Thienyl Pyrazolates - American Chemical Society

Article pubs.acs.org/IC

Tuning the Extended Structure and Electronic Properties of Gold(I) Thienyl Pyrazolates Lyndsey D. Earl,† Jeffrey K. Nagle,‡ and Michael O. Wolf*,† †

Department of Chemistry, University of British Columbia, Vancouver, Bristish Columbia V6T 1Z1, Canada Department of Chemistry, Bowdoin College, Brunswick, Maine 04011, United States

‡

S Supporting Information *

ABSTRACT: A series of thienyl pyrazole proligands and gold(I) thienyl pyrazolate cyclic trinuclear complexes (CTCs) have been synthesized. The relationship between the structure and emission properties of bridging thienyl pyrazolates within gold(I) cyclic trinuclear complexes suggests that the nature of dual emission is sensitive to ligand conjugation length. Density functional theory has been used to support the assignment of metal-sensitized, ligand-localized phosphorescence from monothienyl complexes, while low-lying, ligand-localized LUMOs present in bithienyl systems prohibit metal-sensitized phosphorescence. Soluble n-hexyl derivatives have been synthesized to explore the electrochemical properties of gold(I) thienyl pyrazolates CTCs, and conductive electropolymerized thin films were realized.

■

INTRODUCTION Gold(I) pyrazolate cyclic trinuclear complexes (CTCs) have been widely studied over the past 30 years for basic science research and applications in device fabrication.1−4 The structural5,6 and electronic7 properties associated with coinage metal CTCs have resulted in a rich and ever expanding area of research for this class of materials. The potential uses of CTCs include metal−organic light emitting diodes,8 biomedical applications,9 liquid crystals,2,10 and as building units in metal−organic frameworks.11 Emission in gold(I) CTCs originates from both aurophilic interactions12 and ligand centered states, making these materials suitable candidates for tunable or white light generation. However, little attention has been given to the conjugation length and orientation of the peripheral ligand moiety with respect to tuning emission properties.13,14 Electron rich peripheral groups such as oligothiophenes are suitable candidates for further studying the electronic structure of gold(I) CTCs with applications related to sensors, photovoltaics, and electrochromic devices.15 Additionally, cyclic trinuclear complexes containing thiophene groups may be suitable for electropolymerization16,17 to form conductive thin films. This paper sets out to establish the relationship between the structure of bridging thienyl pyrazolates within gold(I) cyclic trinuclear complexes and the photoluminescent properties of these compounds. We report a series of gold(I) cyclic trinuclear complexes with thienyl moieties exhibiting tunable emission. Computational methods are used to support the assignment of metal-sensitized, ligand-localized phosphorescence from monothienyl complexes, while low lying ligand-localized LUMOs present in bithienyl systems prohibit metal-sensitized phos© XXXX American Chemical Society

phorescence. Soluble n-hexyl derivatives have been synthesized to explore the electrochemical properties of gold(I) thienyl pyrazolates CTCs, and characterization of electropolymerized thin films has been conducted.

■

EXPERIMENTAL SECTION

General. 3-Hexylthiophene, 2,2′-bithiophene-5-boronic acid pinacol ester, 1-boc-pyrazole-4-boronic acid pinacol ester, 1-boc-3,5dimethyl pyrazole-4-boronic acid pinacol ester, tetrabutylammonium hexafluorophosphate ([n-Bu 4 N][PF 6 ]), and [1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene](3-chloropyridyl)palladium(II) dichloride (PEPPSI-IPr) were purchased from Sigma-Aldrich. Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) was purchased from Strem Chemicals. 2-Thienylboronic acid was purchased from Maybridge. THF was purchased from OmniSolve. Triethylamine was purchased from Fisher Scientific. All chemicals were used as received. AuCl(tht) (tht = tetrahydrothiophene),18 2-bromo-3hexylthiophene,19 and 5-bromo-3,3′-dihexyl-2,2′-bithiophene19 were prepared according to literature procedures. Syntheses of 4-iodo-1-[(4methylphenyl)sulfonyl]-1H-pyrazole and 3,5-dimethyl-4-iodo-1-[(4methylphenyl)sulfonyl]-1H-pyrazole were modified from literature procedures:20 iodinated precursors were used instead of brominated precursors. The synthetic details for the proligands and gold(I) complexes are located in the Supporting Information. 1 H and 13C NMR spectra were collected on a Bruker AV-300 or AV-400 spectrometer and were referenced to residual solvent: CDCl3, 7.27 ppm (1H), 77.0 ppm (13C); acetone-d6, 2.05 ppm (1H), 29.8 ppm (13C); CD2Cl2, 5.32 ppm (1H), 53.8 ppm (13C). UV−vis absorption spectra were collected using a Varian Cary 5000 spectrometer. Emission and excitation spectra were obtained on a Photon Technology International fluorimeter using a 75 W arc lamp as a Received: December 24, 2013

A

dx.doi.org/10.1021/ic4031365 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Chart 1

Chart 2

source and were uncorrected for lamp intensity. Low temperature emission spectra were obtained from toluene solutions using an Oxford OptistatDN cryostat. EI mass spectra were obtained using a Kratos MS-50 mass spectrometer coupled to a MASPEC data system. MALDI-TOF mass spectra were obtained on a Bruker Biflex IV MALDI-TOF instrument equipped with a nitrogen laser. CHN elemental analyses were performed using an EA1108 elemental analyzer. Infrared spectra were obtained on a Thermo Nicolet 6700 with a Smart Orbit accessory in the range of 4000−400 cm−1. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Leybold MAX200 spectrometer equipped with an aluminum Kα source. Cyclic voltammetry was performed on a Pine potentiostat with either a glassy carbon or indium tin oxide (ITO) working electrode, silver/silver ion nonaqueous reference electrode, and platinum mesh counter electrode. Decamethylferrocene was used as an internal reference. Tetrabutylammonium hexafluorophosphate ([n-Bu4N][PF6]) was recrystallized three times from ethanol and heated to 80 °C for three days prior to use. Solutions used for cyclic voltammetry contained 0.1 M [n-Bu4N][PF6] and 1 × 10−3 M analyte. Cyclic voltammetry was carried out in CH2Cl2 dried over an activated alumina column. X-Ray Crystallography. All crystals were mounted on glass fibers and were measured on a Bruker APEX DUO diffractometer with graphite monochromated Mo Kα radiation. Data were collected and integrated using the SAINT software package.21 Data were corrected for absorption effects using the multiscan technique (SADABS).22 Structures were solved using direct methods.23 Non-hydrogen atoms were refined anisotropically. All hydrogens were placed in calculated positions. Refinements for AuPzT and AuPz*3HT (see Chart 2 for molecular structures) were performed using SHELXL-9724 via the WinGX25 interface. Compound AuPzT crystallizes as a twocomponent twin and with disorder about one of the thiophene rings. Compound AuPz*3HT crystallizes with disorder in the n-hexyl chains. Restraints on bond lengths for the disordered n-hexyl groups were employed to maintain reasonable geometries. Crystallographic details for compounds AuPzT and AuPz*3HT can be found in Table S1. Visualization of the solid state molecular structures was performed using CrystalMaker. DFT Calculations. The 2013.01 version of the Amsterdam Density Functional (ADF) program was used for all calculations.26−33 All electrons were included in the variational treatment (i.e., no frozencore approximation was applied). C1 (NOSYM) symmetry-unre-

