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Article

Panchromatic Absorbers Tethered for Bioconjugation or Surface Attachment

by
Rui Liu
1,†,
Jie Rong
1,†,
Zhiyuan Wu
1,†,
Masahiko Taniguchi
1,
David F. Bocian
2,*,
Dewey Holten
3,* and
Jonathan S. Lindsey
1,*
1
Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA
2
Department of Chemistry, University of California, Riverside, CA 92521-0403, USA
3
Department of Chemistry, Washington University, St. Louis, MO 63130-4889, USA
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2022, 27(19), 6501; https://doi.org/10.3390/molecules27196501
Submission received: 6 September 2022 / Revised: 25 September 2022 / Accepted: 27 September 2022 / Published: 1 October 2022
(This article belongs to the Special Issue Porphyrin-Based Compounds: Synthesis and Application)
Browse Figures
Versions Notes

Abstract

:
The syntheses of two triads are reported. Each triad is composed of two perylene-monoimides linked to a porphyrin via an ethyne unit, which bridges the perylene 9-position and a porphyrin 5- or 15-position. Each triad also contains a single tether composed of an alkynoic acid or an isophthalate unit. Each triad provides panchromatic absorption (350–700 nm) with fluorescence emission in the near-infrared region (733 or 743 nm; fluorescence quantum yield ~0.2). The syntheses rely on the preparation of trans-AB-porphyrins bearing one site for tether attachment (A), an aryl group (B), and two open meso-positions. The AB-porphyrins were prepared by the condensation of a 1,9-diformyldipyrromethane and a dipyrromethane. The installation of the two perylene-monoimide groups was achieved upon the 5,15-dibromination of the porphyrin and the subsequent copper-free Sonogashira coupling, which was accomplished before or after the attachment of the tether. The syntheses provide relatively straightforward access to a panchromatic absorber for use in bioconjugation or surface-attachment processes.

1. Introduction

Chromophores that absorb across the visible spectrum (400–700 nm) are of essential importance for diverse studies in the photosciences. The chlorophylls of plant photosynthesis exhibit strong absorption in the violet and red regions, with relatively weak absorption across the rest of the visible region [1]. Natural photosynthetic systems use carotenoids and/or other accessory chromophores in complementary fashion to fill the blue–orange region of the solar spectrum [2]. In this approach, the distinct absorption of a given accessory pigment is followed by excited-state energy transfer among a set of pigments, thereby increasing the wavelength expanse of absorption beyond that of chlorophyll alone. Work that began some 45 years ago [3] led to the realization in the early 1990s that the absorption spectrum of porphyrins could be substantially broadened upon the conjugation of ethynyl groups to the macrocycle [4,5,6,7]. Penetrating studies thereafter by the groups of Anderson [8] and Therien [9,10,11,12,13,14,15,16,17,18,19,20,21] chiefly focused on joining porphyrins via butadiyne and ethyne linkers, respectively, eliciting fascinating spectroscopic features of the resulting arrays. The joining of porphyrins and other chromophores via ethynyl linkers quickly became a prominent molecular motif [22,23,24,25], a topic that has been reviewed [26,27].
In a quite separate research thread, we had prepared and characterized a large number of tetrapyrrole-perylene constructs for studies of excited-state energy transfer as part of a program in artificial photosynthesis [28,29,30,31,32,33,34,35,36,37,38,39,40]. In such constructs, we had employed long linkers such as phenyl-ethynyl-phenyl to separate the tetrapyrrole and perylene or ethynylphenyl joining the pigments in the manner perylene-ethynylphenyl-tetrapyrrole. As an intended “last molecular design” in this area [41], we omitted any phenyl groups and prepared a perylene-ethynyl-porphyrin dyad (0). The dyad exhibited such broad and unusual absorption across the visible region that the characteristic Soret band, a hallmark of porphyrins, was almost unrecognizable (Figure 1) [41].
For exacting comparison, we then prepared an analogous perylene-ethynyl-porphyrin dyad (1) [42], which differed from the initial dyad (0) in the nature of the non-linking substituents (p-tolyl versus mesityl groups) on the porphyrin, and also the corresponding ethynyl-porphyrin (2) [43,44] and ethynyl-perylene (3) [40] benchmark compounds bearing nearly identical substituents. The change from mesityl to p-tolyl substituents was inconsequential, as shown in the overlay of the spectra of dyads 0 and 1. While the absorption spectrum of each dyad resembles that of a strongly potentiated perylene and diminished porphyrin (Figure 1), the fluorescence emission spectrum resembles that of a porphyrin, albeit one shifted bathochromically (~40 nm) and with substantially increased intensity. Indeed, the fluorescence quantum yield (Φf) of dyad 1 is 0.38 instead of 0.14 for the phenylethynyl-substituted porphyrin 2 [42].
This unexpected finding in 2013 [41] did not close the experimental program but instead prompted an additional decade of research [45,46,47,48,49,50] to establish the physical basis for the origin of panchromaticity. An ideal panchromatic absorber was found to be a porphyrin bearing two perylene-monoimides each joined via a single ethyne unit bridging the perylene 9-position and a porphyrin meso-position (4, Chart 1). The absorption and fluorescence spectra of 4 are shown in Figure 2, upper panel. Conversion of the free base porphyrin to the zinc chelate (Zn4) alters the spectral features (Figure 2, lower panel). The free base porphyrin triad also has been incorporated as a crossbar unit in a pentad array for solar light capture and charge separation [48].
In this paper, we describe the extension of the panchromatic triad for studies in light-harvesting. One design incorporates a single carboxylic acid for bioconjugation, such as in biohybrid assemblies wherein additional solar light capture is advantageous. A second design incorporates an isophthalic acid terminal unit for attachment to surfaces, such as metal oxides for studies of photoinduced charge injection. The syntheses rely on established rational methods and provide relatively direct access to lipophilic panchromatic triads each bearing a single attachment handle.

