Isomerization between 2-(2, 4-Dihydroxystyryl)-1-benzopyrylium and 7

Article pubs.acs.org/JPCA

Isomerization between 2-(2,4-Dihydroxystyryl)-1-benzopyrylium and 7-Hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium Vesselin Petrov, A. Jorge Parola, and Fernando Pina* REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal S Supporting Information *

ABSTRACT: 2-Phenyl-1-benzopyrylium (flavylium) and 2-styryl-1benzopyrylium (styrylflavylium) cations establish in aqueous solution a series of equilibria defining chemical reaction networks responsive to several stimuli (pH, light, redox potential). Control over the mole fraction distribution of species by applying the appropiate stimuli defines a horizontal approach to supramolecular chemistry, in agreement with the customary bottom-up approach toward complex systems. In this work, we designed an asymmetric styrylchalcone able to cyclize in two different ways, producing two isomeric styrylflavylium cations whose chemical reaction networks are thus interconnected. The chemical reaction networks of 2-(2,4-dihydroxystyryl)-1-benzopyrylium (AH+) and 7-hydroxy-2-(4-hydroxystyryl)1-benzopyrylium (AH+iso) comprise the usual species observed in flavylium-derived networks, in this case, the styryl derivatives of quinoidal bases, hemiketals, and chalcones. The thermodynamics and kinetics of the crossed networks were characterized by the use of UV−vis absorption and NMR spectroscopy as well as time-resolved pH jumps followed by stopped-flow. The two styrylflavylium cations are connected (isomerize) through two alternative intermediates, the asymmetric trans-styrylchalcone (Ct) and a spiropyran-type intermediate (SP). At pH = 1, AH+ slowly evolves (kobs ≈ 10−5 s−1) to a mixture containing 62% AH+iso through the Ct intermediate, while at pH = 5, the SP intermediate is involved. The observed rate constants for the conversion of the styrylflavylim cations into equilibrium mixtures containing essentially Ct follow a pH-dependent bell-shaped curve in both networks. While at pH = 1 in the dark, AH+ evolves to an equilibrium mixture containing predominantly AH+iso, irradiation at λ > 435 nm induces the opposite conversion.

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INTRODUCTION Systems with increasing complexity (multistate) aiming to achieve more functions (multifunctional) in order to fill the gap between simple molecular systems and biology have been designed essentially in the frame of supramolecular chemistry.1−5 The usual approach in supramolecular chemistry is a vertical one, where complexity is reached through a bottom-up strategy. However, beyond this vertical approach and complementary to it, complexity can also be built horizontally by increasing the number of available states of a system (species) and finding ways (stimuli) to control the weight distribution of those states, defining a horizontal supramolecular chemistry or molecular metamorphosis, more close to the recent and rapidly evolving domain of systems chemistry.6 It is possible to conceive molecules that by the action of external stimuli are reversibly transformed in other species exhibiting different physicochemical properties, everything taking place at the molecular level, at the bottom. The behavior of these molecules has some analogy with the life cycle of insects; a molecule suffers successive changes in its structure and properties along time by the action of external stimuli (metamorphosis). This concept fits very well the © 2012 American Chemical Society

description of the chemical reaction networks characteristic of the flavylium compounds,7−9 as illustrated in Scheme 1 for 7-hydroxyflavylium.10 In acidic media, the flavylium cation, AH+, is the thermodynamically stable species. When the pH is raised, other species are produced: the quinoidal base A, formed by deprotonation of the phenol group; the hemiketal B2, obtained through the hydration in position 2 of the flavylium cation; ring−chain tautomerization of B2 leading to the formation of the (Z)- or cis-chalcone Cc, through a ring-opening process; and finally, the cis−trans isomerization resulting in the formation of the (E)- or trans-chalcone, Ct. At more basic pH values, negatively charged species can also be produced upon further deprotonation of phenolic groups, increasing the number of transient or permanent states of the network. The following set of equations accounts for the sequence of reactions in acid to neutral media; see Scheme 1. Received: April 13, 2012 Revised: July 10, 2012 Published: July 10, 2012 8107

dx.doi.org/10.1021/jp303589n | J. Phys. Chem. A 2012, 116, 8107−8118

The Journal of Physical Chemistry A

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Scheme 1. Chemical Reaction Network of Flavylium Compounds Here Exemplified for 7-Hydroxyflavylium Exhibiting Several Species Whose Mole Fraction Distributions Can Be Controlled under Various Stimuli (pH and Light, in This Case)

