A Systematic Approach toward Designing Functional Ionic Porphyrin

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A Systematic Approach toward Designing Functional Ionic Porphyrin Crystalline Materials Ursula Mazur* and K. W. Hipps

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Department of Chemistry and Materials Science & Engineering Program, Washington State University, Pullman, Washington 99163-4630, United States ABSTRACT: Synthetic ionic porphyrin self-assembled structures possess remarkable optoelectronic properties that are useful for energy and sensor related technological application, and there is a continued interest in tailoring their molecular structure and interactions in order to control their various functions. Central to these efforts is the fundamental understanding of the structure, the chemical and physical properties, and the relationship between the structural organization of the chromophores and their functions. In this Feature Article, we demonstrate the capability of a nucleation and growth algorithm to reproducibly synthesize nano- to millimeter size single crystals of several different ionic binary porphyrin (BP) systems under chemically controlled conditions. Single crystal X-ray diffraction analysis, morphology, sensing, and mechanical measurements are presented. UV−vis and X-ray photoelectron spectroscopy of these materials is also discussed. The crystalline porphyrin assemblies have varying degrees of dark conductivity (depending on their chemical and structural composition) but all appear to be sensitive to light; i.e., they absorb radiation and become conductive and/or persistently photoconductive (PPC). In addition, some of these crystalline porphyrins show persistent ion beam induced conductivity. This rich photo and ion response provides opportunities for the selective light control of these semiconductors. Exposure of the BP crystals to small gas molecules such as O2 strongly modifies their photocurrent in sensing experiments. The range of the Young’s modulus values extracted from nanoindentation experiments indicates that the BP crystals have a Young’s modulus intermediate between “soft” polymers and composite materials, making them excellent candidates for deformable optoelectronic devices. Both the experimental photoconductive and mechanical properties of the porphyrin self-assembled structures can be correlated with their molecular organization and morphology. In addition, quantum mechanical calculations provide the electronic band structure and the density of states and help explain experimental prompt photoconductivity measurements. Theory was also used to develop strategies for precise manipulation of the photoresponse of the BP crystals by selective structure modification. This work shows that combining the results from structural and theoretical studies and correlating them with electronic and optoelectronic properties can assist in engineering highly organized functional materials from organic π-conjugated molecules.

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INTRODUCTION Porphyrins are tetrapyrrolic pigments structurally and functionally related to naturally occurring chromophores essential for many biological activities including electron and energy transfer (photosynthesis),1−3 molecular recognition4 and enzymatic catalysis.5 These bioinspired molecules possess panchromatic behavior with large absorption coefficients (of the order of 104 M−1 cm−1)6 in the near-ultraviolet to nearinfrared region. Porphyrin molecules can be readily organized into self-assembled structures whose morphology as well as optical and electronic characteristics can be manipulated by simply changing the substituent groups on the macrocyclic ring and/or introducing different metals into the macrocycle cavity.7−10 There is an ongoing interest in porphyrin research, and it has amassed a great body of literature, including a dedicated journal,11 several volumes of “The Porphyrins”,12 and a “Handbook of Porphyrin Science” series introduced in 2010 which at present consist of 44 volumes.8 © XXXX American Chemical Society

Because of the above delineated properties, porphyrin based self-assembled materials are currently used and are being developed as active components in solar cells,13−16 photovoltaics,17−20 photocatalysts,7,21−23 field-effect devices,24,25 gas storage,26−28 sensors,29−36 and medical applications.37,38 Energy related applications of porphyrins require accurate knowledge and control of the charge carriers and their concentration in the self-assembled structures. A prerequisite for the precise manipulation of the charge and energy transport properties begins with a fundamental understanding of the chemical and physical properties of these structures. For this purpose, single crystal porphyrin structures are preferred because they possess long-range structural order and chemical purity and tend to be free of large structural defects (e.g., grain boundaries) that can limit charge transport mobilities.39 Ionic Received: April 1, 2018 Revised: May 28, 2018

A

DOI: 10.1021/acs.jpcc.8b03091 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C porphyrins are known to readily form novel robust crystalline assemblies in solution that have been shown to be potentially useful as active components of solar cells13,14,16,17 and photovoltaics.16,22,40−42 Typically, single crystals fabricated from ionic free-base porphyrins and their metal derivatives have dimensions of nano- and micrometers and are too small for single crystal measurements. Shelnutt and Medforth,7,9,10 Drain,16,37 Imahori,15,17 Paolesse29,31,32 and Mazur and Hipps,43−45 among others, have demonstrated that ionic porphyrin nanostructures can be made with exciting conductivity, photoconductivity, sensing, and catalytic properties. Nevertheless, progress has been slow in improving the observed properties because of a dearth of knowledge13 about the actual molecular arrangements in these nanocrystals. Recently, we have made a breakthrough in the synthesis of nano- to millimeter size ionic porphyrin crystals using a growth and nucleation model developed in our laboratory. With the aid of this model, one can now fabricate high quality porphyrin (and other molecular) crystals suitable for single crystal X-ray diffraction in a controlled and reproducible manner.46−49 In this work, we highlight an iterative process of tuning functional properties (optical, electronic, and mechanical) of a few different binary ionic porphyrin assemblies by manipulating their molecular organization, crystal structure, and morphology. This scheme has both experimental and theoretical components and provides structure−function correlations. First, we describe the synthetic methodology for producing single crystals of BPs with well-defined shapes and sizes. Then, we present the results of single crystal X-ray diffraction analysis along with microscopic imaging and spectroscopic data. Conductivity, photoresponse, sensing, and mechanical studies follow. Quantum mechanical calculations are used to explain the experimental photoconductivity measurements. The conduction process, sensing, and mechanical properties are related to the molecular structure and organization of ionic tectons within the BP crystalline assemblies. On the basis of the structure−function relationship data, density functional calculations provide guidelines for developing novel ionic porphyrins with optimal charge transport properties which are then implemented. The road map for the above-described investigative process is depicted in Figure 1.

Figure 1. Schematic of stages involved in developing and optimizing structural and optoelectronic properties of binary porphyrin assemblies.

states of the porphyrins in solution can be estimated from available pKa data. We have synthesized a series of stoichiometric crystalline BPs from free-base and metal (Cu2+ and Ni2+) substituted porphyrins with functionalities in the meso position, namely, tetra(4-sulfonatophenyl)porphyrin H4[H2TSPP] (TSPP), tetra(4-aminophenyl)porphyrin H2TAPP (TAPP), tetra(Nmethyl-4-pyridyl)porphyrin H2TMPyP (TMPyP), and tetra(4-pyridyl)porphyrin H2TPyP (TPyP).46−49,55,59 Figure 2 summarizes the protonation states of the porphyrin monomers in solution under different pH conditions. Elemental analysis was performed on (not presented here) all starting materials and the BP solids they produced. The stoichiometry of the BP as well as the protonation states of the ionic tectons were e xa m i n e d b y X - r ay p h o t o e l e c t r o n s p e c t r o s c o p y (XPS),43,45−49,55 and these results will be discussed later. Abbreviated names for the tectons and the BP are used, but their actual protonation state in the figures and the text will be identified, when needed. Controlled growth of the binary porphyrin crystals was achieved using a crystal size distribution prediction model.46 This model was developed for binary porphyrin synthesis, but it can be applied (with appropriate modifications) for controlling the growth of low solubility organic crystalline structures in general. The model assumes a homogeneous nucleation in the crystallization process of a binary porphyrin structure based on classic nucleation theory and incorporates Chiang and Donohue’s growth formula for multicomponent precipitation crystallization.60 The rate of nucleation, J (number of nuclei per unit time per unit volume), is expressed in the form of an Arrhenius equation

