Solid-State and Solution Metallophilic Aggregation of a Cationic [Pt

Article pubs.acs.org/IC

Solid-State and Solution Metallophilic Aggregation of a Cationic [Pt(NCN)L]+ Cyclometalated Complex Vasily V. Sivchik,† Elena V. Grachova,*,‡ Alexei S. Melnikov,∥,⊥ Sergey N. Smirnov,§ Alexander Yu. Ivanov,§ Pipsa Hirva,† Sergey P. Tunik,*,‡ and Igor. O. Koshevoy*,† †

Department of Chemistry, University of Eastern Finland, 80101 Joensuu, Finland Institute of Chemistry, §Center for Magnetic Resonance, and ⊥Department of Physics, St. Petersburg State University, 198504 St. Petersburg, Russia ∥ Institute of Nanobiotechnologies, Peter the Great St. Petersburg Polytechnic University, 195251 St. Petersburg, Russia ‡

S Supporting Information *

ABSTRACT: The noncovalent intermolecular interactions (π−π stacking, metallophilic bonding) of the cyclometalated complexes [Pt(NCN)L]+X− (NCN = dipyridylbenzene, L = pyridine (1), acetonitrile (2)) are determined by the steric properties of the ancillary ligands L in the solid state and in solution, while the nature of the counterion X− (X− = PF6−, ClO4−, CF3SO3−) affects the molecular arrangement of 2·X in the crystal medium. According to the variable-temperature Xray diffraction measurements, the extensive Pt···Pt interactions and π-stacking in 2·X are significantly temperature-dependent. The variable concentration 1H and diffusion coefficients NMR measurements reveal that 2·X exists in the monomeric form in dilute solutions at 298 K, while upon increase in concentration [Pt(NCN)(NCMe)]+ cations undergo the formation of the ground-state oligomeric aggregates with an average aggregation number of ∼3. The photoluminescent characteristics of 1 and 2·X are largely determined by the intermolecular aggregation. For the discrete molecules the emission properties are assigned to metal perturbed IL charge transfer mixed with some MLCT contribution. In the case of oligomers 2·X the luminescence is significantly red-shifted with respect to 1 and originates mainly from the 3MMLCT excited states. The emission energies depend on the structural arrangement in the crystal and on the complex concentration in solution, variation of which allows for the modulation of the emission color from greenish to deep red. In the solid state the lability of the ligands L leads to vapor-induced reversible transformation 1 ↔ 2 that is accompanied by the molecular reorganization and, consequently, dramatic change of the photophysical properties. Time-dependent density functional theory calculations adequately support the models proposed for the rationalization of the experimental observations.

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INTRODUCTION

determining the photophysical parameters of these compounds both in solid state and in solution.2f,5 A significant progress observed in the field has been largely stimulated by successful employment of phosphorescent metal complexes in the development of organic light-emitting devices of improved efficiency.1a,5a,d,6 As a result, numerous organometallic Pt(II) species were rationally designed to achieve exceptionally high luminescence intensity, color tunability, and stability.7 An intrinsic feature of square-planar platinum complexes is a pronounced tendency to undergo solution and solid-state aggregation via weak metallophilic Pt···Pt contacts, which arise due to the effective overlap of dz2 and pz orbitals of the interacting metal centers.8 As a result of this secondary bonding, the formation of excited- and/or ground-state assemblies becomes possible, thus leading to a decrease of

Luminescent transition metal complexes represent a fascinating class of functional compounds that find impressively wide applications in the numerous areas related to light generation, for example, photoluminescent sensing, imaging techniques, and electroluminescent devices.1 Depending on the nature of a metal center, ligand environment, and ionic character of the molecules, modulation of light absorption and/or emission characteristics of these species is possible due to the formation of different aggregates through several association pathways, mostly favored by metal−metal and/or π-stacking interactions.2 The resulting self-assembled entities of variable complexity often demonstrate physical properties that cannot be achieved by using the isolated molecular components only.3 The photoactive platinum(II) complexes have been attracting growing attention since early reports on their solid-state luminescence.4 Extensive investigations conducted during the past two decades provided sufficient knowledge on the factors © 2016 American Chemical Society

Received: November 22, 2015 Published: March 11, 2016 3351

DOI: 10.1021/acs.inorgchem.5b02713 Inorg. Chem. 2016, 55, 3351−3363

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Inorganic Chemistry

C21H16BF4N3Pt: C, 42.60; H, 2.70; N, 7.10. Found: C, 42.50; H, 2.90; N, 7.00%. The complex [Pt(NCN)(py)]CF3SO3 (1·CF3SO3) was prepared analogously using AgCF3SO3 (yield 89%). The NMR data are identical to those of 1. Anal. Calcd for C22H16F3N3O3PtS: C, 40.37; H, 2.46; N, 6.42; S, 4.90. Found: C, 40.02; H, 2.65; N, 6.73; S 5.29%. Synthesis of [Pt(NCN)(MeCN)]X (X = PF6−, ClO4−, CF3SO3−). These complexes were prepared analogously to 1 using the corresponding Ag salt. The crude product was dissolved in acetonitrile (ca. 40 mL) resulting in a reddish solution, which was filtered and evaporated to give a red solid. [Pt(NCN)(MeCN)]PF6 (2·PF6). Bright red needle-shaped crystals were obtained by a gas-phase diffusion of diethyl ether into acetonitrile solution of 2·PF6 at 298 K (68%). 1H NMR (acetonitrile-d3, 298 K, C = 3.01 mmol/L δ): 8.45 ppm (m, JPtH 40.0 Hz; JHH 5.7, 1.5 Hz; NCNpy 2H), 8.11 (dt, 7.7, 1.5 Hz; NCNpy 2H), 7.81 (m, JHH 7.7, 1.5 Hz; NCNpy 2H), 7.47 (d, JHH 7.7 Hz; NCNphen 2H), 7.36 (ddd, JHH 7.7, 5.7, 1.5 Hz; NCNpy 2H), 7.25 (t, JHH 7.7 Hz; NCNphen 1H). 195Pt NMR (acetone-d6, 298 K, δ) −3800 (broad signal). Anal. Calcd for C18H14F6N3PPt: C, 35.30; H, 2.30; N, 6.86. Found: C, 35.32; H, 2.45; N, 6.79%. [Pt(NCN)(MeCN)]ClO4 (2·ClO4). Dark brown needle-shaped crystals were formed upon cooling of hot concentrated acetonitrile solution of 2·ClO4 (55%). The NMR data are identical to those of 2·PF6 at a given concentration. Anal. Calcd for C18H14ClN3O4Pt: C, 38.14; H, 2.49; N, 7.41. Found: C, 37.89; H, 2.53; N, 7.38%. [Pt(NCN)(MeCN)]CF3SO3 (2·CF3SO3). Deep red needle-shaped crystals were obtained by a gas-phase diffusion of diethyl ether into acetonitrile solution of 2·CF3SO3 at 298 K (84%). The NMR data are identical to those of 2·PF6 at a given concentration. Anal. Calcd for C19H14F3N3O3PtS: C, 37.02; H, 2.29; N, 6.82; S, 5.20. Found: C, 36.67; H, 2.22; N, 6.73; S, 5.34%. X-ray Structure Determinations. The crystals of 1, 2·X (X = PF6−, ClO4−, CF3SO3−) were immersed in cryo-oil, mounted in a Nylon loop, and measured on a Bruker Smart Apex II or Bruker Kappa Apex II Duo diffractometers using Mo Kα radiation (λ = 0.710 73 Å). The APEX214 program package was used for cell refinements and data reductions. The structures were solved by direct methods using the SHELXS-201315 program with the WinGX16 graphical user interface. A semiempirical absorption correction (SADABS)17 was applied to all data. Structural refinements were performed using SHELXL-2013.15 The crystallization acetonitrile molecules in 2·X were found around the special positions and were refined with an occupancy of 0.5. Both geometrical and displacement constraints and restraints were applied to these moieties. All hydrogen atoms in 1 and 2·X were positioned geometrically and constrained to ride on their parent atoms with C−H = 0.95−0.98 Å and Uiso = 1.2−1.5 Ueq (parent atom). The crystallographic details are summarized in Table S1. Photophysical Measurements. The pulse laser DTL-399QT “Laser-export Co. Ltd” (351 nm, 50 mW, pulse width 6 ns, repetition rate 1 kHz) was used for excitation luminescence. A monochromator MUM (LOMO, interval of wavelengths 1 nm), photon counting head H10682 (Hamamatsu) and multiple-event time digitizer P7887 (FAST ComTec GmbH) were used for lifetime measurements. Emission spectra were recorded using an HR2000 spectrometer (Ocean Optics). Halogen lamp LS-1-CAL (Ocean Optics) and deuterium lamp DH2000 (Ocean Optics) were used to calibrate the absolute spectral response of the spectral system in the 200−1100 nm range. To measure the emission spectra at 77−273 K samples were placed in a cryostat. Excitation spectra were measured on a Varian Cary Eclipse spectrofluorimeter. Computational Details. All of the models were calculated using the Gaussian09 program package18 at the density functional theory (DFT) level of theory with a hybrid density functional PBE0.19 The basis set consisted of the quasi-relativistic effective core potential basis set def2-TZVPPD20 for metal atoms, and the standard all-electron basis set 6-31G(d) for all other atoms. To obtain the electronic properties of the complexes, we performed topological charge density analysis using the Quantum Theory of Atoms in Molecules method,21 which allowed us to access the nature

