Spectroscopy and Tautomerization Studies of Porphycenes

Review pubs.acs.org/CR

Spectroscopy and Tautomerization Studies of Porphycenes Jacek Waluk* Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Kasprzaka 44/52, Poland Faculty of Mathematics and Science, Cardinal Stefan Wyszyński University, Dewajtis 5, 01-815 Warsaw, Poland S Supporting Information *

ABSTRACT: Tautomerization in porphycenes, constitutional isomers of porphyrins, is strongly entangled with spectral and photophysical parameters. The intramolecular double hydrogen transfer occurring in the ground and electronically excited states leads to uncommon spectroscopic characteristics, such as depolarized emission, viscosity-dependent radiationless depopulation, and vibrational-mode-specific tunneling splittings. This review starts with documentation of the electronic spectra of porphycenes: Absorption and magnetic circular dichroism are discussed, together with their analysis based on the perimeter model. Next, photophysical characteristics are presented, setting the stage for the final part, which discusses the developments in research on tautomerism. Porphycenes have been studied in different experimental regimes: molecules in condensed phases, isolated in supersonic jets and helium nanodroplets, and, recently also on the level of single molecules investigated by optical and scanning probe microscopies. Because of the rich and detailed information obtained from these diverse investigations, porphycenes emerge as very good models for studying the complex, multidimensional phenomena involved in the process of intramolecular double hydrogen transfer.

CONTENTS 1. Introduction 2. Electronic Spectroscopy 2.1. Absorption Spectra 2.1.1. Alkyl-Substituted Porphycenes 2.1.2. Aryl-Substituted Porphycenes 2.1.3. Heteroatom-Containing Porphycenes 2.1.4. Benzoporphycenes and Naphthoporphycenes 2.1.5. Metalloporphycenes 2.1.6. Radical Species, Cations, and Anions 2.1.7. High-Resolution Spectra of Cold, Isolated Molecules 2.2. Magnetic Circular Dichroism and the Perimeter Model 2.3. Two-Photon Absorption 2.4. Calculations of Electronic Structure and Spectra 3. Photophysics 3.1. Fluorescence and Other Energy Relaxation Channels 3.1.1. Models of Excited-State Deactivation 3.2. Triplet−State Parameters 3.3. Singlet Oxygen Generation 4. Tautomerism 4.1. Coherent Double Hydrogen Tunneling in Isolated Molecules 4.2. Rates of Double Hydrogen Transfer in Condensed Phases 4.3. Theoretical Studies of Tautomerization in Porphycenes 4.4. Tautomerization in Single Molecules 4.4.1. Single-Molecule Fluorescence Studies © 2016 American Chemical Society

4.4.2. Single-Molecule Raman Spectra 4.4.3. Single-Molecule Scanning Probe Microscopy 5. Summary Associated Content Supporting Information Author Information Corresponding Author Notes Biography Acknowledgments References

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1. INTRODUCTION The synthesis of porphycene (1), first reported in 1986 by Vogel and co-workers,1 opened up a new direction in studies of porphyrin-related compounds: research on constitutional (structural) isomers of porphyrin (2). As discussed by Waluk and Michl,2 there are eight different ways of rearranging four pyrrole units and four methylene bridges in such way that the “nitrogen-in” cavity is preserved (Scheme 1). In the years after the publication of this theoretical work, syntheses of other structural isomers have been reported: hemiporphycene (3),3,4 corrphycene (4),5,6 and isoporphycene (5).7,8 Three additional isomers (6−8 in Scheme 1) still await successful synthesis. Remarkably, these are the molecules that have been predicted by theory to be the least stable.9 Free-base porphycene, on the other hand, is, according to calculations, the most stable among

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Special Issue: Expanded, Contracted, and Isomeric Porphyrins Received: May 25, 2016 Published: July 28, 2016 2447

DOI: 10.1021/acs.chemrev.6b00328 Chem. Rev. 2017, 117, 2447−2480

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applications include using porphycenes in catalysis,14−17 organic photovoltaics,18−20 and information storage21,22 and as molecular transistors,23 conductors,24,25 switches,26 liquidcrystalline materials,27 and artificial heme components.28−34 The present review is focused on the electronic spectroscopy and tautomerization of porphycenes. First, electronic absorption spectra are described, and then the photophysical behavior is presented. Finally, tautomerism of porphycenes is discussed. Even though some of these subjects have been partially covered previously,35−39 the goal is to include the relevant literature on electronic spectroscopy and tautomerism published since Vogel et al.’s first work on porphycene synthesis. Because the review focuses on tautomerism, free-base porphycenes are discussed in more detail than their metalcomplexed counterparts. However, comprehensive electronic absorption and photophysical data are included for both freebase and metalloporphycenes. Because of space limitations, the so-called “extended” or “stretched” porphycenes40−42 are not included, except for the cases where the extension preserves the basic porphycene skeleton.

Scheme 1

2. ELECTRONIC SPECTROSCOPY 2.1. Absorption Spectra

Free-base porphyrins exhibit a characteristic absorption pattern: In the low energy range, several weak bands are observed, corresponding to two electronic transitions. They are referred to as Q bands. In those porphyrins that retain the D2h symmetry of the parent chromophore, the lower-energy (Qx) transition is polarized along the NH−HN direction, and the higher-energy transition, Qy, is polarized perpendicular to it. These bands are followed by very strong absorption at about 400 nm (Soret bands), comprising at least two electronic, differently polarized transitions. For lower-symmetry porphyrins, a more general nomenclature is appropriate, based on the perimeter model:43−46 In order of increasing energy, the electronic transitions are called L1, L2, B1, and B2, respectively. Figure 1 shows a comparison between the absorption spectra of the parent porphyrin and porphycene. Although qualitatively similar, the spectra differ significantly with respect to both transition energies and intensities. The Q bands of porphycene are red-shifted with respect to those of porphyrin. Because the shift of L2 is larger than that of L1, the energy gap between the

all of the isomers, including porphyrin. This stability has been attributed to exceptionally strong intramolecular hydrogen bonds in porphycene, arising as a result of the favorable, rectangular geometry of the central cavity. When the inner protons are replaced by metal atoms, porphyrin becomes the lowest-energy structure. Other classes of porphyrin isomers include the “inverted” (“confused”) porphyrins reported in 1994 by the groups of Latos-Grażyński10 and Furuta11 and the “neo-confused” porphyrin, obtained in 2011 by Lash et al.12 (9 and 10, respectively, in Scheme 1). The number of literature references shows clearly that, among porphyrin isomers, the most popular ones are porphycene and inverted porphyrin. The interest in the latter is due to the ability to form various coordination complexes and uncommon derivatives. The research on porphycenes is largely stimulated by their two specific features: (a) spectral and photophysical characteristics that are qualitatively similar but quantitatively very different from those of porphyrin and other isomers and (b) hydrogen-bonding properties and tautomerism. Strong links connect these two areas, because the research on hydrogen transfer, occurring in both the electronic ground and excited states, is often based on careful spectral and photophysical studies. Most important, some spectral photophysical properties of free-base porphycenes are often controlled by the energetics of the intramolecular hydrogen bonds and by tautomerization dynamics. The spectral characteristics of porphycenes, particularly their strong absorption in the red spectral region combined with good efficiency of triplet-state formation, make these compounds attractive as sensitizers in photodynamic therapy (PDT). This area has been reviewed.13 Other possible

Figure 1. Room-temperature absorption spectra in toluene: (top) porphyrin, (bottom) porphycene. 2448


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two lowest electronic transitions becomes smaller than in porphyrin (ca. 88047 vs 3100−340048 cm−1; the exact value depends on the solvent). The Q transitions in porphycene, L1 in particular, are much more intense than in porphyrin. The absorption coefficients measured at the frequency of the S1(0− 0) transition differ by a factor of about 50. Integration over the whole energy range of Q bands leads to an area that is a factor of 3 larger for porphycenes. On the contrary, porphyrin exhibits stronger absorption than porphycene in the Soret region: the ratio of maximum absorption coefficients is about 2, whereas a value of 1.4 is obtained from integrated absorption intensities. Interestingly, absorption into B states lies at lower energies in porphyrin. Numerous calculations of the electronic transitions in porphycenes have been reported; they are discussed in section 2.4. Theoretical analysis allows an understanding of the differences in the electronic absorption spectra of 1 and 2, as well as the trends observed in differently substituted porphycenes. The symmetry of porphycene (C2h) allows any in-plane direction of an electronic transition moment corresponding to a 1 ππ* excited state. Measurements of linear dichroism and the experiments that exploit the change in direction of the transition moment as a result of trans−trans tautomerization, described in detail below, made it possible to determine the absolute directions of the transition moments corresponding to the S0−S1 and S0−S2 transitions in porphycene.49,50 These two transitions are nearly orthogonal, with the S0−S1 transition moment approximately parallel to the direction defined by the NH − HN bonds. Thus, the transition moment pattern characteristic for porphyrin is retained in porphycene. Table S1 contains the absorption data reported for roomtemperature solutions for free-base porphycenes. The corresponding data for metalloporphycenes are presented in Table S2. 2.1.1. Alkyl-Substituted Porphycenes. Absorption spectra have been reported for a number of alkyl-substituted porphycenes. Symmetrically tetra-β-substituted (2,7,12,17-) derivatives include tetramethyl- (11),51 tetraethyl- (12),51 tetrapropyl- (13),47,51,52 tetra-n-hexyl-(14),53 tetra-tert-butylporphycene (15)54 (Scheme 2). For all these compounds the

Figure 2. Absorption at 293 K in acetonitrile solution: (a) 2,7,12,17tetrapropylporphycene, (b) 2,3,6,7,12,13,16,17-octaethylporphycene, and (c) 9,10,19,20-tetrapropylporphycene.

2,7,12,-tritert-butylporphycene (18), 2,7,12,17-tetra-tert-butylporphycene (15).55 Adding four more substituents at β′ positions has a larger effect. The spectra of 2,3,6,7,12,13,16,17-octaalkyl porphycenes56−58 (Scheme 3) are red-shifted with respect to that of 1 Scheme 3

by about 30 nm for the Q and Soret bands (Figure 2b). This is due not only to electronic, but also to steric factors, as can be deduced from a comparison of the spectra of nonplanar 19 and nearly planar 20. Both compounds exhibit considerable red shifts, but with the values reduced after the replacement of ethyl by methyl groups at the β′ positions.57 Introduction of four electron-withdrawing CF3 groups at β′ positions results in a highly distorted, nonplanar structure 21, with a strongly red-shifted Q band.59 The position of the Soret band remains similar to that in the corresponding EtioCH3 derivative 20. In contrast, substitution of 19 at the meso position with an electron-withdrawing group, leading to 9(ethyl-3′-prop-2′-enoate)octaethylporphycene 22 lowers the frequencies of both Q and Soret bands.60 Meso-substituted alkyl derivatives 23−25 (Scheme 4) show absorptions similarly red-shifted with respect to parent 1 as the octaalkyl β,β′-substituted porphycenes, with a somewhat different intensity pattern in the Q region (Figure 2c).49,61,62

Scheme 2

absorption spectra are very similar to that of parent porphycene (cf. Figures 1 and 2a). Alkyl substitution causes a small red shift in the transition energies (up to 4−5 nm for the Q and 10−12 nm for the Soret bands), whereas the intensity pattern is practically unchanged. These trends have been illustrated in a more systematic fashion by introducing tert-butyl groups, one by one, into the porphycene skeleton and monitoring absorption changes in the series: porphycene (1), 2-tertbutylporphycene (16), 2,7-di-tert-butylporphycene (17),

Scheme 4

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Absorption spectra have been reported for mixed tetra-βtetra-meso-substituted porphycene 26 (Scheme 5).53 X-ray studies of 26 reveal nonplanarity, which might be responsible for the absorption extending to 800 nm.

