Star-shaped Pt(II) complexes with excellent optical power limiting performance and their flexible optical limiters

Abstract

Efficient optical power limiting (OPL) materials are crucial for protecting optical devices and the eyes. However, achieving strong OPL responses while maintaining high optical transparency and processability remains a challenge. Herein, we report a molecular design strategy to improve the OPL performance of a series of Pt(II) complexes (Pt-1a–Pt-2c) by incorporating trinuclear Pt centers and modulating the core-ethynyl ligand electronic interacations. The trinuclear Pt centers extended the triplet-state lifetimes (0.52–2.05 μs) and enhanced the excited-state absorption (ESA), while the core-ethynyl ligand interacations effectively tuned the ground- and excited-state properties. These complexes exhibited tunable nonlinear optical (NLO) and OPL properties at 532 nm, with a minimum normalized transmittance (T_min) in the range of 0.38–0.78. Notably, Pt-2b showed outstanding OPL performance with a nonlinear absorption coefficient (β_eff) and optical limiting threshold (F_OL) of 192.15 cm GW⁻¹ and 0.71 J cm⁻², respectively. In flexible films, Pt-2b@PDMS-1.00 wt% demonstrated markedly enhanced OPL performance (β_eff = 965.83 cm GW⁻¹, F_OL = 0.26 J cm⁻²), surpassing the performance of many reported OPL materials. This study provides valuable insights into designing transparent Pt(II) complexes with superior OPL properties, and highlights the promise of flexible films for photonic applications.

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Introduction

Nonlinear optical (NLO) materials have garnered significant interest owing to their broad applications in pulsed laser generation, optical power limiting (OPL), optical switching, and bioimaging.^1–4 OPL materials are particularly valuable as they attenuate high-energy laser pulses to protect sensitive optical devices and the human eyes while maintaining high transmittance for low-energy light. Ideal OPL materials should possess rapid response times, low limiting thresholds, high transparency, and good stability. Currently developed OPL materials include inorganic compounds,^5,6 organic π-conjugated molecules,^7,8 organometallic complexes (Pt and Ir systems),^9–12 metal–organic frameworks (MOFs),^13–17 clusters (metal and metal-oxo clusters),^18–22 carbon materials,^23,24 two-dimensional (2D) materials,^25,26 and composites.^27,28 However, many of these materials still suffer from certain limitations. For instance, C₆₀ and porphyrins/phthalocyanines show strong OPL activity but suffer from intense absorption in the 400–700 nm visible range, which reduces its transparency. Although, 2D materials offer broadband OPL responses, they often exhibit poor dispersibility, and materials such as black phosphorus also suffer from limited stability. Thus, developing high-performance OPL materials that combine excellent transparency, stability, and processability, along with solid-state OPL limiters that are suitable for diverse applications remains a critical challenge in this field.

Square-planar Pt(II) complexes exhibit intriguing photophysical properties²⁹ and have been widely explored in organic light-emitting devices (OLEDs),³⁰ OPL,³¹ organic solar cells,³² and self-assembled materials.³³ Notably, Pt-induced efficient intersystem crossing (ISC) generates long-lived triplet states with high quantum yields, facilitating two-photon absorption (TPA) and reverse saturable absorption (RSA), making them promising candidates for OPL applications.^34,35 Cyclometalated Pt(II) complexes show remarkable OPL performance because of their strong spin–orbit coupling (SOC) effect. However, their intense metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT) transitions reduce their optical transparency.^29,36 In contrast, alkylphosphine Pt(II) complexes exhibit weaker MLCT absorption, ensuring better transparency, and phosphine ligands (PR₃) improve solubility. In recent years, Schanze and Zhou et al. have reported numerous alkylphosphine Pt(II) acetylides and Pt(II) polyynes with considerable TPA and RSA effects, successfully demonstrating their OPL applications.^37–41 Some of these molecular systems even outperform C₆₀ and metalloporphyrins. However, current studies remain focused on linear structures. The weak Pt d_π–π orbital mixing limits their OPL activity. Thus, achieving strong OPL responses while maintaining high transparency remains a critical challenge that urgently needs to be addressed in this field.

Star-shaped Pt(II) structures are promising candidates to enhance the OPL performance of alkylphosphine Pt(II) complexes, with several advantages, such as (1) synergistic interactions among the branches improve the nonlinear polarizability (χ);^42,43 (2) trinuclear Pt centers introduce strong SOC effects, promoting ISC and enhancing excited-state absorption (ESA);⁴⁴ and (3) compared with Pt(II)-based dendrimers, star-shaped Pt(II) complexes are more synthetically accessible and cost-effective.⁴⁵ For example, Zhou et al. reported that in V-shaped and star-shaped Pt(II) acetylide systems, high-density Pt centers efficiently enhanced the SOC effect, improving the OPL activity.⁴⁶ Some Pt(II) acetylides even surpassed C₆₀ in OPL performance while maintaining high transparency. Despite their wide use in OLEDs,⁴⁴ ion capture,⁴⁷ and cancer therapy,⁴⁸ star-shaped Pt(II) complexes remain underexplored for OPL applications.

