New donor-acceptor 2-aryl-3-(4-pyridyl)-2,5-norbornadienes as thermally reversible photochromic systems

New donor–acceptor 2,3-diarylnorbornadienes (1N–4N) showing high quantum yields for the blue light induced valence isomerization from the norbornadienes into quadricyclanes and the latters’ thermal reversion are studied in comparison with their protonated counterparts.