Molecular designs with PEG groups for water-solubilization of sparsely substituted porphyrins

Abstract

Molecular designs that achieve solubility of porphyrins in aqueous media are attractive for diverse applications. The presence of 4-sulfophenyl or 4-N-methylpyridinium groups at the four meso-positions is effective, but the use of fewer substituents is desirable for custom tailoring. Here, five target porphyrins (along with selected copper or zinc chelates) were prepared bearing PEG groups to understand how distinct designs affect aqueous solubility (where “PEG” refers to an oligoethylene glycol unit). One objective was to employ only one or two pegylated meso-aryl groups so that other meso-positions would be open for synthetic elaboration while retaining a compact structure. The key design features examined include (i) 2,6- versus 3,5-dipegylated aryl groups; (ii) one versus two 2,6-dipegylated aryl groups; (iii) a nonpolar versus ionizable terminus of the PEG moiety; and (iv) length of the PEG moiety. In each case, the PEG groups were attached on a porphyrin scaffold bearing one or two bis(2-propynyloxy)aryl groups. Assessment entailed octanol–aqueous solution partitioning (log P values) and aggregation over the concentration range of 0.2–200 μM. A trans-A₂ free base porphyrin bearing two 2,6-dialkoxyphenyl groups equipped with methyl-terminated PEG₆ groups gave ∼3 : 1 partitioning in water versus octanol but high overall solubility (at least 12 mM) in water alone; the analogous porphyrin with carboxylic acid-terminated PEG₆ groups gave >100 : 1 partitioning in phosphate-buffered saline (PBS, pH 7.4). A trans-AB porphyrin bearing a single 2,6-dialkoxyphenyl group equipped with carboxylic acid-terminated PEG₆ groups gave 43 : 1 partitioning in PBS versus octanol but was prone to self-aggregation at 20–200 μM in PBS alone. The results point to new molecular designs of potential value in the life sciences.

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Introduction

The design and synthesis of water-soluble tetrapyrrole macrocycles have been of longstanding interest.^1–4 The chief challenge is to attach groups at the perimeter of the hydrophobic, tetrapyrrole macrocycle so that the entire structure can be dissolved in aqueous solution. The tetrapyrrole macrocycle is disk-shaped and essentially planar, where the core free base porphine macrocycle, lacking any peripheral substituents, has formula C₂₀N₄H₁₄. Early designs have relied on incorporation of polar groups at the meso-positions of porphyrins. Indeed, meso-tetrakis(4-sulfophenyl)porphyrin (TPS-por)⁵ and the meso-tetrakis(4-N-methylpyridinium)porphyrin (TPyr-por)⁶ are water soluble and remain in common use (Chart 1). These designs have attractive features, but the occupancy of all four meso-positions leaves diminished opportunities for convenient synthetic elaboration. The uroporphyrins, derived from biosynthesis of tetrapyrrole macrocycles, are water soluble but have been little used for synthetic elaboration or physicochemical studies.⁷

Chart 1 Water-soluble porphyrins.

We have pursued several design features over the years in an effort to create compact, water-soluble, bioconjugatable porphyrins. The features include (1) use of a trans-AB porphyrin architecture, with A and B substituents at the 5- and 15-positions of the macrocycle and no substituents at any other position, (2) emphasis on substituent types that cause polar groups to project above and below the face of the hydrophobic porphyrin, and ideally, (3) reliance on a “single-junction water-solubilization unit”.⁸ The work has been intermittent as objectives and synthetic methods have evolved, key examples of which are shown in Chart 2.⁹

Chart 2 Representative prior trans-AB porphyrins.⁹

• The first design contained a compact, swallowtail substituent, which is a symmetrically branched alkyl group; the preferred conformation contains the C–H unit in the plane of the macrocycle and the two alkyl groups project above and below the plane of the porphyrin.¹⁰ The porphyrins (e.g., Por-A) appeared reasonably soluble in aqueous solution, with the key drawback, however, of the rather lengthy synthesis of the swallowtail units.¹¹

• The second design relied on a 2,4,6-trialkoxyaryl unit, which was attractive given the inexpensive phloroglucinaldehyde (2,4,6-trihydroxybenzaldehyde) and ease of synthesis. The earliest set of porphyrins (e.g., Por-B) exhibited broadened absorption spectra both in aqueous as well as organic media.¹²

• The third design extended the 2,4,6-trialkoxyaryl unit to include PEG groups (e.g., Por-C).^8,9 The resulting porphyrin was water-soluble but in ensuing years uncertainty arose as to how much solubilization was provided by the carboxylic acid in the bioconjugation motif versus the three neutral PEG groups.

The use of PEG groups to impart aqueous solubility of hydrophobic compounds has become widespread across the molecular sciences.^13,14 PEG groups have long been used in the pharmaceutical industry^15–17 to increase aqueous solubility of hydrophobic drugs and to prolong the systemic circulation of rapidly excreted drugs. A striking feature of PEG groups is solubility in both aqueous and organic media.¹³ The origin of the solubility in water is not merely due to the preponderance of oxygen atoms, as the polymer with twice the number of oxygen atoms (repeating unit –CH₂O–) is not soluble in water. The solubility likely stems from the preferred gauche conformation of the –OCH₂CH₂O– moiety and all of the consequences therefrom including local dipole moments, hydrogen-bonding patterns, and kinked or partially coiled conformations.¹⁸ The PEG groups rarely achieve the fully extended conformation as are shown (for clarity) in Chart 2. The synthetic installation of PEG groups has been aided by the advent of heterotelechelic, monodisperse PEG reagents,^15,17,19,20 which are available from commercial sources.

In this paper, the synthesis and characterization of a dozen porphyrins are described. The motivation for the work has been to revisit and extend studies over the past two decades concerning the role of substitution patterns and substituent composition in engendering aqueous solubility. The issues addressed include the advantages and disadvantages of the following: (i) the pattern of 2,6- versus 3,5-dipegylated aryl groups; (ii) the terminus of the PEG moiety (methyl versus carboxylic acid); (iii) one versus two 2,6-dipegylated aryl groups; and (iv) a very short versus intermediate length PEG moiety. Zinc and copper chelates also were prepared given utility in photochemistry^21,22 and radiotherapeutics,²³ respectively. The attributes were assessed by measurements of aqueous–organic partitioning and concentration-dependent self-aggregation in aqueous solution.

Results

Synthesis

(i) Pattern of 2,6- versus 3,5-dipegylated aryl groups. For these architectures, prior synthesis has made available advanced intermediates.²⁴ Thus, tetraethynylporphyrin Zn1²⁴ and mPEG₆-azide (I, where the prefix “m” implies a methyl terminal group) were reacted under conditions^25,26 for copper-mediated click chemistry with tetrapyrroles. The conditions entail the use of CuI, sodium ascorbate, and diisopropylethylamine (DIPEA) in tetrahydrofuran (THF) at reflux. The reaction afforded the corresponding tetra-pegylated porphyrin Zn2 (Scheme 1). The aryl groups are identical and are located at the 5- and 15-positions of the macrocycle, which is denoted as a trans-A₂ architecture. The zinc chelate was employed to block adventitious metalation of the free base porphyrin by copper, which if occurred would likely thwart the click reaction.

