Cyclometalated Platinum(II) - ACS Publications - American Chemical

Nov 2, 2016 - 276 (27 420), 342 sh (6210), 357 (7160),. 376 sh (4710). 457, 489, 519 ..... (9) Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Toche...
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Cyclometalated Platinum(II) Complexes of 1,3-Bis(1‑n‑butylpyrazol3-yl)benzenes: Synthesis, Characterization, Electrochemical, Photophysical, and Gelation Behavior Studies Yeye Ai,†,‡ Yongguang Li,*,† Huiqing Ma,† Cheng-Yong Su,† and Vivian Wing-Wah Yam*,†,‡ †

Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, P. R. China Institute of Molecular Functional Materials [Areas of Excellence Scheme, University Grants Committee (Hong Kong)] and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China

‡

S Supporting Information *

ABSTRACT: A new series of cyclometalated platinum(II) complexes of N^C^N ligands, where N^C^N = 1,3-bis(1-n-alkylpyrazol-3-yl)benzene (bpzb), namely, [Pt(bpzb)Cl] (1 and 2) and [Pt(bpzb)(CC−R)] (3−10) (R = C6H5, C6H4−OCH3-p, C6H4−NO2-p, C6H4−NH2-p, 4-cholesteryl phenyl carbamate, and cholesteryl methylcarbamate) were synthesized and characterized. Their electrochemical and photophysical properties were investigated. Two of the platinum(II) complexes were also structurally characterized by X-ray crystallography, and short intermolecular C−H···Pt contacts were observed. Vibronic-structured emission bands originating from triplet IL (3IL) excited states of the bpzb ligands with mixing of some 3MLCT [dπ(Pt)→π*(bpzb)] character were observed in solution state. Interestingly, complex 5 shows a lowenergy emission that is derived from the involvement of the p-nitrophenylethynyl ligand. Complex 9 with hydrophobic cholesteryl 4-ethynylphenyl carbamate ligand was found to form stable metallogels in several organic solvents, which are responsive to mechanical sonication and thermal stimuli and show circular dichroism activity.

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INTRODUCTION Square−planar platinum(II) complexes have attracted much attention in recent years due to their intriguing spectroscopic and photophysical behaviors arising from the diversity of the metal−ligand chromophores. Moreover, luminescence has been shown to be enhanced by introducing strong-field ancillary ligands such as phosphine and alkynyl moiety to raise the energy of the d−d states and diminishing the deactivating effect of the nonemissive ligand-field (LF) state or by extending the π-conjugation of the pincer ligands.1−5 The platinum(II) complexes with bidentate and tridentate ligands such as those containing 2,2′-bipyridine and terpyridine2−5 with square− planar geometry, which display a tendency toward forming high-ordered oligomeric structures, have been shown to exhibit intriguing spectroscopic and rich luminescence properties. On coordination to tridentate pincer ligands, such as 6-phenyl-2,2′bipyridine (C^N^N) 6,7 and 1,3-di(2-pyridyl)benzene (N^C^N),8a−c the platinum(II) complexes display excellent luminescence properties when compared to that of the 2,2′bipyridine and terpyridine complexes, due to the stronger σdonating effect of the cyclometalating ligands, which further raises the energy of the d−d excited state, effectively decreasing the probability of nonradiative pathway by the nonemissive LF state.8 On the one hand, cyclometalated platinum(II) C^N^N complexes with chloro,8e,9 phosphine,7c,d or alkynyl ligands8e have been demonstrated to show strong luminescence. On the © XXXX American Chemical Society