Based on these considerations, we designed a series of star-shaped trinuclear Pt(II) complexes (Pt-1a–Pt-2c) featuring phenyl and triazine cores to enhance the OPL performance of alkylphosphine Pt(II) complexes (Scheme 1). 1,3,5-Triazine, a strong electron acceptor (ionization potential of 11.67 eV) with an excellent photothermal stability, is widely used in star-shaped π-conjugated systems.^42,49 The incorporation of trinuclear Pt centers markedly enhances the ESA. By modulating the electronic interactions between the cores and the ethynyl ligands (carbazole, fluorene, and naphthalimide), the ground- and excited-state properties of Pt-1a–Pt-2c were precisely tuned. Additionally, bulky alkyl chains suppressed the aggregation and improved the solubility of these complexes. As expected, Pt-1a–Pt-2c exhibited extended triplet state lifetimes and enhanced ESA, with their optical transparency and OPL performance optimized by core-ethynyl ligand interactions. In particular, Pt-2b showed the strongest OPL responses at 532 nm while maintaining high transparency. Moreover, the flexible Pt-2b@PDMS films displayed remarkable OPL performance, making them ideal candidates for practical OPL applications. This study reveals the essential roles of metal centers and ligand engineering in tuning the OPL properties of Pt(II) complexes and provides a new strategy for designing advanced OPL materials.

Scheme 1 Molecular structures of star-shaped Pt(II) complexes Pt-1a–Pt-2c.

Results and discussion

Synthesis and characterization

The synthetic routes for Pt-1a–Pt-2c are shown in Scheme 2, with detailed experimental procedures provided in the ESI.† Briefly, compounds 1a–1c were reacted with trans-[Pt(PBu₃)₂Cl₂] via the Sonogashira cross-coupling reactions to afford precursors Pt-Cl-a–Pt-Cl-c. In the presence of TBAF, Et₃N and CuI, the trimethylsilyl (TMS)-protected compounds 2 or 3 underwent deprotection, followed by in situ base-catalyzed dehydrohalogenation reactions with Pt-Cl-a–Pt-Cl-c to give Pt-1a–Pt-2c in high yields (73%-85%).

Scheme 2 Synthetic routes for Pt-1a–Pt-2c.

Pt-1a–Pt-2c were characterized using ¹H, ¹³C, and ³¹P NMR spectroscopies and MALDI-TOF-MS (see ESI†). In the ³¹P NMR spectra, all complexes displayed sharp singlet signals flanked by two ¹⁹⁵Pt satellites, with ¹J_Pt–P of ∼2350 Hz, indicating a trans-P–Pt–P configuration. These data confirmed the successful synthesis of Pt-1a–Pt-2c. They were highly soluble in common organic solvents (e.g., toluene and THF) and exhibited excellent stability (Fig. S25†), with no sensitivity to light or oxygen. TGA curves indicated that the decomposition temperatures (T_d) exceeded 306 °C, demonstrating their high thermal stability (Fig. S26†).

X-Ray crystal structures

The molecular structures of Pt(II) complexes were determined by single-crystal X-ray diffraction. Single crystals of Pt-1b, Pt-2b, Pt-1c, and Pt-2c were grown by layering ethanol onto a concentrated CH₂Cl₂ solution. Despite multiple attempts, suitable crystals for Pt-1a and Pt-2a could not be obtained. The crystallographic data are summarized in Tables S1 and S2,† with selected bond lengths and bond angles listed in Tables S3–S6.† ORTEP diagrams and packing models are shown in Fig. S27–S30.†

All four complexes crystallized in the triclinic P1̄ space group. The asymmetric units of Pt-1b, Pt-2b, and Pt-1c each contain one molecule, while that of Pt-2c contains two. Fig. 1a–d illustrate that the Pt(II) complexes exhibit the anticipated star-shaped structure with each Pt center adopting a slightly distorted square-planar geometry, where the two PBu₃ groups and ethynyl ligands are in a trans-arrangement. The C–Pt–P bond angles range from 85.2(3)° to 95.7(4)°, which alleviates steric hindrance between the PBu₃ groups. The Pt–P and Pt–C bond lengths (2.25(4)–2.41(7) Å and 1.77(12)–2.07(11) Å, respectively) are consistent with those of reported alkylphosphine Pt(II) complexes.^35,41,50

Fig. 1 (a–d) Single-crystal structures of Pt-1b, Pt-2b, Pt-1c, and Pt-2c (Color codes: cyan-green = C, light gray = H, blue = N, pink = O, green = P, and orange = Pt). (e–h) Crystal packing modes of Pt-1b, Pt-2b, Pt-1c, and Pt-2c. Partial H atoms, C₆H₁₃ chains, and PBu₃ groups are omitted for clarity.

