untitled European Journal of Chemistry 5 (3) (2014) 451‐456 European Journal of Chemistry ISSN 2153‐2249 (Print) / ISSN 2153‐2257 (Online)  2014 Eurjchem Publishing ‐ Printed in the USA http://dx.doi.org/10.5155/eurjchem.5.3.451‐456.1033 European Journal of Chemistry Journal homepage: www.eurjchem.com Effects of chemical structure, solvent and solution pH on the visible spectra of some new methine cyanine dyes Hassan Abazied Shindy *, Mohamed Abdalla El‐Maghraby and Fayez Mohamed Eissa Department of Chemistry, Faculty of Science, Aswan University, Aswan 81511, Egypt *Corresponding author at: Department of Chemistry, Faculty of Science, Aswan University, Aswan 81511, Egypt. Tel.: +2.097.2322725. Fax: +2.097.3480450. E‐mail address: hashindy2@hotmail.com (H.A. Shindy). ARTICLE INFORMATION ABSTRACT DOI: 10.5155/eurjchem.5.3.451‐456.1033 Received: 14 February 2014 Received in revised form: 29 March 2014 Accepted: 29 March 2014 Online: 30 September 2014 KEYWORDS Some new dimethine and bis‐dimethine, cyanine dyes derived from benzo[2,3‐b; 2`,3`‐ b`]bis‐pyrazolo[4,5‐b]‐l,4‐(oxa‐, thia‐, and pyra‐)‐zine‐6,12‐dione were synthesized. Effect of chemical structure on the electronic visible absorption spectra of all the synthesized cyanine dyes was investigated in 95% ethanol solution. Effects of solvent and/or solution pH on the electronic visible absorption spectra of some selected synthesized cyanine dyes were also examined in pure solvents having different polarities and/or in aqueous universal buffer solutions, respectively. Structural confirmations were carried out through elemental analysis, mass spectroscopy, IR and 1H NMR spectra. Synthesis Cyanine dyes Absorption spectra Acid‐base properties Dimethine cyanine dyes Solvent effect on spectra 1. Introduction Cyanine dyes [1‐10] have a very wide range of application, especially in the so‐called “high‐tech” fields, science and modern technology. They are used as luminescent covalent or non‐covalent probes in biological, medicinal and clinical research and assay. Well known is their utilization in optical information storage for fabrication of CD‐R and DVD‐R (optical recording disks), in preparation of security marks over banknotes and documents, as novel materials in non‐linear optics, as highly sensitive analytical reagents, as analytical sensors, in photodynamic therapy of cancer, for single molecule fluorescence detection, as photosensitizers in photographic processes [11‐13] and as photo polymerization initiators [14]. The most numerous and continuously expanding applications of cyanine dyes are in the field of biotechnology. Among those is the visualization of DNA by following different biological processes and quantification of DNA in solution. YOYO‐1 (intercalating dye) labeled complexes may be employed as qualitative DNA markers in intracellular delivery studies [15]. Taking in accounts and consideration the above vital and important benefits of cyanine dyes, the aim of this research paper is synthesis of some novel photosensitizers, solvato‐ chromic and halochromic cyanine dyes as new synthesis contribution and spectroscopic investigation in this field and/or to be used in any of the high‐tech fields of cyanine dyes, particularly as photographic sensitizers, acid‐base indicators and/or as probes for determining solvent polarity. In addition, because cyanine dyes have a multidisciplinary uses and applications in several fields and different areas of science, technology, engineering, pharmacology and medicine, this research paper may be very interesting to read by the many of the large heterogenous community groups of chemists, biologists, physicists, biotechnologists, pharmacologists and medical scientists. 