Structural characteristics that make chlorophylls green: interplay of hydrocarbon skeleton and substituents

Abstract

Understanding the effects of substituents on natural photosynthetic pigments is essential for gaining a deep understanding of why such pigments were selected over the course of evolution for use in photosynthetic systems. This knowledge should provide for a more thoughtful design of artificial light-harvesting systems. The hydrocarbon skeleton of all chlorophylls is phorbine, which contains an annulated five-membered (isocyclic) ring in addition to the reduced pyrrole ring characteristic of chlorins. A phorbine and a 13¹-oxophorbine (which bears an oxo group in the isocyclic ring) were synthesized as benchmark molecules for fundamental spectral and photophysical studies. The phorbine and 13¹-oxophorbine macrocycles lack peripheral substituents other than a geminal dimethyl group in the reduced ring to stabilize the chlorin chromophore. The spectral properties and electronic structure of the zinc or free base 13¹-oxophorbine closely resemble those of the corresponding analogues of chlorophyll a. Accordingly, the fundamental electronic properties of chlorophylls are primarily a consequence of the 13¹-oxophorbine base macrocycle.

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Introduction

Chlorophylls are the most famous members of the chlorin family.¹ The chlorin designation stems from the presence of one reduced pyrrole ring in the tetrapyrrole macrocycle. However, the carbon skeleton of chlorophylls is not simply a cyclic tetrapyrrole, but also contains an annulated five-membered ring. This hydrocarbon framework is known as phorbine.² A phorbine that carries an oxo group at the 13¹-position, as is the case with all chlorophylls, is known as a 13¹-oxophorbine. The structures of chlorin, phorbine, 13¹-oxophorbine, chlorophyll a and chlorophyllb are shown in Chart 1. Inspection of the chlorophyll structure leads naturally to questions concerning the requirements for the fifth ring, the given set of peripheral substituents, and the extent to which such features contribute to the distinct spectral and electronic properties of the chlorophylls.

Chart 1 Building the skeleton of chlorophyll a.

The question of ‘why chlorophyll?’ has been asked a number of times from different perspectives. Mauzerall considered the features of chlorophyll with regards to the biosynthetic pathway, the electronic properties that give rise to spectral and photochemical properties, and how such properties are well matched for photosynthetic function.³ A generation later the same question was addressed from the vantage point of a deeper molecular understanding of the diverse roles of chlorophylls in photosynthetic energy- and electron-transfer processes, including the apparently unique role of chlorophyll a in oxygenic photosynthesis.⁴ Björn considered the spectral matching of the chlorophyll absorption spectrum with solar radiation⁵ and also the prospects for finding habitable exoplanets on the basis of chlorophyll spectral properties.⁶ The spectroscopic properties of chlorophylls have been studied in detail.⁷ Still, the role of the specific substituents about the perimeter of the tetrapyrrole macrocycle in engendering the characteristic features of chlorophylls has largely been unaddressed, at least in a systematic manner.

The presence of specific substituents at sites around the perimeter of the chlorophyll macrocycle can have a significant effect on the spectral, electronic, and photochemical properties. For example, the sole distinction between chlorophyll a and chlorophyllb is the presence of a 7-methylversus 7-formyl group, yet this structural change results in a significant difference in spectral properties. This difference is exploited in nature, where chlorophyllsa and b serve together to broaden spectral coverage in plant photosynthesis.⁸

The traditional route to probe the effects of substituents in chlorophylls has been to modify intact chlorophylls.^9–22 Such semisynthesis methods have resulted in structural changes such as hydrogenation of the 3-vinyl group, reduction of the 13-keto group, and insertion of other metals. However, achieving more global changes such as removing all alkyl groups (2, 7, 8, 12 positions) has not been feasible via semisynthesis. An alternative entails total synthesis, which has been used to prepare analogues of the naturally occurring chlorophylls. Our own work has been focused on the development of de novo synthetic methods for the preparation of stable synthetic analogues of the naturally occurring chlorophylls.^23–43

Herein we report the use of recently developed synthetic methods to gain access to a series of benchmark analogues of chlorophylls that lack any substituents other than a stabilizing geminal dimethyl group in the reduced ring. The compounds include phorbine FbP, chlorin FbC–A¹³, and 13¹-oxophorbine FbOP (Chart 2), as well as their zinc chelates (ZnP, ZnC–A¹³, and ZnOP). The analogous chlorins FbC and ZnC have been synthesized previously,³¹ as has the 17-oxochlorin FbOC.³² The spectral and photophysical properties of each compound are also presented and are accompanied by density functional theory (DFT) calculations aimed at characterizing the frontier molecular orbitals. Collectively, these studies systematically address the issue of how structural modifications to the base chlorin macrocycle contribute to the electronic and spectral characteristics of chlorophylls.

Chart 2 Benchmark molecules for chlorophylls.

Results and discussion

I Synthesis

A Reconnaissance. A method for installing the isocyclic ring was first reported nearly three-quarters of a century ago, when Fischer and Laubereau subjected (hydroxymethylcarbonyl)porphyrin to dehydrating conditions to give “pheoporphyrin” I (Chart 3).⁴⁴ Fischer and Oestreicher also used Dieckmann condensation to convert chlorine₆ trimethyl ester to methyl pheophorbide aII.⁴⁵ Other methods for installing the isocyclic ring have been described.³⁰ To our knowledge, the first mention of the formation of a phorbine (lacking the keto group) appears in Fischer's work, where deoxy-pyropheophorbide aIII was reported (but not characterized) as a side product during esterification of pheophorbidea.⁴⁶ Some 30 years later, Wolf reported the spectral properties of phorbine III.⁴⁷ The first report of the synthesis and characterization of a number of phorbines (IV) was provided by Brockmann and Tacke-Karimdadian, who reduced bacteriopheophorbidedmethyl ester with LiAlH₄ in the presence of zinc chloride to deoxygenate the 13-keto group.⁴⁸ Lash et al. reported the condensation of one dipyrromethane bearing an annulated ring with a second dipyrromethane to form a free base porphyrin containing the five-membered isocyclic ring (V, M = H,H).^49–51 Callot's group, drawing on the work of Kenner and Smith and coworkers,⁵² reported the acid-catalyzed cyclization of a β-pyrrolic vinyl group to give the analogous nickel porphyrin (V, M = Ni).⁵³ However, in each of the studies by Lash and by Callot, the resulting macrocycle was a porphyrin, not a chlorin; in other words the skeleton was a 17,18-dehydrophorbine.

Chart 3 Historical precedents in 13¹-oxophorbine and phorbine chemistry.

Each of the aforementioned syntheses of phorbines, dehydrophorbines, and 13¹-oxophorbines affords a macrocycle that contains a full complement of substituents at the β-pyrrole positions. The de novo synthesis of chlorins that we developed joins an Eastern half (rings B and C) and a Western half (rings A and D), where the latter contains a geminal methyl group in the pyrroline ring to stabilize the resulting chlorin macrocycle toward adventitious dehydrogenation (Scheme 1).^23–43 The de novo route requires far more synthetic effort than semisynthetic methods that begin with chlorophylls,^9–22 yet affords greater versatility in the scope and range of substituents that can be introduced. The available substituent pattern includes a benchmark chlorin (FbC) that differs from chlorin itself only in the presence of the geminal dimethyl group in the reduced ring.³¹ Methodology also has been developed to convert a synthetic 13-acetylchlorin to the corresponding 13¹-oxophorbine (via 15-bromination and intramolecular α-arylation).³⁰ In this manner, seven free base 13¹-oxophorbines have been prepared, each of which bears a mesityl group at the 10-position and one or two other substituents. Four such 13¹-oxophorbines incorporate various substituents at the 7-position.^30,41 Nevertheless, a synthesis of the phorbine and 13¹-oxophorbine lacking any meso or β-pyrrole substituents has heretofore not been accomplished. Access to such macrocycles is fundamentally important because these molecules serve as benchmarks against which to compare the spectral and photophysical properties of the full library of synthetic chlorins, as well as the naturally occurring chlorophylls.

Scheme 1 De novo synthesis of chlorins (top). All prior known synthetic 13¹-oxophorbines incorporate a 10-mesityl group and one or two other substituents (bottom).

B Unsubstituted 13¹-oxophorbine. The de novo method was employed for the synthesis of the unsubstituted 13¹-oxophorbine FbOP. The condensation of 8,9-dibromo-1-formyldipyrromethane 1⁴² (Eastern half) and 2²⁸ (Western half) was carried out in CH₂Cl₂ upon treatment with a solution of TsOH·H₂O in anhydrous MeOH under argon. The resulting crude tetrahydrobilene-a was subjected to zinc-mediated oxidative cyclization in CH₃CN for 20 h at reflux exposed to air.^31,38Filtration through a silica pad and column chromatography afforded chlorin ZnC–Br¹³ in 16% yield, which upon demetalation with TFA in CH₂Cl₂ gave FbC–Br¹³ in 87% yield (Scheme 2).

