Unimolecular Photophysical Processes

The lowest vibrational energy levels of a state are indicated by thick horizontal lines other horizontal lines represent associated vibrational levels. Vertical straight lines represent radiative transitions, wavy lines nonradiative transitions. The orders of magnitude of the first-order rate constants for the various processes are indicated. From R. B. Gundall and A. Gilbert, Photochemistry, Thomas Nelson, London, 1970. Reproduced by permission of Thomas Nelson and Sons Limited. 

The three unimolecular physical processes that originate from SI are internal conversion to emission of lights and transformation of Sx into Tx. If IC occurs, the net change resulting from electronic excitation is heat transference to the solvent. The other two processes are considerably more interesting. 

A substance with a highly probable 50—transition (e x 105) will have r°f of 10 9 sec. A substance whose S0 - S1 transition is space-forbidden (emax X 10-1) will have a t°s of 10 3 sec such long-lived direct fluorescence is only very rarely observed because competing processes destroy or quench before a quantum 

Occasionally absorption occurs from a higher vibrational level of S0. This leads to anti-Stokes lines, in which the fluorescence is at shorter wavelengths than 

The distance between peaks in a fluorescence spectrum is a measure of the energy differences between vibrational levels in the ground state likewise, the peaks in the lowest-energy band of the absorption spectrum give the energy differences between vibrational levels in Sv If the spacings are similar, the absorption and emission spectra are mirror images of one another. 

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The rate constants for unimolecular photophysical processes in few representative organic molecules are given in the Table 5.4. These give an idea of the order of magnitude expected for various processes. 

Late constants for unimolecular photophysical processes in some organic molecules... 

The various unimolecular photophysical processes may be envisaged in a rather illuminating way with the help of the Jablonski diagram shown in Fig-... 

Table 5.1 Relations Between Quantum Yield, Lifetime, and Rate Constcmt of Unimolecular Photophysical Processes...

Figure 4.1 Jablonski diagram of the important unimolecular photophysical processes of excitation energy dissipation absorption (A), fluorescence (F), phosphorescence (P), internal conversion (IC), intersystem crossing (ISC), vibrational relaxation (VR). Sq is the singlet ground state, Sj and S2 are excited singlet states and Tj is the excited...

Rate constants for photophysical unimolecular radiative processes can be obtained from spectral data and fcJSC and kjsc computed therefrom. The rate constants for radiationless processes are important parameters in photochemistry because the lowest singlet and triplet states are seats of photochemical reactions. 

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. 

The photochemistry of phenyl azide and its simple derivatives have received the most attention in the literature. The results of early studies were summarized in a number of reviews. " Over the last decade, modem time-resolved spectroscopic techniques and high level ab initio calculations have been successfully applied and reveal the detailed description of aryl azide photochemistry. This progress was analyzed in recent reviews. Femtosecond time resolved methods have been recently employed to study the primary photophysical and photochemical processes upon excitation of aryl azides. The precise details by which aryl azide excited states decompose to produce singlet arylnitrenes and how rapidly the seminal nitrenes lose heat to solvent and undergo unimolecular transformations were detailed. As a result of the application of modem experimental and theoretical techniques, phenylnitrene (PhN) - the primary intermediate of phenyl azide photolysis, is now one of the best characterized of all known organic nitrenes. " 5 "-2° - ... 

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