Emission solvatochromic, solid-state and aggregation-induced emissive α-pyrones and emission-tuneable 1 H-pyridines by Michael addition-cyclocondensation sequences

Abstract

Starting from substituted alkynones, α-pyrones and/or 1H-pyridines were generated in a Michael addition-cyclocondensation with ethyl cyanoacetate. The peculiar product formation depends on the reaction conditions as well as on the electronic substitution pattern of the alkynone. While electron-donating groups furnish α-pyrones as main products, electron-withdrawing groups predominantly give the corresponding 1H-pyridines. Both heterocycle classes fluoresce in solution and in the solid state. In particular, dimethylamino-substituted α-pyrones, as donor-acceptor systems, display remarkable photophysical properties, such as strongly red-shifted absorption and emission maxima with daylight fluorescence and fluorescence quantum yields up to 99% in solution and around 11% in the solid state, as well as pronounced emission solvatochromism. Also a donor-substituted α-pyrone shows pronounced aggregation-induced emission enhancement.

Keywords: 1H-pyridines; DFT calculations; cyclocondensation; fluorescence; heterocycles; α-pyrones.

Scheme 1

Consecutive three-component alkynylation–Michael addition–cyclocondensation (AMAC) synthesis of α-pyrones from acid chlorides, terminal alkynes and dialkyl malonates.

Scheme 2

Consecutive pseudo-four-component alkynylation–Michael addition–cyclocondensation (AMAC) synthesis of 1H-pyridines 5a and an aniline derivative.

Scheme 3

Consecutive pseudo-four-component alkynylation–Michael addition–cyclocondensation (AMAC) synthesis of 1H-pyridines 5a from acid chlorides 1, terminal alkynes 2 and ethyl cyanoacetate (4).

Scheme 4

Model system for the optimization of the Michael addition–cyclocondensation reaction step to 1H-pyridine 5a or/and α-pyrone 6a.

Scheme 5

Formation of α-pyrone 6a and 1H-pyridine 5a at 20 °C.

Scheme 6

Formation of α-pyrone 6a starting from alkynone 3b having an electron-donating substituent.

Scheme 7

Formation of 1H-pyridine 5b starting from alkynone 3d having an electron-withdrawing substituent.

Scheme 8

Formation of 1H-pyridine 8a by Michael addition–cyclocondensation reaction.

Scheme 9

Mechanistic rationale for the formation of the 1H-pyridine 5a.

Scheme 10

Formation of 1H-pyridine 8a from alkynone 3b and dimer 7.

Figure 1

Molecular structure of 1H-pyridine 5a (50% thermal ellipsoids), showing the intramolecular N–H···O bond as dashed orange line. H-bond details N1–H 0.90(2) Å, H···O1 1.87(2) Å, N1···O2 2.624(2) Å, O1–H···O2 140(1)°.

Figure 2

Supramolecular C–H···N [–39] and C–H···π [–49] interactions around the 6-positioned phenyl ring in 5a. Details of C–H···N bond (dashed orange line) C11–H 0.95 Å, H···N2 2.61 Å, C11···N2 3.263(2) Å, C11–H···N2 127°. Symmetry transformations are i = 1−x, 1−y, 1−z; ii = x, 3/2−y, −1/2+z, iii = 1−x, −1/2+y, −1/2−z.

Figure 3

1H-Pyridine derivatives 5 as solids under daylight (top), under UV light (λ_exc = 365 nm, c(5) = 10⁻⁴ M) in dichloromethane solution (center), and under UV light (λ_exc = 365 nm) in the solid state (bottom).

Figure 4

Selected normalized absorption (solid lines) and emission (dashed lines) spectra of 1H-pyridines 5a–e (recorded in dichloromethane at T = 298 K).

Figure 5

Selected normalized emission spectra of 1H-pyridine 5a and 5b in the solid state at T = 298 K.

Figure 6

Selected normalized absorption (solid lines) and emission (dashed lines) spectra of 1H-pyridines 8a and 8b (recorded in dichloromethane at T = 298 K).

Figure 7

Solid-state luminescence of 1H-pyridines 5a, 8a and 8b (λ_exc = 365 nm).

Figure 8

α-Pyrones 6 as solids under daylight (top), selected derivatives under UV light (λ_exc = 365 nm, c(6) = 10⁻⁴ M) in dichloromethane solution (center), and under UV light (λ_exc = 365 nm) in the solid state (bottom).

Figure 9

Selected normalized absorption spectra of α-pyrones 6a, 6b, 6d, and 6e recorded in dichloromethane at T = 298 K.

Figure 10

Selected normalized absorption (solid lines) and emission (dashed lines) spectra of α-pyrones 6c, 6e, and 6g recorded in dichloromethane at T = 298 K.

Figure 11

Absorption (top) and fluorescence (bottom) of compound 6c with variable solvent polarity (left to the right: toluene, ethyl acetate, acetone, DMF and DMSO, c(6c) = 10⁻⁴ M; λ_exc = 365 nm, handheld UV lamp).

Figure 12

Absorption (solid lines) and emission (dashed lines) spectra of α-pyrone 6c in five solvents of different polarity (recorded at T = 298 K).

Figure 13

Lippert plot for α-pyrone 6c (n = x, r² = 0.970).

Figure 14

Normalized emission spectra of selected α-pyrones 6a–d,f in the solid state at T = 298 K.

Figure 15

Fluorescence of compound 6e in different THF/water fractions (top, λ_exc = 365 nm, handheld UV lamp) and I/I₀ vs %H₂O of α-pyrone 6e in THF/water mixtures containing different water fractions (bottom, recorded at T = 298 K).

Figure 16

Selected DFT-computed (B3LYP 6-311G**) Kohn–Sham FMOs for 1H-pyridines 5f and 5g representing contributions of the longest wavelength Franck–Condon absorption bands.

Figure 17

Selected DFT-computed (B3LYP 6-311G**) Kohn–Sham FMOs for 1H-pyridines 6a, 6c, 6e, 6f, and 6g and representing contributions of the longest wavelength Franck-Condon absorption bands.