Scheme 2 Synthesis of the 13¹-oxophorbines FbOP and ZnOP.

Stille coupling ⁵⁴ of FbC–Br¹³ with tributyl(1-ethoxyvinyl)tin and [(PPh₃)₂Pd]Cl₂ in CH₃CN/DMF (3 ∶ 2)⁴² for 2.5 h at 85 °C followed by acidic workup afforded 13-acetylchlorin FbC–A¹³ in 81% yield. Regioselective 15-bromination of FbC–A¹³ was achieved with 1 molar equivalent of NBS under acidic conditions⁴¹ for 1 h at room temperature to give FbC–A¹³Br¹⁵ in 73% yield. Acidic conditions cause deactivation of ring B of the macrocycle and thus prevent bromination at the 7-position. Intramolecular α-arylation^30,55,56 of the 13-acetyl group in the presence of (PPh₃)₂PdCl₂ and Cs₂CO₃ in toluene installed the isocyclic ring (spanning positions 13 and 15) in 55% yield. Metalation of FbC–A¹³ and FbOP with Zn(OAc)₂·2H₂O gave ZnC–A¹³ and ZnOP in 70% and 79% yield, respectively. The zinc oxophorbine ZnOP was successfully purified by precipitation; however, other attempts to purify by column chromatography using 1% TEA resulted in complete decomposition.

C Conversion to phorbine. The 13-keto group of naturally occurring chlorophylls has been reduced (predominantly in acidic media^{46,48,57–69}) to the methylene group. However, the absence of peripheral substituents on FbOP makes this macrocycle somewhat susceptible to the vigorous conditions generally employed for reduction of the 13-keto group. Hence, we turned to the use of milder conditions via the reduction of an intermediate tosylhydrazone.^70,71 Treatment of oxophorbine FbOP with tosylhydrazide in CHCl₃/ethanol (1 ∶ 1) at 65 °C afforded crude tosylhydrazone–FbP, which upon reduction with sodium cyanoborohydride in the presence of TsOH·H₂O in MeOH/THF gave tosylhydrazide–FbP. The latter was converted to the target phorbine FbP in 42% overall yield in EtOH/DMF for 5 h at 80 °C in the presence of sodium acetate trihydrate. Metalation of phorbine FbP with Zn(OAc)₂·2H₂O gave the zinc chelate phorbine ZnP in 90% yield (Scheme 3).

Scheme 3 Synthesis of the free base phorbine FbP and the zinc phorbine ZnP.

II Characterization

A Molecular identification. All of the phorbines and analogues were characterized by ¹H NMR spectroscopy (including gCOSY and NOESY), ¹³C NMR spectroscopy, laser desorption mass spectrometry [without a matrix or using a matrix of 1,4-bis(5-phenyloxazol-2-yl)benzene],⁷² high-resolution electrospray ionization mass spectrometry, and absorption and fluorescence spectroscopy.^73,74 Key electronic, spectral, and excited-state characteristics of the compounds are described in the following sections.

B Absorption spectra . The absorption spectra of the synthetic phorbine and 13¹-oxophorbine compounds are shown in Fig. 1 in the free base form (top panel) and zinc chelate form (bottom panel). The spectra for ZnC, FbC, ZnOC and FbOC match those reported previously.^32,33 For comparison purposes, the spectral properties of sparsely substituted chlorins lacking any β-pyrrole substituents (FbC and ZnC) or only the 13-acetyl substituent (FbC–A¹³ and ZnC–A¹³) also are provided. For exact comparison with species containing the full complement of substituents characteristic of chlorophylls, also displayed are the free base⁷⁵ and zinc⁷⁶ analogues of chlorophyll a [pheophytina (Pheo aa) and zinc pheophytin a (Zn–Pheo a); Chart 4].

Chart 4 Analogues of chlorophyll a.

Fig. 1 Absorption spectra of synthetic chlorins and phorbinesversuspheophytins^75,76 (normalized at the B-band maxima). Top panel: free base macrocycles. Bottom panel: zinc chelates. The solvent is diethyl ether (Pheo aa and Zn–Pheo a) or toluene (all other compounds).

The absorption spectrum of each compound is comprised of three main spectral regions. The strong red-region feature (600–670 nm) is the Q_y(0,0) band; on average, this band lies ∼30 nm to longer wavelength for the free base compound versus the corresponding zinc chelate. The weaker features in the blue-green to yellow spectral region (470–580 nm) are the Q_x bands; these features are somewhat stronger for the free base compounds than the zinc chelates. The intense bands in the near-ultraviolet region are the B_x and B_y features, also known as the Soret bands; these bands are generally superimposed for the zinc chelates but split for the free base compounds. The positions and relative intensities of the B and Q_y bands for the various macrocycles are listed in Table 1.

Table 1 Photophysical properties of oxophorbines and analogues^a

Compound	λ B [fwhm]/nm	Δ B ^b/cm^−1	λ Qy [fwhm]/nm	Δ Qy ^b/cm^−1	I B/IQy^c	Σ Qy /ΣB^d	HOMO–LUMO^e/eV	Φ f ^f	τ s ^g/ns
a In toluene at room temperature unless noted otherwise. b The redshift of the band relative to the corresponding band of the parent chlorin (FbC or ZnC). c Ratio of the peak intensities of the B and Qy bands. d Ratio of the integrated intensities of the Qy manifold [Qy(0,0) and Qy(1,0) bands] and B manifold [Bx and By origins and first vibronic overtones]. e Energy gap between the HOMO and LUMO orbitals. f Fluorescence quantum yield (error ± 7%). g Lifetime of the singlet excited state measured using fluorescence techniques (error ± 8%). h Absorption data from ref. 75 (in diethyl ether). i Absorption data from ref. 76 (in diethyl ether). j Essentially the same value (2.65 eV) is obtained with an H2O axial ligand. k From ref. 34.
FbC	389 [33]	—	633 [10]	—	2.4	0.19	2.69	0.20	8.8
FbP	404 [30]	−950	629 [9]	100	3.6	0.15	2.72	0.28	10.6
FbOP	408 [55]	−1170	654 [11]	−490	1.5	0.24	2.58	0.30	11.5
FbC –A ^13	405 [34]	−970	654 [12]	−500	1.8	0.24	2.59	0.24	7.8
FbOC	400 [27]	−710	634 [9]	−10	4.6	0.09	2.76	0.12	8.4
Pheo aa ^h	409 [53]	−1200	668 [17]	−810	2.0	0.25	2.47
ZnC	398 [13]	—	602 [11]	—	3.6	0.24	2.67	0.062	1.7
ZnP	402 [2]	−240	601 [12]	11	4.6	0.27	2.69	0.064	1.7
ZnOP	419 [19]	−1230	637 [17]	−910	1.6	0.42	2.55	0.23	5.1
ZnC – A ^13	411 [21]	−760	627 [14]	−680	1.9	0.40	2.56	0.23	4.3
ZnOC	412 [14]	−810	602 [9]	−10	3.4	0.23	2.68	0.030	0.82
Zn –Pheo a ^i	423 [38]	−1480	653 [17]	−1300	1.4	0.40	2.43
MgC	402 [10]	—	607 [12]	—	4.3	0.28	2.63^j	0.26	6.9
Chl aa	432 [40]	−1730	665 [18]	−1440	1.3	0.63	2.40	0.33	6.3^k

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The characteristics of the main absorption features (B_x, B_y, Q_x, Q_y) depend on the nature of the macrocycle and peripheral substituents. Pheo aa bears two major auxochromes, the 13-keto group (embedded in the isocyclic ring) and the 3-vinyl group. The 13¹-oxophorbines FbOP and ZnOP lack the 3-vinyl group; the phorbines FbP and ZnP additionally lack the 13-keto group; the chlorin FbC–A¹³13 further lacks the five-membered ring; the base chlorins FbC and ZnC lack any substituents (except for the geminal dimethyl group in the reduced pyrrole ring common to all of the synthetic compounds examined here). Of major interest is the manner in which the nature of the macrocycle influences the position and intensity of the Q_y(0,0) band. This is so because this feature corresponds to excitation to the lowest singlet excited state, from which key energy- and electron-transfer events of photosynthesis are initiated. The salient points concerning the Q_y(0,0) band are as follows:

(1) In the free base series, FbOP has the most intense Q_y(0,0) band (relative to the Soret maximum). The Q_y(0,0) band of FbC–A¹³ exhibits the same (largest) bathochromic shift as FbOP, but is about 80% as intense. Both compounds nearly mimic the spectrum of Pheo aa. In the zinc-chelate series, ZnOP has the relatively most intense Q_y(0,0) band and the largest bathochromic shift, with ZnC–A¹³ second in the series in both categories.

