1. Introduction
Thin-film organic photovoltaic technology has been the subject of considerable attention because of the advantages it provides, such as light devices and low preparation cost [
1,
2,
3,
4]. Within this field, small-molecule heterojunction solar cells receive more attention because of their clear molecular structure, molecular weight and controllable material purity [
5,
6,
7]. In small-molecule solar cells, the use of fullerenes and phthalocyanines has been widely reported [
8,
9]; nevertheless, there are a large number of small molecules that are also worthy of study. In this regard, organotin(IV) complexes may be a novel option. Organotin(IV) derivatives have been given special consideration because of the stable bonds formed by tin with carbon atoms, as well as with heteroatoms [
10,
11,
12]. Particular consideration has been given to organotin complexes derived from Schiff bases, since these bases provide a wide variety of molecular structural conformations as ligands [
13,
14,
15,
16,
17]. Moreover, complexes with Schiff bases show thermo- and photostability, are easy to synthesize and have the versatility to tune optical and optoelectronic properties through the chemical modulation of the ligands [
18]. The optoelectronic activity of organotin(IV) complexes depends upon the number and nature of the radical groups linked to the metal ion, as well as on the anionic ligand [
19]. Their azomethine C=N group presents an electron-withdrawing character that, combined with tin, results in “push-pull” molecules with non-linear optical and electrical properties [
17,
20,
21,
22,
23], which may represent potential applications in optoelectronics. According to the push–pull model, photoinduced charge transfer is possible and usable in photovoltaic devices [
21,
24]. Organotin(IV) complexes have also been reported as sensitizers [
25], PVC stabilizers [
26], fungicides, catalysts, and active electroluminescent layers in organic light-emitting diodes (OLEDs) [
27]. In addition, organotin(IV) complexes can be used in the manufacture of semiconductor films, in which their electrical conductivity can be increased by adding substituents to their molecular structure [
28,
29,
30]. Charge transport is present in organotin(IV) films due to their π-conjugated structures and the presence of electronegative atoms and substituents coordinated to the tin atom [
31].
Keeping the above in mind, and in order to broaden the study of different tin coordination structures, in this work, we report the synthesis of four pentacoordinated organotin(IV) complexes prepared in a one-pot reaction from 2-hydroxy-1-naphthaldehyde, 2-amino-3-hydroxypyridine and organotin(IV) oxides. This work is divided into three parts: the first one corresponds to the synthesis and characterization of the pentacoordinated organotin(IV) complexes; the second one refers to the fabrication and characterization of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-graphene-organotin(IV) complex hybrid films, and the third one refers to the manufacture and electrical characterization of devices made from the hybrid films. It is important to mention that PEDOT:PSS is one of the best-known conjugate polymers that have proved their value in the development of electronic devices. PEDOT:PSS provides excellent optoelectronic functionalities, as highly transparent and electrochemically stable conducting films can be obtained from its doped state. The combination of PEDOT:PSS with chemically different structures, such as graphene, permits the improvement of their electrical properties and their use in various kinds of photovoltaic applications [
32,
33,
34,
35,
36]. Because of their π-π stacking interactions, the conjugated aromatic chains of PEDOT:PSS can be stably fixed on graphene sheets without destroying the electronic structure of graphene [
37]. The π-π stacking interactions between graphene and PEDOT:PSS results in enhanced electrical conductivity, chemical stability and thermoelectric performance [
38]. In this work, PEDOT:PSS and graphene function as a hole-transport layer (HTL), while the organotin complexes operate as components of the active layer of the device.
The behavior of organotin(IV) complexes is related to their π-conjugated structure, including the presence of electron-donating or electron-accepting substituents. This turns organotin(IV) complexes into anisotropic semiconductors with preferential conduction channels for charge transport. The novelty of this work is related to the synthesis of new pentacoordinated tin complexes and their use in hybrid films with optical and electrical properties.
3. Materials and Methods
All reagents and solvents were obtained from commercial suppliers (Merck KGaA, Darmstadt, Germany) and used without further purification. Cyclohexyl- and bis[(trimethylsilyl)methyl]tin oxides were prepared using the method previously described in the literature [
72].
