Tuning the Excimer Emission of Amphiphilic Platinum(II) Complexes Mediated by Phospholipid Vesicles

Article abstract of DOI:10.1021/acs.inorgchem.6b03100

Two new amphiphilic platinum(II) complexes, [Pt(2-(4-fluorophenyl)-5-(4-dodecyloxyphenyl)pyridine) (acac)] (Pt-1) and [Pt(2-(4-dodecyloxyphenyl)-5-(thien-2-yl)-c-cyclopentenepyridine) (acac)] (Pt-2), where acac is acetylacetonate, were synthesized and characterized. Apart from conventional phosphorescence of single molecules (ME-monomer emission), complexes Pt-1 and Pt-2 also exhibit excimer emission (EE) when embedded into phospholipid vesicles, that is assigned to emissive Pt-Pt excimers. The EE intensity in vesicular media appeared to depend on the viscosity of the vesicles and the concentration of the embedded complex. Differences in the EE properties of complexes Pt-1 and Pt-2 are correlated with the energies of the π-character frontier orbitals defined by the design of the cyclometalating phenylpyridine ligand. Higher energies of the frontier π-orbitals (HOMO and LUMO) naturally promote stronger π-π interactions, thus obstructing the Pt^II-Pt^II interaction.

Full text of DOI:10.1021/acs.inorgchem.6b03100

Article
pubs.acs.org/IC
Tuning the Excimer Emission of Amphiphilic Platinum(II) Complexes
Mediated by Phospholipid Vesicles
Marsel Z. Shafikov,*,†,‡ Alfiya F. Suleymanova,‡ Dmitry N. Kozhevnikov,‡ and Burkhard Konig*,§
̈
†Ural Federal University, Mira 19, Ekaterinburg, 620002, Russia
‡I. Postovsky Institute of Organic Synthesis, Ekaterinburg, 620990, Russia
§Institut fur Organische Chemie, Universitat Regensburg, Universitatsstraße 31, D-93053 Regensburg, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: Two new amphiphilic platinum(II) complexes, [Pt(2-(4-
fluorophenyl)-5-(4-dodecyloxyphenyl)pyridine) (acac)] (Pt-1) and [Pt(2-
(4-dodecyloxyphenyl)-5-(thien-2-yl)-c-cyclopentenepyridine) (acac)] (Pt-2),
where acac is acetylacetonate, were synthesized and characterized. Apart from
conventional phosphorescence of single molecules (MEmonomer
emission), complexes Pt-1 and Pt-2 also exhibit excimer emission (EE)
when embedded into phospholipid vesicles, that is assigned to emissive Pt−Pt
excimers. The EE intensity in vesicular media appeared to depend on the
viscosity of the vesicles and the concentration of the embedded complex.
Differences in the EE properties of complexes Pt-1 and Pt-2 are correlated
with the energies of the π-character frontier orbitals defined by the design of
the cyclometalating phenylpyridine ligand. Higher energies of the frontier π-
orbitals (HOMO and LUMO) naturally promote stronger π−π interactions,
thus obstructing the PtII−PtII interaction.
Pt−Pt distance in these dimers is reported to be of 3.3−3.4
Å.17,18,23
A Pt−Pt interaction between an excited molecule and a
INTRODUCTION
■
Organometallic complexes of transition metals such as Pt(II)
and Ir(III) are known to exhibit dual emission1−4 or
phosphorescence1,2,5 that can be applied in ratiometric oxygen
sensors4,6 and in OLEDs as triplet harvesting emitters,7−9
respectively. Such properties are provided by a strong spin−
orbit coupling (SOC) of the heavy atoms that allows
population of the lowest excited triplet state (T1) and makes
the state emissive.10,11
Apart from the heavy atom effects of Pt, the complexes of
Pt(II) often feature a planar geometry due to the d8 electronic
configuration of Pt(II) providing a square-planar configuration
of the coordination center.12 This planar geometry of Pt(II)
complexes, sterically undemanding along the z-axis, allows
intermolecular metal−metal (PtII−PtII) contact via the overlap
of the two filled 5dz2 orbitals and the two unfilled 6pz orbitals.
In the course of such metal−metal contact between two
molecules in the ground state, the linear combination of the
two filled 5dz2 orbitals results in a filled bonding σd orbital and a
filled antibonding σd* orbital, whereas the two 6pz orbitals
analogously result in an unfilled bonding σp orbital and an
unfilled antibonding σp* orbital. Although, in such a system one
filled antibonding orbital (σd*) counteracts one filled bonding
orbital (σd), stabilization occurs due to configurational
interaction,13−15 which slightly destabilizes the unfilled bonding
σp orbital while the filled σd bonding orbital is being
additionally stabilized, resulting in a weak bonding interaction
between the Pt(II) ions with formation of a dimer.13−22 The
molecule in the ground state often results in formation of a Pt−
Pt excimeran excited aggregate where an electron from the
antibonding σd* orbital is promoted to an orbital of one of the
ligands. This gives the excited state the metal−metal to ligand
charge transfer (MMLCT)12 character.17−21,24,25 In the excited
state of MMLCT character, the bonding effect of the fully
occupied σd orbital prevails the antibonding effect of the semi-
occupied σd* orbital, thus strongly enhancing the Pt−Pt
bonding interaction.17,18,26−30 Accordingly, with formation of a
Pt−Pt excimer, the Pt−Pt distance shortens to 2.8−3.0 Å.28
These excimers manifest themselves by excimer emission
(EE), that is not observable for isolated molecules.17−19,29,31−33
The EE of a Pt(II) complex is typically red-shifted as compared
to the phosphorescence of single molecules (MEmonomer
emission), and appears in the red34 and near-infrared (NIR)
regions,19−21,25,35−37 depending on the structure of the ligands
and other intermolecular interactions such as π−π.38−43 These
red-shifted EEs of Pt(II) complexes can be applied in
bioimaging18,35 and production of NIR-OLEDs19,37,44−46 or
white OLEDs (WOLEDs)18,24,41,47−53 when combined with a
shorter wavelength emission of single molecules.
Given the potential for application, methods allowing to tune
the EE need to be developed. In the 1970s a method was
Received: December 20, 2016
© XXXX American Chemical Society
A
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shown which helps to monitor structural changes in biological
membranes by doping them with organic molecules that form
fluorescent π−π excimers.54−56 The method is based on the
sensitivity of the emission intensity of these π−π excimers to
the viscosity of the host-membrane, as the excimers are formed
more efficiently in a less viscous media.54−62
Interestingly, this method could be used conversely, e.g. to
control the formation of excimers via the viscosity of
phospholipid media. However, to our surprise, we found no
report about this quite obvious approach being applied by
utilizing Pt−Pt excimers of Pt(II) complexes. In this
contribution, we present an approach that allows tuning of
the photophysical properties of amphiphilic Pt(II) complexes
based on formation of intermolecular emissive excimers within
phospholipid membranes (Figure 1).
pyridinium chloride that afforded the corresponding phenols,64
which were finally alkylated with 1-dodecyl bromide (Scheme
1). Coupling proligands HL-1 and HL-2 with K2[PtCl4]
resulted in platinum(II) dimer complexes [Pt2(μ-Cl)2(L-1−L-
2)2], that were cleaved with sodium acetylacetonate to afford
neutral cyclometalated complexes Pt-1 and Pt-2, respectively
(Figure 2 and Scheme 1).
