The step-wise synthesis of 1 – 4 was monitored by UV-Visible spectroscopy (Figure 5.2). The spectral changes after each step are consistent with the addition of new components. A UV/vis spectrum of a slide with 4-aminobenzyl phosphonic acid (TiO2-PPh-NH2, Figure 5.2) shows no difference to the background absorption of the TiO2 thin film due to the fact that 4-aminobenzyl phosphonic acid only absorbs in the UV region, which is dominated by the absorption and scattering of the TiO2 thin film. Addition of PTCA (IA) or PTCA(OPhtBu)2 (IB) in the second step, however, has a marked effect (Intermediate A, IA, and Intermediate B, IB, Figure 5.2). New absorptions in the visible region appear due to the perylene dye. IA has a λmax at 475 nm while IB is red-shifted and broad with a λmax of 510 nm. Final reaction of the exposed anhydride functionality of IA and IB was achieved by dipping the slide into a 70 °C solution of aniline or [Fe(tpy-PhNH2)2]2+ in acetonitrile. Addition of aniline caused a slight loss of structure of the peak in IA, but changes to both IA and IB were minimal (Figure S1 in Appendix D). Addition of [Fe(tpy-PhNH2)2]2+ to give assemblies 3 and 4 also is seen in the UV/vis spectrum. A new peak in both 3 and 4 appeared at 575 nm as well as a slight red-shift in the λmax of IA and IB by ~10
86
nm (Figure 5.2). The λmax of 1 – 4 due to the perylene absorbance is approximately the same as soluble derivatives in CH3Cl (Figure S2 in Appendix D), although 1 – 4 show broader absorptions with a different distribution of the vibronic structure.
Figure 5.2. UV/vis spectra of each step in the reaction to form assemblies 3 and 4 showing the addition of the absorptions of both the perylene dye and the of [Fe(tpy-PhNH2)2]2+. UV/vis spectra were taken by holding the thin-film area of the slide perpendicular to the spectrophotometer beam open to air at 298 ± 3K.
Cyclic voltammetry (CV) of 1 – 4-TiO2 was done by immersing the slides in a 0.1 M perchloric acid solution using a slide of thin film TiO2 coated with 1 – 4 as the working electrode with a Ag/AgCl (3 M NaCl) reference (0.207 V vs. NHE) and a platinum mesh counter electrode at a scan rate of 10 mV/s. On the surface, the perylene moieties in 1 – 4 are electrochemically silent: the expected oxidative couples that were observed in solution (Figure S3 in Appendix D) were not observed in 1 – 4. It should be noted that the two expected reductions of 1 – 4 could not be investigated due to the conduction band edge of TiO2 (~0 V vs. NHE) that gives rise to a large background current. Although no redox couples corresponding to the central perylene unit in 1 – 4 were observed, a wave corresponding to the FeIII/II couple was seen for 3 and 4. The CV of 3 is shown in Figure 5.3 and displays a reversible redox couple with E1/2 = 1.2 V vs. NHE, a value consistent with the FeIII/II potential of [Fe(tpy-PhNH2)2]III/II in solution. Although scans to higher
87
potential fail to reveal the one-electron oxidation of the perylene core, the FeIII/II wave in 3 and 4 provides evidence (vide infra) for the proposed structures of 3 and 4 where Fe is bound to the PDI cores, as [Fe(tpy-PhNH2)2] does not bind appreciably to TiO2.
Figure 5.3. CV of 3 attached to TiO2 in 0.1 M HClO4 with a Ag/AgNO3 reference and Pt mesh counter electrodes taken at 10 mV/s at 298 ± 3 K.
Time-resolved absorption difference spectra and single wavelength kinetic measurements for 1 – 4 on TiO2 and ZrO2 were collected. The full spectrum of 1-TiO2 (Figure 5.4A) shows a bleach at 465 nm consistent with the ground state absorption of 1-TiO2 (Figure S1 in Appendix D) and positive features at 720 and 810 nm at the first observable time (20 ns). These same spectral features are also present in 1-ZrO2(Figure 5.4B). 2-TiO2 shows a similar set of features with a bleach at ~510 nm consistent with the ground state absorption of 2-TiO2 and a positive feature with a peak at 720 nm (Figure S4 in Appendix D). Absorption – time traces following 450 nm excitation for 1-TiO2, 1-ZrO2 and 2-TiO2 at 720 nm are shown in Figure 5.5. The positive feature at this wavelength was long-lived in all cases with ~5 % of the original signal still remaining after 400 µs. The comparison of 1-TiO2 and 1-ZrO2 for the first 10 µs is shown in Figure 5.5A and a longer timescale, 400 µs, for 1-ZrO2 is shown in Figure 5.5B.
