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A Novel n-Type Organosilane – Metal Ion Hybrid of Rhodamine-B and Copper Cation for Low Temperature Thermoelectric Materials John R. Bertram, Aubrey Penn, Matthew Jason Nee, and Hemali Rathnayake ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15857 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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ACS Applied Materials & Interfaces

A Novel n-Type Organosilane – Metal Ion Hybrid of Rhodamine-B and Copper Cation for Low Temperature Thermoelectric Materials John R. Bertram,a Aubrey Penn, a Matthew J. Nee, a* and Hemali Rathnayake. a,b* a

Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101; bDepartment of Nanoscience, University of North Carolina at Greensboro, NC, 27401 ABSTRACT: A n-type organosilane-metal-ion hybrid of Rhodamine-B silane and copper cation (Cu-RBS) was investigated as a low-temperature thermoelectric material. Computational analysis revealed the most likely localized binding site of Cu2+ to the Rhodamine-B core and provided predictions of molecular orbitals and electrostatic potentials upon complexation. The concentrationdependent optical absorption and emission spectra confirmed the effective metal-ligand charge transfer from Cu2+ to the xanthene core of RBS, indicating the potential for improved electrical properties for the complex relative to RBS. The electrical conductivity and Seebeck thermoelectric (TE) behavior were evaluated and compared with its precursor complex of Rhodamine-B and copper cation. While a moderately high electrical conductivity of 4.38 S m-1 was obtained for Cu-RBS complex, the relatively low Seebeck coefficient of -26.2 µV/K resulted in a low TE power factor. However, compared to other organic doped materials, these results were promising towards developing n-type thermoelectric materials with no doping agents. Both phase segregation and thin film heterogeneity remain to be optimized, thus the balance between Cu2+ domains and RBS domain phases will likely yield higher Seebeck coefficients and improved power factors.

KEY WORDS: Rhodamine-B, Thermoelectric materials, Seebeck coefficient, electrical conductivity, organosilane-metal hybrid

INTRODUCTION Organic semiconducting building blocks and their derivatives of organic-inorganic hybrids are promising materials for designing functional devices.1-4 Owing to their flexibility, easy processing, and tunability of optical and electronic properties through simple chemical modifications, organic semiconductors and their composites can offer new opportunities for nextgeneration, flexible, energy-harvesting technologies. The use of organic materials and organic-inorganic hybrids to harvest electricity from waste heat via the Seebeck effect has been promising for improving the efficiency of thermoelectric (TE) devices.5-7 Great progress has been made developing both organic- and organic-inorganic-based thermoelectric materials to overcome limitations of conventional brittle and usually toxic, inorganic thermoelectric materials.8-14 An organic counterpart provides mechanical flexibility with low thermal conductivity, which gives hybrids a potentially significant advantage for use in wearable technologies compared to more brittle inorganicbased thermoelectric devices. An efficient TE material must posses a higher electrical conductivity and a low thermal conductivity. Unfortunately, in most materials, electrical and thermal conductivities are dependent upon each other and proportional to the charge carrier concentration. Therefore, it is necessary to develop new TE materials with high electrical conductivities with no increase in thermal conductivities. The energy conversion efficiency of a TE device is determined by the dimensionless thermoelectric Figure of Merit (ZT), defined as S2σ T/κ where S, σ, T, and κ represent Seebeck coefficient, electrical conductivity, absolute temperature, and

thermal conductivity, respectively and S2σ is the power factor (PF).15 All these key parameters, which must be balanced to gain higher ZT, strongly depend on the structural and morphological features of the material. Materials with a ZT of 1 or higher are expected to be competitive with other methods of electric-power generation. Inorganic semiconductors such as bismuth telluride alloys (Bi2Te3) are clearly efficient thermoelectric materials with higher ZT in the range of 0.8 to 1.1 at room temperature.16 However, fabricating high-performance flexible thermoelectric devices from inorganic alloys is a challenge due to severe degradation owing to the porous microstructure.17,18 Conducting polymers and their composites have emerged as alternatives.3 Among them, p-type polymers and composites, especially derivatives of poly(3,4ethylenedioxythiophene) (PEDOT), show a great promise for p-type TE properties with the highest ZT of 0.42.19 Hybrid composites of carbon nanotubes (CNT) and p-type semiconducting polymers also achieve a high power factor of 25 µW/mK2 as a result of their heterogeneous film morphology.2024 However, TE materials for n-type counterparts have not yet emerged due to the challenges in electron affinity of semiconducting polymers with n-type doping. Among the few examples of n-doped TE materials, metal-polymer coordination complexes containing linkage of 1,1,2,2-ethenetetrathiolate have been shown to have excellent TE properties with a ZT value of 0.2 at 440 K.25 Recent work introduced by Wan et.al. has reported a ZT of 0.28 at 373 K for a n-type hybrid superlattice of TiS2/[(hexylammonium)x⋅(H2O)y⋅(DMSO)z] as a potential TE material for wearable TE devices.26

