Doping of metal–organic frameworks towards resistive sensing

Coordination polymerization leads to various metal–organic frameworks (MOFs) with unique physical properties and chemical functionalities. One of the challenges towards their applications as porous materials is to make MOFs optimally conductive to be used as electronic components. Here, it is demonstrated that Co-MOF-74, a honeycomb nano–framework with one–dimensionally arranged cobalt atoms, advances its physical properties by accommodating tetracyanochinodimethan (TCNQ), an acceptor molecule. Strong intermolecular charge transfer reduces the optical band gap down to 1.5 eV of divalent TCNQ and enhances the electrical conduction, which allows the MOF to be utilized for resistive gas- and photo-sensing. The results provide insight into the electronic interactions in doped MOFs and pave the way for their electronic applications.


The effect of heating as observed with X-ray diffraction
In-situ X-ray diffraction measurements were done upon annealing the TCNQ@Co-MOF-74 at temperatures up to 300 • C, Fig. S1. The MOF's (1 1 0) and (3 0 0) peaks are shifted to higher angles upon annealing at 200 • C above which shifts are subtle up to 300 • C. The post-annealing measurement at 30 • C confirms that the shifts are irreversible. This lattice contraction that is not observed with the empty Co-MOF-74 can be attributed to the guest-host electrostatic attraction emerging due to the removal of toluene.   5 The effect of heating as observed with Raman spectroscopy Figure S5 shows Raman spectra collected at 633 nm before and after XRD measurements in which the TCNQ@Co-MOF-74 sample was heated in vacuum at temperatures up to 300 • C. All major Raman lines assigned to those of TCNQ 2− are visible after heating at 300 • C, proving that TCNQ molecules stay inside the Co-MOF-74.

X-ray photoemission spectroscopy at the Co2p edge
In the Co-MOF-74, cobalt ions in a square pyramidal coordination of oxygen atoms are exposed to the hexagonal voids that accommodate TCNQ. As in other metal-TCNQ salts, TCNQ could be stabilized in the MOF by electron transfer from the cobalt ions or the linker. In the following, we study electronic states of cobalt and nitrogen with regard to the intermolecular charge transfer by means of X-ray photoemission spectroscopy (XPS). XPS data at the Co 2p edge is plotted in Fig. S6. For the Co-MOF-74, the spectral shape and binding energy are similar to those typically observed for some well-studied Co(II) and Co(II,III) oxides [9,10] and Co(III) oxides [11,12]. Upon infiltration with TCNQ, the edge is shifted to a higher binding energy. The spectral shape and binding energy are similar to those of cobalt(II) fluoride [13]. The peak shift didn't change with varying the X-ray intensity, justifying that it is not caused by charging.
The electron configuration of neutral Co 0 , divalent Co 2+ and trivalent Co 3+ ions are 3d 7 4s 2 , 3d 7 4s 0 and 3d 6 4s 0 , respectively. Considering the ligand state L, the initial state of the 2p photoemission in 3d transition metals can be expressed as a linear combination of 3d n , 3d n+1 L −1 and 3d n+2 L −2 states, where n = 7 and 6 for Co 2+ and Co 3+ , respectively. When the orbital hybridization effect and crystal fields are omitted, the energies of the two excited states 3d n+1 L −1 and 3d n+2 L −2 are ∆ CT and ∆ CT + U dd above the energy of the ground state 3d n , where ∆ CT is the charge transfer energy between the 3d and the ligand state L, and U dd the Coulomb energy between 3d electrons.
A peak fit analysis shows two or three spin-orbit coupled pairs with 2p 5/2 (2p 3/2 ) peaks It was reported that ∆ CT < U cd in CoF 2 while ∆ CT > U cd in some cobalt alloys with Co 2+ (n = 7) such as CoCl 2 and CoBr 2 . In the former case, the final state of the lowest energy is the 8 c −1 3d 7 state that lowers the Co 2p binding energies, as observed in CoF 2 , while in the latter case, it is one of the other states.
In the case of Co-MOF-74 in which cobalt ions are reportedly Co 2+ 3d 7 high spin state [1], the final state is a linear combination of c −1 3d 7 , c −1 3d 8 L −1 and c −1 3d 9 L −2 . Tentative assignments of the spin-orbit splits in the case of Co 2+ 3d 7 with ∆ CT < U cd are given in Fig. S6.
The number of spin-orbit splits is reduced from three to two by infiltration with TCNQ, possibly due to changes in ∆ CT , ligand field and/or valence number.

Ohmic vs space charge limited conduction
As shown in Fig. S8, the ratio of V Ohm = aI to V SCL = bI 0.5 is V Ohm /V SCL = a/b × I 0.5 , and V Ohm is much larger than V SCL over the whole range of current measured.