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SYNTHESIS AND CONTROLLED GROWTH Synthesis of binary porphyrin (BP) based crystalline nanostructures by ionic self-assembly (ISA) was first introduced by Shelnutt and co-workers over a decade ago.9 ISA is the coupling of structurally different or structurally similar (or identical) ionic building blocks (charged tectons) by electrostatic interactions.9,10,50 Since then, nano- to microsized crystalline assemblies fabricated from oppositely charged ionic free-base porphyrins and their metal derivatives have been reported.7,9,10,41,45,46,48,51−59 These take the shape of rods, tubes, nanofibers, sheaves, sheets, spheres, or clovers. Binary porphyrin crystals can be built from monomers with strong electron acceptors (−NO2, −COOH, −SO3H, −OH, −CN, etc.) and electron donors (−NR2, −OR, etc.) as substituents. By controlling the acidity/basicity of the reaction medium, one can manipulate the charges on the tectons in solution and thereby influence the stoichiometry and the morphology of the resulting ISA products. The protonation

3 3 2 ji −4β γCL Vm zyz zz J = A expjjjj j 27α 2k3T 3(ln S)2 zz k { and the growth rate, RG, is defined as

RG

(1 + C) − = 2k r kd

1+

(C +

)

kr C 2 kd eq

2k r kd 2

B

4k r kd

(1)

(2) DOI: 10.1021/acs.jpcc.8b03091 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. Protonation states of the TSPP, TMPyP, TPyP, and TAPP porphyrin tectons under different pH conditions. The red dashed line identifies a change in the pH scale. Adapted from a figure in ref 45. Copyright 2016 Handbook of Porphyrin Science.

where γCL is the interfacial tension between crystal and solution (N/m), k is the Boltzmann constant (J/K), T is the temperature in Kelvin, Vm is the volume of the growth unit (m3), α is the volume shape factor, β is the surface shape factor, S is the relative supersaturation, kd is the mass transfer coefficient, C is the concentration of the solute in the bulk of the solution, and kr is the integration rate constant. The above formulas predict that higher temperature and lower concentration as well as changes in solubility will dramatically affect the nucleation rate and, therefore, the final size of the crystals. Once formed, BP crystals are nearly insoluble in water. TMPyP:TSPP, for example, has solubilities of 3.1 × 10−9 mol/ L at 293 K and 6 × 10−8 mol/L at 343 K.46 For comparison, the solubility of a saturated aqueous solution of AgCl is about 1.3 × 10−5 mol/L. To increase the size of the crystals, we lowered both the nucleation and growth rate by increasing the solubility of the ISA solid. (Higher solubility of the product reduced the supersaturation, a factor that strongly influences the final size of the crystals.) Addition of 20% acetonitrile (by

volume) to the reaction mixture increased the solubility of the TMPyP:TSPP solid by a factor of 5, while the nucleation rate is reduced by a factor of 1010 compared to the case where water alone was used as the reaction medium.47 Using methanol reduced the nucleation rate by 30 times. A comparison of the size distribution of TMPyP:TSPP crystals produced in 20% acetonitrile at 60 °C with the predicted size range is illustrated in Figure 3. The simulated histogram represented by solid red bars parallels the experimental (hollow bars) data very well, validating the accuracy of the predictive model. The purple TMPyP:TSPP crystals can be easily observed under a polarizing optical microscope as long uniform rods (inset a in Figure 3) averaging about 1 mm in length. SEM micrographs suggest that the rods have pseudohexagonal shapes (inset b in Figure 3). It should be mentioned that TMPyP:TSPP crystals synthesized in three different solvent environments (pure water and coadded acetonitrile and methanol) all have identical compositions, tecton protonation states, and XRD patterns.47 C

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Figure 3. Comparison of experimental TMPyP:TSPP crystal size distribution (hollow bars) and simulated (red solid bars) size. The ISA product was prepared in acetonitrile and water at 313 K under neutral pH conditions (monomers are +4 and −4, respectively). The insets are micrographs of a large crystal obtained under a polarized light (a) and using SEM (b). Reproduced in part from ref 47. Copyright 2016 Royal Society of Chemistry.

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STRUCTURE, MORPHOLOGY, AND SPECTROSCOPY Structure and Morphology. High quality TMPyP:TSPP crystals were used for the crystallographic measurements. The space group crystal structure was determined to be monoclinic P21/c with cell constants a = 8.3049(11) Å, b = 16.413(2) Å, c = 29.185(3) Å, β = 92.477(9)°, and volume = 3974.4(8) Å3. The reliability factor, R, is 6%, indicating an excellent agreement between the crystallographic model and the experimental X-ray diffraction data.47 We note that this structure was determined independently by Scheldt and coworkers using very different growth methods than those normally employed for property measurements.61 The crystal structure of TMPyP:TSPP presented in Figure 4 is consistent with a 1:1 stoichiometry of the ionic tectons. The a axis is the long (primary growth) axis in the rods. Figure 4a depicts a projection down the crystallographic a axis in which the viewer sees molecules composed of half each of the [H2TMPyP]4+ and the [H2TSPP]4− ion because the molecular slab in that crystal direction is not flat. Five water molecules (per porphyrin dimer) are located in the channels between the molecular stacks.47 The free-base porphyrin macrocycles are planar and form coherent columns (Figure 4b) of alternating [H2TMPyP]4+ and [H2TSPP]4− ions with their centers slightly offset relative to each other with a separation of 3.8 Å. The centroid-to-centroid distance is 4.15 Å.47 In the TMPyP:TSPP crystal, strong ionic attraction between the tectons plays the dominant role in their organization. As seen from the calculated molecular electrostatic potentials (Figure 5), attraction between alternating electron-deficient TMPyP and electron-rich TSPP molecular ions results in what can be referred to as an “aromatic donor−acceptor interaction”.47 Although the π-orbital overlap between the ionic tectons is weak judging by the long interplanar distance of 3.8 Å (π−π bonding for self-assembled porphyrins ranges

Figure 4. Crystal structure of TMPyP:TSPP nanorods in two different orientations: (a) direction normal to the crystallographic a axis; (b) direction normal to the crystallographic b axis, showing the alternating cationic and anionic porphyrin tectons within the columns. Color codes: blue, N; gray, C; yellow, S; red, O. Hydrogens and waters are not shown. Calculated morphology (c) using attachment theory with views presented normal to the a crystallographic axis (left) and normal to the b axis (right). Reproduced in part from ref 47. Copyright 2016 Royal Society of Chemistry.

from 3.4 to 3.8 Å62,63), it contributes to the overall columnar structure and stability of the TMPyP:TSPP crystalline assembly. With the crystal structure parameters for TMPyP:TSPP crystals at hand, their morphology was calculated employing an attachment energy (AE)64−66 computational method. The AE model considers chemical bonding and is used for predicting morphologies of ionic organic molecules.67 The calculated topography of the TMPyP:TSPP system is depicted in Figure 4c. The most important morphological faces are the {002} and {011} step and kink faces, as they make up most of the surface area of the crystal. The unequal {002} to {011} interplanar aspect ratio computed by the model is due to the higher faceto-face attachment energy, leading to a faster crystal growth along the ⟨001⟩ direction.47 Also, as expected, the {011} step D

