Symmetrical and Non-symmetrical meso-meso Directly-Linked Hydroporphyrin Dyads: Synthesis and Photochemical Properties

Abstract

A series of a rigid meso-meso directly-linked chlorin-chlorin, chlorin-bacteriochlorin, and bacteriochlorin-bacteriochlorin dyads, including free bases as well as Zn(II), Pd(II) and Cu(II) complexes has been synthesized, and their absorption, emission, singlet oxygen (¹O₂) photosensitization, and electronic properties have been examined. Marked bathochromic shift of the long-wavelength Q_y absorption band and increase in fluorescence quantum yields in dyads, compared to the corresponding monomers, are observed. Non-symmetrical dyads (except bacteriochlorin-bacteriochlorin) show two distinctive Q_y bands, corresponding to the absorption of each dyad component. A nearly quantitative S₁-S₁ energy transfer between hydroporphyrins in dyads, leading to an almost exclusive emission of hydroporphyrin with a lower S₁ energy, have been determined. Several symmetrical and all non-symmetrical dyads exhibit a significant reduction of fluorescence quantum yields in solvents of high dielectric constants; this is attributed to the photoinduced electron transfer. The complexation of one macrocycle by Cu(II) or Pd(II) enhances intersystem crossing in the adjacent, free base dyad component, which is manifested by a significant reduction in fluorescence and increase in quantum yield of ¹O₂ photosensitization.

Graphical abstract

Introduction

Arrays of tetrapyrrolic macrocycles directly connected between their meso positions represent an interesting class of molecules, since due to orthogonal arrangement of macrocycles there is a little direct π-conjugation between dyad components, however due to their close proximity, there is a substantial through-space electronic interaction between chromophores, which significantly affects electronic, optical, and electrochemical properties of the resulting constructs.^1–14 Meso-meso linked porphyrin,^1–9 chlorin,^10–12 bacteriochlorin,¹² and corrole^13,14 dyads have been reported, and their spectral and electronic properties, as well as energy- and electron transfer processes between array components have been examined. Resulting dyads have been applied, for example, as biomimetic models for photosynthetic light harvesting^2–4 and electron transfer arrays,⁵ fluorescence imaging agents,⁶ molecular wires,^1,7 molecules for information storage,⁸ and triplet state photosensitizers.^9,12

The vast majority of such arrays reported so far are composed of porphyrins, while arrays of hydroporphyrins have been explored far less.^10–12 Hydroporphyrins - chlorins and bacteriochlorins, are partially saturated tetrapyrrolic macrocycles that differ substantially from fully unsaturated porphyrins, in terms of electronic, optical, and electrochemical properties.^15–18 In particular, hydroporphyrins exhibit enhanced absorption in the red (chlorins) or near-IR (bacteriochlorins) spectral window, and they exhibit intense fluorescence in the respective regions.^16–18 Thus, they are more efficient in photon harvesting in visible and near-IR spectral windows, which is utilized by natural light harvesting antenna.¹⁵ Moreover, due to their near-IR absorption and emission, hydroporphyrins are used for a host of photomedical applications, such as photodynamic therapy and in vivo imaging.^19–22 Hydroporphyrins are easier to oxidize than porphyrins,²³ and are therefore more prone to the oxidative photoinduced electron transfer (PET), which is utilized in natural photosynthetic reaction centers, where hydroporphyrins function as primary electron donors.¹⁵ Thus, it can be expected that hydroporphyrin dyads will exhibit set of properties, distinctive from those observed in analogous porphyrin arrays. For example, we recently reported a series of symmetrical meso-meso directly-linked¹² and strongly conjugated^24,25 hydroporphyrin dyads. For the meso-meso directly linked bacteriochlorin dyad, fluorescence quantum yield Φ_f and lifetime τ_f, as well as singlet oxygen ¹O₂ quantum yield Φ_Δ are all progressively reduced when solvent dielectric constant (ε) increases, resulting in nearly complete quenching of fluorescence and ¹O₂ photosensitization in solvents of high ε.¹² These quenching of photochemical activity in bacteriochlorin dyad is much more extensive than in corresponding chlorin¹² and porphyrin dyads.²⁶ Although details of excited state dynamics for these dyads are still being investigated, these observations indicate that directly linked hydroprorphyrin arrays have a complex and rich photochemical properties. This has prompted us to examine the broader set of directly-linked hydroporphyrin arrays, which includes various symmetrical metal complexes, and non-symmetrical arrays i.e. arrays composed of two hydroporphyrins with distinctive electronic, optical, and redox properties. To the best of our knowledge, the only non-symmetrical directly-linked hydroporphyrin dyads have been reported only recently by Borbas et al.¹¹ (non-symmetrical meso-β directly linked porphyrin-chlorin dyad were also reported²⁷).

Here, we report the synthesis and characterization of a series of symmetrical and non-symmetrical chlorin-chlorin, chlorin-bacteriochlorin, and bacteriochlorin-bacteriochlorin dyads (Chart 1). Symmetrical dyads include free base chlorin-chlorin 2C¹² as well as bacteriochlorin-bacteriochlorin 2BC¹² and 2BC1, and chlorin-chlorin metal chelates: 2ZnC,¹² 2PdC, 2CuC, (note, that 2C and 2BC have been previously reported, and they are included here for comparison). Non-symmetrical chlorin-chlorin dyads are composed of one free base and one metal [Zn(II), Pd(II), or Cu(II)] complex of the same chlorin (ZnC-C, PdC-C, and Cu-C respectively). Asymmetry here is achieved by metalation of one chlorin component. It is well known that metalation significantly alters spectral, electronic and redox properties of hydroporphyrins.^28,29 Chlorin-bacteriochlorin dyads include one composed of two-free base macrocycles (C-BC) and that composed of a Pd(II) complex of the same chlorin (PdC-BC). Finally, bacteriochorin-bacteriochlorin dyad BC-BC1 is composed of two bacteriochlorin free-bases with different sets of substituents on the macrocycle perimeter. As benchmarks we included monomers ZnC,^28,30 PdC, H₂C,^28,30 H₂C-Ph,³¹ BC,¹⁸ BC1, BC-Tol,¹² and BC1-Tol.

Chart 1.

Structures of dyads and monomers discussed in this paper (Ms = 2,4,6-trimethylphenyl).

This set of compounds has enabled us to determine several key aspects of photophysics of directly linked hydroporphyrin dyads. First, we sought to determine how metalation and electronic asymmetry between dyads components affects the basic absorption and emission properties of dyads, and how efficient energy transfer is between dyads’ components. Second, we determined to what extent emission properties in dyads are influenced by the solvent dielectric constant, in view of the possible PET between redox non-equivalent hydropoprhyrin components. Finally, we intended to determine how heavy element complexation [i.e. Pd(II) or Cu(II)] by one hydroporphyri in dyad affects fluorescence and singlet oxygen (¹O₂) photosensitization of the adjacent, free base hydroporphyrin. Tetrapyrrolic macrocycles are capable of photosensitizing ¹O₂ (as well as other reactive oxygen species) due to the high quantum yield of intersystem crossing (ISC).³² Complexation of a heavy element, particularly Pd(II) or Pt(II), greatly increases ISC rate and quantum yield, and consequently quantum yield of ROS photosensitization.³² However, insertion of Pd(II) and Pt(II) into chlorins causes substantial hypsochromic shift of the Q_y band, placing it out of the “therapeutic window” and thus rendering them potentially less efficient for deep tissue PDT^33–35 (note however, that complexation of bacteriochlorins with Pd(II) causes a bathochromic shift of the Q_y band²⁹). In addition, metal complexation alters redox properties of the tetrapyrrolic macrocycles, which affects the efficiency of ROS photosensitization.³⁶ Moreover, for Pd(II) and Pt(II) complexes of hydroporphyrins reduction of the T₁ lifetime (to < 10 μs) is observed,^29,36 which makes these chelates less useful for applications where long T₁ lifetime (i.e. hundreds of microseconds) is desirable. Alternative strategies to enhance ISC in tetrapyrrolic macrocycles without complexation of heavy element by central nitrogen atoms include T₁-T₁ energy transfer,^37–43 enhancement of ISC through interaction with an unpaired electron of a metal^44,45 or stable organic radical,⁴⁶ and the “remote” heavy atom effect.^47,48 In principle, these strategies allo ISC rate and T₁ lifetime in tetrapyrrolic macrocycles to be tuned without undesirable alteration of optical and redox properties. We have therefore determined here whether heavy metal complexation of one hydroporphyrin improve ¹O₂ photosensitization (as an indirect measure of T₁ formation) of the free base hydroporphyrin in dyads.