Scheme 1 Pegylation of a meta-aryl substituted trans-A₂ porphyrin.

A facially encumbered porphyrin was prepared by a MacDonald-type condensation^27–31 (Scheme 2). Thus, condensation of 2,6-bis(prop-2-yn-1-yloxy)benzaldehyde (3)³² and dipyrromethane (4)³³ was carried out upon catalysis by BF₃·O(Et)₂ followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and subsequent neutralization of the Lewis acid with triethylamine (TEA). In this manner, the trans-A₂ porphyrin 5,15-bis(2,6-bis(2-propynyloxy)phenyl)porphyrin (5) was obtained in 21% yield. Porphyrin 5 was metalated with Zn(OAc)₂·2H₂O in refluxing N,N-dimethylformamide (DMF)³⁴ to give the zinc chelate Zn5.

Scheme 2 Synthesis of a facially encumbered trans-A₂ porphyrin scaffold.

(ii) Terminus of the PEG moiety (methyl versus carboxylic acid). Zinc porphyrin Zn5 and mPEG₆-azide were subjected to click reaction to give the corresponding tetra-pegylated zinc porphyrin Zn6 in 71% yield (Scheme 3). To explore the effect of the nature of the metalation state on aqueous solubility, and to gauge the ease of copper insertion for studies in radiochemistry with copper radionuclides,³² zinc porphyrin Zn6 was demetalated with trifluoroacetic acid (TFA) at room temperature to give the free base porphyrin 6 in 96% yield. Free base porphyrin 6 was then treated overnight with Cu(OAc)₂·H₂O at room temperature, affording the copper chelate Cu6 in quantitative yield.

Scheme 3 Synthesis of trans-A₂ pegylated porphyrins.

Analogues of porphyrins 6, Zn6, and Cu6 were prepared wherein each PEG group is terminated with an ionizable group. The carboxylic acid was chosen given ionization as the carboxylate at physiological pH as well as commercial availability of the required PEG synthons. Thus, the click reaction of porphyrin Zn5 with azido-PEG₆-CO₂t-Bu (II) gave Zn7 in 81% yield (Scheme 3). Treatment of the latter with TFA caused removal of the tert-butyl protecting groups and the zinc chelate, affording the free base porphyrin bearing four carboxylic acids (8) in 58% yield. Finally, exposure to copper acetate gave the copper porphyrin Cu8 in 62% yield. Zinc porphyrin Zn8 was prepared in small scale for comparative studies by zincation of free base porphyrin 8.

(iii) One 2,6-dipegylated aryl group. A porphyrin scaffold (Zn9) containing two propynyloxy groups and one TIPS-protected ethyne was recently prepared for successive elaboration with PEG and bioconjugatable groups.³² Click chemistry with azido-PEG₆-CO₂t-Bu (II), sodium ascorbate, CuBr, and the ligand tris(hydroxypropyltriazolyl)methylamine (THPTA) followed by removal of the TIPS group with tetra-n-butylammonium fluoride (TBAF) afforded Zn10³² in 70% yield (Scheme 4, left panel). Treatment of Zn10 with TFA in CH₂Cl₂ for 2 h gave, in 94% yield, the free base porphyrin 11 for examination of aqueous solubility. Finally, Cu11 and Zn11 were prepared in small scale by metalation of free base porphyrin 11.

Scheme 4 Synthesis of trans-AB pegylated porphyrins.

(iv) Short PEG chains. The porphyrin scaffold Zn9 also was treated with the short azido-PEG₂-CO₂t-Bu (III) via click chemistry to create porphyrin Zn12 in 73% yield. Removal of the TIPS group with TBAF gave in 93% yield porphyrin Zn13, a homologue of zinc porphyrin Zn10,³² containing PEG₂versus PEG₆ groups. Treatment with TFA in CH₂Cl₂ gave the free base porphyrin 14, a homologue of 11, for studies of aqueous solubility (Scheme 4, right panel).

Characterization

The porphyrins were typically characterized by absorption spectroscopy, ¹H NMR and ¹³C{¹H} NMR spectroscopy, and mass spectrometry. The absorption spectra of trans-A₂ and trans-AB free base porphyrins closely resemble those of well-known A₄ free base porphyrins, with a strong B (Soret) band and a progression of bands in the visible region typically in a phyllo³⁵ pattern. The zinc and copper chelates exhibit absorption spectra, particularly in the visible region, that are distinct from the parent free base porphyrin and also distinct from each other. For 6, for example, the most intense visible band is found at 503 nm versus that of Cu6 at 531 nm and of Zn6 at 545 nm. The ¹H NMR spectrum of each free base or zinc porphyrin exhibited the characteristic resonance of the two meso-protons at δ ∼ 10.12 ppm, and for the free base porphyrins, the NH resonances near δ −3.13 ppm. The β-pyrrole protons resonated as two doublets in the aromatic region (δ ∼ 9.29 and ∼ 8.98 ppm) for trans-A₂ porphyrins and four doublets (δ ∼ 9.36, 9.26, 9.03 and 8.94 ppm) for trans-AB porphyrins. The PEG resonances are described below. The copper chelates were not characterized by NMR spectroscopy given the line broadening caused by relaxation from the copper center.^36,37

The ¹H NMR spectrum of the zinc trans-A₂ porphyrin with 3,5-dipegylated aryl groups (Zn2) is shown in Fig. 1 (top panel). The resonances from the PEG –OCH₂CH₂– units and the terminal methyl group appear in the range δ ∼ 3.7–3.0 ppm. The ¹H NMR spectrum of the zinc trans-A₂ porphyrin with 2,6-dipegylated aryl groups (Zn6) is shown in Fig. 1 (bottom panel). Here, the resonances from the PEG –OCH₂CH₂– units and the terminal methyl group encompass a much larger range, spanning δ 3.7–2.0 ppm, versus that of Zn2. The peaks in the upfield region of Zn6 are attributed to –OCH₂CH₂– units thrust over the faces of the macrocycle, thereby experiencing the aromatic ring current. The –OCH₂– unit linking the meso-aryl and triazole moieties of Zn6 resonates downfield (δ 5.04 ppm) versus that of Zn2 (δ 4.45 ppm) which is attributed also to the ring current but at the outer edge of the macrocycle. Similarly, the triazolyl-H proton of Zn6 resonates downfield (δ 5.99 ppm) versus that of Zn2 (δ 5.12 ppm), again shifted by the porphyrin ring current. The minimum conclusions are that the 2,6-disubstitution pattern (Zn6) affords facial encumbrance by the attached PEG chains that is not present in the 3,5-disubstitution pattern (Zn2).

Fig. 1 ¹H NMR spectra in CDCl₃ at room temperature of zinc porphyrins Zn2 (top) and Zn6 (bottom). Peak assignments: (a) the resonance of the triazolyl–H. (b) The resonance of the protons of the –OCH₂– linking the meso-aryl and triazole moieties. All other peaks derive from the PEG chains.