2. Experimental 2.1. General Melting points were recorded on a Gallenkamp melting point apparatus and are uncorrected. Elemental analyses were carried out at the Microanalytical Center at Cairo University. Infrared spectra were measured with a 1650 FT‐IR instrument, and the 1H NMR spectra on an EM‐390 90 MHz NMR Spectrometer, and mass spectroscopy was recorded on GCMS‐ QP‐1000 EX Spectrometer (Cairo University). The organic solvents were of spectroscopic grades and the new dyes were investigated and purified using TLC chromatographic procedures. 452 Shindy et al. / European Journal of Chemistry 5 (3) (2014) 451‐456 Table 1. Characterization of the prepared compounds. Compound no Nature of products Molecular Formula (M.Wt., g) Analysis % Calculated (Found) Visible absorption spectra in 95% ethanol solution Color Yield, % M.p., C C H N  (nm)  (mol‐1 cm2) 2a Dark brown crystals 63 191‐193 C26H16N6O5 (492.12) 63.41 (63.43) 3.27 (3.83) 17.07 (17.04) ‐ 2b Dark brown crystals 43 186‐188 C26H16N6O3S2 (524.60) 59.53 (59.51) 3.07 (3.11) 16.02 (15.98) ‐ ‐ 2c Pale brown crystals 59 199‐201 C26H18N8O3 (490.50) 63.67 (63.65) 3.70 (3.67) 22.85 (22.87) ‐ ‐ 3a Reddish brown crystals 41 202‐204 C26H14N6O6 (506.43) 61.66 (61.63) 2.79 (2.82) 16.59 (16.62) ‐ ‐ 3b Brown crystals 38 187‐189 C26H14N6O4S2 (538.55) 57.99 (57.94) 2.62 (2.59) 15.60 (15.63) ‐ ‐ 3c Intense brown crystals 47 213‐215 C26H16N8O4 (504.46) 61.90 (61.91) 3.20 (3.22) 22.21 (22.24) ‐ ‐ 4a Reddish brown crystals 71 219‐221 C34H24N7O4I (721.09) 56.58 (56.56) 3.35 (3.31) 13.59 (13.63) 528.5 19700 4b Reddish violet crystals 65 227‐229 C38H28N7O4I (773.12) 58.98 (58.95) 3.65 (3.69) 12.68 (12.65) 393.0, 445, 552.0 25310, 23150, 20970 4c Reddish brown crystals 61 221‐223 C34H24N7O4I (721.09) 56.58 (56.54) 3.35 (3.37) 13.59 (13.60) 533.5 20100 4d Pale violet crystals 36 209‐211 C38H28N7O2S2I (805.08) 56.64 (56.66) 3.51 (3.53) 12.18 (12.13) 421.5, 556.0 27100, 23540 4e Reddish violet crystals 52 217‐219 C38H30N9O2I (771.16) 59.13 (59.17) 3.92 (3.97) 16.34 (16.37) 461.5, 561.0 26890, 25010 5a Dark brown crystals 68 220‐222 C34H22N7O5I (735.07) 55.50 (55.48) 3.02 (3.03) 13.33 (13.35) 524.5 16570 5b Reddish violet crystals 77 231‐233 C38H26N7O5I (787.10) 57.93 (57.95) 3.33 (3.35) 12.45 (12.46) 381.0, 434.5, 535.0 22350, 21150, 19810 5c Dark brown crystals 44 218‐220 C34H22N7O5I (735.07) 55.50 (55.53) 3.02 (3.00) 13.33 (13.31) 528.5 18970 5d Violet crystals 49 211‐213 C38H26N7O3S2I (819.06) 55.67 (55.63) 3.20 (3.18) 11.97 (12.01) 411.0, 551.0 25030, 21960 5e Dark violet crystals 39 228‐230 C38H28N9O3I (785.13) 58.08 (58.11) 3.59 (3.61) 16.05 (16.01) 459.0, 557.5 26380, 25030 6a Reddish violet crystals 60 217‐219 C42H34N8O4I2 (968.08) 52.06 (52.07) 3.54 (3.51) 11.57 (11.61) 543.0 24110 6b Intense violet crystals 61 234‐236 C50H42N8O4I2 (1072.14) 55.96 (55.94) 3.95 (3.97) 10.45 (10.42) 411.0, 491.0, 559.5 27410, 26130, 25810 6c Violet crystals 63 215‐217 C42H34N8O4I2 (968.08) 52.06 (52.10) 3.54 (3.52) 11.57 (11.56) 551.0 27110 6d Intense violet