Excited-State Dynamics of Carbazole and tert-Butyl-Carbazole in Thin Films

Thin films of carbazole (Cz) derivatives are frequently used in organic electronics, such as organic light-emitting diodes (OLEDs). Because of the proximity of the Cz units, the excited-state relaxation in such films is complicated, as intermolecular pathways, such as singlet-singlet annihilation (SSA), kinetically compete with the emission. Here, we provide an investigation of two benchmark systems employing neat carbazole and 3,6-di-tert-butylcarbazole (t-Bu-Cz) films and also their thin film blends with poly(methyl methacrylate) (PMMA). These are investigated by a combination of atomic force microscopy (AFM), femtosecond and nanosecond transient absorption spectroscopy (fs-TA and ns-TA) and time-resolved fluorescence. Excitonic J-aggregate-type features are observed in the steady-state absorption and emission spectra of the neat films. The S-1 state shows a broad excited-state absorption (ESA) spanning the entire UV-Vis-NIR range. At high S-1 exciton number densities of about 4 x 10(18) cm(-3), bimolecular diffusive S-1-S-1 annihilation is found to be the dominant SSA process in the neat films with a rate constant in the range of 1-2 x 10(-8) cm(3) s(-1). SSA produces highly vibrationally excited molecules in the electronic ground state (S-0*), which cool down slowly by heat transfer to the quartz substrate. The results provide relevant photophysical insight for a better microscopic understanding of carbazole relaxation in thin-film environments.