The value of octa acid (OA) as a reaction vessel in steering a photoreaction toward a less favored product was established. Photodimerization of 2-anthracenecarboxylic acid within OA yields exclusively head-to-head dimers, while in media such as solution, cyclodextrins (CD) and related hosts yield predominantly head-to-tail dimers. Further, OA enhances the chiral selectivity on the product dimers. The difference between OA and CD is attributed to the variation in the dimensions of their entry ports.