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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 [D.sub.2]O, 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. Key words: 1-pyrenol, 1-hydroxypyrene, excited state intramolecular proton transfer, quinone methide, photoprotonation, photochemistry. Dans son etat excite; singulet, le 1-hydroxypyrene (1) presente une chimie acide-base inhabituelle. La plupart des hydroxyarenes presentent une augmentation marquee de leur acidite dans leur etat excite; et ils se deprotonent rapidement pour conduire a l'anion pheenolate correspondant; ce n'est pas une voie importante pour 1 meeme si les predictions theeoriques indiquent que 1 devrait aussi voir son aciditee augmenteee. Dans ce travail, on deemontre que dans son etat excite; 1 subit aussi une reaction competitive de transfert intramoleeculaire de proton a partir du groupe OH vers les atomes de carbone dans les positions 3, 6 et 8 du noyau pyrene qui conduisent a des intermediates quinone-methides. Quand on effectue la re;action dans du [D.sub.2]O, la re;version de ces quinone-me;thides vers le produit de de;part conduit au remplacement des atomes d'hydrogene du cycle par du deuterium qui fournit un moyen utile de suivre la reaction a l'aide de la spectroscopie RMN et de la spectromeetrie de masse. Le rendement quantique pour la reaction est de 0.025 et il ne semble pas dependre beaucoup du contenu d'eau lorsqu'on utilise des solutions aqueuses d'aceetonitrile. On a aussi prepare et etudie la reaction du 1-(2-hydroxyphenyl)pyrene (19) et il presente la meeme reactivitee que le 1. Mots-cles: 1-pyrenol, 1-hydroxypyrene, transfert intramoleeculaire de proton dans l'eetat excite;, quinone-methides, photoprotonation, photochimie., [Traduit par la Redaction] Introduction It is well-known that a large variety of aromatic alcohols experience enhanced acidity when excited to [S.sub.1].1 For example, the hydroxyl group of 2-naphthol increases [...]

The 9-(4-methoxyphenyl)-9-fluorenyl cation (2) has been generated in 100% water by laser flash photolysis of 9-(4-melhoxyphenyl)-9-fluorenol (3), representing the first observation of a 9-fluorenyl cation in this solvent with lifetimes in the microsecond timescale. The relatively long lifetime permitted quenching studies with a number of anionic nucleophiles, and bimolecular rate constants for each were determined. For both bromide and iodide, rate data suggest that an equilibrium between the cation and trapped product is rapidly established, followed by slower, irreversible trapping of the cation by water. The bimolecular rate constants obtained show that the generated 9-fluorenyl cation is significantly more reactive towards nucleophilic attack, by two orders of magnitude, than related triary I methyl cations that lack the 4n π-system, lending support to the characterization of fluorenyl cations as antiaromatic. Key words: 9-fluorenyl cation, laser flash photolysis, electrophilicity, carbocation, antiaromaticity. On a genere le cation 9-(4-methoxyphenyl)-9-fluorenyle (2) dans un solvant a 100 % d'eau par photolyse eclair au laser du 9-(4-methoxyphenyl)-9-fluorenol (3); ceci correspond a la premiere observation du cation 9-fluorenyle duns ce solvant avec un temps de vie de l'echelle de la microseconde. Le temps de vie relativement long a permis d'effectuer des etudes de piegeage avec un certain nombre de nucleophiles anioniques et on a determine les constantes de vitesses bimoleculaires pour chacune. Pour le bromure ainsi que pour l'iodure, les donnees de vitesse suggerenl qu'il s'etablit rapidement un equilibre entre le cation el le produil piege, suivi par un piegeage irreversible plus lent du cation par l'eau. Les constantes de vitesses hi molecul aires obtenues montrent que vis-a-vis des attaques nucleophiles, le cation 9-fluorenyle genere est beaucoup plus reactif (an ntoins deux ordres de grandeur) que les reactions apparentees des cations triarylmethyles qui ne component pas de systeme 4n π; cette observation est en accord avec tine caracterisation des cations fluorenyles comme antiaromatiques. Mots-des : cation 9-fluorenyle, photolyse eclair au laser, caractere electrophile, carbocation, antiaromaticite. [Traduit par la Redaction], Introduction Antiaromaticity is a central concept in organic chemistry, and has been defined in thermodynamic terms: 'If the structure is of higher energy (less stable) than such a hypothetical classical [...]

Abstract: The photochemistry of 1,1'-bi-2-naphthol (BINOL, 5) has been studied in aqueous solution and found to undergo rapid deuterium incorporation at the 4 and 5 positions (in D2O-CH3CN). All data [...]