3.1. Physical Measurements
Melting points were measured with a Fischer-Johns MEL-TEMP II (Thermo Scientific, Waltham, MA, USA) apparatus and are not corrected. IR spectra were obtained with a Bruker Tensor 27 apparatus (BRUKER, Ettlingen, Germany) by means of the attenuated total reflection (ATR) technique. The NMR spectra of
1H,
13C and
119Sn were acquired using the Bruker Avance III instrument (BRUKER, Rheinstetten, Germany) at 300 MHz, 400 MHz and HD 500 MHz in CDCl
3. Direct analysis in real time (DART) mass spectra were recorded on a JEOL-JMS-T100LC instrument (JEOL, Tokyo, Japan). The X-ray diffraction study was performed with a BRUKER SMART APEX CCD diffractometer (BRUKER, Karlsruhe, Germany) at a wavelength λ
(Mo-Kα) = 0.71073 Ǻ (graphite monochromator) and temperature T = 298 K for complex
1c. The structure was solved using direct methods in the SHELXS program. All non-hydrogen atoms were anisotropically refined using a full-matrix, least-squares technique, and all hydrogen atoms were placed in idealized positions using the riding hydrogen model approximation. Structural solutions and refinements were performed using SHELXTL [
73]. The molecular structure and packing were drawn with DIAMOND [
74]. Molar conductivity measurements were recorded with a Hanna 6484 system using anhydrous methanol as a solvent. The UV-Vis spectra were recorded using a Cary 50 Varian instrument with anhydrous methanol as a solvent at 2.04530 × 10
−5 M. The study of the thermal degradation was carried out using simultaneous TGA-DSC (Simultaneous Thermal Analyzer STA 449 F3 Jupiter, NETZSCH-Gerätebau GmbH - Selb, Germany). In each trial, a 4 mg constant-mass sample was placed in a 25 μL aluminum crucible and then heated from room temperature to 550 °C under a nitrogen atmosphere and using a total flow rate of 50 mL min
−1.
3.2. General Procedure for the Synthesis of Diorganotin(IV) Compounds 1a–d
2-Hydroxy-1-naphthaldehyde and the corresponding diorganotin oxide were added in a stoichiometric ratio (1:1:1) to a solution of 2-amino-3-hydroxypyridine. The reaction mixture was refluxed for 30 h using a Dean–Stark trap in the case of complexes 1a and 1d, while the corresponding time for complexes 1b and 1c (containing the phenyl and cyclohexyl moieties) was 48 h. Afterward, the solvent was eliminated under reduced pressure to produce the desired complexes. All complexes were purified by methanol recrystallization.
3.3. 2,2-Dibutyl-6-aza-1,3-dioxa-2-stannanaphtho[1,2-h]pyrido[3,2-d]cyclononene (1a)
