Intramolecular electron hopping in double carbazole molecules studied by the fluorescence-detected magnetic field effect

The intramolecular electron hopping in cation dimer radicals of I,n-bis(N-carbazolyl) alkanes for n = 3,4 that are produced via photoinduced electron transfer to 1,4-dicyanobenzene (DCNB) has been studied by means of the exciplex fluorescence detected magnetic field effect (MFE) in a 18 vol % mixture of dimethylformamide with tetrahydrofuran. It has been found that on passing from a single cation radical of N-methylcarbazole to the cation dimers, the parameter B-1/2 decreases from ca. 50 G to ca. 26 G, while saturation values of MFE (at B = 300 G) are almost the same (ca. 3.0% for MeCz and CzB) but strongly decreased for CzP (ca. 0.5%). The experimental results obtained are in fair agreement with the Schulten theory of intramolecular electron exchange. In contrast to the well-known pyrene/N,N-dimethylaniline exciplex system, the carbazole derivatives/DCNB system exhibits a two-exponential decay of exciplex fluorescence and a significantly smaller MFE that might result from the fact that in the latter exciplex system the radical ion pair state is the lowest energy state above the ground state.