Unusual Reverse Photochromic Behavior of Indolinobenzospiropyran 6-Carboxylates in Aqueous Binary Solvents

Abstract

Thermoand photochromic indolinobenzospiropyran dyes recently have become important in connection with the rapid development of information recording systems, such as high-density optical data storage, optical switching, displays and non-linear optics. These molecules are known to be in equilibrium with their metastable colored forms. The absorption of ultraviolet light shifts the equilibrium towards the merocyanine colored form (MC), while the reverse process shifting them toward the colorless form (SP) is induced by visible light, heat, or can occur spontaneously (normal photochromism). The color of the MC form as well as the rate of rearrangement back to the SP form are both dependent on the polarity of the solvent. The stabilization of the MC form in polar solvents leads to it having a larger energy of activation and a slower MC↔SP transition as compared to that in non-polar solvents. Occasionally, the merocyanine form is further stabilized by factors such as hydrogen bonding, combination with a crown ether or cyclodextrin and complexation with a metal. Thus, the initially colorless solutions of spiropyrans become deeply colored upon the spontaneous formation of the merocyanine (reverse photochromism). Continuing our previous works on the solvatokinetic and solvatochromic behaviors of spiropyran derivatives in various neat organic solvents, we examined the photochromic behaviors of spiropyran derivatives in binary aqueous solvent media. Since merocyanine dyes in solution usually exist in dipolar ionic forms, their preferential solvation in binary mixtures was expected to affect their photophysical behavior. Although many studies have been conducted on the photochromic behaviors of spiropyran derivatives in various neat organic solvents, to the best of our knowledge, no papers have previously been published on the photochromic behaviors of spiropyrans in aqueous binary solvent systems. Herein, we describe the unusual photochromic behaviors of indolinobenzospiropyran 6-carboxylates 1-2 in aqueous binary solvent media, as depicted in Scheme 1. The electronic absorption spectra of the spiropyran carboxylates 1-2 showed no absorbance in the visible region on irradiation in neat solvents, whereas, they unexpectedly showed chromotropism in aqueous binary solvent mixtures such as water/methanol, water/DMSO, water/acetonitrile, etc. The UV-Vis. spectral changes for the SP → MC conversion of SP-1 in water-methanol solvent mixtures (50/50 v/v %) are shown in Figure 1, as a representative example. Initially, the water-methanol solution of SP-1 has an absorption band only in the 300 nm region. However, the colorless solution becomes spontaneously deeply colored upon the ring-opening reaction of SP-1 to MC-1. MC-1 has a strong absorption at 522 nm in 50/50% aqueous methanol. Reproducible results for the ring-opening reaction of spiropyran to merocyanine were obtained through the development of a protocol that maximized the formation of the colored form, as indicated by the solid line in Figure 1. The colored merocyanine solutions revert to the colorless spiropyran state upon their exposure to visible light of the appropriate wavelength. In addition to its major absorption at 500-550 nm, SP-2 has an extra absorption band in the 400-450 nm region in aqueous binary solvent mixtures. This extra absorption band is probably due to the proton transferred MC form (MCHCOO−). The proton might migrate from the 6-substituent COOH to the oxygen atom of the phenoxide anion in aqueous solvents, since the COOH group is more acidic than the OH group of the phenyl ring. This was confirmed by the UV-Vis spectral behavior of spiropyran both in HCl/aqueous