Article pubs.acs.org/JPCC
Synthesis and Characterization of Porphyrin-Based GUMBOS and NanoGUMBOS as Improved Photosensitizers Paulina E. Kolic, Noureen Siraj, Suzana Hamdan, Bishnu P. Regmi, and Isiah M. Warner* Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States ABSTRACT: Porphyrin-based GUMBOS and nanoGUMBOS were synthesized for potential application as improved photosensitizing materials. In this study, porphyrin dyes [meso-tetra(4-carboxyphenyl)porphine (TCPP) and zinc(II) meso-tetra(4-carboxyphenyl)porphine (Zn-TCPP)] were selected as anions, and trihexyltetradecylphosphonium (P66614) was employed as a cation. The resulting [P66614]4[TCPP] and [P66614]4[ZnTCPP] GUMBOS (group of uniform materials based on organic salts) provided high photostability and excellent thermal stability for these compounds. NanoGUMBOS, i.e., nanomaterials derived from GUMBOS, were synthesized using reprecipitation and ion association methods. The surface charges of these nanoparticles were tuned from positive to negative through use of an ion association synthetic method without the need for additives or stabilizers. When compared to the parent dyes, nanoGUMBOS exhibited excellent photodynamic properties for potential applications as photosensitizers. Evaluation of the electrochemical properties of these GUMBOS suggest that these compounds can be applied as photosensitizers in optoelectronic devices such as dye-sensitized solar cells.
■
INTRODUCTION Over the last several decades, porphyrin dyes have attracted tremendous interest as photosensitizers in the fields of photodynamic therapy (PDT) and optoelectronics such as organic light emitting diodes (OLEDs) and dye-sensitized solar cells (DSSCs).1−5 Porphyrin dyes are known for their beneficial characteristics such as high molar absorptivity as well as the natural role that these compounds play in light harvesting.6 As molecular dyes, porphyrins can be synthetically modified into novel molecular structures via introduction of beta and meso substituents, to produce dyes with characteristics that are suitable for a specific application. For example, Mathew et al. have synthesized a zinc-porphyrin dye for application as a photosensitizer (photosensitizing dye) in DSSCs. Through precise tailoring of the molecular structure, they have acquired the highest solar efficiency to date in DSSCs technology.2 Nanotechnology has also influenced the development of photosensitizers. For example, porphyrin-based photosensitizers in the form of nanomaterials have been found to be beneficial in applications such as PDT.7,8 The unique properties of nanomaterials include ease of tunability of size and spectral properties, stability, and high surface area.9−11 Thus, improvements to photosensitizers can be made for specific applications through optimization of synthesis and photodynamic properties of nanomaterials. In particular, several mechanisms have been reported for synthesis of porphyrin-based nanoparticles.7,8 However, many of these syntheses consist of multiple steps and/or require the use of additives, stabilizers, or other matrices for nanoparticle formation.7,8 Our research group has recently introduced a new class of tunable materials known as a group of uniform materials based on organic salts (GUMBOS) and the nanomaterials derived © XXXX American Chemical Society
from GUMBOS (nanoGUMBOS). In this regard, we have synthesized potential photosensitizers that can be applied in PDT, DSSCs, and OLEDs.11−14 GUMBOS are solid phase organic salts akin to ionic liquids, but with melting points ranging from 25 to 250 °C.13 As with ionic liquids, the properties of GUMBOS can be tuned simply by changing the cation or anion used to form these materials. Thus, new materials can be easily produced with suitable properties for applications as photosensitizers. Moreover, another benefit of GUMBOS chemistry is the production of photosensitizers with high product yields. Use of a facile ion exchange reaction for synthesis of GUMBOS avoids complex, multistep organic syntheses where product yields are often diminished.13 The hydrophobic properties of GUMBOS allow for facile formation of nanoGUMBOS in aqueous media in the absence of any additives or stabilizers.9,11,15 Furthermore, nanoGUMBOS exploit the beneficial properties of organic nanomaterials including ease of synthesis, utilization of nontoxic organic compounds, and reduction in cost. The study reported herein focuses on design of improved photosensitizer materials. The GUMBOS described here are composed of meso-tetra(4-carboxyphenyl)porphine (TCPP) and the zinc-substituted porphyrin (Zn-TCPP) as anions and a bulky, hydrophobic phosphonium cation, trihexyltetradecylphosphonium (P66614), to provide hydrophobicity. Characteristics of these photosensitizers were evaluated using various techniques, including thermogravimetric analysis, ultraviolet− visible absorption and fluorescence spectroscopies, and cyclic Received: December 8, 2015 Revised: February 17, 2016
A
DOI: 10.1021/acs.jpcc.5b12013 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Formation of GUMBOS via an ion exchange reaction. Trihexyltetradecyl phosphonium [P66614] was used as a cation, and (meso-tetra(4carboxyphenyl)porphine [TCPP] and Zn(II) meso-tetra(4-carboxyphenyl)porphine [Zn-TCPP] were employed as anions.
