Synthesis and absorption spectra of new polymethine cyanine dyes

Dyes and Pigments 52 (2002) 129–136 www.elsevier.com/locate/dyepig Synthesis and absorption spectra of new polymethine cyanine dyes Reda Mahmoud Abd El-Aal Chemistry Department, Aswan Faculty of Science, South Valley University, Aswan 81528, Egypt Received 24 July 2001; received in revised form 27 September 2001; accepted 5 November 2001 Abstract The reaction of a ratio of 1 mol of 2-chloro-1-formyl crotonic ester 1 with 2 mol of an appropriate 2(4)-heterocyclic quaternary salt afforded the novel pentamehine cyanine dyes 2a–2c. Condensation reaction of compound 1 with aromatic amines followed by reaction with 2(4)-heterocyclic quaternary salt resulted in aza-tetramethine cyanine dyes 4a– 4e. Condensation of compound 1 with active methylene compounds and acetophenone derivatives, followed by reaction with 2 mol of 2-methylquinoline methyliodide afforded the corresponding heptamethine cyanine dyes 6a–6f. The structures of the novel compounds were established by elemental analysis, visible/near-infra-red (near-IR) absorption, IR, 1H NMR spectroscopy and mass spectra. Absorption of electromagnetic radiation of some selected new polymethine dyes was studied. The relationship between the constitution and properties of these dyes has been studied. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polymethine; Synthesis; Characterization; Optical recording material; Optical disk and electromagnetic radiation 1. Introduction Cyanine dyes are used as the spectral sensitizer in photographic emulsion [1], optical recording materials [2], laser technologies [3], as well as potential sensitizer for photodynamic therapy [4]. Such studies have motivated a large amount of scientific work in the past three decades. Di- and tri-carbocyanine dyes with long polymethine chains are attractive mainly due to their ability to absorb light in the red and near-infra-red (near-IR) region [5]. Near-IR absorbing functional dyes have attracted the attention of researchers for E-mail address: [email protected] many years as the use of laser diodes has grown. Development of commercially available laser diodes with oscillation wavelengths below 700 nm is technologically very difficult, but a breakthrough in the reduction of the wavelength of the laser diode has now been reported [6]. A 30-mw, 680-nm red diode laser for data recording and digital high-definition picture storage has been demonstrated using a rewritable magnetooptic (MO) disc [7]. Development of new optical disk drivers will be a great incentive with regard to storage media. In addition to having high extinction coefficients, carbocyanines exhibit excellent reflection properties (up to 40% at 800 nm) and thus yield materials with good stability in proportional to number of methine group (S/N ratio) 0143-7208/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0143-7208(01)00088-2 130 R.M. Abd El-Aal / Dyes and Pigments 52 (2002) 129–136 values and high signal contrast [2]. However, as the number of vinylene groups increases, the absorption maxima of cyanine dyes undergoes a bathochromic shift and their stability becomes weak. Pentamethine cyanines show better stability than heptamethine cyanines; their absorption maxima are below 700 nm, so they have potential for high-density for optical recording. An increase in the stability may be achieved by the introduction of electron-withdrawing substituents, on their meso carbon atom. This paper reports the syntheses of near-IR absorbing meso-substituted penta- and heptamethine cyanine dyes and also evaluates the structure–properties relationships of the dyes on the basis of their visible/ near-IR absorption, 1H NMR and IR spectra. 