Enhanced luminescent properties of an open-shell (3,5-Dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical by coordination to gold

Article abstract of DOI:10.1002/anie.201411572

A gold(I) complex containing an open-shell luminescent (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl (PyBTM) radical was prepared. The complex showed fluorescence centered mainly on the coordinated PyBTM ligand. The photophysical and photochemical properties were positively modulated upon coordination to Au^I; the photoluminescence quantum yield, fluorescence wavelength, and the stability in the photoexcited state all increased. A gold(I) complex containing an open-shell luminescent (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl (PyBTM) radical was prepared. The complex has fluorescence that is centered mainly on the coordinated PyBTM ligand. The photoluminescence quantum yield, fluorescence wavelength, and the stability in the photoexcited state all increased upon coordination to Au^I.

Full text of DOI:10.1002/anie.201411572

Angewandte
Chemie
DOI: 10.1002/anie.201411572
Radical Complexes
Enhanced Luminescent Properties of an Open-Shell (3,5-Dichloro-4-
pyridyl)bis(2,4,6-trichlorophenyl)methyl Radical by Coordination to
Gold**
Yohei Hattori, Tetsuro Kusamoto,* and Hiroshi Nishihara*
Abstract: A gold(I) complex containing an open-shell lumi-
nescent (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)-
ambient condition and photoirradiation.[5] The nitrogen
atom of PyBTM, which enhances its photostability (up to
115 times higher than tris(2,4,6-trichlorophenyl)methyl rad-
ical), acts as a proton coordination site. The electronic and
optical properties were controlled by reversible protonation;
the fluorescence intensity decreased largely upon protona-
tion. Thanks to its stimuli-responsive properties and high
photostability, PyBTM is a promising candidate for inves-
tigating the characteristic luminescent properties of doublet
species controlled by external stimuli.
methyl (PyBTM) radical was prepared. The complex showed
fluorescence centered mainly on the coordinated PyBTM
ligand. The photophysical and photochemical properties were
positively modulated upon coordination to AuI; the photo-
luminescence quantum yield, fluorescence wavelength, and the
stability in the photoexcited state all increased.
S
pin multiplicity is an important issue in fundamental and
applied research of luminescent molecules. In closed-shell
luminescent molecules, fluorescence from singlet excited
states (S1) and phosphorescence from the triplet excited
state (T1) have been extensively studied. The introduction of
open-shell radical species into luminescent closed-shell mol-
ecules often quenches their fluorescence. In paramagnetic
Our next aim is to enhance the luminescent properties of
PyBTM through its coordination to metal. Although the
fluorescence properties of perchlorinated triphenylmethyl
radicals have been modulated by chemical modification with
organic moieties,[4] to our knowledge there are no examples of
using coordination chemistry to prepare luminescent open-
shell species with unique photofunctions. This aim is chal-
lenging because the luminescence of a radical ligand is usually
quenched by coordination to a metal; for example, a carbox-
ylate ligand with luminescent radicals loses its luminescent
properties upon coordination to metal ions.[6]
CuII-porphyrin[1] and Zn-porphyrin with
a coordinated
nitronyl nitroxide radical,[2] fluorescence from singlet excited
states of the porphyrin (2S1 = D2) are quenched via quartet
(4T1 = Q1) and doublet (2T1 = D1) states at a lower energy
derived from the triplet excited state of the porphyrin. Spin-
labeled luminogens act as fluorescent probes; the luminescent
off state of the probes is switched to the on state by the loss of
its radical character upon the reaction with other radical
species such as COH.[3] Recently, the luminescence of open-
shell organic molecules, such as doublet monoradicals, has
attracted much interest owing to the characteristic photo-
functions.[4] These radicals show fluorescence from the lowest
excited doublet state (D1) to the ground doublet state (D0).
