Highly Stoke shifted near infrared (NIR) emitting donor-pi-acceptor chromophore: Synthesis and combined experimental and computational studies of photophysical properties

Abstract

A series of novel highly Stokes shifted near infrared (NIR) emitting donor-π-acceptor (D-π-A) based push-pull chromophore BJH1-BJH6 having 4- (diethylamino) aryl as a donor, separated by π-spacer were designed and synthesized. BJH2 and BJH5 have additional cyano group, which acts as electron trap and alters the photophysical properties significantly. The photophysical properties were recorded in nine different solvents, and as the polarity increases the absorption and emission are red shifted. The solvent polarity plots support positive solvatochromism and charge transfer. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) computations were utilized to correlate the effect of the π-linker modification on absorption. Stokes shifts were in the range of 47 nm to 240 nm. The highest Stokes shift of 240 nm was observed for BJH3 in acetonitrile. The higher Stokes shift and larger dipole moment ratio confirm the higher polar the excited state. The HOMO-LUMO energy levels obtained by DFT are utilized to establish a correlation between structural modification and photophysical properties. BJH2 and BJH5 having acetonitrile thiophene as a π-linker show selective stabilization of LUMO level without altering HOMO energy levels.