Scheme 2. Flavylium Isomers Obtained through Ring Closure of Asymmetrically Substituted 2,6-Dihydroxychalcones16

Scheme 3. Symmetric and Asymmetric 2,2′-Styrylchalcones and Respective Isomeric Styrylflavylium Cations Formed upon Different Ring-Closure Processes

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Figure 1. (A) Spectral variations observed upon dissolution of the 2-(2,4-dihydroxystyryl)-1-benzopyrylium chloride in water/ethanol (80:20) at pH = 1.68; (inset) absorbance variation at 537 nm as a function of the reaction time. (B) Spectra of the initial compound, AH+ (full line), of the equilibrated mixture (dotted line), and of the product, AH+iso (dashed line; obtained by mathematical decomposition of the spectrum of the stationary state: 38% AH+ + 62% AH+iso).

Figure 2. (A) 1H NMR spectra of 2-(2,4-dihydroxystyryl)-1-benzopyrylium chloride in D2O/CD3OD (80:20) at pD = 1.87 as a function of time. (B) Time dependence of the normalized areas of the peaks regarding AH+ and AH+iso. The fitting was achieved for a mole fraction of 58% AH+ + 42% AH+iso at the equilibrium through a first-order kinetic process, with a rate constant of 2.5 × 10−5 s−1.

AH+ + H 2O ⇌ A + H3O+

Ka

In the flavylium chemical reaction network, the molecular transformations between the different states can be performed by light, pH, or heat11 and, in some particular cases, by a redox stimulus.12−14 For the flavylium compounds possessing a relatively high cis−trans isomerization barrier, it is very useful to describe the so-called pseudoequilibrium, defined as the transient state formed between all of the species of the network before formation of significant amounts of the trans-chalcone, that is, involving eqs 1−3.

Proton transfer (1)

AH+ + 2H 2O ⇌ B2 + H3O+

Kh2

Hydration (2)

B2 ⇌ Cc

Kt

Tautomerisation

(3)

Cc ⇌ Ct

Ki

Isomerization

(4)

Equations 1−4 can be described by a single apparent acid−base equilibrium9 AH+ + H 2O ⇌ CB + H3O+

K a′ =

AH+ + H 2O ⇌ CB^ + H3O+

[CB][H3O+] [AH+]

[CB^][H3O+] [AH+] (7)

where

(5)

K a^ = K a + Kh2 + Kh2K t

where

and [CB^] = [A] + [B2] + [Cc]

K a′ = K a + Kh2 + Kh2K t + Kh2K tK i and [CB] = [A] + [B2] + [Cc] + [Ct]

K a^ =

(8)

The pseudoequilibrium still behaves as a single acid−base equilibrium with a (pseudo)equilibrium constant equal to

(6) 8109



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flavylium compounds (Scheme 1). 17 These findings were confirmed with more detail for the compounds 7-hydroxy2-styryl-1-benzopyrylium, 7-hydroxy-2-(4-hydroxystyryl)1-benzopyrylium, and 7-hydroxy-2-(4-dimethylaminostyryl)-1-benzopyrylium.19,20 Here, we report the synthesis of 2-(2,4-dihydroxystyryl)-1benzopyrylium chloride and the characterization of the network of chemical reactions involving this compound, the isomeric 7hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium, a common asymmetric trans-2,2′-styrylchalcone (Scheme 3) and also a common intermediate, most probably a bis(benzospiropyran).

