Chandra B. Kc

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.

Phenothiazine–BODIPY–Fullerene Triads as Photosynthetic Reaction Center Models: Substitution and Solvent Polarity Effects on Photoinduced Charge Separation and Recombination

Novel photosynthetic reaction center model compounds of the type donor2 -donor1 -acceptor, composed of phenothiazine, BF2 -chelated dipyrromethene (BODIPY), and fullerene, respectively, have been newly synthesized using multistep synthetic methods. X-ray structures of three of the phenothiazine-BODIPY intermediate compounds have been solved to visualize the substitution effect caused by the phenothiazine on the BODIPY macrocycle. Optical absorption and emission, computational, and differential pulse voltammetry studies were systematically performed to establish the molecular integrity of the triads. The N-substituted phenothiazine was found to be easier to oxidize by 60 mV compared to the C-substituted analogue. The geometry and electronic structures were obtained by B3LYP/6-31G(dp) calculations (for H, B, N, and O) and B3LYP/6-31G(df) calculations (for S) in vacuum, followed by a single-point calculation in benzonitrile utilizing the polarizable continuum model (PCM). The HOMO-1, HOMO, and LUMO were, respectively, on the BODIPY, phenothiazine and fullerene entities, which agreed well with the site of electron transfer determined from electrochemical studies. The energy-level diagram deduced from these data helped in elucidating the mechanistic details of the photochemical events. Excitation of BODIPY resulted in ultrafast electron transfer to produce PTZ-BODIPY(.+) -C60 (.-) ; subsequent hole shift resulted in PTZ(.+) -BODIPY-C60 (.-) charge-separated species. The return of the charge-separated species was found to be solvent dependent. In nonpolar solvents the PTZ(.+) -BODIPY-C60 (.-) species populated the (3) C60 * prior to returning to the ground state, while in polar solvent no such process was observed due to relative positioning of the energy levels. The (1) BODIPY* generated radical ion-pair in these triads persisted for few nanoseconds due to electron transfer/hole-shift mechanism.

Syntheses, Charge Separation, and Inverted Bulk Heterojunction Solar Cell Application of Phenothiazine–Fullerene Dyads

A series of phenothiazine-fulleropyrrolidine (PTZ-C60) dyads having fullerene either at the C-3 aromatic ring position or at the N-position of phenothiazine macrocycle were newly synthesized and characterized. Photoinduced electron transfer leading to PTZ(•+)-C60(•-) charge-separated species was established from studies involving femtosecond transient absorption spectroscopy. Because of the close proximity of the donor and acceptor entities, the C-3 ring substituted PTZ-C60 dyads revealed faster charge separation and charge recombination processes than that observed in the dyad functionalized through the N-position. Next, inverted organic bulk heterojunction (BHJ) solar cells were constructed using the dyads in place of traditionally used [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) and an additional electron donor material poly(3-hexylthiophene) (P3HT). The performance of the C-3 ring substituted PTZ-C60 dyad having a polyethylene glycol substituent produced a power conversion efficiency of 3.5% under inverted bulk heterojunction (BHJ) configuration. This was attributed to optimal BHJ morphology between the polymer and the dyad, which was further promoted by the efficient intramolecular charge separation and relatively slow charge recombination promoted by the dyad within the BHJ structure. The present finding demonstrate PTZ-C60 dyads as being good prospective materials for building organic photovoltaic devices.

Charge Separation in Graphene‐Decorated Multimodular Tris(pyrene)–Subphthalocyanine–Fullerene Donor–Acceptor Hybrids

A new approach to probe the effect of graphene on photochemical charge separation in donor-acceptor conjugates is devised. For this, multimodular donor-acceptor conjugates, composed of three molecules of pyrene, a subphthalocyanine, and a fullerene C60 ((Pyr)3 SubPc-C60 ), have been synthesized and characterized. These systems were hybridized on few-layer graphene through π-π stacking interactions of the three pyrene moieties. The hybrids were characterized using Raman, HRTEM, and spectroscopic and electrochemical techniques. The energy levels of the donor-acceptor conjugates were fine-tuned upon interaction with graphene and photoinduced charge separation in the absence and presence of graphene was studied by femtosecond transient absorption spectroscopy. Accelerated charge separation and recombination was detected in these graphene-decorated conjugates suggesting that they could be used as materials for fast-responding optoelectronic devices and in light energy harvesting applications.

