We report the discovery of a family of organic compounds, substituted 1,3,5-benzenetrisamides, that-depending on subtle details of their chemical structure-are capable of selectively and extremely efficiently nucleating the growth of the common alpha-crystal modification of the major industrial polymer isotactic polypropylene (i-PP), rendering this normally turbid material highly transparent ( "clarified") at exceedingly low concentrations (as low as a weight fraction of 0.0002), inducing the mechanically distinct, tough i-PP P-polymorph, or promoting growth of hybrids of both crystallographic modifications with heretofore unknown combinations of the above desirable optical-mechanical properties. In addition, these trisamide-based compounds exhibit excellent thermal stability and do not feature absorption of visible light, adding to a most favorable set of characteristics that provides marked benefits over the presently employed nucleating/clarifying agents.