One of the goals of this study was also to investigate how the fragment anchored in the cavity modifies the properties of the ligand itself. Thus, in the next step, we removed a cobalt centre from acetone-functionalised azuliporphyrin (2) to get a free ligand (Scheme 5). The cobalt(II) ion coordinating three inner nitrogen atoms can be easily removed if the complex's solution is exposed to hydrochloric acid vapours. The demetallated species is then separated from the metal by basic alumina chromatography. Obtained ligand 13, green when dissolved (in contrast to the amber solution of 1), produced a very unexpected 1 H NMR spectrum (Fig. ). In its neutral state, azuliporphyrin 1 is borderline aromatic ( 1 H NMR signals within 8-7.3 ppm, excluding azulene). Here, the signals from the macrocycle span from 8.3 to 6.5 ppm and the complete asymmetry of the molecule is apparent. In fact, after careful examination, we notice two sets of signals, one from the expected ligand (13a) and another from an asymmetric species (13b). The signals coming from the substituent were diagnostic of the situation. For 13a, two singlets integrate 2:3 with chemical shifts 1.00, 0.96 ppm in 1 H and 42.4, 30.4 ppm in C NMR (HSCQ). For asymmetric 13b, the integration is 1:3, and the 1 H shifts are 4.00, 1.37 ppm and 13 C are 93.3, 20.7 ppm. The NOESY spectrum (Fig. ) indicates that both species are in chemical exchange, and the HRMS analysis (Fig. ) showed an isotopic peak for the single demetallated species. Combining all the data, we should conclude there is a keto-enol tautomerisation at play and that the enol form (13b) activates one meso position. Indeed, there is a correlation in the HMBC spectrum (Fig. ) from one of the meso o-phenyl protons to the carbon atom with a chemical shift of 84.1 ppm, indicating the connection is made through an oxygen atom to a C(sp 3 ) atom. The final confirmation was made by analysing a single crystal diffraction pattern, resulting in a structure shown in Fig. . One can see a reversibly formed azuleno[1,2-c]pyran motif. The former acetone moiety connects to the C(sp 3 ) mezo position [C(5)] through an oxygen atom. The moiety bond lengths are within the expected ranges for a substituted enol (formal C=C, 1.342(3) Å; C(sp 2 )-O, 1.362(3) Å). The whole molecule is heavily distorted to accommodate a newly formed, rigid motif as the C(5) meso atom sticks 2.08 Å below the plane formed by the rest of the C(meso) atoms. As the unexpected reactivity of the C(21) substituent reminds quasi-esterification in its nature, it is fully reversible with acid excess. Thus, the 13 sample was acidified with trifluoroacetic acid (TFA), leading to a single symmetric species ( ) as confirmed with a 1 H NMR spectrum (Fig. ). It presents a typical azulipophyrin feature, namely enhanced macrocyclic aromaticity of the dication due to the positive charge accumulated at the seven-membered ring, introducing more importance to fully aromatic resonance forms. The acetone Scheme 5. Demetallation of 2 and keto-enol equilibria existing for free ligand 13. Enol quenching by protonation of 13 into 14 with enhanced macrocyclic aromaticity. We managed to get a crystal structure for the dication so we can directly compare 13 and 14. For the latter (Fig. ), we can see a symmetric molecule with the acetone moiety wrapped under the highly deflected azulene fragment (similarly as in cobalt(II) complex 2). The C(H2)-C(O) and C=O bond distances equal 1.525(4), 1.215(3) Å, respectively. To evaluate geometrical signs of macrocyclic aromaticity, Harmonic Oscillator Model of Aromaticity (HOMA) can be determined. In the case of 14, it is equal to 0.78 (1 would be for a perfect benzene ring). Meanwhile, 0.28 for nonaromatic 13b, further underlines the fact of switchable macrocyclic aromaticity based on keto-enol equilibrium tuned with acid. Similar procedures can be applied to 8, leading to a free N(2)methylated NCP ligand with the C(21) substituent in two forms (S1a, S1b) and its dication (S2; see ESI, Scheme S1). In this case, the substitution of the meso position forms a 2,4dihydropyrano [3,4-c]pyrrole motif and results in chemical shifts of most pyrrole signals "below chloroform" (7.5-6.4 ppm and 6.13 ppm for H(3) from the inverted pyrrole; Fig. ). The enol substituent with 1:3 intensities is located at 3.72/92.9 ppm and 1.35/19.9 ppm for 1 H/ C NMR, respectively. Meanwhile, the substituent in the keto form yields three proton signals the CH2 group is diastereotopic: 0.35,-0.11/40.4 ppm and 0.86/29.3 ppm for CH3. Interestingly, while the azuliporphyrin derivatives 13a:13b ratio between tautomers in chloroform-d is ca. 1:1.3, it is only 3.3:1 in the case of N(2)-methylated NCP (keto S1a : enol S1b form, respectively). The ratios may slightly depend on the acidity of the particular solvent sample (and, in fact, the used solvent, Fig. ) but rather clearly suggest a higher preference for the keto tautomer with a conjugated πsystem in the NCP derivative.