Porphyrinoids: Highly Versatile, Redox-Active Scaffolds...

Porphyrinoids: Highly Versatile, Redox-ActiveScaffolds for Supramolecular Design andBiomimetic Applications

Jonathan P. Hill1, Francis D’Souza2, and Katsuhiko Ariga1

1WPI-Center for Materials Nanoarchitectonics, Tsukuba, Japan2Wichita State University, Wichita, KS, USA

1 Introduction 17132 Naturally Occurring Porphyrin Pigments 17143 Porphyrin as a Supramolecular Entity 17154 Porphyrinoids 17205 Outlook and Concluding Remarks 1728Acknowledgments 1728References 1728

1 INTRODUCTION

Porphyrins make up an extended family of pigments thatgenerally contain a tetrapyrrole macrocycle (Figure 1).1 Onthe other hand, porphyrinoids are members of the por-phyrin family that bear some distinctive structural features.Thus, while porphine, chlorin and, to an increasing extent,corrole2 might now be considered standard porphyrins,there are a burgeoning number of compounds whose struc-tures might be considered atypical,1, 3, 4 which have beenaccessed usually through synthetic methods. Porphyrinoids(Figure 1) can be recognized as cyclic oligopyrroles,5 N-confused porphyrins,6 heteroatom-substituted porphyrins,7

reduced or oxidized porphyrins8 (e.g., porphyrinogensand oxoporphyrinogens, respectively), as well as many

Supramolecular Chemistry: From Molecules to Nanomaterials.Edited by Philip A. Gale and Jonathan W. Steed. 2012 John Wiley & Sons, Ltd. ISBN: 978-0-470-74640-0.

peripherally substituted tetrapyrroles and core-elaboratedmacrocycles. Scientific interest in porphyrins and porphyri-noids is mainly due to their unique electronic absorptive andelectrochemical properties, which lead to potential applica-tions in electronics,9 catalysis,10 sensing,11 and light energyconversion,12 among others. Porphyrins are also uniquelyplaced in natural systems where they perform functionsappropriate to their structure including solar energy col-lection and conversion,13 biocatalysis,14 electrochemicalfunctions, and oxygen transport.15 In this chapter, we dis-cuss the organizational aspects of porphyrin chemistry,with a special focus on the porphyrinoids and how theycan be used as scaffolds for synthetic and supramolecularelaboration.

1.1 Porphyrins, porphyrinoids, andnanotechnology

The nanometric dimensions of porphyrins (a typical tetra-phenylporphyrin is 1–2 nm in diameter) make them idealfor investigations that include complex nanostructures andtheir applications.16 Research on porphyrin is thereforeconstantly focused on its nanometric size, especially withrespect to increasing the dimensions of the molecules.Furthermore, porphyrins possess excellent electrochemicaland optical properties, making them suitable for use asactive elements in nanoscale devices (e.g., single moleculeswitches, and optical gates) and are among the most stableof common organic molecules, even capable of survivingconditions under which semiconductor silicon is normallyprocessed.17

1714 Supramolecular aspects of chemical biology

NH

NH

NH HN NH HN

NH HNNH HN

NH

HN HN

HN

N

N

NN

NN

N

N

X

X

N

N

H

H

NH

HN

N

N

NH

HN

HN

N

R1

R1

R1

R1

R1

R1R2

R2

R2

R2R1 R1

(a)

(b)

Figure 1 (a) Typical porphyrins: porphine, chlorin, and corrole. (b) Some common porphyrinoids. Note the tetrapyrrole skeletoncommon with the naturally occurring porphyrins.

2 NATURALLY OCCURRINGPORPHYRIN PIGMENTS

A brief study of the structures and some of the general activ-ities of the porphyrinic pigments that are available in naturalbiochemical systems is necessary before comparing themwith compounds developed synthetically.18 “Porphyrin” isa generic term that is applied usually to aromatic tetrapyr-role molecules. In nature, porphyrins perform a variety offunctions almost invariably in the form of metal complexeswhere the tetrapyrrole nitrogen atoms chelate a metal cationsuch as Fe3+ or Mg2+. The tetrapyrrole then becomes a lig-and and the resulting metal complex acts depending on itsredox, photonic, or structural functionality. Both porphyrinand metal cation have particular roles and some of theseare described briefly.

For simplicity, we can classify the naturally occurringporphyrins into two major categories: (i) hemes, whichare contained in hemoglobin and cytochrome proteins and(ii) chlorophylls, contained in green plants and bacteria.Some other complex tetrapyrroles are available, includingvitamin B12, and the chemical structures of some exemplaryporphyrin pigments are shown in Figure 2. In reality,structural variations of porphyrin-containing proteins makean extensive subset of the available natural proteins and

this serves to emphasize their indispensability in terms offunctionality in living systems and helps to explain whyporphyrins (and porphyrinoids) have attracted so muchattention from the research community.19

2.1 Hemoproteins

Heme is perhaps the best known of the porphyrins becauseof its presence at the dioxygen binding site of hemoglobinin the circulatory systems of most multicellular nonplantorganisms. Hemoglobin is composed of four myoglobinsubunits, each of which contains a single heme unit. Atan initial binding event, dioxygen is bound by coordina-tion to the Fe2+ cation of heme, causing the metal atom tobecome coplanar with the tetrapyrrole macrocycle and stim-ulating changes in the myoglobin morphology. Changes inthe myoglobin shape in turn increase the propensity fordioxygen binding at the hemes of the remaining three sub-units. Dioxygen release is promoted when the hemoglobincomplex reaches a region of high carbon dioxide concen-tration as one end of the myoglobin protein unit becomesprotonated. Porphyrin stabilizes specific electronic statesof a metal cation, leading to significant supramoleculareffects over relatively large distances in the hemoglobinmolecule.20

Porphyrinoids: highly versatile, redox-active scaffolds 1715

N N

N N

N N

N NFe

HO OHO O

O

O

O

OO

O

NH

HO

HO

O OO O

O

Mg

phytyl

Chlorophyll aHeme b

H2NOC

H2NOC

H2NOC

CONH2

CONH2

CONH2

N N

NNH

R

Co+

P

N

N

Vitamin B12

[R = 5′-deoxyadenosyl, hydroxyl, methyl, nitrile]

(a) (b)

(c)

Figure 2 Structures of some naturally occurring porphyrin pig-ments. (a) Heme b, (b) chlorophyll a, and (c) the macrocycliccore of vitamin B12.

