Zinc Phthalocyanine-Phosphonic Acid Monolayers on ITO: Influence of

Feb 28, 2019 - Luis E Oquendo , Ramanan Ehamparam , Neal R. Armstrong , S. Scott Saavedra , and Dominic V. McGrath. J. Phys. Chem. C , Just Accepted ...
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Zinc Phthalocyanine−Phosphonic Acid Monolayers on ITO: Influence of Molecular Orientation, Aggregation, and Tunneling Distance on Charge-Transfer Kinetics Luis E. Oquendo, Ramanan Ehamparam, Neal R. Armstrong, S. Scott Saavedra,* and Dominic V. McGrath* Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States

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ABSTRACT: Efficient charge harvesting at electrodes is critical for the effective performance of organic photovoltaics and is strongly influenced by the first molecular monolayer at the transparent conducting oxide electrode. Herein, we present a study of the relationship between molecular orientation and tunneling distance on charge-transfer rates between a tethered sub-monolayer/monolayer of phosphonic acid (PA)-functionalized Zn phthalocyanines (ZnPcs) and an indium tin oxide (ITO) surface by a combination of electrochemical techniques and waveguide tools such as attenuated total reflectance (ATR) UV−vis spectroscopy and potential-modulated ATR (PM-ATR) spectroelectrochemistry. The distance between the main chromophore and PA anchoring group was modulated by an aliphatic carbon of various lengths (n = 3, 9, 10, or 11) that resulted in an extended distance of 7−18 Å between the ZnPcs and the ITO surface. Modified ITO surfaces were composed of monomeric and aggregated subpopulations of ZnPcs with molecular orientations predominantly in-plane (36−39°) and out-of-plane (72−75°), respectively. Charge-transfer rate constants (ks,opt) were measured using PM-ATR. For a given tether length, the aggregated subpopulations exhibit higher ks,opt values compared to the monomeric subpopulations. The observed ks,opt values had an exponential dependence on the effective tunneling distance with a decay constant (β) that ranged from 0.32 to 0.47 Å−1, depending on the chromophore orientation and aggregation state. The fastest charge-transfer rate constants were found for the chromophores with the smallest tunneling distance (n = 3). A ks,opt of 3.9 × 104 s−1 represents the fastest rate constant measured by PM-ATR for a PA-functionalized ZnPc chromophore tethered to an ITO waveguide electrode.

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well as their chemical and thermal stability.20−26 The nature of the ITO/Pc interface, and the resulting effect on contact ohmicity and series resistance, has been shown to be a critical factor in the performance of OPVs using this electrode/donor combination.12,27 Tethering Pc molecules to an ITO surface allows investigation of the influence of molecular orientation, aggregation, and distance between ITO and the Pc chromophores on charge-transfer rate constants at this interface. Recently, we reported that a phosphonic acid-functionalized ZnPc (ZnPcPA) with a flexible linker of 10 carbons between the anchoring group and the main chromophore can be successfully attached to ITO, and that the resultant chargetransfer rates were dictated by the orientation and aggregation states of the chromophores on the surface.28,29 Here, we report how the magnitude of the charge-transfer rates is also related to the distance between the ZnPc donor material and the ITO surface by modifying ITO surfaces with 3 new tethered Pcs that vary in the distance between the chromophore and the surface. The separation between the main chromophore and the surface was controlled by the length of alkyl linkers ranging

INTRODUCTION Harvesting energy from sunlight is an essential component of production of energy for the foreseeable future.1 Organic photovoltaic solar cells (OPVs), a technology with the potential to economically compete with fossil fuels, utilize the versatility of small molecules with high optical absorption coefficients.2−6 Transparent conductive oxides (TCOs), especially indium tin oxide (ITO), play a critical role in OPVs as well.7−10 However, the lack of physical compatibility with organic materials and the ineffective work function of ITO make it a poor electrode for extraction of charge carriers at the TCO/organic interface which leads to a recombination of charge and inefficient OPV device performance.11−13 By understanding and ultimately controlling the surface properties at the ITO/organic hole collection interface, we can better design new organic materials to improve device performance. The modification of ITO by anchoring redox-active organic materials has the ability to effectively tune the work function of the system due to the presence of dipoles, improve compatibility with organic materials, and may improve the charge injection across this interface.7,14−19 Phthalocyanines (Pcs) have been extensively used in devices such as organic field-effect transistors, organic light emitting diodes, and OPVs due to their high molar absorptivity and strong Q-band absorptions in the visible and near IR regions as © XXXX American Chemical Society

Received: October 22, 2018 Revised: February 28, 2019 Published: February 28, 2019 A

DOI: 10.1021/acs.jpcc.8b10301 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