X-ray quality single crystals of Pt-1 and Pt-2 were grown by
slow convectional diffusion of methanol into dichloromethane
solutions. X-ray diffraction analysis of the crystals revealed
distorted square-planar geometries of the coordination cores,
typical for d8 transition metal complexes (Figure 3), with N−
Pt−C1 and O1−Pt−O2 angles of 81.59(11) and 92.17(8) in
Pt-1, and of 81.36(17)° and 91.40(13)° in Pt-2, respectively.
The coordinative bond lengths Pt−C1, Pt−N, Pt−O1, and Pt−
O2, of 1.970(3), 1.993(2), 2.0848(19), and 2.008(2) Å in Pt-1,
and 1.958(5), 1.979(3), 2.089(3), and 2.012(3) Å in Pt-2,
respectively, are typical compared to analogous Pt(II)
complexes.1,2,69 The pendant alkoxy-phenyl group of Pt-1
and the pendant thienyl group of Pt-2 are twisted with respect
to the phenyl-pyridine planes by 32.084° and 22.626°,
respectively. It is noted that no Pt−Pt contact in crystal
packing was found for either of the represented complexes. The
shortest Pt−Pt distance between two layers of molecules in a
crystal is found to be of 12−14 Å for Pt-1 and of 6.90 Å for Pt-
2 (Figure S1 in the Supporting Information (SI)).
Photophysical Characterization. UV−vis and emission
spectra of complexes Pt-1 and Pt-2, and proligands HL-1 and
HL-2 were measured in chloroform solutions at ambient
temperature. HL-1 and HL-2 display intense absorption bands
at 303 and 315 nm, respectively (Figure 4), which probably
arise from π → π* transitions. Emission spectra of HL-1 and
HL-2, centered at 370 and 378 nm, respectively, have mirror
symmetries with small shifts from the corresponding absorption
spectra. Therefore, emissions of proligands HL-1 and HL-2 are
believed to represent the S1 → S0 radiative transition
(fluorescence) that is typical for organic molecules without
heavy atoms.
Figure 1. Schematic representation of a lipid membrane containing the
molecules of an embedded dopant (an amphiphilic Pt(II) complex).
Red stars: excimers showing EE; yellow stars: phosphorescent, single
molecules.
The investigations were carried out with complexes Pt-1 and
Pt-2 (Figure 2), which were designed to differ from each other
in the electron richness of their ligands. The ligand of Pt-1
contains an electron withdrawing fluorine substituent on the
phenyl ring, whereas the ligand of Pt-2 contains a thiophen and
cyclopentene substitution on pyridine. These structural differ-
ences should affect and modulate the intermolecular
interactions of the complexes. Long alkoxy chains in structures
of both of the complexes are supposed to increase the solubility
of the complexes in the phospholipid membranes.
Complexes Pt-1 and Pt-2 show two sets of absorption bands
(Figure 4). Bands of higher energy and intensity are believed to
rise from the π → π* transitions on the phenyl-pyridine ligands,
since these bands appear energetically very close to the
absorption bands of the respective proligands (Figure 4).
The absorption bands of lower energy in both of the
complexes, Pt-1 and Pt-2, appear upon complexation and can
be assigned to π → π* transitions within the π-systems
delocalized over phenyl-pyridine ligand, metal atom, and the
auxiliary ligand (acac)1,2 (Figure S2). Emissions of both
complexes, Pt-1 and Pt-2, featuring decay times of a few tens
of microseconds and structured spectra (Figure 4 and Table 1),
are assigned to the transition from the lowest triplet state, T1 →
Figure 2. Chemical structures of complexes Pt-1 and Pt-2.
3
RESULTS AND DISCUSSION
S0 (phosphorescence), of ππ* character.
■
It is found that complex Pt-2 shows a lower photo-
luminescence quantum yield (ΦPL) and a longer emission
decay time (τ) (lifetime of the emitting stateT1), compared
to Pt-1 under the same conditions (Table 1). The photo-
luminescence quantum yield is cooperatively defined by the
radiative rate (kr), calculated as kr = ΦPL/τ, and the non-
radiative rate (knr), calculated as knr = (1 − ΦPL)/τ, of the T1 →
S0 transition, and is proportional to the ratio kr/knr. For
example, for degassed CHCl3 solution, from the data in Table
1, one obtains kr = 2.16 × 104 s−1, knr = 1.70 × 104 s−1, and kr/
knr = 1.27 for Pt-1, and kr = 1.04 × 104 s−1, knr = 1.56 × 104 s−1,
Synthesis. Amphiphilic proligands HL-1 and HL-2
containing long C12 alkyl chains were synthesized using the
well-known 1,2,4-triazine method.63−66 The method implies
coupling 2-bromo-1-(4-methoxyphenyl)ethanone with 4-fluo-
robenzoic hydrazide for HL-1, and 2-bromo-1-(thiophen-2-
yl)ethanone with 4-methoxybenzyl-hydrazide for HL-2. The
obtained 1,2,4-triazines were converted into pyridines by
inverse electron-demand Diels−Alder reaction using 2,4-
norbornadiene (HL-1) and 1-(4-morpholin)-cyclopentene
(HL-2) as dienophiles.67,68 Demethylation of the methoxy
groups of the obtained pyridines was carried out with
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a
Scheme 1. Syntheses of Phenyl-Pyridine Proligands HL-1 and HL-2 and Platinum Complex Pt-1
a
Complex Pt-2 was prepared analogously to Pt-1 from the corresponding proligand HL-2 with the yield of 54%. 1:2-bromo-1-(4-
methoxyphenyl)ethanone, sodium acetate, ethanol/acetic acid (3:1), 100 °C, 8 h, 76%; (1a) 1-(thienyl-2)-2-bromocarbonyl, sodium acetate,
ethanol/acetic acid (3:1), 100 °C, 8 h, 94%; 2:2,5-norbornadiene, o-xylene, 200 °C, 12 h, 64%; (2a) 1-(4-morpholin)-cyclopentene, 190 °C, 2 h,
67%; (3) pyridinium chloride, 200 °C, 8 h, 53% for HL-1 and 95% for HL-2; (4) 1-bromododecane, potassium carbonate, DMF, 100 °C, 12 h, 91%
for HL-1 and 96% for HL-2; (5) K2PtCl4, acetic acid, reflux, 24 h; (6) sodium acetylacetonate, acetone, reflux, 48 h, 67%.
Figure 3. Perspective views of complexes Pt-1 and Pt-2 (OLEX-270
plots with 50% thermal probability ellipsoids).
and kr/knr = 0.67 for Pt-2. Since the phosphorescence of Pt-2 is
red-shifted compared to that of Pt-1, the obtained smaller kr/knr
ratio value of Pt-2 is probably the effect of a more pronounced
vibrational overlap of states T1 and S0, which favors non-
radiative decay of the excited state as expressed by the energy
gap law71 (Table 1).
