Synthesis of chlorophyll-a derivatives possessing (un)substituted 13-exo-methylene moiety and their optical properties

Chlorophyll-a derivatives possessing an un/mono/disubstituted methylene moiety at the 13¹-position were prepared by (un)substituted methylation of the 13-carbonyl group and successive dehydration. Substitution of the 13¹-oxo to the methylene group slightly blue-shifted electronic absorption and emission bands in a solution and decreased chemical stability to give an oxidation product cleaved at the E-ring. Further mono/disubstitution at the methylene terminal increased wavelengths of absorption and emission maxima as well as oxidative tolerance.     

Synthesis of chlorophyll-a derivatives possessing the 3-(2-acylethenyl) group by cross-aldol condensation and their optical properties

Chlorophyll-a derivatives possessing a trans-2-acylethenyl group at the C3-position were prepared by cross-aldol (Claisen-Schmidt) condensation of methyl pyropheophorbide-d having the C3-formyl group with ketones bearing α-hydrogen atoms including (un)substituted acetophenones, acetylarenes, and alkanones under basic conditions. The Qy absorption and fluorescence emission bands of the synthetic chalcone analogs in dichloromethane were largely dependent on the C3³-substituents. Especially, the introduction of electron-donating and withdrawing groups to the C3³-phenyl ring resulted in bathochromic and hypsochromic shifts of the maxima, respectively. Furthermore, fluorescence quantum yields and lifetimes were largely suppressed by the substitution with a nitro group on the C3³-phenyl ring.     

Synthesis of 20-iodochlorophyll derivatives and their properties including reactivity,electronic absorption,and self-aggregation

Methyl 20-iodopyropheophorbides-a possessing several substituents at the 3-position were prepared by treatment of the corresponding 20-unsubstituted chlorophyll-a derivatives with iodine and [bis(trifluoroacetoxy)iodo]benzene in chloroform and water at room temperature. The sterically demanding 20-iodination was clean, efficient, and regioselective without alteration of the 3-substituents, and also highly reproducible. The resulting 20-iodides were so reactive that they were readily transformed into 20-acetylchlorin (Pd-catalyzed cross-coupling) and 20-unsubstituted chlorin (acidic deiodination). Electronic absorption spectra in dichloromethane showed that the 20-iodination shifted the maxima to longer wavelengths in their monomeric states. Zinc complexes of 3¹-hydroxy-20-iodo-13¹-oxochlorins self-aggregated in non-polar organic solvents to give red-shifted and broadened absorption bands, similarly as in the corresponding 20-unsubstituted molecules, which are good models of the main light-harvesting antenna systems in photosynthetic green bacteria.     

Synthesis of 3,20-disubstituted chlorophyll-a derivatives and reactivity of the substituents

Methyl 20-bromo-pyropheophorbide-a possessing a vinyl group at the 3-position and its analogs having 3-ethyl, 3-(1-hydroxyethyl) and 3-acetyl groups were prepared by modifying naturally occurring chlorophyll(Chl)-a. The 20-bromo-chlorins were cross-coupled with vinyl- and 1-ethoxyvinyl-tributyltins in the presence of a Pd(0) catalyst to afford 20-vinyl- and acetyl-chlorins, respectively. The interconversions among the above four C2-functional groups at the 3-position were readily performed in the semi-synthetic Chl derivatives, but no modification of the 20-vinyl and acetyl groups was observed. The difference of the chemical reactivity is ascribable to the steric factor: the 3¹-position was less sterically hindered than the 20¹-position. Single hydrogenation and hydration of 3,20-divinyl-chlorin and reduction of 3,20-diacetylchlorin proceeded exclusively at the 3-position. The redmost (Qy) bands in electronic absorption spectra of the semi-synthetic Chl derivatives in a diluted dichloromethane solution were controlled by the 3,20-substituents. The 3-substituent effect on their Qy maxima was determined by the electronic factor (their electronegativity), while the 20-substituent effect was primarily dependent on the steric factor (their local size).     

