Photoactivable malachite green-based alkynylplatinum(II) 2,6-bis(N-alkylbenzimidazol-2-yl)pyridine complexes

Abstract

Photo-responsive malachite green moieties have been incorporated into an alkynylplatinum(II) bzimpy system. The photo-caged complexes in acetonitrile solutions exhibit self-assembly properties modulable by photo-removal of the cyano protecting group. Distinct aggregate morphologies, which are facilitated by the non-covalent metal–metal and π–π stacking interactions, have been observed before and after photo-irradiation.

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Malachite green (MG), a photo-responsive dye with a triphenylmethane structure, has received considerable attention in both fundamental¹ and applied research areas.^2–5 Dissolution of malachite green leucocyanide gives a colorless solution, and it becomes deep green upon exposure to UV irradiation.^1a The exclusion of the cyano group and the formation of tertiary carbocations are reported to be due to the photo-induced heterolytic bond cleavage,^1,2 leading to the formation of the colored ionized form. Owing to its drastic responses to UV exposure, malachite green leucocyanide (MGCN) has been used as the actinometer for UV light.¹ In addition, the generation of an extra positive charge and the interplay of amphiphilicity in the MG-based compounds have resulted in the exploration of metal ion-complexation studies,³ micelle formation,^2a,4 self-assembled monolayers^2b and polymeric super-amphiphiles.⁵

Square-planar d⁸ platinum(II) polypyridine complexes have been well reported to exhibit directional metal–metal interactions and/or π–π stacking interactions, giving rise to the intriguing spectroscopic properties and luminescence enhancement.^6–9 With the early report on the solution-state self-assembly based on a cationic alkynylplatinum(II) terpyridine complex, [Pt(terpy)(C[triple bond, length half m-dash]C–C[triple bond, length half m-dash]CH)]⁺,^9a various investigations on the utilization of Pt⋯Pt and/or π–π stacking interactions to probe the changes in the microenvironments such as pH,^9d temperature,¹⁰ and the addition of charged polymers^9b,11 have been reported. More recently, another series of platinum(II) 2,6-bis(benzimidazole-2-yl)pyridine (bzimpy) complexes has also been shown to exhibit interesting Langmuir–Blodgett properties,¹² vapochromic behaviors,¹³ metallogel formation,¹⁴ unusual memory behaviors¹⁵ and the formation of double complex salt (DCS) associated with the switching on of the non-covalent Pt⋯Pt and π–π stacking interactions,¹⁶ as well as aromatic donor–acceptor pair interactions.¹⁷ The exploration of novel water-soluble amphiphilic anionic platinum(II) complexes has opened up a new research focus on tailoring well-defined aggregate morphologies in aqueous media.¹⁸

In light of the unique square-planar geometry of platinum(II) complexes and the linear alkynyl ligand with high rigidity, the incorporation of photo-responsive groups into the platinum(II) polypyridine framework could result in intriguing self-assembly properties that could be modulated via photo-irradiation. Furthermore, it is worth noting that the incorporation of photoactivable groups into the platinum(II) system could give rise to interesting spectroscopic properties and distinct aggregate morphologies that the pure organic moieties could not achieve. Herein, the design, synthesis and characterization of two MGCN-based alkynylplatinum(II) 2,6-bis(N-decylbenzimidazol-2-yl)pyridine (bzimpy) complexes are reported. The photolysis of the MGCN moiety has been monitored using spectroscopic methods and the differences in morphologies formed by the complexes have been probed by means of microscopic experiments. Taking advantage of the change in the geometry and the net charge after photo-removal of the cyano group in the MGCN moiety, the self-assembly properties in the platinum(II) complexes could be photo-modulated. The current study could provide important insights into the self-assembly behaviors of platinum(II) complexes modulated by external stimuli and the future developments of photo-triggered supramolecular architectures.

The MGCN-based alkynylplatinum(II) bzimpy complexes 1 and 2 (Fig. 1) were synthesized¹⁹ (Schemes S1 and S2, ESI†) and characterized by ¹H NMR spectroscopy, high-resolution electrospray ionization (HR-ESI) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, and elemental analysis. The MGCN moiety is attached to the alkynyl ligand without and with a non-conjugated spacer for 1 and 2, respectively.

