Synthesis and Photophysical Properties of Novel Highly Stable Zero / Bis-Zero Methine Cyanine Dyes Based on N-Bridgehead Heterocycles

Cyanine dyes of zero/bis-zero methine incorporating imidazo(1,2-a)Pyridine (quinoline) or pyrazino(1,2-a)pyridine (quinoline) with stable C-N bond were synthesized using keto-oxime methylene C-link heterocyclic quaternary salts [1-phenyl-3-methyl-pyrazolino-4-keto-oxime-α-methylene-bis-pyridin-(quin oin)-1(4)-di-ium-iodide(ethiodide) salts and 1-phenyl-3-methyl-pyrazolino4-ketooxime-α-methylene-N-2-methyl-bis pyridin (quinoin)-1(4)-di-iumiodide(ethiodide) salts]. Such heterocyclic precursors and related dyes were identified by elemental and spectral analyses. The absorption spectra properties of such dyes were investigated in 95% Ethanol to attempt and throw some light on the influence of such new heterocyclic nuclei and to compare or evaluate spectral behaviors. The absorption spectra of dyes in different pure solvents were examined in the visible region showing solvatochromism and the colour changes of dyes with solvents having different polarities. This permits a selection of the optimal solvent (fractional solvent) when such dyes are applied as photosensitizers. The spectral behavior of some selected newly synthesized cyanine dyes is observed in mixed solvents of different polarities and progressively increasing quantities of one solvent over the other were studied and showed an increase in the absorbance of CT band with increasing proportion of that solvent. Evidence for hydrogen bond formation between the solute molecules and solvent molecules allows measurement of certain energies such as hydrogen bonding, orientation, and free energies.


Introduction
Special attention is given to the implementation, preparations, and applications of heterocyclic cyanine dyes to show the various aspects in order to satisfy the great demand in industrial, physiology, biochemistry and various biological fields.Cyanine dyes are colorant compounds used in staining of internal limiting membrane (ILM) [1] [2] [3], as fluorescent dyes in DNA detection [4] [5] [6] [7], optical sensors [8] [9], organic photoconductors [10], vulcanizing accelerator agents, photographic sensitizers [11], solar cell [12].This multi-property and numerous applications of cyanine dyes push the scientist to design and improve synthetic procedure of new cyanine dyes based on N-bridgehead heterocycles [13] [14], which are characterized by outstanding chemical stability, absorb at long wavelength [15] [16], high solubility and change their optical, thermal or electronic properties to meet the requirements for opto-electronic or biological products.
N-Bridgehead heterocyclic cyanine dyes are an important class of dyes which are characterized by the presence of nitrogen atom inside the ring.N-bridge head heterocyclic compounds used as precursors possess high site reactivity susceptible to be attacked by either Electrophile/Nucleophile in the substitution/addition reactions which give high stability nature for the dyes [17].These dyes have many vital general applications back to their higher stability which can be used as bioactive compounds such as the N-methyl-D aspartate antagonists [18].Furthermore, N-Bridgehead heterocyclic cyanine dyes have a wide range of potential applications in fluorescent compounds, DNA-binding dyes [19] and organic materials [20].According to their bioactivity and structural planarity, it is a reasonable idea to develop functional dyes based on these N-Bridgehead heterocycles.A variety of cyanine dyes incorporating different N-bridgehead heterocyclic moieties have been reported [21] [22] [23] [24].
In this paper, we designed and synthesized novel highly stable cyanine dyes.
Keto-oxime methylene C-link heterocyclic quaternary salts were used in the synthesis of N-bridge head heterocyclic incorporating imidazolo(1,2-a)pyridine(quinoline) or pyrazino(1,2-a)pyridine(quinoline) as main entities for zero and bis-zero methine cyanine dyes synthesis.The spectral, solvatochromic behavior and mixed solvent effect are described.

Physical and Chemical Properties Determinations
All melting points are uncorrected Elemental and spectral analysis was carried out at the microanalytical center (Cairo University).The IR (νKBr) spectra were determined with Perkin Elmer Infrared 127ß spectrophotometer (Cai-

Synthesis
Analytical data and molecular Properties for all the starting N-bridge head heterocycles and the target dyes involved in this study was obtained and summarized in (Table 1).
The reaction mixtures were filtrated from unreacted materials.The reaction mixture quenched by water and extracted by chloroform (3 × 50 ml).The combined organic layers were washed with water, dried over MgSO 4 and evaporated under reduced pressure.The product was purified via recrystallization from ethanol.

UV-Vis Spectra Absorption
UV-Vis spectra for all dyes in pure and mixed solvents were recorded at 25˚C in

Solvatochromic Studies
The organic solvents were used of spectroscopic grade which purified according to the recommended methods [25].The electronic absorption spectra of the studied dyes in different organic solvents were recorded within the wavelength (350 -700 nm) on 6405 UV/Visible recording spectrophotometers using 1 cm cell.
The stock solution of the dye was of the order 10 −3 M. Solutions of low molarities used in spectral measurements were obtained by accurate dilution.

Preparation of Working Dye Solutions
For studying the effect of pure solvents in the UV and visible range: An accurate volume of the stock solution (10 -3 M in ethanol) of the dyes were diluted to appropriate volume in order to obtain the required concentration.The spectra were recorded immediately after mixing in order to eliminate as much as possible the effect of time.A range of organic solvents was investigated, including water, dimethylformamide (DMF), ethanol, acetone, carbon tetrachloride, chloroform and benzene.Moreover, to study the spectral behavior in mixed solvents in the visible region: An accurate volume of stock solution (10 -3 M in ethanol) of the dyes were placed in 10 ml measuring flask containing the required volume of ethanol, then completed to the mark with the other solvent.