Synthesis of 7-substituted chlorophyll-a derivatives as chlorophyll-b analogs with specific visible absorption bands

Methyl 3-substituted 13¹-deoxo-pyropheophorbides-a possessing the 7-methyl group were transformed into the corresponding 13¹-deoxo-pyropheophorbides-b bearing the 7-formyl group via the regioselective mono-dehydration of cis-7,8-diols to 7-hydroxymethyl-chlorins under mild acidic conditions. The exclusive production of the 7¹-hydroxy-chlorins without detection of the 8¹-hydroxy-chlorins in the reaction mixture did not depend on the 3-substituents. The regioselectivity was regulated by the 13-functional groups, and the ratio of the 8¹-OH over 7¹-OH products enhanced with an increase of the group electronegativity. Methyl mesopyropheophorbide-b (7-formyl-13¹-oxo-chlorin) was efficiently obtained by modifying methyl mesopyropheophorbide-a, one of the chlorophyll-a derivatives, through the protection of the 13¹-oxo moiety and the aforementioned mono-dehydration. The effects of the 7-substituents on the visible absorption spectra in a solution were comparable to those of naturally occurring, photosynthetically active chlorophylls-a/b and bacteriochlorophylls-c/e bearing the 7-methyl/formyl groups.     

Synthesis of zinc 20-substituted bacteriochlorophyll-d analogs and their self-aggregation

Zinc complexes of methyl 3¹-demethyl-bacteriopheophorbides-d possessing a 2-(un)substituted ethynyl or ethenyl group at the 20-position were prepared through palladium-catalyzed cross-coupling of their 20-bromide with the corresponding alkyne or alkene. The synthetic zinc 20-substituted 3-hydroxymethyl-13¹-oxo-chlorins self-aggregated in an aqueous micellar solution to give similar large oligomers as self-aggregates of bacteriochlorophylls in the core part of the main light-harvesting antenna of photosynthetic green bacteria. The zinc 20-ethynyl-chlorin fully gave the self-aggregates, but its 20²-substitution disturbed the self-aggregation. The 20-(2-phenyl)ethenyl moieties induced the formation of relatively smaller oligomers as well as residual monomers due to their steric bulkiness.     

20-(N-Methylpyridiniumyl)ethynylated chlorophyll-a derivative with an intense Qx absorption band at a green to orange region

Methyl 20-substituted mesopyropheophorbides-a were prepared by the chemical modification of natural chlorophyll-a via palladium-catalyzed cross-coupling reactions. The 20-ethynylation and further 2-pyridylation showed bathochromic shifts in their visible absorption bands as well as hyperchromic effects of the Qx bands in a diluted chloroform solution. The protonation and methylation at the nitrogen atom of the 20²-pyridyl group induced much larger shifts and effects, especially giving red-shifted and intensified Qx bands.     

Synthesis of 3/8-carbonylated chlorophyll derivatives and regiodependent reductivity of their carbonyl substituents

Methyl pyropheophorbide-d possessing a formyl group at the 3-position and its regioisomer having 8-CHO were prepared and their reactivities with a reductant were determined by the ¹H NMR technique: 3-CHO>8-CHO. The regioselective reduction of a synthetic 3,8-diformyl-chlorin also supported the higher reactivity in 3-CHO than in 8-CHO. Regiodependent reduction of the corresponding acetyl-chlorins confirmed that carbonyl groups at the 3-position in chlorophyllous pigments were reduced more rapidly than those at the 8-position. From the reports that reactions of 3-CHO with amines were preferable to those of 7-CHO, the CO functional groups on the pyrrole A-ring of chlorophylls are more reactive than those on the B-ring.     

