Lone pair-π interaction-induced generation of photochromic coordination networks with photoswitchable conductance

Abstract

Lone pair-π interaction-induced charge-transfer was successfully used for switching the conductance of a coordination network, through variation of the degree of charge transfer caused by external photostimulation. The underlying mechanism is attributed to the changes in efficient charge-carriers by photoinduced strong charge transfer, which was investigated by in situ UV-Vis absorption, ESR, and computational studies.

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Functional materials such as photochromic, conductive and catalytic materials, have raised extensive interest due to their particularly interesting and significant importance, especially two-dimensional (2D) materials with excellent mechanical properties.¹ Stimuli-responsive functional materials, in particular some species that display different properties under external stimuli but do not undergo structural changes, can transform between at least two states, for instance, photoconductive switches.² The fabrication of conductive coordination polymers faces enormous challenges, since it is related to long-range energy and charge transfer.^1a,e,h Suitable non-covalent bonds have been considered to be one of the best pathways for energy or charge transfer in large π-conjugate coordination polymer systems.³ Through π–π stacking between organic ligands, an extended charge-transport pathway with a rigid metal–organic framework (MOF) structure can be created that enforces close packing and sufficient orbital overlap between the adjacent ligands to construct conductive materials, which is called the “through-space” approach to charge transport.^1d,h Based on this design concept, functional MOFs with excellent conductivity have emerged,⁴ whereas photo-responsive coordination polymers with switchable conductance were seldom documented.⁵ Coordination polymers with switchable conductance not only can act as electronic materials without altering their components or structure, but develop functional materials in circuit overload protectors and photovoltaic materials.⁶

Naphthalene diimide derivatives (NDIs) are classical n-type organic semiconductors. They possess redox-activity and π-acidity and show potential as radical anions with high conductivity, in artificial photosynthesis and in donor–acceptor systems, of course, they can act as building units as well.⁷ Through supermolecular self-assembly, electron-deficient NDIs are introduced into the coordination polymers to achieve functional materials with ion/molecule recognition, photochromism, electrochromism or electronic conductivity.⁸ Generally, unconventional non-covalent interactions, such as anion-π and lone pair-π interactions, can be observed in these systems which consist of electron-deficient organic tectons.⁹ However, compared with the popular anion-π interaction, lone pair-π interaction that occurs between a neutral electron-rich molecule and a π-acidic aromatic ring is rarely studied, in particular, the specific influence of lone pair-π interaction on its performance involving charge transfer.¹⁰

Here we present a novel 2D coordination network with a redox active naphthalene diimide derivative for potential applications in photo-controlled conductivity switches. The system is very interesting because the color changes accompanying different degrees of charge transfer can be used to trigger an optical or electronic response. Single-crystal structure analysis shows that the coordination polymer (complex 1: Na(TauNDI)_0.5(H₂O)₂) contains sodium cations bound by deprotonated N,N′-di(ethanesulfonic acid)-1,4,5,8-naphthalenediimide (H₂TauNDI) to form an undulating layered network with intermolecular lone pair-π interactions between two neighboring TauNDI molecules; the shortest O-ring distance is 3.380 Å (O4⋯C5) (Fig. 1a).^9b Furthermore, an oxygen atom of a sulfonic-end-group from the trans-TauNDI molecule is located at the edge of an imide ring to form an intramolecular lone pair-π interaction with the shortest distance of 3.009 Å (O3⋯N1) (Fig. 1a).^9b The sodium cation adopts a slightly twisted pentagonal bipyramidal geometry, with two μ²-sulfonic oxygens (O1 and O2), one carbonyl oxygen of TauNDI (O5i) and two oxygens from H₂O molecules (O6 and O7, Fig. S2, ESI†). The μ²-sulfonic oxygens (O1 and O2i) dibridge sodium cations and occupy four coordination sites of each metal, forming a one-dimensional [Na–O–Na]_n chain (Fig. S3a, ESI†) which upon interconnection results in a two-dimensional sheet like architecture (Fig. S3b, ESI†). Moreover, strong lone pair-π interactions have been observed among layers (Fig. 2), consequently the TauNDI molecules exhibit a pseudo-cofacial herringbone packing with abundant lone pair-π and C–H⋯π interactions within complex 1 (Fig. 1b).^1b We found that whereas the carbonyl oxygen atoms are within 2.715 Å from the centroid of the electron-deficient imide rings, the average O-ring distances vary from 2.971 to 3.233 Å (Fig. 2), which indicates that there are strong lone pair-π interactions in 1.^9b Additionally, neighbouring wave-like layers connect each other by C–H⋯π interactions and hydrogen bonds to generate a three-dimensional supermolecular structure (Fig. S4, ESI†); more details of these interactions can be seen in the ESI† (Fig. S5 and Table S1). Remarkably, the deprotonated TauNDI molecules in complex 1 have a dual role: electron acceptors (central rings) and electron donors (the oxygen atoms from the carbonyl or sulfonic acid) in lone pair-π interactions. The charge transfer of lone pair-π interactions may efficiently strengthen the interactions among adjacent layers and hence result in a long-lived charge-separated state to form stable organic radicals. In other words, the cofacial herringbone stacking arrangement of the TauNDI molecules which are connected by moderate lone pair-π and C–H⋯π interactions affects electron transport between the 2D sheets, leading to charge transfer interactions accompanied by peculiar changes in the electronic and optical properties of the complex, which hold great promise for applications in electronic and photonic devices.

