1-styrylanthracene (1SA) undergoes adiabatic cis → trans one-wayisomerization (3c* → 3t*) similarly to 2-anthrylethylenes. However, upon direct irradiation, cis-1SA in the singlet excited state mostly undergoes cyclization to a dihydrophenanthrene-type product (DHP), 4a,4b-dihydrobenzo[b]chrysene, competing with an inefficient intersystem crossing to 3c* followed by one-wayisomerization. The produced DHP, in