Photoluminescent Phosphinine Cu(I) Halide Complexes: Temperature

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Photoluminescent Phosphinine Cu(I) Halide Complexes: Temperature Dependence of the Photophysical Properties and Applications as a Molecular Thermometer Yaqi Li, Zhongshu Li,* Yuanfeng Hou, Ya-Nan Fan, and Cheng-Yong Su* Lehn Institute of Functional Materials (LIFM), School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China

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S Supporting Information *

ABSTRACT: Luminescent thermometers have attracted much attention, because of their fast response, high sensitivity, and noninvasive operation, relative to other traditional thermometers. The extensive studies on the temperature-dependent luminescent properties of Cu(I) complexes make this low-cost metal source a promising candidate as a component of thermometers. Herein, we prepared three luminescent phosphinine Cu(I) complexes whose emission lifetimes are precisely dependent on the temperature variations. For practical utilization, sensor films have been fabricated by doping these Cu(I) complexes into the matrices of polyacrylamide. These films not only exhibit excellent linear correlations between the temperature and emission lifetime over the wide range of 77−337 K, but also show high sensitivity (with the best one to −6.99 μs K−1). These are essential factors for the application in luminescent molecular thermometers. Moreover, the emission mechanism for these Cu(I) complexes are rationalized by the combination of experimental and theoretical results.

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steric hindrance at the metal center will interrupt the flattening distortion of the emissive metal−ligand charge transfer (MLCT) state to lead an increase in lifetime.9 For example, the lifetime of a more steric hindered Cu(I) complex B (5 μs) (see Scheme 1) is almost 55 times higher than that of A (0.09 μs).10 Meanwhile, when a strong π-acceptor ligand, such as an isocyanide group, was applied in the Cu(I) complex C (see Scheme 1), the lifetime exceptionally increased to 1204 μs.11 In comparison to pyridines, phosphinines usually possess an energetically low-lying lowest unoccupied molecular orbital (LUMO) and a highest occupied molecular orbital (HOMO) that is a π-type orbital delocalized over the phosphinine ring.12 Consequently, phosphinines normally act as weak σ-donor but relatively strong π-acceptor ligands, which can efficiently stabilize metal centers in low oxidation states.12a Thus, replacing the pyridines with phosphinines on Cu(I) complexes is expected to increase their luminescent lifetimes. Indeed, ∼20 years after Ford’s report on a Cu(I) cluster D (see Scheme 1) with a lifetime of 0.54 μs,13a Müller, Steffen, and co-workers succeeded in replacing the pyridine derivative with phosphinine (E; see Scheme 1), which resulted in a much longer lifetime of 171 μs.13b In addition, the lifetime of E at 77 K was found to be 1516 μs. This work indicates that the phosphinine-based Cu(I) complexes could be excellent candidates for luminescent thermometer materials. Unfortunately, few examples are known to explore

INTRODUCTION Temperature-dependent luminescence is a common phenomenon that has been utilized to develop luminescent temperature sensors.1 A large group of molecular temperature sensors are luminescent metal−ligand complexes, which are mainly made of noble-metal ions, including Ru,2a,b Ir,2c Pt,2d,e and rare-earth ions such as Eu and Tb.2f,h However, with the rapid development of temperature sensors, the production cost becomes a greater concern. Therefore, it is desired to find an alternative strategy based on less-expensive materials. In this regard, luminescent copper-based complexes appear as promising molecular sensor materials and have recently attracted significant research activities.3 To date, luminescent mononuclear and multinuclear Cu(I) complexes have been extensively studied.4 However, utilization of Cu(I) complexes as potential thermometers is still in its infancy. Since measurement of the lifetime requires sophisticated and expensive techniques, indicators must possess lifetimes that are sufficiently long (preferably above microsecond scale), then the fast and low-cost methods for lifetime determination can be applied.2h,5 However, the luminescent Cu(I) complexes normally suffer from relatively short luminescent lifetimes.3 Rare examples of Cu(I) complexes have been utilized as lifetime-based molecular thermometers,6 although some other Cu(I) complexes have show potential for use in molecular thermometers.7,8 According to previous systematic studies, two main factors of the ligands in metal complexesnamely, the steric and the electronic propertieswill affect their luminescent lifetime. A large © XXXX American Chemical Society

Received: June 22, 2018

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DOI: 10.1021/acs.inorgchem.8b01732 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Selected Luminescent Cu(I) Complexes with Lifetime Value Measured at Room Temperature

