The absorption spectrum and the luminescence properties of a cylindrical macrotricyclic receptor (1), which is made of two diazatetraoxa macrocyclic [18]-N2O4 units linked by two 2,6-dimethylnaphthalene (DMN) bridges, have been investigated. Comparison with the behavior of the 2,6-dimethylnaphthalene reference chromophore shows that in CH2Cl2 solution at room temperature, the covalent bond between the DMN units and the nonabsorbing and nonemitting [18]-N2O4 macrocycles causes the appearance of a charge-transfer (CT) absorption tail below 310 nm, the disappearance of the structured DMN fluorescence at 342 nm, and the appearance of a broad and weak CT emission band at 438 nm. In rigid matrix at 77 K, however, 1 behaves similarly to DMN, showing a structured fluorescence band with maximum at 342 nm and a structured phosphorescence band with maximum at 518 nm and tau = 2.6 s. Addition of CF3COOH to a CH2Cl2 solution of 1 causes the successive protonation of the four amine units of the two [18]-N2O4 macrocycles which are responsible for the CT transitions to the naphthalene rings. As a consequence, the CT absorption tail disappears, the absorption spectrum of the macrotricycle becomes very similar to that exhibited by the isolated DMN chromophore, and the DMN-type fluorescence reappears. The luminescence intensity at 342 nm increases by at least 800 times upon protonation. Therefore, 1 is a fluorescence sensor highly responsive to protons. Upon adduct formation with alpha,omega-alkanediyldiammonium ion NH3+(CH2)(5)NH3+ (cadaverine cation), for which a molecular inclusion into the receptor 1 was previously demonstrated, the intensities of the CT absorption tail below 310 nm and the CT luminescence band at 438 nm decrease by similar to 50%, but the fluorescence DMN band at 342 nm is negligibly small. Similar results are obtained upon adduct formation with NH4+ ions.