Rigid molecular architectures that comprise a 1,3,5-trisubstituted benzene core and three oligoaryleneethynylene arms: light-emitting characteristics and pi conjugation between the arms.

In view of increasing interest in light-emitting materials, we have investigated the light-emitting characteristics and occurrence of conjugation between arms of star-shaped rigid molecules that comprise a 1,3,5-triethynylbenzene core and methoxy group-substituted oligo(p-phenylethynylene) arms. Consequently, we achieved the ultimate goal (Phif approximately 1.0, log epsilon > 5) for organic molecules with respect to light-emitting ability by creating very intense violet-blue (8, Phif = 0.97, log epsilon = 5.11) and blue (9, Phif = 0.98, log epsilon = 5.29) bright light-emitters. Also, pi conjugation was found to occur between the arms of 9 despite the meta-substituted system. We found a linear relationship of kr (with positive slope) and kd (with negative slope) with the number of dimethoxyphenyleneethynylene units for MMPT (4, 6, 8) and DMPT (5, 7, 9) homologues and the contrasting solvent effect on lambdaem of 8 and 9. It is also interesting that lambdaabs, epsilon, lambdaem, and Phif of 9 are greater than those of the corresponding banana- and rod-shaped molecules.

Light-emitting efficiency tuning of rod-shaped pi conjugated systems by donor and acceptor groups.

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.