Ligand and counteranion effects in cyclometalated Pt(II) diphosphine complexes: photophysics, singlet-oxygen generation and photocatalysis

Abstract

This work reports the synthesis, characterization and study of the photophysical properties of two families of cyclometalated [Pt(C^N)(P^P)]X [C^N = 2-phenylbenzothiazolate (pbt), 1-phenylisoquinolinate (piq)] complexes. This study establishes structure–property relationships across variations in the C^N ligand (pbt vs. piq), the diphosphine backbone [P^P = 1,2-bis(diphenylphosphino)benzene (dppbz), 2,3-bis(diphenylphosphino)pyrazine (dpppyz), 2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (BINAP) and bis[2-(diphenylphosphino)phenyl]ether (DPEphos), and the counteranion [X = PF₆⁻, NO₃⁻, [Al(OR^F)₄]⁻]. Single-crystal X-ray diffraction reveals well-defined trends in coordination geometry, with wide-bite-angle diphosphines inducing pronounced distortions that correlate with weakened emissive behaviour. The photophysical study shows that the nature of the C^N ligand predominantly governs the excited-state energy landscape: pbt complexes emit in the yellow region with high efficiencies, whereas the more conjugated piq derivatives display red-shifted phosphorescence with longer emission lifetimes. In solution, several complexes exhibit dual fluorescence–phosphorescence emission. All complexes (except 5) act as efficient triplet sensitizers. Guided by their increased photophysical stability and higher solubility, two Krossing-type anion [Al(OR^F)₄]⁻ based salts were selected as catalysts for the photooxidation of p-bromothioanisole under homogeneous conditions. While the pbt derivative shows limited activity, due to catalyst degradation under near-UV irradiation, the piq analogue operates efficiently under mild blue-light excitation (460 nm), achieving 100% conversion with only 1% catalyst loading in 9 hours.

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Introduction

Cyclometalated platinum(II) complexes have attracted wide attention due to their rich photochemical and photophysical behaviour, which underpins applications in electroluminescent devices,¹ bioimaging,² sensing,³ probes⁴ and photosensitizers.^2a,5 The strong spin–orbital coupling (SOC), associated with the Pt atom, promotes efficient intersystem crossing to triplet excited states and the presence of strong-field aryl groups in the cyclometalated units rises the energy of the deactivating d–d excited states, suppressing non-radiative decay pathways and enabling intense phosphorescence.⁶ Depending on the ligand environment, the emissive states may involve ³MLCT, ³LC/³IL or ³L′LCT transitions, and their energies are strongly influenced by the cyclometalated and ancillary ligands, as well as by aggregation phenomena such as Pt⋯Pt or π–π interactions.⁷

In Pt(II) chemistry, phosphine ligands, with their soft σ/π-donor/acceptor character, raise d–d metal centered transition energies compared to N- or O-donor ligands increasing emission efficiencies.⁸ Furthermore, diphosphines (P^P) may act as chelating or bridging ligands. In Pt(II) chemistry, there are a good number of examples of both; mononuclear cationic Pt(II) complexes containing rigid diphosphines⁹ and neutral binuclear derivatives featuring bridging diphosphine.^9c–f,10 This is understandable given the versatility of these complexes, which range from potential antitumoral agents^9f–i,11 to emitters in solid state lighting devices such as OLEDs^9a,b and light-emitting electrochemical cells (LECs).¹²

In this context, tuning the optical properties of cationic complexes of the type [Pt(C^N)(P^P)]X is commonly achieved by modifying either the diphosphine or the cyclometalated ligand.^{9a,b,d,e,g–j} Generally, the anions used are Cl⁻ (ref. 9g–i) or the classical PF₆⁻, BF₄⁻ or CF₃CO₂⁻,^9a,b,d,e but the influence of the counteranion has rarely been systematically studied.¹³ Among these, PF₆⁻ remains the most widely used “non-coordinating” anion for isolating luminescent cationic Pt(II) species. Nevertheless, several aspects deserve consideration. Solid-state structures frequently reveal F⋯H and F⋯C contacts, indicating that PF₆⁻ can participate in weak interactions with the cationic framework.^9a,14 Furthermore, PF₆⁻ hydrolysis is a well-known issue,¹⁵ which may even be accelerated under light exposure, what is precisely a working condition of the use of these complexes. These factors can be detrimental when targeting photoactive or photochemically robust materials and replacing PF₆⁻ by a more stable weakly coordination anion (WCA)¹⁶ such as Krossing's [Al{OC(CF₃)₃}₄]⁻ ([Al(OR^F)₄]⁻)¹⁷ appears particularly attractive. This is an excellent counterion for stabilizing highly reactive or photosensitive systems, offering minimal coordinating ability together with outstanding chemical resilience.

On the other hand, in contrast to the extensive knowledge in photophysical properties of cyclometalated platinum(II) complexes, their development in photocatalysis is still rather limited.¹⁸ One emergent approach is using the photoactive Pt(II) substrate as triplet photosensitizer (PS) in aerobic oxidation reactions generating reactive oxygen species (ROS). Photosensitization of molecular oxygen involves light-activation of the photoactive substance to a triplet excited state, which can transfer electrons or energy to the oxygen molecules; to give radicals, such as O₂˙⁻ (mechanism type I), or generate singlet oxygen ¹O₂ (type II).¹⁹ Typically, cyclometalated complexes capable of acting as photosensitizers (PS) must have an excited state (³PS*) with enough energy to transfer oxygen molecules in the triplet ground state ³O₂ to give singlet oxygen molecules ¹O₂.^5b,20 The energy difference to T₁ → S₀ transition of oxygen lies in the near IR region (∼1270 nm or 0.98 eV). Therefore, the energy difference between S₀ and T₁ in the PS must be ≥0.98 eV to generate ¹O₂.^19c,21

In this regard, Pt(II) complexes have proved to be efficient in photodynamic therapy (PDT)²² and as photocatalysts in the oxidation of different organic molecules.^20c,23 Among these oxidation reactions, the photooxidation of sulfides to sulfoxides using O₂ as oxidant has interest.^23a,24 Traditionally, chemoselective oxidation of sulfides to sulfoxides has been extensively studied due to the importance of sulfoxides in organic synthesis, medicinal chemistry and natural products,²⁵ but these oxidations using handling peroxide-based reagents are not eco-friendly and cause safety problems.

