Impact of cyclometalation and π-conjugation on photoisomerization of an N,C-chelate organoboron compound

Article abstract of DOI:10.1021/om200534m

N,C-Chelate four-coordinate boron compounds that contain a B(ppy)Mes ₂ unit (ppy = 2-phenylpyridyl, Mes = mesityl) are a new class of photochromic molecules discovered recently by our group that undergo photoisomerization upon exposure to light

Full text of DOI:10.1021/om200534m

ARTICLE
pubs.acs.org/Organometallics
Impact of Cyclometalation and π-Conjugation on Photoisomerization
of an N,C-Chelate Organoboron Compound
Ying-Li Rao and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
S Supporting Information
b
ABSTRACT: N,C-Chelate four-coordinate boron compounds
that contain a B(ppy)Mes2 unit (ppy = 2-phenylpyridyl, Mes =
mesityl) are a new class of photochromic molecules discovered
recently by our group that undergo photoisomerization upon
exposure to light. To examine the influence of a covalently
bound transition metal ion on the photochromic properties of
this class of boron compounds, a new molecule (L1) that
contains two linearly conjugated ppy units has been synthesized.
A BMes2 group was attached to L1 via chelation with one of the
ppy units, producing a new four-coordinate boron compound,
B1. The reactions of B1 with PtPh2(DMSO)2 produced a Pt(II)
cyclometalated compound, Pt1, where a PtPh(DMSO) unit is
bound to the second ppy unit of B1. Replacement of DMSO in
Pt1 by p-t-Bu-pyridine provided a new compound, Pt2. A third Pt(II) compound, Pt3, where a Pt(dpm) group (dpm =
dipivaloylmethane) is chelated to the second ppy site of B1, was also synthesized successfully. The crystal structures of B1 and Pt3
have been determined by single-crystal X-ray diffraction analyses. The photophysical and photochromic properties of B1 and
Pt1ꢀPt3 have been examined. Experimental and computational studies established that Pt(II) cyclometalation to B1 stabilizes a
3LC state that involves πfπ* transitions localized on the ppyꢀppy conjugated backbone. This 3LC state of the Pt(II) compounds is
highly phosphorescent, with quantum efficiencies being 0.16, 0.13, and 0.45 for Pt1, Pt2, and Pt3, respectively, in toluene and at
ambient temperature. The B(ppy)Mes2 chromophore in all three Pt(II) compounds has been found to undergo photoisomerization
in a similar manner to that of B1, but with a much lower quantum efficiency than B1. Deactivation of the photoisomerization process
3
by the LC state has been found to be most likely responsible for the low photoisomerization quantum efficiency of the Pt(II)
compounds.
’ INTRODUCTION
may be directly coupled to the B(ppy)Mes2 chromophore, thus
modulating the photochromic properties of the boron com-
pound. Furthermore, the ancillary ligands on the Pt(II) center
allow the tuning of the energy of the metal-to-ligand charge
transfer (MLCT) transition and its influence on the properties of
the boron center. The ligand we designed for this study is a π-
conjugated molecule, B1 (Chart 1), where the N,C-chelate site
for the Pt(II) center is directly conjugated with the photochro-
mic boron group. The details of our investigation on B1 and its
Pt(II) complexes are presented herein.
