Synthesis and Characterization of Luminescent Cyclometalated

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Synthesis and Characterization of Luminescent Cyclometalated Platinum(II) Complexes with Tunable Emissive Colors and Studies of Their Application in Organic Memories and Organic Light-Emitting Devices Alan Kwun-Wa Chan, Maggie Ng, Yi-Chun Wong, Mei-Yee Chan,* Wing-Tak Wong, and Vivian Wing-Wah Yam* Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China S Supporting Information *

ABSTRACT: A series of luminescent cyclometalated N^C^N [N^C^N = 1,3-bis(N-alkylbenzimidazol-2′-yl)benzene]platinum(II) alkynyl and carbazolyl complexes has been prepared. The structure of one platinum(II) carbazolyl complex has been characterized by X-ray crystallography. The corresponding electrochemical and photophysical properties have been explored and analyzed. The N^C^N platinum(II) complexes displayed rich luminescence in degassed dichloromethane solution, with their emission profiles dependent on the coordinated alkynyl and carbazolyl ligands. Their emission energies are correlated to the electronic properties of the alkynyl and carbazolyl ligands. By varying the electronic properties of the alkynyl and carbazolyl ligands, emission energies could be fine-tuned to cover a wide range of the visible spectrum, as supported by computational studies. A donor−acceptor platinum(II) complex has been utilized to fabricate memory devices that exhibit binary memory performances with low operating voltages, high ON/OFF ratios, and long retention times. Solution-processable OLEDs have been fabricated based on another platinum(II) carbazolyl complex, resulting in a maximum external quantum efficiency of up to 7.2%, which is comparable to that of the vacuum-deposited devices based on the small-molecule counterpart, illustrating the multifunctional nature of the platinum(II)-containing materials.

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INTRODUCTION In recent years, luminescent transition metal complexes have drawn growing research interest regarding their capability to exhibit intense phosphorescence. Among them, luminescent d8 platinum(II) complexes are one of the vital complex systems being extensively studied and explored because of their versatile photophysical properties.1−5 A variety of bidentate1,2 or tridentate3 pyridine-type nitrogen donor ligands have been accommodated in the square-planar geometry. Besides the wellknown example of terpyridines,3d−g,f,h,i other tridentate N-donor ligands such as 2,6-bis(benzimidazol-2-yl)pyridines4 have been coordinated to the platinum(II) center,4c−e some of which exhibit solid-state polymorphism and interesting photophysical properties, mainly arising from metal-to-ligand charge transfer (MLCT) and metal-metal-to-ligand charge transfer (MMLCT) excited states. Besides, platinum(II) complexes with tridentate cyclometalating ligands in C^N^N and N^C^N coordination modes can exhibit intense luminescence and versatile emissive excited states, including not only intraligand (IL) (π−π*) excited states but also the excimeric excited states.5−11 Upon variations in local environment and electronic properties of the complex system, the relative energy of the emissive excited states can be altered. More recently, cyclometalated platinum(II) complexes © XXXX American Chemical Society

of 2,6-bis(N-alkylbenzimidazol-2′-yl)benzene (bzimb) as the N^C^N ligand have been reported to display intense green phosphorescence.11a,b The employment of different cyclometalating N^C^N ligands with gradual changes in electronic properties has given rise to tunable colors of emission.11c The ancillary position has further been incorporated with various ligand systems to enhance their luminescence properties. However, their emissions are still mainly dominated by the 3IL excited state of the cyclometalating pincer ligands,7,11d−f with the incorporation of different ancillary ligands causing only minor to indiscernible changes in their emission energy profiles. Thus, the systematic control of electronic properties of N^C^N ligands has become the most common and predominant strategy as well as the method of choice for the tuning of their excited state energies and redox properties,7,11d−f thus making strategies other than the modification of the pincer ligand for the tuning of emission energy and color almost unexplored. Donor−acceptor conjugated molecules are important components for optoelectronic applications, given their unique photophysical and intramolecular charge transfer (ICT) properReceived: May 13, 2017

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DOI: 10.1021/jacs.7b04952 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society ties,12 resulting from the versatile combinations of donor and acceptor groups to modulate their molecular structures and hence their electronic properties for optoelectronic applications. Oxadiazole, diarylboron, quinoxaline, and pyridopyrazine are commonly used acceptor moieties,13a−c while triarylamine, carbazole, and fluorene moieties are common donor components13d,e in donor−acceptor molecules. These special classes of molecules have been utilized to fabricate different optoelectronic and electronic devices, including light-emitting diodes,13a−f solar cells,13g transistors,13h and organic memory devices.13i−m It is anticipated that upon the incorporation of donor−acceptor moieties into the alkynyl ligand for coordination to the cyclometalated platinum(II) complexes, their emissive behavior would be enriched by the combination of the unique properties of both components; i.e., both the bipolar nature and the luminescence behavior of the current system would lead to very unique optical properties and electrical bistability. Besides, the carbazole moiety has been coordinated to the platinum(II) center through the N-donor atom to perturb the excited state properties since the strongly electron-donating and holetransporting carbazolyl ligand can narrow the [dπ/pπ(Pt)π*(N^C^N)] energy gap and thus facilitate the formation of a low-lying ligand-to-ligand charge transfer (LLCT) excited state. As a result, the emission colors of the complexes are fine-tuned by simply varying the electron-donating properties of the carbazolyl ligand, which serves as an important alternative and orthogonal strategy to tune their emission colors and optical properties. Herein, luminescent cyclometalated N^C^N platinum(II) carbazolyl and alkynyl complexes 1−6 have been reported. Besides the exploration of their electrochemistry and photophysical properties, computational studies have also been conducted for the further understanding of the excited state nature that gives rise to the versatile luminescence properties of the complex system and for providing guiding principles on the design strategy to achieve tunable emission energies. Their applications in both electroluminescence (EL) and organic memory have been investigated, in which solution-processable organic light-emitting devices (OLEDs) with high external quantum efficiencies (EQEs) of up to 7.2% and organic memory cells with binary performance have been realized, the latter representing the first demonstration of organic resistive memories based on the cyclometalated platinum(II) system. The present study may also provide important insights for the future development of smart and multifunctional materials using small-molecule organometallic compounds based on their different desirable electronic features.

The chemical structures of complexes 1−6 are depicted in Figure 1.

Figure 1. Molecular structures of complexes 1−6.

