Origin of the Metastable Stability in Flavylium Multistate Systems - The

Article pubs.acs.org/JPCA

Origin of the Metastable Stability in Flavylium Multistate Systems Vesselin Petrov,*,†,‡ Stoyanka Slavcheva,† Stanislav Stanimirov,† and Fernando Pina*,‡ †

Laboratory of Organic Photochemistry, Faculty of Chemistry and Pharmacy, Sofia University, 1164 Sofia, Bulgaria REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal

‡

ABSTRACT: Metastable states regarding the network of chemical reactions involving flavylium compounds were investigated as well as the role they may play in models for optical memories capable of write−read−erase. A necessary requirement to achieve metastable states in flavylium systems is the existence of a high cis−trans isomerization barrier, as in 4′-hydroxyflavylium described through this paper. In an optical memory, the metastable state could be the signal to be detected upon the write step. In that case the autoerase is prevented by the metastable state. Conversely, the metastable state may be the initial state and prevents the auto and unwanted write step. The compound 4′hydroxyflavylium offers the possibility of achieving both of these two situations, depending on the sequence of the pH stimuli prior to light absorption. In this work the pH dependent distribution of the flavylium species of the network in the presence of β-cyclodextrin was calculated. Improvement of the performance of the photochromic system in the presence of βcyclodextrin was observed.

■

INTRODUCTION

this rate, different applications of photochromic systems can be envisaged. In particular, photochromic systems can be the basis by which to conceive optical memories.4 For this purpose three steps are necessary: write by means of a light impulse that triggers the photochromic system and leads to the formation of the photoproduct (signal formation), read the signal by some analytical tool, and erase back to the initial state for successive applications.5,6 The lifetime of the photoproduct should be very high; otherwise, the signal spontaneously erases with time. In other words, the existence of a metastable state for the photoproduct is needed for an optical memory of type (a) in Scheme 1. Other possible photochromic systems are those where the reagent is in a metastable state. They have the advantage that the signal is thermodynamically stable and thus does not autoerase. Conversely, there is the possibility of a spontaneous and not wanted write step. Independently of the type of optical memory, the metastable state is thus of crucial importance and an indispensable requirement. The network of chemical reactions regarding flavylium derivatives is a peculiar photochromic system. Flavylium derivatives7 constitute an important family of compounds comprising anthocyanins, the colorants responsible for most of the red and blue color of flowers and fruits.8,9 Depending on the substituents of the 2-phenyl-1-benzopyrilium, a pH dependent distribution of species is obtained (Scheme 2).

Metastable states are relatively common in isomerization reactions. Regarding photochromic systems based on cis− trans isomerizations,1 in most of them the photoactive reagent is the thermodynamic stable species and the photoproduct possesses higher energy (Scheme 1a). Under these conditions, the photoproduct tends to revert back to the initial state, and this is a necessary requirement for the definition of a photochromic system.2 Examples of such systems are the azobenzenes and diarylethenes, among many others.3 However, the rate of return of the photoproduct back to the thermodynamic state could change dramatically. According to Scheme 1. (a) Thermodynamics of the Common Photochromic Systems; (b) Photochromic System Where the Reagent Is Metastable

Received: February 12, 2015 Revised: February 25, 2015 Published: February 25, 2015 © 2015 American Chemical Society

2908

DOI: 10.1021/acs.jpca.5b01473 J. Phys. Chem. A 2015, 119, 2908−2918

Article

The Journal of Physical Chemistry A Scheme 2. Network of Chemical Reactions of the Compound 4′,7-Dihydroxyflavylium

Scheme 3. Energy Level Diagram of the Flavylium Network of Chemical Reactions

function as a kinetic barrier. Another useful barrier is the cis− trans isomerization preventing the back reaction immediately after the Cc formation upon irradiation and constituting an additional obstacle for the recovery of Ct from the flavylium cation. The same arguments justify why in flavylium networks it is possible to achieve a metastable state from Ct at pH = 0, for example. The barrier to give the thermodynamically stable species is very high.13 In that case the system behaves as the situation reported in Scheme 1b. In past years we described a way to overcome the need for the metastable stable state. This is made by the introduction of a lock step. After irradiation at a pH, for example equal to pKa in Scheme 3, that leads to Cc, the lock step consists of the addition of acid to bring the flavylium cation to an energy level lower than Ct (for example pH = 0 in Scheme 3) in such a way that at the new pH the signal becomes thermodynamically stable. Now, to erase, it is necessary to unlock the system to a pH value where it is thermodynamically unstable and use light or heat to bring the system to the initial sate (Ct). The strategy is thus write−lock−read−unlock erase. This proved to be an efficient way to obtain models for optical memories at the molecular level, but it has the drawback of needing successive addition of mass in each cycle, which is not convenient for practical applications. In this work we are revisiting compound 4′-hydroxyflavylium, an example of a flavylium system exhibiting metastable states. The main scope of the work is to systematize and treat quantitatively the origin of the metastable states. The possibility of tuning the metastable states and increasing the performance of the photochromic systems upon incorporation of the different species in β-cyclodextrin was also investigated.

