Lowest Triplet State of Anthracene

A study has been made of the electronic transitions between the lowest triplet state of anthracene and its ground state. Low‐temperature spectroscopic study of the phosphorescence revealed a sharp emission band with a 0,0‐frequency (T→S) at 14 927 cm^—1 (EPA rigid glass solution at 77°K). A vibrational analysis in terms of known Raman frequencies indicates that the transition is of a u→g allowed type. The T←S absorption band was observed in anthracene with the 0,0‐frequency at 14 850 cm^—1 (solution at 293°K). The study of the phosphorescence spectra of seven different chlorinated anthracenes confirmed the assignment of the anthracene lowest triplet. This study re‐establishes the earlier value obtained by Lewis and Kasha, and rejects the recent value deduced by Reid. A correlation of the above results for anthracene with calculations and observations on related polyacenes indicates that the lowest triplet of benzene is ³B_1u (in D_6h), and for naphthalene, anthracene, and naphthacene the lowest triplet is ³B_2u (in D_2h). The perturbing singlets which mix with these triplets are obtained from previously derived rules for spin‐orbital coupling in molecules.