1.1. Preparation of meso-substituted penta- and heptamethine cyanine dyes Virtually all meso-substituted penta- and heptamethine cyanine dyes in this work were synthesized from 2-chloro-1-formyl-crotonic ester 1, in turn prepared through the Vilsmeier reaction of ethylacetoacetate treated with excess dimethylformamide–phosphorus oxychloride complex in tetrahydrofurane at 0
C [8]. Reaction of 2-chloro-1formyl-crotonic ester 1 with bimolar ratios of 2- or 4-methyl-substituted heterocyclic quaternary salts (a-picoline, quinaldine and/or g-picoline) in the presence of acetic anhydride [9] afforded the corresponding meso-substituted 2(4)-pentamethine cyanine dyes 2a–2c. Structures of compounds 2a–2c were established based on analytical and spectral data. Thus, IR of compound 2a showed absence of CHO group at 1730 cm1, 1H NMR showed absence of signal of CHO at
9.5 ppm and M+=464 (cf. Table 1). Reaction of equimolar ratios of 1 and aromatic amine compounds in the presence of piperidine as basic catalyst afforded the corresponding 1-anilino-2-chloro crotonic ester 3a–3c as key intermediate compounds. Thus, interaction of equmolar ratios of 3a–3c with 2- or 4methyl-substituted heterocyclic quaternary salts in the presence of acetic anhydride gave the meso-substituted aza-tetramethine cyanine dyes 4a–4e. (Scheme 1). The structure of compound 4b as example was confirmed by analytical and spectral data. Thus, IR showed (CH¼N) at 1485 cm1, (CH¼CH) at 1585 cm1, 1H NMR reveals 6.7– 7.9 (m, 12H, Ar-H+CH¼N), 10.4 (s, 1H,OH), 3.4 (q, 2H, CH2O), 1.4 (t, 3H, CH3), 1.5 (s, 3H, CH3), 0.95 (s, 3H, CH3) and M+=416 (cf. Table 1). Interaction of equimolar ratios of 2 chloro-1-formyl crotonic ester 1 with acetophenone derivatives (p-OCH3 and p-NO2 acetophenone) and active methylene compounds (ethylacetoacetate, acetylacetone and ethyl cyanoacetate) in the presence of piperidine as basic catalyst afforded the corresponding compounds 5a–5f as key intermediate compounds in the synthesis of heptamethine cyanine dyes 6a–6f. Thus, reaction of 5a–5f with bimolar ratios of 2-methylquinolinium mthiodide in acetic anhydride afforded meso-substituted heptamethine cyanine dyes 6a–6f (Scheme 1). The structure of compound 6c as an example was confirmed by analytical and spectral data. Thus, IR showed (CH3I) at 2980–2965 cm1, (CH¼CH) at 1580 cm1, 1H NMR reveal 6.7–7.9 (m, 20H, Ar–H+CH¼CH), 3.9 (s, 3H, CH3I), 3.4 (q, 2H, CH2O), 1.3 (t, 3H, CH3), 1.1 (s, 3H, CH3), 0.95 (s, 3H, CH3) and M+=711. Elemental analyses, IR, 1H NMR spectral data and mass spectra were confirmed the structure of 2a–2c, 3a–3c, 4a–4e, 5a–5f and 6a–6f (Tables 1 and 2). Meso-substituted penta-, aza-tetra- and heptamethine cyanine dyes were highly colored and fairly soluble in polar organic solvents giving a green fluorescence, but they were only sparingly soluble in nonpolar solvents. They readily dissolved in conc. H2SO4, liberating iodide vapor on warming except aza-tetramethine dyes. Their ethanolic solutions gave a violet color in alkaline medium that was discharged on acidification. 