Martin Justinek, Stephan Landgraf and Günter Grampp
Short-lived (< 100 ns) radical ion pairs (RIPs) generated in a spin-correlated state (singlet or triplet) as an intermediate in a photoinduced electron transfer reaction undergo spin conversion driven by electron-nuclear hyperfine interactions. The spin conversion is sensitive to an applied magnetic field of strength B. Consequently, if either the singlet or the triplet manifold yields a product in a luminescent state, the observed luminescence will be a function of B. This phenomenon called magnetic fiel effect (MFE) is the principle of MARY spectroscopy (MAgnetic field effect on Reaction Yield) in which delayed luminescence (i.e., charge recombination fluorescence or exciplex fluorescence) is recorded while an external magnetic field is swept. The equipment used in our lab [1] allows the generation of magnetic fields up to 500 mT. In order to exclude spontaneous fluorescence from detection and to improve the signal-to-noise ratio, B is modulated at a small amplitude Bmod (typically 0.2 mT at a frequency of 225 Hz), and with the help of a lock-in amplifier only the portion of luminescence being in phase with the field modulation is detected. The spectrum obtained by this modulation technique is the first derivative of the normal MARY spectrum gained without modulation. The exemplary modulated spectrum shown in the figure exhibits inversion symmetry about zero field and consists of a broad peak and a narrow low-field line.
Figure Modulated MARY spectrum of 1*10–3 M N-ethylcarbazole + 0.12 M 1,4-dicyanobenzene in tetrahydrofurane, Bmod = 0.2 mT, fmod = 225 Hz.
Two effects contribute to the shape of the MARY spectrum: at B = 0, the singlet state S and the three triplet states T0, T+ and T– of the RIP are degenerate, and HFI-driven spin voncersion occurs to a certain extent. At B > 0, two contrary effects appear: first, the break-down of certain quantum-mechanical selection rules enhances singlet-triplet mixing at small fields giving rise to the narrow low-field feature in the centre of the spectrum. Secondly, upon further increase of B, the T+ and T– levels become more and more separated from S and T0 due to the Zeeman interaction. Consequently, spin mixing is no longer possible between S and T±, and the population of triplet RIPs decreases. The S <--> T0 mixing efficiency is unaffected by B. This explains the broad peak and the observed saturation behaviour of the MFE. The maximum of the broad peak is related to B1/2, the field strength at which the MFE reaches half its saturation value.
Research topic: study of electron self-exchange by MARY spectroscopy
Reference
[1] G. Grampp, M. Justinek and S. Landgraf, Mol. Phys. 2002, 100, 1063.