Tuning Fluorescent Molecules by Inclusion in a Metal-Organic Framework: An Experimental and Computational Study

Fluorescent molecules included within a metalorganic framework (MOF) offer a new opportunity to develop luminescent hostguest materials. Two linear p-conjugated fluorescent molecules, 1,4-bis-p-cyanostyrylbenzene (bpcb) and 1,4-bis(5-phenyloxazol-2-yl)benzene (bpob), incorporated into MOF-5 systems are reported. The as-obtained bpcb@MOF-5 and bpob@MOF-5 displayed tunable blue fluorescent properties (emission wavelength, fluorescent lifetime, and quantum yield) compared with the pure guest molecule and host matrix. Moreover, bpcb@MOF-5 and bpob@MOF-5 thin films were fabricated by the solvent-evaporation method. The MOF-based thin films exhibit well-defined polarized blue emission with a fluorescent anisotropy of approximately 0.15, demonstrating the uniform dispersion and well-oriented bpcb and bpob chromophores within the MOF matrix. Furthermore, a periodic density functional theoretical (DFT) calculation demonstrates that the photoactive molecule assembled MOFs systems feature energy and/or electron transfer between the guest and host framework during the photoexcitation process, thus suggesting that the encapsulated molecule can further modify the electronic structure of the host MOF materials. Based on the combination of experimental and theoretical studies on the fluorescent guests@MOF systems, this study not only presents a feasible method for fabricating MOF-based powders and films with tunable luminescence, but also gives detailed information on the geometric and electronic structures of the photoactive molecule confined within the MOF matrix.