Chinnasamy Muthiah

Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 2: Redox Properties, Optical Spectra and Electronic Structure

The optical absorption spectra and redox properties are presented for 24 synthetic zinc chlorins and 18 free base analogs bearing a variety of 3,13 (β) and 5,10,15 (meso) substituents. Results are also given for a zinc and free base oxophorbine, which contain the keto‐bearing isocyclic ring present in the natural photosynthetic pigments such as chlorophyll a. Density functional theory calculations were carried out to probe the effects of the types and positions of substituents on the characteristics (energies, electron distributions) of the frontier molecular orbitals. A general finding is that the 3,13 positions are more sensitive to the effects of auxochromes than the 5,10,15 positions. The auxochromes investigated (acetyl > ethynyl > vinyl > aryl) cause a significant redshift and intensification of the Qy band upon placement at the 3,13 positions, whereas groups at the 5,10,15 positions result in much smaller redshifts that are accompanied by a decrease in relative Qy intensity. In addition, the substituent‐induced shifts in first oxidation and reduction potentials faithfully track the energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), respectively. The calculations show that the LUMO is shifted more by substituents than the HOMO, which derives from the differences in the electron densities of the two orbitals at the substituent sites. The trends in the substituent‐induced effects on the wavelengths and relative intensities of the major features (By, Bx, Qx, Qy) in the near‐UV to near‐IR absorption bands are well accounted for using Gouterman’s four‐orbital model, which incorporates the effects of the substituents on the HOMO−1 and LUMO+1 in addition to the HOMO and LUMO. Collectively, the results and analysis presented herein and in the companion paper provide insights into the effects of substituents on the optical absorption, redox and other photophysical properties of the chlorins. These insights form a framework that underpins the rational design of chlorins for applications encompassing photomedicine and solar‐energy conversion.

Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 1: Synthesis, Vibrational Properties and Excited-state Decay Characteristics

Understanding the effects of substituents on the spectra of chlorins is essential for a wide variety of applications. Recent developments in synthetic methodology have made possible systematic studies of the properties of the chlorin macrocycle as a function of diverse types and patterns of substituents. In this paper, the spectral, vibrational and excited‐state decay characteristics are examined for a set of synthetic chlorins. The chlorins bear substituents at the 5,10,15 (meso) positions or the 3,13 (β) positions (plus 10‐mesityl in a series of compounds) and include 24 zinc chlorins, 18 free base (Fb) analogs and one Fb or zinc oxophorbine. The oxophorbine contains the keto‐bearing isocyclic ring present in the natural photosynthetic pigments (e.g. chlorophyll a). The substituents cause no significant perturbation to the structure of the chlorin macrocycle, as evidenced by the vibrational properties investigated using resonance Raman spectroscopy. In contrast, the fluorescence properties are significantly altered due to the electronic effects of substituents. For example, the fluorescence wavelength maximum, quantum yield and lifetime for a zinc chlorin bearing 3,13‐diacetyl and 10‐mesityl groups (662 nm, 0.28, 6.0 ns) differ substantially from those of the parent unsubstituted chlorin (602 nm, 0.062, 1.7 ns). Each of these properties of the lowest singlet excited state can be progressively stepped between these two extremes by incorporating different substituents. These perturbations are associated with significant changes in the rate constants of the decay pathways of the lowest excited singlet state. In this regard, the zinc chlorins with the red‐most fluorescence also have the greatest radiative decay rate constant and are expected to have the fastest nonradiative internal conversion to the ground state. Nonetheless, these complexes have the longest singlet excited‐state lifetime. The Fb chlorins bearing the same substituents exhibit similar fluorescence properties. Such combinations of factors render the chlorins suitable for a range of applications that require tunable coverage of the solar spectrum, long‐lived excited states and red‐region fluorescence.

