Electron Paramagnetic Resonance Studies of the Triplet State of Coumarin and Related Compounds

Electron paramagnetic resonance and optical experiments have been performed on the lowest photoexcited triplet state of coumarin and structurally related molecules which were randomly oriented in a rigid matrix at 77°K. The fine structure of the paramagnetic resonance spectrum of coumarin measured at 9.2 GHz can be described by the spin Hamiltonian $\mathrm{H\hspace{0.17em}=\hspace{0.17em}g\beta }H{\mathrm{·S\hspace{0.17em}-\hspace{0.17em}(XS}}_x^2{\mathrm{\hspace{0.17em}+\hspace{0.17em}YS}}_y^2{\mathrm{\hspace{0.17em}+\hspace{0.17em}ZS}}_z^2)$ with the zero‐field‐splitting energies $\text{X\hspace{0.17em}=\hspace{0.17em}0.0094}{\mathrm{cm}}^{\text{−1}}\text{, Y\hspace{0.17em}=\hspace{0.17em}0.0680}{\mathrm{cm}}^{\text{−1}}$ , and $\text{Z\hspace{0.17em}=\hspace{0.17em}0.0774}{\mathrm{cm}}^{\text{−1}}$ , assuming an isotropic $g$ value. These zero field splitting energies and a phosphorescence lifetime of 0.54 sec indicate that the lowest triplet state of coumarin is of a $\mathrm{\pi –\pi *}$ character.