Traditionally the network is presented and named considering the flavylium cation. However, this species is stable only at very acidic pH values. Raising the pH, the quinoidal base is formed immediately after the addition of base, and an acid (AH+)−base (A) equilibrium is established, followed by the disappearance of both species in a slower reaction leading to the hemiketal. This occurs because proton transfer (miliseonds) is much faster than hydration (seconds to minutes depending on pH). One important breakthrough was discovered by Brouillard and Dubois,10 who observed that in acidic medium the hemiketal is formed exclusively through the attack of the flavylium cation. As soon as hemiketal is formed, it equilibrates with cis-chalcone in a fast process (subseconds). The thermodynamic equilibrium is reached from the isomerization, with rate constants between minutes and days (even months) depending on the nature and position of the substituents. Scheme 2 illustrates the sequence of reactions of the flavylium network from the background of anthocynains that privileges the red flavylium cation and the blue quinoidal base. From the point of view of the photochromism it is useful to represent the network by an energy level diagram starting from Ct, Scheme 3.11 Scheme 3 is easily constructed provided that the equilibrium constants of the network are previously calculated.12 In the example of Scheme 3, irradiation of the equilibrated system at pH = 4 (Ct is the thermodynamically stable species) leads to the cis isomer. This species may give back the Ct isomer or go forward, equilibrating within subseconds with hemiketal, followed by the spontaneous formation of the flavylium cation (few seconds). Flavylium cation is not the most stable thermodynamic product and thus tends to give back the initial species Ct. In order to get a more or less permanent memory, the existence of a metastable state is thus necessary. In flavylium chemistry this can be at least partially achieved if the species B and Cc are localized at the top of the diagram in order to

■

EXPERIMENTAL SECTION 4′-Hydroxyflavylium was previously synthesized according to the method described elsewhere.14 All the reagents were used 2909


Article

The Journal of Physical Chemistry A

Figure 1. (a) Spectral changes measured by means of stopped flow, 10 ms after pH jump from 1 to higher pH values; [β-CD] = 8 mM; pKobs(1) is 5.0; reported pKa value for this compound in the absence of β-CD is pKa = 5.4; (b) Evolution of the absorbance upon a pH jump from pH = 1 to 7, [β-CD] = 6 mM. The data can be fitted by a monoexponential with rate constant 0.027 s−1. The same constant in the absence of the host is approximately two times smaller.12

Figure 2. (a) Absorption spectra of 4′-hydroxyflavylium in aqueous solution upon a direct pH jump recorded after 10 min. The inset shows the ̂ ̂ absorption of AH+ maxima (437) nm as a function of pH; pKa = 4.54; (b) the same as (a) for [β-CD] = 8 mM; pKa(obs) = 3.45.

(Oriel). Flash photolysis experiments were carried out as reported previously.16

without any additional purification. The solutions were prepared in Millipore water. The pH of the solutions was adjusted by addition of HCl, NaOH, or 0.1 M universal buffer of Theorell and Stenhagen15 and was measured on a Meterlab pHM240 pH meter from Radio-meter Copenhagen. The ionic strength of the solutions was kept constant at 0.1 M by the addition of NaCl when needed. UV/vis absorption spectra were recorded on a Varian Cary 100 Bio and Varian Cary 5000 spectrophotometers. The stopped flow experiments were conducted in an Applied Photophysics SX20 stopped-flow spectrometer provided with a PDA.1/UV photodiode array detector with a minimum scan time of 0.65 ms and a wavelength range of 200−735 nm. Photoexcitation in continuous irradiation experiments was carried out using a xenon/medium pressure mercury arc lamp, and the excitation band (340 or 365 nm) was isolated with an interference filter

■

RESULTS AND DISCUSSION A convenient way to study the flavylium system is to carry out a series of direct and reverse pH jumps. In the first case an equilibrated solution of the compound at pH = 1.0 (or lower) is made more basic, and in the second case solutions equilibrated or pseudoequilibrated at higher pH values are made more acid. The spectral variations can be monitored by stopped flow or, if the respective kinetic process is sufficiently slow, by a common spectrophotometer.17 The absorption spectra of a solution of 4′-hydroxyflavylium in the presence of β-cyclodextrin monitored 10 ms after a direct pH jump are shown in Figure 1a. The absorption bands fit with an acid−base equilibrium between flavylium cation and 2910


Article

The Journal of Physical Chemistry A quinoidal base with pKobs(1) = 5.0, which compares with pKa = 5.517,18 in the absence of the host. The data can be interpreted by means of the set of eqs 1−4. AH+ ⇌ A + H+

AH+ + β ‐CD ⇌ AH+β ‐CD A + β ‐CD ⇌ Aβ ‐CD

pK a = 5.5

(1)

K1

(2)

K2

(3)

Considering the acid base equilibrium involving the acid species (free and host−guest) and the equivalent basic species, eq 4 AH+ + AH+β ‐CD ⇌ A + Aβ ‐CD + H+

Kobs(1)

(4)

The system behaves as a single acid base equilibrium (Appendix 1) with constant equal to Kobs(1) =

Ka(1 + K 2[β ‐CD]) (1 + K1[β ‐CD])

(5) Figure 3. Spectral variations at the pseudoequilibrium of 3.3 × 10−5 M4′-hydroxyflavylium, upon addition of β-cyclodextrin at pH = 6.8.

