Structure of rhodanine cyanine dyes, spectroscopy and performance in photographic emulsions Zheng-hong Peng," Xiang-feng Zhou," Suzanne Carroll," Herman J. Geise,"" Bi-xian Peng,b Roger Dommisse," Eddy Esmansc and Robert Carleerd "University of Antwerp (UIA), Department of Chemistry, Universiteitsplein 1, B-2610 Wilrijk, Belgium bInstitute of Photographic Chemistry, Academia Sinica, Beijing, 1001 01, Peoples Republic of China "University of Antwerp (RUCA), Department of Organic Chemistry, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium dUniversityof Limburg, Institute for Materials Research, Universitaire Campus, B-3590 Diepenbeek, Belgium The synthesis is reported of eight pyridine-, benzothiazole- and benzimidazole-rhodanine zeromethine merocyanines as blue- sensitising dyes (1-s), and of three benzoxazole-rhodanine dimethine merocyanines as green-sensitising dyes (9-1 1).The compounds were characterised from UV-VIS, mass spectroscopic and NMR data, and the purity of the products was analysed using HPLC. Oxidation potentials were measured using cyclic voltammetry. Ionisation potentials and electron affinities were obtained by semi-empirical MOPAC calculations. The charge-density distributions of the pyridine and benzothiazole rhodanine dyes are in agreement with assignments in NMR spectroscopy. A linear dependence is found between the oxidation potential and ionisation potential, as well as between the reduction potential and electron affinity. Reflection spectra of 6 and 7 coated on photographic emulsions showed J-aggregate absorption at il= 450 and 465 nm, respectively.The sensitising properties of the merocyanine dyes were evaluated in actual photographic T-grain emulsions. Silver halide crystals, which constitute the sensitive elements in most photographic processes, have significant sensitivity only to the ultraviolet, violet and blue region of the spectrum, while the human eye is sensitive to the part of the electromag- netic spectrum between il= 400 and 700 nm. Since silver halide emulsions are unaffected by green, yellow, orange and red light, these 'colour-blind' materials give photographs in which the tone values are distorted and therefore cannot be used as the basis for colour photography. The sensitivity of silver halide must be extended from A = 480 nm (blue light) into the il=500-600 nm (green light) and 600-700 nm (red light) regions of the visible spectrum in order to produce colour photographic products.Rhodanine merocyanine dyes are well known as sensitising dyes in photographic In a continuation of our research programme on functional three pyridine- rhodanine zeromethine merocyanines (1-3), four benzothia- zole-rhodanine zeromethine merocyanines (4-7), and one benzimidazole-rhodanine zeromethine merocyanine dye (8) were synthesized and their efficiencies tested as blue sensitizers in photographic T-grain emulsions. The formulae of the com- pounds are shown in Fig. 1 (u)-(d). Three benzoxazole-rhodan- ine dimethine merocyanine dyes (9-11) were also synthesized and their efficiencies tested as green sensitizers.The formulae of these compounds are given in Fig. l(e)-(f). High-pressure liquid chromatography (HPLC) was used to check the purity of the compounds. Chemical ionization mass spectroscopy proved efficient to confirm the empirical formula and purity of the merocyanine compounds. Fast atom bombardment mass spectrometry (FAB MS) proved useful to check the structure of selected dyes and intermediates. The compounds were further characterized by UV-VIS spectroscopy and NMR measurements. The structures of the zeromethine dyes were also studied by semi-empirical MOPAC 93 calculations, and the charge distributions correlated with the NMR spectra. The ionization potentials and electron affinities were related to the one-electron reduction and the one-electron oxidation poten- tials measured by cyclic voltammetry.These properties were then correlated with dye solubility, sensitizing efficiency and dyeing extent (residual dye density). Experimenta1 Syntheses N-Methyl-2-thiomethyl pyridinium iodide" (QL1). 2-Sulfanylpyridine (5 g, 0.045 mol) and NaHCO, (5.67 g, 0.06 mol) were suspended in 20 ml water, then dimethyl sulfate (13 ml, 0.137 mol) was added dropwise over a period of about 20 min to the solution while stirring vigorously. The solution was stirred at 20-25 "C for a further 100 h. Sodium iodide (16 g) was added to the solution which was cooled in an ice bath. Orange needles of the crude product were precipitated.After filtration, the product was washed with diethyl ether and acetone, and then crystallized from ethanol. Yellow crystals, mp 140-142°C. Yield 50%. 