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Despite these organometallic-polymer hybrid systems, there are ongoing efforts to fabricate flexible thermoelectric devices using small molecular organic semiconductors including some of the most promising fused-arene systems, such as pentacene,27-29 rubrene,30 and phthalocyanines.31 Owing to welldefined molecular structures and their controllable properties, these small molecules have been utilized for designing organometallic molecular systems, in which Zn, Fe, Co, and Ru centers were incorporated into molecular derivatives of porphyrines,32 bis(terpyridine),33 and bis(arylacetylide).34 This has shown that superior long-range electric transport can be realized by self-assembled monolayers of organometallic complexes.35 Here, we present a novel inorganic-organic hybrid material designed from an acceptor, a derivative of Rhodamine-B and a metal ion (Cu2+) as a promising n-type TE material for flexible TE devices. Owing to the materials abundance, the ability to act as a Schiff-base π-acceptor ligand, easy processing in the aqueous phase, and the molecular engineering of side chains, Rhodamine-B and its derivatives are potential candidates as flexible thermoelectric materials and have not yet explored its thermoelectric properties. In this study, we described both experimental and computational analysis to evaluate the thermoelectric properties of this novel organometallic complex. We investigate the effect of the metal-ion interaction with the core of a Rhodamine silane derivative on its electrical conductivity, Seebeck coefficient, and power factor. Computational analyses reveal the most probable binding site of Cu2+ to Rhodamine-B silane and provide insight to its optical behavior and enhanced electrical properties. EXPERIMENTAL SECTION Materials. HPLC grade Rhodamine-B, N,N’dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), chloroform, copper(II) nitrate, calcium nitrate, and dichloromethane (DCM) were purchased from Sigma-Aldrich. 3-aminopropyltriethoxysilane (3-APT) was purchased from Gelest, Inc. Indium tin oxide (ITO) coated glass substrates with resistance of 8-12 Ω/sq were purchased from SPI supplies, PA, USA. Unless otherwise specified, all chemicals were used as received. Characterization. Proton NMR spectra were recorded on a 500-MHz JEOL spectrometer using CDCl3 as a solvent. A Varian 500 Ion Trap LCMS was used with acetonitrile and water as the mobile phase for mass and purity analysis. Optimal LCMS parameters were found to be a needle voltage of 5000 V, RF loading of 72%, and capillary voltage of 100 V. Elemental analysis was performed from a LECO TruSpec CHN analyzer with EDTA as the standard. IR analysis was performed on an ATR Perkin Elmer Spectrum One FT-IR. The photophysical properties in solution were studied using a fluorescence spectrometer (Perkin Elmer LS 55) and UV-visible spectrometer (Perkin Elmer, Lambda 35). IV characterization was performed using a Keithley 2400 source meter controlled by a PC with LabVIEW software. Cyclic voltammetry measurements for Rhodamine-B silane and the complex were performed in solution. A glassy carbon electrode was taken as a working electrode while platinum wire and Ag/AgCl electrodes were used as counter and reference electrodes respectively. As a supporting electrolyte, PBS buffer (0.1M) prepared in de-ionized water was used after purging with dry argon gas for 1 h. To the buffer solution (10 mL), 100 µL of either RBS solution (0.1M) or Cu-RBS solu-

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tion (1:1 M of Cu+2: RBS) in de-ionized water was added. Cyclic voltammetry experiments were conducted using a VMP3 Biological Science Instrument – EC Electrochemical workstation with a scan rate of 100 mV/s. Ferrocene is used as a reference to calculate the energy of HOMO and LUMO levels, including the ferrocene value of -4.4 eV. The energy levels were calculated using the following empirical equations used by Bredas et al.36 E(HOMO) = -e [Eox + 4.4] E(LUMO) = -e [Ered + 4.4] Optical band gaps were also calculated and compared with the band gaps obtained from the electrochemical method. The onset wavelengths of UV thin film spectra were used to calculate the optical band gaps and band gap energy was calculated from the equation given below.

