# Catellani-Inspired BN-Aromatic Expansion: A Versatile Tool toward π‑Extended 1,2-Azaborines with Tunable Photosensitizing Properties

## TL;DR

This paper introduces a new method to create BN-containing aromatic compounds with tunable light-sensitizing properties for optically responsive materials.

## Contribution

A versatile Catellani-inspired synthesis method for π-extended 1,2-azaborines with tunable photosensitizing properties is developed.

## Key findings

- The method enables the synthesis of BN-embedded polyaromatic cores with diverse ring structures.
- The 'boron-up' BN isostere shows over 10-fold higher singlet oxygen generation compared to the inverted isomer.
- The approach allows direct comparison of the effect of BN orientation on photosensitizing properties.

## Abstract

BN-isosterism, the replacement of carbon–carbon
units with
boron–nitrogen pairs in organic frameworks, offers a powerful
means to create novel compounds, yet methods to access larger BN-containing
polyaromatic cores remain scarce. Leveraging our recently developed
multigram-scale synthesis of BN-naphthalene, we now combine it with
a Catellani-type arene extension (Pd­(OAc)2/P­(2-furyl)3, norbornene) to rapidly access diverse extended BN-embedded
polyaromatic cores. This strategy delivers BN-embedded benzo­[c]­phenanthridines and curved 8- and 7-membered ring-fused
derivatives, as well as BN-embedded benzofluorenones in both normal
and inverse BN-vector orientations. Importantly, the ability to access
both directional BN isomers, in addition to the parent CC
core, provides a rare opportunity to directly interrogate the effect
of the presence and sense of the BN moiety. Most notably, light-induced
singlet oxygen (1O2) generation promoted by
the benzofluorenone core shows a more than 10-fold enhancement in
the “boron-up” BN isostere, while dropping to negligible
levels upon inversion of the BN unit. This work thus offers a blueprint
for experimental electronic tuning of optically responsive organic
materials through BN-mapping.