crystals 59 224‐226 C50H42N8O2S2I2 (1104.10) 54.34 (54.31) 3.83 (3.79) 10.15 (10.10) 420.0, 571.0 29830, 28630 6e Intense violet crystals 53 237‐239 C50H44N10O2I2 (1070.14) 56.07 (56.09) 4.14 (4.11) 13.08 (13.11) 501.0, 589.5 29070, 28790 2.2. Synthesis 2.2.1. Synthesis of 4‐formyl‐10‐methyl‐2,8‐diphenyl‐5,ll‐ dihydro‐benzo[2,3‐b; 2`,3`‐b`]bis‐pyrazolo[4,5‐b]‐l,4‐(oxa‐, thia‐, and pyra‐)‐zine‐6,12‐dione (2a‐c) A pure crystallized sample of compounds 1a‐c [16] was heated under reflux in equimolar ratios (0.01 mol) with selenium dioxide in dioxane (50 mL) for 16 hours. The reaction mixtures were filtered on hot to remove selenium metal and the filtrate was cooled, and then poured in ice water mixture. The precipitated products were filtered, dried, collected, and recrystallized from ethanol. The data are shown in Table 1. 2.2.2. Synthesis of 4,10‐diformyl‐2,8‐diphenyl‐5,ll‐dihydro‐ benzo[2,3‐b;2`,3`‐b] bis pyrazolo[4,5‐b]‐l,4‐(oxa‐, thia‐, and pyra‐)‐zine‐6,12‐dione (3a‐c) Two different routes are employed to prepare these compounds. Route 1: This route is carried out in a way that has been described earlier [17]. Route 2: An equimolar ratios (0.01 mol) of both the 4‐mono formyl derivatives (2a‐c) and selenium dioxide were heated under reflux in dioxane (50 mL) for 16 hours. The reaction mixtures were filtered while hot to remove selenium metal, cooled, and poured in ice‐water mixture. The precipitated products were filtered, dried, and recrystallized from ethanol to give the same diformyl compounds 3a‐c obtained by Route 1 characterized by melting points, mixed melting points, same IR and 1H NMR spectral data, Table 1 and 2. 2.2.3. Synthesis of 10‐methyl‐2,8‐diphenyl‐5,ll‐dihydro‐ benzo[2,3‐b; 2`,3`‐b`] bis pyrazolo[4,5‐b]‐l,4‐(oxa‐, thia‐, and pyra‐)‐zine‐6,12‐dione‐4[2(4)] dimethine cyanine dyes (4a‐ e) Reaction mixtures of equimolar ratios (0.01) of compounds 2a‐c and ethyl iodide quaternary salts of 2‐picoline, quinaldine, 4‐picoline were refluxed in ethanol (30 mL) containing few drops (3‐5) of piperidine for 8‐10 hours. During the refluxed time the colors changed from deep brown to intense violet at the end of refluxing. The reaction mixtures were filtered while hot to remove unreacted materials, concentrated by evapo‐ ration, cooled in ice bath, neutralized with glacial acetic acid and precipitated by adding cold water. The precipitates were collected, dried, and recrystallized from ethanol. The relevant data are given in Table 1 and 2. 2.2.4. Synthesis of 10‐formyl‐2,8‐diphenyl‐5,ll‐dihydro‐ benzo[2,3‐b; 2`,3`‐b`] bis pyrazolo[4,5‐b]‐l,4‐(oxa‐, thia‐, and pyra‐)‐zine‐6,12‐dione‐4[2(4)] dimethine cyanine dyes (5a‐ e) N‐Ethyl‐(2‐picolinium, quinaldinium, 4‐picolinium) iodide quaternary salts were heated under reflux in equimolar ratios (0.01 mol) with compounds 3a‐c in ethanol (30 mL) containing piperidine (3‐5 drops) for 8 hours. Shindy et al. / European Journal of Chemistry 5 (3) (2014) 451‐456 453 Table 2. IR and 1H NMR spectral data of the prepared compounds. Compound no IR spectrum (KBr), cm‐1 1H NMR spectrum (DMSO‐d6, , ppm) and Ms data 2a 674‐718 (mono substituted phenyl), 1041, 1172 (C‐O‐C cyclic), 1284 (C‐N), 1441 (C=N), 1585 (C=C), 1677(C=O quinone), 1726 (formyl group), 3424 (NH cyclic) 3.85 (s, 3H., CH3 at C(10)), 5.85 (s, 2H, 2NH of oxazine rings), 6.80‐8.00 (m, 10H, aromatic protons), 9.60 (s, 