E = hc

λ

Where h is Planck’s constant, c is the speed of light, and λ is the onset wavelength. Synthesis of Rhodamine-B silane (RBS). To a twonecked round bottom flask, Rhodamine-B (500 mg, 1.04 mmol), DCC (258.4 mg, 1.25 mmol), DMAP (25.0 mg, 0.205 mmol), and 3-APT (0.2925 mL, 1.25 mmol) were added along with DCM (60 mL) and flushed with argon ~ 20 min. The pink, clear solution was stirred gently at room temperature for 16-20 h. The reaction mixture turned slightly cloudy as the reaction proceeded due to the formation of dicyclohexylurea. The reaction mixture was subjected to suction filtration to remove dicyclohexylurea and the filtrate was concentrated under vacuum at room temperature. The resulting thick, dark purple liquid was washed multiple times with hexanes and separated as a thick liquid. This liquid was dissolved in a minimum volume of dichloromethane and re-precipitated in a large volume of hexane (~50 mL). Repeating this procedure multiple times, excess Rhodamine-B was removed and solid was dried in a vacuum oven at room temperature to yield a flaky purple solid (0.380 g, 56% yield). 1H-NMR in CDCl3 {δ, ppm}: 1.14-1.18 (t, 21H), 1.66 (m, 2H), 1.89 (d, 2H), 3.36 (q, 4H), 3.79-3.83 (q, 6H), 6.36 (d, 1H), 6.61 (d, 3H), 7.03 (m, 1H), 7.57-7.62 (m, 4H), 8.02-8.03 (d, 1H). Molecular weight and purity were determined by LC-MS (positive mode): m/z [M+] = 649.2. Fabrication of Test Devices. A representative device schematic can be found in the Supporting Information (Figure S1). ITO-coated glass substrates were cleaned prior to device fabrication. First, substrates were washed with dichloromethane in an ultrasonic bath for ten minutes, followed by washing with soap solution. Substrates were then rinsed in deionized water and treated with a mixture of ammonium hydroxide and hydrogen peroxide for 15 minutes at 50-70 °C. After the above treatment, the plates were again washed with deionized water under ultrasonication for 15 minutes and blown dry with dry argon. The cleaned ITO substrates were subject to UV cleaning for 35 min prior to the deposition of the active layer. To a vial, Rhodamine-B silane (RBS, 15 µmol) and Cu(NO3)2⋅3H2O (15 µmol) and 2-propanol (2 mL) were added, and stirred steadily for an hour while heating at 70 °C. A portion of the ITO substrate was masked using tape to leave an average cell area of 14 cm2. Upon cooling the solution to room temperature, a layer of the sample mixture was deposited on the ITO substrate by spray-coating the solution (2 mL) using an airbrush (Paasche, H0513) with twenty swift passes maintaining the nozzle pressure at 270 kPa under argon atmos-


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phere. To keep the thickness of the layer at a constant value for all the test devices, the distance between the nozzle and the substrate was maintained at 20 cm. The test devices prepared in this manner were subjected to the deposition of the cathode (Cu electrode) on the active layer area under a vacumm of 7 µTorr to have ~100 nm thick cathode layer. Control devices of RBS only, Rhodamine-B only, and Rhodamine-B with Cu2+ (Cu-RB) were made in the same fashion to compare the performance of test devices made with Rhodmaine-B silane with Cu2+ (Cu-RBS) devices. Ca2+ was also used in place of Cu as a control to assess the ionic strength effect on electrical conductivity. Multiple devices composed of Cu-RBS, Cu-RB, RBS, and Rhodamine-B were fabricated to ensure the data reproducibility. Electrical Conductivity Measurements. Electrical conductivities of all tested devices were measured using ITO as a counter anode and copper as a counter cathode. For this purpose, active layer only devices were fabricated following the method describes for the fabrication of test devices. Copper electrode (~100 nm) was deposited on the sample thin film layer using a thermal evaporation unit. For this measurement, the channel length was kept constant at 1.25 cm while the active cell area maintained at 1.5 cm2. Conductance of samples was calculated from the slope of the ohmic region of I-V curves and conductivities were obtained from

Gl a Where ρ is conductivity, G is conductance, and l and a are channel length and area respectively. Each sample was tested five times to ensure reproducibility of the results. Measurements of Thermoelectric Properties. Thermopower measurements were conducted in a separate custom-built apparatus using a ceramic block heated in a vacuum oven to the desired hot-side temperature. After 15 min at this temperature, the block was placed on the underside of a device’s active layer directly over one of the probes leaving the other end exposed to room temperature (~25 °C). The voltage induced by the temperature gradient (∆T) at zero current density was measured directly from the test devices using the ITO anode and copper cathode directly connected to the source meter while maintaining the temperature of the cold side of the device at ~25 °C (room temperature). The Seebeck coefficient (S) is the slope of the thermovoltage (V) vs temperature gradient (∆T, in K) plots, at current density J = 0. Power factors (PF) were calculated based on the Seebeck coefficient and the conductivity, σ, using

