2 micro molar concentration of pyrene doesn't give a proper fluorescence spectrum, in spite of using the right parameters, do you have any clues to help me figure it out?
I suggest that you check the fundamentals before looking for more exotic explanations, e.g. the emission from the source, the UV transmission of the cells, and the behavior of the fluorescence detector.
That is a pretty high concentration for a fluorescence measurement. You may have saturated the detector. Try reducing the pyrene concentration 10-fold or 100-fold, or lowering the gain on the detector, or using narrower slit widths.
Pyrene at high concentrations will form excimers, which produce a different emission spectrum from monomeric pyrene.