Structural Aspects of Porphyrins for Functional Materials Applications

crystals

Review Structural Aspects of Porphyrins for Functional Materials Applications

Lawrence Cook 1,2,*, Greg Brewer 2 and Winnie Wong-Ng 3 ID 1 Department of Materials Science and Engineering, The Catholic University of America, Washington, DC 20064, USA 2 Department of Chemistry, The Catholic University of America, Washington, DC 20064, USA; [email protected] 3 Materials Measurement Science Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-202-319-5535

Academic Editor: Helmut Cölfen Received: 19 April 2017; Accepted: 12 July 2017; Published: 15 July 2017

Abstract: Porphyrinic compounds comprise a diverse group of materials which have in common the presence of one or more cyclic tetrapyrroles known as porphyrins in their molecular structures. The resulting aromaticity gives rise to the semiconducting properties that make these compounds of interest for a broad range of applications, including artiﬁcial photosynthesis, catalysis, molecular electronics, sensors, non-linear optics, and solar cells. In this brief review, the crystallographic attributes of porphyrins are emphasized. Examples are given showing how the structural orientations of the porphyrin macrocycle, and the inter-porphyrin covalent bonding present in multiporphyrins inﬂuence the semiconducting properties. Beginning with porphine, the simplest porphyrin, we discuss how the more complex structures that have been reported are described by adding peripheral substituents and internal metalation to the macrocycles. We illustrate how the conjugation of the π-bonding, and the presence of electron donor/acceptor pairs, which are the basis for the semiconducting properties, are affected by the crystallographic topology.

Keywords: porphyrin; metalloporphyrin; crystal structure; crystallography; semiconductor; electrical conductivity; molecular electronics; covalent bonding; π-bonding; conjugation

1. Introduction Porphyrins are defined as a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their carbon atoms via methine (=CH–) bridges [1]. Since the porphyrin macrocyclic structure was first proposed by Küster over a century ago [2], research in the field has grown dramatically, resulting in a vast body of literature, which is continuing to expand at a rapid pace. As an indication, the current series of the “Handbook of Porphyrin Science,” initiated in 2010, at present encompasses 44 volumes containing a total of 214 chapters [3]. In addition to purely scientific importance, as evidenced by the award of several Nobel prizes for porphyrin-related research through the years [4], much of the recent growth has been driven by the potential for applications in materials science and engineering [5,6]. A large part of the progress to date has been facilitated by the application of X-ray crystallography (including synchrotron) and neutron crystallography to the determination of porphyrin crystal structures. Currently, more than 4000 porphyrin crystal structures are reported in the Cambridge Structural Database (CSD) [7]. It is the objective of the present paper to provide a brief outline of what is known of porphyrin solid state structures from a crystallographic perspective, in the hope that it may be useful to materials scientists as they become involved with porphyrin research. To ensure adequate focus in this review, in general we do not

Crystals 2017, 7, 223; doi:10.3390/cryst7070223 www.mdpi.com/journal/crystals Crystals 2017, 7, 23 2 of 24 CrystalsCrystals 20172017, ,77, ,23 223 22 of of 24 22 we do not include porphyrin derivatives and related compounds such as corrins, chlorins, phthalocyanines,we do not include metal porphyrin-organic frameworks derivatives (MOFs) and related and covalent compounds organic such frameworks as corrins, (COFs). chlorins, phthalocyanines,include porphyrin metal derivatives-organic frameworks and related (MOFs) compounds and covalent such as corrins,organic chlorins,frameworks phthalocyanines, (COFs). metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). 2. Porphine, the Parent Compound 2. Porphine, the Parent Compound 2. Porphine,The structure the Parent of the Compound porphyrin parent compound, porphine (C20H14N4), which forms a stable The structure of the porphyrin parent compound, porphine (C20H14N4), which forms a stable solid atThe room structure temperature, of the porphyrinis shown in parent Figure compound, 1. The various porphine carbon (Cand HnitrogenN ), whichpositions forms have a stablebeen solid at room temperature, is shown in Figure 1. The various carbon and20 nitrogen14 4 positions have been indicatedsolid atroom in Figure temperature, 2 according is shown to the innomenclature Figure1. The suggested various carbon by IUAPC and nitrogen[8]. The positionscarbons of have the indicated in Figure 2 according to the nomenclature suggested by IUAPC [8]. The carbons of the macrocyclebeen indicated are numbered in Figure 2from according 1 to 20, to and the the nomenclature four interior suggestednitrogens are by at IUAPC positions [ 8]. 2 The1–24 carbonsin Figure of macrocycle are numbered from 1 to 20, and the four interior nitrogens are at positions 21–24 in Figure 2a.the The macrocycle nitrogens are at positions numbered 21 from and 123 to are 20, bonded and the to four hydrogens, interior nitrogenswhich project are attoward positions the interior 21–24 in 2a. The nitrogens at positions 21 and 23 are bonded to hydrogens, which project toward the interior ofFigure the macrocycle.2a. The nitrogens Nume atrous positions sidearm 21 substituents and 23 are bonded are possible to hydrogens, at the β whichand meso project sites toward shown thein of the macrocycle. Numerous sidearm substituents are possible at the β and meso sites shown in Figureinterior 2b, of but the in macrocycle. the basic porphine Numerous structure, sidearm these substituents substitutions are possible are lacking. at the Meanβ and bondmeso anglessites shown and Figure 2b, but in the basic porphine structure, these substitutions are lacking. Mean bond angles and distancesin Figure 2forb, butporphine in the basicmacrocycles porphine are structure, indicated these in Figure substitutions 3. These are macrocycle lacking. Mean dim bondensions angles are distances for porphine macrocycles are indicated in Figure 3. These macrocycle dimensions are relativelyand distances constant for porphinefor all porphyrins, macrocycles including are indicated complex in Figure multiporphyrins,3. These macrocycle as well dimensionsas the simpler are relatively constant for all porphyrins, including complex multiporphyrins, as well as the simpler porphinerelatively derivatives. constant for all porphyrins, including complex multiporphyrins, as well as the simpler porphine derivatives. porphine derivatives.

FigureFigure 1. 1. TheThe structure structure of porphine, of porphine, from from data data of [7,9] of [.7 Dark,9]. Dark gray gray= C, light = C, blue light = blueN, white = N, = whiteH. (Note: = H. Figure 1. The structure of porphine, from data of [7,9]. Dark gray = C, light blue = N, white = H. (Note: This(Note: is the This col isor the scheme color schemeused from used here from on, here unless on, otherwise unless otherwise described) described).. This is the color scheme used from here on, unless otherwise described).

(a) (b) (a) (b) FigureFigure 2. 2. NomenclatureNomenclature of macrocycle of macrocycle atomic atomic positions positions [8]: (a [)8 numerical]: (a) numerical designation; designation; and (b) site and Figure 2. Nomenclature of macrocycle atomic positions [8]: (a) numerical designation; and (b) site designation.(b) site designation. designation.

Crystals 2017, 7, 223 3 of 22 Crystals 2017, 7, 23 3 of 24

FigureFigure 3. Mean 3. Mean bond bond distances distances and and anglesangles in the porphine porphine macrocycle, macrocycle, after after [10] [, 10with], with permission permission fromfrom World World Scientiﬁc Scientific Publishing Publishing C.o. C.o. Note Note thatthat the numbering numbering system system is the is themirror mirror image image of that of in that in FigureFigure2a, with 2a, with the exceptionthe exception of of the the N N positions. positions.

The crystallographic parameters of porphine are given in Table 1. The structure is monoclinic, withThe crystallographicspace group P 21/ parametersc. The unit cell of porphinestructure (not are givenshown) in consists Table1 .of The four structure stacked, isbut monoclinic, offset withmacrocycles space group withP an21/ interlayerc. The unit spacing cell of structure 3.355 Å. The (not macrocycles shown) consists themselves of fourare planar, stacked, coplanar but offset macrocycleswith each with other, an and interlayer stacked in spacing a rotated, of 3.355offset manner. Å. Themacrocycles In more complex themselves porphyrins, are the planar, planarity coplanar withof each the other,individual and macrocycles stacked in amay rotated, undergo offset distortions manner. of Inup more to a few complex tenths porphyrins,of Ångstroms, the termed planarity of the“ruffling” individual or “doming macrocycles”. may undergo distortions of up to a few tenths of Ångstroms, termed “rufﬂing” or “doming”. Table 1. Crystallographic Properties of Porphine [9].

Table 1. CrystallographicCSD No. 609953 Properties (Figure of1) Porphine [9]. Name porphine CSDFormula No. C14H 60995320N4 (Figure1) System monoclinic Name porphine Space gr. P 21/c Formula C H N A (Å) 10.2262(3)14 20 4 System monoclinic B (Å) 11.9060(5) Space gr. P 21/c A (Å)C (Å) 12.3853(4)10.2262(3) Bα (Å) (deg.) 90.0 11.9060(5) Cβ (Å) (deg.) 101.711(3)12.3853(4) α (deg.)γ (deg.) 90.00 90.0 β (deg.)V (Å3) 1476.56101.711(3) γ (deg.)Z 4 90.00 Vρ(Å (g/cm3) 3) 1.396 1476.56 Z 4 ρ (g/cm3) 1.396 3. Peripheral Substituents The porphyrin macrocycle is capable of accommodating several different types of substituents 3. Peripheral Substituents by bonding to the external carbons in the β- and the meso-positions on the ring. Simple substitutions basedThe porphyrin on the porphine macrocycle structure is lead capable to the ofcommon accommodating porphyrins given several in Table different 2 and types shown of in substituents Figure by bonding4a–c; these to theare, externalrespectively, carbons tetraphenyl in the βporphyrin- and the (TPP)meso-positions (C44H30N4), ontetrapropyl the ring. porphyrin Simple substitutions (TPrP) based(C on32H38 theN4), porphine and octaethyl structure porphyrin lead (OEP) to the (C36 commonH46N4). TPP porphyrins and TPrP have given phenyl in Table(–C6H25) andand propyl shown in (-CH2CH2CH3) substitutions, respectively, on the four meso-positions, while OEP has ethyl (-CH2CH3) Figure4a–c; these are, respectively, tetraphenyl porphyrin (TPP) (C 44H30N4), tetrapropyl porphyrin substitutions on the eight β-positions. Somewhat more complicated substitutions on the β-positions (TPrP) (C32H38N4), and octaethyl porphyrin (OEP) (C36H46N4). TPP and TPrP have phenyl (–C6H5) and propyl (-CH2CH2CH3) substitutions, respectively, on the four meso-positions, while OEP has ethyl (-CH2CH3) substitutions on the eight β-positions. Somewhat more complicated substitutions on the β-positions lead to protoporphyrin IX (PP9) (C36H38N4O4), as described in Table2 and Figure4d. Crystals 2017, 7, 223 4 of 22

Crystals 2017, 7, 23 4 of 24 PP9 is a well-known compound closely related to hemin [11]. The β substituents on PP9 are not symmetrical,lead to withprotoporphyrin four methyl IX (PP9) (–CH (C363H)38 groupsN4O4), as on describedβ-positions in Table 3, 2 8,and 12, Figure and 4 18,d. PP9 two is vinyla well- (–CH=CH2) known compound closely related to hemin [11]. The β substituents on PP9 are not symmetrical, with groups on β-positions 2 and 7, and two propionic acid (-CH2CH2COOH) groups on β-positions 13 four methyl (–CH3) groups on β-positions 3, 8, 12, and 18, two vinyl (–CH=CH2) groups on β-positions and 17. Among the substituents, two of the TPP phenyls and the two PP9 propionic acids have the 2 and 7, and two propionic acid (-CH2CH2COOH) groups on β-positions 13 and 17. Among the greatestsubstituents, steric deviations two of the from TPP the phenyls macrocycle and the plane.two PP9 In propionic Table2 ,acids it can have be the seen greatest that thesteric substituents have lowereddeviations the from densities the macrocycle and mostplane. ofIn Table the crystal2, it can be symmetries seen that the relativesubstituents to have the lowered basic porphine of Table1. However,the densities inand general, most of the the crystal presence symmetries of simple relative substituentsto the basic porphine does of not Table substantially 1. However, affect the in general, the presence of simple substituents does not substantially affect the crystallographic crystallographicparameters parameters of the basic macrocycle of the basic itself. macrocycleMost instances itself.of macrocycle Most distortion instances involve of macrocycle metalation distortion involve metalationand/or the presence and/or theof complex presence substituents of complex requiring substituents non-planar requiring inter-porphyrin non-planar bonding inter-porphyrin bondingarrangements. arrangements.

