Promoting Photochromism on Flavylium Derived 2-Hydroxychalcones

J. Phys. Chem. B 2007, 111, 12059-12065

12059

Promoting Photochromism on Flavylium Derived 2-Hydroxychalcones in Aqueous Solutions by Addition of CTAB Micelles Raquel Gomes, A. Jorge Parola, Ce´ sar A. T. Laia, and Fernando Pina* REQUIMTE, Departamento de Quı´mica, Faculdade de Cieˆ ncias e Tecnologia, UniVersidade NoVa de Lisboa, 2829-516 Caparica, Portugal ReceiVed: May 18, 2007; In Final Form: August 8, 2007

A strategy to obtain photochromism from the network of chemical reactions originated by flavylium compounds in solution is described. This strategy is particularly useful for flavylium salts bearing amino groups which give rise to a variety of beautiful colors but lack photochemistry in water. The trans-chalcone of 7-(N,Ndiethylamino)-4′-hydroxyflavylium interacts strongly with CTAB micelles defining a yellow dark state. Upon irradiation, the system switches to a pink-red state emerging from the flavylium cation that is formed inside the micelle and ejected to the bulk aqueous phase. The photochemical product reverts back to the transchalcone adduct with the micelle in the dark. The thermodynamics as well as the kinetics of the photochromic system were studied in detail. The best color contrast is obtained at pH ) 4.25 with Φ ) 0.001 and a recovery lifetime of approximately 3 h. This photochromic system works with no need of changing the pH, which constitutes an important improvement over previously described systems dependent on pH jumps.

Introduction

SCHEME 1

The study of compounds whose color changes permanently or temporarily by the action of light (photochromism)1 is a recurrent topic of research suitable for applications in photonic devices such as optical switches,2,3 erasable memories,3 or even in the mimicking of elementary properties of a neuron.4 Synthetic flavylium salts are among the most promising families of photochromic compounds, in particular due to the possibility of conceiving multistate systems responding to a given combination of multiple stimuli, a step further to the achievement of complex switches (logic gates) at the molecular scale.1d,5 The photochromic properties of flavylium compounds arise from the photochemical reactivity of the trans-chalcone species, which can isomerize to the cis form upon light excitation.6 The cis-chalcone, in turn, spontaneously converts into the colored flavylium cation (in acidic media), defining the flavylium network of species as a photochromic system, Scheme 1. Within the family of flavylium compounds, those bearing strong electron-donating amino groups give rise to a variety of beautiful colors but lack photochemistry in water; only in highly viscous borax-glycerol medium,7 organic solvents,8 or ionic liquids9 photochemistry was reported. Microheterogeneous aqueous environments such as micellar solutions, however, have not been explored, despite their promising properties in terms of microviscosity or local polarity within the micelles. In this work we describe a novel and general strategy to optimize the photochromic response of the network of species originated by flavylium compounds in aqueous solutions, particularly useful for those bearing amino groups. The interaction of the flavylium network with micelles is known to stabilize some species (states) through specific interactions, allowing modulation of the network.10 However, up to now, no exploitation of these effects on the photochromic properties of flavylium compounds has been reported. This work takes profit from the * Corresponding author. Fax: +351212948550. Phone: +351212948355. E-mail: [email protected]..

strong associative interaction of the trans-chalcone of 7-(N,Ndiethylamino)-4′-hydroxyflavylium with cetyltrimethylammonium bromide (CTAB) micelles and the repulsive interaction of flavylium cation with the positively charged surface of the micelle. Experimental Section 7-(N,N-Diethylamino)-4′-hydroxyflavylium tetrafluoroborate was available from previous studies.11 All other chemicals used were of analytical grade. Solutions were prepared using Millipore water. The pH of solutions was adjusted by addition of HCl, NaOH, or universal buffer of Theorell and Stenhagen12

10.1021/jp073859c CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

12060 J. Phys. Chem. B, Vol. 111, No. 42, 2007

Gomes et al.

Figure 1. (A) pH dependent spectral changes of 7-(N,N-diethylamino)-4′-hydroxyflavylium in the presence of CTAB micelles at equilibrium, [AH+] ) 5 × 10-5 M and [CTAB] ) 5 × 10-3 M. (B) Effect of the addition of CTAB micelles to a solution of 7-(N,N-diethylamino)-4′hydroxyflavylium (1 M HCl), [AH+] ) 3 × 10-5 M and [CTAB] ) 5 × 10-3 M. Inset: absorbance changes at 550 nm as a function of time.