The role of hemoproteins (not hemoglobin) as electrontransport elements in metabolic pathways is less widelyappreciated. In these systems, hemoproteins called cyto-chromes may be mobile in the intracellar medium transfer-ring electrons between reactive sites or may be immobilizedwithin membrane-bound macromolecular complexes.21

2.2 Chlorophylls

Chlorophylls (and other similar pigments) are presentin photosynthetic organisms and are responsible for theprimary collection and concentration of incident sunlight(i.e., photons) prior to its conversion into the chemical

energy required for biosynthetic processes. Chlorophyll,present in organisms in the form of pigment–proteincomplexes, is suitable for this process because its specificabsorption characteristics allow it to harvest photons fromsunlight, usually in the 600- to 700-nm region of thespectrum. The collected photonic energy is funneled to acomplex of two chlorophyll molecules (often referred to as“P 680” or the photosynthetic “special pair”) stimulating acharge-separation event whereby an electron is transferredfrom the special pair to a mobile quinone cofactor. Theremaining positively charged P 680+ species is highlyreactive, abstracting an electron from water mediated by anassociated tetranuclear manganese complex. These are theprimary chemical reactions in the photosynthesis process.22

Porphyrins have been selected by evolutionary processesas the optimum available materials for performing the des-ignated processes as described. Their functionality is basedon optical absorption properties (e.g., chlorophyll), elec-trochemistry (e.g., cytochromes), and also on their capa-bilities in stabilizing specific electronic states of transitionmetal cations. The importance of these pigments has beenappreciated for over a century, and, with the advent of theapplication of modern synthetic and supramolecular chemi-cal methods, it is becoming increasingly possible to developsystems that are either analogous with those in nature (i.e.,biomimetic) or which possess properties that are not foundin natural analogs. In the following examples, we describemethods used to prepare self-assemblies of porphyrins andporphyrinoids. Since one aim in preparing self-assembliesof porphyrins has been to mimic the sunlight-harvestingproperties of chlorophyll conglomerates in green plants,there are many synthetic systems that have been stud-ied, which may yield appropriately structured materials forsolar cell applications. However, it is also now possibleto develop porphyrinoids to perform similar tasks with theconsiderable added benefit of enhanced tunability of elec-trochemical, photonic, and structural properties over theporphyrins. Examples of these are discussed later. Thus, weconsider the methods used to form aggregated states of por-phyrins and porphyrinoids and how supramolecular effectscan be used. We later focus on how biomimetic effects havebeen used to generate and influence charge-separated statesin a synthetic supramolecular porphyrinoid system.

3 PORPHYRIN AS ASUPRAMOLECULAR ENTITY

It is apparent from the aforementioned biochemical systemsthat the electrochemical and electronic absorptive propertiesof porphyrins as well as supramolecular effects, includ-ing metal cation chelation, are essential aspects of por-phyrin activity in nature, for instance, either as a component


of coordination complexes or aggregated within proteinscaffolds of photosynthetic light-harvesting antenna. Theseeffects, as they have evolved, have become indispensablefeatures of living organisms and their hierarchies (i.e.,food chains). On the other hand, because of the feasibil-ity to incorporate porphyrins in biochemistry and becauseof their great synthetic flexibility in adaptations these com-pounds are very appealing for incorporation into a vari-ety of man-made, especially supramolecular, systems. Thesynthetic systems can be considered biomimetic becausethey are often aimed at recreating activities observed innature, including sunlight energy harvesting. Since the1980s, methods for the synthesis of porphyrins23 havebeen developed, enabling appendage of virtually any func-tionality with the aim of assembling the highly coloredchromophores. In particular, there have been attempts toassemble porphyrins (and other chromophores) using com-plex biochemicals (such as deoxyribonucleic acids (DNA)24

or even viruses and components thereof25), taking advan-tage of the intrinsic self-assembling properties that por-phyrins have because of the presence of substituents (e.g.,in aggregates26 or gels27) or by other physical techniques(e.g., Langmuir–Blodgett films28). Examples of these aregiven below. The appeal of organizing porphyrins intowell-defined supermolecules or aggregates lies in the hopeof preparing analogs of photosynthetic light-harvestingunits for examining natural mechanisms or as potentialsources of solar energy. With this in mind, the aggregatesare often subjected to analyses using high-level photo-optical techniques to identify their internal excited statesand energy-transfer processes. The aggregated structure ofthe porphyrins is crucial for observing energy-transfer orelectron-transfer reactions analogous with those in the nat-ural light-harvesting systems since the orientations of thechromophores involved in these processes should be main-tained (i.e., proximity and spatial arrangement) to promotethese processes. Another significant advantage of “self-assembly” is that it can be considered an autonomousphenomenon relying on molecular structure and the pre-vailing medium (i.e., solvent or interface), resulting in aneffective concerted and predesignated formation of compo-nents. Aggregated states permit us to maintain molecularcomponents in specific orientations over narrowly definedlength scales, leading us to design appropriate structureson the basis of knowledge of the supramolecular assembly.Overall, in this context, porphyrin/porphyrinoid chemistryintroduces the benefits of synthetic control coupled withaggregated states, which possess properties suitable forlight energy collection. There are some limitations includ-ing occasional unpredictability of aggregate structure or aperceived lack of stability of organic materials under high-intensity light. These limitations are overcome in naturalsystems through corrective mechanisms evolved by nature,

whereas any useful organic man-made systems may bemade recyclable or disposable.