1,3-Dimethyl-2-thiomethyl-3H-benzimidazolium iodide" (QL6). 1,3-Dimethyl-2-thiomethylbenzimidazolium iodide (QL6) was prepared similarly to QL1, but using 2-sulfanylbenz- imidazole (7.5 g, 0.05 mol), NaHCO, (12.6 g, 0.06 mol) and dimethyl sulfate (23.7 ml, 0.25 mol). A yellow powder was obtained with mp 138-140°C. Yield 51%. The other QL salts were synthesized under the conditions reported in our recent work,' some details are given in Table 1. N-Methyl -2 -[( 3-ethyl-4-0x0-2-thioxo-1,3-thiazolidin-5-ylidene)] pyridine (1). N-Methyl-2-thiomethyl pyridinium iodide, QL1 (1 g, 0.00375 mol) and N-ethyl rhodanine (0.6 g, 0.00375 mol) were dissolved in 10 ml water-free alcohol and the solution was heated to 80"C, triethylamine (1 ml, 0.007 mol) was added and the mixture heated at reflux under an N2 atmosphere for 45 min.The solution was then cooled in a refrigerator overnight. The precipitate was filtered and washed with a mixture of acetone and diethyl ether (1 : l),and J. Muter. Chem., 1996,6(8), 1325-1333 1325 ii 1 R=CH3 R‘=C2H5 5 R=C2H, R’=C2H5 2 R=C2H5 R’=C2H5 6 R=CH3 R’=C2H5 3 R = CH3 R’ = CH2COOH 7 R = CH3 R’= CH2COOH -CH 11o2 8 12 CHS CH215 I11 9 CH 1132 S03K( Na) 10 R=OCH3 11 R=CI (91 N I x-I I-RX-R CH3 QLl R=CH3 X=l-QL3 R = (CH2)4S03-QL6 QL2 R=C2H5 X=l-QL4 R=CH3 QL5 R=C2H5 $!2 OM1 R1= C2H5 R2= C6H5 X = I-QM2 R1= (CH2)4S03-R2 =OCH3 X = I-OM3 R1 = (CH2)4S03- R2 = CI Fig.1 Formulae and numbering used for the compounds, together with atomic numbering schemes then recrystallized three times from ethanol and 0.5g final red product was obtained; mp 152-153“C;yield 56%. Table 1 Synthetic details of QL quarternary salts [see Fig. l(g) for The other blue-sensitizing dyes (2-8) were synthesized under formulae] similar conditions to the one described above. reaction conditions 2-Methyl-3-ethyl-5-phenyl-1H-benzoxazoliumiodide (QM1). A mixture of 2-methyl-5-phenylbenzoxazole( 11 g, 0.053mol)mp/”C yield (YO) T/”C t/h and ethyl iodide (11ml, 0.075mol) was heated under a QL1 140-142 50 25 100 nitrogen atmosphere on an oil bath to 120°Cfor 12h.Large QL2 125-126 24 see ref. 9 amounts of solid products were formed during the reaction. QU 221-223 99 see ref. 9 Upon cooling to room temperature, acetone was added to QU 142-144 30 see ref. 9 wash the crude products, which were crystallized from QL5 117-120 30 see ref. 9 ethanol. The white product (9g; 53%) was dried in vacuum, QM 138-140 51 25 100 mp 190-192“C. 1326 J. Muter. Chem., 1996,6(8), 1325-1333 2-[ 2-(N-Acetylanilino)ethenyl]-3-ethyl-5-phenyl-lH-benz-oxazolium iodide (QM4). A mixture of 2-methyl-3-ethyl-5- phenylbenzoxazolium iodide (3.66 g, 0.01 mol) and N,N'-diphenylformamidine (5.88 g, 0.03 mol) was heated under a nitrogen atmosphere on a metal bath to 160°C for 1h. During the reaction a vacuum of about 1 mmHg was maintained to distill off the byproduct aniline.Upon cooling to room tem- perature, acetic anhydride (20 ml) was added and the mixture heated at reflux at 140°C for 1h. After distillation of the solvent, acetone (40ml) was added and the the mixture was heated to reflux. Finally, 70ml diethyl ether was added to precipitate the product. The solution was kept in the refriger- ator overnight. A dark red product (2.4 g) was obtained, and dried in vacuum. Yield 47%; mp 278-280°C. 5 -Met hoxy-Zmet hyl-3 -(4-sul fonatobu ty1)- 1H- benzoxaz- ohm (QM2). A mixture of 4.1 g (0.025 mol) of 5-methoxy-2- methyl-lH-benzoxazole and 3.7 g (0.03 mol) of 1,4-butanesul- fone (1,2-oxathiane-2,2-dioxide)was gradually heated to 120 "C and kept at this temperature for 22 h, during which time the colour changed from yellow to dark brown.After cooling, the product was filtered, washed with a mixture of cold acetone and methanol (1:1, v/v), and crystallized from methanol; mp 258-260 "C, yield 3.6 g (48%). 