Tetraphenylethene-decorated carbazoles: synthesis, aggregation-induced emission, photo-oxidation and electroluminescence

Abstract

In this paper, the bromination of carbazole has been controlled and then subsequent mono, di, tri and tetra-bromocarbazoles have been subjected to Suzuki cross-coupling to yield a series of carbazole-tetraphenylethene (TPE) materials with varying numbers of peripheral TPE groups. Accompanied by 2,7-dibromo-carbazole, five new Cz-TPE materials are successfully synthesized. All the Cz-TPEs are non-fluorescent in THF and emit strong blue-green fluorescence in aggregate states (f_w = 90%) and thin films because of their aggregation induced emission (AIE) nature. In THF (10⁻⁵ M) solution, all the Cz-TPEs emit deep blue fluorescence after UV irradiation and we find that the more the TPE groups, the slower the enhancement of fluorescence. In thin films (40 nm), the emission maximum blue shift decreases and finally becomes non-emissive after about 1.5 h of UV irradiation for all the Cz-TPEs. We suppose these optical properties are attributed to photo-induced irreversible changes of TPE to diphenylphenanthrene derivatives. The photo-oxidation process for Cz-1TPE has been investigated and proved by HPLC analysis, ¹H NMR and LS-MS. Electrochemical properties of all the Cz-TPEs are investigated by cyclic voltammetry and with UV-absorption spectra, and their energy levels are calculated. Finally, we applied the Cz-TPEs to organic light emitting diode (OLED) devices by the spin coating deposition process with OLED devices configuration: ITO/PEDOT (40 nm)/Cz-TPEs (50 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm). All EL devices emit blue-green light with a maximum luminance (L_max) of 2858, 1353, 986, 2088 and 1129 cd m⁻², a maximum current efficiency (η_c,max) of 3.5, 4.9, 3.1, 3.1 and 5.5 cd A⁻¹, a maximum power efficiency (η_P,max) of 2.1, 3.9, 2.2, 1.8 and 3.1 lm W⁻¹ and a turn on voltage (V_on) of 3.5, 3.5, 3.3, 3.6 and 4.2 V for Cz-1TPE, Cz-2TPE, Cz-3TPE, Cz-4TPE, Cz-2TPE (2,7), respectively.