3.1 Biomimetic scaffolding for organizingporphyrins

The properties of porphyrins make them an attractivesource of inspiration in designing biomimetic materialsbut they can also be assembled into interesting structuresby convoluting them with other supramolecular systemsborrowed from nature including portions of viral capsidsor DNA as described below.

Porphyrins are organized in the photosystem II of light-harvesting plants and microbes through various interactionswithin a protein “medium,” which maintains the chro-mophores in environments suitable for photon capture andlight energy funneling toward the photosynthetic reactioncenter (also composed of porphyrins).29 However, from asynthetic organic chemistry viewpoint, there is no reasonwhy other biochemical species cannot be adapted for a sim-ilar purpose.

3.1.1 Porphyrin–virus conjugates

Whole organisms (or components thereof) can also be usedas scaffolding for the organization of porphyrins. Endoet al. used the tobacco mosaic virus (TMV) protein tomake biohybrid–porphyrin assemblies for light-harvestingstudies.30 The approach involved controlled aggregationof the protein depending on the pH conditions. On thebasis of their work, they proposed that similar biohybridscould be developed for photocurrent generation at electrodesurfaces. The use of the virus scaffold illustrates well howa biomolecular species can be adapted for light collectionapplications and how subsequent aggregative processesof the proteins can be applied in a secondary assemblyprocess. As such, this work is an excellent example of howmolecular engineering principles can be applied in light-harvesting applications. Specifically, spectroscopic studiesindicated an advantage of this system in that geneticengineering of the protein scaffold could provide a meansto control energy transfer between appended porphyrinunits.

Nam et al. used a porphyrin-coated M-13 bacteriophageto illustrate the benefits of using a virus in the preparationof a biosynthetic hybrid light-harvesting complex.31 Theystudied the photon absorption and interchromophore energytransfer in the porphyrin-modified M-13 and proposed thatgenetic engineering of the virus could be used to controlthe energy-transfer process (Figure 3b).


FRET

hν

Quenching

PigmentElectronic coupling

HO

HO OH

OH

OHHO O O

N

N N

N

Zn

X

(b)

(a)

Figure 3 Porphyrins appended to biochemical species as supporting scaffolds. (a) Porphyrin–DNA conjugate. (Reproduced withpermission from Ref. 33. American Chemical Society, 2007.) (b) Porphyrin–virus component conjugate. (Reproduced with permissionfrom Ref. 31. American Chemical Society, 2010.)

3.1.2 Porphyrin–DNA conjugates

DNA32 makes an attractive scaffold because of its self-assembly into a helical structure dependent on intermolecu-lar hydrogen bonding of strands of polymeric nucleic acids.Although their use as a scaffold was first developed forother chromophoric species, DNA has recently been usedto organize porphyrins into the respective helical structures(Figure 3a).24, 33 Interestingly, it was found that incorpora-tion of porphyrins on the DNA scaffold in some cases sta-bilized the helix while energy transfer between porphyrinscould occur only with DNA in the helical conformation andnot after DNA denaturation, which emphasizes the require-ment for close proximity and ordering of chromophores inapplications involving these processes.34 One advantage ofthe DNA–porphyrin system is its chemically programmablenature so that nonidentical porphyrins can be assembled,allowing the generation of intramolecular energy cascades,which would perhaps be applicable in solar cell applicationsor molecular electronics.

3.2 Porphyrins as components ofsupramolecular systems

Porphyrins have been extensively studied for their use assupramolecular tectons.35 This is because the structuring of

chromophores at the molecular or nanoscale levels signif-icantly affects the properties of the resulting bulk materialand so strongly influences the potential for application ofthe materials prepared using self-assembly or supramolec-ular methods. Biomimicry, in this case, is often aimed atreproducing light-harvesting antenna in order to funnel sun-light energy into a suitable chemical or electrochemicalsystem.36

3.2.1 J-aggregates

Porphyrins are one of the major families of dyes studiedfor their formation of J-type aggregates. J-type aggrega-tion occurs when organic dyes aggregate in a face-to-facemanner with only a partial overlap of their chromophoresas opposed to H-type aggregation in which chromophoresare completely overlapped in a stack (Figure 4). This phe-nomenon, which is common in dye stuffs, has been pro-posed to lead to unique electronic and/or spectroscopicproperties, including cooperative phenomena such as super-radiance37 as well as photoelectric effects.38 J-aggregatedcyanine dyes were applied in photography because theysensitize silver bromide crystals to certain wavelengths oflight.39 J-aggregates of some dyes have also been appliedin photovoltaic sensitizing applications in heterojunctiondevices.40 In fact, the study of porphyrin self-assembly


−O3S SO3−

−O3S SO3−

SO3−

SO3−

N N

N

N

R

H

H

NH HN

N

N

H

H+

+

R(a) (c)

(b)

Figure 4 (a) Typical water-soluble porphyrins known to undergo J-aggregation. (b) Structure of J-type porphyrin aggregates. (c) Originof helical chirality in porphyrin J-aggregates.

through J-aggregation has almost become a research fieldin its own right since preliminary reports in the 1990s.41

This is again due to the extreme importance of the state ofaggregation of tetrapyrrole pigments as it relates to photonharvesting and photo-activated processes in plants. In addi-tion, the J-aggregate state of porphyrins has recently gainedan enhanced status owing to the observation of nanoscalechirality and chiral induction,42 which has connotations forasymmetric biochemical processes. Porphyrins involved inthis type of aggregation are typically substituted with polargroups (e.g., 4-sulphonatophenyl, 4-methylpyridiniumyl),which facilitate the offset stacking mechanism essential forJ-aggregate formation.