2. Results and Discussion

2.1. Synthesis of a Bioconjugatable Panchromatic Triad

We sought to place a hexanoic acid chain on the central porphyrin for bioconjugation purposes. A meso-dibromo-substituted porphyrin building block was prepared for the construction of the bioconjugatable panchromatic triad. The core porphyrin contains an AB pattern of substituents with two open meso-positions and was prepared in a standard manner [51] from two dipyrromethanes (Scheme 1). The dibutyltin-complexed 1,9-diformyl-5-phenyldipyrromethane 5 [52] was treated with propylamine [51] and then reacted with 1,9-diunsubstituted 5-(4-bromophenyl)dipyrromethane 6 [53] in the presence of Zn(OAc)2 to afford the trans-AB zinc porphyrin 7, which bears a bromine atom at a site for the introduction of a bioconjugatable tether. The bromo-porphyrin 7 thereafter reacted with ethynyl tether 8 [54] via a Sonogashira coupling reaction to form the bioconjugatable zinc porphyrin 9. The Pd-mediated Sonogashira reaction [55] was carried out under copper-free conditions [56,57] using a solvent mixture of toluene and triethylamine (TEA). The bromination [58] of porphyrin 9 using N-bromosuccinimide (NBS) at 0 °C yielded the dibromoporphyrin building block 10. The next step entailed attachment of two perylene-ethyne units.
Several points warrant comment concerning the choice of ethynyl-perylene for coupling with the dibromoporphyrin. The chosen perylene (11) bears two aryloxy groups (one in each bay region) and a 2,6-diisopropylphenyl group at the N-imide site, which together impart structural features that enable good solubility of the perylene in hydrocarbon solvents [46]. The ethyne is located at the perylene 9-position, a site of considerable electron density in the frontier molecular orbitals and a site known to afford substantial electronic communication upon covalent attachment to the porphyrin [42,45,46,47,48,49,50]. The ethynyl-perylene-monoimide 11 is an advanced functional dye that has emerged from extensive studies over several decades beginning with the pioneering work of Langhals [59,60,61,62,63,64,65]. A key distinction between 11 and many other rylene dyes [66,67,68] is the presence of a single imide versus two imides. The presence of two imides causes the resulting dye to be a good photooxidant, whereas the mono-imide is more electroneutral. The synthesis of 11 [46] follows methods [39,69,70,71] established earlier using a 2,5-di-tert-butylphenyl group at the N-imide site. The 2,6-diisopropylphenyl group of 11 is superior in not giving rise to stereoisomers as occur with the 2,5-di-tert-butylphenyl group (as in 1) [60,61,65].
The absorption spectra of ethynyl-perylene-monoimides bearing the 2,6-diisopropylphenyl group (11) or the 2,5-di-tert-butylphenyl (11-tBu [70,71]) are nearly identical, as shown in Figure 3. The similarity of spectra upon use of either solubilization motif is consistent with observation that the 2,6-diisopropyl versus 2,5-di-tert-butyl group has insignificant effects on perylene photophysics [40]. The similar spectra and photophysics are attributed to the presence of a node at the perylene-imide nitrogen atom in both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) [61]. Thus, the changes in porphyrin substituents from mesityl to p-tolyl groups, and perylene N-imide from a 2,5-di-tert-butylphenyl to a 2,6-diisopropylphenyl group, facilitate synthesis and chemical characterization but have hardly any effects on spectral and photophysical properties.
The Sonogashira coupling [55] reaction of ethynyl-perylene 11 and dibromoporphyrin 10 was carried out under copper-free [56,57] Pd-mediated conditions (as in the reaction of 7 and 8). Such a reaction of an ethynylphenyl-chromophore and a meso-bromoporphyrin is facile and has been carried out under nearly identical conditions to form porphyrin-phenylethynyl-porphyrin dyads [44]. The rationale for copper-free conditions in the synthesis was to avoid any transmetalation of the zinc porphyrin given (1) the avidity of porphyrins for Cu(II); (2) the very short-lived singdoublet excited-state lifetime of copper porphyrins (marked by the absence of fluorescence); and (3) the subsequent difficulty of removing copper from porphyrins, unlike the facile removal of zinc achieved upon treatment with mild acid. Thus, the subsequent acid-mediated demetalation of zinc [42] with trifluoroacetic acid (TFA) afforded the target panchromatic triad 12. Purification was achieved via a three-chromatography sequence that includes use of size-exclusion chromatography (SEC) [42,72]. Removal of the tert-butyl protecting group using 40% TFA [73] in CH2Cl2 gave the carboxy-triad 13 (Scheme 1). For bioconjugation purposes, 13 was further transformed to an N-hydroxysuccinimide ester 14 via reaction [74] with N-hydroxysuccinimide in the presence of N,N-dicyclohexylcarbodiimide (DCC).

2.2. Synthesis of a Panchromatic Triad for Surface Attachment

We previously prepared a set of tetrapyrrole macrocycles (15–17) bearing an isophthalic acid tether [75] for surface attachment (Chart 2) [76]. The present work extends this design motif.
The synthesis of the core porphyrin follows that shown for the bioconjugatable triad. Thus, the 1,9-formyldipyrromethane 18 was reacted with propylamine [51] to form the bis(imine), which was then treated with the complementary dipyrromethane 19 [77] to afford the zinc porphyrin 20 in 38% yield (Scheme 2). The bromination [58] of zinc porphyrin 20 afforded the corresponding dibromo zinc porphyrin 21 in 75% yield. Sonogashira coupling [55] of the dibromo zinc porphyrin 21 and ethynyl-perylene 11 under copper-free conditions [56,57] afforded the triad 22 bearing two perylenes and one zinc porphyrin in 90% yield (~200 mg). The reaction progress was monitored by analytical SEC [72], as has been done previously with other panchromatic arrays [42,46]. The analytical SEC traces with absorption spectral determination show the starting materials (Figure 4, panel a), crude mixture after reaction for several hours (panel b), and the purified triad 22 (panel c) following preparative purification using the three-column chromatography process. The cleavage of the trimethylsilyl (TMS) group [48] of 22 afforded triad 23 bearing a free ethynyl group in 93% yield.
The surface-attachment motif was prepared through the DCC-mediated condensation of 5-bromoisophthalic acid (24) and 2-(trimethylsilyl)ethanol (25) in N,N-dimethylformamide (DMF) to give the protected 5-bromo-isophthalate 26 in 23% yield (Scheme 2). The Sonogashira coupling [55] of ethynyl-triad 23 and isophthalate 26 under copper-free conditions [56,57] afforded the protected tethered triad 27 in 69% yield. The removal of the 2-trimethylsilylethyl group [76] of 27 upon treatment with tetrabutylammonium fluoride (TBAF) in tetrahydrofuran (THF) afforded isophthalate-tethered triad 28 in 59% yield. While protected triad 27 was fully characterized, all efforts to characterize 28 by NMR spectroscopy and mass spectrometry were unsuccessful. For confirmation purposes, a small portion of 28 was treated with methanol in the presence of DCC and and 4-dimethylaminopyridine (DMAP) to form the corresponding dimethyl ester 29, which gave the expected mass peak upon matrix-assisted laser-desorption ionization mass spectrometry (MALDI-MS) analysis.
A tethered porphyrin analogue lacking the two perylene-ethynyl groups was prepared. The cleavage of the TMS group [48] of zinc porphyrin 20 afforded zinc porphyrin 30 bearing an ethynyl group in 91% yield (Scheme 3). The Sonogashira coupling [55] of 30 and isophthalate 26 under copper-free conditions [56,57] afforded the protected tethered porphyrin 31 in 45% yield. The removal of the 2-trimethylsilylethyl groups [76] of 31 upon treatment with TBAF afforded isophthalate-tethered porphyrin 32 in 78% yield.

2.3. Chemical Characterization

The triads were generally characterized by 1H NMR spectroscopy, absorption spectroscopy, and MALDI-MS analysis. Limited solubility precluded the collection of 13C NMR spectra for a number of the triads as well as other compounds. Accurate mass data were obtained by electrospray ionization mass spectrometry (ESI-MS) where possible. The absorption and fluorescence spectra of two tethered triads are shown in Figure 5. The carboxy-triad 13 contains a free base porphyrin, whereas the isophthalate-triad 27 contains a zinc porphyrin. The absorption spectrum of carboxy-triad 13 (panel a) is nearly identical to that of the untethered triad 4 shown in Figure 2. The absorption spectrum of isophthalate-triad 27 (panel b) is nearly identical to that of the untethered triad Zn4 shown in Figure 2. For comparison purposes, the absorption and fluorescence spectra of a benchmark perylene (33) [46] and the trans-AB zinc porphyrin 31 are shown in panels c and d, respectively. Perylene 33 includes a phenyl group at the terminus of the ethyne and a 2,6-diisopropylphenyl substituent at the N-imide position (Chart 3) and is displayed correctly in the original report of synthesis and characterization [46], but is shown incorrectly with the 2,5-di-tert-butylphenyl substituent at the N-imide position in a subsequent report [50]. Molar absorption coefficient values are reported for triads 13, 23, 27, and 28, as well as for benchmark ethynyl-perylene-monoimide 11. The Φf values measured in toluene for triad 13, triad 27, perylene 33, and porphyrin 31 are 0.21, 0.24, 0.94 [50], and 0.011, respectively. For comparison, the Φf values for free base porphyrin triad 4 and zinc porphyrin triad Zn4 are 0.26 and 0.30, respectively [45]. The fluorescence emission of each triad exhibits (as expected) the general spectral features of a porphyrin but with an enhanced quantum yield. 1H and 13C{1H} NMR spectra, mass spectra, and absorption spectra (where available) for new compounds are provided in the Supplementary Materials.

