Effect of Acid on the UltravioletâVisible Absorption and Emission

Article pubs.acs.org/JPCA

Effect of Acid on the Ultraviolet−Visible Absorption and Emission Properties of Curcumin Yuval Erez,† Ron Simkovitch,‡ Shay Shomer,‡ Rinat Gepshtein,‡ and Dan Huppert‡,* †

Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel

‡

S Supporting Information *

ABSTRACT: Steady-state and time-resolved emission techniques were employed to study the acid−base effects on the UV−vis spectrum of curcumin in several organic solvents. The fluorescence-decay rate of curcumin increases with increasing acid concentration in all of the solvents studied. In methanol and ethanol solutions containing about 1 M HCl, the short-wavelength fluorescence (λ < 560 nm) decreases by more than an order of magnitude. (The peak fluorescence intensity of curcumin in these solvents is at 540 nm.) At longer wavelengths (λ ≥ 560 nm) the fluorescence quenching is smaller by a factor of ∼3. A new fluorescence band with a peak at about 620 nm appears at an acid concentration of about 0.2 M in both methanol and ethanol. The 620 nm/530 nm band intensity ratio increases with an increase in the acid concentration. In trifluoroethanol and also in acetic acid in the presence of formic acid, the steadystate emission of curcumin shows an emission band at 620 nm. We attribute this new emission band in hydrogen-bond-donating solvents to a protonated curcumin ROH2+ form. At high acid concentrations in acetic acid and in trifluoroethanol, the ground state of curcumin is also transformed to ROH2+ which absorbs at longer wavelengths with a band peak at ∼530 nm compared to 420 nm in neutral-pH samples or 480 nm in basic solutions. In hydrogen-bond-accepting solvents such as dimethyl sulfoxide and also in methanol and ethanol, curcumin does not accept a proton to form the ground-state ROH2+

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INTRODUCTION Curcumin (shown in Scheme 1) is the main curcuminoid of turmeric, which is a member of the ginger family.1 In India,

nm over a wide range of solvents. In the excited state it shows even stronger solvent dependence, with the emission maximum ranging between 460 and 560 nm. The time-resolved optical spectrum of curcumin was recently studied by several groups using a variety of ultrafast spectroscopy techniques. Petrich and co-workers15,16 suggested the occurrence of excited-state intramolecular hydrogen transfer (ESIHT) in methanol and ethylene glycol, after detecting two decay components in the excited-state kinetics with short and long time scales of 12−20 and 100 ps. Deuteration of curcumin led to a pronounced isotope effect in the slow-decay component. Palit and co-workers17,18 used fluorescence up-conversion to study the time-resolved fluorescence of curcumin in 15 solvents. They concluded that, because of its planar structure, the cis-enol conformer exhibits, in the first electronically excited state (S1), a strong intramolecular charge-transfer character. They attributed the solvent dynamics in the S1 electronic state in alcohols to hydrogen-bond reorganization. The solvation dynamics of curcumin in the alcohols used in their work correlates well with the solvation-dynamics data acquired by

Scheme 1. Molecular Structure of Curcumin

turmeric has been used traditionally as a spice, a food-coloring agent, and a dye for fabric as well as for various applications in folk medicine. Recently, the accumulation of data has driven the growth of research on various potential medicinal virtues of curcumin, resulting in clinical trials on humans for some applications.2−4 These, in turn, have ignited renewed interest in some of the more basic properties of this compound. Curcumin exhibits spectacular photophysical and photochemical properties.1,5−11 Because of its low solubility in water, studies have turned to its dynamics in other solvents and thus, in the search for an alternative solvent of choice, have revealed that these photochemical dynamics depend essentially12,13 on all of the Kamlet−Taft14 solvent parameters. The optical absorption maximum of curcumin ranges between 408 and 430 © 2014 American Chemical Society

Received: November 28, 2013 Revised: January 6, 2014 Published: January 9, 2014 872

dx.doi.org/10.1021/jp411686d | J. Phys. Chem. A 2014, 118, 872−884

The Journal of Physical Chemistry A

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Figure 1. (a) Steady-state emission of curcumin in neat acetic acid and in mixtures containing formic acid on a semilog scale. (b) Normalized version shown on a linear scale.

in another attempt to uncouple some of the photophysical processes curcumin may undergo.25 In the current study, we have focused our attention on the effect of acid on the steady-state absorption and emission properties of curcumin, as well as its effect on the time-resolved emission of curcumin. We find a profound acid effect on the absorption and emission of curcumin. The fluorescence of curcumin at short wavelengths (λ < 560 nm) is quenched by more than an order of magnitude in 1 M HCl in methanol solution. At long wavelengths, the fluorescence quenching is ∼3 times smaller and a new band at 620 nm appears. We attribute this band in hydrogen-bond-donating solvents such as trifluoroethanol which lack hydrogen-bond-accepting property to a protonated form of curcumin designated as ROH2+. The absorption spectrum of curcumin in hydrogen-bond-donating solvents is modified at high acid concentrations. A new band with a maximum at ∼530 nm is formed while the 420 nm ROH form decreases. We also argue that, as advocated in the work of Saini and Das,22 the dynamics of curcumin in the excited state cannot be well explained without taking full account of all Kamlet−Taft solvent parameters,14 which thus serve here as the framework for our approach.

