Preferential Through-Space Charge Separation and Charge Recombination in V‑Type Configured Porphyrin–azaBODIPY–Fullerene Supramolecular Triads

Electron transfer is one of the most fundamental and prevalent processes occurring in chemistry, physics, and biology. In donor–acceptor systems with one of the partners’ a photosensitizer, upon photoexcitation, transfer of an electron between the photoexcited and ground-state molecules occurs. Factors affecting the geometry, energetics, and dynamics of this process have been one of the intensively studied scientific topics, often by building model donor–acceptor conjugates or by utilizing natural systems. A wealth of information, applicable to almost all areas of modern science and technology, has been generated from these studies. In the present study, we demonstrate preferential through-space charge separation and charge recombination in supramolecular triads composed of porphyrin (free-base, zinc, or magnesium at the central cavity) as excited-state electron donor, BF₂-chealted azadipyrromethene (azaBODIPY), and fullerene (C₆₀) as electron acceptors. Because of spatial close proximity of the terminal porphyrin and fullerene entities of the triads as a consequence of the V-type configuration, photoinduced charge separation from the singlet excited porphyrin involves fullerene instead of energetically more favorable covalently linked azaBODIPY entity. Interestingly, charge recombination also follows this path of through-space instead of an electron migration from the fullerene anion radical to the covalently linked azaBODIPY entity. The present study highlights the importance of geometric disposition of donor and acceptor entities in governing not only the forward photoinduced electron transfer but also the dark reverse electron transfer in multimodular donor–acceptor conjugates, applicable toward light-energy-harvesting and building optoelectronic devices

Paddle-Wheel BODIPY–Hexaoxatriphenylene Conjugates: Participation of Redox-Active Hexaoxatriphenylene in Excited-State Charge Separation to Yield High-Energy Charge-Separated States

Hexaoxatriphenylene, a scaffold linker often utilized in building covalent organic frameworks, is shown to be electroactive and a useful entity to build light energy-harvesting donor–acceptor systems. To demonstrate this, new donor–acceptor conjugates have been synthesized by employing BODIPY as a sensitizer. Excited-state electron transfer leading to high-energy charge-separated states, useful to drive energy-demanding photocatalytic reactions, from the electron-rich hexaoxatriphenylene to ¹BODIPY*, in the synthesized tri-BODIPY–hexaoxatriphenylene “paddle-wheel” conjugates, has been successfully demonstrated using femtosecond transient absorption spectroscopy. The measured rate of charge separation was in the range of ∼3–10 × 10¹¹ s^–1, revealing ultrafast charge separation

Phenothiazine-Sensitized Organic Solar Cells: Effect of Dye Anchor Group Positioning on the Cell Performance

Effect of positioning of the cyanoacrylic acid anchoring group on ring periphery of phenothiazine dye on the performance of dye-sensitized solar cells (DSSCs) is reported. Two types of dyes, one having substitution on the C-3 aromatic ring (Type 1) and another through the N-terminal (Type 2), have been synthesized for this purpose. Absorption and fluorescence studies have been performed to visualize the effect of substitution pattern on the spectral coverage and electrochemical studies to monitor the tuning of redox levels. B3LYP/6-31G* studies are performed to visualize the frontier orbital location and their significance in charge injection when surface modified on semiconducting TiO₂. New DSSCs have been built on nanocrystalline TiO₂ according to traditional two-electrode Grätzel solar cell setup with a reference cell based on N719 dye for comparison. The lifetime of the adsorbed phenothiazine dye is found to be quenched significantly upon immobilizing on TiO₂ suggesting charge injection from excited dye to semiconducting TiO₂. The performances of the cells are found to be prominent for solar cells made out of Type 1 dyes compared to Type 2 dyes. This trend has been rationalized on the basis of spectral, electrochemical, computational, and electrochemical impedance spectroscopy results

π‑Extended Pyrazinepyrene-Fused Zinc Phthalocyanines: Synthesis and Excited-State Charge Separation Involving Coordinated C₆₀

A series of pyrazinepyrene-fused zinc phthalocyanines (ZnPc-Pyrn) have been newly synthesized by reacting quinoxaline and the corresponding diamino-functionalized phthalocyanines as a new class of π-extended phthalocyanine systems. Bathochromically shifted absorption as a function of the number of pyrazinepyrene entities due to extended π-conjugation and quenched fluorescence due to the presence of fused pyrazinepyrene were witnessed. The electronic structures of these phthalocyanines were probed by systematic computational and electrochemical studies, while the excited-state properties were examined by pump–probe spectroscopies operating at the femto- and nanosecond time scales. Similar to the excited singlet lifetimes, the excited triplet states also revealed diminished lifetimes with an increased number of pyrazinepyrene entities. Further, the coordinatively unsaturated zinc in these molecules was coordinated with phenyl imidazole-functionalized fullerene, ImC60, to form a new series of donor–acceptor conjugates. Upon full characterization of these conjugates, the occurrence of excited-state charge separation was established by transient pump–probe spectroscopy, covering wide temporal and spatial regions. The lifetime of the final charge-separated states was ∼2 ns and decreased with an increase in the number of fused pyrazinepyrene units

Pyrazinacenes: aza analogues of acenes

A series of edge-sharing condensed oligopyrazine analogues of acenes, the pyrazinacenes, were synthesized and characterized. X-ray crystallographic determinations revealed intermolecular interactions that affect the propensity of the molecules to undergo π-π stacking. Increasing heteroatom substitution of the acene framework induces shorter intermolecular π-π stacking distances (shorter than for graphite) probably due to lower van derWaals radius of nitrogen atoms. Hydrogen bonding is also a determining factor in the case of compounds containing reduced pyrazine rings. Combined electrochemical, electronic absorption, and computational investigations indicate the substantial electron deficiency of the compounds composed of fused pyrazine rings. The pyrazinacenes are expected to be good candidates as materials for organic thin film transistors