Forster’s cycle

Sandstrom et al. (65) evaluated the Kj value for 4,5-dimethyl-A-4-thiazoline-2-thione (46) in water (Scheme 19) K-j= 10. A-4-Thiazoline-2-thiones are less basic in the first excited state (61) than in the ground state, so application of Forster s cycle suggests that the thione form is even more favored in the first excited state. Huckel molecular orbital (HMO) calculations suggest that electronic effects due to substitution in... 

Forster s cycle (50MI1) (method 1 in Table VIII, also known as the thermodynamic method ). This cycle is particularly important because it can be used even when the protolytic equilibrium is not reached in the excited state. On the other hand, it has two important limitations (i) the frequencies of the 0-0 transitions in absorption or emission are necessary and (ii) ionization entropy changes are assumed to be the same in the ground and in the excited states. The experimental difficulties involved in determining the 0-0 transition frequencies have led to the use of the frequencies of the absorption maxima (procedure a), emission maxima (procedure b), or the average therefrom (procedure c). 

Singlet excited state acid dissociation constants pK can be smaller or greater than the ground state constant pK by as much as 8 units. Phenols, thiols and aromatic amines are stronger acids upon excitation, whereas carboxylic acids, aldehydes and ketones with lowest >(71, ) states become much more basic. Triplet state constants pKr are closer to those for the ground state. Forster s cycle may be used to determine A pK =pK —pK) from fluorescence measurements if proton transfer occurs within the lifetime of the excited molecule. 

Figure 4.12 Forster s cycles for pK and pK HA=undissociated acid A=conjugated base.

Excited-state proton transfer relates to a class of molecules with one or more ionizable proton, whose proton-transfer efficiency is different in the ground and excited states. The works of Forster [2-4] and Weller [5-7] laid the foundation for this area on which much of the subsequent work was based. Forster s work led to the understanding of the thermodynamics of ESPT. He constructed a thermodynamic cycle (Forster cycle) which, under certain acceptable approximations, provides the excited-state proton-transfer equilibrium constant (pK f,) from the corresponding ground-state value (pKa) and electronic transition energies of the acid (protonated) and base (deprotonated) forms of the ESPT molecule ... 

Adiabatic protolytic equilibria in the triplet state are generally fully established due to the intrinsically longer lifetimes of triplets. Soon after Forster s work, Jackson and Porter determined the acidity of 2-naphthol (10) in the triplet state by flash photolytic titration.382 The triplet triplet absorption of 10 changes from 2max = 432 to 460 nm as the pH is moved above the triplet p/Ta (10) of 8.1. Triplet state acidity can also be predicted using the Forster cycle. The triplet excitation energies ET of the acid and its conjugate base are determined from the 0 0 bands of their phosphorescence spectra. 

In some cases, data obtained through the Forster cycle show similar inconsistencies, depending on whether absorption or emission is used. It may well be that either the equilibrium structure in the excited state is very different from the unrelaxed Franck-Condon one, or that 0-0 frequencies are too poorly estimated. It seems, therefore, that the most reliable results are those generated by method (3). This method has been applied to the study of carbazole (3) acidity in its S, state (85MI5). 

As mentioned in the previous section, the carbonylation of methanol to acetic acid is an important industrial process. Whereas the [Co2(CO)s]-catalyzed, iodide-promoted reaction developed by BASF requires pressures of the order of 50 MPa, the Monsanto rhodium-catalyzed synthesis, which is also iodide promoted and which was discovered by Roth and co-workers, can be operated even at normal pressure, though somewhat higher pressures are used in the production units.4,1-413 The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster.414 The anionic complex m-[RhI2(CO)2]- (95) initiates the catalytic cycle, which is shown in Scheme 26. 

