Linear And Nonlinear Optical Properties Of A Quadrupolar Carbo-Benzene And Its Benzenic Parent: The Carbo-Merization Effect

Herein, the optical properties of thiophene-functionalized quadrupolar carbo-benzenes and a benzenic parent, of generic structure Th-C=C-[core]-C=C-Th, Th = R2C4HS, are comparatively investigated. Beyond the previously unknown dioctylthienylethynylbenzene (core = p-C6H4, R = nOct), two bis-dialkylthienylethynyl-carbobenzenes (core = C18Ph4, R = nOct, nBu) are envisaged for the unique "carbo-aromatic" character of the C-18 macrocycle. The three targets were synthesized from the corresponding ethynylthiophenes in 47, 20 and 10% yield, respectively, then characterized by classical methods such as NMR spectroscopy, and X-ray crystallography for one of the carbo-benzenes. Regarding linear and nonlinear optical properties, our results show that the carbomerization induces a significant shift to lower energies of the one-photon electronic excitations accompanied by an 8-fold increase of the molar extinction coefficient compared to the parent molecule. Intriguingly, these excitations lead to a broad band of photoluminescence comprising decay transitions of the type S-1 -> S-0 but also of the type S-2 -> S-0. This phenomenon of emission from higher excited states, which is contrary to Kasha's rule, is assigned to - or revealed by - a reduction of the internal conversion efficiency between S-2 and S-1. Two-photon induced transitions are also enhanced, the two-photon absorption cross-section (sigma(2PA)) being in average five times larger for the carbo-benzenes than for their benzene parent in the wavelength range 650-950 nm, with a maximum of sigma(2PA) = 1430 GM (1 GM = 10(-50) cm(4) s/photon). Beyond a moderate nonlinearity, this comparative study provides quantitative insights about the way carbo-merization or insertion of a pi-conjugated macrocycle between chromophoric functions (here thiophene rings) can tune optical properties of organic molecules. The optical properties of the bis-dialkylthienylethynyl-carbo-benzenes are also discussed in regard of recent reports on organic chromophores based on other types of pi-conjugated macrocyclic cores.