Storehouse of Peculiar\nSupramolecular Architecture:\nProbing the Charge-Transfer Phenomenon and Effect of Altering the\npH in a Meta-Oriented Single Donor-Double Acceptor Fluorophore

The\ninvestigation of excited-state intramolecular charge\ntransfer\n(ESICT) has been a fascinating area of research. Although the ESICT\nevents have been studied mostly for <i>para</i>-disubstituted\ndonor–acceptor type molecules, the <i>meta</i>-oriented\ndonor–acceptor type molecules have also shown tremendous potential\nas ESICT active molecules. In the current work, a small fluorescent\nprobe diethyl 5-amino isophthalate (<b>DE-5A-IPA</b>) was investigated\nas a potential model to investigate ESICT events in the solution as\nwell as solid phase. <b>DE-5A-IPA</b> was synthesized easily\nstarting from commercially available 5-amino isophthalic acid in excellent\nyield. In the solid state, differing extents of CH···π-type\nbinding led to formation of fully planar and puckered “cyclobutane”\nmimic supramolecular architecture, as evidenced from the crystal structure\nanalysis. In addition, the single crystal structure of <b>DE-5A-IPA</b> shows that the donor (−NH<sub>2</sub>) and acceptor (−CO<sub>2</sub>Et) are situated in the same plane, thereby assuring the prerequisites\nof a through-space charge transfer. <b>DE-5A-IPA</b> undergoes\nnoticeable Stokes shift (in cm<sup>–1</sup>) when the polarity\nof the medium was changed (4820 cm<sup>–1</sup> in hexane;\n9375 cm<sup>–1</sup> in water). The excited-state dipole moment\n(μ<sub>e</sub>) was calculated to be 4.0 units higher than the\nground-state dipole moment (μ<sub>g</sub>). <b>DE-5A-IPA</b> had an appreciable quantum yield (0.10 ± 0.03 to 0.27 ±\n0.04). The ESICT phenomenon was also investigated by ground- and excited-state\nstructural calculations using Gaussian 16 software. The excited-state\nlifetime measured by the time correlated single photon counting technique\nwas found to vary with the polarity of the solvent, thereby providing\nfurther support to the ESICT phenomenon being operative in <b>DE-5A-IPA</b>. Taking advantage of the –NH<sub>2</sub> group (which is\nsusceptible to protonation) in <b>DE-5A-IPA</b>, steady state\nand time-resolved photodynamics were investigated in solutions of\nvarying pH values. Interestingly, the emission quantum yields as well\nas the emission lifetime increase with an increase in pH value, thereby\nestablishing <b>DE-5A-IPA</b> as a pH-sensitive probe. The current\nfindings shall boost the understanding of <i>meta</i>-oriented\nESICT enabled compounds in terms of their excited-state photophysics.