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1. Charge stabilization via electron exchange: excited charge separation in symmetric, central triphenylamine derived, dimethylaminophenyl– tetracyanobutadiene donor–acceptor conjugates

   https://doi.org/DOI: 10.1039/D0SC04648E  

   Yadav, IS; Alsaleh, AZ; Misra, R; D'Souza, F (May 2021, Chemical science)

   Photoinduced charge separation in donor-acceptor conjugates play a pivotal role in technology breakthroughs, especially in the areas of efficient conversion of solar energy into electrical energy and fuels. Extending the lifetime of the charge separated species is a necessity for their practical utilization, and this is often achieved by following the mechanism of natural photosynthesis where the process of electron/hole migration occurs distantly separating the radical ion-pairs. Here, we hypothesize and demonstrate a new mechanism to stabilize the charge separated states via the process of electron exchange among the different acceptor entities in multimodular donor-acceptor conjugates. For this, star-shaped, central triphenylamine derived, dimethylamine-tetracyanobutadiene conjugates have been newly designed and characterized. Electron exchange was witnessed upon electroreduction in conjugates having multiple numbers of electron acceptors. Using ultrafast spectroscopy, occurrence of excited state charge separation, and the effect of electron exchange in prolonging the lifetime of charge separated states in the conjugates having multiple acceptors has been successfully demonstrated. This work constitutes the first example of stabilizing charge-separated states via the process of electron exchange. 

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2. Highly Efficient, Long-Lived Charge Separated States in Star-Shaped Ferrocene-Diketopyrrolopyrrole-Triphenylamine Donor-Acceptor-Donor Conjugates

   https://doi.org/doi.org/10.1002/chem.202000346  

   M. Poddar, Y. Jang (November 2020, Chemistry)

   null (Ed.)

   The significance of multiple number of donor-acceptor entities on a central electron donor in a star-shaped molecular system in improving light energy harvesting ability is reported. For this, donor-acceptor-donor type conjugates comprised up to three entities ferrocenyl (Fc)-diketopyrrolopyrrole (DPP) onto a central triphenylamine (TPA), (4-6) by the Pd-catalyzed Sonogashira cross–coupling reactions have been newly synthesized and characterized. Donor-acceptor conjugates possessing diketopyrrolopyrrole (1 to 3 entities) onto the central triphenylamine, (1-3) served as reference dyads while monomeric DPP and Fc-DPP served as control compounds. Both DPP and Fc-DPP carrying conjugates exhibited red-shifted absorption compared to their respective control compounds revealing existence of ground state interactions. Furthermore, DPP fluorescence in 4-6 was found to be quantitatively quenched while for 1-3, this property varied between 73-65% suggesting occurrence moderate amounts of excited state events. The electrochemical investigations exhibited an additional low potential oxidation in the case of Fc-DPP-TPA based derivatives (4-6) owing to the presence of ferrocene unit(s). This was in addition to DPP and TPA redox peaks. Using spectral, electrochemical and computational studies, Gibbs free-energy calculations were performed to visualize excited state charge separation (GCS) in these donor-acceptor conjugates as a function of different number of Fc-DPP entities. Formation of Fc+-DPP•--TPA charge separated states (CSS) in the case of 4-6 was evident. Using spectroelectrochemical studies, spectrum of CSS was deduced. Finally, femtosecond transient absorption spectral studies were performed to gather information on excited state charge separation. Increasing the number of Fc-DPP entities in 4-6 improved charge separation rates. Surprisingly, lifetime of the charge separated state, Fc+-DPP•--TPA was found to persist longer with an increase in the number of Fc-DPP entities in 4-6 as compared to Fc-DPP-control and simple DPP derived donor-acceptor conjugates in literature. This unprecedented result has been attributed to subtle changes in GCS and GCR and the associated electron coupling between different entities. 

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3. Zinc Phthalocyanine N‐Linked to Pyrroloperylenediimide Dyad: Significance of N‐Connectivity in Modulating Photodynamics Toward Prolonging the Lifetime of Charge Separation

   https://doi.org/10.1002/cptc.202500238  

   Ileperuma, Chamari V; Garcés‐Garcés, Jose; Gutiérrez‐Vílchez, Ana M; Sastre‐Santos, Ángela; Lázaro, Fernando Fernández; D’Souza, Francis (March 2026, ChemPhotoChem)

   Liu, B (Ed.)