3. Materials and Methods

3.1. General Methods

All chemicals obtained commercially were used as received unless noted otherwise. Reagent-grade solvents (CH2Cl2, hexanes, methanol, toluene, ethyl acetate) and HPLC-grade solvents (toluene, CH2Cl2, hexanes) were used as received. THF was freshly distilled from sodium/benzophenone ketyl and used immediately. MALDI-MS was performed with the matrix 1,4-bis(5-phenyl-2-oxazol-2-yl)benzene (POPOP) [78] or α-cyano-4-hydroxycinnamic acid (α-CHCA) as noted. ESI-MS data were obtained in positive-ion mode unless noted otherwise. Known building blocks 5 [52], 6 [53], 8 [54], 11 [46], 18 [51], and 19 [77] were prepared via reported methods.

3.2. Purification Following Sonogashira Coupling Reactions

Following a three-chromatography procedure [42,70], reaction mixtures of arrays were first chromatographed with adsorption column chromatography (flash silica, Baker) to remove catalysts and ligands from the coupling reaction. Then, preparative-scale size exclusion chromatography (SEC) was performed using BioRad Bio-Beads S-X1. A preparative-scale glass column (4.3 × 53 cm) was packed using BioRad Bio-Beads S-X1 in HPLC grade toluene. The chromatography was performed with gravity flow (~0.2 mL/min). Thereafter, a subsequent adsorption column chromatography (flash silica, Baker) procedure was performed (with HPLC-grade CH2Cl2 and hexanes unless noted otherwise) to remove material that may have leached from the SEC resin.
The preparative purification procedure is generally most effective when the reaction affords a change in size; e.g., in instances where the product is substantially larger than the starting materials. Such is the case of forming triads via Sonogashira coupling procedures as described herein. The purification protocol is applicable to both zinc and free base porphyrins. Here, all Sonogashira coupling reactions were carried out under anaerobic, copper-free conditions [56,57] using zinc porphyrins in relatively dilute solution, as required for homogeneous solubilization of each porphyrin reactant.
Analytical-scale SEC was performed to monitor the purification of arrays [42,56]. Analytical SEC columns (styrene-divinylbenzene copolymer) were purchased from Polymer Laboratories. Analytical SEC was performed with a Hewlett-Packard 1100 HPLC using PLgel 100 Å, Plgel 500 Å, and PLgel 1000 Å columns (each ~30-cm in length) in series, eluting with toluene (flow rate = 1.0 mL/min). Sample detection was achieved by absorption spectroscopy using a diode array detector with quantitation at 422, 521, 638, and 726 nm (±8 nm band width), which best captures the peaks of the arrays. In other cases, analytical SEC was performed using PLgel 50 Å, PLgel 100 Å × 2, and PLgel 500 Å columns (each ~30-cm in length) in series, eluting with THF (flow rate = 0.8 mL/min) at room temperature. Sample detection was achieved by absorption spectroscopy using a diode array detector with quantitation at 440, 488, 515, 550, and 710 nm.

