External conversion

Energy level diagram for a molecule showing pathways for deactivation of an excited state vr Is vibrational relaxation Ic Is Internal conversion ec Is external conversion, and Isc Is Intersystem crossing. The lowest vibrational energy level for each electronic state Is Indicated by the thicker line. 

Another form of radiationless relaxation is internal conversion, in which a molecule in the ground vibrational level of an excited electronic state passes directly into a high vibrational energy level of a lower energy electronic state of the same spin state. By a combination of internal conversions and vibrational relaxations, a molecule in an excited electronic state may return to the ground electronic state without emitting a photon. A related form of radiationless relaxation is external conversion in which excess energy is transferred to the solvent or another component in the sample matrix. 

Internal Conversion.—Nonradiative transition between states of like multiplicity. The process is conceived of as involving iso-energetic transitions from a higher electronic state to an upper vibrational level of a lower state (cf. Fig. 1). To consummate the change, transfer of vibrational energy to the environment (external conversion) must occur rapidly. Since some authors feel that the transition between states in solution is directly coupled with solvent phonon states, the distinction... 

External Conversion.—Transfer of excitation energy to the environment. The term is now little used but was originally coined to contrast with internal conversion (vide supra). In the usual context the term has about the same meaning as energy transfer. 

A rise in temperature increases the rate of vibrations and collisions, resulting in increased intersystem crossing, internal and external conversion. Consequently, the fluorescence intensity is inversely proportional to the temperature increase. Additionally, an increased temperature causes a red shift of the emission wavelength. 

Deactivation of an excited electronic stale may involve interaction and energy transfer between the excited molecule and the solvent or other solutes. This process is called cxtcrnulconversion. Hvidence for external conversion includes a marked solvent effect on the fluorescence intensity of most species. Fui Ihcrniorc. those conditions that tend to reduce the number of collisions between particles flow temperature and high viscosity) generally lead to enhanced fluorescence. The details of external conversion processes are not W cll undersiood. 

Radiationless transitions to the ground state from the lowest excited singlet and triplet states (t ig uro t 5-2) prohably involve external conversion, as well as internal conversion. 

Deactivation of electronic excited slates may also involve phosphorescence. After inlersystem crossing to the triplet state, further deactivation can occur cither by internal or external conversion or by phosphores cencc. A triplet — singlet transition is much less probable than a singlet-singlel conversion. Transition probability and excited-state lifetime are inversely related, Thus, the average lifetime of the excited triplet stale with respect to emission is large and ranges from 10 to 10 s or more. Emission from such a transition may persist for some time after irradiation has ceased. 

For external conversion controlled by dynamic quenching with a single quencher, the external conversion rate cxtnsiani can he w ritten as... 

Detine the following terms (a) fluorescence, (b) phosphorescence, (c) resonance fluorescence, (d) singlet state, (e) triplet slate, (f) vibrational relaxation, (g) internal conversion, (h) external conversion, (i) iniersyslem crossing. (j) predissocia-tion, (k) dissociation, (1) quantum yield, (ni) chemiluminescence. 

Because of this dichotomy in the language requirements, several languages must be used. This imposes the condition that either all the languages support the same interface semantics, or the language definitions provide an external conversion methodology between the interface protocols. For both the behavior defined on the interface and the internal behavior of a block, specification of the desired behavior must consider both functionality, sequencing, and timing. 

Figure 1 A Jabtonski diagram for a hypothetical luminescent molecule, where A is absorption, F is fluorescence, P is phosphorescence, VR is vibrational relaxation, IC is internal conversion, EC is external conversion, ISC is intersystem crossing, v" and are vibrational levels associated with each electronic state. So is the electronic ground state. Si and S2 are excited singlet states and Ti and T2 are excited triplet states.

The energy conversion process changes the nature and amounts of energy from one subvariety to another (internal conversion) or from one variety to another (external conversion). In this latter... 

Figure 15-2 and our discussion of deactivation processes suggest that the fluorescence quantum yield 4> for a compound is determined by the relative rate constants k, for the processes by which the lowest excited singlet slate is deactivated. These processes are fluorescence (k,). intersyslem crossing (k ). external conversion (k, ), internal conversion (Jt ), predissociation (fcpd), and dissociation (kj). We can express these relationships by the equation... 

The quantum efficiency of fluorescence in most molecules decreases with increasing temperature because the increased frequency of collisions at elevated temperatures improves the probability for deactivation by external conversion. A decrease in solvent viscosity also increases the likelihood of external conversion and leads to the same result. 

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