We have recently reported the fluorescence of a novel
organic radical, (3,5-dichloro-4-pyridyl)bis(2,4,6-trichloro-
phenyl)methyl radical (PyBTM), which is stable under
Herein we report the synthesis of a AuI complex with
PyBTM as a ligand, [AuI(PyBTM)PPh3]X (1X; X = ClO4,
BF4; Figure 1), and its photophysical and photochemical
[*] Y. Hattori, Dr. T. Kusamoto, Prof. Dr. H. Nishihara
Department of Chemistry, School of Science
The University of Tokyo
Figure 1. Structure of PyBTM and 1+.
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 (Japan)
E-mail: kusamoto@chem.s.u-tokyo.ac.jp
nisihara@chem.s.u-tokyo.ac.jp
Homepage: http://www.chem.s.u-tokyo.ac.jp/users/inorg/
new_en/index.html
properties. We chose AuI as a metal center because it can form
a strong coordination bond with pyridine derivatives, and gold
complexes often show superior luminescence properties.[7]
[AuI(PyBTM)PPh3]ClO4 (1ClO4) was synthesized from
[AuI(PPh3)Cl] and PyBTM by using AgClO4 to remove
a chloro ligand as an AgCl precipitate.[8] The complex was
characterized by ESI-TOF-MS and elemental analysis. 1ClO4
was used for single-crystal X-ray diffraction, ESR measure-
ment, and cyclic voltammetry. [AuI(PyBTM)PPh3]BF4 (1BF4)
was also prepared by a slightly modified method and used for
optical measurements: the perchlorate salts are generally
avoided because they are potentially explosive. No clear
[**] We are grateful to Dr. Reizo Kato at RIKEN for his kind provision of
facilities and laboratory equipment. This work was supported
financially by Grants-in-Aid from MEXT of Japan (Grants 26220801,
24750142, and 21108002; area 2107 (coordination programming)),
and MERIT (Material Education Program for the future leaders in
Research, Industry, and Technology) in the MEXT Leading Graduate
School Doctorial Program.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411572.
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Figure 2. a) Crystal structure of 1ClO4 with ellipsoids set at 50%
probability. One cation and one anion for two sets of crystallographi-
cally independent molecules are shown. Hydrogen atoms are omitted
for clarity. b) ESR spectrum of 1ClO4 in CH2Cl2 at 180 K (top) and
computer simulation (bottom). c) Calculated spin density distribution
of 1+. d) Photographs of PyBTM and 1BF4 solutions in dichlorome-
thane under room light (left) and under UV light at l=365 nm (right).
Figure 3. Molecular orbitals of 1+ calculated using DFT (UM06/
SDD(Au), 6-31G(d)(H, C, N, P, Cl)).
Coordination to AuI lowers the MO energies compared
with those of PyBTM. The lower energies of the frontier
orbitals (MOs 207b and 207a) were confirmed by the cyclic
voltammogram in 0.1m Bu4NClO4/CH2Cl2 (Supporting Infor-
mation, Figure S2). The reversible reduction potential attrib-
difference was observed in the optical properties of 1ClO4 and
1BF4.
Single-crystal X-ray diffraction studies revealed the
molecular structure of 1+ in the crystalline state (Figure 2a;
Supporting Information, Table S1).[11] Single crystals of 1ClO4
contained two sets of crystallographically independent cat-
ions and anions. The two cations of 1 have similar structures:
the N-Au-P angles are both 1768 and the perchlorate ions do
not coordinate to the AuI center, indicating a two-coordinate
uted to the injection of an electron into 207b at E0’(red)
=
ꢀ0.21 V vs. ferrocenium/ferrocene (Fc+/Fc) was more pos-
itive than that of PyBTM (E0’(red) = ꢀ0.74 V). This means that
an electron is more easily accepted in 1+ than in PyBTM
owing to the electrostatic effect of the cationic Ph3PAu+
moiety. The irreversible oxidation peak of 1+ (Ep(ox) =+
1.11 V), which reflects the energy of 207a, was slightly more
positive than that of PyBTM (Ep(ox) =+ 1.04 V).