Scheme 7

Scheme 5

The absorption spectra of 9,20-dimethylporphycene 27 and 10,19-dimethyl-2,7-di-tert-butylporphycene 28 (Scheme 6) are Scheme 6

Scheme 8 63

nearly identical. Once again, this shows the insensivity of the absorption to β-substitution. The peaks are located approximately halfway between those of the parent porphycene and those of the meso-tetramethyl derivative 23. Dodeca-substituted porphycenes (30 and 31) were obtained in the group of Yamada by McMurry coupling of bicyclo[2.2.2]octadiene- (BCOD-) fused 5,5′-diacyl-2,2′-bipyrroles.53,64−66 This strategy has led to various BCOD-fused porphycenes (29−34, Scheme 7). Their absorption spectra are similar to those of 2,3,6,7,12,13,16,17-octaalkyl porphycenes. The large series of 9-substituted derivatives of 2,7,12,17tetrapropylporphycene 13 (Scheme 8), 2,7,12,17-tetrakis(2methoxyethyl) porphycene 46 (Scheme 9), and related derivatives (Scheme 10) were the subject of a photophysical study by Braslavsky and co-workers.35 Some of these compounds, along with additional ones (73−76),67 were successfully tested as photodynamic antitumor agents.68−72 Remarkably, most of these porphycenes exhibited very similar absorption spectra, with the S1 transition occurring at about 640 nm. A considerable shift to the red was observed for amino (42 and 56) and hydroxy (35 and 47) derivatives, with the former displaying a larger effect. Various β′-substituted derivatives of 2,7,12,17-tetrapropylporphycene 13 have been obtained (Scheme 11), including mono-, di-, tri-, and tetrabrominated compounds (73−76),67,73 sulfonatoporphycenes (77−80),74−76 and sulfonamide derivatives (81−85).77 A theoretical study of 73, 74, and 7678 correctly reproduced the red shifts of the Q bands with increasing number of bromine atoms. Other β-tetraalkyl derivatives substituted at β′ with iodine or bromine for which the absorption spectra are available (86, 8755and 88, 8979) are presented in Scheme 12. Taking advantage of selective iodination of 2,7,12,17-tetra-nhexylporphycene 14, followed by Sonogashira coupling with trimethylsilylacetylene, Kuzuhara and Yamada annd colleagues

Scheme 9

obtained the series of porphycenes 90−95 with ethynyl substituents (Scheme 13).79 These molecules served as substrates for acetylene- and butadiene-linked porphycene dimers 80 (96 and 97, Scheme 14) and porphycenediketopyrrolopyrrole conjugates79 (98−100, Scheme 15). The absorption spectra of dimers 96 and 97 are similar in shape to those of the monomeric units, whereas the absorption coefficients are approximately doubled. This indicates a lack of strong interactions between the porphycene subunits. In diketopyrrolopyrrole conjugates, the interactions between the subunits are stronger, as evidenced by broadening of the 2450


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Scheme 10

Scheme 13

Scheme 11

Scheme 14

Scheme 12

The groups of Abe, Majima, and Hisaeda reported the synthesis of two porphycenes with ferrocenyl pendants (103 and 104, Scheme 17).82 The absorption spectrum remains practically unchanged after the first oxidation, occurring at the ferrocene moiety. In contrast, the second oxidation, occurring on the porphycene ring, results in a decrease of the intensities of the Q and Soret bands and the appearance of a broad absorption in the red region (600−800 nm). Haug and Richert attached porphycene 46 to the 3′-terminus of an oligodeoxynucleotide (Scheme 18).83 They found that such a porphycene−DNA hybrid binds more readily to a complementary region of a DNA target than the unsubstituted oligonucleotide. Absorption spectra were used to probe the effect of the porphycene chromophore on DNA. The UVmelting point (the temperature where 50% of the duplex is

absorption bands and by an increase in the absorption coefficient (178000 M−1 cm−1 at 653 nm for 99 in CHCl3).79 Jux, Guldi, and co-workers studied the properties of electronaccepting porphycenes on graphene.81 Porphycene 102, obtained from 101 (Scheme 16), rapidly accepts an electron after photoexcitation of graphene. The combination of nanographene and 102 was successfully tested in dye-sensitized solar cells. 2451


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Scheme 15

Scheme 16

Scheme 17

Scheme 18

dissociated) was found to increase, demonstrating duplex stabilization. Okawara, Abe, and Hisaeda carried out a systematic study of the effect of the acetoxylation of 2,7,12,17-tetra-n-propylporphycene 13 on its spectral properties.84 Comparison of the absorption spectra of 13 and six acetoxy derivatives (38, 106− 110, Scheme 19) showed rather modest changes. The largest red shift of the lowest-energy transition was observed for the tetraacetoxy derivative 110 (645 nm, compared with 633 nm in 13). Much larger shifts were observed in mixed single acetoxy− single amino derivatives 112 and 114 (but not in nitro derivatives 111 and 113).85 The effect of the amino group is 2452


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Scheme 19

Scheme 22

much larger than that of the acetoxy substituent, as can be deduced from the observation that the absorption spectra of compounds 42,35 112, and 114 are very similar. The groups of Panda and Kim reported the syntheses and spectral studies of β-tetrachlorotetramethoxyporphycenes 115 and 116,86 β,β′-octamethoxyporphycene 117,87 and β,β′octakis(methylthio)porphycene 11888 (Scheme 20). Whereas Scheme 20

the absorption spectra of 115−117 are quite similar to those of β,β′-octaalkyl-substituted porphycenes, this is not the case for the sulfur-containing derivative 118: It exhibits an ususual pattern in the Q region, with a low-lying (ca. 750 nm) S1 transition and the maximum intensity for the second band at about 700 nm. Moreover, the absorption intensities in the Q and Soret regions are very similar. The red-shifted absorption in 118 has been explained by a small highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap, caused mainly by stabilization of the LUMO in the sulfur derivative. Porphycene derivatives have been obtained with two pyrrolic nitrogens linked by methano (119)89 or ethano (120, 121)90 bridges (Scheme 21). The absorption spectra do not differ much from those of the parent compounds.

Figure 3. Absorption spectra of (a) 2,7,12,17-tetraphenylporphycene and (b) parent porphycene recorded at 293 K in toluene.

and by 26 nm with respect to that of the tetra-n-propyl analogue. Similar shifts are observed for the S2 and Soret transitions. The calculated one- and two-photon absorption spectra of 122 and isomeric meso-tetraphenylporphyrin agree very well with the experimental data.92 The absorption pattern characteristic for 122 is essentially preserved in derivatives with various substituents on the phenyl group, namely, 126,93128,94129,95 and 130 and 131,96 and for meso-dinitro derivatives 132 and 133.97 It is also maintained in temocene (134, Scheme 23),98 the porphycene analogue of temoporfin (Foscan), a commercially available photosensitizer in PDT. Temocene and its derivatives, 135,98 136 and 137,99 and 138,100,101 are promising in PDT because of their stronger absorption, higher photostability, and lower dark toxicity compared to temoporfin. Other potentially applicable arylsubstituted porphycenes are poly-ortho-functionalizable derivatives 139−141,102 shown in Scheme 24. They have been tested as building blocks in photoelectrochemical cells, showing good photoresponse profiles. The effect of substitution at the meso positions on the absorption strongly depends on the substituent: It is weak for the acetoxy group (123),103moderate for the nitro (124,103 132 and 133)97 and amide (127,97 12894) derivatives, and very strong for the amino substitution (125).103 Calculations of

Scheme 21

2.1.2. Aryl-Substituted Porphycenes. The first arylsubstituted derivative of 1, 2,7,12,17-tetraphenylporphycene (122, Scheme 22), was obtained by Nonell and co-workers in 1995.91 Phenyl substituents at the β positions affect the absorption to a larger degree than alkyl groups. This is illustrated in Figure 3. The lowest-energy transition in 122 is shifted by 30 nm with respect to that of the parent porphycene 2453


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fluorescence properties in the red and near-IR regions (Figure 4).105

Scheme 23

Scheme 25

Scheme 24

Figure 4. Absorption and fluorescence of (top) 142 and (bottom) 144 in acetone solution. Reproduced with permission from ref 105. Copyright 2015 The Royal Society of Chemistry.

Meso-aryl-substituted porphycenes, reported by the groups of Srinivasan (150 and 151),106 Ravikanth (150−155),107 and Jux (156)108 are shown in Scheme 26. The absorption spectra resemble those of aryl-β-substituted porphycenes, with the differences between these two groups being smaller than in the case of alkyl derivatives. 2.1.3. Heteroatom-Containing Porphycenes. Four-fold aza substitution in the porphycene macrocycle resulting in “imidacenes”, the imidazole analogues of porphycene (157,109 158−160,110,111 Scheme 27), leads to significant changes in the absorption. Large bathochromic shifts are observed for the Q bands. The effect is more pronounced in the aryl derivatives, as illustrated by the position of the S0−S1 transitions in 157 and 158: 686 vs 760 nm. Moreover, the aryl derivatives exhibit absorption with maximum intensities in the Q region equal to

transition energies, performed for 125,104 reproduce this trend. Nonell and co-workers carried out the reaction of porphycene isothiocyanate 142 with primary and secondary amines, obtaining a series of thiazolo[4,5-c]porphycenes (143−149, Scheme 25) that exhibit very promising absorption and 2454