With these considerations in mind, the present work aims not merely to describe new Pt(II) complexes, but to disentangle how each structural component governs their properties. To this end, we selected a representative family of cationic cyclometalated Pt(II) systems in which we varied the C^N ligand, the rigid diphosphine P^P ligand and the counteranion X⁻, in a controlled manner. Specifically, the synthesis and characterization of cationic cyclometalated mononuclear Pt(II) complexes with chelating rigid diphosphine ligands [Pt(C^N)(P^P)]X [C^N = 2-phenylbenzothiazolate (pbt), 1-phenylisoquinolinate (piq); P^P = 1,2-bis(diphenylphosphino)benzene (dppbz), 2,3-bis(diphenylphosphino)pyrazine (dpppyz), 2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (BINAP), bis[2-(diphenylphosphino)phenyl]ether (DPEphos) is described. Moreover, this study examines the effect of the anion in the dppbz series by moving from X = PF₆⁻, NO₃⁻ to [Al(OR^F)₄]⁻, and demonstrates that the anion has a significant impact on the stability and solubility. Photophysical studies and theoretical calculations were also carried out on these cationic complexes, evaluating the influence of the cyclometalating, phosphine and counteranion on the photophysical properties. In addition, the ROS generation and use as catalyst in the photocatalytic oxidation of sulfides to sulfoxides has been examined on representative examples.

Results and discussion

Synthesis and characterization

³¹P{¹H} NMR spectroscopy in (CD₃)₂CO confirms that the starting materials [Pt(pbt)Cl(DMSO-κS)] and [Pt(piq)Cl(DMSO-κS)] react with the corresponding P^P to generate, in situ, the cations [Pt(C^N)(P^P)]⁺ (Scheme 1). Unfortunately, all the attempts to isolate the chloride salts [Pt(C^N)(P^P)]Cl failed, with the chloride competing with the phosphine to produce mixtures of different species. Therefore, the PF₆⁻ series were directly isolated (1–7) following the synthesis reported by Sicilia et al. with KPF₆ as halide-abstractor and anion source (see SI).²⁶

Scheme 1 Synthesis of complexes 1–7.

Aiming to explore the influence of replacing PF₆⁻ by the WCA [Al(OR^F)₄]⁻, the salt Li[Al(OR^F)₄] was prepared in multigram scale following Krossing's recipe.^17a,27 As shown in Scheme 1, this lithium salt was employed in the preparation of the [Al(OR^F)₄]⁻ series of complexes with dppbz [1_Al(OR^F)₄, 2_Al(OR^F)₄]. Besides, for comparison, the system piq/dppbz with NO₃⁻ as counteranion (2_NO₃) was also prepared.

All the complexes have been fully characterized by the conventional spectroscopic and analytical techniques (see Experimental section and Fig. S1–S10). Their IR spectra show the most characteristic vibrations for the anions, with very strong bands for PF₆⁻ (∼830, 550 cm⁻¹) and the perfluorinated aluminate (∼725, 970 and 1215, 1350 cm⁻¹).²⁸ The most diagnostic features arise from the ³¹P{¹H} NMR spectra, where the two phosphorus nuclei of the chelating diphosphine become inequivalent upon coordination. In all cases, the P atom trans to the cyclometalated carbon appears downfield and exhibits the smallest ¹J_P–Pt value, whereas the P trans to the nitrogen donor resonates upfield with the largest ¹J_P–Pt constant. These trends, consistent across the series, reflect the stronger trans influence of the Pt–C bond relative to Pt–N. Representative chemical shifts and coupling constants are summarized in Table S1. The ¹⁹⁵Pt{¹H} NMR spectra, available for most derivatives, show the expected doublet-of-doublets pattern arising from coupling to the two inequivalent phosphorus atoms. The magnitudes of the ¹J_P–Pt coupling constants mirror the ³¹P{¹H} trends, with the phosphorus trans to C showing the smaller values.

Complex 7 shows, however, a striking NMR spectroscopic behavior in solution. Its ³¹P{¹H} NMR spectrum in (CD₃)₂CO at room temperature displays a broad signal at δ_P = 10.6, that contrasts with the solid-state structure (see below), thus indicating a dynamic behavior (Fig. S9). This behavior is consistent with the MALDI(+) mass spectra. In solid state, the molecular peak [M]⁺ (m/z 949) is obtained as parent peak, whereas in CH₂Cl₂ or CH₃CN solutions, the intensity of this peak decreases, increasing new peaks assigned mainly to [{Pt(pbt)}₂(μ-DPEphos) + Na]⁺ (m/z 1373) and [Pt(DPEphos)]⁺ (m/z 734) (Fig. S10).

