Impact of the Alkyne Substitution Pattern and Metalation on the Photoisomerization of Azobenzene-Based Platinum(II) Diynes and Polyynes

Article abstract of DOI:10.1021/acs.inorgchem.6b01509

Trimethylsilyl-protected dialkynes incorporating azobenzene linker groups, Me₃SiC ≡ CRC ≡ CSiMe₃ (R = azobenzene-3,3′-diyl, azobenzene-4,4′-diyl, 2,5-dioctylazobenzene-4,4′-diyl), and the corresponding terminal dialkynes, HC ≡ CRC ≡

Full text of DOI:10.1021/acs.inorgchem.6b01509

Article
pubs.acs.org/IC
Impact of the Alkyne Substitution Pattern and Metalation on the
Photoisomerization of Azobenzene-Based Platinum(II) Diynes and
Polyynes
Rayya A. Al-Balushi,† Ashanul Haque,† Maharaja Jayapal,† Mohammed K. Al-Suti,† John Husband,†
Muhammad S. Khan,*,† Jonathan M. Skelton,*,‡ Kieran C. Molloy,‡ and Paul R. Raithby*,‡
†Department of Chemistry, Sultan Qaboos University, P.O. Box 36, Al Khod 123, Sultanate of Oman
‡Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
S
* Supporting Information
ABSTRACT: Trimethylsilyl-protected dialkynes incorporating azoben-
zene linker groups, Me3SiCCRCCSiMe3 (R = azobenzene-3,3′-
diyl, azobenzene-4,4′-diyl, 2,5-dioctylazobenzene-4,4′-diyl), and the
corresponding terminal dialkynes, HCCRCCH, have been
synthesized and characterized. The CuI-catalyzed dehydrohalogenation
reaction between trans-[Ph(Et3P)2PtCl] and the deprotected dialkynes
i
in a 2:1 ratio in Pr2NH/CH2Cl2 gives the platinum(II) diynes trans-
[Ph(Et3P)2PtCCRCCPt(PEt3)2Ph], while the dehydrohalogena-
tion polycondensation reaction between trans-[(nBu3P)2PtCl2] and the
dialkynes in a 1:1 molar ratio under similar reaction conditions affords
the platinum(II) polyynes, [−Pt(PnBu3)2−CCRCC−]n. The
materials have been characterized spectroscopically, with the diynes also studied using single-crystal X-ray diffraction. The
platinum(II) diynes and polyynes are all soluble in common organic solvents. Optical-absorption measurements show that the
compounds incorporating the para-alkynylazobenzene spacers have a higher degree of electronic delocalisation than their meta-
alkynylazobenzene counterparts. Reversible photoisomerization in solution was observed spectroscopically for the alkynyl-
functionalized azobenzene ligands and, to a lesser extent, for the platinum(II) complexes. Complementary quantum-chemical
modeling was also used to analyze the optical properties and isomerization energetics.
processes that occur in the conjugated organic polymers used in
organic light-emitting diodes,6 lasers,7 photovoltaic cells,8 field-
effect transistors,9 surface-acoustic-wave sensors, and nonlinear-
optical materials.10 The incorporation of heavy metals such as
platinum into the polymer backbone introduces large spin−orbit
coupling effects, which allow for light emission from triplet
excited states. Moreover, the role of the spacer group in the
polymer structure is also well established,11 with platinum(II)
polyynes incorporating a wide range of conjugated aromatic and
heteroaromatic spacers having been investigated by us and by
others.12
In this work, we have explored the synthesis and character-
ization of di- and poly(platinaynes) incorporating both meta -
and para-substituted azobenzene spacers. We have performed a
comprehensive structural and spectroscopic characterization of
nine materials and elucidated how the substitution pattern on the
spacer and complexation with platinum affects the absorption
profile, highlighting the difference between the meta and para
forms. The difference in the conjugation in the two series mirrors
the difference in the electronic properties recently reported for
the related diplatinum complexes linked by 2,7- and 3,6-
INTRODUCTION
■
With the rapidly developing interest in molecular switches, a
considerable amount of work is being carried out worldwide to
identify suitable materials. Examples exist of molecular switching
through both physical and chemical means,1 and the materials
have a wide range of applications including in optoelectronics, as
holographic materials, and for multicolor displays and data-
storage and communication technologies.2 To achieve this
switching functionality, many organic chromophore building
blocks such as stilbenes, azobenzenes, spiropyrans, fulgides, and
chromenes are available and readily incorporated into more
complex systems.3 Of these, diazene or azo moieties are well-
established and promising chromophores, being robust, chemi-
cally stable, and cost-effective.4 Many new materials have been
designed based on this chromophore, showing a range of novel
properties.2
We have a long-standing interest in conjugated “rigid-rod”
poly(metallaynes), which represent a class of solution-process-
able materials with tunable optoelectronic properties.5 The rigid
backbone of the polyynes facilitates extended electronic
delocalization along the polymer chain, allowing for efficient
electronic transport and thus enabling the compounds to
function as molecular wires. Metallopolyynes have been used
as model systems for the study of some of the photophysical
Received: June 24, 2016
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Bis(trimethylsilyl)-Protected and Diterminal Alkyne Ligands Incorporating Azobenzene Moieties
Scheme 2. Synthesis of Platinum(II) Diynes and Polyynes Incorporating Azobenzene Spacers
carbazolediyl units, where the carbazole-2,7-diyl units display
enhanced conjugation.13 Preliminary photoirradiation experi-
ments on these diplatinum diynes indicate that the spacers, and,
to a lesser extent, the organometallic species, undergo reversible
photoinduced trans-to-cis isomerization about the azo bond in
solution, as observed for closely related diplatinum diylazoben-
zene linked complexes.14 The experimental measurements are
supported by quantum-mechanical modeling of a subset of
materials, which is used to interpret the spectroscopy and to
investigate the energetics of the isomerization.
systems.12b,f,16 The terminal dialkynes 1b−3b were purified by
silica gel column chromatography and isolated as orange or
brown solids with typical overall yields of between 76 and 91%.
The synthesis and characterization of compound 2b have been
reported previously.14 These diterminal alkynes were found to be
highly reactive and were therefore freshly prepared before
reaction with the platinum(II) precursor complexes.
The dehydrohalogenation reactions between trans-[(Ph-
i
(Et3P)2PtCl] and 1b−3b in a 2:1 stoichiometry, in Pr2NH/
CH2Cl2 in the presence of CuI at room temperature, afforded the
platinum(II) diynes M1−M3, while the polycondensation
reactions between trans-[(nBu3P)2PtCl2] and 1b−3b in a 1:1
ratio under similar reaction conditions yielded the platinum(II)
polyynes P1−P3 (Scheme 2). Purification of the platinum(II)
diynes and polyynes was carried out using silica and alumina
columns, respectively. The platinum(II) polyynes were obtained
in yields of 78−80%, indicating a very high conversion compared
to the platinum(II) diynes, which were obtained in 61−62%
yield. The platinum(II) diynes and polyynes were fully
characterized by UV−visible, IR and NMR spectroscopy, ESI-
MS, and elemental analysis.
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization. A
slightly modified palladium(II)/copper(I)-catalyzed cross-cou-
pling reaction between the diiodo azobenzenes 1−3 with
(trimethylsilyl)ethynyl was performed in iPr2NH/tetrahydrofur-
an (THF) to obtain 1,2-bis[3-[(trimethylsilyl)ethynyl]phenyl]-
diazene (1a), 1,2-bis[4-[(trimethylsilyl)ethynyl]phenyl]diazene
(2a), and 1,2-bis[2,5-dioctyl-4-[(trimethylsilyl)ethynyl]phenyl]-
diazene (3a) (Scheme 1).15 The protected dialkynes (1a−3a)
were found to be stable in air and light and were fully
characterized by IR and NMR (1H and 13C) spectroscopy,
electrospray ionization mass spectrometry (ESI-MS), and
elemental analysis.