cool to rt. The resulting mixture was suspended in MeOH (40 mL) and centrifuged. The supernatant was discarded, and the residue was allowed to air dry. The green residue was dissolved (CH2Cl2) and purified by flash column chromatography (SiO2, 7:3 hexanes/EtOAc) to give 3a as a dark green solid (50 mg, 70%): 1H NMR (499 MHz, CD2Cl2/pyridine-d5 (95:5)) δ 8.87−8.78 (m, 4H), 7.70−7.04 (m, 29H), 4.49 (t, J = 8 Hz, 2H), 4.15 (ddd, J = 9.5, 7.9, 7.0 Hz, 4H), 2.74−2.59 (m, 12H), 1.68 (tt, J = 8.1, 4.0 Hz, 12H), 1.44−1.33 (m, 28H), 1.05−0.70 (m, 24H); 31P NMR (202 MHz, 95:5 CD2Cl2/pyridine-d5) δ 31.83; UV−vis (9:1 CH2Cl2/pyridine) Q-band λmax 682 nm (ε = 2.2 × 10 5 ); MS (MALDI) m/z (M) + calcd for C105H115N8O10PZn 1742.8, found 1742.1. (2,3,9,10,16,17-Hexakis(4-pentylphenoxy)-23-(diethyl(9nonyloxy)phosphonate) phthalocyanine) Zinc(II) (3b). Following the procedure for 2a, Pc 1 (100 mg, 0.064 mmol), DMF (2.5 mL), 2b (32 mg, 0.095 mmol), KI (5.3 mg, 0.031 mmol), and K2CO3 (13.2 mg, 0.095 mmol) yielded 3b as a dark green solid (104 mg, 90%): 1H NMR (499 MHz, 95:5 CD2Cl2/pyridine-d5) δ 9.15 (dd, J = 18.4, 8.2 Hz, 1H), 8.76 (d, J = 31.7 Hz, 1H), 7.28−7.15 (m, 31H), 4.46 (t, J = 6 Hz, 2H), 4.09−3.94 (m, 4H), 2.67 (q, J = 8.7, 8.2 Hz, 14H), 1.68 (d, J = 7.2 Hz, 26H), 1.33−1.25 (m, 14H), 0.98−0.86 (m, 34H); 31P NMR (202 MHz, 95:5 CD2Cl2/pyridine-d5) δ 31.52; UV−vis (9:1 CH2Cl2/pyridine) Q-band λmax 682 nm (ε = 2.4 × 10 5 ); MS (MALDI) m/z (M) + calcd for C111H127N8O10PZn+ 1826.9, found 1826.8. (2,3,9,10,16,17-Hexakis(4-pentylphenoxy)-23-(diethyl(11undecyloxy)phosphonate) phthalocyanine) Zinc(II) (3c). Following the procedure for 2a, Pc 1 (100 mg, 0.064 mmol), DMF (2.5 mL), 2c (35 mg, 0.096 mmol), KI (5.3 mg, 0.031 mmol), and K2CO3 (13.2 mg, 0.095 mmol) yielded 3c as a dark green solid (104 mg, 90%): 1H NMR (499 MHz, 95:5 CD2Cl2/pyridine-d5) δ 9.23 (d, J = 8.3 Hz, 1H), 9.12 (s, 1H), 8.85 (d, J = 2.3 Hz, 1H), 7.46−7.10 (m, 30H), 4.51 (t, J = 6.4 Hz, 2H), 4.11−4.00 (m, 4H), 2.79−2.65 (m, 12H), 1.82−1.66 (m, 18H), 1.51−1.28 (m, 44H), 1.05−0.92 (m, 18H); 31P NMR (202 MHz, 95:5 CD2Cl2/pyridine-d5) δ 32.02; UV−vis (9:1 CH2Cl2/pyridine) Q-band λmax 682 nm (ε = 2.5 × 10 5 ); MS (MALDI) m/z (M) + calcd for C113H131N8O10PZn+ 1854.9, found 1854.9. (2,3,9,10,16,17-Hexakis(4-pentylphenoxy)-23-((3propyloxy)phosphonic acid) phthalocyanine) Zinc(II) (4a). To a dry three-neck round-bottom flask charged with a stir bar were added 3a (50 mg, 0.029 mmol) and anhydrous CH2Cl2 (2 mL). The reaction vessel was purged with Ar, followed by dropwise addition of bromotrimethylsilane (TMS-Br) (63 mg, 0.43 mmol). The reaction mixture was stirred at rt for 12 h. The reaction was quenched with MeOH (20 mL), and the solvent was removed under reduced pressure. The resulting green residue was suspended in MeOH and centrifuged. The supernatant was discarded, and the residue was air-dried, followed by drying under high vacuum to give 4a (10 mg, 21%) as a dark green solid: 1H NMR (499 MHz, 9:1 CD2Cl2/ pyridine-d5) δ 8.21−8.48 (m, 4H), 7.50−6.98 (m, 29H), 4.45 (t, J = 6.4 Hz, 2H), 2.65 (d, J = 37.5 Hz, 12H), 1.62−1.74 (m, 12H), 1.52−1.18 (m, 28H), 0.99 (d, J = 7.1 Hz, 18H); 31P NMR (202 MHz, pyridine-d5) δ 45.37; UV−vis (hexamethylphosphoramide, HMPA) Q-band λmax 680 nm (ε = 2.0 × 105); MS (MALDI) m/z (M)+ calcd for C101H107N8O10PZn+ 1686.7, found 1686.7. (2,3,9,10,16,17-Hexakis(4-pentylphenoxy)-23-((9nonyloxy)phosphonic acid) phthalocyanine) Zinc(II) (4b).

from 3 to 11 carbons which corresponds to distances of 7 to 18 Å between the Pc and ITO surface. We used polarizationdependent attenuated total reflectance (ATR) spectroscopy to characterize these chromophores on ITO, including their aggregation state and relative orientation. A combination of cyclic voltammetry and potential-modulated ATR (PM-ATR) spectroelectrochemistry was used to measure apparent chargetransfer rate constants (ks,app), elucidating that PA-functionalized ZnPcs with the shortest linker length (n = 3; 7.6 Å) exhibited a charge-transfer rate (ks,opt = 104 s−1) that exceeded the minimum rate required at the hole−collection interface for OPV devices exposed to AM 1.5 radiation.

