use have been widely discussed and will be considered in Chapter 5.
— *
Experimentally Determined pK Values
Table 1,1 shows some well-known values of pK(S^) for compounds of
various types plus some more recently published results. More comprehensive tables of pK(S^) values can be obtained in the literature^^’^^'^^’^ ^ ’^^.
72-76
Schulman et al have investigated the excited state acid-base properties of several heterocyclic and substituted heterocyclic compounds. Similar
77
studies have been reported on phenazine by Grabowska and Pakula and on a,a'-Dipyridyl and 1,10-phenanthroline by Lahiri and Aditya^^. Leonhardt
79 58
et al have followed up the work of Lindqvist on fluorescein by an
investigation of the acid-base equilibria of fluorescein and 2'-7 *-Dichloro- fluorescein in the S and S, states.o i
It is evident from Table 1.1 that, in general, A pK(S^-S^) is 56
several units large, Weller divides organic compounds into two groups. Acids belonging to Group A: ArOH, Ar Ar^H^; ArSH
have pK* < pK R R R.
whereas those of Group B ; Ar COOH; Ar C = OH; Ar C = N H
+ +
*
have pK > pK.
If, on the other hand, we consider the relative strength of the conjugate bases, a base of the type A (Ar Nik ) becomes a weaker base in the excited
* R
state (pK < pK) whereas bases of the type B (Ar C = 0) become stronger *
bases in the excited state (pK > pK). This means that bases of type A we would expect to show a first absorption band at longer wavelengths than their conjugate acid, while the opposite is true for type B,
TABLE 1.1 pK(S^) values^obtained by (1) the Fürster Cycle (2) fluorescence intensity measurements.
MOLECULE REACTION PK(S^) PK(S^) Method Referenc»
1-Naphthol deprotonation 9.23 2,0 1 51 2“Naphthylamine protonation 4.07 -1.5 2 48 2 Aminoanthracene protonation 3.4 -4.4 1 61 Acridine protonation 5.45 10.3 1 65 10.65 2 65 Acridone protonation -0.6 0.92 1 66,67 2.0 2 66,67 Acetophenone protonation -6.0 -1.0 2 68
1— Naphthoic acid deprotonation 3.7 7.7 1 70
1-Naphthoic acid protonation -7.7 2 2 69
2-Naphthoic acid deprotonation 4.2 6.6 1 70
2-Naphthoic acid protonation -7.7 0.7 2 69
9-Anthroic deprotonation 3.0 6.5 1 61 1-Naphthaldehyde protonation -6.6 3,8 1 63 1.0 2 63 Methyl-1-naphthyIketone protonation -6.2 5.8 1 63 1.5 2 63 Phthalide protonation - 8 . 0 1.3 1 63 0 2 63 Anthrone protonation -5.0 6.15 1 63 2 Naphthamide protonation -2.35 2.47 1 63 2.5 2:79 § 2-Aminopyridine protonation 6.9 9.4 1 72
majority are obtained by the Fbrster Cycle method. Table 1.2 shows some values of pK(T^) with pK(S^) and pK(8^) values given for comparison. It
is evident that in most cases the pK(T^) value lies much closer to the pK(S^) value than the pK(S^) value does. For some time after Jackson and Porter’s initial results it was thought that all pK(T^) values would lie very close to the value of pK(8^) but several results since have shown pK(T^) values spread over the pK(S^-8^) range,
Vander Donckt and Porter^^ showed this to be true for anthracene© carboxylic acids. For dyes of the thiazine (Thionine, Methylene Blue, Methylene Green, Novomethylene Blue, Azur A, B and C) and azine (safranine and phenosafranine) classes, pK(T^) différés from pK(8^) by more than 6 units^^ A major problem when considering dye molecules which have several different sites where protolytic reactions can occur is whether the same equilibrium is being considered in each excited state.
It has been suggested^^ that in the case of thionine the site of protonation does not remain the same in the different states and therefore the
pK(S^), pK(8^), and pK(T^) values cannot be compared. Grabowska and 82
Pakula report pK values for the second protonation of phenazine which indicate a large pK(8g-T^), No phosphorescence or S^-T^ absorption could be detected for the di-protonated species and an estimate of its 0-0
transition had to be made. The uncertainty in the pK(T^) obtained was considered to be less than t 1.5 pK units, which means that the increase in the pK(T^) value on excitation was of the same order as that observed in the 8^ state.
