Multistimuli-Responsive Properties of Aggregated Isocyanide Cycloplatinated(II) Complexes.

Here, we describe the neutral cyclometalated tert-butylisocyanide PtII complexes, [Pt(C∧N)Cl(CNBut)] 1, the double salts [Pt(C∧N)(CNBut)2][Pt(C∧N)Cl2] 2, and the cationic complexes [Pt(C∧N)(CNBut)2]ClO4 3 [C∧N = difluorophenylpyridine (dfppy, a), 4-(2-pyridyl)benzaldehyde (ppy-CHO, b)]. A comparative study of the pseudopolymorphs 1a, 1a·CHCl3, 1b, 1b·0.5Toluene, 1b·0.5PhF, and 3a·0.25CH2Cl2 reveals strong aggregation through Pt···Pt and/or π···π stacking interactions to give a variety of distinctive one-dimensional (1D) infinite chains, which modulate the photoluminescent properties. This intermolecular long-range aggregate formation is the main origin of the photoluminescent behavior of 1a and 1b complexes, which exhibit highly sensitive and reversible responses to multiple external stimuli including different volatile organic compounds (VOCs), solvents, temperatures, and pressures, with distinct color and phosphorescent color switching from green to red. Furthermore, complex 1b undergoes supramolecular self-assembly via Pt···Pt and/or π···π interactions into a polymer thin polystyrene (PS) film 10 wt % in response to toluene vapors, and 3a exhibits vapochromic and vapoluminescent behavior. Theoretical simulations on the dimer, trimer, and tetramer models of 1a and 1b have been carried out to get insight into the photophysical properties in the aggregated solid state.

Introduction

Currently, there is a great deal of interest in producing chromic PtII complexes, with changes in the color or luminescence upon the application of external stimuli.[1] Most of the studies focus on changes in luminescence using as external stimuli liquids,[2] vapors,[3] temperature,[3d,4] or mechanical grinding[5] of interest in memory, sensors, and optoelectronics.[6] Some excellent reviews about these topics have been published.[7] Many complexes only respond to a single external stimulus, with the systems that respond to multiple external stimuli being rare.[3d,4b,8]

The versatile chromic behavior of the PtII complexes is a consequence of their rigid square planar geometry, which enables noncovalent π···π stacking and intra- or intermolecular Pt···Pt interactions via open axial positions. These interactions endow supramolecular structures generating 3ππ excimeric and metal–metal-to-ligand charge-transfer (3MMLCT) transitions, respectively, remarkably red-shifted in comparison to the intraligand (3IL), ligand–ligand charge-transfer (3LLCT) or metal-to-ligand charge-transfer (3MCLT) transitions present in the monomer.[7a,7g,9] The interplanar and metallophilic interactions, with Pt···Pt separations shorter than the sum of van der Waals radii, can allow the formation of dimers, trimers, oligomers, or one-dimensional (1D)-stacked structures,[3a,7g,10] and some of these systems have been successfully utilized to develop white light, deep-red to near-infrared organic light-emitting diodes (OLEDs) or data storage devices.[11] However, intermolecular interactions can also cause nonradiative deactivation.[12] Considering that the metallophilic Pt···Pt interactions show a bond strength comparable to that of the hydrogen bonding,[13] these relatively weak interactions can be affected by steric effects or other noncovalent intra-/intermolecular interactions such as solvation, hydrogen/halogen interactions, etc.[7b,14] In consequence, noncovalent interactions play key roles in the assembly of platinum systems that respond to external stimuli and in the modulation of the photoluminescence properties of the resulting materials.[7a,15]

Among chromic PtII systems, a number of cyclometalated PtII complexes have been developed as a means to achieve bright, colorful luminescent materials.[1,2,7c,7e,7i,16] On the other hand, isocyanides are versatile synthons for photoluminescent platinum complexes, which show stimuli response properties,[17] and on some occasions, the luminescence can be modulated by noncovalent interactions.[18] In this field, some chromic cycloplatinated complexes containing the strong field ligand isocyanides have been documented,[14a,19] and it has been demonstrated that their photophysical properties can be modulated effectively by adjusting steric effects, the strength of the intermolecular interactions, or the medium. In this line, our group has reported two families of chloro-cyclometalated complexes, the benzoquinolinyl [Pt(bzq)Cl(CNR)],[14a] in which isocyanide was varied, [R = tert-butylisocyanide (Bu); 2,6-dimethylphenyl (Xyl); 2-naphthyl (2-Np)], and the xylylisocyanide compounds, [Pt(C∧N)Cl(CNXyl)], in which the cyclometalated backbone was modified [HC∧N = 2-phenylpyridine (ppy); 2,4-difluorophenylpyridine (dfppy); and phenylquinoline (pq)],[19a] aiming to examine the role of the intermolecular π···π and short- or long-range Pt···Pt stacking interactions on their structural arrays and photophysical properties. The higher self-assembly behavior in the ground and excited state was found for the bzq/CNBu, 2-Np and ppy-based complexes.

In this work, we report the preparation, characterization, and optical study of cyclometalated isocyanide PtII complexes, [Pt(C∧N)Cl(CNBu)] 1, the double salts [Pt(C∧N)(CNBu)2][Pt(C∧N)Cl2] 2, and the cationic complexes [Pt(C∧N)(CNBu)2]ClO43 with difluorophenylpyridine (dfppy, a) or formyl-functionalized phenylpyridine (ppy-CHO, b) cyclometalated ligands. Among them, we discuss in depth the relationship between the structure–optical properties of complexes that display chromism upon the application of one or several stimuli. In particular, we present the differences in the crystal packing and luminescent properties of several pseudopolymorphs of 1a and 1b and the effect of solvents (vapochromism and solvatochromism), temperature (thermochromism), and mechanical force (mechanochromism), together with the vapochromism of 3a.

Synthesis and Characterization of Complexes

The synthesis of the cycloplatinated chloride–isocyanide complexes [Pt(C∧N)Cl(CNBu)] (1) and bis-isocyanide [Pt(C∧N)(CNBu)2]ClO4 (3) complexes was carried out using the corresponding complexes containing HC∧N as an auxiliary ligand as precursors, [Pt(C∧N)(HC∧N)Cl] (C∧N = dfppy Ia,[20] ppy-CHO Ib). Precursor Ib was prepared in this work following a conventional protocol, which involves the reaction of K2PtCl4 with an excess of HC∧N in an ethoxyethanol/water mixture at 80 °C.