1.2. Relation between molecular structure and spectra of the synthesized cyanines The electronic absorption spectral data (lmax and "max values) meso-substituted pentamethine 2a–2c aza-tetramethine 4a–4e and heptamethine 6a–6f cyanine dyes respectively are shown in Table 3. The visible absorption spectra of the synthesized cyanines in 95% ethanol exhibit various absorption bands within the wavelength range 350–700 nm. R.M. Abd El-Aal / Dyes and Pigments 52 (2002) 129–136 131 Table 1 Characterization data of penta- and aza-tetramethine cyanine dyes 2a–2c, 3a–3c and 4a–4e Compd no. Mol formula (mol wt) 1 Calcd (%) Found (%) C H Yield (%) M.p. (
C)
KBr IR  max (cm1) H NMR (CDCl3) M+  Assignment N 2a C21H25N2O2I (464) 54.31 5.39 6.04 54.17 5.43 6.15 73 135–137 1585 (CH¼CH) 6.8–17.9 (m, 11H, Ar–H+CH¼CH), 1685 (C¼O) 3.9 (s, 3H,CH3I), 3.5 (q, 2H, CH2O), 1.3 (t, 3H, CH3), 1.5 (s, 3H,CH3), 2985 (CH3I) 0.9 (s, 3H, CH3N) 2b C29H29N2O2I (564) 61.70 5.14 4.97 61.63 5.11 5.09 87 142–144 2c C21H25N2O2I (464) 54.31 5.39 6.04 54.19 5.45 6.11 76 154–156 1590 (CH¼CH) 1690 (C¼O) 2975 (CH3I) 3a C13H14NO3Cl (267.5) 58.32 5.23 5.23 58.23 5.17 5.31 81 137–139 1735 (C¼O) 3300 (OH) 1475 (CH¼N) 464 6.7–7.9 (m, 15H, Ar–H+CH¼CH), 564 3.95 (s, 3H, CH3I), 3.5 (q,2H, CH2O), 1.4 (t, 3H, CH3), 1.5 (s, 3H,CH3), 0.95 (s, 3H, CH3 ) 464 7.1–7.9 (m, 5H, Ar–H+CH=N), 270 5.5 (s, 1H, OH), 3.6 (q, 2H, CH2O ), 1.3 (t, 3H, CH3 ), 0.95 (s, 3H, CH3) 3b C14H14NO4Cl (295.5) 56.85 4.74 4.74 57.01 4.84 4.79 71 123–125 11730 (C¼O) 3300 (OH) 1475 (CH¼N) 6.8–7.9 (m, 5H, Ar–H+CH=N), 298 10.5 (s, 1H, OH), 3.6 (q, 2H, CH2O ), 1.3 (t, 3H, CH3 ), 0.95 (s, 3H, CH3) 3c C13H13N2O4Cl 52.61 4.39 9.44 (296.5) 52.53 4.47 9.35 67 132–134 11730 (C¼O) 3300 (OH) 1485 (CH¼N) 6.9–7.9 (m, 5H, Ar–H+CH¼N), 3.6 (q, 2H, CH2O), 1.3 (t, 3H, CH3 ), 0.95 (s, 3H, CH3) 4a C24H24N2O3 (388) 74.23 6.19 7.22 74.13 6.25 7.31 73 166–168 3300 (OH) 6.7–7.9 (m, 12H, Ar–H+CH¼N), 1585 (CH¼CH) 5.5 (s, 1H, OH), 3.6 (q, 2H, CH2O), 1680 (C¼O) 1.4 (t, 3H, CH3), 1.6 (s, 3H, CH3), 0.95 (s, 3H, CH3) 389 4b C25H24N2O4 (416) 72.16 5.77 6.73 72.23 5.67 6.87 68 171–173 1585 (CH¼CH) 6.7–7.9 (m, 12H, Ar–H+CH¼N), 1680 (C¼O) 10.4 (s, 1H, OH), 3.6 (q, 2H, CH2O), 1485 (CH¼N) 1.4 (t, 3H, CH3), 1.5 (s, 3H, CH3), 0.95 (s, 3H, CH3) 416 4c C20H21N3O4 (367) 65.40 5.72 11.44 65.37 5.81 11.35 71 180–182 1585 (CH¼CH) 6.7–7.9 (m, 10H, Ar–H+CH¼N), 1690 (C¼O) 3.6 (q, 2H, CH2O), 1.4 (t, 3H, CH3), 1475 (CH¼N) 1.5 (s, 3H, CH3), 0.95 (s, 3H, CH3) 368 4d C24H23N3O4 (417) 69.07 5.52 10.07 68.91 5.63 10.17 83 192–194 1585 (CH¼CH) 6.6–7.8 (m, 12H, Ar–H+CH¼N), 1690 (C¼O) 3.5 (q, 2H, CH2O), 1.4 (t, 3H, CH3), 1480 (CH¼N) 1.5 (s, 3H, CH3), 0.95 (s, 3H, CH3) 417 4e C20H21N3O4 (367) 65.40 5.72 11.4 75 65.47 5.79 411.41 146–148 1585 (CH¼CH) 6.7–7.9 (m, 10H, Ar–H+CH¼N), 1685 (C¼O) 3.5 (q, 2H, CH2O), 1.4 (t, 3H, CH3), 1485 (CH¼N) 1.5 (s, 3H, CH3), 0.95 (s, 3H, CH3) 367 These absorption bands are affected by the nature of heterocyclic residue (A) and their linkage position 2a–2c, substituted in aryl moiety of 4a–4e and the substituents in the polymethine chains of 300 6a–6f. Thus, substituting A=pyridinium-2yl in compound 2a by A=quinolinium-2yl in compound 2b causes bathochromic shift of 30–60 nm, with appearance of three new absorption bands. 