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28) and includes benzo[2,3]porphycenes 166 and 167;68 octaethylbenzochloracene 168; 6 0 dibenzo[cde,mno]porphycenes 169 and 170;117,118 dibenzo[b,f ]porphycenes 17164 and 172;108 tetrabenzo[b,f,l,p]porphycenes 17364 and 174 and 175;53 dinaptho[b,f ]porphycene 176;65 tetranaphto[b,f,l,p]porphycene 177;65 dinaphtho[cde,mno]porphycenes 178,119,120 179,121 180 and 181,119 and 182−184;120 benzo[i]porphycenes 185122 and 186;35 and dibenzo[i,s]porphycene 187.122 Most of these compounds, but not all, exhibit absorption above 700 nm. Fusing one or two benzene rings to the ββ′ bond does not affect the absorption significantly, as can be deduced from the inspection of the spectra of 171, 172, and 176. It is interesting to note that, for a particular attachment position, benzo and naphtha fusions have similar effects. In fact, larger dinaphtho systems 178−184 absorb at shorter wavelengths than their dibenzo counterparts, 169 and 170. This can be explained by distortion from planarity in the latter. Definitely, the largest perturbation is observed in mesodibenzoporphycene 187, which exhibits the first absorption peak at 1047 nm, a wavelength ca. 300 nm or more longer than those observed in other benzoporphycenes (Figure 5). 2.1.5. Metalloporphycenes. Replacement of two inner hydrogen atoms in free-base porphyrin by a metal results in a symmetry increase, from D2h to D4h. This leads to degenerate L(Q) and B(Soret) excited states, and so the absorption bands become “thinner”. In metalloporphycenes, which, for planar structures, have D2h symmetry (C2h in free base), there are no degeneracies, but complexation leads to similar spectral effects as in porphyrins, although the two Q transitions in free-base porphycenes lie closer to each other and so the “thinning effect” is smaller. Thus, the spectra of metalloporphycenes are usually simpler than those of free bases, as can be demonstrated, for example, by comparison of the absorption of free-base 19 (Figure 2b) and its complexes with Zn, Co, and Mn (Figure 6, left). It should be noted that the “simplicity” is lost in singly reduced anions (Figure 6, right). Comparison of the spectrum of the copper complex of 13 with that of octaethylporphyrin (Figure 7) shows that the absorption pattern is quite similar, although the Q/Soret intensity ratio is definitely higher in the former. Table S2 contains the absorption data of metalloporphycenes available in the literature. The largest number of spectra has been reported for various nickel porphycenes: 13,51,115,123 14,64 19,60,123,124 20,123,125 25,61,115 35,126 38,126117,87 118,88 170,125 171,64 173,65 181,119 183,120 186, 188−192125

Scheme 26

or even larger than those of the Soret bands. This effect might be due to a larger energy splitting of the transitions contributing to the Soret bands in the aryl derivatives 158− 160 than in the tetrapropyl porphycene 157. 21,23-Dithiaporphycene 161112 (Scheme 27) shows absorption characteristics typical for porphycenes, with a somewhat larger separation between the energies of the Q transitions. Double 3,13-aza substitution (162)113 leads to bathochromic shifts, but not as strong as was the case for imidacenes 157− 160. Two-fold substitution of 162 at the β positions (2,12-) with tert-butyl (163)113 and phenyl (164)113 groups mainly influences the lowest-energy transition; its red shift is larger in the aryl derivative. Replacement of all four nitrogen atoms by oxygen leads to oxaporphycene dication 165 (Scheme 27).114 The electronic spectrum of this species115,116 is definitely porphycene-like, with a richer vibrational structure, and the S0−S1 transition at 599 nm, thus about 30-nm blue-shifted with respect to the parent compound. 2.1.4. Benzoporphycenes and Naphthoporphycenes. For applications in such areas as PDT and cellular imaging, it is extremely important to use chromophores that absorb and emit efficiently in the so-called “physiological window”, the spectral region between ∼650 and 950 nm, where the tissue is relatively transparent. Such requirements stimulate the design of porphycenes with enlarged π-electron conjugation paths. The list of such compounds obtained so far is quite rich (Scheme Scheme 27

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Scheme 28

(Scheme 29), 193−197127 (Scheme 30), 198 and 19960 (Scheme 31), and 203−205123 (Scheme 32). For 25-Ni, the near degeneracy of the S1 and S2 states was confirmed by magnetic circular dichroism.115 Signals of opposite signs, separated by less than 500 cm−1, were observed. The two transitions lie too close to each other to be distinguished in the absorption spectrum. The list of iron(III) porphycenes investigated using UV/vis spectroscopy includes 13,128,129 19,124 20,130,131 21,132,131 209,28,29 212−21432,34,133 (Scheme 33), 215−217,54,131,134 219 and 220,131,132,135 and 221 and 222136 (Scheme 34). UV− visible spectral studies were crucial for the analysis of redox processes, allowing for the determination of the reduction site.128,129,134,137 Moreover, the absorption is sensitive to the spin state of the metal.138

Insertion of ruthenium into the porphycene skeleton was achieved for 13,139,140 19,140 200−202123 (Scheme 32), 224141 (Scheme 35), and 233142 (Scheme 40, below). Upon irradiation of benzene solutions containing pyridine, ruthenium(II) carbonyl complexes of 13 and 19 undergo photosubstitution and yield bis-pyridine complexes.139,140 The process is faster in porphycenes than in the corresponding 2,3,7,8,12,13,17,18-octaethylporphyrin analogue. Absorption spectra have been measured for zinc complexes of 19,56,57,143,144 29,65 32,65 115−118,86−88 168,60 171,65 173,65 176 and 177,65 discotic liquid-crystalline materials 225 and 22627 (Scheme 36), and self-assembled porphycene−C60 complex 227145 (Scheme 37). Spectrophotometry was used to determine the rate of the self-exchange electron-transfer reaction between 19-Zn and its π-cation radical.143 An 2456


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Figure 5. Electronic absorption spectra of 1 (black), 185 (blue), and 187 (red) in CH2Cl2 at 298 K. Inset: Expanded Q-band absorption region. Reprinted with permission from ref 122. Copyright 2015 Wiley-VCH Verlag. Figure 7. Absorption spectra, recorded in CH2Cl2, of 13-Cu (CuPPC) and (inset) 2,3,7,8,12,3,17,18-octaethylporphyrin (CuOEP). Reprinted with permission from ref 138. Copyright 1991 American Chemical Society.

into 13-Co(III)(alkyl) complexes.147 Alkyl complexes of Co(III) with 13, 19, and 20 were found to be efficient catalysts for the oxidation of vinyl ethers.15 The spectra of palladium complexes have been obtained for 13,150 19,87 35,151 38,151 115−117,86−88 and 122.152,153 Pd complexes of 11787 and 122153 are efficient singlet oxygen generators. Using HeLa tumor cells, photokilling properties of 122-Pd were demonstrated.152 Copper complexes of porphycenes for which absorption has been measured include 13,138,154 19,123,124 20,123 76,138 122,153 178,155 and 206−208.123 Comparison of the spectra of Cu and Ni complexes of 13 with those of the octaethylporphyrin analogues led to the conclusion that the low-lying ππ* states in porphycenes are, in contrast to those in porphyrins, well described by a one-electron function and that the configuration interaction is weak.138 Vibronic coupling, very important for the interpretation of the porphyrin spectra, is not as relevant for porphycenes, which have much simpler absorption spectra. Hexacoordinated complexes of porphycenes with tin have been described for 1,115 13,156 and 19.157,158 Dihalide complexes of 13-Sn(IV) exhibit strong absorption in the Q region. They are also good photosensitizers, which was demonstrated by their ability to catalyze the photooxidation of 1,3-diphenylisobenzofuran.156 Guilard and Kadish and co-workers reported pentacoordinated alumina complexes 218134,159 and 228−230159 (Scheme 38). Typically for porphycenes, the absorption spectra in the visible region are more intense in comparison with those of porphyrins, and the Soret transitions are split. Manganese-containing porphycenes 19124 and 210 and 21117 (Scheme 33) have been described. The latter two compounds were used in the construction of artificial myoglobin, which, in contrast to the native molecule, was able to catalyze alkyl C−H bond hydroxylation. Other less frequently studied metalloporphycenes are those containing rhenium (231160 and 232,161 Scheme 39), osmium (234 and 235,142 Scheme 40), indium (223, Scheme 34),137

Figure 6. (Left) Absorptions of (from bottom to top) Zn, Co, and Mn complexes of 19 in benzonitrile containing 0.1 M tetra-nbutylammonium perchlorate. (Right) Spectra of the singly reduced species. Reprinted with permission from ref 124. Copyright 1994 American Chemical Society.

extremely high rate was found, explained by a small value of the inner-sphere reorganization energy. Various cobalt complexes have been investigated for porphycenes 13,15,128,135,146−149 19,124,135 20,135 and 21.148 UV/vis spectroscopy served as a tool for identifying different redox species.124,135 Stable cobalt(III) hydride species were detected for complexes with 13.148 Reversible binding of molecular oxygen to 13-Co(II) could be monitored by observing changes in the absorption spectra.146,149 Similar absorption changes were observed when dioxygen was inserted 2457


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Scheme 29

shifted with respect to that of the neutral molecule, with the lowest-energy peak appearing well above 800 nm. Absorption spectra were obtained for five redox stages of tetraoxaporphycene 165.116 The spectra are very different for the various species; that of the radical anion shows a low-lying, strong transition at 902 nm. Absorption spectra of neutral and singly reduced metalloporphycenes were compared for complexes of 19 with Co, Ni, Zu, Zn, Mn, and Fe.124 Large red shifts were observed in the anionic species in the Q region, whereas the positions of the Soret bands were similar (Figure 6). Vibrational and electronic spectra of reduced porphycenes have been studied by Teraoka and co-workers for 1,166 13,167 13-Fe(III)Cl,168 and 20.168 Differences between N-deprotonated σ-type dianions and π-type dianions have been discussed.168 Guldi and co-workers used various radiolysis techniques to observe the oxidation169 and reduction170 of 13 and its complexes with various metals (Fe, Co, Ni, Cu, Sn). π-radical cations and anions, dications, and dianions were observed and characterized by UV/vis spectroscopy. In comparison with analogous porphyrins, the oxidized porphycene species are, in most cases, more stable, which can be explained by the lower oxidation potentials of the porphycene. Also, the π-radical anions of porphycenes are significantly more stable in protic solvents than the corresponding porphyrin forms. This indicates more difficult protonation of the reduced porphycenes, because of the less negative reduction potentials. Similar experiments were performed to study the reduction of 2,3,6,7,11,12,17,18-octaethylcorrphycene and its complexes with Sn, Fe, Co, Ni, Cu, and Ag.171 The general pattern of reduction was similar to that observed in porphyrins and porphycenes. In some cases, however, differences were observed, explained by variations in the reduction potentials and in the size of the macrocyle inner cavity. Oxidation of copper complexes of 13 and zinc complexes of 19 was studied, together with the electron self-exchange reaction between the complexes and their π-cation radicals.143

Scheme 30

Scheme 31

Scheme 32

platinum (13),150,162 molybdenum (19),163 and magnesium (19).164 2.1.6. Radical Species, Cations, and Anions. Fajer and co-workers reported the absorption spectrum of 13-Ni π-anion radical.165 The spectrum in tetrahydrofuran (THF) is strongly 2458


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Scheme 33

Scheme 34

Scheme 36

Scheme 37

Scheme 35

Scheme 38

Electronic absorption spectra and their decay kinetics were reported for both the radical cation and the radical anion of 122 and its Pd(II) complex.172 The π-radical species absorb at lower energies than the triplet states, which is a general pattern for both alkyl- and arylporphycenes. 2459