Significant differences are observed in dihedral angles between the cores and ethynyl ligands among the Pt(II) complexes. The dihedral angles between the fluorene units and the core are 22.43°, 24.84°, and 80.78° for Pt-1b, and 8.41°, 27.02°, and 58.75° for Pt-2b. In contrast, Pt-1c and Pt-2c exhibit obviously smaller dihedral angles between the naphthalimide units and the core, suggesting better coplanarity. In the crystal structures, Pt-1b and Pt-2b are primarily stabilized by C–H⋯π interactions (2.92–3.72 Å, Fig. 1e and f). Notably, C–H⋯Pt interactions (2.75 Å) are also observed in the Pt-2b dimer. For the trimers of Pt-1c and Pt-2c, multiple weak interactions are present, including C–H⋯π (2.67–3.61 Å), C–H⋯N (3.09 and 2.62 Å), and C–H⋯O interactions (2.72 and 2.57 Å), as shown in Fig. 1g and h. A summary of these interactions is provided in Table S7.† The PBu₃ groups extend the intermolecular contact distances, resulting in the absence of Pt⋯Pt interactions. Nevertheless, sliding π–π stacking are observed between the naphthalimide units in Pt-1c and Pt-2c, with centroid-to-centroid distances of 3.53 and 3.61 Å, respectively. These results suggest that the fluorene units in Pt-1b and Pt-2b introduce significant distortion, effectively suppressing π–π stacking and promoting homogeneous dispersion within polymer matrices.

Photophysical properties

Absorption spectra. The UV–vis absorption spectra of Pt-1a–Pt-2c were recorded in CH₂Cl₂, with the photophysical parameters summarized in Table 1. As illustrated in Fig. 2a, all the complexes showed intense absorption bands, attributed to Pt center-disturbed ¹π–π* transitions of ethynyl ligands. Pt-1a and Pt-2a displayed two main absorption peaks between 280–390 nm, along with a moderate band at ca. 310 nm, likely from metal-centered (MC) transitions.⁵¹ Pt-1b and Pt-2b exhibited similar absorption features below 400 nm (280–305 nm and 305–380 nm), with maximum absorption wavelengths (λ_abs) of 358 nm and 352 nm, respectively. However, the cut-off wavelength (λ_cut-off) of Pt-2b was red-shifted by 12 nm. Meanwhile, the d_π–π orbital mixing led to red-shifted absorption compared with their precursors Pt-Cl-a–Pt-Cl-c (Fig. S31†). Nevertheless, the λ_cut-off of Pt-1a, Pt-2a, Pt-1b, and Pt-2b remained below 405 nm, ensuring high optical transparency.

Fig. 2 (a) UV–vis absorption spectra of Pt-1a–Pt-2c in CH₂Cl₂ and (b) emission spectra of Pt-1a–Pt-2c in deaerated CH₂Cl₂ (c = 1.0 × 10⁻⁵ mol L⁻¹, r.t.). (c and d) Normalized emission spectra of Pt-2b and Pt-2c in 2-Me THF at r.t. and 77 K, respectively. (e) Emission lifetimes of the T₁ state for Pt-1a–Pt-2c at 77 K. (f) ns-TA spectra of Pt-1b, Pt-2b, Pt-1c, and Pt-2c at zero delay in deaerated toluene (λ_ex = 355 nm).

Table 1 Photophysical parameters for Pt-1a–Pt-2c

Complexes	λabs^a/nm	λem/nm (τem/μs)^b	λem/nm (τem/μs)^c	ΦP^d/%	λT1–Tn/nm (τT/μs)^e	λcut-off/nm
a UV–vis absorption band maxima in CH2Cl2.b Emission band maxima and phosphorescence lifetimes measured in deaerated CH2Cl2.c Emission band maxima and decay lifetimes of T1 state recorded in 2-Me THF at 77 K.d Absolute phosphorescence quantum yields measured in deaerated CH2Cl2 using an integrating sphere.e ns-TA band maxima or triplet state lifetimes in deaerated toluene.
Pt-1a	293, 310, 345	401, 462	460 (78.61)	—	—	388
Pt-2a	95, 309, 324, 356	401, 452	453 (37.59)	—	—	405
Pt-1b	297, 358	380, 507 (0.93)	507 (202.04)	15.31	560 (0.96)	383
Pt-2b	299, 352	389, 506 (0.50)	505 (211.40)	18.28	530 (0.52)	395
Pt-1c	284, 320, 349, 435	512, 639 (1.98)	483, 634 (368.01)	6.77	628 (2.05)	498
Pt-2c	298, 427	491	463, 629 (435.75)	—	640 (1.59)	480

