Synthesis, electrochemical, spectral and DFT study of novel thiazole-annelated subphthalocyanines with inherent chirality

Dyes and Pigments 130 (2016) 24e36

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Synthesis, electrochemical, spectral and DFT study of novel thiazole-annelated subphthalocyanines with inherent chirality
ria Micha
lkova
Ne
a, Peter Magdolen a, Andrea Fülo € pova
a, *, Marek Ciga
n  b, Ma cedova Pavol Zahradník a, Juraj Filo b a b


dolina, Ilkovicova 6, 842 15 Bratislava, Slovak Republic Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University, Mlynska
dolina, Ilkovicova 6, 842 15 Bratislava, Slovak Republic Institute of Chemistry, Faculty of Natural Sciences, Comenius University, Mlynska

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 January 2016 Received in revised form 27 February 2016 Accepted 1 March 2016 Available online 3 March 2016

Five-membered thiazole-annelated subphthalocyanine derivatives have been prepared for the first time. Both regioisomers with C3 and C1 molecular symmetry have been isolated and optical resolution of C3 enantiomers was achieved. Heterocycle annelation resulted in red shifted long wave Q-band with regard to the unsubstituted subphthalocyanines, in the case of linear annelated compounds it is more red shifted (603, 615 nm) than the corresponding band in angular annelated derivatives (590, 594 nm). Analysis of fluorescence quantum yields revealed the heavy atom effect of bromine. Irrespective of the present bromine atom, all complexes were characterized by high singlet oxygen production with quantum yields ranging from 0.62 to 0.80. The HOMO and LUMO energy levels (DFT calculated and electrochemically determined) showed that frontier orbital energies can be effectively tuned by different type of annelation. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Subphthalocyanines Thiazole Electrochemistry Singlet oxygen CD spectra Density functional theory

1. Introduction Given that phthalocyanines belong to a very known macrocyclic ring systems, their contracted homologues subphthalocyanines (SubPcs) have received considerable attention in recent years. Unlike planar 18 p-electron phthalocyanines, subphthalocyanines present an aromatic delocalized 14 p-electron system with a C3v cone-shaped structure. The molecule is composed of three isoindole subunits bearing boron atom within their central cavity with a halogen in axial position [1]. The geometry about the central boron is approximately tetrahedral (Fig. 1). SubPcs received attention as intermediates for the synthesis of unsymmetrically substituted phthalocyanines by the ring expansion reaction [2]. Nonplanar aromaticity of SubPcs results in rather unique spectral and electronic features. During the past few years SubPcs have emerged as functional materials in a variety of applications including organic photovoltaics [3], OLEDs [4], chemical sensors [5] or organic thin film transistors [6]. As a consequence of their octupolar character SubPcs show high second-order nonlinear

* Corresponding author. €pova
). E-mail address: [email protected] (A. Fülo http://dx.doi.org/10.1016/j.dyepig.2016.03.001 0143-7208/© 2016 Elsevier Ltd. All rights reserved.

response [7]. The properties of excited states make the SubPcs good photosensitisers with potential application in photodynamic therapy [8]. SubPcs are synthesized in good to moderate yield by cyclotrimerization reaction of phthalonitrile precursors in the presence of boron trihalide [9]. Required properties of SubPcs may be tuned by functionalizing the positions on the benzene ring [10] or by linking other functional units at the axial position [11]. Various peripherally substituted SubPcs with substituents single bonded to benzene ring were prepared [12]. Another unique property arising from the structural features is an inherent molecular chirality. When SubPcs are prepared from asymmetrically substituted phthalonitrile precursors, two structural isomers with C1 and C3 molecular symmetries with respect to the arrangement of the substituents are obtained. Each structural isomer is inherently chiral due to the absence of any mirror plane in their structures. Claessens and Torres [13] investigated the inherent chirality of SubPcs for the first time, further studies were presented by Kobayashi [14]. Despite of intensive research on SubPcs there is still little general information available on the relationship between their absolute molecular structures and the signs and intensities of their circular dichroism (CD) spectra. Quantum chemical calculations are a useful tool for predicting the structure


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova

25

2. Experimental 2.1. Materials and methods

Fig. 1. Top and side view of the subphthalocyanine molecule structure.

e chirality relationship to a certain extent [1c]. Peripheral annelation presents another eventuality of SubPcs structural modification permitting the extension of the conjugated system. Next ring can be fused to the SubPc by linear or angular fashion leading to several regioisomers depending on the symmetry of the added part (Fig. 2). If the annelated ring is a heterocycle, the thermic, chemical and photochemical stability of the macrocycle can be improved. What is more, the heterocyclic annelation leads to the alteration of optical, physical and eventually biological properties. However, unlike the case of phthalocyanine, heterocycle annelated subphthalocyanines remain unreported thus far, except some derivatives [15]. Herein, we report the design, synthesis and characterization of novel peripherally p-expanded SubPc analogues containing a thiazole unit fused to the benzene ring. SubPcs with linear (LIN) as well as with angular annelation (ANG) have been prepared and both types as a mixture of regioisomers with C1 and C3 symmetry. For the first time a study about heterocyclic congeners of SubNc and about inherent chirality in linear and angular derivatives is presented. Structure assignment of the enantiomers was based on a comparison of the experimentally measured and theoretically calculated CD spectra. The brominated SubPcs (LIN-Br and ANG-Br) have been prepared with the aim of possible next derivatization. Structures and labelling of novel SubPcs are presented in Fig. 3. Each of the eight regioisomers possesses the inherent chirality that gives rise to a pair of enantiomers. Spectral, electrochemical and theoretical study was accomplished to explain the effect of the added thiazole fragment on the SubPc properties. The singlet oxygen production capabilities of the prepared compounds were also evaluated. To better comprehend the impacts of thiazole-annelation the results were compared to unsubstituted SubPc and isoelectronic SubNc.

Reagents were purchased from commercial sources and used without further purification unless stated otherwise. Solvents were purified and dried using common methods. Column chromatography was carried out on silica gel Merck-60 (230e400 mesh, 60 Å), and TLC on aluminium sheets pre-coated with silica gel 60 F254 (E. Merck). Melting points were measured on a Kofler apparatus Electrothermal IA-9200. UV/Vis spectra were obtained on a HP 8452A diode array spectrophotometer (Hewlett Packard, USA). Solution fluorescence was measured in a 1 cm cuvette with a FSP 920 (Edinburgh Instruments, UK) spectrofluorimeter in a right-angle arrangement. The fluorescent quantum yield (FF) of studied compounds in chloroform was determined by Eqs. (1) and (2) using integrating sphere (Edinburgh Instruments):

FXF ¼

LSam ð%Þ ERef  ESam

(1)

corrected to re-absorption by:

FF ¼

FXF


ð%Þ 1  a þ aFXF 100

(2)

where LSam is the area under the detected spectrum in the part of the spectrum where sample emission occurs, ERef is area under the reflection part of the detected spectrum using pure solvent as reference material (diffuse reflectance), ESam is area under the reflection part of the detected spectrum after absorption by sample and a is reabsorbed area. The time-resolved fluorescence measurements were performed on a FSP 920 (Edinburgh Instruments, UK) spectrofluorimeter with a time-correlated single-photon counting (TCSPC) module and a red sensitive high speed photomultiplier in peltier housing, featuring Hamamatsu H5773-04 detector (R928P detector; Edinburgh Photonics, UK). Excitation source was 402.8 nm ps pulsed diode laser (Model EPL-405; Pulse Width: 60.5 ps; Edinburgh Photonics, UK). Reconvolution fit analysis software (F900, Edinburgh Instruments) was used for lifetime data analysis. Singlet oxygen quantum yields (FD) were estimated relatively to a methanol solution of Rose Bengal (FD ¼ 0.79) [16] as standard sensitizer using equation:

Fig. 2. Possible patterns in annelated subphthalocyanines.


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26

Fig. 3. Structure and labelling of the investigated SubPc compounds.