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Scheme 39

Scheme 40

Absorption spectra have been published for doubly protonated dications of 1, 13, and 25,115 as well as 20.173 The changes occurring after protonation are rather minor. 2.1.7. High-Resolution Spectra of Cold, Isolated Molecules. 2.1.7.1. Matrix-Isolated Porphycenes. Electronic absorption spectra have been obtained for the parent porphycene 147,174−176 and derivatives (13,47 15,176,177 38,178 and 169 and 170179) isolated in low-temperature rare gas, nitrogen, or glassy solvent matrixes. The combination of fluorescence, IR, Raman, and neutron scattering measurements and quantum-chemical calculations made it possible to assign nearly all of the vibrations of 1 in the ground electronic state and compare the frequencies in S0, S1, and S2.47,180 Moreover, high spectral resolution enabled the determination of the exact energies of the S0−S1 and S0−S2 transitions and, thus, the S1− S2 separation.47 Because of the proximity of the two lowest excited states, this could not have been done based by standard, condensed-phase measurements. The site structures exhibited by the electronic spectra of porphyrin,181,182 parent porphycene 1,183 and the tert-butyl derivative 15177 in argon and xenon matrixes were analyzed using molecular dynamics and DFT calculations. The geometry and energy characteristics of the main trapping sites were proposed. A general methodology was developed, allowing numerical molecular dynamics (MD) simulation of matrix deposition and thermal annealing effects. 2.1.7.2. Porphycenes Isolated in Supersonic Jets and Helium Nanodroplets. Fluorescence excitation spectra recorded for porphycene 1 isolated in a supersonic jet revealed splitting of the vibronic lines into doublets.184,185 The splitting disappeared when the inner protons were replaced with deuterons. This finding provides evidence for coherent double hydrogen tunneling in a symmetric double-minimum potential. Tunneling splittings have also been observed for other jetisolated porphycenes: 15,186 23, and 25.187 For porphycene 1 embedded in ultracold (0.37 K) superfluid helium nanodroplets, the splittings were not detected (Figure 8).188 They appeared, however, in the fluorescence spectra, indicating that tunneling mainly originates in S0, whereas in S1, the doublet components are very close in energy. Extremely important is

Figure 8. (Top) Fluorescence excitation spectra recorded in supersonic jet (upper curve) and in a helium droplet for 1 (Pc) and its two isotopologues with one or two protons replaced by deuterons (Pc-d1 and Pc-d2, respectively). (Bottom) Fluorescence spectra obtained in helium nanodroplets, with the largest values of tunneling splittings indicated. Adapted with permission from ref 188. Copyright 2009 Wiley-VCH Verlag.

the observation that the value of the splitting crucially depends on which vibrational level is excited.185,188,189 The mode selectivity of tunneling is discussed in more detail in section 4.1 on tautomerism. 2.2. Magnetic Circular Dichroism and the Perimeter Model

Magnetic circular dichroism (MCD) spectra have been published for parent porphycene, its doubly protonated and doubly deprotonated forms,62,115 and several derivatives: 1SnCl2,115 13,115 13-Ni,115 14,161 14-Ni,64 23,62 25,62,115 25Ni,115 165,115 171-Ni,64 173-Ni,64 and 232.161 The MCD spectra can be interpreted in terms of the perimeter model of aromatic systems, originally developed by Platt 43 and Moffit44,190 and extended by Gouterman,45 Heilbronner and Murrel,191 and finally Michl,46,192−194 who obtained the analytical formulas for the Faraday A, B, and C terms, the parameters that characterize MCD signs and intensities. In this model, the molecule of interest is derived from an ideal (CH)n perimeter containing 4N + 2 π electrons by specific perturbations, such as bridging, substitution, or replacement by heteroatoms. Figure 9 shows the derivation of porphyrin and porphycene from a C20H202+ perimeter by bridging with two NH and two N− groups. The same perturbations, applied in a different way to the same perimeter, lead to a completely 2460


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Figure 9. Derivations of porphyrin and porphycene from an ideal C20H202+ perimeter and the ensuing splitting of the HOMO and LUMO.

Figure 10. Absorption and MCD spectra of (a) 1, (b) 13, (c) 19, and (d) 25 recorded in acetonitrile solution at 293 K.

different pattern of energy splitting in the two HOMOs and the two LUMOs. To analyze the Q and Soret transitions, four electronic configurations are taken into account, involving the HOMO and LUMO. These two pairs of orbitals are degenerate in the parent perimeter, but the degeneracy is lifted as a result of perturbations. Crucial for the spectral predictions is the analysis of the relative energy splittings of HOMO and LUMO pairs. The chromophores that exhibit similar splittings (ΔHOMO ≈ ΔLUMO) are called “soft”, whereas those for which the splittings are very different are labeled “hard” (positive-hard if ΔHOMO ≫ ΔLUMO and negative-hard for ΔHOMO ≪ ΔLUMO). This terminology is based on the fact that in soft chromophores even a weak perturbation can significantly affect

the signs and magnitude of MCD spectra, as well as the relative absorption intensities of the Q and Soret bands. This is not the case for hard chromophores, where the strong inequality of HOMO and LUMO energy splitting is very difficult to change. Simple analysis of the shapes of the frontier orbitals of the C20H202+ parent perimeter, supported by calculations,2,38 shows that porphyrin is a soft chromophore, whereas, among all of the isomers, porphycene exhibits the largest inequality of orbital splitings: With ΔHOMO ≪ ΔLUMO it can be classified as a negative-hard chromophore.2,115 This property has large consequences for the spectral behavior of porphycenes, in both absorption and MCD spectra. For the latter, the negative/ positive sign sequence of the Faraday B terms is predicted for the Q transitions. Indeed, this pattern was observed for all of 2461


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the porphycenes measured so far by this technique. Figure 10 illustrates the similarity of MCD spectra of 1, 13, 19, and 25. Using the perimeter model, it can be shown that the relative ratio of Q/Soret absorption intensities scales approximately as |ΔHOMO2 − ΔLUMO2|.115 This ratio should be much larger in porphycenes than in porphyrins, and the absorption pattern of porphycenes should not be strongly affected by various possible substitutions. As can be seen from Tables S1 and S2, this is indeed observed. The perimeter model allows for the prediction of the properties of all porphyrin isomers.2 Thus, corrphycene should be a soft chromophore, whereas for hemiporphycene, a negative-hard character is expected, with the inequality of HOMO and LUMO splitting somewhat smaller than in porphycene. Both predictions have been corroborated by experiments carried out for octaethylhemiporphycene195 and octaethylcorrphycene.196 2.3. Two-Photon Absorption

In view of the potential use of porphycenes in PDT, the values of two-photon absorption (TPA) cross sections are of great importance. Initial research in this relatively unexplored area is encouraging. The comparison of the TPA characteristics of 122 and its Pd(II) complex with its isomeric analogue, mesotetraphenylporphyrin (TPP), showed that, in the 750−850-nm range, the TPA cross sections of porphycenes are much larger than those of TPP.92 This was explained by a near-resonance condition with the strong Q transition in porphycenes. In a series of theoretical works,197−199 it was shown that the TPA parameters can be tuned by replacement of core nitrogen atoms, metal coordination, or addition of electron-withdrawing or -donating substituents. Rao and co-workers estimated two-photon and three-photon properties for dinaphthoporphycenes 178, 180, and 181 and their nickel complexes 178-Ni and 180-Ni, exploiting the resonances at 800 nm.200 The values of the nonlinear absorption coefficients were reported to be superior to or similar to those of molecules that exhibit large nonlinearities.

Figure 11. S0−S1 absorption spectra of 1: (a) spectrum simulated at the B3LYP/def-TZVP level,223 (b) absorption in Ar matrix,47 and (c) LIF excitation spectrum.185 Reprinted with permission from ref 223. Copyright 2014 AIP Publishing LLC.

3. PHOTOPHYSICS 3.1. Fluorescence and Other Energy Relaxation Channels

Porphycene is a good emitter: The quantum yield of roomtemperature fluorescence, 0.36−0.49, depending on solvent (Table S3), is about 10 times higher than the emission yield in porphyrin under the same conditions, even though the decay times are very similar (about 10 ns). Thus, the higher quantum yield in 1 has the same origin as the difference in absorption: a larger radiative constant (3.5 × 107 in 1 vs 4.3 × 106 s−1 in 2). As explained in section 2.2, the origin of the larger value in porphycene lies in its hard chromophore character (ΔHOMO ≪ ΔLUMO), as opposed to the soft chromophore nature of porphyrin (ΔHOMO ≈ ΔLUMO). Table S3 compiles the photophysical data reported over the years for more than 110 different porphycenes, mostly in their free-base form. For those for which both fluorescence quantum yield (ϕ) and decay time (τ) have been measured, the radiative constant can be calculated by using the formula kr = ϕ/τ. This was done based on the data in Table S3, and the results are presented in Figure 12. The histograms were divided into three groups, showing kr values of alkyl- and aryl-substituted porphycenes and a smaller set of differently coordinated zinc complexes of 19. It is remarkable that the values of the radiative constants are very similar. The values for aryl derivatives seem, on average, to be somewhat higher than those for alkylporphycenes. Complexes of 19-Zn exhibit definitively smaller values. It is unfortunate that the value for free-base 19the lowest in Figure 12a and clearly different from the other valuesis probably incorrect, because of an erroneous reported fluorescence lifetime, 5.0 ns.56 The measurements performed at the author’s laboratory for the same molecule yield a value roughly 10 times shorter.

2.4. Calculations of Electronic Structure and Spectra

The electronic structure of parent porphycene 1 has been the subject of many studies that used various theoretical models.2,9,50,115,167,201−227 Other porphycenes for which calculations have been performed include 1-Mg;201,202,228 1Zn;228 1-Fe(II);229 13;78,225 20 and 21;218,230 25;49,225 38;231 73, 74, and 76;78 122;92 125;104 dibenzoporphycenes 169 (without methyl groups)210,232 and 187;210 169-(Fe)II;229 and tetraoxaporphycene 165.204 Some of these works reported calculations of the electronic transitions in 1 (see refs 50, 115, 201, 202, 204−206, 218, and 221−223); 13;78,225 20 and 21;218,230 25;49,225 73, 74, and 76;78 122;92 125;104 tetraoxaporphycene 165;204 169; and 187.210 In general, satisfactory agreement with the observed pattern of Q and Soret transitions has been obtained, with the differences between experimental and predicted band maxima being no larger than 0.2−0.3 eV. Also, the vibrational structure observed in matrix or supersonic-jet spectra can usually be reliably reproduced by calculations. This is illustrated in Figure 11, which compares the observed and simulated high-resolution spectra of 1. Calculations allowed the assessment of the contributions of Duschinsky and Herzberg−Teller effects to the intensity of vibronic transitions in 1, compared with magnesium porphyrin and zinc tetraazaporphyrin.223 2462