Compound 1a was prepared from 0.157 g (0.907 mmol) of 2-hydroxy-1-naphthaldehyde, 0.101 g (0.907 mmol) of 2-amino-3-hydroxypyridine and 0.2264 g (0.907 mmol) of dibutyltin oxide, producing 0.376 g (84%) of a brown solid; m.p. 87–89 °C. (dec); molar conductance, ΛM (1 × 10−3 M, methanol): 8.7 μS cm−1; UV-Vis [methanol, λmáx/nm (ε/M−1 cm−1)]: 219 (30,832), 253 (17,567) π → π* (aromatic), 363 (4034) π → π* (C=N), 483 (13,274) n → π* (C=N); IR (ATR) cm−1: 3043 ν(C-H), 2948 νas(C-H), 2920 νas(C-H), 2865 νsim(C-H), 2846 νsim(C-H), 1601 ν(C=O), 606 ν(Sn-C), 548 ν(Sn-O), 483 ν(Sn-N); 1H NMR (300.18 MHz, CDCl3) δ: 0.85 (6H, t, J = 7.51 Hz, H-δ), 1.34 (4H, Sext, J = 7.51, H-γ), 1.50–1.55 (4H, m, H-α), 1.61–1.72 (4H, m, H-β), 6.89 (1H, d, J = 9.32 Hz, H-2), 7.07–7.08 (2H, H-14, H-15), 7.33 (1H, ddd, J = 0.90, 7.21, 7.21 Hz, H-6), 7.56 (1H, ddd, J = 1.50, 7.21, 7.21 Hz, H-7), 7.67 (1H, dd, J = 0.90, 7.81 Hz, H-5), 7.78 (1H, dd, J = 2.40, 3.91 Hz, H-13), 7.81 (1H, d, J = 9.32 Hz, H-3), 8.26 (1H, d, J = 8.41 Hz, H-8), 10.48 (1H, s, 3J(1H-117/119Sn) = 52 Hz, H-11); 13C NMR (100.62 MHz, CDCl3) δ: 174.2 (C-1), 157.7 (C-11), 153.8 (C-16), 145.2 (C-12), 139.7 (C-3), 135.2 (C-13), 134.9 (C-9), 129.4 (C-5), 128.7 (C-7), 127.1 (C-4), 124.8 (C-2), 124.5 (C-14), 124.3 (C-15), 123.6 (C-6), 119.5 (C-8), 108.8 (C-10), 26.9 (2J(13C-117/119Sn) = 40 Hz, C-β), 26.6 (C-γ), 22.4 (C-α), 13.6 (C-δ); 119Sn NMR (112.04 MHz, CDCl3) δ: −191; MS: (DART+) [m/z] (%): [(M+ + 1), 497] (33). HR-MS (DART+) m/z: 497.12151 (calc. for12C241H2914N216O2120Sn), observed: 497.12384.
3.4. 2,2-Dicyclohexyl-6-aza-1,3-dioxa-2-stannanaphtho[1,2-h]pyrido[3,2-d]cyclononene (1b)
Compound 1b was prepared from 0.236 g (1.365 mmol) of 2-hydroxy-1-naphthaldehyde, 0.151 g (1.365 mmol) of 2-amino-3-hydroxypyridine and 0.410 g (1.365 mmol) of dicyclohexyltin(IV) oxide, producing 0.409 g (55%) of a brown solid; m.p. 69–71 °C; molar conductance, ΛM (1 × 10−3 M, methanol): 7.9 μS cm−1; UV-Vis [methanol, λmáx/nm (ε/M−1 cm−1)]: 214 (104,238), 256 (40,336) π → π* (aromatic), 347 (7285) π → π* (C=N), 485 (20,730) n → π* (C=N); IR (ATR) cm−1: 2916 νas(C-H), 2845 νsim(C-H), 1599 ν(C=O), 601 ν(Sn-C), 548 ν(Sn-O), 488 ν(Sn-N); 1H NMR (500.17 MHz, CDCl3) δ: 1.50–2.17 (22H, m, H-α, H-β, H-γ, H-δ), 6.93 (1H, d, J = 9.20 Hz, H-2), 7.06 (1H, dd, J = 4.50, 8.00 Hz, H-15), 7.11 (1H, dd, J = 1.50, 8.00 Hz, H-14), 7.32 (1H, ddd, J = 0.50, 7.00 Hz, H-6), 7.55 (1H, ddd, J = 1.00, 7.00 Hz, H-7), 7.67 (1H, dd, J = 1.00, 8.00 Hz, H-5), 7.76 (1H, dd, J = 1.50, 4.00 Hz, H-13), 7.81 (1H, d, J = 7.95 Hz, H-3), 8.27 (1H, d, J = 8.50 Hz, H-8), 10.56 (1H, s, 3J(1H-117/119Sn) = 49 Hz, H-11); 13C NMR (100.62 MHz, CDCl3) δ: 174.7 (C-1), 157.6 (C-11), 154.4 (C-16), 145.5 (C-12), 139.6 (C-3), 135.0 (C-9), 134.9 (C-13), 129.3 (C-5), 128.6 (C-7), 127.1 (C-4), 125.0 (C-2), 124.4 (C-14), 124.2 (C-15), 123.5 (C-6), 119.5 (C-8), 108.8 (C-10), 40.2 (1J(13C-119/117Sn) = 600, 574 Hz, C-α), 30.9 (2J(13C-119Sn) = 25 Hz, C-β), 28.6 (3J(13C-119/117Sn) = 94, 81 Hz C-γ), 26.6 (C-δ); 119Sn NMR (149.18 MHz, CDCl3) δ: −258; MS: (DART+) [m/z] (%): [(M+ + 1), 549] (17); HR-MS (DART+) m/z: 549.15640 (calc. for12C281H3314N216O2120Sn), observed: 549.15405.
