Photophysical Study of Blue, Green, and Orange-Red Light-Emitting Carbazoles

Simple synthetic procedures have been developed to prepare suitably substituted stable carbazoles B1-B3, G1-G3, and R1-R3. These compounds emit blue, green, and orange-red light, respectively, and show a red-shifted emission in the solid state relative to that in solution. The extent of the shift is highly dependent on the nature and the positions of the substituents. A red-shift as high as 120 nm can be achieved by a suitable substitution, especially by N-substitution of carbazole. The presence of a carbaldehyde or malononitrile group on the carbazole moiety is found to quench fluorescence severely in solution and in the solid state, as indicated by low fluorescence quantum yields of B1 (phi(F) approximately 0.03), B3 (phi(F) approximately 0.04), and G1-G3 (phi(F) approximately 0.04-0.15). However, the effect is not the same for the fluorescence lifetime (tau(F) approximately 1-5.69 ns). The rate constants of radiative and nonradiative deactivation of B1-R3 have been found to be in the range of 6.40 x 10(6) to 9.50 x 10(8) and 1.38 x 10(8) to 9.84 x 10(8), respectively. Lowering the temperature from 25 to -10 degrees C causes a small but distinct red-shift in the emissions and a systematic increase in the phi(F) values of the blue and green emitters. Solvatochromism and concentration-dependent emissions of the compounds are also discussed.