Table 2. Crystallographic Properties of Selected Substituted Porphines (no metalation). Table 2. Crystallographic Properties of Selected Substituted Porphines (no metalation). CSD No. 1275315 (Figure 4a) 1051569 (Figure 4b) 1225729 (Figure 4c) 1238138 (Figure 4d) Tetraphenyl porphyrin Tetrapropyl porphyrin Octaethyl porphyrin protoporphyrin IX CSDName No. 1275315 (Figure4a) 1051569 (Figure4b) 1225729 (Figure4c) 1238138 (Figure4d) (TPP) (TPrP) (OEP) (PP9) Tetraphenyl porphyrin Tetrapropyl porphyrin Octaethyl porphyrin protoporphyrin IX NameFormula C44H30N4 C32H38N4 C36H46N4 C36H38N4O4 (TPP) (TPrP) (OEP) (PP9) System triclinic monoclinic triclinic triclinic Formula C44H30N4 C32H38N4 C36H46N4 C36H38N4O4 Space gr. P −1 P 21/n P −1 P −1 System triclinic monoclinic triclinic triclinic a (Å) 6.44 (1) 5.0843 (2) 9.791 (2) 11.303 (5) Space gr. P −1 P 21/n P −1 P −1 a (Å)b (Å) 10.426.44 (1) (1)11.6074 5.0843 (6) (2)10.771 (2)9.791 (2)22.553 (10) 11.303 (5) b (Å)c (Å) 12.4110.42 (1) (1)22.1695 11.6074 (11) (6)7.483 (2)10.771 (2)6.079 (3) 22.553 (10) cα (Å) (deg.) 96.0612.41 (5) (1)90.00 22.1695 (11)97.43 (1) 7.483 (2)91.38 (2) 6.079 (3) α (deg.)β (deg.) 99.1496.06 (5) (5)93.53 (2) 90.00 106.85 (1)97.43 (1)94.08 (2) 91.38 (2) β (deg.)γ (deg.) 101.1299.14 (5) (5)90.00 93.53 (2)93.25 (1)106.85 (1)81.96 (1) 94.08 (2) γ (deg.)V (Å3) 798.747101.12 (5)1305.86 (11) 90.00 745.23 93.25 (1)1530.36 81.96 (1) 3 V (Å Z) 1798.747 2 1305.86 (11)1 745.232 1530.36 ρZ (g/cm3) 1.278 11.217 21.192 11.282 2 3 ρ (g/cmTemp) . 2951.278 293 (2) 1.217295 1.192295 1.282 Temp. 295 293 (2) 295 295 Ref. [7,12] [7,13] [7,14] [7,15] Ref. [7,12][7,13][7,14][7,15]

Crystals 2017, 7, 23 5 of 24

(a) (b)

Figure 4. Examples of substituents: (a) TPP; (b) TPrP; (c) OEP; and (d) PP9; from data of [7,12–15]. Figure 4. ShownExamples without of hydrogens. substituents: Red = O. (a ) TPP; (b) TPrP; (c) OEP; and (d) PP9; from data of [7,12–15]. Shown without hydrogens. Red = O. 4. Metalation of the Macrocycle Core Another structural variation possible involves deprotonating the interior nitrogens of the macrocycle at positions 21 and 23 and inserting a metal ion, which is bonded to all four nitrogens. In some cases, to satisfy charge balance, metalation may be accompanied by axial bonding of the metal to a cation such as a halide. In other situations, coordinate covalent bonding between the metal and a neutral ligand may occur. More than 50 metals and semi-metals have been inserted in various porphyrins. Crystallographic data are available for several metalated porphines, of which data for four compounds are included in Table 3. Included in the table are a Ni-metalated variant, a Mg-metalated variant, a Zn-metalated variant, and a Ge-metalated variant. All are monoclinic with space group P 21/c, with the exception of the Mg variant, which is monoclinic with space group C 2/c. In the Ni- metalated variant, the Ni is coplanar with the macrocycle (Figure 5) and the structure is otherwise similar to that of porphine, with an interlayer distance of 3.347 Å. In the Mg-metalated variant (Figure 6a) the Mg is also coplanar with the macrocycle, but is axially bonded to pyridine groups (C5H5N) above and below the plane of the macrocycle. This results in a more complicated stacking arrangement in which the macrocycles of successive layers are not all coplanar, but the pyridine extensions are nearly parallel and coplanar with one another (Figure 6b). The distance between the canted macrocycles, which are closest at the edges, is 3.315 Å, less than the interplanar stacking distance in porphine. In the Zn-metalated variant, the Zn is positioned slightly above the plane of the macrocycle and is axially bonded to the nitrogen of a single pyridine (Figure 7a,b). The plane of the macrocycle is puckered upward slightly (domed) toward the Zn (Figure 7b). In the Ge-variant, which contains two axially bonded methoxide (–CH3O) units on opposite sides of the macrocycle, the Ge is in the plane of the macrocycle (Figure 8). As listed in Table 3, the root-mean-square displacement of atoms from the mean macrocycle plane is as follows: Ni-porphine, 0.0191 Å; Mg-porphine, 0.0242 Å; Zn-porphine, 0.0873 Å; and Ge-porphine, 0.0157 Å [10]. The Zn-porphine atoms have the highest RMS departure from the mean plane due to lifting of the Zn by coordination with a single pyridine. The compounds are all monoclinic; three have the same space group as the basic porphine (Table 1). As expected, all have higher densities than the porphine, although the Mg-variant has substantially lower density than the other three, presumably because the lower atomic mass of Mg is supplemented by the less efficient crystal packing of the bulkier molecules.

Crystals 2017, 7, 223 5 of 22

4. Metalation of the Macrocycle Core Another structural variation possible involves deprotonating the interior nitrogens of the macrocycle at positions 21 and 23 and inserting a metal ion, which is bonded to all four nitrogens. In some cases, to satisfy charge balance, metalation may be accompanied by axial bonding of the metal to a cation such as a halide. In other situations, coordinate covalent bonding between the metal and a neutral ligand may occur. More than 50 metals and semi-metals have been inserted in various porphyrins. Crystallographic data are available for several metalated porphines, of which data for four compounds are included in Table3. Included in the table are a Ni-metalated variant, a Mg-metalated variant, a Zn-metalated variant, and a Ge-metalated variant. All are monoclinic with space group P 21/c, with the exception of the Mg variant, which is monoclinic with space group C 2/c. In the Ni-metalated variant, the Ni is coplanar with the macrocycle (Figure5) and the structure is otherwise similar to that of porphine, with an interlayer distance of 3.347 Å. In the Mg-metalated variant (Figure6a) the Mg is also coplanar with the macrocycle, but is axially bonded to pyridine groups (C5H5N) above and below the plane of the macrocycle. This results in a more complicated stacking arrangement in which the macrocycles of successive layers are not all coplanar, but the pyridine extensions are nearly parallel and coplanar with one another (Figure6b). The distance between the canted macrocycles, which are closest at the edges, is 3.315 Å, less than the interplanar stacking distance in porphine. In the Zn-metalated variant, the Zn is positioned slightly above the plane of the macrocycle and is axially bonded to the nitrogen of a single pyridine (Figure7a,b). The plane of the macrocycle is puckered upward slightly (domed) toward the Zn (Figure7b). In the Ge-variant, which contains two axially bonded methoxide (–CH3O) units on opposite sides of the macrocycle, the Ge is in the plane of the macrocycle (Figure8). As listed in Table3, the root-mean-square displacement of atoms from the mean macrocycle plane is as follows: Ni-porphine, 0.0191 Å; Mg-porphine, 0.0242 Å; Zn-porphine, 0.0873 Å; and Ge-porphine, 0.0157 Å [10]. The Zn-porphine atoms have the highest RMS departure from the mean plane due to lifting of the Zn by coordination with a single pyridine. The compounds are all monoclinic; three have the same space group as the basic porphine (Table1). As expected, all have higher densities than the porphine, although the Mg-variant has substantially lower density than the other three, presumably because the lower atomic mass of Mg is supplemented by the less efﬁcient crystal packing of the bulkier molecules.

Table 3. Crystallographic Properties of Selected Metalated Porphines (no substituents).

CSD No. 1268870 (Figure5) 912742 (Figure6a,b) 912743 (Figure7a,b) 1296339 (Figure8a,b)

Name Ni(porphine) Mg(porphine) (C5H5N)2 Zn(porphine) (C5H5N) Ge(porphine) (OCH3)2 Formula C20H12NiN4 C20H12MgN4 (C5H5N)2 C20H12ZnN4 (C5H5N) C20H12GeN4 (OCH3)2 System monoclinic monoclinic monoclinic monoclinic Space gr. P 21/c C 2/c P 21/c P 21/c A (Å) 10.1066 (7) 12.7579 (9) 9.5746 (4) 15.013 (5) B (Å) 11.945 (9) 15.0501 (12) 14.6945 (6) 14.441 (5) C (Å) 12.229 (2) 12.3850 (8) 14.6410 (6) 8.414 (4) α (deg.) 90.0 90.00 90.0 90.0 β (deg.) 101.56 (3) 92.071 (4) 105.542 (2) 91.85 (2) γ (deg.) 90.0 90.00 90.0 90.0 V (Å3) 1446.38 2376.46 1984.44 1823.23 Z 4 4 4 4 ρ (g/cm3) 1.686 1.372 1.516 1.614 Displ. (Å) 0.0191 0.0242 0.0873 0.0157 Ref. [7,16][7,17][7,17][7,18] Crystals 2017, 7, 23 6 of 24

Crystals 2017, 7Table, 23 3. Crystallographic Properties of Selected Metalated Porphines (no substituents). 6 of 24

CSD No. 1268870 (Figure 5) 912742 (Figure 6a,b) 912743 (Figure 7a,b) 1296339 (Figure 8a,b) Table 3. Crystallographic Properties of Selected Metalated Porphines (no substituents). Name Ni(porphine) Mg(porphine) (C5H5N)2 Zn(porphine) (C5H5N) Ge(porphine) (OCH3)2 CSDFormula No. 1268870C20H (Figure12NiN4 5) 912742C20H12MgN (Figure4 (C 65Ha,b)5N) 2 912743C20H12 ZnN(Figure4 (C 75Ha,b)5N) 1296339C20H12GeN (Figure4 (OCH 8a,b)3)2 NameSystem Ni(porphine)monoclinic Mg(porphine)monoclinic (C5H 5N)2 Zn(porphine)monoclinic (C5H 5N) Ge(porphine)monoclinic (OCH 3)2 FormulaSpace gr . C20HP12 2NiN1/c 4 C20H12MgNC 2/4 (Cc 5H5N)2 C20H12ZnNP 241 (C/c 5H5N) C20H12GeNP 241 (OCH/c 3)2 SystemA (Å) monoclinic10.1066 (7) monoclinic12.7579 (9) monoclinic9.5746 (4) monoclinic15.013 (5) SpaceB (Å g)r . 11.945P 21/c (9) 15.0501C 2/c (12) 14.6945P 21/c (6) 14.441P 21/c (5) AC ( Å(Å) ) 10.106612.229 (7)(2) 12.757912.3850 (9) (8) 9.574614.6410 (4) (6) 15.0138.414 (5)(4) αB (deg.)(Å) 11.94590.0 (9) 15.050190.00 (12) 14.694590.0 (6) 14.44190.0 (5) βC (deg.)(Å) 12.229101.56 (2) (3) 12.385092.071 (8)(4) 14.6410105.542 (6) (2) 8.41491.85 (4) (2) αγ (deg.) (deg.) 90.090.0 90.0090.00 90.090.0 90.090.0 β V(deg.) (Å3) 101.561446.38 (3) 92.0712376.46 (4) 105.5421984.44 (2) 91.851823.23 (2) γ (deg.)Z 90.04 90.004 90.04 90.04 ρV (g/cm (Å3) 3) 1446.381.686 2376.461.372 1984.441.516 1823.231.614 DisplZ . (Å) 0.01914 0.02424 0.08734 0.01574 ρ (g/cmRef.3 ) 1.686[7,16] 1.372[7,17] 1.516[7,17] 1.614[7,18] Displ. (Å) 0.0191 0.0242 0.0873 0.0157 CrystalsRef2017. , 7, 223 [7,16] [7,17] [7,17] [7,18] 6 of 22

Figure 5. Ni-porphine, from data of [7,16]. Green = Ni.

Figure 5. Ni-porphine, from data of [7,16]. Green = Ni. Figure 5. Ni-porphine, from data of [7,16]. Green = Ni.

(a) (b)

CrystalsFigure 2017, 7 6., 23 Mg -porphine, from data of [7,17]: (a) single molecule; and (b) crystal packing. Light green =7 of 24 Figure 6. Mg-porphine,(a) from data of [7,17]: (a) single molecule; and (b) crystal(b packing.) Light green = Mg. Mg. Figure 6. Mg-porphine, from data of [7,17]: (a) single molecule; and (b) crystal packing. Light green =

Mg.

(a) (b)

Figure 7. Zn-porphine, from data of [7,17]: (a) top view, with hydrogens; and (b) side view, without Figure 7. Zn-porphine, from data of [7,17]: (a) top view, with hydrogens; and (b) side view, without hydrogens. Dark blue = Zn. hydrogens. Dark blue = Zn.

(a) (b)

Figure 8. Ge-porphine, after data of [7,18]: (a) top view; and (b) side view. Dark green = Ge, red = O.

Devillers et al. [10] have calculated average bond lengths and bond angles for the pyrrole units among a sampling of eight porphine macrocycles (including both non-metalated and metalated, but with no substituents). The results do not deviate significantly from the mean values. Thus, based on this small sample, the effect of metalation by itself on the crystallographic dimensions of the macrocycle appears to be small. However, the presence of ligands axially bonded to the metals may sterically require differences in the overall stacking mode, which can affect the overall crystal properties. As noted by Fleischer [19], the coordination of the metal in metalated porphyrins, depending on the metal, may be described as: four-coordinated (square-planar), five-coordinated (square pyramidal), six-coordinated (distorted octahedral), or eight-coordinated (square-antiprismatic, as when sandwiched in a dimer). The type of coordination is influenced by the size of the metal ion. Divalent metals that can readily fit into the plane of the macrocycle core can have square-planar coordination, as shown for the nickel in Figure 5. High-spin ferric iron has five-coordination, with a halogen at the apex, and the four nitrogens at the base of the coordination pyramid. The germanium in Figure 8 can be regarded as octahedrally coordinated. Seven-coordination is also possible when the metal is out of the macrocycle plane. The metal coordination can have a substantial effect on the optical absorption spectrum and the electrical and magnetic properties.

