Excited state intramolecular proton transfer in 1-hydroxypyrene

Abstract

1-Hydroxypyrene (1) shows unusual acid–base chemistry in its singlet excited state. Whereas most hydroxyarenes experience a marked enhancement in their acidity when excited, and rapidly deprotonate to give the corresponding phenolate anion, this is not an important pathway for 1, despite theoretical predictions that 1 should experience enhanced acidity as well. In this work, we demonstrate that 1 undergoes a competing excited state intramolecular proton transfer from the OH to carbon atoms at the 3, 6, and 8 positions of the pyrene ring to give quinone methide intermediates. When the reaction is carried out in D2O, reversion of these quinone methides to starting material results in replacement of the ring hydrogens with deuterium, providing a convenient handle to follow the reaction with NMR spectroscopy and mass spectrometry. The quantum yield for the reaction is 0.025 and appears to not be strongly dependent on the water content when aqueous acetonitrile solutions are used. 1-(2-Hydroxyphenyl)pyrene (19) was prepared and studied and shows similar reactivity to 1.

Very Efficient Generation of Quinone Methides through Excited State Intramolecular Proton Transfer to a Carbon Atom

“…Why is the quantum efficiency for ESIPT in 6 much higher than in any other investigated system? [13][14][15][16][17][18][19] For example in 3, the optimization of the S 1 state of S3, leads directly to CI with S 0 as there is no minimum on the S 1 surface. However, the minimum energy structure of 3·W is anti (see the Supporting Information).…”

“…[1] Although some examples are known wherein protonation of a carbon can compete with the protonation of heteroatoms, [12] ESIPT to carbon usually takes place with much lower quantum efficiency. [13][14][15][16][17][18][19] Consequently, it is often overlooked as a possible nonradiative relaxation pathway in biological molecules, [20] or as a viable synthetic route to quinone methides (QM). [21] The later are important reactive intermediates in organic synthesis [22] capable of cross-linking DNA, [23] and involved in the antineoplastic action of some antibiotics.…”

“…[24] To date, several examples of ESIPT to carbon atoms of the aromatic ring have been described. [12][13][14][15][16][17][18][19][20] One prominent example is that of 1-naphthol (1). In S 1 , 1 becomes significantly more acidic (pK* a = 0.4), [13] and in addition to proton transfer (PT) to solvent, a solvent-assisted transfer of the OH proton to the carbon atom at position 5 of the ring takes place giving quinone methide (QM) 2 with a quantum efficiency of 0.11 (Reaction (1)).…”

Excited State Intramolecular Proton Transfer (ESIPT) from Phenol to Carbon in Selected Phenylnaphthols and Naphthylphenols

“…20 ESIPT from the phenol OH to carbon atoms is now recognized as a general reaction as many examples have been documented by us involving phenol adjacent to the following rings: phenyl, 21 naphthyl, 22 anthryl 23 terphenyl 24 or pyrenyl. 25 More recently we reported a very efficient ESIPT reaction in 2-phenyl-1-naphthol (1) that gives QM 2, 4 and zwitterion 3 (eq 1). 26 Formation of QMs 2−4 was inferred due to regiospecific incorporation of deuterium on irradiation in D 2 O (D-exchange Φ = 0.7), the highest efficiency ever reported for such a process.…”

Toward understanding tautomeric switching in hydroxynaphthaldehydes: Characterization of electronic absorption spectra

“…17 In fact the assignment of the 370 nm band in the spectrum of ALD-14 in methanol to the corresponding anion is supported by the fact that 4-hydroxybenzaldehyde, in aqueous solution at pH ¼ 7.4, exists in the anion form. [29][30][31][32][33][34] …”