Abstract
Two isoleptic complexes involving Pd(II) and Pt(II) centres with a tetradentate luminophoric ligand were adsorbed onto Laponite (LAP), yielding H2O-dispersible nanohybrids with remarkable photoexcited state properties. The adsorption promoted rigidification of the coordination compounds and suppression of roto-vibrational deactivation pathways, thus enhancing the performance of the molecular species. Interestingly, room temperature phosphorescence was achieved in the case of the Pd(II)-containing nanohybrid along with a marked 3O2 sensitivity, whereas the photoluminescence of the Pt(II)-based material showed only a mild dependence on the presence of 3O2 while reaching up to 37% quantum yield.
1 Introduction
Among d 8-configured metal complexes, Pt(II)-based coordination compounds have been well-studied concerning their photophysical properties, due to their broad spectrum of uses ranging from optical sensing [1], OLEDs [2], [3], [4], [5], and photocatalysis [6], [7], [8] to biomedical applications like bioimaging [9], [10], [11], [12] and photodynamic therapy [13], among others. Bidentate or tridentate ligands are flexible enough to allow a geometrical distortion out of the coordination plane upon photoexcitation, a process that facilitates non-radiative decay pathways via conical intersections with the ground state [14], [15], [16], [17], [18], whereas tetradentate luminophores with covalent Pt–C bonds in cyclometalated derivatives enhance the rigidity of the coordination environment and the ligand-field splitting by strong σ donation. Hence, dissociative states are energetically destabilized and become thermally inaccessible, which improves the photoluminescence of the complexes and their applicability [19], [20], [21], [22], [23]. While there is a large number of examples involving Pt(II) complexes with tetradentate ligands [4, 20], [21], [22], [23], the relative paucity of reports on Pd(II)-based species restricts their rather scarce spectrum to only few efficient emitters [21, 24, 25]. Particularly, room-temperature phosphorescent (RTP) Pd(II) complexes are quite unusual, due to the significantly smaller ligand-field splitting observed in coordination compounds owing to the relatively compact 4d orbitals instead of the rather diffuse 5d counterparts of Pt(II), despite the fact that the atomic radii of both metal centres are similar (lanthanide contraction). Hence, dissociative states (e. g., excited d–d* or π–d* electronic configurations) are thermally accessible for Pd(II), where the antibonding d x 2 – y 2 orbitals are populated favouring non-radiative deactivation by excited-state distortion of the metal-ligand bonds and leading to conical intersections with the ground state [26], [27], [28]. Besides their potential phosphorescence related to an enhanced spin–orbit coupling, there is a broad range of applications for Pd(II) complexes as anticancer agents [29], oxygen sensors [21, 30], and as DNA modifiers [31], among others. Even though they are usually non-luminescent at room temperature, various supramolecular strategies have been reported in the literature towards RTP from Pd(II) complexes [21, 32, 33]. Besides broad spectral ranges and high photoluminescence quantum yields (Φ L), H2O solubility is needed for practical applications in biomedicine or bioimaging [34], [35], [36]. Within this frame, different approaches have been used in order to impart hydrophilicity to such rather insoluble square-planar complexes. Recently, we have shown that their encapsulation in polystyrene nanoparticles provides H2O dispersability and RTP along with the overall enhancement of the photoexcited properties towards self-referenced oxygen sensors [21]. We have also shown that Laponite® (LAP) can be used as a versatile nanocarrier imparting hydrophilicity to various Pt(II) complexes featuring diverse combinations of tridentate luminophores and monodentate co-ligands [37, 38]. As discussed in the literature, the distinctive photophysical properties of such nanohybrids (complex@LAP) depend on the molecular structure of the adsorbed complexes as well as on the self-assembly performance on LAP surfaces [37], [38], [39], [40]. In the work described herein, we have extrapolated this concept to two isoleptic complexes with Pt(II) and Pd(II) centres chelated by a bis-cyclometallated tetradentate luminophore (herein denoted as PtLF and PdLF, respectively, see Scheme 1) yielding H2O-dispersible nanohybrids based on the non-toxic platform LAP (PtLF@LAP and PdLF@LAP, respectively). The pattern of these tetradentate ligands has been designed by Vezzu et al. [41] and expanded by our group in previous studies regarding dilute solutions, homogeneous matrices, crystalline phases, and metallic interfaces [20], [21], [22], [23]. In the present report, adsorption onto LAP led to a facilitated dispersion in H2O and also enhanced the photophysical properties of both complexes. For PtLF@LAP, a substantial enhancement of the Φ L value was obtained, if compared with PtLF in dilute solutions, while a significant increment was observed regarding the photoexcited state lifetimes (τ) of PdLF@LAP.
