Tuning the photoisomerization of a N^C-chelate organoboron compound with a metal-acetylide unit

Article abstract of DOI:10.1002/chem.201204048

To examine the impact of metal moieties that have different triplet energies on the photoisomerization of B(ppy)Mes₂ compounds (ppy=2-phenyl pyridine, Mes=mesityl), three metal-functionalized B(ppy)Mes ₂ compounds, Re-B, Au-B, and Pt-B, have been synthesized and fully characterized. The metal moieties in these three compounds are Re(CO) ₃(tert-Bu₂bpy)(C≡C), Au(PPh₃)(C≡C), and trans-Pt(PPh₃)₂(C≡C)₂, respectively, which are connected to the ppy chelate through the alkyne linker. Our investigation has established that the Re^I unit completely quenches the photoisomerization of the boron unit because of a low-lying intraligand charge transfer/MLCT triplet state. The Au^I unit, albeit with a triplet energy that is much higher than that of B(ppy)Mes₂, upon conjugation with the ppy chelate unit, substantially increases the contribution of the π→π* transition, localized on the conjugated chelate backbone in the lowest triplet state, thereby leading to a decrease in the photoisomerization quantum efficiency (QE) of the boron chromophore when excited at 365 nm. At higher excitation energies, the photoisomerization QE of Au-B is comparable to that of the silyl-alkyne-functionalized B(ppy)Mes₂ (TIPS-B), which was attributable to a triplet-state-sensitization effect by the Au^I unit. The Pt^II unit links two B(ppy)Mes₂ together in Pt-B, thereby extending the π-conjugation through both chelate backbones and leading to a very low QE of the photoisomerization. In addition, only one boron unit in Pt-B undergoes photoisomerization. The isomerization of the second boron unit is quenched by an intramolecular energy transfer of the excitation energy to the low-energy absorption band of the isomerized boron unit. TD-DFT computations and spectroscopic studies of the three metal-containing boron compounds confirm that the photoisomerization of the B(ppy)Mes₂ chromophore proceeds through a triplet photoactive state and that metal units with suitable triplet energies can be used to tune this system. Copyright

Full text of DOI:10.1002/chem.201204048

DOI: 10.1002/chem.201204048
Tuning the Photoisomerization of a N^C-Chelate Organoboron Compound
ꢀ
with a Metal Acetylide Unit
Nan Wang, Soo-Byung Ko, Jia-Sheng Lu, Leanne D. Chen, and Suning Wang*[a]
Abstract: To examine the impact of
metal moieties that have different trip-
let energies on the photoisomerization
that of B
N
links two BACTHUNGTRENNU(G ppy)Mes2 together in Pt-B,
with the ppy chelate unit, substantially
increases the contribution of the p!p*
transition, localized on the conjugated
chelate backbone in the lowest triplet
state, thereby leading to a decrease in
the photoisomerization quantum effi-
ciency (QE) of the boron chromophore
when excited at 365 nm. At higher ex-
citation energies, the photoisomeriza-
tion QE of Au-B is comparable to
that of the silyl–alkyne-functionalized
thereby extending the p-conjugation
through both chelate backbones and
leading to a very low QE of the photo-
of
phenyl pyridine, Mes=mesityl), three
metal-functionalized B(ppy)Mes2 com-
BACHTUNGTRENNUNG(ppy)Mes2 compounds (ppy=2-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
pounds, Re-B, Au-B, and Pt-B, have
been synthesized and fully character-
ized. The metal moieties in these three
compounds are Re(CO)3(tert-Bu2bpy)-
A
ACTHNUGERTNNUNG ACTHUNTGRENNUNiG ACHTUNGERTNNUNG ACHTNUGTRENNNUG
second boron unit is quenched by an
intramolecular energy transfer of the
excitation energy to the low-energy ab-
sorption band of the isomerized boron
unit. TD-DFT computations and spec-
troscopic studies of the three metal-
containing boron compounds confirm
that the photoisomerization of the
ꢁ
ꢁ
ACHTUNGTRENNUNG(C C), AuACHTUNGTRENNUNG(PPh3)ACHTUNGTRENNUNG
ꢁ
ACHTUNGTRENNUNG(PPh3)2ACHTUNGTRENNUNG
connected to the ppy chelate through
the alkyne linker. Our investigation has
established that the ReI unit complete-
ly quenches the photoisomerization of
the boron unit because of a low-lying
BACTHNGUTRE(NNUG ppy)Mes2 (TIPS-B), which was at-
tributable to a triplet-state-sensitization
effect by the AuI unit. The PtII unit
BACTHNUTRGEN(NUG ppy)Mes2 chromophore proceeds
through a triplet photoactive state and
that metal units with suitable triplet en-
ergies can be used to tune this system.
Keywords: boron
isomerization
transition metals
intraACHTUNGTRENNUNGliACHTUNGTRENNUNGgand charge transfer/MLCT trip-
let state. The AuI unit, albeit with a
triplet energy that is much higher than
Introduction
cited state must be dominated by the CT transition. As a
result, extended p-conjugation of the chelate backbone,
which leads to a low-lying p!p* transition state, must be
avoided.[3] Sterically bulky aryl groups, such as mesityl
groups, around the boron center are necessary to enable the
photoisomerization with moderate-to-high quantum efficien-
cies. We have also shown that the attachment of a PtII che-
late unit onto the backbone can also greatly inhibit the pho-
toisomerization process, owing to the introduction of a low-
lying and highly emissive triplet state that effectively
quenches the isomerization process.[1g] Nonetheless, the role
of the triplet states in the photoisomerization of the N^C-
chelate BMes2 compounds was not understood until a recent
report from our group, in which we demonstrated the possi-
ble involvement of a photoactive triplet state in the isomeri-
zation process[4] by examining the impact of triplet acceptors
with various triplet energies that were covalently attached
onto a N^C-chelate BMes2 chromophore through a non-
conjugated linker on the corresponding photoisomerization
quantum efficiencies. A number of well-established photo-
N^C-chelate organoboranes, such as BACTHNUTRGEN(UNG ppy)Mes2 (ppy=2-
phenyl pyridine, Mes=mesityl), have recently been found to
display thermally reversible photoisomerization, thereby
changing color from colorless or light yellow to deep blue or
green on excitation by light.[1] This property affords new op-
portunities in organoboron-based photochromic materials.