Maroncelli and co-workers on coumarin dyes in the same alcohols.19 In previous studies, we used steady-state and time-resolved emission techniques to study the excited-state dynamics of curcumin. Temperature dependence of the nonradiative decay process of curcumin in both ethanol and 1-propanol was reviewed.20 In both solvents it was found that the decay rate of the curcumin emission strongly depends on the temperature at T > 175 K, and the nonradiative decay rate behaves in a manner similar to that of the dielectric relaxation of the solvent at T < 250 K. Fluorescence of curcumin was also studied in highly concentrated methanol and ethanol solutions of sodium and potassium acetate.21 The mild base was found to have a notable effect on both the time-resolved and the steady-state emissions. At about 1.8 M, the fluorescence intensity of curcumin in a methanol solution of NaAc is lower by a factor of ∼5, as is the average lifetime of the time-resolved emission signals, which are complex and wavelength-dependent. In alcoholic acetate solutions two decay components were observed and attributed to proton transfer: at λ ≤ 530 nm a short-time-decay component was seen with a time constant of 0.7 and 1.2 ps in methanol and ethanol, while, at λ ≥ 550 nm, the main component of the decay, attributed to the acetate, has a decay time of ∼4 and 8 ps in the same solutions. Saini and Das studied the time-resolved emission decay of curcumin in mixtures of toluene and polar solvents.22,23 Analysis of the time-resolved area-normalized emission (TRANE) led them to propose the existence of at least three species in the excited state. They argued that the strength of intermolecular hydrogen bonding has an effect on the ESIHT process and observed the excited-state dynamics of curcumin with the use of the Kamlet−Taft solvent parameters. They suggested further investigation by means of up-conversion fluorescence to resolve early time solvation dynamics. In a most recent attempt to deal with the complications that the coupling of multiple solvent-governed processes pose to the study of the excited-state dynamics of curcumin, Ghosh and Palit studied a partially analogous but asymmetric compound, (N,N-dimethylanilino)-1,3-diketone (DMADK).24 There, they (implicitly) adopt the view that an ESIPT process is taking place (ESIHT for curcumin, suggested by Petrich and coworkers15,16) and the application of the Glasbeek model to account for a twisted intramolecular charge transfer (TICT) process (suggested by us for curcumin20), at least in the case of DMADK. Barik and Priyadarsini studied dimethoxy curcumin

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EXPERIMENTAL SECTION

Curcumin (of 81% purity) was purchased from Sigma-Aldrich. All of the measurements were carried out with the use of fresh solutions of curcumin at the desired concentration and solvent. Methanol, ethanol, and other chemicals used in this study were of analytical grade and were purchased from Aldrich and Merck. The experimental data gathered on curcumin in ethanol are similar to the data presented in a study by Palit and coworkers17 whose sample was purified to more than 99%. We are, therefore, quite confident that the 19% mass ratio of unknown compounds in commercial curcumin does not contribute appreciably to the fluorescence signals. The fluorescence up-conversion technique was employed in this study to measure the time-resolved emission of curcumin at room temperature. The laser used for the fluorescence upconversion was a cavity-dumped Ti:Sapphire femtosecond laser (Mira, Coherent), which provides short, 150 fs, pulses at about 800 nm. The cavity dumper operated with a relatively low repetition rate of 800 kHz. The up-conversion system (FOG100, CDP) operated at 800 kHz. The samples were excited by pulses of ∼8 mW on average at the second-harmonicgeneration (SHG) frequency. The time-response of the up873



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Figure 2. (a) Steady-state emission spectra of curcumin in trifluoroethanol (TFE) excited at 400 nm, close to the absorption peak at 415 nm. (b) Normalized version of panel a. (c) Excitation at 400 nm of curcumin in TFE with formic acid at different molar fractions. (d) Normalized version of panel c.

Panels a and b of Figure 2 show the steady-state emission of curcumin in neat trifluoroethanol (TFE), in trifluoroacetic acid (TFA), and trifluoroethanol mixtures of 0.07, 0.13, and 0.22 mole fraction of TFA excited at 400 nm near the absorption peak of curcumin in neutral pH. Trifluoroethanol is a polar hydrogen-bond-donating solvent, but, unlike ethanol, it lacks hydrogen-bond-accepting properties. In TFE the excited state proton transfer (ESPT) process that takes place in photoacids in hydrogen-bond-accepting solvents such as water and alcohols is unlikely to occur, whereas protonation of a photobase by an excess proton in acidified solution may be promoted. The Kamlet−Taft solvent parameters of TFE are π = 0.73, β = 0, and α = 1.51, whereas, in ethanol, π = 0.54, β = 0.77, and α = 0.83. Similar results are also obtained in acidified curcumin TFE samples by formic acid with a pKa = 3.7 as shown in Figure 2c,d, a much weaker acid than TFA with pKa ∼ 0.7. As seen in the figure, the fluorescence of curcumin is strongly modified when TFA or formic acid is added to the curcumin TFE solution and a new emission band with a band peak at ∼620 nm is seen. Its relative intensity with respect to that of curcumin in neat TFE increases with an increase in the mole fraction of the TFA or formic acid. We suggest that the 620 nm band arises from a protonated curcumin which we designate as ROH2+. A similar acid effect on curcumin fluorescence is achieved when adding mineral acids such as HCl. Figure S1 in the SI shows the steady-state emission of curcumin in methanol solutions with increasing HCl concentrations up to 1 M HCl. We found that water also quenches the curcumin fluorescence. Since HCl solution (∼10 M) is in water, we measured the combined quenching effect of two processes, one by water and the other by excess protons.

conversion system is evaluated by measuring the relatively strong Raman−Stokes line of water shifted by 3600 cm−1. It was found that the full width at half-maximum (fwhm) of the signal is 300 fs. Samples were placed in a rotating optical cell to avoid degradation. The steady-state emission and absorption spectra were recorded by a Horiba Jobin Yvon FluoroMax-3 spectrofluorometer and a Cary 5000 spectrometer.