5. Role of Covalent Inter-Molecular Bonding

Crystals 2017, 7, 23 7 of 24

(a) (b)

Crystals 2017, 7Figure, 223 7. Zn-porphine, from data of [7,17]: (a) top view, with hydrogens; and (b) side view, without 7 of 22 hydrogens. Dark blue = Zn.

(a) (b)

Figure 8. Ge-porphine, after data of [7,18]: (a) top view; and (b) side view. Dark green = Ge, red = O. Figure 8. Ge-porphine, after data of [7,18]: (a) top view; and (b) side view. Dark green = Ge, red = O. Devillers et al. [10] have calculated average bond lengths and bond angles for the pyrrole units Devillersamong a et sampling al. [10] of have eight calculated porphine macrocycles average bond (including lengths both and non- bondmetalated angles and formetalated, the pyrrole but units with no substituents). The results do not deviate significantly from the mean values. Thus, based on among a sampling of eight porphine macrocycles (including both non-metalated and metalated, but this small sample, the effect of metalation by itself on the crystallographic dimensions of the with nomacrocycle substituents). appears The to resultsbe small. do However, not deviate the presence significantly of ligands from axially the bonded mean values.to the metals Thus, may based on this smallsterically sample, require the effect differences of metalation in the overall by itself stacking on the mode, crystallographic which can affect dimensions the overall of the crystal macrocycle appearsproperties. to be small. However, the presence of ligands axially bonded to the metals may sterically require differencesAs noted inby theFleischer overall [19] stacking, the coordination mode, whichof the metal can in affect metalated the overall porphyrins, crystal depending properties. on Asthe noted metal, by Fleischermay be described [19], the as: coordination four-coordinated of the (square metal-planar), in metalated five-coo porphyrins,rdinated (square depending pyramidal), six-coordinated (distorted octahedral), or eight-coordinated (square-antiprismatic, as on thewhen metal, sandwiched may be in described a dimer). The as: type four-coordinated of coordination is (square-planar), influenced by the size five-coordinated of the metal ion. (square pyramidal),Divalent six-coordinated metals that can (distortedreadily fit into octahedral), the plane of or the eight-coordinated macrocycle core can(square-antiprismatic, have square-planar as when sandwichedcoordination, as in shown a dimer). for the The nickel type in Figure of coordination 5. High-spin isferric influenced iron has five by-coordination, the size of thewith metal a ion. Divalenthalogen metals at the that apex, can and readily the four fit nitrogens into the at plane the base of of the the macrocyclecoordination pyramid. core can The have germaniu square-planarm coordination,in Figure as 8 showncan be regarded for the nickelas octahedrally in Figure coordinated.5. High-spin Seven ferric-coordination iron has is five-coordination, also possible when with a the metal is out of the macrocycle plane. The metal coordination can have a substantial effect on the halogen at the apex, and the four nitrogens at the base of the coordination pyramid. The germanium optical absorption spectrum and the electrical and magnetic properties. in Figure8 can be regarded as octahedrally coordinated. Seven-coordination is also possible when the metal5. Role is out of ofCovalent the macrocycle Inter-Molecular plane. Bonding The metal coordination can have a substantial effect on the optical absorption spectrum and the electrical and magnetic properties.

5. Role of Covalent Inter-Molecular Bonding Many porphyrins crystallize as molecular compounds. Intermolecular interactions are important in determining the overall crystallographic properties and include hydrogen bonding, van der Waals bonding, π–π stacking, covalent bonding, coordinate covalent bonding, and electrostatic interactions. As a result of the first two types of interaction, discrete porphyrin molecules are attached to one another in the crystal structure by hydrogen- or van der Waals-bonds. For example, we have described the presence of hydrogen bonding in hemin, an Fe-porphyrin [11]. Coordinate covalent bonding can be important for the configuration of neutral axial ligands. Electrostatic interactions may play a role in the self-assembly of porphyrin molecules. As noted throughout this review, π–π conjugation plays an essential role in determining electronic properties. Covalent intermolecular bonding in porphyrins, by virtue of the geometrical control exerted, has a direct effect on π–π stacking. Relative to hydrogen and van der Waals bonding, covalent bonding results in the linking of molecules by stronger bridges. Such bridges include C2H2,C2H4, –N=N–, =C=C=, –S–S–, –C(OH)–, etc., which link porphyrins to form crystalline dimers, trimers or other oligomers [20]. Covalency can occur as meso-meso, β-β, or meso-β links. The geometries of the linkages are controlled by the bond angles present in the bridges. The C2H2 bridge allows for some flexibility in crystal packing as it can fold to allow more compact crystal packing. In linkages involving C2H4, rotational flexibility of the linear –C–C– single bond also allows for some flexibility in molecular packing. Table4 gives parameters for two examples of covalent Crystals 2017, 7, 223 8 of 22 dimer bonding, as illustrated in Figures9 and 10. In Figure9, the –N=N– meso-meso bridge linking the TPP dimer has a zig-zag geometry that results in a displacement of the macrocycles horizontally and vertically relative to one another, but which nonetheless allows them to remain nearly parallel with one another. In Figure 10, the OEP dimer is β-β linked at a high angle due to the angularity of the –C(OH) – bridge. Thus, there are many possibilities for the formation of 3-D molecular structures based on the packing of covalently-bonded oligomeric macrocycles; structures are influenced and limited by the nature of the linkages, as well as by the steric chemistry of the substituents on the linked macrocycles. Structural possibilities are further extended by the fusion or direct linkage of macrocycles.

Table 4. Crysallographic Properties of –N=N– and –C(OH)– Covalently Bonded Porphyrin Dimers.

CSD No. 627200 (Figure9) 1230159 (Figure 10)

Formula C81H51N11Ni2 C67H80N8Ni2O System monoclinic monoclinic Space gr. P 2/c C 2/c A (Å) 13.0693 (3) 40.31 (2) B (Å) 9.7304 (2) 14.997 (7) C (Å) 24.1344 (5) 21.954 (11) α (deg.) 90.00 90 β (deg.) 104.2710 (10) 108.6 (4) γ (deg.) 90.00 90 V (Å3) 2974.45 (11) 12578.6 Z 2 8 ρ (g/cm3) 1.447 1.309 Temp. 150 (2) 130 Crystals 2017, 7, 23 Ref. [7,21][7,22,23] 9 of 24 Crystals 2017, 7, 23 9 of 24

Figure 9. –N=N– covalent meso-meso linkage in TPP-based dimer, from data of [7,21]. Green = Ni. Figure 9. –N=N–Figure 9. – covalentN=N– covalentmeso meso-meso-mesolinkage linkage in in TPP-basedTPP-based dimer, dimer, from from data of data [7,21] of. Green [7,21 ].= Ni. Green = Ni.

Figure 10.Figure–C(OH)– 10. –C(OH) covalent– covalentβ-β βlinkage-β linkage in in OEP-based OEP-based dimer. dimer. From From data of data [7,22,23] of [7. ,Peripheral22,23]. Peripheral Figure 10. –C(OH)– covalent β-β linkage in OEP-based dimer. From data of [7,22,23]. Peripheral substituentssubstituents are omitted are omitted for clarity. for clarity. Green Green = = Ni, Ni, redred = = O. O. substituents are omitted for clarity. Green = Ni, red = O. 6. Electronic Properties as Related to Structure 6. Electronic6. Electronic Properties Properties as Related as Related to Structureto Structure The semiconducting nature of porphyrins is of great interest to materials scientists for The semiconducting nature of porphyrins is of great interest to materials scientists for The semiconductingapplications ranging nature from artificial of porphyrins photosynthesis is of great to catalysis, interest molecular to materials electronics, scientists sensors, for non applications- applications ranging from artificial photosynthesis to catalysis, molecular electronics, sensors, non- linear optics, and charge separation in solar cell materials. The semiconducting properties arise from ranging fromlinear artiﬁcial optics, and photosynthesis charge separation to in catalysis, solar cell materials. molecular The electronics, semiconducting sensors, properties non-linear arise from optics, and the aromaticity of the macrocycle, which leads to a significant electron mobility. In particular, charge separationthe aromaticity in solar of the cell macrocycle, materials. which The semiconductingleads to a significant properties electron mobility. arise from In particular, the aromaticity of stacking of the macrocycles in crystal structures such that there is close proximity of the π-bonding stacking of the macrocycles in crystal structures such that there is close proximity of the π-bonding the macrocycle,regions which results in leads enhanced to a capability signiﬁcant for electron transfer. mobility. Additionally, In particular, π-bonding stacking conjugation of the may macrocycles regions results in enhanced capability for electron transfer. Additionally, π-bonding conjugation may occur in planes parallel to the macrocycle through covalent linkages, as outlined above. Furthermore, occur in planes parallel to the macrocycle through covalent linkages, as outlined above. Furthermore, in certain structures, porphyrins act as electron donors when bonded to electron acceptors, such as in certain structures, porphyrins act as electron donors when bonded to electron acceptors, such as fullerenes. By varying the linkages in these and other dyads, the kinetics of electron–hole fullerenes. By varying the linkages in these and other dyads, the kinetics of electron–hole recombination can be tuned. Factors important in optimizing porphyrin semiconducting properties recombination can be tuned. Factors important in optimizing porphyrin semiconducting properties include the planarity of the macrocycle, and its parallelism with others in the structure. When include the planarity of the macrocycle, and its parallelism with others in the structure. When discussing electronic properties, it is important to differentiate among different structural regimes, discussing electronic properties, it is important to differentiate among different structural regimes, i.e., whether molecular, single-crystal, thin-film, or bulk-polycrystalline environments are being i.e., whether molecular, single-crystal, thin-film, or bulk-polycrystalline environments are being considered. Thin-film materials may have the added complexity of epitaxy or texturing, which unless considered. Thin-film materials may have the added complexity of epitaxy or texturing, which unless fully characterized, can make precise interpretation of data difficult, as electrical conductivity is a fully characterized, can make precise interpretation of data difficult, as electrical conductivity is a second-rank tensor property. second-rank tensor property. 6.1. Properties at the Molecular Level 6.1. Properties at the Molecular Level Porphyrins, including fused or aromatically linked linear architectures, have a number of Porphyrins, including fused or aromatically linked linear architectures, have a number of properties at the molecular level which make them suitable for electronic applications [24]. They can properties at the molecular level which make them suitable for electronic applications [24]. They can be readily oxidized to π radical cations, or reduced to π radical anions; the HOMO/LUMO energy be readily oxidized to π radical cations, or reduced to π radical anions; the HOMO/LUMO energy gap is generally only about 2 eV; metal chelation at the core allows control over the redox properties; gap is generally only about 2 eV; metal chelation at the core allows control over the redox properties;

Crystals 2017, 7, 223 9 of 22 in crystal structures such that there is close proximity of the π-bonding regions results in enhanced capability for electron transfer. Additionally, π-bonding conjugation may occur in planes parallel to the macrocycle through covalent linkages, as outlined above. Furthermore, in certain structures, porphyrins act as electron donors when bonded to electron acceptors, such as fullerenes. By varying the linkages in these and other dyads, the kinetics of electron–hole recombination can be tuned. Factors important in optimizing porphyrin semiconducting properties include the planarity of the macrocycle, and its parallelism with others in the structure. When discussing electronic properties, it is important to differentiate among different structural regimes, i.e., whether molecular, single-crystal, thin-film, or bulk-polycrystalline environments are being considered. Thin-film materials may have the added complexity of epitaxy or texturing, which unless fully characterized, can make precise interpretation of data difficult, as electrical conductivity is a second-rank tensor property.

6.1. Properties at the Molecular Level Porphyrins, including fused or aromatically linked linear architectures, have a number of properties at the molecular level which make them suitable for electronic applications [24]. They can be readily oxidized to π radical cations, or reduced to π radical anions; the HOMO/LUMO energy gap is generally only about 2 eV; metal chelation at the core allows control over the redox properties; selection of outer ring substituents gives additional control over solubilities and functionalities. Solubilities in various solvents allow for ﬂexibility in the use of chemical deposition methods. Porphyrins are ◦ robust; manyCrystals are 2017 stable, 7, 23 under ambient conditions, as well as having thermal stability10 of up 24 to 300 C or 400 ◦C[25]. Jurow et al. [26] have reviewed the potential applications of porphyrins in functional selection of outer ring substituents gives additional control over solubilities and functionalities. molecularSolubilities electronic in devices various suchsolvents as allow diodes, for rectiﬁers,flexibility in wires, the use capacitors, of chemical anddeposition electronic methods. or magnetic memory. ManyPorphyrins molecular are robust; applications many are stable involve under ambient self-assembled conditions, monolayersas well as having (SAMs) thermal onstability electronically active substrates,up to 300 °C which or 400 can°C [25] then. Jurow be et patterned al. [26] have and reviewed assembled the potential into applications nano- or of micro-scale porphyrins devices. Surface effectsin functional are of molecular prime importance; electronic devices the fabricationsuch as diodes, and rectifiers, performance wires, capacitors, of 2-D and porphyrin electronic structures or magnetic memory. Many molecular applications involve self-assembled monolayers (SAMs) on have beenelectronically recently reviewed active substrates, by [27]. which However can then we be are patterned concerned and assembled here mainly into na withno- or themicro crystallographic-scale controls ondevices. molecular Surface electronic effects are of properties. prime importance; the fabrication and performance of 2-D porphyrin It is knownstructures that have in been molecules recently connectedreviewed by to[27] electrodes. However we with are tunnelingconcerned here barriers, mainly electronwith the currents are determinedcrystallographic by relative controls HOMO on molecular and LUMO electronic spacing, properties. spin, and vibrational modes [28,29]. Figure 11 It is known that in molecules connected to electrodes with tunneling barriers, electron currents shows threeare possible determined transport by relative paths HOMO within and LUMO a porphyrin spacing, macrocycle, spin, and vibrational depending modes on [28,29] whether. Figure connection is made at11meso showssites three or possible at β sites. transport Route paths (1) involveswithin a porphyrin transport macrocycle, through depending the conjugated on whether bonds of the macrocycleconnection and avoids is made the at metal; meso sites route or (2)at β takessites. Route the electrons (1) involves straight transport across through the the macrocycle, conjugated through the metal;bonds and route of the (3) macrocycle involves and conjugated avoids the bondsmetal; route as well (2) takes as the the metal. electrons Theoretical straight across calculations the [30] macrocycle, through the metal; and route (3) involves conjugated bonds as well as the metal. predict thatTheoretical if the porphyrin calculations is[30] connected predict that via if oppositethe porphyrinmeso is sites,connected transport via opposite is most meso likelysites, to occur via the conjugatedtransport is bonds. most likely On to the occur other via the hand, conjugated if linkages bonds. On are the made other via hand,β sites,if linkages the are transport made will be controlledvia by β the sites, central the transport metal. will be controlled by the central metal.