It is noted that the absolute values of both kr and knr of Pt-2
are smaller compared to those of Pt-1, respectively. This
Figure 4. UV−vis absorption and emission spectra of complexes Pt-1
explains the longer lifetime of the T1 state of Pt-2, and is
(top panel) and Pt-2 (bottom), and respective ligands HL-1 and HL-
indicative of the less relaxed spin-forbiddenness of the T1 → S0
transition of Pt-2, compared to Pt-1. The relaxation of the spin-
forbiddenness of a T1 → S0 transition corresponds to the
strength of the spin−orbit coupling (SOC) between the T1
state and the states of the singlet manifold. The strength of
SOC between the states strongly depends on the contribution
of the Pt atom (Pt has a large SOC constant of ζl = 4481
2. The concentrations of the proligands and the complexes were 10−5
M in CHCl3 solutions. PMMA films were prepared with about 1 wt %
of the doped complex.
cm−1)72 to formation of the coupling states. For example,
coupling of the T1 state with a close in energy singlet state of
C
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Table 1. Ambient Temperature Absorption and Emission Data for the Platinum(II) Complexes Pt-1 and Pt-2
absorption
emission
CHCl3 degassed solution
CHCl3 solution
(c ≈ 10−5 M)
PMMA film under nitrogen
λ
max/nm (ε/M−1·cm−1
)
λmax/nm
τ/μs
ΦPL
λmax/nm
τ/μs
ΦPL
Pt-1
Pt-2
400(9778), 385(7966), 362(8981), 317(29932), 301(30936), 255(33495)
410(12490), 396(12854), 321(25287), 305(23677), 275(22606), 253(23743)
531, 570
556, 601
25.9
38.5
0.56
0.40
530, 565
553, 593
19.7
34.5
0.63
0.41
metal to ligand charge transfer (1MLCT) character can be quite
efficient and thus significantly relax the spin-forbiddenness of
the T1 → S0 transition. It is remarked that for an efficient SOC,
apart from having a small energy gap, the coupling states should
be contributed by the heavy atom’s (Pt) orbitals of different
magnetic quantum numbers (mS) (different d-orbitals) in order
to conserve the total angular momentum (orbital + spin) of the
electron when the states couple.73 As the lowest excited states
are formed of transitions between the frontier orbitals,
transitions such as HOMO → LUMO and HOMO−1 →
LUMO, the strength of SOC between them can be correlated
with the amount of Pt contribution to the frontier orbitals.1,2
Thus, it can be that complex Pt-2, having higher energies of
HOMO and LUMO (Figure S2), compared to Pt-1, also has a
smaller amount of Pt contribution to them, resulting in a
weaker SOC between the lowest excited states, and
consequently, Pt-2 shows lower radiative and nonradiative
rates of the T1 → S0 transition. Besides, SOC of the T1 state
with a higher lying singlet state (Sn) can simply be weaker in
the case of Pt-2 because of a larger energy gap between the
states, that also would result in a less relaxed spin-forbiddenness
of the T1 → S0 transition.1,73,74
character such as MLCT; therefore, the effect of different
polarity is not so prominently reflected on the T1 states’ energy
and the emission spectra (Figure 4).
Photoluminescence of Pt-1 and Pt-2 Embedded in
Phospholipid Membranes. Water suspensions of vesicles
(Figure S3) with doped Pt-1 or Pt-2 were prepared according
to procedures reported before54−56 (the procedure is also
described in the Experimental Section) using three different
phospholipids (amphiphiles): 1,2-dioleoyl-sn-glycero-3-phos-
phocholine (DOPC), 1,2-dimyristoyl-sn-glycero-3-phosphocho-
line (DMPC), and 1,2-distearoyl-sn-glycero-3-phosphocholine
(DSPC) (Figure 5). The total concentration of amphiphiles in
We noted that either Pt-1 or Pt-2 exhibits a slightly shorter
emission decay time when doped in a PMMA film compared to
degassed CHCl3 solution (Table 1), meaning that the T1 → S0
transitions are faster in doped PMMA films. Calculating the kr
and knr values for PMMA films using the data in Table 1, one
obtains kr = 3.20 × 104 s−1, knr = 1.90 × 104 s−1, and kr/knr
=
1.68 for Pt-1, and kr = 1.20 × 104 s−1, knr = 1.70 × 104 s−1, and
kr/knr = 0.71 for Pt-2. Thus, the rates kr and knr of either Pt-1 or
Pt-2 doped in PMMA film indeed appear to be slightly higher
when respectively compared to those obtained for chloroform
solutions (see above). It is noted that, in both cases, Pt-1 and
Pt-2, the kr values increase more than knr values, resulting in
higher quantum yields (increased kr/knr ratio values). The
increase of kr and knr values is indicative of a more relaxed spin-
forbiddenness of the T1 → S0 transition in the PMMA film
compared to degassed CHCl3 solution. PMMA film, according
to the polarity of the methyl methacrylate (μ = 1.60−1.97 D)
and the polarity of the ester group (μ = 1.80 D),75 is a medium
of higher polarity compared to chloroform (μ = 1.04−1.15 D).
Considering either Pt-1 or Pt-2, this polarity difference
between the two media changes the stabilizations (energies)
of the higher lying singlet states which the T1 state has SOC
with, especially those with significant admixture of MLCT
character. It is believed that the higher lying excited singlet
states with MLCT character are more stabilized (have lower
energies) in the media of higher polarity (PMMA film) and
therefore are energetically closer to the T1 state of ππ*
character, which results in a stronger SOC between the excited
singlet states and the T1 state that means a more relaxed spin-
forbiddenness of the T1 → S0 transition. It is noted that the
stabilization of ππ* character excited states is not so sensitive to
the polarity of the media as excited states of charge transfer
Figure 5. Chemical structures of the phospholipids used for vesicle
preparation.
water was 10−3 M including 1 or 3 mol % of an embedded
complex, Pt-1 or Pt-2. The obtained vesicle suspensions were
analyzed by dynamic light scattering (DLS), showing size
distributions of the formed vesicles in the range 90−100 nm in
diameter (Figure S4).
When dissolved and concentrated in phospholipid vesicles,
the dopant can form excimers if the diffusion rate within the
vesicles is high enough for an excited single molecule of dopant
to meet another molecule within the excited state’s decay time.
The dopant diffusion rate depends on the viscosity of the
vesicles, which, at a given temperature, is defined by the used
phospholipid and correlates with the phase transition temper-
ature (PTT). The PTT is the melting point of the phospholipid
vesicles into the gel phase where the lipid bilayers (membranes)
become fluid and mobile. The vesicles used in this research
have PTTs of about −20 °C, +25 °C, and +55 °C for DOPC,
D
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DMPC, and DSPC, respectively.55,76−83 A higher phase
transition temperature means a higher membrane viscosity at
a given temperature. Therefore, the diffusion rate of the
embedded complex (dopant) within the phospholipid mem-
branes at a given temperature is expected to be the highest in
DOPC vesicles, lower in DMPC vesicles, and the lowest in
DSPC vesicles.
When embedded in the least viscous DOPC vesicles, Pt-1
exhibits an additional intense emission band at λmax = 670 nm
(Figure 6a), which is not observed for a diluted chloroform
lowest triplet state, S0 → T1, might contribute to the discussed
excitation signals of low intensity (Figure 7a). The two
excitation spectra can be so similar if the emission with λmax
=
670 nm is attributed to the excimers of Pt-1 which form after
excitation of the single molecules. This assignment agrees well
with the experimental fact that the emission decay detected at
λdet = 670 nm, in contrast to the emission decay detected at λdet
= 570 nm, is delayed by growth of intensity, which is thought to
be associated with the formation process of the emissive
excimers (Figure 8). It is noted that the emission decay curves
Figure 8. Emission decay curves of Pt-1 embedded in DOPC vesicles
measured with detection at λdet = 570 nm (a) and at λdet = 670 nm (b)
at 300 K.
measured in vesicular media are not perfectly monoexponential.
This is probably due to the inhomogeneity of the media90 and
quenching processes, including self-quenching. However, by
careful fitting of the obtained decay curves with monoexpo-
nential functions, one can obtain the Pt-1 EE decay time of
about 3 μs, and ME decay time of 2.8 μs (Figure 8).