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Interestingly, the transient signals exhibited quantum oscillations, decaying in 1.6 ps. This value is comparable to the time required for tautomerization, which is promising in view of possible quantum control of the reaction by suitably timeshaped laser beams.231,235 In meso-alkyl porphycenes 23 and 25, ultrafast depopulation (50 fs) of S3/S4 Soret states into S2/S1 Q levels was observed. This time scale was similar to that determined for porphyrin,236 but it should be noted that in porphycenes the energy separation between Q and Soret transitions is about twice larger, so the ultrafast relaxation could not have been predicted a priori. Relaxation processes have been also studied for porphycenes 1,175,176 15,176 and 169 and 170175,179 embedded in cryogenic rare-gas matrixes. For 1, a completely relaxed S1 state, decaying in nanoseconds, could be observed only after more than 100 ps from the moment of excitation. A strikingly different behavior was found in dibenzoporphycenes: the relaxation into and from S1 occurred in picoseconds. The relaxation times, 16−20 ps in argon, were shortened to 9−13 ps for molecules embedded in glassy or liquid environments. Fast relaxation in dibenzoporphycenes was ascribed to the flexibility of their nonplanar macrocyclic skeletons. 3.1.1. Models of Excited-State Deactivation. An intriguing behavior was observed for tetra-meso-alkylated porphycenes 23 and 25. These molecules show extremely low quantum yields in nonviscous solvents, but the emission intensity strongly increases in high-viscosity environments, such as oils, polymers, or glasses.62 The deactivation channel involves internal conversion, not ISC.62,225 A similar behavior was also discovered in 19, for which the quantum yields of deactivation through radiative and ISC channels sum to only 0.06.56 In contrast, this sum amounts to 0.97 in 19-Zn, indicating that the inner hydrogen atoms play a major role in radiationless deactivation. Zinc complexation has also been reported to dramatically enhance the fluorescence of 32.65 A model that can explain the mechanisms of excited-state relaxation has been proposed, based on the above findings and the results of calculations.237 It postulates a conical intersection along the path of single hydrogen transfer in S1, leading from the initially excited trans to the cis2 form (Figure 13). The ground state of the latter is actually an open-shell species, with a closed-shell state located nearby. The structure responsible for efficient S0 ← S1 internal conversion is nonplanar, with two pyrrole rings bearing the hydrogens in cis2 twisted in opposite directions. Thus, the deactivation path involves both hydrogen transfer and large-amplitude twisting motion. The calculations were able to predict the relative efficiencies of radiationless deactivation in the series 1, 27, 19, and 23. A conceptually similar model, involving excited-state trans− cis tautomerization, was proposed earlier to understand the origin of extremely weak fluorescence of dibenzoporphycenes 169-170.118 To explain the dual fluorescence and tautomerism in several 9-substituted porphycenes (42, 44, 47, 56, 125), Nonell and co-workers postulated the population of an upper excited state.238 They assigned this state to a cis species, without, however, specifying its structure. One should note that four different cis forms are possible in an unsymmetrically substituted porphycene. Not much photophysical data exist for metalloporphycenes, especially regarding the triplet formation efficiencies (Table S3). Based on the results available for palladium and copper complexes of 122,153 one can postulate that, in the metal

Figure 12. Histograms of the values of radiative constants obtained for (a) alkyl-substituted porphycenes, (b) various complexes of 19-Zn, and (c) aryl-substituted derivatives.

The similar values of radiative constants in very different porphycenes are in agreement with similar absorption characteristics. They also indicate that no large structural changes occur upon excitation. There are some exceptions, however: 124 and 125, tetraphenylporphycenes substituted at the meso position with strongly electron-accepting and -donating nitro and amino groups, respectively. Also, the porphycenes 137 and 138, bearing a positive 4+ charge, exhibit very small values of kr in water. In methanol, however, the “normal” values are recovered. Most porphycenes exhibit fluorescence quantum yields at least on the order of a few percent or higher. Those that do not can be rapidly deactivated in S1 for various reasons: (a) nonplanarity (75, 76, 80, 96, 97, 169, 170), associated with the conical intersection along the excited-state trans−cis2 tautomerization path, as described in more detail in section 3.1.1; (b) low-lying S1 state (169, 170, 186) (energy-gap law); or (c) specific interactions with the solvent (137, 138). The low fluorescence quantum yields of Pd and Cu complexes of 122 might be due to a heavy-atom effect, but it is rather surprising that the triplet formation efficiency of 122-Cu is only 0.35. It is important to stress that deactivation through internal conversion (IC) cannot be neglected in porphycenes. Contrary to the case of many porphyrins, where the sum of fluorescence and intersystem crossing (ISC) efficiencies is close to 1, the IC and ISC rates are often comparable and similar to the value of the radiative constant. For instance, in the parent compound 1, the values of kr, kIC, and kISC are 3.5 × 107, 2.2 × 107, and 4.1 × 107 s−1, respectively, for toluene solution.38,52 Thus, all three channels participate with comparable probablities in S 1 deactivation. The situation is very similar in 13. In the tetraphenyl porphycene 122, the value of kIC significantly increases (about 5 times), most probably due to a lower rigidity caused by introduction of phenyl substituents.153 Relaxation from higher excited singlet states was studied by transient absorption spectroscopy for 1233 and by time-resolved emission spectroscopy for 23 and 25.234 Parent porphycene, when excited in acetonitrile solution with a femtosecond pulse into S2, exhibits a sequence of three processes: intramolecular vibrational redistribution, lasting for tens of femtoseconds, S1 ← S2 internal conversion (750 fs), and cooling of the hot molecule by energy exchange with the solvent (16 ps). 2463


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9195 cm−1). Nonell and co-workers obtained values of 124, 126, and 126 kJ/mol for 122, 122-Pd, and 122-Cu, respectively.153 For the tetraazaporphycene 158, the triplet lies much lower, 98 kJ/mol,111 which is consistent with the red shift observed in the absorption spectrum. Phosphorescence was measured by the group of Hisaeda for 1 and the brominated derivatives 73−75.67 Peaks were observed at 978, 990, 996, and 1053 nm, respectively. A T1 energy of 10575 cm−1 (126.5 kJ/mol) was also determined for the 19-SnCl2 complex.157 A similar energy (10780 cm −1 = 129.0 kJ/mol) was found for the octaethylhemiporphycene analogue, but for the octaethylporphyrin complex, the value was much higher (14010 cm−1 = 167.6 kJ/mol). The lifetime of the triplet state in porphycenes is, in welldeaerated solutions, on the order of 200 μs. For instance, values of 200 and 270 μs were reported for 1 and 13, respectively.239 Upon saturation with air, the lifetimes decrease to 150 and 190 ns, respectively. For comparison, the triplet-state lifetimes of porphyrin in rare-gas matrixes are 0.63 ms in Xe and 2.6 ms in Ar.240 Metal complexation accelerates triplet decay. Lifetimes of 26, 5, and 0.25 μs were obtained using laser photolysis for 1-Zn, 13-Pd, and 13-Pt, respectively.150 For the same compounds, sensitization experiments yielded lifetimes of 85, 20, and 8 μs, respectively. Additionally, for 13-Ni, a value of 15 μs was obtained. A triplet lifetime of 85 μs was reported for the zinc complex of 19, to be compared with 140 μs in the free base.56 Triplet states of several porphycenes (1,150,242−246 1-Zn,150 13,150,242,243 13-Pd,150 13-Pt,150 13-Ni,150 19,56,243 19-Zn,56 and 25242) have been studied using the electron paramagnetic resonance (EPR), electron−nuclear double resonance (ENDOR), and electron spin−echo techniques. Zero-field splitting parameters and the hyperfine coupling tensor components were determined. The authors concluded that 1 exists in the trans form in both S0 and T1. Interestingly, however, two triplet states were observed, one of them populated at higher temperatures.244,245 This was interpreted in terms of tautomerism.

Figure 13. Possible tautomeric forms of 1 and their relative energies.

complexes, internal conversion might also provide an important S1 deactivation channel. 3.2. Triplet−State Parameters

The energy of the lowest triplet state was estimated from phosphorescence measurements in bromobenzene to be 10110 and 10370 cm−1 in 1 and 13, respectively.239 In porphyrin 2, the T1 state lies about 2500 cm−1 higher.240 This might be due to the lower symmetry of 1 as compared with 2. Another possible explanation is that the lowest triplet state in porphycene corresponds orbitally to the second excited singlet state, because of larger singlet−triplet splitting for S2 than for S1. Such a situation cannot be excluded, because the S1−S2 energy separation in 2, ca. 3500 cm−1,48 is lowered in 1 by 2500 cm−1. Further studies are required to resolve this issue. The calculations predict T1 and T2 in 1 to be spaced by less than 0.3 eV.241 Braslavsky and co-workers estimated the triplet energies for a large series of porphycenes (13, 35−39, 46−48, 50, 56, 58, 60, 64, 65, 67). All of the triplet energies were ca. 110 kJ/mol (=

3.3. Singlet Oxygen Generation

As already mentioned, absorption in the red portion of the visible spectrum is much stronger in porphycenes than in porphyrins. This would make porphycenes better candidates for photosensitizers in PDT, provided, however, that they are able

Figure 14. Symmetric double-minimum potential and the resulting tunneling splittings. Cross sections through the potential energy surface corresponding to the tautomerization-promoting and -inhibiting vibrational modes are shown on the left and right, respectively. 2464


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to efficiently generate singlet oxygen. In parent 1, the values of the triplet formation efficiency, ΦT, determined using various techniques and different solvents, span the range of 0.30− 0.42.52,239,247 As shown in Table S3, these values are typical: In most porphycenes, Φ T is about 0.30−0.40, which is approximately one-half the value found in porphyrins. However, once the triplet state is populated, it generates singlet oxygen with nearly 100% yield. In fact, porphycenes might be much more efficient sensitizers than porphyrins, because the difference in absorption coefficients more than compensates the lower triplet/singlet oxygen generation yield. Moreover, the triplet formation efficiency can still be increased by adding heavy atoms, either as substituents (73−75)67 or by replacing the inner hydrogens with a metal (122-Pd, 122Cu).153

4. TAUTOMERISM Porphycene can exist in three, pairwise degenerate, tautomeric forms: trans, cis1, and cis2 (Figure 13). Theory predicts the lowest energy for the trans form, but the cis1 structure lies only about 2 kcal/mol higher.203 For meso-tetraalkyl-substituted porphycene 23, the trans−cis1 energy separation drops below 1 kcal/mol.62 The energy of the cis2 tautomer is predicted to be much higher, about 30 kcal/mol.237 The symmetric double-minimum character of the potential describing tautomerization makes porphycenes good models for observing and analyzing such quantum effects as tunneling, coherent in isolated molecules and incoherent in condensed phases, or its manifestation in isolated molecules in the form of tunneling splitting (Figure 14). The process of tautomerization is usually referred to in the literature as “proton transfer” or “hydrogen transfer”, the distinction between the two terms often being somewhat arbitrary. Based on an analysis of the charge distribution,216 the term “double hydrogen transfer” seems preferable in the case of porphycene. An interesting question regarding the shape of the potential describing the hydrogen translation is whether, for small N−N distances and, thus, very strong H-bonds, the inner protons might reside halfway between the nitrogens, resulting in a single minimum. Ghosh and co-workers studied X-ray photoelectron spectra of several porphycenes with different N−N separations, along with tetraphenylporphyrin.248 The two X-ray photoelectron spectroscopy (XPS) maxima, corresponding to nitrogen atoms with and without a proton, approach each other in more strongly H-bonded molecules, but they do not converge (Figure 15). The authors discussed the question of whether perfectly symmetrical hydrogen bonds (i.e., single-well potentials) exist. Shortly after the synthesis of porphycene, it became obvious that its tautomeric properties are very different from those of porphyrin. A comparative study of the two isomers, performed for crystalline samples by 15N cross-polarization magic-anglespinning (CPMAS) NMR spectroscopy in a wide temperature range, showed that, in porphyrin, the exchange of two inner hydrogens between nitrogen atoms is fast at room temperature but becomes “frozen” below 192 K. Freezing of the reaction in the ground electronic state at low temperatures was also demonstrated for porphyrin in Shpol’skii249 or rare-gas250 matrixes. In contrast, in crystalline porphycene, the reaction is so fast that, even at temperatures as low as 107 K, the rate could not be measured.251 An extremely high rate of tautomerization was also found for other porphycenes (13, 19, and 25) using

Figure 15. XPS nitrogen 1s maxima. From top to bottom: 170, 13, 19, and meso-tetraphenylporphyrin. Reprinted with permission from ref 248. Copyright 2001 Wiley-VCH Verlag.

