A colorimetric/luminescent benzene compound sensor based on a bis(σ-acetylide) platinum(II) complex: enhancing selectivity and reversibility through dual-recognition sites strategy

Abstract

A square-planar bis(σ-acetylide) Pt(II) complex containing dual-recognition sites was designed, synthesized and used as a colorimetric/luminescent sensor for detecting the vapor of benzene compounds.

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Benzene and its homologues are a class of highly volatile and harmful materials that can cause many human diseases, such as acute and chronic lymphocytic leukaemia, non-Hodgkin's lymphoma and multiple myeloma through inhalation or dermal absorption.¹ The detection of benzene compound vapors in the environment, indoor residential, and workplaces is very important. Traditional methods to detect these harmful vapors usually involve the use of expensive and complicated equipment, such as gas chromatography,² sensor devices based on fiber-optic devices,³ diode laser IR absorption spectroscopy,⁴ cross-reactive array electronic-nose sensing,⁵ and metal oxide semiconductor films.⁶ Currently the development of convenient and high performance methods for detecting these compounds is still a great challenge.

Recently vapor sensors based on luminescent materials have received more and more attention.⁷ This kind of sensor displays a concurrent, selective, reversible and naked-eye perceivable luminescence change upon exposure to specific volatile organic compounds and has been proved to be one of the most effective, cheap and convenient methods in the detection of harmful vapors.⁸ Although the research on it has achieved great progress, significant challenge still exists in detection of benzene compounds. So far, only a few examples based on Ru(II),⁹ Zn(II),¹⁰ Au(I),¹¹ Ag(I),¹² Pt(II)¹³ and Cu(I)¹⁴ complexes have been reported that could detect vapors of benzene compounds. However, in most cases, benzene compounds molecule just lies in the crystal voids instead of forming effective interaction with the chromophore which results in bad selectivity, low sensitivity and/or reversibility of the sensors. Consequently, the rational design of a chromophore exhibiting stronger interactions with guest molecules is crucial for the preparation of benzene compounds sensor with better performance.

Since the pioneering work of Che et al. about diimine platinum(II) bis(σ-acetylide) complexes¹⁵ and their tunable luminescence properties triggered by VOCs, the complexes and their derivatives have been successfully used in detection of various VOC molecules through strong hydrogen bond interaction by their H-bond recognition site (locates between the two acetylene auxiliary ligands), resulting in different packing structures and intermolecular Pt–Pt interaction which eventually lead to dramatic changes in color and/or emission.^8d,16 However, the solely H-bond recognition site present in almost all of the complexes prevent them to recognize and detect aromatic compounds efficiently owning to the lack of strong π⋯π stacking interactions which usually involve in the recognition of such VOCs. Thus the introduction of π–π recognition site into bis(σ-acetylide) platinum(II) system is important to improve their selectivity for benzene compounds.

Based on the above considerations, herein we report the design and synthesis of a new bis(σ-acetylide) platinum(II) complex [Pt(TMSC≡CPhenC≡CTMS) (C≡CC₆H₄–Cl-4)₂] (1) based on 3,8-bis(trimethylsilylethynyl)-1,10-phenanthroline (TMSC≡CPhenC≡C TMS) bearing bigger π-electronics group (Scheme 1). 1 is characterized by two potential host–guest recognition sites, H-bond site that locates between two phenylacetylene auxiliary ligands and π–π stacking site situated above the phenanthroline aromatic rings. On one hand, 1 could use the two sites to selectively recognize different guest molecules. On the other hand, the newly imported π–π stacking site could provide strong π–π interactions for aromatic molecules, which enables 1 to be a potential sensor for detection of benzene compounds vapors. Detailed property study indicates the dual-recognition sites design strategy does work. 1 is an excellent common benzene compounds sensor and exhibits drastic change from yellow to red both in color and luminescence upon exposure to vapor of benzene compounds, and subsequently can be recovered by CH₂Cl₂ vapor.

Scheme 1 Molecular structure and two host–guest recognition sites of complex 1.

