Understanding Excited-State Relaxation in 1,3-Bis(N-carbazolyl)benzene, a Host Material for Organic Light-Emitting Diodes

Abstract

We present a detailed spectroscopic study of 1,3-bis(N-carbazolyl)benzene (mCP), a prototypical host material for blue emitters in organic light-emitting diodes, using time-resolved emission and ultrafast transient absorption spectroscopy. Upon photoexcitation, mCP dissolved in tetrahydrofuran and mCP dispersed at low number density in poly(methyl methacrylate) (PMMA) films show a monoexponential emission decay in the range 6–7 ns. Transient absorption experiments detect the formation of the T1 triplet state from S1 with a quantum yield of ca. 20%, a band maximum at 450 nm, and a lifetime on the microsecond time scale. The strong spectral overlap of S1 stimulated emission and S1 excited state absorption suggests that S1–S1 singlet–singlet annihilation (SSA) based on Förster resonance energy transfer (FRET) should be feasible at high mCP concentrations in mCP:PMMA and neat mCP films. At these concentrations, the intensity of the mCP emission is strongly quenched and the spectra show a dramatic change exhibiting monomer, aggregate, and excimer emission bands. The small Stokes shift and thus the good overlap of the absorption and emission spectra of mCP open up the possibility for efficient diffusive S1 singlet hopping in neat mCP films by means of a multi-step homo-FRET mechanism involving S1 and the respective S0 nearest neighbor. Fluence-dependent transient absorption measurements find pronounced S1–S1 SSA. Kinetic modeling suggests a bimolecular diffusive SSA mechanism with a rate constant kdiff of 1.40 × 10–8 cm3 s–1. In contrast, modeling based on direct S1–S1 FRET results in an unrealistically large Förster radius of 17 nm. These experiments therefore suggest that mCP molecules in S1 efficiently diffuse through the mCP film by multi-step homo-FRET and annihilate upon approaching a critical distance to another mCP molecule in S1. Finally, we find clear spectral evidence for vibrationally hot mCP molecules (S0*) in the transient absorption spectra of neat mCP thin films, which cool down on a time scale from nanoseconds to microseconds.