We have shown that, by varying the N^C-chelate ligands
and substituents, the colors of the dark isomers and the pho-
toisomerization quantum efficiency of this class of boron
compounds can be tuned.[1,2] The key factors that have been
found to have a great impact on the photoisomerization of
the boron compounds are the charge-transfer (CT) transi-
tion from the mesityl groups to the chelate backbone and
the steric congestion imposed by the mesityl groups.[2] For
the photoisomerization to proceed efficiently, the lowest ex-
[a] N. Wang, Dr. S.-B. Ko, J.-S. Lu, L. D. Chen, Prof. S. Wang
Department of Chemistry
Queenꢀs University
chromic chromophores, such as diarylethenes and spiroACHTUNGTRENNUNGpy-
Kingston, Ontario, K7L 3N6 (Canada)
Fax : (+1)613-533-6669
E-mail: wangs@chem.queensu.ca
AHCTUNGTRENNUNG
rans,[5] are known to isomerize through photoactive triplet
states.[6] The attachment of a transition-metal unit as a trip-
let sensitizer in such systems has been extensively used as
an effective approach to modulate the isomerization effi-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204048.
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FULL PAPER
ciency of the system.[7] To further understand the role of the
triplet state and to determine whether transition-metal ions
can be used as triplet sensitizers for the isomerization of
N^C-chelate BMes2 compounds, we initiated an investiga-
tion on the influence of ReI ([Re(CO)3(tert-Bu2bpy)]+), AuI
[Re(tert-Bu2bpy)(CO)3Cl], B1, TlPF6, and Et3N at reflux in
THF, Re-B was obtained in moderate yield. Au-B and Pt-B
were prepared by copper(I)-catalyzed transmetalation reac-
tions between B1 and the corresponding AuI and PtII precur-
sors, respectively. These complexes can be purified by
column chromatography on silica gel. The ReI and PtII com-
plexes are air-stable, whereas the AuI compound undergoes
gradual degradation in air. All of these metal compounds
were fully characterized by NMR spectroscopy, single-crys-
tal X-ray diffraction, and elemental analysis.
([Au
T
(PPh3)2]+2) on the
ACHTUNGTRENNUNG
isomerization of BACHTUNGTRENNUNG
shown in Scheme 1. These three metal units are chosen be-
cause of their distinct triplet energies and coordination ge-
ACHTUNGTRENNUNGomACHTUNGTRENNUNGetries. We have found that these metal ions have a dis-
tinct impact on this system and the details are presented
herein.
The crystal structures of these three metal complexes are
shown in Figure 1, Figure 2, and Figure 3. Selected bond
lengths and angles are given in Table 1. The metal ions in all
Scheme 1. Isomerization of N^C-chelate BMes2 compounds with
metal–acetylide unit.
a
Results and Discussion
Synthesis and structures: The new metal-functionalized
(ppy)Mes2 compounds were synthesized according to
BACHTUNGTRENNUNG
Figure 1. Crystal structure of Re-B with labeling schemes for selected
atoms; thermal ellipsoids are set at 50% probability. Hydrogen atoms
are omitted for clarity.
Scheme 2. The metal moiety was connected to the boron
unit through an alkyne linker by using well-established pro-
cedures for related metal–acetylide compounds.[8,9] Parent
boron molecule B1 was obtained by the initial synthesis of
TIPS-B, followed by the removal of the TIPS group accord-
ing to literature procedures.[1b,c] By heating a mixture of
Figure 2. Crystal structure of Au-B with labeling schemes for selected
atoms; thermal ellipsoids are set at 50% probability.
three compounds display the expected geometry, that is, oc-
tahedral for ReI, linear for AuI, and square planar for PtII.
ꢀ
ꢀ
ꢀ
ꢀ
The metal P, metal C, B C, and B N bond lengths in Au-
ꢀ
B and Pt-B are very similar. Although the B CMes bond
ꢀ
lengths in all three compounds are similar, the B N bond
(1.634(4) ꢂ) in Re-B is significantly shorter than in Au-B
(1.652(5) ꢂ) and Pt-B (1.663(3) ꢂ). This difference can be
explained by the 18e ReI unit, which takes less electron den-
sity away from the pyridyl ring of the N^C chelate com-
Scheme 2. Syntheses of N^C-chelate organoboron compounds with a
metal–acetylide unit.
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5315

S. Wang et al.
Figure 3. Crystal structure of Pt-B with labeling schemes for selected
atoms; thermal ellipsoids are set at 50% probability. Hydrogen atoms
are omitted for clarity.
ꢀ
Figure 4. C H···p interactions (about 2.50 ꢂ) between CHCl3 and the
alkyne bond in Re-B.
Table 1. Selected bond lengths and angles of the metal complexes.
