Tale of a Growth: Marangoni Bursting Phenomenon as a Viable Route for Obtaining Fibers of Organic Chromophores

Abstract

Spreading and emulsification dynamics of a binary droplet of water and alcohol deposited on a rapeseed oil pool or a glass substrate are accompanied by fingering instability that generates a large number of small droplets. The Marangoni bursting process, lasting a few minutes, opens the possibility of growing micro-and nanocrystals of some organic substances that are soluble in alcohols and insoluble in water due to the evaporation of more volatile alcohol. In this work, we used an aqueous 2-propanol (IPA) solution with dissolved organic nonlinear optical dye (3-(2,2-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole), abbreviated as DCNP. The aim of such an approach was to observe the luminescence size effect predicted for DCNP molecular aggregates. Surprisingly, the Marangoni bursting process on the oil bath resulted in the formation of flexible fibers (up to 1.5 cm long and 20−30 μm in width) as well as a wide range of micro-and nanocrystals. Marangoni bursting on a solid glass substrate led not only to the crystallization of dendrimer-like branched structures but also to the fibers that show either red or blue luminescence when excited due to the two-photon absorption of femtosecond laser pulses. The blue luminescence band centered at 430 nm was never observed in DCNP crystals, but its presence was predicted by quantum-chemical calculations as coming from molecular J-or H-aggregates of DCNP. Ab initio quantumchemical calculations demonstrated that cross section for the two-photon absorption in DCNP dimers can be quite large and reach around 100 GM. Based on spectroscopic findings, we concluded that DCNP flexible fibers are built in from crystalline parts and amorphous molecular aggregates.