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bundles of intercrossing ribbons (μm long) with no clear definition of the growth origin at the core. An average single sheaf bundle consists of ribbons 50 nm tall and 180 nm wide. The size of the sheaves increases with elevated growth temperature. Powder X-ray diffraction patterns obtained from TAPP:TSPP sheaves55 and rods are very similar, indicating that both morphological types have the same structural geometry. The arms of the sheaves are homogeneously single-crystalline.55 TAPP:TSPP is a body centered crystal system that belongs to an orthorhombic Pnn2 space group with unit cell dimensions of a = 30.88 Å, b = 15.45 Å, and c = 7.98 Å; α = β = γ = 90°.55,70 The diffraction pattern from TPyP:TSPP single crystals has characteristics of an aperiodic or incommensurate crystal structure very similar to the quasi-orthorhombic Pmm2 system with lattice parameters a = 26.71 Å, b = 20.2 Å, and c = 8.61 and all angles equal to 90° as proposed earlier by Eskelen and co-workers on the basis of their powder pattern data analysis.55 These authors determined that the charge neutral TPyP:TSPP solid can result from the combination of one [H4TSPP]2− and one H2[H2TPyP]2+ ion or from two [H4TSPP]2− with one each of H4[H2TPyP]4+ and H2TPyP.55 While both types of tecton groupings preserve charge neutrality and stoichiometry in the solid state, the latter combination was favored by the authors. The diffraction pattern of the modulated TPyP:TSPP crystals consists of two types of reflections: a strong main reflection that corresponds to the basic structure and a weak satellite reflection that corresponds to the modulation wave.48 The unit cell contains two porphyrin dimers. The solution and structural refinement of the TPyP:TSPP crystal structure is still in progress and will be reported elsewhere. A common structural feature among the BP crystals we discussed earlier is the near face-to-face arrangement of alternating ionic tectons in highly coherent columns propagating along the primary growth direction of the crystals (Figure 4). Such ordered geometrical arrangement of porphyrin macrocycles was not observed in the few crystal structures of BPs prepared by others.13,71−73 Shelnutt and Medforth, for example, determined that the crystal structure of nanosheets composed of ZnTSPP and Sn(IV)-tetra(N-methyl4-pyridiniumyl) porphyrin exhibited severe slipped π−π stacking of the synthons in the crystal framework.13 This arrangement resulted in no significant electronic interactions between the porphyrin ionic dimers, and the material was found to not be photoconductive.13 The single-crystal X-ray structure of a 1:1 combination of meso-tetrakis(N-alkylpyridinium-4-yl)porphyrin and TSPP revealed that this triclinic crystal was composed of slipped-stack chains of tetracationic and tetraanionic porphyrins.71 The length of the alkyl substituents on the monomers significantly influenced the degree of misalignment of the ionic macrocycles. Slipped layers of porphyrin synthons were also observed in the triclinic crystals formed from meso-tetrakis(4-carboxyphenyl)porphyrin, TCPP4−, and meso-tetrakis[4-(pyridiniummethyl)phenyl]porphyrin, TPyPP4+.72 In the crystals composed of a 1:2 ratio of free-base and zinc(II) substituted porphyrin tetrathiafulvalenes, the porphyrin chromophores were offset and bridged by the tetrathiafulvalene moieties.73 Significant structural changes can potentially occur in the BP crystals when they are heated due to the loss of the water molecules which reside within the crystalline channels49 or by chemical decomposition. For example, the TGA profile of H2TMPyP:H2TSPP in Figure 7 shows three discrete temper-

Figure 5. Molecular electrostatic potential (MEP) surfaces of neutrally charged TMPyP:TSPP dimer. The electrostatic potential surface energies range from −67 (red) to +67 (blue) kcal mol−1. The isodensity value for the MEP mapping is 0.03 au.

and kink faces develop more rapidly (larger attachment energy) and have a smaller morphological importance compared to the flat {002} face. This leads to the elongated hexagonal cross section which is consistent with AFM images.47 Using our growth and nucleation model and careful pH control, we can prepare BP crystals with different morphology but with similar molecular organization. TAPP and TSPP monomers were reacted under two different pH conditions: 2.9 and 3.6. Within this hydrogen ion concentration range, the expected charges in solution on the TAPP and TSPP ions are +2 and −2, respectively.68,69 Interestingly, the TAPP:TSPP crystals synthesized at the lower pH (2.9) have a sheaf-like or hyperbranched structure (Figure 6a), while those prepared at pH 3.6 form rods with a rectangular cross-sectional geometry (Figure 6b). The TAPP:TSPP sheaves are composed of

Figure 6. SEM micrographs of two different morphologies of H2TAPP:H4TSPP crystals fabricated under controlled growth conditions at pH 3.6 (a) and pH 2.9 (b). Structures of the ionic tectons are also shown. E

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of H2TMPyP:H2TSPP crystals (and the metalated versions) as gas sensors. XPS Spectra. Charges on the tectons in solution influence the ISA product stoichiometry. Protonation states of the porphyrin ions existing in solution (Figure 2) need not be the same in the solid state. The packing and interactions that occur during the crystallization process can change the protonation state of one or more porphyrin tectons.56 X-ray photoelectron spectroscopy is particularly useful for identifying the coordination environment (including protonation) of nitrogen in porphyrin tectons. The free-base porphyrins exhibit two distinct N 1s signals: the imine (CN) nitrogen with a binding energy near 398 eV and the pyrrole (NH) nitrogen at 400 eV.74,75 Note that in the metal substituted porphyrins the four nitrogen atoms are equivalent with N 1s binding energy at 399 eV. XPS is also an excellent technique for determining the stoichiometry and the elemental composition of porphyrin nanostructures. Friesen et al. were the first to investigate the protonation state of a porphyrin in a nanostructure by XPS.56,76 They studied the nanotubes resulting from TSPP in acidic solution and observed a single N 1s peak at 400 eV43−45 which they attributed to a fully protonated porphyrin core having a +2 center. They saw no signal from chlorine or another common counterion. The presence of only one type of nitrogen, in conjunction with the absence of any counterions, reinforced the idea that the TSPP nanotubes are zwitterionic and form entirely from H2[H4TSPP] molecules while maintaining charge neutrality. Figure 9 compares the N 1s region of three metal-free TSPP based BP crystalline solids. The binding energies of the different types of unprotonated and protonated nitrogens in the tectons are identified in the spectra and in molecular models above. The TMPyP:TSPP XPS spectrum presents three types of nitrogens in equal ratios: imine nitrogen with a binding energy near 398 eV, pyrrole (−NH) nitrogen at 400 eV, and the N-methyl-pyridinium N 1s peak near 402 eV.46 At pH 7, both TMPyP and TSPP are expected to exist in the freebase formsthis is confirmed by the ratio of N 1s peak intensities. On the basis of the XPS results and supported by the elemental analysis (not shown here), a 1:1 composition of TMPyP:TSPP describes these structures.46 In the XPS spectrum of TPyP:TSPP nanorods (Figure 9), the N 1s signal at 401.7 is typical for a pyridinium cation nitrogen. The strong band near 400 eV is assigned to the four protonated pyrrole nitrogens of the TSPP diacid and the two protonated pyrrole nitrogens of the TPyP free base. Also contributing to that signal is the N 1s binding energy of the unprotonated pyridyl nitrogens of the TPyP. The 397.2 eV peak is assigned to the unprotonated imine nitrogens in TPyP. The spectrum was fitted to four bands associated with four types of nitrogens. These bands are linked with the signals from six protonated and two unprotonated pyrrole nitrogens and two protonated and two unprotonated pyridyl nitrogens, with relative areas of 6:2:2:2. At pH 2, TSPP is expected to be in the diacid [H4TSPP]2− form, while TPyP is expected to be a free base with all four of the outer pyridyl nitrogens protonated. Analysis of the N 1s region resulted in the assignment of six protonated pyrrole nitrogens (four from TSPP and two from TPyP), two unprotonated pyrrole nitrogens from TPyP, two protonated pyridyl nitrogens on the TPyP, and two unprotonated pyridyl nitrogens on the TPyP, resulting in either an ionic ratio of H2[H2TPyP]2+: [H4TSPP]2− or a ratio of H4[H2TPyP]4+:[H2TPyP]:2-