Organoboron compounds have important applications in
optoelectronic devices1ꢀ3 including organic light emitting
diodes (OLEDs),2 probes, and sensors.3 Our recent discovery on
an unusual photochromic switching phenomenon of four-
coordinate N,C-chelate organoboron compounds4 with the general
formula B(N,C-chelate)Mes2 (Mes = mesityl) added a new
dimension to organoboron-based materials chemistry. The in-
fluence of transition metal coordination on photochromic pro-
perties of well-established systems such as azobenzene5 and
diarylethene (DTE)6 has been studied extensively. It has been
shown that by choosing appropriate metal ions and the linker, it
is possible to sensitize the photochromic switching through the
excited state of the metal chromophore for systems such as
DTE.5,6 To examine if the attachment of a metal center has any
impact on the photochromic properties of a N,C-chelate boron
compound such as4a B(ppy)Mes2 (ppy = 2-phenylpyridyl), we
have synthesized a new conjugated photochromic organoboron
compound and several of its cyclometalated N,C-chelate Pt(II)
compounds. The N,C-chelate Pt(II) chromophore is chosen
because the energy of its emissive state is usually dominated by
the 3LC (ligand-centered) transition of the chelate ligand,7 which
’ EXPERIMENTAL SECTION
General Procedures. All reactions were performed under N2 with
standard Schlenk techniques unless otherwise noted. All starting materials
were purchased from Aldrich Chemical Co. and used without further
purification. DMF, THF, Et2O, and hexanes were purified using an
Innovation Technology Co. solvent purification system. CH2Cl2 was
freshly distilled over phosphorus pentoxide prior to use. Deuterated
Received: June 21, 2011
Published: July 20, 2011
r
2011 American Chemical Society
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solvents were purchased from Cambridge Isotopes and were used as
received without further drying. NMR spectra were recorded on Bruker
Avance 400 or 500 MHz spectrometers. [PtPh2(DMSO)2] was synthe-
sized according to a literature method.8 UVꢀvis spectra were recorded
on an Ocean Optics UVꢀvisible spectrometer. Excitation and emission
spectra were recorded on a Photon Technologies International Quanta-
Master model C-60 spectrometer. Emission lifetimes were measured on
a Photon Technologies International Phosphorescent spectrometer
(Time-Master C-631F) equipped with a xenon flash lamp and digital
emission photon multiplier tube using a band pathway of 5 nm for
excitation and 2 nm for emission. Fluorescent quantum yields of B1 were
determined in CH2Cl2 using 9,10-diphenylanthracene as the standard at
298 K (Φr = 0.90). The absorbance of all the samples and the standard at
the excitation wavelength is approximately 0.096ꢀ0.109. For the Pt(II)
complexes Pt1ꢀPt3, the phosphorescent quantum efficiency was mea-
sured under N2 using Ir(ppy)3 as the standard (Φ = 0.95) in 2-Me-THF.9
Syntheses. As shown in Scheme 1, B1 was synthesized by a Suzuki
cross-coupling reaction between 2-(2-bromophenyl)-5-iodopyridine
and 4-(pyridin-2-yl)phenylboronic acid to produce L1, followed by
the lithiation and substitution by BMes2F. The synthetic procedures for
the precursor compounds 2-(2-bromophenyl)-5-iodopyridine, 2-(4-
bromophenyl)pyridine, and 4-(pyridin-2-yl)phenylboronic acid of L1
can be found in the Supporting Information.
2-dicylohexylphosphino-20,60-dimethoxybiphenyl (50.0 mg, 0.012 mmol),
and K3PO4 (1.2 g, 6 mmol) under nitrogen. The mixture was stirred at
60 °C for 48 h. Then 20 mL of H2O was added to the reaction mixture,
andthe waterlayerwasseparatedandextractedwithCH2Cl2 (3ꢁ 10 mL).
The combined organic layers were dried over Na2SO4, and the solvents
were evaporated under reduced pressure. Purification of the crude product
by column chromatography (hexane/THF, 4:1) afforded the product as a
white solid in 20% yield (0.23 g). 1H NMR (400 MHz, CDCl3, 25 °C, δ,
ppm): 9.02 (d, J = 1.2 Hz; 1H), 8.73 (d, J = 4.8 Hz; 1H), 8.14 (d, J =
8.0 Hz; 2H), 8.01 (dd, J = 8.0 Hz, J = 2.0 Hz; 1H), 7.77 (m, 4H), 7.71 (m;
2H), 7.61 (d, J = 6.4 Hz; 1H), 7.43 (t, J = 8.0 Hz; 1H), 7.25 (m, 2H).