other hand, Williams and co-workers also reported a series of cyclometalated platinum(II) N^C^N complexes that show strong green emission originating from the 3IL [π→π* (N^C^N)] excited state.10 Apart from 6-phenyl-2,2′-bipyridine and 1,3-dipyridylbenzene as cyclometalating ligands, 2,6-bis(Nalkylbenzimidazol-2′-yl)benzene platinum(II) complexes have also been explored to fabricate organic light-emitting diodes (OLEDs) with high current and external quantum efficiencies.7e Recently, Yam and co-workers reported a new class of cyclometalated platinum(II) N^C^N complexes of 1,3-bisheteroazolylbenzenes with chloro or alkynyl as ancillary ligands that shows diverse luminescence properties.11 Cyclometalated platinum(II) pincer complexes have also been explored as phosphorescent materials for OLEDs.7,8e,11a,c,d In a different aspect of their solid-state properties, square−planar platinum(II) complexes have been explored to construct metallogelators, which have attracted much attention in recent years for the potential application in the fields of drug delivery, sensors, intelligent materials, etc.12−15 Gelation processes in some of these complexes generated by the selfassembly of molecular metal complexes can be based on noncovalent interactions that are in addition to hydrogen bonding, hydrophobic−hydrophobic interactions, and π−π Received: August 21, 2016

A

DOI: 10.1021/acs.inorgchem.6b02033 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthetic Routes of Complexes 3−10

Figure 1. Perspective drawings of complexes 1 (a) and 5 (b) with atomic numbering. Hydrogen atoms and solvent molecules are omitted for clarity. Thermal ellipsoids were shown at the 30% probability level.

stacking interactions, namely, the metal−metal interactions.16,17 By these weak noncovalent interactions as driving forces, the supramolecular gels can become responsive to external stimuli, such as temperature, light, pH, mechanical stress, sonication, and chemicals.17,18 In particular, square−planar platinum(II) complexes have been explored to construct metallogelators due to their propensity to form aggregates through Pt···Pt and π−π stacking interactions.17,18 The metallogels have been shown to display obvious color and emission changes during the gel-tosol phase transition because of the occurrence of aggregation− deaggregation processes in the square−planar platinum(II) complexes.18 In the present work, we report the synthesis and characterization of a new series of cyclometalated N^C^N platinum(II) complexes of 1,3-bis(1-n-alkylpyrazol-3-yl)benzene (bpzb) ligands, with chloro and alkynyl motifs consisting of different substituent groups, [Pt(bpzb)Cl] (1 and 2) and [Pt(bpzb)(CC−R)] (R = C6H5, C6H4−OCH3-p, C6H4−NO2-p, C6H4−NH2-p, 4-cholesteryl phenyl carbamate and cholesteryl methylcarbamate) (3−10). Two of the complexes were structurally characterized by X-ray crystallography, and short intermolecular C−H···Pt contacts in the crystal state were observed. Their electronic absorption and photoluminescence properties were investigated. The complex with cholesteryl 4ethynylphenyl carbamate ligand was found to form stable metallogels that are responsive to mechanical sonication and

thermal stimuli, with the exhibition of circular dichroic (CD) activity.

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RESULTS AND DISCUSSION Synthesis and Characterization. The bpzb ligands were synthesized by an alkylation reaction of 1,3-di(1H-pyrazol-3yl)benzene with the corresponding alkyl bromides.19 The chloroplatinum(II) complexes 1 and 2 were prepared by modifications of previously reported literature procedures for the related [Pt(bpqb)Cl] (bpqb = 1,3-bis(4′-phenyl-2′quinolinyl)benzene; Scheme S1).8c Complexes 3−10 were synthesized by reaction of the chloroplatinum(II) complex precursors with various alkynes in the presence of sodium hydroxide in MeOH or MeOH−CH2Cl2 mixture (Scheme 1).11a All the complexes were characterized by 1H NMR, electrospray ionization mass spectrometry (ESI-MS), and elemental analyses. Complexes 1 and 5 were structurally characterized by X-ray crystallography. X-ray Crystal Structures. Single crystals of complexes 1 and 5 were obtained by diffusion of diethyl ether vapor into a solution of the complexes in dimethyl sulfoxide (DMSO) and dichloromethane, respectively. Their structures were determined by X-ray crystallography, and the perspective drawings of the complexes 1 and 5 are shown in Figure 1. Selected bond distances and bond angles are collected in Table S1. Both structures adopt an essentially distorted square−planar B

DOI: 10.1021/acs.inorgchem.6b02033 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) Crystal packing diagram of complex 1. (b, c) Crystal packing diagrams of complex 5.

