCT character in the excited ZnBph was shied to higher-energy with a slight decrease in
state of a complex was revealed by varying the extent of p- intensity and then enhanced largely upon a continuous addi-
conjugation between the dipyrrinate and arylborane moieties.
tion of TBAF as shown in Fig. 4(b). The maximum wavelength (l
f
¼
503 nm) and band shape in the presence of $3.0 equivalence
of uoride were almost identical to those of ZnBdu, and the k
r
ꢀ
8
ꢀ1
value of ZnBph$2F (1 ꢂ 10 s ) was also similar to that of
Spectroscopic responses to uoride
ZnBdu. Owing to these uorescence characteristics, it can be
ꢀ
Since the electron-decient boron atom in an arylborane expected that the uorescent excited state of ZnBph$2F
derivative can bind with a small Lewis base such as uoride possesses the pure pp* character. Signicantly smaller F
f
(0.02)
we of ZnBph$2F than that of ZnBdu can be explained by enhanced
6
ꢀ1 77,78
ꢀ
(Chart 2) with a binding constant being ꢁ10
M ,
9
ꢀ1
carried out the uoride-addition experiments for the thermal deactivation (knr ¼ 5 ꢂ 10 s ) via a rotation of the
complexes. Fig. 4 shows absorption and uorescence spectra of phenylene moiety as reported for Znph and Znmes (knr ¼ 1.1 ꢂ
1
0
8
ꢀ1
54
the complexes in the absence and presence of tetra-n-buty- 10 and 3.2 ꢂ 10 s in toluene, respectively). On the other
lammonium uoride (TBAF) in toluene. The absorbances of hand, there was no experimental evidence of the uoride
1
11
each spectrum were divided by the total concentration of the binding to ZnBdu in the absorption, uorescence and H,
B
1
17
1
complex (c ) and optical path length (l) so that the vertical axis { H} and F{ H} NMR spectra as shown in Fig. 4(c) and (d), S9,
0
corresponds to the molar absorption coefficient. Upon an S11 and S14† presumably due to the steric hindrance of the
addition of TBAF to ZnBph, the p(aryl)–p(B) CT absorption band durylene moieties. Thus, uoride binding affinity of an aryl-
at around 334 nm disappeared, and a broad absorption band at borane–dipyrrinato zinc(II) complex was controllable by the
around 383 nm appeared with an isosbestic point at 360 nm. In bridging arylene moiety and, upon the uoride binding, the
addition, the uoride binding shortened the maximum wave- excited-state electronic structure of ZnBph was switched from
length of the lowest-energy pp*/ILCT absorption band of pp*/ILCT to pure pp*.
ZnBph from 487 nm to 481 nm, similarly to that observed for
41
the relevant cyclometalated iridium(III) complex. Complete
disappearance of the p(aryl)–p(B) CT band and the complicated
Conclusions
spectral changes in the absorption band at ꢁ480 nm indicate The bridging arylene moieties in novel bis(dipyrrinato)zinc(II)
successive bindings of two uoride, affording a 1 : 1 adduct, derivatives having the arylborane groups at 5-position of the
ꢀ
followed by a 1 : 2 adduct, ZnBph$2F . The spectroscopic dipyrrinato ligands had signicant impacts on the absorption/
changes arising from the binding of uoride to the boron atom uorescence spectra and uoride-binding affinity of the
1
11
1
19
1
were strongly evidenced by H, B{ H} and F{ H} NMR complex. The theoretical calculations suggest that ZnBph with
measurements as shown in Fig. S8, S10 and S13,† respectively. the phenylene linkers possesses smaller dihedral angles
1
1
The B NMR signal of ZnBph was drastically shied from between dipyrrinate–phenylene and phenylene–dimesitylboryl
5.7 ppm to 5.5 ppm upon the addition of uoride as TBAF. The moieties than the relevant values of ZnBdu with the durylene
7
1
9
broad signal was also observed at ꢀ174.41 ppm for the F NMR linkers. The smaller dihedral angles in ZnBph afford the p-
spectrum of ZnBph in the presence of TBAF (4.0 eq.) owing to conjugation in the entire of the ligand and, therefore, the
1
0
11
the coupling to B (I ¼ 3) and B (I ¼ 3/2), indicating the electron-withdrawing arylborane groups participate in the
formation of B–F bond with ZnBph. In addition, the absence of electronic structure of the complex. As a result, the excited state
the signal at ꢀ84.04 ppm (corresponding to the signal of of ZnBph was best characterized by the synergistic pp*/ILCT,
2
CDClF generated by the reaction of uoride with solvent whereas that of ZnBdu was the pure pp*. ZnBph could bind
79
19
molecule CDCl3) in the F NMR spectrum of ZnBph suggests with uoride, and the excited state was switched from pp*/ILCT
that uoride quickly binds to the boron atom before proceeding to pure pp* upon the uoride binding owing to the disap-
the exchange reaction of uoride with chloride in the solvent pearance of the electron-withdrawing ability of the dimesi-
molecule. Consequently, the binding of uoride to the boron tylboryl moieties. Thus, we revealed the importance of the
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molecular design including the linker structure in metal 20 S. T. Lam, N. Zhu and V. W. W. Yam, Inorg. Chem., 2009, 48,
complexes with an arylborane group(s). Tuning of the extent of 9664–9670.
the CT character in an excited-state metal complex will be an 21 W. J. Xu, S. J. Liu, X. Y. Zhao, S. Sun, S. Cheng, T. C. Ma,
important factor to control spectroscopic and photophysical
properties of metal complexes.
H. Bin Sun, Q. Zhao and W. Huang, Chem.–Eur. J., 2010,
16, 7125–7133.
2
2
2 C. R. Wade and F. P. Gabbai, Inorg. Chem., 2010, 49, 714–720.
3 E. Sakuda, Y. Ando, A. Ito and N. Kitamura, Inorg. Chem.,
Conflicts of interest
2
011, 50, 1603–1613.
24 A. Ito, T. Hiokawa, E. Sakuda and N. Kitamura, Chem. Lett.,
010, 40, 34–36.
5 Z. M. Hudson and S. Wang, Organometallics, 2011, 30, 4695–
701.
The authors declare no competing nancial interest.
2
2
Acknowledgements
4
This research was partially funded by the Sasakawa Scientic 26 Y. Sun, Z. M. Hudson, Y. Rao and S. Wang, Inorg. Chem.,
Research Grant from the Japan Science Society. We are grateful 2011, 50, 3373–3378.
to J. Nagaoka for the technical assistance and the X-ray 27 A. Ito, Y. Kang, S. Saito, E. Sakuda and N. Kitamura, Inorg.
measurements.
Chem., 2012, 51, 7722–7732.
2
8 Z. M. Hudson, C. Sun, M. G. Helander, Y. L. Chang, Z. H. Lu
and S. Wang, J. Am. Chem. Soc., 2012, 134, 13930–13933.
9 R. S. Vadavi, H. Kim, K. M. Lee, T. Kim, J. Lee, Y. S. Lee and
M. H. Lee, Organometallics, 2012, 31, 31–34.
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