SUPRAMOLECULAR PHOTOCHEMISTRY AND PHOTOPHYSICS - A CYLINDRICAL MACROTRICYCLIC RECEPTOR AND ITS ADDUCTS WITH PROTONS, AMMONIUM-IONS, AND A PT(II) COMPLEX

The absorption spectrum and the luminescence properties of a cylindric al macrotricyclic receptor (1), which is made of two diazatetraoxa mac rocyclic [18]-N2O4 units linked by two 2,6-dimethylnaphthalene (DMN) b ridges, have been investigated. Comparison with the behavior of the 2, 6-dimethylnaphthalene reference chromophore shows that in CH2Cl2 solut ion at room temperature, the covalent bond between the DMN units and t he nonabsorbing and nonemitting [18]-N2O4 macrocycles causes the appea rance of a charge-transfer (CT) absorption tail below 310 nm, the disa ppearance of the structured DMN fluorescence at 342 nm, and the appear ance of a broad and weak CT emission band at 438 nm. In rigid matrix a t 77 K, however, 1 behaves similarly to DMN, showing a structured fluo rescence 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 ami ne units of the two [18]-N2O4 macrocycles which are responsible for th e CT transitions to the naphthalene rings. As a consequence, the CT ab sorption tail disappears, the absorption spectrum of the macrotricycle becomes very similar to that exhibited by the isolated DMN chromophor e, 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 t ail below 310 nm and the CT luminescence band at 438 nm decrease by si milar to 50%, but the fluorescence DMN band at 342 nm is negligibly sm all. Similar results are obtained upon adduct formation with NH4+ ions . The [Pt(NH3)2(bpy)](2+) complex, which can be used as a guest for a variety of crown ethers, forms a 1:1 adduct with unprotonated 1. The a bsorption spectrum of the adduct is noticeably different from that exp ected for the sum of the two separated components, particularly becaus e of the presence of an absorption in the 340-420-nm region. At room t emperature, the luminescence bands exhibited by the two separated comp onents are no longer observed in the adduct. In rigid matrix at 77 K, the phosphorescence band of 1 can be observed in the adduct regardless of the excitation wavelength, but its lifetime (0.8 ms) is considerab ly shorter than that (2.6 s) of the phosphorescence of 1. As suggested by observation of CPK models, the above results indicate that [Pt(NH3 )2(bpy)](2+) is hosted in the cylindrical cavity of 1 with an amine li gand which interacts with a [18]-N2O4 macrocycle unit via hydrogen bon ds and the Pt(bpy)(2+) electron-deficient moiety involved in a CT inte raction with the DMN chromophoric units.