Scheme 2. Synthesis of Phosphinine Ligand 3 and Dimeric Copper Complexes 4−6

weak or broad NMR spectra were obtained (see Figures S7−S10 in the Supporting Information). Single-crystal X-ray diffraction (XRD) was used to unambiguously identify the molecular constitution of complexes 4−6 (see Figure 1). The two Cu(I) centers are bridged by two halogen ligands to form a binuclear structure with a four-membered Cu2X2 rings for all complexes, for which similar skeletons have been reported.16a−d The Cu−Cu distances (4, 3.099 Å; 5, 3.159 Å; 6, 3.166 Å) observed in these complexes are much longer than their van der Waals radius of 2.8 Å, which indicates no substantial interactions between two Cu(I) centers. We observed that all Cu−Pphosphinine bond lengths (2.29−2.32 Å) are generally longer than the bond to triphenylphosphine ligand (2.22−2.24 Å). Note that the phosphinine bridging in complex F is not present in complexes 4−6, because of the cumbersome appended unit at the phosphinine ring. To satisfy a tetrahedron coordination environment of the Cu(I) center in complexes 4−6, the halide bridging Cu2X2 four-membered cycle is thus obtained. All these complexes are isostructural and crystallized in the orthorhombic space group Pbca. The crystal packings are attributed primarily from the π−π stacking and van der Waals interactions. XRD analysis confirms the successful implementing of bulky environment to the metal center. In addition, no obvious diffraction changes are observed from temperature-dependent powder X-ray diffraction (PXRD) analyses from 25 °C to 225 °C for complexes (see Figures S11−S13 in the Supporting Information). Optical Properties. The UV-vis absorption spectrum of ligand 3 is characterized by a broad absorption at λ = 301 nm and a shoulder at 346 nm (see Figure 2). The phosphinines usually possess an energetically low-lying LUMO and a π-type HOMO,12 the second occupied orbital (HOMO−1) would mainly consist of the nonbonding electron pairs on both of the P atoms. Thus, the absorptions of 3 are assigned to the contributions of π(phosphinine) → π* and n(phosphinine-P) → π* transitions with λ= 346 nm; as well as π(phenyl groups) → π* and n(Ph2P) → π* transitions with λ = 301 nm. The UV-vis absorption spectra of complexes 4−6 in dichloromethane (DCM) solution exhibit mainly two broad bands (see Figure 2): one major band with a maximum at 322 nm for 4, 328 nm for 5, 332 nm for 6, and a shoulder at longer wavelengths of ∼410 nm for 4−6. The high-energy absorptions are assigned to the overlapping π(phosphinine) → π*, while the low energy absorptions

phosphorescence of phosphinine-based Cu(I) complexes. Last year, our group developed a second type of Cu(I) complex (F; see Scheme 1), which exhibits orange to red light, based on a chelating phosphinito phosphinine ligand with an average lifetime of 23 μs in the solid state at room temperature.13c We reasoned that, by introducing steric bulky substituents on the phosphinine ligand, the resulting Cu(I) complexes would show dramatically increased lifetimes. Herein, we report the synthesis and photophysical properties of a series of Cu(I) complexes 4−6 (see Scheme 2) with chelating phosphinito phosphinine ligand 3 containing steric bulky substituents, which show a much longer lifetime at room temperature, compared to Cu(I) complexes of F (Scheme 1). By lowering the temperature from 297 K to 77 K, the emission lifetime increases rapidly. Moreover, after incorporation of the complexes into polyacrylamide (PAN) thin film, an excellent linear relationship between the lifetime and temperature with a large slope over a long temperature range is established. These results show, for the first time, that phosphinine Cu(I) complexes are valuable materials for use in high-performance molecular thermometers.