Bond length [ꢂ]
Such interactions may be re-
Bond angle [8]
sponsible for the bending of the
ꢀ
ꢀ
Re-B
Re(1) C(14)
1.908(4)
1.906(4)
1.961(4)
1.657(5)
1.643(5)
Re(1) C(13)
2.118(3)
2.178(3)
2.187(3)
1.642(5)
1.634(4)
C(16)-Re(1)-C(13)
C(15)-Re(1)-N(3)
C(14)-Re(1)-N(2)
N(1)-B(1)-C(1)
C(26)-B(1)-C(17)
Re-C(12)-C(13)
C(1)-Au-P(1)
172.63(16)
171.85(13)
170.88(14)
95.3(2)
115.1(3)
167.6(3)
ꢀ ꢁ
M C C bond in these two
ꢀ
ꢀ
Re(1) C(15)
Re(1) N(3)
compounds. The chlorine atoms
of the chloroform solvent mole-
cules also form H-bonds with
protons of the mesityl groups in
the Re-B crystal lattice. For Pt-
B, the most significant intermo-
lecular interactions are p-stack-
ing interactions between the
ppy chelate units, with a short-
est separation distance of
3.74 ꢂ. As a result, molecules
of Pt-B all orient along one di-
ꢀ
ꢀ
Re(1) C(16)
Re(1) N(2)
ꢀ
ꢀ
B(1) C(17)
B(1) C(1)
ꢀ
ꢀ
B(1) C(26)
B(1) N(1)
ꢀ
ꢀ
Au-B
Pt-B
Au(1) P(1)
2.2728(9)
1.636(6)
1.655(6)
Au(1) C(1)
2.000(4)
1.638(6)
1.652(5)
171.86(11)
95.3(3)
ꢀ
ꢀ
B(1) C(14)
B(1) C(8)
C(8)-B(1)-N(1)
C(14)-B(1)-C(23)
Au(1)-C(1)-C(2)
C(1)-Pt(1)-P(1)
ꢀ
ꢀ
B(1) C(23)
B(1) N(1)
115.8(3)
169.9(3)
87.69(6)
ꢀ
ꢀ
Pt(1) P(1)
2.3140(5)
2.002(2)
1.653(3)
B(1) C(8)
1.633(3)
1.646(3)
1.663(3)
ꢀ
ꢀ
Pt(1) C(1)
B(1) C(14)
C
(1 A)-Pt(1)-P(1)
92.31(6)
ꢀ
ꢀ
B(1) C(23)
B(1) N(1)
C(8)-B(1)-N(1)
C(14)-B(1)-C(23)
Pt(1)-C(1)-C(2)
95.58(15)
113.76(17)
176.80(19)
pared to the electron-deficient 14e AuI unit and the 16e PtII
unit. The other important feature as revealed from the crys-
tal structure is that, in Pt-B, the two N^C-chelate units are
coplanar and the p-conjugation is extended to the two
alkyne linkers and the PtII atom, similar to the observations
in some previously reported [trans-Pt
molecules.[13b,c] In Re-B, the three carbonyl ligands adopt a
facial geometry, which is typical of Re(CO)3A
(bpy)X units,[8]
ACHTNUGTRNEN(UGN PR3)2(acetylide)2]
CTHUNGTRENNUNG
to minimize the strong mutual trans effects of the carbonyl
ꢀ
ꢀ
groups. The Re C(16) bond length (1.961(4) ꢂ) is signifi-
Figure 5. Relative orientations of the Au-B molecules and the C H···p
interactions (2.90–3.0 ꢂ) in the crystal lattice.
ꢀ
ꢀ
cantly longer than those of the Re C(14) and Re C(15)
bonds (1.908(4) and 1.906(4) ꢂ, respectively), which can be
attributed to the greater trans effect exerted by the alkyne
ꢀ ꢁ
ligand as compared to bpy. The M C C bond angle in Re-B
and Au-B is considerably distorted from linearity (167.6(3)8
rection, thereby forming an extended p-stacked lattice, as
shown in Figure 6.
and 169.9(3)8, respectively). For Re-B, the proton of a
Absorption spectra: All of the absorption spectra were re-
corded in degassed CH2Cl2 at room temperature. The data
are shown in Table 2 and Figure 7. For comparison, the spec-
trum of TIPS-B is also shown in Figure 7. The absorption
spectrum of Re-B can be assigned by comparison with pre-
viously known ReI–a,a’-diimine complexes[11] and N^C-che-
late four-coordinate boron complexes.[1] The absorption
[10]
ꢀ
CHCl3 solvent molecule displays C H···p interactions
with the alkyne bond, as evidenced by a H···C separation of
about 2.50 ꢂ, whereas, for Au-B, the H atoms of the phenyl
ꢀ
ring from PPh3 and the mesityl ring show C H···p interac-
tions with the alkyne bond, with a H···C separation in the
range 2.90–3.00 ꢂ (Figure 4 and Figure 5, respectively).
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Photoisomerization of a N^C-Chelate Organoboron Compound
FULL PAPER
The absorption spectrum of Au-B shows absorption bands
in the regions 270–325 nm and 330–455 nm. By comparing
these values to the absorption spectra of previously known
AuI–acetylide systems[12] and TIPS-B,[1d] the high-energy
bands can be assigned to charge-transfer transitions from
the alkyne to the PPh3 ligand, whilst the low-energy bands
can be attributed to charge transfer from the mesityl groups
to the chelate backbone of the boron unit.
As in many previously reported PtII–phosphine–acetylide
complexes,[13] Pt-B did not display any MLCT bands. In-
stead, an intense absorption band was observed at about
395 nm, which was assigned as a p!p* transition of the
conjugated backbone that involves the two alkyne–
Figure 6. p-Stacking (shortest C···C separation distance is 3.74 ꢂ) of Pt-B
molecules in the crystal lattice. Hydrogen atoms are omitted for clarity.
