Hot fluorescence

Absorption (A in the diagram) of a photon which has a higher energy (shorter wavelength, “blue” photon) excites the molecule to higher excited singlet state (S2 levels) than absorption of a photon which has a lower energy (longer wavelength, “orange” photon) which excites the molecule to lower excited singlet state (S1 levels). Absorption of light radiation usually leads to excitation of the molecule to a higher vibrational energy state within given electronic energy state and the energy of vibrational states is almost immediately lost as heat via vibrational relaxations (VR in the diagram) and related electronic excited state is populated. Energy of the electronic singlet states can be lost again as heat during the internal conversion (IC in the diagram). Moreover, the spin of the excited electron in singlet state S1 can be reversed (called intersystem crossing, ISC in the diagram) and excited triplet state T1 is formed (its vibrational energetic states). This is a non-radiative process during which an energy is lost as a heat again. A loss of triplet state energy can be done either non-radiatively via heat dissipation (IC) or radiatively and the process is known as phosphorescence (P in the diagram). On the other hand, fluorescence is defined as radiative deactivation of higher excited electronic singlet states (S2 but mainly S1) to the ground singlet state (S0). If fluorescence occurs because of deactivation of the S2 state, then it is the so-called hot fluorescence. It implies from the diagram that energy of emitted fluorescence is higher (shorter wavelength) than energy of emitted phosphorescence (longer wavelength). Similarly, hot fluorescence has higher energy than fluorescence.