As shown below and in accordance with the interaction of other flavylium compounds with β-cyclodextrin,19,20 K1[β-CD] ≪ 121 and thus eq 5 can be simplified to eq 6, from which a value of K2 = 200 M−1 is obtained. Kobs(1) = Ka(1 + K 2[β ‐CD])

for the constants reported in Tables 1 and 2, and εA = 41000M−1 cm−1. As shown in Appendix 1, it is possible to treat this complex system by considering a single acid base equilibrium, eq 8, between the species AH+ free and complexed (acidic species) and all the “basic species” free and complexed, with apparent constant given by eq 9

(6)

The evolution of the spectra shown in Figure 1(a) is presented in Figure 1(b) for a direct pH jump to pH = 6.8 [βCD] = 6 mM. At this final pH value essentially quinoidal base is formed, disappearing with a rate constant equal to 0.027 s−1, to reach the pseudoequilibrium. Pseudoequilibrium. The compound 4′-hydroxyflavylium possesses a high cis−trans isomerization barrier,13 and by consequence the so-called pseudoequilibrium, i.e. a (pseudo) equilibrium involving all the network species except transchalcone, can be easily characterized, because formation of this last species is very slow (it takes days for the Ct to be formed). In Figure 2 a series of direct pH jumps have been performed and the respective absorption monitored after 10 min to ensure that the pseudoequilibrium was reached. The spectral variations in the absence and presence of 8 mM β-cyclodextrin are respectively shown in Figure 2(a) and (b). Inspection of these two set of spectra clearly shows that AH+ is destabilized upon interaction with β-cyclodextrin in ̂ comparison with the basic species “CB”. Moreover, the species A is also destabilized (in comparison with B and Cc) in the presence of β-cyclodextrin. The effect of the β-cyclodextrin in the pseudoequilibrium of the 4′-hydroxyflavylium is nicely accounted for by Figure 3. In this figure the absorption of the quinoidal base (A) decreases dramatically by increasing concentration of the host, up to reach the pseudoequilibrium concentration at the respective pH value, as in Figure 2(b). The trace reported in the inset of Figure 3 can be fitted by eq 7 (see Appendix 1). ε C K (1 + K 2[β ‐CD]) A = A +0 a [H ] + Ka ̂ + Ka″[β ‐CD]

AH+ + AH+β ‐CD ⇌ CB + CBβ ‐CD

K a(obs) ̂

(8)

with CB = [A] + [B] + [Cc] and CBβ-CD = [Aβ-CD] + [BβCD] + [Ccβ-CD] Kobs =

Ka ̂ + (K 2Ka + Kh(K3 + K4K t ))[β ‐CD] 1 + K1[β ‐CD]

(9)

where K1, K2, K3, and K4 are respectively the association constants of the species AH+, A, B, and Cc with β-cyclodextrin; see Appendix 1. Considering as above the association with flavylium cation to be negligible21 Kobs = Ka ̂ + (K 2Ka + Kh(K3 + K4K t ))[β ‐CD]

(10)

Representation of the observed apparent acid−base constant as shown in Figure 1(b) for different concentrations of βcyclodextrin is represented in Figure 4. The intercept of Figure ̂ 4 leads to pKa = 4.53, in perfect agreement with the experimental data calculated for this parameter in the absence of β-cyclodextrin. Moreover, the slope gives the product of constants, K2Ka + Kh(K3 + K4Kt). More information regarding the system can be achieved by means of reverse pH jumps. In the presence of β-cyclodextrin the ratio of the amplitudes of the slow/fast exponentials is equal to ratio = K t(obs)

(7)

[Cc] + [Ccβ ‐CC] [B] + [Bβ ‐CD]

(11)

Using the association constants of B and Cc with the host and by definition of Kt

where A is the absorbance at 500 nm, εA is the molar absorption coefficient of the A (500 nm) free and complexed (which is the same within experimental error), and the other constants are as defined in the Appendix. Fitting was achieved

K t(obs) = K t 2911

1 + K4[β ‐CD] 1 + K3[β ‐CD]

(12) DOI: 10.1021/acs.jpca.5b01473 J. Phys. Chem. A 2015, 119, 2908−2918

Article

The Journal of Physical Chemistry A Table 1. Equilibrium Constants of the Compound 4′-Hydroxyflavylium β-CD

2.2 ± 0.1 1.7 ± 0.1

β-CD a

pK∧a

pK′a

Kh (M−1)b

pKaa

4.5 ± 0.1 3.45 ± 0.1

5.4 ± 0.1 5.0 ± 0.1

−6

9.5 × 10 2.3 × 10−4

Ktb

Kib

0.87 0.6

900 175

In the literature the values of pKa reported range from 4.6 to 5.5. bEstimated error ∼15% .

