Isoquinoline and its derivatives, which constitute an important category of heterocyclic compounds and are found in a variety of naturally occurring alkaloids, serve a variety of biological purposes such as a potent agonist for human melatonin receptors 1. This research was conducted in an attempt to develop new dihydroisoquinoline molecules (III and IV). Single-crystal X-ray crystallography study validated their structures. The Hirshfeld surface analysis identifies intermolecular interactions by using a 2-D fingerprint map to recognize each type's relative contribution H center dot center dot center dot H connections are discovered to be dominating. The interaction energies between chemical pairs in crystal structures were found using an energy framework analysis. The DFT investigation demonstrates the electronic stability and reactivity of the compounds using the HOMO-LUMO and global reactivity descriptors, indicating that IV has higher chemical reactivity than III. The derived polarizability (ao) and hyperpolarizability (bo) values were used to calculate the optical and nonlinear optical characteristics of III and IV. The IV's significant bo value (488.94 au) indicates that it has good optical and NLO qualities. Molecular docking simulation using human melatonin receptors 1 was used to better understand the binding interaction mechanism of the title compounds. In addition, ADMET evaluations were performed to establish the therapeutic potential of III and IV.(c) 2023 The Author(s). Published by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).