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1. Concerning the stability of biexcitons in hybrid HJ aggregates of π -conjugated polymers

   https://doi.org/10.1063/5.0090515  

   Bittner, Eric R.; Silva, Carlos (May 2022, The Journal of Chemical Physics)

   Frenkel excitons are the primary photoexcitations in organic semiconductors and are ultimately responsible for the optical properties of such materials. They are also predicted to form bound exciton pairs, termed biexcitons, which are consequential intermediates in a wide range of photophysical processes. Generally, we think of bound states as arising from an attractive interaction. However, here, we report on our recent theoretical analysis, predicting the formation of stable biexciton states in a conjugated polymer material arising from both attractive and repulsive interactions. We show that in J-aggregate systems, 2J-biexcitons can arise from repulsive dipolar interactions with energies E 2 J > 2 E J , while in H-aggregates, 2H-biexciton states with energies E 2 H < 2 E H can arise corresponding to attractive dipole exciton/exciton interactions. These predictions are corroborated by using ultrafast double-quantum coherence spectroscopy on a [poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene)] material that exhibits both J- and H-like excitonic behavior. 

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2. Carbon‐13 chemical shift tensor measurements for nitrogen‐dense compounds

   https://doi.org/10.1002/mrc.5422  

   Holmes, Sean T.; Boley, Cameron M.; Dewicki, Angelika; Gardner, Zachary T.; Vojvodin, Cameron S.; Iuliucci, Robbie J.; Schurko, Robert W. (January 2024, Magnetic Resonance in Chemistry)

   Abstract This paper reports the principal values of the¹³C chemical shift tensors for five nitrogen‐dense compounds (i.e., cytosine, uracil, imidazole, guanidine hydrochloride, and aminoguanidine hydrochloride). Although these are all fundamentally important compounds, the majority do not have¹³C chemical shift tensors reported in the literature. The chemical shift tensors are obtained from¹H→¹³C cross‐polarization magic‐angle spinning (CP/MAS) experiments that were conducted at a high field of 18.8 T to suppress the effects of¹⁴N‐¹³C residual dipolar coupling. Quantum chemical calculations using density functional theory are used to obtain the¹³C magnetic shielding tensors for these compounds. The best agreement with experiment arises from calculations using the hybrid functional PBE0 or the double‐hybrid functional PBE0‐DH, along with the triple‐zeta basis sets TZ2P or pc‐3, respectively, and intermolecular effects modeled using large clusters of molecules with electrostatic embedding through the COSMO approach. These measurements are part of an ongoing effort to expand the catalog of accurate¹³C chemical shift tensor measurements, with the aim of creating a database that may be useful for benchmarking the accuracy of quantum chemical calculations, developing nuclear magnetic resonance (NMR) crystallography protocols, or aiding in applications involving machine learning or data mining. This work was conducted at the National High Magnetic Field Laboratory as part of a 2‐week school for introducing undergraduate students to practical laboratory experience that will prepare them for scientific careers or postgraduate studies. 

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3. ¹ H‐decoupling and Isotopic Labeling for the Measurement of the Longitudinal Relaxation Time of Hyperpolarized ¹³ C‐Methylenes in Choline Analogs

   https://doi.org/10.1002/ijch.201900016  

   Uppala, Sivaranjan; Gamliel, Ayelet; Harris, Talia; Sosna, Jacob; Gomori, J. Moshe; Jerschow, Alexej; Katz‐Brull, Rachel (April 2019, Israel Journal of Chemistry)

   Abstract The T₁of a hyperpolarized site in solution is a key parameter that determines the time‐window in which its NMR signals are observable. For¹³C sites adjacent to protons,¹H‐decoupling has been shown to increase the hyperpolarized signal resolution and SNR. Additionally, polarization transfer to protons has shown utility in increasing the sensitivity of detection. However,¹H‐decoupling could lead to a change in the decay rate of a hyperpolarized¹³C site. Here we tested this possible effect in a case where the protons are directly bound to an sp³hyperpolarized¹³C site (using [1,2‐¹³C₂]choline) and¹H‐decoupling was applied continuously throughout the hyperpolarized decay measurement. We found that¹H‐decoupling did not lead to any significant changes in the¹³C polarization decay time but did result in the expected collapse of J‐coupling and produced sharper signals. This result suggested that¹H‐decoupling did not affect the decay rate of hyperpolarized sp^{3 13}C sites. The deuterium‐substitution approach (using [1,1,2,2‐D₄,1‐¹³C]choline) showed a dramatic prolongation of T₁. Upper bounds on the T₁of all investigated sites were calculated. 

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4. J ‐modulated ¹⁹ F‐ and ¹ H‐detected dual‐optimized inverted ¹ J _CC 1,n‐ADEQUATE: A universal ADEQUATE experiment

   https://doi.org/10.1002/mrc.5324  

   Crouch, Ronald C; Pelmuş, Marius; Raab, Jeffrey G; Tischenko, Evgeny; Frey, Michael; Wang, Yunyi; Reibarkh, Mikhail; Williamson, R Thomas; Martin, Gary E (March 2023, Magnetic Resonance in Chemistry)

   Abstract The recently reported¹⁹F‐detected dual‐optimized inverted¹J_CC1,n‐ADEQUATE experiment and the previously reported¹H‐detected version have been modified to incorporateJ‐modulation, making it feasible to acquire all 1,1‐ and 1,n‐ADEQUATE correlations as well as¹J_CCandⁿJ_CChomonuclear scalar couplings in a single experiment. The experiments are demonstrated usingN,N‐dimethylamino‐2,5,6‐trifluoro‐3,4‐phthalonitrile andN,N‐dimethylamino‐3,4‐phthalonitrile. 

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5. A molecular fluorophore in citric acid/ethylenediamine carbon dots identified and quantified by multinuclear solid‐state nuclear magnetic resonance

   https://doi.org/10.1002/mrc.4985  

   Duan, Pu; Zhi, Bo; Coburn, Luke; Haynes, Christy_L; Schmidt‐Rohr, Klaus (January 2020, Magnetic Resonance in Chemistry)

   Abstract The composition of fluorescent polymer nanoparticles, commonly referred to as carbon dots, synthesized by microwave‐assisted reaction of citric acid and ethylenediamine was investigated by¹³C,¹³C{¹H},¹H─¹³C,¹³C{¹⁴N}, and¹⁵N solid‐state nuclear magnetic resonance (NMR) experiments.¹³C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases.¹⁵N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved¹³C NMR peaks, including an unusual ═CH signal at 84 ppm (¹H chemical shift of 5.8 ppm) and ═CN₂at 155 ppm, and two distinctive¹⁵N NMR resonances near 80 and 160 ppm proved the presence of 5‐oxo‐1,2,3,5‐tetrahydroimidazo[1,2‐a]pyridine‐7‐carboxylic acid (IPCA) or its derivatives. This molecular fluorophore with conjugated double bonds, formed by a double cyclization reaction of citric acid and ethylenediamine as first shown by Y. Song, B. Yang, and coworkers in 2015, accounts for the fluorescence of the carbon dots. Cross‐peaks in a¹H─¹³C HETCOR spectrum with brief¹H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative¹³C and¹⁵N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in¹³C chemical‐shift perturbations and peak widths, with the broadest lines near the COO group of IPCA, indicated at least partial transformation of the carboxylic acid of IPCA by amide or ester formation. 

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