Photophysical processes Fluorescence and phosphorescence

The singlet and triplet electronic levels are labeled S and T, respectively. Subscripts indicate the order of increasing energy the superscript v indicates that a molecule has an excess vibrational energy absence of a superscript indicates that the vibrational energy of the molecule is in thermal equilibrium a zero superscript indicates that a molecule is in the lowest vibrational level. For clarity, the vibrational and rotational levels are shown equally spaced. 

If the system is initially in the ground state, Sq, the only quanta that it can absorb are those which raise it to some level in another singlet state. Si or S2 in the diagram. (Radiative transitions are indicated by solid lines, nonradiative transitions by wavy lines.) Because 

Path 1 Radiative transition with emission of a quantum of fluorescent radiation, hv. The fluorescent radiation has a lower frequency than that of the absorbed light which raised the system from Sq to S2. Since the transition, Si So, is permitted by the selection rules, it is very rapid. Since it drains the excited level very rapidly, it ceases almost immediately after the exciting radiation, which supplies population to the upper state, is extinguished. 

Path 2 Nonradiative crossing to Ti followed by rapid vibrational equilibration to Ti. This is followed by a radiative transition Ti Sq. The radiation emitted is called phosphorescence. The nonradiative intersystem crossing (ISC) is much slower than the vibrational equilibrations, but competes with the fluorescent emission in the molecules that exhibit phosphorescence. The radiative transition, Ti So, is usually very slow since the triplet-singlet transition is spin-forbidden by the selection rules. Consequently, the phosphorescence persists for some time after the exciting radiation is turned off. 

Path 3a Nonradiative internal conversion to Sq and rapid thermalization of the vibrational energy to bring the system to So. 

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