and was measured in a Radiometer Copenhagen PHM240 pH/ ion meter. UV-vis absorption spectra were recorded in a Varian-Cary 100 Bio spectrophotometer or in a Shimadzu VC2501-PC. A concentration of 5 × 10-3 M CTAB was used in all experiments and 7-(N,N-diethylamino)-4′-hydroxyflavylium tetrafluoroborate was used in a concentration of 5 × 10-5 M in all kinetic rate measurement, flash photolysis, and quantum yield determination experiments. Quantum yields were determined by irradiation at 436 nm, using a medium-pressure mercury arc lamp, and the excitation bands were isolated with interference filters (Oriel). Actinometry was made using the ferrioxalate system.13 The flash photolysis experiments were performed as previously described.14 Results and Discussion Network of Chemical Reactions of 7-(N,N-Diethylamino)4′-hydroxyflavylium in Water. In Scheme 1, the network of chemical reactions originated by 7-(N,N-diethylamino)-4′-hydroxyflavylium in acidic media is shown:11 the flavylium cation AH+, the quinoidal base A, resulting from the deprotonation of AH+; the hemiketal species B, obtained by hydration in the 2 position of the flavylium cation; the cis-2-hydroxychalcone (Cc), formed from the hemiketal B through a tautomeric process and the trans-2-hydroxychalcone (Ct), obtained from Cc via a cis-trans isomerization reaction. The several forms (states) can be reversibly interconverted by pH modifications. In very acidic and basic media, protonated species on the amino group and unprotonated species on the phenolic group can be, respectively, formed. At equilibrium, in acidic to slightly basic media, the system can be described by eq 1 because the flavylium cation AH+ and the trans-chalcone Ct behave as a single acid-base reaction.1d The remaining species reported in Scheme 1 are however crucial in the control the kinetic processes (see Supporting Information).

AH+ + H2O h Ct + H+ K′a ) 10-5.9

(1)

The trans-chalcones of the present compound do not exhibit measurable photochemistry in water.11 The lack of significant

photochemistry in chalcones possessing an amino group was previously reported and attributed to the formation of a highly polar state on the S1 hypersurface, presumably a twisted intramolecular charge transfer state (TICT)7 or another ICT mechanism. Such a mechanism implies the presence of an extra reaction coordinate at the chalcone singlet excited state, which would quench the trans-cis photoisomerization reaction (as well as its fluorescence) giving rise to very low photoisomerization quantum yields. Nevertheless, kinetic reasons may also contribute to the absence of measurable photochemistry in water (see Supporting Information). On CTAB micelles, however, this picture is changed (see below). Equilibria of the Flavylium Network in the Presence of CTAB Micelles. Addition of CTAB micelles (monomer 5 × 10-3 M) to solutions of 7-(N,N-diethylamino)-4′-hydroxyflavylium at different pH values leads to dramatic changes in the equilibrium positions. While AH+ is unaffected by addition of CTAB due to electrostatic repulsion with the positively charged micelles, the potentially photochromic chalcone Ct is stabilized in the micellar phase. As a consequence, when pH jumps from basic Ct2- solutions to the acidic-neutral region, with concomitant addition of CTAB, are carried out, the trans-chalcone Ct preferentially partitionates to the micellar phase, defining a pseudo-equilibrium between Ct+ (pKaCt+ ) 4.511) and CtCTAB (see Supporting Information). The final equilibrium is again attained with the presence of only AH+ and Ct-CTAB, according to Figure 1 and eq 2. It is worth to note that even at [H+] ) 1 M, addition of CTAB gives rise to a small fraction of Ct+ going inside the micelle in the forms of Ct-CTAB and eventually Ct+-CTAB, Figure 1B.

AH+ + CTAB + H2O h -1.9 (2) Ct-CTAB + H+ K′m a ) 10

It was possible to determine all the equilibrium constants that define the pH dependent mole fraction distribution of species at equilibrium, Figure 2 (see Supporting Information). When the distributions of species in the presence (Figure 2A) and in the absence (Figure 2B) of CTAB are compared, the results very clearly show that the addition of CTAB shifts the Ct domain until the pH is approximately 4 pH units lower.

Flavylium Derived 2-Hydroxychalcones

J. Phys. Chem. B, Vol. 111, No. 42, 2007 12061

Figure 2. (A) Mole fraction distribution of the of 7-(N,N-diethylamino)-4′-hydroxyflavylium in the presence of CTAB micelles at equilibrium. (B) In the absence of CTAB for comparison purposes.

SCHEME 2

Figure 3. Rate constant for the thermal conversion of AH+ into CtCTAB vs pH at 22 °C. The solid line shows the fit with eq 3.

Kinetics of the Flavylium Network in the Presence of CTAB Micelles. The kinetics of the flavylium disappearance when CTAB is added into the solution was measured at different pH values. The decay of the flavylium cation absorbance follows a first-order kinetic law and is pH dependent, as plotted in Figure 3. In order to explain the behavior reported in Figure 3, Scheme 2 is proposed from which eq 3 can be deduced (see Supporting Information):

kthermal )

[H+] +

(

kikh +

[H ] + Ka ki + k-h[H ]

+

ki+kh+ ki+ + k-h+[H+]

)

+

k-i+k-h+[H+] (3) χCt+ ki + + k-h+[H+]

where χCt+ is given by eq S10 in the Supporting Information and the other constants are reported in Scheme 2, the only

adjustable parameter being kh+. However, the fitting was achieved with kh+ ≈ 0 s-1 (or 3. At such pH values in the bulk, AH+ is still more stable than Ct and is slowly converted in Ct-CTAB. Under thermodynamic equilibrium, irradiation of Ct-CTAB leads to the production of AH+-CTAB which is quickly ejected to the bulk, thus giving rise to a significant photochromism. In water, net photochemistry was not observed, but both quantum yield and efficiency of AH+ formation are increased upon addition of CTAB micelles. This photochromic system works with no need to change pH,

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