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General for Synthesis. All reagents and solvents used were of analytical grade. Mass spectra were run on an Applied Biosystems Voyager PRO spectrometer (MALDI-TOF MS). Elemental analyses were obtained on a Thermofinnigan Flash EA 1112 Series instrument. NMR Spectroscopy. The NMR spectra at 298.0 K were obtained on a Bruker Avance 400 operating at 400.15 (1H) or 100 MHz (13C). For each compound, 1H, 13C, COSY, HSQC or HMQC, HMBC was carried out. Proton assignment was done on the basis of chemical shifts and COSY spectra, and carbon assignments were made on the basis of chemical shifts, HSQC or HMQC, and HMBC NMR spectra. For more details; see the Supporting Information. 2-(2,4-Dihydroxystyryl)-1-benzopyrylium Chloride. This compound was prepared according to a procedure adapted from Robinson, frequently used in the synthesis of 2-phenyl-1-benzopyrylium (flavylium) salts.21 2,4-Dihydroxybenzaldehyde (1.38 g; 10 mmol) and o-hydroxystyrylmethylketone (1.61 g; 10 mmol)20,22 were dissolved in 23 mL of acetic acid. The solution was saturated with dry hydrogen chloride for 2.5 h. The solution became dark purple, and a solid precipitated out. The solid was filtered off, carefully washed with diethyl ether, and dried (2.27 g; 7.6 mmol). Yield: 75.6%. 1H-RMN (D2O/NaOH, pD ≈ 12.0, Ct2−/Ct3− 400.13 MHz) δ (ppm): 8.01 (1H, d, Hβ, 3JHβ−Hα = 15.3 Hz), 7.05−6.99 (3H, m, H4, H5, H3′ or H6′), 6.92 (1H, t, H4′ or H5′ = 7.4 Hz), 6.44 (1H, d, H3′ or H6′ = 8.0 Hz), 6.32 (1H, d, Hα, 3JHα−Hβ = 15.3 Hz), 6.24 (1H, t, H4′ or H5′ = 7.3 Hz), 6.12 (1H, d, H3, 3JH3−H4 = 12.4 Hz), 5.73 (1H, d, H6, 3JH6−H7 = 8.7 Hz), 5.58 (1H, s, H8). MS (MALDI-TOF): m/z (%): 265.04 (100) [M+]; calcd for C17H13O3+: 265.09. EA: calcd. for C17H13O3Cl·2H2O: C 60.63, H 5.09; found: C 60.49, H 4.71. Measurements. Solutions were prepared using Millipore water and absolute ethanol (when needed). The pH of solutions was adjusted by addition of HCl, NaOH, or universal buffer of Theorell and Stenhagen23 and was measured in a Radiometer Copenhagen PHM240 pH/ion meter. UV−vis absorption spectra were recorded in a Varian-Cary 5000 spectrophotometer. Second and subsecond flash photolysis experiments were performed as previously described.24 The stopped-flow experiments were conducted in an Applied Photophysics SX20 stoppedflow spectrometer equipped with a PDA.1/UV photodiode array detector. Light excitation was carried out using a medium-pressure mercury arc lamp (Newport) and the excitation wavelength range isolated with cutoff filters (Oriel).

Figure 3. Absorption spectra of 1.8 × 10−5 M thermally equilibrated solutions of 2-(2,4-dihydroxystyryl)-1-benzopyrylium in water/ethanol (80:20) as a function of pH; (inset) the variation of absorbance at 393 nm with pH can be fitted with pKobs = 3.3.

(eq 7) the difference between the equilibrium constant and the pseudoequilibrium constant leading to the product of three equilibrium constants of the network, eq 9.

K a′ − K a^ = Kh2K tK i

EXPERIMENTAL SECTION

(9)

The multistate/multifunctional character of the flavylium chemical reaction networks has been explored in several ways, in particular, for the development of molecular-level models of optical memories11 and photochromic systems.15 One interesting possibility much less explored that also fits the metamorphosis concept is the possibility of achieving flavylium isomerization. This occurrence is possible in the case of 5-hydroxyflavylium derivatives; see Scheme 2. In this case, the flavylium isomerization involves necessarily a free rotation on the cis-chalcone. This species, once formed, can close either through the hydroxyl substituent in position 2 (recovering the initial flavylium cation) or alternatively through the hydroxyl group in position 6. When the substituents at positions 3 and 5 of the chalcone (6 and 8 of the flavylium) are the same, the two isomers are undistinguishable. However, if they are different, closure of the ring through one hydroxyl or the other leads to different isomers. This type of isomerization was reported in the 1960s by Jurd (Scheme 2).16 The formation of isomers based on a ring-closure reaction can also be conceived in 2,2′-substituted asymmetric (pseudo)chalcones derived from 2-styryl-1-benzopyrylium compounds (hereafter referred to as styrylflavylium); see Scheme 3. While in the case of symmetric chalcones17 the resulting styrylflavylium is the same regardless of the direction of the ring closure, when asymmetric chalcones are considered, it is possible to expect the formation of two different styrylflavylium cations, isomers AH+ and AH+iso in Scheme 3. The synthesis of styrylflavylium compounds was reported by Jurd during the 1960s.18 However, only a few works regarding the chemistry of these compounds have so far been reported.17,19,20 On the basis of the experimental results carried out with the compound 7-hydroxy-2-(4-methoxystyryl)-1-benzopyrylium, it was concluded that styrylflavylium compounds follow a network of chemical reactions identical to the one of 8110



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Figure 4. (A) Spectral variations upon a pH jump from a fresh solution of 2-(2,4-dihydroxystyryl)-1-benzopyrylium in water/ethanol (80:20) at pH = 1 to pH = 3.15; (inset) kinetic traces at 394 and 531 nm and respective fittings. (B) Comparison of the initial spectrum with the one after 16 min. (C) Spectral variations after a pH jump from 1 to 5.45; (dashed line) ∼1 min after the pH jump; (dotted line) ∼5 min after the pH jump; (inset) kinetic traces at 394 and 525 nm. (D) Rate constants of the slowest process as a function of pH.