Figure 7. TGA curves obtained for H2TMPyP:H2TSPP crystals. The broken black, red, and blue lines specify, respectively, temperature regions in which the surface water molecules residing in the crystal channels are removed. Reproduced in part from ref 49. Copyright 2018 Royal Society of Chemistry.

ature regions: 295−320 K, 320−380 K, and above 380 K. The resulting total mass loss corresponds to approximately 7.5 water molecules per porphyrin dimer. Since the diffraction data refinement for H2TMPyP:H2TSPP crystals specifies the presence of five water molecules per ion pair, it is likely that the sample weight loss recorded (295−380 K) corresponds to water desorption from the surface of the rods (2.5 per ion pair) and the internal channels (5 per ion pair). Interestingly, when the H2TMPyP:H2TSPP samples were annealed to 100 °C or less, they could be rehydrated and exhibit the same XRD patterns as the diffraction data acquired from original unheated samples (Figure 8).49 Similar results were obtained for other BP crystals studied.47−49 Strong electrostatic interactions preserve the integrity of the crystal structures even when annealed and minimize the potential impairment of their conductive and photoconductive properties (as demonstrated below). The dehydration/rehydration process is completely reversible, suggesting the potential usage

Figure 8. Comparison of the powder XRD patterns obtained from a H2TMPyP:H2TSPP sample before heating (black), after heating to 100 °C (red), and after rehydration of the crystals (blue). Reproduced from ref 49. Copyright 2018 Royal Society of Chemistry. F

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Figure 9. N 1s region XPS for the different porphyrin nanostructures. The color line fits are related with the binding energies arising from different types of unprotonated and protonated nitrogens, as shown in the associated molecular structures. The relative areas under each curve in the XPS spectra help determine the stoichiometry of the porphyrins in the different ISA solids.

[H4TSPP]2−. Both cases are possible on the basis of the available XPS data.59 Because this tecton combination forms an incommensurate crystal structure, the latter stoichiometry is favored.48 XPS measurements of TAPP:TSPP crystals showed three different N 1s signals with binding energies at 402, 400, and 398 eV in a 1:4:1 ratio. Since the synthesis of TAPP:TSPP proceeds near pH 3, the TSPP core is expected to be fully protonated. To be consistent with the 1:4:1 nitrogen signal ratio in the XPS spectrum, the 398 eV signal is assigned to the unprotonated imine nitrogen N 1s and the pyrrole is assigned as the N 1s peak at 400 eV. The N 1s signal arising from the −NH2 group on the phenyl substituents is also located at 400 eV, while the peak at 402 eV is likely due to the protonated aminophenyl group (−NH3+). Thus, for a stoichiometric TAPP:TSPP system with a total of 12 nitrogens, this assignment must result in a 2:8:2 relationship consistent with the 1:4:1 nitrogen ratio obtained from XPS data. Interestingly, the TAPP parent molecule in a pH 3 solution exists as a diacid, while in the solid state it is in a free-base form. This difference may be attributed to the hyperporphyrin nature of TAPP in solution.68,77 Absorption and Luminescence Spectra. Optical spectra were all acquired from solid-state samples. In Figure 10a, the simulated spectrum of the TMPyP:TSPP crystals (obtained by addition of the [H2TMPyP]Cl4 and Na4[H2TSPP] free-base spectra) is compared to the actual UV−visible diffuse reflectance (DRS) spectrum of the TMPyP:TSPP crystals (normalized intensities). The prominent Soret band present in each spectrum has a maximum absorbance near 400 nm. Similarly, the Q bands of BP crystal and the monomers’ sum spectrum have the same band profile and appear in almost the same region although with slightly different intensities and small shifts in the lowest energy components of the Q band. The absence of new absorption bands unique to the ISA species suggests a weak electronic interaction between the ionic tectons and, therefore, an absence of delocalized states

that may be accessible by photoexcitation. Thus, both porphyrins can be excited nearly independently of the other. However, energy or charge transfer between molecules may still occur in states that are only weakly photon accessible from the ground state; this concept is supported by the luminescence spectrum. It is low energy states, like those from which the luminescence occurs, that participate in the conduction process. The luminescence spectrum of the TMPyP:TSPP rods is depicted in Figure 10b along with the emission arising from an equimolar blend of the solid TMPyP and TSPP monomers (both in free-base forms) for comparison. The intensity from the photoluminescence of the mixed porphyrins at 734 nm is dominated by the Q band emission from TMPyP. The solid state luminesce of TMPyP is similar to its emission spectrum in solution under neutral pH conditions.78 The TSPP free-base emission band from a powder sample is extremely weak, unlike the fluorescence from the TSPP diacid in solution which is much more intense.79 The 807 nm emission band from the TMPyP:TSPP rods is very broad and red-shifted relative to the luminescence originating from its monomers. This suggests that there are lower energy excited states that are unique to the ISA porphyrin system. The fact that the intensity of the luminescence does not increase with time eliminates the possibility that it originates from a photoinduced defect. Like the absorbances of TMPyP:TSPP, the electronic spectra of TPyP:TSPP and TAPP:TSPP crystals resemble the sum spectra of their respective monomers with no unique bands due to the ISA species themselves. The position of the Soret band remains essentially at the same frequency for all three BP crystalline systems. The Q bands, however, gain intensity and shift farther to the near-infrared for the TMPyP:TSPP, TPyP:TSPP, and TAPP:TSPP series (Figure 10). The near IR absorption band in the TAPP:TSPP crystals may be a result of the hyperporphyrin characteristics (charge delocalization onto the peripheral aminophenyl substituents) of the TAPP molecule. The TPyP:TSPP crystals do not G

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luminesce, and the emission from the TAPP:TSPP solid could not be measured because of the limited range of our detector (≤900 nm).

