Delayed Fluorescence from Upper Excited Singlet States Sn (n > 1) of the Aromatic Hydrocarbons 1,2‐benzanthracene, fluoranthene, pyrene, and chrysene in methylcyclohexane

Delayed fluorescence (DF.) spectra of 1,2‐benzanthracene, fluoranthene, pyrene, and chrysene in methylcyclohexane were measured at −80° up to the wavenumber corresponding to the energy of two triplets. With all four compounds a weak DF. Sn → S0 from the highest state Sn accessible by triplet‐triplet annihilation (TTA) was found. Lifetimes of Sn calculated (a) from the ratio of the DF's Sn → S0 and S1 → S0 and (b) from the difference in line‐width of the 0,0‐transitions Sn,0 ← S0,0 and S1,0 ← S0,0 agree reasonably well. This indicates that population of the highest accessible excited singlet state is the dominating primary process in the excited singlet channel of TTA. There is no evidence for excimer formation being the first step in TTA. DF. spectra extend up to the energy of two triplets. With pyrene the intensity distribution in the hot‐band region of the DF. S2 → S0 suggests that in TTA the same vibrational selection rules are valid as in one‐photon absorption S2 ← S0 and that vibrational relaxation within the S2 manifold is slow compared with internal conversion S2 ↝ S1. The experimental technique is described in detail and experimental difficulties arising from impurities and photoproduct formation are discussed.