Photochemistry of 2‘-Hydroxychalcone. One-Way Cis−Trans Photoisomerization Induced by Adiabatic Intramolecular Hydrogen Atom Transfer

Abstract

Intramolecular hydrogen atom transfer and novel one-way photoisomerization of 2′-hydroxychalcone (2HC) were investigated by quantum yield measurements, transient absorption spectroscopy, and semiempirical calculations. The trans isomer of 2HC (trans-2HC) did not give the cis isomer, while cis-2HC underwent one-way isomerization to give the trans-2HC. The triplet-triplet absorption spectra of cis-and trans-2HC were similar and were assigned to the tautomer produced via the intramolecular hydrogen atom transfer in the excited triplet state. The triplet energies of the normal form and the tautomer form of trans-2HC were estimated by quenching experiments to be 229 and 177.5 kJ mol -1 , respectively. Heats of formation, the frontier molecular orbital coefficients, and optimized structures of trans-2HC and cis-2HC in the ground, the excited singlet, and the excited triplet states were calculated by semiempirical (MOPAC93 PM3/4CI) calculations. These calculations agree well with the experimental observations. Based on the experimental observations and theoretical considerations, a novel mechanism and the potential energy surface for the oneway photoisomerization of 2HC were proposed. The photoisomerization of 2HC was induced by hydrogen atom transfer in the excited triplet state. Thus, 2HC can be viewed as a molecule in which the mode of isomerization around the carbon-carbon double bond can be controlled by the remote intramolecular hydrogen bonding. Solvent affected the triplet lifetime without changing the mode of isomerization. On prolonged irradiation, trans-2HC gave flavanone by very low quantum yield (Φ ) 1.9 × 10 -3 in benzene). The cyclization reaction to form flavanone was studied and the quantum yields were determined.

Enantioselective photochemistry through Lewis acid–catalyzed triplet energy transfer

“…In particular, 2 has been experimentally shown to possess a triplet state 54 kcal/mol higher in energy than its closed-shell singlet ground state. (27) This triplet energy (E T ) is well outside of the range that should be reasonably accessible using Ru(bpy) 3 2+ as a triplet sensitizer (E T = 46 kcal/mol). (28) Indeed, an experiment in which 2 and diene 3 were irradiated with visible light in the presence of Ru(bpy) 3 (PF 6 ) 2 showed minimal evidence of productive photoreaction (entry 1).…”

“…These calculations gave a triplet energy of 51 kcal/mol (Figure 4A), in reasonably good agreement with the reported experimental value of 54 kcal/mol. (27) The analogous computation on the Sc(III) complex of 2 , however, suggested that the energy of the lowest optimized triplet state would be dramatically lowered to 32 kcal/mol. (37) This 20 kcal/mol bathochromic shift in triplet energy upon coordination to a Lewis acid would lie easily within a range where triplet energy transfer from Ru*(bpy) 3 2+ would be exergonic, in good accord with our design plan.…”

Chalcone derivatives as fluorescence turn-on chemosensors for cyanide anions

“…The coupling constant of vinylic protons of compounds 1-3 is nearly 15 Hz, which clearly indicates that these compounds are trans-conformation and this was also proved by the single crystal diffraction results. No cis-isomers were detected [35]. To investigate the reactive site of Michael addition, corresponding cyanide bonding compounds were also prepared and purified.…”

“…The weak fluorescence emission and low quantum yields of the compounds 1-3 are maybe due to the ultrafast intramolecular hydrogen atom transfer and/or ultrafast deactivation processes such as intersystem crossing and internal conversion through the hydrogen bonding [30,33,35]. However, their fluorescence intensity and quantum yields increase smartly with the addition of cyanide anion with quantum yields being 0.1 (1 added cyanide anions), 0.05 (2 added cyanide anions) and 0.041 (3 added cyanide anions).…”

Photochemical and complexation studies for new fluorescent and colored chelator

“…Our approach for development of the photoresponsive chelator is based on our recent investigations of the photochemical isomerization and photo-induced intramolecular proton (hydrogen atom) transfer of the intramolecular hydrogen bonded molecules [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57]. We have shown that olefin with a pyrrole ring and an indole or a phenanthroline ring (Scheme 3a and b) exhibited trans-to-cis one-way photochemical isomerization and the obtained cis-isomer underwent intramolecular hydrogen atom transfer in the excited singlet state [42][43][44].…”