Abstract A new class of π-spacer conjugated donor–acceptor polyoxometalate (POM) cluster covalently attached to the side chain of conjugated polymer, based on the strong electron-donating hexamolybdate pendant and an electron-withdrawing acceptor moiety (NO2), exhibits large second-order optical nonlinearity. For the first time, the effect of increasing the length of the polyenic π-spacer (CC) in polymeric chain containing POM cluster and the influence of the nature of the acceptor moiety on the second-order nonlinear optical (NLO) properties have been studied by using the TDDFT calculations. An intense long range charge-transfer has been observed in these D-π-spacer conjugation – A compounds where the degree of the intramolecular charge transfer from the donor (hexamolybdate pendant) to the acceptor (end phenyl ring bearing NO2 moiety) has been enhanced through π-spacers which might be responsible for the large NLO responses. The conjugated polymers with POM (hexamolybdate) as side-chain pendant are anticipated to be better entrants for NLO applications as their large first hyperpolarizability make them attractive candidates for nonlinear optical materials. Such polymers may exhibit both efficient charge transfer and efficient charge transporting properties as increased conjugation is amplifying the NLO response. These features provide a rationale for the NLO response of these materials and are attractive for designing new, highly efficient second-order nonlinear optical inorganic–organic polymeric composites.