Photostability comparison in solution

A remarkable difference between the PF₆⁻ and the [Al(OR^F)₄]⁻ series is their different solubility, which is much higher in derivatives with [Al(OR^F)₄]⁻. In addition, as was confirmed by NMR studies, while complexes bearing PF₆⁻ decompose over time in chlorinated solvents (evidenced by the gradual appearance of additional ³¹P{¹H} resonances) the corresponding [Al(OR^F)₄]⁻ derivatives remain spectroscopically unchanged over extended periods, displaying no detectable evolution by NMR (Fig. S11 and S13). These observations highlight the significantly higher chemical inertness imparted by the weakly coordinating and robust anion [Al(OR^F)₄]⁻. To get further insight into this, photostability experiments were conducted under UV irradiation. As can be seen in Fig. S12 and S14, the PF₆⁻ complexes exhibit the same spectral changes observed in the dark but with a clearly accelerated rate under illumination. The ¹⁹F NMR spectrum of a CDCl₃ solution of 2_PF₆ irradiated with light of λ_exc 385–400 nm showed small signals assigned to HF, PO₃F²⁻ and PO₂F₂⁻ (Fig. S15) produced by hydrolysis of the PF₆.²⁹ Conversely, the [Al(OR^F)₄]⁻ analogue shows negligible photochemical degradation, maintaining intact NMR profiles even under prolonged irradiation. This enhanced photostability reinforces the conclusion that the WCA counteranion not only minimises undesired interactions in solution but also provides a more robust coordination environment under photochemical conditions.

X-ray structural analysis

Single-crystals of suitable quality for X-ray diffraction analysis were obtained by slow vapour diffusion of diethylether (1_PF₆, 7·CH₂Cl₂) or n-hexane (1_Al(OR^F)₄, 2_PF₆, 2_Al(OR^F)₄, 2_NO₃, 3, 4·2CH₂Cl₂, 5·0.75CH₂Cl₂) into CH₂Cl₂ solutions of the corresponding crudes. This represents a good set of structures spanning the different combinations of cyclometalating ligands (pbt, piq), diphosphines (dppbz, dpppyz, BINAP, DPEphos) and anions and enables a systematic comparison of the molecular geometries across the series. Tables S2–S5 in the SI summarize the X-ray crystallographic data, the most relevant bond distances and angles and additional details about the geometrical parameters.

As can be seen in the general view of the molecules collected in Fig. 1, 2 and S16, S17, the X-ray structures reveal the expected distorted square-planar coordination environment around Pt(II) centres, defined by the C^N cyclometalated ligand and the chelating diphosphine, with distances similar to other related Pt(II) complexes.^9a,b,g The Pt–C^N bond lengths fall within narrow ranges across the series (Pt–C ≈ 2.04–2.14 Å; Pt–N ≈ 2.09–2.14 Å), with only modest variations between the pbt- and piq-based derivatives (Table S5). As expected from trans-influence considerations, the Pt–P distances trans to C are systematically longer (ca. 2.31–2.38 Å) than Pt–P distances trans to N bonds (ca. 2.22–2.24 Å). Concerning the angular metrics, the C–Pt–N bite angle remains almost invariant (≈79–80°), characteristic of constrained C^N chelation, but some interesting features emerge in the analysis of the remaining angles of the Pt(II) coordination environment. As expected, the geometry of the cations [Pt(C^N)(dppbz)]⁺ in complexes 1_Al(OR^F)₄ and 2_Al(OR^F)₄ is close to that of the related complexes with PF₆⁻ anion. In these, the [Al(OR^F)₄]⁻ anions adopt a typical branched structure similar to those described in other examples in the literature.³⁰

Fig. 1 Molecular structures of 1_PF₆, 2_PF₆, 3, 4·2CH₂Cl₂, 5·0.75CH₂Cl₂, 7·CH₂Cl₂. PF₆ are omitted for clarity.

Fig. 2 View of the distortion of the square planar environment around the Pt(II) center for the pbt complexes in 1_PF₆, 3, 5·0.75CH₂Cl₂ and 7·CH₂Cl₂. PF₆ are omitted for clarity.

As is evident upon inspection, the distortion around the Pt(II) centers in 5·0.75CH₂Cl₂ and 7·CH₂Cl₂ is pronounced and readily measurable. Obviously, a key factor governing the magnitude of the structural distortion in these complexes is the intrinsic bite angle of the diphosphine ligand. As established in systematic studies of diphosphine geometries,³¹ ligands such as dppbz or dpppyz possess relatively narrow natural bite angles (typically 71–76°), whereas BINAP and DPEphos belong to the class of wide-bite-angle ligands, with preferred P–M–P angles approaching or exceeding 90°. This trend is clearly reflected in the solid-state structures of our complexes, and the P–Pt–P angles increase from ca. 86° in 1–4 to 92° in 5 (BINAP) and reach 100° in 7 (DPEphos). Furthermore, such P–Pt–P expansion enforces a substantial deviation from the ideal square-planar geometry and correlates directly with the larger τ₄ values observed for 5 (0.25) and 7 (0.33) when compared with the other complexes (0.04–0.09) (Table S5). In fact, and as can be seen in Fig. 2 for the comparison between the pbt complexes (1_PF₆, 3, 5, and 7) and in Fig. S17 for the piq complexes (2_PF₆, 4), the angle between the planes defined by both chelated ligands, the C^NPt plane and the P^PPt plane, vary from the relatively coplanar disposition in 1_PF₆ (2.89°), 3 (3.83°), 2_PF₆ (9.70°), 4 (3.17°) to much wider values for 5 (24.54°) and 7 (29.38°). In fact, the P atoms in those latter structures are located up and down the plane C^NPt with distances as long as 0.991 Å, for the P trans- to carbon in complex 7.