FxD

Sx RB ¼ F SRB D

(3)

where: Sx and SRB are the slopes of the actinometer absorbance versus irradiation time plots (Eq. 6 in Ref. [17]) in the presence of the measured subphthalocyanine and Rose Bengal (RB), respectively. Sensitizer absorbed photon flux Ia at the irradiation wavelength l using a monochromatic light source must be equal to Ia absorbed by RB at the same irradiation wavelength (Iax ¼ IaRB ). 1,3Diphenylisobenzofurane (DPIBF) was chosen as actinometric indicator of photooxidative attack of 1O2 in methanol. Measurements of singlet oxygen production were performed using the apparatus described elsewhere [18] (without ultrasonic horn H and lens L1 and using Ocean Optics SD 2000 diode array spectrophotometer). The light sources were four 520 nm LED diodes Thorlabs. The actual actinometer absorbances in air-saturated solutions during irradiation in a 1 cm quartz fluorescence cuvette were measured spectrophotometrically in right-angle arrangement (HP 8452A), and LED light sources were turned off during concentration measurements. IR spectra were recorded on a Thermo Scientific Nicolet iS10 spectrometer (Smart iTR diamond ATR) at room temperature. HRMS analyses were performed on an ORBITRAP VELOS PRO instrument, Thermo Scientific with APPI ionization using krypton lamp and methanol/water 1:1 solution as dopant. 1H NMR and 13C NMR spectra were recorded on a Varian NMR SystemTM 300 (300 MHz) spectrometer or a Varian NMR SystemTM 600 (600 MHz) in CDCl3 or DMSO-d6 with tetramethylsilane (TMS) as an internal standard. Elemental analyses were measured on a CarloErba instrument, model 1106 SCIENCE and Vario MICRO CUBE analyser (Elementar). Separation of the enantiomers was carried out by highperformance liquid chromatography (HPLC) with CHIRALPAK IA column by monitoring absorbance at 580 nm. The HPLC system was coupled to a CD spectrometer specified below, which allowed onthe-fly measurements of CD spectra of the individual mixture components employing the stop-flow technique. CD spectra were recorded on a Jasco J815 instrument at 25  C in the same eluent as the antecedent HPLC measurement. The electrochemical measurements (i.e., cyclic voltammetry and square wave voltammetry) were performed at room temperature using an Autolab PGSTAT101 potentiostat. The measurements were carried out with a three electrode setup consisting of a Pt working electrode, a Pt counter electrode, and an Ag/AgCl reference electrode separated from the bulk solution by an integrated salt bridge. The detailed procedure was as follows: 0.1 M solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6) in anhydrous

CH2Cl2 (5 mL) serving as a supporting electrolyte was added to the cell and purged by nitrogen for 5 min to remove residual oxygen. Then, the studied compound (5 mg) was added and purging continued for next 5 min. Different scan rates were applied at cyclic voltammetry measurement (50 mV/s, 100 mV/s and 200 mV/s) for each sample. Half-wave potentials (E1/2) were recorded from square wave voltammetry with a potential step of 5 mV and a scan rate of 100 mV/s. Ferrocene was added to the solution at the end of each experiment and the potentials listed in Table 4 are all referenced to the ferrocene/ferrocenium couple (Fc/Fcþ). HOMO and LUMO energy levels of compounds were calculated using the HOMO level of ferrocene at 4.8 eV and the first oxidation and first reduction values of subphtalocyanine derivatives in CH2Cl2. Equation (4) and (5) were used for the calculation of HOMO and LUMO energy levels, respectively.

h  i EHOMO ¼  Eox  E1=2 ðferroceneÞ þ 4:8

(4)

h  i ELUMO ¼  Ered  E1=2 ðferroceneÞ þ 4:8

(5)

Computational Details: The optimal geometries for each compound in vacuo were calculated in the Turbomole 6.6 program [19] using the DFT method [20] and the PBE0 exchange-correlation functional [21]. On all atoms the def2-TZVP basis set [22] was employed. Optimal geometries were proved by calculating infrared spectra with no negative frequencies. Absorption spectra, vertical excitation energies (DEe), oscillator strengths (f), ECD spectra and molecular orbital energies (EHOMO, ELUMO, DE(H-L)) using the timedependent DFT (TD-DFT) method [23] were calculated with Gaussian 09 program [24] applying M06 functional [25] and TZVP basis set [26] (for subphthalocyanine derivatives) or B3-LYP functional [27] and 6-31þG(d) basis set [28] (for benzothiazoledicarbonitriles). The Polarizable Continuum Model (PCM) [29] was used to perform the computations with the inclusion of chloroform as solvent. 2.2. Synthesis The starting dinitriles 1 and 2 were prepared and purified according to the procedures described previously [30]. 2.2.1. 4-amino-3,5-dibromophthalonitrile (6) Starting 4-aminophthalonitrile (5) (1 g, 7 mmol) was dissolved in glacial acetic acid (50 mL) by gentle heating to 50  C, then Nbromosuccinimide (2.49 g, 2 eq., 14 mmol) was added in a single portion. The mixture was vigorously stirred for 2 h at r.t., then poured into water (200 mL). The white-yellow precipitate was


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova

collected by filtration, washed with water and dried. The analytical sample was recrystallized from ethanol. Yield: 1.78 g (85%); m.p. 259e261  C; IR (ATR, cm1): n ¼ 3474, 3359 (NH2), 3069 (CeH arom.), 2629, 2224 (C^N), 1613, 1469, 1307, 1235, 1148, 907. 1 H NMR (300 MHz, DMSO-d6): d ¼ 8.27 (s, 1H, HeC6), 6.96 (bs, 2H, NH2) ppm. 13C NMR (75 MHz, DMSO-d6): d ¼ 148.3, 136.6, 117.9, 115.6, 115.4, 110.5, 109.8, 101.8 ppm. Elemental analysis: Found: C, 31.94; H, 0.87; N, 13.82%; C8H3Br2N3 requires C, 31.93; H, 1.00; N, 13.96%. 2.2.2. N-(2,6-dibromo-3,4-dicyanophenyl)pivalamide (7) A suspension of 6 (1.78 g, 5.91 mmol) in pivalic anhydride (10 mL) with catalytic amount of conc. sulphuric acid (2 drops) was stirred and heated to 170  C for 2 h. The unused pivalic anhydride was removed from the cooled solution under reduced pressure. The product was purified by silica gel column chromatography using CHCl3 as eluent to give 7 as a slightly beige solid. Yield: 1.8 g (79%); m.p. 164.5e166  C; IR (ATR, cm1): n ¼ 3201 (NH), 3073 (CeH arom.), 2982, 2236 (C^N), 1665 (C]O), 1505, 1445, 1374 (CeH), 1281, 1030, 769. 1H NMR (300 MHz, DMSO-d6): d ¼ 9.86 (s, 1H, NH), 8.65 (s, 1H, HeC5), 1.27 (s, 9H, CH3) ppm. 13C NMR (75 MHz, DMSO-d6): d ¼ 176.0, 142.9, 137.0, 130.8, 129.3, 118.3, 116.1, 114.8, 114.2, 27.0 ppm. Elemental analysis: Found: C, 40.79; H, 2.80; N, 10.67%; C13H11Br2N3O requires C, 40.55; H, 2.88; N, 10.91%. 2.2.3. 4-bromo-2-tert-butylbenzothiazole-5,6-dicarbonitrile (3) and 4-bromo-2-tert-butylbenzothiazole-6,7-dicarbonitrile (4) To a solution of 7 (2.3 g, 5.97 mmol) in ethanol (25 mL) sulphur powder (0.19 g, 5.97 mmol) and sodium sulfide nonahydrate (0.72 g, 2.98 mmol) was added. The crimson-coloured suspension was stirred and heated to reflux for 3 h and then filtered off while still hot. The solution was cooled to room temperature, diluted with water (100 mL) and made slightly acidic with concentrated HCl to cause precipitation. The resulting suspension was heated to boiling for 5 min and then cooled down. The resulting yellow precipitate was collected after cooling, washed with water and dried (1.5 g). The water layer was extracted with CHCl3 (3  50 mL), and combined organic layers were dried with sodium sulphate. The solvent was removed under reduced pressure to yield 0.35 g of orange solid. The isomers 3 and 4 were separated and purified by silica gel column chromatography using hexane/ ethyl acetate (7:1) as eluent, to yield 0.7 g of 3 (37%) and 0.45 g of 4 (24%). Products were crystallized from methanol. Data for 4bromo-2-tert-butylbenzothiazole-5,6-dicarbonitrile (3): Yield: 0.7 g (37%); m.p. 178e179.5  C (white needles); IR (ATR, cm1): n ¼ 3088 (CeH arom.), 2965, 2926, 2868 (CeH), 2232 (C^N), 1576, 1492, 1415, 1366 (CeH), 1273, 1162, 1063, 1016, 902, 820, 750. 1H NMR (300 MHz, DMSO-d6): d ¼ 8.97 (s, 1H, HeC7), 1.57 (s, 9H, CH3) ppm; 13C NMR (75 MHz, DMSO-d6): d ¼ 189.9, 153.2, 139.3, 128.7, 121.4, 115.7, 115.4, 114.5, 110.7, 30.1 ppm. Elemental analysis: Found: C, 49.08; H, 3.05; N, 13.14; S, 10.00%; C13H10BrN3S requires C, 48.76; H, 3.15; N, 13.12; S, 10.01%. UV/Vis (CHCl3): lmax (log ε) ¼ 249 (4.80), 271 (3.99), 277 (3.93), 287 (3.73), 316 (3.26), 328 nm (3.38). Data for 4-bromo-2-tert-butylbenzothiazole-6,7dicarbonitrile (4): Yield: 0.45 g (24%); m.p. 145e147  C (white powder); IR (ATR, cm1): n ¼ 3067 (CeH arom.), 2969, 2957, 2927, 2868 (CeH), 2239 (C^N), 1540, 1495, 1452, 1350, 1275, 1223, 1064, 1011, 892, 828, 788, 750, 702. 1H NMR (300 MHz, DMSO-d6): d ¼ 8.62 (s, 1H, HeC5), 1.52 (s, 9H, CH3) ppm; 13C NMR (75 MHz, DMSO-d6): d ¼ 188.2, 153.4, 139.1, 133.6, 121.5, 115.0, 114.5, 112.3, 108.6, 30.0 ppm. Elemental analysis: Found: C, 48.65; H, 3.09; N, 13.03; S, 10.13%; C13H10BrN3S requires C, 48.76; H, 3.15; N, 13.12; S, 10.01%. UV/Vis (CHCl3): lmax (log ε) ¼ 252 (4.57), 285 (4.11), 294 (4.16), 316 (3.73), 330 nm (3.72).

