Optical Zeeman and low-field ODMR studies of thenπ* phosphorescence ofp-chlorobenzaldehyde

Abstract

The emissive triplet states ofp-chlorobenzaldehyde and p-chlorobenzaldehydedl 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"00714cm-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 Tz. The d~-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 3nn* assignment for the phosphorescent triplet state.

Isotope effects and proton hyperfine interactions in the lowest 3nπ* state of substituted benzaldehydes

“…The crystal axes in each sample were located by x-ray methods, based on the known structure of the host at Ϸ130 K ͑Pbca, Zϭ4͒. 16 Samples of the guest in p-xylene ͑distilled͒ were outgassed and sealed off under helium gas in quartz tubes.…”

“…1,16,17,22 Our high-resolution results are summarized in Table I, which also includes new results for a d 4 species ͑ring protons͒. Figure 1 compares example phosphorescence spectra of the origin region of p-chlorobenzaldehyde and its d 1 and d 4 species in a p-dimethoxybenzene host at 1.4 K. The extra structure just to the red of the origin of the d 4 species is due to isotopic species with incomplete deuteration of the aromatic ring.…”

“…Monitoring the phosphorescence intensity at zero applied magnetic field, two major ODMR signals, ͉DϮE͉, are detected in each system at 1.4 K. 16,17 Figure 2 shows example spectra, and Table I includes the deduced zero-field splitting parameters, DϵϪ3Z/2 and Eϵ(Y ϪX)/2. The energy ordering of the spin levels is taken as T z ϾT x ϾT y , with the negative sign for D based on optical Zeeman results.…”

“…We have shown recently that certain properties of the 3 n* state in aromatic aldehydes also are sensitive to the energy gap to the 3 * state. 16,17 Analyses of nuclear hyperfine interactions have been reported in the lowest 3 n* state of benzophenone 18 and other ketones, 19 but not in the lowest 3 n* state of any benzaldehyde derivative.…”

“…Our earlier studies 16,17 have established that the lowest energy triplet state of the guest in these systems has mainly 3 n* character, and that its energy gap to the nearby 3 * state of the guest depends on the isotopic species. In this work, we extend the studied isotopic species, and investigate the effects on various properties of the 3 n* state.…”