X-ray Crystal Structures. Upon diffusion of diethyl ether vapor into a concentrated dichloromethane solution of complex 4, single crystals were obtained, and the structure was solved by X-ray crystallography. The perspective view of complex 4 is depicted in Figure 2. The platinum(II) metal center adopts a

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RESULTS AND DISCUSSION Synthesis and Characterization. The donor−acceptor alkynes were synthesized from modified versions of procedures reported in the literature.9b−d The 3,6-bis((4-(trifluoromethyl)phenyl)ethynyl)-9H-carbazole ligand was synthesized by Sonogashira coupling between 3,6-diiodo-9H-carbazole10b and 1-ethynyl-4-(trifluoromethyl)benzene.10c The donor−acceptor alkynyl ligands as well as the carbazolyl ligands were then coordinated to the platinum(II) center upon reacting with the chloroplatinum(II) complexes to afford the target platinum(II) complexes. For all newly synthesized platinum(II) complexes, their identities have been confirmed and supported by different spectroscopic techniques, such as 1H NMR spectroscopy, positive FAB mass spectrometry, and IR spectroscopy, and are in a good agreement with the results from elemental analyses.

Figure 2. Perspective drawing of complex 4 with atomic numbering. Hydrogen atoms and solvent molecules are omitted for clarity. Thermal ellipsoids were shown at the 30% probability level.

distorted square-planar geometry. There is a plane of symmetry along the platinum−carbazolyl ligand bond axis (along the C8 and N1 atoms). Table S1a summarizes the crystal structure determination data, while Table S1b lists the selected bond distances and bond angles. The N−Pt−N and N−Pt−C angles [N1i−Pt1−N1, 160.1°; N1−Pt1−C8, 80.0°; N1i−Pt1−C8, 80.0°] are found to deviate from the respective ideal value of 180° and 90°; such deviations could be attributed to the steric constraints offered by the bite angle of the cyclometalating ligand. The bond length of Pt1−N3 is 2.10 Å, which is comparable to the platinum−alkynyl bond distance of other B


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Journal of the American Chemical Society related alkynylplatinum(II) bzimpy complexes.4d The plane of the carbazolyl ligand is almost orthogonal to that of the platinum N^C^N unit, with an interplanar angle of 85.4° to minimize the steric hindrance. The crystal packing of 4 shows a head-to-tail stacking between pairs of complex molecules (Figure 3a) and an

Table 1. Electrochemical Data for 1−6 complex

oxidation E1/2 (Epa)/V vs SCEa,b (ΔEp/mV)

1 2

+0.71,b +1.39,b +1.87b +0.67,b +1.16,b +1.75b

3 4 5 6

+0.53,b +0.99,b +1.41b +0.58 (91), +1.31 (78), +1.70b +0.37 (80), +1.20 (77), +1.77b +0.71 (78), +1.71b

reduction Epc/V vs SCEc (ΔEp/mV) −1.64 (94), −2.13c −1.29 (78), −1.53 (68), −2.13c −1.78 (89), −2.09c −2.03c −2.04c −2.03c

a

E1/2 refers to the quasi-reversible anodic peak calculated by E1/2 = (Epa + Epc)/2; Epa and Epc are the peak anodic and peak cathodic potentials, respectively. ΔEp of Fc+/Fc ranges from 62 to 64 mV; scan rate, 100 mV s−1. ΔEp = |Epa − Epc |. bValues refer to the anodic peak potential (Epa) for irreversible oxidation waves. cEpc refers to the cathodic peak potential for the irreversible reduction waves.

Figure 4. Cyclic voltammogram of complex 6 in CH2Cl2 in the presence of 0.1 M nBu4NPF6.

benzothiadiazole,9c and oligothienylenevinylene9d for complexes 1−3, respectively. This has been further supported by the absence of such reduction couples in the reductive scan of their chloroplatinum(II) complexes.11a The irreversible reduction wave at about −2.05 V vs SCE is found to be insensitive toward the nature of the alkynyl ligand and is assigned as the reduction of the N^C^N ligand. Similar reductive waves have been observed in related platinum(II) N^C^N complexes.11b On the other hand, the potential for the first irreversible oxidation wave has been found to gradually change with the electronic natures of the alkynyl ligands. With more π-conjugated and π-donating oligothienylenevinylene alkynyl ligand in complex 3, a less positive potential (+0.53 V vs SCE) than those of complex 2 (+0.67 V vs SCE) and 1 (+0.71 V vs SCE) has been observed. This suggests that the oxidation is alkynyl-ligand-based with mixing of metal-centered contribution. The second irreversible oxidation wave, observed at +0.99 to +1.39 V vs SCE for 1−3, is tentatively assigned as oxidation of di(thienyl)bithiazole,9b di(thienyl)benzothiadiazole,9c and oligothienylenevinylene,9d respectively. Platinum(II) carbazolyl complexes 4−6 are generally found to display two quasi-reversible oxidation couples from +0.37 to +1.31 V vs SCE, an irreversible oxidation wave at about +1.72 V vs SCE, and an irreversible reduction wave at about −2.05 V vs SCE. The first quasi-reversible oxidation couple at +0.37 to +0.71 V vs SCE is shown to be closely related to the electron-donating properties of the carbazolyl ligands. Complexes with more electron-donating carbazolyl ligand would give rise to less positive potential such that 6 (+0.71 V) > 4 (+0.58 V) > 5 (+0.37 V), suggesting that the oxidation is carbazolyl ligand-based with mixing of metal-centered character. The quasi-reversible oxidation couple observed for 4 and 5 at +1.31 and +1.20 V vs

Figure 3. Crystal packing diagrams of complex 4 showing (a) the headto-tail configuration and (b) an extended columnar array of molecules.

orientation with alternating long−short molecular distances as shown in the tetrameric unit (Figure 3b) to further enhance the packing efficiency. The shortest Pt···Pt distance between the adjacent molecules is determined to be 9.97 Å, indicating the absence of Pt···Pt interactions. An interplanar distance of 3.67 Å has been determined between the phenyl rings of the N^C^N ligand of two adjacent molecules, indicative of weak π−π interactions. Electrochemistry. The cyclic voltammograms of platinum(II) complexes 1−3 in dichloromethane (0.1 M nBu4NPF6) exhibit irreversible oxidation waves between +0.53 and +1.87 V vs standard calomel electrode (SCE). For the reduction process, quasi-reversible reduction couples at about −1.29 to −1.78 V vs SCE as well as an irreversible reduction wave at about −2.05 V vs SCE could be observed. Table 1 summarizes the electrochemical data and Figure 4 illustrates the cyclic voltammogram of 6. The quasi-reversible reduction couple is tentatively assigned as alkynyl ligand-centered reduction as similar potentials are observed in the reduction of di(thienyl)bithiazole,9b di(thienyl)C


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Journal of the American Chemical Society Table 2. Photophysical Data for the Platinum(II) Complexes electronic absorption λmax/nm (ε /dm3 mol−1cm−1)

complex

medium (T/K)

1

dichloromethane (298)a

300 (52 220), 313 (48 240), 369 sh (31 940), 426 (51 090)

2

solid (298)b solid (77)b glass (77)c thin film (298)d dichloromethane (298)

305 (46 890), 315 (50 560), 365 sh (30 250), 506 (24 500)