stricted ground state geometry optimizations were performed with experimental X-ray crystallographic geometries, where available, as starting points. The ground state-optimized geometries were used as starting points for the TD-DFT excited state geometry optimizations. Energies, geometries, and the orbital electronic structure were calculated using the generalized gradient approximation (GGA) of density functional theory (DFT) at the BP86 level. The GGA proceeds from the local density approximation (LDA), where exchange is described by Slater’s Xa potential28 and correlation is treated in the Vosko−Wilk−Nusair (VWN) parametrization,29 and is augmented with nonlocal corrections to exchange due to Becke30,31 and correlation due to Perdew32 added self-consistently.33 Relativistic effects were taken into account in all cases using the zeroth-order relativistic approximation (ZORA),34 and all-electron TZ2P basis sets from the ADF ZORA basis sets library were used for all atoms.27 ADF calculations at this level have been shown to effectively account for metal−metal interactions in compounds containing related heavy metals.35 The SAOP XC functional and the Davidson method were used for TD-DFT36 calculations of ground state and singlet and triplet excited state excitation energies, and spin-orbit coupling effects were included in some of these calculations.37 All other parameters were the same as for the BP86 ground state geometry optimizations.

■

RESULTS AND DISCUSSION Synthesis and Solid State Molecular Structures. Proligands (Chart 1) were synthesized from cross coupling reactions of the appropriate pyrazole and thiophene molecules (see Supporting Information for synthetic details). The reaction of proligands with AuCl(tht) and a base produces the cyclic trinuclear complexes shown in Chart 2. Compounds with nhexyl chains are soluble in common organic solvents, while compounds lacking aliphatic moieties are very sparingly soluble in common organic solvents. Exposure of a mixture of proligand HPzT and AuCl(tht) to a base gives colorless crystals of the nine-membered trimeric species AuPzT (Figure 1). Thienyl pyrazolate torsion angles of the cyclic trinuclear complex are similar with values of −155.4(11)° (C1−C2−C4−S1), −145.4(12)° (C8−C9− C11−S2), −154.4(14)° (C15−C16−C18−S3), and −152(3)° B


Inorganic Chemistry

Article

Figure 1. (a) Solid state molecular structure of AuPzT. Hydrogens and disorder have been omitted for clarity. Thermal ellipsoids are shown at 50% probability. Views of two molecules of AuPzT along the (b) c-axis and (c) b-axis.

the molecular structure. PXRD was used to confirm the preliminary structure by comparing the predicted and experimental powder patterns (Figure S2). Diffraction associated with the 0n0 or 00n reflections dominates the powder pattern, which may be due to the preferred orientation of either the ac or ab plane with the plane of the sample holder. The discussion of the structure is limited to the nuclearity of the molecule as well as the intermolecular proximity of gold atoms and the bridging linkers to one another. One intermolecular gold(I)−gold(I) contact of 3.216(10) Å is present in AuPzT2, and the interaction extends to form onedimensional chains of the cyclic trinuclear species. It is notable that the presence of a planar biaryl group at the 4-position of the pyrazolate does not prohibit intermolecular aurophilic interactions. The extended structures of compounds AuPzT and AuPzT2 are 1D coordination polymers connected by noncovalent aurophilic interactions. Molecules of AuPzT2 are weakly connected in a staircase structure, while AuPzT adopts a rotationally staggered extended structure.

(C17−C16−C18B−S3B). Intramolecular Au−Au distances are typical with values between 3.30 and 3.38 Å. Intermolecular Au−Au distances are 3.2170(7) Å (Au1−Au2), 3.5841(7) Å (Au3−Au3), and 3.8049(7) Å (Au2−Au1). Both contacts Au1−Au2 and Au3−Au3 fall within the distance requirement for aurophilic interaction.12 Figure 1b shows that molecules of AuPzT stack in an ABAB fashion along the c-axis. The rotational angle between the two orientations is approximately 30°, and there are no intermolecular π−π interactions. This rotation angle minimizes repulsive intermolecular interactions from the organic linker while maximizing ground state aurophilic interactions. Looking at AuPzT along the c-axis and b-axis (Figure 1c) illustrates the interplay of thienyl pyrazolate torsion angle and long-range orientation. Crystals of AuPzT2 were obtained from a synthetic route similar to AuPzT, and a preliminary solid state molecular structure of compound AuPzT2 was determined (Figure S1). Diffuse scattering and severe twinning due to, in part, the similarity of the b and c axis dimensions (see Supporting Information for unit cell parameters) prevented a full analysis of C


Inorganic Chemistry

Article

Figure 2. (a) Solid state molecular structure of AuPz*3HT. Hydrogens and disorder have been omitted for clarity. Thermal ellipsoids are shown at 50% probability. (b) Dimer unit of AuPz*3HT. (c) Packing of AuPz*3HT. (d) Au···S contact of two molecules of AuPz*3HT. n-Hexyl chains have been omitted for clarity.