3.2.2 Porphyrin gels and mesophases

Similar kinds of aggregation can lead to fiber-like struc-tures or gel formation in solutions of porphyrins bearingappropriate substituents such as long alkyl chain amidegroups, which promote stronger noncovalent intermolec-ular interactions and enable higher order networks tobe established. Although J-aggregation is common, H-aggregation where chromophores are stacked with a discoticstructural motif is also observed. Shinkai and coworkershave extensively investigated the structures of porphyrin-containing gels and the influence of molecular structureon solvent gelation ability.43 In the absence of solventgelation, a porphyrin mesophase or liquid crystal may

result from weak intermolecular forces caused by spe-cific substituents such as long aliphatic chains. Porphyrinmesophases have been known for some time, with typicalexamples being derived from meso- or β-substituted por-phyrins (Figure 5).44, 45 There are a variety of mesophasestructures depending on the substituent although discotictendencies are induced by the presence of the flat tetrapyr-role macrocycle. Discotic tendencies tend to confer energy-transfer characteristics suitable for solar energy capture orother applications.46

3.2.3 Porous crystals

Porphyrins can also be easily crystallized, with perhaps themost significant ones being those that form porous net-works. In fact, crystalline porphyrins were some of thefirst porous compounds with many subsequently havingbeen reported as metal–organic frameworks and hydrogen-bonded networks.47 Their ability to operate as materialsstorage or sequestration media has been amply demon-strated, revealing the advantages of the porphyrins as cat-alytic entities or for specific materials storage.48, 49

3.2.4 Self-assembled monolayers

If molecules are constrained in two dimensions at a sub-strate surface, then other phenomena can be observed in theformation of supramolecular structures, sometimes referredto as two-dimensional crystal structures. Porphyrins were


R RR R

R

R

RR

R

R

RR

N

N

N NM N

N

N

NM

(a)

(c)

(b)

aa

b

a

Figure 5 (a) Chemical structures of typical mesomorphic tet-raphenylporphyrins and octaalkylporphyrins. R = CnH(2n+1) forn ≥ 8. (b) Optical micrograph of the texture of mesophase ofoctaalkylsubstituted porphyrin (n = 12) viewed through cross-polarizers. (c) Stacking modes of the crystalline (left) and rect-angular mesophase (right) of an octaalkylporphyrin (n = 12).(Reproduced with permission from Ref. 45. Royal Society ofChemistry, 2009.)

among the first molecules to be studied in scanning tun-neling microscopy (STM) owing to their great thermalstabilities and nanoscale size.50 Archetypal molecules con-taining the 3,5-di-tert-butylphenyl substituent have beenobserved to form a variety of structures at a metal interface(Figure 6). These structures include close-packed struc-tures, networks, and nanowires. By further analogy withtheir 3D crystalline counterparts, the porphyrins have alsobeen observed to undergo 2D crystal–crystal transitions(Figure 6b).52

(i)

(a)

(b)

(ii) (iii)

(i) (ii) (iii)

Figure 6 (a) Scanning tunneling microscopy (STM) imagesof 5,10,15,20-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrinat Cu(111) surface illustrating the different possible conforma-tions including unsymmetrical ones: (i) symmetrical planar con-formation; (ii) unsymmetrical form with “mixed” conformationof substituents; (iii) compact symmetrical conformation. (Repro-duced with permission from Ref. 51. PCCP Owner Societies,2006.) (b) Surface bound two-dimensional crystal–crystal phasetransition observed for the same porphyrin as in (a). (Reproducedwith permission from Ref. 52. Royal Society of Chemistry,2006.)

Porphyrins have also been observed extensively usingelectrochemical STM where the influence of redox eventson structure can be observed.53 Monolayers of porphyrinson electrodes also represent a significant opportunityfor developing applications including sensing (especiallybiosensing).

3.3 Other means of organizing porphyrins

Combined physical/supramolecular methods have also beenapplied for the organization of porphyrins. The layer-by-layer method,54 based on alternating electrostatic assem-bly of materials, has been used to assemble porphyrins(Figure 7), and their potential as sensing elements wasassessed.55 Also, porphyrinoids such as N-confused por-phyrins have been incorporated in LB films and used as aselective sensor for iodide anions based on the incorpora-tion of these anions in the LB films upon their transfer to asubstrate.56 In that work, only iodide anions formed a stable


nc-TPPH+

nc-TPPH+

nc-TPP

nc-TPP

nc-TPPH+

(J-aggregate)

(J-aggregate)

(H-aggregate)

(J-aggregate)

pH 7

pH 7

pH 12

Iodideanions

Iodideanions

Substrate

Substrate

Air–water interface

Other halide anions(Not iodide)

−HX

Langmuir–Blodgett film

Repeat many times

Wash Wash

Final step

++

++ +

+

+ ++

++

+ +− − − − − − − −

−−−

(a)

(b)

Figure 7 (a) Langmuir films with N-confused porphyrins foriodide anion sensing. (Reproduced with permission from Ref. 56. Royal Society of Chemistry, 1996.) (b) Layer-by-layer assem-bly of porphyrins. (Reproduced with permission from Ref. 54. PCCP Owner Societies, 2007.)

enough complex with the J-aggregated monoprotonated N-confused porphyrins, enabling transfer of the complex asan LB film. Other halide anions could not be incorporatedand were excluded in the water subphase with a concur-rent deprotonation of the macrocycle. The authors claimedthis as the first example of “aggregation-mediated” anionrecognition since they found no simple affinity enhance-ment between iodide and N-confused porphyrins over theother halide anions.