Chemical structure of the complexes adsorbed onto LAP.
2 Results and discussion
The complexes reported herein were obtained as described in the previous work carried out by our group [21]. The nanohybrids were prepared in analogy to a well-established protocol mentioned in our recent studies with tridentate ligands [37, 38]. Interestingly, PdLF was comparatively less soluble in toluene than PtLF, probably due to a weaker polarizability of the Pd(II) centre if compared with Pt(II). Further details can be found in Section 4.
Figure 1 (left) summarizes the normalized photoluminescence spectra of the nanohybrids, namely PtLF@LAP and PdLF@LAP, at different concentrations in aqueous dispersions. In analogy to a previous report by Maisuls et al. [21] regarding the nanoconfinement of PtLF, we observed that for an increasing concentration of PtLF@LAP, the emission maximum located at 519 nm drops in relative intensity while the vibrational shoulder at 540 nm grows. Due to the relatively low loading with the complex (10 molecules per disc, abbreviated hereafter as mpd), only marginal aggregation-induced intermolecular interactions can be observed as an incipient metal-metal-to-ligand charge-transfer triplet state (3MMLCT) along with the onset of a weak band at around 613 nm. This 3MMLCT band has been previously observed in the emission spectra of PtLF in its pure crystalline form where the Pt–Pt distance is below 3.5 Å (the spectroscopic data is depicted herein for comparison) [21].
Photoluminescence spectra of the nanohybrids in aqueous dispersions at different concentrations, namely PtLF@LAP (
In the case of PdLF@LAP (Figure 1, right), an enhancement of the emission intensity located at roughly ∼565 nm was evidenced upon increasing the PdLF@LAP concentration, whereas the intensity of the emission band at 480 nm decreased, in analogy to an earlier report where PdLF was enclosed in polystyrene nanobeads [21]. Moreover, a gradual bathochromic displacement of the red-shifted band was observed as the concentration of PdLF@LAP was increased. These results suggest a weak Pd–Pd interaction supported by the fluorine atoms on the ligand and self-assembly of the nanodiscs. Thus, when the concentration of the dispersed material rises, the intermolecular interactions between adjacent discs are promoted, and the relative enhancement of the red-shifted band can be ascribed to an excimeric 3MMLCT state [21, 37, 38]. As shown in Figure 1 (right), the noticeable peak at 420 nm corresponds to the intrinsic emission from the pristine LAP. Since the PdLF@LAP hybrid is weakly emissive and blue-shifted if compared with PtLF@LAP, the band corresponding to the LAP appears relatively prominent. In the case of PtLF@LAP, this band is hidden in the baseline, as the corresponding hybrid is strongly luminescent and red-shifted when compared with the Pd analogue.
Unlike the nanohybrids described in our previous reports concerning Pt(II) complexes with tridentate chromophores [37, 38], reliable UV–vis absorption spectra were not attainable in the present study, most likely due to the high planarity of the tetradentate ligand [21]. For both PtLF@LAP and PdLF@LAP hybrids, the adsorbed conformation is nearly flat and hence the extent of decoupling from the disc surface is negligible (Scheme 2), which probably affects the absorption of light by scattering. Therefore, in the UV–vis absorption spectra of PtLF@LAP and PdLF@LAP, a significant background scattering from the nanoclays was observed and no spectral features due to the complexes could be detected. In contrast, the molecular structures described in our previous reports were bulkier and a larger tilt angle at the surface of the LAP disc has been inferred with a concomitant decoupling of the chromophores from the nanodiscs [37, 38], in agreement with our STM studies on similar complexes deposited onto metallic substrates [23, 42], [43], [44]. In order to understand if a different aggregation state and viscosity can affect the photophysical properties of the hybrids, their corresponding hydrogels as well as the neat hybrids in the form of powders were studied. The hydrogels were prepared by dispersing 30 mg of the respective material (PtLF@LAP or PdLF@LAP) in 1 mL of water and leaving them resting for at least 24 h at room temperature in a closed vial. Due to the relatively high concentration of the nanohybrid, intermolecular interactions between adjacent nanodiscs were enhanced in both cases, as can be inferred from the augmented red-shifted emission bands (Figure 2).