Table 2. Association Constants of the Compound 4′Hydroxyflavylium with β-Cyclodextrina K1 (M−1) 12. At these pH values the species Cc2− is rapidly formed and by a slow thermal process or upon light irradiation the form Ct 2− is obtained as almost the only product at the equilibrium.18 When the cis−trans isomerization barrier is very high, as in anthoycanins and the present compound, the rate of Ct formation is given by eq 15, where Ct are formed from the pseudoequilibrium involving Cc species.

eqs 6, 10, and 12 allow calculation of all the equilibrium constants involved in the system; see Tables 1 and 2. The reverse pH jumps to those reported in Figure 5 constitute a powerful tool to measure the mole fraction distribution of all the species of the network, including the basic medium.22 The solution is equilibrated or pseudoequilibrated at different pH values, a certain amount of acid is added to reach a pH value where flavylium cation is the dominant species, and the spectral variation is followed by stopped flow. The traces have the aspect of the one reported in Figure 5. At each pH value the initial absorbance measures the fraction of the species A, because formation of AH+ from A takes place during the mixing time of the stopped flow (In Figure 5 that fraction was very small in the presence of β-cyclodextrin). If the final pH is sufficiently acidic, the hydration (with a rate proportional to [H+]) becomes faster than tautomerization, the so-called change of regime, and the faster exponential corresponds to the formation of AH+ from B. The last and slower step is the formation of more AH+ from Cc via B (Scheme 2). The rates of these two exponentials are given by eqs 13 and 14.23 k fast = kh

[H+] + k −h[H+] [H+] + Ka

kslow = k −t

kisom = χCc ki + χCcβCD kiCD + k −i + k −iβCD

(15)

where χCc is the mole fraction of the species Cc and χCcβCD the mole fraction of Cc interacting with the host, ki and k−i are the rate constants of the cis to trans isomerization and the trans to cis isomerization, respectively, and kiCD and k−iCD are the equivalent constants for the flavylium interaction with the host. The rate constants for the isomerization process are given in Figure 6. Fitting was achieved with eq 15 for the host concentration 8 mM or 0, for the isomerization rate constants (●) kiCD = 1.75 × 10−5 s−1, k‑iCD = 1.8 × 10−7 s−1 and (○) ki = 1.2 × 10−5 s−1, k‑i ≈ 1.3 × 10−8 s−1. Final equilibrium. Similarly to anthocyanins, the isomerization is much slower than the other steps and the system can

(13) (14) 2912


Article

The Journal of Physical Chemistry A Table 3. Rate Constants of the Compound 4′-Hydroxyflavylium in the Presence and Absence of β-Cyclodextrina

β-CD a

kh/s−1

k−h/s−1

kt/s−1

0.095 0.24

1.0 × 10 1.1 × 103 4

k‑t/s−1

0.59 0.23

ki/s−1

k‑i/s−1

−5

≈1.3 × 10−8 1.0 × 10−7

1.2 × 10 1.75 × 10−5

0.68 0.39

Estimated error 15%.

Scheme 4. Equilibrium between the trans-Chalcones

This implies that the species Ct2− is the less stabilized by the host in comparison with Ct−, probably due to better water solubility of the dianionic form. This result is compatible with the neutral charge density of the β-cyclodextrin cavity.24 In order to measure all the equilibrium constants of Scheme 4, the following relations can be used:

K5KCt(CD) = K 6KCt

(16)

and K 6KCtCD− = K 7KCt −

(17)

Taking into account that the values of the acidity constants are known (Table 4), one more equation is needed to add to

Figure 6. Rate constants of the slowest kinetic process of the network corresponding to the formation of Ct from the pseudoequilibrium. Fitting was achieved with eq 16, using the data in Tables 1 and 2 for (●) kiCD = 1.75 × 10−5 s−1, k‑iCD = 1.8 × 10−7 s−1, (○) ki = 1.2 × 10−5 s−1, k‑i ≈ 1.3 × 10−8 s−1.