The IR spectra of the platinum(II) diynes M1−M3 and the
polyynes P1−P3 show a single, sharp νCC absorption at 2090−
2098 cm−1, which is consistent with a trans configuration of the
ethynylenic units around the platinum(II) center. The bond
stretch frequency is similar to that found in a range of related
materials.12d,e The νCC values for the terminal diynes 1b−3b
(2105−2109 cm−1) are considerably lower than those of the
corresponding trimethylsilyl-protected dialkynes 1a−3a (2152−
2155 cm−1), as is expected.17 The platinum(II) diynes and
Protodesilylation of 1a−3a was accomplished by cleavage of
the trimethylsilyl groups with dilute aqueous KOH in methanol
(MeOH)/THF to generate the diterminal alkynes bis(3-
ethynylphenyl)diazene (1b), bis(4-ethynylphenyl)diazene
(2b), and 1,2-bis(4-ethynyl-2,5-dioctylphenyl)diazene (3b)
under conditions similar to those reported previously for related
B
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polyynes display lower νCC values than those in the
corresponding protected or terminal dialkynes, an observation
that has been attributed to either metalyne π-back-bonding or the
Mδ+−Cδ− polarity.18
Table 1. Crystallographic Data for M1 and M2
M1
M2
CCDC
empirical formula
fw/amu
temperature/K
cryst syst
space group
a/Å
1486091
1486092
C52H78N2P4Pt2
1245.22
C52H78N2P4Pt2
1245.22
In all of the ligand precursors and platinum(II) complexes,
clear 1H NMR resonances from the protons of the aromatic ring
systems were observed, indicating both the platinum(II) diynes
and polyynes to be structurally rigid. The two distinct 13C NMR
resonances of the ethynylenic carbon atoms in the diynes and
polyynes were both shifted downfield relative to the signals from
the diterminal alkynes, in agreement with the expected
structures. The resonances due to the ethyl and butyl groups
of the auxiliary ligands in the platinum(II) diynes and polyynes,
respectively, could be clearly identified. The single resonance in
the 31P NMR spectra of the platinum(II) diynes and polyynes
further confirms the trans arrangement of the phosphine ligands.
150(2)
250(2)
monoclinic
P21/c
triclinic
P1
̅
18.7794(4)
10.5232(2)
14.8114(3)
90.0
8.9899(3)
9.6169(3)
15.8333(7)
92.353(3)
93.954(3)
94.633(3)
1359.65(9)
1
b/Å
c/Å
α/deg
β/deg
110.929(3)
90.0
γ/deg
V/Å3
2733.89(11)
2
Z
1
The JPt−P values range from 2629−2654 Hz for the diynes to
ρcalc/(g cm−3
μ/mm−1
)
1.513
1.521
2338−2347 Hz for the polyynes; these spectral features are
similar to those of previously reported platinum(II) diynes and
polyynes12a,g and again confirm the all-trans configuration
around the metal centers.
5.262
5.290
F(000)
1240
620
cryst size/mm3
0.35 × 0.30 × 0.15
Mo Kα (λ = 0.71073 Å)
6.732−61.958
0.34 × 0.19 × 0.04
Mo Kα (λ = 0.71073 Å)
5.96−61.02
radiation
The MS results confirm the molecular masses expected for the
ethynyl ligands and the platinum(II) diyne complexes. Gel
permeation chromatography (GPC), calibrated against a
polystyrene standard, gave weight-average molecular weights
(Mw) for the polyynes P1−P3 in the range of 41800−84400 g/
mol, corresponding to degrees of polymerization (DPs) between
35 and 55. The polydispersity index (PDI) was found to vary
between 1.2 and 1.4. The relatively narrow polydispersity (PDI <
2) in the molecular weights is consistent with the proposed linear
structure from condensation polymerization. However, the
absolute molecular weights determined by GPC should be
treated with caution; GPC provides a measure of the
hydrodynamic volume, which is different for rigid-rod and
flexible polymers. Thus, the Mw values determined by this
technique using the polystyrene standard are likely to artificially
increase the measured molecular weights to some extent.
However, a high DP in the polyynes was supported by NMR
spectroscopy, in which resonances corresponding to protons on
the terminal end groups were too weak to be observed.
Structural Characterization. In addition to the spectro-
scopic characterization, we were also able to grow crystals of the
platinum(II) diyne complexes M1 and M2, which were suitable
for single-crystal X-ray diffraction. The key crystallographic data
are summarized in Table 1, and images of the two structures are
shown in Figures 1 and 2.
M1 crystallizes in the monoclinic space group P21/c, with half a
dimeric complex in the asymmetric unit and the midpoint of the
azo NN bond corresponding to a crystallographic center of
symmetry. The crystal structure is shown in Figure 1 and
confirms the trans conformation about the azo bond in the spacer
group, the meta substitution of the arene rings on the spacer, and
the trans arrangement of the phosphine ligands on the square-
planar platinum(II) centers. We observed no significant short
intermolecular contacts within the crystal structure, with the
dimeric complexes all being separated by normal van der Waals
distances.
2θ range for data
collection/deg
index ranges
−26 ≤ h ≤ 26, −14 ≤ k ≤ −12 ≤ h ≤ 12, −13 ≤ k ≤
15, −20 ≤ l ≤ 21 13, −22 ≤ l ≤ 22
reflns collected
indep reflns
70107 23252
8186 [Rint = 0.0495; Rσ=
8278 [Rint = 0.0363; Rσ =
0.0433]
0.0329]
data/restraints/
param
GOF on F2
8186/0/277
8278/51/334
1.060
1.042
final R indexes [I ≥ R1 = 0.0287, wR2 = 0.0508 R1= 0.0397, wR2 = 0.0809
2σ(I)]
final R indexes (all
data)
R1 = 0.0396, wR2 = 0.0533 R1 = 0.0545, wR2 = 0.0879
platinum(II) coordination plane, and the plane is itself at an
angle of 37.86° with the phenyl ring of the azobenzene spacer. All
other bond parameters were found to be within the expected
ranges for dimeric platinum(II) diyne complexes.11,12
M2 crystallizes in the lower-symmetry triclinic space group P1
(Figure 2). As in the M1 structure, the asymmetric unit contains
half a molecular complex, with the midpoint of the NN double
bond located on a crystallographic center of symmetry. The X-
ray structure confirms the trans geometry about the azo bond in
the spacer group, the para substitution of the spacer arene rings,
and the square-planar coordination of the metal, with a trans
relationship between the phosphine ligands. Again, there are no
significant short intermolecular contacts less than the sum of the
van der Waals radii.
As in the structure of M1, the phenyl rings in the azobenzene
group are coplanar. In contrast, however, the dihedral angle
between the unique arene ring and the platinum(II) square plane
is 86.77°, compared to the equivalent angle 37.86° in M1. The
angle between the platinum(II) coordination plane and the
terminal phenyl ring is 86.89°, making it almost coplanar with the
arene ring of the azobenzene spacer, as opposed to almost
perpendicular to it as in M1. Since within the molecule there is
free rotation about the Pt−C bond connecting the spacer and the
platinum(II) center, it is not surprising that there is considerable
variation in the dihedral angle between the central spacer and the
platinum(II) coordination plane. Although there is variation in
the angle of the terminal phenyl ring with respect to the phenyl
rings in the spacer group, in both structures the terminal phenyl
̅
The crystallographic symmetry implies that the azobenzene
spacer, which forms the central core of the complex, i.e., the
PtCC-m-C6H4NN-m-C6H4CCPt unit, is planar. The
coordination geometry around each of the two platinum(II)
centers is, as noted above, square-planar; the terminal phenyl
rings are at an angle of 86.90° with respect to the unique
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Figure 1. Crystal structure of M1, with the unique atoms numbered as shown. Displacement ellipsoids are plotted at the 50% probability level. Selected
geometric data: C(1)−Pt(1) 2.017(3) Å, C(21)−Pt(1) 2.069(3) Å, P(1)−Pt(1) 2.2882(7) Å, P(2)−Pt(1) 2.2856(7) Å, C(1)−C(2) 1.212(4) Å,
C(2)−C(3) 1.439(3) Å, N(1)−N(1)i 1.246(4) Å, ∠C(1)−Pt(1)−C(21) 176.06(11)°, ∠C(1)−Pt(1)−P(1) 87.42(7)°, ∠C(1)−Pt(1)−P(2)
92.27(8)°, ∠C(21)−Pt(1)−P(1) 92.27(8)°, ∠C(21)−Pt(1)−P(2) 88.19(8)°, ∠P(2)−Pt(1)−P(1) 179.01(3)°, ∠C(2)−C(1)−Pt(1) 174.2(2)°,
∠C(1)−C(2)−C(3) 176.7(3)°, ∠N(1)i−N(1)−C(7) 114.6(3)°. Symmetry operation i: −x, 1 − y, −z.
Figure 2. Crystal structure of M2, with the unique atoms numbered as shown. Displacement ellipsoids are plotted at the 50% probability level. Selected
geometric data: C(1)−Pt(1) 2.018(4) Å, C(9)−Pt(1) 2.062(4) Å, P(1)−Pt(1) 2.2938(14) Å, P(2)−Pt(1) 2.2851(13) Å, C(1)−C(2) 1.207(6) Å,
C(2)−C(3) 1.441(6) Å, N(1)−N(1)i 1.254(7) Å; ∠C(1)−Pt(1)−C(9) 178.69(17)°, ∠C(1)−Pt(1)−P(2) 87.55(14)°, ∠C(1)−Pt(1)−P(1)
92.62(14)°, ∠C(9)−Pt(1)−P(1) 88.16(12)°, ∠C(9)−Pt(1)−P(2) 91.56(12)°, ∠P(2)−Pt(1)−P(1) 173.87(6)°, ∠C(2)−C(1)−Pt(1) 177.3(4)°,
∠C(1)−C(2)−C(3) 177.7(5)°, ∠N(1)i−N(1)−C(6) 114.1(4)°. Symmetry operation i: 2 − x, 2 − y, −z.
rings sit perpendicular to the platinum(II) plane, as is typically
found to be the case in dimeric diplatinum diyne complexes of
this type. The intramolecular bond parameters in M2 are similar
to those in M1 and in related dinuclear diplatinum
complexes.11,12
Optical-Absorption Spectroscopy. The room temper-
ature solution absorption spectra of the platinum(II) diynes
(M1−M3) and platinum(II) polyynes (P1−P3) in CH2Cl2 are
shown in Figure 3.
