New trans-2-hydroxychalcones bearing a carboxylate group at position 2' (Ct(-)) were synthesized (compounds 2 and 3). These compounds lead to a network of chemical reactions depending on pH value, light, and solvent. In water, when the pH value is lowered, the ionized trans-chalcone is protonated and the flavylium cation AH(+) is formed at very acidic pH values through hemiketal B and cis-chalcone Cc, with global acidity constants of pK'(a) <= -1 and approximate to 0.1, respectively, for 2 and 3. The electron-acceptor character of the carboxylic substituent not only increases the observed acidity of the flavylium cation, but also decreases the rate of the ring-opening/closing from a subsecond timescale to hours relative to model compound 1 (without carboxylate). The photochemistry of the network was studied in detail by means of continuous irradiation, monitored by UV/Vis absorption and H-1 and C-13 NMR spectroscopic analysis. Although compound 3 is only slightly photoactive, compound 2 (Ct(-)) reacts in aqueous solutions (lambda(irr) = 313 nm) to form B- and Cc(-), with a global quantum yield of 0.15, and fully reverts back to Ct(-) with a rate constant of k = 6.7 x 10(-5) s(-1). The flavylium cation is no longer formed in methanol, and irradiation of Ct(-) leads to the formation of B- and the new lactone-trapped chromene species La. The formation of La takes place through a sequence of three photochemical steps: photoisomerization of Ct(-), photo-ring-closing reaction of Cc , and photolactonization of B-. Only the cis/trans isomerization and ring-closing reactions are thermally reversible on a timescale of seconds and hours, respectively. A photochromic system was achieved in rigid matrices of methanol (at 77 K) and 1-dodecanol (5 degrees C) by irradiating lactone La to give a red ortho-quinone allide through a photo-ring-opening reaction; the color disappears with a rate constant of k = 1.25 x 10(-2) s(-1) in 1-dodecanol at 5 degrees C.
ISI Document Delivery No.: 737JTTimes Cited: 2
Cited Reference Count: 48
Leydet, Yoann Gavara, Raquel Cunha-Silva, Luis Jorge Parola, A. Pina, Fernando
Pina, Fernando/C-8161-2011; Cunha-Silva, Luis/A-8765-2012; Parola, A. Jorge/F-4048-2010; Gavara, Raquel/C-6669-2013; Caparica, cqfb_staff/H-2611-2013; REQUIMTE, AL/H-9106-2013; Chaves, Pedro/K-1288-2013
Parola, A. Jorge/0000-0002-1333-9076;
FCT-MCTES, Portugal [PTDC/QUI/67786/2006, PTDC/QUI-QUI/104129/2008]; Ciencia 2008 program; [SFRH/BPD/44230/2008]; [SFRH/BPD/44639/2008]
We thank Dr. V. Petrov (REQUIMTE) for the ab initio calculations, Prof. L. Rodriguez (University Barcelona) and Dr. Conceicao Oliveira for the MALDI and ESI MS measurements, and LabRMN at FCT-UNL and Rede Nacional de RMN (supported with funds from FCT-MCTES, Portugal) for access to the facilities. FCT-MCTES is also acknowledged for financial support through projects PTDC/QUI/67786/2006 and PTDC/QUI-QUI/104129/2008, the Ciencia 2008 program (L.C.-S.), and postdoctoral grants SFRH/BPD/44230/2008 (Y.L.) and SFRH/BPD/44639/2008 (R.G.).
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