Having said all this, a complete understanding of the supramolecular interactions that operate in the solid state is essential for rationalizing the macroscopic behaviour of complexes. In the particular case of ionic complexes, the nature and strength of cation–anion interactions are key factors for modulating their properties and recent studies even point to ion-pairing as critical for dictating some differences in solution.³² Consequently, analysing the organization of these complexes within the crystal is a necessary complement to the discussion of their molecular geometries, and, in our case, the series 1_PF₆ vs. 1_Al(OR^F)₄, and 2_PF₆ vs. 2_Al(OR^F)₄ vs. 2_NO₃ particularly useful for this respect. Views of the crystal packing are collected in Fig. S16. A characteristic difference in the crystal packing is the shorter intercationic C–H⋯C contacts in the PF₆ (∼3.3 Å 1_PF₆, 3.2 Å 2_PF₆) and NO₃ complexes (∼2.8 Å 2_NO₃), which maintain the cations closer together than those in the Al(OR^F)₄ derivatives. Furthermore, the anions stablish the expected C–H⋯F contacts (2.5–2.6 Å) in the PF₆ or Al(OR^F)₄ derivatives or C–H⋯O contacts (2.4–2.6 Å) in 2_NO₃. The distinct packing arrangements likely influence the compounds solubility.

Photophysical properties and theoretical calculations

Detailed photophysical measurement procedures are provided in the SI (section S5), with key data summarized in Table S6 and Tables 1, 2. To gain deeper insight into these properties, theoretical calculations were performed in CH₃CN solution and in gas phase using the Gaussian 16 package (for details on functionals and basis sets, see SI, section S6). Since the change of anion does not significantly affect the absorption spectra and, to reduce the computational requirements, we decided to perform the calculations on the cationic part alone.

Table 1 Emission data for all complexes in solid state at 298 K

Complex	λem (λex)/nm	τ (λem/nm^−1)/μs	Φ^a/%	kr ^b(s^−1)	knr ^c (s^−1)
a λex 400–420 nm.b kr = ϕ/τaverage.c knr = (1 − ϕ)/τaverage.
1_PF6	530, 570max, 615 (370)	35.1 (570)	31	8.7 × 10^3	2.0 × 10^4
1_Al(OR^F)4	527, 566max, 612 (370)	32.9 (565)	30	9.1 × 10^3	2.1 × 10^4
2_PF6	613, 655max, 713sh (420)	12.9 (655)	3	1.9 × 10^3	7.6 × 10^4
2_Al(OR^F)4	606, 648max, 714sh (400)	5.0 (42%), 12.2 (58%) (650)	3	2.8 × 10^3	1.1 × 10^4
2_NO3	613, 655max, 713sh (365)	11.0 (650)	3	2.4 × 10^3	8.9 × 10^4
3	532, 570max, 614 (370)	35.2 (570)	17	4.8 × 10^3	2.4 × 10^4
4	612, 657max, 718sh (430)	8.4 (660)	3	3.3 × 10^3	1.2 × 10^4
5	416sh, 444sh, 546, 576max, 622sh (365)	15.9 (97.1%), 2.3 (2.9%) (620)	<1.0	—	—
	456, 534, 576, 618max, (400)
	634, 710sh (450)
6	446max, 478sh, 564, 616, 732sh (400)	14.1 (98.4%) 1.7 (1.6%) (620)	<1.0	—	—
7	418, 422, 650sh (365)	13.1 (98.8%), 1.5 (1.2%) (630)	<1.0	—	—

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Table 2 Emission data for all complexes in CH₃CN solutions (5 × 10⁻⁵ M) at 298 K

Complex	λem(λex)/nm	τ (λem/nm^−1)
1_PF6	445max, 526, 568, 615sh (400)	2.1 ns (450); 0.2 µs (575)
1_Al(OR^F)4	445max, 529, 572, 620sh (385)	0.18 µs (575)
2_PF6	468, 606, 651max, 710sh (400)	7.4 ns (460); 1.6 µs (650)
2_Al(OR^F)4	467, 612, 650max, 714sh (356)	1.5 µs (650)
2_NO3	444, 610, 650max, 714sh (375)	0.6 µs (650)
3	450, 542, 578max, 626sh (400)	1.1 µs (10%), 0.5 µs (90%) (580)
4	455, 603, 649max, 696sh (400)	1.5 µs (95%), 2.5 µs (5%) (650)
5	436, 532max, 574, 626sh (365)	14 ns (65.6%), 3.7 ns (34.4%) (575)
	496max, 532, 574, 626sh (420)
6	494, 608, 646, 714sh (375–450)	0.38 µs (68.8%), 1.26 µs (31.2%) (650)
7	452, 526, 564max, 616sh (400)	38 ns (81.3%), 20 ns (18.7%) (560)

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Absorption spectra. The UV–Vis absorption spectra of the [Pt(C^N)(P^P)]X complexes were recorded in CH₃CN solution (5 × 10⁻⁵ M) (Fig. 3 and S18–20). All derivatives exhibit intense bands in the high-energy region (λ ≈ 260–330 nm), due to ligand-centred π → π* transitions and weaker and broader bands or shoulders extending into the visible region (below 400 nm) responsible of the yellow orangish colour of the solutions. The most obvious feature is the red-shift of the lowest energy absorption of the piq based systems with respect to the pbt series (see Table S6) as was expected given the extended electronic delocalization of the first.

