Triplet Dynamic Nuclear Polarization of Guest Molecules through Induced Fit in a Flexible Metal–Organic Framework**

Dynamic nuclear polarization utilizing photoexcited triplet electrons (triplet-DNP) has great potential for room-temperature hyperpolarization of spins. However, the transfer to molecules interest remains a challenge due fast spin relaxation and weak interaction with target at room temperature in conventional host materials. Here, we demonstrate first example DNP guest porous material around by induced-fit-type structural transformation crystalline yet flexible metal–organic framework (MOF). In contrast usual hosts, 1H spin-lattice time becomes longer accommodating pharmaceutical model 5-fluorouracil as MOF changes its structure upon accommodation maximize host–guest interactions. Combined triplet-DNP cross-polarization (CP), this system realizes an enhanced 19F NMR signal molecules.