Quantum yields for ISC

Spin-orbit coupling (SOC) plays an important role in determining the likelihood of ISC. If SOC is small, then so is the quantum yield for ISC the lack of SOC thus can inhibit ISC significantly. If, on the other hand... 

If the quantum yield for ISC is high saturation may also occur, as the triplet lifetime is much longer than the singlet excited state lifetime [6,7]. Molecules that... 

The formation of product C, on the other hand, proceeds in two steps with the efficiencies 1 r/isc. (Equation 3.32) and 3rjc 3/cr/(3/cr I 3/qsc I 3/cph + 3kqcq) where 3kr refers to the reaction from the lowest triplet state Ti leading to the photoproduct C, 3/cph is the rate constant of phosphorescence, 3/cisc that of intersystem crossing to the ground state and 3/cq is the second-order rate constant for quenching of I) by a quencher q. The overall quantum yield for the formation of product C is equal to the product of the efficiencies of the two reaction steps, (I visc Vc- Many authors do not distinguish between... 

Photoreduction of aliphatic ketones may involve both singlet and triplet excited states, but the quantum yield for product formation via singlet is usually low because other competing processes, such as radical pair recombination, are involved. The rapid intersystem crossing (ISC) in aryl ketones (Section 2.1.6) allows triplet reactivity. Ketones with n,7t lowest triplets, having an unpaired electron localized in an n-orbital on oxygen, are far more reactive than those with k.k lowest triplets.863... 

For these systems, it was shown that LF excitation is followed by rapid intersystem crossing (d>isc = 1) to the lowest energy LF excited state 3E from which reactive (kp), radiative (/cr) and nonradiative (ka) deactivation occur competitively (Fig. 19) [76,83], The measurement of the phosphorescence lifetimes t and quantum yields quantum yields for halide (<1>X) and NH3 (4>A) solvolysis under identical conditions allowed calculation of the rate constants for the individual ES decay processes according to Eq. (5) [84], These studies also demonstrated that the photoreactions of Rh(NH3)5Cl2+ are dramatically solvent dependent. La-bilization of Cl- predominates in aqueous or formamide solutions NH3... 

The external quantiun yield or I PC E (incident photon to current efficiency) is defined as the quotient of the number of incident photons and the number of charge carriers output to the external circuit. It is smaller than the internal quantum yield for conversion of the absorbed photons into charge carriers within the cell, because it takes into account losses due to reflection, recombination, and scattering. In contrast to the internal quantum yield, which can attain values of nearly 100% (see above), the value of the external quantum yield can be measured directly from the short-circuit current density jsc. with jsc = Isc/A where A is the active area of the cell, and the incident light intensity is lo- At a given wavelength k, we have... 

The quantum yield is an indicator of how efficient a particular process is. However, some care must be taken in comparing quantum yields for different systems, because the quantum yield is always measured relative to other processes in the molecule. For example, pyrene derivative. The reason this is not reflected in the quantum yields is that we must also consider competing processes. The ISC rate for pyrene-3-carboxaldehyde is also much faster than that for benzene due to the ability to access a ( ,tt ) state that is not available for benzene. This competing ISC process limits the amount of fluorescence, and by coincidence the two compounds end up with the same fluorescence quantum yield. Thus, while the quantum yield tells you about the efficiency of a process for a given molecule, it alone cannot tell you why the process is or is not efficient. 

The singlet-triplet intersystem crossing quantum yields (isc) for the compounds with unknown values, sc tit known triplet molar absorption coefficient, 8 , can be obtained by comparing the AODl, in the triplet-triplet absorption maximum of the compounds, with the AODj in the triplet wavelength absorption maximum of a reference compound with known intersystem crossing quantum yield, (j> Q, and triplet molar absorption coefficient, using Eq. (15.14) [17]. 

The photo physical and electrochemical properties ofACN are summarized in Table 21.1. Data for naphthalene (NP) and acenaphthene (ACE), congeners of ACN with a naphthalene moiety, are included in the same table to emphasize the peculiarly short lifetime of the ACN S, state, the low efficiency of fluorescence, the low quantum yield for intersystem crossing (ISC), and the low efficiency for phosphorescence. 

TABLE 4B. 1 Singlet Excited State Lifetimes, Quantum Yields of ISC and ISC Rate Constants for Several Non-Conjugated Ketones... 

Solovjev et al. [8, 21] have shown that the main decay route of the first excited singlet state (l Bia in our notations) of free-base porphin is the intersystem crossing to the lowest triplet state T (1 B2 in our results. Table 1.2) with a quantum yield 4>isc = 0.9. So far no phosphorescence has been observed from free-base porphin in solid solvents usually used for phosphorescence detection only Ti 5o emission induced by the external heavy atom (EHA) effect has been recorded [21-23]. Gouterman and Khalil [22] observed a low-resolution phosphorescence spectram of free-base porphin in ethyl iodide+EPA mixture. The natural radiative lifetime for phosphorescence of various free bases were found to be 70 s and higher. Michl et al. [23] have reported Fourier transform measurements of fluorescence and... 

For a triplet state reaction the quantum yield is not only dependent upon the relative rates of the process and other processes leading to deactivation of the triplet state, but also to the efficiency of population of the triplet state (d>isc) ... 

Because of the forbiddeness of the transition, Tx —> S0 + ho, the natural phosphorescent lifetime, t°, of a triplet state is long—from approximately 10"3 sec for an n,it triplet to 30 sec for a rr,n aromatic triplet. At room temperature in solution, phosphorescence is often not observed because ISC of Tx to S0 and quenching of Tx by impurities and molecular 02 (see below) competes effectively with phosphorescence. Therefore most phosphorescence studies must be carried out at low temperatures in carefully purified, outgassed, rigid media. Under these conditions the quantum yield of phosphorescence, 9P, defined by Equation 13.10, is often high and approaches 1.0 for some aromatic carbonyls. 

Although the triplet spectra and the ISC quantum yield of oligothiophenes are well characterized, the location of the lowest triplet state Ti could not be determined in solution and in isolated molecules so far because of a lack of phosphorescence (95PAC9 96JPC18683). Attempts have also been made to detect an So-Ti transition by absorption measurements in highly concentrated solutions, but these attempts were not very successful. The only So-Ti spectrum that has been reported was measured for 3T (90PP(52)655). 

Marginal fluorescence quantum yields (1%) are generally observed though 25 and 33 fluorescence with 8% and 14% yields, respectively. Such low quantum yields are indicative of the effective competition of radiationless processes such as the Si —> Tj ISC and fast internal conversion (Si —> S0). The rate constants for radiative decay of Si (kF) range from 8 x 106 to 1.3 x 108 s-1, and the nonradiative decay rate constants (fcNR) range from 1.9 x 108 to 3.5 x 109 s / The nonradiative deactivation pathway is thus six times faster than the radiative one for 33 (anti) and about 110 times faster for 32 (syn). 

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