3.3. Synthesis and Characterization

Zinc(II)-5-(4-Bromophenyl)-15-phenylporphyrin (7). Following a general procedure [51], a solution of 5 (126 mg, 247 μmol) in propylamine (0.4 mL, 5 mmol) was stirred at room temperature for 2 h. Then the mixture was concentrated and dried at high vacuum for 5 min. The resulting solid was dissolved in ethanol (30 mL) and then treated with 6 (75.0 mg, 250 μmol) and zinc acetate (0.52 g, 25 mmol). The mixture was refluxed at 90 °C open to the air for 20 h. The mixture was allowed to cool to room temperature and then was concentrated. Column chromatography [silica, hexanes/CH2Cl2 (1:2) to CH2Cl2] afforded a pink solid (67 mg, 45%): 1H NMR (THF-d8, 400 MHz) δ 10.29 (s, 2H), 9.44–9.41 (m, 4H), 9.04–9.02 (m, 4H), 8.26–8.24 (m, 2H), 8.18–8.16 (m, 2H), 7.98–7.96 (m, 2H), 7.80–7.78 (m, 3H); 13C{1H} NMR (THF-d8, 100 MHz) δ 150.7, 149.8, 136.4, 134.8, 132.0, 131.7, 131.5, 129.7, 127.3, 126.5, 105.8; MALDI-MS (POPOP) obsd 603.8, calcd 603.0 [M + H]+; ESI-MS obsd 602.0089, calcd 602.0079 (M+), M = C32H19BrN4Zn; λabs (toluene) 413, 538, 573 nm.
Zinc(II)-5-[4-(7-tert-Butoxy-7-oxohept-1-ynyl)phenyl]-15-phenylporphyrin (9). Following a general procedure [56,57], a mixture of toluene/triethylamine (5:1, v:v) was deaerated with a continuous steam of argon for 1 h. A mixture of zinc porphyrin 7 (22 mg, 36 µmol), P(o-tol)3 (14 mg, 46 μmol), and Pd2(dba)3 (5 mg, 6 µmol) was placed into a 100 mL Schlenk flask and evacuated under high vacuum for 10 min. The flask was refilled with argon thereafter, and the procedure was repeated three times. Then the degassed solvent (10 mL) was added to the flask, whereupon three freeze–pump–thaw cycles were performed. The mixture was allowed to warm to temperature. Ethynyl coupling partner 8 (19 mg, 0.10 mmol) was added dropwise to the mixture under a continuous stream of argon. The mixture was then stirred for 3 h at 60 °C. The mixture was allowed to cool to room temperature and then was quenched by the addition of water. The mixture was extracted with CH2Cl2. The organic layer was washed with water, dried (Na2SO4), concentrated and chromatographed [silica, hexanes/CH2Cl2 (1:2) to CH2Cl2] to afford a dark purple solid (47 mg, 60%): 1H NMR (THF-d8, 400 MHz) δ 10.46 (s, 2H), 9.61–9.58 (m, 4H), 9.24–9.21 (m, 4H), 8.45–8.37 (m, 4H), 8.01–7.96 (m, 5H), 2.81 (t, J = 6.8 Hz, 2 H), 2.55 (t, J = 6.8 Hz, 2H), 2.08 (q, J = 7.2 Hz, 2H), 2.00–1.95 (m, 2H), 1.69 (s, 9H); 13C{1H} NMR (THF-d8, 100 MHz) δ 187.5, 172.0, 150.1, 149.9, 149.8, 149.7, 143.6, 143.0, 142.4, 135.6, 134.9, 134.8, 131.9, 131.7, 131.6, 131.5, 130.2, 129.7, 128.9, 128.4, 127.3, 126.5, 125.8, 123.5, 119.7, 118.8, 105.8, 90.6, 81.1, 79.3, 34.8, 28.6, 19.2; MALDI-MS (POPOP) obsd 705.2, calcd 705.2 [M + H]+; ESI-MS obsd 704.2134, calcd 704.2124 (M+), M = C43H36N4O2Zn; λabs (toluene) 413, 539, 573 nm.
Zinc(II)-10,20-Dibromo-5-[4-(7-tert-butoxy-7-oxohept-1-ynyl)phenyl]-15-phenylporphyrin (10). Following a general procedure [58], a solution of 9 (47 mg, 67 μmol) in CHCl3 (23 mL) was stirred at 0 °C in ice bath, then treated with NBS (38 mg, 21 μmol). The mixture was stirred at 0 °C for 30 min, whereupon acetone was added to quench the reaction. Then the mixture was washed with water and extracted with CH2Cl2. The organic extract was dried (Na2SO4), concentrated, and chromatographed [silica, CH2Cl2] to afford a green solid (41 mg, 71%): 1H NMR (THF-d8, 400 MHz) δ 9.69–9.67 (m, 4H), 8.90–8.87 (m, 4H), 8.19 (d, J = 6.4 Hz, 2H), 8.13 (d, J = 8.0 Hz, 2H), 7.83–7.79 (m, 5H), 2.64 (t, J = 6.8 Hz, 2H), 2.38 (t, J = 8.0 Hz, 2H), 1.91 (p, J = 6.8 Hz, 2H), 1.82 (p, J = 7.2 Hz, 2H), 1.52 (s, 9H); 13C{1H} NMR (THF-d8, 100 MHz) δ 172.0, 151.0, 150.7, 150.2, 142.9, 142.2, 134.7, 134.6, 133.2, 133.0, 132.9, 132.7, 129.6, 127.7, 126.6, 123.9, 122.3, 121.5, 104.6, 90.9, 80.9, 79.3, 34.8, 28.6, 27.6, 19.2; ESI-MS obsd 860.0335, calcd 860.03245 (M+), M = C43H34N4O2Br2Zn; λabs (toluene) 429, 539, 598 nm.
5-[4-(7-tert-Butoxy-7-oxohept-1-ynyl)phenyl]-10,20-bis[2-(3,4-(N-(2,6-diisopropylphenyl)iminodicarbonyl)-1,6-bis(4-tert-butylphenoxy)perylen-9-yl)ethynyl]-15-phenylporphyrin (12). Following a standard procedure [44,46,56,57], a mixture of zinc dibromoporphyrin 10 (3.0 mg, 3.5 μmol), ethynyl-perylene 11 (6.0 mg, 7.5 μmol), P(o-tol)3 (2.2 mg, 7.2 μmol), and Pd2(dba)3 (0.8 mg, 1.0 µmol) in degassed toluene/triethylamine (2.2 mL, 5:1, v:v) was stirred at 60 °C for 3 h. The mixture was allowed to cool at room temperature. The mixture was then washed with water and extracted with CH2Cl2. The organic extract was dried (Na2SO4) and concentrated. The resulting solid was dissolved in CH2Cl2 (2.0 mL) and treated with TFA (14 μL). The mixture was stirred at room temperature for 1 h, whereupon excess triethylamine was added to quench the reaction. The solution was then washed with water, dried (Na2SO4), concentrated, and chromatographed using the standard three-chromatography procedure to afford a black solid (2.3 mg, 29%): 1H NMR (CDCl3, 300 MHz) δ 9.78–9.73 (m, 4H), 9.31 (d, J = 7.8 Hz, 2H), 9.20 (d, J = 8.1 Hz, 2H), 9.06 (d, J = 8.1 Hz, 2H), 8.89–8.83 (m, 4H), 8.34 (s, 2H), 8.26–8.04 (m, 8H), 7.91 (d, J = 8.1 Hz, 2H), 7.84–7.80 (m, 3H), 7.74–7.68 (m, 2H), 7.46–7.39 (m, 10H), 7.32 (d, J = 7.8 Hz, 4H), 7.08 (d, J = 9.0 Hz, 4H), 6.98–6.95 (m, 4H), 2.77 (q, J = 6.6 Hz, 4H), 2.63 (t, J = 6.8 Hz, 2H), 2.39 (t, J = 7.2 Hz, 2H), 1.92 (q, J = 7.2 Hz, 2H), 1.83 (q, J = 6.6 Hz, 2H), 1.52 (s, 9H), 1.34 (s, 36H), 1.18 (d, J = 6.6 Hz, 24H), –2.01 (br, 2H); MALDI-MS (POPOP) obsd 2242.7, calc 2241.0 [M + H]+, M = C155H136N6O10; λabs (toluene) 431, 476, 537, 638, 727 nm.
5-[4-(7-Hydroxy-7-oxohept-1-ynyl)phenyl]-10,20-bis[2-(3,4-(N-(2,6-diisopropylphenyl)iminodicarbonyl)-1,6-bis(4-tert-butylphenoxy)perylen-9-yl)ethynyl]-15-phenylporphyrin (13). A solution of the tert-butyl protected triad 12 (2.3 mg, 1.0 μmol) in CH2Cl2 (1.2 mL) was treated with TFA (0.8 mL). The mixture was stirred at room temperature for 1 h, whereupon excess triethylamine was added to quench the reaction. The mixture was then washed with water, dried (Na2SO4), and concentrated to afford a black solid (2.2 mg, 100%): 1H NMR (CDCl3, 300 MHz) δ 9.73–9.66 (m, 4H), 9.24 (d, J = 8.1 Hz, 2H), 9.18 (d, J = 8.4 Hz, 2H), 8.99 (d, J = 8.4 Hz, 2H), 8.83–8.78 (m, 4H), 8.32–8.28 (m, 2H), 8.16–8.07 (m, 8H), 7.96 (d, J = 8.4 Hz, 2H), 7.83–7.76 (m, 3H), 7.66–7.61 (m, 2H), 7.43–7.38 (m, 10H), 7.31 (d, J = 7.5 Hz, 4H), 7.06 (d, J = 9.0 Hz, 4H), 6.99 (d, J = 9.0 Hz, 4H), 2.77 (q, J = 6.6 Hz, 4H), 2.65 (t, J = 6.6 Hz, 2H), 2.41 (t, J = 7.5 Hz, 2H), 1.86 (q, J = 7.2 Hz, 2H), 1.83 (q, J = 6.9 Hz, 2H), 1.33 (s, 36H), 1.17 (d, J = 7.2 Hz, 24H), –1.97 (br, 2H), a signal due to the carboxylic acid was not observed; MALDI-MS (POPOP) obsd 2187.9, calc 2185.0 (M+), M = C151H128N6O10; λabs (toluene) 430, 476, 537, 638, 727 nm, ε537 nm = 67,800 M−1cm−1, ε727 nm = 56,100 M−1cm−1; λem (toluene, λex = 521 nm) 743 nm.