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In contrast, Pt-1c and Pt-2c, bearing naphthalimide units with strong ¹π–π* transition absorptions near 430 nm, exhibited a λ_cut-off extending beyond 480 nm, resulting in reduced transparency. Furthermore, the absorption spectra in different solvents revealed that the absorption peaks of Pt-2a, Pt-2b, and Pt-1c broadened more noticeably in polar solvents (Fig. S32†), indicating stronger intramolecular charge transfer (ICT) effects than Pt-1a, Pt-1b, and Pt-2c. These enhanced ICT characteristics are expected to improve their OPL responses.

Emission spectra. The emission spectra of Pt-1a–Pt-2c in deaerated CH₂Cl₂ are shown in Fig. 2b. Except for Pt-2c, all the complexes exhibited dual emission properties at room temperature. The short-wavelength bands with lifetimes <2 ns were assigned to the lowest singlet state (S₁) fluorescence.⁵² The long-wavelength band, featuring well-resolved vibrational structures, microsecond-scale lifetimes and large Stokes shifts (>100 nm), corresponded to the lowest triplet state (T₁) phosphorescence.

The ethynyl ligands successfully influenced the emission properties. For Pt-1a and Pt-2a, phosphorescence appeared as weak shoulders beside fluorescence, while for Pt-1b and Pt-2b phosphorescence was the predominant emission. Pt-2c showed only cyan fluorescence with λ_em = 491 nm, which was blue-shifted by 20 nm compared with Pt-1c, with a higher fluorescence quantum yield (Φ_F = 5.40%) than the others (Φ_F < 0.05%). The low Φ_F values were attributed to the heavy-atom effect of Pt centers, which enhanced the SOC effect and facilitated ISC from the S₁ state to T₁ state. The absolute phosphorescence quantum yields (Φ_P) for Pt-1b, Pt-2b, and Pt-1c were 15.31%, 18.28%, and 6.77%, respectively (Table 1). Notably, the incorporation of fluorene units enhanced the d_π–π orbital coupling, increasing the T₁ state populations. Pt-2b showed the highest Φ_P, which could improve the OPL performance via the RSA mechanism.

The fluorescence peaks of Pt-1a–Pt-2c exhibited red-shifts with increasing solvent polarity, suggesting that the S₁ state originated from the mixed ¹π–π*/¹ICT transitions. In contrast, the phosphorescence bands, which were attributed to the predominant ³π–π* transitions, were almost unaffected by solvent polarity (Fig. S33†). These assignments were also supported by theoretical calculations. To further explore the T₁ emissive state properties, low-temperature emission spectra were recorded (Fig. 2c, d, and S34†). At 77 K, the long-wavelength bands intensified, and their lifetimes were greatly extended (Fig. 2e and Table 1), confirming their phosphorescent origin. However, Pt-1c and Pt-2c still displayed intense fluorescence, suggesting lower T₁ state populations despite their long phosphorescence lifetimes.^41,46

Nanosecond transient absorption spectra. The triplet state properties of Pt-1a–Pt-2c were investigated using ns-TA spectroscopy. Except for the weak TA signals of Pt-1a and Pt-2a, the TA spectra at zero delay and time-resolved TA profiles of the Pt(II) complexes in deaerated toluene are presented in Fig. 2f and Fig. S35.† These complexes exhibited broad and intense ESA bands in the visible to NIR region (380–800 nm for Pt-1b and Pt-2b and 450–800 nm for Pt-1c and Pt-2c), which were stronger than their GSA, indicating the potential for RSA effects. Pt-1c and Pt-2c displayed distinct bleaching bands in the range of 380–450 nm, which were consistent with the GSA (Fig. 2a), confirming that their excited-states are localized on the naphthalimide units. Therefore, the observed triplet states predominantly originated from the ligand-centered ³π–π* transitions.⁵³

Fitting the decay curves of the TA signals yielded the triplet state lifetimes (τ_T) for Pt-1b, Pt-2b, Pt-1c, and Pt-2c as 0.96, 0.52, 2.05, and 1.59 μs, respectively (Fig. S36†), indicating their characteristic as long-lived RSA materials. Compared with the precursors (Pt-Cl-b and Pt-Cl-c), they showed significantly enhanced ESA signals and 3–6 times longer triplet lifetimes (Fig. S37†), highlighting the great potential of star-shaped Pt(II) complexes in optimizing the triplet state properties. Although incorporation of the triazine core slightly shortened the triplet state lifetimes of Pt-2b and Pt-2c, it improved their ESA intensity, facilitating the RSA effect. Specifically, Pt-2b showed the strongest ESA across the entire TA spectra, indicating its superior triplet state characteristics and promising OPL responses.

