Solid state luminescence of phosphine-EWO ligands with fluorinated chalcone skeletons and their PdX₂ complexes: metal-promoted phosphorescence enhancement

Abstract

Complexes trans-[PdX₂L₂] (X = Cl and Br), where L is 1-(PR₂),2-(CH=CH–C(O)Ph)-C₆F₄ (R = Ph, Cy, and iPr), display phosphorescent emission in the solid state, whereas due to their substantially lower lifetimes, the free ligands exhibit fluorescent behaviour. Alternatively, structurally identical derivatives with halide replaced by CN⁻ or Pd replaced by Pt are non-emissive. DFT calculations explain this diverse behaviour, showing that the hybridization of orbitals of the MX₂ moiety with those of the chalcone fragment of ligands is significant only for the LUMO of the emissive compounds. In other words, in our complexes, only ML_MCT processes (L_M = Metal-perturbed Ligand-centered orbital) lead to observable luminescence.

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Chalcone derivatives (Ar–CH=CH–CO–Ar’) are mainly known for their biological activity and pharmacological applications,¹ but some of them also display interesting photophysical properties.² These compounds typically exhibit aggregation-induced emission due to lack of planarity that hinders π-stacking interactions and push–pull conjugation effects at the enone moiety,³ which are the subject of very recent studies. Two reviews^3,4 and three relevant reports have been published in the last two years. Thus, the rare crystal jumping behaviour has been found for a highly luminescent chalcone derivative due to a reversible intermolecular [2 + 2] cycloaddition process in the solid state.⁵ Moreover, chemosensors with chalcone-based chromophores have been applied for the selective detection of Cu²⁺ in aqueous media.⁶ Besides, there is a report on the application of chalcone-derived liquid crystals for NH₃(g) sensing.⁷

Our pursuit of hemilabile ligands able to promote challenging Pd-catalysed C–C couplings (e.g. Ar^F–Ar^F’, Ar^F = fluorinated aryl) led us to develop the family of fluorinated PEWO ligands shown in Fig. 1A (PEWO = Phosphine bearing Electron-Withdrawing Olefins). The previously synthesized L¹,⁸L²,⁹ and L³, reported herein for the first time, differ in the PR₂ substituents (R = Ph, Cy, and iPr respectively), while the analogue of L¹ labelled as L⁴, reported by Lei et al., (Fig. 1B), lacks fluorine substitution.¹⁰ All these PEWO ligands have a chalcone skeleton highlighted in red in Fig. 1. The hybrid chelating ligand L¹ is extremely efficient in lowering the C–C reductive elimination barriers and allows for oxidative addition under catalytic conditions with either Ar–I or Ar–Br (Ar–Cl cannot be used).^11,12 Particularly, the Z-chelated complex [PdCl₂L¹] (2L¹) is an active catalyst for the Negishi heterocoupling of perhaloaryls.¹³

Fig. 1 Hybrid phosphine-EWO ligands used in this work with the chalcone group highlighted in red. A) Fluorinated derivatives (*L³ has not been previously reported). B) Non-fluorinated Ph analogue.

The remarkably low activation barrier for E/Z olefin-isomerization (it occurs at room temperature) supports the extraordinary electronic delocalization of the double bond into both extremes (C₆F₄ and COPh) of our fluorinated chalcone phosphines (Scheme 1).^9,14 The latter confirms the special pull–pull conjugation in our R-PEWO-F₄ ligands, which is presumably related to the solid-state luminescence reported herein, displayed by the free ligands and some of their metal complexes, clearly enhanced by fluorination in the EWO.

Scheme 1 Resonance forms contributing to the electron deficiency of the C=C double bond of the chalcone fragment.

Complexes trans-[PdCl₂(Lⁿ)₂] (1L^1–4) show monodentate P-coordination and E configuration of the non-coordinated olefin, similar to the free ligands (Fig. 1). These compounds were selectively prepared by the reaction of trans-[PdCl₂(NCMe)₂] with the appropriate PEWO in a Pd : Lⁿ = 1 : 2 ratio (Scheme 2). Conversely, the reaction using a Pd : L¹ = 1 : 1 ratio eventually leads to the formation of an E-chelated intermediate (E-2L¹), which subsequently isomerizes to the Z-chelated complex cis-[PdCl₂(L¹)] (Z-2L¹). The Z-configuration of Z-2L¹ eventually triggers a C–F activation process, which opens the gate to new reactivity.¹⁵

Scheme 2 Synthesis of trans-[MX₂(Lⁿ)₂] complexes 1Lⁿ (n = 1–4), Z-2L¹, 3L¹, 4L¹ and 5L¹.