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RESULTS AND DISCUSSION Following our previously reported method, with a slight modification,14 sodium phosphaethynolate (Na(OCP))15 reacts with 1 equiv of 4,6-diphenyl-2-pyrone 1 in tetrahydrofuran (THF) to afford sodium 1,3-diphenyl phosphaphenolate 2 as yellowish solid with chemical shift centered at 141 ppm in 31P NMR spectrum (see Scheme 2). Compound 2 reacts further with 1 equiv of diphenylphosphine chloride (Ph2PCl) to yield chelating phosphinito phosphinine ligand 3 as a yellow oil, almost quantitatively. In the 31P NMR spectrum, two doublet resonances at 144 and 114 ppm with coupling constant 3JPP = 113.3 Hz is observed for ligand 3 (see Scheme 2), indicating the successful building of the O−P bond. When mixed with copper halide, the chemical shift of ligand 3 in the 31P NMR spectrum diminishes gradually within 2 h. After workup, the yellowish Cu(I) complexes [X = Cl (4), Br (5), I (6)] were obtained almost quantitatively (see Scheme 2), which are poorly soluble in common organic solvents, such as THF, toluene, CH2Cl2, dimethyl formamide (DMF), or dimethyl sulfoxide (DMSO); therefore, only B


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Inorganic Chemistry

well as Figures S14 and S15 in the Supporting Information), suggesting the charge-transfer character of the emissive excited states.16a,e−h It is consistent with experimental results that the rather long Cu−Cu distance (3.099 Å for 4, 3.159 Å for 5, 3.166 Å for 6) rules out the presence of a cluster-centered (CC) state.4c,e,16a,b By lowering the temperature from 297 K to 77 K, the emission intensity increases dramatically. In the meantime, the emission maxima are gradually blue-shifted from 692 nm to 673 nm for 4, from 690 nm to 684 nm for 5, and from 684 nm to 667 nm for 6 (see Table 1). These results imply the presence of two very close emission centers, which are thermally equilibrated. It is interesting to note that the luminescent lifetime also increases dramatically as the temperature decreases. At 297 K, the average lifetimes were measured to be 144.4, 266.6, and 63.8 μs for complexes 4−6, respectively, while they correspondingly reached 2317.9, 1868.4, and 388.4 μs at 77 K (see Figure 3c, as well as Figures S14c and S15c, and Table 1). It is noticeable the lifetimes of complexes 4−6 are much longer than our previously reported nonsubstituted phosphinine Cu(I)-complexes (τ297 = 20−25 μs),13c verifying that the steric hindrance on the phosphinine would result in longer lifetimes of related Cu(I) complexes. It is supported that the Arrheninus-type model (eq 1) can be used to describe the temperature-dependent luminescent lifetime:17 ÄÅ É−1 ÅÅ ij ΔE yzÑÑÑÑ Å zzÑÑ τ = ÅÅk 0 + k1 expjj− ÅÅÇ k RT {ÑÑÖ

Figure 1. Molecular structures of 4−6 in the solid state (H atoms were omitted for clarity; 50% probability thermal ellipsoids). Selected distances for 4, X = Cl: Cu1−Cu1′, 3.099 Å; Cl1−Cl1′, 3.492 Å; Cu1− Cl1, 2.3172(5) Å; Cu1−Cl1′, 2.3516(5) Å; Cu1−P1, 2.3154(5) Å; Cu1−P2, 2.2204(5) Å. Selected distances for 5, X = Br: Cu1−Cu1′, 3.159 Å; Br1−Br1′, 3.761 Å; Cu1−Br1, 2.4767(3) Å; Cu1−Br1′, 2.4349(3) Å; Cu1−P1, 2.3008(5) Å; Cu1−P2, 2.2315(5) Å. Selected distances for 6, X = I: Cu1−Cu1′, 3.166 Å; I1−I1′, 4.150 Å; Cu1−I1, 2.6357(4) Å; Cu1−I1′, 2.5841(5) Å; Cu1−P1, 2.2942(7) Å; Cu1−P2, 2.2442(7) Å.

(1)