1 was easily obtained in high yield by the reaction of precursor Pt(TMSC≡CPhenC≡CTMS)Cl₂ and 4-chlorophenylacetylene according to Sonogashira's method (CH₂Cl₂/^i-Pr₂NH/CuI) and fully characterized by ¹H NMR, ESI-MS, IR spectroscopy. 1 can dissolve in common organic solvents and exhibits excellent stability in both solution and solid state. Furthermore, 1 presents good thermal stability up to 280 °C (Fig. S1†).

Recrystallization of 1 in CH₂Cl₂ and benzene lead to yellow 1·0.5(CH₂Cl₂) and red 1·0.5(benzene) crystals respectively.‡ Single crystal X-ray analysis reveal that platinum atom exhibits a distorted C₂N₂ square-planar configuration in both structures (Fig. 1 and Table S1†). In 1·0.5(CH₂Cl₂), solvate CH₂Cl₂ molecule is captured by the H-bond site and combines with platinum moiety through multi-hydrogen bonds (Fig. 1b, S2 and Table S2†). Two neighboring platinum(II) moieties display an antiparallel stacking pattern and have a severe slide from one another due to the steric hindrance of CH₂Cl₂ molecule (Fig. 1a and S2†). Although the corresponding Pt⋯Pt separation is 4.3805(5) Å and there is no metallophilic interaction, strong C–H⋯π interactions between the two moieties still connect them into a dimmer (Fig. S3 and Table S2†). In contrast, in 1·0.5(benzene) two neighboring platinum(II) moieties are arranged in closer proximity with Pt⋯Pt distance of 3.5107(6) Å (Fig. 1a). The results of Wiberg bond indices (WBIs) calculation of Pt–Pt contact also reveal a stronger bond order in 1·0.5(benzene) (0.0968) relative to that in 1·0.5(CH₂Cl₂) (0.0169), implying the existence of weak Pt–Pt interaction in 1·0.5(benzene).^8d,15 The Pt–Pt interaction and strong π⋯π stacking interactions between alkynyl groups from phenylacetylene and aromatic rings from phenanthroline, connect two neighboring platinum(II) moieties into a dimmer (Fig. S4 and Table S2†). Solvate benzene molecule is situated in the π–π stacking site between the two phenanthroline groups from two neighboring dimmers and contacts with the groups through face-to-face π–π stacking interactions (Fig. 1b, S5 and Table S2†). The intra- and inter-dimmer separations of 1·0.5(benzene) are 3.35 and 6.66 Å respectively, compared with 3.34 and 3.55 Å in 1·0.5(CH₂Cl₂). The different packing structures in both crystals demonstrate that in the solid state π–π stacking site has a high affinity for benzene, which make 1 a possible vapor sensor for detection of benzene compounds.

Fig. 1 (a) Vertical view of the dimmer structure in 1·0.5(CH₂Cl₂) and 1·0.5(benzene), showing the antiparallel packing patterns in both structures. (b) Side view of the stacking structures in 1·0.5(CH₂Cl₂) and 1·0.5(benzene), showing the solvate molecules recognized by recognition sites.

Fig. 2 gives the absorption and emission spectra for solid-state 1, crystalline 1·0.5(CH₂Cl₂) and 1·0.5(benzene). 1 exhibits high-energy absorption bands due mainly to intraligand transitions, and broad, low-energy absorption bands at 437–532 nm tailing to 680 nm. 1·0.5(CH₂Cl₂) has a similar absorption spectrum. DFT calculation shows its broad, low-energy absorption bands at 429–525 nm tailing to 650 nm can be attributed to a mixture of ¹LLCT and ¹MLCT transitions (Fig. S6 and S7 and Table S3 and S4†). In contrast, the low-energy absorption bands of 1·0.5(benzene) show a distinct redshift to 461–542 nm extending to 780 nm. Such redshift is also correlated with the shortened Pt⋯Pt distance (the increased Pt–Pt interaction). Under irradiation at λ_ex = 350–500 nm, 1 shows a dark-yellow emission peaked at 546 and 585 nm with a shoulder at 629 nm. Crystalline 1·0.5(CH₂Cl₂) exhibits a bright yellow luminescence peaks at 544 and 582 nm with a lifetime of 1.152 μs at ambient temperature. Vibronic-structured emission bands in both solids with the vibrational progressional spacing of 1200 cm⁻¹ is typical for the C=C and C=N stretching frequency of the aromatic rings in phenanthroline, suggesting the involvement of TMSC≡CPhen C≡CTMS in the emission state. Therefore, this yellow luminescence can be assigned to an admixture of ³MLCT and ³LLCT triplet transitions, as supported by TD-DFT studies (Fig. S7 and Table S3 and S4†). In contrast, excitation of 1·0.5(benzene) by near-UV light at room temperature leads to a broad, structureless emission centered at 638 nm with a lifetime of 416 ns. The dramatic red-shift in emission spectra and distinct peak shape strongly suggest different emission mechanism of 1·0.5(benzene). Considering the weak Pt–Pt interaction in 1·0.5(benzene), the red emission can be attributed to ³MMLCT triplet transition which has smaller HOMO–LUMO energy gap than that of the corresponding ³MLCT transition, as supported from DFT studies (Fig. S8 and S9 and Table S5 and S6†).