27

2.2.4. Subphthalocyanines synthesis. General synthetic procedure for the synthesis of ANG, ANG-Br, LIN, LIN-Br In a 25-mL two-necked round-bottomed flask, equipped with a condenser, magnetic stirrer and a rubber seal to the corresponding dry aromatic dicarbonitrile (0.3 g, 1.24 mmol for 1, 2 and 0.94 mmol for 3, 4) the solution of BCl3 (1.24 mL to 1, 2 and 0.94 mL to 3, 4; 1 eq., 1 M solution in p-xylene) was added under argon atmosphere. The reaction mixture was stirred and heated to reflux for 1.5 h. Then the dark solution was cooled to room temperature and flushed with argon. The obtained suspension was dissolved in toluene/THF (10:1) and passed through a short silica plug. The solvent was removed under reduced pressure and the solid residue was subjected to column chromatography on silica gel using appropriate eluent mixture. In this way, the first C3 and then C1 regioisomers were isolated and both characterized independently. In some cases, a second column chromatography was necessary for complete isolation and purification. Each compound was further purified by precipitation from CH2Cl2 solution into methanol to give a dark magenta solid. ANG-C1 and ANG-C3: The C3 and C1 regioisomers were separated by silica gel column chromatography using chloroform/THF (40:1) as eluent. The isolated isomers were further purified using hexane/ ethyl acetate (5:1) for C3 and hexane/CHCl3 (1:1) for C1 isomer. Data for isomer ANG-C1: Yield: 38 mg (12%); m.p. > 250  C; IR (ATR, cm1): n ¼ 2960, 2926, 2853, 1747, 1508, 1456, 1414, 1364, 1320, 1249, 1219, 1189, 1135, 1109, 1049, 1012, 959, 821, 772, 711, 674. 1H NMR (300 MHz, CDCl3): d ¼ 8.99 (d, J ¼ 8.6 Hz, 1H), 8.95 (d, J ¼ 8.6 Hz, 1H), 8.93 (d, J ¼ 8.6 Hz, 1H), 8.51 (d, J ¼ 8.6 Hz, 1H), 8.50 (d, J ¼ 8.6 Hz, 1H), 8.49 (d, J ¼ 8.6 Hz, 1H), 1.72 (s, 18H), 1.71 (s, 9H) ppm. HRMS: (ESI) m/z calculated for C39H33N9S3BCl [M]þ 769.1803, found 769.1820. UV/Vis (CHCl3): lmax ¼ 291, 348, 546, 570, 590 nm. Data for isomer ANG-C3: Yield: 21 mg (6.5%); m.p. > 250  C; IR (ATR, cm1): n ¼ 2959, 2922, 2851, 1722, 1507, 1456, 1416, 1363, 1320, 1254, 1219, 1191, 1132, 1112, 1049, 1012, 958, 822, 771, 711, 675. 1 H NMR (300 MHz, CDCl3): d ¼ 9.01 (d, J ¼ 8.6 Hz, 1H), 8.51 (d, J ¼ 8.6 Hz, 1H), 1.71 (s, 9H) ppm. 13C NMR (75 MHz, CDCl3): d ¼ 185.36, 155.97, 151.22, 149.78, 128.59, 128.17, 125.33, 124.92, 120.27, 39.04, 31.10 ppm. HRMS: (ESI) m/z calculated for C39H33N9S3BCl [M]þ 769.1803, found: 769.1813. UV/Vis (CHCl3): lmax (log ε) ¼ 291 (4.73), 348 (4.49), 540 (4.35), 589 nm (4.90). ANG-Br-C1 and ANG-Br-C3: Firstly, impurities were removed by silica gel column chromatography using hexane/ethyl acetate (10:1) and then the C3 and C1 regioisomers were separated by next chromatography using hexane/CHCl3 (2:1). Data for isomer ANG-Br-C1: Yield: 77 mg (24%); m.p. > 250  C; IR (ATR, cm1): n ¼ 2960, 2927, 2866, 1735, 1506, 1455, 1427, 1362, 1308, 1262, 1236, 1174, 1128, 1052, 1010, 995, 967, 844, 814, 781, 772, 762, 717, 702, 690. 1H NMR (600 MHz, CDCl3): d ¼ 9.17 (s, 1H), 9.14 (s, 1H), 9.13 (s, 1H), 1.73 (s, 9H), 1.73 (s, 9H), 1.72 (s, 9H) ppm. 13C NMR (151 MHz, CDCl3): d ¼ 185.83, 185.62, 153.85, 153.80, 153.71, 150.68, 150.58, 150.35, 149.88, 149.74, 149.54, 128.91, 128.89, 128.67, 128.54, 128.43, 124.28, 124.08, 124.06, 123.61, 123.57, 123.55, 119.84, 119.79, 119.61, 39.47, 39.45, 31.11, 31.10 ppm. HRMS: (ESI) m/z calculated for C39H30N9S3Br3BCl [M]þ 1002.9118, found 1002.9123. UV/Vis (CHCl3): lmax (log ε) ¼ 292 (4.51), 352 (4.48), 550 (4.39), 594 nm (4.94). Data for isomer ANG-Br-C3: Yield: 27 mg (8.6%); m.p. > 250  C; IR (ATR, cm1): n ¼ 2959, 2925, 2866, 1737, 1505, 1456, 1427, 1362, 1310, 1266, 1235, 1174, 1124, 1051, 1004, 984, 968, 849, 815, 772, 761, 717, 688. 1H NMR (600 MHz,CDCl3): d ¼ 9.17 (s, 1H), 1.73 (s, 9H) ppm. 13C NMR (151 MHz, CDCl3): d ¼ 185.78, 153.71, 150.32, 149.55, 128.88, 128.55, 124.21, 123.48, 119.61, 39.45, 31.13 ppm. HRMS: (ESI) m/z calculated for C39H30N9S3Br3BCl [M]þ 1002.9118, found 1002.9122. UV/VIS (CHCl3): lmax (log ε) ¼ 292 (4.74), 355 (4.71), 547