Synthesis of carboxylated chlorophyll derivatives and their activities in dye-sensitized solar cells

Chlorophyll-a derivatives possessing a carboxy group in the substituent at the 3-position were prepared by chemical modification of methyl pyropheophorbide-d bearing the 3-formyl group via a Wittig, Barbier-type, or Knoevenagel reaction. The synthetic carboxylated chlorophyll pigments were employed as dye sensitizers for solar cells and their performances were compared in a conventional device based on a mesoporous titanium dioxide electrode and a liquid electrolyte. The solar power conversion efficiency was suppressed with an increase in the length of the oligomethylene moiety between the chlorin π-system and the carboxy group, while a corresponding π-linked ethenylene spacer enhanced the efficiency.     

Theoretical study of solvent influence on the electronic absorption and emission spectra of kynurenine

Neutral/zwitterionic form equilibrium, excited state wave functions, absorption and emission spectra of kynurenine (KN) in various solvents (water, methanol, ethanol, and dimethylsulfoxide) have been studied theoretically. The ground electronic state geometries have been optimized by density functional theory methods; the geometries of the first two singlets excited electronic states have been optimized using the CASSCF technique. The influence of the solvent was taken into account by the calculation of the solvation free energies using the Polarizable Continuum Model (PCM). The spectra of electronic absorption and fluorescence emission have been calculated by the CS‐INDO S‐CI and SDT‐CI methods [Momicchioli, Baraldi, and Bruni, Chem Phys, 1983, 82, 229]. The calculated data reproduce the experimental positions of maxima and the solvent‐induced shifts of the absorption and emission bands well. The energy gap between the two lowest excited states of KN increases from aprotic to protic solvents. This fact suggests that the “proximity effect” cannot be responsible for the ultrafast decay of KN fluorescence in protic solvents. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Synthesis of a new class of highly fluorescent aryl-vinyl benzo[1,2-b:4,5-b′]difuran derivatives

The synthesis of 2-(2-arylvinyl)- and 2,6-di(2-arylvinyl)dibenzo[1,2-b:4,5-b′]furan derivatives for application in optoelectronics is described. Wittig reaction of the triphenylphosphonium bromides derived from diethyl 2,6-dimethylbenzofuro[5,6-b]furan-3,7-dicarboxylate with aryl aldehydes gave the products in 70-99% yield. The corresponding products derived from furfural and cinnamic aldehyde were also obtained. The prepared products reveal UV-Vis fluorescence with quantum yields, varying from 1% to 100%, and may be used as organic small molecule materials for organic light-emitting devices (OLED).     

Synthesis, modification, and optical properties of C3-ethynylated chlorophyll derivatives

3-Ethynyl-chlorin was prepared from methyl pyropheophorbide-d using Bestmann-Ohira reagent. The mono-substituted acetylene was subjected to copper-free, Pd-mediated coupling to form chlorin derivatives possessing a series of substituted ethynyl groups at the C3-position. Its 1,3-dipolar cycloadditions with azido compounds were also demonstrated.     

New pyrrolo[3,2-b]pyrrole derivatives with multiple-acceptor substitution: Efficient fluorescent emission and near-infrared two-photon absorption

Two new D-π-A organic chromophores, 1,2,4,5-tetra(4-cyanophenyl)pyrrolo[3,2-b]pyrrole (4CNPP) and 1,2,3,4,5,6-hexa(4-cyanophenyl)pyrrolo[3,2-b]pyrrole (6CNPP), were developed by modifying the electron-rich pyrrolo[3,2-b]pyrrole (PP) unit with four and six 4-cyanophenyls respectively. The compounds exhibited bright blue emissions in low- to high-polarity solvents, with high Ф_fs of 88–90% in toluene, as well as impressive two-photon absorption cross-sections at the near-infrared (NIR) 700?nm.     