3

4

5

6

solid (298)b solid (77)b glass (77)c thin film (298)d dichloromethane (298) solid (298)b solid (77)b glass (77)c thin film (298)d dichloromethane (298) solid (298)b solid (77)b glass (77)c thin film (298)d dichloromethane (298) solid (298)b solid (77)b glass (77)c thin film (298)d dichloromethane (298) solid (298)b solid (77)b glass (77)c thin film (298)d

299 (53 000), 312 (47 070), 375 sh (32 080), 426 sh (61 920), 453 (83 850), 480 (71 510)

297 (51 300), 312 sh (41 960), 351 sh (13 150), 374 (25 070), 387 (29 790)

297 (67 910), 312 sh (46 050), 344 sh (11 530), 376 (24 640), 389 (27 710)

305 (52 190), 317 (54 760), 349 sh (30,110), 388 (45 100), 407 sh (36 700)

−5

emission λem/nm (τo/ μs) 506, 546, 581 (2.2)g 560 (1.3)h 567, 600 (0.9) 568, 609 (1.5) 496, 534, 614 (2.4) 564 509, 548, 641 (2.4)g 650 (2.5)h 507, 547 (0.7) 562 (1.6) 597 (3.3) 650 538 (0.9) 488, 514 (0.7) 505, 544 (1.1) 505, 542 (2.4) 560 629 (9.5) 526, 554, 604 (1.1) 536, 601, 657 (9.1) 504, 540, 578 (5.1) 550 656 (8.5) 554, 609 (1.1) 536, 564, 617 (6.1) 548 (3.3) 570 588 (2.6) 521, 558 (0.9) 543, 622 (3.3) 508, 540, 576 (3.2) 512

ΦPL 0.10e

0.02f 0.12e

0.26f 0.02e

0.02f 0.03e

0.64f 0.008e

0.52f 0.22e

0.60f

Measured at concentration = 1 × 10 M. Solid-state emission was recorded after grinding. In butyronitrile glass. 5 wt % of complex in MCP. The relative luminescence quantum yield was measured by the optical dilute method with a degassed aqueous solution of quinine sulfate in 1.0 N sulfuric acid (Φ = 0.546, excitation wavelength at 365 nm) was used as the reference. From ref 14. fAbsolute PL quantum yield of 5 wt % of complex in MCP thin films. gExcited at 365 nm in degassed dichloromethane. hExcited at 436 nm in degassed dichloromethane. a

b

c

d

e

SCE respectively is assigned as the oxidation of carbazolyl moiety.12b For complex 6, the carbazolyl with the strong electron-withdrawing CF3-phenyl substituent attached cannot be oxidized within the potential window studied such that the second quasi-reversible oxidation couple could not be observed. The irreversible oxidation wave shown at ca. +1.72 V vs SCE for 4−6 is found to be rather insensitive to the nature of the carbazolyl ligands, possibly resulting from a PtII→PtIII oxidation. The irreversible reduction waves for 4−6 are found to be insensitive toward the nature of the carbazolyl ligand and are thus assigned as ligand-based N^C^N reduction. Electronic Absorption Spectroscopy. In dichloromethane solution at room temperature, the electronic absorption spectra of platinum(II) complexes 1−3 display intense absorptions at around 299−375 nm, with extinction coefficients on the order of 104 dm3 mol−1cm−1. These high-energy absorptions are assigned as IL [π→π*] transitions of the alkynyl and cyclometalating N^C^N ligands. Table 2 summarizes the electronic absorption data of all complexes and Figure 5 shows the electronic absorption spectra of complexes 1−6. In addition, broad and intense absorption bands of complexes 1 and 2 at about 426−506 nm with extinction coefficients on the order of 104 dm3 mol−1cm−1 are observed. This low-energy absorption

Figure 5. Electronic absorption spectra of complexes 1−6 in CH2Cl2 at 298 K.

band could be assigned as ICT transitions of the donor−acceptor alkynyl ligands, probably with some mixing of the MLCT/LLCT [dπ(Pt)/π(CC−R)→π*(N^C^N)] transition. Further examination of the free donor−acceptor alkynyl ligand associated with complexes 1 and 2 has been performed, which indicated that they absorb strongly in the same region with similar molar extinction coefficients as that of their corresponding platinum(II) complexes. The ICT absorption energies of the complexes D


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Journal of the American Chemical Society are found to be red-shifted from 1 (426 nm) to 2 (506 nm) by switching the bithiazole unit to the stronger benzothiadiazole electron acceptor9b,c,12c while keeping the same thiophene donor. This could be attributed to the stronger push−pull effect between the donor and acceptor.12d,e Complex 3, on the other hand, serves as a control complex containing only the conjugated oligothienylenevinylene moiety on the alkynyl ligand backbone, with its low-energy absorption band at 480 nm assigned as the IL [π→π*] transitions of the cyclometalating N^C^N ligands and the alkynyl ligands, mixed with the MLCT/LLCT [dπ(Pt)/ π(CC−R)→ π*(N^C^N)] transitions.4d,11 The platinum(II) carbazolyl complexes 4−6 exhibit intense absorption bands at about 297−351 nm, with extinction coefficients in the order of 104 dm3 mol−1cm−1, which are attributed to IL [π→π*] transitions of the cyclometalating N^C^N ligands and carbazolyl ligands. Moderately intense absorption bands at about 374−407 nm with extinction coefficients on the order of 104 dm3 mol−1cm−1 are also observed. These absorption bands are also assigned as predominantly IL [π→π*] transition of the cyclometalating N^C^N ligand,4d,11 with some mixing of MLCT [dπ(Pt)→ π*(N^C^N)] and LLCT [pπ(carbazolyl)→π*(N^C^N)] transitions. Luminescence Spectroscopy. Intense luminescence is observed for all the platinum(II) complexes in solution state, in 77 K glass and in solid state upon excitation at λ > 350 nm. Emission lifetimes of the microsecond range and large Stokes shifts observed are both suggestive of triplet emission origin. Table 2 summarizes the emission data of the complexes. In degassed CH2Cl2 solution at 298 K, the platinum(II) complexes 1−6 exhibit structureless emission bands at around 538−656 nm upon excitation at 436 nm (Figure 6). For donor−acceptor

Figure 7. Normalized emission spectra of complex 2 with different excitation wavelengths and its corresponding free alkynyl ligand in degassed CH2Cl2 at 298 K.