Figure 6. Emissive behavior (EE/ME ratio) of Pt-1 embedded in
vesicles: (a) Dependence of Pt-1 emission spectra on the type of the
vesicles at ambient temperature; (b) Temperature dependence of Pt-1
emission spectra in DMPC vesicles; (c) Concentration dependence of
Pt-1 emission spectra in DMPC vesicles; (d) Temperature depend-
ence of Pt-1 emission spectra in DSPC vesicles. The given percentages
represent the molar fraction of Pt-1 out of the total amphiphilic
concentration in water of 10−3 M.
Embedded in DMPC vesicles, complex Pt-1 shows the EE of
a moderate intensity, comparable to the intensity of ME and
much lower compared to that in DOPC vesicles (Figures 6a,
6b, and 6c). Interestingly, the EE/ME intensity ratio in DMPC
vesicles notably depends on temperature. In particular, the
higher EE/ME intensity ratio at 35 °C, compared to that at 5
°C, is quite prominent (Figure 6b). This is indicative of the
emissive excimers of Pt-1 being formed more effectively at
higher temperatures. Also, the dependence of excimer
formation efficiency, as judged on the basis of the EE/ME
intensity ratios, on the concentration of the complex Pt-1
(dopant) can be clearly tracked in DMPC vesicles (Figure 6c),
which is not so well pronounced in DOPC and DSPC vesicles
in comparison (Figure S5). When complex Pt-1 is embedded in
DSPC vesicles, the most viscous vesicles among those
presented, at temperatures about ambient, the EE is barely
pronounced (Figures 6a and 6d).
solution and a doped PMMA film. The excitation spectrum
obtained with detection at λdet = 670 nm is quite similar to the
spectrum obtained with detection at λdet = 570 nm (ME
monomer emission) (Figure 7a). The weak signals observed
When embedded in phospholipid vesicles, Pt-2 also exhibits
an additional emission band with λmax = 630 nm (Figure 9),
which is assigned to the EE of Pt-2 on the basis of excitation
spectra, as in the case of complex Pt-1. Unfortunately, the EE
and ME of complex Pt-2 strongly overlap, which does not allow
detailed investigations with their decay curves in the way it was
carried out for complex Pt-1. The concentration dependence of
the ME/EE intensity ratio of complex Pt-2 in DOPC vesicles
(Figure 9a) is more pronounced compared to that of Pt-1. In
contrast to Pt-1, the EE/ME intensity ratio of Pt-2 embedded
in DMPC or DSPC vesicles slightly decreases with the increase
of temperature (Figures 9b and 9d), which is discussed in the
next section.
Figure 7. Excitation spectra of complexes Pt-1(a) and Pt-2 (b) at 300
K obtained with detection at the EE maximum and at the ME
maximum. Given percentages represent the molar fraction of the
complex within the total amphiphilic concentration of 10−3 M in
water.
after 450 nm (shown in Figure 7a with 15-fold magnification)
appear in both of the excitation spectra, and can be attributed
to excitation by light produced by the second harmonic
generation (SHG) effect, which is reported to take place at the
colloidal interface of the vesicles’ suspension.84−89 Apart from
the SHG effect, also, a spin-forbidden direct excitation to the
It is remarked here that the EE of Pt-1 in a chloroform
solution is observed only at concentrations as high as 10−3
M
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In DSPC vesicles (PTT ≈ 55 °C), an increase of the
temperature from 5 to 35 °C has only a very weak influence on
the formation of Pt-1 excimers. This is because the viscosity of
DSPC vesicles even at 35 °C remains too high for fast diffusion
of Pt-1 molecules and formation of excimers within the excited
state’s decay time, and consequently, formation of the excimers
remains strongly retarded (Figure 6d). On the other side, in
DOPC vesicles (PTT ≈ −20 °C) at temperatures near 0 °C the
diffusion rate of Pt-1 seems to be already so high (low
viscosity) that further increase of the rate at higher temper-
atures barely affects the excimer formation efficiency.
Interestingly, even an increase of the Pt-1 concentration in
DOPC and DSCP vesicles, in contrast to DMPC vesicles, has a
very insignificant effect on the EE/ME intensity ratio. On this
basis, we conclude that at near-ambient temperatures the
excimer formation efficiency (EE intensity) in DOPC and
DSPC vesicles is rather defined by the viscosity of the vesicles
than by the dopant concentration in them.
The photophysical behavior of complex Pt-2 embedded in
vesicles is found to differ from that of Pt-1 discussed above. In
particular, the EE of Pt-2 is found to be more pronounced in
DSPC vesicles (Figure 9d), and to be more concentration
sensitive in DOPC vesicles (Figure 9a), compared to complex
Pt-1. This, we are inclined to think, is due to a stronger
intermolecular π−π interactions of Pt-2 provided by the higher
energy frontier π-orbitals (Figure S2), and the associated better
lateral phase separation in the phospholipid membranes,
compared to Pt-1.
It was noted that Pt-2 embedded in DMPC or DSPC
vesicles, in contrast to Pt-1, shows a slight decrease of the EE/
ME intensity ratio with an increase of temperature. This might
be associated with a higher temperature sensitivity of the
photoluminescence quantum yield (ΦPL) of Pt-2 excimers, so
that when the temperature is increasing, the effect of decreasing
quantum yield of the excimers exceeds the effect of increasing
rate of excimer formation governed by decreasing viscosity.
Also, a possible lower stability of emissive excimers of Pt-2,
compared to those of Pt-1, could result in a higher excimer
dissociation rate at higher temperatures, thus decreasing the EE
intensity with an increase of temperature.
Theoretical Considerations of Intermolecular Inter-
actions in Pt(II) Complexes. The quantum-chemical
calculations were carried out using the ORCA 3.0 programs
package91 at the def2-SVP92,93/BLYP theory level with
Grimme’s D3BJ dispersion correction94,95 applied for dimers
and excimers (see the Experimental Section for the details). In
order to accelerate the calculations, model structures Pt-1′ and
Pt-2′ (Figure 10), where methoxy groups replace the dodecoxy
groups of experimentally investigated complexes Pt-1 and Pt-2,
were used. The optimized molecular geometries of Pt-1′ and
Figure 9. Emission spectra of Pt-2 embedded in vesicles: (a)
Concentration dependence of Pt-2 emission spectra in DOPC
vesicles; (b) Temperature dependence of Pt-2 emission spectra in
DMPC vesicles; (c) Concentration dependence of Pt-2 emission
spectra in DMPC vesicles; (d) Temperature dependence of Pt-2
emission spectra in DSPC vesicles. Given percentages represent the
molar fraction of Pt-2 within the total amphiphilic concentration of
10−3 M.
order, while the EE of Pt-2 in chloroform solution is not
observed even at those high concentrations (Figure S6).
Factors Affecting the Excimer Formation Efficiency in
Vesicles. The case of complex Pt-1, which at equal conditions
shows a very intense EE in DOPC vesicles, EE of moderate
intensity in DMPC vesicles, and barely pronounced EE in
DSPC vesicles, is very indicative of a strong influence of a
medium’s viscosity on the efficiency of excimer formation.
The vesicles consisting of DOPC with unsaturated alkyl
chains feature the lowest viscosity at a given temperature,
among the represented vesicles, which correlates with the
lowest PTT ≈ −20 °C, as discussed above. Therefore, the
dopant diffusion rate within a DOPC vesicle at ambient
conditions is comparatively high, and so is the excimer
formation efficiency, which is reflected in an intense EE of
Pt-1 (Figure 6a). It is noted that having a fast dopant diffusion
within vesicles is of high importance for formation of an
excimer because for this an excited single molecule should meet
another molecule quickly within the excited state’s decay time.