Fig. 2 UV-vis absorption spectra (solid) and emission (dash dot) spectra of solid state 1, crystalline 1·0.5(CH₂Cl₂) and 1·0.5(benzene).

To examine the color and luminescence responses to VOCs, 1 is deposited on quartz slide and then exposed to various saturated VOC vapors at ambient temperature and the result is shown in Fig. 3a. Remarkably, 1 is only sensitive to benzene compounds, including benzene, toluene, chlorobenzene, nitrobenzene, xylene, para-xylene and ortho-xylene, and presents red color under ambient light and red luminescence under UV light (365 nm) respectively. The corresponding emission spectra have a strong unstructured band centered in the range of 634–654 nm (Fig. 3b). In contrast, other VOCs lead to only yellow color and yellow emission. The corresponding emission spectra have vibronic-structured emission bands at 544 and 582 nm, similar to that of solid 1·0.5(CH₂Cl₂) (Fig. 3b). Such distinct changes in color and luminescence of 1 upon exposure to different VOCs reveal that it has an excellent selectivity in detection of benzene compounds.

Fig. 3 (a) Photographic images of 1 deposited on quartz slide upon exposure to various VOC vapors under ambient light and UV light irradiation (365 nm). (b) Solid-state emission spectra of 1 upon exposure to various VOC vapors at ambient temperature.

In order to explore the reversibility of 1 as a benzene compounds sensor, the dynamic process in a vapor absorbing cycle was monitored by emission spectra. Fig. 4 shows the emission spectral changes of 1 in response to CH₂Cl₂ or benzene vapor in the reverse process. When the solid sample of 1·0.5(CH₂Cl₂) was exposed to benzene vapor, the vibronic-structured emission bands at 544 and 582 nm were weakened gradually and a broad, unstructured emission band centered at 638 nm was observed and enhanced progressively. Meanwhile, the color of the sample changes gradually from yellow to red. Powder XRD experiments (Fig. 5) indicate during the process the patterns of 1·0.5(CH₂Cl₂) were gradually weakened whereas the patterns of 1·0.5(benzene) were progressively enhanced. Within several minutes 1·0.5(CH₂Cl₂) is completely transformed into 1·0.5(benzene). Once the resulted red phase was exposed to CH₂Cl₂ vapor, the broad unstructured emission band could be reverted to the original vibronic-structured emission bands and the color also changed from red to yellow gradually, corresponding to the transformation of 1·0.5(benzene) to 1·0.5(CH₂Cl₂), as revealed by XRD patterns. The response time of a process would take several dozen seconds to a few minutes depending on the thickness of the sample. Therefore, the color and luminescence changes of 1 in response to VOCs is fully reversible.

Fig. 4 Emission spectral changes of solid state 1·0.5(CH₂Cl₂) in response to benzene vapor (left) and 1·0.5(benzene) in response to CH₂Cl₂ vapor (right).

Fig. 5 XRD patterns recorded in a vapor absorbing cycle process, 1·0.5(CH₂Cl₂) ⇆ 1·0.5(benzene), showing dynamic variations of XRD patterns from (a)–(c) in the process 1·0.5(CH₂Cl₂) → 1·0.5(benzene) by exposure of 1·0.5(CH₂Cl₂) to benzene vapor, and the XRD patterns from (c)–(f) in the reverse process 1·0.5(benzene) → 1·0.5(CH₂Cl₂) by exposing 1·0.5(benzene) to CH₂Cl₂ vapor at ambient temperature.

