In the past, several studies have shown, experimentally and theoretically, that, in 2D perovskites, the quantum confinement of the electron and hole, and thus the emission wavelength, can be tuned by varying the thickness of the inorganic part, i.e. the quantum well thickness. It is evident that, in our case, it is not the length of the alkyl chain variation which induces the abrupt emission blue shift along the sample series, as this length linearly varies across the series (Figure ). The transition from phase II to phase III in the iodine samples is accompanied by a relatively large abrupt change in the I-Pb-I angle, from around 156° for n ≤ 10 to around 150° for n ≥ 12. As already observed by some of us, this change leads to a decrease of the contributions of Pb orbitals to the CBM and to a band-gap increase and a concomitant blue shift of the PL. We prove therefore the reason of the thermochromism observed in these materials being induced by a polymorphic transition, from phase II to Phase III, involving change in the ammonium cation positioning impacting on the halogen/lead coordination sphere, in turn affecting the halogen/lead atomic orbitals interaction.