Photocycloaddition of Anthracene to trans,trans-2,4-Hexadiene

Article abstract of DOI:10.1021/ja00270a028

Irradiation of anthracene (A) in the presence of trans,trans-2,4-hexadiene (D) gives A dimer (A₂), two adducts corresponding to <4+4> addition of D to the 9,10 positions of A, an adduct corresponding to <2+4> addition, and several unidentified minor adducts.A-14C and isotopic dilution based analyses were used to establish that D reduces rather than enhances anthracene photodimerization quantum yields.A thermal Diels-Alder reaction between the strained trans double bond of the major anthracene/diene adduct (t-<4,4>Ad) and A is shown to account quantitatively for high A losses previously attributed to A dimerization.The structure of the 2:1 A/D adduct (A₂d) and, by inference, the structure of t-<4,4>Ad are established unequivocally by X-ray crystallography.These observations confirm Kaupp's qualitative results and conclusions.The singlet A/D exciplex is not an intermediate leading to either anthracene dimer or to A₂d.The singlet pathway for adducts gives t-<4,4>Ad (81percent), c-<4,4>Ad (<5percent), and the <2+4> adduct (<2,4>Ad) (14percent).Triplet quenching, sensitization, and CH3I experiments show that <2,4>Ad and a fourth unknown minor adduct (x-Ad) derive from triplet-state precursors.