The generalisation has been made for %-tz* states that pK(8 ) > pK(T_) > pK(8.)o 1 J.
or
pK(S^) < pK(T^) < pK(S^)
* and that this system of inequalities is independent of the type of tî-tc
state considered^^. Several approaches have been used to obtain an
understanding of these pK differences, Murrell^^ put forward an explanation based on the charge transfer theory. The hydroxyl group, for example, is
TABLE 1,2 pK values for aromatic compounds in the Sq, 8^, and T^ states.
pK(T^), 1 , Values obtained by Fbrster Cycle pK(T^), 2, Values obtained by flash photolysis
Molecule Reaction PK(S^) pK(S^) PK(T^) pK(T^) Reference
1 2 2-Naphthol D 9.5 3.0 7.7 8.1 57 1-Naphthoic acid D 3.7 7.7 4.6 3.8 57 2-Naphthoic acid D 4.2 6,6 4,2 4,0 57 1-Anthroic D 3,7 6.9 5.6 61 2-Anthroic D 4,2 6.6 6.0 61 9-Anthroic D 3.0 6.5 4.2 61 Quinoline P 4.9 5.6 6.0 57,82 Acridine P 5.5 10.6 5.4 5.6 57,82 2-Naphthylamine P 4,1 -2 3,1 3.3 57 N ,N-dimethy1aniline P 4,9 2.9 2.7 57 ;j 2-Aminoanthracene P 3,4 -4.4 3.3 61 ,| o-Phenanthroline P 4.85 11,4 6.7 75,80 !j *4* o-Phenanthroline-H P -1,4 -0.1 3.8 75,80 I Phenol D 10 4.0 8.5 31 4-bromophenol D 9.3 3.1 7.7 31 1 4-methoxypheno1 D 10.2 5.6 8.6 31 i 1 4-nitrophenol D 10,46 -4.0 9.2 81 I 6-nitroquinoline P 2.10 4.4 2.1 81 i i Phenazine P 1,2 6,0 4.0 82 Phenazine-H P -4.3 4,1 5.7 82 i *4* Thionine-H P 0 9 6.3 83,84 i
In this column D = deprotonation reaction
P = protonation reaction ^
an electron donor, so that any contributions of the charge transfer structure
in the excited state will increase their acidity. The charge transfer state would lie at higher energy than the singlet or triplet, but since the amount of mixing is dependent on the energy gap between the two states it will always be greater for the singlet state which lies at higher energy. This theory explains the direction of the inequalities but fails to explain why the pK(T^) is often very similar to that of the ground state although detailed calculations for aniline have indicated that the contribution of
charge transfer to the first singlet state is 17% whereas it is only 4% to the first triplet^, Jackson and Porter suggested, on the basis of spin correlation considerations that the relative contribution of the CT
structures compared with the diradical structures would be greater in the than in the T^ state.
The resonance theory has also been applied to explain A p K values, ApK(S^-Sj^) for phenols and aromatic amines have been found to be greater than 6 units while the values for dissociation of aromatic carboxylic acids have been only 2-3 units (See Table 1,2), The former compounds all have a resonance formula in which there is a net charge on the actual site of the protolytic reaction.
Sh
For carboxylic acids the resonance structure has a negative charge on the f oxygen of the carbonyl group and thus only effects the OH acidity indirectly, j From these structures it is easy to see why the amino group will be a j weaker base in the excited state and that the carbonyl group ( including that 1 of carboxylic acid) will be affected in the opposite direction. I
30
3. Introduction to the Experimental Investigation
In an attempt to characterise the proton-rdonating ability of strongly acidic solutions Hammett and Deyrup introduced a logarithmic term, similar to pH, and called it the acidity function Referring to the protonation of a base B B + H* ^ BH"*" H is defined as . . . . ^BH+ = PKb h+ - log — (1.5)
where a^, f^ and c^ are the activity, activity coefficient, and concentration of species i and p K ^ ^ = -log where
w ■ ^
Hammett used a series of weak organic bases, mainly nitro anilines, and measured the variation of the indicator ratio [BH^]/[B] with increasing acidity. By using indicators which protonate at different acidities and defining the slope of the graph of log iO LBJ against H as unity, ao complete acidity scale was evaluated up to 100% sulphuric acid. The
scale was anchored in the pH range by the most basic compound 4-nitroaniline. The Hammett treatment assumes that the ratio of activity coefficients for the conjugate acid and its neutral base, f ^ ^ / f g is only a function of
the solvent; i.e. at an acidity in which two of the indicators overlap
^BH+ ^BH+
and