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RESULTS AND DISCUSSION Thermal Equilibration of the Styrylflavylium Cation. A notable difference observed when comparing very acidic solutions of the compound 2-(2,4-dihydroxystyryl)-1-benzopyrylium chloride with its parent flavylium compounds (including natural anthocyanins) is the evolution of its absorption spectra with time (Figure 1A), something not observed in the latter compounds. Figure 1A describes the UV−vis spectral variations after dissolution of the compound 2-(2,4-dihydroxystyryl)-1-benzopyrylium chloride in water/ethanol (80:20) at pH = 1.68. The absorption maximum is blue-shifted from an initial value of 538 to 510 nm at the stationary state. The spectrum of the second component in Figure 1B could be extracted from the data in Figure 1A by mathematical decomposition using the FINAL algorithm (λmax(AH+) = 538 nm; λmax(AH+iso) = 499 nm).25 The absorption maximum wavelength can also be used to measure the percentage of each of these two species; see Figure S1 of the Supporting Information. On this basis,

the composition of the steady state is 38% of the initial 2-(2,4dihydroxystyryl)-1-benzopyrylium and 62% of the product. The shape and position of the product’s band and the fact that the conversion occurs at very acidic pH values suggests that the new species formed is another styrylflavylium cation. Because the synthesized styrylflavylium equilibrates with a new styrylflavylium compound under acidic conditions and acidic conditions are actually present during the synthesis (see Experimental Section), the actual nature of the initially synthesized compound and the one formed upon thermal equilibration under acidic conditions was determined by full assignment of 1H and 13C NMR spectra (see the Supporting Information). It was concluded that the compound isolated by precipitation during the synthesis is 2-(2,4-dihydroxystyryl)-1benzopyrylium (referred as AH+ in Scheme 3 and thereafter), and the product formed upon thermal equilibration is its isomer 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium (referred as AH+iso in Scheme 3 and thereafter). 8111



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Figure 5. (A) Spectral variations observed upon pH jumps from a fresh solution of 2-(2,4-dihydroxystyryl)-1-benzopyrylium in water/ethanol (80:20) at pH = 0.8 to higher pH values, monitored by stopped-flow 4 ms after the mixing; (inset) fitting of the absorbances at 544 and 624 nm with pKa = 5.1. (B) Absorption spectra of AH+ and AH+iso from Figure 1B, together with the respective quinoidal bases, A and Aiso. The absorption spectrum of base A was calculated from the stopped-flow immediately after a pH jump to pH = 6.5, while the spectrum of base Aiso was obtained upon mathematical decomposition of the spectrum of an equilibrated solution.

AH+iso) were used to carry out a series of pH jumps represented in Figure 4. The spectral variations of Figure 4A are compatible with two kinetic processes, the first one converting the initial styrylflavylium cation into AH+iso, as indicated by the absorption spectrum after 16 min (Figure 4B), which is in accordance with the spectral changes of the styrylflavylium cation interconversion presented in Figure 1; in particular, the observed absorption band (after 16 min) can be decomposed in a mixture of these two styrylflavylium cations. Only after this step does the absorption of the trans-chalcone start to increase significantly. This behavior suggests the initial formation of a pseudoequilibrium involving the two styrylflavylium cations that later evolves to a network involving other species. The final equilibrium at pH = 3.15 involves the trans-chalcone and a mixture of the two styrylflavylium cations. In Figure 4C, the spectral modifications obtained after a pH jump from pH = 1.0 to 5.45 are shown. The global process leads also to the formation of the trans-chalcone (identified by 1H NMR) in equilibrium with quinoidal base(s) as the minor component(s). Formation of the trans-chalcone occurs through a first-order reaction only during the slowest kinetic step. Representation of the rates of the slowest process at different pH values leads to a bell-shaped curve (Figure 4D), as previously observed for flavylium compounds lacking the cis−trans isomerization barrier.28 The ascending branch of the bell-shaped curve results from the kinetic control of the cis−trans isomerization because the hydration is faster at low pH values (it is proportional to [H+]), and the concentration of the cis-chalcone available for isomerization increases with increasing pH. On the other hand, the descending branch of the bell-shaped curve is kinetically controlled by the hydration because with increasing pH, more quinoidal base is formed, reducing the concentration of flavylium cation available to the hydrate.29 Therefore, at pH = 5.45, the network of the present system is kinetically controlled by the hydration reaction. There is however a peculiar behavior at the initial times. The absorption spectrum taken immediately after the pH jump is very different from the one observed after