the excited-state energy level that is accompanied by a significant bathochromic shift of emission maximum in comparison to the monomeric species.2f,3i,5a,7a Despite the excimer emission is generally considered to be of a low intensity, certain aggregated complexes demonstrate unusually high quantum yields exceeding 30%.9 Adjusting the concentration of a luminophore in solution of a polymeric matrix allows for a fine balance between the monomeric and aggregated (dimeric in the simplest case) forms in the emitting materials of this sort. Following this strategy, simultaneous monomer and excimer emissions occurring in different regions of the visible spectrum open an intriguing possibility to design a controllable multicolour light-emitting system using a single chemical entity, which was successfully demonstrated by the fabrication of platinum(II)-based white light-emitting devices.5a,10 Additionally, metallophilicity- and/or π-stacking driven aggregation of Pt(II) compounds provides a route to stimuli− responsive functional materials, which exhibit a distinct alteration of absorption and emission properties, for example, upon applied mechanical force, exposure to certain (mainly volatile organic) compounds or temperature variation.1e,i,2j,3j,11 Such dynamic behavior accompanied by easy-to-detect changes of optical properties has a promising potential for the development of selective sensing devices. Among a wide variety of inorganic and organometallic platinum(II) complexes the cationic [Pt(NCN)L]+ species (L is a neutral ligand) considerably lack for attention.3a,12 In this contribution we describe the preparation of cationic cyclometalated complexes [Pt(NCN)L]+ (NCN = dipyridylbenzene, L = acetonitrile, pyridine) and their structural and spectroscopic studies both in fluid and crystalline media. The results obtained reveal the subtle effects that govern the solid-state behavior, and they also made possible investigation of aggregation in solution by means of diffusion coefficients NMR measurements, correlated with variable-temperature (VT) and concentration dependence of the 1H NMR and emission measurements.

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EXPERIMENTAL SECTION

General Comments. Pt(NCN)Cl were prepared according to the published procedure.13 Other reagents were used as received. The reactions with Ag(I) salts were performed in the absence of light. The solution 1H, COSY, 13C{1H}, DEPT, 1D TOCSY, 1H13C HSQC, 1 13 H C HMBC, and 195Pt NMR spectra were recorded on a Bruker Avance III 400 spectrometer. The diffusion coefficients measurements were performed at 296 K using STEbp method. The diffusion coefficients were obtained by processing the experimental data using a standard module of the Topspin 3.2 software. Microanalyses were performed in the analytical laboratory of the University of Eastern Finland. [Pt(NCN)(py)]BF4 (1). Pt(NCN)Cl (53 mg, 0.115 mmol) was dissolved in a mixture of dichloromethane (30 mL) and pyridine (0.3 mL), which was treated with a solution of AgBF4 (26 mg, 0.133 mmol) in methanol (10 mL), causing an immediate precipitation of AgCl. The suspension was stirred overnight. Then the solvents were evaporated, the residue was suspended in dichloromethane (ca. 10 mL), and the insoluble AgCl was removed by filtration. Evaporation of the solvent gave a yellow solid, which was recrystallized by a gas-phase diffusion of diethyl ether into acetone solution of 1 at 298 K (63 mg, 93%). 1H NMR (acetone-d6, 298 K, δ) 9.21 (m, JHH 4.8, 1.6 Hz; py 2H), 8.35 (m, JHH 7.7, 1.6 Hz; py 1H), 8.27 (td, JHH 7.7, 1.6 Hz; NCNpy 2H), 8.20 (m, JHH 7.7, 1.6 Hz; NCNpy 2H), 8.01 (m, JHH 7.7, 4.8 Hz; py 2H), 7.99 (dd, JHPt 42 Hz, JHH 5.7 Hz; NCNpy 2H), 7.81 (d, JHH 7.7 Hz; NCNphen 2H), 7.45 (ddd, JHH 7.7, 5.7, 1.5 Hz; NCNpy 2H), 7.40 (t, J HH 7.7 Hz; NCN phen 1H). Anal. Calcd for 3352


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Inorganic Chemistry of the bonding by calculating the different properties of the electron density at the bond critical points (BCPs). The analysis was performed with the AIMALL program,22 using the wave functions obtained from the DFT calculations based on the geometry from the experimental crystal structures. Time-dependent (TD) DFT calculations were used for further comparison of the electronic properties of the complexes. The UV−vis spectra were simulated on the basis of the geometries from the X-ray diffraction (XRD) data and partially optimized structures with the constraint of the Pt···Pt separations, as well as symmetrized Pt···Pt···Pt and N−Pt···Pt−N torsion angles. The solvent effects of acetonitrile were calculated using the conductor-like polarized continuum model (CPCM)23 for the models.

