Habtom B. Gobeze

Porphyrin-Sensitized Solar Cells: Effect of Carboxyl Anchor Group Orientation on the Cell Performance

The effect of the orientation of the porphyrin sensitizer onto the TiO2 surface on the performance of dye-sensitized solar cells (DSSCs) is reported. Free-base and zinc porphyrins bearing a carboxyl anchoring group at the para, meta, or ortho positions of one of the meso-phenyl rings were synthesized for application in Grätzel-type photoelectrochemical cells. The remainder of the meso-phenyl rings was substituted with alkyl chains of different length to visualize any aggregation effects. Absorption and fluorescence studies were performed to characterize and observe spectral coverage of the thirteen newly synthesized porphyrin derivatives. Photoelectrochemical studies were performed after immobilization of porphyrins onto nanocrystalline TiO2 and compared with DSSC constructed using N719 dye as reference. The performance of DSSCs with the porphyrin anchoring at the para or meta position were found to greatly exceed those with the anchoring group in the ortho position. Additionally, cells constructed using zinc porphyrin derivatives outperformed the free-base porphyrin analogs. Better dye regeneration efficiency for the zinc porphyrin derivatives compared to their free-base porphyrin analogs, and for the meta and para derivatives over the ortho derivatives was evaluated from electrochemical impedance spectroscopy studies. Femtosecond transient absorption spectroscopy studies were performed to probe the kinetics of charge injection and charge recombination with respect to the orientation of porphyrin macrocycle on TiO2 surface. The ortho porphyrin derivative with an almost flat orientation to the TiO2 surface revealed fast charge recombination and suggested occurrence of through-space charge transfer. The overall structure-performance trends observed for the present porphyrin DSSCs have been rationalized based on spectral, electrochemical, electrochemical impedance spectroscopy, and transient spectroscopy results.

Dual Functioning Thieno-Pyrrole Fused BODIPY Dyes for NIR Optical Imaging and Photodynamic Therapy: Singlet Oxygen Generation without Heavy Halogen Atom Assistance.

We discovered a rare phenomenon wherein a thieno-pyrrole fused BODIPY dye (SBDPiR690) generates singlet oxygen without heavy halogen atom substituents. SBDPiR690 generates both singlet oxygen and fluorescence. To our knowledge, this is the first example of such a finding. To establish a structure-photophysical property relationship, we prepared SBDPiR analogs with electron-withdrawing groups at the para-position of the phenyl groups. The electron-withdrawing groups increased the HOMO-LUMO energy gap and singlet oxygen generation. Among the analogs, SBDPiR688, a CF3 analog, had an excellent dual functionality of brightness (82290 m(-1)  cm(-1) ) and phototoxic power (99170 m(-1)  cm(-1) ) comparable to those of Pc 4, due to a high extinction coefficient (211 000 m(-1)  cm(-1) ) and balanced decay (Φflu =0.39 and ΦΔ =0.47). The dual functionality of the lead compound SBDPiR690 was successfully applied to preclinical optical imaging and for PDT to effectively control a subcutaneous tumor.

Ultrafast Photoinduced Electron Transfer and Charge Stabilization in Donor–Acceptor Dyads Capable of Harvesting Near‐Infrared Light

To harvest energy from the near-infrared (near-IR) and infrared (IR) regions of the electromagnetic spectrum, which constitutes nearly 70 % of the solar radiation, there is a great demand for near-IR and IR light-absorbing sensitizers that are capable of undergoing ultrafast photoinduced electron transfer when connected to a suitable electron acceptor. Towards achieving this goal, in the present study, we report multistep syntheses of dyads derived from structurally modified BF2-chelated azadipyrromethene (ADP; to extend absorption and emission into the near-IR region) and fullerene as electron-donor and electron-acceptor entities, respectively. The newly synthesized dyads were fully characterized based on optical absorbance, fluorescence, geometry optimization, and electrochemical studies. The established energy level diagram revealed the possibility of electron transfer either from the singlet excited near-IR sensitizer or singlet excited fullerene. Femtosecond and nanosecond transient absorption studies were performed to gather evidence of excited state electron transfer and to evaluate the kinetics of charge separation and charge recombination processes. These studies revealed the occurrence of ultrafast photoinduced electron transfer leading to charge stabilization in the dyads, and populating the triplet states of ADP, benzanulated-ADP and benzanulated thiophene-ADP in the respective dyads, and triplet state of C60 in the case of BF2 -chelated dipyrromethene derived dyad during charge recombination. The present findings reveal that these sensitizers are suitable for harvesting light energy from the near-IR region of the solar spectrum and for building fast-responding optoelectronic devices operating under near-IR radiation input.

