Optimization of the Substitution Pattern of 1,3‐Disubstituted Imidazo[1,5‐a]Pyridines and ‐Quinolines for Electro‐Optical Applications

Abstract

A series of recently synthesized 1,3‐disubstituted imidazo[1,5‐a]pyridines (IPs) and ‐quinolines (IQs) targeting at increased efficiency of luminescence is investigated. The properties of molecules in solution as well as their change in the solid state are reported and assessed regarding possible application in organic electronics. The influence of increased ring size by substitution, e.g., exchanging phenyl to naphthalenyl, as well as pyridyl to quinolinyl moieties, and by means of a larger IQ fluorophore is discussed. A higher oscillator strength and quantum yield can be achieved. Frontier orbital energies are estimated based on cyclic voltammetry and density functional theory (DFT) calculations. Single crystals of molecules are grown. A red‐shift in the photoluminescence spectra found for crystals of IQs compared with those in solution is proposed to be caused by intermolecular coupling based on the parallel stacking of the enlarged fluorophore units. Thin films deposited by physical vapor deposition exhibit similar effects, showing promise as active layers in organic light‐emitting diodes (OLEDs). An amorphous morphology is inferred for these films from both spectral broadening in photoluminescence and atomic force microscopy. An OLED test structure is prepared, using the most efficient IQ lumophore and demonstrating the feasibility of obtaining electroluminescence from such thin films.

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

“…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 UV-Vis absorption spectrum of 1 in DCM displayed an intense high-energy band at 323 nm, together with a shoulder located at 380 nm.…”

“…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]). In the latter field, two issues are particularly bothersome: the first is the lack of high-energy stable emitters, while the second one is the lack of sustainable ionic emitters [24][25][26][27].…”

“…This class of compounds is prone to aggregate in thin films with a dramatic loss of photoluminescence quantum yields, which affects their performance in electroluminescence devices [22]. Recently, Albrecht et al [21] optimized the substitution of 1,3-disubstituted imidazo [1,5-a]pyridines and -quinolines in thin films and OLED devices, demonstrating the feasibility of obtaining electroluminescence from these fluorophores. This year, O. Y. Vassilyeva et al [33] used imidazo [1,5-a]pyridinium-based cations and halometallate anions as fluorescent agents to ensure excellent dispersion of the luminescent components in a polymeric matrix and prevent solid-state quenching.…”

Luminescent Imidazo[1,5‐a]pyridine Scaffold: Synthetic Heterocyclization Strategies‐Overview and Promising Applications

“…[12][13][14][15] To obtain these structural modifications it is of paramount importance to know the available different synthetic approaches to introduce functional groups, spacers or additional conjugated systems (such as imidazo-quinoline). [16][17][18][19][20][21] The distinctive emission of the impy moiety typically lies in the 450-530 nm range, with a large Stokes shift (up to 150 nm) and good quantum yield (ranging from 10% to 50%). Due to their structural flexibility and tuneable optical behaviours, impy luminescent products represent excellent candidates to be widely employed for different applications such as downshifting [22][23][24] and optoelectronic devices.…”

“…In general, impy skeleton could be designed to obtain luminescent ligands from transition metal complexes, to connect other fluorophores (as BODIPY or rhodamine), [8–11] or to act as a fluorogenic centre itself [12–15] . To obtain these structural modifications it is of paramount importance to know the available different synthetic approaches to introduce functional groups, spacers or additional conjugated systems (such as imidazo‐quinoline) [16–21] …”