Examination of Chlorin-Bacteriochlorin Energy-transfer Dyads as Prototypes for Near-infrared Molecular Imaging Probes †

New classes of synthetic chlorin and bacteriochlorin macrocycles are characterized by narrow spectral widths, tunable absorption and fluorescence features across the red and near‐infrared (NIR) regions, tunable excited‐state lifetimes (<1 to >10 ns) and chemical stability. Such properties make dyad constructs based on synthetic chlorin and bacteriochlorin units intriguing candidates for the development of NIR molecular imaging probes. In this study, two such dyads (FbC‐FbB and ZnC‐FbB) were investigated. The dyads contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. In both constructs, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of ∼(5 ps)−1 and a yield of >99%. Thus, each dyad effectively behaves as a single chromophore with an exceptionally large Stokes shift (85 nm for FbC‐FbB and 110 nm for ZnC‐FbB) between the red‐region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (λf = 760 nm, Φf = 0.19, τ ∼ 5.5 ns in toluene). The long‐wavelength transitions (absorption, emission) of each constituent of each dyad exhibit narrow (≤20 nm) spectral widths. The narrow spectral widths enabled excellent selectivity in excitation and detection of one chlorin–bacteriochlorin energy‐transfer dyad in the presence of the other upon diffuse optical tomography of solution‐phase phantoms.

De Novo Synthesis of Long-Wavelength Absorbing Chlorin-13,15-dicarboximides

Chlorins bearing a six-membered imide ring spanning positions 13-15, commonly referred to as purpurinimides, exhibit long-wavelength absorption yet have heretofore only been available via semisynthesis from naturally occurring chlorophylls. A concise route to synthetic chlorins, which bear a geminal dimethyl group in the pyrroline ring, has been extended to provide access to chlorin-13,15-dicarboximides. The new route entails (i) synthesis of a 13-bromochlorin, (ii) palladium-catalyzed carbamoylation at the 13-position, (iii) regioselective 15-bromination under acidic conditions, and (iv) one-flask palladium-mediated carbonylation and ring closure to form the imide. In some cases the ring closure reaction afforded the isomeric (and readily separable) chlorin-isoimide in addition to the chlorin-imide. The resulting chlorin-imides and chlorin-isoimides exhibit long-wavelength absorption (679-715 nm) and emission (683-720 nm) in the far-red and near-infrared spectral region. The absorption of the chlorin-(iso)imides fills the spectral window between that of analogous synthetic chlorins and 13(1)-oxophorbines (603-687 nm) and bacteriochlorins (707-792 nm). The synthetic versatility of the de novo route complements the existing semisynthetic route from chlorophylls and should enable fundamental spectroscopic studies and photochemical applications.

Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 3: The Distinctive Impact of Auxochromes at the 7- versus 3-Positions

Assessing the effects of substituents on the spectra of chlorophylls is essential for gaining a deep understanding of photosynthetic processes. Chlorophyll a and b differ solely in the nature of the 7‐substituent (methyl versus formyl), whereas chlorophyll a and d differ solely in the 3‐substituent (vinyl versus formyl), yet have distinct long‐wavelength absorption maxima: 665 (a) 646 (b) and 692 nm (d). Herein, the spectra, singlet excited‐state decay characteristics, and results from DFT calculations are examined for synthetic chlorins and 131‐oxophorbines that contain ethynyl, acetyl, formyl and other groups at the 3‐, 7‐ and/or 13‐positions. Substituent effects on the absorption spectra are well accounted for using Gouterman’s four‐orbital model. Key findings are that ( 1 ) the dramatic difference in auxochromic effects of a given substituent at the 7‐ versus3‐ or 13‐positions primarily derives from relative effects on the LUMO+1 and LUMO; (2) formyl at the 7‐ or 8‐position effectively “porphyrinizes” the chlorin and (3) the substituent effect increases in the order of vinyl < ethynyl < acetyl < formyl. Thus, the spectral properties are governed by an intricate interplay of electronic effects of substituents at particular sites on the four frontier MOs of the chlorin macrocycle.

