Method of ``Magnetophotoselection'' of the Lowest Excited Triplet State of Aromatic Molecules

Abstract

The unique intensity distribution as a function of the angles between the principal magnetic axes and the applied magnetic field of the Δm=±1 ESR transitions of a collection of randomly oriented photoexcited triplet‐state molecules has been used as a magnetoselection device, to observe only the molecules whose principal magnetic axes are parallel to the magnetic field. The latter is possible because the shape of the derivative of the Δm=±1 ESR absorption curve gives special prominence to those resonant fields corresponding to transitions of molecules possessing a canonical orientation. Changes in the relative intensities of the derivative ESR lines of the magnetophotoselected collection of molecules were observed as a function of the angle between the external field and the polarization direction of the exciting monochromatic light. The results obtained for glasses of naphthalene‐d₈ and quinoxaline were analyzed in terms of the known ESR spectra and polarization of the electronic absorption bands; the conclusions agree very well with theoretical predictions. The known directions of polarization of the electronic absorption bands of phenanthrene‐d₁₀ were then used to assign the Δm=±1 ESR spectrum of phenanthrene‐d₁₀ in EPA glass at 77°K. The assignment is found to agree with the first‐order spectrum calculated using the published D and E values determined from single crystal studies. A simple procedure based on the magnetoselection method, and making use of the symmetry properties of π,π^* and n,π^* states, is then outlined by which assignments can be made for randomly oriented aromatic hydrocarbon or N‐heterocyclic molecules, of the Δm=±1 ESR lines corresponding to those molecules which have their molecular plane either perpendicular or parallel to the magnetic field. The polarization of the electronic transitions used to excite these molecules to their triplet states need not be predetermined. A discussion is given of the advantages of using the method of magnetophotoselection instead of the normal photoselection method (where optical luminescence is used for analysis) to such problems as the assignment of optical electronic transitions and the mutual orientation of donor and acceptor in triplet—triplet transfer processes.