The 2:1 guest-host complex of 2-aminopyridine hydrochloride with cucurbit[7]uril (CB[7]) undergoes a stereoselective [4+4] photodimerization reaction in aqueous solution to yield exclusively the anti-trans isomer of 4,8-diamino-3,7-diazatricyclo[4.2.2.2(2,5)]dodeca-3,7,9,11-tetraene, and in the absence of CB[7], the photochemical reaction produces the anti-trans and syn-trans photodimers in a 4:1 ratio. In addition, encapsulation of the photodimer product in the CB[7] cavity stabilizes it with respect to the otherwise observed rearomatization to the 2-aminopyridine monomer at room temperature.