where τ is the lifetime, k0 the temperature-independent decay rate for the excited-state deactivation, k1 a pre-exponential factor, R the gas constant, ΔE the energy gap to an upper excited state above the emitting level, and T the absolute temperature. The temperature dependence of the luminescent lifetime of complexes 4, 5, and 6 can be well fit (correlation coefficient R2 ≈ 0.994, 0.994, and 0.996, respectively) with the following parameters: k0 = 426.9, 549.5, and 2658.7 s−1, respectively, k1 = 27460.0, 32216.4, and 284623.7 s−1, respectively, and ΔE = 4.8, 6.3, and 7.7 kJ mol−1, respectively. For comparison, the temperature-dependent luminescent properties of ligand 3 were also investigated systematically (see Figures 3d−f). The emission maximum of the ligand 3 (λmax = 684 nm) is comparable with those of complexes 4−6 (λmax = 684−692 nm) (see Table 1), implying that the luminescence of the Cu(I) complexes may essentially correlate with the ligand.16c,18 Nevertheless, the luminescent lifetime of 3 is 4.8 μs at 297 K. With decreases in temperature, the luminescent lifetime of 3 increases slightly, by 0.6 μs, at 77 K, reaching a value of 5.4 μs (see Table 1). The lifetimes of 3 both at room temperature and at low temperature are enormously less than that of the related Cu(I) complexes 4−6 (see Table 1). Furthermore, the excitation spectra of 3 are essentially distinguished from that of complexes 4−6 at different temperature (see Figures 3a and 3d). There are mainly two photoluminescent mechanisms for the extensively studied copper complexes. One is attributed to the thermal population of CC and MLCT state, which is normally responsible for thermochromic luminescence.4b,c,8a−d,f−h This mechanism is already ruled out as the reason for long Cu−Cu distances. The second one is attributed to the thermal population of singlet MLCT (1MLCT) and triplet MLCT (3MLCT), which is known as thermally activated delayed fluorescence (TADF) (see Figure 4a).4e,16c,19 At low temperature, the emission from the triplet excited state T1 to the ground state S0 is

Figure 2. UV-vis absorption spectra of ligand 3 and complexes 4−6 in dichloromethane (DCM) solution.

are assigned to the n(X) → π* transitions. All the spectra of ligand and complexes are red-shifted, with respect to those of relevant Cu(I) phosphine complex F (see Scheme 1) reported previously.13c Like other reported phosphinine Cu(I) complexes,13b-13c 4−6 exhibit an intense orange luminescence when irradiated with UV light in the solid state at room temperature. The emission spectra of 4−6 (λmax = 692 nm for 4; λmax = 690 nm for 5; λmax = 684 nm for 6) in the solid state after laser excitation at 410 nm are broad without vibronic progressions (see Figure 3, as C


Article

Inorganic Chemistry

Figure 3. Photoluminescent properties in the solid state for (a−c) complex 4 and (d−f) ligand 3. Panels (a) and (d) show temperature-dependent excitation spectra; panels (b) and (e) show temperature-dependent emission spectra; and panels (c) and (f) show plots of temperature-dependent emission lifetime and Arrhenius-type model fitting.

Table 1. Photoluminescent Data at Different Temperatures of Compounds 3−6, Free Film PAN, and Films 4−6@PAN λex [nm] 3 4 5 6 PAN 4@PAN 5@PAN 6@PAN

λem [nm]

τ [μs]

297 K

77 K

297 K

77 K

387 460 461 461 370 367 365 364

381 432 409 423 378 366 363 360

684 692 690 684 660 615 613 627

698 673 684 667 698 590 598 609

Φ [%]

337 K

297 K

77 K

297 K

1710.5 1438.9 1460.3

4.8 144.4 266.6 63.8 8.5 2027.3 1700.7 1612.5

5.4 2317.9 1868.4 388.4 10.1 3538.9 2632.6 2423.3

1.1 4.1 1.8 0.1 2.5 1.9 0.8

(II) The lifetime increased dramatically. It is due to the increased contribution to the total emission from T1 → S0 transition, which is spin-forbidden and thus produces light with longer lifetime.16c,19 (III) The emission intensity decreased to some extend. Because emission rate of S1 → S0 transition is generally higher than that of T1 → S0 transition.10,20 After a detailed analysis of the photoluminescent properties of our complexes, we also observe three common features when the temperature decreases from 297 K to 77 K (Table 1): Figure 4. Simplified temperature-dependent energy level diagram.

(I) The emission maximum is blue-shifted by 19, 6, and 17 nm for complexes 4, 5, and 6, respectively. (II) The lifetime increased dramatically by 2173.5, 1601.8, and 324.6 μs for complexes 4, 5, and 6, respectively. (III) The emission intensity increased apparently by more than an order of magnitude for all studied complexes.

dominant. With increasing temperature, the emission from singlet excited state S1 to the ground state S0 can be thermally activated. Thus, at room temperature, the emission of S1 → S0 is dominant. When the temperature decreases, the TADF mechanism is normally responsible for three characteristic features: (I) The emission maximum is red-shifted.10,20 This is caused by a relatively more stable T1 state, in comparison to the S1 state, resulting an emission light with smaller energy from transition of T1 → S0, relative to the transition of S1 → S0.