ρ=

PF = σ S 2

Computational Analysis. All electronic structure calculations were performed using the Gaussian 09 package37 and their output files analyzed using GaussView05 software.38 First, the molecular geometry of Rhodamine-B was optimized using the B3LYP functional39 and the split-valence 6-31G basis set.40 Geometries were re-optimized using the same approach, but with a Cu2+ ion placed in ten different locations (specified in Figure S2) around the structure. The same method was used but this time with a mixed basis set, applying the LANL2DZ pseudo-potential41 to Cu2+ and 6-31G for all other atoms. Alternative calculations were employed including the use of 2propanol as an implicit solvent, but there was no change in the ordering or relative energy differences between positions nor

the optimized positions of Cu2+ ions and thus was not included in further calculations. A more diffuse basis set (6-31+G) was also applied to inspect the affect of Cu positions taking into account the outer reaches of the atomic radii, but again showed no change in the energy ordering of positions or the optimized locations of Cu and thus was also omitted from further calculations. The RBS molecule was constructed from the optimized structure of Rhodamine-B; its geometry was optimized using B3LYP/6-31G. From this optimized geometry, an electrostatic potential surface (ESP) was generated. Cu2+ ions were then placed in eight various locations (Figure S2) based on the electron distribution in the ESP, followed by geometry optimization using the same method and mixed basis set, applying LANL2DZ to Cu2+ ion and 6-31G for all others, to assess their relative conformation energies. An ESP was generated for the lowest energy structure for each compound. RESULTS AND DISCUSSION It is necessary to achieve a Figure of Merit (ZT) ≥1 for thermoelectric materials to be more economically viable. However, knowing the thermal conductivity (κ) of the substance is necessary to calculate the ZT. For a newly designed material, the thermoelectric performance is described by the power factor (PF) in terms of the Seebeck coefficient and electrical conductivity. This approach is used for the work described here. In the typical practical approach, Seebeck coefficient is inversely proportional to the electrical conductivity, so that PF can be used to estimate TE behavior. Increasing ZT requires an increase in both S and σ, while thermal conductivity for an organic material is expected to remain low as 0.1 W m-1K1 14,42 . It has been shown previously that the S and σ could simultaneously be increased. For example, theoretical analysis of the discrete packing pattern of tetrathiafulvenetetracyanoquinodimethane (TTF:TCNQ) radical cation and anion ion pair shows that TTF acts as a donor and TCNQ acts as an acceptor.43,44 Taking this general concept into account, we considered the possible thermoelectric properties of a metal ion-organic complex, Cu2+-Rhodamine B silane, in which Rhodamine B silane can act as a Schiff-base ligand for Cu2+ providing enhanced optical and electronic properties. The discrete, well-defined, conjugated, fused-arene moiety along with the silane linker connected through an amide bond in this system provides better binding sites for Cu2+ ion. This leads to simultaneous changes in both geometry and conjugation of the core of Rhodamine B. Because there is no prior evidence of increasing both Seebeck and conductivity using a Schiff base ligand-metal ion complex, we performed a thorough investigation to determine if this Schiff base ligand-metal ion complex accommodates enough conjugated proximity to induce thermoelectric behavior. Synthesis and Characterization of Rhodamine B Silane (RBS). Scheme 1 summarizes the synthetic methodology used to make Rhodamine B silane. The Steglich esterification45 of rhodamine B with 3-aminopropyltriethoxy silane (3APT) was performed to make RBS in considerably good yield. Upon washing with hexanes multiple times, excess 3APT was removed. The proton NMR spectrum of RBS and LC-MS analysis confirmed the assigned structure, composition, and its purity. The FT-IR spectrum (Figure S3) further supported the formation of an amide bond, based on carbonyl stretching at 1755 cm-1, and a secondary amine N-H stretching at 3218 cm-1. The



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presence of Si-O-Si stretching (1123–1040 cm-1) and Si-C stretching (1220–1335 cm-1) further confirmed the successful coupling of Rhodamine B with 3-aminopropyltriethoxy silane linker.

