Synthesis and characterization of the ground and excited states of tripodal-like oligothienyl-imidazoles

Abstract

Six new thiophene oligomers, here designated as tripodal-like oligothienyl imidazoles, were synthesized and have been investigated in ethanol solution at room and low temperature. The oligomers bear a common core where two or more thiophenes are linked to one or more imidazole unit which further links through its alpha position to a different number of incremental thiophene units. The study involves a comprehensive spectral and photophysical investigation where the properties of the singlet and triplet states have been investigated regarding absorption, fluorescence and phosphorescence, transient triplet-triplet absorption together with all relevant quantum yields (fluorescence, internal conversion, intersystem crossing and singlet oxygen) and lifetimes. In addition, DFT quantum chemical calculations were performed to gain a detailed understanding of the molecular geometry and optical properties of the investigated oligomers. From the overall data, the radiative (kF) and radiationless (kNR, kIC and kISC) rate constants have been determined and it is shown that, in contrast with the parent oligothiophenes, the radiative competes with the radiationless deactivation channels. The results show that, by comparison with the oligothiophene counterparts, there is an augment of the relative contributions of the internal conversion and fluorescence processes relative to the S1~~T1 intersystem crossing. Phosphosphorescence emission was found for the simplest member of the investigated compounds, with a low quantum yield and a lifetime of 8 picos. The data also show that the introduction of a 4,5-dithienyl-imidazole moiety in a bi- or terthiophene oligomer results in, respectively, a 20-fold and a 3-fold increase of the fluorescence quantum yield relative to their oligothiophene counterparts.