Solvatochromism
The color changes of cyanine dyes with solvents (solvatochromism) was previously discussed by [27] and extended [28] to correlate the effect of structure on molecular orbital energy levels.It is clear that the type of substituents and the solvent polarity change the electron densities of cyanine dyes.The visible absorption spectra of dyes 5b and 6e in the wavelength range 400 -700 nm have been studied in different organic solvents (H 2 O, DMF, EtOH, acetone, CCl 4 , CHCl 3 , & C 6 H 6 ) respectively [29] as shown in (Figure 1).This is constructed with the intention to illustrate the solvatochromic behavior of these dyes, (λ max and ε max ) values of the intramolecular charge transfer bands are given in (Table 2).These   to ethanol and its lower extinction coefficients were mainly ascribed to the ease of interactions of water molecules, through intermolecular hydrogen bonding, with the lone pair of electrons of the nitrogen atoms of the heterocyclic ring system, through intermolecular hydrogen bonding, which intern preclude the charge transfer from the heterocyclic ring system to the positively charged residue along the conjugated bridge.

Absorption Spectra in Mixed Solvent
The absorption spectra of dye (6e) in 1 × 10 −4 M DMF in the presence of different concentrations of benzene are shown in (Figure 2).
It was obvious that in presence of 12.97 M of DMF, the spectrum exhibits a band located at 500 nm.In the presence of 1.16 M of DMF, the band is shifted to 463 nm concomitant with a gradual blue shift.Also, an increase in band intensity at fixed wavelength (500 nm) is observed on increasing of C 6 H 6 concentration as depicted in (Figure 3(a)).The increase in absorbance as well as the gradual International Journal of Organic Chemistry

A
. I. Koraiem et al.DOI: 10.4236/ijoc.2018.83021286 International Journal of Organic Chemistry a 1 cm path length quartz cell on a Cary 3 Spectrophotometer.Ethanolic solution of 1 × 10 −5 M was prepared, and the absorbance was measured and the extinction coefficient was calculated in each case.
dyes are showed positive solvatochromism with increased solvent polarity, which depend on the structure and the type of dye.This indicates that the polar excited states of these cyanine dyes are stabilized by polarization interaction forces as the polarizability of the solvent is increased.This behaviour occurs as a result of electrostatic interactions of the distributed cationic charges with the dipoles of the solvated molecules which lead to formation of specific solvated forms of dyes.The absorption spectra of the dyes in ethanol are characterized by the presence of one or two essential bands which reflects the presence of intermolecular charge transfer.This intermolecular charge transfer had arisen from transferring the electron lone pair of the nitrogen atoms of the heterocyclic ring system towards the positively charged residue along the conjugated chain between both.The representing graphs disclosed that these electronic charge transfer bands exhibit a hypsochromic shifts in ethanol relative to DMF, CHCl 3 , and CCl 4 .This shift can be attributed to the following factors: The bathochromic shift occurred in DMF relative to ethanol is mainly a result of the increase in solvent polarity due to increasing the dielectric constant of the former.The hypsochromic shifts appeared in ethanol relative to CHCl 3 & CCl 4 is generated from the solute-solvent interaction through intermolecular hydrogen bonding between ethanol and the lone pair of electrons within the heterocyclic ring system.Otherwise, this decreases the mobility of the electron cloud over the conjugated pathway towards the positively charged center.It was worth mentioning that the intermolecular hydrogen bonding between CHCl 3 molecules and the lone pair of electrons of nitrogen atoms of the heterocyclic ring system is difficult due to the steric hindrance of the three bulk chlorines.Moreover, the solute solvent interactions in cases of CHCl 3 & CCl 4 generated a residual negative charge on the nitrogen atoms of the heterocyclic ring system which intern facilitated the electronic charge transfer to the positively charged center and this explain the bathochromic shifts in these solvents relative to ethanol.The unexpected hypsochromic shifts in the absorption spectral maxima in water relative

Figure 3 .
Figure 3. Relation between (a) Absorbance and molarity; (b) mole fraction (b) for dye 6e in DMF-C 6 H 6 mixed solvents; Relation between (c) absorbance and D of the medium; (d) Δν and 1 1 D D − + ; Relation between (e) the mole fraction of DMF and E; (f) the mole fraction and ΔE; (g) Relation between Koraiem et al.
DOI: 10.4236/ijoc.2018.83021284 International Journal of Organic Chemistry ro-University). 1 H-NMR spectra were recorded with a Bruker AMX-250 spectrometer (Cairo-University).Mass spectra were recorded on an HpMs 6988 spectrometer (Cairo University).The absorption spectra were recorded immediately after preparation of the solutions within the wavelength range (350 -700) on 6405 UV/Visible recording spectrophotometers, Faculty of Science, Aswan University.

Table 1 )
, IR, 1 H-NMR and Mass spectral data.Thus, IR (ν KBr cm −1 ) showed general absorption bands at 3426.89 cm −1 (OH of oxime), 2963.09cm −1 (heterocyclic quaternary salt), 1601.59cm −1 (C=C) conjugated, 1501.31ion peaks at m/z = 515, and base peak at m/z = 69 for compound 1, a molecular ion peaks at m/z = 719, and base peak at m/z = 77 for dye 2a, a molecular ion peaks at m/z = 362, and base peak at m/z = 77 for dye 3b, a molecular ion peaks at m/z = 574, and base peak at m/z = 69 for dye 4a, a molecular ion peaks at m/z = 435, and base peak at m/z = 77 for compound 5a and a molecular ion peaks at m/z = 635, and base peaks at m/z = 158 & 377 for 5b.