Optical Zeeman and low-field ODMR studies of the nπ* phosphorescence of p-chlorobenzaldehyde

Abstract

The emissive triplet states of p-chlorobenzaldehyde and p-chlorobenzaldehyde-d₁ as guests in p-dimethoxybenzene host crystals are investigated by optical Zeeman and low field optically detected magnetic resonance (ODMR) studies of their phosphorescence at ⩽4·2 K. The active spin level, the sign of the principal fine structure splitting constant, its magnitude and the approximate location of the fine structure axes are deduced from high-field Zeeman perturbations of the phosphorescence origins of the guests. ODMR measurements confirm the conclusions and yield better values for the constants and locations of the principal axes of the fine structure tensor. The results for the perprotonated guest show that D ≡ -3Z/2 = -0·58290 and E ≡ (Y - X)/2 = -0·00714 cm^-1; the fine structure axes are oriented relative to the host with the y axis nearly normal to the molecular plane and the z axis rotated 40° from the long molecular axis; and the phosphorescence arises predominantly from T_z . The d₁-isotope of the guest has a larger magnitude for |D| (≠ 11%), but the other properties of its triplet state are similar to those of the perprotonated molecule. The results are interpreted in terms of a substitutional guest with a ³ nπ* assignment for the phosphorescent triplet state.