Synthesis, Crystal Structure, and Absorption Spectroscopy of a Carbazole-Dinitrofluorene Bichromophore. Identity of Ground-State Molecular Interactions in the Single Crystal and Aggregates in Solution

Article abstract of DOI:10.1021/ja00281a022

The synthesis, crystal structure, conformational analysis, and absorption spectroscopy of the bichromophore 9-(γ-(N-carbazolyl)propyl)-9-methyl-2,7-dinitrofluorene (1) are reported.The torsion angles within the propyl part of the molecule correspond to a gauche-trans-gauche-gauche conformation while the dinitrofluorene and the carbazole groups of atoms are nearly planar and are at 25.6 deg from each other.The observed conformational behavior has been analyzed with the aid of a conformational analysis program based on empirical methods, and a good agreement between calculated and crystallographic minima has been found.It is shown by the comparison of spectra in polar and nonpolar solvents that no ground-state intramolecular interaction takes place in the bichromophore.All observed spectroscopic changes are due to solute-solute intermolecular interactions or to solute-solvent intermolecular interactions.It is clearly shown that the 2,7-dinitrofluorene chromophore is responsible for all of these interactions.A new band appearing at 370 nm in a solution of the bichromophore 1 in 3-methylpentane at high concentrations or at low temperatures is assigned to aggregates by comparison with the spectrum of a single crystal.Furthermore, single-crystal data allow evaluation of the energetics of the ground and first excited electronic states of the crystal.The exciton splitting is calculated as being 887 cm^-1, but this is the sum of the dipole-dipole interactions along with the charge-resonance interactions in the first excited singlet state which is mainly responsible for the 2380-cm^-1 bathochromic shift observed in comparing the spectrum of the free molecule with that of the crystal.