Synthesis and Strong Solvatochromism of Push-Pull Thienylthiazole Boron Complexes

Abstract

The solvatochromic behavior of two donor-π bridge-acceptor (D-π-A) compounds based on the 2-(3-boryl-2-thienyl)thiazole π-linker and indandione acceptor moiety are investigated. DFT/TD-DFT calculations were performed in combination with steady-state absorption and emission measurements, along with electrochemical studies, to elucidate the effect of two different strongly electron-donating hydrazonyl units on the solvatochromic and fluorescence behavior of these compounds. The Lippert-Mataga equation was used to estimate the change in dipole moments (Δ<i>µ</i>) between ground and excited states based on the measured spectroscopic properties in solvents of varying polarity with the data being supported by theoretical studies. The two asymmetrical D-π-A molecules feature strong solvatochromic shifts in fluorescence of up to ~4300 cm^-¹ and a concomitant change of the emission color from yellow to red. These changes were accompanied by an increase in Stokes shift to reach values as large as ~5700-5800 cm^-¹. Quantum yields of ca. 0.75 could be observed for the <i>N</i>,<i>N</i>-dimethylhydrazonyl derivative in nonpolar solvents, which gradually decreased along with increasing solvent polarity, as opposed to the consistently reduced values obtained for the <i>N</i>,<i>N</i>-diphenylhydrazonyl derivative of up to ca. 0.20 in nonpolar solvents. These two push-pull molecules are contrasted with a structurally similar acceptor-π bridge-acceptor (A-π-A) compound.