Figure 11.FigureTheoretically 11. Theoretically possible possible electron electron transport transport pathways pathways in a porphyrin in a porphyrin macrocycle macrocyclewith: (a) with: meso–meso connections; and (b) with β–β connections [30]; diagram after [31], with permission from (a) meso–meso connections; and (b) with β–β connections [30]; diagram after [31], with permission from World Scientific Publishing Co. World Scientiﬁc Publishing Co. Crossley and Burn [24] synthesized molecular wires comprised of a linearly conjugated tetrakisporphyrin, as shown schematically in Figure 12. Based on spectroscopic studies, the HOMO/LUMO gap was reduced by about 0.8 eV, compared with the same gap in simple porphyrins. This effect is largely due to the presence of conjugated bridges of 1,4,5,8-tetra-azaanthracene. The presence of the meta-di-t-butylphenyl substituents on the macrocyles enhances solubility. Reimers et al. [32] performed a quantum-chemical analysis of this type of oligomer and discussed in detail the effect of oligomer size and geometry on electron- and hole-conduction. Based on their analysis, they suggest a design for a molecular switch, a modification of the design of Aviram [33], in which an oligoporphyrin is linked to a polythiophene wire as a side chain, as shown in Figure 13. The conduction between the source and drain electrodes can be switched on or off depending on the location of an excess electron hole on the porphyrin side chain, as controlled by the electric field at the gate electrode. Yoon et al. [34] measured electrical conduction through two types of porphyrin wires using nanoelectrodes. One type consisted of directly meso-meso-linked Zn(II) porphyrin arrays with 48 Zn(II) porphyrin moieties. These arrays have an orthogonal configuration such that adjacent porphyrins are oriented at right angles to one another along the chain. A second type consisted of flat

Crystals 2017, 7, 23 11 of 24 Crystals 2017, 7, 23 11 of 24 tape-like arrays with eight porphyrin units in which the porphyrin macrocycles are triply linked at tape-like arrays with eight porphyrin units in which the porphyrin macrocycles are triply linked at meso-meso, β-β, and β-β positions, resulting in a linear fused array. I–V curves for both types were meso-meso, β-β, and β-β positions, resulting in a linear fused array. I–V curves for both types were measuredCrystals 2017 ,and7, 223 compared. The orthogonal type showed hysteresis, which suggests the possibility10 of of 22 measured and compared. The orthogonal type showed hysteresis, which suggests the possibility of application to memory devices, whereas the fused tape-like type showed no hysteresis. The hysteresis application to memory devices, whereas the fused tape-like type showed no hysteresis. The hysteresis is interpreted as arising from the rotation of the meso-meso carbon-carbon bond in the orthogonal is interpretedCrossley as and arising Burn from [24] the synthesized rotation of molecular the meso-meso wires carbon comprised-carbon of bond a linearly in the orthogonal conjugated configuration, whereas rotation of the meso-meso bonds in the fused array is hindered by the triple configuratitetrakisporphyrin,on, whereas as shownrotation schematically of the meso-meso in Figurebonds in12 the. Basedfused array on spectroscopic is hindered by studies, the triple the linkage. The room-temperature resistance of the orthogonal array was estimated as 125–670 MΩ, linkage.HOMO/LUMO The room gap-temperature was reduced resistance by about 0.8of the eV, comparedorthogonal with array the was same estimated gap in simple as 125 porphyrins.–670 MΩ, whereas that for the fused array was estimated as 50 MΩ, which suggests a higher conductivity for whereasThis effect that is for largely the fused due array to the was presence estimated of as conjugated 50 MΩ, which bridges suggests of 1,4,5,8-tetra-azaanthracene. a higher conductivity for the latter, as would be expected from the co-planar orientation of the fused macrocycles. Porphyrin- theThe latter, presence as would of the be meta-di-expectedt -butylphenylfrom the co-planar substituents orientation on of the the macrocyles fused macrocycles. enhances Porphyrin solubility.- based molecular wires were investigated by Sedghi et al. [35] using STM methods. Three architectures basedReimers molecular et al. [32 wires] performed were investigated a quantum-chemical by Sedghi et analysis al. [35] using of this STM type methods. of oligomer Three and architectures discussed were prepared: (A) meso-butadiene-linked porphyrin chains; (B) twisted meso-meso directly linked werein detail prepared: the effect (A) of meso oligomer-butadi sizeene- andlinked geometry porphyrin on electron-chains; (B) and twisted hole-conduction. meso-meso directly Based on linked their chains; and (C) multiply-fused (at both β and meso positions) chains. Allowing for the length of the chainsanalysis,; and they (C) suggest multiply a- designfused (at for both a molecular β and meso switch, positions) a modiﬁcation chains. Allowing of the design for the of length Aviram of [the33], structures, the fused versions (C) had the greatest molecular conductance, and the twisted versions structures,in which an the oligoporphyrin fused versions is (C) linked had tothe a polythiophenegreatest molecular wire conductance, as a side chain, and as the shown twist ined Figure versions 13 . (B) had the lowest conductance. This is consistent with the higher macrocycle co-planarity in (C) and (B)The had conduction the lowest between conductance. the source This and is consistent drain electrodes with the can higher be switched macrocycle on orco off-planarity depending in (C) on and the the resulting increase in conjugation. thelocation resulting of an increase excess electron in conjugation. hole on the porphyrin side chain, as controlled by the electric ﬁeld at the gate electrode.

FigureFigure 12. An anthracene anthracene-conjugated-conjugated tetrakisporphyrin tetrakisporphyrin with with terttert--butylbutyl substituents, substituents, after after [24] [24],, with with Figure 12. An anthracene-conjugated tetrakisporphyrin with tert-butyl substituents, after [24], with permissionpermission from from the Royal Society of Chemistry . permission from the Royal Society of Chemistry .

Figure 13. An oligoporphyrin-containing molecular switch, after [32], with permission from IOP Publishing Co. “+” indicates location of electron hole on oligoporphyrin side chain.

Crystals 2017, 7, 223 11 of 22

Yoon et al. [34] measured electrical conduction through two types of porphyrin wires using nanoelectrodes. One type consisted of directly meso-meso-linked Zn(II) porphyrin arrays with 48 Zn(II) porphyrin moieties. These arrays have an orthogonal configuration such that adjacent porphyrins are oriented at right angles to one another along the chain. A second type consisted of flat tape-like arrays with eight porphyrin units in which the porphyrin macrocycles are triply linked at meso-meso, β-β, and β-β positions, resulting in a linear fused array. I–V curves for both types were measured and compared. The orthogonal type showed hysteresis, which suggests the possibility of application to memory devices, whereas the fused tape-like type showed no hysteresis. The hysteresis is interpreted as arising from the rotation of the meso-meso carbon-carbon bond in the orthogonal configuration, whereas rotation of the meso-meso bonds in the fused array is hindered by the triple linkage. The room-temperature resistance of the orthogonal array was estimated as 125–670 MΩ, whereas that for the fused array was estimated as 50 MΩ, which suggests a higher conductivity for the latter, as would be expected from the co-planar orientation of the fused macrocycles. Porphyrin-based molecular wires were investigated by Sedghi et al. [35] using STM methods. Three architectures were prepared: (A) meso-butadiene-linked porphyrin chains; (B) twisted meso-meso directly linked chains; and (C) multiply-fused (at both β and meso positions) chains. Allowing for the length of the structures, the fused versions (C) had the greatest molecular conductance, and the twisted versions (B) had the lowest conductance. This is consistent with the higher macrocycle co-planarity in (C) and the resulting increase in conjugation. Wang et al. [36] demonstrated self-assembled synthesis of porphyrin nanotubes based on Sn(IV)-porphyrin with meso-substituted pyridines. The Sn(IV) was axially bonded above and below the macrocycle plane to Cl– or OH–. The nanotubes were micrometers in length, with 50–70 nm diameters and ~20 nm-thick walls and showed moderate crystallinity. Electrostatic forces between the porphyrin building blocks are thought to contribute to the self-assembly process, in addition to van der Waals, hydrogen bonding, and axial coordination interactions. The nanotubes were found to be photocatalytic, and are stable for extended periods (months) when stored in the dark. They are mechanically responsive to light, i.e., when illuminated with strong light, the nanotubes become rods, in a reversible manner. This response may be due either to a softening of the tube walls and a collapse due to local heating, or to the result of photoinitiated intermolecular electron transfer that disrupts the charge balance and the rigidity of the ionic structure.

6.2. Conductivity in Single Crystals Tetragonal porphyrin single crystals with parallel open channels and interconnected columns of macrocycles were synthesized by Li et al. [37]. These features allowed simultaneous ionic (lithium) conductivity and electrical conductivity. Electrical conductivities of 10−9 S cm−1 were reported; by insertion of conducting aniline into the channels, the conductivity was increased to 10−3 S cm−1. Single crystal conductivity data was reported by Chen et al. [38] for a porphyrin salt based on 4+ − 5,10,15,20-tetrakis (4-pyridyl) porphyrin, or H2TPyP. This triclinic salt, H2TEPyP ·4I , contains four ethyl groups and four iodines, with macrocycle stacking in the a crystallographic direction. Conductivity was measured by a four-probe method. Data as a function of temperature are shown in Figure 14 for orientations parallel and perpendicular to the stacking direction. The room temperature conductivity was found to be ~3 orders of magnitude higher along the stacking axis relative to measurements perpendicular to it (3.2 × 10−8 S cm−1 vs. 1.2 × 10−11 S cm−1). The nature of the structure results in higher π-bond conjugation, and thus conductivity, in the direction of stacking. Although phthalocyanines are not directly considered in this review, it is instructive to include an example of single crystal conductivity measurements on a phthalocyanine. The phthalocyanine macrocycle (C32H18N8) is closely related to porphyrin in that it is based on four pyrrole-like moieties which are linked together to form a ring much like the porphyrin macrocycle, and thus it has some similar aromatic properties. The differences are that nitrogens are present instead of carbons at the meso positions (refer to Figure2b), and a benzene is fused to each pyrrole at the β positions, such that Crystals 2017, 7, 223 12 of 22

the four pyrroles become isoindoles (C8H7N). Schramm et al. [39] measured electrical conductivity of single crystals of nickel phthalocyanine iodide, a compound which is composed of stacks of planar nickel phthalocyanine molecules in a one-dimensional configuration parallel with adjacent chains of − −1 I3 molecules. A very high conductivity (250 to 650 S cm at room temperature) was observed in the stacking direction (c-axis), and the temperature dependence confirmed the conductivity as metallic. The carrier mean free path of the most highly conductive crystals was estimated to be ~60 Å, indicating the effectiveness of the stacked macrocyclic electron donor/linear iodine acceptor configuration in promotingCrystals 2017, the 7, 23 metallic conductivity along the stacking direction. 13 of 24

FigureFigure 14.14. ElectricalElectrical conductivityconductivity inin aa triclinictriclinic porphyrinporphyrin singlesingle crystalcrystal (after(after Ref.Ref. [[27]27],, withwith permissionpermission fromfrom TaylorTaylor and and Francis): Francis): ( a(a)) conductivity conductivity perpendicular perpendicular to to the thea a-axis;-axis; and and ( b(b)) parallel parallel with with the thea a-axis.-axis.