(2) The spectral properties of phorbines (FbP and ZnP) closely resemble those of chlorins (FbC or ZnC). Both sets of compounds exhibit a Q_y(0,0) band that lacks the intensity and bathochromic shift of the keto-substituted analogues.

(3) The main absorption characteristics of unsubstituted chlorin MgC³³ closely resemble those of ZnC (Table 1). This similarity parallels that for the native magnesium-containing chlorophyll a (Chl aa) and the zinc-bearing analogue Zn–Pheo a (Table 1).

In summary, the chlorin chromophore alone affords a poor mimic of the spectral properties of chlorophylls. However, the addition of a 13-keto group to the chlorin is essential (and suffices) to closely mimic the absorption spectrum of the naturally occurring chlorin pigment; it is anticipated that the added effect of a 3-vinyl group would give a near complete match to the spectrum of chlorophyll a.

C Fluorescence spectra and quantum yields and excited-state lifetimes. For each compound studied, the fluorescence spectrum (not shown) contains a prominent Q_y(0,0) band and a much weaker Q_y(0,1) band, which together are in approximate mirror symmetry to the Q_y(0,0) and Q_y(1,0) absorption features. The Q_y(0,0) emission band has approximately the same spectral width as, and lies no more than 2 nm to longer wavelength of, the corresponding Q_y(0,0) absorption feature. This very small (<60 cm⁻¹) absorption–fluorescence spacing (Stokes shift) indicates little change in the structure or solvent interactions of these macrocycles upon photoexcitation. The fluorescence quantum yields of FbC, FbP, FbOP, and FbC–A¹³ are in the range 0.20–0.30, while the value for FbOC is somewhat smaller (0.12). For ZnC, ZnP, and ZnOC, the fluorescence yields are each about one-quarter of those for the corresponding free base form, while the values for ZnOP (0.23) and ZnC–A¹³ (0.23) are closer to those for FbOP and FbC–A¹³ (Table 1). The lifetime of the lowest singlet excited state (determined viafluorescence detection) is in the range 7.8 to 11.5 ns for all free base compounds. The values are considerably shorter for ZnC (1.7 ns), ZnP (1.7 ns) and ZnOC (0.82 ns), with the oxochlorin being the shortest. We have previously found such short excited-state lifetimes for peripherally substituted zinc oxochlorins,²⁶ and the current work shows that this effect is inherent to the parent macrocycle and does not stem from substituent effects. The excited-state lifetimes for ZnOP (5.1 ns) and ZnC–A¹³ (4.3 ns) are much longer than for the other zinc chelates and are about half of those for the free base forms, FbOP (11.5 ns) and FbC–A¹³ (7.8 ns).

The fluorescence yield of the magnesium chlorin MgC is larger than those for the free base form FbC and zinc chelate ZnC (0.26, 0.20, 0.062) while the singlet excited-state lifetime is intermediate (6.9 ns, 8.8 ns, 1.7 ns). This trend follows those for the fluorescence yields (0.14, 0.09, 0.030) and excited-state lifetimes (13 ns, 8.9 ns, 2.1 ns) of magnesium, free base, and zinc tetraphenylporphyrins.^77–79

The collective photophysical data show that the addition of a 13-keto group to the chlorin results in a more intense and bathochromically shifted Q_y(0,0) absorption feature, accompanied by fluorescence yields and singlet excited-state lifetimes for the zinc chelates that are not nearly as diminished compared to the free base analogues as for the other macrocycles. The decreased excited-state energy and increased optical-transition strength affords better utilization of the red region of the solar spectrum. The 13-keto group also increases the excited-state lifetime, which is desirable for allowing a higher yield of the light-driven energy- or charge-transfer reactions of photosynthesis. The lengthening of the excited-state lifetime is unexpected in that the lower excited-state energy would normally (based on the energy-gap law⁸⁰) lead to a shortening of the lifetime due to enhancement of the nonradiative (internal conversion) decay pathway. Thus, the incorporation of a 13-keto group in a chlorin or phorbine macrocycle results in a very favorable motif from a photophysical point of view.

D Structural studies. X-Ray crystal structure analysis of the benchmark macrocycles porphine, FbC, FbC–Br¹³, FbP, FbOP and FbOC allowed analysis of structural changes of the core macrocycle lacking any β- and meso-pyrrolic substituents along the series porphyrin, chlorin, phorbine, 13¹-oxophorbine and 17-oxochlorin.^33,81 Such studies were accompanied by resonance Raman spectroscopy and DFT calculations.⁸¹ Moving from chlorin to phorbine (upon installation of the five-membered isocyclic ring) and, finally, from phorbine to oxophorbine (upon introduction of the 13-oxo group) causes numerous alterations in the core macrocycle size, shape and framework. The macrocycle changes from kite-shaped (FbC) to more trapezoid-like (FbP and FbOP), and the size decreases along the series FbC > FbP ≈ FbOP. Surprisingly, alterations (e.g., shortening of the C13–C13¹ bond length) caused by introduction of the keto group are very small indicating that the unsymmetrical nature of the isocyclic ring (ring E) is probably due to annulation with the macrocycle rather than conjugation with the 13-keto group.

E Frontier molecular orbitals and electronic properties. The energies and electron-density distributions of the frontier molecular orbitals (MOs) of all the newly synthesized phorbines and chlorins were obtained from DFT calculations. Such methods were also applied to several fictive chlorins, including the 13,15-dimethylchlorin FbC–Me^13,1513,15 and the 13-acetyl-15-methylchlorin FbC–A¹³Me¹⁵, as well as the corresponding zinc chelates ZnC–Me^13,15 and ZnC–A¹³Me¹⁵ (Chart 5). Examination of such fictive chlorins provides deeper insight into the origin of the effects caused by the isocyclic ring and the 13-keto group. The key results of the DFT calculations for both the synthetic and fictive free base compounds are summarized in Fig. 2, and the analogous results for the zinc chelates in Fig. 3. These figures show the characteristics of the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), the HOMO-1, and LUMO+1. Below, we first discuss the relationship between the MO energies and the structure of the macrocycle, with connections to the anticipated redox properties of the molecules. We then discuss the relationship between the MO energies and position and intensity of the lowest-energy absorption feature of the molecules.

Chart 5 Fictive chlorins for which DFT calculations were performed.

Fig. 2 Electron-density distributions and energies (eV) of the frontier MOs of the free base compounds.

Fig. 3 Electron-density distributions and energies (eV) of the frontier MOs of the zinc chelates.

Macrocycle structure and MO energies. The key findings from the MO calculations are as follows:

(1) The conversion of the chlorin to a phorbine results in a small (0.08–0.14 eV) shift in the HOMO and LUMO energies to less negative values. Such shifts correspond to the phorbine being slightly easier to oxidize and slightly harder to reduce than the chlorin. This connection follows from our prior studies on about two dozen synthetic zinc chlorins that showed linear correlations between the calculated MO energies and measured redox potentials.^34,35 Moreover, the calculations reported herein on the fictive chlorins FbC–Me^13,1513,15 and ZnC–Me^13,15 suggest that about half (free base) or almost the entire (zinc chelate) MO-energy shifts can be accounted for simply by the presence of the 13- and 15-substituents and that closure to the five-membered ring (and associated ring strain) has only a small effect.

(2) Incorporation of a 13-keto group in a chlorin to produce FbC–A¹³ or ZnC–A¹³ gives a 0.25 eV shift in the HOMO energy and a 0.35–0.36 eV shift in the LUMO energy to more negative values. The analogous incorporation of an oxo group in a phorbine to produce the 13¹-oxophorbine (e.g., FbP to FbOP) gives an even larger 0.41–0.42 eV shift in the HOMO energy and a larger still 0.55–0.56 eV shift in the LUMO energy to more negative values. The direction of the shifts is such that the presence of the 13-keto group will make the molecule harder to oxidize and easier to reduce. The calculations on the fictive chlorin ZnC–A¹³M¹⁵, together with those described above for ZnC–M^13,15, suggest that closure to the five-membered ring (and associated ring strain) has a somewhat greater effect on the frontier-MO energies when the 13-keto group is present (e.g., FbP to FbOP) than in its absence (e.g., FbC to FbC–A¹³).

