3-1 Syntheses
The synthetic route of DTCPiFBT and DTCPoFBT is illustrated in Scheme 3-1.
The procedure was similar to that of thiophene-based electron donors reported in Chapter 2, except that 4-(N,N-ditolylamino)-1-(tri-n-butylstannyl)phenylene (dtap-tin) was used as starting material. Stille coupling of dtap-tin and FBTCN directly afforded DTCPiFBT in 81% yield. Stille coupling of dtap-tin and FBTBr afforded DTPoFBTBr in 73% yield, which subsequently underwent Palladium-catalyzed cyanation to give DTCPoFBT in 76% yield.
Scheme 3-1. Synthetic route for DTCPiFBT and DTCPoFBT.
3-2 Optical Properties
Optical properties of DTCPiFBT and DTCPoFBT are shown in Figure 3-1.
Photophysical and electrochemical properties of their and DTCPB are summarized in Table 3-1. In solution state, these molecules perform two bands absorption located at 300–350 nm and 400–600 nm. It is noteworthy that the extinction coefficient of the former band is larger than that of the latter band, which was also found for DTCPB.
Both DTCPiFBT and DTCPoFBT show slightly bathochromic shifts in absorption compared to DTCPB due to the introduction of a fluorine atom. In thin films, all molecules exhibit bathochromic shifts (19–23 nm) in their absorption maxima in the same trend as in solution state. However, all together, the phenylene-bridged electron donors exhibit 63–71 nm hypsochromic shifts as compared to thiophene-bridged counterparts because of the higher aromaticity of phenylene.6-7
(a) (b)
Figure 3-1. Absorption spectra of DTCPiFBT (square) and DTCPoFBT (circle) in (a) CH2Cl2 solutions and (b) vacuum deposited thin films.
Table 3-1. Photophysical and electrochemical parameters for DTCPiFBT, DTCPoFBT, and DTCPB.
3-3 Electrochemical Properties
Cyclic voltammograms of DTCPiFBT and DTCPoFBT are shown in Figure 3-2.
As shown in Table 3-1, both molecules perform lowered HOMO and LUMO as compared to DTCPB in both solution state and vacuum deposited thin films.
Furthermore, DTCPiFBT shows narrowed bandgap, resulting from the much lowered LUMO energy level. DTCPoFBT exhibits same bandgap as DTCPB; however, the decrement of HOMO and LUMO energy levels are the highest among all newly synthesized electron donors in this thesis. The outward FBT linked to another cyano group could account for stronger electron withdrawing ability and thus stronger propensity for lowering HOMO and LUMO. Such propensity suggests that a higher Voc can be achieved for DTCPoFBT.
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
DTCPoFBT DTCPiFBT
Current (A)
Potential (V vs Fc+/Fc)
Figure 3-2. Cyclic voltammograms of DTCPiFBT and DTCPoFBT.
3-4 Theoretical Calculations
DFT and TD-DFT calculations were performed for DTCPiFBT and DTCPoFBT in CH2Cl2. The optimized geometries for both molecules are showed in Table 3-3. Unlike thiophene-based electron donors mentioned in Chapter 2, all phenylene-based electron donors exhibit highly twisted conformation. The dihedral angles between phenylene (P) and benzothiodiazole (BT) units are 40.9o for
DTCPiFBT, 35.0o for DTCPoFBT, and 36.1o for DTCPB. The non-copolanirity is due to ortho–ortho steric interaction between P and BT units, which is greater for DTCPiFBT due to the larger size of an inward fluorine atom (1.35 Å ) than a hydrogen atom (1.20 Å ).
Front Flank
DTCPiFBT
DTCPoFBT
Figure 3-3. DFT-optimized geometries for DTCPiFBT and DTCPoFBT.
The dipole moments of DTCPiFBT, DTCPoFBT, and DTCPB are summarized in Table 3-2. The total dipole moments for all molecules are smaller as compared to the counterparts of thiophene-based molecules. Also, inward fluorination gives smaller dipole moment in both S0 and S1, whereas outward fluorination gives larger dipole moment than non-fluorinated DTCPB. At the first excited state, because of charge separation caused by charge transfer, all molecules show larger dipole moment in same trend.
Table 3-2. Dipole moment parameters for DTCPiFBT, DTCTPoFBT, and DTCPB. overall dipole moment overlap between S0 and S1.
The absorption located at 400–600 nm corresponds mainly to transition from S0
to S1, which is prominently constituted by the charge transfer excitation from HOMO to LUMO. However, the main absorption spanning 300–350 nm comprises several electronic transitions, which sums to larger oscillator strength. Such observation could be correlated to the twisted conformation phenylene-based molecules possess.