BACHTNUTRGNE(NUG ppy)Mes2 units with contributions from the mesityl-to-
chelate charge-transfer transitions. The strong absorption in-
tensity and the red-shift of the absorption energy compared
to those of TIPS-B and Au-B were clearly caused by the
presence of two boron chromophores and by the extended
conjugation of the backbone through the PtII center.
Luminescence spectroscopy: Luminescence spectra were re-
corded at room temperature and at 77 K for all of these
complexes in degassed toluene (Figure 8). Upon excitation
at 388 nm, Re-B displayed a broad, featureless emission
peak at lmax =615 nm at room temperature (Figure 8, top).
By comparing to the spectra of previously reported ReI–
acetACHNTUGRENNUyG AHCTNUGTRENNUNG
lide complexes,[11] this oxygen-sensitive orange–red
emission is assigned as a 3MLCT [dp(Re)!p*
ACHTUNGTRENUN(NG diimine)]
Figure 7. UV/Vis spectra of solutions of the metal complexes (ca. 1.0ꢃ
10ꢀ5 m) in CH2Cl2 at ambient temperature.
transition (see the Supporting Information), as confirmed by
the time-resolved phosphorescence spectrum (Figure 8,
band at 270–360 nm is attributed to a mixture of intra
G
G
middle). At 77 K, Re-B shows bright luminescence at lmax =
p!p* transitions centered on the diimine and alkynyl li-
gands. The low-energy absorptions at about 370–420 nm
may be assigned to a mixture of charge-transfer transitions
from the mesityl groups to the p-conjugated backbone and
520 nm (Figure 8, bottom). Because the emission spectrum
has a very long decay lifetime and resolved vibrational fea-
tures, this peak can be assigned as phosphorescence local-
ized on the backbone of the B1 unit. The dramatic blue-
shift of the phosphorescent emission band from ambient
temperature to 77 K may be explained by the high efficiency
[dp(Re)!p*
(diimine)]
metal-to-ligand
charge-transfer
(MLCT) transitions with moderate intensity. This enhanced
MLCT transition can be explained by the incorporation of
an acetylide ligand, which is known to raise the d–d state
energy of the rhenium(I) center, thus enhancing the popula-
tion of the MLCT state.[8] The absorption tail in the region
420–460 nm may be assigned to alkynyl!bpy charge-trans-
fer transitions that are mixed with the MLCT transition.
3
of the p!p* emission of the alkyne–chelate backbone at
77 K, compared to the MLCT/CT phosphorescence of the
ReI unit.
At room temperature, Pt-B displays a dual emission at
480 and 520 nm upon excitation at 390 nm (Figure 8, top).
Time-resolved phosphorescence spectroscopy (Figure 8,
Table 2. Photophysical data for the metal compounds.
Absorption
lmax [nm]
Emission
Phosphorescence
decay lifetime [ms][e]
77 K[c]
lmax [nm] (F[d]
)
(e
N
298 K[b]
77 K[c]
Re-B
Au-B
Pt-B
290 (31100), 335 (18800), 390 (18700)
305 (30400), 345 (16500), 370 (15700)
305 (34400), 345 (36700), 395 (66600)
295 (24800), 345 (12900), 375 (8090)
615 (0.015)
520
493
516
482
2500 (1)
1428 (1)
294 (1)
–
493, 500 sh (0.082)
452, 520 sh (0.015)
495 (0.42)[f]
TIPS-B
[a] All of the spectra of solutions of the metal complexes (ca. 1.0ꢃ10ꢀ5 m) were recorded in degassed CH2Cl2 at ambient temperature. [b] All of the spec-
tra of solutions of the metal complexes (ca. 1.0ꢃ10ꢀ5 m) were recorded in degassed toluene at ambient temperature. [c] All of the spectra of solutions of
the metal complexes (ca. 1.0ꢃ10ꢀ5 m) were recorded in 2-methyltetrahydrofuran at 77 K. [d] A degassed solution of [fac-Ir
ACTHNUTRGEN(UNG ppy)3] in 2-methyltetrahydro-
furan (F=0.97)[14a] was used as a reference for quantum-efficiency measurements. [e] The decay lifetimes were obtained by fitting the decay curve with
a single exponential function. [f] Determined by using 9,10-diphenylanthracene as a standard.[14b]
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S. Wang et al.
assigned to the 3p!p* transition that is localized on the
conjugated backbone.
Upon excitation at 365 nm at ambient temperature, Au-B
displays a broad emission band at 490 nm with a shoulder at
about 500 nm (Figure 8, top). Time-resolved phosphores-
cence spectroscopy (Figure 8, middle) indicates that the
band at 490 nm is fluorescence, whilst the shoulder band at
about 500 nm is phosphorescence. By comparison with pre-
viously reported AuI–acetylide complexes[12] and related
boron compounds,[1] the fluorescence band may be assigned
to a mesityl-to-chelate-backbone CT transition, whilst the
phosphorescent band may be assigned to the alkyne–chelate
backbone. At 77 K, the emission spectrum of Au-B resem-
bles those of Re-B and Pt-B, with a very long decay lifetime
(Figure 8, bottom), which is assigned as a p!p* transition
that is localized on the conjugated backbone.