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PHOTOSEPONSE, SENSING, AND MECHANICAL PROPERTIES Photoresponse. Conduction and photoconduction measurements on BP crystals were performed under vacuum conditions.47−49 Porphyrins were deposited onto interdigitated gold electrodes, and in that sample configuration, we measured parallel conductivity (σ∥) where charge flow proceeded along the columns of stacked porphyrins. Some of the porphyrin materials we prepared thus far are nonconducting (TMPyP:TSPP)47,49 in the dark, while others are weakly conducting (TAPP:TSPP).70 However, every BP we have studied thus far is an n-type photoconductor.47−49 The action spectra for the free base BP crystals (Figure 10) mirror their absorption spectra, suggesting that the number of electrons contributing to the photoconductivity is proportional to the number of photons absorbed (Figure 11). A further implication is that the mechanism of the immediate photoconductivity is the same for excitation into the Soret or Q bands. With illumination (typically into the Soret or Q bands of the BP), the photocurrent shows a sharp increase followed by an increase (sometimes slow) over time until a saturation photocurrent is obtained (Figure 12). Once the light source is turned off, the current drops, but for some BPs, there is a slow decrease in conductivity over time in the darkthis is referred to as persistent photoconductivity (PPC) (Figure 11). PPC has been mainly observed in III−V or II−VI semiconductors and in a few organic crystalline semiconductors.80,81 The physical origin of PPC is still not universally understood and likely differs in different materials. PPC has been proposed to be useful in optoelectronic and memory devices.81−84 Both the onset of conductivity and PPC processes can be described by a stretched exponential85,86 ÄÅ É ÅÅi t y β ÑÑÑ Å j z Å I(t ) = IPPC expÅÅjj zz ÑÑÑÑ + INPCF(t ) ÅÅk τ { ÑÑ (3) ÅÇ ÑÖ

Figure 10. Overlaid UV−vis diffuse reflectance spectra obtained from the TMPyP:TSPP rods and the sum of the free-base spectra of the monomers (a). Luminescence spectra of the TMPyP:TSPP nanorods and of a stoichiometric mixture of the parent tecton solids (b). Reproduced in part from ref 47. Copyright 2016 Royal Society of Chemistry.

where IPPC is the initial current (nA) from persistent photoconductivity, τ is a weighted average lifetime (s), β is a stretching factor, INPC is the nonpersistent photocurrent, and F(t) is a function describing the time dependence associated

Figure 11. Comparison of the UV−vis diffuse reflectance spectra of the TMPyP:TSPP, TPyP:TSPP, and TAPP:TSPP rods (black trace) and their photoconductivity action spectra (red squares). H

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cause the energy levels of the HOMOs and LUMOs of those molecules to shift, producing states near the Fermi level of the Au electrodes. The formation of these states allows for a hopping conductivity mechanism. When the excitation source is shut off, these PID states persist for some time but eventually return to their initial equilibrium geometry. In the H2TMPyP:H2TSPP rods,47 persistence was only observed with excitations in the Soret band, suggesting that the EA for the formation of the PIDs was quite high. The activation energy for PID formation for the TPyP:TSPP system,48 on the other hand, is expected to be lower, as PPC occurred with illumination into both the Soret and Q bands. As the BP samples are irradiated, the PID concentration increases, resulting in an increase in mobility. When the light source is removed, the PID concentration slowly decreases, resulting in the related decrease in electron mobility over time. Eventually, enough of the defects have relaxed that hopping is no longer possible and the crystals are once again insulating. In this case, the EA for the PPC would be the activation energy associated with hopping between sites. The values for the room temperature dark conductivity of porphyrin crystals are very small, ranging between 10−8 and 10−13 Ω−1 m−1, while their photoinduced conductivities measure up to 5 orders of magnitude higher (see the last section of the article for more discussion). The conductivity and photoconductivity values for these ionically self-assembled (ISA) porphyrins are comparable to the amounts reported for other ionic and neutral porphyrin based solids (the actual structures for most of these materials have not been determined). Electrical conductivity of a tetragonal single crystal of 5,10,15,20-tetrakis(4-N-ethylpyridyl) porphyrin salt, [H2TEPyP4+][4I−], exhibited high anisotropy, in that the dark conductivity along the stacking column (σ∥) equaled 3.2 × 10−8 Ω−1 cm−1 and was 3 orders of magnitude larger than that perpendicular to the stacking column.93 The photoconductivity of Langmuir−Blodgett films composed from chlorophyll a investigated by Jones was also directional with σ⊥ = 1.0 × 10−10 Ω−1 m−1, while σ∥ ≤ 1.0 × 10−8 Ω−1 m−1. The dark conductivity for these films averaged 5 × 10−13 Ω−1 m−1.94 Nanorods of zwitterionic H2[H2TSPP] studied by Schwab42 and Riley36 while insulating in the dark displayed a much higher photoconductivity signal of 6.4 × 10−5 Ω−1 m−1 (on exposure to 488.0 nm laser) than our binary ionic porphyrin systems. Sensing. Detection and monitoring of gas concentrations is of tremendous industrial and environmental importance. There is a high demand for developing selective, sensitive, portable, and cost-effective gas sensors. Porphyrins are attractive chemical sensing materials because they detect a wide range of analytes by diverse sensing mechanisms.29−36 One of the detection mechanisms is a change in conductivity. For example, Riley and co-workers first demonstrated that the conductivity of zwitterionic TSPP nanorods decreases in the presence of O2.36 In what follows, we illustrate the utility of BP crystals as O2 detectors. In particular, we studied the time dependent photoresponse of H 2 TMPyP:H 2 TSPP, H2TPyP:H4TSPP, and H2TAPP:H4TSPP rods exposed to oxygen gas in a controlled environment. The rods were deposited onto interdigitated electrodes illuminated by 671 nm laser light under flow of 20% O2 in argon (PO2 = 400 mTorr) at different temperatures. The experimental setup is described elsewhere.47 Figures 13 and 14 exemplify the results of the

Figure 12. Time dependence of the photoconductivity at different BP crystalline solids acquired at room temperature with laser illumination into the Soret and Q bands.

with INPC, primarily arising from the time constant of the electronics used to measure the current. Two important fit parameters are τ (the effective weighted average lifetime) and β (the stretching factor, also known as the dispersion factor).85 In what follows, we only consider the dark process in our analysis. It is generally accepted that β is related to the dimensionality of the charge transport process. In a 1D processes, 0 < β < 0.5, and in 3D processes, 0.5 < β < 1.87,88 An average β value of 0.45 for our systems is consistent with 1D transport of electrons along the lengths of porphyrin cofacial assemblies via π−π interactions.89,90 β varies linearly with temperature, while ln(τ) varies linearly with 1/T. We interpret these T dependences as being due to activation barriers. Depending on the BP system and the excitation wavelengths, τ can vary from tens to thousands of seconds for room temperature measurements. Activation energies are ≤400 mV. Persistent photoconductivity is not observed in all porphyrin nanostructures. Interestingly, we recently observed persistent conduction (nA to μA) in TPyP:TSPP nanorods after exposure to an Ar+ ion beam (in UHV) with low ion energies.91 This behavior has not been shown for any known organic nanomaterial. It has, however, been recently demonstrated in ZnO nanowires and termed persistent ion beam induced conduction (PIC).92 The PIC and PPC of ZnO nanostructures (bombarded with noble gas ions) have similar excitation efficiencies, decay rates, and chemical sensitivities.92 PIC in TPyP:TSPP nanorods also exhibits the same characteristics as its PPC.48,91 Johannes and co-workers have predicted that materials which display PPC should also show PIC.92 There are a few likely explanations for the origin of the PPC. In our work on the H2TMPyP:H2TSPP rods,47 we proposed that the source of the PPC was the formation of photoinduced metastable defects (PIDs), which likely arise from changes in the equilibrium geometry of either individual porphyrins or individual dimers within the nanostructure. These changes I

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Figure 14. Mathematical fit to the time response of the photoconductivity of TMPyP:TSPP in Figure 1. Solid curves are data, and broken curves are the fit.

