Electron‐Rich Phenothiazine Congeners and Beyond: Synthesis and Electronic Properties of Isomeric Dithieno[1,4]thiazines

Abstract

A series of isomeric dithieno[1,4]thiazines is accessible through an intermolecular–intramolecular Buchwald–Hartwig amination starting from dihalodithienyl sulfides. The electronic properties of dithieno[1,4]thiazine isomers differ conspicuously over a broad range depending on the thiophene–thiazine anellation: a large cathodic (340 mV) or an anodic shift (130 mV) of the redox potentials relative to corresponding phenothiazines is possible. Structure–property relationships of the dithieno[1,4]thiazine constitution derived from DFT calculations and cyclic voltammetry not only reveal increased electron density but also different delocalization of the radical cations that determines the electrochemical properties significantly. In addition, photophysical properties (absorption and emission) qualify dithieno[1,4]thiazines as promising substitutes of phenothiazine and beyond due to increased tunable electron richness.

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

“…Phenothiazine 6,t he heterocyclic topological analogue of 3a-aa and 3b-ss,i sa bout2 00 mV anodically shifteda gainst 3a-aa and 3b-ss,w hich is in line with the unfunctionalized systems [9] and the calculated HOMO energy levels ( Figure 2). The HOMO energy levels scale with the electron richness and the ionization potentialo facompound.…”

“…The stabilities of radical cations can be comparedb yt heir semiquinone formation constants K SEM (equilibrium constants of the comproportionation of dications and neutral ground states). [21] As indicated by K SEM , the radicalc ations formed upon oxidation of the anti-anti isomers, in contrastt ot he unsubstituted systems, [9] are alwayso ne order of magnitude lower than those of the corresponding syn-syn isomers ( Table 1). This can well be ac onsequence of stronger substituent effects in the anti-anti isomers resulting in ah igher destabilization of the radical cations by the acceptors in 3a-aa and 3c-ss and a higher stabilization of the dicationi n3e-aa (DE + 1/ + 2 (3f-ss-3e-aa) = 84 mV) relative to the synsyn isomers, respectively.M oreover,t he K SEM values of all substituted dithieno [1,4]thiazines 3 are several magnitudes lower than the K SEM values of the unsubstituted mothers ystems 2.…”

“…This can well be ac onsequence of stronger substituent effects in the anti-anti isomers resulting in ah igher destabilization of the radical cations by the acceptors in 3a-aa and 3c-ss and a higher stabilization of the dicationi n3e-aa (DE + 1/ + 2 (3f-ss-3e-aa) = 84 mV) relative to the synsyn isomers, respectively.M oreover,t he K SEM values of all substituted dithieno [1,4]thiazines 3 are several magnitudes lower than the K SEM values of the unsubstituted mothers ystems 2. [8,9] However,i nc omparisont oo ther organic redox systems [21] K SEM % 10 10 -10 12 are still remarkably high.…”

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

“…However, in contrast to benzene, thiophene can be anellated with the 1,4-thiazine in three different ways, thus leading to six dithieno [1,4]thiazine regioisomers 2 (Figure 1). For simplification, the anellation mode is abbreviated by the prefixes syn (2,3-b anellation), anti (3,2-b anellation) and exo (3,4-b anellation), depending on the orientation of the thiophene and thiazine sulfur atoms [30]. Figure 1.…”

“…Figure 1. The six regioisomers 2 of the dithieno [1,4]thiazines derived from phenothiazine 1 by benzothieno exchange [30].…”