Theoretical calculations

Molecular orbital analysis. To understand the observed photophysical phenomena, DFT and TD-DFT calculations were performed to analyze the frontier molecular orbitals (FMOs) of Pt-1a–Pt-2c (Fig. 3a and S38–S43†). The contributions of each fragment to the FMOs are listed in Tables S8–S13,† and the excited-state parameters for the S₁ and T₁ states are summarized in Table 2. As illustrated in Fig. 3a, after introducing triazine core, the LUMO (L) levels of Pt-2a and Pt-2b are greatly reduced compared with Pt-1a and Pt-1b, with the energy gaps (E_g) decreasing by 0.47 and 0.35 eV, respectively. In contrast, the triazine of Pt-2c, lowers the HOMO(H) level by 0.20 eV relative to Pt-1c, increasing E_g to 3.35 eV and resulting in a 40 nm blue-shift in λ_abs.

Fig. 3 (a) Contour plots of HOMO, HOMO−1, LUMO, and LUMO+1 for Pt-1a–Pt-2c based on optimized S₀ geometries. (b) Distribution patterns of NTOs for S₀ → T₁ excitation of Pt-1a–Pt-2c based on optimized T₁ geometries.

Table 2 Excitation energies (eV), wavelengths (nm), oscillator strengths (f), and dominant contributing configurations of the S₁ and T₁ states for Pt-1a–Pt-2c based on their optimized S₀ geometries

Complexes	State	Excitation energy/eV (Wavelength/nm)	f	Orbital contribution (>10%)	Assignment
Pt-1a	S1	3.60 (344)	0.440	H → L (38%), H → L+3 (11%)	^1IL/^1MLCT
	T1	2.67 (463)	0.000	H−3 → L+3 (15%), H−4 → L+4 (13%)	^3IL/^3MLCT/^3LLCT
Pt-2a	S1	3.23 (383)	0.088	H → L (68%), H−2 → L (17%)	^1IL/^1LLCT
	T1	2.69 (460)	0.000	H → L+2 (57%)	^3IL/^3MLCT
Pt-1b	S1	3.58 (346)	0.963	H → L+2(18%), H → L+3 (25%)	^1IL/^1LLCT/^1LMCT
	T1	2.60 (476)	0.000	H−1 → L (62%)	^3IL/^3LLCT
Pt-2b	S1	3.26 (380)	0.0366	H → L (81%)	^1IL/^1LLCT/^1LMCT
	T1	2.61 (475)	0.000	H → L+2 (62%)	^3IL/^3MLCT
Pt-1c	S1	2.77 (447)	0.001	H → L (68%), H−1 → L (19%)	^1IL/^1MLCT/^1LLCT
	T1	1.97 (629)	0.000	H−4 → L (57%), H−1 → L (21%)	^3IL/^3MLCT/^3LLCT
Pt-2c	S1	3.04 (407)	1.138	H → L (67%)	^1IL/^1MLCT
	T1	1.98 (626)	0.000	H → L (82%)	^3IL/^3MLCT

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Upon photoexcitation, Pt-1a–Pt-2c exhibit intraligand ¹π–π* transitions, accompanied by varying degrees of ¹ICT effects. Specifically, the H and H−1 of Pt-1a and Pt-1b were delocalized over the entire molecule, while the L and L+1 were mainly localized on the carbazole or fluorene units, exhibiting predominantly ¹π–π* transitions with minor ¹MLCT/¹LLCT features. In Pt-2a, Pt-2b, and Pt-1c, the H and H−1 were mainly localized on the carbazole, fluorene, or phenyl, while the L and L+1 were distributed on the triazine core or naphthalimide units, indicating pronounced ¹LLCT transitions (e.g., carbazole or fluorene → triazine, phenyl → naphthalimide). For Pt-2c, the electron density was entirely localized on the naphthalimide units, exhibiting predominately ¹π–π* transitions. Additionally, the Pt atoms contribute differently to their FMOs. For Pt-1a, Pt-1c, and Pt-2c, the Pt atoms contribute 10.50–25.80% to the H and H−1, while their contribution to the L and L+1 is negligible, suggesting some ¹MLCT (Pt → ethynyl ligands) features. In contrast, Pt atoms contribute more (>3.07%) to the L and L+1 of Pt-1b and Pt-2b, imparting slight ligand-to-metal charge transfer (¹LMCT) transitions, which are beneficial for enhancing their OPL responses.⁴¹

Natural transition orbital analysis. To investigate the origin of phosphorescent emission in Pt-1a–Pt-2c,⁴⁵ natural transition orbitals (NTOs) were calculated based on the optimized T₁ state geometries. As presented in Fig. 3b, the hole → particle transitions contributed over 97.2% to the T₁ state. The ethynyl ligands accounted for more than 82.17% to both hole and particle, while the contribution from Pt atoms ranged from 2.29% to 11.14%, with a higher contribution to the hole (Table S14†). These results indicated that the T₁ state of Pt-1a–Pt-2c primarily originated from ³π–π* transitions, with a minor ³MLCT character, consistent with the experimental observations. The contribution of Pt atoms to both the hole and particle in Pt-1c and Pt-2c was lower than the other complexes, indicating a weaker d_π–π coupling and consequently inhibiting the ISC process. The intense fluorescence observed for Pt-1c and Pt-2c at 77 K further supported this conclusion.

