Photochromism of Dihydroindolizines Part XI : Synthesis of Novel Carbon-Rich Photochromic Dihydroindolizines-Based Potential Electronic Devices

Novel carbon-rich photochromic dihydroindolizine (DHI) derivatives substituted in the fluorene part (region A) in addition to the new spirocyclopropene 6 have been synthesized. The synthesis of dimethyl 2′,7′-diethynylspiro[cycloprop[2]ene-1,9′-fluorene]2,3-dicarboxylate precursor 6 was accomplished in five steps, starting with the literature known conversion of fluorene to 2,7dibromo-9H-fluoren-9-one in 56% yield over three steps. The chemical structures of the new synthesized materials have been elucidated by both analytical and spectroscopic tools. Three alterative synthetic pathways for the synthesis of DHI 9 have been established.


Introduction
Molecules that respond to the application of external stimuli by undergoing reversible transformations between two distinct structures have the potential to significantly influence the development of numerous important materials science and structural biology technologies [1,2].This potential is based on the fact that, because the molecules typically undergo dramatic changes in their electronic and topological characteristics, they can act as switching elements and other dynamic components in various optoelectronic devices and functional materials.Photons are particularly appealing stimuli because modern lasers can be used to achieve fast response times, to focus a fine-tuned stimulus on small localized domains without significant diffusion and to trigger photochemical events under conditions mild enough to pose minimal danger to sensitive biomaterials.Compounds that interconvert between different isomers having unique absorption spectra when stimulated with light are referred to as photochromic, and the process is called photochromism.In these systems, the changes in the electronic patterns responsible for the changes in color also result in variations in other practical physical properties such as luminescence [3], electronic conductance [4,5], refractive index [6], optical rotation [7], and viscosity [8,9].These materials, based on the 1,5-electrocyclization between two distinct isomeric states: ring-opening form (betaine-form) and ring-closed form (DHI-form), are promising candidates for optical storage media and electronic devices [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].
As a continuation of our research on photochromic dihydroindolizines (DHIs), this manuscript is devoted to the synthesis of carbon-rich fluorenyl-dihydroindolizines derivatives and represents the first step toward the application of photochromic dihydroindolizines in electronic devices.Different synthetic approaches will be described.

Conclusion
We have successfully extended the photochromism of photochromic DHI through the coupling reactions in the fluorene part.New spirocyclopropene and photochromic dihydroindolizines (DHIs) substituted in the fluorene part (region A) with acetylenic bridge for future using in electronic devices have been furnished.Further modification of the chemical structure of DHI system and their photochromic properties as well as supporting onto the surface of metals such as gold, silicon, and titanium will be discussed in details in the forthcoming paper.

Experimental
Spirocyclopropene derivatives were obtained via photolysis of the corresponding pyrazoles prepared according to reported procedures [10][11][12][13][14][15][16][17][18][19][20][21][22][23].Photolysis was carried out in the photochemical reactor of Schenck [31] made from Pyrex (λ > 290 nm).The source of irradiation was a high-pressure mercury lamp Philips HPK 125 W. Solutions to be photolyzed were flushed with dry nitrogen for 30 minutes before switching on the UV lamp.The progress of the reaction and the purity of the products isolated were monitored using TLC.Separation and purification of all synthesized photochromic materials were carried out using column chromatography (80 cm length × 2 cm diameter) on silica gel and CH 2 Cl 2 as eluent.Melting points were determined on (Electrothermal Eng. Ltd., Essex, UK) melting point apparatus and are uncorrected.All NMR spectra were collected on a Brüker DRX-400 spectrometer (400 MHz) in CDCl 3 using TMS as the internal standard.Chemical shifts (δ) are reported in ppm.Experimental details, procedures, and full characterizations of the new synthesized compounds will be described elsewhere.

Scheme 3 :
Scheme 3: Another reaction pathway for the synthesis of the target photochromic DHI 9.