Synthesis and Study of Novel Coumarin Derivatives Potentially Utilizable as Memory Media

Novel coumarin derivatives, 2-oxo-2H-chromenecarbaldehyde hydrazones were prepared by reaction of substituted 2-oxo-2H-chromenecarbaldehydes with N-aminoimides in ethanol in the presence of 4-toluenesulfonic acid as catalyst. The photochromic and thermochromic properties of the prepared compounds were investigated.


Introduction
Information technology has revolutionized daily life and the continuosly increasing amount of data to be stored and manipulated has strongly stimulated the search for switching and memory elements as tiny as a single molecule. Molecular switches can be converted from one state to another by an external stimulus such as light, temperature, electricity or a chemical reaction. The operation of any optically controlled device is critically dependent upon the efficiency of light absorption by the input chromophore, and its ability to undergo electron transfer. Three characteristics make an excellent input chromophore: a large extinction coefficient, long-lived excited state, and a high fluorescence quantum yield. Photochromic compounds, where the reversible switching process is based on photochemically induced interconversions, are particularly attractive. Photochemical switching of mechanical devices [1], catalysts [2], transport systems [3], sensors [4] and surface properties of materials [5] are only a OPEN ACCESS few applications that can be envisioned. Another important advantage of this type of systems is that light can be used for writing and processing information and, comparable to glass fiber technology, this would offer a highly increased speed compared to electronic addressing and switching.
Chromenes, especially 2-oxo-2H-chromenes (coumarins), have been extensively studied due to their commercial applications in several fields. These compounds have outstanding optical properties, including an extended spectral range, high quantum yield, superior photostability and good solubility in common solvents. Typical feature of chromene derivatives is that photophysical and spectroscopic properties can be readily modified by introduction of substituents in the chromene ring, giving themselves more flexibility to fit to various applications [6][7][8]. Coumarin derivatives are widely used as laser dyes [9], optical brighteners [10] and fluorescence markers [11].
The aim of this work was to synthesize new coumarin derivatives with C=N double bonds, which can potentially be used as a memory media. E-Z isomerization around C=N double bond can serve as a process on which a molecular switch is based [12]. The conversion from one isomer to the other can be initiated by light or temperature [13,14]. Besides E-Z isomerization around the C=N double bond, the possibility of conformer and tautomer formation should be considered, depending on the structure of adjacent fragments. Similar systems, 4-oxo-4H-chromene (chromone) derivatives with C=N bonds in position 3, were already synthesized [12].
Alcohols were found as the most suitable solvents for the synthesis. The isolation of products is very simple because they precipitate from a refluxing reaction mixture. Hydrazones 4 or 5 are stable crystalline products and can be easily purified by recrystallization, resp. trituration from alcohol.

Photochromism and thermochromism
2-Oxo-2H-chromene derivatives, depending on their structure, absorb light in a spectral range which is very suitable and attractive because readily available light sources can be used to store, resp. delete information. The photochromic (solvatochromic and fluorosolvatochromic) and thermochromic properties of synthesized compounds 4, 5 were studied using UV-VIS and fluorescence spectroscopy. All studied compounds absorb light in the range λ = 200-520 nm. Their absorption maxima are bathochromically shifted (positive solvatochromism) few nanometers with increasing polarity of solvent (e.g. for 4c λ max-MeOH = 444 nm, λ max-PhMe = 434 nm, Figure 1). This absorption corresponds to π→π * band. Introduction of electron donating substituent into position 7 of the 2-oxo-2H-chromene fragment induces a large shift of absorption maximum (e.g. for 4c λ max-MeOH = 444 nm, for 4a λ max-MeOH = 354 nm, Figure 1).  Intensity of fluorescence emission of prepared compounds derived from unsubstituted aldehyde 1a in methanol ( Figure 2) is several-fold lower than for compounds derived from 7-N,N-dimethylamino-2-oxo-2H-chromene-3-carbaldehyde ( Figure 3). 2-Oxo-2H-chromene-3-carbaldehyde hydrazones emit light with maximum of emission in the range of wavelenghts 420-460 nm. The maximum of the fluorescence emission is bathochromically shifted with increasing electron donating ability of substituent bound in position 7 of 2-oxo-2H-chromene fragment as well as with increasing solvent polarity. This shift confirms that the excited state of these compounds is more polar than their ground state. The dipole moment of the molecule increases after excitation, particularly of derivatives with dimethylamino substituent. Structure of molecule influences charge separation. Such "bipolar" structures ( Figure 4) are better stabilized by more polar solvents.
Irradiation of studied derivatives at λ ≅ 310 nm in methanol or toluene at room temperature induces hypsochromic shift of absorption maximum as well as decrease of intensity of long-wave absorption band. This change of long-wave band was assigned to E-Z isomerization around C=N bond ( Figure 5).  Isomerization around the C=N bond is a reversible process. Retro-Z-E isomerization occurs photochemically with visible light or thermally in polar solvents ( Figure 6). Thermal back isomerization takes place in non-polar solvents (Figure 7).  When the compounds are dissolved, an increase of absorbance of the long-wave band is observed after standing a few minutes at room temperature. This change is probably due to reaching equilibrium between isomers, eventually conformers. This equilibrium is reached faster in polar than in non-polar media. Heating of solutions also causes fast reaching of equilibria between geometrical isomers. Irradiation of samples with UV-light induces hypsochromic shift of absorption maximum (the less stable isomer is formed in solution). Irradiation of this mixture with visible light causes bathochromic shift of absorption maximum. This process was repeated several times. The evidence for photochromic and thermochromic behaviour of studied compounds is, for examples of compound 4d, also decrease of intensity of long-wave band (at λ = 450 nm) simultaneously with increase of intensity of absorption band at λ= 330 nm at irradiation of sample with UV-light followed by heating ( Figure 8).