These phenomena are in sharp contrast to that of the extensively applied TADF mechanism, although the relationship of lifetime and temperature is similar between our observation and the cases which the TADF mechanism is applied. We arbitrarily D


Article

Inorganic Chemistry

Figure 5. Molecular orbitals of 5: (a) LUMO, (b) HOMO (isovalue = 0.04) of the optimized ground state, and (c) spin density distribution of optimized T1 state (isovalue = 0.005).

assign the emission of complexes 4−6 solely from the 3MLCT state, based on the observations of very long lifetimes, from 297 K to 77 K and rather low quantum yield at room temperature ( I−).27 Obviously, the luminescent 3XMLCT excited state is affected by the nature of the halogen ions directly bound to the Cu atoms. Note that the emission intensities for these films are also linearly correlated with the temperatures changes (see Figure 7c, as well as Figures S16 and S17), which can serve as the second temperature sensing output. When fresh powders are exposed in air, the quenching rates of the pristine samples of complexes 4, 5, and 6 are 23%, 25%, and 30%, respectively. In contrast, when the three sensor films are exposed to air for weeks, no obvious changes were detected for their emission lifetimes, verifying the good stability of these films. Moreover, we tested the reliability of these sensor films by applying them for real temperature measurements and found that the emission lifetimes at the tested temperature agree well with the linear fitting function with only small relative standard deviations of 6.07 μs for 4@PAN, 3.73 μs for 5@PAN, and 5.61 μs for 6@PAN. When the indicators cannot be wellprotected from the air, a completely different emission spectra were observed, and the similar temperature-dependent photophysical properties cannot be observed (for instance, 4@PAN vs 4@PMMA; see Figure S19 in the Supporting Information) The unique photoluminescence properties of the present complexes, including long lifetimes in a wide range of temperature, precise temperature dependence of emission lifetime and intensity, and

a concentration quenching effect, which is likely caused by the nonradiative transfer of energy from an excited molecule to another unexcited molecule.26 We noticed that the sensitivity of the sensor films decreases from −6.99 μs K−1 to −1.54 μs K−1 as the indicator concentration increases from 0.9 wt % to 26.7 wt %, as elucidated in Table 2. Furthermore, the emission lifetime of film 4@PAN reaches 2027.3 μs at 297 K with a loading of 4 (0.5 mg, first run in Table 2). Thus, the sensor film 4@PAN with concentration of 0.9 wt % was prepared and studied in detail below. In comparison, the films with 0.5 mg loadings of complexes 5 or 6 as indicators in 55.0 mg of PAN (0.9 wt %) were prepared, offering sensor films 5@PAN or 6@PAN. All these fabricated sensor films are optically transparent and colorless under daylight. The thickness of the sensor film is ∼180 nm, and it is quite homogeneous, as observed from SEM measurements (see Figure 6). A relatively strong orange emission light is observed under irradiation with UV light (∼365 nm), which is remarkably enhanced when the temperature decreases to 77 K. To show their potentials in practical application, we evaluate the photoluminescence property in a wide temperature range from 77 K to 337 K, which is also the temperature limit of our instrument. The excitation maximum centers at ∼370 nm at different temperatures for all of these complexes (see Figure 7a and Table 1). The emission spectra recorded for these films show broad without vibronic progressions, similar to these pure F


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Inorganic Chemistry

Figure 6. (a) Sensor film 4@PAN on glass slide under daylight, under UV at room temperature, and under UV at 77 K are shown (top of figure); (b) SEM photomicrograph from the side view of sensor film 4@PAN.

Figure 7. (a) Temperature-dependent excitation spectra, (b) temperature-dependent emission spectra, (c) temperature-dependent correlation of emission intensity with linear fitting results, and (d) temperature-dependent correlation of emission lifetime with Arrhenius-type model fitting and linear fitting results of 4@PAN.