Nonlinear optical properties

Third-order nonlinear optical properties. Based on the broad and intense ESA signals observed in the ns-TA spectra, the NLO properties were evaluated using an open-aperture Z-scan technique (Fig. 4a). Detailed experimental procedures and data fitting for the Z-scan are included in the ESI.† Under 532 nm pulsed laser irradiation (4.0 ns, 100 μJ), the Z-scan curves of Pt-1a–Pt-2c in toluene exhibited obvious RSA characteristics (Fig. 4b and S44†), with the minimum normalized transmittance (T_min) at the focal point (Z = 0) ranging from 0.38 to 0.78, indicating tunable NLO properties. As expected, Pt-2b showed the lowest T_min value of 0.38, demonstrating the best NLO properties.

Fig. 4 (a) Schematic of the Z-scan setup. (b) Open-aperture Z-scan (points) and theoretical fit (solid lines) curves of Pt-1a–Pt-2c in toluene (c = 1.0 mM) at 532 nm. (c) Comparison of the T_min and β_eff value for Pt-1a–Pt-2c. (d) Output fluence versus input fluence curves for Pt-1a–Pt-2c. (e) Nonlinear transmittance versus input fluence curves of Pt-1b and Pt-2b. (f) Open-aperture Z-scan curves and (g) output fluence versus input fluence curves for Pt-Cl-b, Pt-1b, and Pt-2b. (h) Jablonski diagram for Pt-1a–Pt-2c based on the RSA mechanism.

To quantitatively compare their NLO properties, the effective nonlinear absorption coefficients (β_eff) were extracted by fitting the Z-scan data. Pt-2a (31.60 cm GW⁻¹), Pt-2b (192.15 cm GW⁻¹), and Pt-1c (83.52 cm GW⁻¹) showed higher β_eff values than Pt-1a (26.49 cm GW⁻¹), Pt-1b (125.33 cm GW⁻¹), and Pt-2c (67.45 cm GW⁻¹), which can be attributed to their stronger ICT effects (Fig. 4c). The β_eff value of Pt-2b surpassed those of most alkylphosphine Pt(II) complexes,^29,54,55 many cyclometalated Pt(II) complexes^9,28 and Pt(II)-incorporated copolymers.⁵⁶ The outstanding NLO response of Pt-2b originated from the strong SOC-facilitated ISC, along with ICT effects, which synergistically enhanced its ESA.

Optical power limiting performance. OPL is a key application in the NLO field. At low incident fluence, Pt-1a–Pt-2c showed a linear input–output relationship with high transmittance. As fluence increased, the output fluence gradually decreased and tended to saturate, displaying a typical OPL behavior (Fig. 4d). The OPL performance followed the trend of Pt-2b > Pt-1b > Pt-1c > Pt-2c > Pt-2a > Pt-1a, which was consistent with the Z-scan results. Compared with electron-donating carbazole and electron-withdrawing naphthalimide units, the weakly donating fluorene units in Pt-1b and Pt-2b enhanced their OPL performance. As shown in Fig. 4e, the optical limiting thresholds (F_OL, defined as the incident fluence at 50% linear transmittance) for Pt-1b and Pt-2b were 0.88 and 0.71 J cm⁻², respectively. Owing to the strong ESA, they exhibited markedly enhanced NLO and OPL performances compared with their precursor Pt-Cl-b (Fig. 4f and g).

The OPL mechanism is illustrated in the Jablonski diagram (Fig. 4h). Under 532 nm excitation, the molecule underwent rapid ISC from the S₁ state to the T₁ state. The long triplet state lifetimes enabled the accumulation of T₁ state populations and promoted the subsequent T₁–T_n absorption (ESA), thereby inducing OPL effects. Therefore, the OPL performance of Pt-1a–Pt-2c closely correlated with their ESA intensity. Pt-2b exhibited the strongest ESA at 532 nm, thus achieving the best OPL performance. Although Pt-1c and Pt-2c showed lower T₁ state populations, their long triplet state lifetimes (1.59 and 2.05 μs) and moderate ESA intensities still provided considerable OPL responses. In contrast, the undetectable ESA signals of Pt-1a and Pt-2a in the TA spectra accounted for their weak OPL activities.

