Photochromism of metal-organic frameworks based on carbazole-dicarboxylic acid and bipyridine: sensing adjustment by controlling strut-to-strut energy transfer.

In this paper, two energy-transfer photochromic metal-organic frameworks (MOFs) {[Zn(L)(0.5)(bpy)]center dot H2O center dot DMF}(n)(1) and {[Zn(L)(0.5)(bpe)]center dot 2H(2)O center dot DMF}(n)(2) (H4L = 9,9 '-(1,4-phenylenebis(methylene))bis(9H-carbazole-3,6-dicarboxylic acid), bpy = 4,4 '-bipyridine, bpe = 4,4 '-vinylenedipyridine) were designed and synthesized. Both1and2showed similar pillared-paddle wheel type frameworks with bpy and bpe as the chromophore, respectively, and L(4-)as the antenna-type light harvester, yielding strut-to-strut energy transfer (antenna behavior) within the well-ordered structures. Among them,1displayed excellent energy-transfer photochromic behavior under UV light accompanied by color transformation from colorless to purple. In addition, the photochromic behavior of1has obvious, fast, controllable and reversible characteristics. On the other hand,2showed a different energy-transfer photochromic behavior in the aspects of color changing, gamut, and sensitivity. The variation has been ascribed to the substitution of chromophore bpy in1with bpe in2, which influences the efficiency of energy transfer within the MOFs. Therefore, with the structural diversity and tunability of MOFs, the sensitivity, color, and gamut of energy-transfer of the photochromic MOFs can be tuned by the appropriate choice of the constitutions of MOFs. This work will provide useful guidance for developing novel energy-transfer photochromic MOF materials.