Table 3. Photoluminescence Data at Different Temperatures of Films 4−6@PAN λex [nm] 4@PAN 5@PAN 6@PAN

λem [nm]

τ [μs]

337 K

77 K

337 K

77 K

337 K

77 K

S [μs K−1]

S [K−1]

R2

364 362 355

366 363 360

632 617 630

590 598 609

1710.5 1438.9 1460.3

3538.9 2632.6 2423.3

−6.99 −4.46 −3.71

−0.20% −0.17% −0.15%

0.997 0.996 0.997

G


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Inorganic Chemistry

UV-vis: λ1 = 341 nm (ε = 5803 Μ−1 cm−1), λ2 = 407 nm (ε = 2424 Μ−1 cm−1). IR (ATR, [cm−1]): 3025 (w, C−H str.), 1975 (w), 1945 (w), 1686 (w), 1600 (w), 1574 (w), 1555, 1490, 1470, 1433 (w), 1369 (m), 1344 (m), 1291 (w), 1238 (w), 1220 (w), 1130 (s, C−O str.), 1098 (m), 1077 (w), 1050 (w), 1026 (w), 999 (w), 953 (s), 909 (w), 861 (m), 813 (w), 788 (w), 756 (s), 689 (s), 648 (m), 618 (w), 574 (m), 524 (w), 473 (s), 439 (m). Preparation of 3. Chlorodiphenyl phosphine (308 mg, 1.40 mmol) were added to a stirred solution of 2 (400 mg, 1.40 mmol) in toluene (10 mL). After 4 h stirring, the precipitate was removed by filtration, and the residue was washed with another portion of toluene (5 mL). The filtrate was dried under reduced pressure to yield 3 as a yellow oil (608 mg, 1.35 mmol, 97% yield). 1H NMR (C6D6, 400 MHz): δ = 7.86 (m, 2 H, C5), 7.73 (m, 4 H, Carom), 7.53 (d, 2 H, JPH = 7.5 Hz, Carom), 7.34 (d, 2 H, JPH = 7.5 Hz, Carom), 7.10 (m,12 H, Carom); 13C{1H}NMR (C6D6, 100.5 MHz): δ = 192.2 (dd, 1JPC = 47.5 Hz, PCOP; 2JPC = 12.9 Hz, PCOP), 170.0(dd, 1JPC = 50.5 Hz, CPCOP; 4JPC = 8.9 Hz, CPCOP), 145.6, 143.6, 140.6, 140.5, 131.0, 129.8, 128.6, 127.9, 127.7, 127.4, 123.5; 31P{1H} NMR (C6D6, 161.9 MHz) δ = 144 (d, 3JPP = 113.3 Hz, phosphinine - P), 114 (d, 3JPP = 113.3 Hz, diphenylphosphine - P). UV-vis: λ1 = 301 nm (ε = 14773 Μ−1 cm−1), λ2 = 346 nm (ε = 2827 Μ−1 cm−1). IR (ATR, [cm−1]): 3057 (w, C−H str.), 3020 (m), 2967 (m), 2853 (w), 2104 (w), 1949 (w), 1887 (w), 1804 (w), 1729 (m), 1623 (w), 1571 (s), 1483 (ss), 1432 (s), 1373 (s), 1330 (w), 1304 (w), 1263 (s), 1180 (w), 1125 (s, C−O str.), 1093 (s), 1026 (s), 1000 (s), 952 (s), 914 (m), 864 (s), 783 (s), 744 (ss), 689 (s), 652, 633, 619, 575, 551, 495 (s), 436. Preparation of Complex 4. Cu(I) chloride (30.7 mg, 0.31 mmol) was added to a solution of 3 (140 mg, 0.31 mmol) in THF (5 mL). After 2 h stirring, the precipitate was collected by filtration and washed with another portion of THF (5 mL) and hexane (10 mL). Drying the residue in vacuo afforded 4 as a yellow powder (142.0 mg, 0.13 mmol, 86% yield). The crystals of the complex 4 was obtained from saturated solution in CH2Cl2 via slow evaporation. Elemental analysis (%): Calcd for C58H44P4O2Cu2Cl2: C, 63.63%; H, 4.05%; Found: C, 63.37%; H, 3.97%. UV-vis: λ1 = 322 nm, λ2 = 410 nm. IR (ATR, [cm−1]): 3058 (w, C−H str.), 1966 (w), 1900 (w), 1822 (w), 1777 (w), 1571 (m), 1530, 1493, 1475, 1450 (m, −C6H5), 1437 (m), 1384 (m), 1353 (w), 1335 (w), 1312 (w), 1273 (w), 1251 (w), 1183 (w), 1139 (s, C−O str.), 1108 (s), 1079 (m), 1027 (w), 999 (w), 957 (s), 915 (w), 898 (w), 883 (m), 871 (m), 794 (m), 759 (s), 742 (m), 727 (m), 703 (s), 690 (ss). Preparation of Complex 5. Cu(I) bromide (18.6 mg, 0.13 mmol) was added to a solution of 3 (58.0 mg, 0.13 mmol) in THF (5 mL). After 2 h of stirring, the precipitate was collected by filtration and washed with another portion of THF (5 mL) and hexane (10 mL). Drying the residue in vacuo afforded 5 as a yellow powder (65.0 mg, 0.06 mmol, 85% yield). The crystals of the complex 5 was obtained from saturated solution in CH2Cl2 via slow evaporation. Elemental analysis (%): Calcd for C58H44P4O2Cu2Br2: C, 58.85%; H, 3.75%; Found: C, 58.82%; H, 3.88%. UV-vis: λ1 = 328 nm, λ2 = 410 nm. IR (ATR, [cm−1]): 3057 (w, C−H str.), 1965 (w), 1900 (w), 1814 (w), 1777 (w), 1570 (m), 1531, 1492, 1472, 1450 (m, −C6H5), 1436 (m), 1383 (m), 1353 (w), 1334 (w), 1312 (w), 1272 (w), 1249 (w), 1184 (w), 1138 (s, C−O str.), 1107 (s), 1079 (m), 1027 (w), 999 (w), 957 (s), 915 (w), 898 (w), 883 (m), 871 (m), 794 (m), 759 (s), 742 (m), 727 (m), 704 (s), 690 (ss). Preparation of Complex 6. Cu(I) iodide (13.6 mg, 0.07 mmol) was added to a solution of 3 (32.0 mg, 0.07 mmol) in THF (5 mL). After 2 h of stirring, the precipitate was collected by filtration and washed with another portion of THF (5 mL) and hexane (10 mL). Drying the residue in vacuo afforded 6 as a yellow powder (40.0 mg, 0.03 mmol, 88% yield). The crystals of the complex 6 was obtained from saturated solution in CH2Cl2 via slow evaporation. Elemental analysis (%): Calcd for C58H44P4O2Cu2I2: C, 54.52%; H, 3.47%; Found: C, 54.21%; H, 3.45%. UV-vis: λ1 = 332 nm, λ2 = 410 nm. IR (ATR, [cm−1]): 3054 (w, C−H str.), 1956 (w), 1900 (w), 1817 (w), 1776 (w), 1570 (m), 1532, 1492, 1471, 1450 (m, −C6H5), 1436 (m), 1381 (m), 1353 (w), 1334 (w), 1313 (w), 1271 (w), 1248 (w), 1185 (w), 1135 (s, C−O str.), 1107 (s), 1079 (m), 1068 (w), 1028 (w), 999 (w), 957 (s), 917 (w), 899 (w), 883 (m), 871 (m), 791 (m), 759 (s), 742 (m), 727 (m), 701 (s), 690 (ss).

