Styryl-based new organic chromophores bearing free amino and azomethine groups: synthesis, photophysical, NLO, and thermal properties

Abstract

Herein we report the synthesis and characterization of a new series of styryl-based push-pull dyes containing a free amino group and their Schiff base derivatives. The dyes include the dicyanomethylene group as an acceptor and different para-substituted alkylamines as donors. Morever as a proton-sensitive group a pyridin-2-yl substituent was attached to the para-position of the phenyl moiety in both series of compounds. The photophysical properties of the dyes were examined in various solvents with different polarities and showed absorption in the visible region and green-red emission with low quantum yields. The absorption and the emission maxima were shifted bathocromically by increasing the solvent's polarity. However, there was no correlation with the polarity parameters of the solvents. The pH-sensitive properties of all prepared Schiff bases were examined against TBAOH in DMSO, via deprotonation of the OH group in the salicylidene moiety and their reverse protonation was also investigated using TFA. The Schiff bases exhibited a bathochromic shift upon the addition of TBAOH to their solutions in DMSO. Therefore, they showed potential to be utilized as colorimetric and luminescence pH sensors. The second-order nonlinear optical (NLO) responses of the dyes were measured by the electric field-induced second harmonic (EFISH) generation method. The highest μβ values were obtained for the dyes bearing the julolidine donor as 1430 × 10^-48 esu (for free amino derivative) and 1950 × 10^-48 esu (for Schiff base derivative), respectively. The structural and electronic properties of the dyes as well as their NLO properties were further studied using DFT calculations. The thermal stabilities of all dyes were evaluated by thermogravimetric analysis (TGA). The TGA data showed that all dyes were thermally stable up to 250 °C.

Keywords: DFT calculations; NLO; Schiff base; pH sensitive dyes; solvent effect; styryl dyes.

Scheme 1

Synthetic pathways of dyes 3–7 and Schiff base analogs 8–12.

Figure 1

The optimized geometry of dyes 3 and 8.

Figure 2

Absorption spectra of dyes 3 (a, left) and 8 (b, right). Inset: Color of dyes 3 and 8 in the given solvents of different polarities in ambient light, c = 10 μM.

Figure 3

Emission spectra of dyes 3 (a, left) and 8 (b, right). Inset: Color of dyes 3 and 8 in the indicated solvents of different polarities under UV irradiation (λ_ex = 365 nm, c = 1 μM).

Figure 4

Red shift phenomena with changing substituents in absorption (a, left) and emission (b, right) spectra of dyes 3–6 (top) and 8–11 (bottom) in DMSO.

Figure 5

Absorption (a, left) and emission (b, right) change of dye 12 upon addition of 15 equiv of TBAOH and reverse protonation by adding 5 equiv of TFA in DMSO solution.

Figure 6

Photographs of dye 12 (left, ambient light), without, after the addition of 15 equiv of TBAOH (middle), and reverse protonation by 5 equiv of TFA in DMSO solution (right photograph: the same solutions under UV light (UV lamp, λ_ex = 365 nm)).

Figure 7

Absorption (a, left) and emission (b, right) change of 8 in Britton–Robinson buffer solutions at different pH values.

Figure 8

Photographs of dye 8 in Britton–Robinson buffer solutions at different pH values.

Figure 9

Sigmoid function obtained from dye 8 UV–vis absorption spectra during pH investigation.

Figure 10

TGA curves of all synthetized dyes.