Complex 1 adopts a near-orthogonal arrangement with neighboring complex molecules, with a dihedral angle of 85.8° between the tridentate bpzb platinum(II) planes of two neighboring molecules through three nonclassical intermolecular hydrogen bonds associated with two intramolecular C− H···Cl interactions (Figure 2a). The value of the H···Cl separations are 2.60 and 2.42 Å, respectively.20c Two of the intermolecular hydrogen bonds are C−H···C with angles of 154.6° and 123.0°, and the values of the H···C separations are 2.74 and 2.73 Å, respectively. Interestingly, short intermolecular C−H···Pt contacts have also been observed with the H···Pt separation of 2.92 Å. The value of C−H···Pt angle of ∼163.0° suggests that the interaction is nearly colinear.22 To the best of our knowledge, such arrangement of short hydrogen bond contacts in an intermolecular manner is unusual, since in most cases the X−H···Pt (X = N or C) hydrogen bond contacts are intramolecular in nature.22a,b Only few examples of structurally characterized neutral complexes showing short intermolecular X−H···Pt contacts (X = N or C) have been reported.22c For complex 5, the phenyl ring of the alkynyl ligand is twisted relative to the plane of the tridentate bpzb ligand with a dihedral angle of 29.3°. It shows a head-to-tail stacking mode and a near-orthogonal arrangement between pairs of complex molecules. The shortest Pt···Pt distances between the adjacent molecules are found to be 4.48 Å, indicating that there are no obvious Pt···Pt interactions between the two metal centers, and only weak π−π interactions exist (Figure 2b,c). Electrochemistry. The cyclic voltammograms of all the complexes were performed in dichloromethane (0.1 M [nBu4N][PF6]) versus standard calomel electrode (SCE), and the electrochemical data are collected in Table S2. The cyclic voltammograms of complex 5 and the p-nitrophenylacetylene free ligand are shown in Figure S1. All the complexes display one irreversible anodic wave at ca. +0.84 to +1.15 V, and no reduction wave is observed in the dichloromethane solution within the solvent window (from +1.5 to −2.0 V vs SCE), in which the lowest unoccupied molecular orbital (LUMO) in

coordination geometry, which is similar to that of typical cyclometalated platinum(II) N^C^N complexes.8,10 The N− Pt−N and N−Pt−C angles display deviations from the idealized values of 90° [N(2)−Pt−C(9) 78.7° and N(3)− Pt−C(9) 78.6° for complex 1 and N(2)−Pt−C(9) 77.5° and N(3)−Pt−C(9) 78.3° for complex 5] and 180° [N(2)−Pt− N(3) 157.3° and C(9)−Pt−Cl(1) 179.7° for complex 1 and N(2)−Pt−N(3) 155.9° and C(9)−Pt−C(21) 177.4°for complex 5], respectively. The distortions can be ascribed to the steric demand exerted by the bite angles of the N^C^N ligand. The platinum(II) metal center coordinates to the tridentate 1,3-bis(1-n-butyl-1H-pyrazol-3-yl)benzene ligand through two pyrazolyl nitrogen atoms and one benzene carbon atom to form an essentially coplanar motif.8b,d,e,10 The Pt−C bond distances (1.95 and 1.98 Å for complexes 1 and 5, respectively) are similar to those in other related cyclometalated platinum(II) N^C^N complexes8a,10a,20a,b but are shorter than the Pt−N bonds in the platinum(II) N^N^N complexes [Pt(N5Cn)Cl]+[X]− (N5Cn = 2,6-bis(1-alkylpyrazol-3-yl)pyridyl with n denoting the number of carbon atoms in the alkyl chain) (1.96 Å) and [Pt(N5Cn)(CCR)]+[X]− (1.99 Å),21 as well as the Pt−C bonds in other cyclometalated platinum(II) C^N^N complexes (typically ∼2.04 Å).7a The crystal-packing diagrams of complexes 1 and 5 are depicted in Figure 2. The fourth coordination site is occupied by a chloro ligand with a distance of 2.42 Å for the Pt−Cl bond in complex 1, which is comparatively longer than those of [Pt(N5Cn)Cl]+[X]− (2.29−2.30 Å) 21 and the chloroplatinum(II) terpyridine complexes.5e This can be explained by the stronger trans influence of the anionic phenyl ring of the tridentate N^C^N ligand than the N^N^N ligand, which would weaken the Pt−Cl bond.20a,b Similarly, the fourth site can be coordinated to an alkynyl ligand with a distance of 2.06 Å for the Pt−C bond in complex 5, which is also comparatively longer than those of [Pt(N5Cn)(CCR)]+[X]−21 and the platinum(II) N^N^N complexes11b for the same reason. C