General
Melting points (uncorrected) were measured on a Kofler hot stage. The 1 H-NMR and 13 C-NMR spectra were recorded at 300 MHz and at 75 MHz, respectively, on a Varian Gemini 200 spectrometer in CDCl 3 or DMSO-d 6 with tetramethylsilane as internal standard. Elemental analyses were performed on a Carlo Erba Strumentacione 1106 apparatus. Chemicals and solvents were purchased from the major chemical suppliers as highest purity grade. All solvents were dried by standard methods and distilled prior to use. Reactions with moisture-sensitive chemicals were performed under argon in an oven-dried flask. N-Aminobenz[de]isoquinoline-1,3-dione (3b) [15] and 2-oxo-2H-chromene-3carbaldehyde (1) [16] were prepared according to literature procedures. For column chromatography Merck silica gel 60, 230-400 mesh was used with the indicated eluents.

General procedure for the preparation of 2-oxo-2H-chromene-3-carbadehyde hydrazones 4
Aldehyde 1 was dissolved in 10 mL of hot absolute ethanol. N-Aminoimide 3 was added to the solution and after its dissolution a crystal of 4-toluenesulfonic acid was added to the reaction mixture. The mixture was refluxed for 15 minutes. After cooling, the precipitate was filtered off, washed with ethanol, dried and recrystallized from ethanol.

General procedure for the preparation of 2-oxo-2H-chromene-8-carbadehyde hydrazones 5
Aldehyde 2 was dissolved in hot absolute ethanol (10 mL). N-Aminoimide 3 was added to the solution and after its dissolution a crystal of 4-toluenesulfonic acid was added to the reaction mixture. The mixture was refluxed for 15 minutes. After cooling, the solvent was partially evaporated, the precipitate was filtered off, washed with ethanol, dried and recrystallized from ethanol.

Photochromism and thermochromism
UV-VIS spectra were recorded on Hewlett-Packard 'Diode Array 8254' spectrometer. Fluorescence spectra were taken in metanol on a Hitachi F2000 spectrometer (S 1 , S 2 = 10 nm). All measurements were performed in a 1-cm thick absorption cell, resp. fluorescence cell, at room temperature. Photolysis experiments were performed in a quartz spectrophotometric cell using the filtered irradiation (λ ≅ 310 nm) from an UVP, Inc. Chromato VUE transilluminator, Model TM-15 or using unfiltered light from an Osram halogen lamp (150 W) in degassed methanol or toluene under nitrogen at room temperature.

Conclusions
2-Oxo-2H-chromenecarbaldehydes 1 or 2 react with N-aminoimides 3 to give 2-oxo-2Hchromenecarbaldehyde hydrazones 4 or 5 under mild conditions. It was found that E-Z isomerisation around the C=N bond takes place with ultraviolet light. This isomerization is a reversibile process. The back-Z-E isomerization occurs photochemically with the visible light in polar and in non-polar solvents and thermally only in non-polar solvents.