Fig. 1 Structures of MGCN-based alkynylplatinum(II) bzimpy complexes 1 and 2.

The electronic absorption spectra of 1 and 2 in MeCN at 298 K displayed intense high-energy absorption bands at 270–400 nm and less intense low-energy absorption bands at 400–500 nm (Fig. S1 and Table S1, ESI†). The bands at ca. 275, 350, and 450 nm were tentatively assigned to the intraligand (IL) [π → π*] transitions of the MGCN-based alkynyl ligands, the IL [π → π*] transitions of the bzimpy ligands, and the admixture of the metal-to-ligand charge transfer (MLCT) [dπ(Pt) → π*(bzimpy)] and ligand-to-ligand charge transfer (LLCT) [π(alkynyl) → π*(bzimpy)] transitions, respectively.^14b,18a,19 Upon increasing the concentration of 1 or 2, the absorption tail at ca. 515 nm was found to deviate from Beer's Law (Fig. S2 and S3, ESI†), which implied the formation of ground-state aggregations. The low-energy absorption tails were assigned to metal–metal-to-ligand charge transfer (MMLCT) transitions associated with intermolecular Pt⋯Pt and π–π stacking interactions. Upon photo-irradiation at 254 nm, 1 displayed a color change from yellow to green. The UV-visible spectral changes showed a drop in the absorption band at 275 nm and a growth of the low-energy absorption bands at ca. 315, 485, and 620 nm with an isosbestic point at 300 nm, which indicated a clean conversion from the neutral MGCN to the cationic MG form^2a,3a,5 (Fig. 2a). Likewise, 2 showed a drop in the absorption band at 275 nm and a growth of the low-energy absorption bands at ca. 350, 445, and 625 nm with an isosbestic point at 295 nm (Fig. S4, ESI†). Similar absorption features were also observed in the spectra of L1 and L2 under the same experimental conditions, confirming the successful photolysis of the MGCN moiety for their corresponding complexes 1 and 2 (Fig. S5 and S6, ESI†). The cations were stable for at least 48 hours under ambient conditions (Fig. S7 and S8, ESI†). With reference to the reported molar extinction coefficient of MG,^1d the percentage conversion of the photolysis was determined to be 57% and 61% for 1 and 2, respectively. The Fourier-transform infrared (FT-IR) spectra of 1 and 2 in MeCN at 298 K showed strong bands of C[double bond, length half m-dash]C stretching mode assignable to the aromatic rings at 1550 cm⁻¹ (Fig. 2b and Fig. S9, ESI†).^20a After photo-irradiation at 254 nm, the spectra showed an additional band at 1712 and 1710 cm⁻¹ for 1 and 2, respectively, characteristic of the C[double bond, length half m-dash]N stretching mode of the MG moiety,²⁰ which were consistent with the UV-visible spectral changes observed in the photolysis studies. The photo-irradiated 1 and 2 were also subsequently subjected to MALDI mass spectrometry. In addition to the [M–PF₆]⁺ molecular ion peak at m/z 1165 and m/z 1345, a new set of signals were observed at m/z 1139 and m/z 1319 for 1 and 2, respectively, which correspond to the molecular ion peaks of [M–CN–PF₆]⁺ after the removal of the cyano group, suggesting that photolysis has taken place (Fig. S10 and S11, ESI†). Attempts were also made to probe the photolysis by ¹H NMR spectroscopy, but the difference before and after photo-irradiation was insignificant, which was probably due to the low sensitivity of the chemical shifts towards the change from MGCN to MG (Fig. S12, ESI†).^1c

Fig. 2 (a) UV-visible spectral changes of 1 in MeCN upon photo-irradiation at 254 nm ([1] = 1.23 × 10⁻⁵ M). Spectra were taken at time intervals of 60 s. Inset shows the colour change of the solution. (b) FT-IR spectra of 1 before and after photo-irradiation at 254 nm for 15 min.