The structure of Zn5 was confirmed by single-crystal X-ray diffraction upon crystallization from THF at −20 °C (Fig. 2). The zinc ion is hexacoordinate with a THF molecule at each apical site. The meso-aryl dihedral plane is ca. 71° (70.98(4)°). The 2-propynyl groups project over the two faces of the macrocycle. Other analogues include the following substituents on the oxy moieties of 2,6-dialkoxyarylporphyrins: methyl,^38–45 ethyl,⁴⁶ butyl,^47–49 octyl,⁵⁰ dodecyl,⁵¹ 3,3-dimethylbutyl,⁵² 4-hydroxybutyl,⁵³ 4-oxa-3-oxoheptyl,⁵⁴tert-butyldimethylsilyl,⁵⁵ and pentafluorophenylmethyl.⁵⁶ Strapped porphyrins with 2,6-dialkoxyaryl units include doubly strapped meso–meso-linked arrays⁵⁷ and basket-handle thiolate Fe(III) porphyrins.⁵⁸

Fig. 2 Crystal structure of Zn5 (crystallized from THF at −20 °C). Disorder is omitted for clarity. Labels: C = grey, N = blue, O = red, Zn = cyan, and H = white.

Assessments in aqueous solution

A set of tests was carried out with selected porphyrins to gauge suitability of the designs for use in aqueous solution.

(i) Log P measurements. The partitioning of an organic compound between octanol and aqueous solution is a standard proxy to gauge aqueous-membrane partitioning in biological systems.⁵⁹ The partition coefficient (P) for a given porphyrin was examined by allowing a minute quantity of porphyrin to partition between deionized water (or PBS) and octanol at room temperature (∼10⁻⁴ M total porphyrin concentration).⁶⁰ A sample was removed from each phase and examined by absorption spectroscopy in dimethylsulfoxide (DMSO) to determine the concentration. The results are shown in Table 1. Representative photographs are provided in Fig. 3. Two porphyrins can be regarded as benchmarks: TPS-por is found entirely in the aqueous layer (log P < −2) whereas the fully tert-butyl protected tetraester zinc porphyrin Zn7 is found entirely in the octanol layer (log P > 2).

Table 1 Aqueous-organic partitioning of porphyrins

Compd	Type	Projection	PEG n	Terminus	Log P
a Deionized water as the aqueous phase. b PBS as the aqueous phase. c Not determined because of insolubility in aqueous solution.
TPS-por	A4	—	0	—	<−2
Zn2	A2	3,5-	6	–OMe	ND^c
6	A2	2,6-	6	–OMe	−0.46
Zn6	A2	2,6-	6	–OMe	−0.40
Cu6	A2	2,6-	6	–OMe	−0.37
Zn7	A2	2,6-	6	–CO2tBu	>2
8	A2	2,6-	6	–CO2H	<−2
Zn8	A2	2,6-	6	–CO2H	<−2
Cu8	A2	2,6-	6	–CO2H	<−2
11	AB	2,6-	6	–CO2H	−1.6
Zn11	AB	2,6-	6	–CO2H	−1.1
Cu11	AB	2,6-	6	–CO2H	ND^c
14	AB	2,6-	2	–CO2H	ND^c

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Fig. 3 Photographs of results upon log P evaluations with free base porphyrins (TPS-por, 8), copper porphyrin Cu8, and zinc porphyrins (Zn7, Zn8, Zn11).

Porphyrin Zn2, which bears neutral PEG groups at the 3,5-positions of the two aryl rings, exhibited an extensively broadened Soret band (see the ESI†) in water, which is suggestive of aggregation; hence, a log P value was unobtainable. All of the other porphyrins examined bear PEG groups at the 2,6-positions and gave sharp absorption bands at very dilute concentration (2 μM) in aqueous media. The three porphyrins 6, Zn6, and Cu6 each showed preferential partitioning in water versus octanol by a ratio of ∼2.5 : 1. No significant difference was observed as a function of the metalation state: zinc(II), copper(II), or free base. Similar phenomena were observed for the series of 8, Zn8, and Cu8; however, the aqueous partitioning decreased along the series 11 (log P = −1.6), Zn11 (−1.1), and Cu11 (insoluble in aqueous media). The lack of solubility of the copper chelate Cu11versus the zinc chelate Zn11 and free base 11 was surprising and may indicate beneficial solubilization interactions in the latter two cases that are not available in the copper chelate. Such interactions could include aqueous (or PEG carboxylate/carboxylic acid) oxygenic coordination to the apical zinc site of Zn11, or aqueous (or PEG carboxylate/carboxylic acid) hydrogen-bonding with the free base N–H moieties of 11.

Porphyrin 8, which bears four PEG groups terminated with carboxylic acid moieties (as opposed to methyl groups), was not detected in the organic layer. We denote the ratio as >100 : 1 and the log P value as <−2. For porphyrins with only two PEG groups, the PEG₆ groups imparted preference for aqueous solution (11, log P −1.6) whereas the PEG₂ groups (14) resulted in aggregation in PBS; hence, a log P value could not be determined.

(ii) Self-aggregation in aqueous solution. The second test examined self-aggregation as a function of concentration in aqueous solution. The occurrence of self-aggregation can be examined by absorption spectroscopy upon reciprocal change of porphyrin concentration (0.2–200 μM) and cuvette pathlength (100–0.1 mm).⁶⁰ The results with porphyrin 6, Cu6, 8 and 11 in aqueous solution over the 1000-fold concentration range are shown in Fig. 4. Porphyrins 6, Cu6 and 8 showed no change in absorption spectral features consistent with absence of self-aggregation over this range. On the other hand, 11 showed slight spectral broadening at 20 μM and extensive broadening at 200 μM, indicative of self-aggregation. The inclusion of 3% bovine serum albumin (BSA), a protein that can solubilize organic molecules in aqueous solution,⁶⁰ resulted in little or no spectral broadening of porphyrin 11 even at 200 μM (see the ESI†).

Fig. 4 Absorption spectra of porphyrins in PBS at room temperature.

During the course of experimentation, stock solutions of porphyrins 6 and 11 were prepared in aqueous solution (see ESI†). Porphyrin 6 in water (12 mM) gave an optically clear, bright red solution. Porphyrin 11 (1 mM) in PBS gave a transparent albeit muddy red solution. A muddy appearance is consistent with broadened absorption bands,⁶¹ and the aforementioned concentration-dependent studies indicated aggregation even at substantially lower concentrations.

Discussion

The development of water-soluble porphyrins has been an ongoing activity since the 1950s. Early studies required only a simple porphyrin devoid of substituents other than those that impart aqueous solubility, which could be satisfied by uroporphyrins and the synthetic porphyrins shown in Chart 1. In the decades since, designs have been sought that achieve aqueous solubilization while maintaining sites on the porphyrin macrocycle for other substituents such as auxochromes and bioconjugatable groups. In some cases, very compact designs are desired for biomedical applications.

