Assemblies of Boron Dipyrromethene/Porphyrin, Phthalocyanine, and C60 Moieties as Artificial Models of Photosynthesis: Synthesis, Supramolecular Interactions, and Photophysical Studies

A series of light-harvesting conjugates based on a zinc(II) phthalocyanine core with either two or four boron dipyrromethene (BODIPY) or porphyrin units have been synthesized and characterized. The conjugation of BODIPY/porphyrin units can extend the absorptions of the phthalocyanine core to cover most of the visible region. Upon addition of an imidazole-substituted C₆₀ (C₆₀ Im), it can axially bind to the zinc(II) center of the phthalocyanine core through metal-ligand interactions. The resulting complexes form photosynthetic antenna-reaction center mimics in which the BODIPY/porphyrin units serve as the antennas to capture the light and transfer the energy to the phthalocyanine core by efficient excitation energy transfer. The excited phthalocyanine is then quenched by the axially bound C₆₀ Im moiety by electron transfer, which has been supported by computational studies. The photoinduced processes of the assemblies have been studied in detail by various steady-state and time-resolved spectroscopic methods. By femtosecond transient absorption spectroscopic studies, the lifetimes of the charge-separated state of the bis(BODIPY) and bis(porphyrin) systems have been determined to be 3.2 and 4.0 ns, respectively.

Modelling excitation energy transfer in covalently linked molecular dyads containing a BODIPY unit and a macrocycle

We model the singlet–singlet Excitation Energy Transfer (EET) process in a panel of large BODIPY–macrocycle dyads, including some azacalixphyrin derivatives.

An artificial photosynthetic model based on a molecular triad of boron dipyrromethene and phthalocyanine

A boron dipyrromethene (BDP) unit and its monostyryl derivative (MSBDP) were introduced at the axial positions of a silicon(iv) phthalocyanine (SiPc) core. The absorption spectrum of this compound virtually covered the entire visible region (300-700 nm) and could be interpreted as a superposition of the spectra of individual components. The intramolecular photoinduced energy and charge transfer processes of this triad were studied using steady-state and time-resolved spectroscopic methods in polar and nonpolar solvents. Upon BDP-part excitation, a fast and highly efficient excitation energy transfer (EET) occurred resulting in strong quenching of its fluorescence and the formation of the first excited singlet state of SiPc or MSBDP. It was found that both EET and charge transfer (CT) processes competed with each other in the depopulation of the first excited singlet state of the MSBDP moiety. The former strongly superseded CT in nonpolar toluene, whereas the latter was dominant in a polar environment. Direct or indirect (via EET) excitation of the SiPc-part of the triad was followed by CT yielding the charge-separated (CS) species BDP-SiPc(˙-)-MSBDP(˙+). The energy gap between the CS state and the S1-state of the SiPc moiety was found to be only 0.06 eV in toluene, which facilitated the back CT process and resulted in the appearance of thermally activated delayed fluorescence. With increasing solvent polarity, the energy of the CS state reduced resulting in the disappearance of the delayed fluorescence in CHCl3, tetrahydrofuran or N,N-dimethylformamide. The charge recombination rate, k(CR), was very fast in polar DMF (3.3 × 10(10) s(-1)), whereas this process was two-orders of magnitude slower in nonpolar toluene (k(CR) = 4.0 × 10(8) s(-1)).

Photoinduced energy and charge transfer in a bis(triphenylamine)–BODIPY–C60 artificial photosynthetic system

The bis(triphenylamine)–BODIPY–C₆₀ artificial photosynthetic system has been prepared and studied for its photoinduced transfer processes in polar and nonpolar solvents using various steady-state and time-resolved spectroscopic techniques.

Energy-transfer studies on phthalocyanine–BODIPY light harvesting pentad by laser flash photolysis

A molecular pentad, comprised of zinc phthalocyanine ( ZnPc ) with four boron dipyrromethene units (BODIPY) have been examined by femtosecond and nanosecond laser flash photolysis to explore its photoinduced intramolecular events from the excited BODIPY. The geometry optimization showed that the phthalocyanine moiety is completely symmetric and form perfect square planar complex with zinc. The absorption spectrum of ZnPc -BODIPY pentad covers most of the visible region (ca. 300–750 nm), which clearly is an advantage for capturing solar energy. The excitation transfer from the singlet BODIPY to ZnPc is envisioned due to good spectral overlap of the BODIPY emission and ZnPc absorption spectra. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of energy transfer from the singlet excited BODIPY to ZnPc in tetrahydrofuran. The kinetic study of energy transfer measured by monitoring the decay of the BODIPY emission revealed fast energy transfer (5.90 × 1010 s-1) in the molecular pentad. Since the electron transfer from the singlet ZnPc to BODIPY is thermodynamically not feasible, the singlet ZnPc decayed to populates the triplet ZnPc , in addition to the grounds state. These findings suggest the potential of the examined ZnPc -BODIPY pentad to be efficient photosynthetic antenna in the artificial photosynthetic systems.