Dendrimer structural motifs have been used to pre-pare very large oligochromophoric arrays of porphyrins.Porphyrin dendrimers are particularly attractive since theycombine an essentially chemically discrete structure withproperties such as high solubilities and processabilities. Por-phyrin dendrimers are the subject of a recent comprehensivereview by Li and Aida.57

4 PORPHYRINOIDS

Now that we have introduced several of the known methodsof assembling the porphyrin chromophores, let us considerthe aspects that make porphyrinoids unique as scaffolds.Probably the most important aspect is the novel syntheticconnectivity that can be introduced by varying the tetrapyr-role framework. For instance, in the N-confused porphyrins,a macrocylic nitrogen atom becomes available for modifica-tion at the molecules’ periphery. Furthermore, porphyrinswith peripherally oxidized pyrrole groups can have theirconjugated π-electronic clouds significantly extended, lead-ing to new potential applications in molecular electronicsand nonlinear optics. In addition, if macrocyclic planarityis attenuated by formal oxidation or reduction, then com-pletely novel species become available with properties suit-able for sensing or incorporation in oligochromophoricsystems.

We have chosen to divide the porphyrinoids intotwo families: the peripherally functionalized porphyrinoids(e.g., N-confused porphyrins, π-extended porphyrins, oligo-porphyrins) and the core-functionalized (nonplanar) por-phyrinoids (e.g., porphyrinogens, alkylidene porphyrins,oxoporphyrinogens, and some oligopyrroles). The formercontains mostly molecules that have complex elaboration attheir periphery, which leads to special supramolecular prop-erties. Core-substituted porphyrinoids include N-substitutedoxoporphyrinogens and even some metalloporphyrins andporphyrin dications.

Porphyrinoids, in the context described here, have theirorigins in the synthetic development of the porphyrins.However, this has allowed us not only to assess the effectsof structural variations but also to observe properties notusually associated with common porphyrins.

4.1 Peripheral functionalization

By peripheral functionalization, we refer to those por-phyrins whose structures have been modified at the pyrroleβ-position or at the phenyl rings in tetraphenylporphyrins,with the aim of incorporating them into self-assembledsystems. The examples presented are not meant to bean exhaustive list of peripherally augmented porphyrins


but are a selection to illustrate the flexibility available tothe porphyrin chemist in terms of nanometric synthesis.The briefly discussed N-confused porphyrins are perhapsthe simplest form of peripheral variation since the struc-tural position of only a single atom is exchanged. Otherexamples are fused conjugated porphyrins (as peripher-ally functionalized molecules) and N-alkylated porphyrino-gens (as core-funtionalized porphyrins). Metalloporphyrinswould be examples of core-functionalized porphyrins butthey are far too numerous to be covered here.

4.1.1 N-confused porphyrins

A particularly important case of peripheral functionalizationof TPP occurs in the N-confused porphyrins58 where one(or more) nitrogen atom(s) is (are) positioned at a peripheralposition owing to the reaction at a pyrrole 3-position (ratherthan 2-position) during synthesis (Figure 1b). These com-pounds can then be modified for predesignated purposes.

4.1.2 π-Extended porphyrins

This section introduces some examples where the delo-calized electronic system of porphyrin has been extendedsynthetically. There are two now well-established methodsfor achieving the expansion of the porphyrin electronic sys-tem. The first, introduced by Crossley and coworkers,59

involves a sequence of reactions starting with nitration ofpyrrole groups and oxidation to a 1,2-diketone. The lat-ter can then be condensed with a 1,2-diamine or itself beconverted to a 1,2-diamine. Using this strategy, large nano-metric multichromophoric systems have been prepared, anexample of which is shown in Figure 8(a).

Another method of preparing extended conjugated mul-tiporphyrin arrays was introduced by Osuka and cowork-ers and it has been applied by several other groups.60

Meso–meso-linked porphyrin oligomers are oxidativelyfused, leading to extremely large fully conjugated por-phyrin polymers. The synthetic methodology can be appliedwherever pyrrole β-protons of meso-linked porphyrins are

Ar

Ar

Ar

Ar

Ar

Ar Ar

Ar Ar

Ar Ar

Ar Ar

Ar

Ar =

Ar

N N N

N N

Zn Ar

Ar

ArN N

N N

Zn

Ar ArN N

N N

Zn

Ar ArN N

N N

Zn

Ar ArN N

N N

Zn

Ar ArN N

N N

Zn

Ar Ar

DDQ

N N

N N

Zn

Ar ArN N

N N

Zn

Ar ArN N

N N

Zn

Ar Ar

Ar Ar

N N

N N

Zn

N

N N

N N

N

N

NN

NN

N

N

NH HN

NH HN

NH HN

tBu

tBu

Sc(OTf)3

(a) (b)

Figure 8 Extended conjugated porphyrins after (a) Crossley and coworkers and (b) Osuka and coworkers.


closely adjacent, although the reagents can also be appliedin the preparation of, for instance, fused porphyrin dimersfrom discrete monomers. The process is a developmentof the Scholl reaction, which has been used to preparepolyphenylenes for more than 50 years. Another notableapplication of this reaction (albeit using different reagents)has been in the development of hexabenzocoronene andrelated compounds by Mullen and coworkers.61 Scan-dium(III) triflate has emerged as the reagent of choice inthe porphyrin condensation step to fused porphyrins, oftenbeing applied in conjunction with an oxidant such as DDQ(2,3-dichloro-5,6-dicyanobenzoquinone). The structure of atypical fused porphyrin is shown in Figure 8(b).

One of the key advantages of these fused porphyrinoids istheir increased stabilities due to their large delocalized aro-matic systems and greater polymeric nature, making thesecompounds more suitable in photochemical applications.In addition, the fused porphyrinoids have larger absorptivecapacities over wider regions of the solar spectrum leadingto better potential in photon absorption applications.