(3) The effect of the 13-keto group on the LUMO energy is larger than the effect on the HOMO energy by an average of 0.11 eV for the four cases (13-keto addition to chlorin or 13¹-oxo addition to phorbine and for free base and zinc forms). This effect can be understood by the generally larger electron density on the 13-keto group for the LUMO compared to the HOMO (Fig. 2 and 3). The unequal effect of the 13-keto group on the LUMO and HOMO alters the HOMO − LUMO energy gap and thus affects the energy/wavelength of the Q_y(0,0) absorption band, as described further below.

(4) The 13-keto substituted chlorins FbC–A¹³13 and ZnC–A¹³ have HOMO energies that are only slightly (0.02–0.03 eV) less negative and LUMO energies that are modestly (0.14–0.16 eV) less negative than those of the corresponding natural pigments Pheo aa and Zn–Pheo a. Thus, the simple presence of a 13-keto group, without closure to the five-membered ring to give the oxophorbine, is sufficient to give a macrocycle which is slightly easier to oxidize and only modestly harder to reduce than the related photosynthetic pigment (i.e., Pheo aa). Carrying the comparison one step further, the unsubstituted oxophorbines FbOP and ZnOP have HOMO energies that are actually slightly (0.04–0.05 eV) more negative and LUMO energies that are slightly (0.07 eV) less negative than those of Pheo aa and Zn–Pheo a. Thus, the oxophorbine macrocycle encodes the primary electronic properties that dictate the redox potentials of the native photosynthetic pigments, with only small net adjustments due to the substituents at the six β-pyrrole positions.

Absorption spectra and MO energies. Fig. 4 (top) plots the energies of the HOMO, LUMO, HOMO-1, and LUMO+1 versusQ_y(0,0) absorption energy/wavelength (Table 1) for the various macrocycles, along with linear fits to the data and values indicating the slopes of the trend lines. For both the free base macrocycles (open symbols and dashed lines) and zinc chelates (closed symbols and solid lines), the LUMO exhibits a much greater dependence on macrocycle characteristics than the HOMO or other orbitals (HOMO-1 and LUMO+1). In particular, the slope of the trend lines encompassing the addition of a 13-keto group to a chlorin (e.g.FbC to FbC–A¹³) or a 13¹-oxo group to a phorbine (e.g.FbP to FbOP) is twice as large for the LUMO than for the other three frontier MOs.

Fig. 4 Calculated frontier MO energies (top panel) and energy gaps (bottom panel) versus the measured Q_y(0,0) absorption-band energy (bottom axis) and wavelength (top axis) for free base macrocycles (open symbols) and zinc chelates (closed symbols). Top: energies LUMO+1 (diamonds), LUMO (circles), HOMO (squares), HOMO-1 (stars). Bottom: energy gaps HOMO-1 − LUMO+1 (up triangles), and HOMO − LUMO (down triangles). The indicated values are the slopes of the trend lines for free base macrocycles (dashed lines) and zinc chelates (solid lines). The points for the different macrocycles are labeled as follows: Pheo aa (1), FbC–A¹³ (2), FbOP (3), FbC (4), FbOC (5), and FbP (6), and similarly for the zinc chelates.

Close inspection of the orbital energies listed in Fig. 2 and 3 shows that the trends depicted in Fig. 4 (top) reflect a correlation between a decrease in the Q_y(0,0) absorption-band energy (an increase in wavelength) and a decrease in the HOMO − LUMO energy gap upon the presence of a 13-keto group (in either a chlorin or phorbine). This tracking of the Q_y(0,0) energy/wavelength with the HOMO − LUMO energy gap is shown explicitly in Fig. 4 (bottom) for the free base macrocycles (open downward triangles and dashed line) and zinc chelates (closed downward triangles and solid line). The bottom panel of Fig. 4 also shows that, in contrast, the energy gap between the LUMO+1 and HOMO-1 increases slightly as the Q_y(0,0) energy decreases (wavelength increases), and with a trend-line slope that is about one-third that of the HOMO −LUMO energy gap.

The collective results depicted in Fig. 4 show that (1) the bathochromic shift in the position of the Q_y(0,0) absorption band is driven by a reduction in the HOMO − LUMO energy gap. (2) The reduction in the HOMO − LUMO energy gap is in turn driven by a stronger effect of the 13-keto substituent on the LUMO energy than the HOMO energy. (3) These effects can be understood by the greater electron density on the 13-keto group for the LUMO compared to the other frontier MOs.

The results above can be taken one step further by application of Gouterman's four-orbital model.^82–84 Within this model, the energy/wavelength of the Q_y(0,0) absorption band depends on the quantity ΔE_avg defined in eqn (1), namely the average of the HOMO − LUMO and LUMO+1 − HOMO-1 energy gaps, whereas the intensity of the band depends on the quantity ΔE_dif defined in eqn (2), namely the difference in these two orbital energy gaps:

ΔE_avg = [(HOMO-1 − LUMO+1) + (HOMO − LUMO)]/2 (1)

ΔE_dif = [(HOMO-1 − LUMO+1) − (HOMO − LUMO)] (2)

Fig. 5 (top) shows a good overall correlation between ΔE_avg and the Q_y(0,0) absorption energy/wavelength for both the free base macrocycles and zinc chelates. Fig. 5 (bottom) shows a good correlation between ΔE_dif and the integrated intensity of the Q_y(0,0) absorption band (Table 1), especially for the free base compounds (open symbols and dashed line). We have previously utilized the four-orbital model to understand how the macrocycle-substituent pattern controls the MO energies and the spectral characteristics of a large set of synthetic free base and zinc chlorins.^34,35

Fig. 5 (top) Calculated average of the HOMO-1 − LUMO+1 and HOMO − LUMO energy gaps versus the measured Q_y(0,0) absorption-band energy (bottom axis) and wavelength (top axis) for free base macrocycles (open symbols and dashed trend line) and zinc chelates (closed symbols and solid trend line). (bottom) Calculated difference in the HOMO-1 − LUMO+1 and HOMO − LUMO energy gaps versus the ratio of the integrated intensities of the Q_y and B absorption manifolds for the free base macrocycles (open symbols and dashed trend line) and zinc chelates (closed symbols and solid trend line). The points for the different macrocycles are labeled as follows: Pheo aa (1), FbC–A¹³ (2), FbOP (3), FbC (4), FbOC (5), and FbP (6), and similarly for the zinc chelates.

In the present study of unsubstituted parent macrocycles (chlorin, phorbine, oxophorbine), the use of the four-orbital framework (involving simple sums and differences of MO energy gaps) provides additional fundamental insights into the intimate relationship between the hydrocarbon skeleton, electronic structure, and characteristics of the long-wavelength absorption band of the natural photosynthetic pigments. Moreover, this framework and understanding has predictive value in aiding the design of synthetic analogues of the parent macrocycles and of the fully substituted photosynthetic pigments to allow better coverage of the solar spectrum for use in artificial systems for solar-energy conversion. The ability to mimic the spectral properties of chlorophylls with far simpler and synthetically accessible macrocycles augurs well for the use of such hydroporphyrins in a variety of artificial photosynthetic architectures.

Finally, we note that the studies here have employed zinc chelates rather than magnesium as is present in the chlorophylls. Compared with zinc, magnesium affords a bathochromic shift of the Q_y band of about 10 nm, a 3–4-fold longer singlet excited-state lifetime, more facile ground-state oxidation, a more potent excited-state reductant, and tighter binding of oxygenic apical ligands. On the other hand, magnesium is more susceptible to demetalation. Any application would need to balance the advantageous photophysical properties of magnesium chlorinsversus the greater chemical stability of the zinc chelates.

Experimental section

I General synthetic methods

All ¹H NMR (300 MHz and 400 MHz) and ¹³C NMR (100 MHz) spectra were obtained in CDCl₃ unless noted otherwise. Mass spectra of chlorins were obtained vialaser desorption ionization mass spectrometry without a matrix or using a matrix of 1,4-bis(5-phenyloxazol-2-yl)benzene. Electrospray ionization mass spectrometry (ESI-MS) data are reported for the molecule ion or protonated molecule ion. Column chromatography was performed with flash silica. NBS was recrystallized (water). All other commercially available materials were used as received. Absorption and fluorescence spectra were obtained in toluene at room temperature unless noted otherwise. All of the Pd-mediated coupling reactions were carried out under argon using standard Schlenk-line procedures (e.g., three freeze–pump–thaw cycles were performed prior to and after the addition of the palladium reagent, for a total of six such cycles).

