Investigation on molecular modification of luminescent methoxylated imidazo[1,5-a]pyridines

Luminescent and electroluminescent small organic molecules are widely employed in everyday products, like digital displays of televisions, smartphones and wearable devices. Defined emission properties, in a selected spectral region, such as high quantum yield and intense absorption, optical tunability and large Stokes shift are essential requirements for any fluorophore designed for fluorescence microscopy, down-converting technologies, lighting devices or any other field in which luminescent molecules are employed [1]. Among the different families of luminophores, the imidazo[1,5-a]pyridine derivatives are receiving an increasing interest due to their peculiar features: i) intense blue emission with large Stokes shift, ii) good optical tunability, and iii) moderate photo-luminescence quantum yields [2]. In a few previous works, differently substituted imidazo[1,5-a]pyridines were synthetized and characterized to find a relationship between optical properties and chemical modifications in position 1 of the imi-dazo[1,5-a]pyridine skeleton [3]. As clearly reported, the methoxy group is regarded as the best substituent to increase the emissive behaviour of the imidazo[1,5-a]pyridine nucleus. For such reason, herein, we present a systematic study on the synthesis and optical characterization of differently methoxylated imidazo[1,5-a] pyridine derivatives. The fluorophores have been designed to investigate the relationship between the chemical structure (i.e. the position and the number of the conjugated methoxy groups on the dye skeleton) and the electronic behaviour, in order to further optimize the optical properties. The key role of the 1,3 substitutions on the imidazo[1,5-a] pyridine nucleus on rotational barriers and on frontier molecular orbitals is discussed in relation to the experimental hyperchromic effect, photoemission quantum yield and electrochemical properties. Depending on the position of the introduced methoxy substituents on the imidazo[1,5-a]pyridine nucleus, we are able to tune the Stokes shift and to increase the emission quantum yield from 22% to 50% [4]. These results pave the way, also for this class of simple and low-cost emitters, to a real application in sensors, in fluorescence microscopy imaging or even in energy related devices (from lighting to photovoltaic applications).