Experimental Section

Known compounds ZnC,³⁰ H₂C,³⁰ 2C,¹² 2ZnC,¹² BC-B,¹² BC-Tol,¹² 2BC,¹² and C-Br,³¹ were synthesized following the reported procedures. Pyrrole S3⁴⁹ was previously synthesized, and here it was prepared essentially followed reported procedure. Pyrrole S4⁵⁰ was also reported, and here was prepared by a different procedure.

General procedure for palladium catalyzed reactions

Schlenk flask was charged with all reagents and solvent was added. The resulting mixture was degassed by freeze-thaw cycles (two times), and filled with nitrogen. A palladium catalyst was added to the reaction flask, and the mixture was degassed again by freeze-thaw cycles (two times), and filled with nitrogen. Reaction flask was warmed to the room temperature and placed in the pre-heated oil bath (85 °C). TLC, UV-Vis spectroscopy, and LD-MS were used to monitor the progress of the reaction. When consumption of the hydroporphyrin starting material was determined, the reaction mixture was cooled to room temperature, diluted with ethyl acetate, washed (water and brine), dried (Na₂SO₄), and concentrated. The final purification was done by column chromatography, as specified below.

C-BC

Samples of BC-B (19.7 mg, 0.037 mmol), C-Br (20.1 mg, 0.037 mmol), Cs₂CO₃ (121.7 mg, 0.37 mmol) and Pd(PPh₃) ₄ (12.8 mg, 0.011 mmol) in toluene/DMF (12 mL, 2:1), were reacted overnight as described in General procedure. Column chromatography (silica, hexanes/DCM 1:1) afforded a green brown solid sample; 11.4 mg, 36% ¹H NMR (CDCl₃, 400 MHz) δ −1.88 (s, 1H), −1.70 (s, 1H), −1.39 (s, 2H), 1.72 (s, 3H), 1.76 (s, 3H), 1.83 (s, 3H), 1.84 (s, 3H), 1.92 (s, 3H), 1.94 (s, 3H), 2.06 (s, 6H), 2.55 (s, 3H), 3.61 (d, J = 17.3 Hz, 1H), 3.64 (d, J = 17.3 Hz, 1H), 3.77 (d, J = 17.3 Hz, 1H), 3.85 (d, J = 17.3 Hz, 1H), 4.53 (s, 2H), 4.58 (s, 3H), 7.20 (s, 2H), 7.74 (dd, J = 1.8, 4.6 Hz, 1H), 7.80 (d, J = 4.8 Hz, 1H), 8.38 (d, J = 4.8 Hz, 1H), 8.55 (d, J = 4.3 Hz, 2H), 8.77–8.80 (m, 3H), 8.96 (s, 1H), 8.98 (d, J = 4.3 Hz, 1H), 9.01 (d, J = 4.6 Hz, 1H), 9.05 (dd, J = 1.9,4.4 Hz, 1H), 9.29 (d, J = 4.6 Hz, 1H), 9.87 (s, 1H); ¹³C NMR (CDCl₃, 125 MHz) δ 21.5, 21.6, 30.8, 31.3, 31.6, 31.8, 45.4, 46.0, 46.1, 47.8, 52.3, 52.6, 65.5, 94.8, 97.3, 97.9, 106.8, 111.1, 112.6, 118.0, 121.2, 121.3, 122.9, 123.4, 123.8, 127.1, 127.8, 128.2, 131.6, 131.9, 132.8, 134.6, 135.1, 135.4, 136.1, 136.4, 137.6, 138.3, 139.2, 139.3, 141.0, 142.1, 152.0, 152.6, 153.9, 161.9, 165.1, 169.0, 169.2, 175.2; MS ([M]⁺, M = C₅₆H₅₆N₈O): Calcd: 856.4572, Obsd: (HRMS, ESI) 856.4539.

PdC-BC

A solution of C-BC (5.0 mg, 5.3 μmol) and Pd(OAc) ₂ (7.9 mg, 0.035 mmol) was stirred in chloroform/MeOH (6 ml of 5:1) was stirred at 50 °C, for 5h. The resulting mixture was concentrated. Column chromatography (silica, hexanes/CH₂Cl₂ 2:1) afforded a brown green solid; 2.2 mg, 40%; ¹H NMR (CDCl₃, 400 MHz) δ −1.96 (s, 1H), −1.49 (s, 1H), 1.73 (s, 3H), 1.77 (s, 3H), 1.79 (s, 6H), 1.90 (s, 6H), 2.03 (s, 3H), 2.04 (s, 3H), 2.50 (s, 3H), 3.60 (d, J = 17.2 Hz, 1H), 3.70 (d, J = 17.3 Hz, 1H), 3.76 (d, J = 16.9 Hz, 1H), 3.86 (d, J = 16.8 Hz, 1H), 4.50 (s, 2H), 4.55 (s, 3H), 7.15 (s, 2H), 7.61 (d, J = 4.8 Hz, 1H), 7.75 (dd, J = 1.7, 4.6 Hz, 1H), 8.14 (d, J = 4.8 Hz, 1H), 8.43(d, J = 4.6 Hz, 1H), 8.54 (dd, J = 1.8, 4.6 Hz, 1H), 8.71 (s, 1H), 8.75–8.78 (m, 4H), 8.86 (d, J = 4.6 Hz, 1H), 9.02 (dd, J = 2.1, 4.5 Hz, 1H), 9.73 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 21.3, 21.5, 22.8, 29.5, 29.9, 30.5, 30.8, 31.3, 31.5, 32.1, 45.1, 45.5, 46.0, 47.8, 50.6, 52.3, 65.5, 95.9, 97.3, 97.3, 97.9, 100.1, 100.9, 111.7, 112.0, 118.0, 121.3, 122.9, 123.3, 123.7, 126.9, 127.0, 127.1, 127.8, 127.9, 130.8, 131.9, 132.1, 135.4, 136.1, 136.3, 137.3, 137.6, 137.8, 137.9, 138.7, 138.9, 139.0, 145.9, 147.1, 151.2, 153.9, 161.6, 161.7, 169.0, 169.3;MS ([M]⁺, M = C₅₆H₅₄N₈OPd): Calcd: 960.3469, Obsd: (HRMS, ESI) 960.3456.

PdC

A solution of H₂C (10.0 mg, 0.022 mmol) and Pd(OAc) ₂ (14.7 mg, 0.065 mmol) in chloroform/MeOH mixture (10 mL, 5:1) was stirred at room temperature, for 7h. The resulting solution was concentrated. Column chromatography (silica, hexanes/CH₂Cl₂ 2:1) afforded a red solid; 10.9 mg, 89% ¹H NMR (CDCl₃, 400 MHz) δ 1.84 (s, 6H), 2.02 (s, 6H), 2.60 (s, 3H), 4.58 (s, 2H), 7.23 (s, 2H), 8.40 (d, J = 4.5 Hz, 1H), 8.45 (d, J = 4.7 Hz, 1H), 8.54 (d, J = 4.7 Hz, 1H), 8.68 (s, 1H), 8.71 (d, J = 4.5 Hz, 1H), 8.74 (s, 1H), 8.81 (d, J = 4.5 Hz, 1H), 8.96 (d, J = 4.6 Hz, 1H), 9.67 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 21.2, 21.6, 31.0, 45.6, 50.2, 95.5, 97.7, 110.0, 122.5, 126.8, 126.9, 127.0, 127.8, 127.9, 131.1, 131.9, 137.1, 137.5, 137.6, 137.7, 138.5, 138.5, 139.0, 144.6, 145.6, 149.9, 161.6; MS ([M]⁺, M = C₃₁H₂₈N₄Pd): Calcd: 562.1355, Obsd: (HRMS, ESI) 562.1354.