Table 3-3. TD-DFT calculated oscillator strengths, absorption wavelengths, molecular orbital compositions, and transition characters for DTCPiFBT and DTCPoFBT.
electronic transition
oscillator strength
λexp /λcalc
(nm) MO Composition character
DTCPiFBT
Though both molecules possess several electronic transitions, the most significant transition is from HOMO to LUMO via charge transfer. As shown in Figure 3-4, the benign separation between HOMO and LUMO is similar with thiophene-based molecules.
DTCPiFBT DTCPoFBT
Figure 3-4. Isodensity surface plots of the HOMO (green) and LUMO (red) of DTCPiFBT and DTCPoFBT.
3-5 Crystal Structures and Packings
The crystals of DTCPiFBT and DTCPoFBT were easily obtained by slow evaporation of dichloromethane. Their crystal packings are illustrated in Figure 3-5.
DTCPiFBT shows antiparallel fashion in dimer with an average distance in the range of 3.64–3.70 Å , indicating benign intermolecular −interactions (the residue solvent was omitted for clarity.) However, no obvious packing pattern in crystal grain was found. Unlike the antiparallel fashion for DTCPiFBT and DTCPB, the crystal of DTCPoFBT exhibits zig-zag orientation both in dimer and in crystal packing. Such observation is presumably correlated to vdW interaction between a fluorine atom and two hydrogen atoms on P unit, which was only found in DTCPoFBT. As a result, the distance between each dimer ranges 3.30–3.57 Å . Introduction of a fluorine atom can
provide more interaction in crystal packing concomitant with morphology alteration for the active layer in BHJ device.
The ortho–ortho steric interaction between P and BT induces the dihedral angles more than 27o (34.8o for DTCPiFBT, 29.7o for DTCPoFBT, and 27.3o for DTCPB26). The larger dihedral angle of DTCPiFBT is due to the steric hindrance from the inward fluorine atom. The observation is similar to the results in Section 3-4.
DTCPiFBT DTCPoFBT
Figure 3-5. Crystal structures of DTCPiFBT and DTCPoFBT in molecular packing.
3-6 Thermal Properties
Thermal stabilities and morphological properties of DTCPiFBT and DTCPoFBT are shown in Fig. SB5 – Fig. SB8 and summarized in Table 3-4. The decomposition temperatures are above 270 oC for all molecules, which are suitable for vacuum fabrication. However, no distinct correlation for fluorination and the change in Td and Tm was found.
Table 3-4. Thermal parameters for DTCPiFBT, DTCPoFBT, and DTCPB.
dyes Td (oC) Tm (oC)
DTCPiFBT 209
DTCPoFBT 191
DTCPB26 204
3-7 Photovoltaic and Electrochemical Impedance Characteristics
DTCPiFBT and DTCPoFBT were subject to vacuum deposition with the OPV structure: ITO / MoO3 (20 nm) / dye: C60 (x nm) / BCP (7 nm) / Al (100 nm). The photovoltaic and electrochemical impedance characteristics were measured under simulated AM 1.5 G (100 mW cm-2) illumination. The device performances are summarized in Table 3-5, and their J-V curves along with EQE spectra are shown in Figure 3-6. DTCPoFBT/C60 OPV cell exhibited a Voc of 0.88 V, a Jsc of 6.50 mA/cm2, and a high FF of 0.63, achieving a PCE of 3.62%. DTCPiFBT/C60 OPV cell exhibited a Voc of 0.89 V, a Jsc of 4.90 mA/cm2, and a FF of 0.58, achieving a PCE of 2.65%. As for EQE analyses, responses centered at 380–400 nm contributed mainly by C60 were 53.9% and 54.5% for DTCPiFBT and DTCPoFBT, respectively; responses centered at 530–535 nm mainly from dyes were 32.1%, and 42.7%, respectively. The former band is similar in intensity, whereas the latter band shows significant difference, as expected. Due to the highly twisted backbone of DTCPiFBT, its CT harvesting efficiency is lower than that of DTCPoFBT, resulting in inferior EQE response.
In summary, the tendency in Voc of the new OPV devices in this thesis was coincident with the HOMO levels determined from cyclic voltammogram or AC-2.
The low-lying HOMO level of DTCPoFBT resulted in a high Voc of 0.88 V. However, the Jsc of DTCPoFBT was lower than that of DTCToFBT (6.85 mA/cm2) due to the larger bandgap of phenylene-based electron donors. The results demonstrated a trade-off between Voc and Jsc for a series of D-A-A based devices. Among all,
DTCPoFBT stroke a balance between the photovoltage and the photocurrent, achieving a remarkable PCE of 3.62%.
dyes dye: DTCPoFBT (circle) for C60-based OPV.