Synthesis of 2-(2-(Dimesitylboryl)phenyl)-5-(4-(pyridin-2-yl)phenyl)-
pyridine (B1). To a stirred solution (diethyl ether/THF, 40/80 mL) of
2-(2-bromophenyl)-5-(4-(pyridin-2-yl)phenyl)pyridine (0.23 g, 0.6 mmol)
at ꢀ78 °C was added dropwise via a syringe an n-BuLi solution (1.60 M;
0.6 mL, 1.0 mmol) over 1 min. The resulting solution was stirred for 1 h
at ꢀ78 °C. Then a solution of dimesitylboron fluoride (0.36 g, 1.2
mmol) in Et2O was quickly added. The reaction mixture was kept
at ꢀ78 °C for another 1 h and then allowed to reach ambient tem-
perature and stirred overnight. After the removal of the solvent, purification
of the crude product by column chromatography (CH2Cl2/hexane, 1:1)
afforded the product as a yellow solid in 25% yield (0.08 g). The re-
sulting yellow solid was recrystallized avoiding light from CH2Cl2/
1
Synthesis of 2-(2-Bromophenyl)-5-(4-(pyridin-2-yl)phenyl)pyridine
(L1). This compound was synthesized using a Suzuki coupling procedure.
A 10 mL amount of dry toluene was added to a solid mixture of 4-
(pyridin-2-yl)phenylboronic acid (0.8 g, 4.0 mmol), 2-(2-bromophenyl)-
5-iodopyridine (1.1 g, 3.0 mmol), Pd(OAc)2 (13.0 mg, 0.06 mmol),
hexane to give yellow crystals of B1. H NMR (400 MHz, CD2Cl2,
25 °C, δ, ppm): 8.93 (d, J = 1.2 Hz; 1H), 8.71 (d, J = 4.8 Hz; 1H), 8.32
(dd, J = 8.0 Hz, J = 2.0 Hz; 1H), 8.17 (d, J = 8.0 Hz; 2H), 8.10 (d, J =
8.0 Hz; 1H), 7.95 (d, J = 6.4 Hz; 1H), 7.80 (t; 2H), 7.78 (d, J = 6.4 Hz;
1H), 7.63 (d, J = 8.0 Hz; 2H), 7.32 (m; 3H), 6.68 (s; 4H), 2.18 (s; 6H),
1.87 (s; 12H). 13C{1H}NMR (100 MHz, C6D6, δ, ppm): 158.16,
156.46, 150.24, 144.55, 140.10, 138.45, 136.50, 136.33, 135.07, 134.38,
134.21, 132.16, 131.62, 130.82, 129.14, 127.08, 125.64, 122.49, 121.75,
120.12, 119.56, 117.50, 114.45, 113.18, 25.70, 20.99. Anal. Calcd for
C40H37BN2: C 86.32, H 6.70, N 5.03. Found: C 86.28, H 6.87, N 5.01.
Synthesis of Pt1. To a stirred THF (20 mL) solution of [PtPh2-
(DMSO)2] (51.0 mg, 0.10 mmol) was added B1 (50 mg, 0.09 mmol).
The mixture was stirred at 50 °C for 6 h, and the solvent was removed
under reduced pressure. Pure Pt1 was obtained after the residue solid
was washed with hexane and ether, with 90% yield. 1H NMR (400 MHz,
CD2Cl2, 25 °C, δ, ppm): 9.70 (d, J = 4.8 Hz; 1H), 8.55 (s; 1H), 7.96 (m;
Chart 1
2H), 7.84 (m; 3H), 7.71 (d, J = 8.0 Hz; 2H), 7.47 (d, satellites, JPtꢀH
=
60.0 Hz, J = 8.0 Hz; 2H), 7.73 (m; 3H), 7.18 (dd, J = 8.0 Hz, J = 1.6 Hz;
1H), 6.98 (t, J = 7.2 Hz; 2H), 6.87 (d; J = 6.8 Hz; 1H), 6.81 (d, J = 2.0 Hz;
1H), 6.64 (s; 4H), 2.94 (s, satellites; 6H), 2.18 (s; 6H), 1.76 (s; 12H).
