Effect of chain length and donor–acceptor substitution on the electrical responsive properties of conjugated biphenyls: a DFT-based computational study

Abstract

ABSTRACTThe effect of donor–acceptor (D-A) substituent and chain length on the electrical polarisabilities and first hyper polarisability of cis and trans biphenyl oligomeric compounds have been investigated using density functional theory-based hybrid functional CAM-B3LYP with 6-311G (2d,2p) basis set. Our extensive computational study reveals that both average polarisability and first hyper polarisability of the studied compounds increase with the increasing ethylene spacer chain length. Again the substitution of donor (NMe2) and acceptor (C≡N) at the para position of the phenyl rings to each oligomer shows order of magnitude increase of both αav and βav value compared to the unsubstituted one. This increased αav and βav values have been explained due to increasing charge transfer contribution resulting from decreasing optical energy gap (ΔE = S1 − S0) upon D-A substitution. It is also observed that the charge transfer contribution to first hyperpolarisability (βCT) is more than the polarisability (αCT) for the studied molecules. The electronic spatial extent (<R2>) which is a measure of electron density volume around the molecule is found to play a major role along with the intramolecular charge transfer character to explain the non-linear variation of first hyperpolarisability (βav) as a function of ethylene spacer chain length (n) and D-A substitution.