Photoacid

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Photoacids are molecules that become more acidic upon absorption of light. Either the light causes a photodissociation to produce a strong acid, or the light causes photoassociation (such as a ring forming reaction) that leads to an increased acidity and dissociation of a proton.

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There are two main types of molecules that release protons upon illumination: photoacid generators (PAGs) and photoacids (PAHs). PAGs undergo proton photodissociation irreversibly, while PAHs are molecules that undergo proton photodissociation and thermal reassociation. [1] In this latter case, the excited state is strongly acidic, but reversible.

Photoacid generators

An example due to photodissociation is triphenylsulfonium triflate. This colourless salt consists of a sulfonium cation and the triflate anion. Many related salts are known including those with other noncoordinating anions and those with diverse substituents on the phenyl rings.

The triphenylsulfonium salts absorb at a wavelength of 233 nm, which induces a dissociation of one of the three phenyl rings. This dissociated phenyl radical then re-combines with remaining diphenylsulfonium to liberate an H+ ion. [2] The second reaction is irreversible, and therefore the entire process is irreversible, so triphenylsulfonium triflate is a photoacid generator. The ultimate products are thus a neutral organic sulfide and the strong acid triflic acid.

[(C6H5)3S+][CF3SO
3
] + hν → [(C6H5)2S+.][CF3SO
3
] + C6H.
5
[(C6H5)2S+.][CF3SO
3
] + C6H.
5
→ (C6H5C6H4)(C6H5)S + [CF3SO
3
][H+
]

Applications of these photoacids include photolithography [3] and catalysis of the polymerization of epoxides.

Photoacids

An example of a photoacid which undergoes excited-state proton transfer without prior photolysis is the fluorescent dye pyranine (8-hydroxy-1,3,6-pyrenetrisulfonate or HPTS). [4]

The Förster cycle was proposed by Theodor Förster [5] and combines knowledge of the ground state acid dissociation constant (pKa), absorption, and fluorescence spectra to predict the pKa in the excited state of a photoacid.

The name photoacid can be abbreviated PAH, where the H does not stand for a word starting with H, but rather for a hydrogen atom which is lost when the molecule reacts as a Brønsted acid. This use of PAH should not be confused with other meanings of PAH in chemistry and in medicine.

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In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula CH3. In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in many organic compounds. It is a very stable group in most molecules. While the methyl group is usually part of a larger molecule, bonded to the rest of the molecule by a single covalent bond, it can be found on its own in any of three forms: methanide anion, methylium cation or methyl radical. The anion has eight valence electrons, the radical seven and the cation six. All three forms are highly reactive and rarely observed.

<span class="mw-page-title-main">Phenyl group</span> Cyclic chemical group (–C₆H₅)

In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6H5, and is often represented by the symbol Ph or Ø. The phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen, which may be replaced by some other element or compound to serve as a functional group. A phenyl group has six carbon atoms bonded together in a hexagonal planar ring, five of which are bonded to individual hydrogen atoms, with the remaining carbon bonded to a substituent. Phenyl groups are commonplace in organic chemistry. Although often depicted with alternating double and single bonds, the phenyl group is chemically aromatic and has equal bond lengths between carbon atoms in the ring.

<span class="mw-page-title-main">Leaving group</span> Atom(s) which detach from the substrate during a chemical reaction

In chemistry, a leaving group is defined by the IUPAC as an atom or group of atoms that detaches from the main or residual part of a substrate during a reaction or elementary step of a reaction. However, in common usage, the term is often limited to a fragment that departs with a pair of electrons in heterolytic bond cleavage. In this usage, a leaving group is a less formal but more commonly used synonym of the term nucleofuge. In this context, leaving groups are generally anions or neutral species, departing from neutral or cationic substrates, respectively, though in rare cases, cations leaving from a dicationic substrate are also known.