5-[4-(7-(N-succinimidooxy)-7-oxohept-1-ynyl)phenyl]-10,20-bis[2-(3,4-(N-(2,6-diisopropylphenyl)iminodicarbonyl)-1,6-bis(4-tert-butylphenoxy)perylen-9-yl)ethynyl]-15-phenylporphyrin (14). Following a general procedure [74], a solution of carboxy-triad 13 (5.5 mg, 2.5 μmol), N-hydroxysuccinimide (2.8 mg, 25 μmol), and DCC (5.0 mg, 25 μmol) in CH2Cl2 (0.25 mL) was stirred under argon at room temperature for 4 h. Then the mixture was washed with water, dried (Na2SO4), concentrated, and chromatographed [silica, CH2Cl2] to afford a black solid (5.6 mg, 98%): 1H NMR (CDCl3, 300 MHz) δ 9.78 (d, J = 4.5 Hz, 4H), 9.47–9.42 (m, 4H), 9.10 (d, J = 8.4, 2H), 8.89–8.87 (m, 4H), 8.36 (d, J = 4.2 Hz, 4H), 8.31 (d, J = 8.4 Hz, 2H), 8.13 (d, J = 8.1 Hz, 2H), 7.86–7.80 (m, 7H), 7.46–7.43 (m, 10H), 7.31 (d, J = 7.8 Hz, 4H), 7.14–1.09 (m, 8H), 2.87 (s, 4H), 2.84–2.71 (m, 6H), 2.68 (t, J = 6.6 Hz, 2H), 2.17–2.06 (m, 2H), 1.95–1.85 (m, 4H), 1.35 (s, 36H), 1.17 (d, J = 6.6 Hz, 24H), –1.78 (br, 2H); MALDI-MS (POPOP) obsd 2283.5, calc 2283.0 (M+), M = C155H131N7O12.
Zinc(II) 15-p-Tolyl-5-[4-(2-(trimethylsilyl)ethynyl)phenyl]porphyrin (20). Following a reported method [51], a solution of diformyldipyrromethane 18 (438 mg, 1.5 mmol) in THF (5 mL) was treated with propylamine (1.8 g, 30 mmol) at room temperature for 1 h. The solution was concentrated to dryness. The resulting solid was dissolved in ethanol (150 mL) and treated with dipyrromethane 19 (480 mg, 1.5 mmol) and Zn(OAc)2 (2.8 g, 15 mmol). The mixture was refluxed open to the air for 18 h. The reaction mixture was allowed to cool to room temperature, then concentrated to dryness and purified by chromatography [silica, hexanes/CH2Cl2 (4:1 to 1:1)] to afford a purple solid (351 mg, 38%): 1H NMR (600 MHz, CDCl3) δ 10.28 (s, 2H), 9.41 (d, J = 4.4 Hz, 4H), 9.16 (d, J = 4.4 Hz, 2H), 9.08 (d, J = 4.4 Hz, 2H), 8.19 (d, J = 7.7 Hz, 2H), 8.14 (d, J = 7.4 Hz, 2H), 7.91 (d, J = 7.7 Hz, 2H), 7.61 (d, J = 7.4 Hz, 2H), 2.75 (s, 3H), 0.41 (s, 9H); 13C{1H} NMR (151 MHz, CDCl3) δ 150.2, 149.6, 149.4, 149.3, 142.9, 137.1, 134.5, 134.4, 132.6, 132.0, 131.8, 131.6, 130.2, 127.3, 122.2, 120.4, 118.9, 106.2, 21.4; MALDI-MS (α-CHCA) obsd 634.2, calcd 634.2 (M+), M = C38H30N4SiZn; λabs (toluene) 414, 540 nm.
Zinc(II) 5,15-Dibromo-20-p-tolyl-10-[4-(2-(trimethylsilyl)ethynyl)phenyl]porphyrin (21). Following a standard bromination method [58], a solution of zinc porphyrin 20 (127 mg, 0.20 mmol) in CH2Cl2 (70 mL) containing pyridine (1.4 mL) was treated with NBS (85 mg, 0.48 mmol) at 0 °C for 1 h. The reaction mixture was quenched by the addition of acetone (3.0 mL). The mixture was washed with water (80 mL) and brine (80 mL), dried, concentrated to dryness, and purified by chromatography [silica, hexanes/CH2Cl2 (10:1 to 3:1)] to afford a greenish purple solid (119 mg, 75%): 1H NMR (600 MHz, CDCl3) δ 9.68–9.62 (m, 4H), 8.89 (d, J = 4.6 Hz, 2H), 8.82 (d, J = 4.6 Hz, 2H), 8.09 (d, J = 7.9 Hz, 2H), 8.02 (d, J = 7.5 Hz, 2H), 7.86 (d, J = 7.9 Hz, 2H), 7.55 (d, J = 7.4 Hz, 2H), 2.72 (s, 3H), 0.39 (s, 9H); MALDI-MS (α-CHCA) obsd 790.2, calcd 790.0 (M+), M = C38H28Br2N4SiZn; λabs (toluene) 432, 560, 600 nm.
Zinc(II) 10,20-Bis[2-(3,4-(N-(2,6-diisopropylphenyl)iminodicarbonyl)-1,6-bis(4-tert-butylphenoxy)perylen-9-yl)ethynyl]-5-p-tolyl-15-(4-(2-trimethylsilylethynyl)phenyl)porphyrin (22). Following a standard procedure [44,46,56,57], a Schlenk flask containing zinc porphyrin 21 (79 mg, 0.10 mmol), ethynyl-perylene 11 (192 mg, 0.24 mmol), and tri(o-tolyl)phosphine (73 mg, 0.24 mmol) was flushed with argon, treated with degassed toluene/TEA (50 mL, 5:1, v:v), and subjected to three freeze–pump–thaw cycles. The mixture was then treated with Pd2(dba)3 (27 mg, 0.030 µmol) and subjected to two additional freeze–pump–thaw cycles. The resulting mixture was heated to 60 °C for 4 h. The crude mixture was purified following the standard three-chromatography procedure [silica (hexanes/CH2Cl2 (2:1 to 1:1)), SEC (toluene), silica (hexanes to hexanes/CH2Cl2 (1:1))] to afford a purple solid (161 mg, 72%): 1H NMR (600 MHz, CDCl3) δ 9.80–9.69 (m, 4H), 9.11–8.98 (m, 4H), 8.87 (d, J = 4.2 Hz, 2H), 8.81 (d, J = 4.3 Hz, 2H), 8.58 (s, 2H), 8.26 (s, 4H), 8.06–7.99 (m, 2H), 7.99–7.93 (m, 2H), 7.91–7.85 (m, 4H), 7.82 (br, 2H), 7.58 (d, J = 7.2 Hz, 2H), 7.47–7.38 (m, 8H), 7.31 (d, J = 8.1 Hz, 4H), 7.27 (s, 2H), 6.97 (d, J = 8.3 Hz, 4H), 6.49 (s, 4H), 2.82–2.72 (m, 7H), 1.35 (s, 18H), 1.30 (s, 18H), 1.23–1.13 (m, 24H), 0.43 (s, 9H); MALDI-MS (α-CHCA) obsd 2232.7, calcd 2232.9 (M+), M = C150H128N6O8SiZn; λabs (toluene) 440, 488, 515, 549, 706 nm.
Zinc(II) 10,20-Bis[2-(3,4-(N-(2,6-diisopropylphenyl)iminodicarbonyl)-1,6-bis(4-tert-butylphenoxy)perylen-9-yl)ethynyl]-5-p-tolyl-15-(4-ethynylphenyl)porphyrin (23). A solution of triad 22 (197 mg, 88 µmol) in toluene (75 mL) and methanol (75 mL) was treated with K2CO3 (1.21 g, 8.8 mmol) at room temperature for 3 h. The reaction mixture was poured into water (300 mL) and extracted with CH2Cl2 (200 mL × 2). The combined organic extract was washed with brine (200 mL), dried, and concentrated to dryness to afford a dark red solid (178 mg, 93%): 1H NMR (600 MHz, CDCl3) δ 9.81 (d, J = 4.4 Hz, 4H), 9.24 (s, 2H), 9.12 (d, J = 8.0 Hz, 2H), 8.96 (d, J = 4.4 Hz, 2H), 8.88 (d, J = 4.4 Hz, 2H), 8.32 (s, 4H), 8.16 (d, J = 8.0 Hz, 2H), 8.06 (d, J = 7.5 Hz, 2H), 8.03 (s, 2H), 7.99 (d, J = 7.2 Hz, 2H), 7.93 (d, J = 7.6 Hz, 2H), 7.65–7.59 (m, 4H), 7.46–7.42 (m, 6H), 7.34 (d, J = 8.6 Hz, 4H), 7.31 (d, J = 8.0 Hz, 4H), 7.05 (d, J = 8.5 Hz, 4H), 6.73 (s, 4H), 3.37 (s, 1H), 2.80–2.74 (m, 7H), 1.35 (s, 18H), 1.32 (s, 18H), 1.21–1.13 (m, 24H); MALDI-MS (α-CHCA) obsd 2160.9 (M+), calcd 2160.8 (M+), M = C147H120N6O8Zn; λabs (toluene) 440, 488, 515, 548, 706 nm, ε548 nm = 103,000 M−1cm−1.
Bis(2-(trimethylsilyl)ethyl) 5-bromoisophthalate (26). A solution of 5-bromoisophthalic acid (24, 490 mg, 2.0 mmol) and 2-(trimethylsilyl)ethanol (25, 497 mg, 4.2 mmol) in DMF (12 mL) was treated with DCC (866 mg, 4.2 mmol) and DMAP (489 mg, 4.0 mmol) at room temperature for 16 h. The reaction mixture was diluted with ethyl acetate (80 mL) and filtered. The filtrate was washed with water (50 mL) and a saturated NH4Cl aqueous solution (50 mL), dried, concentrated to dryness, and then purified by chromatography [silica, hexanes to hexanes/CH2Cl2 (10:1)] to afford a white amorphous solid (204 mg, 23%): 1H NMR (600 MHz, CDCl3) δ 8.59 (t, J = 1.5 Hz, 1H), 8.33 (d, J = 1.5 Hz, 2H), 4.47–4.42 (m, 4H), 1.18–1.13 (m, 4H), 0.09 (s, 18H); 13C{1H} NMR (151 MHz, CDCl3) δ 166.2, 137.9, 134.3, 130.5, 123.9, 65.6, 18.9, 1.5; ESI-MS (negative-ion mode) obsd 442.92573, calcd 443.0715 [M – H]–, M = C18H29BrO4Si2.