The interconversion between both styrylflavylium cations was monitored by 1H NMR at pD = 1.87. A relevant result is the fact that in both the initial and the final compounds, the styryl double bond is in the trans configuration, as indicated by the high coupling constants between protons α and β; see Figure 2A and the Supporting Information (Table S1 and Figure S2). The normalized area of the peaks regarding AH+ plotted against time (Figure 2B) allows calculation of a firstorder rate constant of 2.5 × 10−5 s−1, slightly lower than the value of 4.0 × 10−5 s−1 obtained at pH = 1.68 (Figure 1A). At pD ≈ 0.5, the reaction rate is much slower, with a rate constant of 4.0 × 10−6 s−1; see the Supporting Information (Figure S6). The data from Figure 2 leads to a ratio of [AH+iso]/[AH+] = 0.72, which compares with 1.63 for the UV−vis absorption measurements shown in Figure 1.26 The use of different solvents and concentrations in the two determinations may explain this apparent discrepancy. Anthocyanins and related compounds are known to assemble by π−π stacking, leading to significant changes in the mole fraction distribution of the network species. The formation of aggregates is expected to be much more significant at the higher concentrations used in the NMR experiments.27 Thermal Equilibrium As a Function of pH. The spectral variations after equilibration of 2-(2,4-dihydroxystyryl)-1benzopyrylium chloride solutions in water/ethanol (80:20) as a function of pH are represented in Figure 3. Despite its complexity, the system behaves like a single acid−base equilibrium with an observed pKobs = 3.3. The shape and position of the spectra are compatible with pH-dependent equilibria involving the two styrylflavylium cations and the basic species CB consisting of trans-chalcone (major) and quinoidal bases (minor). The behavior of the system is globally very similar to that exhibited by the hydroxyflavylium parent compounds.12 Kinetic Processes. Due to the slow conversion of the two styrylflavylium compounds at acidic pH values, fresh solutions of 2-(2,4-dihydroxystyryl)-1-benzopyrylium chloride at very low pH values (in order to avoid significant formation of 8112



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Figure 6. (A) Spectral variations observed upon pH jumps from a fresh solution of 2-(2,4-dihydroxystyryl)-1-benzopyrylium in water/ethanol (80:20) at pH = 0.8 to pH = 6.25 followed by stopped-flow; (inset) trace at 527 nm (considering all experimental points, taken each at 1.28 ms). (B) Spectral variations of the same experiment regarding the second kinetic process. (C) (top left) Observed rate constants of the fast process as a function of pH; (top right) absorbance after completion of the first process as a function of pH; (bottom) traces at 540 nm of the first process as a function of pH. (D) pH jump from 0.8 to 4.

mixing time of the solutions. This base can be assigned to the quinoidal base A (see Scheme 5), allowing calculation of Ka = 10−5.1. It is worth noticing the vibrational structure of its absorption spectra with relative maxima at 544 , 577, and 624 nm, a spectral feature that is commonly observed in quinoidal bases derived from flavylium cations. Figure 6 shows the kinetic processes that follows the one of Figure 5 in the case of a pH jump from pH = 0.8 to 6.25. In Figure 6A, the quinoidal base disappears in a process that is pH-dependent (see Figure 6C, bottom), and the respective rate constants follow a sigmoid function, as shown in Figure 6C (top left). This sigmoidal dependence is compatible with a firstorder reaction approaching equilibrium where one of the species is the basic form of an acid−base pair, kobs = χbasek1 + k−1, and can be fitted according to eq 10.

