Optoelectronic properties of a self-assembling rigidly-linked BF2-curcuminoid bichromophore

We describe the synthesis and spectroscopic study of the bichromophoric molecule DA-meso consisting of two boron difluoride curcuminoid (BF2-curcuminoid) units tethered by the rigid diacetylenic bridge. This structural feature imparts the quadrupolar-like DA-meso with a strong ability to self-assemble in solution, which induces a profound change in the optical properties. The UV–vis absorption spectrum of the aggregate is characterized by the appearance of a redshifted band at low energy and its fluorescence emission occurs in the near infrared (λmax = 700 nm) with a low quantum yield (5%). This is in contradiction with the strong emission expected for J-aggregates considering the Frenkel exciton model. From concentration-dependent UV–vis absorption spectroscopy we demonstrate the formation of a dimer composed of four BF2-curcuminoid units. Single-crystal X-ray diffraction structure analysis and in-depth quantum chemical calculations based on density functional theory (DFT) and subsequent decomposition of the electronic excited states on a diabatic basis enable to discuss the geometry of the DA-meso dimer, and to establish the presence of excited states close in energy to that of the unperturbed monochromophore, which are not predicted by the classical Frenkel exciton model. Different dimer structures characterized by π-stacks of two and up to four chromophores are investigated, in which low-lying excited states with vanishing oscillator strength are found to be redshifted with respect to the lowest-energy allowed state of the aggregate. From these experimental and theoretical data, the spectral features of the DA-meso dimer are rationalized by considering the formation of a “non-fluorescent J-aggregate”, illustrating the trend in non-Kasha behavior observed for quadrupolar-like dyes.