Zinc(II) 10,20-Bis[2-(3,4-(N-(2,6-diisopropylphenyl)iminodicarbonyl)-1,6-bis(4-tert-butylphenoxy)perylen-9-yl)ethynyl]-5-(4-(2-(3,5-bis((2-(trimethylsilyl)ethoxy)carbonyl)phenyl)ethynyl)phenyl)-15-p-tolylporphyrin (27). Following a standard procedure [56,57], a Schlenk flask containing zinc porphyrin triad 23 (51.9 mg, 24 µmol), bromoisophthalate 26 (16.0 mg, 36 µmol), and tri(o-tolyl)phosphine (11.0 mg, 36 µmol) was flushed with argon, treated with degassed toluene/TEA (12 mL, 5:1, v:v) and subjected to three freeze–pump–thaw cycles, and then treated with Pd2(dba)3 (6.6 mg, 7.2 µmol) and subjected to two additional freeze–pump–thaw cycles. The resulting mixture was heated to 60 °C for 5 h. The crude mixture was purified following the standard three-chromatography procedure [silica (hexanes/CH2Cl2 (2:1 to 1:1)), SEC (toluene), silica (hexanes to hexanes/CH2Cl2 (1:1))] to afford a dark red solid (42.0 mg, 69%): 1H NMR (600 MHz, CDCl3) δ 9.85–9.75 (m, 4H), 9.21–9.14 (m, 2H), 9.11 (d, J = 8.1 Hz, 2H), 8.99–8.94 (m, 2H), 8.91–8.87 (m, 2H), 8.67 (s, 2H), 8.51–8.43 (m, 3H), 8.30 (s, 4H), 8.16–8.11 (m, 2H), 8.10–8.06 (m, 2H), 8.03–7.99 (m, 2H), 7.98–7.94 (m, 2H), 7.61 (d, J = 7.4 Hz, 2H), 7.59–7.54 (m, 2H), 7.47–7.40 (m, 6H), 7.35–7.28 (m, 8H), 7.02 (d, J = 8.3 Hz, 4H), 6.69 (s, 4H), 4.53–4.46 (m, 4H), 2.82–2.72 (m, 7H), 1.34 (s, 18H), 1.31 (s, 18H), 1.22–1.12 (m, 28H), 0.14 (s, 18H); MALDI-MS (α-CHCA) obsd 2525.8, calcd 2525.0 (M+), M = C165H148N6O12Si2Zn; λabs (toluene) 441, 486, 516, 550, 713 nm, ε549 nm = 99,800 M−1cm−1; λem (toluene, λexc = 549 nm) 733 nm.
Zinc(II) 10,20-Bis[2-(3,4-(N-(2,6-diisopropylphenyl)iminodicarbonyl)-1,6-bis(4-tert-butylphenoxy)perylen-9-yl)ethynyl]-5-(4-(2-(3,5-dicarboxyphenyl)ethynyl)phenyl)-15-p-tolylporphyrin (28). Following a reported method [76], a solution of 27 (30 mg, 12 µmol) in THF (30 mL) was treated with a tetrabutylammonium fluoride solution (50 µL, 1.0 M in THF) at room temperature for 2 h. The reaction mixture was diluted with CH2Cl2 (100 mL) and then washed with water (100 mL × 2) and brine (100 mL). The organic extract was dried, concentrated to dryness, and purified by chromatography [silica, CH2Cl2 to CH2Cl2/MeOH (5:1)] to afford a purple solid (16 mg, 59%): λabs (toluene) 454, 515, 756 nm, ε454 nm = 44,400 M−1cm−1 (broad spectrum).
10,20-Bis[2-(3,4-(N-(2,6-diisopropylphenyl)iminodicarbonyl)-1,6-bis(4-tert-butylphenoxy)perylen-9-yl)ethynyl]-5-(4-(2-(3,5-dimethoxycarbonylphenyl)ethynyl)phenyl)-15-p-tolylporphyrin (29). A solution of 28 (2.0 mg, 0.86 µmol) and methanol (100 µL) in DMF (900 µL) was treated with DCC (1.0 mg, 4.8 µmol) and DMAP (1 mg, 8 µmol) at room temperature for 16 h. The reaction mixture was diluted with CH2Cl2 (20 mL) and then filtered. The filtrate was washed with water (20 mL) and a saturated NH4Cl aqueous solution (20 mL), dried, concentrated to dryness, and then dissolved in CH2Cl2 (500 µL). The resulting solution was used for MALDI-MS characterization without further purification: MALDI-MS (α-CHCA) obsd 2352.6, calcd 2352.9 (M+), M = C157H128N6O12Zn.
Zinc(II) 5-(4-Ethynyl)phenyl-15-p-tolylporphyrin (30). A solution of porphyrin 20 (63.6 mg, 0.10 mmol) in toluene (30 mL) and methanol (30 mL) was treated with K2CO3 (1.38 g, 10 mmol) at room temperature for 3 h. The reaction mixture was poured into water (120 mL) and extracted with CH2Cl2 (100 mL × 2). The combined organic extract was washed with brine (100 mL), dried, and then concentrated to dryness to afford a purple solid (51.1 mg, 91%): 1H NMR (600 MHz, CDCl3) δ 10.32 (s, 2H), 9.46–9.41 (m, 4H), 9.18 (d, J = 4.4 Hz, 2H), 9.11 (d, J = 4.4 Hz, 2H), 8.23 (d, J = 7.8 Hz, 2H), 8.15 (d, J = 7.4 Hz, 2H), 7.94 (d, J = 7.7 Hz, 2H), 7.61 (d, J = 7.4 Hz, 2H), 3.34 (s, 1H), 2.75 (s, 3H); MALDI-MS (α-CHCA) obsd 562.2, calcd 562.1 (M+), M = C35H22N4Zn; λabs (toluene) 414, 539, 576 nm.
Zinc(II) 5-(4-(2-(3,5-Bis((2-(trimethylsilyl)ethoxy)carbonyl)phenyl)ethynyl)phenyl)-15-p-tolylporphyrin (31). Following a standard procedure [56,57], a Schlenk flask containing zinc porphyrin 30 (37.9 mg, 65 µmol), bromoisophthalate 26 (44.0 mg, 98 µmol), and tri(o-tolyl)phosphine (29.8 mg, 98 µmol) was flushed with argon, treated with degassed toluene/TEA (35 mL, 5:1, v:v), and subjected to three freeze–pump–thaw cycles, and then treated with Pd2(dba)3 (18.3 mg, 20 µmol) and subjected to two additional freeze–pump–thaw cycles. The resulting mixture was heated to 60 °C for 16 h. The crude mixture was concentrated to dryness and then purified by chromatography [silica, hexanes/CH2Cl2 (1:1) to CH2Cl2] to afford a purple solid (22.4 mg, 45%): 1H NMR (600 MHz, CDCl3) δ 10.34 (s, 2H), 9.47 (d, J = 4.4 Hz, 2H), 9.45 (d, J = 4.4 Hz, 2H), 9.19 (d, J = 4.4 Hz, 2H), 9.16 (d, J = 4.4 Hz, 2H), 8.69 (t, J = 1.6 Hz, 1H), 8.52 (d, J = 1.6 Hz, 2H), 8.29 (d, J = 7.9 Hz, 2H), 8.16 (d, J = 7.6 Hz, 2H), 8.00 (d, J = 7.9 Hz, 2H), 7.61 (d, J = 7.6 Hz, 2H), 4.55–4.48 (m, 4H), 2.75 (s, 3H), 1.25–1.20 (m, 4H), 0.14 (s, 18H); MALDI-MS (α-CHCA) obsd 926.6, calcd 926.3 (M+), M = C53H50N4O4Si2Zn; λabs (toluene) 415, 540, 575 nm; λem (λexc = 415 nm) 584, 633 nm.
Zinc(II) 5-(4-(2-(3,5-Dicarboxyphenyl)ethynyl)phenyl)-15-p-tolylporphyrin (32). Following a reported method [76], a solution of porphyrin 31 (18.5 mg, 20 µmol ) in THF (6.0 mL) was treated with a tetrabutylammonium fluoride solution (200 µL, 1.0 M in THF) at room temperature for 2 h. The reaction mixture was diluted with CH2Cl2 (20 mL) and then washed with water (20 mL × 2) and brine (20 mL). The organic extract was dried, concentrated to dryness, and purified by chromatography [silica, CH2Cl2/MeOH (20:1 to 1:1) to afford a purple solid (9.8 mg, 67%): 1H NMR (600 MHz, dimethylsulfoxide-d6) δ 10.38 (s, 2H), 9.54 (d, J = 4.4 Hz, 2H), 9.50 (d, J = 4.3 Hz, 2H), 9.01 (d, J = 4.3 Hz, 2H), 8.97 (d, J = 4.4 Hz, 2H), 8.27 (d, J = 7.7 Hz, 2H), 8.23 (d, J = 7.4 Hz, 1H), 8.15–8.09 (m, 4H), 8.04 (d, J = 7.6 Hz, 2H), 7.66 (d, J = 7.5 Hz, 2H), 2.71 (s, 3H); ESI-MS (negative-ion mode) obsd 725.1157, calcd 725.1173 [M – H]–; ESI-MS obsd 727.1301, calcd 727.1318 [M + H]+, M = C43H26N4O4Zn; λabs (dimethylsulfoxide) 419, 548, 587 nm.