an unoccupied orbital on Au3. Near-perpendicular thienyl pyrazolate torsion angles of −84(2)° (C2−C3−C6−S1), 90.4(19)° (C17−C18−C21−S2), and −70(2)° (C32−C33− C36−S3) are attributed to repulsion of the methyl and n-hexyl groups. Electronic Absorption and Emission Spectra. A summary of the solution state electron absorption spectra data in CH2Cl2 for the proligands and their corresponding gold(I) complexes is shown in Table 1. Thienyl pyrazole π−π* transitions are the primary features of the spectra. An increase in conjugation causes bathochromic shifts, and methylation of the 3 and 5 positions of the pyrazoles causes hypsochromic shifts. Coordination of the n-hexyl derivative proligands to gold(I) centers does not induce significant hypsochromic or bathochromic shifts in the absorption maxima. In addition, no significant solvent or concentration dependence in the electronic absorption profiles was observed. The emission spectra of the proligands show broad or weakly structured fluorescence at room temperature in both solution and the solid state. Upon cooling, the emission features become

Unlike the 1D chains of compounds AuPzT and AuPzT2, crystals of AuPz*3HT (Figure 2a) grown from layering methanol on a CH2Cl2 solution of AuPz*3HT form dimers in a chair geometry (Figure 2b). Gold ions Au1 and Au2 engage in intermolecular aurophilic interactions, and the atoms are separated by 3.109(4) Å. Two n-hexyl chains per molecule are oriented perpendicular to the CTC plane of the molecule, one pointing toward the other molecule within the dimer and one pointing away from the dimer, and one chain wraps around and resides adjacent to the dimeric unit. The orientation of the nhexyl chains and formation of dimers in the solid state is similar to that observed in the crystalline phase of tris(μ-N,N′-(3,5dimethyl-4-octylpyrazolato)trigold(I)).2 As seen in the extended structure of AuPz*3HT in Figure 2c, the presence of n-hexyl chains is a driving force for the formation of isolated dimers rather than one-dimensional polymeric structures. Figure 2d shows that there is an intermolecular Au3···S3 contact of 3.290(7) Å which is less than the sum of the van der Waals radii for these two atoms. Electron density from the lone pair on the S3 may interact with D


Inorganic Chemistry

Article

HPzT and HPz*T and are assigned to ligand-based fluorescence. Upon cooling, the fine structure of the fluorescence becomes apparent. Emission intensity increases for AuPzT and remains the same for AuPz*T. Excitation of AuPzT and AuPz*T at 315 nm and 298 K generates no emission features. Broad, weakly structured emission with a lifetime of 2 ms (at 77 K) and centered at 540 and 520 nm, respectively, grows in upon cooling. The Stokes shifts for these low temperature emission bands are 12 000−13 000 cm−1, slightly less than the Stokes shifts of T1 → S 0 transitions reported for other cyclic trinuclear complexes.7,43 The magnitude of the Stokes shifts suggests some geometric distortion occurs between the triplet excited and singlet ground states. Two primary geometric changes to consider are contractions of inter- and intramolecular gold− gold distances43,44 and rotation of the aryl groups.39,41 Structured features spaced 1400−1500 cm−1 apart are typical for pyrazole and thiophene vibronic coupling.38 The long lifetimes and magnitudes of the Stokes shifts are typical for ligand-centered phosphorescence from this class of complexes.7 In the present case, population of the ligand-localized T1 states is postulated to occur from higher-lying Tn states that have substantial gold character. A recent report of a gold(I) 3-(2thienyl) pyrazolate complex shows similar behavior to that of AuPzT and AuPz*T at low temperatures.14 The authors assigned sharp, structured emission at low temperatures to multiple states having significant gold character. However, their data, particularly the energy spacing between structured features, are more consistent with ligand-centered transitions. Complexes AuPz3HT and AuPz*3HT exhibit similar variable temperature emission behavior compared to complexes AuPzT and AuPz*T. A detailed study of AuPz*3HT (Figure 4) was conducted because of its known solid state molecular structure. At room temperature, structured solution state emission is centered at 360 nm, and weakly structured and broad emission is centered at 415 nm in the solid state. Structured emission centered at 505 nm with a Stokes shift of 11 950 cm−1 grows in gradually in the solid state (Figure 4b) and appears suddenly near the glass transition temperature in toluene (Figure 4a). The appearance of lower energy emission only upon transitioning to a glassy or solid solution suggests there may be specific structural requirements such as dimer formation or reduced rotational freedom in addition to reduced nonradiative decay pathways for this radiative process to occur. Unlike monothienyl pyrazolate gold(I) cyclic trinuclear complexes AuPzT, AuPz*T, AuPz3HT, and AuPz*3HT, fluorescence dominates the variable temperature emission spectra of bithienyl complexes AuPzT2 and AuPz*T2 . Inspection of the emission spectra of AuPzT2 and AuPz*T2 (Figure 5 and λmax data in Tables 2 and 3) shows the emissions from these complexes are bathochromically shifted compared to proligands HPzT2 and HPz*T2, are featured at room temperature, and have spectra that become slightly more resolved when cooled to 77 K. No new emission features develop upon cooling to cryogenic temperatures. For both AuPzT2 and AuPz*T2, excitation between 300 and 400 nm produces emission spectra that are of similar shape and intensity. The emission spectra of n-hexyl bithienyl compounds AuPz3HT2 and AuPz*3HT2 are hypsochromically shifted compared to AuPzT2 and AuPz*T2 and have fewer structured features at room temperature. The emission spectra of

Table 1. Solution State Absorption Data for the Proligands and Their Corresponding Gold(I) Complexes in CH2Cl2 at 298 K compound HPzT HPzT2 HPz3HT HPz*3HT HPz3HT2 HPz*3HT2

λmax (nm) 313, 345, 305, 245 294, 285,

270, 245 324, 248 255 245 245

compound HPz*T HPz*T2 AuPz3HT AuPz*3HT AuPz3HT2 AuPz*3HT2

λmax (nm) 264, 340, 305, 245, 305, 300,

247 245 235 230 255, 235 250, 230

more structured owing to vibronic coupling within the pyrazole and thiophene moieties.7,38 No evidence for phosphorescence is present at room temperature or at 77 K. Phosphorescence has been observed in functionalized terthiophenes at 80 K with laser excitation and gated detection39 as well as in thin films of poly(3-hexylthiophene) at 18 K using typical detection methods.40 However, phosphorescence of conjugated bi- and triaryl thiophenes is often difficult to observe due to slow rates for intersystem crossing and nonradiative decay pathways that are accessible even at 77 K.39,41,42 The variable temperature emission spectra of complexes AuPzT and AuPz*T are shown in Figure 3. Broad emission features centered at 445 and 425 nm, respectively, are present at room temperature when AuPzT and AuPz*T are excited at 375 nm. These emission features are similar to proligands

