Delayed Fluorescence

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Delayed Fluorescence [K K Rohatgi-Mukkerjee, Fundamentals of Phy. Chem., p150-160]:

Delayed Fluorescence [K K Rohatgi-Mukkerjee , Fundamentals of Phy . Chem., p150-160] The long-lived emission as phosphorescence has spectral characteristics very different from fluorescence. But there are delayed emissions whose spectra coincide exactly with the prompt fluorescence from the lowest singlet state, the only difference is in their life time. These processes are known as delayed fluorescence Two most important types of delayed fluorescence are: E-type delayed fluorescence P-type delayed fluorescence

E-type delayed fluorescence :

E-type delayed fluorescence Fig 1: Jablonski diagram for E-type delayed fluorescence

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E-type delayed fluorescence is observed when the singlet-triplet energy gap E ST is fairly small. The molecules, initially excited to the singlet energy level, cross over to the triplet level by intersystem crossing mechanism. After vibrational relaxation, if E ST is small, some of the molecules may be again promoted, with the help of thermal energy from the surrounding, with the isoenergetic, point and cross back to the first excited singlet state. Then, the molecules are returning to the ground state by radiative process in Fig 2. The various stages in the process can be expressed as given in Fig 2 indicating the possibility of emission from each relaxed energy.

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Fig 2: E-type delayed fluorescence pathways indicating possibility of emission from each relaxed state.

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If the rate constants are competitive, we expect the emission of prompt fluorescence, phosphorescence and delayed fluorescence of energy quanta h  f , h  p , h  ED respectively Although h  f is equal to h  ED , the lifetime of delayed fluorescence will match the lifetime of triplet decay. The rate constant for h  ED for E-type delayed fluorescence is temperature dependent and can be expressed as: k ED = Aexp (-E ST /RT) where A is the frequency factor and E ST , the activation energy difference between the singlet and the triplet level.

P-type delayed Fluorescence:

P-type delayed Fluorescence This type of emission was first observed in deoxygenated solution of eosin in glycerol and ethanol at room temperature. Thus, it is designated as E-type or eosin type. P-type delayed fluorescence is so called because it was first observed in pyridine and phenanthrene solutions. In aromatic hydrocarbons singlet- tiplet splitting is large. Therefore, thermal activation to excited singlet sate at room temperature is not possible. The intensity of emission of the delayed fluorescence I PD was proportional to the square of the intensity of absorption of the exciting light I a

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Since  f is independent of I a , the ratio of the intensity of P-type delayed emission to that of prompt fluorescence should show linear dependence on the intensity of absorption. The square law indicates the necessity of two photons for the act of delayed emission and is hence known as biphotonic process. It has been observed in fluid solutions and also in the vapour state of many compounds.

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