2-{2-(N-Acetylanilino)ethenyl)-5-methoxy-3-( 4-sulfonato-butyl )-1H-benzoxazolium (QM5). A mixture of 5-methoxy- 2-methyl-3-(4-sulfonatobutyl)-1H-benzoxazolium (QM2) (3.0 g, 0.01 mol) and N,N'-diphenylformamidine (5.88 g, 0.03 mol) was heated under a nitrogen atmosphere at 140°C for 1 h. During the reaction, a vacuum of about 1 mmHg was maintained to distill off the byproduct aniline. Upon cooling to room temperature, acetic anhydride (10 ml) was added and the mixture was heated at reflux at 140°C for 1h.After distillation of the solvent, 40ml acetone was added and the mixture was heated at reflux for 1h. Finally, 70ml diethyl ether was added to precipitate the product. The solution was kept in a refrigerator overnight; mp 296-298"C7 yield 1.6g (35%). QM3 was prepared in the same way as QM2, starting from 5-chloro-2-methyl-lH-benzoxazole.QM6 was synthesized fol- lowing the procedure described for QM5, starting from QM3. Synthetic details of all QM compounds used are collected in Table 2. N-Ethyl-5-phenyl-2-[2'-( 3-et hyl-4-oxo-2-thioxo-l,3-t hiaz-olidin-5-ylidene)ethenyl]-1H-benzoxazole (g). In 10 ml acetic anhydride, a mixture of 5-phenyl-3-ethyl-2-( 2'-N-acetanilinoe- theny1)benzoxazolium iodide (0.5 g, 0.001 mol) and N-ethyl rhodanine (0.16 g, 0.001 mol) was heated to 140°C on an oil bath for 1 h in the presence of a tertiary amine (NEt,).Upon cooling, 50 ml ice was added to precipitate the products. After recrystallisation three times from methanol, 0.1 g of red, needle- like crystals was obtained. Yield 25%, mp 220-222 "C. Compounds 10 and 11 were synthesized similarly. Equimolar amounts of the ICI intermediate compound and N-ethyl rhodanine were reacted using the same conditions as described Table2 Synthetic details of QM quarternary salts [see Fig. l(h) and (i) for formulae] reaction conditions mp/"C yield (YO) TIT tlh QM1 190- 192 53 120 12 QM2 258-260 48 120 22 278-280 76 140 24 QM4QM5 QM3 296-298 278-280 47 35 140 140 1 1 QM6 285-287 50 140 1 above. The crude dye was precipitated with a cold NaI solution and purified with vacuum liquid chromatography at a pressure of 10-20 mmHg, using silica gel (type H, TLC grade, particle size 10-40 mm) as the sorbent, and eluting with butanol-acetic acid-H,O (4 :1:5 vvv).Mass spectra Direct probe chemical ionization (DCI) spectra of the dyes 1-8 were recorded on a RIBER-1-1OB mass spectrometer equipped with a SIDAR DATA system. Compounds were dissolved and placed on a 60 pm tungsten filament with the aid of a micro-syringe. After evaporation of the solvent, the DCI probe was inserted in the NH3 reagent gas, and the wire was heated at a rate of 9mA s-' until ions were observed. Primary ionisation of the reagent gas (NH,) was achieved with the aid of 70eV electrons.The ionisation current was 0.08 mA and the source temperature was 150 "C. The pressure in the ion source was 0.1 Torr. The DCI spectra of the zeromethine dyes showed in all cases the molecular ion peak [M +H] + , proving the correctness of the empirical formula. With the exception of the spectra of 6 and 8, no spurious peaks indicative of impurities were noted. Fast atom bombardment (FAB) mass spectra of selected intermediates and dyes were obtained on a Finnigan TSQ 70 mass spectrometer equipped with an ION TECH FAB gun operating at 8 kV on xenon. Positive-ion FAB mass spectra were recorded and controlled by the Finnigan data system by repetitive scanning over the range m/z =50-1000 with a scan rate of 1 s.The products were mixed with trifluoracetic acid- glycerol prior to analysis. NMR Measurements 'H and I3C NMR spectra of compounds in deuteriated dimethyl sulfoxide solution or deuteriated methanol solution were recorded at 30°C on a JEOL FX 100 spectrometer. A Varian Unity spectrometer operating at 400 MHz for protons and at 100 MHz for carbons, respectively, was used in conjunc- tion with a Sun Spark (Palo Alto, CA) data system. Tetramethylsilane was used as the internal standard. The atomic numbering schemes of the rhodanine dyes are shown in Fig. 1. UV-VIS and reflection spectra UV-VIS spectra were recorded at room temperature on an UV-8415A spectrophotometer.The absorption spectra of the dyes were determined from methanol solutions (dye concen- tration ca. 5 x mol I-'). Reflection spectra were recorded at room temperature on a Hitachi 557 UV-VIS reflection spectrometer. Coated Ag'Br T-grain emulsions without dyes were used as reference. MOPAC calculations and electrochemical measurements Since no experimental geometries are available for most of the rhodanine dyes, we used the semi-empirical MOPAC 93 pack- age in the PM3 approach to calculate the geometries of some blue-sensitizing dyes. The geometries were fully optimised without any constraints. In case different configurations around the C(2)=C(5') bond are possible, the configuration of lowest calculated energy was used in the further analyses.