C and 15N CPMAS NMR spectroscopies.252 Indications of fast tautomerization were also provided by the crystallographic data for 11 and by the directions of the principal axes of the zero-field splitting tensor in 1, 13, and 25, determined by using time-resolved EPR spectroscopy.242 Further studies by the groups of Limbach, Kühn, and coworkers led to a picture of two strong and correlated intramolecular hydrogen bonds, suggesting a concerted double hydrogen transfer.214,253,254 The temperature dependence indicated tunneling as the major reaction pathway, with a barrier of 32 kJ/mol.253 A similar mechanism has been postulated for other porphycene derivatives: 13, 19, and 170.254 Such behavior is in contrast to porphyrin, where tautomerization is postulated to occur as a stepwise process, through thermally activated tunneling of a single hydrogen atom.255−257 Very unusual effects were reported for polycrystalline porphycene studied by high-resolution solid-state 15N and 2H NMR spectroscopies and longitudal relaxometry.258 The tautomerization rate increased by 4 orders of magnitude (from microseconds to nanoseconds) at low temperatures. This was interpreted as an indication of a phase transition at about 225 K and the switching from a concerted mechanism at high temperatures to a stepwise process at low temperatures. Bernatowicz determined the tautomerization rate for porphycene in CD2Cl2 solution using the 15N longitudinal relaxation rate, 15N and 1H nuclear Overhauser effect (NOE) enhancement, and the rotational relaxation tensor.259 The tautomerization rate constant, k = 1 × 1012 s−1 at 298 K, is in 13

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tautomerization rate is due to weaker intramolecular H-bonds in this molecule. A methodology was proposed that allowed the rate of excited-state trans−trans tautomerization to be determined from either stationary or time-resolved anisotropy measurements.263 Both procedures were based on the fact that, upon double hydrogen transfer, the transition moment is reflected in the plane perpendicular to that of the molecule (Figure 16). In

good agreement with the results obtained using optical spectroscopy techniques, as described below. One should note that NMR measurements on crystalline porphycene yield a much lower value k = 5 × 107 s−1 for the same process.253 This discrepancy between the two sets of data was noted by Smedarchina et al.213 Its origin might lie in the lower symmetry of 1 in a crystalline environment. 4.1. Coherent Double Hydrogen Tunneling in Isolated Molecules

As already mentioned in section 2.1.7.2, high-resolution absorption and fluorescence spectra of 1,184,185,187−189 15,186 and 23 and 25187,188 exhibit splittings of vibronic lines due to coherent double hydrogen tunneling. These splittings are vibrational-mode-specific, indicating that tautomerization in porphycenes is a multidimensional process, in which excitation of a particular vibration can increase or decrease an effective reaction barrier (cf. Figure 14). A spectacular manifestation of this effect is provided by comparison of the two lowest-energy totally symmetric modes of 1 observed in its high-resolution fluorescence spectrum (Figure 8). The fluorescence band corresponding to 1ag (140 cm−1) exhibits no splitting, whereas that corresponding to 2ag (180 cm−1) shows a splitting of 12 cm−1, almost three times larger than that of the 0−0 band (4.4 cm−1). Most vibrational modes observed in emission have a “neutral” character, showing tunneling splittings similar to that of the 0−0 transition. It is remarkable that even a weak structural perturbation can dramatically change the contribution of a particular mode to the tautomerization coordinate. This was shown for Pc-d12, parent porphycene deuterated at the peripheral carbon atoms, which exhibits an unusual “reversed” isotopic effect.189 The tunneling splitting observed for the 4Ag mode, 9 cm−1, is twice as large as that observed for the corresponding mode in 1, indicating switching between “neutral” and “tautomerization-promoting” behaviors. Coexcitation of two vibrations of opposite character results in intermediate tunneling splittings.185 A particularly interesting case is the combination of the most enhancing (2ag) and the most inhibiting (1ag) modes of 1. Unfortunately, this effect has not been observed experimentally. Carr−Parinello molecular dynamics simulations260,274 predict that the promoting character wins, so that the splitting should be higher than that observed for the zero-point energy level. From the values of the tunneling splittings, residence times of the hydrogens in one well can be calculated. These values can be compared with those based on the tautomerization rates determined in the condensed phase. Indeed, the rates obtained in solution for 1, 15, and 2363 correlate well with the tunneling splittings observed for these molecules isolated in supersonic jets.185−188

Figure 16. Change in direction of the S0−S1 transition moment occurring as a result of double hydrogen transfer in porphycene.

fact, this allows for the determination of the absolute direction of the transition moment in the molecular plane, which is not trivial, given the low symmetry of the porphycene chromophore. A value of α = 71° was obtained for the S0−S1 transition in the parent porphycene. Practically the same value was subsequently determined using two other techniques, namely, pump−probe femtosecond transient absorption264 and singlemolecule fluorescence spectroscopy using polarized light.265 Measurements of fluorescence anisotropy for 1 at temperatures between 60 and 200 K led to an unusually low activation energy for tautomerization (0.5 kcal/mol, much less than the calculated barrier).263 This was interpreted as evidence of vibrational gating of double proton tunneling. Such an explanation is in agreement with the measurements of 1 in superonic jets and helium nanodroplets: The 2a1g, 180 cm−1, vibration corresponding to 0.5 kcal/mol (1 kcal/mol = 349 cm−1) is the one that exhibits the largest tunneling splitting among all of the observed vibronic transitions. A more general technique was developed that exploits the change in transition moments upon tautomerization.264,266,267 It is based on using a polarized femtosecond pulse (pump), followed by time-delayed probe pulses with parallel (par) and perpendicular (perp) polarization. The measured temporal profile of the anisotropy of the transient absorption (ΔA) signal, r(t) = [ΔA(t)par − ΔA(t)perp]/[ΔA(t)par + 2ΔA(t)perp], illustrated in Figure 17, contains three contributions, due to excited-state absorption, ground-state bleaching, and stimulated emission. Using an appropriate combination of pump and probe wavelengths allows for the determination, in a single experiment, of the rates of double hydrogen transfer for both the ground and S1 electronic states. Such measurements have now been performed for over 20 different porphycenes, in different environments and at different temperatures.63,261,264,268−270 The measured tautomerization times span 4 orders of magnitude, from less than 100 fs to more than 100 ps (Figure 18). For a given porphycene, the reaction rates are several times lower in S1 than in S0; this is explained by the molecule “expanding” in the excited state, which leads to larger N−N distances and, thus, to weakening of intramolecular hydrogen bonds. The measured rates correlate well with such

4.2. Rates of Double Hydrogen Transfer in Condensed Phases

The depolarization of the emission of 1 in a glassy solvent at 77 K115 suggested that tautomerization occurs in the lowest excited singlet state on a time scale comparable to or shorter than the fluorescence lifetime, about 10 ns at 298 K and about twice as long at low temperatures. Investigations of substituted derivatives revealed significant differences: Fluorescence was also depolarized for 13 and 25 in n-propanol glass at 113 K, but for the octaethyl derivative 19, “normal”, textbook behavior (i.e., polarized emission) was observed.50,262 Because the NH− N distance is the largest in 19 among the four studied derivatives,57 the observation implied that a lower excited-state 2466


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Scheme 41

isomeric 2,3,7,8,12,13,17,18-octaethyl-5,10-diphenylporphyrin 237, for which the geometry of the inner cavity is typical for porphyrins. Using femtosecond transient spectroscopy, tautomerization rates have been studied for 1 and 15 in a wide temperature range, using both nondeuterated molecules and the isotopologues with two inner protons replaced by deuterons.261 It was demonstrated that three different channels contribute to tautomerization: (a) “deep” tunneling, occurring from the zero vibrational level; (b) vibrationally activated tunneling, operating through excitation of the 180 cm−1 mode; and (c) a channel with a larger activation energy. Even at room temperature, channels a and b dominate (Figure 19), showing

Figure 17. Time-resolved transient absorption anisotropy profiles recorded for porphycenes 14, 28, 29, and 34 in paraffin oil after excitation into S1 and probing of the (a) S0−S1 and (b) S0−S2 transitions. Adapted with permission from ref 63. Copyright 2015 American Chemical Society.