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in the azobenzene spacer in the M1/P1 series being meta to the
azo bond as opposed to para in the M2/P2 and M3/P3 series.
Photoirradiation. Azobenzenes are well-known for their
ability to show cis-to-trans isomerization following thermally
induced or photoinduced excitation.21 The resulting large
geometric change can potentially be harnessed for a number of
applications including liquid crystals,22 molecular photo-
switches,23 and photochromic ligands for optochemical genet-
ics.24 The isomerization of azobenzene-containing materials
depends on several factors, including the chemical structure (e.g.,
to what extent the isomerization is restricted by steric
constraints), the degree of conjugation, and the influence of
the solvent.25
We therefore performed preliminary photoirradiation experi-
ments on the 1a and 2a series of compounds, with these six
materials being chosen in order to examine the effects of the
alkyne groups being para or meta to the azo bond. A series of
experiments were performed in which CH2Cl2 solutions of 1a,
2a, and the corresponding platinum(II) diynes and polyynes
(M1/M2 and P1/P2, respectively) were irradiated with a 254
nm UV lamp (6 W) for 60 min, and the isomerization was
followed spectroscopically by recording UV−visible spectra at 10
min intervals. Although the spectra possess features at longer
wavelengths, we found that we were only able to observe spectral
changes at this wavelength. To check for the recovery of spectral
changes induced by UV irradiation, following the experiments,
the solutions were allowed to stand in ambient light for at least 48
h, and a final spectrum was recorded to compare against the time
zero measurement. The recorded time series are shown in Figure
4.
Figure 3. Normalized solution UV−visible absorption spectra of the
protected ligands 1a−3a and the corresponding platinum(II) diynes
M1−M3 and polyynes P1−P3 in CH2Cl2.
The series of spectra of the protected ligands in Figure 4a,d
show consistent spectral changes as a function of the irradiation
time, with a fall in the intensity of the primary absorption bands
at 320 and 370 nm, together with a corresponding growth in a
shorter-wavelength feature. With reference to the simulated
spectra in the Computational Modeling section, these changes
can be ascribed to formation of the cis isomers. While the time
series thus indicates that both protected ligands undergo
photoinduced isomerization, the spectral changes appear to
saturate at long irradiation times, suggesting that the photo-
chemical conversion does not go to completion.
Whereas 1a shows significant photoisomerization, the spectral
changes in the absorption profiles of M1 and P1 are rather less
marked, with the main spectral change being a growth in the
short-wavelength part of the absorption profile. With reference
to the calculations, the two absorption maxima at 323 and 295
nm in the spectrum of M1 might be attributable to the cis and
trans isomers of this complex, which would suggest that a small
amount of photoisomerization occurs under UV irradiation.
There is also a similar trend visible in the spectra of P1, although
it is not clear whether this can be attributed to an increase in the
proportion of cis spacer units in the polymer backbone.
Presuming it can, the changes are less marked than those in
the spectrum of M1, which would suggest that isomerization
happens less readily in the polymer than in the diyne complex.
In contrast, the spectra of the platinum(II) species
incorporating 2a, viz., M2 and P2, do appear to undergo
systematic changes under UV irradiation, with a blue shift and a
decrease in the intensity of the prominent absorption bands at
∼440 and 460 nm. Energetics calculations (see the Computa-
tional Modeling section) suggest that the cis-azo form of M2 is
more energetically unfavorable than the corresponding isomer of
M1, and, of course, both are considerably less stable than the
The spectra of the protected ligands show intense primary
absorption bands at ∼320 nm (1a), 370 nm (2a), and 390 nm
(3a), which were assigned as π → π* transitions, together with
weaker absorptions at longer wavelength, which are ascribed to
weak n → π* transitions.19
For the platinum(II) diynes and polyynes of 2a and 3a (M2/
M3 and P2/P3, respectively; Figures 3b/c), there is a clear red
shift in the absorption profile compared to the corresponding
protected ligands. The absorption maxima in the spectra of the
diynes M2 and M3 occur at ∼440 nm, while the maxima in the
spectra of the corresponding polyynes P2 and P3 are further red-
shifted to approximately 460 nm. All four spectra also appear to
exhibit a prominent longer-wavelength shoulder feature, leading
to a significantly enhanced absorption out to ∼500−550 nm
compared to the protected ligands.
The fact that there is a red shift in the spectra of both P2 and
P3 compared to the platinum(II) diyne complexes (M2 and M3)
suggests enhanced electronic delocalization along the polymer
chains, which would require conjugation through the platinum-
(II) centers. That such conjugation is possible, which was
confirmed by supporting calculations (see the Computational
Modeling section). The enhanced absorption at visible wave-
lengths can, as in related materials, be attributed to metal-to-
ligand charge-transfer (MLCT) excitations.20
In contrast, the trend for M1 and P1 (Figure 3a) is markedly
different. In the absorption profiles of M1 and P1, the long-
wavelength maxima occur at ∼320 and 335 nm, respectively,
which are substantially blue-shifted compared to the maxima of
the other diynes and polyynes. The bands also appear to have a
considerably narrower bandwidth than the corresponding
features in the spectra of the other platinum(II) species. Either
or both of these observations may be related to the alkyne groups
E
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∼280 nm. Bearing in mind the loss of signal at short wavelengths
due to absorption from the solvent, the spectra of 1a, M1, and P1
in toluene are fairly similar to those recorded in CH2Cl2, in both
the shapes and positions of the spectral bands (cf. Figures 3a and
4a−c). The longest-wavelength absorption maxima in the
profiles of 1a/M1 and P1 occur at approximately 320 and 330
nm in toluene, respectively, which are very similar to the values in
CH2Cl2. The progressive reduction in the intensity of the 320 nm
absorption band upon irradiation in the spectrum of 1a again
provides clear evidence for photoinduced isomerization. The
spectra of M1 and P1 also display the same changes under
illumination as in CH2Cl2. The changes are again much less
marked than those for 1a, which suggests that isomerization in
the platinum(II) species may be restricted.
Whereas the time series of 2a appears to demonstrate behavior
similar to that in the spectra recorded in CH2Cl2, M2 and P2
appear to undergo very little change under irradiation in toluene.
Although this could be taken to imply that the isomerization of
these platinum(II) species is sensitive to the solvent, it could also
be due simply to the strong absorption of toluene below 280 nm
attenuating the excitation lamp. In this regard, we note that our
energetics calculations do not predict the solvent to have a strong
effect on the energy difference between the isomers of M2, and so
a large difference in behavior with respect to the experiments in
CH2Cl2 would be unexpected. When the spectra of P2 in the two
solvents are compared, there appears to be a notable change to
the relative magnitude of the signals forming the primary
absorption bands, the origin of which is unclear.
In summary, these photoisomerization experiments provide
evidence that both ethynyl-functionalized m- and p-azobenzene
ligands display photoinduced trans-to-cis isomerization in
solution and that the photochemical switching may also be
retained in the platinum(II) diynes and polyynes, albeit with a
low photoisomerization yield. The switching of the para-
substituted azobenzene complexes appears to be more facile
than that of the meta-substituted analogues, which we suggest
may be due to more efficient absorption and/or vibronic
coupling in the former. The experiments provide little evidence
for solvent effects, although we cannot draw any firm conclusions
here based on the present results. While these preliminary studies
suggest that the diynes and polyynes may show interesting
photoisomerization behavior, more quantitative studies would
need to be carried out to investigate this further, e.g., to test the
cyclability or to explore the solvent effects in more detail. This is
beyond the scope of the present study.
Figure 4. Time evolution of the UV−visible absorption spectra of the
trimethylsilyl-protected azobenzene ligands 1a and 2a (a and d) and the
corresponding platinum(II) diynes M1 and M2 (b and e) and polyynes
P1 and P2 (c and f) in CH2Cl2 under irradiation with 254 nm UV light.
The red lines, marked “Rest” on the color scale, are spectra recorded
after allowing the solutions to stand in ambient light for a minimum of
48 h, to check for recovery of the spectral features after the
photoirradiation experiment.
respective trans configurations; from this, one might infer that
the apparently more facile isomerization of M2 is due to stronger
absorption and/or a more efficient coupling of the electronic
excitations to the vibrational modes that effect the rearrange-
ment. We also note that, in contrast to the spectra of the other
species, the spectrum of M2 has a clear isosbestic point at ∼420
nm, although the significance of this feature in the present
context, and of the absence of similar features in the spectra of the
other compounds, is not clear.