(The E configuration calculated for 5 is supported by a single-crystal X-ray determination.16) Oxidation potentials (EOx)were measured using a Princeton Applied Research Corp. EG&G PARC model 175 potentiostat in conjunction with a Universal Program, EG&G PARC 174A Polarographic Analyser. The scanning speeds were 100 and 200mV s-' and the sensitivities were 0.1 and 0.2mAcm-'. The working electrode was a Pt disk of area ca. 0.1 cm2. Cyclic voltammetry curves of the dyes were recorded on a EG&G J. Muter. Chem., 1996,6(8), 1325-1333 1327 PARC RE 0074 X-Y recorder All potentials were measured us the NaCl saturated calomel electrode (SCE) Photographic emulsions The tabular grain emulsion was precipitated using the double- jet method with automated control of pAg It was then coagulated, washed, redispersed and chemically sensitized with S +Au sensitizers, spectrally sensitized with 1-11 at various concentrations, coated on the base, and finally dried Film strips were exposed using a tungsten lamp (5500 K), and exposure times stepped up with 1/20 s employing a Xang Fong (Shanghai, P R China) exposure meter The strips were devel- oped in D-19 b developer for 5 min at 20 "C, followed by fixation in F-5 fixer for 40min Density, fog and maximum density of the dried films were measured Transmission densities were used to construct the characteristic curve from which sensitivity (speed) and contrast followed l7 Results and Discussion Syntheses The zeromethine merocyanines (blue-sensitizing dyes) 1-8 were prepared by the reaction of N-ethyl rhodanine (N-ethyl-2- QL1 "S 1 Scheme 1 Synthetic route towards 1 c CH2-S II thioxo-4-thiazolidinone) with a heterocyclic quaternary salt carrying in the 2-position a thioalkyl leaving group [QL salts, Fig l(g)] Scheme 1 gives as an example the steps in the preparation of 1 Starting from 2-sulfanylpyridine, the quat- ernization at N and the formation of the thioether function are best performed in a single step The one-step synthesis leads to better homogeneity of the intermediate QL salts as we observed previously' It also means that when diethyl sulfate is used to produce N-C2H, (as for 2), or 1,4-butanesul- tone to produce N-(CH2),S03- (as for 4), the QL intermediate contains S-C2H5 and S-(CH,),SO, -,respectively, which fortu- nately are as good leaving groups as SCH, The final step towards the merocyanine dyes was carried out in alcohol or acetic anhydride solution in the presence of a tertiary amine [eg N(C2H,),] The products were recrys- tallized two or three times from alcohol or acetic acid The dimethine merocyanines (green-synsitizing dyes) 9-1 1, were prepared by a multi-stage reaction First, the appropriate heterocyclic compound carrying an active methyl group in the 2-position was quaternized at N The resulting salt CQMl-QM3, Fig 1 (h)] was then reacted with diphenylforma- midine to produce the corresponding intermediate, an anilido- vinyl compound [often called ICI intermediate, QM4-QM6, Fig 1(1)] Then, the ICI intermediate was acylated in acetic anhydride to obtain more reactivity Finally, the acylated ICI intermediate was reacted with N-ethylrhodanine in the pres- ence of a tertiary amine to obtain the final dimethine merocyan- ine dye Details regarding the synthesis of 9 are depicted in Scheme 2, and are described later To check the punty, compounds 1-11 were analysed by HPLC, using a Lichropep 25-40 RP 18 column, and CH,OH-H20 (2 1 v/v) as eluent at a flow rate of 1 0 ml min-' at 3500 psi pressure A Waters Associates Model 440 UV detector was employed, operating at 214nm Retention times, yields, melting points and calculated purities are given in Table 3 The elemental analyses of some dyes are shown in Table4 In the cases of 1 and 5 a capillary electropherogram was made, giving a purity of 100%, thus confirming the result obtained from HPLC The experiment was performed on a QM2 HNEt3 9 Scheme 2 Synthetic route towards 9 1328 J Muter Chem, 1996, 6(8), 1325-1333 Table 3 Yields after purification, resulting melting points, purity from HPLC [using CH30H-HzO (2: 1) as eluents] and retention times of 1-11 (see Fig.1 for numbering of compounds) dye yield (YO) mp/"C purity (YO) retention time/min 1 56 152- 153 99 12.0" 2 42 148-149 100 3.7 3 60 220-222 100 3.2 4 28 249-252 100 4.4 5 63 244-245 98 4.2 6 65 280-282 85 4.6 7 50 31 1-313 90 4.2 8 52 228-230 85 3.2 9 25 220-222 95 8.8 10 43 2 18-220 90 8.1 11 51 >300 86 8.5 "Eluted with CH,OH-H,O =50 :50 (v/v).Table 4 Elemental analyses of some dyes (see Fig. 1 for numbering of the compounds) dye formula ~~~~~~ 1 