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EXPERIMENTAL SECTION Synthesis, Materials, and Methods. All chemicals were purchased from commercial suppliers and used as received unless otherwise noted. Compound 2a was commercially available and used without any further purification. Compounds 2b and 2c were synthesized according to the literature.30 Pyridine was dried over freshly activated 4 Å molecular sieves. Anhydrous CH2Cl2 was distilled from CaH2 under argon atmosphere. NMR spectroscopic data were collected on a commercial instrumentation operated at 500 MHz (1H). Chemical shifts were referenced to the deuterated solvent resonance for 1H (7.26 ppm for CDCl3; 5.32 ppm for CD2Cl2) and 13C (77.0 ppm for CDCl3). Chemical shifts for 31 P NMR were referenced to H3PO4 (0 ppm). UV−vis measurements were performed using a Shimadzu UV-2401PC UV−vis spectrophotometer. Molar absorptivity (ε) values were calculated from Beer’s law plots using three data points. Mass spectra were obtained from the Mass Spectrometry Facility, Department of Chemistry and Biochemistry, University of Arizona. Diethyl (9-Bromononyl)phosphonate (2b).30 A mixture of 1,9-dibromononane (5.0 g, 17.47 mmol) and triethyl phosphite (0.96 g, 5.82 mmol) was sealed in a microwave vessel and maintained at 120 °C for 1 h. After cooling to room temperature, the mixture was purified by flash chromatography (SiO2, 9:1 CH2Cl2/acetone) to provide 2b as a colorless oil (1.28 g, 64%): 1H NMR (499 MHz, CDCl3) δ 4.15−4.01 (m, 4H), 3.38 (t, J = 7 Hz, 2H), 1.88−1.80 (m, 2 H) 1.73−1.65 (m, 2H), 1.62−1.53 (m, 2H), 1.42−1.23 (m, 16H); 13C NMR (126 MHz, CDCl3) δ 61.4, 61.3, 33.9, 32.7, 30.6, 30.4, 29.1, 28.9, 28.9, 28.6, 28.1, 26.2, 25.13, 22.4, 22.3, 16.5, 16.4, 11.9. Diethyl (11-Bromoundecyl)phosphonate (2c).30 A mixture of 1,11-dibromoundecane (2.5 g, 7.96 mmol) and triethyl phosphite (264 mg, 1.59 mmol) was sealed in a microwave vessel and reacted at 120 °C for 1 h. After cooling to room temperature, the mixture was purified by flash chromatography (SiO2, 9:1 CH2Cl2/acetone) to provide 2c as a colorless oil (520 mg, 88%): 1H NMR (499 MHz, CDCl3) δ 4.15−4.01 (m, 4H), 3.38 (t, J = 7 Hz, 2H), 1.86−1.79 (m, 2 H) 1.73−1.64 (m, 2H), 1.61−1.51 (m, 2H), 1.36−1.25 (m, 20H); 13C NMR (126 MHz, CDCl3) δ 63.8, 61.6, 61.5, 34.2, 33.0, 30.8, 30.7, 29.6, 29.5, 29.5, 29.2, 28.9, 28.3, 26.4, 25.3, 22.6, 22.5, 16.7, 16.6, 16.3. (2,3,9,10,16,17-Hexakis(4-pentylphenoxy)-23-(diethyl(3propyloxy)phosphonate) phthalocyanine) Zinc(II) (3a). A mixture of Pc 1 (80 mg, 0.051 mmol), dimethylformamide (DMF) (1.5 mL), 2a (19 mg, 0.076 mmol), KI (4.00 mg, 0.03 mmol), and K2CO3 (11 mg, 0.076 mmol) was allowed to react for 10 h while maintained at 100 °C with stirring under N2. The reaction mixture was removed from heat and allowed to B

DOI: 10.1021/acs.jpcc.8b10301 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

v/v). After 3 h, the electrodes were rinsed with pure solvent to remove dissolved and weakly adsorbed ZnPcs, dried under a stream of N2, and annealed at 110 °C for 2 h to remove pyridine residues from the surface. A CH420A potentiostat (CH Instruments, Inc.) was used to perform cyclic voltammetry with a standard three-electrode configuration. The electroactive area of the ITO working electrode was 0.071 cm2, a Pt wire was used as a counter electrode, and a Ag wire immersed in 0.01 M AgNO3 in acetonitrile served as a nonaqueous pseudoreference electrode (Bioanalytical Systems, Inc.). The electrolyte was 0.3 M tetrabutylammonium perchlorate (TBAP) in acetonitrile. Potential Controlled Spectroelectrochemistry. Measurements were performed using an ATR spectroelectrochemical flow cell with a three-electrode configuration, using the counter and pseudoreference electrodes and electrolyte solution described above. A solution (100 μM) of ZnPc (4a, 4b, or 4c) in acetonitrile/pyridine (7:3 v/v) was incubated with the cleaned and plasma-activated ITO slide for 3 h. Excess material was rinsed with pure solvent from the ITO slide which was then dried with a stream of N2. The electroactive area (2.1 cm2) of the ITO working electrode was defined by a silicone gasket. The electrolyte solution (0.1 M TBAP in acetonitrile) was degassed with Ar for 1 h prior to experiments. A collimated and polarized beam of light was coupled in and out of the ITO-coated slide and detected as described above. Potentials were applied from −0.1 to 0.75 V vs Ag/Ag+ using a potentiostast (Pine AFRDE5 bipotentiostat), and spectra were acquired in TE and TM polarizations. A measurement taken at a potential of 0.75 V vs Ag/Ag+, at which the ZnPc molecules are completely bleached and do not absorb light, was used as a blank. In another type of experiment, a blank solution of 0.1 M TBAP in acetonitrile was injected into the ATR spectroelectrochemical flow cell. After the reference spectra were taken in TE and TM polarizations, a solution (100 μM) of PAfunctionalized ZnPc (4a, 4b, or 4c) in acetonitrile/pyridine (7:3 v/v) was injected and incubated with the electrode for 3 h. The flow cell was then rinsed with dichloromethane, and the electrolyte solution was injected into the cell. Potentials were applied from −0.1 to 0.7 V vs Ag/Ag+, and spectra were acquired in both TE and TM polarizations. Both sample preparation methodologies produced similar spectroelectrochemical responses. Furthermore, there was no difference in the steady-state spectroscopic response of ITO-coated slides used without annealing or annealed for 2 h at 110 °C. Potential-Modulated ATR (PM-ATR) Spectroelectrochemistry. Measurements were performed using an ATR spectroelectrochemical flow cell with a three-electrode configuration, using the counter and pseudoreference electrodes, bipotentiostat, and electrolyte solution described above. The electrolyte solution was degassed for 30 min with Ar before measurements were made. A collimated and polarized beam of light was coupled in and out of the ITO-coated slide as described above. The outcoupled beam was passed through a monochromator (Newport MS260i) or a bandpass filter and detected using a photomultiplier tube (PMT, Newport 77529). A DS335 Function Generator (Stanford Research) was used to apply a sinusoidally modulated voltage (Eac) to the ITO working electrode. The electroreflectance signal was measured in both TE and TM polarizations using an integration time of 30 s.

