In our attempts of tuning the molecular packing of tetracene with H-bonding, we found that tetracenediamide was much more vulnerable to photooxidation than tetracene in crystals despite their similar sensitivity to photooxidation in solution. Unexpectedly photooxidation of tetracenediamide in solution and in crystals show different regioselectivities. To explain the regioselectivity, a mechanism involving H-bonding is proposed. This study indicates that molecular packing in solid state can play an important role in solid-gas reactions.