The T1 state of 7 has the dominant radical character at the two terminal C atoms of the conjugated diene segment (Fig. ), and this diene gives the highest yield of dimers after 24 h of irradiation (93%). After 12 h, the yield of dimers was already 80 wt.%, while it reached 95 wt.% after 36 h. Clearly, 7 has feature(s) not accessible to the acyclic 1,3-dienes which lead to faster kinetics. Indeed, the much faster photodimerization of 7 than of 2 -6 should (in part) result from its long T1 state lifetime of 1.3 -6.0 μs, which is two orders of magnitude longer than those of 2 and 5 being 27 and 32 ns, respectively. Moreover, because of the planar T1 state structure, 3 7 is best described as two localized radicals at C1 and C4, localized to these atoms due to Pauli repulsion between the two same-spin electrons in the planar -orbital system. Thus, there being two causes of the rapid photodimerization of 7; (i) it has a long triplet lifetime since it has fewer deactivation pathways due to the geometrically confined 6-MR, and (ii) its two radicals are fairly localized in a 1,4-diradical leading to a higher reactivity. Cyclic conjugated dienes with larger and more flexible rings should have shorter T1 lifetimes, and for 8 and 9 they are 400 -550 and 40 -120 ns, respectively. From the calculated T1 state structures of the cyclic dienes 7 -9 (Fig. ), the gradual change towards the structure of an acyclic diene becomes clear.