trans versus geminal electron delocalization in tetra- and diethynylethenes: a new method of analysis

trans-Diethynylethene [(E)-hex-3-ene-1,5-diyne (1 a)], geminal-diethynylethene [3-ethynyl-but-3-ene-1-yne (1 b)], and tetraethynylethene [3,4-diethynyl-hex-3-ene-1,5-diyne (2)] are flexible molecular building blocks for pi-conjugated polymers with interesting electronic and photonic properties. The type of functionalization, the length of the polymer chain, and the choice of pi-conjugation pattern, play a crucial role in determining the properties of these compounds. To rationalize the impact of the different delocalization pathways in the various types of isomers (trans or geminal) on the molecular and electronic structure, a detailed theoretical investigation is presented. We develop a method based on the natural bond orbital (NBO) analysis of Weinhold, which allows one to correlate electron delocalization with molecular and electronic structure observables. The method reveals that the difference between trans (or through) and geminal (or cross) conjugation is not only due to the vertical pi conjugation, but also to the in-plane sigma hyperconjugation. The method is used to correlate the changes in molecular and electronic observables, such as the bond lengths or the absorption frequencies, with the electronic structure of the compounds under investigation. Moreover, this method allows us to predict how a certain substituent will affect the molecular structure and the electronic properties of a given backbone.