28


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova

(4.65), 573 (4.89), 593 nm (5.19). LIN-C1 and LIN-C3: Firstly, impurities were removed by silica gel column chromatography using hexane/THF (2:1) and then the C3 and C1 regioisomers were separated by next chromatography using chloroform/THF (100:1). Data for isomer LIN-C1: Yield: 53 mg (17%); m.p. > 250  C; IR (ATR, cm1): n ¼ 2960, 2926, 2865, 1728, 1615, 1514, 1437, 1372, 1301, 1259, 1231, 1152, 1109, 1039, 1008, 964, 880, 788, 721, 696. 1H NMR (600 MHz, CDCl3): d ¼ 9.45 (d, J ¼ 0.6 Hz, 2H), 9.43 (d, J ¼ 0.7 Hz, 1H), 9.38 (d, J ¼ 0.7 Hz, 1H), 9.34 (bs, 2H), 1.640 (s, 9H), 1.638 (s, 9H), 1.634 (s, 9H) ppm. 13C NMR (151 MHz, CDCl3): d ¼ 185.41, 185.34, 185.31, 154.81, 154.77, 149.95, 149.73, 149.66, 149.10, 148.99, 148.78, 138.21, 138.15, 138.12, 130.04, 129.99, 129.93, 128.10, 128.03, 127.99, 116.47, 116.43, 116.34, 115.68, 115.50, 115.48, 39.20, 30.86 ppm. HRMS: (ESI) m/z calculated for C39H33N9S3BCl [M]þ 769.1803, found 769.1817. UV/Vis (CHCl3): lmax (log ε) ¼ 291 (4.94), 335 (4.93), 558 (4.75), 603 nm (5.27). Data for isomer LIN-C3: Yield: 17 mg (5.3%); m.p. > 250  C; IR (ATR, cm1): n ¼ 2959, 2925, 2862, 1729, 1615, 1515, 1437, 1366, 1298, 1259, 1232, 1151, 1106, 1040, 1008, 964, 878, 793, 778, 720, 697. 1H NMR (300 MHz, CDCl3): d ¼ 9.41 (s, 1H), 9.37 (s, 1H), 1.64 (s, 9H) ppm. 13C NMR (75 MHz, CDCl3): d ¼ 185.37, 154.78, 149.85, 148.86, 138.16, 129.94, 128.03, 116.28, 115.70, 39.19, 30.85 ppm. HRMS: (ESI) m/z calculated for C39H33N9S3BCl [M]þ 769.1803, found 769.1809. UV/Vis (CHCl3): lmax (log ε) ¼ 293 (4.42), 335 (4.44), 549 (4.26), 583 (4.49), 602 nm (4.78). LIN-Br-C1 and LIN-Br-C3: The C3 and C1 regioisomers were separated by silica gel column chromatography using hexane/THF (40:1) and then isomers were purified separately using hexane/ CHCl3 (2:1) for C3 and hexane/THF (40:1) for C1 isomer. Data for isomer LIN-Br-C1: Yield: 73 mg (23%); m.p. > 250  C; IR (ATR, cm1): n ¼ 2961, 2926, 2862,1730,1602,1508,1429,1387,1346, 1219, 1169, 1114, 1050, 1009, 994, 976, 881, 867, 812, 789, 762, 718, 699. 1H NMR (600 MHz, CDCl3): d ¼ 9.34 (s, 1H), 9.23 (s, 1H), 9.21, (s, 1H), 1.66 (s, 9H), 1.65 (s, 9H), 1.64 (s, 9H) ppm. 13C NMR (151 MHz, CDCl3): d ¼ 185.93, 185.84, 153.57, 153.56, 153.47, 149.08, 148.59, 148.32, 148.28, 147.73, 137.84, 137.75, 129.32, 129.10, 129.07, 128.62, 128.54, 128.42, 114.86, 114.67, 114.59, 112.56, 112.38, 111.95, 39.59, 39.57, 30.91, 30.90. HRMS: (ESI) m/z calculated for C39H30N9S3Br3BCl [M]þ 1002.9118, found 1002.9123. UV/Vis (CHCl3): lmax (log ε) ¼ 290 (4.87), 338 (4.65), 558 (4.50), 615 nm (5.05). Data for isomer LIN-Br-C3: Yield: 10.7 mg (3.4%); m.p. > 250  C; IR (ATR, cm1): n ¼ 2958, 2924, 2854, 1732, 1602, 1510, 1431, 1388, 1348, 1219, 1169, 1113, 1051, 1014, 992, 876, 860, 813, 799, 757, 719, 698. 1H NMR (600 MHz, CDCl3) 9.38 (s, 1H), 1.66 (s, 9H) ppm. 13C NMR (151 MHz, CDCl3): d ¼ 185.94, 153.49, 149.29, 147.38, 137.85, 129.44, 128.43, 115.03, 111.85, 39.58, 30.90 ppm. HRMS (ESI) m/z calculated for C39H30N9S3Br3BCl [M]þ 1002.9118, found 1002.9127. UV/Vis (CHCl3): lmax (log ε) ¼ 291 (4.91), 337 (4.88), 563 (4.75), 591 (4.96), 614 nm (5.28).

Fig. 4. Structure of starting dicarbonitriles.

Scheme 1. Preparation of brominated dicarbonitriles 3 and 4.

Cyclotrimerization reactions were carried out by heating to reflux a p-xylene solution of corresponding dicarbonitrile precursor (1e4) in an excess of BCl3 [9]. Finally, all target compounds were obtained in 19e33% yields (Scheme 2). Moderate to low yields are typical for the preparation of subphthalocyanines. Higher reaction temperatures and/or longer reaction times led to an increase in the SubPc decomposition degree. SubPcs possess good solubility in common organic solvents and could be purified readily by the column chromatography. Non symmetric starting aromatic dicarbonitriles gave rise to the mixtures of C1- and C3-symmetric SubPcs, which were separated by subsequent column chromatography on silica gel. A ratio of both isomers was estimated by HPLC of the reaction mixture (Table 1) and the C1- and C3-isomers were obtained nearly in statistical ratio (3:1) except LIN-Br where C1-isomer was predominating in a ratio 5:1. All the isomers were fully characterized

3. Results and discussion 3.1. Synthesis and characterization Benzothiazole dinitriles 1 and 2 (Fig. 4) were prepared from commercially available 4-amino-phthalonitrile in good overall yields via the recently described route [30]. Brominated dinitriles 3 and 4 were prepared in a three-step synthesis by a similar synthetic sequence (Scheme 1). Thus, bromination of 4-aminophthalonitrile (5) with 2 equivalents of NBS afforded 6. The acylation of 6 with pivalic anhydride catalysed by concentrated sulphuric acid and followed by one-pot thionation and cyclization [31] led to the required brominated dinitriles 3 and 4.

Scheme 2. Preparation of studied SubPcs from 1 to 4 by cyclotrimerization reactions (BCl3, p-xylene, 1.5 h, 120  C).


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova Table 1 C1/C3 Selectivity observed in the formation of SubPcs determined by HPLC.

ANG a ANG-Br LIN LIN-Br

C1

C3

nd 2.3 2.5 5

nd 1 1 1

a Selectivity cannot be determined by HPLC nor by 1H NMR due to the overlap of signals.

by HRMS, 1H and 13C NMR, IR and electron absorption spectroscopy, the individual structure of each isomer was determined by 1 H NMR spectroscopy. The 1H NMR spectra of C3-symmetric diastereoisomers exhibited typical pattern in accordance with the symmetry of the molecule, whereas the C1-symmetric diastereoisomers showed two or three sets of aromatic proton signals at similar chemical shifts to those of the C3-symmetric diastereoisomers. The unsymmetrically arranged thiazole moieties in these compounds cause also the splitting of tert-butyl group signals into two sets with 2:1 integral ratio or into three sets with the same ratio. The effect of ring current is more prominent for a-benzo protons closest to the SubPc core for ANG at 9.01e8.99 ppm and LIN at 9.46e9.41 and 9.38e9.34 ppm. This phenomenon is also responsible for the downfield shift of the tert-butyl proton signals, which were found at around 1.71 ppm for ANG and 1.64 ppm for LIN. The 1 H NMR spectra of all final products as well as starting materials together with their 13C NMR spectra are shown in ESI. 3.2. Electronic absorption and fluorescence spectra, singlet oxygen generation UV/Vis spectra of these new kind of SubPcs measured in chloroform are presented in Fig. 5 and data in Table 2. For comparison the spectral data of nonannelated SubPc and SubNc are also listed. The absorption spectra show two main transitions analogous to phthalocyanines. The weaker absorption at the 260e400 nm region is equivalent to the split Soret band, a higher-energy one below 300 nm and a second band shifted above 300 nm. An intensive absorption in the 460e650 nm region represents the Q-band with a shoulder typical for SubPcs. Nonplanar structure of SubPcs and the smaller conjugated system (14 vs 18 p-electrons) are the reasons for the blue shift (ca. 130 nm) of both absorption bands compared to planar phthalocyanines [30]. Extension of the conjugated system in thiazole-annelated SubPcs clearly results in red shifted long wave Q-band when compared to the unsubstituted SubPc. Furthermore, the Q-band of linear annelated compounds is more red shifted than