6.3.6.3. Thin-FilmThin-Film ConductivitiesConductivities InIn earlyearly work,work, LivshitsLivshits andand Blyumenfel’dBlyumenfel’d [[40]40] measuredmeasured thethe darkdark electricalelectrical conductivityconductivity ofof PP9PP9 thin-films,thin-films, andand reportedreported aa value value of of ~10 ~10−−1111 S cmcm−11 atat 20 20 °C.◦C. In In an attempt to improve conductivity,conductivity, KobayashiKobayashi et et al. al. [41 [41]] explored explored increasing increasing the the size size of the of porphyrinthe porphyrin ring complexring complex by extending by extending the length the oflength the substituents, of the substituents, thereby enlargingthereby enlarging the conjugation the conjugation system, and system, in principle and inlowering principle the lowering activation the energyactivation for conduction.energy for Theyconduction. prepared They 50 nm prepared thin films 50 of nm four thin compounds films of with four progressively compounds larger with ringprogressively complexes: larger zinc tetraphenylporphyrin, ring complexes: zinc zinc tetraphenylporphyrin, tetrabenzoporphyrin, zinc zinc tetrabenzoporphyrin, tetranaphthaloporphyrin, zinc andtetranaphthaloporphyrin, zinc tetraanthraporphyrin. and Thezinc measured tetraanthraporphyrin. room temperature The conductivities measured room were, respectively,temperature <10conductivities−11 S cm−1 ,were, 4 × 10 respectively,−10 S cm−1, < 310×−1110 S− cm4 S−1 cm, 4 −× 110, and−10 S 1cm×−110, 3− 7× S10 cm−4 S− 1cm, suggesting−1, and 1 × a10 ring−7 S cm size−1, limitsuggesting above whicha ring size the conductivitylimit above which no longer the conductivity increases, but no rather, longer decreases. increases, HOMO/LUMO but rather, decreases. gaps decreasedHOMO/LUMO as the gaps size ofdecreased the π-conjugation as the size systemof the π increased.-conjugation Since system the filmsincreased. were Since prepared the films in air, were the conductionprepared in mechanism air, the conduc is thoughttion mechanism to involve is oxygen thought doping to involve and electronoxygen doping transfer and from electron the porphyrin transfer ringfrom to the an oxygenporphyrin acceptor ring molecule.to an oxygen Jones acceptor et al. [42 molecule.] investigated Jones dark et conductivityal. [42] investigated in multilayer dark Langmuir–Blodgettconductivity in multilayer films and Langmuir showed–Blodgett that an Ag-porphyrin films and showed similar that in an structure Ag-porphyrin to metalated similar PP9 in hadstructure relatively to metalated high conductivity PP9 had relatively (2.5 × 10 high−3 S conductivity cm−1 at 18 ◦ C)(2.5 in × the10−3 planeS cm−1 of at the18 °C) film, in openingthe plane up of possibilitiesthe film, opening for device up applications.possibilities Thefor conductiondevice applications. was observed The toconduction be anisotropic, was andobserved two models to be wereanisotropic, suggested and for two the models conduction: were suggested (1) structural for defectsthe con induction: the films; (1) andstructural (2) a semiconductor-type defects in the films; depletionand (2) a semiconductor layer at the interface.-type depletion Analogs layer with atCu(II), the interface. Ni(II), Analogs Cd(II), and with Au(III) Cu(II), as Ni(II), the metal Cd(II), were and substantiallyAu(III) as the less metal conductive were substantially than the Ag less version. conductive than the Ag version. Liu et al. [43] observed an electrooptical memory effect in thin films of the photoconductor zinc- octakis(β-decoxyethyl) porphyrin (ZnODEP). Information, as trapped charge, can be written, read, and erased by simultaneous application of an electric field and a light pulse. During irradiation and the simultaneous application of an electric field, electron–hole pairs are generated. These become “frozen” within the films when irradiation is interrupted. Subsequent re-irradiation releases charge and generates a transient photocurrent. The structure of ZnODEP, a liquid crystal, is shown schematically in Figure 15.

Crystals 2017, 7, 223 13 of 22

Liu et al. [43] observed an electrooptical memory effect in thin films of the photoconductor zinc-octakis(β-decoxyethyl) porphyrin (ZnODEP). Information, as trapped charge, can be written, read, and erased by simultaneous application of an electric field and a light pulse. During irradiation and the simultaneous application of an electric field, electron–hole pairs are generated. These become “frozen” within the films when irradiation is interrupted. Subsequent re-irradiation releases charge and generates a transient photocurrent. The structure of ZnODEP, a liquid crystal, is shown schematically in FigureCrystals 15. 2017, 7, 23 14 of 24

FigureFigure 15. Structure 15. Structure of the of the photoconductor photoconductor ZnODEP,ZnODEP, after after [43] [43, with], with permission permission from the from American the American Association for the Advancement of Science. Association for the Advancement of Science. 6.4. Conductivity in Bulk-Polycrystalline Porphyrins 6.4. Conductivity in Bulk-Polycrystalline Porphyrins We use the combined term bulk-polycrystalline to clearly differentiate these materials from thin Wefilm use polycrystalline the combined or termsingle bulk-polycrystalline crystal materials. Wood to et clearly al. [44] differentiatestudied pressed these powders materials of eight from thin film polycrystallinedifferent porphyrins or single under crystalpressures materials. up to 90 kbar. Wood These et compounds al. [44] studied included: pressed (a) protoporphyrin powders of eight (PP9), and five related compounds, i.e., hematoporphyrin free base, hematoporphyrin different porphyrins under pressures up to 90 kbar. These compounds included: (a) protoporphyrin dihydrochloride, copper protoporphyrin, hemin, and hematin; and (b) etioporphyrin and vanadyl (PP9), andetiopo fiverphyrin related (relative compounds, to PP9, etioporphyrins i.e., hematoporphyrin have a methyl free and base, an ethyl hematoporphyrin group at the β positions dihydrochloride, of coppereach protoporphyrin, pyrrole). At low hemin,pressure, and the compounds hematin; andin Group (b) etioporphyrin(a) had higher conductivities and vanadyl thanetioporphyrin Group (relative(b), to which PP9, the etioporphyrins authors attribute have to overlap a methyl of the and carbonyls an ethyl in Group group (a). at Conductivities the β positions for the of hemin, each pyrrole). At lowhematin, pressure, and the vanadyl compounds etioporphyrin in Group show (a)marked had higherincreases conductivities with pressure, whereas than Group the others (b), do which the −10 authorsnot. attribute For example, to overlap it is reported of the that carbonyls the room in temperature Group (a). conductivity Conductivities of hemin for increases the hemin, from hematin,~10 and S cm−1 at ambient pressure to ~10−6 S cm−1 at 90 kbar. On the other hand, with increasing pressure for vanadyl etioporphyrin show marked increases with pressure, whereas the others do not. For example, PP9, hematoporphyrin free base, hematoporphyrin dihydrochloride, and copper protoporphyrin, −10 −1 it is reportedconductivities that theremained room relatively temperature constant conductivity at 10−11 to10 of−10 heminS cm−1. increasesA full exp fromlanation ~10 of theseS cm at −6 −1 ambientdifferences pressure was to not~10 givenS, as cm the crystalat 90 structures kbar. On were the not other adequately hand, known. with A increasing possibility could pressure for PP9, hematoporphyrininvolve oxidation/reduction free base, of the hematoporphyrin iron and vanadium,dihydrochloride, as the experiments were and not copper sealed. protoporphyrin, conductivitiesCollman remained et al. [45,46] relatively described constant a strategy at in 10 which−11 to10 conductive−10 S cmmetalloporphyrins−1. A full explanation with a linear of these differencesso-called was “shish not given,-kebab” as structure the crystal were structures designed by were combining not adequately three variables: known. the nature A possibility of the could macrocycle, the bridging group between macrocycles, and the transition metal in the macrocycle core. involveAccordingl oxidation/reductiony, they synthesized of the Fe(II/III) iron and-, Ru(II/III) vanadium,-, and Os(II/III) as the experiments-octaethylporphyrins were not bridged sealed. with Collmanpyrazine, et 4,4 al.′-bipyridine, [45,46] described or 1,4-diazabicyclo a strategy[2,2,2] in whichoctane, conductive and found that metalloporphyrins pressed powders of with the a linear so-calledpartially “shish-kebab” oxidized ruthenium structure and were osmium designed versions bywere combining highly conductive. three variables:Conductivity the increased nature of the macrocycle,in polymers the bridging relative to group monomers. between The macrocycles,conductivity was and found the to transition found to be metal affected in theby molecular macrocycle core. Accordingly,weight theyaccording synthesized to Os > Ru Fe(II/III)-, > Fe, possibly Ru(II/III)-, due to a progressive and Os(II/III)-octaethylporphyrins decrease in the size of the band bridged gaps with with increasing molecular weight. It was found that doping with oxidants, namely ferricenium pyrazine, 4,40-bipyridine, or 1,4-diazabicyclo[2,2,2]octane, and found that pressed powders of the hexafluorophosphate, nitrosyl hexafluorophosphate, or iodine, resulted in a substantial increase in partiallyconductivity. oxidized rutheniumESR and electrochemistry and osmium were versions used to were investigate highly the conductive. conduction Conductivity path, which was increased in polymersdetermined relative to be to influenced monomers. primarily The conductivityby the metal/ligand was backbone found to of found the structure, to be affected rather than by the molecular weightmacrocycle. according Room to Os temperature > Ru > Fe, conductivities possibly due ranged to a from progressive 10−11 S cm decrease−1 for undoped in the pyrazine size of Fe the band gaps withmonomers, increasing to 10 molecular−2 S cm−1 for doped weight. pyrazine It was-linked found Os that polymers. doping with oxidants, namely ferricenium hexafluorophosphate, nitrosyl hexafluorophosphate, or iodine, resulted in a substantial increase in 6.5. Ferroelectric Porphyrins conductivity. ESR and electrochemistry were used to investigate the conduction path, which was Suslick and Chen [47] and Chen and Suslick [48] have described work on linear “shish-kebab” porphyrins in which compounds with possible ferroelectric properties were synthesized. Of

Crystals 2017, 7, 223 14 of 22 determined to be inﬂuenced primarily by the metal/ligand backbone of the structure, rather than the macrocycle. Room temperature conductivities ranged from 10−11 S cm−1 for undoped pyrazine Fe monomers, to 10−2 S cm−1 for doped pyrazine-linked Os polymers.

6.5. Ferroelectric Porphyrins Suslick and Chen [47] and Chen and Suslick [48] have described work on linear “shish-kebab” porphyrinsCrystals 2017 in which, 7, 23 compounds with possible ferroelectric properties were synthesized. Of15 particular of 24 III interest is the polymeric compound [Fe (TPP)(ImPhO)]∞, where Im = imidazole and Ph = phenyl, and particular interest is the polymeric compound [FeIII(TPP)(ImPhO)]∞, where Im = imidazole and Ph = ImPhO is the composite axial ligand linking the macrocycles. This compound, which crystallizes in phenyl, and ImPhO is the composite axial ligand linking the macrocycles. This compound, which the non-centrosymmetric space group Pna2 , is expected to have a permanent dipole moment parallel crystallizes in the non-centrosymmetric space1 group Pna21, is expected to have a permanent dipole with themomentc-axis. parallel In this with compound the c-axis. the In ironthis compound is pulled out the of iron the is macrocycle pulled out of plane the macrocycle by bonding plane to the by axial ligand.bonding A small to the doming axial ligand. effect isA thesmall result. doming An effect applied is the electric result. An field applied could electric be expected field could to have be the effectexpected illustrated to ha inve Figure the effect 16 illustrated, in which in the Figure direction 16, in which of the the dipole direction can of be the reversed dipole can by be reversing reversed the electricby field.reversing In principle, the electric such field.materials In principle, can such be materials fabricated can into be fabricated electronic into oscillators, electronic high-frequency oscillators, filters,high converters,-frequency and filters, non-linear converters, capacitors. and non-linear capacitors.

FigureFigure 16. Schematic 16. Schematic representation representation of of dipole dipole momentmoment switching switching in ina ferroelectric a ferroelectric porphyrin porphyrin polymer. polymer. D = donor,D = donor, X = acceptor.X = acceptor. After After Ref. Ref. [48 [48]], with, withpermission permission from from Elsevier Elsevier..