132 R.M. Abd El-Aal / Dyes and Pigments 52 (2002) 129–136 Scheme 1. Substituents (2a–2c): A=1-methylpyridinium-2yl salt (a); A=1-methylquinolinium-2yl salt (b); A=1-methylpryidinium4-yl salt (c); (3a–3c): X=p-OH (a); p-CO2H (b); p-NO2 (c); (4a–4e): X=p-OH, A=1-methylquinolinium-2-yl salt (a); X=p-CO2H, A=1-methylquinolinium-2-yl salt (b); X=p-NO2, A=1-methylpyridinium-2-yl salt (c); X=p-NO2, A=1-methylquinolinium-2-yl salt (d); X=p-NO2, A=1-methylpryidinium-4-yl salt (e); (5a–5f) and (6a–6f): R=H, R1=ph (a); R1=C6H4–p–OCH3 (b); R1=C6H4–p– NO2 (c); R=COCH3; R1=OEt (d); R1=CH3 (e); R=CN, R1=OEt (f). This can be attributed to more extensive p-delocalization within quinolinium-2yl salt. Changing the linkage position of the pyridinium residue from 2yl to the 4-yl in compounds 2a, 2c, 4c and 4e resulted in bathochromic shifts due to increase in the conjugation of the pyridinium in the 4-yl linking (Table 3). The visible absorption spectra of aza-tetramethine cyanine dyes 4a–4e are influenced by the substituted in the aryl moiety X. Thus, substituting X=OH in compound 4a by X=NO2 in compound 4d causes bathocromic shifts of 10–75 nm. The absorption spectra of heptamethine cyanine dyes 6a–6f reveal four bands located in the wavelength 377–700 nm. The absorption spectra showed bathochromic or hypsochromic shifts, depending on the nature of the substituents R and R1. Thus, substituting R=H, R1=ph in compound 6a by R=H, R1=C6H4–pNO2 in compound 6c causes bathochromic shifts of 2–15 nm. Also, substituting R=COCH3, R1=OEt in compound 6d by R=CN, R1=OEt in compound 6f causes bathochromic shifts of 15–25 R.M. Abd El-Aal / Dyes and Pigments 52 (2002) 129–136 133 Table 2 Characterization data of intermediate compounds 5a–5f and heptamethine cyanine dyes 6a–6f
1 IR KBr Compd no. Mol formula Calcd (%) Yield M.p. H NMR (CDCl3) max
1 (cm ) (mol wt) Found (%) (%) ( C) C H N
Assignment M+ 5a C15H15O3Cl (278.5) 64.63 5.39 64.57 5.43 75 122–124 1585 (CH¼CH) 1735 (C¼O) 1670 (C¼O) 6.9–7.5 (m, 7H, Ar–H+CH¼CH), 3.5 (q, 2H, CH2O), 1.3 (t, 3H, CH3), 0.9 (s, 3H, CH3) 280 5b C16H17O4Cl (308.5) 62.24 5.51 62.13 5.41 87 144–146 1585 (CH¼CH) 1735 (C¼O) 1670 (C¼O) 6.9–7.5 (m, 6H, Ar–H+CH¼CH), 3.5(q, 2H, CH2O), 1.3 (t, 3H, CH3), 2.9 (s, 3H, OCH3), 0.9 (s, 3H, CH3) 310 5c C15H14NO5Cl (323.5) 55.64 4.33 4.33 67 55.73 4.45 4.29 166–168 1590 (CH¼CH) 1735 (C¼O) 1670 (C¼O) 6.7–7.5 (m, 6H, Ar–H+CH¼CH), 3.4 (q, 2H, CH2O), 1.3 (t, 3H, CH3), 0.95 (s, 3H, CH3) 323 5d C13H16O5Cl (287.5) 54.26 5.57 54.39 5.49 79 125–127 1670 (C=O) 1585 (CH¼CH) 1735 (C¼O) 6.7 (s, 1H, CH¼C), 3.6 (q, 4H), 2.3 (s, 3H, COCH3), 1.3 (t, 6H, CH3), 0.95 (s, 3H, CH3) 288 5e C12H14O4Cl (257.5) 55.92 5.44 56.11 5.39 72 136–138 1670 (C¼O) 1590 (CH¼CH) 1735 (C¼O) 6.7 (s, 1H, CH¼C), 3.5 (q, 2H, CH2O), 260 2.3 (s, 3H, COCH3), 1.3 (t, 3H, CH3), 1.1 (s, 3H, CH3), 0.95 (s, 3H, CH3) 5f C11H11O3Cl (226.5) 58.28 4.86 58.37 4.93 65 120–122 1670 (C¼O) 1590 (CH¼CH) 2250 (CN) 6.7 (s, 1H, CH¼C), 