O2 is the result of a kinetically controlled process with a significant activation barrier. The decrease in current with time while the BPs are exposed to O2 is best described by a biexponential function of the form

ji −t zy ji −t zy I(t ) = Imin + A1 expjjj zzz + A 2 expjjj zzz jτ z jτ z (5) k 1{ k 2{ where I(t) is the current at time t, Imin is the asymptotic value of the current after O2 exposure, A1 and A2 are the magnitudes of the decrease in current as a result of the two O2 binding mechanisms, and τ1 and τ2 are the time constants associated with the two mechanisms of O2 binding. Figure 14 displays the relative goodness of the fitting function to the photoresponse decay of TMPyP:TSPP with O2 exposure at different sample temperatures. The values of A1, A2, τ1, and τ2 at 25 °C for all three BP systems studied are displayed in Table 1. The time

Table 1. Comparison of the Room Temperature Response, Lifetime, and Activation Energy of the O2 Binding Process for the H2TAPP:H4TSPP, H2TMPyP:H2TSPP, and H2TPyP:H4TSPP Crystals

Figure 13. Time response of the photoconductivity of TMPyP:TSPP, TPyP:TSPP, and TAPP:TSPP nanorods to exposure to 20% O2 gas (in argon) at different temperatures.

sensing experiments. Because the current measured is temperature dependent, the data presented is a relative photoconductive signal defined as I S = 100 × t I0 (4)

system

τ1 (s)

τ2 (s)

EA1 (meV)

EA2 (meV)

H2TAPP:H4TSPP H2TMPyP:H2TSPP H2TPyP:H4TSPP

63 194 569

2194 2544 2819

249 239 435

457 368 717

constants associated with the response to O2 decrease as temperature increases and follow the Arrhenius behavior

where S is the signal (%), It is the current at time t (pA), and I0 is the current at t = 0 s. The photoconductivity of all three BPs decreases significantly upon exposure to O2. At 25 °C, the sensitivity of the H2TPyP:H4TSPP rods is the lowest and the sensitivity of the H2TAPP:H4TSPP is the greatest. Additionally, the response to O2 exposure is fairly slow in all three systems. The sensitivity of all three systems increases with rising temperature, further indication that the response is due to more than simple physisorption of gas molecules to the surface. In fact, the increase in sensitivity with increasing temperature suggests that the conductivity change induced by

ln(τ ) =

EA + ln(τ0) kBT

(6)

where EA is the activation energy for the O2 binding process, kB is Boltzmann’s constant, and τ0 is the inverse of the attempt frequency of the O2 binding reaction. The general trend for τ and EA values, for the three BP systems studied (Table 1), indicates that the second process for O2 adsorption is slower and requires higher activation energies than process one. Because the decrease in current is slow relative to the rate of impingement of O2 on the surface of the crystals at the J

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corresponding indentation. Young’s modulus values were extracted from the slope of the unloading curve. Figure 15 shows a typical AFM force (nN) versus indent (nm) curve for a TMPyP:TSPP nanocrystal for the load

pressure employed, the rates measured are not the rates of O2 adsorbing to the surface but rather the rate of the O2 already adsorbed on the surface reacting with the crystal surface. The decrease in measured current is likely the result of the formation of O2− at the surface, a behavior reported for other n-type semiconductors upon exposure to oxidizing gases.95−97 When oxygen binds to the surface of the crystal, it extracts an electron from the molecules at the surface and forms O2. This causes the number of free electrons within the crystal to decrease, resulting in a decrease in conductivity. There are clearly two mechanisms associated with the formation of O2−, likely resulting from the presence of two different kinds of sites to which the O2 can bind to form O2−. It is well-known that the sensitivity of semiconductor gas sensors is highly dependent on the crystal faces to which the gases are binding.98−100 The morphology calculations performed on the H2TMPyP:H2TSPP crystals show that the surfaces of those crystals are primarily composed of {002} and {011} faces. Considering that different faces will have different surface potentials and can contain different types of defects, it is possible that the two rates result from the binding of O2 to the different crystal faces on the surface. An alternative mechanism is one in which one of the binding sites is located at the surface of the crystals while the second site is located within the bulk. The crystal structure for the H2TMPyP:H2TSPP and the proposed structure for the H2TPyP:H4TSPP both show channels between the columns of porphyrins that are filled with water molecules.45,46 TGA and temperature dependent powder XRD studies revealed that annealing the BP crystals causes the water within their channels to desorb.48,49 It is, therefore, possible that one of the mechanisms involves the diffusion of O2 within the channels of the crystals where it then adsorbs. A similar mechanism was used to describe the binding of NO gas to βNi(II) phthalocyanine.101 In this case, the faster rate was attributed to the adsorption of NO to the surface of the crystals, while the slower rate was associated with diffusion of the NO into the bulk of the film. Likewise, it makes sense that the faster rates associated with the decrease in current for the BPs correspond to the formation of O2− on the surfaces of the crystals, while the slower rates describe the formation of O2− after the O2 gas molecules have diffused into the crystalline channels. Elasticity. Practical utility of porphyrin crystals in optoelectronic devices requires that they possess high charge carrier mobility combined with flexibility to mechanical deformation. For high performance practical applications of porphyrin nanostructures (sensors, photovoltaics, and solar cells), fast and efficient carrier mobility needs to be coupled with low internal stress and superior tensile characteristics. Thus, accurate evaluation of the mechanical properties of porphyrin structures is of great practical value. Such measurements, however, are complicated by their small physical dimensionsthey are generally nanocrystals. The AFM indentation method is extremely useful for quantifying mechanical properties such as elasticity (or stiffness), hardness, adhesion, and viscosity of materials with nanometer scale spatial resolution.102 One of the more important of these properties is the elasticity (stiffness or Young’s modulus, E) which determines the material’s ability to deform reversibly under applied loading.103−105 Elasticity measurements were made by applying a force to the crystals while detecting the

Figure 15. Representative AFM force (nN) versus indent (nm) curve for TMPyP:TSPP crystals on HOPG. The red points identify the retract curve, and the blue, the approach. Zero separation is set at the point of maximum adhesion on the retract curve. The dashed black vertical lines identify the region of the indent curve, which is used in the DMT fitting program to extract the Young’s modulus value.