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RESULTS Steady-State measurements. Acid Effect on Curcumin in Weak Acid and in Hydrogen-Bond-Donating Solvents. Figure S2 in the Supporting Information (SI) shows the normalized steady-state emission of curcumin in ethanol and in acetic acid. The spectrum of curcumin in acetic acid clearly shows that a second band emitting at ∼620 nm with an intensity at 650 nm of about twice that of the main emission band that exists in both ethanol and acetic acid. Figure 1 shows the steady-state emission of curcumin in neat acetic acid and in mixtures containing formic acid. Acetic acid is a weak acid with pKa (in water) of 4.7, whereas formic acid is a stronger acid with pKa ∼ 3.7. As the formic acid concentration increases, the emission intensity ratio (IFRO−)/ (IFROH) increases. This acid effect is similar to the effect shown in Figure S1 of the SI for curcumin in methanol solutions containing HCl. In the experiments displayed in Figure S1, the HCl added to the methanol solution is dissolved in H2O and thus the solution actually contains water. Water also strongly affects the fluorescence intensity of curcumin and thus somewhat complicates the quantitative analysis of the acid effect shown in Figure S1. The data shown in Figure 1 are not affected by the presence of water. The results shown in Figure 1 clearly show the proton quenching of the ROH* population. 874



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Effect of a Base on the Fluorescence of Curcumin in Methanol and Ethanol. Figure S3 in the SI shows the absorption spectra of curcumin in methanol solutions containing various concentrations of NaOH. The neutral-pH form (ROH) of curcumin has an absorption band with a peak at about 420 nm, whereas the basic form, RO−, has a band peak at about 500 nm in methanol and ethanol. Figure 3a shows the normalized emission spectra of curcumin in neutral-pH ethanol and in two ethanol solutions containing low concentrations of NaOH. The emission of curcumin in basic solutions of methanol and ethanol is red-shifted with a band peak at ∼620 nm, and its intensity is much weaker than that of a neutral-pH ethanol solution where the absorption band peak is at 538 nm. We assign the two emission bands to the ROH and RO− forms of

curcumin, respectively. Figure 3b shows the excitation spectra of curcumin in neutral-pH and in basic solutions. The two excitation spectra are similar in their shape and position to the absorption spectra of curcumin in the ROH (neutral-pH) and RO− (basic solution) forms. An important point to notice is that the intensities of the two excitation spectra differ, at their peak wavelengths, by more than an order of magnitude. This implies that the ROH emission is more than an order of magnitude stronger than that of the RO− emission, since the ratio of the absorbance band peaks (ROH and RO−) is only ∼1.5 in favor of the ROH band. As we will show later, the 10times weaker RO− band intensity arises from the much shorter fluorescence-decay time in alcohols of the RO− (tens of picoseconds) than that of the ROH emission (hundreds of picoseconds). Figure 4a shows, on a semilogarithmic scale, the steady-state emission spectrum of curcumin in neutral-pH ethanol solution excited at 420 nm (the absorption band peak of the ROH form) and in basic solution excited at 500 nm. The fluorescence intensity of curcumin in the neutral-pH sample is about 10-fold larger than that of the basic solution. A point to notice is that the red sides of the spectra of the two emissions coincide. When we subtract the emission spectrum of the basic solution from that of the neutral solution, we obtain the ROH spectrum. Figure 4b shows, on a semilogarithmic scale, the steady-state spectrum of curcumin in neutral and basic ethanol solutions along with a computer fit of the spectra. Figure S4 in the SI shows the results shown in Figure 4b, but on a normalized linear scale. The spectral fit was performed by the use of a lognormal line-shape function as described in section G of the Supporting Information Table S1 in the SI provides the band-fitting parameters for both the ROH and RO− bands. The vibrational spacing of the ROH and RO− bands is about 1300 cm−1, which we attribute to the C−C skeleton-stretching vibration. The vibration substructure is seen in the steady-state emission of curcumin in nonpolar solvents such as dioxane (see Figure S5 in the SI). Note that the bandwidths of the subbands differ from −2500 cm−1 for ROH to 1600 cm−1 for the RO− band. This large difference in the widths of ROH and RO− emission bands has also been found for many photoacids such as 2-naphthol sulfonate derivatives, 8-hydroxy-1,3,6-pyrene trisulfonate, and even for the green fluorescent protein, GFP, chromophore. The fitting procedure begins with the fit of the steady-state emission of the basic solution, and the spectrum is assigned to the RO− band. The next step is the fit of the steady-state emission spectrum of the neutral-pH solution. We multiply the RO− synthetic spectrum by ∼0.1 and then add three log-normal bands at the high-energy side with spacing of 1300 cm−1 to fit the ROH band position at the blue side of the neutral-pH emission spectrum. The results are shown in Figure 4b, and the fitting parameters are given in Table S1 of the SI. Absorption Spectrum of Curcumin in the Presence of an Acid or Base. Figure 5a shows the absorption spectra of curcumin in acetonitrile in neutral pH and basic pH (NaOH) and in the presence of trifluoroacetic acid (TFA), a mild acid with pKa ≈ 0.7 which dissolves well in organic solvents. The absorption spectrum of curcumin shows that, in acidic acetonitrile solution, a new band is formed with a peak at 530 nm. This band is well separated and distinguished from the absorption spectra of curcumin in both neutral-pH and basicpH solutions in acetonitrile. We attribute this band to a

Figure 3. (a) Normalized emission spectra of curcumin in neutral-pH ethanol and in two ethanol solutions containing low concentrations of NaOH. (b) Excitation spectra of curcumin in neutral-pH and in basic solutions (where 1E7 represents 1 × 107). (c) Modification of panel b where the signal of 0.017 M NaOH (red line) was multiplied by 1.5. 875



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Figure 4. (a) Steady-state emission spectrum of curcumin in neutral-pH ethanol solution excited at 420 nm and in basic solution excited at 500 nm, shown on a semilogarithmic scale. (b) Steady-state emission spectrum of curcumin in neutral-pH and in basic ethanol solutions and a computer fit of the spectra, shown on a semilogarithmic scale.