13C NMR was not recorded due to the poor solubility of Pt1. Anal. Calcd
for C49H51BN2OPtS: C 63.84, H 5.58, N 3.04, S 3.48. Found: C 63.68, H
5.34, N 3.11, S 3.14.
Scheme 1a
a Conditions: (i) Pd(OAc)2, K3PO4, 2-dicyclohexylphosphino-20,60-dimethoxybiphenyl in toluene, 60 °C.
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Synthesis of Pt2. To a CH2Cl2 (20 mL) solution of Pt1 (45 mg, 0.05
mmol) was added excess t-Bu-4-pyridine (0.2 mL). The mixture was
kept standing for several hours. After the remove of solvent and washing
with hexanes, the yellow powder of Pt2 was obtained in 90% yield
(40 mg). 1H NMR (400 MHz, C6D6, 25 °C, δ, ppm): 9.35 (s; 1H), 8.57
(dd, J=5.2Hz, J= 1.6 Hz; 2H), 8.18 (d, J= 8.0 Hz, 2H), 8.14 (d, J=2.0Hz;
1H), 7.67 (m; 2H), 7.60 (d, J = 8.0 Hz; 1H), 7.40 (m; 3H), 7.34 (d, J =
8.0 Hz; 1H), 7.12 (m; 1H), 7.05 (m; 2H), 6.94 (s; 4H), 6.65 (dd, J =
4.8 Hz, J = 1.6 Hz; 2H), 6.43 (t, J = 4.8 Hz; 1H), 2.27 (s; 12H), 2.21
(s; 6H), 0.87 (s; 9H). 13C NMR was not recorded due to low solubility
of the compound. Anal. Calcd for C55H54BN3Pt 0.5CH2Cl2: C 66.30,
3
H 5.51, N 4.18. Found: C 66.29, H 5.56, N 4.13.
Synthesis of Pt3. Complex Pt3 was synthesized using a modified
procedure reported by M. E. Thompson and co-workers for the syn-
thesis of Pt(ppy)(acac).7a A Pt(II) μ-dichloro-bridged dimer was syn-
thesized first by heating the solution of K2PtCl4 with 2.2 equiv of B1 in a
3:1 mixture of 2-ethoxyethanol and water at 80 °C for 30 h. This dimer
was used directly in the next step without purification by reacting with
3 equiv of the chelating diketone (dpm) and 10 equiv of Na2CO3 in
2-ethoxyethanol at 100 °C for 30 h. Compound Pt3 was isolated by
column chromatography using CH2Cl2. A yellow solid of Pt3 was
recrystallized from CH2Cl2 and hexanes. The isolated yield for Pt3 is
30%. 1H NMR (400 MHz, CD2Cl2, 25 °C, δ, ppm): 9.02 (d, J = 4.8 Hz;
1H), 8.73 (d, J = 1.6; 1H), 8.26 (dd, J = 8.4 Hz, J = 2.0 Hz; 1H), 8.08 (d,
J = 8.08; 1H), 7.86ꢀ7.92 (m; 2H), 7.82 (d, J = 2.0; 1H), 7.84 (dd, J = 5.6,
J = 1.6; 1H), 7.69 (d, J = 7.6; 1H), 7.56 (d, J = 4.0 Hz; 1H); 7.29ꢀ7.32
(m; 2H), 7.17ꢀ7.23 (m; 2H), 6.62 (s; 4H), 5.85 (s; 1H), 2.14 (s; 6H),
1.82 (s; 12H), 1.29 (s; 9H), 1.21 (s, 9H). 13C NMR (100 MHz, CD2Cl2,
25 °C, δ, ppm): 195.9, 194.3, 167.7, 157.9, 147.6, 146.1, 144.5, 141.7,
139.7, 138.8, 136.6, 136.5, 135.4, 134.1, 131.2, 131.1, 130.3, 129.3, 125.7,
124.0, 122.5, 122.4, 122.1, 119.3, 118.2, 111.3, 93.8, 41.9, 41.3, 35.0, 28.8,
Figure 1. Diagrams showing the crystal structure of Pt3 with 35%
thermal ellipsoids (top) and the π-stacked dimer (bottom).