Computational Modeling. In order to better understand
the isomerization behavior and optical properties of the
azobenzene ligands and the dinuclear platinum(II) complexes,
we carried out computational modeling on the cis and trans
forms of the deprotected ligands 1b and 2b and the
corresponding model complexes M1 and M2 using hybrid
density functional theory (DFT).
Initial models of the trans conformations of M1 and M2 were
made from the X-ray structures, while models of the cis
configurations were prepared by manual rotation about the azo
bond. Models of 1b and 2b were generated from models of the
platinum(II) complexes. Geomerty optimization was performed
in the gas phase and also using the conductor-like screening
model (COSMO)26 to mimic the dielectric environments of the
solvents in which the spectra were recorded and the photo-
isomerization experiments performed, viz., CH2Cl2 (ε = 8.93)
and toluene (ε = 2.38). On the basis of our findings, we also
carried out further COSMO calculations with the higher
For 1a, 2a, M1, and M2, the spectral changes are largely
reversible under ambient conditions, with the absorption profiles
of both solutions recorded after resting for >48 h being very
similar to the time-zero spectra. On the other hand, the spectra of
P1 and P2 recorded after 48 h differ more markedly from the
time-zero measurements. However, we do not observe the
formation of new bands, as would be expected from sample
degradation, and we found that longer irradiation of the solutions
did indeed give rise to persistent new absorption peaks. Thus, the
origin of the difference between the spectra before the
experiment and after resting could be due to small amounts of
degradation but might also be due to a drift in the spectrometer
calibration and/or slow evaporation of the solvent during the
experiment. In particular, the latter could explain the increased
absorption of 1a and P1 after resting.
To check for possible influences of the solvent, we performed a
second set of experiments in toluene (Figure S1), which was
chosen for its lower polarity despite its strong absorption below
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Figure 5. Optimized gas-phase geometries of platinum(II) diyne complexes with the meta-substituted (a and b; M1) and para-substituted (c and d; M2)
azobenzene spacers in the trans-azo (a and c) and cis-azo (b and d) configurations. These images were generated with the VESTA software.27
dielectric constant of H2O (ε = 80.1). To confirm the stability of
the structures, particularly the four cis ones, we computed the
nuclear Hessians of the gas-phase models and observed no
imaginary modes, indicating all eight to be energetic minima.
The four optimized gas-phase molecular models of the
platinum(II) complexes are shown in Figure 5, and the calculated
isomerization energies of 1b, 2b, M1, and M2, in the gas phase
and in the three solvent continuua, are summarized in Table 2.
of a continuum has comparatively little effect on the energy
differences in the corresponding platinum(II) complex M2.
Taken as is, these results suggest that both 1b and 2b would
show similar isomerization behavior, whereas the m-azo complex
M1 might show more facile photoisomerization than the p-azo
complex M2. However, these energetics calculations do not
predict the activation barriers for the photochemical reactions,
the energy differences are all considerably lower than the energy
of a UV−visible photon, and the ease of isomerization will
depend strongly on both the absorption strength and the
coupling of the electronic excited states to the vibrations that
effect the change in geometry. As noted above, this may explain
the apparently opposing trend seen for M1 and M2 in the
photoisomerization experiments. There have also been reports in
the literature that the presence of MLCT excitations can in some
cases block photoinduced isomerization in azobenzene-contain-
ing transition-metal complexes.29
Table 2. Calculated Isomerization Energies ΔEcis−trans of the
Meta- and Para-Substituted Azobenzene Spacer Ligands 1b
and 2b and the Corresponding Platinum(II) Diyne
Complexes M1 and M2, after Optimization in the Gas Phase
and with the COSMO Solvation Model26 Using the Dielectric
Constants of Toluene, CH2Cl2, and H2O
ΔEcis−trans/(kJ mol−1
)
1b
M1
2b
M2
To investigate the electronic structure and optical properties
of the spacers and complexes, time-dependent DFT (TD-DFT)
calculations30 were carried out on the CH2Cl2-optimized models,
with the same dielectric continuum present to mimic the
conditions under which the absorption profiles were measured.
The simulated absorption spectra of the cis and trans isomers
of 1b and 2b are compared to the measured profiles of 1a and 2a
in Figure 6, and the electronic excitations giving rise to the main
absorption features are tabulated in Table S1. Given the assumed
constant peak broadening of 0.2 eV, the agreement between the
calculated and measured spectra is good, with a small degree of
offset that can be ascribed to approximations in the TD-DFT
method used in the calculations.
From the calculations, the trans forms of both azobenzenes
give rise to strong absorption bands between ∼300 and 450 nm.
The absorption in 1b is formed by two transitions involving the
frontier highest occupied and lowest unoccupied molecular
orbitals (HOMO/LUMO) and the lower-lying HOMO−3,
leading to an asymmetric peak shape. The primary absorption
band of 2b, on the other hand, is due to the HOMO → LUMO
excitation, and the HOMO−3 orbital contributes to a much
weaker excitation at 315 nm.
gas phase
63.35
57.71
54.26
53.02
64.28
59.39
54.19
55.07
64.83
59.86
56.49
55.32
71.39
71.42
72.21
73.39
toluene (ε = 2.38)
CH2Cl2 (ε = 8.93)
H2O (ε = 80.1)
From these energetics calculations, it can be seen, as is
expected, that, for both substitution patterns, the cis form of the
azobenzene is consistently higher in energy than the trans
configuration, both in the ligands and in the platinum(II)
complexes, in agreement with the spectroscopic and structural
characterization.
In the gas phase, the differences are 63.35 and 64.83 kJ mol−1
for the m- and p-azo ligands 1b and 2b, respectively. These are on
the same order as those calculated for azobenzene itself.28 The
energy difference for the m-azo platinum(II) complex is similar at
64.28 kJ mol−1, whereas that for the p-azo complex is ∼10%
higher at 71.39 kJ mol−1. This implies that the cis configuration of
the p-azo complex is less favorable than that of the m-azo one,
presumably because of greater steric strain in the former.
The presence of a dielectric continuum significantly reduces
the energy difference between the two isomers of 1b and M1, by
up to around 15%, although in all cases the cis isomer remains
higher in energy than the trans confuguration by a considerable
margin. A similar trend is observed for 2b, whereas the presence
The HOMO → LUMO transitions in the cis isomers are lower
in energy, both being above 450 nm, but have much smaller
oscillator strengths. In cis-1b, the HOMO−3 → LUMO
transition produces a comparatively strong absorption at 298
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around 460 nm. The same excitation gives rise to a relatively
much more intense absorption at a slightly red-shifted
wavelength of 472 nm in the trans-M2 spectrum. A comparison
of the frontier orbitals of trans-M1/M2 (Figure 7f,g) shows a
higher degree of electronic delocalization in the frontier orbitals
of the latter complex, particularly the LUMO, suggesting that the
enhanced oscillator strength arises from a greater spatial overlap
between the frontier orbitals. This explains the much stronger
long-wavelength absorption features in the spectra of M2 and
M3 (see Figure 3), both of which incorporate p-alkynylazo-
benzene spacers, compared to those in the spectrum of M1, in
which the spacer has the platinum-coordinating alkyne groups in
the meta position.
The strong primary absorption band at 339 nm in the trans-M1
spectrum corresponds to an excitation between the HOMO−9
to LUMO orbitals, the former of which has an electron density
delocalized over the azobenzene spacer and the two platinum
centers.
The calculations indicate the shorter-wavelength features in
the spectra to be composed of multiple transitions, with
contributions from a number of occupied and virtual orbitals.
We should also note that the larger discrepancy between the
experiment and theory in this part of the spectrum is due to our
inclusion of only the first 50 singlet excited states in the
simulation of the spectra. However, the present calculations are
sufficient to assign and interpret the longer-wavelength
absorption features, which are of primary interest here.
Figure 6. Simulated absorption spectra of the m- and p-alkynylazo-
benzene ligands 1b (a) and 2b (b) in the cis-azo (blue) and trans-azo
(orange) configurations using TD-DFT. The experimental spectra of 1a
and 2a, which are expected to possess comparable features, are overlaid
as shaded black curves. All simulations were carried out using the
COSMO solvation model26 to mimic a solvent of CH2Cl2, as was used in
the measurement of the experimental spectra.
CONCLUSIONS
■
nm, while in cis-2b, a collection of transitions from the HOMO−
3, HOMO−2, and HOMO−1 orbitals produce an asymmetric
band between ∼300 and 350 nm.
A series of platinum(II) diynes and polyynes with meta- and
para-substituted azobenzene spacers have been successfully
synthesized using a palladium/CuI-catalyzed Sonogashira
coupling.
