Studies on curcumin and curcuminoids. XXXIX. Photophysical properties of bisdemethoxycurcumin.
Bottom Line: The steady-state absorption and fluorescence, as well as the time-resolved fluorescence properties of bisdemethoxycurcumin dissolved in several solvents differing in polarity and H-bonding capability were measured.The bisdemethoxycurcumin decay mechanisms from the S(1) state were discussed and compared with those of curcumin.The differences in S(1) dynamics observed between bisdemethoxy-curcumin and curcumin could be ascribed to a difference in H-bond acceptor/donor properties of the phenolic OH and a difference in strength of the intramolecular H-bond in the keto-enol moiety within the two molecules.
Affiliation: Department of Physics and Mathematics, University of Insubria and C.N.I.S.M.-C.N.R., Via Valleggio, 11- 22100 Como, Italy. email@example.comShow MeSH
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Mentions: The results of the above-mentioned works can be used as a guideline in ascribing a decay mechanism to each of the exponential components of the bisDMC fluorescence decays detected in the various solvents. The quantum yields and decay data suggest that bisDMC and CURC decay through the same deactivation pathways in cyclohexane. Moreover, like CURC, bisDMC displayed minimum Stokes shifts in cyclohexane, which is consistent with formation of KEHB which prevents out-of-plane vibrations. Hence, in analogy to CURC, the three decay components observed for bisDMC in cyclohexane are ascribed to: (a) bisDMC molecules initially in the closed cis enol conformer that are excited to S1 without undergoing cis-trans isomerization and very rapidly decay to S0 by direct ESIPT according to the scheme in Fig. 5 (, with relative amplitude 0.81); (b) molecules initially in the closed cis enol conformer, that undergo cis-trans isomerization upon excitation to S1  and de-excitation by reketonization (, with relative amplitude 0.17); (c) molecules in the trans (anti) diketo conformer (, with relative amplitude 0.02). However, the bisDMC average decay time was much longer than that of CURC and each of the τi values (i = 1,2,3) was higher for bisDMC than for CURC. This indicates that in the case of bisDMC both direct ESIPT and reketonization occur on slower time scales. The crystal structure of solvated bisDMC shows that (at least in polar environment) the enol proton is tightly bound to the enol oxygen, and remains quite distant from the keto oxygen [49, 50]. This is very different from CURC, in which the enol proton is very mobile and equally distributed between the two oxygens of the keto-enol moiety . If this also pertains to non-polar environments, it indicates that the KEHB must be quite loose, and consequently ESIPT rather slow: this is probably the reason why τ1 in cyclohexane was approximately twice as long for bisDMC than for CURC. Moreover, slower reketonization rates compared to CURC are expected and observed for bisDMC in cyclohexane.Fig. 5
Affiliation: Department of Physics and Mathematics, University of Insubria and C.N.I.S.M.-C.N.R., Via Valleggio, 11- 22100 Como, Italy. firstname.lastname@example.org