13C isotope shifts of anthracene and naphthalene isomers in triplet state optical spectra

Position-specific 13C isotope shifts in the T 1←S 0 photoexcitation spectra of anthracene and naphthalene crystals containing 13C substituted molecules in natural abundance and of material enriched with various synthesized 13C isomers are compared. From the spectra of matrix isolated A-h10 in A-d10 and N-h8 in N-d8 the individual isotope shifts U 0 of all the 13C monosubstituted isomers are determined (with values U 0 between 0·7 and 3·5 cm-1). An additivity rule is established for the U 0 of the twofold substituted 13C 13C isomers similar to that found for multiple deuteration. Applying these results also the 13C substructure of the excitonic Davydov components in anthracene and naphthalene crystals is understood quantitatively. A correlation between the specific 13C isotope shifts and the triplet state spin density distribution is discussed.