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RESULTS AND DISCUSSION Synthesis and Structural Characterization. Che and coworkers recently described a two-phase water−dichloromethane system to obtain cationic isocyanide complexes [Pt(NCN)(CNR)]Cl (R = −C6H3−2,6-Me2).12a To avoid possible recoordination of the chloride anion and regeneration of the starting Pt(NCN)Cl compound3a we performed the reaction of Pt(NCN)Cl with pyridine in dichloromethane in the presence of AgBF4 to generate the cationic complex [Pt(NCN)(py)]BF4 (1), which was isolated as yellow crystalline solid in nearly quantitative yield.

Figure 1. Molecular view of 1 showing the stacking structure. Counterions and hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at 30% probability level.

Scheme 1. Synthesis of Complexes 1 and 2

formation of the polymorphs. Recently it was shown that counterions can also determine the intermolecular arrangement and consequently the physical characteristics of the solid Pt(II)based materials.2j,11c These reports prompted us to perform systematic structural studies of a family of 2·X species (X = PF6−, ClO4−, CF3SO3−), which show a counterion-dependent behavior in the solid state. Crystallization of 2·PF6 by a gas-phase diffusion of diethyl ether into an acetonitrile solution of the complex at room temperature gave red needles of the NCMe solvate. When a crystal of 2·PF6 was cooled to 120 K a dramatic color change to dark green was observed (Figure 2). The structure determined at 120 K (Figure 2, the selected parameters are given in Table S1, Supporting Information) displays a stacking arrangement of the planar cations [Pt(NCN)(NCMe)]+ to form infinite polymeric chains via effective Pt···Pt contacts along the a axis with two altering intermetallic distances of 3.3906 and 3.3132 Å, which are shorter than the sum of two van der Waals radii of Pt (3.50 Å). The molecules are found in a staggered configuration with torsion angles of −57.66 and 58.26°, while the Pt−Pt−Pt angle equals 169.32°, which is comparable to the structural data found for the [Pt(NCN)(CCPh)] relative.3b The distances between the planes of the NCN ligands are 3.33 Å indicative of some π−π interactions operating in this system. Gradually heating the crystal of 2·PF6 results in a substantial growth of intermetallic distances (Figure 3), which exhibit a pronounced increase in the temperature range of 200−250 K accompanied by a distinct color change (see inset in Figure 2). Elongation of the Pt···Pt separations is achieved to a large extent due to a linear shift of adjacent molecules with respect to each other and significant decrease of Pt−Pt−Pt angles, which is reflected by a visibly smaller change of the spacing between the planes of neighboring NCN ligands (ca. 3.33 Å at 120 K, ca. 3.40 Å at 280 K) in comparison with metal−metal contacts.

Its crystal structure (Figure 1) features an absence of appreciable Pt···Pt interactions presumably due to noncoplanarity of the pyridine ring and the [Pt(NCN)] system with an angle of 64.6° between the corresponding planes. Such an orientation of the pyridine ligand is reminiscent of the [Pt(NCN)(CCPh)] complex3b and is obviously favorable to minimize a possible repulsion between the hydrogen atoms of the cyclometalated NCN and pyridine ligands (i.e., belonging to C(7)/C(12) and C(17)/C(21) carbon atoms, respectively). The energetically most favorable conformer with a noncoplanar conformation of the pyridine ring was also verified computationally by performing a torsional scan (Figure S1). Thus, the observed stereochemical arrangement sterically prevents the formation of intermolecular metallophilic bonds. The cations of 1 form stacks in which the molecules are placed pairwise in a head-to-tail mode being linked through the weak π−π interactions between the NCN ligands; the interplanar distances are in the range of 3.4−3.6 Å, which agrees well with the characteristics of the previously studied [Pt(NCN)(CNR)]+ and [Pt(NNN+)Cl]+2 species.2j,12a The dissolution of 1 in acetonitrile results in a very fast substitution of the pyridine ligand for NCMe to give the [Pt(NCN)(NCMe)]+ complex (2). It has been documented that sterically nonhindered organoplatinum complexes exhibit different colors and photoluminescent properties depending on the crystal packing. This phenomenon is mainly attributed to the variations in the metal−metal contacts,8 which can be altered, for example, via modification of ligand environment3b or crystallization conditions,8,12a,24 the latter of which results in 3353


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Figure 4. (A) Molecular view of 2·ClO4 showing the stacking structure at 120 K. Counterions and hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at 30% probability level. Symmetry transformations used to generate equivalent atoms (′): x, 0.5 − y, z − 0.5. (B) Overlay of the structures obtained at 120 K (blue) and 270 K (red).

Figure 2. (A) Molecular view of 2·PF6 showing the stacking structure at 120 K. (inset) The crystal at 120 K. Counterions and hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at 30% probability level. Symmetry transformations used to generate equivalent atoms (′): x − 1, y, z. (B) Overlay of the structures obtained at 120 K (blue) and 250 K (red). (inset) The crystal at 250 K. Pt−Pt distances at 120 and 250 K are given for comparison.

to-tail mode as the torsion angle around the Pt−Pt axis is 140.87° (120 K) versus 57.66 and 58.26° in 2·PF6. Variation of temperature from 120 to 270 K has a much smaller effect on the structural features of 2·ClO4 than that observed for 2·PF6 and therefore has no visual appearance. As shown in Figure 4, heating the sample to 270 K leads to a rather small elongation of the Pt···Pt contacts (3.3296 Å at 120 K; 3.3879 Å at 270 K), which is comparable to that one in the Pt(NCN)Cl polymorphs. 24b The Pt−Pt−Pt angle also decreases to a small extent with rising the temperature, from 166.12° at 120 K to 163.07° at 270 K. It must be noted that further heating the crystalline sample of 2·ClO4 to 350 K results in color change to red that can be assigned to the further increase in intermetallic distances. Unfortunately, no structural data could be obtained at this temperature due to the loss of crystallization solvent and the apparent destruction of the crystal. Acetonitrile solvate of the complex 2·CF3SO3 crystallizes as red blocks. The structure is shown in Figure 5 and reveals virtually linear tetramer fragments (Pt−Pt−Pt angle is 172.72°) formed via Pt···Pt bonds, which are the shortest ones among the compounds studied (3.2333 and 3.2706 Å at 120 K).

Figure 3. Dependence of Pt···Pt distances (blue and red lines) and Pt−Pt−Pt angle (green line) on temperature in the crystal of 2·PF6.