Following the procedure for 4a, Pc 3b (50 mg, 0.027 mmol), anhydrous CH2Cl2 (2 mL), and TMS-Br (63 mg, 0.41 mmol) gave 4b (30 mg, 63%) as a dark green solid: 1H NMR (499 MHz, 9:1 CD2Cl2/pyridine-d5) δ 8.21−8.48 (m, 4H), 7.50− 6.98 (m, 29H), 4.45 (t, J = 6.4 Hz, 2H), 2.65 (d, J = 37.5 Hz, 12H), 1.62−1.74 (m, 12H), 1.52−1.18 (m, 28H), 0.99 (d, J = 7.1 Hz, 18H); 31P NMR (202 MHz, pyridine-d5) δ 40.21; UV−vis (HMPA) Q-band λmax 680 nm (ε = 2.1 × 105); MS (MALDI) m/z (M)+ calcd for C107H119N8O10PZn+ 1770.7, found 1770.1. (2,3,9,10,16,17-Hexakis(4-pentylphenoxy)-23-((11undecyloxy)phosphonic acid) phthalocyanine) Zinc(II) (4c). Following the procedure for 4a, Pc 3c (50 mg, 0.027 mmol), anhydrous CH2Cl2 (2 mL), and TMS-Br (62 mg, 0.40 mmol) gave 4c (25 mg, 52%) as a dark green solid: 1H NMR (499 MHz, 9:1 CD2Cl2/pyridine-d5) δ 8.21−8.48 (m, 4H), 7.50− 6.98 (m, 29H), 4.45 (t, J = 6.4 Hz, 2H), 2.65 (d, J = 37.5 Hz, 12H), 1.62−1.74 (m, 12H), 1.52−1.18 (m, 28H), 0.99 (d, J = 7.1 Hz, 18H); 31P NMR (202 MHz, pyridine-d5) δ 42.25; UV−vis (HMPA) Q-band λmax 680 nm (ε = 2.3 × 105); MS (MALDI) m/z (M)+ calcd for C101H107N8O10PZn+ 1798.8, found 1798.8. ITO Substrate Preparation. ITO with a layer thickness of ∼100 nm on a 1 mm polished soda lime float glass and a sheet resistance of ∼15 Ω cm−2 was purchased from Thin Film Devices, Inc. The ITO electrodes were cut into 1 in. × 3 in. pieces for ATR and 1/2 in. × 1/2 in. pieces for electrochemical experiments. The electrodes were cleaned by sonication with detergent (diluted Triton X-100), deionized water (Barnsted Nanopure, measured resistivity of 18.3 MΩ cm), and ethanol for 15 min each. ITO electrodes were stored in ethanol and dried under N2 before use. Immediately before use, ITO electrodes were activated in an air plasma (PDC-3XG, Harrick Scientific) at a medium radio frequency level of 10.5 W for 15 min. Attenuated Total Reflectance Spectroscopy.27,29,31,32 A 75 W Xe arc lamp (Newport 67005) coupled to an optical fiber was used a source of white light. A collimated and polarized beam was coupled into and out of the ITO-coated slide, which served as the waveguide, with two BK-7 glass prisms (η = 1.51) from Edmund Optics. The prisms were placed 44.5 mm apart producing 10 total internal reflections at the ITO/solution interface with an internal reflection angle of 69−72°. The outcoupled light was directed to another optical fiber, sent to a monochromator (Newport MS260i), and detected using a CCD (Andor iDus420A). A solution (100 μM) of PA-functionalized ZnPc (4a, 4b, or 4c) in acetonitrile/pyridine (7:3 v/v) was prepared. A blank solution containing acetonitrile/pyridine (7:3 v/v) was injected into the ATR flow cell, and the reference spectra were acquired in transverse electric (TE) and transverse magnetic (TM) polarizations. The solution of the ZnPc derivative was injected into the ATR flow cell, and spectra were acquired every 5 min for 3.5 h in TE polarization. After the adsorption period, the flow cell was rinsed with acetonitrile/pyridine (7:3 v/v) and dichloromethane to remove dissolved and weakly adsorbed ZnPcs. A fresh solution of acetonitrile/pyridine (7:3 v/v) was injected again in the flow cell, and ATR spectra were acquired in TE and TM polarizations. Electrochemical Characterization on ITO. Cleaned and plasma-activated ITO electrodes were immersed in a 100 μM solution of ZnPc (4a, 4b, or 4c) in acetonitrile/pyridine (7:3 C

DOI: 10.1021/acs.jpcc.8b10301 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Synthesis of Pcs 4a−4c

vis spectroscopy. In CH2Cl2, phosphonate ester Pcs 3a−3c exhibited solution-phase aggregation as evidenced by an apparent increase of the absorbance of the vibronic band to the blue of the Q-band, accompanied by a slight hypsochromic shift of the Q-band absorbance (Figure 1a).33 As expected for