Schematic representation of the planar conformation adopted by the adsorbed complexes at the LAP surface.
Upper row – photographs of PtLF@LAP (left) and PdLF@LAP (right) under UV light including (I) aqueous dispersions with 2 mg mL−1, (II) hydrogels, and (III) neat powders. Lower row – photoluminescence spectra of PtLF@LAP (left) and PdLF@LAP (right) in the form of hydrogels (green solid lines), aqueous dispersions with 2 mg mL−1 (red dotted lines) and neat powders (blue dashed lines). The emission spectrum of an aqueous PdLF@LAP dispersion with 8 mg mL−1 (magenta dotted line) is also depicted.
As observed in Figure 2 (left), a yellowish emission from the PtLF@LAP hydrogel is observed, and appears red-shifted when compared with the neat powder and with dilute aqueous dispersions. This is mainly due the fact that the hydrogel based on PtLF@LAP is highly concentrated (30 mg mL−1), which increases the probability of Pt–Pt interactions between molecules adsorbed onto adjacent nanodiscs and leading to a clear 3MMLCT emission. In the case of the neat powder, due to the relatively low loading with the complexes (10 mpd), the emission arises mostly from monomeric species as the Pt–Pt interactions are hindered in the absence of water. As we discussed in our previous reports, water plays a crucial role in bringing adsorbed molecules closer by stabilizing, organizing and properly arranging the electrostatic interactions between the LAP nanodiscs; if the nanoparticles are not assembled with the support from the aqueous environment, the intermolecular interactions between the metal centres from adjacent coordination chemical entities adsorbed onto different discs are very weak or do not exist at all [37, 38]. On the other hand, the hydrogel involving PdLF@LAP also shows an enhancement of the red-shifted emission due to intermolecular interactions, as can be observed in the 8 mg mL−1 dispersion. In contrast, both the diluted dispersion as well as the neat powder appear strongly quenched, as evidenced by the emission spectra under UV excitation; this implies that the photoluminescence of the monomers is not efficient, even upon deoxygenation (vide infra).
Both Φ L and τ were also determined for air-equilibrated and Ar-purged (i. e., deoxygenated) dispersions as well as for neat powders and hydrogels, and compared with the results previously reported for the complexes in different conditions. Table S1 shows the photophysical properties of the pure complexes at RT in fluid dichloromethane solutions (DCM) and in a frozen glassy matrix of DCM:MeOH at 77 K [21]. Table 1 shows the photophysical properties of PtLF@LAP nanohybrids dispersed in H2O, as a hydrogel, and in its neat powder form. It is clear that the photophysical properties were enhanced upon adsorption onto LAP, if compared with the pure complexes dissolved in air-equilibrated DCM solutions. In dilute fluid solutions, PtLF shows 3O2 sensitivity, as evidenced by the significant changes in both Φ L and τ upon deaeration (Table S1); contrastingly, PtLF@LAP appears relatively insensitive to diffusional quenching by 3O2, as only minor changes were detected upon purging with Ar (Table 1). Particularly, the adsorption led to a significant enhancement of the Φ L value for the aerated sample, mainly due to the reduced rotovibrational relaxation modes as well as by the suppression of solvent-related quenching pathways, which also cause a considerable prolongation of the τ values. In condensed phases, diverse microenvironments and conformational changes can affect the excited state properties, which generally leads to multiexponential decays [37, 38, 45], as also observed in the case of PtLF@LAP and PdLF@LAP.