Energy transfer properties of a novel boron dipyrromethene–perylenediimide donor–acceptor dyad

The boron dipyrromethene–perylenediimide dyad has been prepared and studied for its light-harvesting properties with various steady-state and time-resolved spectroscopic methods.

Electron vs energy transfer in arrays featuring two Bodipy chromophores axially bound to a Sn(IV) porphyrin via a phenolate or benzoate bridge

In this report we describe the synthesis of multichromophore arrays consisting of two Bodipy units axially bound to a Sn(IV) porphyrin center either via a phenolate (3) or via a carboxylate (6) functionality. Absorption spectra and electrochemical studies show that the Bodipy and porphyrin chromophores interact weakly in the ground state. However, steady-state emission and excitation spectra at room temperature reveal that fluorescence from both the Bodipy and the porphyrin of 3 are strongly quenched suggesting that, in the excited state, energy and/or electron transfer might occur. Indeed, as transient absorption experiments show, selective excitation of Bodipy in 3 results in a rapid decay (τ ≈ 2 ps) of the Bodipy-based singlet excited state and a concomitant rise of a charge-separated state evolving from the porphyrin-based singlet excited state. In contrast, room-temperature emission studies on 6 show strong quenching of the Bodipy-based fluorescence leading to sensitized emission from the porphyrin moiety due to a transduction of the singlet excited state energy from Bodipy to the porphyrin. Emission experiments at 77 K in frozen toluene reveal that the room-temperature electron transfer pathway observed in 3 is suppressed. Instead, Bodipy excitation in 3 and 6 results in population of the first singlet excited state of the porphyrin chromophore. Subsequently, intersystem crossing leads to the porphyrin-based triplet excited state.

Synthesis and properties of styryl-substituted tetrapyrazinoporphyrazines [St8PyzPzM], M = 2NaI, MgII(H2O) and ZnII

New styryl-substituted tetrapyrazinoporphyrazines [St8PyzPzM] (M = 2NaI , MgII(H2O) and ZnII ; St = -CH=CHAr , where Ar = phenyl or cumyl) were prepared by template cyclotetramerization of 5,6-distyrylpyrazine-2,3-dicarbonitriles available from condensation of diaminomaleodinitrile with the substituted cinnamils (StCO)2 obtained from biacetyl and arylaldehydes ArCHO . The new porphyrazine macrocycles were characterized by electronic absorption and emission spectra, which provide evidence of efficient π-interaction of the peripheral styryl moieties with the central pyrazinoporphyrazine framework. The electronic excitation of the stylbenoid chromophore in the near UV-region (400 nm) leads to strong emission of the porphyrazine chromophore near 700 nm, allowing to consider the styryl-substituted porphyrazines as efficient light-harvesting species. Their spectral properties and electrochemical behavior are compared with those of stylbenoid phthalocyanines and peripherally substituted tetrapyrazinoporphyrazines.

Facile synthesis of pegylated zinc(II) phthalocyanines via transesterification and their in vitro photodynamic activities

Treatment of 4,5-bis[4-(methoxycarbonyl)phenoxy]phthalonitrile and 4,5-bis[3,5-bis(methoxycarbonyl)phenoxy]phthalonitrile with an excess of 1,3-diiminoisoindoline in the presence of Zn(OAc)(2)·2H(2)O and 1,8-diazabicyclo[5.4.0]undec-7-ene in triethylene glycol monomethyl ether or polyethylene glycol monomethyl ether (with an average molecular weight of 550) led to "3 + 1" mixed cyclisation and transesterification in one pot, affording the corresponding di-β-substituted zinc(II) phthalocyanines in 7-23% yield. As shown by absorption spectroscopy, these compounds were essentially non-aggregated in N,N-dimethylformamide and could generate singlet oxygen effectively. The singlet oxygen quantum yields (Φ(Δ) = 0.53-0.57) were comparable with that of the unsubstituted zinc(II) phthalocyanine (Φ(Δ) = 0.56). These compounds in Cremophor EL emulsions also exhibited photocytotoxicity against HT29 human colorectal adenocarcinoma and HepG2 human hepatocarcinoma cells with IC(50) values in the range of 0.25-3.72 μM. The analogue with four triethylene glycol chains was the most potent photosensitiser and localised preferentially in the mitochondria of HT29 cells. The bis(polyethylene glycol)-counterpart could form surfactant-free nanoparticles both in water and in the culture medium. The hydrodynamic radii, as determined by dynamic laser light scattering, ranged from 6.3 to 79.8 nm depending on the preparation methods and conditions. The photocytotoxicity of these nanoparticles (IC(50) = 0.43-0.56 μM) was comparable with that of the Cremophor EL-formulated system (IC(50) = 0.34 μM).