Storehouse of Peculiar Supramolecular Architecture: Probing the Charge-Transfer Phenomenon and Effect of Altering the pH in a Meta-Oriented Single Donor-Double Acceptor Fluorophore.

The investigation of excited-state intramolecular charge transfer (ESICT) has been a fascinating area of research. Although the ESICT events have been studied mostly for -disubstituted donor-acceptor type molecules, the -oriented donor-acceptor type molecules have also shown tremendous potential as ESICT active molecules. In the current work, a small fluorescent probe diethyl 5-amino isophthalate () was investigated as a potential model to investigate ESICT events in the solution as well as solid phase. was synthesized easily starting from commercially available 5-amino isophthalic acid in excellent yield. In the solid state, differing extents of CH···π-type binding led to formation of fully planar and puckered "cyclobutane" mimic supramolecular architecture, as evidenced from the crystal structure analysis. In addition, the single crystal structure of shows that the donor (-NH) and acceptor (-COEt) are situated in the same plane, thereby assuring the prerequisites of a through-space charge transfer. undergoes noticeable Stokes shift (in cm) when the polarity of the medium was changed (4820 cm in hexane; 9375 cm in water). The excited-state dipole moment (μ) was calculated to be 4.0 units higher than the ground-state dipole moment (μ). had an appreciable quantum yield (0.10 ± 0.03 to 0.27 ± 0.04). The ESICT phenomenon was also investigated by ground- and excited-state structural calculations using Gaussian 16 software. The excited-state lifetime measured by the time correlated single photon counting technique was found to vary with the polarity of the solvent, thereby providing further support to the ESICT phenomenon being operative in . Taking advantage of the -NH group (which is susceptible to protonation) in , steady state and time-resolved photodynamics were investigated in solutions of varying pH values. Interestingly, the emission quantum yields as well as the emission lifetime increase with an increase in pH value, thereby establishing as a pH-sensitive probe. The current findings shall boost the understanding of -oriented ESICT enabled compounds in terms of their excited-state photophysics.