The reaction of the corresponding precursor Ia or Ib with 1 equiv of CNBu in CH2Cl2 at room temperature and further treatment of the dry residue with toluene gave a minority solid, insoluble in toluene, and a major product soluble in toluene (Scheme i). The main product, identified as [Pt(C∧N)Cl(CNBu)], was obtained from the toluene solution in high yields (85–88%) as a yellow-orange (C∧N = dfppy 1a) or a red solvated solid (C∧N = ppy-CHO 1b·0.5Toluene). Interestingly, 1b·0.5Toluene keeps its color in solid for more than 1 month, finally changing to a yellow solid on standing, suggesting that a structural transformation takes place. Indeed, it is also isolated as unsolvated yellow solid (1b) by dissolution in CH2Cl2 and evaporation to dryness. These complexes 1 are rather soluble in common organic solvents. Spectroscopic analysis and X-ray diffraction (XRD) of crystals of 1a and 1b from different solvents (see below) reveal that complexes 1 display the stereochemistry shown in Scheme with CNBu occupying the trans position to the nitrogen of the C∧N ligand, similarly to other chloride–isocyanides published[14a,19a,21] and consistent with the lower trans influence of Cl– compared to the CNBu ligand. They exhibit one ν(Pt–Cl) absorption at 289 cm–1, consistent with a terminal Pt–Cl bond trans to C, and one absorption at 2207 1a and 2205 cm–11b, assignable to ν(C≡N) of terminal CNBu, which is shifted to higher frequencies with respect to the free ligand (2125 cm–1). Spectroscopically, the microcrystals obtained from different solvents exhibit almost identical ν(C≡N) bands to the pristine solids (2207–2211 1a, 2202–2205 cm–11b). Their electrospray ionization (ESI)(+)-mass spectrometry (MS) spectra show the peak corresponding to the loss of chloride ([M – Cl]+) and that corresponding to the dimer [2M – Cl]+, suggesting a strong tendency to form aggregates. 1H and 13C{1H} NMR spectra of complexes 1 display the expected signals for C∧N and CNBu in a 1:1 intensity ratio, which were assigned on the basis of 1H–1H and 13C–1H correlations (see the Experimental Section and Figures S1 and S2). The minority solids were proposed as the double salts [Pt(C∧N)(CNBu)2][Pt(C∧N)Cl2] (red solid, C∧N = dfppy, 2a; yellow, C∧N = ppy-CHO 2b) in agreement with their matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) spectra, which show peaks due to the corresponding anion [Pt(C∧N)Cl2]− and cation [Pt(C∧N)(CNBu)2]+ operating in a negative and positive mode, respectively (Figures S3 and S4), and two characteristic ν(C≡N) absorptions due to the two terminal CNBu ligands. To corroborate this proposal, red complex 2a was obtained alternatively by a reaction of (NBu4)[Pt(dffpy)Cl2] (generated in situ from [Pt(dfppy)(μ-Cl)]2 with 2 equiv of NBu4Cl in refluxing MeOH) and complex 3a, [Pt(dffpy)(CNBu)2]ClO4. Its insolubility in common organic solvents prevents the characterization by NMR spectroscopy and X-ray spectroscopy. 2a does not show solid-state evolution by thermal treatment in an oven (100 °C) for 24 h into the corresponding neutral complex [Pt(dfppy)Cl(CNBu)], a common feature in double salts.[14a,22] A similar reaction between (NBu4)[Pt(ppy-CHO)Cl2] and 3b also evolves with the formation of the expected salt 2b but mixed with 1b due to a relatively fast rearrangement of the salt. This type of behavior is not unusual. Indeed, the salt [Pt(bzq)(CN-2-Np)2][Pt(bzq)Cl2] was also formed as a subproduct together with [Pt(bzq)Cl(CN-2-Np)] by reacting [Pt(bzq)(μ-Cl)]2 and CN-2-Np in a 1:2 molar ratio.[14a]

Scheme 1

Synthesis of 1–3 (a, b), (i) 1 equiv of CNBu, CH2Cl2; Dryness and Treatment with Toluene, (ii) Excess of KClO4 and 2 equiv of CNBu, Acetone/CH2Cl2 (1/1), and 298 K

Cationic complexes [Pt(C∧N)(CNBu)2]ClO4 (C∧N = dfppy 3a, ppy-CHO 3b) were prepared by treatment of a suspension of the corresponding [Pt(C∧N)(HC∧N)Cl] (Ia, Ib) precursor with an excess of KClO4 and the subsequent addition of 2 equiv of CNBu in a mixture of acetone/CH2Cl2 (1/1) (Scheme ii). These complexes were isolated as pale-yellow air-stable solids in good yields (84–88%). They exhibit two terminal ν(C≡N) absorptions at 2236, 2215 3a, 2248, and 2223 cm–13b, shifted to higher energies than in 1, and peaks at 1085 and 622–624 cm–1, assigned to the ionic ClO4–. The ESI(+) mass spectra show the corresponding [Pt(C∧N)(CNBu)2]+ peaks and the NMR spectra (1H and 13C{1H}) show the expected signals of C∧N/CNBu in a 1:2 intensity ratio. It is worth noting that in 3, the most deshielded H2 proton appears high-field shifted in relation to complexes 1 (δ H2 8.90 3avs 9.47 1a; 8.78 3bvs 9.80 1b) (Figures S5 and S6), as a consequence of the substitution of Cl– by CNBu.

Crystallographic Studies

For 1a, crystals suitable for X-ray diffraction were obtained by slow diffusion of n-hexane into a CH2Cl2 solution (1a) or by evaporation from a concentrated CHCl3 solution (1a·CHCl), and for 1b, by slow diffusion at 298 K of n-hexane into tetrahydrofuran (THF), toluene, or fluorobenzene solutions, respectively (1b, 1b·0.5Toluene, 1b·0.5PhF), and they were subjected to single-crystal X-ray analysis. The basic crystallographic data and selected bond distances and angles are summarized in Tables and S1–S3. Their structural analyses reveal the expected distorted square planar geometry with the isocyanide ligand in the trans position with respect to the nitrogen of the cyclometalated groups (Figures and 2). All distances and angles are comparable to those observed in related complexes.[14a,19a,21]

Table 1

Color, Color Emission, and Selected Distances (Å) and Angles (deg) of Crystals 1a, 1a·CHCl, 1b, 1b·0.5Toluene, and 1b·0.5PhF

	1aa	1a·CHCl3	1b	1b·0.5Toluenea	1b·0.5PhFa
color	yellow	pale-yellow	yellow	orange-red	orange-red
emission	orange	green-yellow	yellow	red	red
Pt(1)–Cl(1) (Å)	2.385(2)	2.399(12)	2.3952(8)	2.4054(9)	2.4001(9)
Pt(1)–N(1) (Å)	2.054(5)	2.054(3)	2.057(3)	2.057(3)	2.061(3)
Pt(1)–Cort (Å)	1.986(3)	1.986(4)	1.992(3)	1.992(3)	1.988(4)
Pt(1)–CCNBut (Å)	1.921(7)	1.898(4)	1.894(3)	1.913(4)	1.908(4)
Pt···Pt (Å)	3.818(dimer)/3.688	5.305/6.541	5.337	3.362(dimer)/3.896	3.370(dimer)/3.864
dinterplanar (Å)b	3.448(dimer)/3.531	3.467/3.523	3.434	3.329(dimer)/3.306	3.298(dimer)/3.331
Pt···Pt···Pt (deg)	∼145	73.33	79.70	∼147	∼146
Cα–Pt–Pt–Cα (deg)	75.93/72.84	180.0	180.0	64.79/58.51	63.80/57.78

Pair of molecules with similar distances and angles were found in the crystals of 1a, 1b·0.5Toluene, and 1b·0.5 PhF. Data are given for molecule A.

Shortest interplanar distance.

Figure 1

(a) Single infinite crystal stacking along the a-axis of molecules AB showing the π···π interplanar and Pt···Pt distances. Hydrogen atoms are omitted for clarity. (b) Pictures of crystals of 1a under daylight or UV irradiation. (c) Top view from the a-axis of four stackings showing also C–H···F contacts between columns. (d) Packing structure of 1a·CHCl along the a-axis showing the π···π interplanar and Pt···Pt distances. (e) Pictures of crystals of 1a·CHCl under daylight or UV irradiation. (f) Axial visualization of solvent interactions along the stacking direction. (g) View of the packing structure with the solvent channels marked in yellow along the a-axis. This picture was illustrated using the Mercury computer program.[24]

Figure 2

(a) Single infinite crystal packing of 1b along the a-axis showing the π···π interplanar and Pt···Pt distances. (b) Pictures of crystals of 1b under daylight or UV irradiation. (c) Top view from the b-axis of four columns, showing also H(ppy–CHO)···O(ppy–CHO) interactions. (d) Columnar stacking along the a-axis of 1b·0.5Toluene showing the π···π interplanar and Pt···Pt distances. (e) Pictures of crystals of 1b·0.5Toluene under daylight or UV irradiation. (f) Top view from the a-axis of five stackings showing the toluene solvent localization and secondary contacts. (g) View of the packing structure with the solvent channels marked in red along the a-axis.

