Proton dissociation excited molecules

Proton dissociation in the excited states commonly occurs much easier than in the ground states, and the great difference in proton dissociation constants by several orders of magnitude is characteristic for photoacids [47]. These dyes exist as neutral molecules and their excited-state deprotonation with the rate faster than the emission results in new red-shifted bands in emission spectra [48]. Such properties can be explored in the same manner as the ground-state deprotonation with the shift of observed spectral effect to more acidic pH values. 

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. 

The rate of proton dissociations from the excited states of molecules can be measured directly by nanosecond fluorimetry.189... 

It should be stressed that the reversible chemical reactions give us better chance to observe many-particle effects since there is no need here to monitor vanishing particle concentrations over many orders of magnitude. Indeed, the fluctuation-controlled law of the approach to the reaction equilibrium similar to (2.1.61) was observed recently experimentally [85] for the pseudo-first-order reaction A + B AB of laser-excited ROH dye molecules which dissociate in the excited state to create a geminate proton-excited anion pair. The solvated proton is attracted to the anion and recombines with it reversibly. After several dissociation-association cycles it finally diffuses to long distances and further recombination becomes unobservable. 

Dynamics of Proton Dissociation from Excited Molecules. 4... 

The rate of proton dissociation can be obtained, either by steady-state or time-resolved measurements. The reaction describing the proton dissociation from the excited molecule is summarized in Scheme I... 

Apparently neither electrostatic interactions nor reduced diffusibility of protons is the major cause for the decrease in the proton transfer rate. As these effects are dominating in ion-pair recombination and ion-pair separation, we have to focus our attention to the primary event in proton dissociation ion-pair formation. In this reaction, the hydrogen of the OH bond of the excited parent molecule forms a hydrogen bond with the nearest H2O molecule, which itself is hydrogen bonded to other water... 

Figure 12 (line A) depicts the emission spectrum of hydroxy pyrene trisulfonate dissolved in diluted buffer (pH 5.0). At this pH, the ground state is fully protonated (pK0 = 7.7), but not so the first excited singlet state (pK = 0.5). The excited molecules dissociate and 95% of the emission is at the wavelength of the excited anion (515 nm). The dissociation can be prevented if the compound is dissolved in acid solution, pH < pK., such as 2MHC1 (line B). Under such conditions, we observe the emission of the neutral form with maximum at 445 nm. Upon ligation to apomyoglobin, the fluorescence of hydroxypyrene trisulfonate consists of two... 

Proton transfer is a fundamental process in both chemistry [1-3] and biology [4]. In particular, proton dissociation namely, proton transfer to solvent, from aromatic dye molecules in their excited electronic state [5] can be easily studied by virtue of their strong fluorescence signal [6]. The older fluorescence measurements did not possess time resolution It was only possible to obtain steady-state quantum yields under conditions of constant illumination [6]. The conventional interpretation of the experimental data assumed a chemical kinetic scheme, such as [3]... 

ABSTRACT. Kinetics of proton transfer photoreactions in simple model systems is analyzed from the point of view of reaction kinetics in microphases. Protolytic photodissociation of some hydroxyaromatic compounds ArOH ( 1- and 2-na-phthol, chlorosubstituted naphthols ) was studied in micellar solutions and phospholipid vesicles by fluorescence spectra and kinetics. Experimental results give evidence of at least two localization sites of naphthols in the microphase of these systems. In lipid bilayer membranes of vesicles there are two comparable fractions of ArOH molecules, one of which undergo photodissociation, but another do not dissociate. In micelles only minor fraction ( few per cent ) of ArOH molecules do not take part in excited-state proton transfer reaction. These phenomena reflect heterogeneous structure and dynamic properties of lipid bilayer membranes and micelles. A correlation between proton transfer rate constants and equilibrium constants in microphases similar to that in homogeneous solutions is observed. Microphase approach give a possibility to discuss reactions in dynamical organized molecular systems in terms of classical chemical kinetics. 

In proton dissociation of excited hydroxyarenes, the geminate recombination strongly depends on the coulomb potential between the ejected proton and the parent anion molecule, as well as the mutual diffusion coefficient. In contrast, in proton dissociation of protonated amines, the contribution of geminate recombination is expected to be less important, because the reaction produces a geminate pair between proton and excited neutral amines [92-94]. 

Collision-induced dissociation mass spectrum of tire proton-bound dimer of isopropanol [(CH2)2CHOH]2H. The mJz 121 ions were first isolated in the trap, followed by resonant excitation of their trajectories to produce CID. Fragment ions include water loss mJz 103), loss of isopropanol mJz 61) and loss of 42 anui mJz 79). (b) Ion-molecule reactions in an ion trap. In this example the mJz 103 ion was first isolated and then resonantly excited in the trap. Endothennic reaction with water inside the trap produces the proton-bound cluster at mJz 121, while CID produces the fragment with mJz 61. 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

When pyridoxamine with a dipolar ionic ring structure (Fig. 14-9) and an absorption peak at 30,700 cm-1 (326 ran) is irradiated, fluorescence emission is observed at 25,000 cm 1 (400 ran). When basic pyridoxamine with an anionic ring structure and an absorption peak at 32,500 cm 1 (308 nm) is irradiated, fluorescence is observed at 27,000 cm-1 (370 nm), again shifted 5500 cm 1 from the absorption peak. However, when the same molecule is irradiated in acidic solution, where the absorption peak is at 34,000 cm 1 (294 nm), the luminescent emission at 25,000 cm 1 is the same as from the neutral dipolar ionic form and abnormally far shifted (9000 cm ) from the 34,000 cm-1 absorption peak.185186 The phenomenon, which is observed for most phenols, results from rapid dissociation of a proton from the phenolic group in the photoexcited state. A phenolic group is more acidic in the excited state than in the ground state, and the excited pyridoxamine cation in acid solution is rapidly converted to a dipolar ion. 

Electronically excited benzene, however, can accept a proton to form a kind of complex which eventually dissociates back to the ground state molecule (Figure 4.49). 

---