3.6(q, 4H, CH2O), 1.3 (t, 6H, CH3), 0.95 (s, 3H, CH3) 6a C37H33N2O2I (664) 66.87 4.97 4.22 77 66.73 5.07 4.31 6.7–7.9 (m, 21H, Ar–H+CH¼CH), 137–139 1585 (CH¼CH) 3.9 (s, 3H, CH3I), 3.5 (q, 2H, CH2O), 1680 (C¼O) 2980–2965 (CH3I) 1.3 (t, 3H, CH3), 1.05 (s, 3H, CH3) 0.9 (s, 3H, CH3) 667 6b C38H35 N2O3I (694) 65.71 5.04 4.04 85 65.79 5.17 4.15 182–184 1585 (CH¼CH) 6.8–7.9 (m, 20H, Ar–H+CH¼CH), 1680 (C¼O) 3.9 (s, 3H, CH3I), 3.5 (q, 2H, CH2O), 2980–2965 (CH3I) 2.3 (s, 3H, OCH3), 1.3 (t, 3H, CH3), 1.1 (s, 3H, CH3), 0.9 (s, 3H, CH3) 697 6c C37H32N3O4I (709) 62.62 4.51 5.92 71 62.57 4.63 5.85 6.8–7.9 (m, 20H, Ar–H+CH¼CH), 196–198 1585 (CH¼CH) 3.9 (s, 3H, CH3I), 3.5 (q, 2H, CH2O), 1680 (C¼O) 2980–2965 (CH3I) 1.3 (t, 3H, CH3), 1.1 (s, 3H, CH3), 0.95 (s, 3H, CH3) 711 6d C35H37N2O4I (676) 62.13 5.47 4.14 69 62.21 5.39 4.23 6.7–7.9 (m, 15H, Ar–H+CH¼CH), 145–147 1585 (CH¼CH) 3.8 (s, 3H, CH3I), 3.5 (q, 4H, CH2O), 1685 (C¼O) 2980–2965 (CH3I) 2.3 (s, 3H, COCH3), 1.3 (t, 6H, CH3), 1.05 (s, 3H, CH3), 0.95 (s, 3H, CH3) 677 6e C34H35N2O3I (646) 63.16 5.42 4.33 87 63.07 5.79 4.39 6.8–7.9 (m, 15H, Ar–H+CH¼CH), 177–179 1585 (CH¼CH) 3.75 (s, 3H, CH3I), 3.5 (q, 2H, CH2O), 1680 (C¼O) 2980–2965 (CH3I) 1.3 (t, 3H, CH3), 2.3 (s, 3H, COCH3), 1.1(s, 6H, CH3), 0.95 (s, 3H, CH3) 646 6f C34H34N3O3I (659) 61.91 5.16 6.37 74 62.03 5.07 6.43 151–153 1585 (CH¼CH) 1685 (C¼O) 2980–2965 (CH3I) 2250 (CN) 659 6.7–7.9 (m, 15H, Ar–H+CH¼CH), 3.9 s, 3H, CH3I), 3.5 (q, 4H, CH2O), 1.3 (t, 6H, CH3), 1.1 (s, 3H, CH3), 0.95 (s, 3H, CH3) 229 134 R.M. Abd El-Aal / Dyes and Pigments 52 (2002) 129–136 Table 3 Visible spectra of the synthesized polymethine cyanine dyes lmax (nm)/log "max (mol1 cm1) Pentamethine cyanine dyes 2a–2c 2a 350 (4.20) 420 (3.70) & & & 2b – 480 (4.97) 520 (4.93) 580 (4.88) 690 (4.48) 2c 360 (4.23) 450 (3.65) & & & Aza-tetramethine cyanine dyes 4a–4e 4a & 510 (4.79) 560 (4.90) 605 (4.70 4b & 550 (4.78) 585 (4.88) 675 (4.52) 4c 375 (4.08) 460 (3.40) & & 4d & 520 (4.65) 550 (4.70) 680 (4.04) 4e 390 (4.11) 480 (3.55) & & Heptamethine cyanine dyes 6a–6f 6a 518 (4.95) 558 (5.04) 580 (4.98) 690 (4.47) 6b 520 (4.93) 560 (5.00) 585 (4.88) 690 (4.23) 6c 522 (5.15) 565 (5.11) 595 (5.18) 692 (4.60) 6d 510 (5.70) 540 (5.13) 600 (4.92) 700 (4.30) 6e 520 (5.04) 550 (5.11) 610 (4.90) 700 (4.48) nm. This may be attributed to increasing the conjugation through the accepting ability of withdrawing groups [2]. 1.3. Absorption of electromagnetic radiation of some selected polymethine cyanine dyes in the visible region (400–750 nm) This experiment is a study of the visible spectra of several cyanine dyes. Absorption of electromagnetic radiation (EMR) in the visible (range 400–750 nm) region is associated with the promotion of a valence electron from its lowest energy (ground state) level to a higher energy level. Absorption or emission of visible light by a molecule depends on electron transition between molecular orbital energy levels, just as absorption or emission of electromagnetic radiation by an atom is determined by electron transition between levels in the atom and the E for those transitions. Molecular orbital theory provides a model for the way electromagnetic radiation interacts with molecules. Molecular orbital theory predicts that the energy