Treatment of 1L¹ with KBr or AgCN yields trans-[PdBr₂(L¹)₂] (3L¹) and trans-[Pd(CN)₂(L¹)₂] (4L¹), respectively. The platinum compound trans-[PtCl₂(L¹)₂] (5L¹) was synthesized analogously to 1L¹ (Scheme 2). In all compounds (free ligands and trans-[MX₂(Lⁿ)₂] complexes), the uncoordinated olefin group has the E-EWO configuration. The only exception is the chelate complex [PdCl₂(Z-L¹)] (Z-2L¹), which is coordinated by the P atom and the double bond, which display a Z-EWO conformation. The X-ray structures of 1L¹ (Fig. 2),⁸Z-2L¹,⁸L²,⁹ and 1L⁴,¹⁶ are available in the literature, whereas those of ligand Ph-PEWO-F₄ (L¹) and complexes 1L² and 1L³ (Fig. S1, ESI†) are first reported herein.

Fig. 2 X-ray structures of 1L¹, 4L¹ and 5L¹. H atoms omitted for clarity. Relevant distances in Å: 1L¹ : Pd–Cl = 2.2903(7), Pd–P = 2.3231(7), C=C(olefin) = 1.314(4). 4L¹ : Pd–C = 1.990(4), Pd–P = 2.3388(10), C=C(olefin) = 1.320(6). 5L¹ : Pt-Cl = 2.3029(11), Pt–P = 2.3241(11), C=C(olefin) = 1.303(7).

Neither the free ligands nor their complexes display emitting behaviour in solution,¹⁷ whereas Table 1 presents the experimental data (excitation and emission maxima, average lifetimes, and quantum yield percentages) measured for the crystalline solids, where some remarkably intense emissions are observed. The emission spectra as well as the decay profiles are given in the ESI (Fig. S4–S13†).

Table 1 Excitation and emission data of ligands and complexes in the solid state. Instability of L³ under air (when non-coordinated) precludes its corresponding luminescence analyses

Entry/Comp.	λ exc (nm)^18	λ emis (nm)^a	Φ (%)	τ av (ns)^b
a Very broad emission bands. b τ av = (A1τ1^2 + A2τ2^2 + ⋯)/(A1τ1 + A2τ2 + ⋯).
1/L^1	413	606	10.1	2.5
2/L^2	460	530	4.1	3.5
3/L^4	472	552	<3	3.0
4/1L^1	405	649	28.0	1.3 × 10^3
5/1L^2	412	676	21.8	1.6 × 10^3
6/1L^3	405	643	15.6	1.4 × 10^3
7/1L^4	469	680	<3	6.4 × 10^2
8/Z-2L^1	Undetected emission
9/3L^1	415	666	15.7	5.1 × 10^3
10/4L^1	Undetected emission
11/5L^1	Undetected emission

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The fluorinated ligands L¹ and L² show yellow emission in the solid state upon UV irradiation, while the E-complexes 1L^1–3 and 3L¹ show eye-catching orange luminescence under similar conditions (vide infra, Fig. 3). However, the chelated Z-complex 2L¹, showing a drastic structural divergence, does not exhibit emissive properties. The lack of fluorination is clearly detrimental to the luminescence (see quantum yield percentages of L⁴ and 1L⁴ presented in Table 1), and more interestingly, both 4L¹ and 5L¹ complexes, which do not show any remarkable structural differences with 1L¹, are not luminescent. Fig. 2 compares those three X-ray structures, including relevant distances within the chalcone ring (i.e. –C=C–) and involving the metal centres.

Fig. 3 Emission spectra, with normalised intensity, of the fluorinated ligand L² (yellow line) and its PdCl₂ complex 1L² (red line); dashed lines represent the maximum of excitation (λ_exc in Table 1).¹⁸ Photographs of a sample in a mortar, taken upon UV irradiation, are also shown.

In the free phosphine ligands, the chalcone moiety is clearly affected by the orbital influence of the R substituents at the P atom, as supported by the variations in their λ_emis values (Table 1, entries 1–3). In the complexes, additional dependence on the MX₂ metal fragment at which phosphine is coordinated is obvious (Table 1, entries 4–11).

