Fluorescence quantum yield oligothiophenes

In a study on substituted terthiophenes, if methoxy substitution induces a bathochromic shift of the absorption and emission bands, it has little or no effect on fluorescence quantum yields and lifetimes of the excited states (93JPC513). The bathochromic shifts induced by the end disubstitution by bromine atoms can be rationalized as previously described for the methoxy groups and thus this substituent effect is only limited at the five first terms of the oligothiophene series. 

Only the nitro-substituted oligothiophenes display large bathochromic shifts, large Stokes shifts, high fluorescent quantum yields, and long lifetimes for excited states. As for the other substituents, the trend is mostly noticeable for the short oligomers like terthiophenes and seems to disappear for sexithiophenes. As can be inferred from their solvatochromic effect, an intramolecular charge transfer takes place in the excited states of these molecules. 

Recent time-resolved fluorescence measurements have shown a sharp increase in the fluorescence quantum yield of unsubstituted oligothiophenes when going from two to seven units [58-60]. Such an evolution has been closely related to a decrease in the nonradiative decay rate A nr since almost no change is observed for the radiative decay rate r. Furthermore, the main nonradiative process was found to originate from singlet-to-triplet intersystem crossing [59]. We have therefore tried to rationalize the evolution with chain size of the nonradiative decay rate in oligothiophenes this rate has been expressed in PT3 as a sum of two separate contributions [58], as shown below ... 

The low fluorescence quantum yield and the fact that no phosphorescence could be observed in oligothiophenes leads to the conclusion that most of the electronic excitation energy decays radiationless. There are different views of the participation of triplet states in this decay process in thin films [39, 236, 247, 249, 257, 259, 267-274]. In solution, however, the relaxation via triplet states is well agreed. 

---