PdC-C

A mixture of 2C (21.9 mg,0.024 mmol) and Pd(OAc) ₂ (5.4 mg, 0.024 mmol) in chloroform/MeOH (20 mL, 5:1) was stirred at 50 °C, for 5h. The resulting mixture was concentrated. Column chromatography (silica, hexanes/CH₂Cl₂ 2:1) afforded a green solid; 7.8 mg, 32% ¹H NMR (CDCl₃, 400 MHz) δ −1.74 (s, 1H), −1.41 (s, 1H), 1.80 (s, 3H), 1.80 (s, 3H), 1.84 (s, 3H), 1.86 (s, 1H), 1.88 (s, 6H), 1.92 (s, 6H), 2.50 (s, 3H), 2.53 (s, 3H), 3.76–3.92 (m, 4H), 7.14 (s, 1H), 7.16 (s, 1H), 7.18 (s, 1H), 7.19 (s, 1H), 7.62 (d, J = 4.8Hz, 1H), 7.78 (d, J = 4.8 Hz, 1H), 8.16 (d, J = 4.8 Hz, 1H), 8.38 (d, J = 4.8 Hz, 1H), 8.45 (d, J = 4.6 Hz, 1H), 8.52 (d, J = 4.3 Hz, 1H), 8.72 (s, 1H), 8.76 (d, J = 4.6 Hz, 1H), 8.87 (d, J = 4.6 Hz, 1H), 8.95 (s, 1H), 8.96 (d, J = 4.4 Hz, 1H), 9.00 (d, J = 4.7 Hz, 1H), 9.03 (d, J = 4.6 Hz, 1H), 9.28 (d, J = 4.7 Hz, 1H), 9.74 (s, 1H), 9.85 (s, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 21.4, 21.5, 21.6, 29.9, 30.5, 31.0, 31.3, 31.7, 45.1, 46.2, 50.9, 52.8, 94.8, 96.0, 106.8 110.0, 110.9, 112.0, 121.5, 123.8, 123.9, 126.9, 127.1, 127.2, 127.8, 128.0, 128.3, 130.9, 131.7, 132.2, 132.8, 134.5, 135.2, 137.2, 137.6, 137.7, 138.0, 138.2, 138.7, 138.9, 139.0, 139.2, 139.2, 141.1, 141.9, 146.0, 147.2, 151.4, 152.1, 152.6, 161.8, 165.0, 175.3; MS ([M+H]⁺, M = C₆₂H₅₆N₈Pd): Calcd: 1019.3756, Obsd: (HRMS, ESI) 1019.3762

2PdC

A mixture of 2C (12.9 mg, 0.014 mmol) and Pd(OAc) ₂ (19.0 mg, 0.084 mmol) in chloroform/MeOH (10 mL, 5:1) was stirred at 50 °C for 5h. The resulting mixture was concentrated. Column chromatography (silica, hexanes/CH₂Cl₂, 2:1) afforded a green solid; 14.2 mg, 90%; ¹H NMR (CDCl₃, 500 MHz) δ 1.81(s, 6H), 1.84 (s, 6H), 1.88 (s, 6H), 1.93 (s, 6H), 2.52 (s, 6H), 3.81 (d, J = 16.8 Hz, 2H), 3.92 (d, J = 16.8 Hz, 2H), 7.16 (s, 2H), 7.18 (s, 2H), 7.66 (d, J = 4.8 Hz, 2H), 8.19 (d, J = 4.8 Hz, 2H), 8.44 (d, J = 4.6 Hz, 2H), 8.72 (s, 2H), 8.76 (d, J = 4.6 Hz, 2H), 8.86 (d, J = 4.6 Hz, 2H), 9.01 (d, J = 4.6 Hz, 2H), 9.72 (s, 2H); ¹³C NMR (CDCl₃, 125 MHz) δ 21.3, 21.4, 21.5, 30.6, 31.2, 45.2, 50.9, 96.0, 109.9, 111.5, 124.0, 126.9, 127.1, 127.2, 127.8, 128.0, 130.8, 132.2, 137.2, 137.6, 137.7, 138.0, 138.7, 138.9, 139.0, 146.0, 147.0, 151.2, 161.8; MS ([M+H]⁺, M = C₆₂H₅₄N₈Pd₂): Calcd: 1125.2643, Obsd: (HRMS, ESI) 1125.2640.

ZnC-C

A mixture of 2C (30.0 mg, 0.033 mmol) and Zn(OAc) ₂ (6.0 mg, 0.033 mmol) in chloroform/MeOH (30 mL, 5:1) was stirred at 50 °C, for 3h. The resulting solution was concentrated. Column chromatography (silica, hexanes/CH₂Cl₂ 3:2) afforded a green solid; 13.6 mg, 42% ¹H NMR (CDCl₃, 500 MHz) δ −1.72 (s, 1H), −1.41 (s, 1H), 1.79 (s, 3H), 1.81 (s, 3H), 1.84 (s, 3H), 1.88 (s, 3H), 1.89 (s, 3H), 1.91 (s, 3H), 1.93 (s, 1H), 1.94 (s, 1H), 2.51 (s, 3H), 2.54 (s, 3H), 3.66–3.75 (m, 2H), 3.82–3.94 (m, 2H), 7.15 (s, 1H), 7.16 (s, 1H), 7.18 (s, 1H), 7.20 (s, 1H), 7.62 (d, J = 4.6Hz, 1H), 7.81 (d, J = 4.8 Hz, 1H), 8.25 (d, J = 4.6 Hz, 1H), 8.38 (d, J = 4.8 Hz, 1H), 8.44 (d, J = 4.2 Hz, 1H), 8.53 (d, J = 4.3 Hz, 1H), 8.64 (s, 1H), 8.82 (d, J = 4.4 Hz, 1H), 8.89 (d, J = 4.3 Hz, 1H), 8.96–8.97 (m, 2H), 9.00 (d, J = 4.6 Hz, 1H), 9.16 (d, J = 4.3 Hz, 1H), 9.28 (d, J = 4.6 Hz, 1H), 9.67 (s, 1H), 9.85 (s, 1H); ¹³C NMR (CDCl₃, 125 MHz) δ 21.4, 21.5, 21.6, 30.6, 31.0, 31.5, 31.8, 44.9, 46.2, 51.4, 53.2, 94.7, 94.8, 106.7, 109.4, 111.4, 112.0, 121.3, 123.6, 123.9, 127.0, 127.4, 127.5, 127.7, 127.8, 128.2, 128.3, 128.8, 131.6, 131.8, 132.8, 133.3, 134.5, 135.2, 137.3, 137.6, 138.3, 138.8, 138.9, 139.2, 139.3, 141.0, 142.2, 145.8, 146.5, 147.2, 147.3, 152.1, 152.6, 154.6, 156.0, 160.9, 165.3, 171.2, 175.1; MS ([M+H]⁺, M = C₆₂H₅₆N₈Zn): Calcd: 977.3992, Obsd: (HRMS, ESI) 977.4012.

