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1. Photochemistry of carbon nitrides and heptazine derivatives

   https://doi.org/10.1039/D1CC02745J  

   Hwang, Doyk ; Schlenker, Cody W. ( September 2021 , Chemical Communications)

   We explore the photochemistry of polymeric carbon nitride (C 3 N 4 ), an archetypal organic photocatalyst, and derivatives of its structural monomer unit, heptazine (Hz). Through spectroscopic studies and computational analysis, we have observed that Hz derivatives can engage in non-innocent hydrogen bonding interactions with hydroxylic species. The photochemistry of these complexes is influenced by intermolecular nπ*/ππ* mixing of non-bonding orbitals of each component and the relative energy of intermolecular charge-transfer (CT) states. Coupling of the former to the latter appears to facilitate proton-coupled electron transfer (PCET), resulting in biradical products. We have also observed that Hz derivatives exhibit an extremely rare inverted singlet/triplet energy splitting (Δ E ST ). In violation of Hund's multiplicity rules, the lowest energy singlet (S 1 ) is stabilized relative to the lowest triplet (T 1 ) electronic excited state. Exploiting this unique inverted Δ E ST character has obvious implications for transformational discoveries in solid-state OLED lighting and photovoltaics. Harnessing this inverted Δ E ST , paired with light-driven intermolecular PCET reactions, may enable molecular transformations relevant for applications ranging from solar energy storage to new classes of non-triplet photoredox catalysts for pharmaceutical development. To this end, we have explored the possibility of optically controlling the photochemistry of Hz derivatives using ultrafast pump–push–probe spectroscopy. In this case, the excited state branching ratios among locally excited states of the chromophore and the reactive intermolecular CT state can be manipulated with an appropriate secondary “push” excitation pulse. These results indicate that we can predictively redirect chemical reactivity with light in this system, which is an avidly sought achievement in the field of photochemistry. Looking forward, we anticipate future opportunities for controlling heptazine photochemistry, including manipulating PCET reactivity with a diverse array of substrates and optically delivering reducing equivalents with, for example, water as a partial source of electrons and protons. Furthermore, we wholly expect that, over the next decade, materials such as Hz derivatives, that exhibit inverted Δ E ST character, will spawn a significant new research effort in the field of thin-film optoelectronics, where controlling recombination via triplet excitonic states can play a critical role in determining device performance. 

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2. Quenching of ketone triplet excited states by atmospheric halides

   https://doi.org/10.1039/d0ea00011f  

   Gemayel, R. ; Emmelin, C. ; Perrier, S. ; Tomaz, S. ; Baboomian, V. J. ; Fishman, D. A. ; Nizkorodov, S. A. ; Dumas, S. ; George, C. ( January 2021 , Environmental Science: Atmospheres)

   null (Ed.)

   The photosensitized chemistry of three aromatic ketones (xanthone, flavone, and acetophenone) and also of secondary organic aerosol (SOA) arising from the photo-oxidation of naphthalene was investigated by means of transient absorption spectroscopy. Halide ions were selected to probe the reactivity of the generated triplet states. The quenching rate constants ranged from 10 9 M −1 s −1 with iodide ions to less than 10 5 M −1 s −1 with chloride ions. The halide-triplet state interactions produced the corresponding radical anion (X 2 ˙ − ) along with halogenated and more oxidized organic compounds as identified by liquid chromatography and mass spectrometry. Deoxygenated naphthalene SOA solutions showed strong transient absorption at 420 nm when excited at 355 nm, and were also quenched by iodide ions similar to the single compound experiments indicating that compounds in naphthalene SOA can act as photosensitizers. Combining the study of these individual and known photosensitizers with those formed in the atmosphere (in this case through the oxidation of naphthalene) demonstrates that tropospheric photosensitization may involve a large variety of compounds of primary or secondary nature and will introduce new, unconsidered chemical pathways that impact atmospheric multiphase chemistry. 

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3. Aromaticity of the triplet states of corannulene and coronene

   https://doi.org/10.1002/poc.4464  

   Govorov, Dmitrii ; Pitawela, Niroodha R. ; Gudmundsdottir, Anna D. ( November 2022 , Journal of Physical Organic Chemistry)