The viscosity of DMPC vesicles, with PTT ≈ 25 °C, is
expected to be significantly higher compared to the DOPC
vesicles that should decrease the dopant diffusion rate and,
consequently, the efficiency of excimer formation. Indeed, the
intensity of the EE of Pt-1 in DMPC vesicles is found to be
moderate and comparable to that of ME (Figures 6a, 6b, and
6c). The negative effect of vesicles’ high viscosity on excimer
formation efficiency is even more pronounced in DSPC, with
PTT ≈ 55 °C, where the EE of Pt-1 is barely pronounced
(Figure 6d).
The PTT of DMPC vesicles practically is the ambient
temperature. This fact allows adjustment of the viscosity of
DMPC vesicles easily through temperature, thus affecting the
excimer formation efficiency and the EE intensity, conse-
quently. Indeed, the EE/ME intensity ratio of Pt-1 is found to
be reversibly dependent on temperature in the range 5−35 °C,
having a higher value at 35 °C (lower viscosity) and a lower
value at 5 °C (higher viscosity) (Figure 6b).
Figure 10. Structures of model complexes Pt-1′ and Pt-2′ optimized
under gas phase conditions in the ground state (S0) at the def2-SVP/
BLYP level of theory. Hydrogen atoms are omitted for clarity.
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Pt-2′ are in good agreement with the structures of Pt-1 and Pt-
2, respectively, found by single crystal X-ray diffraction analysis
(Table 2).
Scanning the electronic energy stabilization of the dimer
along the Pt−Pt distance revealed a minimum of a relaxed
dimer at a Pt−Pt distance of about 3.3 Å for both of the model
complexes, Pt-1′ and Pt-2′. Since the dimer geometries fit well
for interaction of the 5dz2 orbitals of two Pt atoms, it is believed
that the stabilization at these minima is strongly contributed by
Pt−Pt interactions, although the π−π interactions could
contribute as well,23 especially in the case of Pt-2′ with the
higher energy frontier π-orbitals (Figure S2). Further we will
refer to these dimers as Pt−Pt dimers.
In contrast to Pt-1′, for Pt-2′ an additional and broader
minimum of a dimer is found at a Pt−Pt distance of 4.8−5.0 Å
(Figure 11) with a relaxed geometry that does not allow any
Pt−Pt contact, but π−π interactions between the ligands.
Further, we will refer to this dimer as a π−π dimer (Figure 12).
It is noted that with the Pt−Pt distance increasing, the
molecular planes in the calculated dimers are not splitting up,
but are sliding with respect to each other, and therefore even at
Pt−Pt distance of 9 Å the π−π interactions remain active and
cause some stabilization (Figure 11).
Scanning the electronic energy stabilizations of the excimers,
of Pt-1′ and Pt-2′, reveals a deep minimum at the Pt−Pt
distance 2.8−2.9 Å for both of the model complexes (Figure
11). The geometries of the excimers in these minima are close
to those of the respective Pt−Pt dimers. The strong
stabilization of these excimers and the decrease of the Pt−Pt
distances in them compared to the dimers is indicative of
significant enhancement of the Pt−Pt interactions that is
associated with the 3MMLCT character of the T1 state (Figure
S7). Therefore, these excimers are believed to be the Pt−Pt
excimers.
In contrast to Pt-1′, complex Pt-2′ has an additional
minimum of relaxed excimer with a Pt−Pt distance of about 4.7
Å. According to the geometry, the excimer can be stabilized
only by π−π interactions, and represents a π−π excimer
(Figures 11 and 12), which is the descendent of the respective
π−π dimer of Pt-2′ with a Pt−Pt distance of 4.8−5.0 Å (see the
SI for the relaxed geometries, representing all the calculated
minima, in XYZ format).
Table 2. Calculated Gas Phase Geometry Parameters of Pt-1′
and Pt-2′ (def2-SVP/BLYP) Compared to Values Obtained
from Single Crystal X-ray Diffraction Analysis of Pt-1 and Pt-
2
Pt-1′
Pt-1
Pt-2′
Pt-2
Calc.
Found
Calc.
Found
Bond length (Å)
1.970(3)
Pt−C1
Pt−N1
Pt−O1
Pt−O2
1.997
2.025
2.141
2.044
1.994
2.023
2.144
2.049
1.958(5)
1.979(3)
2.089(3)
2.012(3)
1.993(2)
2.0848(19)
2.008(2)
Angle (deg)
81.59(11)
173.32(10)
93.97(10)
92.27(9)
C1−Pt−N
C1−Pt−O1
C1−Pt−O2
N−Pt−O1
N−Pt−O2
O1−Pt−O2
81.47
175.11
94.0902
93.6505
175.56
90.80
81.22
174.977
94.57
81.36(17)
175.18(15)
93.38(17)
93.87(14)
174.71(14)
91.40(13)
93.77
175.56(8)
92.17(8)
175.77
90.45
We investigated the stabilizations of the dimers and excimers
of Pt-1′ and Pt-2′ along the Pt−Pt distance that is shown in
Figure 11. Herein ΔE1 shows stabilization of the electronic
energy compared to the doubled electronic energy of the
respective optimized monomer in the ground state. ΔE2 shows
stabilization of the electronic energy compared to the sum of
the electronic energies of the corresponding monomer in the
ground state (S0) and in the lowest triplet state (T1).
Photophysical Properties of Pt-1 and Pt-2 in
Conjunction with Results of the Calculations. As shown
above, scanning the electronic energy stabilizations of the dimer
(ΔE1) and excimer (ΔE2) of Pt-1′ along the Pt − Pt distance
revealed one minimum for the dimer and one minimum for the
excimer, corresponding to a relaxed Pt−Pt dimer and a relaxed
Pt−Pt excimer, respectively (Figures 11 and 12). Thus, the
calculations suggest that the Pt−Pt interactions are probably
the dominating intermolecular forces between molecules of Pt-
1. Evidently, the intense EE of Pt-1 observed from DOPC and
DMPC vesicles belongs to the Pt−Pt excimers, which are often
found to be emissive.18−22,24−50 Besides, the Pt−Pt excimers
are more stabilized, compared to π−π excimers, as an electron
from the Pt−Pt antibonding orbital is transferred to a π* orbital
of the ligands (3MMLCT character of the T1 state; Figure S7),
resulting in an enhanced Pt−Pt bonding interaction. The last
statement is supported by the conducted theoretical calcu-
lations with Pt-1′, where the Pt−Pt distance is shortened from
3.30 Å in the Pt−Pt dimer to 2.86 Å in the Pt−Pt excimer, with
the latter value being considerably smaller than two van der
Waals radii of Pt (3.5 Å).
Figure 11. Stabilization energies of excimers (top panel) and dimers
(bottom) of complexes Pt-1′ (blue lines) and Pt-2′ (red lines)
depending on the Pt−Pt distance (def2-SVP/BLYP). Black dots on
the graphs represent the energy of an optimized geometry with fixed
Pt−Pt distance, except for minima, which represent fully relaxed
geometries.
Scanning the stabilization of Pt-2′ dimer (ΔE1), along the
Pt−Pt distance, revealed two minima of roughly equal
stabilization. These two minima correspond to a Pt−Pt dimer
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Figure 12. Relaxed dimers and excimers of complexes Pt-1′ and Pt-2′ corresponding to the calculated minima on graphs on Figure 11.
and a π−π dimer (Figures 11 and 12). Analogous scanning on
Pt-2 excimer (ΔE2) revealed a local minimum, corresponding
to the π−π excimer, and a much deeper (global) minimum
corresponding to the Pt−Pt excimer (Figures 11 and 12).