use of 2-dicyclohexylphosphino-20,60-dimethoxybiphenyl in this
cross-coupling reaction gave a better yield than other phosphines
such as triphenylphosphine. Lithiation of L1 followed by sub-
stitution with BMes2F provided B1 (Scheme 1). The cyclome-
talated Pt(II) compound Pt1 was obtained quantitatively by the
reaction of B1 and PtPh2(DMSO)2 in THF at 50 °C for 5 h
through intramolecular CꢀH bond activation. Substitution of
DMSO in Pt1 with the stronger σ donor 4-t-Bu-pyridine
quantitatively afforded Pt2. The Pt(dpm) cyclometalated com-
pound Pt3 was obtained in 30% yield using a modified
procedure7a as reported by M. E. Thompson and co-workers
for Pt(ppy)(acac), ppy = phenylpyridyl, by heating K2PtCl4 with
2.2 equiv of B1 in a 3:1 mixture of 2-ethoxyethanol and water at
80 °C for 30 h, followed by the addition of 3 equiv of
dipivaloylmethane (dpm) and 10 equiv of Na2CO3 in 2-ethox-
yethanol at 100 °C for 30 h. B1 and all Pt(II) complexes have
28.5, 25.6, 25.4, 20.7. Anal. Calcd for C51H55BN2O2Pt 0.5C6H14: C
3
66.59, H 6.40, N 2.87. Found: C 66.48, H 6.74, N 2.80.
Molecular Orbital Calculations. DFT calculations were per-
formed for B1 and Pt1ꢀPt3. The geometrical parameters obtained for
B1 and Pt3 from X-ray diffraction experiments were used as the starting
point for their individual geometry optimization. The calculations were
performed using the Gaussian03 package with the B3LYP/6-31G* basis
set10 for all atoms except Pt, for which LAN2LDZ was used. Time-
dependent DFT (TD-DFT) calculations at the B3LYP level of theory
were performed at the optimized ground-state geometry with the 6-31G*
basis set for all atoms except Pt, for which LAN2LDZ was used. The TD-
DFT calculation results can be found in the Supporting Information.
X-ray Crystallographic Analysis. Single crystals of B1 and Pt3
were obtained from a mixed solvent solution of CH2Cl2 and hexanes and
mounted on glass fibers for data collection. Data were collected on a
Bruker Apex II single-crystal X-ray diffractometer with graphite-mono-
chromated Mo KR radiation, operating at 50 kV and 30 mA and at 180
K. Data were processed on a PC with the aid of the Bruker SHELXTL
software package (version 5.10)11 and corrected for absorption effects.
All non-hydrogen atoms were refined anisotropically. Pt3 cocrystallizes
with two CH2Cl2 solvent molecules in the crystal lattice, which were
modeled and refined successfully. One of the tert-butyl groups in Pt3
shows a rotational disordering with ∼50% occupancy for each disordered
site. This disordered group was modeled and refined successfully. Com-
plete crystal structural data can be found in the Supporting Information.
The crystal data of B1 and Pt3 have been deposited at the Cambridge
Crystallographic Data Center (CCDC Nos. 830615, 830616).
1
been characterized by H NMR and elemental analyses. The
structures of B1 and Pt3 have been characterized by single-crystal
X-ray diffraction analysis.
