Electron spin resonance of phosphorescent fluorine-substituted aromatics

The lowest excited triplet state of the three compounds naphthalene, 1-fluoronaphthalene and 2-fluoronaphthalene, has been investigated by electron spin resonance spectroscopy. The phosphorescent molecules were oriented and diluted in durene single crystals. Zero field energies, principal values of the g tensors and hyperfine coupling constants for hydrogen and fluorine nuclei in the three canonical orientations are reported. A detailed comparative analysis of these results shows that the presence of fluorine has little effect on the π-electron distribution in the excited state. Upon substitution, some of the spin density originally present in the carbon 2pπ orbital spreads onto the fluorine atom. Spin densities on other carbon atoms remain constant. This local perturbation model is used to analyse in detail the magnetic properties of phosphorescent 1- and 2-fluoronaphthalene. From the spin densities and the experimental coupling constants, the hyperfine tensor of a C-F fragment bearing unit spin density is derived. This tensor is then broken down into four contributions which are computed using a minimum number of assumptions. The results are: (i) accurate experimental data pertaining to the nature of the C-F bond and its spin delocalization and polarization mechanisms; (ii) a firmly-established hyperfine tensor for fluorine in a planar, sp ² hybridized fragment. A simple method to determine spin densities using fluorine hyperfine structure as a probe is outlined.