<span class="mw-page-title-main">Sulfonate</span> Organosulfur compound of the form R–S(=O)2–O (charge –1)

In organosulfur chemistry, a sulfonate is a salt, anion or ester of a sulfonic acid. Its formula is R−S(=O)2−O, containing the functional group −S(=O)2−O, where R is typically an organyl group, amino group or a halogen atom. Sulfonates are the conjugate bases of sulfonic acids. Sulfonates are generally stable in water, non-oxidizing, and colorless. Many useful compounds and even some biochemicals feature sulfonates.

<span class="mw-page-title-main">Triphenylmethane</span> Chemical compound

Triphenylmethane or triphenyl methane (sometimes also known as Tritan), is the hydrocarbon with the formula (C6H5)3CH. This colorless solid is soluble in nonpolar organic solvents and not in water. Triphenylmethane is the basic skeleton of many synthetic dyes called triarylmethane dyes, many of them are pH indicators, and some display fluorescence. A trityl group in organic chemistry is a triphenylmethyl group Ph3C, e.g. triphenylmethyl chloride (trityl chloride) and the triphenylmethyl radical (trityl radical).

<span class="mw-page-title-main">Triflate</span> Chemical group (–OSO2CF3) or anion (charge –1)

In organic chemistry, triflate, is a functional group with the formula R−OSO2CF3 and structure R−O−S(=O)2−CF3. The triflate group is often represented by −OTf, as opposed to −Tf, which is the triflyl group, R−SO2CF3. For example, n-butyl triflate can be written as CH3CH2CH2CH2OTf.

<span class="mw-page-title-main">Enolate</span> Organic anion formed by deprotonating a carbonyl (>C=O) compound

In organic chemistry, enolates are organic anions derived from the deprotonation of carbonyl compounds. Rarely isolated, they are widely used as reagents in the synthesis of organic compounds.

<span class="mw-page-title-main">Biphenyl</span> Chemical compound

Biphenyl is an organic compound that forms colorless crystals. Particularly in older literature, compounds containing the functional group consisting of biphenyl less one hydrogen may use the prefixes xenyl or diphenylyl.

<span class="mw-page-title-main">Phosphoric acids and phosphates</span> Class of chemical species; phosphorus oxoacids and their deprotonated derivatives

In chemistry, a phosphoric acid, in the general sense, is a phosphorus oxoacid in which each phosphorus (P) atom is in the oxidation state +5, and is bonded to four oxygen (O) atoms, one of them through a double bond, arranged as the corners of a tetrahedron. Two or more of these PO4 tetrahedra may be connected by shared single-bonded oxygens, forming linear or branched chains, cycles, or more complex structures. The single-bonded oxygen atoms that are not shared are completed with acidic hydrogen atoms. The general formula of a phosphoric acid is Hn+2−2xPnO3n+1−x, where n is the number of phosphorus atoms and x is the number of fundamental cycles in the molecule's structure, between 0 and n + 2/2.

<span class="mw-page-title-main">Diazonium compound</span> Group of organonitrogen compounds

Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group [R−N+≡N]X where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide. The parent compound where R is hydrogen, is diazenylium.

Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons with one target molecule that dissociates into two fragments.

<span class="mw-page-title-main">Persistent carbene</span> Type of carbene demonstrating particular stability

A persistent carbene is an organic molecule whose natural resonance structure has a carbon atom with incomplete octet, but does not exhibit the tremendous instability typically associated with such moieties. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC), in which nitrogen atoms flank the formal carbene.

<span class="mw-page-title-main">Triflic acid</span> Chemical compound

Triflic acid, the short name for trifluoromethanesulfonic acid, TFMS, TFSA, HOTf or TfOH, is a sulfonic acid with the chemical formula CF3SO3H. It is one of the strongest known acids. Triflic acid is mainly used in research as a catalyst for esterification. It is a hygroscopic, colorless, slightly viscous liquid and is soluble in polar solvents.