1H, ‐CHO group at C(4)) (m/z 492) 3a 626‐675 (mono substituted phenyl), 1026, 1176 (C‐O‐C cyclic), 1326 (C‐N), 1481 (ON), 1582(C=C), 1682(C=O quinone), 1737 (formyl group), 3459 (NH cyclic) 5.82 (s. 2H. 2NH of oxazinc rings), 7.00‐7.82 (m. 10H, aromatic protons), 9.90 (s. 2H, 2‐(CHO at C(4), C(10)) (m/z 506 ) 4b 658, 742 (mono substituted phenyl), 885, 939 (o. di substituted phenyl), 1068, 1178 (C‐O‐C cyclic), 1339 (C‐N), 1472 (C=N), 1605 (C=C), 1680 (C=O quinone), 2919 (quaternary N), 3427 (NH cyclic) 1.30‐1.70 (t, 3H, CH3, of ethyl at N‐quinoline), 3.5 (s, 3H, CH3, at C(10)), 3.70‐4.10 (q, 2H, CH2 of ethyl at N‐quinoline), 5.81 (s, 2H, 2NH of oxazine rings), 6.58‐8.61 (m, 18H, aromatic, heterocyclic & ‐CH=) 5b 684, 743 (mono substituted phenyl), 827, 911 (o. di substituted phenyl), 1060, 1149 (C‐O‐C cyclic), 1273 (C‐N), 1492 (C=N), 1605 (C=C), 1680 (C=O quinone), 1739 (formyl group), 2900 (quaternary N), 3380 (NH cyclic) 1.25‐1.55 (t, 3H, CH3, of ethyl at N‐quinoline), 3.55‐4.1 (q. 2H, CH2 of ethyl at N‐quinoline), 5.92 (s, 2H, 2NH of oxazine rings), 6.72‐8.65 (m, 18H, aromatic, heterocyclic &‐CH=), 9.51 (s, 1H, ‐CHO at C(10)) 6b 693, 767 (mono substituted phenyl), 886, 955 (o. di substituted phenyl), 1068, 1186 (C‐O‐C cyclic), 1337 (C‐N), 1477(C=N), 1596 (C=C), 1695 (C=O quinone), 2929 (quaternary N), 3437 (NH cyclic) 1.27‐1.71 (t, 6H, 2CH3 of ethyl at N‐quinoline), 3.92‐4.27 (q, 4H, 2CH2 of ethyl at N‐quinoline), 5.96 (s, 2H, 2NH of oxazine rings), 6.65‐8.95 (m. 26H, aromatic, heterocyclic &‐CH=) The reaction mixtures attained dark brown color which turns pale violet at the end of the reflux. It were filtered off on hot, concentrated, neutralized with glacial acetic acid and precipitated by adding ice‐water mixture. The separated cyanine dyes were filtered, washed with water, dried, and recrystallized from ethanol. The results are listed in Table 1 and 2. 2.2.5. Synthesis of 2,8‐diphenyl‐5,ll‐dihydro‐benzo[2,3‐b; 2`,3`‐b`] bis pyrazolo‐ [4,5‐b]‐l,4‐(oxa‐, thia‐, and pyra‐)zine‐ 6,12‐dione‐4,10[2(4)] bis‐dimethine cyanine dyes (6a‐e) Two different routes are employed to prepare these cyanine dyes. Route 1: Piperidine (3‐5 drops) was added to a mixture of ethanolic solution (50 mL) of compounds 3a‐c (0.01 mol) and 1‐ethyl(2‐picolinium‐, ‐quinaldinium‐, 4‐picolinium‐) iodide quaternary salts (0.02 mol). The reaction mixtures were heated under reflux for 10 hours and changed their colors from brown to fluorescent violet colors at the end of the reflux. It was filtered off while hot, concentrated to half its volume, cooled, poured in ice‐water mixture, and neutralized with acetic acid. The precipitated dyes (6a‐e) were filtered, washed with water, dried and recrystallized from ethanol. The data are given in Table 1 and 2. Route 2: A mixture of equimolar ratios (0.01 mol) of the dimethine cyanine dyes (5a‐e) and iodoethane quaternary salts of 2‐methyl pyridine, 2‐methyl quinoline, 4‐methyl pyridine were dissolved in ethanol (30 mL), to which piperidine (3‐5 drops) was added. The reaction mixtures were refluxed for 5 hours exhibiting permanent intense fluorescent violet colour at the end of refluxing. It was filtered while hot, concentrated, cooled, precipitated by adding cold water, and neutralized by acetic acid. The precipitates were collected, dried and recrystallized from ethanol to give the same dyes (6a‐e) obtained by Route 1, characterized by melting points, mixed melting points, same IR and 1H NMR spectral data, Table 1 and 2. 