   A zinc phthalocyanine–pyrroloperylenediimide dyad connected through a nitrogen heteroatom (PDI‐N‐ZnPc) has been newly synthesized and characterized. Solvent‐polarity‐dependent singlet–singlet energy transfer and electron‐transfer quenching were envisioned from absorption and steady‐state fluorescence studies. Electrochemical and spectroelectrochemical studies enabled the assessment of the redox potentials of the donor and acceptor entities, as well as the spectral characterization of the one‐electron oxidation and reduction products. Density functional theory (DFT) studies were performed to investigate the geometry and electronic structure, as well as the role of N‐connectivity in determining the relative orientation of the entities. Further, time‐dependent DFT studies helped establish the different excited states responsible for promoting charge separation. An energy diagram was subsequently established to visualize different photophysical events. Finally, femtosecond transient absorption spectral studies were performed in both nonpolar and polar solvents to observe energy‐ and electron‐transfer events. The kinetic data were subsequently analyzed using global and target analyses. The persistence of the charge‐separated state in the present dyad, compared with earlier reported ZnPc–PDI dyads featuring carbon–carbon connectivity, was the primary outcome of the present study, highlighting the role of heteroatom linkage in regulating electron‐transfer dynamics in donor–acceptor conjugates. 

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4. Excited Charge Separation in a π-Interacting Phenothiazine–Zinc Porphyrin–Fullerene Donor–Acceptor Conjugate

   https://doi.org/10.1021/acs.jpca.4c00976  

   Yadagiri, B; Kaswan, Ram Ratan; Tagare, Jairam; Kumar, Vinay; Rajesh, Manne Naga; Singh, Surya Prakash; Karr, Paul A; D’Souza, Francis; Giribabu, Lingamallu (May 2024, The Journal of Physical Chemistry A)

   Shea, JE (Ed.)

   A zinc phthalocyanine-pyrroloperylenediimide dyad connected through a nitrogen heteroatom (PDI-N-ZnPc) has been newly synthesized and characterized. Solvent polarity-dependent singlet-singlet energy transfer and electron transfer quenching were envisioned from absorption and steady-state fluorescence studies. Electrochemical and spectroelectrochemical studies enabled the assessment of the redox potential of the donor and acceptor entities, as well as the spectral characterization of the one-electron oxidation and reduction products. DFT studies were performed to investigate the geometry and electronic structure, as well as the role of N-connectivity in determining the relative orientation of the entities. Further, time-dependent DFT studies helped establish the different excited states responsible for promoting charge separation. An energy diagram was subsequently established to visualize different photo-physical events. Finally, femtosecond transient absorption spectral studies were performed in both nonpolar and polar solvents to observe energy and electron transfer events. The kinetic data were subsequently analyzed using global and target analyses. The persistence of the charge-separated state in the present dyad, compared with earlier reported ZnPc-PDI dyads featuring carbon-carbon connectivity, was the primary outcome of the present study, highlighting the role of heteroatom linkage in regulating electron transfer dynamics in donor-acceptor conjugates. 

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5. Engineering opposite electronic polarization of singlet and triplet states increases the yield of high-energy photoproducts

   https://doi.org/10.1073/pnas.1901752116  

   Polizzi, Nicholas F.; Jiang, Ting; Beratan, David N.; Therien, Michael J. (June 2019, Proceedings of the National Academy of Sciences)

   Efficient photosynthetic energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states. However, energies of high-lying excited states are rarely extracted, in part because the congested density of states in the excited-state manifold leads to rapid deactivation. Conventional photosystem designs promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor (“one-way” ET), a form of positive design. Curiously, negative design strategies that explicitly avoid unwanted side reactions have been underexplored. We report here that electronic polarization of a molecular chromophore can be used as both a positive and negative design element in a light-driven reaction. Intriguingly, prudent engineering of polarized excited states can steer a “U-turn” ET—where the excited electron density of the donor is initially pushed away from the acceptor—to outcompete a conventional one-way ET scheme. We directly compare one-way vs. U-turn ET strategies via a linked donor–acceptor (DA) assembly in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor. Ultrafast spectroscopy of DA pinpoints the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing. These results demonstrate that oppositely polarized electronically excited states can be employed to steer photoexcited states toward useful, high-energy products by routing these excited states away from states that are photosynthetic dead ends. 

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