4.2 Core functionalization

4.2.1 Porphyrinogens

Porphyrinogens make excellent frameworks for syntheticelaboration toward a variety of applications because theycan be prepared in high yields from commonly avail-able precursors (substituted pyrroles and ketones). Theyhave been exploited from a variety of viewpoints, start-ing with the work of Floriani and coworkers who devel-oped several porphyrinogens as scaffolds for transitionmetal complexes.62 Of particular note are the dimeric andorganometallic species generated through their work.

The porphyrinogens have also been developed as sensingelements (Figure 9) and this has involved its use asa scaffold in the preparation of selective anion sensormolecules. For this purpose, it has been necessary toincorporate chromophores into the molecules so that theycan exhibit colorimetric responses (porphyrinogen itself isa white solid and is colorless in solution). It is also apreparatively simple task to “strap” the porphyrinogensby using appropriately substituted diketones, leading toattractive host molecules, which can exhibit sensing activityin the presence of particular anions or other analytes.63

Unsaturated porphyrinogens can be prepared and theseare also suitable for sensing purposes based on their colorchanges in the presence of some analytes.64 Anion bindingin the simple porphyrinogen systems is through hydrogenbonding between the amine groups of pyrrole groupsand an electronegative atom in the anion species. In theunsaturated porphyrinogens (i.e., oxoporphyrinogens; see

O

O

O

O

OS

S

S S

S

S

S S

S

S S

S S

S

S

S

O O

OO

O

NH

NH

NH

NH

NH HN

HNNH

(a) (b)

Figure 9 Porphyrinogens developed by Sessler and coworkersfor sensing applications. (a) Calix[4]pyrrole-calixarene cage-typereceptor. (b) Tetrathiafulvalene augmented calix[4]pyrrole.

the following section), color changes can be related to thestrength of anion binding and to the radius of the anion.For instance, small highly electronegative fluoride anionsare bound strongly by H-bonding at the pyrrole aminegroups, reducing the noncoplanarity of the porphyrinogenunit but increasing the conjugative interaction betweenthe unsaturated elements of the molecule (pyrrole groupsand alkylidene substituents) leading to a shift in theelectronic absorbance maximum to longer wavelength. Incontrast, larger, weakly bound anions lead to only smallperturbations of the host tetrapyrrole and a less importantvariation in the color of the host. One of the key advantagesof anion binding in the case of the oxoporphyrinogens is theconcurrent variation in electrochemical properties, whichcan lead to an as much as 600-mV cathodic shift in theoxidation potentials of the chromophore.

RR R

R

X

NH HN

NH HN

S S

X

RR R

R

X = CH, NR = CO2CH2CH3

Figure 10 Attractive expanded alkylidene macrocycles firstsynthesized by Lee and coworkers.


4.2.2 Alkylidene porphyrins

Alkylidene porphyrins (including oxo-type porphyrinogens)occupy a special category since they are formed throughsynthetic modifications at both the periphery and the macro-cyclic core and can have properties that depart significantlyfrom those reported for the porphyrins. Several macrocycles

containing alkylidene functionality have been prepared,a notable example being the TCNQ analog prepared byBlake, Anderson, and coworkers.65 The alkylidene motifcan also be used to promote the formation and stability ofspecies such as cumulenes. Expanded alkylidene macrocy-cles are also available including those prepared by Lee andcoworkers, as shown in Figure 10.66

HO

HO OH

O

O O

OOH

HN

NH

N N

N

N

N NR1

R3R4 R2

HN

NH

NH HN

X X OX =

X X

T(DtBHP)P(a)

(c)

(d)

(b) Ox[T(DtBHP)P] (= OxP)

OxPBz2: R1 = R3 = Bz; R2 = R4 = HOxPBz4: R1 = R2 = R3 = R4 = Bzrac-OxPR4: R1 = n-C12H25; R2 = 4-NBz

R3 = 2-Methylnaphthyl; R4 = 4-BrBz

Figure 11 Structures of derivatives of an antioxidant phenol bearing porphyrin. (a) Tetrakis-5,10,15,20-(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin, T(DtBHP)P. (b) Two-electron oxidation product, Ox[T(DtBHP)P] (which can be further abbreviated as OxP).(c) Chemical structures of representative N-alkylated OxP including rac-OxPR4, a chiral derivative. (d) X-ray crystal structure ofrac-OxPR4.


The beauty of these usually nonaromatic compounds liesnot only in their structure but also in the availability ofsubstantial electronic absorptions and well-defined redoxbehaviors. One key example of this lies in the proper-ties of the compound Ox[T(DtBHP)P], which is derivedfrom the two-electron oxidation of the porphyrin 5,10,15,20-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin,(T(DtBHP)P). Ox[T(DtBHP)P] bears a porphyrinogen-typestructure of its macrocycle, although this depends on solu-tion conditions.67 This highly puckered macrocycle makesan ideal highly colored, redox-active porphyrinoid scaffoldbecause of the accessibility of the central nitrogen atomsfor N-alkylation (Figure 11).68 N-alkylation occurs regio-selectively so that its structure can be controlled simplyby performing this reaction in a stepwise manner, evenpermitting synthesis of chiral derivatives.69 Further, onceN-alkylated, the resulting compounds are extremely stableover long periods (at least 15 years in the solid state) underambient conditions.