Fig. 3 Experimental and theoretical (bar) UV-Vis absorption spectra of (a) pbt complexes (1_PF₆, 3 and 5) (b) piq complexes (2_PF₆ and 4) in CH₃CN (5 × 10⁻⁵ M) and schematic representation of selected frontier orbitals and excitations of the corresponding cations.

Varying the nature of the diphosphine within each C^N series reveals interesting differences. As can be seen in Fig. 3 and S19, for the dppbz complexes (1, 2) the lowest energy absorption fits well with the calculated S₁ (376 nm 1⁺, 397 nm 2⁺), which arises from the HOMO to LUMO transition, and is mainly ¹IL(C^N) in nature. The next most intense transition is the S₃ (324 1⁺, 345 nm 2⁺), with mixed contributions from ¹IL/¹MLCT/¹L′LCT in both species. The introduction of the electron-accepting pyrazine ring in the diphosphine shifts the lowest energy absorption slightly to the red (S₁ 381 3⁺ vs. 376 nm 1⁺; 401 4⁺ vs. 397 nm 2⁺) and changes its nature, which now has their origin in the transition S₁, of strong ¹LL′CT (C^N → pyrazine) nature, with somewhat of ¹IL(C^N)/¹MLCT contribution. The next most intense transition S₂ (353 3⁺, 366 nm 4⁺) reinforces the ¹LL′CT character. This is also in agreement with the structural optimization, that reveals a marked tendency of the pyrazine ring to sit in a more parallel disposition with respect to the coordination plane of Pt(II) both in the gas phase and in solution (Tables S15 and S19).

With their wider bite angles and the associated geometry distortion, BINAP and DPEphos based complexes have important particularities. Complexes 5, 6 and 7 show broad and lower intensity low energy absorption bands, which could be related with the geometrical distortion imposed by the wider bite angle of the diphosphine (Fig. S19 and S20). In the case of the BINAP series, we did calculations only for complex 5, as we had its X-ray structure as an input and not so for 6. As can be seen in Fig. 3, the calculations reveal a clear participation of the naphthyl rings of the BINAP in S₁ (392 nm) and S₂ (374 nm) transitions, that therefore can be considered mainly ¹L′(naphthyl)L(pbt)CT in nature. For the DPEphos complex 7, the calculated low-energy S₁ excitation (389 nm) fits well with the lowest energy absorption whose origin can be ascribed to ¹IL with small ¹MLCT contribution. Nevertheless, the presence of dynamic processes in solution, described before, precludes any secure consideration about the absorption spectra of complex 7 in CH₃CN.

Photoluminescent properties. As can be read in Table 1 and is shown in Fig. 4, the complexes show intense photoluminescence in the solid state at 298 K.

Fig. 4 Excitation and emission spectra of (a) 1_PF₆, 1_Al(OR^F)₄ and 2_PF₆ in solid state. (b) Energy and plots of the frontier MOs at the optimized T₁ state in the gas phase for 1⁺ and 2⁺.

The dominant influence of the cyclometalating ligand on the emissive behaviour of these Pt(II) systems can be easily visualized in the direct comparison between complexes 1_PF₆ and 2_PF₆ in the solid state (Fig. 4a). Thus, the pbt-based complex 1_PF₆ exhibits yellow phosphorescence, with a structured band at 530 nm, a relatively long lifetime of 35.1 µs, and a high quantum yield of 31%, indicative of an efficient and rigid emissive triplet state. In contrast, the piq-derived analogue 2_PF₆ displays a markedly red-shifted emission profile, with the high energy peak at 613 nm, together with a significantly shorter lifetime (12.9 µs) and a much lower quantum yield (3%). The lowest energy triplet vertical excitation T₁ with the S₀ geometry, the SOMOs and SOMOs–1 and the spin density surfaces at the optimized T₁ state in the gas phase (Fig. 4b) confirm for both complexes that the phosphorescence is associated mainly to ³IL(C^N) transitions with a low ³MLCT contribution. Therefore, the bathochromic shift, reduced efficiency and shorter lifetime for 2_PF₆ reflects the lower T₁ energy associated with the extended π-system of the phenylisoquinoline ligand, which both stabilises the emissive state and enhances non-radiative decay pathways.

The influence of the counteranion on the solid-state emission is generally minimal. However, the slight blue shifts observed for 1_Al(OR^F)₄ relative to 1_PF₆ (Fig. 4a), and similarly for 2_Al(OR^F)₄ compared with 2_PF₆ and 2_NO₃ (Fig. S21), can be attributed to weaker secondary packing interactions in the Al-based samples. Although subtle, these spectral differences are accompanied by systematic small reductions in the emission lifetimes, a behaviour typically associated with less efficient supramolecular aggregation and hence with a weakly constrained emissive triplet state.

Modifying the electronic nature of the diphosphine from dppbz to dpppyz has only a minor impact on the solid-state emission, and a comparison between 1_PF₆ and 3 (both pbt-based) or 2_PF₆ and 4 (both piq-derived) shows that the position and overall shape of the emission bands remain essentially unchanged, with only modest variations in intensity and lifetime (Fig. S22). This behaviour aligns with the outcome of the spin density surfaces at the optimized T₁ state, which consistently predict that the lowest-energy triplet states are ³IL(C^N), and the vertical excitation T₁ in the S₀ with also limited phosphine contribution ³LL′CT.

The BINAP- and DPEphos-containing complexes (5–7) display weak solid-state luminescence (Φ < 1%) with broad profiles (Fig. S23 and S24) and significantly shortened lifetimes. This correlates with the larger structural distortions that facilitates non-radiative decay pathways, reducing both the efficiency and lifetime of the emissive triplet state. Thus, the three complexes exhibit dual fluorescence (region 400–500 nm) and phosphorescence (500–750 nm) emissions, which are overlapped depending on the λ_exc. This behaviour is reproduced in solution and it is discussed next.