A Zn(II)-13-bromo-17,18-dihydro-18,18-dimethylporphyrin (ZnC–Br¹³). Following a standard procedure,^31,38 a solution of 1 (151 mg, 0.452 mmol) and 2 (86 mg, 0.45 mmol) in anhydrous CH₂Cl₂ (12 mL) was treated with a solution of TsOH·H₂O (430 mg, 2.26 mmol) in anhydrous methanol (3 mL) under argon. The reaction mixture changed immediately to orange-red. The mixture was stirred for 30 min under argon, then treated with 2,2,6,6-tetramethylpiperidine (845 μL, 4.97 mmol) and concentrated to dryness, thereby affording a brown solid. The solid was suspended in acetonitrile (45 mL), whereupon 2,2,6,6-tetramethylpiperidine (1.92 mL, 11.3 mmol), Zn(OAc)₂ (1.25 g, 6.78 mmol) and AgOTf (349 mg, 1.36 mmol) were consecutively added. The resulting suspension was refluxed for 20 h exposed to air. The crude mixture was filtered through a silica pad with CH₂Cl₂, and the filtrate was chromatographed [silica, CH₂Cl₂/hexanes (2 ∶ 1)] to afford a green solid (35 mg, 16%): ¹H NMR (400 MHz, THF-d₈) δ 2.06 (s, 6H), 4.63 (s, 2H), 8.72 (s, 1H), 8.80 (d, J = 4.1 Hz, 1H), 8.89 (s, 1H), 8.94–8.96 (m, 2H), 9.09 (d, J = 4.1 Hz, 1H), 9.15 (s, 1H), 9.58 (s, 1H), 9.64 (s, 1H); ¹³C NMR (THF-d₈) δ 31.5, 46.4, 51.7, 94.0, 95.1, 109.08, 109.17, 115.8, 128.6, 129.4, 129.8, 133.0, 133.7, 144.6, 147.6, 147.8, 148.3, 149.3, 155.1, 159.8, 171.6; MALDI-MS obsd 479.9; ESI-MS obsd 481.00126 (M + H)⁺ corresponds to 479.99398 (M), calcd 479.99281 (M = C₂₂H₁₇BrN₄Zn); λ_abs 401, 610 nm; λ_em 614 nm (λ_ex 401 nm).

B 13-Bromo-17,18-dihydro-18,18-dimethylporphyrin (FbC–Br¹³). A solution of ZnC–Br¹³ (33 mg, 0.069 mmol) in CH₂Cl₂ (11 mL) was treated with TFA (0.256 mL, 3.44 mmol). The reaction mixture was stirred for 3 h at room temperature, and then was quenched by the addition of saturated aqueous NaHCO₃. Following extraction with CH₂Cl₂, the organic phase was dried (Na₂SO₄) and concentrated to afford a green solid. Purification by column chromatography [silica, CH₂Cl₂/hexanes (1 ∶ 1)] afforded a purple solid (25 mg, 87%): ¹H NMR (400 MHz, THF-d₈) δ −2.45 (br, 2H), 2.06 (s, 6H), 4.70 (s, 2H), 9.015–9.017 (m, 2H), 9.06 (d, J = 4.4 Hz, 1H), 9.11 (s, 1H), 9.19 (s, 1H), 9.31 (d, J = 4.4 Hz, 1H), 9.39 (s, 1H), 9.82 (s, 1H), 9.89 (s, 1H); ¹³C NMR (THF-d₈) δ 31.3, 47.4, 52.7, 95.0, 95.9, 106.9, 107.5, 112.4, 125.0, 128.8, 129.1, 133.5, 133.7, 133.9, 136.4, 136.9, 141.8, 153.1, 153.5, 164.1, 176.1; LD-MS obsd 417.9; ESI-MS obsd 419.08744 (M + H)⁺ corresponds to 418.08017 (M), calcd 418.07931 (M = C₂₂H₁₉BrN₄); λ_abs 391, 641 nm.

C 13-Acetyl-17,18-dihydro-18,18-dimethylporphyrin (FbC–A¹³). Following a procedure for Stille coupling of aromatic compounds⁵⁴ with some modifications,⁴² a mixture of FbC–Br¹³ (52 mg, 0.12 mmol), tributyl(1-ethoxyvinyl)tin (169 μL, 0.500 mmol), and (PPh₃)₂PdCl₂ (13 mg, 0.018 mmol) in the solvent mixture of acetonitrile (3.5 mL) and DMF (2.5 mL) was heated at 85 °C for 2.5 h. The reaction mixture was treated with 10% aqueous HCl (2 mL) at room temperature for 40 min and then diluted with CH₂Cl₂. The organic phase was washed with saturated aqueous NaHCO₃, water, and brine. The organic layer was dried (Na₂SO₄), concentrated and chromatographed [silica, CH₂Cl₂/hexanes (2 ∶ 1)] to afford a purple solid (37 mg, 81%): ¹H NMR (300 MHz) δ −1.80 (br, 2H), 2.03 (s, 6H), 3.24 (s, 3H), 4.61 (s, 2H), 8.82 (s, 1H), 8.91 (d, J = 4.4 Hz, 1H), 8.92 (d, J = 4.2 Hz, 1H), 9.00 (d, J = 4.2 Hz, 1H), 9.14 (d, J = 4.4 Hz, 1H), 9.53 (s, 1H), 9.64 (s, 1H), 9.76 (s, 1H), 10.1 (s, 1H); ¹³C NMRδ 29.9, 31.2, 47.0, 51.9, 95.0, 97.7, 106.1, 109.5, 125.7, 128.9, 129.4, 130.4, 132.6, 134.0, 136.9, 137.3, 142.3, 151.6, 154.3, 165.0, 177.6, 197.0, 203.0; MALDI-MS obsd 381.9; ESI-MS obsd 383.18648 (M + H)⁺ corresponds to 382.1792 (M), calcd 382.1794 (M = C₂₄H₂₂N₄O); λ_abs 405 (log ε = 4.91), 654 (log ε = 4.64) nm.

D 13-Acetyl-15-bromo-17,18-dihydro-18,18-dimethylporphyrin (FbC–A¹³Br¹⁵). Following a procedure for regioselective 15-bromination,⁴¹ a solution of FbC–A¹³ (34 mg, 0.089 mmol) in CH₂Cl₂ (40 mL) and TFA (4 mL) was treated with NBS (890 μL, 0.100 M in CH₂Cl₂, 0.0890 mmol) at room temperature. The reaction mixture was stirred for 1 h at room temperature. CH₂Cl₂ was added, and the mixture was washed with saturated aqueous NaHCO₃, water, and brine. The organic layer was dried (Na₂SO₄), concentrated, and chromatographed [silica, CH₂Cl₂/hexanes (2 ∶ 1)] to give a purple solid (30 mg, 73%): ¹H NMR (300 MHz) δ −2.14 (br, 1H), −1.90 (br, 1H), 2.04 (s, 6H), 3.19 (s, 3H), 4.59 (s, 2H), 8.82 (s, 1H), 8.88 (d, J = 4.3 Hz, 1H), 8.91 (d, J = 5.0 Hz, 1H), 8.97 (d, J = 4.3 Hz, 1H), 9.13 (d, J = 5.0 Hz, 1H), 9.13 (s, 1H), 9.61 (s, 1H), 9.73 (s, 1H); ¹³C NMRδ 31.6, 34.8, 46.8, 55.2, 95.0, 95.7, 105.9, 109.7, 126.37, 126.46, 129.2, 130.5, 132.7, 133.8, 134.6, 136.9, 137.6, 141.5, 151.3, 155.0, 163.1, 177.4, 202.3; LD-MS obsd 460.0; ESI-MS obsd 461.09695 (M + H)⁺ corresponds to 460.08968 (M); calcd 460.08987 (M = C₂₄H₂₁BrN₄O); λ_abs 399, 664 nm.

E Zn(II)-13-acetyl-17,18-dihydro-18,18-dimethylporphyrin (ZnC–A¹³). A solution of FbC–A¹³ (7 mg, 0.02 mmol) in CHCl₃ (2 mL) was treated with a solution of Zn(OAc)₂·2H₂O (61 mg, 0.28 mmol) in MeOH (1 mL). The reaction mixture was stirred overnight at room temperature. Then the reaction mixture was concentrated and chromatographed [silica, CH₂Cl₂/hexanes (2 ∶ 1)] to afford a green solid (5 mg, 70%): ¹H NMR (400 MHz, THF-d₈) δ 2.02 (s, 6H), 3.08 (s, 3H), 4.55 (s, 2H), 8.58 (s, 1H), 8.73 (d, J = 4.4 Hz, 1H), 8.85 (d, J = 4.4 Hz, 1H), 8.93 (d, J = 4.4 Hz, 1H), 9.01 (d, J = 4.4 Hz, 1H), 9.48 (s, 1H), 9.60 (s, 1H), 9.63 (s, 1H), 9.87 (s, 1H); MALDI-MS 444.2; ESI-MS obsd 445.09909 (M + H)⁺ corresponds to 444.09181 (M), calcd 444.09286 (M = C₂₄H₂₀N₄OZn); λ_abs 412, 628 nm; λ_em 632 nm (λ_ex 412 nm).