Summary of the photophysical data (i. e., photoluminescence quantum yields Φ L and decay lifetimes τ L) for PtLF@LAP hybrids. The time-resolved photoluminescence decay data including the individual fitting components and their relative amplitudes are shown in the Supplementary material available online, Figures S1–S10.
| PtLF@LAP | Φ L(air) ± 2/% | Φ L(Ar) ± 2/% | τ L(air)/µsa | τ L(Ar)/µsa |
|---|---|---|---|---|
| 1 mg mL−1 | 30 | 32 |
τ
av_amp = 4.40 ± 0.06; τ 1 = 8.6 ± 0.6 (33%); τ 2 = 4.0 ± 0.7 (32%); τ 3 = 0.80 ± 0.09 (35%) |
τ
av_amp = 6.20 ± 0.05; τ 1 = 10.3 ± 0.4 (39%); τ 2 = 5.2 ± 0.7 (37%); τ 3 = 1.06 ± 0.13 (24%) |
| 2 mg mL−1 | 31 | 36 |
τ
av_amp = 3.4 ± 0.03; τ 1 = 8.3 ± 0.2 (27%); τ 2 = 3.0 ± 0.3 (27%); τ 3 = 0.76 ± 0.05 (46%) |
τ
av_amp = 5.30 ± 0.05; τ 1 = 4.5 ± 0.5 (34%); τ 2 = 10.3 ± 0.3 (33%); τ 3 = 1.0 ± 0.1 (33%) |
| 4 mg mL−1 | 30 | 34 |
τ
av_amp = 2.70 ± 0.02; τ 1 = 8.40 ± 0.11 (18%); τ 2 = 3.3 ± 0.2 (20%); τ 3 = 0.71 ± 0.02 (60%) |
τ
av_amp = 4.03 ± 0.05; τ 1 = 3.9 ± 0.3 (29%); τ 2 = 10.17 ± 0.18 (25%); τ 3 = 0.79 ± 0.06 (46%) |
| 8 mg mL−1 | 31 | 37 |
τ
av_amp = 2.34 ± 0.01; τ 1 = 8.04 ± 0.09 (17%); τ 2 = 2.70 ± 0.16 (19%); τ 3 = 0.68 ± 0.02 (64%) |
τ
av_amp = 3.3 ± 0.03; τ 1 = 9.7 ± 0.2 (22%); τ 2 = 3.2 ± 0.3 (24%); τ 3 = 0.75 ± 0.04 (54%) |
| Hydrogel | 25 | – |
τ
av_amp = 1.48 ± 0.02; τ 1 = 0.50 ± 0.04 (68%); τ 2 = 7.60 ± 0.20 (10%); τ 3 = 1.60 ± 0.3 (22%) |
– |
| Neat powder | 9 | – |
τ
av_amp = 5.5 ± 0.3; τ 1 = 1.08 ± 0.19 (42%); τ 2 = 8.6 ± 0.6 (58%) |
– |
-
Air: air-equilibrated dispersion. Ar: Ar-purged sample. aFor multi-exponential decays, the amplitude-weighted average lifetimes (τ av_amp) are given along with the different decay components and the corresponding relative amplitudes as percentages.
On the other hand, no significant changes in the Φ L values were observed upon adsorption of PdLF onto LAP. This could be due to the population of thermally accessible dissociative excited states, which favour non-radiative relaxation via conical intersections with the ground state, making the nanohybrid weakly emissive at RT under any circumstances. However, while the τ values for PdLF@LAP in the aerated dispersions resemble those obtained in air-equilibrated DCM solutions (Table S1), the deoxygenated dispersions displayed significantly prolonged lifetimes (up to 58 µs), as detailed in Table 2. This outcome suggests that there could be a considerable amount of diffusion-controlled Dexter-type energy transfer to 3O2, even though the radiative decay remains spin-forbidden [46, 47], while the rotovibrational deactivation pathways are suppressed due to rigidification upon adsorption.
