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1. One‐Photon Excitation Followed by a Three‐Step Sequential Energy–Energy–Electron Transfer Leading to a Charge‐Separated State in a Supramolecular Tetrad Featuring Benzothiazole–Boron‐Dipyrromethene–Zinc Porphyrin–C 60

   https://doi.org/10.1002/chem.202004262  

   Badgurjar, Deepak ; Seetharaman, Sairaman ; D'Souza, Francis ; Chitta, Raghu ( December 2020 , Chemistry – A European Journal)

   A panchromatic triad, consisting of benzothiazole (BTZ) and BF₂‐chelated boron‐dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using¹H NMR spectroscopy, ESI‐MS, UV‐visible, steady‐state fluorescence, electrochemical, and femtosecond transient absorption techniques. The absorption band of the triad, BTZ‐BODIPY‐ZnP, and dyads, BTZ‐BODIPY and BODIPY‐ZnP, along with the reference compounds BTZ‐OMe, BODIPY‐OMe, and ZnP‐OMe exhibited characteristic bands corresponding to individual chromophores. Electrochemical measurements on BTZ‐BODIPY‐ZnP exhibited redox behavior similar to that of the reference compounds. Upon selective excitation of BTZ (≈290 nm) in the BTZ‐BODIPY‐ZnP triad, the fluorescence of the BTZ moiety is quenched, due to photoinduced energy transfer (PEnT) from¹BTZ^*to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the¹BODIPY* moiety to ZnP, resulting in the appearance of the ZnP emission, indicating the occurrence of a two‐step singlet–singlet energy transfer. Further, a supramolecular tetrad, BTZ‐BODIPY‐ZnP:ImC₆₀, was formed by axially coordinating the triad with imidazole‐appended fulleropyrrolidine (ImC₆₀), and parallel steady‐state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from¹ZnP* to ImC₆₀. Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely,¹BTZ^*‐BODIPY‐ZnP:ImC₆₀→BTZ‐¹BODIPY^*‐ZnP:ImC₆₀→BTZ‐BODIPY‐¹ZnP^*:ImC₆₀→BTZ‐BODIPY‐ZnP^.+:ImC₆₀^.−in the supramolecular tetrad. The evaluated rate of energy transfer,k_EnT, was found to be 3–5×10¹⁰ s⁻¹, which was slightly faster than that observed in the case of BODIPY‐ZnP and BTZ‐BODIPY‐ZnP, lacking the coordinated ImC₆₀. The rate constants for charge separation and recombination,k_CSandk_CR, respectively, calculated by monitoring the rise and decay of C₆₀^.−were found to be 5.5×10¹⁰and 4.4×10⁸ s⁻¹, respectively, for the BODIPY‐ZnP:ImC₆₀triad, and 3.1×10¹⁰and 4.9×10⁸ s⁻¹, respectively, for the BTZ‐BODIPY‐ZnP:ImC₆₀tetrad. Initial excitation of the tetrad, promoting two‐step energy transfer and a final electron‐transfer event, has been successfully demonstrated in the present study.

    

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2. Distance‐Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine‐Fullerene Conjugates

   https://doi.org/10.1002/cphc.202000578  

   Martín‐Gomis, Luis ; Seetharaman, Sairaman ; Herrero, David ; Karr, Paul A. ; Fernández‐Lázaro, Fernando ; D'Souza, Francis ; Sastre‐Santos, Ángela ( September 2020 , ChemPhysChem)

   The effect of donor‐acceptor distance in controlling the rate of electron transfer in axially linked silicon phthalocyanine‐C₆₀dyads has been investigated. For this, two C₆₀‐SiPc‐C₆₀dyads,1and2, varying in their donor‐acceptor distance, have been newly synthesized and characterized. In the case of C₆₀‐SiPc‐C₆₀1where the SiPc and C₆₀are separated by a phenyl spacer, faster electron transfer was observed withk_csequal to 2.7×10⁹ s⁻¹in benzonitrile. However, in the case of C₆₀‐SiPc‐C₆₀2, where SiPc and C₆₀are separated by a biphenyl spacer, a slower electron transfer rate constant,k_cs=9.1×10⁸ s⁻¹, was recorded. The addition of an extra phenyl spacer in2increased the donor‐acceptor distance by ∼4.3 Å, and consequently, slowed down the electron transfer rate constant by a factor of ∼3.7. The charge separated state lasted over 3 ns, monitoring time window of our femtosecond transient spectrometer. Complimentary nanosecond transient absorption studies revealed formation of³SiPc* as the end product and suggested the final lifetime of the charge separated state to be in the 3–20 ns range. Energy level diagrams established to comprehend these mechanistic details indicated that the comparatively high‐energy SiPc^.+‐C₆₀^.−charge separated states (1.57 eV) populated the low‐lying³SiPc* (1.26 eV) prior returning to the ground state.