Photoisomerization: Re-B does not show any photoisomeri-
zation at all and remains intact after irradiation at 365 nm
1
for more than 2 h, as shown by H NMR spectroscopy (see
the Supporting Information). The high photostability of
Re-B can be attributed to a low-lying MLCT/CT state that
involves the ReI unit, which dominates the lowest excited
state, as indicated by the absorption and phosphorescence
spectra. Such a low-energy MLCT state can effectively
quench the isomerization of the boron chromophore.
In contrast to the inactivity of Re-B upon excitation at
365 nm in toluene, Au-B undergoes rapid isomerization,
thereby forming isomer Au-B’ in quantitative yield and
changing color from colorless to dark-blue, as shown in
Figure 9. This photoisomerization process was monitored by
Figure 8. Top: Full emission spectra of solutions of At-B, Pt-B, and Re-B
(ca. 1.0ꢃ10ꢀ5 m) in toluene at 298 K under a N2 atmosphere. Middle:
Time-resolved phosphorescence spectra of the metal compounds in tol-
uene at 298 K under a N2 atmosphere with a delay time of 140–160 ms.
Bottom: Time-resolved phosphorescence spectra of the compounds in 2-
Figure 9. Changes in the UV/Vis spectra of Au-B (ca. 1.0ꢃ10ꢀ5 m) in tol-
uene upon irradiation at 365 nm. Inset: Structures and colors of Au-B
and its dark isomer, Au-B’.
methyltetACHTUNGTRENNUNGrahydrofuran at 77 K with a delay time of 160–200 ms.
middle) and oxygen-sensitivity tests support the assignment
of the peak at 520 nm as phosphorescence and the peak at
480 nm as fluorescence. Both peaks likely arise from transi-
1H and 31P NMR and UV/Vis spectroscopy. The UV/Vis
spectrum of Au-B shows a new, broad band at lmax ꢂ620 nm
upon irradiation, which is attributed to the Au-B’ isomer.
The 1H NMR spectrum of Au-B in C6D6 shows that it under-
tions that involve the two conjugated B
77 K, Pt-B shows a phosphorescence emission peak at lmax
516 nm, which resembles that of Re-B (Figure 8, top) and is
ACHTUNGTRNE(NGNU ppy)Mes2 units. At
=
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Photoisomerization of a N^C-Chelate Organoboron Compound
FULL PAPER
goes clean conversion into Au-B’ upon irradiation (see the
Supporting Information). The structure of Au-B’ is similar
to that of the “dark isomer”[1a] of B
ACTHNUTRGNENU(G ppy)Mes2, because of
their similar and characteristic 1H NMR spectra. The
31P NMR spectra also confirmed the quantitative conversion
of Au-B into Au-B’ because it showed a distinct upfield shift
of the 31P signal of the phosphine group from d=42.96 to
42.83 ppm (see the Supporting Information). This upfield
shift of the 31P signal is consistent with an increased electron
density on the chelate backbone of the dark isomer,[1]
which, in turn, increases the donation from the acetylide
ligand to the AuI center and decreases the donation from
the phosphine ligand. The photoisomerization process of
Au-B is fully thermally reversible, with an activation energy
of 117 kJmolꢀ1, similar to that[1a] observed for B
(see the Supporting Information).
ACHTUNGTNER(NUNG ppy)Mes2
The quantum yield[15] of the photoisomerization of Au-B
at 298 K was determined to be about 0.46 on excitation at
365 nm, which was significantly less than those of B
ACHTUNGTRENNUNG
ACHTUNGTRNE(NUNG ppy)-
Figure 11. Bottom: Changes in the aromatic region of the 1H NMR spec-
tra of Pt-B in C6D6 upon UV irradiation under a N2 atmosphere. Top:
Colors of the solution and the structures of the compound before and
after UV irradiation.
when excited at 310 nm, the photoisomerization quantum ef-
ficiency of Au-B greatly increased and the relative quantum
efficiency ratio of Au-B to TIPS-B became about 1:1 (see
the Supporting Information), which may be attributed to
triplet-energy transfer from the AuI unit onto the boron
unit.
Pt-B was found to undergo photoisomerization upon irra-
diation at 365 nm in toluene. However, as shown in Fig-
ures 10 and 11, the change in the UV/Vis spectrum and the
chromophore, respectively. The single boryl-isomerization
phenomenon can be explained by a fast intramolecular
energy-transfer process in which the excited-state energy is
absorbed by the isomerized boryl unit, owing to its low-
energy absorption band, thus quenching the isomACHTUNGTRENNUNGerACHTUNGTRENNUNGization
of the second boryl unit. The same phenomenon has also
been observed in polyboron compounds that we reported
earlier.[1c,e] The 31P resonance of Pt-B also experiences a
slight upfield shift from d=20.40 to 20.34 ppm upon isomer-
ization, as with Au-B (see the Supporting Information). The
photoisomerization of Pt-B is very slow and much less effi-
cient compared to Au-B, with a quantum yield of about
0.001. This low photoisomerization quantum efficiency may
be attributed to the low-lying p!p* state of the conjugated
chelate backbone of the two BACTHNUTRGNEG(NU ppy)Mes2 unit.