Thermogravimetric analysis (TGA) measurements were collected using a TA Instruments 2950 TGA HR V6.1A (New Castle, DE). Upon increasing temperature, the change in masses of our GUMBOS compounds were monitored while the sample was heated from room temperature to 600 °C, using a rate of 10 °C per minute. Electrochemical properties of GUMBOS were evaluated using an Autolab PGSTAT 302 Potentiostat from Eco. Chemie (Metrohm, Schiedam, Netherlands). Measurements were conducted using a Ag/Ag + reference electrode, a platinum working electrode, and a platinum wire counter electrode. All compounds were dissolved in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte. Ferrocene was used as an internal reference (0.63 V vs NHE). Synthesis and Characterization of NanoGUMBOS. NanoGUMBOS were prepared using two different methods. In the first method, nanoGUMBOS were formed by use of a reprecipitation method, similar to methods that have already been reported.10,14,15,17 Briefly, a small aliquot of concentrated stock solution (1 mM, 50 μL) of GUMBOS, dissolved in ethanol, was slowly added into water (5 mL) while under sonication.14 The final concentration of nanoGUMBOS suspension was 10 μM. In the second method, nanoGUMBOS were formed by use of ion association that has been previously reported.18 However, nanoGUMBOS were prepared without the addition of any additives or stabilizers.10 In this procedure, varying amounts of cation (P66614 in ethanol) were mixed with a fixed solution of anion (TCPP or Zn-TCPP) in water under sonication to obtain a final concentration of 10, 20, 30, or 40 μM of cation and 10 μM of anion. In both methods, sonication of the solution was continued for 30 min, and nanoGUMBOS were allowed to age for 15 min. A Branson 3510RDTH bath ultrasonicator (40 kHz) was utilized for sonication in order to synthesize nanoGUMBOS. Characterization of nanoGUMBOS was performed using dynamic light scattering for measurement of the hydrodynamic diameter of nanoGUMBOS prepared by reprecipitation and ion association synthetic methods. The zeta potentials of these nanoGUMBOS were investigated for further understanding the stability of nanoGUMBOS using a Zetasizer Nano ZS (Malvern
voltammetry. Furthermore, nanoGUMBOS derived from TCPP and Zn-TCPP were also studied as potential photosensitizers. These nanomaterials were prepared by reprecipitation and ion association methods in an effort to control surface charges and spectral properties for production of more efficient photosensitizers.
■
EXPERIMENTAL METHODS Materials. Meso-tetra(4-carboxyphenyl)porphine and zinc(II) meso-tetra(4-carboxyphenyl)porphine were purchased from Frontier Scientific, Inc. (Logan, UT). Trihexyltetradecylphosphonium chloride, dichloromethane, and ethanol were obtained from Sigma-Aldrich (St. Louis, MO), Fisher Scientific (Waltham, MA), and EMD (Billerica, MA), respectively. Triply deionized ultrapure water (18.2 MΩ cm) was obtained using an Aries high-purity water system (West Berlin, NJ). Ultrathin carbon-coated copper grids were purchased from Ted Pella, Inc. (Redding, CA) for characterization of nanoparticles by transmission electron microscopy. Synthesis and Characterization of GUMBOS. GUMBOS derived from meso-tetra(4-carboxyphenyl)porphine (TCPP) and Zn(II) meso-tetra(4-carboxyphenyl)porphine (Zn-TCPP) were synthesized using a metathesis reaction, which has previously been reported.14,16 In this method, the porphyrin dye was dissolved in water with the aid of sodium hydroxide, and trihexyltetradecylphosphonium chloride (P66614) was dissolved in dichloromethane, separately. The aqueous and organic layers were combined using equimolar concentrations (1.1:4, v/v) and stirred, in the dark, for 48 h. Upon completion of the reaction, the organic layer containing [P66614]4[TCPP] or [P66614]4[Zn-TCPP] GUMBOS was washed five times with deionized water in order to remove sodium chloride byproduct. The organic layer was then evaporated under reduced pressure to remove dichloromethane and subsequently freeze-dried to remove residual water. The melting point of GUMBOS was determined using a Digimelt MPA 160 (Stanford Research Systems, Sunnyvale CA). An Agilent 6210 electrospray time-of-flight mass spectrometer (Agilent, Santa Clara, CA) was utilized for GUMBOS characterization. B