Figure 3. Variable temperature solid state emission spectra of AuPzT and AuPz*T. (a) Emission spectra of AuPzT at λex = 375 nm (solid line) and λex = 315 nm (dotted line). (b) Emission spectra of AuPz*T at 298 and 77 K. E


Inorganic Chemistry

Article

Table 3. Emission Data for Gold(I) Complexes AuPzT− AuPz*3HT2 compound AuPzT AuPz*T AuPzT2 AuPz*T2 AuPz3HTa AuPz3HTb AuPz*3HTa AuPz*3HTb AuPz3HT2a AuPz3HT2b AuPz*3HT2a AuPz*3HT2b a

λem (nm), 298 K 455 428 440, 432, 423 380 420 350, 437, 410, 418 403

505, 534 460, 490, 528

365 486 483

λem (nm), 77 K 439, 399, 420, 410, 416, 490, 418, 370, 420, 415, 403, 400,

468, 424, 451, 433, 484, 515, 472, 470, 441, 438, 421, 431

500, 453, 476, 462, 508 555 497 505 470, 469, 444

530, 509, 505, 492,

570 536 540 528

552, 593 496, 545, 588

Solid state. bSolution state, toluene.

Figure 4. Variable temperature emission spectra of compound AuPz*3HT in (a) solution; (b) solid state. λex = 315 nm.

Figure 6. Solid state emission spectra at 77 K (solid) and 298 K (dotted) of (a) AuPz3HT2; (b) AuPz*3HT2.

Figure 5. Variable temperature solid state emission spectra of AuPzT2 and AuPz*T2, λex = 300−400 nm.

Excitation of AuPz3HT2 at 370 nm results in a broad emission feature at 430 nm that is structured at 77 K (Figure 6a). Similarly, excitation at 405 nm generates emission centered at 485 nm. For both excitations, new features centered at 565 nm appear upon cooling to 77 K. These low energy features are similar in structure and energy to the metal-sensitized, ligandlocalized phosphorescence of their monothienyl analogues. However, monitoring the emission at 565 nm shows a weak, broad excitation feature centered at 400 nm, with a Stokes shift that is much smaller than for the monothienyl complexes (7000 cm−1 vs 12 000−13 000 cm−1). The lifetime for the 565 nm emission could not be determined due to its low intensity and overlap with the strong emission features at 430 and 485 nm.

Table 2. Emission Data for Proligands HPzT−HPz*3HT2

a

compound

λem (nm)

compound

λem (nm)

HPzTa HPzT2a HPz3HTb HPz3HT2b

337, 388, 420, 440 419, 448, 490, 523 390 405

HPz*Ta HPz*T2a HPz*3HTb HPz*3HT2b

338, 415, 430 418, 440, 482, 512 380 400

Solid state. bSolution state, toluene.

AuPz3HT2 and AuPz*3HT2 in the solid state at 298 and 77 K are shown in Figure 6. F


Inorganic Chemistry

Article

from about 20° to 40°. Since the calculated differences in orbital properties and transition energies showed only minor differences within these ranges, and since there was generally good agreement between calculated and experimental geometries for the gold(I) complexes with both Pz*T and PzT ligands, no attempt was made to locate more precise global minima. Intermolecular Au−Au distances were all calculated to be about 3.4 Å, close to the experimental values. The energy difference between the ligand-localized LUMO and the lowest vacant metal-based orbital as well as Pz-T and T-T interannular torsion angles for selected complexes were calculated (Table 5). As the dihedral angle increases, the energy

As shown in Figure 6b, excitation of AuPz*3HT2 at 385 nm causes emission centered at 420 nm that has more structure at low temperatures. The 420 nm band is assigned as fluorescence based on the wavelength of the emission relative to the proligand. Interestingly, when AuPz*3HT2 is excited at 450 nm at 77 K, structured emission centered at 525 nm is present. The small Stokes shift of 3200 cm−1 in conjunction with the similarity in excitation and emission energy to other conjugated triaryl systems39 suggests emission at 525 nm is ligand-based phosphorescence that results from direct excitation to the lowest-lying, ligand-localized triplet state. It is not clear why AuPz3HT2 shows two emission bands having different excitation energies. Similar behavior for some related trinuclear copper(I) complexes has been reported.7 We speculate that the bands are both fluorescence and originate from ligand-localized singlet states that are close in energy but not in thermal equilibrium with each other. Overall, ligand-based fluorescence dominates the emission spectra of the gold(I) complexes at room temperature. Metalsensitized, ligand-localized phosphorescence is present at low temperatures in monothienyl complexes in addition to ligandlocalized fluorescence. This dual emission behavior suggests coinage metal thienyl-pyrazolate complexes are suitable targets for white light generation applications.14 Neither phosphorescence nor significant changes in the emission spectra of AuPzT2 and AuPz*T2 are observed, while weak emission assigned to ligand-based phosphorescence from the complex is present at 77 K in AuPz3HT2 and AuPz*3HT2. Because these features are present in the solid state, the low temperature transitions are not attributed to a dissociated and oxidized ligand. Computational Studies. Computational studies were conducted to provide insight into the geometry of proligands and complexes as well as to support the assignment of metalsensitized, ligand-localized phosphorescence for monothienyl metal complexes. Table 4 summarizes the interannular torsion

Table 5. Calculated Energy Difference between the LigandLocalized LUMO and the Gold-Localized Unoccupied Molecular Orbital and Dihedral Angles of Pz-T and T-T for Selected Complexes compound AuPzT AuPzT (dimer) AuPz*T AuPz*3HTa AuPzT2 AuPz*3MT AuPz*3MT (dimer) a

compound

C−C−C−S (deg) 12.0 23.9 60.4 67.2 73.6

dihedral angle, average (deg)

0.44 0.32 0.22 −0.03 0.98 0.19 0.23

dihedral angle, range (deg)

18.0 14.7

17−19 1−24

38.9 81.0 Pz-T: 14.5; TT:15.6 60.0 59.2

38−40 70−90 Pz-T: 11−17; T-T:13−18 58−63 58−60

From experimental geometry.

