Asymmetric indolylmaleimides as non-dopant type red color emitting dyes

The photophysical properties of asymmetric indolylmaleimide derivatives (4a–4f) with a variety of aryl substituents at C(4) were analyzed. Their absorption spectra vary in the region of aryl substituents, whereas their emission spectra are nearly identical. The red emission is derived from a charge transfer transition along the indole-maleimide chromophore, which is not significantly disturbed by the introduction of a nearby aryl substituent at C(4). All compounds displayed intensive emissions both in solutions as well as in the form of solid films. Light emitting diodes (LED) were fabricated successfully by using the neat film of these materials as emitters. The performance of devices among the series varied slightly as a result of different film morphology. The best performance was found on the compound with a pyrene substituent, which exhibited optimal external quantum efficiency of 1.1%. This device could be turned on at 4 V and reached to optimal brightness of 9750 cd/m2. The red color, according to the Commission Internationale de l’Eclairage (CIE) coordinates at (0.65, 0.34), was very close to pure red (0.67, 0.33).