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: In reference  we proposed a model to explain the data on the CURC decay from the S1 state. The following radiationless decay mechanisms were considered to concur with fluorescence emission: (a) direct excited-state intra-molecular proton transfer (ESIPT) from the enol to the keto group, which was postulated to be the fastest possible non-radiative decay mechanism [39, 40] and to take place only if the intra-molecular keto-enol H-bond (KEHB) illustrated in Fig. 4a (see closed cis enol conformer) were formed; (b) reketonization [41, 42]; (c) charge/energy transfer to the solvent molecules ; (d) slow, solvent-rearrangement moderated ESIPT. The latter occurs in case a trans enol or open cis enol molecule isomerizes to the closed cis enol conformer while in the S1 state, and then decays to S0 by means of ESIPT . The different enol conformers are shown in Fig. 4a. On the basis of the photodegradation results, photodegradation was excluded to be the driving force in the S1 decay. It was postulated that CURC in solution at room temperature is essentially present in its enol conformers, in agreement with previous studies [29, 44–47]. The H-bonded closed cis enol structure is dominant in non-polar environments, while either the open cis enol or the trans enol conformers, which cannot form the KEHB, are dominant in polar weakly-H-bonding and polar strongly-H-bonding solvents, respectively . Tiny amounts of the minimally polar trans (anti) diketo conformer (see Fig. 4b) can be found in non-polar environments . In  we have elucidated the very different multi-exponential fluorescence decays of DCMeth, a curcumin analogue lacking both the methoxy and hydroxyl phenolic substituents of CURC, by invoking the concurrence of the same mechanisms outlined above. We only made the further hypothesis that tiny amounts of the very polar cis diketo conformer (see Fig. 4b) of DCMeth exists in polar environments. This conformer has been shown to be unstable for CURC due to either steric interactions  or unfavorable dipole-dipole alignment .Fig. 4
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