Ultrafast Dynamics of the Excited States of Curcumin in Solution

Abstract

Dynamics of the excited singlet (S(1)) state of curcumin has been investigated in a wide varieties of solvents using subpicosecond time-resolved fluorescence and absorption spectroscopic techniques. As a consequence of extra stability of the cis-enol conformer due to the presence of an intramolecular hydrogen bond, it is the major form existing in the ground-state and the excited-state processes described here has been attributed to this form. Steady-state absorption and fluorescence spectra suggest significant perturbation of the intramolecular hydrogen bond and the possibility of formation of intermolecular hydrogen-bonded complex with the hydrogen-bonding solvents. Both the time-resolved techniques used here reveal that solvation is the major process contributing to the relaxation dynamics of the S(1) state. Solvation dynamics in protic solvents is multimodal, and the linear correlation between the longest component of the solvation process and the longitudinal relaxation time of the solvent suggests the specific hydrogen-bonding interaction between the solute and the solvent. However, a good correlation between the experimentally determined average solvation time and that predicted by the dielectric continuum model in all kinds of solvents also suggests that the dielectric relaxation of the solvent is also an important contributor to the solvation process. The lifetime of the S(1) state is very short in nonpolar solvents (∼44 ps in 1,4-dioxane) because of efficient nonradiative deactivation of the S(1) state, which is an important consequence of the ultrafast excited-state intramolecular hydrogen transfer (ESIHT) reaction in the six-membered hydrogen-bonded chelate ring of the cis-enol form. However, it has not been possible to monitor the ESIHT reaction in real time because of the symmetrical structure of the molecule with respect to the hydrogen-bonded chelate ring. In polar solvents, dipole-dipole interaction perturbs the intramolecular hydrogen bond leading to the reduced efficiency of the nonradiative deactivation process. However, stretching vibration in the intermolecular hydrogen bonds formed in the hydrogen-bonding (both donating and accepting) solvents induces another efficient channel for the nonradiative relaxation of the S(1) state of curcumin.

Femtosecond Transient Absorption Spectroscopy of the Medicinal Agent Curcumin in Diamide Linked γ-Cyclodextrin Dimers

“…In addition to the growth components, the ESA signals of curcumin in MeOH show a single decay component with a time constant τ 3 of 116 ps (51%), as previously reported. 40,47 The decay component in the ESA signal of curcumin in MeOD-d 4 results in a time constant τ 3 of 231 ps, showing a significant deuterium isotope effect of 2.0. This decay component is related to ESIHT, as previously investigated using femtosecond fluorescence upconversion spectroscopy.…”

“…In addition to the timescale of ESIHT, there have been some debates on whether the excited-state hydrogen atom transfer reaction of curcumin is intramolecular or intermolecular in nature in polar protic solvents. 41 47 Although it appears that there are two different pathways, these two proposals are in fact very similar, if not identical. This is because the intramolecular pathway proposed by Adhikary et al actually involves interactions of curcumin with the polar protic solvent, which is consistent with the conclusions drawn by Kasha and Maroncelli on the ESIHT reactions of 3-hydroxyflavone and 7-azaindole, respectively.…”

Nanoprecipitation and Spectroscopic Characterization of Curcumin-Encapsulated Polyester Nanoparticles

“…[14][15][16][17] In particular, previous work has demonstrated that solvent reorganization and excited-state intramolecular hydrogen atom transfer (ESIHT) in the keto-enol moiety of curcumin are major relaxation pathways of its excited state. [14][15][16][17] The ESIHT of curcumin exhibits a deuterium isotope effect while solvation remains unaffected. 14,15 In addition to having a low solubility in water, curcumin undergoes rapid degradation.…”

Excited State Dynamics of Curcumin and Solvent Hydrogen Bonding

“…Most recently, Ghosh et al used subpicosecond time-resolved transient absorption (TA) and fluorescence upconversion spectroscopy to study the excited state dynamics of curcumin, 21 and reached basically the same conclusion as Adhikary et al's regarding the shorter time constant. For the longer time constant, however, they suggested that intermolecular hydrogen bonding between protic solvent and curcumin may sufficiently perturb the intramolecular hydrogen bonding of curcumin.…”

“…3 and Table 2 that a non-hydrogen bonding solvent (chloroform) gives a significantly shorter decay time constant τ 2 than hydrogen bonding solvents. It is to be noted that a very fast decay of curcumin in non-hydrogen bonding solvents such as cyclohexane and 1,4-dioxane (each with a time constant of 44 and 55 ps) in earlier studies 18,21 was attributed to the fact that these solvents do not form intermolecular hydrogen bond with curcumin that would have weakened its intramolecular hydrogen bond, which would decrease the rate of its decay through ESIHT.…”