DCC/ CH2Cl2 R.T/Under Argon ~20 hrs

Scheme 1. Preparation of Rhodamine-B Silane (RBS) Photophysical Properties. The UV-visible spectra of the precursor Rhodamine B silane (RBS) and metal-ion-RhodamineB-silane complex (Cu-RBS), along with their controls of Rhodamine B (RB), Rhodamine B-Cu2+ complex (Cu-RB), and a solution of Cu2+, were recorded in solution phase (2-propanol) as well as in thin films. As depicted in Figure 1(a), the solution

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phase absorption spectra of RBS and Cu-RBS exhibit typical spectral characteristic features similar to Rhodamine B precursor and Cu-RB having a main absorption peak at ~536 nm and a shoulder peak at ~500 nm. A negligible shift in the absorption maximum is observed in the solution absorption spectra of RB and Cu-RB, but there is no evidence of detectable changes in either geometry or conjugation of the RhodamineB core upon binding to Cu2+ ions. This suggests that the Rhodamine-B core in either RB or RBS is intact and non-favorable for facilitating Cu2+ on the fused arene core in the solution phase. However, the absorption maximum in thin films depicted in Figure 1 (b) show a considerable red shift of 25-34 nm for all species except for RBS. This difference suggests that RBS molecules packed loosely in thin films whereas the other three species tend to pack closely. Both solution and solid phase spectra show a similar vibronic spectral pattern evidencing that there is no detectable electronic structural changes upon binding to Cu2+ in both solid-state and the solution phase.

Figure 1. UV-visible spectra of RB, Cu-RB, RBS, and Cu-RBS (a) in solution and (b) thin films; (c) UV-visible spectra of Cu-RBS and (d) Fluorescence emission spectra of RBS in 2-propanol upon addition of different concentrations of Cu2+ solution; Excitation wavelength = 538 nm.

In order to understand the effect of interactions between Cu2+ and the Rhodamine core on optical properties, the concentration dependence absorption and fluorescence emission spectral traces of Rhodamine-B, Cu-RB, RBS, and Cu-RBS were collected in solution. For this purpose, the photoluminescence behavior of Cu-RBS complex in solution with increasing Cu2+ concentration by mol% of 25, 50, 75, and 100 was performed. As shown in Figure 1(c) and Figure 1(d), the absorbance and fluorescence intensities gradually increased with Cu2+ concentration. The increase of absorbance and emission observed

here is mainly a result of strong interactions of the Rhodamine moiety with the Cu2+ ion, leading to energy transfer from Cu2+ to the rhodamine core via metal-to-ligand charge transfer (MLCT). This observation also provides evidence that RBS is acting as a Schiff-base ligand for Cu2+: the resulting increase in fluorescence emission is similar to reports in the literature, where Rhodamine derivatives were used as optical sensors for Cu2+ in various biological environments.46,47 The shift in UVvisible absorption maxima of Cu2+ from 804 nm to 796 nm in the Cu-RBS complex (Figure S4) compared to the absorption


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maximum of Cu2+ solution further supports the increase in fluorescence emission intensity upon addition of Cu2+ ions by lowering absorption energy transitions in Cu2+ ions. This demonstrates that upon formation of the Schiff-base complex RBS acts as a π-acceptor ligand, effectively increasing the magnitude of the Cu2+ ∆0 transition by lowering the energy of the t2g orbitals, thereby increasing the amount of energy required to initiate the absorption transition from the t2g orbitals to the eg orbitals.48 Computational Simulations. The computational chemistry results provide insight into the structure of the copperRhodamine complexes, and support the photophysical and electrical conductivity measurements. The optimized geometries for Cu-RB and Cu-RBS complexes are shown in Figure S2 of the Supplemental Information. Figure 2 depicts the ESP visualized in GaussView for Cu-RBS (Cu2+ indicated by orange arrow) in its lowest energy conformation of the eight various Cu locations optimized. Remarkably, this configuration of Cu-RBS was found to be at least 0.87 eV lower in energy than other chosen positions as summarized in Table S1. The optimized geometry suggests that the phenyl-amide nitrogen and the silane oxygen show great affinity for the Cu2+ ion, strongly suggesting that RBS may indeed act as a Schiff base ligand,14 binding to Cu2+ with its lone pair electrons. The calculated torsion angle of the phenyl amide group (or benzoic acid of Rhodamine-B) relative to the fused structure for Rhodamine-B, RBS, and Cu-RBS was examined to investigate strain that the silane tail of RBS may impose on its πconjugated structure, which could impact the likelihood of π stacking in thin films. The dihedral angles were found to be 93.01°, 79.44°, 96.83°, and 94.96° for Rhodamine-B, RBS, Cu-RBS, and Cu-RB respectively. The slight decrease in the dihedral angel of RBS indicates that the attachment of the silane tail to Rhodamine-B does in fact cause some torsion of the xanthene ring relative to the phenyl ring, and provides more freedom to arrange xanthene moieties with void spaces in the solid phase, thus, in turn resulted a blue shift in the thin film absorption maxima of RBS compared to the absorption maxima of Cu-RBS. The larger dihedral angle for Cu-RBS was observed upon complexation with Cu2+, indicating that the geometry of RBS at the phenyl amide group twists back to a relaxed conformation similar to Rhodamine-B thus, leading to a closely packed xanthene moieties in the solid phase. Overall, these theoretical findings support the UV-thin film spectral conclusions that there are no major electronic and geometrical changes to the Rhodamine-B core structure upon binding to Cu2+ while providing some insight into solid-state packing of the xanthene core.

