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1. 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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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. Near‐IR Capturing N‐Methylbenzene Sulfonamide‐Phenothiazine Incorporating Strong Electron Acceptor Push‐Pull Systems: Photochemical Ultrafast Carrier Dynamics

   https://doi.org/10.1002/chem.202304313  

   Kumar_Gupta, Pankaj; Das, Somnath; Misra, Rajneesh; D'Souza, Francis (May 2024, Chemistry – A European Journal)

   Abstract Unraveling the intriguing aspects of the intramolecular charge transfer (ICT) phenomenon of multi‐modular donor‐acceptor‐based push–pull systems are of paramount importance considering their promising applications, particularly in solar energy harvesting and light‐emitting devices. Herein, a series of symmetrical and unsymmetrical donor‐acceptor chromophores1–6, are designed and synthesized by the Corey‐Fuchs reaction via Evano's condition followed by [2+2] cycloaddition retroelectrocyclic ring‐opening reaction with strong electron acceptors TCNE and TCNQ in good yields (~60–85 %). The photophysical, electrochemical, and computational studies are investigated to explore the effect of incorporation of strong electron acceptors 1,1,4,4‐tetracyanobuta‐1,3‐diene (TCBD) and dicyanoquinodimethane (DCNQ) with phenothiazine (PTZ) donor. An additional low‐lying broad absorption band extended towards the near‐infrared (NIR) region suggests charge polarization after the introduction of the electron acceptors in both symmetrical and asymmetrical systems, leading to such strong ICT bands. The electrochemical properties reveal that reduction potentials of3and6are lower than those of2and5, suggesting DCNQ imparts more on the electronic properties and hence largely contributes to the stabilization of LUMO energy levels than TCBD, in line with theoretical observations. Relative positions of the frontier orbitals on geometry‐optimized structures further support accessing donor‐acceptor sites responsible for the ICT transitions. Eventually, ultrafast carrier dynamics of the photoinduced species are investigated by femtosecond transient absorption studies to identify their spectral characteristics and target analysis further provides information about different excited states photophysical events including ICT and their associated time profiles. The key findings obtained here related to excited state dynamical processes of these newly synthesized systems are believed to be significant in advancing their prospect of utilization in solar energy conversion and related photonic applications. 

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4. Zinc Tetrapyrrole Coordinated to Imidazole Functionalized Tetracyanobutadiene or Cyclohexa‐2,5‐diene‐1,4‐diylidene‐expanded‐tetracyanobutadiene Conjugates: Dark vs. Light‐Induced Electron Transfer

   https://doi.org/10.1002/chem.202302665  

   Guragain, Manan; Pinjari, Dilip; Misra, Rajneesh; D'Souza, Francis (December 2023, Chemistry – A European Journal)

   Abstract Using the popular metal‐ligand axial coordination self‐assembly approach, donor‐acceptor conjugates have been constructed using zinc tetrapyrroles (porphyrin (ZnP), phthalocyanine (ZnPc), and naphthalocyanine (ZnNc)) as electron donors and imidazole functionalized tetracyanobutadiene (Im‐TCBD) and cyclohexa‐2,5‐diene‐1,4‐diylidene‐expanded‐tetracyanobutadiene (Im‐DCNQ) as electron acceptors. The newly formed donor‐acceptor conjugates were fully characterized by a suite of physicochemical methods, including absorption and emission, electrochemistry, and computational methods. The measured binding constants for the 1 : 1 complexes were in the order of 10⁴–10⁵ M⁻¹in o‐dichlorobenzene. Free‐energy calculations and the energy level diagrams revealed the high exergonicity for the excited state electron transfer reactions. However, in the case of the ZnNc:Im‐DCNQ complex, owing to the facile oxidation of ZnNc and facile reduction of Im‐DCNQ, slow electron transfer was witnessed in the dark without the aid of light. Systematic transient pump‐probe studies were performed to secure evidence of excited state charge separation and gather their kinetic parameters. The rate of charge separation was as high as 10¹¹ s⁻¹suggesting efficient processes. These findings show that the present self‐assembly approach could be utilized to build donor‐acceptor constructs with powerful electron acceptors, TCBD and DCNQ, to witness ground and excited state charge transfer, fundamental events required in energy harvesting, and building optoelectronic devices. 

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5. Triplet photodynamic and up-conversion luminescence in donor–acceptor dyads with slip-stacked vs. co-facial arrangement

   https://doi.org/10.1039/D1TC05122A  

   Yun, Young Ju; Peccati, Francesca; Wiederrecht, Gary P.; Gosztola, David J.; Diroll, Benjamin T.; Jiménez-Osés, Gonzalo; Ayitou, A. Jean-Luc (May 2022, Journal of Materials Chemistry C)

   The design/synthesis and characterization of organic donor–acceptor (D–A) dyads can provide precious data allowing to improve the efficiency of classical photo-induced bimolecular interactions/processes. In this report, two novel triplet D–A dyads (4 and 5) were synthesized and fully characterized. While the optical absorption and emission profiles of these new systems exhibit similar spectral structures as that of the triplet donor/sensitizer quinoidal thioamide (QDN), the transient absorption (TA) spectra of these two dyads produced new features that can be associated with triplet transients and charge transfer species. However, the kinetics of the excited-state processes/dynamics is significantly influenced by the geometrical arrangement(s) of donor/acceptor chromophores. Further analysis of the TA data suggests that the dyad with slip-stack geometry (4) is less effective in undergoing both intra- and inter-dyad triplet energy transfer than the dyad with co-facial geometry (5). Subsequently, triplet sensitization of 9,10-diphenylanthracene (DPA) using both dyads led to upconverted photoluminescence via triplet–triplet annihilation of DPA triplet transients. But, it was found that a maximum upconversion quantum yield could be achieved at a low power density using the co-facial type dyad 5. Altogether, these results provide valuable guidance in the design of triplet donor–acceptor dyads, which could be used for light-harvesting/modulation applications. 

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