For measurements on ZnPcs 4b and 4c, the electroactive area of the ITO working electrode was 2.1 cm2. Measurements on monomeric Pc subpopulations were acquired using a 680 nm bandpass filter with a full width at half maximum (FWHM) = 10 nm (Thor Labs) and an applied direct current (DC) potential (Edc) of 0.375 V vs Ag/Ag+. Measurements on aggregated Pc subpopulations were acquired using a 630 nm bandpass filter with a FWHM = 10 nm (Thor Labs) and an Edc of 0.25 V vs Ag/Ag+. The Eac was 40 mV(rms) (0.056 V peak-topeak), and the range of applied frequencies was 0.1−1600 Hz For measurements on ZnPc 4a, the electroactive area of the ITO working electrode was 0.8 cm2. The BK-7 glass coupling prisms (η = 1.51) were positioned 12.5 mm apart, producing two total internal reflections at the ITO/solution interface with an internal reflection angle of 72−74°. Measurements on the monomeric Pc subpopulations were acquired at a center wavelength of 690 nm (selected using the monochromator) and an Edc of 0.35 V vs Ag/Ag+. Measurements on the aggregated Pc subpopulations were acquired with the monochromator set at a center wavelength of 630 nm and an Edc of 0.20 V vs Ag/Ag+. The Eac was 30 mV(rms) (0.042 V peak-to-peak), and the range of applied frequencies was 0.1− 5000 Hz. The solution resistance (R s ) and the double-layer capacitance (Cdl) were obtained from the high- and lowfrequency regions, respectively, of Bode plots (as described previously28,31). The measurements were performed using bare, cleaned ITO electrodes at an applied potential of 0.35 V (except for ZnPcPA, which was measured at 0.38 V). For ZnPcPA, Pc 4b, and Pc 4c, the electrode area was 2.1 cm2. For Pc 4a, the electrode area was 0.8 cm2.

Figure 1. (a) Solution-phase UV−vis spectra of phosphonate esterfunctionalized ZnPc 3a in CH2Cl2 (dashed) and 9:1 CH2Cl2/pyridine (red, ε682 = 2.2 × 105 cm−1 M−1). (b) Solution-phase UV−vis absorbance spectra of phosphonic acid-functionalized ZnPc 4a in acetonitrile/pyridine (7:3 v/v) (blue, ε680 = 1.0 × 105 cm−1 M−1), pyridine (red), and HMPA (black). All solutions are [Pc] ∼ 10−6 M, and the spectra are normalized to the Q-band λmax.

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RESULTS AND DISCUSSION Synthesis of PA-Functionalized Asymmetric Zn Phthalocyanine Derivatives. Pc 1 was synthesized according to previous published methods.28 We incorporated the tethered PA functionalities by condensation of Pc 1 with diethyl(3-bromopropyl) phosphonate (2a), diethyl(9bromononyl)phosphonate (2b), or diethyl(11bromoundecyl)phosphonate (2c) under basic conditions to achieve Pcs 3a−3c after purification by column chromatography (Scheme 1). To hydrolyze the phosphonate esters 3a− 3c, we used the relatively mild method of treatment with bromotrimethylsilane (TMS-Br) in CH2Cl2 at room temperature to afford the desired phosphonic acid Pcs 4a−4c via the corresponding bis(trimethylsilyl)phosphonates that are readily hydrolyzed upon workup with methanol or water. UV−Vis Characterization of Pc Derivatives. Solutionphase absorbance properties of 3a−3c were assayed by UV−

ZnPcs, aggregation was disrupted in 9:1 CH2Cl2/pyridine in which the molar absorptivity was 2.2 × 105 cm−1 M−1, and linearity was observed in the Beer−Lambert plot over the 5− 40 μM concentration range (Figure S2). Weak coordination of pyridine with Zn inhibits π−π stacking interactions between Pc macrocycles.34 Phosphonic acid Pc derivatives 4a−4c in CH2Cl2 showed a higher degree of solution-phase aggregation than 3a−3c as evidenced by more severe broadening in the Q-band absorption accompanied by a 5 nm hypsochromic shift (Figure S1).28 This can be explained by the ability of PA to form hydrogen bonding interactions35 allowing Pc macrocycles to interact via π−π stacking to a higher degree. The degree of aggregation as a function of solvent was investigated with these compounds. In acetonitrile/pyridine (7:3 v/v), we observed both monomeric and aggregate species at wavelengths of 680 and 630 nm, respectively (Figure 1b), due to the difference in D

