Knowledge Bank

At temperatures for which the electron spin-lattice relaxation times between the zero-field levels of the lowest triplet state are comparable to or longer than the phosphorescence lifetimes, the relative intensity of the different vibronic bands in the phosphorescence spectrum and the observed decays are sensitive to changes in $temperature,1$ application of magnetic $fields,1$ and saturation of the zero-field transitions with microwave $radiation.2$ From the changes in the spectrum and in the decays upon applying these perturbations, the zero-field level(s) responsible for the intensity of each vibronic band can be determined, as well as the complete description of the spin-orbit coupling scheme. The above techniques are demonstrated for 2,3-dichloroquinoxaline in a durene host crystal. The intersystem crossing route in the guest molecule is established to be $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1(\pi \pi \ast )\delta fr\to 3B1(n,\pi \ast )(\tau y)\delta fr\to 3B2(\pi \pi \ast )(\tau y)$. By means of optical-detection of the zero-field transitions, the usual spin Hamiltonian parameters for the lowest triplet state of 2,3-dichloroquinoxaline are found to be $|D|=0.09955cm-1,|E|=0.01760cm-1$.