3.4. Fluorescence Yield Determinations

The Φf values were determined in the standard manner by a comparison of integrated spectra (corrected for instrument sensitivity) with a known fluorophore of similar absorption and fluorescence spectral features. For 13, the standard was dyad 1 (Φf = 0.38) [42]. For 27, the standard was triad 4 (Φf = 0.41) [42]. For 31, the standard was meso-tetraphenylporphyrin (Φf = 0.070) [79]. The resulting Φf values for 13, 27, and 31 are 0.21, 0.24, and 0.011, respectively.

4. Outlook

Building block routes have been established for the preparation of triads comprised of two perylene-monoimides, one porphyrin, and a single tether. All of this work has emanated from the unexpected observation a decade ago that a perylene-ethynyl-porphyrin (Dyad 0) exhibits an absorption spectrum essentially lacking a characteristic porphyrin Soret band [41], as shown in Figure 1 [45,46,47,49,50]. The panchromatic absorption provided by the perylene-ethynyl-porphyrin construct differs profoundly from that of the constituent parts. Among all other chromophore-tetrapyrrole constructs subsequently examined [42,45,46,47,48,49,50], including the exploration of the type and number of chromophores, the attachment site on the tetrapyrrole, and the composition of the tetrapyrrole, the linear (i.e., trans) arrangement of perylene-ethynyl-porphyrin-ethynyl-perylene with attachment at the porphyrin meso-positions has proven superior for panchromaticity and photophysical features. The triads described herein provide absorption across the 350–750 nm region and fluorescence in the near-infrared region. Such spectral features closely resemble those of triads prepared previously that lack tethers. The triads described herein are hydrophobic and may be best suited for use in membraneous assemblies and other lipophilic environments. For perspective, a phenazinyl-ethynyl-porphyrin that bears a benzoic acid tether has been prepared [80]. Triads 13 and 27 provide broader spectral coverage but also are substantially larger. The building block chemistry described herein should enable the preparation of a variety of porphyrin constructs with a range of tethers for studies of panchromatic absorbers in diverse applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules27196501/s1, 1H and 13C{1H} NMR spectra, mass spectra, and absorption spectra (where available) for new compounds, comprising 59 pages.

Author Contributions

R.L. and J.R. carried out the synthesis of triads 13 and 27, respectively, and companion compounds. Z.W. carried out analysis. M.T. performed spectral comparisons. Z.W. and J.S.L. wrote the paper. D.F.B., D.H. and J.S.L. designed the compounds. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DE-FG02-05ER15661). Compound characterization was performed in part by the Molecular Education, Technology and Research Innovation Center (METRIC) at NC State University, which is supported by the State of North Carolina.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data are contained within the paper and Supplementary Materials.

