Surface modification is an important method to functionalize micro-/nanostructures, but substrates are mainly confined to robust inorganic compounds. We develop here a facile method to modify the surface of a fragile organic 1D microstructure. The bulk molecules and surface modifier were designed with orthogonal solubility to protect the molecular crystals from destruction under the reaction conditions. As a proof of concept, white-light-emitting 10 microstructures were obtained by grafting red chromophores onto the surface of self-assembled blue-emissive microwires via a heterophase S(N)2 reaction. Spatial distribution of the two species is visualized by fluorescent lifetime mapping, which reveals a core shell structure. The ability to postfunctionalize organic 1D structures enables many applications, where the surface property plays key roles, such as an organic P-N junction and a biosensor.