LUMINESCENCE PROPERTIES OF N-ISOPROPYLCARBAZOLE SINGLE-CRYSTAL

A systematic study of photoluminescence properties of N-isopropylcarbazole (NIPC) single crystal is reported in this paper. The emission spectra and radiative lifetimes measured within the temperature range 1.7-300 K reveal short- and long-lived luminescence components, the first equivalent to prompt fluorescence (PF) and the second composed of delayed fluorescence (DF) and phosphorescence (PH). Whereas the PF intensity does not show a conclusive temperature dependence, the DF and PH intensities exhibit anomalous temperature dependences involving several distinct maxima. These experimental observations are interpreted in terms of exciton behaviour in real organic solids containing exciton traps. It is shown that: (i) PF originates from Frenkel-type excitons (S1) of energy E(S)1 = 3.53 eV; (ii) DF is due to emission from the S1 states produced by triplet-triplet exciton annihilation with E(T) = 3.0 eV; (iii) PH characterizes triplet exciton trapping levels with two discrete traps of depths about epsilon-t(s) = 6 x 10(-4) eV (shallow trap) and epsilon-t(d) = 0.3 eV (deep trap). Similar to tetracene single crystals the structural transition in NIPC, known to occur at T(t) congruent-to 140 K, does not show up in the integral emission intensity, indicating its presence by a red shift in the spectra and a weak feature in the monotonic temperature dependence in the singlet exciton lifetime.