The changes in electrical conductivity upon complexation with copper can be understood in terms of changes to the anisotropy of electron distribution in the compounds. Dipole moments of each species were calculated upon completion of geometry optimizations. It was found that the introduction of Cu2+ ion had some effect on the dipole moment of RB, but a more dramatic effect on the dipole moment of RBS, increasing from 11.248 D to 16.621 D for the optimized structure of Cu-RBS. A larger dipole moment may increase the likelihood of electron transfer between sites in the thin films. The increase in dipole moment can be seen visually in the electrostatic potential diagrams of Figure 2. To understand the substantial increase in fluorescence intensity seen in Cu-RBS compared to RB, RBS, and Cu-RB, we investigate the contours of the occupied and unoccupied molecular orbitals. From Figure 1a, both Cu-RB and Cu-RBS show an increase in visible light absorption compared to the same compounds without the Cu2+ complex. The partial molecular orbital diagram of the Cu-RBS species and some contours of the occupied and unoccupied molecular orbitals according to B3LYP/6-31G /LANL2DZ in the gas phase are shown in Figure 3. Due to the doublet spin-multiplicity, calculations involving the open-shell Cu-RBS species yield two sets of singly occupied molecular orbitals (SOMOs) for the alphaand beta-spin electrons (α-spin and β-spin). Despite spincontamination, the contours of both α- and β-spin orbitals provided by DFT exhibit MLCT for the lowest energy transition, displayed in Figure 3. The highest SOMOs possess dz2 character from Cu2+, while the lowest, singly occupied molecular orbitals are located on the xanthene structure of RBS. The additional electronic energy level made available by Cu2+ complexation results in an enhanced fluorescence observed experimentally (Figure 1(d)). For RB, RBS, and Cu-RB (Figure S5), the lowest energy transition is localized to the xanthene system of the rhodamine compound; thus, an enhance-

ment of fluorescence is not observed for these species (Figure S6). Figure 2. (a) ESP of RBS generated by B3LYP calculations used to investigate potential binding sites of the Cu2+ ion. (b) Generated ESP of the optimized geometry of RBS with single Cu2+ ion in its lowest of eight energy conformations.

Figure 3. The energy (eV) and molecular orbital contours of the occupied and unoccupied orbitals of the Cu-RBS complex. Positive values of the contours are shown in orange while negative values are shown in blue.



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Computational chemistry calculations predict a HOMOLUMO energy gap of 2.80 eV and 2.29 eV (by averaging the α- and β-spin SOMO gaps) for RBS and Cu-RBS respectively. The smaller band gap upon complexation observed theoretically agrees with the red-shifted UV thin film absorption maxima of Cu-RBS. To provide further evidence, the electrochemical behavior of RBS and Cu-RBS was investigated by cyclic voltammetry (CV) measurements to obtain the energy levels of the HOMO and LUMO and their band gaps. Optical band gaps were also calculated using the onset absorption wavelength as described in the experimental section. The oxidation and reduction onsets of cyclic voltammograms (Figure S6) were used to calculate the HOMO and LUMO energy levels and band gap energies of RBS and Cu-RBS following the method described in the experimental section. The cyclic voltammogram obtained for RBS shows both oxidation and reduction potentials with an oxidation potential onset (Eox) of 0.73 eV and a reduction potential onset (Ered) of -1.17 eV whereas Cu-RBS shows Eox of 0.78 eV with a poorly resolved redox potential onset of -1.10 eV. This suggests that the presence of Cu+2 slightly increases the oxidation and reduces the reduction potential. Table 1 summarizes the band gap energies obtained from all three methods. The energy levels and the band gap energies obtained from the electrochemical method are in good agreement with their respective energy levels and band gap energies obtained from the thin film optical spectra. The band gap energy for RBS calculated from the CV curve is 1.90 eV, which is 0.17 eV lower compared to the band gap (2.07 eV) estimated from the onset of optical absorption. The band gap energy for Cu-RBS is estimated to be 1.88 eV from the CV curve and 2.00 eV from its optical onset. Typically, the band gap obtained from the CV measurements should be more meaningful as it corresponds directly to an electron being excited from the HOMO level to the LUMO level. Regardless of the slight difference in band gap energies obtained from the electrochemical method and from the thin film absorption optical onset, there is a clear reduction in the band gap of CuRBS compared to the band gap of RBS. The low band gap of Cu-RBS implies the efficient charge transfer from Cu2+ to Rhodamine core by lowering energy gap between HOMO and LUMO of Rhodamine B. Table 1. Summary of redox potentials and band gap energies. Materials