(approach) and unload (retract) curve.106 Stress was applied to the {002} face of the nanocrystal, i.e., perpendicular to the direction where ionic and π−π bonds dominate the packing (Figure 4c). Dashed vertical lines identify the regions of interest. Note that the load−unload (approach−retract) curves are completely reversible with no hysteresis at loads of 1000 nN or less. The average penetration depth at these applied forces is less than 4 nm. The BP crystal deforms elastically under these loads (when the applied load is removed, the material reverts to its original undeformed condition). The model used for fitting the elastic (retract curve) in the figure is the Derjaguin, Muller, Toporov (DMT) model.45,59 Young’s modulus values reported here (Table 2) were obtained from indentation experiments using BP nanocrystals 50 to 250 nm high and 100 nm to 1 μm wide. The underlying substrates HOPG (E = 19 GPa) or mica (E = 137 GPa) did not affect the measurements. Both the TMPy:TSPP and TPyP:TSPP nanorods have a higher elastic modulus than CuPc nanowires,110 almost certainly because, in addition to the Table 2. Comparison of Young’s Modulus Values for Different Molecular Materials

K

material

Young’s modulus (GPa)

polypyrrole films107 polyester108 polyethylene109 H2Pc (film)110 CuPc (film)110 TMPyP:TSPP (nanorod)106 TPyP:TSPP (nanorods)59

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Table 3. Comparison of Optical, Dark Conductivity, and Photoconductivity (NPC and PPC) Behavior for the Free-Base and Metalated Binary Porphyrinsa porphyrin system H2TMPyP:H2TSPP H2TMPyP:NiTSPP NiTMPyP:H2TSPP H2TMPyP:CuTSPP CuTMPyP:H2TSPP H2TPyP:H4TSPP H2TAPP:H4TSPP

luminescence π−πb band gap (eV) yes no no weak weak no nob

1.72 1.76 1.77 1.79 1.78 1.63 1.33

dark conductivity (Ω−1 m−1) no 7.7 5.9 6.7 3.5 no 1.0 −1

Included are room temperature photoconductance values, σ∥ (Ω infrared, and the detector limit is 900 nm. a

× × × ×

10−11 10−12 10−11 10−11

× 10−8

NPC

PPC

yes yes yes yes yes yes yes

Soret no no Q and Soret Soret (weak) Q and Soret no

photoconductivity, 445 nm laser (Ω−1 m−1) 4.0 7.7 6.3 7.7 1.1 9.3 1.6

× × × × × × ×

10−8 10−9 10−9 10−8 10−8 10−8 10−7

m−1). The laser intensity was 1.0 W cm−2. bThe Q band extends into the near-

Figure 16. Projected density of states and band structure of H2TMPyP:H2TSPP (left) and H2TMPyPCuTSPP (right) crystals computed from DFT. The Fermi level (Ef) is set at zero. The high symmetry points of the Brillouin zone are as follows, Γ = (0, 0, 0), Z = (0, 0, 0.5), Y = (0, 0.5, 0), X = (0.5, 0, 0), R = (0.5, 0.5, 0.5).

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THEORETICAL STUDIES AND STRATEGIES FOR PHOTORESPONSE MODIFICATION Quantum molecular calculations were performed in order to help explain the prompt photoconductive behavior of the BPs.47,49 Theory was also used to develop and apply strategies for improving photoresponse by selectively modifying the molecular structure of the ionic tectons. To this end, we performed periodic DFT and extended Hückel tight binding (EHTB) calculations on the crystal structures of the BPs.47,49 The calculated band structure and respective density of states (DOS) for the TMPyP:TSPP example system are presented in Figure 15. Here, the top of the valence band is populated by the TSPP orbitals, while the bottom of the conduction band is predominantly TMPyP states. There is weak mixing/hybridization of TSPP and TMPyP orbitals in the states above the band gap. The band gap is calculated by DFT to be 0.90 eV (DFT usually underestimates the band gap111,112). Extended Huckel theory, on the other hand, predicts band gaps of about 1.3 eV.47 The π−π* band gaps for the BPs obtained from their optical spectra (Table 3) are closer to 1.7 eV, which is a typical Eg value for porphyrins.113

aromatic interactions, they also exhibit electrostatic and hydrogen bonding interactions. Like CuPc nanowires, however, the TPyP:TSPP nanorod stiffness values are 1 order of magnitude smaller than those of inorganic nanowires, indicating that organic nanowire crystals are softer and hence may be more appropriate candidates for deformable optoelectronic devices. Elastic properties of other binary porphyrins discussed in this article are currently being studied. Data, that will be published elsewhere, indicates that the Young’s modulus values of larger mm size BP crystals are greater in magnitude than those for smaller nanosize rods. This is due most likely to fewer defects present in the larger crystals grown under more controlled conditions and slower nucleation rate. In addition, the elasticity of the BPs was found to be modulated by the nature of the substituents on the porphyrin tectons and their molecular arrangement in the crystalline solids. L

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carriers to a nickel d−d transition state causing the lifetime of the photocarriers to decrease, thereby decreasing the photocurrent.49,114,115 The dark and photoconductivities of all of the TMPyP/TSPP derived systems are both strongly temperature dependent.47 The observed direct photoresponse in the Ni2+ and Cu2+ substituted BPs can be attributed to conventional band conduction.49 Table 3 shows that the H2TAPP:H4TSPP crystals are conductive with and without exposure to light. The photoconductivity of the H2TAPP:H4TSPP crystals is much higher (order or two of magnitude) than the photoresponse of H2TMPyP:H2TSPP and its metalated modifications and the H2TPyP:H4TSPP as well.49 The H2TAPP:H4TSPP dark current is more than 3 orders of magnitude higher than that of the other BP crystals listed in the table. The higher conductivity and photoconductivity values of the H2TAPP:H4TSPP can be related to its low band gap (Table 3) and the reduced distance between the ionic tectons in the crystals (based on the crystallographic data discussed earlier) compared to the same parameters for the other BPs studied. The smaller size of the cationic ring substituents (−HN2) facilitates smaller internuclear separation, and the direct faceto-face π−π overlap of the chromophores increases charge transport. TAPP may have a greater electron affinity, which also lowers the band gap. The H2TPyP:H4TSPP crystalline solid has the second highest photocurrent of the BPs studied (Table 3), yet the chromophore separation distance in this incommensurate structure is the largest (see earlier discussion). One plausible explanation for this result may be the reduced local proximity between the [H2TPyP]4+ or [H2TPyP] tectons and the distorted shape of the [H4TSPP]2− diacid. In its saddled conformation, the [H4TSPP]2− can form strong hydrogen bonds and π−π interactions with the [H2TPyP]4+ and [H2TPyP] tectons. The low rotational barriers for charged groups on the porphyrin rings allow the sulfonate and the pyridyl substituents to easily adjust their orientation in the TSPP:TPyP to maximize the ionic and H-bonding interactions. A complete single crystal structure analysis for the H2TPyP:H4TSPP will help further advance the rationale for the charge transport mechanism in this system. Finally, we note that, while the calculated band structure provides insight into the electronic transitions responsible for the prompt photoconductivity, it does not provide an explanation for the observed PPC behavior of the BPs studied. The observed lifetime associated with the PPC is on the order of hundreds of seconds and is therefore too long to be attributed to simple electron−hole recombination.