Conflicts of Interest

The authors declare no competing financial interest.

Sample Availability

Samples have generally been consumed during the course of research.

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Molecules 27 06501 g001 550
Figure 1. Absorption spectra of a perylene-ethynyl-porphyrin dyad (1, solid red line), to be compared with phenylethynyl-porphyrin (2, solid blue line) and perylene-ethyne (3, solid orange line) constituents (in toluene at room temperature). The fluorescence spectrum of 1 also is shown (dashed red line; λem 695 nm) [42]. The absorption spectrum of the perylene-ethynyl-porphyrin dyad 0 (dotted black trace [41]) is essentially identical with that of analogous dyad 1 (solid red line).
Figure 1. Absorption spectra of a perylene-ethynyl-porphyrin dyad (1, solid red line), to be compared with phenylethynyl-porphyrin (2, solid blue line) and perylene-ethyne (3, solid orange line) constituents (in toluene at room temperature). The fluorescence spectrum of 1 also is shown (dashed red line; λem 695 nm) [42]. The absorption spectrum of the perylene-ethynyl-porphyrin dyad 0 (dotted black trace [41]) is essentially identical with that of analogous dyad 1 (solid red line).
Molecules 27 06501 g001
Molecules 27 06501 ch001 550
Chart 1. Structure of panchromatic triad (4) and the zinc chelate Zn4.
Chart 1. Structure of panchromatic triad (4) and the zinc chelate Zn4.
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Molecules 27 06501 g002 550
Figure 2. Absorption spectra (solid line) and fluorescence spectra (dashed line) of triads in toluene at room temperature. The molar absorption coefficient values for free base porphyrin triad 4 are as follows: 431 nm (73,200 M−1cm−1), 537 nm (104,000 M−1cm−1), 637 nm (77,400 M−1cm−1), and 728 nm (83,200 M−1cm−1) [42,45]. The molar absorption coefficient values for zinc porphyrin triad Zn4 are as follows: 439 nm (64,700 M−1cm−1), 548 nm (102,000 M−1cm−1), and 707 nm (103,000 M−1cm−1) [45]. The Φf value of 4 is 0.26, whereas that of Zn4 is 0.30 [45].
Figure 2. Absorption spectra (solid line) and fluorescence spectra (dashed line) of triads in toluene at room temperature. The molar absorption coefficient values for free base porphyrin triad 4 are as follows: 431 nm (73,200 M−1cm−1), 537 nm (104,000 M−1cm−1), 637 nm (77,400 M−1cm−1), and 728 nm (83,200 M−1cm−1) [42,45]. The molar absorption coefficient values for zinc porphyrin triad Zn4 are as follows: 439 nm (64,700 M−1cm−1), 548 nm (102,000 M−1cm−1), and 707 nm (103,000 M−1cm−1) [45]. The Φf value of 4 is 0.26, whereas that of Zn4 is 0.30 [45].
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Molecules 27 06501 sch001 550
Scheme 1. Synthesis of the bioconjugatable panchromatic absorber.
Scheme 1. Synthesis of the bioconjugatable panchromatic absorber.
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Molecules 27 06501 g003 550
Figure 3. Absorption spectra (solid lines) in toluene at room temperature of ethynyl-perylene 11 (λabs 522 nm, ε522 nm = 40,500 M−1cm−1, solid line) and ethynyl-perylene 11-tBu (λabs 520 nm, dashed line).
Figure 3. Absorption spectra (solid lines) in toluene at room temperature of ethynyl-perylene 11 (λabs 522 nm, ε522 nm = 40,500 M−1cm−1, solid line) and ethynyl-perylene 11-tBu (λabs 520 nm, dashed line).
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Molecules 27 06501 ch002 550
Chart 2. Tetrapyrrole dyes bearing an isophthalate tether.
Chart 2. Tetrapyrrole dyes bearing an isophthalate tether.
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Molecules 27 06501 sch002 550
Scheme 2. Synthesis of a perylene-porphyrin-perylene triad (28) for surface attachment.
Scheme 2. Synthesis of a perylene-porphyrin-perylene triad (28) for surface attachment.
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Molecules 27 06501 g004 550
Figure 4. Analytical SEC traces (λdet = 550 nm) for the synthesis of a triad array 22. (a): Mixture of starting materials 11 and 21. (b): Crude mixture after reaction for 4 h. (c): Purified triad 22.
Figure 4. Analytical SEC traces (λdet = 550 nm) for the synthesis of a triad array 22. (a): Mixture of starting materials 11 and 21. (b): Crude mixture after reaction for 4 h. (c): Purified triad 22.
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Molecules 27 06501 sch003 550
Scheme 3. Synthesis of tethered porphyrin 32 lacking perylenes.
Scheme 3. Synthesis of tethered porphyrin 32 lacking perylenes.
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Molecules 27 06501 g005 550
Figure 5. Absorption spectra (solid lines) and emission spectra (dashed lines) in toluene at room temperature (normalized displays). (a): free base porphyrin carboxy-triad 13 shows λabs 537 nm, ε537 nm = 67,800 M−1cm−1; fluorescence λem 743 nm upon λex = 521 nm). (b): zinc porphyrin isophthalate-triad 27 shows λabs 548 nm, ε548 nm = 99,800 M−1cm−1; fluorescence λem 733 nm upon λex = 549 nm). (c): perylene 33 (fluorescence λem 566 nm upon λex ~500 nm). (d): trans-AB zinc porphyrin 31 (fluorescence λem 633 nm upon λex = 415 nm).
Figure 5. Absorption spectra (solid lines) and emission spectra (dashed lines) in toluene at room temperature (normalized displays). (a): free base porphyrin carboxy-triad 13 shows λabs 537 nm, ε537 nm = 67,800 M−1cm−1; fluorescence λem 743 nm upon λex = 521 nm). (b): zinc porphyrin isophthalate-triad 27 shows λabs 548 nm, ε548 nm = 99,800 M−1cm−1; fluorescence λem 733 nm upon λex = 549 nm). (c): perylene 33 (fluorescence λem 566 nm upon λex ~500 nm). (d): trans-AB zinc porphyrin 31 (fluorescence λem 633 nm upon λex = 415 nm).
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Molecules 27 06501 ch003 550
Chart 3. Benchmark perylene.
Chart 3. Benchmark perylene.
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Liu, R.; Rong, J.; Wu, Z.; Taniguchi, M.; Bocian, D.F.; Holten, D.; Lindsey, J.S. Panchromatic Absorbers Tethered for Bioconjugation or Surface Attachment. Molecules 2022, 27, 6501. https://doi.org/10.3390/molecules27196501

AMA Style

Liu R, Rong J, Wu Z, Taniguchi M, Bocian DF, Holten D, Lindsey JS. Panchromatic Absorbers Tethered for Bioconjugation or Surface Attachment. Molecules. 2022; 27(19):6501. https://doi.org/10.3390/molecules27196501

Chicago/Turabian Style

Liu, Rui, Jie Rong, Zhiyuan Wu, Masahiko Taniguchi, David F. Bocian, Dewey Holten, and Jonathan S. Lindsey. 2022. "Panchromatic Absorbers Tethered for Bioconjugation or Surface Attachment" Molecules 27, no. 19: 6501. https://doi.org/10.3390/molecules27196501

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