These calculations help to account for the spectral changes
observed in the photoisomerization experiments carried out in
CH2Cl2 (Figure 4a,d): an increase in the proportion of the cis
isomer of 1a should produce a reduction in the intensity and a
blue shift of the band centered around 320 nm, whereas the most
prominent changes for 2a are predicted to be a reduction in the
intensity of the primary absorption band and the appearance and
growth of a peak in the shoulder of the asymmetric feature
around 250 nm.
The major difference between the simulated spectra of the cis
and trans isomers of M1 and M2 (Figure 7a−d) and those of the
ligands in Figure 6 can be surmised as a red shift of the most
prominent absorption features, which is again consistent with the
conclusions drawn from the spectroscopy.
A comparison of the oscillator strengths of the main transitions
giving rise to the absorption bands (Table S2) suggests that the
oscillator strengths in the platinum(II) complexes are enhanced,
again as observed in the experiments. The major excitations in
the trans forms of both platinum(II) species generally show
higher oscillator strengths than the corresponding transitions in
the cis isomers, as observed for the azobenzenes.
From the spectra of the calculated orbital energy levels of the
four complexes (Figure 7e), the HOMO/LUMO gaps of the M1
complexes are relatively larger than those of the para-substituted
analogues M2, at 3.14/3.41 and 2.87/2.99 eV for the trans/cis-
azo geometries, respectively. This appears to be mainly due to
stabilization of the HOMO in the para compounds. The larger
gaps of the cis isomers compared to those of the trans
configurations are in both systems due to higher-energy LUMOs.
In the spectrum of the trans-M1 complex, the HOMO →
LUMO excitation gives rise to a weak absorption centered
From a comprehensive spectroscopic investigation coupled
with structural characterization of two model compounds, we
have obtained valuable insight into the structure−property
relationships for the different azobenzene substitution patterns.
Photoisomerization measurements indicate that the acetylide-
functionalized azobenzene ligands and, to a limited extent, the
dinuclear and polymeric platinum(II) species incorporating
them, can undergo reversible cis-to-trans isomerization under
irradiation at UV wavelengths. From spectroscopic measure-
ments and computational modeling, the trans geometry about
the azo group is confirmed to be the most stable configuration for
both the bare ligands and the platinum(II) complexes, with the
cis isomer of the p-alkynylazobenzene being energetically
destabilized by bulky substituents and incorporation into
platinum complexes/polymers. A comprehensive analysis of
the optical properties using TD-DFT calculations has allowed us
to characterize the major features in the absorption profiles of the
materials and to interpret them in terms of the underlying
electronic structure.
We expect the results from this systematic study to be a
valuable addition to the growing body of work on molecular
switches, providing insight into the future development of
functional materials with tunable optical and electronic proper-
ties.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were performed under a dry
argon atmosphere using standard Schlenk-line techniques. Solvents
were predried and distilled before use according to standard
procedures.31 All chemicals, except where stated otherwise, were
H
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Figure 7. Electronic structure and optical properties of platinum(II) diyne complexes with the meta- and para-substituted azobenzene spacers in the cis-
and trans-azo configurations. (a−d) Simulated absorption spectra from TD-DFT calculations (blue) compared to solution absorption spectra of M1 (a
and b) and M2 (c and d). The vertical blue lines and red lines/gold stars indicate the positions of singlet and triplet excitations, respectively. (e)
Electronic energy levels in the vicinity of the gap between HOMO and LUMO, with the gaps as marked. (f and g) Frontier orbitals of trans-M1 and trans-
M2, respectively. All simulations were carried out using the COSMO solvation model26 to mimic a solvent of CH2Cl2, as was used in the measurement of
the experimental spectra.
obtained from Sigma-Aldrich and used as received. The compounds 2,5-
dioctyl-4-iodoaniline,32 1,2-bis(3-iodophenyl)diazene,32 1,2-bis(4-
iodophenyl)diazene,32 bis(4-iodo-2,5-dioctylphenyl)diazene,32 trans-
[Ph(Et3P)2PtCl],33 and trans-[(nBu3P)2PtCl2]33 were prepared accord-
ing to reported procedures.
wavelength window was used for these experiments. Spectra were
recorded at intervals of 10 min.
The molar masses of the platinum(II) polyynes were determined by
GPC/light scattering analysis. GPC34 was carried out using two PL Gel
30 cm, 5 μm mixed C columns at 30 °C, running in THF at 1 cm3 min−1
with a Roth model 200 high-precision pump. This was coupled to a
DAWN DSP Wyatt Technology multiangle laser light scattering
apparatus with 18 detectors and an auxiliary Viscotek model 200
differential refractometer/viscometer, which were used to calculate the
molecular weights.
Microanalyses were performed at the University Chemical
Laboratory, University of Cambridge. Elemental analysis for carbon,
hydrogen, and nitrogen was performed using a PerkinElmer 2400
CHNS/O series II elemental analyzer.
Caution! All chemicals used in the current work are irritants to skin, eyes,
and the respiratory system. Therefore, all reactions were performed in a well-
ventilated fume hood. Inhalation of silica/alumina and low-boiling-point
solvents like DCM and hexane may cause injuries to internal organs. Safety
glasses, gloves, masks, and laboratory coats were worn during the
experiments.
Column chromatography was performed either with Kieselgel 60
(230−400 mesh) silica gel or alumina (Brockman Grade II-III).
NMR spectra were recorded in CDCl3 on a Bruker WM-250, WM-
300, or AM-400 spectrometer or on a Bruker Avance III HD 700 MHz
1
spectrometer equipped with a 5 mm TCI H/C/N cryoprobe. H and
13C NMR spectra were referenced to solvent resonances, and 31P NMR
spectra were referenced to external 85% H3PO4 (31P{1H}) for M1, M2,
P1, and P2 or PPh3 for M3 and P3. Attenuated-total-reflectance IR
spectra were recorded on pure samples on diamond using a Cary 630
FT-IR spectrometer. Mass spectra were obtained using a VG Autospec
magnetic sector instrument using ESI.
Absorption spectra were recorded on a Varian-Cary 50 UV−visible
spectrophotometer using a quartz cuvette with a 1 cm path length. The
photoirradiation experiments were carried out in toluene or dichloro-
methane (DCM) under a multiband UV lamp (Mineralight Lamp
UVGL-55; 230 V and 6 W). This type of lamp emits radiation at both
short (254 nm) and longer (365 nm) wavelengths; the shorter-
Ligand Synthesis. 1,2-Bis[3-[2-(trimethylsilyl)ethynyl]phenyl]-
diazene (1a). To a solution of 1,2-bis(3-iodophenyl)diazene (2.00 g,
I
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i
4.61 mmol) in 1:4 (v/v) Pr2NH/THF (120 cm3) under an argon
atmosphere were added catalytic amounts of CuI (10 mg), Pd(OAc)2
(10 mg), and PPh3 (60 mg). The solution was stirred for 30 min at room
temperature followed by the addition of (trimethylsilyl)ethyne (1.64
mL, 11.5 mmol) with vigorous stirring. The reaction mixture was then
refluxed overnight. Completion of the reaction was confirmed by silica
thin-layer chromatography (TLC) and IR spectroscopy. After being
cooled to room temperature, the mixture was quenched with water (60
mL) and extracted with CH2Cl2 (60 × 2 mL). The organic layer was
collected and dried over MgSO4 and filtered and the solvent removed
under reduced pressure. The solid residue was redissolved in CH2Cl2
and purified by silica column chromatography, eluting with 1:1 (v/v)
hexane/CH2Cl2 to obtain the title compound as an orange solid (1.41 g,
Bis(4-ethynyl-2,5-dioctylphenyl)diazene (3b). A procedure similar
to that used in the synthesis of 1b using 3a was followed to obtain 3b as a
brown powder (75.6% yield). IR (CH2Cl2, cm−1): ν 2105 (CC), 3302
(CCH). 1H NMR (700 MHz, CDCl3): δ 7.46 (s, 2 H, H-6,6′), 7.43
(s, 2 H, H-3,3′), 3.37 (s, 2 H, CCH), 3.07 (t, 4 H, J = 7.0 and 14.0 Hz,
methylene), 2.79 (t, 4 H, J = 7.0 Hz, methylene), 1.67 (p, 4 H, J = 7.0 and
14.0 Hz, methylene), 1.36 (p, 4 H, J = 7.0 and 14.0 Hz, methylene),
1.32−1.22 (m, 40 H, methylene), 0.87−0.85 (m, 12 H, methyl). 13C
NMR (176 MHz, CDCl3): δ 82.43, 82.34 (CC), 150.31, 143.70,
140.30, 134.93, 124.00, 115.32 (aromatic C), 34.23, 34.19, 31.92, 31.90,
29.71, 29.62, 29.49, 29.45, 29.34, 29.28, 29.23 (methylene), 14.11, 14.09
(methyl). ESI-MS: m/z 679.6 (M+). Anal. Calcd for C48H74N2: C, 84.89;
H, 10.98; N, 4.12. Found: C, 84.79; H, 10.91; N, 4.16.
