화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.139, No.16, 5973-5983, 2017
Ground-State versus Excited-State Interchromophoric Interaction: Topology Dependent Excimer Contribution in Metal-Organic Framework Photophysics
Metal-organic frameworks (MOFs) define emerging materials with unique optoelectronic properties that stem from the highly organized chromophoric linkers within their frameworks. The extent of ground- and excited-state interchromophoric interaction among the pi-conjugated macro-cyclic linkers was studied within three tetraphenyl-pyrene (1,3,6,8-tetrakis (p-benzoic acid) pyr en e; H(4)TBAPy)-based MOFs: ROD-7 (In-2(OH)(2)TBAPy, frz), NU-901 (scu), and NU-1000 (csq) via steady-state and time-resolved spectroscopic techniques. These experimental data along with computational results indicate that the extent of the interchromophoric interaction, leading to a reduced optical band gap, varies across the series of MOFs and is a function of the relative orientation of the TBAPy linkers determined by their respective framework topology. The trend in the S-1 -> S-0 emission lifetime is consistent with their relative optical bandgap. Analyses of the transient emission decay profiles and time-resolved emission spectroscopic data, recorded in low dielectric media, reveal that a long-lived emissive excimer state appears similar to 1850 +/- 150 cm(-1) lower in energy relative to their corresponding S-1 -> S-0 transitions. The emissive contribution from this excimer state, as well as its corresponding transition energy and time constants, are also found to be dependent on MOF identity. Such variation in properties are particularly influenced by the number density of the TBAPy linkers presented by the topology of a given MOF that are primed to form such an excited state complex. The present work shows how the specific arrangement of the linkers can play a key role in the photophysical properties of MOFs.