Consequently, a linear expansion along the a axis is rather moderatethe unit cell a edge lengthens from 6.6747(10) Å at 120 K to 6.8452(11) Å at 280 K (see Table S1). This structural modulation weakens metal−metal interactions as it affects the overlap of the filled dz2 and empty pz metal orbitals,4c leading to the loss of the Pt(1)−Pt(2′) contact at ∼250 K (3.5074 Å), which ultimately breaks the polymeric chain into dimeric units. This high-temperature arrangement closely resembles dimeric structure of the red form of the [Pt(NNN)(CC−CCH)]+ complex, in which alternating distances of 3.394 and 3.648 Å and the Pt−Pt−Pt angle of 154.3° were determined at 301 K.24a Dark brown crystals of 2·ClO4 were grown from a concentrated acetonitrile solution. The packing of the perchlorate form is illustrated in Figure 4 and consists of the polymeric chains uniformly composed of the [Pt(NCN)(NCMe)]+ units. The stacking of the cations has somewhat different orientation than that in 2·PF6 and is closer to a head-

Figure 5. Molecular view of 2·CF3SO3 at 120 K. Counterions and hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at 30% probability level. Selected interatomic distances (Å) are Pt(1)− Pt(1′) 3.2333(9), Pt(1′)−Pt(2) 3.2706(10), Pt(2)−Pt(2′) 3.8993(11). Symmetry transformations used to generate equivalent atoms (′): 0.5 − x, 0.5 − y, −z; (′): 1 − x, y, 0.5 − z. 3354


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significant increase in the lifetime value (τav = 55 μs) together with poorly resolved vibrational spacing, which is ca. 1200 cm−1 and also points to intraligand nature of the emission. The complex 2·PF6, which has a dimer-like structure determined at 250 K, did not allow for the correlation of the photophysical and structural parameters at room temperature due to the loss of crystallization solvent. Drying of the red crystalline sample yields orange powder probably because of crystal lattice destruction. The solid-state luminescence spectrum of 2·PF6 at room temperature (Figure 7) shows a

The Pt(2)−Pt(2′) metal−metal separation between the tetranuclear blocks is 3.8993 Å, which points to negligible metallophilic interactions between these fragments. Moreover, the Pt(1′)−Pt(2)−Pt(2′) angle of 158.86° is not favorable for the effective overlap of metal orbitals along z axis. Raising the temperature has no visual effect on the crystals of 2·CF3SO3 and is mainly reflected by a slight growth of the nonbonding distance between the tetramers (3.951 Å at 200 K), while the short Pt−Pt contacts lengthen to a smaller extent (Pt(1)− Pt(1′) is 3.2616 Å, and Pt(1′)−Pt(2) is 3.2872 Å at 200 K). The difference in arrangement of [Pt(NCN)(NCMe)]+ cations in a crystalline phase must be determined by the nature of counterions. The anions PF6−, ClO4−, and CF3SO3− used in this work were found to form extensive networks of hydrogen bonds, which involve the cations and crystallization solvent molecules (see Figures S2−S4 showing the packing of 2·X species). The variations of anion stereochemistry and charge distribution apparently has fine influence on the energetics of intermolecular interactions and therefore is responsible for the alteration of the structures observed. Solid-State Luminescence. The solid-state photophysical data for the compounds 1 and 2·X (X = PF6−, ClO4−, CF3SO3−) are summarized in Table 1. Complex 1 is moderately Table 1. Solid-State Photophysical Properties of 1 and 2·X (X = PF6−, ClO4−, CF3SO3−) 298 K 1 2·PF6 2·ClO4 2·CF3SO3

Figure 7. Normalized solid-state emission spectra of desolvated 2·PF6 (λexc = 351 nm).

77 K

λem, nma

τav, μsb

λem, nma

τav, μsb

580 650 755 695c

5.4 0.17 0.27 0.31

550 665 785 780c

55 0.59 0.58 1.3

structureless band at 650 nm that is bathochromically shifted with respect to the emission of discrete molecules observed in dilute solution (see below). This band can be assigned to the dσ*−π* (MMLCT) transition of the dimeric unit, in which the dσ* orbital of the metal−metal bond serves as the highest occupied molecular orbital (HOMO)24b,25 and causes a decrease in emission energy. A large perturbation of the crystal packing of 2·PF6 obviously determines different physical properties of the dried sample in comparison with the solvated single crystals. The solvent-free material does not show appreciable color change upon temperature variation, which might indicate a random distribution of the dimeric fragments in the solid. This prevents formation of the Pt···Pt bonded polymeric chains, which are responsible for lowering the HOMO−LUMO (LUMO = lowest unoccupied molecular orbital) gap and appearance of low-energy absorption bands giving dark green color of the 2·PF6 crystals at low temperature. Consequently, the emission of desolvated 2·PF6 also demonstrates a very minor temperature dependence (Figure 7) featured in a small red shift of the emission band at 77 K (665 nm), while heating the sample mainly gives a decrease in emission intensity (Figure S5). Dark brown 2·ClO4 with linear arrangement of the cations linked via Pt···Pt interactions exhibits a clear dependence of the emission energy on the temperature due to alteration of the metal−metal distances. Similarly to 2·PF6, luminescence of the perchlorate complex originates from the 3MMLCT excited state. The polymeric structure of 2·ClO4 accounts for a decrease of absorption energy compared to an isolated molecule or a dimer. Their discrete HOMO/LUMO levels can be replaced by broad bands of Pt 5dz2 and 6pz orbitals of the linear chain, which have a low energy gap between them.26 As a result, the excitation and emission energies of 2·ClO4 also demonstrate a substantial decrease with respect to the 2·PF6

λexc = 351 nm. bAverage lifetimes τav = (A1τ12 + A2τ12)/(A1τ1 + A2τ1), Ai − weight of i exponent. cλexc = 530 nm.

a

luminescent at room temperature showing a broad band maximized at 580 nm (Figure 6). In the absence of appreciable

Figure 6. Normalized solid-state emission spectra of 1 (λexc = 351 nm).

intermolecular interactions in the crystal of 1 the emission may be assigned to metal-disturbed intraligand (IL) π−π* transitions mixed with some metal-to-ligand charge transfer (MLCT). The lifetime found in the microsecond range (τav = 5.4 μs) indicates that the excited state has a triplet origin. Cooling to 77 K results in a minor blue shift of emission and a 3355


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easy substitution of pyridine ligand in 1 for acetonitrile prompted us to explore possible solvato-/vapochromic behavior due to 1 ↔ 2 transformation. Indeed, when bright yellow solid of 1·CF3SO3 (prepared analogously to 1 using the corresponding AgCF3SO3 salt) was exposed to acetonitrile vapor for ca. 30 min, a distinct change to deep red color was observed, indicating ligand replacement at the Pt center and formation of extensive metal−metal bonding to give the 2· CF3SO3 complex (Figure 10).

dimeric form (λem is 755 and 650 nm at room temperature, respectively). The VT emission spectra of 2·ClO4 are shown in Figure 8. At low temperature a weak broad band appears at ca.

Figure 8. VT solid-state emission spectra of 2·ClO4 (λexc = 351 nm).

