Synthesis and Photochromism of Quinolines and Benzo [ h ] quinolines of 1 , 3-Diazabicyclo [ 3 . 1 . 0 ] hex-3-ene

1,3-Diazabicyclo[3.1.0]hex-3-ene systems represent a very interesting class of organic materials, possessing unique photochromic properties in the solid state. It has been demonstrated that these compounds form deeply colored, fairly stable materials under UV radiation. Here, for the first time we report synthesis of several 1,3-diazabicyclo[3.1.0]hex-3-enes with premade substituted quinoline and benzo[h]quinoline rings.


Introduction
Photochromism is defined as light-induced reversible transformation of a chemical species between two isomers having different absorption spectra [1][2][3].During photoisomerization, along with absorption spectra, the chemical and physical properties of isomers such as refraction coefficient di-electric constant, viscosity, and color apparently changed.Such change in the molecular properties may usefully be utilized in many optic devices such as removable optic memories and within components of light switching device [4,5].Although many types of photochromic compounds have been reported so far, crystals that show photochromic reactions in the crystal state are very rare [6,7].1,3-Diazabicyclo[3.1.0]hex-3enederivatives posses exclusive photochromic properties.These compounds show photochromic behaviour even in a crystalline state.Depending on the particular structure of crystals, 1,3-diazabicyclo[3.1.0]hex-3-enederivative turns yellow, pink, blue, or green upon UV irradiation.Solid-state organic photochromic molecules have attracted much attention due to their potential applications in various optoelectronic devices such as optical memory, optical switch, electronic display, and information storage.Typical examples include N-salicylideneanilines, dinitrobenzylpyridines, diphenylmaleonitriles, triarylimidazole dimers, aziridines, diarylethenes, diarylperfluorocyclopentenes, and biindenylidenedione derivatives.Among them, 1,3-diazabicyclo[3.1.0]hex-3-enederivatives that are synthesized from aziridine moiety are a class of unique photochromic compounds, which generate bistable molecules and undergo photochromism in the crystalline state.This property allows us to consider bicyclic aziridines as possible candidates in the search for radiochromic materials [8][9][10][11][12].
Here, for first time we report the synthesis of new 1,3diazabicyclo[3.1.0]hex-3-enederivatives that possess quinoline and benzo[h]quinoline rings.

Experiments
2.1.General Procedure.All of the chemicals were purchased from Merch and were used without further purification.Melting points were measured on an Electrothermal 9100 apparatus and are uncorrected.All 1 H and 13 C NMR data were recorded in CDCl 3 using a Bruker Avance 500-MHZ spectrometer, the chemical shifts are reported in ppm (δ) using deuterated solvents as internal references.UV spectra were recorded on a Shimadzu UV-2100 spectrophotometer.Elemental analyses were performed with a Carlo-Erba International Journal of Photoenergy EA1110 CNNO-S analyzer.The IR spectra were recorded on a Shimadzu IR-470 spectrophotometer.

Results and Disscuton
Ketoaziridines 1 and 2 were prepared starting from corresponding chalcones, which were brominated at the double bond utilizing bromine in chloroform, followed by aziridination with solution of ammonia in ethanol at room temperature [8,10,12].Quinoline and benzo[h]quinoline aldehydes were prepared starting from acylation of aromatic amines in the presence of acetic anhydride and glacial acetic acid and refluxing for 2-3 h followed by formylation with POCl 3 and DMF for 16-18 h at 70-80 • C [13,14].The general preparation of photochromic compound 3-10 is depicted in Scheme 1.All of these compounds all except compound 10 showed photochromic behavior in the crystalline state.However the UV spectra for all of these synthesis compounds 3-10 in the solution of ethanol confirmed their photochromic properties.
These 1,3-diazabicyclo[3.1.0]hex-3-enesphotochromic compounds can undergo reversible photocyclization between their closed-ring isomer (c) and open-ring isomer (o) under irradiation using a light source of appropriate wavelength.Photoisomerization of these photochromic compounds via nitrile ylides is illustrated in Scheme 2.
All synthesized 1,3-diazabicyclo[3.1.0]hex-3-enes3A-10A undergoes reversible photochromic reactions in cyclohexane, ethanol, acetone, and ethyl acetate by irradiation with 254 nm UV-Vis light.However in the solvent phase undergo reversible photochromic reaction with a shorter half-life compared to the solid phase with more than 10000 turnovers.By interpreting the 1 H and 13 C NMR of several 1,3-diazabicyclo[3.1.0]hex-3-enesystems in CDCl 3 , we found that all of them except 3 and 8 undergo reversible photochromic reaction.However in the crystalline state and in the presence of sun light, all of them except 10 involve 2 + 2 ring opening process, and their color after ∼5-20 sec was changed, see Experiments.Both the c-isomer (colorless) and the photogenerated isomer (o-form, colored) are stable at the ambient condition and the coloration/decoloration cycles could be repeated without destruction of the crystals.By rising the duration of irradiation, the population of the o-form in the colored crystals increases resulting in deeper color of photochromic crystals.The changes of absorption for 3-10 in ethanol at 254 nm are shown in Table 1.
The changes of absorption for 3 and 4 in ethanol at 254 nm are shown in Figure 1.In general, the absorption of the closed-ring form appears at a shorter wavelength, while the absorption of the open-ring form takes place at a longer wavelength.When 3A was irradiated, three new absorption Scheme 2: Photochromic reactions of 3 and 4 before and after irradiation with UV light.

Conclusion
A facile and efficient protocol for the synthesis of 1,3diazabicyclo[3.1.0]hex-3-enephotochromic compounds with premade substituted quinoline and benzo[h]quinoline 6 International Journal of Photoenergy ring has been developed.This method is readily accessible to a variety of these intelligent compounds.These compounds show photochromic behavior in solid state and as well in the solution state.Although this system undergoes reversible photochromic reaction in the solvent phase, the major photochromic activity is practical in solid phase with more than 10000 turnovers.

Scheme 1 :
Scheme 1: Synthetic route for the preparation of compounds.

Table 1 :
The absorption changes for 3-10 in ethanol at 254 nm.Spectra and λ A max λ B max λ ex ε, and percent yield of 3-10.