PEG groups are widely used in seemingly ad hoc fashion to impart aqueous solubility without studies to explore the effects of the number of PEGs, the length of PEGs, and the terminal groups of PEGs. Here, three porphyrin scaffolds bearing 2-propynyloxy groups (Zn1, Zn5, Zn9) have been derivatized with PEG groups via click chemistry. It may be little appreciated beyond the aficionado that PEG molecules can afford both organic and aqueous solubility, although a core text states that “PEG will partition in favor of water in a water–benzene system and in favor of methylene chloride in a water–methylene chloride system.”¹³

Assessments pertaining to aqueous solubilization entailed partitioning between octanol and aqueous solution and examination of the absorption spectra as a function of concentration. Methods for calculation of c log P values often give wildly disparate results with porphyrins;⁶² hence, measured values are essential. It warrants emphasis that a log P value is not an indication of solubility per se but rather a partitioning in lipophilic versus aqueous media. Aqueous–organic partitioning and solubility are related yet distinct phenomena. The former represents competition between solubilization in two distinct liquid phases, whereas the latter represents competition between homogeneous dispersion in a liquid phase versus the affinity for the aggregated solid state. Aqueous–organic partitioning is given by a dimensionless value whereas solubility has units such as g cm⁻³. The overall solubility in water could be low or high but give the same ratio for aqueous–organic partitioning. The key findings are as follows:

(i) 2,6-diaryl substitution (Zn6) is superior to 3,5-diaryl substitution (Zn2) for aqueous solubilization. An interpretation is provided in Fig. 5. The 2,6-dialkoxyaryl group has constrained motion causing the groups to project above and below the plane, whereas the 3,5-dialkoxyaryl group is not so constrained and can rotate toward planarity with the macrocycle.⁶³ The latter motion opens the possibility of intermolecular hydrophobic interactions leading to self-aggregation and therefore limited solubility. We had surmised that facial encumbrance was invaluable for solubilization but had heretofore not done a direct test in aqueous solution with non-ionizable substituents.

Fig. 5 Wide conformational motion is available in the 3,5-disubstituted aryl porphyrin (top) versus limited motion for the 2,6-disubstituted aryl porphyrin (bottom).

(ii) PEG groups terminated with carboxylic acid groups provide enhanced aqueous solubility versus simple methyl termination at physiological pH, where the former are predominantly in the carboxylate form. Both trans-A₂ free base porphyrins 6 (methyl termini) and 8 (carboxylate termini) are aqueous soluble, as evidenced by the log P values (Table 1) and the retention of sharp absorption spectra over a 1000-fold concentration range (Fig. 4). The log P values show a key distinction, however the aqueous:octanol ratio is 100 : 35 for 6versus >100 : 1 for 8.

(iii) One rather than two water-solubilization motifs suffices to impart a degree of aqueous solubility, which enables use of a trans-AB porphyrin (e.g., 11, two PEG-carboxylates) rather trans-A₂ porphyrins (e.g., 8, four PEG-carboxylates). Both 8 and 11 exhibit quite negative log P values (<−2, −1.6; i.e., high aqueous partitioning) but 11 does show signs of self-aggregation at concentrations in the range of 20–200 μM.

(iv) The porphyrin bearing two carboxylate-terminated PEG₂ groups (14) rather than PEG₆ groups (11) was insufficiently soluble to obtain a log P measurement. Despite the self-aggregation of 11 in the 20–200 μM range, a derivative thereof that contains a folic acid moiety tethered via a PEG₅ group (clicked onto the 4-ethynylphenyl moiety) did not show signs of self-aggregation across the concentration range of 4.8–480 μM in PBS at room temperature.³² Such results indicate that incorporation of a polar motif with 11 substantially increases the aqueous solubility.

In summary, the studies reported herein point to molecular designs for achieving aqueous solubilization with porphyrins. The use of only one or two meso-substituents leaves other sites available for substitution. More extensive comparisons beyond the pairwise evaluations reported herein are required to explore the generality of the molecular design heuristics.

Experimental section

General methods

¹H and ¹³C{¹H} NMR spectra were recorded (Bruker Ascend™ 500 and Bruker Ascend™ 700 MHz instruments) in CD₂Cl₂ or CDCl₃ at room temperature unless noted otherwise. Absorption spectra were collected in toluene or DMSO at room temperature. Matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) was recorded using a Bruker autoflex® max with the matrix α-cyano-4-hydroxycinnamic acid (α-CHCA). Electrospray ionization mass spectrometry (ESI-MS) data were recorded using a Thermo Fisher Scientific Exactive Plus MS, a benchtop full-scan orbitrap mass spectrometer using heated electrospray ionization. Data are reported for the molecular ion or cationized molecular ion. Single crystal X-ray diffraction (SCXRD) data were collected using a Bruker D8 Venture, running at 100 K with MoKα (λ = 0.71073 Å). All crystal data and graphics were refined and generated by OLEX2 and Mercury software, respectively. THF was freshly distilled from sodium/benzophenone ketyl and used immediately. Commercially available compounds were used as received. Silica gel (40 μm average particle size) was used for column chromatography. PBS buffer (1×) was employed at pH 7.4. Compounds Zn1,²⁴4,³³Zn9,³² and Zn10³² were prepared as described in the literature.

Determination of log P values

Values were determined at room temperature following a reported method.⁶⁰ The quantity of porphyrin employed was ∼0.2–0.4 mg in a total octanol–aqueous solution of 2 mL giving concentrations of 40–150 μM (i.e., ∼10⁻⁴ M). In each case, octanol refers to 1-octanol.

Examination of concentration-dependent self-aggregation

Values were determined by reciprocal change of concentration and cuvette pathlength in aqueous media at room temperature following a reported method.⁶⁰

Synthesis procedures and characterization data

Zn(II) 5,15-bis{3,5-bis[1-(1-(3,6,9,12,15,18-hexaoxanonadecyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}porphyrin (Zn2). Following a reported method²⁶ with some modifications, a solution of zinc porphyrin Zn1 (14 mg, 0.019 mmol) and mPEG₆-azide (I, 101 mg, 0.314 mmol) in dry THF (8.0 mL) was treated sequentially with sodium ascorbate (9.3 mg, 0.047 mmol), DIPEA (200 μL, 1.1 mmol), and CuI (5.7 mg, 0.030 mmol). The reaction mixture was allowed to reflux overnight (oil bath). The solution was concentrated under reduced pressure and then diluted in CH₂Cl₂. The organic phase was washed with water and brine, dried (Na₂SO₄), and concentrated to dryness. The crude product was chromatographed using a gradient [silica, CH₂Cl₂/MeOH (25 : 1) to (19 : 1) to (15 : 1)] to afford a red oil (5.4 mg, 15%): ¹H NMR (500 MHz, CDCl₃) δ 10.21 (s, 2H), 9.32 (d, J = 4.4 Hz, 4H), 9.08 (d, J = 4.4 Hz, 4H), 7.81 (s, 4H), 7.45 (s, 4H), 7.04 (s, 2H), 5.13 (s, 4H), 4.45 (s, 8H), 3.73 (s, 8H), 3.37–2.86 (m, 96H); ¹³C{¹H} NMR (175 MHz, CDCl₃) δ 156.3, 148.6, 148.5, 144.0, 142.4, 131.2, 130.6, 123.2, 118.0, 114.5, 104.9, 100.2, 70.4, 69.3–68.2, 61.0, 57.6, 53.2, 49.3; λ_abs (toluene) 416, 544, 580 nm; MALDI-MS obsd 2024.02, calcd 2024.90 [M⁺]; ESI-MS obsd 1035.4384, calcd 1035.4393 [(M + 2Na)²⁺], M = C₉₆H₁₃₆N₁₆O₂₈Zn.

