It has recently been found that 2,7-diazapyrenes upon interaction with nucleic acids form stacked (''intercalation'') complexes, which for the methylated derivatives exhibit new absorption features assigned as charge-transfer (CT) transitions.' To better understand the basis of these interactions and associated optical properties, the geometries and electronic spectra of complexes of adenine (A) with 2,7-diazapyrene (DAP), N-methyl-2,7-diazapyrenium (MDAP(+)), and N,N-dimethyl-2,7-diazapyrenium (DMDAP(2+)) have been modeled using semiempirical AM1 and PM3 geometry optimizations, ab initio (vacuum and Onsager model) energy calculations, and ZINDO/S calculations. In addition, absorption spectra, fluorescence quenching, and H-1 NMR spectra for the complexes in aqueous solution have been measured. For the A-DAP complex, a coplanar, hydrogen-bonded complex is predicted by the calculations, while A-MDAP(+) and A-DMDAP(2+) complexes should have edge-to-face geometry. The association is predicted to be of electrostatic nature, mainly between the pyridinium nitrogen (MDAP(+), DMDAP(2+)) and N-1/NH2 of adenine. There seems to be a preference (6 kcal/mol) for the hydrogen-bonded A-DAP complex, and the energetic difference between face-to-face and edge-to-face A-MDAP(+) and A-DMDAP(2+) complexes is 3 and 8 kcal/mol, respectively (Onsager ab initio, epsilon = 79.5). By contrast, the H-1 NMR data and experimental absorption spectra in conjunction with calculated spectra instead indicate that all three adenine-diazapyrene complexes assume face-to-face arrangement in water because of hydrophobic effects. In agreement with the putative CT absorption of diazapyrenium-DNA complexes, absorption tails are also observed for A-DMDAP(2+) and A-MDAP(+), however not for the A-DAP complex. Most satisfactorily, charge-transfer transitions are predicted by the calculations to occur in the correct wavelength region for A-DMDAP(2+) (strongest) and A-MDAP(+), while A-DAP is predicted not to have any CT transitions. Correspondingly, the observation of quenching of fluorescence of MDAP(+) and DMDAP(2+) (but not DAP) by adenine can explained by charge transfer from adenine to the diazapyrenium.