Table 4. Equilibrium and Acidity Constants of the trans Chalcones of Compound 4′-Hydroxyflavyliuma

be treated from the pseudoequilibrium. The final equilibrium in basic to neutral pH values is reported in Figure 7. In the absence or presence of cyclodextrins, the spectral variations are compatible with the existence of equilibrium between Ct, Ct−, and Ct2−, free and interacting with the host, Scheme 4. According to Figure 7 the first pKa between Ct and Ct− is only slightly affected by the inclusion in the host, while the second one between Ct− and Ct2−, changes ∼0.5 pH units.

pKCt/Ct‑

pKCt‑/Ct2‑

pKCt/Ct‑(CD)

pKCt‑/Ct2‑(CD)

K5 (M−1)

K6 (M−1)

K7 (M−1)

7.86

8.95

7.81

9.45

900

1010

320

a

Estimated error ∼10%

the system constituted by eqs 16 and 17. This relation can be obtained by carrying out the titration of the species Ct2− with the host at pH > 12 (Figure 8). In Figure 9 the spectra of the compound 4′-hydroxyflavylium in acidic medium are presented. A diagram regarding the mole

Figure 7. Spectral variations of equilibrated solutions of the compound 4′-hydroxyflavylium; (a) in the absence and (b) presence of 8 mM βcyclodextrin; (c) variation of the trans-chalcones acidity constants as a function of β-cyclodextrin concentration. Acidity constants extrapolated to the complete association of the species with the host were obtained by fitting at the pKa values 7.81 and 9.45. 2913


Article


AH + species (black) gives after a few minutes the pseudoequilibrium species (blue on Figure 10). It is possible to predict for each pH value the reactions taking place from the pseudoequilibrium to the equilibrium: for example, in the presence of the host at pH = 6.0 the species B, A, and Cc give Ct at the end of a slow process; at pH = 14, Cc2− gives Ct2−. Regarding the photochromism, the starting point is the transchalcone. At pH = 3 (Figure 10b), the thermodynamic species is Ct. Irradiation of Ct will lead to a very high conversion to AH+. However, this is not the thermodynamic state and in a much slower process will give back Ct, as in Scheme 1(a). At pH = 0 it is possible to have Ct as metastable state, and this would give AH+ upon irradiation, but now the last species is the thermodynamic one as in Scheme 1(b). Figure 12 reports the traces of the flash photolysis of the trans-chalcone form of the compound 4′-hydroxyflavylium at pH = 1.87 in the presence of [β-cyclodextrin] = 8 mM. Formation of the cis analogue takes place during the lifetime of the flash (50 ms). Both kinetic processes followed at the flavylium cation (437 nm top) and trans-chalcone (340 nm bottom) were fitted with the same rate constant (monoexponential). The slight decreasing of the absorbance at the bottom picture is due to the conversion of Cc into B, because the former absorbs more at 340 nm than the latter. Concomitantly, the flavylium absorbance rises. This result can be explained by the existence of a kinetic process controlled by the tautomerization.25 In other words, at this pH hydration is much faster than tautomerization and the kinetic step gives the rate constant k−t (Table 5). Metastable state. The most interesting metastable state in this system is the one of Ct at very acidic pH values. When formation of Ct is followed upon a pH jump from the flavylium cation, the system reaches the pseudoequilibrium by means of eq 18. In this equation the term k−i was neglected because it is much smaller than the other.

Figure 8. Titration of Ct2− in the presence of β-cyclodextrin with the host at pH = 12. Fitting was achieved for K6 = 320 M−1.

kobs(direct ) = XCc(pseudo)ki

(18)

kobs(reverse) = XCc(eq)ki + XCt(eq)k −i

(19)

When the formation of AH+ from Ct is considered, eq 19 is used. Both equations are represented in Figure 13 using the experimental values in the absence and presence of βcyclodextrin. Inspection of Figure 13 clearly shows that in theory it takes years to convert Ct into AH+. The βcyclodextrin decreases the lifetime of the metastable state, but it is still very high (around 7 years at pH = 0). On the other hand, β-cyclodextrin increases the quantum yield of the photochromic system, and the overall result is a better performance. The energy level diagram shown in Scheme 5 illustrates the reason why metastable states are formed in this and other compounds bearing a high cis−trans isomerization barrier. When a direct pH jump from the flavylim cation takes place, a pseudoequilibrium is attained after a few minutes. The isomerization to reach the final state (essentially Ct) depends on the high isomerization barrier and is very slow. However, when the equilibrated solutions of Ct are made very acidic, besides the isomerization barrier there is an extra kinetic barrier, because the reaction goes toward AH+ and toward Cc and its mole fraction distribution at the equilibrium is very small, due to the great stability of Ct in comparison with Cc.

Figure 9. Absorption spectra of the compound 4′-hydroxyflavylium in acidic medium in the presence of the host 8 mM, pKa = 1.7. Identical spectra in the absence of the host give similar variations with pK′a = 2.2.

fraction distribution of the species at the equilibrium, pseudoequilibrium, and metastable conditions can be constructed from the data presented in Tables 1−4, Appendix 1, and Figure 10. The black curves represent the mole fraction distribution at the equilibrium (basically flavylium cation and the trans-chalcones) and in blue the pseudoequilibrium upon a direct pH jump. In theory all the blue species are more or less metastable, but the one leaving more time is Ct2−, which concerns the reverse pH jumps; for example, from Ct2−, the Ct is a metastable species which can be stored for a long period in the dark. Photochemistry. Due to the high cis−trans isomerization barrier of the compound 4′-hydroxyflavylium it is possible to keep solutions of Ct for long periods in the dark. However, irradiation of these solutions gives rise to the formation of the flavylium cation as shown in Figure 11. In a direct pH jump the 2914


Article


Figure 10. Mole fraction distribution of all the network species regarding 4′-hydroxyflavylium in the absence (a) and presence (b) of β-cyclodextrin.