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been employed to image the morphologies of the aggregates formed by the complexes (Fig. 3 and Fig. S13–S17, ESI†). Before photo-irradiation, the TEM images of 1 displayed some small nanorods with a diameter of ca. 100 nm and a length of ca. 800 nm (Fig. 3a and Fig. S13a, ESI†), while the TEM images of 2 showed a network of aggregates with worm-like structures (Fig. S14, ESI†). The observed morphologies were in line with the designed molecular structure, as one could expect that 2 with a non-conjugated and flexible hydrocarbon spacer would pack in a less well-defined manner. Upon prolonged photo-irradiation at 254 nm, the TEM images of 1 displayed a network of larger nanorods with sharper edges, suggesting a better alignment of the molecular nanoaggregates (Fig. 3b and Fig. S13b, ESI†). The diameter was found to be ca. 300 nm and the length was found to be up to a few micrometers. The SAED experiment revealed the crystalline nature of the aggregates of 1, indicating a highly-ordered packing of the molecules in the aggregate state (Fig. 3c). The d-spacing parallel to the long axis of the rod was found to be 23.9 Å and the d-spacing perpendicular to the long axis of the rod-like aggregates was found to be 3.50 Å. Similar nanorods were observed in the corresponding SEM experiment (Fig. S15, ESI†). This could be attributed to the planarization and the extra positive charge of the alkynyl ligand that could lead to more extensive one-dimensional molecular stacking and result in the formation of more highly-ordered self-assembled nanostructures upon photolysis. In sharp contrast to 1, the TEM images of 2 revealed that the majority of the aggregates were resembling the observed worm-like aggregates, and only some newly formed spherical aggregates were identified after UV irradiation (Fig. S16a, ESI†). The SAED experiment of the spherical aggregates in 2 showed no signal, which could be due to the poor crystallinity and/or the possible amorphous nature, while that of the worm-like aggregates gave some diffused rings, with a d-spacing of ca. 3.49 Å (Fig. S16b, ESI†). Similar observations were found in the corresponding SEM experiment (Fig. S17, ESI†). The relatively small morphological changes in 2 upon UV irradiation could be attributed to the longer distance between the MG moiety and the platinum(II) center. Thus, the overall distortion caused by the photolysis would be minimized, leading to a minute change in the self-assembled aggregates. As for L1 and L2, there are no observable nanoaggregates before and after UV irradiation in the electron microscopy studies.

Fig. 3 TEM images of the aggregates prepared from an MeCN solution of 1 (a) before and (b) after UV irradiation. (c) The SAED pattern of the rod-like aggregates and (d) the corresponding TEM image ([1] = ca. 1 × 10⁻³ M).

Given the relatively significant changes of the aggregate morphologies of 1 upon photo-irradiation, X-ray diffraction experiments were performed for 1 in a cast film prepared in the MeCN solution before and after UV irradiation (Fig. S18, ESI†). The d-spacings of 22.63, 16.17, 11.17 and 8.14 Å were found to be in a ratio of ca. 1 : √2 : √4 : √8, characteristic of the formation of tetragonal packing.²¹ It was also found to exhibit a broad diffraction peak at 2θ = 22.02° (d-spacing = 4.00 Å), suggesting the possible involvement of hydrophobic–hydrophobic interactions.²² The X-ray diffraction (XRD) patterns of the samples prepared before and after UV irradiation were found to be similar, implying that the overall packing structures had no significant changes. This could be explained by the fact that molecules have much restricted degrees of freedom in cast films than in solution. In addition, in the 2D NOESY NMR spectrum of 1 in [D₃]-MeCN solution, there were cross peaks between the alkyl chains on the bzimpy ligand and the NMe₂-substituted phenyl group on the MGCN moiety (Fig. S19a, ESI†). These cross-peaks were absent in the corresponding 2D COSY NMR spectrum (Fig. S19b, ESI†), indicating the signals with intermolecular nature and the plausible head-to-tail conformation of 1 in the solution state. After prolonged photo-irradiation at 254 nm, the above-mentioned cross-peaks disappeared (Fig. S20, ESI†), which prompted computational studies on its molecular packing.

