Scheme 1. Synthetic approach towards acridinium/carbene hybrids. X -= OTf -or BF4 -. In case of IMes hybrid 3a, two well separated reversible redox-events at E 1 1/2 = -1.24 V and E 2 1/2 = -0.45 V (vs. Fc/Fc + ) were observed to give the radical cation and dication, respectively. In stark contrast CAAC and DAC hybrids 4a and 5a only feature a single two-electron oxidation at -0.36 V and -0.18 V. The general trend of more negative redox potentials for the IMes hybrid is in agreement with the low π-accepting properties as observed for pyrylium and pyridinium hybrids. Note, in those cases no potential compressions/inversions have been observed in which the second electron transfer proceeds at the same redox event. In order to investigate the difference in redox properties, we performed spectroelectrochemical measurements for all systems (Figure ). As indicated by the spectroelectrochemistry (SEC) data in the case of IMes hybrid 3a (for 3b see Figure ), the radical cation appears with a very characteristic strongly bathochromic shifted UV-Vis absorption at λ ~ 600 nm. In case of 4a and 5a, only the UV-Vis absorption of the neutral and dicationic oxidation states could be observed in agreement with the CV data. Next, the stoichiometric synthesis of all oxidation states was targeted for the hybrid molecules. Since the CV data of 3a and 3b indicated the possibility to generate the radical cations 3a •+ and 3b •+ , we first attempted their stoichiometric synthesis. Upon addition of one equivalent of AgSbF6 to 3a, 3a •+ SbF6 -could be isolated as an intensely purple colored solid in 92% yield. In case of neutral 3a and its radical cation 3a •+ we were able to obtain single crystals suitable for Xray diffraction (Figure ). Quite strikingly, neutral 3a is distorted in a butterfly fashion with a strongly pyramidalized N-Me group (sum of angles around N3: 349.1°). In the radical cation 3a •+ a major structural reorganization took place in which the imidazolium heterocycle is twisted by 59.1(3)° to the acridinium plane. The simulated EPR spectra of the respective radical cations are in good agreement with the experimentally measured EPR spectra of the monomeric radical cations in solution (Figure ; for the EPR of 3b •+ and 3c •+ see Figures ). The calculated SOMO shows spin density distributed largely over the acridine heterocycle. Oxidation of 3a or 3b with two equivalents of AgSbF6 or AgBF4 cleanly afforded the dications 3a 2+ /3b 2+ as SbF6 -or BF4 - salts in high yields (see SI). ), N3-C5 1.450( ), N1-C1-C2-C3 -3.1(4); Top right (3a 94.8(3). C: Calculated (B3LYP/def2TZVPP//B3LYP-D3BJ/def2SVP) SOMO (isovalue of 0.4); EPR-spectrum of radical cation 3a •+ SbF6 -fitted EPR parameters in [MHz]: 1xN 12.9, 1xN 9.6, 1xN 8.3, 1xH 4.0, 1xH 6.3; 1xH 8.0, 1xH 4.8; 3xH 6.2. Next, we investigated the oxidation of 4a/4b and 5a/5b by two equivalents of silver salts (AgBF4/AgSbF6). In analogy, all dications 4a 2+ /4b 2+ as well as 5a 2+ /5b 2+ could be isolated and characterized. In the 1 H NMR all the acridinium related signals are significantly downfield shifted compared with the neutral compound, which is in line with an increase of aromaticity upon oxidation. In case of the CAAC system 4a, we were able to obtain single crystals of the neutral and dicationic oxidation states that were suitable for X-ray diffraction (Figure ). Again, a striking large structural reorganization is detected upon oxidation, which agrees with a potential compression/inversion mechanism. The neutral oxidation state is highly distorted in a butterfly shape while in the dication the two heterocyclic planes are oriented in a perpendicular fashion.