DOI: 10.1021/acs.jpcc.8b10301 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C optical band gap between monomeric and aggregates species.36,37 In this solvent mixture, the molar absorptivity of 4a was 1.0 × 105 cm−1 M−1, and linearity was observed in the Beer−Lambert plot over the 5−100 μM concentration range (Figure S3). In 100% pyridine, aggregation was further reduced but not completely suppressed. We observed apparent complete suppression of aggregation for Pc 4a in hexamethylphosphoramide (HMPA) solution (Figure 1b) with observation of a single and sharp Q-band absorption. The suppression of the aggregation using HMPA may be due to the ability of the solvent to act as a hydrogen bond acceptor from the phosphonic acid solute.38−40 Moreover, HMPA has the ability to strongly coordinate with Zn, preventing the aggregation of the molecules.41 Adsorption Kinetics, Aggregation, Surface Coverage, and Molecular Orientation of ZnPcs on ITO. ITO surfaces were modified with functionalized Pcs 4a−4c to initially study the difference in optical properties between solution-phase and surface-confined chromophores. We monitored the adsorption kinetics of the chromophores on ITO using ATR UV−vis spectroscopy.29 This technique has the ability to detect molecules tethered to an ITO-coated waveguide surface at the sub-monolayer regime with a much greater sensitivity than a transmission geometry.32 To modify the surface, we exposed the ITO electrodes to 100 μM solutions of each Pc derivative in acetonitrile/pyridine (7:3 v/v). We chose this solvent mixture: (1) to reduce the refractive index of the solution to achieve total internal reflection inside the ITO-coated waveguide; (2) to monitor adsorption for sufficient periods of time without the concentration of the mixture changing due to solvent evaporation; and (3) to enhance the self-assembly of the molecules onto the ITO surface due to the relatively low solubility of 4a−4c in the solvent system. PA was an effective anchoring group due to its strong ability to chemisorb to oxide surfaces.7,15 We monitored self-assembly of Pcs 4a−4c on ITO by observing the increase in the absorbance at the Q-band wavelength (Figures 2a and S5), while the 100 μM solution was in contact with the ITO surface for a period of 3.5 h. During this period of time, the absorbance reached saturation. The ATR flow cell was then flushed with pure solvent to remove bulk dissolved and weakly surface-adsorbed molecules. The absorbance was maintained after rinsing the film with pure solvent, showing a strong molecular attachment of these PA derivatives. The Q-band absorbance at 690 nm for Pc 4a and 680 nm for Pcs 4b and 4c plotted as a function of time demonstrated a saturation time of adsorption about 2.5 h for all derivatives (Figure 2b). As a control, we exposed the ITO surface in an ATR flow cell to a 500 μM solution of Pc 1, lacking an anchoring PA group, in acetonitrile/pyridine (7:3 v/v). The higher solution concentration was selected to enhance binding of Pc 1 to the ITO. As suggested from previous studies,29 molecules that do not have a PA anchoring group do not strongly bind to the ITO surface. Indeed, after observing an increase in the absorbance over the course of 3 h, rinsing the surface with the pure solvent removed the bulk dissolved and weakly surfaceadsorbed Pc molecules (Figure S4). We note that the absorption profile during this experiment was indicative of monomeric (i.e., nonaggregated) Pc chromophore. The surface coverage of ZnPc films at saturation was determined using transmission UV−vis spectroscopy (see

Figure 2. (a) UV−vis ATR spectra of Pc 4a on ITO in TE polarization. The presence of monomeric (690 nm) and aggregate (630 nm) forms of Pc chromophores attached to the ITO surface is observed. (b) Q-band-absorbance at 690 nm for Pc 4a and 680 nm for Pcs 4b and 4c was monitored as a function of time (squares) during the adsorption process. After flushing the cell to remove dissolved and weakly adsorbed species at ca. 200 min, the residual Qband absorbance from strongly adsorbed Pc molecules is observed (circles). No desorption was observed during subsequent experiments, and thus all data reported herein were acquired on strongly adsorbed Pc films.

Supporting Information). For Pcs 4b and 4c, the coverage was 1.9 × 10−10 mol cm−2 which is equivalent to ca. one closepacked monolayer when the molecules are adsorbed on a smooth, flat surface in an edge-on orientation with a projected area of 85 Å2 per molecule. For Pc 4a, a coverage of 1.6 × 10−10 mol cm−2 was calculated which represents a 0.8 closepacked monolayer. In our previous studies,13,29 we demonstrated that the adsorption of PA-functionalized Pc derivatives to ITO produced films containing both monomer and aggregate subpopulations of Pcs.29 The same result was observed in the current work regardless of the length of the linker between the Pc and PA group; the presence of monomeric (680−690 nm) and aggregate (630 nm) subpopulations attached to the ITO surface was observed in all cases. We compared the absorbance of the monomer and aggregate subpopulations in solution with that of the chromophores in the self-assembled film on ITO and observed a ∼10 nm bathochromic shift for 4a (Figure 3a) and a ∼5 nm shift for 4b and 4c (Figure S6). These modest reductions in the optical band gap energy of ∼0.03 eV for 4a and ∼0.01 eV for 4b and 4c after adsorption on the ITO surface are consistent with the published data on the adsorption of organic dyes on oxide surfaces causing shifts in the energetics of ground and excited states.42,43 We assessed the molecular orientation of Pcs 4a−4c on ITO by measuring the dichroic ratios of the monomeric and aggregated absorbance bands using polarized ATR UV−vis spectroscopy (Figure S7).44 We recently published a detailed study on the effect of surface roughness on tilt angle measurements of Pcs tethered to ITO,31 which includes AFM images of the ITO used in the present study and supports our premise that the ITO surface is sufficiently flat on molecular length scales for the orientation results to be valid. E

DOI: 10.1021/acs.jpcc.8b10301 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

the excited state, producing a hypsochromic shift of the Qband absorbance.36,45 Electrochemical and Spectroelectrochemical Characterization of ZnPcs on ITO. Background-subtracted cyclic voltammograms (CVs) of the first oxidation of adsorbed Pcs 4a−4c on ITO electrodes are shown in Figure 4a. The

Figure 3. (a) UV−vis absorbance spectra of 4a in acetonitrile/ pyridine (7:3) solution (blue) and the ATR spectrum of 4a adsorbed on ITO (black) in TE polarization. (b) Overlaid plots of TE- and TM-polarized ATR spectra of 4a adsorbed on ITO showing the presence of monomeric and aggregated subpopulations.

