Microwave-Assisted Synthesis, Optical and Theoretical Characterization of Novel 2-(imidazo[1,5-a]pyridine-1-yl)pyridinium Salts

Luca M Cavinato,Claudia Barolo,Claudio Garino,Andrea Fin,Elisa Fresta,Giorgio Volpi

doi:10.3390/chemistry3030050

Luca M Cavinato, Claudia Barolo + Show 4 more

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https://doi.org/10.3390/chemistry3030050

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Abstract

In the last few years, imidazo[1,5-a]pyridine scaffolds and derivatives have attracted growing attention due to their unique chemical structure and optical behaviors. In this work, a series of pyridylimidazo[1,5-a]pyridine derivatives and their corresponding pyridinium salts were synthesized and their optical properties investigated to evaluate the effect of the quaternization on the optical features both in solution and polymeric matrix. A critical analysis based on the spectroscopic data, chemical structures along with density functional theory calculation is reported to address the best strategies to prevent aggregation and optimize the photophysical properties. The obtained results describe the relationship between chemical structure and optical behaviors, highlighting the role of pendant pyridine. Finally, the presence of a positive charge is fundamental to avoid any possible aggregation process in polymeric films.

Highlights

Imidazo[1,5-a]pyridines are a promising family of fluorophores that can be functionalized to tune their photophysical properties, characterized by a high photoluminescent quantum yield and a large Stokes shift [1,2,3,4]
Since imidazo[1,5-a]pyridines are a well-established class of fluorophores, we investigated the effect of the quaternization reaction on the photophysical features [2,3,20,21,49]
The presence of a second suitable site for quaternization led to a drastic change in the reactivity of the substrates, namely the pyridinic heteroatom was the preferential reaction site in all the considered cases

Summary

Introduction

Imidazo[1,5-a]pyridines are a promising family of fluorophores that can be functionalized to tune their photophysical properties, characterized by a high photoluminescent quantum yield (up to 50%) and a large Stokes shift (up to 150 nm) [1,2,3,4]. Despite the large interest in these compounds, depicted by numerous works on the quaternization onto the imidazo[1,5-a]pyridine moiety in mono- or dimeric forms, a systematic study about the quaternization reaction onto pyridylimidazo[1,5-a]pyridine derivatives and their optical properties is still lacking This is of utter importance as both pyridylimidazo[1,5-a]pyridine derivatives and their salts are gaining considerable interest for optoelectronic applications such as solution-processed light-emitting devices (OLEDs [20,21] and LECs [22,23]). This is of utter relevance to define strategies to rule out aggregation in the solid state In this context, we provide a full study on the impact of both the introduction of double imidazo[1,5-a]pyridine moieties and charge sites, as well as on the impact of the quaternization on the photophysical features in both solution and thin film—Figure 1. This was performed on pristine thin films of the emitters and on emitters mixed with common polymeric matrices, giving highlights on the best strategies to reduce aggregation and retain optimal photophysical properties

Synthesis

Photophysical Properties

Notes about the Photophysical Properties

Findings

Conclusions