Figure 19. Relative contributions to S0 tautomerization in 1 and 15 as a function of temperature: (a) vibrational ground-state tunneling, (b) vibrationally activated tunneling, and (c) other channels. Reprinted with permission from ref 261. Copyright 2016 American Chemical Society. Figure 18. Correlation between tautomerization rates in the ground (red) and S1 (green) electronic states with chemical shifts of the inner protons. Adapted with permission from ref 63. Copyright 2015 American Chemical Society.

that double hydrogen transfer in porphycenes is mainly governed by tunneling. An independent experiment leading to the same conclusion was based on a comparison of the tautomerization rates determined at room temperatures for two similar porphycenes: 13 and 38.269 The symmetry of the double-minimum potential for trans−trans tautomerization is perturbed in the latter. This leads to a reduction in the reaction rate, even though the calculations predict a smaller barrier for the process. An interesting case regarding the anisotropy is provided by the amino-substituted porphycene 125.270 Even though the tautomerization is rapid, the molecule, unlike all other porphycenes studied so far, does not exhibit significant anisotropy changes. The “locking” of the direction of the

parameters as the proton NMR chemical shift, N−N distance, and NH stretching frequency, showing that they can be used as good indicators of H-bond strength.63 The extreme sensitivity of tautomerization dynamics to the N−N distance was exploited in the design of a porphyrin that exhibited a reaction rate several orders of magnitude higher than other porphyrins. 2 7 1 This was achieved for 2,3,7,8,12,13,17,18-octaethyl-5,15-diphenylporphyrin 236 (Scheme 41), a molecule with a rectangular inner cavity and small NH···N distances, similar to those in porphycenes. No acceleration of hydrogen transfer could be detected for an 2467


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transition moment has been attributed to a large energy splitting between two HOMOs. As a result, their ordering does not change upon tautomerization, as is the case in other porphycenes. Experiments using polarized light demonstrate that tautomerization in porphycenes usually corresponds to trans−trans conversion, as no evidence of a cis tautomer was found. However, exceptions to this rule exist. Meso-alkyl-substituted porphycenes 23 and 25 exhibit biexponential fluorescence decay, attributed to the existence of both the trans and cis1 forms.62 Remarkably, the rate of trans−trans double hydrogen transfer is higher than the rates of possible single hydrogentransfer trans−cis1 and cis1−trans conversions, even though the latter is thermodynamically “downhill”. The explanation for this behavior is that tautomerization occurs through tunneling, with the reaction coordinate for single hydrogen transfer strongly coupled to the rotation of the alkyl substituent. In contrast, double hydrogen transfer does not require substantial rotation of the alkyl groups. Another case in which the cis1 form appears and can even become dominant occurs for porphycene 1 placed on a specific metal surface, as discussed below. Very high rates of intramolecular tautomerization in porphycene contrast with extremely slow intermolecular hydrogen exchange. It was shown that the replacement of both protons with deuterons, when 1 is dissolved in EtOD or BuOD, takes about 20 h.266 The reason for such a slow process is the strong intramolecular hydrogen bond. Equal rates were found for the exchange of the first and second protons.

the contribution of stepwise process increases with temperature. Also, the excitation of out-of-plane vibrations can decrease the contribution from the concerted channel. Car−Parrinello molecular dynamics simulations274 predicted that, for 1 in the gas phase at 300 K, approximately 70% of the proton-transfer events correspond to asynchronous double proton transfers, which can be described as two single-transfer events, rapidly taking place one after the other. This work also examined the effect of vibrational excitations on the tautomerization rate, taking into account tautomerizationenhancing and -inhibiting modes, as well as the simultaneous excitation of both. To describe double proton transfer in hydrogen-bonded complexes, McKenzie developed a diabatic state model, in which the key parameters are the donor−acceptor separation R and the ratio between the proton affinities of a donor with one or two protons.275 These parameters are well-defined in the case of intermolecular tautomerization (e.g., involving carboxylic acid dimers), but the situation is less clear for an intramolecular process. It was shown that decreasing R can result in a change in the tautomerization mechanism, from a synchronous concerted mechanism, through an asynchronous concerted mechanism, to a sequential mechanism. Homayoon et al. carried out a theoretical study of modespecific tunneling splitting in 1 using a novel, mode-specific “Qim path method”.276 Their calculations reproduced the experimentally observed splitting values, also for a singly deuterated porphycene. Predictions were made for modes not yet investigated experimentally.

4.3. Theoretical Studies of Tautomerization in Porphycenes

4.4. Tautomerization in Single Molecules

4.4.1. Single-Molecule Fluorescence Studies. Observation of tautomerization in single porphycene molecules was first achieved using confocal fluorescence microscopy.265 Spatial images of the fluorescence of 1 embedded in a polymer film (Figure 20) were obtained using excitation with an azimuthally polarized laser beam. To explain the ring-shaped pattern of the emission intensity observed for some molecules, the presence of two equal transition dipoles forming an angle of 72° had to be assumed. This value is in perfect agreement with the angle estimated from stationary and pump−probe experiments on bulk samples. The two transition dipoles correspond to two trans tautomeric forms that exist in one molecule during the experiment. As shown in Figure 20, not only a ring but also a double-lobe shape was observed for some molecules, because the spatial pattern of the emission depends both on the polarization mode employed for excitation and on the three-dimensional orientation of the molecule. This fact was exploited in the study, which allowed the determination of the orientation of single molecules of porphycene 15 in poly(methyl methacrylate).277 Differently oriented molecules give rise to completely different spatial patterns (Figure 21). This work demonstrated that a chemical reaction that leads to a change in the direction of the transition moment (even if the substrate and product are identical!) can be used as a tool for obtaining precise information about the orientation of single molecules. In a single-molecule study of 122,278 a small fraction of the investigated chromophores (5% of the total number of 871) exhibited an usual behavior: “locking” of the direction of the transition moment (and, thus, stopping of tautomerization) on the time scale of minutes (Figure 22). Given that the tautomerization rate determined for bulk samples is about

Tautomerization in porphycene provides an extremely challenging case for the theory. The experimental results leave no doubt that the reaction is a multidimensional process, and therefore, the main problem is to find a correct potential for the description of the reaction path, a formidable task in view of the fact that parent 1 has 108 vibrational modes. The calculations should account not only for tunneling but also for the mode selectivity of tunneling splitting. Regarding the dynamics, distinction should be made between stepwise and concerted mechanisms; in addition, the latter can be synchronous or asynchronous. Using instanton theory, Smedarchina and co-workers carried out theoretical studies of the potential and dynamics of double proton or hydrogen transfer in various systems, including porphycene.213,272 Regarding the issue of stepwise versus concerted transfer, it was postulated that the two mechanisms can be distinguished by analyzing kinetic isotope effects.272 The same authors also investigated tunneling splittings in porphycene using a multidimensional imaginary-mode Hamiltonian; satisfactory agreement with experiment was obtained.273 Studies by Kühn and co-workers,208,214 performed in close cooperation with experimentalists,254 discussed geometric isotope effects as indices of cooperativity. It was concluded that the two H-bonds in porphycene are cooperative, but this does not necessarily imply the concerted mechanism, as long as the relation between the quantum effects on geometry and the kinetics is not determined. Other works231,235 described quantum dynamics simulations of laser control of double proton transfer in porphycene 38. Path-integral molecular dynamics simulations for 1217,220 suggested that the concerted mechanism dominates and that 2468


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Figure 22. Confocal images of single molecules of 122 in a PMMA matrix, recorded in three consecutive 20-min scans. The molecule labeled with a circle undergoes transitions from the nontautomerizing to the fast-tautomerizing regime and back (to the other trans tautomer). Schematic picture of the changes in the double-minimum potential are shown at the bottom. Reprinted with permission from ref 278. Copyright 2013 American Chemical Society.

Figure 20. Confocal fluorescence images of two single porphycene molecules embedded in a thin PMMA layer. Emission was excited with an azimuthally polarized laser beam. (Left) Experiment, (right) simulation assuming a 72° angle between the transition moments in two tautomeric trans forms, orientation of molecule Σ parallel to the support plane, and orientation of molecule Π perpendicular to it. Scan area = 2.5× 2.5 μm2. Reprinted with permission from ref 265. Copyright 2005 American Chemical Society.

1012 s−1, the tautomerization is slowed by many orders of magnitude. This effect was explained as being due to slow relaxation of the polymer matrix, coupled with the twisting of the phenyl groups of 122. Thus, monitoring tautomerization in single molecules (but not in large ensembles) might be a good way to probe polymer relaxation dynamics. 4.4.2. Single-Molecule Raman Spectra. Not only fluorescence but also surface-enhanced resonance Raman spectra (SERRS) of single molecules could be recorded for porphycene 1 placed on gold and silver nanostructures.279,280 In a few cases, Raman spectra collected consecutively at 0.3-ns intervals exhibited spectral changes that were explained as being due to the transient, short-time presence of the cis1 tautomeric species (Figure 23). As discussed in the next section, the metal support might indeed induce the trans−cis1 conversion.

Figure 23. Single-molecule SERRS spectra of 1 (Pc-d0) and its isotopologue deuterated at the peripheral carbon atoms (Pc-d12), located on Au nanostructures and excited with a 633-nm laser beam. The asterisks show the bands that appear and disappear over time, indicating transitions between the trans and cis1 forms. The calculated Raman spectra of both forms are also included (blue and red for trans and cis1, respectively). Reprinted with permission from ref 279. Copyright 2016 The Royal Society of Chemistry.

Figure 21. (Left) Experimental and (right) simulated confocal fluorescence images of three single molecules of 15 differently oriented in a thin PMMA film: (1) Molecule lying flat on the surface, (2,3) molecules oriented perpendicular to the support plane. (Top) azimuthal and (bottom) radial polarizations of the exciting laser beam. Reprinted with permission from ref 277. Copyright 2009 American Chemical Society. 2469


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Figure 24. Porphycene 1 on a Cu(110) surface. Top: (b) Experimental STM images, revealing the (a) cis1 tautomers. Scan area = 1.49 × 1.42 nm2. Middle: Optimized (c) trans, (d) cis1, and (e) cis2 forms and the calculated relative energies. Bottom: Simulated STM images of the three forms. Adapted from ref 26.

Figure 25. (Left) High- and low-current states, with the hydrogens closer and farther from the STM tip, respectively. (Middle) Time traces of the STM current, allowing for the determination of tautomerization rates. (Right) Current histogram. Adapted from ref 26.

26).26 Thus, a single-molecule switch controlled by a single atom was created. Tautomerization was also observed in porphycene molecular assemblies, including dimers, trimers, tetramers, and pentamers, with the rate depending on the position of the molecule within an oligomer.26 For 1 placed on a Cu(111) surface, the trans form was found to be the most stable. However, it could be converted to cis1 using voltage pulses, which resulted in vibrational excitation through inelastic electron scattering (Figure 27).282 In the same way, reversible cis1−cis1 tautomerization could be induced. The trans−cis1 conversion, however, was unidirectional; the reverse process could be achieved only by heating the sample from 5 to 30 K. The same reaction, unidirectional trans−cis1 conversion on a Cu(111) surface, could also be realized by photoirradiation (Figure 28).284 The proposed mechanism involves photoexcitation of copper d-band electrons, followed by hot carrier generation and energy transfer to molecular vibrations. A huge

4.4.3. Single-Molecule Scanning Probe Microscopy. Low-temperature, ultrahigh-vacuum scanning tunneling microscopy experiments performed for single molecules of 1 adsorbed on a copper surface exhibited complex, crucially environmentdependent single and double hydrogen-transfer processes, involving both trans and cis1 tautomeric forms.26,281−285 When adsorbed on a Cu(110) surface, the molecule exists in the form of cis1 tautomer, contrary to the situation in the gas and condensed phases (Figure 24). The reason for stabilization of cis1 is the interaction of the non-hydrogenated nitrogen atoms with the copper atoms underneath. The double hydrogen cis1−cis1 transfer could be induced thermally or by inelastic electron scattering. The latter process was shown to be vibrational-mode-specific.281 By recording time traces of the STM signal, it was possible to determine the tautomerization rate (Figure 25). Most interestingly, the rate of cis1−cis1 interconversion could be controlled by shifting a single copper adatom in the vicinity of the molecule (Figure 2470


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Figure 28. (Top) STM images of a single molecule of 1 on Cu(111) at 5 K, photoconverted from the (a) trans into the (b) cis1 form. (Bottom) Larger-area STM image (25 × 25 nm2) (c) after deposition of 1 and (d) after irradiation with a 405-nm diode laser. White circles mark the molecules that switched to the cis1 form. White lines show high-symmetry axes of the surface. Reprinted with permission from ref 284. Copyright 2016 American Chemical Society.