7. Structural Examples of Complex Multiporphyrins 7. Structural Examples of Complex Multiporphyrins The long-term technological potential of porphyrins stems from the large number of ways Theindividual long-term molecules technological can be linked potential together of to porphyrins form larger stemsmolecular from structures, the large or numberarrays. In ofthe ways individualfollowing molecules discussion, can we be are linked guided together by the to work form of larger Aratani molecular and Osuka structures, [49], who or categorize arrays.d In the followingporphyrinic discussion, arrays based we are on the guided type of by linkage, the work both ofin terms Aratani of chemistry and Osuka and structure. [49], who We categorized do not porphyrinicinclude arraysporphyrinic based MOFs on the and type COFs, of linkage,which are both primarily in terms of interest of chemistry for their and sorption structure. properties, We do not includeand porphyrinic have been reviewed MOFs elsewhere and COFs, (e.g., which [6,50] are). The primarily compounds of in interest Table 5 for provide their examples sorption of properties, some and haveof the been multiporphyrin reviewed elsewhere structures (e.g.,that have [6,50 been]). The determined compounds by X in-ray Table diffraction5 provide and examples illustrate ofthe some of thestructural multiporphyrin features that structures influence that porphyrin have been molecular determined semiconducting by X-ray properties. diffraction In and Table illustrate 5, the the compounds are referenced to the CSD database and to the figures following the tables. The figures structural features that influence porphyrin molecular semiconducting properties. In Table5, the have been prepared using the Mercury crystal structure software suite [7]. It is noteworthy that the compounds are referenced to the CSD database and to the figures following the tables. The figures majority of the compounds in Table 5 are triclinic or monoclinic, with the highest symmetry havebelonging been prepared to the sole using orthorhombic the Mercury compound crystal structurein the table. software This is most suite likely [7]. due It is to noteworthy the complexity that the majorityof the of crystal the compounds packing required in Table for5 theare bulky, triclinic asymmetric or monoclinic, molecules with constituting the highest these symmetry compounds. belonging to the sole orthorhombic compound in the table. This is most likely due to the complexity of the crystal packing7.1. requiredEthyne-Bridged for the Multiporphyrins bulky, asymmetric molecules constituting these compounds. When the aromatic macrocycles are linked by appropriate ligands, conjugation occurs, resulting in extension of the π–π bonding [24]. As noted, this influences the electronic properties. Ethyne- bridges (based on the acetylene bond) can have an important modulating effect on electronic coupling when attached to either the meso- or the β- sites of the macrocycle. Typically, ethyne-bridged porphyrins show strong exitonic coupling, and large optical absorption bands in the visible and near- IR regions [49]. The selection of bonding sites can be important. For example, a structure with vinylene-meso-bridged porphyrin dimers exhibits a large electronic interaction [51], in contrast to β-

Crystals 2017, 7, 223 15 of 22

7.1. Ethyne-Bridged Multiporphyrins When the aromatic macrocycles are linked by appropriate ligands, conjugation occurs, resulting in extension of the π–π bonding [24]. As noted, this influences the electronic properties. Ethyne-bridges (based on the acetylene bond) can have an important modulating effect on electronic coupling when attached to either the meso- or the β- sites of the macrocycle. Typically, ethyne-bridged porphyrins show strong exitonic coupling, and large optical absorption bands in the visible and near-IR regions [49]. The selection of bonding sites can be important. For example, a structure with vinylene-meso-bridged porphyrin dimers exhibits a large electronic interaction [51], in contrast to β-bridged analogs, which allow twisting of the macrocycles out of conjugation. The vinylene-meso-bridged structure also shows a substantial non-linear optical effect. A longer linkage comprised of double β-to-β butadiyne (H–C≡C–C≡C–H) linkages is shown in Figure 17. This structure is relatively planar, with only 0.053 Å deviation from the average plane formed by the porphyrin cores and the bridges. The planarity results in significant enhancement of two-photon absorption properties. A cyclic ethyne-meso-bridged mixed-metal OEP porphyrin tetramer is shown in Figure 18. This structure contains Rh atoms at the center of the two laterally-positioned macrocycles which are bonded axially in both directions to a phosphine group and thence to ethyne bridges connected to zinc porphyrins at the bottom and top of the cyclic tetramer. The zincs of the upper and lower macrocycles are bonded across the interior of the tetramer to each other with pyridyl groups. Mixed metal porphyrins such as this can conceivably allow fine-tuning of redox and other useful properties by varying the identity of the metals.

7.2. Cofacial Dimers An important attribute of cofacial porphyrin dimers is the ability to capture other structural groups internally, in particular, fullerenes. These combinations can result in enhanced electron and energy transfer [52]. The ability to capture fullerenes depends on the metal atoms of the macrocycle. In Figure 19, a structure is illustrated with cofacial zinc porphyrins bound to an internal C60 molecule. The distance between the pyrrole carbon atoms of the porphyrins and the nearest carbons of the fullerene is shorter than the typical interlayer distance of graphite (3.354 Å), suggesting that a π interaction is present between the two moieties. A cofacial pillared zinc bisporphyrin is shown in Figure 20. This type of architecture, known to have a “Pac-Man” effect, is characterized by the ability under chemical stimulation to open and close its binding pocket by over 4 Å longitudinally. The rufﬂed porphyrin moieties at the top and bottom are not equivalent; the rotation of the porphyrins to maximize π interactions is prevented by the inﬂexibility of the dibenzofuran (C12H8O) spacer with regard to twisting in this direction.

Table 5. Crystallographic Properties of Selected Multiporphyrins.

CSD No. 635463 (Figure 17) 213358 (Figure 18) 150074 (Figure 19) 141402 (Figure 20) F. wt. 2947.42 5186.85 2449.64 1397.20 Formula C331H179N8O6Zn2 C386H291CI2N18P4Rh2Zn2 C168H124N8O4Zn2 — System triclinic monoclinic triclinic monoclinic Space gr. P −1 P 21/n P −1 C 2/c A (Å) 9.287 (5) 25.1838 (9) 13.6550 (11) 23.0808 (2) B (Å) 20.381 (5) 16.5456 (6) 15.3346 (12) 24.9458 (9) C (Å) 21.529 (5) 32.5020 (11) 16.2865 (13) 13.4593 (5) α (deg.) 77.218 (5) 90.00 109.187 (2) 90.0 β (deg.) 80.967 (5) 93.902 (2) 107.904 (2) 110.503 (2) γ (deg.) 80.500 (5) 90.00 97.732 (3) 90.0 V (Å3) 3889 (2) 13511.6 (8) 2957.7 (4) 7549.5 (4) Z 1 2 1 4 ρ (g/cm3) 1.259 1.275 1.375 1.229 Temp. 90 (2) 150 (2) 163.2 183 (2) Ref. [7,53][7,54][7,55][7,56] Crystals 2017, 7, 23 17 of 24

Crystals 2017, 7, 23 17 of 24 CrystalsCSD 2017No. , 7,728863 223 (Figure 21) 685451 (Figure 22) 173782 (Figure 23) 712832 (Figure16 24) of 22 F. wt. 1310.84 3308.31 1825.03 --- CSD No. 728863 (Figure 21) 685451 (Figure 22) 173782 (Figure 23) 712832 (Figure 24) Formula C82H51C1Cu2N8 C200H216C17N12Ni3S3 C120H112N10Zn2 --- F. wt. 1310.84 3308.31 1825.03 --- System orthorhombic triclinicTable 5. Cont. monoclinic monoclinic Formula C82H51C1Cu2N8 C200H216C17N12Ni3S3 C120H112N10Zn2 --- Space gr. Pbca P−1 P21/n C 2/c System orthorhombic triclinic monoclinic monoclinic A (ÅCSD) No.26.455 728863 (5) (Figure 21)18.846 685451 (5) (Figure 22)13.7485 173782 (Figure(2) 23) 71283238.482 (Figure (6) 24 ) Space gr. Pbca P−1 P21/n C 2/c B (Å) 13.095 (5) 27.950 (5) 18.3200 (3) 27.205 (5) A (Å)F. wt. 26.455 (5)1310.84 18.846 (5) 3308.31 13.7485 1825.03 (2) 38.482 — (6) C (ÅFormula) 35.226C (5)H C Cu N 27.989C H(5) C N Ni S 40.8739C H (7)N Zn 17.098— (5) B (Å) 13.09582 (5)51 1 2 8 27.950200 (5)216 17 12 3 3 18.3200120 112 (3)10 2 27.205 (5) α (deg.)System 90.000 orthorhombic(5) 112.376 (5) triclinic monoclinic90 monoclinic90.0 C (ÅSpace) gr. 35.226 (5) Pbca27.989 (5) P−1 40.8739P2 (7)/n 17.098 C 2/c (5) β (deg.) 90.000 (5) 103.171 (5) 84.1947 (3)1 111.967 (5) α (deg.)A (Å) 90.000 (5)26.455 (5)112.376 (5) 18.846 (5) 13.748590 (2) 38.48290.0 (6) γ (deg.) 90.000 (5) 103.126 (5) 90 90.0 β (deg.)B (Å) 90.000 (5)13.095 (5)103.171 (5) 27.950 (5)84.1947 18.3200 (3) (3) 27.205111.967 (5) (5) V (Å3) 12203 (5) 12445 (5) 10242.2 (2) 15306.3 γ (deg.)C (Å) 90.000 (5)35.226 (5)103.126 (5) 27.989 (5) 40.873990 (7) 17.09890.0 (5) Z α (deg.) 8 90.000 (5)2 112.376 (5)8 90 90.0--- V (Å3) 12203 (5) 12445 (5) 10242.2 (2) 15306.3 ρ (g/cmβ 3(deg.)) 1.427 90.000 (5)0.883 103.171 (5)2.367 84.1947 (3) 111.967---(5) Z (deg.) 8 2 8 --- Temp.γ 90 (2) 90.000 (5)90 (2) 103.126 (5)296.2 90 90.0--- ρ (g/cmV3(Å) 3) 1.427 12203 (5)0.883 12445 (5)2.367 10242.2 (2) 15306.3--- Ref. [7,57] [7,58] [7,59] [7,60] Temp. Z 90 (2) 890 (2) 2296.28 —--- 3 Ref.ρ (g/cm ) [7,57] 1.427[7,58] 0.883[7,59] 2.367 —[7,60] Temp. 90 (2) 90 (2) 296.2 — Ref. [7,57][7,58][7,59][7,60]

Figure 17. A doubly β-to-β 1,3-butadiyne-bridged TPP-based dimer, from data in [7,53]. Each zinc has oneFigure axially 17. Abound doubly isopropanolβ-to-β 1,3-butadiyne-bridged ligand. Each phenyl TPP-based ring has two dimer, tert- frombutyl datagroups in [7at,53 3,5]. Eachpositions. zinc Figure 17. A doubly β-to-β 1,3-butadiyne-bridged TPP-based dimer, from data in [7,53]. Each zinc has Solvateshas one axiallyand peripheral bound isopropanol groups are ligand.omitted Each in this phenyl and the ring following has two diagramstert-butyl as groups necessary at 3,5 for positions. clarity. one axially bound isopropanol ligand. Each phenyl ring has two tert-butyl groups at 3,5 positions. DarkSolvates blue and = Zn. peripheral groups are omitted in this and the following diagrams as necessary for clarity. DarkSolvates blue and = Zn. peripheral groups are omitted in this and the following diagrams as necessary for clarity. Dark blue = Zn.

Figure 18. 18. MixedMixed me metalloporphyrintalloporphyrin cage, cage, from from data data in [7,54] in [7., 54Dark]. Dark blue = blue Zn, =dark Zn, green dark = green Rh, orange = Rh, =orange P. Cl is = not P. Cl shown. is not shown. Figure 18. Mixed metalloporphyrin cage, from data in [7,54]. Dark blue = Zn, dark green = Rh, orange = P. Cl is not shown.

Crystals 2017, 7, 23 18 of 24 Crystals 2017, 7, 223 17 of 22 Crystals 2017, 7, 23 18 of 24

Figure 19. Inclusion complex of C60 in cyclic Zn-porphyin dimer, from data in [7,55]. Dark blue = Zn, Figure 19. Figure 19. Inclusion complex of C60 in cyclic Zn-porphyin dimer, from data in [7,55]. Dark blue = Zn, redInclusion = O. complex of C60 in cyclic Zn-porphyin dimer, from data in [7,55]. Dark blue = Zn, red = O. red = O.

Figure 20. Dibenzofuran-bridged cofacial bisporphyrin, from data in [7,56]. Dark blue = Zn, red = O. Figure 20.FigureDibenzofuran-bridged 20. Dibenzofuran-bridged cofacial cofacial bisporphyrin, bisporphyrin, from from data data in [7,56] in [.7 Dark,56]. blue Dark = Zn, blue red == O. Zn, red = O. 7.3. Directly-Linked Porphyrins. 7.3. Directly-Linked Porphyrins. 7.3. Directly-LinkedIn many Porphyrins. directly-linked architectures, the neighboring porphyrins are linked together by meso- In many directly-linked architectures, the neighboring porphyrins are linked together by meso- In manymeso directly-linked bonds. This arrangement architectures, results the in neighboringhigher molecular porphyrins symmetry and are larger, linked more together regular by meso-meso mesoelectronic bonds. interactions,This arrangement due to theresults continuous in higher porphyrinic molecular π-electronic symmetry network. and larger, Such architectures more regular bonds. Thiselectroniccan arrangement lead interactions, to 3-D arrays results due useful to in the higherin continuous light molecularharvesting porphyrinic [61] symmetry. Furthermore, π-electronic and network.individu larger,al Such moredirectly architectures- regularlinked electronic interactions,cansubunits duelead toto can the3- Dbe continuous combinedarrays useful by linking porphyrinicin light with harvesting spacersπ-electronic to form [61] .larger Furthermore, network.units. Large individu porphyrin Such architecturesal directlywheels have-linked can lead to 3-D arrayssubunits usefulbeen prepared can in be light combined by harvestingcombining by linking directly [61 with].-linked Furthermore, spacers porphyrin to form subunits larger individual units. via mesoLarge directly-linked-meso porphyrin-linked phenylene wheels subunits have can be beenspacers prepared [62]. Inby addition combining to direct directly linkage-linked by mesoporphyrin-meso bonding, subunits linkages via meso based-meso on- linkedβ-β, and phenylene meso-β combinedspacers bycombinations linking [62]. In with additionhave spacers been to reported. direct to form linkage An largerexample by meso units.of the-meso latter Large bonding, with porphyrin two linkages porphyrins based wheels is shownon haveβ-β ,in and Figure been meso prepared-β by combiningcombinations directly-linked21. The mean have planes been porphyrin of reported.the macrocycles subunits An example are viaalmost ofmeso the orthogonal -lattermeso with-linked due two to theporphyrins phenylene steric requirements is spacersshown inof [ Figure62the]. In addition bonding, and therefore it is expected that the inter-porphyrin conjugation is limited for this particular to direct linkage21. The mean by meso planes-meso of thebonding, macrocycles linkages are almost based orthogonal on β -dueβ, andto themeso steric-β requirementscombinations of the have been architecture. reported. Anbonding, example and therefore of the latterit is expected with twothat the porphyrins inter-porphyrin is shown conjugation in Figure is limited 21 .for The this mean particular planes of the architecture. macrocycles are almost orthogonal due to the steric requirements of the bonding, and therefore it is expected thatCrystals the 2017 inter-porphyrin, 7, 23 conjugation is limited for this particular architecture.19 of 24

Figure 21.FigureDirectly 21. Directlymeso meso-β linked-β linked Cu-diporphyrin, Cu-diporphyrin, from from data datain [7,57] in. [Red7,57 =]. Cu. Red = Cu.