Figure 12. Flash photolysis of the Ct species of the compound 4′hydroxyflavylium at pH = 1.87 in the presence of [β-cyclodextrin] = 8 mM.

Figure 11. Irradiation of the trans-chalcone of the compound 4′hydroxyflavylium at pH = 2.0 in the presence of 8 mM β-cyclodextrin. The quantum yield is 0.08, which compares with 0.03 in the absence of the host.

Table 5. Rate Constants of the Compound 4′Hydroxyflavylium in the Presence and Absence of βCyclodextrin, Obtained by Flash Photolysis

Scheme 6, adapted from Scheme 5, illustrates the existence of two possible metastable states originated from 4′-hydroxyflavylium. The best way to prepare Ct is from Ct2− if the pH of the system is around pH = 12.0. From this point a pH jump from pH = 3 leads to a metastable state as defined in Scheme 1(a), while if the jump is made up to pH = 0, for example, the metastable state is of the type 1(b).

■

[β-CD] mM

8

8

8

-

-

-

pH kobs/s−1

1.87 0.47

3.01 0.33

5.97 0.047

2.29 0.52

3.06 0.7

5.92 0.059

write−read−erase, where the metastable state prevents the spontaneous appearance of the signal. Contrary to most optical memories based on photochromic systems, the present one gives a signal that does not erases spontaneously. The interaction of 4′-hydroxyflavylium with β-cyclodextrin while decreasing the lifetime of the metastable state increases the irradiation quantum yield and thus exhibits a better global performance which regards the readout capacity of the model memory.

CONCLUSIONS Metastable states with theoretical lifetimes of years have been calculated in the compound 4′-hydroxyflavylium. In this compound the Ct species at pH = 0 have a lifetime of ca. 160 and 7 years, respectively, in the absence and presence of 8 mM β-cyclodextrin. The metastable state is photochromic and upon irradiation of Ct gives a thermodynamically stable product. This system defines an optical memory capable of 2915


Article

The Journal of Physical Chemistry A AH+ + H 2O ⇌ B + H3O+ B ⇌ Cc

Kh

(A2)

Kt

(A3)

AH+ + β ‐CD ⇌ AH+βCD

K1

(A4)

A + β ‐CD ⇌ AβCD

K2

(A5)

B + β ‐CD ⇌ BβCD

K3

(A6)

Cc + β ‐CD ⇌ CcβCD

K4

(A7)

A balance mass gives C0 = [AH +] + [A] + [B] + [Cc] + [AH +CD] + [ACD] + [BCD] + [CcCD]

(A8)

⎛ Ka Kh KhK t C0 = [AH +]⎜1 + + K1[β ‐CD] + + + + ⎝ [H ] [H ] [H +] K K + K 2[β ‐CD] a+ + K3[β ‐CD] +h [H ] [H ] KK ⎞ + K4[β ‐CD] h + t ⎟ [H ] ⎠ (A9)

Figure 13. Lifetimes for the formation of Ct from AH+ (through pseudoequilibrium, in the dark) and formation of AH+ from Ct: (traced line) in the absence of β-cyclodextrin; (full line) in the presence of [β-cyclodextrin] = 8 mM.

Scheme 5. Energy Level Diagram of 4′-Hydroxyflavylium in the Absence (black) and Presence of β-Cyclodextrin 8 mM (red)

Rearranging ⎡ K ̂ + Ka″[β ‐CD] ⎤ C0 = [AH +]⎢1 + K1[β ‐CD] + a ⎥ ⎣ ⎦ [H +]

(A10)

with Ka ̂ = Ka + Kh(1 + K t ) Ka″ = K 2Ka + Kh(K3 + K4K t )

(A11)

The mole fraction of AH+ is thus given by 1

χAH +

[H +] [AH +] (1 + K1[β ‐CD]) = = K ̂ + K ″[β ‐CD] C0 [H +] + (1a + K [aβ ‐CD])

(A12)

1

On the other hand the mole fraction of the complex AH β-CD is given by +

K [β ‐CD]

χAH +β ‐CD

1 [H +] [AH +β ‐CD] (1 + K1[β ‐CD]) = = K ̂ + K ″[β ‐CD] C0 [H +] + (1a + K [aβ ‐CD]) 1

(A13)

Regarding the “basic species”

Scheme 6. Dual Metastable States from the Compound 4′Hydroxyflavylium To Be Used in Cycles to Write−Read− Erase

