Energy Transfer in Surface Anchored Metal-Organic Thin Films

Abstract

To achieve high efficiency energy harvesting system, long range diffusion, reduced non-radiative paths, and directional transport are the key parameters to control. Organic polymers and organic-inorganic hybrids with certain nanostructures perform well in regard to diffusion length and directionality; however those often lack mesoscopic order. Hence, further investigations on long-range ordered crystalline structures are essential to improve exciton diffusion length and directionality. Here, I would like to demonstrate surface anchored metal-organic frameworks (SURMOFs) as a potential candidate to address anisotropic energy transport feature. These surface grown thin films possess following advantages over powder crystalline MOF structures/organic ordered films: a) These are highly crystalline and oriented; b) can be grown on any transparent conductive surface; c) films thickness and roughness can be controlled in a straight forward fashion; and d) heterostructures with organic-organic interface can be constructed; e.g. MOF-on-MOF structures [1-2]. A suitably constructed SURMOF-2, made of 4,4′-(anthracene-9,10-diyl)dibenzoicacid (ADB) linkers showed existence of two excited states, a monomer-like PLMon and an excimer-like PLExc. Both the excited states are found to be mobile and their diffusion and related anisotropy are studied using steady-state and time-resolved fluorescence spectroscopy. Constructing two different type of acceptor doped heterostructures, it is shown that the low energy excimer state (PLExc) transports energy anisotropically along the stacking direction of the linker chromophores, as shown in the below schematic [3]. Such directionality of the exciton motion is introduced by the structural organization of the SURMOF-2.