Summary of the photophysical data (i. e., photoluminescence quantum yields Φ L and decay lifetimes τ L) for PdLF@LAP hybrids. The time-resolved photoluminescence decay data including the individual fitting components and their relative amplitudes are shown in the Supplementary material available online, Figures S11–S20.
| PdLF@LAP | Φ L(air) ± 2/% | Φ L(Ar) ± 2/% | τ L(air)/µsa | τ L(Ar)/µsa |
|---|---|---|---|---|
| 1 mg mL−1 | <2 | <2 |
τ
av_amp = 1.4 ± 0.1; τ 1 = 8.8 ± 0.8 (9%); τ 2 = 0.17 ± 0.04 (58%); τ 3 = 1.60 ± 0.17 (33%) |
τ
av_amp = 40.6 ± 0.8; τ 1 = 184 ± 8 (8%); τ 2 = 48 ± 3 (42%); τ 3 = 11.4 ± 1.2 (50%) |
| 2 mg mL−1 | <2 | <2 |
τ
av_amp = 0.46 ± 0.04; τ 1 = 2.5 ± 0.2 (13%); τ 2 = 0.32 ± 0.06 (30%); τ 3 = 0.07 ± 0.02 (57%) |
τ
av_amp = 51.6 ± 0.6; τ 1 = 67 ± 3 (41%); τ 2 = 253 ± 7 (6%); τ 3 = 18.4 ± 1.2 (53%) |
| 4 mg mL−1 | <2 | <2 |
τ
av_amp = 1.22 ± 0.02; τ 1 = 1.18 ± 0.05 (29%); τ 2 = 6.00 ± 0.12 (13%); τ 3 = 0.15 ± 0.01 (58%) |
τ
av_amp = 55.2 ± 1.4; τ 1 = 284 ± 9 (8%); τ 2 = 63 ± 5 (37%); τ 3 = 15.0 ± 1.9 (55%) |
| 8 mg mL−1 | <2 | <2 |
τ
av_amp = 1.26 ± 0.03; τ 1 = 5.8 ± 0.2 (14%); τ 2 = 1.10 ± 0.07 (32%); τ 3 = 0.15 ± 0.01 (54%) |
τ
av_amp = 58.6 ± 0.6; τ 1 = 53 ± 3 (39%); τ 2 = 255 ± 7 (12%); τ3 = 13.2 ± 0.9 (48%) |
| Hydrogel | <2 | – |
τ
av_amp = 2.70 ± 0.06; τ 1 = 11.8 ± 1.0 (9%); τ 2 = 3.0 ± 0.3 (44%); τ 3 = 0.80 ± 0.09 (47%) |
– |
| Neat powder | <2 | – |
τ
av_amp = 5.6 ± 1.5; τ 1 = 2.3 ± 1.0 (79%); τ 2 = 18 ± 8 (21%) |
– |
-
Air: air-equilibrated dispersion. Ar: Ar-purged sample. aFor multi-exponential decays, the amplitude-weighted average lifetimes (τ av_amp) are given along with the different decay components and the corresponding relative amplitudes as percentages.
Further, in order to figure out the colloidal properties of the nanohybrids in aqueous dispersions, the hydrodynamic diameters (d h) and zeta potentials (ξ) were measured by means of dynamic light scattering (DLS, the results are shown in Table 3). Interestingly, the d h values were only moderately concentration-dependent for both nanohybrids; in fact, they dropped at higher concentrations. This could be attributed to the asymmetry in size (height vs. diameter) and anisotropic charge distribution of the LAP affecting the aggregation process [48]. In addition, the similarity of ξ for both nanohybrids (even at different concentrations) suggests that both self-assembly and net electrostatic charges are comparable for both hybrids; the marginal differences can be attributed to the higher polarizability of Pt(II) if compared with Pd(II).
Colloidal properties of the nanohybrids (i. e., hydrodynamic diameters d h and zeta potentials ξ) determined by dynamic light scattering.