high stability, make these complexes adequate material candidates for luminescent thermometers.

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CONCLUSION In summary, a series of Cu(I) complexes with novel steric crowded substitute bidentate phosphinito phosphinine ligands are prepared and fully characterized. These complexes show a strong orange color emission at maxima wavelengths of ∼690 nm in the solid state after laser excitation at 410 nm. The single broad emission bands without vibronic progressions for all three complexes suggest the charge-transfer character of the emissive excited states. These bands are attributed to the XMLCT character, according to theoretical calculations and experimental results. The emission lifetimes are highly temperature-dependent, making them potential candidates for use in luminescent thermometers. Complexes 4−6 were incorporated into gasblocking polymer PAN to afford temperature sensing films. The lifetimes of the resulting films are in the microsecond range and the lifetime sensitivities are measured to be −6.99, − 4.46, and −3.71 μs K−1 for 4@PAN, 5@PAN, and 6@PAN films, respectively, over an unprecedentedly large temperature range from 337 K to 77 K. The lifetime sensitivities of our thermometer are only lower than those of the Tb-based thermometers (−7.8 μs K−1 to −13.8 μs K−1),2h but are higher than most of these reported examples (see Table S18 in the Supporting Information).2a,c,5,28 Meanwhile, the suitable temperature range for these Tb-based thermometers is limited to 273−343 K. The high performance of these molecule thermometers and the low-cost copper source will make these materials valuable candidate in real applications.

