The dianthracene molecule 9-anthracenecarboxylic acid, methylene ester (9AC-ME) was synthesized and used to form molecular crystal nanorods using anodized aluminum oxide templates. A high-temperature solvent annealing process generated a crystal polymorph where neighboring monomers had overlapping anthracene moieties. Subsequent exposure to ultraviolet light caused the monomers to undergo a [4 + 4] photocycloaddition reaction, forming highly crystalline polymer nanorods of poly(9AC-ME). The polymer nanorods are flexible, resistant to breakage, and insoluble in organic solvents and strong acid/base solutions. Their molecular structure is characterized by powder X-ray diffraction, solid-state NMR, and UV-vis spectroscopy. Different diameter crystalline polymer nanorods could be fabricated using alumina templates of variable pore diameter. © 2007 American Chemical Society.