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J. Phys. Chem. C 2008, 112, 2170-2176
Photoconductivity of Self-Assembled Nanotapes Made from meso-Tri(4-sulfonatophenyl)monophenylporphine Andrew L. Yeats,† Alexander D. Schwab,‡ Brittany Massare,† Danvers E. Johnston,§ Alan T. Johnson,§ Julio C. de Paula,⊥ and Walter F. Smith*,† Physics Department, HaVerford College, HaVerford, PennsylVania 19041, Chemistry Department, Appalachian State UniVersity, Boone, North Carolina 28608, Physics Department, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, and Chemistry Department, Lewis and Clark College, Portland, Oregon 97219 ReceiVed: August 15, 2007; In Final Form: NoVember 2, 2007
meso-Tri(4-sulfonatophenyl)monophenylporphine (TPPS3) has one fewer sulfonate group than meso-tetra(4sulfonatophenyl)porphine (TPPS4), which has been shown to form well-defined, photoelectronically active nanorods. TPPS3, when deposited via immersion and spin-drying, forms tapelike aggregates of two distinct heights. The larger width of these nanotapes (compared to TPPS4 nanorods) is expected from the smaller charge of the monomer when it is dissolved in acidic solution. The deposition of nanotapes onto a substrate can be patterned to a limited extent by scratching the substrate prior to aggregation. The nanotapes exhibit photoconductive properties very similar to those of TPPS4 nanorods, including growth of photoconductivity over hundreds of seconds, as well as photovoltaic activity with trainable polarity. However, they show a stronger memory of the slow growth of photoconductivity. The quantum efficiency of photoconductivity is approximately 10 times lower than that for TPPS4 nanorods.
Introduction Porphyrins are common in nature and present rich possibilities for use in nanoelectronics. For instance, the photosynthetic process used by plants and some bacteria involves one of several modified porphyrins known as chlorophylls. As pointed out by Escudero et al.,1 porphyrins with hydrophilic meso-substituents (and a hydrophobic central macrocycle) offer a different amphipathic geometry than the more common head/tail amphiphiles and therefore may offer new possibilities for selfassembled structures. Photoconductivity measurements on continuous thin films of porphyrins date back at least to 1957.2 Recently,3 we reported on the photoelectronic behaviors of rod-shaped aggregates formed by self-assembly of meso-tetra(4-sulfonatophenyl)porphine (TPPS4) from acidic solution.4 In this study we investigate the properties of aggregates formed from meso-tri(4-sulfonatophenyl)monophenylporphine (TPPS3); the TPPS3 monomer is identical to that of TPPS4, save for the omission of one sulfonate group (see Figure 1). By observing how the aggregates of slightly different monomers differ, we can gain insight into the mechanisms which drive their self-assembly and which determine their photoelectronic properties. TPPS3 was first shown to self-assemble by Pasternack et al. using spectroscopic techniques;5 additional spectroscopic studies have been done by Rubires et al.6 The morphology of TPPS3 aggregates has been studied by Crusats et al.7 who observed clusters of “nanoribbons” folded into helix-like structures whose chirality was correlated with the rotation of the rotary evaporator they used to deposit aggregates from solution. “Whiskers” were * To whom correspondence should be addressed. E-mail: wsmith@ haverford.edu. † Haverford College. ‡ Appalachian State University. § University of Pennsylvania. ⊥ Lewis and Clark College.
Figure 1. Structural comparison of the diacid forms of TPPS4 and TPPS3.
also reported in samples made without rotation. Escudero et al.8 report that TPPS3 aggregates grown in pure water solution, and deposited by drop casting and blow drying, initially form “straight tapes with constant height (3.2 ( 0.1 nm)” but then tend to fold over themselves after longer aggregation times, a process which is hastened by heating and agitation. They performed similar experiments on TPPS4 but did not observe folding even after 2 months of agitation. They propose a doublelayer structure of tilted (45°) molecules for both types of aggregate in order to explain the shorter-height (1.5 nm) borders and corners they observe on some TPPS3 aggregates. Poderys et al.9 studied TPPS3 aggregates deposited by drop casting followed by blow drying. They observed clumps of “stripe like” aggregates, with a width of about 50 nm and height of about 4 nm. Although TPPS4 aggregates are thought to be tubelike in solution,10 existing literature on TPPS3 is agnostic on this point. There is reason to believe that TPPS3 should be electrochemically active. Rong and Mallouk11 report that FeTPPS3 can be arranged in a molecular bilayer film which acts as a “pHsensitive gate” wherein the protonation of the metalloporphyrin modulates the flow of electric current across the film.
10.1021/jp0765695 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/23/2008
Photoconductivity of TPPS3 Nanotapes
J. Phys. Chem. C, Vol. 112, No. 6, 2008 2171
Experimental Methods Interdigitated AuPd electrodes were fabricated by electron beam lithography as previously described.3 Immediately prior to use, substrates were cleaned for 30 min in a UV/O3 cleaner (Bioforce Nanosciences Tipcleaner) and then baked in air at 300 °C for 1 h. Deionized water was obtained from a Millipore Simplicity 185 ultrapure water source. A stock solution of TPPS3 was made by dissolving several micrograms of the monomer (in sodium salt form, Frontier Scientific) in 1 mL of 6 mM NaOH. The porphyrin concentration of this solution was determined by characterization of the 413 nm absorbance peak of a 1:400 dilution of the stock solution in deionized water (Jasco V-570 UV-vis-NIR spectrophotometer), assuming an extinction coefficient5 of M ) 4.00 × 105 M-1 cm-1. From the original stock solution, a 10 µM porphyrin solution was made by dilution with deionized water. Equal volumes of this porphyrin solution and 0.6 M hydrochloric acid stock solution were added to a poly(methyl methacrylate) cuvette with a 1 cm path length, resulting in a 0.3 M hydrochloric acid and 5 µM TPPS3 working solution. Substrates were immersed in this working solution either immediately or after a 2 h aging time and then allowed to incubate at room temperature for 5 h. After removal from solution, the substrates were quickly transferred to a spin-coating platform and dried for 40 s at 4000 rpm, under relative humidity of less than 15% (as measured by a Lufft C200 humidity meter). Solutions used for spectroscopic time course studies were filtered using Pall Acrodisc filters with 0.2 µm pore size (PN 4454). Aggregate deposition, as well as electrode structure, was verified by atomic force microscopy (AFM) imaging using a Digital Instruments Bioscope and/or a Digital Instruments Multimode AFM in tapping mode, using BudgetSensor force modulation (75 kHz) probes. We modified some probes by attaching single-wall carbon nanotubes,12 so as to obtain higher resolution AFM images. Scanning electron microscopy (SEM) imaging (without additional metal coating) was performed on an FEI Strata DB235 at an accelerating voltage of 3 kV. Conductivity measurements were made using a Keithley 6517A electrometer, computer-controlled with LabVIEW. The samples were illuminated by a Spectra Physics 177-G02 Ar ion laser, passed through a monochromator to pick out the 488 nm wavelength. The laser beam was directed through various neutral-density filters before being focused into an ∼0.5 mm spot on the sample surface by an achromatic doublet lens (focal length ) 300 mm). To measure fluctuations in laser power in situ, a small fraction of the beam was reflected onto a calibrated, amplified Si photodiode (Thor Labs PDA50). The beam intensity was measured using a Thor Labs S20MM power meter with a 100 µm diameter aperture above the sensor. All conductivity measurements were performed under nitrogen gas, with an O2 level inside the measurement chamber of