DOI: 10.1021/acs.inorgchem.6b02033 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Photophysical Data for Complexes 1−10 medium (T [K]) 1

2

3

4

5

CH2Cl2 (298)a acetone (298)b solid (298) solid (77) glass (77)c CH2Cl2 (298)a acetone (298)b solid (298) solid (77) glass (77)c CH2Cl2 (298)a acetone (298)b solid (298) solid (77) glass (77)c CH2Cl2 (298)a acetone (298)b solid (298) solid (77) glass (77)c CH2Cl2 (298)a acetone (298)b solid (298) solid (77) glass (77)c CH2Cl2 (298)a acetone (298)b

electronic absorption

emission

λmax [nm] (ε [dm3 mol−1 cm−1])

λem [nm] (τ0 [μs])

264 (26 560), 286 sh (17 720), 338 (7230), 354 (8000) 338 sh (5840), 354 (6900)

264 (43 640), 286 sh (28 780), 338 (11 230), 354 (12 720) 337 (6380), 354 (7360)

270 (78 230), 296 sh (32 660), 344 sh (17 110), 354 (18 620), 367 (11 030) 343 sh (9710), 355 (10 710), 366 sh (5980)

269 (50 440), 297 sh (20 700), 343 sh (10 390), 356 (12 020), 374 (5960) 345 sh (9550), 356 (10 980), 370 sh (5890)

265 (33 670), 296 sh (17 560), 341 sh (18 720), 355 sh (23 770), 382 (26 380) 343 sh (18 670), 357 (22 050), 381 (24 040)

medium (T [K])

e

454, 491 (−)

electronic absorption

emission

λmax [nm] (ε [dm3 mol−1 cm−1])

λem [nm] (τ0 [μs])

solid (298) solid (77)

467, 497 (−)e 459, 482, 511, 550 (−)e 450, 483, 518 (−)f 450, 482, 510, 552 (−)f (−)d

7

(−)d 454, 484, 517, 544 (−)e 450, 486, 512 (−)f 453, 487, 520, 564 (−)f 460, 490, 520 (−)e 468, 498, 533 (4.0) 454, 485, 513, 560 (−)e 454, 489, 501 (−)f 452, 485, 513, 554 (12.7) 459, 488, 522 (−)e 467, 497, 540 (4.7) 470, 487, 512, 550 (−)e 455, 486, 513 (−)f 453, 485, 514, 555 (13.4) 583 (−)e

8

9

glass (77)c CH2Cl2 (298)a acetone (298)b solid (298) solid (77) glass (77)c CH2Cl2 (298)a acetone (298)b solid (298) solid (77) glass (77)c CH2Cl2 (298)a acetone (298)b solid (298)

276 (51 290), 341 sh (10 870), 356 sh (13 400), 372 (7570) 343 sh (11 840), 356 (13 570), 369 sh (8650)

267 (30 910), 342 sh (8940), 354 (8210) 342 (7690), 355 (7870)

277 (52 510), 341 sh (11 540), 356 (13 490), 372 sh (7570) 342 sh (10 710), 356 (12 480), 367 sh (8320)

solid (77)

10

593 (0.7) 553, 586 (−)e

glass (77)c CH2Cl2 (298)a acetone (298)b solid (298) solid (77) glass (77)c

267 (28 760), 295 sh (13 280), 342 (7810), 355 (7520) 340 (7730), 354 (8640)

