Linearly polarized surface warm-yellow LED based on orientated organic dyes in rod-like metal-organic framework crystal arrays

Abstract

We present an LED, which emits linearly polarized warm-yellow light on a surface with arrayed metal-organic framework (MOF) crystals. We first synthesize porous MOFs based on zinc nitrate and mixed ligands, which is comprised of the acid 1,4-benzenedicarboxylic (H2BDC) and the alkaline triethylenediamine (TED). During the synthesis, the direction of the MOF pore channel is the growth direction of the TED. By adjusting the growth environment to be appropriately acidic, we can have a high concentration of the TED and a large growth rate along the TED ligand, and finally obtain the rod-like MOF-BDC crystal. The long-axis of the rod-like crystal is parallel to the direction of the pore channel. Then, via in situ reaction, we introduce the guest rhodamine-B (RhB) dye molecules into the MOF channel, where the RhB molecules are aligned along the direction of the pore channel due to the steric hindrance effect. The obtained MOF⊃RhB crystals exhibit a polarization ratio of 3.67 at the peak fluorescence wavelength and a fluorescence lifetime of 5.8ns. Indeed, by comparing the dichroic ratio and the polarization ratio, we find that the emission bands of RhB molecules within this rod-like crystal have a much preferred orientation along the long axis of the crystal than the absorption bands. Furthermore, we introduce the rod-like MOF⊃RhB crystals within grooves parallel arrayed in a quartz plate; where, by utilizing the gravitational force associated with the anisotropic geometry of rod-like MOF⊃RhB crystals, the long axes of these crystals and also the direction of the RhB molecules are parallel to the orientation of the grooves. Thus, the MOF⊃RhB crystal arrays on the surface emit warm-yellow light at the same linear state of polarization.