Solid-state optical limiter

Solid-state optical limiters offer many advantages for practical applications. Various OPL materials have been successfully integrated into polymer matrices, such as poly(methyl methacrylate) (PMMA), methyltriethoxysilane (MTES), and PDMS, to fabricate doped films or gel glasses.^57–60 Given the high transparency and superior OPL performance of Pt-2b, it was incorporated into PDMS to fabricate flexible and transparent Pt-2b@PDMS films (Fig. 5a). As a chemically inert material with excellent optical transparency, thermal stability, and flexibility, PDMS ensured uniform dispersion of Pt-2b without any chemical interactions.^14,59 Pt-2b@PDMS films with various concentrations (0–1.00 wt%) displayed uniform color and smooth, defect-free surfaces that are suitable for optical measurements (Fig. 5b). SEM-EDS analysis confirmed the homogeneous distribution of Pt-2b within the PDMS matrix (Fig. S45†). Under visible light, the films maintained high optical transparency, clearly revealing the underlying patterns. The pristine PDMS film displayed a linear transmittance (T₀) above 90% across the visible spectrum. With increasing Pt-2b content, T₀ gradually decreased; for Pt-2b@PDMS-1.00 wt%, the T₀ value at 532 nm remained 67%, meeting the practical OPL requirements (Fig. 5c). Even at an increased concentration of 5.00 wt%, Pt-2b remained uniformly dispersed in the PDMS matrix (Fig. S46†) owing to its twisted molecular structure and lack of π–π stacking, which effectively prevented aggregation, along with bulky alkyl chains that enhanced its compatibility with PDMS.

Fig. 5 (a) Schematic of the preparation process for Pt-2b@PDMS films. (b) Optical photographs of Pt-2b@PDMS films at various concentrations (diameter: 40 mm; thickness: 2 mm). (c) Linear transmittance spectra of Pt-2b@PDMS films. (d) Open-aperture Z-scan curves and (e) output fluence versus input fluence curves for Pt-2b@PDMS films at various concentrations at 532 nm. (f) Nonlinear transmittance versus input fluence curve of Pt-2b@PDMS-1.00 wt%. (g) Comparison of the β_eff and F_OL values of Pt-2b@PDMS-1.00 wt% with those of representative solid-state optical limiters.

Under pulsed laser irradiation at 100 μJ, the pristine PDMS film exhibited no detectable NLO response (Fig. S47†), confirming that the polymer matrix has negligible influence on the NLO properties. As shown in Fig. 5d and e, the NLO and OPL responses of Pt-2b@PDMS increased with Pt-2b concentration. The T_min dropped from 0.46 (0.05 wt%) to 0.11 (1.00 wt%), while the β_eff value increased accordingly, indicating that the NLO performance was concentration-dependent. Table S15† summarizes the linear and NLO parameters of Pt-2b@PDMS films at various concentrations. At 1.00 wt%, β_eff reached 965.83 cm GW⁻¹, rivaling or surpassing the β_eff values of the state-of-the-art NLO materials (Table 3).^14,61,62 Besides, Pt-2b@PDMS-1.00 wt% demonstrated excellent OPL performance, with an optical limiting onset threshold (F_ON, defined as the incident fluence at 95% linear transmittance) and F_OL value of only 0.037 and 0.26 J cm⁻², respectively (Fig. 5f). As illustrated in Fig. 5g and Table 3, its performance surpassed those of many representative solid-state OPL limiters, including TTF-Pt(bzimb),⁹ metal clusters (V₉Mo₁ and InOC-19),^22,63 Al molecular rings (AlOC-93 and AlOC-135),^53,54 MOF (ZnTPy-1),¹⁴ COF (CuPc-PI-COF-1),⁶⁴ and composites ((MX@NCS)₄ and MoS₂-PAN).^65,66

Table 3 Comparison of the third-order NLO properties of representative solid-state optical limiters at 532 nm