<span class="mw-page-title-main">Diphenylmethane</span> Chemical compound

Diphenylmethane is an organic compound with the formula (C6H5)2CH2 (often abbreviated CH
2
Ph
2
). The compound consists of methane wherein two hydrogen atoms are replaced by two phenyl groups. It is a white solid.

<span class="mw-page-title-main">Sodium tetraphenylborate</span> Chemical compound

Sodium tetraphenylborate is the organic compound with the formula NaB(C6H5)4. It is a salt, wherein the anion consists of four phenyl rings bonded to boron. This white crystalline solid is used to prepare other tetraphenylborate salts, which are often highly soluble in organic solvents. The compound is used in inorganic and organometallic chemistry as a precipitating agent for potassium, ammonium, rubidium, and caesium ions, and some organic nitrogen compounds.

Unlike its lighter congeners, the halogen iodine forms a number of stable organic compounds, in which iodine exhibits higher formal oxidation states than -1 or coordination number exceeding 1. These are the hypervalent organoiodines, often called iodanes after the IUPAC rule used to name them.

<span class="mw-page-title-main">Triphenylborane</span> Chemical compound

Triphenylborane, often abbreviated to BPh3 where Ph is the phenyl group C6H
5
, is a chemical compound with the formula B(C6H5)3. It is a white crystalline solid and is both air and moisture sensitive, slowly forming benzene and triphenylboroxine. It is soluble in aromatic solvents.

<span class="mw-page-title-main">Dimethylphenylphosphine</span> Chemical compound

Dimethylphenylphosphine is an organophosphorus compound with a formula P(C6H5)(CH3)2. The phosphorus is connected to a phenyl group and two methyl groups, making it the simplest aromatic alkylphosphine. It is colorless air sensitive liquid. It is a member of series (CH3)3-n(C6H5)2P that also includes n = 0, n = 2, and n = 3 that are often employed as ligands in metal phosphine complexes.

<span class="mw-page-title-main">Vinyl cation</span> Organic cation

The vinyl cation is a carbocation with the positive charge on an alkene carbon. Its empirical formula of the parent ion is C
2
H+
3
. Vinyl cation are invoked as reactive intermediates in solvolysis of vinyl halides, as well as electrophilic addition to alkynes and allenes.

<span class="mw-page-title-main">Triphenylcarbenium</span> Chemical compound

In chemistry, triphenylcarbenium, triphenylmethyl cation, tritylium , or trityl cation is an ion with formula [C19H15]+ or (C6H5)3C+, consisting of a carbon atom with a positive charge connected to three phenyl groups. It is a charged version of the triphenylmethyl radical (C6H5)3C•. The name is often abbreviated to triphenylmethyl or trityl in salts, although these names also denote the chemical group in compounds like triphenylmethyl chloride that do not contain the cation.

References

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  2. W. D. Hinsberg, G. M. Wallraff, Lithographic Resists, Kirk-Othmer Encyclopedia of Chemical Technology, Wiley-VCH, Weinheim, 2005. (Published online: 17 June 2005) doi : 10.1002/0471238961.1209200808091419.a01.pub2
  3. J. V. Crivello The Discovery and Development of Onium Salt Cationic Photoinitiators, J. Polym. Sci., Part A: Polym. Chem., 1999, 37, 4241−4254. doi : 10.1002/(SICI)1099-0518(19991201)37:23<4241::AID-POLA1>3.0.CO;2-R
  4. N. Amdursky, R. Simkovitch and D. Huppert, Excited-state proton transfer of photoacids adsorbed on biomaterials, J. Phys. Chem. B., 2014, 118, 13859−13869. doi : 10.1021/jp509153r
  5. Kramer, Horst E. A.; Fischer, Peter (9 November 2010). "The Scientific Work of Theodor Förster: A Brief Sketch of his Life and Personality". ChemPhysChem. 12 (3): 555–558. doi:10.1002/cphc.201000733. PMID   21344592.