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1. Temperature-Dependent Photoreactivity of 2-Azidomethylbenzophenone: Insights into the Triplet Imine Biradical Pathway

   https://doi.org/10.1021/acs.joc.4c02985  

   Thenna-Hewa, Kosala; Sriyarathne, H_Dushanee M; Weisfelder, Jonathan K; Mendis, W Dinindu; Borah, Anindya; Engels, Connor J; Muthukrishnan, Sivaramakrishnan; Krause, Jeanette A; Abe, Manabu; Ault, Bruce S; et al (February 2025, The Journal of Organic Chemistry)

   Photoenols, formed through photoinduced intra-molecular H atom abstraction in o-alkyl-substituted arylketones,typically have limited utility as reactive intermediates owing to fastreversion to the starting material. Herein, we introduced an azidogroup on the o-alkyl substituent to render the photoreactionirreversible. Irradiation of 2-azidomethylbenzophenone (1) inmethanol yielded 2-(hydroxy(phenyl)methyl)benzonitrile (2). Laser flash photolysis of 1 revealed the formation of biradical 3Br1followed by intersystem crossing to photoenols Z-3 (τ ∼ 3.3 μs) and E-3 (τ > 45 μs), both of which reverted to 1. Alternatively, 3Br1could lose N2 to form 3Br2 (not detected), which decays to 2. In cryogenic argon matrices, irradiation of 1 yielded nitrene 31N and 2but no photoenols, likely because Z-3 regenerated 1. Both ESR spectroscopy and absorption analysis in methyltetrahydrofuran (80K) confirmed 31N formation. Upon prolonged irradiation, the absorbance of 31N decreased, whereas that of 3 remained unchangedand that of 2 increased. Thus, TK of 1 is proposed to form 3Br1 via H atom abstraction, with subsequent intersystem crossing to 3competing with the loss of N2 to generate 3Br2. DFT calculations revealed a small energy gap (∼2 kcal/mol) between the triplet andsinglet configurations of Br2, supporting a mechanism in which 3Br2 intersystem crosses to yield 2 

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2. Generation and direct observation of a triplet arylnitrenium ion

   https://doi.org/10.1038/s41467-022-31091-z  

   Du, Lili; Wang, Juanjuan; Qiu, Yunfan; Liang, Runhui; Lu, Penglin; Chen, Xuebo; Phillips, David Lee; Winter, Arthur H. (June 2022, Nature Communications)

   Abstract Nitrenium ions are important reactive intermediates in both chemistry and biology. Although singlet nitrenium ions are well-characterized by direct methods, the triplet states of nitrenium ions have never been directly detected. Here, we find that the excited state of the photoprecursor partitions between heterolysis to generate the singlet nitrenium ion and intersystem crossing (ISC) followed by a spontaneous heterolysis process to generate the tripletp-iodophenylnitrenium ion (np). The triplet nitrenium ion undergoes ISC to generate the ground singlet state, which ultimately undergoes proton and electron transfer to generate a long-lived radical cation that further generates the reduced p-iodoaniline. Ab Initio calculations were performed to map out the potential energy surfaces to better understand the excited state reactivity channels show that an energetically-accessible singlet-triplet crossing lies along theN-Nstretch coordinate and that the excited triplet state is unbound and spontaneously eliminates ammonia to generate the triplet nitrenium ion. These results give a clearer picture of the photophysical properties and reactivity of two different spin states of a phenylnitrenium ion and provide the first direct glimpse of a triplet nitrenium ion. 

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3. Isotope Effect in the Magneto‐Optoelectronic Response of Organic Light‐Emitting Diodes Based on Donor–Acceptor Exciplexes

   https://doi.org/10.1002/adma.202004421  

   Liu, Xiaojie; Popli, Henna; Kwon, Ohyun; Malissa, Hans; Pan, Xin; Park, Bumwoo; Choi, Byoungki; Kim, Sunghan; Ehrenfreund, Eitan; Boehme, Christoph; et al (October 2020, Advanced Materials)

   Abstract The isotope effect is studied in the magneto‐electroluminescence (MEL) and pulsed electrically detected magnetic resonance of organic light‐emitting diodes based on thermally activated delayed fluorescence (TADF) from donor–acceptor exciplexes that are either protonated (H) or deuterated (D). It is found that at ambient temperature, the exchange of H to D has no effect on the spin‐dependent current and MEL responses in the devices. However, at cryogenic temperatures, where the reverse intersystem crossing (RISC) from triplet to singlet exciplex diminishes, a pronounced isotope effect is observed. These results show that the RISC process is not governed by the hyperfine interaction as thought previously, but proceeds through spin‐mixing in the triplet exciplex. The observations are corroborated by electrically detected transient spin nutation experiments that show relatively long dephasing time at ambient temperature, and interpreted in the context of a model that involves exchange and hyperfine interactions in the spin triplet exciplex. These findings deepen the understanding of the RISC process in TADF materials. 

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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)

   Abstract 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. 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)

   ABSTRACT 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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