Sample	Parameters^a	L^b (mm)	c^c (wt %)	T0^d (%)	β^e (cm GW^−1)	FOL^f (J cm^−2)	Ref.
a Laser pulse width and repetition rate of 532 nm laser.b Thickness of the film.c Dopant concentration.d Linear transmittance at 532 nm.e Nonlinear absorption coefficient.f Optical limiting threshold.
Pt-2b@PDMS	4 ns, 10 Hz	2	1.00	67	965.83	0.26	This work
AlOC-135@PDMS	8.5 ns, 10 Hz	—	—	51	41	0.62	Inorg. Chem. Front., 2024, 11, 462–469^67
AlOC-93@PDMS	8.5 ns, 10 Hz	2	—	38	71.80	0.12	Aggregate, 2023, 4, e264^68
ZnTPyP-1@PDMS	5 ns, 5 Hz	2	0.25	62	1350	0.32	J. Am. Chem. Soc., 2021, 143, 17162–17169^14
CuPc-PI-COF-1@MTES	8 ns, 10 Hz	—	—	—	10.72	0.44	Adv. Funct. Mater., 2024, 34, 2404289^64
V9Mo1@MTES	10 ns, 10 Hz	—	1.00	<40	691.79	1.22	Inorg. Chem. Front., 2022, 9, 4413–4424^63
TTF-Pt(bzimb)@MTES	8 ns, 10 Hz	—	3.33	41	393	0.30	J. Mater. Chem. C, 2018, 6, 8495–8501^9
(MX@NCS)4/PMMA	6 ns, 1 Hz	1	4.00	—	387	0.21	Small, 2025, 2411146^65
DAST@PMMA	6 ns, 2 Hz	0.1	0.125	85	744	0.37	ACS Appl. Mater. Interfaces, 2022, 14, 33787–33796^57
MoS2-PAN@PMMA	8 ns, 2 Hz	—	—	∼64	1151.01	1.56	J. Mater. Chem. C, 2017, 5, 11920–11926^66

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Compared with the solution state, Pt-2b@PDMS exhibited significantly enhanced OPL performance, with a nearly 5 times higher β_eff value (Fig. S48†). This enhancement can be attributed to the suppressed molecular distortion and reduced ³O₂-induced T₁ state quenching in the solid matrix, thereby increasing T₁ state populations. To highlight the advantages of Pt-2b, a comparison with C₆₀@PDMS showed that Pt-2b@PDMS not only displayed a stronger OPL performance but also a higher transparency (Fig. S49 and S50†). These findings suggest the potential of Pt-2b@PDMS films for laser protection applications.

Stability of Pt-2b@PDMS films

Benefiting from its low Young's modulus, Pt-2b@PDMS-1.00 wt% retained its original shape after 100 cycles of stretching, twisting, and folding (Fig. 6a). The stress–strain curves showed that Pt-2b induced a negligible effect on the mechanical properties of PDMS, with the tensile strength increasing slightly from 8.09 to 8.11 MPa and the modulus from 7.50 to 7.60 MPa (Fig. 6b). TGA analysis revealed that both pristine PDMS and Pt-2b@PDMS-1.00 wt% exhibited T_d values above 391 °C, confirming their excellent thermal stability (Fig. 6c). Moreover, the NLO responses of the film remained nearly unchanged over time, indicating good durability (Fig. 6d). The combination of strong OPL performance, high transparency, and exceptional stability makes Pt-2b@PDMS a promising candidate for OPL devices.

Fig. 6 (a) Illustration of the mechanical flexibility of Pt-2b@PDMS-1.00 wt%. (b) Stress–strain curves of pristine PDMS and Pt-2b@PDMS-1.00 wt%. (c) TGA curves of Pt-2b, pristine PDMS, and Pt-2b@PDMS-1.00 wt%. (d) Open-aperture Z-scan curves for Pt-2b@PDMS-1.00 wt% after storage for various durations.

Conclusions

In summary, six star-shaped Pt(II) complexes were synthesized to tune their photophysical properties and enhance their OPL performance. Spectroscopic and theoretical analyses showed that the S₁ states of Pt-1a–Pt-2c involved mixed ¹π–π*/¹ICT transitions, while the T₁ states were mainly localized ³π–π* transitions on ethynyl ligands. The incorporation of trinuclear Pt centers extended the triplet state lifetimes and enhanced the ESA, with core-ethynyl ligand electronic effects playing crucial roles in regulating the photophysical properties. Pt-2b exhibited outstanding OPL responses while maintaining excellent transparency, surpassing many representative OPL materials. The corresponding Pt-2b@PDMS-1.00 wt% film exhibited a remarkable OPL performance, featuring a high β_eff of 965.83 cm GW⁻¹ and a low F_OL of 0.26 J cm⁻², demonstrating its potential for practical OPL applications. This work highlights the structure–property relationships in Pt(II) complexes and provides a promising strategy for designing next-generation OPL materials. Future work will explore their broadband OPL properties in the NIR region and further investigate their application in flexible photonic devices.

Author contributions

Lai Hu: methodology, data curation, writing – original draft; Zhiyuan Chen: investigation; Zhi Zhao: data curation; Ruiqi Chen: conceptualization; Senqiang Zhu: conceptualization, writing – review and editing; Rui Liu: funding acquisition, supervision; Hongjun Zhu: funding acquisition, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

The authors greatly acknowledge the National Key R&D Program of China (2023YFA0913600), the Natural Science Foundation of Jiangsu Province (BK20220351) and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No. 22KJB150027) for the financial support.

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Footnote

† Electronic supplementary information (ESI) available. CCDC: 2288730–2288732 and 2213945. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi00986c

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