Non‐linear absorption of 2,5‐dialkynyl thiophenes

Three diarylalkynyl‐substituted thiophenes were synthesized and the optical power limiting (OPL) effect at 532 nm was investigated for solutions of these compounds. The relationship between the OPL and parameters obtained from molecular‐orbital‐based calculations is discussed. Semi‐empirical calculations indicate that the compounds can have a broad distribution of conformations due to inter‐ring twisting, but that the second hyperpolarizability (γ) can be significant despite a ring twist. The calculations imply that substitution by alkyl groups can lead to enhanced γ values. The measurements and calculations show a greater increase of the OPL and γ effects from compound 5 to 6 than from 6 to 7. For these compounds, which differ mainly by the length of the π‐electron system, the magnitude of non‐linear absorption seems to be well correlated with several properties of the electronic ground state estimated by standard ab initio molecular orbital calculations, as well as with γ values from the semi‐empirical calculations.