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Structural Aspects of Porphyrins for Functional Materials Applications

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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 Meanb 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.
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