3.5. 2,2-Diphenyl-6-aza-1,3-dioxa-2-stannanaphtho[1,2-h]pyrido[3,2-d]cyclononene (1c)
Compound 1c was prepared from 0.157 g (0.910 mmol) of 2-hydroxy-1-naphthaldehyde, 0.101 g (0.910 mmol) of 2-amino-3-hydroxypyridine and 0.2631 g (0.910 mmol) of diphenyltin(IV) oxide, producing 0.275 g (57%) of an orange solid; m.p. 148–150 °C; molar conductance, ΛM (1 × 10−3 M, methanol): 10.6 μS cm−1; UV-Vis [methanol, λmáx/nm (ε/M−1 cm−1)]: 220 (77,225), 255 (31,242) π → π* (aromatic), 371 (7529) π → π* (C=N), 481 (21,488) n → π* (C=N); IR (ATR) cm−1: 3044 ν(C-H), 2948 νas(C-H), 2921 νas(C-H), 2866 νsim(C-H), 2847 νsim(C-H), 1601 ν(C=O), 607 ν(Sn-C), 550 ν(Sn-O), 485 ν(Sn-N); 1H NMR (400.13 MHz, CDCl3) δ: 7.11 (1H, dd, J = 4.40, 8.00 Hz, H-14), 7.19 (1H, d, J = 9.20 Hz, H-2), 7.28 (1H, dd, J = 0.40, 6.80 Hz, H-6), 7.32 (1H, d, J = 1.29, 8.00 Hz, H-15), 7.37–7.42 (6H, m, H-m, H-p), 7.50 (1H, ddd, J = 1.20, 7.20, 7.20 Hz, H-7), 7.64 (1H, dd, J = 1.20, 8.00 Hz, H-5), 7.80 (1H, dd, J = 1.20, 4.40 Hz, H-13), 7.87 (1H, d, J = 9.20 Hz, H-3), 7.90–7.96 (4H, m, H-o), 8.20 (1H, d, J = 8.40 Hz, H-8), 10.46 (1H, s, 3J(1H-117/119Sn) = 65 Hz, H-11); 13C NMR (100.62 MHz, CDCl3) δ: 174.1 (C-1), 157.8 (C-11), 153.3 (C-16), 144.7 (C-12), 140.2 (C-3), 139.6 (1J(13C-119Sn) = 1062 Hz, C-i), 136.4 (2J(13C-117/119Sn) = 60.4, C-o), 135.6 (C-13), 134.7 (C-9), 130.5 (C-m), 129.5 (C-5), 128.9 (4J(13C-117/119Sn) = 90.5, C-p, C-7), 127.4 (C-4), 125.1 (C-15), 124.8 (C-2), 124.5 (C-14), 124.0 (C-6), 119.6 (C-8), 109.2 (C-10); 119Sn NMR (112.04 MHz, CDCl3) δ: −332; MS: (DART+) [m/z] (%): [(M+ + 1), 537] (85). HR-MS (DART+) m/z: 537.06250 (calc. for 12C281H2114N216O2120Sn), observed: 537.05877.