In addition, the electronic absorption spectra ofOx[T(DtBHP)P] compounds possess maxima with highabsorption coefficients in the visible light region (Figure 12)suggesting their activity as light-harvesting antenna units.These compounds also have potential in nonlinear opti-cal applications.70 N-alkylation of Ox[T(DtBHP)P] permitstuning of its electrochemistry (Figure 12) and many dif-ferent groups can be accommodated without compromisingthe stability. Up to four tetraphenylporphyrin groups canbe introduced in the preparation of molecular clefts or two-faced clefts (Figure 13). These are unique in that they areporphyrins linked to quinonoids, where the quinonoid unitis incidentally also a tetrapyrrole.71

Here, we illustrate the utility of supramolecular effectsfor biomimicry in this porphyrinoid system. In particular,we note that charge separation is one of the key pro-cesses in photosynthetic systems and so is a valid tar-get for imitation in synthetic systems with a view todeveloping more efficient organic photovoltaic devices. Forpurposes of generating a charge-separated state, it is appro-priate that a molecule or supramolecular complex containsan electron donor moiety accompanied by an electronacceptor so that transfer of electron(s) is facilitated. Anexample of a porphyrin–quinonoid donor–acceptor triadcontaining tetraphenylporphinatozinc(II) groups linked tooxoporphyrinogen through its macrocycle nitrogen atomsis available.72 This configuration of chromophoric unitsleads to interactions between the electron-donating zincporphyrin(s) and the electron-accepting oxoporphyrinogen.Initially, the term P–OxP was used to describe thesecompounds, that is, porphyrin–oxoporphyrinogens. Later,the term alligator porphyrins was applied to describe thejaws-like disposition of two porphyrin units when linkedcofacially to the oxoporphyrinogen macrocycle.73 Direct

1.2

1.0

1.0 0.5

0.8

0.6

0.4

0.2

0.0400 600 800 1000

Wavelength (nm)

Abs

orba

nce

(i)

(i)

(ii)

(ii)

(iii)

(iii)

(iv)

(iv)

(v)

(v)

(vi)

(vi)

0.0 −0.5 −1.0 −1.5 −2.0 −2.5

Potential (V vs. Fc/Fc+)

*

(a)

(b)

Figure 12 (a) Electronic absorption spectra and (b) cyclicvoltammetry of increasingly N-substituted OxP. (i) OxP; (ii)OxPR1; (iii) OxPR2; (iv) OxPR3; (v) OxPR4; and (vi) startingporphyrin, T(DtBHP)P. R = 2-methylnaphthyl. Labeling scheme(i–vi) is applicable to both (a) and (b). Note that colors of thetraces in (a) do not reflect the respective solution colors.

structural data could not be obtained for these compounds;so, computational methods were used to investigate theproperties and to probe the electronic structures of theOxP derivatives and their supramolecular conjugates.71, 73

In fact, electrochemical and other properties could bepredicted using these methods in the initial studies onthe N-alkylated OxP derivatives, with excellent agreementbetween experimental and calculated quantities [including(highest occupied molecular orbital) HOMO–LUMO (low-est unoccupied molecular orbital) gap].74 These methodsalso present optimized geometries and electronic struc-tures of the multichromophoric hosts. As an example,the energy-minimized structure of one of the doublyporphyrin-linked oxoporphyrinogen compounds is shownin Figure 13. The structures and locations of the HOMOand LUMO for these compounds are also shown. From


O O

N

N N

N

Zn

N

N N

N

Zn

O O

HN

N

NH

N

O O

O O

OO

OO(a)

(b)

(c) (i) (ii)

Figure 13 (a) Chemical structure of a bis-porphyrin–oxoporphyrinogen conjugate with a unique N-alkyl linkage of porphyrins.(b) Calculated energy-minimized structure of the conjugate (3-methylbutyl groups substituted for methyl groups for clarity). (c) Locationof (i) highest occupied molecular orbital, (ii) lowest unoccupied molecular orbital, in P–OxP compounds, illustrating the potential forthe occurrence of intramolecular electron-transfer processes.

this data, we can estimate the likelihoods of energy-transferand electron-transfer events. Optical absorption spectrumof the triad contains bands corresponding to donor andacceptor entities, while the redox states of the triad couldbe established from comparative electrochemical analysisof the triad and reference compounds. Steady-state andtime-resolved fluorescence emission indicated quenchingof the singlet excited state of the zinc porphyrin moi-eties in the triad, and free-energy calculations indicatedthat electron transfer from the singlet excited zinc por-phyrin group to the oxoporphyrinogen is likely in polarsolvents. In addition, time-resolved fluorescence measure-ments revealed that excited-state energy transfer to oxo-porphyrinogen from metalloporphyrin occurs in nonpolarsolvents. Nanosecond transient absorption studies (with

time-resolved fluorescence data) strongly suggested thatphotoinduced charge separation from the singlet excitedzinc porphyrin to the oxoporphyrinogen occurs in polarsolvents, which is consistent with the aforementioned free-energy calculations.

Bis(zinc porphyrin)–oxoporphyrinogen [(ZnTPP)2–OxP]is an example of a donor–acceptor-type triad. Excitationof the zinc porphyrin groups leads to an efficient energyor electron transfer in the triad and this process dependson the polarity of the solvent used. Further elaboration ofthis system was accomplished by complexing this com-pound and other derivatives with fullerene-containing com-pounds capable of coordinating at the porphyrin zinc metalions.71, 73 The structures of the complexes are shown inFigure 14.


N

N N

N NZn

N

R

O

OO

O

N

NN N

HH

R

N

N N

N NR RZn

H

R N N

N NNH

R

OO

N

N N

N

O

R ZnN N

N

N

N

NN

N N

NN R

Zn

H

R R

O

RN N

NNZnN N R

Zn

(a)

(b)

Figure 14 Chemical structures and corresponding energy-minimized structures of supramolecular complexes of the por-phyrin–oxoporphyrinogens with bis-pyridyl-substituted fullerene, Py2C60. (a) OxP–(ZnPp)2: Py2C60. (b) OxP–(ZnPm)4: (Py2C60)2.R = 3,5-bis(3-methylbutyloxy)phenyl.