780 nm with a shoulder extending further into near-IR region. Raising the temperature causes a considerable hypsochromic shift (to 675 nm at 413 K) and increase in intensity that reaches its maximum at ∼343 K, which is accompanied by a change of the sample color to red. According to the XRD structural studies, the intermetallic contacts in 2·ClO4 get elongated as temperature increases, which very likely leads to a cleavage of the polymeric chains into the oligomeric fragments, thus stabilizing the Pt···Pt dσ* orbital (HOMO) to give a blue shift of emission.24b The triflate derivative 2·CF3SO3 has the shortest Pt···Pt contacts among the title compounds. However, its color is different than that of 2·ClO4 (red vs dark brown) meaning that absorption bands of 2·CF3SO3 appear at a higher energy presumably because of its tetrameric, not polymeric, structure. At room temperature the emission band of the tetrameric complex was found at 695 nm (Figure 9), which is in between

Figure 10. Normalized solid-state emission spectra of (a) 1·CF3SO3; (b) treated with acetonitrile; (c) treated with pyridine; (d) treated with acetonitrile again (λexc = 351 nm). (inset) A photograph of 2· CF3SO3 and its response to pyridine vapor under ambient (top) and UV (365 nm, bottom) lights.

The reverse transition 2·CF3SO3 → 1·CF3SO3 occurs more effectively and is completed within a few minutes. The emission spectrum of 1 exposed to NCMe vapor is virtually identical to that of independently obtained 2·CF3SO3. The observed solidstate transformation and the corresponding alteration of spectroscopic characteristics of the sample is induced by the alteration of steric hindrance that allows for aggregation− deaggregation of the planar [Pt(NCN)]+ fragments. Introduction of a weakly coordinated pyridine ligand, which can be substituted by an incoming guest to initiate molecular reorganization and to provide a detectable signal, illustrates a route to develop photoactive functional materials based on selfassembly processes, which are driven by weak noncovalent interactions. Solution Behavior. Nuclear Magnetic Resonance Spectroscopic Studies. The solution 1H NMR data obtained for 1 and 2 are compatible with their molecular solid-state structures, see Experimental. Complete assignment was done on the basis of 1H−1H COSY experiments. The 1H NMR spectrum of 1 recorded in acetone-d6 displays a set of multiplets in the aromatic region (Figure S6), which correspond to the protons of the coordinated pyridine and the metalated NCN ligands. In contrast to 1 the chemical shifts of the 1H resonances of 2· X species show clear concentration dependence (Figure 11). Increase in the complex concentration results in a gradual upfield shift and a moderate broadening of all the signals. Somewhat similar effect was noticed upon lowering the temperature (Figure 12). These observations can be attributed to the formation of polynuclear aggregates through stacking of

Figure 9. Normalized solid-state emission spectra of 2·CF3SO3 (λexc = 530 nm).

the maxima observed for dimer 2·PF6 (650 nm) and polymer 2· ClO4 (755 nm) structures. Analogously to its congeners described above, the luminescence of 2·CF3SO3 can be assigned to 3MMLCT transition, which is mainly localized within the tetramer unit found in the crystal cell. It has been reported that various platinum(II) complexes exhibit color and/or photoluminescence variation upon alteration of Pt···Pt contacts in the solid state.1e,i,2c,11b,27 An 3356


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the estimation of the size of the assemblies composed of monometallic PtII blocks. Kotzé et al. used the diffusion coefficients NMR measurements to study the nanosized structures containing up to ca. 735 [Pt(phen)(L1-S,O)]+ cations, which were formed in the course of cation−π induced aggregation in water solutions above a critical aggregation concentration.2i It was also recently demonstrated that the [Pt(Fmdpb)CN] complex (FmdpbH = 4-fluoro-1,3-di(4methyl-2-pyridyl)benzene) forms an excimer (excited-state dimer) upon irradiation at 355 nm, and an increase in solution concentration leads to the formation of excited-state trimeric species.3i However, on the basis on the TD-DFT geometry optimizations Amar et al. stated that “trimeric species are highly improbable in solution” for at least the neutral [Pt(CNN)(C CR)] complexes.29 Similarly, Yam at el. indicated that the aggregation of the cationic terpyridine complex [Pt(tpy)(C C−CCH)]+ “by varying the concentration in acetonitrile is unlikely to occur”.24a The absence of related experimental studies concerning the ground-state associates based on metallophilic bonding, and the NMR observations described above, prompted us to get a deeper insight into behavior of the complex 2 in solution. A very useful approach to investigate the assembly processes in solution consists in measurements of diffusion coefficient (D) of the molecule under study by the NMR spectroscopic methods. The D values show good sensitivity to the effective size of polymeric aggregates, and a number of constituting blocks can be approximately estimated taking advantage of the Stokes−Einstein equation (D = kT/6πηrH; where k is the Boltzmann constant, η is the viscosity of the fluid, and rH is the hydrodynamic radius of the particle). Under the conditions of a fast exchange between aggregates of different size the observed diffusion coefficient (Dobs) is an average of the species presented in solution weighted with their relative amount. The equation is valid for the spherical objects, the size of which is exceedingly larger than that of the solvent. Therefore, the rH values of square-planar complexes (i.e., nonspherical molecules) calculated using the Stokes−Einstein equation is a rough approach. Nevertheless, the dependence of an average rH(av) value (derived from Dobs) on concentration is an indicative assessment of the aggregation degree, as it points to the average number of individual units within the aggregates.2i,31 To eliminate the influence of variable η on the Dobs, tetramethylsilane (TMS) was used as an internal standard for the calculation under the framework of this model. Assuming that TMS is not liable to aggregation and its rH is constant, the ratio DTMS/Dobs is proportional to rH(av) of the aggregate (Figure 13). The aggregation number N as a measure of associated constituting blocks is defined as follows:31

Figure 11. Variable-concentration 400 MHz 1H NMR spectra of 2·PF6 (acetonitrile-d3, 298 K).

Figure 12. VT 400 MHz 1H NMR spectra of 2·PF6 (acetonitrile-d3, c = 3.01 mM; see Figure 11 for the assignment of the signals).

the complex molecules in solution, which occurs via noncovalent π−π or/and Pt···Pt interactions.28 The complexes 2 with ClO4− and CF3SO3− counterions demonstrate similar trends, and all the compounds 2·X exhibit essentially identical spectra at the same concentration, indicating an absence of appreciable cointer-anion effect on the aggregation processes. The degree of aggregation expectedly grows upon increase in concentration and lowering the temperature. Because of the presence of only one set of the signals in the spectra of 2, it is reasonable to conclude that the exchange between monomeric form of 2 and its aggregates is rather fast under the conditions of the experiments. The resonances corresponding to the HF nuclei demonstrate the largest alteration of δ values among the NCN ligand signals similarly to the [Pt(phen)(L1-S,O)]Cl cationic species, which form nanoaggregates through cation−π interactions.2i The shift of the signals to higher frequencies is associated with higher shielding of the aromatic protons due to extensive stacking of the planar PtII complex cations, as the interligand π−π interactions together with the metallophilic bonding may also play an important role in the aggregates formation and additional shielding of adjacent to metal protons.29 Unfortunately, these data do not provide sufficient information for the reliable estimation of the number of cations presented in the aggregates. It is widely accepted that the neutral and cationic PtII species tend to form assemblies in solution in the absence of steric hindrance. Change of the solvent polarity or decrease in solubility was demonstrated to induce aggregation to give oligomers or even the nanostructures (metallogels).2i,3a,24a,30 However, there is a very limited number of the systematic experimental reports that allows for

⎛ rH(av) ⎞3 ⎛ Do ⎞3 VH N= o =⎜ o ⎟ =⎜ ⎟ VH ⎝ rH ⎠ ⎝ Dobs ⎠

where VH and VoH are the hydrodynamic volumes of the observed aggregate and that of the monomer, respectively; roH is the hydrodynamic radius of the monomer, and Do corresponds to the formal absence of aggregation at infinite dilution, obtained by extrapolating the DTMS/Dobs to c = 0 (see Figure 13). The calculated average aggregation numbers and Dobs values are listed in Table 2. According to the data obtained at the lowest acceptable for the diffusion coefficients NMR measurements concentration of 0.37 mM the complex 2·PF6 3357


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Inorganic Chemistry

Photophysical Properties in Solution. The absorption and emission spectra of the complex 1 are given in Figure 14. In

Figure 13. Dependence of the DTMS/Dobs ratio on the concentration of 2·PF6, 298 K.