29

the corresponding band in angular annelated derivatives. The same effect, but less intensive, is observed by the introduction of bromine atoms in the LIN-Br and ANG-Br compounds. Similar trend was also noticed in SubNc, but significantly larger red shift was recorded for the linear annelated 2,3-SubNc compared to the angular 1,2SubNc [14b]. Expectantly, removal of the fused benzene rings from SubPc skeleton produces a remarkable hypsochromic shift of the Q-bands (to ca. 500 nm) [32]. As can be seen in Fig. 5 molecular symmetries defined by the arrangement of the thiazole rings have not a remarkable effect on the shape of the absorption Q-band and on the band maxima values of SubPc molecules. Table 2 brings also the experimental data from fluorescence spectroscopy. The shape of emission spectra as well as the positions of fluorescence band maxima (lF) exhibit similar trends as absorption maxima (lA). All values are red shifted compared to the nonannelated analogue. The largest shift of lF to the 628 nm is achieved by the linear annelation and bromine substitution (see Fig. S4 in ESI). Quantum yields of fluorescence FF in the case of nonbrominated SubPcs are greater compared to the (t-Bu)3SubPc (FF ¼ 0.16) [1a]. Heavy atom effect of bromine leads to decrease in fluorescent quantum yield to ca. 0.1 and decrease in excited singlet state lifetime in brominated SubPcs. Interestingly, these annelated SubPcs indicate the presence of two fluorescence lifetimes, the first in the typical range of 2.5 ns and the second atypical long lifetime in the range ca. 10 ns. This phenomenon was also observed for related annelated phthalocyanines [30] but contribution to the overall fluorescence is lower (~5%) in SubPcs than in Pcs (~20%). Encouraging results have been obtained by measuring singlet oxygen generation quantum yield FD in methanol (Table 2). Irrespective of bromine atom presence, all studied SubPcs are highly efficient in the production of singlet oxygen and the obtained values are comparable with reference dye Rose Bengal (FD (RB) ¼ 0.80). Although high FD values indicate effective intersystem crossing (ISC) which is further enhanced by the presence of bromine atom (FF significantly decreases in bromine derivatives), it surprisingly does not yield to higher FD values in ANG-Br. The rate of triplet state deactivation by ISC in ANG-Br is probably sufficiently high and successfully competes with oxygen quenching of triplet state. SubPcs and related macrocycles are known to undergo photodecomposition upon irradiation with light and this is influenced by structural factors [1a]. During the singlet oxygen production measurements, we have not observed the photodegradation of studied SubPcs (Fig. 6). Fig. 7 shows the normalized absorption, excitation and fluorescence spectra measured in chloroform as well as the calculated absorption spectra of angular subphthalocyanine with C3

Fig. 5. Electronic absorption spectra of studied compounds for C1 (left) and C3 (right) isomers in CHCl3.


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova

30 Table 2 Selected spectral data for SubPcs in CHCl3. Compound

lAa [nm]

lFb [nm]

ESc [kJ mol1]

Dnd [cm1]

SubPci SubNc-linj ANG-C1

564 663 590

569 677 600

211 179 201

156 312 282

0.25 0.22 0.27

ANG-C3

589

598

201

280

0.25

LIN-C1

603

614

197

297

0.29

LIN-C3

602

614

197

325

0.28

ANG-Br-C1

594

604

200

279

0.18

ANG-Br-C3

593

603

200

280

0.11

LIN-Br-C1

615

628

192

337

0.06

LIN-Br-C3

614

627

193

338

0.07

a b c d e f g h i j k

FF e

Lifetime t[ns]f e c2g

3.3 2.5 t1 ¼ t2 ¼ t1 ¼ t2 ¼ t1 ¼ t2 ¼ t1 ¼ t2 ¼ t1 ¼ t2 ¼ t1 ¼ t2 ¼ t1 ¼ t2 ¼ t1 ¼ t2 ¼

2.5 8.5 2.5 11.0 2.6 9.6 2.7 10.5 1.1 11.0 1.1 10.7 0.8 9.5 0.9 9.8

(1.094) (1.145) (1.129) (1.190) (1.186) (1.156) (1.171) (1.028)

Results of bi-exponential fitting

t1 (96%) t2 (4%) t1 (96%) t2 (4%) t1 (95%) t2 (5%) t1 (96%) t2 (4%) t1 (96%) t2 (4%) t1 (96%) t2 (4%) t1 (95%) t2 (5%) t1 (95%) t2 (5%)

FDh

0.61 0.68 0.68 0.70 0.68 0.73 0.68 0.62 0.80 -

k

Maximum of main longwavelength absorption band. Maximum of the fluorescence band. Energy of the excited singlet state S1. Stokes shift. Quantum yield of fluorescence - determined using integrating sphere. Fluorescence lifetime. Quality of fitting. Quantum yield of singlet oxygen generation measured in methanol. Measured in benzene [33]. Measured in toluene [8]. Not soluble.

symmetry. The spectra of angular and linear isomer are very similar (Fig. S3). Excitation spectra correspond well with the absorption spectra indicating that no aggregation occurred. The small Stokes shift suggests minor geometry differences between the ground and excited state. To obtain a more detailed insight into the electronic structure and spectral properties of prepared compounds, the quantum-chemical MO study within the framework of the DFT method was carried out. The red shift of the higher-energy band in Soret region and of the Q-band can be observed in the experimental spectrum when compared to the calculated values. The lowerenergy Soret band is mainly represented by the transitions from HOMO-3 (for angular derivatives) and HOMO-5 (for linear

derivatives) to LUMO and LUMOþ1 and the intensive long-wave Qband corresponds to transitions from HOMO to doubly degenerated LUMOs. Calculated values of absorption maxima, oscillator strengths and the contributions of corresponding transitions are collected in Table S1. Frontier MO diagrams of studied compounds are presented in Fig. 8 and the values of calculated orbital energies in Table 3. The HOMOs of all isomers are equally located on the carbon atoms of the central subphthalocyanine core as well as on the attached benzene rings. In all compounds the degenerate LUMO and LUMOþ1 orbitals can be seen. They are spread over two from the three arms and the central unit, the C3 isomers LUMO and LUMOþ1

Fig. 6. Changes in absorption of LIN-Br-C1 solution during irradiation by 520 nm LEDs in the presence of DPIBF in methanol (slight increase in SubPc Q-band absorbance during measurement probably results from limited SubPc solubility). Inset: Time dependence of DPIBF absorbance measured at 405 nm (green dots). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova

Fig. 7. Experimental absorption (blue), excitation (black) and fluorescence spectra (red) measured in chloroform as well as the calculated absorption spectra (green) of ANG-C3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

31

have closer energies as those of C1 isomers, and for ANG-Br-C3 they are identical. The insertion of bromine into the structure causes decrease in HOMO as well as in LUMOs energies and the resulting HOMO-LUMO gap is only slightly smaller (0.02e0.04 eV) in comparison to the nonbrominated compounds. The thiazole ring annelation reduces the calculated energy gap by 0.06e0.21 eV when compared to the tert-butylated subphthalocyanine. Overall, these results are consistent with a stabilisation of the LUMO and a destabilisation of the HOMO when thiazole moieties are inserted into the structure. Computed total energies of studied molecules can be considered as a criterion of stability. The difference in total energy (DFT) between isomers with C1 and C3 symmetry is negligible. The angular isomers are more stable than the linear ones (the total energy difference is about 34 kJ/mol for brominated isomers and 10 kJ/mol for nonbrominated subphthalocyanines). The distance between the central boron atom and the mean plane defined by the three adjacent nitrogen atoms known as “bowl depth” [14b] is a quantitative measure of the subphthalocyanine core nonplanarity. This value is influenced mainly by the axial ligand and varies from 0.59 to 0.66 Å in different SubPcs [1a]. Bowldepth values of our thiazole-annelated subphthalocyanines, obtained from the DFT optimized structures, are in the range 0.600e0.605 Å and the difference from the reference tert-butylated subphthalocyanine (0.599 Å) is minimal. This means that thiazoleannelation does not influence the nonplanarity of the SubPc core.

Fig. 8. Calculated frontier orbitals and their energies for the studied compounds.