χA =

Ka [A] = Ka ̂ + Ka″[β ‐CD] + C0 [H ] + (1 + K [β ‐CD])

(A14)

Kh [B] = K ̂ + K ″[β ‐CD] + C0 [H ] + (1a + K [aβ ‐CD])

(A15)

1

χB =

1

■

χCc =

APPENDIX 1 Considering the pseudoequilibrium of the flavylium network with β-cyclodextrin AH+ + H 2O ⇌ A + H3O+

Ka

KhK t [Cc] = K ̂ + K ″[β ‐CD] + C0 [H ] + (1a + K [aβ ‐CD]) 1

χAβCD =

(A1)

KaK 2[β ‐CD] [ACD] = K ̂ + K ″[β ‐CD] + C0 [H ] + (1a + K [aβ ‐CD]) 1

2916

(A16)

(A17)


Article

The Journal of Physical Chemistry A χBβCD =

KhK3[β ‐CD] [BCD] = K ̂ + K ″[β ‐CD] C0 [H +] + (1a + K [aβ ‐CD])

χAβCD = KaK 2[β ‐CD]

(A18)

1

+ 3

χCcβCD =

[H ] + Kobs[H ] + KobsKobs(1)[H +] + KobsKobs(1)Kobs(2)

KhK t K4[β ‐CD] [CcCD] = K ̂ + K ″[β ‐CD] C0 [H +] + (1a + K [aβ ‐CD])

+ 2

(A30)

For the chemical system presented in Scheme 4, the following chemical equations could be written:

(A19)

1

The set of eqs A1−A7 can be simplified considering a single equilibrium involving the flavylium cation and its complex as the acidic species and the “basic” species CB and their respective complexes. AH+ + AH+β‐CD ⇌ CB + CBβ‐CD + H+

Kobs (A20)

Kobs

K ̂ + (K 2Ka + K3Kh + K4KhK t )[β ‐CD] = a 1 + K1[β ‐CD]

For the observed constants respectively: ([Ct −] + [Ctβ ‐CD−]) + [H ] [Ct ] + [Ctβ ‐CD] (1 + [β ‐CD]K 6) = KCt (1 + [β ‐CD]K5)

obs KCt =

(A21)

Assuming K1[βCD] ≪ 1, which is a valid approximation due to the small value expected for K1, a linear relation between the experimental observed rate constant is obtained. Kobs = Ka ̂ + (K 2Ka + K3Kh + K4KhK t )[β ‐CD]

([Ct 2 −] + [Ctβ ‐CD2 −]) + [H ] = KCt − [Ct −] + [Ctβ ‐CD−] (1 + [β ‐CD]K 7) (1 + [β ‐CD]K 6)

(A22)

obs − = KCt

When the experiments are followed 10 ms upon the direct pH jump, only the species AH+ and A and a are present AH+ + AH+β‐CD ⇌ A + Aβ‐CD + H+

K a(obs)

And eq A22 is transformed in eq A24

χCt = [H +]2 /{[H +]2 (1 + [β ‐CD]K5) + [H +]2 KCt

(A24)

(1 + [β ‐CD]K 6) + KCt KCt −(1 + [β ‐CD]K 7)}

Extension to the basic region

The system can be extended to the basic region considering the following equilibria to add to eq A20 −

−

CB + CBβ‐CD ⇌ CB + CB β‐CD + H

+

(A39)

χCt − = [H +]KCt /{[H +]2 (1 + [β ‐CD]K5) + [H +]2 KCt

Kobs(1)

(1 + [β ‐CD]K 6) + KCt KCt −(1 + [β ‐CD]K 7)}

(A25) −

−

2−

CB + CB β‐CD ⇌ CB

(A38)

Taking into account eqs A31−A36, the following equation for the mole fractions of the trans-chalcone species can be deduced:

(A23)

Ka(obs) = Ka + K 2Ka[β ‐CD]

(A37)

2−

+ CB β‐CD + H

+

(A40)

Kobs(2)

χCt 2− = KCt KCt −/{[H +]2 (1 + [β ‐CD]K5) + [H +]2 KCt

(A26)

(1 + [β ‐CD]K 6) + KCt KCt −(1 + [β ‐CD]K 7)} (A41)

χAH + = [H +]3 +

1 [H +]3 (1 + K1[β ‐CD]) Kobs[H +]2 + KobsKobs(1)[H +]

{

χCtβCD = [H +]2 [CD]K5/ [H +]2 (1 + [β ‐CD]K5)

+ KobsKobs(1)Kobs(2)

+ 2

+ [H ] KCt (1 + [β ‐CD]K 6) + KCt KCt −

(A27)

(1 + [β ‐CD]K 7)

χAH +CD = [H +]3 +

K1[β ‐CD] [H +]3 (1 + K1[β ‐CD]) Kobs[H +]2 + KobsKobs(1)[H +]

}

(A42)

