Abstract
Synthesis, structural characterization and photophysical properties for a series of new trans-A2B2- and A3B-type ethynyl functionalized meso-phenothiazinyl-phenyl porphyrin derivatives are described. The new compounds displayed the characteristic porphyrin absorption spectra slightly modified by weak auxochromic effects of the substituents and fluorescence emission in the range of 651–659 nm with 11–25% quantum yields. The changes recorded in the UV-vis absorption spectra in the presence of trifluoroacetic acid (TFA) are consistent with the protonation of the two internal nitrogen atoms of the free-base porphyrin (19 nm bathochromic shift of the strong Soret band and one long wave absorption maxima situated in the range of 665–695 nm). Protonation of the phenothiazine substituents required increased amounts of TFA and produced a distinct hypsochromic shift of the long wave absorption maxima. The density functional theory (DFT) calculations of a porphyrin dication pointed out a saddle-distorted porphyrin ring as the ground-state geometry.
Highlights
The UV-Vis absorption spectroscopy was widely applied as a suitable analytical procedure for the investigation of the electronic structure of the porphyrins characterized by typical absorption and fluorescence emission bands situated in the visible region
Encompassing our study on the topic of synthesis, metal complexation and optical properties of meso-phenothiazinyl-phenyl porphyrin (MPP) dyes previously reported by our research group [31,32], in this work we introduce a series of additional new MPP derivatives with extended-conjugation π-electron systems brought by peripheral/bridging ethyne auxochrome units capable of inducing favorable steric orientation of the aromatic rings
4-ethynyl-benzaldehyde) andaryl-carbaldehydes (7-bromo-10-methyl-10H-phenothiazin-3carbaldehyde [34] or 4-bromo-benzaldehyde); mixtures of A3 B- (2a, 2b, 2c, 2d) and trans-A2 B2 -type (3a, 3b, 3c, 3d) ethynyl-MPP dyes were obtained in a 2:1 molar ratio (Scheme 1)
Summary
A large variety of scientific articles describing tetrapyrrolic compounds and their metal derivatives were stated in the literature, most of them documenting synthetic procedures [1,2], chemical, photophysical properties and applications as photosensitizers in photodynamic therapy [3,4] including two-photon excitation [5], active materials for organic solar cells [6,7] or light emitting diodes [8,9,10], chemical/biosensors [11,12,13] and biologically active compounds [14,15]. The chemical and sensing properties could be tuned by introducing different acceptor/donor moieties on meso and/or β positions of the porphyrin or by core complexation with transition metals. Disturbances were reported for the characteristic absorption spectra of porphyrin derivatives as a result of different factors such as: conjugation pathway and symmetry [17]. Ethynyl groups were attached to the porphyrin core as a linker capable for inducing an extensive conjugation through π−π orbital interactions favourable to charge delocalization between the macrocycle and (hetero)aromatic units with the goal of achieving subsequent tailored electronic properties of porphyrin-based materials. The ethynyl bridges were grafted in the β [18] but mainly in the meso positions of the porphyrine core [19,20,21,22]
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