Triplet cyclopentane-1,3-diyl diradical (T-DR) was generated via photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene (AZ) under low-temperature matrix conditions. Temperature independency of T-DR decay and the kinetic isotope effect of T-DR-d6 provided experimental evidence in favor of heavy-atom (carbon) tunneling process during the decay of T-DR to bicyclo[2.1.0]pentane (CP) via singlet S-DR. For the first time, the formation of CP was confirmed using low-temperature infrared spectra. Computations of the heavy-atom tunneling process using the small-curvature tunneling method demonstrated a fast reaction from S-DR to CP. Moreover, we observed heavy-atom tunneling during denitrogenation of AZ. Stereoselectivity in the tunneling process of T-DR-d6 was observed at 7 K to form retention-CP-d6 in higher amounts compared to inversion-CP-d6. Photolysis of AZ-d6 yielded inv-CP-d6 and ret-CP-d6 in environment- and temperature-dependent ratios. Moreover, because of the prominent matrix effect, T-DR decayed more rapidly in Ar than in glassy organic matrices.