469, 484, 509, 543 (−)e 463, 491, 519 (−)f 452, 484, 512, 554 (13.4) 459, 490, 521 (−)e 467, 498, 536 (−)e 459, 486, 516, 554 (−)e 456, 487, 519 (−)f 453, 485, 514, 554 (−)f 455, 488 (−)e 465, 498, 538 (−)e 452, 481, 513, 551 (−)e 457, 488, 517 (−)f 451, 484, 512, 554 (−)f 460, 491, 520 (−)e 468, 500, 538 (−)e 456, 484, 512,550 (−)e 469, 492, 503, 552 (−)f 454, 487, 520, 564 (−)f 453, 486 (−)e 467, 498 (−)e 458, 483, 517, 550 (−)e 469, 495, 525 (−)f 457, 487, 523, 565 (−)f

572, 614 (−)f 529, 561 (547.3) 457, 489, 519 (−)e 468, 500, 536 (5.6)

Sample concentration in the range of (3.1−5.2) × 10−5 M. bSample concentration in the range of (0.9−3.2) × 10−5 M. cIn ethanol− methanol−dichloromethane (4:1:1 v/v). dNonemissive. eLuminescence was too weak for the emission lifetime to be measured with certainty. fEmission lifetime was not measured.

these complexes except for complex 5 is probably dominated by the N^C^N ligand, which has a relatively higher energy. The irreversible oxidation wave is primarily assigned to metalcentered oxidation from Pt(II) to Pt(III), probably mixed with some bpzb and alkynyl ligand character, which has similarly been observed in other cyclometalated N^C^N platinum(II) complex systems in previous studies.11a,d,23 The mixing of an alkynyl oxidation character is substantiated by the fact that, while the oxidation of chloroplatinum(II) complexes 1 and 2 occurs at a similar potential of ca. +1.16 V versus SCE, the

complexes containing different alkynyl ligands show observed shifts of the oxidation waves owing to the sensitivity of the potential of the oxidation waves toward the electronic effect of the alkynyl ligands. Assuming an almost constant overpotential for the structurally related class of metal complexes, the potentials for the oxidation waves are found to decrease in the order of 5 (+1.07 V) > 3 (+0.87 V) > 4 (+0.84 V), in which complexes with less electron-rich alkynyl ligands occur at a more positive potential with CCC6H4OMe-p (4) < C CC 6 H 5 (3) < CCC6 H 4 NO2 -p (5).11a,21 Complex 5

6

276 (27 420), 342 sh (6210), 357 (7160), 376 sh (4710) 343 sh (8410), 359 (9760), 377 sh (6290)

a

D

DOI: 10.1021/acs.inorgchem.6b02033 Inorg. Chem. XXXX, XXX, XXX−XXX

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The high-energy absorption bands are attributed to intraligand (IL) transitions of the tridentate bpzb ligand. The chloroplatinum(II) complexes 1 and 2 show two absorption bands at 338 and 354 nm in the low-energy electronic absorption region (Figure 3 and Table 1). After the chloro group is substituted by various alkynyl ligands (complexes 3− 10), the relatively high-energy absorption band at 338 nm is shifted to 341−344 nm, and the relatively lower-energy absorption band at 354 nm remains at 354−357 nm that is independent of the alkynyl ligands (Figure 3 and Table 1). For complexes 3, 4, 6, 7, and 9 with the phenylethynyl ligand derivatives, they show additional low-energy absorption bands at 367 (3), 372 (7 and 9), 374 (4), and 376 nm (6) in dichloromethane, with a shift of absorption maxima to longer wavelengths with more electron-rich alkynyl ligands, C CC6H5 (3) < cholesteryl 4-ethynylphenyl carbamate (7 and 9) < CCC6H4OMe-p (4) < CCC6H4NH2-p (6). On the basis of the previous spectroscopic work on the related platinum(II) 2,6-bis(N-alkylbenzimidazol-2′-yl)pyridine, 2,6-bis(1-alkylpyrazol-3-yl)pyridine, and cyclometalated 1,3-di(2-pyridyl)benzene platinum(II) complexes,8,10 the low-energy absorption bands of the cyclometalated chloroplatinum(II) complexes 1 and 2 at 338 nm are tentatively assigned as the [dπ(Pt)→π*(bpzb)] metal-to-ligand charge transfer (MLCT) transition, and the absorption band at 354 nm is assigned as primarily IL transitions of the tridentate bpzb ligand. The absorption band at 341−344 nm of complexes 3−10 is also tentatively assigned as the [dπ(Pt)→π*(bpzb)] MLCT transition, and the absorption band at 354−357 nm, which is less sensitive to the alkynyl ligands, is assigned as primarily IL transitions of the tridentate bpzb ligand. The low-energy absorption band at 370−380 nm for complexes 3, 4, 6, 7, and 9, which is sensitive to the ancillary substituents on the phenylethynyl ligand, is assigned as primarily ligand-to-ligand charge transfer (LLCT)