All the above experiments indicate 1 can use H-bond and π–π stacking recognition sites to selectively and reversibly recognize CH₂Cl₂ and benzene compounds vapors respectively. The strength of weak interactions of the two recognition sites and their favorite guest molecules should be close. Thus the two phases, 1·0.5(CH₂Cl₂) and 1·0.5(benzene), can be conversed reversibly into each other in the presence of excess benzene or CH₂Cl₂ vapors respectively, accompanying with adjustment of Pt⋯Pt separation inside the dimmers, and finally dramatic color and luminescence change. Remarkably, no detectable degradation was observed even after twenty vapor absorbing cycles (Fig. S10†), indicating that 1 has good reversibility and reproducibility in sensing benzene compounds.

Several platinum(II) bis(σ-acetylide) complexes based on 1,10-phenanthroline and its derivatives had been published, however, their sensing property to benzene compounds were not reported.^16b,17 In order to check whether dual-recognition sites is a sufficient condition for the Pt(II) complex to sense benzene compounds, we synthesized one of the such compounds (Pt(phen)C≡CC₆H₄) and used it as an example to check the response. Interestingly, no changes in color or luminescence were observed when it was exposed to vapors of different benzene compounds. A detailed comparison of structures of the compound and 1 indicates the former exhibits a layer packing structure with strong and complicated hydrogen bond networks between adjacent platinum moieties. In contrast, 1 adopts a column packing structure with simple and weaker hydrogen bonds between adjacent molecules which allow the π–π interactions between aromatic guest molecule and phen unit of the Pt(II) molecule to play an efficient role to change the packing structure. Thus the two kinds of weak interactions, namely hydrogen bond and π–π interaction, being close to each other in strength is another determining factor to affect the recognition function.

Conclusions

In conclusion, a new vapochromic and vapoluminescent platinum(II) complex containing dual-recognition sites for detection of benzene compounds vapor was designed and synthesized. Compared with other reported benzene compounds vapor sensors,^9–14 1 has obvious advantages of concurrent, selective, naked-eye perceivable, sensitive, reversible, reproducible, and easy performance. Thus the dual-recognition sites strategy opens up fresh opportunities to design new benzene compounds vapor sensors with better performance.

Acknowledgements

This work was supported by the NSFC (No. 21101020, 21071025 and 21471024) and the Fundamental Research Funds for the Central Universities (No. DUT15LK20 and DUT15ZD118).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Detailed synthetic procedures, TGA diagram, tables and figures giving DFT calculation data, crystal data and additional crystallographic diagrams. CCDC 1055045 and 1055046 . For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra13987b

‡ Crystal data for 1·0.5(CH₂Cl₂): C_38.50H₃₃Cl₃N₂PtSi₂, M = 881.29, monoclinic, a = 7.1264(4) Å, b = 38.160(2) Å, c = 14.3796(10) Å, β = 96.337(4)°, V = 3886.6(4) ų, T = 296(2) K, space group P2₁/c, Z = 4, 30 480 reflections measured, 6824 independent reflections (R_int = 0.037). The final R₁ values were 0.0371 (I > 2σ(I)). The final wR(F²) values were 0.0974 (I > 2σ(I)). The final R₁ values were 0.0484 (all data). The final wR(F²) values were 0.1029 (all data). Crystal data for 1·0.5(benzene): C₄₁H₃₅Cl₂N₂PtSi₂, M = 877.88, monoclinic, a = 20.638(3) Å, b = 10.9657(16) Å, c = 19.312(3) Å, β = 103.687(7)°, V = 4246.3(10) ų, T = 296(2) K, space group P2₁/c, Z = 4, 25 213 reflections measured, 6879 independent reflections (R_int = 0.082). The final R₁ values were 0.0691 (I > 2σ(I)). The final wR(F²) values were 0.1468 (I > 2σ(I)). The final R₁ values were 0.1326 (all data). The final wR(F²) values were 0.1780 (all data).

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