Figure 26. (a−f) STM images of 1 on a Cu(110) surface with a Cu adatom and (g) current histograms for the corresponding locations of the adatom, demonstrating the possibility of locking the inner hydrogens at one position (image f). Adapted from ref 26.

5. SUMMARY The results of research in spectroscopy and tautomerism performed on porphycenes during the 30 years that have passed since the synthesis of the parent compound allow for some statements of general nature. First, there can be little doubt that, when it comes to absorption intensity in the Q region, porphycenes do better than all other porphyrin isomers (including those that await synthesis). This is due to the pattern of frontier orbital splitting, making porphycenes the “hardest” among all of the isomers. Strong absorption and fluorescence, also when using the two-photon absorption regime, is a prerequisite for applications in such areas as cellular imaging and photodiagnosis. In addition, some porphycenes exhibit a strong dependence of the fluorescence quantum yield on viscosity, a feature that can be exploited for characterizing specific cellular regions. Use in photodynamic therapy also requires good efficiency of singlet oxygen generation, which is fulfilled by many porphycenes. Regarding tautomerization, porphycenes emerge as role models for studying hydrogen-bonding properties along with intramolecular single and double hydrogen-transfer processes in a molecular structure that is well-defined in terms of hydrogenbond distances and angles. The results obtained so far reveal an extremely complex multidimensional character of tautomerization, governed by vibrational-mode-specific tunneling, involving the presence of several species (trans and cis), different reaction channels, possible cooperativity/anticooperativity between the two transferring hydrogens, and sensitivity to the environment. All of these features create a huge challenge for theoreticians attempting to work out a detailed description of tautomerization. Experimentalists can also feel challenged, especially when it comes to the exact monitoring of tautomerization dynamics. The results obtained so far strongly suggest that quantum control of tautomerization in porphycenes is feasible. It would be fascinating to observe the ultrafast reaction dynamics on the level of single molecules. Finally, the combination of submolecular spatial resolution of scanning probe microscopies

Figure 27. STM image of single molecules of 1 on a Cu(111) surface at 5 K (a) before and (b) after a voltage pulse, applied in the position indicated by the white star. White circles indicate molecules that were converted to the cis1 form. White lines show high-symmetry axes of the surface. Reprinted with permission from ref 282. Copyright 2015 American Chemical Society.

isotope effect, ∼100, was observed when the inner protons were replaced with deuterons. A postulate that should be checked in future studies is that the trans configuration might be the more stable than cis1 at the zero level but vibrational excitation might reverse this ordering. Finally, it was demonstrated that it is possible to induce cis1−cis1 conversion in a single molecule by mechanical force, using the tip of the scanning tunneling microscope.285 In summary, STM experiments have enabled a deep insight into the intricacies of the potential responsible for tautomerization in single porphycene molecules. Regarding applications, these experiments showed the possibility of constructing singlemolecule switches that use tautomerization as the operating principle. It should be mentioned in this context that the interconversion between different tautomeric forms in porphycene can now be induced by four different external stimuli: (i) temperature,281 (ii) light,284 (iii) electrons,281,282 and (iv) mechanical force.26,285 2471


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(2) Waluk, J.; Michl, J. The Perimeter Model and Magnetic Circular Dichroism of Porphyrin Analogues. J. Org. Chem. 1991, 56, 2729− 2735. (3) Callot, H. J.; Metz, B.; Tschamber, T. A Novel Porphyrin Isomer: Hemiporphycene. Formation and Single-Crystal X-ray Diffraction Structure Determination of a Hemiporphycene Nickel Complex. New J. Chem. 1995, 19, 155−159. (4) Vogel, E.; Bröring, M.; Weghorn, S. J.; Scholz, P.; Deponte, R.; Lex, J.; Schmickler, H.; Schaffner, K.; Braslavsky, S. E.; Müller, M.; et al. Octaethylhemiporphycene: Synthesis, Molecular Structure, and Photophysics. Angew. Chem., Int. Ed. Engl. 1997, 36, 1651−1654. (5) Sessler, J. L.; Brucker, E. A.; Weghorn, S. J.; Kisters, M.; Schäfer, M.; Lex, J.; Vogel, E. Corrphycene: A New Porphyrin Isomer. Angew. Chem., Int. Ed. Engl. 1994, 33, 2308−2312. (6) Aukauloo, M. A.; Guilard, R. The “Etioporphycerin”: Synthesis and Characterization of a New Porphyrin Isomer. New J. Chem. 1994, 18, 1205−1207. (7) Vogel, E.; Scholz, P.; Demuth, R.; Erben, C.; Bröring, M.; Schmickler, H.; Lex, J.; Hohlneicher, G.; Bremm, D.; Wu, Y. D. Isoporphycene: The Fourth Constitutional Isomer of Porphyrin with an N4 CoreOccurrence of E/Z Isomerism. Angew. Chem., Int. Ed. 1999, 38, 2919−2923. (8) Vogel, E.; Bröring, M.; Erben, C.; Demuth, R.; Lex, J.; Nendel, M.; Houk, K. N. Palladium Complexes of the New Porphyrin Isomers (Z)- and (E)-IsoporphycenePdII-Induced Cyclization of Tetrapyrrolealdehydes. Angew. Chem., Int. Ed. Engl. 1997, 36, 353−357. (9) Wu, Y. D.; Chan, K. W. K.; Yip, C. P.; Vogel, E.; Plattner, D. A.; Houk, K. N. Porphyrin Isomers: Geometry, Tautomerism, Geometrical Isomerism, and Stability. J. Org. Chem. 1997, 62, 9240−9250. (10) Chmielewski, P. J.; Latos-Grażyński, L.; Rachlewicz, K.; Glowiak, T. Tetra-p-Tolylporphyrin with an Inverted Pyrrole Ring: A Novel Isomer of Porphyrin. Angew. Chem., Int. Ed. Engl. 1994, 33, 779−781. (11) Furuta, H.; Asano, T.; Ogawa, T. “N-Confused Porphyrin”: A New Isomer of Tetraphenylporphyrin. J. Am. Chem. Soc. 1994, 116, 767−768. (12) Lash, T. D.; Lammer, A. D.; Ferrence, G. M. Neo-Confused Porphyrins, a New Class of Porphyrin Isomers. Angew. Chem., Int. Ed. 2011, 50, 9718−9721. (13) Stockert, J. C.; Cañete, M.; Juarranz, A.; Villanueva, A.; Horobin, R. W.; Borrell, J.; Teixidó, J.; Nonell, S. Porphycenes: Facts and Prospects in Photodynamic Therapy of Cancer. Curr. Med. Chem. 2007, 14, 997−1026. (14) Lo, W. C.; Che, C. M.; Cheng, K. F.; Mak, T. C. W. Catalytic and Asymmetric Cyclopropanation of Styrenes Catalysed by Ruthenium Porphyrin and Porphycene Complexes. Chem. Commun. 1997, 1205−1206. (15) Hayashi, T.; Okazaki, K.; Urakawa, N.; Shimakoshi, H.; Sessler, J. L.; Vogel, E.; Hisaeda, Y. Cobaltporphycenes as Catalysts. The Oxidation of Vinyl Ethers via the Formation and Dissociation of Cobalt−Carbon Bonds. Organometallics 2001, 20, 3074−3078. (16) Berlicka, A.; König, B. Porphycene-Mediated Photooxidation of Benzylamines by Visible Light. Photochem. Photobiol. Sci. 2010, 9, 1359−1366. (17) Oohora, K.; Kihira, Y.; Mizohata, E.; Inoue, T.; Hayashi, T. C(sp3)-H Bond Hydroxylation Catalyzed by Myoglobin Reconstituted with Manganese Porphycene. J. Am. Chem. Soc. 2013, 135, 17282− 17285. (18) Saeki, H.; Kurimoto, O.; Misaki, M.; Kuzuhara, D.; Yamada, H.; Ueda, Y. Thermal Conversion Behavior and Morphology Control of Benzoporphycene from a Novel Soluble Precursor. Appl. Phys. Express 2013, 6, 035601-1−035601-3. (19) Saeki, H.; Misaki, M.; Kuzuhara, D.; Yamada, H.; Ueda, Y. Fabrication of Phase−Separated Benzoporphycene/[6,6]-Phenyl-C61Butyric Acid Methyl Ester Films for Use in Organic Photovoltaic Cells. Jpn. J. Appl. Phys. 2013, 52, 111601-1−111601-5. (20) Saeki, H.; Kurimoto, O.; Nakaoka, H.; Misaki, M.; Kuzuhara, D.; Yamada, H.; Ishida, K.; Ueda, Y. Effect of Crystallinity in Small

with the spectral and temporal resolution of vibrational and/or time-resolved techniques would certainly bring new insights into the fascinating world of porphycenes. Naturally, progress in porphycene research will be tightly connected to the development of synthetic strategies.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemrev.6b00328. Tables containing electronic absorption and photophysical data (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 48 22 3433333. Tel.: 48 22 3433332. Notes

The author declares no competing financial interest. Biography Jacek Waluk graduated from the Chemistry Department of the University of Warsaw and received his Ph.D. in 1979 and D.Sci. in 1987 at the Institute of Physical Chemistry, Polish Academy of Sciences. Since 1991, he has been Head of the Photochemistry and Spectroscopy Laboratory of this institute; since 2001, he has been Professor of Chemistry at Cardinal Stefan Wyszyński University, Warsaw, Poland. His scientific interests include various aspects of physical organic chemistry, spectroscopy, photophysics, hydrogen bonding, conformational equilibria, single-molecule studies, polarized spectroscopy techniques, and theoretical chemistry. He has been visiting researcher at the University of Utah, University of Texas at Austin, University of Colorado−Boulder, Royal Danish School of Educational Studies, Copenhagen, University of Roskilde, Technical University of Berlin, and National Renewable Energy Laboratory (NREL). In 2000 and 2002, he was elected Chairman of the European Photochemistry Association, and in 2013, he was elected a corresponding member of the Polish Academy of Sciences.

ACKNOWLEDGMENTS I have now been working with porphycenes for more than a quarter of a century, and the list of co-workers is therefore too large to fit in this section (83 names!). I express my gratitude to all of the collaborators who shared with me an interest in these fascinating molecules. Two names must be mentioned explicitly: Josef Michl and Emanuel Vogel, who stimulated and initiated my interest in and enthusiasm for porphycene research. This work was partially supported by the Polish National Science Centre (NCN) through Grants DEC-2011/ 02/A/ST5/00443 and DEC-2013/10/M/ST4/00069, by a PLGrid Infrastructure grant, and by a computing grant from the Interdisciplinary Centre for Mathematical and Computational Modeling. REFERENCES (1) Vogel, E.; Köcher, M.; Schmickler, H.; Lex, J. Porphycenea Novel Porphin Isomer. Angew. Chem., Int. Ed. Engl. 1986, 25, 257− 259. 2472


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Spectroscopy and Tautomerization Studies of Porphycenes

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