Chlorin–Bacteriochlorin Energy-transfer Dyads as Prototypes for Near-infrared Molecular Imaging Probes: Controlling Charge-transfer and Fluorescence Properties in Polar Media

The photophysical properties of two energy‐transfer dyads that are potential candidates for near‐infrared (NIR) imaging probes are investigated as a function of solvent polarity. The dyads (FbC‐FbB and ZnC‐FbB) contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. The dyads were studied in toluene, chlorobenzene, 1,2‐dichlorobenzene, acetone, acetonitrile and dimethylsulfoxide (DMSO). In both dyads, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of ∼(5–10 ps)−1 and a yield of >99% in nonpolar and polar media. In toluene, the fluorescence yields (Φf = 0.19) and singlet excited‐state lifetimes (τ∼5.5 ns) are comparable to those of the benchmark bacteriochlorin. The fluorescence yield and excited‐state lifetime decrease as the solvent polarity increases, with quenching by intramolecular electron (or hole) transfer being greater for FbC‐FbB than for ZnC‐FbB in a given solvent. For example, the Φf and τ values for FbC‐FbB in acetone are 0.055 and 1.5 ns and in DMSO are 0.019 and 0.28 ns, whereas those for ZnC‐FbB in acetone are 0.12 and 4.5 ns and in DMSO are 0.072 and 2.4 ns. The difference in fluorescence properties of the two dyads in a given polar solvent is due to the relative energies of the lowest energy charge‐transfer states, as assessed by ground‐state redox potentials and supported by molecular‐orbital energies derived from density functional theory calculations. Controlling the extent of excited‐state quenching in polar media will allow the favorable photophysical properties of the chlorin–bacteriochlorin dyads to be exploited in vivo. These properties include very large Stokes shifts (85 nm for FbC‐FbB, 110 nm for ZnC‐FbB) between the red‐region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (λf = 760 nm), long bacteriochlorin excited‐state lifetime (∼5.5 ns), and narrow (≤20 nm) absorption and fluorescence bands. The latter will facilitate selective excitation/detection and multiprobe applications using both intensity‐ and lifetime‐imaging techniques.

Synthesis and Excited-state Photodynamics of a Chlorin–Bacteriochlorin Dyad—Through-space Versus Through-bond Energy Transfer in Tetrapyrrole Arrays

Understanding energy transfer among hydroporphyrins is of fundamental interest and essential for a wide variety of photochemical applications. Toward this goal, a synthetic free base ethynylphenylchlorin has been coupled with a synthetic free base bromobacteriochlorin to give a phenylethyne‐linked chlorin–bacteriochlorin dyad (FbC‐pe‐FbB). The chlorin and bacteriochlorin are each stable toward adventitious oxidation because of the presence of a geminal dimethyl group in each reduced pyrrole ring. A combination of static and transient optical spectroscopic studies indicate that excitation into the Qy band of the chlorin constituent (675 nm) of FbC‐pe‐FbB in toluene results in rapid energy transfer to the bacteriochlorin constituent with a rate of ∼(5 ps)−1 and efficiency of >99%. The excited bacteriochlorin resulting from the energy‐transfer process in FbC‐pe‐FbB has essentially the same fluorescence characteristics as an isolated monomeric reference compound, namely a narrow (12 nm fwhm) fluorescence emission band at 760 nm and a long‐lived (5.4 ns) Qy excited state that exhibits a significant fluorescence quantum yield (Φf = 0.19). Förster calculations are consistent with energy transfer in FbC‐pe‐FbB occurring predominantly by a through‐space mechanism. The energy‐transfer characteristics of FbC‐pe‐FbB are compared with those previously obtained for analogous phenylethyne‐linked dyads consisting of two porphyrins or two oxochlorins. The comparisons among the sets of dyads are facilitated by density functional theory calculations that elucidate the molecular‐orbital characteristics of the energy donor and acceptor constituents. The electron‐density distributions in the frontier molecular orbitals provide insights into the through‐bond electronic interactions that can also contribute to the energy‐transfer process in the different types of dyads.

Regioselective Bromination Tactics in the de Novo Synthesis of Chlorophyll b Analogues