gap between the two orbitals diminishes. In the case of AuPz*3HT, the lowest vacant metal-based orbital is the LUMO, residing 0.03 eV below the ligand-localized unoccupied molecular orbital. A longer conjugation length in AuPzT2 significantly lowers the energy level of the ligand-localized LUMO and raises the energy gap to 0.98 eV. This energy difference is inaccessible to conventional UV excitation, and emission from an excited state having substantial gold character is not observed experimentally. A Jablonski diagram that includes the molecular orbitals that contribute to the fluorescence and phosphorescence of AuPzT is shown in Figure 7. The contours of the molecular orbitals of AuPzT are representative of the ligand-localized orbitals and lowest vacant metal-based orbital of the other thienylpyrazolate gold(I) complexes. The lowest energy states are composed of HOMO−1 → LUMO, HOMO → LUMO+1, and HOMO → LUMO transitions that are all ligand-localized. The lowest vacant gold-based orbital with greater than 10% gold character is the LUMO+5, which is 95% gold-based. Excitation at 375 nm (left) populates the LUMO and results in 460 nm fluorescence. The lowest spin-allowed transition is calculated by TD-DFT to occur at 361 nm and corresponds to mixtures of HOMO and HOMO−1 to LUMO and LUMO+1 orbital transitions. These four orbitals are all ligand-localized. Excitation at 315 nm (right) populates the LUMO+5, with a TD-DFT calculated wavelength of 310 nm. This is followed by rapid intersystem crossing and relaxation to the lowest energy triplet excited state that results in 540 nm phosphorescence. It should be noted that the frontier orbitals in the S1 and T1 excited state geometries of AuPzT are similar in shape and relative energies to their ground state analogues. The difference between the lowest lying singlet−singlet and singlet−triplet

Table 4. Calculated Pz-T (C−C−C−S) Interannular Torsion Angles of Selected Proligands and Model Proligands HPzT HPz*T HPz*3MT HPz*3ET HPz*3HT

ΔE (eV)

angles for the optimized ground state of selected proligands and model proligands. As expected, compounds bearing methyl groups on the pyrazole (Pz*) have larger torsion angles compared to the Pz derivatives. The presence of both methyl groups on the pyrazole and an aliphatic group at the 3 position of the thiophene causes interannular twisting of 60.4°, 67.2°, and 73.6° for methyl, ethyl, and n-hexyl substituents, respectively. ADF geometry optimizations of both the proligands and the gold(I) complexes revealed shallow energy minima for rotations of the thiophene-pyrazole rings relative to each other. Specifically, for HPzT and the AuPzT complexes, virtually no energy differences were observed for Pz-T ring rotations ranging from coplanar (0°) to about 20°, while for HPz*T and the AuPz*T complexes, a similar insensitivity of energy to ring rotation was observed for dihedral angles ranging G


Inorganic Chemistry

Article

Figure 7. Jablonski diagram showing the radiative pathways present in AuPzT at 77 K. Excitation at 375 nm (left) populates the LUMO and results in fluorescence emission. Excitation at 315 nm (right) populates the LUMO+5, where intersystem crossing and relaxation to the lowest triplet excited state generates phosphorescence emission.

Au−Au 6pz bonding character that accounts for the small decrease in the Au−Au distances accompanying the small increase (0.17) in formal Au−Au bond order (3.254 Å, 3.262 Å, and 3.332 Å) in the T 14 excited state. Interestingly, planarization of all three ligands also occurs with the torsional angles decreasing from about 17°, 18°, and 19° in the ground state to 0.1°, 0.1°, and 3.0° in the excited state. These calculations support the assignment of the lowest energy emission to a gold-sensitized, ligand-localized phosphorescence from the lowest-lying triplet state (T1) populated from a higher energy, triplet state having significant gold character. A study of a series of 2-pyridyl pyrrolide/phosphine complexes containing d10 coinage metal ions found compounds with MLCT character in the LUMO of the S1 state had enhanced intersystem crossing rates.46 This effect was larger compared to the influence of the atomic number of an “external” heavy metal ion or systems with MLCT character in higher order singlet excited states. Systems with accessible unoccupied orbitals containing significant metal character such as monothienyl pyrazolate gold(I) CTCs exhibit intense ligand phosphorescence when intersystem system crossing is fast and nonradiative decay pathways become less available at lower temperatures. Bithienyl analogues have very weak or no phosphorescence intensity because of the relatively high energy of the lowest vacant metalbased orbital that is required for population of the lowest ligand-localized triplet excited state. The computational results show that the energy of the unoccupied metal-localized orbital relative to the ligand-localized LUMO is sensitive to both the ligand conjugation path length and, in turn, the thiophene− pyrazole and thiophene−thiophene interannular torsion angles. Cyclic Voltammetry and Electropolymerization. Electropolymerization of oligothiophene complexes can give conductive materials with longer conjugation path lengths.47 Compounds with solubilizing n-hexyl chains were synthesized specifically to investigate the solution state electrochemical

ligand-centered states is 0.30 eV. This corresponds well to the experimental difference in emission maxima of fluorescence and phosphorescence of 0.29 eV and further supports the assignment of phosphorescence from a ligand-localized state. At room temperature, excitation at 315 nm (experimental excitation) into the lowest vacant metal-based orbital generates observable emission. Excitation of AuPzT at 315 nm is postulated to lead to rapid Sn to Tn singlet−triplet intersystem crossing within the electronic state involving the metal-localized LUMO+5. This is then followed by efficient population of the lowest, ligand-localized triplet state responsible for the observed phosphorescence below room temperature. This rapid intersystem crossing presumably occurs to the exclusion of a significant population of the ligand-localized singlet state responsible for the observed fluorescence. Encouragingly, the lowest calculated phosphorescence band for AuPzT, corresponding to decay from the LUMO to HOMO, is calculated to occur at 540 nm. This is in excellent agreement with the observed phosphorescence wavelength maximum (Figure 3). An ADF TD-DFT calculation for the AuPzT monomer that includes spin−orbit effects in a self-consistent way reveals that spin−orbit coupling has little effect on the calculated transition energies that include only scalar relativistic effects. The calculated energy of the spin-allowed transition that involves promotion of an electron from the ligand-localized HOMO to the lowest vacant metal-based orbital decreases by only 0.05 eV, from 4.02 eV (308 nm) to 3.97 eV (312 nm). Optimized excited state geometry calculations of AuPzT show that population of the lowest-lying triplet state leads to a planarization (within about 0.1°) of one of the three thienylpyrazolate groups within the gold(I) complex, with little or no change in the Au−Au distances (3.394 Å, 3.382 Å, and 3.402 Å). This type of geometric change in the excited state is typical of oligothiophenes and systems with ligand-centered phosphorescence.15,45 The lowest unoccupied, gold-localized orbital is the LUMO+5. As seen in Figure 7, this orbital has substantial H