F 17,18-Dihydro-18,18-dimethyl-13¹-oxophorbine (FbOP). Following a standard procedure,^30,55,56 a mixture of FbC–A¹³Br¹⁵ (27 mg, 0.058 mmol), Cs₂CO₃ (96 mg, 0.29 mmol), and (PPh₃)₂PdCl₂ (8 mg, 0.02 mmol) in toluene (6 mL) was refluxed in a Schlenk flask. After 22 h CH₂Cl₂ was added, and the organic layer was washed with water and brine. The organic layer was isolated, dried (Na₂SO₄) and concentrated. The resulting residue was purified by column chromatography [silica, hexanes/ethyl acetate (2 ∶ 1)] to afford a purple solid (12 mg, 55%): ¹H NMR (300 MHz) δ −2.23 (br, 1H), −0.17 (br, 1H), 1.98 (s, 6H), 4.18 (s, 2H), 5.03 (s, 2H), 8.64 (s, 1H), 8.73 (d, J = 4.3 Hz, 1H), 8.81 (d, J = 4.3 Hz, 1H), 8.85–8.87 (m, 1H), 9.91 (s, 1H), 9.03–9.06 (m, 1H), 9.37 (s, 1H), 9.50 (s, 1H); ¹³C NMRδ 31.1, 47.8, 48.57, 48.74, 95.1, 103.8, 106.1, 111.1, 115.6, 126.8, 130.3, 132.5, 132.8, 134.3, 138.1, 138.9, 143.1, 149.2, 152.6, 154.9, 157.3, 177.1, 195.8; MALDI-MS obsd 380.0; ESI-MS obsd 381.17088 (M + H)⁺ corresponds to 380.16360 (M); calcd 380.16371 (M = C₂₄H₂₀N₄O); λ_abs 408 (log ε = 4.74), 654 (log ε = 4.53) nm; λ_em 657 nm (λ_ex 408 nm).

G Zn(II)-17,18-dihydro-18,18-dimethyl-13¹-oxophorbine (ZnOP). A solution of FbOP (11 mg, 0.029 mmol) in CHCl₃/MeOH (4 ∶ 1, 6 mL) was treated with Zn(OAc)₂·2H₂O (95 mg, 0.43 mmol). The reaction mixture was stirred for 20 h at room temperature. CH₂Cl₂ was added, and the mixture was concentrated. The resulting solid was dissolved in CH₂Cl₂, and the resulting organic solution was washed (saturated aqueous NaHCO₃, water), dried (Na₂SO₄), and concentrated to afford a green solid. The solid was washed with hexanes three times and then was dissolved in the minimal amount of CH₂Cl₂. A double volume of hexanes subsequently was added, which afforded a precipitate that was isolated by centrifugation (10 mg, 78%): ¹H NMR (400 MHz, THF-d₈) δ 2.05 (s, 6H), 4.34 (s, 2H), 5.04 (s, 2H), 8.59 (s, 1H), 8.78–8.80 (m, 2H), 8.95 (d, J = 4.4 Hz, 1H), 9.01, (d, J = 4.2 Hz, 1H), 9.05 (s, 1H), 9.39 (s, 1H), 9.74 (s, 1H); LD-MS obsd 442.1; ESI-MS obsd 443.08407 (M + H)⁺ corresponds to 442.07679; calcd 442.07587 (C₂₄H₁₈N₄OZn); λ_abs 420, 637 nm; λ_em 640 (λ_ex 420 nm).

H 18,18-Dimethylphorbine (FbP). A sample of FbOP (30 mg, 0.079 mmol) in ethanol/CHCl₃ (5 mL, 1 ∶ 1) was treated with tosylhydrazide (44 mg, 0.24 mmol) at 65 °C. The reaction mixture was stirred for 2 h at 65 °C and then 2 h at room temperature. The reaction mixture was concentrated, and the solid was washed three times [H₂O/methanol (1 ∶ 4)] and dried under high vacuum. The crude solid was dissolved in THF (4 mL), and the resulting solution was treated with sodium cyanoborohydride (22 mg, 0.32 mmol) followed by a solution of TsOH·H₂O (48 mg, 0.25 mmol) in MeOH (1 mL). After 2 h, the reaction mixture was diluted with CH₂Cl₂ and quenched by the addition of saturated aqueous NaHCO₃. The organic phase was washed with water, dried over Na₂SO₄, and filtered. The filtrate was concentrated to afford a green solid. The solid was dissolved in DMF/EtOH (5 mL, 1 ∶ 2), and the solution was treated with sodium acetate trihydrate (54 mg, 0.40 mmol). After heating at 80 °C for 5 h, the mixture was allowed to cool to room temperature, whereupon CH₂Cl₂ and water were added. The organic phase was washed (water and brine), dried (Na₂SO₄) and concentrated. Column chromatography (silica, CH₂Cl₂/hexanes [1 ∶ 1]) afforded a green solid (12 mg, 42%): ¹H NMR (400 MHz) δ −3.42 (br, 1H), −1.67 (br, 1H), 2.10 (s, 6H), 4.17–4.19 (m, 2H), 4.46 (s, 2H), 4.78–4.80 (m, 2H), 8.79 (s, 1H), 9.04–9.06 (m, 2H), 9.08 (d, J = 4.1 Hz, 1H), 9.16 (d, J = 4.1 Hz, 1H), 9.31–9.33 (m, 1H), 9.73 (s, 1H), 9.98 (s, 1H); ¹³C NMRδ 25.9, 31.9, 37.6, 47.4, 49.9, 95.1, 103.4, 105.8, 112.3, 116.2, 122.6, 127.4, 131.1, 132.5, 133.5, 139.2, 143.0, 145.2, 147.9, 149.2, 152.7, 160.1, 171.4; LD-MS obsd 366.1; ESI-MS obsd 367.19219 (M + H)⁺ corresponds to 366.18492 (M), calcd 366.18445 (M = C₂₄H₂₂N₄); λ_abs 405 (log ε = 5.16), 630 (log ε = 4.59) nm.

I Zn(II)-18,18-dimethylphorbine (ZnP). A solution of FbP (12 mg, 0.033 mmol) in CHCl₃ (4 mL) was treated with a methanolic solution (1 mL) of Zn(OAc)₂·2H₂O (108 mg, 0.492 mmol). The reaction mixture was stirred for 1 h at room temperature. CH₂Cl₂ was added, and the mixture was concentrated. The resulting solid was dissolved in CH₂Cl₂, and the resulting organic solution was washed (saturated aqueous NaHCO₃ and water), dried (Na₂SO₄), and concentrated to afford a green solid. The solid was dissolved in CH₂Cl₂ (1 mL), and hexanes (1 mL) were added. The resulting precipitate was isolated by centrifugation (13 mg, 90%): ¹H NMR (400 MHz) δ 2.07 (s, 6H), 3.74–3.76 (m, 2H), 4.28 (s, 2H), 4.36–4.39 (m, 2H), 8.27 (s, 1H), 8.75 (s, 1H), 8.86 (d, J = 4.1 Hz, 1H), 8.89 (d, J = 4.4 Hz, 1H), 8.93 (d, J = 4.1 Hz, 1H), 9.17 (d, J = 4.4 Hz, 1H), 9.30 (s, 1H), 9.67 (s, 1H); LD-MS obsd 428.0; ESI-MS obsd 428.0971, calcd 428.0974 (C₂₄H₂₀N₄Zn); λ_abs 402 (log ε = 4.89), 601 (log ε = 4.23) nm; λ_em 605 (λ_ex 410 nm).