Yellow needles of 1a show orange emission similar to 1a powder, whereas the pale-yellow crystals of 1a·CHCl display a green-yellow emission due to their different molecular stacking (Figure ). 1a presents two nearly identical molecules (A and B) in the asymmetric unit (data given for A in Table ) that form head-to-head slightly twisted parallel dimers (AB), which stack along the a-axis (Figure a), with alternating Cα–Pt–Pt–Cα(CNBu angles of 75.93/72.84°, interplanar dfppy π···π interactions of 3.448 (dimer)/3.531 Å and Pt···Pt distances of 3.818 (dimer)/3.688 Å, close to the van der Waals limit, which is around 3.5 Å.[23] The Pt atoms in the columns are rather aligned with a Pt–Pt–Pt angle of ∼145°. Inside the columns, there are also secondary C–H(Bu···Cl interactions (2.770 Å) and the columns interact through C–H···F contacts (2.554 Å) (Figure c).

The 1a·CHCl crystals also show a staggered columnar packing. The molecules stack along the a-axis in a head-to-tail manner with an antiparallel arrangement, which minimizes the repulsion between the tert-butyl moieties (Cα–Pt–Pt–Cα angle of 180°, Figure f), and alternating π···π interactions, similar to those found in 1a (3.467 and 3.523 Å, Figure d and Table ). The Pt–Pt distances are longer than in 1a (5.305 and 6.541 Å) with a zigzag Pt–Pt–Pt angle of 73.33°. Interestingly, the molecules of the solvent are occluded in continuous channels along the a-axis, with two rows of CHCl3 per cavity interacting with the complex [Cl···Cl/H(solvent) 2.663 Å, F···Cl(solvent) 3.160 Å, and H(Bu···Cl(solvent) 2.822 Å] (Figures f,g and S7). The channels have an aperture size of 11.1 Å × 10.3 Å, and the total void volume occupies 26.3% of the unit cell. The green-yellow emission of these crystals changes to orange on standing in the air (4 h), suggesting that the CHCl3 molecules are lost, reversing to 1a. The shorter Pt–Pt distances in 1a could account for the lower-energy emissions of the crystals of 1a in relation to 1a·CHCl. The vapochromic behavior of the powder 1a (see below) could be related to the occurrence of a structural transformation from the initial framework of 1a, in which the molecules present a head-to-head disposition to the porous channel structure of 1a·CHCl, with the molecules adopting a head-to-tail arrangement. The solvent channels appear to be more accessible to absorb vapors of solvent molecules, thereby resulting in a rapid vapochromic response (see below).

Yellow needles of 1b display a staggered columnar packing, which resembles 1a·CHCl (Figure ), with a regular antiparallel head-to-tail arrangement of neighboring molecules with π···π (ppy-CHO) interactions of 3.434 Å and a long Pt···Pt distance (5.337 Å), indicative of negligible metallophilic interactions. The Pt–Pt–Pt and torsional Cα–Pt–Pt–Cα angles are 79.70 and 180.0°, respectively (Figure a,c).

The red needles crystals of 1b·0.5Toluene and 1b·0.5PhF display very similar crystallographic data and structural arrangement, with small variations that fit within the experimental error (Figures and S8). Therefore, only the structure of 1b·0.5Toluene will be discussed. In 1b·0.5Toluene, the two nearly identical molecules (A and B) found in the asymmetric unit form a head-to-head dimer with a short Pt···Pt distance of 3.362 Å and a π···π (ppy-CHO) interplanar distance of 3.329 Å (Figure d,f). The short distances[23] imply a strong Pt–Pt interaction within the dimeric unit, which could account for the lower-energy emissions of crystals 1b·0.5Toluene at 298 K, intensified by cooling. The dimers stack along the a-axis to form a columnar structure with a Cα–Pt–Pt–Cα torsion angle of 64.79/58.51° and a slight zigzag Pt–Pt–Pt arrangement (angle ∼147°), thus increasing the Pt···Pt distance (3.896 Å) between dimers. These columns are mainly supported by interplanar ppy-CHO π···π (3.306 Å) and secondary Cl···H(Bu (2.893 Å) interactions. A notable structural feature is the presence of a channel with an approximately 6 Å pore diameter along the a-axis, running parallel with the Pt···Pt stacks, partly occupied by toluene molecules in this red crystalline form, supported by weak contacts such as O(ppy-CHO)···HPh/Me(Tol) (2.500 Å), H(ppy-CHO/Bu)···CMe(Tol) (2.739 Å), and H(ppy-CHO)···HMe(Tol) (2.172 Å) (Figures g and S9). The solvent-accessible volume occupies 22.7% of the total volume of the unit cell for 1b·0.5Toluene and 18.1% for 1b·0.5PhF. Different solvents were studied (THF, xylene, acetone, CHCl3, and MeCN), and only red crystals were obtained from toluene and fluorobenzene. This fact suggests that the formation of channels seems to be driven by the required solvent space and the interactions between solvent molecules and the platinum moieties. These structures are stable with time, as solvent molecules are “trapped”, as evidenced by the fact that the red crystal lasted more than 1 month to lose the solvent and its color into the air. Structural analysis of 1b·0.5Toluene and 1b·0.5PhF provides insight into the solvent exchange mechanism, enabling rationalization of the vapochromic response that will be detailed in the corresponding section (see below).

Microcrystals of 3a·0.25CHCl and pale-yellow blocks of 3b were obtained by slow diffusion of n-hexane into a solution of the corresponding compound in CH2Cl2 at 298 K (Figures and S10 and Table S4). For 3b, the quality of the data collection was not good enough, and only the connectivity and the packing were established (Figure S11). Crystals of 3a·0.25CHCl contain four nearly identical molecules in the asymmetric unit, and only selected parameters for molecule A are given in Table S5. The cation exhibits the expected distorted square planar geometry formed by the dfppy and two tert-butylisocyanide ligands. In the crystal lattice, the cations form columnar structures along the a- and b-axis via partial π-stacking between two dfppy units from neighboring molecules, which have an antiparallel arrangement with short π···π interactions (3.391 Å) and a long Pt···Pt separation (5.160 Å). As shown in the Supporting Information (Figure S10), the dimers packed with other twisted units through a longer π···π (3.499, 3.538 Å) and a shorter Pt···Pt distance (4.558, 4.707 Å) and also show secondary C–H(Bu···F(dfppy) (2.472 Å) and H(dfppy/Bu)···O(ClO4) (2.494–2.525 Å) contacts.

Figure 3

(a) Single infinite crystal packing of 3a·0.25CHCl along the b-axis showing the π···π interplanar and Pt···Pt distances. (b) Pictures of crystals of 3a·0.25CHCl under daylight or UV irradiation.

Photophysical Properties and Theoretical Calculations

Absorption Spectra in Solution and Density Functional Theory (DFT) Calculations

The absorption spectra of the complexes 1a, 1b, 3a, and 3b were recorded in a CH2Cl2 solution (5 × 10–5 M, Figure ), and also in THF for compounds 1 (Figure S12), and the data are listed in Table S6. All complexes display intense high-energy absorption bands till ∼340 nm, assigned to a mixed intraligand 1IL(π–π*), located on the metalated C∧N ligand and charge-transfer (1MLCT/1L′LCT/1LL′CT) transitions. They also show a characteristic low-energy (LE) broad feature, red-shifted for the ppy-CHO compounds (b) in relation to that of dffpy (a) (389, 408 1bvs 362, 379 1a; 360, 375 3bvs 351, 368 nm 3a) and for the neutral Cl/CNBu (1) in relation to the cationic (CNBu)2 (3), as expected on the basis of the lower energy of the corresponding lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), respectively (ppy-CHO vs dfpy on the LUMOs and cationic vs neutral on the HOMOs). The absorption spectra in CH2Cl2 and THF display similar patterns, with a slight negative solvatochromism for the LE band with a red shift on decreasing the polarity of the solvent (362, 379 1a, 389, 408 1b nm in CH2Cl2; 382 1a, 392, 412 nm 1b THF), evidencing a certain charge-transfer (CT) nature (Figure S12).

