Wavelength-dependent selectivity in [4 + 4]-cycloaddition reactions

Selective excitation of a variety of anthracene derivatives in the presence of anthracene led to the formation of only the cross-cyclomer products, while irradiating these mixtures at wavelengths where anthracene also absorbs led to both dianthracene and the mixed cyclomers being formed. This method was shown to be general, so long as the chromophore was inert toward forming its homodimer and could be irradiated at wavelengths at which anthracene does not absorb.