CuC-C

A mixture of 2C (15.0 mg, 0.016 mmol) and Cu(OAc) ₂ (3.0 mg, 0.016 mmol) in chloroform/MeOH (15 mL, 5:1) was stirred at 50 °C, for 1h. The resulting solution was concentrated. Column chromatography (silica, hexanes/CH₂Cl₂ 2:1) afforded the titled compounds as a green solid (8.9 mg, 57%) as well as 2CuC (green solid 4.0 mg, 24%). Data for CuC-C, ¹H NMR (CDCl₃, 400 MHz) δ −1.89 (s, 1H), −1.66 (s, 1H), 1.85–1.88 (br, 12H) 2.31 (s, 3H), 2.58 (s, 3H), 6.91 (br, 2H), 7.21 (s, 2H), 8.48 (s, 2H), 8.88–8.95 (m, 3H), 9.25 (s, 1H), 9.82 (s, 1H); MS ([M]⁺, M = C₆₂H₅₆N₈Cu): Calcd: 976.3997, Obsd: (HRMS, ESI) 976.4006. Data for 2CuC; MS ([M]⁺, M = C₆₂H₅₄N₈Cu₂): Calcd: 1036.3058, Obsd: (HRMS, ESI) 1036.3074.

5-Methoxy-8,8,18,18-tetramethyl-2,12-diphenylbacteriochlorin (BC1)

A solution of 1 (291 mg, 0.896 mmol) and 2,6-DTBP (4.0 mL, 17.92 mmol) in CH₂Cl₂ (180 mL) was for stirred for 0.5 h and then treated with TMSOTf (0.81 mL, 4.48 mmol) at room temperature. The reaction mixture was stirred for 16 h at room temperature. The reaction mixture concentrated and column chromatography was performed [silica, CH₂Cl₂/hexanes (1:1)] afforded a green solid (116 mg, 47%): ¹H NMR (CDCl₃, 500 MHz) δ −1.77 (s, 1H), −1.66 (s, 1H), 1.95 (s, 6H), 1.99 (s, 6H), 4.40 (s, 2H), 4.47 (s, 2H), 4.56 (s, 3H), 7.64–7.68 (m, 2H), 7.80–7.85 (m, 4H), 8.26 (d, J = 7.0, 2H), 8.32 (d, J = 7.0, 2H), 8.69 (s, 1H), 8.74 (d, J = 2.0, 1H), 8.86 (s, 1H), 8.89 (s, 1H), 9.06 (d, J = 2.2, 1H); ¹³C NMR (CDCl₃, 125 MHz) δ 14.3, 22.8, 30.3, 31.1, 31.8, 45.8, 46.2, 47.6, 51.9, 65.2, 95.8, 97.9, 116.5, 121.4, 127.3, 127.7, 129.2, 129.9, 131.2, 131.4, 133.0, 134.1, 134.9, 135.4, 135.7, 136.6, 137.3, 137.4, 153.0, 159.9, 169.7, 170.2; MS ([M]⁺, M = C₃₇H₃₆N₄O): Calcd: 552.2884, Obsd: (HRMS, ESI) 552.2888.

BC1-Br

A solution of BC1 (21.5 mg, 0.027 mmol) in THF (15 mL) was treated room temperature with NBS (4.8 mg, 0.027 mmol). The resulting mixture was stirred for 15 min. Saturated aqueous NaHCO₃ solution was added. The resulting mixture was extracted with ethyl acetate. The organic extract was washed with brine, dried (Na₂SO₄), and concentrated. Column chromatography [silica, hexanes/CH₂Cl₂ (1:1)] afforded a green solid; 16.0 mg, 94%; ¹H NMR (CDCl₃, 400 MHz) δ −1.95 (s, 1H), −1.74 (s, 1H), 1.93 (s, 12H), 4.42 (s, 2H), 4.51(s, 3H), 4.52 (s, 2H), 7.61–7.67 (m, 2H), 7.76–7.82 (m, 4H), 8.21–8.25 (m, 4H), 8.79 (s, 1H), 8.84 (s, 1H), 9.03 (d, J = 2.1 Hz, 1H), 9.09 (d, J = 2.2 Hz, 1H); MS ([M+H]⁺, M = C₃₇H₃₅BrN₄O): Calcd: 632.604, Obsd: (MALDI-MS) 632.443.

BC1-B

A mixture of BC1-Br (20.2 mg, 32 μmol), 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (46 μL, 320 μmol), triethylamine (89 μL, 640 μmol), and (PPh₃)₂PdCl₂ (2.2 mg 3.2 μmol) in 1,2-dichloroethane (8 mL) was reacted as described in General Procedure at 85 °C for 16 hours. The reaction mixture was concentrated, and the residue was purified by column chromatography [silica, hexane/CH₂Cl₂, (1:1)] to provide a green solid (19.1 mg, 88%); ¹H NMR (CDCl₃, 500 MHz) δ −1.37 (s, 1H), −1.04 (s, 1H), 1.72 (s, 12H), 1.91 (s, 6H), 1.92 (s, 6H), 4.36 (s, 2H), 4.51 (s, 3H), 4.60 (s, 2H), 7.60–7.67 (m, 2H), 7.78–7.81 (m, 4H), 8.21 (d, J = 7.6 Hz, 2H), 8.27 (d, J = 7.6 Hz, 2H), 8.74 (s, 1H), 8.81 (s, 1H), 8.92 (s, 1H), 9.30 (s, 1H); ¹³C NMR (CDCl₃, 125 MHz) δ 25.4, 29.9, 30.8, 31.5, 45.3, 46.6, 46.9, 53.3, 65.0, 84.4, 95.9, 96.8, 115.6, 123.3, 127.1, 127.6, 129.0, 129.1, 129.5, 131.4, 132.3, 133.3, 134.8, 136.2, 136.9, 137.4, 138.6, 141.4, 151.9, 166.4, 169.1, 169.7; MS ([M]⁺, M = C₄₃H₄₇BN₄O₃): Calcd: 678.3743, Obsd: (HRMS, ESI) 678.3739.

BC1-Tol

Samples of BC1-Br (15.5 mg, 0.024 mmol), 4-tolylboronic acid pinacol ester (5.2 mg, 0.024 mmol), (Pd(PPh₃)₄ (2.8 mg, 0.0024 mmol), K₂CO₃ (33.2 mg, 0.240 mmol), in toluene/DMF (6 mL, 2:1), was reacted as described in General Procedure at 85 °C under N₂ for 18 hours. Column chromatography (silica, hexanes/CH₂Cl₂, 2:1) afforded a reddish green solid: 14.3 mg, 91%; ¹H NMR (CDCl₃, 400 MHz) δ −1.79 (s, 1H), −1.56 (s, 1H), 1.84 (s, 6H), 1.93 (s, 6H), 2.64 (s, 3H), 4.04 (s, 2H), 4.42 (s, 2H), 4.53 (s, 3H), 7.47–7.51 (m, 2H), 7.54- 7.64 (m, 2H), 7.69–7.72 (m, 2H), 7.75–7.81 (m, 4H), 8.10–8.12 (m, 2H), 8.22 (d, J = 2.1 Hz, 1H), 8.26–8.28 (m, 2H), 8.82 (s, 1H), 8.85 (s, 1H), 8.99 (d, J = 2.2 Hz, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 21.7 31.0, 31.2, 47.5, 52.0, 65.3, 96.0, 96.2, 116.4, 121.7, 127.3, 127.5, 128.7, 129.0, 129.1, 130.3, 131.3, 131.4, 132.2, 133.0, 134.5, 134.6, 135.2, 136.2, 136.6, 136.8, 137.0, 140.0, 153.7, 159.6, 169.6; MS ([M]⁺, M = C₄₄H₄₂N₄O): Calcd: 642.3353, Obsd: (HRMS, ESI) 642.3359.