Synthesis and Photoinduced Electron Transfer Studies of a Tri(Phenothiazine)–Subphthalocyanine–Fullerene Pentad

A novel donor-acceptor pentad featuring subphthalocyanine and fullerene as the primary electron donor and acceptor, and three phenothiazine entities as secondary hole transferring agents, have been newly synthesized and characterized as an photosynthetic reaction center model compound. Occurrences of ultrafast photoinduced electron transfer (PET) and slower charge recombination are witnessed in the pentad from the femtosecond and nanosecond transient absorption studies.

Decorating single layer graphene oxide with electron donor and acceptor molecules for the study of photoinduced electron transfer

Graphene oxide decorated with an electron donor, zinc(II) phthalocyanine, and an electron acceptor, fullerene, has been synthesized, and photoinduced electron transfer leading to charge-separation is reported.

Studies on the Photocatalytic Electron Pooling of Graphene Oxide Hybrids Decorated with Electron Donor and Electron Acceptor Molecules

Photocatalytic behavior of a recently synthesized, single layer graphene oxide (SLGO) decorated with an electron donor, zinc phthalocyanine (ZnPc) and an electron acceptor, fulleropyrrolidine (C60) donor-acceptor hybrid is demonstrated. Electron accumulation in the form of one-electron reduced product of methyl viologen was obtained in high yields in an electron pooling experiment involving the ZnPc-SLGO-C60 hybrid and a sacrificial electron donor compared with control hybrids involving either ZnPc-SLGO or SLGO-C60 hybrids. This novel property of ZnPc-SLGO-C60 hybrid has been ascribed to the proximity effect offered by GO with covalently linked donor and acceptor entities on its surface. The present studies reveal that the ZnPc-SLGO-C60 hybrid is a suitable catalyst for solar fuel production.

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.

Effect of Spacer Connecting the Secondary Electron Donor Phenothiazine in Subphthalocyanine-Fullerene Conjugates in Promoting Electron Transfer Followed by Hole Shift Process.

Sequential electron/hole transfer between energetically well-positioned entities of photosynthetic reaction center models is one of the commonly employed mechanisms to generate long-lived charge-separated states. A wealth of information, applicable towards light energy harvesting and building optoelectronic devices, has been acquired from such studies. In the present study, we report on the effect of spacer (direct or via phenoxy linkage) connecting the hole shifting agent, phenothiazine (PTZ), on photoinduced charge stabilization in subphthalocyanine-fullerene donor-acceptor conjugates. In these conjugates, the subphthalocyanine (SubPc) and fullerene (C60 ) served as primary electron donor and acceptor, respectively, while the phenothiazine entities act as hole shifting agents. The newly synthesized compounds were characterized by optical absorption and emission, computational, and electrochemical methods. The redox potentials measured using differential pulse voltammetry were used to estimate free-energy changes for charge separation, hole migration, and charge recombination processes. Using femto- and nanosecond transient absorption techniques, evidence for charge separation, and kinetics of charge separation and recombination were obtained in polar benzonitrile and nonpolar toluene solvents. In the conjugate where the phenothiazine entities are directly linked to SubPc, evidence for sequential electron transfer followed by hole shift leading to long-lived charge separated state was weak, primarily due to the delocalization of HOMO on both SubPc and PTZ entities. However, in case of the conjugate where the PTZ and SubPc are linked via phenoxy spacers, sequential electron transfer/hole shift was observed leading to the formation of long-lived charge-separated states. The present study brings out the importance of the spacer group connecting the hole shifting agent in model donor-acceptor conjugates to generate long-lived charge-separated states.

Peripheral phenothiazine induced suppression of charge separation from the singlet excited zinc phthalocyanine to coordinated C60 in supramolecular donor–acceptor conjugates

Self-assembled donor–acceptor conjugates featuring zinc phthalocyanine carrying four entities of peripheral phenothiazine entities, (PTZ)4ZnPc, axially coordinated to either phenyl imidazole or pyridine functionalized fulleropyrrolidine (C60Im or C60Py) has been newly designed, synthesized and characterized. Due to the direct connectivity of the phenothiazine entities to the ZnPc π-system, efficient charge transfer type interactions suppressing fluorescence of ZnPc in (PTZ)4ZnPc was observed. Axial coordination of C60Im or C60Py to the metal center of (PTZ)4ZnPc served as an electron acceptor in the conjugates. Optical absorption studies revealed stable complex formation wherein the evaluated binding constants K were found to be 3.9 × 104 M-1 for (PTZ)4ZnPc:ImC60 and 3.3 × 104 M-1 for (PTZ)4ZnPc:PyC60 conjugates with 1:1 molecular stoichiometry. Computational studies performed at the HF/[6-311G(d,p) for H, C, and N, and 6-311G(2df) for S and Zn] level revealed stable structures of the conjugates. The eval...