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1. Meso ‐Biphenyl‐Linked, Near‐ and Far‐Infrared Emitting, Chlorin and Bacteriochlorin Dimers: Synthesis, Excitation Transfer, and Singlet Oxygen Production

   https://doi.org/10.1002/cplu.202100120  

   Seetharaman, Sairaman ; Dukh, Mykhaylo ; Tabaczynski, Walter A. ; Ou, Zhongping ; Karr, Paul A. ; Kadish, Karl M. ; Pandey, Ravindra K. ; D'Souza, Francis ( April 2021 , ChemPlusChem)

   A series ofmeso‐biphenyl linked chlorin and bacteriochlorin dimers, derived from naturally occurring chlorophyll (Chl‐a) and bacteriochlorophyll (BChl‐a) were synthesized in 32 % to 44 % yields and characterized, as photosynthetic antenna mimics, and a new class of singlet oxygen producing agents. The dimers are characterized by absorption, fluorescence, electrochemical, spectroelectrochemical and computational methods to evaluate their physico‐chemical properties, and to identify ground and excited state interactions. Evidence of excited energy exchange among the chromophores in the dimer is derived from femtosecond transient absorption spectral studies. Rate constants for excitation hopping were in the order of 10¹¹ s⁻¹, indicating occurrence of efficient processes. Nanosecond transient absorption studies confirmed relaxation of the singlet excited chlorin and bacteriochlorin dimers to their corresponding triplet states (³Chl* and³Bchl*). As predicted by the established energy level diagrams, both³Chl* and³Bchl* are shown to be capable of producing singlet oxygen with appreciable quantum yields (ϕ_SO∼0.3).

    

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2. Panchromatic Light‐Capturing Bis‐styryl BODIPY‐Perylenediimide Donor‐Acceptor Constructs: Occurrence of Sequential Energy Transfer Followed by Electron Transfer

   https://doi.org/10.1002/chem.202301686  

   Ileperuma, Chamari V. ; Garcés‐Garcés, José ; Shao, Shuai ; Fernández‐Lázaro, Fernando ; Sastre‐Santos, Ángela ; Karr, Paul A. ; D'Souza, Francis ( August 2023 , Chemistry – A European Journal)

   Two wide‐band‐capturing donor‐acceptor conjugates featuring bis‐styrylBODIPY and perylenediimide (PDI) have been newly synthesized, and the occurrence of ultrafast excitation transfer from the¹PDI* to BODIPY, and a subsequent electron transfer from the¹BODIPY* to PDI have been demonstrated. Optical absorption studies revealed panchromatic light capture but offered no evidence of ground‐state interactions between the donor and acceptor entities. Steady‐state fluorescence and excitation spectral recordings provided evidence of singlet‐singlet energy transfer in these dyads, and quenched fluorescence of bis‐styrylBODIPY emission in the dyads suggested additional photo‐events. The facile oxidation of bis‐styrylBODIPY and facile reduction of PDI, establishing their relative roles of electron donor and acceptor, were borne out by electrochemical studies. The electrostatic potential surfaces of the S₁and S₂states, derived from time‐dependent DFT calculations, supported excited charge transfer in these dyads. Spectro‐electrochemical studies on one‐electron‐oxidized and one‐electron‐reduced dyads and the monomeric precursor compounds were also performed in a thin‐layer optical cell under corresponding applied potentials. From this study, both bis‐styrylBODIPY⋅⁺and PDI⋅⁻could be spectrally characterizes and were subsequently used in characterizing the electron‐transfer products. Finally, pump–probe spectral studies were performed in dichlorobenzene under selective PDI and bis‐styrylBODIPY excitation to secure energy and electron‐transfer evidence. The measured rate constants for energy transfer,k_ENT, were in the range of 10¹¹ s⁻¹, while the electron transfer rate constants,k_ET, were in the range of 10¹⁰ s⁻¹, thus highlighting their potential use in solar energy harvesting and optoelectronic applications.