Concerning the free ligands, the emissive transitions are necessarily intraligand. The effect of R substituents at P, with different electronic and steric features, is appreciable. The highest maximum wavelength (λ_emis = 606 nm) is found for the fluorinated L¹ (R = Ph, entry 1). Note that the emission bands given in Table 1 are very broad and λ_emis is not enough to guess the colour (L¹ is yellowish upon irradiation too, see Fig. S3, ESI†). For the fluorinated L² (R = Cy, λ_emis = 530 nm, Fig. 3), the wavelength is closer to the value obtained for the non-fluorinated Ph-PEWO-H₄ ligand L⁴ (entry 3) than for L¹, as if the higher inductive effect of Cy acting as a σ-donor towards the P atom was compensating the electron-withdrawing effect of fluorination at the chalcone aryl group. Apparently, the two effects that should increase the σ-electron density at the chalcone ring (non-fluorination and inductive effect from R) diminish the quantum yield Φ values noticeably, to almost extinction in L⁴.

Remarkably, the colour change upon ligand coordination (yellow to orange) is linked to a substantially higher ν_exc − ν_emis difference (i.e. 9.5 × 10³ cm⁻¹vs. 2.9 × 10³ cm⁻¹ for L² and 1L² respectively) and longer lifetimes (in the order of μs for the complexes and ns for the ligands). Fig. 3 shows the comparison between 1L² and L². These observations suggest a luminescence switch upon coordination to PdCl₂, from fluorescence in the free ligands to phosphorescence in the complexes.¹⁹

Considering now the metal compounds, a remarkable increase in the quantum yield percentages (Φ > 15%, up to 28% for 1L¹), compared to the free ligands (Φ < 10%), is observed upon complexation to PdCl₂ in 1L^1–3 (Table 1, entries 4–6) or to PdBr₂ in the case of 3L¹ (Table 1, entry 9). In contrast, the non-fluorinated 1L⁴ complex (Table 1, entry 7) is scarcely luminescent. This observation supports the important effect of the fluorine substituents of the chalcone ring on the emissive properties.²⁰

In the case of the non-luminescent complexes 4L¹ and 5L¹, obviously their structural similarity with 1L¹ (Fig. 2) does not impede significant orbital differences when Pd or the halides are replaced by Pt or CN. The PdCl₂ (1L^1–3) and the PdBr₂ (3L¹) complexes have two medium-strength σ-donor and slightly π-donor halo ligands, and show similar λ_emis (nm) and Φ (%) values, as given in Table 1. Complex 4L¹ (Table 1, entry 10) has more σ-donor and very strongly π-acceptor cyano ligands. However, 5L¹ (Table 1, entry 11) has Pt replacing Pd, and the lanthanide contraction plus the relativistic effects in Pt produce large orbital energy differences between the Pd and Pt isoelectronic metal centres. Qualitatively, these two cases (4L¹ and 5L¹) are roughly similar, because both compounds are expected to have a metal centre greedier of electron density and, consequently, less prone to be polarized by π back-donation towards the phosphine ligand.

In order to identify the orbitals involved in the absorption and emission, as well as to support our qualitative hypotheses, DFT and TD-DFT calculations were carried out (see ESI for details†). We focused the analysis on the ligand L¹ and the complexes 1L¹, 4L¹, and 5L¹. As the emissions of L¹ and 1L¹ are only observed in the solid state, their X-ray structures are used as initial guesses for geometry optimizations in the gas phase, which led to minimal modifications of the solid structures.

The computed HOMO–LUMO gap fits well the experimental λ_exc measured for L¹ (λ_calc = 402 nm; λ_exp = 413 nm). The HOMO orbital is mainly located in the Ph₂P moiety and the LUMO is a chalcone orbital with high participation of fluorinated C atoms and those of the double bond (Fig. S17, ESI†). However, the computed wavelengths of the deexcitations from triplet states are far from the experimentally observed emission (L¹, λ_exp = 606 nm), discarding them to assign a phosphorescent behaviour. There is, however, a S₁ → S₀ transition from a singlet state, where slight structural bending of the chalcone group is suggestive of loss of sp² character of the double bond, which predicts an emission from π*(EWO) at λ_calc = 580 nm, much closer to the experimental value. Its fluorescent nature is strongly supported by the short lifetime of ca. 2 ns. In this transition, the participation of the R₂P fragment in the orbitals involved is significant (Table S12†). This is consistent with the experimentally observed effect of the R substituents at P.

