Manipulating triplet states: tuning energies, absorption, lifetimes, and annihilation rates in anthanthrene derivatives

Abstract

The photophysical properties of anthanthrene, four anthanthrene derivatives containing varying phenyl and p-tBu-phenyl substituents, and two anthanthrones with phenyl and p-tBu-phenyl substituents are examined. In general, as the anthanthrenes and anthanthrones become more substituted, red-shifts are observed in the peak maxima of the ground- and excited-state absorption and fluorescence spectra. The anthanthrones have large (>0.8) intersystem crossing (ISC) quantum yields (Φ_T) likely caused by nπ* character in the ground or excited states. A bromo-substituted anthanthrene has a unity ISC yield due to an ISC rate constant of 2.5 × 10¹⁰ s⁻¹ caused by heavy-atom induced, spin–orbit coupling. This leads to low fluorescence quantum yields (Φ_F) in these three derivatives. The parent anthanthrene and remaining derivatives behave much differently. All have Φ_F values from 0.58–0.84 with lower Φ_T values as radiative decay outcompetes ISC. The anthanthrones have remarkable excited-state absorption with strong, broad transitions across the visible region with weaker transitions extending to nearly two μm. The anthanthrenes have very similar-shaped, broad transitions in the visible which can be shifted ∼60 nm by controlling the substituents. The triplet lifetimes range from 31–1200 μs and increase as the ISC yields decrease; the bromo-substituted anthanthrene is the shortest, followed by the anthanthrones then the other anthanthrenes. The rate of triplet–triplet annihilation is also affected by the presence of substituents; as the amount of steric bulk is increased, the rate of annihilation decreases.