88
-2.0 0.0 2.0 4.0 6.0 8.0 450 500 550 600 650 700 750 800 850 Del ta O D/10 -3 Wavelength/nm 1 20.00 ns 2 100.00 ns -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 400 450 500 550 600 650 700 750 800 850 Del ta O D/1 0 -2 Wavelength/nmA
B
Figure 5.4. Full TA spectra of 1-TiO2 (A) at 20 ns (green) and 100 ns (pink) and 1-ZrO2 (B) at 20 ns in 0.1 M HClO4 under Argon with a 4.6 mJ excitation at 450 nm at 298 ± 3 K.
0.0 0.2 0.4 0.6 0.8 1.0 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Del ta O D/10 0 Time/ns ZrO2-PPh-PDI-Ph (low l TiO2-Ph-PDI-Ph 720nm -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0 50 100 150 200 250 300 350 Del ta O D/10 -2 Time/us
A
B
Figure 5.5. Absorption – time traces comparing 1-ZrO2 (green, A) and 1-TiO2 (pink, A) for the first 10 µs and 1-TiO2 (B) for the first 400 µs in Ar-deaerated 0.1 M HClO4 at 298 ± 3 K.
TA results of 3 and 4 on ZrO2 are shown in Figure S4 in Appendix D. The spectra of 3-ZrO2 (Figure S4A in Appendix D) and 4-ZrO2 (Figure S4B in Appendix D) are qualitatively similar to the dyes without Fe, 1-ZrO2 and 2-ZrO2. Both spectra show bleaches of the UV/vis ground state spectra at the earliest observable times with positive features centered at 720 nm that decay back to baseline over time. The same behavior was also observed for TiO2-3 and TiO2-4 (Figure 5.6A and B respectively), although the signals for both bleach and the positive feature are more pronounced. Unlike 1 and 2, the bleaches in 3 and 4 display an additional peak at 575 nm. This is
89
consistent with a ground state absorption spectrum bleach of [Fe(tpy-PhNH2)2]2+. Both the bleach of PDI ground state as well as bleach for FeII are seen at earliest observable times, indicating rapid oxidation of FeII to FeIII.
-1.5 -1.0 -0.5 0.0 0.5 1.0 400 450 500 550 600 650 700 750 800 850 Del ta O D/1 0 -2 Wavelength/nm
B
A
Figure 5.6. Full TA difference spectra of 3-TiO2 (A) at 20 ns (green) and 100 ns (pink) and 4-TiO2 (B) at 20 ns (green), 100 ns (pink), and 1 µs (blue) in 0.1 M HClO4 under Argon with a 4.6 mJ excitation at 450 nm at 298 ± 3 K.
Comparisons of non-derivatized PDI (1-TiO2 and 2-TiO2) with Fe-derivatized PDI (3-TiO2 and 4-TiO2) reveal similar kinetics for the decay of the 720 nm peak (Figures S5 – S8 in Appendix D). Qualitatively, the full spectra of 3-TiO2 and 4-TiO2 are the same as 1-TiO2 and 2- TiO2 with an additional bleach due to the FeII. The absorption-time spectra reveal that the positive feature decays at approximately the same rate both with and without Fe present.
DISCUSSION
The new synthetic method for 1 – 4 directly bound to metal oxide surfaces is a versatile approach to molecular assemblies containing a perylene as the dye unit. Reactions on the surface of thin-film metal oxides were effectively monitored by UV/vis spectroscopy. While the λmax is indicative of the addition of components to the surface, the vibronic structure gives a clue as to the actual assembly of 1 – 4 on the surface. Broadening of the perylene transitions as well as a
90
relative intensifying of the higher energy vibration as compared to the solution spectra of the perylene dyes may indicate π-stacking or association between perylene centers on the surface. This behavior is well known in solution, where 1D structures of PDI molecules can be generated in polar solvents.12,40-42 ATR-IR spectra of 1 – 4 on the surface of TiO2 were obtained in an attempt to identify the succinimide or anhydride C=O stretches, however these data are inconclusive; no stretches were seen that match the powder samples of perylene-3,4,9,10- tetracarboxylic dianhydride or perylene-3,4,9,10-tetracarboxylic diimide.
