High brightness in bis(tri-isopropylsilyl)ethynyl-functionalized polycyclic aromatic hydrocarbons: localized representation versus Clar's model

Abstract

Recent studies on acenes have highlighted a localized representation (LR) of aromatic sextets, challenging Clar's classical model of migrating sextets. Here, we extend this framework to peri-fused systems by investigating bis(tri-isopropylsilyl)ethynyl (TIPS)-functionalized polycyclic aromatic hydrocarbons (TIPS-PAHs) namely TIPS-anthanthrene (TIPS-ATT), TIPS-pyranthrene (TIPS-PYR) and TIPS-dibenzo[pyranthrene] (TIPS-DBPYR) as test beds for LR. Crystal structures provide direct evidence for determining the Fries canonical structures, with bond lengths and bond length alternation (BLA) at the zig-zag edge offering robust criteria for positioning aromatic sextets. While NICS and HOMA indices are broadly consistent with these assignments, their interpretation should be considered with some caution. Optical studies reveal progressive bathochromic shifts (491–599 nm) with increasing conjugation, but absorption strength and fluorescence quantum yield depend critically on the mode of π-extension: catacondensation enhances brightness, whereas linear extension diminishes it. Remarkably, TIPS-PYR combines a record molar absorption coefficient (>200 000 M⁻¹ cm⁻¹) with a high fluorescence quantum yield (Φ_F = 0.93), yielding a molar brightness comparable to giant nanoribbons. Stability assays show that TIPS-ATT and TIPS-PYR are bench stable, whereas TIPS-DBPYR, despite reduced fluorescence efficiency, exhibits enhanced photostability relative to TIPS-pentacene (TIPS-PEN) and satisfies the energetic criterion for singlet fission. These findings establish LR as a robust framework for describing peri-fused PAHs, identify TIPS-PYR as an exceptionally bright dye, and position TIPS-DBPYR as a promising singlet fission material.