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Results for “nucleus independent chemical shift (NICS)”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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DOE OSTI2025

Probing Effects of Electron Addition to Polycyclic Conjugated Hydrocarbons Containing Four‐Membered Rings

The rectangular cyclobutadiene (CBD, C 4 H 4 ) is a unique moiety for building nonbenzenoid polycyclic conjugated hydrocarbons with interesting electron‐accepting properties. Herein, the investigation on chemical reduction of several CBD‐containing polycyclic hydrocarbons with increasing conjugation length is reported: biphenylene (C 12 H 8 ), dimethyl[2]naphthalene (C 22 H 16 ), and tetramethyl‐dibenzo‐[3]phenylene (C 30 H 22 ). The two‐step sequential reduction is first demonstrated by in situ spectroscopic investigation and then confirmed by the isolation of single crystals of the reduced products. The X‐ray crystallographic analysis reveals the formation of several mono‐ and doubly reduced products in solvent‐separated and complexed forms. The crystal structures for both neutral parents and corresponding reduced products unravel the changes in bond alternation in each ring of the fused systems. Density functional theory (DFT) and nucleus‐independent chemical shift (NICS) scan calculations reveal that the two‐electron addition reduces the aromatic character in the benzenoid rings but has minor influence on the antiaromatic CBD rings.

Zhu, Yikun [Department of Chemistry University at ↗
DOE OSTI2025

Substituent effect on napthodithiophene-fused porphyrins: Understanding the unusual trend of fluorescence quantum yield quenching

A series of π-extended porphyrins containing thiophene units was prepared to investigate an unusual trend observed in the fluorescence quantum yield of naphtho[2,1-b:3,4-b']dithiophene-fused porphyrins. Monobenzoporphyrins carrying 2-thiophenyl (vinyl thiophene porphyrin) groups (2VTP, 2VTBr and Br2VTP) (numbering 2 and 3 refers to the position of sulfur on the thiophene ring) were synthesized through a Heck-based cascade reaction followed by a newly developed bromination method. These were converted to naphtho[2,1-b:3,4-b']dithiophene-fused porphyrin derivatives (FBr2VTP and F2VTBr) via ring closure with an intramolecular Scholl reaction. Increasingly larger Stokes shifts with a higher number of bromo groups were observed in the unfused π-extended molecular systems, reflecting the heavy atom effect. Fluorescence spectroscopy further confirmed the unusual trend seen in the previous work: structural rigidification in naphtho[2,1-b:3,4-b']dithiophene-fused porphyrins leads to longer fluorescence lifetimes but unexpectedly lowers the quantum yield. Adding one bromo group to the naphtho[2,1-b:3,4-b']dithiophene unit does not change this trend. Furthermore, the presence of two bromo groups prevents the quantum yield from dropping. DFT, TDDFT, and NICS analyses suggest a drastic change in aromaticity in the pyrrole ring where the naphtho[2,1-b:3,4-b']dithiophene is fused in FBr2VTP, which might contribute to the quantum yield reductions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Naphthodithiophene‐Fused Porphyrins: Synthesis, Characterization, and Impact of Extended Conjugation on Aromaticity

The fusion of tetrapyrroles with aromatic heterocycles constitutes a useful tool for manipulating their opto-electronic properties. In this work, the synthesis of naphthodithiophene-fused porphyrins was achieved through a Heck reaction-based cascade of steps followed by the Scholl reaction. The naphthodithiophene-fused porphyrins display a unique set of optical and electronic properties. Fusion of the naphtho[2,1-b:3,4-b’]dithiophene to porphyrin (F2VTP) leads to a ~20% increase in the fluorescence lifetime, which is accompanied, unexpectedly, by a more than two-fold drop in the emission quantum yield (ϕ=0.018). In contrast, fusion of the isomeric naphtho[1,2-b:4,3-b’]dithiophene to porphyrin (F3VPT) results in a ~1.5-fold increase in the fluorescence quantum yield (ϕ=0.13) with a concomitant ~30 % increase in the fluorescence lifetime. This behavior suggests that fusion of the porphyrin with the naphthodithiopheno-system mainly affects the radiative rate constant in the Q-state deactivation pathway, where the effects of the isomeric naphtho[2,1-b:3,4-b’]dithiophene- versus naphtho[1,2-b:4,3-b’]dithiophene-fusion are essentially the opposite. Interestingly, nucleus-independent chemical shifts analysis revealed a considerable difference between the aromaticities of these two isomeric systems. Furthermore, our results demonstrate that subtle structural differences in the fused components of the porphyrin can be reflected in rather significant differences between the photophysical properties of the resulting systems.

Chemistry↗