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1. A new era of LMCT: leveraging ligand-to-metal charge transfer excited states for photochemical reactions

   https://doi.org/10.1039/D3SC05268K  

   May, Ann Marie; Dempsey, Jillian L (May 2024, Chemical Science)

   Ligand-to-metal charge transfer (LMCT) excited states are capable of undergoing a wide array of photochemical reactions, yet receive minimal attention compared to other charge transfer excited states. This work provides general criteria for designing transition metal complexes that exhibit low energy LMCT excited states and routes to drive photochemistry from these excited states. General design principles regarding metal identity, oxidation state, geometry, and ligand sets are summarized. Fundamental photoreactions from these states including visible light-induced homolysis, excited state electron transfer, and other photoinduced chemical transformations are discussed and key design principles for enabling these photochemical reactions are further highlighted. Guided by these fundamentals, this review outlines critical considerations for the future design and application of coordination complexes with LMCT excited states. 

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2. Photocyclization of Alkenes and Arenes: Penetrating Through Aromatic Armor with the Help of Excited State Antiaromaticity

   https://doi.org/10.3390/chemistry7030079  

   Dos_Santos, Nikolas R; Wu, Judy I; Alabugin, Igor V (June 2025, Chemistry)

   This review focuses on photocyclization reactions involving alkenes and arenes. Photochemistry opens up synthetic opportunities difficult for thermal methods, using light as a versatile tool to convert stable ground-state molecules into their reactive excited counterparts. This difference can be particularly striking for aromatic molecules, which, according to Baird’s rule, transform from highly stable entities into their antiaromatic “evil twins”. We highlight classical reactions, such as the photocyclization of stilbenes, to show how alkenes and aromatic rings can undergo intramolecular cyclizations to form complex structures. When possible, we explain how antiaromaticity develops in excited states and how this can expand synthetic possibilities. The review also examines how factors such as oxidants, substituents, and reaction conditions influence product selectivity, providing useful insights for improving reaction outcomes and demonstrating how photochemical methods can drive the development of new synthetic strategies. 

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3. 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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4. Assembly of Pyrenes through a Quadruple Photochemical Cascade: Blocking Groups Allow Diversion from the Double Mallory Path to Photocyclization at the Bay Region

   https://doi.org/10.1021/jacs.4c14486  

   Dos_Santos, Nikolas R; Schober, João Vitor; Laconsay, Croix J; Palazzo, Alexandria M; Kuhn, Leah; Chu, Angel; Hanks, Benjamin; Hanson, Kenneth; Wu, Judy; Alabugin, Igor V (January 2025, Journal of the American Chemical Society)

   We present a six-step cascade that converts 1,3-distyrylbenzenes (bis-stilbenes) into nonsymmetric pyrenes in 40–60% yields. This sequence merges photochemical steps, E,Z-alkene isomerization, a 6π photochemical electrocyclization (Mallory photocyclization); the new bay region cyclization, with two radical iodine-mediated aromatization steps; and an optional aryl migration. This work illustrates how the inherent challenges of engineering excited state reactivity can be addressed by logical design. An unusual aspect of this cascade is that the same photochemical process (the Mallory reaction) is first promoted and then blocked in different stages within a photochemical cascade. The use of blocking groups is the key feature that makes simple bis-stilbenes suitable substrates for directed double cyclization. While the first stilbene subunit undergoes a classic Mallory photocyclization to form a phenanthrene intermediate, the next ring-forming step is diverted from the conventional Mallory path into a photocyclization of the remaining alkene at the phenanthrene’s bay region. Although earlier literature suggested that this reaction is unfavorable, we achieved this diversion via incorporation of blocking groups to prevent the Mallory photocyclization. The two photocyclizations are assisted by the relief of the excited state antiaromaticity. Reaction selectivity is controlled by substituent effects and the interplay between photochemical and radical reactivity. Furthermore, the introduction of donor substituents at the pendant styrene group can further extend this photochemical cascade through a radical 1,2-aryl migration. Rich photophysical and supramolecular properties of the newly substituted pyrenes illustrate the role of systematic variations in the structure of this classic chromophore for excited state engineering. 

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5. Using Restricted Bond Rotations to Enforce Excited State Behavior of Organic Molecules

   https://doi.org/10.1055/a-1785-6910  

   Kandappa, Sunil Kumar; Ahuja, Sapna; Singathi, Ravichandranath; Valloli, Lakshmy Kannadi; Baburaj, Sruthy; Parthiban, Jayachandran; Sivaguru, Jayaraman (April 2022, Synlett)

   This account highlights the role of restricted bond rotations to influence excited state reactivity of organic molecules. It highlights photochemical reactivity of various organic molecules and the design strategies that could be exploited by chemists to utilize restricted bond rotations to uncover new excited state reactivity and achieve selectivity. 

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