Excited-state acidity table

Excited state proton transfer is very likely according to the excited state acidities estimated according to eq. 6. The ground and excited state pK data of HB and 4-methoxy HB are compiled in Table 9. They follow the general order PK3 < pK.j. <... 

Interestingly, Forster-cycle calculations of the pAla in methanol (Table 2) seem to confirm the substituent effect on the polarity of the emitting state of 1-naphthol as discussed above the less polar the emitting state of the acid compared to the emitting state of its conjugate base, the larger the Forster-cycle acidity of the photoacid. The calculated Forster-cycle difference between the ground-state and excited-state acidities in methanol was 12.3, 11.3, 10.9, 9.3 and 8.8 for the 2-substituted, 3-substituted, unsubstituted 4- and... 

Forster photoacidity to the total (absolute) acidity of photoacids in their electronic excited state may be estimated directly from the corresponding pfQ and values. Additional questions are the extent to which photoacidity may be tuned by suitable substituents which also affect the ground state acidity of the photoacid and, alternatively, by the choice of the solvent. Table 12.3 compares the effect of several substituents on the ground- and excited-state acidities of several photoacids. The first conclusion that may be drawn from this table is that ring substituents cause the fQ and K of aromatic photoacids to change in the same direction. 

Table 6 Energy values (in a.u.) of M(n,7r ) excited state of formic acid for the two minima and two planar conformations.

Table 7 Geometrical parameters for four conformations of the A n n ) singlet excited state of formic acid.

All the nucleic acid bases absorb UV radiation, as seen in Tables 11-1, 11-2, 11-3, 11-4, and 11-5, making them vulnerable to the UV radiation of sunlight, since the energy of the photons absorbed could lead to photochemical reactions. As already mentioned above, the excited state lifetimes of the natural nucleobases, and their nucleotides, and nucleosides are very short, indicating that ultrafast radiationless decay to the ground state takes place [6], The mechanism for nonradiative decay in all the nucleobases has been investigated with quantum mechanical methods. Below we summarize these studies for each base and make an effort to find common mechanisms if they exist. 

Remarkable positive shifts of the °red values of the singlet excited states of the metal ion-carbonyl complexes as compared to those of the triplet excited states of uncomplexed carbonyl compounds (Table 2) result in a significant increase in the redox reactivity of the Lewis acid complexes versus uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. For example, photoaddition of benzyltrimethylsilane with naphthaldehydes and acetonaphthones proceeds efficiently in the presence of Mg(C104)2 in MeCN, although... 

In the last few years more information on excited state pA-values has accumulated and the present review contains extensive reference tables of the experimental results in the literature available up to August 1974. We have confined our attention throughout to Br nsted acids and bases, though work on Lewis acid (donor-acceptor) systems continues (Weller, 1961 Birks, 1970 Ottolenghi, 1973) and may prove directly relevant to a deeper understanding of prototropic reactions, which must often be preceded by the formation of hydrogen-bonded complexes. Such interactions also play a role in solvent effects upon the absorption frequencies of acid and base molecules. 

Table 1 is included for quick reference and shows the expected direction of pA-change on excitation to Sj for various functional groups. Molecules with a negative ApA [i.e., pA(S,) — pA(S0)] become stronger acids (weaker bases) in the excited state and those with a positive ApA stronger bases (weaker acids). 

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. 

The excited state pA-behaviour of quinoline derivatives has received considerable attention (see Tables 6.3 and 6.4). Since the heterocyclic nitrogen atom of quinoline is expected to become a stronger base in the excited state while the acidity of hydroxyl or amino-substituents increases, different ionization sequences can be obtained in the S0 and states. 3-Hydroxyquinoline is a typical example of this behaviour as shown in Scheme 2 (Haylock et al., 1963 Mason et al., 1968). 

The luminescence properties of polynucleic acids and proteins have been reviewed in detail previously. However recent time-resolved studies have important implications for existing concepts of the excited states of biopolymers. The readily observable luminescence from proteins at room temperature has made these systems particularly attractive to study. The fluorescence properties of the arrxnatic amino acid zwitter-ions which determine the emission from proteins are summarised in Table 11. The wide range of emission maxima observed in proteins (ranging from 308 nm in azurin to 342 nm in lysozyme) has been exfdained by many authors to reflect the different environments of the tryptophan residues in the proteins However, it is now... 

Negligible photoreaction was observed for p,p -dichlorobenzophenone (DCB), a DDT oxidation product, in air-saturated, distilled water (half-fife >15 h at 313 nm). Nevertheless, this halocarbon photoreacted (313 nm) with half-lives corrected for light attenuation of about 3 h in a filtered natural-water sample and a solution of Contech fulvic acid (Table II). The greater than four- to fivefold enhancement in photoreaction rate in this case probably results from hydrogen atom abstraction from the natural organic matter by the DCB in its excited triplet state (eq 14). 

Several properties of these ruthenium(II) complexes are shown in Table 2. Apparently, the absorption and emission maxima of [Ru((-)-menbpy)3l and [Ru(S( - )-PhEtbpy)3l + exhibit considerably large red shifts, compared to those of [Ru(bpy)3l. A similar red shift was observed in [Ru(dmp)n(dcbpy)3 n] (dcbpy = 2,2 -bipyridyl-4,4 -dicarboxylic acid), as was shown in Table 1. These red shifts are easily understood in terms of the introduction of electron-withdrawing substituents at the 4 and 4 positions of 2,2 -bipyridine. The other important feature is that the lifetime of the MLCT excited state becomes much longer than that of [Ru(bpy)3l +. One of the important reasons is the increase in the energy difference between the triplet d-d ( d-d) and MLCT excited states, as follows [13,29] Since the electron-withdrawing substituent of 2,2 -bipyridine stabilizes the TT orbital of 2,2 -bipyridine, the MLCT excited state becomes lower in energy, but the d-d excited state is little influenced in energy by the substiment. 

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