Light-Emitting Efficiency Tuning of Rod-Shaped π Conjugated Systems by Donor and Acceptor Groups

Abstract

In view of the increasing importance of highly efficient light-emitting materials in chemistry, biological science, and materials science, we investigated the light-emitting efficiency tuning of rod-shaped oligo(p-phenylene ethynylene)s (OPEs, trimeric to pentameric systems) by donor and acceptor groups, so that they emit the very intense fluorescence (Phif approximately 1.0, log epsilon approximately 5) at 460 nm as the desired wavelength region. This goal was achieved by side modification by MeO (donor) groups and end modification by a CN-substituted benzene ring or CF3-substituted pyridine ring (acceptor) of tetrameric p-phenylene ethynylene rod-shaped molecules (Phif = 0.96, lambdaem = 458 nm, log epsilon = 4.96 for the former and Phif = 0.99, lambdaem = 459 nm, log epsilon = 4.92 for the latter). The high Phif values for 11 and 12 are interpreted in terms of kr (radiative rate constant) and kd (radiationless rate constant). The linear relationship with a positive slope between Phif and the Hammett sigma constant was found for the first time. It is found that kd rather than kr varies with sigmap-X. The photophysical properties (Phif, lambdaem, lambdaabs, log epsilon) were not so altered with the solvent polarity, which could be explained by the dipole moments in the excited and ground states. The results would be valuable for the molecular design of highly efficient light-emitting materials.