Directly Connected AzaBODIPY–BODIPY Dyad: Synthesis, Crystal Structure, and Ground- and Excited-State Interactions

Directly connected, strongly interacting sensitizer donor-acceptor dyads mimic light-induced photochemical events of photosynthesis. Here, we devised a dyad composed of BF2-chelated dipyrromethene (BODIPY) directly linked to BF2-chelated tetraarylazadipyrromethene (azaBODIPY) through the β-pyrrole position of azaBODIPY. Structural integrity of the dyad was arrived from two-dimensional NMR spectral studies, while single-crystal X-ray structure of the dyad provided the relative orientation of the two macrocycles to be ∼62°. Because of direct linking of the two entities, ultrafast energy transfer from the (1)BODIPY* to azaBODIPY was witnessed. A good agreement between the theoretically estimated Förster energy transfer rate and experimentally determined rate was observed, and this rate was found to be higher than that reported for BODIPY-azaBODIPY analogues connected with spacer units. In agreement with the free-energy calculations, the product of energy transfer, (1)azaBODIPY* revealed additional photochemical events such as electron transfer leading to the creation of BODIPY(•+)-azaBODIPY(•-) radical ion pair, more so in polar benzonitrile than in nonpolar toluene, as evidenced by femtosecond transient spectroscopic studies. Additionally, the spectral, electrochemical, and photochemical studies of the precursor compound azaBODIPY-dipyrromethane also revealed occurrence of excited-state events. In this case, electron transfer from the (1)azaBODIPY* to dipyrromethane (DPM) yielded DPM(•+)-azaBODIPY(•-) charge-separated state. The study described here stresses the role of close association of the donor and acceptor entities to promote ultrafast photochemical events, applicable of building fast-response optoelectronic and energy-harvesting devices.

Covalent decoration onto the outer walls of double walled carbon nanotubes with perylenediimides

The outer walls of double walled carbon nanotubes (DWCNTs) have been selectively functionalized with different substituted perylenediimides (PDIs) leaving the inner walls intact. The spacer connecting DWCNTs and PDIs, and the PDI macrocycle position has been varied to visualize the DWCNT–PDI interactions in the hybrids. Evidence of outer wall functionalization and the degree of PDI substitution on DWCNTs were arrived from HR-TEM, AFM, FTIR, TGA, XPS and Raman techniques while nanotube–photosentisizer interactions were probed from studies involving optical absorbance and emission, and electrochemical techniques. Fine-tuning of the electronic states of PDIs in the hybrids was possible with the present covalent approach. The fluorescence of PDIs in the hybrids was found to be quenched (60–70%) due to interactions with DWCNTs. Further, femtosecond transient absorption and photocatalytic electron pooling studies were performed to seek evidence of charge separation in these hybrids. In agreement with earlier studies, evidence of charge separation from the transient studies was bleak, and accordingly, yields of photocatalytic electron pooling were much lower than those reported earlier for fullerene and single walled carbon nanotube based hybrids. The present study is suggestive of further tuning of donor–acceptor energy levels in DWCNT derived hybrids for efficient charge separation and stabilization.

Ultrafast Photoinduced Charge Separation in Wide-Band-Capturing Self-Assembled Supramolecular Bis(donor styryl)BODIPY-Fullerene Conjugates.

A new series of self-assembled supramolecular donor-acceptor conjugates capable of wide-band capture, and exhibiting photoinduced charge separation have been designed, synthesized and characterized using various techniques as artificial photosynthetic mimics. The donor host systems comprise of a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) containing a crown ether entity at the meso-position and two styryl entities on the pyrrole rings. The styryl end groups also carried additional donor (triphenylamine or phenothiazine) entities. The acceptor host system was a fulleropyrrolidine comprised of an ethylammonium cation. Owing to the presence of extended conjugation and multiple chromophore entities, the BODIPY host revealed absorbance and emission well into the near-IR region covering the 300-850 nm spectral range. The donor-acceptor conjugates formed by crown ether-alkyl ammonium cation binding of the host-guest system was characterized by optical absorbance and emission, computational, and electrochemical techniques. Experimentally determined binding constants were in the range of 1-2×10(5)  M(-1) . An energy-level diagram to visualize different photochemical events was established using redox, computational, absorbance, and emission data. Spectral evidence for the occurrence of photoinduced charge separation in these conjugates was established from femtosecond transient absorption studies. The measured rates indicated ultrafast charge separation and relatively slow charge recombination revealing their usefulness in light-energy harvesting and optoelectronic device applications. The bis(donor styryl)BODIPY-derived conjugates populated their triplet excited states during charge recombination.

Peripheral versus axial substituted phthalocyanine-double-walled carbon nanotube hybrids as light harvesting systems

Selective functionalization of the outer walls of double-walled carbon nanotubes (DWCNTs) with zinc and silicon phthalocyanines leaving the inner walls undamaged has been accomplished. The mode of metal phthalocyanine (MPc) connection was varied, that is, through the macrocycle periphery in the case of zinc phthalocyanines (ZnPcs) and through the silicon axial position in the case of silicon phthalocyanines (SiPcs) to visualize its effect on phthalocyanine–DWCNT interactions. Evidence of outer wall functionalization and the degree of phthalocyanine substitution on DWCNTs were arrived from HR-TEM, AFM, TGA, XPS and Raman techniques. The sensitizer-nanotube interactions were probed from studies involving optical absorbance, steady-state and time resolved emission, and electrochemical studies. The fluorescence of phthalocyanines in these hybrids was found to be almost quantitatively quenched (>95%) due to the interactions with DWCNTs, and such interactions were more for the SiPc derived hybrids. The femtosecond transient absorption studies performed revealed that the interactions are dynamic involving the singlet excited phthalocyanines and the nanotubes. In agreement with earlier studies, evidence for charge separation in these hybrids was bleak revealing weak signature bands of MPc˙+ in the 850 nm range. Additionally, the presence of trace amounts of single-walled carbon nanotubes in the hybrids further hampered detailed spectral analysis.