careful comparison with the emission spectra at different excitation energies and the free ligand emission. The emission of the control complex 3 has been assigned to be of 3IL [π→ π*(N^C^N)] origin. The structureless emission bands at around 588−656 nm of the platinum(II) carbazolyl complexes 4−6 are assigned to be mainly originated from the 3MLCT [dπ(Pt)→ π*(N^C^N)] excited state with certain extent of 3LLCT [pπ(carbazolyl)→π*(N^C^N)] character. The increase in electron-donating ability of the carbazolyl ligand from CF3phenyl-carbazolyl to carbazolyl to tert-butyl-carbazolyl causes the emission energies to be red-shifted correspondingly from 6 (588 nm) to 4 (629 nm) to 5 (656 nm), attributed to the destabilization of the pπ/dπ orbital which causes the narrowing of the HOMO−LUMO energy gap. In comparison to the typical vibronic-structured emissions from the platinum(II) N^C^N complexes,5,8,11 the incorporation of the strongly π-donating carbazolyl ligand could probably alter the electronic properties of the complexes, rendering the 3MLCT/3LLCT excited state more energetically accessible than the 3IL state. The low concentration (10−5 M) for emission measurement and the orthogonal orientation between the planes of the carbazolyl ligand and the platinum(II) N^C^N moieties as revealed in the X-ray structure both suggest the absence of molecular aggregation. These suggest that the possibility of excimeric emission for complexes 4−6 could be ruled out. In butyronitrile glass at 77 K, complexes 1, 3−4 and 6 exhibit vibronic-structured emission bands at around 496−614 nm with vibrational progressional spacings of around 1300 cm−1. These indicate the involvement of cyclometalating ligands in the excited states and therefore the emissions are assigned to derive from the 3IL [π→π*(N^C^N)]/3MLCT [dπ(Pt)→π*(N^C^N)] excited state (Figure S3). As for 2 and 5, structureless emission bands at 597 and 548 nm are attributed to the ICT excited state of the di(thienyl)benzothiadiazole alkynyl ligand and the 3MLCT/ 3 LLCT [dπ(Pt)/pπ(carbazolyl)→π*(N^C^N)] excited state, respectively. With the exception of complex 2, the emission profiles of the complexes at both 298 and 77 K in the solid state (Figures S7 and S8) are similar to those observed in the glass state. Complex 2 exhibits low-energy emission bands at 547 nm at 298 and 562 nm at 77 K, that is attributed to ICT excited state of the di(thienyl)benzothiadiazole alkynyl ligand. Therefore, it is possible to tune and manipulate the emission colors and profile of the N^C^N platinum(II) system to capture nearly the entire visible spectrum by a simple modification of the electronic properties of the ancillary ligand. Solvatochromic Studies. To fully realize the nature of ICT transitions between the donor and the acceptor of 1 and 2, their

Figure 6. Normalized emission spectra of complexes 1−6 in degassed CH2Cl2 at 298 K upon excitation at 436 nm.

platinum(II) complexes 1 and 2, the structureless emission bands are mainly derived from the ICT excited state arising from the donor−acceptor moiety on the alkynyl ligand, with mixing of 3 MLCT [dπ(Pt)→π*(N^C^N)] character. Interestingly, their emission energies are found to be closely related to the donor− acceptor strength. With stronger benzothiadiazole acceptor on the alkynyl backbone, the emission energies are red-shifted with 1 (560 nm) > 2 (650 nm). The emission profiles of 1 and 2 have also been carefully compared with that of their free ligands, which show similar structureless emission bands but are relatively blueshifted (Figure 7 and Figures S1 and S2). In addition, with higher excitation energy at 365 nm, both emissions from the ICT excited state and the 3IL [π→π*(N^C^N)] state are observed in 1 and 2 (Figure 7 and Figures S1 and S2), which can be decomposed into the ICT and IL bands correspondingly upon E


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Figure 8. (a) Electronic absorption spectra of 2 (5 × 10−5 M) in acetone, acetonitrile, ethyl acetate, 1,4-dioxane, toluene, dichloromethane, and N,Ndimethylformamide at 298 K (left), and plot of absorption maximum as a function of empirical solvent polarity parameters [ET(30)] (right). (b) Emission spectra of 2 (5 × 10−5 M) in acetone, acetonitrile, ethyl acetate, 1,4-dioxane, toluene, and N,N-dimethylformamide at 298 K (left) upon excitation at 436 nm, plot of Stokes shift as a function of empirical solvent polarity parameters [ET(30)] (right).

spectral dependence on solvent polarity was studied and the data have been analyzed by the use of the empirical solvent polarity model.16 Upon increasing the solvent polarities from toluene to acetonitrile, both the absorption maximum of the ICT transition and the emission maximum from the ICT excited state of 1 and 2 exhibit hypsochromic shifts, which indicate a negative solvatochromism (Figure 8), suggesting the charge-transfer nature of the absorption and emission bands and that the excited state is less stabilized than the ground state in more polar solvents. The absorption maximum of the ICT transition band of both 1 and 2 could also be plotted as a function of the empirical solvent polarity parameters16a of the solvents used, which are 33.9 (toluene), 36.0 (1,4-dioxane), 38.1 (ethyl acetate), 41.1 (dichloromethane), 42.2 (acetone), 43.8 (N,N-dimethylformamide), and 46.0 (acetonitrile), to obtain a straight line of positive slope. Similarly for the solvatochromic emission studies, a straight line of positive slope can also be obtained by plotting the Stokes shift (from 3900 cm−1 in ethyl acetate to 5600 cm−1 in acetonitrile) against the empirical solvent polarity parameters. These huge Stokes shift changes further verify and support the assignment of an intramolecular charge-transfer nature16c in these donor−acceptor alkynylplatinum(II) systems. Fabrication of OLEDs with Tunable Emission Colors. To explore the optoelectronic properties of the current system, complex 4 has been selected as phosphorescent dopant to fabricate solution-processable OLEDs. Devices with the configuration of indium-tin-oxide (ITO)/poly(ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS; 70 nm)/x % platinum(II) complex:N,N′-dicarbazolyl-3,5-benzene (MCP; 60 nm)/tris(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)-borane (3TPyMB; 5 nm)/1,3,5-tri[(3-pyridyl)phen-3-yl]benzene (TmPyPB; 30 nm)/LiF (0.8 nm)/Al (100 nm) have been prepared and characterized. Figure 9 shows the normalized EL spectra of devices made of 4. Apparently, the 5% doped device exhibits vibronic-structured emission. The intensity of the low-

Figure 9. Normalized EL spectra of devices made with 4 doped at different concentrations.

energy emission band increases with increasing dopant concentration. Particularly, the emission energies show a significant red shift of 902 cm−1 (i.e., from 524 to 550 nm) when the dopant concentration increases from 5% to 20%. This concentration dependence can be attributed to the change of excited state origin from 3IL [π→π*(N^C^N)] to the 3MLCT/ 3 LLCT [dπ(Pt)/pπ(carbazolyl)→π*(N^C^N)] excited state in the solid-state thin films. This is in good agreement with the hypsochromic shift observed in glass. It is worth noting that devices made with 4 show promising performance. The optimized device doped with 20% of 4 exhibits high current efficiency of 24.0 cd A−1 and EQE of 7.2% (Table 3). The current result demonstrates that the present platinum(II) system holds great promises as phosphorescent dopants for solutionprocessable OLEDs. Fabrication of Organic Memory Devices. To further illustrate the potential utilization of these cyclometalated platinum(II) complexes in organic electronics, complex 1 has been fabricated into a memory device. As illustrated in Figure 10, complex 1 has been sandwiched between an aluminum top electrode and ITO bottom electrode. The film thicknesses of F