   Sunlight‐driven photochemical reactions are an important tool for sustainable organic synthesis. However, compared with ground states, for which the effects of structure on properties and reactivity are well established, the understanding of excited states is limited. In particular, an improved understanding of aromaticity and antiaromaticity in excited states is necessary to develop strategic photochemical methods for synthesizing polycyclic aromatic compounds. Herein, using density functional theory (DFT)‐optimized structures, the ground singlet (S₀) and lowest triplet (T₁) states of coronene and corannulene were compared. Bond length analysis demonstrated that both triplet corannulene and triplet coronene bear a partial resemblance to benzene. Nucleus‐independent chemical shift (NICS(0), NICS(1.7)_ZZ, NICS scans) and anisotropy of the induced current density (ACID) calculations were carried out to compare the induced magnetic currents in these molecules. This analysis demonstrated rather weak π‐conjugation and partial antiaromaticity in the S₀state of each molecule. In contrast, a combination of circular induced currents and pronounced antiaromaticity was found in the T₁state of each molecule. However, the T₁of corannulene exhibited higher stability, which should facilitate functionalization. Consequently, corannulene is considered more suitable for photochemical applications.

    

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4. In silico prediction of annihilators for triplet–triplet annihilation upconversion via auxiliary-field quantum Monte Carlo

   https://doi.org/10.1039/d0sc03381b  

   Weber, John L. ; Churchill, Emily M. ; Jockusch, Steffen ; Arthur, Evan J. ; Pun, Andrew B. ; Zhang, Shiwei ; Friesner, Richard A. ; Campos, Luis M. ; Reichman, David R. ; Shee, James ( January 2021 , Chemical Science)

   null (Ed.)

   The energy of the lowest-lying triplet state (T1) relative to the ground and first-excited singlet states (S0, S1) plays a critical role in optical multiexcitonic processes of organic chromophores. Focusing on triplet–triplet annihilation (TTA) upconversion, the S0 to T1 energy gap, known as the triplet energy, is difficult to measure experimentally for most molecules of interest. Ab initio predictions can provide a useful alternative, however low-scaling electronic structure methods such as the Kohn–Sham and time-dependent variants of Density Functional Theory (DFT) rely heavily on the fraction of exact exchange chosen for a given functional, and tend to be unreliable when strong electronic correlation is present. Here, we use auxiliary-field quantum Monte Carlo (AFQMC), a scalable electronic structure method capable of accurately describing even strongly correlated molecules, to predict the triplet energies for a series of candidate annihilators for TTA upconversion, including 9,10 substituted anthracenes and substituted benzothiadiazole (BTD) and benzoselenodiazole (BSeD) compounds. We compare our results to predictions from a number of commonly used DFT functionals, as well as DLPNO-CCSD(T 0 ), a localized approximation to coupled cluster with singles, doubles, and perturbative triples. Together with S1 estimates from absorption/emission spectra, which are well-reproduced by TD-DFT calculations employing the range-corrected hybrid functional CAM-B3LYP, we provide predictions regarding the thermodynamic feasibility of upconversion by requiring (a) the measured T1 of the sensitizer exceeds that of the calculated T1 of the candidate annihilator, and (b) twice the T1 of the annihilator exceeds its S1 energetic value. We demonstrate a successful example of in silico discovery of a novel annihilator, phenyl-substituted BTD, and present experimental validation via low temperature phosphorescence and the presence of upconverted blue light emission when coupled to a platinum octaethylporphyrin (PtOEP) sensitizer. The BTD framework thus represents a new class of annihilators for TTA upconversion. Its chemical functionalization, guided by the computational tools utilized herein, provides a promising route towards high energy (violet to near-UV) emission. 

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5. Energy- and conformer-dependent excited-state relaxation of an E / Z photoswitchable thienyl-ethene

   https://doi.org/10.1039/c9cp01226e  

   Young, Jamie D. ; Honick, Chana R. ; Zhou, Jiawang ; Pitts, Cody R. ; Ghorbani, Fereshte ; Peters, Garvin M. ; Lectka, Thomas ; Tovar, John D. ; Bragg, Arthur E. ( July 2019 , Physical Chemistry Chemical Physics)

   Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E / Z photoisomerization following absorption of visible light. cis -to- trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565–420 nm) transient absorption (TA) studies to probe the photophysics of cis -to- trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (∼4 ps) and slow (∼40 ps) components of spectral dynamics observed at low excitation energies (>470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (P A and P B ) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the P B conformer is found to contribute to triplet population and the switching of cis -4TCE: high-energy excitation (<470 nm) of P B involves direct excitation to S 2 , relaxation from which prepares an ISC-active S 1 geometry (ISC QY 0.4–0.67, k ISC ∼ 1.6–2.6 × 10 −9 s −1 ) that is the gateway to triplet population and isomerization. We ascribe low cis -to- trans isomerization yield to excitation of the nonreactive P A conformer (75–85% loss) as well as loses along the P B S 2 → S 1 → T 1 cascade (10–20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive “antiparallel” ring conformation. 

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