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EXPERIMENTAL SECTION

General Procedures and Materials. All manipulations were performed under an inert atmosphere of dry nitrogen, using standard Schlenk techniques and glovebox. Dry, oxygen-free solvents were employed, unless otherwise mentioned. Sodium phosphaethynolate was prepared by following literature procedures,15 while all other starting materials were purchased from commercial sources. Nuclear magnetic resonance (NMR) spectra were recorded on Bruker Avance 400 MHz spectrometers. All spectra were obtained in the solvent indicated at 25 °C. The chemical shifts (δ) were measured according to IUPAC and expressed in ppm, relative to SiMe4 (1H, 13C), and 85% H3PO4 (31P). Coupling constants J are reported in Hertz [Hz] as absolute values. Single-crystal XRD data were collected on a Agilent Technologies SuperNova X-RAY diffractometer system that was equipped with a sealed copper tube (λ = 1.54178 nm) at 40 kV and 30 mA. UV-vis absorption spectra were recorded using a Shimadzu, Model UV-2450 spectrophotometer. IR spectra were obtained on a PerkinElmer Model Spectrum ATR 2000 FT-IR-Raman spectrometer with a KBr beam splitter (range = 500−4000 cm−1). Photoluminescence spectra and lifetime data were collected on an Edinburgh Model FLS980 spectrophotometer. Scanning electron microscopy images of the film were obtained via ultrahigh-resolution field-emission scanning electron microscopy (FE-SEM) (Model SU8010, Hitachi). Preparation of 2. 4,6-Diphenyl-2-pyrone (2.0 g, 8.0 mmol), sodium phosphaethynolate ([Na(OCP)·(dioxane)2.5], 2.90 g, 9.6 mmol) and THF (50 mL) were successively added into a Teflon sealed flask. After 24 h of stirring at 90 °C, the precipitate was filtered off and the filtrate was dried under reduced pressure. The remaining solid then was extracted with diethyl ether and washed with hexane. Drying the residue in vacuo afforded 2 as a yellowish powder (1.69 g, 5.90 mmol, 73% yield). Melting point (mp) = 185 °C. 1H NMR (CD3CN, 400 MHz): δ = 7.65 (d, 4 H, JPH = 7.6 Hz, C5/Carom), 7.37 (t, 2 H, Carom), 7.29 (m, 5 H, Carom), 7.00 (d, 1 H, JPH = 4.0 Hz, Carom); 13C{1H}NMR (CD3CN, 100.5 MHz): δ = 209.5 (d, 1JPC = 43.7 Hz, PCO), 171.5 (d, 1JPC = 53.8 Hz, CPCO), 145.8, 145.2, 144.2, 128.8, 127.6, 127.3, 127.1, 122.2, 119.2. 31P{1H} NMR (CD3CN, 161.9 MHz) δ = 140.5. H


Article

Inorganic Chemistry Preparation of Film 4@PAN. Complex 4 (0.5, 2.5, 5.0, 10.0, 15.0, and 20.0 mg) and PAN (55.0 mg) were mixed in DMF (1 mL). After 8 h of stirring, this “cocktail” was spin-coated onto a glass slide. The coated glass slides then were dried at 100 °C for 4.5 h to afford 4@PAN film. Preparation of Film 5 or 6@PAN. Complex 5 or 6 (0.5 mg) and PAN (55.0 mg) were mixed in DMF (1 mL). After 8 h of stirring, this “cocktail” was spin-coated onto a glass slide. The coated glass slides then were dried at 100 °C for 4.5 h to afford 5 or 6@PAN film. Preparation of Blank PAN Film. PAN (55.0 mg) was dissolved in DMF (1 mL). After 8 h of stirring, this “cocktail” was spin-coated onto a glass slide. The coated glass slides then were dried at 100 °C for 4.5 h to afford blank PAN film.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01732. Synthetic details and characterization data for compounds 2−6; supplementary crystallographic and photophysical data; description of the computational protocol; supplementary computational results (PDF) Accession Codes

CCDC 1505303, 1505304, and 1829011 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z. Li). *E-mail: [email protected] (C.-Y. Su). ORCID

Zhongshu Li: 0000-0001-5599-6018 Cheng-Yong Su: 0000-0003-3604-7858 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21603280 and 21720102007), the Fundamental Research Funds for the Central Universities (No. 171gpy78).

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REFERENCES

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Photoluminescent Phosphinine Cu(I) Halide Complexes: Temperature

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