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1. 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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2. Photophysics of Oligothiophenes End‐Capped with Platinum(II) Auxochromes

   https://doi.org/10.1002/cptc.202000123  

   Gundogan, Ali S. ; Jagadesan, Pradeepkumar ; Schanze, Kirk S. ( October 2020 , ChemPhotoChem)

   A series of four oligothiophenes end‐capped with −Pt(PBu₃)₂Cl moieties on both ends of the oligomers was synthesized, and their excited state properties were investigated. The observation of low fluorescence quantum yield (<2 %) for the oligomers indicates the strong effect of platinum on the intersystem crossing (ISC) efficiency. No phosphorescence was detected for any of the oligomers; however, strong triplet‐triplet absorption was observed by nanosecond transient spectroscopy for oligomers with more than one thiophene unit. The oligomers displayed short triplet lifetimes (∼1–2 μs) compared to the unmetallated oligomers, due to large spin‐orbit coupling induced by the platinum atom. The lower limits of the ISC yields were indirectly determined by measuring the singlet oxygen quantum yields. Femtosecond–picosecond transient absorption studies revealed that the ISC rate ranges from 10¹²–10¹⁰ s⁻¹, decreasing with increasing oligomer length. Electrochemical studies showed that the oligomers exhibit relatively low oxidation potentials (ca. 0.1 V vs. Fc/Fc⁺). Quenching of the oligomers’ triplet state absorption, simultaneously with the rise of their corresponding cationic radical absorption band in nanosecond transient spectra in the presence of methyl viologen, as an electron acceptor, established that the electron transfer occurs from their triplet state.

    

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3. Formation of Stilbene Azo‐Dimer by Direct Irradiation of p‐Azidostilbene †

   https://doi.org/10.1111/php.13659  

   Osisioma, Onyinye ; Patton, Leanna J. ; Merugu, Rajkumar ; Govorov, Dmitrii ; Milbrandt, Margaret A. ; Jarus, Cassandra ; Karney, William L. ; Gudmundsdottir, Anna D. ( July 2022 , Photochemistry and Photobiology)

   Triplet arylnitrenes may provide direct access to aryl azo‐dimers, which have broad commercial applicability. Herein, the photolysis ofp‐azidostilbene (1) in argon‐saturated methanol yielded stilbene azo‐dimer (2) through the dimerization of tripletp‐nitrenostilbene (³1N). The formation of³1Nwas verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λ_max ~ 375 nm) in cryogenic 2‐methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of1in methanol formed³1N(λ_max ~ 370 nm, 2.85 × 10⁷ s⁻¹). On shorter timescales, a transient absorption (λ_max ~ 390 nm) that decayed with a similar rate constant (3.11 × 10⁷ s⁻¹) was assigned to a triplet excited state (T) of1. Density functional theory calculations yielded three configurations for T of1, with the unpaired electrons on the azido (T_A) or stilbene moiety (T_Tw, twisted and T_Fl, flat). The transient was assigned to T_Twbased on its calculated spectrum. CASPT2 calculations gave a singlet–triplet energy gap of 16.6 kcal mol⁻¹for1 N; thus, intersystem crossing of¹1Nto³1Nis unlikely at ambient temperature, supporting the formation of³1Nfrom T of1. Thus, sustainable synthetic methods for aryl azo‐dimers can be developed using the visible‐light irradiation of aryl azides to form triplet arylnitrenes.

    

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4. Density Functional Theory and ab Initio Computational Evidence for Nitrosamine Photoperoxides: Hammett Substituent Effects in the Photogeneration of the Nitrooxide Intermediate

   https://doi.org/10.1111/php.12941  

   Greer, Edyta M. ; Kwon, Kitae ( July 2018 , Photochemistry and Photobiology)

   Little attention has been focused on diradical and zwitterionic photoperoxides formed from nitrosamine compounds. Here, an attempt is made to probe the electronic character of the nitrooxide intermediate formed in photochemical reactions with triplet oxygen (³O₂). Theoretical studies have been conducted to screenpara‐substituted phenyl nitrosamines. In particular, we find that electron‐withdrawing substituents produce low‐lying triplet nitrooxide diradicals. A clear electronic dependence in theS₀–T₁andS₀–S₁energy gaps of nitrooxides was found using Hammett plots. Computed geometries show a twisted diradical triplet nitrooxide moiety, which contrasts to the nearly flat singlet zwitterionic ground state nitrooxide moiety; analyses of charges (natural bond order), molecular orbitals (HOMO/LUMO) and spin densities enable these assignments. Calculations predict the former triplet species is photogenerated initially from nitrosamine with O₂. The conversion of the triplet nitrooxide diradical to the singlet ground state is an example where longer‐lived zwitterionic nitrooxide structures become possible. The reaction mechanism is consistent with a zwitterionic ground state nitrooxide playing an important role in the bimolecular oxygen‐transfer reaction with phosphine and phosphite trapping agents as has been observed experimentally.

    

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5. 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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