In CH₃CN solution, the general energy ordering dictated by the cyclometalating ligand is preserved, but the emission profiles of 1 and 2 reveal a markedly different behaviour from the solid state, most notably the appearance of clear mixtures of ¹IL(C^N) fluorescence and ³IL(C^N) phosphorescence. However, the balance between fluorescence (F) and phosphorescence (P) is delicate and caution is required, given that, despite all the measurements have been made in deoxygenated conditions, these complexes are extremely sensitive to dissolved oxygen and small differences in the manipulation of the samples can provide experimental errors. As expected, the excitation spectra at the F and P emission maxima match well with their absorption in the low region showing that these two emitting states share the same origin that absorbs photoexcitation (Fig. 5). Nevertheless, for complexes 1_PF₆ and 1_Al(OR^F)₄ in deaerated conditions, the profile is very similar, with a phosphorescence at ∼525 nm (τ ∼ 0.2 µs) and a dominant fluorescence at 445 nm with shot lifetime (2.13 ns) (Fig. 5a). For the piq derivatives, 2_PF₆, 2_Al(OR^F)₄ and 2_NO₃, the phosphorescence band (∼610 nm) is clearly dominant being the residual fluorescence somewhat more intense in 2_PF₆ (Fig. 5b). Both, the phosphorescence lifetime (0.6–1.6 µs) as the fluorescence lifetime (7.4 ns) are longer than in complexes 1, being therefore more oxygen sensitive. In both series, the phosphorescence emission decreases considerably under aerated conditions, but with higher efficiency in 2_Al(OR^F)₄ in relation to 1_Al(OR^F)₄ (Fig. 5). This behaviour indicates notable sensitization of ³O₂ among other quenching processes. Dual fluorescence-phosphorescence behaviour has been described for a handful of platinum(II) complexes,³³ and also specifically for the related [Pt(thpy)(P^P)]⁺ (thpy = 2-(2′-thienyl)pyridine) complexes.^9j Calculations confirm a ³IL character for the phosphorescent emission in solution (section S6 in SI). In contrast, for complexes 3 and 4, the phosphorescence is practically the only component (Fig. 6a), with lifetimes of ∼0.5 µs for 3 and ∼1.5 µs for 4.

Fig. 5 Excitation and emission spectra in deoxygenated CH₃CN (5 × 10⁻⁵ M) of compounds (a) 1, (b) 2 and in oxygenated solutions for (a) 1_Al(OR^F)₄, (b) 2_Al(OR^F)₄.

Fig. 6 Emission spectra in deoxygenated CH₃CN (5 × 10⁻⁵ M) of compounds (a) 3 and 4, (b) 5 and 6.

The photoluminescence of compounds 5 and 6 is somewhat more complex (Fig. 6b). BINAP ligand is strongly fluorescent (λ_em ∼ 360 nm) at 298 K with a low-energy structured phosphorescence band (λ_em 490 nm) at 77 K.³⁴ As can be seen in Fig. 6b, upon excitation at 365 nm, complexes 5 or 6 display the expected profile with a dual ¹IL(C^N)/³IL(C^N) emission (F 436, P 532 nm 5; F 494, P 608 nm 6) associated to the cyclometalated ligand. Excitation of 6 at 420–450 nm produces the same pattern. However, excitation of complex 5 at low energy (420 nm) generates a band, with maximum at 496 nm that closely resembles the phosphorescence of the BINAP ligand,³⁴ suggesting a different emissive state. Therefore, in complex 5, the BINAP and pbt behave as isolated chromophores rendering, depending on the λ_exc, the dual ¹IL(C^N)/³IL(C^N) pathway located on the pbt or the mixed ³L′LCT/³IL′ (L′ = BINAP) located on the BINAP. There are antecedents of different chromophoric states independently populated or multiple states which may coexist in equilibrium.³⁵ Theoretical calculations on 5⁺ predict that the T₁ → S₀ excitation has a clear BINAP to C^N (³L′LCT) origin, whereas T₂ has a complex mixed nature, including ³IL′ (BINAP). The optimized T₁ reveals a ³IL′ (BINAP) character with low contribution of the platinum (0.0409Pt), which favours dual F and P emissions. The DPEphos complex 7 shows a very weak emission (<1%), likely due to its dynamic behaviour in solution, as a vibronic band at 526 nm (Fig. S25), associated to the ³IL located to the pbt ligand, as confirmed by theoretical calculations (Table S50).

Irradiation aerated solutions of phosphorescent complexes that efficiently sensitize singlet oxygen, treated with a drop of SMe₂ (DMS), provoke a continuous enhancement of the phosphorescent band. This behaviour is explained by the occurrence of a local sensitization caused by energy transfer from the low energy triplet to ³O₂ producing singlet ¹O₂. Oxygen is therefore consumed in the oxidation of DMS, thus creating a oxygen-free microenvironment that enhances the phosphorescence.^20c This was tested for complexes 1_Al(OR^F)₄ and 2_Al(OR^F)₄, that show dual emission, as a further confirmation of the nature of each band. As can be seen in Fig. 7, an aerated CH₃CN solution containing DMS (2 × 10⁻² M) of 1_Al(OR^F)₄ (5 × 10⁻⁴ M) shows a moderated enhancement of the phosphorescence emission upon irradiation of light of 375 nm for 60 min and no visible to the naked eye. However, irradiation of the CH₃CN solution of 2_Al(OR^F)₄ at 375 nm for 30 min causes a dramatic increase of the phosphorescence intensity. This quick test encouraged us to evaluate the potential in photocatalysis of selected examples of these complexes.