The crystal structure of B1 can be found in the Supporting
Information, and that of Pt3 is shown in Figure 1. The geometry
around the boron center in Pt3 is similar to that of B1 and those
of our previously reported molecules such as B1A.4 Compared to
B1, the BꢀMes bond distances in Pt3 are notably longer
(1.659(5) Å in Pt3, 1.645(2) Å in B1, on average). The BꢀN
bond (1.654(4) Å) in Pt3 is also notably longer than that of B1
(1.6446(19) Å). This may be attributed to the greater steric
congestion in the Pt(II) compound. Nonetheless, the B(ppy)
unit and the Pt(ppy)(dpm) unit in Pt3 adopt an approximately
syn arrangement, with a dihedral angle of 40.9° between the two
ppy units (the same dihedral angle is 36.0° for B1), as shown by
the side view of the molecule in Figure 1 (bottom). The Pt(II)
center has a typical square-planar geometry, with the Pt(1)ꢀ
O(1) bond (2.068(2) Å) trans to the Pt(1)ꢀC(12) bond being
much longer than the Pt(1)ꢀO(2) bond (2.006(2) Å) trans to
the PtꢀN bond, due to the greater trans influence exerted by the
’ RESULTS AND DISCUSSION
Syntheses and Structures. B1 was synthesized by a Suzuki
cross-coupling reaction between 2-(4-bromophenyl)pyridine
and 4-(pyridin-2-yl)phenylboronic acid to produce L1. The
carbon donor. The Pt(1) B(1) separation distance is 8.04(1) Å.
3 3 3
Molecules of Pt3 form a π-stacked dimer between the two Pt-ppy
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Figure 4. Time-resolved phosphorescence spectra of B1 and Pt(II)
compounds in toluene (∼10ꢀ5 M) at 77 K under nitrogen. The delay
time used for each spectrum is indicated in the legend.
Figure 2. Absorption spectra of B1 and complexes Pt1ꢀPt3 in toluene
at 298 K.
originates from the mesityl to the chelate backbone CT transi-
tion. All Pt(II) compounds exhibit bright and oxygen-sensitive
phosphorescence (Figure 3). A phosphorescent emission band at
550 nm was observed for Pt1 (Φ = 0.16, τ = 14.67(2) μs). In
addition, Pt1 has a high-energy shoulder band at 480 nm, which
is attributed to the singlet emission of the chelate ligand due to its
resemblance to the B1 emission spectrum and its insensitivity
toward oxygen (see Supporting Information). Only one intense
emission band at 550 nm was observed for both Pt2 (Φ = 0.13,
τ = 9.64(5) μs) and Pt3 (Φ = 0.45, τ = 14.5(1) μs). The well-
resolved vibrational feature and the close resemblance of the
emission spectra of the three Pt(II) compounds indicate that
phosphorescence of these compounds is mostly ligand-centered
emission (3LC) with little involvement of MLCT transitions.
This assignment is further supported by the time-resolved phos-
phorescence spectra recorded at 77 K (Figure 4). The phosphor-
escence of B1 at 77 K has well-resolved vibrational features, thus
may be attributed to a πfπ* transition localized on the
conjugated backbone (3LC), instead of the mesityl to backbone
CT transition. The emission spectra of the metal compounds at
77 K have the same profile and are in a similar energy region to
the phosphorescence spectrum of B1 except a small red shift
(∼5 nm for Pt1 and Pt2, ∼20 nm for Pt3) due to cyclometalation,
thus supporting that the low-energy emission band in all Pt(II)
compounds is indeed ligand-centered phosphorescence (3LC).
Furthermore, the high phosphorescence efficiency of the Pt(II)
compounds indicates that the backbone-based 3LC emission is a
highly effective deactivation pathway for the excited-state energy.
Photoisomerization. Upon excitation at 365 nm in toluene,
the color of the B1 solution becomes dark green with the
appearance of a broad absorption band at 615 nm (Figure 5,
Scheme 2), characteristic of photoisomerization around the boryl
chromophore, forming the species B10, as shown in Scheme 1.
