Metal-free dual singlet-triplet organic light-emitting diode (OLED) emitters can provide direct insight into spin statistics, spin correlations and spin relaxation phenomena, through a comparison of fluorescence to phosphorescence intensity. Remarkably, such materials can also function at room temperature, exhibiting phosphorescence lifetimes of several milliseconds. Using electroluminescence, quantum chemistry, and electron paramagnetic resonance spectroscopy, we investigate the effect of the conjugation pathway on radiative and nonradiative relaxation of the triplet state in phenazine-based compounds and demonstrate that the contribution of the phenazine nπ* excited state is crucial to enabling phosphorescence.

Molecular magnets Cu(hfac)(2)L(R) represent a new type of photoswitchable materials based on exchange-coupled clusters of copper(II) with stable nitroxide radicals. It was found recently that the photoinduced spin state of these compounds is metastable on the time scale of hours at cryogenic temperatures, similar to the light-induced excited spin state trapping phenomenon well-known for many spin-crossover compounds. Our previous studies have shown that electron paramagnetic resonance (EPR) in continuous wave (CW) mode allows for studying the light-induced spin state conversion and relaxation in the Cu(hfac)(2)L(R) family. However, light-induced spin dynamics in these compounds has not been studied on the sub-second time scale so far. In this work we report the first time-resolved (TR) EPR study of light-induced spin state switching and relaxation in Cu(hfac)(2)L(R) with nanosecond temporal resolution. To enhance spectral resolution we used high-frequency TR EPR at W-band (94 GHz). We first discuss the peculiarities of applying TR EPR to the solid-phase compounds Cu(hfac)(2)L(R) at low (liquid helium) temperatures and approaches developed for photoswitching/relaxation studies. Then we analyze the kinetics of the excited spin state at T = 5-21 K. It has been found that the photoinduced spin state is formed at time delays shorter than 100 ns. It has also been found that the observed relaxation of the excited state is exponential on the nanosecond time scale, with the decay rate depending linearly on temperature. We propose and discuss possible mechanisms of these processes and correlate them with previously obtained CW EPR data.

In this work, the cage structure of a family of europium metallofullerenes, Eu@C 74 and three isomers of Eu@C 82 , were investigated. The analogy of the electronic states of three isomers of Eu@C 82 with those of three isomers of Ca@C 82 was confirmed by UV-vis-near-IR absorption spectra. The symmetries C s , C 2 , and C 2V were assigned for three isomers of Eu@C 82 by comparing each corresponding isomer of Ca@C 82 . The D 3h cage structure of Eu@C 74 , which is expected only for the C 74 from the isolated pentagon rule, was also confirmed by comparing its photoabsorption spectrum with that of Ca@C 74 . Multifrequency electron paramagnetic resonance (EPR) spectroscopy was employed to determine the zero-field splitting (ZFS) parameters of the metallofullerenes, which are closely related to the surrounding cage structures. The experimental X-and W-band EPR spectra were completely reproduced by computer simulations based on a spin Hamiltonian considering the ZFS terms up to fourth order. The introduction of the fourth-order ZFS terms into the Hamiltonian resulted in a precise determination of the second-order ZFS terms. A nonvanishing rhombicity parameter E of Eu@C 74 demonstrated the reduction of the symmetry from D 3h to C 2V due to the positioning of Eu 2+ ion at the off-center of C 74 cage. Moreover, in this work, the semiphenomenological superposition-exchange model (SPEM) was invoked to examine a relationship between the molecular structures and ZFS parameters. All of the experimental ZFS parameters were satisfactory reproduced in terms of the SPEM, leading to confirmation of the molecular symmetries. The semiphenomenological analysis also enabled us to select the most appropriate cage for Eu@C 82 with C 2 symmetry from among three possible isomers.