Fig. 7 Enhancement of the phosphorescent emission of (a) 1_Al(OR^F)₄, (b) 2_Al(OR^F)₄ in CH₃CN (5 × 10⁻⁴ M) + DMS in the presence of O₂ under different irradiation times and photographs showing the corresponding visual changes in their emission.

Singlet oxygen generation and photocatalysis

The capability of the complexes to populate long-lived triplet states and to transfer energy efficiently to molecular oxygen prompted us to examine their singlet-oxygen generation. It was examined in CH₃CN solution (5 × 10⁻⁵ M) using a near-infrared detector upon excitation at 395 nm, detecting the characteristic emission profile of ¹O₂ at λ_em 1274 nm (section S7 in SI). To test our method, the standard phenalenone (PN) was measured with the same experimental approach. As summarised in Table 3, the ratio of 1274 nm emission/absorbance (ψ) suggests that all members of the series (except 5) generate large ammounts of ¹O₂, with complex 2_Al(OR^F)₄ exhibiting particularly high signal. Some examples with high ¹O₂-generating efficiency for PDT agents have been reported before.³⁶

Table 3 Integral of singlet oxygen emission in the range 1274 nm per absorbance at 395 nm in acetonitrile (CH₃CN) 5 × 10⁻⁵ M. ψ (CPS × 10⁻³)

	PN	1_PF6	1_Al(OR^F)4	2_PF6	2_Al(OR^F)4	2_NO3	3	4	5	6
ψ	89.4	28.4	41.8	67.1	107.2	73.4	30.8	43.5	—	20.2

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Complexes with piq as cyclometalated ligand show higher values compared with pbt analogues. This can be related to the emission lifetimes of both families, with piq-containing complexes giving longer-lived triplet emissive states. Thus, for example, complex 2_Al(OR^F)₄ presents a lifetime of 1.5 µs, whereas the analogue pbt complex 1_Al(OR^F)₄ shows a lifetime of 0.2 µs. A longer lifetime implies a more favourable energy transfer from the triplet state of the molecule to ³O₂, and therefore higher ¹O₂ production. In addition, introducing Al(OR^F)₄ as the counteranion increases the signal, most likely due to a better dissociation of the ions in solution, which facilitates the energy transfer towards O₂. The modification of the diphosphine cannot be rationalized in a straightforward manner. For example, complex 3 (dpppyz, pbt) shows slightly higher values compared to 1_PF₆ (dppbz, pbt), but complex 4 (dpppyz, piq) shows lower values than 2_PF₆ (dppbz, piq).

Given these results, and with the aim of exploring the photocatalytic potential of the complexes, we selected 1_Al(OR^F)₄ and 2_Al(OR^F)₄ as representative examples of the pbt and piq series, respectively. The choice of the weakly coordinating anion was motivated by two practical considerations already highlighted above: first, the enhanced photophysical stability of the aluminium salts, and second, their substantially higher solubility in a variety of solvents. The photooxidation of sulfides to sulfoxides using molecular oxygen is a reaction of significant interest due to its relevance in synthesizing biologically active compounds and has been broadly studied,^23a,24 what makes it ideal as a model. This transformation typically proceeds via the generation of reactive oxygen species (ROS), such as singlet oxygen (¹O₂) or superoxide (O₂˙⁻), through energy or electron transfer from an excited photocatalyst to dissolved oxygen. Besides, we already had our quick test of singlet oxygen generation mentioned before to know that the oxidation of SMe₂ was effective, therefore encouraging us to test other sulfides. To do so, and taking advantage of the solubility of the picked complexes, we tested the photooxidation of p-bromothioanisole in methanol-d₄ and followed the conversion by ¹H NMR (see section S9 in the SI). Importantly, the irradiation wavelength required for the photocatalysis differs between the two complexes. For 1_Al(OR^F)₄ excitation had to be carried out in the 385–400 nm region, owing to the higher-energy of its low-lying absorption band. In contrast, the broader and red-shifted absorption profile of the piq derivative 2_Al(OR^F)₄ allowed the use of blue light (λ ∼ 460 nm), a more sustainable and safer option.

Under these respective irradiation conditions, 1_Al(OR^F)₄ displays moderate photocatalytic activity, reaching ∼30% conversion before the reaction halts. Control experiments and spectroscopic monitoring suggest that this plateau arises from decomposition of the catalyst, which is likely favoured under the more energetic near-UV excitation required for the pbt system. In contrast, the piq-based analogue 2_Al(OR^F)₄ behaves as an excellent photocatalyst, achieving up to 100% conversion in 9 hours and with 1% of catalyst loading. With 0.5% catalyst, the reaction is completed in 24 h while a double amount of catalyst or double concentration in the reaction do not arise with better results, pointing to saturation of the solution and therefor worst energy absorption (Fig. 8). Under hypoxic conditions (N₂) (Table 4, entry 8) or in absence of light (Table 4, entry 7) no conversion was observed and similarly, without a catalyst in controlled O₂ atmosphere the reaction does not take place (Table 4, entry 9).

Fig. 8 Photosensitized conversion of p-bromothioanisole into the corresponding sulfoxide with the Pt complexes 1_Al(OR^F)₄ and 2_Al(OR^F)₄ in methanol-d₄ with different reaction conditions under light irradiation.

