Synthesis and characterization of imidazole substituted trityl radicals and its optical properties

Recently organic luminescent radical materials have attracted much attention as a class of novel luminescent materials which can produce doublet emission. Here we report two modified organic luminescent radicals TTM-Imz2 and TTM-Imz3 derived from tris(2,4,6-trichlorophenyl)methyl radical (TTM) through introduction of imidazole. Various measuring methods have been carried out to characterize the structure and radical properties of the two products, such as gas chromatography-mass spectrometer (GC-MS), Fourier transform infrared spectroscopy (FT-IR) and electron paramagnetic resonance spectrometer (EPR). The existence of single electron on two products has been confirmed by the EPR spectra with gTTM-Imz2=2.0040 and gTTM-Imz3=2.0038, which were close to the free electron (g=2.0023). Then, photo- physical properties have been characterized through ultraviolet-visible spectroscopy (UV-vis), steady state fluorescence spectroscopy, transient state fluorescence spectroscopy, and so on. Absorption spectra of TTM-Imz2 and TTM-Imz3 both have a strong absorption band at short wavelength and a week absorption band at long wavelength. For TTM-Imz2, its absorption peaks are λ=382 and 552 nm, and for TTM-Imz3, λ=390 and 558 nm. Emission spectra of TTM-Imz2 and TTM-Imz3 in various solvents do not show significant change. The emission peak of TTM-Imz2 is λ=581 nm in hexane, and for TTM-Imz3, λ=584 nm. Compared to the emission peak of TTM, they have bathochromic shift about 20 nm. Using rhodamine 6G in ethanol as a standard, the relative fluorescence quantum yields of TTM-Imz2 and TTM-Imz3 in chloroform have been determined under excitation with λex=380 nm, which are up to 5.9% and 10.5% respectively. The excited luminescence lifetime (τ ) of TTM-Imz2 and TTM-Imz3 in chloroform were 12.8 and 16.1 ns respectively determined by transient fluorescence decay. Furthermore, theoretical calculations have been carried out to study the molecular orbitals, electron distribution and the effect of imidazole moiety of the radicals. The DFT calculations (UB3LYP/6-31G(d, p)) results show that both of the radicals have propeller type planner structure with sp2 hybridization central carbon atom. And the dihedral angle between imidazole moiety and its adjacent benzene ring is 33°–35°. The singly occupied molecular orbital (SOMO) is mainly distributed on the central carbon atom and partially delocalized onto the benzene rings and the imidazole rings. The energy gap between SOMO and the lowest unoccupied orbital of TTM-Imz2 and TTM-Imz3 are 2.22 and 2.19 eV, respectively, which are lower than that of TTM (2.32 eV). Thus, compared to TTM, both of the absorption peaks and the emission peaks of the products have bathochromic shift. Finally, TGA measurements were performed to test the thermostability of the radicals. The thermal-decomposing temperature of TTM-Imz2 and TTM-Imz3 are 129°C and 182°C respectively. Moreover, the fluorescence intensity decay of the radicals in toluene was recorded under excitation of a pulse laser with λex=355 nm. The fluorescence intensity of TTM decreased rapidly with a half-time of 27.9 s, by contrast, TTM-Imz2 and TTM-Imz3 are more stable with a half-time of 1156.1 and 661.4 s, which are up to TTM′s 41.4 and 21.9 times. Finally, OLEDs of TTM-Imz2 and TTM-Imz3 have been fabricated. Both of the devices realized orange light emission. The maximum luminance of TTM-Imz3 reaches 147 cd/m2, and the maximum EQE of TTM-Imz2 and TTM-Imz3 reaches 1.1% and 2.9% respectively. In conclusion, the introduction of imidazole moiety in TTM-Imz2 and TTM-Imz3 increase the fluorescence quantum yields and enhanced the stability of the TTM radicals effectively, and orange light emission has been realized both in solution and OLEDs.