Panchromatic Light Capture and Efficient Excitation Transfer Leading to Near-IR Emission of BODIPY Oligomers.

All-BODIPY-based (BODIPY=boron-dipyrromethene) donor-acceptor systems capable of wide-band absorbance leading to efficient energy transfer in the near-IR region are reported. A covalently linked 3-pyrrolyl BODIPY-BODIPY dimer building block bearing an ethynyl group at the meso-aryl position is synthesized and coupled with three different monomeric BODIPY/pyrrolyl BODIPY building blocks with a bromo/iodo group under Pd(0) coupling conditions to obtain three covalently linked 3-pyrrolyl-BODIPY-based donor-acceptor oligomers in 19-29 % yield. The oligomers are characterized in detail by 1D and 2D NMR spectroscopy, high-resolution mass spectrometry, and optical spectroscopy. Due to the presence of different functionalized BODIPY derivatives in the oligomers, panchromatic light capture (300-725 nm) is witnessed. Fluorescence studies reveal singlet-singlet energy transfer from BODIPY monomer to BODIPY dimer leading to emission in the 700-800 nm range. Theoretical modeling according to the Förster mechanism predicts ultrafast energy transfer due to good spectral overlap of the donor and acceptor entities. Femtosecond transient absorption studies confirm this to be the case and thus show the relevance of the currently developed all-BODIPY-based energy-funneling supramolecular sytems with near-IR emission to solar-energy harvesting applications.

Axially Assembled Photosynthetic Antenna-Reaction Center Mimics Composed of Boron Dipyrromethenes, Aluminum Porphyrin, and Fullerene Derivatives

Sequential photoinduced energy transfer followed by electron transfer leading to the formation of charge separated states in a newly assembled series of supramolecular triads comprised of boron dipyrromethenes (BODIPY or BDP), aluminum porphyrin (AlTPP) and C60 is demonstrated. In the present strategy, the energy donor (BDP) and electron acceptor (C60) were axially positioned to the plane of AlTPP via the central metal. The structural integrity of the newly synthesized compounds and self-assembled systems were fully established using spectral, electrochemical and computational methods. Thermodynamic feasibility of energy transfer from 1BDP* to AlTPP and subsequent electron transfer from 1AlTPP* to generate BDP-AlTPP•+-C60•- charge separated states was derived from free-energy calculations. Occurrence of ultrafast energy transfer from 1BDP* to AlTPP was established from studies involving steady-state and time-resolved emission, as well as femtosecond transient spectroscopic techniques. The BDP-AlTPP•+-C60•- charge separated states persisted for several nanoseconds prior returning to the ground state.

Langmuir–Blodgett Films of Self-Assembled (Alkylether-Derivatized Zn Phthalocyanine)–(C60 Imidazole Adduct) Dyad with Controlled Intermolecular Distance for Photoelectrochemical Studies

A multilayer Langmuir-Blodgett (LB) film of the self-assembled electron donor-acceptor dyad of Zn phthalocyanine, appended with four long-chain aliphatic ether peripheral substituents, and an imidazole adduct of C60 was prepared and applied as a photoactive material in a photoelectrochemical cell. Changes in the simultaneously recorded surface pressure and surface potential vs area per molecule compression isotherms for Langmuir films of the dyad and, separately, of its components helped to identify phase transitions and mutual interactions of molecules in films. The Brewster angle microscopy (BAM) imaging of the Langmuir films showed circular condensed phase domains of the dyad molecules. The determined area per molecule was lower than that estimated for the dyad and its components, separately. The multilayer LB films of the dyad were transferred onto hydrophobized fluorine-doped tin oxide-coated (FTO) glass slides under different conditions. The presence of both components in the dyad LB films was confirmed with the UV-vis spectroscopy measurements. For the LB films transferred at different surface pressures, the PM-IRRAS measurements revealed that the phthalocyanine macrocycle planes and ether moieties in films were tilted with respect to the FTO surface. The AFM imaging of the LB films indicated formation of relatively uniform dyad LB films. Then, the femtosecond transient absorption spectral studies evidenced photoinduced electron transfer in the LB film. The obtained transient signals corresponding to both Zn(TPPE)(•+) and C60im(•-) confirmed the occurrence of intramolecular electron transfer. The determined rate constants of charge separation, kcs = 2.6 × 10(11) s(-1), and charge recombination, kcr = 9.7 × 10(9) s(-1), indicated quite efficient electron transfer within the film. In the photoelectrochemical studies, either photoanodic or photocathodic current was generated depending on the applied bias potential when the dyad LB film-coated FTO was used as the working electrode and ascorbic acid or methylviologen, respectively, as the charge mediator in an aqueous solution.