The geometry and absorption of diketo-pyrrolo-pyrroles substituted with various aryls

A series of five symmetrical and four unsymmetrical diaryl substituted diketo-pyrrolo-pyrroles was synthesized; two unsymmetrical derivatives are reported for the first time. The relationship between the theoretical excitation energies of the S-0 -> S-1 transition, computed by time dependent density functional theory and the experimental positions of 0-0 vibronic bands in the visible absorption (or fluorescence excitation) spectra was studied Experimental data were obtained from either Solution Or from low temperature organic solvent glass, in which the progressions of the vibrational structure enabled correct assignment of vibronic sub-bands in some cases Theoretical calculations predicted that a linear bathochromic and hyperchromic shift would accompany substitution of each phenyl in the parent 3,6-diphenyl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione by providing a more extensively conjugated aryl centre (2-naphthyl, biphenyl. stilbenyl) for the ensuing planar derivatives Qualitatively. the experimental bathochromic shifts of the 0-0 vibronic sub-bands were in exact agreement with theory, whilst hyperchromic shifts were affected by the very low solubility of the planar, symmetrical derivatives. Deviations from this ideal behaviour were observed for non-planar derivatives (aryl = stilbenyl or 1-naphthyl), for which the dihedral angles describing the aryl, out-of-plane torsions were probably underestimated by DFT (C) 2009 Elsevier Ltd All rights reserved