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

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 (ΦT) 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 (ΦF) in these three derivatives. The parent anthanthrene and remaining derivatives behave much differently. All have ΦF values from 0.58–0.84 with lower ΦT 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.