Article Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
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Symmetrical and Nonsymmetrical Meso−Meso Directly Linked Hydroporphyrin Dyads: Synthesis and Photochemical Properties Nopondo N. Esemoto, Andrius Satraitis, Linda Wiratan, and Marcin Ptaszek* Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, United States S Supporting Information *
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 shifts of the long-wavelength Qy absorption band and increase in fluorescence quantum yields in dyads, in comparison to the corresponding monomers, are observed. Nonsymmetrical dyads (except bacteriochlorin−bacteriochlorin) show two distinctive Qy 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, has been determined. Several symmetrical and all nonsymmetrical dyads exhibit a significant reduction in 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.
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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 antennas.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 porphyrins23 and are therefore more prone to 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 a set of properties distinct from those observed in analogous porphyrin arrays. For example, we recently reported a series of symmetrical meso− meso directly linked12 and strongly conjugated26,25 hydroporphyrin dyads. For the meso−meso directly linked bacteriochlorin dyad, the fluorescence quantum yield Φf and lifetime τf, as well as singlet oxygen 1O2 quantum yield ΦΔ, are all progressively reduced when the solvent dielectric constant (ε) increases, resulting in nearly complete quenching of fluorescence and 1O2 photosensitization in solvents of high ε.12 This quenching of photochemical activity in the bacteriochlorin dyad is much more extensive than in corresponding chlorin12 and porphyrin dyads.24 Although
INTRODUCTION Arrays of tetrapyrrolic macrocycles directly connected between their meso positions represent an interesting class of molecules, since due to the orthogonal arrangement of macrocycles there is 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 the electronic, optical, and electrochemical properties of the resulting constructs.1−14 Meso− meso directly linked porphyrin,1−9 chlorin,10−12 bacteriochlorin,12 and corrole13,14 dyads have been reported, and their spectral and electronic properties, as well as energy- and electron-transfer processes between array components, have been examined. The resulting dyads have been applied, for example, as biomimetic models for photosynthetic light harvesting2−4 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 Hydroporphyrinschlorins and bacteriochlorinsare 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 © XXXX American Chemical Society
Received: August 25, 2017
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DOI: 10.1021/acs.inorgchem.7b02200 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Chart 1. Structures of Dyads and Monomers Discussed in This Papera
a
Ms = 2,4,6-trimethylphenyl.
of substituents on the macrocycle perimeter. As benchmarks we included the monomers ZnC,28,30 PdC, H2C,28,30 H2C-Ph,31 BC,18 BC1, BC-Tol,12 and BC1-Tol. This set of compounds has enabled us to determine several key aspects of the photophysics of directly linked hydroporphyrin dyads. First, we sought to determine how metalation and electronic asymmetry between dyad components affects the basic absorption and emission properties of dyads and how efficient energy transfer is between dyad 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-nonequivalent hydropoprhyrin components. Finally, we intended to determine how heavyelement complexation (i.e., Pd(II) or Cu(II)) by one hydroporphyrin in the 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 the ISC rate and quantum yield and consequently the quantum yield of ROS photosensitization.32 However, insertion of Pd(II) and Pt(II) into chlorins causes a substantial hypsochromic shift of the Qy band, placing it out of the “therapeutic window” and thus rendering them potentially less efficient for deep tissue PDT33−35 (note, that complexation of bacteriochlorins with Pd(II) causes a bathochromic shift of the Qy band29). In addition, metal complexation alters the 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
details of excited state dynamics for these dyads are still being investigated, these observations indicate that directly linked hydroprorphyrin arrays have 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 nonsymmetrical arrays, i.e. arrays composed of two hydroporphyrins with distinctive electronic, optical, and redox properties. To the best of our knowledge, the only nonsymmetrical directly linked hydroporphyrin dyads have been reported only recently by Borbas et al.11 (nonsymmetrical meso-β directly linked porphyrin− chlorin dyads were also reported27). Here, we report the synthesis and characterization of a series of symmetrical and nonsymmetrical chlorin−chlorin, chlorin− bacteriochlorin, and bacteriochlorin−bacteriochlorin dyads (Chart 1). Symmetrical dyads include free base chlorin− chlorin 2C12 as well as bacteriochlorin−bacteriochlorin species 2BC12 and 2BC1 and chlorin−chlorin metal chelates 2ZnC,12 2PdC, and 2CuC (note that 2C and 2BC have been previously reported, and they are included here for comparison). Nonsymmetrical 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 the 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, the bacteriochorin−bacteriochlorin dyad BC-BC1 is composed of two bacteriochlorin free bases with different sets B
DOI: 10.1021/acs.inorgchem.7b02200 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry the T1 lifetime (to