DOI: 10.1021/acs.jpcc.5b12013 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Instruments, Malvern, Worcestershire, U.K). Characteristics of the synthesized nanomaterials such as size and morphology were determined using a JEOL JEM-1011 transmission electron microscope (TEM) (JEOL USA, Inc., Peabody, MA). For TEM studies, a 3 μL aliquot of nanoGUMBOS, suspended in water, was drop-casted onto a carbon-coated copper TEM grid and allowed to air-dry overnight before analysis. Finally, spectral properties of the GUMBOS and nanoGUMBOS were evaluated using a Shimadzu UV-3101PC UV−vis scanning spectrometer (Shimadzu, Columbia, MD). Ultraviolet−visible (UV−vis) spectra were acquired using a quartz cuvette with 0.4 cm path length (Starna Cells, Atascadero, CA). An identical cell filled with solvent was used as a blank. Fluorescence emission and photostability were measured using a Fluorolog-3 spectrofluorimeter (model FL3−22TAU3; Horiba Scientific, Edison, NJ). Slit widths of 2 and 3 nm were utilized for TCPP and Zn-TCPP compounds, respectively. All spectroscopic measurements were acquired at room temperature (20 °C).
Figure 2. Thermal stability of GUMBOS was monitored from room temperature to 600 °C using thermogravimetric analysis.
(Figures 3A and 4A). Size information on nanoGUMBOS was obtained using TEM which suggests that the average size of nanoparticles was 55 ± 21 and 52 ± 15 nm for [P66614]4[TCPP] and [P66614]4[Zn-TCPP], respectively. Zeta potential, or the charge at the Stern layer of [P66614]4[TCPP] and [P66614]4[Zn-TCPP] nanoGUMBOS, was determined to be 42.6 ± 5.5 and 55.1 ± 9.3 mV, respectively. These high magnitudes of zeta potential demonstrate that these nanoparticle suspensions were very stable in aqueous media (Table 1).24 Furthermore, positive zeta potential values indicated that GUMBOS assembled into nanoGUMBOS, with cations at the surface of the nanoparticles, thus producing positive surface charge. An important criterion for utilization of nanoGUMBOS in DSSCs as photosensitizers is that photosensitizers should be negatively charged to allow favorable interactions with the titanium dioxide surface of the photoanode.19 To attain nanoGUMBOS with negatively charged surfaces, anionic moieties (e.g., carboxylic groups) should assemble on the surface. The presence of carboxylic acid groups contained in the anionic porphyrin dyes, at the nanoGUMBOS surface, will allow for efficient linking of the photosensitizer with the semiconductor (titanium dioxide). In this regard, the molar ratio of cation to anion was controlled in nanoGUMBOS synthesis using an ion association method. In this study, the molar ratio of cation to anion was 1:1, 2:1, 3:1, and 4:1, which was achieved by maintaining a constant amount of TCPP or Zn-TCPP (anion), at a concentration of 10 μM, while tailoring the amount of P66614 (cation) (10, 20, 30, and 40 μM). NanoGUMBOS were synthesized at various molar ratios using an ion association strategy, and zeta potential measurements were utilized to investigate the effect of cationic concentration on surface charge. The pH of the nanoGUMBOS solutions was between 7.0 and 7.4 for zeta potential measurements. Values of zeta potential, for both TCPP and Zn-TCPP-based nanoGUMBOS, changed from positive to negative as the cationic concentration was decreased (Table 1). At a 1:1 molar ratio of cation to anion for TCPP and Zn-TCPP nanoGUMBOS, the zeta potential was negative and the magnitude of zeta potential supports these nanoparticles being highly stable in aqueous media. This negative value of zeta potential at 1:1 molar ratio indicated that constituent ions assembled to provide excess negatively charged dye (anion) on the surface of the nanoGUMBOS. When nanoGUMBOS were prepared at 2:1 and 3:1 molar ratios, the value of zeta potential tended toward more positive values in comparison to 1:1 nanoGUMBOS. However, low magnitudes of zeta potential (
