Floating layers of BODIPY-C18 demonstrate high molecular extinction with absorption shifted by 10 nm as compared to diluted chloroform solution (Figure ). UV-Vis absorption band shape of a floating layer resemble that of a monomeric dye all the way through compression process (as assumed from the Young modulus). Compression after the bend in isotherm leads to fast emergence of 565 nm high extinction batochromically shifted band, probably devoted to J-aggregate species. Normalization by sum intensity of two peaks transforms graphs to the form with isobestic point, further approving that two bands are related and 1 to 1 conversion is happening. Interestingly, J-aggregate band was never observed in experiments in solution, approving that Langmuir technique is a proper choice for our examinations.