For 3 and 4 [Fe(tpy-PhNH2)2]2+ was used as a hole-transfer acceptor to generate a charge separated species analogous to what would be needed in a DSPEC. [Fe(tpy-PhNH2)2]2+ was chosen due to both a favorable FeIII/II potential (1.2 V vs. NHE) and the very distinct UV/vis spectrum that has a sharp peak at 575 nm for FeII, but no absorption at this wavelength when oxidized to FeIII. This absorption can act as a probe to the redox state of the Fe. Addition of Fe to the perylene intermediates IA and IB gives rise to a clear peak indicating Fe is incorporated into the assembly. Since [Fe(tpy-PhNH2)2]2+ is not functionalized with metal oxide binding groups, the Fe is presumed to bind through the generation of the succinimide functionality on the perylene core. To the amount of Fe that reacted with the perylene, molar absorptivities from solution UV/vis spectra for both the soluble PDI derivative and [Fe(tpy-PhNH2)2]2+ were used to fit the spectra of 3 and 4. For 3, about 30 % of the perylene units are decorated with Fe and for 4, about 80 % are decorated with Fe (Figure 5.7).
The origin of the lack of electrochemical response associated with the perylene moiety for 1 – 4-TiO2 is unclear. The Fe wave in 3 and 4 is distorted due to the need for electrons to travel from the electrode to the Fe via a hopping mechanism around the TiO2 particles, but it does look similar to other surface couples on TiO2 as this distortion is due to the semiconductor surface.
The bleach that is seen at the earliest observable time in the TA spectra for 1 – 4 bound to both TiO2 and ZrO2 is not readily explained. No fluorescence was observed for 1 – 4 bound to a metal oxide, leading to the conclusion that fluorescence is quenched either by interactions between the
91
dyes or injection into the metal oxide. Since 1 – 4-ZrO2 also show fluorescence quenching where electron injection into the ZrO2 conduction band is energetically not possible, the fluorescence quenching is likely due to interactions between dyes. This and the fact that the bleaches last >400
µs also suggests the bleach of UV/vis spectral features for 1 – 4-TiO2 and 1 – 4-ZrO2is also not due to excited state either 1PDI* or 3PDI*. An important spectral feature for deconvoluting the TA spectra is the 720 nm feature present in 1 – 4.
IA
Figure 5.7. UV/vis spectra of IA-TiO2 (orange), [Fe(tpy-PhNH2)2]2+ (in CH3CN, purple), 4-TiO2 (green), and an addition of IA and [Fe(tpy-PhNH2)2]2+ (blue). Molar absorptivities of IA and 4 were estimated from the molar absorptivity of the soluble PDI unit of 4 in CH3Cl.
This positive absorption as well as the positive feature seen ~810 nm is consistent with PDI and has been previously reported.42 It is also known that the radical mono- and di-anions of PDI are stable in deaearated water43 which could be part of the reason why this feature is long-lived in the TA spectroscopy. Strangely, the PDI spectral feature at 720 nm is present in all cases (1 – 4-TiO2 and 1 – 4-ZrO2). The only way to generate PDI on ZrO2 is through a rapid charge separation within the PDI film, an event that is not without literature precedent. 1D columnar structures of PDI in solution can act as excellent electron transport materials and can give charge separation rates of 1.4 x 1012 s-1; faster than the earliest observation times of the TA (10 ns).
92
Further, it has been noted that in these 1D structures, charge recombination is significantly slower, on the order of 4.6 x 108 s-1.41,42 What is most curious about the presence of the PDI , however, is the fact that there is no evidence for electron injection into TiO2 from this species in 1 – 4, a reaction that is downhill by 0.2 eV (PDI-/0 ≈ -0.7 V vs. NHE and TiO2 CB = -0.5 V vs. NHE at pH = 1).