Encouraged by the promising results, we decide to replace the electron acceptor in pursuit of better device performance. C70 is adopted to replace C60 because of its broader optical absorption and higher extinction coefficient, which can provide better light-harvesting capabilities.28 Besides, we expect a much higher Voc can be reached for DTCPoFBT as optimizing conditions, which will improve morphology and reduce charge recombination. Further investigation of all newly synthesized electron donors in this thesis is in process.
400 500 600 700 800
Conclusions
Our work characterized the effect of F-substituted benzothiadiazole unit on D-A-A structured small molecule electron donors. Though DTCTiFBT and DTCToFBT as well as DTCPiFBT and DTCPoFBT have regioisomeric nature, introduction of an inward or outward fluorine atom can alter dihedral angles and crystal packings by providing additional interactions or steric hindrance. All molecules show lowered HOMO and LUMO level and slightly red-shifted absorption as compared to those of the non-fluorinated counterparts, which are beneficial for pursuing higher Voc and Jsc, respectively. Furthermore, all molecules possess benign thermal stability, high extinction coefficient, and appropriate energy levels suitable for OPV application. Among all, thiophene-bridged DTCToFBT/C60 OPV cell exhibited a Voc of 0.83 V, a Jsc of 6.85 mA/cm2, and a FF of 0.58, achieving a PCE of 3.28%.
Pheneylene-bridged DTCPoFBT/C60 OPV cell exhibited a Voc of 0.88 V, a Jsc of 6.50 mA/cm2, and a high FF of 0.63, achieving a remarkable PCE of 3.62%. All molecules are subject to further investigation.
Experiment Session
Syntheses and Materials.
All chemicals and reagents were used as received from commercial sources without purification. THF was dried by molecular sieves, and toluene was distilled by P2O5 as the drying agent. 4,7-dibromo-5-fluorobenzo[c][1,2,5]thiadiazole (FBTBr) was synthesized according to previous literature.15 followed by extraction with EA and brine several times to eliminate NMP. The crude product was purified by column chromatography with dichloromethane/hexane as eluent (v/v, 1:1) to afford FBTCN as a yellow solid (2.03 g, 39%), mp 143-144 oC; IR 256.9059 found 256.9055, Calcd for C7H81BrFN3S 258.9038 found 258.9037.
Synthesis of DTCTiFBT
A mixture of 4-(N,N-ditolylamino)-1-(tri-n-butylstannyl)thiophene (7 mmol), FBTCN (1716 mg, 6.65 mmol), and PdCl2(PPh3)2 (246 mg, 0.35 mmol) in anhydrous toluene (22 mL) was stirred and heated at 120 oC for 3 h under argon atmosphere. After cooled down to room temperature, the solvent was removed by rotary evaporation.
The crude product was further purified by column chromatography with dichloromethane/hexane as eluent (v/v, 1:2). DTCTiFBT was obtained as a purple solid (2342 mg, 77%), mp 211 oC (DSC); IR (KBr) 1156, 1328, 1360, 1389, 1434, 1507, 2222, 2860, 2921, 3040 cm−1; 1H NMR (CDCl3, 400 MHz) δ 8.25 (d, J = 4 Hz, 1H), 7.85 (d, J = 12 Hz, 1H), 7.14-7.20 (m, 8H), 6.54 (dd, J = 4.5, 1.5 Hz, 1H), 2.36 (s, 6H) ; 13C NMR (CDCl3, 100 MHz) δ 160.5 (d, J = 9.1 Hz), 155.3 (d, J = 252.4 Hz), 151.9 (d, J = 11.1 Hz), 151.0, 144.4, 134.9, 134.0 (d, J = 10.1 Hz), 130.2, 126.9 (d, J
= 33.2 Hz), 124.8, 119.7 (d, J = 7.0 Hz), 119.3 (d, J = 14.1 Hz), 115.0, 114.2, 98.5 (d, J = 12.1 Hz), 21.0; 19F NMR (CDCl3, 376 MHz) δ -112.0 (d, J = 12.4 Hz, 1F) HRMS (m/z, FAB+) Calcd for C25H17FN4S2 456.0879 found 456.0883.