The structures of the model complex 1′ and its photocleaved form 1′′ were optimized using density functional theory (DFT). To simplify the calculation, the decyl groups of 1 on the bzimpy ligand were replaced by the methyl groups in 1′ and 1′′. To investigate the intermolecular interactions of the complex, the dimers of 1′ and 1′′ were optimized at the same level of theory, and then non-covalent interaction (NCI) analysis was carried out on the dimers. The dimer of 1′ was found to adopt a head-to-tail conformation with a Pt⋯Pt distance of 3.409 Å, suggesting the existence of Pt⋯Pt interactions (Fig. S21a, ESI†). From the corresponding NCI plot (Fig. 4a), the green surfaces, which indicated attractive interactions, between the two coordination planes and that between the bzimpy moiety and the phenyl ring on the alkynyl ligand in the adjacent complex suggested the presence of π–π stacking interactions, while the surface between the Pt(II) centers showed the presence of Pt⋯Pt interactions. Owing to the steric bulk of the MGCN with a tetrahedral geometry, there were no significant non-covalent interactions observed between the MGCN moieties. The head-to-tail conformation of the complex could also be ascribed to the steric bulkiness of the MGCN, which was found to be in line with the 2D NOESY NMR study. Interestingly, the dimer of 1′′ was found to adopt the head-to-head staggered conformation with dihedral angles of 28.25° (Fig. S21b, ESI†). The Pt⋯Pt distance is determined to be 3.283 Å, suggesting the strengthening of Pt⋯Pt interactions upon photo-irradiation. The interplanar distance between the bzimpy moieties within the dimer was found to be 3.45 Å, indicating the presence of substantial π–π stacking interactions. From the corresponding NCI plot (Fig. 4b), upon the formation of the charged MG species, there is no observable red surface, corresponding to repulsive interactions, located at the interface of the carbocation. This observation might indicate the substantial delocalization of the positive charge throughout the whole complex, minimizing the charge–charge repulsion. On the other hand, the electron-deficient nature of the complex would enhance the intermolecular associations. The green surfaces between the two coordination planes and that between the 4-dimethylaminobenzene moieties in the adjacent complex as well as between the two Pt(II) centers have further supported the presence of π–π stacking and Pt⋯Pt interactions. Based on the computational studies of the model compounds, together with the support from the 2D NOESY experiment, the complexes would exhibit a head-to-tail conformation in the MeCN solution in the absence of UV. After UV irradiation, the removal of the cyano group would planarize the alkynyl ligand and reduce the steric bulkiness, leading to the head-to-head conformation (Fig. S22, ESI†).

Fig. 4 Noncovalent interaction (NCI) plot of the dimer of (a) 1′ and (b) 1′′ optimized at the M06 level.

Two MGCN-based alkynylplatinum(II) bzimpy complexes have been synthesized and characterized. Upon photo-irradiation at 254 nm, the complexes in MeCN solutions displayed drastic color changes from yellow to green, indicating the formation of the cationic MG form. The extra positive charge and the planarization of the MG moiety on the alkynyl ligand were believed to lead to more extensive intermolecular associations in the platinum(II) complexes, resulting in the formation of larger and highly-ordered nanoaggregates. Computational studies of the model complexes have also revealed the enhancement of Pt⋯Pt interactions after removal of the cyano group on the MG-based alkynyl ligand, which served as one of the driving forces in directing platinum(II) complexes to form highly-ordered supramolecular assemblies. The present work has demonstrated that light can act as an external stimulus to trigger and modulate the self-assembly properties of the alkynylplatinum(II) bzimpy complexes. The influence of the length of the spacer between the MGCN and the alkynyl ligand on the overall self-assembly process has been visualized by electron microscopy experiments. These results have illustrated that a subtle modification of the molecular structure would already be sufficient to perturb the aggregate morphologies, which may provide insights into the rational molecular design of stimuli-responsive materials with functional properties.

V. W.-W. Y. acknowledges support from The University of Hong Kong. The work described in this paper is supported by a General Research Fund (GRF) grant (HKU17309220) from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region, People's Republic of China. Support from the Collaborative Research Fund (CRF) (C7075-21GF) of the RGC is also acknowledged. A. S.-H. C. acknowledges the receipt of a postgraduate studentship and a university postgraduate fellowship from The University of Hong Kong. We also thank Mr Frankie Yu-Fee Chan at the Electron Microscope Unit of The University of Hong Kong for his helpful technical assistance.

Data availability

The supplementary data, figures, and tables are available in the ESI.†

Conflicts of interest

There are no conflicts of interest.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc03558e

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