emission spectrum and on the time-resolved emission at all wavelengths. Figure 7 shows the time-resolved emission of curcumin in neat acetic acid, measured at several wavelengths, and in solvent mixtures of acetic acid and formic acid of 0.2, 0.6, and 0.75 mole fraction of formic acid. The fluorescence-decay rate in neat acetic acid is rather small at all wavelengths. At short wavelengths (λ ≤ 530 nm), the fluorescence-decay rate is nonexponential. At long wavelengths (λ ≥ 570 nm), the fluorescence signals show a rise followed by a long, nearly exponential decay. When formic acid is added to acetic acid, the fluorescence-decay rate is much larger than in neat acetic acid. The higher the mole ratio of formic acid, the greater the decay rate. This is observed at all wavelengths. We attribute this effect to proton fluorescence quenching of curcumin in both forms, ROH and ROH2+. Formic acid (pKa = 3.7) is 10 times stronger an acid than acetic acid (pKa = 4.7). If the values of pKa of both acids in the absence of water are about the same as in aqueous solution, then the proton concentration in neat acetic acid (∼20 M) is relatively small, ∼10−3, whereas in formic acid it is 10−2 M. The diffusioncontrolled rate of reaction of a proton with excited curcumin is estimated to be ∼5 × 109 M−1 s−1. The pseudo-first-order rate for 10−2 M of protons is only 5 × 107 s−1, whereas the fluorescence-decay time in formic acid is ∼100 ps and the corresponding rate constant is 1010 s−1. We therefore conclude, that the fluorescence quenching rate by protons arises from the direct contact of a formic acid molecule with an oxygen atom of curcumin rather than from protons that diffuse through the long distance to a curcumin molecule. Thus, in a mixture of acetic and formic acids, the fluorescence-decay rate of curcumin is larger than in neat acetic acid. Figure 8 shows the time-resolved emission of curcumin measured at several wavelengths in neat trifluoroethanol (TFE) and in TFE acidified with formic acid at two mole fractions, χformic acid = 0.14 and χformic acid = 0.26. TFE is a hydrogen-bond-donating solvent with a pKa value of 12.4, and thus it promotes the protonation of curcumin in both the ground and excited states. In both acetic acid pKa = 4.7 and in TFE, an excited-state-protonation reaction occurs. Each panel in the figure shows four decay curves. The longest decay time is that of neat ethanol. The decay rate increases when the solvent is neat TFE, since a protonation reaction takes place in TFE which probably does not take place in ethanol. We believe the difference arises from the hydrogen-bond-accepting

protonated form of curcumin in the ground state, and we designate it as ROH2+. The protonation probably occurs at the diketone moiety. Similar neutral-pH and acid−base spectra of curcumin also exist in TFE and are shown in Figure 5b. Trifluoroethanol is a hydrogen-bond-donating solvent with Kamlet−Taft solvent parameters of π = 0.73, β = 0, and α = 1.5. The acidic ROH2+ band peak in TFE is at ∼540 nm. The steady-state emission of curcumin in both TFE and acetonitrile in acidic media shows the existence of an emission band at 620 nm which we attribute to the emission of the ROH2+ form. Methanol and ethanol have both hydrogen-bond-donating and -accepting properties parameters of π = 0.54, β = 0.77, and α = 0.83 for ethanol and π = 0.6, β = 0.62, and α = 0.93 for methanol. The absorption spectrum of curcumin in an acidified solution of methanol and ethanol does not show an ROH2+ band at mild acid concentrations. These surprising results may arise from the strong hydrogen-bond-accepting properties of both methanol and ethanol. Figure 5c shows the curcumin absorption spectrum in ethanol in the presence of TFA up to χTFA ≈ 0.3. As can be seen, the spectrum consists only of the neutralpH ROH band. Time-Resolved Emission of Curcumin in Acidic Solutions. Figure 6 shows, on a semilogarithmic scale, the time-resolved fluorescence, measured at several wavelengths, of curcumin in neat methanol, in a methanol solution containing 1 M HCl and a methanol solution containing ∼10% H2O by volume. The fluorescence time-resolved signals were measured by the fluorescence up-conversion technique with a system-response function of 300 fs, fwhm. The fluorescence shows a nonexponential decay at short wavelengths of λ ≤ 550 nm, and a nearly exponential decay at long times t > 20 ps is observed in methanol at a long wavelength of 620 nm. In 1 M HCl methanol solution the average decay rate of the signal is the greatest at all wavelengths, whereas in neat methanol is the smallest. At λ ≥ 570 nm, the signals at short times show a rise followed by a nearly exponential long-time decay. Since the HCl we added to the solution was in aqueous solution, we also measured the time-resolved emission of a mixture of methanol containing ∼10% H2O, which is the water content of a 1 M HCl methanol solution. The water itself has a large effect on the decay rate of the signals, as shown in Figure 6. The point we wish to make is that proton-fluorescence quenching has a profound effect on both the steady-state 876



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The fits are rather good at short and long times. The fitting parameters are given in Table 1 The fluorescence decay of curcumin in these solvents at short wavelengths λ = 520 nm could be reasonably fitted by two or three exponents. The fast-decay component is attributed to the protonation reaction, whereas the long-time component is attributed to the fluorescence lifetime. The fluorescence lifetime strongly depends on the proton quenching rate constant kqH+ (see Scheme 2), which affects the lifetime of the ROH and ROH2+ in a similar way (see Table 1). The protonation reaction decay time is 36, 16, and 9 ps in ethanol, TFE, and TFE/formic acid mixture χformic acid = 0.14, respectively. The fluorescence-decay times are 210, 90, and 48 ps in these solvents, respectively. In summary, curcumin emission is strongly modified when acid is introduced to the solution. A new red emission band with a peak at 620 nm, which is attributed to the protonation reaction, is formed ROH* + H+ → ROH 2+ *