5,15-Bis(2,6-bis(2-propynyloxy)phenyl)porphyrin (5). Following a reported method²⁴ with some modifications, a solution of aldehyde 3 (2.2 g, 10 mmol) and dipyrromethane 4 (1.5 g, 10 mmol) in CH₂Cl₂ (500 mL) was treated dropwise with BF₃·O(Et)₂ (190 μL, 1.54 mmol) over 1 minute. The mixture was stirred at room temperature. Aliquots of the solution were analyzed by absorption spectroscopy to monitor the progress of the reaction. After 1.5 h, the reaction mixture was oxidized by the addition of DDQ (2.3 g, 10 mmol), stirred at room temperature for 15 min, and then neutralized by the addition of triethylamine (2.0 mL, 14 mmol). After 10 min, the resulting mixture was poured onto a silica pad (5 cm × 30 cm) and eluted with CH₂Cl₂. The filtrate was concentrated to dryness under reduced pressure. The crude product was triturated with hexanes/CHCl₃ to afford a dark-red solid (0.72 g, 21%): ¹H NMR (700 MHz, CD₂Cl₂) δ 10.25 (s, 2H), 9.37 (d, J = 4.4 Hz, 4H), 8.99 (d, J = 4.4 Hz, 4H), 7.84 (t, J = 8.8 Hz, 2H), 7.29 (d, J = 8.8 Hz, 4H), 4.43 (s, 8H), 2.37 (t, J = 2.6 Hz, 4H), −3.17 (s, 2H); ¹³C{¹H} NMR (175 MHz, CD₂Cl₂) δ 158.5, 131.5, 130.3, 130.1, 120.3, 110.3, 106.8, 104.5, 78.5, 75.1, 56.3; λ_abs (toluene) 408, 501, 534, 579 nm; MALDI-MS obsd 678.13, calcd 678.23 [M⁺]; ESI-MS obsd 679.2335, calcd 679.2339 [(M + H)⁺], M = C₄₄H₃₀N₄O₄.

Zinc(II) 5,15-bis(2,6-bis(2-propynyloxy)phenyl)porphyrin (Zn5). A sample of Zn(OAc)₂·2H₂O (1.3 g, 5.8 mmol) was added to a solution of porphyrin 5 (78 mg, 0.12 mmol) in DMF (20.0 mL). The reaction mixture was placed in an oil bath to reflux for 6 h. The mixture was concentrated under reduced pressure. The crude product was dissolved in CH₂Cl₂, washed (with saturated aqueous NaHCO₃, water, and brine), and then dried (Na₂SO₄) and concentrated to give a red solid (55 mg, 64%): ¹H NMR (500 MHz, CD₂Cl₂) δ 10.29 (s, 2H), 9.43 (d, J = 4.4 Hz, 4H), 9.06 (d, J = 4.4 Hz, 4H), 7.84 (t, J = 8.6 Hz, 2H), 7.29 (d, J = 8.6 Hz, 4H), 4.41 (d, J = 2.4 Hz, 8H), 2.33 (t, J = 2.4 Hz, 4H); ¹³C{¹H} NMR (125 MHz, CD₂Cl₂) δ 158.5, 150.2, 149.4, 131.8, 131.5, 129.9, 121.6, 111.0, 106.8, 105.5, 78.5, 75.0, 56.3; λ_abs (toluene) 414, 539, 575 nm; MALDI-MS obsd 740.05, calcd 740.15 [M⁺]; ESI-MS obsd 741.1453, calcd 741.1475 [(M + H)⁺], M = C₄₄H₂₈N₄O₄Zn. Porphyrin Zn5 upon slow evaporation from tetrahydrofuran at −20 °C afforded a crystalline sample that was then examined by single-crystal X-ray diffraction.

Zn(II) 5,15-bis{2,6-bis[1-(1-(3,6,9,12,15,18-hexaoxanonadecyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}porphyrin (Zn6). Following a reported method²⁶ with some modifications, a solution of zinc porphyrin Zn5 (15 mg, 0.020 mmol) and mPEG₆-azide (I, 93 mg, 0.29 mmol) in dry THF (8.0 mL) was treated sequentially with sodium ascorbate (10 mg, 0.050 mmol), DIPEA (200 μL, 1.10 mmol), and CuI (5.6 mg, 0.030 mmol) under an argon atmosphere. The reaction mixture was refluxed overnight (oil bath). The solution was concentrated under reduced pressure and then dissolved in CH₂Cl₂. The organic layer was washed with water and brine, dried (Na₂SO₄), and concentrated to dryness. The crude product was chromatographed using a gradient [silica, CH₂Cl₂/MeOH (25 : 1) to (15 : 1) to (9 : 1)] to afford a red oil (29 mg, 71%): ¹H NMR (500 MHz, CDCl₃) δ 10.12 (s, 2H), 9.29 (d, J = 4.4 Hz, 4H), 8.97 (d, J = 4.4 Hz, 4H), 7.79 (t, J = 8.5 Hz, 2H), 7.25 (d, J = 8.5 Hz, 4H), 5.99 (s, 4H), 5.04 (s, 8H), 3.66 (t, J = 5.2 Hz, 8H), 3.47–3.35 (m, 32H), 3.26 (s, 12H), 3.20 (t, J = 5.0 Hz, 8H), 3.05–3.03 (m, 8H), 2.83 (t, J = 5.0 Hz, 8H), 2.58 (t, J = 5.0 Hz, 8H), 2.28 (t, J = 5.0 Hz, 8H), 2.15 (t, J = 4.0 Hz, 8H), 2.06 (t, J = 4.5 Hz, 8H); ¹³C{¹H} NMR (175 MHz, CD₂Cl₂) δ 159.5, 150.4, 149.2, 143.5, 131.7, 131.6, 130.3, 122.9, 121.9, 111.6, 106.9, 105.1, 72.0–63.2, 58.7, 58.6, 50.9, 49.5; λ_abs (toluene) 414, 545, 581 nm; MALDI-MS obsd 2025.40, calcd 2024.90 [M⁺]; ESI-MS obsd 2047.8895, calcd 2047.8894 [(M + Na)⁺], M = C₉₆H₁₃₆N₁₆O₂₈Zn.