difference, E, between levels will decrease if the double bond is conjugated with another double bonds. The predicted decrease in E for 6f 525 (4.95) 565 (5.08) 615 (4.94) 700 (4.32) conjugated structures is also observed in Section 2. The greater the number of conjugated multiple bonds in compound, the longer the wavelength of the light that the compound will absorb. Absorption of EMR of some selected newly synthesized polymethine cyanine dyes 2a, 4a, 4d, 6b and 6e as example can be determined by using the De Broglie relationship. E ¼ hc=l ð1Þ where h is Planck’s constant, c is the velocity of light and l is the longest wavelength (2a at lmax=690 nm; 4a at lmax=605 nm; 4d at lmax=675 nm; 6b at lmax=690 nm and 6e at lmax=700 nm). The free electron model as proposed by Kuhn can treat this system theoretically. He assumed that solely the pi electrons, which are ‘‘freely’’ in motion along the chain, determine the spectral characteristics in the visible region. This is effectively an example of one-dimensional ‘‘particle-in-a-box’’ system, one of the simplest applications of quantum mechanics. According to Kuhn’s model [10], the ‘‘box’’ is the chain of conjugated double bonds with ‘‘walls’’ at the two nitrogen atoms. Thus, Eq. (2) can be used to predict the energies of the levels R.M. Abd El-Aal / Dyes and Pigments 52 (2002) 129–136 available to the pi electrons. The lowest energy (longest wavelength) transition corresponds to promotion of an electron from the highest filled level (n1) to the lowest vacant level (n2=n1+1). For a system with N pi electrons a total of N/2 levels is occupied, so that Etheo ¼ h2 =8mL2 n22 n21
  ¼ h2 =8mL2 ðN=2 þ 1Þ2ðN=2Þ2 ¼ h2 =8mL2 ðN þ 1Þ ð2Þ where h is Plank’s constant, m is the mass of electron, L is sum of the bond lengths between the two nitrogen plus one additional bond distance at each end, and N is the number of double bonds. Eqs. (1) and (2) indicate that E is proportional to N/L. As the chain length increases both N and L increase. However, because L is squared, the net effect is a decrease in E as the chain become longer. The data of Eexp and Etheo were recorded discuss the effect of the chain length on the observed at lmax and related this effect to the ‘‘particle in the box’’ (cf. Table 4). 2. Experimental Melting points (m.p.) were recorded on a Gallenkamp melting point apparatus and are uncorrected. Elemental analyses were carried out at the micro analytical center at Cairo University. Infrared were determined on a Perkin-Elmer Infra-red 1650 FT–IR instrument, visible spectra (300–700 nm) were recorded on a Shimadzu UV–visible-240 Table 4 Characterization data of electromagnetic radiation energies of some selected polymethine cyanine dyes Compound no. 2a 4a 4b 6a 6b Åexp (kJ/mol) 2.7512 3.1412 2.8012 2.7512 2.7112 Åtheo (kJ/mol) 1.113 8.7513 2.5212 5.514 7.514 135 spectrophotometer. 1H NMR spectra were recorded on an EM-390 90 MHz NMR spectrometer and mass spectra were recorded on an HPMs 6988 spectrometer. 2.1. Synthesis of 2-chloro-1-formyl crotonic ester 1 To a mixture of POCl3 (118 ml, 1.27 mol) and DMF (97.3 g, 1.3 mol) in THF (300 ml), ethylacetoacetate (42.9 g, 0.33 mol) was added at 0
C in portions during 30 min. The mixture is stirred at 0