Scheme 4. Formation of a Symmetric Bis(benzospiropyran) from a Quinoidal Base Structure30

a few minutes (compare dashed and dotted lines in Figure 4C and note the kinetic trace at 525 nm in the inset), indicating that between these two reaction times, other chemical transformations take place. This behavior requires the analysis of the kinetic process at shorter reaction times and by consequence the use of the stopped-flow technique. Stopped-Flow. In order to get more insight into the faster kinetic processes, stopped-flow experiments were performed (Figures 5 and 6). The spectral variations after 4 ms are shown in Figure 5. The data are compatible with formation of a quinoidal base in equilibrium with the 2-(2,4-dihydroxystyryl)1-benzopyrylium cation, the process occurring during the

kobs1 = 8113

10−5.1 3.0 + 0.63 s−1 [H+] + 10−5.1

(10)



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Scheme 5. General Kinetic Scheme Coonecting the Two Styrylflavylium Cations AH+ and AH+iso

The variation in absorbance after the end of the fast process reported in Figure 6C, top right inflects for pH = 4.2. Considering a pseudoequilibrium between AH+, A, and an intermediate SP (see Scheme 5), the inflection point should be equal to 10−4.2, leading to a value of K1 = 6.9. On the basis of these three measurements, a reliable value of this constant can be calculated, K1 = 6.2. This last kinetic step is followed by a slower one (Figure 6B), showing an increase of the absorption in the visible region where the quinoidal bases are expected to absorb. The shape of this new absorption is different from that of the initial A and could be attributed to the formation of the quinoidal base of the isomer Aiso. It is worth noting that in this step, also no significant amount of trans-chalcone is formed (Figure 6B). The amount of the Aiso formed in Figure 6B is ∼37% of the total amount of base as calculated from the ratio of the mole absorption coefficients of the two bases; see Figure 5B. The mole fraction of Aiso at the pseudoequilibrium is given by eq 11 (see the Supporting Information), the constants being defined in Scheme 5. Using pKa(iso)(AH+iso/Aiso) = 4.3

The disappearance of the quinoidal base A cannot be attributed to a process via hemiketal B2 because (i) the hydration is the rate-determining step at pH values on the right branch of the bell-shaped curve (as pH = 6.25) and (ii) in case of disappearance via B2, the pH dependence of the rate constant would be the reverse of the one observed (see Figure 6C, top left), that is, a decrease with increasing pH would be expected (the hydration is proportional to [H+]) instead of the experimentally observed increase. The inflection point at pH = 5.1 (a value coincident with the pKa of AH+/A) points to a process controlled by the prior formation of the quinoidal base, and thus, the more base available, the higher the rate (Figure 6C, top left). Whatever the nature of this intermediate state, an estimation of the equilibrium constant can be made from the ratio of the forward and backward reaction rate constants, K1 = k1/k−1 = 3.0/0.63 = 4.8 M−1. On the other hand, from the ratio between the initial and final absorbance values at 624 nm (where only the quinoidal base absorbs), a value of K1 = 6.8 can be estimated in reasonable agreement with the previous value. 8114



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intermediate (SP), as recently reported in the literature (Scheme 4).30,31 Formation of the SP intermediate is compatible with the observed decrease in absorbance in the visible, as well as with the increase of the rate and amplitude of the faster process with increasing pH. Any other isomer or rotamer from the 2-(2,4dihydroxystyryl)-1-benzopyrylium would absorb in the visible and would not explain the behavior of Figure 6A. Moreover, the SP intermediate has two possibilities of ring opening, each leading to one of the two styrylflavylium compounds. The global situation is summarized in Scheme 5, considering the SP intermediate. It is important to stress that besides the kinetic study presented here, we were not able to find further experimental evidence to confirm the spiropyran hypothesis. Reverse pH Jumps. Figure 7 reports the spectral variations upon a sequence of two pH jumps as follows: (i) from a fresh solution of 2-(2,4-dihydroxystyryl)-1-benzopyrylium at pH = 0.8 to pH = 6.25, allowing to pseudoequilibrate during ∼2 min, and (ii) a reverse pH jump back to pH = 1.2, monitored after by stopped-flow. The spectral variations reported in Figure 7 can thus be explained by considering that immediately after the pH jump back to acidic conditions, both bases A and Aiso (that coexist as shown in Figure 6C) are transformed into the respective styrylflavylium cations and the spiropyran leads to more styrylflavylium cation in a slower process. The initial amplitude (regarding essentially Aiso) and the one of the trace (concerning SP) are compatible with a mole fraction distribution at pH = 6.25 calculated from K1 = 6.2 and K2 = 0.7. Moreover, the transformation of SP into the flavylium cation is accompanied by a red shift of the absorption maximum, indicating that SP is mainly transformed into AH+. The energy level diagram of all of the species present at the pseudoequilibrium can be calculated from the respective equilibrium constants (Scheme 6).

