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

Abstract

The zero-field splittings, principal spin axes, kinetic parameters, and nuclear hyperfine interactions of the 3nπ* state of p-chloro- and p-methylbenzaldehyde and several of their deuterated derivatives are investigated by zero- and low-field optically detected magnetic resonance (ODMR) at 1.4 K in a p-dimethoxybenzene host. The zero-field splittings show large isotope effects. These are interpreted in terms of spin–orbit interaction with the nearby but higher lying 3ππ* state, yielding the energy gap between the two states in both benzaldehyde derivatives. The locations of the spin axes are approximately along the local symmetry axes of the carbonyl group and are insensitive to isotope. But, the spin axis most nearly normal to the plane of a host molecule deviates from the normal by an angle of 7°–13°. The kinetic parameters of the 3nπ* state also are relatively insensitive to isotope. The dominant hyperfine interactions are associated with the aldehyde hydrogen and indicate that the 3nπ* state is largely localized on the aldehyde moiety. Various properties of the 3nπ* and 3ππ* states are compared.

Orientational anisotropic studies by field rotation technique: Near zero-field pulsed EPR experiments of pentacene doped in p-terphenyl

“…The signal intensity is expected to be proportional to the population difference between two spin sub states and the pulse rotation angle as given in Eqs. (4) and (5). We shall discuss the effects of small external field on the spectral patterns of single crystals and powdered solids, and those of site symmetry in single crystals.…”

“…A series of experiments with field orientations covering an entire hemisphere can be easily carried out. It is further proven to be very useful in pinpointing the molecular symmetry axes within the laboratory frame [4]. This is a potentially valuable tool to sort out the complicated HFI within the field range where electronic Zeeman energy and HFI have comparable magnitude.…”