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‐component one‐pot processes based on lithiation‐electrophilic trapping sequences. As supported by structure–property relationships, the thiophene anellation mode predominantly controls the photophysical and electrochemical properties and the electronic structures (as obtained by DFT calculations). From molecular geometries and redox potentials 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, strong acceptors introduced in anti–anti dithieno[1,4]thiazines nearly induce a planarization of the ground‐state geometry and a highly intense NIR fluorescence (ΦF=0.52), whereas an equally substituted syn–syn dithieno[1,4]thiazine exhibits a much stronger folded molecular structure and fluoresces poorly (ΦF=0.01). In essence, electrochemical and photophysical properties of dithieno[1,4]thiazines can be tuned widely and outscore the compared phenothiazine with cathodically shifted oxidation potentials and redshifted and more intense absorption bands.

Commercial grade reagents were purchased from Sigma Aldrich, Alfa Aesar, ABCR, IR spectra were recorded on a Shimadzu IR Affinity-1 with ATR technique. The intensities of IR signals are abbreviated as s (strong), m (medium) and w (weak).
Due to the C s symmetry of all compounds, the Onsager radii were measured between the dithienothiazines' centers and acceptor substituents respectively.

Data of Quantum Chemical Calculations
The ground state geometries of the compounds 3 and 6 were optimized using the Gaussian09 program package, [12] the PBE1PBE hybrid-functional [13] and the 6-31G** basis set. [14] The Excitation energies of compounds 3 and 6 and the excited state geometries (S 1 ) of 3a-aa, 3b-ss, 3c-aa and 6 were calculated with TDDFT [15] methods implemented in the Gaussian09 program package using the same PBE1PBE functional [13] and either the 6-31G** or the 6-31+G** basis set [14] as indicated. The polarizable continuum model (PCM) with dichloromethane or toluene as a solvent was applied for the calculations. [16] For the calculation of redox potentials (see chapter 6.2) the ground state geometries of the radical cations and dications of compounds 3 and 6 were optimized using the Gaussian09 program package, [12] the uB3LYP functional [17] and the 6-311G* basis set [14] in the gas-phase. Then, single point calculations on the gas phase geometries was applied to determine the solvation enthalpies using the SMD solvation model with dichloromethane as a solvent. [18] All optimized geometries were confirmed as minima (NImag = 0) or as saddle points (transition states, NImag = 1) by analytical frequency analyses. r² = 0.98108  p = 2.74591 ms

Computed xyz-Coordinates, excitations of compounds 3 and 6 and selected
properties derived from the DFT-calculations Table 3. S,N-folding angles of the S0-and S1-geometries  and the HOMO-and LUMO-energies EHOMO and ELUMO derived from the optimized geometries of compounds 3 and 6 (PBE1PBE/6-31G**, PCM CH2Cl2).

Total Energy, E(TD-HF/TD-KS) = -2432.86421280
Copying the excited state density for this state as the 1-particle RhoCI density. Figure S47. Optimized ground state geometry of radical cation of 3c-aa (uB3LYP/6-311G* PCM CH2Cl2 Sum of electronic and thermal Free Energies= -2434.249863

DFT-Calculation of the redox potentials of compounds 3 and 6
In addition to the experimental determination via cyclic voltammetry the redox potentials were also calculated adapting a literature procedure (equation 3) based on DFT methods (Tables 4   and 5). [19] = G solv (gas), G solv (solv) and G redox (gas) were calculated from the values of the free enthalpies obtained from the geometry optimizations given in chapter 6.1 (uB3LYP/6-311G* SMD CH 2 Cl 2 ). The SMD solvation model [18] with dichloromethane as a solvent was applied to determine the solvation enthalpies, since all experimental determined oxidation potentials were measured in dichloromethane solutions. The calculated redox potentials reproduce the experimental data correctly as indicated by linear correlation (Figure 63).