| 1 mg mL−1 | 2 mg mL−1 | 4 mg mL−1 | 8 mg mL−1 | ||
|---|---|---|---|---|---|
| PtLF@LAP | d h/nm | 100 ± 70 | 70 ± 30 | 46 ± 2 | 49 ± 9 |
| ξ/mV | −55 ± 7 | −62 ± 8 | −61 ± 2 | −61 ± 3 | |
| PdLF@LAP | d h/nm | 75 ± 13 | 43 ± 4 | 43 ± 4 | 42 ± 1 |
| ξ/mV | −72 ± 6 | −62 ± 8 | −54 ± 3 | −51 ± 4 |
3 Conclusions
Two new nanohybrids based on isoleptic d 8-configured M(II) complexes (M = Pt or Pd) bearing tetradentate luminophores were prepared and evaluated in terms of their photoexcited properties. The photoluminescence of these nanohybrids (PtLF@LAP and PdLF@LAP) was investigated in aqueous dispersions, as hydrogels, and as dry neat powders. Due to rigidification of the molecules, an improvement of their luminescence properties was observed when compared with the pure complexes dissolved in air-equilibrated solutions. Surprisingly, while the Φ L value in aerated DCM solutions of PtLF is below the experimental uncertainty of 2%, PtLF@LAP showed a striking Φ L value reaching up to 37% when dispersed in H2O, demonstrating that an enhancement of the photoluminescence can be achieved while providing an elegant way to disperse these otherwise insoluble complexes in aqueous media. As illustrated by the photoluminescence spectra of the nanohybrids, intermolecular interactions between complexes adsorbed onto adjacent nanodiscs become relevant upon self-assembly of the nanoparticles in concentrated dispersions, which causes the emergence of a red-shifted phosphorescence, in excellent agreement with our previous reports on the nanoconfinement of the corresponding Pt(II) and Pd(II) complexes in soft polystyrene nanospheres. In addition, while the adsorption of PdLF leads to a Φ L value below the experimental uncertainty for PdLF@LAP, the sensitivity of the still measureable photoluminescence lifetime towards dissolved triplet molecular dioxygen suggests that these hybrids could be used as optical 3O2 sensors. In general, it is clear that adsorption leads to a suppression of radiationless deactivation pathways regarding the photoexcited triplet states, which we attribute to a rigidified microenvironment for the complexes with a reduced density of rotovibrational states; the lower phosphorescence intensity observed for PdLF@LAP (if compared with PtLF@LAP) is related to the reduced spin–orbit coupling.
4 Experimental section
4.1 Preparation of the nanohybrids (powders and hydrogels)
The nanohybrids were prepared as described in our previous reports [37, 38]. Laponite XLG® (LAP) was obtained from BYK additives and used as received. Briefly, for adsorbing 10 molecules per LAP disc (10 mpd), 400 mg of LAP was dispersed in toluene (10 mL) and sonicated for 30 min. Afterwards, 3.5 µmol of the complex (PtLF or PdLF) dissolved in 5 mL of toluene was added, and the mixture sonicated for further 5 min. In order to complete the adsorption, the dispersion was stirred overnight at room temperature. Later, the mixture was centrifuged (10,000 rpm, 20 min), washed and air-dried in order to obtain the solid nanohybrid samples.
By choosing the desired material accordingly, 30 mg of the dried hybrid was dispersed in 1 mL of H2O and left resting for at least 24 h at room temperature in a closed vial to yield the corresponding hydrogel.
4.2 Photophysical and colloidal measurements
Fresh dispersions of a nanohybrid (PtLF@LAP or PdLF@LAP) were prepared by dissolving and gently shaking a specific amount (2, 4, 8 or 16 mg) of sample in 2 mL of double-distilled H2O in order to obtain the desired concentrations (1, 2, 4 and 8 mg mL−1). The colloidal properties of the dispersions, such as hydrodynamic size and zeta potential were determined with the same solution by using dynamic light scattering (DLS). Further details of the used equipment are presented in the Supplementary material.
5 Supporting information
Instrumentation details and time-resolved photoluminescence decays are given as supplementary material available online (https://doi.org/10.1515/znb-2021-0136).
Dedicated to: Professor Richard Dronskowski of the RWTH Aachen on the occasion of his 60th birthday.
Funding source: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
Award Identifier / Grant number: STR 1186/6-1
Acknowledgements
S.C.G. gratefully acknowledges the Westfälische Wilhelms-Universität Münster for a doctoral fellowship. M.E.G.S. acknowledges a doctoral fellowship from the DAAD. I.M. is thankful for the post-doctoral fellowship from the Alexander von Humboldt Foundation.
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Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Research funding: C.A.S. greatly appreciates the generous financial support from the Deutsche Forschungsgemeinschaft (Priority Programme 2102 “Light-controlled Reactivity of Metal Complexes” STR 1186/6-1; Cluster of Excellence Cells in Motion EXC 1003).
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Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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