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova

32

Table 3 TD-DFT (M06/TZVP/PCM chloroform) calculations of vertical excitation energies (DEe), frontier orbital energies (EHOMO, ELUMO) and energy difference (DE(H-L)) of studied compounds.

DEe

Compound

(tBu)3SubPc-C1 (tBu)3SubPc-C3 ANG-C1 ANG-C3 LIN-C1 LIN-C3 ANG-Br-C1 ANG-Br-C3 LIN-Br-C1 LIN-Br-C3

EHOMO [eV]

[nm]

[eV]

528 528 543 543 564 560 549 548 575 572

2.35 2.35 2.28 2.28 2.20 2.21 2.25 2.26 2.16 2.17

5.62 5.62 5.81 5.81 5.53 5.53 5.95 5.95 5.62 5.63

ELUMO [eV] DE(H-L) [eV]

2.67 2.67 2.92 2.92 2.75 2.73 3.08 3.08 2.88 2.87

2.95 2.95 2.89 2.89 2.78 2.80 2.87 2.87 2.74 2.76

3.3. Circular dichroism spectra Each of C1 and C3 regioisomers is characterized by the inherent chirality that gives rise to a pair of enantiomers. The enantiomers of C3-regioisomers were resolved by chiral HPLC using a mixture of hexane and CH2Cl2. The separation of C1-regioisomers into enantiomers was successful only for the angular derivatives. Complete separation of remaining C1-regioisomers was prevented by a lower solubility and it was not possible to link the CD signs. A perfect mirror image is observed in the experimental CD spectra of all enantiomers over the entire absorption region, clearly indicating the inherent molecular chirality. When comparing the experimental and simulated (obtained by DFT calculation) CD spectra, good agreement can be found in the character of the spectrum (the sign, position and intensity of spectral signals). The configuration of particular enantiomers was assigned according to the character of the calculated CD spectra. Fig. 9 presents the structure and CD spectra (measured and simulated) of ANG-C3 enantiomers, where the thiazole fragment is annelated by the clockwise (c) or anticlockwise (a) manner. It holds for our compounds with C3 symmetry that the negative CD sign of Q-band corresponds to three clockwise arranged thiazole moieties and the corresponding enantiomer is assigned as ccc. CD spectrum

of the opposite enantiomer presents a perfect mirror image of CD signal and this enantiomer is assigned as aaa. The enantiomer conformations of compound ANG-Br-C3 have been assigned in the same way (Fig. S1). The sign of the CD spectra Q-band signal is consistent with data published in Ref. [14b]. For comparison the absorption spectrum (measured in chloroform and calculated) is also presented in Fig. 9. The higher-energy band in Soret region and the Q-band in experimental absorption and CD spectrum are a little red shifted when compared to the calculated spectrum. It is known that, in the absence of aggregation, the shapes of the CD spectra of allowed transitions are similar to those of the electronic absorption spectrum, apart from the sign [34]. In angular isomers with C1 symmetry the sign in CD Q-band is negative for the enantiomer with two clockwise and one anticlockwise arrangement of the annelated thiazole rings. This enantiomer is assigned as cac. The mirror-imaged molecular structure with two anticlockwise and one clockwise arrangement is attributed to aca-enantiomer. Similar relation between CD spectra and enantiomer configuration has been used for angular subnaphthalocyanines in Ref. [14b]. The linear isomers configuration assignment is not so straightforward and as a main criterion the sign of the CD spectra Q-band was used. In compound LIN-C3 a negative sign of CD Q-band satisfied the principle with the clockwise arrangement of the annelated thiazole rings if S-heteroatom is preferred to N-heteroatom and this enantiomer was assigned as ccc. Opposite enantiomer with anticlockwise annelated thiazole rings was assigned as aaa (Fig. 10). In compound LIN-Br-C3 the Q-band is less intensive. Regarding the Br-atom as the most preferred, the clockwise arrangement of three linearly annelated thiazole fragments with respect to bromine atoms corresponds with ccc-enantiomer. This arrangement belongs to the enantiomer with a negative sign of CD Q-band (Fig. 11), while the opposite enantiomer is assigned as aaa. CD spectra and assignation of configurations of all isomers are presented in ESI (Fig. S1). It can be concluded that structurechirality relationship can be applied to the studied subphthalocyanine systems as follows. Negative CD signs in the Q-

Fig. 9. Configuration of enantiomers, experimental and calculated CD spectra, experimental (blue) and calculated (red) absorption spectra of ANG-C3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova

33

Fig. 10. Configuration of enantiomers, experimental and simulated CD spectra of LIN-C3.

Fig. 11. Configuration of enantiomers, experimental and simulated CD spectra of LIN-Br-C3.

band region are indicative of the ccc and cac enantiomers and the positive ones of aaa and aca enantiomers, respectively. The CD signal intensity is evaluated by the rotational strength R given by the Rosenfeld equation: R ¼ Im(m$m) [35], which denotes the imaginary part of the direct coupling of electronic transition dipole moment m and the magnetic transition dipole moment m. Considering that circular redistribution of charges in transitions generates transition magnetic dipole moments, the unsymmetrical arrangement of benzothiazole moieties influences the intensity of CD signals. From the calculated rotational strengths (Table S1) and the available experimental CD spectra, some structural influences on signal intensity can be deduced. All C1 isomers show smaller rotational strengths throughout the full spectra as the corresponding C3 isomers due to one oppositely arranged benzothiazole ring. This trend was also witnessed by chiral 1,2-subnaphthalocyanines [14b]. Angular derivatives have more intense Q-band signals as the linear ones, most likely because in the case of the latter the chirality is based only on the position of nitrogen, sulphur and bromine atoms and not the thiazole ring as a whole. The influence of bromine itself is more evident for linear derivatives where the Q-band intensity decreases by ca. 50% for brominated derivatives, whereas for angular ones only by ca. 10%. 3.4. Electrochemical properties The electrochemical behaviour of annelated SubPcs was investigated by cyclic voltammetry (CV) and square wave voltammetry

(SW) in CH2Cl2 at room temperature with ferrocene as an internal standard. The half-wave redox potentials (E1/2) are summarized in Table 4 and given vs (Fc/Fcþ). Since the boron atom is not electrochemically active, redox properties of SubPcs are dependent on the nature of the macrocyclic aromatic core mainly on the character of the peripheral substituents [1a].

Table 4 Half-wave redox potentials Compound ANG-C3 LIN-C1 LIN-C3 ANG-Br-C1 ANG-Br-C3 LIN-Br-C1 LIN-Br-C3 a

E1/2 (Red3)

1.84 1.81

a

of the compounds in [V] vs Fc/Fcþ couple in CH2Cl2. E1/2 (Red2)

1.64 1.61

E1/2 (Red1)

E1/2 (Ox1)

1.31 1.43 1.43 1.17 1.15 1.31 1.28

þ0.73 þ0.50 þ0.50 þ0.83 þ0.83 þ0.59 þ0.60

E1/2 (Ox2) þ1.08 þ1.09

þ1.07

Potentials were recorded from SW voltammetry.

When cyclic voltammograms (V vs Ag/Agþ) of C1 and C3 isomers are compared, the differences are negligible (Fig S8). All compounds in the series underwent one irreversible reduction in the electrochemical window for CH2Cl2, except the most electrondeficient compound ANG-Br that recorded three reductions (Fig. 12.) The current results showed that annelated thiazole rings


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova

34

Fig. 12. Cyclic voltammogram (CH2Cl2 containing 0.1 M tetrabutylammonium hexafluorophosphate at a scan rate of 100 mV/s) of ANG-Br-C1, only reduction presented.

first oxidation and the first reduction, which reflects the HOMOLUMO gap, was found to be between 1.89 and 2.04 V, a little smaller in comparison to that of unsubstituted SubPc (2.09 V) but bigger than SubNc (1.53 V) [1b]. When values of potential differences calculated from electrochemical data are lined up from unsubstituted SubPc through angularly and linearly annelated SubPcs to SubNc, they are directly related to the red shifts of the lowest energy absorption bands: 564 nm (SubPc), 590 nm (ANG), 594 nm (ANG-Br), 603 nm (LIN), 615 nm (LIN-Br) and 663 nm (SubNc), all in accordance with the theoretical predictions. From the intersection between the two Q-bands (absorption and emission spectra), singlet excited state energies of 2.0e2.1 eV are typically derived for SubPcs and are consistent with the HOMO-LUMO gaps derived from cyclic voltammetry measurements.

