The thermodynamics of light absorption for a two-level system

The thermodynamics of light absorption is analysed using, as model system, a two-level molecular system characterised by mirror symmetry between its absorption and emission spectra. This means that the structure of the HOMO-1 and HOMO (highest occupied molecular orbital) ground state and the LUMO, LUMO + 1 (lowest unoccupied molecular orbital) excited states are nearly identical in their vibronic structures. This is a good model for many dyes and, in particular, for chlorophyll molecules, the main chromophore presents in the photosynthetic apparatus of plants. When a photon is absorbed by this two-level system, a transition from the HOMO ground state to the LUMO excited state occurs. Then, the system rapidly relaxes to an equilibrium distribution over the excited state sublevels and this equilibrium distribution is defined by the temperature of the bath that contains the molecule. In the described scenario, taking into account the changes in internal energy and the partition function before and after the photon absorption event, it is demonstrated that no entropy change occurs upon electronic transition from the ground state to the excited state, upon photon absorption. Furthermore, the idea that the photon-induced electronic transition can be described in terms of system heating is considered. We conclude that photon energy cannot be considered as heat. Instead, when thermal excitation is taken into account, the increased system temperature leads inexorably to entropy changes.