TD-DFT calculations: To gain a better understanding of the
photophysical and photochemical properties of Re-B, Au-B,
and Pt-B, TD-DFT computational studies were performed
for all three compounds. In addition, we also performed cal-
ꢁ
ꢁ
culations on Au
TMS-B (the analogue of TIPS-B), and BAHCUTNGTRENNUNG
G
G
ACHTUNGTREN(NUG PPh3)2ACHTUNGTNER(NUGN C CH)2,
Figure 10. Changes in the UV/Vis spectra of Pt-B in toluene upon irradi-
derstand the impact of the metal component on the elec-
tronic properties of the boron chromophore. The data of the
two lowest singlet excited states that have appreciable oscil-
lator strengths and the lowest triplet state for all three metal
compounds are provided in Table 3. The relevant molecular-
orbital diagrams are shown in Figure 12. These TD-DFT re-
sults show that the S1 energy of the metal compounds is
much lower for Re-B compared to TMS-B, whilst it is some-
what higher for Au-B and Pt-B, which is in good agreement
with the absorption spectra (Figure 8). The T1 energy fol-
lows a similar trend, which agrees well with the observed
ation at 365 nm.
color of the solution is much less dramatic compared to that
of Au-B, with a weak broad peak at lmax ꢂ600 nm. To estab-
lish whether both boron units are involved in the photoiso-
1
merization or not, we used H NMR spectroscopy to moni-
tor the process. As shown in Figure 11, upon irradiation, sin-
glet proton Ha (dꢂ8.35 ppm) was replaced with two new
peaks, Hb and Hb’, which maintained a 1:1 ratio and could
be assigned to the isomerized and the non-isomerized boron
Chem. Eur. J. 2013, 19, 5314 – 5323
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5319

S. Wang et al.
Table 3. TD-DFT results for Re-B, Au-B, and Pt-B.[a]
(51%) than the p!p* transition of the backbone (40%).
Because the mesityl!chelate charge transfer is the key driv-
ing force of the photoisomerization, the greater Mes!che-
late CT contribution to the T1 state by TIPS-B compared to
Au-B explains why, on excitation at 365 nm, the quantum ef-
ficiency of Au-B is lower than that of TIPS-B. The impor-
tance of the Mes!chelate CT transition in the T1 state can
be further illustrated by comparing the S1 and T1 states of
State
Contribution
(%)
Energy
[eV]
l
f[b]
[nm]
Re-B
Au-B
Pt-B
T1
S1
S2
H!L (95)
2.01
2.04
2.46
617
608
505
–
H!L (94)
0.0017
0.0328
Hꢀ3!L (15)
Hꢀ2!L (74)
Hꢀ4!L (50)
H!L (34)
T1
2.61
474
–
S1
S2
T1
H!L (92)
2.89
3.22
2.63
429
385
471
0.0409
0.0041
–
TMS-B and BACTHNUGTRNEUNG(ppy)Mes2 in Table 3 and Figure 12. For the S1
state, these two compounds have similar contributions from
Hꢀ1!L (87)
Hꢀ9!L+1 (17)
Hꢀ6!L (13)
Hꢀ2!L (25)
H!L (18)
the Mes!chelate CT transition. However, for the T1 state,
the CT transition is 83% for BACHTNURGTNEUNG(ppy)Mes2 and 51% for
TMS-B, which is responsible for the higher photoisomeriza-
tion quantum efficiency of B(ppy)Mes2 (0.85) compared to
that of TIPS-B (0.70). TD-DFT results indicate that the Au-
(PPh3) unit contributes significantly to the higher triplet
states (e.g., T8, 3.55 eV; see the Supporting Information).
Thus, the enhanced photoisomerization quantum efficiency
S1
S3
Hꢀ1!L+1 (36)
H!L (62)
3.01
3.26
413
381
0.1180
0.0507
AHCTUNGTRENNUNG
Hꢀ4!L (10)
Hꢀ3!L+1 (37)
Hꢀ2!L (44)
Hꢀ4!L (40),
H!L (51)
AHCTUNGTRENNUNG
TMS-B
T1
2.54
488
–
AHCTUNGTRENNUNG
of Au-B on excitation at 310 nm, as compared to that of
TIPS-B, can be attributed to the triplet-state-sensitization
effect exerted by the AuACTHNUTRGNEUNG(PPh3) unit.
S1
S2
T1
H!L (97)
2.74
2.99
2.75
453
415
450
0.0253
0.0081
–
Hꢀ1!L (80)
Hꢀ4!L (10)
H!L (83)
BACHTUNGTRENNUNG(ppy)Mes2
For Pt-B, the S1 state is dominated by the Mes!backbone
CT transition, with high oscillator strength. However, the T1
state is dominated by p!p* transitions that involve the con-
jugated backbone and significant contributions from the
d orbitals of the Pt atom, as shown in Table 3 and Figure 12,
which explains the low photoisomerization quantum effi-
ciency of Pt-B. The dual-emission peaks at ambient temper-
ature can be attributed to the S1 and T1 states.
S1
S2
H!L (98)
2.89
3.15
430
394
0.0129
0.0067
Hꢀ1!L (79)
[a] TD-DFT calculations were performed at the B3LYP level of theory
by using the LANL2DZ basis set for the metal atoms and the 6-31G(d)
basis set for all other atoms. [b] Oscillator strength.
phosphorescent spectra at ambient temperature (Figure 9,
middle).