This takes place within the excited-state lifetime of the neutralpH ROH form. Curcumin in Basic Solution (Comparison with Neutral pH). Figure 10 shows the time-resolved emission of curcumin in basic-pH solutions (pH ∼ 9) of methanol and ethanol measured at 580 and 620 nm and in neutral-pH solutions of both solvents. The fluorescence-decay rate in a basic solution is rather high in comparison with that of the neat solvents. Tables 2 and 3 provide the values of τav = ∫ If(t) dt for the curcumin fluorescence at several wavelengths in both methanol and ethanol, neutral-pH solutions, as well as in basic-pH solutions. As can be clearly seen, τav in basic solutions of ethanol is about four times smaller than in neat solutions at long wavelengths λ ≥ 580 nm and about eight times at shorter wavelengths λ ≤ 560 nm. In this study, we maintain that the large difference in the average fluorescence-decay times between RO− and the ROH, in addition to the rather low proton-transfer rate of curcumin in alcohols, and many other solvents are responsible for the ambiguous picture of curcumin photophysics and photochemistry. The combined effect of both ROH −1 RO − −1 parameters, (k eff ) > (k eff ) , where keff−1 is the fluorescence lifetime and a rather low proton-transfer rate, kPT < kROH eff , mask the existence of two emission bands in the steady-state emission spectrum of curcumin in organic solvents. We assign the strong band with a maximum at ∼540 nm to the protonated ROH form. In hydrogen-bond-accepting solvents such as methanol and ethanol, a second weak band of the deprotonated form (RO−), with a maximum at long wavelengths (λmax ∼ 620 nm) is buried under the strong ROH band. The steady-state-intensity ratio in alcohols such as methanol and ethanol is IROH/IRO− ∼ 10. At such a large ratio and such broad ROH and RO− bands and large overlap, it is almost impossible to observe the contribution of the RO− band to the steady-state fluorescence spectrum. We further discuss this issue in the Discussion.

Figure 5. Absorption spectra of curcumin in (a) acetonitrile in neutral pH and basic pH (NaOH) and in the presence of TFA. (b) Same as for panel a but with trifluoroethanol as solvent. (c) Curcumin absorption spectrum in ethanol with added acidsTFA and HCl.

property of the solvents where ethanol (β = 0.7) accepts hydrogen bonds, and TFE (β = 0) does not. When formic acid (pKa = 3.7) is added, the protonation reaction of curcumin proceeds at a much higher rate, and the higher the concentration of formic acid, the higher the fluorescencedecay rate. Similar results are obtained when TFE is acidified by trifluoroacetic acid (TFA, pKa = 0.7); the time-resolved emission of curcumin in TFE/TFA mixtures is shown in Figure S6 in the SI. Figure 9 shows a multiexponential fit to the time-resolved emission of curcumin measured at 520 nm, near the peak of the ROH* emission band, and at 620 nm, near the ROH2+* emission peak, in ethanol and in TFE solutions as well as in a mixture of TFE with formic acid χformic acid = 0.14.

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DISCUSSION Main Findings. (1) Addition of acid to curcumin solutions in several organic solvents strongly affects the steady-state emission spectrum.

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Figure 6. Time-resolved fluorescence of curcumin in neat methanol, a methanol solution containing 1 M HCl, and a methanol solution containing ∼10% H2O by volume, measured at several wavelengths.

Figure 7. Time-resolved emission of curcumin in neat acetic acid and in solvent mixtures of acetic acid and formic acid of 0.2,0.6 and 0.75 mole fraction of formic acid.

(5) The time-resolved emission of curcumin in acidic solutions shows an increase of the decay rate at short wavelengths λ ≤ 540 nm and the appearance of the new band at 620 nm. (6) The above findings indicate that protonation of one or more of the four oxygen atoms occurs within the short excitedstate lifetime of curcumin. In hydrogen-bond-donating solvents in the presence of acid, the photoprotic reaction leads to a new emission band at 620 nm, which we assign to the ROH2+*.

(2) The emission intensity is reduced as the acid concentration is increased. (3) A new emission band is observed at a longer wavelength (∼620 nm). (4) At high acid concentrations the curcumin absorption spectrum in hydrogen-bond-donating solvents such as trifluoroethanol shows a new band at longer wavelengths with a peak at 520 nm compared to the neutral-pH band, with a peak at 420 nm. 878



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Figure 8. Time-resolved emission of curcumin measured at 520, 540, 600, and 620 nm in neat trifluoroethanol (TFE) and in TFE acidified with formic acid at two mole fractions, χformic acid = 0.14 and χformic acid = 0.26.

Figure 9. Time-resolved emission of curcumin and computer multi-exponential fitting (see Table 1) measured at (a) 520 and (b) 620 nm in neat trifluoroethanol (TFE) and in TFE acidified with formic acid χformic acid = 0.14 and χformic acid = 0.26.

Table 1. Exponential Fitting Parameters for the Time-Resolved Emission of Curcumina wavelength (nm)

solvent

a1

τ1 (ps)

a2

τ2 (ps)

a3

τ3 (ps)

τF (ps)

520

ethanol TFE TFE/formic acid (χformic acid = 0.14) ethanol TFE TFE/formic acid (χformi acid = 0.14)

0.6 0.74 0.17 0.24 (rise) 0.2 0.5

36 16 3.0 1 0.1 0.5

0.4 0.26 0.68 0.09 0.08 0.29

210 86 9.0 10 3.3 3.3

0.15 0.67 0.72 0.21

46 38 16 8

230 90 46

620

a

For 620 nm the fit function is I620(t) = [∑3i=1ai(1 − exp(t/τi)] exp(−t/τF).

parameters scale to differentiate between two types of solvents. The first group possesses hydrogen-bond-accepting properties. From this group we used, in the current study, dimethyl sulfoxide as well as methanol and ethanol. The second group includes solvents which possess hydrogen-bond-donating properties. The solvents used from this group are trifluoroethanol, acetic acid, and acetonitrile. Effect of Acid on the Emission of Curcumin in MeOH and EtOH and in Mixtures of These with Water. Excess protons in in methanol and ethanol solutions of curcumin lead

(7) The protonation reaction occurs in the ground state as well as in the excited state, but only at a much higher acid concentration; thus the ground- and excited-state pKs of the protonation reaction differ by several log units. The positions, shapes, and intensities of the emission bands of the deprotonated form, RO−, of curcumin and of ROH2+ are similar, but not identical. These rather small differences in the relative steady-state emission intensities (lifetimes) and band positions of RO− and ROH2+ make it rather difficult to interpret the acid effects. We used the Kamlet−Taft solvent 879



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Figure 10. Time-resolved emission of curcumin in basic-pH solution (pH ∼ 9) of methanol and ethanol measured at 580 and 620 nm and in neutralpH solutions of both solvents.