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ances with low operating voltages of around 3.5 V, high ON/ OFF ratios of over 105 and long retention times of over 104 s, which are comparable to the gold(III) system reported previously.13k Computational Studies. To obtain additional insights on the nature of their photophysical properties, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations associated with the conductor-like polarizable continuum model (CPCM) at the PBE0 level of theory have been performed for complexes 4−6. For complex 4, the optimized ground-state (S0) geometry is in good agreement with the experimental X-ray crystal structure, as the calculated bond lengths and bond angles (Table S1c) are close to the experimental values. Table S2 lists the first 10 singlet excited states of complexes 4−6. Selected molecular orbitals of complex 4 are shown in Figure 12, and those of complexes 5 and 6 are shown in Figures S7 and S8. The S0→S1 transition of complexes 4−6 is mainly contributed by the HOMO→LUMO transition, which can be assigned as LLCT [π(carbazolyl)→ π*(N^C^N)]/MLCT [dπ(Pt)→π*(N^C^N)] transition, as the HOMO is the π orbital localized on the carbazolyl ligand mixed with the Pt dπ orbital, while the LUMO is the π* orbital on the N^C^N ligand. The computed energy of the HOMO→LUMO transition is found to be red-shifted from 6 (463 nm) to 4 (496 nm) to 5 (529 nm), as the HOMO is destabilized to a greater extent with increasing electron-donating ability of the carbazolyl ligand, whereas the LUMO localized on the N^C^N ligand is less perturbed by the substituent groups on the carbazolyl ligand, and this consequently leads to a smaller HOMO−LUMO energy gap. From the TDDFT calculations, the low-energy absorptions of complexes 4−6 at ca. 350−400 nm are mainly attributed to the IL [π→π*(N^C^N)] transition with some mixing of MLCT [dπ(Pt)→π*(N^C^N)] and LLCT [π(carbazolyl)→π*(N^C^N)] characters, while the high-energy absorption bands at ca. 270−350 nm are attributed to the IL [π→π*] transitions of both the N^C^N and the carbazolyl ligands, with mixing of MLCT [π→π*(N^C^N)] character for complex 6. These results further support the assignment of the electronic absorption spectra. At room temperature, all the Pt(II) complexes exhibit intense phosphorescence in CH2Cl2 solution. To gain more insight into the nature of the emissive state, geometry optimizations of the lowest-lying triplet excited states (T1) have been performed in the unrestricted formalism for complexes 4−6. Shown in Figure 13 are the plots of spin density of their T1 states. The spin density is localized on the metal center, and both the N^C^N and the carbazolyl ligands, supporting that the T1 state has an admixture of 3MLCT and 3LLCT character. The calculated emission energies of these complexes, which are approximated from the energy difference between the S0 and T1 states at their corresponding optimized geometries, are listed in Table 4. The calculated emission energy is found to be red-shifted from 6 (510 nm) to 4 (549 nm) to 5 (591 nm), due to the increase in the electron-donating ability of the substituent on the carbazolyl ligand. This is in good agreement with the trend in the emission studies, and thus further supports the 3MLCT/3LLCT origin of the emissive state.

Table 3. Key Characteristics of OLEDs Made with Complex 4 concentration (%)

max current efficiency/cd A−1

max power efficiency/ lm W−1

max EQE/%

CIE x, y

5 10 15 20

16.1 16.5 22.1 24.0

2.8 3.5 5.0 7.5

4.9 5.0 6.8 7.2

0.39, 0.58 0.40, 0.57 0.41, 0.57 0.41, 0.57

Figure 10. (a) Schematic diagram of the device configuration. (b) SEM image of the cross section of a memory device.

complex 1 and aluminum are determined to be 79 and 94 nm respectively from the SEM image of a cross section of the device (Figure 10b). The memory behavior of the as-fabricated device of Al/complex 1/ITO has been studied through their current− voltage characteristics. The forward bias is defined as a positive bias applied to the ITO electrode. The current−voltage (I−V) characteristics of the device are illustrated in Figure 11. This device initially exhibits a high-resistance state (OFF state). When a voltage of 0 to +8 V (sweep 1) is applied to the device, a sharp increase in the current from 10−7 to 10−2 A is observed at the switching threshold voltage (Vth) of around 3.5 V, indicating that the memory device is switched to a low-resistance state (ON state) from an OFF state. This process can be regarded as the “writing” process in a memory device. To further realize the energy states of complex 1, corresponding electrochemical properties have been correlated as shown in Figure 11e. The first irreversible oxidation wave at +0.71 V vs SCE has been attributed to alkynyl ligand-based oxidation9b with mixing of metalcentered contribution while the first quasi-reversible reduction couple at −1.64 V vs SCE has been tentatively assigned as the di(thienyl)bithiazole ligand-centered reduction.9b The energy levels of the HOMO and the LUMO are found to be at −5.51 and −3.16 eV, respectively (Figure 11e). Together with the electrochemical data determined from cyclic voltammetry, the energy barriers for hole injection and electron injection are found to be 0.71 and 1.14 eV, which are calculated by the energy difference between the work function of the ITO electrode (−4.8 eV) and the HOMO energy of platinum(II) complex 1 and between the work function of Al electrode (−4.3 eV) and the LUMO energy of 1, respectively. This analysis indicates that hole injection from the electrode to complex 1 should be more energetically favorable when compared to electron injection. The current platinum(II) system exhibits binary memory perform-

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CONCLUSION To summarize, a new series of N^C^N platinum(II) carbazolyl and alkynyl complexes has been synthesized and characterized. The electrochemical and photophysical properties of the system have been explored, giving rise to in-depth understanding of their G


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Figure 11. (a) I−V curves of the memory device of 1. (b) Retention time characteristics of the ON and OFF states of the memory device. (c) I−V curves measured from four memory cells. (d) Bar chart of the number of devices against threshold voltage among 15 devices. (e) Schematic diagram of the charge injection process in the memory device.

