Synthesis and characterisation of long wavelength-absorbing donor/acceptor-substituted methine dyes

1 Introduction

In recent years, a large variety of dyes of the general structure D-A, in which D represents a strong electron donor and A a strong electron acceptor moiety, have been prepared and used for different kinds of applications. For instance, such dyes have been used for determination of the solvent polarity [1–3], as light absorbers in organic solar cells [4, 5] or as molecular nanoprobes for biomedical imaging [6, 7]. The last application field could be opened due to the ability of the appropriate dyes to form highly fluorescent nanodots which are able to bind strongly at several biomaterials.

The synthetic methods for preparing the mentioned dyes are rather simple, using for example the Knoevenagel condensation of a donor-substituted aromatic aldehyde, such as 4-(dimethylamino)benzaldehyde (1), with an active methylene compound of the general structure 2, in which one of the acceptor substituents in many cases is a cyano moiety [8].

2 Results and discussion

The simple route to the dyes 3 stimulated us to synthesise certain new D-A compounds by starting, instead of 1, with a variety of aromatic and heteroaromatic aldehydes, such as julolidine-carbaldehyde (4a), 3-guaiazulenecarbaldehyde (5a), or their corresponding dimethyliminium salts 4b and 5b as well as few N,N-disubstituted 2-aminothiophene-5-carbaldehydes 7. Whereas the aromatic aldehydes and their iminium salts 4–5 are easily available by the Vilsmeier formylation of their formyl-free precursors, the thiophenecarbaldehydes 7 are available by Vilsmeier formylation of N,N-disubstituted 2-aminothiophene-5-carboxylic acids 6b which decarboxylate in the course of the formylation reaction yielding intermediate N,N-disubstituted 2-aminothiophenes 6a. The 2-aminothiophene-5-carboxylic acids 6b have been simply prepared by saponification of the corresponding N,N-disubstituted methyl 2-aminothiophene-5-carboxylates 6c [9].

Active methylene compounds 2a–h and as N,N-disubstituted 2-aminothiophene-5-carbaldehydes the compounds 7a–c have been used as coupling partners for the Knoevenagel condensation. Most of the active methylene compounds 2a–h are well known and could be ordered from suited suppliers or prepared by reported methods.

By heating equimolar amounts of a compound 2 with an appropriate aldehyde 4a, 5a or 7a in acetic anhydride, under consumption of the starting material, deeply coloured solutions were observed and by the time crystalline precipitates of the corresponding Knoevenagel condensation products were formed. All the products so obtained could be isolated by suction and were generally obtained subsequently in good to excellent yields.

The structures of the prepared compounds are given in the Experimental section and follow unambiguously from their analytical data. Thus, all compounds exhibit distinct mass spectra and, except for the compound 8h build up from 2-diphenylaminothiophene-carbaldehyde (7a) and benzo[b]thiophen-3[2H]-one 1,1-dioxide (2g), characteristic ¹H NMR signals between δ ≈ 7 and 8 ppm as well as at about 9 ppm which are typical for their carbocyclic or heterocyclic moieties as well as for the methine-bonded H atoms, respectively. In the ¹H NMR spectrum of the compound 8h, two sets of signals are recorded. They indicate the existence of two isomers A and B in the ratio 2:1.

In the visible spectral range, the methine dyes exhibit intense absorption bands in the red and near-infrared (NIR) spectral ranges. These absorptions are responsible for the deep blue or green colour of these compounds. Owing to their D-A character, the absorption maxima of these compounds exhibit, as demonstrated in Fig. 1 for the compound prepared from 2-(diphenylamino)thiophene as donor part and 2-(5-methylene-4-phenylthiazol-2(5H)-ylidene)malononitrile as acceptor part, a positive solvatochromism which ranges from 629 nm in toluene up to 675 nm in dichloromethane.

Fig. 1:

Absorption spectra of a donor/acceptor dye composed of 2-diphenylaminothiophene as donor part and 2-(5-methylene-4-phenylthiazol-2(5H)-ylidene)malonitrile as acceptor part in different solvents.

3 Experimental section

The ¹H NMR spectra were recorded with a Bruker DRX 500 P instrument at 500.13 MHz and the signals are recorded in Hz. The UV/Vis spectra were measured with a Perkin Elmer Lambda 900 UV/VIS/NIR spectrometer and the high-resolution mass spectra (HRMS) with a MAT 8200 Finnigan spectrometer. The melting points were estimated with a Boetius heating-table microscope.