Figure 14. Absorption and emission spectra of 1 in degassed CH2Cl2 (298 K, λexc = 351 nm).

Table 2. Diffusion Coefficient (Dobs) and Average Aggregation Numbers (N) for 2·PF6 (Acetonitrile) and 1 (Pyridine) Concentration Dependences (298 K)

accordance with NMR data, no concentration dependence was detected, which confirms absence of aggregation of 1 both in the ground and excited states. The spectral profiles resemble closely those of [Pt(NCN)Cl] and the other related compounds.3i,12a The intense absorption bands at 320−380 nm can be assigned to the π−π* transitions localized on the metalated NCN ligand. Analogously, the emission showing a vibronic structure (ν ≈ 1300 cm−1, CC and CN modes in the aromatics) originates from the NCN intraligand transitions. The intensity of luminescence of 1 (Φem = 5.6 ± 0.03% in degassed CH2Cl2) is weaker than that of the parent chloride complex (Φem = 60%).3i The lifetime in the microsecond domain (τobs = 1.3 μs) and considerable Stokes shift point to phosphorescence nature of the emission observed. The complex 2·PF6 demonstrates a very different behavior. Increase of the complex concentration leads to appreciable changes of the UV−vis absorption spectra, which show a substantial growth of the low-energy absorption bands with the tail extended up to 600 nm (Figure 15A). The bathochromic shift can be attributed to the formation of oligomeric aggregates via metallophilic bonding and π-stacking interaction that causes the emergence of metal−metal-to-ligand charge-transfer (MMLCT) transitions. In contrast, the [Pt(Fmdpb)CN] (FmdpbH = 4-fluoro-1,3-di(4-methyl-2-pyridyl)benzene) complex, which forms a trimer in the photoexcited state, does not show noticeable absorption changes upon concentration variations, which is indicative of negligible aggregation in the ground state.3i The steady-state emission spectra of 2·PF6 exhibit a gradual change from greenish to deep red color together with the concentration growth (Figures 15B and S7) that is consistent with the aggregation of the [Pt(NCN)(NCMe)]+ units into oligomeric species detected by the NMR spectroscopic measurements. The spectrum of a dilute solution (c = 0.07 mM) is almost identical to that of 1 (though visibly weaker, Φem = 1.4 ± 0.1%) and therefore corresponds to the photoluminescence of the monomer. Increase in the complex concentration is accompanied by successive growth of poorly structured broad bands (see Figure 15B), which can be attributed to the emission of oligomeric species with progressively increased size.3i,32 In accordance with the literature data these bands are bathochromically shifted relative

2·PF6

1

c, mM

Dobs (1 × 10−10 m2 s−1)

N

12.88 10.17 7.58 5.29 3.10 1.49 0.37 11.82 2.25

9.12 9.41 10.4 11.1 11.9 12.1 13.0 4.9 5.1

3.30 2.82 2.12 1.78 1.39 1.19 1.04 1 1

virtually exists in its monomeric form at 298 K (N = 1.04), while at the highest concentration studied N = 3.30 strongly suggests formation of ground-state oligomeric aggregates with an “average species size” (see definition above) of ∼3. Though further concentration increase could not be attained due to spontaneous crystallization of the complex from the solution, we believe that larger aggregates might be formed in the case of a better solubility. Nevertheless, the data available forms solid experimental evidence that proves the existence of the groundstate aggregates (N ≥ 3) achieved via simple increase in the solution concentration. For the sake of a comparison the analogous NMR experiments were performed for the pyridine complex 1 (Table 2). Expectedly, neither Dobs nor the chemical shifts of the 1H NMR signals display any concentration dependence. Note that the Dobs values for 2·PF6 and 1 differ substantially, as they were obtained in the solvents (acetonitrile and pyridine, respectively), which have different viscosities. This observation agrees well with the solid-state structure of 1, where no Pt···Pt interaction was found because of steric hindrance induced by the pyridine ligands that prevents short intermolecular contacts. The structures of the di- and trinuclear aggregates were also optimized using the quantum chemical DFT methods, which show that the species are energetically rather favorable, thus verifying that ground-state aggregation via metallophilic bonding in solution is a plausible option (see details in the Computational Results section). 3358


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Figure 15. Absorption (A) and emission (B) spectra of 2·PF6 in acetonitrile at different concentrations (298 K, λexc = 351 nm). (inset) A photograph of the emission under UV excitation (acetonitrile, 298 K, λexc = 365 nm).

Figure 16. (A) Excitation and emission profiles, (B) kinetics traces for the solution of 2·PF6 (0.07 M) monitored at 483 nm (monomer) and 620 nm (dimer/excimer), acetonitrile, room temperature.

Table 3. Lifetimes (μs) for 1 and 2·PF6 (Deaerated Acetonitrile, 298 K)

to the starting one. These observations fit well the results of diffusion NMR study, vide supra, and estimation of “average size” of aggregates presented in the equilibrium mixture of species. Note that the absence of a detectable monomer emission from the concentrated solutions of 2·PF6 may be caused by two major reasons: (a) a large degree of aggregation that diminishes the amount of the monomer; (b) significant self-absorption by the aggregates in the high-energy region (see Figure 15A). The emission lifetimes of 2·PF6 under deaerated conditions in diluted (Figure 16) solution were measured at different complex concentrations at the wavelengths of 483 and 620 nm; the former evidently corresponds to emission of monomer, whereas the latter could be tentatively assigned to the emission of dimeric species (Table 3). The emission of monomer (423, 514, and 550 nm) demonstrates a clear dependence of τobs on the concentration, an increase in which leads to a significant decrease in the lifetime as a result of self-quenching, often attributed to excimer formation.3i,32a,33 The decay constant can be expressed by the following equation: kobs = 1/τobs= 1/τ0 + kQ·[2·PF6], where τ0 is the monomer emission lifetime in the absence of quenching (i.e., at zero concentration with virtually no aggregation) and kQ is a quenching constant.33 Analysis of the reciprocal plot of the lifetime versus concentration (Figure S8) gives the τ0 = 0.63 μs that is markedly shorter than the corresponding values for the congener [Pt(NCN)X] species being as long as 13 μs.3i,32a The quenching rate constant kQ for 2·PF6 equals 1.57 × 109 M−1 s−1

