Abstract

Donor–acceptor–π-conjugated (D–π–)₂A fluorescent dyes OUY-2, OUK-2 and OUJ-2 with two (diphenylamino)carbazole-thiophene units as D (electron-donating group)–π (π-conjugated bridge) moiety and a pyridine, pyrazine or triazine ring as electron-withdrawing group (electron-accepting group; A) have been designed and developed and their photophysical and electrochemical properties were investigated based on the photoabsorption and fluorescence spectroscopy, Lippert–Mataga plots, cyclic voltammetry and density functional theory calculations. The photoabsorption maximum (λ^abs_max) and the fluorescence maximum (λ^fl_max) for the intramolecular charge-transfer characteristic band of the (D–π–)₂A fluorescent dyes show bathochromic shift in the order of OUY-2 < OUK-2 < OUJ-2. Moreover, the photoabsorption spectra of the (D–π–)₂A fluorescent dyes are nearly independent of solvent polarity, while their fluorescence maxima bathochromically shifted with increasing solvent polarity (i.e., positive fluorescence solvatochromism). The Lippert–Mataga plots for OUY-2, OUK-2 and OUJ-2 indicate that the Δμ (= μ_e ‒ μ_g) value, which is the difference in the dipole moment of the dye between the excited (μ_e) and the ground (μ_g) states, increases in the order of OUY-2 < OUK-2 < OUJ-2, that is, the fact explains our findings that OUJ-2 shows large bathochromic shifts in their fluorescence maxima in polar solvents and that the Stokes shift values for OUJ-2 in polar solvents are much larger than those in nonpolar solvents. The cyclic voltammetry of OUY-2, OUK-2 and OUJ-2 demonstrated that there is little difference in the HOMO energy level among the three dyes, but the LUMO energy levels decrease in the order of OUY-2 > OUK-2 > OUJ-2. Consequently, this work reveals that for the (D–π–)₂A fluorescent dyes OUY-2, OUK-2 and OUJ-2 the bathochromic shifts of λ^abs_max and λ^fl_max and the lowering of the LUMO energy level are dependent on the electron-withdrawing ability of azine ring, which increases in the order of OUY-2 < OUK-2 < OUJ-2.

Keywords: fluorescent dyes; pyridine; pyrazine; triazine; D-π-A structure

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Imato, K.; Enoki, T.; Uenaka, K.; Ooyama, Y. Beilstein Arch. 2019, 201922. doi:10.3762/bxiv.2019.22.v1

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