Abstract
Photophysical properties of fluorophores of structurally similar D - π - A cationic compounds with dual emissions in solution are reported. The studied fluorophores have 2,5-diphenylthiophene as a π-bridge and dimethyl amino group as electron donating group and amidine group as electron withdrawing group (I) in addition to a fluorine at the ortho position to the amidine group (II) for further enhancement of the intramolecular charge transfer process. The photophysical properties were examined with steady-state absorption, fluorescence emission, and time-resolved fluorescence spectroscopy in a series of solvents. The fluorescence emission exhibited dual characteristics, originating from both the locally excited (LE) state and the intramolecular charge transfer (ICT) state. The emission spectra demonstrated a dependence on the excitation wavelength; specifically, increasing the excitation wavelength resulted in the emergence of a red-shifted emission band accompanied by a decrease in quantum yield. The fluorescence emission maxima exhibit significant solvatochromism. For the locally excited (LE) state, the emission undergoes a hypsochromic shift from 497 nm in methanol to 450 nm in cyclohexane for I, and from 489 nm to 455 nm for II. Conversely, the emission from the intramolecular charge transfer (ICT) state displays a more pronounced blue-shift, decreasing from 650 nm in methanol to 511 nm in cyclohexane for I, and from 666 nm to 536 nm for II. Furthermore, the fluorescence excitation spectra were observed to vary as a function of the emission wavelength, indicating a complex interplay between excitation and emission processes. Fluorescence emission decay profiles were recorded at various excitation wavelengths, revealing that the relative contributions of the two emitting states are dependent on the excitation wavelength. The effect of solvent on the nature of dual emission was examined. The photophysical properties of (I) and (II) were correlated to the solvent polarity, ε, reveals the charge transfer process in the ground and excited states, while their correlation with the solvent polarity parameter (E) shows two different dependencies when the solvents are divided into aprotic and protic solvents. To rigorously investigate the influence of individual solvent parameters on the photophysical properties, four-parameter linear solvation energy relationships (LSERs) proposed by Catalán and Laurence were employed. The analyses revealed that non-specific solvent interactions predominantly govern the observed photophysical behavior, as evidenced by the outcomes of both Catalán's and Laurence's models. TD-DFT computations were carried out in a subset of representative solvents and were conducted to predict the dipole moments and molecular geometries in both the ground and excited states.