5,15-Bis{2,6-bis[1-(1-(3,6,9,12,15,18-hexaoxanonadecyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}porphyrin (6). A solution of zinc porphyrin Zn6 (18 mg, 8.8 μmol) in CH₂Cl₂ (6.0 mL) was treated with TFA (40 μmol, 0.52 mmol). The reaction mixture was stirred overnight at room temperature under an argon atmosphere. The solution was neutralized by the addition of saturated aqueous NaHCO₃, then washed with water and brine, and dried (Na₂SO₄). The combined organic extract was concentrated under reduced pressure to give a red oil (17 mg, 96%): ¹H NMR (700 MHz, CDCl₃) δ 10.18 (s, 2H), 9.31 (d, J = 4.4 Hz, 4H), 8.96 (d, J = 4.4 Hz, 4H), 7.80 (t, J = 8.7 Hz, 2H), 7.27 (s, 4H), 6.10 (s, 4H), 5.07 (s, 8H), 3.71 (t, J = 5.2 Hz, 8H), 3.56–3.43 (m, 32H), 3.37–3.35 (m, 8H), 3.31 (s, 12H), 3.27–3.26 (m, 8H), 3.07–3.05 (m, 8H), 2.93 (t, J = 4.4 Hz, 8H), 2.80 (t, J = 4.4 Hz, 8H), 2.47–2.43 (m, 16H), −3.15 (s, 2H); ¹³C{¹H} NMR (175 MHz, CDCl₃) δ 159.2, 143.5, 131.4, 130.8, 122.8, 119.9, 111.3, 106.9, 104.2, 77.2–69.5, 68.4, 63.2, 59.0, 50.7, 49.5; λ_abs (toluene) 410, 503, 537, 578, 632 nm; MALDI-MS obsd 1963.56, calcd 1963.99 [(M + H)⁺]; ESI-MS obsd 1985.9752, calcd 1985.9759 [(M + Na)⁺], M = C₉₆H₁₃₈N₁₆O₂₈.

Cu(II) 5,15-bis{2,6-bis[1-(1-(3,6,9,12,15,18-hexaoxanonadecyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}porphyrin (Cu6). A solution of free base porphyrin 6 (5.2 mg, 2.7 μmol) in CH₂Cl₂/MeOH (3.0 mL, 1 : 2) was treated with Cu(OAc)₂·H₂O (54 mg, 0.27 mmol). The reaction mixture was stirred overnight at room temperature under an argon atmosphere. The mixture was concentrated under reduced pressure, diluted in CH₂Cl₂, and neutralized by the addition of saturated aqueous NaHCO₃. The organic phase was washed with water and brine, and then dried (Na₂SO₄). The combined organic extract was concentrated to give a red oil (5.4 mg, 100%) λ_abs (toluene) 407, 531, 563 nm; MALDI-MS obsd 2046.98, calcd 2046.90 [(M + Na)⁺]; ESI-MS obsd 2046.8931, calcd 2046.8898 [(M + Na)⁺], M = C₉₆H₁₃₆CuN₁₆O₂₈.

Zn(II)-5,15-bis{2,6-bis[1-(1-(21-tert-butyloxy-21-oxo-3,6,9,12,15,18-hexaoxaheneicosanyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}porphyrin (Zn7). Following a reported method²⁶ with some modifications, a solution of zinc porphyrin Zn5 (15 mg, 0.020 mmol) and azido-PEG₆-CO₂t-Bu (II, 70 mg, 0.16 mmol) in dry THF (8.0 mL) was treated with sodium ascorbate (10 mg, 0.050 mmol), DIPEA (200 μL, 1.10 mmol), and CuI (6.0 mg, 0.032 mmol). The reaction mixture was allowed to reflux overnight (oil bath). The mixture was concentrated under reduced pressure and then dissolved in CH₂Cl₂. The organic phase was washed with water and brine, dried (Na₂SO₄), and concentrated to dryness under reduced pressure. The crude product was chromatographed using a gradient [silica, CH₂Cl₂/MeOH (19 : 1) to (16 : 1)] to afford a red oil (40 mg, 81%): ¹H NMR (700 MHz, CDCl₃) δ 10.13 (s, 2H), 9.29 (d, J = 4.3 Hz, 4H), 8.98 (d, J = 4.3 Hz, 4H), 7.78 (t, J = 8.7 Hz, 2H), 7.27–7.17 (m, 4H), 5.99 (s, 4H), 5.05 (s, 8H), 3.69–3.64 (m, 16H), 3.53–3.49 (m, 16H), 3.44 (t, J = 4.9 Hz, 8H), 3.34 (t, J = 4.9 Hz, 8H), 3.18 (t, J = 4.9 Hz, 8H), 2.99 (t, J = 4.9 Hz, 8H), 2.81 (t, J = 4.9 Hz, 8H), 2.47–2.45 (m, 16H), 2.13 (t, J = 4.9 Hz, 8H), 2.04 (t, J = 4.9 Hz, 8H), 1.92 (t, J = 4.9 Hz, 8H), 1.43 (s, 36H); ¹³C{¹H} NMR (175 MHz, CDCl₃) δ 170.9, 159.3, 150.3, 149.0, 143.5, 131.8, 131.5, 130.3, 122.8, 121.7, 111.5, 106.8, 104.9, 80.5, 70.4–68.8, 68.1, 66.8, 63.3, 49.2, 36.2, 28.1; λ_abs (toluene) 414, 544, 582 nm; MALDI-MS obsd 2481.87, calcd 2481.17 [M⁺]; ESI-MS obsd 2504.1593, calcd 2504.1617 [(M + Na)⁺], M = C₁₂₀H₁₇₆N₁₆O₃₆Zn.

5,15-Bis{2,6-bis[1-(1-(21-hydroxy-21-oxo-3,6,9,12,15,18-hexaoxaheneicosanyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}porphyrin (8). A solution of zinc porphyrin Zn7 (19 mg, 7.7 μmol) in THF (2.0 mL) was treated with TFA (300 μL). The reaction mixture was stirred at 40 °C (oil bath) for 3 days, then concentrated under reduced pressure. The crude residue was dissolved in CH₂Cl₂ (25 mL), and water (25 mL) was added followed by triethylamine until pH = 9. The aqueous phase was washed once with CH₂Cl₂. The aqueous phase was acidified with aqueous 1 M HCl until pH = 2, then extracted with CH₂Cl₂ (2 × 25 mL). The combined organic extract was washed with water (25 mL) and brine (25 mL), dried (Na₂SO₄), and concentrated to give a red oil (9.9 mg, 58%): ¹H NMR (700 MHz, CDCl₃) δ 10.21 (s, 2H), 9.32 (d, J = 4.4 Hz, 4H), 8.96 (d, J = 4.4 Hz, 4H), 7.79 (t, J = 8.6, 2H), 7.27 (m, 4H), 6.14 (s, 4H), 5.09 (s, 8H), 3.73–3.36 (m, 64H), 3.28–2.89 (m, 16H), 2.69–2.68 (m, 8H), 2.54–2.48 (m, 8H), 2.39–2.32 (m, 16H), −3.21 (s, 2H); ¹³C{¹H} NMR (175 MHz, CDCl₃) δ 175.0, 159.1, 143.5, 131.5, 130.9, 130.8, 123.0, 119.8, 111.3, 106.9, 104.3, 70.3–69.3, 68.3, 66.8, 63.1, 49.6, 35.5; λ_abs (toluene) 410, 503, 531, 574, 630 nm; ESI-MS obsd 2193.9992, calcd 2194.0013 [(M − H)⁻], M = C₁₀₄H₁₄₆N₁₆O₃₆.