Synthesis of zinc 20-ethenylated bacteriochlorophyll-d analogs and their self-aggregation in an aqueous micelle solution

Zinc complexes of 3-hydroxymethyl-13¹-oxo-chlorins possessing a vinyl or its trans-substituted group at the 20-position were prepared through regioselective bromination and successive palladium-catalyzed cross-coupling reactions from naturally occurring chlorophyll-a. The synthetic zinc chlorophyll-a derivatives self-aggregated in an aqueous Triton X-100 micelle solution to give red-shifted and broadened electronic absorption bands, which were similar to those of bacteriochlorophyll-c/d self-aggregates in the main light-harvesting antenna systems of green photosynthetic bacteria. Because of the sterically demanded perpendicular orientation of the (un)substituted ethenyl group to the chlorin π-system, the present model compounds had less J-aggregation ability in a slipped π–π interaction fashion than the corresponding saturated ethyl-type analogs, while halogenations at the terminal of the 20-vinyl group enhanced the ability due to their electron-withdrawing effect.     

Acid-induced tunable white light emission based on triphenylamine derivatives

A series of triphenylamine (TPA) derivatives with various substituent groups were prepared and showed different absorption and fluorescence characteristics due to the substituent effect. On account of the existence of pyridine units, these TPA derivatives exhibited acid-induced tunable multicolor fluorescence emission including white light emission. In addition, acid-induced fluorescence regulation of these compounds has been also realized in the solid state, which enable them to be successfully constructed the stimuli-responsive fluorescent films and fluorescent inks for inkjet printing.     

Long-wavelength boradiazaindacene derivatives with two-photon absorption activity and strong emission: versatile candidates for biological imaging applications

Novel D-π-D-type boradiazaindacene dyes exhibit considerable two-photon absorption cross-section and strong red emission. Cell stained with these dyes show bright intracellular fluorescence. These properties qualify them as competitive candidates for fluorescent bioimaging applications     

Synthesis and solvent effects on the electronic absorption and fluorescence spectral properties of substituted zinc phthalocyanines

The synthesis and spectroscopic properties of the following tetra- and octa-substituted aryloxy zinc(II) phthalocyanines are reported for the first time: 1,(4)-(tetrabenzyloxyphenoxyphthalocyaninato) zinc(II) (7); 2,(3)-(tetrabenzyloxyphenoxyphthalocyaninato) zinc(II) (8); 2,3-(octabenzyloxyphenoxyphthalocyaninato) zinc(II) (9). The new compounds have been characterized by elemental analysis, IR, ¹H NMR spectroscopy and electronic spectroscopy. Spectroscopic properties of these compounds were investigated in different solvents. Protonation of non-peripherally substituted complex 7 resulted in the splitting and red-shifting of the Q-band. The peripherally substituted derivatives 8 and 9, did not show the split in the Q-band. Fluorescence spectra of the derivatives show Stokes shifts typical of MPc complexes.     

Synthesis and photoluminescence study of benz[f]indene derivatives

The synthesis and optical properties of a series of benz[f]indenes as new building blocks for electronic and optoelectronic materials are described.     

Synthesis of furan-substituted aza-BODIPYs having near-infrared emission

Development of near-infrared-emissive aza-boron dipyrromethene (aza-BODIPY) derivatives having furanyl groups is reported. From the optical measurements, it was clearly indicated that the emission bands were presented in the longer wavelength region than those of the conventional aza-BODIPYs. The emission bands with the peaks at 730 nm and 758 nm were observed from the bis- and tetra-substituted furanyl aza-BODIPYs with similar extents of emission efficiencies, respectively. According to the computer calculations, it was proposed that molecular planarity could be enhanced in the case of the furan groups. As a result, band-gap energy could be lowered comparing to those of the conventional benzene and thiophene-substituted aza-BODIPYs.     

Synthesis of novel fluorescent 3-aryl- and 3-methyl-7-aryl-[1,2,3]triazolo[1,5-a]pyridines by Suzuki cross-coupling reactions

Two series of compounds, 3-aryl- (series A, compounds 2a-j) and 3-methyl-7-aryl-[1,2,3]triazolo[1,5-a]pyridines (series B, compounds 3a-j) have been synthesized by Suzuki cross-coupling reactions, with a triazolopyridine halide and an aryl or heteroaryl boronic acid in moderate to good yields. All compounds obtained are fluorescents, the quantum yields, particularly those of compounds 3f-j, are very high.