Fig. 1 (a) The TauNDI molecules connecting adjacent metal-oxide networks to form a ladder-type network (purple dotted line: intermolecular lone pair-π interaction; green dotted line: intramolecular lone pair-π interaction); (b) cofacial herringbone packing modes of TauNDI molecules in complex 1 through lone pair-π and C–H⋯π interactions (metal ions and the methanesulfonic acid of TauNDI molecules are omitted for clarity except for the middle one).

Fig. 2 Adjacent layers connected with each other by strong lone pair-π interactions; the O⋯ring distances are listed above, varying from 2.971 to 3.233 Å.

Intriguingly, complex 1 displays abnormal photosensitivity, which undergoes a rapid photochromic transformation under blue light irradiation (460–465 nm). It is found that, after several minutes of illumination (∼2 min), the color becomes obviously darker. The darkening of the color points to photosensitivity of the crystals, which is associated with the formation of organic radical anions. This darkened single crystal can be recovered by dark treatment. In addition, all major peaks of experimental PXRD of the sample match well with a simulated PXRD pattern of the single crystal 1, which demonstrates that the crystal structure of 1 is left unchanged during its coloration-decoloration process (Fig. S6, ESI†). To investigate the photochromic mechanism of complex 1, we have carried out room-temperature in situ UV-Vis absorption, electron spin resonance (ESR) and electrical conductivity measurements.

The in situ UV-Vis absorption spectra of the sample are shown in Fig. 3a. The result reveals that the major effect of the irradiation by blue light is to enhance the absorptions in the region from about 430 to 780 nm rather than forming a new absorption band. With the extension of the irradiation time, the absorptions of these bands gradually increase. This result is caused by the fact that the electron-deficient TauNDI rings accept electrons from the donor moieties containing sulfonic oxygens and carbonyl with abundant lone pair-π interactions. Based on a previous report, this was attributed to the absorption of a photoinduced charge-transfer transition relating to the formation of organic radical anions.^8a–d,11 From the in situ ESR spectra (Fig. 3b), a strong peak at g = 2.004 is present for the yellow fresh sample without irradiation. The observed ESR signal originates from the lone pair-π interaction-induced charge transfer of the TauNDI moieties to generate stable organic radical anions,^8a–d,11 which leads to a long-lived charge-separated state. Notably, the degree of charge transfer increases with the prolonged irradiation of the sample. The change degree of charge transfer can be inferred from the increase of in situ ESR in the presence of the UV irradiation (Fig. 3b); presumably, with the increase of illumination time, the amount of organic radical anions will finally reach saturation according to the trend of the changes. Consistent with the results of in situ UV-Vis absorption spectra, the amount of charge transfer increases in the complex in relation to the lone pair-π and C–H⋯π interactions, indicating the strengthening of the charge transfer interactions by photoexcitation. This means that these non-covalent interactions induce generation of organic radical anions to form a stable localized electron state, and the charge transfer interaction will be enhanced upon illumination until finally radicals exist in homeostasis in the complex.

Fig. 3 (a) In situ UV-Vis spectra of 1 (black line: before irradiation; blue line: after irradiation); the inset shows the changes in UV-Vis spectra irradiated by LED light with a power of 3 W (460–465 nm) (irradiation time: 0 s, 4 min, 8 min, 12 min, 16 min and 32 min, respectively); (b) in situ ESR spectra at g = 2.004 for complex 1 subjected to prolonged irradiation by UV light with a power of 16 mW (irradiation time: 0 min, 2 min, 6 min, 10 min, 14 min, 18 min, 22 min, 26 min, 30 min, 34 min, 38 min, 42 min and 46 min, respectively); the inset shows the relationship between the ESR signal intensity and illumination time.