containing the nitro group exhibits one quasi-reversible couple, which is similar to that of the free p-nitrophenylacetylene ligand.1a,24 The shift in reduction potential between complex 5 and free p-nitrophenylacetylene ligand is consistent with the observed shift in their electronic absorption spectra for the intense low-lying band (vide infra). Electronic Absorption Spectroscopy. The electronic absorption spectra of the cyclometalated platinum(II) complexes 1−10 in dichloromethane solution at room temperature display high-energy absorption bands at λ ≈ 264−297 nm, with extinction coefficients (ε) on the order of 1 × 104 dm3 mol−1 cm−1, and low-energy absorption bands at λ = 338−382 nm, with extinction coefficients (ε) on the order from 1 × 103 to 1 × 104 dm3 mol−1 cm−1. In addition, the electronic absorption spectra of the cyclometalated platinum(II) complexes 1−10 in acetone show similar low-energy absorption bands at 337−381 nm. The electronic absorption data of the complexes are summarized in detail in Table 1, and the electronic absorption spectra of complexes 1 and 3−6 are shown in Figures 3 and S2.

Figure 3. Electronic absorption spectra of complexes 1 and 3−6 in dichloromethane at room temperature.

Figure 4. (a) Normalized emission spectra of complexes 3, 4, and 6 in degassed acetone solution at room temperature. (b) Emission spectrum of bpzb ligand in 77 K ethanol−methanol−dichloromethane (4:1:1 v/v) glass state. (c) Normalized emission spectrum of complex 5 in degassed acetone solution at room temperature. (d) Emission spectrum of p-nitrophenylacetylene ligand in 77 K ethanol−methanol−dichloromethane (4:1:1 v/v) glass state. E

DOI: 10.1021/acs.inorgchem.6b02033 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Temperature-dependent electronic absorption spectral changes of wet gel of complex 9 prepared from cyclohexane (1.88 × 10−3 M) with (a) temperature increase from 20 to 60 °C and (b) temperature decrease from 60 to 20 °C.