Table 5 HOMO and LUMO energy level values determined from experimental cyclic voltammetry (cv) and theoretical calculations (th), and band gap energy values found from UV/Vis measurements. All data are given in [eV]. Determination methods

Electrochemistry

Compound

HOMOcv

LUMOcv

Ecva g

HOMOth

LUMOth

Ethb g

ANG-C3 LIN-C1 LIN-C3 ANG-Br-C1 ANG-Br-C3 LIN-Br-C1 LIN-Br-C3

5.53 5.30 5.30 5.63 5.63 5.39 5.40

3.49 3.37 3.37 3.63 3.65 3.49 3.51

2.04 1.93 1.93 2.00 1.98 1.90 1.89

5.81 5.53 5.53 5.95 5.95 5.62 5.63

2.92 2.75 2.73 3.08 3.08 2.88 2.87

2.89 2.78 2.80 2.87 2.87 2.74 2.76

a b c

Theoretical calculations

From UV/Vis absorption band edge Eoptc g 2.09 2.04 2.04 2.07 2.07 2.00 2.00

Potential difference between the first oxidation and the first reduction processes. Measurements in CH2Cl2 solution. The difference between the calculated energies (M06/TZVP/PCM chloroform) of HOMO and LUMO. Optical band gap, estimated from intersection of normalized absorption and emission spectra recorded in CHCl3 solution.

significantly strengthened the electron-deficient character of the macrocycle, what is clearly demonstrated by the position of the first reduction potentials (Table 4). This phenomenon is more significant for the angular pattern of annelation. Annelated subphthalocyanines are reduced easier relative to the unsubstituted or tert-butyl derivatized SubPc in the following order: (tBu)3SubPc < SubPc < LIN < ANG z LIN-Br < ANG-Br (Table S2, ref. [33,36]). Consequently, they are less prone to oxidation than the SubPc and their homologue SubNc. The potentials are shifted to more positive values due to the acceptor character of the thiazole ring in which the oxidation becomes more difficult. On the other hand, the oxidation appears to be reversible or quasi-reversible for both brominated subphthalocyanine (LIN-Br and ANG-Br) and irreversible for LIN and ANG as shown in Fig S8. Obviously, introducing electron-withdrawing bromine atoms in the periphery of the SubPc has an impact on the reversibility of the first oxidation. A second irreversible oxidation was manifested only for linear derivatives. Compared to another SubPcs, a more difficult reversible oxidation is observed for ANG-Br at þ0.83 V. In addition, the small variation in the value of potentials for LIN and ANG (0.23 eV for oxidation and 0.12e0.13 eV for reduction) also inferred interaction both in linear and angular type of annelation. Cyclic voltammetry measurements on SubPc derivatives provided further information about the frontier MOs and their relative energies (Table 5). The potential difference between the

4. Conclusion A series of SubPcs with annelated thiazole rings have been designed, synthesized and characterized. The symmetry and inherent molecular chirality arising from the pattern of substitution were also investigated. Effect of the different type of thiazole ring annelation on the HOMO-LUMO energy gaps was studied based on DFT calculations. Starting from four altered benzothiazole dinitriles all possible isomers were prepared with two different symmetries. Those with C3 symmetry were further resolved to enantiomers with inherent chirality using chiral HPLC techniques. All the enantiomers were determined by CD spectra which were in an excellent accordance with computer aided simulations. Introduction of the electronwithdrawing thiazole units to the periphery of SubPc caused a sizeable absorption and fluorescence Q-band red shift. The presence of bromine atoms in target compounds was manifested by substantial lowering of fluorescence quantum yields compared to unsubstituted SubPcs. On the other side both bromo-substituted and parent SubPcs showed unique singlet oxygen production at the levels that reached or surpassed the values for standard RB dye. Therefore, annelated SubPcs can be suitable for design of new photosensitisers providing photoproduction of singlet oxygen. Significant impact of different annelation type was observed in the electrochemical behaviour. The angular annelated thiazole is a stronger electron acceptor with lower E1/2 (Red1) value compared


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova

to the linear annelated analogue. Prepared subphthalocyanines enable the synthesis of higher order derivatives by coupling reactions in the case of brominated SubPcs. Acknowledgements This work has been supported by the Slovak Research and Development Agency (grant numbers APVV 0424-10, APVV 062212 and APVV-14-0716) and by the Grant from Comenius University No. UK/548/2012 and UK/481/2013. This research is also the result of the project implementation: Comenius University in Bratislava Science Park supported by the Research and Development Operational Programme funded by the ERDF, Grant Number ITMS 26240220086. The authors thank to Assoc. Prof. Veronika Novakova for performing cyclic voltammetry measurements. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2016.03.001. References
lez-Rodríguez D, Torres T. Subphthalocyanines: sin[1] a) Claessens CG, Gonza gular nonplanar aromatic compounds e synthesis, reactivity, and physical properties. Chem Rev 2002;102:835e54.
lez-Rodríguez D, Rodríguez-Morgade MS, Medina A, b) Claessens CG, Gonza Torres T. Subphthalocyanines, subporphyrazines and subporphyrins: singular nonplanar aromatic systems. Chem Rev 2014;114:2192e277. c) Shimizu S, Kobayashi N. Structurally-modified subphthalocyanines: molecular design towards realization of expected properties from the electronic structure and structural features of subphthalocyanine. Chem Commun 2014;50:6949e66. [2] a) Kobayashi N, Kondo R, Nakajima S, Osa T. New route to unsymmetrical phthalocyanine analogs by the use of structurally distorted subphthalocyanines. J Am Chem Soc 1990;112:9640e1. b) Kobayashi N, Ishizaki T, Ishii K, Konami H. Synthesis, spectroscopy, and molecular orbital calculations of subazaporphyrins, subphthalocyanines, subnaphthalocyanines, and compounds derived therefrom by ring expansion. J Am Chem Soc 1999;121:9096e110. c) Claessens CG, Torres T. Synthesis of unsymmetrically substituted subphthalocyanines. Chem Eur J 2000;6:2168e72. [3] a) Morse GE, Bender TP. Boron subphthalocyanines as organic electronic materials. ACS Appl Mater Interfac 2012;4:5055e68. b) Takao Y, Masuoka T, Yamamoto K, Mizutani T, Matsumoto F, Moriwaki K, et al. Synthesis and properties of novel fluorinated subnaphthalocyanines for organic photovoltaic cells. Tetrahedron Lett 2014;55:4564e7. [4] Morse GE, Helander MG, Maka JF, Lu ZH, Bender TP. Fluorinated phenoxy boron subphthalocyanines in organic light-emitting diodes. ACS Appl Mater Interfaces 2010;2(7):1934e44. [5] Xu S, Chen K, Tian H. A colorimetric and fluorescent chemodosimeter: fluoride ion sensing by an axial-substituted subphthalocyanine. J Mater Chem 2005;15:2676e80. [6] Renshaw CK, Xu X, Forrest SR. A monolithically integrated organic photodetector and thin film transistor. Org Electron 2010;11:175e8.
lez-Rodríguez D, Torres T, Martín G, Agullo
-Lo
pez F, [7] Claessens CG, Gonza Ledoux I, et al. Structural modulation of the dipolar-octupolar contributions to the NLO response in subphthalocyanines. J Phys Chem B 2005;109:3800e6. [8] Nonell S, Rubio N, Del Rey B, Torres T. Synthesis, optical absorption and photophysical properties of cone-shaped subnaphthalocyanine. J Chem Soc Perkin Trans 2000;2:1091e4. [9] Claessens CG, Gonz
alez-Rodríguez D, del Rey B, Torres T, Mark G, Schuchmann H-P, et al. Highly efficient synthesis of chloro- and phenoxysubstituted subphthalocyanines. Eur J Org Chem 2003:2547e51. [10] a) Gonz
alez-Rodríguez D, Torres T. Peripheral functionalization of subphthalocyanines. Eur J Org Chem 2009:1871e9.
lez-Rodríguez D, Claessens CG, Torres T, Liu S, Echegoyen L, Vila N, b) Gonza et al. Tuning photoinduced energy- and electron-transfer events in subphthalocyanineephthalocyanine dyads. Chem Eur J 2005;11:3881e93.
lez-Rodríguez D, Torres T. Triflate-subphthalocyanines: [11] a) Guilleme J, Gonza versatile, reactive intermediates for axial functionalization at the boron atom. Angew Chem Int Ed 2011;50:3506e9. b) Morse GE, Bender TP. Aluminum chloride activation of chloroboronsubphthalocyanine: a rapid and flexible method for axial functionalization with an expanded set of nucleophiles. Inorg Chem 2012;51:6460e7. [12] Martínez-Díaz MV, Quintiliani M, Torres T. Functionalisation of phthalocyanines and subphthalocyanines by transition-metal-catalysed reactions. Synlett 2008;1:1e20.

