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

Chemistry An Asian Journal

2016

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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.

Section: Introductionsupporting

confidence: 54%

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

Lim

Chemistry An Asian Journal

2016

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“…Various conjugated macromolecules including porphyrins, phthalocyanines, and BF 2 -chelatedd ipyrromethenes (BODIPY)h ave been widely used as the primary buildingb locks because of their excellent photophysical and photochemical properties. [39][40][41][42][43][44][45][46][47][48][49][50][51][52] Recently,s ignificante fforts have been made to develop various types of near-infrared( near-IR) sensitizers due to their potentiala pplications in variousf ield in chemistry,p hysics, and biology. Because of the different p-conjugated skeleton, shape, and structure of SubPcf rom the commonly used porphyrins and phthalocyanines,t hese molecules show distinct optical ande lectronic properties.…”

Section: Introductionmentioning

confidence: 99%

Tuning Optical and Electron Donor Properties by Peripheral Thio–Aryl Substitution of Subphthalocyanine: A New Series of Donor–Acceptor Hybrids for Photoinduced Charge Separation

Lim

2016

Chemistry A European J

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Subphthalocyanine (SubPc), a unique ring-reduced member of the common phthalocyanines family, although known for its higher absorptivity, reveals narrow absorption with peak maxima around 570 nm thus limiting its utility in light-energy-harvesting applications. In the present study, by peripheral thio-aryl substitution of SubPc macrocycle, the spectral properties have been modulated to extend the absorption and emission well into the visible/near-IR region. Additionally, for α-ring-substituted derivatives, facile oxidation of SubPc was witnessed, thus making these derivatives better electron donors. Next, the preparation of donor-acceptor dyads containing the well-known electron acceptor C60 connected to the central boron atom of SubPc was accomplished by making use of the 1,3-dipolar cycloaddition reaction. Control experiments and free-energy calculations using the redox and spectral data suggested that the observed fluorescence quenching of SubPc in these dyads is due to electron transfer. Accordingly, transient spectral studies performed both in polar and nonpolar solvents conclusively proved electron transfer to be the quenching mechanism in these dyads. The measured rate constants by fitting kinetic data revealed efficient charge separation and charge recombination processes, suggesting that these dyads could be useful materials for the construction of light-to-electricity or light-to-fuel production devices.

“…Recently, a multi-modular donor-sensitizer-acceptor system with three PTZ moieties appending to subphthalocyanine (SubPc) with axial C 60 (PTZ 3 -SubPc-C 60 ) is reported as shown in Fig. 8a [ 20 ]. The visible light excites mainly the central SubPc having strong absorption in the visible region.…”

Section: Charge Separation Via Acceptor Excitation In Covalently Connmentioning

confidence: 99%

“…This newly synthesized -at 1020 nm. Reconstructed using data from reference [ 20 ] multimolecular system is composed of a porphyrin ring with three covalently linked diphenylamine (Ph 2 N) entities at the meso -phenyl positions of the porphyrin ring ((Ph 2 N) 3 ZnP). Delocalization of the π-system over the meso -substituents and porphyrin is anticipated from the HOMO in ( c ) Nanosecond transient absorption spectra obtained by 532-nm laser-light irradiation (6-ns laser pulse).…”

Section: Charge Separation Via Donor Excitation For Substituted Porphmentioning

confidence: 99%

Functionalized Nanocarbons for Artificial Photosynthesis: From Fullerene to SWCNT, Carbon Nanohorn, and Graphene

Ito

From Molecules to Materials

2015

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This chapter describes the recent advances in photosensitized electron transfer processes on nanocarbon surfaces functionalized with electron-donating or electron-accepting molecules. As nanocarbons, well-characterized fullerenes, size (diameter)-sorted single-walled carbon nanotubes (SWCNTs), carbon nanohorns (CNHs), graphene, and graphene oxide are mainly employed. These nanocarbons act as excellent electron acceptors, when they are linked to the light-absorbing electron donors such as porphyrins or phthalocyanines. Employing diameter-sorted SWCNT makes it possible to reveal the relationship between nanotube size and electronic structure, which strongly affect the photoelectron transfer properties. In these studies, for the confi rmation of electron transfer processes, time-resolved spectroscopic methods have been widely used. The kinetic data obtained in solution are found to be quite useful to understand the electron transfer mechanisms. Photovoltaic cells constructed by employing these photosensitizing electron transfer systems are indicative of their potential uses to construct artifi cial photosynthetic systems for the light-to-electricity and light-to-fuel productions.