The formation of B10 via photolysis was also confirmed by NMR
spectral data that have the characteristic chemical shifts of the
dark isomer in related systems,4 as we reported previously (see
Supporting Information). The quantum yield for this photoi-
somerization process of B1 at 298 K was determined to be ∼0.03
at 365 nm excitation, which is much lower than that of B1A
(ϕphotoisomerization = 0.85).4b,c Previously we have shown that
the CT transition from a mesityl to the chelate backbone of
B(ppy)Mes2 (B1A) and its derivative is a key driving force for the
isomerization process.4 Extending the π-conjugation on the
Figure 3. Fluorescence spectrum of B1 and phosphorescence spectra of
Pt1ꢀPt3 in toluene (∼10ꢀ5 M) at 298 K under nitrogen.
chelate rings with the shortest separation distance being 3.392(4)
Å. The Pt Pt separation distance within the dimer is 5.181(2)
3 3 3
Å. For B1, a π-stacked dimer was also observed in the crystal
lattice, although the stacking is between the nonchelate ppy ring
and the B-ppy chelate ring with much greater separation distances
than those observed for Pt3 (see Supporting Information).
Photophysical Properties. The absorption spectra of B1 and
the Pt(II) compounds are shown in Figure 2. B1 has an intense
band at ca. 320 nm, corresponding to π to π* transition of the π-
conjugated backbone, and a shoulder band at ca. 370 nm ascribed
to the charge transfer (CT) transition from the mesityl groups to
the π-conjugated backbone. On coordination to the Pt com-
plexes system, this CT band shifts slightly to the red due to the
PtꢀC and PtꢀN bond formation with the backbone, which
extends the π conjugation of the backbone and lowers the
LUMO level. Except the Pt2 compound, which has a well-
resolved low-energy MLCT band at ∼430 nm, the mesityl to
the π-backbone CT band and the MLCT band are not resolved.
On the basis of TD-DFT calculation results, the lowest energy
band of Pt1 at 410 nm is mainly CT from the mesityl to the
backbone. The lowest energy transition of Pt2 is a mixture of CT
from the mesityl to the backbone and MLCT transitions, while
for Pt3 it is mostly CT from the mesityl to the backbone.
Compound B1 displays blue-green fluorescence in toluene at
ambient temperature with λmax = 480 nm (Φ = 0.18) that
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Figure 5. UVꢀvis spectral change of B1 (left, total exposure time, 30 s) and Pt3 (right, total exposure time, 35 min) in toluene (1.5 ꢁ 10ꢀ5 M) upon
irradiation at 365 nm with a hand-held UV lamp.
Scheme 2.
Figure 6. Comparison of the UVꢀvis spectral change of B1 and
complexes Pt1ꢀPt3 in toluene (1.5 ꢁ 10ꢀ5 M) after irradiation at
365 nm for various time periods.
Figure 7. HOMO and LUMO (the major contributors in the lowest
excited state) diagrams for B1 and Pt3 from DFT computational
results. Details and diagrams for other compounds can be found in
the Supporting Information.
backbone stabilizes the πfπ* transition state of the π-conju-
gated backbone such that it can become or approach the lowest
excited state, thus competing with or partially quenching the CT
state of the boryl chromophore and inhibiting the photoisome-
rization process. Indeed, TD-DFT computational results show
that the lowest excited state of B1 (HOMO to LUMO transi-
tion), albeit dominated by a mesityl to ppy charge transfer, has a
significant contribution from the πfπ* transition of the π-
conjugated backbone (Figure 7). We therefore believe that
because of this the photoisomerization of B1 is much less
efficient than that of B1A. The photoisomerization of B1 to
B10 is fully thermally reversible.