7.4. Multiply-Linked Structures An important benefit of multiple linkages is the enhanced π-conjugation that can be achieved [35]. In general, this results from the co-planarity enforced by multiple bonding of the macrocycles, or in some cases enforced by direct fusion of the pyrroles. Effects such as narrowing of the HOMO- LUMO gaps have been observed, as well as high two-photon absorptions. An example of a multiply- linked structure is the β-thienylene-bridged nickel triporphyrin in Figure 22. In this triangular porphyrin trimer, the porphyrin cores are ruffled, with a maximum carbon displacement from the mean plane of 0.915 Å. The thiophenes (C4H4S) have tilts of 26.6° to 36.1° relative to the adjacent pyrroles. The structural distortion caused by the double-bridging of the macrocycles is reflected by the perturbation of the optical absorption spectrum.

Figure 22. Thienylene-bridged triangular Ni-porphyrin tetramer, from data in [7,58]. Green = Ni, yellow = S.

Crystals 2017, 7, 23 19 of 24

Crystals 2017, 7, 223 18 of 22

7.4. Multiply-Linked Structures An important benefitFigure of 21. multiple Directly meso linkages-β linked Cu is-diporphyrin, the enhanced from dataπ-conjugation in [7,57]. Red = Cu. that can be achieved [35]. In general, this7.4. Multiply results-Linked from Structures the co-planarity enforced by multiple bonding of the macrocycles, or in some cases enforcedAn important by direct benefit fusion of multiple of the linkages pyrroles. is the Effectsenhanced such π-conjugation as narrowing that can be of achieved the HOMO-LUMO gaps have been[35]. In observed, general, this as results well from ashigh the co two-photon-planarity enforced absorptions. by multiple bonding An example of the macrocycles, of a multiply-linked structure is theor in βsome-thienylene-bridged cases enforced by direct nickel fusion of triporphyrin the pyrroles. Effects in Figure such as narrow22. Ining this of the triangular HOMO- porphyrin LUMO gaps have been observed, as well as high two-photon absorptions. An example of a multiply- trimer, the porphyrinlinked structure cores is the are β- ruffled,thienylene with-bridged a maximumnickel triporphyrin carbon in displacementFigure 22. In this fromtriangular the mean plane porphyrin trimer, the porphyrin cores are ruffled, with a ◦maximum carbon◦ displacement from the of 0.915 Å. The thiophenes (C4H4S) have tilts of 26.6 to 36.1 relative to the adjacent pyrroles. The structuralmean distortion plane of 0.915 caused Å. The thiophenes by the double-bridging(C4H4S) have tilts of 26.6° of to the 36.1 macrocycles° relative to the isadjacent reflected by the pyrroles. The structural distortion caused by the double-bridging of the macrocycles is reflected by perturbationthe of perturbation the optical of absorptionthe optical absorption spectrum. spectrum.

Figure 22. Thienylene-bridgedFigure 22. Thienylene- triangularbridged triangular Ni-porphyrin Ni-porphyrin tetramer, tetramer, from from data data inin [[7,58]7,58].. Green Green = =Ni, Ni, yellow = S. yellow = S. 7.5. Self-Assembled Arrays Crystals 2017, 7, 23 20 of 24 For efﬁcient7.5. Self-Assembled synthesis Arrays of more complicated porphyrins, self-sorting methods that utilize mixtures ofcomponents with desired structural attributes that combine spontaneously have proved For efficient synthesis of more complicated porphyrins, self-sorting methods that utilize beneﬁcial.mixtures Tsuda of et components al. [59] used with this desired approach structural to attributes synthesize that combine 3-D porphyrin spontaneously boxes. have Theproved structure of 5-p-pyridyl-15-(3,5-di-beneficial. Tsudatert et -butylphenyl)al. [59] used this approach Zn(II) porphyrin to synthesize is 3-D shown porphyrin in Figureboxes. The 23 .structure The inter-porphyrin of 5- electronicp interactions-pyridyl-15-(3,5 in-di this-tert structure-butylphenyl) are Zn(II) decidedly porphyrin weak is shown by comparison in Figure 23.with The inter other-porphyrinmeso-meso linked electronic interactions in this structure are decidedly weak by comparison with other meso-meso diporphyrins, due to the perpendicular orientations of the macrocycles. linked diporphyrins, due to the perpendicular orientations of the macrocycles.

FigureFigur 23. Porphyrine 23. Porphyrin box box structure, structure, from data data in [7,59] in [7., 59Dark]. Darkblue = blueZn. = Zn.

7.6. Metal-Bridged Arrays The linking of porphyrins by metal bridges adds another dimension to their structural diversity. A Pt(II)-bridged cofacial nickel diporphyrin is shown in Figure 24. The Pt (II) is square-planar- coordinated. The macrocycles have a ruffled configuration, with mean plane deviations of 0.323 Å for the nitrogens. The compound has an inherent helical chirality. Yoshida et al. [63] have explored the reactivity of related Pt (II) and Pt (IV) porphyrinic compounds, which show interesting redox properties. Potentially, there are applications in catalysis.

Figure 24. Pt (II)-linked Ni-porphyrin dimer, from data in [7,60]. Green = Ni, white = Pt.

8. Conclusions Many of the promising materials applications for porphyrins will rely on optimization of solid state semiconducting properties, which in turn depend on the underlying crystallography. Of foremost importance are the planarity of the macrocycle and the conjugation of π-bonding through

Crystals 2017, 7, 23 20 of 24

7.5. Self-Assembled Arrays For efficient synthesis of more complicated porphyrins, self-sorting methods that utilize mixtures of components with desired structural attributes that combine spontaneously have proved beneficial. Tsuda et al. [59] used this approach to synthesize 3-D porphyrin boxes. The structure of 5- p-pyridyl-15-(3,5-di-tert-butylphenyl) Zn(II) porphyrin is shown in Figure 23. The inter-porphyrin electronic interactions in this structure are decidedly weak by comparison with other meso-meso linked diporphyrins, due to the perpendicular orientations of the macrocycles.

Crystals 2017, 7, 223 19 of 22 Figure 23. Porphyrin box structure, from data in [7,59]. Dark blue = Zn.

77.6..6. Metal Metal-Bridged-Bridged Arrays Arrays TheThe linking linking of ofporphyrins porphyrins by metal by metal bridges bridges adds another adds another dimension dimension to their structural to their diversity. structural Adiversity. Pt(II)-bridged A Pt(II)-bridged cofacial nickel cofacial diporphyrin nickel diporphyrinis shown in Figure is shown 24. inThe Figure Pt (II) 24 is. square The Pt-planar (II) is- coordinated.square-planar-coordinated. The macrocycles The have macrocycles a ruffled have configuration, a rufﬂed conﬁguration, with mean plane with meandeviations plane of deviations 0.323 Å forof 0.323the nitrogens. Å for the The nitrogens. compound The compoundhas an inherent has an helical inherent chirality. helical Yoshida chirality. et Yoshidaal. [63] have et al. explored [63] have theexplored reactivity the reactivityof related of Pt related (II) and Pt Pt (II) (IV) and porphyrinic Pt (IV) porphyrinic compounds, compounds, which show which interesting show interesting redox properties.redox properties. Potentially, Potentially, there are there applications are applications in catalysis. in catalysis.

Figure 24. Pt (II)-linked Ni-porphyrin dimer, from data in [7,60]. Green = Ni, white = Pt. Figure 24. Pt (II)-linked Ni-porphyrin dimer, from data in [7,60]. Green = Ni, white = Pt. 8. Conclusions 8. Conclusions Many of the promising materials applications for porphyrins will rely on optimization of solid Many of the promising materials applications for porphyrins will rely on optimization of solid state semiconducting properties, which in turn depend on the underlying crystallography. Of foremost state semiconducting properties, which in turn depend on the underlying crystallography. Of importance are the planarity of the macrocycle and the conjugation of π-bonding through the proximity foremost importance are the planarity of the macrocycle and the conjugation of π-bonding through of adjacent macrocycles or the interconnection of macrocycles via appropriate covalently-bonded ligands. For some structures, conductivity also depends on the presence of electron donor/acceptor pairs. We have described for selected examples how semiconducting properties originate at the molecular level. Starting with the basic porphine parent molecule, we have shown how structures can be modified by addition of substituents and by metalation of the macrocycle core. Complex 3-D multiporphyrins have been illustrated which show interesting and varied topologies. A major need is further understanding of the relation between semiconducting properties at the molecular level and at the macroscopic level (single crystals, and polycrystalline materials). Most of the porphines, porphyrins, and multiporphyrins discussed have relatively low symmetry (triclinic or monoclinic), due to the complexity of the crystal packing required for the porphyrin moieties. Given the large number of possible metal insertions, substituent attachments, and linkages, the number of unstudied crystallographic combinations appears to be very large; obviously, not all will have useful properties. Devising efficient synthesis methods for processing of sufficient quantities to measure properties and evaluate potential applications continues to be a challenge, but one with major potential overall payoff.

Author Contributions: L. C. researched the literature, organized the results, and was the principal author of the paper. G. B. provided oversight on the structural organic chemistry and assisted with writing the paper. W. W.-N. assisted with writing the paper and prepared the majority of the illustrations. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Porphyrin. Available online: https://en.wikipedia.org/w/index.php?title=Porphyrin&oldid=781762373 (accessed on 10 June 2017). 2. Küster, W. Beitrage zur Kenntnis des Bilirubins und Hämins. Hoppe Seyler’s Z. Physiol. Chem. 1912, 82, 463–483. [CrossRef] Crystals 2017, 7, 223 20 of 22

3. Kadish, W.; Smith, K.M.; Guilard, R. (Eds.) Handbook of Porphyrin Science; World Scientiﬁc Publishing Co.: Singapore, 2010–2016; Volume 44. 4. Moore, M.W. An historical introduction to porphyrin and chlorophyll synthesis. In Tetrapyrroles: Birth, Life, and Death; Warren, M., Smith, A., Eds.; Landes Bioscience and Springer Science+Business Media: Berlin, Germany, 2009; pp. 1–28. 5. Suslick, K.S.; Rakow, N.A.; Kosal, M.E.; Chou, J.H. The materials chemistry of porphyrins and metalloporphyrins. J. Porphyr. Phthalocyanines 2000, 4, 407–413. [CrossRef] 6. Cook, L.P.; Wong-Ng, W.; Brewer, G. Porphyrin-based chemistry for carbon capture and sequestration. In Advances in Materials Science for Environmental and Energy Technologies V; Ohji, T., Kanakhala, R., Matyáš, J., Marijooran, N., Pickrell, G., Wong-Ng, W., Eds.; Wiley & Sons: Hoboken, NJ, USA, 2016; pp. 201–221. 7. Cambridge Crystallographic Data Center. Available online: https://www.ccdc.cam.ac.uk/ (accessed on 31 March 2017). 8. International Union of Pure and Applied Chemistry. Available online: http://www.chem.qmul.ac.uk/ iupac/tetrapyrrole/ (accessed on 31 March 2017). 9. Saltsman, I.; Goldberg, I.; Balasz, Y.; Gross, Z. Porphine and pyrrole-substituted porphyrin from cyclocondensation of tripyrrane with mono-substituted pyrolles. Tetrahedron Lett. 2007, 48, 239–244. [CrossRef] 10. Devillers, C.H.; Fleurat-Lessard, P.; Lucas, D. “Porphine,” the fully unsubstituted porphyrin: A comprehensive overview. In Handbook of Porphyrin Science; Kadish, W., Smith, K.M., Guilard, R., Eds.; World Scientiﬁc Publishing Co.: Singapore, 2014; Volume 37, pp. 75–231. 11. Kaduk, J.A.; Wong-Ng, W.; Cook, L.P.; Chakraborty, B.; Lapidus, S.H.; Ribaud, L.; Brewer, G. Synchrotron