Figure 4

(a) Normalized absorption spectra of complexes 1a, 1b, 3a, and 3b in CH2Cl2 at 298 K. (b) Low-energy region of the UV–vis absorption spectra of 1b in CH2Cl2 at different concentrations. (c) Representation of the absorbance at the 427 nm band vs concentration. Green box: the expansion of the low concentration range (<0.01 M).

DFT/time-dependent (TD)-DFT calculations were carried out for monomeric species of 1a, 1b, and 3a in CH2Cl2 (Tables S7–S9 and Figures S13–S16). In complexes 1, the low-energy feature is mainly assigned to a 1IL/1MLCT admixture with some 1XLCT (X = Cl) character, whereas bis-isocyanide complex 3a has an important 1IL character, with some ligand-to-ligand (1LL′CT, C∧N → C≡N) and ligand-to-metal 1LMCT character. The red shift observed for the ppy-CHO complexes (b) in relation to the dfppy (a) can be related to the stabilization of the LUMO induced by the presence of the aldehyde moiety (−1.98 1avs −2.43 eV 1b), whereas the HOMO remains similar (−6.16 1avs −6.14 eV 1b), which reduces the HOMO–LUMO gap. As is shown in Figure , the substitution of the chloride by other isocyanide ligand displays a blue shift in the LE absorption maxima (362, 379 1avs 351, 368 nm 3a), which can be ascribed to the major C∧N contribution in 3a (94%) with respect to 1a (55%) in the HOMO and to the π-acidity of the isocyanide that stabilizes the HOMO, leading to a larger HOMO–LUMO gap.

Figure 5

Schematic representation of selected frontier orbitals and excitations of 1a, 1b, and 3a.

A concentration dependence study in CH2Cl2 was carried out for 1a and 1b. In both complexes (Figure b,c for 1b, and Figure S17 for 1a), the lowest absorption band follows Beer’s law in the concentration range from 5 × 10–6 to 5 × 10–3 M, suggesting that no obvious ground-state aggregation occurs within these concentrations range. However, at higher concentrations (>5 × 10–3 M), a clear deviation from Beer’s law is observed for 1b (monitored at 427 nm). Weaker absorptions are also discernible at lower energy (∼475 and 510 nm), indicating the formation of aggregates. This agrees with color and emission changes from pale-yellow to dark orange on increasing the concentration from 5 ×10–3 to 0.4 M. This aggregation is also reflected in the clear upfield shift of all aromatic proton signals upon increasing the concentration (Figure S18). However, complex 1a exhibited lower solubility at concentrations >5 × 10–3 M, and the study is less conclusive.

Emission Spectra in the Film, Solution, and TD-DFT Calculations

Photophysical properties of complexes 1a, 1b, 3a, and 3b were studied in doped polystyrene (PS) films (1–10 wt %) and CH2Cl2 and THF solutions (298, 77 K) and in the solid state; the relevant data are listed in Tables S10 and 2. Calculations on the lowest-lying (S0–T1, SI) and spin density distribution of the triplet excited states (T1) based on their corresponding optimized S0 and T1 geometries of the monomers are detailed in the SI.

Table 2

Photophysical Data for 1–3 (a, b) in the Solid State at 298 and 77 K

298 K				77 K
compound	λem/nm (λex/nm)	τ/μs	ϕ	λem/nm (λex/nm)	τ/μs
1a	625 (420)	1.1 (63%), 0.5 (37%)	0.51	650 (420)	7.5
1a-CHCl3-R	662 (550)	0.24 (80%), 1.05 (20%)	0.21	660, 720max (470)	9.5 (660)
				730 (580)	3.7 (717)
					4.2 (730)
1a-CHCl3-G	474sh, 534a (420)	0.26 (51%), 1.68 (49%) (474); 22.7 (534)	0.11	474a (440)	12.4
1a-CHCl3-G-ground	632 (460)	0.31 (35%), 1.35 (65%)	0.38	677 (468)	5.5
1a(THF)	627 (470)	0.3 (31%), 1.0 (69%)	0.43	654 (470)	8.9
1a(MeOH)	650 (525)	0.3 (70%), 0.9 (30%)	0.14	722 (530)	3.9
1a(EtOH)	651 (550)	0.4 (50%), 1.3 (50%)	0.26	718, 748sh (500)	4.3 (718)
					2.5 (748)
1a(CHCl3)	660 (570)	0.3 (67%), 0.7 (33%)	0.22	724 (570)	3.2
1a(CH2Cl2)	689 (570)	0.2 (54%), 0.9 (46%)	0.21	739 (570)	2.3
1b	545a (420)	0.3 (53%), 2.8 (48%)	0.06	550 (420)	22.6
1b·0.5Toluene	658	<1%	0.20	550sh, 596, 706 (420)	14.8 (550)
				706 (550)	12.6 (596)
					9.8 (706)
1b-ground	550a (420)	0.32 (64%), 1.96 (36%)	0.03 (420)	557, 598, 660 (420)	17.2 (557)
	660 (500)	(585)	0.05(500)		10.2 (660)
2a	640 (550)	0.22	0.05	717 (550)	2.1
2b				650 (515)	2.0
3a	474,a 595max (385)	1.4 (595)	0.57	478a, 585max (385)	40.7 (478)
				478a, 617max (420)	8.9 (617)
3a·0.25CH2Cl2	490, 509max (420)	24.8 (509)	0.15	488 (420)	47.4 (488)
3a-CHCl3	627 (430)	1.09	0.26	482a, 624max, 638sh, 721sh (430)	32.5 (482)
					10.8 (624)
				650 (475)	9.3 (650)
				725 (590)	1.4 (700)
3a-Acetone	612 (430)	0.4	0.15	480a, 514, 613max, 645sh, 704sh (430)	67.3 (476)
				640, 659, 696sh (475)	10.7 (613)
				630, 690max (550)	9.5 (650)
					1.5 (690)
3a-THF	623 (430)	0.6	0.16	476, 510, 616max, 705sh (430)	54.6 (476)
					12.5 (616)
				645 (470)	6.8 (645)
				630, 700 (540)	2.4 (700)
3b	530 (440)	0.1 (58%), 0.8 (43%)	0.10	560, 628 (480)	15.8

ν0→0 transition of the structured emission.

Both chloride–isocyanide derivatives 1a and 1b display in a diluted PS film (1 wt %) structured phosphorescence bands (λem = 467 1a, 524 nm 1b, Figures and S19), attributed to monomer emission (3IL/3MLCT) involving the cyclometalated ligand and the Pt atom, with a predominant 3IL character (see calculations below). At higher concentrations, only 1a forms aggregates. Thus, at a doped concentration of 10 wt %, a broad unstructured red-shifted emission band appears at ∼580 nm, increasing in intensity relative to the peak at 467 nm by excitation from 365 to 410 nm and only a broad red-shifted band at 635 nm with λexc > 460 nm. These bands are characteristic of the formation of partial ground-state aggregates formed by π···π stacking and/or Pt···Pt interactions, which are supported by the different excitation spectra obtained by monitoring the distinct emission maxima. Because of these contributions, the emission color shifts gradually from green to yellow-orange with increasing concentration (Figure ), and the quantum yields increase from ϕ = 5% (PS 1 wt %, monomer) to 19% (PS 10 wt %), although without a significant effect in the lifetimes, which fit to two components (τaverage = 0.6–0.9 μs).

