Abstract
In the present investigation, a variety of quinoidal oligomers with different arrangements and distinctive fused shapes of thiophene rings are studied. These quinoidal oligomers were further explored for the systematic heteroatom substitution using state-of-art quantum computational methods. The substitution of S atoms by O and N in oligomer compounds is done to see change in optical and nonlinear optical properties while moving from series
1
to series
4
. The substitution of oligomer chain shows a significant effect on linear polarizability, first and second hyperpolarizabilities as calculated at M06/6-311G** level of theory. All the studied compounds showed relatively average first hyperpolarizability while reasonably large second hyperpolarizability. Among all the studied compounds, the largest second hyperpolarizability of 1003 × 10
–36
esu is shown by
4c
which is ~ 138 times greater than standard
p
-NA molecule. Interestingly, TD-DFT analysis indicates that the series
4
has larger oscillator strengths and lower transition energy values which contributes to its enlarged amplitude of second hyperpolarizability response. Furthermore, the transition density matrix, frontier molecular orbitals and molecular electrostatic potential diagrams showed that the charge transfer occurs from oligomer chain to cyanide groups and this charge transfer is highly dependent on the substitution patterns. The density of states diagrams of the selected class of compounds
4a
,
4b
and
4c
show that 4c has significant partial contributions of oligomer chain in HOMO–LUMO orbitals. Thus, the comparative study of four series of compounds indicates not only the importance of shape and size of oligomer molecules but also indicates the importance of heteroatom substitutions to attain a vigorous second hyperpolarizability response for their potential use in optoelectronic and NLO applications.