•Knoevenagel condensation of OMPA with dimethylformamide dimethyl acetal (DMF-DMA).•The chemical modification induce a high changes on the photophysical properties of the prepared oligomer.•The chemical modification induce a change on the electronic properties improved by DFT computations. The synthesized oligo 4-(methoxyphenyl) acetonitrile (OMPA) was chemically modified by Knoevenagel condensation of OMPA with dimethylformamide dimethyl acetal (DMF-DMA). The obtained oligomer is composed of short chains of poly 3-(dimethylamino)-2-(4-methoxyphenyl)acrylonitrile and denoted OMFA. Experimental measurements: ultraviolet-visible (UV-vis), steady-state and time-resolved photoluminescence spectroscopies, infrared spectroscopy, and thermogravimetric analysis (TGA), were combined with theoretical calculations, based on density functional theory (DFT) methodologies, to highlight the effect of the grafting of the dimethyl-amine group on the photo-physical and electronic properties of the as-synthesized oligomer. Thus, a redshift of the absorption and photoluminescence spectra is observed upon the chemical grafting of the functional group. Added to that, a decrease of the optical bandgap (Egopt) and the energy gap EH-L=EHOMO-ELUMO occurs upon the chemical modification. DFT computations show that the chemical insertion of the dimethyl-amine group into the monomer and the oligomer induces a drastic change on their frontier orbitals HOMO and LUMO.