Inorganic Chemistry

Article

oxidation of HPz3HT2 and HPz*3HT2 shows redox couples with anodic peak maxima at 1.05 V. An oxidative peak at 0.86 and 0.87 V, respectively, grows in after the first cycle, and a small increase in current density occurs with successive scans. This behavior indicates the formation of a more conjugated quarterthienyl bis-pyrazole species. Deposition of oligomers or polymers on the working electrode was not observed by UV− vis or fluorescence spectroscopy of the ITO surface. Anodic scans past 1.5 V cause overoxidation and a drop-off in the current density of bithienyl molecules, whereas monothienyl proligands are stable. Cyclic voltammograms of monothienyl complexes AuPz3HT and AuPz*3HT are shown in Figures S3 and 9, respectively,

properties of gold(I) thienyl pyrazolates. The calculated structures of n-hexyl proligands HPz3HT−HPz*3HT2 and the solid state molecular structure of AuPz*3HT suggest that the proligands and bridging linkers of metal complexes AuPz3HT−AuPz*3HT are not planar, and the metal complexes may not be suitable candidates for electropolymerization. Nonetheless, films of AuPz3HT, AuPz3HT2, and AuPz*3HT2 were grown via oxidative electropolymerization. Table 6 summarizes the electrochemical data obtained for compounds HPz3HT−HPz*3HT 2 and AuPz3HT− Table 6. Electrochemical Data of Compounds HPz3HT− HPz*3HT2 and AuPz3HT−AuPz*3HT2 vs SCE (Saturated Calomel Electrode)a compound

Epa (V)

Epc (V)

HPz3HT HPz*3HT HPz3HT2 HPz*3HT2 AuPz3HT AuPz*3HT AuPz3HT2 AuPz*3HT2

1.35 1.46 0.86,b 1.05 0.87,b 1.05 0.89,b 1.07, 1.25 1.14,b 1.53, 1.65, 1.96 0.90,c 1.05 0.90,b 1.05

0.78 0.80, 0.96 0.85, 1.14 0.59,c 1.01,1.25 0.77, 0.92 0.69, 0.86

a Data were collected at a scan rate of 100 mV s−1 in CH2Cl2 with 0.1 M [n-Bu4N][PF6] as the supporting electrolyte. bNot present on first scan. cFinal scan.

AuPz*3HT2. In general, the reported redox potentials reflect the degree of conjugation within these compounds. Molecules with methylated pyrazoles have higher redox potentials compared to the nonmethylated analogues, and bithienyl compounds have lower oxidation potentials than the monothienyl counterparts. Cyclic voltammograms of HPz3HT and HPz3HT2 are shown in Figure 8 and have similar features to the methylated counterparts HPz*3HT and HPz*3HT2. HPz3HT and HPz*3HT both have irreversible oxidations at 1.35 and 1.46 V, respectively. No new features grow in with successive scans, suggesting oxidative coupling of the proligand to form a bithienyl bis-pyrazole species does not occur. The initial

Figure 9. Successive cyclic voltammograms of AuPz*3HT on ITO. Data were collected at a scan rate of 100 mV s−1 in CH2Cl2 with 0.1 M [n-Bu4N][PF6] as the supporting electrolyte.

and do not reflect the behavior of proligands HPz3HT and HPz*3HT. Oxidative scans of AuPz3HT beyond 1.25 V generate a new anodic peak at 1.06 V that signifies the formation of species with a longer conjugation length, presumably oxidatively coupled units of AuPz3HT. An anodic shift of the peak centered at 1.75 V occurs with successive cycles and suggests AuPz3HT becomes less conjugated with increasing scans. A yellow film deposits on the ITO working electrode with successive cycles. Overoxidation (beyond 2.2 V) results in a purple film and decreased current intensity. This behavior is assigned to the irreversible oxidation of the polymer.15 AuPz*3HT exhibits similar electrochemical behavior to AuPz3HT during initial voltammetry cycles including the appearance of an oxidative wave at 1.14 V after the first cycle (black trace). The current increases with successive scans (brown and purple traces), and shifts of both anodic and cathodic waves occur (purple, blue, and green traces). A decrease in the current, further shifts of redox waves, and the formation of a purple residue on the ITO suggests irreversible oxidation or decomposition of the material transpired. Gold(I) complexes AuPz3HT2 and AuPz*3HT2 have similar cyclic voltammograms as shown in Figures 10 and S4. Two oxidative waves are present at 0.90 and 1.05 V for both compounds. Both peaks increase in current intensity with repeated scans, which indicates electropolymerization occurred. The peak at 0.90 V, which is not present during the first scan cycle, increases at a faster rate than the peak at 1.05 V. The wave at 0.90 V is attributed to an oxidatively coupled metal

Figure 8. Cyclic voltammograms of HPz3HT (blue) and HPz3HT2 (red) on glassy carbon electrodes. Data were collected at a scan rate of 100 mV s−1 in CH2Cl2 with 0.1 M [n-Bu4N][PF6] as the supporting electrolyte. I


Inorganic Chemistry

Article

pounds AuPz3HT2 and AuPz*3HT2, the Au/S and Au/N elemental composition ratios are close to the monomer formula units of 1:1 and 1:2, respectively. The variable temperature emission of polymers of AuPz3HT2 and AuPz*3HT2 were investigated. The emission spectra of yellow thin films of poly-AuPz3HT2 and poly-AuPz*3HT2 in their undoped state are shown in Figure 11. Polymers were grown by oxidative coupling on ITO-coated quartz substrates. The spectra at 298 and 77 K are identical with no significant enhancement in emission intensity or appearance of new features at 77 K. Compared to the monomer in the solid state, the emission maximum is red-shifted and single-featured. The emission at 500 nm is assigned as ligand-localized fluorescence. An increased conjugation length of the bridging linker unit would cause the observed shift in ligand-localized emission. While care was taken not to overoxidize the films, a shoulder at 610 nm is present in poly-AuPz*3HT2 that suggests some overoxidized polymer may be present in the film.

Figure 10. Cyclic voltammograms of AuPz3HT2 on ITO. Data were collected at a scan rate of 50 mV s−1 in CH2Cl2 with 0.1 M [nBu4N][PF6] as the supporting electrolyte.