1 2 (0.07 mM) 2 (1.66 mM) 2 (11.27 mM) a

monomera

dimerb

1.3 0.57 0.24 0.053

0.55 0.37

Measured at 423, 514, and 550 nm. bMeasured at 620 nm.

and serves as a crude estimation of the bimolecular association rate leading to an excimer. The obtained value is in a good agreement with the previously reported data on efficient excimer formation (kQ ≈ 1 × 109 M−1 s−1),3i,32a which may be interpreted in terms of major contribution of this relaxation channel into monomer deactivation pathway, and therefore the red-shifted emission at ca. 620 nm originates to a significant extent from the excimer species. The kinetics of excited-states relaxation traced at 483 and 620 nm (Figure 16) in diluted solution (0.07 mM) show a complicated behavior that indicates the presence of multicomponent mixture of monomeric/dimeric/eximeric emitters, which give major contribution to the emissions detected at these particular wavelengths. It is worth noting that the presence of oligomers higher than dimers in the diluted solution is hardly possible, and kinetics model described below is based on the monomer−dimer equilibrium exclusively under the conditions of a dilute solution. 3359


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Inorganic Chemistry

two was minimal, which can be explained with a similar “polymeric” arrangement within the tetrametric models for 2· ClO4 and 2·CF3SO3, since the latter was found to exist in solid state as tetramers. Further analysis of aggregates of a higher nuclearity, for example, [(Pt(NCN)L)8]8+, would probably show the real differences, but the models were too large for a TD-DFT calculation at the present level of theory. However, the topological charge-density analysis of the larger [(Pt(NCN)L)8]8+ assemblies shows the difference in Pt···Pt interactions of the two tetramers in 2·CF3SO3 versus the polymer in 2·ClO4 (Table 4). The nature of the interaction in

Monitoring of the emission kinetics trace at 620 nm band shortly after the excitation pulse (t = 6 ns) shows that its intensity is ca. 50% of its maximum. The subsequent growth of emission followed by the decay stage both show monoexponential character and the same rate as that obtained for the monomer emission (τ ≈ 0.6 μs). This type of behavior may be explained in terms of two processes, which give excited dimers. The emission after excitation (t > 6 ns) could be assigned to the direct excitation of the ground-state dimers D (first stage): D + hν → D*

that involves ∼50% of the total amount of D* species and is a fast process. The second (slow) stage might be assigned to the excimer formation pathway with possible contribution of the excitation collisional transfer:

Table 4. Selected Properties of the Electron Density According to Quantum Theory of Atoms in Molecules Analysis at the Pt···Pt Bond Critical Points in Models [(Pt(NCN)L)8]8+ of Complex 2a

M * + M → D*, M * + D → D* + M

Thus, the balance equation for monomers and dimers in the excited state can be written as

d(Pt···Pt), Å

BCP

2·ClO4

3.330

2·ClO4

3.330

2·PF6

3.313

2·PF6 2·CF3SO3

3.391 3.271

2·CF3SO3

3.233

2·CF3SO3

3.899

Pt(1)− Pt(1′) Pt(1′)− Pt(1) Pt(1)− Pt(2′) Pt(2)−Pt(1) Pt(1′)− Pt(2) Pt(1)− Pt(1′) Pt(2)− Pt(2′)

complex

dM * 1 = − M* − kQ M* dt τ0 dD* 1 = − D* + αk Q M * τd dt

where τd is the lifetime of the excited dimers, and α is a probability of the dimer formation in the quenching process. As mentioned above, time dependence of the concentration of excited monomers is monoexponential with the lifetime τobs. The initial condition D* (t = 0) is determined by direct excitation from the ground state. The position of the maximum and its value on the curve of decay is determined by the ratio τd, τobs, and αkQ. Satisfactory agreement with experimental data can be obtained when τobs = 0.57 μs, τd = 0.23 μs. This model adequately describes experimental data that are clearly confirmed by the data fit given in Figure 16. This also means that experimentally observed lifetime of the D* species is presumably determined mainly by the secondary effects (quenching of the excited monomers M*) and cannot be considered as a real lifetime of the excited dimers. Increase in the concentration gives a complicated emission pattern (Figure 15), which is evidently generated by a complex mixture of oligomers with a systematic red shift of the “mixed band” maximum that is compatible with both NMR measurements, vide supra, and results of theoretical calculations, vide infra. Computational Results. Effect of Crystal Packing in Complexes 2·X. To study the effect of different solid-state packing of the complexes 2·X, UV−vis absorption spectra were simulated for the models, which have the geometries from the experimental XRD data. For this procedure the tetrameric [(Pt(NCN)L)4]4+ units were chosen (Figure S9, the lowestlying absorption bands are depicted on Figure S10) following the formal nuclearity of 2·CF3SO3; application of the same size to all 2·X compounds would enable comparison of the energy values also. The lowest-energy absorptions, which consists mainly of the HOMO → LUMO transition, is clearly blueshifted in the case of 2·PF6. This is in agreement with XRD structural analysis, which shows that 2·PF6 has the longest Pt··· Pt bonding distance (3.3906(6) Å at 120 K) among the 2·X species. As indicated in Figure S11, the lowest-energy transitions show clear dependence on the metal−metal separation, the elongation of which causes a substantial increase of the excitation energy. However, the difference in the other

ρ(BCP), e/Å3

|V|/G

Eint, kJ mol−1

0.137

1.10

−17.5

0.139

1.11

−17.8

0.143

1.12

−18.3

0.123 0.152

1.08 1.12

−15.2 −20.0

0.164

1.14

−22.1

0.058

0.91

−5.7

a

The experimental coordinates from the crystal structures of 2·ClO4, 2·PF6, and 2·CF3SO3 (120 K) were used for the corresponding computational models. ρ(BCP) = electron density at the BCP, |V|/G = ratio of the potential energy density and the kinetic energy density, Eint = interaction energy at the BCP.

all models was interpreted to be a weak metallophilic interaction, as can be seen in the typical small electron density, |V|/G value of ∼1.1, and interaction energy ranging from −15 to −22 kJ mol−1. Although the properties of the electron density were very similar within the tetramers, the Pt···Pt interaction was clearly weaker between the two tetramers in the model of 2·CF3SO3, explaining the difference in the lowestenergy transitions. Aggregation in Solution. To verify the experimental suggestion of forming larger aggregates in solution, we created computational models by stacking separately optimized monomeric units together based on the torsional behavior of the ligands in the experimentally studied crystal structure of 2· PF6. Figure 17 presents the model of a [(Pt(NCN)L)n]n+ configuration with n = 4; models with n = 1−8 were also calculated. Since in solution the Pt···Pt distance can be expected to average out, an experimental room-temperature distance of the crystal structure (3.426 Å) was selected for all metal−metal distances. The models were also somewhat symmetrized with Pt···Pt···Pt angle of 180° and the torsional angle between the two auxiliary acetonitrile ligands set to 60°. These constrained models in the presence of the acetonitrile solvent field were used for the estimation of energy parameters. At the first stage, relative stabilization energies were calculated to find out how plausible the formation of larger aggregates would be. Stabilization energy was defined via the 3360


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Inorganic Chemistry

Figure 18. Lowest-energy excitations in the computationally simulated absorption spectra of the [(Pt(NCN)L)n]n+ models with n = 1−4 in acetonitrile solution.

