BN/BO Doping of peri-Acenoacenes: Modulating Excited States in Trapeziumene Congeners.

Abstract

The rational design of polycyclic aromatic hydrocarbons that combine chemical and physical robustness with finely tuned optoelectronic properties remains a key challenge in materials science. As an initial step toward this goal, we report the synthesis and comprehensive characterization of a new class of boron-, nitrogen-, and oxygen-doped peri-acenoacenes, termed (2,5,4)-trapeziumene congeners. Analysis of these systems provides chemical descriptors that could guide the rational tailoring of their properties through peripheral doping. The target trapeziumene congeners were obtained via a sequence of Suzuki-Miyaura and Buchwald-Hartwig couplings, followed by directed borylation, giving both phenylborane and borinic derivatives with diverse peripheral doping sequences. Single-crystal X-ray diffraction revealed planar to slightly twisted backbones, with peripheral heteroatomic motifs that modulate π-conjugation and intermolecular packing. Photophysical studies showed bright fluorescence (Φ_F up to 0.99), narrow Stokes shifts, and structured phosphorescence at 77 K. Electrochemical analysis demonstrated p-type behavior and a progressive HOMO-LUMO gap widening upon N→O substitution. Theoretical investigations revealed that N→O substitution asymmetrically affects the excited states, blue-shifting fluorescence while red-shifting phosphorescence, through an asymmetric charge stabilization in the S₁ and T₁ excited states. This is accompanied by a progressive widening of the T₁-T₂ energy gap.