Figure 12. Spatial plots (isovalue = 0.03) of selected molecular orbitals of complex 4.

structure−property relationships. The variation of donor− acceptor properties in the alkynyl ligand as well as electrondonating abilities of the carbazolyl ligands can be employed to fine-tune the emission energies. The cyclometalated N^C^N

platinum(II) complexes are found to exhibit rich luminescence properties spanning the visible region with their emission profiles dependent on the coordinated alkynyl and carbazolyl ligands, which are assigned to be originated from the 3ICT excited state of H


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reaction mixture was then heated to reflux overnight. The brown suspension was removed by filtration and washed with diethyl ether. The filtrate was evaporated to dryness and then further purified using silica gel column chromatography with n-hexane-dichloromethane (3:1 v/v) as the eluent to give the product as a white solid. Yield: 0.82 g, 67%. 1H NMR (400 MHz, acetone-d6, 298 K, δ/ppm): δ 7.65 (m, 4H, −C6H3−), 7.78 (m, 8H, −C6H4−), 8.49 (d, J = 2.0 Hz, 2H, −C6H3−). Positive FAB-MS: m/z 503 [M]+. HRMS (positive EI): m/z calcd for C30H15F6N: 503.1109; found: 503.1089. Complex 1. Sodium hydroxide (0.01 g, 0.40 mmol) was added into a well-stirred solution of ethynyldi(thienyl)bithiazole (0.28 g, 0.47 mmol) in methanol (50 mL) and the mixture was stirred for 25 min. A solution of chloroplatinum(II) bzimb11 (0.15 g, 0.23 mmol) in dichloromethane (50 mL) was gradually added. Afterward, the mixture was heated to reflux overnight. The orange suspension was then isolated by filtration and the solid was washed successively with deionized water, methanol and diethyl ether to give a yellowish orange solid. Recrystallization of the product was performed by diffusion of diethyl ether vapor into its dichloromethane solution to give the orange crystals. Yield: 0.17 g, 70%. 1 H NMR (400 MHz, CD2Cl2, 298 K, δ/ppm): δ 0.94 (t, J = 1.4 Hz, 6H, −CH3), 1.04 (t, J = 7.4 Hz, 6H, −CH3), 1.39 (m, 12H, −CH2−), 1.43 (m, 4H, −CH2−), 1.88 (m, 4H, −CH2−), 2.03 (t, J = 7.6 Hz, 4H, −CH2−), 2.99 (t, J = 7.9 Hz, 2H, −CH2−), 3.06 (t, J = 8.1 Hz, 2H, −CH2−), 4.65 (t, J = 7.6 Hz, 4H, −NCH2−), 7.17 (m, 2H, −C4H2S−), 7.22 (d, J = 3.7 Hz, 2H, −C4H3S−), 7.28 (d, J = 3.7 Hz, 1H, −C4H3S−), 7.36 (t, J = 7.6 Hz, 1H, −C6H3−), 7.42 (d, J = 3.8 Hz, 4H, −C6H4−), 7.47 (m, 2H, −C6H4−), 7.68 (d, J = 7.8 Hz, 2H, −C6H3−), 8.88 (d, J = 8.2 Hz, 2H, −C6H4−). IR (KBr disk): v = 2116 cm−1 (w; v(CC)). Positive FAB-MS: m/z 1140 [M]+, 616 [M−alkynyl]+. Elemental analyses. Found (%): C 58.24, H 5.32, N 7.27; Calcd (%) for 1·H2O: C 58.06, H 5.39, N 7.25. Complex 2. The compound was synthesized with similar procedure as 1, except ethynyldi(thienyl)benzothiadiazole (0.27 mg, 0.70 mmol) was used instead of ethynyldi(thienyl)bithiazole to give the product as purple crystals. Yield: 155 mg, 70%. 1H NMR (400 MHz, CD2Cl2, 298 K, δ/ppm): δ 1.00 (t, J = 7.4 Hz, 6H, −CH3), 1.47 (m, 4H, −CH2−), 1.98 (t, J = 7.6 Hz, 4H, −CH2−), 4.62 (t, J = 7.6 Hz, 4H, −NCH2−), 7.21 (m, 2H, −C4H2S−), 7.34 (t, J = 8.2 Hz, 1H, −C6H3−), 7.41 (m, 3H, −C4H3S−), 7.48 (m, 4H, −C6H4−), 7.67 (d, J = 7.8 Hz, 2H, −C6H3−), 7.89 (m, 2H, −C6H4−), 8.11 (d, J = 3.8 Hz, 1H, −C6H2−), 8.17 (d, J = 3.8 Hz, 1H, −C6H2−), 8.91 (d, J = 8.0 Hz, 2H, −C6H4−). IR (KBr disk): v = 2111 cm−1 (w; v(CC)). Positive FAB-MS: m/z 939 [M]+, 616 [M−alkynyl]+. Elemental analyses. Found (%): C 55.52, H 4.06, N 8.81; Calcd (%) for 2·H2O: C 55.16, H 4.00, N 8.77. Complex 3. The compound was synthesized with similar procedure as 1, except ethynyloligothienylenevinylene (0.2 mg, 0.50 mmol) was used instead of ethynyldi(thienyl)bithiazole to give the product as orange crystals. Yield: 160 mg, 74%. 1H NMR (400 MHz, CD2Cl2, 298 K, δ/ppm): δ 0.98 (t, J = 7.4 Hz, 6H, −CH3), 1.47 (m, 4H, −CH2−), 1.96 (m, 4H, −CH2−), 4.58 (t, J = 7.4 Hz, 4H, −NCH2−), 6.94 (m, 2H, −C4H2S−), 7.02 (m, 6H, −C6H4−, −C2H2−), 7.06 (t, J = 2.4 Hz, 1H, −C4H2S−), 7.20 (d, J = 5.6 Hz, 1H, −C4H2S−), 7.30 (t, J = 7.8 Hz, 1H, −C6H3−), 7.35 (m, 4H, −C6H4−, −C2H2−), 7.44 (m, 3H, −C4H3S−), 7.62 (d, J = 7.8 Hz, 2H, −C6H3−), 8.81 (d, J = 8.2 Hz, 2H, −C6H4−). IR (KBr disk): v = 2106 cm−1 (w; v(CC)). Positive FAB-MS: m/z 940 [M]+, 616 [M−alkynyl]+. Elemental analyses. Found (%): C 58.73, H 4.31, N 5.93; Calcd (%) for 3: C 58.77, H 4.29, N 5.96. Complex 4. Potassium hydroxide (0.18 g, 3.21 mmol) was added into a well-stirred solution of 9H-carbazole (0.1 g, 0.60 mmol) in acetone (20 mL). After stirring for 10 min, chloroplatinum(II) bzimb11 (0.08 g, 0.12 mmol) (50 mL) was slowly added. The reaction mixture was then stirred at room temperature for 24 h. Upon removal of solvent, the orange suspension was isolated by filtration and then washed successively with deionized water, methanol and diethyl ether to give a yellowish orange solid. Recrystallization of the crude product was performed by slow vapor diffusion of diethyl ether into its dichloromethane solution to give the final product as yellow crystals. Yield: 80 mg, 83%. 1H NMR (400 MHz, CDCl3, 298 K, δ/ppm): δ 1.04 (t, J = 7.3 Hz, 6H, −CH3), 1.37 (m, 4H, −CH2−), 2.04 (m, 4H, −CH2−), 4.61 (t, J = 7.3 Hz, 4H, −NCH2−), 6.04 (d, J = 8.3 Hz, 2H, −C6H4−), 6.71 (t, J = 8.1 Hz, 2H,

Figure 13. Plots of spin density (isovalue = 0.001) of the lowest-lying triplet excited states for complexes 4−6.