The spectra in Figure 3b of an adsorbed 4a film acquired in TE and TM polarizations show that the monomeric species are tilted predominately in-plane with respect to the ITO surface, whereas the aggregated species are tilted predominantly out-ofplane. The mean tilt angles between the normal to the Pc molecular plane and the ITO surface normal (Table 1) for the Table 1. Mean Tilt Angle for Subpopulations of PAFunctionalized ZnPcs Adsorbed on ITO Pc

monomer (±1°)

aggregate (±1°)

4a 4b 4c

36 38 39

72 73 75

Figure 4. (a) Overlaid, background-subtracted cyclic voltammograms showing the first oxidation of monomeric and aggregated forms of PA-functionalized ZnPc films adsorbed on ITO. The presence of two oxidation peaks indicates the presence of monomeric and aggregated species. (b) Potential-dependent ATR spectra of 4a on an ITO-coated waveguide acquired using TE polarization. (c) Potential-dependent ATR spectra of 4a on an ITO-coated waveguide acquired using TM polarization. The hypsochromically shifted Q-band was bleached at less positive potentials, consistent with the lower oxidation potential of the aggregate species.

monomeric subpopulation were 36−39°. For the aggregated species, the mean tilt angles were 72−75°. For both subpopulations, there was a slight increase in tilt angle with increasing linker length. There is a discrepancy between the tilt angles measured for the 4a−4c aggregate subpopulations, and the tilt angle measured in our previous work for the ZnPcPA aggregate subpopulation, which was 57.8 ± 0.7°.29 The films in the previous work were prepared on differently sourced ITO that has a higher surface roughness, and the films in the current work were annealed, unlike the ZnPcPA film. Both factors are known to affect the measured tilt angle.27,31 Compared with the TM spectra, the maximum absorbance for both monomeric and aggregate chromophores in the TE (in-plane) spectra was shifted ∼20 nm hypsochromically (Figure 3b). The presence of aggregated species on the surface alters the molecular frontier orbital energies, shifting the maximum absorbance for both monomeric and aggregate subpopulations.42,43 The molecular exciton model describes the excited state resonance interactions between aggregated chromophores.37 Aggregated Pc species exhibit different spectral properties compared with monomeric species due to their molecular interactions. H-aggregates result in parallel transition dipoles with in-phase and out-of-phase configurations. The in-phase transition dipoles are higher in energy, resulting in an allowed energy transition from ground state to

voltammetric responses are much broader than predicted for the one-electron oxidation of an immobilized monolayer. However, these data are consistent with published electrochemical data for Pcs tethered to ITO.27−29 Electrochemical reactions on ITO are inherently slow which is thought to be due to its low and spatially heterogeneous surface conductivity, where the dimensions of the electrically active and inactive regions are on nanometer to micrometer length scales.11,12,46−48 These voltammograms are resolved into two peaks indicative of an electrochemical response for the two distinct subpopulations in the film.29,49 The midpoint potentials (Table 2) show that the aggregated subpopulations are more easily oxidized than monomeric forms. This is consistent with π−π stacking interactions which help to stabilize the oneelectron oxidation product.50 Mezza and Armstrong 49 investigated the redox activity of SiPc monomer, dimer, and trimer using diffusion-controlled electrochemistry and obF

DOI: 10.1021/acs.jpcc.8b10301 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Table 2. Electroactive Surface Coverage (Γ), Midpoint Potential (E°′), Apparent Charge-Transfer Rate Constant (ks,app), and Calculated Distance (d) between Surface and Chromophore for PA-Functionalized ZnPcs on ITO Electrodes Pc

Γ × 10−10 (mol cm−2)

4a 4b 4c ZnPcPAb

1.2 1.8 1.8 2.0

E°′(V vs Ag/Ag+) 0.45 0.46 0.46 0.42

(0.19)a (0.17)a (0.15)a (0.23)a

ks,app (s−1) 6.9 2.6 2.4 1.6

(7.5)a (3.7)a (3.2)a (2.4)a

d (Å)c 7.6 15.3 17.8 16.6

a

Values in parentheses are the results for the aggregated species. bValues of ZnPcPA with a 10-carbon linker length were obtained from ref 28. Obtained from the energy-minimized structures (DFT B3LYP/6-31G*) as through space distances between the Pc carbon attached to the linker and the furthest oxygen of the PO3H residue. c

combination with the solution resistance (Rs) and the doublelayer capacitance (Cdl) to calculate the rate constant from ks,opt = 0.5ω2RsCdl. The waveguided light is TE or TM polarized which allows ks,opt to be measured in each polarization and correlated to the molecular orientation of both the monomeric and aggregated subpopulations of Pc molecules in the film. Edc was selected by measuring the electroreflectance as a function of applied potential and light polarization (Figure S11). For Pc 4a, the optical response was monitored at 690 and 630 nm for the monomeric and aggregated subpopulations, respectively. The largest difference between the Re(9AC) and Im(9AC) signals was used to select Edc, which was 0.20 and 0.35 V for aggregate and monomeric subpopulations, respectively. The respective values for Pcs 4b and 4c were 0.25 and 0.37 V. The Eac was 30 and 40 mVrms for Pc 4a and Pcs 4b and 4c, respectively. Figure 5 shows example complex plane

