W-band transient EPR and photoinduced absorption on spin-labeled fullerene derivatives

Abstract

We investigated by W-band (94 GHz) transient electron paramagnetic resonance (TREPR) and photoinduced absorption (PIA) spectroscopy two fullerene derivatives bearing a nitroxide radical unit. After pulsed laser photoexcitation of the molecules in liquid toluene solution, complex EPR spectra are recorded, with lines in absorption and emission. The intrinsic higher spectral and temporal resolution of the W-band frequency leads to the assignment of all the lines in the spectrum and the determination of the sign and the absolute value of the exchange coupling between the fullerene in its photoexcited triplet state (S(T) = 1) and the radical (S(R) = 1/2). The two compounds with different fullerene-nitroxide spacers show opposite-ferromagnetic and antiferromagnetic-exchange couplings. The time evolution of the spectra and the polarization of the lines are interpreted in terms of several possible spin polarization mechanisms. The EPR measurements are complemented with PIA experiments.

Photo-induced radical polarization and liquid-state dynamic nuclear polarization using fullerene nitroxide derivatives

“…17 and 21, for FN we observe signatures of a polarized nitroxide radical S = 1/2 corresponding to the D S0 state and of the photo-excited Q T1 state. 17,21,23,24 These two components can be distinguished according to their g-factors and nitroxide hyperfine splittings and are also observed here in FN-1a and FN-2a ( Fig. 1 and Fig.…”

“…20 Besides, the photo-excited state properties of fullerene-nitroxide (FN) have been extensively investigated by Corvaja and coworkers. 17,[21][22][23] Scheme 1B illustrates a simplified electronic-state diagram of photo-excited FN. 22 The C 60 and nitroxide moieties of FN can be considered as two separate electronic entities with electronic excitation localized on the fullerene.…”

Excited‐State Dynamics of Non‐Luminescent and Luminescent π‐Radicals

“…Figure b shows the time‐resolved ESR spectrum of a quartet state with a fine‐structure splitting observed for tetraphenylporphilinato‐zinc(II) coordinated by p ‐pyridyl nitronyl‐nitroxide (Zn‐TTP‐nipy) diluted in a toluene glass matrix. Later, additional homologous systems have been also reported . Furthermore, Ishii et al.…”

Enhanced Intersystem Crossing due to Resonant Energy Transfer to a Remote Spin

“…In strongly coupled systems, mixing of the trip-doublet and trip-quartet sublevels by the zero-field splitting (ZFS) of the chromophore is sometimes invoked to explain the observed spin polarization. , In the moderately coupled case, the trip-doublet and trip-quartet states are determined not only by the ZFS of the chromophore but also by the exchange and Zeeman interactions . These systems have attracted strong interest due to their compact, rigid structures and rich photophysical properties ,,,,,− that can be exploited for producing nuclear hyperpolarization in some cases …”