For 3 and 4-TiO2 and –ZrO2, there is an additional bleach in the TA spectra due to the loss of the FeII MLCT absorption. The FeII bleach indicates oxidation of the Fe center to FeIII; a very rapid charge separation and hole collection by the attached FeII center since this bleach is at a maximum at 20 ns. Ultrafast TA spectra are able to capture the growth of FeIII, which occurs in the first 10 – 20 ps. The resultant charge separated state is long lived (>450 µs). Rapid charge separation and slow recombination kinetics are known in the 1D π-stacked systems when a Zn porphyrin was incorporated.42
The nature of the CS state remains unclear as kinetics of charge recombination for 1 – 4 are similar. The thermodynamic driving force for charge recombination between 1 and 2 and 3 and 4 are drastically different with the PDI potential (1.6 V vs. NHE) 400 mV higher than the FeIII potential (1.2 V vs. NHE). The electron seems to be “trapped” with the back electron transfer dynamics dependent on the energy penalty required for the electron to escape this trapped state.
CONCLUSIONS
We have reported a new synthetic procedure that allows easy access to surface-bound PDI units in a step-wise manner. These chromophoric materials were functionalized with a variety of substituents, including redox active metal centers. Upon photo-excitation of the films, rapid charge separation occurs leading to a long-lived charge separated state in 3 and 4 with the electron delocalized in the perylene layer and the holes on the Fe center. For 1 and 2 the fate of the hole and the nature of the charge-separated state is unclear.
93
The long-lived charge separated states reported here are promising for use of PDI as a dye in DSPEC devices. The oxidation of the FeII center to FeIII indicates that hole collection is possible. Although the PDI does not inject electrons into TiO2, the PDI film is effectively acting as a semiconductor; undergoing exciton formation and charge separation when excited with an appropriate wavelength of light.
ACKNOWLEGEMENTS
Funding by the Chemical Sciences, Geosciences and Biosciences Division of the Office of Basic Energy Sciences, U.S. Department of Energy, Grant DE-FG02-06ER15788, and UNC Energy Frontier Research Center (EFRC) “Center for Solar Fuels,” an EFRC funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE- SC0001011, supporting M.R.N, M.K.B. J.J.C., L.A. and D.L.A. (T.J.M., J.L.T.). We acknowledge support for the purchase of instrumentation from the UNC EFRC (Center for Solar Fuels, an EFRC funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001011) and UNC SERC (“Solar Energy Research Center Instrumentation Facility” funded by the U.S. Department of Energy, Office of Energy and Efficiency & Renewable Energy, under Award DE-EE0003188).
ASSOCIATED CONTENT
94
REFERENCES(1) Nelson, J. Current Opinion in Solid State and Materials Science2002, 6, 87.
(2) Kearns, D.; Calvin, M. The Journal of Chemical Physics1958, 29, 950.
(3) McGehee, M. D.; Topinka, M. A. Nat. Mater.2006, 5, 675.
(4) Hoppe, H.; Sariciftci, N. S. Journal of Materials Research2004, 19, 1924.
(5) Ghosh, A. K.; Morel, D. L.; Feng, T.; Shaw, R. F.; Charles A. Rowe, J. Journal of Applied Physics1974, 45, 230.
(6) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science1995, 270, 1789.
(7) Song, W.; Chen, Z.; Brennaman, M. K.; Concepcion, J. J.; Patrocinio, A. O. T.; Iha, N. Y. M.; Meyer, T. J. Pure & Applied Chemistry2011, 83, 749.
(8) Norris, M. R.; Concepcion, J. J.; Harrison, D. P.; Binstead, R. A.; Ashford, D. L.; Fang, Z.; Templeton, J. L.; Meyer, T. J. J. Am. Chem. Soc.2013, 135, 2080.
(9) Ashford, D. L.; Song, W.; Concepcion, J. J.; Glasson, C. R. K.; Brennaman, M. K.; Norris, M. R.; Fang, Z.; Templeton, J. L.; Meyer, T. J. J. Am. Chem. Soc. 2012, 134, 19189.
(10) Youngblood, W. J.; Lee, S.-H. A.; Kobayashi, Y.; Hernandez-Pagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E. J. Am. Chem. Soc.2009, 131, 926.
(11) Youngblood, W. J.; Lee, S.-H. A.; Maeda, K.; Mallouk, T. E. Acc. Chem. Res. 2009, 42, 1966.
(12) Kim, J. W.; Kim, H. S.; Yu, K. H.; Fujishima, A.; Kim, Y. S. Bull. Korean Chem. Soc. 2010, 31, 2849
(13) Ford, W. E.; Kamat, P. V. The Journal of Physical Chemistry1987, 91, 6373.
(14) Qvortrup, K.; Bond, A. D.; Nielsen, A.; McKenzie, C. J.; Kilsa, K.; Nielsen, M. B.
Chemical Communications2008, 0, 1986.