The research was to consider derivatives bearing an anthracenyl substituent in the photochemistry of O-hydroxybiaryls. Hydrogen substituted phenyalthracenes have received lot of attention because of their ability to form hydrogen-bonded networks.

The photochemistry of naphthols 1, 2, 4, 5 and 9, and phenol 10 has been studied in aqueous solution with the primary aim of exploring the viability of such compounds for naphthoquinone and quinone methide photogeneration, along the lines already demonstrated by our group for phenol derivatives. 1-Naphthol (1) is known to be substantially more acidic than 2-naphthol (2) in the singlet excited state (p[K.sub.a]* = 0.4 and 2.8, respectively) and it was expected that this difference in excited-state acidity might be manifested in higher reactivity of 1-naphthol derivatives for photochemical reactions requiring excited-state naphtholate ions, such as quinone methide formation. Our results show that three types of naphthoquinone methides (26a, 26b, and 27) are readily photogenerated in aqueous solution by irradiation of 1-naphthols. Photolysis of the parent 1-naphthol (1) in neutral aqueous solution gave 1,5-naphthoquinone methide 26a as well as the non-Kekule 1,8-naphthoquinone methide 26b, both via the process of excited-state (formal) intramolecular proton transfer (ESIPT), based on the observation of deuterium exchange at the 5-and 8-positions, respectively, on photolysis in [D.sub.2]O-C[H.sub.3]CN. A transient assignable to the 1,5-naphthoquinone methide 26a was observed in laser flash photolysis experiments. The isomeric 2-naphthol (2) was unreactive under similar conditions. The more conjugated 1,5-naphthoquinone methide 27 was formed efficiently via photodehydroxylation of 4; isomeric 5 was unreactive. The efficient photosolvolytic reaction observed for 4 opens the way to design related naphthol systems for application as photoreleasable protecting groups by virtue of the long-wavelength absorption of the naphthalene chromophore. Key words: photosolvolysis, excited-state intramolecular proton transfer, quinone methide, photorelease, photoprotonation.

The kinetics of the reactions of nine carbanions each stabilized by two acyl, ester or cyano groups with benzhydrylium ions in water are examined photometrically. Correlations between the nucleophilic reactivities of carbanions with the pKa values of the corresponding CH acids disclose that the malodinitrile anion is significantly more nucleophilic than was expected.

Synthesis and a study of the first group of 'extended' hydroxystyrenes (with a benzene ring as a conjugated 'spacer') is presented and the photochemistry of the related hydroxybiphenyl alcohols is examined. It was concluded that fluorescence quenching of the hydroxybiphenyl alcohols needed a much higher content of water.

Data consistent with a mechanism of exchange that involves excited-state intramolecular proton transfer (ESIPT) from the phenol to the 2'-carbon position of the benzene ring not containing the phenol, to generate the corresponding keto tautomer is described. The new type of ESIPT might is a general photochemical pathway for many unconstrained o-arylphenols and o-aryl hydroarenes.

A new photolabile protecting group (PPG) for carboxylic acids and amines is developed based on the rapid single state photodecarboxylation of xanthone acetic acids with several features that are superior to many other systems. The studies have shown that the xanthonate PPG can photorelease carboxylic acids and amines quantitatively in neutral phosphate buffer solution with an uncaging cross section.

The development of a new photocage that is superior to the o-nitrobenzyl (oNB) group in several respects, including a release rate improvement of over 1000-fold, a higher photorelease quantum yield, excellent aqueous solubility, and formation of benign photoproducts is reported. The design of this new photocage is capable of effecting rapid and efficient photorelease of carboxylic acids, halides, and alcohols and potentially many other functional groups.

Enhanced acidity and basicity are experienced when the organic molecules undergoes electronic excitation, as many of them possess both acidic and basic functionalities. This simultaneous change in proton denoting and accepting power is significant enough in many occasions to promote the transfer of the proton from the acidic group to the basic group, via the process known as excited-state intramolecular proton transfer (ESIPT).

[reaction: see text] Irradiation of 2- and 4-xanthone acetic acid in aqueous buffer (pH 7.4) leads to efficient (Phi = 0.67 and 0.64, respectively) photodecarboxylation to give the corresponding methyl products, consistent with an intermediate benzylic carbanion. Fluorescence and laser flash photolysis (LFP) studies suggest singlet state reactivity, which is unusual for aromatic ketones. 3-Xanthone acetic acid is photoinert under the same conditions. The results suggest that the reactive xanthone acetic acids are promising precursors for carbanion-mediated photocages.