Quantification of the push–pull character of azo dyes and a basis for their evaluation as potential nonlinear optical materials

The push–pull characters of a large series of donor–acceptor substituted azo dyes—71 structures in all—have been quantified by the Ndouble bond; length as m-dashN double bond lengths, dNdouble bond; length as m-dashN, the 15N NMR chemical shift differences, Δδ15N, of the two nitrogen atoms and the quotient, π∗/π, of the occupations of the antibonding π∗, and bonding π orbitals of this partial Ndouble bond; length as m-dashN double bond. The excellent correlation of the occupation quotients with the bond lengths strongly infers that both π∗/π and dNdouble bond; length as m-dashN are excellent parameters for quantifying charge alternation in the push–pull chromophore and the molecular hyperpolarizability, β0, of these compounds. By this approach, selected compounds can be appropriately considered as viable candidates for nonlinear optical (NLO) applications.