On-off conduction photoswitching in modelled spiropyran-based metal-organic frameworks

Abstract

Materials with photoswitchable electronic properties and conductance values that can be reversibly changed over many orders of magnitude are highly desirable. Metal-organic framework (MOF) films functionalized with photoresponsive spiropyran molecules demonstrated the general possibility to switch the conduction by light with potentially large on-off-ratios. However, the fabrication of MOF materials in a trial-and-error approach is cumbersome and would benefit significantly from in silico molecular design. Based on the previous proof-of-principle investigation, here, we design photoswitchable MOFs which incorporate spiropyran photoswitches at controlled positions with defined intermolecular distances and orientations. Using multiscale modelling and automated workflow protocols, four MOF candidates are characterized and their potential for photoswitching the conductivity is explored. Using ab initio calculations of the electronic coupling between the molecules in the MOF, we show that lattice distances and vibrational flexibility tremendously modulate the possible conduction photoswitching between spiropyran- and merocyanine-based MOFs upon light absorption, resulting in average on-off ratios higher than 530 and 4200 for p- and n-conduction switching, respectively. Further functionalization of the photoswitches with electron-donating/-withdrawing groups is demonstrated to shift the energy levels of the frontier orbitals, permitting a guided design of new spiropyran-based photoswitches towards controlled modification between electron and hole conduction in a MOF.

Fig. 1. Photoswitching of spiropyran.

The SP-MC isomerization with the visualization of the HOMO. The SP-to-MC isomerization typically occurs by UV light (λ_UV), while MC-to-SP occurs by thermal relaxation or irradiation with visible light (λ_vis). The C atoms are shown in brown, O in red, N in light blue, and H in white.

Fig. 2. Schematic representation of investigated pillared-layer metal-organic frameworks.

To show the change of electronic properties of MOFs upon photoisomerization, both SP-based and their MC-based MOF counterparts were calculated. Several layer linker functionalizations (R₁, R₂ and R₃) were considered. Dabco and bipyridine were used as pillar linkers, while Cu-based paddlewheel node were used as secondary building unit (SBU). The MOF analysis was performed for the SP and MC linkers and linkers with R₁= H, OH and CHO.

Fig. 3. Dependence of the electronic coupling on the distance between the parallel layer linkers in the pillared-layer MOF.

Distance screening from the initially DFT optimized a Cu₂(SP)₂(dabco) and b Cu₂(MC)₂(dabco). The electronic coupling between c HOMO and d LUMO orbitals of two neighboring SP (in red) and MC (in blue) layer linkers extracted from the MOF and moved on a specific distance (center-of mass distance, COM) using Achmol code.

Fig. 4. Dynamics of linkers in the MOF.

Vibrational flexibility of the layer linkers in (a) Cu₂(SP)₂(dabco) and (b) Cu₂(MC)₂(dabco). Due to its rigidity, the SP linkers interact only on a relatively large distances, of e.g. 8.6 Å, while the MC linkers form various π···π interactions pairs and channels. The parallel neighboring linkers, separated by the pillar linker, as depicted in the dashed circles, were extracted from the MOF MD trajectories and used for the calculation of the electronic coupling matrix elements depicted in Fig. 5. Linkers extracted for direct electronic coupling and electronic coupling via the superexchange-like mechanism are marked inside the blue- and orange-colored circles, respectively.

Fig. 5. Electronic coupling between linkers.

The time evolution of the electronic coupling between HOMO and LUMO orbitals of SP (red) and MC (blue) linkers at 298 K, see labels. a Direct coupling calculated between neighboring linkers extracted from Cu₂(SP)₂(dabco) and Cu₂(MC)₂(dabco), see extracted pair of molecules in blue circles in Fig. 4. b Comparison of direct coupling between SP linkers and the total electronic coupling with superexchange-like process (defined in Eq. 7) between MC linkers (shown in orange circles in Fig. 4).

Fig. 6. The change in electronic coupling upon linker functionalization.

The time evolution of the electronic coupling between HOMO orbitals of unmodified SP and MC linkers in Cu₂(SP)₂(dabco) and Cu₂(MC)₂(dabco) (in black), and their OH- and CHO-modified analogs from Cu₂(SP-OH)₂(dabco) and Cu₂(MC-OH)₂(dabco) (in green) and Cu₂(SP-CHO)₂(dabco) and Cu₂(MC-CHO)₂(dabco) (in violet), respectively. Only direct electronic couplings are considered.

Fig. 7. Visualization of HOMO orbitals of linkers considered in the present study (see Fig. 2 for clarity).

Orbitals are given with the respective orbital energy. Isovalue of 0.02 a.u. was used for visualization. The unmodified SP and MC linkers are highlighted in light blue. MOFs with SP, MC, SP-OH, MC-OH, SP-CHO and MC-CHO layer linkers were simulated.