SOLUTION AND SOLID-STATE PHOTOCHEMISTRY OF 2-ANILINO-5-PHENYL-2,4-PENTADIENENITRILES

2-(N-Methylanilino)-5-phenyl-2,4-pentadienenitrile (3) undergoes either isomerization or oxidative cleavage at the 2,3-double bond upon irradiation in solution, whereas it dimerizes in a regiospecific manner to give the [2 + 2] cycloaddition product 12 upon irradiation in the solid state. From the fluorescence spectra and semiempirical calculation, the excited state of 3 is proposed to be a twist intramolecular charge transfer species. The X-ray analysis indicates that molecules 3b orient appropriately with a reasonable distance for the [2 + 2] cycloaddition in the solid state. The distance between C2 and C3' is 525 pm, the longest distance known in solid-state [2 + 2] cycloadditions. 2-(N-Methyl-p-bromoanilino)-5-phenyl-2,4-pentadienenitrile (4) shows behavior similar to that of 3 in the solution and solid-state photochemistry, whereas analogs 7-10 having nitro substituents on the 5-phenyl rings do not dimerize upon irradiation in the solid state. Semiempirical calculation of the electronic structures is used to interprete the discrepancy of reactivity.