Linear and Angular Distyrylpyrazines with Terminal Donor Groups: Synthesis, Solvatochromism, and Acidochromism of the Electronic Spectra

Abstract

A series of linear and angular distyrylpyrazines and lateral donor groups has been prepared by aldol condensation between dimethylpyrazines and the appropriate aromatic aldehyde. The optical absorption and emission properties of these systems were studied in different solvents and media. The materials display a strong solvatochromism of the emission that is reflected by large red shifts of their fluorescence emission maxima on increasing the solvent polarity. This behaviour suggests a highly polar emitting state, which is characteristic of compounds that undergo an internal charge transfer upon excitation. Upon protonation, the UV-vis spectra are altered, and the fluorescence intensity of the neutral compound vanishes. These molecules can be used as colorimetric and luminescence polarity and pH sensors.

Donor‐substituted distyrylpyrazines: influence of steric congestion on UV–Vis absorption and fluorescence

“…The UV–Vis absorption spectra of these DSPs are dominated by a low energy band with a maximum around λ max = 400 nm for the DSPs 10 , 11 , 12 , 13 , 14 , only compound 9 without ortho ‐substitution around the amino group absorbs with a red shifted maximum at λ max = 442 nm in cyclohexane. Similar to other quadrupolar distyrylpyrazines, increasing solvent polarity (cyclohexane to ethanol) has only minor effect on the absorption spectra of 10 , 11 , 12 , 13 , 14 (Δλ ≤ 10 nm), only the absorption maximum of 9 is shifted about Δλ = 21 nm (Δν̃ = 1026 cm –1 ) to the red. The maximal solvatochromic shift is reached in dichloromethane and inverts for all compounds to a negative solvatochromism in more polar solvents.…”

“…). As judged by λ max = 395–405 nm, the donor effects of the amino groups in 10 , 11 , 12 , 13 , 14 are comparable to a methoxy‐ or 9‐carbazolyl substitution . Contrary to 10 , 11 , 12 , 13 , 14 , and to its molecular symmetry, compound 9 shows a significant positive solvatochromism of the absorption (Δν̃ = 1026 cm –1 from cyclohexane to ethanol).…”

“…The substitution of 2,5‐distyrylpyrazines with amino groups results in chromophores with a quadrupolar donor–acceptor–donor (D–A–D) electronic structure. The fundamental chromophore, unsubstituted distyrylpyrazine 15 absorbs with λ max = 382 nm and its emission peaks at λ F max = 424 nm in cyclohexane . Absorption and emission are only weakly positive solvatochromic, with TFA ≥ 10 –2 M protonation of the pyrazine starts and shifts absorption and emission to the red side (1 M TFA: λ max = 450 nm, λ F max = 535 nm).…”

“…Absorption and emission are only weakly positive solvatochromic, with TFA ≥ 10 –2 M protonation of the pyrazine starts and shifts absorption and emission to the red side (1 M TFA: λ max = 450 nm, λ F max = 535 nm). Substitution of 15 with donors of moderate strength (methoxy, 9‐carbazolyl) shifts the electronic spectra about Δλ ≤ 30 nm to the red side (in cyclohexane) and provokes a pronounced solvatochromism of the emission (Δν̃= 2030 cm –1 ; 2900 cm –1 ) …”

“…As part of our interest in fluorophores with switchable optical properties and efficient two‐photon absorption, we report here the synthesis of linear 2,5‐distyrylpyrazines (DSP) with lateral amino groups but steric congestion around these donors. With the central electron acceptor pyrazine, a donor–acceptor–donor electronic structure results.…”