Effect of the substituting groups and their positions on the optical properties of phenylacetylenic compounds

A series of phenylacetylenic compounds were synthesized. Some substituting groups with different electronic effects were introduced into the phenylene core of the m-bis(phenylethynylene) benzene (3a) at different positions. The introduction of Ph-CC, Br-, NO2- or NH2- into the 5-(meta-) position of 3a do not significantly increase the positive value of Qd, the charge density of the carbon atom which connects the phenylene core and the phenylethynylene branch in 3a, and the conjugation is still interrupted by the phenylene core and their UV absorptions are similar to that of 3a. Introducing NH2- into the 4-position of 3a decreases the Qd from positive to negative value, which makes the UV absorption blue-shifting. However, introducing NO2- into 4-positon of 3a increases the positive value of Qd much significantly, which makes the conjugation extending to the whole molecule and the UV absorption red-shifting obviously. The compound 4a with a NH2- substituent in the 4-position of 3a can be used as a blue-light-emitting material.