Synthesis and spectroscopic studies of highly fluorescent, solvatochromic diastereomers with differentially stacked bithiophene-substituted quinoxaline rings

Abstract

Diastereomeric C-shaped molecules containing closely stacked bithiophene-substituted quinoxaline rings were synthesized and characterized by NMR, UV-vis absorption, and fluorescence spectroscopy. The unique geometry of each diastereomer resulted in different degrees of -overlap between the bithiophene-substituted quinoxaline ring chromophores, modulating their spectroscopic properties. The donor-acceptor nature of this chromophore gave rise to its positive solvatochromism. 1 H NMR and UV-vis absorption spectroscopy confirmed the existence of - interactions in the ground state between the quinoxaline rings in both molecules but between the bithiophene rings only in the syn isomer. They exhibited significant emission maxima bathochromic shifts, a strong, positive solvatochromism, increased band broadening, and larger Stokes shifts when compared to a compound with an unstacked chromophore. Additionally, the syn isomer consistently showed  max,em value red-shifts and larger band broadening and Stokes shifts compared to the anti isomer due to the greater -overlap in the syn isomer.

Crystal structure of a fluorescent C-shaped molecule containing closely stacked bithiophene-substituted quinoxaline rings

“…These differences suggest that theoverlap between the quinoxaline rings in anti-BBT is slightly less than in anti-BT, which is probably due to increased steric hindrance between the pendant bithiophene rings (vide infra). Nonetheless, our previously published emission studies showed that both anti isomers are highly fluorescent and appear to emit from an intramolecular excimer-like state resulting fromoverlap between the quinoxaline rings, compared to molecules containing the thiophene-quinoxaline or bithiophenequinoxaline chromophore with no overlap (DeBlase et al, 2014;Ciufo et al, 2017). Additionally, the absorption and emission spectra of the anti isomers were consistently red-research papers 2Notes: (a) measured between the six-membered aromatic rings (Dolomanov et al, 2009); (b) DeBlase et al (2014).…”

“…The crystal structure of anti-BT shows one dominant conformer that has its pendant thiophene rings attached to the quinoxaline rings such that the S atoms are oriented outward away from one another, as shown in Scheme 1. 1 H NMR spectroscopy experiments revealed a similar conformational preference for both anti-BT and anti-BBT in solution, which was determined by examining the 1 H anisotropic shielding effects observed for the aromatic protons (DeBlase et al, 2014;Ciufo et al, 2017). By contrast, the crystal structure of anti-BBT shows a clear preference for the conformer with the thiophene rings attached to the quinoxaline rings being rotated such that atoms S1 and S3 are oriented inward toward each other (Fig.…”

“…-Overlap between chromophores in fluorescent molecules can result in emission from an intramolecular excimer-like state, rather than from the locally excited state of the single chromophore, leading to broad featureless red-shifted emission bands (Ohkita et al, 2002;Giaimo et al, 2008;Alamiry et al, 2011). Recently, the synthesis and characterization of a series of C-shaped molecules that are highly fluorescent and solvatochromic, with either thiophene-(anti-and syn-BT; DeBlase et al, 2014) or bithiophene-substituted (anti-and syn-BBT; Ciufo et al, 2017) quinoxaline rings held in a closely stacked arrangement, were reported (see Scheme 1).…”

Iron-Catalyzed Hydrogen Transfer Reduction of Nitroarenes with Alcohols: Synthesis of Imines and Aza Heterocycles

“…13 7-Bromo-1,2,3,4-tetrahydrophenazine 8k. 51 Following the general procedure, 8k was purified by silica gel column chromatography using a mixture of ethyl acetate and heptane (1:10) as the eluent. Brown solid (37 mg, 28%).…”