Dimethylbutene-2, reaction + singlet

The addition of /ran -stilbene to 2,3-dimethylbutene-2 appears to be a singlet reaction ... 

Aromatic alkynes, such as 1-naphthylbuta-1,3-diyne (120) in the presence of 2,3-dimethylbutene, undergo a [2 + 2] photocycloaddition reaction to form a cyclobutene derivative 121 in 77% chemical yield (Scheme 6.52). This particular addition occurs exclusively on the C1-C2 triple bond. Quenching, fluorescence and exciplex emission studies suggested that this reaction proceeds from both singlet and triplet excited states.717... 

---