Table 2. τav of Curcumin in Methanol Neutral and Basic Solutions λ (nm)

τav of MeOH neutral soln (ps)

τav of MeOH basic soln (ps)

τneutral /τbasic av av

560 580 620

101 117 124

11 13 21

9.20 8.91 5.75

F F −1 −/ I kPT ∼ (IRO ROH)τF −1

where τF is the RO emission lifetime. When the band intensity ratio is ∼20 and τF = 5 ns, then kPT = 4 × 109 s−1 and τPT = 250 ps. As seen in Figure 10 and in Tables 2 and 3, the curcumin RO− form emission lifetime is much smaller than that of the ROH form. The shorter lifetime (by a factor of 10) of the RO− form of curcumin lowers the RO− steady-state emission band intensity. This leads us to propose that the broad emission band of curcumin in organic protic solvents, such as methanol and ethanol, with hydrogen-bond-accepting properties is actually composed of two emission bands, a strong emission band of ROH and a much weaker band of RO−. In such a case, curcumin may undergo an ESPT process in protic organic solvents with hydrogen-bond-accepting properties, such as H2O/(organic solvent) mixtures and also methanol and ethanol. Previous photoacid studies revealed that ESPT occurs in methanol and ethanol at much lower rates than in water. For photoacids with pKa* > 0 in water, the ESPT in methanol will be smaller by more than 3 orders of magnitude. The Förstercycle provides an estimate of the pKa* and can be used to estimate the pKa* value of curcumin. Förster-Cycle Calculation. With the Förster-cycle calculation,39 it is possible to estimate the change in acidity upon excitation of the molecule. This calculation is based on the position of the optical absorption or emission bands of the protonated and deprotonated forms of a photoacid. The Förster energy cycle leads to a simple relation between band positions and change in acidity:

Table 3. τav of Curcumin in Ethanol Neutral and Basic Solutions λ (nm)

τav of EtOH neutral soln (ps)

τave of EtOH basic soln (ps)

τneutral /τbasic av av

560 580 600 620

226 265 268 291

32 51 71 88

7.13 5.18 3.76 3.32

(1)

−

to strong quenching of the ROH emission of curcumin at short wavelengths (λ ≤ 560 nm; see Figures 5 and 6). The quenching in the steady-state spectrum measured in methanol and ethanol (see Figures 1 and S1 of the SI) enables the uncovering of the deprotonated form, the RO− emission of curcumin, which emits at about 600 nm. This leads us to suggest that an intermolecular ESPT occurs with rather low efficiency in curcumin in both methanol and ethanol and in water mixtures of these solvents and that curcumin is a mild photoacid in water, like many other hydroxyaryl compounds that have been investigated in the past.26−38 Common photoacids show a dual band emission when excited in their neutral ROH form. The photoprotolytic reaction of photoacids is given in Scheme S1 in the SI. For common photoacids with pKa* < 2.5 and fluorescence lifetimes of ∼5 ns (in the absence of the photolytic process), the RO− band intensity position at longer wavelengths is much greater than that of the ROH band. In fact, the steady-state fluorescence band intensity ratio IFRO−/IFROH is used to estimate the ESPT rate constant kPT.

ΔpK a* = C·Δν

(2)

where C is the product of universal constants: C=

NAh = 2.09 × 10−3 cm ln(10)RT

Δν is the difference, in wavenumber units, between the positions of the ROH* and RO−* absorption or emission bands. 880



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The maxima of the emission bands of curcumin ROH and RO− in methanol are at 543 nm (18400 cm−1) and 620 nm (16100 cm−1), so Δν ≈ 1700 cm−1. This leads to a ΔpKa* = 3.55. The pKa of curcumin in methanol is estimated to be about ∼7.2, and thus pKa* ∼ 3.6. This value is rather small, and the expected ESPT rate constant should also be small. This should limit the ESPT efficiency. For 2-naphthol with pKa* ∼ 2.7 in water, kPT = 108 s−1 and τF = 5 ns, so the ESPT efficiency in water is about 0.3. The time-resolved emission of curcumin in 50% acetonitrile−H2O shows that the ROH band disappears at ∼20 ps. A photoacid with a pKa* ∼ 0 has an ESPT time constant of about 20 ps. We therefore estimate that the pKa* of curcumin in water is approximately 0 and in methanol it should be about 3. The logarithms of the ESPT rates of photoacids are inversely proportional to the pKa* values for pKa* > 0.5. We estimate therefore that in methanol the ESPT rate of curcumin should be 3 orders of magnitude smaller than in water. If the ESPT rate in water is indeed the inverse of the fluorescence lifetime, then it is expected that in methanol the ESPT time constant is ∼20 ns. The pure radiative lifetime of curcumin is about 5 ns, and thus the upper value of the ESPT quantum efficiency is 0.2, but, in fact, nonradiative processes limit the radiative lifetime of curcumin and thus may reduce it by an order of magnitude, i.e., 0.02. This small number may explain the weak RO− band intensity deduced in the analysis given in Figures 3 and 4. Excess Proton Effect on Curcumin in Weak Acids and in Hydrogen-Bond-Donating Solvents. When we discovered that in hydrogen-bond-donating solvents such as trifluoroethanol and also in weak acids such as acetic acid, the addition of acid modifies the emission spectrum. A new emission band is formed, positioned slightly more to the red than that of the RO− band, and we looked for a different explanation for the photoreaction of curcumin in these solvents. In the past we studied the photophysics and photochemistry of 7-hydroxy-4-methylcoumarin (also known as coumarin-4) which has been found to have dual photochemical properties.40 In water, its neutral form (ROH*) is a strong photoacid and transfers a proton to the solvent with pKa* ∼ 0. It is also a strong photobase capable of accepting a proton from protic solvents or reacting with excess protons (acid) in solution. In the study mentioned above, we measured the rate and dynamics of the reaction of excited-state coumarin-4 with excess protons in neat monols and water−glycerol solutions. The reaction shows diffusion-controlled reactivity and fits the Smoluchowski model.41 The reaction can be described by the kinetic scheme ROH* + H+ → ROH2+*, where ROH2+* and ROH* are the protonated and neutral forms of the photobase, respectively. Scheme S2 in the SI shows the possible groundstate protolytic equilibria involving the addition of protons to coumarin-4 and the removal of protons from it. The steady-state emission of coumarin-4 strongly depends on the pH of the solution. Three structureless emission bands exist, and their band peaks in methanol and ethanol are at 380, 440, and 485 nm. The emission bands are assigned to ROH at 380 nm and RO− at 440 nm, and the most striking emission band is that of ROH2+ which appears in acidic solutions with a maximum at 485 nm. Figure 11 shows the steady-state emission of 7-hydroxy-4methylcoumarin (coumarin-4 shown in Scheme S2 in the SI), in neat ethanol (neutral pH) and in solvent mixtures, of χTFA acid = 0.18, χethanol = 0.82 and χTFA acid = 0.3, χethanol = 0.7.