II Photophysical measurements

Static absorption (Varian Cary 100) and fluorescence (Spex Fluorolog Tau 2 or PTI Quantamaster 40) measurements were performed as described previously.^73,74Argon-purged solutions of the samples in toluene with an absorbance of ≤0.10 at the excitation wavelength were used for the fluorescence spectral, quantum yield, and lifetime measurements. Fluorescence lifetimes were obtained using a phase modulation technique and Soret-band excitation⁷⁴ (Spex Fluorolog Tau 2) or via decay measurements using excitation pulses at 337 nm from a nitrogen laser and time-correlated-single-photon-counting detection (Photon Technology International LaserStrobe TM-3). Emission measurements employed 2–4 nm excitation- and detection-monochromator bandwidths and 0.2 nm data intervals. Emission spectra were corrected for detection-system spectral response. Fluorescence quantum yields were determined relative to chlorophyll a in benzene (Φ_f = 0.325)⁸⁵ or chlorophyll a in toluene, which was found here to have the same value as in benzene.

III Density functional theory calculations

DFT calculations were performed with Spartan '08 for Windows (Wavefunction, Inc.) on a PC (Dell Optiplex GX270) equipped with a 3.2 GHz CPU and 3 GB of RAM.⁸⁶ The hybrid B3LYP functional and the 6-31G* basis set were employed. The equilibrium geometries were fully optimized using the default parameters of the Spartan '08 program. A methyl group was substituted for the phytyl chain in the calculations on Pheo aa and Zn–Pheo a.

Acknowledgements

This work was supported by grants from the Division of Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences of the U.S. Department of Energy to D.F.B. (DE-FG02-05ER15660), D.H. (DE-FG02-05ER15661) and J.S.L. (DE-FG02-96ER14632). Mass spectra were obtained at the Mass Spectrometry Laboratory for Biotechnology at North Carolina State University. Partial funding for the facility was obtained from the North Carolina Biotechnology Center and the National Science Foundation.