■

CONCLUSIONS Photoluminescent gold(I) cyclic trinuclear complexes containing thienyl pyrazolate bridging linkers were synthesized and characterized. The X-ray data demonstrate that the presence of aryl, biaryl, or alkylated aryl groups at the 4 position of the pyrazolate does not prohibit the formation of dimeric species in the solid state. The orientation of n-hexyl groups prevented the formation of a polymeric species in AuPz*3HT as a consequence of the thienyl pyrazolate torsion angle. All metal complexes emit ligand-localized fluorescence that is similar to proligand fluorescence. For monothienyl complexes, dual emission from ligand fluorescence and ligand-based phosphorescence is observed when samples are cooled. Computational studies showed that phosphorescence is sensitized by excitation to the Au-localized LUMO+5. Facilitation of such intersystem crossing to a metal-sensitized, ligand-localized state does not occur in bithienyl complexes due to low lying ligand excited state contributions. Oligomeric or polymeric thin films of gold(I) thienyl pyrazolates have been synthesized from soluble precursors, and the presence of gold(I) in the films was confirmed by X-ray photoelectron spectroscopy. The redshifted singlet emission from these films is assigned to fluorescence from oxidatively coupled linkers. This study is the first systematic look at the role of a series of conjugated bridging linkers on the photoluminescence of cyclic

complex species. Successive scans with ITO as the working electrode results in the deposition of yellow films that turn blue when oxidized. The electrochemical properties of polyAuPz3HT2 and poly-AuPz*3HT2 are comparable to other gold(I) bithienyl compounds.16 X-ray photoelectron spectroscopy (XPS) was performed on ITO working electrodes after successive cyclic voltammetry scans of compounds AuPz3HT−AuPz*3HT2. Selected elemental compositions and ratios are shown in Table 7. No Table 7. X-Ray Photoelectron Spectroscopy Data for Material Deposited on ITO after Successive Voltammetry Cycles of AuPz3HT−AuPz*3HT2 compound

% Au

%S

%N

Au/S

Au/N

poly(AuPz3HT) poly(AuPz*3HT) poly(AuPz3HT2) poly(AuPz*3HT2)

3.65 0.13 3.52 3.36

3.86 4.29 5.36 4.81

4.38 6.14 5.91 5.82

1:1.20 1:36 1:1.52 1:1.43

1:1.05 1:52 1:1.68 1:1.73

evidence for other oxidation states of gold besides gold(I) are present. As the cyclic voltammogram of compound AuPz*3HT suggests, an electropolymerized film is not deposited on ITO. For monothienyl compound AuPz3HT and bithienyl com-

Figure 11. Excitation (dotted) and emission (solid) spectra of polymers of AuPz3HT2 and AuPz*3HT2. J


Inorganic Chemistry

Article

(20) Allin, S. M.; Barton, W. R.; Bowman, W. R.; (née Mann), E. B.; Elsegood, M. R.; McInally, T.; McKee, V. Tetrahedron 2008, 64, 7745−7758. (21) SAINT, version 7.60A; Bruker AXS Inc.: Madison, WI, 1997− 2009. (22) SADABS; Bruker AXS Inc.: Madison, WI, 2008. (23) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115−119. (24) Sheldrick, G. M. SHELXL-97, Release 97-2; Institüt für Anorganische Chemie der Universität Göttingen: Göttingen, Germany, 1998. (25) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838. (26) (a) Guerra, C. F.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998, 99, 391−403. (b) Fonsca Guerra, C.; Visser, O.; Snijders, J. G.; te Velde, G. In Methods and Techniques for Computational Chemistry; Clementi, E., Corongiu, G., Eds.; STEF: Cagliari, Italy, 1995; pp 305−395. (c) Baerends, E.; Ellis, D.; Ros, P. Chem. Phys. 1973, 2, 41−51. (d) Boerrigter, P. M.; te Velde, G.; Baerends, E. J. Int. J. Quantum Chem. 1988, 33, 87−113. (e) te Velde, G.; J, B. E. J. Comput. Phys. 1992, 99, 84−98. (27) (a) Snijders, J. G.; Baerends, E. J.; Vernooijs, P. At. Nucl. Data Tables 1981, 26, 483−509. (b) Krijn, J.; Baerends, E. J. Fit Functions in the HFS Method; Free University: Amsterdam, The Netherlands, 1984. (28) Slater, J. C. Quantum Theory of Molecules and Solids; McGrawHill: New York, 1974; Vol. 4. (29) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200− 1211. (30) Becke, A. D. J. Chem. Phys. 1986, 84, 4524−4529. (31) Becke, A. D. J. Phys. Rev. A 1988, 38, 3098−3100. (32) Perdew, J. P. Phys. Rev. B 1986, 33, 8822−8824. (33) (a) Fan, L.; Ziegler, T. J. Chem. Phys. 1991, 94, 6057−6063. (b) Schipper, P. R. T.; Gritsenko, O. V.; van Gisbergen, S. J. A.; Baerends, E. J. J. Chem. Phys. 2000, 112, 1344−1352. (c) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931−967. (34) (a) Chang, C.; Pelissier, M.; Durand, P. Phys. Scr. 1986, 34, 394−404. (b) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99, 4597−4610. (c) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1994, 101, 9783−9792. (d) van Lenthe, E.; van Leeuwen, R.; Baerends, E. J.; Snijders, J. G. Int. J. Quantum Chem. 1996, 57, 281−293. (e) van Lenthe, E.; Ehlers, A.; Baerends, E. J. J. Chem. Phys. 1999, 110, 8943−8953. (35) Maliarik, M.; Nagle, J. K.; Ilyukhin, A.; Murashova, E.; Mink, J.; Skripkin, M.; Glaser, J.; Kovacs, M.; Horváth, A. Inorg. Chem. 2007, 46, 4642−4653. (36) (a) Rosa, A.; Baerends, E. J.; van Gisbergen, S. J. A.; van Lenthe, E.; Groeneveld, J. A.; Snijders, J. G. J. Am. Chem. Soc. 1999, 121, 10356−10365. (b) van Gisbergen, S. J. A.; Snijders, J. G.; Baerends, E. J. Comput. Phys. Commun. 1999, 118, 119−138. (37) (a) Wang, F.; Ziegler, T.; van Lenthe, E.; van Gisbergen, S. J. A.; Baerends, E. J. J. Chem. Phys. 2005, 122, 204103. (b) Wang, F.; Ziegler, T. J. Chem. Phys. 2005, 123, 194102. (38) Ovejero, P.; Mayoral, M. J.; Cano, M.; Lagunas, M. C. J. Organomet. Chem. 2007, 692, 1690−1697. (39) Wasserberg, D.; Marsal, P.; Meskers, S. C. J.; Janssen, R. A. J.; Beljonne, D. J. Phys. Chem. B 2005, 109, 4410−4415. (40) Xu, B.; Holdcroft, S. J. Am. Chem. Soc. 1993, 115, 8447−8448. (41) Belletête, M.; Césare, N. D.; Leclerc, M.; Durocher, G. Chem. Phys. Lett. 1996, 250, 31−39. (42) Becker, R. S.; de Melo, J. S.; Maçanita, A. L.; Elisei, F. Pure Appl. Chem. 1995, 67, 9−16. (43) Grimes, T.; Omary, M. A.; Dias, H. V. R.; Cundari, T. R. J. Phys. Chem. A 2006, 110, 5823−5830. (44) Vorontsov, I. I.; Kovalevsky, A. Y.; Chen, Y.-S.; Graber, T.; Gembicky, M.; Novozhilova, I. V.; Omary, M. A.; Coppens, P. Phys. Rev. Lett. 2005, 94, 193003.