The simulation of the absorption spectra allows us to assign the electronic transition from HOMO-2 to the LUMO orbital as responsible for the excitation maxima observed for 1L¹. The calculated transition (λ_calc = 395 nm) matches reasonably well the experimentally observed λ_exc maximum (405 nm, Table 1, entry 4). The contribution of the different molecular moieties is 58% Ph₂P + 22% PdCl₂ + 20% EWO for HOMO-2 (Fig. S18†) and 42% PdCl₂ + 38% EWO + 20% Ph₂P for LUMO (Fig. 4). Both orbitals are symmetrical, with equal contribution of the two ligands. Remarkably, similar transitions (HOMO-2 to LUMO) are computed to be markedly more energetic either in the Pt analogue 5L¹ (λ_calc = 349 nm) or in the Pd(CN)₂ complex 4L¹ (λ_calc = 333 nm), confirming the qualitative interpretation of similar behaviour of these two complexes (see Fig. S19 and S20 for MOs†).

Fig. 4 FMOs of 1L¹ (top), 5L¹ (middle) and 4L¹ (bottom). Contributions (%) of the fragments are included, with the main participation in each case highlighted in bold. Relevant metal atom contributions (higher than 10%) are also shown.

Using TD-DFT calculations, the experimentally observed emission for 1L¹ (λ_emis = 649 nm, Table 1, entry 4) is assigned to the computed monodeexcitation from the first triplet state (T₁), with a much similar wavelength (λ_calc = 667 nm, Table S9, ESI†). This emission band mainly involves the LUMO → HOMO phosphorescent transition. This is perfectly consistent with the high ν_exc − ν_emis difference and the long lifetime values provided in Table 1 and commented previously. It is worth remarking that geometry relaxation was discarded due to the very limited deformability allowed in the solid state.²¹ Moreover, the fluorescent emissions as well as other potentially phosphorescent ones were also computed and then discarded because of numerical results far from the experimental data (details in ESI†).

Fig. 4 gathers the frontier molecular orbitals (FMOs) of 1L¹, 4L¹ and 5L¹ for comparison. Evidently, deexcitation from the LUMO (42% PdCl₂, 38% EWO, 20% Ph₂P) to the PdCl₂ HOMO (Fig. 4, top), responsible for the orange-emitting behaviour of 1L¹, is drastically affected either in the PtCl₂ or in the Pd(CN)₂ analogues. The computed phosphorescent emissions (decay from triplet states) become experimentally unobservable for 4L¹ and 5L¹ (Tables S10 and S11 respectively†), as a direct consequence of the changes in the FMOs. For 5L¹ (Fig. 4, middle), the PtCl₂ HOMO resembles its PdCl₂ analogue but the contribution of the PtCl₂ fragment in the LUMO drops to 7% (vs. 42% for 1L¹) and the deexcitation from the first triplet is computed to be much more energetic (i.e. 555 nm). The effect of Cl by CN substitution in 4L¹ FMOs is also clear (Fig. 4, bottom), with reduced participation of the Pd(CN)₂ fragment in the LUMO of 4L¹ (casually almost identical to the PtCl₂ compound) and negligible in the HOMO.

After the identification of the key orbitals involved in the luminescent behaviour (with relevant participation of the PdCl₂ fragment), the data provided in Table 1 can be better rationalized: (i) both the PdBr₂ substitution in 3L¹ (Br is electronically quite similar to Cl) or the R₂P modification in 1L² and 1L³ move in the range of variations of orbital polarization; (ii) totally different emission wavelengths must be expected from the Pd(CN)₂ (4L¹) or PtCl₂ (5L¹) congeners, with negligible contribution of the MX₂ moieties in their LUMOs. In fact, these happen not to be experimentally detectable. While many examples of either MLCT (MLCT = Metal to Ligand Charge Transfer) or ILCT (IL = Intra-Ligand Charge Transfer) emitting metal compounds can be found in the literature; in our system, only ML_MCT processes (L_M = Metal-perturbed Ligand-centered LUMO)²² lead to observable phosphorescence.

In summary, we reported herein a family of yellow fluorescent 1-(PR₂),2-(CH=CH–C(O)Ph)-C₆F₄ chalcone-derived phosphines (Lⁿ) and some intensely orange phosphorescent trans-[PdX₂Lⁿ₂] palladium(II) complexes (X = Cl and Br) with them displaying quantum yields (Φ) above 15%. The luminescence is only observed in the solid state and is enhanced by the fluorination in chalcone, both in the ligand and in the PdCl₂ complex. Using TD-DFT calculations, the emission of the free Ph-PEWO-F₄ ligand (L¹) is assigned to the S₁ → S₀ transition presumably associated with a loss of sp² character of the olefinic C atoms. Computational studies reproduce the excitation and emission data of the PdCl₂ compound 1L¹ satisfactorily and reveal the decisive involvement of the LUMO orbital, which features a clear metal/chalcone connection, in the luminescent behaviour. Conversely, the non-emitting Pd(CN)₂ and PtCl₂ analogues, structurally identical to 1L¹, display LUMO orbitals with scarce participation of the metal centres, supporting the synergistic contributions of both the MX₂ core and the chalcone skeleton in the emissive properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Spanish MCINN (Projects PID2020-118547GB-I00 and PGC 2018-093863-B-C21) for the funding provided and both Eric Mates-Torres and Marta Mansilla (UBU) for help. J. P.-de-L. also acknowledges MCINN for a FPI studentship (BES-2017-080726). M. N. P-D. thanks the UVa for a Margarita Salas postdoctoral fellowship (ref. CONVREC-2021-221).

