A Deep Blue B,N-Doped Heptacene Emitter That Shows Both Thermally Activated Delayed Fluorescence and Delayed Fluorescence by Triplet–Triplet Annihilation

An
easy-to-access, near-UV-emitting linearly extended B,N-doped
heptacene with high thermal stability is designed and synthesized
in good yields. This compound exhibits thermally activated delayed
fluorescence (TADF) at ambient temperature from a multiresonant (MR)
state and represents a rare example of a non-triangulene-based MR-TADF
emitter. At lower temperatures triplet–triplet annihilation
dominates. The compound simultaneously possesses narrow, deep-blue
emission with CIE coordinates of (0.17, 0.01). While delayed fluorescence
results mainly from triplet–triplet annihilation at lower temperatures
in THF solution, where aggregates form upon cooling, the TADF mechanism
takes over around room temperature in solution when the aggregates
dissolve or when the compound is well dispersed in a solid matrix.
The potential of our molecular design to trigger TADF in larger acenes
is demonstrated through the accurate prediction of ΔEST using correlated wave-function-based calculations.
On the basis of these calculations, we predicted dramatically different
optoelectronic behavior in terms of both ΔEST and the optical energy gap of two constitutional isomers
where only the boron and nitrogen positions change. A comprehensive
structural, optoelectronic, and theoretical investigation is presented.
In addition, the ability of the achiral molecule to assemble on a
Au(111) surface to a highly ordered layer composed of enantiomorphic
domains of racemic entities is demonstrated by scanning tunneling
microscopy.