3.6. 2,2-Bis(trimethylsilyl)methyl-6-aza-1,3-dioxa-2-stannanaphtho[1,2-h]pyrido[3,2-d]cyclononene (1d)
Compound 1d was prepared from 0.235 g (1.364 mmol) of 2-hydroxy-1-naphthaldehyde, 0.1503 g (1.364 mmol) of 2-amino-3-hydroxypyridine and 0.4210 g (1.364 mmol) of bis[(trimethylsilyl)methyl]tin(IV) oxide, producing 0.7156 g (95%) of a red solid; m.p. 88–90 °C; molar conductivity, ΛM (1 × 10−3 M, methanol): 23.6 μS cm−1; UV-Vis [methanol, λmáx/nm (ε/M−1 cm−1)]: 219 (131,423), 256 (75,897) π → π* (aromatic), 348 (20,665) π → π* (C=N), 488 (52,559) n → π* (C=N); IR (ATR) cm−1: 3046 ν(C-H), 2948 νas (C-H), 2890 νsim(C-H), 1602 ν(C=O), 603 ν(Sn-C), 549 ν(Sn-O), 488 ν(Sn-N); 1H NMR (400.13 MHz, CDCl3) δ: 0.00 (18H, H-β), 0.48 ( (4H, d, J = 4.00, H-α), 6.85 (1H, d, J = 9.20 Hz, H-2), 7.05–7.08 (2H, m, H-14, H-15), 7.34 (1H, ddd, J = 0.80, 7.60, 7.60 Hz, H-6), 7.57 (1H, ddd, J = 1.20, 7.20, 7.20 Hz, H-7), 7.68 (1H, dd, J = 1.20, 8.00 Hz, H-5), 7.78 (1H, dd, J = 2.20, 3.80 Hz, H-13), 7.82 (1H, d, J = 9.20 Hz, H-3), 8.28 (1H, d, J = 8.40 Hz, H-8), 10.51 (1H, s, 3J(1H-117/119Sn) = 56 Hz, H-11); 13C NMR (125.78 MHz, CDCl3) δ: 172.4 (C-1), 156.5 (C-11), 151.9 (C-16), 143.7 (C-12), 138.8 (C-3), 134.2 (C-13), 133.7 (C-9), 128.3 (C-5), 127.6 (C-7), 126.0 (C-4), 123.8 (C-2), 123.5 (C-14), 123.2 (C-15), 122.6 (C-6), 118.4 (C-8), 107.6 (C-10), 7.1 (1J(13C-117/119Sn) = 519, C-α), 0.0 (C-β); 119Sn NMR (149.18 MHz, CDCl3) δ: −154; MS: (DART+) [m/z] (%): [(M+ + 1), 557] (16). HR-MS (DART+) m/z: 557.11025 (calc. for 12C241H3314N216O2120Sn), observed: 557.10326.
3.7. Hybrid Film Fabrication and Characterization
The hybrid films were deposited by the spin-coating technique, and Smart Coater 200 equipment was used. The dispersion used for the manufacture of the films consisted of 6 mL of a graphene-PEDOT:PSS hybrid dispersion in dimethylformamide. Subsequently, a saturated dispersion was generated with the pentacoordinated organotin(IV) complex. The graphene-PEDOT:PSS-organotin(IV) complex mixture was dispersed with the G560 shaker of Scientific Industries Vortex-Genie. The dispersion was later deposited on the substrate, and the equipment was operated at a constant angular speed of 900 rpm for 20 s and then at an accelerated speed of 300 rpm/s and dried at 80 °C for 3 min. Thin films were deposited on glass, silicon wafers (c-Si) and indium tin oxide (In2O3·(SnO2)x)-coated glass (glass-ITO) substrates. Previously, the glass and glass-ITO substrates were sequentially washed in an ultrasonic bath with dichloromethane, methanol and acetone. The silicon substrate was washed with a p solution (10 mL HF, 15 mL HNO3 and 300 mL H2O) to remove surface oxides. In order to obtain the quinoid form in the PEDOT:PSS polymer, the films were post-treated by exposure to isopropanol (IPA) vapor while heated at 40 °C for 10 min. For the hybrid films on Corning glass, the UV-Vis spectra were obtained in the 200–1100 nm wavelength range with a UV-Vis 300 Unicam spectrophotometer. Topographic and mechanical characteristics were investigated with an atomic force microscope (AFM) using a Ntegra platform. Finally, the devices were fabricated by using ITO as an anode and silver as a cathode: glass/ITO/organotin(IV) complex/Ag. For this evaluation, a programmable voltage source, a sensing station with lighting and a temperature-controller circuit from Next Robotix and an auto-ranging Keithley 4200-SCS-PK1 pico-ammeter were employed.