References

1. H. Scheer, in Chlorophylls and Bacteriochlorophylls. Biochemistry, Biophysics, Functions and Applications, ed. B. Grimm, R. J. Porra, W. Rüdiger and H. Scheer, Springer, Dordrecht, 2006, vol. 25, pp. 1–26.
2. G. P. Moss, Pure Appl. Chem., 1987, 59, 779–832.
3. D. Mauzerall, Ann. N. Y. Acad. Sci., 1973, 206, 483–494.
4. L. O. Björn, G. C. Papageorgiou, R. E. Blankenship and Govindjee, Photosynth. Res., 2009, 99, 85–98.
5. L. O. Björn, Photosynthetica, 1976, 10, 121–129.
6. L. O. Björn, G. C. Papageorgiou, D. Dravins and Govindjee, Curr. Sci., 2009, 96, 1171–1175.
7. M. Kobayashi, M. Akiyama, H. Kano and H. Kise, in Chlorophylls and Bacteriochlorophylls. Biochemistry, Biophysics, Functions and Applications, ed. B. Grimm, R. J. Porra, W. Rüdiger and H. Scheer, Springer, Dordrecht, 2006, vol. 25, pp. 79–94.
8. W. L. Butler, in The Chlorophylls, ed. L. P. Vernon and G. R. Seely, Academic Press, New York, 1966, pp. 343–379.
9. W. Flitsch, Adv. Heterocycl. Chem., 1988, 43, 73–126.
10. P. H. Hynninen, in Chlorophylls, ed. H. Scheer, CRC Press, Boca Raton, FL, 1991, pp. 145–209.
11. K. M. Smith, in Chlorophylls, ed. H. Scheer, CRC Press, Boca Raton, FL, 1991, pp. 115–143.
12. F.-P. Montforts, B. Gerlach and F. Höper, Chem. Rev., 1994, 94, 327–347.
13. F.-P. Montforts and M. Glasenapp-Breiling, Prog. Heterocycl. Chem., 1998, 10, 1–24.
14. L. Jaquinod, in The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic Press, San Diego, CA, 2000, vol. 1, pp. 201–237.
15. M. G. H. Vicente, in The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic Press, San Diego, 2000, vol. 1, pp. 149–199.
16. R. K. Pandey and G. Zheng, in The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic Press, San Diego, 2000, vol. 6, pp. 157–230.
17. F.-P. Montforts and M. Glasenapp-Breiling, Fortschr. Chem. Org. Naturst., 2002, 84, 1–51.
18. V. Y. Pavlov and G. V. Ponomarev, Chem. Heterocycl. Compd. (N. Y.), 2004, 40, 393–425.
19. M. O. Senge, A. Wiehe and C. Ryppa, in Chlorophylls and Bacteriochlorophylls. Biochemistry, Biophysics, Functions and Applications, ed. B. Grimm, R. J. Porra, W. Rüdiger and H. Scheer, Advances in Photosynthesis and Respiration, Springer, Dordrecht, The Netherlands, 2006, vol. 25, pp. 27–37.
20. S. Fox and R. W. Boyle, Tetrahedron, 2006, 62, 10039–10054.
21. M. Galezowski and D. T. Gryko, Curr. Org. Chem., 2007, 11, 1310–1338.
22. A. M. G. Silva and J. A. S. Cavaleiro, in Progress in Heterocyclic Chemistry, ed. G. W. Gribble and J. A. Joule, Elsevier, Amsterdam, 2008, vol. 19, pp. 44–69.
23. J.-P. Strachan, D. F. O'Shea, T. Balasubramanian and J. S. Lindsey, J. Org. Chem., 2000, 65, 3160–3172.
24. T. Balasubramanian, J.-P. Strachan, P. D. Boyle and J. S. Lindsey, J. Org. Chem., 2000, 65, 7919–7929.
25. M. Taniguchi, D. Ra, G. Mo, T. Balasubramanian and J. S. Lindsey, J. Org. Chem., 2001, 66, 7342–7354.
26. M. Taniguchi, H.-J. Kim, D. Ra, J. K. Schwartz, C. Kirmaier, E. Hindin, J. R. Diers, S. Prathapan, D. F. Bocian, D. Holten and J. S. Lindsey, J. Org. Chem., 2002, 67, 7329–7342.
27. M. Taniguchi, M. N. Kim, D. Ra and J. S. Lindsey, J. Org. Chem., 2005, 70, 275–285.
28. M. Ptaszek, J. Bhaumik, H.-J. Kim, M. Taniguchi and J. S. Lindsey, Org. Process Res. Dev., 2005, 9, 651–659.
29. (a) J. K. Laha, C. Muthiah, M. Taniguchi, B. E. McDowell, M. Ptaszek and J. S. Lindsey, J. Org. Chem., 2006, 71, 4092–4102; (b) J. K. Laha, C. Muthiah, M. Taniguchi, B. E. McDowell, M. Ptaszek and J. S. Lindsey, J. Org. Chem., 2009, 74, 5122.
30. J. K. Laha, C. Muthiah, M. Taniguchi and J. S. Lindsey, J. Org. Chem., 2006, 71, 7049–7052.
31. M. Ptaszek, B. E. McDowell, M. Taniguchi, H.-J. Kim and J. S. Lindsey, Tetrahedron, 2007, 63, 3826–3839.
32. M. Taniguchi, M. Ptaszek, B. E. McDowell and J. S. Lindsey, Tetrahedron, 2007, 63, 3840–3849.
33. M. Taniguchi, M. Ptaszek, B. E. McDowell, P. D. Boyle and J. S. Lindsey, Tetrahedron, 2007, 63, 3850–3863.
34. H. L. Kee, C. Kirmaier, Q. Tang, J. R. Diers, C. Muthiah, M. Taniguchi, J. K. Laha, M. Ptaszek, J. S. Lindsey, D. F. Bocian and D. Holten, Photochem. Photobiol., 2007, 83, 1110–1124.
35. H. L. Kee, C. Kirmaier, Q. Tang, J. R. Diers, C. Muthiah, M. Taniguchi, J. K. Laha, M. Ptaszek, J. S. Lindsey, D. F. Bocian and D. Holten, Photochem. Photobiol., 2007, 83, 1125–1143.
36. C. Muthiah, J. Bhaumik and J. S. Lindsey, J. Org. Chem., 2007, 72, 5839–5842.
37. K. E. Borbas, V. Chandrashaker, C. Muthiah, H. L. Kee, D. Holten and J. S. Lindsey, J. Org. Chem., 2008, 73, 3145–3158.
38. C. Muthiah, M. Ptaszek, T. M. Nguyen, K. M. Flack and J. S. Lindsey, J. Org. Chem., 2007, 72, 7736–7749.
39. C. Muthiah, H. L. Kee, J. R. Diers, D. Fan, M. Ptaszek, D. F. Bocian, D. Holten and J. S. Lindsey, Photochem. Photobiol., 2008, 84, 786–801.
40. C. Ruzié, M. Krayer and J. S. Lindsey, Org. Lett., 2009, 11, 1761–1764.
41. C. Muthiah, D. Lahaye, M. Taniguchi, M. Ptaszek and J. S. Lindsey, J. Org. Chem., 2009, 74, 3237–3247.
42. O. Mass, M. Ptaszek, M. Taniguchi, J. R. Diers, H. L. Kee, D. F. Bocian, D. Holten and J. S. Lindsey, J. Org. Chem., 2009, 74, 5276–5289.
43. M. Ptaszek, D. Lahaye, M. Krayer, C. Muthiah and J. S. Lindsey, J. Org. Chem., 2010, 75, 1659–1673.
44. H. Fischer and O. Laubereau, Justus Liebigs Ann. Chem., 1938, 535, 17–37.
45. H. Fischer and A. Oestreicher, Justus Liebigs Ann. Chem., 1940, 546, 49–57.
46. H. Fischer and H. Siebel, Justus Liebigs Ann. Chem., 1932, 499, 84–108.
47. H. Wolf, Justus Liebigs Ann. Chem., 1966, 695, 98–111.
48. H. Brockmann Jr. and R. Tacke-Karimdadian, Liebigs Ann. Chem., 1979, 419–430.
49. T. D. Lash, R. P. Balasubramaniam, J. J. Catarello, M. C. Johnson, D. A. May Jr., K. A. Bladel, J. M. Feeley, M. C. Hoehner, T. G. Marron, T. H. Nguyen, T. J. Perun Jr., D. M. Quizon, C. M. Shiner and A. Watson, Energy Fuels, 1990, 4, 668–674.
50. B. Zhang and T. D. Lash, Tetrahedron Lett., 2003, 44, 7253–7256.
51. T. D. Lash, W. Li and D. M. Quizon-Colquitt, Tetrahedron, 2007, 63, 12324–12342.
52. G. W. Kenner, J. Rimmer, K. M. Smith and J. F. Unsworth, J. Chem. Soc., Perkin Trans. 1, 1978, 845–852.
53. C. Bauder, R. Ocampo and H. J. Callot, Tetrahedron, 1992, 48, 5135–5150.
54. M. Kosugi, T. Sumiya, Y. Obara, M. Suzuki, H. Sano and T. Migita, Bull. Chem. Soc. Jpn., 1987, 60, 767–768.
55. H. Muratake and M. Natsume, Tetrahedron Lett., 1997, 38, 7581–7582.
56. H. Muratake, M. Natsume and H. Nakai, Tetrahedron, 2004, 60, 11783–11803.
57. H. Brockmann Jr. and J. Bode, Justus Liebigs Ann. Chem., 1974, 1017–1027.
58. K. M. Smith and D. A. Goff, J. Am. Chem. Soc., 1985, 107, 4954–4964.
59. K. Maruyama, H. Yamada and A. Osuka, Chem. Lett., 1989, 833–836.
60. N. W. Smith and K. M. Smith, Energy Fuels, 1990, 4, 675–688.
61. K. Maruyama, H. Yamada and A. Osuka, Photochem. Photobiol., 1991, 53, 617–626.
62. R. J. Abraham, A. E. Rowan, N. W. Smith and K. M. Smith, J. Chem. Soc., Perkin Trans. 2, 1993, 1047–1059.
63. H. Tamiaki, M. Amakawa, Y. Shimono, R. Tanikaga, A. R. Holzwarth and K. Schaffner, Photochem. Photobiol., 1996, 63, 92–99.
64. R. K. Pandey, S. Constantine, T. Tsuchida, G. Zheng, C. J. Medforth, M. Aoudia, A. N. Kozyrev, M. A. J. Rodgers, H. Kato, K. M. Smith and T. J. Dougherty, J. Med. Chem., 1997, 40, 2770–2779.
65. H. Tamiaki, T. Miyatake and R. Tanikaga, Tetrahedron Lett., 1997, 38, 267–270.
66. S. Mettath, M. Shibata, J. L. Alderfer, M. O. Senge, K. M. Smith, R. Rein, T. J. Dougherty and R. K. Pandey, J. Org. Chem., 1998, 63, 1646–1656.
67. H. Tamiaki, S. Yagai and T. Miyatake, Bioorg. Med. Chem., 1998, 6, 2171–2178.
68. M. Katterle, A. R. Holzwarth and A. Jesorka, Eur. J. Org. Chem., 2006, 414–422.
69. T. Miyatake, S. Tanigawa, S. Kato and H. Tamiaki, Tetrahedron Lett., 2007, 48, 2251–2254.
70. R. O. Hutchins, C. A. Milewski and B. E. Maryanoff, J. Am. Chem. Soc., 1973, 95, 3662–3668.
71. V. Nair and A. K. Sinhababu, J. Org. Chem., 1978, 43, 5013–5017.
72. N. Srinivasan, C. A. Haney, J. S. Lindsey, W. Zhang and B. T. Chait, J. Porphyrins Phthalocyanines, 1999, 3, 283–291.
73. F. Li, S. Gentemann, W. A. Kalsbeck, J. Seth, J. S. Lindsey, D. Holten and D. F. Bocian, J. Mater. Chem., 1997, 7, 1245–1262.
74. H. L. Kee, C. Kirmaier, L. Yu, P. Thamyongkit, W. J. Youngblood, M. E. Calder, L. Ramos, B. C. Noll, D. F. Bocian, W. R. Scheidt, R. R. Birge, J. S. Lindsey and D. Holten, J. Phys. Chem. B, 2005, 109, 20433–20443.
75. E. Zass, H. P. Isenring, R. Etter and A. Eschenmoser, Helv. Chim. Acta, 1980, 63, 1048–1067.
76. I. D. Jones, R. C. White, E. Gibbs and C. D. Denard, J. Agric. Food Chem., 1968, 16, 80–83.
77. S. I. Yang, J. Seth, J.-P. Strachan, S. Gentemann, D. Kim, D. Holten, J. S. Lindsey and D. F. Bocian, J. Porphyrins Phthalocyanines, 1999, 3, 117–147.
78. A. T. Gradyushko, A. N. Sevchenko, K. N. Solovyov and M. P. Tsvirko, Photochem. Photobiol., 1970, 11, 387–400.
79. P. G. Seybold and M. Gouterman, J. Mol. Spectrosc., 1969, 31, 1–13.
80. J. B. Birks, Photophysics of Aromatic Molecules, Wiley-Interscience, London, 1970, pp. 140–192.
81. M. Taniguchi, O. Mass, P. D. Boyle, Q. Tang, J. R. Diers, D. F. Bocian, D. Holten and J. S. Lindsey, J. Mol. Struct., 2010, 979, 27–45.
82. M. Gouterman, in The Porphyrins, ed. D. Dolphin, Academic Press, New York, 1978, vol. 3, pp. 1–165.
83. M. Gouterman, J. Chem. Phys., 1959, 30, 1139–1161.
84. M. Gouterman, J. Mol. Spectrosc., 1961, 6, 138–163.
85. G. Weber and F. W. J. Teale, Trans. Faraday Soc., 1957, 53, 646–655.
86. Except for molecular mechanics and semi-empirical models, the calculation methods used in Spartan have been documented in Y. Shao, L. F. Molnar, Y. Jung, J. Kussmann, C. Ochsenfeld, S. T. Brown, A. T. B. Gilbert, L. V. Slipchenko, S. V. Levchenko, D. P. O'Neill, R. A. DiStasio Jr., R. C. Lochan, T. Wang, G. J. O. Beran, N. A. Besley, J. M. Herbert, C. Y. Lin, T. Van Voorhis, S. H. Chien, A. Sodt, R. P. Steele, V. A. Rassolov, P. E. Maslen, P. P. Korambath, R. D. Adamson, B. Austin, J. Baker, E. F. C. Byrd, H. Dachsel, R. J. Doerksen, A. Dreuw, B. D. Dunietz, A. D. Dutoi, T. R. Furlani, S. R. Gwaltney, A. Heyden, S. Hirata, C.-P. Hsu, G. Kedziora, R. Z. Khalliulin, P. Klunzinger, A. M. Lee, M. S. Lee, W.-Z. Liang, I. Lotan, N. Nair, B. Peters, E. I. Proynov, P. A. Pieniazek, Y. M. Rhee, J. Ritchie, E. Rosta, C. D. Sherrill, A. C. Simmonett, J. E. Subotnik, H. L. Woodcock III, W. Zhang, A. T. Bell, A. K. Chakraborty, D. M. Chipman, F. J. Keil, A. Warshel, W. J. Hehre, H. F. Schaefer III, J. Kong, A. I. Krylov, P. M. W. Gill and M. Head-Gordon, Phys. Chem. Chem. Phys., 2006, 8, 3172–3191.

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