Synthesis of DTToFBTBr
A mixture of 4-(N,N-ditolylamino)-1-(tri-n-butylstannyl)thiophene (1.5 mmol),
FBTBr (468 mg, 1.5 mmol), and PdCl2(PPh3)2 (53 mg, 0.075 mmol) in anhydrous 120 oC for 2 h under argon atmosphere. After cooled down to room temperature, the solution was extracted with EA and brine several times to remove NMP. The crude product was purified by column chromatography with dichloromethane/hexane as eluent (v/v, 1:1) to afford DTCToFBT as a purple solid (240 mg, 89%), mp 192 oC (DSC); IR (KBr) 1054, 1156, 1217, 1348, 1442, 1507, 1552, 2227, 2860, 2921, 3032
cm−1; 1H NMR (CDCl3, 400 MHz) δ 8.17 (d, J = 4 Hz, 1H), 7.29 (d, J = 12 Hz, 1H),
A mixture of 4-(N,N-ditolylamino)-1-(tri-n-butylstannyl)phenylene (9 mmol), FBTCN (2207 mg, 8.55 mmol), and PdCl2(PPh3)2 (316 mg, 0.45 mmol) in anhydrous toluene (28 mL) was stirred and heated at 120 oC for 3 h under argon atmosphere.
After cooled down to room temperature, the solvent was removed by rotary evaporation. The crude product was further purified by column chromatography with dichloromethane/hexane as eluent (v/v, 1:2). DTCPiFBT was obtained as a black
Synthesis of DTPoFBTBr
A mixture of 4-(N,N-ditolylamino)-1-(tri-n-butylstannyl)phenylene (1.5 mmol), FBTBr (468 mg, 1.5 mmol), and PdCl2(PPh3)2 (53 mg, 0.075 mmol) in anhydrous toluene (4.7 mL) was stirred and refluxed overnight under argon atmosphere. After cooled down to room temperature, the solvent was removed by rotary evaporation.
The crude product was further purified by column chromatography with dichloromethane/hexane as eluent (v/v, 1:4). DTPoFBTBr was obtained as an orange solid (550 mg, 73%), mp 153-156 oC; IR (KBr) 816, 1208, 1273, 1295, 1481, 1506, 1597, 2919, 3026 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.77-7.80 (m, 2H), 7.53 (d, J = 8.0 Hz, 1H), 7.07-7.13 (m, 10H), 2.35 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 161.0 (d, J = 252.4 Hz), 154.4 (d, J = 7.4 Hz), 150.3, 149.4, 144.5, 134.3 (d, J = 10.2 Hz), 133.6, 130.1, 129.9, 127.3, 125.5, 120.9, 117.6 (d, J = 30.3 Hz), 95.6 (d, J = 24.2 Hz), 20.9; 19F NMR (CDCl3, 376 MHz) δ -104.1 (d, J = 7.5 Hz, 1F); HRMS (m/z, FAB+) Calcd for C26H1979BrFN3S 503.0467 found 503.0457, Calcd for C26H1981BrFN3S 505.0447 found 505.0448.
Synthesis of DTCPoFBT
A mixture of DTPoFBTBr (2600 mg, 5.15 mmol), Zn(CN)2 (424 mg, 3.61 mmol),
Pd(PPh3)4 (477 mg, 0.41 mmol) in degassed NMP (52 mL) was stirred and heated at 120 oC for 2 h under argon atmosphere. After cooled down to room temperature, the solution was extracted with EA and brine several times to eliminate NMP. The crude product was purified by column chromatography with dichloromethane/hexane as eluent (v/v, 1:1) to afford DTCPoFBT as a red solid (1758 mg, 76%), mp 191 oC (DSC); IR (KBr) 813, 1221, 1336, 1483, 1520, 1556, 1601, 2231, 2864, 2917, 3040 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.86-7.88 (m, 2H), 7.54 (d, J = 8.0 Hz, 1H), 7.08-7.15 (m, 10H), 2.35 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 166.9 (d, J = 265.5 Hz), 153.9 (d, J = 8.0 Hz), 150.6, 150.1, 144.0, 140.6 (d, J = 11.1 Hz), 134.3, 130.4, 130.2, 126.0, 125.9, 120.0, 115.6 (d, J = 27.2 Hz), 111.3, 88.8 (d, J = 18.1 Hz), 20.9;
19F NMR (CDCl3, 376 MHz) δ -98.9 (d, J = 12.0 Hz, 1F); HRMS (m/z, FAB+) Calcd for C27H19FN4S 450.1314 found 450.1316.
General Experiment
Nuclear Magnetic Resonance Spectroscopy, NMR
1H, 13C, and 19F NMR were measured by Varian 400 unity plus (400 MHz), using CDCl3 as solvent. The Chemical shift δ was reported in ppm. The definition of splitting pattern was: s, singlet; d, doublet; t, triplet; q, quartet; qn, quintet; sex, sextet;
m, multiplet. Coupling constant was represented by J and reported in Hz.