81.4% yield, mp 120.5 °C). IR (CH2Cl2, cm−1): ν 2155 (CC). H
1
Synthesis of Platinum(II) Diynes and Polyynes. trans-[(Ph)-
(Et3P)2PtCCRCCPt(PEt3)2(Ph)] (M1; R = Diphenyldiazene-3,3′-
diyl). To a stirred mixture of 1b (0.0500 g, 0.217 mmol) and trans-
[Pt(PEt3)2PhCl] (0.236 g, 0.434 mmol) in iPr2NH (3 mL) and CH2Cl2
(20 mL) was added CuI (0.5 mg). The solution was stirred at room
temperature under an argon atmosphere over a period of 18 h, after
which all volatile components were removed under reduced pressure.
The crude product was taken up in CH2Cl2 and passed through a silica
column with 1:1 (v/v) hexane/CH2Cl2 as the eluent. The product was
obtained as a yellow solid (0.164 g, 60.7% yield, mp 173.9 °C). IR
(CH2Cl2, cm−1): ν 2093 (CC). 1H NMR (700 MHz, CDCl3): δ 7.83
(s, 2 H, H-2,2′), 7.64−7.62 (m, 2 H, H-6,6′), 7.39 (d, 2 H, J = 7.6 Hz, H-
4,4′), 7.36−7.26 (m, 2 H, H-5,5′), 7.06 (d, 4 H, J = 7.7 Hz, Hortho of Ph),
6.99−6.95 (m, 4 H, Hmeta of Ph), 6.81 (t, 2 H, J = 6.9 Hz, Hpara of Ph)
1.80−1.08 (m, 24 H, PCH2), 1.13−1.06 (m, 36 H, methyl). 13C NMR
(176 MHz, CDCl3): δ 152.98, 152.65 (C-1,1′), 139.15, 139.09 (C-4,4′),
NMR (700 MHz, CDCl3): δ 8.01 (s, 2 H, H-2,2′), 7.87 (d, 2 H, J = 8.0
Hz, H-6,6′), 7.57 (d, 2 H, J = 7.7 Hz, H-4,4′), 7.46 (t, 2 H, J = 7.8 Hz, H-
5,5′), 0.27 (s, 18 H, SiMe3). 13C NMR (176 MHz, CDCl3): δ 153.03 (C-
1,1′), 134.43, 131.13 (C-2,2′,6,6′), 129.12, 126.09, 124.28 (C-3,3′,5,5′),
104.21, 103.76, 95.91, 95.34 (CC), 0.42 (SiMe3). ESI-MS: m/z 375.0
(M+). Anal. Calcd for C22H26N2Si2: C, 70.53; H, 7.00; N, 7.48. Found:
C, 70.62; H, 7.07; N, 7.43.
1,2-Bis[4-[2-(trimethylsilyl)ethynyl]phenyl]diazene (2a). This com-
pound was synthesized in a manner similar to that of 1a using 1,2-bis(4-
iodophenyl)diazene to afford the product as an orange-brown solid
(77.5% yield, mp 184.5 °C). IR (CH2Cl2, cm−1): ν 2155 (CC). H
1
NMR (700 MHz, CDCl3): δ 7.87 (d, 4 H, J = 7.9 Hz, H-2,6,2′,6′), 7.57
(d, 4 H, J = 7.6 Hz, H-3,5,3′,5′), 0.07 (s, 18 H, SiMe3). 13C NMR (176
MHz, CDCl3): δ 152.31 (C-1,1′), 129.18, 128.71 (C-2,2′,6,6′), 124.33,
123.65 (C-3,3′,5,5′), 104.26, 95.39 (CC), 0.05 (SiMe3). ESI-MS: m/z
374.9 (M+). Anal. Calcd for C22H26N2Si2: C, 70.53; H, 7.00; N, 7.48.
Found: C, 70.42; H, 7.01; N, 7.41.
̀
130.88, 130.27, 130.09, 129.81 (C of Ph), 128.80, 128.45 (C-5,5′),
123.89 (C-6,6′), 109.58, 109.24 (CC), 38.72, 31.92, 30.35, 29.96,
Bis[4-[(trimethylsilyl)ethynyl]-2,5-dioctylphenyl]diazene (3a). This
compound was synthesized in a manner similar to that of 1a using bis(4-
iodo-2,5-dioctylphenyl)diazene to afford the product as an orange-
28.92, 23.73, 22.98, 22.69 (methylene), 15.19, 15.10, 15.06, 15.00, 14.12,
14.05 (methyl). 31P{1H} NMR (400 MHz, CDCl3): δ 9.97 (1JPt−P
=
2653.78 Hz). ESI-MS: m/z 1245.4 (M+). Anal. Calcd for
C52H78N2P4Pt2: C, 50.16; H, 6.31; N, 2.25. Found: C, 50.18; H, 6.37;
N, 2.29.
brown solid (76.1% yield). IR (CH2Cl2, cm−1): ν 2152 (CC). H
1
NMR (700 MHz, CDCl3): δ 7.42 (s, 2 H, H-6,6′), 7.41 (s, 2 H, H-3,3′),
3.05 (t, 4 H, J = 7.0 Hz, methylene), 2.76 (t, 4 H, J = 7.0 Hz, methylene),
1.66−1.21 (m, 48 H, methylene), 0.88−0.84 (m, 12 H, methyl), 0.085
(s, 18 H, SiMe3). 13C NMR (176 MHz, CDCl3): δ 150.13 (C-1,1′),
143.68 (C-3,3′), 140.35 (C-5,5′), 134.42 (C-2,2′), 124.95 (C-6,6′),
115.30 (C-4,4′), 104.05, 100.03 (CC), 34.56, 32.29, 31.92, 31.00,
30.96, 30.54, 29.65, 29.54, 29.51, 29.36, 22.70 (methylene), 14.11
(methyl), 1.04 (SiMe3). ES-MS: m/z 823.5 (M+). Anal. Calcd for
C54H90N2Si2: C, 78.76; H, 11.02; N, 3.40. Found: C, 78.72; H, 11.11; N,
3.36.
trans-[(Ph)(Et3P)2PtCCRCCPt(PEt3)2(Ph)] (M2; R = Diphenyl-
diazene-4,4′-diyl). A procedure similar to that for the synthesis of M1
was followed using 2b. The product was obtained as an orange solid
(62.4% yield, mp 190.2 °C). IR (CH2Cl2, cm−1): ν 2090 (CC). 1H
NMR (700 MHz, CDCl3): δ 7.77 (d, 4 H, J = 8.4 Hz, H-2,6,2′,6′), 7.38
(d, 4 H, J = 9.1 Hz, H-3,5,3′,5′), 7.33 (d, 4 H, J = 7.0 Hz, Hortho of Ph),
6.975 (t, 4 H, J = 7.4 Hz, Hmeta of Ph), 6.92 (t, 2 H, J = 7.5 Hz, Hpara of
Ph), 1.77 (m, 24 H, PCH2), 1.08 (m, 36 H, methyl). 13C NMR (176
MHz, CDCl3): δ 150.03 (C-1,1′), 139.25 (C-2,6,2′,6′), 136.93 (C-
3,5,3′,5′), 131.60 (C-4,4′), 128.01, 127.49, 122.78, 122.03, 121.47 (C of
Ph), 111.33 (CC), 40.12, 33.52, 31.20, 29.59, 29.01, 25.46, 23.90,
23.16 (methylene), 15.44, 15.34, 15.24, 15.14, 13.55 (methyl). 31P{1H}
NMR (400 MHz, CDCl3): δ 10.07 (1JPt−P = 2648.57 Hz). ESI-MS: m/z
1245.6 (M+) Anal. Calcd for C52H78N2P4Pt2: C, 50.16; H, 6.31; N, 2.25.
Found: C, 50.12; H, 6.29; N, 2.31.
trans-[(Ph)(Et3P)2PtCCRCCPt(PEt3)2(Ph)] (M3; R = 2,5-Dio-
ctyldiphenyldiazene-4,4′-diyl). A procedure similar to that for the
synthesis of M1 using 3b was followed. The product was obtained as an
orange viscous liquid (61.3% yield). IR (CH2Cl2, cm−1): ν 2095 (C
C). 1H NMR (700 MHz, CDCl3): δ 7.47 (s, 4 H, H-3,6,3′,6′), 7.35 (d, 4
H, J = 7.0 Hz, Hortho of Ph), 6.97 (t, 4 H, J = 7.4 Hz, Hmeta of Ph), 6.81 (t,
2 H, J = 7.2 Hz, Hpara of Ph), 3.05 (t, 8 H, J = 7.0 Hz, methylene), 2.83 (t,
24 H, J = 7.0 Hz, PCH2), 1.82−1.64 (m, 8 H, methylene), 1.44−1.36 (m,
24 H, methylene), 1.35−1.20 (m, 16 H, methylene), 1.14−1.06 (m, 12
H, methyl), 0.87 (dt, J = 7.0 and 10.7 Hz, 36 H, methyl). 13C NMR (176
MHz, CDCl3): δ 147.90, 141.08, 139.80, 139.26, 133.31, 131.07, 127.33,
121.23 (aromatic C), 114.74, 110.04 (CC), 34.67, 32.42, 32.04, 32.00,
31.36, 30.54, 29.98, 29.88, 29.85, 29.73, 29.64, 29.50, 29.48, 22.76, 22.74
(methylene), 15.29, 15.19, 15.09, 14.15 (methyl). 31P{1H} NMR (121.5
MHz, CDCl3): δ 9.38 (1JPt−P = 2629.3 Hz). ESI-MS: m/z 1695.3 (M+).
