Photoacids and Photobases

Where an asterisk on a concentration symbol indicates that the species is in the 

4) [5] with the equilibrium constant of the photoacid in the ground state, which should be independently known to facilitate the calculation. 

The Forster acidity, ApfC, may be found with the aid of the Forster cycle (Fig. 

1 -naphthol-3,6-disulfonate (1 N3,6diS), 1 -naphthol-4-chlorate (1 N4CI), 1-naphthol-5-cyano (lN5CN),and 1-naphthol-5-tetrabutyl (INStBu). 

2-naphthol-5-cyano- (2N5CN), 2-naphthol-8-cyano (2N8CN), 

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Fig. 3. Decay of the photoacid band at 1486 cm 1 (left panel), the rise of the conjugated photobase band at 1503 cm 1 (centre panel) and acetic acid band at 1720 cm 1 (right panel) as function of acetate concentration. The photoacid and photobase signals are in addition affected by rotational diffusion.

Over the years the field of photoacids (and photobases) has been reviewed many times . The most extensive list of photoacids appeared, so far, in a 1976 review by Ireland and Wyatt The hydroxyarenes are the most widely used photoacids. In polar solutions they may undergo an excited-state proton-transfer reaction according to the general reaction scheme of equations 2-5. 

The total error in carrying out the Forster cycle with the averaged transition energies of the photoacid and photobase rather than with the thermodynamic AGppG and AG ppQ values may be estimated by the following simple algebraic consideration, Eq. (12.8). 

Proton transfer dynamics of photoacids to the solvent have thus, being reversible in nature, been modelled using the Debye-von Smoluchowski equation for diffusion-assisted reaction dynamics in a large body of experimental work on HPTS [84—87] and naphthols [88-92], with additional studies on the temperature dependence [93-98], and the pressure dependence [99-101], as well as the effects of special media such as reverse micelles [102] or chiral environments [103]. Moreover, results modelled with the Debye-von Smoluchowski approach have also been reported for proton acceptors triggered by optical excitation (photobases) [104, 105], and for molecular compounds with both photoacid and photobase functionalities, such as lO-hydroxycamptothecin [106] and coumarin 4 [107]. It can be expected that proton diffusion also plays a role in hydroxyquinoline compounds [108-112]. Finally, proton diffusion has been suggested in the long time dynamics of green fluorescent protein [113], where the chromophore functions as a photoacid [23,114], with an initial proton release on a 3-20 ps time scale [115,116]. 

SHI 96] Shirai M., Tsunooka M., Photoacid and photobase generators Chemistry and applications to polymeric materials , Progress in Polymer Science, vol. 21, pp. 1-45, 1996. 

Equation 17 may be viewed as an explicit form of the Forster cycle. It depends on both intramolecular and intermolecular factors which determine the extent of the photoacidity. The first factor is the difference between the excited-state and the ground-state proton affinities of the photobase. This difference will be equal to the difference in the intramolecular stabilization of the proton upon the electronic excitation of the acid, and will depend, in general, on the quantum-mechanical properties of the first excited electronic state of the photoacid. The second factor is the difference in the solvation energies of the base and the photoacid upon electronic excitation. The magnitude of the solvation-energy terms will depend in general both on the solvent and the solutes and will depend on the nature of the first electronic state of the photoacid and its conjugate base. 

In conclusion, it is the opinion of this review that photoacidity manifests itself in both the photoacid and the photobase sides, the reactant side becoming a stronger acid and the product side becoming a weaker base in the excited state. It is still a matter of additional experimental and theoretical studies to establish if general rules may be drawn up concerning the relative importance and generality of these processes. Similarities to ground-state... 

For many molecules, due to extensive redistribution of electron densities, acid-base property in the excited state differs considerably from that in the ground state [33 For instance, aromatic amines are weakly basic in the ground state. But many of them become acidic in the excited state and readily donate a proton to a proton acceptor to produce the anion in the excited state. Such a molecule, which behaves as an acid in the excited state, is called a photoacid similarly, photobases are those that display basic properties in the excited state. In many cases, excited state proton transfer (ESPT) results in dual emission bands. One of these emission bands arises om the neutral excited state and bears mirror image relation with the absorption spectrum. The other emission band is due to the excited deprotonated (anion) or protonated species and exhibits a large Stokes shift. 

Recently, the usefulness of fs-resolved mid-IR measurements of some vibrational markers of the photoacid and the photobase was demonstrated by Nibber-ing et al. [97-100]. Direct mid-IR absorption spectroscopy has thus proved to be an additional tool for directly monitoring the proton-transfer kinetics of photoacids while in the excited state. 

Partial charge transfer to the aromatic ring may be important both in the photoacid and the photobase side. In the acid side this process is usually of a smaller magnitude than in the photobase side and will make the acidic proton more positively charged and more susceptible to hydrogen bonding. 

Acids are in equilibrium with their conjugate bases in protic solvents, where the relative concentrations depend on the pK value. The observed dynamics of an electronically excited photoacid, typically interpreted as the proton transfer rate to the (protic) solvent [77, 78], is thus governed by the equilibration dynamics to the new configuration - as long as the photoacid and conjugate photobase remain in the electronically excited state - as dictated by the new excited state pJ a value. Depending on the pFI of the solvent one can observe the reversible time-depen-dent geminate recombination of the photobase with the released proton [79-83], or even the reaction of the photobase with other protons present in solution. 

Recent experimental results on the acid-base neutralization reaction between HPTS and the carboxylic bases mono-, di- and trichloracetate have revealed the underlying mechanisms of proton transfer of the loose complexes [138]. It turns out that a sequential, von Grotthuss-type of hopping occurs through a water molecule bridging the HPTS photoacid and the carboxylic base. Figure 14.9 shows the transient spectra obtained with a solution of 20 mM HPTS in D2O with 1 M of monochloroacetate OAc-Cl added. At early pulse delays about 20% of HPTS has released its deuteron, as is indicated by the appearance of the HPTS photobase marker band at 1435 cm i within the time resolution. A vibrational marker band at 1850 cmr indicates the transient existence of hydrated deuterons. Comparison with literature values for hydrated proton species with well-defined surroundings... 

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