Cu(II)-5,15-bis{2,6-bis[1-(1-(21-hydroxy-21-oxo-3,6,9,12,15,18-hexaoxaheneicosanyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}porphyrin (Cu8). A solution of free base porphyrin 8 (4.1 mg, 1.9 μmol) in CH₂Cl₂/MeOH (3.0 mL, 1 : 2) was treated with Cu(OAc)₂·H₂O (54 mg, 0.27 mmol). The mixture was stirred overnight at room temperature, then concentrated under reduced pressure. The crude residue was dissolved in CH₂Cl₂ and neutralized by the addition of saturated aqueous NaHCO₃. The organic layer was washed with water and brine, dried (Na₂SO₄), and concentrated to give a red oil (2.6 mg, 62%): λ_abs (toluene) 408, 530, 568 nm; ESI-MS obsd 2254.9144, calcd 2254.9152 [(M − H)⁻], M = C₁₀₄H₁₄₄CuN₁₆O₃₆.

5-{2,6-Bis[1-(1-(21-hydroxy-21-oxo-3,6,9,12,15,18-hexaoxaheneicosanyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}-5-(4-ethynylphenyl)porphyrin (11). A solution of zinc porphyrin Zn10 (3.0 mg, 2.0 μmol) in CH₂Cl₂ (200 μL) was treated with TFA (200 μL). The reaction mixture was stirred at room temperature for 2 h, then concentrated under reduced pressure. The crude residue was dissolved in CH₂Cl₂ (15 mL), and water (15 mL) was added followed by saturated aqueous NaHCO₃/NaOH (20 : 1) until pH = 11. The aqueous phase was washed once with CH₂Cl₂. The aqueous phase was acidified with aqueous 1 M HCl until pH = 2, then extracted with CH₂Cl₂ (2 × 15 mL). The combined organic extract was dried (Na₂SO₄) and concentrated to give a red solid (2.5 mg, 94%): ¹H NMR (700 MHz, CDCl₃) δ 10.28 (s, 2H), 9.41 (d, J = 4.4 Hz, 2H), 9.34 (d, J = 4.4 Hz, 2H), 9.05 (d, J = 4.4 Hz, 2H), 8.99 (d, J = 4.4 Hz, 2H), 8.23 (d, J = 7.8 Hz, 2H), 7.96 (d, J = 7.8 Hz, 2H), 7.80 (t, J = 8.7 Hz, 1H), 7.28 (d, J = 8.7 Hz, 2H), 5.91 (s, 2H), 5.11 (s, 4H), 3.66 (t, J = 5.2 Hz, 4H), 3.61 (t, J = 6.0 Hz, 4H), 3.50–3.37 (m, 12H), 3.36 (s, 1H), 3.32–3.27 (m, 4H), 3.21–3.14 (m, 4H), 3.05–2.99 (m, 4H), 2.87 (t, J = 5.1 Hz, 4H), 2.76 (t, J = 4.7 Hz, 4H), 2.48–2.44 (m, 8H), 2.29–2.24 (m, 4H), 2.15–2.7 (m, 4H), −3.11 (s, 2H); ¹³C{¹H} NMR (175 MHz, CDCl₃) δ 174.0, 159.1, 143.7, 142.0, 134.7, 131.9, 131.6, 131.1, 131.0, 130.8, 130.7, 122.8, 121.8, 119.8, 118.1, 111.4, 107.0, 105.0, 83.6, 70.4, 70.2, 70.1, 70.0, 70.0, 69.6, 69.4, 69.3, 69.3, 68.4, 66.5, 63.2, 49.5, 35.1; ESI-MS obsd 1375.5843, calcd 1375.5857 [(M + Na)⁺], M = C₇₀H₈₄N₁₀O₁₈.

Zn(II)-5-(2,6-bis[1-(1-(9-tert-butyloxy-9-oxo-3,6-dioxanonyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl)-15-(4-(2-(triisopropylsilyl)ethynyl)phenyl)porphyrin (Zn12). Following a reported method²⁶ with some modifications, a solution of zinc porphyrin Zn9 (20 mg, 25 μmol), tert-butyl 1-azido-3,6-dioxanonan-9-oate (III, 52 mg, 0.10 mmol), sodium ascorbate (20 mg, 0.10 mmol), and CuI (9.5 mg, 50 μmol) in dry THF (5.0 mL) was treated with DIPEA (100 μL, 0.57 mmol) under an argon atmosphere. The reaction mixture was stirred at room temperature for 2 h then eluted through a silica pad (2.5 cm × 1 cm) with ethyl acetate. The filtrate was concentrated and chromatographed [silica, CH₂Cl₂/ethyl acetate (9 : 1) to (1 : 9) then pure ethyl acetate] to afford a red non-crystalline solid (24 mg, 73%): ¹H NMR (500 MHz, CDCl₃) δ 10.05 (s, 2H), 9.28 (d, J = 4.5 Hz, 2H), 9.13 (d, J = 4.4 Hz, 2H), 8.99 (d, J = 4.4 Hz, 2H), 8.78–8.65 (m, 2H), 8.14 (d, J = 7.5 Hz, 2H), 7.89 (d, J = 7.5 Hz, 2H), 7.60 (t, J = 8.5 Hz, 1H), 6.79–6.61 (m, 2H), 4.88 (s, 2H), 3.91 (s, 4H), 2.99 (s, 4H), 2.36–2.11 (m, 8H), 1.75–1.50 (m, 8H), 1.39–1.31 (m, 4H), 1.28 (s, 21H), 1.16 (s, 18H); ¹³C{¹H} NMR (125 MHz, CDCl₃) δ 170.3, 158.6, 150.3, 149.4, 149.1, 149.0, 143.4, 143.1, 134.5, 131.8, 131.7, 131.4, 130.2, 122.5, 121.6, 121.3, 119.0, 111.1, 107.3, 106.4, 105.3, 91.6, 80.3, 68.5, 68.4, 67.4, 65.6, 62.2, 48.6, 35.0, 27.8, 18.9, 11.5; ESI-MS obsd 1331.5653, calcd 1331.5662 [(M + H)⁺], M = C₇₁H₈₆N₁₀O₁₀SiZn.