Table 4. Emission Energies of the Lowest-Lying Triplet Excited States (T1) of Complexes 4−6 complex

ΔE(T1−S0)/cm−1 (λ/nm)a

4 5 6

18200 (549) 16922 (591) 19600 (510)

a Calculated from the difference in the solvent-corrected energies at the optimized S0 and T1 geometries.

the donor−acceptor alkynyl ligand of the platinum(II) alkynyl complexes or the 3MLCT/LLCT [dπ(Pt)/pπ(carbazolyl)→ π*(N^C^N)] state of the platinum(II) carbazolyl complexes. The donor−acceptor properties of the cyclometalated platinum(II) complexes have also been utilized to fabricate organic memory devices which exhibit binary memory performances with low operating voltages, high ON/OFF ratios of over 105 and long retention times of over 104 s, while the highly emissive platinum(II) carbazolyl complexes have been fabricated into solution-processable OLEDs, in which the optimized device doped with 20% of the complex exhibits high current efficiency of 24.0 cd A−1 and EQE of 7.2%. The work has provided new opportunities for the development of various organic electronic devices by structure−property control of the versatile metal− ligand chromophores.

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EXPERIMENTAL SECTION

Materials and Reagents. Di(thienyl)bithiazole-,9b di(thienyl)benzothiadiazole-,9c and oligothienylenevinylene-containing9d alkynes were synthesized with respect to modification of literature procedures. 9H-Carbazole and 3,6-di-tert-butyl-9H- carbazole were purchased from Sigma-Aldrich. 3,6-Bis((4-(trifluoromethyl)phenyl)- ethynyl)-9H-carbazole was synthesized by Sonogashira coupling between 3,6-diiodo9H-carbazole10b and 1-ethynyl-4-(trifluoromethyl)benzene.10c The cyclometalated chloroplatinum(II) complexes were prepared with reference to a literature procedure upon modification.11 The solvents were purified and distilled according to the standard procedures before their use for reaction. All other reagents were of analytical grade and were used as received. Synthesis. 3,6-Bis((4-(trifluoromethyl)phenyl)ethynyl)-9H-carbazole. A solution of 1-ethynyl-4-(trifluoromethyl)benzene (1.42 g, 8.35 mmol) in triethylamine (50 mL) was added into a mixture of 3,6-diiodo9H-carbazole (1.00 g, 2.39 mmol) with [Pd(PPh3)2Cl2] and CuI. The I


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Journal of the American Chemical Society −C6H4−), 7.06 (m, 4H, −C6H4−), 7.19 (t, J = 8.0 Hz, 2H, −C6H4−), 7.29 (m, 1H, −C6H3−), 7.41 (m, 2H, −C6H4−), 7.67 (d, J = 7.8 Hz, 2H, −C6H3−), 7.86 (d, J = 8.1 Hz, 2H, −C6H4−), 8.32 (d, J = 7.8 Hz, 2H, −C6H4−). Positive FAB-MS: m/z 814 [M]+, 616 [M−carbazole]+. Elemental analyses. Found (%): C 61.31, H 4.86, N 8.83; Calcd (%) for 4: C 61.37, H 4.76, N 8.95. Complex 5. The compound was synthesized with similar procedure as 4, except 3,6-di-tert-butyl-9H-carbazole (0.05 g, 0.18 mmol) was used instead of 9H-carbazole to give the product as yellow crystals. Yield: 60 mg, 87%. 1H NMR (400 MHz, CDCl3, 298 K, δ/ppm): δ 1.03 (t, J = 7.4 Hz, 6H, −CH3), 1.45 (s, 18H, −CMe3), 1.52 (m, 4H, −CH2−), 2.03 (m, 4H, −CH2−), 4.59 (t, J = 7.4 Hz, 4H, −NCH2−), 6.12 (d, J = 8.3 Hz, 2H, −C6H3−), 6.72 (t, J = 7.7 Hz, 2H, −C6H3−), 7.08 (t, J = 7.7 Hz, 2H, −C6H3−), 7.29 (m, 1H, −C6H3−), 7.39 (m, 4H, −C6H4−), 7.64 (d, J = 8.0 Hz, 2H, −C6H3−), 8.07 (d, J = 7.9 Hz, 2H, −C6H4−), 8.31 (d, J = 8.1 Hz, 2H, −C6H4−). Positive FAB-MS: m/z 894 [M]+, 616 [M− carbazole]+. Elemental analyses. Found (%): C 65.16, H 6.41, N 7.35; Calcd (%) for 5·0.5C6H14: C 65.29, H 6.45, N 7.47. Complex 6. The compound was synthesized with similar procedure as 4, except 3,6-bis((4-(trifluoromethyl)phenyl)ethynyl)-9H-carbazole (0.08 g, 0.16 mmol) was used instead of 9H-carbazole to give the product as yellow crystals. Yield: 80 mg, 93%. 1H NMR (400 MHz, CDCl3, 298 K, δ/ppm): δ 1.05 (t, J = 7.4 Hz, 6H, −CH3), 1.56 (m, 4H, −CH2−), 2.04 (m, 4H, −CH2−), 4.60 (t, J = 7.4 Hz, 4H, −NCH2−), 6.00 (d, J = 7.8 Hz, 2H, −C6H3−), 6.74 (t, J = 8.0 Hz, 2H, −C6H3−), 7.12 (t, J = 8.2 Hz, 2H, −C6H3−), 7.30 (d, J = 8.0 Hz, 2H, −C6H4−), 7.41 (m, 3H, −C6H4−, −C6H3−), 7.65 (m, 10H, −C6H4−, −C6H3−), 7.87 (d, J = 7.8 Hz, 2H, −C6H4−), 8.54 (d, J = 8.2 Hz, 2H, −C6H4−). Positive FAB-MS: m/z 1118 [M]+, 616 [M−carbazole]+. Elemental analyses. Found (%): C 62.02, H 3.98, N 6.46; Calcd (%) for 6: C 62.25, H 3.87, N 6.26. Physical Measurements and Instrumentation. The 1H NMR spectra of the complexes were obtained from a Bruker DPX-300 (300 MHz) or Bruker DPX-400 (400 MHz) Fourier transform NMR spectrometer and the chemical shifts were recorded with tetramethylsilane (Me4Si) as reference. Positive FAB mass spectra of the complexes were obtained from a Thermo Scientific DFS high resolution magnetic sector mass spectrometer. The IR spectra were obtained as KBr disks on a Bio-Rad FTS-7 FTIR spectrometer (4000−400 cm−1). Elemental analyses of the newly synthesized complexes were performed on a Flash EA 1112 elemental analyzer. A Varian Cary 50 UV−vis spectrophotometer was used to obtain the electronic absorption spectra with concentrations of the sample solutions in the range of 1 × 10−6 to 2 × 10−4 M. The steady state excitation and emission spectra were recorded on a Spex Fluorolog-3 model FL3-211 fluorescence spectrofluorometer equipped with an R2658P PMT detector. Solid-state emission studies were performed with solid samples confined in a quartz tube. For the measurements of emission of samples in the butyronitrile glass at 77 K, sample concentrations of 10−6 M were used. The degassing process of solution samples for emission measurements was performed in a twocompartment cell with 1 cm path length quartz cuvette on a highvacuum line for at least four successive freeze−pump−thaw cycles. Emission lifetimes were measured by the use of a conventional laser system with excitation source of 355 nm output (third harmonic) of a Spectra-Physics Quanta-Ray Q-switched GCR-150-10 pulsed Nd:YAG laser. The emission decays were detected by a Hamamatsu R928 PMT and the signals were captured by a Tektronix Model TDS-620A (500 MHz, 2GS/s) digital oscilloscope and analyzed by exponential fits. A Hamamatsu C9920-03 absolute PL quantum yield measurement system was utilized to determine the absolute luminescence quantum yields of thin films. The relative luminescence quantum yields in solution were measured by the optical dilute method previously reported by Demas and Crosby.14a A degassed solution of quinine sulfate in 1.0 N sulfuric acid (Φ = 0.546, excitation wavelength at 365 nm) was used as the reference.14b Cyclic voltammetric measurements were carried out by the use of a CH Instruments, Inc. model CHI 600A electrochemical analyzer in dichloromethane solutions with 0.1 M nBu4NPF6 as supporting electrolyte under an argon atmosphere. The working electrode was a glassy carbon electrode (CH Instruments, Inc.) with a platinum wire as the counter electrode, and the reference electrode was a Ag/AgNO3 (0.1