X-ray investigation of α-chlorohemin, C34H32ClFeN4O4, an Fe-porphyrin. Solid State Sci. 2016, 53, 63–70. [CrossRef] 12. Silvers, S.J.; Tulinsky, A. The crystal and molecular structure of triclinic tetraphenyl porphyrin. J. Am. Chem. Soc. 1967, 89, 3331–3337. [CrossRef][PubMed] 13. Codding, P.W.; Tulinsky, A. Structure of tetra-n-propylporphine. Average structure for the free base macrocycle from three independent determinations. J. Am. Chem. Soc. 1972, 94, 4151–4157. [CrossRef] [PubMed] 14. Lauher, J.W.; Ibers, J.A. Structure of Ocaethylporphyrin. A comparison with other free base porphyrins. J. Am. Chem. Soc. 1973, 95, 5148–5152. [CrossRef][PubMed] 15. Caughey, W.S.; Ibers, J.A. Crystal and molecular structure of the free base porphyrin, protoporphyrin IX dimethyl ester. J. Am. Chem. Soc. 1977, 99, 6639–6645. [CrossRef][PubMed] 16. Jentzen, W.; Trowska-Tyrk, I.; Scheidt, W.R.; Shetnutt, J.A. Planar solid-state and solution structures of (porphinato) nickel(II) as determined by X-ray diffraction and Raman spectroscopy. Inorg. Chem. 1996, 35, 3559–3567. [CrossRef] 17. Devillers, C.H.; Dime, A.K.D.; Cattey, H.; Lucas, D. Crystallographic, spectroscopic and electrochemical characterization of pyridine adducts of magnesium(II) and zinc(II) porphine complexes. C. R. Chim. 2013, 16, 540–549. [CrossRef] 18. Mavridis, A.; Tulinsky, A. Crystal and molecular structure of dimethoxy-porphinatogermanium (IV). Inorg. Chem. 1976, 15, 2723–2727. [CrossRef] 19. Fleischer, E.B. The structure of porphyrins and metalloporphyrins. Acc. Chem. Res. 1970, 3, 105–112. [CrossRef] 20. Arnold, D.P.; Goh, M.S.; Harper, S.R.; McMurtie, J.C. Oligoporphyrins with one- and two-atom covalent bridges: Synthesis, and reactivity, solid-state, solution and electronic structures, spectroscopy and applications. In Handbook of Porphyrin Science; Kadish, W., Smith, K.M., Guilard, R., Eds.; World Scientiﬁc Publishing Co.: Singapore, 2014; Volume 31, pp. 277–302. 21. Esdaile, L.J.; Jensen, P.; McMurtie, J.C.; Arnold, D.P. Azoporphyrin: The porphrin analogue of azobenzene. Angew. Chem. Int. Ed. 2007, 46, 2090–2093. [CrossRef][PubMed] 22. Arnold, D.; Johnson, A.W.; Winter, M. Bis-porphyrin derivatives. Part 1. Reaction of meso-hydroxymethyl- porphyinatometal derivatives with acids. J. Chem. Soc. Perkin Trans. 1977, 14, 1643–1647. [CrossRef] 23. Senge, M.; Vicente, M.G.; Gerzevske, K.R.; Forsyth, T.P.; Smith, K.M. Models for the photosynthetic reaction center: Preparation, spectroscopy, and crystal and molecular structures of cofacial bisporphyrins linked by cis-1-2- and trans-1-2-ethene bridges and of 1,1-carbinol-bridged bisporphyrins. Inorg. Chem. 1994, 33, 5625–5638. [CrossRef] Crystals 2017, 7, 223 21 of 22

24. Crossley, M.J.; Burn, P.L. An approach to porphyrin-based molecular wires: Synthesis of a bis(porphyrin)tetraone and its conversion to a linearly conjugated tetrakisporphyrin system. J. Chem. Soc. Chem. Commun. 1991, 21, 1569–1571. [CrossRef] 25. Perlovich, G.L. Thermodynamics of porphyrin sublimation. J. Porphyr. Phthalocyan. 2000, 4, 699–706. [CrossRef] 26. Jurow, M.; Schuckman, A.; Batteas, J.D.; Drain, C.M. Porphyrins as molecular components of functional devices. Coord. Chem. Rev. 2010, 254, 2297–2310. [CrossRef][PubMed] 27. Aüwarter, W.; Écija, D.; Klappenberger, F.; Varth, J.V. Porphyrins at interfaces. Nat. Chem. 2015, 7, 105–119. [CrossRef][PubMed] 28. Kubatkin, S.; Danilov, A.; Hjort, M.; Cornil, J.; Brédas, J.-L.; Stuhr-Hanson, N.; Hedegard, P.; Bjornholm, T. Single-electron transistor of a single organic molecule with access to several redox states. Nature 2003, 425, 698–701. [CrossRef][PubMed] 29. Averin, D.V.; Likharev, K.K. Single electronics: A correlated transfer of single electrons and Cooper pairs in systems of small tunnel junctions. In Mesoscopic Phenomena in Solids; Altshuler, B.L., Lee, P.A., Webb, R.A., Eds.; Elsevier: Amsterdam, The Netherlands, 1991; pp. 173–270. 30. Wang, N.; Liu, H.; Zhan, J.; Cui, Y.; Xu, Z.; Ye, Y.; Kiguchi, M.; Murakoshi, K. Theoretical investigation on the electron transport path through the porphyrin molecules and chemisorption of CO. J. Phys. Chem. C 2009, 113, 7416–7423. [CrossRef] 31. Galloni, P.; Vecchi, A.; Coletti, A.; Gatto, E.; Floris, B.; Conte, V. Porphyrins as active components for electrochemical and photoelectrochemical devices. In Handbook of Porphyrin Science; Kadish, W., Smith, K.M., Guilard, R., Eds.; World Scientific Publishing Co.: Singapore, 2014; Volume 33, pp. 225–415. 32. Reimers, J.R.; Lu, T.X.; Crossley, M.J.; Hush, N.S. Molecular electronic properties of fused rigid porphyrin-oligomer molecular wires. Nanotechnology 1996, 7, 424–429. [CrossRef] 33. Aviram, A. A view of the future of molecular electronics. Mol. Cryst. Liq. Cryst. 1993, 234, 13–28. [CrossRef] 34. Yoon, D.H.; Lee, S.B.; Yoo, K.-H.; Kim, J.; Lim, J.K.; Aratani, N.; Tsuda, A.; Osuka, A.; Kim, D. Electrical conduction through linear porphyrin arrays. J. Am. Chem. Soc. 2003, 125, 11062–11064. [CrossRef][PubMed] 35. Sedghi, C.; Esdaile, L.J.; Anderson, H.L.; Martin, S.; Bethell, D.; Higgins, S.J.; Nichols, R.J. Comparison of the conductance of three types of porphyrin-based molecular wires: β,meso,β-fused tapes, meso-butdiyne-linked and twisted meso-meso linked oligomers. Adv. Mater. 2012, 24, 653–657. [CrossRef][PubMed] 36. Wang, Z.; Medforth, C.J.; Shelnutt, J.A. Porphyrin nanotubes by ionic self-assembly. J. Am. Chem. Soc. 2004, 126, 15954–15955. [CrossRef][PubMed] 37. Li, L.L.; Yang, C.-J.; Chen, W.-H.; Lin, K.-J. Towards the development of electrical conduction and lithium-ion transport in a tetragonal porphyrin wire. Angew. Chem. Int. Ed. 2003, 42, 1505–1508. [CrossRef][PubMed] 38. Chen, Y.C.; Lee, L.L.; Lin, K.J. Electrical and optical properties of porphyrin single crystals. J. Macromol. Sci. Part B Phys. 2008, 47, 955–966. [CrossRef] 39. Schramm, C.J.; Stojakovic, D.R.; Hoffman, B.M.; Maarks, T.J. New low-dimensional molecular metals: Single-crystal electrical conductivity of nickel phthalocyanine iodide. Science 1978, 200, 47–48. [CrossRef] [PubMed] 40. Livshits, V.A.; Blyumenfel’d, L.A. Semiconductor properties of porphyrins. J. Struct. Chem. 1968, 8, 383–388. [CrossRef] 41. Kobayashi, N.; Nevin, W.A.; Mizunuma, S.; Awaji, H.; Yamaguchi, M. Ring-expanded porphyrins as an approach towards highly conductive molecular semiconductors. Chem. Phys. Lett. 1993, 205, 51–54. [CrossRef] 42. Jones, R.; Tredgold, R.H.; Hoorfar, A. Electrical conductivity in Langmuir-Blodgett films of porphyrins: In-plane and thorough-the-film studies. Thin Solid Films 1984, 113, 115–128. [CrossRef] 43. Lui, C.-Y.; Pan, H.-L.; Fox, M.A.; Bard, A.J. High-density nanosecond charge trapping in thin films of the photoconductor ZnODEP. Science 1993, 261, 897–899. 44. Wood, D.W.; Andersen, T.N.; Eyring, H. Electrical properties of some porphyrins under high pressure. J. Phys. Chem. 1966, 70, 360–366. [CrossRef] 45. Collman, J.P.; McDevitt, J.T.; Yee, G.T.; Leidner, C.R.; McCullough, L.G.; Little, W.A.; Torrance, J.B. Conductive polymers derived from iron, ruthenium, and osmium metalloporphyrins: The shish-kebab approach. Proc. Natl. Acad. Sci. USA 1986, 83, 4581–4585. [CrossRef][PubMed] 46. Collman, J.P.; McDevitt, J.T.; Leidner, C.R.; Yee, G.T.; Torrance, J.B.; Little, W.A. Synthetic, electrochemical, optical and conductivity studies of coordination polymers of iron, ruthenium, and osmium octaethylporphyrin. J. Am. Chem. Soc. 1987, 109, 4606–4614. [CrossRef] Crystals 2017, 7, 223 22 of 22

47. Suslick, K.S.; Chen, C.-T. Polymeric metalloporphyrins for field responsive materials. Polym. Mater. Sci. Eng. 1990, 63, 272–278. 48. Chen, C.-T.; Suslick, K.S. One-dimensional coordination polymers: Applications to materials science. Coord. Chem. Rev. 1993, 128, 293–322. [CrossRef] 49. Aratani, N.; Osuka, A. Synthetic strategies toward multiporphyrinic architectures. In Handbook of Porphyrin Science; Kadish, W., Smith, K.M., Guilard, R., Eds.; World Scientific Publishing Co.: Singapore, 2010; Volume 1, pp. 1–132. 50. Gao, W.-Y.; Chrzanowski, M.; Ma, S. Metal-metalloporphyrinframeworks: A resurging class of functional materials. Chem. Soc. Rev. 2014, 43, 5841–5866. [CrossRef][PubMed] 51. Frampton, M.J.; Akda, H.; Cowley, A.R.; Rogers, J.E.; Slagle, J.E.; Fleitz, P.A.; Drobizhev, M.; Rebane, A.; Anderson, H.L. Synthesis, crystal structure, and nonlinear optical behavior of β-unsubstituted meso-meso-E-vinylene-linked porphyrin dimers. Org. Lett. 2005, 7, 5365–5368. [CrossRef][PubMed] 52. Imahori, H.; Fukuzumi, S. Porphyrin- and fullerene-based molecular photovoltaic devices. Adv. Funct. Mater. 2004, 14, 525–536. [CrossRef] 53. Hisaki, I.; Hiroto, S.; Kim, K.S.; Noh, S.B.; Kim, D.; Shinokubo, H.; Osuka, A. Synthesis of doubly β-to-β 1,3-butadiene-bridged diporphyrins: Enforced planar structures and large two-photon absorption cross sections. Angew. Chem. Int. Ed. 2007, 46, 5125–5128. [CrossRef][PubMed] 54. Stulz, E.; Scott, S.M.; Bond, A.D.; Teat, S.J.; Sanders, J.M. Selection and amplification of mixed-metal porphyrin cages from dynamic combinatorial libraries. Chem. Eur. J. 2003, 6039–6048. [CrossRef][PubMed] 55. Zheng, J.-Y.; Tashiro, K.; Hirabayashi, Y.; Kinbara, K.; Saigo, K.; Aida, T.; Sakamoto, S.; Yamaguchi, K. Cyclic dimers of metalloporphyrins as tunable hosts for fullerenes: A remarkable effect of rhodium(III). Angew. Chem. Int. Ed. 2001, 40, 1858–1861. [CrossRef] 56. Deng, Y.; Chang, C.J.; Nocera, D.G. Direct observation of “Pac-Man” effect from dibenzofuran-bridged cofacial bisporphyrins. J. Am. Chem. Soc. 2000, 122, 410–411. [CrossRef] 57. Tokuji, S.; Yurino, T.; Aratani, N.; Shinokubo, H.; Osuka, A. Palladium-catalyzed dimerization of meso-bromoporphyrins: Highly regioselective meso-β coupling through unprecedented remote C-H bond cleavage. Chem. Eur. J. 2009, 15, 12208–12211. [CrossRef][PubMed] 58. Song, J.; Jang, S.Y.; Yamaguchi, S.; Sankar, J.; Hiroto, S.; Aratani, N.; Shin, J.Y.; Easwaramoorthi, A.; Kim, K.S.; Kim, D.; et al. 2,5-thienylene-bridged triangular and linear porphyrin trimers. Angew. Chem. Int. Ed. 2008, 47, 6004–6007. [CrossRef][PubMed] 59. Tsuda, A.; Nakamura, T.; Sakamoto, S.; Yamaguchi, K.; Osuka, A. A self-assembled porphyrin box from meso-meso-linked bis [5-p-pyridyl-15-(3,5-di-octyloxyphenyl)porphyrinato zinc(II)]. Angew. Chem. Int. Ed. 2002, 41, 2817–2821. [CrossRef] 60. Yamaguchi, S.; Katoh, T.; Shinokubo, H.; Osuka, A. Pt(II)- and Pt(IV)-bridged cofacial diporphyrins via carbon—transition metal σ-bonds. J. Am. Chem. Soc. 2008, 130, 14440–14441. [CrossRef][PubMed] 61. Nakano, A.; Yamazaki, T.; Nishimura, Y.; Yamazaki, I.; Osuka, A. Three-dimensionally arranged windmill and grid porphyrin arrays by AgI-promoted meso-meso block oligomers. Chem. Eur. J. 2000, 6, 3254–3271. [CrossRef] 62. Hori, T.; Peng, X.; Aratani, N.; Takagi, A.; Matsumoto, T.; Kawai, T.; Yoon, Z.S.; Yoon, M.C.; Yang, J.; Kim, D.; et al. Synthesis of nanometer-scale porphyrin wheels of variable size. Chem. Eur. J. 2008, 14, 582–595. [CrossRef][PubMed] 63. Yoshida, K.; Yamaguchi, S.; Osuka, A.; Shinokubo, H. Platinum(II) and Platinum(IV) porphyrin pincer complexes: Synthesis, structures, and reactivity. Organometallics 2010, 29, 3997–4000. [CrossRef]

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