Figure 6

Normalized excitation (dashed line) and emission (solid line) spectra in PS at 1–10 wt % at 298 K in air of 1a.

Only 1b is emissive in fluid CH2Cl2 (5 × 10–4 M) at 298 K (Figure S20), displaying a typical structured phosphorescent emission, indicative of an emissive state of mixed 3IL/3MLCT nature with a dominant 3IL character, which was further supported by calculations (see below). No significant changes neither in emission maxima nor the quantum yield were observed in a THF solution at 298 K (Table S10). Complex 1b shows an aggregation of the PtII monomer in glasses of CH2Cl2 even in a diluted solution (5 × 10–5 M, λem 640 nm), whereas in THF glasses, the presence of the monomer (λem 517 nm, Figure S20) is dominant. The effect of the concentration on the emissive behavior was examined in CH2Cl2 at 298 and 77 K. The distinct emission profiles and color change can be visualized in Figure S21. At 298 K, increasing the concentration (5 × 10–5 to 0.4 M) affects the emission color. With concentrations of 5 × 10–5 to 0.01 M, only the monomer is observed. From 0.01 to 0.4 M, an aggregation broad peak (λem ∼ 710 nm), attributed to metal–metal-to-ligand charge transfer (3MMLCT), gradually gains intensity, and the emission changes from green to red (Figure S21a). Upon cooling to 77 K, the monomer emission dominates in diluted solutions (5 × 10–5 to 10–3 M), whereas that aggregated emission peak is observable from 0.01 to 0.4 M, together with the monomer (Figure S21b).

Complex 1a is nonemissive in solution probably due to deactivation through molecular motions in fluid or the ease of the thermal activation of the 3MC excited states. In a glassy solution at 77 K, it exhibits intense emission profiles, which depend on the solvent, the concentration, and the wavelength excitation. Upon cooling, diluted CH2Cl2 solutions (5 × 10–5 M) show three different emission bands depending on the excitation wavelengths (Figure ). By excitation to low wavelengths (λexc 365 nm), a minority band of the monomer (λem 465 nm), a broad band at 552 nm, and a shoulder at 645 nm are observed, whereas with λexc > 440 nm, the band at 645 nm dominates. The excitation spectra monitored at the three peaks differ, also suggesting the aggregation processes. At higher concentrations (5 × 10–4 M), only two broad emission bands (λem 570 and 650 nm) are observed by excitation at λexc < 400 nm, the LE emission band being selectively obtained using LE excitation wavelengths (λexc 450 nm) (Figure S22). Considering the unstructured emission profiles and their crystal packing, we tentatively ascribe the origin of these LE bands to ligand-centered 3ππ* and 3MMLCT transitions, respectively. Similar behavior was observed in THF at 77 K (Figure S23). In the glasses, the aggregate lifetimes [τ = 9.7 μs (552), 6.4 μs (645 nm) 1a, CH2Cl2 5 × 10–5 M; 10.1 μs (640 nm) (5 × 10–5 M), 9.7 μs CH2Cl2 0.01 M, 1b] are shorter than the corresponding monomer decay [τ = 14.9 μs (465), 1a, CH2Cl2; 14.8 μs (540) CH2Cl21b], in agreement with 3MMLCT contribution for the lowest-energy band (645 1a, 710 nm 1b) or some triplet–triplet annihilation (TTA)[25] in the 3ππ* excimeric-like feature (552 1a, 640 nm 1b).

Figure 7

Normalized excitation and emission spectra of 1a in CH2Cl2 5 × 10–5 M at 77 K. Photographs taken under lamps of the ∼λexc indicated and the N2 atmosphere.

For the bis-isocyanide compounds in PS films (1 wt %), 3b displays a typical 3IL-structured emission (524 nm), similar to 1b, while in the dfppy derivative (3a) dominates an unstructured emission peak [600 nm, τ = 1.8 (61%), 3.0 (39%) μs], characteristic of aggregate emission, in relation to the small intensity of the monomer (474 nm) (Figure ). In a fluid CH2Cl2 solution (5 × 10–4 M) (Figures S24a and S25a) both compounds show monomer emission (471 3a, 511 nm 3b) with lower quantum yields (ϕ 1% 3a, 2% 3b) than in PS films (ϕ 27% 3a, 9% 3b). In agreement with its behavior in PS films, compound 3a shows a higher tendency to aggregate in glasses at 77 K (Figure S24b), whereas 3b displays essentially monomeric emission with an increase in the LE shoulder (Figure S25b). As in complexes 1, the aggregate lifetimes [τ = 6.6 (580) 3a, 14.5 (618) μs 3b, CH2Cl2 77 K] are shorter than the corresponding monomer [τ = 39.1 (475) 3a, 24.5 (524) μs 3b, CH2Cl2 77 K].

Figure 8

Normalized emission spectra of 3a (λex 365 nm) and 3b (λex 420 nm) in PS (1 wt %) at 298 K in air.

The analysis of the monomer emission of these compounds indicates that the variation of the cyclometalated ligand has a notable effect on the emission maximum, with the ppy-CHO complexes (b) red-shifted with respect to the dfppy (a) derivatives, consistent with the higher energy gap for the π–π* orbitals of the dfppy complexes. The nature of the emissions was studied in 1a, 1b, and 3a through calculations of the lowest-lying T1 (S0 → T1) (Table S9) and spin density distribution for the triplet excited states (T1) (Figure ), based on their corresponding optimized S0 and T1 geometries, respectively. The calculated lowest-lying T1 state involves mainly the HOMO → LUMO transition (66% 1a, 77% 1b, 83% 3a), resulting in a 3IL transition with a minor contribution of 3MLCT and 3XLCT for complexes 1 and 3LL′CT (C∧N → CNBu) and 3LMCT (C∧N → Pt) for 3a. However, the calculated spin density distribution in the optimized T1 state (Figure ) is located mainly on the cyclometalated ligand and to a lesser extent in platinum (Pt, 0.1377 1a, 0.1912 1b, 0.0850 3a), with a negligible contribution of the other coligands, thus supporting a predominant 3IL state with 3MLCT contribution higher in the neutral complexes 1 than in 3. This result is in coherence with numerous works on cyclometalated isocyanide metal complexes, in which the large separation between the 3MLCT and 3IL states lead to a weak configuration interaction and a predominant 3IL character in T1.[26] In agreement with this, the calculated contribution of the Pt center in the SOMO-1 (Figure S26 and Table S11) decreases (Pt 9% 1a, 16% 1b, 3% 3a) in the optimized T1 state in relation to the optimized S0 geometry, especially for 1 (HOMO, 30% 1a, 34% 1b, 5% 3a), suggesting distortion upon excitation. The calculated emission wavelengths [1a (521), 1b (619), 3a (531 nm)] are consistent with the monomer emission observed [1a (467), 1b (524), 3a (474 nm), PS 1 wt %], although with expected overestimated values.

Figure 9

Spin density distribution for the lowest triplet excited state in 1a, 1b, and 3a.

Emission Spectra in the Solid State

The photophysical characteristics in the solid state are compiled in Tables (emission) and S12 (absorption spectra calculated from their reflectance spectra). Interestingly, complexes 1a and 1b exhibit multistimuli behavior, and 3a shows vapochromic behavior. As is seen in the structural section, the different possibility of packing of the complexes seems to be decisive in the observed vapochromic (1a, 1b, 3a), solvatochormic, and mechanochromic (1a, 1b) behavior.