Infrared Spectroscopy, IR
The IR spectra were measured by OMNIC 8.0 spectrometer, and the recorded wavenumbers were reported in cm-1. Samples were dissolved in CH2Cl2 and dropped onto KBr and dried prior to use.
Ultraviolet–Visible Spectroscopy, UV
The absorption spectra in CH2Cl2 were measured by JASCO V-670 spectrophotometer.
Three different concentrations ranging from 10-6 to 10-7 M in CH2Cl2 were measured in order to obtain the UV-Vis spectrum and the extinction coefficient. The absorption spectra of thin films were measured by Hitachi U4100.
Cyclic Voltammetry, CV
Cyclic voltammetry spectra were recorded in 0.1 M of substrate on a CHI619B electrochemical analyzer. Oxidation CV measurements were carried out in anhydrous CH2Cl2 containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte. Reduction CV measurements were conducted in anhydrous THF with 0.1 M tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte
and purging with argon for 5 minutes prior to experiment. Carbon electrode was used as the working electrode and a platinum wire as the counter electrode. All potentials were recorded versus Ag/AgCl (sat’d) as a reference electrode and ferrocenium/ferrocene redox couple was used as an internal reference.
Theoretical Calculations
The electronic and optical properties and optimized conformation were estimated by density function theory (DFT) and time-dependent density function theory (TD-DFT) calculations in CH2Cl2 using CAM-B3LYP function with the 6-311G(d,p) basis set.
Mass Spectroscopy, MS
The high resolution mass spectra were measured by spectrometer JEOL SX-102A using fast atom bombardment as the ionization method.
Differential scanning calorimetry, DSC
Differential scanning calorimetry analyses were performed on a TA Instrument 2920 MDSC V2.6A Low-Temperature Difference Scanning Calorimeter. The spectra were recorded at a heating rate of 10 °C/min with nitrogen flushing.
Thermogravimetric analyses, TGA
Thermogravimetric analyses were performed on a TA Instrument Dynamic Q500 at a heating rate of 10 °C/min with nitrogen flushing.
Photocurrent-Voltage Measurement
The photocurrent-voltage characteristics of the photovoltaics were measured under illumination of AM1.5G solar light (100 mW/cm2) from a solar simulator (Newport
model 91195A). An electrical source meter (Keithley 2400) was employed to record the photocurrents of the device under different voltage.
External Quantum Efficiency Measurement, EQE
The external quantum efficiency measurement system consisted of a source meter (Keithley 2400), a solar simulator, a wavelength selectable arc lamp (A-1010B Arc Lamp Housing), and a monochromator (Newport model 74100). The signals were amplified by a lock-in amplifier (Signal Recovery model 7265). The photocurrents from the OPV device were measured through the lock-in amplifier, and the number of electrons and holes generated in different wavelength can be calculated.
References
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Appendix A
1
H and
13C NMR Spectra
Fig. SA1. 1H NMR spectrum of DTCTiFBT.
Fig. SA2. 13C NMR spectrum of DTCTiFBT.
Fig. SA3. 1H NMR spectrum of DTCToFBT.
Fig. SA4. 13C NMR spectrum of DTCToFBT.
Fig. SA5. 1H NMR spectrum of DTCPiFBT.
Fig. SA6. 13C NMR spectrum of DTCPiFBT.
Fig. SA7. 1H NMR spectrum of DTCPoFBT.
Fig. SA8. 13C NMR spectrum of DTCPoFBT.
Fig. SA9. 1H NMR spectrum of DTToFBTBr.
Fig. SA10. 13C NMR spectrum of DTToFBTBr.
Fig. SA11. 1H NMR spectrum of DTCPoFBTBr.
Fig. SA12. 13C NMR spectrum of DTPoFBTBr.
Fig. SA13. 1H NMR spectrum of FBTCN.
Fig. SA14. 13C NMR spectrum of FBTCN.
Appendix B
TGA and DSC Thermogram
Fig. SB1. DSC thermogram of DTCTiFBT.
Fig. SB2. TGA thermogram of DTCTiFBT.
Fig. SB3. DSC thermogram of DTCToFBT.
Fig. SB4. TGA thermogram of DTCToFBT.
Fig. SB5. DSC thermogram of DTCPiFBT.
Fig. SB6. TGA thermogram of DTCPiFBT.
Fig. SB7. DSC thermogram of DTCPoFBT.
Fig. SB8. TGA thermogram of DTCPoFBT.
Appendix C
X-ray Crystallography Data
Table S1. Crystal data for DTCTiFBT.
Table S2. Crystal data for DTCPiFBT.
Table S3. Crystal data for DTCPoFBT.
Table S4. Crystal data for FBTCN.