served that the ensemble was easier to oxidize compared with the monomeric species upon increasing the number of layers. Assuming one-electron oxidation for both monomeric and aggregated forms of Pc 4a−4c on ITO, we calculated the electroactive surface coverage to be 0.7−0.9 close-packed monolayer with the molecules adsorbed in an edge-on orientation (Table 2). The greater surface coverage is consistent with increasing the length of the linker as the longer linker reduces screening of the surface by the chromophore during deposition. By fitting the anodic portion of the CVs, we calculated the subpopulation composition of the film. Pc 4a films were composed of 39:61% aggregate/ monomer subpopulations, whereas Pcs 4b and 4c films were composed of 35:65% aggregate/monomer. We determined the apparent heterogeneous electrontransfer rate constants (ks,app) for the monomeric and aggregated forms of Pcs 4a−4c from the CV data using the Laviron method.51 Peak separations were determined by computing the first derivative of the CV and calculating the difference between the respective minima. The aggregated form of 4a, having the shortest linker, exhibited the fastest apparent rate constant observed, whereas those for the longer linkers were smaller (Table 2). This is consistent with the Marcus theory which predicts a faster electron-transfer rate when the donor−acceptor distance is shorter,52 which in the present case corresponds to Pc 4a. Waveguide ATR spectroelectrochemistry was used to study optical changes associated with oxidation of Pc 4a−4c films. The potential applied to the ITO working electrode was stepped from −0.1 to 0.6 V vs Ag/Ag+, and spectra were acquired using both TE and TM-polarized light (Figures 4b,c and S9). A decrease in both monomer and aggregate absorbance bands was observed due to the oxidation of the Pc macrocycle. In agreement with the CV data, aggregated Pcs oxidize at less positive potentials (ca. 0.2 V) than the monomeric Pcs (ca. 0.4 V), regardless of the length of the linker (Figure S10). Determination of Charge-Transfer Rate Constants by PM-ATR. PM-ATR was employed as a second method to determine electron-transfer rate constants for Pc films.27,29 In this technique, a sinusoidally modulated potential (Eac) is applied about a DC bias (Edc) near the midpoint potential of the redox-active film tethered to the ITO-coated waveguide.53−57 Due to differences in the molar absorptivity of the oxidized and neutral Pc film, the intensity of light outcoupled from the waveguide (the electroreflectance) is modulated at the angular frequency (ω) at which Eac is modulated. The real (Re(9AC)) and imaginary (Im(9AC)) components of the electroreflectance are measured over a range of ω values, and polynomial fitting is used to determine the ω at which Re(9AC) equals zero. This ω is used in

Figure 5. Complex plane plots for the aggregated subpopulation of a Pc 4a film measured at 630 nm in TE (blue) and TM (red) polarizations.

plots (Re(9AC) vs Im(9AC) over an ω range) of the aggregated subpopulation of Pcs 4a measured using TE and TM-polarized light, respectively. The ks,opt, Rs, and Cdl values for all Pc films are listed in Table 3. For all Pc films, the charge-transfer rates measured by PMATR (Table 3) were faster than those measured by CV (Table 2) which is likely due to the difference in the fraction of the film that contributes to the measured signal.29 In the CV experiment, the electrochemical response of all electroactive molecules is measured, whereas in the PM-ATR experiment, G

DOI: 10.1021/acs.jpcc.8b10301 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Table 3. Charge-Transfer Rate Constants (ks,opt) for PA-Functionalized ZnPcs Subpopulations on ITO Determined by PMATR, and Solution Resistance (Rs) and Double-Layer Capacitance (Cdl) Values Measured by Electrochemical Impedance Spectroscopy (EIS) ks,opt (× 102 s−1) monomer Pc 4a 4b 4c ZnPcPAa

TE 150 4.1 1.5 2.0

± ± ± ±

aggregate TM

20 0.2 0.2 0.6

120 3.4 1.2 1.7

± ± ± ±

TE 30 0.3 0.1 0.5

390 33 14 21

± ± ± ±

Rs (Ω cm2)

TM 60 4 3 5

230 11 5.6 7

± ± ± ±

30 1 0.4 2

7.4 31 28 54

± ± ± ±

0.1 3 3 6

Cdl (μF cm−2) 10.4 8.0 7.4 7

± ± ± ±

0.4 1 0.3 1

a

Ref 29.

ks,opt values exceeded those of the monomeric population; this is attributed to a lower reorganization energy for the aggregate relative to the monomer.58,59 This trend was also present in the ks,app results listed in Table 2. Focusing on the aggregate subpopulation, the ks,opt values measured in TE polarization were consistently greater than those measured in TM polarization for all four Pc films. A distribution of upright and in-plane aggregates is likely to form on the polycrystalline, chemically heterogeneous surface of ITO, producing a distribution of Pc aggregate-electrode separation distances.11,12,29,46,48 In a Pc film for which D4h symmetry and a uniaxial orientation distribution in the electrode surface plane are assumed, molecules at all tilt angles will contribute to the rate constants measured in TE and TM polarizations, but their contributions are weighted by the extent to which their absorption transition dipoles project onto the electric fields of TE and TM-polarized light.60 The Pc molecules with more in-plane orientations will have shorter separation distances and thus undergo more rapid electron transfer, leading to a higher rate constant measured in TE polarization.27 In contrast, the ks,opt values for the monomer subpopulation did not exhibit a strong dependence on polarization. Consistent with previous studies,27,29 we attribute this result to a narrower orientation distribution for monomers relative to aggregates. When the tilt angle distribution is narrow, then subpopulations with different orientation distributions are not present, and there is no structural basis for a difference in the rate constants measured in TE and TM polarizations. The Marcus relationship that describes the distance dependence of a heterogeneous electron-transfer reaction is expressed as ks = k0 exp(−βd) where d is tunneling distance for electron transfer, k0 is the extrapolated value of the rate constant for d = 0, and β is the exponential decay coefficient.52 Exponential decay coefficients were obtained from the slope of the natural logarithm of ks,opt vs d for monomeric and aggregated Pc subpopulations in Pc films (Figure 7). We obtained values of β = 0.46 Å−1 for monomeric subpopulations and β = 0.32 and 0.37 Å−1 for the TE and TM aggregated subpopulations, respectively. These values are lower than those frequently reported for electron transfer through alkane monolayers, which typically exhibit a β value of ca. 1 Å−1.61−63 The β values obtained here are more characteristic of electron transfer through unsaturated bridges64−66 of molecules tethered to electrode surfaces. For example, β values of 0.57 and 0.36 Å−1 were reported for oligo(phenylethynyl)-linked ferrocene monolayers on gold.65,66 We posit that the lower than expected β values in our system result from both the distribution of chromophore orienta-

only those molecules that are oxidized within