2BC1

A solution of BC1-Br (10.0 mg, 16 μmol), BC1-B (11.0 mg, 16 μmol), Cs₂CO₃ (52.1 mg, 160 μmol), and Pd(PPh₃)₄ (5.5 mg, 4.8 μmol) in toluene/DMF (9 mL, 2:1) was reacted as described in General Procedure at 85 °C under for 18 hours. The mixture was diluted with ethyl acetate, washed (water and brine), dried (Na₂SO₄), and concentrated. The residue was purified by column chromatography (hexane/CH₂Cl₂, 1:1) to give a pink solid (7.2 mg, 41%); ¹H NMR (CDCl₃, 500 MHz) δ −1.52 (s, 2H), −1.09 (s, 2H), 1.70 (s, 6H), 1.73 (6H), 1.98 (s, 6H), 2.02 (s, 6H), 3.65 (d, J = 17.1 Hz, 2H), 3.76 (d, J = 17.1 Hz, 2H), 4.51 (s, 4H), 4.58 (s, 6H), 7.34 (t, J = 7.4 Hz, 2H), 7.44 (t, J = 7.7 Hz, 4H), 7.62 (t, J = 7.4 Hz, 2H), 7.78–7.81 (m, 6H), 7.85 (d, J = 7.3 Hz, 4H), 8.30 (d, J = 7.3 Hz, 4H), 8.91 (s, 2H), 8.93 (s, 2H), 9.08 (d, J = 2.0 Hz, 2H). ¹³C NMR (CDCl₃, 125 MHz) δ 29.9, 30.5, 31.1, 31.2, 31.6, 45.6, 46.2, 47.7, 52.2, 65.4, 96.1, 96.7, 112.3, 116.8, 121.8, 127.3, 127.4, 128.8, 129.2 130.6, 131.1, 131.4, 133.5, 134.2, 134.8, 135.4, 136.4, 136.9, 137.1, 137.8, 154.0, 162.1, 169.8. HRMS ([M+H]⁺, M = C₇₄H₇₀N₈O₂): Calcd: 1103.5695, Obsd: (HRMS, ESI) 1103.5676.

BC-BC1

A solution of BC1-Br (7.6 mg, 12 μmol), BC-B (6.3 mg, 12 μmol), Cs₂CO₃ (39.1 mg, 120 μmol), and Pd(PPh₃)₄ (4.2 mg, 3.6 μmol) in toluene/DMF (6 mL, 2:1) was reacted as described in General Procedure at 85 °C under N₂ for 18 hours. The mixture was diluted with ethyl acetate, washed (water and brine), dried (Na₂SO₄), and concentrated. The residue was purified by column chromatography (hexane/CH₂Cl₂, 1:1) to give a pink solid (5.5 mg, 48%); ¹H NMR (CDCl₃, 400 MHz) δ −1.92 (s, 1H), −1.52 (s, 1H), −1.46 (s, 1H), −1.10 (s, 1H), 1.67 (s, 3H), 1.69 (s, 3H), 1.74 (s, 3H), 1.75 (s, 3H), 1.99 (s, 3H), 2.01 (s, 3H), 2.03 (s, 3H), 2.04 (s, 3H), 3.57–3.75 (m, 4H), 4.50 (s, 4H), 4.56 (s, 3H), 4.58 (s, 3H), 7.29–7.34 (m, 1H), 7.41–7.44 (m, 2H), 7.59–7.63 (m, 1H), 7.72 (d, J = 2.1 Hz, 1H), 7.77–7.83 (m, 5H), 8.29–8.31 (m, 2H), 8.53 (dd, J = 1.9, 4.6 Hz, 1H), 8.75–8.78 (m, 3H), 8.90 (s, 1H), 8.92 (s, 1H), 9.02 (dd, J = 1.9, 4.4 Hz, 1H), 9.07 (d, J = 2.2 Hz, 1H). Due to the low solubility, no satisfactory ¹³C NMR spectra have been obtained. MS ([M]⁺, M = C₆₂H₆₂N₈O₂): Calcd: 950.4990, Obsd: (HRMS, ESI) 950.4992.

Results and Discussion

Synthesis

Preparation of chlorin dyads relies on a PIFA-mediated dimerization of the ZnC monomer^10,12 and subsequent demetalation of the resulting 2ZnC leading to the free-base 2C.¹² Non-symmetrical metal complexes were prepared by a statistical metalation of 2C with corresponding metal acetates, in CHCl₃/MeOH (Scheme 1) which affords mono-metalated ZnC-C, PdC-C, and CuC-C in moderate yield (42%, 32%, and 57% yield, respectively). Metalation, even when conducted with the 1 equiv of metal salt, leads to the formation of the mixture of mono- and di-metalated dyads, which can be separated by a column chromatography, however, formation of di-metalated product significantly diminished the yield of the target mono-metalated dyads. Synthesis of 2PdC was accomplished by refluxing of the 2C with an excess of Pd(OAc)₂ in 90% yield, while 2CuC was isolated as a side product in synthesis of CuC-C in 24% yield.

Scheme 1.

Metallation of 2C.

For synthesis of chlorin-bacteriochlorin and bacteriochlorin-bacteriochlorin dyads we applied a strategy employed previously for synthesis of analogous bacteriochlorin dyads, which entails Suzuki reaction as a key step.^4,11,12,51 The required chlorins and bacteriochlorin building blocks, were either known compounds (i.e. C-Br³¹ and BC-B¹²), or were prepared analogously to those reported previously (Schemes 2–4). Thus, BC1 was prepared through TMSOTf-mediated self-condensation⁵² of dihydrodipyrrin 1, (the latter was prepared following the route reported previously for analogous dihydrodipyrrins, see Scheme S1). Bromination of BC1 by NBS⁵² afforded BC1-Br in 94% yield (Scheme 2). Subsequent Suzuki reaction of BC1-Br with BC-B¹² afforded dyad BC-BC1 in 48% yield. Similarly, Suzuki reaction of C-Br³¹ with BC-B afforded C-BC in 36% yield (Scheme 3). Metalation of the C-BC with Pd(OAc)₂ provides selectively PdC-BC in 40% (Scheme 3). No bacteriochlorin metallation was observed under this condition, which is consistent with the previous report that metallation of synthetic bacteriochlorins required harsher condition (i.e. strong base) than metallation of chlorins.²⁹

Scheme 2.

Synthesis of BC-BC1.

Scheme 4.

Synthesis of symmetrical 2BC1 dyad.

Scheme 3.

Synthesis of chlorin-bacteriochlorin dyads C-BC and PdC-BC.

Symmetrical 2BC1 dyad was prepared in Suzuki reaction of BC1-B (prepared in Miyaura reaction of BC1-Br with 2, in 88% yield) with BC1-Br in 41% yield (Scheme 4). Note, that attempted synthesis of symmetrical bacteriochlorin dyads by PIFA-mediated dimerization of corresponding monomers (either free base or Zn(II) complex) were unsuccessful. Benchmark monomer BC1-Tol was prepared in Suzuki reaction of BC1-Br with corresponding boronic ester in 91% yield (Scheme 5), while PdC were prepared by the metallation of H₂C³⁰ with Pd(OAc)₂ in 89% yield (Scheme 6).

Scheme 5.

Synthesis of benchmark monomer BC1-Tol.

Scheme 6.

Synthesis of PdC

Characterization

Resulting dyads were characterized by ¹H_, ¹³C NMR and HRMS. The data are consistent with the proposed structures. In particular, each ¹H NMR spectra of non-symmetrical dyads consist of resonances of protons from two different hydroporphyrins. Resonances of protons at the 17-postions of each hydroporphyrin in each dyad are significantly shifted upfield (~1 ppm) compared to monomers (Figure S1). This effect is consistent with the orthogonal mutual arrangement of the macrocycles, which places the protons at 17 position within a shielding cone of neighboring macrocycle. Moreover, each dyad (symmetrical and non-symmetrical) is axially chiral, which is manifested by a differentiation of proton shifts of the respective diastereotopic protons, observed in ¹H NMR (specifically H¹⁷ and these of geminal CH₃ groups, see Figure S1). The DFT calculations confirm a nearly perpendicular arrangement of macrocycles in dyads (Figure S2), with the dihedral angles between macrocycles 180° ± 2°. The orthogonal geometry of these dyads is relatively stable, as the temperature-dependent ¹H NMR spectra for 2C and 2BC in CDCl₃ do not show noticeable changes in the chemical shift of protons up to 50 °C. Moreover, DFT calculations suggest that the energy barrier for rotation around C¹⁵-C¹⁵ single bond is at least 150 kJ/mol (Table S1).

