The absorption spectra of naphthalene can change upon rotation around the C-C bond in its excited state due to the alteration of its electronic properties. Naphthalene is a polycyclic aromatic hydrocarbon PAH consisting of two fused benzene rings. The electronic properties of naphthalene are mainly determined by the -electron system formed by the overlapping p-orbitals of the carbon atoms in the molecule.When naphthalene absorbs light, it undergoes a transition from its ground state S0 to an excited state S1 or S2 . The excited state is characterized by a different electron distribution in the -system, which can lead to changes in the molecular geometry. Rotation around the C-C bond in the excited state can cause a change in the relative orientation of the two benzene rings, which in turn can affect the electronic properties of the molecule.The absorption spectra of naphthalene can be analyzed using techniques such as UV-Vis spectroscopy. The changes in the absorption spectra upon rotation around the C-C bond in the excited state can provide insights into the electronic properties of the molecule, such as the nature of the excited states, the energy levels involved, and the extent of conjugation in the -system.For instance, if the rotation around the C-C bond leads to a decrease in the overlap of the p-orbitals and a reduction in the conjugation, the energy gap between the ground state and the excited state may increase, resulting in a blue shift in the absorption spectra. Conversely, if the rotation enhances the conjugation, the energy gap may decrease, leading to a red shift in the absorption spectra.Moreover, the changes in the absorption spectra can also provide information about the potential energy surface of the excited state, which can be useful for understanding the photophysical and photochemical properties of naphthalene. By analyzing the changes in the absorption spectra upon rotation around the C-C bond, researchers can gain insights into the factors that govern the electronic properties of naphthalene and other PAHs, which can be valuable for the design of new materials and applications in areas such as organic electronics, photovoltaics, and sensors.