ꢀ
linear structure. No intermolecular Au Au contacts (auro-
philic bonds) are observed, probably because of the bulkiness
of the triphenylphosphine and PyBTM moieties.
1BF4 displayed three characteristic transition bands at
labs = 566, 434, and 380 nm in the UV/Vis absorption spec-
trum (Figure 4). Each transition was assigned based on the
time-dependent (TD)-DFT calculations, which confirmed
that the transition processes in the visible region were
attributed to the PyBTM ligand (Supporting Information,
Figure S3). The excitation of the transition band at the longest
wavelength, attributed to the 206b!207b transition, forms
the D1 state, from which the fluorescence of 1BF4 arises. The
An ESR spectrum in CH2Cl2 was recorded at 180 K to
examine the spin density distribution (Figure 2b). The g value
(2.004) was similar to that of PyBTM. The spectrum showed
1
hyperfine coupling with H, 14N, 13C, and 31P atoms, and was
fitted by computer simulation. The obtained hyperfine
coupling constants (hccs) were qualitatively similar to those
calculated using density functional theory (DFT; Supporting
Information, Table S2). The results confirmed that the spin
density in 1+ was distributed mainly on the PyBTM ligand
(Figure 2c).
MOs of 1+ were calculated using DFT (UM06/SDD(Au),
6-31G(d)(H, C, N, P, Cl)) (Figure 3). The singly occupied
molecular orbital of 1+ (SOMO; 207a) and the lowest-
unoccupied spin orbital (207b) are delocalized mainly on the
PyBTM ligand; the electron density distributions are similar
to those of PyBTM. The MOs 206a, 208a, 206b, and 208b are
all distributed on the PyBTM moiety, with small electron
densities on the AuI atom in 208a and 208b. Namely, the MOs
of 1+ around its frontier orbitals are centered on the PyBTM
moiety. These results indicate that the PyBTM moiety plays
a main role in the electrochemical and optical properties of
the complex. PPh3-centered and AuI-centered orbitals are
observed in the lower energy region (201b and 191b;
Supporting Information, Figure S1).
Figure 4. Absorption spectra of PyBTM (gray line) and 1BF4 (black
line) and corrected emission spectra of PyBTM (gray dashed line,
lex =370 nm) and 1BF4 (black dashed line, lex =435 nm) in CH2Cl2.
Enlarged portions of the absorption spectra (ten-fold) are shown from
l=500 to 650 nm (gray dotted line for PyBTM and black dotted line
for 1BF4).
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maximum peak wavelengths of the transition band and the
resulting fluorescence band (lem = 653 nm) were bathochrom-
ically shifted compared with those of PyBTM (lem = 585 nm)
owing to the lower energy of 207b (Figure 2d and Figure 4). It
should be noted that 1BF4 is the first example of a luminescent
metal complex that contains a luminescent organic radical as
a ligand. This result is in contrast with previously reported
AuIPPh3 complexes that exhibit luminescent properties with
shorter emission wavelengths.[9] The absorption band at
434 nm is assigned as mainly from the 207a!208a transition,
which corresponds to the transition at 370 nm for PyBTM; the
band was also bathochromically shifted. The band at 380 nm
probably originates from transitions such as 207a!213a and
195b!207b.
to AuI, a heavy atom, could decrease the thermal fluctuation
and vibration of the molecule.
Improvement of photostability is an important issue to be
overcome for luminescent radicals. We have reported that the
photostability of PyBTM is up to 115 times higher than that of
tris(1,3,5-trichlorophenyl)methyl radical,
a
previously
reported luminescent radical. The decay of the fluorescence
intensity was investigated for 1BF4 in dichloromethane upon
irradiation with light at l = 370 nm to elucidate the effect of
the coordination to AuI on the photostability of the com-
pound. The estimated half-life (t1/2) was (5.0 ꢁ 1.1) ꢀ 104 s; this
value was three times larger than that of PyBTM ((1.5 ꢁ 0.2) ꢀ
104 s) in the same condition, indicating that 1BF4 has superior
photostability to PyBTM (Figure 5; Supporting Information,
The coordination of PyBTM to AuI improved the photo-
luminescence quantum yield in solution. 1BF4 in dichloro-
methane showed an absolute photoluminescence quantum
yield (f) of 8% (Supporting Information, Table S3). The
value was four times that of PyBTM (f= 2%). The fluores-
cence lifetime measurements revealed that 1BF4 showed
a single-component exponential decay with a lifetime t of
13.1 ꢁ 0.2 ns; this value was about twice that of PyBTM (t =
6.4 ꢁ 0.2 ns). The prolonged lifetime would contribute to
enhancing the luminescence quantum yield.
