Substituent effects on the triplet-triplet absorption spectra of coumarin and its derivatives

Preliminary work on the triplet-triplet (T-T) absorption of coumarin and two of its 4-hydroxy substituted derivatives is extended to four additional substituted coumarins in order to investigate substituent effects on T-T absorption in these molecules. A steady cross Illumination technique is used in EPA glass at 77° K. Measurements of the decay of T-T absorption are carried out with a flash technique and a comparison made with phosphorescence lifetimes to verify the triplet nature of the absorption. The observed T-T spectra show a marked dependence on the presence and position of hydroxy substitution. An explanation is developed based on the mixing of $\text{nπ}{}^1$ character into the lowest $\text{ππ}{}^1$ triplet state. The effect of methyl substitution is also investigated and, although it leads to relatively small changes in the T-T spectra, the results suggest that it plays a role in the susceptibility of the molecules to photochemical change.