Photoenolization as a convenient driver for the synthesis of plasmonic nanostructures

Substituted tetralones such as 3,3,6,8-tetramethyl-1-tetralone undergo photoenolization to produce a photoenol excited state with a lifetime around ~ 3 μs, which involves the carbonyl triplet state of the ketone (τ ~ 1.9 ns), as a precursor; the excited photoenol also has biradical character and is useful for the fast synthesis of gold nanostructures. In the case of excited photoenols like this one, if metal ion trapping fails, they return to the original ketone precursor and remain available for future events that can lead to the target nanoparticles. This study includes the characterization of the photochemistry of the substituted tetralone, and the dual behavior of reaction intermediates, as biradicals and excited states, in energy and electron transfer processes.

Photochromic Dibenzobarrlenes: Long-Lived Triplet Biradical Intermediates

Upon exposure to UV light, the disubstituted dibenzobarrelene derivative 1a turns green in the solid phase and reverts back to its original pale-yellow color within several hours in the dark. The lifetime of the colored species in degassed benzene at room temperature is 37 +/- 2 s (Ea for decoloration is 14.5 +/- 0.7 kcal mol-1 and log A is 8.92 +/- 0.5 s-1) and highly sensitive to molecular oxygen; the Stern-Volmer quenching constant is 6.9 +/- 0.2 x 108 M-1 s-1. Similarly, the disubstituted dibenzobarrelenes 1b and 1c exhibited pink coloration when exposed to UV light in the solid phase. On the basis of combined experimental and theoretical evidence, it is proposed that upon photoexcitation the excited singlet state of 1a undergoes rapid intersystem crossing to its triplet state, followed by intramolecular delta-H abstraction, to yield the triplet biradical intermediate (3)2. Upon prolonged irradiation, 2 undergoes cyclization to the alcohol 3, which affords the enone 4 as the final photoproduct. The delta-H abstraction on the triplet-state potential energy surface, calculated at the B3LYP/6-31G* level of density functional theory (DFT), has an activation energy of 18.5 kcal/mol. Further, the absorption spectrum of the triplet biradical (3)2, obtained from time-dependent DFT calculations, displays an intense absorption maximum at 670 nm, which is in good agreement with the observed absorption peak at 700 nm. The molecular-orbital analysis of the triplet diradical (3)2 suggests that its long-wavelength absorption involves the transition of the unpaired electron from the comparatively localized benzyl-type HOMO to the extensively conjugated benzoyl-type LUMO. The present experimental and theoretical results strongly support the intervention of a long-lived triplet biradical (3)2 in the photochromism of appropriately substituted dibenzobarrelenes.

Substituent Electronic Effects on the Persistence and Absorption Spectra of (Z)-o-Xylylenols. A Nanosecond Laser Flash Photolysis Study

[reaction: see text] A systematic investigation on a broad set of aldehydes reveals that the lifetimes of (Z)-photoenols can be modulated by variation of the substituents. We have found that the lifetimes of (Z)-enols (in benzene) can be varied by more than 1 order of magnitude with a judicious choice of the substituents that exert mesomeric and inductive effects as, for example, in the case of pentamethylbenzaldehyde (tau = 35 ns) and dicyanomesitaldehyde (tau = 760 ns). This study thus points to the fact that the electronic factors in conjunction with hydrogen bonding stabilization can considerably broaden the uni- as well as bimolecular chemistry based on photoenolization. Further, we have shown that the photoenols exhibit dramatic shifts in their absorption properties with variation of the substituents; although the photoenols have long been considered to be colored, their absorption properties have not been heretofore comprehensively examined.

Mechanistic studies on the photochromism of [e]-annelated dimethyldihydropyrenes

The photochemistry of several photochromic arene [e]-annelated dimethyldihydropyrenes (DHPs) was studied. These compounds have much larger photochemical ring opening quantum yields than the simple DHPs, making them potentially more useful as building blocks for photoswitchable materials.

Novel Photochemical Hydrolysis of o-Acetylphenylacetonitriles to Amides

Irradiation of o-acetylphenylacetonitriles 1a,b in methanol (lambda >280 nm) leads to essentially quantitative photohydrolysis with formation of the corresponding ketal amides 3a,b. The mechanism proposed for this novel reaction is supported by the qualitatively different photochemical behavior of 1c and by a labeling experiment with 1a-L that confirms transfer of the carbonyl oxygen atom of 1a to the amide oxygen atom of 3a.