Zn(II)-5-(2,6-bis[1-(1-(9-tert-butyloxy-9-oxo-3,6-dioxanonyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl)-15-(4-ethynylphenyl)porphyrin (Zn13). A solution of zinc porphyrin Zn12 (24 mg, 18 μmol) in THF (3.6 mL) was treated with TBAF (1.0 M solution in THF, 36 μL, 36 μmol) for 5 min. The reaction mixture was concentrated and dissolved in CH₂Cl₂ (20 mL). The solution was washed with brine (30 mL × 2), saturated aqueous NaHCO₃ (30 mL × 2), and then brine (30 mL). The organic phase was then dried (Na₂SO₄), concentrated, and chromatographed [silica, CH₂Cl₂/ethyl acetate (9 : 1 to 1 : 2)] to afford a dark red solid (20 mg, 93%): ¹H NMR (500 MHz, CDCl₃) δ 10.04 (s, 2H), 9.28 (d, J = 4.4 Hz, 2H), 9.11 (d, J = 4.4 Hz, 2H), 8.98 (d, J = 4.4 Hz, 2H), 8.70 (d, J = 4.4 Hz, 2H), 8.20–8.14 (m, 2H), 7.94–7.87 (m, 2H), 7.57 (t, J = 8.5 Hz, 1H), 6.65 (d, J = 8.6 Hz, 2H), 4.88 (s, 2H), 3.74 (s, 4H), 3.34 (s, 1H), 2.95 (t, J = 5.4 Hz, 4H), 2.37–2.24 (m, 8H), 1.81–1.67 (m, 8H), 1.45 (t, J = 6.5 Hz, 4H), 1.18 (s, 18H); ¹³C{¹H} NMR (125 MHz, CDCl₃) δ 170.4, 158.7, 150.4, 149.5, 149.3, 149.1, 144.1, 143.1, 134.7, 131.9, 131.8, 131.6, 130.4, 130.2, 121.5, 121.4, 121.2, 118.8, 111.2, 106.3, 105.5, 84.0, 80.4, 78.2, 68.8, 68.6, 67.5, 65.8, 62.1, 48.7, 35.2, 28.0; ESI-MS obsd 1175.4308, calcd 1175.4328 [(M + H)⁺], M = C₆₂H₆₆N₁₀O₁₀Zn.

5-(2,6-Bis[1-(1-(9-hydroxy-9-oxo-3,6-dioxanonyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl)-15-(4-ethynylphenyl)porphyrin (14). A solution of zinc porphyrin Zn13 (11 mg, 10 μmol) in CH₂Cl₂ (500 μL) was treated with TFA (500 μL). The reaction mixture was stirred at room temperature for 2 h, then concentrated under reduced pressure. The crude residue was dissolved in CH₂Cl₂ (15 mL), then water (15 mL) was added followed by saturated aqueous NaHCO₃/NaOH (20 : 1) until pH = 11. The aqueous phase was washed once with CH₂Cl₂. The aqueous phase was acidified with aqueous 1 M HCl until pH = 2, then extracted with CH₂Cl₂ (2 × 15 mL). The combined organic extract was dried (Na₂SO₄) and concentrated to give a red solid (4.4 mg, 44%): ¹H NMR (500 MHz, CD₂Cl₂) δ 10.29 (s, 2H), 9.41 (d, J = 4.5 Hz, 2H), 9.36 (d, J = 4.5 Hz, 2H), 9.05 (d, J = 4.4 Hz, 2H), 8.95 (d, J = 4.4 Hz, 2H), 8.24 (d, J = 7.9 Hz, 2H), 7.96 (d, J = 7.9 Hz, 2H), 7.78 (t, J = 8.7 Hz, 1H), 7.25 (d, J = 8.7 Hz, 2H), 6.16 (s, 2H), 5.06 (s, 4H), 3.72 (t, J = 5.0 Hz, 4H), 3.42 (s, 1H), 2.95 (t, J = 5.0 Hz, 4H), 2.82 (t, J = 6.1 Hz, 4H), 2.49–2.42 (m, 4H), 2.39–2.33 (m, 4H), 2.03 (t, J = 6.1 Hz, 4H); ¹³C{¹H} NMR (125 MHz, CD₂Cl₂) δ 175.4, 159.0, 143.5, 142.0, 134.8, 132.0, 131.6, 130.8, 122.9, 121.6, 119.7, 118.2, 111.2, 106.8, 105.0, 83.5, 78.2, 69.3, 69.2, 68.4, 65.6, 63.0, 49.6, 34.4; ESI-MS obsd 1023.3746, calcd 1023.3760 [(M + Na)⁺], M = C₅₄H₅₂N₁₀O₁₀.

For log P measurements, the following porphyrins were prepared in small scale by zincation or cupration of the corresponding free base porphyrin. The products were characterized by absorption spectroscopy, fluorescence spectroscopy (for the zinc chelates), and mass spectrometry prior to examination in log P measurements.

Zn(II)-5,15-bis{2,6-bis[1-(1-(21-hydroxy-21-oxo-3,6,9,12,15,18-hexaoxaheneicosanyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}porphyrin (Zn8). Following a reported method,²⁴ Zn(OAc)₂·2H₂O (34.5 mg) was added to a solution of free base porphyrin 8 (1.0 mg) in CH₂Cl₂/MeOH (570 μL, 9 : 1). The reaction mixture was stirred at room temperature for 3 h. The mixture was diluted with CH₂Cl₂ (10 mL). The organic phase was washed with water (10 mL) and brine, dried (Na₂SO₄), and concentrated to dryness: λ_abs (DMSO) 427, 556, 571 nm; λ_em 600, 652 nm; MALDI-MS obsd 2257.65, calcd 2257.92 [(M + H)⁺], M = C₁₀₄H₁₄₄N₁₆O₃₆Zn.

Zn(II)-5-{2,6-bis[1-(1-(21-hydroxy-21-oxo-3,6,9,12,15,18-hexaoxaheneicosanyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}-5-(4-ethynylphenyl)porphyrin (Zn11). Following a reported method,²⁴ Zn(OAc)₂·2H₂O (60.0 mg) was added to a solution of free base porphyrin 11 (1.0 mg) in CH₂Cl₂/MeOH (570 μL, 9 : 1). The reaction mixture was stirred at room temperature for 3 h. The mixture was diluted with CH₂Cl₂ (10 mL). The organic phase was washed with water (10 mL) and brine, dried (Na₂SO₄), and concentrated to dryness: λ_abs (DMSO) 419, 549, 584 nm; λ_em 591, 643 nm; MALDI-MS obsd 1415.46, calcd 1415.51 [(M + H)⁺], M = C₇₀H₈₂N₁₀O₁₈Zn.

Cu(II)-5-{2,6-bis[1-(1-(21-hydroxy-21-oxo-3,6,9,12,15,18-hexaoxaheneicosanyl)-1H-1,2,3-triazol-4-yl)methoxy]phenyl}-5-(4-ethynylphenyl)porphyrin (Cu11). A sample of Cu(OAc)₂·H₂O (45.0 mg) was added to a solution of free base porphyrin 11 (1.0 mg) in CH₂Cl₂/MeOH (570 μL, 9 : 1). The reaction mixture was stirred at room temperature for 30 min. The mixture was diluted with CH₂Cl₂ (10 mL). The organic phase was washed with water (10 mL) and brine, dried (Na₂SO₄), and concentrated to dryness: λ_abs (DMSO) 407, 528, 559 nm; MALDI-MS obsd 1436.66, calcd 1436.51 [(M + Na)⁺], M = C₇₀H₈₂N₁₀O₁₈Cu.

Conflicts of interest

The authors declare competing financial interests.

Acknowledgements

Funding was provided by the National Science Foundation to Oncurie, Inc. (NSF 2136700), and by NC State University. NMR spectroscopy and mass spectrometry measurements were carried out in the Molecular Education, Technology, and Research Innovation Center (METRIC) at NC State University.

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, ¹³C{¹H} NMR, and MALDI-MS spectra for new compounds; absorption spectra and photographs of selected compounds in solution; single-crystal X-ray diffraction data. CCDC 2321457 (Zn5). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nj01178c

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