Optical Eg (eV)

Voltammetry Eg (eV)

Theoretical Eg (eV)

RBS

2.07

1.90

2.80

Cu-RBS

2.00

1.88

2.29

Electrical Conductivity and Thermopower Measurements. Electrical conductivities of all four species were evaluated using ITO as an anode and copper as a cathode by maintaining the cell area and channel length at 1.5 cm2 and 1.25 cm, respectively. The detailed experimental set up is described in the experimental section. The current-voltage (IV) curves were collected and are depicted in Figure 4. The IV curves of thin films of Rhodamine-B, Cu-RB, RBS, and Cu-RBS show ohmic behavior with the ohmic conduction from -1 to +1 V, indicating the number of free carriers remains the same throughout the applied voltage. Addition of Cu2+ into Rhodamine-B solution showed only a minor improvement in resistance reduction. In contrast, the Cu-RBS device shows a

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significant increase in conductance over RBS devices, more than doubling its current under an applied voltage range of -1 to +1 V. The respective electrical conductivities at room temperature are summarized in Table 2. The reproducibility of conductivities was evaluated by fabricating multiple devices and measuring their resistances. The effect of Cu2+ addition on Rhodamine-B showed a slight improvement in the average conductivity, from 2.75 S m-1 to 2.87 S m-1, but standard deviations indicate that the differences in these results are not statistically significant. Thus, addition of Cu2+ ions had minimal effect on the conductivity of Rhodamine-B. This further supports our spectral and computational analyses: the lowest energy transition is localized on the xanthene ring rather than on the Cu2+ moiety in Cu-RBS. On the other hand, there is no effective charge transfer from Cu2+ to Rhodamine- B.

Figure 4. Representative IV curves of test devices of Rhodamine-B (black, dotted line), Cu-RB (green, dotted line), RBS (black, solid line), and Cu-RBS (green, solid line) at room temperature (25 °C). Similarly, RBS devices showed a comparable conductivity to that of Rhodamine-B. However, upon the addition of Cu2+ the conductivity of RBS more than doubled, increasing from 2.12 S m-1 to 4.38 S m-1. The increase in conductivity supports the changes in optical properties upon addition of Cu2+ to the RBS solution and analysis of HOMO and LUMO molecular orbital electron density distributions. The fluorescence emission measurements of RBS varying the amount of Cu2+ in solution (Figure 1(d)) exhibit an increase in fluorescent emission, which correlates to an effective charge transfer from the metal cation to the xanthene ring. Thus, the population of excitable electrons made available increases through complexation of RBS to Cu2+. To confirm that conductivity increase is due to the effective charge transfer and not due to the effect of ionic strength, conductivity of Rhodamine-B and RBS with the addition of a non-transition metal cation, Ca2+ was investigated. The conductivity data obtained for these two films showed only minor influences, exhibiting a slight decrease in the conductivity of Rhodamine-B and slightly increasing the conductivity of RBS. Therefore, these findings support that the higher conductivity of Cu-RBS complex is due to the effective charge transfer from Cu2+ to the xanthene ring with no geometrical perturbation to the π-π stacking of xanthene cores in thin films.


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Table 2. Comparison of the electrical conductivities of Rhodamine-B, Cu-RB, Rhodamine-B and RBS with Ca(NO3)2, as well as RBS and Cu-RBS devices at room temperature. Active Layer

σ (S m-1)

Rhodamine-B

2.75 ± 0.7

Cu-RB

2.87 ± 0.5

RBS

2.12 ± 0.2

Cu-RBS

4.38 ± 0.2 2+

Rhodamine-B with Ca

2.56 ± 0.4

RBS with Ca2+

3.04 ± 0.6

The temperature dependent conductivities were also calculated form the IV curves collected from the same set of devices for Cu-RBS; results are depicted in Figure 5(a). The respective electrical conductivities gradually decrease with respect to temperature gradient. The moderate conductivity of 3.68 S m-1 was maintained at 90 °C, above which conductivities dropped significantly. We speculate that the film defects and phase segregation are the main causes for the surprisingly low electrical conductivity at temperatures above 90 °C. While this will remain an active area of investigation for this project, we are highly encouraged by the effects of copper complexation, which suggest a promising new direction for low temperature thermoelectric devices.