The ability to introduce substituents at designated sites about the perimeter of the chlorin or 13(1)-oxophorbine macrocycle is essential for fundamental studies related to chlorophylls. A chlorin is a dihydroporphyrin, whereas a 13(1)-oxophorbine is a chlorin containing an annulated oxopentano ring spanning positions 13 and 15. 13(1)-Oxophorbines bearing auxochromes at the 7-position of the macrocycle are valuable targets given their resemblance to chlorophyll a or b, which contains the 13(1)-oxophorbine skeleton and bears a 7-methyl or 7-formyl group, respectively. A rational route to 7-substituted 13(1)-oxophorbines was developed that relies on a new method for regioselective bromination. Under neutral conditions, a 13-acetyl-10-mesitylchlorin (FbC-M(10)A(13)) undergoes bromination (with 1 molar equiv of NBS in THF) both in ring B (7-position) and at the 15-position (42% versus 28% isolated yield), thereby thwarting installation of the isocyclic ring (ring E, spanning the 13-15 positions). Under acidic conditions (10% TFA in CH(2)Cl(2)), ring B is deactivated, and bromination occurs preferentially at the 15-position (87% yield). The capability for preferential 15-bromination is essential to install the isocyclic ring, after which bromination can be directed to the 7-position of ring B (neutral conditions, 86% yield). The ability to suppress bromination in ring B (under acidic media) has been exploited in syntheses of sparsely substituted analogues of chlorophyll b. The analogues contain a 7-substituent (acetyl, formyl, or TIPS-ethynyl), a 10-mesityl group, and the 18,18-dimethyl group as the only substituents in the 13(1)-oxophorbine skeleton. The three analogues exhibit absorption spectral features that closely resemble those of free base analogues of chlorophyll b. Taken together, the facile access to chlorins and 13(1)-oxophorbines bearing substituents at distinct sites should enable fundamental spectroscopic studies and diverse applications.

Synthesis and photophysical characterization of porphyrin, chlorin and bacteriochlorin molecules bearing tethers for surface attachment.

The ability to tailor synthetic porphyrin, chlorin and bacteriochlorin molecules holds promise for diverse studies in artificial photosynthesis. Toward this goal, the synthesis and photophysical characterization of five tetrapyrrole compounds is described. Each compound bears a surface attachment group. One set contains three meso‐substituted porphyrins that differ only in the nature of a surface‐binding tether—isophthalic acid, ethynylisophthalic acid or cyanoacrylic acid. The other set includes a porphyrin, chlorin and bacteriochlorin each of which bears an ethynylisophthalic acid tether. The ester derivative of each compound was prepared for solution photophysical characterization studies. The photophysical studies include determination (in toluene or acetonitrile) of the electronic absorption and fluorescence spectra, fluorescence yield and lifetime of the lowest excited singlet state. The excited‐state lifetimes range from 1 to 5.6 ns for the five compounds. The radiative rate constant for the excited‐state decay was estimated from the photophysical data (fluorescence yield and excited‐state lifetime) and from Strickler–Berg analysis of the absorption and fluorescence spectra. The synthesis and characterization of the tetrapyrrole compounds underpin their use as sensitizers in molecular‐based solar cells.

Near-infrared molecular imaging probes based on chlorin-bacteriochlorin dyads

Chlorin-bacteriochlorin dyads as a new class of near-infrared fluorophores were synthesized and spectroscopically characterized. Each dyad is comprised of a chlorin macrocycle (free base or zinc chelate) as an energy donor (and absorber) and a free base bacteriochlorin as an energy acceptor (and emitter). Excitation of the chlorin (λ= 650 nm, zinc chelate; 675 nm, free base) results in fast (5 ps) and nearly quantitative (>99%) energy transfer to the adjacent bacteriochlorin moiety, and consequently bacteriochlorin fluorescence (λ= 760 nm). Thus, each chlorinbacteriochlorin dyad behaves as a single chromophore, with a large effective Stokes shift (85 or 110 nm), a significant fluorescence quantum yield (Φf = 0.19), long excited-state lifetime (τ = 5.4 ns), narrow excitation and emission bands (<20 nm), and high chemical stability. Imaging experiments performed using phantoms show that the chlorin-bacteriochlorin dyads exhibit a range of superior properties compare with commercially available imaging dyes. While the latter are six-fold brighter (comparing ε•Φf values), the chlorin-bacteriochlorin dyads exhibit narrower excitation and emission bands and larger Stokes shift, therefore allowing more efficient and selective excitation and detection of fluorescence. The high selectivity is further demonstrated with in vivo imaging studies using mice. This selectivity together with the tunability of absorption and emission wavelengths using substituent effects under synthetic control make the chlorin-bacteriochlorin dyads ideal candidates for multicolor imaging applications. In addition, the long fluorescence lifetimes make those probes suitable for lifetime-imaging applications.