C for 1 h and then heated for 30 min in a water bath. The reaction mixture poured into ice (2 kg) and left overnight. The precipitated product was filtered and crystallized from EtOAC. M.p. 47
C, yield 67%. Analytical data for C7H9O3Cl (mol wt. 176.5). Calc. (%) C: 47.59, H: 5.10. Found (%) C: 47.53 H: 5.19. IR (KBr): 1730 cm1 (CHO), 2765 cm1 (CH str.) and 1610 cm1 (C¼O). 1H NMR (CDCl3):
1.0 (s, 3H, CH3), 1.3 (t, 3H, CH3) and 2.5 (q, 2H, CH2O), 9.5 (s, 1H, CHO). M+=178. 2.2. Synthesis of pentamethine cyanine dyes 2a–2c A mixture of 1 (0.01 mol) and appropriate 2- or 4-methyl-substituted heterocyclic quaternary salts (0.02mol) was dissolved in acetic anhydride (20 ml). The reaction mixture was refluxed for 15 min, the excess of acetic anhydride was distillated, then the residue was dissolved in ethanol. The precipitated products, after dilution with water, were collected and recrystallized from methanol. Characterization data are listed in Table 1. 2.3. Synthesis of aza-tetramethine cyanine dyes 4a–4e 2.3.1. Method A A mixture of 1 (0.01 mol) and aromatic amine derivatives (p-aminophenol, p-aminobenzoic acid and p-nitroaniline) was dissolved in ethanol (20 ml) and piperidine (3–5 drops) was added. The reaction mixture was refluxed for 6–8 h, filtered hot, concentrated and cooled. The resultant products were crystallized from methanol to give the intermediate compounds 3a–3c. Characterization data are listed in Table 1. 136 R.M. Abd El-Aal / Dyes and Pigments 52 (2002) 129–136 2.3.2. Method B A mixture of 3a–3c (0.01 mol) and the appropriate 2-or 4-methyl-substituted heterocyclic quaternary salts (0.02 mol) was used. The reaction was essentailly the method as that for 2a–2c. Characterization data for aza-tetramethine cyanine dyes 4a–4e are listed in Table 1. reaction was essentially the method as that for 2a– 2c. Characterization data for heptamethine cyanine dyes 6a–6f are listed in Table 2. 2.4. Synthesis of heptamethine cyanine dyes 6a–6f [1] (a) Watanabe S, Tani TJ. Imaging Sci Technol 1995;39(1):181; (b) Peng ZH, Peng BX. Dyes and Pigments 1998;6(4):559. [2] (a) Dia ZF, Peng BX. Dyes and Pigments 1998;36(2):169; (b) Dia ZF, Peng BX. Dyes and Pigments 1998;36(3):243. [3] Ivri J, Buyshtein Z, Miron E. Appl Opt 1991;30(18):2484. [4] Krieg M, Pedmond PW. Photochem Photobiol 1993; 57:472. [5] Chibisov AK, Zakharova GV. J Chem Soc, Faraday Trans 1996;92(24):4917. [6] Carlin DB, Tsunoda Y. Proceeding of the IEEE 1994; 82(4):469. [7] Tatsuske K, Yasuo K. Japanese patent, 1995, 07–70 453. [8] Bartmann W, Konz E, Rüger W. Synthesis 1988:680. [9] Abd El-Aal RM, Koraiem AIM, Shindy HA. Heteroatom Chem 1997;8(3):259. [10] Roy DK. Quantum mechanical tunnelling and its application. World Scientific, 1986. 2.4.1. Method A A mixture of 1 (0.01 mol) and acetophenone derivatives or active methylene compounds (0.01 mol) was dissolved in ethanol (20 ml) and piperidine (3–5 drops) was added. The reaction mixture was refluxed for 8–10 h, filtered hot, concentrated and cooled. The precipitated product was crystallized from methanol to give the intermediate compounds 5a–5f. Characterization data are listed in Table 2. 2.4.2. Method B A mixture of 5a–5f (0.01 mol) and 2-methylquinolinium methiodide (0.02 mol) was used. The References