95
(16) O'Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L.; Wasielewski, M. R. Science1992, 257, 63.
(17) Zang, L.; Liu, R.; Holman, M. W.; Nguyen, K. T.; Adams, D. M. J. Am. Chem. Soc. 2002, 124, 10640.
(18) Huang, C.; Sartin, M. M.; Siegel, N.; Cozzuol, M.; Zhang, Y.; Hales, J. M.; Barlow, S.; Perry, J. W.; Marder, S. R. J. Mater. Chem.2011, 21, 16119.
(19) Miura, T.; Carmieli, R.; Wasielewski, M. R. J. Phys. Chem. A2010, 114, 5769.
(20) Ramos, A. M.; Beckers, E. H. A.; Offermans, T.; Meskers, S. C. J.; Janssen, R. A. J. J. Phys. Chem. A2004, 108, 8201.
(21) Marchanka, A.; Maier, S. K.; Hoeger, S.; van, G. M. J. Phys. Chem. B2011, 115, 13526.
(22) Gomez, R.; Segura, J. L.; Martin, N. Org. Lett.2005, 7, 717.
(23) Ahrens, M. J.; Sinks, L. E.; Rybtchinski, B.; Liu, W.; Jones, B. A.; Giaimo, J. M.; Gusev, A. V.; Goshe, A. J.; Tiede, D. M.; Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 8284.
(24) Dinçalp, H.; Aşkar, Z.; Zafer, C.; İçli, S. Dyes and Pigments2011, 91, 182.
(25) Erten-Ela, S.; Turkmen, G. Renewable Energy2011, 36, 1821.
(26) Tachikawa, T.; Cui, S.-C.; Tojo, S.; Fujitsuka, M.; Majima, T. Chem. Phys. Lett. 2007,
443, 313.
(27) Langhals, H.; Lona, W. European Journal of Organic Chemistry1998, 1998, 847.
(28) Che, Y.; Datar, A.; Balakrishnan, K.; Zang, L. J. Am. Chem. Soc.2007, 129, 7234.
(29) Huang, H.; Che, Y.; Zang, L. Tetrahedron Letters2010, 51, 6651.
(30) Nagao, Y.; Naito, T.; Abe, Y.; Misono, T. Dyes and Pigments1996, 32, 71.
96
(32) Mikroyannidis, J. A.; Stylianakis, M. M.; Roy, M. S.; Suresh, P.; Sharma, G. D. Journal of Power Sources2009, 194, 1171.
(33) Sermet, K.; Mahmut, K.; Serafettin, D.; İsmet, K.; Eyup, O.; Siddik, I. Journal of Polymer Science Part A: Polymer Chemistry2008, 46, 1974.
(34) Zhang, X.-C.; Huo, Z.-M.; Wang, T.-T.; Zeng, H.-P. J. Phys. Org. Chem.2012, 25, 754.
(35) Manning, S. J.; Bogen, W.; Kelly, L. A. J. Org. Chem.2011, 76, 6007.
(36) Storrier, G. D.; Colbran, S. B. J. Chem. Soc., Dalton Trans.1996, 2185.
(37) Lee, S.-H. A.; Abrams, N. M.; Hoertz, P. G.; Barber, G. D.; Halaoui, L. I.; Mallouk, T. E.
J. Phys. Chem. B2008, 112, 14415.
(38) Song, W.; Glasson, C. R. K.; Luo, H.; Hanson, K.; Brennaman, M. K.; Concepcion, J. J.; Meyer, T. J. The Journal of Physical Chemistry Letters2011, 2, 1808.
(39) Gillaizeau-Gauthier, I.; Odobel, F.; Alebbi, M.; Argazzi, R.; Costa, E.; Bignozzi, C. A.; Qu, P.; Meyer, G. J. Inorg. Chem.2001, 40, 6073.
(40) Wang, H.; Schaefer, K.; Pich, A.; Moeller, M. Chem. Mater.2011, 23, 4748.
(41) Supur, M.; Yamada, Y.; El-Khouly, M. E.; Honda, T.; Fukuzumi, S. The Journal of Physical Chemistry C2011, 115, 15040.
(42) Supur, M.; Fukuzumi, S. The Journal of Physical Chemistry C2012, 116, 23274.
(43) Shirman, E.; Ustinov, A.; Ben-Shitrit, N.; Weissman, H.; Iron, M. A.; Cohen, R.;