Tuning the Excimer Emission of Amphiphilic Platinum(II) Complexes

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Tuning the Excimer Emission of Amphiphilic Platinum(II) Complexes Mediated by Phospholipid Vesicles Marsel Z. Shafikov,*,†,‡ Alfiya F. Suleymanova,‡ Dmitry N. Kozhevnikov,‡ and Burkhard König*,§ †

Ural Federal University, Mira 19, Ekaterinburg, 620002, Russia I. Postovsky Institute of Organic Synthesis, Ekaterinburg, 620990, Russia § Institut für Organische Chemie, Universität Regensburg, Universitätsstraße 31, D-93053 Regensburg, Germany ‡

S Supporting Information *

ABSTRACT: Two new amphiphilic platinum(II) complexes, [Pt(2-(4fluorophenyl)-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 πorbitals (HOMO and LUMO) naturally promote stronger π−π interactions, thus obstructing the PtII−PtII interaction.

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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 © 2017 American Chemical Society

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 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 semioccupied 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 Published: April 7, 2017 4885

DOI: 10.1021/acs.inorgchem.6b03100 Inorg. Chem. 2017, 56, 4885−4897

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Inorganic Chemistry

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−L2)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 Pt2 (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. 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 → S0 (phosphorescence), of 3ππ* character. It is found that complex Pt-2 shows a lower photoluminescence 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 photoluminescence quantum yield is cooperatively defined by the radiative rate (kr), calculated as kr = ΦPL/τ, and the nonradiative 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,

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).

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 differences 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.

Figure 2. Chemical structures of complexes Pt-1 and Pt-2.

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RESULTS AND DISCUSSION 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-fluorobenzoic hydrazide for HL-1, and 2-bromo-1-(thiophen-2yl)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 2,4norbornadiene (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 4886


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Inorganic Chemistry Scheme 1. Syntheses of Phenyl-Pyridine Proligands HL-1 and HL-2 and Platinum Complex Pt-1a

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-(4methoxyphenyl)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 nonradiative 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 explains the longer lifetime of the T1 state of Pt-2, and is indicative of the less relaxed spin-forbiddenness of the T1 → S0 transition of Pt-2, compared to Pt-1. The relaxation of the spinforbiddenness 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

Figure 4. UV−vis absorption and emission spectra of complexes Pt-1 (top panel) and Pt-2 (bottom), and respective ligands HL-1 and HL2. 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 4887


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Inorganic Chemistry Table 1. Ambient Temperature Absorption and Emission Data for the Platinum(II) Complexes Pt-1 and Pt-2 absorption

emission CHCl3 degassed solution (c ≈ 10−5 M)

CHCl3 solution λ Pt-1 Pt-2

max/nm

−1

−1

(ε/M ·cm )

400(9778), 385(7966), 362(8981), 317(29932), 301(30936), 255(33495) 410(12490), 396(12854), 321(25287), 305(23677), 275(22606), 253(23743)

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 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 spinforbiddenness 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 spinforbiddenness 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

PMMA film under nitrogen

λmax/nm

τ/μs

ΦPL

λmax/nm

τ/μs

ΦPL

531, 570 556, 601

25.9 38.5

0.56 0.40

530, 565 553, 593

19.7 34.5

0.63 0.41

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-phosphocholine (DOPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) (Figure 5). The total concentration of amphiphiles in

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 temperature (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, 4888


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Inorganic Chemistry

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

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 membranes 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

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 monoexponential functions, one can obtain the Pt-1 EE decay time of about 3 μs, and ME decay time of 2.8 μs (Figure 8). 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). 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. 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

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 dependence 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.

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

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 4889


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Inorganic Chemistry

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 temperatures 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 Interactions 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, consequently. 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. 4890


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Inorganic Chemistry

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). 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 Pt1. 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 calculations 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). 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

Pt-2′ are in good agreement with the structures of Pt-1 and Pt2, respectively, found by single crystal X-ray diffraction analysis (Table 2). 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 Pt2

Pt−C1 Pt−N1 Pt−O1 Pt−O2 C1−Pt−N C1−Pt−O1 C1−Pt−O2 N−Pt−O1 N−Pt−O2 O1−Pt−O2

Pt-1′

Pt-1

Pt-2′

Pt-2

Calc.

Found

Calc.

Found

Bond length (Å) 1.970(3) 1.993(2) 2.0848(19) 2.008(2) Angle (deg) 81.47 81.59(11) 175.11 173.32(10) 94.0902 93.97(10) 93.6505 92.27(9) 175.56 175.56(8) 90.80 92.17(8) 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)

81.22 174.977 94.57 93.77 175.77 90.45

81.36(17) 175.18(15) 93.38(17) 93.87(14) 174.71(14) 91.40(13)

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).

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. 4891


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Inorganic Chemistry

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. 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 larger [Pt−Pt excimer − Pt−Pt dimer] energy gap of Pt2, 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 destabilizes 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

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.

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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 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 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.

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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 4892


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Inorganic Chemistry

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-Fluorophenyl)-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). 2-(4-Fluorophenyl)-5-(4-dodecyloxyphenyl)pyridine. 2-(4Fluorophenyl)-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 filtered out, washed with methanol, and dried. Yield = 91%. 1H 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-(4dodecyloxyphenyl)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 chromatography (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]+·. 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). 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, dichloromethane). Yield = 67%. 1H 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),

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. 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. 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 Phospholipid 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 Synthesis and Structural Characterization of the Represented Compounds. 1H 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-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-Fluorophenyl)-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, 4893


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Inorganic Chemistry Funding

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). 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 dried. Yield = 95%. 1H NMR (DMSO-d6): δ (ppm) = 10.18 (s, 1H), 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). 2-(4-Dodecyloxyphenyl)-5-(thienyl-2)-c-cyclopenthylpyridine. 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 out, washed with cold methanol, and dried. Yield = 96%. 1H 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 chromatography (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]+.

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M.Z.S. is grateful to the German Academic Exchange Service (Deutscher Akademischer Austauschdienst − DAAD) for financial support. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.Z.S. is grateful to Dr. Rafał Czerwieniec (Regensburg) for discussions and critical remarks.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03100. Supporting figures, relaxed geometries of model complexes Pt-1′ and Pt-2′, and their dimers and excimers in XYZ format. (PDF) X-ray crystallographic data of Pt-1 in CIF format (CIF) X-ray crystallographic data of Pt-2 in CIF format (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

* E-mail: shafikoff@gmail.com (M.Z.S.). * E-mail: [email protected] (B.K.). ORCID

Marsel Z. Shafikov: 0000-0003-0495-0364 4894


Article

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Tuning the Excimer Emission of Amphiphilic Platinum(II) Complexes

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