The manipulation of the photophysical properties of molecular emitters can be realized by composing the close environment of the metal center with the "heavier pnictogen atom" effect. Replacing a nitrogen atom with a heavier phosphorus atom in otherwise isostructural molecular systems results in a significant change of the photophysical parameters. Herein, we report on the synthesis of four pairs of novel phosphinine-based and isostructural diimine-based Cu(I) complexes, which feature peculiar photophysical properties, and show how these parameters depend on the "heavier pnictogen atom" effect. The obtained Cu(I) complexes show triplet luminescence with MLCT character, which was investigated by means of spectroscopic and computational methods. It has been found that the photophysical properties of the coordination compounds show a dependency on the rigidity of the ancillary phosphine ligand in an unexpected manner. Replacing the nitrogen atom with a heavier phosphorus atom in otherwise isostructural molecular systems results in a significant change in emission energy and especially in the lifetime of the excited state. The results obtained demonstrate an efficient approach to the design of emissive molecular materials, which allows the construction of luminescent complexes with controlled photophysical properties.