Synthesis, characterization, and solvent-independent photochromism of spironaphthooxazine dimers

Abstract

Specifically angled, conjugated spiroindolinonaphthooxazine dimers (SNOD) have been synthesized. The photochromic reactions of two types of SNOD were studied under continuous UV irradiation in solvents of different polarity. Comparison of these results with the single unit provides the examination of the specific effect of substituents on their photochromic properties and relaxation kinetics. The photomerocyanine isomers showed positive solvatochromism, supporting the premise for a less polar quinoidal structure. The thermal closing rate at 25 °C ranged from 0.2 to 1.6 s⁻¹ depending on the compound and solvent. Photochromism of these new compounds showed little dependency on solvent polarity and stable cyclability.

Introduction

Photochromic materials have shown practical applications in the latest technologies such as optical switching, sensor, drug delivery, data storage, and recording [1], [2]. The use of spirooxazine based materials has become of increasing interest due to their excellent photostability, compatibility in a variety of matrices, and distinctive changes in structure and absorption spectra upon irradiation. The photoresponsive site of the spirooxazine is the center sp³ spiro carbon. When irradiated with UV light, the carbon oxygen bond cleaves and achieves sp² hybridization, yielding a conjugated zwitterionic or quinoidal system [3], [4]. This conjugated isomer, called merocyanine (MC) is metastable and readily isomerizes via ring closing into the spiro (SP) form either thermally in dark conditions or under visible light [5].

As an approach to improve the applicability of spirooxazine based compounds, joining two of the species through a spacer offers unique bi-photochromic molecules with distinctive photokinetic properties [6], [7], [8]. A challenge faced with designing bi-photochromic molecules is that these dimer systems often exhibit different or deficient photochemical properties from those of the single components. Photochromic properties and the rate of thermal closing are affected by the nature of the spacer, e.g., inducing a loss of thermal reversibility, yielding different isomers from the starting material, and/or extensive photodegradation [7], [8]. In addition, bi-photochromic systems have shown to be either too rigid or flexible for further application.

Recently, we reported a shape specific photochromic dimer, called SPOD, consisting of two spiro-phenanthro-oxazine moieties connected via triple bond at the indoline portion of the spirooxazine moieties [9]. This SPOD has shown interesting binding ability with palladium catalyst. The open MC form of this molecule showed a bathochromic shift at the maximum wavelength of absorption and higher colourability due to the increased conjugation. It exhibited excellent reversibility; however, the data from thermal bleaching illustrates that the rate of thermal closure increased dramatically with increasing solvent polarity. Thermal closing rates ranged from 0.12 s⁻¹ in nonpolar methylcyclohexane to 4.48 s⁻¹ in polar solvent, dimethylformamide. Reports suggest that the solvent dependency for phenanthrene systems is due to a loss in pseudo pi-conjugation between the indoline and oxazine moieties in its transition state. As the electronic charge on the indoline nitrogen and oxazine oxygen atoms increases, the molecular dipole moment increases causing the accelerated closing with polarity of solvent [10], [11].

In this work, we have improved our bi-photochromic systems by replacing the phenanthro-oxazine with naphtho-oxazine to create molecules having environmentally independent optical properties. We report the synthesis and photochromic properties of two types of spiro-naphtho-oxazine dimers: Debald-SNOD(2) and Deba-SNOD(3). The chemical structures and typical photoreactions for these compounds are given in Fig. 1. Two SNO units in these dimers were connected to different positions of a phenyl ring spacer, which would create different conjugation systems. Additionally, both dimers possess a functional group on the center phenyl ring to offer further modifications such as grafting on the surface and polymers. The investigation in this report includes studies on the effects of solvent and structure on the absorption properties, fatigue resistance, and closing rate kinetics of the merocyanine of the dimers as compared to the single unit, Iodo-SNO(1).