0.5–4.5THz band terahertz spectroscopy of thermally activated delayed fluorescence molecules

In this work we performed an expanded THz range spectroscopic analysis of organic electroluminescence chromophores; specifically the thermally activated delayed fluorescence (TADF) compounds 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN), 2,3,5,6-tetrakis(carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN) and 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as amorphous powders. THz parametric oscillator, injection seeded THz parametric generator, THz time-domain and Fourier transform infrared spectroscopy techniques were all used to assess these compounds over the frequency ranges of 0.50–4.5 and 18–54THz. The resulting experimental spectra were compared with the results of density functional theory (DFT) calculations at the M06-2X/6-31G(d,p) level of theory. THz absorption bands in the experimental spectra were found to be very weak because of the lack of crystallinity in these specimens, even though the DFT calculations predicted absorbance in the THz range. Based on the DFT results, the absorbance lines that were observed are attributed to the wagging, rocking, rotational and out-of-plane bending motions of the carbazole moieties.