Therefore, the EE emission observed for Pt-2 from vesicles is
thought to come from Pt−Pt excimers, as in the case of Pt-1.
Evidently, the emissive Pt−Pt excimers of Pt-2 can be quickly
formed by metal−metal (PtII−PtII) interactions between an
excited molecule and a molecule in the ground state when the
molecules are dissolved and concentrated in the phospholipid
membranes.
represent the HOMO and the LUMO of the complexes.
Calculations suggest that the HOMO and LUMO of Pt-2′ are
by 0.3 and 0.1 eV, respectively, higher in energy compared to
those of Pt-1′ (Figure S2). Accordingly, the higher energies of
the frontier π-orbitals of Pt-2 produce a stronger π−π
disturbance, destabilizing the Pt−Pt excimer and, consequently,
resulting in a smaller red-shift of the EE from the respective
ME.
CONCLUDING REMARKS
■
A significant influence of the organic ligand’s structure on the
photophysical properties of the excimers formed by Pt(II)
complexes is seen throughout this research. A π-conjugated
system of a higher absolute energy naturally promotes stronger
π−π interactions. When excited, these π−π interactions (i)
promote formation of π−π excimers and (ii) counteract
stabilization of Pt−Pt excimers, consequently diminishing the
desired bathochromic shift of their emission. Therefore, an
electron-richer organic ligand of Pt-2, providing a lower energy
phosphorescence of Pt-2, compared to Pt-1, plays for the
reverse trend when it comes to the respective EEs.
The EE maximum of Pt-2 (λmax = 630 nm) is by 766 cm−1
red-shifted from the maximum of ME (λmax = 600 nm), whereas
in the case of Pt-1 the analogous shift of the EE maximum (λmax
= 670 nm) from the maximum of the ME (λmax = 570 nm)
includes 2620 cm−1. This can be explained by comparison of
the energy gaps between the corresponding dimers and
excimers. The relaxed Pt−Pt dimer of Pt-2′ is by 24 kJ·mol−1
more stabilized than the Pt−Pt dimer of Pt-1′, by comparison
of ΔE1 stabilization energies (Figure 11). However, the relaxed
Pt−Pt excimer of Pt-2′ is only by 13 kJ·mol−1 more stabilized
than the Pt−Pt excimer of Pt-1′, by comparison of ΔE2
stabilization energies (Figure 11). Thereby, according to
calculations, the [Pt−Pt excimer − Pt−Pt dimer] energy gap
is smaller in the case of Pt-1′, compared to Pt-2′. This smaller
energy gap means a longer wavelength emission, when the Pt−
Pt excimer radiatively relaxes to the Pt−Pt dimer. Indeed, the
EE of Pt-1 is experimentally found to be of longer wavelength
(λmax = 670 nm) as compared to the EE of Pt-2 (λmax = 630
nm).
The sensitivity of the EE’s intensity of Pt-1 in DMPC
vesicles to viscosity shows that in some media Pt-1 can be
adapted as a luminescent sensor, which, once calibrated, might
signal the local temperature or viscosity.96 Besides, in a media
of viscosity as that of DOPC vesicles at ambient temperature,
excimers of complex Pt-1 can be used for optical imaging in the
red regime. This can be of a particular actuality if the media
contains organic dopants that absorb the light of shorter
wavelengths.
The larger [Pt−Pt excimer − Pt−Pt dimer] energy gap of Pt-
2, compared to that of Pt-1 can be explained by stronger π−π
interactions interfering with the enhancement of Pt−Pt
interactions in the Pt−Pt excimer, thus causing some
destabilization. This explanation can be found plausible if one
takes into account the fact that the Pt−Pt distance in the
MMLCT state (excimer) shortens compared to the dimer while
the π-systems of the ligands are enriched with an additional
(MMLCT transferred) electron. The enriched π-systems of the
molecules enrolled in the excimer are expected to modify the
π−π interactions actual for a dimer. This shall require
reorganization of the two molecules, and even cause π−π
repulsion at a shortening Pt−Pt (intermolecular) distance, that,
consequently, counteracts the Pt−Pt interactions and desta-
bilizes the Pt−Pt excimer. The scale of this effect is believed to
depend on the energies of the frontier π-orbitals, which are, in
our cases, mostly localized on the phenylpyridine ligands and
The multiplicity of the emitting state of the investigated Pt−
Pt excimers, thanks to the heavy atoms, is triplet (T1). Since the
singlet ↔ triplet transitions (T1 ↔ S0, in particular) are
formally forbidden (spin-forbidden), the Pt−Pt excimers
feature decay times on the microsecond time scale, which is
relatively long compared, for example, to excimer fluorescence
(S1 → S0) of pyrene with a decay time of ∼40 ns.97 The
relatively long decay time of the Pt−Pt excimers makes their
emission intensity more sensitive to quenchers such as oxygen.
Therefore, applied in a suitable media, Pt−Pt excimers of
complexes Pt-1 and Pt-2, in principle, can work as oxygen
sensors functioning in the red regime.
EXPERIMENTAL SECTION
■
Instrumentation for Optical Spectroscopy. Absorption spectra
were measured on a CaryBio 50 UV/vis spectrometer, using quartz
cuvettes of 1 cm path length. Steady-state luminescence spectra were
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measured using a Jobin Yvon FluoroMax-4 spectrofluorometer; the
spectra shown are corrected for the wavelength dependence of the
detector’s sensitivity, and the quoted emission maxima refer to the
values after correction. Photoluminescence quantum yields were
determined using a Hamamatsu C9920-02 system. Presented emission
lifetimes were determined using a PicoBright PB-375L pulsed diode
laser (λexc = 378 nm, pulse width 100 ps) as an excitation source, while
the emission signal was detected with a cooled photomultiplier
attached to a FAST ComTec multichannel scalar PCI card with the
time resolution of 250 ps.
J = 8.25, 2.50 Hz), 7.73 (dd, 1H, J = 8.25 Hz, J = 0.57 Hz), 7.57 (m,
2H), 7.16 (m, 2H), 7.03 (m, 2H), 3.87 (s, 3H).
2-(4-Fluorophenyl)-5-(4-hydroxyphenyl)pyridine. 2-(4-Fluo-
rophenyl)-5-(4-methoxyphenyl)pyridine (1.8 mmol, 500 mg) and
pyridinium chloride (27 mmol, 3.19 g) were alloyed at 200 °C in a
round-bottom flask equipped with a magnetic stirrer and a condenser
and kept at this temperature for 8 h under nitrogen. Cold water (50
mL) was added at vigorous stirring while the mixture is still liquid
(110−120 °C). After 1 h, the precipitated product was filtered out,
washed with methanol, and dried. Yield = 53%. Product was used in
the next step without further purification. 1H NMR (CDCl3): δ (ppm)
= 8.80 (dd, 1H, J = 2.30, 0.54 Hz), 7.95 (m, 2H), 7.83 (dd, 1H, J =
8.25, 2.50 Hz), 7.66 (dd, 1H, J = 8.25, 0.57 Hz), 7.45 (m, 2H), 7.10
(m, 2H), 6.89 (m, 2H), 5.01 (s, 1H).
Dynamic Light Scattering (DLS). DLS measurements were
performed on a Malvern Zetasizer Nano at 25 °C after the
temperature equilibration time of 120 s using disposable sizing
polystyrene cuvettes (VWR) with 1 cm path length.