For the individual components of compounds Au-B and
Pt-B, TD-DFT computational results showed that the T1 en-
ꢁ
The key finding from the TD-DFT calculations of Re-B is
that the S1 and T1 states are all charge-transfer transitions
from p (involving orbitals from the chelate backbone, the
alkyne bond, the CO ligand that is trans to the acetylide,
and the Re atom) to p* (bpy), thus confirming the assign-
ment of the lowest-energy absorption band and the ambi-
ergies of AuACHTUGNTERN(NNUG PPh3)ACHTUNRTGENGUN(N C CH) (3.66 eV) and trans-PtACHTUNGTRENNUNG(PPh3)2-
ꢁ
G
B
Therefore, in principle, connecting these metal components
with the B(ppy)Mes2 unit should enhance the photoisomeri-
AHCTUNGTRENNUNG
ent-temperature phosphorescence of Re-B to intra
li
N
zation quantum efficiency through triplet-state sensitization
and energy transfer. The fact that the Au-B and Pt-B have
either a lower or similar photoisomerization quantum effi-
ciency, relative to that of the parent boron chromophore
and MLCT transitions. In addition, the TD-DFT results indi-
cate that the T1 energy (2.01 eV) of Re-B is much lower
than the triplet energy of the mesityl!chelate CT transition
of the B
N
(TIPS-B or BACTHGUNETRNNU(G ppy)Mes2), can be explained by the enhance-
firming that the photostability of the boryl chromophore in
ment of the conjugation in the alkyne–boron chelate back-
bone by the metal unit, which lowers the energy of the p!
p* transitions that are localized on the p-conjugated back-
bone, thus leading to a greater contribution of these transi-
tions to the T1 state and the partial quenching of the photo-
Re-B is indeed caused by the low lying intraACTHNUTRGENNlUG iACHUTGTNRENNgGU and CT/
MLCT of the ReI unit.
For Au-B, the S1 state is a typical mesityl-to-alkyne–che-
late charge-transfer transition with moderate oscillator
strength, which is responsible for the lowest-energy absorp-
tion band and the fluorescence peak at ambient tempera-
ture. The T1 state is dominated by a p!p* transition
(HOMOꢀ4!LUMO, 50%) of the alkyne–chelate back-
bone with about 34% mesityl!chelate CT transition, which
is responsible for the observed phosphorescence peak of
Au-B. Appreciable contributions from the AuI atom are
present in HOMOꢀ4. The key difference between TMS-B
(to simplify the computations, we calculated TMS-B instead
of TIPS-B) and Au-B is the T1 state. For TMS-B, the mesi-
tyl!chelate CT contribution to the T1 state is much greater
AHCTUNGTRENNiGUN somerization of the boron chromophore.
Conclusion
We have demonstrated that a metal–acetylide unit that is
connected to the chelate backbone of the B(ppy)Mes2 com-
AHCTUNGTRENNUNG
pound can have a significant impact on the photoactivity of
the boron compound. The introduction of a Re(CO)3(tert-
Bu2bpy) unit affords a low-energy MLCT/intra
ACHUTGTNRNEUGlN iACTHNGUTRENgUNG and CT
triplet state around the ReI center, which can completely
5320
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 5314 – 5323

Photoisomerization of a N^C-Chelate Organoboron Compound
FULL PAPER
Figure 12. Isodensity surface plots for frontier orbitals of N^C-chelate BMes2 compounds in the ground states (isodensity contour=0.03).
Experimental Section
shut off the photoactivity of the boron unit. The trans-Pt-
ꢁ
(PPh3)
E
General procedures: All reactions were performed under a dry N2 at
AHCTUNGTRENNUNG
ACHTUNGTRENNUNGmo-
the chelate backbone of both boron units through the Pt
center and the contribution of the p!p* transitions that are
localized on the backbone to the T1 state, thereby leading to
a dramatic quenching of photoisomerization of the boron
ꢁ
starting materials were purchased from Aldrich Chemical Co. and used
without further purification. Solvents were freshly distilled over appropri-
ate drying reagents under a N2 atmosphere. NMR spectra were recorded
on a Bruker Avance 400 MHz spectrometer. Excitation and emission
spectra were recorded on a Photon Technologies International Quanta-
Master Model C-60 spectrometer. UV/Vis spectra were recorded on a
Varian Cary 50 Bio UV/Vis spectrophotometer. Phosphorescent lifetimes
were measured on a Photon Technologies International (PTI) phos-
phorimeter (Time-Master C-631F) that was equipped with a Xe flash
lamp and a digital emission photon-multiplier tube at a band pathway of
5 nm for excitation and 2 nm for emission. Elemental analysis was per-
formed at the Elemental Analysis Service, Department of chemistry, Uni-
versity of Montreal (Montreal, Quebec, Canada). Starting materials B1[8c]
(Scheme 2) and [Re(tert-Bu2bpy)(CO)3Cl][8] were synthesized according
to literature procedures.
chromophore. The AuACTHNUTRGENNUG(PPh3)CAHTUNGTRNE(NUGN C CH) unit also enhances the
p-conjugation of the backbone and the contribution of the
p!p* transitions of the backbone to the T1 state, albeit to a
much lesser extent. Thus, a significant decrease in the photo-
isomerization quantum efficiency of Au-B was also ob-
served, compared to that of TIPS-B, when excited at the
lowest-energy absorption band. At higher excitation energy,
the photoisomerization quantum efficiency of Au-B be-
comes comparable to that of TIPS-B because of the triplet-
state-sensitization effect. This study has established that the
photoisomerization of BACTHNUTRGNE(UGN ppy)Mes2 indeed involves a photo-
active triplet state. Furthermore, the CT transition from the
mesityl groups to the chelate backbone must make a major
contribution to the T1 state to achieve efficient photo
zation in this system. Finally, to use the metal units that
have a high triplet energy as effective triplet sensitizers,
p-conjugation between the metal unit, the linker, and the
chelate backbone of the boron unit should be avoided.