Absorption and emission properties

Absorption spectra of dyads are presented in Figures 1–2, and summarized in Table 1 (spectra of corresponding monomers are presented in Figures S3–S4, SI). The spectrum of each symmetrical dyad shows features characteristic of given hydropoprhyrins,^18,28 with the maximum of long-wavelength Q_y band shifted bathochromically, compared to both corresponding monomers and 15-aryl-substituted monomers (Table 1. Note, that formally, use of Q_y, B, etc. descriptions for absorption bands in dyads is incorrect, however, for simplicity, we use this notation to describe absorption spectra of both monomers and dyads). Metal complexation of the dyads causes a significant hypsochromic shift of the Q_y band [by 27 nm, 46 nm, and 30 nm, for Zn(II), Pd(II), and Cu(II), respectively], which is similar to those observed for monomers.^28,33 Note that for 2PdC, 2CuC, 2ZnC, and 2C, the Q_y band shows a slight shoulder on the short wavelength side, whereas for 2BC and 2BC1, a new distinctive band appears at the shorter wavelength to the Q_y band. This splitting is probably due to the exciton coupling between macrocycles.⁵⁶ For non-symmetrical chlorin-chlorin and chlorin-bacteriochlorin dyads, absorption spectra show two distinctive Q_y bands, for which the maxima are slightly shifted hypsochromically, compared to that of symmetrical dyads (e.g. compare 2ZnC with ZnC-C, 2BC with C-BC, and 2BC1 with BC-BC1). The metal complexation has a negligible effect on the absorption wavelength of the neighboring free-base hydroporphyrin component (e.g. compare absorption of the free base chlorin in ZnC-C with that of PdC-C and C-BC with that in ZnC-BC). Chlorin-bacteriochlorin dyads also exhibit distinctive, clearly resolved the B bands (i.e. shortest wavelength bands), whereas for chlorin-chlorin dyads B band of individual dyads components overlap.

Figure 1.

Absorption spectra of symmetrical dyads in toluene: 2PdC (black), 2ZnC (blue), 2CuC (orange), 2C (red), 2BC (green), and 2BC1 (purple). All spectra were taken at the room temperature and normalized at the maximum of their B bands.

Figure 2.

Absorption spectra of non-symmetrical dyads in toluene: PdC-C (black), ZnC-C (blue), CuC-C (orange), C-BC (red), PdC-BC (green), and BC-BC1 (purple). All spectra were taken at the room temperature and normalized at the maxima of their B bands.

Table 1.

Absorption and photochemical properties of hydroporphyrin dyads and benchmark monomers.

Compound	λabsa	λema	Φf (toluene)b	Φf (DMF)	ETEc	ΦΔd
2Ce	421, 508, 650	654	0.31	0.29	-	0.53
2ZnC	408, 419, 623	628	0.15	0.032	-	0.48
2PdC	408, 604	-	< 0.005	< 0.005	-	1.00
2CuC	415, 620	-	< 0.005	< 0.005	-	< 0.03
2BCe	372, 510, 706, 731	738	0.28	0.003	-	0.65
2BC1	386, 524, 729, 754	760	0.27	0.013	-
ZnC-C	408, 419, 506, 613, 646	646	0.31	<0.005f	> 0.95g	0.54
PdC-C	417, 596, 646	645	0.05	0.04	> 0.95g	0.90
CuC-C	419, 506, 610, 646	-	< 0.005	< 0.005	-	0.86
C-BC	366, 399, 512, 642, 719	719	0.24	<0.005	0.99	0.65
PdC-BC	366, 398, 510, 595, 718	718	0.11	<0.005	0.86	0.74
BC-BC1	381, 520, 713, 746	754	0.29	0.007	-	0.62
H2Ch	393,399, 498, 636	636	0.22	0.22	-	0.44
ZnCh	403, 604	605	0.08	0.08	-	-
PdC	396,588	589	< 0.005	< 0.005	-	-
BC1	372, 510, 731	737	0.21	0.20	-	-
BC-Tol	369, 508, 712	715	0.22	0.20	-	-
BC1-Tol	375, 518, 734	736	0.24	0.21	-	-
H2C-Phi	411, 640	-	-	-	-	-

^aAll data taken in toluene at the room temperature.

^bFluorescence quantum yield Φ_f was determined in air-equilibrated solvents upon excitation at the maximum of the B band of corresponding chlorin. Φ_f was determined using tetraphenylporphyrin in air-equilibrated toluene (Φ_f = 0.070⁵³) as a reference. Estimated error ± 5%. A long bandpass filter (with cut off 600 nm) was used for each Φ_f determination to eliminate subharmonic band from excitation.

^cEnergy transfer efficiency ETE (all data in toluene) was determined as a ratio of Φ_f of the component with longer emission wavelength (energy acceptor) upon excitation at the chlorin B band to the same Φ_f obtained upon excitation at the maximum of the bacteriochlorin B band (unless noted otherwise).

^dQuantum yield of ¹O₂ photosensitization Φ_Δ was determined by comparing the ¹O₂ luminescence intensity (λ_em = 1270 nm) to that generated by tetraphenylporphyrin in air-equilibrated toluene (Φ_Δ = 0.67⁵⁴). Samples were excited at the maximum of the B band. Estimated error ± 10%.

^eData taken from ref. 12).

^fΦ_f is approximate, due to the very weak emission and presence of an additional weak peak at 660 nm of unknown origin (most likely a trace amount of 2C impurity).

^gETE was determined by comparing absorption and fluorescence excitation spectra, monitored at the wavelength where energy acceptor emits exclusively.

^hAll data (except Φ_Δ) for H₂C,^28,55 and ZnC,^28,55 were reported previously. For consistency, all data for these compounds reported here were re-determined in our laboratory, and are consistent with those reported previously.

ⁱData taken from ref. 31.

The absorption spectrum of BC-BC1 is different from those of other non-symmetrical hydroporphyin dyads reported here, since it consists of a strong band at 746 nm, and a much weaker shoulder at 713 nm, and no distinguishable Q_y bands from individual bacteriochlorins. This pattern resembles those observed for symmetrical 2BC1 and 2BC.

Emission spectra of symmetrical dyads 2C, 2ZnC, 2BC and 2BC1, as well as BC-BC1 in toluene (Figure 3, Table 1; for spectra of corresponding monomers see Figures S5 and S6) show a strong 0–0 band with a small Stokes shift (0 – 5 nm), similar to the analogous monomers.⁵⁷ Emission spectra of non-symmetrical dyads ZnC-C, PdC-C, C-BC, and PdC-BC predominantly show one strong band, with the wavelength corresponding to emission of the longer-wavelength absorbing component, regardless of the excitation wavelength. There is a second, very weak emission band at the shorter wavelength, corresponding to the short-wavelength absorbing component. Excitation spectra of each dyad, monitored at the main emission peak, closely resemble the absorption spectra. These results indicate a very efficient energy transfer from the shorter to the longer-wavelength absorbing hydroporphyrin components. Comparison of excitation and absorption spectra allowed estimation of the energy transfer efficiency (ETE) as > 0.95 for most of the dyads, except for PdC-BC, for which ETE of 0.86 was determined. It is interesting to note, that high ETE is observed even from the Pd(II)-coordinated chlorin, despite very fast intersystem crossing (ISC) in the monomer PdC, as well as 2PdC (which is manifested by a significant reduction of the fluorescence quantum yield Φ_f, see below). Apparently, S₁-S₁ energy transfer from the Pd(II) chlorin to the directly linked hydroporphyrin is faster than ISC (S₁-S₁, rather than T₁-T₁ as a dominant ET mechanism, is indicated by the similarity of Φ_f of the free base component regardless of the excitation wavelength). 2PdC shows a very weak fluorescence, while both 2CuC and CuC-C are essentially non-fluorescent, regardless of excitation wavelength.

Figure 3.

