Metal–Organic Framework Photoconductivity via Time-Resolved Terahertz Spectroscopy

Abstract

While metal–organic frameworks (MOFs) have been under thorough investigation over the past two decades, photoconductive MOFs are an emerging class of materials with promising applications in light harvesting and photocatalysis. To date, there is not a general method to investigate the photoconductivity of polycrystalline MOF samples as-prepared. Herein, we utilize time-resolved terahertz spectroscopy along with a new sample preparation method to determine the photoconductivity of Zn2TTFTB, an archetypical conductive MOF, in a noncontact manner. Using this technique, we were able to gain insight into MOF photoconductivity dynamics with subpicosecond resolution, revealing two distinct carrier lifetimes of 0.6 and 31 ps and a long-lived component of several ns. Additionally, we determined the frequency dependent photoconductivity of Zn2TTFTB which was shown to follow Drude–Smith behavior. Such insights are crucially important with regard to developing the next generation of functional photoconductive MOF materials.

A conductive metal–organic framework photoanode

“…The signal from the Zn 2 TTFTB-ZrO 2 lm (Fig. S16 †) is due to the intrinsic photoconductivity of Zn 2 TTFTB as we reported previously, 54 and its negligible contribution to the injection signal is not considered in modeling.…”

“…Interestingly, the distances observed from EXAFS analysis are shorter than those from the reported crystal structure. This is potentially because the reported bulk synthesis method (which produces particles on the order of 10 mm  3 mm) 54 differs from the reported synthesis to grow larger X-ray crystallography quality crystals. 42 With conrmation that the Zn 2+ ions measured on lms have local structure consistent with Zn 2 TTFTB formation, SEM was used to investigate the morphology of the samples.…”

“…The Zn 2 TTFTB MOF was chosen for development of photosensitizing arrays due to its visible light absorption, stability under irradiation, and sub-picosecond photoconductivity dynamics. 42,54,61 It was synthesized using tetrathiafulvalene tetrabenzoate (TTFTB) and Zn 2+ and computationally characterized by using model systems of varying complexity, including a half-unit cell of the MOF structure consisting of a columnar stack of TTFTB ligands (denoted as the 3-layer model; Fig. 1a and b), a single TTFTB ligand coordinated to Zn 2+ nodes (denoted as the 1-layer model; Fig.…”

“…3c). 54 As the optical penetration depth is dened as the distance at which incident radiation intensity decays to 1/e (about 37%), the MOF features can be excited throughout their structure, even at their largest radii. TTFTB-sensitized TiO 2 (Fig.…”

“…[43][44][45][46][47][48][49][50][51][52][53] Conductive MOFs oen have spatially overlapping ligands that form continuous electronic bands and mediate through-space charge transfer. 39,40,42,54 In such systems, the overlapping ligands form a photosensitizing array that allows charge carrier migration along crystallographically-oriented chromophores. Such a design enables holes to migrate away from the metal oxide surface, or alternatively allow electrons to migrate to the surface for interfacial charge transfer.…”