2.3. Visible absorption spectra The electronic visible absorption spectra for all the prepared cyanine dyes were examined in 95% ethanol solution. Also, the spectra for some selected synthesized cyanine dyes were investigated in pure solvents of spectroscopic grade [18] and/or in aqueous universal buffer solutions of varying pH values. All the measurements recorded on a Shimadzu UV/Visible 160‐A Spectrophotometer. The stock solution was about 1×10‐3, the lower molarities were obtained by accurate dilution. 3. Results and discussion 3.1. Synthesis Selenium dioxide oxidation of 4,10‐dimethyl‐2,8‐diphenyl‐ 5,11‐dihydro‐benzo[2,3‐b;2`,3`‐b`]bis‐pyrazolo‐[4,5‐b]‐l,4‐(oxa, thia‐, and pyra‐)‐zine‐6,12‐dione (1a‐c) in 1:1 molar ratios resulted the mono formyl (2a‐c) compounds, Scheme 1, Table 1. Other equimolar selenium dioxide oxidation of the mono formyl compounds 2a‐c in dioxane as organic solvent gave the diformyl compounds (3a‐c), Scheme 1, Table 1. Interaction of equimolar ratios of the 4‐formyl compounds 2a‐c and 1‐ethyl (2‐picolinium, quinaldinium, 4‐picolinium) iodide quaternary salts in ethanol containing piperidine gave 4[2(4)] mono cationic dimethine cyanine dyes (4a‐e). Also interaction of the 4,10‐diformyl compounds 3a‐c with equimolar and/or bimolar ratios of the iodoethane quaternary salts of 2‐picoline, quinaldine and/or 4‐picoline in ethanol containing piperidine yielded 4[2(4)]dimethine and/or 4,10 [2(4)] bis‐dimethine cyanine dyes (5a‐e) and/or (6a‐e), respectively, Scheme 1, Table 1. Chemical confirmations take place via route 2, through reaction of the 4[2(4)] dimethine cyanine dyes (5a‐e) and equimolar ratios of N‐ethyl‐(2‐picolinium, quinaldinium, and/or 4‐picolinium) iodide quaternary salts in ethanol catalyzed by piperidine to achieve the same 4,10[2(4)] bis‐ dimethine cyanine dyes (6a‐e), characterized by same melting points, mixed melting points, same IR and 1H NMR spectra, Scheme 1, Table 1 and 2. The structures of the prepared compounds were identified by elemental analysis, Table 1, mass spectroscopy, IR [19] and 1H NMR [20] spectra, Table 2. 3.2. Effect of chemical structure on the visible spectra Effect of chemical structure on the visible spectra of all the prepared cyanine dyes were investigated by measuring their electronic visible absorption spectra in 95% ethanol solution, Table 1. The electronic visible absorption spectra of the dimethine cyanine dyes (4a‐e), (5a‐e), and the bis‐dimethine cyanine dyes (6a‐e) in 95% ethanol solution showed absorption spectra bands in the visible regions, (393.0‐561.0 nm), (381.0‐557.5 nm), and (411.0‐589.5 nm), respectively. These bands undergoes to give bathochromic and/or hypsochromic shifts depending largely on the nature of the heterocyclic quaternary salts, their linkage position and the type of the benz‐bis biheterocyclic system present, Table 1. Thus, the absorption spectrum of the dimethine cyanine dyes 4a, 5a, and 6a (X = O, A = 1‐ethyl pyridinium‐2‐yl salt) showed bands at  528.5, 524.5, and 543.0 nm, respectively. 