Figure 11. Steady-state emission spectra of coumarin-4 in neat ethanol and in solvent mixtures of ethanol and trifluoroacetic acid.

We used TFA to acidify the coumarin-4 samples (Figure 11) since it was also used in this study to acidify the curcumin samples. The steady-state emission spectrum of coumarin-4 in basic ethanol solution (pH ∼ 9) is also shown in the figure. The band positions of ROH, RO−, and ROH2+ of coumarin-4 in this solvent mixture are at 382, 450, and 485 nm, respectively. The point we stress here is that the emission bands of RO− and ROH2+ of coumarin-4 differ in their position and shape and are visually well separated. The position of the ROH2+ emission band is at a longer wavelength than that of RO−. A similar (but somewhat different) situation is found in the steady-state emission spectra of curcumin in neutral-pH, basic, and acidic solutions. In acidic solutions, we find a large reduction in the fluorescence intensity at short wavelengths (λ < 560 nm) as the solution acidity increases, whereas at long wavelengths, we find a new band with a maximum at ∼620 nm in acetonitrile, TFE, and acetic acid. We attribute this new band to the emission of ROH2+. Scheme 2 shows the excited-state protonation reaction Scheme 2. Excited-State Protonation Reaction of Curcumin as a Photobase

of a photobase ROH* and the possible relaxation pathways to the ground state. In acidic medium, curcumin in an excited state reacts with the excess proton in solution This reaction is also accompanied by a nonradiative channel that leads to large fluorescence quenching in acidic solution of ROH* and, to a lesser extent, of the fluorescence of ROH+2 *. This can be clearly seen in the intensity of the steady-state emission spectrum of curcumin as a function of the acid concentration, shown on a semilogarithmic scale and by the time-resolved emission of curcumin, shown in Figure 7. Figure 12a shows the steady-state emission spectra of curcumin in neutral-pH, acidic, and basic solutions. Figure 12b shows the steady-state emission spectra in TFE and also in acidic and basic mixtures of TFE. The position of the RO− and ROH2+ emission bands of curcumin in acetonitrile are at 600 and 620 nm, respectively. This difference in the bands maxima is much smaller than in coumarin-4 where the band maxima are at 450 and 485 nm, and, in addition, their shapes are rather different (see Figure 881



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Figure 12. Steady-state emission spectra of curcumin in (a) different solutions of ACN: neutral and acidic solutions excited at 400 nm and basic solutions excited at 460 nm. (b) Neutral-pH trifluoroethanol (TFE), basic solution (pH ∼ 9) excited at 490 nm and an acidic solution of a mixture of TFE and TFA excited at 400 nm and another TFE/TFA solution excited at 530 nm.

The suggested structure is stabilized by the six-membered ring structure and the symmetrical dual conjugated systems on both sides of the diketone (see Scheme 3). A somewhat similar

11). The curcumin ROH band position is blue-shifted in TFA/ acetonitrile mixtures of λTFA ≥ 0.2, since TFA lacks hydrogenbond-accepting properties (β-parameter equals zero). When we superimpose the emission spectrum of curcumin in this solvent mixture to coincide at short wavelength (∼450 nm) with that in neat acetonitrile, the band which we assigned to the ROH2+ emission is red-shifted with respect to the RO− emission band (basic solution) excited at 520 nm. This small difference in curcumin RO − and ROH 2 + fluorescence may confuse and lead to wrong identifications of the long-wavelength band in hydrogen-bond-donating solvents such as trifluoroethanol and in weak acids such as acetic acid. In neat acetic acid (pKa = 4.7) the steady-state fluorescence spectrum of curcumin shows a small bump at about 620 nm (see Figure 1). When a stronger organic acid such as formic acid (pKa = 3.7) is added to the solution, or a much stronger acid such as trifluoroacetic acid (pKa = 0.75), a new band at 620 nm is seen (Figure 2). The higher the mole fraction of the acid in the solvent mixture, the lower the fluorescence intensity of curcumin at short wavelengths (λ < 560 nm) and the higher the band-intensity ratio (IF620/IF530). We assign the 620 nm band in these solvents to ROH2+ rather than to RO−. In acetic acid we were unable to determine the position of the RO− band, but we also measured the effect of acid on curcumin fluorescence in acetonitrile mixtures. Acetonitrile is a polar liquid with mild hydrogen-bonding-accepting (β) and -donating (α) properties. The π, β, and α values are 0.75, 0.31, and 0.19, respectively. Figure 12b shows the steady-state emission spectra of curcumin in neutral-pH TFE, in basic solution (pH ∼ 9) excited at 490 nm, and in an acidic solution of a mixture of TFE and TFA excited at 400 nm and another spectrum, excited at 530 nm, which is the peak of the curcumin ROH2+ form absorption band. As seen in the figure, the emission spectrum of the ROH2+ form and that of RO− are almost the same. The intensities of the two bands differ by a factor of ∼40 in favor of the ROH2+ form (measured in acidified TFE), when curcumin is excited at both ROH and ROH2+ absorption wavelengths. Protonated ROH2+ Form of Curcumin. A plausible description of the protonated ROH2+ form of curcumin is given in the following discussion. The proton is equally attracted by the two oxygen atoms of the diketone.

