Widely Electronically Tunable 2,6‐Disubstituted Dithieno[1,4]thiazines—Electron‐Rich Fluorophores Up to Intense NIR Emission

Abstract

2,6-Difunctionalized dithieno[1,4]thiazines were efficiently synthesized by (pseudo)five-or (pseudo)three-componento ne-pot processes based on lithiation-electrophilic trappings equences. As supported by structure-property relationships, the thiophene anellationm ode predominantly controls the photophysical and electrochemical properties and the electronic structures (as obtained by DFT calculations). From molecular geometries and redoxp otentials to fluorescence quantum yields in solution, the interaction of the dithieno[1,4]thiazine-core with the substituents causes striking differences within the series of regioisomers. Most interestingly,s trong acceptorsi ntroduced in anti-anti dithieno[1,4]thiazines nearly induceaplanarizationo ft he groundstate geometry and ah ighly intense NIR fluorescence (F F = 0.52),w hereas an equallys ubstituted syn-syn dithieno[1,4]thiazinee xhibits am uch stronger folded molecular structure and fluorescesp oorly (F F = 0.01). In essence, electrochemical and photophysical propertieso fd ithieno[1,4]thiazines can be tuned widely and outscore the compared phenothiazine with cathodically shiftedo xidation potentials and redshifted and more intense absorption bands. Scheme1.Tuning of the widely used phenothiazine moiety 1:gaining accesstoi mproved electronic properties by construction of the dithieno[1,4]thiazines 2 by topological benzo-thieno exchange and further2,6-difunctionalization of the dithieno[1,4]thiazine core.

Dithieno[1,4]thiazines and Bis[1]benzothieno[1,4]thiazines—Organometallic Synthesis and Functionalization of Electron Density Enriched Congeners of Phenothiazine

“…α-Lithiation of thiophenes using n-butyllithium sets the stage for various 2,6-functionalization. Thereby, di-as well as monofunctionalizations are readily realized [36,44,45].…”

“…Other one-pot sequences, similarly based on a lithiation-formylation sequence, focus on the use of dimethyl amine, which is present in equilibrium after the formylation, as an organocatalyst for subsequent Knoevenagel condensations [47]. Hence, dithieno [1,4]thiazines were also efficiently functionalized via a lithiation-formylation-Knoevenagel sequence in a one-pot fashion (Scheme 12) [45]. These building blocks set the stage for a diverse range of follow-up chemistry.…”

“…Ultimately, dithieno [1,4]thiazine 6b exhibited an outstanding intense NIR fluorescence at λ max,em = 719 nm (Φ F = 52%), whereas 6a fluoresces significantly less intensively (λ max,em = 750 nm, Φ F = 1%). [45].…”

“…Besides electron-donating (hetero)aryl substituents, electron-withdrawing substituents can be introduced and ortho-substituents are also tolerated (Scheme 15) [37]. In addition, monoarylations and the diarylation of the anti-anti dithieno [1,4]thiazines 2e proceed similarly [44,45]. Polymerization of syn-syn dithieno [1,4]thiazines was accomplished by polymer analogous Negishi couplings using a diiodide as the dihalogen substrate.…”

“…However, 2,6-substituted syn-syn dithieno [1,4]thiazines mostly do not fluorescence intensely, whereas the anti-anti isomers do indeed. Furthermore, the π-system of the anti-anti isomers interacts with the π-system of the substituents by full conjugation more efficiently than in syn-syn isomers triggering the different electronic properties of the isomers [45]. Substituents in the 2,6-positions have a stronger influence on the redox potentials than the substituents in the N-position, which was demonstrated for syn-syn isomers by a different response to the substituent effects (correlation with Hammett parameters) [35,36].…”