Powdered as-obtained yellow-orange samples or crystals of 1a show a bright orange structureless emission band (625 nm), narrower and red-shifted (650 nm) to 77 K (Figures and S27), which is associated with a mixed 3MMLCT/3ππ* manifold due to the formation of dimers in the ground-state stacking in columns with relatively close Pt···Pt and π···π contacts. However, crystals of 1a·CHCl display a yellow-green monomer 3IL/3MLCT emission (534 nm) with a minor structured 3IL band at 474 nm, which dominates at 77 K (Figure S28), in accordance with their staggered columnar packing with longer Pt···Pt separations. Interestingly, upon fuming the yellow-orange powder 1a with vapors of CHCl3 for only ∼5 min, the sample changes to a kinetic red form, with a red emission (1a-CHCl-R form) (Figures a and S29). The elemental and thermogravimetric analyses (TGA) of the 1a-CHCl-R form revealed a phase transition at 75 °C arising from the desorption of CHCl3 from the lattice (Figure S30a). When a red sample of 1a-CHCl-R was heated up to 175 °C, the lost weight (ca. ∼19%) corresponds to about one lattice CHCl3 molecule per complex, giving 1a. This 1a-CHCl-R form displays a red shift structureless band centered at 662 nm, which narrows and considerably shifts to lower energies (λmax 730 nm, λexc 580 nm) at 77 K (Figure S29). The emission is attributed to an 3MMLCT transition from long-range kinetic aggregates (trimers, tetramers, etc.) formed by fast incorporation of the solvent and likely having shorter Pt···Pt intermolecular interactions than that found in 1a. Interestingly, CHCl3 is easily removed in this kinetic red form (1a-CHCl-R), and upon treatment with acetone or CH2Cl2 vapors, the unsolvated 1a is recovered. By contrast, if the fuming time of 1a with vapors of CHCl3 increases to ∼30 min at 298 K, we observed that the color changes gradually from red (1a-CHCl-R) to a final thermodynamically more stable yellow form (1a-CHCl-G) (Figure a), with a green-yellow structured emission identical to that measured for crystals 1a·CHCl. TGA analysis shows a phase transition from 1a-CHCl-G at 75 °C, corresponding to the transformation to 1a, by the loss of CHCl3 (Figure S30b). If the solid 1a-CHCl-G is kept in a close vial, it is stable for weeks, but if it is heated to 65 °C, it reverts to 1a, although without passing through the previous kinetic red form. Interestingly, the color of the phase 1a-CHCl-G changed from pale-yellow to dark yellow by manual grinding in a mortar (Figure S31). The 1a-CHCl-G-ground form is an amorphous orange emissive phase, which displays a broad unstructured band at 632 nm, similar to the as-prepared powder 1a, which is red-shifted at 77 K (677 nm). This indicates that mechanical stress in the yellow phase favors the ground-state interchromophore interactions. The emission quantum yields of these forms range from ϕ 11% in 1a-CHCl-G, 21% 1a-CHCl-R, and 38% 1a-CHCl-G-ground to 51% 1a (Table ). These values are comparable to or higher than those in polystyrene.

Figure 10

(a) Scheme and photographs of the external stimuli-responsive color and emission changes in 1a; left: under ambient light and right: under UV light irradiation (λex 365 nm). Emission color changes of 1a after exposure to CHCl3 vapors for 5 min (1a·CHCl-R) or more than 30 min (1a·CHCl-G). (b) Normalized excitation (red, λem 660 nm; orange λem 625 nm; green λem 535 nm) and emission spectra of 1a powder (λex 420 nm), 1a·CHCl-R (λex 550 nm), 1a·CHCl-G (λex 400 nm), and 1a·CHCl-G-ground (λex 450 nm) in air. (c) Normalized absorption spectra calculated from their reflectance spectra in the solid state. (d) Powder XRD (PXRD) patterns of different forms of 1a.

The absorption properties of all forms in the solid state are shown in Figure c and Table S12. The yellow-orange pristine solid 1a shows a broad absorption band up to 550 nm, whereas the 1a-CHCl-G form is characterized by a blue-shifted absorption band up to 500 nm, attributed to mixed 1LLCT/1MLCT transitions. The 1a-CHCl-R phase shows an absorption spectrum extending to 600 nm, most likely due to 1MMLCT transitions based on Pt···Pt interactions, in coherence with its color. As expected, the absorption spectrum of 1a-CHCl-G-ground and 1a-CHCl-R treated with acetone vapors are similar to that obtained for pristine 1a. The powder X-ray diffraction (PXRD) patterns show that pristine 1a is crystalline in nature, the PXRD peaks being consistent with the major diffraction peaks simulated from the cif files of the structure of larger single crystals of 1a (Figure d), suggesting that microcrystalline solid has a molecular packing similar to that of their larger crystalline samples. After exposing the sample of 1a to CHCl3 fumes for 5 min to obtain the red form 1a-CHCl-R, new peaks appeared (red line), indicating that the incorporation of a molecule of CHCl3 induces a structural transformation. The peaks of the pristine sample 1a can be mainly recovered once the red sample 1a-CHCl-R is exposed to acetone or CH2Cl2 vapors or is heated above 75 °C (yellow line, Figure d), indicating that both phases are dynamically related and that the positions of CHCl3 are not occupied by the incoming solvents. This behavior suggests the ease of loss of CHCl3 either by displacement with more volatile vapors or by heating above the boiling point of CHCl3 (61 °C). The vapochromic luminescent behavior of 1a in CHCl3 and 1a-CHCl-R in acetone is reversible for at least five cycles without perceivable performance degradation (Figure S32). Furthermore, when the red sample 1a-CHCl-R is exposed to air without stimuli, it gradually changes its color and emissive behavior from red to yellow-orange (more than 2 weeks), indicating that the desolvated 1a species is recovered. On the other hand, the PXRD peaks of the pale-yellow solid 1a-CHCl-G, obtained by treatment of 1a with vapors of CHCl3 for ∼30 min (green line, Figure d), show consistency with the major diffraction peaks simulated from the cif file of crystals of 1a·CHCl (blue line). Finally, the grinding process of 1a-CHCl-G induces a considerable decrease in its crystallinity, transforming it into an amorphous phase.

In addition to responding to vapors and pressure, complex 1a displays reversible color, color emission, and quantum efficiency change depending on the crystallization solvents (Figure a). The colors of the solids obtained by evaporation after the dissolution of 1a in different solvents vary from yellow to red and the emission ranges from 627 nm (THF) to 689 nm (CH2Cl2) (Figure b), red-shifted at 77 K (654 nm, THF to 739 nm, CH2Cl2) (Figure S33 and Table ). With MeOH or EtOH, the shift observed is ∼25 nm (298 K)/∼70 nm (77 K) and with CHCl3 or CH2Cl2 it reaches up to ∼64 nm (298 K)/∼90 nm (77 K). This discernible behavior can be also observed in the UV–vis absorption solid spectra. The solids obtained from alcohols show, in relation to 1a, a red-shifted LE band up to 550 nm and from chloride solvents up to 630 nm (Figure c and Table S12). The notable red shift of the red solvates could be mainly ascribed to 1MMLCT transitions favored by the presence of different aggregates with distinct π···π/Pt···Pt interactions, which seems to be stronger in chloride solvents. The emission color changes are accompanied by a dramatic decrease in the emission brightness from ϕ = 51%, 1a to 14%, 1a(MeOH), typically attributed to aggregation caused quenching. Interestingly, the formed phases in these solvents are not stable with time and the initial form 1a (emission and quantum efficiencies) can be restored on standing ca. 72 h or by stirring the solids in hexane for 1 h. This fact suggests that crystallization from chloride solvents probably gives rise to structures with short Pt···Pt distances, which slowly undergo switching to more stable structures surely with longer Pt···Pt separations.