Emission spectra of symmetrical dyads in toluene: 2ZnC (blue), 2C (red), BC (purple), 2BC (green), and 2BC1 (purple). All spectra were taken at room temperature and normalized at the maxima of their emission bands.

Φ_f for symmetrical chlorin and bacteriochlorin dyads in toluene are markedly higher (1.2–1.9-fold) than for the corresponding monomers (Table 1). For non-symmetrical dyads ZnC-C, and BC-BC1, Φ_f in toluene is similar to that determined for the respective symmetrical dyads (2C, and 2BC1), while Φ_f for C-BC is more similar to monomer BC. Note, that insertion of Zn(II) in chlorin does not affect Φ_f of non-symmetrical dyads. In contrast, Φ_f for PdC-C and PdC-BC is significantly reduced compared to the corresponding free base analogs (6.2-fold and 2.5-fold, respectively). This Φ_f reduction is most likely due to the “remote” heavy atom effect, i.e. enhancement of ISC of the free base hydroporphyrin component by Pd(II) complexation of the neighboring macrocycle. Similar reduction of Φ_f, attributed to enhanced ISC, has been reported previously for the chlorin-bispyridine-Ru(II) dyad.⁴⁸ We cannot however rule out other processes, such as photoinduced electron transfer, being responsible for reduction of Φ_f in PdC-C and PdC-BC. More detailed spectroscopic studies are required to clarify this issue.

Effect of solvent dielectric constant on dyads fluorescence

Φ_f for non-symmetrical dyads ZnC-C, C-BC, PdC-C, and BC-BC1 as well as for symmetrical bacteriochlorin dyads 2BC and 2BC1, exhibit a dramatic reduction (more than 40 times) in DMF compared to that observed in toluene (Table 1; for Φ_f of ZnC-C and BC-BC1 in range of solvents of different ε, see Table 2). Reduction of Φ_f in DMF is also observed, albeit to a lesser degree for 2ZnC (4.7 fold) and PdC-C (1.25 fold). Φ_f for 2C and all examined monomers (chlorins and bacteriochlorins) are nearly the same in DMF and toluene. Observed reduction of Φ_f in polar solvents is consistent with the PET, which may occur in polar solvents (such as DMF). Solvents of high ε stabilizes charge-separated state, formed upon electron transfer and thus make PET energetically possible (in solvents of low ε energy of charge-separated state is higher than energy of S₁).²⁷ It is know that bacteriochlorins are easier to oxidize and harder to reduce than analogous chlorins,²³ and similarly Zn(II) chlorins are significantly easier to oxidize and slightly harder to reduce than chlorin free bases.²⁸ Therefore, it is reasonable to assume that in ZnC-C, the metalated chlorin is an electron donor, while the free base chlorin is an electron acceptor. Similarly, in C-BC the bacteriochlorin component is an electron donor. On the other hand, Pd(II) chelates of chlorins are only slightly easier to oxidize and slightly harder to reduce than corresponding free bases.^10,58 Thus, the observed reduction of Φ_f in DMF for PdC-C is much less pronounced (1.25 fold, compared to toluene) than that for ZnC-C. For BC1-BC the direction of electron transfer is less obvious, however a inspection of available redox data for monomers, analogous to that composing BC-BC1, lead to conclusion that photoexcited 2,12-diphenylbacteriochlorin component is an electron donor.^29,59 Further detailed studies are required to fully understand PET processes occurring in all dyads reported here.

Table 2.

Φ_f for ZnC-C and BC-BC1 in solvents of different dielectric constants.

Dyad	Solvent	Φf
ZnC-C	Toluene	0.31
	THF	0.061
	CH2Cl2	0.17
	PhCN	~ 0.006
	DMF	< 0.005
BC-BC1	Toluene	0.29
	THF	0.14
	CH2Cl2	0.028
	DMF	~ 0.007

Note, that for ZnC-C Φ_f in THF (ε = 7.58) is much lower than in CH₂Cl₂ (ε = 8.93). We speculate, that coordination of THF to Zn(II) alters significantly redox properties of ZnC component of dyad and facilitates PET.

Effect of metalation on the singlet oxygen (¹O₂) photosensitization

The relative quantum yield of ¹O₂ photosensitization was determined by comparing ¹O₂ luminescence intensity at 1270 nm generated by dyads, to that observed for tetraphenylporphyrin (Φ_Δ = 0.67⁵⁴). Φ_Δ determined for ZnC-C and C-BC (0.56 and 0.65, respectively) correspond well with those reported previously for analogous chlorins⁶⁰ and bacteriochlorins.¹² Insertion of Pd(II) into the macrocycle enhances Φ_Δ significantly for 2PdC (up to 1.00), and PdC-C (1.7-fold compared to 2C), but only slightly for PdC-BC (1.1-fold compared to C-BC). Direct excitation at the longest absorption band indicates that ¹O₂ photosensitization efficiency of the free base component in PdC-C increases 1.5-fold (compared to 2C), and 1.2-fold for the BC component in PdC-BC (compared to C-BC). Combining both fluorescence data and ¹O₂ photosensitization experiments together, it is reasonable to assume, that excitation of the PdC component in PdC-C and PdC-BC leads to the (predominantly) S₁-S₁ energy transfer to the free base chlorin or bacteriochlorin ¹O₂ photosensitization mainly occurs through ISC in the dyads free base component, where ISC is enhanced due to the “remote” heavy atom effect of Pd in the neighboring macrocycle (enhanced ISC in dyad free base hydroporphyrin components is inferred by both substantial reduction of Φ_f and enhanced Φ_Δ). Note that the extent of Φ_f reduction (5-fold for PdC-C vs. ZnC-C and 2.2-fold for PdC-BC vs. C-BC) is much larger than the respective increase in Φ_Δ (1.7-fold for PdC-C and 1.1 for PdC-BC). However, increase in ISC quantum yield does not always result in the same increase in Φ_Δ, as the latter depends on both yield and lifetime of T₁,⁶¹ and enhanced ISC is usually accompanied by a decrease in T₁ lifetime.^29,36 In polar solvent (DMF) Φ_Δ is reduced to ~0 for ZnC-C and C-BC, while for PdC-C and PdC-BC ¹O₂ emission could still be seen in DMF, though we did not attempt to quantify Φ_Δ in DMF.

For 2CuC excitation in toluene leads to a negligible ¹O₂ production, while for CuC-C, efficient ¹O₂ photosensitization is observed (Φ_Δ = 0.86; direct excitation of free base component shows 1.4-fold increase in ¹O₂ photosensitization, compared to 2C). It has been previously reported that excitation of the Cu(II) complexes of porphyrins and bacteriochlorins leads to the ultrafast formation of tripdoublet ²T or tripquartet ⁴T states, which decay to the ground state within nanosecond time range.^29,62 Due to this short-lived triplet state, Cu(II) complexes of tetrapyrrolic macrocycles usually show very low Φ_Δ.⁵⁴ On the other hand, in non-symmetrical porphyrin dyads containing Cu(II) and free-base porphyrins, enhanced ISC in the free base component has been observed due to the through-bond exchange electron interaction.^42,43 This latter mechanism most likely operates for CuC-C.

Electronic properties

The absorption and emission properties indicate substantial electronic interactions between macrocycles in both symmetrical and non-symmetrical directly-linked dyads. This interaction is manifested by the bathochromic shift of the absorption Q_y and emission bands of dyad components, and increase in dyad Φ_f, compared to monomers (both unsubstituted and 15-arylsubstituted). In the non-symmetrical chlorin-chlorin and chlorin-bacteriochlorin, each dyad component maintains its individual identity, as indicated by the presence of clearly distinctive Q_y bands for each hydroporphyrin. In case of BC-BC1, interactions between macrocycles leads to significant alteration of the absorption spectrum, which no longer preserves the features of individual chromophores. Based on the above consideration we concluded, that the strength of interchromophoric interactions decreases in the following order: symmetrical bacteriochlorin-bacteriochlorin ~ symmetrical chlorin-chlorin > non-symmetrical bacteriochlorin-bacteriochlorin > non-symmetrical chlorin-chlorin > chlorin-bacteriochlorin. This trend is additionally supported by the degree to which Pd(II) coordination reduces Φ_f and increases Φ_Δ in the adjacent macrocycle (i.e. PdC-C > PdC-BC).

