Carbon-nitrogen based two dimensional frameworks have attracted significant attention due to their great potential for environmental applications. Among the g-CxNy family, the C3N monolayer (ML) is one of the promising candidates, which exhibits semiconducting behavior. The structural, electronic and optical properties of the polycyclic aromatic hydrocarbons (PAHs) and their nitroderivatives adsorbed on the C3N ML have been investigated using first principles calculations. We chose six representative molecules from the PAH family, benzene (B), anthracene (A), and benzo[a]pyrene (BaP), and their nitroderivatives, nitrobenzene (NB), 9-nitroanthracene (NA), and 6-nitro-benzo[a]pyrene (NBaP). These molecules are severely affecting human beings and aquatic systems. The optimal adsorption orientation of these molecules on the C3N ML is determined and the adsorption energies (E-ads) are calculated. The E-ads of these PAH molecules with C3N ML are found to be higher than that of graphene. The PAH molecules with the nitro group show higher E-ads as compared to that of molecules without the nitro group. Further, we have checked the implicit water solvation effect on the adsorption behavior of PAHs on the C3N ML. In a water solvent, E-ads further increases. The energy gap of the C3N ML is sensitive to the adsorption of these nitroderivatives of the PAH molecules, which implies the possibility of its use as a sensor. We have noticed an increase in the calculated work function upon the adsorption of PAH molecules which further explains the affinity of the C3N ML towards these molecules. The static dielectric constant epsilon(1)(0) and the refractive index (n(0)) of the pristine C3N ML are 4.24 and 2.06, respectively. Adsorption of B, A and BaP molecules on C3N does not have a significant impact on the values of epsilon(1)(0) and n(0). But adsorption of their nitroderivatives shows a notable increase in the values of epsilon(1)(0) and n(0). We believe that our results could trigger further theoretical and experimental work towards constructing highly sensitive nanosensors and optical devices to detect PAHs.