References

1. (a) C. Zhuang, W. Zhang, C. Sheng, W. Zhang, C. Xing and Z. Miao, Chem. Rev., 2017, 117, 7762–7810; (b) N. A. A. Elkanzi, H. Hrichi, R. A. Alolayan, W. Derafa, F. M. Zahou and R. B. Bakr, ACS Omega, 2022, 7, 27769–27786; (c) S. L. Gaonkar and U. N. Vignesh, Res. Chem. Intermed., 2017, 43, 6043–6077.
2. K. G. Komarova, S. N. Sakipov, V. G. Plotnikov and M. V. Alfimov, J. Lumin., 2015, 164, 57–63.
3. S. Kagatikar and D. Sunil, Chem. Pap., 2021, 75, 6147–6156.
4. P. Mahesha, N. S. Shetty and S. D. Kulkarni, J. Fluoresc., 2022, 32, 835–862.
5. X. Cheng, F. Yang, J. Zhao, J. Ni, X. He, C. Zhou, J. Z. Sun and B. Z. Tang, Mater. Chem. Front., 2020, 4, 651–660.
6. L. J. Gomes, T. Moreira, L. Rodriguez and A. J. Moro, Dyes Pigm., 2022, 197, 109845.
7. A.-T. Mohammad and W. R. Abbas, RSC Adv., 2021, 11, 38444–38456.
8. E. Gioria, J. M. Martinez-Ilarduya, D. Garcia-Cuadrado, J. A. Miguel, M. Genov and P. Espinet, Organometallics, 2013, 32, 4255–4261.
9. M. N. Peñas-Defrutos, A. Vélez, E. Gioria and P. Espinet, Organometallics, 2019, 38, 4701–4707.
10. X. Luo, H. Zhang, H. Duan, Q. Liu, L. Zhu, T. Zhang and A. Lei, Org. Lett., 2007, 9, 4571–4574.
11. E. Gioria, J. del Pozo, J. M. Martínez-Ilarduya and P. Espinet, Angew. Chem., Int. Ed., 2016, 55, 13276–13280.
12. E. Gioria, J. del Pozo, A. Lledós and P. Espinet, Organometallics, 2021, 40, 2272–2282.
13. J. Ponce-de-León and P. Espinet, Chem. Commun., 2021, 57, 10875–10878.
14. A. Roque, J. C. Lima, A. J. Parola and F. Pina, Photochem. Photobiol. Sci., 2007, 6, 381–385.
15. M. N. Peñas-Defrutos, A. Vélez and P. Espinet, Organometallics, 2020, 39, 841–847.
16. H. Zhang, X. Luo, K. Wongkhan, H. Duan, Q. Li, L. Zhu, J. Wang, A. S. Batsanov, J. A. K. Howard, T. B. Marder and A. Lei, Chem. – Eur. J., 2009, 15, 3823–3829.
17. For an example of luminescent chalcones in the solid state but not in solution see: L. Zhang, J. Liu, J. Gao, R. Lu and F. Liu, RSC Adv., 2017, 7, 46354–46357.
18. Excitation spectra of metal complexes are scarcely informative (Fig. S14 and S15†). λ_exc is assigned as the wavelength that maximizes the emission intensity.
19. J. M. Forward, D. Bohmann, J. P. Fackler Jr. and R. J. Staples, Inorg. Chem., 1995, 34, 6330–6336.
20. For a different case see: J. Ponce-de-León, R. Infante and P. Espinet, Chem. Commun., 2021, 57, 5458–5461.
21. Reoptimization of both S₁ and T₁ states led to profound structural change, presumably not accessible in the crystal.
22. This notation has been recently used in: T. Theiss, S. Buss, I. Maisuls, R. López-Arteaga, D. Brünink, J. Kösters, A. Hepp, N. L. Doltsinis, E. A. Weiss and C. A. Strassert, J. Am. Chem. Soc., 2023, 145, 3937–3951.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization of the complexes including NMR spectra, computational details and X-ray data. CCDC 2205013–2205017. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt00408b

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