35

[13] Claessens CG, Torres T. Subphthalocyanine enantiomers: first resolution of a C3 aromatic compound by HPLC. Tetrahedron Lett 2000;41:6361e5. [14] a) Kobayashi N, Nonomura T. First observation of the circular dichroism spectra of chiral subphthalocyanines with C3 symmetry. Tetrahedron Lett 2002;43:4253e5. b) Shimizu S, Miura A, Khene S, Nyokong T, Kobayashi N. Chiral 1,2subnaphthalocyanines. J Am Chem Soc 2011;133:17322e8. c) Zhao L, Wang K, Furuyama T, Jiang J, Kobayashi N. Synthesis and spectroscopic properties of chiral binaphthyl-linked subphthalocyanines. Chem Commun 2014;50:7663e5. [15] a) Shang H, Zhao L, Qi D, Chen C, Jiang J. The first five-membered-heterocyclefused subphthalocyanine analogues: chiral tri(benzo[b]thiopheno)subporphyrazines. Chem Eur J 2014;20:16266e72. b) Shimizu S, Yamazaki Y, Kobayashi N. Tetrathiafulvalene-annulated subphthalocyanines. Chem Eur J 2013;19:7324e7. [16] Mathai S, Smith TA, Ghiggino KP. Singlet oxygen quantum yields of potential porphyrin-based photosensitisers for photodynamic therapy. Photochem Photobiol Sci 2007;6:995e1002.  M, Ga
plovský A, Sigmundov
dic R, Hromadov
[17] Cig
an a I, Zahradník P, De a M. Photostability of DepeA nonlinear optical chromophores containing a benzothiazolium acceptor. J Phys Org Chem 2011;24:450e9.
plovský A, Toma S, Donovalova
J. Reactors for simultaneous application of [18] Ga UV light and ultrasound on the reaction mixture and their modifications for kinetic studies. J Photochem Photobiol A 2007;191:162e6. [19] TURBOMOLE V6.6. In: A development of university of karlsruhe and forschungszentrum karlsruhe GmbH, 1989-2007, TURBOMOLE GmbH, since 2007; 2014. available from: http://www.turbomole.com. [20] Treutler O, Ahlrichs R. Efficient molecular numerical integration schemes. J Chem Phys 1995;102:346e54. [21] a) Dirac PAM. Quantum mechanics of many-electron systems. Proc R Soc Lond Ser A 1929;123:714e33. b) Slater JC. A simplification of the Hartree-Fock method. Phys Rev 1951;81: 385e90. c) Perdew JP, Wang Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys Rev B 1992;45:13244e9. d) Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. e) Perdew JP, Ernzerhof M, Burke K. Rationale for mixing exact exchange with density functional approximations. J Chem Phys 1996;105:9982e5. €ser M, Patzelt H, Ahlrichs R. RI-MP2: optimized auxiliary [22] a) Weigend F, Ha basis sets and demonstration of efficiency. Chem Phys Lett 1998;294:143e52. b) Weigend F, Ahlrichs R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: design and assessment of accuracy. Phys Chem Chem Phys 2005;7:3297e305. [23] a) Bauernschmitt R, Ahlrichs R. Treatment of electronic excitations within the adiabatic approximation of time dependent density functional theory. Chem Phys Lett 1996;256:454e64. b) Scalmani G, Frisch MJ, Mennucci B, Tomasi J, Cammi R, Barone V. Geometries and properties of excited states in the gas phase and in solution: theory and application of a time-dependent density functional theory polarizable continuum model. J Chem Phys 2006;124:1e15. [24] Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. [25] Zhao Y, Truhlar DG. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor Chem Acc 2008;120:215e41. [26] a) Schaefer A, Horn H, Ahlrichs R. Fully optimized contracted Gaussian-basis sets for atoms Li to Kr. J Chem Phys 1992;97:2571e7. b) Schaefer A, Huber C, Ahlrichs R. Fully optimized contracted Gaussian-basis sets of triple zeta valence quality for atoms Li to Kr. J Chem Phys 1994;100: 5829e35. [27] Becke AD. Density-functional thermochemistry. III. The role of exact exchange. J Chem Phys 1993;98:5648e52. [28] a) Ditchfield R, Hehre WJ, Pople JA. Self-consistent molecular orbital methods. 9. Extended Gaussian-type basis for molecular-orbital studies of organic molecules. J Chem Phys 1971;54:724e8. b) Hehre WJ, Ditchfield R, Pople JA. Self-consistent molecular orbital methods. 12. Further extensions of Gaussian-type basis sets for use in molecular-orbital studies of organic-molecules. J Chem Phys 1972;56:2257e61. c) Hariharan PC, Pople JA. Influence of polarization functions on molecularorbital hydrogenation energies. Theor Chem Acc 1973;28:213e22. d) Hariharan PC, Pople JA. Accuracy of AH equilibrium geometries by single

36


et al. / Dyes and Pigments 130 (2016) 24e36 M.M. Necedova

determinant molecular-orbital theory. Mol Phys 1974;27:209e14. e) Gordon MS. The isomers of silacyclopropane. Chem Phys Lett 1980;76: 163e8. f) Francl MM, Pietro WJ, Hehre WJ, Binkley JS, DeFrees DJ, Pople JA, et al. SelfConsistent Molecular Orbital Methods. 23. A polarization-type basis set for 2nd-row elements. J Chem Phys 1982;77:3654e65. g) Binning Jr RC, Curtiss LA. Compact contracted basis-sets for 3rd-row atoms - GA-KR. J Comp Chem 1990;11:206e16. h) Blaudeau J-P, McGrath MP, Curtiss LA, Radom L. Extension of Gaussian-2 (G2) theory to molecules containing third-row atoms K and Ca. J Chem Phys 1997;107:5016e21. i) Clark T, Chandrasekhar J, Spitznagel GW, Von Schleyer PR. Efficient diffuse function-augmented basis-sets for anion calculations. 3. The 3-21þG basis set for 1st-row elements, LieF. J Comp Chem 1983;4:294e301. [29] a) Miertus S, Scrocco E, Tomasi J. Electrostatic interaction of a solute with a continuum. A direct utilization of ab initio molecular potentials for the prevision of solvent effects. Chem Phys 1981;55:117e29. b) Cossi M, Barone V. Solvent effect on vertical electronic transitions by the polarizable continuum model. J Chem Phys 2000;112:2427e35.
M, Magdolen P, Novakova V, Ciga
n  M, Vl [30] Ne cedova ckov
a S, Zahradník P, et al.

[31]

[32]

[33]

[34] [35]

[36]

Synthesis and photophysical, electrochemical and theoretical study of thiazole-annelated phthalocyanines. Eur J Org Chem 2015:7053e68. The method for one-pot thionation and cyclization described. In: Spieler I, Prijs B, editors. Zur Kenntnis des 5-Amino-benzothiazols. Helv Chim Acta33; 1950. p. 1429e33 [was modified]. Rio Y, Rodríguez-Morgrade MS, Torres T. Modulating the electronic properties of porphyrinoids: a voyage from the violet to the infrared regions of the electromagnetic spectrum. Org Biomol Chem 2008;6:1877e94.
-Lo
pez F, Nonell S, et al. Synthesis Del Rey B, Keller U, Torres T, Rojo F, Agullo and nonlinear optical, photophysical, and electrochemical properties of subphthalocyanines. J Am Chem Soc 1998;120:12808e17. Moffitt W, Moscowitz A. Optical activity in absorbing media. J Chem Phys 1959;30:648e59. a) Rosenfeld L. Quantenmechanische Theorie der Natürlichen optischen Aktivit€ at von Flüssigkeiten und Gasen. Z Phys 1928;52:161e74. b) Roger A, Norden B. Circular dichroism and liner dichroism. Oxford: Oxford University Press; 1997.
nez-Banzo A, Torres T, Nonell S. Spectral and kinetic properties Rubio N, Jime of the radical ions of chloroboron(III) subnaphthalocyanine. J Photochem Photobiol A Chem 2007;185:214e9.