    

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3. meso ‐ and β‐Pyrrole‐Linked Chlorin‐Bacteriochlorin Dyads for Promoting Far‐Red FRET and Singlet Oxygen Production

   https://doi.org/10.1002/chem.202003042  

   Dukh, Mykhaylo ; Tabaczynski, Walter A. ; Seetharaman, Sairaman ; Ou, Zhongping ; Kadish, Karl M. ; D'Souza, Francis ; Pandey, Ravindra K. ( October 2020 , Chemistry – A European Journal)

   A series of chlorin‐bacteriochlorin dyads (derived from naturally occurring chlorophyll‐a and bacteriochlorophyll‐a), covalently connected either through themeso‐aryl or β‐pyrrole position (position‐3) via an ester linkage have been synthesized and characterized as a new class of far‐red emitting fluorescence resonance energy transfer (FRET) imaging, and heavy atom‐lacking singlet oxygen‐producing agents. From systematic absorption, fluorescence, electrochemical, and computational studies, the role of chlorin as an energy donor and bacteriochlorin as an energy acceptor in these wide‐band‐capturing dyads was established. Efficiency of FRET evaluated from spectral overlap was found to be 95 and 98 % for themeso‐linked and β‐pyrrole‐linked dyads, respectively. Furthermore, evidence for the occurrence of FRET from singlet‐excited chlorin to bacteriochlorin was secured from studies involving femtosecond transient absorption studies in toluene. The measured FRET rate constants,k_FRET, were in the order of 10¹¹ s⁻¹, suggesting the occurrence of ultrafast energy transfer in these dyads. Nanosecond transient absorption studies confirmed relaxation of the energy transfer product,¹BChl*, to its triplet state,³Bchl*. The³Bchl* thus generated was capable of producing singlet oxygen with quantum yields comparable to their monomeric entities. The occurrence of efficient FRET emitting in the far‐red region and the ability to produce singlet oxygen make the present series of dyads useful for photonic, imaging and therapy applications.

    

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4. 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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5. Ultrafast branching in intersystem crossing dynamics revealed by coherent vibrational wavepacket motions in a bimetallic Pt( ii ) complex

   https://doi.org/10.1039/D2FD00009A  

   Kim, Pyosang ; Valentine, Andrew J. ; Roy, Subhangi ; Mills, Alexis W. ; Castellano, Felix N. ; Li, Xiaosong ; Chen, Lin X. ( June 2022 , Faraday Discussions)

   Ultrafast excited state processes of transition metal complexes (TMCs) are governed by complicated interplays between electronic and nuclear dynamics, which demand a detailed understanding to achieve optimal functionalities of photoactive TMC-based materials for many applications. In this work, we investigated a cyclometalated platinum( ii ) dimer known to undergo a Pt–Pt bond contraction in the metal–metal-to-ligand-charge-transfer (MMLCT) excited state using femtosecond broadband transient absorption (fs-BBTA) spectroscopy in combination with geometry optimization and normal mode calculations. Using a sub-20 fs pump and broadband probe pulses in fs-BBTA spectroscopy, we were able to correlate the coherent vibrational wavepacket (CVWP) evolution with the stimulated emission (SE) dynamics of the 1 MMLCT state. The results demonstrated that the 145 cm −1 CVWP motions with the damping times of ∼0.9 ps and ∼2 ps originate from coherent Pt–Pt stretching vibrations in the singlet and triplet MMLCT states, respectively. On the basis of excited state potential energy surface calculations in our previous work, we rationalized that the CVWP transfer from the Franck–Condon (FC) state to the 3 MMLCT state was mediated by a triplet ligand-centered ( 3 LC) intermediate state through two step intersystem crossing (ISC) on a time scale shorter than a period of the Pt–Pt stretching wavepacket motions. Moreover, it was found that the CVWP motion had 110 cm −1 frequency decays with the damping time of ∼0.2 ps, matching the time constant of 0.253 ps, corresponding to a redshift in the SE feature at early times. This observation indicates that the Pt–Pt bond contraction changes the stretching frequency from 110 to 145 cm −1 and stabilizes the 1 MMLCT state relative to the 3 LC state with a ∼0.2 ps time scale. Thus, the ultrafast ISC from the 1 MMLCT to the 3 LC states occurs before the Pt–Pt bond shortening. The findings herein provide insight into understanding the impact of Pt–Pt bond contraction on the ultrafast branching of the 1 MMLCT population into the direct ( 1 MMLCT → 3 MMLCT) and indirect ISC pathways ( 1 MMLCT → 3 LC → 3 MMLCT) in the Pt( ii ) dimer. These results revealed intricate excited state electronic and nuclear motions that could steer the reaction pathways with a level of detail that has not been achieved before. 

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