454 Shindy et al. / European Journal of Chemistry 5 (3) (2014) 451‐456 Scheme 1 Substituting A = 1‐ethyl quinolinium‐2‐yl salt to give dyes 4b, 5b, and 6b causes a bathochromic shifts of 23.5, 10.5, and 16.5 nm with intensification of the absorption bands in addition to the appearance of new absorption bands at (393.0, 445.0 nm), (381.0, 434.5 nm) and (411.0, 491.0 nm) for the latter dyes, respectively. This is related to extra conjugation in the quinoline moiety in the latter dyes compared with the pyridine moiety in the former dyes, Scheme 1, Table 1. Also, changing the linkage position in the pyridinium residue from 2‐ yl salts in dyes 4a, 5a, and 6a, to 4‐yl salts in dyes 4c, 5c, and 6c, resulted in a bathochromic shift of 5, 4, and 8 nm, respectively, accompanied by increasing the intensities of the absorption bands. This is due to increasing the ‐delocalization conjugation in the latter 4‐yl salt dyes compared with the former 2‐yl salts, Scheme 1, Table 1. Additionally, the dimethine and bis‐dimethine cyanine dyes 4b, 5b, and 6b incurporating pyrazolo‐oxazine nucleus showed hypso‐ chromically shifted bands by (4 and 9 nm); (16.0 and 22.5 nm); and (11.5 and 30.0 nm) if compared by its analogous, dyes, (4d, 4e); (5d, 5e); and (6d, 6e) which contain pyrazolo‐thiazine and pyrazolo‐pyrazine nuclei, respectively. This is due to increasing the electron attracting character of the oxazine ring in dyes 4b, 5b, and 6b compared with thiazine and pyrazine rings in dyes (4d, 4e), (5d, 5e), and (6d, 6e), respectively. This is leads to decreasing the electronic charge transfer pathways to the heterocyclic quaternary nitrogen atom of quinolinium salt residue, and consequently a hypsochromic shift is resulted in oxazine dyes. On the other side it was observed that substituting the 10‐methyl group in dyes (4a‐e) by 10‐formyl group to give dyes (5a‐e), made hypsochromic shifts in addition to decreasing the intensities of the absorption spectra bands. This could be attributed to the electron donating character of methyl group in dyes (4a‐e) and electron attracting character of the formyl group in dyes (5a‐e), Scheme 1, Table 1. Moreover, it is noticed that the number of electronic charge transfer pathways have a remarkable effect on the spectral behavior of these dyes, so, the bis‐dimethine cyanine dyes (6a‐e) showed absorption spectra bands which are bathochromically shifted if compared with its analogous dimethine cyanine dyes (5a‐e) and/or (4a‐e) with inten‐ sification of the absorption spectra bands, Table 1. This is can be explained by the presence of two electronic charge transfer pathways in the bis‐dimethine cyanine dyes (6a‐e) in correspondence to one electronic charge transfer pathways in the dimethine cyanine dyes (5a‐e) and/or (4a‐e), Scheme 2 (A), (B), Table 1. 3.3. Effect of solvent on the visible spectra Effect of solvent in the visible spectra of the prepared bis‐ dimethine cyanine dye 6b was investigated by measuring its electronic visible absorption spectra in pure solvents having different polarities, Table 3. The electronic visible absorption spectra of the bis‐ dimethine cyanine dye 6b in pure solvents of different dielectric constant via water (78.54), DMF (36.70), ethanol (24.3), chloroform (4.806), carbontetrachloride (2.238) and dioxane (2.209) [21] are recorded. The λ (wavelengths) and ε (molar extinction coefficients) values of the absorption bands due to different electronic transitions within the solute molecule in these solvents are represented in Table 3. From Table 3, its clearly that, the electronic visible absorption