We estimated the radiative and non-radiative rate con-
stants, kf and knr, of 1BF4 and PyBTM by using Equations (1)
Figure 5. Plots showing the emission decay of PyBTM and 1BF4 in
dichloromethane under continuous excitation with light at
lex =370 nm. The half-lives (t1/2(PyBTM) =(1.5ꢁ0.2)ꢀ104 s, t1/2-
ꢀ ¼ kf=ðkf þ knrÞ
ð1Þ
(1BF ) =(5.0ꢁ1.1)ꢀ104 s) were estimated from repeated experiments
and (2), to reveal the origin of the improvement in the
4
(Supporting Information, Figure S4, Table S3).
t ¼ 1=ðkf þ knrÞ
ð2Þ
Figure S4, Table S3). The lower energy levels of the frontier
orbitals involved in the photophysical process, as shown by
cyclic voltammetry and DFT, contribute to the enhanced
photostability.[5] The results show that the coordination
positively affected the photophysical (f) and photochemical
(t1/2) properties.
In conclusion, we prepared the first luminescent metal
complex (1+) with a coordinated luminescent radical. The
complex exhibited fluorescence from the D1 state to the D0
state transition that was attributed to the PyBTM moiety. The
coordination of PyBTM to AuI had a positive effect on the
photophysical and photochemical properties by increasing
the fluorescence wavelength, quantum yield, and photo-
stability.
quantum yield (Table 1). The quadrupling of the quantum
yield of 1BF4 (from 2% to 8%) originated from two factors:
Table 1: Photophysical parameters of compounds.
f
t [ns]
kf [ꢀ106 sꢀ1
]
knr [ꢀ106 sꢀ1
]
PyBTM
1BF4
0.02
0.08
6.4ꢁ0.2
3
6
153
70
13.1ꢁ0.2
the doubling of kf and the halving of knr. The increase of kf was
explained by the greater oscillator strength (absorption
coefficient) at the lowest-excited transition (206b!207b
transition; Figure 4). This transition corresponds to a forbid-
den transition for a triarylmethyl radical with an ideal three-
fold symmetric structure.[10] For PyBTM and 1+, the three-fold
symmetries are broken and the transitions are partly allowed.
Because of the coordination of PyBTM to the AuIPPh+
moiety, the symmetry is lower in 1+ than in PyBTM, resulting
in the greater absorption coefficient in 1+. The higher
oscillator strength in 1+ was supported by the TD-DFT
calculations.
Received: December 1, 2014
Published online: && &&, &&&&
Keywords: gold · luminescence · N ligands · photostability ·
.
radicals
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4
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Radical Complexes
A gold(I) complex containing an open-
shell luminescent (3,5-dichloro-4-pyri-
dyl)bis(2,4,6-trichlorophenyl)methyl
(PyBTM) radical was prepared. The com-
plex has fluorescence that is centered
mainly on the coordinated PyBTM ligand.
The photoluminescence quantum yield,
fluorescence wavelength, and the stability
in the photoexcited state all increased
upon coordination to AuI.
Y. Hattori, T. Kusamoto,*
H. Nishihara*
&&& — &&&
Enhanced Luminescent Properties of an
Open-Shell (3,5-Dichloro-4-
pyridyl)bis(2,4,6-trichlorophenyl)methyl
Radical by Coordination to Gold
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These are not the final page numbers!