{

χCtβCD− = [H +][β ‐CD]K 6KCt / [H +]2 (1 + [β ‐CD]K5) + 2

+ [H ] KCt (1 + [β ‐CD]K 6) + KCt Kct −

+ KobsKobs(1)Kobs(2)

(1 + [β ‐CD]K 7)

(A28)

χA =

}

(A43)

χCtCD2− = [β ‐CD]K 7KCt KCt −/{[H +]2 (1 + [β ‐CD]K5)

Ka

+ [H +]2 KCt (1 + [β ‐CD]K 6) + KCt KCt −

[H +]3 + Kobs[H +]2 + KobsKobs(1)[H +] + KobsKobs(1)Kobs(2)

(1 + [β ‐CD]K 7)}

(A29) 2917

(A44) DOI: 10.1021/acs.jpca.5b01473 J. Phys. Chem. A 2015, 119, 2908−2918

Article


■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Photochromism: Molecules and Systems; Elsevier: 2003; ISBN: 0444-51322-1. (2) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and Thermochromic Compounds; Kluwer Academic/Plenum Publishers: New York, 1999. (3) Ercole, F.; Davis, T. P.; Evans, R. A. Polym. Chem. 2010, 1, 37− 54. (4) Molecular Switches; Wiley: 2011; ISBN: 978-3-527-31365-5. (5) Kawai, S. H.; Gilat, S. L.; Lehn, J. M. J. Chem. Soc., Chem. Commun. 1994, 1011−1013. (6) Lehn, J. M. Angew. Chem., Int. Ed. 1990, 29, 1304−1319. (7) Bueno, J.; Sáez-Plaza, P.; Ramos-Escudero, F.; Jiménez, A. M.; Fett, R.; Asuero, A. Crit. Rev. Anal. Chem. 2012, 42, 126−151. (8) Castaneda-Ovando, A.; Pacheco-Hernández, M. D. L.; PáezHernández, M. E.; Rodríguez, J. A.; Galán-Vidal, C. A. Food Chem. 2009, 113, 859−871. (9) Cavalcanti, R.; Santos, D.; Meireles, M. Food Res. Int. 2011, 44, 499−509. (10) Brouillard, R.; Dubois, J. E. J. Am. Chem. Soc. 1977, 99, 1359− 1364. (11) Pina, F. Thermodynamic and kinetic processes of anthocyanins and related compounds and their bio-inspired applications” Recent Advances in Polyphenols Research; Wiley-Blackwell: 2014; Vol. 4, Chapter 11, ISBN: 978-1-118-32967-2. (12) Pina, F.; Melo, M. J.; Maestri, M.; Ballardini, R.; Balzani, V. J. Am. Chem. Soc. 1997, 119, 5556−5561. (13) Roque, A.; Lima, J. C.; Parola, A. J.; Pina, F. Photochem. Photobiol. Sci. 2007, 6, 381−385. (14) Michaelis, C.; Wiesinger, R. Helv. Chim. Acta 1951, 34, 1761− 1770. (15) Kuster, F. W.; Thiel, A. Tabelle per le Analisi Chimiche e ChmicoFisiche, 12th ed.; Hoepli: Milano, 1982. This buffer is prepared in the following way: dissolve 2.3 cm3 of phosphoric acid (85% (w/w)), 7.00 g of mono-hydrated citric acid, and 3.54 g of boric acid in 343 mL of 1 M NaOH and dilute with water until 1 dm3. (16) Pina, F.; Melo, M. J.; Ballardini, R.; Flamigni, L.; Maestri, M. Flash photolysis of 4′,7′-dihydroxyflavylium. New J. Chem. 1997, 21, 969−976. (17) Pina, F.; Melo, M. J.; Laia, C. A. T.; Parola, A. J.; Lima, J. C. Chem. Soc. Rev. 2012, 41, 869−908. (18) Pina, F.; Roque, A.; Melo, M. J.; Maestri, M.; Belladelli, L.; Balzani, V. Chem.Eur. J. 1998, 4, 1184−1191. (19) Petrov, V.; Stanimirov, S.; Petrov, I. K.; Fernandes, A.; de Freitas, V.; Pina, F. J. Phys. Chem. A 2013, 117, 10692−10701. (20) Basílio, N.; Fernandes, A.; de Freitas, V.; Gago, S.; Pina, F. New J. Chem. 2013, 37, 3166−3173. (21) Mirzoian, A.; Kaifer, A. E. Chem.Eur. J. 1997, 3, 1052−1058. (22) McClelland, R. A.; Gedge, S. J. Am. Chem. Soc. 1980, 102, 5838−5848. (23) Petrov, V.; Pina, F. J. Math. Chem. 2010, 47, 1005−1026. (24) Lee.; et al. Acc. Chem. Res. 2003, 36, 621−630. (25) Petrov, V.; Gomes, R.; Parola, A. J.; Pina, F. Dyes Pigments 2009, 80, 149−155.

2918


Origin of the Metastable Stability in Flavylium Multistate Systems - The

Recommend Documents