Scheme 3. Diketone Form

situation takes place in 1,8-bis(dimethylamino)naphtalane, the proton sponge that can form a structure similar to that of a protonated form RN2H+ that emits at a longer wavelength than the RN2 compound.42 Effect of Acid on Curcumin in a Hydrogen-BondAccepting Solvent. Figure S7 in the SI shows the steady-state emission of curcumin in neutral-pH, basic-pH, and acidic-pH DMSO solutions. DMSO is a polar, basic solvent. The Kamlet−Taft π, β, and α parameters are 1, 0.76, and 0, respectively. Therefore, the addition of acid to curcumin in DMSO mixtures has a rather small effect on the emission spectra. The intensity of the ROH2+* emission band is rather small and appears at only a very large mole fraction of TFA in DMSO mixtures (λTFA ∼ 0.6). This is in accord with the basic properties of DMSO.

■

SUMMARY AND CONCLUSIONS We have employed steady-state (time-integrated) and timeresolved emission techniques in order to study the effect of acid on curcumin emission and absorption. We find that acid quenches the fluorescence of curcumin dissolved in six organic solvents. We measured the fluorescence quenching of curcumin by acid in the following solvents: dimethyl sulfoxide, methanol, ethanol, acetonitrile, trifluoroethanol, and acetic acid. We used three different acids in order to measure the acid effect trifluoroacetic acid (TFA) (pKa = 0.75), formic acid (pKa = 3.7), and hydrochloric acid dissolved in water (pKa∼ −8). We find that the intensity of the steady-state emission spectrum at short wavelengths that covers about two-thirds of the spectrum area is strongly quenched by all three acids used in the current study. A solution of 1 M HCl quenches the fluorescence of curcumin in methanol at λ ≤ 560 nm by more than an order of 882



Article

Notes

magnitude. Similar results are observed for the much weaker organic acids used in this study. The fluorescence quenching of curcumin takes place in all solvents used. At long wavelengths (λ ≥ 560 nm) the fluorescence quenching is smaller by a factor of 3 than that at shorter wavelengths (λ < 560 nm). In about 0.2 M HCl, in both methanol and ethanol (hydrogen-bondaccepting solvents), an emission band with a maximum at about 620 nm is observed. We attribute this band to the RO− form of curcumin that is formed by the process of ESPT to the solvent. In acetic acid and trifluoroethanol, acidification of the solution by the weak and strong acids used in this study led to a new emission band at ∼620 nm and a reduction in the intensity of the main emission band at 520−540 nm. We attribute the new band in both these acidic solvents to the emission of ROH2+, the protonated (cationic) form of curcumin. It is most likely that the protonation occurs on an oxygen atom. We suggest that it is on the diketone moiety. The protonation reaction occurs in these liquids in both the ground and the excited states of curcumin. Similar amphyprotic behavior in both the ground and excited states was observed in the past40 with 7-hydroxy-4methylcoumarin (coumarin-4) dissolved in alcohols. Coumarin4 in the excited state reacts with protons to form the ROH2+* form that emits at longer wavelengths than does the RO− form (see Discussion and Figure 11). The photoprotic reaction in both coumarin-4 and curcumin leads to the quenching of ROH2+ at high acid concentrations. The quenching of curcumin fluorescence by acid can be seen in Figure 7, which shows the time-resolved fluorescence of curcumin in solvent mixtures of acetic acid and formic acid at several wavelengths. The fluorescence lifetime of ROH2+ is shorter than that of the neutral ROH form of curcumin. The actual lifetime of ROH2+ is not known since the high proton concentration in the solution quenches the fluorescence of both ROH and ROH2+. In the presence of acid in the solution, the absorption of curcumin also changes in polar hydrogen-bond-donating solvents such as trifluoroethanol and acetonitrile. A new band is formed with a peak at 530 nm at the expense of the neutral form with a peak at 420 nm. This new band is well separated from the deprotonated RO− absorption band with a peak at about 490 nm in acetonitrile.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. M. Fridman for helpful suggestions and discussion. This work was supported by grants from the James-Franck German-Israeli Program in Laser-Matter Interaction and by the Israel Science Foundation.

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ASSOCIATED CONTENT

S Supporting Information *

Text describing steady-state emission of curcumin in methanol rich water solutions and in ethanol and acetic acid, adsorption of circumin in basic methanol solutions, reversible and irreversible photoprotolytic cycles of the photoacids, groundstate properties of coumarin 4, curcumin emission spectrum fit by log-normal band shape function, and accompanying references, figures showing time-integrated emission spectra, normalized steady-state emission spectra, absorption spectra of curcumin in methanol solutions, steady-state spectra of curcumin in neutral-pH enthanol and basic solutions and computer fit of the spectra, steady-state emission spectra of curcumin in dioxane, and time-resolved emission and steadystate spectra of curcumin in different solvents, and tables listing the log-normal band fit of curcumin. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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