2-(4-Fluorophenyl)-5-(4-dodecyloxyphenyl)pyridine. 2-(4-
Fluorophenyl)-5-(4-hydroxyphenyl)pyridine (0.47 mmol, 125 mg),
dodecyl bromide (0.7 mmol, 0.17 mL), potassium carbonate (2.35
mmol, 324 mg), and DMF (25 mL) as a solvent were placed in a
round-bottom flask, and the mixture was stirred at 100 °C for 12 h. 50
mL of water was added, and the product was extracted with
dichloromethane (3 × 30 mL). The solvent was evaporated, and
methanol was added for product to precipitate. The precipitate was
Computational Details. Calculations were performed using the
ORCA 3.0.391 programs package at the def2-SVP92,93/BLYP theory
level with ECP92 applied on Pt atom. To improve the estimation of
long-range interactions, calculations on dimers and excimers were
carried out with Grimme’s D3BJ dispersion correction94,95 applied.
Dimers, of model complexes Pt-1′ and Pt-2′, were built using the
optimized geometry of the corresponding monomer. The optimal
initial monomer configuration in the dimers was obtained through
scanning the electronic energy by rotation of two monomer molecules
with respect to each other on the Pt−Pt axis with 20° steps at the Pt−
Pt distance about 3.1−3.4 Å. Since the molecules are not symmetric,
such scanning was carried out in both face to face and face to back
configurations of the monomers around full circle. The monomer
configuration with the thus found lowest electronic energy was chosen
for further optimization to find the totally relaxed geometry of the
dimer.
Preparation Procedure of Water Suspensions of Phospho-
lipid Vesicles. Suspensions of vesicles in water were prepared in small
reaction vessels (5 mL) with 10−3 M total amphiphilic concentration,
which consists of 99 or 97% of a phospholipid and 1 or 3% of Pt-1 or
Pt-2 (given are the molar fractions). The lipid/complex ratio was
achieved by mixing the appropriate quantities of stock chloroform
solutions of a phospholipid (DOPC or DMPC or DSPC)) and a
Pt(II) complex (Pt-1 or Pt-2) prepared in advance. Then the solvent
was removed under reduced pressure and the appropriate amount of
distilled water was added, yielding 10−3 M of total amphiphilic
concentration. Subsequent sonication at 25 °C (DOPC), 35 °C
(DMPC), and 60 °C (DSPC) for 2 h gave a transparent suspension of
turbid multilamellar vesicles (Figure S3).54−56
1
filtered out, washed with methanol, and dried. Yield = 91%. H NMR
(CDCl3): δ (ppm) = 8.87 (dd, 1H, J = 2.30, 0.54 Hz), 8.02 (m, 2H),
7.90 (dd, 1H, J = 8.25, 2.50 Hz), 7.72 (dd, 1H J = 8.25, 0.57 Hz), 7.55
(m, 2H), 7.16 (m, 2H), 7.01 (m, 2H), 4.01 (t, 2H, J = 6.61 Hz), 1.82
(m, 2H), 1.57−0.85 (m, 21H). Calculated for C29H36FNO, %: C,
88.33; H, 8.37; N, 3.23. Found, %: C, 88.48; H, 8.21; N, 3.11.
[Pt(L-1)(acac)], HL-1
= 2-(4-Fluorophenyl)-5-(4-
dodecyloxyphenyl)pyridine. Ground powder of K2[PtCl4] (0.33
mmol, 137 mg) was added to a stirring solution of HL-1 (0.33 mmol,
143 mg) in acetic acid (30 mL). The mixture was heated at reflux
under nitrogen for 24 h. The precipitated solid, representing dimeric
complex [Pt2(μ-Cl)2(L-1)2], was filtered out, washed with acetic acid
(5 mL) and ethanol (5 mL), and dried under reduced pressure. The
resulting product and sodium acetylacetonate (10 equiv) were heated
at reflux in acetone for 48 h. The solvent was removed under reduced
pressure, and the title complex was purified by column chromatog-
raphy (silica gel, dichloromethane). Yield = 67%. 1H NMR (CDCl3): δ
(ppm) = 9.15 (d, 1H, J = 1.91 Hz), 7.94 (dd, 1H, J = 8.34, 2.15 Hz)
7.53 (m, 3H), 7.42 (dd, 1H, J = 8.55, 5.48 Hz), 7.27(dd, 1H, J = 9.43,
2.68 Hz), 7.02 (d, 2H, J = 8.82 Hz), 6.81 (td, 1H, J = 8.66, 2.56 Hz),
5.49 (s, 1H), 4.01 (t, 2H, J = 6.48 Hz), 2.02 (d, 6H, J = 1.00 Hz), 1.82
(m, 2H), 1.57−0.85 (m, 21H). Calculated for C34H42FNO3Pt, %: C,
56.19; H, 5.82; N, 1.93. Found, %: C, 56.58; H, 5.77; N, 1.78. Mass:
Positive-ion LIFDI-MS: m/z 726.22 M+, m/z 727.22 [M + H]+, m/z
1452.46 [2M]+·.
Synthesis and Structural Characterization of the Repre-
1
sented Compounds. H NMR spectra were recorded on a Bruker
AVANCE II NMR spectrometer operating at 400 MHz (1H) with the
residual protic solvent used as the internal standard. Elemental
analyses were carried out using an Exeter Analytical Inc. CE 440
analyzer.
3-(4-Methoxyphenyl)-6-(thienyl-2)-1,2,4-triazine. A solution
of commercially available 2-bromo-1-(thiophen-2-yl)ethanone (14.14
mmol, 2.89 g), 4-methoxybenzohydrazide (28.28 mmol, 4.70 g) and
sodium acetate (21.17 mmol, 2.88 g) in a mixture of ethanol (60 mL)
and acetic acid (20 mL) was refluxed for 8 h. The mixture was cooled
down to room temperature and the precipitate of the titled product
was filtered out. The product was used in the next step without further
purification. Yield = 94%. 1H NMR (CDCl3): δ (ppm) = 8.92 (s, 1H),
8.50 (d, 2H, J = 9.02 Hz), 7.76 (dd, 1H, J = 3.70, 1.07 Hz), 7.57 (dd,
1H, J = 5.26, 1.00 Hz), 7.21 (dd, 1H, J = 4.92, 3.65 Hz), 7.05 (d, 2H, J
= 9.07 Hz), 3.91 (s, 3H).
3-(4-Fluorophenyl)-6-(4-methoxyphenyl)-1,2,4-triazine. A
solution of commercially available 2-bromo-1-(4-methoxyphenyl)-
ethanone (4.35 mmol, 1.00 g), 4-fluorobenzohydrazide (8.70 mmol,
1.34 g), and sodium acetate (8.70 mmol, 714 mg) in a mixture of
ethanol (60 mL) and acetic acid (20 mL) was stirred at 100 °C for 8 h.
The solution was cooled to room temperature, affording the title
compound as a yellow precipitate, which was isolated by filtration,
washed with cold ethanol, and dried. The crude product was used in
the next step without further purification. Yield = 76%. 1H NMR
(CDCl3): δ (ppm) = 8.99 (s, 1H), 8.57 (m, 2H), 8.12 (d, 2H, J = 9.00
Hz), 7.23 (m, 2H), 7.09 (d, 2H, J = 8.92 Hz), 3.91 (s, 3H).