CllH12NzOS2 2 ClzH,,NzOS2 3 Cl1Hl0NzO3Sz 4 Cz2H3,NZO6S4 expt. (YO) calc. (YO) NCHNCH ~ 10.77 51.81 3.92 11.11 52.38 4.76 10.33 53.57 5.00 10.53 54.14 5.26 9.14 44.66 3.53 9.93 46.81 3.55 7.39 46.60 5.60 7.49 47.06 5.53 CZ2H2002NZS2 6.78 64.75 4.96 6.86 64.71 4.90 Waters Quanta 4000 CE system, applying a 20 kV electric field, and using a sodium phosphate buffer solution at pH=7. The retention time of 1 was 10.5min, and that of 5 was 11.0 min. The retention times are in agreement with the neutral character of the compounds. Mass spectra FAB mass spectra of intermediate QL1 showed the molecular ion M+ at m/z= 140, the cluster ion [MI +MI+ at m/z=407, and the loss of CH3 from M+ at m/z= 125. The tandem mass spectra (MS/MS) of the ion at m/z=407 and the parent spectra of the ions at m/z =125 and 140 showed the same fragmentation as the direct FAB mass spectra.FAB mass spectra of QL6 indicated: M+ at m/z= 193, the loss of CH3 from M+ at m/z= 178. MS/MS spectra of the ion at m/z= 193 showed the loss of CH3 and SCH3 groups. The molecular masses of 3 and 4 were confirmed again from the FAB mass spectra. Moreover, MS/MS experiments on their molecular ions showed that they fragment by losing HCOOH and SC(=S)NCH,COOH. Turning to the green-sensitizing dyes, the FAB mass spec- trum of 11 showed the molecular ion of the intact dye cation (M') at m/z=474, and also revealed the cluster ions [M +Na] + and [M +2Na]+ at m/z=497 and 520, respect- ively.Furthermore, it showed the loss of sulfur and of H2S03, thus confirming the main features of the molecular structure. Finally, the FAB mass spectra of the acylated ICI intermedi- ate QM4 (see Scheme 2) indicated the molecular ion M+ at m/z =383, and the expulsion of the ethyl group. Also we noted the loss of CH2 =C=O from M + ,leaving the ICI intermediate. Characteristic fragmentation of the latter shows in peaks at m/z=118 and 169. The peak at m/z =118 can be rationalized by formation of the [CH=CH-NH-Ph]' ion via a-fission with respect to the oxazole ring, and that at m/z= 169 by framentation via the generalized RCN-type fission of oxazole heterocycles, as indicated in our previous work.* NMRmeasurements We start the interpretation of the NMR spectroscopic measure- ments of 1-11 with the 'H NMR signals of 1.In the aromatic region (7.2 <6 <8.6) four peaks are found: two doublets and two triplets. The two doublets are easily assigned to H(3) and H(6) [Fig. l(a)]. H(6) is assigned to the signal at low field, i.e. at 6 8.55, J(ortho)=J(H,-H,)=9 Hz, because H(6) is in the ortho position to the N atom. Then H( 3) is assigned to 6 8.07, J(ortho)=J(H,-H,) =7 Hz. The triplet at 6 7.68, J(ortho)= 9 Hz, is assigned to H(5), and the other triplet with 6 6.90, J(ortho)=7 Hz, is assigned to H(4). The signals of the alkyl hydrogen atoms in the spectrum are easily recognized.With reference to the 13C NMR spectra of 1, the peak at 6 184.0 is assigned to C2'=S, the one at 6 161.8 is assigned to C4'=0, and the peak at 6 80.21 to C5'. Other signals were assigned according to the previous NMR spectra of blue sensitizing dyesg and reference NMR data." 'H and 13C NMR assignments of 1 were then transferred to the other blue-sensitizing compounds included in Tables 5-8. Chemical shift assignments for the green-sensitizing dyes are based on carbon-hydrogen 2D correlation spectra combined with assigned carboxazole spectra and standard substituent increments." The results are collected in Tables 9 and 10. Shift values, coupling patterns and coupling constants unequivocally prove the structure of 9.Moreover, the vicinal coupling constant 3J[H(8)-H(9)] =13.1 Hz shows the E configuration of the C(8)-C(9) moiety [see Fig. l(e) for numbering]. Turning to the averaged I3C NMR shifts in the merocyan- ines, one notes (Fig. 2) that the central atom in the zeromethine Table 5 'H and 13Cchemical shifts (8, ref. Me,Si) to zeromethine pyridine rhodanine dyes in deuteriated Me2S0 at 30 "C [atomic numbering is given in Fig. l(u); atom H(i) is attached to C(i)] pyridine moiety pyridine-alkyl side chain rhodanine moiety rhodanine-alkyl side chain 'H NMR 13C NMR" atom 1 2 3 atom 1 2 8.07 8.09 8.30 137.50 137.46 6.90 6.93 7.13 123.33 123.81 7.68 7.68 7.89 142.67 141.54 8.55 8.72 8.60 114.64 115.12 149.02 148.24 CHZ 4.39 52.00 CH3 4.02 1.41 4.27 45.84 15.45 184.04 183.84 161.75 162.06 80.21 79.45 CHZ CH3 4.03 1.14 4.04 1.15 4.81 39.0gb 12.03 39.0gb 12.03 "The solubility of 3 was too low for its 13C NMR spectrum to be measured.bThe