Figure 11

(a) Photographs under ambient light, air atmosphere, and UV light irradiation (λex = 365 nm) of 1a (solid) and solids obtained by the evaporation of the appropriate solution of 1a in different solvents. (b) Normalized emission spectra. (c) Normalized absorption spectra calculated from their reflectance spectra in the solid state.

Complex 1b also exhibits remarkable luminescence switching properties as a result of their self-assembly behavior under application of different external stimuli. The two pseudopolymorphs, 1b and 1b·0.5Toluene, can be exchanged under vapor/solvent exposure or heating. Furthermore, a new phase (1b-ground), obtained by mechanical grinding of the monomer-emissive form 1b, is described. The related luminescence images and emission spectra of the three phases are shown in Figure a,b. The yellow powder 1b (and also microcrystals of 1b) exhibits a yellow structured emission at 545 nm (ϕ = 6%), only slight red-shifted at 77 K (550 nm) and, as expected, with more prolonged lifetime (τav = 1.5 μs, 298 K; 22.6 μs, 77 K) (Figures and S34). Red microcrystals of 1b·0.5Toluene show a broad red emission at 658 nm with a ϕ value of 20% and a short lifetime (<1 μs) at 298 K, which is red-shifted at 77 K (706 nm; 9.8 μs) (Table and Figure S35). This emission is ascribed to 3MMLCT in accordance with the short Pt··Pt distance (3.362 Å) found in the crystals of 1b·0.5Toluene. After 1b was ground, the resulting amorphous orange-yellow powder (1b-ground) developed a lesser structured band at 550 nm upon excitation at 420 nm, and a broad LE band at 660 nm by exciting to 500 nm (Figure S36), which are red-shifted at 77 K, indicating the formation of an amorphous solid with closer Pt units. This solid is not stable and 1b is recovered on standing by 2 h.

Figure 12

(a) Scheme and photographs of the external stimuli-responsive color and emission changes in 1b; left: under ambient light and right: under UV light irradiation (λex 365 nm). The emission color of 1b changes under treatment with toluene vapors for 2 h or addition of a drop of toluene (1b·0.5Toluene) and by mechanical grinding to afford 1b-ground. (b) Normalized excitation (yellow, λem 544 nm; orange, λem 585 nm; red, λem 660 nm) and emission spectra of 1b (λex 430 nm), 1b·0.5Toluene (λex 480 nm), and 1b-ground (λex 420 nm) in air. (c) Normalized absorption spectra calculated from their reflectance spectra in the solid state. (d) Powder XRD patterns of different forms of 1b.

When samples of 1b or 1b-ground were exposed to toluene liquid or vapors (around 2 h), the yellow powder turned vivid red with red luminescence. The emission spectra displayed an identical pattern to those of the crystals of 1b·0.5Toluene. The diffuse reflectance spectrum showed a broad band reaching 610 nm, clearly red-shifted in relation to those obtained with samples of 1b or 1b-ground tail to 550 nm) (Figure c). TGA of 1b·0.5Toluene revealed the phase transition arising from desorption of the lattice toluene molecules because the TGA curve of up to ∼170 °C represented a weight loss of ca. 7.8% corresponding to a half molecule per complex, as the crystal structure of 1b·0.5Toluene (Figure S37). However, on standing in the solid, the toluene molecules are also lost over time, and after 1 month of exposure to the air, it showed a gradual change to 1b (Figure S38). Moreover, 1b is recovered by exposition to acetone liquid/vapors or heating over 110 °C, as is reflected in its color change and its diffuse reflectance spectrum (Figure c).

The powder X-ray diffraction pattern of 1b coincides well with the simulated powder pattern of 1bcrystal (Figure d). Mechanical force destroys or changes the ordered molecular packing and induces a crystal-to-amorphous transformation in the PXRD pattern of 1b-ground, forming a much looser packing where more active molecular motions can occur. A direct crystal-to-crystal phase transition from 1b to 1b·0.5Toluene was observed for the PXRD of 1b·0.5Toluene, obtained from 1b under saturated toluene vapor or 1b·0.5Toluenecrystal, as shown in Figure d, which coincides with the simulated PXRD pattern of 1b·0.5Toluenecrystal. Toluene fuming can facilitate molecular motions and lead to an arrangement of molecules to form ordered packing again.

Aggregation of PtII compounds embedded in polymeric matrices can lead to sensory properties, which are not present in the starting compound. This type of process has been studied in depth for its use as dyes in the fabrication of smart materials.[16a,27] In this sense, we decided to evaluate the self-assembly behavior of 1b inside the polymer matrix. Two films of polystyrene (PS) were obtained after doping them with 1 and 10%, respectively, of 1b in CH2Cl2. As is shown in Figure a, one drop of each mixture was deposited in a glass holder followed by slow evaporation at 298 K (∼5 h). To investigate if the compound 1b inside the polymer exhibits vapochromic response, the two polymer thin PS films were exposed to vapors of toluene for 20 min, and then their luminescence was measured. For that, the glass with the thin film was placed on top of a vial containing toluene, allowing the vapor to come into contact with the polymer. In the case of the film doped at 1%, it exhibits the typical yellow emission of the monomer species (λ 524 nm). It seems that the low doping is not enough to self-assemble and form aggregates. However, for the higher concentrated film (10%), the change is notable in the presence of toluene vapor both in the naked eye and in the microscope (Figure b,c). In the beginning, the polymer film is a yellow-emitting (λmax 566 nm) translucent sheet with some imperfect points. Within 3 min of being exposed to the solvent, orange-red crystalline needles grow inside the matrix from a starting nucleation point. After 15 min, multiple needles with a width of ∼50 μm have grown alongwith the film, showing a remarkable red shift of the emission with the aggregation band centered at 675 nm. This indicates that molecules are self-assembled via intermolecular π···π and/or Pt···Pt interactions. The emission of these orange-red needles resembles that of the 1b·0.5Toluene crystals and the pristine solid 1b·0.5Toluene, suggesting that the needles are formed by incorporation of toluene channels, as in the crystal packing. The needles are stable for more than 2 months inside the polymer matrix, but after that time, presumably, the toluene gradually is lost (Figure S39), losing its crystallinity and changing its color and emission to yellow.

Figure 13

(a) Schematic drawings showing the fabrication of the PS film and the exposition to toluene vapors. Naked eye images of the PS film of 1b doped at 10% under room light before and after the toluene vapors. (b) Microscopy images of the PS film while the crystals grow by exposure to the solvent for more than 20 min. Images up to 16 min have a magnification of 40× (numerical aperture 0.95), while the image of 20 min has a magnification of 10× (numerical aperture 0.45). (c) Emission color changes of 1b in a film of PS at 10% (line yellow, λex 400 nm) and after exposition to toluene vapors (line red, λex 500 nm) at 298 K.

The double salt 2a is emissive in the solid state at 298 and 77 K and 2b only at low temperature. 2a shows a broad band centered at 640 nm (τ = 0.22 μs, ϕ = 5%), red-shifted to 77 K (717 nm, Δ = 77 nm; τ = 2.1 μs) and in relation to 2b at 77 K (649 nm) that can be ascribed to a 3MMLCT in the salt (Figure S40). Crystals of the cationic bis-isocyanide complex 3a·0.25CHCl displays a monomer green phosphorescence (λem = 490 nm, ϕ = 15%, τ = 24.8 μs) at 298 K, more structured at 77 K (Table and Figure S41a). However, 3a-pristine solid shows a bright orange phosphorescence, with very high efficiency (ϕ = 57%) at 298 K, formed by a minor structured band corresponding to the monomer, together with a structureless band at 595 nm, red-shifted at 77 K (617 nm), ascribed to an 3MMLCT emission. 3b-pristine exhibits a broad feature, blue-shifted (λem = 530 nm) in relation to that observed in 3a-pristine and with a lower quantum yield (ϕ = 10%), which might be attributed to excimer-like 3ππ* emission (Figure S41b). At 77 K, it exhibits a dual emission with maxima at 560 and 628 nm, which is tentatively assigned to excimer-like 3ππ* and 3MMLCT, respectively. The solid 3a-pristine showed reversible changes in its color and emission response to vapors of THF, acetone, and CHCl3 at room temperature for 6 h. In all cases, there is a change in its color from pale-yellow to pinkish-orange, reflected in the diffuse reflectance spectra, which show new bands extending to ∼600 nm in relation to the 3a-pristine (480 nm). In addition, the emission band is slightly red-shifted from 595 (3a-pristine) to 612 (3a-acetone), 623 (3a-THF), and 627 nm (3a-CHCl) (Figure S42). At 77 K, the solvate species exhibit a different pattern depending on the excitation wavelength (from 615 to 725 nm), suggesting the formation of different aggregates at low temperatures (Figure S43).