The electronic band structure of complex 1 was calculated using the VASP (Vienna ab initio simulation package) program.¹² The estimated band gap of complex 1 based on the Kubelka–Munk Function from the UV/Vis diffuse reflectance spectrum is 2.65 eV (Fig. S7, ESI†), which agrees reasonably well with the calculated value of the bandgap (∼1.90 eV). Through analyzing the electronic band structure (Fig. S8, ESI†), it was obvious that the electron-deficient imide ring of TauNDI was the predominant contributor to the lowest conduction bands, while the oxygen atoms from the sulfonic acid were the main contributors to the highest valence bands, which was consistent with structural analysis that TauNDI molecules acted as electron acceptors as well as electron donors in lone pair-π interactions. Thus this complex was of a narrow band-gap semiconductor with a donor–acceptor structure. The generation of optimum crystalline packing modes in complex 1 was induced by lone pair-π and C–H⋯π interactions, which offered effective pathways for charge transfer to generate stable organic radical anions, thus leading to a long-lived charge-separated state. Besides, once complex 1 was illuminated by light, the degree of charge transfer could be increased. This special variation of the degree of charge transfer resulted in the reversible photochromic process of complex 1.

Electrical conductivity measurements help to reveal the long-lived charge-separated nature of the complex. In a two-probe configuration (for details, see the ESI,† Fig. S9), the tablet compressing fresh sample exhibits linear current-voltage characteristics yielding a conductivity value of 10⁻⁶ S m⁻¹ (1.04 × 10⁻⁶ S m⁻¹) that is within the order of magnitude of semiconductivity. This may be derived from lone pair-π interaction-driven charge transfer, which creates an extended charge-transport pathway via non-covalent interactions engendering stable radicals. It is well-known that stable organic radicals can provide free charge carriers, and hence improve the charge density to facilitate the conductivity of the complex.^1h,4a–c In addition, photoswitchable conductance was observed in this tablet compressing sample. It can be seen from Fig. 4 that the conductivity value drops sharply from 1.04 × 10⁻⁶ S m⁻¹ (before irradiation) to 2.31 × 10⁻⁷ S m⁻¹ (after irradiation for 20 min). Such a phenomenon can be used for the development of a photo-controlled conductive switch and photovoltaic materials. The substantial decrease of the conductance may arise from the spatial transfer of charges, where photoexcited charge transfer among the TauNDI molecules enhances the degree of charge transfer, the resultant decrease of the efficient charge-carrier substantially diminishes the conductivity.¹³ Accordingly, the material owes its distinctive performance to the moderate lone pair-π and C–H⋯π interactions that provide an effective charge-transport pathway for achieving charge delocalization resulting in reversible photochromism and photoswitchable conductance. Unlike the conventional photocontrollable isomerization functional materials, such as light-driven molecular switches, which can undergo transformation of two isomers under light with different wavelengths,¹⁴ these unique photoswitchable properties of complex 1 stemmed from the variation of the degree of charge transfer caused by external photostimulation. The thermogravimetric analysis curve of complex 1 is shown in Fig. S10 (ESI†). In the temperature range of 80–146 °C, the amount of weight loss was ascribed to the loss of coordinated water molecules (experimental value: 11.99%, calculated value: 12.03%). A sharp weight loss occurred at a temperature of about 430 °C, which was attributed to the decomposition of the 2D structure.

Fig. 4 The I–V curve of the pressed sample of complex 1.

In conclusion, this communication presents a reversible photochromic 2D coordination polymer with unique photoswitchable conductance. The lone pair-π and C–H⋯π interactions can effectively induce redox-active molecules to adopt a suitable stacking route that results in an optimum crystalline packing motif, which is beneficial for achieving long-lived charge-separated states, making it possible to display photo-switching effects, such as changes in absorption spectra, ESR intensity, charge-transfer interactions, and photoinduced alteration of conductivity. The reversible photochromism and photoswitchable conductance of this material are dependent on its reduction degree by photo-induced charge transfer. This work provides an avenue for designing and preparing functional materials from donor–acceptor coordination networks for applications in photodetection, sensing, displays, and switches.

There are no conflicts to declare.

We acknowledge Professor Guo-Cong Guo for providing the computer resources. This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000), the Key Program of Frontier Sciences, CAS (QYZDJ-SSW-SLH033), the National Natural Science Foundation of China (21373221, 21521061, 51672271, 21671190, 21403236, 21401192, 21403176), the Natural Science Foundation of Fujian Province (2006L2005) and the Chinese Postdoctoral Science Foundation (2016M600509).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, crystallographic data, additional figures, computational studies and characterization (NMR, XRD, TGA, etc.). CCDC 1554405. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cc05150f

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