Abstract
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.
Highlights
The high charge carrier mobility of graphene[1, 2] is an extremely appealing property that can be exploited in electrode design[3] for optoelectronic devices
While delayed fluorescence results mainly from triplet-triplet annihilation at lower temperatures in THF solution, where aggregates form upon cooling, the thermally activated delayed fluorescence (TADF) mechanism takes over around room temperatures in solution when the aggregates dissolve, or when the compound is well dispersed in a solid matrix
The broadness and lack of feature, along with the red-shift compared to the fluorescence band near 400 nm and the intermediate lifetime of less than 1 μs is suggestive of an origin from a physical dimer or aggregate that may have formed when cooling; the driving force for aggregate formation at high dilution may be due in part to intermolecular hydrogen bonding as was observed by scanning tunneling microscopy (STM) on gold surfaces, vide infra
Summary
The high charge carrier mobility of graphene[1, 2] is an extremely appealing property that can be exploited in electrode design[3] for optoelectronic devices. For a 7.6×10-5 M solution at room temperature (Figure 5a), we observe essentially the fluorescence spectrum for all delay times for short gate times of 10 ns, albeit there is some small hypsochromic shifting of the emission with increasing delay times.
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