Further insight into electronic structure and the interchromophoric interaction in dyads has been obtained from DFT calculations. Here, only a basic discussion is presented; detailed computational characterization and comprehensive discussion of results will be presented in due publication. Four MOs are considered (HOMO-1, HOMO, LUMO, and LUMO+1), since according to the Gouterman theory the configuration interactions of electronic transitions between these MOs (i.e. HOMO → LUMO, HOMO → LUMO+1, etc.), give rise to the absorption features of tetrapyrrolic macrocycles.⁶³ The energies and shapes of the relevant MOs (HOMO-1, HOMO, LUMO, and LUMO+1) for representative symmetrical and non-symmetrical dyads and monomers are given in Table 3. For symmetrical 2ZnC and 2BC1, linear combination of MOs of monomers results in a new set of MO, fully delocalized on the entire dyad. Thus, for 2ZnC combination of HOMO of both monomers results in new MOs (now formally HOMO and HOMO-1 of dyad) with an energy difference ΔE = 0.07 eV. Similarly, combinations of LUMO of both ZnC components leads to LUMO and LUMO+1 of 2ZnC (ΔE = 0.15 eV), combination of HOMO-1 of ZnC gives HOMO-2 and HOMO-3 of 2ZnC (ΔE = 0.06 eV), and combination of LUMO+1 of both monomers gives LUMO+2 and LUMO+3 of dyad (ΔE = 0.04 eV). The identical pattern of splitting is observed for 2BC1, where ΔE(HOMO) = 0.06 eV, ΔE(LUMO) = 0.11 eV, ΔE(HOMO-1) = 0.03, and ΔE(LUMO+1) = 0.01 eV. Similar combination of MOs of monomer has been observed in strongly conjugated hydropoprhyrin dyads.²²

Table 3.

DFT calculated FMOs for representative dyads and benchmark monomers.^a

^aAll calculations was conducted employing the DFT B3LYP 6–31G* method. For symmetrical dyads as “HOMO”, “HOMO-1”, etc., two orbitals, formed from linear combination of respective MOs of corresponding monomers are listed. For non-symmetrical dyads, pairs of MOs, corresponding to respective MOs localized on each individual macrocycle, are listed (e.g. as a “HOMO” for ZnC-C, a MOs with the highest energy localized on free base and Zn complex, are listed, etc.).

^bEnergies for H₂C,²⁶ ZnC,²⁶ and BC¹⁶ were reported previously. Here, for consistency, we repeated this computation, obtaining identical results as reported.

For ZnC-C and C-BC, MOs are localized nearly entirely on the single macrocycle; thus HOMO is localized on Zn chelate and bacteriochlorin components of the ZnC-C and C-BC, respectively. Shapes and energies of these MOs resemble that of corresponding HOMO of ZnC and BC benchmarks, respectively (both MOs are stabilized by 0.04 eV for C-BC and 0.06 eV for ZnC-BC, compared to the benchmark monomers). Similarly, formal HOMO-1 for ZnC-C and C-BC are localized at the chlorin moiety, and their distribution and energies are similar to HOMO in H₂C. Conversely, LUMOs for ZnC-C and C-BC maintain LUMO character of H₂C and BC, respectively (energies of LUMO in dyads are lower by 0.09 eV for C-BC and 0.11 eV for C-BC, compared to their counterparts in monomers). Similarly, HOMO-1 and HOMO-2 as well as LUMO+1 and LUMO+2 in ZnC-C and C-BC essentially possess a characteristic of the relevant orbitals in monomers. This situation is different in BC-BC1, where both HOMO and HOMO-1 resulted from the linear combination of HOMOs of the monomers. Both MOs are delocalized over the entire molecule, and the difference of energy difference between them ΔE = 0.07 eV. The same is true for HOMO-2 and HOMO-3 of BC-BC1, which resulted from a linear combination of HOMO-1 of monomers (ΔE = 0.04 eV). In this regard, BC-BC1 resemble more symmetrical dyads. However, LUMO and LUMO+1 (as well LUMO+2 and LUMO+3) are mostly localized on the one macrocyle, and maintain characteristics of relevant MOs in monomers (i.e. LUMO of BC-BC1 resembles LOMO of BC1, LUMO+1 resembles LUMO of BC, etc.).

The DFT calculations thus support our conclusion regarding the strength of the electronic interactions between hydroporphyrins, drawn from the spectroscopic data. For C-BC and ZnC-C individual macrocycles preserve to a large extent, their electronic structures characteristic for monomers, while for BC-BC1, new delocalized MOs resulted from a linear combination of HOMOs (as well as HOMO-1) are present. This difference in electronic structure (as well as optical properties) of BC-BC1, compared to other non-symmetrical dyads, is most likely a result of (coincidentally) the same energies of HOMOs (and nearly the same of HOMO-1) of the macrocycles constituting BC-BC1, whereas there is substantial difference in MOs energies in hydroporphyrins of which other dyads are composed (as well as between LUMO and LUMO+1 of BC and BC1). Therefore, BC-BC1 behaves more closely to analogous symmetrical dyads, despite the distinctive optical properties of its constituents. This conclusion emphasizes the importance of the monomers electronic structure to the properties of the resulting dyads. MOs energies in bacteriochlorins (as well as chlorins) can be broadly tuned by the peripheral substituents, and as such, the proper choice of monomers should allow for construction of directly-linked bacteriochlorin-bacteriochlorin dyads, where each macrocycle retain its individual characteristics, as in case of ZnC-C and (Pd)C-BC.

Conclusion

A range of directly-linked hydroporphyrin-hydroporphyrin dyads can be synthesized using either PIFA-mediated oxidative coupling (for chlorin-chlorin) or the Suzuki reaction (for chlorin-bacteriochlorin and bacteriochlorin-bacteriochlorin dyads), and subsequent symmetrical or non-symmetrical metalation of the resulting dyads. Hydroporphyrin dyads feature a range of interesting properties, depending on the electronic structure of constituent monomers. Symmetrical (or nearly symmetrical) arrays exhibit a marked bathochromic shift of the long-wavelength absorption bands, and enhanced Φ_f. In non-symmetrical dyads, each macrocycle maintains its individual properties, and a very efficient S₁-S₁ energy transfer is observed, even for dyads in which energy donor contains a heavy element (Pd). The influence of heavy elements (Pd(II) or Cu(II) on the properties of the adjacent macrocycle, manifested by reduction of Φ_f and increase in Φ_Δ, has been clearly observed. Moreover, the photochemical properties of most of the dyads exhibit a strong dependence of their photochemical properties on the solvent dielectric constant, which is attributed to the PET. Overall, directly linked hydroporphyrin arrays constitute a useful platform for development of photonic agents for a variety of biomedical and energy-related applications. The dyads reported here are also useful tools for fundamental studies on energy and electron transfers in electronically coupled systems, the “remote” heavy atom effect, metal-metal interactions, etc.

Figure 4.

Emission spectra of non-symmetrical dyads in toluene: PdC-C (black), ZnC-C (blue), C-BC (red), PdC-BC (green), and BC-BC1 (purple). All spectra were taken at room temperature and normalized at the maxima of their emission bands.

Synopsis.

Symmetrical and non-symmetrical meso-meso linked chlorin-chlorin, chlorin bacteriochlorin and bacteriochlorin-bacteriochlorin dyads, (free base and Zn(II), Pd(II) or Cu(II) complexes) are reported. For non-symmetrical dyads an efficient singlet-singlet energy transfer is observed. Insertion of heavy element [Pd(II) or Cu(II)] significantly enhances ¹O₂ photosensitization. Most of the dyads exhibit a significant reduction of the fluorescence in highly polar solvents, suggesting a charge transfer character of the excited state.