Isophthalate with the simple chemical structure is identified as an effective phosphor for room temperature phosphorescent (RTP) materials. With -Br, -CH3, and -CH=CH2 at the 5-position of the benzene ring of didecyl isophthalate, the three crystalline small molecules of DDIP-Br, DDIP-CH3, and DDIP-CH=CH2 demonstrate ultralong RTP with lifetime of 236, 650, and 184 ms, respectively, although no specific interactions are recognized in the crystals. Radical polymerization of DDIP-CH=CH2 readily results in Poly-1, which is the first RTP liquid crystalline polymer. Direct attachment of isophthalate phosphor to every repeating unit of polyethylene backbone through a single carbon-carbon bond leads to a significant side-chain jacketing effect, greatly reducing the motion of phosphor moieties. Poly-1 renders a columnar LC phase constructed by parallel packing of the supramolecular column that is composed of two Poly-1 chains. In the column, the isophthalate phosphor is confined in between the core of polymer backbones and the shell of decyl tails, and thus the nonradiative process is further suppressed. The materials design of Poly-1 can be widely applied for developing new RTP polymers.