Figure 7. Absorption changes observed in a fresh solution of 2-(2,4dihydroxystyryl)-1-benzopyrylium in water/ethanol (80:20) after a sequence of two pH jumps, one from pH = 0.8 to 6.35, allowing to equilibrate for ∼2 min, followed by a second reverse pH jump back to pH = 1.2.

based on the energy diagram of Scheme 6, a value of K2 = 0.7 can obtained from eq11. K aK1K 2 χA = iso K KK [H+] 1 + Ka 1 2 + K a + K aK1 + K aK1K 2

(

a(iso)

= 0.37

)

(11)

The nature of the intermediate is a matter of discussion. One possible explanation is the formation of a bis(benzospiropyran)

Scheme 6. Energy Level Diagram of the Two Styrylflavyium Reaction Networks at the Equilibriuma

a

See Scheme 5 for structures. 8115



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Figure 8. (A) Equilibrated solutions of 2-(2,4-dihydroxystyryl)-1-benzopyrylium in water/ethanol (80:20) at pH = 13 (unprotonated transchalcone) were reacidified to pH = 1.3 and the spectra recorded as a function of time; (inset) time evolution of the absorption maximum at 504 nm. (B) The same upon a pH jump equilibrated solutions at pH = 6.2 to pH = 1.1.

centered at 504 nm, corresponding to 82% AH+iso (see Supporting Information, Figure S1). This is followed by the isomerization of AH+iso to give the AH+ (not shown), a process that is much slower. The absorption maximum at 504 nm indicates that the isomerization of the trans-chalcone occurs preferentially through the right branch of Scheme 5 because AH+iso is the main species formed. In Figure 8B, the acidification of equilibrated solutions at pH = 6.2 (first full line) shows the presence of Ct and AH+iso together with a small fraction of AH+, as expected from the higher stability of Aiso in comparison with A. Further evolution toward the equilibrium (at pH = 1.1) shows two kinetic processes, a fast component (6.6 × 10−4 s−1; inset Figure 9B) concerning the conversion of Ct into AH+iso and a slower component regarding the conversion of AH+ into AH+iso (with a rate constant < 4 × 10−5 s−1; see Figure 1A). The overall view of the system is summarized in Scheme 8. In acidic media, the spiropyran intermediate leads to AH+, while Ct leads to AH+iso. On this basis, the slow observed conversion of AH+ into AH+iso under acidic conditions should take place through the Ct intermediate. The reaction is very slow because the mole fraction of Ct at acidic pH values is very small; at pH = 1.0, its mole fraction is ∼4 × 10−4. Extension to the Basic Medium. In Figure 9, the titration of the fully unprotonated trans-chalcone obtained upon stabilization of the solution at pH = 13.0 (all trans, the respective structure being confirmed by 1H NMR) is shown. The spectral variations are compatible with the existence of three values of pKa, 8.4, 9.9, and 10.9, Scheme 7. Photochemistry. Irradiation of equilibrated solutions of 2-(2,4-dihydroxystyryl)-1-benzopyrylium at pH = 2.1, containing essentially styrylflavylium cations (λirr > 450 nm), and pH = 4.9, containing essentially trans-chalcone (λirr > 385 nm), is presented in Figure 10.

Figure 9. pH titration of the unprotonated trans-chalcone from equilibrated solutions at pH = 13; (inset) simultaneous fitting of the absorbances at 394, 445, and 540 nm allows determination of the three pKa’s of Ct, pKCt1 = 8.4, pKCt2 = 9.9, and pKCt3 = 10.9.

As shown in Scheme 6, the trans-chalcone has two isomerization alternatives, and the question is which are the relative rates of formation of the two styrylflavylium cations. In Figure 8A, are shown the spectral variations of a solution of the unprotonated trans-chalcone at pH = 13 (at this pH value, its formation is much faster) was reacidified to 1.3. Protonation is extremely fast, and thus, Figure 8 shows the evolution of the trans-chalcone to form the styrylflavylium cation, a process that is complete in ∼2 h. The absorption maximum at this stage is Scheme 7. Three Unprotonated Forms of Ct

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Scheme 8. Overall Inputs Operating in the AH+/ AH+iso Systema

a

The thermal reactivity (open linear arrows) at pH = 1 leads to AH+ through SP and to AH+iso through Ct, while at pH = 5, SP leads to AH+iso. The photochemical reactivity (curved bold arrow) transforms AH+iso into AH+ via Ct.

Figure 10. (A) Absorption spectra of an equilibrated solution of 2-(2,4-dihydroxystyryl)-1-benzopyrylium in water/ethanol (80:20) at pH = 2.1 upon irradiation at λirr > 435 nm. (B) The same at pH = 4.9 and λirr > 385 nm.