spectra of the dye 6b in ethanolic medium are characterized by the presence of three essential absorption bands. These bands can be assigned to intramolecular charge transfer transitions [22]. These charge transfer is due to transfer of lone pair of electrons from the N‐Ph pyrazole nitrogen atom to the positively charged quaternary nitrogen atom of the quinolinium salt, Scheme 2, (B). The data given in Table 3 show that the charge transfer band exhibits a hypsochromic shift in ethanol relative to DMF, dioxane, chloroform and carbontetrachloride. This effect may be attributed to the following factors: (a) The bathochromic shift in DMF relative to ethanol is a result of the increase in solvent polarity due to the increasing of dielectric constant of DMF relative to ethanol. (b) The hypsochromic shift occur in ethanol relative to dioxane, chloroform and carbontetracloride is a result of the solute solvent interaction through inter‐ molecular hydrogen bond formation between ethanol and the lone pair electrons of N‐Ph pyrazole nitrogen atom, Scheme 3 (A). This decreases slightly the electron density on the N‐Ph pyrazole nitrogen atom, and consequently decreases to some extent the mobility of the attached π‐electrons over the conjugated pathway to the positively charged quaternary nitrogen atom of the quinolinium quaternary salt. Shindy et al. / European Journal of Chemistry 5 (3) (2014) 451‐456 455 Table 3. Electronic visible absorption spectra of the bis‐dimethine cyanine dye (6b) in pure solvents. Solvent H2O EtOH DMF CHCl3 CCl4 Dioxane Dye no  *  **  *  **  *  **  *  **  *  **  *  ** (6b) 484 20220 411 27410 503 26680 507 19200 504 18070 453 19160 551 19220 491 26130 579 28830 569 16910 576 15160 581 14110 ‐ ‐ 559.5 25810 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ * Unit: nm. ** Unit: mol‐1 cm2. Scheme 2 Also, from the data given in Table 3, it is observed that occurrence of unexpected hypsochromic shifts in water relative to ethanol and the other solvents as well as decreasing the number and intensity of the absorption bands. This can be mainly ascribed to the possible interaction of water molecules with the lone pair electrons of N‐Ph pyrazole nitrogen atom, Scheme 3 (A). This makes difficult the transfer of electronic charge from the N‐Ph pyrazole nitrogen atom to the quaternary heterocyclic residue of the quinolinium salt, and consequently there is observed a hpsochromic shift in water relative to ethanol and the other solvents. 3.4. Effect of solution pH on the visible spectra Effect of solution pH on the visible spectra of the synthesized bis‐dimethine cyanine dye 6b was investigated by measuring its spectra in aqueous universal buffer solutions. The solutions of the bis‐dimethine cyanine dye 6b have a permanent cationic charge in basic media that then discharged on acidification. This prompted and encouraged us to study its spectral behavior in different buffer solutions in order to select a suitable pH for use of this dye as photosensitizers. The acid dissociation or protonation constant of this dye has been determined. The effect of the compound as photosensitizers increases when they are present in the ionic form, which has a higher planarity [23], and therefore more conjugation. The electronic absorption spectra of the bis‐dimethine cyanine dye 6b in aqueous universal buffer solutions of varying