With the aim of understanding the observed red shift in the UV–vis absorption and emission spectra of the aggregated species, the geometries in the ground (S0) and the first triplet excited (T1) states of the dimer, trimer, and tetramer models were optimized in the gas phase based on 1a and 1b crystal structures. Two main intermolecular forces drive the dimer and tetramer assembly, the π···π and Pt···Pt intermolecular interactions. The geometries of calculated structures of [1a], [1a], and [1b] with their Pt···Pt distances and Cα–Pt–Pt–Cα angles and the orbitals involved in the electronic transitions (HOMO/LUMO) as well as the spin density plots are plotted in Figures and S44. For 1a, the computed Pt···Pt distance of the lowest-energy dimer [1a] and trimer [1a] in the S0 is 3.34 and 3.31 Å, respectively, which is in agreement with the data of X-ray structural analyses. The metallophilic interactions are more evident in the T1-optimized geometries ([1a] 2.83, [1a] 2.95 Å), being shorter than in the X-ray structure. The S1 transitions of simulated [1a] and [1a] were mainly derived of the HOMO → LUMO transitions (>97%, Table S13). The HOMO is located at the Pt atoms (84% [1a], 86% [1a]), whereas the LUMO is mainly localized in the dffpy cyclometalated ligand (84% [1a], 82% [1a]), highlighting the prominent 1MMLCT of the low-energy band, red-shifted in relation to the monomer (S1 417 [1a], 488 [1a]vs 383 nm 1a, Table ). However, in the tetramer model [1a], although the Pt···Pt distances are coherent with the X-ray structure, the energy of the low-energy transition with stronger oscillator strength (S2 391 nm) does not fit, and thus this tetramer model is not considered. With respect to the emission, the character of the lowest triplet excited state changes from 3IL/3MLCT character in the monomer to a mixed 3MMLCT/3IL in the dimer and trimer, with a higher 3MMLCT character for these systems, as is seen in their spin density plots (Figure ). In agreement with the experimental evidence, the predicted emission maxima are red-shifted in the trimer in relation to the dimer and the monomer, respectively (762 [1a], 624 [1a]vs 519 nm 1a). The value of the trimer, 762 nm, can be compared to the emission of the kinetic red form 1a-CHCl-R (662 nm), whereas the value of the dimer fits better to the orange emission of unsolvated crystals 1a (625 nm, also pristine solid).

Figure 14

Optimized geometries of [1a]and [1a] models at the S0 and T1 states. Contour plots of HOMO and LUMO at the S0 and spin density at the T1-optimized geometries [B3LYP/6–31G(d,p)].

Table 3

Calculated S1 Vertical Excitation Energies and Emission Energy Computed in the Gas Phase (SI)

complex	state	λ (nm)	assignment	ΔE emission (T1–S0 opt) (nm)
1a	S1	383	HOMO → LUMO (98%)	519
[1a]2	S1	417	HOMO → LUMO (98%)	624
[1a]3	S1	488	HOMO → LUMO (98%)	762
1b	S1	430	HOMO → LUMO (97%)	621
[1b]4	S1	527	HOMO → LUMO (95%)	943

For 1b, the optimized geometries of [1b] and [1b] models do not fit with the experimental X-ray diffraction structure of 1b·0.5Toluene. [1b] and [1b] show Pt···Pt distances in their S0 of 3.73 and 3.81 Å, respectively, longer than in the 1D chain 1b·0.5Toluene, although shorter in the T1 (2.87 [1b], 2.97 Å [1b]). However, the computed Pt···Pt distances of the lowest-energy tetramer [1b] are 3.26 and 3.78 Å, with an orientation of the monomers in perfect agreement with the data of X-ray structural analyses of 1b·0.5Toluene. As expected, the metallophilic Pt···Pt distances turn significantly shorter in the T1-optimized geometry (2.94, 3.03, 3.60 Å). The S1 transition of [1b] is derived from the HOMO → LUMO transition (95%), the HOMO being composed mainly of Pt (81%) and the LUMO of the ppy-CHO ligand (88%). This evidences the red-shifted low-energy absorptions in [1b] (527 nm) with significant MMLCT in relation to the monomer (S1 430 nm). In relation to the emission, the predicted emission maxima of 943 nm, arising from the lowest-energy tetramer [1b]vs 621 nm, obtained for the monomer evidence a high 3MMLCT contribution for the lowest triplet excites state, as is reflected in its spin density (Figure S44). These results confirm that the calculated red-shifted absorption and emission are induced by a packing arrangement better in dimers and trimers in 1a and tetramers in 1b, which improves the intermolecular interactions, leading to an increased MMLCT character.

Conclusions

In summary, we report the synthesis, characterization, and photophysical investigations of new phosphorescent tert-butylisoyanide PtII complexes with difluorophenylpyridine or formyl-functionalized phenylpyridine cyclometalated (C∧N = dfppy a, ppy-CHO b) ligands: neutral, [Pt(C∧N)Cl(CNBu)] (1), double salts [Pt(C∧N)(CNBu)2][Pt(C∧N)Cl2] (2), and cationic [Pt(C∧N)(CNBu)2]ClO4 (3). A comparative study of the crystal packing of the pseudopolymorphs of 1a (1a, 1a·CHCl), 1b (1b, 1b·0.5Toluene, 1b·0.5PhF), and 3a·0.25 CHCl shows a great ability to be assembled by intermolecular Pt···Pt and/or π···π interactions in the solid state, so generating 1D columnar stackings. The structures with head-to-tail packing exhibit larger Pt···Pt distances (4.556–6.541 Å) than the structures with a head-to-head disposition (3.362–3.896 Å), which is reflected in a change of the emission from greenish or yellow to orange/red or red. We discovered that complexes 1a and 1b exhibit multistimuli-responsive properties, displaying vapochromism, vapoluminescence, solvatochromism, thermochromism, and mechanochromism in the solid state, which can be attributed to modulation of Pt···Pt and/or π···π interactions with distinct color switching from green to red. The solvates 1a·CHCl, 1b·0.5Toluene, and 1b·0.5PhF contain channels running parallel to the 1D columns, occupied by the corresponding solvent, thus rationalizing the vapochromic/vapoluminescent/solvatochromic response of solvent-free 1a and 1b materials. The interactions can be established even in a polymer matrix. Thus, 1b undergoes also supramolecular self-assembly via Pt···Pt and/or π···π interactions into a polystyrene film (PS, 10 wt %) in response to toluene vapors. Grinding the pristine solids 1a and 1b affects their molecular packing in the solid state, producing a color emission change to orange with the loss of crystallinity (PXRD). Theoretical simulations confirm that Pt···Pt contacts are relevant in dimer and trimer geometries in 1a and tetramers in 1b, especially in their T1 states, leading to an increased MMLCT character in the solid-state aggregates.