Photo-and thermochromic properties of 1′,3′,3′-trimethyl-6-nitro-8-pyridiniomethylspiro[2H-[1]benzopyran-2,2′-indoline] chloride in the crystalline state

Abstract

1´,3´,3´ Trimethyl 6 nitro 8 pyridiniomethylspiro[2H [1]benzopyran 2,2´ indoline] chlo ride (1) was synthesized. Single crystals (crystal hydrates) of the open merocyanine form of compound 1 were obtained under the dark conditions, and their crystal structure was studied. On irradiation with visible light compound 1 crystallizes from propan 2 ol in the polycrystal line closed form. The spectral and photochemical properties of compound 1 in the polycrystal line state and the kinetic characteristics on thermal transformation of the closed form to the open form were studied.Hybrid polyfunctional compounds of a principally new class have recently been synthesized. 1,2 They combine two sublattices in one crystal structure: the photo chromic cationic sublattice of spiropyrans and magnetic anionic sublattice, for instance, of bimetallic oxalate [Cr III Mn II (ох) 3 ] -. Spiropyran (SP) acts as a photochemi cal molecular switcher perturbing the magnetic sublattice. The main problem of design of these polyfunctional com pounds is to collect different functional "building" blocks in one crystal structure retaining specific features of the crystal structures of the blocks, which determine their properties in individual compounds. The design of hybrid materials is also impeded by the fact that the photo chromic transformations of SP require a considerable free volume and, hence, they are hindered in the crystalline state. It has recently been shown 3,4 that the structure of the magnetic anionic sublattice of bimetallic oxalates [M III M II (ох) 3 ] -allows cation size variation to perform in a wide range, and the crystal structure of the spiropyran cation in salts with chloride and iodide anions is found to be favorable for reversible photochromic transformations of spiropyrans. 5,6 These features of the structures of the cationic and anionic sublattices allowed crystal chemical modeling of photochromic ferromagnets to be per formed. 1,2 In this case, it was important that the SP + cation was involved in the reaction in the closed form. Therefore, the equilibrium "closed form of the spiropyran cation SP + open cationic merocyanine form MC + " in solution was shifted to the closed SP form using irra diation with the visible light. No hybrid structure can be obtained from the merocyanine form.In the present work, we repeated the synthesis of pho tochromic 1´,3´,3´ trimethyl 6 nitro 8 pyridiniomethyl spiro[2H [1]benzopyran 2,2´ indoline] chloride (1) con taining the pyridinium fragment in the lateral aliphatic chain and prepared two forms: closed SP form (1a) (poly crystals) and open MC form (1b) (single crystals) as shown Scheme 1

A Solution- and Solid-State Investigation of Medium Effects on Charge Separation in Metastable Photomerocyanines

“…To date, the only metastable photomerocyanines that have been structurally characterized are a handful of nitro-and pyridinium-substituted spiropyran or spirothiopyran derivatives, which were all shown to exhibit a predominantly zwitterionic structure in the crystalline state. [38][39][40][41][42][43] These molecules also display negative solvatochromism, which corroborates their zwitterionic structure in solution. 41,72,119 Only a few permanent spirooxazine-based PMCs have been characterized, and these have been described as predominantly quinoidal in character, in some cases on the basis of the CdO bond length alone.…”

“…To date, the only metastable photomerocyanines that have been structurally characterized are a handful of nitro- and pyridinium-substituted spiropyran or spirothiopyran derivatives, which were all shown to exhibit a predominantly zwitterionic structure in the crystalline state. – These molecules also display negative solvatochromism, which corroborates their zwitterionic structure in solution. ,, Only a few permanent spirooxazine-based PMCs have been characterized, and these have been described as predominantly quinoidal in character, in some cases on the basis of the CO bond length alone. , Solvatochromic studies of spirooxazine-based PMCs have indicated that these exhibit positive solvatochromic behavior, and it was concluded that they possess a predominantly quinoidal structure. ,,, Computational studies have also predicted a quinoidal structure for spirooxazine-based PMCs . The interpretation of these observationslimited to specific derivatives and conditionsled in some instances to ambiguity regarding the nature of the molecular structure of spiropyrans and spirooxazines and, , in some cases, to the belief that spirooxazines as a class are quinoidal while spiropyrans are zwitterionic .…”

“…Structural analysis in the solid state by X-ray diffraction (XRD) through crystallization of metastable photomerocyanines has been impeded by their transient nature , and the spontaneous formation of molecular aggregates upon irradiation. , In contrast, reports of structural characterization of the metastable photomerocyanine form of the related class of spiropyrans exist, but are rare. XRD analyses of a thiospiropyran PMC form and a small number of spiropyran PMC forms − suggest that spiropyran photomerocyanines with electron-withdrawing nitro and pyridinium substituents on the oxazine moiety exhibit dominant zwitterionic character. XRD analyses of two permanently open (i.e., nonphotochromic) spirooxazines indicate that the merocyanines exhibit a predominantly quinoidal structure. , To date, the structural characterization of the open form of the photochromic spirooxazine class of photochromic molecules has not been carried out by XRD analysis.…”

“…36,37 In contrast, reports of structural characterization of the metastable photomerocyanine form of the related class of spiropyrans exist, but are rare. XRD analyses of a thiospiropyran PMC form 38 and a small number of spiropyran PMC forms [39][40][41][42][43] suggest that spiropyran photomerocyanines with electron-withdrawing nitro and pyridinium substituents on the oxazine moiety exhibit dominant zwitterionic character. XRD analyses of two permanently open (i.e., nonphotochromic) spirooxazines indicate that the merocyanines exhibit a predominantly quinoidal structure.…”