pH values (1.87, 2.51, 3.92, 5.06, 6.89, 8.13, 9.56 and 11.41) showed bathochromic shifts with intensification of their absorption bands at high pH (alkaline media) and hypsochromic shifts with quenching the intensity of the bands at low pH media (acidic media). So, the mentioned dye which has lone pair of electrons on the N‐Ph pyrazole nitrogen atom undergoes to protonation in low pH (acidic media). This leads to a criterion of positive charge on N‐Ph pyrazole nitrogen atom, and consequently the electronic charge transfer pathways to the quaternary heterocyclic quinolinium salt will be difficult resulting a hypsochromc shift for the absorption band, Scheme 3, (B). On increasing the pH of the media, the absorption bands are intensified and bathochromically shifted due to deprotonation of the N‐Ph pyrazole nitrogen atom, and consequently the electronic charge transfer pathways to the quaternary heterocylic quinolinium salt will be easier and facilitated, Scheme 3 (B). Several methods have developed for spectrophotometric determination of the dissociation constants of weak acids. The variation of absorbance with pH can be utilized. On plotting the absorbance at fixed λ vs. pH, S‐shaped curves are obtained, Table 4. An all of the S‐shaped curves obtained, the horizontal portion to the left corresponds to the acidic form of the indicator, while the upper portion to the right corresponds to the basic form, since the pka is defined as the pH value for which one half of the indicator is in the basic form and the other half in the acidic form. This point is determined by intersection of the curve with a horizontal line midway between the left and right segments [24]. The acid dissociation or protonation constants values of the dye 6b are given in Table 4. 456 Shindy et al. / European Journal of Chemistry 5 (3) (2014) 451‐456 Table 4. The variation of absorbance ( = 550 nm) with pH at settled  for the cyanine dye (6b) in different buffer solution *. pH 1.87 2.51 3.92 5.06 6.89 8.13 9.56 11.41 Absorbance at settled wavelength 1.376 1.409 1.657 1.900 1.957 2.188 2.351 2.387 * pKa= 4.380 and 8.235. N N N O O N N N H PhO O H Ph N (6b) N I I H O X HO X X = C2H5 (in ethanol); H (in water) N N N O O N N N H PhO O H Ph N (6b) N I N N N O O N N N H PhO O H Ph N N I I H H I (High pH) OH H (Low pH) (A) (B) Scheme 3 4. Conclusion From the results discussed in this study we can conclude that, theses cyanine dyes can be used as (i) Photographic sensitizers for silver halide emulsions in photographic industry due to their spectral and/or photosensitization properties, (ii) Indicators for solvent polarity due to their solvatochromic properties, (iii) Acid‐base indicators in analytical chemistry due to their halochromic properties. Acknowledgements We are thankful to the Chemistry Department, Faculty of Science, Aswan University, Aswan, Egypt for supporting this work. References [1]. Kovalska, V. B.; Volkova, K. D.; Manaev, A. V.; Losytskky, M. Y.; Okhrimenko, I. N.; Traven, V. T.; Yarmoluk, S. M. Dyes Pigments 2010, 34, 159‐164. [2]. Kabatc, J.; Paczkowski, J. Dyes Pigments 2010, 86, 133‐142. [3]. Cao, D.; Liu, Z.; Zhang, G.; Cao, F.; Chen, H.; Li, G. Dyes Pigments 2008, 76, 118‐124. [4]. Gerasov, A. O.; Shandura, M. B.; Kovtun, Y. B. Dyes Pigments 2008, 79, 252‐258. [5]. Yagupolski, L. 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