Synthesis and Spectral Properties of a New Spirophotochromic Molecule ^†

Abstract

Due to their ability to reversibly isomerize under the influence of external stimuli, spiropyrans represent the most interesting class of organic photochromic molecules. The photochromic properties of the isomeric forms of spiropyrans differ significantly from each other, which makes it possible to use these photochromes as sensors, optoelectronic and holographic devices, memory elements, etc. Also, an undoubted advantage of spiropyrans compared to other classes of organic photochromes is the relative ease of their preparation and chemical transformation. At the same time, modification of the structure of spiropyrans by introducing various functional groups opens up great synthetic possibilities for obtaining new photochromic molecules with various spectral-kinetic characteristics. In the development of research aimed at expanding the boundaries of the use of spirophotochromic compounds, in order to obtain new light-controlled materials with different characteristics, as well as to study the influence of functional groups in the spirophotochromic molecule on the spectral and photochromic properties, we have synthesized a new spiropyran. In this work, we synthesized a new salt of photochromic spiropyran containing various functional groups (–CHO, –NO₂, –OCH₃, –(CH₂)₅N(CH₃)₂*HBr), capable of reversibly responding to external influences. Photoinduced transformations and the spectral and kinetic characteristics of the synthesized compound were studied.

1. Introduction

The synthesis and design of functional molecules in information technology, medical applications, opto-and optobioelectronics, transport systems, photocontrolled nanostructured materials, catalysis, biomaterials, and molecular photodynamic sensors is an actively developing area of research [1,2,3]. These compounds include spiropyrans [4]. Due to the relative ease of their preparation and structural transformation and the possibility of directed changes in optical characteristics over a wide range when changing the structure, spiropyrans have attracted special attention as one of the most important and widely studied classes of photochromic organic molecules.

2. Results and Discussion

Taking into account the above and trying to expand the class of photochromic compounds and to study the effect of substituents on the electronic properties and stability of the new molecule, we synthesized a new photochromic salt of spiropyran containing various functional groups. The target spiropyran was synthesized using methods described in the literature [5,6] according to Scheme 1.

The resulting reaction product 6 was purified via silica column chromatography using 10:1 CHCl₃:EtOH as the eluant. The structures of compound 6 were determined using ¹H and ¹³C NMR spectroscopy and high-resolution mass spectrometry.

The position of the signals in the ¹H NMR spectra of compound 6 in a mixture of chloroform and DMSO, along with the spin–spin interaction constants, is completely consistent with the presented structure (Figure 1). In the region of aliphatic protons, two singlets at 0.45 and 0.57 ppm correspond to the protons of the methyl groups at the nitrogen atom of the indole fragment of the molecule. The singlet signal at 2.09 ppm belongs to protons of two methyl groups at the ammonium nitrogen atom. The signal of O-CH₃ was detected at 3.07 ppm. A doublet in the aromatic region at 5.17 ppm with a spin–spin interaction constant of 10.3 Hz corresponds to the CH group in the pyran moiety, which indicates the presence of a spirocyclic structure. The singlet signal at 9.04 ppm belongs to the proton of the formyl group.

The signal at 105.06 ppm, which correlates in the ¹H–¹³C HMBC spectrum with the signal of the protons of the methyl groups and the signals of the protons at the C3’ and C4’ carbon atoms, is characteristic and belongs to the spiro carbon atom (Figure 2). The signal at 189.29 ppm belongs to the proton of the formyl group.

When a solution of spiropyran 6 in ethanol is exposed to UV radiation, a change in the absorption spectrum is observed (Figure 3a). In this case, a new absorption band appears in the visible range with a maximum in the visible region at 572 nm, indicating the formation of the merocyanine form. During the subsequent dark relaxation, a gradual decrease in the intensity of the photoinduced absorption band occurs, indicating the isomerization of the open form into the closed form.

The change in the intensity of the optical density, namely, the decrease in the amplitude at the maximum of the long-wave absorption band of the photoinduced merocyanine form of compound 6, indicates that this spiropyran is subject to photochromic transformations, which is accompanied by the gradual photodegradation of the molecule (Figure 3b).

With the continuous exposure of a solution of compound 6 to UV light, a decrease in optical density in the open-form absorption spectrum is observed, which indicates the gradual destruction of spirophotochrome. Complete photodecomposition is observed after 2 h (Figure 4).

3. Conclusions

In summary, we synthesized a new salt photochromic spiropyran comprising various functional groups that can react reversibly to external influences and also studied photoinduced transformations and the spectral and kinetic characteristics of the synthesized compound. It was found that the resulting spiropyran exhibits positive photochromism and is quite resistant to irreversible photochemical transformations. This opens up prospects for the use of such molecules as molecular switches.