Anal. Calcd for C84H142N2P4Pt2: C, 59.55; H, 8.45; N, 1.65. Found: C,
59.52; H, 8.51; N, 1.66.
1,2-Bis(3-ethynylphenyl)diazene (1b). 1a (0.400 g, 1.07 mmol) was
protodesilylated in 4:1 (v/v) THF/MeOH (20 mL) using aqueous
KOH (0.180 g, 3.21 mmol). The reaction mixture was stirred at room
temperature for 2 h, after which time TLC and IR revealed that all
protected compound had been converted to the terminal dialkyne
ligand. The solvent mixture was then removed and the residue dissolved
in CH2Cl2 and purified by column chromatography on silica using 1:1
(v/v) hexane/CH2Cl2 as the eluent, affording 1b as a pale-orange
powder (0.225 g, 91.3% yield). IR (CH2Cl2, cm−1): ν 2109 (CC),
3299 (CCH). 1H NMR (400 MHz, CDCl3): δ 8.09 (s, 2 H, H-2,2′),
7.91 (dd, 2 H, J = 7.1 and 15.3 Hz, H-6,6′), 7.62 (dd, 2 H, J = 7.6 and 15.8
Hz, H-4,4′), 7.43 (m, 2 H, H-5,5′), 3.06 (s, 2 H, CCH). 13C NMR
(100.6 MHz, CDCl3): δ 152.30 (C-1,1′), 134.54, 132.13 (C-2,2′,6,6′),
128.7, 127.97, 123.65, 122.59 (C-3,3′,5,5′), 82.4, 79.9 (CC). ESI-MS:
m/z 231.1 (M+). Anal. Calcd for C16H10N2: C, 83.46; H, 4.38; N, 12.17.
Found: C, 83.52; H, 4.41; N, 12.09.
1,2-Bis(4-ethynylphenyl)diazene (2b). A procedure similar to that in
the synthesis of 1b using 2a was followed to afford the product as a pale-
brown powder (88.2% yield). IR (CH2Cl2, cm−1): ν 2106 (CC), 3294
(CCH). 1H NMR (400 MHz, CDCl3): δ 7.90 (dd, 4 H, J = 7.3 and
15.0 Hz, H-2,6,2′,6′), 7.60−7.64 (m, 4 H, H-3,5,3′,5′), 3.06 (s, 2 H, C
CH). 13C NMR (100.6 MHz, CDCl3): δ 152.4 (C-1,1′), 132.6 (C-
2,2′,6,6′), 124.9 (C-4,4′), 122.6 (C-3,3′,5,5′), 82.4, 79.9 (CC). ESI-
MS: m/z 230.9 (M+). Anal. Calcd for C16H10N2: C, 83.46; H, 4.38; N,
12.17. Found: C, 83.42; H, 4.31; N, 12.26.
trans-[−(Bu3P)2PtCCRCC−]n (P1; R = Diphenyldiazene-3,3′-
diyl). P1 was synthesized by mixing 1b (0.0500 g, 0.217 mmol), trans-
J
DOI: 10.1021/acs.inorgchem.6b01509
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
[Pt(PBu3)2Cl2] (0.146 g, 0.217 mmol), and CuI (0.5 mg) in 1:1 (v/v)
iPr2NH/CH2Cl2 (20 mL). After stirring at room temperature overnight
under argon, the solution mixture was evaporated to dryness. The
residue was dissolved in CH2Cl2 and filtered through a short alumina
column using 1:1 (v/v) hexane/CH2Cl2 as te eluent to remove ionic
impurities and residual catalyst. After removal of the solvent, the crude
product was purified twice by precipitation from MeOH in CH2Cl2.
Subsequent washing with hexane and drying under reduced pressure
gave an orange solid as the product (0.114 g, 80.0% yield, mp 139.8 °C).
IR (CH2Cl2, cm−1): ν 2098 (CC). 1H NMR (700 MHz, CDCl3): δ
8.22 (s, 2 H, H-2,2′), 7.88 (d, 2 H, J = 8.2 Hz, H-6,6′), 7.84−7.75 (m, 2
H, H-4,4′), 7.70−7.60 (m, 2 H, H-5,5′), 2.22−1.96 (m, 12 H, PCH2),
1.69−1.56 (m, 24 H, methylene), 0.98−0.80 (m, 18 H, methyl). 13C
NMR (176 MHz, CDCl3): δ 152.73, 152.33 (C-1,1′), 133.87, 133.27
(C-4,4′), 130.25, 130.13 (C-5,5′), 128.75, 128.61 (C-2,2′), 124.89,
123.57 (C-3,3′), 120.23, 119.90 (C-6,6′), 110.68 (CC), 26.55, 26.52,
26.25, 25.94, 24.61, 24.57, 24.54, 24.47, 24.44, 24.23, 24.18, 24.13, 24.08,
24.03, 22.15 (methylene), 14.02, 13.98 (methyl). 31P{1H} NMR (500
MHz, CDCl3): δ 3.27 (1JPt−P = 2346.70 Hz). Anal. Calcd for
[C40H62P2N2Pt]n: C, 57.96; H, 7.66; N, 3.38. Found: C, 58.12; H,
7.51; N, 3.36. GPC (THF): M = 29500 g mol−1 (n = 36); M = 41800 g
cycles of refinement, a weighting scheme was used that gave a relatively
flat analysis of variance.
Computational Modeling. Computational modeling was carried
out on the azobenzene ligands 1b and 2b, together with the
corresponding platinum(II) diynes M1 and M2, within the DFT
formalism, as implemented in the NWChem code.39 We used the B3LYP
hybrid functional for all calculations.40 The electronic wave functions
were expanded in split-valence Gaussian basis sets of 6-31G and 6-
31G** quality for the hydrogen and main-group atoms, respectively.41
The core electrons of the platinum atoms were described by the
LANL2DZ effective-core pseudopotential,42 and the valence electrons
were modeled with the corresponding double-ζ basis set. During the
electronic-wave function minimization, convergence criteria of 10−6,
10−5, and 5 × 10−4 atomic units (a.u.) were applied to the energy,
density, and gradients, respectively.
Initial models of the m- and p-azo platinum(II) systems with the
central azobenzenes in the trans configuration were extracted from the
crystal structures and models of the corresponding cis isomers made by
rotation around the central bond. Models of the azobenzene ligands
were subsequently prepared from the four platinum complexes. The
initial coordinates were optimized until the maximum and root-mean-
square (rms) gradients on the ions were less than 4.5 × 10−4 and 3 ×
10−4 a.u., respectively, and the maximum and rms Cartesian steps in the
last iteration fell below 1.8 × 10−3 and 1.2 × 10−3 a.u. The optimized gas-
phase structures were confirmed to be stationary points by calculating
the nuclear Hessian matrices using analytical gradients. For these
calculations, the tolerances on the energy, density, and gradients applied
during the wave-function optimization were tightened respectively to
10−8, 10−6, and 10−5 a.u.
To approximately model the effect of a solvent environment on the
energetics and optical absorption spectra, the relaxed structures were
further optimized using the COSMO solvation model26 with dielectric
constants of ε = 2.38 (toluene), 8.93 (CH2Cl2), and 80.1 (H2O).
The optical properties of the DCM models were studied using TD-
DFT calculations39 using adiabatic B3LYP, and with a dielectric
continuum with ε = 8.93 as in the geometry optimization. The 50
lowest-energy singlet (spin-allowed) and triplet (spin-forbidden)
transitions of each model were computed, and the spin-allowed
excitations were used to simulate UV−visible absorption spectra
according to
̅
̅
n
w
mol−1; PDI = 1.4.
trans-[−(Bu3P)2PtCCRCC−]n (P2; R = Diphenyldiazene-4,4′-
diyl). P2 was synthesized by following the same procedure as that for P1
using 2b. The title compound was obtained as an orange solid (78.3%,
decomposition temp 211.5 °C) . IR (CH2Cl2, cm−1): ν 2097 (CC).