Figure 5. (a) Measured electrical conductivity (σ) of Cu-RBS device at various elevated temperatures and (b) voltage difference measured (∆V) under a given temperature gradient (∆T) for Cu-RB and Cu-RBS at room temperature. Slopes indicated by the linear fits are used elucidate the reported Seebeck coefficients.

Table 3. Seebeck Coefficients and Power Factors for Cu-RB and Cu-RBS. Materials

PF (x 10-3 µW/K2 m)

S (µV/K)

RT (± 0.50)

At 90 °C (± 0.35)

Cu-RB

-5.3

0.08

-

Cu-RBS

-26.2

3.0

2.4

The Seebeck coefficients for Cu-RBS and Cu-RB obtained from the voltage measurements as a function of temperature (see Figure 5b) were found to be -26.2 µV/K and -5.3 µV/K respectively. Although the overall Seebeck coefficient of CuRBS is lower than our previously reported silsesquioxanebased organic-inorganic hybrid materials,7 we observed a considerably larger Seebeck value for Cu-RBS complex compared to Cu-RB, which suggests that other similar compounds may yield substantially better Seebeck coefficients. We have not performed experiments to evaluate temperature dependency of Seebeck coefficients because the electrical conductivity at higher temperatures was significantly lower than at room temperature. The power factor, which measures the thermoelectric performance of a material, was also calculated for Cu-RBS at room temperature and at 90°C. Generally, a good balance between conductivity and Seebeck coefficient is required to obtain a maximum power factor. The power factors obtained for Cu-RBS are summarized in Table 3, along with Seebeck coefficients of Cu-RBS and Cu-RB. The reported uncertainties are standard deviations propagated from electrical conductivity measurements. The power factors obtained for Cu-RBS are significantly lower compared to PFs of polycarbazole derivatives,47 organometallic coordination polymer thin films of copper and 7,7,8,8-tetracyano-p-quinodomethane (2.5 µW m-1 K-2 at 370 K),10 and polyaniline and Bi2Te3 nanoparticles (~85 µW m-1 K-2 at 370 K).11 We speculate that this performance gap for PF might be due to phase segregation and thin-film heterogeneity leading to poor intermolecular charge hopping among RBS-rich phases, suggesting more detailed studies and technique evaluations could result in improved TE parameters. CONCLUSION The conductivity and thermoelectric parameters for a novel ntype organosilane-metal hybrid (Cu-RBS) derived from Rhodamine-B and Cu2+ ion were studied. The complex exhibited moderate electrical conductivity, but due to the small Seebeck coefficient, power factors were too small to be immediately ready for practical applications even at lower temperature. However, the inclusion of metal ions to complex with hybrid organic-inorganic materials as n-type TE active layer components is, in itself, novel, and many experimental parameters including coordination with other potential transition metals will be explored over the coming years to bring this direction into competitive performance with other organic-inorganic hybrid TE materials. As this is the first investigation into the thermoelectric performance of rhodamine based organosilanemetal hybrid derivatives, these findings offer a new strategy for developing organic-inorganic hybrid thermoelectric materials from readily available and low-cost dye compounds like Rhodamine-B.

ASSOCIATED CONTENT Supporting Information



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Experimental procedures and other supplemental figures are available in supporting information section. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *a Matthew Nee, Department of Chemistry, Western Kentucky University, Bowling Green, KY, 42101, E mail: [email protected], Tel: 270-745-0114 *a,b Hemali Rathnayake, Current Address: Nanoscience Department, University of North Carolina at Greensboro, NC, 27410, [email protected], Tel. Phone: 336-285-2860

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources Kentucky Science & Engineering Foundation (KSEF-3030-RDE017), NSF-CHE-MRI under the Award ID of 1338072, NSF-MRI under the Award ID of 1429563, NASA-KSGC-UF-16-004, and Western Kentucky University Facultly-Undergraduate Student Engagement Grant under the Award ID of 16-SP240.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from Kentucky Science & Engineering Foundation (KSEF-3030-RDE017), NSF-CHE-MRI under the Award ID of 1338072, and NSFMRI under the Award ID of 1429563, NASA Kentucky Space Grant Consortium Undergraduate Fellowship. This work has been supported in part by the Western Kentucky University Office of Research and Office of the Provost through a FacultyUndergraduate Student Engagement (FUSE) grant.

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