in line with the additional electronic absorption band at 382 nm when compared to that of other complexes. It also shows a reduction couple characteristic of the nitro group reduction according to the cyclic voltammetry (CV) results (Figure S1 and Table S2). Moreover, a similar structureless emission band at 504 nm was also observed of the free p-nitrophenylacetylene ligand in the 77 K glass state (Figure 4d). According to the electronic absorption, CV, free-ligand emission results, and some previous studies on the related platinum(II) complexes,1a,24 the emission is tentatively assigned as derived from predominantly metal-perturbed π→π* 3IL state of p-nitrophenylacetylene ligand with some mixing of metal-to-pnitrophenylethynyl MLCT state. This was further confirmed by the much longer excited-state lifetime of complex 5 than those of complexes 3, 4, and 6 (Table 1) in the glass state. Gelation Properties. Complexes 7−10 containing cholesterol-based alkynyl ligand were tested for gelation property in various organic solvents by a “stable-to-inversion of a test tube” approach.12,17 The results of gelation properties are summarized in Table S3. It is found that only complex 9 can form stable metallogels in organic solvents such as n-butanol, DMSO, and cyclohexane with the critical gelation concentration (CGC) at 7.5, 3.9, and 2.6 mg mL−1, respectively (Figure S4). Complex 9 underwent stable thermoreversible gelation in n-butanol, DMSO, and cyclohexane with the gel-tosol phase transition temperature at 45, 63, and 50 °C, respectively. Interestingly, the metallogel prepared from complex 9 also shows mechanical sonication responsive property. The hot solution is accelerated to form stable metallogel under sonication. The metallogel prepared from cyclohexane is transparent compared with that from DMSO and n-butanol (Figure S4). Therefore, the cyclohexane gel was used to study the temperature-dependent electronic absorption spectra. The low-energy absorption band at 362 nm is assigned as primarily IL transitions of the tridentate bpzb ligand, with a shoulder that is tentatively assigned as the [dπ(Pt)→π*(bpzb)] metal-to-ligand charge transfer (MLCT) transition. The absorption tail beyond 400 nm may be due to the aggregation of weak Pt···Pt and π−π stacking interactions. On increasing the temperature from 20 to 60 °C, the intensity of the IL and MLCT absorption bands are slightly increased and show hypsochromic shift. Additionally, the absorption tail beyond 400 nm gradually disappears until the temperature is increased to above the gel-to-sol phase-transition temperature (Figure 5). The isosbestic points at 372 and 390 nm indicate the clean interconversion between the assembly and disassembly processes during the gel-to-sol phase transition. The electronic absorption spectrum was found to recover after cooling to 20

[π(C≡C)→π*(bpzb)] transitions. Complexes 8 and 10 show no LLCT [π(C≡C)→π*(bpzb)] transition band, which may be due to the more lower-lying highest occupied molecular orbital (HOMO) of the 2-propyn-1-yl ligand when compared to that of the various phenylethynyl and bpzb ligands. Complex 5 shows an extra low-energy absorption band at 382 nm with an ε value of 2.6 × 104 dm3 mol−1 cm−1. As complex 5 contains a nitro group with strongly reduced donor ability for the alkynyl ligand, the band cannot be a result of LLCT [π(C≡C)→ π*(bpzb)] or MLCT [dπ(Pt)→π*(bpzb)] transition. According to some previous studies on the related platinum(II) complexes,1a,24 the absorption band of complex 5 is attributed to an admixture of metal-perturbed π→π* IL transition based on the p-nitrophenylethynyl ligand and metal-to-p-nitrophenylethynyl MLCT transition. Luminescence Spectroscopy. The luminescence properties were measured in degassed acetone and dichloromethane solutions, the solid states at room temperature and at 77 K, and the 77 K glass state. All the complexes show well-resolved vibronic-structured emission at ∼467, 500, 535 nm in acetone and weaker emission at 460, 490, 520 nm in dichloromethane except complexes 2 and 5 (Table 1 and Figures 4a and S3a). The vibrational progressional spacings of ∼1300 cm−1 are characteristic of the vibrational stretching frequencies of the aromatic CC and CN modes of the tridentate N^C^N ligand. After careful comparison of the emission properties of all the complexes except 5, it is found that the emission energy is not much influenced by both the alkynyl ligands and the electron-donating moieties on the alkynyl ligands (Table 1 and Figure 4a). A similar well-resolved vibronic-structured emission band at ca. 411, 440, and 463 nm was also observed for the free bpzb ligand in the glass state (Figure 4b). Therefore, the vibronic-structured emission, with much longer excited-state lifetime in the range of microsecond (Table 1), is tentatively assigned as originating from metal-perturbed π→π* 3IL excited states of the tridentate bpzb ligand with mixing of some dπ(Pt)→π*(bpzb) 3MLCT character.11a,d,24 The emission wavelengths in acetone show bathochromic shifts compared to those in dichloromethane solution, which further confirm the emission originating from metal-perturbed π→π* 3IL excited states of the tridentate bpzb ligand. Complex 2 is nonemissive in the degassed solution state, which may be ascribed to the floppiness of the complex containing the long alkyl chains, leading to facile nonradiative decay, dissipating the energy of the emissive 3IL excited state. Interestingly, complex 5 with nitro group on the alkynyl ligand shows a structureless emission band at 593 and 583 nm in degassed acetone and dichloromethane solutions, respectively (Figures 4c and S3b), F