1H NMR (700 MHz, CDCl3): δ 7.81−7.74 (m, 4 H, H-2,6,2′,6′), 7.40−
7.32 (m, 4 H, H-3,5,3′,5′), 2.13 (t, J = 7.2 Hz, 12 H, PCH2), 1.63 (dd, J =
3.4 and 7.0 Hz, 12 H, methylene), 1.47 (dq, 12 H, J = 7.3 and 14.5 Hz,
methylene), 0.94 (t, J = 7.2 Hz, 18 H, methyl). 13C NMR (176 MHz,
CDCl3): δ 152.4 (C-1,1′), 133.11 (C-2,6,2′,6′), 132.18 (C-3,5,3′,5′),
122.53 (C-4,4′), 111.40 (CC), 29.84, 26.74, 26.74, 26.53, 24.72,
24.68, 24.60, 24.56, 24.53, 24.22, 24.13, 24.03 (methylene), 13.98
(methyl). 31P{1H} NMR (500 MHz, CDCl3): δ 3.31 (1JPt−P = 2337.50
Hz). Anal. Calcd for [C72H62P2N2Pt]n: C, 57.96; H, 7.66; N, 3.38.
Found: C, 58.17; H, 7.52; N, 3.34. GPC (THF): M = 28800 g mol−1 (n
̅
n
= 35); M = 50500 g mol−1; PDI = 1.3.
̅
w
trans-[−(Bu3P)2PtCCRCC−]n (P3; R = 2,5-Dioctyldiphenyldia-
zene-4,4′-diyl). P3 was synthesized by following the procedure for P1
using 3b to afford the product as a dark-orange solid (79.3%). IR
(CH2Cl2, cm−1): ν 2095 (CC). 1H NMR (700 MHz, CDCl3): δ 7.41
(s, 2 H, H-6,6′), 7.39 (s, 2 H, H-3,3′), 3.04−2.60 (m, 8 H, methylene),
2.79−2.69 (m, 12 H, PCH2), 1.68−1.44 (m, 48 H, methylene), 1.43−
1.10 (m, 24 H, methylene), 0.92−0.74 (m, 30 H, methyl). 13C NMR
(176 MHz, CDCl3): δ 148.11, 141.03, 139.96, 133.58, 130.97, 127.64
(aromatic C), 114.78 (CC), 34.59, 32.62, 32.18, 32.13, 31.48, 30.48,
30.03, 29.86, 29.79, 29.65, 29.58, 26.60, 26.53, 26.30, 24.61, 24.57, 24.53,
24.41, 24.19, 24.09, 24.00, 22.88, 22.18 (methylene), 14.28, 14.07, 14.02
(methyl). 31P{1H} NMR (121.5 MHz, CDCl3): δ 2.76 (1JPt−P = 2341.3
Hz). Anal. Calcd for [C40H126P2N2Pt]n: C, 67.68; H, 10.02; N, 2.19.
2
⎛
⎝
⎞
⎟
⎠
fi
⎡ v − v ⎤
i
ε(v) =
1.3062974 × 108 exp⎜−
∑
⎢
⎣
⎥
⎦
σ
σ
i
where the energies are in cm−1, ε is the molar extinction coefficient in L
mol−1 cm−1, fi are the (dimensionless) oscillator strengths, vi are the
band positions, and σ is the bandwidth, here set to 0.2 eV.
ASSOCIATED CONTENT
■
Found: C, 67.76; H, 9.96; N, 2.20. GPC (THF): M = 70200 g mol−1 (n
S
* Supporting Information
̅
n
= 55); M = 84400 g mol−1; PDI = 1.2.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01509. Crystallographic data from structural analysis
have been deposited with the Cambridge Crystallographic Data
Centre as CCDC1486091−1486092 and are available free of
charge from https://summary.ccdc.cam.ac.uk/structure-
summary-form. Data from the calculations, including the
coordinates of the optimized models and simulated spectro-
scopic data, are available from an online repository 10.15125/
BATH-00287, and is available from https://doi.org/10.15125/
BATH-00287. Any other data may be obtained upon request
from the authors.
̅
w
X-ray Crystallography. Single-crystal X-ray diffraction experiments
were performed at 150 K (for M1) and 250 K (for M2) on an Oxford
Diffraction Gemini A Ultra CCD diffractometer using monochromatic
Mo Kα radiation (λ = 0.71073 Å). The sample temperature was
controlled using an Oxford Diffraction Cryojet apparatus. CrysAlis Pro
was used for collecting frames of data, indexing reflections, and
determining lattice parameters. Structures were solved by direct
methods using SHELXS-8635 and refined using full-matrix least squares
on F2 with SHELX-9736 (for M2) and SHELX201437 (for M1) within
the OLEX2 package.38 A multiscan absorption correction was applied in
both cases. For M2, the phosphine ligands were disordered over two
sites, the ethyl groups were refined with partial occupancies summed to
unity, and bond parameter restraints were used to maintain chemically
sensible geometries for these groups. For both structures, hydrogen
atoms were included using rigid methyl groups or a riding model and,
again, partial occupancies were used as appropriate. Refinement for both
structures continued until convergence was reached, and in the final
UV−visible spectra from photoisomerization experiments
on 1a/M1/P1 and 2a/M2/P2 in toluene and orbital
assignments of the most intense transitions obtained in the
TD-DFT calculations on 1b, 2b, M1, and M2(PDF)
K
DOI: 10.1021/acs.inorgchem.6b01509
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
X-ray crystallographic data in CIF format for CCDC
Article
conjugated polymers as a new avenue towards high-efficiency polymer
solar cells. Nat. Mater. 2007, 6 (7), 521−527.
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X-ray crystallographic data in CIF format for CCDC
1486092 (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: msk@squ.edu.om.
*E-mail: j.m.skelton@bath.ac.uk.
*E-mail: p.r.raithby@bath.ac.uk.
■
(11) Khan, M. S.; Al-Suti, M. K.; Maharaja, J.; Haque, A.; Al-Balushi, R.;
Raithby, P. R. Conjugated poly-ynes and poly (metalla-ynes)
incorporating thiophene-based spacers for solar cell (SC) applications.
J. Organomet. Chem. 2016, 812, 13−33.
Notes
The authors declare no competing financial interest.
(12) (a) Khan, M. S.; Al-Mandhary, M. R.; Al-Suti, M. K.; Ahrens, B.;
ACKNOWLEDGMENTS
■
Mahon, M. F.; Male, L.; Raithby, P. R.; Boothby, C. E.; Kohler, A.
̈
M.S.K. thanks The Research Council (TRC), Oman (Grant
ORG/EI/SQU/13/015), His Majesty's Trust Fund for Strategic
Research (Grant SR/SQU/SCI/CHEM/16/02) and Sultan
Qaboos University (Grant SQU/IG/SCI/CHEM/16/07) for
funding. R.A.A. acknowledges SQU for a Ph.D. scholarship, A.H.
acknowledges TRC for a postdoctoral fellowship, and M.J.
acknowledges TRC for a Ph.D. scholarship. J.M.S. and P.R.R. are
supported by an EPSRC programme grant (Grant EP/K004956/
1). The computational modeling was carried out using the UK
Archer facility, accessed through the UK Materials Chemistry
Consortium, which is funded by the EPSRC (Grant EP/
L000202). Some modelling was also performed using the Balena
HPC system at the University of Bath, which is maintained by
Bath University Computing Services.
Synthesis, characterisation and optical spectroscopy of diynes and poly-
ynes containing derivatised fluorenes in the backbone. Dalton Trans.
2003, No. 1, 74−84. (b) Khan, M. S.; Al-Mandhary, M. R. A.; Al-Suti, M.
K.; Al-Battashi, F. R.; Al-Saadi, S.; Ahrens, B.; Bjernemose, J. K.; Mahon,
M. F.; Raithby, P. R.; Younus, M.; Chawdhury, N.; Kohler, A.; Marseglia,
̈
E. A.; Tedesco, E.; Feeder, N.; Teat, S. J. Synthesis, characterisation and
optical spectroscopy of platinum(II) di-ynes and poly-ynes incorporat-
ing condensed aromatic spacers in the backbone. Dalton. Trans. 2004,
15, 2377−2385. (c) Khan, M. S.; Al-Mandhary, M. R. A.; Al-Suti, M. K.;
Corcoran, T. C.; Al-Mahrooqi, Y.; Attfield, J. P.; Feeder, N.; David, W. I.
F.; Shankland, K.; Friend, R. H.; Kohler, A.; Marseglia, E. A.; Tedesco,
̈
E.; Tang, C. C.; Raithby, P. R.; Collings, J. C.; Roscoe, K. P.; Batsanov, A.
S.; Stimson, L. M.; Marder, T. B. Synthesis and optical characterisation
of platinum(II) poly-yne polymers incorporating substituted 1,4-
diethynylbenzene derivatives and an investigation of the intermolecular
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2003, 27 (1), 140−149. (d) Khan, M. S.; Al-Mandhary, M. R. A.; Al-Suti,
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DOI: 10.1021/acs.inorgchem.6b01509
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