# Merging Pyrrole with Boron into Versatile Di(2‐pyrryl)borane Building Blocks: π‐Extension, Polymerization, and Coordination

## TL;DR

This paper introduces a new method to synthesize di(2-pyrryl)boranes, which are useful for creating materials that emit blue light efficiently and show enhanced emission when aggregated.

## Contribution

A high-yield synthesis of di(2-pyrryl)boranes, including an unprotected N–H derivative, and their application in optoelectronic materials.

## Key findings

- The new di(2-pyrryl)boranes exhibit bathochromically shifted absorption and intense fluorescence.
- They show aggregation-induced emission enhancement (AIEE) and solid-state emission.
- Time-dependent DFT calculations reveal TICT states responsible for dual emission.

## Abstract

Conjugated di(hetaryl)boranes—particularly those of thiophene, but also of furan—have recently proven to be extremely useful building blocks for organic optoelectronic materials. The importance of the combination of boron with pyrrole, on the other hand, becomes particularly apparent when considering the well‐known, versatile BODIPY dyes, in which the boron atom is tetracoordinated. However, synergistic effects of combining pyrrole with tricoordinate boron have been hardly exploited to date, which is probably due to synthetic challenges associated with the special reactivity of this five‐membered heterocycle. Herein, a high‐yield, gram‐scale synthesis of two di(2‐pyrryl)boranes is presented, including the previously elusive unprotected NH derivative. Their versatility is demonstrated by their application as building blocks for π‐extended tetrahetarene boranes, a polymer, and a zirconium(IV) complex. The crystallographically determined molecular structures of both di(2‐pyrryl)boranes and the Zr complex show largely planar dipyrrylborane moieties. Compared to their thiophene and furan congeners, the new conjugated boranes show bathochromically shifted absorption bands and more intense fluorescence emission, with solvatochromic behavior, aggregation‐induced emission enhancement (AIEE), and solid‐state emission. Time‐dependent DFT calculations reveal that this is due to the population of twisted intramolecular charge transfer (TICT) states, giving rise to dual emission in the unprotected triarylborane.

The synthesis of di(2‐pyrryl)boranes, including the previously elusive unprotected N–H derivative, has been achieved that serve as versatile building blocks for π‐extended tetrahetarene boranes, a poly(dipyrrole borane), and a zirconium(IV) complex. The new π‐conjugated boranes are efficient blue‐light emitters, due to a twisted intramolecular charge transfer (TICT) character of the excited state. Aggregation causes emission enhancement, resulting from the formation of highly emissive nanoaggregates.