The optical properties of dye molecules in confined spaces can differ from the solution phase due to confinement effects. Pre-organized mesoscale channels of metal-organic frameworks (MOFs) are very suited for hosting various dyes, and the robust frameworks often render the encapsulated dyes with certain preferential geometries, which are different from those found in solution. Furthermore, pre-organized open channels can efficiently guide the uniform and unique spatial distribution of dye molecules in a controlled manner, which are otherwise difficult to achieve. Thus, sufficiently large dye molecules can avoid the formation of complex aggregates when captured inside open channels. In contrast, small dye molecules can form well-defined dimers or aggregates. The resulting dye-encapsulated MOFs can display unusual photophysical properties of the captured dyes. An anionic framework of In-BTB with mesoscale 3D channels is utilized for the efficient encapsulation of various cationic dyes through cation-exchange processes. Six different cationic dyes are encapsulated in the anionic framework of In-BTB, and their crystal structures are completely solved. Novel photophysical properties of these spatially distributed dye molecules in dye@In-BTBs are investigated.

The pseudotetrahedral node, [In(O 2 CR) 4 ] − , often found in In III-based metal-organic frameworks (MOFs) without a cluster-based secondary building unit (SBU) is a negatively charged center due to charge mismatch between an 8-coordinate In III ion and four anionic carboxylate bridging ligands. Thus, In-MOFs with this pseudotetrahedral node tend to bear a counter-cation near each In III center in the frameworks. Generally, dialkylammonium-based cations such as Me 2 NH + 2 and Et 2 NH + 2 directly derived from N,N-dimethylformamide (DMF) or N,N-diethylformamide (DEF) solvents during MOF formation reactions play a significant role to form a stable framework through charge matching. If these cations thermally derived from DMF or DEF were not suitable for crystal growth of In-MOFs, it becomes very challenging to obtain high quality single crystals for X-ray structure determination of the frameworks. In this context, high quality crystals of In-ABDC MOF were not easily prepared from a ditopic azobenzene-4,4'-dicarboxylic acid (H 2 ABDC) through a thermal reaction in DMF or DEF. We successfully overcome this problem by employing a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF 4 ]), and the resulting three-dimensional (3D) In-ABDC MOF, [EMIM][In(ABDC) 2 ]•DEF•H 2 O (I), was structurally characterized by X-ray diffraction. The 3D framework indicates a 4-connected uninodal net with Schläfli symbol of 6 6 (dia). The gas sorption properties of solvent-free I were also investigated in detail.