Weller s work [5-7] on the kinetics of ESPT brought out the importance of competition between the rates of deactivation of the excited states and the rates of proton transfer. In cases where the deactivation rates are slow enough for a complete establishment of excited-state equilibrium, fluorimetric titrations provide a method for experimental determinations of pK a. However, it has been realized that for a fairly large number of ESPT molecules, there is a frequent mismatch of pA"f, values obtained from Forster cycle and fluorimetric titrations methods. There are also examples of extended fluorimetric titration curves resulting from low proton availability in the mid-pH region (4-10). Various modifications of the Forster cycle and extensions of Weller s original kinetic considerations have been made from time to time and have been reviewed periodically. Some of the earlier important ones include those by Weller [7] in 1961, Vander Donckt [8] in 1970, Schulman [9] in 1974, and Klopffer [10] in 1977. The review by Ireland and Wyatt [11] contains extensive references of experimental results available in the literature until 1976. 

Excited state pX-values are most easily accessible through the use of the Forster cycle which has been described in the introduction. To perform this calculation for a particular molecule it is necessary to know the ground state equilibrium constant for the reaction in question and to have some measure of the energy difference between the lowest vibrational level of the ground and the excited state in both the B and BH+ forms. Thus to calculate pi Sj) we need the 0-0 energy of the S0-S transition and for pX(Tt) that of the Sq-T transition. 

The kinetic isotope effects shown in Fig. 11 (Forster, 1972) resemble those reported for 2-naphthol by Stryer (1966). Like 2-naphthylamine, 2-naphthol shows an increased quantum yield and protonation of Sj occurs at lower acidities in D20 than in H20. For 2-naphthol, p-KJSj )-values of 3 0 in H20 and 3-4 in D20 are calculated from the measured excited state rate constants in H20 k.j = 5-29 x 107 s 1 and k2 = 5-5 x 101 0 dm3 mole-1 s-1, while in D20 k1 = 1 3 x 107 s-1 and k2 = 3-5 x 1010 dm3 mole-1 s-1. These results confirmed the earlier p/ (S )-values calculated by Wehry and Rogers (1966) using the Forster cycle (Table 9), which show incidentally that the pK-values are closer by about 0 1 unit in the Sj state. 

In order to understand the striking difference between the S, and T states, one needs to go beyond a simple consideration of the charge densities (Constanciel, 1972). From the FOrster cycle shown in Figure 1.21 it is clear that the critical quantity is the difference in the substituent effect on the... 

Calculated from pK(S )-value and Ap/C(Si —So) obtained using the Forster cycle, t Azaphenanthrenes are numbered using the IUPAC numbering of phenanthrene. 

Reinvestigation of the excited state acid-base properties of 2-naphthyIamine (Schulman and Capomacchia, 1972) showed that a reported change of hybridization from sp3 to sp2 on excitation had little effect on the entropy of protonation of nitrogen in the S state and that therefore the Forster cycle was still applicable. A p/qSj )-value, calculated from the fluorescence maxima of the B and BH+ form, of —8-1 is in poor agreement with the value, —2, obtained from fluorescence titration measurements. From the acidity dependence of fluorescence intensity for 1- and 2-naphthylamine Liedke and Schulman (1973a) found that the decrease in emission of the B form occurred at lower acidities than the appearance of BH+ fluorescence. [A similar titration curve for the fluorescence of the neutral molecule was obtained by Seliskar and Brand (1971), who obtained a value of 0-64 for p/s((S1) from the decrease of the... 

Colpa et al. (1963) calculated p/ (S )-values for a series of aromatic hydrocarbons, but could not detect fluorescence changes in the regions of acidity indicated by the Forster cycle, although fluorescence spectra attributed to proton complexes of 3,4-benzo-pyrene and 1,2-benzanthracene were observed in some solutions containing only the neutral molecule in the ground state. Flurry and co-workers (1963, 1966, 1967) have carried out theoretical and Forster cycle calculation on the excited state basicities of poly-methylbenzenes and Kuz min et al. (1967) have also calculated p/sT(S1)- and p7 (T )-values for polycyclic aromatic hydrocarbons increases in base strength of from 7 to 30 powers of ten were derived for Sj. 

Figure 12.5 Forster cycle of photoacids in solution. Energy levels are for a general photoacid A H and its conjugated base (A ) - S,> is the excited-state of the acid and S, > of the base, g> is the ground-state ofthe acid and g > ofthe anion, respectively, hv, and hv i, are the energy of the absorption transition and hv f, are the energy ofthe fluorescence transition ofthe acid and base, respectively. ACp,c and (the Forster...

---