Table 4 Data for the photooxidation of p-bromothioanisole to the corresponding sulfoxide with the Pt complexes 1_Al(OR^F)₄ and 2_Al(OR^F)₄

Entry	PS	% PS (%)	Light (nm)	Atmosphere	Time (h)	Conversion (%)
a Double amount of reagent (p-bromothioanisole) and photosensitizer than in the entry.b In the presence of DABCO.c In the presence of BQ.
1	1_Al(OR^F)4	1	+ (385–400)	Air	16	34
2	1_Al(OR^F)4	2	+ (385–400)	Air	16	30
3	2_Al(OR^F)4	0.5	+ (460)	O2	24	>99
4	2_Al(OR^F)4	1	+ (460)	O2	9	100
5^a	2_Al(OR^F)4	1	+ (460)	O2	16	>99
6	2_Al(OR^F)4	2	+ (460)	O2	12	>99
7	2_Al(OR^F)4	1	—	O2	24	—
8	2_Al(OR^F)4	1	+ (460)	N2	24	—
9	2_Al(OR^F)4	—	+ (460)	O2	24	—
10^b	2_Al(OR^F)4	1	+ (460)	O2	9	17
11^c	2_Al(OR^F)4	1	+ (460)	O2	9	7

---

To elucidate the mechanism, we sought to identify the reactive oxygen species involved. The photocatalytic reaction was evaluated in the presence of specific quenchers: the singlet oxygen quencher 1,4-diazabicyclo[2.2.2]octane (DABCO, 10 equiv.; Table 4, entry 10) and the superoxide radical quencher 1,4-benzoquinone (BQ, 10 equiv.; Table 4, entry 11). In both cases, a significant decrease in reaction yield was observed. This inhibition by both quenchers suggests that both ¹O₂ and O₂˙⁻ are generated by 2_Al(OR^F)₄ and contribute to the oxidation process. Finally, we also carried out the reaction with no control of the atmosphere, but the results obtained suggest that the kinetic become second order due to the consumption of the dissolved O₂. Anyway, in the right conditions we could reach a 100% conversion at about 20 h of irradiation.

Taken together, these results reveal that although the entire family is competent at generating ROS, the photocatalytic efficiency is strongly dependent on the nature of the cyclometalating ligand, with the piq systems clearly outperforming their pbt counterparts. Besides, the anion is also critical as it provides higher solubility in different solvents and extra photostability. With all these results it can be said that the combination of high photostability and excellent catalytic performance places 2_Al(OR^F)₄ among the most promising sensitizers in the series and highlights the potential of these Pt(II) complexes for applications in oxidative photocatalysis.

Conclusions

The combined structural, photophysical and photocatalytic investigation of this family of [Pt(C^N)(P^P)]X complexes highlights the central role of the cyclometalating ligand in governing both the electronic structure and the excited-state behaviour. The pbt derivatives display higher triplet energies and more rigid coordination environments, whereas the piq complexes show markedly red-shifted emissions arising from their lower-lying triplet states. Modulation of the electronic nature of the diphosphine ligand, by moving from dppbz to dpppyz, affects the absorption spectra, as the central ring of these rigid diphosphines is involved in the frontier orbitals of the lowest energy absorption transitions. Wide-bite-angle ligands such as BINAP and DPEphos introduce significant distortions that enhance non-radiative decay pathways and weaken the emissive properties. In the case of DPEphos, the distortion is high enough to give dynamic processes in solution. However, the BINAP complex shows particularly rich properties, with mixing of IL and L′LCT/IL′ transitions. The choice of counteranion exerts only minor influence on the electronic structure, but proves crucial in determining photostability and solubility, with the weakly coordinating [Al(OR^F)₄] anion providing clear advantages.

All complexes are effective singlet-oxygen sensitizers what translate into promising photocatalytic activity under homogeneous conditions in methanol-d₄. Precisely, among the two selected representatives, 1_Al(OR^F)₄ (pbt) displays moderate activity and limited operational stability, while 2_Al(OR^F)₄ (piq) stands out as a robust and efficient photocatalyst, achieving full photooxidation of p-bromothioanisole under mild blue-light irradiation with low catalyst loadings in. The superior performance of the piq derivative underscores the importance of triplet-state energetics and the photochemical robustness, increased by the introduction of the WCA, in dictating catalytic efficiency.

Overall, these results establish clear structure–property relationships within this family of Pt(II) complexes and identify the piq-based systems—particularly 2_Al(OR^F)₄ as highly promising candidates for applications in photochemical oxidation processes and triplet-state-mediated catalysis. Besides, the results presented here encourage us to study the potential of WCA in photocatalysis but also in electroluminescent devices based on this and other types of complexes.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.

Supplementary information (SI): experimental section, NMR spectra, details about X-ray crystallography, photophysical properties, theoretical calculations, singlet oxygen measurements and photocatalysis (.docx). See DOI: https://doi.org/10.1039/d6dt00853d.

CCDC 2540427–2540435 contain the supplementary crystallographic data for this paper.^37a–i

Acknowledgements

This work was supported by the Spanish MICIU/FEDER (Project PID2023-149547NB-I00) funded by MICIU/AEI/10.13039/501100011033. J. F.-C holds a Ramón y Cajal Fellowship (RYC2021-034075-I), and is funded by MICIU/AEI/10.13039/501100011033 and the European Union “NextGenerationEU/PRTR”. We would like to thank the Single-Crystal X-ray Diffraction Laboratory of the Interdepartmental Research Service (SIdI) at the Universidad Autónoma de Madrid (UAM) and the staff of the SAI–Unit at the University of La Rioja for the technical support and assistance provided.

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