All Pt(II) compounds undergo a similar photoisomerization
process to that for B1, as confirmed by UVꢀvis and NMR
spectra (see Supporting Information). Upon excitation of com-
plexes Pt1, Pt2, and Pt3 at 365 nm, a broad band that covers the
480 to 800 nm region arises as shown in Figures 5 and 6 (for complete
photolysis data of each compound, please see Supporting Information),
but at a much lower efficiency, compared to B1, despite the
greater absorbance by the Pt(II) compounds at 365 nm. The
quantum yields for photoisomerization of the Pt(II) compounds
at 365 nm and 298 K were estimated to be ∼0.0015 for Pt1,
<0.0001 for Pt2, and ∼0.0005 for Pt3. The low photo-
isomerization quantum efficiencies of the Pt(II) compounds
3
can be attributed to the low-lying LC state that becomes an
effective deactivation pathway due to the efficient spinꢀorbit
coupling and the singletꢀtriplet intersystem crossing imposed by
Pt(II) chelation, as supported by the highly efficient 3LC
phosphorescence of these compounds. This finding also sup-
ports that photoisomerization of the B(ppy)Mes2 chromophore
is most likely through the singlet CT state. Furthermore, on the
basis of the absorption spectral data and TD-DFT computational
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Organometallics
ARTICLE
results, it appears that the greater the involvement of MLCT in
the lowest electronic transition, the less efficient the photoi-
somerization since Pt2 is the least efficient in photoisomerization
but with the most significant contributions of MLCT to the
lowest excited state, compared to Pt1 and Pt3 (see Supporting
Information). Hence a low-lying MLCT state is also a highly
effective deactivation pathway for photoisomerization of the
boryl chromophore in the metal compounds.
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The behavior of the B(ppy)Mes2 system is in sharp contrast to
that of dithienylethene compounds, which have been shown to
undergo photoisomerization from a LC excited state that may be
sensitized by the attachment of a metal ion via an energetically
favorable MLCT excited state, as demonstrated by a number of
research groups.6
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’ CONCLUSIONS
A new photochromic boryl compound that is capable of
forming cyclometaled Pt(II) compounds has been achieved.
Three cyclometalated Pt(II) compounds that share a conju-
gated chelate backbone with the photochromic chromophore
B(ppy)Mes2 have been synthesized. These Pt(II) compounds
are brightly phosphorescent at ambient temperature. The Pt(II)
unit in these compounds has been found to be highly effective in
quenching the photoisomerization process of the B(ppy)Mes2
chromophore by enhancing the 3LC state localized on the chelate
backbone. On the basis of this study, enhancing the mesityl to the
chelate charge transfer transition and destabilizing the πfπ*
transition of the chelate backbone are the key factors that should
be considered for achieving new and highly efficient photo-
chromic systems based on N,C-chelate four-coordinate boron
compounds.
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’ ASSOCIATED CONTENT
S
(7) (a) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba,
I.; Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055. (b) Hudson,
M. Z.; Sun, C.; Helander, M. G.; Amarne, H.; Lu, Z.-H.; Wang, S. Adv.
Funct. Mater. 2010, 20, 3426. (c) Rao, Y. L.; Wang, S. Inorg. Chem. 2009,
48, 7698.
(8) Klein, A.; Schurr, T.; Kn€odler, A.; Gudat, D.; Klinkhammer,
K.-W.; Jain, V. K.; Zalis, S.; Kaim, W. Organometallics 2005, 17, 4125.
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W. A., III; Thompson, M. E. J. Am. Chem. Soc. 2009, 131, 9813.
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Supporting Information. Synthetic details for the pre-
b
cursor compounds of L1, phosphorescence spectral change of
Pt(II) compounds toward oxygen, TD-DFT computational data
and MO diagrams, photoisomerization experimental details and
data, and complete crystal structural data. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: wangs@chem.queensu.ca.
(11) SHELXTL Version 6.14; Bruker AXS, 2000ꢀ2003.
’ ACKNOWLEDGMENT
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support.
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dx.doi.org/10.1021/om200534m |Organometallics 2011, 30, 4453–4458