Synthesis of Re-B: A mixture of [Re(tert-Bu2bpy)(CO)3Cl] (140 mg,
0.24 mmol), TlPF6 (100 mg, 0.28 mmol), Et3N (5 mL), and B1 (150 mg,
0.35 mmol) in THF (100 mL) was heated at reflux under a nitrogen at-
mosphere for 2 days. Then, the insoluble inorganic salt was removed by
filtration and the yellow filtrate was concentrated under vacuum. The
residue was purified by column chromatography on silica gel (hexanes/
CH2Cl2, 1:1) to give the analytically pure material. Recrystallization of
the yellow product in CHCl3/hexanes afforded the compound Re-B as
yellow crystals (25% yield). 1H NMR (400 MHz, CDCl3, 298.0 K): d=
8.96 (d, J=5.9 Hz, 2H), 8.10 (s, 1H), 8.08 (d, J=1.7 Hz, 2H), 7.66–7.61
ACHTUNGTRENNUNGiACHTUNGTRENNUNG ACHTUNGTRENNUNG
Chem. Eur. J. 2013, 19, 5314 – 5323
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5321

S. Wang et al.
(m, 4H), 7.47 (dd, J=5.9, 1.9 Hz, 2H), 7.20–7.13 (m, 2H), 6.56 (s, 4H),
2.15 (s, 6H), 1.63 (s, 12H), 1.46 ppm (s, 18H); 13C{1H} NMR (100 MHz,
CDCl3): d=197.7, 192.4, 164.6, 162.9, 155.6, 154.4, 152.8, 148.1, 145.9,
143.1, 140.2, 135.3, 134.7, 133.4, 130.8, 130.4, 129.5, 124.9, 124.2, 123.0,
120.8, 119.2, 116.3, 101.2, 35.5, 30.3, 24.7, 20.6 ppm; elemental analysis
calcd (%) for C52H53BN3O3Re: C 64.72, H 5.54, N 4.35; found: C 64.58,
H 6.17, N 3.99.
Acknowledgements
We thank the Natural Sciences and Engineering Research Council for fi-
nancial support. S.W. thanks the Canada Council for the Arts for a
Killam Research Fellowship.
Synthesis of Au-B: A mixture of [AuACHTNUGRTENUNG(PPh3)Cl] (112 mg, 0.23 mmol), B1
(100 mg, 0.23 mmol), CuI (2 mg), and Et3N (6 mL) in degassed CH2Cl2
(30 mL) was stirred under a nitrogen atmosphere in the dark for 2 days.
Then, the mixture was concentrated under vacuum and partitioned be-
tween CH2Cl2 and water. The aqueous layer was further extracted with
CH2Cl2 (2ꢃ20 mL) and dried with MgSO4. The crude mixture was fur-
ther purified by column chromatography on silica gel (hexanes/CH2Cl2,
4:1) to give the gold(I) compound as a light-yellow powder (65% yield).
1H NMR (400 MHz, CD2Cl2, 298.0 K): d=8.48 (s, 1H), 7.99 (dd, J=8.3,
1.5 Hz, 1H), 7.89 (d, J=8.3 Hz, 1H), 7.83 (dd, J=6.5, 2.8 Hz, 1H), 7.71
(dd, J=5.8, 1.8 Hz, 1H), 7.57–7.46 (m, 15H), 7.29–7.22 (m, 2H), 6.62 (s,
4H), 2.16 (s, 6H), 1.77 ppm (s, 12H); well-resolved 13C NMR spectra
could not be obtained for this compound, owing to its poor solubility; el-
emental analysis calcd (%) for C49H44AuBNP: C 66.45, H 5.01, N 1.58;
found: C 66.37, H 4.98, N 1.55.
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Synthesis of Pt-B: This PtII compound was prepared in a similar manner
to Au-B, by using [trans-PtACHTNUGTRNEUGN(PPh3)2Cl2] (108 mg, 0.14 mmol), B1 (122 mg,
0.28 mmol), CuI (2 mg), and Et3N (6 mL) in degassed CH2Cl2 (30 mL).
After the completion of the reaction, the mixture was purified by column
chromatography on silica gel (hexanes/CH2Cl2, 4:1) to give Pt-B as a
light-yellow powder (62% yield). 1H NMR (400 MHz, C6D6, 298.0 K):
d=8.22 (s, 1H), 8.00 (d, J=7.3 Hz, 1H), 7.85 (dd, J=12.8, 6.8 Hz, 6H),
7.41 (d, J=7.6 Hz, 1H), 7.11–6.93 (m, 11H), 6.88 (d, J=8.3 Hz, 1H), 6.77
(s, 4H), 6.66 (dd, J=8.3, 1.5 Hz, 1H), 2.23 (s, 6H), 1.94 ppm (s, 12H);
13C{1H} NMR (100 MHz, CDCl3): d=155.4, 147.2, 146.2, 142.8, 140.5,
135.7, 135.4, 135.3, 135.2, 134.1, 131.3, 131.1, 131.0, 130.7, 130.3, 128.6,
128.5, 128.4, 125.6, 121.7, 117.0, 25.2, 20.9 ppm; elemental analysis calcd
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N 2.36.
X-ray diffraction analysis: Single crystals of all three metal compounds
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Apex II single-crystal X-ray diffractometer with graphite-monochromat-
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tures were solved by using direct methods. All non-hydrogen atoms were
refined anisotropically. Disordered CH2Cl2 solvent molecules were locat-
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Au-B) and were modeled and refined successfully. CHCl3 solvent mole-
cules were located in the crystal lattice of Re-B (2CHCl3 molecules per
molecule of Re-B) and were refined successfully. Benzene solvent mole-
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molecule of Pt-B) and were refined successfully. The Pt atom in Pt-B
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5322
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Chem. Eur. J. 2013, 19, 5314 – 5323

Photoisomerization of a N^C-Chelate Organoboron Compound
FULL PAPER
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Received: November 13, 2012
Published online: March 4, 2013
Chem. Eur. J. 2013, 19, 5314 – 5323
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