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In contrast with flavylium compounds, which are not photoreactive,32 unless for long irradiation times leading to decomposition products, in Figure 10A, the changes on the UV−vis absorption spectra upon irradiation of equilibrated mixtures clearly show the formation of AH+ from AH+iso, the opposite of the observed thermal conversion. The photostationary state is characterized by the wavelength maximum of 535 nm, corresponding to 90% AH+. The formation of AH+ probably occurs via a small mole fraction of the trans-chalcone that is present at pH = 2.1. However, a mechanism through the SP intermediate cannot be excluded due to the reported photoactivity of spiropyrans.33,34 The photochemistry carried out at pH = 4.9 (Figure 10B) is similar to the one observed for chalcones derived from flavylium compounds, exhibiting as photoproducts the quinoidal bases in equilibrium with the respective styrylflavylium cations.

CONCLUSIONS Through this work, the concept of metamorphosis in chemistry was illustrated by means of the network of chemical reactions followed by the compounds 2-(2,4-dihydroxystyryl)-1-benzopyrylium and 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium. The trans-chalcone leads directly to 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium from only one of the routes. On the other hand the spiropyran intermediate gives preferentially 2-(2,4-dihydroxystyryl)-1-benzopyrylium at acidic pH values and 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium at higher pH values. On this basis, the slow transformation of the initial flavylium 2-(2,4-dihydroxystyryl)-1-benzopyrylium into 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium in acidic media seems to take place through the trans-chalcone intermediate. This is also compatible with the decreasing of the isomerization rate with decreasing pH, following the variation of the mole fraction of the trans-chalcone at the equilibrium. 8117



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Article

(22) Buck, J. S.; Heilbron, I. M. J. Chem. Soc., Trans. 1922, 121, 1500−1515. (23) Küster, F. W.; Thiel, A. Tabelle per le Analisi Chimiche e ChimicoFisiche, 12th ed.; Hoepli: Milano, 1982; pp 157−160. This universal buffer is prepared in the following way: 2.3 cm3 of 85% (w/w) phosphoric acid, 7.00 g of monohydrated citric acid, and 3.54 g of boric acid are dissolved in water; 343 mL of 1 M NaOH are then added and the solution diluted to 1 dm3 with water. (24) Maestri, M.; Ballardini, R.; Pina, F.; Melo, M. J. J. Chem. Educ. 1997, 74, 1314−1316. (25) Antonov, L.; Petrov, V. Anal. Bioanal. Chem. 2002, 374, 1312− 1317. (26) The small difference between the two equilibrium constants in Figures 1 and 2 can, in principle, be attributed to the solvent, ethanol, and deutered methanol. (27) Houbiers, C.; Lima, J. C.; Maçanita, A. L.; Santos, H. J. Phys. Chem. B 1998, 102, 3578−358. (28) Pina, F.; Melo, M. J.; Laia, C. A. T.; Parola, A. J.; Lima, J. C. Chem. Soc. Rev. 2012, 41, 869−908. (29) As shown by Brouillard and Dubois, the quinoidal base does not hydrate at acidic or neutral pH values. (30) Lu, N. T.; Nguyen, V. N.; Kumar, S.; McCurdy, A. J. Org. Chem. 2005, 70, 9067−9007. (31) Kumar, S.; Chau, C.; Chau, G.; McCurdy, A. Tetrahedron 2008, 64, 7097−7105. (32) Roque, A.; Lodeiro, C.; Pina, F.; Maestri, M.; Ballardini, R.; Balzani, V. Eur. J. Org. Chem. 2002, 2669−2709. (33) Bertelson, R. C. In Organic Photochromic and Thermochromic Compounds, Crano, J. C. C., Guglielmetti, R. J., Eds.; Plenum Press: New York, 1998; Vol. 2, Chapter 6. (34) Gaude, D.; Gautron, R.; Gugielmetti, R. Bull. Soc. Chim. Belg. 1991, 100, 299−313.

ASSOCIATED CONTENT

S Supporting Information *

Detailed NMR data for AH+ and AH+iso as well as the deduction of mole fraction expressions for all species are presented. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +351212948550. Tel: +351212948355. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Fundaçaõ para a Ciência e Tecnologia through Projects PTDC/QUIQUI/104129/2008, PTDC/CTM-NAN/120658/2010, and Pest-C/EQB/LA0006/ 2011 and Grant SFRH/BPD/18214/2004 (V.P.). We acknowledge LabRMN at FCT-UNL and Rede Nacional de RMN, Portugal, for access to the facilities.

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Isomerization between 2-(2, 4-Dihydroxystyryl)-1-benzopyrylium and 7

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