DOI: 10.1021/acs.inorgchem.6b02033 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. SEM images of the xerogels prepared from complex 9 in (a) n-butanol, (b) DMSO, (c) cyclohexane, (d) n-butanol with mechanical sonication treatment, (e) DMSO with mechanical sonication treatment, and (f) cyclohexane with mechanical sonication treatment.

°C. The metallogels of complex 9 prepared from n-butanol, DMSO, and cyclohexane are emissive at room temperature and show well-resolved vibronic-structured emission spectra (Figure S5). The vibrational progressional spacings of ∼1300 cm−1 are characteristic of the vibrational stretching frequencies of the aromatic CC and CN modes of the tridentate N^C^N ligand. The corresponding wet gels and xerogels also show luminescent properties that are similar to that in acetone and dichloromethane solutions and the solid state, respectively, which is tentatively assigned as metal-perturbed 3IL emission of the tridentate bpzb ligand with mixing of some dπ(Pt)→ π*(bpzb) 3MLCT character (Figures 4a, S3a, S5, and S6). The supramolecular metallogel shows CD activity with two positive peaks at 315 and 363 nm and a shoulder at 348 nm (Figure S7). The strong peak at 315 and 363 nm with a shoulder at 348 nm might correspond to the cholesterol-induced chirality of the tridentate bpzb platinum(II) complex, in line with the electronic absorption data (Figure S7). The tail from 400 to 600 nm is caused by the cholesterol-induced chirality due to the aggregation via weak Pt···Pt and π−π stacking interactions.18g To investigate the morphologies of the metallogels, scanning electron microscopy (SEM) was performed with the corresponding xerogels. The SEM images of xerogels of complex 9 were shown to consist of entangled three-dimensional fibrous network made of helical nanofibers with a width in the range of 100−300 nm (Figure 6). The interactions of the gelator− gelator and solvent−gelator result in the gelation of lowmolecular-weight molecules, and the gelator molecules tend to further assemble them into bigger aggregates with higher solvent−gelator interactions.

solution state. Interestingly, complex 5 shows an additional low-lying IL/MLCT absorption band and emission that is derived from the involvement of the p-nitrophenylethynyl ligand. Complex 9 with cholesteryl 4-ethynylphenyl carbamate ligand has been found to form stable metallogels, which are responsive to mechanical sonication and thermal stimuli, as well as to show CD activity.

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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.6b02033. Experimental details, electronic absorption and emission spectra, electrochemical data, gelation data, CV spectra, CD spectra, photographs of metallogels. (PDF) X-ray crystallographic data (CCDC 1499692 and 1499693). (CIF) X-ray crystallographic data. (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: (+)(852) 2857-1586. E-mail: [email protected]. edu.cn. (Y.L.) *E-mail: [email protected]. (V.W.-W.Y.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.W.-W.Y. acknowledges support from The University of Hong Kong under the URC Strategic Research Theme on New Materials and the Sun Yat-Sen University. Y.L. acknowledges support from the National Natural Science Foundation of China (Grant No. 21503284). Dr. A. K.-W. Chan and Dr. E. Y.H. Hong in the Department of Chemistry at The University of Hong Kong are gratefully acknowledged for their kind assistance in excited-state lifetime measurements and helpful

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CONCLUSION A series of cyclometalated tridentate platinum(II) complexes has been synthesized. Two of the platinum(II) complexes have also been structurally characterized by X-ray crystallography. Vibronic-structured emission bands, originating from 3IL transition of bpzb ligands with mixing of some 3MLCT [dπ(Pt)→π*(bpzb)] excited states, have been observed in G

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DOI: 10.1021/acs.inorgchem.6b02033 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02033 Inorg. Chem. XXXX, XXX, XXX−XXX