Effect of core twisting on self-assembly and optical properties of perylene bisimide dyes in solution and columnar liquid crystalline phases

A series of highly soluble and fluorescent core-twisted perylene bisimide dyes (PBIs) 3a-f with different substituents at the bay area (1,6,7,12 positions of the perylene core) were synthesized and fully characterized by H-1 NMR, UV/Vis spectroscopy, MS spectrometry, and elemental analysis. The pi-pi aggregation properties of these new functional dyes were investigated in detail both in solution and in condensed phase by UV/Vis and fluorescence spectroscopy, vapor pressure osmometry (VPO), differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction. Concentration-dependent UV/Vis measurements and VPO analysis revealed that these core-twisted a-conjugated systems show distinct self-dimerization equilibria in apolar solvent methylcyclohexane (MCH) with dimerization constants between 1.3 x 10(4) and 30m(-1). The photoluminescence spectra of the dimers of PBIs 3a-f exhibit bathochromic shifts of quite different magnitude which could be attributed to different longitudinal or rotational offsets between the dyes as well as differences in the respective pi-pi stacking distance. In condensed state, quite a few of these PBIs form luminescent rectangular or hexagonal columnar liquid crystalline phases with low isotropization temperatures. The effects of the distortion of the a systems on their a-a stacking and the optical properties of the resultant stacks in solution and in LC phases have been explored in detail. In one case (3a) a particularly interesting phase change from crystalline into liquid crystalline could be observed upon annealing that was accompanied by a transformation from nonfluorescent H-type into strongly fluorescent J-type packing of the dyes.