chemical shift of the CH2 group coincides with those of the solvent Me,SO. J. Muter. Chem., 1996, 6(8), 1325-1333 1329 Table 6 'H and 13Cchemical shifts (6, ref Me4&) and 3J(H-H) coupling constants (in Hz) of 4 in deutenated Me2S0 at 30 "C [atomic numbenng is given in Fig l(b), atom (H(i) is attached to C(i)] atom 6 area JIHz atom 6 benzothiazole moiety 153 93 139 59 7 91 lH, dd 124 12 7 50 lH, dt 125 40 7 34 lH, dt 127 29 7 76 lH, dd 112 38 122 33 benzothiazole-alkyl side chain H(8) 4 36 2H, t 44 89 H(9) 188 2H, m 27 77 H( 10 178 2H, m 22 09 H(ll 2 50 2H, t 50 46 H( 12 8 94 1H H(13) 3 08 6H, 9 45 77 H( 14) 117 9H, t 8 50 rhodanine moiety 187 10 167 45 81 61 rhodanine-alkyl side chain H(6) 4 71 2H 46 90 164 15 Table7 'H and 13C chemical shifts (6, ref Me4Si) of zeromethine benzothiazole rhodanine dyes 5 7 in deutenated Me2S0 at 30°C [atomic numbenng is given in Fig l(c), atom H(i) is attached to C(i)] 'H NMR 13C NMR" atom 5 6 7 atom 5 7 benzothiazole moiety 7 93 153 51 154 76 7 52 139 26 140 16 7 35 124 09 12400 7 68 125 67 125 30 127 31 127 15 111 85 11207 122 50 122 34 benzothiazole-alkyl side chain CH2 4 41 42 23 CH3 1 42 14 06 40 12b rhodanine moiety 186 72 187 32 164 58 164 04 82 04 82 57 rhodanine-alkyl side chain CHZ 4 10 39 96' 40 12' CH3 120 11 95 "The solubility of 6 was too low for its 13C NMR spectrum to be measured bThe chemical shift of the CH2 group coincides with those of the solvent Me2S0 Table 8 'H and 13C chemical shifts (6, ref Me&) of zeromethine benzimidazole rhodanine dye 8 in deuteriated Me,SO at 30°C [atomic numbenng is gven in Fig l(d), atom H(i) is attached to C(i)] 'H NMR 13CNMR bennmidazole moiety 7 72 150 22 744 147 46 110 95 124 32 benzimidazole-alkyl side chain 3 81 39 90" rhodanine moiety 184 49 160 39 80 15 rhodanine-alkyl side chain 4 04 32 99 118 11 94 "The chemical shift of the CH3 group coincides with those of the solvent MezSO 1330 J Mater Chew, 1996,6(8), 1325-1333 Table 9 'H and 13C chemical shifts (6, ref.Me,Si) and ,J(H-H) coupling constants (in Hz) of 9 in deuteriated Me,SO at 30 "C [atomic numbering is given in Fig. l(e); atom H(i) is attached to C(i)] 'H NMR 13CNMR atom 6 area J/Hz atom 6 ~~~ 160.56 132.08 7.76 lH, d 107.93 137.84 7.51 lH, dd 122.10 7.63 lH, d 110.21 145.76 5.31 lH, d 76.82 7.82 lH, d 132.15 139.24 7.72 2H,dd 126.90 7.48 2H, t 128.77 7.40 lH, tt 127.53 4.01 1.31 2H, q 3H, t 38.76 12.28 189.65 165.04 104.01 4.19 2H 38.88 1.14 3H 11.95 Table 10 'H and 13C chemical shifts (6, ref. Me4Si) of dimethine benzoxazole rhodanine dyes 10 and 11in deuteriated Me,SO at 30 "C [atomic numbering is given in Fig.10;atom H(i) is attached to C(i)] 161.18 160.82 132.59 133.28 7.19 7.69 96.35 110.03 157.39 129.26 6.76 7.22 110.25 122.77 7.46 7.55 109.09 110.98 140.16 144.90 5.35 5.36 77.17 77.00 7.78 7.75 132.25 131.84 4.07 4.06 42.76 45.57 1.72 1.78 25.86 25.78 1.72 1.73 22.06 21.98 2.51 2.55 50.44 50.45 OCH, 3.80 56.04 56.04 189.54 190.03 164.97 165.13 103.38 105.25 H(6') 4.00 4.00 42.76 42.91 H(7') 1.13 1.15 11.96 11.96 zeromethine rhodanines N,$=q-c,To: '. I 1-3 (pyridine) 137.5 79.8 161.9 4-7 (benzothiazole) 154.1 82.1 165.4 8 (benzimtdazole 150.2 80.2 160.4 dimethine rhodanines N-C=,C-C=C,-C_o" ,..* ,'I ' , H '\\ '*., '\ I 9-11 (benzoxazole) 160.6 76.8 132.2 104.0 165.0 Fig.2 Comparison of chemical shifts (ppm) in merocyanine dyes chain of compounds 1-8 has a lower shift value (upfield shift) and in the dimethine chain of compounds 9-11 it has a higher shift value (downfield shift). It follows that the n-electron density in the dimethine chain is lower than that in the zeromethine hai in.^^'^ This conclusion is in accordance with a simple count of the number of n-electrons per C atom present in each methine cyanine system. Moreover, the vinylogous elongation of the cyanine system and the accompanying changes in the electron distribution can be rationalized with the electron-in-a-box des~ription.l~-'~In a zeromethine chain the number of n-electrons is 6, and hence in the highest occupied molecular orbital (HOMO) there is a maximum n-electron density at the position of the central C atom.In contrast, in the dimethine chain the number of n-electrons is 8, and thus in the HOMO there is a minimum n-electron density at the position of the central C atom. In accordance with this view one observes in the dimethine series significantly higher shift values (downfield shifts)in the central and adjoining C, H. The differences induced in the pyridine ring when N-CH, is