M in acetonitrile) electrode. The ferrocenium/ferrocene couple (FeCp2+/0) was used as the internal reference.14c Solution-Processable OLED Fabrication. For PHOLED fabrication, devices with the structure of ITO/PEDOT:PSS (70 nm)/ emissive layer (60 nm)/3TPyMB (5 nm)/TmPyPB (30 nm)/LiF (0.8 nm)/Al (100 nm) were fabricated, in which the emissive layer was formed by mixing the platinum(II) complexes with MCP to prepare a 10 mg cm−3 solution in chloroform via a spin-coating technique. Current density−voltage−luminance characteristics of devices were simultaneously measured by a programmable Keithley 2420 source meter and a PR-655 colorimeter. All devices were measured under ambient conditions without encapsulation. Fabrication and Characterization of Memory Devices. The indium tin oxide (ITO)-coated glass substrate was precleaned by sonicating successively with deionized water, acetone, isopropanol and absolute ethanol for 15 min in each step. A chloroform solution of complex 1 (10 mg mL−1) was spin-coated onto the ITO glass substrate. The thin film was baked on a hot plate at 70 °C for 10 min. Aluminum top electrodes were thermally evaporated and deposited onto the films. Devices with area of ca. 0.25 mm2 were obtained. The devices were characterized under ambient conditions in a probe station equipped with a Keithley 4200-SCS semiconductor characterization system. Crystal Structure Determination. The X-ray diffraction data of complex 4 were collected on a Bruker Smart CCD 1000, using graphite monochromatized Mo Kα radiation (λ = 0.71073 Å). The structure was then solved by direct methods using the SHELXS-97 program.15 In the structure refinement, full-matrix least-squares refinement on F2 was employed. The positions of H atoms were calculated based on the riding mode with thermal parameters equal to 1.2 times those of the associated C atoms and participated in the calculation of final R indices. All nonhydrogen atoms were then refined anisotropically in the final stage of least-squares refinement. Tables S1 and S2 summarize the crystallographic and structural refinement data. CCDC 1054157 contains the crystallographic data of 4. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif. Computational Details. Calculations were performed with the Gaussian 09 program suite.17 The Stuttgart effective core potentials (ECPs) and the associated basis set were employed to describe Pt18 with f-type polarization functions (ζ = 0.993),19 while the 6-31G(d,p) basis set20 was used for all other atoms. The ground-state (S0) geometries of complexes 4−6 were fully optimized in CH2Cl2 by using DFT calculations with the hybrid Perdew, Burke, and Ernzerhof functional (PBE0)21 in conjunction with the conductor-like polarizable continuum model (CPCM).22 Based on the optimized S0 geometries, TDDFT23 calculations at the same level associated with the CPCM (CH2Cl2) were carried out to compute the singlet−singlet and singlet−triplet transitions. To gain more insight into the emissive states, geometry optimizations of the lowest-lying triplet excited states were performed with the unrestricted UPBE0/CPCM (CH2Cl2) method. All DFT and TDDFT calculations were performed with a pruned (99,590) grid for numerical integration. All stationary points were verified to be potential energy minima by vibrational frequency calculations as there is no imaginary frequency (NIMAG = 0). The calculated emission energies of the T1 state were approximated from the difference in the solventcorrected energies at the optimized S0 and T1 geometries. The Cartesian coordinates of the optimized structures of 4−6 at the S0 and T1 states are given in Tables S3−S8.

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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/jacs.7b04952. Crystal and structure determination data (CCDC 1054157); selected singlet excited states (Sn) of 4−6 computed by TDDFT/CPCM (CH2Cl2) at the optimized ground-state geometries; emission spectra in solid state and butyronitrile glass; electronic absorption spectra of 1 J


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in various solvents and plot of absorption maxima as a function of empirical parameters of solvent polarity ET(30); spatial plots (isovalue = 0.03) of selected molecular orbitals of complex 5 and 6; Cartesian coordinates of the optimized structures of 4−6 at the S0 and T1 states, including Tables S1−S8 and Figures S1−S7 (PDF) X-ray crystallographic data for 4 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Vivian Wing-Wah Yam: 0000-0001-8349-4429 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 and the URC Strategic Research Theme on New Materials. This work has been supported by a grant from the Theme-Based Research Scheme (TRS) (Project No. T23-713/11) and a General Research Fund (GRF) grant (HKU 17302414) from the Research Grants Council of the Hong Kong Special Administrative Region, China. A.K.-W.C. acknowledges the receipt of a University Postdoctoral Fellowship from The University of Hong Kong. Dr. Nelson C. T. Poon and Dr. Eugene Y. H. Hong are gratefully acknowledged for their help in the fabrication of the memory device and the measurement of memory behavior. Dr. Anthony Yiu-Yan Tam is gratefully thanked for his helpful discussion of the project. Dr. Elizabeth Suk-Hang Lam is sincerely acknowledged for her preliminary work in the computational calculations. We are grateful to Dr. L. Szeto for her assistance in X-ray crystal structure data collection and determination, and the Information Technology Services of The University of Hong Kong for providing computational resources.

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