2-(4-Fluorophenyl)-5-(4-methoxyphenyl)pyridine. 3-(4-Fluo-
rophenyl)-6-(4-methoxyphenyl)-1,2,4-triazine (2.84 mmol, 800 mg),
o-xylene (10 mL), and 2,5-norbornadiene (14.22 mmol, 3 mL) were
placed in an autoclave equipped with a magnetic stirrer, and the
mixture was left for 12 h in an oil bath at 200 °C. The solvent was
evaporated and methanol was added for the product to precipitate,
which then was filtered out and purified by column chromatography
(silica gel, dichloromethane). Yield = 64%. 1H NMR (CDCl3): δ
(ppm) = 8.99 (dd, 1H, J = 2.30, 0.54 Hz), 8.02 (m, 2H), 7.90 (dd, 1H,
2-(4-Methoxyphenyl)-5-(thienyl-2)-c-cyclopentylpyridine. A
mixture of 3-(4-methoxyphenyl)-6-thienyl-1,2,4-triazine (1.85 mmol,
500 mg) with 1-(4-morpholin)cyclopentene (5.55 mmol, 0.92 mL)
was stirred at 190 °C under nitrogen for 1 h and an additional portion
of 1-(4-morpholin)cyclopentene (3.7 mmol, 0.63 mL) was added and
the mixture was stirred under the same conditions for an additional
hour. The resulting oily mixture was treated with ethanol and placed in
a freezer overnight. The precipitation formed was filtered out, and the
compound was purified by column chromatography (silica gel,
1
dichloromethane). Yield = 67%. H NMR (CDCl3): δ (ppm) = 8.71
(s, 1H), 7.76 (d, 2H, J = 8.96 Hz), 7.39 (dd, 1H, J = 5.05, 1.00 Hz),
7.30 (dd, 1H, J = 3.57, 1.00 Hz), 7.15 (dd, 1H, J = 5.11, 3.64 Hz),
I
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Article
7.00(d, 2H, J = 8.96 Hz), 3.87 (s, 3H), 3.18 (td, 4H, J = 7.41, 4.00
Hz), 2.13 (m, 2H).
Funding
M.Z.S. is grateful to the German Academic Exchange Service
(Deutscher Akademischer Austauschdienst − DAAD) for
financial support.
2-(4-Hydroxyphenyl)-5-(thienyl-2)-c-cyclopenthylpyridine.
2-(4-Methoxyphenyl)-5-(thienyl-2)-c-cyclopenthylpyridine (0.976
mmol, 300 mg) and pyridinium chloride (4.88 mmol, 1.5 g) were
alloyed at 200 °C for 8 h. 50 mL of cold water was added under
vigorous stirring while mixture is still liquid (110−120 °C).
Precipitated product was filtered out, washed with methanol, and
Notes
The authors declare no competing financial interest.
1
dried. Yield = 95%. H NMR (DMSO-d6): δ (ppm) = 10.18 (s, 1H),
ACKNOWLEDGMENTS
8.69 (s, 1H), 7.73 (dd, 1H, J = 5.10, 1.00 Hz), 7.69 (d, 2H, J = 8.79
Hz), 7.64 (dd, 1H, J = 3.63, 1.00 Hz), 7.24 (dd, 1H, J = 5.17, 3.90 Hz),
6.98 (d, 2H, J = 8.55 Hz), 3.37 (t, 2H, J = 7.50 Hz), 3.25 (t, 2H, J =
7.50 Hz), 2.24 (m, 2H).
■
M.Z.S. is grateful to Dr. Rafał Czerwieniec (Regensburg) for
discussions and critical remarks.
2-(4-Dodecyloxyphenyl)-5-(thienyl-2)-c-cyclopenthylpyri-
dine. 2-(4-Hydroxyphenyl)-5-(thienyl-2)-c-cyclopenthylpyridine (1
mmol, 293 mg), dodecyl bromide (1.5 mmol, 0.38 mL), potassium
carbonate (5 mmol, 690 mg), and DMF (25 mL) as a solvent were
placed in a round-bottom flask equipped with a condenser and a
magnetic stirrer, and the resulting mixture was stirred at 100 °C under
nitrogen for 12 h. 50 mL of water were added, and the product was
extracted with dichloromethane (3 × 50 mL); then solvent was
evaporated and methanol (10 mL) was added. Precipitate was filtered
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1
out, washed with cold methanol, and dried. Yield = 96%. H NMR
(CDCl3): δ (ppm) = 8.71 (s, 1H), 7.74 (d, 2H, J = 8.82 Hz), 7.39 (dd,
1H, J= 5.09, 1.00 Hz), 7.30 (dd, 1H, J = 3.60, 1.00 Hz), 7.15 (dd, 1H, J
= 5.09, 3.60 Hz), 6.99 (d, 2H, J = 8.83 Hz), 4.01 (t, 2H, J = 6.70 Hz),
3.17 (td, 4H, J = 7.25, 2.74 Hz), 2.12 (m, 2H), 1.81 (m, 2H), 1.56−0.8
(m, 21H). Calculated for C30H39NOS, %: C, 78.04; H, 8.51; N, 3.03.
Found, %: C, 77.93; H, 8.38; N, 3.19.
[Pt(L-2)(acac)], HL-2 = 2-(4-Dodecyloxyphenyl)-5-(thienyl-2)-
c-cyclopenthylpyridine. Ground powder of K2[PtCl4] (0.67 mmol,
280 mg) was added to a stirring solution of HL-2 (0.67 mmol, 309
mg) in acetic acid (30 mL). The mixture was heated at reflux under
nitrogen for 24 h. The precipitated solid representing the dimeric
complex [Pt2(μ-Cl)2(L-2)2] was filtered out, washed with acetic acid
(5 mL) and ethanol (5 mL), and dried under reduced pressure. The
resulting product and sodium acetylacetonate (10 equiv) were heated
at reflux in acetone for 48 h. The solvent was removed under reduced
pressure, and the title complex was purified by column chromatog-
raphy (silica gel, dichloromethane). Yield = 54%. 1H NMR (CDCl3): δ
(ppm) = 9.09 (s, 1H), 7.50 (d, 2H, J = 8.50 Hz), 7.40 (dd, 1H, J =
5.16, 1.11 Hz), 7.29 (dd, 1H, J = 3.80, 1.20 Hz), 7.18 (d, 1H, 2.75 Hz),
7.15 (dd, 2H, J = 5.18, 3.68 Hz), 4.08 (t, 2H, J = 6.60 Hz), 3.37 (t, 2H,
J = 7.77 Hz), 3.19 (t, 2H, J = 7.62 Hz), 2.2 (m, 2H), 2.01 (d, 6H, J =
2.14 Hz), 1.81 (m, 2H), 1.6−0.8 (m, 21H). Calculated for
C35H45NO3PtS, %: C, 55.69; H, 6.01; N, 1.86. Found, %: C, 55.77;
H, 5.88; N, 1.80. Mass: Positive-ion LIFDI-MS: m/z 754.26 M+, m/z
755.26 [M + H]+, m/z 1508.53 [2M]+.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
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AUTHOR INFORMATION
■
Corresponding Authors
(15) Cho, Y.-J.; Kim, S.-Y.; Son, H.-J.; Cho, D. W.; Kang, S. O. Steric
effect on excimer formation in planar Pt(II) complexes. Phys. Chem.
Chem. Phys. 2017, 19, 5486.
* E-mail: shafikoff@gmail.com (M.Z.S.).
* E-mail: burkhard.koenig@chemie.uni-regensburg.de (B.K.).
ORCID
(16) Kobayashi, K.; Sato, H.; Kishi, S.; Kato, M.; Ishizaka, S.;
Marsel Z. Shafikov: 0000-0003-0495-0364
Kitamura, N.; Yamagishi, A. Spectroscopic Evidence for Pt−Pt
J
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