replaced by N-C2H5may be studied by comparison of 1 and 2 [Fig. l(a)]. It is seen that in 2, with the exception of H(5), all H atoms have higher 6 values (downfield shifts), whereas C(2) and C(6), which are directly attached to N, have lower 6 values (upfield shifts).This behaviour suggests a more positive charge on the N-ethyl substituent and is in accordance with our earlier research on other blue-sensitizing, pyridine-contain-ing monomethine cyanine sensitisers.' UV-VIS and reflectionspectra The absorption maxima and molar extinction coefficients for the dye monomers 1-11 are shown in Table 11.It can be seen that change of the N-substituents has little effect on the spectra of the dyes. Significant hypsochromic shifts are seen when pyridine is replaced by benzothiazole. Further hypsochromic shifts are seen when benzothiazole is in turn replaced by benzimidazole. This is due to the decrease of the n-electron density in the heterocyclic rings.In the series 9-11, batho-chromic shifts are seen, caused by substituents on the 5-posi-tion of the heterocyclic ring, following the sequence OCH, >Ph >Cl. Dyes 6 and 7 were added to coated Ag'Br T-grain emulsions, and the reflection spectra were recorded. Both spectra showed J-aggregation behaviour. Dye 6 showed a molecular peak at 430 nm and a J-aggregation peak at 450 nm (Fig. 3), and 7 showed a molecular peak at 435 nm and a J-aggregation peak at 465 nm (Fig. 4). MOPAC calculations and electrochemical measurements Based on the optimised geometries obtained from the MOPAC calculations, the ionization potentials (I:h) and electron affin-ities (Eaff)of dyes 1-3, 5 and 7 were calculated (see Table 12), which may be related to the energy levels of the HOMO and Table 11 UV-VIS data for 1-11 in the monomeric state (dye concentration ca.5 x lop6moll-' in methanol) 1 455 6.4 2 455 1.3 3 453 4.1 4 426 1.8 5 428 4.1 6 427 0.6 7 425 0.7 8 408 1.4 9 496 5.0 10 500 3.4 11 491 3.0 J. Muter. Chem., 1996, 6(8), 1325-1333 1331 85L 400 500 600 A /nm Fig. 3 Reflectance spectrum of 6 95 400 500 600 A/nm Fig. 4 Reflectance spectrum of 7 Table 12 Oxidation potentials (Eox), ionization potentials (Zth),reduction potentials (&) and electron affinities (Eafl) of the zero- methine merocyanine compounds 1 0.583 8.12 -1.622 1.379 2 0.609 8.11 -1.640 1.391 3 0.668 8.22 -1.584 1.551 5 0.862 8.47 -1.641 1.573 7 0.900 8.53 -1.736 1.696 LUMO, respectively.In fact, the difference Isth-Eaffin 1-3 amounts to ca. 6.7 eV, compared to ca. 6.9 eV in 5 and 7. This agrees with the larger A,,, values (ca.455 nm) observed in the UV-VIS spectra of 1-3, compared to the smaller A,,, values (ca. 428 nm) measured for 5 and 7. Charge distributions, calculated for 2, 5 and 8 are given in Fig. 5. A comparison with NMR data (Fig. 2, and Tables 5-8) shows the following. First, the charges on the central C atoms [i.e. C(5’)] of the merocyanine systems (N-C=C-C=O) are highly negative and indicate maximum n-electron density. This result is in line with the NMR measurements and the electron- in-a-box theory. Furthermore, the I3C chemical shifts of C( 5’) increase in the same order as the charge densities decrease when going from 2 (pyridine dye) to 8 (benzimidazole dye) to 5 (benzothiazole dye).Secondly, the charge on the N atom of the rhodanine heterocycle is always smaller than that on the N atom of the other heterocycle, a result also reflected in the available NMR data. In fact, in the 13C NMR spectra of 2 and 5 the ethyl group attached to the N position of the pyridine and benzothiazole moieties, respectively, shows higher 6 values (downfield shifts) than the ethyl group attached to the 1332 J. Muter. Chem., 1996, 6(8), 1325-1333 / -0 172 -0 153 C-N -0 272 g5 41 -0 164 c/ I0 104\C=C<549 c-O 186 -0 129 -00&-c/ ON?C-~~~~~/~~~ -0 189 // CH2 0‘ -0 069 -0 365 I-0 102 CH3 -0 127 Fig.5 Charge distnbution of the merocyanine dyes: (a)2, (b) 5, (c) 8 N position of the rhodanine heterocycle. We have earlier interpreted this observation to reflect the lower positive charge on the rhodanine N atom. Table 12 also lists the measured oxidation potentials (Eox) of dyes 1-3, 5 and 7. When we assume that the bandgap is equal to the optical excitation energy E,, of the merocyanine (the longest wavelength of the UV absorption), and ignore solvatation energies, the reduction potential (Erd) can be 85 84 z-83 5* 82 81 8 05 06 0.7 0.8 E,,N (vs. SCE) Fig. 6 Relation between the oxidation potential (E,,/V us. SCE) and ionization potential (Zth,/eV) Table 14 Sensitometric properties of the green-sensitizing dyes 9-11" 1.7 1.6 15 14 13 -1 8 -1.7 -1.6 -1.5 -1.4 -1.3 Er,N (VS.SCE) Fig.7 Relation between the reduction potential (Erd/Vus. SCE) and ionization potential (Eaff/eV) Table 13 Sensitometric properties of the blue-sensitizing dyes 1-5, 7 and 8" Sd dye Ab/ml D,' (DIN) ye D,,, solubility control 0 0.05 20 2.4 4.05 1 2 3 0.06 0.08 21 21 2.0 2.2 4.24 3.37 good 2 2 3 0.06 0.06 22 22 2.8 2.4 4.63 4.25 good control 0 0.08 21.5 2.4 6.03 3 0.5 1 0.18 0.24 21.0 17.5 2.4 2.0 5.89 5.90 poor 2 0.30 18.0 2.0 5.74 control 0 0.09 21 3.0 5.67 4 2 3 0.24 0.41 20 18 2.4 2.3 5.74 5.44 good 4 0.64 15 2.3 5.41 control 0 0.08 21.5 2.4 5.28 5 0.5 1 0.08 0.08 20.0 20.0 2.0 2.0 4.34 5.49 poor 2 0.07 18.5 2.0 4.40 control 0 0.09 21 3.0 5.54 7 2 3 0.14 0.20 20 20 2.8 2.8 5.54 5.71 poor 4 0.23 19 2.6 5.44 control 0 0.05 20 2.4 4.05 8 2 3 0.08 0.09 20 20 2.0 2.2 5.90 4.55 good ~ ~~~~ "The solubility in H20 of 6 is too low to measure its sensitizing properties.bA=number ofml added of a 0.2% solution of dye in MeOH to 50 g emulsion. 'Do, D,,, =minimum and maximum optical density of photographic plate. dS=sensitivity of photographic plate in DIN (Deutsche Industrie Norm; see ref. 17). ey=contrast. evaluated6 by Calculated one-electron reduction values are also reported in Table 12. A least-squares fit of the oxidation potentials (EJ found electrochemically us.the MOPAC calculated ionization poten- tials (Isth)gave a linear relation with correlation coefficient y= 1.00, as depicted in Fig. 6: I:h= 1.355E0,+7.308 (2) The correlation between the reduction potential (&) and the electron affinity (Eaff) was also enumerated, giving a straight line with y=O.98, as depicted in Fig. 7: E,ff=0.997Erd +3.050 (3) S dye A/ml Do (DIN) y D,,, solubility control 0 0.08 21.5 2.4 5.28 9 0.5 0.07 21.5 2.4 1 0.10 21.5 2.4 5.08 5.11 poor 2 0.10 23.0 2.4 4.85 10 0.5 0.07 21.5 2.3 1 0.08 20.0 2.2 4.53 5.68 poor 2 0.07 21.5 2.2 5.36 control 0 0.08 21.5 2.4 5.82 11 0.5 0.17 20.0 2.5 1 0.20 19.0 2.5 4.05 4.89 good 2 0.32 15.5 2.2 4.70 "For definitions, see footnotes to Table 13.Furthermore, the oxidation potential of 2, which showed sensitizing properties, was reversible, whereas those of the desensitizers (3, 5, 7) were irreversible which suggests that when the desensitizers lose an electron, the remainder of the compound is not stable. Performance in photographic emulsions The results of the experiments with photographic emulsions for dyes 1-8 are given in Table 13, and those for dyes 9-11 are reported in Table 14. It follows that 2 and 9 show sensitizing properties in emulsions, while most other dyes are desensitizers. These results coincide with former work on rhodanine merocy- anine dyes,4 except for 7, which does not show any sensitizing properties in our work. References 1 Ullmann's Encyclopedia of Industrial Chemistry, ed.W. Gerhartz, VCH Verlag, Weinheim, 1992,vol. A 20, pp. 1-151. 2 G. F. Ficken, in The Chemistry of Synthetic Dyes, Academic Press, New York, 1971, vol. IV, pp. 218 ff. 3 F. M. Hamer, The Cyanine Dyes and Related Compounds, Wiley-Interscience, New York, 1964. 4 D. Fabricius and J. J. Welter, US Pat., 5 108 887, 1992. 5 J. Vavrova, M. Vavra and Z. Babak, Coll. Czech. Chem. Commun., 1979,44,1413. 6 T. B. Tang, H. Yamamoto, K. Imaeda, H. Inokuchi, K. Seki, M. Okazak and T. Tani, J. Phys. Chem., 1989,93,3970. 7 X-F. Zhou, PhD Dissertation, University of Antwerp, 1990. 8 X-F. Zhou, H. J. Geise, B-X. Peng, Z-X. Li, M. Yan, R. Dommisse and R. Carleer, J. Imaging Sci. Technol., 1994,36, 18. 9 X-F. Zhou, Z-H. Peng, H. J. Geise, B-X. Peng, Z-X. Li, M. Yan, R. Dommisse and R. Carleer, J. Imaging Sci. Technol., 1995, 39, 244. 10 H. Quast and E. Schmitt, Chem. Ber., 1968,101,4012. 11 P. Clerc and S. Simon, Strukturaufklurung organischer Verbindungen mit spektroskopischen Methoden; 3.Aujlage, Springer Verlag, Berlin, 1990. 12 E. Breitmaier and G. Bauer, I3C NMR Spectroscopy, a Working Manual with Exercises, Harwood Academic, Switzerland, 1984. 13 H. Kuhn, Helv. Chim. Acta, 1951,34, 1308. 14 H. Kuhn, Helv. Chim. Acta, 1951,34, 2371. 15 H. Meier, Die Photochemie der Organischen Farbstofen, Springer Verlag, Berlin, 1963. 16 G. Wu, Z-H. Peng and H. J. Geise, unpublished work. 17 The Manual of Photography (formerly the Ilford Manual of Photography), ed. A. Horder, Lund Humphries, London, 6th edn., 1971. Paper 6/00215C; Received 10th January, 1996 J. Muter. Chem., 1996,6(8), 1325-1333 1333