    

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3. Far‐Red Excitation Induced Electron Transfer in Bis Donor‐AzaBODIPY Push‐Pull Systems; Role of Nitrogenous Donors in Promoting Charge Separation

   https://doi.org/10.1002/chem.202301659  

   Alsaleh, Ajyal Z. ; Pinjari, Dilip ; Misra, Rajneesh ; D'Souza, Francis ( August 2023 , Chemistry – A European Journal)

   A far‐red absorbing sensitizer, BF₂‐chelated azadipyrromethane (azaBODIPY) has been employed as an electron acceptor to synthesize a series of push‐pull systems linked with different nitrogenous electron donors, viz.,N,N‐dimethylaniline (NND), triphenylamine (TPA), and phenothiazine (PTZ) via an acetylene linker. The structural integrity of the newly synthesized push‐pull systems was established by spectroscopic, electrochemical, spectroelectrochemical, and DFT computational methods. Cyclic and differential pulse voltammetry studies revealed different redox states and helped in the estimation of the energies of the charge‐separated states. Further, spectroelectrochemical studies performed in a thin‐layer optical cell revealed diagnostic peaks of azaBODIPY⋅⁻in the visible and near‐IR regions. Free‐energy calculations revealed the charge separation from one of the covalently linked donors to the¹azaBODIPY* to yield Donor⋅⁺‐azaBODIPY⋅⁻to be energetically favorable in a polar solvent, benzonitrile, and the frontier orbitals generated on the optimized structures helped in assessing such a conclusion. Consequently, the steady‐state emission studies revealed quenching of the azaBODIPY fluorescence in all of the investigated push‐pull systems in benzonitrile and to a lesser extent in mildly polar dichlorobenzene, and nonpolar toluene. The femtosecond pump‐probe studies revealed the occurrence of excited charge transfer (CT) in nonpolar toluene while a complete charge separation (CS) for all three push‐pull systems in polar benzonitrile. The CT/CS products populated the low‐lying³azaBODIPY* prior to returning to the ground state. Global target (GloTarAn) analysis of the transient data revealed the lifetime of the final charge‐separated states (CSS) to be 195 ps for NND‐derived, 50 ps for TPA‐derived, and 85 ps for PTZ‐derived push‐pull systems in benzonitrile.

    

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4. Supramolecular Porous Organic Nanocomposites for Heterogeneous Photocatalysis of a Sulfur Mustard Simulant

   https://doi.org/10.1002/adma.202001592  

   Beldjoudi, Yassine ; Atilgan, Ahmet ; Weber, Jacob A. ; Roy, Indranil ; Young, Ryan M. ; Yu, Jierui ; Deria, Pravas ; Enciso, Alan E. ; Wasielewski, Michael R. ; Hupp, Joseph T. ; et al ( June 2020 , Advanced Materials)

   Efficient heterogeneous photosensitizing materials require both large accessible surface areas and excitons of suitable energies and with well‐defined spin structures. Confinement of the tetracationic cyclophane (ExBox⁴⁺) within a nonporous anionic polystyrene sulfonate (PSS) matrix leads to a surface area increase of up to 225 m²g⁻¹in ExBox•PSS. Efficient intersystem crossing is achieved by combining the spin‐orbit coupling associated to Br heavy atoms in 1,3,5,8‐tetrabromopyrene (TBP), and the photoinduced electron transfer in a TBP⊂ExBox⁴⁺supramolecular dyad. The TBP⊂ExBox⁴⁺complex displays a charge transfer band at 450 nm and an exciplex emission at 520 nm, indicating the formation of new mixed‐electronic states. The lowest triplet state (T₁, 1.89 eV) is localized on the TBP and is close in energy with the charge separated state (CT, 2.14 eV). The homogeneous and heterogeneous photocatalytic activities of the TBP⊂ExBox⁴⁺, for the elimination of a sulfur mustard simulant, has proved to be significantly more efficient than TBP and ExBox⁺⁴, confirming the importance of the newly formed excited‐state manifold in TBP⊂ExBox⁴⁺for the population of the low‐lying T₁state. The high stability, facile preparation, and high performance of the TBP⊂ExBox•PSS nanocomposites augur well for the future development of new supramolecular heterogeneous photosensitizers using host–guest chemistry.

    

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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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