Photochemistry of “Super” Photoacids. 2. Excited-State Proton Transfer in Methanol/Water Mixtures

Solntsev, Kyril M.; Huppert, Dan; Agmon, and Noam; Tolbert, Laren M.

doi:10.1021/jp994454y

Cited by 159 publications

(183 citation statements)

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“…This finding agrees with the results of optical studies on the proton transfer dynamics of HPTS and related photoacids. [5][6][7][8][9][10][11] In the case that only HPTS is dissolved in water ͓no base, Fig. 2͑a͔͒, the proton is transferred to the water solvent leading to the formation of hydrated protons.…”

Section: Resultsmentioning

confidence: 99%

“…The excitation leads to an enhancement in the acidity of the molecule by a factor of 10 6 . HPTS has been used to study the dynamics of acid dissociation [5][6][7][8][9][10][11] in water using a number of time-resolved spectroscopic techniques. The excited state of HPTS, HPTS * , deprotonates ͑PT to solvent͒ with a time constant of 90/ 220 ps in H 2 O / D 2 O.…”

Section: -3mentioning

confidence: 99%

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Parallel proton transfer pathways in aqueous acid-base reactions

Cox

Bakker

2008

The Journal of Chemical Physics

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We study the mechanism of proton transfer ͑PT͒ between the photoacid 8-hydroxy-1,3, 6-pyrenetrisulfonic acid ͑HPTS͒ and the base chloroacetate in aqueous solution. We investigate both proton and deuteron transfer reactions in solutions with base concentrations ranging from 0.25M to 4M. Using femtosecond midinfrared spectroscopy, we probe the vibrational responses of HPTS, its conjugate photobase, the hydrated proton/deuteron, and chloroacetate. The measurement of these four resonances allows us to follow the sequence of proton departure from the acid, its uptake by the water solvent, and its arrival at the base. In recent studies it was shown that proton transfer to carboxylate bases proceeds via Grotthuss conduction through a water wire connecting the acid and the base ͓Mohammed et al., Science 310, 83 ͑2005͒;Agnew. Chem. Int. Ed. 46, 1458 ͑2007͒;Siwick and Bakker, J. Am. Chem. Soc. 129, 13412 ͑2007͒; J. Phys. Chem. B 112, 378 ͑2008͔͒. Here we show that, for the weaker base chloroacetate, an alternative channel for proton transfer arises. In this channel the proton is first transferred to the water solvent and only later taken up from the water by the base. We study the base concentration dependence of the two competing channels.

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Section: Resultsmentioning

confidence: 99%

Section: -3mentioning

confidence: 99%

Parallel proton transfer pathways in aqueous acid-base reactions

Cox

Bakker

2008

The Journal of Chemical Physics

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“…In methanol-water mixtures where ESPT is observed already in the absence of water, we separated the methanol-and water-dependent components of the protolytic dissociation rate constant for several photoacids [41]. As in THF-water mixtures, the water-dependent component has a power-law dependence on water concentration.…”

Section: Dynamics In Water and Mixed Solventsmentioning

confidence: 99%

Design and Implementation of “Super” Photoacids

Tolbert

Solntsev

2006

Hydrogen‐Transfer Reactions

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Electron and proton transfer (see Eqs. (13.1) and (13.2), involve obvious similarities. When the starting materials are neutral, both result in the formation of charge and in the necessity for separation of charge to stabilize the product states. In principle, both can occur in the ground state, but transfer that is exoergic only in the excited state allows the use of time-resolved spectroscopic techniques to determine the details of solvation and structural reorganization. For electron transfer, the development of such techniques and the accompanying theoretical rationale, most especially the Marcus theory, has been one of the triumphs of modern mechanistic chemistry.The relationship between driving force and proton transfer has been much more elusive despite considerable evidence that the vast photosynthetic electron transfer machinery mainly exists to set up a charge gradient to drive proton transfer. This is due to a combination of two factors. First, there is a vast reservoir of readily available materials with which to examine electron transfer. Second, the relationship between rates and driving force for electron transfer, based upon the excitation energies and the relevant redox potentials (the Rehm-Weller equation [1]) is reasonably straightforward. In contrast, the relationship between rates and driving force for excited-state pK a * ¼ pK a À DE o;o =2: 3RT (13.3) proton transfer (based upon relative pK a s and calculated through the Förster equation, Eq. (13.3) [2]) is less straightforward. For instance, the existence of a so-called "inverted" region for proton transfer has been the subject of much controversyHydrogen-Transfer Reactions. Edited by

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“…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][85][86][87] and naphthols [88][89][90][91][92], with additional studies on the temperature dependence [93][94][95][96][97][98], and the pressure dependence [99][100][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 10-hydroxycamptothecin [106] and coumarin 4 [107].…”

Section: Proton Recombination and Acid-base Neutralizationmentioning

confidence: 99%