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1. Heavy‐Atom Tunneling Through Crossing Potential Energy Surfaces: Cyclization of a Triplet 2‐Formylarylnitrene to a Singlet 2,1‐Benzisoxazole

   https://doi.org/10.1002/ange.202006640  

   Nunes, Cláudio M. ; Viegas, Luís P. ; Wood, Samuel A. ; Roque, José P. L. ; McMahon, Robert J. ; Fausto, Rui ( August 2020 , Angewandte Chemie)

   Not long ago, the occurrence of quantum mechanical tunneling (QMT) chemistry involving atoms heavier than hydrogen was considered unreasonable. Contributing to the shift of this paradigm, we present here the discovery of a new and distinct heavy‐atom QMT reaction. Triplet syn‐2‐formyl‐3‐fluorophenylnitrene, generated in argon matrices by UV‐irradiation of an azide precursor, was found to spontaneously cyclize to singlet 4‐fluoro‐2,1‐benzisoxazole. Monitoring the transformation by IR spectroscopy, temperature‐independent rate constants (k≈1.4×10⁻³ s⁻¹; half‐life of ≈8 min) were measured from 10 to 20 K. Computational estimated rate constants are in fair agreement with experimental values, providing evidence for a mechanism involving heavy‐atom QMT through crossing triplet to singlet potential energy surfaces. Moreover, the heavy‐atom QMT takes place with considerable displacement of the oxygen atom, which establishes a new limit for the heavier atom involved in a QMT reaction in cryogenic matrices.

    

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2. 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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3. Photophysical Insights of Halogenated Dipyrrolonaphthyridine‐Diones as Potential Photodynamic Therapy Agents †

   https://doi.org/10.1111/php.13757  

   Morgan, Jayla ; Yun, Young Ju ; Jamhawi, Abdelqader M. ; Islam, Shahidul M. ; Ayitou, A. Jean‐Luc ( January 2023 , Photochemistry and Photobiology)

   We report the synthesis and photophysical characterization of novel halogenated dipyrrolonaphthyridine‐diones (X₂–DPNDs, X = Cl, Br, and I), as candidates for photodynamic therapy (PDT) application. Apart from the heavy atom‐induced spin‐orbit coupling (SOC) dynamics in the investigated X₂–DPNDs, it was found that the position of the halogen atom (relative to the nitrogen of the pyrrole ring) also influenced the triplet excited state behavior. Interestingly, the faster/efficiency sensitization of³O₂to¹O₂using X₂–DPND correlates with the rate of triplet population,k_ISC >1.6 × 10⁸s⁻¹for I₂–DPNDvs k_ISC >2.9 × 10⁹s⁻¹for Cl₂–DPND and Br₂–DPND (whereτ_ISC = 343 ± 3 ps for I₂–DPND andτ_ISC = 5–6 ns for Cl₂–DPND and Br₂–DPND are the lowest time constants/values for ISC). Furthermore, the heavy atom‐induced SOC in Cl₂–DPND and Br₂–DPND did not lead to a reduction of the corresponding fluorescence (ca75%vs67% for the parent DPND). The attractive photophysical characteristics of Cl₂/Br₂–DPND put them on the landscape as not only promising PDT agents but also as fluorescence probes. The present study is a stepping stone in the development of novel organic photosystems for synergistic photomedicinal applications.

    

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4. Photolysis of 3‐Azido‐3‐phenyl‐3 H ‐isobenzofuran‐1‐one at Ambient and Cryogenic Temperatures †

   https://doi.org/10.1111/php.13500  

   Thenna‐Hewa, Kosala ; Sebastien, William ; Lemen, Elaine M. ; Karney, William L. ; Abe, Manabu ; Gudmundsdottir, Anna D. ( August 2021 , Photochemistry and Photobiology)

   Although alkyl azides are known to typically form imines under direct irradiation, the product formation mechanism remains ambiguous as some alkyl azides also yield the corresponding triplet alkylnitrenes at cryogenic temperatures. The photoreactivity of 3‐azido‐3‐phenyl‐3H‐isobenzofuran‐1‐one (1) was investigated in solution and in cryogenic matrices. Irradiation (λ = 254 nm) of azide 1 in acetonitrile yielded a mixture of imines 2 and 3. Monitoring of the reaction progress using UV‐Vis absorption spectroscopy revealed an isosbestic point at 210 nm, indicating that the reaction proceeded cleanly. Similar results were observed for the photoreactivity of azide 1 in a frozen 2‐methyltetrahydrofuran (mTHF) matrix. Irradiation of azide 1 in an argon matrix at 6 K resulted in the disappearance of its IR bands with the concurrent appearance of IR bands corresponding to imines 2 and 3. Thus, it was theorized that azide 1 forms imines 2 and 3 via a concerted mechanism from its singlet excited state or through singlet alkylnitrene¹1N, which does not intersystem cross to its triplet configuration. This proposal was supported by CASPT2 calculations on a model system, which suggested that the energy gap between the singlet and triplet configurations of alkylnitrene 1N is 33 kcal/mol, thus making intersystem crossing inefficient.

    

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5. 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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