The band structure for TMPyP:TSPP in Figure 16 shows little dispersion at the top of the valence band and weak dispersion in the conduction band. Together, the DOS and band structure support the experimental analysis from absorption spectra47,49 (vide supra) of TSPP, TMPyP, and TMPyP:TSPP that there is a minimal electronic interaction between the porphyrin species. The weak dispersion that was observed in both DFT47,49 and EHTB47,49 (not shown here) band calculations along the k-path in the Brillouin zone (Figure 16) corresponds to propagation along the long a axis of the TMPyP:TSPP crystal structure, which is also the π−π stacking axis of ionic tectons (Figure 4)the direction of charge transport. The primary path for electronic communication in these systems is through π−π interactions between adjacent porphyrins. One strategy that presents itself for improving the π−π communication responsible for photoconductivity in TMPyP:TSPP crystalline structures is to reduce the intermolecular distance between the ions.47 Calculated band structures (using the TMPy:TSPP crystal model) showed that decreasing the stacking distance between the porphyrin cores does not affect the band gap but does increase the band dispersion. The calculated effective masses (m*) obtained at the gamma point show that decreasing the π-stacking distance significantly decreases m*. Since the carrier mobility (μ) in a semiconductor is inversely proportional to the effective mass of the charge, lowering m* results in better mobility. Decreasing the size of cationic and anionic ring substituents should reduce the approach distance between the porphyrin synthons and increase their π−π interactions, thus increasing both conductivity and photoconductivity. One could also select a porphyrin with a greater electron affinity than TMPyP or a smaller ionization potential than TSPP to reduce the band gap. Another strategy for increasing charge transport in the binary porphyrin systems is to narrow the band gap and/or introduce midgap bands by adding metal ions into the cores of their monomers. The route of selectively metallating the ISA synthons proved to be advantageous in that it allowed the functional properties of the BP to be altered and tuned systematically without actually changing the spatial arrangement of their free-base monomers. We studied nickel(II) and copper(II) substituted H2TMPyP:H2TSPP single crystals where a metal ion was present in one or both of the synthons.49 Metal inclusion in one of the ionic porphyrin synthons resulted in the formation of crystals with nearly identical crystal structures to the free-base analogue H2TMPyP:H2TSPP monoclinic system. Dual metal core substitution significantly modified the crystal geometry.49 Consistent with the presence of metal bands in the π−π* band gap calculated for the metalated complexes, the dark conductivities of the nickel and copper substituted complexes are much greater than those for the parent free base. Both of the Cu2+ based BPs exhibit stronger direct photoconductivity than their Ni2+ counterparts and similar photoconductivity to the parent free base. The H2TMPyP:CuTSPP system yields the highest photoresponse of the metalated and parent H2TMPyP:H2TSPP series (Table 3). The metalated and free base nanocrystals all have computed effective masses of the order of 11me in the conduction band,49 which is consistent with the similar photoconductivity of copper and free base systems. We interpret the low photoconductivity of the Ni2+ based BPs as the result of energy transfer from photogenerated charge

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CONCLUSIONS AND OUTLOOK Porphyrins are excellent building blocks for crystalline solids with favorable light absorption and charge- and energytransport functions for energy related applications. Over the years, a number of researchers have reported on conductivity, photoconductivity, sensing, and catalytic properties.7,23,36,41,47−49 In addition, some of these BPs show persistent ion beam induced conductivity.91 Critical to the commercial application of these materials is precise control of their composition and structure through synthetic strategies and processing, since those factors determine the useful behavior of these materials. The work reported to date has had little success in producing commercially viable materials because the dearth of crystal structures of the active material M

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has made progress an Edisonian venture. What has eluded researchers until recently has been the ability to do true structure−function studies. We have demonstrated that determination of nucleation and growth kinetics of a particular BP can allow one to tailor growth conditions to produce crystals of a desired size and size distribution. This includes growing crystals of sufficient size for conventional X-ray diffraction studies. With a structure in hand, one can make significant progress in understanding and even predicting electronic and mechanical properties of these BPs. The crystal structure also allows one to perform computational studies to explore how modifications in the composition of the tectons might modify the electronic and mechanical properties in a desirable way. We have already shown that significant improvements in photoconductivity of the BP crystalline assemblies can be realized by manipulating the ring substituents in the meso position. This strategy should be taken further by designing porphyrins with diverse electron donating/withdrawing functionalities in the β position or a combination of meso and β substitutions. The objective is to decrease the band gap of the porphyrin ion pair by increasing the electron affinity of the negative tecton and increasing the ionization potential of the cation. Such electronic tuning may not yield a greater dispersion in the band structure but will create an increase in thermal electrons in the conduction band. The one property that is still difficult to understand on a molecular scale (and thus impossible at this point to predict) is the persistent photoconductivity. Since the active agents are metastable, they will need to be studied by methods appropriate for transient defects rather than equilibrium properties. One approach we would recommend is through isotopic substitution in order to affect the rates of the formation and relaxation processes. For example, proton transfer may be playing a significant role in the defect formation. In that case, deuteration would significantly change measured PPC rates. Persistently photoconductive porphyrin materials may have potential uses as nanosized optical switches, photodetectors, electro-optical information storage devices, and chemical sensors. On the other hand, there are applications where PPC is diagnostic for poor device performance.

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Ursula Mazur received her BS in Chemistry and Mathematics from Wayne State University in Detroit and her PhD in Physical Inorganic Chemistry from the University of Michigan in Ann Arbor where her dissertation was on microwave spectroscopy of ozonides and their decomposition products. She currently holds the rank of Full Professor of Chemistry and Materials Science and Engineering and is an Affiliate Professor of the School of Mechanical and Materials Engineering at Washington State University. Her past interests included applications of inelastic electron tunneling spectroscopy to organic and inorganic materials, synthesis and properties of cyanocarbons, and vibrational and luminescence spectroscopy. Her current interests are kinetics and thermodynamics of chemical reactions at the solution/solid interface, 2D metal−organic selfassembled structures on surfaces, and the structure−function relationship of ionic porphyrinic crystalline semiconductors. She is a Fellow of the ACS and of the AAAS.

K. W. Hipps received his BS in Chemistry from the University of Texas at El Paso. His PhD is in Chemical Physics, and his dissertation was on magnetically induced circular polarization of emission. He won an NSF Postdoctoral Fellowship that he took to the University of Michigan where he studied microwave-optical double resonance spectroscopy. He is a Regents Professor of Chemistry and of Materials Science and Engineering at Washington State University in Pullman, Washington. Over the years, his interests have included microwaveoptical double resonance, thermal modulation spectroscopy, determining excited state geometries from vibronic structuring of emission, Raman spectroscopy, inelastic electron tunneling spectroscopy for probing optically forbidden transitions, scanning tunneling microscopy, elastic tunneling spectroscopy, and XPS. His current interests include processes at the solution−conductor interface and nanocrystalline porphyrinic materials.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ursula Mazur: 0000-0002-3471-4883 K. W. Hipps: 0000-0002-5944-5114 Notes

The authors declare no competing financial interest. N

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ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation under grant CHE-1403989. We gratefully acknowledge their support. We also acknowledge the generous support of the M. J. Murdock Charitable Trust for providing the XPS instrumentation used in this study. Computational work was performed using the Cascade supercomputer at EMSL (DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research) located at the Pacific Northwest National Laboratory (PNNL) under grant 48783 and the resources from Kamiak HPC provided by the Center for Institutional Research Computing (CIRC) at Washington State University. We also thank the X-ray diffraction group at Bruker AXS Inc. for solving the crystal structures and the Franceschi Microscopy and Imaging Center at Washington State University for the use of their fluorescence microscope.

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