Synthesis of Pyridyl and N‐Methylpyridinium Analogues of Rosamines: Relevance of Solvent and Charge on Their Photophysical Properties

Abstract

A series of pyridyl analogues of rosamines was prepared by employing two methodologies: (i) the conventional‐heating condensation of a pyridinecarboxaldehyde with 3‐(diethylamino)phenol in propionic acid, and (ii) the novel ohmic‐heating assisted condensation under “on water” conditions, followed by oxidation. The 4‐pyridyl substituted rosamine was further converted into the N‐methylpyridinium derivative through N‐alkylation using methyl iodide. The influence of the position and cationization of the nitrogen atom of the pyridyl ring in the physicochemical properties of fluorophores was investigated by 1H, 13C, 15N NMR spectral analysis, UV/Vis and fluorescence spectroscopy, single‐crystal X‐ray diffraction (4‐pyridyl and N‐methylpyridinium derivatives) and thermal‐behavior analysis. Curiously, for ethanolic solutions of 4‐pyridyl and N‐methylpyridinium derivatives an extinction of color and fluorescence over time was observed. This phenomenon was further studied and the data revealed that it is the result of nucleophilic addition of ethoxide ion to the central 9‐position of the xanthene. The kinetics of the process is slower for the 4‐pyridyl rosamine, which emphasizes the importance of the charge in the N‐methylpyridinium analogue in the reactivity of the molecule towards a nucleophile agent. This phenomenon is reversible, meaning that the compounds can be rapidly recovered by decreasing the pH, opening new avenues in the sensing applications of this class of rosamines.

Targeted Synthesis of NIR Luminescent Rhenium Diimine cis,trans‐[Re()(CO)2(L)2]n+ Complexes Containing N‐Donor Axial Ligands: Photophysical, Electrochemical, and Theoretical Studies

“…Both C⋅⋅⋅I − and C⋅⋅⋅O interactions are not very common and usually occur in electron deficient π‐systems. In 2Re(BiPy‐Me) 2 the carbon atoms adjacent to the positively charged nitrogen can obviously act as acceptors of electron density; a similar behavior has been previously described for the other N‐methylpyridinium containing compounds [54,55] …”

“…In 2Re(BiPy-Me) 2 the carbon atoms adjacent to the positively charged nitrogen can obviously act as acceptors of electron density; a similar behavior has been previously described for the other N-methylpyridinium containing compounds. [54,55] Structure and composition of these complexes in solution were elucidated by ESI mass-spectrometry and NMR spectroscopy. The mass-spectra of 2Re(Py) 2 , 2Re(Pz) 2 , 2Re(BiPy) 2 and 2Re(BiPy-Me) 2 display dominant signals of molecular ions with m/z values and isotopic patterns corresponding to the [(N N)Re (CO) 3 (L) 2 )] + stoichiometry (Figs.…”

Advances in Pyridyl-Based Fluorophores for Sensing Applications

“…In 2019, our work group designed a series of pyridyl analogs of rosamines (rhodamine derivatives lacking the carboxylic group at position 2′ of the benzenic ring) and studied the influence of solvent and charge on their photophysical properties [30]. It was found that the structural variation involving the position of the N-atom in the pyridine did not influence the absorption and fluorescence properties of dyes, the same could not be said about the charge -the introduction of a positive charge at the N-atom (18) in the pyridinium analog promoted a significant bathochromic shift in the absorption and fluorescence quenching, both effects associated to d-PET mechanism.…”

Valkyrie Probes: A Novel Class of Enzyme‐Activatable Photosensitizers based on Sulfur‐ and Seleno‐Rosamines with Pyridinium Unit**

“…The low fluorescence of the corresponding pyronin‐pyridinium hybrid structure was explained in terms of oxidative photo‐induced electron transfer (d‐PeT) process from the excited state of fluorescent pyronin group to the N ‐alkylpyridinium substituent LUMO (Figure 1). [22] Analyte‐triggered self‐immolative N ‐dealkylation process leads to the release of strongly fluorescent rosamine with pyridine unit, the effects of d‐PeT being prohibited. Several biomarkers including enzymes (nitroreductase, NTR), [21c] gasotransmitters (hydrogen sulfide, H 2 S) [21a,b] and ROS (hydrogen peroxide, H 2 O 2 ) [21b] have been successfully detected in solution and in cellular imaging, using this reaction‐based approach.…”