Figure 17. A computational model of the stacking aggregates [(Pt(NCN)L)n]n+ with n = 4. Pt···Pt distance is represented with an experimental distance in complex 2·PF6, d = 3.426 Å. Color coding: Pt = green, N = blue, C = gray, H = white.

the Pt d-orbitals increases both in HOMO and in LUMO, which stabilizes their energies and also reduces the energy gap. The appearance of the frontier molecular orbitals for the monomer and the trimer can be seen in Supporting Information (Figure S12). The given analysis of the electronic structures unambiguously shows that the Pt···Pt interactions in the aggregates are responsible for the observed red shift in both absorption and emission spectra.

total energy of the aggregate relative to the total energy of the monomer according to equation:

En − E1 n Table 5 shows Estab values for the aggregates with n = 1−8 in acetonitrile solution, together with the orbital energies of HOMO and LUMO and the corresponding HOMO−LUMO gap. Estab =

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CONCLUSIONS In the current study we performed a systematic structural and spectroscopic study of luminescent cationic cyclometalated complexes [Pt(NCN)L]+ (NCN = dipyridylbenzene, L = pyridine (1), acetonitrile (2)) making a particular emphasis on the aggregation processes via intermolecular noncovalent interactions (π−π and metallophilic bonding). Compound 1 does not demonstrate any tendency for self-assembly due to the orientation of the pyridine ligand that sterically prevents association of the cationic units. On the contrary, the 2·X species (X = PF6−, ClO4−, CF3SO3−) easily give aggregates both in the solid state an in acetonitrile solution. In the crystalline state the complexes 2·X form different types of polymeric modifications, stabilized by the effective Pt···Pt interactions and π-stacking of the aromatic fragments. The VT XRD experiments performed for 2·X show a clear dependence of the intermetallic distances on temperature, variation of which is accompanied by a distinct color change, particularly pronounced for 2·PF6 species. The complexes 2·X undergo the reversible assembly in solution, the degree of which is independent of the counterion X−, but is governed by the temperature and concentration of the 2·X. Varying the concentration of 2·PF6, which has the best solubility, it is shown by the diffusion coefficients NMR measurements that the monomeric form, dominating at low concentration at 298 K, undergoes the formation of ground-state oligomeric aggregates with an average aggregation number of ∼3. The luminescent behavior of the compounds 1 and 2·X is largely determined by the intermolecular aggregation. The photophysical properties of complex 1 are assigned to the metal-perturbed IL with some MLCT contribution. Expectedly, in the absence of appreciable association the emission properties of 1 are not affected by the concentration and temperature and therefore correspond to the discrete molecules. The luminescence of 2·X is significantly red-shifted with respect to 1 and originates predominantly from the 3 MMLCT excited state. The energies of excitation and emission of 2·X in solid state depend on the structural arrangement of

Table 5. Stabilization Energies (Estab) for the [(Pt(NCN)L)n]n+ Aggregates (n = 1−8) and the Corresponding Orbital Energies of HOMO and LUMO n

Estab, kJ mol−1

e(HOMO), a.u.

e(LUMO), a.u.

gap, eV

1 2 3 4 5 6 7 8

0 4 9 12 14 15 17 18

−0.238 99 −0.234 88 −0.231 15 −0.228 80 −0.227 97 −0.228 88 −0.228 91 −0.229 06

−0.076 75 −0.089 74 −0.098 15 −0.103 53 −0.107 30 −0.109 16 −0.111 56 −0.113 48

4.41 3.95 3.62 3.41 3.28 3.26 3.19 3.14

The very small positive stabilization energies show that it is energetically not very demanding to form larger aggregates with respect to the monomer. Therefore, it can be concluded that the formation of dimers, trimers, tetramers, etc. can occur in the solution under favorable conditions (e.g., high concentration), as the experimental NMR data suggest. The same conclusion is further supported by the small differences in the energy gap of the frontier molecular orbitals, which indicates that the stability of the larger aggregates only slightly decreases compared to that of the monomer. The trends in the absorption spectra of the stacking aggregates were further studied by simulating the spectra for models with n = 1−4 by TD-DFT calculations (Figure 18). The increase of the aggregation number n in [{Pt(NCN)L}n]+ leads to a clearly visible red shift in the lowest-energy transitions. These bands (for n = 2−4) originate from the HOMO → LUMO transitions, where the HOMO corresponds to antibonding combination of the dz2 orbitals of the Pt atoms. The analogous absorption is missing in the monomer (n = 1), which shows its lowest energy band at much shorter wavelengths, because the HOMO demonstrates a dominating ligand contribution. Notably, in aggregates the contribution of 3361


Article

Inorganic Chemistry the chromophore [Pt(NCN)(NCMe)]+ units in the crystal cell and particularly on the Pt···Pt distances, which can be altered by temperature variations. The complexes 1 and 2 undergo reversible transformation 1 ↔ 2 in solid, which is induced by exposure to the corresponding vapors of acetonitrile or pyridine. This molecular reorganization accompanied by easily detectable change of the photophysical properties represents an example of photoactive functional materials, which involve weak noncovalent interactions. The photoemission characteristics of 2·X in solution are consistent with NMR spectroscopic observations and can be described in terms of concentration-dependent aggregation, which leads to the appearance of a number of oligomeric species and variation of the emission color from greenish to deep red. The theoretical analysis of the electronic structures of 1 and oligomeric species 2·X supports the experimental observations pointing to (a) the energetic favorability of the aggregates formed via the metallophilic bonding and (b) the principal role of Pt···Pt interactions in the modulation of absorption and emission spectra.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02713. Packing diagrams of 2·X, torsion scan of complex 1, excitation and emission spectra of 2·PF6, selected bond distances and angles, additional NMR and computational data. (PDF) X-ray crystallographic data in CIF format for 1 and 2·X. (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (E.V.G.) *E-mail: [email protected]. (S.P.T.) *E-mail: igor.koshevoy@uef.fi. (I.O.K.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by the University of Eastern Finland (strategic funding, Russian−Finnish projects), the Academy of Finland (Grant No. 268993 to I.O.K.), St. Petersburg State University research Grant No. 0.37.169.2014, grants of the Russian Foundation for Basic Research 13-04-40342 and 13-03-12411. The work was performed using equipment of the Analytical Center of Nano- and Biotechnologies of SPbSPU with financial support of Ministry of Education and Science of Russian Federation; Centers for Magnetic Resonance and for Optical and Laser Materials Research, Research Park of St. Petersburg State University.

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Solid-State and Solution Metallophilic Aggregation of a Cationic [Pt

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