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1. 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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2. 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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3. Photoelectron spectroscopy and thermochemistry of o -, m -, and p -methylenephenoxide anions

   https://doi.org/10.1039/C8CP05403G  

   Nelson, Daniel J. ; Gichuhi, Wilson K. ; Nichols, Charles M. ; Bierbaum, Veronica M. ; Lineberger, W. Carl ; Lehman, Julia H. ( October 2018 , Physical Chemistry Chemical Physics)

   The anionic products following (H + H + ) abstraction from o -, m -, and p -methylphenol (cresol) are investigated using flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometry and anion photoelectron spectroscopy (PES). The PES of the multiple anion isomers formed in this reaction are reported, including those for the most abundant isomers, o -, m - and p -methylenephenoxide distonic radical anions. The electron affinity (EA) of the ground triplet electronic state of neutral m -methylenephenoxyl diradical was measured to be 2.227 ± 0.008 eV. However, the ground singlet electronic states of o - and p -methylenephenoxyl were found to be significantly stabilized by their resonance forms as a substituted cyclohexadienone, resulting in measured EAs of 1.217 ± 0.012 and 1.096 ± 0.007 eV, respectively. Upon electron photodetachment, the resulting neutral molecules were shown to have Franck–Condon active ring distortion vibrational modes with measured frequencies of 570 ± 180 and 450 ± 80 cm −1 for the ortho and para isomers, respectively. Photodetachment to excited electronic states was also investigated for all isomers, where similar vibrational modes were found to be Franck–Condon active, and singlet–triplet splittings are reported. The thermochemistry of these molecules was investigated using FA-SIFT combined with the acid bracketing technique to yield values of 341.4 ± 4.3, 349.1 ± 3.0, and 341.4 ± 4.3 kcal mol −1 for the o -, m -, and p -methylenephenol radicals, respectively. Construction of a thermodynamic cycle allowed for an experimental determination of the bond dissociation energy of the O–H bond of m -methylenephenol radical to be 86 ± 4 kcal mol −1 , while this bond is significantly weaker for the ortho and para isomers at 55 ± 5 and 52 ± 5 kcal mol −1 , respectively. Additional EAs and vibrational frequencies are reported for several methylphenyloxyl diradical isomers, the negative ions of which are also formed by the reaction of cresol with O − . 

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4. Combined Crossed Molecular Beams and Ab Initio Study of the Bimolecular Reaction of Ground State Atomic Silicon (Si; 3 P) with Germane (GeH 4 ; X 1 A 1 )

   https://doi.org/10.1002/cphc.202100235  

   Krasnoukhov, Vladislav S. ; Azyazov, Valeriy N. ; Mebel, Alexander M. ; Doddipatla, Srinivas ; Yang, Zhenghai ; Goettl, Shane ; Kaiser, Ralf I. ( June 2021 , ChemPhysChem)

   The chemical dynamics of the elementary reaction of ground state atomic silicon (Si;³P) with germane (GeH₄; X¹A₁) were unraveled in the gas phase under single collision condition at a collision energy of 11.8±0.3 kJ mol⁻¹exploiting the crossed molecular beams technique contemplated with electronic structure calculations. The reaction follows indirect scattering dynamics and is initiated through an initial barrierless insertion of the silicon atom into one of the four chemically equivalent germanium‐hydrogen bonds forming a triplet collision complex (HSiGeH₃;³i1). This intermediate underwent facile intersystem crossing (ISC) to the singlet surface (HSiGeH₃;¹i1). The latter isomerized via at least three hydrogen atom migrations involving exotic, hydrogen bridged reaction intermediates eventually leading to the H₃SiGeH isomeri5. This intermediate could undergo unimolecular decomposition yielding the dibridged butterfly‐structured isomer¹p1(Si(μ‐H₂)Ge) plus molecular hydrogen through a tight exit transition state. Alternatively, up to two subsequent hydrogen shifts toi6andi7, followed by fragmentation of each of these intermediates, could also form¹p1(Si(μ‐H₂)Ge) along with molecular hydrogen. The overall non‐adiabatic reaction dynamics provide evidence on the existence of exotic dinuclear hydrides of main group XIV elements, whose carbon analog structures do not exist.

    

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5. 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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