R RN N

N NZn R

R

N N

N NZn

O

OO

O

N

N NH

HN

F−

NN NH

HS

CS

hν

OxP

OxP

OxP

[F−:OxP]+.

OxP

OxP−.

1OxP*

1ZnP*

ZnP

ZnP

ZnP

ZnP

ZnP

ZnP

ZnP

ZnP

ZnP

ZnP+.

ZnP+.

ZnP

ZnP

C60

1C60*

C60−.

C60

C60−.

C60

C60(2.05 eV)

(1.75 eV)

(1.40 eV)

(<0.83 eV)

(1.60 eV)

(1.90 eV)kENII

kENI

kCSII

hνkCR

I

kCR

kCSI

kCS

(c)

(b)(a)

Figure 15 Heterotropic allostery in a supramolecular system. (a) Chemical structure of a host–guest complex containing porphyrin-N-substituted oxoporphyrinogen host with guests (dipyridyl-substituted fullerene; fluoride anions) bound at their respective recognitionsites. R = 3,5-bis(3-methylbutyloxy)phenyl. (b) Space-filling model of the complex showing proposed route of charge separation uponirradiation. (c) Energy-level diagram illustrating the stabilization of charge separation by fluoride anion complexation. (Adapted withpermission from Ref. 76. American Chemical Society, 2009.)

These supramolecular aggregates, composed of zincporphyrin(s), fullerene(s), and oxoporphyrinogen redox/photoactive groups, underwent photoinduced electrontransfer from singlet excited porphyrin to fullerene asdemonstrated during time-resolved emission and transientabsorption studies. Charge-separation rates (kCS) weregreater for the pentad, (C60Py2:ZnP)2-OxP than for thecorresponding triad, C60Py2:ZnP–OxP. Lifetimes of theradical ion pair (τRIP) were hundreds of nanoseconds,indicating that there is some charge stabilization occur-ring within these supramolecular systems. An analysis of

this system reveals that control of energy/electron transferprocesses in these porphyrin–fullerene complexes can beachieved by carefully designing the oligochromophore sys-tem. For the doubly-(4-pyridyl)-substituted C60 and host,we employed a two-point binding mode so that chargetransfer occurring in the noncomplexed OxP–(ZnPx)n hostmolecules could be switched to efficient photoinducedelectron-transfer processes from singlet excited porphyrinto fullerene upon donor–acceptor complex formation, andsubsequent transient absorption studies indicated chargestabilization.


The most recent advance in this system involves theoxoporphyrinogen moiety as both scaffold and host foran incoming (anionic) guest. It is known that binding offluoride anions to OxP causes an almost 600-mV anodicshift in its first reduction wave (Figure 15).75 This featuregives us a method for varying the route of electron transferwithin the previously mentioned supramolecular manifold,thus illustrating the benefits of making host moleculeswith multiple nonidentical guest binding sites. This hassome analogies with natural systems. In fact, many protein-based enzymes operate in the presence of a cofactor. Thepresence of the cofactor often critically affects the rates ofenzymatic processes, a feature which is sometimes referredto as heterotropic allostery. In the OxP system describedhere, we observe an almost 100-fold increase in stability ofthe charge-separated state between porphyrin and fullerenewhen fluoride anions are added into solution.76 Thus, thissystem can be said to exhibit heterotropic allostery, andit is possible that similar effects will be observed whenother substituents such as catalytic metal complexes areincorporated at one face of the OxP molecule.

Overall, this system demonstrates that processes oper-ating in natural systems can be mimicked using appropri-ately designed supramolecular systems. In particular, wecan show that a combination of synthetic organic chemistry(cf. biosynthesis) followed by coassembly of supramolecu-lar complexes leads to complexes that exhibit propertiesanalogous with protein and enzyme activities, includinghigh-level effects such as allostery involving the binding ofdifferent supramolecular guests at different sites of the samehost molecule. Of course, the latter example here includesporphyrins and a porphyrinoid although the function of thelatter is beyond the capability of the porphyrin macrocy-cle since it involves the stable introduction of substituentsat the pyrrole nitrogen atoms. This aspect emphasizes thatporphyrinoids have been developed to a level beyond thecapabilities of the porphyrins although incorporation of por-phyrin elements in this system confirms their importance asfunctional dyes in light-harvesting operations. Thus, whilethe porphyrinoids are being constantly updated in the searchfor novel compounds and physical effects, the porphyrinsalso remain at the forefront of research into the chemistryand applications of organic chromophores.

5 OUTLOOK AND CONCLUDINGREMARKS

The world of supramolecular chemistry is certainly a biggerand better place because of the existence of the porphyrinsand porphyrinoids. While the porphyrins have long occu-pied a special position in the minds of chemists, it is the

porphyrinoids that are currently emerging as a class ofnew molecules exhibiting adaptable and unusual proper-ties. Thus, the outlook for the study of the chemistry andself-assembly of these species is very promising. In par-ticular, there are several current scientific questions thatmight find answers through the development of porphyri-noids. First, can a cheap yet operating synthetic light-harvesting complex be prepared using a chromophore andself-assembly techniques alone? Also, can operating molec-ular circuits be constructed by a similar combination ofsynthesis and supramolecular methods? These problems arecurrently being addressed. Because of the properties of theporphyrinoids, it is highly likely that such molecular mani-fold devices will, if not consist entirely of, at least containthese highly colored, redox-active supramolecular scaffolds.

ACKNOWLEDGMENTS

This work was partly supported by World Premier Inter-national Research Center Initiative (WPI), MEXT, Japan,Core Research for Evolutional Science and Technol-ogy (CREST) program of Japan Science and TechnologyAgency (JST), Japan, National Science Foundation (GrantNos. 0804015 and EPS-0903806), and matching supportfrom the State of Kansas through Kansas Technology Enter-prise Corporation.

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