trinuclear complexes. The low lying triplet state of the thienyl moiety in combination with the good electronic communication between the peripheral linker and trinuclear core prevents photoluminescence from a LMCT excited state. Future studies will further address tuning the triplet energy of the thiophene group, allowing for triplet metal−metal luminescence to coincide with ligand based radiative transitions.

■

ACKNOWLEDGMENTS We thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding this research. J.K.N. thanks Bowdoin College for sabbatical leave support.

■

ASSOCIATED CONTENT

S Supporting Information *

Synthetic details, crystallographic data, computational data, cyclic voltammograms, and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: (604) 822-1702. Fax: (604) 822-2847. E-mail: [email protected] Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Raptis, R. G.; Fackler, J. P. Inorg. Chem. 1988, 27, 4179−4182. (2) Kim, S. J.; Kang, S. H.; Park, K.-M.; Kim, H.; Zin, W.-C.; Choi, M.-G.; Kim, K. Chem. Mater. 1998, 10, 1889−1893. (3) Yang, G.; Raptis, R. G. Inorg. Chem. 2003, 42, 261−263. (4) Zhan, S.-Z.; Li, M.; Zhou, X.-P.; Li, D.; Weng Ng, S. RSC Adv. 2011, 1, 1457−1459. (5) Mohamed, A. A. Coord. Chem. Rev. 2010, 254, 1918−1947. (6) Burini, A.; Mohamed, A. A.; Fackler, J. P. Comments Inorg. Chem. 2003, 24, 253−280. (7) Dias, H. V. R.; Diyabalanage, H. V. K.; Eldabaja, M. G.; Elbjeirami, O.; Rawashdeh-Omary, M. A.; Omary, M. A. J. Am. Chem. Soc. 2005, 127, 7489−7501. (8) Lo, S.-C.; Burn, P. L. Chem. Rev. 2007, 107, 1097−1116. (9) Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr.; Burini, A.; Galassi, R.; de Luzuriaga, J. M. L.; Olmos, M. E. Coord. Chem. Rev. 2009, 253, 1661−1669. (10) Barberá, J.; Elduque, A.; Giménez, R.; Lahoz, F. J.; López, J. A.; Oro, L. A.; Serrano, J. L. Inorg. Chem. 1998, 37, 2960−2967. (11) (a) He, J.; Yin, Y.-G.; Wu, T.; Li, D.; Huang, X.-C. Chem. Commun. 2006, 0, 2845−2847. (b) Zhang, J.-P.; Kitagawa, S. J. Am. Chem. Soc. 2008, 130, 907−917. (c) Zhang, J.-X.; He, J.; Yin, Y.-G.; Hu, M.-H.; Li, D.; Huang, X.-C. Inorg. Chem. 2008, 47, 3471−3473. (12) Pyykkö, P. Angew. Chem., Int. Ed. 2004, 43, 4412−4456. (13) Gong, F.; Wang, Q.; Chen, J.; Yang, Z.; Liu, M.; Li, S.; Yang, G.; Bai, L.; Liu, J.; Dong, Y. Inorg. Chem. 2010, 49, 1658−1666. (14) Ni, W.-X.; Li, M.; Zheng, J.; Zhan, S.-Z.; Qiu, Y.-M.; Ng, S. W.; Li, D. Angew. Chem., Int. Ed. 2013, 52, 13472−13476. (15) Perepichka, I. F.; Perepichka, D. F. Handbook of ThiopheneBased Materials: Applications in Organic Electronics and Photonics; Wiley: West Sussex, England, 2009. (16) Powell, A. B.; Bielawski, C. W.; Cowley, A. H. J. Am. Chem. Soc. 2009, 131, 18232−18233. (17) (a) Kuchison, A. M.; Wolf, M. O.; Patrick, B. O. Inorg. Chem. 2010, 49, 8802−8812. (b) Lardies, N.; Romeo, I.; Cerrada, E.; Laguna, M.; Skabara, P. J. Dalton Trans. 2007, 5329−5338. (18) Uson, R.; Laguna, A.; Vicente, J. J. Organomet. Chem. 1977, 131, 471−475. (19) Higuchi, H.; Nakayama, T.; Koyama, H.; Ojima, J.; Wada, T.; Sasabe, H. Bull. Chem. Soc. Jpn. 1995, 68, 2363−2377. K


Inorganic Chemistry

Article

(45) Bäuerle, P. In Electronic Materials: The Oligomer Approach; Müllen, K., Wegner, G., Eds.; WILEY-VCH: Amsterdam, 1998; Chapter, Sulfur-Containing Oligomers: Oligothiophenes, pp 105−197. (46) Hsu, C.-W.; Lin, C.-C.; Chung, M.-W.; Chi, Y.; Lee, G.-H.; Chou, P.-T.; Chang, C.-H.; Chen, P.-Y. J. Am. Chem. Soc. 2011, 133, 12085−12099. (47) Bobacka, J.; Ivaska, A. Synth. Met. 1991, 43, 3053−3058.

L


Thienyl Pyrazolates - American Chemical Society

Recommend Documents